IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 36, NO. 9, SEPTEMBER 2000 1035 Ultrashort Pulse Generation by Intracavity Spectral Shaping and Phase Compensation of External-Cavity Modelocked Semiconductor Lasers S. Gee, Gerard A. Alphonse, Fellow, IEEE, John C. Connolly, Member, IEEE, C. Barty, and Peter J. Delfyett, Senior Member, IEEE Abstract—Intracavity spectral shaping and external chirp com- pensation techniques were employed to generate nearly transform- limited optical pulses with a temporal duration of 250 fs from an ex- ternal-cavity modelocked semiconductor laser. It was also demon- strated that intracavity spectral shaping techniques can be used for artificially tailoring the chirp of the output pulses. Index Terms—Intracavity spectral shaping, modelocked diode lasers, optical phase measurements, pulse shaping, ultrafast pulse generation. I. INTRODUCTION C OMPACT efficient sources of ultrashort, high-repeti- tion-rate optical pulses are necessary for a variety of commercial applications. Modelocked semiconductor lasers are very promising in ultrashort optical pulse generation [1]. It was demonstrated for semiconductor lasers that generating relatively temporally broad optical pulses directly from a laser cavity is more advantageous than generating short trans- form-limited pulses directly from a laser cavity since one can avoid the pulsewidth-dependent gain saturation effect arising from dynamic carrier heating in semiconductor diodes [2]–[4]. In other words, for shorter optical pulses propagating in semi- conductor optical amplifiers, the greater is the effect of dynamic carrier heating and pulsewidth-dependent gain saturation. The chirp impressed on the pulses can then be removed externally to generate short output pulses using a dispersion-compensating element such as a grating pair containing a one-to-one telescope [5]. Having this as a standard strategy, efforts to create shorter pulses were pursued in two directions: the first step is to gen- erate a broader optical spectrum from a laser cavity, while the second step is to develop means to compensate for nonuniform spectral phase externally to generate transform-limited short pulses. In the second step, the characterization of the spectral phases of the optical pulses is an important prerequisite for optimum dispersion compensation. Manuscript received January 5, 2000; revised May 24, 2000. This work was supported in part by the National Science Foundation under Grant ECS 96-29066. S. Gee and P. J. Delfyett are with the School of Optics, Center for Research and Education in Optics and Lasers (CREOL), University of Central Florida, Orlando, FL 32816-2700 USA. G. A. Alphonse and J. C. Connolly are with Sarnoff Corporation, Princeton, NJ 08543 USA. C. Barty is with the University of California at San Diego, La Jolla, CA 92093-0339 USA. Publisher Item Identifier S 0018-9197(00)07264-X. In this paper, an intracavity spectral shaping technique is introduced in order to increase the spectral width of intracavity pulses. Both temporal information via intensity (TIVI) and Gerchberg–Saxton (GS) algorithms are employed to charac- terize the spectral phase of the output pulses. Then, the spectral phase compensation is performed by using a conventional pulse-shaping setup with a liquid crystal spatial light modulator (SLM). It was found that the intracavity spectral shaping tech- nique allows not only the control of spectral intensity, but also the control of chirp, to some degree, owing to the nonlinearity of the semiconductor gain media. II. INTRACAVITY SPECTRAL SHAPING Since the temporal pulsewidth of intracavity pulses (10 ps) is much longer than the intraband transition time ( 100 fs) of semiconductors, semiconductor optical amplifiers (SOAs) can be regarded simply as homogeneously broadened, two-level gain media [6]. For homogeneously broadened gain media, the gain-narrowing effect due to mode competition is the major obstacle in obtaining a broad optical spectrum. Typically, SOAs have a gain bandwidth of 10 Hz. Modelocked semicon- ductor laser spectra, however, generally have bandwidths less than 2.5 10 Hz. One way to avoid the gain-narrowing effect is to utilize an intracavity spectral shaping technique, where artificial loss is introduced into the laser cavity. By controlling the loss profile in the spectral domain, it is possible to control the laser output spectrum to produce an arbitrary spectral shape. In our case, in order to generate a broad optical spectrum, an intracavity spectral shaping element that mimics the inverse spectral gain profile is necessary. In other words, the loss is high (low) where the gain is high (low) so that net gain has a flat spectrum to suppress the gain-narrowing effect. This gain-flattening filter was realized by employing a Fabry–Perot etalon. It is well known that the etalon has a periodic loss profile in the spectral domain. By adjusting the shape of the loss spectrum, the gain and loss product can be made spectrally flat. This technique has been used to reduce gain-narrowing effects in an ultrafast regenerative amplifier system [7]. It should be noted that the gain of the SOA is large enough to overcome the extra loss caused by the etalon. Two optical flats were separated by an air gap to form an etalon and the reflectivity of the interface between air and glass determined the finesse of this etalon. In order to match the gain spectrum of the SOA and the loss spectrum of etalon, it is necessary 0018–9197/00$10.00 © 2000 IEEE