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Abstract
Mode-division multiplexing (MDM) for on-chip interconnect, as a degree of freedom to enable further scaling the communication capacity, has attracted wide attention. However, selective loading information to the multimode light carriers of MDM systems is not as simple as the situation in wavelength-division multiplexing (WDM). In this paper, we demonstrate a scalable mode-selective modulation device for on-chip optical interconnect. It consists of two functional blocks. In one block, we use carrier-depletion add-drop silicon microring resonators to implement the simultaneous mode de-multiplexing from the multimode bus waveguide and high-speed modulation function. In the other block, we use asymmetric directional coupler based mode multiplexers to restore the modulated signals from fundamental mode to original mode sequences. By this structure, each mode channel from input port is separated and can be processed individually. In other words, we can selectively modulate arbitrary mode channels as requirement. The structure could be scaled to numerous mode channels. As a proof of concept, we design and fabricate a device with four microring resonators and a four-channel mode multiplexer. The insertion losses for all modes are less than 2.1 dB, and the inter-mode crosstalk is lower than −19.7 dB. 25 Gbps on-off key (OOK) electrical signals are utilized to drive the microring resonators, the optical eye-diagrams derived from every mode channels are clear and open. The preliminary demonstration of the device with a 50 Gbps OOK signals is also investigated. Our approach can provide more manipulation flexibility to the multimode optical interconnect.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optical interconnect has advantages such as large capacity, low latency, low power consumption, and it has been a promising alternative to traditional metallic interconnect in many aspects [1]. Among these applications, to meet the explosive bandwidth requirement [2–4], one tremendous challenge is to continuously increase the communication capacity transmitted in mono physical channel. Consequently, wavelength-division multiplexing (WDM) technique has been maturely developed and deployed in optical communication systems [2,3]. Mode-division multiplexing (MDM), as a new form of optical parallelism, is one of the promising technologies to increase the information capacity seamlessly. It allows multiple channels of information to be transmitted using the orthogonal spatial modes in a physical channel [5,6], and is very suitable for on-chip optical interconnect. Various mode multiplexers and de-multiplexers based on silicon waveguides [7–19] have been proposed and demonstrated, other important multimode functional devices such as multimode waveguide crossings [20], multimode waveguide bends [21], and multimode optical switches have also been reported [22–25]. These components guarantee the MDM transmissions [26–28].
Loading signals to multiple optical carriers is a pivotal process in optical interconnect system. In most demonstrations of multimode signal generation and switching, the multiple fundamental-mode optical carriers are first modulated simultaneously before the multiplexing process, then coupled to the multimode bus waveguide by mode multiplexers for subsequent processing [29–31]. However, in real applications, there exist cases that not all of the carrier mode channels coming from laser sources are required to be pre-modulated. As the multimode bus waveguide would pass through a series of functional modules, modules in some regions of the chip require signals at certain mode carriers, while in other regions may not need the signals. In other words, signals should be able to upload or superimpose to arbitrary mode channels temporarily to fulfill the necessity of network. In this circumstance, flexible mode-selective modulation devices are in demand. Nevertheless, different from the WDM system, in which wavelength-selective modulation is convenient to realize. It is not easy to selectively modulate mode carriers in multimode bus waveguide.
In this paper, we demonstrate a mode-selective modulation device by two functional blocks. We utilize carrier-depletion add-drop microring resonators to combine the function of mode de-multiplexer and optical modulators. They enable simultaneous mode de-multiplexing and high-speed signal modulation. Asymmetric directional coupler (ADC) based mode multiplexer is cascaded to restore the modulated signals to the original mode sequences. By this structure, each mode channel from input port is separated and can be processed individually, so we can selectively modulate arbitrary mode channels as requirement. As a proof of concept, we design and fabricate a device constituted by four microring resonators and a four-channel mode multiplexer. The insertion losses are less than 2.1 dB, and the inter-mode crosstalk is lower than −19.7 dB. 25 Gbps on-off key (OOK) electrical signals are utilized to drive the microring resonators, the optical eye-diagrams derived from every mode channels are clear and open. The preliminary demonstration of the device with 50 Gbps OOK signals is also investigated. The structure can be straight-forwardly scaled to numerous mode channels as requirement and can be used to increase the flexibility of optical networks.
2. Principle and design
Figure 1 shows the schematic of a four-channel mode-selective modulation device. It is constituted by two functional blocks. In the left block, four cascaded carrier-depletion add-drop microring resonators with different parameters of waveguides are labeled as MRR1, MRR2, MRR3, MRR4. On the one hand, they are in charge of de-multiplexing the input light carriers at TE3, TE2, TE1, and TE0 modes from multimode input bus waveguide to the fundamental modes at their corresponding drop ports, respectively. In this process, they act as a four-channel mode de-multiplexer. On the other hand, they can realize the modulation function by suitably implementing PN-junction to embed in the microring cavity regions. Plasma-dispersion effect is utilized to tune their refractive index, which endows them high-speed signal modulation functionality at the same time. In detail, high-speed electrical signals from pulse-pattern generator (PPG) are used to drive the microring resonators. The static resonance wavelengths of the microring resonators are tuned to match the wavelengths of the multiple input carriers by the integrated micro-heaters. For one specific microring resonator, if there is no modulation signal, input light will be directly de-multiplexed to the fundamental mode of its drop port, then sent to the mode multiplexer. In contrast, input light would be modulated by the electrical signals and de-multiplexed simultaneously. In the right block, four selectively modulated fundamental mode signals are restored to their original mode sequences at the multimode output bus waveguide by the suitable waveguide connection of a four-channel mode multiplexer. The length of multimode bus waveguide can be prolonged as requirement. Synthesizing the function of two blocks, by inserting the device into any position of multimode interconnect system, it can selectively modulate arbitrary mode channels. As our device works in the region of linear optics, the structures are all reciprocal. We can also achieve the identical function by reversing the physical sequence of the two functional blocks. In such circumstance, input multimode carriers are first de-multiplexed, then modulated and restored to their original mode sequences simultaneously. We cascade ADCs to construct the four-channel mode multiplexer since it has relatively large optical bandwidth. When the input wavelength is changed, we only need to tune the resonance wavelength of microring resonator. It is unnecessary to actively tune the mode multiplexer to match the new wavelength, which decreases the extra power consumption and the difficulty to align the wavelength.
Fig. 1 Schematic of the four-channel mode-selective modulation device (MRR: microring resonator, CW: continuous wave laser, MUX: multiplexer).
Moreover, the structure is easy to scale. To expand the number of mode channels, we can increase the number of cascaded microring resonator units and the channel of ADCs following the structure mentioned above. No additional re-design would be made.
In the following part we briefly introduce the parameter design procedure of the functional blocks. First, we introduce the model of an add-drop microring resonator and the selection of waveguide parameters to realize mode conversion.
The scattering matrix model of an add-drop microring resonator is shown in Fig. 2(a). It is constituted by two straight waveguides, one microring cavity, and has two coupling regions [32]. We label the gap between through-port waveguide and microring waveguide as gap 1, and the gap between drop-port waveguide and microring waveguide as gap 2. The widths of the two straight waveguides are designed to be different to fulfill the phase matching condition among different mode orders. The microring cavity is set to support only fundamental mode to avoid extra optical loss and undesired mode coupling. To simplify the design, we consider the situation of symmetric coupling and ignore the energy loss of the coupling regions. To make a balance between the free spectrum range (FSR, which limits the wavelength channel spacing of signals in WDM applications) and the footprint, we choose the radius of the microring as 20 μm. Considering the loss of certain doping level and radius, we set the attenuation of per unit length in the microring cavity as 10 dB/cm. By the scattering matrix model, the relationship of insertion loss and extinction ratio versus coupling coefficient can be depicted in Figs. 2(b) and 2(c). Here the insertion loss is defined by the loss of signal traveling into the microring from input port and out of the microring from through port or drop port. From the curves of drop port of these two figures we can observe that lower insertion loss and higher extinction ratio at a certain coupling coefficient are antagonistic. To make a tradeoff, we choose the coupling coefficient value as 0.4. The theoretical insertion loss is lower than 1.4 dB and the extinction ratio is about 20 dB at this value.
Fig. 2 (a) Scattering matrix model of an add-drop microring resonator. (b) The relationship of the insertion loss and the coupling coefficient. (c) The relationship of the extinction ratio and the coupling coefficient. (d) The relationship of the effective indices and waveguide widths for different modes.
The microring resonators are functioned as the mode de-multiplexer. To achieve as higher mode conversion efficiency as possible, the phase matching condition should be obeyed. Considering the compatibility of doping structure, the silicon layer is chosen as 220 nm and the rib region with 70 nm in slab thickness is utilized to construct waveguide. Under these geometry structures, the effective indices of different spatial modes in rib waveguides with different widths at the wavelength of 1.55 μm are calculated by finite element method, as shown in Fig. 2(d). The width of the rib waveguide carrying the fundamental mode (TE0) is chosen to be 400 nm, which is denoted by the first circle in the horizontal dotted line. The widths of the waveguides carrying higher-order modes are then determined according to the phase matching condition, which are denoted by the other circles in the horizontal dotted line. The widths of the bus waveguides carrying the TE1, TE2 and TE3 modes are chosen to be 896 nm, 1416 nm and 1916 nm, respectively. Under these parameters, to fulfill the selected coupling coefficient (in our case, the value is 0.4) for different coupling region, their gaps between the microring cavity and bus waveguides are different. We use the finite-different time-domain (FDTD) method to optimize the gap in each situation. The result is given in Table 1. The ADCs of the mode multiplexer have the same width parameters. When the gap of narrow and wide waveguide is chosen as 240 nm, the optimized coupling length for TE1, TE2 and TE3 are 13 μm, 15μm, and 19μm.
Table 1. Structural parameters of the microring resonators
To realize the high-speed modulation functionality, we tune the refractive index of the microring cavity by the plasma dispersion effect [33]. The PN junctions are embedded in the microring cavity. The P-doping concentration and N-doping concentration are 2.3 × 1017 cm−3 and 1.4 × 1017 cm−3 respectively. 2.0 × 1020 cm−3 P-doping concentration and N-doping concentration are formed for Ohmic-contact. In theory, higher Q-factor is required to achieve a relatively large dynamic extinction ratio (ER) with a moderate driving voltage. However, higher Q-factor means a larger photon lifetime, which affects the modulation speed of the microring modulator. The Q-factor is mainly decided by the propagation loss of the ring waveguide and the coupling coefficient between the ring waveguide and the straight waveguide. In order to achieve moderate Q-factors, distance from the heavily-doped regions to the side of rib waveguide and the gap between the straight waveguide edge and the ring waveguide edge is chosen as 600 nm.
3. Fabrication and experimental characterization
The device is fabricated on an 8-inch silicon-on-insulator wafer with a 220-nm-thick top silicon layer and a 3-μm-thick buried silicon dioxide layer at Institute of Microelectronics, Singapore. 248-nm deep ultra-violet photolithography is used to define the patterns and inductively coupled plasma etching is employed to form the silicon waveguides. Corresponding P-doping and N-doping are implemented. 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 titanium nitride with 200-nm-thick is sputtered on the silicon dioxide, and 1-µm-wide “Ω” shaped titanium nitride micro-heaters are fabricated on microring resonators to tune their static resonance wavelengths and compensate wavelength deviations caused by fabrication imperfection. Via holes are etched after depositing a 300-nm-thick silica layer by PECVD. Finally, aluminum wires and pads are fabricated. Figure 3 shows the micrograph of the fabricated device. Except for the two functional blocks shown in Fig. 1, auxiliary ADC-based mode de-multiplexer and multiplexer are set before the multimode input bus waveguide and after the multimode output bus waveguide. The input and output coupling regions are both made to be fundamental-mode to facilitate the functional verification. We label the multimode input and output bus waveguide for the device as I and O, and label the different mode channels in the multimode waveguides as ITEj and O TEj (j = 0, 1, 2, 3).
The experimental setup for characterizing the device is shown in Fig. 4. First, we characterize the static spectra. An amplified spontaneous emission source and an optical spectrum analyzer are connected to the input and output of the device. In the coupling region of the chip, 200-μm-long linearly inverse tapers with 180-nm-width in tip are used to couple the light into and out of the device. DC power supply is utilized to offer voltage to the micro-heaters and PN-junctions by probes. It is used to characterize the thermal tuning efficiencies of the micro-heaters and the modulation efficiencies of the microring modulators.
Fig. 4 Experimental setup (ASE: amplified spontaneous emission, TL: tunable laser, PC: polarization controller, DCPS: direct-current power supply, DUT: device under test, PPG: Pulse pattern generator, OSA: optical spectrum analyzer, DCA: digital communication analyzer, RTO: real-time oscilloscope, VOA: variable optical attenuator).
We fabricated a reference structure which include input/output coupling regions, the auxiliary mode multiplexer and de-multiplexer. The parameters are identical to our demonstrated chip. We can use the spectra of reference device to deduct the influence of coupling region, auxiliary mode multiplexer and de-multiplexer. The normalized transmission spectra illustrate the characteristic from multimode input port I to multimode output port O, as Fig. 5 shown. In detail, in Fig. 5(a), broadband light is first coupled to the TE0 mode of multimode input port I, then flow through the mode-selective modulation device. At the multimode output port O, we can derive the transmission characteristics of the mode channels TE0, TE1, TE2 and TE3. The results are illustrated by the four curves. Among them ITE0 to OTE0 is the transmitted signal, the other three are the crosstalk to the other mode channels. There are periodic peaks in spectra characteristic, which are caused by the microring resonators. The period equals to the FSR of the microring resonators, which is approximately 5 nm. The Q-factors at 1550 nm is approximately 6000. Similarly, Figs. 5(b)-5(d) are the results for broadband light coupled into the TE1, TE2 and TE3 mode of the multimode input port I. It is observed that the light is mainly guided from the TEi channel of input port to the TEi channel of output port (i = 1, 2, 3) as desired. The insertion losses of the structures in charge of TE0, TE1, TE2 and TE3 mode, as the curves of signals shown, are ~1.5 dB, 1.5 dB, 1.8 dB and 2.1 dB, respectively. As the limited extinction ratios and fabrication non-idealities of the microring resonators and mode de-multiplexer, a fraction of input light is leaked to other modes and becomes crosstalk at the corresponding mode channels of output port. From these figures we can see that the inter-mode optical crosstalk of the device is lower than −19.7 dB in the wavelength range from 1525 nm to 1565 nm.
Fig. 5 Normalized static transmission spectra of the mode-selective modulation device for the different mode channels at the multimode output port O when broadband light (1525-1565 nm) is coupled into the mode channels TE0 (a), TE1 (b), TE2 (c) and TE3 (d) of the multimode input port I, respectively.
The structural parameters of micro-heater and doping parameters of the microring resonators are identical for each mode channel, so they have identical thermal tuning efficiencies and modulation efficiencies. The experimental results are illustrated in Fig. 6. When DC voltage is applied to the “Ω” shaped micro-heater, the resonance wavelength of the microring modulator is shifted. The thermal tuning efficiency can be calculated by linear fitting of the relationship of the resonance wavelength shift and the tuning power, which is 12.6 mW/nm. With the reversed-bias voltage is applied to the PN-junction of the microring modulator, its resonance wavelength is also red-shifted under different voltages. The modulation efficiency, which is represented by VπLπ , can be calculated by the following equation [34–36]:
Where δV is the applied voltage, δλ is the resonance wavelength shift, FSR is the free spectral range and L is the total length of the phase shifter (in our case, this value is 90% circumference of the microring cavity). The VπLπ of the microring modulator varies from 0.85 V · cm to 1.64 V · cm with the increase of the applied voltage [Fig. 6(d)].Fig. 6 (a) Resonance wavelength shift of the microring modulator by thermal tuning. (b) Linear fitting and calculation of the thermal tuning efficiency. (c) Spectral responses under different applied voltages. (d) VπLπ under different applied voltages.
After that we demonstrate the dynamic mode-selective modulation function. Experimental setup is shown in the lower part of Fig. 4. Continuous-wave light is generated by tunable laser and coupled into the device. 25 Gbps pseudorandom binary sequence electrical signal with the pattern length of 29-1 is applied to the microring resonators through RF probe. Based on the DC response and the limitation of our electrical signal amplifier, the Vp-p of the RF driving signal is set to be 5.0 V with a −2.5 V DC bias voltage. The output modulated optical signals from the chip is sent to the digital communication analyzer and real-time oscilloscope for eye-diagram observation and bit-error-rate (BER) measurement. Figure 7(a) shows the eye diagrams and the BER curves of the four different mode channels flowing through device. The wavelengths are all set at 1550 nm. Clear and open eye diagrams verify the mode-selective modulation functionality for the four different mode channels of the multimode bus waveguide. We also investigated the WDM manipulation by injecting four wavelengths with the channel spacing of the microring’s FSR. Output signals are de-multiplexed by an external wavelength filter. The eye-diagrams are shown in Fig. 7(b), there are no observable deterioration in quality. Moreover, the preliminary demonstration of the device with 50 Gbps OOK driving signal is characterized at 1550 nm, the result is shown in Fig. 7(c), at this speed the eye-diagrams still don’t have too much deterioration.
Fig. 7 (a) Measured 25 Gbps eye-diagrams and BER characteristic of the mode-selective modulation device for four different mode channels at 1550 nm. (b) Measured 25 Gbps eye-diagrams at four wavelengths for four different mode channels. (c) Measured 50 Gbps eye-diagrams of the mode-selective modulation device for four different mode channels at 1550 nm.
4. Conclusion
In conclusion, we demonstrate a scalable mode-selective modulation device for optical interconnect. It consists two functional blocks. In the first block we realize the simultaneous high-speed signal modulation and mode de-multiplexing functionality exploiting carrier-depletion add-drop silicon microring resonators. In the second block we configure a mode multiplexer with suitable waveguide connections of first block to restore the multiple de-multiplexed fundamental modes to their original mode sequences. As a proof of concept, the device with four mode channels are constructed by using four microring resonators and a four-channel mode-multiplexer. The on-chip insertion losses for all modes are less than 2.1 dB, and the inter-mode crosstalk of the device is lower than −19.7 dB. 25 Gbps on-off key (OOK) signals are implemented to drive the microring resonators. Clear and open eye-diagrams can be observed. The device can be flexibly inserted into the multimode optical links to selectively process signals at arbitrary mode channels in multimode application scenario.
Funding
National Key R&D Program of China (2017YFA0206402, 2016YFB0402501); China National Funds for Distinguished Young Scientists (61825504); National Natural Science Foundation of China (NSFC) (61704168, 61575187, 61535002, 61505198); The Opened Fund of the State Key Laboratory of Integrated Optoelectronics (IOSKL2018KF15).
Acknowledgment
We acknowledge Dr. Huifu Xiao for assistance in figure preparation and Dr. Wei Chang for useful discussion.
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