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