An optical modulator is a device that modulates or varies the amplitude, phase, or polarization of an optical signal in a controlled manner. It generates desired intensity and color in light by changing optical parameters like transmission, refractive index, or reflection according to a modulating signal. Optical modulators are used when directly modulating a laser would cause issues like frequency chirp or loss of information. There are different types of optical modulators including acousto-optic, phase, polarization, and amplitude modulators that work using various effects like Pockels or acousto-optic. Modulation can be done directly on the laser by changing its current or externally using a separate modulator.

1. Opto-modulator
Ali Nargesi Khojasteh

2. 1- What is Optical Modulator?
2- Why do we use optical modulator?
3- Types of optical modulator
4- Modulation Techniques
5- Direct Modulation
6- External Optical Modulation
Outlines
- Phase
- Polarization
- Amplitude
Opto-modulator

3. What Is Optical Modulator?
A device that modulates or varies the amplitude, phase or polarization of an optical
signal in a controlled manner.
Generates desired intensity, color in the passing light by changing optical
parameters such as the transmission factor, refractive index, reflection factor,
degree of deflection and coherency of light in the optical system according to the
modulating signal.
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4. Why Do we use an Optical Modulator?
Directly modulating the laser causes frequency chirp, pulse spreading in optical
fibers, and loss of information;
We may also use an optical modulator when we cannot easily or rapidly vary the
output of a laser;
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5. Types of Optical Modulator
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6. Acousto-Optic
Opto-modulator
An acousto-optic modulator (AOM), also called a Bragg cell, uses
the acousto-optic effect to diffract and shift the frequency of light
using sound waves (usually at radio-frequency)

7. Types of Electro-optic Modulators
• Phase Modulators
The simplest type of electro-optic modulator is a phase
modulator containing only a Pockels cell, where an electric field
changes the phase delay of a laser beam sent through the crystal.
The polarization of the input beam often has to be aligned with
one of the optical axes of the crystal, so that the polarization
state is not changed.
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8. Opto-modulator
The Pockels effect (after Friedrich Carl Alwin Pockels who studied
the effect in 1893), or Pockels electro-optic effect, changes or
produces birefringence in an optical medium induced by an
electric field

9. Types of Electro-optic Modulators
• Polarization Modulators
Depending on the type and orientation of the nonlinear
crystal, and on the direction of the applied electric field, the
phase delay can depend on the polarization direction.
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10. Types of Electro-optic Modulators
• Amplitude Modulators
A beam splitter divides the laser light into two paths, one of
which has a phase modulator as described above.
The beams are then recombined. Changing the electric field on
the phase modulating path will then determine whether the two
beams interfere constructively or destructively at the output,
and thereby control the amplitude or intensity of the exiting
light.
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11. Modulation Techniques
Direct modulation of laser diode:
•Vary the current supply to the laser diode
•Directly modulates the output power of the laser
•Output frequency drifts
•Carrier induced (chirp)
•Temperature variation due to carrier modulation
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12. Direct Modulation
• The message signal (ac) is superimposed on the bias current (dc) which
modulates the laser;
• Robust and simple, hence widely used;
• Disadvantages: laser resonance frequency chirp, turn on delay, clipping and laser
nonlinearity;
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13. External Optical Modulation
• Modulation and light generation are separated;
• Offers much wider bandwidth: up to 60 GHz;
• More expensive and complex;
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14. References
• Robert G. Hunsperger, “Integrated Optics”, Springer, 2009.
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15. Thanks