Laser Frequency Sweep Research Articles

SNR Improvement of Optical Frequency Domain Polarimetry Against Laser Frequency Sweep Nonlinearity

A high-precision measurement of distributed polarization crosstalk (DPC) is necessary for developing the device with an ultrahigh polarization extinction ratio of even more than 80 dB. Optical frequency domain polarimetry (OFDP) is a novel DPC measurement method with a potential ultra-wide dynamic range. The degradation of dynamic range caused by interference phase error is a common issue in optical frequency domain measurement techniques. In this work, we enhanced the dynamic range by solving the phase error problem induced by the laser frequency sweep nonlinearity to improve the measurement precision of weak polarization crosstalk. The aliasing issue caused by the fluctuation of the frequency sweep rate is the primary reason for the limited dynamic range. We proposed a modified dynamic range limit model of OFDP considering the aliasing issue. The experimental results agree with the inflection points predicted by the modified dynamic range limit model. Under the guidance of the model, we achieved a dynamic range of more than 115 dB with different optical path differences of the interferometer and various sweep rates of the laser. Compared to the DPC measurement method with a dynamic range of 100 dB, the measurement repeatability is improved from 7.0 dB to <1.0 dB for polarization crosstalk less than -95 dB.

Laser frequency scanning interference nonlinear correction method based on Lomb-Scargle algorithm

Laser frequency scanning interference technology has become a research hotspot due to its high precision and strong anti-interference capability and other advantages. The nonlinear problem of laser frequency modulation has always been a key factor affecting the accuracy of the measurement system. The most direct result of the nonlinearity of frequency modulation is that the spectrum of the beat signal is severely broadened, resulting in a decrease in the ranging accuracy. In order to solve this problem, this paper proposes a nonlinear correction method based on the Lomb-Scargle algorithm, and builds a laser frequency sweep interferometry system with an auxiliary interferometer. The phase is extracted by performing Hilbert transform on the auxiliary path beat signal, thereby generating a new time series based on the extracted phase information. The generated time series carries the phase change information of the auxiliary path beat signal, and it is combined with the Lomb-Scargle algorithm to perform the nonlinear correction of the measurement system and the frequency calculation of the beat signal simultaneously. As a verification, the targets in the range of 0.5–1.3 m are measured with a maximum error of 14 μm. The traditional frequency sampling method is limited by the Nyquist sampling theorem, and the laser emission and reception need to travel a round-trip distance, which means that the frequency sampling method must meet the requirement that the distance of the measured target cannot exceed a quarter of the optical path difference of the auxiliary interferometer. Therefore, the range of distance measurement is limited when the optical path difference of the auxiliary interferometer is constant. Different from the correction principle of the traditional frequency sampling method, the correction method proposed in this paper does not use the beat signal of the auxiliary path to resample the measurement path, so there is no need to satisfy the condition that the optical path difference of the auxiliary interferometer is greater than four times the measuring distance. Therefore, in the case of a certain optical path difference of the auxiliary interferometer, it can provide a way to increase the ranging range of the system.

Frequency sweep extension using the Kerr effect for static temperature measurement range enhancement in Chirped Pulse φ-OTDR.

In Chirped Pulse φ-OTDR systems used for sensing temperature or strain along an optical fiber, the largest disturbance between two single-shot measurements that can reliably be detected depends on the range of frequencies swept by the chirped pulse. If electrical modulation is used to generate the laser frequency sweep, the achievable sweeping range is limited by the electrical components, leading to a narrow measurement range for static measurements. In this work, we demonstrate the extension of the frequency range of a chirped laser pulse by all-optical means using evenly spaced frequency sidebands generated via the Kerr effect to improve the Chirped Pulse φ-OTDR measurement range. We report chirp extensions by factors up to 13 and apply the effect to achieve a sixfold increase in the measurement range of a Chirped Pulse φ-OTDR system measuring the temperature of a random fiber grating array. The method described in this paper can be applied to other optical systems utilizing chirped laser pulses and allow for variable extension of their chirping range.

Nonlinear correction of frequency-modulated continuous-wave lidar frequency modulation based on singular value decomposition-least square algorithm

Frequency-modulated continuous-wave (FMCW) lidars combine the advantages of lasers and FMCW radars and have a wide range of applications in three-dimensional imaging and meteorological observation. However, its spatial resolution is directly limited by the nonlinearity of laser frequency sweep. A nonlinear correction method is proposed using a singular value decomposition (SVD)-least squares algorithm, which can calculate the nonlinear coefficient of each order accurately by minimizing the error between the actual phase and the fitted phase. In the simulation, the results demonstrate that the proposed method can achieve significantly better performance on nonlinearity and root-mean-square error (RMSE) than that of the conventional high-order ambiguity function (HAF) algorithm. Calculation results show that, for the SVD algorithm, the nonlinearity and RMSE are reduced by 50.37% and 52.55%, respectively, compared with those of the HAF algorithm under the signal-to-noise ratio of 25 dB, proving the performance improvement of the proposed algorithm.

40 GHz continuous, precise, and low power-loss laser frequency sweep using an electro-optic modulator.

We propose and demonstrate a novel technique for precisely and widely sweeping the frequency of a continuous-wave laser. One of the modulation sidebands of a slave laser generated with an electro-optic modulator is phase-locked to a master laser; in this situation, the slave carrier component can be swept by sweeping the modulation frequency. It does not require beat signal detection at varying and/or high frequency, thus providing a robust and reliable laser frequency sweep. Also, it requires neither a frequency comb for the sweep nor a large power loss. We successfully swept an 852 nm laser over 20 GHz; we confirmed that a second harmonic 426 nm laser could be continuously swept over 40 GHz.

Laser frequency sweep linearization by iterative learning pre-distortion for FMCW LiDAR.

We report on a laser frequency sweep linearization method by iterative learning pre-distortion for frequency-modulated continuous-wave (FMCW) light detection and ranging (LiDAR) systems. A pre-distorted laser drive voltage waveform that results in a linear frequency sweep is obtained by an iterative learning controller, and then applied to the FMCW LiDAR system. We have also derived a fundamental figure of merit for the maximum residual nonlinearity needed to achieve the transform-limited range resolution. This method is experimentally tested using a commercial vertical cavity surface-emitting laser (VCSEL) and a distributed feedback (DFB) laser, achieving less than 0.005% relative residual nonlinearity of frequency sweep. With the proposed method, high-performance FMCW LiDAR systems can be realized without expensive linear lasers, complex linearization setups, or heavy post-processing.

Nonlinear error correction for FMCW ladar by the amplitude modulation method.

FMCW ladar is a kind of absolute distance measurement technology with high spatial resolution. However, the advantage of high spatial resolution is significantly covered up by the non-linearity of laser frequency sweep. One of the typical approaches for the nonlinearity is resample technology, which has residual phase error from the sample time delay mismatch between the clock signal and the measurement signal. We have proposed and demonstrated a novel amplitude modulation method for correcting the nonlinear error of FMCW technology. The optical structure of the method is comprised of two tandem fiber interferometers. The first interferometer is used to produce a carrier signal and the second one is used to load the range information on the amplitude of the carrier signal. In the end, the experimental result verifies that the nonlinear error can be suppressed effectively, the phase error from the mismatch has been eliminated observably, and the range resolution can be notably improved to 69μm; the stability is 2.9μm and the measurement precision is 4.3μm.

A Low Bandwidth DFB Laser-Based Interrogator for Terahertz-Range Fiber Bragg Grating Sensors

This letter reports an all-electronic, low bandwidth swept frequency laser used to interrogate terahertz-range fiber Bragg gratings (THz FBGs) for distributed strain sensing applications. A distributed feedback laser with current injection modulation was employed as the swept frequency laser source. Using the resulting narrow bandwidth (~110 GHz) laser frequency sweep, high accuracy distributed strain measurements were achieved. In order to experimentally investigate this concept, a strain test was conducted using the THz FBGs. During the test, the laser sweeping time was limited to less than 2 ms. Highly linear results were found (R 2 = 0.996), with an observed sensitivity of -0.142 GHz/μe and the standard deviation of 1.167 μe. A multiplexing test was also conducted, and no cross-talk found between sensor elements. These results demonstrate that this interrogation system holds substantial potential as a method of rapid distributed optical fiber frequency-domain sensing.

Investigation of measurement sensitivities in cross-correlation Doppler global velocimetry

Cross-correlation Doppler global velocimetry (CC-DGV) is a flow measurement technique based on the estimation of Doppler frequency shift of scattered light by means of cross-correlating two filtered intensity signals. The signal characteristics of CC-DGV result in fundamental limits for estimation variance as well as the possibility for estimator bias. The current study assesses these aspects theoretically and via Monte Carlo signal simulations. A signal model is developed using canonical numerical functions for the iodine absorption cell and incorporating Poisson and Gaussian signal noise models. Along with consideration of the analytical form of the Cramér–Rao lower bound, best practices for system settings are discussed. The CC-DGV signal processing routine is then assessed by a series of Monte Carlo simulations studying the effect of temperature mismatch between flow signal and reference detector cells, velocity magnitude, and discretization error in the frequency modulation. A measurement bias was observed; the magnitude of the bias is a weak function of the cell temperature mismatch, but it is independent of the flow velocity magnitude. The measurement variance was found to approach the Cramér–Rao lower bound for optimized conditions. A cyclical bias error resulting from the discrete nature of the laser frequency sweep is also observed with maximum errors of ±1.0% of the laser frequency scan step size, corresponding to peak errors of ±0.61ms−1 for typical settings. Overall, the signal estimator is found to perform best for matched cell temperatures, small frequency step size, and high velocity regimes, where the relative bias errors are collectively minimized.

Active Compensation Method for Light Source Frequency Drifting in &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$\Phi $ &lt;/tex-math&gt;&lt;/inline-formula&gt;-OTDR Sensing System

Phase-sensitive optical time-domain reflectometry ( $\Phi $ -OTDR) has the ability to make a measurement on external disturbance dynamically along the sensing fiber by monitoring the interference fading pattern variance of the Rayleigh backscattering waveform through continuous probe pulse injection. However, even if there is no disturbance, the received $\Phi $ -OTDR trace will still show slow but noteworthy distortion due to light source frequency drifting (LSFD). This trace-to-trace slow distortion makes it challenging to capture low-frequency event. In this letter, an active compensation method based on laser frequency sweep and cross-correlation calculation has been proposed to suppress the influence of LSFD. With the proposed active compensation method, the tracking of LSFD can be realized, and the trace-to-trace distortion can be greatly suppressed. The experimental result has shown that a vibration event of 0.5 Hz has been identified successfully under mean frequency drifting speed of 1.68 MHz/s, which extends the application realm of the $\Phi $ -OTDR sensing system to quasi-static measurement.

Results from the Stanford 10 m Sagnac interferometer

The design of a 10 m all-reflective prototype Sagnac interferometer with suspended optics is described and the experimental results are presented. It uses a polarization scheme to allow detection of the dark fringe on the symmetric port of the beamsplitter for optimal interference contrast. The necessary low-frequency response of the interferometer requires delay lines in the arms. To deal with the noise introduced by scattered light in the delay lines, a laser frequency sweep frequency shifts the scattered light so that it does not produce noise near zero frequency. This results in a shot-noise-limited phase sensitivity of Δϕ = 1.6 × 10−9 rad Hz−1/2 at frequencies as low as 200 Hz. Scaling this prototype to several kilometres with kilowatts of circulating power requires several technical improvements in high-power solid-state lasers, second harmonic generation and the fabrication of large mirrors, which are likely to be made in the next 10 years.

Thermally induced self-locking of an optical cavity by overtone absorption in acetylene gas

Strong self-locking phenomena are observed when laser power is converted into heat by a weakly absorbing medium within a high-finesse cavity. Deposited heat leads to increased temperature and, for the case of weakly absorbing intracavity gases studied here, to an associated reduction of density and refractive index. This thermal change in refractive index provides self-acting cavity tuning near resonant conditions. In the experiments reported here a Fabry–Perot cavity of finesse 274 was filled with acetylene gas and illuminated with a titanium:sapphire laser tuned to the P(11) line of the ν1 + 3ν3 overtone band near 790 nm. The dependencies of maximum frequency-locking range on gas pressure, laser power, and laser frequency sweep rate and direction were measured and could be well unified by analysis based on the thermal model. In the domain of strong self-tuning an interesting self-sustained oscillation was observed, with its several sharp frequencies directly and quantitatively linked to the acoustic boundary conditions in our cylindrical cell geometry. The differences between the behavior of acetylene near 790 nm and molecular oxygen with electronic transition near 763 nm are instructive; whereas the absorbed powers were similar, they differed strongly in their rates for internal to translational energy conversion by collisional relaxation.

Laser Polarization Effect on Adiabatic Inversion of Degenerate Two-Level System

Coherent excitation dynamics of a degenerate two-level system by a frequency-swept laser were analyzed by solving the optical Bloch equations. Physical parameters (Rabi frequency, laser frequency sweep rate and laser frequency sweep width or pulse width) giving a high excitation efficiency were obtained. Laser polarization also affects the excitation efficiency because the variation of dipole matrix elements depends on the laser polarization and ΔJ, the difference between the J values of the upper and lower levels. It was found that circular polarization is preferable when ΔJ=0 and linear polarization is preferable when ΔJ=±1. This polarization effect was qualitatively analyzed.

A synthesized method to improve coherence in semiconductor lasers by electrical feedback

A method for improving coherence in semiconductor lasers by negative electrical feedback is proposed for stabilization of the center frequency of the field spectrum, linewidth reduction of the field spectrum, frequency tracking to another highly coherent laser, and stable and wideband frequency sweep. Experimental center frequency stabilization of the master laser showed that the magnitude of frequency fluctuations was reduced to 50 kHz at the integration time tau =3 s. The linewidth of the master laser was reduced to 100 kHz, which was 1/50 that of the free running laser. Under these frequency control conditions, the frequency of the slave laser was controlled so that the phase of the heterodyne signal between the master and the slave lasers could be locked to that of a stable microwave synthesizer. The slave laser frequency tracked accurately to the master laser frequency.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>