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Complexities of the underwater environment can seriously affect many underwater detection means, especially the influence of light scattering by water. To solve this problem, a three-dimensional (3D) morphology measurement method is proposed based on the photoacoustic effect. In this method, a measurement object is irradiated with pulsed laser light to produce ultrasonic waves via the photoacoustic effect. A probe collects the ultrasonic signal and subsequent data processing can yield complete object detection. This approach can make full use of the advantages of high precision and good directivity of laser ranging and completely avoid the influence on the laser of backscattering from water. The results yield a displacement measurement accuracy of less than 0.5 mm and an average error of 3D reconstruction of 0.21 mm, demonstrating great application potential.

Interferometric fiber optic gyroscopes are widely used in spacecraft, navigation, and autopilot and missile systems due to their high accuracy and fast speed. However, the dynamic measurement range is generally limited to a small value. In this paper, an angular velocity measurement system based on a dual-comb Sagnac interferometer is proposed. The theoretical dynamic range can reach the order of 10 8 deg/s, which is remarkably superior to that of the conventional fiber optic gyroscopes. To the best of our knowledge, this is the first demonstration of a dual-comb-based passive fiber optic gyroscope system, offering an attractive tool for measuring angular velocity with large dynamic range.

A fabrication method for large-area gratings with uniform duty without using a spatial beam modulator is introduced in this study. The inhomogeneity of gratings caused by flaws of the lens and stray light was solved by controlling exposure time within an appropriate range and selecting a suitable beam expansion aperture in the optical path. A model for representing this process was established by analyzing the effects of exposure and development time length, and experimental results exhibited good agreement with the simulation results. Finally, a grating with a period of 550 nm, a uniform duty cycle, and a diameter larger than 30 mm was achieved using a Mach-Zehnder interferometer optical path without a spatial beam modulator. The uniformity of this grating was observed via atomic force microscopy, and the results were highly desirable.

In this Letter, we demonstrate a passive all-fiber pulse delay method for repetition rate multiplication of dual-comb spectroscopy. By combining a cascaded Mach-Zehnder interferometer and digital error correction, a mode-resolved spectrum with improved acquisition speed and sensitivity can be obtained. This technique has the strengths of compact, broadband, high energetic efficiency, and low complexity. Due to the use of an adaptive post-processing algorithm, sophisticated closed-loop feedback electronics are not required, which provides a simple and effective scheme to break through the physical limitation of the repetition frequency of the frequency comb for phase-stable dual-comb applications.

Dual-comb spectroscopy (DCS) is an emerging and promising spectrometric technique with high resolution, high sensitivity, broad spectral range, and fast acquisition speed. For the recovery of the information encoded on comb modes without resolution loss, two continuous wave lasers are commonly utilized as optical intermedia to track the real-time jitter of dual-comb interferograms. This paper presents a simplified error correction method based on single optical intermedium for quasi-free-running fiber DCS. This method combines the strengths of conventional optical referencing and self-referencing error correction. We acquired whole P branch H 13 C 14 N transmittance spectra in the near infrared as a demonstration. In contrast to that of conventional dual intermedium error correction, the standard deviation of our method was merely 0.01 over the 4 THz spectral range. Our method provides a balanced and practical postprocessing routine for high-performance mode-resolved DCS applications.

A two-axis Lloyd's mirrors interferometer based optical fabrication system was theoretically investigated and constructed for patterning high-uniformity nanoscale crossed grating structures over a large area with a high throughput. The current interferometer was configured with two reflected mirrors and a grating holder, which are placed edge by edge and orthogonal with each other. In such a manner, the two beams reflected from the two mirrors interfere with the incident beam, respectively, forming a crossed grating patterns with only one exposure. Differing from the conventional solution for elimination of unexpected interference between the two reflected beams, a systematical analysis, that is based on the proposed index indicating the non-orthogonality between the two beams at different incident angles, was conducted by using a spatial full polarization tracing method. Without polarization modulation to eliminate the additional interference, an optimal exposure condition with small non-orthogonality between reflected beams was found at a certain incident angle range, while the two required interferences to construct cross grating still remain high. A pattern period of ∼1 µm-level crossed grating structure could be obtained through balancing the structure area and the non-orthogonality. Finally, the exposure setup with orthogonal two-axis Lloyd's mirrors interferometer is established, and the crossed grating structure with the periods of 1076 nm along X-direction and 1091 nm along Y-direction was successfully fabricated on a silicon substrate via microfabrication technology over a large area of 400 mm 2 . The uniformity of crossed grating array over the whole area was evaluated by an atomic force microscope, and the standard deviations of structure periods along X- and Y-directions smaller than 0.3% are achieved. It is demonstrated that the orthogonal two-axis Lloyd's mirrors interferometer based on single-beam single-exposure scheme with non-orthogonality systematic analysis is an effective approach to fabricate crossed grating patterns of 1 µm-level period with high uniformity over a large area.

An absolute optical encoder with nanometric positioning repeatability is introduced and investigated. The encoder consists of an improved scale grating and a compact two-probe reading head. The scale grating contains multiple unevenly spaced distance marks that are superimposed onto grating grooves. The two probes, which are located in the sensor head, read the marks and the grating grooves to determine the position and displacement simultaneously. For one of the two probes, a mask with the same code design with the marks on the scale grating generates a pulse signal to locate the position of the marks approximately-with an accuracy of less than about half a grating period (0.5 μm). For the other probe, light beams are configured for grating interferometry to measure displacement with nanometer-scale resolution. In this Letter, high-quality sinusoidal signals of grating interferometry are used to accurately locate the marks on the scale grating, with stable 100-subdivision phase information. The testing results show that the positioning repeatability can reach 10 nm for a motion range of several tens of millimeters.

Long-term mutual coherence is a key factor that affects the signal-to-noise ratio and resolution of a dual-comb interferometer. To realize a phase-stable dual-comb interferometer configuration, tightly phase-locked loop systems or digital error correction methods with external optical reference are commonly used. This paper presents a self-referencing digital error correction method based on the short-term spectral characteristics of interferograms to reduce the cost and complexity of the phase-stable dual-comb interferometer configuration. In our experiment, fully mutual coherence of a dual-comb interferometer is reconstructed and 1 Hz theoretical linewidth in 1 s acquisition time is achieved by digitally compensating for time jitter, center frequency jitter, and carrier-envelope-phase jitter, offering an effective technique for advanced dual-comb applications.

To improve the positioning capabilities of a chromatic confocal displacement measurement system, we introduce a super-resolving pupil filtering element (referred to as spatial-bandpass-filter), an X-shaped fiber-coupler, and a modified peak-extraction algorithm. The spatial-bandpass-filer decreases the spherical aberration of conventional refractive lenses and transmission loss, without changing the dispersion range. The new optical design minimizes the effect of light-source-variation as well as reflectance-variation of the sample surface against the wavelength by adding a reference arm with an X-shaped fiber-coupler and normalization algorithm. This enables rapid re-purposing to obtain data for crosstalk, reference and measurement signals. A prototype setup was built and the experimental results show that the new chromatic dispersion system can reach a position stability of ± 0.36 μm using a commercial cost-effective white-light source. It reaches an axial displacement resolution of up to 1 μm and a peak-fit error below ± 0.2% for a range of 1.05 mm.

High-precision long geometrical distance measurement performs a vital role in large-scale manufacturing and future light detection and ranging (LIDAR) for tight formation. Its high precision, fast measurement rate, and large ambiguity range have traditionally made dual-comb ranging (DCR) a powerful tool for absolute distance measurement. However, DCR experiences the same issues caused by the refractive index of air as other laser-based ranging systems. The conventional method used to compensate refractive index of air is through using empirical equations by monitoring environment parameters. This real-time compensation method relies on precise sensors and cannot be easily applied to long-distance measurement. Thus, a two-color compensation method is proposed that requires only two co-propagating lights at different wavelengths, without specific identification of the refractive index of air. In this paper, the two-color method is combined with a low-noise DCR realized by a digital correction method. Mode resolved and phase-stable comb spectra are available for ranging at both two wavelengths with ~200 THz difference. The experimental result demonstrates 46 nm precision and 2.7 m ambiguity range by two-color DCR (TC-DCR) with 0.1 s coherent averaging at 1 kHz repetition rate difference. This method achieves a precision of the order of ~10 -8 and an accuracy of the order of ~10 -7 , which is comparable to the single-color DCR results by empirical equations with environmental sensing. The proposed two-color DCR demonstrates great potential for long-distance measurement in open air.

We first investigate the polarization coexistence and switching between the orthogonal π- and σ-polarization states in a free running optical bistability Tm,Ho:LLF laser. The output performances of the optical bistability Tm,Ho:LLF laser is numerically simulated, based on a new rate equation theory model taking into account the influences of polarization-dependent gain and losses. The simulation results show the polarization coexistence and switching of output laser in the process of decreasing pump power. The physical mechanism of polarization coexistence and switching is clarified by analyzing the polarization-dependent net-gain coefficients. Furthermore, the polarization coexistence and switching are experimentally realized in the optical bistability Tm,Ho:LLF laser, which validates the theoretical analysis. The theoretical model can be applied to analyze the polarization coexistence and switching in other kinds of (quasi-) three-level optical bistable lasers.

A phase-stable dual-comb interferometer measures materials' broadband optical response functions, including amplitude, frequency, and phase, making it a powerful tool for optical metrology. Normally, the phase-stable dual-comb interferometer is realized via tight phase-locking methods. This paper presents a post-correction algorithm that can compensate for carrier wave phase noise and interferogram timing jitter. The compensating signal is a beat between two combs using a free-running continuous wave laser as an optical intermediary. In our experiment, sub-hertz relative linewidth, ~1 ns relative timing jitter, and 0.2 rad precision in the carrier phase is demonstrated.

We present an absolute distance measurement system using a phase-stable dual-comb system with 56.09 MHz repetition rate and 2 kHz repetition rate difference. A relative phase stability of 0.1 rad in 0.5 ms between two combs is achieved using a mutual locking scheme. The dual-comb ranging system combines the time-of-flight (TOF) method, synthetic-wavelength interferometry (SWI), and carrier wave interferometry (CWI). Each method provides a particular ambiguity range and resolution, and they can be applied simultaneously and linked to enhance the precision and measurement rate of the ranging system. The experimental results demonstrate that a precision of 1.2 μm is obtained without time averaging, and the precision can be improved to 3 nm with only 10 ms averaging time using the SWI method described in this study. The precision reaches a sub-nanometer when the averaging time exceeds 0.1 s. A system with high accuracy and short averaging time would enhance fast measurement performance in various industrial applications. The ambiguity range is about 2.67 m in our system, we test the performance of the system with 1.5 mm range at 1.5 m distance.

We propose a new technique to fabricate a highly specialized optical element, a hybrid planar Grating/Fresnel lens (G-Fresnel), which is particularly useful to improve or enable more-affordable miniature/portable spectrometers. Both the Fresnel and the grating surface are fabricated simultaneously by sandwiching soft PDMS between a hard grating and a pre-replicated negative Fresnel surface. Several adhesion reduction techniques are also investigated that help improve both fabrication and cost efficiency (by reducing the solidification time) as well as the lifetime of the mold. Alignment errors are systematically analyzed, and their effects on the G-Fresnel lens evaluated. A compact fabrication platform was built, which is smaller than a volume of 160☓140☓106 mm 3 to fit into a conventional vacuum drying oven, for the fabrication of a G-Fresnel lens with a diameter of 25.4 mm, an equivalent focal length of 25 mm, and a blazed grating pattern with 600 lines/mm spacing. The solidification time was reduced to 2 hours thanks to the improved adhesion reduction technique that permits a PDMS drying-temperature as high as 65 °C. The fabricated G-Fresnel lens was evaluated with regard to both geometrical fabrication precision and optical performance. The measured results, using a step gauge and atomic force microscopy, confirm that this replication technique produces high-quality replicates of the master surface-profile. Furthermore, a prototype spectrometer that uses a G-Fresnel lens was built and evaluated. The spectrometer fits within a volume of about 100 mm☓50 mm☓30 mm, and it operates across a wide wavelength spectrum (450 nm to 650 nm). Both the calculation based on the optical software ZEMAX and the experimental measurements are consistent and confirm that the spectrometer with the G-Fresnel lens can provide a spectral resolution of better than 1.2nm.

We present a method for the fabrication of an arrayed one-axis scale grating for a two-probe optical linear encoder using a dual-beam interference lithography (IL) system with a compact diode laser source. We employ a multiple-exposure stitching method to form an arrayed scale grating. This allows a scale grating with small line spacing to be achieved over a large width. This stitched scale grating integrates well with a newly developed two-probe optical encoder, allowing the measurement results to be numerically connected. Since neither the gap width nor the grating phase of the two adjacent gratings must be controlled, the fabrication process is both simplified and made more robust. This flexible and cost-effective fabrication technique can benefit many precision measurement applications. Experiments are carried out to demonstrate the feasibility of this technology.

A novel method enabling estimation of not only the screen surface as the conventional one, but the depth information from two-dimensional coordinates in an interactive projection system was proposed in this research. In this method, a one-shot black-and-white stripe pattern from a projector is projected on a screen plane, where the deformed pattern is captured by a charge-coupled device camera. An algorithm based on object/shadow simultaneous detection is proposed for fulfillment of the correspondence. The depth information of the object is then calculated using the triangulation principle. This technology provides a more direct feeling of virtual interaction in three dimensions without using auxiliary equipment or a special screen as interaction proxies. Simulation and experiments are carried out and the results verified the effectiveness of this method in depth detection.

An imprinting process that enables fabrication of blazed concave gratings with small radius and large curvature is presented. In this process, a combination-type convex grating substrate is used as a master to replace the single part used in the traditional process. The two parts of the combined convex grating are independently separated from the resist layer. In this manner, the concave blazed grating pattern in the resist can maintain maximum consistency with that on the combined convex grating. Experiments are conducted to demonstrate the feasibility of a two-step fabrication technology.

In this paper, a novel optical encoder enabling the simultaneous measurement of displacement and the position of precision stages is presented. The encoder is composed of an improved single-track scale grating and a compact two-probe reading head. In the scale grating, multiple reference codes are physically superimposed onto the incremental grooves, in contrast to conventional designs, where an additional track is necessary. The distribution of the reference codes follows a specific mathematical algorithm. For the reading head, a two-probe structure is designed to identify the discrete reference codes by means of the superimposition of the codes with a stationary mask and to read the continuous incremental grooves by means of a grating interferometry, respectively. A prototype encoder was designed, constructed and evaluated, and experimental results show that the distance code precision achieved is 0.5 μm, while the linearity error of the linear displacement measurement is less than 0.06%.

We introduce a novel dual-beam interference lithography (IL) method that makes it possible to fabricate a concave grating with a large line spacing. A concave lens is placed between two point sources for spatial interference and a concave substrate to produce the grating pattern. The original positions of the two point sources are separated by the concave lens, which permits the IL method to fabricate a concave grating that bypasses the line spacing limitation of the conventional IL system. A concave grating with a line spacing of about 3.8 μm was fabricated and fitted inside a miniature spectrometer. The enlarged line spacing reduces the detector length by 66.5%, while keeping the resolution better than 1.5 nm over a wide spectral band (360 - 825 nm).

In this Letter, we propose a variable-line-spacing (VLS) grating pattern for a hybrid diffractive device termed a grating Fresnel (G-Fresnel) lens, which is used in spectrometers to improve spectral resolution over a wide spectral range. The VLS grating pattern disperses light of specific wavelengths with a different angle and position such that the aberration caused by the Fresnel surface can be compensated for. In this manner, high resolution can be achieved over a relatively wide spectral range. The VLS grating pattern is designed based on the least wave-change principle and simulated by ZEMAX. Results reveal that the VLS G-Fresnel device allows a subnanometer resolution over a spectral range of 200 nm.

We present a new design for the fabrication of concave gratings with large grating constants for flat-field miniature spectrometers with a wide spectral band. In this new design, one of the two optical paths for the holographic lithography of a curved grating structure with variable line spacing is modified by adding a concave lens in front of the point source. The addition of the concave lens allows the real point source, as well as the spatial filter for generating this point source, to be moved back. In this manner, the two spatial filters for generating two point sources are separated. Avoiding the physical conflict between these two spatial filters reduces the difficulty of fabricating large-constant concave gratings. Experimental results verify the feasibility of the proposed design in fabricating concave gratings with large grating constants. The resolution of a spectrometer using the fabricated concave grating is evaluated and found to be better than 1.1 nm across a spectral band ranging from 360 nm to 825 nm.

Dual-comb system parameters have significant impacts on the ranging accuracy. We present a theoretical model and a numerical simulation method for the parameter optimization of a dual-comb ranging system. With this method we investigate the impacts of repetition rate difference, repetition rate, and carrier-envelope-offset frequency on the ranging accuracy. Firstly, the simulation results suggest a series of discrete zones of repetition rate difference in an optimal range, which are consistent with the experimental results. Secondly, the simulation results of the repetition rate indicate that a higher repetition rate is very favorable to improve the ranging accuracy. Finally, the simulation results suggest a series of discrete optimal ranges of the carrier-envelope-offset frequency for the dual-comb system. The simulated results were verified by our experiments.

In this study, a new flat-field concave grating miniature spectrometer is proposed with improved resolution across a wide spectral band. A mirror is added to a conventional concave grating spectrometer and placed near the existing detector array, allowing a wide spectral band to be divided into two adjacent subspectral bands. One of these bands is directly detected by the detector, and the other is indirectly analyzed by the same detector after being reflected by the mirror. These two subspectral bands share the same entrance slit, concave grating, and detector, which allows for a compact size, while maintaining an improved spectral resolution across the entire spectral band. The positions of the mirror and other parameters of the spectrometer are designed by a computer procedure and the optical design software ZEMAX. Simulation results show that the resolution of this kind of flat-field concave grating miniature spectrometer is better than 1.6 nm across a spectral band of 700 nm. Experiments based on three laser sources reveal that the measured resolutions are comparable to the simulated ones, with a maximum relative error between them of less than 19%.

We present a simple photonic crystal fiber interferometer (PCFI) that operates in reflection mode for pH measurement. The sensor is made by coating polyvinyl alcohol/polyacrylic acid (PVA/PAA) hydrogel onto the surface of the PCFI, constructed by splicing a stub of PCF at the distal end of a single-mode fiber with its free end airhole collapsed. The experimental results demonstrate a high average sensitivity of 0.9 nm/pH unit for the 11 wt.% PVA/PAA coated sensor in the pH range from 2.5 to 6.5. The sensor also displays high repeatability and stability and low cross-sensitivity to temperature. Fast, reversible rise and fall times of 12 s and 18 s, respectively, are achieved for the sensor time response.

A relative humidity (RH) sensor in reflection mode is proposed and experimentally demonstrated by using a polyvinyl alcohol (PVA)-coated tilted-fiber Bragg grating (TFBG) cascaded by a reflection-band-matched chirped-fiber Bragg grating (CFBG). The sensing principle is based on the RH-dependent refractive index of the PVA coating, which modulates the transmission function of the TFBG. The CFBG is properly designed to reflect a broadband of light spectrally suited at the cladding mode resonance region of the TFBG, thus the reflected optical signal passes through and is modulated by the TFBG again. As a result, RH measurements with enhanced sensitivity of ∼1.80  μW/%RH are realized and demodulated in the range from 20% RH to 85% RH.

A low-cost yet high-sensitivity temperature fiber sensor is proposed and demonstrated in this paper. A single-mode fiber with coating is simply bent in a droplet-like circle with a radius of several millimeters. The strong bending induces mode interferences between the silica core mode and the excited modes propagating in the polymer coating. Many resonant dips were observed in the transmission spectra and are found to shift to a shorter wavelength with the increase of environmental temperature. Our linear fitting result of the experimental data shows that the proposed sensor presents high temperature sensitivity up to -3.102  nm/°C, which is even comparable with sensors based on selective liquid-filled photonic crystal fibers. Such high temperature sensitivity results from the large thermo-optical coefficient difference between the silica core and the polymer coating. The influence of a circle radius to the sensitivities is also discussed.

Extraneous thermal motion can limit displacement sensitivity and radiation pressure effects, such as optical cooling, in a cavity-optomechanical system. Here we present an active noise suppression scheme and its experimental implementation. The main challenge is to selectively sense and suppress extraneous thermal noise without affecting motion of the oscillator. Our solution is to monitor two modes of the optical cavity, each with different sensitivity to the oscillator's motion but similar sensitivity to the extraneous thermal motion. This information is used to imprint "anti-noise" onto the frequency of the incident laser field. In our system, based on a nano-mechanical membrane coupled to a Fabry-Pérot cavity, simulation and experiment demonstrate that extraneous thermal noise can be selectively suppressed and that the associated limit on optical cooling can be reduced.

An optical fast associative filtering method based on a multi-channel volume holographic correlator to search the database as a front-end filter is described. The features of searching query are parallelly extracted by a volume holographic correlator based on two-dimensional discrete Walsh transform, and are used to measure the similarity between the query and all the database records. The best matches are picked up for further searching. An experiment is carried out, and the experimental results prove the validity of the method.

We propose a hybrid multiplexing method for volume holographic correlators based on the combination of orthogonal random phase modulation of an object beam and two-dimensional angular multiplexing of a reference beam. This method implements storing multiple groups of library images in a single storage location and outputting multiple groups of correlation spots by switching the modulation phase masks. Three experiments are designed to test the validity of this method.

We introduce a two-dimensional interleaving method to improve the output accuracy of volume holographic correlators. The method redistributes the pattern of every input and stored image without changing their inner products, so it can eliminate the impact of correlation pattern difference on calculated inner product values. Experimental tests show that this method can achieve more accurate output intensity for every channel, which agrees much better with the theoretical value than without this method.