Scheme for Extending the Bandwidth of Injection-current-induced Laser Diode Optical Frequency Modulation

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Abstract
It is shown that optical frequency modulation of laser diodes via control of their injection current is dominated by thermal effects for modulation frequencies in the range up to about 1 MHz, and that the modulation bandwidth is determined by the complex thermal structure of the laser. A theoretical model is developed, based on the thermal properties and the heat transfer between the laser chip, its submount and its heat sink. The theory is validated by experiments which indicate that the 1 dB modulation bandwidth of the typical laser diode used was about 10 kHz. The design of a simple equalization circuit for the laser diode is described, and its application was shown to increase the modulation bandwidth to about 300 kHz. It is shown that the equalization technique is valuable in extending the bandwidth of phase demodulation techniques commonly employed in interferometric fibre optic sensors.