Haoyun Wei

Absolute distance measurement using frequency-sweeping heterodyne interferometer calibrated by an optical frequency comb.

We present a frequency-sweeping heterodyne interferometer to measure an absolute distance based on a frequency-tunable diode laser calibrated by an optical frequency comb (OFC) and an interferometric phase measurement system. The laser frequency-sweeping process is calibrated by the OFC within a range of 200 GHz and an accuracy of 1.3 kHz, which brings about a precise temporal synthetic wavelength of 1.499 mm. The interferometric phase measurement system consisting of the analog signal processing circuit and the digital phase meter achieves a phase difference resolution better than 0.1 deg. As the laser frequency is sweeping, the absolute distance can be determined by measuring the phase difference variation of the interference signals. In the laboratory condition, our experimental scheme realizes micrometer accuracy over meter distance.

Absolute distance measurement by dual-comb nonlinear asynchronous optical sampling.

A dual-comb nonlinear asynchronous optical sampling method is proposed to simplify determination of the time interval and extend the non-ambiguity range in absolute length measurements. Type II second harmonic generation facilitates curve fitting in determining the time interval between adjacent pulses. Meanwhile, the non-ambiguity range is extended by adjusting the repetition rate of the signal laser. The performance of the proposed method is compared with a heterodyne interferometer. Results show that the system achieves a maximum residual of 100.6 nm and an uncertainty of 1.48 μm in a 0.5 ms acquisition time. With longer acquisition time, the uncertainty can be reduced to 166.6 nm for 50 ms and 82.9 nm for 500 ms. Moreover, the extension of the non-ambiguity range is demonstrated by measuring an absolute distance beyond the inherent range determined by the fixed repetition rate.

Design and performance of an absolute gas refractometer based on a synthetic pseudo-wavelength method

We present a refractometer that is capable of measuring the refractive index of gases with an unambiguous range of 1.000395 and an uncertainty of 3.14×10(-8) at 633 nm. The measurement range was extended via the combination of the vacuum cells according to the proposed synthetic pseudo-wavelength (SPW) method. The basic principles of the SPW method and the design of the gas refractometer are presented in detail. The performance of the refractometer was verified in the measurements of dry air, nitrogen gas, and ambient air under different environmental conditions. No gas-filling or pumping processes were required during the measurements; so one measurement could be completed within 70 s. Compared with existing refractometers, the method reported here holds advantages in its large unambiguous measuring range, fast speed, high accuracy, and simple instrumentation design.

Dual-soliton Stokes-based background-free coherent anti-Stokes Raman scattering spectroscopy and microscopy

We propose an all-fiber-generated, dual-soliton, Stokes-based scheme for background-free coherent anti-Stokes Raman scattering (CARS) under the spectral focusing mechanism. Owing to the strong birefringence and high nonlinearity of a polarization-maintaining PCF (PM-PCF), two soliton pulses can be simultaneously emitted along different eigenpolarization axes and both serve as Stokes pulses, while allowing feasible tunability of frequency distance and temporal interval between them. This proposed scheme, based on an all-fiber light source, exploits a unique combination of slight frequency-shift temporal walk-off of these two solitons to achieve efficient suppression of the nonresonant background and beat the inaccessibility and complexity of the excitation source. Capability is experimentally demonstrated by background-free CARS spectroscopy and unambiguous CARS microscopy in the fingerprint region.We propose an all-fiber-generated dual-soliton pulses based scheme for the background-free detection of coherent anti-Stokes Raman spectroscopy under the spectral focusing mechanism. Due to the strong birefringence and high nonlinearity of a polarization-maintaining photonic crystal fiber (PM-PCF), two redshifted soliton pulses can be simultaneously generated relying on high-order dispersion and nonlinear effects along two eigenpolarization axes in the anomalous dispersion regime, while allowing feasible tunability of the frequency distance and temporal interval between them. This proposed scheme, termed as DS-CARS, exploits a unique combination of slight frequency-shift and advisable temporal walk-off of this two soliton pulses to achieve robust and efficient suppression of nonresonant background with compact all-fiber coherent excitation source. Capability of the DS-CARS is experimentally demonstrated by the background-free CARS spectroscopy and unambiguous CARS microscopy of polymer beads in the fingerprint region.

A Raman peak recognition method based automated fluorescence subtraction algorithm for retrieval of Raman spectra of highly fluorescent samples

Intense fluorescence background is a major problem in the application of Raman spectroscopy. An appropriate algorithm which can faithfully retrieve weak tissue Raman signals is required. In this article, we propose a new algorithm for automated and artifact-free recovery of Raman spectra which combines a novel Raman peak recognition method (RPR method) with an improved iterative smoothing method (SG-SR method). The SG-SR method, based on the modified Savitzky–Golay iterative process, substantially improves its convergence speed. By applying a novel negative relaxation factor to the successive relaxation iterative method, automatic recognition of Raman peaks is realized. In the proposed algorithm (RIA-SG-RPR algorithm), a real Raman peak position is first detected by the RPR method to serve as the intrinsic criterion of convergence for the SG-SR method to avoid human interference. Then, real Raman signals are recovered from the iterative procedure of the SG-SR method. This algorithm has been optimized and validated with mathematically simulated Raman spectra as well as experimentally recorded Raman spectra from various fluorescent samples, resulting in a significant improvement in the rejection of both high fluorescence background and direct human intervention. This algorithm drastically avoids false Raman features to benefit the utilization of Raman spectroscopy to characterize molecular specifics in more challenging Raman applications.

Five-degrees-of-freedom measurement system based on a monolithic prism and phase-sensitive detection technique

This paper presents a method for measuring five-degrees-of-freedom errors of a moving stage with a monolithic prism and phase-sensitive detection technique. It consists of a pigtailed laser diode, three position-sensitive detectors (PSDs), a monolithic prism, and additional optical and electronic components. The monolithic prism mounted on the moving stage generates three beams that are detected by three PSDs, respectively, so that the straightness, pitch, yaw, and roll errors can be simultaneously measured. Theoretical analysis of each error measurement process is presented. To reduce the influence of disturbing light, the laser diode is modulated by a sinusoidal wave current, and a phase-sensitive detection technique is developed to demodulate the signals. Compared with a laser interferometer, the deviation errors when measuring the horizontal and vertical straightness errors are better than ±0.25 and ±0.4 μm, respectively. The deviation errors for the pitch, yaw, and roll are better than ±0.5, ±0.3, and ±2 arc sec, respectively, in comparison with an autocollimator. The system can be assembled to measure five error components of machine tools in an industrial environment.

Phase-shifting interferometer using a frequency-tunable diode laser calibrated by an optical frequency comb

We present a phase-shifting interferometer based on a frequency-tunable diode laser calibrated by an optical frequency comb and the Carré algorithm. By use of the frequency control strategies of locking the diode laser to different comb modes and scanning the repetition rate, an arbitrary single optical frequency synthesizer is obtained. The relative laser frequency uncertainty is 5.7 × 10(-12) for 1 s averaging time with tracing to an Rb clock and accurate phase steps are achieved by optical frequency tuning. The surface topography of a standard sphere is measured by this phase-shifting interferometer based on a flat reference. The phase measurement repeatability is λ/200. With this technique, phase measurement uncertainties from the laser frequency and phase steps are negligible.

Reliable non-ambiguity range extension with dual-comb simultaneous operation in absolute distance measurements

We report an absolute distance measurement scheme using two simultaneous optical frequency combs with different repetition rates for reliable non-ambiguity range extension. Since the non-ambiguity range extension is susceptible to distance drift during the repetition rate adjustment, pulse trains with two different repetition rates are coupled and directed simultaneously onto a target for coincident distance measurement. The simultaneous measurement avoids the process of adjusting the repetition rate and suppresses the influence of distance drift therein. The distances measured by the two repetition rates are distinguished by type II second harmonic generation. Target movement and atmospheric variation are made to imitate the distance drift, and experimental results show that the non-ambiguity range extension remains effective along the measurement.

Active laser ranging with frequency transfer using frequency comb

A comb-based active laser ranging scheme is proposed for enhanced distance resolution and a common time standard for the entire system. Three frequency combs with different repetition rates are used as light sources at the two ends where the distance is measured. Pulse positions are determined through asynchronous optical sampling and type II second harmonic generation. Results show that the system achieves a maximum residual of 379.6 nm and a standard deviation of 92.9 nm with 2000 averages over 23.6 m. Moreover, as for the frequency transfer, an atom clock and an adjustable signal generator, synchronized to the atom clock, are used as time standards for the two ends to appraise the frequency deviation introduced by the proposed system. The system achieves a residual fractional deviation of 1.3 × 10−16 for 1 s, allowing precise frequency transfer between the two clocks at the two ends.

Improved system calibration for specular surface measurement by using reflections from a plane mirror.

In this paper, we introduce a flexible and simple system calibration method for specular surface metrology based on the combination of reflection rays determined by the varied points on a screen and reflection images of a plane mirror without fiducials placed at three different locations. This calibration procedure involves three steps. The camera is first calibrated based on plane patterns. Then the reflection ray directions are measured via correspondence matching. The last calibration step is the pose estimation by the orthogonal iteration algorithm and reflections in a plane mirror. Basically, the concept of replacing the coordinates of the camera center with the reflection ray can alleviate the trouble of imaging aberration. Then global optimization can be operated with the orthogonal projection defined by the reflection ray, providing precise initial values for the process of bundle adjustment, compared to the classical calibration approach directly using the local optimization algorithm. Simulations and experiments both demonstrate the validity, efficiency, and robustness of the proposed improved method. In the simulations, the proposed method achieves the absolute errors of the camera parameters within 3 pixels and the relative errors of the screen pose are below 0.5% when the noise level is 0.6 pixel. Furthermore, the calibration method shows strong anti-noise ability, relying on the application of the reflection rays and the global optimization before the final bundle adjustment. In addition, the reconstruction accuracy in our experiment improves by 60.11% by the proposed method compared with the calibration procedure, which only utilizes the bundle adjustment optimization. In general, this novel calibration method can make the measurement achieve high accuracy and robustness at a low cost and with a simple setup, providing an efficient, economical, and flexible approach for a phase measuring deflectometry system in practical situations.