We present the design and fabrication of thermally-efficient tuning structures integrated into a narrowband reconfigurable radio-frequency (RF)-photonics filter using silicon-on-insulator waveguide optical delay lines. By introducing thermal isolation trenching, we are able to achieve IIR, FIR or arbitrary mixed response with less than 120mW average tuning power in a single RF-photonic unit cell filter.
© 2010 OSA
Many signal processing applications, such as radar, laser remote sensing and electronic sensing require high performance digital signal conversion or operation over a wide RF frequency band. Optics offers a promising method to address a wide frequency range or to reduce the demands on analog to digital conversion technology [1–3]. To realize this promise, reconfigurable filter, correlator, and equalizer structures based on highly integrated programmable structures are a subject of current research [1–7]. To realize narrowband applications, a long optical delay is required, for instance, >500ps. The optical delay can be realized by using true optical delay lines  or by resonant structures [5–7]. In some cases, multiple stages of such filters may be required to construct arbitrary filter shapes with high out of band rejection. Therefore, such filters should possess extremely flexible reconfigurability and cascadability properties and, importantly, low power consumption.
Silicon photonics is an emerging technology that is compatible with CMOS processing and has the capability to monolithically integrate optical and microelectronic components . Here, we describe the realization of power efficient tuning structures and the fabrication of a reconfigurable narrowband RF-silicon-photonics filter using true optical delay lines fabricated using 0.25μm thick silicon-on-insulator (SOI) waveguides. The device incorporates optimized thermally-efficient tuning structures enabling full reconfigurability with low power consumption. The similar isolation trench idea has been used for microring resonator tuning previously [14,15]. By introducing a thermal isolation trench, we are able to achieve infinite impulse response (IIR), finite impulse response (FIR) or arbitrary mixed response with less than 120mW average tuning power with a single RF-photonic unit cell filter. This filter is a building block, namely a unit cell, which can be reconfigured and cascaded to fulfill much more complicated RF signal processing tasks.
2. Device design and fabrication
An optical microscopic image of the reconfigurable unit cell is shown in Fig. 1 . The unit cell uses 0.25μm thick SOI waveguides fabricated with standard CMOS compatible technology. The dimensions of the device are approximately 2mm x 8mm. The device consists of three tunable couplers, three phase tuning elements and two optical delay lines with a total 500ps optical delay line. As illustrated in Fig. 1, the unit cell is a mixed structure of the Mach-Zehnder Interferometer (MZI) and a feedback loop. To match the FSR of IIR and FIR paths, two optical delays δ and τ are included. A previous publication contains design details . From the requirement of the FSR of the filter response, it is easy to calculate, where is the group index of the waveguide, the ΔFIR and ΔIIR are the path length differences of FIR and IIR filters. The optical delay line lengths δ and τ can be calculated accordingly.
The optical delay lines are the most important element and their phase shifts are the key parameters that, along with the coupler settings, determine the frequency response of the unit cell. Unlike optical delay realized by some nonlinear phase shifters, such as a ring resonator [5–7], here we use true optical delay induced by the true optical path length, which provides wide-band operation as well as control advantages. The delay line is realized by fabricating silicon waveguide spirals on a SOI wafer. The optical loss of the delay lines has significant impact of the device performance because their length can be up to tens of centimeters. To minimize the optical loss of the optical delay line, we fabricated shallow-etched ridge waveguides that were optimized for low waveguide loss. Using test spiral structures fabricated on chips without heater elements, we were able to characterize the waveguide loss and achieved 0.3dB/cm and 0.5dB/cm loss for 2μm and 1μm wide waveguides, respectively. Due to correctable processing-induced effects associated with fabrication of the heater elements with deep-etched trench, the waveguide loss increased to around 0.5dB/cm and 0.9dB/cm, respectively, after full processing was completed. With further processing optimization, it is likely that the waveguide loss can be reduced significantly for the full processed devices. The long spiral waveguide delay lines used here were fabricated using 2μm wide SOI waveguides to take the advantage of the lower propagation loss. The tunable couplers are the key elements for achieving reconfigurability. As shown in Fig. 1, three tunable couplers are necessary to fulfill the full functionality of the unit cell. Each of them needs to independently tune with a 0 to 100% coupling ratio. To cover such a large tuning range with the lowest power consumption, an MZI structure was used. Inside the MZI, multimode-interferometer (MMI) elements are used as splitter/combiners with thermal tuning sections (micro-electrical heaters) on one or both arms. Tuning through thermo-optic effects has the lowest associated optical loss, but power efficiency and speed are both concerns [9,10]. Alternative tuning methods are also possible, including free-carrier electro-optic effects, which can exhibit high efficiency and high speed, but also have higher optical losses due to free carrier effects [6,11–13].
After waveguide etching, a thin film oxide and nitride cladding is deposited followed by a thin Ti metal on top of the waveguide cladding. The resistance of the metal heater ranged from 500 to 1000 ohms depending on the designed values. To improve the efficiency of the heater, heat dissipation characteristics need to be carefully studied. Since silicon is an excellent heat conductor, controlling the heat dissipation paths from the silicon waveguide and substrate is the key to improve the tuning efficiency, as well as the interaction length over which thermal effects are controlled. We fabricated 40μm deep-etched heat isolation trenches surrounding the heating sections at various offset distance from the heated waveguides (shown in Fig. 2(a) ). Figure 2(b) illustrates a typical simulation result of the temperature distribution of the tuning section of the tunable coupler with trenches 5μm away from the waveguide edge. Using the data obtained from Fig. 2(b), we are able to calculate the phase shift induced by the temperature change of the tuning section. The phase shift versus tuning power consumption is shown in Fig. 2(c) for the structures with and without thermal isolation trench. As illustrated by the simulation, a π phase shift can be achieved with power consumption about 20mW and 36mW for the cases with and without the isolation trench, respectively. The values match our measurement results 20mW and 32mW very well. The transient simulation result is shown in Fig. 2(d) for the structure with isolation trench, where the temperature change induced by a 5kHz square wave electric signal is plotted as function of time. The rise and fall times of the temperature response to the applied signal can be estimated from the figure as being about 19μs and 23μs, respectively.
The measurement results of the MZI-based tunable coupler shown in Fig. 3 demonstrate the effectiveness of this approach. The power required to adjust the tunable coupler required for a π phase shift versus the trench offset distance (gap between the waveguide and trench edge) is shown in Fig. 3(a). As expected, the smaller the trench distance, the lower the tuning power required and the higher the tuning efficiency. At a 5μm trench offset, only 20mW is needed to realize a transition from bar state to cross state as illustrated in Fig. 3(b), where an extinction ratio larger than 20dB was achieved. The results agree with the simulation result shown in Fig. 2(c). The power consumption compares favorably with other techniques such as the one described in . The transient response of the MZI tunable coupler with isolation trench 5μm away from the waveguide edge was measured and the result is shown in Fig. 3(c). The rise and fall times of the device response to the applied square wave were measured to be around 16μs and 21μs, respectively. They are very close to the simulation results.
3. Filter Response
The unit cell configuration illustrated in Fig. 1 allows independent tuning of the tunable couplers (θa, θb, and θc) to realize FIR/IIR response. The unit cell performance was measured using a high-resolution tunable laser with a wavelength resolution of 0.01pm to resolve the ultra-narrowband filter response. By configuring the feedback coupler (θc) to the pure bar state (100% coupling ratio), the FIR response can be obtained by tuning the input and output couplers (θa, θb) to balance the optical losses of the upper and lower paths. In Fig. 4(a) , we illustrate a FIR filter response of the unit cell. In this design, a 500ps optical delay line was fabricated (about 4cm long spiral waveguide). As designed, the FSR of the filter is 2GHz. The extinction ratio (ER) of the FIR filter response can reach 20dB.
Due to the small waveguide cross-section, the device can experience a fiber coupling loss of ~7dB/facet without mode expansion structures. To improve the fiber coupling loss, we have fabricated inverse taper structures at the input and output ends of the chip. Instead of E-beam lithography used in , we used a standard deep-UV stepper combined with oxidation. We achieved 1.5-2.0dB/facet coupling loss from a lensed fiber (with ~3μm spot size) to the 0.25μm thick SOI waveguide using this process, and believe continued improvement is possible with further process optimization. Taking the fiber coupling loss into account, it leads to an estimation of the insertion loss of the device of ~4dB, which includes ~0.4dB MMI loss and ~3.6dB waveguide loss from the optical delay line. The result is consistent with the waveguide loss measurement of 0.9dB/cm for a trenched waveguide.
When the input coupler (θa) is tuned to the cross state (100% coupling ratio) and the upper input port is energized, the upper path of the MZI is isolated, then the IIR response can be obtained by tuning the output and feedback couplers (θb, θc) to obtain the critical coupling condition of the feedback loop. Accordingly, the measured IIR response is illustrated in Fig. 4(b). With an FSR of 2GHz, the ER of the IIR filter response can be larger than 35dB. The 3dB bandwidth of this IIR notch filter response is 400MHz, corresponding to a quality factor Q of ~3 x 105. The FIR passband filter has a 3dB bandwidth of 800MHz. The total tuning power for realizing the FIR and IIR responses is the contribution from the all heaters (including the phase shifters), primarily from the tunable couplers. Considering the worst case scenario, i.e. all three fixed couplers (θa,b,c) requiring a π phase shift, the total tuning power would be around 60mW. The adjustable couplers (ϕa,b,c) will also require a variable power dissipation to tailor the transfer function to the desired response. The specific offset of each coupler is uncertain, leading to a variable power dissipation depending on the ‘zero-bias’ configuration of the unit cell that can be expected to be less than 120 mW averagely. For the configurations described in , thermo-optic phase shifter dissipation ranging between 75 and 180 mW were observed. To realize the FIR and IIR states shown in Fig. 4(a) and 4(b), total power consumption from the three tunable couplers was 45mW and 42mW, respectively. The power consumption reported here is much lower than for previous reports (~160 mW) using similar thermo-optic tuning mechanisms [4,7]. These results provide evidence of the effectiveness of the thermal isolation technology described here. Further optimization of speed and power is possible with an appropriate combination of thermal and junction-based methods.
We have presented the design and fabrication of a thermally optimized reconfigurable RF-optical mixed FIR/IIR unit cell filter fabricated using 0.25μm SOI waveguides. A thermally-efficient phase tuning structure was incorporated into the device to enable the full reconfigurability of the filter response with less than 120 mW average power dissipation for the entire unit cell. By tuning the combination of the three tunable couplers and the phase tuning elements, the filter can independently realize FIR, IIR or mixed FIR/IIR filter responses. The presented unit cell filter can form a building block for the realization of much more complicated RF-optical signal processing devices. An ultra-narrowband filter response (<50MHz) can be realized by cascading multiple stages of the unit cells.
This material is based upon work supported by the Defense Advanced Research Projects Agency PhASER program under Contract No. HR0011-08-C-0026. The views, opinions, and/or findings contained in this article are those of the authors and should not be interpreted as representing the official views or policies, either expressed or implied, of the Defense Advanced Research Projects Agency or the Department of Defense. Approved for Public Release, Distribution Unlimited.
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