1. Introduction

The capacity of single-mode based optical transmission systems is approaching the theoretical Shannon limit [1]. Alternatively, the mode division multiplexing (MDM) technology has been explored in recent years, for the platforms of both optical fibers and integrated chips. Optical fiber based researches are concentrated on multi-core and multi-mode fibers [2, 3 ], while the integrated chip based researches are targeting on the design of high-performance, multifunctional mode convertor and multiplxer. The silicon photonic is commonly regarded as a promising solution for next generation optical network due to its low cost, high refractive index contrast, and compatibility with matured Complementary Metal-Oxide-Semiconductor (CMOS) technologies [4]. In reported silicon integrated mode convertor/multiplexer schemes, the multimode interference (MMI) [5, 6 ] and Y –branch [7, 8 ] based schemes are always in large footprint, and can only support 2 modes. The directional coupler (DC) based mode multiplexer [9–11 ] is compact and supports numerous modes, it however suffers from the poor fabrication tolerance due to its sensitivity to the coupling length and waveguide width. In contrast, by introducing the resonator structure, the microring resonator (MRR) based mode multiplexer reduces the sensitivity to the coupling length [12, 13 ], and additional wavelength division multiplexing (WDM) filtering functionality can be simultanously realized. Furthermore, advanced compenent such as integrated multimode switch which can simultaneously process different wavelengths and modes is reported based on this scheme [14]. However, the reported MRR based mode multiplexer shows relatively large mode crosstalk and low extinction ratio, which limit its application in optical communication network.

On the other hand, in order to reduce the number of the discrete devices and achieve high integration level, photonic interconnection between long haul transmited fiber and integrated chip with high multiplexing level and additional advanced optical signal processing functionality is highly desired [15, 16 ]. As the phase encoded signal is widely utilized in long-haul fiber-optic communication networks because of its high robustness to propagation nonlinearities [17], while in some occasions, the intensity modulation formats combined with direct detection are preferred for on-chip optical communication, due to the low cost and easy implementation. Hence, an on-chip interconnection supporting both multi-dimensional multiplexing technology and optional phase to intensity modulation format conversion (defined as demodulation) is quite desirable.

In this paper, we propose and demonstrate an on-chip WDM compatible MDM interconnection between the long-haul fiber communication network and short-reach silicon chip circuit. By improving and optimizing the structure of the MRR based mode multiplexer, WDM compatible and multi-functional MDM system with high performance is successfully realized. The proposed circuit is fabricated using standard CMOS technology, which is promising for large-scale integration. Four wavelengths non return-to-zero differiential phase shift keying (NRZ-DPSK) signals (each at 10Gb/s) from 3 single-mode-fibers (SMFs) are lunched into the chip and successfully mode multiplexed, and the wavelength conflict for multipath WDM signal can be avoided. An optional phase to intensity demodulation function can also be achieved simultaneously. Measured results show open and clear eye diagrams for both switching states, and the bit error rate (BER) measurements indicate reasonable power penalties and performance variation for all wavelengths and modes. The proposed flexible scheme can be used at the interface of long-haul and on-chip communication systems. To be noted, both devices proposed in [14] and this work can be utilized as flexible multifunctional components in advanced MDM networks, but the functionalities, device structures and application occasions demonstrated are different. Reference [14] proposed a channel waveguide racetrack MRR structure to perform the switching function between different modes and wavelengths, which is utilized as an advanced switching compenent within on-chip MDM networks. The device structure proposed in this work is optimized into ridge waveguide MRR without racetrack, to improve the performance of the MRR mode convertor. Additionly, optional phase to intensity demodution function can be realized, along with the mode conversion/multiplexing, which is utilized at the interface of long-haul fiber network and short reach on-chip multimode network. Furthermore, the function here we proposed can be flexiblely controlled according to different application occasions. The preliminary result was first reported in Conference and Lasers and Electro-Optics 2015 [18] and this paper presents more comprehensive design and results.

2. Design

2.1 Principle

The schematic diagram of the proposed circuit is explained in Fig. 1(a) . The WDM DPSK signals at network nodes A, B and C transmitted from the fiber are lunched into the proposed integrated interconnection which can convert these WDM single-mode signals at same wavelengths into WDM-MDM signals. Furthermore, a flexible DPSK demodulation can be achieved within the circuit by switching the mode converters. The structure of the proposed interconncertion is presented in Fig. 1(b). Signals are lunched into the input ports (I-1, I-2 or I-3) and coupled into the chip via the grating coupler as TE₀ mode. Then those TE₀ mode signals transmit into the MRR based mode converter and convert into the desired mode according to the input port. Adiabatic tapers are utilized to connect the waveguides supporting different modes. By combining the output signals from each drop port, mode multiplexed signals can be achieved.

 

Fig. 1 (a) Shematic and (b) structure of the proposed circuit; (c) structure and parameter description of the TE₀-TE₁ MRR based mode convertor.

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Taking the TE₀-TE₁ MRR as an representative, the structure and parameter description are shown in Fig. 1(c). According to the phase matching condition, the input waveguide and the ring waveguides are set to be single-mode, and the width of the output waveguide (the MRR drop port), is chosen to support higher mode, achieving the TE₀-TE₁ mode conversion. For the case of TE₀-TE₂ conversion, the output waveguide width is accordingly set wider. Based on these designs, the MRRs for MDM multiplexing/de-multiplexing can be designed with identical parameters (Q factor, free spectral range, et.al) but different mode conversion functions.

The proposed MRR based circuit can simultaneously demodulate the DPSK signals optionally. This optional demodulation functionality can be achieved by slightly tuning the resonance wavelength of the MRR, which can be realized by the thermo-optic effect. As seen in the Fig. 2 , when the resonance wavelength is aligned with the input DPSK signals, the input DPSK signals will transmit through the MRR only experiencing a filtering processing, without demodulation. When the resonance wavelength is detuned appropriately with the input DPSK signals, the input DPSK signals will be demodulated into on-off-keying (OOK) formats. By precisely detuning the resonator wavelength of the MRR, the carrier of the NRZ-DPSK signal, which corresponds to the phase persistence part of the waveform, is suppressed deeply while the high order optical frequencies, which correspond to the phase transition part of the waveform, are extracted. Consequently, the phase information is translated into amplitude pulses. In order to ensure the performance for both cases, the 3 dB bandwidth of the MRR should be appropriately designed, according to the demand of the bit rate and the WDM spacing [17]. For WDM DPSK signals (each at 10G/s) demodulation, a 3 dB bandwidth of ~0.1nm will be suitable.

 

Fig. 2 Principle of the optional demodulation function.

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2.2 Optimization

In order to realize a high performance interconnection, the mode conversion efficiency, mode dependent loss and mode crosstalk should be comprehensively considered. Compared to the DC based mode convertor, MRR based scheme introduces the extra resonator structure, which reduces the sensitivity of the coupling length in the DC based scheme [13]. However, introducing such resonator structure will cause insufficient coupling strength between the straight waveguide and the ring bend waveguide, resulting in low mode conversion efficiency. A straightforward solution is to introduce the racetrack structure to compensate the insufficient coupling strength [12]. However, there are two drawbacks. Firstly, the racetrack structure will introduce excess loss in the resonator loop due to modes between the racetrack and bend waveguides cannot be perfectly matched. Consequently, the high conversion efficiency, high ER and narrow 3 dB bandwidth can hardly be realized. Secondly, as the coupling length of the asymmetrical waveguides increased at the output, more undesired modes may be excited due to the imperfect fabrication, resulting in excess mode crosstalk.

Based on the above considerations, the racetrack is removed in our design. In order to achieve a sufficient coupling strength in no-racetrack ring structure, we utilized the modified ridge waveguide in Fig. 3(a) instead of the conventional channel waveguide structure in Fig. 3(b). Due to the coupling strength between the ridge waveguides is much larger than the case of channel waveguides, the sufficient coupling strength can be achieved while coupling length can be significantly reduced. Furthermore, compared to the channel waveguide, the ridge waveguide relates to a flatter relationship between effective index and waveguide width thanks to the phase matching condition, indicating larger fabrication tolerance of the waveguide width. The comparasions for the channel and ridge waveguides are calculated and shown in Fig. 3(c).

 

Fig. 3 (a) Modified ridge waveguide (b) conventional channel waveguide (c) simulated effective index to waveguide width relationship for both two waveguide designs.

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3. Fabrication

The proposed circuit is designed and fabricated based on the silicon-on-insulator (SOI) wafer with top silicon layer of 220 nm and SiO₂ layer of 2 µm. 248 nm deep ultraviolet photolithography and inductively coupled plasma (ICP) etching are used to form the waveguide structure. Etching depth of the ridge waveguide is 130 nm. The widths of single-mode waveguide are chosen as 500 nm, while the multimode waveguide are designed as 1.1 and 1.7 μm for TE₁ and TE₂ mode, respectively. The radius and the gap of the MRRs are chosen to be 50 μm and 0.3 μm, respectively. In order to tune the MRRs to align with the WDM channels and thereby optimize the performance of the optional demodulation function, an integrated TiN heater is fabricated on top of each microring. The thickness and the width of the heaters are 200 nm and 5 µm, respectively. The grating couplers (GCs) are used to couple light into and out of the chip, with coupling loss of ~4.5 dB and 1 dB bandwidth of ~20 nm. The layout of the fabricated device is illustrated in Fig. 4 . In order to evaluate the function and performance of the circuit, an extra mode multiplexer is designed to de-multiplex the obtained MDM signals. The circuit were fabricated at the Insititute of Microelectronics (IME), Singapore.

 

Fig. 4 Microscope image of the fabricated device. Inset: image of the single MRR to show the structure in details.

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4. Measurement

4.1 Device characterization

In order to verify the performance of the fabricated circuit, the transmission spectra at the three output ports for signal injection on each of the three input ports are measured, as shown in Fig. 5. I1, I2 and I3 stand for the inputs of TE₀, TE₁ and TE₂ mux, and O1, O2 and O3 stand for the outputs of TE₀, TE₁ and TE₂ demux, respectively. The positions of the ports are indicated in Fig. 4. The free spectral range (FSR) and 3 dB bandwidth of the MRR are measured to be 2 and 0.1 nm, respectively. More than 20 dB ER can be found for all three MRRs. Insertion loss is measured to be 1.5, 3.5 and 3 dB for TE₀-TE₀, TE₀-TE₁ and TE₀-TE₂ mode conversions, respectively, and the mode crosstalk is measured to be −32, −26 and −20 dB for cases of aboved mentioned conversions.

 

Fig. 5 The measured transmission spectra at (a) O1 (TE₀); (b) O2 (TE₁); (c) O3 (TE₂) for signal injection on each of the three input ports.

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4.2 Signal demodulation switching experimental results

The experimental setup is illustrated in Fig. 6 . The WDM NRZ-DPSK signals are used to test the proposed circuit. Four-channel continuous wave laser lights (1548, 1550, 1552 and 1554 nm) are separately modulated in NRZ-DPSK format via the Mach-Zehnder modulators (MZMs), and combined by the array waveguide grating (AWG). The bit rate of each wavelength is 10 Gb/s, and the bit pattern length is 2³¹-1. The Erbium Doped Fibre Amplifier (EDFA) and attenuator (ATT) are utilized to optimize the output power for a fair comparison, and the band pass filter is used to filter out wavelength of interest for measurement.

 

Fig. 6 The experimental setup for optional WDM DPSK signals demodulation. The insets show eye diagram of the signal before and after processing in the chip. Two situations are demonstrated (a) ring resonating wavelength aligned to the DPSK signals; (b) ring resonating wavelength detuning to the DPSK signals. (PC: Polarization Controller, OSA: Optical Spectrum Analyzer, CSA: Communication Signal Analyzer)

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By aligning the resonance wavelength with the input WDM DPSK signals, as shown in Fig. 6(a), the proposed circuit works only as a mode multiplexer for multipath WDM signals. On the other hand, when the ring resonance wavelength appropriately detuned with the input signal, as shown in Fig. 6(b), the WDM DPSK signals will be further demodulated. The detuned value is set to be ~0.11 nm. The measured eye diagrams of the output signals for 4 wavelengths and 3 modes are shown in Fig. 7 , on both detuned and aligned states. For the detuned state, the input phase modulated signals can be demodulated to intensity ones with clear and open eyes in the cases of different mode conversions. For the aligned state, the output signals remain in the DPSK modulation formats with clear and open eyes. These indicate a good optional demodulation performance of the proposed circuit. The input and output optical spectra for three mode cases in two states are measured in Figs. 8(a)-8(c) and Figs. 9(a)-9(c) , respectively. Figures 8(a)-8(c) are the results of ring detuned state, for TE₀ TE₁ and TE₂ cases, respectively. In Fig. 8, by precisely detuning the ring resonance wavelength with the input signals, the input DPSK signals (black line) can be demodulated into the output on-off-keying (OOK) format (red line). On the other hand, Figs. 9(a)-9(c) are the results of ring aligned state, for TE₀ TE₁ and TE₂ cases, respectively. Similar curves can be observed for all wavelengths and mode conversions, indicating a similar performance.

 

Fig. 7 The measured input and output eye diagrams for different modes, wavelengths and states.

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Fig. 8 WDM DPSK signals demodulation (detuned state). The measured (a) to (c): corresponding transmission spectra for TE₀-TE₀, TE₀-TE₁ and TE₀-TE₂ mode conversions; (d) measured BER results.

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Fig. 9 WDM DPSK signals filtering (aligned state). The measured (a) to (c): corresponding transmission spectra for TE₀-TE₀, TE₀-TE₁ and TE₀-TE₂ mode conversions; (d) measured BER results.

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The BER measurements in detuned state are performed for the output signal with different modes and wavelengths, respectively. The results are plotted in Fig. 8(d). Less than 1 dB variation of the received powers can be found for three modes and four wavelengths, indicating a good demodulation performance for all three modes, on top of the mode conversions. Additionally, since BER measurements can only work on the intensity signal, a commercial delay interferometer (DI) was utilized to demodulate the DPSK signal in aligned state. The results are plotted in Fig. 9(d). The back-to-back case is also measured as a reference. The average power penalty on 4 wavelengths is measured to be −1.5, −3 and −3.5 dB for TE₀-TE₀, TE₀-TE₁ and TE₀-TE₂ mode conversions, respectively.

5. Conclusion

In summary, we have proposed and experimentally demonstrated the on-chip WDM compatible MDM interconnection with optional demodulation function. High performance MDM circuit has been designed and successfully achieved on the standard CMOS line, which is promising for large-scale integration. Furthermore, optional phase encoded signal demodulation can be simultaneously achieved within the proposed circuit. Four wavelengths NRZ-DPSK signals (each at 10Gb/s) from 3 SMFs are lunched into the chip and successfully processed. Measured results show open and clear eye diagrams for both cases. The proposed flexible scheme can be used at the interface of long-haul and on-chip communication systems.