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Injection-locking in fabry-perot quantum-well lasers

Optoelectronic devices and fiber optic communication have revolutionized the Internet, longdistance phone calls, and cable TV for the past few years. The massive amounts of data transmission require large amounts of bandwidth, achievable through high-speed modulation of the laser sources. In this study, we report experiment results and theoretical calculations of dc and small-signal modulation of a side-mode injection-locked Fabry-Perot (FP) laser. We first measure the lasing spectra of the master laser and the injection-locked quantum-well test laser. The master laser is a tunable laser source emitting at 1557.45 nm, which is close to the fifth side-mode on the longer wavelength side of the lasing mode of the test laser. In Fig. 1, the dashed line shows the laser emission spectrum of the test laser with the main peak wavelength at 1551.2 nm. This is a typical FP laser spectrum, and the side-mode suppression ratio (SMSR) of the laser is around 20 dB. When the external master laser light is injected into the test laser biased above threshold, the injected light competes with the spontaneous emission of the laser for amplification. When the external light is strong enough and close to the eigen-frequency of the test laser, it is amplified since there is gain available. At the same time it saturates the gain of the other modes and extinguishes all the other free-running modes. Once a perfect locking state is reached, all of the power of the test laser is emitted at the optical frequency of the master laser, as shown by the solid line in Fig. 1. Injection locking greatly improves the SMSR of the test laser and shifts the lasing wavelength from 1551.2 nm to 1557.45 nm. This side-mode injection locking of a FP laser has the advantage of mode selection over that of a single-mode DFB laser. This intermodal injection locking can switch the information from the free-running mode to any side mode as long as the injection-locking condition is satisfied, and can be used for optical wavelength conversion in wavelength division multiplexing (WDM) channel selection [1][2].