Negative Feedback Semiconductor Optical Amplifiers and All-Optical Triode

The field of optical communications is moving toward the realization of photonic networks with wavelength division multiplexing (WDM) utilizing the full bandwidth of optical fibers. Conventionally, an erbium-doped fiber amplifier (EDFA) and a semiconductor optical amplifier (SOA) are used for amplifying an optical signal in optical communications. SOAs can contribute to this and offer the key advantages of small size and ease of mass production, but their practical adoption has been largely precluded by their inferior polarization dependence and noise characteristics as compared those of with optical fiber amplifiers doped with erbium and other rare earths. For eliminating noise generated in such amplifiers, the optical signal that is transmitted at a high speed is once converted into an electrical signal, so as to be subjected to noise elimination and signal processing in an electronic circuit, and the processed signal is then reconverted into an optical signal to be transmitted. This incapability to achieve direct processing of an optical signal without its conversion into an electrical signal limits the speed of the optical signal processing. Therefore, there has been demanded a technique which enables an optical signal to be processed without its conversion into an electrical signal. However, in a field of optoelectronics, there have not yet realized high-performance signal amplifiers corresponding to a negative feedback amplifier or an operational amplifier known in a field of electronics. The negative feedback amplifier in electronics is capable of providing an output signal whose gain, waveform and baseline are stabilized without generating large noise. Negative feedback amplification is widely used in electronics and readily enables gain stability and low-noise electric signal amplification, as the existence of negativeand positivevalued entities facilitate its design and implementation. For optical signals, however, the absence of negative-valued entities poses the need for special techniques. One technique for SOA gain stabilization which has been the subject of research and development at many institutions is the use of a clamped-gain SOA (Bachmann et al., 1996), which utilizes a lasing mode generated outside the signal band. An SOA with gain control obtained by an experimental feedback loop system utilizing a bandpass filter (Qureshi et al., 2007), which is conceptually similar to the technique we have proposed, has also been reported (Maeda, 2006). In the present study, we utilized phase mask interferometry to fabricate an optical fiber filter (a fiber Bragg grating; FBG) having reflection wavelength characteristics specially designed for surrounding light feedback, formed a lens in the optical fiber tip, and coupled the fiber