Integrated optical driver for interferometric optical gyroscopes

Excitation of surface plasmon mode in bulk semiconductor lasers

We propose a realistic process for the excitation of surface plasmon polariton (SPP) modes in a silicon photonic waveguide (WG). The process involves the placement of buried oxide (BOX) composed of silica between a WG and silicon substrate. When the BOX thickness is manipulated, different amounts of modal power leak toward the BOX into the substrate and simultaneously acquire compensation from a semiconductor located on the WG. The compensation related to the leakage can be used to infer transparency gain. Similar to the case for a semiconductor laser cavity, the lowest transparency gain among WG modes can be favored; thus, only one mode can survive in the WG, and it is in the region with the specified BOX thickness. Finally, we propose a credible mechanism suitable for demonstrating the region requirements of the existence of SPP modes.

A Novel Closed-Loop Control to Solve Light Source Power Fluctuations in the Fiber-Optic Gyroscope

The performance of a gyroscope is directly affected by the fluctuations in the light source power (LSP) in an interferometric fiber-optic gyroscope (IFOG). Therefore, it is important to compensate for fluctuations in the LSP. When the feedback phase generated by the step wave completely cancels the Sagnac phase in real-time, the error signal of the gyroscope is linearly related to the differential signal of the LSP, otherwise, the error signal of the gyroscope is uncertain. Herein, we present two compensation methods to compensate for the error of the gyroscope when the error is uncertain, which are double period modulation (DPM) and triple period modulation (TPM). Compared with the TPM, DPM has better performance, but it increases the requirements for the circuit. TPM has lower requirements for the circuit and is more suitable for small fiber- coil applications. The experimental results show that, when the frequency of the LSP fluctuation is relatively low (1 kHz and 2 kHz), DPM and TPM do not differ significantly in terms of performance; both of them can achieve an improvement of about 95% in bias stability. When the frequency of the LSP fluctuation is relatively high (4 kHz, 8 kHz and 16 kHz), DPM and TPM can achieve about 95% and 88% improvement in bias stability, respectively.

High-performance lasers for fully integrated silicon nitride photonics

Silicon nitride (SiN) waveguides with ultra-low optical loss enable integrated photonic applications including low noise, narrow linewidth lasers, chip-scale nonlinear photonics, and microwave photonics. Lasers are key components to SiN photonic integrated circuits (PICs), but are difficult to fully integrate with low-index SiN waveguides due to their large mismatch with the high-index III-V gain materials. The recent demonstration of multilayer heterogeneous integration provides a practical solution and enabled the first-generation of lasers fully integrated with SiN waveguides. However, a laser with high device yield and high output power at telecommunication wavelengths, where photonics applications are clustered, is still missing, hindered by large mode transition loss, non-optimized cavity design, and a complicated fabrication process. Here, we report high-performance lasers on SiN with tens of milliwatts output power through the SiN waveguide and sub-kHz fundamental linewidth, addressing all the aforementioned issues. We also show Hertz-level fundamental linewidth lasers are achievable with the developed integration techniques. These lasers, together with high-Q SiN resonators, mark a milestone towards a fully integrated low-noise silicon nitride photonics platform. This laser should find potential applications in LIDAR, microwave photonics and coherent optical communications.

Mode-evolution-based TE mode magneto-optical isolator using asymmetric adiabatic tapered waveguides

As an indispensable component in the photonic integrated circuits, the design and fabrication of optical isolators, particularly in the transverse electric (TE) polarized mode, is a long-standing challenge. Herein, we present a TE mode magneto-optical isolator using adiabatic tapered waveguides to realize conversions between designated modes. The isolator exhibits an ultranarrow structure of 1.27 μm × 1498 μm. We demonstrate that the device functions under a TE mode input with a maximum isolation ratio of 15 dB and an insertion loss of 5 dB at a wavelength of 1537.3 nm.

Compact solid-state optical phased array beam scanners based on polymeric photonic integrated circuits

Optical phased array (OPA) devices are being actively investigated to develop compact solid-state beam scanners, which are essential in fields such as LiDAR, free-space optical links, biophotonics, etc. Based on the unique nature of perfluorinated polymers, we propose a polymer waveguide OPA with the advantages of low driving power and high optical throughput. Unlike silicon photonic OPAs, the polymer OPAs enable sustainable phase distribution control during beam scanning, which reduces the burden of beamforming. Moreover, by incorporating a tunable wavelength laser comprising a polymer waveguide Bragg reflector, two-dimensional beam scanning is demonstrated, which facilitates the development of laser-integrated polymeric OPA beam scanners.

Interferometric optical gyroscope based on an integrated silica waveguide coil with low loss

An interferometric optical gyro (IOG) based on integrated devices are a promising alternative for miniaturized inertial sensors. However, improving their accuracy, which is determined by the sensing coil insertion loss, is crucial. In this work, an IOG is built using an integrated sensing coil produced from a 2.14-m-long SiO₂ waveguide, the minimum bend radius and spacing of which are chosen to minimize the sensing coil insertion loss. The coil length is chosen by considering optimal detection limit constraints. Sinusoidal wave biasing modulation improves the system detection sensitivity. Finally, the IOG realizes the best yet reported bias drift of 7.32°/h.

Hybrid Integrated Semiconductor Lasers with Silicon Nitride Feedback Circuits

Hybrid integrated semiconductor laser sources offering extremely narrow spectral linewidth, as well as compatibility for embedding into integrated photonic circuits, are of high importance for a wide range of applications. We present an overview on our recently developed hybrid-integrated diode lasers with feedback from low-loss silicon nitride (Si 3 N 4 in SiO 2 ) circuits, to provide sub-100-Hz-level intrinsic linewidths, up to 120 nm spectral coverage around a 1.55 μ m wavelength, and an output power above 100 mW. We show dual-wavelength operation, dual-gain operation, laser frequency comb generation, and present work towards realizing a visible-light hybrid integrated diode laser.

Mode-assisted Silicon Integrated Interferometric Optical Gyroscope

The increasing demands in consumer electronics markets have promoted the development of chip-scale optical gyroscopes. In this study, a mode-assisted on-chip silicon-on-insulator interferometric optical gyroscope is proposed and assessed. The proposed gyroscope uses two different spatial modes propagating oppositely in the sensing waveguide coil to form a fixed phase difference that ensures the system operating at the best sensitive point. Compared with conventional schemes, it avoids the phase modulator and the circulator, which are not easy to be integrated in the same platform. The simulated results show that the detectable angular rate reaches 0.64 deg/s with a footprint of 3.85 × 10⁻³ m². The experimental results validate the realization of the highly sensitive phase bias of the fabricated device.

Asymmetry Analysis of the Resonance Curve in Resonant Integrated Optical Gyroscopes

The Resonant Integrated Optic Gyroscope (RIOG) is a type of high accuracy gyroscope based on the Sagnac effect. A symmetrical resonance curve is very important to the performance of the RIOG. To further investigate and design a RIOG with a waveguide ring resonator, an in-depth research of the asymmetric resonance curve and its influence on the RIOG is fully developed. Four possible optical noises inducing the resonance curve asymmetry are analyzed and their mathematic models are established. These four optical noises are the normal mode effect, the backscattering noise, the backreflection noise and the polarization noise. Any asymmetry of the resonance curve will not only induce a large output bias error into the gyro output, but also seriously decrease the frequency discrimination parameter of the demodulation curve. By using a tunable fiber laser, the high aspect ratio silicon nitride WRR and the silicon dioxide WRR were tested. The experiment measured resonance curves can be well fitted with the theoretical simulation results. The experimental results show that a high aspect ratio silicon nitride waveguide can effectively suppress the polarization noise in the RIOG.

Application of the Wiener filter for intensity noise reduction in fiber optic gyroscopes

An essential issue for the low-noise system application of the fiber optic gyroscope (FOG) is to reduce its noise level. The relative intensity noise (RIN) of the light source is the dominant noise of the FOG when the light power on the detector reaches a certain level. The noise subtraction method is effective for RIN reduction and easy to implement in a FOG. This paper theoretically analyzes the factors that influence the result of the method and deduces the function to calculate the noise suppression ratio that can be achieved. A method that uses an optimum filter design based on the Wiener filter in the reference detector signal is proposed to improve the subtraction result. A FOG system is set up to test the feasibility of the method. The experiment results meet with the theoretical analysis, and by using the Wiener filter, the achieved noise subtraction factor reaches the limitation that restrains the optical system and detection circuit.

Polarimetry fiber optic gyroscope

We report a different mechanism for rotation sensing by analyzing the polarization of light exiting from a Sagnac loop. Unlike in an interferometric fiber optic gyroscope (I-FOG), here the counter-propagating waves in the Sagnac loop are orthogonally polarized at the loop exit and, consequently, cannot directly interfere with each other when recombined at the exit. We show that the Stokes parameters s₂ and s₃ of the combined waves are simply the cosine and sine functions of the phase difference between the counter propagation waves, which is linearly proportional to the rotation rate, allowing precise determination of the rotation rate by polarization analysis. We build such a proof-of-concept polarimetry FOG and achieved key performance parameters comparable to those of a high-end tactical-grade gyroscope. In particular, the device shows a bias instability of 0.09°/h and an angular random walk of 0.0015°/h, with an unlimited dynamic range, demonstrating its potential use for rotation sensing. This new approach eliminates the need for phase modulation required in I-FOGs, and promotes easy photonics integration, enabling the development of low-cost FOGs for price-sensitive applications, such as autonomous and robotic vehicles.

Research on the Frequency-Dependent Halfwave Voltage of a Multifunction Integrated Optical Chip in an Interferometric Fiber Optic Gyroscope

The multifunction integrated optical chip (MIOC) is one of the most critical parts of the interferometric fiber optic gyroscope (IFOG), and research on the halfwave voltage of the MIOC is meaningful for a high-precision IFOG. In this paper, the correlation between the frequency and halfwave voltage, which affects the interference light intensity of IFOG, is presented theoretically. A widespread measurement method for frequency dependence of the halfwave voltage, based on lock-in amplification and sinusoidal modulation, is proposed. Further, the measurement result and the oscillation of interference light intensity in the Sagnac interferometer are presented, which are in great agreement with the theory. This paper proposes the frequency dependence of the halfwave voltage and provides a new error research direction for the improvement of the MIOC in a high-precision IFOG.

Heterogeneous silicon photonics sensing for autonomous cars

Heterogeneous silicon photonics is uniquely positioned to address the photonic sensing needs of upcoming autonomous cars and provide the necessary cost reduction for widespread deployment. This is because it allows for wafer-scale active/passive integration, including optical sources. We present our recent research and the development of interferometric optical gyroscopes and LiDAR sensors. More specifically, we show a fully integrated gyroscope front-end occupying an area of only 4.5 mm². We also show the first dense pitch optical phased array using heterogeneous phase shifters. The 4 µm pitch heterogeneous phase shifters provide very low V_2π of only 0.35-1.4 V across 200 nm, low residual amplitude modulation of only 0.1-0.15 dB for 2π phase shift, extremely low static power consumption (<3 nW), and high speed (> 1 GHz). All of these factors make them ideal for next-generation LiDAR systems that employ optical phased arrays.

Sub-wavelength-pitch silicon-photonic optical phased array for large field-of-regard coherent optical beam steering

This paper reports on large field-of-regard, high-efficiency, and large aperture active optical phased arrays (OPAs) for optical beam steering in LIDAR systems. The fabricated 5 mm-long silicon photonic OPA with a 1.3 μm waveguide pitch achieved adjacent waveguide crosstalk below -12dB. A relatively large and uniform emission aperture has been achieved with a low-contrast silicon nitride assisted grating (~20 dB/cm) whose emission profile can be further optimized using an apodized design. The fabricated silicon-photonic OPA demonstrated > 40° lateral beam steering with no sidelobes in a ± 33° field-of-regard and 3.3° longitudinal beam steering via wavelength tuning by 20 nm centered at 1550 nm. We have fully integrated the silicon photonic OPA device with electronic controls and successfully demonstrated 2-dimensional coherent optical beam steering of pre-planned far-field patterns. Future improvements include placement of a distributed Bragg reflector (DBR) underneath the grating emitter in order to achieve nearly a factor of two improvement in emission efficiency.

Novel on chip rotation detection based on the acousto-optic effect in surface acoustic wave gyroscopes

An Acousto-Optic Gyroscope (AOG) consisting of a photonic integrated device embedded into two inherently matched piezoelectric surface acoustic wave (SAW) resonators sharing the same acoustic cavity is presented. This constitutes the first demonstration of a micromachined strain-based optomechanical gyroscope that uses the effective index of the optical waveguide due to the acousto-optic effect rather than conventional displacement sensing. The theoretical analysis comparing various photonic phase sensing techniques is presented and verified experimentally for the cases based on a Mach-Zehnder interferometer, as well as a racetrack resonator. This first prototype integrates acoustic and photonic components on the same lithium niobate on insulator (LNOI) substrate and constitutes the first proof of concept demonstration of the AOG. This approach enables the development of a new class of micromachined gyroscopes that combines the advantages of both conventional microscale vibrating gyroscopes and optical gyroscopes.

Ultra-Low-Loss Silicon Waveguides for Heterogeneously Integrated Silicon/III-V Photonics

Integrated ultra-low-loss waveguides are highly desired for integrated photonics to enable applications that require long delay lines, high-Q resonators, narrow filters, etc. Here, we present an ultra-low-loss silicon waveguide on 500 nm thick Silicon-On-Insulator (SOI) platform. Meter-scale delay lines, million-Q resonators and tens of picometer bandwidth grating filters are experimentally demonstrated. We design a low-loss low-reflection taper to seamlessly integrate the ultra-low-loss waveguide with standard heterogeneous Si/III-V integrated photonics platform to allow realization of high-performance photonic devices such as ultra-low-noise lasers and optical gyroscopes.

O-band electrically injected quantum dot micro-ring lasers on on-axis (001) GaP/Si and V-groove Si

We report statistical comparisons of lasing characteristics in InAs quantum dot (QD) micro-rings directly grown on on-axis (001) GaP/Si and V-groove (001) Si substrates. CW thresholds as low as 3 mA and high temperature operation exceeding 80 °C were simultaneously achieved on the GaP/Si template template with an outer-ring radius of 50 µm and a ring width of 4 μm, while a sub-milliamp threshold of 0.6 mA was demonstrated on the V-groove Si template with a smaller cavity size of 5-μm outer-ring radius and 3-μm ring width. Evaluations were also made with devices fabricated simultaneously on native GaAs substrates over a significant sampling analysis. The overall assessment spotlights compelling insights in exploring the optimum epitaxial scheme for low-threshold lasing on industry standard Si substrates.