Silica Planar Lightwave Circuit Research Articles

Simplified decoy-state quantum key distribution based on planar lightwave circuit integration module

The BB84 protocol with decoy states for quantum key distribution (QKD) has generated significant interest due to its theoretical unconditional security. Presently, the most of QKD systems demonstrated rely on bulk optics. Nevertheless, in upcoming large-scale commercial availability, it is imperative to have highly integrated, consistently performing, and cost-effective systems. Photonic integrated circuits (PICs) based on silica planar lightwave circuits (PLCs) play a crucial role in QKD due to its mature manufacturing process, excellent stability, low loss, and high integration level. We develop a simplified BB84 time-bin QKD experiment system that can significantly improve the key rate because it does not require time basis decoding. And two silica PLC asymmetric Mach-Zehnder interferometer (AMZI) module are used with a high-precision temperature control system (±0.01 °C). Three quantum states |ψ0⟩, |ψ1⟩ and |ψ+⟩ are encoded and decoded with the module. The system achieves an estimated key rate about 3.4 Mbps using an analytical model with one decoy state at a clock repetition rate of 1.25 GHz. Finally, we experimentally obtain a key rate of 531 kbps over a 20 km single-mode fiber (SMF) at a clock repetition rate of 156.25 MHz.

Study of hybrid integrated PLC-AWG chip for FBG demodulation

Fiber Bragg grating (FBG) sensors have been widely used in various fields. To develop a miniaturized, high-precision, and high-response FBG interrogation system, arrayed waveguide grating (AWG) has been widely used as a key device. In this paper, a 40-channel arrayed waveguide grating based on silica planar lightwave circuits (PLCs) is designed fabricated, coupled to a photodetector array using hybrid integration techniques. The hybrid-integrated demodulation chip enables precise demodulation of the FBG sensor wavelength in the C-band. The test results show that the AWG has a good transmission spectrum with a 3-dB bandwidth of 1.98 nm, insertion loss of about 3.5–4.1 dB, and adjacent channel crosstalk of about −7.84 dB. The established FBG interrogation system can realize continuous interrogation in the dynamic range of 1514.6–1578.6 nm, the wavelength resolution is 1 pm and the demodulation precision is 2.11 pm in the dynamic range of 1555.3–1556.1 nm.

Broadband design of silica-PLC mode-dependent-loss equalizer for 2LP-mode transmission systems.

A new configuration of mode-dependent-loss (MDL) equalizer for two linearly-polarized mode transmission systems using the silica planar lightwave circuit platform is proposed. This device acts as an LP01-mode attenuator (precisely, LP01/LP21 mode converter) to adjust the MDL keeping a high transmission of the LP11 modes. Almost all components constructing the device are based on the adiabatic mode conversion, which brings broadband operation. Especially, a newly proposed E12/E22 mode converter plays a key role in broadband MDL equalization. It is numerically revealed that the flattened spectra with designated transmission can be obtained for the wavelength from 1200 nm to 1650 nm.

Efficient and scalable edge coupler based on silica planar lightwave circuits and lithium niobate thin films

Recently, lithium niobate on insulator (LNOI) has emerged as a promising photonic integration platform for optical communication due to its outstanding material properties. However, an optical coupler that features high-efficiency is essential to improve the practicability of LN devices. In this work, we proposed a high coupling efficiency and scalable edge coupler, which consists of silica-based planar lightwave circuit (PLC) and LN bilayer taper. The silica-based PLC is positioned on the bottom of LN bilayer taper, which serves as the optical interface connecting the LNOI waveguide to the fiber. The silica-based PLC is interfaced to optical fibers through butt-coupling. Then, LN bilayer taper transfers the light form silica PLC to LN ridge waveguide through adiabatic coupling and converts it gradually into the fundamental mode. When coupled to an ultra-high numerical aperture fiber (UHNA), the simulation results show a low coupling loss of 0.11 dB and 0.128 dB per facet for TE and TM mode, respectively. The scalability of the coupler for large numbers of channels has been proven, which meets the performance requirements of LN devices employed in high-bandwidth, multi-channel systems. These results open a path for lithium niobate photonic integration circuit (PIC) and low-cost optical packaging.

6-mode and 10-mode photonic lantern mode multi/demultiplexers based on silica planar lightwave circuit

We propose a 6-mode and 10-mode photonic lantern mode multiplexer based on two- and three-layer silica planar lightwave circuitry, respectively. The 6-mode multiplexer has transmission greater than −0.19 dB at a wavelength of 1550 nm for the E11, E21, E12, E22, E31, and E13 modes. The 10-mode multiplexer is designed to only excite selected modes (E11, E21, E12, E22, E31, E13, E32, E23, E41, and E14 modes), used for fiber transmission. Each mode has transmission greater than −1.15 dB, at a wavelength of 1550 nm.

Temperature characterizations of silica asymmetric Mach-Zehnder interferometer chip for quantum key distribution

Quantum key distribution (QKD) system based on passive silica planar lightwave circuit (PLC) asymmetric Mach–Zehnder interferometers (AMZI) is characterized with thermal stability, low loss and sufficient integration scalability. However, waveguide stresses, both intrinsic and temperature-induced stresses, have significant impacts on the stable operation of the system. We have designed silica AMZI chips of 400 ps delay, with bend waveguides length equalized for both long and short arms to balance the stresses thereof. The temperature characteristics of the silica PLC AMZI chip are studied. The interference visibility at the single photon level is kept higher than 95% over a wide temperature range of 12 °C. The delay time change is 0.321 ps within a temperature change of 40 °C. The spectral shift is 0.0011~nm/0.1 °C. Temperature-induced delay time and peak wavelength variations do not affect the interference visibility. The experiment results demonstrate the advantage of being tolerant to chip temperature fluctuations.

Wavelength (DE)MUX-and-Switch Based on 5.5%-Δ-Silica PLC/Silicon Photonics Hybrid Platform

Silicon-based optical media present several desirable properties for a wide range of applications. Herein, we propose a novel hybrid integration scheme that combines a silicon photonics platform and a 5.5%--silica planar-lightwave circuit (PLC) platform. By exploiting the performance advantages of each platform, we fabricated a polarization-insensitive low-crosstalk 8 8 silicon photonics switch butt-jointed with a compact 5.5%--silica PLC-based 100-GHz 8-channel arrayed waveguide grating (AWG). The device was driven by a smartphone-sized (9 cm 13.5 cm) control board. The fabricated device exhibits a fiber-to-fiber insertion loss of 12.6 dB, an average polarization-dependent loss of less than 0.57 dB, and less than -40 dB leakage to non-target output ports. We also demonstrate two uses of the proposed device. The first is the DEMUX and Switch operation, in which the spectrally divided light by the AWG is routed to an arbitrary output port by a subsequent switch. The second is the Switch and MUX operation, in which arbitrary wavelengths from arbitrary input ports are merged by the AWG. No spectral degradation was observed in either operation. These results demonstrate the potential of the 5.5%--PLC/silicon photonics hybrid platform for compact, low-power, and fast-switching applications.

Quantum key distribution transmitter chip based on hybrid-integration of silica and lithium niobates

A quantum key distribution transmitter chip based on hybrid-integration of silica planar light-wave circuit (PLC) and lithium niobates (LN) modulator PLC is presented. The silica part consists of a tunable directional coupler and 400-ps delay line, and the LN part is made up of a Y-branch, with electro-optic modulators on both arms. The two parts are facet-coupled to form an asymmetric Mach–Zehnder interferometer. We successfully encode and decode four BB84 states at 156.25-MHz repetition rate. Fast phase-encoding of 0 or π is achieved, with interference fringe visibilities 78.53% and 82.68% for states |+〉 and |–〉, respectively. With the aid of an extra off-chip LN intensity modulator, two time-bin states are prepared and the extinction ratios are 18.65 dB and 15.46 dB for states |0〉 and |1〉, respectively.

Polarization-insensitive interferometer based on a hybrid integrated planar light-wave circuit

Interferometers are essential elements in classical and quantum optical systems. The strictly required stability when extracting the phase of photons is vulnerable to polarization variation and phase shift induced by environment disturbance. Here, we implement polarization-insensitive interferometers by combining silica planar light-wave circuit chips and Faraday rotator mirrors. Two asymmetric interferometers with temperature controllers are connected in series to evaluate the single-photon interference. Average interference visibility over 12 h is above 99%, and the variations are less than 0.5%, even with active random polarization disturbance. The experiment results verify that the hybrid chip is available for high-demand applications like quantum key distribution and entanglement measurement.

Six-mode scrambler based on cascaded side-wall grating waveguides

A six-mode scrambler based on cascaded side-wall long-period-grating waveguides on a silica planar lightwave circuit (PLC) platform is proposed for mode-division-multiplexing (MDM) transmission. The device can be fabricated with only one-step etching since it does not use a surface grating. A mode-dependent loss (MDL) of the system is estimated and the MDL can be reduced with a multiple scrambling operation. Two gratings proposed here and a fiber-pigtailed module containing these gratings are fabricated and the expected proof-of-concept mode scrambling operation is confirmed. It is found that the connection between few-mode fiber (FMF) and PLC chip seems to be the dominant loss of the module. Considering these losses, the measured results are in good agreement with the calculated results.

High-Q-Factor Silica-Based Racetrack Microring Resonators

In this paper, ultrahigh-Q factor racetrack microring resonators (MRRs) are demonstrated based on silica planar lightwave circuits (PLCs) platform. A loaded ultrahigh-Q factor Qload of 1.83 × 106 is obtained. The MRRs are packaged with fiber-to-fiber loss of ~5 dB. A notch depth of 3 dB and ~137 pm FSR are observed. These MRRs show great potential in optical communications as filters. Moreover, the devices are suitable used in monolithic integration and hybrid integration with other devices, especially in external cavity lasers (ECLs) to realize ultranarrow linewidths.

Interference at the single-photon level based on silica photonics robust against channel disturbance

Quantum key distribution (QKD) provides a solution for communication of unconditional security. However, the quantum channel disturbance in the field severely increases the quantum bit-error rate, degrading the performance of a QKD system. Here we present a setup comprising silica planar light wave circuits (PLCs), which is robust against the channel polarization disturbance. Our PLCs are based on the asymmetric Mach–Zehnder interferometer (AMZI), integrated with a tunable power splitter and thermo-optic phase modulators. The polarization characteristics of the AMZI PLC are investigated by a novel pulse self-interfering method to determine the operation temperature of implementing polarization insensitivity. Over a 20 km fiber channel with 30 Hz polarization scrambling, our time-bin phase-encoding QKD setup is characterized with an interference fringe visibility of 98.72%. The extinction ratio for the phase states is kept between 18 and 21 dB for 6 h without active phase correction.

Design of small mode-dependent-loss scrambling-type mode (de)multiplexer based on PLC.

A general design rule of scrambling-type mode (de)multiplexers (mode scramblers) based on silica planar lightwave circuit (PLC) with small mode-dependent-loss (MDL) is presented for a mode-division multiplexing (MDM) system. First, we consider four- and eight-mode scramblers and demonstrate that if the number of modes is 2N, it is possible to construct small-MDL mode scramblers by using Y-branch waveguides and mode rotators. Next, a 6-mode scrambler, which can be used for four linearly polarized (LP) mode transmission in MDM system, is considered, and the MDL is large if Y-branch waveguides are cascaded simply, originating from the radiation loss of unwanted modes at the Y-branch. We propose a 2 + 4-type mode scrambler by combining 2- and 4-mode scramblers and demonstrate that it is possible to design a small MDL 6-mode scrambler.

Design of Transmission-Type Refractive Index Sensor, Based on Silica Planar Lightwave Circuit Using Combination of Refractive Angle and Phase Measurements.

A transmission-type refractive index sensor, based on planar lightwave circuit (PLC) technology is proposed. In the proposed structure, we introduce a combination of coarse measurements, using the dependence of the angle of refraction and fine measurement, and the dependence of the phase on the refractive index to measure the absolute refractive index precisely, without expensive optical measurement equipment. The theoretical model of the proposed refractive index sensor is derived based on Fourier optics and transfer function to simulate its performance. The simulation results for the use of the 2.5%-Δ silica-based PLC technology indicate that the proposed structure has the potential to achieve a refractive index error of approximately 1 × 10−6 RIU or less when a monitored power deviation of ±0.05 dB is accepted.

Fourier-transform spectrometer chip covering visible band on silica planar waveguide

To minimise a spectrometer maximally, using a micro-electro-mechanical system or planar waveguide chip, has aroused interest in both academia and industry. This is because a portable and inexpensive spectral analysis tool has tremendous potential in various applications including scientific exploration, industrial inspection, and environmental protection. Here, we design, fabricate, and characterise a Fourier-transform spectrometer (FTS) chip consisting of 101 Mach–Zehnder interferometers (MZIs) with gradually varying optical path differences on a silica planar waveguide platform, aiming to cover the entire visible band. To ensure a wide-band operation while simultaneously compressing the size of the chip, wide-bending arm waveguides that can support high-order modes in a short wavelength range are employed in the design of the MZI array. The experimental result of our FTS chip shows its ability to analyse the visible spectrum with a minimum resolution of better than 10 nm and to reproduce the original spectrum with an accuracy of less than 0.5 nm. The successful implementation of the FTS chip based on a matured and low-cost silica planar lightwave circuit process makes it quite promising to realise mass production and widespread application.

Low-mode-crosstalk coupler using multistage mode conversion

We developed a low-mode-crosstalk coupler using silica planar lightwave circuit (PLC) technology. The coupler has two mode conversion stages. One is a mode converter for converting the fundamental mode to the second-order mode, and the other is for converting the second-order mode to the first-order mode, which is the target mode. Isolation between fundamental mode and first-order mode can be enhanced by using indirect conversion with the second-order mode instead of direct conversion. In the fabricated coupler, the mode crosstalk between the fundamental mode and first-order mode was under -30 dB, and the conversion ratio was -0.8 dB at the wavelength of 1050 nm. The coupler can be used for sensitive 1 μm-band mode-detection optical time-domain reflectometry.

Low-index-contrast waveguide bend based on truncated Eaton lens implemented by graded photonic crystals

Low-index contrast waveguides such as silica waveguides are indispensable part of passive integrated optical components. Reducing the bending radius of silica waveguides with low bend loss is vital in miniaturizing silica planar lightwave circuits. The Eaton lens is a gradient index lens that can bend parallel light rays by 90{\deg}, 180{\deg}, or 360{\deg}. We present a low-loss and compact 90{\deg} waveguide bend with a radius of 9 $\mu$m designed by truncating the Eaton lens. The performance of the designed bend, implemented by graded photonic crystal, was evaluated by full-wave two-dimensional finite element method. The average bend loss in the C-band is 0.9dB while the average bend loss for the wavelength range of 1260-1675nm is 1.05dB. The proposed design strategy can be applied to other low-index contrast waveguides.

Cost-effective 400-Gbps micro-intradyne coherent receiver using optical butt-coupling and FPCB wirings.

We present a cost-effective and bandwidth-enhanced 64-Gbaud micro-intradyne coherent receiver based on hybrid integration of InP waveguide-photodetector (WG-PD) and silica planar lightwave circuit (PLC). InP waveguide-photodetector (WG-PD) arrays are simply chip-to-chip bonded and optically butt-coupled to a silica-based dual-polarization optical hybrid chip. Multiple flexible printed circuit boards are adapted for electrical RF and DC wirings, which provide low-cost integration and good RF performance of the receiver. A 3-dB bandwidth of the fabricated coherent receiver is extended to ~36 GHz by optimization of bondwire inductance between the WG-PD array and the transimpedance amplifier (TIA), even when commercial TIAs with a typical bandwidth of ~29 GHz are used. Through optimization of the silica hybrid integrated coherent receiver, 64-Gbaud DP-16QAM signal transmission over 1050-km standard single-mode fiber is successfully demonstrated below a bit error rate of 2 × 10-3. This is the threshold for a soft decision-based forward error correction, at the optical signal to noise ratio of 23.8 dB.

Shape-optimized multi-mode interference for a wideband visible light coupler

Abstract We developed a wideband visible light coupler based on multi-mode interference (MMI) using silica planar lightwave circuit (PLC) technology. The MMI shape was designed by using analytical and numerical optimization techniques to expand the transmission range around the target wavelength. We used this method for green (G) and blue (B) coupler to expand G and B transmission range. The transmittances of the fabricated coupler designed by the hybrid design method were −1.5 to −1.0 dB at wavelengths of 430 to 465 nm and −0.5 to −0.1 dB at wavelengths of 508 to 625 nm, respectively. The coupler length is about 1 mm without an input/output circuit. The coupler can be used for the whole wavelength range of G and B as a GB coupler.

Scrambling-Type Three-Mode PLC Multiplexer Based on Cascaded Y-Branch Waveguide With Integrated Mode Rotator

A novel scrambling-type mode multiplexer (MUX) is proposed for future mode-division multiplexing with large number of modes. The proposed MUX does not aim at high modal extinction ratio as in a conventional-mode MUX, rather, all the modes are moderately mixed assuming that an MIMO-based receiver is used. The proposed configuration is superior to the conventional-mode MUX based on asymmetric directional couplers in terms of the fabrication tolerance of the waveguide width and scalability to a large number of modes. The wavelength dependence is also small since no coherent coupling devices are used, except for mode rotators. A three-mode multiplexer design based on a cascaded Y-branch waveguide and an integrated mode rotator on a silica planar lightwave circuit shows that low-loss and small wavelength dependence multiplexing is possible and a proof-of-concept device is fabricated.