Optical Receivers Theory and Operation

Photo Detectors Optical receivers convert optical signal (light) to electrical signal (current/voltage) Hence referred ‘O/E Converter’ Photodetector is the fundamental element of optical receiver, followed by amplifiers and signal conditioning circuitry There are several photodetector types: Photodiodes, Phototransistors, Photon multipliers, Photo-resistors etc.

Requirements Compatible physical dimensions (small size) Low sensitivity (high responsivity) at the desired wavelength and low responsivity elsewhere  wavelength selectivity Low noise and high gain Fast response time  high bandwidth Insensitive to temperature variations Long operating life and low cost

Photodiodes Photodiodes meet most the requirements, hence widely used as photo detectors. Positive-Intrinsic-Negative (pin) photodiode No internal gain, robust detector Avalanche Photo Diode (APD) Advanced version with internal gain M due to self multiplication process Photodiodes are sufficiently reverse biased during normal operation  no current flow without illumination, the intrinsic region is fully depleted of carriers

Physical Principles of Photodiodes As a photon flux Φ penetrates into a semiconductor, it will be absorbed as it progresses through the material. If αs(λ) is the photon absorption coefficient at a wavelength λ, the power level at a distance x into the material is   Absorbed photons trigger photocurrent Ip in the external circuitry

Quantum Efficiency The quantum efficiency η is the number of the electron–hole carrier pairs generated per incident–absorbed photon of energy hν and is given by Ip is the photocurrent generated by a steady-state optical power Pin incident on the photodetector.

Avalanche Photodiode (APD) APD has an internal gain M, which is obtained by having a high electric field that energizes photo-generated electrons. These electrons ionize bound electrons in the valence band upon colliding with them which is known as impact ionization The newly generated electrons and holes are also accelerated by the high electric field and gain energy to cause further impact ionization This phenomena is the avalanche effect

APD Vs PIN  

Responsivity () Quantum Efficiency () = number of e-h pairs generated / number of incident photons APD’s have an internal gain M, hence where, M = IM/Ip IM : Mean multiplied current mA/mW M = 1 for PIN diodes

Photodetector Noise SNR Can NOT be improved by amplification In fiber optic communication systems, the photodiode is generally required to detect very weak optical signals. Detection of weak optical signals requires that the photodetector and its amplification circuitry be optimized to maintain a given signal-to-noise ratio. The power signal-to-noise ratio S/N (also designated by SNR) at the output of an optical receiver is defined by SNR Can NOT be improved by amplification

Response Time in pin photodiode Transit time, td and carrier drift velocity vd are related by For a high speed Si PD, td = 0.1 ns 4

Rise and fall times Photodiode has uneven rise and fall times depending on: Absorption coefficient s() and Junction Capacitance Cj 5

Junction Capacitance εo = 8.8542 x 10(-12) F/m; free space permittivity εr = the semiconductor dielectric constant A = the diffusion layer (photo sensitive) area w = width of the depletion layer Large area photo detectors have large junction capacitance hence small bandwidth (low speed)  A concern in free space optical receivers

Comparisons of pin Photodiodes NOTE: The values were derived from various vendor data sheets and from performance numbers reported in the literature. They are guidelines for comparison purposes.

Comparisons of APDs NOTE: The values were derived from various vendor data sheets and from performance numbers reported in the literature. They are guidelines for comparison purposes only.

Part B Optical receiver

Signal Path through an Optical Link 1

Fundamental Receiver Operation The first receiver element is a pin or an avalanche photodiode, which produces an electric current proportional to the received power level. Since this electric current typically is very weak, a front-end amplifier boosts it to a level that can be used by the following electronics. After being amplified, the signal passes through a low-pass filter to reduce the noise that is outside of the signal bandwidth. The also filter can reshape (equalize) the pulses that have become distorted as they traveled through the fiber. Together with a clock (timing) recovery circuit, a decision circuit decides whether a 1 or 0 pulse was received,

Noise Sources in a Receiver The term noise describes unwanted components of an electric signal that tend to disturb the transmission and processing of the signal The random arrival rate of signal photons produces quantum (shot) noise Dark current comes from thermally generated eh pairs in the pn junction Additional shot noise arises from the statistical nature of the APD process Thermal noises arise from the random motion of electrons in the detector load resistor and in the amplifier electronics

Probability of Error (BER) BER is the ratio of erroneous bits to correct bits A simple way to measure the error rate in a digital data stream is to divide the number Ne of errors occurring over a certain time interval t by the number Nt of pulses (ones and zeros) transmitted during this interval. This is the bit-error rate (BER) Here B is the bit rate. Typical error rates for optical fiber telecom systems range from 10–9 to 10–12 (compared to 10-6 for wireless systems) The error rate depends on the signal-to-noise ratio at the receiver (the ratio of signal power to noise power).

Receiver Sensitivity A specific minimum average optical power level must arrive at the photodetector to achieve a desired BER at a given data rate. The value of this minimum power level is called the receiver sensitivity.

Eye Diagrams Eye pattern measurements are made in the time domain and immediately show the effects of waveform distortion on the display screen of standard BER test equipment. The eye opening width defines the time interval over which signals can be sampled without interference from adjacent pulses (ISI). The best sampling time is at the height of the largest eye opening. The eye opening height shows the noise margin or immunity to noise. The rate at which the eye closes gives the sensitivity to timing errors. The rise time is the interval between the 10 and 90% rising-edge points

Stressed Eye Tests The IEEE 802.3ae spec for testing 10-Gigabit Ethernet (10-GbE) devices describes performance measures using a degraded signal. This stressed eye test examines the worst-case condition of a poor extinction ratio plus multiple stresses, ISI or vertical eye closure, sinusoidal interference, and sinusoidal jitter. The test assumes that all different possible signal impairments will close the eye down to a diamond shaped area (0.10 and 0.25 of the full pattern height). If the eye opening is greater than this area, the receiver being tested is expected to operate properly in an actual fielded system. The inclusion of all possible signal distortion effects results in a stressed eye with only a small diamond-shaped opening