Miniature Two-Photon Microscopic Imaging Using Dielectric Metalens.

Miniature two-photon microscopy has emerged as a powerful technique for investigating brain activity in freely moving animals. Ongoing research objectives include reducing probe weight and minimizing animal behavior constraints caused by probe attachment. Employing dielectric metalenses, which enable the use of sizable optical components in flat device structures while maintaining imaging resolution, is a promising solution for addressing these challenges. In this study, we designed and fabricated a titanium dioxide metalens with a wavelength of 920 nm and a high aspect ratio. Furthermore, a meta-optic two-photon microscope weighing 1.36 g was developed. This meta-optic probe has a lateral resolution of 0.92 µm and an axial resolution of 18.08 µm. Experimentally, two-photon imaging of mouse brain structures in vivo was also demonstrated. The flat dielectric metalens technique holds promising opportunities for high-performance integrated miniature nonlinear microscopy and endomicroscopy platforms in the biomedical field.

Resonant fiber optic gyroscope with three-frequency differential detection by sideband locking.

A new scheme of three-frequency differential detection with a sideband locking technique is firstly proposed to suppress backscattering noise for improving the accuracy of resonator fiber optic gyroscope (RFOG). In the system we proposed, one light path is divided into three paths and sinusoidal wave modulations of different frequencies are respectively applied to generate the sideband. The first-order sidebands of the three channels of light in the cavity are locked to the adjacent three resonance peaks by sideband locking technique. The carrier and the remaining sidebands of the three channels of light are moved to a position away from the resonance peak, thereby achieving the purpose of being suppressed by the cavity itself. As a result, the frequency difference between the CW light and the other two CCW lights reaches one free spectral range (FSR), eliminating the expected backscattering noise. The experimental results demonstrate that the RFOG has a bias stability 0.9°/h based on the Allan deviation, and the corresponding angular random walk (ARW) 0.016°/√h, which validate that our scheme can effectively suppress backscattering noise to promote performance of RFOG in practical applications.

Linewidth-related residual intensity modulation in lithium niobate phase modulators.

We present a modified model for residual intensity modulation (RIM) observed in lithium niobate phase modulators, which is suitable for both narrow linewidth and wide linewidth lasers. This model is based on two key points leading to RIM: one is the optical propagation loss, which is proportional to the applied voltage, and the other is the interference between an injected wave and its reflection from the lithium niobate substrate. In order to verify the model, the RIM is measured accurately with different linewidths of input lasers respectively. The experimental results are in good agreement with the theoretical model as the values of fitting determination coefficient R-square are all above 0.995. The results have revealed that the chief reasons causing RIM are different. When using a narrow linewidth laser, the interference is the dominant reason leading to RIM as the ratio of the reflection-related coefficient including linewidth effects to optical loss reaches 34.33. However, the optical loss is the dominant reason leading to RIM with the ratio mentioned above reaching 0.31 when using a wide linewidth laser.

Realization of Hollow-Core Photonic-Crystal Fiber Optic Gyro Based on Low-Noise Multi-Frequency Lasers with Intermediate-Frequency Difference.

Mainly focusing on the demand for a novel resonator optic gyro based on a hollow-core photonic-crystal fiber (HC-RFOG), we achieve a multi-frequency lasers generation with low relative phase noise via an acousto-optic modulation of light from a single laser diode. We design a homologous heterodyne digital optical phase-locked loop (HHD-OPLL), based on which we realize the low-noise multi-frequency lasers (LNMFLs) with an intermediate frequency difference. The noise between the lasers with a 20 MHz difference is 0.036 Hz, within the bandwidth of 10 Hz, in a tuning range of 120 kHz, approximately 40 dB lower than that produced without the HHD-OPLL. Finally, based on the LNMFLs, an HC-RFOG is realized and a bias stability of 5.8 °/h is successfully demonstrated.

Design of low-crosstalk polarizing resonator and homologous multi-frequency differential detection for hollow-core photonic-crystal fiber optic gyro.

In order to suppress the undesired polarization in the hollow-core photonic-crystal fiber (HCPCF) resonator and reduce the loss of the resonator, we realize a low-crosstalk polarizing resonator with the polarization-correlated phase modulation technique (PCPM). In addition, we put forward a homologous multi-frequency differential detection scheme, with which the backscattering noise and the backreflection noise of the gyro can be well suppressed. Finally, we realize a hollow-core photonic-crystal fiber optic gyro based on the low-crosstalk polarizing resonator and the homologous multi-frequency differential detection. With this novel gyro system, a bias stability of 1.23°/h is successfully demonstrated.

Free spectral range measurement using homologous heterodyne optical phase-locked loop based on acousto-optic modulation.

We demonstrate a homologous heterodyne optical phase-locked loop for free spectral range measurement of a fiber ring resonator. In this loop, the frequency noise within the 10 Hz bandwidth is reduced by more than 40 dB from 147.350 to 0.014 Hz, and the power spectral density of the frequency noise reaches 7.69×10-8 Hz2/Hz at 10 Hz. Finally, the relative measurement accuracy of 1.39×10-9 is achieved by this loop and the free spectral range coefficient of thermal expansion is measured as -174.1±0.2 Hz/°C with a cavity finesse of 26.65. This work provides a method to measure free spectral range by tracking the resonance modes of the resonator and reducing frequency noise, especially for two signals with frequency offset.

Improvement of long-term stability of hollow-core photonic-crystal fiber optic gyro based on single-polarization resonator.

To improve long-term stability, we present a single-polarization resonator optic gyro based on a hollow-core photonic-crystal fiber (HCPCF), utilizing a micro-optical polarizing coupler formed by pairs of collimators and a series of polarization-dependent devices. We build the mathematical model of the polarization noise of the proposed gyro and experimentally validate the elimination of the undesired polarization eigenstate, which is the basis of the system's improved long-term stability. We use multi-modulation to suppress the backscattering noise and the closed-loop detection method to eliminate the effect of fluctuating output power on the gyro bias. A long-term bias stability of 20°/h is successfully demonstrated.

Real-time free spectral range measurement based on optical single-sideband technique.

We demonstrate a real-time scheme for measuring the free spectral range (FSR) of a high-aspect-ratio Si3N4 waveguide ring resonator with a fiber-based hybrid unbalanced Mach-Zehnder modulator (MZM) using an optical single-sideband technique. Resonance-tracking loops were established with the Pound-Drever-Hall technique for locking resonance modes. A relative precision of 3.25 × 10-6 was achieved for a 35-mm waveguide ring resonator with FSR = 1,844,628 kHz and Q = 3.211 × 106. Furthermore, the Si3N4 resonator FSR coefficient of thermal expansion was measured as -16.735±0.002 kHz/°C. This method will provide a flexible photonic interface for realizing advanced photonic systems.

Suppression of backscattering-induced noise by sideband locking based on high and low modulation frequencies in ROG.

Backscattering-induced noise is a dominant noise in a resonant optic gyroscope (ROG). This paper proposes a method to suppress the carrier and backscattering-induced noise with a sideband locking technique. The resonant cavity can be taken as a band-pass filter, and the carrier frequency component can be located at the stop-band while one sideband is locked to the cavity resonance. Then, the carrier will be suppressed by the cavity itself, which will reduce the interference with carrier backscattering. For the adoption of different modulation frequencies in clockwise (CW) and counter-clockwise (CCW) directions, the first-order sidebands of CW and CCW have a frequency offset to each other. Therefore, the first-order sideband backscattering can be eliminated when the sideband is locked to the cavity resonance. Also, both high and low modulation frequencies are applied to the phase modulator, which will further suppress the carrier, and the demodulation of low frequency will reduce the sensitivity to phase fluctuation noise in the system. The method has low requirements for parameter accuracy or device performance.

Passive compensation of intensity and polarization-induced noise by a quadrature demodulation technique in a resonator optic gyroscope.

Polarization-induced noise is a dominant noise that seriously hinders the progress of resonator optic gyroscopes. Many countermeasures have been developed but showed insufficient performance. In this paper, we propose a quadrature demodulation technique (QDT) that adopts references of both sine and cosine to demodulate the signal. Theoretical analyses of the polarization effect and QDT are shown, and experimental results are listed and compared. Experimental results are consistent with theoretical analyses. QDT shows great performance in suppressing environment- and polarization-induced phase fluctuation of signal. We also demodulate the intensity-dependent coefficient of 2Ωt term, which is demonstrated effective for compensating polarization-induced intensity noise together with QDT. The scheme shows significant progress in improving long-term stability.

Analysis of polarization noise in transmissive single-beam-splitter resonator optic gyro based on hollow-core photonic-crystal fiber.

We realize a transmissive single-beam-splitter resonator optic gyro based on a hollow-core photonic-crystal fiber (HCPCF), utilizing a micro-optical coupler formed by pairs of lenses and one filter, which is a new type of resonator fiber optic gyro based on the HCPCF (HC-RFOG). We build a mathematical model of the polarization noise based on the transfer function of this novel transmissive single-beam-splitter resonator. We construct a HC-RFOG and simulate and validate the effects of polarization noise on the gyro system. In addition, we apply an effective method to suppress the polarization noise and prove its efficacy through experiments. The bias stability of the gyro system is successfully improved from 25 °/h to 2 °/h, which indicates a remarkable advance of performance of HC-RFOG.

Transmissive single-beam-splitter resonator optic gyro based on a hollow-core photonic-crystal fiber.

We propose a transmissive single-beam-splitter resonator optic gyro based on a hollow-core photonic-crystal fiber (HCPCF), which is a new regime of resonator fiber optical gyro based on an HCPCF, a hollow-core resonator fiber optical gyro (HC-RFOG), for the first time, to the best of our knowledge. We evaluate the transfer function of this novel transmissive single-beam-splitter resonator, and the resonance characteristics are simulated to optimize the element parameters. We utilize a micro-optical coupler formed by pairs of lenses and one filter to realize this novel resonator. Our approach yields a fineness of 12 and a transmittance of about 5%. Based on this novel resonator, we construct a HC-RFOG, and we simulate and analyze the effects of polarization noise on the gyro system. In addition, an effective method to suppress polarization noise is employed, and the efficacy of this method is confirmed by experiments. A bias stability of 1.3°/h is successfully demonstrated, which is the best result reported to date, to the best of our knowledge, for RFOGs based on an HCPCF resonator.

Reduction of angle random walk by in-phase triangular phase modulation technique for resonator integrated optic gyro.

In a resonator integrated optic gyro (RIOG) employing a planar optical waveguide ring, the interference between backreflected light and signal light will not only cause nonreciprocal drift of cw and ccw resonance frequencies, but also deteriorate the original signal waveform of the resonator output. If contra-phase triangular phase modulation (CPM) were applied, a cosine-like ripple, whose initial phase varies randomly, would superpose upon the quasi-square waveform of the resonator output, resulting in increment of noise and the gyro's angle random walk (ARW). Therefore, in-phase triangular phase modulation (IPM) technique is proposed and used to eliminate the ripple and improve the waveform quality of the resonator output, and the gyro's ARW is obviously reduced from 3 to 0.8 deg/h1/2 compared to that of CPM. This enlightens a new way to design the scheme of backreflection/backscattering suppression.

Suppression of backreflection error in resonator integrated optic gyro by the phase difference traversal method.

The phase difference traversal (PDT) method is proposed to suppress the backreflection-induced error in resonator integrated optic gyro (RIOG). Theoretical analysis shows that the backreflection-induced zero-bias fluctuation is periodical and sine/cosine-like. By forcing the phase difference between the CW and CCW incident light to traverse the interval [0, 2π] repeatedly and rapidly enough, the fluctuation can be low-pass filtered and, hence, the backreflection-induced error can be effectively suppressed. A RIOG apparatus is built up, with multi-wave hybrid phase modulation to traverse the phase difference and in-phase modulation to set the operation point. A short-term bias stability of 0.0055 deg/s and a long-term bias stability of 0.013 deg/s are successfully demonstrated which, to the best of our knowledge, are the best results reported to date for the buried-type silica waveguide ring resonator-based RIOG.