Unit I: Fundamentals of Photonics

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Questions and Answers

What is the condition for a wave to be considered temporally coherent?

The phase difference φ2 should change continuously.

The phase difference φ2 should not exist.

The wave must have multiple wavelengths.

The phase difference φ2 should remain constant over time. (correct)

What does a finite bandwidth (∆ν) indicate about a light source?

Light waves will remain in phase indefinitely.

The light source produces a single wavelength.

Different frequencies can fall out of phase. (correct)

Coherence length will be infinite.

What is the relationship between coherence time (∆τ) and bandwidth (∆ν)?

Coherence time is directly proportional to bandwidth.

There is no relation between coherence time and bandwidth.

Coherence time increases with bandwidth.

Coherence time is inversely proportional to bandwidth. (correct)

What is the coherence length (∆L) of a laser beam with a bandwidth of 1 MHz?

300 m Signup and view all the answers

Which type of light source exhibits the highest degree of spatial coherence?

Laser light Signup and view all the answers

What happens to the phase difference at fixed points in a plane with spatial coherence?

It remains constant over time. Signup and view all the answers

If a light source has a bandwidth of 10^14 Hz, what is its coherence time (∆τ)?

10^(-14) s Signup and view all the answers

What does a laser beam's spatial coherence depend on?

The uniformity of the wave in perpendicular planes. Signup and view all the answers

What occurs during the absorption process in an atom?

The atom becomes excited to a higher energy state by absorbing a photon. Signup and view all the answers

Which statement accurately describes spontaneous emission?

It occurs when an atom returns to ground state and emits radiation. Signup and view all the answers

What is a key characteristic of photons emitted through stimulated emission?

They are coherent, with the same frequency and phase as the incident photon. Signup and view all the answers

What does the rate of spontaneous emission depend on?

The number of atoms in the excited state. Signup and view all the answers

Who predicted the mechanism of stimulated emission?

Albert Einstein Signup and view all the answers

How is the transition during stimulated emission represented mathematically?

A* + hν = A + 2(hν) Signup and view all the answers

Which of the following describes the nature of light produced by stimulated emission?

It is coherent, monochromatic, and directional. Signup and view all the answers

What determines the energy conservation during the absorption process?

The difference between the excited and ground state energy levels. Signup and view all the answers

What is the primary purpose of low power semiconductor lasers in devices like CD players and bar code scanners?

To emit light for data reading Signup and view all the answers

How does optical energy travel through fiber optics?

By bouncing off the walls of the fiber at certain angles Signup and view all the answers

Which of the following describes a hologram in comparison to a photograph?

Holograms record both intensity and phase distribution. Signup and view all the answers

What is the significance of the reference wave in hologram recording?

It helps in the formation of a clearer three-dimensional image. Signup and view all the answers

Which application of lasers is associated with guiding missiles in warfare?

Very high power lasers Signup and view all the answers

Why is fiber optics preferred over traditional radio waves in telecommunications?

Fiber optics can carry significantly more information. Signup and view all the answers

What happens to a hologram when it is cut into multiple pieces?

Each piece still retains a view of the entire image. Signup and view all the answers

What are the typical diameters of fibers in a bundle used in fiber optics?

2 * 10^{-4} cm to 10^{-3} cm Signup and view all the answers

What is the primary reason that a LASER beam exhibits a high degree of coherence?

Photons are emitted in phase with incoming photons. Signup and view all the answers

What does the concept of directionality in a LASER imply?

A LASER emits light primarily in a focused manner. Signup and view all the answers

What role does the active medium play in a LASER?

It amplifies light through stimulated emission. Signup and view all the answers

Which of the following best describes the brightness of LASER light?

It is concentrated in a narrow beam with high intensity. Signup and view all the answers

What is an Active Center in the context of a LASER?

The specific atoms or ions responsible for light emission. Signup and view all the answers

Why is coherence not ideal in practice for LASER light?

Not all photons start in phase with one another. Signup and view all the answers

What distinguishes a CO2 LASER from other types of lasers?

It relies on transitions between vibrational and rotational levels. Signup and view all the answers

What happens to light not traveling parallel to the axis of a LASER?

It is reflected back and forth in the cavity. Signup and view all the answers

What is the primary purpose of pumping in laser technology?

To raise atoms to a higher energy level Signup and view all the answers

Which method of pumping involves an external light source?

Optical pumping Signup and view all the answers

In optical pumping, what happens after atoms are raised to a higher energy level E2?

Some transition to the metastable state E1 Signup and view all the answers

What is the key advantage of the long lifetime of atoms in the metastable state?

Reduced probability of spontaneous emission Signup and view all the answers

What initiates electrical pumping in gas lasers?

High voltage pulse ionization Signup and view all the answers

What is a characteristic of a resonant cavity in laser systems?

It facilitates multiple reflections of light Signup and view all the answers

In a laser resonant cavity, how does the laser beam emerge?

It escapes through a partially silvered mirror Signup and view all the answers

What is the result of photons travelling along the axis in the resonant cavity?

They enhance beam unidirectionality Signup and view all the answers

Study Notes

Fundamentals of Lasers

Photonics involves the study of laser technology focusing on atomic transitions.

Atomic State Transitions

Absorption:
- An atom transitions from energy state E1 to E2 when it absorbs a photon of energy hν (E2 – E1 = hν).
Spontaneous Emission:
- Excited atoms return to ground state E1 with radiation release, characterized by their lifetime in the excited state.
Stimulated Emission:
- An external photon with energy matching the gap between energy levels can induce an excited atom to emit a second photon, resulting in coherent light.

Lasing Process

Pumping:
- Required to maintain population inversion in laser material to continue stimulated emissions.
- Optical Pumping: Uses external light (like flash lamps) to excite atoms to high energy levels, leading to population inversion, commonly used in solid-state lasers.
- Electrical Pumping: In gases, high voltage ionizes the gas, enabling electrical excitation to achieve population inversion.

Resonant Cavity

Mirrors form a resonant cavity allowing light to bounce and amplify, ensuring high directionality and coherence.
Light emitted through a partially silvered mirror provides high unidirectionality and narrow beam width.

Coherence

Temporal Coherence: Light retains phase relationship over time; ideal for lasers with narrow bandwidths (e.g., 1 MHz results in 1μs coherence time).
Spatial Coherence: Uniformity of wave phase across a perpendicular plane; ideal point sources exhibit high spatial coherence.

Directionality and Brightness

Lasers emit light with extremely low divergence, allowing for concentrated energy in a narrow beam, making them suitable for applications like cutting and drilling.

Active Medium and Active Center

Active Medium: Material amplifying light through stimulated emission upon energy input.
Active Center: Specific atoms or ions within active medium responsible for photon emission.

Types of Lasers

CO2 Laser: Utilizes molecular transitions; notable for simplicity and continuous output.
Applications:
- Low power semiconductor lasers in devices (CD players, printers).
- High power lasers in thermonuclear fusion and military applications for missile guidance.

Applications of Lasers in Communication

Optical fibers use total internal reflection to transmit light over long distances, facilitating high-frequency data transfer, surpassing radio and microwave capabilities.

Holography

Technique producing 3D images via interference patterns recorded using laser beams, differing from traditional photography by capturing both intensity and phase information, enabling true 3D representation.
Holograms retain full image quality even if cut, unlike conventional photographs.

Hologram Recording

Achieved through superposition of object and reference waves, leading to complex interference patterns on photographic plates, capturing detailed information about the object's surface.

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Description

This quiz focuses on the fundamental concepts of photonics related to lasers, specifically examining atomic transitions and photon absorption. It will explore how photons interact with atoms and the conditions required for these processes. Test your understanding of energy states and laser mechanics.

Unit I: Fundamentals of Photonics - Laser

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