Optical modulation can be done directly by modulating the driving current of the light source, or externally by modulating light in the fiber. Direct modulation has limitations including turn-on delay and nonlinear distortions that limit modulation speed. External modulation using a Mach-Zehnder interferometer separates modulation and light generation, allowing wider bandwidth up to 60 GHz without limitations from the laser's properties.

1. Optical Modulation
of light sources

2. Modulation of light sources
1. Optical sources can be modulated either
directly or externally.
2. Direct modulation is done by modulating the
driving current according to the message signal
(digital or analog)
3. In external modulation, the laser emits
continuous wave (CW) light and the
modulation is done in the fiber

3. Types of Optical Modulation
1. Direct modulation is done by superimposing the
modulating (message) signal on the driving current
2. Indirect (external modulation)

4. Direct Modulation
The message signal (ac) is superimposed on the
bias current (dc) which modulates the laser

5. Direct Analog Modulation

6. Analog LED Modulation

7. Limitations of direct
modulation
1. Turn on delay and resonance frequency are the two
major factors that limit the speed of digital laser
modulation
2. Saturation and clipping introduces nonlinear
distortion with analog modulation
3. Nonlinear distortions introduce second and third
order intermodulation products

8. Laser Digital Modulation (Clipping
Problem)

9. Analog Laser Modulation

10. Laser Digital Modulation (delay on
problem)
When the driving current suddenly jumps from low
(I1 < I th) to high (I1 > I th) , (step input), there is a finite
time before the laser will turn on.
This delay limits bit rate in digital systems

11. External optical modulation
1. Modulation and light generation are separated
2. Offers much wider bandwidth up to 60 GHz
3. The modulation frequency is not limited by the
laser’s relaxation oscillation frequency
4. More expensive and complex

12. External optical modulation
Mach-Zehnder Intensity Modulator
1. Entering light is split and
propagates through both arms
of a Mach-Zehnder
interferometer
2. A voltage applied at the
contacts changes the refractive
index of one or both arms,
producing a phase difference
when the beams are or both
arms, producing a phase
recombined
3. If the phase difference is π,
then no light is output

13. External optical modulation
Mach-Zehnder Intensity Modulator