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Modal Gain of 2.4-um InGaAsSb–AlGaAsSb Complex-Coupled
Distributed-Feedback Lasers
Gupta, J. A.; Barrios, P. J.; Lapointe, J.; Aers, G. C.; Storey, C.; Waldron, P.
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1532 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 20, OCTOBER 15, 2009
Modal Gain of 2.4-
m InGaAsSb–AlGaAsSb
Complex-Coupled Distributed-Feedback Lasers
J. A. Gupta, P. J. Barrios, J. Lapointe, G. C. Aers, C. Storey, and P. Waldron
Abstract—High-resolution spectroscopy was used to ex-amine gain characteristics of Cr-grating complex-coupled distributed-feedback (DFB) lasers near 2.4 m. The single-mode lasers contain InGaAsSb–AlGaAsSb active regions grown by molecular beam epitaxy on GaSb. Modal gain was extracted from the measured amplified spontaneous emission spectra and compared with reference Fabry–Pérot lasers. The material gain is similar in both cases, having a value near 1300 cm 1, while the internal losses are quite different. The DFBs have an additional loss, approximately equal to the lateral Cr grating coupling coef-ficient. This indicates a fundamental performance limitation for complex-coupled DFBs.
Index Terms—Distributed-feedback (DFB) lasers, semicon-ductor lasers, GaSb, InGaAsSb.
I. INTRODUCTION
S
EMICONDUCTOR lasers are ideal for sensing in the 2–4 m range where numerous industrial gases exhibit strong absorption features. For such applications it is desirable to have a single-mode laser with linewidth comparable to the target absorption line. These wavelengths are easily accessed using (Al)(In)GaAsSb laser diodes on GaSb substrates [1], [2], although single-mode operation is more challenging. Etch and regrowth distributed-feedback (DFB) laser processes are unsuitable because the high Al-content AlGaAsSb cladding layers oxidize rapidly upon exposure to the atmosphere. An alternative GaSb DFB technology was reported [3] in which lateral Cr gratings are deposited adjacent to narrow ridge waveguides (RWGs). The evanescent optical mode interacts with the periodic grating structure to produce feedback through complex coupling because of the large extinction coefficient of the metal. Devices thus fabricated have been used to measure absorption spectra of CH and NH [4]; and 3- m DFB lasers have even been demonstrated recently [5].In spite of these successful deployments of Cr-grating DFBs, few works have addressed fundamental device characteris-tics, especially below threshold. Coupling calculations have not been presented in detail, although the principles follow from coupled-wave DFB theory applied to mixed index- and loss-coupled gratings. Detailed spectroscopic measurements have generally focused on the above-threshold region most relevant for applications. Amplified spontaneous emission (ASE) spectra for 2.4- m DFBs were discussed in [6], but the measurements used a device with very limited DFB modulation
Manuscript received May 29, 2009; revised July 23, 2009. First published August 18, 2009; current version published September 30, 2009.
The authors are with the Institute for Microstructural Sciences, National Research Council of Canada, Ottawa, K1A 0R6, Canada (e-mail: james. [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]).
Digital Object Identifier 10.1109/LPT.2009.2029244
effects. In this letter, we report a detailed modal gain analysis of GaSb-based complex-coupled DFBs. The gain was extracted from the ASE spectra measured below threshold for a DFB laser and a reference Fabry–Pérot (FP) laser. The results illus-trate the strong coupling effects and the nature of the complex coupling mechanism.
II. DEVICESTRUCTURE ANDFABRICATION
The epiwafers were grown on (100) GaSb : Te substrates in a V90 molecular beam epitaxy system. The laser structure uses 2- m-thick Te- and Be-doped Al Ga As Sb claddings with digital-alloy grading between the cladding layers and the 0.5- m GaSb : Te buffer ( cm ) and the top 0.15 m GaSb : Be ( cm ) contact. The n-cladding was doped uniformly ( cm ), while the p-cladding doping was graded near the waveguide to reduce optical losses. The active region nominally contains three 9.42-nm In Ga As Sb quantum wells (QWs) separated by 30 nm, with Al Ga As Sb barrier and waveguide layers.
Lateral Cr-grating DFBs were fabricated using the approach of [3]. RWGs were fabricated using BCl –Ar inductively coupled plasma reactive ion etching (ICP-RIE) producing smooth waveguide sidewalls inclined at 12 from the vertical for uniform e-beam resist coating around the ridges. E-beam lithography with fine-pitch control defined the grating lift-off pattern across the ridges using ZEP resist. First-order gratings (50% duty cycle, 40 nm thick, pitch nm) were deposited using e-beam evaporation. A first planarization step followed by a Cl -O ICP-RIE etch removed the Cr around the top of the ridge to isolate the top p-contact from the grating. A 150-nm SiO layer was then deposited across the entire sample using plasma-enhanced chemical vapor deposition. Windows were patterned on the ridges, the top oxide was removed using CHF –O RIE and a TiPtAu top p-contact stack was deposited. After wafer thinning, NiGeAu backside n-contact metal was deposited and the samples were cleaved for testing.
Two devices will be discussed in this letter, a DFB laser and a reference FP laser fabricated from a separate epiwafer with identical layer structure. The devices have ridge widths of 4 m, cavity lengths of 400 m, and were mounted epi-side-up onto Au-electroplated Cu–W (15%/85%) carriers using Ag epoxy for testing on a Cu thermoelectric stage.
III. DEVICECHRARACTERISTICS
High-resolution interferograms were acquired using a Bruker IFS66/S Fourier Transform Infrared (FTIR) instrument (resolution 0.25 cm ) with a KBr beamsplitter and liquid nitrogen-cooled InSb detector. The ASE spectra were obtained from the interferograms using Mertz phase correction (resolu-tion 4 cm ) and a Blackman–Harris three-term apodization
1041-1135/$26.00 © 2009 British Crown Copyright
GUPTA et al.: MODAL GAIN OF 2.4- m InGaAsSb–AlGaAsSb COMPLEX-COUPLED DFB LASERS 1533
function which successfully reduced Fourier transform arti-facts, particularly near laser threshold.
Two-dimensional (2-D) effective index modeling of the Cr grating RWG laser structures found the 2-D QW optical con-finement in the lateral and transverse waveguide directions to be for a 4- m ridge width. The effective index was calculated to be at the 2396-nm target wavelength.
The Cr-grating DFB coupling was calculated using a similar approach to [7], based on the coupled-wave theory [8]. The cou-pling coefficient is given by
(1) with grating order ; waveguide effective index ; wave-length ; grating duty cycle ; evanescent field overlap with the grating region ; and where and are the complex re-fractive indices of the grating materials. For etched index-cou-pled gratings [7], the field overlap in the grating region is cal-culated using a virtual layer with a weighted average dielectric constant of the materials providing the grating contrast, e.g., semiconductor and air. In that case, the longitudinal field in the virtual material is considered homogenous along the grating length.
For metal gratings, the situation is different because of the large extinction coefficient of Cr at 2396 nm . Since the optical field penetra-tion into the metal is limited to the skin depth , the field is strongly squeezed into the spaces between the metal and the longitudinal field component is related to , where is the metal thickness. The coupling coefficient was calculated to be cm for the RWG structure in-cluding first-order Cr gratings. This suggests a very strong coupling for the target wavelength, and an optimum coupling factor could be obtained for this DFB laser with a cavity length of 803 m.
Fig. 1 shows the ASE spectra near threshold. The 20 C CW threshold currents for the FP and DFB devices were measured to be 25 and 45 mA, respectively. In spite of the large differ-ence in the injection current, the two spectra of Fig. 1 look sim-ilar, having a broad spectrum distinguished by FP ASE reso-nances. For the DFB device, however, a single narrow resonance is observed near 2389 nm, at slightly longer wavelength than the peak of the ASE spectrum. In the below-threshold spectrum of Fig. 1, this feature exhibits an intensity 6 dB greater than the ad-jacent sidemodes. Above threshold, the strong grating coupling becomes even more evident and the sidemode suppression ex-ceeds 30 dB throughout most of the operational range.
According to coupled-mode theory [8] a pure gain- (or loss)-coupled DFB laser has a resonance at exactly the Bragg condi-tion, while a pure index-coupled DFB exhibits a stopband with symmetrical resonances on either side. Our complex-coupled device is a mixture of these two ideal cases since the grating provides contrast for both real and imaginary parts of the refrac-tive index. The measured spectrum does not have a well-defined stopband, although a slight perturbation of the ASE envelope occurs on the short wavelength side of the DFB peak.
The gain spectra extracted with the Cassidy method of [9] are presented in Fig. 2. The modal gain depends on the
Fig. 1. High-resolution ASE spectra acquired in CW mode at 20 C. Top: FP laser with 22-mA drive current. Bottom: DFB laser with 42-mA drive current.
Fig. 2. CW net modal gain spectra at 20 C. Top: FP laser, spectra acquired at 1-mA intervals but displayed at 2-mA intervals. Bottom: DFB laser, spectra acquired and displayed at 3-mA intervals.
material gain , while the net modal gain is , and at long wavelength the gain spectra converge to the in-ternal loss . As shown in Fig. 2, the internal loss of the FP laser has a value of about cm . At threshold, the net modal gain clamps at the mirror loss, calculated for this de-vice to be 28.9 cm , where ; is the cavity length and the cleaved facet reflectivity. The net modal gain increases with increasing drive current and near threshold the gain curves become slightly distorted near the peak. This is partly due to FTIR apodization artifacts but also in-dicates that the threshold gain is reached by several FP modes si-multaneously. Indeed, competition between these modes above threshold results in mode-hopping, making the FP devices un-suitable for gas sensing applications.
1534 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 20, OCTOBER 15, 2009
Fig. 3. Dependence of the peak modal gain on injection current for DFB (squares) and FP (circles) lasers. The solid lines indicate the logarithmic gain model best-fit curves.
Although the general shape of the DFB gain curves is similar to the FP case, the gain extraction algorithm produces a distor-tion near the DFB resonance. We emphasize that the algorithm was not modified for DFB lasers and Fourier transform apodiza-tion artifacts may affect the spectrum near this resonance. Nev-ertheless, the indicated gain near the resonance provides qualita-tive insights into the lasing mechanism. Notably, the DFB mode has larger gain than adjacent FP modes. Below transparency, the DFB mode is close to the FP gain peak wavelength. However, the DFB mode redshifts slightly at higher injection levels due to the heating-induced increase in the waveguide effective index while the FP gain blueshifts with the increase in carrier den-sity. On the shorter wavelength side of the resonance the gain exhibits a small dip, related to the dip observed in the spectra. Near transparency , the calculated gain near the DFB mode is only slightly greater than the peak gain of the nearby FP modes. At higher currents, the peak DFB gain be-comes much greater, and the DFB mode reaches threshold with a clear advantage over the competing FP modes. It is evident that the DFB mode exceeds the mirror loss at threshold, while the nearby FP modes have noticeably lower gain. This selec-tivity obviously persists above threshold, since this device ex-hibited single-mode behavior throughout its operational range (to 200 mA). For the DFB device, the internal loss is signifi-cantly greater than for the FP device. The long-wavelength data converge to a value around cm . This is comparable to the sum of the FP internal loss ( cm ) and the grating coupling (12.5 cm ).
Fig. 3 shows the variation in the peak modal gain with in-creasing injection current for both devices, after correcting for the measured internal loss. For the DFB device, we considered the peak gain of the spontaneous emission spectrum which is more representative of the gain provided by the active material, rather than the gain of the DFB mode. The solid lines indicate the nonlinear least-squares best-fits to the experimental data as-suming a logarithmic dependence on the injection current [10] according to the expression . From the
analysis, the gain coefficient can be used to determine the material gain . The resulting values of
cm and cm are similar for the FP and DFB devices, respectively. Thus the Cr grating intro-duces significant optical loss which is approximately equal to the coupling coefficient. Since the available gain from the QW material is the same, the DFB threshold current is greater than for the FP laser because of the larger current required to over-come the losses. The larger current also blueshifts the material gain peak away from the DFB resonance, decreasing the device efficiency. This could be resolved by using a slight negative de-tuning (DFB mode at shorter wavelength than gain peak).
IV. CONCLUSION
High-resolution spectra were used to analyze the gain char-acteristics of GaSb-based FP and DFB lasers fabricated using a regrowth-free lateral Cr grating process. The values of mate-rial gain were similar for the two devices, while the DFB de-vice exhibited a much larger loss due to the large Cr grating ex-tinction coefficient. The complex grating has clear processing advantages over conventional index-grating processes because no epitaxial regrowth is required, and facet coatings are not re-quired to break symmetry across the stopband. The disadvan-tage of the Cr gratings is the larger loss which must be carefully considered in the device design. In the future, purely index-cou-pled DFB lasers will be processed from the same epiwafers to allow a direct comparison between the losses due to index- and complex-coupling.
REFERENCES
[1] C. Lin, M. Grau, O. Dier, and M.-C. Amann, “Low threshold room-temperature continuous-wave operation of 2.24–3.04 m GaInAsSb-AlGaAsSb quantum-well lasers,” Appl. Phys. Lett., vol. 84, pp. 5088–5090, 2004.
[2] M. Rattunde, J. Schmitz, G. Kaufel, M. Kelemen, J. Weber, and J. Wagner, “GaSb-based 2.Xm quantum-well diode lasers with low beam divergence and high output power,” Appl. Phys. Lett., vol. 88, p. 081115, 2006.
[3] M. Hümmer, K. Rössner, A. Benkert, and A. Forchel, “GaInAsSb–Al-GaAsSb distributed feedback lasers emitting near 2.4 m,” IEEE
Photon. Technol. Lett., vol. 16, no. 2, pp. 380–382, Feb. 2004. [4] S. Civi˘s, V. Horká, T. ˘Sime˘cek, E. Hulicius, J. Pangrác, J. Oswald, O.
Pet˘ri˘cek, Y. Rouillard, C. Alibert, and R. Werner, “GaSb based lasers operating near 2.3m for high resolution absorption spectroscopy,”
Spectrochimica Acta A, vol. 61, pp. 3066–3069, 2005.
[5] T. Lehnhardt, M. Hümmer, K. Rössner, M. Müller, S. Höfling, and A. Forchel, “Continuous wave single mode operation of GaInAsSb/GaSb quantum well lasers emitting beyond 3m,” Appl. Phys. Lett., vol. 92, p. 183508, 2008.
[6] A. Salhi, D. Barat, D. Romanini, Y. Rouillard, A. Ouvrard, R. Werner, J. Seufert, J. Koeth, A. Vicet, and A. Garnache, “Single-frequency Sb-based distributed-feedback lasers emitting at 2.3m above room temperature for application in tunable diode laser absorption spec-troscopy,” Appl. Opt., vol. 45, pp. 4957–4965, 2006.
[7] W.-Y. Choi, J. C. Chen, and C. G. Fonstad, “Evaluation of coupling coefficients for laterally-coupled distributed feedback lasers,” Jpn. J.
Appl. Phys., vol. 35, pp. 4654–4659, 1996.
[8] H. Kogelnik and C. V. Shank, “Coupled-wave theory of distributed feedback lasers,” J. Appl. Phys., vol. 43, pp. 2327–2335, 1972. [9] D. T. Cassidy, “Technique for measurement of the gain spectra of
semi-conductor diode lasers,” J. Appl. Phys., vol. 56, pp. 3096–3099, 1984. [10] L. A. Coldren and S. W. Corzine, Diode Lasers and Photonic
Inte-grated Circuits. New York: Wiley, 1995.
