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Abstract
We report a novel microwave photonic phase and amplitude control structure based on a single microring resonator with a tunable Mach Zehnder interferometer reflective loop, which enables the realization of a continuously tunable microwave photonic phase shifter with enhanced phase tuning range while simultaneously compensating for the RF power variations. The complimentary tuning of the phase and amplitude presents a simplistic approach to resolve the inherent trade-off between maintaining a full RF phase shift while eliminating large RF power variations. Detailed simulations have been carried out to analyze the performance of the new structure as a microwave photonic phase shifter, where the reflective nature of the proposed configuration shows an effective doubling of the phase range while the amplitude compensation module provides a parallel control to potentially reduce the RF amplitude variations to virtually zero. The phase range enhancement, which is first verified experimentally with a passive only chip, demonstrates the capability to achieve a continuously tunable RF phase shift of 0–510° with an RF amplitude variation of 9 dB. Meanwhile, the amplitude compensation scheme is incorporated onto an active chip with a continuously tunable RF phase shift of 0–150°, where the RF power variations is shown to be reduced by 5 dB while maintaining a constant RF phase shift.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The field of microwave photonics (MWP) elegantly combines the use of photonic techniques to generate, distribute, and process radio frequency (RF) signals in the optical domain. Enabling superior performance enhancement over classical microwave techniques, this has spurred efforts to pursue MWP phase shifters to achieve large bandwidth, fast tunability, immunity towards electromagnetic interference, reconfigurability for phased-array beamforming networks, ultrahigh-speed signal processing, and microwave photonic filters for radar, defense, and satellite communication systems [1–3]. Whilst initial efforts of implementing MWP phase shifters have been realized using discrete photonic components such as 2-dimensional liquid crystal on silicon [4], fiber based stimulated Brillouin scattering [5], vector-sum [6], optical filters [7,8], semiconductor optical amplifiers [9], fiber Bragg gratings [10], and electro-optic modulators [11,12], the last decade has seen a paradigm shift towards miniaturizing these MWP devices. Various integration based platforms such as silicon-on-insulator (SOI) [13], silicon nitride [14], chalcogenide waveguides [15], and aluminum nitride [16] have since been explored to realize compact and lightweight MWP phase shifters.
Across these various platforms, ring resonators remain a favorable structure to realize MWP phase shifting on a nanoscale level [17–19] due to its simplistic and compact design as well as straightforward tuning scheme. To achieve a full 360° phase shift range, multiple cascaded ring resonator schemes have been proposed [20,21] to extend the phase tuning range. However, the use of multiple ring resonators poses two drawbacks. The first is the omnipresent amplitude variations which are exacerbated further due to the cascaded notch responses of the ring resonator. Thus, large phase range and low power variations still remain elusive. To minimize these variations, the ring resonator could be designed with a lower quality (Q) factor. However, the fundamental design trade-off between a low Q ring response and slow phase transient irrevocably impacts the performance of the MWP phase shifters at lower RF frequencies, thereby limiting the operation frequency range. Recognizing this conflict, an electrically tunable feedback-coupled ring resonator has recently been proposed, which has experimentally demonstrated a phase tuning range of 172° while reducing the amplitude variations by 4 dB [22]. Meanwhile, the second challenge lies in the rigidness in tolerating fabrication errors arising between different ring resonators. One possible approach to resolve the inconsistencies between the fabricated microrings is by incorporating microheaters to compensate for the fabrication errors. However, such a scheme would typically involve separate and precise synchronization between the microheaters to alter the resonance characterization of the individual ring resonators. Consequently, thermal crosstalk becomes a major issue as it is difficult to isolate the thermal effects of the microheaters employed for the fabrication correction from the ones responsible for controlling the tuning of the MWP phase shifts due to the close proximity of the microheaters located on the same microring.
In this paper, we propose and demonstrate a new MWP phase shifter which aims to address the aforementioned limitations, demonstrating the capability to double the phase range by using only a single microring resonator while simultaneously tackling the problem of large RF power variations via an amplitude compensation scheme. The novel structure features two main modules: (i) a ring resonator with a reflective loop to provide a return route for light to travel back across the same ring resonator twice, thus effectively doubling the achievable phase shift and (ii) a tunable Mach Zehnder interferometer (MZI) to equalize the amplitude variations. Experimental results demonstrate the capability to achieve continuously tunable RF phase shift of 0–510° from 3 GHz–40 GHz and up to 5dB of RF power compensation while keeping the RF phase constant.
2. Operation principle, design, and simulations
The schematic diagram of the proposed MWP phase shifter is shown in Fig. 1. The key feature of the setup is characterized by a tunable MZI reflective resonator (TMZI-RR), which consists of a single all-pass microring resonator to perform the phase shift, a tunable coupler based on MZI which serves to compensate for the amplitude variations, and a reflective phase enhancement loop to double the achievable phase range. The MWP phase shifter operation follows the simple conventional single sideband (SSB) modulation scheme to achieve a direct relationship between the optical and microwave phase shift [20]. Light from the laser diode (LD) is sent to the Mach Zehnder modulator where an RF signal is modulated onto the light to generate an optical signal consisting of an optical carrier and an optical SSB at angular frequencies w0 and wRF, respectively. Launching this SSB signal into the TMZI-RR, the light first passes through a single all-pass microring resonator whose intensity transmission is given as [23]
where a is the single-pass amplitude transmission taking into account the propagation loss in the ring. φ = βL is the single-pass phase shift with L and β denoting the round-trip length and propagation constant of the circulating mode, respectively. r is the self-coupling coefficient of the coupling region between the bus waveguide and the microring resonator. The light is then propagated through the MZI-based tunable coupler before reaching the reflective loop where it circulates along the reflective path, guiding light back to the microring resonator and exiting via its original point of entry. One arm of the MZI is designed to be tunable to enable controllability of the ratio of transmitted and reflected light. With this reflective mechanism, the output signal would thereby appear to have travelled the single all-pass ring resonator twice and the resulting optical field of the reflected light can thus be expressed by a second order transfer function of the single all-pass phase shifter aswhere rc1,2 and kc1,2 are defined as the self- and cross-coupling coefficients of the first and second couplers, respectively. Assuming a lossless coupler, the power splitting ratios of the coupler are assumed to satisfy the relation rc1,22 + kc1,22 = 1. φ1 and φ2 represent the phase shift at the top and bottom arms of the MZI, respectively, while φc represents the accumulated round-trip phase shift in the reflective loop. The transfer function of the proposed TMZI-RR in Eq. (2), which encompasses a second order term (A2) to illustrate the phase doubling effect, also contains an amplitude compensation term which is governed by the coupling coefficients of the two couplers (rc1,2, kc1,2) and the phase difference between the two MZI arms (φ1, φ2). Treating φc as a constant, it can therefore be understood that the length of the reflective loop carries no effect on the relative amplitude compensation.
Fig. 1 Schematic diagram of the MWP phase shifter based on single microring reflector with tunable MZI coupler showing the phase doubling effect and amplitude compensation range denoted by Δ|H(ω)|.
As light exits the TMZI-RR, a circulator is used to guide the back-reflected light into the photodetector (PD). The beating of the optical carrier and the optical SSB at the PD provides a direct relationship between the optical and microwave interaction, generating an output RF photocurrent described as
where is the responsivity of the photodiode, |H(ωc)| and |H(ωRF)| are the amplitudes of the optical carrier and the optical sideband, respectively, which takes into consideration the initial amplitude of the two frequency components and the acquired transmission gains at the corresponding frequencies of the TMZI-RR response. θc and θRF are the respective phase shifts imparted onto the optical carrier and optical sideband after passing through the TMZI-RR. The resulting phase of the output RF signal is therefore determined by the effective optical phase difference between the optical carrier and its sideband, which can be varied by detuning the resonance wavelength of the all-pass ring resonator relative to the optical carrier wavelength to generate different θc. However, in conventional MWP phase shifters which typically provide tunability for only one degree of freedom that is phase tuning, the change in θc will inevitably induce a corresponding amplitude change Δ|H(ωc)|, causing the amplitude of the RF response to vary with the RF phase, which is a highly undesirable trait. In contrast, the novel TMZI-RR structure, which offers two degrees of freedom for both phase and amplitude control, features two distinct advantages: (i) phase doubling to achieve full 360° RF phase tuning range and (ii) amplitude compensation to maintain minimal RF power variations.We first perform simulation to investigate the phase doubling effect of the proposed structure by comparing the attainable phase range at different RF frequencies with the reflective structure as opposed to using conventional MWP phase shifters based on single microring resonators. The two structures are simulated with the same ring circumference of 130 µm and same optical notch depth of about 8.7 dB for a fair comparison of the phase shifting operation. Figure 2 shows the phase shifts that can be obtained at various operating frequencies investigated from 2.25 – 40 GHz. The two structures are plotted together for comparison with circle and diamond markers representing the single all-pass microring resonator and TMZI-RR, respectively. The red horizontal dashed line shows the targeted range for a complete MWP phase shifter operation of 360°. It is evident that by employing the reflective TMZI-RR structure, the operating frequency range can be extended significantly, showing a full 360° phase shift even at 2.25 GHz which is far superior than the performance of the single all-pass microring resonator displaying only 250° phase shift for the same frequency evaluated. Moreover, the phase shifter with the single all-pass microring resonator also failed to achieve the desirable 360° phase shift even for frequencies up to 40 GHz, showing only up to a maximum phase shift of 350°. On the other hand, owing to the doubled phase range offered by the reflective structure, the RF phase shift for all frequencies greater than 2.25 GHz is shown to exceed 360°. As such, the operational frequency range of the phase shifter is improved tremendously covering lower RF frequencies, thus providing the capability to extend the bandwidth of the MWP phase shifter.
Fig. 2 Comparison between the simulated RF phase shifting range at different RF frequencies using conventional single all-pass microring resonator and the proposed TMZI-RR.
Next, we address the problem in typical microring resonator based MWP phase shifters which exhibit large optical amplitude variations. This issue is prevalent especially in the case of such reflective structure with cascaded response where we anticipate an optical notch of twice the depth, thus potentially doubling the RF power variations. The TMZI-RR proposes to overcome this detrimental drawback by incorporating an amplitude compensation module based on a tunable MZI reflective loop to equalize the RF power variations. As can be seen from Eq. (2), the design of the coupling sections (rc1,2, kc1,2) of the TMZI-RR is critical to obtain the desired amplitude compensation. Using microheaters to tune the lower arm of the MZI, the change in phase Δφ due to the thermo-optic effect is given by
where λ is the optical wavelength, Δneff is the change in effective index due to the change in temperature ΔT, dnsi/dT is the thermo-optic coefficient of silicon (1.86 × 10−4 K−1 at λ = 1550nm) [24], and L is the length of the microheater.Assuming a modest temperature variation of 10.75 K which results in a maximum variation of Δneff,max = 0.002, Fig. 3 investigates the relationship between the amount of reflected power under different combinations of coupling parameters kc1 and kc2. The simulation result in Fig. 3(a) shows the maximum range of optical power variations that can be achieved for different coupling parameters ranging from 0.01 to 0.9. The symmetrical mapping of the coupling coefficients suggests that the optical feedback power experiences the most dramatic power variations when both coupling parameters are designed either about the middle range with more or less equal strength or with complementary opposing values i.e. increasing kc1 with decreasing kc2 or vice versa. It should be noted that in order to provide compensation for a ring resonator with, for example, an optical notch depth of 8.7 dB, the amplitude compensation module of the TMZI-RR only needs to provide half the equivalent amount of required optical power variance. This can be observed from Eq. (3), where introducing the same amount of optical attenuation to both the optical carrier and optical sideband results in a collective effect of doubling the overall electrical photocurrent. Consequently, this leads to double the amount of RF power variations which is in turn equivalent to the optical notch depth. The shaded regions in Fig. 3(a) highlights the range of values for kc1 and kc2 marked in red, which enable a minimum optical power variance of 4.35 dB in order to compensate for the 8.7 dB optical notch depth introduced by the ring resonator. Meanwhile, Fig. 3(b) illustrates the corresponding power attenuation that is involved to equalize the RF power variations. Large power compensation is available but at the expense of greater attenuation, thus posing a trade-off between RF power variations and the resulting intensity of the RF signal. However, this can be overcome by using optical amplifiers to boost the overall power of the RF signal. Thus, it is desirable to determine the optimum coupling parameters which introduce just enough compensation without severely degrading the signal.
Fig. 3 Simulated optical power variations for different combinations of coupling parameters kc1 and kc2. (a) Maximum range of optical power variations (b) Maximum optical signal attenuation.
Using values kc1 = 0.41 and kc2 = 0.7, we investigate the effectiveness of the TMZI-RR in compensating for the RF amplitude while performing the MWP phase shifter operation. Figure 4 maps the relationship between the RF phase shift and the corresponding RF power under different refractive index variations Δneff. By tuning the arm of the MZI, we are able to shift the RF power spectrum to achieve different power variations. As predicted, RF power variation of up to 8.7 dB is achieved when Δneff = 0.002. The inset of Fig. 4 shows an example for maintaining a constant RF power of about −19 dB, as depicted by the blue horizontal dashed line across the plot, where different RF phase shifts can be obtained under this same RF power, thus effectively equalizing the RF power variations. This successfully demonstrates that the proposed MWP phase shifter based on the novel TMZI-RR structure is able to achieve the full 360° RF phase tuning range while providing amplitude controllability to theoretically decrease the RF power variations to zero.
Fig. 4 RF power variations at different RF phase shifts under different refractive index variations Δneff.
3. Experimental measurements
The experimental testing of the proposed TMZI-RR was carried out in two stages. First, we fabricated a passive only TMZI-RR device on a SOI wafer via ePIXfab where the height of the silicon core waveguide is 220 nm and the waveguide width is 450 nm for both the bus and racetracks. Figure 5(a) displays the scanning electron microscope (SEM) image of the device, which consists only of the reflective loop and a single all-pass microring resonator. This allows us to investigate independently the role of the reflective loop in establishing the phase doubling effect. The measured optical response is displayed in Fig. 5(b), showing an optical notch depth of around 8.7 dB. Next, the fully active tuning platform with the inclusion of the tunable MZI coupler is integrated and fabricated at the University of California, Santa Barbara, as illustrated in Fig. 5(c). To ensure isolation of the thermo-optic effects, the distance between the successive microheaters on top of the microring and the MZI arm is placed far apart enough (>100 μm) to prevent thermal crosstalk. Figure 5(d) shows the measured optical response of the fabricated device. Both chips comprise an all-pass microring resonator with a ring circumference of 130 µm and designed to have a coupling coefficient of 0.02565. For simplicity, the first and second couplers of the amplitude compensation module were designed with 3-dB couplers, translating to a power coupling coefficient of 0.5.
Fig. 5 SEM images and measured optical responses of the fabricated TMZI-RR devices (a)-(b) Passive only device (c)-(d) Active device with microheaters.
Due to the absence of active tuning elements on the passive device, the phase shifter operation was accomplished by placing the chip under a heat sink with a temperature controller (Newport 325) to emulate the thermo-optic effects of a microheater. By inducing a change in the controlled temperature, the wavelength of the optical resonance is shifted accordingly, thereby changing the phase of the optical carrier. Alternatively, different phase shifts can also be achieved by fixing the resonance location and varying the wavelength of the laser source instead. Figure 6(a) shows the experimental results of the MWP phase shifter operation based on the passive only device which was produced at different temperature settings. It is clearly evident that the RF phase range has been successfully extended beyond 360° with a phase shifter operation of 0-510°, while the corresponding RF power variations, which is depicted in Fig. 6(b), display maximum variations of up to 9 dB.
Fig. 6 MWP phase shifter operation based on passive only device. (a) RF phase shifts (b) RF power variations.
Next, the fully active TMZI-RR device with amplitude compensation module was tested. Figure 7(a) shows the realization of the MWP phase shifter of the active device via thermal tuning of the ring resonator to achieve different RF phase shifts while Fig. 7(b) shows the corresponding RF amplitude variations. A total RF phase shift of 0-150° is achieved which is accompanied by 7 dB of RF amplitude variations for a frequency range of 12 – 26 GHz. Based on the measured performance of the active MWP phase shifter, there are two major issues that need to be addressed. Firstly, the operation range of the RF frequency presented in the case of the active chip was smaller. This is mainly attributed to the higher insertion loss incurred in the active chip thereby causing degradation in the RF signal, thus limiting the operating RF frequency range. Secondly, the phase shift achieved by the active device was not as large as the phase shift measured using the passive device. Despite both structures exhibiting similar notch depths, further investigation of the optical phase response of the active device shows different phase characteristics. The inset of Fig. 7(a) shows the measured optical phase profile of the active TMZI-RR device using an optical vector network analyzer, which reveals a phase shift of about 185°. This small transient in the optical phase response exhibited by the active device suggests that the microring resonator is operating under the under-coupling regime [25]. One possible reason could be due to the unanticipated increase in loss incurred in the active device, thus causing the actual coupling conditions of the ring resonator to differ from the intended designed values. To achieve a larger phase shift, the additional waveguide loss incurred in the active device should be taken into consideration in order to ensure that the microring resonator works in the over-coupling regime. In view of the reflective nature of the proposed structure, the challenge of incorporating circulators on an integrated platform needs to be considered in order to fulfill the realization of a fully integrated MWP phase shifter. Recently, the integration prospects of providing on-chip isolations which breaks the light reciprocity by bonding magneto-optic materials demonstrates the capability of achieving large-scale integration of nonreciprocal components such as isolators and circulators with extremely small footprint [26]. Furthermore, with the advent of heterogenous integration, the future of silicon photonics integration holds promising solutions to include laser sources, modulators, and photodetectors on a single photonic platform [27], thereby greatly reducing the insertion loss resulting from the fiber-to-chip coupling in between optical devices.
Fig. 7 MWP phase shifter operation based on active device. (a) RF phase shifts (b) RF power variations.
The active device provides the essential proof-of-concept validation of the independent tuning of the phase shifter and amplitude compensation module via the individual tuning elements based on microheaters. With the amplitude compensation module, we demonstrate that the initial RF amplitude variations of up to 7 dB exhibited by the active device can subsequently be reduced. This is illustrated in Fig. 8(a), which displays the maximum observed RF power variations when a DC voltage is applied to the microheater on one arm of the MZI, hence giving rise to a maximum difference in the relative RF power output of approximately 5 dB which is indicative of the range of RF power available for compensation. Whilst obvious changes in the RF power is evident, Fig. 8(b) shows that the RF phase on the other hand remains unchanged, thereby enunciating the ability of the proposed structure to achieve up to 5 dB of observed amplitude compensation while maintaining a constant RF phase shift. This thus validates the concept of the amplitude control scheme which is capable of implementing a phase-independent tuning of the RF power to compensate for the accompanying power variations resulted from the different RF phase shifts, without interrupting the MWP phase shifter operation.
Fig. 8 MWP phase shifter operation based on active device. (a) RF power variations (b) RF phase shifts.
4. Conclusion
In conclusion, we have reported a novel MWP phase shifter configuration which provides two degrees of freedom tuning for phase and amplitude control. The proposed structure demonstrates the capability to double the phase range by using only a single microring resonator to resolve the challenging issue confronting MWP phase shifters which is the large RF power variations. We present simulation analysis to investigate optimal design which enables the sufficient compensation of the RF power variations in MWP phase shifters. Experimental results show that the device has the capability to achieve continuously tunable RF phase shifts of 0-510° using the passive only chip while the isolated tuning of the MZI arms on the active chip shows phase-independent RF power change, thus greatly reducing the RF power variations by 5 dB for a constant RF phase shift.
Funding
Australian Research Council Discovery Project.
Acknowledgments
This work was supported in part by the Australian Research Council Discovery Project. Suen Xin Chew and Xiaoke Yi would like to acknowledge valuable discussions with Dr. L. Nguyen and Jianfu Wang from University of Sydney.
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