Diode Laser Lab

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Diode Laser Lab
Anthony Mannino
Lab Partners: Adam Keegan, Chris Dahdouh
Abstract:
The purpose of the following four experiments were to observe the properties of the 808 nm
diode laser using such equipment as a function generator, an oscilloscope, and the Nd:YAG
material, and taking note of how certain changes affect the laser. First, to familiarize ourselves
with the oscilloscope, we made it a task to find the time constraint of an RC circuit with the
oscilloscope. Then, we commenced our experiments concerning the diode laser itself. We first
analyzed the relationship between the power of the laser and its diving current in order to find
the threshold current of the laser. Then we observed how the output power was altered by
changes in the temperature at which the laser operated. Our final experiment details our findings
when measuring the fluorescence lifetime of the Nd:YAG material. These experiments and their
results are explained in detail in the report that follows.
Objective:
Observe the properties of a diode laser and the YAG crystal with the use of lab equipment and
familiarize ourselves with said equipment in the process.
Equipment:
-
LEOI-50A Diode-Pumped YAG Laser an Frequency-Doubling – Enhanced Model with
current driver and temperature controller
Tektronix Oscilloscope
RC circuit
Introduction and Theory
Common light emission is caused by two processes: stimulated absorption and spontaneous
emission. The medium of the common light source is affected by an external source of energy in
such a way that the energy is absorbed (stimulated absorption). This, in turn, “excites” the
electrons so much so that they move to an upper energy state. Since the lifetime of electrons in
an upper energy state is approximately a nanosecond, they are spontaneously released to a lower
energy state. This essentially immediate change in energy states results in the emission of a
photon with an energy of
ℎ = 2 − 1
where h is Planck’s constant,  is the frequency of the light emitted in Hertz, and 2 and 1 are
the respective energies of the upper and lower states. This phemonenon is know as spontaneous
emission.
In contrast to common light emission, LASER (light amplification by stimulated emission of
radiation) is caused by a process known as stimulated emission. When an atom in energy state 2
is stimulated by a photon possessing an energy of ℎ = 2 − 1 , the atom is released to the
lower state 1 . This results in the emission of a photon with energy, phase, polarization, and
directivity identical to that of the stimulated photon. The emission of the two identical photons
results in the amplification of light. The processes is depicted in the diagram below:
This stimulated photon, however, will often cause stimulated absorption rather than emission
under standard conditions and thermal equilibrium unless the population of atoms in the upper
state is greater than that of the lower energy state. This is known as population inversion and is
necessary for the generation of laser light.
The gain medium used for this experiment was the Neodymium-doped Yttrium Aluminum
Garnet (Nd:YAG) material. Using thisgain medium, population inversion is achieved so that
laser light can be generated. The Nd3+ ions are pumped by the 808-nm laser from the ground
state (4I9/2) to the excited state (4F5/2). These ions are then transferred to the upper energy level
(4F3/2). The subsequent stimulated emissions from the upper level to the lower level (4I11/2) give
off photons at a wavelength of approximately 1064 nm. The following diagram thoroughly
depicts this process:
A crucial component for any laser is the resonator. Lasers consist of three primary components: a
pumping source, a gain medium, and a resonator. The resonator is a specific arrangement of
mirrors configured around the gain medium in such a way that feedback is created. The pumped
laser enters the resonating cavity at a certain wavelength, 808 nm in our case, and mode.
Standing waves are then produced inside the cavity at a certain mode and wavelength (1064 nm)
depending on the configuration and types of mirrors used. This resultsin the emission of laser
light. It is important to match the modes of the pumped laster and 1064-nm light for an efficient
pumping outcome. Below is a diagram further demonstrating how the resonating cavity is set up.
Part One: Finding the RC Time Constant (Τ) of a Circuit Using an Oscilloscope
Procedure:
In order to determine the time constant of an RC circuit, we ran an RC circuit through an
oscilloscope. The oscilloscope generated a waveform depicting how the voltage varied as a

function of time. Using the relationship V(t) = V0  − and the observed maximum/initial
voltage, we calculated the time constant.
Observations:
After observing that the initial voltage (Vmax) of the circuit was approximately 19.40 Volts
(±0.01 Volts), we then divided Vmax by e, which gave us VΤ =
Vmax

= 7.14 Volts (±0.01 Volts).
As was previously defined, Τ is the RC time constant of the circuit and VΤ is the total electric
potential when time (t) is equivalent to the RC time constant. Thus, after calculating VΤ , we were
able to measure the time, Τ, from the point at which the electric potential of the circuit was at its
maximum (t = 0.00s) to the point at which the electric potential was VΤ (t = Τ). Using the
oscilloscope, we observed that Τ = 124.0 μs. Thus, every value of the circuit’s electric potential,
V, dissipates by a factor of
1

every 124.0 μs.
Since we know the resitance and capacitance of the circuit with which we were working, we can
calculate the theoretical value of Τ and the percent error. With a resistance of 1000 Ω and a
capacitance of 0.14 μF, the theoretical value of the time-constant is 140 μs. Thus, there was a
percent error of 11.4%. This is most likely due to a slight inaccuracy using the oscilloscope on
our part or due to the inherent error of the labeled capacitance on a capacitor, which usually is
±20% in terms of error. This would account for the small error we calculated. Having more
advanced equipment might allow us to receive a value closer to the theoretical constant.
Part Two: Electro-Optic Characteristics of a Diode Laser
Procedure:
In this experiment, our objective was to find the current threshold of the 808 nm diode laser. To
do so, we observed and graphed the relationship between the driving current and the observed
power of the diode laser for 24 different measurements of current.
Data and Observations:
Power (mW)
0.50
0.40
0.60
23.00
60.00
86.00
116.00
153.50
190.00
234.00
327.00
441.00
480.00
520.00
560.00
590.00
618.00
653.00
684.00
720.00
733.00
750.00
754.00
767.00
Power-Current Curve
900
800
700
Power (mW)
Current (A)
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
1.10
1.20
1.30
1.40
1.50
1.60
1.70
1.80
1.90
2.00
2.10
2.20
2.30
2.40
600
500
400
300
200
100
0
0
0,5
1
1,5
2
2,5
3
Current (A)
The recorded data show that the output power of a laser is
defined as a function of the operating current. This
information is most useful in calculating an approximate
threshold current for the diode laser. At this threshold
current, the diode will begin to steadily emit laser light. This
is due to a steep increase in power. From our data, it is a safe
assumption to say that the threshold current is 0.40 A (±0.01
A). As seen in the data table to the left, the point at which the
power begins to increase is when the diode-operating current is 0.40 A.
The accuracy of our observed relationship between power and current, however, seems to be
questionable. The power-current curve we obtained is seemingly slightly different from other P-I
curves for other diode lasers in that the maximum output power was far smaller than that
reported even in the manual (2000 mW). This could be due to a number of reasons, the most
likely of which is equipment malfunction. The data was taken again, yielding the following
results.
Power (mW)
0
0
0
25
59
76
102
133
165
199
281
305
419
454
490
517
546
573
590
604
630
643
679
412
411
Power-Current Curve
800
700
600
Power (mW)
Current (A)
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2
2.3
2.4
2.5
500
400
300
200
100
0
0
0,5
1
1,5
2
2,5
3
Current (A)
The similarity between this data set and the former is
indicative of our general accuracy in taking the data (any
reltive errors may be due to slight changes in set up). This
may further be indicative of malfunctioning equipment. This
is also supported by the fact that the laser should be
transmitting approximately 2.00 Watts of power at 2.5 A,
which neither of our sets show.
Part Three: Absorption of Nd:YAG Material
Procedure:
In this experiment, we further analyzed how the laser’s output power varied in relation to other
variables. In the following case, our goal was to record the relationship between the outpout
power and temperature in order to analyze how the laser’s absorption spectrum changed with
regards to temperature. We took measurements for the 23 temperatures below. Since the power
reading fluctuated when taking each measurement, we took the average of the maximum and
minimum power readings for each temperature. We then used these averages to generate a
power-temperature curve. Using this power-temperature and the absorption spectrum provided in
the manual, we observed how the absorption spectrum changed as a result of changes in
temperature.
Data and Observations:
Temperature (C)
40.0
39.0
38.0
37.0
36.0
35.0
34.0
33.0
32.0
31.0
30.0
29.0
28.0
27.0
26.0
25.0
24.0
23.0
22.0
21.0
20.0
19.0
18.0
Max Power (mW)
44.0
42.0
51.0
47.0
70.0
92.0
89.5
85.0
83.0
84.0
100.0
107.0
100.0
107.0
120.0
110.0
123.0
121.0
117.0
117.0
123.0
141.0
145.0
Minimum Power (mW)
32.0
33.0
31.0
40.0
36.0
69.0
60.7
62.0
64.0
66.0
72.0
74.0
88.0
97.0
102.0
100.0
103.0
107.0
112.0
117.0
119.0
135.0
135.0
Average Power (mW)
38.0
37.5
41.0
43.5
53.0
80.5
75.1
73.5
73.5
75.0
86.0
90.5
94.0
102.0
111.0
105.0
113.0
114.0
114.5
117.0
121.0
138.0
140.0
Power-Temperature Curve
Average Relative Power (mW)
160
140
120
100
80
60
40
20
0
15
20
25
30
Temperature (C)
35
40
45
Despite the above curve presenting an accurate depiction of the data we recorded, I noticed an
unusually stark difference between this set of data and the data we took in a second trial for the
purpose of determining the consistency and reproducibily of our results. The second set of data is
represented by the following power-temperature curve.
Power-Temperature Curve
300
Power (mW)
250
200
150
100
50
0
10
15
20
25
30
35
40
45
Temperature (C)
The obvious inconsistency may be due to an equipment malfunction. Human error may have
played some small roll, but taking this along with the results for the power-current curve into
consideration is evidence enough to believe the equipment was malfunctioning in some way.
However, since the second curve is less linear and more realistic, we will be referring to that
when discussing the effect on the absorption coefficient.
Using the above plot of the absorption spectrum for diode lasers, we can compare the graphs and
analyze how the absorption coefficient is affected when temperature is altered. First, it is
noteworthy to state that we are assuming the wavelength to be 808 nm at 25 degrees Celsius
since that was the temperature at which we operated the laser for the last experiment. We are also
assuming that the wavlength is only undergoing slight alterations (± 7.0 nm) throughout this
experiment. So comparing both plots and analyzing the absorption coefficients for the
appropriate wavelengths, we see, for example, at 40 degrees Celsius, the wavelength could very
well be approximately 815 nm and the absorption coefficient approximately 1.0 cm-1. We can
make this assumption because we know that output power and wavelength share an inverse
relationship. Furthermore, from our data, we know that power decreases as temperature
increases; thus, wavelength increases as temperature increases. Now our assumptions concerning
the coefficients could be very inaccurate because we have no means of calculating the exact
coefficient as of now. The one piece of information of which we are certain is, based upon the
absorption plot, the absorption coefficient when the wavelength of the diode laser is 808 nm is
approximately 12.0 cm-1.
Part Four: Fluorescence Lifetime of Nd:YAG Material
Procedure:
For the final part of this lab, we used the oscilloscope to measure the flourescence lifetime of the
Nd:YAG crystal. Since voltage and intensity are directly proportional, this process was, for the
most part identical to finding the time constant of the RC circuit with the oscilloscope. We used
the oscilloscope to generate voltage-time plots and find the grime constants for two cases: the
laser in the presence and in the absence of the YAG. Then, by finding the difference between the
two time consants, this gives us the fluorescence lifetime of the crystal itself.
Data and Observations:
*Note: The following pictures do not represent the actual numerical results; rather, they
represent the relationships in the presence and absence of the Nd:YAG material.
When measuring the fluorescence lifetime of the diode laser without the YAG crystal in place,
we received the following reading:
In the above case, the response time of the setup in the absence of the Nd:YAG material (τ) was
measured as 100 μs (±1.00 μs). Furthermore, we now know that the intensity decreases as a

function of time described by  = 0  −100, where 0 = 6.08 mW/nm2.
In the presence of the YAG, we observed the following reading on the oscilloscope:
In this case, τ = 690 μs (±1.00 μs) and an 0 of 5.08 mW/nm2. Thus, the the function describing

this behavior is  = 5.08 −690 .
With this information, we can now calculate our observed fluorescence lifetime of the YAG
crystal itself, which is nothing by the difference of the two response times we measured.
Accordingly, the fluorescence lifetime of the YAG crystal is 590 μs.
Conclusion:
Though our results at times seemed skewed for an unknown reason, this lab was very
informational. We came away with more knowledge on how to use the equipment and how to
apply the theory we learned. The equipment may need investigation as there may have been
malfunctions which may have skewed our data. More measurements must be taken to improve
the accuracy of our results. Despite the apparent inaccuracy of our data, however, the
experiments were successful. We were able to accurately depict the relationships between certain
quantites and characteristics related to how the laser functioned such as the output power,
operating temperature, and driving current. For that reason, the overall experiment was a success,
however, to truly determine the consistency of our methods, we would have to perform this lab
again with equipment that functions in the manner it should. Only then will we know if this was
truly a success.
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