Variable Optical Delay Line Research Articles

Time-domain Vernier effect-based optical fiber sensor.

In this Letter, we demonstrate an easy-to-fabricate time-domain Vernier-effect-based sensor. An all-fiber variable optical delay line (VODL) is utilized to drive an OPD scan of two interferometers simultaneously, and fiber Bragg gratings are used to filter out two slightly detuned time-domain interferometric signals. Then two normalized interferograms with different spatial frequencies can be achieved and utilized to generate an envelope modulation, viz., a Vernier envelope, with enhanced sensitivity in comparison to the native state of the interferometers used. The sensitivity magnification factor of our structure can be regulated simply via altering the resonant wavelength difference of FBGs rather than optimizing the OPDs of the interferometers. The proposed sensor is independent of the precise and complicated fabrication procedures. The Vernier signal can be demodulated without a broadband light source and spectrometer. We argue that the proposed structure may inspire a new concept for constructing simple and cheap Vernier effect-based sensors that are well suited for practical applications.

Bit-tunable XOR operation based on photonic spiking neuron

The XOR logic operation is an important element for photonic computing, but its implementation is limited by high power consumption, complex determination rule of the output signal and polarization sensitive. By utilizing the spiking excitation characteristics of photonic spiking neuron, we propose a polarization-insensitive XOR operation architecture which has the advantage of the tunable bit interval by changing the delay of feedback loop. We simulatively demonstrate that XOR operation with arbitrary bit interval can be realized with zero-error rate by tuning the variable optical delay line. Besides, when introducing a noise with an SNR = 10 dB or a time jitter of 2μs (10% of the unit length of input spike) to the input spike, the error rate of XOR can still reach zero, showing a strong robustness of our system. Experiment for adjacent-bit XOR operation is further implemented and zero error-rate is achieved, verifying the feasibility of our proposed system.

Synchronized triple-wavelength fiber lasers at 1, 1.55, and 1.9 µm.

Synchronized lasers working at different wavelengths are of great significance for numerous applications, such as high-energy femtosecond pulse emission, Raman microscopy, and precise timing distribution. Here, we report synchronized triple-wavelength fiber lasers working at 1, 1.55, and 1.9 µm, respectively, by combining the coupling and injection configurations. The laser system consists of three fiber resonators gained by ytterbium-doped fiber, erbium-doped fiber, and thulium-doped fiber, respectively. Ultrafast optical pulses formed in these resonators are obtained by passive mode-locking with the use of a carbon-nanotube saturable absorber. A maximum cavity mismatch of ∼1.4 mm is reached by the synchronized triple-wavelength fiber lasers in the synchronization regime by finely tuning the variable optical delay lines incorporated in the fiber cavities. In addition, we investigate the synchronization characteristics of a non-polarization-maintaining fiber laser in an injection configuration. Our results provide a new, to the best of our knowledge, perspective on multi-color synchronized ultrafast lasers with broad spectral coverage, high compactness, and a tunable repetition rate.

Phase-Derived Ranging Based Fiber Transfer Delay Measurement Using a Composite Signal for Distributed Radars with Fiber Networks

Fiber transfer delay (FTD) variations influence the coherence of distributed radars with fiber networks, resulting in a performance degradation in target detecting and imaging. To measure and compensate for the variation, a phase-derived ranging based FTD measurement using a composite signal is proposed. The composite signal comprises a sinusoidal component and a linear frequency modulation (LFM) component. As the composite signal passes through a fiber under test (FUT), the sinusoidal component generates a phase shift that corresponds to the FTD. The phase shift can be represented by two parameters: the number of complete periods of 2π that can be estimated by using the LFM component, and a phase shift less than 2π that be measured employing the sinusoidal component. When using the proposed measurement system to measure FTD variations in a distributed radar, only an additional sinusoidal component is needed, which minimizes interference with radar signals. Moreover, the proposed measurement system can share core function modules such as signal generation and process modules with distributed radars, which enhances the compatibility and reduces the overall complexity. Experiments are carried out to measure a variable optical delay line and a long optical fiber. The experiment results verify the feasibility of the measurement system and show that a measurement range of more than 15 km, an accuracy of ±0.1 ps and a measurement time of 105 ms can be achieved.

Real-time adaptive optical self-interference cancellation for in-band full-duplex transmission using SARSA(λ) reinforcement learning.

Self-interference (SI) due to signal leakage from a local transmitter is an issue in an in-band full-duplex (IBFD) transmission system, which would cause severe distortions to a receiving signal of interest (SOI). By superimposing a local reference signal with the same amplitude and opposite phase, the SI signal can be fully canceled. However, as the manipulation of the reference signal is usually operated manually, it is difficult to ensure a high speed and high accurate cancellation. To overcome this problem, a real-time adaptive optical SI cancellation (RTA-OSIC) scheme using a SARSA(λ) reinforcement learning (RL) algorithm is proposed and experimentally demonstrated. The proposed RTA-OSIC scheme can automatically adjust the amplitude and phase of a reference signal by adjusting a variable optical attenuator (VOA) and a variable optical delay line (VODL) achieved through an adaptive feedback signal, which is generated by evaluating the quality of the received SOI. To verify the feasibility of the proposed scheme, a 5 GHz 16QAM OFDM IBFD transmission experiment is demonstrated. By using the proposed RTA-OSIC scheme, for an SOI at three different bandwidths of 200, 400, and 800 MHz, the signal can be adaptively and correctly recovered within 8 time periods (TPs), which is the required time of a single adaptive control step. The cancellation depth for the SOI with a bandwidth of 800 MHz is 20.18 dB. The short- and long-term stability of the proposed RTA-OSIC scheme is also evaluated. The experimental results indicate that the proposed approach could be a promising solution for real-time adaptive SI cancellation in future IBFD transmission systems.

Microwave-photonic optical fiber interferometers for wavelength interrogation of fiber Bragg grating sensors with high sensitivity and a tunable dynamic range.

We propose and demonstrate a new, to the best of our knowledge, microwave interference-based scheme with high sensitivity and tunable measurement range, which is realized by a Mach-Zehnder interferometer (MZI). A chirped fiber Bragg grating and single-mode fiber serve as the two unbalanced arms of the RF interferometer. The induced differential chromatic dispersion transfers the wavelength shift of the fiber Bragg gratings to the change of the RF phase difference between the two interferometric carriers, which ultimately leads to the variation of the RF signal intensity. The phase sensitivity can be improved by adjusting the power ratio of the two beams in the interferometer and coarse adjustment of the optical variable delay line (OVDL). The OVDL is also employed to tune the measurement range of the system by adjusting the time delay difference between the two arms of the MZI. The system effectively solves the problem of unavoidable attenuation of the sensitivity of the optical carrier-based microwave interferometry system caused by the change of phase difference due to the change of measurement parameters, avoiding the mutual constraint between the measurement range and high sensitivity.

Variable optical true-time delay line breaking bandwidth-delay constraints.

Continuously variable true-time optical delay lines are typically subject to a constraint of the bandwidth-delay product, limiting their use in several applications. In this Letter, we propose an integrated topology that breaks the bandwidth-delay product limit. The device is based on multiple Mach-Zehnder Interferometers (MZIs) arranged in parallel, providing easier control and a larger bandwidth compared to ring resonator-based solutions. The functionality of this architecture is demonstrated with a 4-stage delay line by performing measurements in both the time and frequency domains. The delay line introduces a delay of 90 ps over a bandwidth of more than 22 GHz with a negligible group delay distortion, operates on a wavelength range of about 60 nm, and is scalable to a higher number of MZI stages.

Arbitrary distance measurement without dead zone by chirped pulse spectrally interferometry using a femtosecond optical frequency comb.

We demonstrate an arbitrary distance measurement method by chirped pulse spectrally interferometry (CPSI) using femtosecond optical frequency comb (OFC). In this paper, the chirped fiber Bragg grating (CFBG) is used to investigate the mapping relationship between displacement and the center frequency of the chirped spectral interferogram. We overcome the direction ambiguity of dispersive interferometry (DPI) ranging and expand the range of distance measurement to 18 cm. Besides, we achieve a full range of dead-zone free ranging by introducing a variable optical delay line (VODL). And through principles simulation and experiment, it is demonstrated that the measurement accuracy is 12 µm in comparison with an incremental He-Ne laser interferometer and the minimum Allen deviation is 52 nm at an average time of 1.76 ms. Similarly, in the experiment with long-distance of ∼30m, the accuracy reaches 20 µm, and 2.51 µm repeatability is achieved under harsh environment.

An adaptive microwave photonic filter with LMS algorithm

A microwave photonic filter (MPF) with an adaptive algorithm is proposed and theoretically analyzed. The structure combines the finite impulse response (FIR) MPF with the adaptive algorithm. The FIR MPF includes variable optical attenuators (VOAs) and variable optical delay lines (VDLs). The least mean square (LMS) algorithm can calculate the weight coefficient of each tap in FIR MPF, which can influence the VOA to realize the adaptive filtering process. This adaptive MPF can extract the desired signal from noise precisely. The noise signal can be well suppressed and the signal-to-noise ratio (SNR) can be developed from 4.15 to 25.5 dB. In addition, this adaptive MPF can be suitable for a wide range of signals.

Compensation of inter-core skew in multi-core fibers with group velocity dispersion.

This work proposes an approach for the compensation of inter-core skew in homogeneous single-mode multi-core fiber links. We adjust the wavelengths of the transmitted spatial channels in such a way that the skew induced by group velocity counters inter-core skew. This approach is demonstrated experimentally using a 111 Gb/s spatial super channel (4 spatial channels at 27.8 Gb/s) on a 10.1 km 19-core multi-core fiber. It is shown that inter-core skew may be compensated without the need for devices such as variable optical delay lines or electronic buffers.

Photonic Generation of Multiple Shapes and Sextupled Microwave Signal Based on Polarization Modulator

In this article, a photonic technique is proposed for generating the microwave signal of multiple shapes. Furthermore, this is modified to give a microwave signal with frequency multiplying capability, which does not require an optical filter. The proposed methodology is relied on the polarization modulator (PM), the Mach-Zehnder modulator (MZM), and the variable optical delay line (VODL), where the transmission point of PM has been selected by static phase of polarizer controller and MZM by the dc bias voltage. An RF signal of 6-GHz frequency is applied at PM to modulate the optical carrier signal. Frequency-dependent VODL has been used to provide dynamic phase shifts in the harmonics of the modulated signal. Firstly, 12-GHz full-wave rectified (FWR) and 6-GHz half-wave rectified (HWR) sinusoidal signals have been generated using PM and VODL. For the generation of FWR and HWR, respective phase shifts of - π/2 and π have been introduced by VODL. Then, a sextupled microwave signal of 36-GHz frequency has been generated which is further processed for the generation of triangular-shaped HWR of 6-GHz frequency. This has been achieved by modulating one of the split powers of PM with the help of an MZM. The generation of such multishape rectified waveforms has not been reported before. Also, the frequency sextupling capability of the proposed technique defines the multifunctionality of the system. Theoretical analysis of the considered approach has been given which is supported by simulated results and validated by the experimental results.

Actively mode-locked fiber laser at 2-μm for free-space data transmission with tunable repetition rate

We experimentally demonstrate a repetition rate controllable actively mode-locked fiber laser for 2-μm free-space data transmission. The actively mode-locking can be produced by a Mach-Zehnder modulator (MZM) driven by an arbitrary waveform generator (AWG). A Lyot filter and a polarization dependent isolator (PD-ISO) are used to suppress the super-mode noise and the ambient noise. The signal-to-noise ratio (SNR) of the mode-locked pulse with repetition rate of about 690MHz can be increased from 42.64dB to 49.45dB. By inserting a high-precision variable optical delay line (VODL) in the cavity, the fundamental frequency of the cavity can be adjusted from 18.21MHz to 18.13MHz with a tuning accuracy of 73.52Hz. The RF spectra of the 330th order harmonic mode-locked pulses with continuously tunable repetition rate can be observed. In addition, the mode-locked pulse sequence can be driven by sine wave, triangular wave and square-wave, and the duration of actively mode-locked pulse can be tuned independently by changing the type of the driven signal.

Frequency-modulated continuous-wave laser ranging using low-duty-cycle signals for the applications of real-time super-resolved ranging.

A frequency-modulated continuous-wave laser ranging method using low-duty-cycle linear-frequency-modulated (LFM) signals is proposed. A spectrum consisting of a dense Kronecker comb is obtained so that the frequency of the beat signal can be measured with finer resolution. Since the dense comb is provided, super-resolved laser ranging can be achieved using a single-parametric frequency estimation method. Therefore, the run times of the estimation are reduced which promises real-time applications. A proof-of-concept experiment is carried out, in which an LFM signal with a bandwidth of 5 GHz and a duration of 1 µs is used. The duty-cycle of the LFM signal is 10%. The time delay of a scanning variable optical delay line is obtained in real time from the frequency of the highest comb tooth, of which the measurement resolution is 20 ps. Moreover, a single-parametric nonlinear least squares method is used to fit the envelope so that the time delay can be estimated with super-resolution. The standard deviation of the estimation displacements is 2.3 ps, which is 87 times finer than the bandwidth-limited resolution (200 ps). Therefore, the variation of the time delay can be precisely monitored. The proposed method may be used to achieve real-time high-resolution laser ranging with low-speed electronic devices.

Out-of-band suppression improved tunable microwave photonic Hilbert transformer based on optical spectral shaping

An out-of-band suppression improved microwave photonic Hilbert transformer (MPHT) with adjustable bandwidth and central frequency based on optical spectral shaping is theoretically analyzed and experimentally demonstrated. The MPHT consists of a broadband optical source, a Mach–Zehnder interferometer (MZI), a dispersion compensating fiber and a photodetector. The MZI incorporates an intensity modulator in one arm and a programmable optical filter (POF) and a variable optical delay line in the other. The frequency response of the MPHT is determined by the Fourier transformation of the optical spectral shaping response of the POF. By tailoring the optical phase profile, a π phase shift can be introduced into the center of the MPHT. By shaping the optical amplitude profile, a MPHT with adjustable bandwidth and improved sidelobe suppression can be achieved. In the experiments, MPHTs with bandwidths varying from 100 MHz to 1 GHz and the central frequency varying from 2.5 GHz to 16.5 GHz are demonstrated. The out-of-band suppression is more than 20 dB, which is the best result ever reported for a MPHT. The impulse response of the MPHT is verified and the result proves that the proposed MPHT can be applied to microwave analytic signal processing systems.

Photonic-assisted single system for microwave frequency and phase noise measurement

We propose a photonic-assisted single system for measuring the frequency and phase noise of microwave signals in a large spectral range. Both the frequency and phase noise to be measured are extracted from the phase difference between the signal under testing and its replica delayed by a span of fiber and a variable optical delay line (VODL). The system calibration, frequency measurement, and phase noise measurement are performed by adjusting the VODL at different working modes. Accurate frequency and phase noise measurement for microwave signals in a large frequency range from 5 to 50 GHz is experimentally demonstrated.

Wideband Microwave Frequency Distribution for Multi-Access Along a Single Fiber Link

In this paper, a fiber-optic signal access method is proposed for stable wideband microwave frequency distribution. Incorporating with the tapped counter-propagating reference signals and variable optical delay lines (VODLs), a stable wideband microwave frequency signal can be obtained at any access point (AP) along the main fiber link. Moreover, all the phases of received wideband microwave frequency can be aligned. In this way, the whole system complexity can be dispersed as the central station only stabilizes the main fiber link delay, and the access ends (ANs) operate independently. Experimentally, a multi-access wideband microwave frequency distribution is demonstrated over a 50 km dispersion-compensated fiber link. A system figure of merit is investigated in detail. The results show that the fiber-optic wideband microwave frequency transmission system can operate for the frequency over 40 GHz. The additive phase noises of -125 dBc/Hz for the remote end (RN), and -131 dBc/Hz for the AN, respectively, are achieved at the 1 kHz offset frequency for a 1 GHz frequency signal. Thanks to the active delay stabilization, the Allan deviation (ADEV) of 1.3 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-13</sup> /s and 2.4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-15</sup> /100 s for the RN, and the one of 8.8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-14</sup> /s and 4.4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-15</sup> /100 s for the AN, respectively, are acquired.

Photonic-Assisted Leakage Cancellation for Wideband Frequency Modulation Continuous-Wave Radar Transceiver

Frequency modulated continuous-wave (FMCW) radar systems suffer from permanent leakage from the transmitted signal into the receiver, which will reduce the dynamic range of the system and degrade the receiver sensitivity. Herein, a photonic-assisted leakage cancellation method for a wideband FMCW radar transceiver is proposed and experimentally demonstrated. At the remote antenna unit (RAU), a local oscillator (LO) signal, and a leakage signal are converted to optical signals with orthogonally polarization states by using a polarization-division-multiplexed Mach–Zehnder modulator (PDM-MZM). After the transmission of 10 km single mode fiber (SMF), the leakage signal is cancelled by using a variable optical attenuator (VOA), a variable optical delay line (VODL), and a balanced photodetector (BPD) at central office (CO). Compared with other analog cancellation methods, the proposed one has wider sweeping bandwidth and simpler structure. In the experiment, the LO signal and leakage signal are emulated by using an arbitrary waveform generator (AWG), a power divider, and two different length radio frequency lines. Then, experimental results show that a leakage cancellation depth of 17.5 dB at the carrier frequency of 15 GHz with a sweeping rate of 2 GHz/4 us is achieved.

Instantaneous microwave frequency measurement based on non-sliced broadband optical source

We propose and experimentally demonstrate a reconfigurable instantaneous frequency measurement (IFM) system. A non-sliced broadband optical source (BOS) is used to realize two tunable microwave photonic filters (MPFs) with equivalent infinite taps. The two MPFs allow us to have an amplitude comparison function (ACF) which maps the unknown frequency to microwave power. The IFM system is reconfigurable by simply adjusting two variable optical delay lines (VODLs). The principle behind the proposed IFM systems is theoretically analyzed. Experimentally, unknown frequency measurement in a frequency range of 8–16.5 GHz and 11.5–20 GHz with a measurement error of less than 110 MHz is realized.

Stable photonic transmitting link based on passive phase compensation with active‐assisted control method

The authors demonstrate a photonic link for stable radio frequency (RF) dissemination over fibre based on passive phase compensation with an active-assisted control method. A variable optical delay line is introduced in the transmitting link for active-assisted phase compensation. After active-assisted compensation, the residual jitter is <500 fs. Experimentally, an RF signal at a frequency of 5 GHz is successfully transmitted over a 20-km fibre link, with the stability of 2.49 × 10−14 at 1 s and 9.19 × 10−18 at 104 s.

Optical Fiber Transfer Delay Measurement Based on Phase-Derived Ranging

High-accuracy optical fiber transfer delay (OFTD) measurement with a broad measurement range is essential for the development of high-speed or parallel optical devices and systems. In this letter, we propose a system for OFTD measurement using the phase-derived ranging. A microwave signal is converted into an optical signal, which is transmitted through a fiber under test. After photodetection, the OFTD is converted into the phase shift of the microwave signal, which can be detected using a microwave phase detector. When calculating the OFTD, the phase error is divided by the highest microwave frequency, which ensures the high measurement accuracy. The integer ambiguity caused by the phase detection can be removed by sweeping the microwave frequency in a range of 1 MHz. An experiment is performed. The measurement of a motorized variable optical delay line and a long fiber shows that a dynamic range of more than 20 km and an accuracy of ±0.1 ps can be achieved with no dead zone.