Optical injection locking of vertical cavity surface emitting lasers

Semiconductor optoelectronics are a major enabling component in optical communications. Directly modulated diode lasers are widely deployed in today's high-speed digital optical communications. They are compact, low cost and consume low power. However, their inherent nonlinearity and noise made them, thus far, unsuitable for analog transmission. Additionally, the frequency response has prevented directly modulated lasers to being usable for high speed digital transmission at bit-rates above 10 Gb/s. Instead, high performance links have used continuous wave lasers in conjunction with external optical modulators, which can be made to have a higher linearity and higher speed. The disadvantages are in size, cost in materials and integration, and power consumption.; For low-cost, high-performance optical data links, it is of interest to increase the performance of directly modulated semiconductor lasers. It was found that a technique called optical injection-locking can drastically improve the performance of a transmitter laser. Optical injection-locking uses a second laser to inject photons at a similar wavelength into the transmitter laser. The transmitter laser is thus locked (in wavelength and phase) to the master due to the coherent non-linear interaction inside the laser cavity. The advantage of injection locking is that the transmitter laser characteristics may change fundamentally resulting in a far better device performance, achieving large signal modulation at a far higher frequency than achieved today. The work has demonstrated an experimental record modulation bandwidth of over 30 GHz for a directly modulated vertical cavity laser.; With the use of a 1.55 um vertical cavity surface emitting laser (VCSEL), we recently showed that optical injection locking is a very promising scheme to enhance the analog transmission characteristics of a directly modulated laser. The unique high Q cavity in a VCSEL makes the injection locking far more efficient, stable and requires much less injection power. A record 25 dB increase in spur free dynamic range (SFDR), 7X relaxation oscillation frequency and 5--10 dB increase in modulation efficiency have been experimentally demonstrated. Additionally, a laser noise reduction is achieved. The theoretical modeling confirms the experimental results.; With the demonstrated improvements in directly modulated laser transmission, high performance transmitters using injection locking can be envisioned. A novel 1-to-N locking scheme is proposed, where one master laser could wavelength lock an array of lasers at a reproducible wavelength spacing. This would substantially reduce the size and cost of a multi-channel transmitter. Eliminating the temperature and wavelength controllers in a transmitter is very desirable, since these are the most bulky and power intensive components in a transmitter module. Using tunable VCSELs with injection-locking, we have shown that the devices become temperature insensitive with a performance enhancement due to locking. Their wavelength remains locked over an ambient temperature range, with a nearly constant resonance frequency and linearity.; The injection-locking technique has been theoretically and experimentally shown to be extremely effective at improving the performance of semiconductor lasers, which to date, have not been possible otherwise. This technique will enable higher speed communications, paving the way for a new higher performance transmitter. Furthermore, theoretical and experimental work at the much higher frequencies is expected to reveal a further understanding of laser physics.