• Document: OptiSystem applications: Photodiode sensitivity modelling
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OptiSystem applications: Photodiode sensitivity modelling © 2009 Optiwave Systems, Inc. 7 Capella Court +1 (613) 224-4700 Nepean, ON, Canada www.optiwave.com K2E 7X1 Introduction Photodetector sensitivity modelling  One of the main working parts of any optical receiver is the photodetector (which converts optical power to an electric current). Either a PIN or APD (avalanche photodiode) photodetector can be used depending on the system performance objectives.  The bit error rate (BER) is the primary metric used to specify the reliability of a communication transmission system and is normally linked to a Receiver sensitivity value which defines the minimum average optical power that must reach the photodetector to achieve a desired BER performance. Alternatively the Q of the channel can be calculated from sampled signal statistics and used to estimate the system BER (OptiSystem supports both calculation methods).  The photodetector plays an important role in defining the ultimate sensitivity of a basic communication system as it contributes statistical perturbations in the form of shot (quantum-based) and thermal noise. It also introduces a dark current (which can be viewed as DC noise) and has a defined Responsivity (a measure of how much electrical output is obtained per unit of input power) which depends on the wavelength of the incident light and the sensor’s material properties and physical design. In addition to these effects, photodetectors also exhibit a frequency-dependent transfer function due to the presence of a junction capacitance and the need to connect to a load resistor to measure the received signal (for this analysis the transfer function is assumed to be ideal)  The following four examples demonstrate how to setup and measure (using OptiSystem) the receiver sensitivities of PIN and APD intensity-modulated direct-detection (IM-DD) systems, specifically:  Quantum-limited ideal PIN photodetector  Thermal-noise limited PIN photodetector  Thermal and shot-noise APD performance  PIN photodetector with optical pre-amplification  The reference file for this application note is: PIN and APD Receiver Sensitivity Analysis Version 1_0 24 Jan 17.osd. 2 Ideal photodetector (PIN) Photodetector sensitivity modelling  The test configuration is as follows : Bit rate: 10 Gb/s; Wavelength = 1550 nm; PIN responsivity: 1 A/W; Dark current = 0 nA; Sequence length = 1048576.  As the receiver is ideal its only noise source is the PIN shot (quantum) noise - the thermal noise contribution has been disabled. The receiver makes an error when an expected Logic 1 (ON signal) sees no photons (Poisson statistics). The absolute threshold of the Data Recovery component is set to 1E-12 to verify this condition.  The minimum number of photons/bit required to achieve a given BER can be calculated as follows: BER = 1/2 * exp (- 2*N) where N is the average number of photons per bit. For the example below the attenuator was set to 58.1 dB (average photons per bit ≈ 6). The resulting expected quantum limit performance is LOG(BER) = -5.51.  For the simulation run below, the BER Test Set shows that three bit errors were detected (LOG(BER) = -5.54) REF: L. Kazovsky, S. Benedetto, A. Willner, Optical Fiber Communication Systems, Artech House (1996), pp. 199-200 Layout: Ideal optical receiver (PIN) To model quantum-limited performance the Shot Noise Distribution has been set to the Poisson statistical noise model A Component script has been set up to calculate the average photons arriving per bit interval (=Sampled Signal Power (Photons)/Bandwidth) The BER Test Set is the best tool to model quantum-limited performance as we need to count (over a very long sequence) the number of times that no photons are received over a specified bit interval (but expecting a Logic 1 ON state) 3 Thermal noise limited PIN (1) Photodetector sensitivity modelling  In this example the receiver sensitivity of a PIN photodiode (for Q=7, BER=1E-12) is determined based on the following configuration: Bit rate: 100 Mb/s; Wavelength = 1550 nm; Load resistance: 100 ohm; Temp = 300K; PIN responsivity: 0.95 A

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