We present a compact 4x8 wavelength-routing switch that monolithically integrates fast tunable wavelength converters (TWCs) and an arrayed-waveguide grating (AWG) for optical packet switching. The TWC consists of a double-ring-resonator-coupled tunable laser which allows rapid and stable switching, and an optical gate based on a parallel amplifier structure which prevents an input optical signal from being routed through the AWG (filter-free operation). A deep-ridge waveguide technology, employed for the AWG and ring resonators, facilitates the fabrication of the switch and makes the device compact. The filter-free TWCs achieve low crosstalk of the input optical signal of less than −22 dB. The wavelength routing operation of a non-return-to-zero (NRZ) signal at 10 Gbit/s is achieved with a switching time of less than 5 ns.
© 2010 OSA
The capacity of optical communications networks continues to grow strongly due to the Internet and Internet-related services. This growth has required large-capacity and low-power-consumption nodes with flexible traffic engineering capability for high-level services. Photonic multiprotocol label switching (MPLS) networks of generalized MPLS, optical burst switching, and optical packet switching (OPS) have emerged as potential solutions for the above requirements [1,2]. Of these three, OPS maximizes the flexibility and throughput of the network due to its packet-level data granularity. However, the implementation of OPS within the network will require the realization of optical routers that surpass the performance of current electrical routers.
In high-capacity electrical routers, an incoming packet is first segmented into a number of fixed-length data cells at a line card. The cells are routed through a switch fabric consisting of multiple parallel switching planes and then re-assembled into a packet at the line card . This forwarding method is very complex and becomes a bottleneck to improving performance: throughput, power consumption, latency, and size. A potential solution is the use of a high-speed optical switch that can operate on a packet-by-packet basis while maintaining data in the optical domain.
For an optical switch for optical routers, the requirements are low power consumption, small size, scalability, and data-format transparency, as well as low cost. A variety of optical switch technologies have been proposed for use in OPS. These technologies include Lithium Niobate, GaAs, or InP crosspoint switches in a multistage configuration , broadcast-and-select switches based on semiconductor-optical-amplifier (SOA) gate arrays , and wavelength-routing switches (WRSs) based on tunable wavelength converters (TWCs) and an arrayed-waveguide grating (AWG) . Of these technologies, the WRS demonstrates the most potential for low power consumption, high scalability, and larger throughput . In previous work, we have developed a WRS subsystem  with double-ring-resonator-coupled tunable lasers (DRR TL)  and an NxN cyclic AWG (silica planar lightwave circuit), and demonstrated a prototype hybrid optoelectronic router in which the WRS was installed . To achieve further reduction of power consumption, size, and cost of the WRS, integrating the TWCs and the AWG monolithically on a single chip would lead to dramatic improvements. An 8x8 monolithic all-optical switch on InP has been previously reported , but serious issues remain, such as transient thermal effects that cause the lasing wavelength of the tunable laser to drift due to the large current swing required for wavelength tuning. In addition, the optical signal input into the TWC must be removed before the AWG, or it will also be routed through the AWG.
In this paper, we report a WRS capable of high-speed switching and removal of the aforementioned input optical signal. The switch incorporates all-optical TWCs we have previously developed , which employ the DRR TL and optical gates based on a parallel amplifier structure (PAS) . The PAS enables the spatial separation of the converted signal and input signal to different output ports. Therefore, the input signal can be output in the opposite direction from the converted signal. The crosstalk of the input optical signal at the output ports of the WRS is suppressed to less than −22 dB. The 1x8 wavelength routing operation of a non-return-to-zero (NRZ) signal at 10 Gbit/s is achieved. We demonstrate for the first time, dynamic switching operation between four output ports for a monolithic WRS, with a switching time of several nano-seconds.
2. Device Structure and fabrication
Figure 1(a) shows a photograph of the fabricated WRS consisting of an array of four TWCs and an 8x8 AWG. The device has a footprint of 5.1 mm x 2.1 mm. Connections between the four input and eight output ports are accomplished by changing the TWC output wavelength to connect to the desired output port. An enlarged view of the TWC is shown in Fig. 1(b). The TWC consists of the DRR TL and the optical gate. An etched gap mirror placed between the tunable laser and the optical gate makes it possible to realize the laser cavity and to integrate the two devices . Wavelength conversion is performed by modulating the CW light from the DRR TL with cross-gain modulation (XGM) caused by the input signal injected into the optical gate. The optical gate based on the PAS is a symmetric Mach-Zehnder interferometer (MZI), which consists of two 3-dB multimode interference (MMI) couplers and a semiconductor optical amplifier (SOA) in each arm. Since the MZI is set in the cross state, the input signal after the XGM and converted signal are output to different output ports when the input and DRR TL output signals are input into different input ports.
Compared with the sampled-grating-distributed-Bragg-reflector (SG-DBR) often employed in tunable lasers, the ring-resonator filters employed within the cavity of the DRR-TL exhibit superior filter characteristics as well as a compact structure. These characteristics include a narrower transmission bandwidth with a Lorentzian-type filter response and an infinite number of resonant peaks. When using the Vernier tuning mechanism, the maximum injection current required for tuning can be reduced by reducing the free-spectral ranges (FSRs) of the filters. The ring-resonator filter enables reduction of the FSR while expanding a tuning range, resulting in low tuning current operation. The low tuning current operation is critical for reducing wavelength drift due to thermal transients . The small wavelength drift is highly advantageous for high-speed and stable switching operation of the WRS, as the wavelength drift exhibits a much longer response time (millisecond order) than the mechanism employed for fast tuning and will cause loss and crosstalk in the WRS.
The FSRs of the ring resonators were set at 400 and 444 GHz, which correspond to ring radii of around 20 μm. As a result, the total FSR of the two ring resonators was 4 THz (Vernier effect), which sufficiently covers the FSR of the AWG. The channel spacing of the 8x8 AWG was set to 400 GHz, that is, equal to the FSR of one of the ring resonators. This enables simple control of the switching operation, since any output port can be selected by changing the injection current into the ring resonator with an FSR of 444 GHz.
For the device, a stack-layer structure was used, which enables fabrication to be achieved with only a single regrowth step. The detailed InGaAsP/InP layer structure can be found in Refs [9,14]. The gain sections of the DRR TL and the two SOA sections have a shallow-ridge waveguide structure, whereas the ring resonators and the AWG have a deep-ridge waveguide structure. The deep-ridge waveguide has a very large refractive index difference in the lateral direction, which minimizes the allowable bending radius and makes the device compact. The deep-ridge waveguide structure was formed by Cl2-based inductively coupled plasma reactive ion etching (ICP-RIE) with only a lithography step. This simplifies the fabrication process of the WRS. After the structure had been fabricated, the device was coated with benzocyclobutene (BCB) and etched back for planarization. Electrodes were then formed by a liftoff process. The lengths of the gain and SOA sections are 400 and 1200 μm, respectively. An anti-reflection (AR) film was formed on both the input and output sides of the WRS.
3. Experimental results
The static routing characteristics of the fabricated WRS were measured first. Figure 2 shows superimposed output spectra of the WRS for every output port using each TWC. The injection current for the ring resonator with an FSR of 444 GHz was changed from 0 to 8.4 mA, whereas the currents for the gain section, both SOAs, and the other ring resonator were kept constant at 100, 150, and 0 mA, respectively. Output ports were easily selected by changing the injection current for only a single ring resonator, because the channel spacing of the AWG and the FSR of the other ring resonator were designed to be equal. The maximum injection currents are around one-fourth of those of an SG-DBR laser designed for fast and accurate tuning . The small injection currents needed for switching suppress the thermal wavelength drift of the laser . The connection between each TWC output signal and WRS output through eight different wavelengths is based on the cyclic nature of the 8x8 AWG.
Figure 3 shows the wavelength-converted eye diagrams for every output port of the WRS. In this experiment, only a single input port was tested. The 10-Gbit/s NRZ input optical signal had a wavelength of 1545 nm with a pseudo-random bit sequence (PRBS) of length 231-1. The signal was fed into input port 2 with an average power of 10 dBm. The wavelength of the converted signal was tuned so that the converted signal was output at one of eight output ports. The currents for the gain section, both SOAs, and the other ring resonator of the TWC were kept constant at 100, 250, and 0 mA, respectively. A clear eye opening was observed for every output port, confirming the 1x8 wavelength routing operation of the NRZ signal at 10 Gbit/s. Although wavelength conversion is performed only with the XGM of the SOAs in this work, it is also possible to utilize cross phase modulation (XPM) to achieve higher bit-rate operation. This has been demonstrated by integrating the SOA with an asymmetric MZI filter, for example .
As described in the previous section, an optical gate based on the PAS was employed in the TWC to remove the input signal before the AWG. However, due to the imperfection of the symmetric MZI, a small amount of the input signal leaked through the AWG, which was observed at the output ports of the WRS. Figure 4 shows the output spectra of the WRS from output ports 1 and 5. In this experiment, the operating conditions were the same as those in the previous experiment. The wavelength of the converted signal was tuned so that the converted signal was output at output port 5 as shown in Fig. 4(a). The wavelength of the input signal was 1545 nm, which coincides with the transmission-peak wavelength of the AWG at output port 1 when the input signal is fed into input port 2. Therefore, the input signal after XGM was mainly observed at output port 1 as shown in Fig. 4(b). Compared with the peak power of the converted signal at output port 5, the crosstalk was less than −22 dB. The low crosstalk of the WRS is attributed to the spatial-separation effect of the PAS.
Figure 5 shows signal waveforms from four output ports of the WRS when the input signal was fed into input port 3. The 10-Gbit/s NRZ input signal had a wavelength of 1545 nm with an average power of 13 dBm. Dynamic switching between the four output ports (ports 1, 3, 5, and 7) was performed with a period of 160 ns. High-speed and stable wavelength routing with a switching time of less than 5 ns was achieved. The switching time between output ports is sufficiently fast for application to OPS. High-speed switching capability of the monolithic WRS can be attributed to the fast, stable, and low-power switching characteristics of the DRR TL.
We have demonstrated an all-optical wavelength-routing switch with monolithically integrated filter-free TWCs and an 8x8 AWG. To make the device compact, we employed a deep-ridge waveguide structure for the AWG and ring resonators. By using filter-free TWCs, the input optical signal is effectively removed before the AWG. We achieved 1x8 wavelength routing operation of an NRZ signal at 10 Gbit/s and rapid and stable wavelength routing with a switching time of less than 5 ns. These characteristics make the device very promising for use in OPS.
This work was partially supported by the National Institute of Information and Communications Technology (NiCT).
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