Takahashi Hiroshi

We describe the fabrication method and characteristics of hybrid external cavity lasers composed of a spot-size converter integrated LD (SS-LD) and a UV written Bragg grating in a planar lightwave circuit (PLC) on a Si substrate. The SS-LD is passively aligned on a Si platform formed in the PLC, and the UV grating is created in the PLC with ArF laser irradiation through a phase mask. This structure enables us to fabricate a stable single-mode laser with a precisely controllable oscillation wavelength. By using the above techniques, we obtained a threshold current of 7-8 mA and a side mode suppression of 37 dB for an external cavity laser operating at 1.3 mu m. Moreover, we successfully demonstrated a four-channel external cavity laser with a wavelength interval of 2 nm +/-0.1 nm by integrating 4 SS-LDs on a PLC and controlling the Bragg wavelengths with ArF laser irradiation without a phase mask.

A new low loss groove design for athermal silica-based AWG multi/demultiplexers is proposed. The insertion loss was < 3.2dB with an excess loss of 0.4dB. The temperature dependence of the channel wavelength change was suppressed to be below 2.5GHz in the 0-85 degrees C range.

This paper describes a silica-based 8 x 8 strictly nonblocking thermooptic matrix switch with a high-extinction ratio and low loss. We realized this matrix switch by using a configuration which combines a double Mach-Zehnder interferometer (MZI) switching unit and an N x N matrix arrangement thus reducing the total waveguide length. The 8 x 8 matrix switch was fabricated on a 4-in wafer using planar lightwave circuit technology. We obtained an average extinction ratio of 60.3 dB and an average insertion loss of 5.1 dB, The operating wavelength bandwidth completely covers the gain band of practical erbium-doped fiber amplifiers (EDFA's).

The authors propose a new integrated external cavity laser which eliminates temperature dependent mode hopping by employing silicone between the LD and the grating. Operation without mode hopping is experimentally confirmed from 18 to 34 degrees C.

External cavity lasers [1] composed of a UV written waveguide grating and an LD are promising light sources for WDM systems because their oscillation wavelength is stabilized to the Bragg wavelength of the grating and is less dependent on temperature than that of conventional DF'B LDs. We have fabricated integrated external cavity lasers where an LD chip is integrated with a grating written in a silica waveguide [2]-[5] and we also confirmed 2.5 Gb/s direct modulation [6]. However, our lasers have mode hopping every several °C [2] caused by the difference between the thermo-optic (TO) coefficients of the LD and the silica waveguide. In this paper, we propose a new integrated external cavity laser which eliminates temperature dependent mode hopping, and report its oscillation characteristics.

We propose a new low loss groove design for athermal silica-based arrayed-waveguide grating multiplexers. The insertion loss was 3 dB and the cross talk was -35 dB. The temperature dependence of the channel wavelength change was suppressed to below 0.4 nm in the 0-85°C temperature range.

We report a hybrid integrated multiwavelength light source in which eight DFB-LD's and an MMI coupler are integrated with a silica PLC platform. We also report 2.5 Gb/s operation using LD drivers assembled module.

This paper describes the operation principle, design and characteristics of arrayed-waveguide grating wavelength multiplexers and also presents techniques for eliminating polarization dependence and flattening the spectral response. It is shown that the multiplexer can be used in practical WDM systems and applied to wavelength selective switches and multiwavelength devices.

A temperature dependent channel wavelength shift in a silica-based arrayed-waveguide grating multiplexer is successfully suppressed from 0.95 to 0.05 nm in the 0-85 degrees C temperature range, which means that it can be used in practical WDM systems without the need for temperature control.

This paper proposes a low-loss technique for eliminating polarization sensitivity in a silica-based planar lightwave circuit (PLC) which uses a polarization mode converter formed at the center of the circuit, This converter consists of a waveguide gap housing a polyimide half waveplate. The excess loss of the converter was drastically reduced to 0.26 dB with a Delta = 0.75% waveguide by employing an 18 mu m-wide waveguide gap and a 14.5 mu m-thick polyimide half waveplate, A polarization mode conversion crosstalk of -37 dB was achieved at 1.55 mu m. Using this converter, we successfully eliminated the polarization sensitivity in some silica-based PLC-type wavelength division multiplexers. The converter is also insensitive to temperature and offers long term stability.

A 1.4Tbit/s (200Gbit/s OTDM x 7 channel WDM) signal is successfully transmitted in a 50km dispersion shifted fibre. A supercontinuum pulse source and arrayed waveguide grating are used to generate WDM channels with 600 GHz spacing and highspeed TDM techniques are used to multiplex/demultiplex a 200Gbit/s signal.

The fabrication of a UV written grating in silica-based waveguide and an integrated external cavity laser composed of the grating and a spot-size converter integrated LD on planar lightwave circuit platform is described. The proposed structure provides a low loss laser cavity, which enables a stable single mode oscillation with threshold current of 12mA. The side mode suppression ratio is 37 dB and the average thermal coefficient is as low as-1.7 GHz/degree. Multiwavelength light source composed of the external cavity lasers with 2 nm spaced four wavelength are also realized with the same technique. It is shown that the proposed laser has a potential to be used in practical WDM systems.

Two novel configurations for digitally tunable optical filters based on arrayed-waveguide grating (AWG) multiplexers are described in detail with emphasis on the connection of the AWG multiplexer and optical switches. Performance comparisons show that conventional configurations are disadvantaged by the switch size required and loss imbalance among the optical frequency-division-multiplexed (FDM) channels; the proposed configurations require only O(root N) switch elements to select one of N FDM channels, and the loss imbalance is lower by up to 75% in decibel.

The dynamic performance of an integrated multiwavelength light source based on a UV written waveguide grating external cavity laser is described for the first time. It has 4 wavelength channels, each of which is successfully modulated at 2.5 Gb/s.

Temperature dependent channel wavelength shift in a silica-based arrayed-waveguide grating multiplexer is successfully suppressed from 0.95 nm to 0.05 nm in the 0 - 85 degrees C temperature range, which means it can be used in practical WDM systems without temperature control.

The silica-based Planar Lightwave Circuits (PLCs) offers versatile optical signal processing functions such as wavelength division multiplexing (WDM). Therefore, the hybrid integration of the PLCs and the semiconductor opto-electronic devices is promising technology to realize advanced optical communication systems such as an optical access networks and a WDM optical networks. In order to realize the hybrid integration, we developed a PLC platform which consists of the device assembly region and the electrical wire region together with a PLC region. This paper describes the recent progress of our hybrid integration technology.

A 128-channel arrayed-waveguide grating multiplexer with 0.2nm (25GHz) channel spacing al 1.55 mu m has been fabricated using a planar lightwave circuit (PLC). The authors obtained a crosstalk of less than -16dB to neighbouring and all other channels. The on-chip insertion loss ranged from 3.5 to 5.9dB for central and peripheral output ports, respectively.

The impact of crosstalk in an arrayed-waveguide N x N wavelength multiplexer is investigated precisely in relation to its application to wavelength-routing N x N all optical networks, In such systems multiple crosstalk light which has the same wavelength as the signal results in signal-crosstalk beat noise, We confirm that the noise is Gaussian and obtain the relation between crosstalk and power penalty, It is shown that the crosstalk must be less than -38 dB for a 16 x 16 system to keep the power penalty below 1 dB at a bit error rate of 10(-9).

The authors report for the first time an external cavity laser composed of a spot-size converted LD and a UV written waveguide grating, both integrated on Si. The laser operates in a singlemode with a side-mode suppression of 37 dB. The threshold current is 12 mA and the average thermal coefficient is as low as -1.7 GHz/degrees.

100 Gbit/s x 10 channel (1 Tbit/s) optical signals from a single supercontinuum WDM source are successfully transmitted over a 40 km dispersion-shifted fibre using a 400 GHz channel-spaced arrayed-waveguide grating WDM MUX/DEMUX as well as a 100 Gbit/s-10 Gbit/s all-optical TDM MUX/DEMUX.

We describe a low loss and high-extinction ratio 8 x 8 thermo-optic matrix switch with a reduced 8 stage configuration composed of silica-based planar lightwave circuits. The average insertion loss and the extinction ratio were 5.1 dB and 60.3 dB, respectively.

We have developed an optical path cross-connect demonstrator designed to achieve a 320-Gbit/s throughput. Measured performance confirms the viability of 320-Gbit/s throughput from a full scale implementation.

Wavelength conversion using a light injected DFB-LD and a Mach-Zehnder filter with a ring resonator (r-MZ) is described, It utilizes frequency modulation of the DFB-LD induced by an external light and the FM/IM conversion function of the r-MZ filter, Owing to a unique characteristic of the r-MZ filter, the converter has good tolerance for input signal level and an effect of suppressing signal fluctuation, compared with a conversion scheme using a conventional MZ filter.

How to connect arrayed-waveguide-grating (AWG) multiplexers in cascade is discussed with the goal of equalizing the loss imbalance among frequency-division-multiplexed (FDM) channels, This paper proposes to average the FDM-channel loss over the cascaded multiplexers by shifting port connections between each adjacent multiplexer. A simulation predicts that, in a cascade of M periodic N x N multiplexers, shifting the connections by N/M reduces the loss imbalance from M Delta alpha to Delta alpha/M where Delta alpha [dB] denotes the loss imbalance per multiplexer, This improvement will extend the cascadable node number in all-optical FDM networks, The prediction is confirmed experimentally in an optical add-drop filter (M = 2) constructed with a silica-based AWG 16 x 16 multiplexer; the largest loss difference among 15 FDM channels is reduced from 5.0 dB to 1.5 dB, This paper also reports that imperfect multiplexer periodicity due to waveguide dispersion restricts the equalizable frequency bandwidth to less than several free spectral ranges (FSR's).

A simple, effective technique to cancel waves generated by four-wave mixing is proposed and experimentally confirmed. A single-mode fiber, which has large dispersion at 1.55 ¼m, is simply inserted into the middle of a dispersion-shifted fiber transmission line. Dispersion in the single-mode fiber is effectively utilized to cause π phase shift in the light waves generated by four-wave mixing in the first and second halves of the transmission line so that the four-wave mixing is totally canceled. © 1995, Optical Society of America.

To realize practical wavelength division multiplexing (WDM) systems, a high-performance N x N wavelength multiplexer is introduced that is based on an arrayed-waveguide grating, Its transmission characteristics are theoretically derived and experimentally confirmed. A prototype is constructed using the previously proposed techniques that attain low insertion loss and polarization independent operation, It has 16 channels (N = 16) with a spacing of 0.8 nm, or 100 GHz, in the 1.55-mu m band, Frequency relation between input and output ports, free spectral range, and passband width are determined, A demonstration of IM-DD pulse transmission shows that there is no degradation of bit error rate resulting from the finite passband width and crosstalk of the multiplexer, It is confirmed that the multiplexer can realize highly reliable N-channel WDM and WDM-based N x N interconnect optical networks.

A novel optical add-drop multiplexer based on an arrayed-waveguide wavelength multiplexer is proposed The foldback configuration is employed to avoid the bit error rate degradation caused by the light crosstalk common in arrayed-waveguide multiplexers. Penalty free transmission is demonstrated.

The optimized configuration of an N-channel frequency-selective 2 x 2 switch based on an arrayed-waveguide grating (AWG) (2N + 1) x (2N + 1) multiplexer is proposed that offers significant crosstalk reduction and loss-imbalance equalization. The optimization is achieved by employing fold-back optical paths instead of loop-back paths. A switch constructed with a silica-based AWG 16 x 16 multiplexer demonstrates coherent crosstalk of less than -30 dB in the transmitting frequency-division-multiplexed (FDM) channels; the gain in optimization is more than 15 dB. The largest loss-difference among the seven FDM channels is simultaneously reduced from 4.5 dB to 1.6 dB. These improvements will extend the cascadable number of nodes in all-optical FDM networks.

A novel channel selection filter based on an arrayed-waveguide grating multiplexer is reported for optical frequency-division-multiplexed (FDM) networks. The filter employs fold-back optical paths and optical switches, and can be operated digitally with multiple switches. A 16 channel selection filter employing a silica-based 16 x 16 multiplexer achieves a 20GHz bandwidth and a crosstalk of less than 27dB.

An FSK-direct detection (FSK-DD) based optical FDM distribution, 1 channel selection experiment using a waveguide-type double ring resonator (DRR) is reported. In this experiment, the DRR is used as both a channel selective filter and an FSK to ASK converter. The finesse and insertion loss of the fabricated DRR are 200 and 6.1 dB, respectively. In order to verify the effectiveness of the DRR, a 10 GHz spaced, 8-channel distribution experiment over 40 km is demonstrated. All channels were successfully received with no power penalty.

Loss imbalance in an (N-1)-channel arrayed-waveguide grating add-drop multiplexer is equalised by optimising the offset of the loop-back optical-path connections. The largest loss-difference among 15 wavelength channels is successfully reduced from 5.0 dB to 1.5dB when the loop-back paths connect the output ports to the N/2-shifted input ports of a silica-based N x N multiplexer (N = 16).

The characteristics of a wavelength multiplexer based on an arrayed-waveguide grating are carefully investigated by using the grating theory and related experiments. A 28-channel multiplexer is designed and fabricated as SiO2-Ta2O5 waveguides on a 1 cm x 2 cm substrate. The designed wavelength channel spacing of 1 nm is obtained. The crosstalk to an adjacent channel is -15 dB. The measured minimum loss is 4.2 dB, which is composed of 3.4 dB excess loss and 0.8 dB grating loss.

A TE/TM polarization mode converter, with an excess loss of 0.26 dB, is constructed by inserting a polyimide half waveplate into a groove formed in a silica-based single mode waveguide. The polarization dependence of an arrayed-waveguide grating multiplexer is completely eliminated with this mode converter.

A novel configuration of an Er-doped fibre ring laser for simultaneous multiwavelength lasing is proposed. An arrayed-waveguide N x N wavelength multiplexer selects lasing wavelengths and forms plural ring laser cavities. Independently controllable four-wavelength lasing with a stable spacing of 1.6 nm is demonstrated.

A novel configuration for a multiplexer which can drop and add desired wavelength channels while transmitting other channels is proposed based on an arrayed-waveguide grating geometry with loopback optical fibre paths. A 15 channel multiplexing function with a 0.8 nm wavelength spacing is experimentally confirmed by using silica-based planar lightwave circuit technology.

The polarization dependence of an arrayed-waveguide wavelength multiplexer for a dense WDM system is successfully eliminated by depositing stress applying film on the waveguide. ne film compensates for waveguide birefringence induced by thermal stress. 16 wavelength channels with a 0.8 nm spacing are operated with no polarization dependence.

An integrated-optic 1 x 128 power splitter is demonstrated. It is composed of a slab waveguide, funnel-shaped waveguides, and output waveguides. Input light from a fiber diffracted in the slab waveguide and received by funnel waveguides. To achieve uniform power splitting, the width of the funnel waveguides is designed based on the Gaussian diffraction pattern; that is, the funnel widths increase progressively away from the center. The splitter is fabricated with silica waveguides on silicon. 1 x 128 splitting is achieved with a low excess loss of 2.3 dB and a small standard deviation of 0.63 dB.

GeO2-doped silica waveguides with a high refractive index difference (DELTA) of 2% and a propagation loss of 0.1 dB/cm are fabricated on Si substrates. 200 cm-long test circuits and ring resonators are demonstrated, both with a minimum curvature of 2 mm.