IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 3, FEBRUARY 1, 2014 253

Multi-Wavelength Operation of a Single Broad AreaDiode Laser by Spectral Beam Combining

Christof Zink, Nils Werner, Andreas Jechow, Axel Heuer, and Ralf Menzel

Abstract— Stabilized multi-wavelength emission from a singleemitter broad area diode laser (BAL) is realized by utilizingan external cavity with a spectral beam combining architecture.Self-organized emitters that are equidistantly spaced across theslow axis are enforced by the spatially distributed wavelengthselectivity of the external cavity. This resulted in an array likenear-field emission although the BAL is physically a single emitterwithout any epitaxial sub-structuring and only one electricalcontact. Each of the self-organized emitters is operated at adifferent wavelength and the emission is multiplexed into onespatial mode with near-diffraction limited beam quality. Withthis setup, multi-line emission of 31 individual spectral linescentered around and a total spectral width of 3.6 nm is realizedwith a 1000 µm wide BAL just above threshold. To the bestof our knowledge, this is the first demonstration of such aself-organization of emitters by optical feedback utilizing aspectral beam combining architecture.

Index Terms— Laser resonators, semiconductor lasers, opticalfeedback.

I. INTRODUCTION

MULTI-WAVELENGTH lasers are used for several appli-cations in metrology like multi-wavelength interfer-

ometry [1]. The broadband emission of these devices leadsto a speckle reduction even in continuous wave emission,which is desirable in the emerging field of laser projection.Furthermore, the near-diffraction limited beam quality andhigh average output powers result in higher brightness thanthe commonly used superluminescence diodes (SLDs) [2].

Broad area diode lasers (BALs) are very efficient laser lightsources that can reach electro-optical conversion efficienciesof more than 70% [3]. Their compact size, low manufacturingcosts and broad spectral gain make them ideal devices tosetup compact multi-wavelength light sources. However, BALssuffer from poor beam quality and low frequency stability.One way to overcome these drawbacks and realize stable andtailored emission from a BAL is to operate it in an externalcavity with beam shaping optics and frequency selectiveelements [4], [5].

Recently, multi-wavelength operation of a single BAL wasdemonstrated but the different wavelengths were not emittedin the same spatial mode resulting in a non diffraction-limited

Manuscript received October 24, 2013; revised November 12, 2013;accepted November 14, 2013. Date of publication November 20, 2013; dateof current version January 9, 2014.

The authors are with the Institute of Physics and Astronomy, Universityof Potsdam, Photonics Department, Potsdam D-14476, Germany (e-mail:photonics@uni-potsdam.de).

Color versions of one or more of the figures in this letter are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LPT.2013.2291963

Fig. 1. Sketch of the spectral beam combining (SBC) external cavity setupconsisting of a broad area diode laser (BAL), a fast axis collimator (FAC),a cylindrical lens, a diffraction grating, a slit aperture, and an outcoupling(OC) mirror.

quality [6]. With a single emitter diode laser operated in aLittmann type external cavity comprising a liquid crystal arraydual wavelength operation was realized [7].

Another way to realize a multi-wavelength laser source isto utilize spectral beam combing (SBC). Usually SBC is usedfor power scaling of diode laser arrays or fiber lasers [8]–[13].In the original work by Daneu et al. [14] a diffraction gratingand a two lens f-f telescope was used to stabilize each emitterof a diode laser array at a different wavelength. The cavitywas designed to superimpose the emission from the individualemitters into one spatial mode. All previously reported SBCsetups have utilized either individual lasers or laser arrays withphysically separated emitters.

Here, we utilize a single stripe BAL in an SBC architecturefor the first time, achieving multi-wavelength emission fromone physical emitter. Our results show that multiple individualand physically separated emitters are not required for this typeof SBC architecture and that an array structure can be realizedby optical feedback, only.

II. EXPERIMENTAL SETUP

The experimental setup is shown in Fig. 1. The cavityconsists of a fast axis collimator (FAC), a cylindrical lensin slow axis, a diffraction grating, a mirror, a slit apertureand an outcoupling mirror. The BAL (manufactured by AxcelPhotonics) has a width of 1000 µm (slow-axis) and a heightof about 1 µm (fast-axis) and did not possess an anti-reflection(AR) coating at the front facet. The BAL had only oneelectrical contact and no additional gain guiding or waveguiding substructure along the slow axis.

The emission of the BAL in the fast-axis is collimated bythe FAC which is a cylindrical lens with a focal length of

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254 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 3, FEBRUARY 1, 2014

f = 0.9 mm and a high numerical aperture N A = 0.8. Thewaveguide structure in this axis allows only one transversalmode, therefore, no beam shaping is necessary in fast-axisdirection and the diffraction-limited beam quality is just con-served.

The slow-axis emission of the BAL, which had a top-hatlike transversal profile below threshold, is collimated with acylindrical lens with f = 40 mm which is placed at a distanceof the focal length f from the BAL and also with the distancef in front of the diffraction grating. By this lens the spatialdistribution of the emission at the front facet of the BAL istransformed into an angular distribution at the grating. Thisangular distribution is related to a wavelength distribution andtherefore, each spatial position at the front facet of the BALis linked to a specific wavelength.

The grating with a line density of 2400 mm−1 is placed withthe grooves perpendicular to the slow-axis under the Littrowangle αlit t so that the diffracted light passes the cylindrical lensagain. However, the grating is slightly tilted at an angle βt ilt inthe fast-axis direction to realize a quasi Littrow arrangementas described in [15]. This geometry requires a highly reflectivemirror to redirect the light towards the outcoupling mirror.

The second pass through the slow-axis lens transforms theangular distribution at the diffraction grating into a spatialdistribution at the outcoupling mirror which is placed at adistance of f from the slow axis lens. The slit aperture witha width of 25 µm at the outcoupling mirror defines the widthof the emitted beam and selects the transversal mode.

Thus, the light reflected by the outcoupling mirror (R =50%) has only a narrow spatial distribution. When passingthrough the lens again the reflected light will consist ofmultiple wavelengths but only one spatial mode is incidentat one specific angle at the grating.

Due to the angular wavelength dependency of the grating,the fourth pass through the slow axis lens will lead to a spatialseparation of the individual wavelength components at theBAL front facet. Therefore, each lateral position of the BAL islinked to a specific wavelength. Due to the f-f telescope eachspatial mode incident at the BAL is defined by the width ofthe aperture.

III. EXPERIMENTAL RESULTS

In Fig. 2 a) the spectrum of the BAL emission at a pump cur-rent of 1.1 × Ith,S BC (Ith,S BC = 6.0 A) with the SBC externalcavity is shown. The spectrum possesses a comb-like structureof 31 nearly equidistant wavelengths. The bandwidth of thewhole spectral distribution is 3.6 nm with an average peak topeak distance of 0.1 nm. The corresponding near field intensitydistribution is plotted in Fig. 2 b). Near threshold both distribu-tions, the spectrum and the near field intensity have a matchingcomb-like structure. Thus, each wavelength is emitted from aspecific position along the slow axis of the device. In the nearfield intensity distribution the peak width corresponds to theaperture width of 25 µm as shown in Fig. 4 a).

In Fig. 2 the spectrum c) and the near field intensitydistribution d) of the free running laser diode just abovethreshold 1.1 × Ith, f ree (Ith, f ree = 7.5 A) is shown. The near

Fig. 2. Spectral (red) and near field (black) intensity distribution of theBAL emission just above threshold current (1.1 × Ith,SBC ) for a) and b) theBAL operated in the external SBC cavity and c) and d) the free running BAL(1.1 × Ith, f ree ).

field intensity distribution has no intrinsic structure in contrastto the near field of the SBC setup. The spectrum is irregularand rather narrow by comparison with the spectra of Fig. 2 a).No correlation between near field intensity distribution andspectral distribution is perceptible.

In Fig. 3 both the near field intensity distribution and thespectrum of the BAL emission at a pump current of 10 Aare plotted for SBC setup a) and b) and for the free runninglaser c) and d). The device itself has an intrinsic asymmetryas well as some defective areas (at near field positions 0.1,0.5, and 0.7 mm) due to front facet damages, which becomesclearly evident in both the spectrum and near field. Even at thehigher pump current, the improved and more homogeneousnear field and the broad spectral distributions remain stillprominent while the free running device has no correlationbetween spectrum and near field emission.

We attribute this behavior to the fact that after some roundtrips in the cavity stable self-organized emitters are enforcedacross the slow axis of the BAL although it is only a singleemitter device. The self-organization process is complex andrequires further study. We present here a possible simpleexplanation.

Below threshold, the BAL emits amplified spontaneousemission (ASE) with a top-hat like spatial profile. Due to theSBC cavity design described above the ASE reflected backinto the BAL after one roundtrip in the cavity will still have atop-hat like spatial profile but possess a wavelength distribu-tion where only a specific wavelength λi will be incident at agiven spatial position xi of the BAL.

ZINK et al.: MULTI-WAVELENGTH OPERATION OF A SINGLE BAL 255

Fig. 3. Spectral (red) and near field intensity (black) distribution of the BALemission at a pump current of (10 A) for a) and b) the BAL operated in theexternal SBC cavity and c) and d) the free running BAL.

Just above threshold, lasing operation will start at somespatial position xn at the front facet linked to a specificwavelength λn . Thus, some portion of the gain is removedaround the spatial position centered at xn corresponding toλn with a width �λ given by the SBC cavity design. If thisradiation propagates in the SBC cavity a spatial filtering at theoutcoupling mirror will occur by the aperture.

However, radiation that is incident at spatial positions adja-cent to xn that have a spatial overlap with the mode at xn

will experience loss. Therefore, radiation incident at spatialpositions xm having no or only very small overlap with themode xn will have no loss and can be amplified.

The SBC cavity is designed in a way, that all radiation thatdoes not have the wavelength λi linked to xi will not be fedback to the BAL at all. The interplay between the spatiallydistributed hole burning process and the wavelength selectivityof the SBC cavity will lead to the self-organized formation ofthe emitters.

At higher pump currents however, the internal resonator willstart to compete with the external cavity and modes disallowedby the SBC cavity will start to lase internally but will not befed back to the BAL due to the SBC design.

To validate the operation of these induced emitters in anSBC like behavior, spectrally resolved near field intensitydistributions were captured.

In Fig. 4 the spectrally resolved near field for the SBCsetup (a, c) and for the free running laser (b, d) are shownjust above threshold (a, b) and at a pump current of 10 A(c, d). In the near field distribution of the SBC setup almostall peaks are aligned on a single diagonal line at both pumpcurrents. Thus, each peak in the near field has only a single

Fig. 4. Spectrally resolved near field intensity distribution of the BALemission with (a, c) and without (c, d) external cavity at 1.1 × Ith thresholdcurrent (a, b) and at a pump current of 10 A (c, d).

corresponding peak in the spectrum. This behavior is a clearevidence for the correlation of near field and spectrum dueto the enforcement by the external SBC cavity. Especiallyjust above threshold (Fig. 4 a)) the self-organized emitterspossessing an individual wavelength are clearly resolved. Evenat higher pump current (Fig. 4 c)) when the internal resonatorcompetes against the external resonator there is still a definedspectral distribution visible with only a slight deviation at adefect of the front facet at near field position of 0.1 mm.

256 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 3, FEBRUARY 1, 2014

Fig. 5. Spectral bandwidth as a function of pump current for the free runningBAL and the external cavity stabilized SBC setup.

Without feedback, no such correlation between spectrum andnear field is present. Therefore, the free running BAL emitsdifferent wavelengths from all or rather random positions ofthe front facet as expected.

Fig. 5 shows the total spectral bandwidth of the BALemission with and without external cavity as a function ofthe pump current. The spectral bandwidth of the SBC setupis defined by the parameters of the used components andis constant ≈ 3.6 nm over a wide range of pump currents.In contrast, irregular multi-mode emission and mode jumpbehavior was observed for the free running BAL.

The output power of the free running diode laser was 2.5 Wat 10 A. The maximum output power of the SBC setup was430 mW at the same pump current. The beam quality for theSBC setup was determined to be M2

slow < 3.0 ± 0.5 in theslow axis and M2

f ast < 1.5 ± 0.3 in the fast axis in all casesup to a pump current of 10 A. Due to the large divergenceit was not possible to measure the beam quality in the slowaxis for the free running BAL. We estimated the beam qualityby the use of the far field divergence and the emitter widthto be about M2

slow ≈ 125. Therefore, estimating an overallbrightness improvement by a factor of 7.

IV. CONCLUSION

We have achieved multi-wavelength operation of a singleemitter BAL by using an SBC external cavity setup. Nearthreshold both the near field intensity and the spectral distribu-tion show a distinct comb-like profile with a strong correlationbetween spectral and spatial distribution. Self-organized emit-ters enforced by the external cavity were realized by opticalfeedback and lead to an array like emission pattern from asingle emitter device. Although the comb-like structure in thespectrum and the near field intensity distribution smoothesout slightly at higher pump currents, the correlation betweenspectral and spatial distribution remains evident even at highpump currents. No such correlation was observed for thefree running BAL. Thus, compared to the free running BALfilamentation and transversal mode structure are stabilized.

To our knowledge, this is the first time that an SBCarchitecture is realized with a single emitter. Our results show,

that the use of multiple individual lasers or a laser array isnot a requirement for SBC. We have to point out that inour experiment stabilized operation was even possible witha standard BAL without proper AR-coating.

As with other SBC setups we expect, that the spectralbandwidth can be tailored by reducing or increasing the linedensity of the refraction grating or changing the focal lengthof the slow axis lens. This can also lead to a broader orrespectively smaller peak to peak spacing of the wavelengthcomb depending on the application. Furthermore a smaller slitaperture width could lead to a better slow axis beam quality.

The good beam quality in combination with broad and stablespectral emission make our setup a good alternative to SLDscommonly used for applications that require reduced spectralor temporal coherence with a good beam quality.

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