Design of C-band Fiber Source

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AbstractWe demonstrate a simple design of Erbium doped C-band Superfluorescent fiber Source (SFS). The one stage erbium

SFS source has erbium doped fiber (EDF) length of 8m, used as

an active medium to generate amplified spontaneous emission

(ASE). The source is pumped with a 976nm Bragg grating

stabilized laser diode to provide co-propagating pump in the

system. The design is polarization independent based on single-

mode scheme with polarization extinction ratio of 0.8dB at the

output. Backward travelling ASE is reflected using a circulator

and incorporating ASE filter in the feedback to inject photons of

desired band for amplification. The source gives maximum

output power of 21mW at a pump power of 100mW and has a

full width half maximum (FWHM) of 21nm around a centre

wavelength of 1555nm, and can be used in CWDM applications.

Keywords Amplified spontaneous emission (ASE), erbium

doped fiber (EDF), Superfluorescent fiber source (SFS).

I. INTRODUCTIONDifferent semiconductor based broadband light sources are inuse today, ranging from light emitting diodes to

superluminescent diodes. However, the requirement of high

output power and efficient coupling has drawn much researchtowards the area of fiber sources [1-2]. Such light sources are

made with rare-earth doped single mode fiber; generating high

amplified spontaneous (ASE) output powers.The ASE spectrumof an EDF is spread over C-band (1525-1565nm) and L-band (1565-

1605nm), which is the most suitable wavelength range in present day

optical networks. Superfluorescent fiber source has been

demonstrated to offer the highest output power, better mean

wavelength stability, and broader line width[3].Fiber basedsource finds its application in optical testing [4], optical

sensing[5] and optical communication [6] .In this paper, we

present a source which has been optimized for maximum

output power and its spectrum has been tailored to remove

non-uniform spectral characteristics, normally associated withSFS source[7] ,which may lead to an undesirable coherence

function of the source. The filtering of backward ASE and

passing through SFS for amplification has been reported tooffer wavelength stability, smooth spectrum and high output

powers in such sources [8].

II. THEORY OF OPERATIONErbium doped fiber amplifier (EDFA), which consists oferbium doped fibre (EDF) with a certain level of population

inversion, can be used to generate a broad spectrum of ASE

over the C- and L-band. The population inversion is achievedthrough a pump source that excites Er3+ ions to higher energy

states. Although Er3+ ions can be excited at a few possible

pump wavelengths, a laser diode at 980nm is preferred as a

pump source because this wavelength provides relativelyhigher gain, low noise and very low excited state absorption

(ESA) [9]. Absorption and emission spectra of an EDFA are

broadened because the energy levels of Er3+ ions are split by

the electric fields of the glassy network (Stark splitting) whendoped into glass [10]. Although the EDFA is a quasi 3-level

system, including Stark-split sub-levels [11], a relatively

simpler two-level model [12] is sufficient to explain theamplification process of the EDFA. Depending upon the

inversion level, photons of the input signal are amplified over

the length of EDFA as:

( )g t lout inP P e

= (1)

whereP is the power (photons / second) of the input signal of

optical frequency and ( )g t is the length-averaged gaincoefficient of the EDF defined by:

( ) ( ) ( ) ( ) ( )2 1 2g t N t N t N t = = + (2)

( )1N t , ( )2N t are respectively the length-averagednormalized population factors in the ground and excited

energy levels, and

and

are the emission and

absorption coefficient of EDF. The absorption and emission

spectra is shown in figure 1. An important principle of

quantum theory states that the spontaneous emission (SE) rate

at each frequency is the same as the stimulated emission ratethat would be produced by one signal photon (the so-called

extra photon) at that frequency [13]. Therefore the SE

power produced over a small band of frequencies , by a

very small length dz of the EDF is given as [30]:

( ) ( )( )2, 2SEP t N t dz = (3)

The factor 2 is indicates two polarization states for the extra

photon. The SE power emitted at any point z on the EDF is

amplified over the remaining length (l-z). Hence the total

ASE power generated in one direction, over the frequencyband , from all points of the EDF can be calculated as:

Saad Rizvi*, Fida Hussain, Mohammad Aleem Mirza*[email protected]

Centres of Excellence in Science and A lied Technolo ies CESAT Islamabad Pakistan

Design of a C-band Erbium doped superfluorescent fiber source using

backward ASE filtering technique

Proceedings of International Bhurban Conference on Applied Sciences & TechnologyIslamabad, Pakistan, 10 13 January, 2011 115


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( ) ( )( ) ( ) ( )

( ) ( )

2

0

, 2

,

lg t l z

ASE

SE

P t N t e dz

P t A t

=

=

(4)

where ( ) ( )

( )( )

( )1

g t lA t e g t l

= represents the

amplification factor for the SE power (PSE) produced from the

entire length of the EDF. Equation (4) gives the ASE power

produced in one direction and the bidirectional ASE power,

over the given frequency band, can be simply computed by

multiplying it by 2. The ASE will be generated by the

EDFA at all the frequencies within its emission spectrum and

hence the total ASE power (photons / second) is given by

replacing by d and integrating equation (4) over the full

SE bandwidth.

Figure 1: Fiber gain and absorption spectra near 1550nm for the fiber

used in our experiment (ThorLabs) versus wavelength

III. DESIGN AND CONSTRUCTIONThe Erbium superfluorescent fiber source was optimized in

three stages shown in figure 2 (a), (b), (c). It mainly consists

of erbium doped fiber (EDF), a 980nm pump source,

980/1550nm wavelength division multiplexing (WDM)

coupler and an isolator at the output. Erbium doped fiber was

pumped with a 976nm laser diode as forward propagating

pump. The pump source is Bragg stabilized laser whose output

power can easily be varied from 0-100mW by controlling the

input current. Pumping power against input current is shownin figure 3. We used 8 m length of erbium doped fiber that has

peak absorption of 7dB/m at 1530nm.Its gain-absorption

spectra is shown in figure 1. We optimized the length by

taking into account the residual pump power at the output and

in our setup; it is between 7-8 m. Using lengths longer than 8

m, the output power of SFS started decaying. Since the round

trip gain in a typical SFS is very high, even minor reflections

at the transmitting end will turn SFS into a laser. The isolator

is important to avoid lasing in the active medium due to back

reflections.

Figure 2: Schematics of Er-SFS (a) basic design (b) design

using circulator for ASE feedback (c) design of Er-SFS using

backward ASE filtering configuration.

Figure 3: DFB pump laser output power vs. input current.

First we constructed a simple design to observe the ASE

spectrum both in the forward and backward directions. Figure

2(a) illustrates the basic design. Using this configuration we

measured the forward and backward ASE spectra generated by

976nm pump in EDF. Figure 4 shows the measured forwardand backward ASE spectra. The backward ASE power

measured at the 1550nm port of WDM coupler was around 2.5

mW. In this case, the backward ASE had less power levelscompared to forward ASE. The forward ASE power was

measured to be 3.8mW at the output. The wastage of

backward ASE did not provide high output power levels

desired. We incorporated optical circulator to process the

backward traveling ASE, to form a double pass configuration.

Proceedings of International Bhurban Conference on Applied Sciences & Technology

Islamabad, Pakistan, 10 13 January, 2011

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The reflected signal is amplified as it passes through erbium

fiber, which also minimizes the spectral nonuniformity.

Figure 2(b) illustrates the configuration using a circulator.

Optical circulator is used in the scheme to provide a simplereflection mechanism without designing reflectors like fiber

optic loop mirrors, mostly utilized. Three-port polarization

insensitive optical circulator is used, splicing its ports in a way

to reflect the incoming signal at the input port. Port 2 and 3 are

spliced together so that the incoming signal at port 1 isreflected back .The reflected ASE signal when passes through

erbium fiber, produces stimulated emission thus amplifying

the signal. Output power levels were found to be 7.2mW,nearly two folds compared to scheme in which circulator was

not used. Output for this scheme is shown in figure 5.To get

high output power from the source, we further filtered the

backward ASE, using ASE filter to flatten the output spectrumand amplify certain portion of desired spectrum. The ASE

filter is a micro optics device based on thin film technology,

used in CWDM systems.ASE filter is integrated with

circulator between its ports 2 and 3, to filter and reflect thebackward ASE to the active medium, shown in figure 2(c).

Figure 4: Measured ASE spectrum in the forward (blue) and

backward (yellow) directions, for figure 2(a).

We used ASE filter with pass band of 21nm (1544~1565nm).

In this case the excited erbium ions are not wasted byundesired wavelengths, and maximum power of 21mW was

achieved. We used fusion splicing and recoating techniques to

reduce losses in our design. The output spectrum is shown in

figure 6.The output spectrum has a flattened shape on higher

powers.

IV. EXPERIMENTAL RESULTS

We have demonstrated a high output power all fiber source

with variable output power. Its power is controlled by

controlling the input pump power. Figure 7 illustrates the

measured response between the output power of the SFS and

input pump power. This relationship between the output and

pump power gives the slope efficiency of 25 percent. We

measured the extinction ratio of the source to check for its

depolarized nature. Using PER meter, measured ER was

nearly 0.8dB, which proved its depolarized nature. This small

extinction ratio is present due to the net effect of various

components used in the design.

Figure 5: Forward ASE spectrum using configuration of figure

2(b)

Figure 6: Output spectrum of the SFS using proposed

configuration of figure 2(c).

We computed the coherence length and spectral width of

our source using autocorrelation function of the source,

Figure 7: Measured Relationship between output and

pump power in the SFS



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passing the light through a scanning Michelson interferometer.

Figure 8 shows the autocorrelation function of the SFS output

spectra measured as a function of the optical path difference in

a scanning Michelson interferometer. The measured value of

coherence length at 3dB points is 0.11mm (0.055x2, a factor

of 2 is used because the imbalance of Michelson

interferometer is twice the mirror displacement).

Corresponding spectral width can simply be calculated from

coherence length, which comes out to be ~22nm, matchingwell with the measured spectral width. The central wavelength

variation of our source was measured over a limited range of

temperature. Central wavelength variation of 0.006nm was

recorded over temperature cycles (20 to 40 oC).Such a small

change in wavelength is close to the resolution of optical

spectrum analyzer. Temperature cycles were applied over a

period of 2 hour. This shows that our SFS exhibits the inherent

property of wavelength stability. We also tested the variation

of sources output power over temperature cycles between 20

to 40 oC. The graph in Figure 9 shows the power variation in

relation with temperature over time. Temperature cycling was

applied for two hour. We calculated a variation of 0.94% in

the output power, determining the stability of the source.

V. CONCLUSIONWe have experimentally demonstrated a simple design of C-

band Erbium SFS by using backward ASE filtering technique

to achieve high output power and flat broadband spectrum.

The characteristics of high output power of 21mW, broad

spectral width of 21nm and polarization extinction ratio of less

than 0.8dB have been obtained. The key advantages of thisdesign are its inherent wavelength and power stability,

polarization insensitivity and no coupling loss to fiber.

Thermal results have shown that the output power variation ismuch smaller and wavelength change is negligible when

temperature varies. With such benefits over semiconductor

superluminescent diodes, the all fiber SFS can be used

effectively in CWDM networks, fiber sensing and opticaltesting.

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Figure 8: Autocorrelation function of SFS using

Michelson Interferometer

Figure 9: Thermal testing of SFS to check for power stability

over 2 hour time period.



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Design of C-band Fiber Source

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