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Abstract
We propose a 2 × 2 multimode optical switch, which is composed of two mode de-multiplexers, n 2 × 2 single-mode optical switches where n is the number of the supported spatial modes, and two mode multiplexers. As a proof of concept, asymmetric directional couplers are employed to construct the mode multiplexers and de-multiplexers, balanced Mach-Zehnder interferometer is utilized to construct the 2 × 2 single-mode optical switches. The fabricated silicon 2 × 2 multimode optical switch has a broad optical bandwidth and can support four spatial modes. The link-crosstalk for all four modes is smaller than −18.8 dB. The inter-mode crosstalk for the same optical link is less than −22.1 dB. 40 Gbps data transmission is performed for all spatial modes and all optical links. The power penalties for the error-free switching (BER<10−9) at 25 Gbps are less than 1.8 dB for all channels at the wavelength of 1550 nm. The power consumption of the device is 117.9 mW in the “cross” state and 116.2 mW in the “bar” state. The switching time is about 21 μs. This work enables large-capacity multimode photonic networks-on-chip.
© 2017 Optical Society of America
1. Introduction
The rapid evolution of multi-core processors puts forward tremendous demands on a systematic solution to on-chip interconnect. Photonic network-on-chip (NoC) is considered as a promising solution to large-capacity, low-latency and low-power-consumption data exchange among multiple processor cores [1,2]. Lots of architectures for photonic NoC have been studied, such as Mesh, Fat-Tree, Clos and Crossbar [3–6].
In order to meet the explosively increasing demands on the data transmission capacity, wavelength division multiplexing (WDM) can be utilized and wavelength multiplexer and de-multiplexer are necessary to be integrated into the photonic NoC [7–9]. Correspondingly, as a building block of the photonic NoC, optical router is required to be capable of switching the WDM data among different optical links simultaneously. Such optical routers based on Mach-Zehnder optical switches or microring optical switches have been reported [10–20]. Compared with the microring optical switch, Mach-Zehnder optical switch has a larger optical bandwidth, which makes the optical routers based on it more suitable for WDM application. To further expand the data transmission capacity, mode division multiplexing (MDM) is considered to be introduced as another degree of freedom, which allow multiple channels of information to be transmitted by utilizing the orthogonal eigen-modes of a waveguide. Many multimode devices like mode multiplexers/de-multiplexers, multimode waveguide crossings and multimode waveguide bends have been reported [21–33]. For the photonic NoCs adopting WDM and MDM, the optical router is required to have the ability of switching the WDM and MDM data among different optical links simultaneously. As a constituent of the optical router, a 1 × 2 silicon multimode optical switch based on microring optical switches and mode multiplexer and de-multiplexer has been demonstrated [34]. While, the optical router constructed by 2 × 2 multimode optical switches is more compact and more power-efficient as it is able to manipulate two optical links simultaneously. In this paper, we propose a 2 × 2 multimode optical switch, which is composed of two mode de-multiplexers, n 2 × 2 single-mode optical switches and two mode multiplexers. The demonstrated silicon 2 × 2 multimode optical switch is able to manipulate multiple WDM channels and four spatial modes simultaneously.
2. Optical switch design
A 2 × 2 multimode optical switch has two input multimode waveguides marked as I1, I2 and two output multimode waveguides marked as O1, O2. As shown in Fig. 1(a), two groups of mode-multiplexed optical signals are guided from the input multimode waveguides I1/I2 to the output multimode waveguides O2/O1 when the optical switch is in the “cross” state. Two groups of mode-multiplexed optical signals are guided from the input multimode waveguides I1/I2 to the output multimode waveguides O1/O2 when the multimode optical switch is in the “bar” state.
Fig. 1 (a) General scheme of the 2 × 2 multimode optical switch. (b) Detailed architecture of the 2 × 2 multimode optical switch (M-MUX: mode multiplexer; M-DEMUX: mode de-multiplexer; MM-OS: multimode optical switch; SM-OS: single-mode optical switch; I1Mi and I2Mi: the auxiliary input single-mode waveguide for the ith spatial mode of the input multimode waveguides I1 and I2; O1Mi and O2Mi: the auxiliary single-mode waveguide for the ith spatial mode of the output multimode waveguides O1 and O2).
To realize the “group switching” function, we propose a 2 × 2 multimode optical switch, as shown in the grey rectangle in Fig. 1(b). Two groups of multiplexed optical signals with n spatial modes is injected from the input multimode waveguides I1 and I2. Then they are de-multiplexed to 2n fundamental-mode optical signals by two mode de-multiplexers. After that n 2 × 2 single-mode optical switches marked as SM-OSi (i = 1, …, n) are utilized to switch data between two input mode de-multiplexers and two output mode multiplexers. Note that all the single-mode optical switches are placed to their “cross” or “bar” states simultaneously so that the “group switching” between two input multimode waveguides and two output multimode waveguides can be accomplished. Finally, 2n fundamental modes are converted to the corresponding spatial modes by two mode multiplexers and the mode sequences are kept constant.
The auxiliary mode multiplexers and de-multiplexers outside the 2 × 2 multimode optical switch are unnecessary and only utilized for device characterization. I1Mi and I2Mi (i = 1, …, n) denote the auxiliary input single-mode waveguide for the ith spatial mode of the input multimode waveguides I1 and I2. O1Mi and O2Mi denote the auxiliary single-mode waveguide for the ith spatial mode of the output multimode waveguides O1 and O2.
As a proof of concept, the multiplexed mode number is chosen to be four. Several structures have been utilized to realize the mode multiplexer and de-multiplexer, such as adiabatic coupler [25], asymmetric Y-junction [26] and asymmetric directional coupler (ADC) [28]. Here, we use ADC to construct the mode multiplexer and de-multiplexer as it is more compact and has a better scalability. Considering the compatibility with the electro-optic tuning or modulation structures in the future, rib waveguide with 70 nm in slab thickness is utilized to construct the ADCs. The width of the rib waveguide carrying the TE0 mode is chosen to be 400 nm and the widths of the rib waveguides carrying the TE1, TE2 and TE3 modes are chosen to be 916 nm, 1416 nm and 1916 nm, respectively. The optimized coupling lengths for the TE1, TE2 and TE3 modes are 13 μm, 15 μm and 19 μm, respectively. Adiabatic tapers with the lengths of 10 μm is utilized to connect the waveguides with different widths. Previous work indicates that the mode multiplexer and de-multiplexer based on ADC have relatively large optical bandwidths, which are suitable for WDM application [28]. Although both Mach-Zehnder optical switch and microring optical switch can manipulate WDM optical signals [11, 13], the former has a better performance in fabrication tolerance and temperature sensitivity. Two MMI couplers with 6 μm in width and 43 μm in length and two balanced thermo-optic phase shifters with 200 μm in length are utilized to construct the 2 × 2 Mach-Zehnder optical switch.
3. Device fabrication
The device is fabricated on an 8-inch silicon-on-insulator (SOI) wafer with a 220-nm-thick top silicon layer and a 3-μm-thick buried silicon dioxide layer at the Institute of Microelectronics, Singapore. 248-nm deep ultraviolet photolithography is used to define the patterns and inductively coupled plasma etching is employed to form the silicon waveguides. Single-mode rib waveguide is 400 nm in width, 220 nm in height and 70 nm in slab thickness which only supports the fundamental quasi-TE mode. A 1500-nm-thick silica layer is deposited on the silicon layer by plasma-enhanced chemical vapor deposition (PECVD), which is used to prevent the absorption of the optical field by the metal. Then a 150-nm-thick titanium nitride is sputtered on the separate layer, and 1-µm-wide titanium nitride metals are fabricated on the two arms of the Mach-Zehnder optical switch as the micro-heaters to realize the function of thermal tuning. Via holes are etched after depositing a 300-nm-thick silica layer by PECVD. Finally, aluminum wires and pads are fabricated. Figure 2 shows the micrograph of the fabricated device, which has a footprint of 740 µm × 700 µm.
4. Experimental characterization
The experimental setup for characterizing the device is shown in Fig. 3. The spectrum response of the device is measured by an amplified spontaneous emission source and an optical spectrum analyzer. Direct-current powers are utilized to calibrate the voltages and states of each single-mode switch.
Fig. 3 Experimental setup for characterizing the device (ASE: amplified spontaneous emission; TL: tunable laser; PC: polarization controller; DCP: direct-current powers; DUT: device under test; PPG: pulse pattern generator; AFG: arbitrary function generator; OSA: optical spectrum analyzer; DCA: digital communication analyzer; EDFA: erbium-doped fiber amplifier; MD: modulator; RTO: real-time oscilloscope).
Inverse taper with a 180-nm-wide tip is used to increase the coupling efficiency between the waveguide and the lensed fiber. The coupling loss between the device and the lensed fiber with the spot size of 5 μm is about 3.0 dB. To eliminate the influence of the auxiliary mode multiplexers and de-multiplexers on the transmission spectra of the multimode optical switch, we fabricate a reference structure only including one mode multiplexer and one mode de-multiplexer which have the same structural parameters with the auxiliary parts outside the multimode optical switch. Figure 4(a) shows the transmission spectra of the reference structure. Note that the noise refers to the sum of the noise leaked from other modes to the destination mode.
Fig. 4 Transmission spectra for the reference structure including one mode de-multiplexer and one mode multiplexer (a). Normalized transmission spectra for the optical links I1 to O1 (b), I1 to O2 (c), I2 to O1 (d) and I2 to O2 (e).
We use the measured transmission spectra of the reference structure to normalize those of the device including the auxiliary mode multiplexers and de-multiplexers. The normalized transmission spectra for the optical links I1 to O1, I1 to O2, I2 to O1 and I2 to O2 are shown in Figs. 4(b)-4(e). The crosstalk for the specific mode in one optical link is mainly decided by the inter-mode crosstalk for the same optical link, the link-crosstalk for the same mode and the inter-mode crosstalk from the other optical link. The inter-mode crosstalk for the same optical link of the multimode optical switch is caused by the inter-mode crosstalk of the mode de-multiplexers and multiplexers and should be as the same as that of the reference structure. We can judge the performance of the multimode optical switch in this aspect from Fig. 4(a). Moreover, the inter-mode crosstalk from the other optical link is almost negligible compared with the link-crosstalk for the same mode. As shown in Figs. 4(b)-4(e), the link-crosstalk for all four modes in the four optical links I1 to O1, I1 to O2, I2 to O1 and I2 to O2 is smaller than −18.8 dB, which is mainly caused by the single-mode optical switches and the waveguide crossings.
The transmission spectra for all optical links of the multimode optical switch show slight wavelength dependence, which is not only caused by the dispersion of the ADC based mode multiplexers/de-multiplexers and the Mach-Zehnder optical switches but also caused by the dispersion of the silicon waveguide. Moreover, manufacturing imperfection makes the central wavelengths of the ADC based mode multiplexers/de-multiplexers and the Mach-Zehnder optical switches deviate from their theoretical values, which further results in the transmission fluctuation. The propagation loss of the waveguide crossing is about 0.3 dB and the propagation loss of the Mach-Zehnder optical switch is about 0.5~1.3 dB in the wavelength of 1525-1565 nm. The detailed propagation loss and optical crosstalk for all optical links of the multimode optical switch in the wavelength of 1525-1565 nm are shown in Table 1.
Table 1. Propagation loss and optical crosstalk for all optical links of the multimode optical switch in the wavelength of 1525-1565 nm (PL: propagation loss, OCT: optical crosstalk).
The experimental setup for the data transmission is shown in the bottom part of Fig. 3. 40 Gbps pseudo-random binary sequence with a length of 231-1 is generated by a pulse pattern generator and then adopted to drive a LiNbO3 optical modulator. Continuous-wave light generated by a tunable laser is first modulated by a LiNbO3 optical modulator and then coupled into the input waveguide one by one. The output optical signal from the device is amplified by an erbium-doped fiber amplifier and a tunable filter is used to reduce the background noise of the optical signal before it is sent into a digital communication analyzer for eye diagram observation.
Figure 5 shows the eye diagrams for the data transmission through the optical links I1Mi to O1Mi and I2Mi to O2Mi in the “bar” state and the optical links I1Mi to O2Mi and I2Mi to O1Mi in the “cross” state (i = 1, …, 4) at the wavelength of 1550 nm. The eye diagrams are clear and open at the speed of 40 Gbps and their extinction ratios (ERs) are not much deteriorated compared with that of the back-to-back signal. Among them TE3 mode has the lowest ER and TE2 and TE1 modes have similar ERs. The tendency is the same with the propagation losses of the corresponding optical links. WDM data transmission through the optical link I1M1 to O1M1 is also performed. Based on ITU-T G. 692 standard, there are 50 wavelength channels with the channel spacing of 100 GHz in the wavelength of 1525-1565 nm. The ERs of the eye diagrams for the 40 Gbps data transmission at the 50 wavelength channels fluctuate from 13.6 dB to 16.6 dB.
Fig. 5 40 Gbps eye diagrams for the data transmission through the optical links I1Mi to O1Mi and I2Mi to O2Mi in the “bar” state and the optical links I1Mi to O2Mi and I2Mi to O1Mi in the “cross” state (i = 1, …, 4) at the wavelength of 1550 nm.
The bit error rates (BERs) for the 25 Gbps data transmission through the optical links I2Mi to O1Mi in the “cross” state and the optical links I1Mi to O1Mi in the “bar” state (i = 1, …, 4) at the wavelength of 1550 nm are measured [Fig. 6(a)]. The power penalties for the error-free switching (BER<10−9) are 0.7 dB and 0.4 dB for the TE0 mode, 1.6 dB and 0.8 dB for the TE1 mode, 1.6 dB and 1.0 dB for the TE2 mode, 1.8 dB and 1.4 dB for the TE3 mode in the “cross” and “bar” states. Further, the power penalties for the optical link I2M4 to O1M4 in the “cross” state are 2.3 dB, 1.8 dB and 1.9 dB at the wavelengths of 1530 nm, 1550 nm and 1565 nm [Fig. 6(b)], respectively.
Fig. 6 (a) BERs for the 25 Gbps data transmission through the optical links I2Mi to O1Mi in the “cross” state and the optical links I1Mi to O1Mi in the “bar” state (i = 1, …, 4) at the wavelength of 1550 nm. (b) BERs for the 25 Gbps data transmission through the optical link I2M4 to O1M4 at the wavelength of 1530 nm, 1550 nm and 1565 nm.
The power consumptions of the four single-mode optical switches in different working states are listed in Table 2. The tuning power variances are caused by different initial phase differences between the two arms for different single-mode optical switches, which is caused by the fabrication imperfection. The power consumption of the multimode optical switch is 117.9 mW in the “cross” state and 116.2 mW in the “bar” state.
Table 2. Driving voltages and power consumptions of the single mode optical switches.
Square-wave electrical signal from an arbitrary function generator is applied to the single-mode optical switch and its response time can be achieved directly by a real-time oscilloscope. The response speed of each optical link is decided by the specific single-mode optical switch and the response speed of the multimode optical switch is decided by all single-mode optical switches. More specifically, the response speed of the multimode optical switch is decided by the slowest single-mode optical switch. We characterize all single-mode optical switches and find that their response times are around 21 μs.
5. Conclusion
In conclusion, we propose an optical switch compatible with wavelength-division multiplexing and mode-division multiplexing for photonic networks-on-chip. The demonstrated silicon 2 × 2 multimode optical switch has a broad optical bandwidth and can support four spatial modes. The link-crosstalk for all four modes is smaller than −18.8 dB. The inter-mode crosstalk for the same optical link is less than −22.1 dB. 40 Gbps data transmission is performed for all spatial modes and all optical links. The power penalties for the error-free switching (BER<10−9) at 25 Gbps are less than 1.8 dB for all channels at the wavelength of 1550 nm. The power consumption of the device is 117.9 mW in the “cross” state and 116.2 mW in the “bar” state. The switching time is about 21 μs. This work enables large-capacity multimode photonic networks-on-chip.
Funding
Program 863 (2015AA017001, 2015AA015503); National Key R&D Program of China (2016YFB0402501, 2017YFA0206402); National Natural Science Foundation of China (NSFC) (61505198, 61535002, 61235001, 61575187, 61377067).
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