doped fibers are cascaded and the external injected C-band signal is employed as a secondary pump source. Approximately 7.5-dB gain improvement is achieved for the short EDF. The gain improvement of L-band EDFA through the C-band laser injection depends on the injection wavelength, namely, the power transfer efficiency from C-band to L-band. A tradeoff necessary to realize gain enhancement and noise characteristic improvement is confirmed from the experimental results. The experimental results imply that the suggested EDFA approach is capable of implementing the gain enhancement in the short erbium-doped fiber and a lower 980-nm pump power. From our viewpoint, therefore, this scheme will provide a promising and cost-effective approach in the development of practical L-band EDFAs.
ACKNOWLEDGMENTThis work was supported by Chinese Natural Science Foundation under grant no. 60177015.
INTRODUCTIONThe local oscillator in a wireless transceiver has to satisfy many challenging requirements, such as low phase noise, wide-tuning, and quadrature signal generation [1,2]. Recently, wide tuning features have been in high demand, due to the generalization of multi-band transceivers and the wideband natures of the future generation transceivers, such as software defined radios. Satellite and cable-TV receivers are another application that requires wide tuning VCO. The ever-increasing wireless communication market drives the need for better integration of personal communication transceivers in order to meet lower cost, smaller form factor, and lower power dissipation.Ring oscillators are one of the most popular wide tuning topologies. They are generally used in monolithic form, and provide a wide range of oscillation frequency by controlling the bias currents or the value of passive components, such as resistance or capacitance. They are also simpler than other oscillators and can easily operate at high frequencies implemented in standard CMOS technology. However, ring oscillators tend to show high phase noise compared to, for example, the LC-tuned oscillator. Lately, ring oscillators that adopt LC-tuning have been reported [3,4]. Although the LC-tuning improves phase noise considerably, their performance is still worse than that of conventional LC-tuned oscillators. Besides, the tuning ranges of the LC-tuned ring oscillators are poorer than the classical ring oscillators.The typical monolithic LC-tuned oscillators tend to offer poor tuning range, as the tuning relies on the on-chip varactors. Commonly used monolithic varactors are the p-n junction, and inversion-and accumulation-mode MOS capacitors [5]. A typical p-n junction provides about Ϯ10% of linearly varying capacitance, while the recently reported accumulation-mode MOS varactors provide about Ϯ30% [6,7]. With Ϯ30%, the maximum oscillation frequency range is below 40%, and the tuning range can be given by where f max and f min are the maximum and minimum oscillation frequency, and C max and C min are the maximum and minimum capacitance of the varactor, respectively. Thi...
