Integrated Optical Waveguide Device for Picosecond Pulse Compression

We report on the fabrication and characterization of a new device, named IRIS. It is designed to increase the coherent optical bandwidth of a picosecond pulse train for subsequent pulse compression. The IRIS device consists of a concatenated array of semiconductor optical amplifiers and saturable absorbers. It is realized in the InP/InGaAsP material system for the 1550 nm range. Measurements on the realized IRIS devices show increased broadening of the pulse spectrum as compared to an SOA of equivalent length and a smoother optical power spectrum. Consequent compression of the chirped pulses using standard single mode fiber shows decreased pulse pedestals as compared to an SOA, indicating increased linearity of the chirp. Pulse compression up to 50 % is observed. Introduction Trains of picosecond optical pulses with wavelengths around 1550 nm generated by semiconductor lasers have many applications, most notably in high-speed optical time division-multiplexed multiplexed (OTDM) systems [1]. Future 640 Gbit/s and Tbit/s systems will even require pulses with sub-picosecond duration. Integrated mode-locked semiconductor laser sources (MLLs) are typically able to generate transform limited pulse trains with durations down to a few picoseconds [2]. Further pulse compression can for example be obtained using soliton effects in fiber amplifiers and the subsequent use of dispersive fibers [3]. However these are bulk components. For reasons of compactness, stability and cost, integration of a pulse compressing system with the pulse source is preferable. Several options have been proposed as an integrated optical pulse compressor. In [4] a waveguide device with concatenated semiconductor optical amplifiers (SOAs) and saturable absorbers (SAs) has been simulated and it was shown to be able to compress initially transform limited picosecond pulses. Chirped, or non-transform limited, pulses can be compressed further using integrated pulse compressing devices [5, 6]. Chirping or adding bandwidth to an initially transform limited pulse can be done using SOAs, though the amount of spectral broadening is limited and the power spectrum shows a modulation [7]. The latter will result in pulse pedestals or even satellite pulses when the pulses are compressed. In [7] we showed experimentally that an integrated optical waveguide device consisting of multiple equal SOA/SA pairs, which we named IRIS (Integration of Regeneration, Isolation and Spectral Shaping), seems promising for abovementioned application. Compared to a single SOA the obtained bandwidth after picosecond pulse propagation through the device is larger and showed significantly less modulation in the spectrum for specific operation conditions. In this work we study this IRIS device with respect to both its temporal pulse shaping and spectral shaping effects on a picosecond pulse train. A number of configurations of IRIS devices have been realized by us. We experimentally compare the performance of these devices with an SOA of the same length. IRIS device design and realization The IRIS device consists of a series of equal pairs of one SOA section and one SA section, as schematically depicted in Fig. 1(a). Its most important feature is that the saturation energy of the SA is lower than the saturation energy of the SOA. As a result a picosecond pulse that is being amplified by the device will first saturate the SA and only after that the SOA, much like in passive mode-locking.