Terence S.-Y. Chen
Latitude Design Systems
Abstract
This article outlines the use of pSim, the photonic circuit simulation tool in PIC Studio toolchain, to analyze the key linear performance metrics and sensitivity of PIN photodetectors used in optical receivers. It covers the theoretical background of noise sources, linearity characteristics, and factors affecting receiver sensitivity. Practical simulation examples are provided to demonstrate how to evaluate the linearity of the photodetector via third-order intercept point and the sensitivity via eye diagrams for a direct detection system. The complementary theoretical and simulation content offers a comprehensive guide to analyzing the performance of PIN photodetectors in optical communication systems.
Introduction
Photodetectors are one of the main components in optical receivers, used to convert optical power into current. According to the system performance objectives, PIN or APD (avalanche photodiode) photodetectors can be selected. Bit error rate (BER) is the primary metric used to specify the reliability of a communication transmission system, and is typically related to the receiver sensitivity value, which defines the minimum average optical power that must reach the photodetector to achieve the required BER performance. In addition, the channel's Q value can be calculated from the statistics of the sampled signal and used to estimate the system's BER. Photodetectors play an important role in defining the ultimate sensitivity of a basic communication system, because they provide statistical disturbances in the form of shot noise and thermal noise and introduce dark current and responsivity to measure how much electrical output is obtained per unit of optical input power. These characteristics depend on the wavelength of the incident light and the material properties and physical design of the sensor.
In a typical RF photonic link operating at an optical power level of around 10 dBm, low-noise microwave amplifiers are used to combat the inherent insertion loss of the photonic link (no pre-amplifier for the modulator or no post-amplifier for the photodiode). These amplifiers introduce nonlinear distortion in the RF signal, thus limiting the link’s spurious-free dynamic range (SFDR). Such amplifiers can be avoided by using a photodiode that exhibits highly linear operation when the optical power level significantly exceeds 10 dBm. Operating the photodiode at high optical power levels, and thus having a larger DC photocurrent, has the additional benefit of reducing the link's noise figure (NF). In this section, we focus on the linearity of the photodiode in continuous wave (CW) operation, characterized by its third-order output intercept point (OIP3), and demonstrate how to set up and measure the receiving sensitivity of an intensity-modulated direct detection (IM-DD) photodetector system.
Circuit Functional Description
Linearity and sensitivity are two important parameters for measuring PIN photodetector performance. This is divided into two parts: the first part introduces how to obtain the linearity of the PIN photodetector, and the second part discusses the sensitivity of the PIN photodetector.
For linearity, as shown in the figure, the user easily builds a schematic containing both optical and electrical signals in the pLogic schematic tool by dragging and dropping components from the pSim system component library. Two continuous wave lasers (CWL_1 and CWL_2) are used as the light source emitting into a 2x2 directional coupler (C_1). A small frequency difference is applied between the two continuous wave lasers to produce beating. After C_1, an optical attenuator (ATT_1) is used to attenuate the optical power. A PIN photodetector (Pin_1) is used to detect the received signal and convert the signal type from an optical signal to an electrical signal (O-E conversion). A DC block (HPF_1) is used to filter out low frequency noise. An electrical amplifier (AMP_1) is used to enhance the optical power and generate first-order and third-order nonlinear effects. An electrical fork (fork_1) is used to split the signal from one port to two ports. Finally, two rectangular bandpass filters (BPF_1 and BPF_2) are used to filter out the first-order and third-order signals, respectively. Two power meters (PWM_1 and PWM_2) are used to measure the electrical power.
For sensitivity, as shown in the figure, the user continues to easily build a schematic in the pLogic schematic tool by dragging and dropping. The schematic shows a pseudorandom bit sequence (PRBS_1) and non-return-to-zero code (NRZ_1) used to generate random on-off keying (OOK) signals. After NRZ_1, part of the signal is sent to the eye diagram as a reference signal, and the other part is sent as a received signal to the optical amplitude modulator (AM_1) and passes through the entire system to the eye diagram. A continuous wave laser (CWL_1) serves as the light source emitting into the optical port of the optical amplitude modulator. An optical attenuator (ATT_1) is used to attenuate the optical power. An optical power meter (OPWM_1) is used to monitor the optical power. After signal modulation, a PIN detector (pin_1) is used to convert the signal from an optical signal to an electrical signal. A low pass Bessel filter (LPF_1) is used to filter out high frequency signals. Finally, the eye diagram analyzes the quality of the transmitted signal.
From Component Design to Circuit Practice
For the circuit theory, component parameters, and simulation parameter settings, please refer to Latitude Design Automation's tutorial material: "SiPh IC Design with pSim". After connecting each component, run the simulation. The results window will display the first-order power as 2.736 dBm and the third-order power as -99.191 dBm. After processing, the HOIP3 is obtained, as shown in the pSim simulation figure, with the intersection of the red and blue lines being the HOIP3.
For sensitivity analysis, select the time domain and set the simulation parameters according to the tutorial. After connecting each device, run the simulation as shown in the pLogic screenshot of the pSim simulation results. The measured optical power after the attenuator (OPWM_1) is -31.377 dBm.
Click the EYE_1 icon, then right-click to open the viewer, and the eye diagram will be displayed as shown in the figure.
The eye diagram results show a decision instant of 5.47 ns, a threshold level of 0.68 uV, an extinction ratio of 16.03 dB, an eye opening of 0.86, an eye width of 7.43 ns, a minimum bit error rate of 2.20e-12, and a maximum Q factor of 6.92.
Summary
This paper outlined the use of pSim, the optoelectronic chip link simulation tool in Latitude Design Automation's PIC Studio platform, to perform simulation analysis of the key linear performance metrics and sensitivity of PIN photodetectors used in optical receivers. The introduction introduces the role of photodetectors in optical receivers, as well as key performance parameters such as linearity and sensitivity.
The main part first introduces how to build system-level schematics for linearity and sensitivity simulations using pLogic, the schematic design tool in PIC Studio, through drag and drop.
For linearity analysis, two lasers and a directional coupler are used to generate two-tone signals, and amplifiers introduce nonlinear effects. Finally, the first-order and third-order intercept points are analyzed to obtain the linearity metrics of the photodetector.
For sensitivity analysis, a PRBS sequence modulates the laser, and after transmission through the system, the eye diagram is used to analyze metrics such as bit error rate and Q factor to evaluate sensitivity performance.
Through simulation analysis, the two key performance metrics of linearity and sensitivity of PIN photodetectors are examined, verifying pSim's simulation capabilities as a chip-level system simulation tool. Latitude Design Automation's tutorials provide important reference with supplementary theory and simulation settings for analyzing photodetector performance.
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