Self..mode",locking of a semiconductor laser

Active-mode locking of semiconductor lasers has pro­ duced optical pulses as short as 0.58 ps.! This result was achieved using the positive portion of a 16 GHz sinusoid as an electrical drive signal. For many applications, such as electro-optic sampling, it is more useful to have the mode­ locked optical pulses at a much lower repetition rate. This requires an electrical drive source that produces short elec­ trical pulses at this lower rate. Step recovery diodes can be used, but they typically have pulse widths greater than 50 ps. Since the mode-locked optical pulses themselves are short, they are good candidates to drive the mode-locking action. This letter describes a new mode-locking technique caned self-mode-locking that uses a high-speed optical to electrical (0/E) converter in a positive feedback configuration to con­ vert the output optical pulses back into electrical drive sig­ nals in a regenerative process. Previous works,3 have used positive feedback in a self-gain-switching configuration. Self-mode-Iocking is different in that both coordinated opti­ cal and electrical feedback are involved and much shorter pulses can be obtained. Advances in high-speed electrical components allow the short optical pulse to generate a short electrical drive pulse. Photodetectors have been shown to be capable of producing impulse responses ofless than 10 pS. Since the photocurrent is too small to directly drive the laser, an amplifier is necessary to boost the feedback signaL Broad­ band distributed amplifiers have produced 3 dB bandwidths of over 30 GHz and new device technologies promise to extend this bandwidth even further. Consequently, this tech­ nique can produce short mode-locked pulses at lower repeti­ tion rates, and it can do this without the need for an external electrical drive source. The block diagram ofthe self-mode-Iocked semiconduc­ tor laser is shown in Fig. 1. A semiconductor laser is placed in an external ring cavity configuration with a round trip delay time of 2 ns. The laser is a high-speed 1300 nm semi­ insulating planar buried heterostructure laser with antire­ flection coatings on both facets. The light from the laser is collimated by the use of two antireflection-coated graded index lenses. The 0 IE convcrter consists of a high-speed p-i-n photodetector and a four-stage metal-semiconductor field-effect transistor distributed amplifier. The 0 IE con­ verter has an overall responsivity of 20 a/W. Impulse re­ sponse measurements ofthe 0/E converter using 1060 nm, 3 ps optical pulses show that it is capable of producing 50 ps full width at half maximum (FWHM) pulses with a peak amplitude of 3.5 V into a 50 n system. The self-mode-locking action occurs as follows. The la­ ser is first biased above the cw lasing threshold. Counterpro­ pagating optical signals start to build up in the ring cavity. One of the two output signals is fed into an 0/E converter. This optical signal is then converted into an electrical drive signal and applied to the direct modulation input ofthe laser. Ifthe found trip delay ofthe optical cavity and the round trip delay of the signal that goes through the 0/E converter are properly chosen, a regenerative process will build up a mode­ locked pulse in the laser, In order for self-made-locking to occur, the loop gain of the 0/E converter feedback path, Rl]hvlq, must be greater than 1. R is the 0 IE converter responsivity, 17 is the laser differential quantum efficiency, h is Planck's constant, v is the optical frequency, and q is the electronic charge. The other condition necessary for self­ oscillation is that the optical path time delay To must be related to the electrical path time delay Te by the following expression: