© (2006) OSA

Naoto Namekata and colleagues at Nihon University in Tokyo1 have developed an InGaAs/InP avalanche photodiode (APD) operated with a sine wave gating — compared with the square wave form in present systems — allowing detection of single photons at 1,550 nm, and at high repetition frequency. APDs detect such low light intensities by multiplying the charge carriers created by photon absorption until they produce a detectable current. InGaAs-based devices are commonly used at telecommunication wavelengths but are limited by an effect known as afterpulsing — in which some of the generated carriers recombine to create light, which in turn is absorbed and starts further avalanches. Afterpulsing can be controlled by switching the APD on and off — so called gating: if the time between gates is longer then the relaxation time of the carriers, the afterpulses are not detected. This, however, limits the gate frequency to a few megahertz. Also, the gate produces a charge pulse through the device, which competes with the real avalanche signal. Although reducing the level of carrier multiplication, or gain, can reduce afterpulsing, it makes it difficult to discriminate the avalanche signal from this charge pulse.

Now, Namekata and colleagues have shown that using a sinusoidal gate allows discrimination between very small avalanche signals and those due to afterpulsing and, as a consequence, a higher repetition frequency can be used. The sine wave still generates a charge pulse, but this can easily be removed using filtering electronics, a process that is much more difficult for a gate with a square wave form. Using this technique, an InGaAs avalanche photodiode was gated at a frequency of 800 MHz and a detection efficiency of 8.5% for 1,550 nm wavelength light. Such a detector could significantly increase the data transfer rate in quantum cryptography systems.