Silica-based Planar Lightwave Circuit Research Articles

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.

Development of Terabit-Scale Compact Hierarchical Optical Cross-Connect System Using Planar Device Integration

The first practical terabit-scale hierarchical optical cross-connect system is realized by employing silica-based planar lightwave circuit devices. The system is scalable to 320 line cross connects + 160 client cross connects, and fully implemented equipment would require a 16-U-high chassis and only a 350-W power consumption. The potential of node cascadability is discussed and experimentally confirmed using partially implemented equipment that has four wavelength-division-multiplexed ports and one waveband switching capability. Stable operation was also confirmed using field-installed fibers.

Tunable Lattice-Form Mach–Zehnder Interferometer for Arbitrary Binary Code Generation at 40 GHz

We use the direct temporal domain approach to design spectrally periodic optical filters for pulse repetition rate multiplication (PRRM) with envelope shaping. In particular, we demonstrate a tunable lattice-form Mach-Zehnder interferometer using Silica-based planar lightwave circuit (PLC) for arbitrary 4-bit binary amplitude code generation at 40 GHz and to increase the repetition rate of a 10 GHz input pulse train to 20 GHz or 40 GHz. In addition to PRRM and envelope shaping, the device also has the capability of arbitrary phase coding.

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.

Generating 4 $\times$ 20 GHz and 4 $\times$ 40 GHz Pulse Trains From a Single 10-GHz Mode-Locked Laser Using a Tunable Planar Lightwave Circuit

We demonstrate a tunable pulse repetition rate multiplier using a silica-based planar lightwave circuit (PLC). The PLC is a six-stage lattice-form Mach-Zehnder interferometer which can be tuned thermally to generate 20- and 40-GHz output pulse trains from a 10-GHz input. We use a nonlinear optical loop mirror as a wavelength converter to perform intensity-to-field conversion to eliminate pulse-to-pulse phase fluctuations in the higher-rate output pulse trains. Moreover, we use wavelength multicasting (simultaneous wavelength conversion) to generate multiple pulse trains (4 × 20 GHz and 4 × 40 GHz in the example shown here) from a single 10-GHz input.

64QAM Modulator With a Hybrid Configuration of Silica PLCs and LiNbO$_{3}$ Phase Modulators

Quadrature amplitude modulation (QAM) is a promising approach for increasing the spectral efficiency of wavelength-division-multiplexing transmission, but its optical implementation is challenging. We demonstrate a 64-level QAM (64QAM) modulator, which uses a signal synthesis in the optical domain. We achieved an integration of low-loss asymmetric couplers and three quadrature-phase-shift-keying modulation circuits using a hybrid configuration of silica-based planar lightwave circuits and an array of LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> phase modulators. The modulator successfully operated at 60 Gb/s (10 Gbaud).

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.

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.

2×2 photonic crystal fiber splitter based on silica-based planar lightwave circuits

A 2x2 photonic crystal fiber (PCF) planar lightwave circuit (PLC) splitter, which splits optical power between two PCF channels, has been made by PCF-to-PLC connections. PCF array blocks were lithographically fabricated to have fiber V grooves and used to firmly hold PCFs and align them to the PLC splitter. The proposed splitter showed a rather flat splitting ratio over a wide wavelength range from 1250 nmto1750 nm. With the implemented splitter, we obtained a low excess loss of 1.6 dB, a low polarization-dependent loss of 0.1 dB, and a high return loss of 52 dB. The ultrabroadband operation of the proposed splitter is expected to find applications in optical performance monitoring, Ethernet passive optical networks, and biomedical optics including optical coherence tomography.

Reduction of Polarization Dependence of PLC Mach-Zehnder Interferometer Over Wide Wavelength Range

Differential phase shift keying are today considered key solutions for the WDM long-haul transmission infrastructure, and a Mach-Zehnder interferometer (MZI) for its demodulation needs to have very low polarization dependent frequency shift (PDf). MZIs based on silica-based planar lightwave circuits can offer a compact reliable demodulator, but it suffers from a large PDf. In particular, they exhibit a large wavelength dependence on the PDf. It results in a narrow wavelength range where the PDf is sufficiently low, and this limits the operating range of the demodulator. In this paper, we propose a novel method for reducing the PDf over a wide wavelength range. We reveal that the large wavelength dependence of the PDf is due to both the polarization crosstalk of the half-wave plate and the birefringence of the waveguide. Using a mathematical approach, we derived a way to suppress the PDf by controlling the birefringence and the direction of the optic axis of the half-wave plate in the MZI. We confirmed the effectiveness of our proposed method experimentally and achieved a low PDF of < 1% for an FSR of over 140 nm in the wavelength.

Waveguide Interconnection in Silica-Based Planar Lightwave Circuit Using Femtosecond Laser

The interconnection of waveguides inside a silica-based planar lightwave circuit (PLC) is demonstrated both two- and three-dimensionally by using a femtosecond laser. The waveguides written with a femtosecond laser can be successfully connected to waveguides inside a PLC with low loss. Unlike previous work on the direct writing of 2-D and 3-D waveguides in bulk glass, the waveguide must be written a few tens of micrometers beneath the surface of a PLC composed of multilayers of different glasses. To realize a low-loss waveguide, we studied the sensitivity difference for femtosecond pulses in each layer in detail and developed a multiple-scanning technique with a femtosecond laser for writing waveguides. In addition, we investigated a mode-field (MF) diameter control technique to allow us to achieve the low-loss interconnection of PLC and written waveguides. We also studied particular problems caused by nonlinear optical effects, such as the positional displacement of written waveguides from the focal point. As a result, we achieved a low-loss waveguide with almost the same MF diameter as a PLC waveguide and performed the first demonstration of interconnection between PLC waveguides. The excess losses at 1550 nm were 1.4 and 2.3 dB for 2-D and 3-D interconnection, respectively.

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>

Arrayed Waveguide Gratings and Their Application Using Super-High-.DELTA. Silica-Based Planar Lightwave Circuit Technology

This paper reviews our recent progress on arrayed wave-guide gratings (AWGs) using super-high-Δ silica-based planar lightwave circuit (PLC) technology and their application to integrated optical devices. Factors affecting the chip size of AWGs and the impact of increasing relative index difference Δ on the chip size are investigated, and the fabrication result of a compact athermal AWG using 2.5%-Δ silica-based waveguides is presented. As an application of super-high-Δ AWGs to integrated devices, a flat-passband multi/demultiplexer consisting of an AWG and cascaded MZIs is presented.

An athermal delay interference circuit using trenches filled with low-refractive index material

Recently, various types of silica-based planar lightwave circuits (PLCs) have been used for optical networks. However, silica exhibits large temperature dependence, so the temperature of silica-based PLCs needs to be controlled. We can reduce the cost and the energy budget of optical equipment by using athermal waveguides. We propose an athermal optical waveguide comprised of trenches filled with low-refractive index material. The propagation constant of the waveguide is determined by the refractive indices of the core and the low-refractive index material; their thermo-optic coefficients have opposite signs, and therefore the temperature dependence of the waveguide can be suppressed. A 1-bit delay circuit was designed for a 12.5Gbit/s differential phase shift keying receiver using the athermal waveguide, and it displayed a wide operating temperature range.

40-channel in-line optical power monitor integrated on 2.5%- silica PLC using out-of-plane micromirror arrays

A compact 40-channel in-line optical power monitor was integrated on a 2.5%-Delta silica-based planar lightwave circuit (PLC). Two 20-channel chip-scale-packaged photodiodes were surface-mounted on the PLC, in which out-of-plane micromirror arrays were integrated. A high responsivity of 0.9-1.1 A/W was obtained.

Demonstration of Compact and Low-Loss Athermal Arrayed-Waveguide Grating Module Based on 2.5%-Δ Silica-Based Waveguides

We demonstrated a compact and low-loss athermal arrayed-waveguide grating (AWG) module utilizing silica-based planar lightwave circuit (PLC) technology. Spot-size converters based on a vertical ridge-waveguide taper were integrated with a 2.5%-Δ athermal AWG to reduce the loss at chip-to-fiber interface. Spot-size converters based on a segmented core were formed around resin-filled trenches for athermalization formed in the slab to reduce the diffraction loss at the trenches. A 16-channel athermal AWG module with 100-GHz channel spacing was fabricated. The use of a 2.5%-Δ athermal chip with a single-side fiber array enabled a compact package of the size of 41.6×16.6×4.5 mm3. Athermal characteristics and a small insertion loss of 3.5–3.8 dB were obtained by virtue of low fiber-to-chip coupling loss and athermalization with low excess loss.

Multichip Integration on a PLC Platform for 16$\,\times\,$16 Optical Switch Module Using Passive Alignment Technique

We have integrated eight sets of four-channel semiconductor optical amplifier (SOA) array chips on a silica-based planar lightwave circuit (PLC) platform for 16 times 16 optical switch module by flip-chip bonding technique. To accurately bond eight sets of the arrayed SOA chips on a PLC platform, two methods of two-step assembly and one-step assembly were tried. Among the results of two assembly methods, one-step assembly was estimated to be desirable in respect of the horizontal and vertical bonding accuracy. The bonding accuracy was measured to be within 0.5 mum. Also, we demonstrated 16 times 16 optical switch module.