IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 12, NO. 2, MARCH/APRIL 2006 181 Femtosecond Optical Parametric Amplification With Dispersion Precompensation Peng Yuan, Liejia Qian, Hang Luo, Heyuan Zhu, and Shuangchun Wen Abstract—In this paper, we study a midinfrared femtosecond optical parametric amplifier (OPA) that is severely affected by group velocity dispersion (GVD). Both theoretical and experimen- tal results show that GVDs in nonlinear crystals will significantly degrade the performance of a femtosecond OPA. By introducing a prechirp to the pump pulse, the effect of GVD can be effec- tively compensated. A lithium-niobate-crystal-based femtosecond OPA demonstrates that the conversion efficiency with optimally prechirped pumping is nearly twice that of the nonchirp case, and the output pulses can be further compressed to nearly their Fourier-transform limit by prism pairs. Index Terms—Femtosecond pulses, group-velocity dispersion (GVD), optical parametric amplifier (OPA). I. INTRODUCTION O VER the last decade, there have been spectacular devel- opments in ultrafast laser technology due to the introduc- tion of broadband solid-state laser materials, and to Kerr lens mode-locking and amplification techniques. These advances, along with the new nonlinear optical crystals, have made op- tical parametric amplification (OPA) a routine technique for generating tunable femtosecond pulses covering visible through midinfrared spectral ranges. [1] Basically, femtosecond OPAs were developed along with the experience of long-pulse pumped OPAs, by imposing special requirements on nonlinear interac- tions of femtosecond pulses; i.e., the spectral acceptances of the crystal need to be sufficiently large compared with the pump pulse bandwidth. Equivalently, in the temporal domain, the over- all group-velocity mismatches (GVMs) among the three waves in the crystal need to be small compared with the pump pulse duration. Thus, knowledge of long-pulse OPAs can be directly applied to the prediction of performance of femtosecond OPAs in the case when GVMs are negligible [2]. One crucial emphasis in a femtosecond OPA design is to search for an appropriate crystal with both a large effective nonlinear coefficient and small GVMs in the spectral range of interest. To date, β-barium borate (BBO) is a commonly used Manuscript received July 14, 2005; revised January 24, 2006. This work was supported in part by Science and Technology Commission of Shanghai Grant 05SG02 and Grant 05JC14005 and in part by Natural Science Foundation of China Grant 60538010 and Grant 10376009. P. Yuan, L. Qian, H. Luo, and H. Zhu are with the State Key Labo- ratory for Advanced Photonic Materials and Devices, Department of Op- tical Science and Engineering, Fudan University, Shanghai 200433, China (e-mail: ljqian@fudan.edu.cn; kingcoco@gmail.com; luohang@fudan.edu.cn; hyzhu@fudan.edu.cn). S. Wen was with the State Key Laboratory for Advanced Photonic Ma- terials and Devices, Department of Optical Science and Engineering, Fudan University, Shanghai 200433, China. He is now with the School of Computer and Communications, Hunan University, Changsha 410082, China (e-mail: scwen@vip.sina.com). Digital Object Identifier 10.1109/JSTQE.2006.872723 crystal for visible and near infrared OPAs [3]–[6]. Pulse dura- tions of both the signal and idler are usually comparable to that of the pump pulses. By resorting to the advantage of perfect group-velocity matching in a noncollinear phase-matching con- figuration, BBO-based noncollinear OPA is one of the effective approaches to the generation of few-cycle optical pulses. [5]–[9] More recently, stabilizations of the carrier-envelope offset phase (CEP) in OPAs were demonstrated, which revealed the unique features of CEP in OPAs [10]–[12], Midinfrared (MIR) fem- tosecond OPAs, on the other hand, rely mainly on nonlinear crystals like LiNbO 3 (LN), KTiOPO 4 (KTP), and their iso- morphs (KNB, RTA, etc.), and either seeded OPA configuration or difference-frequency generation was adopted to improve the efficiency. [13]–[15] These MIR OPA crystals show a com- mon property that GVMs are much smaller than that of BBO in the visible and near infrared and, hence, the crystal lengths can be designed to be longer. Unlike a BBO-based OPA, group- velocity dispersion (GVD) in a MIR OPA would be as important as GVM, and affects the performance of the device considerably. Consequently, the design criteria of a MIR femtosecond OPA should be somewhat different than the conventional OPAs. For example, the typical length of BBO crystal is only 1 mm (non- collinear) to 7 mm (collinear), limited by GVMs, and thus GVD is negligible in many common situations with a ∼100-fs pump pulse, while crystal lengths of ∼2–4 cm are generally adopted in MIR OPAs. Surprisingly, the fundamental issues concerning the impact of GVD on femtosecond OPAs have not been clearly discussed in the literature. In this paper, we study MIR OPA where GVD plays a sig- nificant role. We show that GVDs in crystals will degrade the performance of OPA. It is demonstrated both numerically and experimentally that introducing an appropriate prechirp to the pump pulse may provide an effective means to compensate the effect of GVD, and to achieve high conversion efficiency. The results obtained in this paper can be a useful guideline in designing and optimizing a femtosecond OPA in MIR. In Section II, we present a detailed theoretical study of fem- tosecond OPA with GVDs, which focuses on the optimization of OPA efficiency by dispersion precompensation. The experimen- tal results of a lithium-niobate-crystal-based femtosecond OPA are presented in Section III, which demonstrate that prechirp of the pump pulse is a new degree of freedom for optimizing a femtosecond OPA in MIR. Finally, conclusions are given in Section IV. II. NUMERICAL SIMULATIONS OPA with femtosecond pulses can be treated using the nonlinear-coupled wave equations in the time domain. 1077-260X/$20.00 © 2006 IEEE