Strong Optical Injection Locking of Edge-Emitting Lasers and Its Applications


Semiconductor lasers are essential components that enable high-speed long-haul communication and have been widely used for various applications in photonics technology. Semiconductor lasers under optical injection locking exhibit superior performance over free-running lasers and provide useful applications not achievable through the free-running lasers. The performance of injection-locked lasers has been found to be significantly improved with stronger injection.

In this dissertation, the characteristics and applications of semiconductor lasers under strong optical injection locking are presented and analyzed in various aspects. First, ultra-strong (injection ratio ~ 10 dB) optical injection locking properties are investigated experimentally and theoretically. Direct modulation responses of ultra-strong optical injection-locked distributed feedback (DFB) lasers show three distinctive modulation characteristics depending on frequency detuning values. These different optical properties and electric modulation characteristics can be utilized in various applications such as analog fiber optic link, broadband digital communications, RF photonics and opto-electronic oscillators (OEOs). Using the strong injection-locked lasers, a novel single sideband generation has been demonstrated. A modulation sideband on the longer wavelength side is enhanced due to the resonant amplification by the slave laser’s cavity mode, resulting in a 12-dB asymmetry at 20-GHz RF modulation. The dispersion- limited RF bandwidth has been greatly increased by maintaining the variation of fiber transmission response within 7 dB up to 20-GHz RF carrier frequency over 80-km fiber transmission.

Second, to improve fiber optic link performances, gain-lever distributed Bragg reflector (DBR) lasers have been fabricated. With a gain-lever modulation, 9-dB increase of a link gain has been achieved compared with a standard modulation. By combining the gain-lever modulation with optical injection locking, nonlinear distortion reduction and modulation bandwidth enhancement have been achieved as well as maintaining the improved link gain.

Finally, we have proposed two-section DFB lasers for simple optical injection locking systems. The two-section DFB lasers show similar locking / unlocking phenomena as conventional injection locking systems using external master lasers. Electrically isolated gain sections in monolithic lasers can support independent lasing modes. The independent modes are mutually locked under certain current bias conditions. With a direct modulation of the monolithic lasers, we achieved resonance frequency enhancement, modulation bandwidth increase and chirp reduction. We have also demonstrated high optical extinction ratio and millimeter-wave generation using the monolithic lasers.

For future works, ultra-strong injection-locked lasers might enable broadband (> 40 Gb/s) digital signal transmission of directly-modulated lasers and be employed in high- frequency (> 60 GHz) OEOs. The integration of the two-section lasers with modulators will enable high optical extinction ratio (> 65 dB) with modulation bandwidth up to several GHz. The ultra-strong injection locking technique and monolithic injection- locked lasers can significantly extend the performance of directly-modulated lasers for high frequency applications.

Constance Chang-Hasnain
Costas P. Grigoropoulos
Publication date: 
December 31, 2006
Publication type: 
Ph.D. Dissertation
Sung, H-K. (2006). Strong Optical Injection Locking of Edge-Emitting Lasers and Its Applications. United States: University of California, Berkeley.

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