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Optical Receivers Theory and Operation

- Xavier Fernando
- Ryerson Communications Lab
- http//www.ee.ryerson.ca/fernando

Photodetectors

- Part A

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.

Photodetector Requirements

- Good sensitivity (responsivity) at the desired

wavelength and poor responsivity elsewhere ?

wavelength selectivity - Fast response time ? high bandwidth
- Compatible physical dimensions
- Low noise
- Insensitive to temperature variations
- Long operating life and reasonable cost

Photodiodes

- Due to above requirements, only photodiodes are

used as photo detectors in optical communication

systems - Positive-Intrinsic-Negative (pin) photodiode
- No internal gain
- Avalanche Photo Diode (APD)
- An internal gain of M due to self multiplication
- Photodiodes are sufficiently reverse biased

during normal operation ? no current flow, the

intrinsic region is fully depleted of carriers

Physical Principles of Photodiodes

- As a photon flux F penetrates into a

semiconductor, it will be absorbed as it

progresses through the material. - If as(?) is the photon absorption coefficient at

a wavelength ?, the power level at a distance x

into the material is

Absorbed photons trigger photocurrent Ip in the

external circuitry Photocurrent ? Incident Light

Power

Examples of Photon Absorption

pin energy-band diagram

Cut off wavelength depends on the band gap energy

Quantum Efficiency

- The quantum efficiency ? is the number of the

electronhole carrier pairs generated per

incidentabsorbed photon of energy h? 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 obtained by having a

high electric field that energizes

photo-generated electrons and holes - These electrons and holes ionize bound electrons

in the valence band upon colliding with them - This mechanism is known as impact ionization
- The newly generated electrons and holes are also

accelerated by the high electric field and they

gain enough energy to cause further impact

ionization - This phenomena is called the avalanche effect

APD Vs PIN

- APD has high gain due to self multiplying

mechanism, used in high end systems - The tradeoff is the excess noise due to random

nature of the self multiplying process. - APDs need high reverse bias voltage (Ex 40 V)
- Therefore costly and need additional circuitry

Responsivity (?)

- Quantum Efficiency (?) number of e-h pairs

generated / number of incident photons - Avalanche PDs have an internal gain M

mA/mW

IM average value of the total multiplied

current M 1 for PIN diodes

Responsivity

When ?ltlt ?c absorption is low When ? gt ?c no

absorption

Light Absorption Coefficient

- The upper wavelength cutoff is determined by the

bandgap energy Eg of the material. - At the lower-wavelength end, the photo response

cuts off as a result of the very large values of

as.

Photodetector Noise

- 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

Notation Detector Current

- The direct current value is denoted by, IP

capitol main entry and capital suffix. - The time varying (either randomly or

periodically) current with a zero mean is denoted

by, ip small main entry and small suffix. - Therefore, the total current Ip is the sum of the

DC component IP and the AC component ip .

Quantum (Shot Noise)

Due optical power fluctuation because light is

made up of discrete number of photons

F(M) APD Noise Figure F(M) Mx (0 x 1)

Ip Mean Detected Current B Bandwidth

Dark/Leakage Current Noise

There will be some (dark and leakage ) current

without any incident light. This current

generates two types of noise

Bulk Dark Current Noise

ID Dark Current

Surface Leakage Current Noise

(not multiplied by M)

IL Leakage Current

Thermal Noise

The photodetector load resistor RL contributes

to thermal (Johnson) noise current

KB Boltzmanns constant 1.38054 X 10(-23) J/K

T is the absolute Temperature

- Quantum and Thermal are the important noise
- mechanisms in all optical receivers
- RIN (Relative Intensity Noise) will also appear

in analog links

Signal to Noise Ratio

Detected current AC (ip) DC (Ip)

Signal Power ltip2gtM2

Typically not all the noise terms will have equal

weight. Often the average signal current is much

larger than the leakage and dark currents

Noise Calculation Example

Limiting Cases for SNR

- When the optical signal power is relatively high,

then the shot noise power is much greater than

the thermal noise power. In this case the SNR is

called shot-noise limited or quantum noise

limited. - When the optical signal power is low, then

thermal noise usually dominates over the shot

noise. In this case the SNR is referred to as

being thermal-noise limited.

Limiting Cases of SNR

- In the shot current limited case the SNR is
- For analog links, there will be RIN (Relative

Intensity Noise) as well

SNR vs. Received Power

Noise-Equivalent Power

- The sensitivity of a photodetector is describable

in terms of the minimum detectable optical power

to have SNR 1. - This optical power is the noise equivalent power

or NEP. - Example Consider the thermal-noise limited case

for a pin photodiode. Then

To find the NEP, set the SNR 1 and solve for P

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

Rise and fall times

- Photodiode has uneven rise and fall times

depending on - Absorption coefficient ?s(?) and
- Junction Capacitance Cj

Junction Capacitance

eo 8.8542 x 10(-12) F/m free space

permittivity er 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

Various pulse responses

Absorbed optical power at distance x

exponentially decays depending on ?s

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. Detailed values on specific

devices for particular applications can be

obtained from photodetector and receiver module

suppliers.

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. Detailed values on specific

devices for particular applications can be

obtained from photodetector and receiver module

suppliers.

Optical receiver

- Part B

Signal Path through an Optical Link

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,

Optical receiver schematic

Bandwidth of the front end CT Total

Capacitance CdCa RT Total Resistance Rb //

Ra Try Example 6.7 in Keiser

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 109 to 1012 (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).

Derived BER Expression

- A simple estimation of the BER can be calculated

by assuming the equalizer output is a gaussian

random variable. - Let the mean and variance of the gaussian output

for a 1 pulse be bon and s2on, respectively, and

boff and s2off for a 0 pulse. - If the probabilities of 0 and 1 pulses are

equally likely, the bit error rate or the error

probability Pe becomes

Logic 0 and 1 probability distributions

Asymmetric distributions

Select Vth to minimize Pe

Deciding Threshold Voltage

Probability of error assuming Equal ones and

zeros

Where,

Depends on the noise variance at on/off levels

and the Threshold voltage Vth that is decided to

minimize the Pe

Question Do you think Vth ½ Von Voff ?

Probability of Error Calculation

- The factor Q is widely used to specify receiver

performance, since it is related to the SNR

required to achieve a specific BER. - There exists a narrow range of SNR above which

the error rate is tolerable and below which a

highly unacceptable number of errors occur. The

SNR at which this transition occurs is called the

threshold level.

BER as a Function of SNR

- BER as a function of SNR when the standard

deviations are equal (son soff) and when boff

0

Receiver Sensitivity

- To achieve a desired BER at a given data rate, a

specific minimum average optical power level must

arrive at the photodetector. The value of this

minimum power level is called the receiver

sensitivity. - Assuming there is no optical power in a received

zero pulse, then the receiver sensitivity is

Where, including an amplifier noise figure Fn,

the thermal noise current variance is

Receiver Sensitivity Calculation

- The receiver sensitivity as a function of bit

rate will change for a given photodiode depending

on values of parameters such as wavelength, APD

gain, and noise figure.

The Quantum Limit

- The minimum received optical power required for a

specific bit-error rate performance in a digital

system. - This power level is called the quantum limit,

since all system parameters are assumed ideal and

the performance is limited only by the detection

statistics.

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

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