Acousto-optic Frequency Shifter Research Articles

Drift Error Compensation Algorithm for Heterodyne Optical Seawater Refractive Index Monitoring of Unstable Signals.

The refractive index measurement of seawater has proven significance in oceanography, while an optical heterodyne interferometer is an important, highly accurate, tool used for seawater refractive index measurement. However, for practical seawater refractive index measurement, the refractive index of seawater needs to be monitored for long periods of time, and the influence of drift error on the measurement results for these cases cannot be ignored. This paper proposes a drift error compensation algorithm based on wavelet decomposition, which can adaptively separate the background from the signal, and then calculate the frequency difference to compensate for the drift error. It is suitable for unstable signals, especially signals with large differences between the beginning and the end, which is common in actual seawater refractive index monitoring. The authors identify that the primary cause of drift error is the frequency instability of the acousto-optic frequency shifter (AOFS), and the actual frequency difference was measured through experimentation. The frequency difference was around 0.1 Hz. Simulation experiments were designed to verify the effectiveness of the algorithm, and the standard deviation of the optical length of the results was on the scale of 10-8 m. Liquid refractive index measurement experiments were carried out in a laboratory, and the measurement error was reduced from 36.942% to 0.592% after algorithm processing. Field experiments were carried out regarding seawater refractive index monitoring, and the algorithm-processing results are able to match the motion of the target vehicle. The experimental data were processed with different algorithms, and, according to the comparison of the results, the proposed algorithm performs better than other existing drift error elimination algorithms.

Attosecond-precision balanced timing detector with a single photodiode.

We experimentally demonstrate a novel and practical timing detector based on a double-pass acousto-optic frequency shifter. With time and frequency multiplexing, for the first time to our knowledge, a balanced detection is realized using only a single photodiode, which greatly decreases the excess electronic noise during photodetection. With a total input optical power of 1.4 mW (0.35 mW per pulse train), an almost shot-noise-limited detection floor of 28.3 zs/√Hz is achieved, and the timing jitter integrated from 1 kHz to 1 MHz is reduced from 99.0 as (without eliminating the photodetector electronic noise) to only 30.4 as. Even with an input power of 50 µW per pulse train, 221.4 zs/√Hz detection floor and 268.0 as integrated timing jitter at [1 kHz and 1 MHz] are still maintained. This timing detector provides a powerful tool for high-precision metrology, ultra-long-distance ranging, and large-scale timing synchronization.

Pseudorandom modulation continuous-wave narrowband sodium temperature and wind lidar

Abstract. We report the first pseudorandom modulation continuous-wave (PMCW) narrowband sodium temperature and wind lidar developed at the University of Science and Technology of China (USTC). The laser system uses a 1178 nm diode seed laser and a fiber Raman amplifier with a fiber-coupled acousto-optic frequency shifter (AOFS) to generate a narrowband 589.158 nm light with an output power of 1.5 W at three frequencies of v0, v+, and v−. Based on an innovative technique and algorithm, the main beam and the residual beam modulated by electro-optic modulator (EOM) with M pseudorandom sequence code (M-code) are separately directed to the vertical and eastward directions. The three-frequency light is designed in timing with the multiple-period 127-bit M-code groups. The uncertainties of the temperature and wind with the vertical and temporal resolutions of 1 km and 30 min under clear-sky conditions are estimated to be 5.0 K and 10 m s−1, respectively, at the sodium peak. The temperature and wind results are in good agreement with those observed by satellite and nearby ground-based meteor radar, demonstrating the reliability of the PMCW narrowband sodium lidar system for measuring the mesopause region's temperature and wind.

Stable frequency transfer for clock synchronization for telecom networks

A stable and reliable clock synchronization system that transfers reference clocks to nodes is essential for telecom networks. Mainly, the clock synchronization of telecom networks is carried out through satellite links. However, for the evolving ubiquitous wireless communications, especially in some harsh environments, such as underground and trenches, the satellite-based clock synchronization scheme bears inevitable obstacles, which may cause strategic risks for the telecom networks, since no control and manipulation can be imposed on the satellite systems. Therefore, to ensure the robust operation of telecom networks with performance guarantees, some auxiliary means are desirable to assist clock synchronization that is solely based on satellite links. In this article, we report a phase-stabilized frequency signal transmission on branching optical fiber for clock synchronization for telecom networks. Particularly, the phase fluctuation due to optical carriers separation link and fiber link is compensated by a feedback network, which employs a high-precision voltage-controlled oscillator in the phase-locked loop to drive the acousto-optic frequency shifter for fast phase correction. Furtherly, the factors that cause performance limitations of the frequency distribution system are analyzed. Eventually, the experiment results obtained show that a stabilized fiber-optic frequency transfer scheme can be used as a reliable method for clock synchronization with high accuracy.

Broadband Microwave Photonic Channelizer with 18 Channels Based on Acousto-Optic Frequency Shifter

A microwave photonic channelizer can achieve instantaneous reception of ultra-wideband signals and effectively avoid electronic bottleneck; therefore, it can be perfectly applied to a wideband radar system and electronic warfare. In channelization schemes based on an optical frequency comb (OFC), the number of comb lines usually depends on that of the sub-channels. In order to improve the utilization rate of the comb lines of OFC, we propose a scheme to shift the frequency of OFC by using an acousto-optic frequency shifter (AOFS), which can obtain three times the number of sub-channels of the comb lines of an OFC. In order to simplify the experiment, only a three-line OFC is used in the experiment. A three-line local oscillator (LO) OFC is frequency-shifted up and down by two AOFSs, and nine optical LO signals with different frequencies are obtained, thereby realizing the simultaneous reception of eighteen sub-channels. The proposed scheme enjoys a large number of sub-channels and minimal channel crosstalk. Experimental results demonstrate that a 9-GHz bandwidth RF signal covering 10–19 GHz is divided into 18 sub-channels with a sub-bandwidth of 500 MHz. The image rejection ratio of the sub-channels is about 23 dB, and the spurious-free dynamic range (SFDR) of the receiver can reach 98 dB·Hz2/3.

Dual-comb interferometry using frequency shifting loops

Dual-comb systems have demonstrated their potential for metrology, e.g. spectroscopy vibrometry or ranging [1], However, the implementation of dual-combs is often complex and generally requires substantial optical and optoelectronic hardware Here, we propose a simple and compact architecture based on a bidirectional frequency shifting loop, that provides more than I 00 mutually coherent comb lines The system makes use ofa CW laser and a slow electronic detection chain (10 MSa/s) We have implemented two configurations enabling dynamic multi-heterodyne interferometry at 20kHz: the first one makes use of acousto-optic frequency shifters, and allows highly sensitive distributed acoustic sensing along a fiber, The second one involves electro-optic frequency shifters, and enables ranging with a sub-mm resolution.

Passive quadrature demodulation of multiplexed interferometric sensors using a CW correlation reflectometer with a single DFB diode laser.

In this Letter, we report a novel, to the best of our knowledge, and simple approach for passive quadrature-phase demodulation of relatively long multiplexed interferometers based on two-channel coherence correlation reflectometry. Two-wavelength channels are generated using a single unmodulated CW-DFB diode laser and an acousto-optic frequency shifter. The introduced frequency shift determines the optical lengths of the interferometers. In our experiments, all interferometers have the same optical length of 32 cm corresponding to the π/2 phase difference between channel signals. An additional fiber delay line was introduced between channels to destroy coherence between initial and frequency-shifted channels. Demultiplexing of channels and sensors was performed using correlation-based signal processing. Amplitudes of cross correlation peaks obtained for both channels were used to extract the interferometric phase for each interferometer. Phase demodulation of relatively long multiplexed interferometers is experimentally demonstrated. Experimental results prove that the proposed technique is suitable for interrogating a serial array of relatively long interferometers dynamically modulated with phase excursions exceeding 2π. Simultaneous interrogation and phase demodulation were experimentally demonstrated using an in-line array of low-finesse Fabry-Perot interferometric sensors.

Influence of acousto-optic frequency shifter's thermal-induced birefringence on laser frequency-shifted feedback system

The acousto-optic frequency shifter (AOFS) is the key component of the laser frequency-shifted feedback (LFSF) system, and its thermal-induced birefringence (TIB) seriously affects the stability of the system. In this paper, the influence of the AOFS's TIB on LFSF system is analyzed, and the method of suppressing this influence is proposed. The theoretical model of LFSF considering the AOFS's TIB is established, and the influence on both amplitude and phase of frequency-shifted signal is analyzed. When the AOFS starts working, its TIB effect is obvious due to the accumulation of working heat, which makes the polarization state of its diffraction light change dynamically in most cases. The laser feedback imaging system and laser feedback displacement measurement system are built separately to evaluate the amplitude and phase stability. By principal axis alignment or quasi-common-path compensation, the amplitude variation, phase fluctuation, phase jump and phase temperature drift caused by the AOFS's TIB can be eliminated or suppressed. This study improves the startup stability and anti-ambient temperature interference ability of the LFSF system, which also has guiding significance to other heterodyne interference systems.

Stable terahertz wave dissemination over underground fiber network with optical phase correction

We report a phase-stabilized photonic terahertz wave distribution over an urban underground fiber network. The optical carriers of the terahertz signal are generated by filtering out optical spectral lines of an optical frequency comb (OFC). The phase fluctuation due to the optical carriers separation link and fiber link is compensated by an optical phase actuation network, which employs a high-precision voltage-controlled oscillator (VCO) in the phase-locked loop to drive the acousto-optic frequency shifter (AOFS) for fast phase correction We demonstrated the stabilized transfer of a 1000.04 GHz terahertz signal over a 42 km urban telecom fiber link, and achieved residual phase noise of -37 dBc/Hz at 1 Hz offset frequency, and the long-term frequency stability at 4000 s averaging time reaches 2 × 10−16. The factors that cause the performance limitations of the photonic terahertz wave distribution system are further analyzed.

Acousto-optic frequency shifter-based microwave photonic channelized receiver using a single optical frequency comb.

A microwave photonic channelization receiver is a promising technology for broadband radio frequency (RF) signal monitoring and reception. In this Letter, by exploiting acousto-optic frequency shifters (AOFSs), a microwave photonic channelization receiver is proposed. The proposed microwave photonic channelized receiver can reduce dual coherent optical frequency comb (OFC) generators with detuning frequency spacing into a single OFC generator. To verify the feasibility of the proposed channelization scheme, a broadband RF signal with 3.2 GHz bandwidth is channelized into eight narrowband RF signals with 0.4 GHz bandwidth. Moreover, we investigate the effect of the tuning error imposed by AOFSs, where the error vector magnitude (EVM) of subchannels is obtained by channelizing the four 16-quadrature amplitude modulation (16-QAM) signals into four subchannels.

Broadband photonic RF channelizer with 66 channels based on acousto-optic frequency shifter

Microwave photonic channelized receiver is one of the important means to realize broadband radio frequency (RF) signal reception, and it also has great application potential in the fields of radar systems and electronic warfare. In view of the above, a microwave photonic channelizer with 66-subchannels based on acousto-optic frequency shifter (AOFS) is proposed. The 11-line optical frequency comb (OFC) used in this scheme is generated via one phase modulator. The 33 optical local oscillators can be obtained using two AOFSs to shift the frequency of 11-line OFC up and down. The 33 optical local oscillators and broadband RF signal are image-rejected and downconverted in the I / Q receiver and output by 66 subchannels with a bandwidth of 500 MHz. The simulation results verify the complete reception of 3 to 36 GHz wideband RF signal, and the spurious-free dynamic range of the system can reach 116.3 dB · Hz2/3.

All-fiber beat-frequency laser heterodyne absolute distance measurement system

To realize precise absolute distance measurement, an all-fiber beat-frequency laser heterodyne distance measurement system based on a fiber-optic interferometric structure was proposed and demonstrated. An acousto-optic frequency shifter is introduced into one arm of a fiber-optic Mach–Zenhder interferormeter to generate the beat-frequency laser beams in two fiber-optic paths. By a ternary sinusoidal curve fitting method to extract the initial phases of the two beat-frequency laser signals and their phase difference, a distance measurement precision of several tens micrometers can be realized in a distance range of several meters. Experiments showed a maximum relative error of 0.0548% and a resolution of 83.333 μm in a distance range of 0 to 600 mm.

Acousto-optic modulators/frequency shifters with single-mode optic fibers

Acousto-optic modulators/frequency shifters based on TeO2 crystals with single-mode optical fibers supporting and not supporting polarization for collimated and focused light beams at radiation wavelengths of 785, 1064, 1550 nm have been developed, produced and experimentally investigated. The mechanisms of formation and methods of expanding the working band of the modulator are determined. A double-crystal acousto-optic laser emission frequency shifter with an working bandwidth of ≈40 MHz has been created. Single-crystal modulators based on collimated beams with a frequency band of ≈10 MHz are considered. A single-crystal modulator with a focused light beam with a switching time of ≈ 18 ns and an extended reception band of ≈ 40 MHz is investigated. It is shown that a light beam focusing makes it possible to implement a modulator with a minimum switching time of ≈ (2-3) ns. This value is limited by electrical breakdown of the ultrasonic wave transmitter.

Microwave Photonic Channelizer With Large Instantaneous Bandwidth Based on AOFS

The microwave photonic channelizer can convert the RF signal into the optical domain for transmission and procession, effectively avoiding the limitation of electronic bottleneck, and realizing the instantaneous reception of ultra-wideband signal or multi-frequency signals, which can be perfectly applied to radar system and electronic warfare. In this paper, a microwave photonic channelizer based on dual-output image-reject mixer is proposed, both the signal path and the local oscillator path are divided into three paths by using an optical coupler. An acousto-optic frequency shifter (AOFS) is used by the optical local oscillator to shift the frequency to the left and right and then enters the image rejection mixer with the signal path. Finally, a 6 GHz bandwidth RF signal is divided into six subchannels with a bandwidth of 1 GHz to achieve simultaneous reception. This scheme needs no optical frequency comb and doubles the channelization efficiency, the image rejection ratio of the sub-channels is about 22 dB, and the spurious-free dynamic range of the system can reach 103.2 dB·Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2/3</sup> .

An All-Fiber Mode-Locked Pulse Laser by Fiber Bragg Grating-Based Acousto-Optic Frequency Shifter

An all-fiber frequency shifted feedback laser is developed to export mode-locked pulses with a narrow spectral bandwidth (238 MHz). A simple cavity structure is established by employing a fiber Bragg grating-based acousto-optic frequency shifter. An exotic reflection peak in a large spectral separation from the Bragg reflection peak is generated by applying flexural acoustic waves, and is utilized for narrow-band filtering and frequency shifting in the laser cavity. Stable mode-locked pulse output can be obtained with the cavity length as short as 16.5 m, whereas increasing the cavity length can reduce the pump power to 37 mW for mode-locked pulse output. A maximum power conversion efficiency of 17% with an average output power of 103 mW were obtained experimentally at a pump power of 600 mW, with corresponding pulse width, pulse energy and peak power of 2 ns, 88 nJ and 4.5 W, respectively. A time-bandwidth product of mode locked pulses of 0.476 denotes nearly Fourier-transform-limited performance. This work provides a new technique to implement efficient, compact and low-cost all-fiber FSFLs for narrow spectral bandwidth mode-locked pulse output.

Dynamic Range Enhanced Optical Frequency Domain Reflectometry Using Dual-Loop Composite Optical Phase-Locking

We report on a dynamic range enhanced optical frequency domain reflectometry distributed backscattering interrogator based on dual-loop composite optical phase-locked loop (OPLL). Exploiting simultaneously an acousto-optic frequency shifter based an external modulation loop and a piezo based direct modulation loop, the proposed composite OPLL allows offering a larger loop bandwidth and gain, permitting a more efficient coherence enhancement as well as sweep linearization. A high fidelity frequency sweep of ~8.2 GHz at 164 GHz/s sweep rate is generated with a peak-to-peak frequency error as low as ~ 120 kHz. It leads to a dynamic range enhancement of more than 3 dB for the measured power loss compared to the case when only piezo loop is applied. This corresponds to ~15 km extension for the measurement range of Rayleigh backscattering without any spatial resolution penalties. Fourier transform-limited spatial resolution has been demonstrated at a range window more than about 28 times of the intrinsic coherence length of the adopted fiber laser. The proposed method provides a straightforward optimization of the real-time sweep control and is expected to be a useful tool in industrial and commercial applications.

Rapid and precise partial pressure measurement of multiple gas species with mid-infrared electro-optic dual-comb spectroscopy

In order to overcome the shortcoming of the TDLAS method, a partial pressure measurement method based on dual comb spectroscopy is proposed. The comb-based partial pressure measurement system comes with excellent performance in nearly all criteria: high detection sensitivity, high measurement speed, and synchronous survey of multiple-gas partial pressures. The experimental system was built in the mid-infrared band of 88.42–90.10 THz with a spectral resolution of 54.5 MHz and used for partial pressure measurement of CH4 and C2H4. Two electro-optics (EO) optical frequency combs (OFC) with a repeat frequency of ～25 GHz are spectrally interpolated to increase spectral sampling ratio using an acousto-optic frequency shifter, so that they could be utilized for high-resolution gas sensing. The measurement result demonstrates the excellent performance of this system in partial pressure measurement. The deviation of the measurement results of two gases CH4 and C2H4 from the true value are about 3%. Moreover, once the gas absorption line strength is accurately calibrated, the accuracy of partial pressure measurement will be further improved.

All-Fiber Frequency Shifter Based on an Acousto-Optic Tunable Filter Cascaded with a Tapered Fiber-Coupled Microcavity

An all-fiber acousto-optic frequency shifter (AOFS) based on an acousto-optic tunable filter (AOTF) cascaded with a packaged tapered fiber (TF)-coupled microsphere was proposed and demonstrated in both theory and experiment. The configuration has the advantages of easy alignment, robustness, compact size, and low cost, which will improve its further application, such as in the optical heterodyne detection technique (OHDT).

Impact of Position Mismatch Between Crystal and Electrode on Performance of Acousto-Optic Frequency Shifter

Impact of Position Mismatch Between Crystal and Electrode on Performance of Acousto-Optic Frequency Shifter

Broadband fibre-pigtailed acousto-optic frequency shifter

A two-crystal fibre-pigtailed acousto-optic frequency shifter has been developed and investigated. An optimal configuration of acousto-optic cells is chosen. A two-crystal acousto-optic frequency shifter of 785-nm laser radiation in the range from 156 to 196 MHz, with single-mode polarisation maintaining fibres at the input and output, is described. Some recommendations are given how to improve the shifter transmission coefficient and increase the frequency shift range.