As the bit rate of optical FSK modulation increases,1 it is important to develop techniques for measuring the FM response of semiconductor lasers at modulation frequencies up to tens of gigahertz. Both the magnitude and phase of the FM response can be measured using an interferometer as an optical frequency discriminator.2 A highly birefringent fiber, such as a polarization-maintaining (PM) fiber, followed by a polarizer acts as an interferometer when equal optical power is launched along the fiber’s optical axes. Since both optical paths of the interferometer exist in the same fiber, spatial alignment is automatically achieved, the path length difference is insensitive to vibrations, and the free spectral range can be easily set by selecting the fiber length.

© 1989 Optical Society of America

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