SUMMARY. In conclusion, we have demonstrated lasing under pulsed excitation at low temperature for microdisk lasers fabricated from InGaAs/InAlGaAs/InAlAs ...
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the InGaAs/InAlGaAs material system could be due to the higher trap density and surface recombination in this material system. These properties will be further studied.
REFERENCES
i450
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wavelength (nm) Fig. 5. ,Two modes lasing spectrum for some 20-pm microdisk laser. The intermode spacing is 10 nm.
SUMMARY In conclusion, we have demonstrated lasing under pulsed excitation at low temperature for microdisk lasers fabricated from InGaAs/InAlGaAs/InAlAs material system. Similar experiments done with the InP/InGaAsP material system [ l ] give a laser threshold intensity about four times lower than ours (consider 25% of incident power excites the lasing and our mode area is 12 times larger). We believe that the higher-threshold intensitv of
S. L. McCall, A. F. J. Levi, R. E. Slusher, S. J. Pearton, and R. A. Logan, “Whispering-gallery mode microdisk lasers,” Appl. Phys. Lett., vol. 60,pp. 289-291, 1992. A. F. J. Levi, R. E. Slusher, S. L. McCall, T. Tanbun-Ek, D. L. Coblentz, and S. J. Pearton, “Room temperature operation of microdisk lasers with submilliamp threshold current,” Electron. Left., vol. 28, pp. 1010-1012, 1992. T. Gauss, P. J. R. Laybourn, and J. Roberts, “CW operation of semiconductor ring lasers,” Electron. Len., vol. 26, pp. 2095-2097, 1990. D. Y. Chu, and S. T. Ho, “Spontaneous emission from excitons in cylindrical dielectric waveguides and the spontaneous-emission factor of microcavity ring lasers,” J . Opt. Soc. Amer. E., vol. 10, pp. 381-390,1993. D. Y. Chu, M. K. Chin, and S. T. Ho, “Spontaneous emission factors of microring and microdisk lasers,” Ann. Meet. Opt. Sociefy Amer., Toronto, Ontario, Can., Dig. Tech. Paper WN2, Oct. 1993. G. Bjork and Y. Yamamoto, “Analysis of semiconductor microcavity lasers using rate equations,” IEEE J . Quantum Electron., vol. 27, pp. 2386-2396, 1991. S. T. Ho, S. L. McCall, and R. E. Slusher, “Spontaneous emission from excitons in thin dielectric layers,” Opt. Lett., vol. 18, pp. 909-911, 1993. N. J. Sauer and K. B. Chough, “A selective etch for InAlAs over InGaAs and for different InGaAlAs quartemaries,” J. Electrochem. Soc., vol. 139, pp. L10-L11, 1992.
Mode-Locked Multisegment Resonant-Optical-Waveguide Diode Laser Arrays Alan Mar, Roger Helkey, Member, IEEE, Thomas Reynolds, John Bowers, Fellow, ZEEE, D a n Botez, Fellow, ZEEE, Charles Zmudzinski, Member, ZEEE, Chan Tu, Member, ZEEE, and Luke Mawst, Member, IEEE Abstract-We report the first mode-locked operation of resonant optical waveguide (ROW) semiconductor laser arrays. The well-behaved emission patterns of such arrays allow coupling to external cavities with efficiencies comparable to those achieved by using single-element lasers. Single and multisegment lasers Manuscript received July 28, 1993; revised September 27, 1993. This work was supported by the Office of Naval Research. A. Mar, R. Helkey, T. Reynolds, and J. Bowers are with the Department of Computer and Electrical Engineering, University of Califomia, Santa Barbara, CA 93106. D. Botez, C. Zmudzinski, C. Tu, and L. Mawst are with the TRW Research Center, Redondo Beach, CA 90278. IEEE Log Number 9214115.
are employed to achieve active, passive, and hybrid mode-locking. The use of an arrayed gain region is effective in increasing the saturation energies of gain and absorber segments, resulting in high pulse energies. Pulses are generated that have well-suppressed secondary pulsations, with pulsewidths as short as 5.6 ps and peak powers of over 3 W in a collimated beam with a single main lobe.
I. INTRODUCTION
M
ODE-LOCKED semiconductor lasers are attractive as compact sources of short optical pulses for
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use in physics measurements, for instrumentation systems, and for telecommunications applications. Such lasers operate with average output powers of, typically, a couple milliwatts, which is a problem for certain applications where higher power is required. The use of gain-guided - laser arrays results in higher output powers, but such arrays tend to emit in multilobed far-field patterns. This makes their incorporation into external cavities with high coupling efficiencies difficult. Fiber nonlinearities were employed to produce bursts of multiple subpicosecond pulses from such an array [11. Resonant-optical-waveguide (ROW) diode array lasers have recently demonstrated in-phase (single main-lobed) diffraction-limited operation at total output powers of up to 2.1 W [2]. Such arrays are therefore good candidates for high-power semiconductor laser mode-locking in external cavities. In this letter, we report the first mode-locked operation of such devices, with external cavity coupling efficiencies comparable to that typically obtained by using single-element lasers. Mode-locking without multiple pulsations is achieved with increased output power proportional to the number of array elements. The power output and pulse energy from a mode-locked laser with an intra-waveguide saturable absorber is limited by the saturation energy of the absorber and gain regions, with ineffective net pulse shortening occurring for larger values of pulse energy [3]. Arrays have the advantage of increased pulse energies because the saturation energy, expressed as follows:
h vA ‘sat
= -
dg/dn
being proportional to the mode cross-sectional area A , can be made relatively large for an arrayed gain region. For a single stripe laser with a mode cross-section of 6. m2 and d g / d n of 4 . lo-’’ m2, E,,, is about 4 pJ. If an array of 20 such emitters is used, the saturation energy should scale by this factor, resulting in an E,,, of approximately 80 pJ. The saturation energy limits the pulse energies that can be generated from a passively mode-locked laser. Actively mode-locked lasers rely on gain modulation for pulse formation and are not subject to the absorber saturation energy limitation. In addition to potentially higher pulse energies, actively mode-locked lasers also provide the advantage of synchronizing the output to an external signal. However, it is more difficult to generate short pulses by using this technique, because the pulse-shortening velocity decreases as the optical pulsewidth becomes shorter than the gain modulation pulsewidth, and also because it requires the use of high-speed electrical modulation and laser structures with minimized electrical parasitics. For this reason, in hybridly mode-locked lasers, the dominant pulse-shaping element is often the saturable absorber. The absorber saturation energy also limits the pulse energies generated from hybridly mode-locked lasers, but such lasers offer the advantages of both the
pulsewidth performance provided by saturable absorbers and of synchronized pulse output due to active gain modulation. 11. SEGMENTED ROW ARRAYDEVICES The use of segmented lasers for external cavity modelocking has been demonstrated to result in improved performance and operational flexibility. Multisegment lasers allow for the separation of the functions of dc gain, gain modulation, and saturable absorption within a single device. This results in shorter pulses and suppression of the secondary pulsations seeded by reflections from the antireflection-coated facet at the interface between the laser and the external cavity [4], [5]. To demonstrate analogous benefits with arrayed lasers, two-segment ROW devices were fabricated by etching the p-contact layer and metalization, resulting in lasers with electrically isolated absorbing sections of both 25- and 50-pm lengths at the output facet, with the balance of the device used as a dc gain segment. The lasers were 1000-pm long overall, and the electrical isolation between segments was typically 70 a. To allow separate contacting of the device segments in the p-side down mounting configuration, an electrically insulating diamond heat spreader is used between the laser and the copper heatsink. This diamond heat spreader has a patterned solder metalization to match the device’s segmented contact design. 111. EXTERNAL CAVITYPERFORMANCE
The experiments employed 20-element arrays with a lasing wavelength of 850 nm. The devices were fabricated to operate close to the lateral resonance condition, which, for this device geometry, occurs with A n 0.05 between the active regions and the passive interelement waveguides [6]. The A n used thus corresponds to a nearly resonant waveguide, resulting in beam patterns with central lobe widths approximately two times the diffraction limit. Wafers with A n both higher and lower than the resonant value were tested. The devices with lower A n had more sidelobe energy than the higher A n devices did. Device yields were about 50%. Half-wave A Z O 3coatings were evaporated onto both diode facets to increase the catastrophic facet damage output power limit. The facet used to couple to the external cavity was then additionally antireflection-coated with a reactively sputtered antireflective (AR)quarterwave SiN,O, layer (index 1.83). Such coatings reproducibly reduce laser facet reflectivities to less than 0.1%
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F‘I. The ROW arrays are coupled to an external air cavity by using three intra-cavity lenses, as shown in Fig. 1. An AR-coated GRINROD lens is used at the laser because of its high collection efficiency and numerical aperture. The cylindrical lens is used to compensate for an astigmatism in the laser emission, which may be due to thermal lensing. The beam is focused onto the external cavity mirror by using an achromatic doublet. This doublet is used because the noncircular beam profile fills most of
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GRlNROD
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Fig. 1. Schematic of the ROW array external cavity mode-locked laser.
the lens and would be subject to the off-axis aberrations characteristic of a singlet lens, which are compensated in an achromat. Especially for the devices with lower An, the external cavity may serve to couple and reinforce more strongly the central portion of the array emission pattern, suppressing the energy of the sidelobes. No deliberate spatial filtering is employed in the cavity, because spatial mode control is dominated by the mode discrimination in the array itself. We did not observe any obvious effects on the far-field patterns that were due to the external cavity coupling. Fig. 2 shows the light versus current dependence of an array laser with and without feedback from the extemal cavity. The coupling reduces the threshold current from 570 to 330 mA, which is virtually the same as the threshold before AR-coating. This suggests that the cavity coupling efficiency is approximately 30%. The output beam is collimated by a GRINROD and cylindrical lens with a similar collection efficiency. The ROW array (in this case, a higher An-type device), which is very effective in discriminating against lateral modes that result in highly multilobed emission patterns, exhibits external cavity coupling that compares well with what is typically achieved by using single element lasers [8]. This is in contrast with previous results using arrays with highly multilobed beams [91-[121.
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Fig. 2. Light versus current characteristics of the ROW array extemal cavity laser.
22.9 ps pulsewidth O.50.
.‘ .. .
0 ’
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IV. ACTIVEMODELOCKING Active mode-locking was implemented by modulating the current at 1 GHz to a 500-pm ROW laser coupled to a 15-cm cavity by using a frequency synthesizer and a 20-W R F amplifier through an impedance-matching stub tuner. A high-speed (impulse response 22 ps) GaAs PIN photodetector and 40 GHz oscilloscope and an autocorrelator are used to monitor the pulse output. Fig. 3(a) shows such an autocorrelation measurement. Pulses as short as 23 ps have been generated, with slightly broader pulses being obtained at higher output powers. The maximum power of 51 mW was limited by the current capacity of the bias tee used. Taking into account the external coupling of 70% and a lens collection efficiency of approximately 30%, this corresponds to a 242-pJ pulse energy in the laser itself, demonstrating that pulses of energy larger than E,,, can be generated by using active gain modulation. Because this laser structure is not optimized for high-speed modulation, the pulses are relatively long in duration and exhibit the “coherence spikes” indicative of excess optical bandwidth. To generate shorter pulses by
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D e h Y (ps)
(b)
Fig. 3. (a) Autocorrelation measurement of actively mode-locked ROW array pulses. (b) Autocorrelation of pulses measured from passively mode-locked ROW array laser.
using this structure, it is necessary to employ a saturable absorber to provide a stronger pulse-shaping force. V. PASSIVE MODE-LOCKING The multisegment devices were coupled to an external cavity similar to that described above, of approximately 19 cm length, corresponding to a repetition rate of 775 MHz. Passive mode-locking was initiated by reverse-biasing (typically, -0.5 to - 1.0 V) the short laser section and forward-biasing the gain section above threshold. The
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introduction of this absorption did not adversely affect the external cavity coupling or noticeably affect the far-field characteristic of the devices, presumably because this absorption is bleached for most of the duration of the optical pulse output. Measurements on single-stripe devices have shown that such short reverse-biased sections act as intra-waveguide saturable absorbers with fast ( 15 ps) recovery times and lower saturation energies than the forward-biased gain segments due to the sublinearity of the differential gain versus carrier density characteristic [3]. Essentially functioning as a waveguide photodetector, such an absorber also provides a useful source of electrical signals that are synchronized with the pulse output of the passively mode-locked laser. The absorber electrical output was amplified and used to trigger the time base of the sampling oscilloscope, providing a low-jitter measurement as in the actively mode-locked case, where the modulation source itself was used as the trigger signal. Such a measurement results in pulsewidths of 23 ps, which is the impulse response of the measurement system. Autocorrelation measurements show that the pulses generated are of 9-10 ps autocorrelation width, corresponding to pulsewidths of 6-7 ps using a deconvolution factor of 1.55 (appropriate for hyberbolic secant squared pulses). Fig. 3(b) shows the autocorrelation of the shortest pulses measured thus far, with an autocorrelation width of 8.6 ps corresponding to a pulsewidth of 5.6 ps. These pulses were obtained at an average power of 13.4 mW, corresponding to peak powers of over 3 W. The measurement also shows good suppression of the trailing pulses that occur from reflections from the AR-coated laser facet. A maximum pulse energy of 21.9 pJ is obtained at 800-mA gain bias current, with attempts to operate at higher pulse energies resulting in the cessation of mode-locked pulsing. This corresponds to a 104-pJ maximum pulse energy in the laser itself, in reasonable agreement with the calculated value of saturation energy. This demonstrates the effectiveness of increasing the saturation energy of the laser by increasing the mode cross-sectional area in an arrayed structure.
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VI. HYBRID MODE-LOCKING
For certain applications, it is necessary to synchronize the optical pulse output with an external electrical signal. We therefore investigated hybrid mode-locking as a technique to combine the strong pulse-shortening effect of the saturable absorber with external gain modulation. Electrical pulses were injected into the gain segment of the laser along with the dc forward bias, with the short absorber section reverse-biased as in the passively mode-locked case. When the modulation frequency was tuned to match the roundtrip time of the laser, short pulses with characteristics similar to that of the passively mode-locked case were generated, with pulsewidths less than 6.5 ps as measured by autocorrelation at a pulse energy of 16 pJ. This is due to the fact that the saturable absorption is the dominant pulse-shaping mechanism in this configuration. Fig. 4 shows the sampling oscilloscope measurement of
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Fig. 4. Sampling oscilloscope measurement of hybridly mode-locked pulses. The pulsewidth is limited by the impulse response of the measurement system.
pulses generated in this manner, with the time base triggered by the R F modulation source. The oscilloscope displays the pulsewidth to be 23 ps, which again is the impulse response limit of the measurement, indicating that the pulses are short and have low timing jitter with respect to the drive signal. VII. CONCLUSIONS We have demonstrated the first active, passive, and hybrid mode-locking of ROW laser arrays. The in-phase operation of such arrays results in well-behaved emission patterns that allow for coupling into external cavities with high efficiencies comparable to those of single-stripe laser designs. Active mode-locking of such lasers resulted in > 50-pJ pulse energies that are not limited by the gain saturation energy, but were relatively long in duration (22 ps) because high-speed gain modulation was not feasible. By using multisegment devices with a short saturable absorber segment, passively mode-locked pulses as short as 5.6 ps were generated without multiple pulsations. The maximum passively mode-locked pulse energy was 21.9 pJ, with peak powers of over 3 W. This is about the value of pulse energy one would expect, based on a scaling of the saturation energy by the number of array elements, demonstrating that ROW arrays are effective for increasing the output powers from mode-locked semiconductor lasers with intra-waveguide saturable absorbers. Hybrid mode-locking resulted in pulse characteristics similar to that of the passively mode-locked case, with the output synchronized to the external drive signal. These characteristics of mode-locked ROW array lasers indicate a strong potential for use in applications requiring high output powers. REFERENCES [l] L. Y. Pang, J. G . Fujimoto, and E. S. Kintzer, “Ultrashort-pulse
generating from high-power diode arrays by using intracavity optical nonlinearities,” Opt. Left., vol. 17, pp. 1599-1601, Nov. 1992. [2] L. J. Mawst, D. Botez, C. Zmudzinski, M. Jansen, C. Tu, T. J. Roth, and J. Yun. “Resonant self-aligned-stripe antiguided diode laser array,” Appl. Phys. Lett., vol. 60, no. 6, Feb. 1992. [31 D. J. Derickson, R. J. Helkey, A. Mar, J. R. Karin, J. G. Wasserbauer, and J. E. Bowers, “Short pulse generation using multi-segment mode-locked lasers,” J . Quantum Electron., vol. 28, pp. 2186-2202, Oct. 1992. [4] A. Mar. D. J. Derickson, R. Helkey, J. E. Bowers, R. T. Huang,
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and D. Wolf, “Actively mode-locked external-cavity semiconductor lasers with transform-limited single pulse output,” Opt. Lett., vol. 17, pp. 868-870, June 1992. D. J. Derickson, R. J. Helkey, A. Mar, and J. E. Bowers, “Suppression of multiple pulse formation in external cavity mode-locked lasers using intra-waveguide saturable absorbers,” IEEE Photon. Technol. Lett., vol. 4, pp. 333-335, Apr. 1992. D. Botez, L. J. Mawst, G. L. Peterson, and T. J. Roth, “Phaselocked arrays of antiguides: Modal content and discrimination,” J. Quantum Electron., vol. 26, pp. 482-495, Mar. 1990. A. Mar, J. D. Dudley, E. L. Hu, and J. E. Bowers, “Reactively sputtered silicon oxynitride for anti-reflection optical coatings,” Electronic Materials Conference, Santa Barbara, CA, Oct. 1990. J. E. Bowers, P. A. Morton, S. Corzine, and A. Mar, “Actively mode locked semiconductor lasers,” J . Quantum Electron., vol. 25, pp. 1426-1439, June 1989.
191 J. P. van der Ziel, H. Temkin, R. D. Dupuis, and R. M. Mikulyak, “Mode-locked picosecond pulse generation from high power phase-locked GaAs laser arrays,” Appl. Phys. Lett., vol. 44, Feb. 1984. [lo] H. Masuda and A. Takada, “Picosecond optical pulse generation from mode-locked phased laser diode array,” Electron. Lett., vol. 25, pp. 1418-1419, Oct. 1989. [ l l ] M. Segev, Y. Ophir, B. Fischer, and G. Eisenstein, “Mode locking and frequency tuning of a laser diode array in an extended cavity with a photorefractive phase conjugate mirror,’’ Appl. Phys. Lett., vol. 57, Dec. 1990. [12] J. C. Kuo, C. S. Chang, and C. L. Pan, “Buildup of steady-state picosecond pulses in an actively mode-locked laser-diode array,” Opt. Lett., vol. 16, pp. 1328-1330, Sept. 1991.
High-speed Vertical-Cavity Surface Emitting Laser G. Shtengel, H. Temkin, Fellow, ZEEE, P. Brusenbach, Member, ZEEE, T. Uchida, M . Kim, C . Parsons, W. E. Quinn, and S. E. Swirhun, Member, ZEEE Abstract-We describe planar, gain-guided vertical-cavity surface emitting lasers with a modulation bandwidth of 14 GHz. This bandwidth is reached at a drive current of only 8 mA. The intrinsic bandwidth of these devices is estimated to be greater than 50 GHz. Nonlinear light-current characteristics of these lasers may lead to a high level of nonlinear harmonic distortion of the high-frequencyoutput. We show that the relative intensity of the second harmonic response decreases rapidly at higher drive currents, in agreement with a phenomenological model based on the dc characteristics of the laser.
modulation bandwidths achieved thus far have been limited to approximately 8 GHz for gain-guided lasers [61 and about 5 GHz for index-guided lasers [71. We report a significant improvement in the modulation bandwidth, of up to 14 GHz at a bias current of only 8 mA, of completely planar, gain-guided VCSEL’s. We also study the modulation efficiency and the effects of nonlinear harmonic distortion arising in such lasers from the nonlinear light-current characteristics. Two types of lasers, emitting at 0.96 and 0.78 pm, were used in this work. The epitaxial structures consisted of a H E POSSIBILITY of high-speed operation at low bias currents is one of the most attractive features of bottom n-type Bragg reflector, a single-wavelength cavity, vertical-cavity surface emitting lasers (VCSEL‘s). This and a top p-type reflector. The bottom quarter-wavelength arises from the small active layer volume and high photon layers comprising the high-reflectivity mirrors consisted of density. Considerable progress has been achieved recently AlAs/GaAs (23.5 periods) and AIAs/AIo,,Gao,,As (30.5 in improving the continuous wave (CW) parameters of periods) for the 0.96- and 0.78-pm lasers, respectively. VCSEL‘s such as peak optical power, operating voltage, These mirrors were doped with Si to n = 1.5 . 1OI8 ~ m - ~ . and thermal resistance [l],[2]. However, the modulation The p-type top mirrors (the output couplers) consisted of properties of VCSEL‘s remain restricted by parasitics 16.5 and 23.5 periods, respectively, and were doped with C resulting from the high p-type mirror resistance and, to p = 3 * 1018 cm-,. The p-type mirrors had composiprobably, transport effects [31, [4]. Although relaxation tionally graded interfaces. The active region of the 0.96oscillations at frequencies as high as 70 GHz have been p m laser contained three quantum-wells (QW’s) of observed, confirming the high intrinsic bandwidth [5], the In,,Ga,,,As, each being 8 nm thick and separated by 10-nm thick barriers of GaAs. The active layer of the 0.78-pm laser was a 52-nm thick layer of ~,,,Ga,,,,As. Proton implantation was used for isolation and gain guidManuscript received July 29, 1993; revised October 4, 1993. G. Shtengel and H. Temkin are with the Department of Electrical ing. Both wafers received an initial implant that extended from the top surface through the active layers. The 780-nm Engineering, Colorado State University, Ft. Collins, CO 80523. P. Brusenbach, T. Uchida, M. Kim, C. Parsons, W. E. Quinn, and S. E. wafer also received a second implant centered at the Swirhun are with Bandgap Technology Corporation, Broomfield, CO active layer and defining a slightly smaller aperture (by 80021. approximately 20%) than the first implant. The structures IEEE Log Number 9214045.
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