Integrated Optical Chip Research Articles

Ultrafast laser direct writing of in-line polarizers based on nano-gratings.

In-line polarizers play an important role in the emerging quantum computing, integrated photonics, and ultrafast science. However, to our knowledge, no one has actually used in-line polarizers based on nano-gratings (NGs) in those research fields. Here, we present a novel approach to write a fiber polarizer that utilizes nano-gratings directly within the fiber core for the first time to our knowledge. An optical fiber polarizer measuring 1 mm in length, 2.5 dB in polarization-dependent loss (PDL), and 2 dB in insertion loss was developed and initially utilized for the artificially saturable absorber nonlinear polarization evolution (NPE) experiment. In the experiment, several nonlinear transmission curves are measured. In-line polarizers based on nano-gratings are expected to be used in future integrated optical chips because of their small size and easy direct writing.

Reduction in Temperature-Dependent Fiber-Optic Gyroscope Bias Drift by Using Multifunctional Integrated Optical Chip Fabricated on Pre-Annealed LiNbO3

The refractive index change obtained after annealed proton exchange (APE) in lithium niobate (LiNbO3) crystals depends on both the proton exchange process carried out in hot acid and the structure of the crystals. In devices produced by the APE method, dislocations and lattice defects within the crystal structure are considered to be primary contributors to refractive index discontinuities and waveguide instability. In this study, the effects of pre-annealing LiNbO3 crystals at 500 °C on multifunctional integrated optical chips (MIOCs) were investigated through interferometric fiber-optic gyroscope (IFOG) system-level tests. It was observed that the pre-annealing process resulted in an improvement in the optical throughput of MIOCs (from %34 to %51) and the temperature-dependent bias drift stability of the IFOG (from 0.031–0.038°/h to 0.012–0.019°/h). The angle random walk (ARW) was measured as 0.0056 deg/√h.

Sub-micron pixel polarization-sensitive photodetector based on silicon nanowire

Silicon nanowire is a potential candidate to be used as polarization-sensitive material, but the relative mechanism of polarization response must be carried out. Herein, a sub-micron metal-single silicon nanowire-metal photodetector exhibits polarization-sensitive characteristics with an anisotropic photocurrent ratio of 1.59 at 780 nm, an excellent responsivity of 24.58 mA/W, and a high detectivity of 8.88 × 109 Jones at 980 nm. The underlying principle of optical anisotropy in silicon nanowire is attributed to resonance enhancement verified by polarizing light microscopy and simulation. Furthermore, Stokes parameter measurements and imaging are all demonstrated by detecting the characteristics of linearly polarized light and imaging the polarizer array, respectively. Given the maturity of silicon processing, the sub-micron linearly polarized light detection proposed in this study lays the groundwork for achieving highly integrated, simplified processes, and cost-effective on-chip polarization-sensitive optical chips in the future.

Heterogeneous integration of an on-chip Nd:YAG whispering gallery mode laser with a lithium-niobate-on-insulator platform.

The integration of heterogeneous optical components onto an optical platform is crucial for the advancement of photonic chips. To achieve this, efficient coupling of optical signals between components and the platform is essential. Here, we have successfully integrated a Nd:YAG microdisk laser with a lithium-niobate-on-insulator (LNOI) photonic platform by modulating the propagation modes of LNOI. Ridge waveguides are fabricated on the LNOI by carefully adjusting the cross-sectional dimensions to enable the propagation of higher-order propagation modes. This ridge waveguide ensures that the effective refractive index of the higher-order mode closely matches that of the fundamental mode of the Nd:YAG microdisk, ensuring efficient waveguide-microdisk coupling. This on-chip laser, consisting of an Nd:YAG microdisk and LNOI integration, achieves a maximum output power of 23 µW, and a mode suppression ratio of 53.6 dB. This research presents an efficient approach for constructing highly functional heterogeneous integrated optical chips.

Silicon based integrated photonic chip technology for datacenter optical interconnection

With the rise of the modern Internet of Things, big data and other industries, the human society’s demand for information increases rapidly, and the demand for broadband network capacity shows a dramatic growth trend. As its core support, optical communication system and data center are facing major challenges. With the characteristics of small volume, light weight and low power consumption, optoelectronic integrated devices are the key devices to break through the bottleneck of optical communication system and data center optical interconnection. In this paper, silicon-based integrated optical switching technology and multiplexing technology applied for datacenter optical interconnection are studied. The implementation methods and technical bottlenecks of silicon-based integrated optical switching chips and mode multiplexing/demultipelxing chips are discussed. Moreover, the future of silicon-based integrated photonic technology is prospected.

Optimization of the annealed proton exchange method with controlled annealing for multifunctional integrated optical chip production.

The main objective of our study is to develop a new approach to the annealed proton exchange (APE) method for the fabrication of the multifunctional integrated optical chip (MIOC) used in fiber-optic gyro systems and to eliminate the loss of time and material, especially in mass production applications. In this work, self-polarized waveguides, which are the basic components of a MIOC device, were produced by the APE method and studied. With the developed method, controlled annealing trials have been carried out from a certain region on the LiNbO3 substrate used in waveguide production, and the annealing time specific to the annealing process was determined. By utilizing a special setup for the hot acid process, the proton exchange process was accomplished without a sudden temperature change of the substrate. Using prism coupling measurements of the fabricated waveguides, annealing times were determined to obtain index change values suitable for 45%-50% optical throughput. Mode profiles of devices with high optical throughput that were produced by the proposed method were measured, and it was seen that devices from different proton exchange runs had similar profiles. As a result, many undamaged substrates were fabricated, and their optical quality was found to be within the expected values.

Interferometric beam combination with a triangular tricoupler photonic chip

Beam combiners are important components of an optical/infrared astrophysical interferometer, with many variants as to how to optimally combine two or more beams of light to fringe-track and obtain the complex fringe visibility. One such method is the use of an integrated optics chip that can instantaneously provide the measurement of visibility without temporal or spatial modulation of the optical path. Current asymmetric planar designs are complex, resulting in a throughput penalty, and so here, we present developments into a three-dimensional triangular tricoupler that can provide the required interferometric information with a simple design and only three outputs. Such a beam combiner is planned to be integrated into the upcoming Pyxis interferometer, where it can serve as a high-throughput beam combiner with a low size footprint. Results into the characterization of such a coupler are presented, highlighting a throughput of 85 ± 7 % and a flux splitting ratio between 33:33:33 and 52:31:17 over a 20% bandpass. We also show the response of the chip to changes in the optical path, obtaining an instantaneous complex visibility and group delay estimate at each input delay.

Low-Drift Closed-Loop Fiber Optic Gyroscope of High Scale Factor Stability Driven by Laser With External Phase Modulation

In view of the poor scale factor stability of the interferometric fiber optic gyroscope (IFOG), it is a creative method to use laser to drive the IFOG for its better frequency stabilization characteristics instead of the broadband light source. As the linewidth of laser is narrow, the errors of coherent backscattering, polarization coupling, and Kerr effect are reintroduced which cause more noise and drift. This paper studies laser spectrum broadening based on external phase modulation of Gaussian white noise (GWN). The theoretical analysis and test results indicate that this method has a good effect on spectrum broadening and can be used to improve the performance of the laser-driven IFOG. In the established closed-loop IFOG, a four-state modulation (FSM) is adopted to avoid temperature instability of the multifunction integrated-optic chip (MIOC) and drift caused by the electronic circuit in demodulation. The experimental results show that the IFOG driven by broadened laser has the angular random walk noise of 0.003 8 °/√h and the drift of 0.017 °/h, which are 62% and 66% better than those without modulation respectively, of which the drift has reached the level of the broadband light source. Although the noise still needs further reduction, its scale factor stability is 0.38 ppm, which has an overwhelming advantage compared with the traditional IFOG.

Ultrafast multi-target control of tightly focused light fields

The control of ultrafast optical field is of great interest in developing ultrafast optics as well as the investigation on various light-matter interactions with ultrashort pulses. However, conventional spatial encoding approaches have only limited steerable targets usually neglecting the temporal effect, thus hindering their broad applications. Here we present a new concept for realizing ultrafast modulation of multi-target focal fields based on the facile combination of time-dependent vectorial diffraction theory with fast Fourier transform. This is achieved by focusing femtosecond pulsed light carrying vectorial-vortex by a single objective lens under tight focusing condition. It is uncovered that the ultrafast temporal degree of freedom within a configurable temporal duration (~400 fs) plays a pivotal role in determining the rich and exotic features of the focused optical field at one time, namely, bright-dark alternation, periodic rotation, and longitudinal/transverse polarization conversion. The underlying control mechanisms have been unveiled. Besides being of academic interest in diverse ultrafast spectral regimes, these peculiar behaviors of the space-time evolutionary beams may underpin prolific ultrafast-related applications such as multifunctional integrated optical chip, high-efficiency laser trapping, microstructure rotation, super-resolution optical microscopy, precise optical measurement, and liveness tracking.

Improving the Selected Stages of Integrated-Optic Chip Structure Formation and Its Interfacing with Optical Fibers

The paper considers experimental technological regimes at different stages of integrated optical chips creation for modern applications: optoelectronics, fiber-optic sensors, microfluidics. The considered experimental regimes demonstrate not only qualitative, but also quantitative efficiency: at the stage of the ridge waveguides formation, there is a possibility of achieving high etching rates of lithium niobate - up to 950 nanometers per minute in fluorine-containing plasma with an increased consumption of plasma gases without forced thermostating of the sample. Using scanning electron microscope, structural changes in the surface have been demonstrated: clustering of surface etching foci, indicating the uniformity of etching at the macroscale, and changes in the thickness and structure of the near-surface defect layer, indicating the possibility of its complete removal by the plasma-chemical method. The method can be applied both for the manufacturing of ridge waveguides and optical elements based on them, and for microfluidics elements. Then, the efficiency of the waveguides and optical fibers interfacing method has been confirmed with frequency domain reflectometry. The algorithmic details of the interfacing by the modified Newton's method are also described. The modified Newton’s method allowed to simplify the alignment process by performing coarse alignment based on reflections from the far end face of the waveguide, speed up the alignment process at fine mode, and fully automate the alignment of the waveguide and optical fiber at any initial displacements. All the proposed technological process modernization leads to the improvement of technological characteristics and the individual stages production time reduction.

Pyroelectric drift of integrated-optical LiNbO3 modulators

The pyroelectric response has been studied for electro-optic modulators utilizing X-cut LiNbO3 integrated-optical chips. Since this response induces the modulator drift that appears only at fast change of a chip temperature, it causes the temperature and temporal instabilities of integrated-optical devices utilizing these chips. This drift was significantly reduced with the aid of extra electrodes providing the significant shielding of the pyroelectric field.

Simulating the effects of refractive index difference on the coupling efficiency of periodically segmented waveguide mode converter

Efficient coupling of micro/nano-optical waveguides with single-mode fibers is the premise for the efficient operation of the integrated photonic chip, which directly determines its optical performance. In this paper, the design principles of periodically segmented waveguide (PSW) structure used for high-efficiency fiber-chip coupling are proposed, and the effects of refractive index difference Δ on coupling efficiency and structural parameters are studied by simulation. It is found that as the Δ of the PSW increases, the period of the PSW tends to be smaller, and the coupling efficiency decreases continuously, reduced by around 0.673 dB in the range of Δ = 3% to Δ = 7%. Through the analysis of PSW optical mechanisms, it demonstrates that the main reason for the decrease of coupling efficiency is that the transmission loss of the tapered section increases sharply with the increase of Δ. High-Δ PSW is difficult to apply to highly integrated silica optical chips due to the unignorably insertion loss.

Tactical-grade interferometric fiber optic gyroscope based on an integrated optical chip

We present a tactical-grade interferometric fiber optic (IFOG) based on integrated optical chip (IOC). The chip comprises a light source, a photodiode , a polarizer, a double Y-junction and a phase modulator. By using a small-diameter sensing coil, and a signal-detection circuit, an IFOG based on IOC is realized. This allows for a significant reduction in the IFOG’s size, weight, power consumption and cost (SWaP-C). Preliminary performance data of a gyro prototype exhibits 0.12 deg/h bias instability over 1000 min. For the first time, to our knowledge, tactical-grade IFOG is realized based on integrated optical components except fiber coil.

Использование высокочастотной шумоподобной модуляции для снижения дрейфовых процессов в LiNbO-=SUB=-3-=/SUB=--фазовом модуляторе волоконно-оптического гироскопа

In this paper, the phase modulator optical phase drift reduction method is proposed and researched for LiNbO3 multi-functional integrated optical chip as a part of a fiber-optic gyroscope. This approach consists of applying a very high-frequency noise signal to a phase modulator in the range from 20 MHz to 100 MHz, which is outside of a fiber-optic gyroscope operating frequency range. The presented method showed that the increase in very high-frequency noise signal voltage up to around 1.7 half-wave voltages of the phase modulator allows decreasing optical phase drift by more than 4 times in the frequency range below 64.5 kHz.

Broadened-Laser-Driven Polarization-Maintaining Hollow-Core Fiber Optic Gyroscope

A state-of-the-art fiber optic gyroscope (FOG) using a 250-m quadrupolar-wound coil of polarization-maintaining hollow-core fiber (HCF) interrogated by a broadened laser is demonstrated. The fiber coil is directly connected to the multi-function integrated optics chip (MIOC) to reduce instabilities and reflections. The HCF FOG is first driven with a broadband source, yielding an angular random walk (ARW) of 2.12 μrad/√Hz and a drift of 0.375 μrad. This drift is 3.4 times lower than reported in any previous HCF FOG. Measuring the loss and ARW versus wavelength of the HCF FOG driven by a tunable narrow-linewidth laser reveals that the HCF supports a dense spectrum of high-scattering surface modes (∼70 modes/nm). The HCF FOG was also interrogated with a 22.5-GHz broadened-laser source, yielding an ARW of 6 μrad/√Hz. This ARW is most likely limited by backscattering mediated by coupling to the high-scattering surface modes. The drift of the HCF FOG driven by the broadened laser was 0.88 μrad. This drift is limited by polarization coupling, and confirms the holding parameter given by the fiber provider for this fiber (10−3 m−1).

Temperature modeling of modulation phase error in the integrated optical chip for closed-loop interferometric fiber optic gyroscope

An integrated optical chip (IOC) is the phase modulation actuator in a closed-loop interferometric fiber optic gyroscope (IFOG). Research on temperature characteristic of IOC is meaningful for high-precision IFOG working in harsh environment. We focus on the temperature modeling of the IOC modulation phase error. In theoretical analysis, based on the temperature dependence of the IOC equivalent capacitance, a model between IOC modulation phase error and temperature is proposed. Through experiments, the variation tendency of the IOC equivalent capacitance with temperature is first presented. Subsequently, the IOC modulation phase error is demonstrated to be linear with temperature, which verifies the effectiveness of the proposed model. We provide an error research direction of IOC for high-precision fiber optic gyroscopes.

Distributed Measurement of Polarization Characteristics for a Multifunctional Integrated Optical Chip: A Review

Multifunctional integrated optical chip (MFIOC) has the function of phase modulation, beam split, and polarization rejection, thus is widely used in fiber-optic sensing, in particular, the interferometric fiber-optic gyroscope (IFOG). In this paper, we reviewed the distributed measurement of polarization characteristics for an MFIOC, including the measurement principle based on the optical coherence domain polarimetry (OCDP), the modeling methods based on the Jones matrix and optical path, and the measurement performance. Also, the system errors that result from the optical delay line in the OCDP instrument or the birefringence dispersion in the MFIOC, the relevant suppression techniques, and the calibration technique are discussed. The technique meets the measurement requirement of MFIOC that with the polarization extinction ratio of more than 80 dB. Such MFIOC is the key component of the IFOG that with the strategic-grade performance.

Efficient Plasmonic 2D Arrangement and Manipulation System, Suitable for Controlling Particle–Particle Interactions

Investigating interactions between biological or artificial micro and nanoparticles (NPs) are vital for understanding processes such as intercellular communications, and quantum and optical effects in an array of NPs. In this line of research, methods to precisely trap and manipulate NPs are essential to investigate such inter-particle interactions. Exciting higher order counter-propagating leaky surface plasmon modes (LSPMs) of a thin gold stripe, we present a simple and efficient method to arrange NPs into a two-dimensional (2D) periodic pattern, allowing precise control over inter-particle interactions. We demonstrate that the 2D lattice of traps is generated by interference of totally reflected surface plasmons from the gold stripe edges and also interference of two counter-propagating LSPMs. Analytical calculations and numerical simulations are in good agreement with the observed modal behavior of LSPMs. Due to the modal dependency of LSPMs on the incident wavelength, we show that in this system different inter-particle distance with nanometer resolution can be achieved by adjusting the wavelength of the incident laser beams. Moreover, we show that the trapped NP array shows waveguiding behavior, enhances the mode intensities and the resulted potential wells consequently. We expect that this self-induced enhancement of trapping efficiency is beneficial for the periodic arrangement of NPs and array formation. The proposed approach implemented in a simple plasmonic optophoresis system, opens the possibility to develop integrated optical manipulation chips, which allows simple and low-cost approach for controlling and studying the inter-particle interactions. Furthermore, the proposed approach is also promising for controllable assembling of NPs without the need for complicated and time consuming nano-lithography techniques.

Study of Wavelength Temperature Stability of Multifunctional Integrated Optical Chips Applied on Fiber Optic Gyroscopes

In this paper, we theoretically analyze the wavelength temperature stability of multifunctional integrated optical chips (MIOCs) applied on fiber optic gyroscopes (FOGs). This study is critical for improving the temperature stability of the FOG scale factor, which is a significant parameter used to evaluate the dynamic characteristics of FOGs. Analytical and multiphysics simulations uncover two significant new predictions. First, changes in environmental temperature can lead to misalignment of the coupling structure between the chip and fiber, which is the main reason for the deterioration of the MIOC wavelength temperature stability. Second, misalignment caused by temperature changes is related to the initial alignment parameters of the coupling structure. Therefore, we can effectively improve the wavelength temperature stability of an MIOC by correctly choosing the initial alignment state of the coupling structure. Space coupling experiments are also presented to verify these predictions, and the experimental and simulation results are in good agreement.

Influence of the modulation phase error on bias error in interferometric fiber optic gyroscope

The integrated optical chip (IOC) is one of the most critical parts of the interferometric fiber optic gyroscope (IFOG). Unfortunately, the integral implementation of the IOC has a number of undesirable effects, which includes the modulation phase error. The inflexible influence of the modulation phase error on the bias error is analyzed in theory. For demonstrating, a method is found to measure the modulation phase error quantitatively. By experiment, the quantized parameter and its effect on bias error are presented. In addition, we offer a practical method to compensate the mentioned effect in IFOG, which is proved by the quantitative measurement. This study proposes a direction for IOC manufacturing inspection and evaluation.