Abstract

External modulation is advantageous in digital large signal¹ as well as in small signal modulation applications. The effects of the dynamics of direct modulation (chirping, patterning) are avoided (of specific significance in digital applications which require large extinction ratios) and the basic bandwidth limiting factors of the direct modulation process (gain coefficient, photon density and lifetime) are replaced by modulator parameters such as "walk off" (difference in optical and microwave velocities) and microwave attenuation in traveling wave type modulators. A further factor of importance in microwave applications is that the high speed modulator and the laser can be optimized separately for high speed and high output power, respectively, such that increased modulated optical power can be achieved with unchanged microwave power. In LiNbO₃ baseband traveling wave modulators, the difference in propagation velocities of the optical signal and the microwave signal sets a limit to the bandwidth. However, microwave applications do not require baseband response and generally only require small signal modulation. A number of techniques have been devised to circumvent the walk off problem; closer matching of the propagation velocities², and different velocity mismatch compensation schemes^{3, 4, 5, 6.} Baseband, bandpass or periodic frequency responses are obtained, with improved figures of merit. We report here a bandpass response modulator based on a two section phase reversal electrode scheme, and compare its performance as a small signal microwave modulator to a conventional baseband modulator.

© 1988 Optical Society of America