IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 15, AUGUST 1, 2009 1093

Two-Color Output From InGaAs Laser WithMultiplexed Reflective Bragg Mirror

S. A. Zolotovskaya, V. I. Smirnov, G. B. Venus, L. B. Glebov, and E. U. Rafailov, Senior Member, IEEE

Abstract—An optical scheme for stable two-color operationfrom a single laser diode (LD) by means of a multiplexed reflectiveBragg mirror is proposed and demonstrated. A mode spectralseparation of 3 nm (1 THz) and laser linewidth less than 80 pm,with sidemode suppression ratio over 24 dB, and an output powerof 1.7 W are achieved with a monolithic dual-wavelength Braggmirror. Temperature stability of spectral locking of an LD indual-wavelength operation is presented.

Index Terms—Dual-wavelength operation, semiconductor laser,volume Bragg grating (VBG).

S PECTRAL shaping of laser output is of interest in a broadrange of applications including optical communications,

spectroscopy, device characterisation, and imaging systems.Dual-wavelength operation with a mode separation rangingfrom hundreds of picometers to several nanometers, with cor-responding difference frequency beating between 0.1 to severalterahertz (THz), is particularly desirable for difference signalgeneration by photomixing of an optical beat in photoconduc-tive antennas [1]. Over the years, several approaches have beenimplemented in order to achieve simultaneous two-color gen-eration from a compact laser system. Some of the techniquesare represented by monolithic dual-wavelength semiconductorlasers [2]–[4]. These systems offer all the benefits of mutuallycoherent two-color operation [5] but low output power leveland high fabrication cost significantly restrict their fields ofapplication. Dual-wavelength operation from a single laserdiode (LD) or LD array has also been achieved using variousfrequency selective filters in an external-feedback cavity con-figuration, providing relative flexibility in the choice of LDand cavity design. Two-color external cavity LDs, based on amulticomponent cavity design, have been demonstrated withsurface diffractive gratings as frequency selective elements.A continuous frequency difference tuning from 0.15 THz(0.32 nm) to more than 7 THz (17 nm) was demonstrated witha continuous-wave (CW) two-wavelength LD array at 800 nm[6]. Dual-wavelength operation with a mode separation up to2.5 THz in a CW and up to 4.2 THz in a mode-locked regimewas achieved from a ridge waveguide LD emitting at 835 nm[7]. A broadband tuning of dual-wavelength output, with a

Manuscript received March 09, 2009; revised April 29, 2009. First publishedJune 02, 2009; current version published July 17, 2009.

S. A. Zolotovskaya and E. U. Rafailov are with the School of Engineering,Physics and Mathematics, University of Dundee, Dundee DD1 4HN, U.K.(e-mail: S.A.Zolotovskaya@dundee.ac.uk; E.U.Rafailov@dundee.ac.uk).

V. I. Smirnov is with the OptiGrate, Orlando, FL 32826 USA (e-mail:vsmirnov@optigrate.com).

G. B. Venus and L. B. Glebov are with the College of Optics and Photonics/CREOL, University of Central Florida, Orlando, FL 32816-2700 USA (e-mail:gvenus@mail.ucf.edu; lbglebov@creol.ucf.edu).

Digital Object Identifier 10.1109/LPT.2009.2023207

spectral separation ranging from 3.5 to 53 nm was attained witha gain-switched LD at around 1 m [8]. However, these opticalschemes are relying on the use of bulk optics and cannot be re-alized in either compact or intrinsically simple configurations.Recently, an external cavity design for stable dual-wavelengthoperation by means of two single-wavelength volume Bragggratings (VBGs) has been reported. A maximum 83% of theLD total output power, with a difference frequency step offsetin the range of 0.16–2.05 THz, was attained in dual-wavelengthregime within a compact external cavity setup [9].

VBGs recorded in bulk photo-thermo-refractive (PTR) glassare a type of wavelength selective device, containing Braggplanes with varying index of refraction that penetrate the entirevolume of the material. A refractive index modulation of ,with corresponding diffraction efficiency exceeding 99%, canbe achieved in PTR glass which is comparable to that of theconventional fiber Bragg gratings imprinted in fused silica[10]. In contrast with surface diffraction gratings, the spectralselectivity of a VBG is determined by the number of planeswith modified refractive index, equidistantly spaced throughthe volume of PTR glass [11]. Thus, significant spectral nar-rowing can be attained with a diffractive element of reasonablethickness which can also be integrated with collimating opticsor LD directly. Unique properties of the PTR glass, such aslow induced loss, high laser damage threshold and temperaturestability [12]–[14], have also determined a wide range ofhigh-power laser applications for these devices [15]–[19].

Here, we report an optimized optical scheme for externalcavity dual-wavelength semiconductor lasers by means of mul-tiplexed reflective Bragg gratings, fabricated in a single wafer.A frequency difference of 1 THz and bandwidths of 21 GHz,with sidemode suppression ratio (SMSR) over 24 dB and anoutput power of up to 1.7 W, have been achieved from a broadarea InGaAs LD in two-color operation. Stability of the spectrallocking of the LD in dual-wavelength operation was assessed foran LD operation temperature range from 20 C to 40 C.

A monolithic multiplexed Bragg grating, with a thickness of1.4 mm, designed to reflect at separate wavelengths of 977 and980 nm, was fabricated by recording two reflection VBGs ina single plate of PTR glass. This differed to a single gratingrecording technique [12], [13], in that a PTR glass wafer wassubjected to two consecutive exposures of interference fringes,with a slightly changed different period between them whichsubsequently defines the Bragg wavelengths of the gratings. Athermo development procedure at high temperature ( 500 C)was further required to yield the formation of the two gratingsin the same volume of the glass. It should be highlighted thatthe two reflection VBGs, which appeared during the thermodevelopment process, are absolutely independent, with no spe-cific behavior that could contradict with theoretical predictions

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1094 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 15, AUGUST 1, 2009

Fig. 1. Schematic of a two-color external cavity with monolithicdual-wavelength Bragg mirror. FL: focal length.

for a single grating [11]. Moreover, controlling the exposureof each grating allows recording of multiplexed gratings withinthe same volume with different parameters, e.g., diffraction ef-ficiency, that can be preferable to compensate gain difference.The dual-wavelength grating sample used in this work had anequal diffraction efficiency of 30%, with precision at a level of2%, and bandwidth of 100 pm for each reflection VBG. Thespectral characterization of the monolithic multiplexed Bragggrating was carried out in retro-reflection geometry, using acollimated output beam from a tunable single frequency lasersource, with a linewidth less then 1 MHz. In order to preventparasitic back-reflection, the grating planes were tilted at 1to the grating glass surface.

Two-color external cavity operation with a multiplexedBragg mirror, acting as a reflective narrow band filter and anoutput coupler, was investigated with a commercially availablebroad area stripe quantum-well InGaAs–GaAs LD in the CWregime (see Fig. 1). The laser source employed was a 150- mbroad area LD with a cavity length of 1 mm, corresponding toa Fabry–Pérot mode separation of 0.14 nm at the LD centralemission wavelength of 978.5 nm. The spectral bandwidth

in free running geometry was estimated to be 4-nmfull-width at half-maximum (Fig. 2). This had an output powerof 2 W for a dc forward bias of 3 A and a threshold currentof 420 mA at the LD operating temperature of 20 C. Thefront facet of the wide stripe LD was antireflection (AR)-coatedwith reflection below 1%. The LD was mounted on a copperheat sink ( ) and maintained at a constant temperature using athermoelectric cooler. In order to achieve optimal coupling withthe grating sample and improve stability of the external cavity,the LD output was focused into the VBG using an AR-coatedaspherical lens ( ), with a focal length of 6.2 mm and a numer-ical aperture of 0.4. This resulted in a strongly asymmetricalbeam with dimensions of about 5 740 m along the fast andslow axes of the LD, respectively. It should be noted that nodamage of the grating sample or degradation, e.g., thermal shiftof the Bragg wavelengths or changes in the spectral selectivityof the VBG, was observed during the experimental work. Theexternal cavity length was approximately 4.5 cm and waslimited only by the size of the cavity components.

The dual-wavelength laser output, with a mode separationof 3 nm (1 THz) and laser bandwidths of 21 GHz with

Fig. 2. Emission spectra of 150-�m stripe LD: free running geometry (graycurve) and two-color regime (black curve) at an injection current of 3 A, regis-tered with a spectral resolution of 0.02 nm.

SMSR of 24 dB, was observed without any additional spectralstabilization. The dual-wavelength operation was stably main-tained, with the intensity of oscillations nearly equal, when theLD emission spectra overlapped with the Bragg wavelengthsof the grating sample. This led to an approximately doubledthreshold of the two-color operation to the one observed in thefree-running geometry. The spectral characteristics of the LDin the free running geometry and dual-wavelength VBG-lockedare represented in Fig. 2. A maximum output power of 1.7 Win two-color operation regime, corresponding to 85% of totaloutput power in free running geometry, was obtained withthis arrangement, at an injection current of 3 A. The resultwas achieved with a Bragg mirror without AR coatings andan efficiency of 30% which can be improved by applyingappropriate AR coatings and further reduction of the gratingsdiffraction efficiency down to 3%–5% [20]. In comparison witha previously reported, two single-wavelength Bragg mirrorexternal cavity configuration [9], a reduction in the numberof the external cavity components and total optical loss inthe system is demonstrated by utilizing multiplexed reflectiveBragg gratings, fabricated in a single wafer, as an externalcavity output coupler.

Temperature stability of the spectral locking of the broad areaLD in dual-wavelength operation was also investigated in thesame experimental setup by adjusting the LD operation temper-ature while holding the Bragg mirror at room temperature. Thetwo-color operation of the LD remained stable in the investi-gated temperature range from 20 C to 40 C (Fig. 3), which waslimited only by the LD temperature controller used. It shouldbe mentioned that the conversion efficiency in dual-wavelengthregime remained the same, at about 85% of total output powerin free running geometry, within the examined temperature in-terval.

The intensity spectrum of the dual-wavelength laser output,depicted in Fig. 2, reveals the presence of several externalcavity longitudinal modes, with calculated mode separation of0.011 nm, packed within the lasing peaks, with bandwidths ofabout 0.08 nm. This indicates the multimode character of thetwo-color lasing regime. It is important to mention that the pre-sented two-color generation by means of the multiplexed VBGwas kept stable, with equal oscillation intensities of each modeenvelope, in the investigated LD operation temperature range.

ZOLOTOVSKAYA et al.: TWO-COLOR OUTPUT FROM InGaAs LASER WITH MULTIPLEXED REFLECTIVE BRAGG MIRROR 1095

Fig. 3. Emission spectra of 150-�m stripe LD in free running geometry anddual-wavelength VBG-locked at 20 C and 40 C, and an injection current of3 A.

It should be emphasized that no gain competition between thetwo mode packets was observed. This is attributable to the factthat in multimode two-color regime, the net gains in the modesare already compensated for by the presence of several internalmodes within each frequency packet, reducing the need for aperfect balance of the model gain differences for the two filteredcolors, as shown in [5]. Although this regime is not ideal forgeneration of high-quality THz signals, it is well suited formany applications for which incoherent or broadband radiationsources are desired, such as THz imaging or spectroscopy [21],[22]. It is our belief that the proposed optical scheme for stabledual-wavelength operation, by means of multiplexed Braggmirror, demonstrates the potential for further improvementof the two-color operation by optimization of the grating pa-rameters, e.g., thickness, diffraction efficiency. Additionally,adjustment of particular cavity modes with angular acceptanceof a Bragg mirror, will result in a compact dual-wavelengthlaser system without any restrictions on the cavity aspectratio. In addition, temperature stability of the spectral lockingdemonstrates relaxed requirements for temperature control ofthe dual-wavelength laser system. Furthermore, technologicalflexibility of the hologram formation in PTR glass matrix,allowing fabrication of multiwavelength monolithic Braggmirrors in combination with a chirped period in the transversedirection, will enable continuous difference frequency tuningby a single diffraction element, employing the whole availablenet gain region of quantum well or quantum-dot-based LDs.As has been recently reported, the direct THz emission out ofa two-color external cavity LD [7] also confirms the potentialof the proposed dual-wavelength laser system as a tunable THzradiation source.

In conclusion, an optical scheme for stable dual-wavelengthoperation from a single LD by means of multiplexed reflectiveBragg gratings, fabricated in a single wafer, was proposed anddemonstrated. A frequency difference of 1 THz and band-widths of 21 GHz, with SMSR over 24 dB and an outputpower of up to 1.7 W, corresponding to 85% of total outputpower in free running geometry, has been achieved in two-coloroperation from a broad area InGaAs LD with a monolithic dual-wavelength Bragg mirror. Temperature stability of the spectral

locking of the broad area LD in dual-wavelength operation wasdemonstrated for the LD temperatures ranging from 20 C to40 C.

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