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Abstract
Recent advances in silicon photonic components operating in the thulium-doped fiber amplifier (TDFA) wavelength regime around 2-µm have shown that these wavelengths hold great promise for on-chip photonic systems. Here we present our work on characterizing a Mach-Zehnder interferometer coupled silicon photonic ring resonator operating in the TDFA window for optical time delay applications. We describe the optical transmission and variable time delay properties of the resonator, including a detailed characterization and comparison of the directional coupler and Mach-Zehnder interferometer base components at both 1930 and 1550 nm wavelengths. The results show tuning of a ring from a 190-ps peak time delay at a resonant extinction ratio of 5.1-dB to a 560-ps peak time delay at an extinction ratio of 11.0-dB, in good agreement with optical models of the device. These results demonstrate significant promise towards the future application of TDFA band devices in optical time delay systems.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Over the last several years there has been a significant growth in the performance capabilities and manufacturability of on-chip integrated photonic devices and systems, especially in those using silicon waveguides (‘silicon photonics’) [1–3]. While a primary driver of these advances is silicon photonic integrated circuit deployment in datacenters for telecommunications applications [4,5], the advances in silicon photonics technology has seen it proposed as a solution for various applications, including, biological sensing [6,7], quantum computing [8,9], and wireless communications networks [10,11], among others [12,13].
An area of silicon photonics that has seen a large amount of growth recently is the exploration of components designed for operation in the thulium doped fiber amplifier (TDFA) wavelength band, typically referred to as the 2-µm band and considered between 1840 to 2010 nm wavelengths [14]. Some of the major recent milestones reported in this area include the characterization of many basic waveguide properties [15] and the demonstrations of a high speed all silicon PIN detector [16], high speed optoelectronic ring and Mach-Zehnder modulators [17,18], and a thulium laser monolithically fabricated by coating doped gain layers onto a silicon microdisk resonator [19]. However, despite the device demonstrations that have been made around the 2-µm wavelength, very little has been explored regarding the use of this wavelength window in systems other than a 2-µm designed grating coupler connected to a ring resonator [20] and a modulator connected to a detector for data processing applications [21]. Regardless of these limited results, there are many promising aspects of silicon photonic systems operating in this wavelength regime, including lower waveguide losses from reduced sidewall scattering [15,22], greater efficiency of optical modulators and attenuators due to the larger plasma dispersion coefficients [23–25] and improved handling of high-powered signals due to lower two-photon absorption rates [26].
The unique advantages of operating at wavelengths near 2-µm become particularly beneficial in large-scale, multicomponent system applications, where electrical and optical power budgets can become strained [27–29]. For example, phased array antenna networks [30] for beamforming applications in satellite and wireless communications networks [31] have recently become a growing application of interest for integrated photonics [32–35] due to their potential for higher data rates and lower power consumption [36] as well as their insensitivity to electrical interference [37,38]. The primary component needed in such systems is a tunable time delay element [39], which has been demonstrated on integrated platforms using photonic crystals [40,41], optical path switches [42–44], and increasingly ring resonators for their ability to achieve precise time delay tuning in a compact form [45–48]. While various on and off-chip time delay elements and architectures have demonstrated and studied [49], and progress is being made on the integrated tunable time delay systems, with recent demonstrations of 1×4 [50–52] and 1×8 [53] beam forming tree networks, there are many ongoing challenges in realizing large-scale on-chip systems integration. We propose designing such systems for operation around 2-µm wavelengths can provide an effective pathway towards large-scale phased array systems integration, while compatibility concerns of working at non-standard wavelengths are mitigated by the optical to electrical conversion for radio frequency (RF) antennas.
Here we discuss the design, fabrication and characterization of a Mach-Zehnder coupled silicon ring resonator [54,55] tunable time delay element operating near 2-µm. This work provides a pathway for the future application of silicon photonic beamforming systems with a high level of on-chip systems integration due to the established optical amplification, lasing, detection and modulation functions available in this wavelength window.
2. Fabrication and design
We fabricated the true time delay circuits using a silicon photonics multiproject wafer (MPW) run at the Advanced Micro Foundry (AMF) in Singapore with a 220-nm-thick silicon waveguiding layer on a 2-µm-thick buried oxide. The silicon layer can be patterned either to be etched the full 220-nm to fabricate strip waveguides, or only 130-nm-deep to leave a 90-nm slab around the waveguides. The process permits dopants of either donor or acceptor type at various concentrations to then be added through ion implantation of the silicon layer to enable optoelectronic modulation and detection capabilities. After implantation and thermal activation, an SiO2 top cladding layer of 3-µm-thickness is deposited in several steps as conductive metal layers and vias are fabricated, including a titanium nitride (TiN) resistive heater layer that lies 2-µm above the top of the silicon layer and ending with the exposure of bond pads to allow electrical connections. Following device fabrication, the wafer undergoes a deep etch process to create smooth edge coupling facets before being diced into chips.
All circuits designed on the chip use edge couplers with 0.18-µm-wide strip waveguide nano-tapers at opposite ends of the chip. Over a 70-µm-long taper the waveguides are expanded to a standard 0.5-µm-wide strip waveguide and then routed to the on-chip device of interest, where they are again tapered into 0.5-µm-wide ridge waveguides surrounded by the 90-nm-thick silicon slab. The primary device of interest for this study is the Mach-Zehnder interferometer (MZI) tunable coupler microring resonator, referred to simply as the tunable coupler ring (TCR), which follows the microscope layout seen in the diagram of Fig. 1(a). The TCR was designed using two evanescent field directional couplers (DCs), which are connected to make a symmetric MZI structure. Both the DC and MZI base components of the TCR are highlighted in Fig. 1(a). The two DCs of the TCR were identically designed, with a gap of 0.2 µm separating the interior walls of the two waveguides over a nominal coupling length of 5 µm, and then separated by sinusoidal bends [56] with a 15-µm-amplitude and 45-µm-length on each side. The MZI structure connects the respective pass and drop arms of the two DCs with 230-µm-long straight waveguides, with a 3-µm-wide, which is the minimum dimension for this layer, 230-µm-long TiN heater centered over the pass arm of the MZI connecting the two DCs, noted as ϕc in the diagram. For the TCR the input and output of the pass arms for the first and second DCs are then routed to the edge tapers, forming a continuation of the bus waveguide, while the drop arm of the MZI at the output of the second DC loops back to the drop arm at the input of the first DC, forming the ring resonator. The loop uses two 180° radial bends with a 20-µm radius separated by a 420-µm-long straight waveguide. This then creates a ring resonator which can have a dynamically tuned ring/bus coupling condition based on using the TiN phase shifting heater to modify the interference condition of the MZI. Along the path of the ring there is second 230-µm-long and 3-µm-wide TiN phase shifter, noted as ϕr in the diagram, which allows for tuning of the resonant wavelength of the ring without affecting the coupling condition, as well as a 60-µm-long integrated silicon PIN photodiode [57] to enable resonant monitoring of the ring [58], a feature which is not used in the course of this study.
Fig. 1. (a) Microscope image of tunable coupler ring with Mach-Zehnder interferometer and directional coupler base components. Simulated optical electric-field mode profiles of the waveguide at a wavelength of (b) 1550 nm and (c) 1930nm.
We designed on-chip circuits to explore the characteristics of the TCR as well as its directional coupler and MZI base components, for which direct characterization of their operation at 2-µm wavelengths had not been previously explored. The circuits were measured optically by fiber chip coupling from a 1900–1950 nm tunable wavelength laser onto and off the bus waveguide of the device under test and measuring the transmitted power on a photodetector. Measurements focused primarily around a 1930-nm wavelength, which is near the optimal emission wavelength for thulium-doped devices [59]. To provide a context for the results, the devices were also characterized at the more common 1550-nm wavelength window as a comparison. The optical electric field mode profiles of the 0.5-µm-wide ridge waveguide at 1550 and 1930-nm wavelengths based on eigenmode simulations can be seen in Fig. 1(b) and (c) respectively, where a noticeable relative expansion of the mode at 1930-nm can be observed. This effect has a significant impact on several of the optical properties of the waveguide, where between 1550 and 1930-nm wavelengths, the optical confinement in the silicon drops from 76 to 61%, the mode area expands from 0.18 to 0.34-µm2, and the group index decreases from 3.89 to 3.66, as found from the eigenmode simulations. For the circuits designed here, a 0.5-µm-wide waveguide is used to maintain single mode operation around 1550 nm, while a circuit designed specifically for 1930 nm could use up to 0.6-µm-wide waveguides [15], which would increase the optical confinement in the silicon to 70%, reduce the mode size to 0.30-µm2, and increase the group index to 3.71.
3. Component characterization
3.1 Directional couplers
The cross-over characteristics of the silicon waveguide DCs around both 1550- and 1930-nm wavelengths were characterized with a series of test circuits. The DC test circuits use couplers with 0.2-µm gaps between the interior walls of the 0.5-µm-wide ridge waveguides at nominal coupler lengths of 0 to 20 µm with sinusoidal bends similar to those described earlier. For the DC test circuits all four ports of the device are connected to edge coupling tapers. The devices were measured in the fiber-chip coupling test setup by launching incident light into an edge facet of one of the DC arms, and measuring the transmitted power detected at the edge facets connected to the nominal pass and drop arm of the DC. Assuming near identical fiber-chip coupling and waveguide losses in the different ports of the device, the cross-over ratio of the directional coupler (KDC) can then be found by taking the ratio of the transmitted power when aligned to the nominal drop arm compared to the transmitted power when aligned to the nominal drop and pass arms combined. The results are shown in Fig. 2(a). At 1550 nm the cross-over ratio is 0.54 for a nominal coupler length of 0 µm (‘point coupler’), with all coupling resulting from the modal interaction that occurs as the pass and drop waveguides approach each other along the sinusoid bends towards the coupling point. This then acts as a constant offset for longer couplers, which follow an expected sinusoidal relationship [60] with a nominal cross-over length of 30.2 µm. At 1930 nm the cross-over ratio for the point coupler was measured to be 0.93. The evolution of KDC with coupling length towards lower cross-over ratios at this wavelength indicates that a full beat into the drop waveguide has already occurred as a result of the sinusoidal bends and light is already starting to couple back into the pass waveguide. According to the data, the nominal cross-over length at 1930 nm was found to be 17.7 µm, where the much more rapid coupling at this wavelength is a result of the larger optical mode area, creating greater evanescent interaction between the two waveguides. The cross-over characteristics of the DCs were modeled using ‘supermode’ analysis [61] of the two waveguides when separated by gaps of between 0.1 and 5.0 µm to determine the coupling coefficient at each gap. Integration of the coupling coefficient along the length of the sinusoidal bend combined with the straight coupler section [61] were then used to predict expected cross-over ratios for the DCs of different length. The simulation results for both wavelengths are plotted with the solid lines of Fig. 2(a). These results show very good agreement with the measured data at both 1550 and 1930 nm. Figure 2(b) shows the measured cross-over ratios versus wavelength around both 1550 and 1930 nm for a fixed coupler length of 5 µm. The cross-over ratio increases from 0.88 to 1.00 from 1510 to 1610 nm, while the cross-over ratio was seen to drop from 0.19 to 0.08 from 1905 to 1945 nm.
Fig. 2. (a) Comparison between the measured and simulated cross-over ratios for directional couplers of different lengths at 1550 and 1930nm wavelengths, and (b) measured cross over ratio for a 5 µm long coupler versus wavelength around both 1550 and 1930nm.
3.2 Mach-Zehnder interferometers
A set of standalone MZI test circuits were designed using a pair of two DCs, separated by 230-µm-long waveguides connecting their drop and pass arms, with a TiN heater integrated with one of the arms. The MZI test circuits were varied by using DC pairs of different nominal coupler lengths. Similar to the DC test circuits, all four ports of the MZI were routed to edge tapers. The MZI circuits were characterized by connecting electrical probes from a power-supply to the bond pads of the on-chip integrated heater while performing transmission measurements across the nominal pass and drop arm of the MZI. By convention, light was launched into the arm over which the integrated TiN heater lies and this was considered the pass arm of the device, although the results are expected to be invertible. Measurements were taken for a fixed input wavelength, while the electrical power applied to the integrated heater was swept from 0 to 120 mW with the output fiber aligned to the edge facet of the nominal pass arm, and then was repeated when the output fiber was aligned to the nominal drop arm.
Figure 3(a) shows an example of the transmitted optical power measured in the pass and drop arms for an MZI test circuit using DCs with a 3-µm-long nominal coupler length versus different electrical powers applied to the heater at a signal wavelength of 1930 nm. Here the laser was outputting 3 dBm of optical power, and approximately 3 dB of coupling loss per facet was estimated. As the electrical power applied to the integrated heater changes the transmitted optical power measured at the two arms changes, although the total transmitted power measured by the combination of both the drop and pass arms remains constant, indicating minimal excess loss. The cross-over ratio of the MZI (KMZI) versus applied heater power (PAppl) was calculated similarly as done in the DC case, by taking the transmitted power measured when aligned to the drop arm of the MZI compared to the total transmitted power for both arms. The measured cross-over ratios at 1930 nm for the MZIs with 3 and 5-µm-long nominal coupler lengths are shown in Fig. 3(b). The results for both cases demonstrate a sinusoidal relationship of the MZI cross-over ratio with respect to the applied power, with similar periodicity but different amplitudes. Assuming the pair of DCs in the MZI are identical with cross-over ratios of KDC, the cross-over ratio of the MZI can be estimated based on analysis of the interference condition between the two arms of the MZI to be:
Fig. 3. (a) Transmitted power across chip measured at 1930nm in the drop and pass arm of an MZI test circuit with 3-µm-long directional couplers versus power applied to the phase shifter. Comparison of measured and simulated MZI cross-over ratio versus power applied to the heater for (b) 3 and 5 µm long directional couplers at 1930nm and (c) 5 µm long couplers at 1550 and 1930nm.
For the MZI with 5-µm-long DCs the same measurement method was applied using a 1550-nm wavelength light source and compared to the results measured at 1930 nm, as shown in Fig. 3(c). Here the MZI measurements were best fit to the model with a KDC of 0.94, again agreeing well with the measured results for the same wavelength and coupler length in the standalone DCs, with a Pπ of 26 mW. This significant improvement in the efficiency of the heater at 1550 nm is primarily accounted for by the larger optical overlap of the mode with the silicon waveguide core, as well as the small decrease in silicon’s thermo-optic coefficient of 1.87×10−4 K-1 to 1.78×10−4 K-1 from 1550 to 1930 nm [62]. While for most system applications it would be non-ideal to have heaters of lower efficiency, several modifications to the design, including thermal isolation trenches and undercutting, folded waveguide routing, doped silicon heaters, and increasing the bus waveguide width could be used to bring heater efficiencies for 1930 nm to significantly lower values [63–66]. However, when comparing similar heater designs at 1550 nm a heater efficiency loss of approximately 10 to 15% at 1930 nm, resulting from lower mode confinement and thermo-optic coefficient, is always to be expected.
4. Tunable coupler ring characterization
4.1 Optical characterization
A TCR resonator circuit was designed as described and displayed in Fig. 1(a) and characterized optically by fiber-chip coupling with a tunable laser around 1930 nm. The 3-mm-long bus waveguide contributes 0.5 dB of insertion loss, while the TCR itself introduces negligible off-resonance loss. Initially measurements were taken with an electrical power supply connected to the ring phase shift heater and used to tune the resonant wavelength of the ring. The relative transmission spectrum, characterizing the TCRs interferometric related loss versus wavelength, for several different applied heater powers can be seen in Fig. 4(a), showing a red shifting of the resonance wavelength, with minimal effect on the other properties of the device. It was observed that 72 mW of power was required to shift the resonant wavelength by a full FSR, which was measured to be 1.02 nm, corresponding to a Pπ of 36-mW, in agreement with the DC and MZI results reported above. Another set of transmission measurements were made with the electrical power supply connected to the MZI heater to influence the ring/bus coupling condition of the device, with a sample set of spectra shown in Fig. 4(b). It can be seen in this case that as the heater power is increased the resonance mode experiences a change in both extinction ratio and spectral resonant width as a result of the changes in the coupling condition, as well as experiencing a small red shift in wavelength resulting from the effect of the MZI heater on the roundtrip path length of the ring. The relative optical transmission around the resonant mode of each measured spectrum was then fit with a Lorentzian function [67] to determine the ring waveguide loss related intrinsic Q factor (Qi), ring/bus coupler related extrinsic Q factor (Qe), and optical extinction ratio, with results versus the applied heater power and expected phase shift in the MZI plotted in Fig. 4(c) and (d). As the waveguide loss is independent of the coupling condition the intrinsic Q factor maintained an almost constant value of 1.3×105 for all different tuning conditions, as plotted in the black circles of Fig. 4(c), corresponding to an estimated 3.9 dB/cm of background waveguide loss. However, tuning of the MZI heater does have a significant effect on the measured extrinsic Q factor of the ring due to the changes in the ring/bus coupling condition, as plotted in the green squares of Fig. 4(c). Initially when no heat was applied to the MZI the ring was fit to have an extrinsic Q factor of 3×104, corresponding to a ring/bus cross-over ratio of 0.32, in good agreement with the cross-over ratio expected from the standalone MZI measurements and model. As more power is applied to the MZI heater the ring/bus cross-over ratio evolves in accordance with the relationship established in the previous section, whereby using this relationship with a KDC of 0.08 and Pπ of 38 mW, the expected Qe for the different heater powers was modelled and is plotted by the solid green line in Fig. 4(c). As the extrinsic Q factor is dependent on the coupling losses, it is inversely proportional to the ring/bus cross-over ratio, starting at a low value when the cross-over ratio is large and then increasing asymptotically towards infinity as the cross-over ratio decreases towards 0 at a π phase shift in the MZI arm. Between a π and 2π phase shift the trend is mirrored as it enters the second half of the sine-wave coupling relation, before repeating the cycle. With regard to extinction ratio (ER), it follows similar trends relative to phase shift/applied heater power as the extrinsic Q factor, because the ER is primarily determined by the relationship between Qe and Qi as:
Initially, with no applied heater power, the ring has an extinction ratio of 4.6 dB and the Qe of the ring is lower than Qi, placing the ring in an overcoupled state where ring/bus coupling is the dominant form of loss. As the Qe increases with heater power the extinction ratio of the resonances increases asymptotically, reaching the critical coupling condition, where Qe is equal to Qi around a heater power of 23 mW, corresponding to a ring/bus cross-over ratio of approximately 0.08. As the power is increased beyond this the extinction ratio decreases towards 0 at the point where the heater reaches a π phase shift, before mirroring and repeating in the 2π cycle. The estimated extinction ratio of the ring for different heater powers was calculated using Eq. (2) based on the extrinsic Q factor model and a constant intrinsic Q factor of 1.3×105 plotted against the measured results in Fig. 4(d)
Fig. 4. Transmission spectrum of tunable coupler ring for different applied (a) ring phase shift and (b) MZI heater powers. Measured and simulated ring (c) intrinsic and extrinsic Q factors and (d) extinction ratios versus applied MZI heater power and phase shift, assuming a Pπ of 38 mW.
At 1550 nm the ring operates similarly in terms of trends, but is scaled by different parameters. Here, the ring was found to have an intrinsic Q factor of 1.6×105, which although is slightly higher than that measured at 1930 nm, corresponds to a larger waveguide loss of 4.5 dB/cm. When no power was applied to the MZI heater the extrinsic Q factor was found to be 6×104 corresponding to a ring/bus coupling ratio of 0.22, which was then able to be tuned with a periodicity corresponding to a Pπ of 26 mW, in agreement with the values found in the standalone MZI device. The extinction ratio was then initially 7.5 dB with no applied power to the heater and required approximately 13 mW to reach critical coupling
4.2 Tunable time delay
Following optical characterization, the time delay performance of the ring around 1930 nm was measured based on the phase delay of a modulated signal travelling through the chip using the setup shown in Fig. 5(a). Although this is an indirect method to measure time delay it was implemented because of its wavelength insensitivity and ability to be performed at relatively low RF speeds such that high bandwidth electrical components are not required. In this setup the signal from the 1930-nm tunable laser was initially left at a fixed wavelength which was modulated with a fiber-coupled LiNbO3 amplitude modulator by an RF sine-wave at a frequency of 150 MHz (fRF) generated by an electrical vector network analyzer (VNA). The modulated signal was then launched into the bus waveguide of the TCR circuit and collected on the opposite side where it was routed into an extended InGaAs photodetector (EOT ET-5000F). The VNA was then used to measure the phase difference (ψDiff) between the RF signal sent to the modulator and the RF signal measured by the photodetector. The phase difference can then be used to calculate the time delay, tDelay, generated in the ring by:
Fig. 5. (a) Diagram of setup used to measure time delay through the device. (b) Measured and simulated time delay dispersion profiles versus relative wavelength around resonance with the ring at a cross-over ratio of 0.28 and 0.15. (c) Measured and simulated peak on resonance time delay versus tunable coupler ring cross-over ratio.
This method was used to calculate the time delay of the ring versus wavelength for various powers applied to the MZI heater. From Refs. [45–47,50] the spectral time delay response of the ring is expected to follow a dispersive delay profile, with a peak delay at the resonant wavelength and a decay at wavelengths away from resonance. The delay profile can be tuned by altering the ring/bus coupling condition through the MZI heater to achieve tunable time delay. The measured and simulated time delay for the TCR versus relative wavelength at cross-over ratios of 0.28 and 0.15 are shown in Fig. 5(b). For a cross-over ratio of 0.28, where the extinction ratio of the resonant mode is 5.1 dB, the time delay peaks at a value of 190 ps near the resonant wavelength and broadly reduces to no delay away from resonance. As the cross-over ratio is brought closer towards critical coupling, the time delay has a larger peak value as well as a narrower wavelength bandwidth, as can be seen for a cross-over ratio of 0.15, where the extinction ratio is 11.0 dB and the peak measured time delay is 560 ps. This measurement was performed for several different cross-over ratio values between 0.28 and 0.15, with the peak time delay values plotted in Fig. 5(c) against the expected theoretical values using Eq. (1) in [50], showing good agreement between the two. The dynamic delay range achieved here of 370 ps would be suitable (for example by using cascaded devices) for use in a low-earth orbit satellite link, which would require approximately 540 ps of dynamic delay to achieve a beam steering angle of ±50° in a 512 antenna element array [Ref. 68]. These results demonstrate effective operation of the tunable-coupler ring as a time delay element operating in the TDFA wavelength window.
5. Conclusion
We have demonstrated and characterized a Mach-Zehnder interferometer ring resonator operating in the TDFA wavelength window. The results provide a thorough understanding of the operation of the ring and its base components, with the performance accurately modeled and compared to their operation around a wavelength of 1550 nm. The demonstration of tunable time delay in this structure at these extended wavelengths motivates the future development of silicon photonic phased array and beamforming networks built around the TDFA window, taking advantage of performance benefits and integration with recently developed sources, modulators and detectors.
Funding
MacDonald, Dettwiler and Associates (MDA); High-throughput and Secure Networks Challenge program; Canada Foundation for Innovation; National Research Council Canada; Natural Sciences and Engineering Research Council of Canada; Mitacs.
Acknowledgements
The authors would like to acknowledge Alex Strong and Khaled Ahmed from MDA for helpful discussions. Also, authors would like to thank Jessica Zhang, Dan Deptuck, Sarah J. Neville, Susan Xu and Gayathri Singh from CMC Microsystem for their assistance in chip fabrication.
Disclosures
The authors declare no conflicts of interest.
Data Availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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