Fig. 1. (a) Schematic of the optical layer of the LLO-CV-QKD system with an on-chip III-V/Si3N4 external cavity laser. The integrated system contains two parties, Alice and Bob, as, respectively, the transmitter and receiver. Alice’s side consists of an isolator (Iso), two attenuators (ATTs), an amplitude modulator (AM), a beam splitter (BS), an IQ-modulator (IQ MOD), and a DC supply. Bob’s side has two polarization controllers (PCs) and an integrated coherent receiver (ICR). The photon detectors (PDs) are used for optical power monitoring. The inset shows the schematic structure of the external cavity lasers (ECLs). (b) Photograph of the packaged ECLs. The RSOA is butt-coupled to an extension chip. The laser is supplied with electric current via wires. The on-chip light is measured and used in the QKD system using an optical fiber. (c) Microscope photo of the ECL. The footprint of the external chip is 2.4 mm×1.27 mm.

Fig. 2. (a) L-I curve at the wavelength of 1545 nm. The lasing threshold is 38.9 mA, and the slope efficiency is about 238 mW/A. The maximum on-chip output of 47.3 mW is obtained at a 236 mA injection current. The fiber-to-chip coupling loss is measured to be about 3 dB. (b) Superimposed output spectra of continuous frequency tuning by synchronously driving two Vernier MRRs and a phase shifter. The continuous frequency tuning range is 30 GHz. (c) Output spectrum showing the single-mode lasing with an SMSR above 75 dB. (d) Measured superimposed lasing spectra of the ECL during coarse wavelength tuning. When it turns on the RSOA, the initial center wavelength is around 1498 nm. By tuning one of the MRRs, the lasing wavelength shifts from 1498 to 1531 nm with a step equalling the FSR of the MRR. When tuning the other MRR, the lasing wavelength first shifts to 1491 nm step by step, then jumps to 1564 nm, corresponding to the tuning range of the laser, and finally shifts to 1532 nm. The current of the RSOA is adjusted to make the output power close to the same level over the whole range. The wavelength tuning range is around 73 nm from 1491 to 1564 nm with an SMSR more than 65 dB.

Fig. 3. (a) Optical spectrum of two lasers before (blue) and after (orange) adjustment. (b) Histogram of the measured beat frequency. When measuring the change of the beat frequency, we use a data volume of 5 Mb per frame and a sampling frequency of 10 Gb/s to measure the frequency offset of the beat frequency. After analyzing 800 frames of data, we found that the beat frequency of more than 70% of the frames is below 40 MHz, which sufficiently satisfies the demand of the system. (c) Frequency noise spectrum of the signal and the LO laser.

Fig. 4. (a) Cross-correlation results of Bob’s measurement and Alice’s modulation on corresponding quadratures. Inset is the raw secret key shared by Alice and Bob. (b) Measured excess noise at 50 km. The error bars represent the measured excess noise at 50 km in the experiment, and the blue line represents the average excess noise of 60 frames of experimental data. (c) Secret key rate.

Fig. 5. Cross section of the Si3N4 waveguide tuned by a metallic heater.

Fig. 6. (a) Simulated add-drop transmission spectra of two MRRs. The FSR is, respectively, 1.93 nm and 1.99 nm for the two MRRs. The two sets of transmission spectra are aligned at 1.55 and 1.488 μm. (b) Simulated round-trip transmission spectrum of the external cavity. The extended FSR is more than 60 nm due to a slight difference in the circumference between the two MRRs.

Fig. 7. (a) Simulated PCC of the MRR as a function of wavelength. (b) Power reflectivity of the TSL changes with PCC for various phase differences. (c) Power reflectivity of the TSL changes with phase difference for various PCCs. (d) Illustration of the TSL transfer function derivation.

Fig. 8. EPR scheme. The EPR pair is on the left. Heterodyne detection is done on part of it. The other part enters the channel through the right transmission, and is affected by the channel and the eavesdropper Eve. The receiving end adopts two detection methods: heterodyne detection and homodyne detection. To express the detection noise and quantum efficiency of the receiving end, it is equivalent to another EPR pair interfering with the transmission part through the BS with the transmission rate η.