Efficient and Low Noise Single-photon Detection in 1550-nm Communication Band by Frequency Upconversion in Periodically Poled LiNbO3 Waveguides

Efficient and Low Noise Single-photon Detection in 1550-nm Communication Band by Frequency Upconversion in Periodically Poled LiNbO₃ Waveguides
　
Hidehiko Kamada
Optical Science Laboratory
　　In recent years, the increasing demand for secure communication has accelerated the development of a new generation of telecommunication techniques based on quantum mechanics. Specifically, quantum key distribution (QKD) is expected to be a key technology; practical fiber-based QKD systems have been intensively studied in the 1500-nm wavelength band. For distributing keys over a long distance at a high rate, efficient and low noise single-photon detection is important. We demonstrate 1500-nm band single photon detection with low dark count noise and a potentially high efficiency. By developing frequency up-conversion devices based on sum-frequency generation (SFG) in periodically poled LiNbO₃ (PPLN) waveguide, which are specifically designed to use a pump wavelength longer than that of communication-band photons, we eliminate the dark count noise caused by parasitic nonlinear processes in the waveguide [1].
　　Periodic poling relaxes the wavenumber mismatch among three waves, thus realizing quasi-phase matching (QPM), and tight mode confinement in a LN waveguide enhances the SFG. Our devices are specifically designed to use a pump wavelength at 1810 nm. The PPLN wafers were fabricated by directly bonding a thin periodically poled wafer to a LiTaO₃ cladding wafer. Subsequently, the bonded wafer was cut into pieces, then a series of 7-µm thick, 6～8-µm wide and 35 or 50-mm long ridge stripes were defined with a dicing saw.
　　We observed internal conversion efficiency as high as 40 % (Fig. 1, 2), and demonstrated scaling down to the single photon level. Favored by long wavelength pump, which never induces parasitic χ² process thus eliminates the noise photons in 1500-nm band. By carefully eliminate noise photons from the pump laser, a background dark count rate less than 10² sec^-1 was achieved (Fig. 2). Using the as-measured coupling of 65～70 % of a 1500-nm wave into the waveguide, and a Si-APD efficiency of ～57 %, we predict an overall photon detection efficiency of about 34～40 %.
　　This work was partly supported by NICT.
[1] H. Kamada, M. Asobe, T. Honjo, H. Takesue, Y. Tokura, Y. Nishida, O. Tadanaga, and H. Miyazawa, Opt. Lett. 33 (2008) 639.

Fig. 1. SFG output power (left), transmitted signal (middle) and pump (right). The temperature was set at 24 ℃.

Fig. 2. Efficiency and dark count rate as a function of coupled 1810-nm pump power: throughput was ～20 %. The DC rate is reduced to below 100 cps.

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