Fig. 1. Schematic of the QKD transmitter module. The entire module fulfills the function of QKD encoding and transmission.

Fig. 2. (a) Schematic diagram of the PIC optical circuit. The optical signal is coupled into the chip via a grating coupler (GC). It is then modulated by an IM and a POM before being output through a two-dimensional grating coupler (2D-GC). (b) Schematic diagram of the equivalent circuit of the MZI. The MZI consists of two PN junctions placed opposite each other in a push-pull structure. (c) A photo of the QKD PIC. The high-speed and the DC signal interface are located on the left and top of the chip, respectively, and an NTC thermistor is placed on the upper right of the chip.

Fig. 3. (a) Schematic diagram of chip connection and power supply. Under the control of the V-set and PWC configuration signals, the MDC converts the input data signal into a drive signal, which is then amplified by an AMP and input into the MZI modulator. (b), (c) Photos of MDC and AMP chip binding wires. AMP, amplifier.

Fig. 4. Cross-section view of the COB module. Each chip is placed in a cavity of the corresponding depth and connected to the back heat-dissipation structure through vias. The PIC is placed on a ceramic substrate to meet the requirements of heat dissipation and bandwidth.

Fig. 5. PCB layout design of the COB module. (a) The chip and resistor components are placed on the top side. To minimize the interconnect distance and reduce the bonding wire length, the chip pads have been rearranged into the layout shown in the figure. (b) Space for the TEC is reserved on the bottom side.

Fig. 6. Photo of the COB module installation. The COB module is first mechanically fixed to the system control board through screws and then electrically connected through castellated holes. The TEC and heat sink are attached to the back of the module.

Fig. 7. (a)–(f) Simulated thermal distributions with different TEC currents. The colors in the figure represent different temperatures, with color bar scales from 20°C to 80°C.

Fig. 8. Simulated PIC temperature (dashed line and data points) and TEC cooling/heating power (columns) versus TEC current.

Fig. 9. Thermal imaging photos of the module during operation. Panels (a)–(l) represent the temperature when TEC is off, and panels (m)–(p) represent the temperature when TEC is on. The color bars represent the temperature in Celsius degrees, ranging from 20°C to 80°C.

Fig. 10. Read temperatures of the NTC thermistor in different states. The blue and red data indicate TEC off and on, respectively.

Fig. 11. Output signals of discrete packaged MDC and RF amplifier chips. Panel (a) is the MDC output signal at 1.25 GHz, and panel (b) is its amplified signal. Panels (c) and (d) represent these signals at 2.5 GHz. A 36-GHz-bandwidth oscilloscope operating at 80 GSa/s is used for data acquisition. The data points in the figure are displayed using the oscilloscope persistence effect.

Table 1. Average modulation extinction ratio.