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 10
8 (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 = 360X

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
D
f = 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)

 

2. Typical He-Ne Laser

 


C. Types of Lasers

1. Gas

2. Solid

3. Liquid

 


D. Laser Beam Output

1. Divergence-diffraction effect/beam waist

2. Cavities

Modes

Typical output