Laser Diodes;

Light Emitting Diodes;

Photodetectors

EE4035 Optical Communications

Semester A 2019-20

Lecture 7

1
 Intended Learning Outcomes (ILOs)
• Explain the operation principles of laser diodes.
• Calculate the basic transmission characteristics of laser diodes.
• Explain the transient effects and noise effects in laser diodes.
• Explain the principles of narrow-line lasers.
• Describe the transmission characteristics of light emitting diodes.
• Explain the operation principles of p-n photodiodes, PIN
photodiodes, and APDs.
• Calculate the basic characteristics of photodetectors.

2
 Laser Diodes
Homostructure Double Heterostructure
I
(DH Laser)
-y I

d
x p p

z
n n
L
Light Light

Buried Double Heterostructure Surface Emitter

(BH Laser)
I Light I

p w
d p
L

n n
L
Light
3
 Laser Diodes

https://www.photonics.com/EDU/Handbook.aspx?AID=25099

http://www.qphotonics.com/
Fiber-Coupled-Single-
Mode-Laser-Diodes/

http://laser-industrial.com/principles-of-operations/ 4
Laser Diodes
P-I Curve

5
 Laser Operation
Three basic conditions:
• Active medium
• Population inversion Stimulated emission in the optical
• Optical feedback spectrum
Threshold condition for laser oscillation:
 : Loss coefficient in cm-1
g : Gain coefficient in cm-1
Active Medium
r1 r2
g,  Fractional gain = r1r2 exp( -2 L)
Fractional loss = exp( 2 gL)
L
Steady-state condition for laser oscillation is achieved when the gain in the active
medium exactly balances the total losses.
exp( 2 gL)  r1r2 exp( -2 L)  1 or r1r2 exp[ 2( g -  ) L]  1

Threshold gain per unit length: 1 1

gth    ln
2 L r1r2

Transmission loss through the mirrors 6

 Laser Modes
Condition for setting up standing waves in the laser cavity

q q : Integer
L
2n  : Emission wavelength
qc n : Refractive index
or f 
2nL c : Speed of light

Each value of q defines an axial or longitudinal mode.

Frequency spacing or wavelength spacing between two modes:
c
f  2
2nL
i.e.,  
2nL
There may also be transverse modes.

Finite linewidth due to the change in refractive index produced by a change of carrier
density in laser diode.
The refractive index of a semiconductor is inversely proportional to the carrier density.

7
 Laser Modes
Finite linewidth due to the
c change in refractive index
2nL produced by a change of
carrier density in laser diode
Intensity

Frequency

Intensity
Gain Curve

Review: Fabry-Perot filter

Frequency
8
 Homostructure Light Emitter
Depletion
region
p-type n-type

D
Free carriers: A A D Free carriers:
holes A A A D D D
D electrons
A D
A A D

Depletion
region

Energy

D D D D D
Potential
Fermi level
D
DD barrier
A A
A

A A A A A
Ef

9
 Homostructure Light Emitter
Forward
biasing

Injection current
(pump) Population
inversion
Potential
Emitted wavelength: barrier
 = hc/Ef
hf
h : Planck's constant Fermi level
hf Ef
Ef : Bandgap energy

Energy vs position in the

case of forward biasing
10
 Semiconductor Material for Laser Diodes
In1-xGaxAsyP1-y Quaternary III-V Semiconductor Alloys (x, y: Mole fractions)

Major considerations:
 Direct bandgap semiconductors (efficiency)
 Bandgap energy (wavelength)
 Lattice matching (fabrication)

Examples:
GaAs/AlxGa1-xAs on GaAs for  ~ 0.8 - 0.9 mm
InyGa1-yAs/InxGa1-xP on GaAs for  ~ 0.85 - 1.1 mm
Ga1-yAlyAs1-xSbx/GaSb on GaSb for  ~ 1.0 - 1.7 mm
In1-xGaxAsyP1-y/InP on InP for  ~ 0.92 – 1.7 mm

Most lasers at 1.3 and 1.55 mm are InGaAsP/InP devices.

11
 Double Heterojunction Lasers
Two heterojunctions
p-cladding n-cladding
Conduction band Active

Fermi level

Valence band Eg

hf = Eg’ Eg’

d
A heterojunction is a junction between two materials of different bandgaps.
The electron and hole concentrations build up to much higher values in the active region than they
would have in the depletion region of a simple homojunction. In addition to confining the carriers
tightly within the active region, the heterostructure has another useful property. The refractive
index of the active-region material is slightly greater than that of the extrinsic material of the
cladding, so that not only is the injected current concentrated, but the light, too, is concentrated.
These two forms of confinement are referred to as gain guiding and index guiding, respectively.
Double heterojunction lasers have low threshold currents (Ith ~ 5 – 10 mA). Not only is power
saved and device lifetime increased, but also the control circuitry is much simpler. If Ith is a small
fraction of the maximum drive current, an essentially biasless mode of operation can be used.
12
 Quantum-Well Lasers
The active layer of the laser is made only a few tens of Angstrom (Å) thick (by using MBE –
molecular beam epitaxy). The carrier motion normal to the active layer is restricted, resulting in a
quantization of the kinetic energy into discrete energy levels for the carriers moving in that direction.
Multi-quantum-well (MQW) structure
Barrier Active Cladding
Conduction band

hf
Valence band

Quantum well
energy
In a quantum-well laser diode, the top of the valence
bulk
band and the bottom of the conduction band are
squared-off steps instead of rising parabolas. This
results in a sharper onset in the gain curve. The
sharpness of the band gap means less temperature density of
sensitivity and lower threshold current (a few mA) as permitted states
well as narrower linewidth.
Also, quantum-wire and quantum-dot lasers

13
 Temperature Effect
InGaAsP
double channel
planar BH laser

The threshold current Ith is a function of temperature:

I th (T )  I 0 exp(T / T0 )
where I0 is a hypothetical threshold current at absolute zero (T = 0) and T0 is some fictitious
temperature reference. It is desired to maximize T0, which has proved to be easier for short-
wavelength lasers (T0 ~ 120 K) than for those at 1.3 or 1.55 mm. (T0 ~ 50 – 70 K). Quantum-
well lasers can be made with higher values of T0.
It is necessary to pay substantial attention to thermal dissipation in order to provide efficient
heat sinking arrangements (e.g. thermal electric cooling etc.) to achieve low operating current.
14
 Gain Curve
N: carrier density (which increases with
injected current)

As N increases, more of lower energy states

become occupied and higher energy states
become stronger candidate.

Gain curve broadening by heavy pumping

15
 Transient Effects
Turn-on delay
If the diode is biased too low, for example, below threshold, then when I suddenly goes well
above Ith, it takes some time before there is a great enough density of injected carriers to start
the stimulated emission process. Since the onset of lasing is triggered by spontaneous emission,
the turn-on time is a random variable.

Relaxation oscillation
Once stimulated emission commences, it consumes carriers, the light level drops, carriers build
up sufficiently for strong light emission to recommence, and so forth. This oscillation
eventually dies out and radiation tends to be in that cavity mode where frequency is nearest the
gain peak.

Light output
Relaxation
Oscillation

Time
td
Turn-on delay

16
 Transient Effects
Instantaneous (transient) chirp
As the carrier density swings back and forth during relaxation oscillation, the refractive index
also changes accordingly. This produces frequency modulation of the laser output until the
frequency eventually settles down to a constant value.
Adiabatic frequency chirp
The final frequency depends on the pump current. With growth of the carrier density, there is a
bulk frequency shift due to index change. Changing the bias current between two current
values, for example, can produce a frequency shift. This is often used as a simple way of doing
frequency modulation.
Mode hopping
When there is more than one cavity mode with frequencies lying very near the top of the gain
curve, random fluctuations of various kinds can cause one mode to capture the gain for a while
and then another.

Output Output
I decreases

 
1.3 mm 1.3 mm

Thermal effects: time constant ~ 0.1 msec (~10 MHz).

17
 Narrow-Line Lasers
Grating in a slab waveguide
n1
L Distributed Bragg region (DBR) laser
n2 y
Guiding layer y(Lg)
y- p-type Grating
+

n1 Substrate z
z = Lg n-type DBR
z=0

Coupling between forward and backward waves

Lg
y Pumped region
y-
y

y-
s Distributed feedback (DFB) laser

z=0 z = Lg p-type Grating

Coupling in an active medium Active region

Grating region n-type
y- with gain

y
Review: fibre Bragg grating
z=0 z = Lg
18
 Light Emitting Diodes (LED)
LEDs have similar structures as laser diodes except that the facet reflectivities are low so that
lasing does not occur. Under the conditions of low facet reflectivity, continued increase of the
pump current I does not lead to lasing, but to continued emission of more and more spontaneous
radiation. There are surface-emitter LEDs (SLED) and edge-emitter LEDs (ELED). When
antireflection coating and other artifices are used, together with tight confinement and high current
densities, in order to generate high light output power, one has superluminescent diodes (SLDs).

Drawbacks (compared to laser diodes)

• low optical power coupled into a fibre (μW)
• lower modulation bandwidth

Advantages
• simpler fabrication
• low cost
• reliable
• less temperature dependence
• simpler device circuitry (low drive current and reduced temperature sensitivity)
• better linearity
• no threshold

LEDs are useful sources for multimode fibre communication links.

19
 Light Emitting Diodes (LED)

Surface emitter

Edge emitter
20
 LED Characteristics

The spectral width of an LED operating at room temperature in the 0.8 to 0.9 mm
wavelength band is usually between 25 and 40 nm - the full width at half-maximum
(FWHM) points. For materials with small bandgap energies operating in the 1.1 to
1.7 mm wavelength region, the spectral width tends to increase to around 50 to 160
nm. The FWHM points are around 1.6 times smaller for the ELED than the SLED.

21
LED Characteristics

 = 1.3 mm

22
 LED Characteristics
Output spectrum tends to broaden with increase in temperature.

SLED

23
LED Characteristics
LEDs tend to be slower
devices with significantly
lower output power than
injection lasers because of the
longer lifetime of electrons in
their donor regions resulting
from spontaneous
recombinations rather than
stimulated emission (SLD is
an exception), coupled with
the increased numbers of non-
radiative centres at higher
doping levels. At high
modulation bandwidths, the
optical output power from
conventional LED structures
decreases as shown in the
figure.

24
 Photodetectors
An photodetector is a device to convert the received optical signal into an electrical
signal, which is then amplified before further processing. Improvement of detector
characteristics and performance allows the installation of fewer repeaters and lowers
both the capital investment and maintenance costs.

Criteria when choosing a photodetector:

1. High sensitivity at the operation wavelength
First generation 0.8 – 0.9 mm;
At present, 1.3 mm and 1.55 mm where attenuation and material dispersion can be
minimized.
2. High fidelity
For analogue transmission, the response of an optical detector must be linear with
respect to the optical source over a wide dynamic range.
3. Large electrical response to the received signal waveform
Maximum conversion efficiency
4. Short response time to obtain a suitable bandwidth
At present 10 GHz and above

25
5. Minimum noise
Low dark current, leakage current and shunt conductance
Gain mechanism must be of low noise.
6. Stability of performance characteristics
Because characteristics (i.e. sensitivity, noise, internal gain) vary with temperature,
temperature compensation is necessary.
7. Small size
Efficient coupling to fibre and easy packaging
8. Low bias voltage
No excessive bias voltage and current
9. High reliability
Continuous stable operation at room temperature for many years
10. Low cost
Photodetectors

Photodiodes
26
 Principle of Operation
hole
electron
(b) - p n +
Ec
Depletion
hf > Eg Eg region
p n
Ec

Ev (c)
Ev
(a)
hf EF (Fermi level)

Operation of the p-n photodiode: (a) photo-generation of an electron-hole pair in an intrinsic

semiconductor; (b) the structure of the reverse biased p-n junction illustrating carrier drift in the
depletion region; (c) the energy band diagram of the reverse biased p-n junction showing photo-
generation and the subsequent separation of an electron-hole pair.

A photon incident in or near the depletion region that has energy larger than or equal to Eg will
excite an electron from the valence band into the conduction band. This process leaves an empty
hole in the valance band and is known as photo-generation of an electron-hole (carrier) pair.
Carrier pairs are separated and swept under the influence of the electric field to produce a
displacement by the current in the external circuit in excess of any reverse leakage current.
27
 Characteristics of Photodetectors (PDs)
 Absorption coefficient
 Quantum efficiency
 Responsivity
 Spectral Response
 Rise Time
 Noise
 Uniformity
 Area
Absorption coefficient
The absorption of photons in a PD depends on the absorption coefficient, 0, of light
in the semiconductor, which depends strongly on the wavelength.
P0e(1 - r )
Ip  1 - exp -  0d 
hf
Ip : Photocurrent
P0 : Incident optical power
e : Electron charge
r : Fresnel reflection coefficient at the semiconductor-air interface
d : Width of the absorption region
h : Planck’s constant
f : Optical frequency
28
 Optical absorption
curves for some
photodiode materials
(silicon, germanium,
gallium arsenide,
indium gallium
arsenide and indium
gallium arsenide
phosphide):

Quantum efficiency
Quantum efficiency h is defined as the fraction of the incident photons that are absorbed
by the photodetector and generate useful electrons.
number of electrons collected re rp : Incident photon rate
h 
number of incident photons rp re : Electron rate

The quantum efficiency h is determined by the absorption coefficient and hence a function of
wavelength.
29
Responsivity
Responsivity r is defined as
Photocurrent ( I p ) (in AW-1)
r
Input optical power ( P0 )

Photons are converted into electrons with certain efficiency. If the quantum efficiency
is 100%, then for every incident photon, one electron is produced.
Incident photon rate rp:
P
r  o
p hf

hPo
Since re  hrp 
hf

hPo e
hence, Ip 
hf

he
thus, r
hf
he
With f = c/, we obtain r
hc

30
Spectral response

Long wavelength cutoff

As wavelength increases, photon energy decreases. When the wavelength reaches the
material’s cutoff wavelength c, the photon energy can no longer liberate electrons from
the atoms in the material. c is determined by the material’s bandgap energy Eg:
hc 1.24
c  
Eg Eg
Silicon : c = 1.1 mm
Germanium : c = 1.85 mm
InGaAs : c = 1.7 mm
31
Rise time
90%

10%

Tr
Input power
Output detector
waveform
current waveform

Photodetector has a finite rise time Tr due to

 Charge carrier transit times
 Capacitance
The 3-dB modulation bandwidth is
0.35
F3dB, Elect 
Tr

32
Noise

1. Thermal noise
Detector does not cause thermal noise.
It is due to random movements of electrons in the detector’s load resistor.
2. Shot noise
Shot noise is due to the photocurrent being made up of individual electrons.
Both signal current and dark current contribute to shot noise.
The rms value of the shot-noise current is given by

(is 2 )1 2  2eBI 
12

i2 : Mean squared current variation

s
I : Detector average current
B : Detector bandwidth

3. Excess noise
Some detectors (avalanche photodiodes) have internal gain due to a random process.
The variations in this random process cause excess noise.

33
Uniformity
 The responsivity of a detector may vary from point to point across its surface.
 The effect is only important when making very accurate loss measurement.
 One obviously unresponsive area is where the bonding wire contacting the
surface casts a shadow over the detector surface.

Area
Large area detector (~5 mm2)
 Used in power meters, especially for measuring free space beams
 Good connection repeatability
 Slow rise time (large capacitance)
 High dark current
Small area detector (0.5 mm2)
 Used in receivers
 Fast rise time (low capacitance)

34
 p-n Photodiodes
hf
E-field

p Depletion
region
Absorption
- region
+

n Diffusion
region

x
Load
 The absorption region’s position and width depend upon the energy of the incident photons
and on the material from which the photodiode is fabricated. In case of the weak absorption of
photons, the absorption region may extend completely throughout the device.
 Electron-hole pairs are generated in both the depletion and diffusion regions.
 In the depletion region, the carrier pairs separate and drift under the influence of the electric
field, whereas outside this region, the holes diffuse towards the depletion region in order to be
collected. The diffusion process is very slow compared to drift and thus limits the response
time of the photodiode.
 To make sure that the photons are absorbed in the depletion region, the depletion region is
made as long as possible by reducing the doping in the n-type material.
 Typical width of depletion region is 1 to 3 mm.
35
 p-n Photodiodes

Typical p-n photodiode output characteristics

Drawbacks:
 Low responsivity because most photons are absorbed outside the junction.
 Long rise time because photons absorbed near the junction generate electrons and
holes that diffuse slowly to the junction. Rise Time > 1ms.

36
 PIN Photodiodes
hf
E-field

-
Depletion region
+ i
Absorption region

x
Load
p-i-n photodiode showing combined absorption and depletion regions

 To allow operation at longer wavelengths, a wider depletion region is necessary.

 n-type material is doped so lightly that it can be considered intrinsic, and to make a
low resistance contact a highly doped n-type layer is added.
 The intrinsic layer is thick and has a large response.
 Thick layer gives good chance of absorbing photons and results in high responsivity.
 High resistance gives high electric field, which allows the holes and electrons to
move faster, and results in short rise time.
 Tr < 1 ns.
37
 PIN Photodiodes

The ultimate bandwidth of the device is limited by the drift time tdrift of the carriers
through the depletion region. The maximum photodiode 3dB bandwidth Bm or the gain-
bandwidth product is given by
1 v
Bm   d
2tdrift 2w
vd : Maximum drift velocity
w : Depletion layer width
Maximum possible quantum efficiency 100% is assumed (no internal gain).
38
 Avalanche Photodiodes (APDs)
Semiconductor Photodiodes with Internal Gain
hf
E-field

n
p Gain region

Electron
i Absorption region
Ionizing collision
Hole
p+

x
Load (a) (b)
(a)Avalanche photodiode showing high electric field (gain) region. (b) Carrier pair
multiplication in the gain region.
APDs are strongly reversed bias. When a photon arrives, an electron-hole pair is generated in the
usual way, but the electron (or sometimes hole) is accelerated by the strong electric field inside
the device to such a high speed that when it ‘collides’ with another atom, another electron-hole
pair may be generated. This process may repeat itself many times resulting in an avalanche of
electrons, hence the name AVALANCHE photodiode. As the voltage is increased, the current gain
becomes larger. 39
 Avalanche Photodiodes (APDs)
If the reverse bias voltage is increased too much, the gain approaches infinity, so that
even when there is no light on the diode, a few thermally generated electrons will be
multiplied into a large current. The device effectively becomes a conductor and the
current flowing does not depend on the presence or absence of light. The voltage at
which this happens is the Reverse Breakdown Voltage (VBR).
The response time is limited by:
• The transit time of the carriers across the absorption region
• The time taken by the carriers to perform the avalanche multiplication process
• The RC time constant incurred by the junction capacitance of the diode and its load
The current gain M of an APD is given by:
1
M M increases with bias voltage
n
V 
1- B 
 VBR 
VB : Bias voltage
VBR : Break down voltage
n : Device constant related to the geometry (n < 1)

40
 Avalanche Photodiodes (APDs)
APD Characteristics
Breakdown voltage : 20 to 500 V
Responsivity : 20 to 80 A/W
Gain (M) : 10 to 200

Excess Noise
Electrical signal power is improved by a factor of M2 over a PIN, but this gain is not
without cost. Note that M is the average current gain. Actually there are some random
variations in M that cause extra shot noise. The shot noise power is increased by a factor
larger than that for the signal power. The factor for the noise is Mn, where 2 < n < 3.
Mn n-2
The excess noise factor =  M
M2

The trade-off between signal and excess noise means that there exists an optimum gain
and, hence, bias voltage in any particular situation.

41
 Avalanche Photodiodes (APDs)
Advantages:
 APD provides an increase in sensitivity over PD without internal gain by 5 to 15 dB.
 APD gives a wider dynamic range due to their gain variation with response time and
reverse bias.
 APD can offer a smaller minimum detectable optical power for direct detection.

Receiver sensitivity comparison

of PIN photodiode and APD at a
BER of 10-9 using InGaAs
detectors operating at a
wavelength of 1.55 mm.

42
 Avalanche Photodiodes (APDs)
Drawbacks:
 Fabrication difficulties due to
their more complex structures
and hence increased cost.
 The random nature of the gain
mechanism, which gives an
additional noise contribution.
 The high bias voltage required
(50 to 400 V), which is
wavelength dependent.
 The variation of the gain with
temperature; thus temperature
compensation is necessary to
stabilize the operation of the
device.

Current gain against reverse bias for a silicon APD

operating at a wavelength of 0.825 mm.

43
 Comparison of Photodiodes
Typical Characteristics of Photodiodes

Material Structure Rise Wavelength Responsivity Dark Gain

Time (nm) (A/W) Current
(ns) (nA)
Silicon PIN 0.5 300-1100 0.5 1 1
Germanium PIN 0.1 500-1800 0.7 200 1
InGaAs PIN 0.3 1000-1700 0.6 10 1
Silicon APD 0.5 400-1000 77 15 150
Germanium APD 1 1000-1600 30 700 50

NOTE: APDs are generally not popular with receiver designers because they require a high
voltage power supply.

44
 Exercise
1. The longitudinal modes of a gallium arsenide injection laser eimitting at a
wavelength of 0.87 mm are separated in frequency by 278 GHz. Determine the
length of the optical cavity and the number of longitudinal modes emitted. The
refractive index of gallium arsenide is 3.6.
2. An injection laser has a GaAs active region with a bandgap energy of 1.43 eV.
Estimate the wavelength of optical emission from the device and determine its
linewidth in Hertz when the measured spectral width is 0.1 nm.
3. The threshold current density for a stripe geometry AlGaAs laser is 3000 A/cm2 at
a temperature of 15 C. Estimate the required threshold current at a temperature
of 60 C when the threshold temperature coefficient To for the device is 180 K,
and the contact stripe is 20 x 100 mm.
4. A detector operating at 850 nm produces 80 mA of output current for a 500 mW
input beam. Calculate (i) the responsivity of the detector, and (ii) the quantum
efficiency.
5. A detector has a responsivity of 0.5 A/W at the wavelength of interest and a dark
current of 1 nA. Calculate the mean-square noise current and the RMS noise
current due to shot noise if the noise bandwidth is 50 MHz and the incident power
is 100 mW.

45
6. A silicon APD has a multiplication factor of 103 when operating at a wavelength
of 0.82 mm. At this operating point, the quantum efficiency of the device is 90%
and the dark current is 1 nA. Determine the number of photons per second at the
wavelength 0.82 mm required in order to register a light input to the device
corresponding to an output current (after avalanche gain) which is greater than the
level of the dark current (i.e., I > 1 nA).
7. Given that the following measurements were taken for an APD, calculate the
multiplication factor for the device.
Received optical power at 1.35 mm: 0.2 mW
Corresponding output photocurrent: 4.9 mA
(after avalanche gain)
Quantum efficiency at 1.35 mm: 40%

46