Broadband radiofrequency design of a laser diode bias circuit

Laser diodes are used in broadband communications including the radiofrequency range. Broadband design of a laser diode bias circuit is presented. The biasing circuit must have a high input impedance for radiofrequency signals, in order to prevent them from entering the DC power supply. The desired frequency bandwidth of the biasing circuit is up to 5 GHz. In this paper, two different biasing circuit designs are explored. Each design is simulated and optimized using electromagnetic simulation software. Based on the simulations, a series of printed circuit boards are designed and measured. It is shown that the power supply input impedance has a very small effect on the bias circuit input impedance value at high frequencies. Decreasing the resistor solder pad size and adding a slot in the ground plane achieves a significant increase in the input impedance value at high frequencies. It is shown that the new bias circuit design has a significantly higher differential impedance in the whole measurement bandwidth, and especially at high frequencies.