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Abstract
We propose and demonstrate a reconfigurable and tunable chip-scale comb filter and (de)interleaver on a silicon platform. The silicon-based photonic integrated device is formed by Sagnac loop mirrors (SLMs) with directional couplers replaced by multi-mode interference (MMI) assisted tunable Mach-Zehnder interferometer (MZI) couplers. The device can be regarded as a large SLM incorporating two small SLMs which form a Fabry–Perot (FP) cavity. By appropriately adjusting the micro-heaters in tunable MZI couplers and cavity, switchable operation between comb filter and (de)interleaver and extinction ratio and wavelength tunable operations of comb filter and (de)interleaver are achievable by thermo-optic tuning. Reconfigurable comb filter and (de)interleaver is demonstrated in the experiment. The central wavelength shifts of comb filter and (de)interleaver are demonstrated with wavelength tuning efficiencies of ~0.0224 nm/mW and ~0.0193 nm/mW, respectively. The 3-dB bandwidth of the comb filter is ~0.032 nm. The 3-dB and 20-dB bandwidths of the (de)interleaver passband are ~0.225 nm and ~0.326 nm. The obtained results indicate that the designed and fabricated device provides switchable comb filtering and interleaving functions together with extinction ratio and wavelength tunabilities. Reconfigurable and tunable silicon-based comb filter and (de)interleaver may find potential applications in robust wavelength-division multiplexing (WDM) optical communication systems.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The rapid growth of data traffic has fueled the development of optical communications ranging from long-distance fiber transmission links to short-distance access networks and even shorter-reach rack-to-rack, backplanes, chip-to-chip, and on-chip optical interconnects [1, 2]. Exploiting wavelength-division multiplexing (WDM) technology in optical interconnect networks, assisted by high-performance silicon photonic chips with a large bandwidth and low power consumption, provides a potential solution to the electrical interconnect bottleneck [3–6]. To achieve this laudable goal, high-speed and low-power silicon-based devices such as modulators and photodetectors have been reported [7–10]. Moreover, interleavers and comb filters are indispensable for routing a large number of WDM channels with high data-rate signals from one location to another location in a network [11]. A ring-assisted interleaver has been demonstrated utilizing Mach-Zehnder interferometers (MZIs), having relatively large radii of ring resonators to meet the requirement of narrow channel spacing [12]. A waveguide-enabled interleaver based on cascaded MZIs and a simplified design with ring resonators coupling to the MZI have been demonstrated, showing some limitations in channel crosstalk and operation range [13, 14]. Recently, compact silicon photonic interleavers using loop-mirror-based Michelson-Gires-Tournois interferometer or interfering loop containing a Fabry-Perot cavity formed by Sagnac loop have been demonstrated [15–17]. In addition to interleavers, optical comb filters are also key components in WDM optical networks [18]. For compact applications, various schemes have been proposed to realize on-chip comb filters based on Bragg gratings [19, 20], microring resonators [21, 22], and Sagnac loop mirrors (SLMs) [23–25]. To meet the requirements of flexible WDM optical networks, dynamic tuning of comb filter is highly desired [23–25]. Note that most of the previously demonstrated devices showing impressive operation performance focus on either interleaver or comb filter, i.e. single filtering function [15–17, 23–25]. Remarkably, both interleaver and comb filter are essentially filtering components with similar functions but different spectral responses. Besides the tunablity of separate interleaver and comb filter, robust WDM optical networks may also require flexible switching between interleaving and comb filtering functions, i.e. reconfigurable comb filter and interleaver. In this scenario, one would expect to see a silicon photonic device that has tunability and incorporates both interleaving and comb filtering functions with reconfigurability.
In this paper, we propose, fabricate and experimentally demonstrate a silicon photonic device, which is tunable and easily switchable between comb filter and (de)interleaver. The device fabricated on silicon platform is formed by SLMs, the directional couplers of which are replaced by multi-mode interference (MMI) facilitating relaxed fabrication tolerance. By properly adjusting micro-heaters in tunable MZI couplers and cavity, switchable operation between the comb filter and (de)interleaver is achievable. Moreover, tunable extinction ratio and central wavelength of the comb filter and (de)interleaver are also available. The free-spectral ranges (FSRs) of the comb filter and the (de)interleaver are ~0.22 nm and ~0.45 nm, respectively. The 3-dB bandwidth of the comb filter is ~0.032 nm. The 3-dB and 20-dB bandwidths of the (de)interleaver passband are ~0.225 nm and ~0.326 nm, respectively, leading to a 20-to-3dB bandwidth ratio of ~1.45. The extinction ratio of (de)interleaver is tuned from 11.8 to 24.0 dB. The central wavelengths of comb filter and (de)interleaver are shifted with tuning efficiencies of ~0.0224 and ~0.0193 nm/mW, respectively.
2. Concept, principle and theory of reconfigurable and tunable comb filter and (de)interleaver
The schematic structure of the proposed and designed device is illustrated in Fig. 1(a). The device can be regarded as a large SLM incorporating two small SLMs. The two embedded small SLMs (SLM 1, SLM 2) form an FP cavity (SLM 1↔L3↔SLM 2). Remarkably, traditional SLM is based on a 2x2 directional coupler, the two outputs of which are connected to form a loop structure. Instead of directional coupler, here a tunable MZI coupler is employed to construct the SLM, i.e. tunable MZI 1 for SLM 1, tunable MZI 2 for SLM 2 and tunable MZI 3 for the large SLM. As illustrated in the inset of Fig. 1, each tunable MZI consists of two 2x2 multi-mode interference (MMI) waveguides cascaded with each other by two interference arms. The tunable MZI is realized simply by putting a micro-heater on one interference arm to introduce a phase shift. For the SLM, light fed into Port1 is split into two arms by the first MMI, which are combined by the second MMI after introducing a relative phase shift between the two arms by thermal tuning. The outputs of the second MMI, i.e. Port3 and Port4, are connected to form the Sagnac loop. After clockwise (CW) and counterclockwise (CCW) propagation, the CW and CCW lights are combined again and output after passing through a second time the tunable MZI. For the designed device, as shown in Fig. 1, light launched into the input waveguide (Input) is spit by a tunable MZI (MZI 3). After passing through the FP cavity, the lights interfere and output though the tunable MZI either from the same input waveguide (Reflection) or from the other waveguide (Transmission). Due to the coupled interference effects between the large SLM and FP cavity, the designed device can function as either a comb filter or an (de)interleaver. By appropriately adjusting the micro-heaters (Micro-Heater 1, 2, 3) in the tunable MZIs (MZI 1, 2, 3) and another micro-heater (Micro-Heater 4) in the cavity of large SLM, switchable and tunable comb filter and (de)interleaver are achievable.
Fig. 1 (a) Schematic illustration of reconfigurable and tunable comb filter and (de)interleaver formed by Sagnac loop mirrors (SLMs) with tunable Mach-Zehnder interferometer (MZI) couplers. Inset shows the details of tunable MZIs assisted by multi-mode interference (MMI) waveguides and micro-heaters. (b) Operation principle of reconfigurable comb filter and de-interleaver.
Figure 1(b) shows the principle details of reconfigurable comb filter and (de)interleaver. When light is input from port 3 of MZI 3, generally there are four possible light paths to each output of the device (port 3 or port 4 of MZI 3) as follows.
- 1) Output from Port 3 of MZI 3
- ▪ Path 1: port 3 (MZI 3)→port 1 (MZI 3)→L4 (left)→SLM 1→L4 (left)→port 1 (MZI 3)→port 3 (MZI 3)
- ▪ Path 2: port 3 (MZI 3)→port 1 (MZI 3)→L4 (left)→SLM 1→L3→SLM2→L4 (right)→port 2 (MZI 3)→port 3 (MZI 3)
- ▪ Path 3: port 3 (MZI 3)→port 2 (MZI 3)→L4 (right)→SLM 2→L4 (right)→port 2 (MZI 3)→port 3 (MZI 3)
- ▪ Path 4: port 3 (MZI 3)→port 2 (MZI 3)→L4 (right)→SLM 2→L3→SLM1→L4 (left)→port 1 (MZI 3) →port 3 (MZI 3)
- 2) Output from Port 4 of MZI 3
- ▪ Path 1: port 3 (MZI 3)→port 1 (MZI 3)→L4 (left)→SLM 1→L4 (left)→port 1 (MZI 3)→port 4 (MZI 3)
- ▪ Path 2: port 3 (MZI 3)→port 1 (MZI 3)→L4 (left)→SLM 1→L3→SLM2→L4 (right)→port 2 (MZI 3)→port 4 (MZI 3)
- ▪ Path 3: port 3 (MZI 3)→port 2 (MZI 3)→L4 (right)→SLM 2→L4 (right)→port 2 (MZI 3)→port 4 (MZI 3)
- ▪ Path 4: port 3 (MZI 3)→port 2 (MZI 3)→L4 (right)→SLM 2→L3→SLM1→L4 (left)→port 1 (MZI 3)→port 4 (MZI 3)
These available light paths directly deliver the resultant transmission (T) and reflection (R) spectral responses of the device, as depicted in Fig. 1(b). To clearly describe the input-output field relationship of the designed device, the Transmission (T) and Reflection (R) spectral responses, which can be derived by synthesizing the above four path outputs based on the transfer matrix method [26], are written by
where tsi and rsi (i = 1, 2) are the field transmission and reflection functions of the two tunable SLMs (SLM 1, SLM 2). tFPi and rFPi are the transmission and reflection functions of the FP cavity formed by two SLMs (SLM 1↔L3↔SLM 2), and i = 1, 2 denote inputs from the SLM 1 and SLM 2, respectively. ti and ki (ti2 + ki2 = 1, i = 1, 2, 3) are the transmission (same side ports) and coupling (cross ports) coefficients of the tunable MZIs (MZI 1, MZI 2, MZI 3) formed by cascaded MMIs. ai = exp(−αLi − jβLi) (i = 1, 2, 3, 4) are the transmission factors of the waveguides and Li (i = 1, 2, 3, 4) denote the lengths of the waveguides. α and β = 2πnG/λ are the loss factor and propagation constant of the silicon waveguides, respectively, and nG denotes the group index of the transverse electric (TE) mode. Note that the four terms in Eq. (1) and Eq. (2) correspond to the transfer functions of four paths for the cases of output from port 3 and port 4 of MZI 3, respectively. Reconfigurable and tunable comb filter and (de)interleaver can be achieved through appropriate tuning of three micro-heaters (Micro-Heater 1, 2, 3) in the tunable MZIs (MZI 1, MZI 2, MZI 3) and another micro-heater (Micro-Heater 4) in the cavity of large SLM as follows.- 1) Adjusting Micro-Heater 3 to change the coupling coefficient of tunable MZI 3 can switch the comb filtering and (de)interleaving functions, i.e. reconfigurable comb filter and (de)interleaver. Proper adjustments of Micro-Heater 1 in MZI 1 and Micro-Heater 2 in MZI 2 are accompanied to optimize the performance of the comb filter and (de)interleaver. For example, the device functions as a comb filter when t3 = 1 and k3 = 0, as shown in Fig. 1(b).
- 2) Adjusting Micro-Heater 1/Micro-Heater 2 to change the coupling coefficient of tunable MZI 1/MZI 2 and transmission of SLM 1/SLM 2 can tune the extinction ratio of the comb filter/(de)interleaver.
- 3) Adjusting Micro-Heater 4 to change the equivalent cavity length can tune the central wavelength of the comb filter/(de)interleaver.
3. Simulation results
According to the Eqs. (1)-(7), we first simulate the proposed device. In the simulations, nG is 4.68 and α is 10.2 dB/cm. The lengths of the waveguides are L1 = 95.82 μm, L2 = 95.82 μm, L3 = 383.67 μm, and L4 = 137.12 μm, respectively.
Figure 2 shows the simulation results of transmission spectra of reconfigurable comb filter and (de)interleaver by adjusting the tunable MZI 3. As the transmission coefficient t3 (coupling coefficient k3) of the tunable MZI 3 is changed from 0 (1) to 1 (0), the device first functions as a comb filter, then an (de)interleaver, and finally a comb filter again. During the tuning of MZI 3, proper adjustments of tunable MZI 1 and MZI 2 are accompanied to achieve optimized extinction ratio for each tuning state of the MZI 3.
Fig. 2 Simulation results of reconfigurable comb filter and (de)interleaver by adjusting the transmission coefficient t3 (coupling coefficient k3) of tunable MZI 3. Proper tuning of transmission coefficients (t1, t2) of tunable MZI 1 and MZI 2 is accompanied to achieve optimized extinction ratio for each tuning state of the MZI 3.
Figure 3(a) shows the simulation results of extinction ratio tunable (de)interleaver by adjusting the tunable MZI 2. The coefficients t2 and t3 are 0.238 and 0.31, respectively. Tuning MZI 2 varies the transmission of SLM 2, resulting in the change of the extinction ratio of the (de)interleaver. Figure 3(b) depicts the simulated transmission and reflection spectra of the (de)interleaver for both output ports (bar and cross). The coefficients t1, t2 and t3 are chosen as 0.244, 0.24, and 0.382, respectively, to achieve the same extinction ratio of the (de)interleaver at the two output ports.
Fig. 3 (a) Simulation results of extinction ratio tunable (de)interleaver by adjusting the tunable MZI 2. (b) Simulated transmission and reflection spectra of the (de)interleaver for both output ports (bar and cross)
Figure 4 shows the simulation results of transmission spectra of wavelength tunable comb filter and (de)interleaver. As shown in Fig. 4(a), the central wavelength of the comb filter increases with the thermal tuning induced refractive index difference. Similarly, the central wavelength of the (de)interleaver also increases with the thermal tuning induced refractive index difference, as shown in Fig. 4(b).
Fig. 4 Simulation results of wavelength tunable (a) comb filter and (b) (de)interleaver under different thermal tuning (Micro-Heater 4) induced refractive index differences.
4. Device fabrication
We design the proposed device on silicon platform. The designed device is fabricated on a silicon-on-insulator (SOI) wafer with a 220-nm-thick top silicon layer and a 2-μm-thick buried oxide layer. In the fabrication process, 248-nm deep ultraviolet (DUV) photolithography is employed to define the pattern and an inductively coupled plasma (ICP) etching process is followed to etch the top silicon layer. The etched silicon waveguides have a cross section of 500 nm width and 220 nm thickness. A 1-μm-thick SiO2 layer is deposited over the whole device as upper cladding by plasma enhanced chemical vapor deposition (PECVD). Seven TiN micro-heaters with length of 100 μm are fabricated along the MZI arms (two micro-heaters for each MZI) and the large SLM cavity (one micro-heater) to tune the phase shifts. Note that the two micro-heaters along the two arms of each MZI provide the same functions. Hence, only one micro-heater for each MZI is actually used. As a result, only four micro-heaters (Micro-Heater 1, 2, 3, 4 in Fig. 1) are adjusted in the experiment to perform reconfigurable and tunable comb filtering and interleaving operations. Figure 5 depicts the measured microphotograph of the device fabricated on silicon platform. The footprint of the device is 736.1 × 522.8 μm2. Grating-assisted vertical coupling is employed to enable fiber-chip-fiber performance characterization. Shown in the inset of Fig. 5 is the enlarged microphotograph of vertical grating couplers.
Fig. 5 Measured microphotograph of the fabricated device. Inset shows the enlarged microphotograph of grating coupler for fiber-chip-fiber vertical coupling.
5. Experimental results
In the experiment, a tunable continuous wave (CW) laser is employed to scan the fabricated device with a step size of 5 pm. Grating couplers (inset of Fig. 5) for TE polarization are used to couple light in/out of the chip with single-mode fibers. The fiber coupling loss is ~6 dB at input and ~6 dB at output. The insertion loss of the chip is ~6 dB. Hence, the total fiber-chip-fiber loss is assessed to be ~18 dB.
Figure 6(a) and 6(b) show the measured typical transmission spectra, respectively. The heating powers of Micro-Heater 1, 2, 3 and 4 are set to 15.8, 9.5, 26.8 and 2.5 mW for comb filter and 3.9, 8.2, 9.3 and 1.5 mW for (de)interleaver, respectively. The FSR of the comb filter is ~0.22 nm, while the FSR of the (de)interleaver is ~0.45 nm. The 3-dB bandwidth of the comb filter is ~0.032 nm. The 3-dB and 20-dB bandwidths of the (de)interleaver passband are ~0.225 nm and ~0.326 nm, respectively. Therefore, boxlike passbands with a 20-to-3dB bandwidth ratio of ~1.45 are obtained. The extinction ratio of the comb filter and (de)interleaver is measured to be ~14.3 dB and ~20 dB, respectively. Note that the heating powers applied to Micro-Heater 1 and Micro-Heater 2 are not equal to each other, which might be ascribed to the slight difference between MZI 1 and MZI 2 induced different initial states. Figure 7 depicts the output power at Port3 of the MZI as a function of the heating power of Micro-Heater when the light wavelength fed into Port1 is 1550 nm. Changing the heating power of Micro-Heater and adjusting the phase difference between two arms of MZI facilitate the switching between the comb filtering and interleaving functions. When the heating power of Micro-Heater 3 is changed to 26.8 mW at 1550 nm, the device is reconfigured from the (de)interleaver to the comb filter.
Fig. 6 Measured typical transmission spectra of reconfigurable (a) comb filter and (b) (de)interleaver.
We then demonstrate the extinction ratio tunable (de)interleaver by adjusting Micro-Heater 2 to change the coupling coefficient of tunable MZI 2 and transmission of SLM 2. As shown in Fig. 8, when adjusting the heating power applied to Micro-Heater 2 in MZI 2 from 8.2 to 9.9 mW, the extinction ratio of the (de)interleaver is tuned from 11.8 to 24.0 dB. The heating powers applied to Micro-Heater 1 and Micro-Heater 3 are set to 3.9 and 8. 2mW, respectively.
We further demonstrate the wavelength tunable comb filter and (de)interleaver by adjusting Micro-Heater 4 to change the equivalent cavity length. Shown in Fig. 9(a) and 9(c) are measured absolute wavelength under absolute heating power. Shown in Fig. 9(b) ad 9(d) are extracted central wavelength shift versus incremental heating power (∆P). Here, the central wavelength shift is defined as the difference between the central wavelength under current heating power (Micro-Heater 4) and that under a reference heating power of 0.5 mW for comb filter and 11.8 mW for (de)interleaver. The incremental heating power (∆P) is defined as the difference between the current heating power (Micro-Heater 4) and a reference heating power of 0.5 mW for comb filter and 11.8 mW for (de)interleaver. Figure 9(a) shows measured transmission spectra of wavelength tunable comb filter. The central wavelength of the comb filter is shifted by ~0.044 nm when the heating power applied to Micro-Heater 4 changes from 0.5 to 2.5 mW. The heating powers applied to Micro-Heater 1, 2 and 3 are set to 15.8, 9.5 and 26.8 mW, respectively. Figure 9(b) plots measured central wavelength shift as a function of incremental heating power (∆P). A linear fit to the measured data is given, showing a linear relationship between the central wavelength shift and incremental heating power (∆P). From Fig. 9(b), the wavelength tuning efficiency is assessed to be ~0.0224 nm/mW. Similarly, shown in Fig. 9(c) and 9(d) are measured transmission spectra of wavelength tunable (de)interleaver and measured central wavelength shift as a function of the incremental heating power (∆P), respectively. The heating powers applied to Micro-Heater 1, 2 and 3 are set to 3.2, 3.5 and 12.1 mW, respectively. The central wavelength of the (de)interleaver is shifted by ~0.092 nm when the heating power applied to Micro-Heater 4 changes from 11.8 to 16.5 mW. The wavelength tuning efficiency is evaluated to be ~0.0193 nm/mW.
Fig. 9 (a)(c) Measured transmission spectra of wavelength tunable (a) comb filter and (c) (de)interleaver. (b)(d) Measured central wavelength shift versus incremental heating power applied to Micro-Heater 4 and linear fit curves for (b) comb filter and (d) (de)interleaver
The obtained experimental results shown in Figs. 6-9 are in good agreement with simulation ones show in Figs. 2-4, indicating successful implementation of switchable and tunable silicon-based comb filter and (de)interleaver with favorable operation performance.
Strictly speaking, the demonstrated function is actually not an interleaver but a de-interleaver that could be used to de-multiplex a WDM signal into transmitted even channels and reflected odd channels (or vice versa). The interleaver, on the other hand, would combine even and odd WDM channels together. Remarkably, based on the principle of reciprocity, the same device can also be used to combine even and odd WDM channels, i.e. an interleaver.
To further improve the overall performance of the device, stability, efficiency, fiber compatibility, packaging and cost should be considered. With future improvement, precise thermo-optic tuning with high temperature stability is highly desired. The fabrication technique could be further improved to reduce the loss. Grating-assisted vertical coupling for easy lab testing can be replaced by end face coupling. The coupling in/out should be compatible with single-mode fibers. Compact packaging including both electric drive and control (wire bonding the chip to a printed circuit board) as well as input/output optical fiber is of great importance. Finally, mass production with low cost is another determining factor for practical applications.
6. Conclusion
In summary, we design, fabricate and demonstrate chip-scale reconfigurable and tunable compact silicon-based comb filter and (de)interleaver. The device can be regarded as a large SLM incorporating two embedded small SLMs. The traditional directional couplers of SLMs are replaced by MMI-assisted tunable MZI couplers, which helps to relax the fabrication tolerance. Thermo-optic tuning, i.e. adjusting the heating powers applied to micro-heaters in tunable MZI couplers and large SLM cavity, facilitate switchable operation, extinction ratio tunable operation and wavelength tunable operation. Reconfigurable and tunable comb filter and (de)interleaver are demonstrated both in theory and in experiment. The FSRs of the measured comb filter and (de)interleaver transmission spectra are ~0.22 nm and ~0.45 nm, respectively. The 3-dB bandwidth of the comb filter is ~0.032 nm. The 3-dB and 20-dB bandwidths of the (de)interleaver passband are ~0.225 nm and ~0.326 nm, respectively, and therefore the 20-to-3dB bandwidth ratio is ~1.45. The extinction ratio of (de)interleaver is tuned from 11.8 to 24.0 dB. The wavelength tuning efficiency is assessed to be ~0.0224 nm/mW for comb filter and ~0.0193 nm/mW for (de)interleaver. The power/FSR of comb filter and (de)interleaver is ~9.82 mW/FSR and ~23.32 mW/FSR. These demonstrations may open up a door to develop more silicon-based photonic integrated devices for superior WDM optical networks.
Funding
National Program for Support of Top-notch Young Professionals; Royal Society-Newton Advanced Fellowship; National Natural Science Foundation of China (NSFC) (61761130082, 61222502, 11574001, 11774116, 11274131); Yangtze River Excellent Young Scholars Program; Program for HUST Academic Frontier Youth Team.
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