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Questions and Answers
What type of device changes the phase of optical signals by applying voltage?
What type of device changes the phase of optical signals by applying voltage?
Phase modulator
What are the two main device types mentioned for modulators?
What are the two main device types mentioned for modulators?
Optical amplitude modulators are typically made of ________.
Optical amplitude modulators are typically made of ________.
Lithium Niobate (LiNbO3)
Phase modulators are used to compensate for degradation in long-distance optical transmission.
Phase modulators are used to compensate for degradation in long-distance optical transmission.
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What is the driving voltage needed to induce a phase change of π in an amplitude modulator?
What is the driving voltage needed to induce a phase change of π in an amplitude modulator?
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What is the phenomena where the transmission curve gradually shifts in the long term?
What is the phenomena where the transmission curve gradually shifts in the long term?
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What type of modulator uses the acousto-optic effect to diffract and shift the frequency of light using sound waves?
What type of modulator uses the acousto-optic effect to diffract and shift the frequency of light using sound waves?
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For binary data, the phase changes between 0 and π, and the output intensity changes from 2𝐸0 to 0.
For binary data, the phase changes between 0 and π, and the output intensity changes from 2𝐸0 to 0.
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The intensity of light is proportional to 𝐸, which is proportional to ______________.
The intensity of light is proportional to 𝐸, which is proportional to ______________.
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What is modulation in the context of lightwaves?
What is modulation in the context of lightwaves?
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What is the difference between direct modulation and external modulation?
What is the difference between direct modulation and external modulation?
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What is the principle of optical interference?
What is the principle of optical interference?
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Analog modulation of laser diodes requires a linear relation between light output and current input.
Analog modulation of laser diodes requires a linear relation between light output and current input.
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Why is external modulation preferred over direct modulation for higher data rates?
Why is external modulation preferred over direct modulation for higher data rates?
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Study Notes
Optoelectronics Course Overview
- Course schedule: 16 weeks, covering topics including introduction to optoelectronics, wave nature of light, dielectric waveguides, stimulated emission devices, optical modulation, and more.
- Lecturers: MHHM, MSDZ, and AAAB.
- Mid-semester break and final exam dates.
Optoelectronics Course Outcomes
- HPK1: Ability to design active components and systems based on optoelectronic technology.
- HPK2: Ability to assemble and measure optical devices and systems using modeling software and optical tools.
- HPK3: Ability to solve problems associated with fundamental optoelectronic technology.
- HPK4: Ability to propose solutions and solve problems related to optoelectronic applications.
Modulation of Laser Diodes
- Modulation: Process of putting information onto a lightwave.
- Direct modulation: Varying laser drive current with electrically formatted information stream to produce a varying optical output power.
- External modulation: Using a device to temporally modify a steady optical power level emitted by the laser.
- Categories of modulation: Direct and external.
Integrated Optical Devices
- Also known as Photonic Integrated Circuits (PIC).
- Devices used in high-speed coherent optical communication.
- Examples: Modulators (phase, amplitude, and polarization), filters.
- Mainly used material: Lithium Niobate (LiNbO3).
Optical Interference
- Definition: Interaction of two or more lightwaves yielding a resultant irradiance that deviates from the sum of the component irradiance.
- Principle of Superposition: Electric field intensity at a point in space, arising from the separate fields of various contributing sources.
Optical Modulation
- Direct modulation: Modulating the laser drive current with the data signal.
- External modulation: Using a modulator to impose information on the optical carrier.
- Advantages of external modulation: Higher data rates, fewer chirp problems.
Electro-Optic Effect
- Definition: Change in dielectric constant of material due to electric field (Pockel effect).
- Causing a change in the refractive index of the material.
- Most widely used material: Lithium Niobate.
- Examples of devices: Phase modulator, intensity modulator, Mach-Zehnder interferometer.
Practical Limitations of Devices
- Optical power handling capability: Limited by photorefractive damage.
- Piezoelectric effect: Electrical signal produces phase modulation and vibrations, altering the indices of refraction.
- Residual amplitude modulation: Unwanted amplitude modulation caused by back-reflections.
External Modulation
- For higher-rate applications, an external modulator is used.
- Produces a time-varying optical signal.
- Two main device types: Electro-optical phase modulator and electro-optical amplitude modulator.
Limitations of Direct Modulation
- Modulation frequency cannot be larger than the frequency of the relaxation oscillations of the laser field.
- Relaxation oscillation frequency depends on both the spontaneous lifetime and the photon lifetime.
Analog Modulation
- Carried out by making the drive current above threshold proportional to the baseband information signal.
- Requires a linear relation between the light output and the current input.
Signal Degradation
- Nonlinearities in the transient response characteristics of laser diodes make analog intensity modulation susceptible to intermodulation and cross-modulation effects.
- Special compensation techniques can alleviate these nonlinear effects.
Optical Phase Modulator
- Changes the phase of optical signals by applying voltage.
- Phase change between the two states is π.
- Required voltage (Vπ) to obtain phase change of π: Vπ = λd / (n r LΓ)
Optical Amplitude Modulator
- Employs INTERFEROMETER principle.
- Important parameters: Switching voltage-current, extinction ratio, frequency response, optical frequency chirp, distortion, bias voltage stability, and more.### Optical Amplitude Modulator
- Two commonly used optical amplitude modulators are:
- Mach-Zehnder interferometer
- Directional coupler
Mach-Zehnder Modulator
- Basic structure consists of:
- Two waveguides
- Two Y-junctions
- RF/DC electrode
- Operation:
- When voltage is not applied to the RF electrode, the two signals are in phase and the output signal is recognized as "ONE".
- When voltage is applied, the phase of the optical signal in one arm is advanced or retarded, and the output signal is recognized as "ZERO".
- Transfer function: I(t) = aIqcos²(V(t)π/2Vπ), where I(t) = transmitted intensity, a = insertion loss, Iq = input intensity, V(t) = applied voltage, and Vπ = driving voltage.
- Importantly, the driving voltage induces the "ZERO" and "ONE" states.
- It is necessary to set the static bias on the transmission curve through the Bias electrode, typically at the 50% transmission point (Quadrature Bias point).
Y-Junction
- Splits and combines power equally at both input and output
- Introduces a phase shift of Δφ in the optical signals in the two branches
- The intensity of light is proportional to cos²(Δφ/2)
Acousto-Optic Modulator (AOM)
- Uses the acousto-optic effect to diffract and shift the frequency of light using sound waves
- Consists of an AO crystal with a piezoelectric transducer attached to one of the crystal faces
- The ultrasonic transducer generates acoustic waves, which propagate along the crystal and lead to a periodic variation in the refractive index in synchronization with the acoustic wave amplitude.
Modes of AOM Operation
- Bragg regime: The width L of the diffraction grating is sufficiently large, and we need to consider the interference of waves flowing from points along the width L.
- Raman-Nath regime: The width L of the diffraction grating is small, and the interaction of the incident light and the grating occurs almost along a line in the x-direction.
Bragg Regime
- Condition: L > Λ, where Λ is the acoustic wavelength given by Va/f, where Va is the acoustic velocity and f is the acoustic frequency.
- The diffracted beam is modulated by the voltage applied to the transducer.
Frequency Shift
- Due to the Doppler effect, there is a shift in frequency: w' = w ± W, where w is the incident light frequency, W is the acoustic frequency, and w' is the diffracted light frequency.
Example of AO Modulator
- Generating 150 MHz acoustic waves on a TeO2 crystal, with a length (L) of 10 mm and a height (H) of 5 mm, modulating a red-laser beam from a He-Ne laser, λ = 632.8 nm.
- Acoustic wavelength Λ = 2.8 × 10^-5 m, which satisfies the Bragg regime condition.
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This quiz is about the profile of Prof. Ir. Dr. Ahmad Ashrif A Bakar, an academic in the Electrical and Electronic Engineering Programme at Universiti Kebangsaan Malaysia. It covers his background, affiliations, and contact information.
