A multiple frequency-swept source based on a recirculating frequency shifter loop (RFSL) is established. Three distributed feedback (DFB) lasers are used as a seed source and sweep in the RFSL synchronously. The swept spectra of the separate DFB lasers are precisely controlled and stitch together without overlap. The key significance of this technique is that the swept range increases as the number of the multiple seed wavelengths increase. Experimentally, the swept range of our system can broaden to 4.5 nm/3.9 nm with an 8.6 GHz/7.5 GHz swept step. The swept rate is 200 kHz. The output of the source is sent into a Mach-Zehnder interferometer and the interference signal is detected to measure the length difference of interferometer arms.
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.
In recent years, an approach of photonic true-time delay (TTD) unit which employs a linear chirped fiber grating (LCFG) and a tunable multi-wavelength laser (TMWL) for phased array antennas (PAAs) has been reported. In this paper, several key parameters of the LCFG and TMWL are analyzed. The beam-pointing resolution is effected by the parameters including the group delay slope of LCFG, the tuning accuracy of the wavelength spacing of TMWL. The relationship between the beam-pointing angle and the wavelength spacing of TMWL is an inverse trigonometric function. So linear output of the beam-pointing angle could be obtained by tuning the wavelength spacing as sine function. Furthermore, the relationship of the beam-pointing resolution and the beam-pointing angle with different tuning accuracies of the wavelength spacing of TMWL and different group delay slopes of LCFG are also analyzed.
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.
Photonic true-time delay (TTD) beamforming has been considered as a promising technique for future wide-band phased array antenna systems. A TTD beamforming system based on a linear chirped fiber grating (LCFG) requires a tunable multi-wavelength laser source, in which the wavelengths are tuned simultaneously with equally increased or decreased wavelength spacing. We present a novel approach of a TTD laser source employing a DQPSK modulator and a highly nonlinear fiber (HNLF). The wavelength tuning is achieved by tuning the frequency of the radio frequency signal generator that drives the DQPSK modulator. Up to 41 lasing wavelengths with 0.08 nm spacing, 29 lasing wavelengths with 0.06 nm spacing and 31 lasing wavelengths with 0.12 nm spacing in 3 dB bandwidth were obtained. In theory, the multi-wavelengths can be simultaneously tuned from 10 MHz to 20 GHz in our experiments based on the fact that the frequency range of the RF signal source is from 10 MHz to 20 GHz and the bandwidth of DQPSK modulator is up to 22 GHz. The tuning precision of the RF signal generator is up to 1Hz and phase noise is about -104dBc/Hz. Therefore, the wavelengths can be tuned precisely and stably.
We proposed and demonstrated a linearly frequency-swept multi-wavelength laser source for optical coherence tomography (OCT) eliminating the need of wavenumber space resampling in the postprocessing progress. The source consists of a multi-wavelength fiber laser source (MFS) and an optical sweeping loop. In this novel laser source, an equally spaced multi-wavelength laser is swept simultaneously by a certain step each time in the frequency domain in the optical sweeping loop. The sweeping step is determined by radio frequency (RF) signal which can be precisely controlled. Thus the sweeping behavior strictly maintains a linear relationship between time and frequency. We experimentally achieved linear time-frequency sweeping at a sweeping rate of 400 kHz with our laser source.
Proc. SPIE. 9747, Terahertz, RF, Millimeter, and Submillimeter-Wave Technology and Applications IX
KEYWORDS: Laser sources, Fiber Bragg gratings, Sensors, Multiplexing, Demodulation, Optoelectronics, Sensing systems, Time division multiplexing, Data conversion, Data centers, Frequency conversion, Signal detection, Optical data conversion, Intelligent sensors
In this paper, we propose an all-optical real-time data format conversion system to realize the efficient-utilization of the spectrum resource in the FBG sensing network. In the data format conversion unit, frequency domain sensing signals reflected by FBGs are converted into optical time division multiplexing signals with a specified wavelength in real time. Format converted data from each node are sent back to the control center for demodulation. Experimentally, data format conversion and demodulation of one sensing node is carried out with different temperature and static strain. The spectra of sensing pulses before and after data format conversion indicates that the data format conversion is successful and the spectrum resource in this node is released.
In this paper, wavelengths parallel swept technique is proposed by using an optical swept loop outside the laser cavity. A frequency shifter controlled by RF signal in the loop shifts the frequencies of the incident optical signals simultaneously with a constant value in every circulation. Experimentally, the outputs of two distributed feed back (DFB) lasers were parallel swept by using the optical swept loop. The swept step is tunable from 300MHz(24pm) to 15GHz(0.12nm) depending on the electro-optical bandwidth of the frequency shifter used in our experiments. Dual-wavelength swept output at 214 kHz swept rate with 0.08nm swept step and 122 kHz swept rate with 0.04nm swept step were achieved respectively. The swept span of the swept source was 1.6nm with a flatness of ±1.5dB.