KEYWORDS: Avalanche photodetectors, Free space optics, Telecommunications, Signal to noise ratio, Receivers, Optical amplifiers, Interference (communication), Signal detection, Single mode fibers, Oscilloscopes
In general, the excess noise of avalanche photodiodes (APDs) will increase with incident mean optical power (Pin). However, it is found that the excess noise of InGaAs/InP APD decreases rather than increases with Pin in the case of high gain. The experimental and simulated results reveal that this phenomenon can be attributed to the nonlinearity effects, which results in the degradation of gain of APDs. And the nonlinearity effect is more significant with larger gain. Consequently, the excess noise will decrease due to the decreases of gain. For free space optical (FSO) communication systems which commonly operates the APDs at high gain, this discovery can be used to determine the optimum threshold of receivers to achieve the minimum bit error rate (BER).
Device-level optimization of sensitivity in separated absorption, grading charge, and multiplication avalanche photodiodes (SAGCM APDs) has been performed. The impulse response was derived using the modified random path length (RPL) model for nonuniform electric field. The impact ionization in the charge layer was also taken into consideration with a modified RPL model based on the ionization coefficients deduced from Monte Carlo simulation. The bit error rate was calculated based on the derived impulse response, considering tunneling current, intersymbol interference, and bandwidth-limited gain. The bandwidth has significant effect on response, and sensitivity of APDs was optimized.
We propose a Schottky photodetector with tapered thin metal strip on SOI platform. Schottky photodetector can detect photons below the semiconductor bandgap energy by exploiting the internal photoemission. In the internal photoemission process, the hot carriers generate in the tapered thin metal strip where light absorption occurs, and part of these carriers can be emitted over the Schottky barrier and collected as photocurrent. The small thickness of the tapered metal strip contributes to a high internal quantum efficiency of 11.25%. This metal-semiconductor structure acts as a photonics-plasmonics mode convertor. According to 3D-FDTD simulation, about 95.8% of the incident optical power can be absorbed in the absorption area within 4.5μm at wavelength of 1550 nm. The responsivity is estimated to be 0.135A/W at 1550 nm. This compact design with a low dark current has a minimum detectable power of -23.15 dβm. We argue that this design can promote the progress of all-Si photo-detection in near-infrared communication band.
This paper reviews the recent advances in telecommunication avalanche photodiodes with nano-sized multiplication region. A new low noise avalanche photodiode based on InP traditional material has been proposed for high speed optical communication.
2.5 and 10 Gb/s InP/InGaAs avalanche photodiodes (APDs) have been widely used in optical communication systems. However, the study on InP/InGaAs APDs above 10 Gb/s is insufficient. Recently, high-speed and high-sensitivity APDs for 100 Gb/s or even 400 Gb/s optical communication systems have drawn a lot of attention. On basis of the physical model for frequency response of APD including the dead space effect, a waveguide separate absorption, grading, charge, and multiplication (WG-SAGCM) InP/InGaAs APD has been designed for 25 Gb/s operation. Also, the frequency response of WG InAlAs/InGaAs APD was also simulated, which is perfectly in accordance with the experimental data. The comparison between InP/InGaAs APD and InAlAs/InGaAs APD with the same thickness of multiplication layer shows that the speeds of carriers in the nonionization layers are also important for the gain-bandwidth characteristics of SAGCM WG-APD. The higher drift velocity of carriers returned from multiplication layer and the lower drift velocity of carriers injected into multiplication layer result in a higher gain-bandwidth product and a higher dc gain. This work is helpful for the design of high-speed APDs.
It is well known that the performance of modern photodiodes is determined by three
basic parameters: sensitivity, response time and noise equivalent power (NEP). In
practical situations it is almost always necessary to achieve the maximum product of
sensitivity and reciprocal response time (bandwidth). At low Continuous Wave (CW)
light incidence, a photodiode gives a signal proportional to optical intensity. But with
increasing light power, photocurrent deviates from a linear behavior. High-linear
performance is becoming increasingly important for photodiodes because high
photocurrents directly translate into an increased dynamic range and reduced noise figure.
This is crucial for many photonic systems, including photonic analog-to-digital
converters and high-bit-rate digital receivers. The increase of photocurrent depends on
two primary factors: one is space-charge limitations, which are influenced by device
physical dimensions, structure type, illumination conditions, maximum electric field, and
the other is thermal considerations. From the view of long-term reliability of photodiodes,
thermal effects are crucial because it results in device failure due to dark current runaway.
As known to us, there have many reports on the thermal effect originating from InGaAs
intrinsic region. However, nonlinearity originating from both InGaAs intrinsic region and
contact resistance is still unclear. In some cases, the contact resistance between metal
electrode and semiconductor is not negligible. In this report, N type heavily doped InP
single crystal is used as a substrate, on which a buffer layer of n doped InP is grown.
Then the intrinsic absorbing layer of InGaAs, and finally a transparent InP cap layer are
deposited with MOCVD technique. A circuit model has been developed and the
nolinearities of PIN photodiodes has also been discussed.
In this report, the multiplication characteristics of InP/InGaAs avalanche
photodiode (APD) with thick multiplication and charge layer have been studied
theoretically and experimentally, considering the electric field distribution, carrier
concentration, and different multiplication layer thickness. We find that ionization in
the charge layer is very sensitive to avalanche multiplication (M) and breakdown
voltage (Vbr). Partial ionization in the charge layer has been suggested, which gives a
good description of experimental results.
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