Vertical-cavity semiconductor optical amplifiers have promised enhanced coupling efficiencies and no polarization dependency for long-distance optical communications, but they have not demonstrated the performance to compete with existing solutions. A new vertical-cavity semiconductor optical amplifier structure may now enable them to do so. The outer layers of the device, which was developed at the University of California, are built on GaAs substrates. Two GaAs/GaAlAs distributed Bragg reflectors form a 5/2l cavity. Three quantum-well stacks are positioned at the peaks of the standing wave in the cavity. A 980-nm diode laser source focused to an 8-µm spot serves as a pump. In principle, the amplifier can be operated in either transmission or reflection mode, although the prototype device is optimized for reflection-mode operation. The input beam enters the amplifier from the side opposite the pump beam, is reflected off the back-side distributed Bragg reflector and exits along the entrance path. With an input power of 225 dBm at 1328 nm and a pump power of 110 mW, the device displays a fiber-to-fiber gain of 13 dB and a bandwidth of 100 GHz. E. Staffan Björlin, a graduate student working with John E. Bowers' group at the university, believes the gain is good enough to be competitive, but the researchers would like to demonstrate saturation powers of up to —3.5 dBm. They first reported the amplifier in the August 2000 issue of IEEE Photonics Technology Letters and are working to improve its efficiency and saturation power. Björlin believes that if they are successful, the vertical-cavity semiconductor optical amplifiers will offer an attractive solution wherever compactness, cost and power consumption are important issues. The technology is flexible enough that Björlin anticipates that vertical-cavity semiconductor optical amplifiers will find applications as amplifying filters and modulators and as preamplifier arrays. "[They] typically also have a narrow-gain bandwidth, which makes them attractive as preamplifiers in receiver modules," he said.