Contents I. Introduction
Not only the expected future increase in the capacity of dense wavelength-division-multiplexing (WDM) systems, but also the recently increasing use of coarse WDM for low-cost systems has accelerated the development of various optical amplifiers for wavelengths outside the erbium-doped fiber amplifier (EDFA) bands. Fiber Raman amplifiers (FRAs) [1], Pr-doped [2] and Tm-doped [3] fiber amplifiers, Tellurite-based EDFAs [4], and semiconductor optical amplifiers (SOAs) [5]–[8] are representative of those amplifiers. In order to fill the gap of the wavelength bands not covered by the rare-earth-doped fiber amplifiers, FRAs and/or SOAs, having a large degree of freedom in wavelength choice, are required. Device structure. To provide a reasonable gain, however, FRAs require high pump powers and long optical fibers on the order of kilometers, which makes their current practicality in low-cost systems questionable. Also, as for SOAs, the degradation of signal quality coming from both ASE noise addition [large noise figure (NF)] and signal distortion in the gain-saturated regime (pattern effect), is a critical problem, which is the main reason that SOAs have been kept away from practical commercial applications. In SOAs, the input fiber-coupling loss, as well as the degree of population inversion and the ratio of the internal loss to the gain, has been the main origin of the large NF. At present, it is not difficult to reduce this coupling loss down to 1 dB [9]. As for signal distortion, output power has to be, typically, at least 4 dB below the 3-dB saturation output power to be immune from pattern effect, which severely limits the usable output power. Gain clamping [8], [10], [11] is effective in increasing the usable output power, and recently a very simple scheme utilizing only one vertical-cavity surface-emitting laser chip has been proposed [12]. Our approach to this problem is completely different, which drastically increases the pattern-effect-free output power by replacing the active material with quantum dots (QDs) [13]–[18]. Taking into account the requirement of cost effectiveness, covering as wide a range of transmission bands as possible with the simplest and minimum number of optical amplifiers is of great importance. QDs are also effective in drastically increasing the bandwidth of SOAs, i.e., the wavelength range where the values of gain, NF, and satisfy the requirements. We have successfully realized a single-chip optical amplifier having the record-widest bandwidth among all kinds of optical amplifiers, and also having a penalty-free output power of 23 dBm, 8-dB improvement compared to the best result obtained with a conventional SOA. Bandwidths of (a) gain, (b) NF, and (c) saturation output power. The dark gray areas represent the bandwidth where gain of 25 dB, NF of 5 dB, and saturation output power of 19 dBm can be obtained at the same time. The light gray areas are for the bandwidth where gain of 20 dB, NF of 7 dB, and saturation output power of 19 dBm can be obtained. Comparison of bandwidth.
