A theory of small signal frequency response of four-wave mixing (FWM) in a traveling wave semiconductor optical amplifier has been developed. The bandwidth of FWM is >300 GHz. For large wavelength separation between the input signals, where the nonlinear gain effects dominate the generation of FWM, the bandwidth of the FWM signal exceeds 1 THz.
Mode locked semiconductor lasers are important for a wide range of fiber transmission systems. These include soliton generation and clock recovery in very high speed transmission systems. Compact, single chip, mode locked lasers are desirable for practical systems since they are likely to be more stable than the off-chip external cavity designs. Colliding pulse mode locked semiconductor lasers that operate at -300 GHz have been reported. However, electronic circuits do not exist to encode such a high speed stream of pulses with data. Hence, for many applications including optical transmission, short pulses (few ps wide) at close to transmission rates (-10 GHz) is important. The light from these sources could be encoded with data and then multiplexed to generate high data rate ( > 100 Gb/s ) bit stream. The fabrication and performance characteristics of a colliding pulse mode locked laser which operates near 10 GHz repetition rate is reported here.In the colliding pulse mode locked laser, a saturable absorber is placed in the middle of the optical cavity. The mode locked pulses from opposite directions arrive at the saturable absorber at the same time, which saturates the absorber faster and hence a shorter pulse could be obtained. The laser has four amplifier sections. Two sections are in the middle and two sections are at each end of the optical cavity. These sections are joined by a waveguide layer through which the light propagates and are pumped with enough current to produce gain. The saturable absorber is in the middle. All sections have both d.c. and rfcontacts. The modulation current is applied to the saturable absorber section. The waveguide below the active region exists over the entire length of the chip. The light generated in the active region travels through this waveguide and is reflected back. The optical cavity is formed by the cleaved facets. The four active gain sections are -350 pm long each, and, the saturable absorber section is -150 pm long. The total length of the chip is -8.6 mm long. The estimated round trip time is 0.1 ns which corresponds to a repetition rate of about 10 GHz for the mode locked pulses.The threshold current of the laser is -65 mA ( with no current in the absorber section ) and fiber coupled differential efficiency is 0.04 mW/mA. Under CW operation the laser emits in a single frequency near 1.55 pm. The mode locking behavior of the laser is investigated by modulating the laser at close to 10 GHz. The pulse width is measured using an autocorrelator. The measured mode locked pulse width plotted as a function of modulation frequency is shown in Fig 1. The total bias current on center amplifier is 44mA, lOOmA on each facet amplifier, 0 mA on saturable absorber. Minimum pulse width is 3.9ps at frequency 9.594GHz, spectral width 1.2nm, so the Az*Au=0.59The laser performance was examined further by modulating the end amplifiers in addition to the saturable absorber with the same modulation current. In this case, the pulse width as a function of the phase difference between the ...
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