unit iv fiber optic receiver and measurements materials/7/ocn/unit4.pdf · • an electric signal...

UNIT IV

FIBER OPTIC RECEIVER AND

MEASUREMENTS

FUNDAMENTAL RECIEVER

OPERATION

• The design of an optical receiver is much more complicated

because it must detect the weak, distorted signals and then

make decision on what type of data was sent based on the

amplified version of the distorted signal.

FUNDAMENTAL RECIEVER OPERATION

Digital signal transmission

• Pre amplifiers used in fiber optic communciation receivers can

be classified as

–The Low Impedance Preamplifier

–The High Impedance Preamplifier

–The Trans Impedance Preamplifier

PREAMPLIFIERS

• The noise in optical receivers is caused by the spontaneousfluctuations of current or voltage in electric circuits.

• The two common samples of these spontaneous fluctuationsare shot noise and thermal noise.

• Shot noise arises in electronic devices because of the discretenature of the current flow in the device.

• Thermal noise arises from the random motion of electrons in aconductor.

ERROR SOURCES

• The random arrival rate of signal photons produces a quantum (shot noise)

noise at the photo detector.

• Since this noise depends on the signal level, it is of particular importance

for pin receivers that have large optical input levels.

• For Avalanche photodiode, an additional shot noise arises from the

statistical nature of the multiplication process.

ERROR SOURCES

• Thermal noises arises from the detector load resistor and from amplifierelectronics tend to dominate in applications with low signal to noise ratiowhen a pin photodiode is used.

• The primary photocurrent generated by the photo detector is a time varyingpoison process resulting from the random arrival of photons at the detector.

• If the detector is illuminated by an optical signal p(t), then the averagenumber of electron – hole pairs N generated in a time τ is

• η is the detector quantum efficiency

hv is the photon energy

E is the energy received in a

time interval τ

ERROR SOURCES

hv

Edttp

hvN

0

)(

• The three basic stages of the receiver are a

– Photo detector,

– Amplifier and

– Equalizer.

RECIEVER CONFIGURATION

• The photodiode can be either an avalanche photo diode with

mean gain M or pin photodiode with gain M=1.

• The photodiode has a quantum efficiency η and a capacitance

Cd

• The detector bias resistor has a resistance Rb which generates a

thermal noise current ib(t).


• The amplifier has an input impedance represented by the

parallel combination of resistance Rd and a shunt capacitance

Cd

• Voltages appearing across this impedances causes current to

flow in the amplifier output.

• This amplifying function is represented by voltage controlled

current source which is characterized by a trans conductance

gm


• There are two amplifier noise sources, the input noise current

ia(t) arises from the thermal noise of the amplifier input

resistance Ra, whereas the noise voltage source ea(t) represents

the thermal noise of the amplifier.

• These noise sources are assumed to be Gaussian in statistics,

flat in spectrum and uncorrelated.

• The Equalizer that follows the amplifier is normally a linear

frequency shaping filter that is used to mitigate the effects of

the signal distortion and inter symbol interference.


PROBABILITY OF ERROR

t

E

t

e

B

N

N

NBER

Typical error rates for optical fiber telecommunication systems range

from 10-9 to 10-12.

This error rate depends on the SNR of the reciever

PROBABILITY DISTRIBUTIONS FOR RECEIVED

LOGICAL 0 AND 1 SIGNAL PULSES

• To compute the bit error rate at the receiver, we have to know

the probability distribution of the signal at the equalizer

output.

• The signal probability distribution is important because it is

here the decision is made as to whether a 0 or 1 is sent.

• Which is the probability the equalizer output voltage is less

than v when a logic 1 pulse is sent


dyy

pvP

V

11

dyy

pvPv

00

• If the threshold voltage Vth then the error probability Pe isdefined as

• Plot of BER vs factor Q

• Factor Q is widely used to specify receiver

• performance, since it is related to the signal

• to noise ratio required to achieve a specific

• bit error rate.


)()( 01 ththe vbPvaPP

• An Ideal photo detector which has unity quantum efficiency

and which produces no dark current; that is no electron hole

pairs are generated in the absence of an optical pulse.

• Given this condition, it is possible to find the minimum

received optical power required for a specific bit error rate

performance in a digital system.

• This Minimum received optical power level is known as the

quantum imit.

QUANTUM LIMIT

• Attenuation in optical fiber waveguide is a result of absorption

processes, scattering mechanisms, and waveguide effects.

• Three basic methods are available in fiber attenuation

measurements

FIBER ATTENUATION MEASUREMENTS

• The most common approach involves measuring the optical

power transmitted through a long and a short length of the

same fiber using identical input couplings. This method is

known as the cut back technique.

• A less accurate but a non destructive method is the insertion –

loss method, which is useful for cables with connector on

them.

• OTDR – Optical Time Domain Reflectometer


• The cutback technique which is a destructive method requiringaccess to both ends of the fiber.

• Measurements may be made at one or more specific wavelengths oralternatively a spectral response may be required over a range ofwavelengths.

• To find the transmission loss, the optical power is first measured atthe output of the fiber.

• Then without disturbing the input condition, the fiber is cutoff fewmeters from the source and the output power at this near end ismeasured.


THE CUTBACK TECHNIQUE

• If PF and PN represents the output powers at the far and near

ends of the fiber, respectively, the average loss α in decibels

per kilometer is given by

• Where L is the separation of the two measurement points


THE CUTBACK TECHNIQUE

F

N

P

P

Llog

10

• This is less accurate than the cutback method, but is intended

for field measurements to give the total attenuation of a cable

assembly in decibels.

• The wavelength tunable light source is coupled to a short

length of the fiber that has the same basic characteristics as the

fiber to be tested.


INSERTION LOSS METHOD

• To carry out the attenuation tests, the connector of the short length

launching fiber is attached to the connector of the receiving system

and the launch power level is P1(λ) is recorded.

• Next, the cable assembly to be tested is connected between the

launching and receiving systems, and the received power level P2(λ)

is recorded.

• The attenuation of the cable in decibel is then

• The attenuation is the sum of the loss of the cabled fiber and the

connector between the launch connector and the cable.


INSERTION LOSS METHOD

2

1log10P

PA

• Three basic forms of dispersion produce pulse broadening of

light wave signals in optical fibers, thereby limiting the

information – carrying capacity.

• Intermodal Dispersion

• Chromatic Dispersion

• Polarization-mode Dispersion


DISPERSION MEASUREMENTS

• In evaluating intermodal dispersion, the fiber can e considered

as a filter characterized by an impulse response h(t) or by a

power transfer function H(f), which is the fourier transform of

the impulse response.

• Either of these can be measured to determine the pulse

dispersion .

• The impulse response measurements are made in time domain,

whereas the power transfer function is measured in the

frequency domain.

INTERMODAL DISPERSION

• Both the time domain and frequency domain dispersion

measurements assume that the fiber behaves quasi linearly in

power, that is the individual overlapping output pulses from an

optical waveguide can be treated as adding linearly.

INTERMODAL DISPERSION

TIME DOMAIN INTERMODAL


FREQUENCY DOMAIN INTERMODAL


)(

)()(

fP

fPfH

in

out

As the modulation frequency is increased, the optical power level at the fiber

output will eventually start to decrease.

• Modulation phase – shift method

• An electric signal generator intensity modulates the output of a narrowband, tunable

optical source by means of an external modulator.

• After detecting the transmitted signal with a photo diode receiver, a vector

voltmeter is used to measure the phase of the modulation of the receiver signal

relative to the electrical modulation source.

CHROMATIC DISPERSION

• The difference in propagation times between the two

orthogonal polarization modes at a given wavelength will

result in pulse spreading which is called as polarization mode

dispersion.

POLARIZATION MODE DISPERSION

unit iv fiber optic receiver and measurements materials/7/ocn/unit4.pdf · • an electric signal...

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