Wavelength selective injection-locking of a 1545.59-/1548.18-nm dual-mode DFBLD with a transient-current-switched single-mode master DFBLD is performed for differential phase-shift-keying and delay-interferometric on-off-keying at 192 MHz that enables secure and complex quantum-key-distribution via alternative wavelength switching.
In comparison with the rapidly developing progress on the optical C-band (1525-1565 nm) master-to-slave injection-locked transmitter to perform the coherent single-photon quantum key distribution (QKD), the development on the O-band (1250-1350 nm) QKD transmitter is somewhat delayed as the commercially available wavelength-matched narrow-linewidth distributed feedback laser diode (DFBLD) pair is hardly accessible up to now. By using the DFBLDs with only sub-MHz linewidth and relatively deviated wavelength for the first time, this work demonstrates the optically differential-phase-shift-keying (DPS) QKD by an O-band master-to-slave injection-locked DFBLD pair. The master and slave DFBLDs with wavelength fluctuations of ±0.05% and ±0.2 pm are controlled by a thermo-electric cooler with a feedback gain of 100. The 1-bit delay interferometer (DI) under thermo-insulation maintains its visibility at >96% with dP o /P o and dP o /dt measured below ±0.1% and ±1 × 10 −3 mW/s. By RZ-OOK modulating the master DFBLD with step-like power coding at 150 µW to induce π phase shift in the injection-locked slave DFBLD, the rising-/falling-edge DPS envelope distortion of the slave DFBLD diminishes by decreasing the bias current of the master DFBLD from 7I th (35 mA) to 2I th (10 mA). This phenomenon enables the 128-bit DPS-QKD transmission with a quantum bit-error rate (QBER) of 3.57% and a secure key rate of 3.524 kbit/s in the 6-km SMF link. The O-band injection-locked single-photon DPS-QKD bit-stream with a mean photon number of 0.2 #/bit minimizes its decoding QBER to 3.88% and 4.84% for 512-bit and 1024-bit, respectively.
A single‐chip dual‐wavelength coherent photon source is proposed to introduce additional parametric uncertainty and improve the security scaling of single‐photon differential binary‐phase‐shift‐keying quantum key distribution (D‐BPSK‐QKD). Simultaneous lasing of two longitudinal modes in the same semiconductor laser cavity generates two coherent photon groups at different wavelengths, and attenuation of their combined output to the single‐photon level inevitably produces wavelength uncertainty at the receiver. Optical injection control enables stabilized D‐BPSK encoding on one selected mode while the other mode remains free‐running, which supports mode‐selective coding and a decoy scenario during single‐photon streaming. This intermodal wavelength switching/selection provides another degree of freedom for QKD security and increases the difficulty of eavesdropping attacks. The dual‐wavelength single‐photon bits deliver D‐BPSK‐QKD codes with a QBER of <4.1% under 2 10 ‐bit encryption, a sifted key rate of 408.6 kbit/s, and a secure key rate (SKR) of 10.5 kbit/s at an average photon number of 0.22 photons#/bit. The transmission distance can be extended to more than 13.25 km with a SKR of 3.6 kbit/s under a nearly equivalent QBER. This versatile wavelength‐uncertain coding or decoying operation enables flexible and secure D‐BPSK‐QKD transmission over wavelength‐division‐multiplexed channelized single‐mode fiber links.