detectors

The University of Texas at Dallas Erik Jonsson School ofEngineering & Computer Science

c© C. D. Cantrell (06/2003)

OPTICAL DETECTORS AND RECEIVERS

Notes prepared for EE 6310

by

Professor Cyrus D. Cantrell

August–December 2003



DETECTORS FOR OPTICAL COMMUNICATIONS (1)

• All detectors for optical communications use optical absorption in a depletionregion to convert photons into electron-hole pairs, and then sense the numberof pairs

Because of the electric field in the depletion region, the electron-hole pairsgive rise to a photocurrent, Ip

One figure of merit is the responsivity, defined as the ratio of thephotocurrent to the optical power, Pin:

R =Ip

Pin=

ηQq

ω=

ηQq

hν(units: A/W)

where ηQ = quantum efficiency and q = charge generated per photon

ηQ =electron-hole pair generation rate

photon incidence rate



DETECTOR MATERIALS

• Bandgaps and emission wavelengths (at 300 K) of semiconductors used asdetectors for optical communications

Material Bandgap, eV Wavelength Wavelength of Responsivityrange (nm) peak response (nm) (max) (A/W)

Si 1.17 300–1100 800 0.5Ge 0.775 500–1800 1550 0.7

InGaAs 0.75–1.24 1000–1700 1700 1.1



DETECTORS FOR OPTICAL COMMUNICATIONS (2)

• p-n photodiodes

Electron-hole pairs are created in the depletion region of a p-n junctionin proportion to the optical power

Electrons and holes are swept out by the electric field, leading to a current

• p-i-n photodiodes

Electric field is concentrated in a thin intrinsic (i) layer

• Avalanche photodiodes

Like p-i-n photodiodes, but have an additional layer in which an average ofM secondary electron-hole pairs are generated through impact ionizationfor each primary pair

Leads to a responsivity

R = Mηq

ω



p-n PHOTODIODES

• Operated in reverse-biased regime for detection, instead of forward-biasedregime for emission

• Wide depletion region

Advantage: High quantum efficiency

Problem: Diffusion of carriers created in the boundary p and n regionslimits the detector bandwidth

Problem: Transit time across the depletion region also limits the detectorbandwidth

RC time constant:τRC = (RL + Rs)Cp

RL = load resistance, Rs = internal series resistance, Cp = parasiticcapacitance


Joseph C. Palais, Fiber Optic Communications, 4th Edition

PHOTOCONDUCTIVE vs. PHOTOVOLTAIC OPERATION

• Photoconductive regime: Reverse-biased

• Photovoltaic regime: Unbiased



p-i-n PHOTODIODES

• Basic idea: Eliminate diffusion of carriers created outside the depletionregion by:

Sandwiching a thin layer of a different semiconductor material (of intrinsicconductivity) between the outer p and n layers

Choosing the outer p and n layers to be transparent to light in the workingwavelength range

• Typical sensitivities for a BER of 10−10 are −26 dBm at a bit rate B = 2.5Gb/s, or −18 dBm at a bit rate B = 10 Gb/s


Agilent 5988-5927EN.pdf

InGaAs p-i-n DC RESPONSIVITY vs. WAVELENGTH

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800

Wavelength (nm)

Res

pons

ivit

y (A

/W

)


Agilent 5988-5927EN.pdf

p-i-n SENSITIVITY vs. BIT RATE

-27

-25

-23

-21

-19

-17

-15

100 125 155 310 622 1065 1250 1500 1750 2125 2488 2700

Bit rate (Mb/s)O

ptic

al S

ensi

tivity

(dB

m)


Joseph C. Palais, Fiber Optic Communications, 4th Edition

LOAD-LINE ANALYSIS OF A p-i-n CIRCUIT

• Photoconductive regime: Voltage across load resistor is proportional tooptical power

For optical powers above a certain critical value (40 µW in this example),the voltage across the load grows very slowly as a function of optical power



AVALANCHE PHOTODIODES

• Internal gain

Electron-hole pairs created by absorption of photons are accelerated to en-ergies at which more pairs are created, then the new pairs are acceleratedand create more pairs, in an “avanlanche”

Overall gain is M pairs generated for each pair created optically

M ≈ 1

1 − (vd/VBR)n

where vd = reverse bias voltage, VBR = breakdown voltage, and n > 1

Avalanche multiplication creates excess noise

Scales nonlinearly with M , while the signal scales linearly

Therefore there’s an optimal value, Mopt

Typically 3 Mopt 9

Much better signal-to-noise ratio than with external amplification

• Typical sensitivities for a BER of 10−10 are −32 dBm at a bit rate B = 2.5Gb/s, or −22 dBm at a bit rate B = 10 Gb/s


Govind P. Agrawal, Fiber-Optic Communication Systems, 2nd Edition, Fig. 4.11

OPTICAL RECEIVERS (1)

• Block diagram of a digital optical receiver

Vertical dashed lines separate various functional units of the receiver




• Front end

High-impedance preamplifier

Bandwidth is

∆f =1

2πRLCT

where CT = total capacitance and load impedance RL Rs

Conflicting design goals: RL must be high for high sensitivity, but mustbe low for high bandwidth

Transimpedance preamplifier

High sensitivity and high bandwidth


Govind P. Agrawal, Fiber-Optic Communication Systems, 2nd Edition, Fig. 4.12


• Equivalent circuits for optical receiver front ends

(a) High-impedance

(b) Transimpedance




• Linear channel

Transfer functionHout(ω) = HT (ω)Hp(ω)

HT = transfer function of linear channel

Hp = transfer function of photodetector

Intersymbol interference (ISI) occurs if the time-domain output signal fora “one” bit extends beyond the bit-slot boundaries

To minimize ISI, try to ensure that Hout is the transfer function of araised-cosine filter,

Hout(ω) =

12[1 + cos(ω/2B)], if ω < 2πB;

0, if ω ≥ 2πB




• Data recovery

Clock recovery

Purpose: Isolate a spectral component at the line rate of the signal(f = B) in order to synchronize the sampling times with the bit slotsof the received bit stream

For RZ, can pass the signal through a narrow bandpass filter

For NRZ, have to square and rectify the signal spectral componentat f = B/2

The spectral component recovered from the signal is used in a phase-locked loop to control the frequency of a local oscillator in such a waythat there is negligible drift when some data transitions are absent

Frequency recovery is not enough; one must also have an edge detector

Decision circuit

Compares the sampled linear-channel output to a threshold level


Behzad Razavi, Monolithic Phase-Locked Loops and Clock Recovery Circuits, Theory and Design (IEEE Press, 1996)

PHASE-LOCKED CLOCK RECOVERY CIRCUIT

• Block diagram for phase-locked clock recovery

LPF = low-pass filter

VCO = voltage-controlled oscillator

⊗

= multiplier

Produces an error voltage at the difference frequency between the edgedetector and the VCO, which is fed back to control the VCO




• Features and impairments of real receivers

Noise ⇒ vertical eye closure and increased bit error rate

Electronic noise sources

Thermal noise (mostly from the preamplifier)

Shot noise

Fluctuations in APD gain also contribute noise

Noise is discussed in greater detail in another section of the course

Dark current

Timing jitter ⇒ horizontal eye closure and increased bit error rate

Caused by imperfect clock recovery

P r o d u c t B u l l e t i n

ERM 5772.5 Gb/s High GainAvalanche PhotodiodeOptical Receiver Modules

EPITAXX ERM 577 series are high gain, highbandwidth, differential output, AvalanchePhotodiode (APD) receivers with GaAstransimpedance amplifiers. The high gain of thereceivers provides system designers with a largeoutput at low optical power levels. Also, thedifferential output can be used for added gain orfor signal monitoring.

Key Features

Electro-optical• InGaAs photodiode with Transimpedance

Amplifiers• High gain: 50,000 V/W typical• High dynamic range: 31 dB typical• Low dark current: 10 nA typical

Packaging• 14-pin butterfly with single mode 900 µm loose

jacketed fiber pigtail

or

AMG package with single mode 900 µm loosejacketed fiber pigtail

• Both packages available with LC, SC or FCconnectors

Applications• High sensitivity digital receivers• Long haul SONET/SDH receivers

SpecificationsConditions (unless noted):Temperature = 25°C, λ = 1550 nm, R

L = 50Ω, V

ss = -5.2V

All specifications without connector.

Parameter Measurement Min Typ Max UnitsConditions

Sensitivity 2.5 Gb/s -34 -32 dBm1E-10 BERR

APD = 8.5 A/W

Small Signal Single-ended 30 50 kV/WGain f = 1.2 GHz

RAPD

= 8.5 A/W

Bandwidth RAPD

= 2.5 1.5 1.8 GHzto 10 A/W

Overload RAPD

= 2.5 A/W -7.0 -3.0 dBm

Optical Back -40 -30 dBReflection

Output Impedance Single-ended 50 ΩMaximum Output Single-ended 550 mVVoltage Voltage (p-p) (p-p)

Maximum Ratings

Parameter Min Typ Max Units

Operating Temperature 0 70 °C

Storage Temperature -40 85 °C

Supply Voltage (-) -6 VDC

APD Supply Voltage Vb

V

Optical Input Power 1.0 mW

Note: APD breakdown voltage is equal to Vb

DC Electrical Characteristics

Parameter Measurement Min Typ Max UnitsConditions

APD Breakdown Id = 10 µA 40 50 70 V

Voltage, Vb

APD Responsivity 1 µW Optical 8.5 A/WR

APDPower,V

APD = V

b -1.5

Dark Current VAPD

= Vb -1.5 10 40 nA

Thermistor 3 kΩSupply Voltage (-) -4.95 -5.2 -5.45 V

Supply Current 130 mA

19.0

7

8 14

1

TOP VIEW

22.53

0.9 O.D. Fiber3.71

7.60

25.4 1.25 meters

10.0 min

2.54

Gold Plated Kovar

0.39 ±0.05

0.25 ±0.04

Detail A

1.91

Pins 8 through 14 shown.Pins 1, 3, 5, 7, 8, 9 and 12grounded through devicepackage as shown.

Pin Configuration1 - Gnd2 - VPD (Bias: +)3 - Gnd4 - V supply (-5.2 V)5 - Gnd 6 - Thermistor7 - Gnd8 - Gnd9 - Gnd10 - Output (inverted)11 - Output (non-inverted)12 - Gnd13 - NC14 - NC

Mechanical Dimensions - ERM 577

All dimensions in mm (nominal)

12.70

7

8 14

1

BOTTOM VIEW

22.53

0.9 O.D. Fiber

5.13 7.98

1.25 meters

2.54

Gold Plated Kovar

0.39 ±0.05

0.25 ±0.04

Detail A

1.91

Pins 8 through 14 shown.Pins 1, 3, 5, 7, 8, 9, 12 and 14

grounded through devicepackage as shown.

Pin Configuration1 - Gnd2 - VPD (Bias: +)3 - Gnd4 - V supply (-5.2 V)5 - Gnd 6 - Thermistor7 - Gnd8 - Gnd9 - Gnd10 - Output (inverted)11 - Output (non-inverted)12 - Gnd13 - NC14 - Gnd

15.24

22.5

13.1 RIGID

4.50

19.05

25.35

3.17

12.70

M3 x 0.52.54 DEEP4 PLCS

8 14

See Detail A

1.04

3.71

Mechanical Dimensions - ERM 577AMG

All dimensions in mm (nominal)

ERM 577 2.5 Gb/s High Gain | 2Avalanche Photodiode Optical Receiver Module

Ordering Information

Product Model Description

ERM 577 2.5 Gb/s APD, 14-pin Butterfly900 µm buffer without connector

ERM 577AMG 2.5 Gb/s APD, AMG Package900 µm buffer without connector

ERM 577xxx FJS LC/SPC 900 µm buffer with LC/SPC connector

ERM 577xxx FJS SC/SPC 900 µm buffer with SC/SPC connector

ERM 577xxx FJS SC/APC 900 µm buffer with SC/APC connector

ERM 577xxx FJS FC/SPC 900 µm buffer with FC/SPC connector

ERM 577xxx FJS FC/APC 900 µm buffer with FC/APC connector

100.5mV

20mV /div.

trig'd.

T

-99.5mV55.61ns 100ps/div 56.61ns

500.5mV

100mV/div.

trig'd.

T

-500mV55.61ns 100ps/div 56.61ns

10

11

0.033µF//10µF

4

TIA

APD

ERM577

2

P

0.1µF

0.1µF

0.033µF//10µF

To Post-ampor

Clock DataRecovery

VAPD

VSS

0

ERM 577 Typical Transfer Function

Optical Power (µW)

Out

put V

olta

ge (

mV

)

60 100804020

0

100

200

300

400

500

600

Figure 1 Figure 2

Figure 3


Typical Eye DiagramOptical Power = -32 dBm

Typical Eye DiagramOptical Power = -3 dBm

ERM 577 Application Circuit

All information contained herein is proprietary and confidential, believed to be accurate and is subject tochange without notice. No responsibility is assumed for its use. JDS Uniphase Corporation, its subsidiariesand affiliates, or manufacturer, reserve the right to make changes, without notice, to product design, productcomponents, and product manufacturing methods. Some specific combinations of options may not beavailable. Please contact JDS Uniphase, EPITAXX Division, for more information. ©JDS Uniphase Corporation.All rights reserved. 08/13/00 Printed in USA

JDS Uniphase Corporation Tel 609 538-1800EPITAXX Division Fax 609 538-16847 Graphics Drive [email protected] Trenton, NJ 08628 www.jdsuniphase.com

Quality VisionEPITAXX has a leadership position in the optoelectronicindustry with a vision for excellence in quality. The division iscommitted to providing customers with the highest levels ofquality and reliability in design and manufacturing. The toppriorities remain continuous process improvement and totalcustomer satisfaction. EPITAXX obtained ISO 9001 certificationin 1996 for both design and manufacturing operations. Inaddition, EPITAXX maintains a strict quality control programto ensure that all products meet or surpass customer require-ments.

Precautions for UseESD protection is imperative. Use of grounding straps,anti-static mats, and other standard ESD protectiveequipment is recommended when handling or testingan InGaAs PIN or any other junction photodiode.

Soldering temperature of the leads should not exceed260 oC for more than 10 seconds.

Fiber feed through tube temperature should not exceed120 oC.

Fiber pigtails should be handled with less than 10 N pulland with a bending radius greater than 1“.


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