We present a novel broadband continuously tunable microwave photonic delay line consisting of a modulator, a four-stage microring resonator delay line, a tunable optical bandpass filter, and a photodetector. Unlike the traditional microring delay lines working at the on-resonant wavelength, the microring resonators in our chip work at the anti-resonant wavelengths, leading to a large delay bandwidth and a small delay ripple. The experimental results show that relative group delay can be continuously tuned from 0 to 160 ps for microwave frequencies in the range of 0 to 16 GHz. The delay ripple is less than 6.2 ps. These results represent an important step towards the realization of integrated continuously tunable delay lines demanded in broadband microwave phased array antennas.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Microwave photonics technology combines the advantages of microwave and photonic technologies to overcome the electronic bottlenecks of traditional microwave systems in terms of transmission length and processing bandwidth. Besides, in microwave photonics systems, the microwave signal is modulated onto an optical beam for signal transmission and processing. The high frequency of the optical signal and the good confinement of the transmission medium make it difficult for other electromagnetic waves to interfere with it. Therefore, it is considered as a destructive technology with the advantages of large bandwidth, low loss, and immunity to electromagnetic interference. It has great potential for development in the microwave and optical wave fields [1,2]. Optical true-time delay lines (OTTDLs) are one of the key components in microwave photonics [3,4], which can effectively avoid beam squinting issues in electronic phased array antennas (PAAs) [5,6]. For PAA applications, OTTDLs with a flat-top delay response and continuously tunable delay is highly required. In recent years, various integrated optical delay line structures have been proposed, such as microring resonators (MRRs) [7–9], gratings , photonic crystal waveguides [11,12], switchable delay lines [13,14], and so on. Among them, optical delay lines based on MRRs and optical switches have become a research hotspot in these years due to their simple structures and high scalability, which have been demonstrated on various material platforms.
Delay lines based on optical switches digitally select optical paths of progressively incremental lengths to obtain variable time delays. This scheme possesses a wide operating bandwidth and a large delay tuning range. Besides, the delay control algorithm is relatively simple. Recently, we have demonstrated a monolithically integrated microwave photonic beamformer on the silicon platform . It is composed of eight 5-bit digitally switchable OTTDLs, one electro-optic modulator, and eight photodetectors (PDs), which gives 31 distinguishable beam steering angles in the range of more than 150°. The delay amount is only discretely tuned, and the tuning resolution is limited by the minimum delay unit.
Compared with optical path switching-based OTTDLs, MRRs can provide continuously tunable delay by changing the coupling coefficient or the resonant wavelength of the MRRs. However, the time delay of a single MRR is limited by the delay-bandwidth product . It is difficult to obtain a large delay while maintaining a broad bandwidth. To increase the delay-bandwidth product, various cascaded MRRs are proposed and demonstrated, including the side-coupled integrated spaced sequence of resonators (SCISSOR) [17,18] and the coupled-resonator optical waveguides (CROW) [19–21]. Besides, the delay can be doubled via circulating through the MRRs twice using an integrated end mirror . Currently, most of the proposed MRR-based optical delay lines are working at the on-resonant wavelength. The delay spectrum around the resonance of the MRR exhibits a peak, rendering the delay to vary with the operating wavelength. Although a flat-top delay spectrum can be obtained by using SCISSORs or CROWs, the coupling coefficients and the resonant wavelengths of the MRRs should be precisely tuned, increases the control complexity. To get low delay ripples, the number of MRRs should be large. Compared to the on-resonant MRRs, the optical delay at the anti-resonant wavelength is relatively flat and broadband, despite a reduced delay tuning range. The maximum delay is the round-trip time of the MRR, which can be easily increased by using a larger MRR. However, the bandwidth for the flat delay is reduced as the free spectral range (FSR) of the MRR decreases.
In this paper, we report a novel microwave photonic tunable delay line based on four cascaded MRRs on the SOI platform. One of the MRRs is working at the anti-resonant wavelengths for continuous delay fine-tuning. The other three MRRs are digitally tuned to realize large delay coarse tuning, like the switchable OTTDLs. In this way, it works as a broadband long-range continuously tunable delay line. Besides, we also integrate a high-speed electro-optic modulator, a tunable bandpass filter (BPF), and a PD with the delay line to fully leverage the monolithic integration capability of silicon photonics. Our chip exhibits the advantages of compact size, wide operating bandwidth, and continuous delay tuning.
2. Device structure and working principle
Figure 1(a) shows the schematic diagram of our proposed microwave photonic delay line. It consists of a Mach-Zehnder modulator (MZM), an OTTDL, a tunable BPF, and a germanium PD. The modulator is based on a 2×2 single-drive push-pull carrier-depletion balanced MZM. One of the output ports is used as a test port to individually characterize the MZM performance. The cross-section of the MZM arms is shown in Fig. 1(b). The length of the modulation arm is 3 mm. The doping concentrations of the p- and n-type regions are ∼3 × 1017 cm−3 and ∼5 × 1017 cm−3, respectively. The heavy p++-doped regions with a doping concentration of ∼1 × 1020 cm−3 are located outside the MZM and connected to the signal (S) and the ground (G) electrodes of the traveling wave electrode. The heavily n++-doped region with a doping concentration of ∼1 × 1020 cm−3 is located between the MZM arms and connected to the DC bias electrode . A thermo-optic (TO) phase shifter is also positioned in the upper arm of the MZM for optical bias point control.
The OTTDL is composed of four MRRs, denoted as R1, R2, R3, and R4. The round-trip time τi (i = 1, 2, 3, 4) of the four MRRs are designed to be 20 ps, 20 ps, 40 ps, and 80 ps, corresponding to an FSR of 0.4 nm, 0.4 nm, 0.2 nm, and 0.1 nm, respectively. To flexibly adjust the coupling coefficient of MRRs, a tunable coupler based on a 2 × 2 symmetric Mach-Zehnder interferometer (MZI) is used at the MRR coupling point, as shown in Fig. 1(c). The equivalent coupling coefficient of the MRR is dependent on the phase difference between the two arms of the MZI as well as the performance of the two 3-dB couplers. A titanium nitride (TiN) TO phase shifter is placed in the bottom arm of the MZI coupler. The phase difference between the two arms can therefore be tuned by the phase shifter, which enables the adjustment of the coupling coefficient of the MRR resonator. As long as the two 3-dB couplers have a 50:50 power splitting ratio, the equivalent coupling coefficient of the MRR can be tuned from 0 to 1. The MRR waveguide is also integrated with another TO phase shifter to adjust the resonant wavelength. Air trenches are placed beside the phase shifters to improve thermal isolation.
The tunable BPF is based on a 2 × 2 dual-ring assisted MZI (DR-MZI) structure . Two identical racetrack-type MRRs are over-coupled with the interference arms of the 2 × 2 symmetrical MZI. These two MRRs are designed with a radius of 20 μm, a coupling length of 24 µm, and a coupling gap of 200 nm. TiN micro-heaters are integrated in MRRs and MZI arms. Etched air trenches around the micro-heaters are used to improve TO tuning efficiency while suppressing thermal crosstalk. In principle, when the two arms of the MZI are exactly equal in length, the two output ports have the maximum extinction ratio. Due to the fabrication-induced phase errors, the extinction ratio is limited, which can be compensated by the micro-heaters in the MZI arm. The micro-heaters in the MRRs are used to tune the resonant wavelengths. The optical bandwidth of the filter gradually increases when the resonant wavelengths are detuned. However, if the resonances are far apart, a dip appears in the passband, degrading the performance of the filter . Therefore, the filter passband width and extinction ratio can be tuned to a certain degree by the micro-heaters in the two MRRs and the two MZI arms of the DR-MZI, respectively. We use the BPF to filter out one of the sidebands and the optical carrier to realize microwave delay. If there were no BPF, we would need twice the delay bandwidth to achieve a flat delay response.
The working principle of the microwave photonic delay line is illustrated by insets A, B, and C in Fig. 1(a). The microwave signal is converted to the optical domain by double sideband modulation using the MZM. The modulated optical signal is then delayed by the OTTDL working at the anti-resonance. An optical BPF is followed to filter out the carrier and one sideband. Finally, the delayed optical signal is converted back to a microwave signal by a germanium PD with a vertical p-i-n diode.
3. Simulation of MRR delay element
We use the transfer matrix method to analyze the transmission characteristics of the MRR . Figures 2(a) and 2(b) show the simulated transmission and group delay spectra of an MRR with a round-trip time of 20 ps for different coupling coefficients, respectively. When the MZI is at the bar state (κ = 0), the microring is completely isolated, and the entire structure becomes a single waveguide with zero delays. When the MZI is at the cross-state (κ = 1), light passes the MRR waveguide only once. It is thus equivalent to a 20-ps delay line. In these two limiting cases, the delay response is flat. However, for other coupling coefficients, the delay is no longer flat around the on-resonant frequency. As shown in Fig. 2(b), a delay peak is present at the resonance wavelength. In contrast, at the anti-resonant wavelength of the MRR, the delay is almost flat over a large bandwidth of more than 30 GHz. Therefore, we can set the operating wavelength near the anti-resonant wavelength of the MRR to get a flat-delay response. By adjusting the coupling coefficient of the MRR, we can obtain a continuously tunable delay from 0 to 20 ps. Figures 2(c) and (d) show the simulated transmission loss and delay varying with the coupling coefficient at the resonant and anti-resonant wavelengths, respectively. It can be seen that the amount of delay and loss show a positive relationship. Although the delay is larger at the on-resonant wavelength, it is accompanied by a higher loss. On the other hand, the loss at the anti-resonant wavelength is relatively lower due to the smaller delay. It should be noted that for the same delay, the loss at the on-resonant and anti-resonant wavelengths is close because the effective waveguide length that the optical wave passes is equivalent.
The trade-off between bandwidth and delay still exists for an MRR working at the anti-resonant wavelengths. To obtain a large amount of delay in a wide bandwidth, we design four stages of MRRs with different FSRs. R1 is working at the anti-resonant wavelength. By adjusting the coupling coefficient, the delay is continuously tuned from 0 to 20 ps. The other three MRRs (R2∼R4) are working at the two special states with a coupling coefficient of either 0 or 1, corresponding to a delay of 0/20 ps, 0/40 ps, and 0/80 ps, respectively. Therefore, by combining with R1, we can obtain a continuously tunable delay from 0 to 160 ps. In general, the delay tuning range for an N-stage MRR delay line is 0∼2N-1τ1,2, where τ1,2 is the round-trip time of the first two MRRs.
4. Device fabrication and package
Figure 2(a) illustrates the layout of the optical delay line. The chip was fabricated on an 8-inch SOI wafer with a top silicon layer thickness of 220 nm and a buried oxide (BOX) layer thickness of 2 µm using complementary metal-oxide-semiconductor (CMOS) compatible processes. The chip was designed for transverse electrical (TE) polarization. Figure 3(b) shows the microscope image of the fabricated delay line. The chip footprint is 3.7 mm × 2.15 mm. The chip was attached to a specially designed package case using UV curable adhesive, as illustrated in Fig. 3(c). All the RF and DC pads were wire-bonded to a printed circuit board (PCB). The traveling wave electrode of the MZM was terminated to a 50-ohm resistor on the PCB. The microwave signal and DC control voltages were applied to the chip through the end launch connectors and the I-PEX cables, respectively. Two lens fibers were edge-coupled to the chip and fixed by UV curable adhesive. The coupling loss is 2.6 dB per facet at the 1550 nm wavelength. A thermo-electric cooler (TEC) was used to stabilize the chip temperature during the experiment.
5. Experimental results
5.1. Characterization of key elements
Figure 4(a) shows the normalized transmission spectra of the on-chip modulator under various reverse-bias voltages on one modulation arm. The on-chip insertion loss of the modulator is about 3.6 dB. The modulation efficiency Vπ·L is 1.8 V·cm. The static extinction ratio (ER) of the MZM is about 21 dB. We used a 43 GHz vector network analyzer (VNA, Anritsu, MS46522B) to measure the electro-optic (EO) S-parameter of the modulator. The microwave signal was applied to the modulator by a 40-GHz microwave probe. As shown in Fig. 4(b), the 3-dB EO bandwidth is approximately ∼11.3 GHz at 0 V bias. When the DC bias changes from −1 V to −3 V, the EO bandwidth increases from 15.8 GHz to 17.8 GHz.
Due to fabrication errors, the initial coupling coefficients of the four MRRs in the OTTDL are relatively random. Therefore, we first calibrate the voltages required to get the special coupling states (κ = 0, 1) one by one using a software-controlled multi-channel voltage source. For these two cases, there are no resonance dips in the transmission spectrum. Figure 5(a) shows the measured transmission spectra of the shortest and longest paths in the OTTDL when the coupling coefficients of the four MRRs are all adjusted to 0 and 1, respectively. The loss difference is about 2.1 dB, which means the largest path-dependent loss is 2.1 dB. With the assistance of the test port, we can get the loss difference of each MRR when the coupling coefficient is tuned from 0 to 1. The corresponding loss differences between the shortest (κ=0) and the longest (κ=1) delay of the four MRRs are 0.44 dB, 0.45 dB, 0.56 dB, and 0.65 dB, respectively. The loss can be reduced through further optimization of the design.
To demonstrate the continuous delay tuning, we set the coupling coefficients of R2, R3, and R4 to zero and applied different voltages (coupling tuning voltage VM1 and ring resonance tuning voltage VR1) to R1. Figure 5(b) shows the transmission spectra. When VM1 is 5.6 V and 7.2 V, the coupling coefficient of R1 is close to 1 and 0, respectively, and therefore, no resonance dips are observed. Under the other voltages, the resonance dips are discernable and they change with the applied voltage. In contrast, the transmission around the anti-resonant wavelength is almost the same, which can be employed to get low-loss optical delay.
Figure 6 shows the measured bar-port transmission spectra of the DR-MZI filter with different voltages applied on one MZI arm and one MRR. The FSR of the passband is 0.64 nm. The insertion loss at the filter center wavelength is about 1.3 dB. The sideband rejection ratio and bandwidth of the filter can be adjusted by the voltage applied on the MZI arm and the MRR of the DR-MZI, respectively. As we can see, when the voltage applied to the MZI arm Vm changes from 3.5 V to 5.4 V, the sideband rejection ratio increases from 12.8 dB to 20 dB. The filter passband roll-off rate also increases from 70.1 dB/nm to 99.8 dB/nm. When the voltage applied to the MRR Vr changes from 4 V to 5.8 V, the 3-dB bandwidth of the optical filter is tuned from 0.149 nm (18.62 GHz) to 0.175 nm (21.87 GHz). If we further expanded the 3-dB bandwidth by increasing the applied voltage, a dip would gradually appear in the passband, generating in-band ripples.
5.2. Characterization of the entire chip
Figure 7 depicts the experimental setup for microwave signal delay characterization. Continuous-wave (CW) light was generated by a tunable laser source (TLS, Santec-710) with a wavelength of 1549.85 nm and output power of 0 dBm. It was adjusted to TE polarization by a polarization controller (PC) and amplified by an erbium-doped fiber amplifier (EDFA) to ∼23 dBm before it is coupled to the chip. We used a multi-channel programmable voltage source to adjust the delay. The on-chip modulator worked at the quadrature point to produce double sidebands. It was driven by a microwave signal from the vector network analyzer (VNA, Anritsu, MS46522B). The modulated optical signal was then delayed by the OTTDL working at the anti-resonance wavelength. The optical carrier and one sideband were selected by the DR-MZI bandpass filter (BPF). Finally, the optical signal was converted back to the electrical domain by the on-chip PD and received by the VNA to retrieve the amplitude and phase responses of the microwave signal. The group delay was obtained from the derivative of phase to angular frequency.
Figures 8(a) and 8(b) show the measured delay and amplitude responses when the 4 MRRs are adjusted to the longest delay one by one, corresponding to the delay amount of 20 ps, 20 ps, 40 ps, and 80 ps. The measured delay amount is consistent with the theoretical calculations. The delay spectral response curve is relatively flat over the frequency range from 0 to 18 GHz. The maximum delay ripple is less than 4.5 ps, limited by the uncertainty of the phase measurement by the VNA. For different delay amounts, the variation in the microwave transmission loss is less than 1 dB. The loss difference for each ring is very low. We next fixed R2, R3, and R4 to the maximum delay and continuously tuned R1 by adjusting the applied voltages on the two phase shifters. In this case, the delay is continuously tuned from 140 ps to 160 ps. Figures 8(c) and 8(d) show the measured delay and amplitude responses. The delay ripple is less than 6.2 ps for frequencies lower than 16 GHz. The delay response degrades when the frequency exceeds 16 GHz possibly due to the passband limitation of the DR-MZI BPF. The amplitude response curves in Figs. 8(b) and 8(d) have similar characteristics, but the loss difference is about 3.4 dB. Considering that the microwave loss is twice the optical loss for a microwave photonic link, the microwave loss increment for the longer path measured in Fig. 8 is consistent with the optical loss increment measured in Fig. 5.
We compare our work with other MRR-based OTTDL chips, as illustrated in Table 1. Our device exhibits the largest operating bandwidth and the smallest delay ripple. Besides, our chip has a high level of integration including the modulator and PD in the same chip, which makes the system more compact and also more robust to the environment vibrations. To further increase the delay tuning range, we can cascade more stages of digitally switched MRRs. Because the MRRs work at the anti-resonate wavelengths, loss and delay ripple will not increase significantly with the number of MRRs, unlike the MRR delay lines working at the resonant wavelengths.
We have demonstrated a monolithically integrated tunable microwave photonic delay line on the SOI platform, including a modulator, a cascaded MRR-based OTTDL, a DR-MZI bandpass filter, and a PD. At the anti-resonant wavelengths of the MRR, we simultaneously achieve a large delay bandwidth and a small delay ripple. The experimental results show that continuous delay tuning from 0 to 160 ps is achieved. The delay bandwidth is 16 GHz and the delay ripple is less than 6.2 ps. The delay line can be easily scaled up by cascading more digitally switched MRRs. A larger operating bandwidth can be obtained by increasing the 3-dB bandwidth of the bandpass filter and reducing the size of MRRs but at the expense of a reduced delay tuning range.
National Key Research and Development Program of China (2018YFB2201702, 2019YFB1802903, 2019YFB2203200); National Natural Science Foundation of China (61535006, 61705129, 62075128); Science and Technology Commission of Shanghai Municipality (2017SHZDZX03); Wuhan National Laboratory for Optoelectronics (2019WNLOKF004).
The authors declare no conflicts of interest.
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