618 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 6, JUNE 2000
Short-Pulse Generation with Broad-Band Tunability
from Semiconductor Lasers in an
External Ring Cavity
Bor-Lin Lee and Ching-Fuh Lin
Abstract—Short optical pulses are generated by actively mode locking semiconductor lasers in an external ring cavity with a very broad tuning range from 795 to 857 nm. The wide tunability is pos-sible because the gain bandwidth is broadened by the use of asym-metric dual quantum wells for the semiconductor laser material. Assuming a Gaussian shape, the generated pulses have pulsewidths of 13–21 ps and spectral widths of 2–4.5 Å for the tuning range. The mode-locked spectrum contains almost no amplified spontaneous emission noise.
Index Terms—Asymmetric dual quantum wells, ring cavity, semiconductor laser, short-pulse generation.
O
PTICAL pulses generated from semiconductor lasers have potential applications in areas such as optical com-munications, physics measurement, and photonic switching, due to their compact sizes and low costs. In short-pulse generation, wide-gain bandwidth of laser materials is usu-ally desired because it provides the possibility of achieving ultrashort pulses and broad-wavelength tunability. So far, wide-gain bandwidth has not been fully utilized to achieve ultrashort-pulse generation. In contrast, it could easily offer broad-band tunability. For example, the corresponding spectral width of mode-locked pulses in semiconductor lasers is usually less than 20 nm [1], [2], while the tuning range of mode-locked semiconductor lasers could easily reach over 25 nm [3], [4]. In other words, wide-gain bandwidth of semiconductor laser materials is more promising in broad-band tunability than in extremely short pulse generation. Recently, the gain bandwidth of semiconductor laser materials is further broadened using multiple quantum wells (MQW’s) of different widths [5]–[9]. Its broad-band tunability on single-wavelength oscillation has already been experimentally demonstrated [7]–[10]. In this work, we further report that extremely wide-range tunability can also be achieved for short-pulse generation from semi-conductor lasers using asymmetric dual quantum wells. The tuning range could be as large as 62 nm and the pulsewidth is 13–21 ps.The semiconductor laser material used for the experiment has two QW’s with widths of 40 Å and 75 Å, respectively. The gain bandwidth can then be broadened more than two times. Design
Manuscript received April 5, 1999; revised December 21, 1999. This work was supported in part by the National Science Council, Taipei, Taiwan, R.O.C., under Contract NSC88-2215-E-002-021 and Contract NSC2112-M-002-038.
The authors are with the Institute of Electro-Optical Engineering and the De-partment of Electrical Engineering, National Taiwan University, Taipei, Taiwan, R.O.C.
Publisher Item Identifier S 1041-1135(00)04577-8.
Fig. 1. A schematic diagram of the setup for actively mode-locking experiment.
consideration on the QW structure had been given in [5]. Semi-conductor laser amplifiers (SLA’s) were fabricated on such ma-terial using standard processing techniques. The SLA’s have a ridge waveguide tilted at 7 from the normal of the cleaved facet. The waveguide is approximately 6 m wide and 500 m long. Fig. 1 schematically shows the experimental setup for short-pulse generation using such an SLA, in which the Fabry–Perot resonance of the SLA chip is significantly reduced [11]. Also, a ring cavity was used in this experiment for the following rea-sons. First, because the tilted-stripe SLA has both facets with very small retro-reflectivity, the linear-cavity configuration typ-ically needs two arms. In order to synchronize the RF modula-tion with the pulse train, the lengths of the two arms should be equal or with a ratio of rational number, which is very difficult to align. In contrast, it is relatively easy to synchronize the RF modulation signal with the pulse train in the ring cavity. Second, even though the two arms of the linear cavity is aligned at the re-quired lengths, the small fluctuation of each arm will cause spec-tral instability due to the coherence interference effects [12]. For stability consideration, a ring cavity is also better than a linear cavity. Third, amplified spontaneous emission (ASE) noise of output (a) in the ring-cavity configuration is significantly sup-pressed [10].
Using an SLA with both facets of small reflectivity has the advantage of suppressing the secondary pulses [12], but it also has a drawback. Because the counterpropagating pulses collide in the SLA, gain competition exists between the pulses. How-ever, because the two pulses usually have quite different energy in the semiconductor gain medium, the pulse-colliding effect is not significant. Therefore, the actively mode-locked pulses
1041–1135/00$10.00 © 2000 IEEE
LEE AND LIN: SHORT-PULSE GENERATION WITH BROAD-BAND TUNABILITY FROM A SEMICONDUCTOR LASER 619
Fig. 2. The measured average power of the laser output (a) versus RF modulation frequency.
[12] are not obviously worse than conventional ones using laser diodes with one AR-coated facet.
Fig. 2 shows the measured laser output power versus the mod-ulation frequency for RF power fixed at 16 dBm and dc bias at threshold. This figure shows the overall frequency response of the laser. Very strong response at the harmonics of the cavity fre-quency (295.3 MHz) is observable. High frefre-quency of RF mod-ulation could usually result in shorter pulsewidths [13] because it provides a narrower gain range in the time domain. However, as shown in Fig. 2, the frequency response decreases with the increasing harmonics. The reason is twofold. First, because the device was not well impedance-matched, the high-frequency RF power could not be delivered well to the SLA. Second, in the ex-ternal-cavity configuration, the frequency of relaxation oscilla-tion is usually reduced to less than 1 GHz due to the long cavity length, leading to rapid roll-off of response at high modula-tion frequencies. In order to have a sufficient modulamodula-tion depth, TIA 900-10 was used to amplify the RF signal from the HP 83732B synthesized signal generator. Because TIA 900-10 has a fixed amplification gain of 34 dB within the frequency range 100–900 MHz, the third harmonic, 886 MHz, was used.
In the beginning, the grating was adjusted to have the laser oscillating near 830 nm. The measured autocorrelation shape and width are not strongly affected by the dc biased level as long as it is between and . On the other hand, the RF modulation frequency has a very significant influence. In order to maintain the measured autocorrelation trace with a well-be-haved shape, the RF modulation frequency can only be varied within 0.3 MHz, as shown in Fig. 3. Because the modulation fre-quency coincides with the cavity round-trip characteristics and the pulses sensitively vary with the frequency change, the short pulses are most likely generated by a mode-locking mechanism instead of gain-switching.
The above behavior is similar for the tuning range from 810 to 857 nm. Tuning was achieved by rotating the grating angle, which inevitably changed the cavity length. As a result, the round-trip frequency was slightly different, therefore the modu-lation frequency had to be changed. Fig. 4 shows the RF modula-tion frequency, dc bias, and lasing threshold versus tuning wave-length in the experiment. The lasing threshold for 810–857 nm is approximately between 45 and 70 mA. The RF modulation depth was maintained at for the entire tuning range.
Fig. 3. The autocorrelation trace of the mode-locked pulses at different modulation frequencies.
Fig. 4. The RF modulation frequency, dc biased current, and threshold current versus tuning wavelength for the actively mode-locked lasers in the experiment.
Under the above biasing condition, the output power is around 2 mW, measured from the output beam (a). For wavelengths below 810 nm, the lasing threshold increases to above 70 mA. The RF modulation depth at then causes an extra gain for the tail of the pulses, leading to significant broadening of the pulses. The extra gain could be reduced by lowering the dc biased level. Therefore, the experiment has shown that the well-behaved autocorrelation trace is retained by decreasing the dc bias.
Although the tuning is demonstrated only for 795–857 nm, a wider tuning range for such actively mode-locked pulses is pos-sible with a proper adjustment of dc bias and RF modulation depth. The wide tuning range is because of the broad gain band-width provided by the asymmetric dual QW structure. In ad-dition, unlike passive mode-locking using saturable absorbers, active mode-locking does not require a broad-band absorption spectrum, so it is particularly suitable for mode-locking of semi-conductor lasers with a wide tuning range.
Fig. 5 shows the mode-locked pulsewidth and spectral width versus the tuning wavelength. The pulsewidth is calculated by assuming the Gaussian pulse shape. The pulsewidth is 13–21 ps and, in general, decreases with decreasing laser wavelength. One reason for the tendency may be due to the increasing differ-ential gain with decreasing wavelength [14]. The corresponding spectrum from beam (a) contains almost no ASE noise [10]. The
620 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 6, JUNE 2000
Fig. 5. The mode-locked pulsewidth, spectral width, and the time–bandwidth product versus the tuning wavelength.
FWHM of the spectrum is between 2 and 4.5 Å, showing no ob-vious spectral dependency. This spectral width is much wider than that of single-wavelength operation of the ring laser cavity without RF modulation [10]. The spectra are slightly undulated by the Fabry–Perot resonance of the SLA chip. With the RF modulation, because the instantaneous gain is larger near the peak of the modulation cycle, the Fabry–Perot resonance of the SLA chip is probably enhanced, leading to the undulated spec-trum. The measured spectrum may cover either more convex than concave features or vice versa, depending on the tuning of the grating. As a result, the spectral width randomly varies with the tuning wavelength.
The variation of the time–bandwidth product is also shown in Fig. 5. The excess bandwidth more than transform limit is common in active mode-locking [13] due to the existence of chirp. Because the mode-locking force is not strong enough for the present modulation frequency, increasing the spectral width does not result in a reduced pulsewidth. Instead, it only increases the time–bandwidth product. Therefore, the variation of this product with the tuning wavelength is almost the same as the spectral width. This variation also shows that there is no evidence of decreased chirp with decreasing lasing wavelength. This is different from the past observation [3]. The reason is probably because the spectral width randomly varies with the wavelength. The broad gain bandwidth provided by the asym-metric dual QW’s may also reduce the spectral dependence of the chirp.
To see if a wider spectrum of the laser with asymmetric dual QW’s could be actively mode-locked, the grating was also replaced with a mirror. The spectral width then significantly increased to about 50 Å when the modulation signal was applied. However, no well-behaved auocorrelation shapes were
measured as a result of insufficient mode-locking force for such a broad spectrum.
In conclusion, short optical pulses are generated from semi-conductor lasers in an external ring cavity with a tuning range as large as 62 nm. To our knowledge, this is the broadest tuning range for actively mode-locked semiconductor lasers near the 0.8- m wavelength range. The wide tuning range is possible due to the broadened gain bandwidth using asymmetric dual QW’s for the laser material. The pulsewidths are between 13 and 21 ps with the spectral width 2–4.5 Å. The beam is accompanied with almost no ASE noise.
REFERENCES
[1] P. J. Delfyett, L. Florez, N. Stoffel, T. Gmitter, N. Andreadakis, G. Alphonse, and W. Ceislik, “200-fs optical pulse generation and intracavity pulse evolution in a hybrid mode-locked semiconductor diode-laser/amplifier system,” Opt. Lett., vol. 17, pp. 670–672, 1992. [2] A. Azouz, N. Stelmakh, P. Langlois, J.-M. Lourtioz, and P. Gavrilovic,
“Nonlinear chirp compensation in high-power broad-spectrum pulses from single-stripe mode-locked laser diodes,” IEEE J. Select. Topics
Quantum Electron., vol. 1, pp. 577–582, 1995.
[3] M. Serenyi, J. Kuhl, and E. O. Gobel, “Pulse shortening of actively mode-locked diode lasers by wavelength tuning,” Appl. Phys. Lett., vol. 50, pp. 1213–1215, 1987.
[4] T. Schrans, S. Sanders, and A. Yariv, “Broad-band wavelength tunable picosecond pulses from CW passively mode-locked two-section multiple quantum-well lasers,” IEEE Photon. Technol. Lett., vol. 4, pp. 323–325, 1992.
[5] C.-F. Lin, B.-L. Lee, and B.-J. Lin, “Broad-band superluminescent diodes fabricated on a substrate with asymmetric dual quantum wells,”
IEEE Photon. Technol. Lett., vol. 8, pp. 1456–1458, 1996.
[6] C.-F. Lin and B.-L. Lee, “Extremely broadband AlGaAs/GaAs superlu-minescent diodes,” Appl. Phys. Lett., vol. 71, pp. 1598–1600, 1997. [7] H. S. Gingrich, D. R. Chumney, S.-Z. Sun, S. D. Hersee, L. F. Lester,
and S. R. J. Brueck, “Broadly tunable external cavity lasers diodes with staggered thickness multiple quantum wells,” IEEE Photon. Technol.
Lett., vol. 9, pp. 155–157, 1997.
[8] X. Zhu, D. T. Cassidy, M. J. Hamp, D. A. Thompson, B. J. Robinson, Q. C. Zhao, and M. Davies, “1.4-m InGaAsP-InP strained mul-tiple-quantum-well laser for broad-wavelength tunability,” IEEE
Photon. Technol. Lett., vol. 9, p. 1202, 1997.
[9] T. F. Krauss, G. Hondromitros, B. Vogele, and R. M. De La Rue, “Broad spectral bandwidth semiocnductor lasers,” Electron. Lett., vol. 33, pp. 1142–1143, 1997.
[10] B.-L. Lee and C.-F. Lin, “Wide-range tunable semiconductor lasers using asymmetric dual quantum wells,” IEEE Photon. Technol. Lett., vol. 10, pp. 322–324, 1998.
[11] G. A. Alphonse, J. C. Connolly, N. A. Dinkel, S. L. Palfrey, and D. B. Gilbert, “Low spectral modulation high-power output from a new Al-GaAs superluminescent diode/optical amplifier structure,” Appl. Phys.
Lett., vol. 55, pp. 2289–2291, 1989.
[12] N. A. Olsson and C. L. Tang, “Active mode locking of linear and ring external-cavity semiconductor lasers,” IEEE J. Quantum Electron., vol. QE-17, pp. 1977–1982, 1981.
[13] J. E. Bowers, P. A. Morton, A. Mar, and S. W. Corzine, “Actively mode-locked semiconductor lasers,” IEEE J. Quantum Electron., vol. 25, pp. 1426–1439, 1989.
[14] M. Hofmann, M. Koch, J. Feldmann, W. Elsaber, E. O. Gobel, W. W. Chow, and S. W. Koch, “Picosecond gain dynamics of an actively mode-locked external-cavity laser diodes,” IEEE J. Quantum Electron., vol. 30, pp. 1756–1762, 1994.
