February 2001 Meeting

ADVANCES IN PHOTONIC DEVICES FOR ANALOG FIBER LINK APPLICATIONS

The Optical Society of San Diego and this evening's host, Steve Pappert, are pleased to present Professor Paul Yu, Department of Electrical and Computer Engineering, UCSD.

Abstract:  This presentation surveys the requirements to be met by photonic components in applications such as CATV, antenna remoting and base station connections in wireless communication systems. Demands for greater bandwidth have led to the increasing use of photonic approaches to what had been exclusively electronic applications. We will review the advances in the state-of-the-art photonic devices such as laser diodes, high power lasers, external modulators, optical amplifiers, and photodetectors. Comparisons will also be made regarding the direct modulation scheme versus the external modulation scheme.

Speaker:   Dr. Yu is a professor in the Department of Electrical and Computer Engineering at the University of California at San Diego (UCSD) where he teaches courses in fiber-optics and optoelectronics and conducts research on semiconductor materials and devices for photonic and microwave applications. His research interests include: lasers in the near infrared wavelength region for optical communication and optical interconnection; optical/RF scheme for narrow-band, high center frequency microwave transmission; high speed, high power optical detectors and high speed waveguide modulator devices for both digital and analog modulation; high power semiconductor optical switches for microwave generation; and electronic analog-to-digital conversion using high speed optical switches. He is one of the founding members of the IEEE AP/ED/MTT Chapter of San Diego, a Senior member of IEEE and a member of OSA.

February Meeting Review:
Advances in Photonic Devices for Analog Fiber Link Applications
by Paul Young, IPITEK

Dr. Yu's goals were to discuss advantages and applications of certain link architectures and important devices. He was, in fact, very successful. He started with a review of analog fiber optic links. In particular, a model for an externally RF modulated laser coupled through a fiber optic waveguide to a receiving photodetector. The important link parameters being RF link gain, bandwidth, noise figure and spurious-free dynamic range (SFDR).

He then discussed the two most important modulation methods: direct modulation of the laser's cavity, and modulation by a modulating device placed external to the laser. External modulation ("ex-mod") improves the third order intermodulation distortion caused by laser diode frequency chirping combined with fiber dispersion; especially for non-1310 nm wavelength transmitters since this wavelength is in the minimum dispersion window for standard singlemode fiber. Also, "ex-mods" can use a higher power laser source (e.g., 200-mW YAGs). Dr. Yu pointed out that with ex-mods, a broader-linewidth source has the advantage of reducing stimulated Brillouin scattering (SBS) in the fiber.

A very important parameter of ex-mods, which exhibit a sinusoidal transfer function, is the Vp. In fact, the RF link gain is inversely proportional to the square of the Vp. Thus, researchers are hard at work to minimize Vp For example, Mach-Zehnder (MZ) external modulators made from certain polymers exhibit lower Vp than the more common Lithium Niobate modulators. Dr. Yu discussed the importance of incorporating automatic bias control with MZ modulators, in order to keep the modulator operating precisely at the one-half- Vp point on the MZ transfer function.

Please Note: Vp represents V(subscript pi). The character pi may not be available in all browser font files.