# II. Introduction to Lasers and Light

(See Notes on Lasers and Light by D. L. Jaggard)

## A. Laser Light vs. Natural Light

### 1. Directionality

Laser:

Beamwidth ~ l/D ~ 10–3 radians Thermal:

Beamwidth ~ p radians ### 2. Focusing

Laser:

Minimum spot size ~ l Thermal:

Minimum spot size >> l Wavelength: l = c/f (in free space) l = wavelength (m) c = speed of light = 3 x 108 (m/s) f = frequency (Hz. or cps) = w/2p

### 3. Intensity

Laser is brighter than the sun!
Sun brightness ˜ 1.5 x 105 lumens/cm2-str.
Laser brightness ˜ 108 lumens/cm2-str.
On a per frequency basis, laser is ~ 106 more intense

(Safety)

 Units: Intensity 1 watt = 680 lumens Angles Planar Angle q 2p radians = all of a circle = 360˘X Solid Angle W 4p steradians = all space Length 1Å = 10–10 m 1 micron = 10–6 m light travels 1 foot in one nanosecond (= 10–9 sec)

### 4. Monochromaticity

Laser:

Energy spread over l of < 1Å = 10–10 m

Sun:

Energy spread over l from 4,000 Å - 7,000 Å

 Differential Wavelength/Differential Frequency: Dl = change in wavelength c = speed of light = 3 x 108 (m/s) f = frequency Df = change in frequency

### 5. Coherence

 Coherence: A measure of the ability of a wave to interfere with a delayed or displaced version of itself.

Laser:

Light can interfere with itself over significant distances (speckle)

Sun:

Light cannot interfere with itself easily

Coherence length lc coherence time t:

## B. Overview of Laser Operation

### 1. Laser Architecture

Three components needed:

Source of power (pump)
Amplifier (active medium)
Feedback (cavity or reflector) 