1. Ch.3 OPTICAL TRANSMITTERS
General Block Diagram of the Optical Link
Note: Light sources that can be used on the transmitter side are light emitting diodes and laser diodes.

2. General requirements for a light source for use in optical communications
The emission wavelength compatible with the loss spectrum of glass fibers, 820nm, 1300nm & 1550nm.
2. The sources should be capable of modulation at rates in excess of 1GHz for high data rate transmission.
3. The spectral width of the sources should be narrow in order to minimize the bandwidth limiting pulse dispersion in the fibers.
4. The average emitted power of the source that is needed is typically few milliwatts, although higher power values are needed for very long continuous fiber links or if high loss fibers are used.
5. The radiance of the source should be as high as possible for effective coupling into the low-loss fiber with small NA ( ~0.2). This means that the beam spread of the sources must be minimized.
6. The sources must have long lifetime and it must be possible to operate the device continuously at room temperature.
7. The sources must be highly reliable.
8. The sources should be reasonably low cost.

3. TYPES OF OPTICAL SOURCES
Wideband continuous spectra sources (Incandescent Lamps).
Monochromatic incoherent sources (Light Emitting Diodes - LED).
Monochromatic coherent sources (Light Amplification by Stimulated Emission of Radiation - LASER)
Note
The principal light sources used for fiber optic communications applications are heterojunction-structured semiconductor laser diodes or injection laser diodes (ILDs) and light-emitting diodes (LEDs).
SPECTRAL LINEWIDTHS

4. OPTICAL TRANSMITTERS
The starting point of the optical communication system is the transmitter where the electrical signal convert to the optical signal by modulate the optical source. Converting the electrical signal into the optical signal is using an electronic circuit. The circuit is a driving circuit.
The light sources are mounted in a package that enables an optical fiber to be placed in very close proximity to the light emitting region in order to couple as much light as possible into the fiber.
In some cases, the emitter is even fitted with a tiny spherical lens to collect and focus “every last drop” of light onto the fiber and in other cases, a fiber is “pigtailed” directly onto the actual surface of the emitter.
There are few types of semiconductor sources in communication system. The most commonly use in the communication system is Light Emitting Diode (LED) and Laser Diode (LD). These two types of sources has different characteristic and the driving circuit for each type of sources should be different.
The difference between LEDs and laser diodes is that LEDs produce incoherent light, while laser diodes produce coherent light

5. OPTICAL TRANSMITTERS (Cont’d)
The basic optical transmitter converts electrical input signals into modulated light for transmission over an optical fiber. Depending on the nature of this signal, the resulting modulated light may be turned on and off or may be linearly varied in intensity between two predetermined levels, as shown in Figure 1.
On-OFF Modulation
Linear Modulation
Figure 1. Basic Optical Modulation Methods Intensity

6. Transmitter Component
Light source
Driving Circuit
Protection and voltage supply
Light Source
The sources for transmitter is semiconductor Diode.
Most common semiconductor diode in the transmitter are Light Emitting Diode (LED) and Laser Diode (LD).
Semiconductor Diode
Semiconductor laser is developed by two of semiconductor materials that are p-type and n-type materials in which n-type material contains more electrons and p-type material contains more holes.
The materials producing the energy gap between the two materials that is called band gap.

7. Semiconductor Diode (Cont’d)
Semiconductor material have conduction properties between insulator and metals
The conduction properties creates the energy-band that call band gap.
There two energy bands: valance – lower, meaning less energy and conduction – upper, meaning higher energy.
There are separate by energy gap as shown in figure below
This band gap is the place where the recombination and excitation process occur.
The electron can be either at the valance band or conduction band.
When some external energy – either through temperature or by an external electric – is provided to the electron at the valance band, some electron acquire enough energy to leap over energy gap and occupy energy level at the conduction band
This process call excitation process where electron leaves holes (positive charge carries) at the valance band
Electron-hole recombination process

8. How Semiconductor Diode Works
The p-n junction on semiconductor material create the depletion region.
This region create when electron from n material filled holes at p material.
When the equilibrium state achieved its prevent from net movement of charges.
The junction or the depletion now has no mobile carrier, since electron and holes locked into covalent structure.
Distribution of carrier across a p-n junction without an externally applied biasing.

9. How semiconductor Diode works (Cont’d)
If external battery is connected to the junction in reversed biased the depletion region will increase and the minority carrier flow across the junction.
If the external battery supply connected to the junction in forward biased, then the magnitude of the barrier potential reduced and allow the electron to diffuse.
Reverse biased widen the depletion region
Forward biased lowering the barrier potential

10. 1. Light Emitting Diodes (LED)
LED is a semiconductor diode; the construction of the LED is same as other diode but the other regular diode is loss the recombination energy in the thermal.
LED is used the recombination energy into radiation spectrum of light.
LEDs have relatively large emitting areas and as a result are not as good light sources as LDs.
However, they are widely used for short to moderate transmission distances because they are much more economical, quite linear in terms of light output versus electrical current input and stable in terms of light output versus ambient operating temperature.
LEDs are of interest for fiber optics because of five inherent characteristics:
1. They are small.
2. They possess high radiance (i.e., They emit lots of light in a small area).
3. The emitting area is small, comparable to the dimensions of optical fibers.
4. They have a very long life, offering high reliability.
5. They can be modulated (turned off and on) at high speeds.

11. LED Structures and Configuration
There are two possible structure in LED: Homostructure and heterostructure.
Homostructure configuration have drawback where the active region is too defuse which makes the device’s efficiency very low.
Homostructure makes the device radiates a broad light beam and make coupling light into fiber inefficient.
Most LED is design using heterostructure because its gives good confinement of recombination process.
Two type LED configurations
Edge Emitting LED (ELED)
Surface Emitting LED (SLED)
Typical spectrum of edge emitting and surface emitting LEDs.
As seen from there, edge emitting LED has narrower spectrum.

12. LED Configuration Energy Gaps in LEDs (a) (b)
The peak emission wavelength in an LED is expressed as a function of the band gap energy, Eg in electron volts (ev) as follows:
LED Configuration
(a)
(b)
LED structures a) Edge emitters b) Surface emitter

13. LED Materials
There are many material in construct LED, for example GaAlAs (gallium aluminum arsenide) for short-wavelength devices. Long-wavelength devices generally incorporate InGaAsP (indium gallium arsenide phosphide).
These material gives different energy gap as shown in table 1 below.
Different material also will gives different wavelength for different application.
The material composition is chosen depending on wavelength of operation.
For instance, to operate in 800 to 900 nm, ternary alloy of Ga1-xAlxAs is used. The fraction ratio x of aluminum to gallium arsenide determines the wavelength of peak emitted radiation.
By choosing x = 0.08, we obtain the spectrum in figure below, where the peak wavelength is λ = 810 nm and the spectral width is, σλ = 36 nm.
At longer wavelengths, we have to switch to the quaternary alloy By varying x and y of In1-xGaxAsyP1-y . With this quaternary allow, it is possible optical power outputs at wavelengths between 1 µm and 1.7 µm.

14. LED Materials (Cont’d)
For a ternary alloy, the relationship between the band gap energy, Eg and fraction ratio, x , when 0 ≤ x ≤ 0.37 is given by
Eg = x x2
For a quaternary alloy, when 0 ≤ x ≤ 0.47 is given by, Eg will be given in terms of y as:
Eg = y y2
Table 1: Energy gap and wavelengths
Material
Energy Gap Eg (eV)
Wavelength (nm) Si
1.17
1067 Ge
0.775
1610
GaAs
1.424
876
InP
1.35
924
InGaAs
AlGaAs
InGaAsP
Spectral view of an LED made of ternary alloy, Ga1-xAlxAs with x = 0.08

15. LED Materials (Examples)
Example: Consider a ternary alloy of Ga1-xAlxAs, if x = 0.07 , find the wavelength of operation.
Solution:
Example: For a quaternary alloy of In1-xGaxAsyP1-y In0.74Ga0.26As0.57P0.43 find the wavelength of operation.
Solution:

16. Characteristic of LEDs
There are five major characteristics of LED
1. Peak wavelength Spectral Width 3. Emission pattern
4. Power Speed Linearity
Peak wavelength: The wavelength at which the source emits the most power. It should be matched to the wavelengths that are transmitted with the least attenuation through optical fiber. The most common peak wavelength are 780, 850, and 1310 nm.
Spectral width: Ideally, all the light emitted from an LED would be at the peak wavelength, but in practice the light is emitted in a range of wavelengths centered at the peak wavelength. This range is called the spectral width of the source.
Emission Pattern: The pattern of emitted light affects the amount of light that can be coupled into the optical fiber. The size of the emitting region should be similar to the diameter of the fiber core.
Power: The key requirement is that the output power of the source be strong enough to provide sufficient power to the detector at the receiving end, considering fiber attenuation, coupling losses and other system constraints. In general, LEDs are less powerful than lasers.

17. Characteristic of LEDs (Cont’d)
Speed: A source should turn on and off fast enough to meet the bandwidth limits of the system. The speed is given according to a source’s Rise or fall time, the time required to go from 10% to 90% of peak power. LEDs have slower rise and fall times than lasers.
Linearity is another important characteristic. Linearity represents the degree to which the optical output is directly proportional to the electrical current input. Most light sources give little or no attention to linearity, making them usable only for digital applications. Analog applications require close attention to linearity. Nonlinearity in LEDs causes harmonic distortion in the analog signal that is transmitted over an analog fiber optic link.
The input-output characteristic or the P-I curve of LED is linear compare to the laser diode.
P-I curve for LED

18. LED Driver Circuits
The figure (a) below is the simplest of the three configurations. It uses a transistor, Q1, and a limited amount of resistors to convert an analog input voltage into a proportional current flowing through the LED, D1. Also referred to as a transconductance amplifier, this configuration converts a voltage into a current.
In LEDs, the light output equates proportionally to the drive current, not the drive voltage. LEDs exhibit a peak drive current at about 100 mA, and the voltage drop is typically 1.5 Volts.
Digital circuit and the analog circuit is does not have much different where the only data signal is in digital where the threshold level is fixed as shown in figure (b).
This figure is a simple series driver circuit. The input voltage is applied to the base of transistor Q1 through resistor R1. The transistor will either be off or on. When transistor Q1 is off, no current will flow through the LED, and no light will be emitted. When transistor Q1 is on, the cathode (bottom) of the LED will be pulled low.
(b) Series digital LED circuit
(a) Simple analog LED circuit

19. 2. Laser Diode (LD)
Laser diode (LD) is different to LED, even though the material used in construction laser diode is similar to the LED, but the radiate light is came from the other process, stimulated emission process.
Laser diode light is monochromatic and the spectral width of the light is small.
LD is a semiconductor that emits coherent light when forward biased.
Laser diode also produces coherent light where all oscillations are in phase and provide better detection for receiver of an information signal.
Since the laser diode is monochromatic, the light is easy directed especially directed into the fiber.
Compare to the LED, laser diode is highly intense and power efficient. LED need 150mA of current to achieve power radiate at 1mW but laser diode only need 10mA current to achieve same power level.

20. Laser Diode (Cont’d)
Five inherent properties make lasers attractive for use in fiber optics:
They are small.
They possess high radiance (i.e., They emit lots of light in a small area).
The emitting area is small, comparable to the dimensions of optical fibers.
They have a very long life, offering high reliability.
They can be modulated (turned off and on) at high speeds.

21. Laser Diode Working Principle
Laser from LD is create from the stimulated emission radiation.
We can talk about three processes to describe the act of lasing. These are absorption, spontaneous emission and simulated emission.

22. Laser Diode Working Principle (Cont’d)
Lasing effect and input-output characteristic occur when below process met:
Population inversion.
Stimulated emission.
Positive feedback.
According to Planck’s law, if an electron is in ground state, i.e. E1 in Figure above, can be excited to an upper energy level of E1 by supplying an (photon) external energy of hf12 , where is h Planck’s constant, f12 is the frequency of the external optical energy such that the energy of hf12 is sufficient to raise the electron from the ground state, E1 to the excited state, E2 .

23. Laser Diode Working Principle (Cont’d)
The radiation in a laser diode is generated within a Fabry-Perot resonator cavity. Its mechanical construction is shown in figure below.
As seen from there, laser radiation is emitted in the longitudinal direction from the lasing spot. Since we are mainly interested in what goes on in the resonator cavity, we show in the following figure a simplified side view together with the mirrors at the cavity ends. To describe how this lasing takes place, we express the electric field in the longitudinal direction as follows:
I(z) is the optical intensity, ω is the optical radial frequency and β is the propagation constant.

24. Laser Diode Working Principle (Cont’d)
Population inversion must be achieved so that optical amplification and thus the lasing can start. For this to happen, gains in the cavity must overcome losses. After traversing a distance of z in the cavity, the optical intensity will become
Where Гg is called the confinement factor, g represents the gain, ά is the absorption coefficient. As seen from figure above, one round trip covers a length of z = 2L and involves reflections from mirrors with reflectivity coefficients of R1 and R2 which are given by
Where n1 and n2 are the refractive indices of the first and second mirrors, rn is the refractive index of the resonator cavity medium. Hence after being reflected from the mirrors, I(z) will become

25. Laser Diode Working Principle (Cont’d)
For oscillations to occur, the magnitude and the phase of the returned wave must be equal at z = 0 and z = 2L , thus
Example
Solution

26. Laser Diode Characteristic
Laser diode light that can characterized as below
Monochromatic: The spectral width of the radiated light is very narrow. The line width of a laser diode can be in tenth or hundred of nanometer
Well directed: A laser diode radiates narrow , well directed beam that can be easily launched into optical fiber.
Input-output characteristic of LD

27. Laser Diode Characteristic (Cont’d)
Highly intense and power efficient: A laser diode can radiate hundreds of milliwatts of output power. LD making the current to light conversion 10 times more efficient than it is in the best LEDs.
Coherent: Light radiates by a laser diode is coherent; where all oscillation are in phase.
Threshold current of laser diode depend on the temperature.
The threshold current will increase when the temperature increase as shown in the following figure.
There are two types of laser diode design that provide solution for the temperature: Cooled laser diode and uncooled laser diode.
Uncooled laser diode means, laser diode does not require any cooling.
Cooled laser diode means, laser diode needs a heat pump to transfer heat from the one place to another.

28. Laser Diode Characteristic (Cont’d)
Cooled laser diode always include a thermoelectric cooler (TEC) which function to keep laser diode at operating temperature.
Emission Pattern: The pattern of emitted light affects the amount of light that can be coupled into the optical fiber. The size of the emitting region should be similar to the diameter of the fiber core.
The threshold current at different temperature.

29. Laser Diode Driver Circuit
Laser Diode Material
The material inside laser diode will present the wavelength of laser diode.
Different material will present different wavelength, and also provide different energy gap. Material in laser diode will provide how long the transmitter can support and the wavelength mode and type.
Laser Diode Driver Circuit
The design of laser diode driver circuit should be suitable to the characteristic of laser diode.
The laser diode has certain threshold level when its start in linear region.
The laser diode driver should make the laser operate in linear region where the output power will be linear to the input current.

30. Laser Diode Driver Circuit (Cont’d)
The modulation of the laser signal is the done by the driving circuit.
The driving circuit will modulate the electrical signal into optical source.
The driving circuit will make the laser flash according to the data modulation.
The rise time and fall time of the signal is depend on the driving circuit and the source itself.
If the signal is in the form of analog the analog driving circuit is to modulate used and if the signal is in digital the digital driving circuit is used to modulate.