Transponder-type laser interferometer prototype for spaceborne gravitational wave detectors.

Transponder-type laser interferometry is essential in spaceborne gravitational wave detection missions. This paper presents a transponder-type laser interferometer prototype for potential noise calibration of spaceborne gravitational wave detectors. Using a digital optical phase-locked loop, we successfully locked the phase of the slave laser to the master laser (∼200p W). Once the link between the master laser and the slave laser is established, the two satellites (essentially two lasers) form a transponder-type laser interferometer. We carefully analyze the measurement stability and noise characteristics of the interferometer, and the results show that the Allan deviation of the zero drift can reach 243.2 pm at t=0.429s, while the noise spectral density has a typical 1/f line shape with a floor of 21p m/H z 1/2 at 1 Hz. The coherence analysis shows that the temperature drift is an important factor limiting the performance of the interferometer below 2 mHz, while the frequency noise of the master laser is not dominant in the experiment. Transponder-type laser interferometers have a wide range of applications in intersatellite communication and measurement. Our design can serve as a valuable reference for gravitational wave detection missions such as LISA.

Time-domain fit for improved contrast in quantitative coherent anti-Stokes Raman spectroscopy.

Among the multiple coherent anti-Stokes Raman scattering (CARS) techniques that provide important quantitative molecular microscopic contrast, Fourier-transform CARS (FT-CARS) stands out with the immunity to nonresonant background and high-speed detection capacity. However, by using FFT for the exponentially decaying signal, FT-CARS faces the dilemma of choosing the delay range of the signal for high SNR or high resolution, the lack of either of which is detrimental to the quantitative contrast of imaging. Here, time-domain fit (TDF) is proposed to fully utilize the time-domain information of FT-CARS, providing optimized SNR and vibrational feature distinguishment. The capacity of noise restriction and feature distinguishment of the traditional FFT and the proposed TDF is analysed with theoretical examination and simulation. Exploiting the matrix pencil extraction of vibrational parameters, TDF is performed for quantitative analysis for simulated FT-CARS signal, and shows more accurate and consistent performance than the FFT method. FT-CARS coupled with TDF intensity evaluation holds the promise to provide micro-spectroscopic contrast with higher SNR and free of spectral overlapping, contributing to a more powerful diagnostic tool.

Rapid coherent Raman hyperspectral imaging based on delay-spectral focusing dual-comb method and deep learning algorithm.

Rapid coherent Raman hyperspectral imaging shows great promise for applications in sensing, medical diagnostics, and dynamic metabolism monitoring. However, the spectral acquisition speed of current multiplex coherent anti-Stokes Raman scattering (CARS) microscopy is generally limited by the spectrometer integration time, and as the detection speed increases, the signal-to-noise ratio (SNR) of single spectrum will decrease, leading to a terrible imaging quality. In this Letter, we report a dual-comb coherent Raman hyperspectral microscopy imaging system developed by integrating two approaches, a rapid delay-spectral focusing method and deep learning. The spectral refresh rate is exploited by focusing the relative delay scanning in the effective Raman excitation region, enabling a spectral acquisition speed of 36 kHz, ≈4 frames/s, for a pixel resolution of 95 × 95 pixels and a spectral bandwidth no less than 200 cm-1. To improve the spectral SNR and imaging quality, the deep learning models are designed for spectral preprocessing and automatic unsupervised feature extraction. In addition, by changing the relative delay focusing region of the comb pairs, the detected spectral wavenumber region can be flexibly tuned to the high SNR region of the spectrum.

High-precision digital optical phase locking for 10-12 W order weak light for a spaceborne gravitational wave interferometer.

We developed a digital optical phase-locking loop (DOPLL) for weak light phase locking in spaceborne gravitational wave interferometers (SGWIs). Using the system, we successfully locked the phase of the slave laser to the master laser with the power of only several picowatts, much smaller than the LISA requirement (100 pW). The system does not introduce steady-state errors, and the Bode diagram shows its stability. The out-loop phase noise floor (2.3×10-4 and 5.2×10-4 r a d/H z 1/2) is very close to the shot noise limit. The Allan standard deviation of the heterodyne signal reaches 3.1×10-17 at 1000 s. With the previous automatic locking program designed by other researchers, the results demonstrate that DOPLLs have bright application prospects and can be applied in the transducer of the SGWI.

Sensitive impulsive stimulated Brillouin spectroscopy by an adaptive noise-suppression Matrix Pencil.

Impulsive stimulated Brillouin spectroscopy (ISBS) plays a critical role in investigating mechanical properties thanks to its fast measurement rate. However, traditional Fourier transform-based data processing cannot decipher measured data sensitively because of its incompetence in dealing with low signal-to-noise ratio (SNR) signals caused by a short exposure time and weak signals in a multi-peak spectrum. Here, we propose an adaptive noise-suppression Matrix Pencil method for heterodyne ISBS as an alternative spectral analysis technique, speeding up the measurement regardless of the low SNR and enhancing the sensitivity of multi-component viscoelastic identification. The algorithm maintains accuracy of 0.005% for methanol sound speed even when the SNR drops 33 dB and the exposure time is reduced to 0.4 ms. Moreover, it proves to extract a weak component that accounts for 6% from a polymer mixture, which is inaccessible for the traditional method. With its outstanding ability to sensitively decipher weak signals without spectral a priori information and regardless of low SNRs or concentrations, this method offers a fresh perspective for ISBS on fast viscoelasticity measurements and multi-component identifications.

Improved opposition-based self-adaptive differential evolution algorithm for vibrational hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering thermometry.

We propose an improved opposition-based self-adaptive differential evolution (IOSaDE) algorithm for multi-parameter optimization in vibrational hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (CARS) thermometry. This new algorithm self-adaptively combines the advantages of three mutation schemes and introduces two opposite population stages to avoid premature convergence. The probability of choosing each mutation scheme will be updated based on its previous performance after the first learning period. The IOSaDE method is compared with nine other traditional differential evolution (DE) methods in simulated spectra with different simulation parameters and experimental spectra at different probe time delays. In simulated spectra, both the average and standard deviation values of the final residuals from 20 consecutive trials using IOSaDE are more than two orders of magnitude smaller than those using other methods. Meanwhile, the fitting temperatures in simulated spectra using IOSaDE are all consistent with the target temperatures. In experimental spectra, the standard deviations of the fitting temperatures from 20 consecutive trials decrease more than four times by using IOSaDE, and the errors of the fitting temperatures also decrease more than 18%. The performance of the IOSaDE algorithm shows the ability to achieve accurate and stable temperature measurement in CARS thermometry and indicates the potential in applications where multiple parameters need to be considered.

Hybrid fs/ps vibrational coherent anti-Stokes Raman scattering for simultaneous gas-phase N2/O2/CO2 measurements.

Single-shot, 1-kHz measurements of temperatures and mole fraction ratios along with theoretical modeling for gas-phase N2, O2, and CO2 are demonstrated using hybrid femtosecond/picosecond (fs/ps) vibrational coherent anti-Stokes Raman scattering (CARS). The combination of broadband pump and Stokes pulses covers a spectral range over 1800cm-1 while the narrowband probe pulses generated from a quasi-common-path second harmonic bandwidth compressor (QCP-SHBC) resolves the molecular structures with a bandwidth of ∼7cm-1. Temperature results of 1700-2000 K in methane/air fuel-lean flames show state-of-the-art inaccuracies of less than ∼3% and precision less than 2%. Mole fraction ratios inaccuracy at room temperature is ∼5%, and precision at flame temperatures are 6%-8%.

Dual-wavelength excitation combined Raman spectroscopy for detection of highly fluorescent samples.

As fluorescence is the major limitation in Raman scattering, near-infrared excitation wavelength (>780nm) is preferred for fluorescence suppression in Raman spectroscopy. To reduce the risk of fluorescence interference, we developed a dual-wavelength excitation combined Raman spectroscopy (DWECRS) system at 785 and 830 nm. By a common optical path, each laser beam was focused on the same region of the sample by a single objective lens, and the dual-wavelength excitation Raman spectra were detected by a single CCD detector; in addition, 785 and 830 nm excitation Raman spectra can be directly constructed as combined Raman spectrum in the DWECRS system. The results of pure peanut oil and glycerol indicate that the combined Raman spectrum cannot only reduce fluorescence interference but also keep a high signal-to-noise ratio in the high-wavenumber region. The results of dye-ethanol solutions with different concentrations show that the handheld DWECRS system can be used as a smart method to dodge strong fluorescence. Furthermore, we developed a peak intensity ratio method with the DWECRS system to distinguish different types of edible oils. The peak intensity ratio distribution chart of edible oils showed each oil normalized center was relatively independent and nonoverlapped, which can be used as the basis of edible oil classification analysis. In the future, the DWECRS system has potential to be used as a tool for more complex applications.

Dual-wavelength wide area illumination Raman difference spectroscopy for remote detection of chemicals.

Remote Raman instruments have become powerful analytical tools in some special environments. However, ambient daylight is the main limitation in the data acquisition process. To suppress daylight background interference and obtain a high signal-to-background ratio (SBR), we develop a dual-wavelength wide area illumination Raman difference spectroscopy (WAIRDS) system for daytime remote detection. In the WAIRDS system, a wide area illumination scheme and shifted-excitation Raman difference spectroscopy method are used to improve the reliability of collected Raman spectra. Measurements of polystyrene indicate that the WAIRDS system can be operated to obtain background-free Raman spectra under different levels of daylight background interference. The remote results show that the improvement in SBR is about three- to fivefold, and the system can work at distances of up to 9.2 m on a sunny afternoon. Moreover, to be close to the actual detection, the system is used for mineral and explosive raw material detection during daytime measurement. Measurements show that the WAIRDS system will be a useful tool for many remote applications in the future.

Surface-enhanced shifted excitation Raman difference spectroscopy for trace detection of fentanyl in beverages.

In recognition of the misuse risks of fentanyl, there is an urgent need to develop a useful and rapid analytical method to detect and monitor the opioid drug. The surface-enhanced shifted excitation Raman difference spectroscopy (SE-SERDS) method has been demonstrated to suppress background interference and enhance Raman signals. In this study, the SE-SERDS method was used for trace detection of fentanyl in beverages. To prepare the simulated illegal drug-beverages, fentanyls were dissolved into distilled water or Mizone as a series of test samples. Based on our previous work, the surface-enhanced Raman spectroscopy detection was performed on the beverages containing fentanyl by the prepared AgNPs and the SE-SERDS spectra of test samples were collected by the dual-wavelength rapid excitation Raman difference spectroscopy system. In addition, the quantitative relationship between fentanyl concentrations and the Raman peaks was constructed by the Langmuir equation. The experimental results show that the limits of quantitation for fentanyl in distilled water and Mizone were 10 ng/mL and 200 ng/mL, respectively; the correlation coefficients for the nonlinear regression were as high as 0.9802 and 0.9794, respectively; and the relative standard deviation was less than 15%. Hence, the SE-SERDS method will be a promising method for the trace analyses of food safety and forensics.

Active modulation of intracavity laser intensity with the Pound-Drever-Hall locking for photoacoustic spectroscopy.

Here we report a novel, to the best of our knowledge, method of active intracavity intensity modulation for cavity-enhanced photoacoustic spectroscopy (PAS) without the need for any external optical modulators. Based on the Pound-Drever-Hall (PDH) locking technique, a dither is added to the PDH error signal to periodically vary the locking point between the laser frequency and optical cavity within a sub-MHz frequency range. While significantly enhancing the intracavity laser intensity, the optical cavity also acts as an intensity modulator. As a proof-of-principle, we demonstrated the PAS of ${{\rm C}_2}{{\rm H}_2}$C2H2 by placing a photoacoustic cell ($Q$Q-factor $\sim{10}$∼10) inside a Fabry-Perot cavity (finesse $\sim{628}$∼628) and adopting the proposed intracavity intensity modulation scheme. By detecting the weak ${{\rm C}_2}{{\rm H}_2}$C2H2 line at ${6412.73}\;{{\rm cm}^{ - 1}}$6412.73cm-1, the sensor achieves a normalized noise equivalent absorption (NNEA) coefficient of ${1.5} \times {{10}^{ - 11}}\;{{\rm cm}^{ - 1}}{{\rm WHz}^{ - 1/2}}$1.5×10-11cm-1WHz-1/2. This method enables the continuous locking of laser frequency and optical cavity, and it achieves the intracavity intensity modulation with an adjustable modulation depth as well.

Baseline correction method based on improved asymmetrically reweighted penalized least squares for the Raman spectrum.

We present a baseline correction method based on improved asymmetrically reweighted penalized least squares (IarPLS) for the Raman spectrum. This method utilizes a new S-type function to reduce the risk of baseline overestimation and speed up the reweighting process. Simulated spectra with different levels of noise and measured spectra with strong fluorescence background from different samples are used to validate the performance of the proposed algorithm. Considering the drawbacks of the weighting rules for the asymmetrically reweighted penalized least squares (arPLS) method, we adapt an inverse square root unit (ISRU) function, which performs well in baseline correction. Compared with previous penalized least squares methods, such as asymmetric least squares, adaptive iteratively reweighted penalized least squares, and arPLS, experiments with the simulated Raman spectra have confirmed that the proposed method yields better outcomes. Experiments with the measured Raman spectra show that the IarPLS method can improve real Raman spectra within 20 ms. The results show that the proposed method can be successfully applied to the practical Raman spectrum as a strong basis for quantitative analysis.

Sensitivity improvement by optimized optical switching and curve fitting in a cavity ring-down spectrometer: publisher's note.

This publisher's note amends an award ID in the Funding section of Appl. Opt.57, 8487 (2018)APOPAI0003-693510.1364/AO.57.008487.

Segment-Resolved Gas Concentration Measurements by a Time Domain Multiplexed Dual Comb Method.

Locating gas concentration changes in widespread locations can be conducive to environmental atmospheric detection, gas emissions monitoring, production process control, etc. A time domain multiplexed dual-comb system for segment-resolved gas concentration measurement is reported in this work. Both absorption spectra and path lengths for multiple path-segments in a target path can be derived from the time domain separated interferograms and then the equivalent gas concentrations in each segment can be retrieved separately. A benchtop experiment aiming at a target path with three path-segments of different gases has been demonstrated. The relative deviation of gas concentration retrieval is 1.08% in 1 s. Besides, additional numerical simulations prove that the crosstalk between the interference signals affects the spectrum analysis by no more than 0.1% for a kilometer-long atmospheric absorption detection. Therefore, achieving a gridded measurement of regional gas concentration in the open air can be foreseen using this method.

Ultrasensitive photoacoustic detection in a high-finesse cavity with Pound-Drever-Hall locking.

We demonstrate an ultrasensitive photoacoustic sensor using a low laser power (4 mW) and high-finesse (>9000) optical cavity. The Pound-Drever-Hall (PDH) method is adopted to lock the external cavity diode laser at 1531.58 nm to the Fabry-Pérot cavity. By placing a photoacoustic cell inside the 130-mm-long optical cavity, we obtain an enhancement of more than 630 times in laser power for acetylene (C2H2) detection. The present photoacoustic spectroscopy (PAS) sensor achieves a normalized noise equivalent absorption coefficient of 1.1×10-11 cm-1 WHz-1/2, which is unprecedented sensitivity among all the current PAS sensors. Our results demonstrate the feasibility of merging PAS with a high-finesse cavity using PDH locking for ultrasensitive trace gas detection.

Beam drift reduction by straightness measurement based on a digital optical phase conjugation.

One of the greatest challenges of long distance measurement is the beam drift caused by the air refractive index gradient. It has been established in many researches that optical phase conjugation (OPC) can be used to compensate for the beam bending. However, this method is limited to responding speed, phase conjugate reflectivity, flexibility, and specific source and medium. To reduce beam drift, instead of OPC, this study applies a digital OPC (DOPC) method, which is also creatively applied to collimation and flatness measurements. The main devices in the wavefront correction unit are the spatial light modulator and the Shack-Hartmann wavefront sensor. For the straightness measurement unit, the collimation and flatness of the optical rail are measured through the prism system and a position-sensing detector. After wavefront compensation, the root mean square is decreased from 0.0029λ to 0.0005λ. The beam drift is decreased from 1.22 mm to 0.70 mm in the x direction and from 2.49 mm to 1.55 mm in the y direction. The experimental data indicate that the straightness measurement system based on DOPC can effectively decrease the beam drift.

Flexible dual-soliton manipulation for coherent anti-Stokes Raman scattering spectroscopy.

Though dual soliton is used as the Stokes source in coherent anti-Stokes Raman scattering (CARS) experiments, the mechanism behind it is still not fully investigated. In this paper, dual-soliton pulses generation with a highly birefringent photonic crystal fiber (PCF) is numerically explored and experimentally verified. The simulated pulse exhibits various characteristics in the temporal and spectral domains with respect to the input linear polarization angle, and can be classified into four regions based on distances in the temporal and spectral domain of the soliton pair. By tuning input power and polarization angle, a soliton pair with desirable spectral coverage and temporal delay for CARS applications can be flexibly generated before the two solitons overlap in time domain. The simulated results are then experimentally validated by comparing their temporal and spectral characteristics. In addition, the benefit of using dual-soliton sources, such as Stokes beam, in CARS experiments is also demonstrated. The results and methods presented in this work can improve CARS spectroscopy and microscopy techniques, and may also be beneficial to other dual-wavelength source applications.

Comb-referenced frequency-sweeping interferometry for precisely measuring large stepped structures.

A precise 3D surface measurement method for large stepped structures without height ambiguity is proposed based on optical-frequency-comb-referenced frequency-sweeping interferometry and Fourier-transformed fractional phase retrieval. Unlike other interferometry that depends on the absolute phase value for several certain wavelengths, this method obtains results from the phase change during frequency sweeping and thus remains free from the confined non-ambiguity range. By reference to an optical frequency comb, the relative uncertainty from the tunable laser frequency was reduced by three orders of magnitude, and the sweeping frequency range can be precisely determined. Besides, the fractional phase can be rapidly retrieved in only one step using a Fourier transform method, with advantages of high accuracy and immunity to light intensity fluctuation and mechanical vibration noise. Samples of step heights from 1 µm to 1 mm were measured, and the standard uncertainty was 45 nm. This permits applications such as quality assurance in microelectronics production and micro-electro-mechanical system (MEMS) manufacture.

Sensitivity improvement by optimized optical switching and curve fitting in a cavity ring-down spectrometer.

We presented methods for the improvement of the sensitivity of a cavity ring-down spectrometer other than modifying the cavity length and the mirrors. As for the light switching, a fast driving scheme was proposed to address the slow switching speed of the boost optical amplifier, which makes it have only half of the switching time of that for the common acoustic-optical modulators and electro-optical modulators, as well as have higher extinction ratios. This effectively suppressed the distortions of the ring-down signals. We further adopted a realistic non-exponential curve-fitting method, taking into account the switching speed and the delayed triggering of the optical switch. These methods help accurately determine the ring-down time constants, which in turn reduced the Allan variance of the measurement results and increased the sensitivity. We performed tests at different repetition rates and all of them revealed more than 30% sensitivity improvement. At a rate of 16 kHz, we increased the minimal detectable absorption of 9.1×10-11 cm-1 to 5.7×10-11 cm-1. The effectiveness of these upgrades could benefit many spectroscopic applications of the cavity ring-down spectroscopy, especially for frontier research that requires sensitive measurement and high-quality spectral data.

Simply-integrated dual-comb spectrometer via tunable repetition rates and avoiding self-referencing.

Dual-comb system with simplified integration acts a crucial role in the next stage of field-deployable application. Here we demonstrate a home-made dual-comb spectrometer that meets this requirement based on optically stabilized erbium-fiber frequency combs. The simplified integration and operability are highlighted by the embedded large motion-span automatic optical delay line that resulted in 1.7 MHz of continuous repetition rate tuning range and avoid multiple fiber cutbacks. This relaxes the second similar frequency comb duplication, therefore simplifies dual-comb system integration. The mutual coherence was enhanced by optical phase lock to a common narrow-linewidth reference laser via pump current modulation. We acquired the absorption feature of acetylene 12C2H2 molecule in 1.5 µm region with an apodized instrument linewidth of 0.076 cm-1. The mean signal-to-noise ratio within a spectral bandwidth of 90 cm-1 achieved 280, and the corresponding noise equivalent absorption coefficient was 3.7 × 10-6 cm-1·Hz-1/2 for an absorption path of 7.5 cm and a pressure of 100 Torr. The accuracy of the retrieved spectrum was further confirmed by comparing with HITRAN database.