Multimode interference (MMI) waveguide switches show promise for switch in optical packet switching (OPS). In this work, we fabricated 1 × 4 InGaAsP/InP MMI waveguide switch device which consists of a 1 × 4 MMI splitter, 4 equally spaced single-mode waveguides with phase shifters, and a 4 × 4 MMI combiner. Good crosstalk and extinction ratio of −14.47 dB and 23.39 dB, respectively, are obtained. In addition, we experimentally demonstrate dynamic switching, and the rise and fall time of 1.4 ns and 1.2 ns, respectively, are obtained.
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Low-power consumption is demanded for electric IP router as well as high bit rate interface beyond 40 Gbit/s. Optical packet switching (OPS) might be a promising solution to these issues. Optical delay line is only an option for buffer memory in the optical domain . There has been an optoelectronic approach to optical buffer, which consists of static random access memory (RAM), optical serial-to-parallel converter, and optical parallel-to-serial converter . Lack of optical RAM for the buffer is one of obstacles to realize OPS. A challenge to all-optical RAM remains. In the R&D program where we are involved, nano-structured optical bit memory is a promising device because the retention time of the memory has been drastically improved up to a few hundred nanoseconds recently . Hence, the two-dimensional array of the optical bit memory can be constructed for the all-optical RAM.
An optical addressor is required in order to store a series of bits of optical packet, an optical pulse sequence, in the all-optical bit memories. As shown in Fig. 1 , the optical addressor can spatially steer and position a light beam of optical pulse to a designated array of optical bit memory. We will present optical switch devices, which can be used for the addressing to the optical bit memory. Requirements of the optical addressor are large port count and high speed response. To meet these requirements, many types of the optical space switch have been proposed because they enable us to obtain a bit rate free and signal format free operations . Examples are the broadcast and select type switch using the SOA gates  and the waveguide switch using the phased-array [6,7]. About the latter switch, it is reported that the average crosstalk is −15.5 dB, the dynamic extinction ratio is larger than 13 dB, and the switching time is less than 10 ns . We have also proposed and demonstrated the optical space switches using the multimode interference (MMI) structure, because of their potentials of large port count, small footprint, and good fabrication tolerance . There have been proposed a wide variety of MMI waveguide switch devices such as type of Si waveguides  and GaAs/AlGaAs waveguides . About the latter type, it is reported that the average crosstalk is −13.6 dB for 1 × 10 device. However, there are few studies on the MMI waveguide switch which consists of InGaAsP/InP waveguides, which could be monolithically integrated with a semiconductor optical amplifier (SOA) to compensate the loss in the switch. Furthermore, there are no experimental results on dynamic switching characteristics.
In this paper, we prepare and experimentally demonstrate the dynamic switching of 1 × 4 InGaAsP/InP MMI waveguide switch. First, the operation principle of optical switch is introduced, followed by the device design and fabrication. Finally, the experimental results of the test optical switch will be presented. Good crosstalk and extinction ratio of −14.47 dB and 23.39 dB, respectively, are obtained. The rise and fall times are 1.4 ns and 1.2 ns, respectively for dynamic switching.
2. Operation principle
A schematic diagram of 1 × N MMI waveguide switch is shown in Fig. 2 . In general, 1 × N switch consists of a single-mode input waveguide, a 1 × N MMI splitter, N equally spaced single-mode waveguides with phase shifters, an N × N MMI combiner, and N equally spaced single-mode output waveguides.
Here, we discuss the operation principle of the proposed MMI waveguide switch. In general, N multiple images are exited at the output of the MMI section in N × N MMI structure based on the self-imaging property when the light signal is launched from the input arms j . The phase relations at the output arms k in relation to the input arms j are given by following expression:
Now, we consider that N light signals with phase relationship of are inserted from N input arms. The field distribution of these light signals at the input of the MMI section is denoted . By assuming as the sum of the field distribution of the single light signal , can be expressed as follows:
For example, when switching the input light to output port 1 in 1 × 4 switch, the relative phases of the four input lights of 4 × 4 MMI combiner set to be (0, −5π/4, -π/4, 0) by using the phase shifters, because the relative phases at the front of 4 × 4 MMI combiner are (0, 5π/4, π/4, 0) when the light is launched reversely from output port 1 . To switch the input light to port 1, required phase changes by the phase shifters are (-π/4, -π, 0, -π/4). In the same way, to switch the input light to port 2, required phase changes are (0, -π/4, -π/4, -π). To switch the input light to port 3, required phase changes are (-π, -π/4, -π/4, 0). To switch the input light to port 4, required phase changes are (-π/4, 0, -π, -π/4).
3. Design and device fabrication
In order to design the proposed switch device, we perform numerical modeling using the finite-difference beam propagation method (FD-BPM). To simplify designing the MMI waveguide switch device, we performed 2-D modeling of the waveguide by the effective index method. Then, we used FD-BPM to calculate the field evolution on the basis of the 2-D switch structure. By using FD-BPM, we got designed 1 × 4 MMI switch device parameter as follows. MMI length (Lmmi), MMI width (We), and waveguide width is 411.8 μm, 14.40 μm, and 1.6 μm, respectively. Each arm is separated by a lateral distance of 2.0 μm to decrease the current outflow among the arms in the actual device. The designed high-mesa waveguide structure is shown in Fig. 3 . Refractive index of InP in clad layer is 3.168, and InGaAsP (1.4Q) in core layer is 3.440. The light intensity pattern in the 1 × 4 MMI waveguide switch device for two different output ports is illustrated in Fig. 4 . Figure 4 shows the output lights, which are switched from input port 1 to output port 1 in (a) and port 2 in (b), respectively, by changing amount of phase shift in phase shifters. Figure 5 shows the light intensity distribution at the output for two different output ports. We obtained good extinction ratio of more than 23 dB for each port in theory.
We fabricated 1 × 4 MMI waveguide switch device on InP with the array structure designed in the previous section. Figure 6 shows the top view of the fabricated switch device. It consists of a single-mode input waveguide, a 1 × 4 MMI splitter, 4 equally spaced single-mode waveguides with phase shifters, a 4 × 4 MMI combiner, and 4 equally spaced single-mode output waveguides. An electrode is deposited on top of each arm in waveguide array for current injection to change the refractive index. By injecting current, we can induce desired phase shift at each arrayed waveguide through the carrier-induced refractive index change in the InGaAsP core layer. The length of the phase shifters is 500 μm.
4. Experimental results
We conducted static switching operation. To switch the light to desired output port of 4 × 4 MMI combiner, the phases of light split by 1 × 4 MMI splitter are changed by phase shifters. Figure 7(a) shows the near field pattern of the output light without applying any current to the phase shifters. Figures 7(b), (c), (d), and (e) show the near field patterns with applying current to the phase shifters for switching to port 1, 2, 3, and 4, respectively. Table 1 summarizes the crosstalks and extinction rations, measured in experiment. The wavelength of the tunable laser diode is set at 1550 nm. The input light is launched into the input port by using a lensed single mode fiber, and the light emerging from output ports are focused into a lensed single mode fiber array. The fiber array outputs are terminated in photodetectors to measure output power. The phase shifters are operated by using four independent current sources. The output power variation for each output port was ± 14%. The typical values of crosstalk and extinction ratio were −14.47 dB and 23.39 dB, respectively.
Figure 8 shows the output power from port 3 against the wavelength of the input light. The output power is normalized so that the maximum power is 0 dB. Figure 8(a) shows the experimental result when we changed only the wavelength of the tunable laser diode. The output power has a peak at 1550 nm, and decreases as the wavelength gets shorter or longer from 1550 nm. The experimental wavelength dependence is in good agreement with the simulation results denoted by (b). This means the fabricated device has wavelength dependence. Figure 8(c) shows the experimental result when we adjusted the injection current values optimally, in order to obtain constant output power against the wavelength. We can see that it is possible to mitigate the decrease of the output power depending on the wavelength by adjusting the injection current values. The decrease of the output power reduced from −2.0 dB to −1.3 dB at 1530 nm, and from −1.7 dB to −1.3 dB at 1570 nm. Figure 8(d) shows the experimental result when we used the device whose MMI length is 8 μm shorter than optimal length. The wavelength where the output power has a peak is longer than that of (a). This means that center wavelength of MMI waveguide switch depends on the MMI length.
Dynamic switching operation by applying the electrical signal pattern to the phase shifter will be practically important. The laser diode outputted at 1550 nm is launched into the input port, and the light from output ports are converted into electrical signals by photodetectors to be observed by an oscilloscope. The phase shifters are operated by using a 4-channel arbitrary waveform generator (AWG) (Tektronix, AWG5004B). The electrical signal pattern from AWG changes at intervals of 20 ns to switch cyclically the light to ports 1, 2, 3, and 4, in order. Figure 9(a) shows the timing chart of the output power with changing the electrical signal pattern to the phase shifters dynamically. We can find that the signal from each output port is high level when the light is switched to the port, and it becomes low when the light is switched to the other port. The typical value of extinction ratio is 10 dB. Figure 10(a) shows the timing chart of the electrical signal pattern to the phase shifters. The voltage of electrical signal changes from 0 mV to 930 mV. Figure 9(b) and (c) show the time variation of the output power at the moment of rising and falling with dynamic switching, respectively. The typical values of rise time and fall time are 1.4 ns and 1.2 ns, respectively. These values of switching time are limited by the changing speed of electrical signal pattern from AWG. Figure 10(b) and (c) show the time variation of the electrical signal to the phase shifter 1 at the moment of rising (port 1 to port 2) and falling (port4 to port 1) with dynamic switching, respectively. Both the rise and fall time of electrical signal are 0.8 ns. The switching time of 1.4 ns and 1.2 ns would be shorter if the changing speed of electrical signal pattern from AWG were higher. We could reduce the switching times by using carrier plasma effect to change the refractive index at phase shifters. The refractive index changes in only a few microseconds by carrier plasma effect.
We have experimentally demonstrated dynamic switching of 1 × 4 InGaAsP/InP MMI waveguide switch. First, the operation principle of optical switch was introduced, followed by the device design and fabrication. Finally, the experimental results of the test optical switch were presented. Good crosstalk and extinction ratio of −14.47 dB, and 23.39 dB, respectively, are obtained. The rise and fall times were 1.4 ns and 1.2 ns, respectively for dynamic switching. These characteristics are adequate to spatially steer and position the light beam of optical packet to the designated array of optical bit memory.
This research has been conducted under the program contract financially supported by the National Institute of Information and Communications Technology (NICT).
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