Multichannel frequency-selective switch employing an arrayed-waveguide grating multiplexer with fold-back optical paths

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/spl times/N multiplexers, shifting the connections by N/M reduces the loss imbalance from M/spl Delta//spl alpha/ to /spl Delta//spl alpha//M where /spl Delta//spl 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/spl times/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).< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Utilizing T-band (1000 nm to 1260 nm) for optical communications is promising for short reach, and large capacity networks, such as data centers or access networks. It is feasible to use this with low-cost coarse wavelength division multiplexing (WDM). However, a tunable wavelength light source is necessary for such applications. In this paper, we propose a new configuration for an external cavity laser, which uses a silica-based arrayed waveguide grating (AWG) for the wavelength selecting element. The external cavity laser consists of a gain chip with high reflection (HR) and anti-reflection (AR) coated facets, coupling lenses, an AWG with AR/HR coatings, and an output fiber. The AWG has 17 connection ports, which correspond to 17 wavelengths with a channel spacing of 1.67 nm. The width of the connection port waveguides was optimized to achieve high coupling efficiency. The AWG chip size is 15 mm x 30 mm. The active layer in the gain chip has InAs quantum dots. The spontaneous emission 3-dB bandwidth was 48 nm (1108 nm to 1156 nm) when a current of 150 mA was injected into the gain chip. The lasing wavelength of the external cavity laser was successfully tuned from 1129.9 nm to 1154.4 nm by selecting the connection ports of the AWG. The typical threshold current was about 130 mA.

We propose a new compensation technique for the second-order temperature dependence in a silica-based arrayed-waveguide grating (AWG) multi/demultiplexer with a resin-filled groove that realizes a wide operating temperature range. We newly employ an additional interferometer in the input port and control the optical field perturbation by using a first-mode lightwave at the entrance to the first slab waveguide. We employ the design to fabricate a 32-channel 100-GHz-spacing athermal AWG that is as compact as a conventional AWG, and demonstrate a reduction in the passband wavelength variation from 70 to 22 pm over an extended - 40degC to 80degC temperature range.

Using low-temperature plasma-enhanced chemical vapor deposition, a silica-based arrayed-waveguide grating (AWG) filter has been monolithically integrated with germanium photodiodes on a silicon-based photonic platform. All the outputs of a 16-channel AWG with 200- GHz spacing have been equipped with germanium photodiodes. All wavelength channels of the integrated device were confirmed to function as a wavelength-division-demultiplexing receiver for 1.25-Gbps PRBS signals. In addition, we have successfully mounted multi-channel transimpedance amplifiers and limiting amplifiers on the integrated photonic device using flipchip bonding technology.

We demonstrate the regrowth-free integration of an InP-based arrayed waveguide grating (AWG) with a 1 /spl times/ 8 In/sub 0.53/Ga/sub 0.47/As p-i-n photodiode array. Low-loss polarization-independent adiabatic tapers transfer light from a passive waveguide optimized for external fiber coupling into a higher index waveguide for absorption in the In/sub 0.53/Ga/sub 0.47/As layer. The AWG and individual photodiodes have an external responsivity of 0.46 A/W and a polarization-dependent loss of /spl les/0.76 dB. The integrated device shows a worst-case crosstalk of -12.9 dB and a best-case crosstalk of -16.2 dB between channels, 9-dB total loss (including coupling losses), and -1.5-dB channel nonuniformity. The photodiode 3-dB electrical bandwidth is (25 /spl plusmn/ 5) GHz.

PHASARs or Arrayed Waveguide Grating (AWG) multiplexers for application in Dense Wavelength Division Multiplexing optical networks have been realized using silica-waveguide technology. Using waveguide tapers, a flat-top passband characteristic is obtained. Using Fourier transform spectroscopy employing a low coherence interferometer, the power-distribution coefficients and the phase error distribution in such components is evaluated. Because of the coherence length of the used light source being smaller than the optical path length difference in the PHASAR, it is possible to resolve individual waveguides of the array. Thus, the transfer functions of the individual optical paths are accessible to the measurement, enabling the full simulation of the PHASAR transmission spectrum. It is demonstrated that the measured phase error distribution in tapered PHASAR devices consists of contributions from both tapered waveguides and arrayed waveguide grating regions. An evaluation method suitable for investigating both contributions separately is developed. Phase shift introduced by the waveguide tapers is in good agreement with simulation calculations based on the Beam Propagation Method. An analytic transmission calculation based on measured phase and power-distribution coefficients enables a full simulation of PHASARs including insertion loss, bandwidth and crosstalk performance. Excellent agreement with transmission measurements is performed.

This paper proposes a cascade-connected arrayed-waveguide grating (AWG) as a solution to the problem of crosstalk accumulation in a large-scale AWG multiplexer/demultiplexer (MUX/DEMUX) and demonstrates a 64-channel cascaded AWG module with a very low background crosstalk of less than -80 dB and a total crosstalk of about -34 dB. In this paper, the authors densely integrate 64 additional compactly designed crosstalk-suppressing AWGs whose bandwidths were carefully optimized and directly attach them to a conventional 64-channel AWG. Consequently, in addition to a very low crosstalk, a low insertion loss and a compact size without passband shape distortion are achieved with this module. Based on the performance of the cascaded AWG module, it is then estimated that it is possible to realize a 1000-channel AWG MUX/DEMUX that is free from the problem of crosstalk accumulation.

Both the 4×20 GHz coarse wavelength division multiplexing and LAN-WDM receiver optical sub-assemblies (ROSAs) were developed. The ROSA package was hybrid integrated with a planar lightwave circuit arrayed waveguide grating (AWG) with 2% refractive index difference and a four-channel top-illuminated positive-intrinsic-negative photodetector (PD) array. The output waveguides of the AWG were designed in a multimode structure to provide flat-top optical spectra, and their end facet was angle-polished to form a total internal reflection interface to realize vertical coupling with a PD array. The maximum responsivity of ROSA was about 0.4 A/W, and its 3 dB bandwidth of frequency response was up to 20 GHz for each transmission lane. The hybrid integrated ROSA would be a cost-effective and easy-assembling solution for 100 GbE data center interconnections.

A multichannel optical wavelength selective switch is investigated, which can switch signals at arbitrary wavelengths between multichannels in wavelength division multiplexing systems. It is based on the combination of an arrayed-waveguide grating (AWG) multiplexer and an array of optical space-division switches. A monolithically integrated 2/spl times/2 optical wavelength selective switch is successfully fabricated for four multiplexed signals in the 1.55 /spl mu/m wavelength region. Here, an AWG and four 2/spl times/2 thermooptic switches are laid out on a 30/spl times/60 mm/sup 2/ chip. Wavelength selective switching operations for four wavelength signals between two lines are demonstrated in a time slot of 40 ms with a switching speed of 1 ms.

In future optical networks, versatile functionalities will be required for the optical network subsystems to fully utilize the spectral resources with low energy consumption. The key technologies are the spectral efficient MUX/DEMUX technique and flexible control of optical channels with high frequency granularity. An orthogonal frequency division multiplexing (OFDM) and Nyquist wavelength division multiplexing (N-WDM) are the most promising candidates of spectral efficient multiplexing techniques, and all-optical (AO) processing is expected to reduce the energy consumption. In an AO-OFDM systems, discrete Fourier transform (DFT) and inverse DFT (IDFT) are performed in optical domain by specially designed arrayed waveguide gratings (AWGs). In our experiment, 12.5 GHz spaced AO-OFDM system has been successfully demonstrated with no guard interval. In N-WDM systems, the Nyquist signal is generated by using carrier-suppressed return-to-zero (CS-RZ) signal and optical Nyquist filtering, which is achieved with two flat-top AWGs and optical interleaver, and the 25 Gbaud signals are successfully multiplexed in the experiment. Although both AO-OFDM and N-WDM can achieve the highest spectral efficiency, N-WDM is more suitable for flexible optical networks. This is because the N-WDM channels have less spectral overlap with the other channels than AO-OFDM, owing to its rectangular shaped compact spectrum. Therefore, N-WDM channel can be easily multiplexed and demultiplexed by optical filters. At an optical network node, channel defragmentation is indispensable technology to flexibly control the optical channels. We have experimentally demonstrated a format independent optical channel defragmentation with N-WDM signal. We believe these technologies are promising for future flexible optical networks.

We present packet switching applications based on extended spectral-amplitude-coding (SAC) labels in generalized multi-protocol label switching (GMPLS) networks. The proposed approach combines the advantages of wavelength-division multiplexing (WDM) and optical code-division multiple access (OCDMA). The extended SAC labels preserve the orthogonal property to avoid the effect of multiple access interference (MAI) shown at the decoder. We investigate the node architecture of label generation/recognition based on arrayed waveguide grating (AWG). Combining cyclic-shifted maximal length sequence (MLS) codes with the wavelength routed property of AWG simplifies the node structure. The simulation results show that the proposed labels achieve good performances against receiver noise due to the low average cross-correlation values. Under a given bit-error-rate (BER), the switching efficiency of the extended SAC labels outperforms the previous OCDMA schemes, as the network nodes are capable of processing a large number of labels simultaneously.

A compact hybrid-integrated 100 Gb/s (4 lane × 25.78125 Gb/s) transmitter optical sub-assembly (TOSA) has been developed for a 100 Gb/s transceiver for 40-km transmission over a single-mode fiber. The TOSA has a simple configuration in which four electro-absorption modulator-integrated distributed feedback (EADFB) lasers are directly attached to the input waveguide end-face of a silica-based arrayed waveguide grating (AWG) multiplexer without bulk lenses. To achieve a high optical butt coupling efficiency between the EADFB lasers and the AWG multiplexer, we integrated a laterally tapered spot-size converter (SSC) for the EADFB laser and employed a waveguide with a high refractive index difference of 2.0% for the AWG multiplexer. By optimizing the laterally tapered SSC structure, we achieved a butt-coupling loss of less than 3 dB, which is an improvement of around 2 dB compared with a laser without an SSC structure. We also developed an ultracompact AWG multiplexer, which was 6.7 mm × 3.5 mm in size with an insertion loss of less than 1.9 dB. We achieved this by using a Mach–Zehnder interferometer-synchronized configuration to obtain a low loss and wide flat-top transmission filter spectra. The TOSA body size was 19.9 mm (L) × 6.0 mm (W) × 5.8 mm (H). Error-free operation was demonstrated for a 40-km transmission when all the lanes were driven simultaneously with a low EA modulator driving voltage of 1.5 V at an operating temperature of 55 °C.

We report a simple method for fabricating a metal mask with wet etching designed to reduce the adjacent crosstalk of arrayed-waveguide gratings (AWGs) by using a photoinduced refractive index change. This process utilizes a commercially available inkjet printer and a copy machine, and so the resolution of the fabricated window pattern is not very high. However, the fabricated mask enables us to achieve a great reduction in adjacent crosstalk because it depends solely on a slowly varying component in the phase error distribution of an AWG.

We present a novel architecture for strictly non-blocking multistage photonic switches implemented using tunable wavelength converters and arrayed waveguide gratings. Clos networks, studied mostly in electronic domain, are required when the physical switching needs exceed the capacity of the largest feasible single crossbar switch. Unfortunately, straightforward extensions of these networks to the photonic domain show that the switch size has to be severely limited by the coherent crosstalk in each of the AWGs. We describe how recursive Clos networks free of coherent crosstalk can be built taking into account the current physical limitations of arrayed waveguide gratings.

The software radio architecture has extended the digital realm into the intermediate frequency (IF) signal processing functions such as band-selective filtering and up/down frequency conversion. In this paper, we proposed a computationally efficient scheme for the digital implementation of the multiplexing and the frequency up conversion functions in a frequency division multiplexing (FDM) system. Instead of performing the interpolation and frequency up conversion functions for each FDM channel independently, the proposed algorithm utilizes the common features of these functions for each channel and processes these signals jointly with less computation. Several additional applications of this scheme are also described.