Widely Tunable Short-Pulse Generation With Ultralong Semiconductor Optical Amplifiers

Ultralong bulk semiconductor optical amplifiers (bulk UL-SOAs) have tremendous four-wave mixing (FWM) efficiencies so that due to two continuous wave (CW) input signals, a broad mode comb at the output of the UL-SOA can be obtained. When using tunable single mode lasers as input signals, the generated FWM mode comb is tunable in the mode spacing and the wavelength. As a result in time domain, widely tunable pulses in the repetition rate and the carrier frequency are obtained. Since the phase signature of the FWM modes is not linear over the whole mode comb, different filtering techniques can be applied for filtering modes with linear phase in order to efficiently generate short pulses. With the optimal filtering technique, pulses with full width at half maximum of less than 2 ps and a pulse repetition rate of 20 GHz can be obtained.

I. Introduction

With increasing demand for higher transmission rates in optical communication systems, the transmission capacity has to be optimised. In [1] a combination of wavelength division multiplexing (WDM) and time division multiplexing (TDM) has been applied to transmit 2.56 Tb/s over a 160 km fiber link. For the TDM part, add-drop multiplexing plays a key role, where short pulses as a clock or for switching are needed. Moreover, a trend towards optical packet switched networks can be observed where again short pulses are needed for all-optical signal processing. Furthermore, in packet switched networks the components should be agile, in order to fulfil operations at different wavelengths and different data rates [2]. In general, the Kerr effect in highly nonlinear fibers (HNLFs) is used for generating a supercontinuum [3] which in turn can be used for the generation of short pulses [4], [5]. Typically, the Kerr effect creates self-phase modulation (SPM) due to its dependence of the input signal's optical power. The additional modulation in combination with a dispersion managing component creates a broader spectra so that in time domain short pulses can be obtained. But due to the HNLFs, the source cannot be easily integrated. Another device that creates short pulses and can be integrated, is the mode-locked laser, but its possibilities for tuning the repetition rate or the carrier frequency of the pulses are very limited, because these properties are defined by the material and the geometry [6]. Opposite to the passively mode-locked laser, the monolithic fundamental actively mode locked laser can be tuned in the repetition rate of the pulses, but needs a complex electronic driving stage in order to obtain high repetition rates [7]. Furthermore, when reaching with the pulse duration the terahertz regime (0.6–6 THz) also non-telecommunication applications in the field of biomedical or imaging applications are possible [8], [9].