MAJOR REQUIREMENTS - OPTICAL FIBER

EMITTER

1. LIGHT 0/P SHOULD BE HIGHLY DIRECTIONAL.

2. SOURCE SHOULD BE ‘LINEAR’ (MIN. DISTORTION AND
NOISE)
3 SHOULD EMIT LIGHT AT WAVELENGTHS WHERE THE
3.
FIBER HAS LOW LOSSES & LOW DISPERSION.
4 SHOULD BE CAPABLE OF SIMPLE SIGNAL
4.
MODULATION OVER A WIDE BW (AUDIO TO GHz)
5. MUST COUPLE SUFFICIENT OPTICAL POWER INTO
THE OFC.
6 SHOULD HAVE A NARROW LINEWIDTH (SO
6.
AS TO MINIMISE DISPERSION IN THE FIBER)
7 0/P SHOULD NOT BE TEMP DEPENDENT
7. DEPENDENT.
8. SOURCE SHOULD BE CHEAPER &
RELIABLE.
RELIABLE
• FIRST GENERATION OPTICAL SOURCES -
0 85 µm (WAVELENGTH) .
0.85
• SECOND GENERATION OPTICAL SOURCES -
1.1 to 1.6 µm (WAVELENGTH) •
 ENERGY STATE DIAGRAM

INITIAL STATE FINAL STATE

E2
PHOTON ABSORPTION

E1
ATOM
EMISSION PROCESS SPONTANEOUS EMISSION (A)
STIMULATED EMISSION (B)

ATOM
E2 ---------------------------- ---------------------------
(A)

E1 ----------------------------
• ATOM RETURNS TO LOWER ENERGY STATE IN AN
ENTIRELY RANDOM MANNER (INCOHERENT LIGHT
RADIATION)----LED !
ATOM
• E2

(B)

E1

A PHOTON HAVING AN ENERGY EQUAL TO (E2 –E1) INTERACTS WITH THE

ATOM (IN UPPER ENERGY STATE) CAUSING IT TO RETURN TO LOWER
STATE WITH THE CREATION OF A SECOND PHOTON – (LASER !)

COHERENT RADIATION !
OPTICAL SOURCE - ‘LED’

• OPTICAL SOURCE CONVERTS ELECTRICAL

ENERGY (CURRENT) INTO OPTICAL ENERGY
(LIGHT).
• THREE TYPES OF OPTICAL SOURCES
- WIDEBAND CONTINUOUS SPECTRA SOURCES
(INCANDESCENT LAMP).
LAMP)
- MONOCHROMATIC INCOHERENT SOURCES
(LEDs)
- MONOCHROMATIC COHERENT SOURCES
(LASERS).
(LASERS)
LED’S ADVANTAGES :-
• SIMPLE CONSTRUCTION & OPERATION
• LOWER COST
• TROUBLE FREE LIFE (HIGH RELIABILITY)
•. LESS TEMP DEPENDANCE
• LINEARITY
DISADVANTAGES:-
• LOWER OPT POWER CAN BE COUPLED INTO OFC
• LOWER MODULATION BANDWIDTH
• HARMONIC DISTORTION

HOWEVER,LEDs CONTINUE TO REMAIN A

PROMINENT OPTICAL FIBER COMMUNICATION
SOURCE FOR MANY SYSTEM APPLICATIONS.
 OPT. EMISSION FROM
OPT
SEMICONDUCTOR
THE P-N JUNCTION

Impurities and charge carriers at the PN junction

BARRIER POTENTIAL : 0.3V (Ge), 0.7V (Si) AT 250C

 FORWARD BIAS

• THE APPLIED FIELD OPPOSES THE DEPLETION LAYER FIELD .

• THUS IT PUSHES ELECTRONS & HOLES TOWARDS THE
JUNCTION.
• EDGES OF DEPLETION LAYER GET DE-IONISED .
• THIS NARROWS THE DEPLETION LAYER.
• THUS GREATER THE EXTERNAL VOLTAGE NARROWER THE
DEPLETION LAYER.
• RECOMBINATION BETWEEN ELECTRONS AND HOLES
OCCUR AROUND THE JUNCTION.
 CARRIER COMBINATION GIVING SPONTANEOUS EMISSION OF LIGHT

An illustration of carrier recombination giving spontaneous emission of

light in a p—n junction diode.

THE AVERAGE TIME THE MINORITY CARRIER REMAINS IN A FREE STATE

BEFORE RECOMBINATION IS SHORT,
SHORT 10-88 TO 10-10
10 SEC.
SEC (MINORITY
CARRIER LIFETIME)
PN JUNCTION WITH FORWARD BIASING
• INCREASED CONCENTRATION OF MINORITY CARRIERS IN
THE OPPOSITE TYPE REGION IN FORWARD BIASED P-N
DIODE LEADS TO RECOMB1NATION OF CARRIERS.

• ENERGY RELEASED BY ELECTRON HOLE RECOMBINATION IS

APPROX. EQUAL TO BAND GAP ENEGY Eg.

• ENERGY IS RELEASED WITH THE CREATION OF A PHOTON.

E
Eg= hf = hc/λ
h /λ WHERE h h=6.626
6 626 x10
10-34
34 J (PLANCK’S

CONSTANT)

THIS SPONTANEOUS EMISSION OF LIGHT FROM DIODE IS

CALLED- ELECTROLUMINESCENCE.
 LED’S POWER & EFFICIENCY

INTERNAL QUANTUM η = NO OF PHOTONS GENERATED

NO OF ELECTRONS INJECTED

RECOMBINATION RADIATIVE (PHOTON IS GENERATED)

NON-RADIATIVE(ENERGY
NON RADIATIVE(ENERGY RELEASED
IN THE FORM OF HEAT)
INTERNAL QUANTAM η = RADIATIVE RECOMBINATION RATE
TOTAL RECOMBINATION RATE
= rr/rr+ rnr = rr /rt
NON-RADIATIVE RECOMBINATION TAKES PLACE WITHIN THE
STRUCTURE DUE TO CRYSTALLINE IMPERFECTIONS AND
IMPURITIES GIVING AN EFFICIENCY OF 50% (MAX)
• LED
LED’S
S POWER & EFFICIENCY (contd)

• THE ENERGY RELEASED BY THIS ELECTRON –

HOLE RECOMBINATION IS APP. EQUAL TO
BANDGAP ENERGY Eg = hf.
• LET Δn = EXCESS DENSITY OF ELECTRONS IN p –
TYPE MATERIAL .
Δp = EXCESS DENSITY OF HOLES IN n-
n TYPE
MATERIAL.
Δ n = Δ p (FOR CHARGE NEUTRALITY)
 RATE = n FOR CARRIER RECOMBINATION

d/dt (Δn)= J/ed – Δn/ τ (m-3s-1)

At eq
equilibrium
ilibri m ,rate
rate of change of densit
density = 0
or J/ed = Δn/ τ or Δn = Jτ/ed (m-3) (1)
=n(1) GIVES STEADY STATE ELECTRON DENSITY
WHEN A CONSTANT CURRENT IS FLOWING INTO
JUNCTION

AT STEADY STATE,TOTAL NO OF CARRIER

RECOMBINATIONS PER SECOND, = rt = J/ed =
rr+rnr
 RATE = n FOR CARRIER RECOMBINATION
• Δ n = Δ n (o) e-t/τ
• WHERE Δ n (o)= Initial injected excess electron
density :
τ = total carrier recombination life time.

At equilibrium
ilib i ((constant
t t currentt flflows iinto
t jjunction
ti didiode)
d )
TOTAL RATE (carrier generation)= EXT SUPPLIED + THERMALLY
GENERATED
Let J = CURRENT DENSITY (amp/m2)
= J/ed= ELECTRONS PER CUBIC METRE PER SEC.
(where d = thickness of recombination region)
FURTHER,Rt = total number of
recombinations p
per sec= i/e ( i = forward
bias current)
LED INTERNAL QUANTAM EFFICIENCY
η int = Radiative Recombination rate = rr = rr
Total recombination rate rt rr+rnr
= Rr/ Rt
Or Rr = η int x Rt = η int x i/e
= total no of p
photons generated
g per
p sec.

ENERGY IN EACH PHOTON Eg = hf joules

 OPT POWER GENERATED BY LED (Pint)
= No of photons generated x energy /photon
= η int x i/e x hf Watts
P int = ηint x hci/eλ
NOW τr = RADIATIVE MINORITY CARRIER LIFE TIME.
= Δn / rr = electrons /m3
electrons /m3 /sec.
τnr = Δn / rnr
η int = rr / rr + rnr
η int = rr / rr+rnr = 1/1+(rnr / rr ) = 1/1+(τr/ τnr)
rnr /rr = Δn/ τnr x τr / Δn = τr / τnr
Hence η int = 1/1+(τr / τnr)
Also τ = Total recomb.life time = Δn/rt
= Δn/ rr+rnr = Δn/ (Δn/ τr ) + (Δn/ τnr)
= 1/ (1/ τr ) + (1/ τnr)
1/ τ =1/ τr + 1/ τnr
Further η int = rr / rr + rnr
= rr / rt
= (Δn / τr ) = τ/τr
(Δn/ τ)
Hence η int i = τ/τr
THE DOUBLE HETROJUNCTION LED

Layered Structure With Applied Forward Bias

 THE DOUBLE HETROJUNCTION LED
• p- TYPE GaAs IS SANDWITCHED BETWEEN A p–TYPE Al Ga As
AND AN n- TYPE Al Ga As .

• ON APPLICATION OF FORWARD BIAS

• ELECTRONS FROM n TYPE ARE INJECTED THR’

p-n JUNCTION, INTO p TYPE GaAs LAYER.
• THESE MINORITY CARRIERS RECOMBINE WITH MAJORITY
CARRIERS (HOLES).
• PHOTONS ARE PRODUCED WITH ENERGY CORRESP TO
BAND GAP ENERGY OF p- TYPE GaAs LAYER.
• THE INJECTED ELECTRONS ARE INHIBITED FROM
DIFFUSING INTO p – TYPE Al Ga As LAYER BECAUSE OF
POTENTIAL BARRIER PRESENTED BY p p-p
p HETROJUNCTION
 THE DOUBLE HETROJUNCTION LED(contd)

– THUS ELECTRO LUMINESCENCE OCCURS ONLY

IN GaAs LAYER PROVIDING GOOD INTERNAL
QUANTUM EFFICIENCY AND’ HIGH RADIANCE
EMISSION.

• THE DH STRUCTURE IS MOST EFFICIENT

INCOHERENT SOURCE FOR OPT.FIBER
COMM.
COMM
LED STRUCTURES
FIVE MAJOR TYPES OF LED STRUCTRE
• PLANNAR LED’S
• DOME LED’S
• SURFACE EMITTER LED
LED’S
S
• EDGE EMITTER LED’S
• SUPER LUMINESCENT LED’S

PLANAR LED

Ohmic
Oh i
contacts

The structure of a planar LED showing the emission of light from all surfaces.

-P TYPE DIFFUSION OCCURS INTO N TYPE SUBSTRATE

-FORWARD CURRENT FLOWS THR’ JUNCTION AND DEVICE EMITS LIGHT .
-HOWEVER, RADIANCE IS LOW (light emitted from entire surface)
 DOME LED

• DIA OF DOME IS SO CHOSEN TO MAXIMISE AMOUNT OF

INTERNAL EMISSION REACHING THE SURFACE (WITHIN CRITICAL
ANGLE OF GaAs- AIR INTERFACE).

• HIGHER EXT EFFICIENCY THAN PLANAR LED

• DOME SIZE IS FAR GREATER THAN THE ACTIVE RECOMBINATION

AREA . SO EFFECTIVE EMISSION AREA IS GREATER ,THEREBY
REDUCING THE RADIANCE.
SURFACE EMITTER LED (SLED)

• GIVES HIGH RADIANCE

- DUE TO LOW INTERNAL ABSORPTION

- HIGHER REFLECTION COEFFICIENT AT BACK CRYSTAL FACE
(GIVING GOOD FORWARD RADIANCE)

• POWER COUPLED INTO MULTIMODE STEP INDEX FIBER.

Pc =π(1-r)A RD(NA)2 — (1)

r-FRESNEL COEFFICIENT AT FIBER SURFACE

A-EMISSION AREA OF THE SOURCE
RD- RADIANCE OF THE SOURCE
POWER COUPLED ALSO DEPENDS UPON

- DISTANCE AND ALIGNMENT BETWEEN

EMISSION AREA & FIBER
- SLED EMISSION PATTERN
- MEDIUM BETWEEN EMITTING AREA & FIBER

- DOUBLE HETROJUNCTION LED SURFACE

EMITTERS GIVE MORE COUPLED OPTICAL
POWER THAN GIVEN BY =n(1)
 SLED (contd)
• MUCH OF THE LIGHT COUPLED INTO A MM FIBER
FROM A LED IS LOST WITHIN A FEW HUNDRED
METRES.

• HENCE MORE POWER IS COUPLED INTO SHORTER

LENGTH THAN LONGER LENGTH.

• THE SLED’S SUFFER FROM CURRENT SPREADING

RESULTING IN REDUCED CURRENT DENSITY &
EFFECTIVE EMISSION AREA GREATER THAN
CONTACT AREA.
 Al Ga As DH SURFACE EMITTING LED (0.8-0.9 µm WAVE
LENGTH)

The structure of an AIGaAs DH surface-emitting LED (Burrus type)

• INTERNAL ABSORPTION OF THIS DEVICE IS LOW DUE TO LARGE

BAND GAP CONFINING LAYERS.
• THE ADDITION OF EPOXY RESIN IN THE ETCHED WELL TENDS TO
REDUCE THE REFRACTIVE INDEX MISMATCH AND INCREASE THE
EXTERNAL POWER EFFICIENCY OF THE DEVICE.
Small area InGaAsP mesa-etched surface-emitting LED structure
 In Ga As P MESA ETCHED SELED
STRUCTURE(
STRUCTURE(contd)
td)
• MESA STRUCTURE ((mesa dia 20 to 25 µ
µm at the
active layer) REDUCES THE CURRENT SPREADING

• WAVE LENGTH = 1.3 µm

• THE STRUCTURE IMPROVES THE COUPLING η DUE

TO FORMATION OF INTEGRAL LENS AT EXIT FACE.
FACE

• TYPICAL DATA : WITH A DRIVE CURRENT OF 50 mA,

IT COUPLES 2 µw POWER INTO A SINGLE MODE
FIBER.
• COUPLING η UPTO 15% CAN BE ACHIEVED WITH
OPTIMISED DEVICES.
DEVICES
EDGE EMITTING LED
LED’S
S (ELED)

Stripe Geometry DH AlGaAs Edge Emitting LED

 ELED (contd)
• ACTIVE LAYER (50 TO 100 µm) WITH TRANSPARENT GUIDING
LAYERS REDUCES SELF ABSORPTION IN THE ACTIVE LAYER.

• O/P WITH HALF POWER WIDTH OF 30º & 120º

• MOST OF LIGHT EMISSION IS AT ONE END FACE ONLY

• EDGE EMITTERS COUPLE MORE OPTICAL POWER INTO LOW

NA < 0.3 THAN SELED, AND OPPOSITE IS TRUE FOR NA > 0.3.

• COUPLING η IS 3.5 to 6 TIMES THAN SELED.

• USE OF LENS COUPLING INCREASES COUPLING η ( 5 TIMES).

• EDGE EMITTERS ALSO GIVE BETTER MODULATION BW

(HUNDREDS OF MHz) THAN COMPARABLE SELED WITH THE
SAME DRIVE LEVEL.

• ELED’S HAVE LESSER SPECTRAL LINE WIDTH THAN SELED.

TRUNCATED STRIPE In Ga As P EDGE EMITTING LED

Truncated Stripe InGaAsP Edge Emitting LED

• TRUNCATED STRIPE In Ga As P EDGE EMITTING
LED ( Contd )

• OPERATING WAVE LENGTH = 1.3 µm.

• THE DEVICE IS DH EDGE EMITTING LED HAVING
RESTRICTED LENGTH ,STRIPE
STRIPE GEOMETRY p –
CONTACT ARRANGEMENT.
• THE EXTERNAL EFFICIENCY OF THE ELED IS
HIGHER DUE TO LESSER INTERNAL ABSORPTION
OF CARRIERS.
• SILICA LAYER GIVES THE ISOLATION BETWEEN
THE p TYPE LAYERS.
• STRIPE 100 µµm LENGTH
20 µm WIDTH
HIGH SPEED In Ga As EDGE EMITTING LED’S

Mesa Structure High Speed LED

 HIGH SPEED In Ga As EDGE EMITTING LED
LED’S
S

• MESA STRUCTURE (8 µm WIDTH x 150 µm

LENGTH FOR CURRENT CONFINEMENT).
• TILTED BACK FACE TO AVOID LASING
ACTION .

• ACTIVE LAYER IS HEAVILY DOPED (WITH Zn)

TO REDUCE MINORITY CARRIER LIFE TIME
& IMPROVE MODULATION BW.
• MODULATION BW OF 600 MHz IS POSSIBLE .
HIGH
G SPEED
S In Ga As
s EDGE
G EMITTING
G LED’S
S

• 4 µw to 6 µw POWER CAN BE
LAUNCHED AT 100 mA AND 240 mA
DRIVE CURRENT RESPECTIVELY INTO
A SINGLE MODE FIBER.
• 7µw POWER IN BURIED
HETROSTRUCTURE WITH 20 mA
DRIVE CURRENT LAUNCHED INTO SM
FIBER
V GROOVED SUBSTRATE BH ELED
V-GROOVED

V-grooved substrate BH ELED

V-GROOVED SUBSTRATE BH ELED
FRONT FACE IS AR COATED
REAR FACE ETCHED SLANTLY TO SUPPRESS LASING

λ→1.3 µm, 3dB Mod BW ≈ 350 MHz

OPT. POWER ≈ 30 µW (INTO SINGLE MODE FIBER)
BY LENS COUPLING
COUPLING, POWER UPTO 200 µw CAN BE
LAUNCHED WITH DRIVE CURRENT OF 100 mA.

SPECTRAL WIDTH = 50 nm ( narrow )

Al Ga As CONTACT STRIPE SLD

AlGaAs contact stripe SLD

Al Ga As CONTACT STRIPE SLD (contd)
PROVIDES SIGNIFICANT BENEFITS OVER ELED &
SLED
Advantages :
1.
1 HIGH OUTPUT POWER
2. DIRECTIONAL BEAM
3
3. NARROW SPECTRAL LINE WIDTH
4. HIGHER MODULATION BW.
 Al Ga As CONTACT STRIPE SLD(contd)

• p
p- n JUNCTION IN THE FORM OF A LONG
RECTANGULAR STRIPE .
• ONE END OF THE DEVICE IS MADE LOSSY
IN A MANNER TO PREVENT REFLECTIONS
(TO SUPRESS LASING)
• .OUTPUT
OUTPUT IS FROM THE OTHER END.DEVICE
END DEVICE
GIVES PEAK O/P POWER OF 60 mw AT 0.87
µm WAVELENGTH
• ANTI REFLECTION COATING APPLICATION
REDUCES LASER RESONANCE POSSIBILITY
.
Al Ga As CONTACT STRIPE SLD(contd)

• DEVICE PARAMETERS
• 550 µw POWER – 50 µm DIA MMGI
FIBER-25O mA
• 250 µw POWER – SINGLE MODE FIBER
– 100 mA

• LINEWIDTH : 30 TO 40 nm COMPARED
TO 60 TO 90 nm ASSOCIATED WITH
CONVENTIONAL ELED’S
 lnGaAsP / lnP SLD
• STRUCTURE EMITS AT 1
1.3
3 µm

¾ BURRIED ACTIVE LAYER WITHIN V-SHAPED GROOVE ON p -

TYPE InP SUBSTRATE.

¾ LOW LEAKAGE CURRENT

¾ A LIGHT DIFFUSION SURFACE IS PLACED WITHIN THE

DEVICE WHICH SCATTERS THE BACKWARD LIGHT.THIS
SCATTERING FROM THE ACTIVE LAYER DECREASES
FEEDBACK INTO THIS LAYER

¾ AN AR COATING IS PROVIDED ON THE 0/ P FACET.

¾ HIGH 0 / P POWER OF 1 mw CAN BE COUPLED INTO A
SINGLE MODE FIBER.
FIBER
 lnGaAsP SLD / lnP SLD

High output power lnGaAsP SLD

DRAWBACKS - SLD

-HIGH DRIVE CURRENT

-NON – LINEAR O/P CHARACTERISTIC.

-INCREASED TEMP. DEPENDECE OF O/P POWER.

LENS
S COUPLING
COU G TO
O FIBER

COUPLING η = POWER COUPLED (INTO THE FIBRE)

TOTAL POWER EMITTED

COUPLING EFFICIENCY IS GENERALLY POOR (1 T0 2%)

USE OF LENSES IMPROVES THE COUPLING EFFIIENCY

BY 2 TO 5 TIMES.

FOR BETTER COUPLING FIBER CORE DIA >> WIDTH OF

EMISSION REGION.
 LENS COUPLING
CONFIGURATIONS
.a)SPHERICAL
a)SPHERICAL POLISHED STRUCTURES
b) SPHERlCALLY ENDED OR TAPERED
FIBER COUPLING
c) TRUNCATED MICROLENSES
d)GRIN-ROD LENSES
e)) INTEGRAL LENS STRUCTURE
Note :LED output is not fully coupled into the
fiber because of narrow acceptance angle
of the fiber.
Spherical-Ended Fiber Coupled
AlGaAs LED
 SPHERICALLY ENDED FIBER COUPLED Al Ga As LED

• EMITTING DIA = 35 µm RATIO OF

• CORE DIA (OF FIBER)= 75 – 110 µm 1: 2 (min)

• COUPLING EFFICIENCY OF 2 TO 5 TIMES CAN BE ACHIEVED

THR THE USE OF SPHERICAL FIBER LENS.

• THE DEVICE IS A PLANAR SURFACE EMITTING STRUCTURE

WITH THE SPHERICAL ENDED FIBER ATTACHED TO THE CAP
BY EPOXY
O RESIN.
S
• COUPLING EFFICIENCY = 6 %
USE OF TRUNCATED SPHERICAL MICROLENS FOR COUPLING THE
EMISSION FROM AN InGaAsP surface EMITTING LED
• OPERATING wave length = 1.3 µm
• EMISSION REGION DIA SHOULD BE MUCH SMALLER THAN CORE
DIA OF THE FIBER.
• TYPICAL VALUES ( for a step Index fiber )
• ACTIVE DIA : 14 µm
• CORE DIA : 85 µm
NUM APERTURE : 0.16
COUPLING η INCREASED BY A FACTOR OF 13.
OVERALL POWER CONVERSION EFFICIENCY (η PC)

= OPT POWER COUPLED INTO FIBER IS STILL LOW=

ELECT. POWER APPLIED AT TERMIALS 0.4%

NOTE : THEORY SUGGESTS POSSIBLE INCREASE OF UPTO 30

TIMES IN THE COUPLING η )
 LENS COUPLING WITH EDGE EMITTING LED’S

Lens coupling with edge

edge-emitting
emitting LEDs: (a) lens-ended
lens ended fiber coupling;
(b) tapered (pIano-convex) GRI N-rod lens coupling to single-mode fiber.
 LENS COUPLING WITH EDGE EMITTING LED’S

• HIGHER POWER CAN BE COUPLED INTO SINGLE

MODE FIBERS IN CASE OF EDGE EMITTING LED’S
THAN SELED’S.
• TAPERED FIBER – LENSES YIELD A COUPLING
EFFICIENCY OF 15%
• COUPLING η = COUPLED POWER
TOTAL EMITTED POWER
A ) LENS – ENDED FIBER COUPLING
B ) TAPERED(PLANO - CONVEX) GRIN – ROD LENS
COUPLING TO SINGLE MODE FIBER.
 LED CHARACTERISTICS

An ideal light output against current

(a) an AIGaAs surface emitter with a 50 µm diameter

(b) an AIGaAs edge emitter with a 65 µm wide stripe and 100 µm length.
SURFACE EMITTER LED RADIATES SIGNIFICANTLY MORE OPTICAL POWER
THAN EDGE EMITTER LED LED.
BOTH ARE REASONABLY LINEAR AT MODERATE CURRENTS
 LIGHT OUTPUT TEMP DEPENDENCE-LED

• THE INTERNAL QUANTUM EFFICIENCY OF LED’S DECREASES

EXPONENTIALLY WITH INCREASING TEMPERATURE & SO THE LIGHT
OUTPUT DECREASES AS P-N JUNCTION TEMPERATURE INCREASES.

• ELED EXHIBITS GREATER TEMPERATURE DEPENDENCE THAN SLED

• OUTPUT OF SLD WITH ITS STIMULATED EMISSION IS STRONGLY

DEPENPANT ON THE JUNCTION TEMPERATURE.
 CURVES FOR AN Al Ga AS SURFACE
EMITTING LED

OUTPUT SPECTRA TENDS TO BROADEN AT A RATE 0.1 TO 0.3 nm/deg

INCREASE IN TEMP. (DUE TO GREATER ENERGY SPREAD IN CARRIER
DISTRIBUTIONS)
 OUTPUT SPECTRUM

LED output spectra: (a) for an AlGaAs surface emitter with doped active region
.(b) for an lnGaAsP surface emitter showing both the lightly doped and heavily doped cases.

-SPECTRAL LINEWIDTH OF LED OPERATING AT ROOM TEMP IN THE 0.8

TO 0.9 µm WAVELENGTH BAND IS 25 – 40 nm AT HALF POWER POINTS .
LINE WIDTH INCREASES DUE TO INCREASED DOPING LEVELS.
-TYPICAL VALUES FOR ELED & SLED IS 75 nm & 125 nm RESP at 1.3 µm
 MODULATION
• TO TRANSMIT INFORMATION ,IT IS NECESSARY TO MODULATE A
PROPERTY OF THE LIGHT ,WITH THE INFORMATION SIGNAL.
• PROPERTY : INTENSITY , FREQUENCY, PHASE , POLARISATION
(DIRECTION)

INTENSITY MODULATION (IM) OF THE SOURCE IS THE MAJOR

MODULATION STRATEGY.

• IM IS EASY TO IMPLEMENT (variation of drive current of the source)

• ANALOG INTENSITY MODULATION IS USUALLY EASIER TO APPLY
BUT REQUIRES LARGE S/N RATIO , AND HENCE LIMITED TO SHORT
DISTANCE APPLICATIONS (NARROW BW).
• DIGITAL INTENSITY MODULATION GIVES IMPROVED NOISE
IMMUNITY , BUT REQUIRES WIDER BW’S. IDEALLY SUITED FOR OFC,
AS LARGE BW IS AVAILABLE .
MODULATION
O O BANDWIDTH
ELECTRICAL DEFINITION :
ELECT SIGNAL POWER HAS DROPPED TO
HALF ITS CONSTANT VALUE DUE TO
MODULATED PORTION OF THE OPTICAL
SIGNAL (3 DB DOWN). THIS CORRESPONDS
TO THE FREQ AT WHICH ELECT POWER IS
REDUCED BY 3 db wrt I/P ELECT POWER ie
WHEN OUTPUT CURRENT HAS DROPPED
TO 0.707 OF INPUT CURRENT.
 MODULATION BANDWIDTH

MODULATION BANDWIDTH (OPTICAL) :

FREQUENCY RANGE BETWEEN ZERO
AND THIS HIGH FREQUENCY 3 DB
POINT WHEN OUTPUT CURRENT HAS
DROPPED TO 00.5
5 OF THE INPUT
CURRENT.
OPTICAL BANDWIDTH IS NORMALLY
√2 TIMES GREATER THAN THE
ELECTRICAL BANDWITH.
 MODULATION BANDWIDTH (ELECT & OPTICAL)

RATIO OF ELECT. O/P POWER TO ELECT I/P POWER IN db = RE db

R E db = 10 Log10 Elect POWER OUT (DET)/ Elect POWER IN(SOURCE)

α10 Log10 [Iout/I in]2 AT 3db (Iout/I in )2= ½
Iout/I in = 1/√2 = 0.707
 OPT power out (DET)
RO = 10 log10 OPT power in (SOURCE)

I out At 3 db I out = 1/2

/
=10 log10
I in I in

(Due to linear light/ current relationship) of the source and detector

Opt. BW = √ 2( Elect BW)
 RELIABILITY OF LED’S
• LED’S EXHIBIT GRADUAL DEGRADATION IN
ADDITION TO RAPID DEGRADATION.
• RAPID DEGRADATION IS DUE TO GROWTH OF
DISLOCATIONS AND PRECIPITATE – TYPE
DEFECTS IN ACTIVE REGION (CALLED DLDs
& DSDs)
• THESE DEFECTS DEPEND UPON INJECTION
CURRENT DENSITY, TEMP & IMPURITY
CONCENTRATION .
• LONG TERM DEGRADATION COULD BE DUE
TO MIGRATION OF IMPURITIES INTO THE
ACTIVE REGION.
REGION
 RELIABILITY(contd)
• Output power
Pout : INITIAL O/P POWER
βr : DEGRADATION RATE = β0e-Ea/KT
e Ea/KT
Where β0 – PROPORTIONALITY CONSTANT
K- BOLTZMAN’S CONSTANT

OPT. POWER O/P Pe(t) = Pout e-βr t

T- ABS. TEMP. OF THE EMITTING REGION.

T

Ea – ACTIVATION ENERGY ≈ 0.56 TO 1.0 ev FOR SLED’S

((dependant upon material and structure of device )
AVG. LIFE OF SLED’S

-106 TO 107 HOURS(100 TO 1000 YRS)

( FOR
O CWC OPERATION
O O AT ROOM
OO TEMP FOR
O AlGaAs
G
DEVICES)
- IN EXCESS OF 109 HRS FOR SURFACE EMITTING
InGaAsP LED’S.

- DEVICE LIFE TIME IS OFTEN DETERMINED FOR A 50%

DROP IN LIGHT OUTPUT FROM THE DEVICE

- JUNCTION TEMP, EVEN FOR A DEVICE OPERATING

AT ROOM TEMP. IS LIKELY TO BE WELL IN EXCESS
OF ROOM TEMP, WHEN SUBSTANTIAL DRIVE
CURRENTS ARE PASSED
BEHAVIOUR AT HIGH FREQUENCIES –LED’S.
T study
To t d the
th behavior
b h i att hi
high
h ffreq, llett us consider
id eqvt.
t circuit
i it off a LED.
LED

L,R (INDUCTANCE & RESISTANCE OF SEMICONDUCTOR)

CJ = JUNCTION CAPACITANCE
CD=diff. capacitance
Rd=shunt resistance
rd,cd & cj are non-linear components
Cj=ΔQ/ ΔV
Where Q=charge stored in depletion layer
BEHAVIOUR AT HIGH FREQUENCIES –LED’S LED’S (contd)
( td)
CJ=C0/(1-V/Vd)1/2
Where C0 =cap.of
=cap of unbiased junction
V=applied voltage
Vd=diffusion p
potential
Yd=admittance of forward biased diode =1/rD+jw
Cd=J1A/V= I1/V1
Where A=junction area
I1=AC Component of current
We need to find values of rD & Cd to know behavior of
LED at high freq.
BEHAVIOUR AT HIGH FREQUENCIES –
LED’S(contd)
YD=1/rD+j w CD
There is time varying
y g diffusion length
g ,that is
a complex function of freq.
Similarly there are time varying components
of current density J1 and optical o/p power
density P1, that result from sinusoidal
component of driving voltage V1.
J1= J0(1+jwt)1/2 ev1/KT
• Admittance
d a ce
Yd=J1A/V1=J0(1+jwt)1/2eA/KT=eJ0A/KT(1
j )1/2
+jwt)
• Putting I0=J0A & squaring both sides
• YD2=1/rD2+j2wcD/rD-w2cD2=(eI0/KT)2(1+jwt)
• Equating real parts
• 1/rD2-w2cD2=(eI0/KT)2
• Neglecting
g g w2cD2((being g very
y small))
• rD=KT/eI0
• Equating imaginary parts 2cD/rD =
J(eI0/KT)2
2 D = rD
2cD D .J(eI
J( I0/KT)2
= KT/eI0.J(eI0/KT)2 =J.(eI0/KT)
Cd =J/2rD
AT HIGH FREQ., Cd FALLS [Cd α 1/ω2 ], JN
BEHAVIOUR BECOMES DOMINATED
BY DEPLETION LAYER CAPACITANCE
THE TOTAL OPTICAL POWER DENSITY
= TOTAL NO OF RADIATIVE RECOMBINATIONS PER SEC PER UNIT AREA
MULTIPLIED BY MEAN PHOTON ENERGY.
P=0∫XEph(dn/dt)r dx
Po(ss comp) + P1 (time varying comp)
BY INTEGRATION & SUBSTITUTION
|P1/J1 = | Po/Jo * 1/(1+ω2 J2)1/2
PLOT OF THE ABOVE =n n
I1 THE CURRENT FLOWING IN rd & Cd VARIES WITH
FREQUENCY AND FALL IN OPT. POWER ABOVE FREQ. OF
½πJ IS SNPERIMPOSED ON THIS CKT RESPONSE.
- AT FREQUENCIES ABOVE fm=1/2πj, SOURCE η FALLS.
-DECREASING t INCREASES THE UPPER CUT OFF FREQ.
-REDUCING RADIATIVE LIFE TIME tr, MAXIMISES THE
QUANTAM η AT LOW MOD.
MOD FREQ.
FREQ AND HIGH FREQ.
FREQ
CUTOFF.--