Multi-channel thermo-optic polymer digital optical switch (DOS) and variable optical attenuator (VOA) arrays for a 40-channel planar lightwave circuit (PLC) reconfigurable optical add/drop multiplexing (ROADM) switch have been studied by taking advantage of exceptional benefits of a polymer PLC technology. For both cost-effective and high-productive polymer based array modules, we focus on two main concepts as follows: each optical component for the ROADM switch is separated discretely, and it is partially grouped as 10 channels instead of fully integrating 40 channels. Ultimately, the purpose of this study is to realize highly competitive pol-ymer based array devices in terms of functionality, compactness, performance, and reliability for commercialization. Multi-channel polymer optical array devices showing excellent optical properties are designed, fabricated, and characterized, and we propose new types of DOSs using the total-internal reflection (TIR) effect. The optimization of the polymer array devices are accomplished using the well-known finite difference method (FDM) and beam propagation method (BPM).
For the improvement of fabrication tolerances of conventional 1×2 polymer Y-branch DOSs, a single channel crosstalk-enhanced Y-branch DOS with a low power consumption is proposed, which uses modified radiation-type attenuators. The fabricated DOS shows a good switching property with an optical crosstalk of below -45 dB at a switching power of 60 mW, which is far less power consumption compared to the convention-al DOS. Using this basic structure, the proposed DOS is expanded to a 10-channel polymer DOS array. To sup-press stray light, i.e., attenuated or radiated lights by heater operation, and to secure an optical crosstalk level of below -45 dB in the DOS array with a narrow channel pitch, we introduce a quartz substrate on which grid pat-terns are formed to disperse the stray light. As a result, we fabricate 10-channel polymer DOS array showing a low crosstalk of less than -45 dB at a low electrical power of 50 mW. Using the same substrate, we also fabricate the 10-channel polymer multimode filtering VOA array, which shows excellent optical characteristics such as an attenuation of less than -41 dB at a low power consumption of 17 mW, a low PDL of below 0.5 dB at 20-dB attenuation, and an adjacent channel crosstalk of -40 dB. To reduce overall size and packaging cost of the ROADM switch, the multimode filtering VOA array and the Y-branch DOS array are monolithically integrated into one polymer PLC chip to realize the 10-channel DOS/VOA array module. Even after this monolithic inte-gration, it exhibits good optical characteristics similar to those of each 10-channel DOS and VOA array. The high productivity and fabrication tolerances of the DOS and VOA arrays can lead to successful monolithic inte-gration.
To reduce the number of control heaters of the 1×2 Y-branch DOS, a novel 1×2 DOS using the TIR effect is proposed. The 1×2 TIR-DOS is created by connecting a reflection port to the conventional multimode filtering VOA. The fabricated TIR-DOS shows a low power consumption of 18 mW for an on-off extinction ratio of above 35 dB, and a switching isolation during a turned-off state is around 39 dB. As a multi-channel expansion of the TIR-DOS, a 2×2 TIR-DOS is proposed, and it has a cross cascaded configuration of four 1×2 TIR-DOSs, with a thermo-optic reflector located at each node of the branched multimode waveguides. The DOS has a distinctive feature of bend-free waveguides, which allows a very compact integration in the 2×2 DOS. The switching isolations during the turned-off (bar) and turned-on (cross) states are even higher than 50 dB. The 2×2 DOS shows a low insertion loss of about 1.4 dB and a PDL of below 0.1 dB, for both the bar and cross states within the whole C-band wavelength range. Its wavelength-insensitivity opens a possibility for application of the 2×2 DOS to the O-band wavelength range without any design modifications. Furthermore, the new types of 1×2 and 2×2 TIR-DOSs can be expanded to a multi-channel array structure for use in add/drop switching of the ROADM switch, and the 2×2 DOS is well suited for use in protection switching in optical networks.
Owing to the simplicity of the switching structure using the TIR, the 1×2 TIR-DOS is easily scaled up to very compact 4×4 and 8×8 optical matrix switches, which are realized using the 1×2 TIR-DOS as a unit switch-ing element. The matrix switches distinguishingly consist of a plurality of the crossed multimode waveguides, the offset heater electrodes in the switching nodes, and the single-mode and tapered waveguides in only the in-put/output region. From the optimization of the waveguide dimension, cross angle, heater angle, and heater off-set, excellent optical characteristics can be achieved in terms of the insertion loss, switching isolation, PDL, and power consumption. For large scale matrix switches of more than four channels, a compact electrode configura-tion is also developed using a two-layer wiring structure. The TIR matrix switches have such a simple structure as to be sufficiently used for the cost-effective and high performance optical switching systems.
In terms of reliability for the polymer based array devices, basic performance of the 10-channel polymer DOS/VOA array are examined, exhibiting dependences on wavelength, polarization, and temperature. From the result, a thermo-electric cooler (TEC) is used to reduce the temperature dependence of the devices. We first per-form a short-term temperature cycling test of the 10-channel polymer DOS/VOA array, and then a certified reli-ability test of the 10-channel VOA array, based on the Telcordia requirements, is accomplished. Satisfactory results fully meeting the Telcordia requirements can be obtained, and they prove that the thermo-optic polymer array devices can be well employed for the PLC ROADM switch.
As one application of the polymer optical array devices, a 40-channel PLC ROADM switch is successful-ly realized using four 10-channel polymer DOS/VOA array modules including two C-band 40-channel thermal AWG-WDMs. By employing 5% tap-photodiodes, optical power monitoring of each channel is performed. Con-troller and firmware with functions of the DOS/VOA control, optical power monitoring, TEC and AWG tem-perature control, and data communication interfaces are also developed. As expected, the total low power con-sumption of 10.6 W can be obtained by using the unique 10-channel DOS/VOA array modules showing low power consumption. It is demonstrated that the 10-channel polymer DOS/VOA array with both low optical crosstalk and low power consumption is well applicable to the ROADM systems. In addition, the thermo-optic polymer array devices and their resulting ROADM switch using them could be commercialized in the near fu-ture, and they would lead to a competitive edge for relevant optical devices.