High power pulsed 976 nm DFB laser diodes

Wolfgang Zeller*a, Martin Kampb, Johannes Koetha, Lukas Worschechb ananoplus GmbH, Oberer Kirschberg 4, 97218 Gerbrunn, Germany;

bTechnische Physik, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany

ABSTRACT

Distributed feedback (DFB) laser diodes nowadays provide stable single mode emission for many different applications covering a wide wavelength range. The available output power is usually limited because of catastrophical optical mirror damage (COD) caused by the small facet area. For some applications such as trace gas detection output powers of several ten milliwatts are sufficiently high, other applications like distance measurement or sensing in harsh environments however require much higher output power levels. We present a process combining optimizations of the layer structure with a new lateral design of the ridge waveguide which is fully compatible with standard coating and passivation processes. By implementing a large optical cavity with the active layer positioned not in the middle of the waveguide layers but very close to the upper edge, the lasers' farfield angles can be drastically reduced. Furthermore, the travelling light mode can be pushed down into the large optical cavity by continuously decreasing the ridge waveguide width towards both laser facets. The light mode then spreads over a much larger area, thus reducing the surface power density which leads to significantly higher COD thresholds. Laterally coupled DFB lasers based on this concept emitting at wavelengths around 976 nm yield hitherto unachievable COD thresholds of 1.6 W under pulsed operation. The high mode stability during the 50 ns pulses means such lasers are ideally suited for high precision distance measurement or similar tasks.

Keywords: laser diodes, distributed feedback lasers, large optical cavity, COD, high power lasers, taper

1. INTRODUCTION Semiconductor laser diodes are key elements in a multitude of different applications such as e.g. solid state laser pumping1, data transfer in fiber optics2 or laser based gas sensing3. For many applications multimode emission as provided by Fabry-Pérot laser diodes is sufficient, others however as e.g. frequency doubling, sensing or metrology require laser diodes emitting in only one spectral mode. Such single-mode emitting devices typically contain mode selective structures, usually either distributed bragg reflectors (DBR)4 or distributed feedback gratings (DFB)5.

Conventional DFB lasers yield output powers of up to several hundred milliwatts6,7 however they are based on a complicated epitaxial overgrowth step. By implementing DFB gratings positioned laterally to the ridge waveguide (RWG)8,9, this critical step can be avoided. Instead, the layer structure is grown in a single epitaxial growth step and common etching, lithography and evaporation processes are executed afterwards. The output power of these devices however is limited to rather moderate levels due to the narrow mode profile that is required to achieve sufficient coupling between the light mode and the lateral grating.

Generally, the output power of laser diodes is limited by two different effects, thermal roll-over and catastrophic optical damage (COD). Thermal roll-over can be decreased by using submounts with a high thermal conductivity and optimizing the mounting of the devices, e.g. by mounting them epi-side down. Furthermore, if continuous wave (CW) operation is not required, the laser diodes can be operated in pulsed mode resulting in less thermal stress and therefore delaying the thermal roll-over considerably.

Although COD also shows a dependence on pulse length, it cannot be avoided by simply switching from CW to pulsed operation. In fact, COD manifests mainly during pulsed operation since the high power densities required to initiate COD are hardly achievable during CW operation10. It is caused by a thermal runaway process culminating in thermal microexplosions11-15 and local melting of the semiconductor material due to increased absorption at the laser facet under high photon flux. Since surface carrier recombination is one of the major sources for facet heating16, the status of the facet surface is a major parameter influencing COD behavior12,13,17.

*wolfgang.zeller@nanoplus.com; phone +49 931 9082710; fax +49 931 9082719; nanoplus.com

Hence there have been made many attempts at decreasing both the recombination velocity18,19 and the carrier density close to the laser facets19-22. The other major effect responsible for triggering COD is reabsorption of light generated by stimulated emission.

Recent studies indicate that the COD of broad area lasers originates in spatially very confined regions of the laser facet in which the nearfield intensity exhibits local maxima10,17. These results show that very high surface power densities have to be avoided in order to manufacture laser diodes exhibiting high COD thresholds. In the case of laterally coupled DFB laser diodes the nearfield distribution shows only one intensity peak, however the optical mode is confined to significantly smaller areas due to the mode guiding provided by the very narrow RWG structure. Thus high output powers result in very high surface power densities.

One possibility to decrease the optical power density on the laser facet is the use of tapered lasers. A narrow RWG provides a well defined optical mode which is amplified in a second section of the device in which the RWG width increases towards the laser facet. Combined with feedback gratings such devices yield spectrally narrow or even single-mode emission23. Because of lensing and filamentation effects in the tapered section however, the beam quality is usually worse than that of standard RWG lasers. Furthermore, the farfield distribution of these devices is highly astigmatic and several lenses are required for beam collimation or coupling to an optical fiber.

We present devices based on a special asymmetrical layer structure combined with RWGs whose width decreases towards the laser facets. This concept guarantees good coupling between the light mode and the lateral grating while increasing the width of the light mode on the laser facets, therefore decreasing the farfield divergence angles as well as improving the COD thresholds of the devices.

2. DEVICE DESIGN 2.1 Vertical laser structure

Typically the waveguide layer of 976 nm laterally coupled DFB lasers consists of two 0.2 µm to 0.3 µm thick AlxGa1-xAs layers positioned symmetrically around the active region. Depending on whether their index of refraction is constant or graded, one speaks of separate confinement heterostructures or graded index separate confinement heterostructures (GRINSCH). The spatial confinement of the light mode is determined by two parameters: vertically it is confined mainly to the waveguide core, i.e. to a height of about 0.5 µm. The lateral confinement is determined by the width of the RWG which has to be narrow (typically about two microns) in order do provide sufficient coupling to the lateral grating and to suppress higher order lateral modes. Consequently the mode extends over an area of about 0.5 µm by 2.0 µm.

The left part of figure 1 displays a simulation of the normalized nearfield distribution of a laser diode with a 0.55 µm thick GRINSCH waveguide and a 2.0 µm wide RWG. To enable a quantitative comparison of different designs we use the effective mode area Aeff which is defined as the total emitted intensity divided by the maximum power density on the facet. In this case, Aeff amounts to 0.63 µm.

Because the field distribution’s vertical boundaries are determined by the waveguide’s dimensions, a large optical cavity (LOC) design would result in a much larger effective mode area. In the case of laterally coupled DFB lasers however, a standard LOC is not the optimal solution since the increased distance between active area and lateral grating result in weaker coupling. Therefore we chose to use an asymmetrical LOC design with an active region positioned close to the upper edge of the LOC waveguide. The active region is composed of three InGaAs quantum dot layers separated by 25 nm thick GaAs barriers. The waveguide core consists of a 2400 nm thick Al0.3Ga0.7As layer. P- and n-side cladding layers are composed of Al0.35Ga0.65As layers with a thickness of 1200 nm and 1000 nm respectively. On the p-side, a 25 nm thick graded AlxGa1-xAs layer (x=0.35 → 0.00) follows before a 200 nm thick highly doped GaAs cap-layer completes the structure.

The right-hand part of figure 1 shows the simulated nearfield distribution of a laser diode based on this material with a 2.0 µm wide RWG. The high refractive index of the active region concentrates the optical intensity mainly around the active layers, so that the major part of the optical power is still confined to a rather small area. However, due to the asymmetrical LOC, the evanescent tail falling off exponentially into the lower Al0.3Ga0.7As layer is much longer than in the case of the standard structure. This field distribution is optimized to realize a good coupling to the lateral grating and a large confinement factor of the active region, but its effective mode area of 0.62 µm2 is almost identical to that of the reference device described above. In order to increase the effective mode area on the laser facets, the lateral design described in the next section was used.

Figure 1. Calculated nearfield distributions on the facet of a RWG laser diode with standard epitaxial design (left side)

and an asymmetrical LOC (right side). In case of the standard design, the ridge width and the thickness of the GRINSCH waveguide layers define an area of approximately 2.0 µm by 0.5 µm to which the major part of the optical power is confined. The asymmetrical LOC leads to an extension of the evanescent tail deep into the lower part of the LOC.

2.2 Ridge design

By decreasing the dimensions of a light guiding structure, the light mode can be forced out of it. This effect is used in twin-waveguide lasers where a tapered ridge is used to guide light from one waveguide layer into another one situated below24. The same effect is used here to push the light mode, which is centered mainly around the active region, deep into the asymmetrical LOC structure. Figure 2 shows a schematic view of the structure that was used. Laterally to the 2.0 µm wide ridge waveguide, a first-order chromium grating is defined to select a single longitudinal mode.

Figure 2. Schematic view of the laser structure. The hatched area indicates the LOC with the asymmetrically positioned

active region. The ridge width is tapered exponentially towards the laser facets, the lateral DFB grating is present only along the untapered region of the ridge waveguide.

Towards both laser facets, the width of the RWG is tapered down exponentially to a width of only several hundred nanopmeters. In the untapered region of the RWG the light mode is close to the DFB grating, thus providing good coupling as in standard DFB lasers. Once the ridge width decreases, the mode is pushed down deep into the asymmetrical LOC waveguide. There it spreads over a significantly larger area than before, consequently increasing the COD threshold of the devices. Simulated nearfield distributions of such lasers with 2.0 µm wide RWGs tapered down to different widths ranging from 1.5 µm to 0.5 µm are shown in figure 3. At a ridge width of 1.5 µm, the effective mode area is 0.55 µm2, i.e. slightly smaller than in case of the 2.0 µm wide standard ridge. Once the ridge is tapered down to

widths of 1.0 µm and less, the mode starts to move down into the LOC. Consequently Aeff increases. In case of a 2.0 µm wide RWG tapered down to a width of 0.5 µm at the laser facets, the effective mode area amounts to 8.70 µm2.

Figure 3. Simulation of the normalized field distributions on the facet of laser diodes with an asymmetrical LOC and ridge waveguides tapered down exponentially to different widths ranging from 1.5 µm to 0.5 µm. The corresponding effective mode areas are 0.55 µm2, 0.77 µm2, 1.86 µm2, 6.55 µm2 and 8.70 µm2.

3. DEVICE FABRICATION Due to the narrow taper structures that are crucial to the design, the ridge level had to be defined by electron beam lithography. The etch mask was composed of titanium and nickel, as this combination offers a sufficiently fine texture for the accurate etching of the narrow parts of the tapered ridges. The ridge structures were etched close to the upper edge of the LOC by using an Ar-Cl2 gas-mixture and an inductively coupled plasma etch process. Afterwards the DFB gratings were implemented by applying chromium lines defined laterally to the RWG in its untapered regions. To this end a resist was structured using high definition electron beam lithography. Chromium was deposited subsequently, followed by a lift-off process. Afterwards, the structure was simultaneously planarized and insulated with benzocyclobutene. In the following step, the metallic p-contact was defined by evaporation, optical definition and etching. Then the sample thickness was decreased to 150 µm in a backside lapping step before applying the n-contact in a further evaporation step. Finally, the laser chip was cleaved into bars. Aside from standard anti- and high-reflection coatings on the front and rear facets respectively, no further facet treatment was applied.

4. RESULTS Several devices were manufactured using the epitaxial layer and the design described above. The total cavity length of the laser diodes was 1800 µm. The 2.0 µm wide ridges were tapered down to a width of about 0.4 µm at the facets over a length of 200 µm. Additionally some lasers with a constant RWG width of 2.0 µm were manufactured in order to enable reference measurements. The lateral DFB grating extends only along the central 1400 µm long untapered section of the RWG. The devices were mounted p-side up on c-mounts and attached to a heat-sink for the measurements.

4.1 Farfield distributions

The farfield distributions of the devices were measured using a beam profiler. The laser diodes with standard 2.0 µm wide RWGs yield FWHM values of 14.0° in lateral and 19.0° in vertical direction (figure 4). Particularly the vertical farfield distribution is considerably narrower than those of standard laser diodes. This is caused by the very long evanescent tail of the optical field reaching deep into the optical cavity as displayed in figure 1. The almost circular farfield distribution allows for very high fiber coupling efficiencies.

Figure 4. Measured farfield distributions of a 1800 µm long laser with an asymmetrical LOC and a standard 2.0 µm RWG.

The lateral FWHM is 14.0°, the vertical FWHM is 19.0°.

As shown in figure 5, the FWHM values of laser diodes with tapered RWGs amount to even smaller values of 5.2° in lateral direction and 13.0° in vertical direction. Thus the farfield of these laser diodes is significantly narrower than those of standard DFB laser diodes, confirming the influence of the combination of asymmetrical LOC and tapered RWG on the nearfield distribution.

Figure 5. Measured farfield distributions of a 1800 µm long laser with an asymmetrical LOC and a tapered RWG. The

lateral FWHM is 5.2°, the vertical FWHM is 13.0°.

4.2 Spectral characteristics

Both current and temperature dependence of the laser diodes were measured in CW mode. For this purpose emission spectra of a mounted laser were recorded at different temperatures and currents yielding a temperature dependency of the emission wavelength of 0.085 nm/K and a current dependency of 0.004 nm/mA (figure 6).

Figure 6. Temperature (squares) and current (diamonds) dependency of the emission wavelength of a mounted 1800 µm

long laser with asymmetrical LOC and tapered ridges. Δλ/ΔT amounts to 0.085 nm/K, Δλ/ΔI is 0.004 nm/mA.

The mode stability during pulsed operation of the lasers was measured using a Michelson interferometer setup. The pulse length was 50 ns, the repetition rate 1 MHz. The emitted light was collimated and guided into the interferometer setup. The mirror positions were fixed, consequently interference occurs only because of intra-pulse wavelength tuning. The interference signal was recorded using a photodiode connected to an oscilloscope. Figure 7 shows a superposition of several thousand measurements recorded at a pulse current of 1000 mA. Due to the lateral DFB grating the devices exhibit single-mode emission. The continuous sinus signal shows that no intra-pulse mode hops occurred. The lack of aberrations in the superimposed signals proves that no inter-pulse mode hops occurred either.

Figure 7. Superimposed interferometer signals measured at pulse currents of 1000 mA. During the 50 ns pulses, the

emission wavelength of the laser increases slightly. The continuous sinus signal proves that no intra-pulse mode hops occur. Furthermore the superposition of several thousand pulse cycles shows no aberration, thus also ruling out inter-pulse mode hops.

4.3 COD

In CW mode an output power of 200 mW was achieved without any sign of COD before the onset of thermal roll-over. Higher CW output powers are expected for devices mounted epi-side down due to better heat dissipation. In order to determine the COD threshold of the devices, pulsed measurements using the same pulse parameters as for the interferometer measurements were performed. In order to obtain reference data, a laser with constant RWG width of 2.0 µm was measured. The results are shown in the left-hand part of figure 8. At a heat sink temperature of 23 °C, the slope efficiency is 0.8 W/A, the threshold current is 31 mA. The laser exhibits COD at an output power of 0.28 W. The right-hand part of figure 8 displays the measurement results of a laser with a RWG width tapered down to 0.4 µm. The threshold current and slope efficiency of this laser diode are 40 mA and 0.76 W/A respectively. Although the laser starts to show signs of thermal roll-over at currents exceeding 2000 mA, COD can be observed at an output power of 1.6 W. The COD threshold of the tapered device is approximately 5.8 times higher than that of the standard RWG laser. It exceeds any values achievable with standard RWG designs and no special treatment of the facets.

Figure 8. Output characteristics of two 1800 µm long laser diodes with asymmetrical LOC. The RWG widths at the

facets are 2.0 µm (left) and 0.4 µm (right) respectively. Under pulsed conditions, the laser with the standard RWG exhibits a COD threshold of 0.28 W. The device with the 0.4 µm wide tapered RWG however shows COD at an output power of 1.6 W.

5. CONCLUSION The COD threshold of RWG laser diodes is limited by the relatively narrow nearfield distribution at the laser facet which leads to very high surface power densities. These trigger a thermal runaway process resulting in local microexplosions at the laser facet.

The combination of a newly developed asymmetrical LOC design with a tapered RWG enables very large nearfield distributions at the laser facet. These in turn result in both narrow and homogeneous farfield distributions and high COD thresholds in laterally coupled DFB lasers emitting single-mode light. The farfield distributions of the laser diodes were measured and FWHM values of 14.0° and 19.0° in lateral and vertical direction could be determined for the combination of an asymmetrical LOC and a standard 2.0 µm RWG. A laser with an asymmetrical LOC and a RWG tapered down to 0.4 µm at the facets yielded lateral and vertical FWHM values of 5.2° and 13.0°. Interferometer measurements show both high intra- and inter-pulse mode stability. The COD threshold of the devices was quantified in pulsed mode, yielding maximum output powers of 0.28 W for the standard RWG laser and 1.6 W for the laser with the tapered RWG. Although no further COD preventing techniques as e.g. specialized facet passivation processes or quantum well intermixing were applied, the concept presented here is fully compatible with those and can be applied to every compound semiconductor material. The combination of such high output powers with the spectral characteristics of

conventional laterally coupled DFB laser diodes makes these lasers perfectly suited for applications requiring stable single-mode output at powers higher than those achievable with standard laterally coupled DFB lasers.

ACKNOWLEDGEMENTS

We would like to acknowledge the expert technological assistance of B. Eberhardt, B. Hubert, C. Koenig and S. Kuhn.

REFERENCES

1. Fan, T. Y., Byer, R. L., "Diode laser-pumped solid-state lasers," IEEE J. Quant. Electron. 24, 895-912 (1988). 2. Sugawara, M., Hatori, N., Ishida, M., Ebe, H., Arakawa, Y., Akiyama, T., Otsubo, K., Yamamoto, T. and Nakata,

Y., "Recent progress in self-assembled quantum-dot optical devices for optical telecommunication: temperature-insensitive 10 Gbs-1 directly modulated lasers and 40 Gbs-1 signal-regenerative amplifiers," J. Phys D: Appl. Phys. 38, 2126-2134 (2005).

3. Werle, P., [Lasers in Environmental and Life Sciences – Modern Analytical Methods], Springer, Heidelberg, 223-243 (2004).

4. Koyama, F., Arai, A., Suematsu, Y. and Kishino, K., "Dynamic spectral width of rapidly modulated 1.58 µm GaInAsP/InP buried-heterostructure distributed-Bragg-reflector integrated-twin-guide lasers," Electron. Lett. 17, 938-940 (1981).

5. Kogelnik, H. and Shank, C. V., "Coupled-wave theory of distributed feedback lasers," J. Appl. Phys. 43, 2327-2335 (1972).

6. Klehr, A., Wenzel, H., Braun, M., Bugge, F., Fricke, J., Knauer, A. and Erbert, G., "Five-hundred-milliwatts distributed-feedback diode laser emitting at 940 nm," Opt. and Laser Technol. 39, 333-337 (2007).

7. Wenzel, H., Fricke, J., Klehr, A., Knauer, A. and Erbert, G., "High-power 980-nm DFB RW lasers with a narrow vertical far field," IEEE Photon. Technol. Lett. 18, 737-739 (2006).

8. Kamp, M., Hofmann, J., Forchel, A., Schäfer, F. and Reithmaier, J. P., "High performance laterally gain coupled InGaAs/AlGaAs DFB lasers," Conference proceedings of the 10th Conference on Indium Phosphide and related Materials, 831-834 (1998).

9. Kamp, M., Hofmann, J., Schäfer, F., Reinhard, M., Fischer, M., Bleuel, T., Reithmaier, J. P. and Forchel, A., "Lateral coupling – a material independent way to complex coupled DFB lasers," Optical Materials 17, 19-25 (2001).

10. Ziegler, M., Tomm, J. W., Elsaesser, T., Matthiesen, C., Sanayeh, M. B. and Brick, P., "Real-time thermal imaging of catastrophic optical damage in red-emitting high-power diode lasers," Appl. Phys. Lett. 92, 103514 (2008).

11. Henry, C. H., Petroff, P. M., Logan, R. A. and Meritt, F. R., "Catastrophic damage of AlxGa1−xAs double-heterostructure laser material," J. Appl. Phys. 50, 3721 (1979)

12. Nakwaski, W., "Thermal analysis of the catastrophic mirror damage in laser diodes," J. Appl. Phys. 57, 2424-2430 (1985).

13. Schatz, R. and Bethea, C. G., "Steady state model for facet heating leading to thermal runaway in semiconductor lasers," J. Appl. Phys. 76, 2509 (1994).

14. Eliseev, P. G., [Reliability Problems of Semiconductor Lasers], Nova Science, Commack, 213 (1991). 15. Smith, W. R., "Mathematical modeling of thermal runaway in semiconductor laser operation," J. Appl. Phys. 87,

8276 (2000). 16. Rinner, F., Rogg, J., Kelemen, M. T., Mikulla, M., Weimann, G., Tomm, J. W., Thamm, E. and Poprawe, R., "Facet

temperature reduction by a current blocking layer at the front facets of high-power InGaAs/AlGaAs lasers," J. Appl. Phys. 93 (3), 1848-1850 (2003).

17. Ziegler, M., Tomm, J. W., Reeber, D., Elsaesser, T., Zeimer, U., Larsen, H. E., Petersen, P. M. and Andersen, P. E., "Catastrophic optical mirror damage in diode lasers monitored during single-pulse operation," Appl. Phys. Lett. 94, 191101 (2009).

18. Strite, T. and Harder, C., "Uniphase's 980 nm pump lasers show their reliability, "III-Vs Rev. 12, 24-26, (1999). 19. Horie, H., Yamamoto, Y., Arai, N. and Ohta, H., "Thermal rollover characteristics up to 150°C of buried-stripetype

980-nm laser diodes with a current injection window delineated by aSiNx layer," IEEE Photonics Technol. Lett. 12, 13-15 (2000).

20. Knauer, A., Bugge, F., Erbert, G., Wenzel, H., Vogel, K., Zeimer, U. and Weyers, M. "Optimization of GaAsP/AlGaAs-Based QW Laser Structures for High Power 800 nm Operation," IEEE J. Electron. Mater. 29, 53-56 (2000).

21. Piva, P. G., Goldberg, R. D., Mitchell, I. V., Fafard, S., Dion, M., Buchanan, M., Charbonneau, S., Hillier, G. and Miner, C., "Reduced 980 nm laser facet absorption by band gap shifted extended cavities," J. Vac. Sci. Technol. B 16, 1790-1793 (1998).

22. Hiramoto, K., Sagawa, M., Kikawa, T. and Tsuji, S., "High-power and highly reliable operation of Al-Free InGaAs-InGaAsP 0.98-μm lasers with a window structure fabricated by Si ionimplantation," IEEE J. Sel. Top. Quantum Electron. 5, 817-821 (1999).

23. Bisping, D., Pucicki, D., Fischer, M., Koeth, J., Zimmermann, C., Weinmann, P., Hofling, S., Kamp, M. and Forchel, A., "GaInNAs-Based High-Power and Tapered Laser Diodes for Pumping Applications," IEEE J. Sel. Top. Quantum Electron. 15, 968-972 (2009).

24. Studenkov, P. V., Gokhale, M. R. and Forrest, S. R., "Efficient coupling in integrated twin-waveguide lasers using waveguide tapers," IEEE Photon. Technol. Lett. 11, 1096-1098 (1999).