2000 J. Opt. Soc. Am. B/Vol. 15, No. 7 /July 1998 Lo”bel et al.

Tunable single-mode operation of a high-powerlaser-diode array by use of an external

cavity with a grating and a photorefractivephase-conjugate mirror

Martin Lo/bel, Paul M. Petersen, and Per M. Johansen

Riso” National Laboratory, Department of Optics and Fluid Dynamics, DK-4000 Roskilde, Denmark

Received September 11, 1997; revised manuscript received December 19, 1997

We demonstrate a technique for tunable single-mode operation of high-power laser-diode arrays. A spatiallyfiltered part of the far field from a gain-guided GaAlAs laser-diode array is directed to a diffractive grating andis coupled to a phase-conjugating self-pumped barium titanate crystal. The interaction among the grating,the dynamic gratings in the photorefractive crystal, and the spatial filter forces the laser-diode array with poorspatial and temporal coherence to oscillate in a single spatial and single longitudinal mode. At a drive currentof two times the threshold, the array operates in a single-lobed far-field pattern that is only 1.5 times the dif-fraction limit. The bandwidth of the enhanced output beam is measured to be less than 0.03 nm, and thecoherence length of the output is increased 45 times, to at least 16 mm. Once single-mode operation has beenobtained, one can continuously tune the frequency over a range of 5 nm around a center wavelength of 811 nmby tilting the grating. More than 50% of the radiated energy of the free-running laser is coupled out of thesystem. The significantly enhanced coherence properties of the output may lead to a number of new or im-proved applications in areas such as second-harmonic generation, coupling into single-mode fiber, and pump-ing of solid-state lasers. © 1998 Optical Society of America[S0740-3224(98)02507-7]

OCIS codes: 190.5040, 140.2010, 190.5330, 140.3570, 140.3520, 050.1950.

1. INTRODUCTIONSemiconductor laser-diode arrays can produce impressiveamounts of optical power, and they are attractive becauseof their compactness and simplicity of operation. Thesehigh-power sources, however, suffer from a non-diffraction-limited radiation pattern with low spatial co-herence and an optical bandwidth of 1–2 nm, which yieldsa very low coherence length of a few hundred microme-ters. The poor coherence characteristics of these devicesmake them ill suited for applications such as the launch-ing of light into single-mode waveguides, as a pumpingsource for other laser systems, as lasers for material pro-cessing and for second-harmonic generation. In an at-tempt to enhance the output characteristics, differenttechniques such as injection locking with an externalsource,1–4 with an external cavity,5,6 and with direct opti-cal feedback from a diffractive grating7,8 have been ap-plied. Many of the injection-locking schemes with an ex-ternal source provide an almost diffraction-limited single-lobed far-field pattern from gain-guided diode arrays, butthe extra laser adds complexity to these schemes. Otherexternal-cavity configurations have demonstrated anearly diffraction-limited output by use of spatial filteringto discriminate the many spatial modes of the arrays.9,10

Laser arrays can be forced to operate at a single spatialmode by means of geometric feedback methods that pro-duce a nearly diffraction-limited output pattern. In gen-eral, however, several longitudinal modes are oscillatingsimultaneously. The longitudinal-mode spacing of the

0740-3224/98/072000-06$15.00 ©

arrays is of the order of 0.1 nm, and, consequently, theoutput will always have a short coherence length unlesssingle-mode oscillation can be achieved. Single-longitudinal-mode operation of a GaAlAs array with adrive current below threshold has been demonstrated pre-viously by coupling it to an external cavity formed by adiffractive grating and a photorefractive phase-conjugatemirror (PCM).11 This system has proved to be widelytunable with respect to the frequency owing to the pres-ence of the diffractive grating. If operated far abovethreshold, this system of coupled resonators is in generalnonstable because of nonlinear processes in the photore-fractive PCM, and self-induced frequency scanning cantake place.12–14 An exact understanding of the mecha-nisms of the processes that lead to the frequency scanningis yet to be gained. Experiments have shown that theorigin of the mechanisms may be the material frequencydispersion of the photorefractive crystal itself and thatthe frequency scanning can be avoided if the dispersion iscompensated by a prism or a grating placed between thelaser and the PCM.12 To yield an even more stable sys-tem, however, only one half of the far field must be ret-roreflected back into the array so that it behaves like aninjection locked amplifier.1,9 With this technique an al-most diffraction-limited output from an array has beendemonstrated with the use of a photorefractive PCM.15

This type of mirror offers an ideal way to feed light backinto an array; it has a large field of view and is self-aligning. Despite the diffraction-limited output profile

1998 Optical Society of America

Lo”bel et al. Vol. 15, No. 7 /July 1998 /J. Opt. Soc. Am. B 2001

that results from this technique, the coherence length isin general still low. This technique was also applied to alaser bar, which is a number of arrays placed next to oneanother, and a narrow spectral output was obtained.16

In that case the narrow spectrum is due to gratingsshared among the radiation wavelengths from the indi-vidual arrays produced by mutually self-pumped phaseconjugation (as well as by the self-pumped phase conjuga-tion of the individual arrays themselves), which eventu-ally leads to a single shared wavelength.17,18

In the present paper we demonstrate how an array oflasers can be forced to oscillate simultaneously in a singlespatial and longitudinal mode. We couple the array withan external adaptive photorefractive PCM. We reportwhat are the first, to our knowledge, experiments of tun-able single-mode operation of a laser array driven with adrive current far above threshold. The coherence lengthof the phase-locked output is increased significantly.Only one of the two lobes of the far field is directed to thePCM. Furthermore, a spatial filter is included in the ex-ternal cavity, allowing only a limited number of spatialmodes to interact with the adaptive PCM. Before theoutput beam from the array reaches the PCM it is dif-fracted in a grating. The angular dispersion of the grat-ing makes the feedback system much more frequency se-lective and forces the array to oscillate in one single-longitudinal mode. The interaction among the array, thedynamic gratings of the adaptive PCM, the spatial filter,and the diffractive grating permits phase locking of thelaser array and causes all the radiated energy to be trans-ferred into only a single mode. This technique allows alarge fraction of the radiated energy to be extracted fromthe laser system with a single-lobed far-field pattern thatis close to diffraction limited. One can tune the centerfrequency of the output of the array more than 5 nm bytilting the grating.

The structure of the paper is as follows. Section 2deals with the experimental setup, Section 3 contains apresentation and a discussion of the results obtained, andSection 4 is the conclusion and summarizes the results.

2. EXPERIMENTThe experimental setup is shown in Fig. 1. The laser ar-ray is a GaAlAs 10-stripe proton-implanted gain-guidedlaser array (model SDL-2432), also known as a broad-area diode laser, with a 100-mm-wide emitting junction.It is temperature controlled with a peltier element. Thearray has a threshold of 0.28 A and a maximum outputpower of 0.5 W at 0.9 A (3.2 3 Ith). The center wave-length, l0 , of the multimode spectrum at 13 °C is 811 nm.The longitudinal-mode spacing is 0.11 nm. When the ar-ray runs freely a band of many spatial modes is oscillat-ing at several longitudinal modes. At a drive current of3 3 Ith the spectrum has a full width at half-maximum(FWHM) of 1.2 nm corresponding to approximately 10longitudinal modes. The output beam of the array is col-limated with a lens (L1), and a second lens (L2) generatesa pseudo-far-field at a distance of 400 mm from the array.The collimated light is focused with a lens (L3) that gen-erates a 2-mm spot at the front face of the PCM made upof a 0°-cut rhodium-doped (800 parts in 106) BaTiO3 crys-tal in which a phase-conjugation process takes place andreturns the phase-conjugate wave front toward the array.The crystal is cut along its crystallographic axes andmeasures 5.11 mm 3 5.53 mm 3 5.33 mm (a 3 a 3 caxes). The crystal is arranged in a self-pumped cat con-figuration with an angle of incidence of 60°.19 To yieldthe largest photorefractive response both the coherenceaxis,15 the polarization, and the crystal c axis must lie inthe same plane. This requirement can be accomplished ifwe rotate the beam polarization through 90° while keep-ing the coherence axis fixed. We do this by rotating thearray 90° around its optical axis and inserting an half-wavelength wave plate in the path between the array andthe PCM. A 2° wedge is used as beam splitter. The tworeflections are used for beam diagnostics: One is formonitoring the pseudo-far-field generated with a lens; thesecond reflection is directed to a spectrometer with a reso-lution of 0.02 nm. A coupling loss of approximately 20%occurs between the array and the collimating lens (L1),and an additional 10% is lost at the beam splitter, the

Fig. 1. Experimental setup. A gain-guided GaAlAs diode array is coupled to a PCM. L1, collimating lens (Thorlab, Newton, N.J.,model C230TM-B): N.A., 0.55; f1 5 4.5 mm. L2, plano-convex cylindrical lens; f2 5 60 mm. L3, spherical singlet lens; f35 100 mm. BS, beam splitter (2° wedge). WP, l/2 wave plate (zero-order) at 815 nm. G, 1200 lines/mm ruled grating with blazeangle of 26.4 deg (750 nm). SF, spatial filter (two razorblades mounted on translation stages). P, indication of polarization. Thecomponents in the dashed box shall be rotated 90 deg around the optical axis of the array.

2002 J. Opt. Soc. Am. B/Vol. 15, No. 7 /July 1998 Lo”bel et al.

wave plate, and the lenses. The external cavity has around-trip length of two times 995 mm. As previouslymentioned, the nature of the phase-conjugate feedbackmechanism of the BaTiO3 crystal can cause self-inducedfrequency scanning of the array. To avoid this scanningand to obtain a more stable configuration, only one of thetwo far-field lobes is directed to the PCM; a mirror isplaced at the position of the generated pseudo-far-field topick out one half of the radiated far-field pattern. Thebeam reflected off this mirror is the output beam of thesystem. The lobe that is directed to the PCM is dif-fracted in a ruled grating with 1200 lines/mm before it en-ters the BaTiO3 crystal. The angular orientation of thegrating is controlled by a piezo element. When coupledto a self-pumped PCM, this grating adds angular disper-sion to the system and has a strong influence on the dy-namic behavior and optical bandwidth of a multimodelaser-diode array.12 The interaction between the angulardispersion of the grating and the dynamic gratingsformed in the BaTiO3 crystal makes the external feedbacksystem much more frequency selective. The differentspatial modes of the array radiate its far-field lobes inslightly different angles with respect to the normal of theoutput facet, and they can be discriminated by a spatialfilter (SF) placed in the plane of the generatedpseudo-far-field.20 The SF consists of two razorbladesmounted on translation stages, and by adjusting their po-sitions one can control the number of spatial modes thatcan interact with the PCM.

3. RESULTSAfter the crystal is illuminated the reflectivity of the PCMbuilds up, and a significant narrowing of the spectrum en-sues. The far field changes from a broad radiation pat-tern to the well-known pattern with two lobes.21 As timeelapses the reflectivity increases further, and the inten-sity of the output lobe increases at the expense of the in-tensity of the lobe that is directed to the PCM. Often,however, it is necessary to open the SF slightly so thatmore energy will reach the PCM to start the self-pumpedconjugator; after typically 10 s the distance between thetwo razorblades can then be diminished again. The re-flectivity of the PCM is typically 10–12% (not correctedfor Fresnel reflections) in steady state. The phase-conjugate feedback will often force the phase-locked spec-trum to shift up to 1 nm toward the red with respect to l0before it reaches an equilibrium with stable single-modeoutput. At higher drive currents the shift can be asmuch as 2 nm. This shift is a part of the self-induced fre-quency scanning. As previously described, one avoidsthe constant change of frequency by feeding back only onelobe of the far field. For this configuration the PCM iscoupled only weakly to the array that behaves like aninjection-locked amplifier. We believe that, as the phase-locked spectrum starts to shift, the system will reach anequilibrium owing to the material frequency dispersion ofthe BaTiO3 crystal. The fact that a narrow spectrum andeven single-mode operation can be obtained is due to theangular dispersion of the grating, which disperses thebeam inside the crystal and enhances this process.

Figure 2 shows the optical spectrum of the output beam

of the array. The spectrum when the array runs freely isdisplayed in Fig. 2(a). At a drive current of 2 3 I th sev-eral longitudinal modes are present, and the FWHMbandwidth is 0.7 nm. The spectrum of the output beamat the same drive current when the feedback is appliedfor different tilts of the grating is shown in Figs. 2(b)–2(e). Once single-mode operation has been obtained, onecan tune the frequency by tilting the grating. Betweenthe recording of Figs. 2(b) and 2(e) the grating is tilted0.43°, and the sensitivity is 12 nm/deg. A change of thetilt of the grating results in a change of the point of inci-dence and angle of incidence at the air–crystal interface.If the grating is tilted, the array will optimize the oscil-lating frequency for the best Bragg match to the existinggratings in the crystal and thereby again obtain high re-flectivity from the PCM. By tilting the grating one tunesthe frequency by discrete steps corresponding to alongitudinal-mode spacing of the array (0.11 nm); how-ever, two longitudinal modes can oscillate simulta-neously, since the energy from one longitudinal mode isslowly transferred to the next mode as the grating istilted. When the tilting of the grating stops, the adaptivePCM will adjust and optimize for highest reflectivity, andsingle-mode operation will, within seconds, again be ob-tained. For fixed grating position one can continuouslytune the frequency over a range of 0.1 nm correspondingto one longitudinal mode spacing by changing the tem-perature of the junction of the array less than 1 °C. Byadjusting the grating and the temperature of the junctionone can achieve any wavelength within the 5-nm range.The gain bandwidth of the GaAlAs array is huge, so thelimited wavelength range may be explained as follows:As the grating is tilted, the beam is transversely shiftedout of the regions that have gratings inside the crystal,and this leads to a reduced reflectivity and consequently alimited scanning range. For a fixed grating tilt and a

Fig. 2. Optical spectrum at I 5 0.55 A (2 3 Ith). (a) With thearray running freely, (b) with feedback applied. Angle of inci-dence u at the grating, 19.73 deg; bandwidth (FWHM), 0.03 nm.(c) Same as (b), but u 5 19.60 deg; (d) same as (c), but u5 19.47 deg; (e) same as (d), but u 5 19.30 deg.

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drive current of 2 3 Ith the FWHM bandwidth of thesingle-mode spectrum is measured to be less than 0.03nm. We verified that the array has only one lasing spa-tial mode by recording the frequency-resolved near-fieldpattern with a setup similar to one publishedpreviously.20 This verifies true single-mode operation.

The frequency can be automatically tuned by applica-tion of a sawtooth modulation voltage to the piezo ele-ment. At a drive current of up to 2 3 Ith for the laser ar-ray and a modulation frequency for the piezo element ofless than 0.2 Hz the phase-locked spectrum scanssmoothly back and forth in a single mode that transfersits energy to the next mode as described previously.During the scanning the reflectivity of the PCM decreases10–20% as compared with the case in which the grating isstationary. If the modulation frequency is higher than0.5 Hz, the bandwidth of the phase-locked spectrum willincrease to more than 0.3 nm, and at 5–10 Hz the band-width is the same as it is when the array runs freely. Ifthe modulation frequency is larger than approximately0.2 Hz, the reflectivity of the PCM will continuously fall tozero as time elapses. The response time for the BaTiO3crystal is of the order of 1 s, and only for very low modu-lation frequencies (,0.2 Hz) can the crystal respond tothe changes of the incident beam and thereby maintainhigh phase-conjugate reflectivity.

The far field of the array when it runs freely at a drivecurrent of 2 3 Ith is displayed in Fig. 3. The far field of

Fig. 3. Measured far field from array (measured in the plane ofthe junction, all curves on the same scale). (a) With the arrayrunning freely at I 5 0.55 A (2 3 Ith); (b) with feedback appliedat I 5 0.55 A (2 3 Ith); (c) same as (b), but drive current I5 0.83 A (3 3 Ith).

the array when the feedback is applied at a drive currentof 2 3 Ith and 3 3 Ith , respectively, is shown in Figs. 3(b)and 3(c). The radiation angle is measured between thenormal of the output facet of the array and the radiationdirection in the plane of the junction (vertical plane). Inthe plane perpendicular to the junction (horizontal plane)the far field is close to the diffraction limit and has aGaussian shape that is unaffected by the number of oscil-lating spatial modes. It is the negative angle lobe that isdirected to the PCM and then fed back to the array. Theoutput peaks are centered at 12.0° and 12.1° for 23 Ith and 3 3 Ith , respectively. The most efficient per-formance of the system is always with the output beamradiated at 2–2.1°. The edges of the two razorbladesforming the SF are positioned at 21.8° and 22.4° and al-low only radiated energy between the razor blades toreach the PCM. With this setting maximum two spatialmodes are transmitted through the filter. For a 100-mm-wide junction of an array lasing at an 811-nm wavelengththe diffraction limit is 0.55°.9 The FWHM of the outputbeam (positive angle) is 0.84° (2 3 Ith), corresponding to1.5 times the diffraction limit. As can be seen in Fig.3(c), the main peak is still narrow as the drive current isincreased to 3 3 Ith ; however, additional spatial modesstart to emerge and increase the bandwidth to 0.1 nm.

The total radiated energy from the array is increasedby approximately 3% with the phase-conjugate feedbackapplied, as compared with the case in which the arrayruns freely. With the configuration shown in Fig. 1 alarge fraction of the radiated energy may be contained inthe output beam that is nearly diffraction limited if cou-pling loss and transmission loss at the lenses, etc. areeliminated. The power measured in the output beam is110 and 220 mW for drive currents of 2 3 Ith and 33 Ith , respectively. When the array runs freely the to-tal power after the cylindrical lens (L2) is 150 and 320mW for 2 3 Ith and 3 3 Ith , respectively. At 2 3 Ith and3 3 Ith and when the phase-conjugate feedback is ap-plied, the total radiated energy from the array is 222 and465 mW, respectively, i.e., 70% of the available energy af-ter lens L2 is contained in the output beam, and 50% ofthe total radiated energy (before lens L1) is contained inthe output beam. The amount of phase-conjugate feed-back measured at the beam splitter is typically 0.4–0.7%(highest at lower drive current). The amount of powerthat is fed back into the array is estimated to be 0.5–1.4mW (highest at lower current). That is an overall feed-back level of 0.1–0.6%.

The improved coherence of the output from the array ismeasured by a standard Michelson interferometer con-sisting of two mirrors (one mounted on a translationstage) and a beam splitter. The reference arm has afixed round-trip length of two times 110 mm. The visibil-ity of approximately five fringes of the interference pat-tern is recorded by a 25-mm-wide photodetector arraywith 2048 pixels. The coherence degree is taken in theform g 5 @(Imax 2 Imin)/(Imax 1 Imin)#@(I1 1 I2)/(2AI1I2)#,where Imax and Imin are the maximum and minimum in-tensity in the interference pattern, respectively. I1 is theintensity generated at the detector with the mirror in arm2 covered, and I2 is the intensity with the mirror in arm 1covered. Figure 4 displays the coherence degree versus

2004 J. Opt. Soc. Am. B/Vol. 15, No. 7 /July 1998 Lo”bel et al.

the path difference of the two arms in the interferometer.By comparison of the path difference for the case in whichthe array runs freely and the case in which feedback isapplied, it is observed that the coherence length of the ar-ray with the phase-conjugate feedback is increased by afactor of 45 to at least 16 mm for a drive current of 23 Ith . At a drive current of 3 3 Ith the coherencelength is increased by a factor of 28.

Once the PCM has been turned on and single-mode op-eration has been obtained, the output becomes very stablewith respect to frequency and output power. Oneachieves the highest stability if the wavelength is shifted1–2 nm toward the red with respect to the center wave-length l0 of the multimode spectrum by tilting the grat-ing. At a drive current of 2 3 Ith the power and thewavelength of the output beam are recorded continuouslyover 3 h of operation. The standard deviations of the de-tected power and wavelength are 0.7% and 0.01 nm (reso-lution limited), respectively.

4. CONCLUSIONA technique that uses a phase-conjugate mirror and a dif-fraction grating to enhance the spatial and temporal co-herence of high-power laser-diode arrays has been dem-onstrated. For a drive current of 2 3 Ith single-modeoperation is achieved. The spectral bandwidth is re-duced to less than 0.03 nm, the coherence length is in-creased by a factor of 45 to at least 16 mm, and the farfield is narrowed to as low as 1.5 times the diffractionlimit. One can tune the frequency of the output over arange of 5 nm by tilting the grating. Even at almostmaximum drive current (3 3 Ith) the coherence length isstill increased by a factor of 28. If a better antireflectioncoating is applied to the output facet of the array, an evengreater coherence length may be possible at high drivecurrent. More than 50% of the radiated energy from the

Fig. 4. Coherence degree versus the path difference of the armsin a standard Michelson interferometer.

array is extracted in the experimental setup, but a muchlarger fraction can be extracted with an optimized setup.The output beam of the configuration is very stable withrespect to center frequency and optical output power.Over a period of 3 h of operation the wavelength and theoutput power vary less than 0.01 nm and 0.7%, respec-tively.

ACKNOWLEDGMENTSM. Lo”bel thanks the Danish Research Academy for finan-cial support in relation to a Ph.D. dissertation (grant 95-0058-SAM), and P. M. Petersen acknowledges the supportprovided by the Danish Technical Research Council(grant 9502694).

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