Moderate power cw fibre lasers

Moderate-power cw fibre lasers

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Abstract. A review of the development and investigation ofmoderate-power (10ÿ1 ÿ 102 W) cw ébre lasers is presented.The properties of optical ébres doped with rare-earth ions andmethods for fabricating double-clad ébres are considered.The methods for fabrication of ébre Bragg gratings used asselective reêectors are discussed and the grating propertiesare analysed. The main pump schemes for double-clad ébrelasers are described. The properties of ébre lasers doped withneodymium, ytterbium, erbium, thulium, and holmium ionsare also considered. The principles of fabrication of Ramanconverters of laser radiation based on optical ébres of

different compositions are discussed and the main results oftheir studies are presented. It is concluded that ébre lasersdescribed in the review can produce moderate-power radiationat any wavelength in the spectral range from 0.9 to 2 lm.

Keywords: ébre laser, active optical ébres, stimulated Ramanscattering.

1. Introduction

Fibre lasers are one of the most spectacular achievements ofmodern quantum electronics. This line of investigation hasappeared in the intermediate éeld between laser physics andébre optics. Fibre lasers offer a number of advantages overconventional lasers and can be used along with conven-tional lasers and even replace them in some cases. Note thatthe main task of ébre optics at the beginning was thedevelopment of optical ébres as a passive medium forcommunication. However, as in any developing éeld ofscience, new possibilities of ébre optics were found during

A.S. Kurkov, E.M. Dianov Fiber Optics Research Center,A.M. Prokhorov General Physics Institute, Russian Academy of Sciences,ul. Vavilova 38, 119991 Moscow, Russia

Received 15 April 2004Kvantovaya Elektronika 34 (10) 881 ë 900 (2004)Translated by M.N. Sapozhnikov

PACSnumbers: 42.55.Wd; 42.60.By; 42.60.Jf; 42.60.LhREVIEW

DOI:10.1070/QE2004v034n10ABEH002739

Moderate-power cw ébre lasers

A.S. Kurkov, E.M. Dianov

600/725 ëKAI ë 30/ix-07 ë VERSTKA ë 20 ÒÑÎÑÔ ÍÑÏÒ. å 1Quantum Electronics 34 (10) 881 ë 900 (2004) ß2004 Kvantovaya Elektronika and Turpion Ltd

Contents

1. Introduction 881

2. Elements of a ébre laser 882

2.1. Active dopants in optical ébres

2.2. Photoinduced refractive-index gratings

2.3. Active optical ébres

2.4. Pump schemes for active optical ébres

2.5. Speciéc features of optical ébres as ampliéers

3. Fabrication of Raman lasers 888

4. Characteristics of ébre lasers 889

4.1. Nd3�-doped ébre lasers

4.2. Yb3�-doped ébre lasers

4.3. Er3�-doped ébre lasers

4.4. Tm3�-doped ébre lasers

4.5. Ho3�-doped ébre lasers

5. Raman ébre lasers 894

5.1. Single-stage Raman lasers

5.2. Multistage Raman lasers

5.3. Composite Raman lasers

6. Conclusions 898References 898

solving the initial task, which resulted in the expansion ofthe scope of objects and phenomena being studied and, inturn, expanded the éeld of applications of optical ébres anddevices based on them.

One of the éelds of ébre optics is the development andinvestigation of ébre lasers. The active medium of theselasers are optical ébres doped with various impurities, érstof all rare-earth elements. Note that the érst ébre laser wasfabricated by Snitzer in 1961, who demonstrated lasing in aglass doped with Nd3� ions [1]. The active element of thelaser was a glass élament covered with a cladding made ofglass with a lower refractive index. The development oftechnologies for production of optical ébres and semi-conductor pump sources inspired interest in ébre lasersat a qualitatively new level. This éeld of research began todevelop rapidly in the late 1980s, when its was shown thatoptical ébres doped with Er3� ions could énd applicationsin ébre ampliéers in the wavelength range between 1.53 and1.56 mm [2], i.e., in the spectral region of minimal opticallosses of silica ébres. Studies of ébre ampliéers stimulatedthe development of high-power semiconductor pump sour-ces. The discovery of the photorefractive effect in ébres [3]and the development of the ébre-Bragg-grating writingprocess [4] made it possible to produce Bragg mirrorsforming a resonator directly in optical ébres and to obtaina variety of the all-ébre laser conégurations. All thisresulted in a rapid development of ébre lasers.

Note that the concept of `ébre lasers' encompassesnumerous laser conégurations, which are characterised bysubstantially different output powers and different spectraland temporal properties of output radiation. In this paper,we consider cw ébre lasers, which attract great recentattention due to a variety of the applications, both realisedand promising.

The classiécation of ébre lasers according to their outputpower is rather conditional, reêecting at the same time thefeatures of the laser design. Thus, low-power lasers arepumped into the ébre core. This restricts the parameters of apump semiconductor laser, whose emitting region shouldhave a dimension of the order of 5 ë 10 mm. In this case, thepump power does not exceed a few hundred milliwatts, sothat the typical output power of such lasers is in the rangefrom 10ÿ1 to 102 mW. Among these devices are ébre lasersemitting a single longitudinal mode with the linewidth of�20 kHz [5, 6]. Low-power ébre lasers are used as radiationsources in optical communication systems and ébre sensors,as well in spectroscopy.

The development of high-power semiconductor laserswith a broad emitting region stimulated the elaboration ofmoderate-power cw ébre lasers emitting from a few hundredmilliwatts to a few ten watts. These lasers use double-cladoptical ébres with a core doped with active ions. As mirrorsin moderate-power lasers, ébre Bragg gratings (FBGs) arecommonly used. In this case, there are no bulk elements inthe laser and, as a rule, no nonlinear-optical effects appearin the resonator. These lasers do not require any specialprovisions for eliminating the radiation breakdown of theébre material. The conventional maximum output powerof such lasers is a few ten watts in a single transversemode.

Note that the output power exceeding 1 W is sufécientfor exciting stimulated Raman scattering (SRS) in opticalébres to convert eféciently radiation of the ébre laser toradiation at a different wavelength [7]. Moderate-power

ébre lasers and Raman converters are used for pumpingerbium and Raman ampliéers operating in various spectralregions. In addition, they can be used in medicine, materialmachining, optical ranging, guidance systems, and wirelessoptical communication. Note that such a broad scope ofapplications is possible to a great extent due to the use ofvarious rare-earth dopants in combination with Ramanconverters, which permit the fabrication of ébre lasersemitting virtually at any wavelength in the spectral range0.9 ë 2 mm.

For the last few years, high-power ébre lasers have beenextensively developed. The output power of such lasersexceeds 100 W in a single transverse mode [8 ë 11] and 1 kWin the multimode regime [12, 13]. A speciéc feature of theselasers is the use of bulk mirrors, which assumes the necessityof laser alignment. Obviously, the advantages of the all-ébredesign, in which all the elements are connected by standardsplicing of ébres, are lost in this case. In addition, activeébres, used in high-power single-mode lasers, have a ratherlarge diameter of the ébre core (up to 30 mm), whichcomplicates their joining with commercial optical ébres.High-power multimode lasers consist of single-mode ébrelasers assembled in a bunch. High-power ébre lasers aremainly used for material machining and in medicine.

Therefore, ébre lasers, which have been initially deve-loped for the use in ébreoptic communications, begin toacquire now their own importance. This allows one toconsider the development and investigation of such lasers asan independent éeld in quantum electronics and ébre optics.The fact that ébre lasers have already found a variety ofapplications, which can be further expanded, makes studiesin this éeld urgent.

In this paper, we present a review of the main results inthe development and investigation of moderate-power cwébre lasers. Their design, which offer all advantages of theall-ébre conéguration, is now generally accepted. It is theselasers that énd now the widest application. We consider theproperties of active media for moderate-power cw ébrelasers, the fabrication of the lasers and their pumping, aswell as Raman lasers based on optical ébres with ébre coresof different chemical compositions.

2. Elements of a ébre laser

Figure 1 shows a simplest conéguration of an end-pumpedébre laser consisting of a pump semiconductor laser with apig tail, an optical ébre doped with active ions, and FBGs(other pump schemes will be considered below). Thereêectivity of the input FBG is typically close to unity atthe laser wavelength, while the required reêectivity of the

Splicing

pointActive ébre Output

Semiconductor

laser

Fibre Bragg gratings

Figure 1. Simplest conéguration of a ébre laser.

882 A.S. Kurkov, E.M. Dianov

output grating is determined by the gain and optical lossesin the active ébre. FBGs can be written both directly in theactive ébre and in photosensitive ébres spliced to the activeébre. Consider individual elements of the ébre laser andtheir properties.

2.1 Active dopants in optical ébres

As in the case of solid-state lasers, lanthanide and rare-earth ions are most often used as active dopants in opticalébres. The speciéc optical properties of these ions aredetermined by the fact that they have occupied outer shells,whereas the inner f shell is not completely élled. Thepresence of the unélled inner shell leads to a distinctdiscrete structure of electronic transitions, which determinesthe use of rare-earth ions as active impurities.

The applicability of an active ion for doping silica ébresis determined by the following factors: érst, it should have aradiative transition in the near-IR region, where a silicaglass is transparent. Second, the phonon energy in a silicaglass is 400 ë 1100 cmÿ1, so that the presence of energy levelswith a narrow energy gap in the optical transition leads tononradiative relaxation, which prevents the appearance ofluminescence. For this reason, the ions presented in Table 1are most often used as active dopants in optical ébres.Table 1 also gives the luminescence regions of these ions.Note that in optical ébres based on glasses of differentcompositions, for example êuoride and chalcogenideglasses, lasing was obtained using other active ions, forexample, praseodymium. The energy level diagrams of ionspresented in Table 1 are shown in Fig. 2.

Neodymium (Nd 3�). Neodymium ions in a silica glasshave a number of strong absorption bands in the visible andnear-IR regions; however, selective excitation to the 4F5=2

level is most often performed using a 0.8-mm semiconductorlaser. Three main luminescence bands are located at 0.92,1.06, and 1.34 mm. The most intense is the 1.06-mm bandcorresponding to the 4F3=2 ! 4I11=2 transition. (The excited-state lifetime is � 0.5 ms.) The laser operates at thiswavelength according the four-level scheme. For this reason,neodymium was the érst active impurity used in a silica glassébre laser [14].

Generation at the 4F3=2 ! 4I9=2 transition (l � 0.92 mm)in ébre lasers is complicated due to competition withluminescence at 1.06 mm, while generation at 1.34 mm[the ( 4F3=2 ! 4 I13=2) transition] is prevented due toexcited-state absorption. Nevertheless, the efécient ébrelaser emitting at 0.92 mm was fabricated by suppressingluminescence at 1.06 mm [15].

Holmium (Ho 3�). As shown in Ref. [16], Ho3� has the5I7 ! 5I8 laser transition corresponding to emission at 2 mm.

The excited-state lifetime is about 0.5 ms. The holmiumlaser should operate according to the three-level schemebecause ground-state absorption occurs at the same wave-length. Therefore, to obtain the population inversion in suchlasers, pumping should be performed into an intenseabsorption band. For this reason, solid-state holmium laserspumped by êashlamps into absorption bands in the visibleregion found the most common use. The 0.9-mm absorptionband corresponding to the 5I8 ! 5I5 transition was not usedfor pumping by a semiconductor laser because of the lowintensity of the band. At the same time, the presence of theintense absorption band at 1.15 mm corresponding to the5I8 ! 5I6 transition suggests the possibility of improving thecharacteristic of the holmium-doped ébre laser by pumpinginto this band. As will be shown below, this was achievedupon pumping by an ytterbium-doped ébre laser [17].

Erbium (Er 3�). Erbium ions in a silica glass exhibit the4I13=2 ! 4I15=2 laser transition corresponding to emission at1.53 ë 1.6 mm, the metastable-level lifetime being 10 ë 12 ms.This spectral range coincides with the range of minimumoptical losses in silica ébres and, hence, with the spectralrange used in modern ébreoptic communication links. Incombination with pump semiconductor lasers emitting at0.98 and 1.45 ë 1.48 mm, this resulted in wide applications oferbium-doped ébres and devices based on them. Note thatthe quantum eféciency of pumping these ébres approaches100%, while a long lifetime of the metastable level(� 10 ms) provides high gains. The properties of erbium-doped ébres are considered in several monographs, inparticular in Ref. [18, 19].

Thulium (Tm 3�). Lasing of a silica glass doped withthulium ions was demonstrated in Ref. [20]. Lasing wasobserved at the 3H4 ! 3H6 transition by pumping into theabsorption bands corresponding to the transition to the 3F4

level (� 790 nm) or to the 3H5 level (1060 ë 1250 nm). Thespectral range of lasing was 1850 ë 2100 nm, and themetastable-level lifetime was � 0.2 ms. Lasing in a Tm3�-doped ébre was érst demonstrated in Ref. [21].

Table 1. Rare-earth elements used for fabrication of active optical ébresand their luminescence regions.

Active ion Luminescence region�mm

0.92 ë 0.94

Nd3� 1.05 ë 1.1

1.34

Ho3� 1.9 ë 2.1

Er3� 1.53 ë 1.6

Tm3� 1.7 ë 1.9

Yb3� 0.98 ë 1.16

Energy� 103

cmÿ1

Wav

elength� mm

Nd3� Ho3� Er3� Tm3� Yb3�

4F5=2

4F3=2

4I9=2

4I11=2

4I13=2

4I15=2

3F4

2F5=2

3H5

3H4

3H6

4I13=2

4I15=2

4I11=2

4I9=2

0.8

1.0

1.5

2.0

5.0

14

12

10

8

6

4

2

0

5I5

5I6

5I7

5I82F7=2

Figure 2. Energy level diagram of rare-earth ions.

Moderate-power cw ébre lasers 883

Ytterbium (Yb 3�). The energy level diagram of Yb3�

ions is very simple. Except the 2F7=2 ground-state level, theytterbium ion has only one level 2F5=2. Therefore, theabsorption spectrum of Yb3�-doped ébres consists ofonly one absorption band, which has a complicated shapedue to the Stark splitting. This band exhibits two maxima at915 and 976 nm, and pumping is performed by semi-conductor lasers emitting in these spectral ranges. Theluminescence spectrum corresponding to the 2F5=2 !2 F7=2

transition has maxima in the regions 978 ë 982 nm and1030 ë 1040 nm, extending up to 1.15 ë 1.2 mm. The meta-stable-level lifetime in an aluminosilicate glass is � 0.8 ms.This makes it possible to fabricate various ébre lasers withdifferent spectral and energy parameters emitting in therange from 0.98 to 1.2 mm [22, 23].

2.2 Photoinduced refractive-index gratings

Photoinduced refractive-index FBGs are used in ébre lasersas selective mirrors forming the laser resonator. Theproduction of FBGs became possible after the discoveryof the photosensitivity of optical ébres, i.e., a stablevariation in the refractive index in the core of an opticalébre induced by UV radiation at certain wavelengths. Atpresent many studies devoted to this problem were reportedand monograph [24] published. In the general case, aphotoinduced FBG is a piece of an optical ébre with themodulated refractive index of the ébre core with theamplitude at a level of 10ÿ5 ÿ 10ÿ3 and modulation periodof the order of the wavelength of light.

The main parameters of the FBG are the index-modu-lation period L, the induced variation dn in the refractiveindex, the number N of lines or the length L of the FBG.Two modes interact on the grating with the period L if theirpropagation constants b1 and b2 satisfy the phase-matchingcondition

b2 ÿ b1 �2pmL

; (1)

where m is an integer characterising the grating order inwhich the intermode interaction occurs. The coupling of thefundamental mode with the counterpropagating modeappears at a certain wavelength lBr determined by therelation

2neffL � lBr; (2)

where neff is the effective refractive index for thefundamental mode of the ébre. As a result, reêectionappears at the wavelength lBr. The parameters of gratings(spectral width and reêectivity) can be varied in a broadrange depending on the writing conditions and photo-sensitivity of the ébre. The reêectivity R of a uniform FBGof length L is determined by the expression [25]

R � th 2�kL�; (3)

where k � pZdn=lBr is the coupling coefécient; Z is thefraction of radiation power propagating in the region withmodulated refractive index. The FWHM of the resonance isdescribed by the expression [26]

Dl � la��

dn2n

�2��

1

N

�2 �1=2; (4)

where a is the parameter that is approximately equal tounity for deep gratings (R � 1) and 0.5 for gratings with asmall depth.

Because the FBG period is less than 1 mm for reêectionin the near-IR region, the grating is produced by theinterferometric method using UV radiation. Figure 3 showsthe scheme for FBG writing using a Lloyd interferometer.Note that such a scheme requires a high spatial coherence ofradiation and for this reason it uses, as a rule, the 244-nmsecond harmonic of an argon laser.

Another method for FBG writing is the phase-maskmethod [27]. This method is based on the interference of the�1st and ÿ1st diffraction orders of radiation propagatedthough a silica phase mask with a relief designed to suppressthe zero and other diffraction orders. The requirements tothe coherence of a radiation source in this method are notvery strict, which allows the use of KrF (248 nm) and ArF(193 nm) excimer lasers.

Figure 4 shows the typical transmission spectrum of aFBG with the parameters L � 5 mm, and dn � 8� 10ÿ4,L � 0:4 mm. The grating provides the reêectivity R � 0:99at a wavelength of 1136 nm, the width of the reêectionspectrum being � 0.4 nm. Such gratings, having a high

Spectrometer

UV radiation

Cylindrical

quartz lens

Mirror

Fibre

Lens

Radiation

source

(lamp or LED)

a

Figure 3. Scheme for FBG writing using a Lloyd interferometer.


reêectivity and a large width of the reêection spectrum, arecommonly used as input mirrors in ébre lasers.

2.3 Active optical ébres

Optical ébres doped with ions having optical transitions arecalled active ébres. The active ion can be doped both intothe core of a ébre and into its reêecting cladding if anoticeable fraction of optical power propagates in thelatter. Active ébres are produced using a number oftechnological processes: MCVD (modiéed chemical vapourdeposition) [28, 29], OVD (outer vapour deposition) [30],VAD (vapour axial deposition) [31], PCVD (plasmachemical vapour deposition) and SPCVD [32, 33]. Theactive impurity is doped in these processes, as a rule, by theimpregnation method, when a non-melted porous corematerial is impregnated with the solution of the activeimpurity salt, or by doping from volatile compounds.

Note that the maximum concentration of active ions inthe silica glass network is low and is restricted by theirsolubility and cooperative effects. As a result, the length of aébre laser can achieve a few ten meters. Therefore, active

optical ébres should have suféciently low non-resonanceoptical loses. The acceptable value of losses is 5 ë20 dB kmÿ1. In this case, their inêuence on the laser efé-ciency does not exceed a few percent. Figure 5 shows thespectrum of optical losses for an optical ébre doped withYb3� ions at a concentration of 8� 1019 cmÿ3 [34]. The coreof this ébre was co-doped with Al2O3, which allows one toincrease the solubility limit for rare-earth ions in a silicaglass and to decrease the probability of their clustering,resulting in cooperative up-conversion [35].

As mentioned above, high-power semiconductor sourcesfor pumping ébre lasers require the use of double-cladoptical ébres. As the active medium for high-power ébrelasers, optical ébres are used which consist of a single-modecore doped with active rare-earth ions and with impuritiesforming the refractive-index proéle, as well as of an innersilica cladding and outer cladding with the refractive indexlower than that of a silica glass. The model proéle of therefractive index for such ébres is shown in Fig. 6a.

The inner cladding (together with the outer cladding)forms a multimode ébre in which pump radiation propa-gates. The typical diameter of the inner cladding is 0.1 ë

Wavelength�nm

Transm

ission� dB

0

ÿ5

ÿ10

ÿ15

ÿ201133 1134 1135 1136 1137 1138

Figure 4. Typical transmission spectrum of a FBG with the parametersL � 5 mm and dn � 8� 10ÿ4.

Yb3� band

OH band

Wavelength�nm

Opticallosses� dB

kmÿ1

60

40

20

0800 1000 1200 1400

Figure 5. Spectrum of optical losses in the ébre core at the Yb3�-ionconcentration equal to 8� 1019 cmÿ3.

Doped core

Outer

cladding

Inner

cladding

Single-mode

lasing

a

b

Outer

cladding

Refractiveindex

Inner cladding Doped core

Multimode

pump

Figure 6. Model refractive-index proéle (a) and the principle of conversion of multimode pump radiation to single-mode radiation of a ébre laser (b).


1 mm, which provides coupling of up to a few ten watts ofpump radiation from semiconductor lasers. The pumpradiation propagating in a multimode ébre is absorbedby active rare-earth ions, resulting in their luminescence,which can develop to lasing in the presence of feedback. Theregion of lasing is localised in a single-mode core, i.e., itstypical transverse size is 5 ë 10 mm [36, 37]. The principle ofconversion of multimode pump radiation to single-moderadiation of a ébre laser is illustrated in Fig. 6b. Therefore,the cladding-pumped ébre laser can be considered as adevice providing the enhancement of the brightness of asemiconductor laser by a few hundred times (of course, at adifferent wavelength).

The outer cladding of double-clad optical ébres isnormally made of polymers with a low refractive index.In particular, silicon rubber is used, which provides thenumerical aperture of a multimode ébre equal toNA � 0:38, and AF Teêon which increases NA to 0.6.As a rule, ébres with a polymer coating have an outerdiameter of 100 ë 300 mm.

A disadvantage of silicon coatings is the high level ofoptical losses for pump radiation, which can exceed50 dB kmÿ1. The use of Teêon reduces optical losses inthe cladding down to � 10 dB kmÿ1. In addition, Teêon hasa high transmission in the UV region, which allows FBGwriting without polymer removing. However, a Teêoncoating is thin (10 ë 20 mm) and thus cannot reliably protectthe ébre from damage.

To provide the efécient coupling of modes in the innercladding with the activated core, it is necessary to use ébreswith the noncircular inner cladding [38], because otherwise agreat part of radiation power will propagate in modes thatdo not intersect the ébre core. The eféciency of radiationabsorption in optical ébres with inner claddings of differentshapes was studied for several samples made of the sameperform with the core doped with ytterbium [34]. Theoptical ébres had the following shape and parameters ofthe inner cladding: circular (diameter 125 mm), D-like withone polished face (125� 100 mm), rectangular (150�75 mm), and square (125� 125 mm). The absorption bandof ytterbium was measured in straight ébres and ébres bentin the form of égure eight with the radius of curvature of1 cm. The use of the latter conéguration should provide themixing of cladding modes and the increase in the intensity ofthe absorption band of Yb3� if cladding modes are notcoupled with the ébre core in a straight ébre. The results ofthe measurements presented in Table 2 show that theirregular ébre bending leads to a change in absorptiononly for a ébre with a circular inner cladding. Therefore, wecan conclude that each noncircular inner cladding providesthe eféciency of pump radiation absorption close to 100%.The D-like shape seems simplest to fabricate because onlyone face of a preform should be ground off. However, thesplicing of such a ébre with a ébre with a circular claddingused for FBG production leads to high losses because of itsasymmetric shape. Therefore, the optimal shape of thecladding is a square, which provides both a high eféciencyof radiation absorption and low optical losses in splicingwith circular ébres. Note that other cladding shapes can bealso used, in particular, a hexagonal shape.

In some cases, it is necessary to use active ébres with asmall diameter (30 ë 60 mm) of the inner cladding, which arepumped by higher-brightness semiconductor lasers. The useof ébres with a polymer coating is complicated due to their

small diameter, which precludes the use of welders forsplicing with other optical ébres. In this case, double-clad ébres made of silica glasses of different compositions[39] can be used. In such ébres, a silica supporting tube isused as the outer cladding, and the inner cladding is made ofa silica glass containing GeO2 at high concentration. Adisadvantage of this structure is the restriction of thenumerical aperture of the ébre because of the preformdestruction due to a difference between the thermal expan-sion coefécients of materials of the inner and outercladdings. The probability of preform destruction increasesat large sizes of the deposited cladding. For this reason, thenumerical aperture for pump radiation in practice is0:2ÿ 0:25, which considerably restricts the pump radiationcoupled to the ébre.

Another variant is the use of an activated ébre with amicrostructure cladding [40], whose cross section is shown inFig. 7. The numerical aperture of such ébres is typically 0.5and is limited by the leakage of higher-order modes throughconnectors supporting the inner cladding. A high value ofthe NA allows one to increase the pump density by severaltimes compared to the previous design of ébres by using thesame pump sources and the same diameter of the innercladding.

2.4 Pump schemes for active optical ébres

Several methods were proposed for pumping active opticalébres. The simplest of them is end-pumping, when radiationfrom a semiconductor laser is coupled to the active ébrethrough its end (Fig. 1). An advantage of this method isthat it can be used for all double-clad ébres describedabove. A disadvantage of the method is that only one pumpsource can be used (a laser diode or a diode array), so that

Table 2. Effect of the inner-cladding geometry on the absorption ofpump radiation in a Yb3�-doped ébre.

Cladding geometryAbsorption at 978 nm

�dB mÿ1

straight ébre `égure eight'

Circular 0:3� 0:05 0:6� 0:05

D-like 2:2� 0:05 2:2� 0:05

Rectangular 3:5� 0:05 3:5� 0:05

Square 3:3� 0:05 3:3� 0:05

Doped

coreInner

cladding

Supporting

tube

Microstructure

cladding

Figure 7. Scheme of a ébre with a microstructure cladding. Dark circlesare air gaps.


optical power coupled to the ébre is restricted by theparameters of modern semiconductor lasers.

Two other pump schemes use the principle of distribu-tion of coupled pump radiation over the active ébre length.Thus, it was proposed in Ref. [41] to couple pump radiationby using a set of V-grooves made on the side surface of theébre. The principle of pump coupling is shown in Fig. 8. Anobvious advantage of this method is the possibility ofadding radiation sources along the active ébre lengthwith decreasing power from a previous source. A disadvant-age of the method is the necessity of éxing a semiconductorlaser with respect to the active ébre and of protecting theregion of radiation coupling from external perturbations.

A speciéc feature of another pump method is the use of aspecial double ébre consisting of two ébres with a commonpolymer jacket having the refractive index lower than that ofa silica glass (GTW ébre). On of the ébres (active) has thecore doped with ytterbium ions, while the second one(passive) is made of a high-purity silica glass [42](Fig. 9). Pump radiation from a semiconductor laser iscoupled into the passive ébre. The pump power coupledinto the passive ébre is redistributed between the ébres inthe region of their contact, the ratio of pump powers in bothébres being determined by the ratio of areas of theircladdings. Absorption of a part of radiation in the activeébre core is compensated for by additional transfer ofradiation from the passive ébre, which is required tomaintain a constant ratio of pump powers in the activeand passive ébres. Therefore, pump radiation is distributedover the entire length of the active ébre. In this scheme, twopump sources can be used for coupling radiation from theopposites ends of the passive ébre, and the passive ébre canbe ruptured to add a new radiation source in the region ofpump power depletion (Fig. 9). In addition, the number ofpassive ébres in the assembly can be increased, which makesit possible, in turn, to increase the number of pump sources.

2.5 Speciéc features of optical ébres as ampliéers

Active optical ébres as ampliéers have a number of speciécfeatures compared to laser crystals and glasses. Thus, theguided propagation of pump and signal photons excludes,as a rule, radiative losses through the ébre surface.Radiative losses can appear only when the ébre parametershave been selected inappropriately or when the ébre has asmall-radius bending. This is an obvious advantage of ébre

lasers over traditional solid-state lasers. However, becausethe typical length of the active medium of ébre lasers is afew metres or ten metres, they eféciency should be analysedby taking into account the non-resonance optical lossesboth for pump and signal radiation.

The other features of ébre lasers are determined by thefact that radiation propagates not only in the core of asingle-mode ébre but also in its reêecting cladding, and thefractions of the optical power in the core and cladding canbe comparable. In addition, an active impurity can be dopednot into the entire core but only into its part or into a part ofthe cladding. In principle, different regions of a ébre can bedoped with different active impurities. In this case, the ionsdo not interact with each other in the accepted sense, but areonly optically coupled.

Therefore, classical expressions for the gain in the case ofoptical ébres should be modiéed. Let us present therelations for the evolution of the signal and pump powersfor the three-level ampliécation scheme according to whichmost ébre lasers operate:

dPs

dz� ws�N1se�ls� ÿN0sa�ls��Ps ÿ asPs; (5)

dPp

dz� ÿwpN0sa�lp�Pp ÿ apPp; (6)

where Ps and Pp, ls and lp are powers and wavelengths ofthe signal and pump, respectively; sa and se are theabsorption and luminescence cross sections; N0 and N1 arethe populations of the ground and metastable levels; as andap are the coefécients of non-resonance losses for the signaland pump; ws and wp are the overlap integrals of the

V-groove

Doped core

Inner claddingMicrolens

Diode laser

Figure 8. Scheme of pumping through a V-groove.

Pump Passive ébre

Active ébrePump

Figure 9. Pump scheme using a double ébre.


radiation éelds of the signal and pump with the activeregion,

ws;p �

� r2

r1

Es;p�r�2prdr�10

Es;p�r�2prdr, (7)

where E(r) is the radial distribution of the correspondingéeld; r1 and r2 are the boundaries of the ébre region dopedwith active ions.

Upon pumping into the ébre cladding, the value of wpcan be estimated approximately as

wp 'Sd

Scl; (8)

where Sd and Scl are the cross sections of the doped regionand cladding, in which pump radiation propagates. Notethat relation (8) is only approximate because the overlapintegral for the pump radiation depends in this case on thedistribution of the pump radiation intensity, the excitationmethod, the cladding shape, etc. Nevertheless, it is clearthat pumping into the ébre cladding is equivalent to asubstantial decrease in the absorption cross section at thepump wavelength.

3. Fabrication of Raman lasers

Raman ébre converters can eféciently convert pump laserradiation to lower-frequency radiation (Stokes radiation) byusing stimulated Raman scattering (SRS) of light in anoptical ébre. The SRS of laser radiation in a glass ébre wasobserved in 1971 [43]. Glass ébres with low optical lossesdeveloped shortly before by Corning Glass were used inexperiments.

In the case of cw pumping, the initial growth of theStokes wave intensity is described by the expression

dIsdz� gRIpIs; (9)

where Is is the Stokes wave intensity; Ip is the pump waveintensity; and gR is the SRS gain.

A speciéc feature of optical ébres as a SRS medium istheir relatively low gain. Thus, for fused silica, from whichoptical ébres are mainly produced, the gain is� 10ÿ13 m Wÿ1 [43]. In addition, the number of dopantsthat can be used for changing the spectrum and SRS gain inoptical ébres is limited by technological possibilities and therequirement of preserving low optical losses. On the otherhand, silica ébres have a unique property such as a longinteraction length, which reduces the SRS threshold. A silicaglass itself as an amorphous material has a broad Ramanspectrum with a maximum at 440 cmÿ1. Doping with GeO2

at concentrations used for fabricating optical ébres weaklychanges the shape of the Raman spectrum. The Ramanspectrum of a germanosilicate ébre is shown in Fig. 10[curve ( 1 )] [44].

By placing the active medium of the Raman converter ina resonator formed by narrow-band mirrors, we obtain aRaman laser. In the érst works devoted to the developmentof Raman lasers, dielectric or metal mirrors deposited on amassive substrate were used, which made the laser rather

bulky and required adjustment. The development of Ramanlasers was considerably stimulated by the demonstration oftheir application for pumping Raman ampliéers [45, 46].The use of ébre couplers and FBGs to obtain feedback inthe resonator substantially simpliéed the Raman laserdesign, made this laser acceptable for applications andmade it possible to develop multistage Raman converters.

The Raman laser design was further simpliéed by usingan active ébre with the core doped with phosphorous oxide[47]. The Stokes shift DnSt in this ébre is 1330 cmÿ1, which isthree times larger than that in a germanosilicate ébre(Fig. 10). The improvement of characteristics of phospho-silicate ébres [48] resulted in the development of Ramanlasers competing in eféciency with Raman germanosilicateébre lasers, but having a simpler design due to a smallernumber of conversion stages and, hence, a smaller numberof FBGs.

Figure 11 shows a simpliéed scheme of a multistageRaman laser. It is assumed in simulation that the ébre issingle mode at the pump wavelength. In addition, it isassumed that other nonlinear effects, in particular stimu-lated Brillouin scattering, are not excited. The latterapproximation is valid when pumping is performed by aébre laser with a rather broad emission line. A Raman laserconsists of an optical ébre and a set of FBGs with resonancewavelengths corresponding to Stokes shifts in the ébre. Thegratings tuned to the intermediate wavelengths have thereêectivity close to 100%.

Raman lasers were theoretically studied in a number ofpapers [49 ë 54]. The behaviour of a multistage converter canbe described by the relations

Frequency�cmÿ1

Intensity

(arb.u

nits) 1 2

200

150

100

50

0 200 400 600 800 1000 1200 1400

Figure 10. Raman spectra of germanosilicate ( 1 ) and phosphosilicate( 2 ) ébres.

lsn : : lsk : : ls1 lp : : ls1 : : lsk : : lsn

BG BG

Pin�lp� Pout�lsn�

Figure 11. Simpliéed scheme of a multistage Raman converter (Pin is theinput pump power; Pout is the output power at the nth conversion stage;k � 1; :::; n).


dP f; bp

dz� �apP f; b

p �vpvs1

g 1R

A 1eff�P f

s1 � Pbs1�P f; b

p ;

dP f; bsk

dz� �askP f; b

sk �vskvsk�1

gk�1R

Ak�1eff�P f

sk�1 � Pbsk�1�P f;b

sk

� gkR

Akeff�P f

skÿ1 � Pbskÿ1�P f;b

sk ;

(10)

dP f; bsn

dz� �asnP f;b

sn �gnR

Aneff�P f

snÿ1 � P bsnÿ1�P f; b

sn ;

where Pp, and Psk are the pump and signal powers atintermediate frequencies; Psn is the signal power at the énalwavelength; indices f and b describe the forward andbackward waves; vp and vsk are the pump and signalfrequencies; the vp=vsk ratio takes into account the diffe-rence in the energies of photons; ap, and ask are thecoefécients of linear losses at the pump and signal waves;and Aeff is the effective area occupied by the mode.

Note that it is difécult to measure the parameters gR andAeff with a sufécient accuracy. Because of this, the integralcharacteristic ë the gain G of the optical ébre at a speciéedwavelength,

G � gRAeff

(11)

which can be readily measured, can be used in calculations.The numerical calculation based on the above relations

is used to optimise the parameters of the converter byselecting the ébre length inside the converter and thereêectivity of a FBG with the resonance wavelengthcorresponding to the output signal for the speciéed powerof the input signal and parameters of the optical ébre.

4. Characteristics of ébre lasers

In this section, we consider the characteristics of ébre lasersbased on ébres doped with different rare-earth elements.

4.1 Nd3�-doped ébre lasers

A Nd3�-doped ébre laser emitting at 1.06 mm was the érstlaser in which pumping into cladding was used [55]. Thiswas done because the érst high-power semiconductor lasershad a low brightness, and to couple pump radiation it wasnecessary to use active ébres with the inner claddingdiameter of a few hundred micrometres. As a result, thepopulation inversion was low, and lasing could be achievedonly in four-level systems, where signal reabsorption isabsent.

During several years after the érst publication, the out-put power of neodymium lasers [57] exceeded 30 W [56].Figure 12 shows the typical characteristics (spectrum andpower) of the Nd3�-doped ébre laser having the 290� 290-mm ébre cladding and the activated core of diameter 5 mm.The laser was pumped at 0.81 mm by a diode array.

In Ref. [58], a ébre laser emitting simultaneously at twowavelengths of 1060 and 1090 nm was demonstrated. Theébre core was doped with aluminium and germanium. Theactive ions were located in different surroundings and haddifferent luminescence spectra. Upon 8.5-W pumping, thelaser emitted more than 3 W at 1060 nm and more than 1 Wat 1090 nm.

Of great interest is the development of a neodymiumlaser emitting at 0.92 mm. Such lasers can be used forpumping an ytterbium-doped ébre laser emitting at0.98 mm. In turn, the ytterbium laser, having a ratherhigh power, can be used for pumping erbium-doped ébreampliéers. The neodymium laser also can be used forfrequency doubling to obtain blue emission.

As mentioned above, it is difécult to produce lasing atthe 4F3=2 !4 I9=2 transition in the Nd3�-doped laser due tothe competition with luminescence at 1.06 mm. Therefore,the main problem in this case is the suppression ofluminescence in this region. In Ref. [15], a special opticalébre was used with the waveguide structure having highradiation losses in the spectral region >1 mm. This allowedthe authors [15] to obtain lasing with the 0.5-W outputpower at 925 nm. The emission spectrum of this laser isshown in Fig. 13. One can see that the suppression ofluminescence at 1060 nm was about 60 dB. Figure 14 showsthe dependence of the output power on the pump power.The slope lasing eféciency was about 35%.

l�mm

Pout

�W

3.0

2.5

2.0

1.5

1.0

0.5

0 1 2 3 4 5 6 7 Pp

�W

1.04 1.06 1.08

I

Figure 12. Dependence of the output power of a Nd3�-doped ébre laseron the pump power. The inset shows the emission spectrum of the laser.

3

c

b

a

Power� dB

800 850 900 950 1000 1050 1100 1150

1 4 5 2

0

ÿ10

ÿ20

ÿ30

ÿ40

ÿ50

ÿ60

ÿ70

ÿ80

Wavelength�nm

Figure 13. Emission spectrum of a Nd3�-doped ébre laser at 925 nm:pump line (a), the 925-nm laser line (b), emission at 1060 nm (c). Theinset shows the laser scheme: ( 1 ) pump radiation; ( 2 ) output radiation;( 3 ) active ébre; ( 4, 5 ) FBGs.


4.2 Yb3�-doped ébre lasers

The energy level diagram of ytterbium ions in a silica glassis very simple, consisting of the 2F7=2 ground level and theonly excited 2F5=2 level (Fig. 2). The absence of other energylevels up to the UV region means that both excited-stateabsorption and cooperative up-conversion should be absentin this system. This allows one to use ytterbium ions atmuch higher concentrations than concentrations of neo-dymium and erbium ions employed in corresponding ébrelasers. The use of ébres with high concentrations of activeions permits one, in turn, to reduce the length of the activeébre and, therefore, to decrease additional optical losses.

The absorption spectrum of such ébres, determined byelectronic transitions between split energy levels, consists ofthe absorption band with two maxima at 915 and 976 nm.The luminescence spectrum consists of a narrow line at980 nm and a band at 1035 nm, extending approximately upto 1200 nm (Fig. 15). The maximum absorption and lumi-nescence cross sections virtually coincide and are2:5� 10ÿ20 cm2 [23] (silica glass was doped with aluminiumand germanium). Note that pumping at 976 nm is moreefécient due to a large absorption cross section. At the sametime, pumping into the 915-nm band produces lasing at976 nm. In addition, in this case the requirements to thestability of the pump wave with changing temperature arenot strict due to a large width of the 915-nm band.

At present ytterbium lasers are the most popular clad-pumped ébre lasers, and they were described in numerous

papers (see, for example, [59, 60]). In this review, weconsider mainly the results obtained at the FORC,A.M. Prokhorov General Physics Institute, RAS.

The ytterbium-doped ébre laser uses a scheme withFBGs written both in the active ébre itself [61] and anotherébre spliced then with the active ébre [62]. The parametersof active ébres used in these papers were as follows: thedifference of the refractive indices of the core and claddingDn � (9ÿ 11)� 10ÿ3, the core diameter 2a � 4:5ÿ 5:5 mm,and the concentration of ytterbium ions (5ÿ 10)�1019 cmÿ3. The size of the inner square cladding was120� 120 mm, which provided 100% eféciency of ébrecoupling with the ébre output of a pump semiconductorlaser.

Figure 16a shows the typical dependence of the outputpower of an ytterbium-doped ébre laser at 1100 nm on thepump power. The slope eféciency of the laser is 80%,corresponding to the quantum eféciency of about 90%.Figure 16b shows the laser emission spectrum measuredwith a resolution of 0.01 nm; the width of the emission bandis 0.1 nm.

As mentioned above, a speciéc feature of clad-pumpedébre lasers is that it is difécult to produce complete inversepopulation in them because the effective pump cross sectiondecreases proportionally to the ratio of areas of the ébrecladding and core. Therefore, the pump conversioneféciency strongly depends on the reabsorption of the signalby the part of non-inverted active ions. Absorption at thewavelengths 1000 and 1060 nm in the long-wavelength wingof the 976-nm absorption band of ytterbium ions is

600Outputpower

at92

5nm� mW

100

200

300

400

500

Absorbed power at 805 nm�mW

0 400 800 1200 1600 2000

Figure 14. Dependence of the 925-nm output power of a Nd3�-dopedébre laser on the absorbed pump power.

Intensity

(arb.u

nits)

Wavelength�nm

1.0

0.8

0.6

0.4

0.2

0850 900 950 1000 1050 1100 1150

Figure 15. Absorption (solid curve) and luminescence (dotted curve)spectra of ytterbium ions in a silica glass.

6Outputpower� W 5

4

3

2

1

0 1 2 3 4 5 6 7 8Pump power

�W

a

b

Intensity

(arb.u

nits)

Wavelength�nm

1.0

0.8

0.6

0.4

0.2

01100.0 1100.2 1100.4

Figure 16. Dependence of the output power of a Yb3�-doped ébre laseron the pump power (a) and the laser emission spectrum (b).


approximately 2% and 0.2%, respectively, of absorption at976 nm. At the same time, the luminescence band ofytterbium ions is located at 1035 nm and also has theextended long-wavelength tail. Therefore, the eféciency ofthe ébre laser is determined by the competition betweenluminescence and reabsorption, determining, in turn, thespectral dependence of the pump conversion eféciency.

Figure 17 shows the spectral dependence of the slopelasing eféciency for seven ébre lasers [63]. One can see thatthe maximum slope eféciency amounts to 80% in thespectral region between 1.08 and 1.11 mm. It decreasesdown to 57% at a wavelength of 1.049 mm due toreabsorption of radiation. The decrease in the lasingeféciency in the long-wavelength region is explained by adrastic decrease in the luminescence cross section.

Note that the above results are valid for the selectedgeometry of the active ébre, i.e., for the given ratio of areasof the core and cladding equal to � 500. The lasingeféciency in the short-wavelength region can be enhancedby increasing the core diameter. The use of ébres withcladdings of a smaller size also seems promising. This ispossible, taking into account the current trend for increasingthe brightness of semiconductor lasers.

The use of active ébres with a diminished diameter of theinner cladding is especially important for the developmentof lasers emitting at 980 nm. Such lasers attract interestbecause they can be used for pumping high-power erbium-doped ébre ampliéers. One can see from Fig. 15 thatabsorption in this spectral region competes with lumines-cence, which requires the production of a high degree ofpopulation inversion to obtain lasing.

In [64], double-clad active ébres made of glasses ofdifferent compositions were used to obtain lasing at 980 nm.Figure 18 shows the refractive index proéle of the preformof such a ébre fabricated by the MCVD method. The innercladding of the ébre is made of a silica glass co-doped withGeO2. The molar concentration of the impurity in thecladding was about 10%, which provided the numericalaperture for pump radiation equal to 0.22. A supportingsilica glass tube served as the outer cladding. The ébre corewas doped with ytterbium and aluminium ions fromsolution. The inner-cladding and core diameters of theébre were 25 and 10 mm, respectively. The concentrationof Yb3� ions was about 2� 1019 cmÿ3, which provided theabsorption coefécient of 2.5 dB mÿ1 at 915 nm. The inputFBG of the laser was written directly in the preliminary

hydrogen-loaded active ébre. The ébre laser was pumped bya semiconductor laser at 915 nm, by coupling � 5 W ofpump power into the ébre of diameter 50 mm with theNA � 0:22.

The inset in Fig. 19 shows the emission spectrum of the977.5-nm ébre laser. The maximum output power wasobtained when an optical ébre of length 1.5 m was used.Figure 19 also shows the dependence of the output poweron the pump power. The slope eféciency of the pumpradiation conversion was 53%, and the eféciency withrespect to the absorbed pump power was 82%.

To couple higher pump powers for obtaining higheroutput powers, a ébre with a microstructure cladding wasused [65]. The numerical aperture of the inner cladding ofdiameter 20 mm was 0.7 and the output power at 980 nmwas 1.4 W for the absorbed pump power of 2.5 W. Adrawback of this laser design is the use of bulky mirrors inthe resonator.

Optical ébres doped with Yb3� ions were used in the érstmultimode laser with a resonator formed by multimodeFBGs written in a multimode gradient ébre with the corediameter of 50 mm [66]. (Such FBGs were produced for theérst time in Ref. [67].) An optical ébre with the core ofdiameter 16 mm doped with Yb3� ions and the inner-

Slopeeféciency

(%)

85

75

65

55

451.04 1.06 1.08 1.10 1.12 1.14 1.16

Wavelength�mm

Figure 17. Spectral dependence of the slope eféciency of a Yb3�-dopedébre laser pumped into a cladding.

SiO2 : Al2O3 : Yb2O3

SiO2 : GeO2

Refractive-index

proéle

0.02

0.01

0

ÿ4 ÿ2 0 2 4Radius

�mm

Figure 18. Refractive-index proéle for an active-ébre preform withcladdings made of glasses of different compositions.

975 976 977 978 979

Pump power�W

Outputpower� W

Wavelength�nm

0Intensity

(arb.u

nits)0.8

0.6

0.4

0.2

0 0.4 0.8 1.2 1.6 2.0

1.0

0.8

0.6

0.4

0.2

Figure 19. Dependence of the 977.5-nm output power of the laser on thepump power. The inset shows the laser emission spectrum.


cladding diameter 50 mm was used as an active medium inthe laser. Because of a small difference between therefractive indices of the activated core and inner cladding(Dn � 0:003), lasing at the ébre ends occurred not only inthe fundamental mode but also in modes of the innercladding, whose éelds well overlapped with the core.Two multimode ébre lasers emitting at 1.03 and 0.98 mmwere fabricated and studied. The output end of the activeébre was used as the output mirror in these lasers. Thedependence of the output power of the 1036-nm laser on thepump power is shown in Fig. 20. The slope eféciency of thelaser was 80%, which corresponds to the 90% slopequantum eféciency. The output power of the 978-nm laserwas 1.5 W. These results demonstrate that multimodegratings can be eféciently used as mirrors in multimodelasers.

Recently, special optical ébres, combining the active andpassive ébres, énd increasing applications in the develop-ment of high-power ébre lasers [42]. A laser using a tripleébre consisting of an active ébre and two passive ébres wasdemonstrated in Ref. [68] (Fig. 21). Four pump sources of atotal power up to 100 W were used. The output cw power ofthe laser was 65 W at a wavelength of 1072 nm. Thedependence of the output power of this laser on thepump power is shown in Fig. 22.

4.3 Er 3�-doped ébre lasers

The Er 3� ions in a silica glass have the luminescence bandat 1.53 mm, which make it possible to fabricate ébre lasersand ampliéers operating in the spectral range from 1.53 to

1.6 mm. The luminescence spectrum of Er 3� ions in analuminogermanosilicate glass is shown in Fig. 23. Er 3�-doped ébre lasers operate according to the three-levelscheme, which requires a high degree of populationinversion.

The maximum of the absorption band virtually coincideswith the luminescence maximum. Because of this, an opticalébre with a small diameter of the inner cladding allowingthe achievement of high inversion degree was used in theérst paper demonstrating a clad-pumped Er 3�-doped ébrelaser [39]. The inner-cladding diameter was 22 mm and thenumerical aperture was 0.18. The output power of 300 mWat 1540 nm was obtained upon 900-mW pumping at980 nm. The slope eféciency was 40%.

It is obvious that the output power in this case isrestricted by the eféciency of pump power coupling intothe inner cladding of a small diameter. At the same time, asone can see from Fig. 23, the luminescence band in thespectral region from 1.56 to 1.6 mm (the L band) dominatesover absorption. This reduces the requirements on thedegree of population inversion and allows one to use active

Pump power�W

Outputpower� W l � 1036 nm

2.5

2.0

1.5

1.0

0.5

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Figure 20. Dependence of the 1036-nm output power on the pumppower for a multimode ébre laser.

L2

Ytterbium-doped ébreHR

1072 nmL1Output

Fibre end cleave

Figure 21. Simpliéed scheme of a triple-ébre laser. L1 and L2: branchingpoints of the ébre; HR: highly reêecting FBG with the reêectivity closeto 100% at 1072 nm. A ébre end cleave perpendicular to its axis was usedas the output mirror.

Outputpower� W

0 20 40 60 80

Pump power�W

60

40

20

Figure 22. Dependence of the output power on the pump power for aYb3�-doped ébre laser pumped through passive ébres.

Luminescence

spectrum

Absorption

spectrum

Intensity

(arb.u

nits)

1.0

0.8

0.6

0.4

0.2

01450 1500 1550 1600

Wavelength�nm

Figure 23. Absorption and luminescence spectra of erbium ions in analumogermanosilicate glass.


ébres with greater diameters of the inner cladding and,hence, to employ higher-power pump sources. Thisapproach was realised in paper [69], where an activeébre with the inner-cladding diameter of 50 mm was usedand the 1-W ampliéed signal in the L band was obtained.We can assume that such ébres can be also used for thedevelopment of lasers emitting in this spectral range.

To obtain lasing in the 1.53 ë 1.6-mm range, optical ébresco-doped with Yb3� and Er3� ions are most often used. Insuch ébres, pump radiation at 976 nm is eféciently absorbedby Yb3� ions, which transfer their excitation energy to Er3�

ions [70]. The energy transfer is possible because of thenearness of the 2F5=2 level of ytterbium ions and the 4I11=2level of erbium ions.

One of the problems of manufacturing such a ébre is theappropriate selection and reproduction of the chemicalcomposition of the ébre core. Thus, to reduce the proba-bility of reverse energy transfer, it is necessary to decreasethe lifetime of Er 3� ions at the excited 4I11=2 level, which isachieved by using a phosphosilicate glass in the core. Inaddition, it is important to select properly the ratio ofconcentrations of the active ions. It was shown in Ref. [71]that the maximum lasing eféciency (� 50%) was achievedwhen the concentration of ytterbium ions exceeded that oferbium ions by a factor of 30.

At present, IPG produces commercial ébre lasers emit-ting more than 100 W in the spectral range from 1.53 to1.62 mm. Note also that optical ébres doped with Er3� andYb3� ions are used for the development of high-powersingle-frequency ébre lasers. Thus, a ébre laser emitting 5 Win the line of width less than 30 kHz (pumped by 25-Wradiation) was demonstrated in Ref. [72].

4.4 Tm3�-doped ébre lasers

As in the previous case, the efécient use of pumping intothe cladding of ébres doped with Tm3� ions is complicatedbecause lasing in the 1.8 ë 2-mm region occurs according tothe three-level scheme. However, the situation is alleviateddue to the presence of a strong absorption band centered at787 nm caused by the 3H6 ! 3F4 transition. The absorptionspectrum of a ébre doped with Tm3� ions is shown inFig. 24. In addition, the absorption and luminescencebands corresponding to the 3H4 ! 3H6 transition are well

separated in a silica glass doped with Tm3� ions, beinglocated at 1600 and 1800 nm, respectively. The width of theluminescence band is � 300 nm [73]. This allows a sufécientdegree of population inversion to be obtained, at which thesignal reabsorption is negligible in the spectral region>1.8 mm.

In the last years a number of papers devoted to thedevelopment of a clad-pumped Tm3�-doped ébre laser werepublished. Such a laser pumped by 36.5 W at 787 nm andemitting 14 W at 2 mm with the slope eféciency of 46% wasdemonstrated in Ref. [74]. A tunable Tm3�-doped ébre laserwas built in Ref. [75]. The tuning range of this laser was230 nm (between 1860 and 2090 nm) and the maximumoutput power achieved 7 W at 1950 nm, decreasing down to1 W at the tuning range boundaries.

4.5 Ho3�-doped ébre lasers

The Ho3� ions have the 5I7 ! 5I8 transition that can beused to obtain lasing at 2 mm. Figure 25 shows theluminescence spectrum of an optical ébre doped withHo3� ions excited by a krypton laser at 676 nm. Thiswavelength corresponds to the 5I8 ! 5I4 transition. Theluminescence lifetime was 0.5 ms.

In Ref. [17], a ébre laser was developed based on analuminogermanosilicate ébre doped with Ho3� ions by theimpregnation method. The estimated concentration of Ho3�

ions was 1:2� 1019 cmÿ3. The difference of the refractiveindices of the ébre core and cladding was � 0.08, and thecutoff wavelength for the érst higher mode was 1.5 mm.Figure 26 shows the spectrum of optical losses of the ébre.

Because holmium-doped ébre lasers operate accordingto the three-level scheme, to achieve the required populationinversion, the pump radiation should be efécientlyabsorbed. Because the most intense absorption bands arelocated in the visible region, the érst holmium-doped ébrelaser was pumped by an argon laser at 457.9 nm [76]. Theébre laser had a relatively low slope eféciency (1.7%) and ismaximum output power was 0.67 mW upon pumping by85 mW.

It is obvious that the efécient practical application ofholmium-doped ébre lasers requires pumping by semi-conductor lasers. However, available high-power lasersemit at 750 nm and cannot be used for pumping into theabsorption bands of holmium in the visible region. At thesame time, pumping into the 900-nm absorption bandcannot be efécient because of weak absorption in this band.

Absorption� dB

mÿ1 12

8

4

0800 1000 1200 1400 1600

Wavelength�nm

Wavelength�nm

1.0

0.8

0.6

0.4

0.2

0Intensity

(arb.u

nits)

1500 1600 1700 1800 1900 2000 2100

Figure 24. Absorption spectrum of a ébre doped with thulium ions. Theinset shows the luminescence spectrum.

Intensity

(arb.u

nits)

1.2

1.0

0.8

0.6

0.4

0.2

01800 1900 2000 2100 2200 2300

Wavelength�nm

Figure 25. Luminescence spectrum of holmium ions in a silica glass.


The efécient holmium-doped ébre laser was developedby using pumping by high-power ytterbium-doped ébrelasers pumped by diodes into cladding [17]. The holmiumlaser is pumped into the strong 1.15-mm absorption bandcorresponding to the 5I8 ! 5I6 transition [17]. The laserdesign is shown in Fig. 27. The laser uses a holmium-dopedébre with parameters presented above. A FBG had theresonance wavelength at 2001 nm with the linewidth 1 nmand the reêectivity above 99%. The output end of the ébrewas used as the output mirror with the reêectivity 4%. Theholmium laser was pumped into the core by a 1150-nmytterbium-doped ébre laser with the maximum outputpower 3 W.

Figure 28 illustrates the dependence of the output powerof the holmium laser on the absorbed pump power from theytterbium laser for the resonator of length 4.5 m. Themaximum output power 280 mW was achieved at theabsorbed pump power equal to 2 W, and the slope eféciencywas 20%. The inset in Fig. 28 shows the emission spectrumof the holmium laser measured with the resolution of0.2 nm. The lasing wavelength was determined by theresonance wavelength of the FBG, and the laser line widthwas � 0.4 nm [17].

The lasing eféciency of the holmium laser describedabove was considerably reduced due to high additionallosses in the active ébre (0.65 dB mÿ1 at 1.3 mm). Inprinciple, ébres with optical losses of the order of0.01 dB mÿ1 can be fabricated, which will increase the slopelasing eféciency up to � 25%. Note that the output power

of modern semiconductor lasers with a ébre output with theébre core diameter 100 mm achieves 20 W.

In recent years, holmium lasers co-doped with thulium[77] and ytterbium [78] ions were developed and studied.The lasers are pumped into the absorption bands of co-dopants, with the subsequent energy transfer to holmiumions. This allowed pumping into the cladding of the activeébre. The output power of the Tm : Ho-doped ébre laserwas 5 W at 2.1 mm upon pumping by 20 W. The outputpower of the Yb : Ho-doped ébre laser was 0.85 W uponpumping by 11 W.

5. Raman ébre lasers

Fibre lasers based on ébres doped with rare-earth ions emitonly in certain spectral ranges which do not cover the entireIR region. SRS in optical ébres can be used for the efécientwavelength conversion to obtain lasing virtually at anywavelength of the near-IR region. The design of a Ramanlaser and parameters of the ébre used for wavelengthconversion depend on the pump wavelength and thespeciéed wavelength of the converter. It is preferable touse an ytterbium-doped ébre laser for pumping due to itshigh eféciency in a broad spectral range.

The érst efécient Raman lasers were developed in papers[59, 79 ë 81]. In [82], an efécient Raman converter based onébre multiplexers providing feedback was described. In thissection, we consider different types of lasers developed andstudied at FORC.

5.1 Single-stage Raman lasers

The simplest type of a Raman laser is a single-stage Ramanlaser. An optical ébre with the germanosilicate core (DnSt �440 ë 480 cmÿ1) converts radiation from an ytterbium-doped ébre laser to radiation in the spectral range between1.1 and 1.22 mm.

In Ref. [54], the 1.09-mm radiation from an ytterbiumlaser was converted to the 1.15-mm radiation by using astandard Flexcore-1060 ébre in the Raman laser withoptical losses equal to 0.8 dB kmÿ1 at 1.06 mm. The Ramangain at 1.15 mm was 5:5� 0:5 dB kmÿ1 Wÿ1. The length ofthe ébre was 500 m, and the reêectivity of the output FBG

Losses� dB

mÿ1

8

6

4

2

00.6 1.0 1.4 1.8 2.2

Wavelength�mm

Figure 26. Absorption spectrum of a ébre doped with holmium ions.

976-nm

diode pump

Output

Ytterbium-doped

ébre

Holmium-doped

ébre

R � 100%

1.15 mmR � 40%

1.15 mmR � 100%

2 mm

Figure 27. Scheme of a Ho3�-doped ébre laser. Splicing points areindicated by crosses.

Outputpower� mW

300

200

100

Absorbed pump power at 1.15 mm�mW

0 500 1000 1500 2000

Wavelength�nm

Intensity

(arb.u

nits)

2000 2001 2002

1.0

0.8

0.6

0.4

0.2

0

Figure 28. Dependence of the output power of a Ho3�-doped ébre laserat 2 mm on the pump power. The inset shows the laser emissionspectrum.


was 20%. Figure 29 shows the experimental dependence ofthe output power of the converter on the output power ofthe ytterbium laser for the ébre length of 500 m. Also, thetheoretical dependence calculated using the model describedin section 2.4 is presented. It follows from Fig. 29 that theconversion eféciency for radiation from the ébre laserexceeds 70%.

Optical ébres with the phosphosilicate glass core exhibitthe additional Raman line at 1330 cmÿ1 (Fig. 10). As a rule,the molar content of phosphorous oxide in such ébres is13%ë15% and optical losses are � 1.8 dB kmÿ1 at1.06 mm and decrease approximately down to 1 dB kmÿ1

at 1.5 mm. Single-stage Raman converters of radiation froman ytterbium-doped ébre laser based on these ébres giveradiation in the region between 1.22 and 1.35 mm [83]. Byusing a neodymium-doped ébre laser for pumping, a Ramanlaser emitting at 1.24 mm with the conversion eféciency of� 70% was developed [80].

Figure 30 shows the dependence of the output power ofa Raman laser at 1234 nm on the power of a ébre laseremitting at 1060 nm. The emission spectrum of the Ramanlaser is shown in the inset in Fig. 30. A speciéc feature of thelaser design is that the FBGs forming the resonator of theRaman laser were written directly in the phosphosilicateébre. This reduced intracavity losses and enhanced theconversion eféciency.

Of great interest is the development of Raman lasersbased on optical ébres with a very high concentration(above 50%) of GeO2 in the ébre core. In this case, dueto a higher nonlinearity of a germania glass compared tosilica and a small diameter of the ébre core, the Ramangains amounting to a few hundreds dB kmÿ1 Wÿ1 can beobtained, which allows one to reduce considerably the activeébre length in the Raman laser. Thus, a Raman laser basedon an optical ébre with the molar concentration ofgermanium dioxide in the core equal to 75% and theRaman gain � 300 dB kmÿ1 Wÿ1 was demonstrated inRef. [84]. The resonator length of the ébre laser wasonly 3 m. The eféciency of conversion of the 1.07-mmradiation from the ytterbium-doped ébre laser to radiationat 1.12 mm was 70%. Figure 31 shows the correspondingdependence of the output power of the Raman laser on thepump power from the Yb-doped ébre laser.

5.2 Multistage Raman lasers

To obtain lasing in the spectral region above 1.35 mm, it isnecessary to use multistage Raman lasers. Thus, by usingfour-stage Raman conversion in a germanosilicate ébre,one can produce lasing in the spectral range between 1.35and 1.45 mm upon pumping the ébre by radiation from anytterbium-doped ébre laser. However, the increase in thenumber of stages considerably complicates the laser designdue to the increase in the number of FBG pairs, whichshould have certain resonance wavelengths maintained witha high accuracy. In addition, FBGs introduce additionalexcess losses (� 0.05 dB per grating), which reduces theconversion eféciency. Therefore, the search for new activemedia for Raman lasers is an urgent task.

A substantial step forward was achieved by using a ébrewith the phosphosilicate core, in which ampliécation wasobtained not only at the Stokes frequency of 1330 cmÿ1 butalso at the frequency shifted by 440 cmÿ1 and correspondingto SRS in pure silica. Therefore, it became possible to usetwo different frequency shifts in the same optical ébre [85].

In Ref. [86], a Raman laser emitting at 1407 nm wasdeveloped, which used one conversion with the 1330-cmÿ1

shift and two conversions with the 440-cmÿ1 shift. Themaximum output power at 1407 nm was 1 W and theconversion eféciency was 25%. In Ref. [87], the same design

Outputpower� W Experiment

Calculation

Input power�W

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

3.0

2.5

2.0

1.5

1.0

0.5

Figure 29. Dependence of the output power of a single-stage Ramangermanosilicate ébre converter on the ytterbium laser power.

Outputpower� W

1000 1100 1200 l�nm

2.5

2.0

1.0

0.50 1 2 3

Neodymium laser power�W

1.5

I�dB

ÿ40

ÿ60

ÿ80

Figure 30. Dependence of the output power of a single-stage Ramanlaser on the pump power at the input to a phosphosilicate ébre. The insetshows the output emission spectrum of the Raman laser.

Output power emitted to both sides

Output power emitted forward

Outputpower

at1.12

mm� W

12

9

6

Pump power at 1.07 mm�W

0 3 6 9 12 15

3

Figure 31. Dependence of the output power of a Raman germanate ébrelaser on the pump power.


was used in the laser emitting the maximum power of 1.3 Wat 1430 nm (Fig. 32).

Of special interest are Raman lasers emitting in the1.45 ë 1.6-mm range corresponding to the spectral regionused in optical communications. In particular, such laserscan be applied to pump Raman and erbium-doped ébreampliéers. Lasing in this spectral range can be mostconveniently obtained by using two-stage frequency con-version in phosphosilicate ébres. The dependence of theeféciency of such a laser on the pump power, the ébre lengthand the reêectivity of the output FBG was theoreticallyanalysed in Ref. [53]. The calculations were performed usingthe parameters of a phosphosilicate ébre with the concen-tration of P2O5 equal to 13%. The initial wavelength was

1061 nm and the énal wavelength was 1480 nm. The opticallosses of the phosphosilicate ébre were 1.55, 0.92, and0.75 dB kmÿ1 at the wavelengths 1061, 1240, and1480 nm, respectively. The gains were 5.5 and4 dB kmÿ1 Wÿ1 at the wavelengths 1240 and 1480 nm,respectively. In addition, the calculation took into accountintracavity losses caused by splicing of the phosphosilicateébre with a ébre in which FBGs were written and byadditional scattering from FBGs. These additional losseswere estimated as 8%. Figure 33 shows the results ofcalculations. Figure 34 shows the experimental and calcu-lated dependences of the output power of the Raman laseron the pump power from the ytterbium-doped ébre laser.

To increase the eféciency of the Raman converter, FBGsin Ref. [88] were written directly in a phosphosilicate ébre,which reduced intracavity losses, resulting in the 45%conversion eféciency.

Intensity

(dB)

Wavelength�nm

1100 1200 1300 1400 1500ÿ80

ÿ60

ÿ40

ÿ20

0I(arb.u

nits)

1

01431 1433 1435

l�nm

Figure 32. Emission spectrum of a Raman phosphosilicate ébre laserwith the use of the 440-cmÿ1 and 1330-cmÿ1 Stokes shifts.

L�m L

�m

L�mL

�m

a b

c d

600

400

2005 10 15 20 Rout (%)

600

400

2005 10 15 20 Rout (%)

Pin � 5W

Pin � 3W

Pin � 4W

Pin � 2W

33.8%33.1%

32.4%

600

400

2005 10 15 20 Rout (%)

600

400

2005 10 15 20 Rout (%)

46%

41%

43.2%

44.6%

45.3%

43.5%42.9%42.2%40.9%

38.9%

39.7%38.9%38.2%

36.6%

Figure 33. Dependences of the conversion eféciency (in %) of a two-stage Raman laser on the phosphosilicate ébre length L and the reêectivity Rout ofthe output grating for different input pump powers Pin.

Experiment

Calculation

Outputpower� W

1.2

0.8

0.4

0 1 2 3 4

Ytterbium laser power�W

Figure 34. Dependence of the output power of a 1480-nm two-stageRaman phosphosilicate laser on the 1061-nm input power of anytterbium laser.


5.3 Composite Raman lasersThe long-wavelength boundary of the efécient wavelengthconversion in lasers described in section 5.2 is 1.6 mm. Inthe case of Raman lasers, it is determined by the two-stageRaman conversion in a phosphosilicate ébre and by thelong-wavelength emission edge (1.12 mm) of ytterbium-doped ébre lasers, where the lasing eféciency exceeds 60%.At the same time, the spectral range between 1.6 and1.8 mm is also of interest for a number of applications.

Emission in this spectral region can be obtained by usinga Raman laser based on a phosphosilicate optical ébre withtwo frequency shifts by 1330 cmÿ1 related to the P2O5

component and one frequency shift by 440 cmÿ1 related to asilica glass.

Unfortunately, phosphosilicate ébres have at presentmuch higher optical losses than germanosilicate ébres (0.8 ë1 dB kmÿ1 against 0.2 dB kmÿ1 at 1.5 mm). As the wave-length further increases, optical losses in phosphosilicateébres related to the edge of phonon absorption increasefaster than losses in germanosilicate ébres. Therefore, thescheme, which uses simultaneously Stokes shifts caused bythe phosphorous and silica components of the ébre, doesnot seem promising. At the same time, optical losses ingermanosilicate ébres grow much slower, and in telecom-munication optical ébres they are � 0.25 dB kmÿ1 in thisspectral region.

The use of germanosilicate optical ébres in Raman lasersrequires the seven-stage frequency conversion to obtainradiation at wavelengths longer than 1.6 mm. Becausesuch a laser design is complex and low-efécient due to ahigh total threshold pump power, it was proposed [89] to usea composite Raman laser, whose scheme is shown in Fig. 35.The laser consists of two parts: a two-stage phosphosilicateébre converter and a single-stage germanosilicate ébreconverter. The érst and second parts of the converterproduce radiation at 1533 and 1649 nm, respectively. Themaximum output power at 1533 nm was 2.07 W uponpumping by a 8-W semiconductor laser. A standard tele-communication ébre with the zero chromatic dispersionwavelength shifted to 1.55 mm was used as an active mediumat the second stage. Optical losses in this ébre were 0.2 and0.25 dB kmÿ1 at the wavelengths 1533 and 1649 nm,respectively. The small-signal gain at 1640 nm was2.2 dB kmÿ1 Wÿ1. The gain was measured with the helpof a semiconductor laser emitting in this spectral range.Figure 36 shows the dependence of the output power of the1649-nm Raman laser on the pump power.

Despite a low gain, the conversion eféciency for the1533-nm radiation to the 1649-nm radiation was 63%, the

slope eféciency of conversion of the 1089-nm radiation fromthe ytterbium-doped ébre laser was 32%, and the linewidthof the Raman laser was 0.35 nm (Fig. 36).

Note that the use of the composite Raman laser providesemission in the spectral range between 1.6 and 1.75 mm. Theexpected output power is approximately the same becausethe required radiation wavelength of the ytterbium-dopedébre laser lies in this case in the 1.07 ë 1.12-mm range, wherethe eféciency of this laser changes weakly.

To obtain laser radiation at 2 mm and at longer wave-lengths, GeO2 glass ébres should be used. The GeO2 glasshas a minimum of optical losses at 2 mm, where the Ramancross section exceeds that for a silica glass by an order ofmagnitude. In addition, the germania glass has a highphotosensitivity, which allows FBG writing in a hydro-gen-unloaded ébre [90].

In Ref. [91], a four-stage Raman germania glass ébrelaser (Raman laser 1) emitting at 2.06 mm was developed.The laser was pumped by a two-stage Raman phosphosi-licate ébre laser emitting at 1472 nm, which, in turn, waspumped by a 1057-nm ytterbium-doped ébre laser. Fig-ure 37 shows the scheme of this Raman laser, and itsemission spectrum is presented in Fig. 38.

Ytterbiumébre

Germanosilicateébre

Phosphosilicateébre

HR1.089

HR1.273

HR1.273

HR1.089

HR1.649

HR1.533

HR1.649

HR1.533

R�20%1.089

R�20%1.533

Diode

pump

Figure 35. Scheme of a composite Raman laser. The crosses indicatesplicing points; the resonance wavelengths are indicated (in mm) neareach of the gratings.

Outputpower

at16

50nm� W

1.2

1.6

0.8

0.4

0 1 2 3 4 5

Input radiation power�W

I(arb.u

nits)

1648.5 1649.0 1649.5 l�nm

1.00.80.60.40.2

0

Figure 36. Dependence of the output power of a composite Raman laseron the ytterbium laser power at 1089 nm. The inset shows the emissionspectrum of the composite laser.

Raman laser 2

R � 5%1057

Yb laser

HR1057

R � 60%1472

HR1231

HR1472 HR

1057HR1231

P2O5

Raman laser 1

Output

GeO2R � 50%

°³2062 1853 1695 1575 1472 1575 1695 1853 2062

HR HR HR HR HR HR HR HR

Figure 37. Scheme of a 2.06-mm four-stage Raman ébre laser. OC:output coupler. The resonance wavelengths are indicated (in nm) neareach of the gratings.


6. Conclusions

We showed in the review that optical ébres doped withvarious rare-earth ions have been used for the developmentof ébre lasers emitting in various spectral regions. Thus,Nd3�-doped ébre lasers emit at 0.92 and 1.06 mm; Yb3�-doped ébre lasers emit in the region 0.98 ë 1.15 mm; Er 3�-and Er3� : Yb3�-doped ébre lasers emit in the region 1.53 ë1.6 mm; and Tm3�- and Ho3�-doped ébre lasers emit in theregions 1.5 ë 2 mm and 2 ë 2.1 mm, respectively. To obtainmoderate lasing powers, it is necessary to use pumping intothe ébre cladding. In this case, the ébre structure for thepump radiation can be made of a silica glass and a polymer,silica glasses of different compositions, and also a micro-structure cladding can be used.

The use of photoinduced FBGs as mirrors substantiallysimpliéed the design of lasers. Note here that multimodegratings also can be used.

Raman lasers based on optical ébres of differentcompositions and FBGs are used for conversion of radia-tion from ébre lasers to radiation at any wavelength in thespectral region between 1.15 and 1.65 mm. The conversioneféciency is 30%ë 70% depending on the number ofconversion stages. The use of a germania glass ébre expandsthis region up to 2 mm.

Therefore, ébre lasers considered in the review producemoderate-power radiation at any wavelength in the spectralrange between 0.9 and 2 mm, covering virtually the entirenear IR-region.

Acknowledgements. The authors thank I.A. Bufetov forplacing the experimental results at our disposal.

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2nd order

1st order

Power� dB 0

ÿ10ÿ20

ÿ30

ÿ40ÿ50

ÿ601050 1200 1350 1500 1650 1800 1950

2100 2400 2700 3000 3300 3600 3900

6

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4328

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Wavelength�nm

Figure 38. Output emission spectrum of a four-stage Raman laser: ( 1 )1057-nm pump radiation from an ytterbium laser; ( 2, 3 ) emission fromtwo stages of Raman phosphosilicate ébre laser 2 (at 1231 and 1472 nm);( 4, 5, 6 ) emission from the érst three stages of four-stage Raman laser 1(at 1575, 1695, and 1853 nm); ( 7 ) emission at the maximum of theRaman gain at 2017 nm corresponding to the 440-cmÿ1 frequency shiftfrom 1853 nm; ( 8 ) 2062-nm emission from the fourth stage of Ramanlaser 1.


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