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The optical frequency combs (OFCs) with widely and precisely tunable frequency spacing have several unique applications such as generation of microwave to terahertz signals, high-precision phase-coherent wavelength conversion, coherent wireless and wavelength division-multiplexed (WDM) communications. In recent years, a number of approaches have been proposed for OFCs generation (OFCG). Mode-locked lasers and microresonator can generate OFCs with large bandwidth and high stability but suffer from poor tunability because of their fixed resonator. An OFCG based on an optoelectronic oscillator (OEO) can generate OFCs with good tunability but has a complex configuration. Another typical type of OFCG is based on modulators. It is a potential and economic method due to its advantages of simplicity, stability and tunability. In this paper, a novel approach to generating optical frequency combs with widely and precisely tunable frequency spacing based on a double quadrature phase shift key (DQPSK) modulator and highly nonlinear optical fibers (HNLFs) is proposed and experimentally demonstrated. A DFB-LD seed laser at 1550nm is modulated by the DQPSK modulator which is driven by RF signals. 5-line OFCs are generated as the seed OFCs at the output of DQPSK modulator and then sent into a segment of HNLFs. In this scheme, the frequency spacing of OFCs is directly decided by the RF signals’ frequency, which can be widely and precisely tuned. Four-wave mixing (FWM) effect in HNLFs can effectively increase the number of comb lines and expand bandwidth of the seed OFCs without influence on frequency spacing. The configuration is relatively simple and adjustable. The frequency spacing can be precisely tuned from 10 MHz to 20 GHz in our experiments. The typical 25-line OFCs are experimentally generated with 432 GHz bandwidth at 16 GHz frequency spacing.
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