Comparison of All-Optical and Electro-Optic Phase Encoding Quantum Key Distribution at O-Band

O-band carrier has long been ignored for optical quantum key distribution (QKD) application owing to the broader laser linewidth, larger fiber loss, and lower wavelength tolerance, despite the only advantage of its dispersion-free transmission in standard single-mode fiber (SSMF). Nonetheless, the secure data transmission at O-band is still an intriguing target as most of the short-reach SMF-based metropolitan or data-center networks demand the optical cryptography solutions. By comparing four types of all-optical or electro-optic differential-phase-shift (DPS) encoding techniques, this study details the key-rate/error/distance performances by using a pulsated carrier generated from a TO-can packed distributed feedback laser diode (DFBLD). At a carrier wavelength of 1308.4 nm with a limited encoding bandwidth of 1.5 GHz, the comparison on temporal stability, coding efficiency, and cost effectiveness is performed for four DFBLD-based DPS-QKD systems using external or direct modulation schemes for the QKD-bit generation. The adiabatic-controlled master and slave (M and S) DFBLD carrier maintains its stability with ΔP/P variation below 0.1% at an average power of 0.68 mW for 30 minutes. After passing through the delay interferometer with -3.2-dB loss, the wavelength-induced transient power fluctuation significantly reduces to 0.006 mW/s with ΔP/P of 1.89%. The DPS-QKD using a cost-ineffective DFBLD+EOM-IM/PM (M→IM→PM) scheme is treated as a DPS-QKD reference, degrading its QBER from 2.3% to 3.7% with allowable SKR decreasing from 50 to 0.18 kbit/s in a 15-km SSMF link. Using the M-to-S OIL-DFBLDs with external PM (M→PM→S) can provide a QBER of 2.94% and an SKR of 27 kbit/s under back-to-back transmission, which degrades to QBER = 3.45% and SKR = 4.37 kbit/s after 12-km SSMF. The M-to-S OIL-DFBLDs with external IM (M→IM→S) degrade QBER and SKR from 3.03% and 24 kbit/s (at back-to-back link) to 3.45% and 5.34 kbit/s over 9-km SSMF. In contrast, the low-cost compact M-to-S OIL-DFBLDs with DM-PM/IM scheme (M→S) reveal the back-to-back QBER and SKR of 3.13% and 20 kbit/s, but only allow 6-km SSMF transmission with degraded QBER and SKR of 3.6% and 2.7 kbit/s, respectively.