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.
Dynamic stress sensing interrogation has become into a potential research for its wide applications. In order to test and monitor the dynamic stress in real-time, new types of fiber Bragg gratings (FBGs) sensing interrogation systems have been gradually developed. In this paper, a high interrogation rate dynamic stress sensing interrogation system by using a wavelength-swept laser with high sweeping rate and flexible-controlled sweeping step is proposed and demonstrated. In this system, a single sideband (SSB) modulator driven by radio-frequency (RF) signal is used to realize the abovementioned parameters of the wavelength-swept laser, which overcomes the limitation of traditional mechanical sweeping devices. The interrogation rate of the whole system is decided by the sweeping rate of the source, which is significantly improved and tunable with a broad range. Owing to the advantages of the sweeping source, our sensing interrogation system is effective to deal with different vibration sensing interrogations. Experimentally, the sweeping rate of the wavelength-swept laser can be tuned from 40 kHz to 200 kHz as sweeping step tuned from 5 GHz to 15 GHz. The highspeed vibration generated by a piezoelectric transducer (PZT) on the FBGs is sensed and interrogated in real-time. The interrogation of frequency vibration sensing from 5 Hz to 11 kHz are obtained. A 25 μs transient vibration mutation is also successfully interrogated by this system.
In this paper, a multi-wavelength parallel swept light source (MWPSS) is proposed for fiber Bragg grating (FBG) interrogation. The MWPSS has two main parts: a multi-wavelength light source used as a seed source and a synchronous lightwave synthesized frequency sweeper (SLSFS). The multi-wavelength of the seed source is shifted with a constant frequency sweeping step synchronously every circulation in the SLSFS. There is a one-to-one correspondence between the swept range of the seed source and the central reflection wavelength of each FBG sensor. By interrogating the data of reflection intensity in time-domain to calculate the difference between the center wavelength of each sensing FBG and the reference FBG, 10.055±0.005pm/°C temperature sensitivity and 1.614±0.002pm/με static strain sensitivity were obtained synchronously without cross-talk.