35 mW CW single-frequency injectionlaser with an external dispersive cavity

V. Yu. Bazhenov, A.P. Bogatov, P.G. Eliseev, O.G. Okhotnikov, G.T. Pak,M.P. Rakhvalsky, M.S. Soskin, V.B. Taranenko, K.A. Khairetdinov

Indexing terms: Semiconductor lasers, Lasers

Abstract: Using a highly selective external cavity for a TS laser, the oscillation in a single longitudinal mode ofthe external cavity, with output power of up to 35 mW, was obtained.

1 Introduction

The use of an external dispersive cavity in injection lasersallows one to achieve single-longitudinal-mode oscillationof the external cavity, i.e. single-frequency operation [1-5].The laser linewidth measured by the optical heterodynedetection [5, 6] was found to be not larger than a few kHz.These data were later confirmed in References 7-10; butthe output power in those works was as low as 1-4 mW ina single-frequency operation regime.

In References 2 and 3 it was shown that the outputpower enhancement in a single-frequency regime waslimited by nonstationary laser oscillations which appearwith increasing pumping current. Starting from some criti-cal value of the current, low-frequency pulsations appearin the output of a laser with frequencies between 1 and100 MHz, that is one to two orders of magnitude lowerthan the resonance frequencies of the external cavity [11-13]. As a result, a great number of external cavity modesare excited. The linewidth of each of them is significantlybroadened, due to a chirp of longitudinal mode resonancesat changing electron concentration (light intensity in theactive region). Such behaviour, typical of lasers with a low-selective external cavity utilising a reflective grating or aplane mirror, has also been observed in References 8-10and 14. In Reference 13 it was shown that, using highlyselective dispersive elements, such as a holographic selectoror a diffraction grating in combination with a Fabry-Perotetalon, in the external cavity of the laser, one can suppresslow-frequency pulsations and achieve single-frequencyoperation with an output intensity of up to 13 mW. In thatwork a conventional GaAlAs planar stripe diode was usedas an active element. The output power of the laser diodewas saturated at 13 mW due to the kink. This limited thepower of single-frequency operation.

In this paper we investigate a terraced-substrate (TS)GaAs-(GaAl)As diode [16] with an external highly selec-tive dispersive cavity.

TS diodes have noticeable optical confinement, and thelateral distribution of the electromagnetic field is stabilisedmuch better than, for example, in planar stripe diodes. TheTS diodes, therefore, are of interest as active elements,where the threshold of transverse field instability is shiftedto larger intensities. Therefore, by using these diodes in anexternal dispersive cavity, one can obtain much higherpowers of a single-frequency operation than with conven-tional planar stripe laser diodes.

Paper 3609J (E13, E3), first received 12 July and in revised form 30th October 1984Mr. Bazhenov and Prof. Taranenko are with the Institute of Physics, UkranianAcademy of Sciences, Kiev, USSR. Dr. Bogatov, Dr. Eliseev, Prof. Okhotnikov, Dr.Pak, Mr. Rakhvalsky, Dr. Soskin and Mr. Khairetdinov are with the P.N. LebedevPhysical Institute, USSR Academy of Sciences, Kiev, USSR

2 Experimental procedure

The TS laser diodes were produced by a liquid-phaseepitaxy on an n-GaAs substrate having terraces with rec-tangular ridges (~2 ^m high) formed by chemical etching.The stripe contacts, nearly 7 /an wide, were made justabove the bend region of the active layer. The thickness ofthe active layer was ~0.3 jtim, and the diodes were 250 fimlong. All the experiments were carried out at room tem-perature in a CW operation regime. Typical threshold cur-rents for the laser oscillation in solitary diodes were about150 mA.

A scheme of the studied laser is given in Fig. 1A. As acollimating objective we used an objective with an NA of

Fig. 1A Block diagram of a laser with external dispersive cavity

0.5 and a focal length of 20 mm; the cavity was 10 cmlong. An external mirror was a holographic selector [17]with a 3100 grooves/mm phase grating and ~70% effi-ciency. The total power feedback ratio (with regard to thediode coupling losses) from the holographic selector isestimated from the optical parameters to be ~0 .1 . Theexternal cavity selectivity is estimated as 10 GHz, that isan order better than in the case of a conventional planereflection grating used as an external mirror. The radiationspectrum was measured by a 30 cm grating diffractionmonochromator and 2 and 15 cm base-scanning confocalinterferometers, as well as a 0.4 cm base-scanning FP inter-ferometer. The output power was measured by a calibratedsilicon photodiode in a parallel beam formed by an outputobjective with 0.5 NA.

3 Experimental

Single-frequency oscillation was achieved by an optimaladjustment of an external cavity (from the viewpoint of thehighest output intensity) at a fixed pumping current. Fig.IB illustrates the oscillation spectrum obtained by a dif-fraction spectrometer; the spectrum consists of a singlelongitudinal mode of the external cavity. This is evidentfrom an FP spectrum (Fig. 1C). Linewidth is determinedby resolution of interferometer of 18 MHz. The linewidthcan correctly be measured by the optical heterodyningmethod only, and we did not measure it. Such measure-ments were taken in Reference 17 for the laser with acavity analogous to ours, with the output power around3 mW; the value of linewidth was ~500 Hz.

Fig. 1 shows superluminescent noise modes of the diode.The intensity of the single noise superluminescent modewas measured to be not higher than 1 /iW, with the spec-tral width ~20 GHz. This corresponds to the maximumspectral density of the noise radiation of 5 x 10" 8 W/GHz,

IEE PROCEEDINGS, Vol. 132, Pt. J, No. I, FEBRUARY 1985

which is, as a rule, a few orders less than for super-luminescent noise in a single-frequency solitary laser.

The light-output/current characteristics of a laser withan external dispersive cavity in the single-frequency oper-

—HI*—resolution

x10J

(i)

JJJLJLU (ii)

7960 7980 8000

Fig. 1 B Oscillation spectra obtained with a diffraction spectrometer

(i)—lasing spectrum(ii)—low-intensity superluminescent modes

500 MHz

scanning FP spectrum

Fig. 1C High-resolution scanning FP spectrum (two orders of FP areshown)

ation regime is shown in Fig. 2A (top solid line). For eachvalue of the current the wavelength tuning was performed.But in all cases the lasing wavelength was close (with anaccuracy of up to a few intermode distances of the diode)to the free-lasing wavelength.

Fig. 2B shows, for comparison, the light-output/currentcharacteristics of a solitary laser diode (the holographicselector is blocked). The oscillation spectrum, in this case,represents a few longitudinal modes of a solitary lasercavity. From Fig. 2 it is seen that, for a given pumpingcurrent, the output intensity for single-frequency operationwith an external cavity is always higher than the oscil-lation power of a solitary laser. One can also see that thedifference in powers is not only due to the difference in theoscillation thresholds. For example, for an output power ofhigher than 10 mW, the oscillation efficiency for a solitarylaser is decreased, whereas the efficiency of the single-frequency oscillation in the external cavity is kept thesame.

For an output power of less than 10 mW, the structureof the far field for the external cavity oscillation is almostindependent of the cavity adjustment and coincides withthat for the solitary laser. As a rule, this structure rep-resents a single lobe of the far-field pattern. For largeroutput intensities (more than 10 mW) the far-field pattern(in the y direction parallel to the p-n junction) becomessensitive to the external mirror adjustment angle in theplane of the p-n junction (Fig. 2). The far field for thesingle-frequency operation at a power of more than10 mW, in our case, has the form of two lobes. The funda-mental lobe, which had been the only one at small powers,is added with a broad satellite lobe. With the cavity mis-adjustment, the single-frequency oscillation fails. Simulta-neously, the far-field pattern changes quite drastically, forexample its additional broad lobe might disappear. Thedashed region in Fig. 2 depicts the power of the mis-

adjusted external cavity. With an increase in the pumpingcurrent, the adjustment region, in which one could obtainthe far-field pattern for the single-frequency operation,became narrower (and for the current above 340 mA it

30

E 20

10

200 300

DC current , mAFig. 2A Light-output-current characteristics for a laser with andwithout an external dispersive cavityUpper curve—with external dispersive cavityLower curve—without external dispersive cavity

far-fieldfringes

ho-

lasing spectra

2GHz 2GHz

Fig. 2B Fragments of the far-field and oscillation spectra at points aand b on graph 2A

entirely disappeared). The maximum recorded power in asingle longitudinal external-cavity mode oscillation was35 mW (see Fig. 2).

Thus, using a terraced laser diode as the active element,we essentially increased the output intensity of the single-frequency laser oscillation with the external dispersivecavity. The previous value for the output power ofexternal-cavity lasers operating in the single-frequencyregime at 77°K was 17 mW [1]. In the present paper wedoubled that value, but for uncooled lasers.

4 Discussion and conclusion

Our observations confirmed the results of work describedin Reference 3, where the failure of single-frequency oscil-lation with increasing output power had been explained by

10 IEE PROCEEDINGS, Vol. 132, Pt. J, No. 1, FEBRUARY 1985

a transverse-mode instability in the active region of thelaser. In the present paper we observed that the single-frequency oscillation was correlated with the far-fieldpattern. At large intensities of the light flux (for anoutput power of 10 mW the intensity was higher than5 x 105 W/cm2) the single-frequency operation regimeexisted as long as the transverse field configuration couldbe maintained as optimal for inversion-light interaction inthe active volume of the diode. The transverse field dis-tribution is optimal at single-frequency operation since thelight-output/current characteristic is linear and has thelargest differential efficiency. In this case it is difficult tointerpret deformations of the transverse field distributioncaused by self-focusing. One can change the form of theself-sustained spatial field distribution by means of cavityadjustment. It is known that the laser efficiency decreases ifthe transverse field distribution is deformed in such a waythat its maximum does not coincide with the distributionmaximum of the electron concentration in the activeregion of the laser. On the contrary, the highest efficiencyobtainable in our laser means a considerable spatial over-lapping of field and inversion. This provides conditions forthe most efficient depletion of inversion by the field. So,improving the active element from the viewpoint of thetransverse-mode stabilisation, or using cylindric opticsinside the cavity, one can further increase the outputpower of the single-frequency oscillation.

Thus, to obtain high laser power in a single longitudinalmode of the external cavity, two conditions must be ful-filled:

(i) high selectivity of the external cavity (10 GHz)(ii) control for the transverse field structure inside the

active diode region.

Note that, unlike References 8-10, the external cavityfeedback efficiency in our case is rather high, about 0.1.From our observations it follows that an increase in thefeedback efficiency improves the characteristics of thesingle-frequency oscillation. The low value of the feedbackused in References 8-10 seems to be caused by the usage ofa low-selectivity external mirror (grating or mirror). Thebasic selectivity of such cavities is determined by the soli-tary diode cavity serving as an intracavity FP etalon (withrespect to the external cavity). With a decrease in the feed-back value, the selectivity of the diode cavity increases, i.e.the spectral width of the superluminescent diode modesdecreases.

The single-frequency oscillation of the laser, where thecoupling efficiency from the external mirror is low, isunstable (one can observe frequent external-cavity modejumpings). Besides, a low Q-factor of the cavity with lowcoupling efficiency leads to broadening of the oscillationlinewidth. In our case, on the contrary, rather high effi-ciency of the feedback is accompanied by an increasingstability of the single-frequency oscillation with theincreasing output power of the laser (if the transversal fielddistribution of the diode is somehow stabilised). Thisregime of the oscillation has been called 'self-stabilisationof the single-frequency operation' [18]. At 'self-stabilisation', fields are suppressed at frequencies close tothe laser mode. One can perform a continuous wavelengthtuning by changing the length of the external cavity by adistance more than an intermode interval (for example, upto six intermode intervals). As shown recently in Reference19, the oscillation linewidth at a 'self-stabilisation' oper-ation regime of the laser is not wider than 500 Hz.

Thus, rather high powers of the order of tens of milli-watts, small width of the spectral line, and a possibility of

spectral tuning make such lasers attractive for such appli-cations as nonlinear spectroscopy of super-high resolutionand coherent optical communication.

5 References

1 LUDEKE, R., and HARRIS, E.P.: Tunable GaAs laser in an exter-nal dispersive cavity', Appl. Phys. Lett., 1972, 20, pp. 499-510

2 VOUMARD, G: 'External-cavity-controlled 32 MHz narrow-bandCW GaAIAs-diode lasers', Opt. Lett., 1977, 1, pp. 61-63

3 BOGATOV, A.P., GUROV, YU. V., ELISEEV, P.G., OKHOTNI-KOV, O.G., PAK, G.T., and KHAIRETDINOV, K.A.: 'Single-frequency CW injection tunable heterolaser by an external dispersiveresonator', Sov. J. Quantum Electron., 1979, 9, (6), pp. 743-747

4 BOGATOV, A.P., GUROV, YU. V., ELISEEV, P.G., OKHOTNI-KOV, O.G., PAK, G.T., and KHAIRETDINOV, K.A.: 'Low-frequency pulsations in the output intensity and multimode operationof GaAs-AlGaAs cw diode laser coupled to an external dispersivecavity', IEE J. Solid State & Electron Devices, 1979, 3, (3), pp. 72-74

5 VELICHANSKY, V.L., ZIBROV, A.S., KARGOPOLTSEV, V.S.,MOLOCHEV, V.I., NIKITIN, V.V., SAUTENKOV, V.A., KHARI-SOV, G.G., and TYURIKOV, D A : 'About minimal linewidth ofinjection laser oscillation', Zh. Tekh. Phys. Lett., 1978, 4, pp. 1087-1090

6 BAZHENOV, V. YU., BOGATOV, A.P., GUROV, YU. V.,ELISEEV, P.G., OKHOTNIKOV, O.G., PAK, G.T., RAKH-VALSKY, M.P., SOSKIN, M.S., TARANENKO, V.B., and KHAI-RETDINOV, K.A.: 'Optical heterodyning of radiation from aninjection laser with an external dispersive cavity', Sov. J. QuantumElectron., 1980, 10, (12), pp. 1546-1547

7 SAITO, S., and YAMAMOTO, Y.: 'Direct observation of Lorentzianlineshape of semiconductor laser and linewidth reduction with exter-nal grating feedback', Electron. Lett., 1981, 17, (9), pp. 325-327

8 GALDBERG, L., TAYLOR, H.F., DANDRIDGE, A., WELLER,J.F., and MILES, R.O.: 'Spectral characteristics of semiconductorlasers with optical feedback', IEEE J., 1982, QE-18, (4), pp. 555-564

9 FAVRE, F , LE GUEN, D , and SIMON, J.C.: 'Optical feedbackeffects upon laser diode oscillation field spectrum', ibid., 1982, QE-18,(10), pp. 1712-1717

10 OLESEN, H.," SAITO, S., MUKAI, T., SAITON, T., and MIKAMI,O.: 'Solitary spectral linewidth and its reduction with external gratingfeedback for a 1.55 /an InGaAsP BH laser', J. Appl. Phys., 1983, 22,(10), pp. L664-L666

11 MORIKAWA, T., MITSUHASHI, Y., SHIMODA, J., and KOJIMA,Y.: 'Return-beam-induced oscillations in self-coupled semiconductorlasers', Electron. Lett., 1976, 12, (17), pp. 435-436

12 FISCH, CH., and VOUMARD, C : 'Self-pulsation in the outputintensity and spectrum of GaAs-AlGaAs CW diode lasers coupled toa frequency-selective external optical cavity', J. Appl. Phys., 1977, 48,(5), pp. 2083-2085

13 BACHERT, H.-J., BOGATOV, A.P., GUROV, YU. V., ELISEEV,P.G., OKHOTNIKOV, O.G., PAK, G.T., RAKHVALSKY, M.P.,and KHAIRETDINOV, K.A.: 'Radiofrequency spectra of mode beatsand fluctuations of an intensity of radiation emitted by an injectionlaser with an external resonator', Sov. J. Quant. Electron., 1981, 11, (9),pp. 1184-1187

14 MILES, R.O., DANRIDGE, A., TVETEN, A.B., TAYLOR, H.F., andGIALLORENZI, T.G.: 'Feedback-induced line broadening in CWchannel-substrate planar laser diodes', Appl. Phys. Lett., 1980, 38, (11),pp. 998-992

15 MILES, R.O., DANDRIDGE, A., TVETEN, A.B., GIALLORENZI,T.G., and TAYLOR, H.F.: 'Low-frequency noise characteristics ofchannel substrate planar GaAlAs laser diodes', ibid., 1980, 38, (11), pp.848-850

16 SUGINO, T , WADA, M., SHIMIZU, H., ITOH, K., and TERA-MOTO, I.: 'Terraced-substrate GaAs-(GaAl)As injection lasers', Appl.Phys. Lett., 1979, 34, (4), pp. 270-272

17 SOSKIN, M.S., and TARANENKO, V.B.: 'Holographic total-internal-reflection selector for tunable lasers', Sov. J. Quantum Elec-tron., 1977, 7, (2), pp. 298-302

18 BOGATOV, A.P., ELISEEV, P C , OKHOTNIKOV, O.G., RAKH-VALSKY, M.P., and KHAIRETDINOV, K.A.: interaction of modesand self-stabilization of single-frequency emission from injectionlasers', ibid., 1983, 13, (9), pp. 1221-1229

19 AKULYSHIN, A.M., BASOV, N.G., VELICHANSKY, V.L.,ZIBROV, A.S., ZVERKOV, M.V., NIKITIN, V.V., OKHOTNI-KOV, O.G., SENKOV, N.V., SAUTENKOV, V.A., TIURIKOV,D.A., and YURKIN, E.K.: 'Heterodyne determination of the width ofthe emission lines of injection lasers in the beat frequency stabilizationregime', ibid., 1983, 13, (8), pp. 1003-1005

IEE PROCEEDINGS, Vol. 132, Pt. J, No. 1, FEBRUARY 1985 11