Reflectometry Study of the Pyroelectric Effect on Proton-Exchange Channel Waveguides in Lithium Niobate

Liu J., Zhang C., Gao F., Song J., Xu X., Guo Z.

The polarization extinction ratio (PER) of the multifunction integrated optic circuit (MIOC) is significant in maintaining polarization reciprocity in the fiber-optic gyroscope (FOG), and a high PER value is required, particularly in high-precision FOGs. Practically, the value of the PER decreases owing to the recoupling of the TM mode to the output port, thereby degrading the performance of the FOG. To improve the PER, the propagation of the leaking TM mode in the substrate is analyzed first. The variation of the PER with the chip structure is simulated based on the overlap integral algorithm of the optical mode. According to the analysis results, a structure of double absorption trenches at the bottom of the MIOC is proposed to block the TM mode from reflecting to the output port. In comparison with the traditional design, the optimized MIOC exhibits a higher PER that increases by approximately 25 dB and the average value of the PER reaches 75 dB. The MIOC design proposed in this study has good potential for application in high-precision FOGs.

Rahim A., Hermans A., Wohlfeil B., Petousi D., Kuyken B., Van Thourhout D., Baets R.

Optical links are moving to higher and higher transmission speeds while shrinking to shorter and shorter ranges where optical links are envisaged even at the chip scale. The scaling in data speed and span of the optical links demands modulators to be concurrently performant and cost-effective. Silicon photonics (SiPh), a photonic integrated circuit technology that leverages the fabrication sophistication of complementary metal-oxide-semiconductor technology, is well-positioned to deliver the performance, price, and manufacturing volume for the high-speed modulators of future optical communication links. SiPh has relied on the plasma dispersion effect, either in injection, depletion, or accumulation mode, to demonstrate efficient high-speed modulators. The high-speed plasma dispersion silicon modulators have been commercially deployed and have demonstrated excellent performance. Recent years have seen a paradigm shift where the integration of various electro-refractive and electro-absorptive materials has opened up additional routes toward performant SiPh modulators. These modulators are in the early years of their development. They promise to extend the performance beyond the limits set by the physical properties of silicon. The focus of our study is to provide a comprehensive review of contemporary (i.e., plasma dispersion modulators) and new modulator implementations that involve the integration of novel materials with SiPh.

Kostritskii S.M., Korkishko Y.N., Fedorov V.A., Yatsenko A.V.

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 ch...

Shang K., Lei M., Xiang Q., Na Y., Zhang L.

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. • A tactical-grade interferometric fiber optic (IFOG) based on integrated optical chip (IOC) is realized. • The chip comprises a light source, a photodiode , a polarizer, a double Y-junction and a phase modulator, except the fiber coil. • A small-diameter sensing coil, and a miniature signal-detection circuit are used in IFOG. • Preliminary performance data of a gyro prototype exhibits 0.12 deg/h bias instability over 1000 min.

Xu M., He M., Zhang H., Jian J., Pan Y., Liu X., Chen L., Meng X., Chen H., Li Z., Xiao X., Yu S., Yu S., Cai X.

The coherent transmission technology using digital signal processing and advanced modulation formats, is bringing networks closer to the theoretical capacity limit of optical fibres, the Shannon limit. The in-phase/quadrature electro-optic modulator that encodes information on both the amplitude and the phase of light, is one of the underpinning devices for the coherent transmission technology. Ideally, such modulator should feature a low loss, low drive voltage, large bandwidth, low chirp and compact footprint. However, these requirements have been only met on separate occasions. Here, we demonstrate integrated thin-film lithium niobate in-phase/quadrature modulators that fulfil these requirements simultaneously. The presented devices exhibit greatly improved overall performance (half-wave voltage, bandwidth and optical loss) over traditional lithium niobate counterparts, and support modulation data rate up to 320 Gbit s−1. Our devices pave new routes for future high-speed, energy-efficient, and cost-effective communication networks. In-phase/quadrature (IQ) electro-optic modulators are underpinning devices for coherent transmission technology. Here the authors present IQ modulators in the lithium-niobate-on-insulator platform, which provide improved overall performance and advanced modulation formats for future coherent transmission systems.

Ponomarev R.S., Shevtsov D.I., Karnaushkin P.V.

It is shown that the termination of the channeling of the fundamental radiation mode in the waveguide can be observed upon heating of an optical integrated circuit based on proton exchange channel waveguides formed in a lithium niobate single crystal. This process is reversible, but restoration of waveguide performance takes tens of minutes. The effect of the waveguide disappearance is observed upon rapid heating (5 K/min) from a low temperature (minus 40 °C). This effect can lead to a temporary failure of navigation systems using fiber optic gyroscopes with modulators based on a lithium niobate crystal.

Liang L., Liang J., Yao Q., Zheng M., Xie X., Liu H., Zhang Q., Pan J.

We demonstrate a compact all-fiber polarization-independent up-conversion single-photon detector based on integrated reverse proton exchanged periodically poled lithium niobate waveguides. The horizontally and vertically polarized components of randomly polarized signals are separated with a fiber-coupled polarization beam splitter, launched into two orthogonally polarized polarization maintaining fibers and fetched into two adjacent independent waveguides on the same device. The up-converted outputs from both waveguide channels are then combined with a multi-mode fiber combiner and detected by a silicon detector. With this configuration, the polarization-independent single-photon counting at 1.55 um is achieved with a system detection efficiency of 29.3%, a dark count rate of 1600 counts per second, and a polarization dependent loss of 0.1dB. This compact all-fiber system is robust and has great application potential in practical quantum key distribution systems.

Yu B., Tu G., Zhao M., Lin J.

As a kind of distributed optical fiber sensor, Optical Frequency-Domain Reflectometry (OFDR) can realize high spatial resolution distributed strain/temperature measurement. A method of measuring Rayleigh backscatter spectrum shift by cross-correlation calculation is widely adopted in OFDR sensor system. The other approach is based on the phase shift induced by the strain/temperature variation. In this paper, we propose a digital demodulation method to achieve it. Firstly the output of the photon detector is Fourier transformed and the phase information is obtained. The cross-correlation method and phase demodulation method are compared based on the theoretical and numerical analysis. The result shows that the spatial resolution (SR) of strain/temperature sensing is decided by the sweep range of the tunable laser source, while this parameter is much larger in traditional scheme. However, better stability can be achieved in cross-correlation scheme for sharp varying strain/temperature.

Roberts G.L., Pittaluga M., Minder M., Lucamarini M., Dynes J.F., Yuan Z.L., Shields A.J.

Quantum key distribution (QKD) is a technology that allows two users to exchange cryptographic keys securely. The decoy state technique enhances the technology, ensuring keys can be shared at high bit rates over long distances with information theoretic security. However, imperfections in the implementation, known as side-channels, threaten the perfect security of practical QKD protocols. Intensity modulators are required for high-rate decoy-state QKD systems, although these are unstable and can display a side channel where the intensity of a pulse is dependent on the previous pulse. Here we demonstrate the superior practicality of a tunable extinction ratio Sagnac-based intensity modulator (IM) for practical QKD systems. The ability to select low extinction ratios, alongside the immunity of Sagnac interferometers to DC drifts, ensures that random decoy state QKD patterns can be faithfully reproduced with the patterning effects mitigated. The inherent stability of Sagnac interferometers also ensures that the modulator output does not wander over time.

Klamkin J., Zhao H., Song B., Liu Y., Isaac B., Pinna S., Sang F., Coldren L.

A summary of photonic integrated circuit (PIC) platforms is provided with emphasis on indium phosphide (InP). Examples of InP PICs were fabricated and characterized for free space laser communications, Lidar, and microwave photonics. A novel high-performance hybrid integration technique for merging InP devices with silicon photonics is also discussed.

Wang C., Zhang M., Chen X., Bertrand M., Shams-Ansari A., Chandrasekhar S., Winzer P., Lončar M.

Electro-optic modulators translate high-speed electronic signals into the optical domain and are critical components in modern telecommunication networks1,2 and microwave-photonic systems3,4. They are also expected to be building blocks for emerging applications such as quantum photonics5,6 and non-reciprocal optics7,8. All of these applications require chip-scale electro-optic modulators that operate at voltages compatible with complementary metal–oxide–semiconductor (CMOS) technology, have ultra-high electro-optic bandwidths and feature very low optical losses. Integrated modulator platforms based on materials such as silicon, indium phosphide or polymers have not yet been able to meet these requirements simultaneously because of the intrinsic limitations of the materials used. On the other hand, lithium niobate electro-optic modulators, the workhorse of the optoelectronic industry for decades9, have been challenging to integrate on-chip because of difficulties in microstructuring lithium niobate. The current generation of lithium niobate modulators are bulky, expensive, limited in bandwidth and require high drive voltages, and thus are unable to reach the full potential of the material. Here we overcome these limitations and demonstrate monolithically integrated lithium niobate electro-optic modulators that feature a CMOS-compatible drive voltage, support data rates up to 210 gigabits per second and show an on-chip optical loss of less than 0.5 decibels. We achieve this by engineering the microwave and photonic circuits to achieve high electro-optical efficiencies, ultra-low optical losses and group-velocity matching simultaneously. Our scalable modulator devices could provide cost-effective, low-power and ultra-high-speed solutions for next-generation optical communication networks and microwave photonic systems. Furthermore, our approach could lead to large-scale ultra-low-loss photonic circuits that are reconfigurable on a picosecond timescale, enabling a wide range of quantum and classical applications5,10,11 including feed-forward photonic quantum computation. Chip-scale lithium niobate electro-optic modulators that rapidly convert electrical to optical signals and use CMOS-compatible voltages could prove useful in optical communication networks, microwave photonic systems and photonic computation.

Rao A., Fathpour S.

Lithium niobate (LN), spurred by its success for fiber-optic communications, has remained the material of choice for high-performance electrooptic (EO) modulators. The past decade has seen a surge in efforts aimed at miniaturizing LN EO modulators with higher order modulation formats, data centers, and optical interconnect applications in mind. The state-of-the-art of these compact modulators, with a focus on fabrication, design, and high-speed performance is reviewed. Guidelines for design optimization and key performance metrics of these important integrated photonic devices are presented. Furthermore, an outlook on the road toward commercial viability, along with potential novel applications is provided.

Fieberg S., Streit L., Kiessling J., Becker P., Bohaty L., Kühnemann F., Buse K.

The thermo-optic coefficient of lithium niobate (LiNbO3) has been measured in the temperature range from 10 to 160 °C using an interferometric setup. Undoped and magnesium-doped congruently melting LiNbO3 and undoped stoichiometric LiNbO3 were studied over a wide wavelength range in the visible and near infrared (450 – 600 nm and 900 – 1130 nm) using a frequency-doubled cw optical parametric oscillator. Experimental results for congruently grown lithium niobate were aggregated using a Schott equation to describe the wavelength and temperature dependence of the thermo-optic coefficient.

Pignatiello F., De Rosa M., Ferraro P., Grilli S., De Natale P., Arie A., De Nicola S.

A moiré interferometer is used to measure the thermal expansion of two ferroelectric crystals, LiNbO 3 and KTiOPO 4 . The crystal samples are patterned with a chromium reflective grating and used as a diffractive component in a reflective grating interferometer. The thermal expansion of all the three axes of congruent LiNbO 3 and of x and y axes of the flux-grown KTiOPO 4 were measured from room temperature to 200 °C. For this temperature range the thermal expansion coefficient has been modeled by a second-order polynomial and its coefficients have been estimated by accurate analysis of the resulting moiré fringe pattern .

Froggatt M.E., Gifford D.K., Kreger S., Wolfe M., Soller B.J.

Optical-frequency-domain reflectometry is used to measure the group-index difference and the refractive-index difference (i.e., beat length) between the fast and slow modes in polarization-maintaining optical fiber. The Rayleigh scatter normally present in the fiber is measured in reflection. This measurement, in turn, enables a distributed measurement of the fiber's birefringence that is rapid and completely nondestructive

Sun P., She X., Zhang L., Han H., Bi R., Shen H., Huang F., Wang L., Shu X.

Taranov M.A., Gorshkov B.G., Alekseev A.E., Konstantinov Y.A., Turov A.T., Barkov F.L., Wang Z., Zhao Z., Zan M.S., Kolesnichenko E.V.

The presented literature review was prepared by a team of authors united by the Program and Organizing Committees of the “Optical Reflectometry, Metrology, and Sensing” (ORMS) conference in 2023. It is intended to assess the state and prospects in this area for the coming years. The review covers the following topics: distributed acoustic sensors, fiber-optic measurement systems based on Brillouin scattering, research methods based on the principles of optical reflectometry in the frequency domain, and low-coherence approaches to distributed temperature and strain monitoring.

Belokrylov M.E., Claude D., Konstantinov Y.A., Karnaushkin P.V., Ovchinnikov K.A., Krishtop V.V., Gilev D.G., Barkov F.L., Ponomarev R.S.

Simple measures to improve the signal-to-noise ratio of optical frequency domain reflectometer (OFDR) readings are described. After applying a two-stage optical amplification of the backscattered signal, as well as eliminating the source of spurious reflections, it was possible to increase the signal-to-noise ratio of the frequency domain trace from 8 to 19 dB. This technique can be applied in fiber optic sensors and metrology of fiber optic and integrated optical elements.

Белокрылов М.Е., Клод Д., Константинов Ю.А., Карнаушкин П.В., Овчинников К.А., Криштоп В.В., Гилев Д.Г., Барков Ф.Л., Пономарев Р.С.

Таранов М.А., Горшков Б.Г., Алексеев А.Э., Константинов Ю.А., Туров А.Т., Барков Ф.Л., Wang Z., Zhao Z., Zan M.S., Колесниченко Е.В.

Ponomarev R.S., Konstantinov Y.A., Belokrylov M.E., Shevtsov D.I., Karnaushkin P.V.

A system has been developed for studying the pyroelectric effect in integrated optical modulators (IOMs) based on proton-exchange channel waveguides on a lithium niobate substrate. The system is also able to control the docking of an IOM chip with an optical fiber. A laboratory-certified optical frequency domain reflectometer has been integrated into the system to provide sufficient accuracy in determining the spatial coordinate of the test sample and the high sensitivity in measuring backscattering and reflections. The use of certified metrological instrumentation makes it possible to certify the temperature drift of the refractive index in IOM waveguides and qualitatively observe the variation in the phase state of radiation at each point of the waveguide. The use of an automated signal-processing system, which allows the user to observe all the desired parameters of the test sample with a varying spatial coordinate along the IOM length, has made it possible to reduce the number of routine research procedures in data analysis and to focus on their content. The advantages and drawbacks of replacing the self-made prototype based on a tunable laser with a commercial optical frequency domain reflectometry system are discussed. The created and applied filter based on the algorithm of dynamic nonlinear averaging in space has made it possible to increase the signal-to-noise ratio of data by 6−10 dB.