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We propose a cyclic NxN AWG router that can multi/demultiplex upstream and downstream signals with different channel spacings in different wavelength regions. We fabricated a 4x4 AWG router for our novel λ-tunable WDM/TDM-PON using silica-based PLC technology and showed that it can cyclically multi/demultiplex both 20 nm-spaced and 200 GHz-spaced signals at 1.3 and 1.5 μm, respectively.
©2012 Optical Society of America
1. Introduction
Passive optical network (PON) systems are now being installed worldwide to establish high-speed optical access networks, and 10 Gbit/s-class PON systems are already being standardized for next generation optical access networks [1]. A number of systems have been proposed for higher speed future optical access networks including time division multiplexing (TDM)-PON, wavelength division multiplexing (WDM)-PON, WDM/TDM-PON, and orthogonal frequency division multiplexing (OFDM)-PON. In particular, the WDM/TDM-PON system, which uses wavelength routing and the dynamic wavelength and bandwidth allocation (DWBA) algorithm, offers high bandwidth utilization efficiency and bandwidth allocation flexibility for inhomogeneous subscriber distribution [2]. The system employs optical splitters and a 1xN arrayed-waveguide grating (AWG) to multi/ demultiplex signals at an optical line terminal (OLT). Implementing an NxN AWG in this system can increase bandwidth allocation flexibility.
In this paper, we propose a novel cyclic NxN AWG router to realize a novel λ-tunable WDM/TDM-PON system with higher bandwidth allocation flexibility. We fabricated the proposed router with 1.5%-Δ silica-based planar lightwave circuit (PLC) technology and demonstrated that the proposed AWG router can cyclically multi/demultiplex upstream and downstream WDM signals with different channel spacings in different wavelength regions.
2. Proposed configuration
2.1 Wavelength allocation
The wavelength allocation of our novel λ-tunable WDM/TDM-PON system [3] employing an NxN AWG is shown in Fig. 1 . To make it possible to achieve smooth migration from the current PON systems, the system allocates 20 nm-spaced WDM signals at 1.3 μm for upstream transmission and 200 GHz-spaced WDM signals at 1.5 μm for downstream transmission. Moreover, upstream transmission uses coarse channel spacing to implement a cost-effective transmitter in an optical network unit (ONU) while downstream transmission uses dense channel spacing to take account of the tuning range capability of a tunable laser source in the line card (LC) at an OLT. Four wavelengths, λu,1 - λu,4, for upstream signals were set at 1331, 1271, 1291, and 1311 nm, respectively. Four wavelengths, λd,1 - λd,4, for downstream signals were set at 1575.37, 1577.03, 1578.69, and 1580.35 nm, respectively.
To use such a wavelength allocation, a novel NxN AWG router is required that can multi/demultiplex both 20 nm-spaced and 200 GHz-spaced WDM signals simultaneously.
2.2 Cyclic AWG router
The configuration of the novel NxN AWG router that we propose is shown in Fig. 2 . It consists of a first AWG, a second AWG, and two multichannel waveguide crossings with a thin film filter (TFF). The first AWG is designed as an N-port 20 nm channel spacing cyclic AWG operating at 1.3 μm with an FSR of 20 x N nm and with parabolic-shaped [4] input waveguides to produce flat-top transmission spectra. The second AWG is designed as an N-port 200 GHz channel spacing cyclic AWG operating at 1.5 μm with a free spectral range (FSR) of 200 x N GHz and with conventional Gaussian transmission spectra.
2.3 TFF-embedded multichannel waveguide crossing
The TFF-embedded multichannel waveguide crossing that we propose is shown in Fig. 3 [5]. N parallel waveguides tilted at θ and another N parallel waveguides tilted at -θ cross each other and form N insertion points angled at 2θ. The N filter insertion points are arranged in a straight line so that embedding one TFF can separate 1.3/1.5 μm signals simultaneously. The pitch of the insertion points is set at approximately 0.1 mm to prevent crosstalk from adjacent waveguides. In addition, the waveguides at the insertion points are laterally tapered to enlarge the mode field of the incident light entering the TFF with no wave-guiding structure to suppress diffraction loss [6].
With a previously reported TFF-embedded silica-waveguide WDM filter, light is input almost perpendicularly in relation to the TFF surface [7–9]. However, our multichannel waveguide crossing requires a larger crossing angle to separate the 1.3 and 1.5 μm lights and input them into the two AWGs in a compact area, and to prevent the waveguide crossings from exhibiting any excess loss. In addition, although a lower refractive index difference Δ is generally desirable as regards effectively decreasing the diffraction loss [10], it is desirable to use a relatively large refractive index difference to integrate the AWGs and the waveguide crossings compactly into one chip.
We used a Δ of 1.5% and set the crossing angle at 2θ = 60 degrees to obtain a compact chip and to keep the insertion loss below 0.05 dB. Therefore, we adopted a TFF for a silica waveguide that can operate even at a large incident angle of θ = 30 degrees and a high Δ of 1.5%. Note that we have optimized the TFF-embedded waveguide structure and used a smaller incident angle than we used in our initial experiment [5] to improve the transmission spectra of the TFF-embedded waveguide, and thus reduce the insertion loss. The TFF is designed to transmit 1.3 μm signals to and from the first AWG and reflect 1.5 μm signals to and from the second AWG. With this configuration, we can separate the 1.3/1.5 μm wavelengths for all N channels with only one TFF, and can easily increase the system scale by increasing the number of channels when needed.
3. Device fabrication
The proposed AWG router was fabricated on a silica-based PLC using flame hydrolysis deposition and reactive ion etching. The refractive index difference Δ was 1.5%, and the waveguide core size was 4.5 x 4.5 μm. The chip size was 27.0 x 16.5 mm, which is 30% smaller than that of our initial design [5]. A groove was formed on the fabricated chip with a dicing saw, and two 25 μm thick TFFs were inserted into the groove at the first and second multichannel waveguide crossings and fixed in place with adhesive. Four-channel fiber blocks were connected to the ONU side and OLT side input/output ports of the AWG router chip. Figure 4 shows the fabricated AWG router chip.
The AWG router chip was mounted on a temperature-controlled package to maintain it at a constant room temperature specifically to stabilize the downstream wavelengths. By employing athermal AWG technology [11], we can eliminate the need for a temperature control unit and reduce device cost, size, and power consumption.
4. Experimental results
A test circuit consisting of a four channel TFF-embedded waveguide crossing was also fabricated to evaluate the transmission characteristics of the TFF-embedded waveguide. The transmission spectra of the test circuit were measured using an optical spectrum analyzer, and the relative transmittance and excess loss was calculated by subtracting the transmittance of a waveguide with and without a TFF.
Figures 5 and 6 show the measured relative transmittance and excess loss of the light transmitted through and reflected from the TFF-embedded waveguide in the test circuit. The excess loss was < 0.5 dB, the polarization dependent loss (PDL) was < 0.1 dB, and the isolation was > 21 dB within the 1250-1350 nm wavelength range (Figs. 5(a) and 6(a)). The excess loss was < 0.25 dB, the PDL was < 0.1 dB, and the isolation was > 22 dB within the 1500-1600 nm wavelength range (Figs. 5(b) and 6(b)). The transmission characteristics of the TFF-embedded waveguide successfully improved, and the 1.3 μm wavelengths transmitted through the TFF and the 1.5 μm wavelengths reflected from the TFF both showed a low loss and high isolation with low polarization dependence.
Fig. 5 (a) Measured relative transmittance of the light transmitted through a TFF in the test circuit. (b) Measured relative transmittance of the light reflected from a TFF in the test circuit.
Fig. 6 (a) Measured loss of the light transmitted through a TFF in the test circuit. (b) Measured loss of the light reflected from a TFF in the test circuit.
Figure 7 shows the upstream transmission spectra of the fabricated AWG router module measured near 1.3 μm when light was input from the ONU side and output from the OLT side. The center wavelength accuracy was ± 2.7 nm (13.5% of the channel spacing), the adjacent channel crosstalk was < −22 dB, and the 3 dB bandwidth was 12.6 - 15.9 nm. The insertion loss was < 8.4 dB including an AWG center channel insertion loss of 1.3 dB, a loss imbalance of 3.0 dB among the port combinations of the cyclic AWG caused by nonuniform diffraction efficiency [12], an excess loss of 2.1 dB to produce flat-top transmission spectra, a fiber coupling loss of 0.5 dB x 2, and a TFF transmission loss of 0.5 dB x 2.
Figure 8 shows the downstream transmission spectra of the fabricated AWG router module measured near 1.57 μm when light was input from the OLT side and output from the ONU side. The center wavelength accuracy was ± 0.05 nm (3.1% of the channel spacing), the adjacent channel crosstalk was < −23 dB, and the 3 dB bandwidth was 114.4 – 117.7 GHz. The insertion loss was < 5.4 dB including an AWG center channel insertion loss of 1.3 dB, a loss imbalance of 2.6 dB among the port combinations of the cyclic AWG, a fiber coupling loss of 0.5 dB x 2, and a TFF reflection loss of 0.25 dB x 2.
The measurement confirmed that the 1.3 μm spectra exhibited a wide bandwidth while the 1.5 μm spectra maintained good wavelength accuracy as designed. For utilization in future time and wavelength division multiplexed passive optical network (TWDM-PON), it is necessary to further reduce the total insertion loss. By employing a Mach-Zehnder interferometer synchronized AWG structure with a low loss and a wide passband [13], and an optical spotsize converter with a low fiber coupling loss [14], the insertion loss can be reduced by 1.6 and 0.4 dB, respectively.
The upstream and downstream wavelengths are summarized in Tables 1 and 2 . When light was input from port 1, lights with wavelengths of λ1, λ2, λ3, and λ4 were output from ports 1, 2, 3, and 4, respectively. When light was input from port 2, the output wavelengths were shifted by one channel and lights with wavelengths of λ2, λ3, λ4, and λ1 were output from ports 1, 2, 3, and 4, respectively. It was thus confirmed that the fabricated device functioned as a cyclic AWG in both the 1.3 and 1.5 μm bands.
Table 1. Upstream wavelengths.
Table 2. Downstream wavelengths.
5. Conclusion
We proposed and demonstrated a novel integrated NxN AWG router that can cyclically multi/demultiplex 1.3 μm upstream and 1.5 μm downstream WDM signals with different channel spacings. We fabricated a 4x4 AWG router on a 1.5%-Δ silica waveguide and obtained 20 nm spacing flat-top spectra at 1.3 μm and 200 GHz spacing Gaussian spectra at 1.5 μm. Since our proposed configuration has excellent design flexibility and scalability, the device will be useful for realizing a novel WDM/TDM-PON system for future optical access networks.
Acknowledgments
We thank M. Yanagisawa, Y. Inoue, and M. Ishii for valuable discussions.
References and links
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