Linearized integrated microwave photonic circuit for filtering and phase shifting

The development of cognitive and intelligent radio frequency (RF) systems calls for RF front-ends with multiple programmable functions.^1,2 Microwave photonic (MWP) circuits with high bandwidth and flexible reconfigurability can play an important role in those modern RF systems.^3–5 A number of approaches have been proposed to achieve programmable RF photonic circuits. Functionalities including filtering,^6–10 phase shifting,^11–15 and beamforming^16–18 have been realized in application-specific photonic integrated circuits (PIC) that can achieve high performance. Another approach is to create general-purpose integrated MWP circuits that can be configured for a high number of functionalities. These circuits have been demonstrated in the form of waveguide mesh or micro-disk resonator arrays.^19–22 Finally, cascaded MWP systems that can simultaneously perform notch filtering and phase shifting have also been demonstrated in Refs. 23 and 24.

To be relevant for real-world applications, MWP circuits need to be more than just programmable. Sufficient RF performance, such as a low noise figure and high spurious-free dynamic range (SFDR), is also required so that they can be placed at the beginning of the RF front-end chain, where the broad bandwidth is most needed.

However, to date, most demonstrated integrated MWP systems are plagued by poor SFDR.^6,10,25,26 There is an opportunity to circumvent this problem by implementing linearization techniques.^27–30 However, only very recently can RF photonic functionality and linearization be simultaneously achieved in the same system, for example, in fiber-based phase shifters³¹ or in a chip-based programmable filter.³² Extending this concept to the multi-functional circuit will be important for the MWP field.

In this work, we present a reconfigurable integrated MWP circuit that can perform two distinct functionalities, i.e., a tunable notch filter and a 2π-range phase shifter, while maintaining an ultrahigh SFDR. The tunable notch filter function can achieve a rejection of more than 50 dB with a 3-dB bandwidth of 315 MHz from 6 to 16 GHz. In the meantime, the tunable phase shifter function can realize 2π continuous tuning range from 12 to 20 GHz. Moreover, both functions can provide an SFDR larger than 120 dB · Hz^4/5. This work shows the important first step toward high-performance integrated MWP subsystems with a large number of RF functions.

The concept of our reconfigurable integrated MWP circuit is illustrated in Fig. 1. It can serve as a notch filter or a phase shifter with different configurations. By manipulating the phases and amplitudes of multi-order optical sidebands independently, both functionalities can also exhibit an ultrahigh dynamic range.

The multi-functional integrated MWP circuit is fabricated with the Si₃N₄ TriPleX process.^33,34 Figure 1(a) shows the micrograph of the chip. It consists of two spectral deinterleavers, a tunable attenuator, an optical phase shifter, and an array of all-pass ring resonators, as illustrated in Fig. 1(b). The spectral deinterleavers are implemented with an asymmetric Mach–Zehnder interferometer (aMZI) loaded with 3 ring resonators,³⁵ exhibiting flat-top complementary filter response with a free spectral range (FSR) of 160 GHz.²⁹ The array of all-pass ring resonators (FSR of 50 GHz) between the two deinterleavers has a tunable coupling ratio and round trip phase that can be controlled via thermo-optic tuning. By applying different voltages to the microheaters, this chip can be configured as a notch filter or a phase shifter while maintaining an ultrahigh SFDR, as indicated in Figs. 1(c) and 1(d).

The signal processing of both functionalities, as shown in Figs. 1(e) and 1(f), follows a similar procedure. First, the deinterleaver separates the input optical spectrum into two paths. The attenuator and phase shifter in the upper path change the relative phase and amplitude between the two paths for wideband spectrum shaping. In the lower path, an array of all-pass ring resonators performs narrowband phase and amplitude tailoring with high precision. The signals in the two paths are then recombined and directly coupled out of the chip or coupled into the second deinterleaver for further processing. By tailoring the amplitudes and phases of the optical carrier and multi-order sidebands in such a manner, the third-order intermodulation distortion (IMD3) from beating products between different optical sidebands can destructively interfere with each other, leading to an ultrahigh dynamic range.

We first configure this multi-functional MWP circuit into a high-rejection notch filter with enhanced SFDR. A phase-modulated optical signal is sent to the programmable photonic chip for spectral shaping. Because of the nonlinearity of the phase modulator, the modulated optical signal consists of multi-order sidebands, among which the ±first-order sidebands have a π phase difference. We use the spectral deinterleaver to first separate the optical carrier and lower sidebands from the upper sidebands. Then, the lower sidebands and optical carrier are attenuated to generate the asymmetric double sideband (aDSB) spectrum while simultaneously satisfying the linearization condition.³² Meanwhile, an under-coupled ring resonator imposes a shallow notch at the upper sideband to meet equal amplitude and anti-phase conditions at the notch frequency. After that, the signal is recombined, and a high rejection MWP notch filter based on RF interference is created.³⁶ Because of the destructive interference between different IMD3 components, the linearity of the notch filter can be significantly improved (see the supplementary material for more experimental details).

The performance of the linearized MWP notch filter is shown in Fig. 2. To highlight the advantages of our filter, we compared it with a standard single sideband (SSB) modulated MWP notch filter using the same ring response without linearization. Our demonstrated notch filter has a notch rejection of 52 dB and a 3-dB bandwidth of 315 MHz (at the center frequency of 12 GHz), as shown in Fig. 2(a). We also performed the two-tone test to evaluate the linearity of the demonstrated notch filter. The results shown in Fig. 2(b) exhibit an IMD3 suppression of 28.7 dB when the two-tone RF frequency is at 9 GHz with a spacing of 10 MHz. The large suppression of the IMD3 makes the fifth-order nonlinearity dominate, leading to SFDR improvement from 103.8 dB · Hz^2/3 to 123.6 dB · Hz^4/5 with a noise floor of −164.5 dBm/Hz, as observed in Fig. 2(c). Moreover, the filter maintains high rejection and a large SFDR during the tuning of the notch frequency, as shown in Fig. 2(d). The filter shows a link gain of −26.3 dB and a noise figure (NF) of 35.8 dB. This moderate gain and NF are because of the partially destructive interference between two first-order sidebands (see the supplementary material for extended measurement results).

The chip can also be reconfigured as a 2π-range tunable phase shifter while maintaining the ultrahigh dynamic range. To realize the phase shifting function, we first use an external bandpass filter (BPF) to remove the lower sideband of the phase-modulated optical signal. We also attenuate the optical carrier with the BPF by positioning the optical carrier at the transition band for the purpose of linearization. The preprocessed single-sideband (SSB) optical signal is then coupled into the chip. We separate the optical carrier and the +first-order sideband from the +second-order sideband with the spectral deinterleaver. By applying two over-coupled ring resonators at the optical carrier, the 2π-range phase shift is introduced with negligible amplitude variation. The optical phase shift is then converted to the RF phase shift when the optical carrier beats with the +first-order sideband at the photodetector (see the supplementary material for the experimental setup). To suppress the IMD3 and improve the linearity of the phase shifter, both amplitude and phase conditions between the optical carrier and the +second-order sideband need to be satisfied (see the supplementary material for the linearization method). On the one hand, we adjust the power ratio between the optical carrier and +second-order sideband by controlling the passband location of the external BPF and the coupling coefficients of the ring resonators. On the other hand, we tune the phase of the +second-order sideband with an optical phase shifter to satisfy the phase condition, while the MWP phase shifter tunes over the 2π range.

The performance of the demonstrated phase shifter is presented in Fig. 3. We can realize continuous phase tuning over the 2π range by adjusting the resonance frequencies of the two over-coupled rings. Because of the relatively flat stopband response of the two cascaded ring resonators, the amplitude fluctuation of the phase shifter is less than 1 dB over the whole 2π range, as shown in Fig. 3(a). To evaluate the linearity of the demonstrated phase shifter, we also conducted a two-tone test and compared the results with a phase shifter without linearization for the same phase shift (0.6 π in this case). As shown in Fig. 3(b), the linearized phase shifter exhibits an improved fundamental to IMD3 ratio of 20 dB. As a result, the SFDR of the phase shifter increases from 105.2 dB · Hz^2/3 to 121.2 dB · Hz^4/5, which is shown in Fig. 3(c). The noise floor of both phase shifters is −146.8 dBm/Hz. The phase shifter exhibits a link gain of −5.8 dB and an NF of 33 dB. The NF of the proposed phase shifter is mainly deteriorated by the phase noise-to-intensity noise conversion of the laser when the optical carrier is attenuated by the external BPF.

In this work, we demonstrated a multi-functional integrated MWP circuit that can be reconfigured between a high-rejection notch filter and a 2π-range phase shifter. By vectorially manipulating the multi-order optical sidebands, an ultrahigh dynamic range over 120 dB · Hz^4/5 is achieved in both functions. Table I compares this work with the state-of-the-art MWP circuits. While application-specific MWP circuits already show a wide operating frequency band, RF performances, including link gain, NF, and SFDR, are still relatively poor or not available in most demonstrations. Very recently, reconfigurable MWP circuits with relatively high RF performance have been realized in Refs. 10, 32, and 40; however, only the filter functions are demonstrated.

On the contrary, we demonstrated two distinct functionalities, namely, filter and phase shifting, while maintaining high linearity in this work. The array of microring resonators and two spectral deinterleavers could potentially unlock even more functionalities, for example, true time delay. Moreover, the possibility to realize multiple different functions in one circuit also opens the path to the cascaded integrated MWP system, which would be a major leap toward integrated RF photonic front-ends that can be directly applied in real RF environments.

Currently, the reconfigurable MWP circuit in this work exhibits relatively low link gain and a high noise figure. In principle, the link gain can be improved by increasing the optical power of a high power-handling modulator. The strategy for noise figure reduction, on the other hand, is more intricate. The use of the phase modulator in our experiments prevents NF reduction techniques such as low-biasing a Mach–Zehnder intensity modulator³² from being employed. Emulation of such a technique using carrier processing by a ring resonator has been previously considered,⁸ but the power builds up in the high-Q ring used for carrier processing, preventing effective NF reduction. We believe that achieving simultaneous high link gain and low NF together with the features demonstrated in this work will require an entirely new topology potentially using a more complex interferometric modulator (for example, dual-drive or dual-parallel MZM) with on-chip linearization. Several recent studies have shown encouraging results in this direction.^30,41

Another direction to improve our reconfigurable MWP circuit is to integrate active devices with passive devices on a single chip.¹⁰ Currently, our system still utilizes discrete components for the laser, modulator, amplifier, and photodetector. However, encouraging progress has been made in the development of the low-RIN lasers in the hybrid silicon nitride-InP platform⁴² as well as the high-gain erbium-doped amplifiers in silicon nitride waveguides.⁴³ Although the integration of all these advancements will require considerable effort and dedicated designs, it will ultimately lead to the creation of an all-integrated, multi-functional MWP circuit with superior RF performance.

See the supplementary material for the setup and experimental details.

The authors acknowledge funding from the European Research Council Consolidator Grant (101043229 TRIFFIC) and the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) Vidi (15702) and Start Up (740.018.021).

The authors have no conflicts to disclose.

Gaojian Liu and Kaixuan Ye contributed equally to this work. D.M. and G.L. developed the concept and proposed the physical system. O.D. designed the photonic circuits. G.L. and K.Y. developed and performed numerical simulations. G.L. and K.Y. performed the experiments with input from O.D. D.M., G.L., and K.Y. wrote the article with input from Q.T. and H.Y. D.M. led and supervised the entirev project.

Gaojian Liu: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Kaixuan Ye: Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Okky Daulay: Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Validation (equal). Qinggui Tan: Formal analysis (equal); Investigation (equal); Methodology (equal); Writing – review & editing (equal). Hongxi Yu: Formal analysis (equal); Investigation (equal); Methodology (equal); Writing – review & editing (equal). David Marpaung: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Project administration (equal); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding authors upon reasonable request.