Microwave photonics (MWP) is a new interdisciplinary field. By combining microwaves and light waves, MWP can overcome the challenges and troubles in the generation, transmission, and processing of microwave signals (namely, the three key areas of MWP), which is due to the inherent broadband and low loss characteristics of modern photonics. Traditionally, MWP systems use discrete components that have been fully developed and implemented, such as in arbitrary waveform generators, wireless communication, signal processing and detection, and radar. However, the scalability of functionality and cost has hindered the popularity of MWP. Therefore, it is necessary to implement compact and easy-to-handle photonic integrated circuits (PICs) to accelerate the maturity of integrated MWP, as it can greatly reduce footprint and cost and significantly improve stability and energy efficiency.

Recently, many PICs have been developed to implement different microwave devices or systems, including signal generators^[1–5], filters^[6–8], true delay lines and beamformers^[9–12], signal processors^[13–19], front-end transceivers^[20,21], and systems for signal characterization and interference cancellation. Some PICs have been developed to implement general-purpose programmable processors or processor cores in the field of signal processing^[14–18]. However, these PICs are currently developed to perform one or more functions covering a single domain (i.e., microwave signal generation, transmission, or processing). PIC auxiliary functions are limited to a single domain. The weaknesses behind single-domain operations result in long development time and high casting costs, thereby limiting applications. In addition, although PICs have been widely used in the field of MWP, most of them have only been conceptually proven in laboratory environments with good conditions or in laboratory testing under offline conditions^[14–19]. There is a serious gap between laboratory demonstration and practical application, of which stability and robustness are two problems that must be solved. At present, the transceiver module is mainly focused on digital communication modules, and there are few reports on MWP modules.

Therefore, we propose an eight-channel microwave photonics transceiver photonic integrated circuit (MWPTPIC) that can cover the three key areas of MWP, namely modulation, transmission, and detection. This PIC consists of 4 transmission channels and 4 reception channels. It aims to achieve some multifunctional or switchable on-chip components, modulation architecture, and optical signal flow. Our PIC is stable and reliable and can be directly deployed in real-world applications. This work demonstrates a milestone in the development and maturity of integrated MWP, thus taking a step towards future large-scale applications.

As shown in Figs. 1 and 2, our transceiver module has a total of 4 transmission channels and 4 reception channels. Each transmission channel is composed of a tunable laser chip, a thermal electrical refrigerator (TEC), a modulator chip, and a fiber array (FA). The laser chip is soldered onto a ceramic substrate, which is then soldered onto the TEC. The modulator chip is glued onto a metal heat sink. The laser chip is end coupled with the modulator chip. The modulator chip is end coupled with the output fiber. Each receiving channel is composed of an FA, a detector chip, and an electrical amplifier. The detector chip is soldered onto a ceramic substrate, which is soldered onto a heat sink. The detector chip is end coupled with the input fiber. The electrical connection between the radio frequency (RF) electrode of the chip and the RF circuit, the direct current (DC) electrode of the chip and the DC circuit, and the DC circuit and the shell pins are completed through gold wire bonding.

Figure 3(a) shows the optical field transmission simulation diagram. The distance between the laser chip and lithium niobate chip is 2 µm. Due to the large refractive index difference, there is an obvious standing wave in the air gap between the two, that is, the influence of the air resonator, which seriously affects the coupling efficiency. On the other hand, because there is only a 2-µm-thick insulation layer, the vertical mode field of the lithium niobate chip is about 600 nm, and the divergence angle is large. Some light inevitably leaks into the substrate, further affecting the coupling efficiency. Figure 3(b) shows the simulation diagram of the optical field transmission between the laser and the lithium niobate chip after filling the optical adhesive. The standing wave strength in the gap between the two is significantly weakened, indicating that the resonance between the end faces is weakened, thus significantly improving the coupling efficiency. The actual measured coupling loss is about 6 dB.

In the actual electrical packaging process, considering the mode field matching of the electromagnetic field, we will inevitably generate air gaps in the connection between the coaxial connector and the transmission line, which will lead to impedance discontinuity and resonance. Therefore, we simulated and discussed the effect of the spacing a between the coplanar waveguide (CPW) used for transition and the coaxial adapter on the transmission line S parameters.

We used HFSS to simulate the effect of the spacing a between the ground coplanar waveguide (GCPW) and coaxial adapter on the transmission line S parameters, as shown in Fig. 4. The specific simulation structure is shown in Fig. 5. We set the spacing a to 0, 0.05, 0.1, 0.15, and 0.2 mm, respectively. Because the coaxial adapters and the GCPW are generally electrically connected using a solder during packaging, excessive spacing can cause significant difficulties in soldering. Therefore, we have set the maximum spacing to 0.2 mm. From Fig. 5, when the spacing a increases by 50 µm steps, the S11 (b) and S21 (a) parameters of the entire structure significantly deteriorate, and resonance also occurs at low frequencies. Therefore, during the packaging process, it should be ensured that the GCPW is tightly attached to the coaxial joint in the tube shell to minimize the deterioration of the S parameter caused by resonance caused by gaps.

For packaged amplifiers, we first place the microstrip circuit board structure into a cavity and then use a solder to weld the circuit board to the RF substrate of the amplifier. After completion, we use gold wires to connect the RF substrate, adapter board, and modulator chip together.

When packaging the detector, we first place the microstrip circuit board structure into the cavity, aligning the microstrip line signal line with the glass insulator needle. We use conductive adhesive to smoothly bond the circuit board with the heat sink and bake it in an 80°C constant temperature oven for 3 h. After completion, we connect the RF insulator to the microstrip line with the solder. Pay attention to controlling the amount of solder during welding, and then clean it with ultrasonic waves. Use conductive adhesive to sinter chips, capacitors, resistors, and other components onto microstrip circuit boards in the same way for sintering fixation.

The modulator is designed and fabricated on the lithium niobate on insulator (LNOI) material platform. The manufacturing process is shown in Fig. 6. Hydrogen silsesquioxane (HSQ) is a negative tone resist, which is spin-coated on the substrate as a mask. Then, the electron beam lithography (EBL) is used to transfer the pattern on the lithium niobate (LN) wafer. The LN layer is etched by the inductively coupled plasma-reactive ion etching (ICP-RIE) system with CHF3 and Ar plasma. Next, the SiO2 layer is formed by plasma-enhanced chemical vapor deposition (PECVD). Subsequently, the AZ4620 photoresist is spin-coated and patterned. The gold layer is manufactured by electron beam evaporation coating. Finally, we use smart-cut to manufacture gold electrodes.

The LNOI layer thickness is 600 nm, the LNOI waveguide width is 1 µm, the etching depth is 300 nm, and the inclination angle is about 75°. The fabricated LNOI modulators with arm lengths of 6 mm are measured in detail. We first measured the direct current transmission with thermal optical (TO) phase shifters. Figure 7 shows the TO transmission curve as a function of the applied voltage. The length of the TO phase shifter is 250 µm with a resistance of about 800 Ω, and the required voltages for biasing at quadrature is 5.2 V, corresponding to power dissipations of about 34 mW.

We integrated the same four transmission channels in one module. Figure 8 shows the normalized small signal optical-to-optical (OO) bandwidth (S21 parameter) and electrical reflection (S11) of the manufactured device. The measured 6 dB OO bandwidth is greater than 50 GHz. The input return loss (S11 parameter) of this module is less than −10dB at up to 67 GHz. We also measured the crosstalk between different channels, and the crosstalk between two adjacent channels is also less than −40dB, which is small enough for practical use.

We tested the NF of the entire MWP link (including lasers, modulators, detectors, and amplifiers) using a noise figure analyzer. The NFs of the four channels are 16.5, 12.5, 25, and 18.4 dB, respectively. The link gain is greater than −26dB. As shown in Fig. 9, the spurious-free dynamic ranges (SFDRs) of the four channels are 94.1 dBm/Hz, 104.8 dBm/Hz, 96.6 dBm/Hz, and 97.9 dBm/Hz^2/3, respectively.

We also measured the transmission of our device on a logarithmic scale, indicating that the measured extinction ratio is greater than 16 dB. The measured link gain is greater than −26dB and the measured S parameter indicates good channel uniformity. These voltage and bandwidth parameters indicate that the devices we manufacture have excellent performance and are well-suited for high-speed operation. The success of such a multi-channel PIC marks a crucial step forward in the implementation of large-scale MWP.

Our PIC integrates lasers, modulators, amplifiers, and detectors in the module, successfully manufacturing an eight-channel array transceiver module. We conduct performance tests on the encapsulated transceiver module and find that the cascaded bandwidth of the eight-channel transceiver module is greater than 40 GHz and the SFDR of the broadband array receiver module is greater than 94 dBm · Hz^2/3. The NF is less than −35dB and the link gain is greater than −26B. The success of the multi-channel PIC marks a crucial step forward in the implementation of large-scale MWP.