Michael Chau
Scott MacFarlane
3.11.03
PH 464
Light Amplification by Stimulated Emission of Radiation
Rudimentary Laser Concept
The concept of a laser involves the amplification of light into an intense ray
consisting of photons. These particles have the same direction, frequency, and
polarization as well as identical or a phased with a fixed differential with respect to other
photons. Because of these attributes, the beam is monochromatic, a very pure light
source characterized by a single color. Laser light is substantially different from ordinary
light from, say, an incandescent light source in that the latter emits white light that
disperses and is of considerably less intensity than the former. The photons from a light
bulb do not share a fixed phase relationship, identical direction and frequency as do those
from a laser emitter. The terminology used to identify a fixed phase difference is
coherence. Conversely, incoherence, as in the incoherent light from a fluorescent light
bulb, the photons are not in phase and have variant phase differences.
(Source: http://users.aber.ac.uk/tej/concpts5.doc)
Michael Chau
Scott MacFarlane
3.11.03
PH 464
Processes of Lasers
What is stimulated absorption?
Stimulated absorption is the number of atoms having distinct energy levels, which the
energy levels are equal to the amount of energy a photon is carrying.
Figure 1: Another illustration of stimulated absorption
 dN i 
   Bij N i u v is the rate of change of the
 dt  ab
Stimulated absorption equation: 
number of atoms in some initial state
Ni  Noe
-E i
k BT
, where Ei is the excited energy state
kB is the Boltzmann constant equal to 1.38 x 10-23 J / K , where K is the temperature in
Kelvin
T is the temperature in Kelvin
No is a constant for a given temperature
Ni is a number of atoms per unit volume
uv is the spectral energy density (J * s / m2)
Bij is a constant of proportionality
Minus sign mean that Ni is decreasing
What is Stimulated emission?
Michael Chau
Scott MacFarlane
3.11.03
PH 464
Stimulated emission is “the atom can then dump its excess energy in-step with the
incoming photon”, which will emit its energy (Hecht, p.586).
Figure 2: Stimulated emission
 dN j
Stimulated emission equation: 
 dt

   B ji N j u v
 st
-E j
N j  Noe
k BT
, where Ej is the excited energy state
Bji is a constant of proportionality
uv is the spectral energy density (J * s / m2)
Nj is the number of atoms per unit volume
Minus sign mean that Nj is decreasing
What is spontaneous emission?
It is the where an atom has high energy level, which shorten its life for 10ns or so,
without any external interference,”… the atom will emit its overload of energy as a
photon.” (Hecht, p586).
Figure 3: Spontaneous emission
Michael Chau
Scott MacFarlane
3.11.03
PH 464
 dN j 
   A ji N j
Spontaneous emission equation: 
 dt  sp
A ji   sp , where p is the probability per second of spontaneous emission
N sp  NA ji 
A ji 
N
 , where Aji is Einstein’s coefficient
1

-E j
N j  Noe
k BT
, where Ej is the excited energy state
Nj is the number of atoms per unit volume
Minus sign mean that Nj is decreasing
Experimental Results
Figure 4: Block diagram of audio and video for modulated laser apparatus
Michael Chau
Scott MacFarlane
3.11.03
PH 464
Potentiometer Resistance
Voltage Across Laser
0.961 kΩ
1.827 V
0.695 kΩ
1.904 V
490.4 Ω
1.971 V
287.5 Ω
2.081 V
92.9 Ω
3.05 V
96.1 Ω
2.992 V
Table 1: Voltage across laser with respect to varying potentiometer (2 kΩ ) resistance
Using Fig. 4 and Table 1 as reference, it is clear that the voltage of the laser
increases (with the exception of the last entry in the table) with increasing resistance of
the 2 kΩ potentiometer. It can be inferred from the data that the current increases
through laser as a result of the variant resistance of the potentiometer because of Ohm’s
Law (V = I x R).
We also found that simply turning on the modulator as it was hooked in parallel
with the laser emitter decreased the voltage from 3 V to 2.46 V across the laser.
Consequently, it appeared necessary to increase the voltage appreciably to compensate
for the voltage reduction.
Initially, we intended to demonstrate both video and audio using the modulated
laser but the former became increasingly difficult to stabilize an image on the CCD using
the same circuit that was used for the audio in Fig. 4. The most workable configuration is
shown in the lower half of the aforementioned figure but it does not involve the
potentiometer but rather, a battery-powered laser as opposed to the 6 V brick-shaped
battery that was used in the circuit for the audio. It was an ad hoc decision to exhibit the
video part of our experiment so there were no additional measurements or waveforms.
Essentially, the modulated laser used for the video signal is being powered with two
AAA batteries at 1.5 V each. The results for the fully functional audio setting were 76.8
Ω for the potentiometer and 2.274 V for the laser.
Michael Chau
Scott MacFarlane
3.11.03
PH 464
Figure 5: Output through Metrologic modulator without audio
Figure 6: Waveforms of pure audio from MP3 player
Michael Chau
Scott MacFarlane
3.11.03
PH 464
Figure 7: Output through Metrologic modulator with audio from MP3 player
Figure 8: Closer view of previous waveform
Michael Chau
Scott MacFarlane
3.11.03
PH 464
Work Cited
Hecht, Eugene. OPTICS. San Francisco: Addison Wesley, 2002.
http://www.tfh-wildau.de/oloeffl/download/laserppt.pdf
http://users.aber.ac.uk/tej/concpts5.doc