1、Design and Characterization of Single Photon APD Detector for QKD Application Abstract Modeling and design of a single photon detector and its various characteristics are presented. It is a type of avalanche photo diode (APD) designed to suit the requirements of a Quantum Key Distribution (QKD) dete
2、ction system. The device is modeled to operate in a gated mode at liquid nitrogen temperature for minimum noise and maximum gain. Different types of APDs are compared for best performance. The APD is part of an optical communication link, which is a private channel to transmit the key signal. The en
3、crypted message is sent via a public channel. The optical link operates at a wavelength of 1.55m. The design is based on InGaAs with a quantum efficiency of more than 75% and a multiplication factor of 1000. The calculated dark current is below 10-12A with an overall signal to noise ratio better tha
4、n 18dB. The device sensitivity is better than -40dBm, which is more than an order of magnitude higher than the dark current, corresponding to a detection sensitivity of two photons in pico-second pulses. I. INTRODUCTION Photon detectors sensitive to extremely low light levels are needed in a variety
5、 of applications. It was not possible to introduce these devices commercially several years ago because of the stringent requirements of QKD. Research efforts however resulted in photon detectors with reasonably good performance characteristics. The objective here is to model a single photon detecto
6、r of high sensitivity, suitable for a QKD system. The detector is basically an APD, which needs cooling to very low temperature (77K) for the dark current to be low. The wavelength of interest is 1.55m. Different applications may impose different requirements, and hence the dependence of the various
7、 parameters on wavelength, temperature, responsivity, dark current, noise etc, are modeled. Comparison of the results from calculations based on a suitable model provides amenable grounds to determine the suitability of each type of APD for a specific application. Attacks on communication systems in
8、 recent years have become a main concern accompanying the technological advances. The measures and counter measures against attacks have driven research effort towards security techniques that aim at absolute infallibility. Quantum Mechanics is considered one of the answers, due to inherent physical
9、 phenomena. QKD systems which depend on entangled pairs or 1 polarization states will inevitably require the usage of APDs in photon detection systems. How suitable these detectors may be, depends on their ability to detect low light level signals, in other words “ photon counting”. It is therefore
10、anticipated that the application of high security systems will be in high demand in a variety of fields such as banking sector, military, medical care, e-commerce, e-government etc. . AVALANCHE PHOTO DIODE A. Structure of the APD Fig. 1 shows a schematic diagram of the structure of an APD. The APD i
11、s a photodiode with a built-in amplification mechanism. The applied reverse potential difference causes accelerates photo-generated carriers to very high speeds so that a transfer of momentum occurs upon collisions, which liberates other electrons. Secondary electrons are accelerated in turn and the
12、 result is an avalanche process. The photo generated carriers traverse the high electric field region causing further ionization by releasing bound electrons in the valence band upon collision. This carrier generation mechanism is known as impact ionization. When carriers collide with the crystal la
13、ttice, they lose some energy to the crystal. If the kinetic energy of a carrier is greater than the band-gap, the collision will free a bound electron. The free electrons and holes so created also acquire enough energy to cause further impact ionization. The result is an avalanche, where the number
14、of free carriers grows exponentially as the process continues. The number of ionization collisions per unit length for holes and electrons is designated ionization coefficients n and p, respectively. The type of materials and their band structures are responsible for the variation in n and p. Ioniza
15、tion coefficients also depend on the applied electric field according to 2 the following relationship: , e x p np ba E (1) For n = p = , the multiplication factor, M takes the form 11aWM (2) W is the width of the depletion region. It can be observed that M tends to when W 1, which signifies the cond
16、ition for junction breakdown. Therefore, the high values of M can be obtained when the APD is biased close to the breakdown region. The thickness of the multiplication region for M = 1000, has been calculated and compared with those found by other workers and the results are shown in Table 1. The la
17、yer thickness for undoped InP is 10m, for a substrate thickness of 100m. The photon-generated electron-hole pairs in the absorption layer are accelerated under the influence of an electric field of 3.105V/cm. The acceleration process is constantly interrupted by random collisions with the lattice. T
18、he two competing processes will continue until eventually an average saturation velocity is reached. Secondary electron-hole pairs are generated at any time during the process, when they acquire energy larger than the band gap Eg. The electrons are then accelerated and may cause further impact ioniz
19、ation. Impact ionization of holes due to bound electrons is not as effective as that due to free electrons. Hence, most of the ionization is achieved by free electrons. The avalanche process then proceeds principally from the p to the n side of the device. It terminates after a certain time, when the electrons arrive at the n side of the depletion layer. Holes moving to the left create electrons that move to the right,
