Linear Optical Amplifier Generates Multiple Wavelengths Simultaneously

Breck Hitz

The previous report describes a technique to finesse mode competition in a homogeneously broadened erbium-fiber laser. Researchers at Hong Kong Polytechnic University in Kowloon, China, have taken the opposite approach to obtaining multiple wavelengths from a single laser. They used a linear optical amplifier (LOA) as the ultra-inhomogeneous gain medium in a ring laser.

An LOA is a semiconductor optical amplifier designed such that its output power is highly linear with respect to input power. It is similar to a conventional semiconductor optical amplifier, except that a vertical-cavity surface-emitting laser operates simultaneously in the same gain medium as the amplifier (Figure 1). The circulating power in the vertical laser acts as a ballast and ensures a linear relationship between the amplifier’s input and output.

Figure 1. A linear optical amplifier looks like a conventional semiconductor optical amplifier, except that a vertical-cavity surface-emitting laser operates with the same gain medium. The gain saturation produced by the vertical laser acts as a ballast, ensuring constant gain for the linear amplifier independent of the amplifier’s input power. ©OSA.

LOAs are designed primarily to serve as amplifiers in wavelength division multiplexing, where they provide nearly constant gain despite signals switching on and off in adjacent channels. But the almost total lack of coupling between channels — that is, the high degree of bandwidth inhomogeneity — makes the device an ideal gain medium for a multiwavelength laser.

The Hong Kong researchers used a commercial LOA from Finisar Corp. of Sunnyvale, Calif., in a ring resonator with a Fabry-Perot interferometer (Figure 2). The Fabry-Perot had a free spectral range of 100 GHz and served as a comb filter that forced the laser to oscillate only at the transmission frequencies of the filter. The laser output consisted of more than 38 such frequencies across the C- and L-bands (Figure 3).

Figure 3. The laser’s output consisted of more than 38 wavelengths, separated by the 100-GHz free spectral range of theintracavity Fabry-Perot interferometer. The spectral profile of the laser could be tuned by adjusting the loss of the intracavity variable optical attenuator; for example, 0 dB (a), 3 dB (b), 5 dB (c) and 7 dB (d). ©2005 IEEE.

Gain in the C-band (1530 to 1565 nm) was strongly saturated by the resonator’s high feedback, and as a result, the gain profile was shifted to longer wavelengths in the L-band. By increasing intracavity loss with the variable optical attenuator, the researchers tuned the peak profile by ~22 nm into the C-band. All other laser parameters — e.g., the bias current driving the LOA and the output coupling — remained constant during the tuning process.

They also measured the stability of their laser. They separated an output channel and observed its power over a three-hour period using an optical power meter. The power fluctuation during the period was less than 0.2 dB, and the signal-to-noise ratio was better than 50 dB.