Silica-based Planar Lightwave Circuit Technology Research Articles

A Transponder Aggregator With Efficient Use of Filtering Function for Transponder Noise Suppression

Colorless, directionless, and contentionless reconfigurable optical add/drop multiplexing (CDC-ROADM) provides highly flexible physical layer network configuration. Such CDC-ROADM must operate in multiple wavelength bands which are being increasingly implemented in optical transmission systems. The operation in C+L bands requires switch devices used in CDC-ROADM to also be capable of multiband operation. Recent studies on wavelength division multiplexing (WDM) systems have pointed out the impact of amplified spontaneous emission (ASE) noise generated by signals of different wavelengths, which causes OSNR degradation. Therefore, it is desirable to filter out the ASE noise from different transponders when multiplexing multiple wavelengths at the transmitter side, especially in a system with non-wavelength selective combiners such as directional couplers and multicast switches. The use of transponder aggregators with filtering functions, such as the M × N wavelength selective switch (WSS), is preferable for this filtering. However, the downside of these devices is that it is difficult to provide economical multiband support. Therefore, we propose an economical transponder aggregator configuration by allowing a certain amount of ASE superposition and reducing the number of filtering functions. In this paper, we fabricated a prototype of the proposed transponder aggregator by combining silica-based planar lightwave circuit technology and C+L band WSS, both commercially available, and verified its feasibility through transmission experiments. The novel transponder aggregator is a practical solution for a multiband CDC-ROADM system with improved OSNR performance.

Low-loss 1 × 256 optical power splitter

A low-loss 1 × 256 optical power splitter is designed and fabricated using silica-based planar lightwave circuit technology on a 6-in. quartz substrate. The measured results show that the insertion loss, uniformity, and wavelength-dependence loss (WDL) of a 1 × 256 splitter are <26.2, 1.97, and 1.83 dB, respectively, in the wavelength range from 1.26 to 1.63 μm. The insertion loss, uniformity, and WDL of a 1 × 256 splitter are <25.62, 1.33, and 1.03 dB, respectively, in the normal communication wavelength range from 1.31 to 1.55μm. The polarization dependence loss is <0.16 dB in the wavelengths of 1.31 and 1.55 μm.

Single-photon interference using silica-based AMZI with phase modulation

An asymmetric Mach-Zehnder interferometer (AMZI) which comprised a variable optical attenuator (VOA), a thermo-optic phase modulator (TOPM) and a delay line of 740 ps was designed and fabricated using silica-based planar lightwave circuit technology. Two identical AMZI chips were connected in series to demonstrate the single photon interference in the middle pulse. The experimental results showed the TOPM can modulate the phase difference between the double pulses. The single photon interference visibility was dependent on the temperature and the polarization of the input light. The observed maximum visibility was 98.38% when the temperature of temperature controller (TEC) was 10 °C. And the interference visibility was dependent on the polarization of the input light. The AMZI could form the basis of the passive decoder in the future quantum cryptography systems.

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.

Integrated four-channel directly modulated O-band optical transceiver for radio over fiber application.

We have fabricated a compact and integrated 4-channel analog optical transceiver for radio over fiber application. In the fabricated module, the transmitter optical sub-assembly is composed of four directly modulated DFB laser chips integrated with an optical multiplexer based on an arrayed waveguide grating (AWG) using silica-based planar lightwave circuit (PLC) technology. The receiver optical sub-assembly consists of a PIN photodiode array integrated with an AWG-PLC-type optical de-multiplexer. For all the lanes, the 3 dB bandwidth exceeds 19.1 GHz and the measured spurious-free dynamic range (SFDR) is up to 90.5 dB⋅Hz2/3 when the input RF frequency is from 2 GHz to 14 GHz. Meanwhile, the electrical inter-channel crosstalk of the transceiver is less than -20 dB when the carry frequency is below 18.5 GHz. This module shows a great transmission performance in radio over fiber system. Under simultaneous 4-channel different 600 Mb/s 5-band 64QAM-OFDM RF signal operation, the measured error vector magnitude (EVM) performance below 8% is achieved after 15.5 km single-mode fiber propagation for all lanes. This scheme has potential applications in guiding high-dense, cost-effective and high-linearity analog optical transceiver design to realize high-capacity radio over fiber transmission systems.

Wavelength selective switch array employing silica-based waveguide frontend with integrated polarization diversity optics.

We propose a waveguide frontend with integrated polarization diversity optics for a wavelength selective switch (WSS) array with a liquid crystal on silicon switching engine to simplify the free space optics configuration and the alignment process in optical modules. The polarization diversity function is realized by the integration of a waveguide-type polarization beam splitter and a polarization rotating half-wave plate in a beam launcher using silica-based planar lightwave circuit technology. We confirmed experimentally the feasibility of using our proposed waveguide frontend in a two-in-one 1 × 20 WSS. The experimental results show that the fabricated waveguide frontend provides a polarization diversity function without any degradation in optical performance.

Design and fabrication of a low-loss and asymmetric 1 × 5 arbitrary optical power splitter.

In this paper, a low-loss and wavelength-insensitive asymmetric 1×5 arbitrary optical power splitter is designed and optimized, which is composed of a three-branch and two Y-branch elements. A low-loss and wavelength-independent arbitrary 1×5 optical power splitter can be obtained. Asymmetric 1×5 optical power splitters of different output ratios are fabricated using silica-based planar lightwave circuit (PLC) technology on a 6-inch quartz substrate. The measured results show that the IL_output1, IL_output2∼5, UNIF_output2∼5, PDL, and WDL of the output ratio 33%:67%, 50%:50%, and 95%:5% are less than (5.302 dB, 8.483 dB, 0.422 dB, 0.191 dB, 0.345 dB), (3.46 dB, 9.71 dB, 0.28 dB, 0.119 dB, 0.231 dB), and (1.52 dB, 14.15 dB, 0.44 dB, 0.366 dB, 0.561 dB), respectively, in the wavelength of 1.27 μm, 1.31 μm, 1.49 μm, 1.55 μm, and 1.625 μm.

Optical Chirp &lt;italic&gt;Z&lt;/italic&gt;-Transform Processor: Design and Application

This paper presents, to the author's knowledge, for the first time, a novel all-optical signal processing functionality, namely, a reconfigurable optical chirp z -transform (OCZT) processor. It is shown that the well-known non-tunable discrete Fourier transform (DFT) processors computed using the fast Fourier transform algorithm are a special case of the chirp z -transform (CZT) algorithm. A novel tunable DFT processor is proposed as a special case of the CZT algorithm. The tunable OCZT processor is designed using the CZT algorithm and synthesized using the silica-based planar lightwave circuit technology. To demonstrate its effectiveness, the design and simulation results of a novel optical discrete Fourier transform processor as a special case of the synthesized OCZT are presented.

Optical chirp z-transform processor with a simplified architecture.

Using a simplified chirp z-transform (CZT) algorithm based on the discrete-time convolution method, this paper presents the synthesis of a simplified architecture of a reconfigurable optical chirp z-transform (OCZT) processor based on the silica-based planar lightwave circuit (PLC) technology. In the simplified architecture of the reconfigurable OCZT, the required number of optical components is small and there are no waveguide crossings which make fabrication easy. The design of a novel type of optical discrete Fourier transform (ODFT) processor as a special case of the synthesized OCZT is then presented to demonstrate its effectiveness. The designed ODFT can be potentially used as an optical demultiplexer at the receiver of an optical fiber orthogonal frequency division multiplexing (OFDM) transmission system.

Design and fabrication of optical power splitters with large port count

We design and fabricate compact, low loss, and high port-count optical power splitters of 1 × 128 and2 × 128 using silica-based planar lightwave circuit (PLC) technology on 6 inch quartz substrate. PLC tech­nology is mainly based on plasma enhanced chemical vapor deposition, photolithography, and etching. The measured results show that the insertion loss, uniformity, and wavelength-dependent loss of 1 × 128 and 2 × 128 optical power splitters are less than 23, 1.43, and 0.92 dB and 23.3, 1.8, and 1.3 dB, respectively, in the wavelength range from 1.26 to 1.65 mm. The polarization-dependent losses are less than 0.16 and 0.2 dB, respectively, in the wavelengths of 1.31, 1.49, and 1.55 mm.

Optimization of a Birefringence-Enhanced-Waveguide-Based Polarization Beam Splitter

We present the optimization of a birefringence-enhanced-waveguide (BWG)-based polarization beam splitter (PBS) in a Mach-Zehnder interferometer (MZI) configuration and analyze the structure-dependent or polarization-dependent phase difference, using a delay-line MZI (DL-MZI). We fabricate the DL-MZI using silica-based planar lightwave circuit technology and, using the DL-MZI, demonstrate the ability to optimize a PBS by measuring the birefringence of the BWG and structure-dependent phase offset.

All-in-One 112-Gb/s DP-QPSK Optical Receiver Front-End Module Using Hybrid Integration of Silica-Based Planar Lightwave Circuit and Photodiode Arrays

We propose a new hybrid integrated optical front-end module for a 100-Gb/s-class coherent receiver using silica-based planar lightwave circuit (PLC) technology. We have developed an integrated structure composed of multichannel microcollimator optics between PLC waveguides and photodiodes (PDs), and compact hermetically sealed optical-to-electrical converters with a PD array and a transimpedance amplifier array to maintain reliability. We used the technology to fabricate an all-in-one optical front-end module for a dual polarization quadrature phase-shift-keying (DP-QPSK) coherent receiver, and confirmed that the module operated successfully for 112-Gb/s DP-QPSK signal detection.

Experimental demonstration of multi-degree colorless, directionless, contentionless ROADM for 127-Gbit/s PDM-QPSK transmission system

We experimentally demonstrate the feasibility of a multi-degree colorless, directionless, and contentionless (C/D/C-less) ROADM node composed of high port count wavelength-selective switches and transponder aggregators using silica-based planar lightwave circuit technology. The experimental results show that the introduction of a C/D/C-less function to a multi-degree ROADM node induces no significant penalty in a 127-Gbit/s PDM-QPSK signal transmission.

Design of Hilbert transformers with tunable THz bandwidths using a reconfigurable integrated optical FIR filter

We present the design and analysis of a wideband and tunable optical Hilbert transformer (OHT) using a tunable waveguide-based finite-impulse response (FIR) filter structure by using the digital filter design method and the Remez algorithm. The tunable Nth-order waveguide-based FIR filter, which simply consists of N delay lines, N tunable couplers, N tunable phase shifters and a combiner, can be tuned, by thermally adjusting the tunable couplers and tunable phase shifters, to tune the bandwidth of an OHT using silica-based planar lightwave circuit (PLC) technology. To demonstrate the effectiveness of the method, the simulation results have an excellent agreement with the theoretical predictions. The tunable OHT can function as a wideband and tunable 90° phase shifter and thus has many potential applications. The two unique features of wideband characteristic (up to ~ 2 THz) and tunable bandwidth (THz tuning range) of the proposed OHT cannot be obtained from the existing OHTs.

Dual polarisation optical hybrid using silica-based planar lightwave circuit technology for digital coherent receiver

Described is a compact dual polarisation optical hybrid (DPOH) that has been achieved by integrating polarisation beam splitters and 90°-hybrids fabricated with silica-based planar lightwave circuit technology. The fabricated DPOH worked completely passively without any adjustments and its optical characteristics were sufficient for the coherent detection of 111°Gbit/s DPQPSK signals.

One-chip integrated polarisation-multiplexed DQPSK demodulator using silica-based planar lightwave circuit technology

A compact polarisation-multiplexed differential quadrature phase-shift keying (DQPSK) demodulator achieved by integrating a polarisation beam splitter and two DQPSK demodulators consisting of a delay interferometer and a 90°-hybrid using silica-based planar lightwave circuit technology is demonstrated. The optical characteristics of the fabricated demodulator were sufficient for the direct detection of 100 Gbit/s polarisation-multiplexed DQPSK signals.

Compact and Athermal DQPSK Demodulator with Silica-Based Planar Lightwave Circuit

We present a full description of a polarization-independent athermal differential quadrature phase shift keying (DQPSK) demodulator that employs silica-based planar lightwave circuit (PLC) technology. Silica-based PLC DQPSK demodulator has good characteristics including low polarization dependence, mass producibility, etc. However delay line interferometer (DLI) of demodulator had the large temperature dependence of its optical characteristics, so it required large power consumption to stabilize the chip temperature by the thermo-electric cooler (TEC). We previously made a quick report about an athermal DLI to reduce a power consumption by removing the TEC. In this paper, we focus on the details of the design and the fabrication method we used to achieve the athermal characteristics, and we describe the thermal stability of the signal demodulation and the reliability of our demodulator. We described two athermalization methods; the athermalization of the transmission spectrum and the athermalization of the polarization property. These methods were successfully demonstrated with keeping a high extinction ratio and a small footprint by introducing a novel interwoven DLI configuration. This configuration can also limit the degradation of the polarization dependent phase shift (PDf) to less than 1/10 that with the conventional configuration when the phase shifters on the waveguide are driven. We used our demodulator and examined its demodulation performance for a 43Gbit/s DQPSK signal. We also verified its long-term reliability and thermal stability against the rapid temperature change. As a result, we confirmed that our athermal demodulator performed sufficiently well for use in DQPSK systems.

Loss Reduction of Waveguide Crossings by Wavefront Matching Method and Their Application to Integrated Optical Circuits

This paper describes a waveguide crossing design that uses the wavefront matching (WFM) method. The WFM method enables us to design waveguide crossings with lower loss than simple crossings composed of two straight waveguides, without any crosstalk degradation. We report the procedure for designing waveguide crossings based on the WFM method and some experimental results. For experimental confirmation, we made a waveguide crossing test circuit using silica-based planar lightwave circuit (PLC) technology. By comparing the results obtained with samples constructed with different Delta waveguides, we show that our design method is very efficient for higher Delta waveguides suitable for high-density integration. In addition, we describe an example application of our designed crossings to an integrated PLC device, namely a wavelength multiplexer with a variable optical attenuator. We show that waveguide crossings designed by the WFM method are useful for improving the loss characteristic of highly integrated PLC devices.

Low Power Consumption DQPSK Demodulator Using Silica-Based Planar Lightwave Circuit

We demonstrate a low power consumption differential quadrature phase-shift keying (DQPSK) demodulator using silica-based planar lightwave circuit (PLC) technology. A reduction of the power consumption is achieved by suppressing the temperature dependence of the circuit characteristic and eliminating a thermo-electric cooler (TEC) device. The maximum power consumption of the fabricated demodulator with an operating temperature range of -5degC to 70degC is 1.4 W, which is 60% lower than that of a conventional PLC demodulator with TEC. We report experimental results that demonstrate its good demodulation performance for a 43-Gb/s DQPSK system.

Low-Loss Y-Branch Waveguides Designed by Wavefront Matching Method

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> In this paper, we describe the procedure for designing low-loss Y-branch waveguides by the wavefront matching (WFM) method, and report experimental results as proof of concept. The designed Y-branches were fabricated using silica-based planar lightwave circuit (PLC) technology. A Y-branch fabricated with a 0.45%-<formula formulatype="inline"><tex Notation="TeX">$\Delta$</tex> </formula> waveguide exhibited a low excess loss of less than 0.2 dB over a wide wavelength range of 1250 to 1650 nm. In addition, we demonstrate that the WFM-designed Y-branches enable us to provide compact 1 <formula formulatype="inline"> <tex Notation="TeX">$\times$</tex></formula> 32 splitters with an average insertion loss of 16.0 dB at a wavelength of 1550 nm. In addition, we present some experimental results obtained using samples with different <formula formulatype="inline"> <tex Notation="TeX">$\Delta$</tex></formula> values, and show that our design method is more efficient for a higher <formula formulatype="inline"><tex Notation="TeX">$\Delta$</tex> </formula> waveguide suitable for functional PLC devices. </para>