In the first week of the guided Gaussian Beams lab, you
learn about mounting optics and photodetectors and try out some
techniques that are generally useful in optics labs and
elsewhere. In particular, you will set up a simple optics
system for measuring the width of a laser beam and in the
process will have to mount and align the laser and optics.
2 Lab Notebook
Your lab notebook will play an important role in this
course. You will use your notebook for keeping records of many
things including:
Answering prelab questions from the lab guides
Answering in-lab question from the lab guides
Recording data
Including plots of data
Analysis and results
Diagrams and pictures
Procedures of experiments that you design
The lab notebook will be an important part of your grade
because learning to keep a good lab notebook is an important
part of your professional development.
3 Definitions
Optic – Any optical component that
manipulates the light in some way. Examples include lenses,
mirrors, polarizing filters, beam splitters, etc.
Optomechanics – This category includes
optics mounts and the components to align them. Examples in the
lab include post, post holders, bases, lens mounts, adjustable
mirror mounts, rotation mounts, and translation stages.
4 Setting up the Laser and
Mounting Optics
Your group should find an empty optical breadboard
workstation in one of the five color coded optics bays in
G-214. Spend some time to explore the space and familiarize
yourself with the components. The shelf above the breadboard
should have:
An oscilloscope
A waveform generator
Triple output DC Power Supply
Set of ball drivers
Optics caddy to hold optics already mounted on 0.5”
posts
Set of 1/4-20 and 8-32 screws, setscrews, washers, and
nuts
4.1 Cleaning and handling
of optical components
It’s very important that you do not handle optical
components (lenses, mirrors, polarizers, wave plates,
beam splitters, etc.) with your bare hands.
The oils on your skin can damage the optics and degrade the
light in your experiment. Always handle these components while
using latex/nitrile gloves or finger cots,
which can be found on the top of the tool cabinets in your
optics bay, or on top of the black drawer cabinets in the main
lab. This technical
note from Newport discusses how to go about cleaning dirty
optics, and is also demonstrated in this
video. For a more in-depth treatment on cleaning optics, we
have a copy of “The
Proper Care of Optics: Cleaning, Handling, Storage, and
Shipping” by Robert Schalck for reference in the lab (it
should be located on top of the black tool cabinets in the main
lab).
4.2 Mounting your
optomech: bases, post holders, and posts
The two videos linked below provide information and tips on
how to use and mount components on the optical breadboards that
we have in the lab. Please watch these before proceeding.
Get a laser and power supply from the “He-Ne Laser” drawer
in the colored drawer cabinet found in your chosen or assigned
optics bay. You’ll also need two sets of 3D-printed laser tube
mounts which you can find in the same drawer (see Figure 1). The bottom mount first gets
mounted to an optical post and then the top of the mount can be
assembled with ¼-20 socket head cap screw and nut. After both
sets of tube mounts are attached to the laser, insert the
optical posts into post holders and attach the assembly to the
optical table.
Each person in your group is responsible for assembling a
mirror as shown in Figure 2.
In the end, you will need at least 2 mirrors to complete the
next task. Remember to wear latex/nitrile gloves or
finger cots while handling optical components.
As you are mounting the optics, choose the heights so
that the laser hits the center of each optic and the beam is
parallel to the table.
4.3 Walking a beam
Next, you will align your laser such that beam is aligned
parallel to the table. You will do this by using two mirrors
and a technique that is called “walking a beam”. This
video provides an overview on how to do so. The most
relevant information for our lab starts at 4:09 (which the
above link should take you to). Also, we have 3D printed beam
alignment tools that are used in place of the iris in the video
(these can be found in the colored drawer cabinet in your
optics bay).
Mount two 3D printed beam alignment discs onto the optical
table at the same height above the table.
Use only two mirrors to get the beam to pass through the
center of each disc. Having two mirrors allows you to
independently adjust the angle and position of your beam.
Draw a diagram in your lab notebook of the configuration of
your laser, mirrors, and alignment discs.
5 Modeling Characteristics
of the Photodetector
The goal of this part of the lab is to understand a lot
about the specifications given on the datasheet for the
Thorlabs PDA36A
(or PDA36A2)
Switchable Gain Amplified Photodetectors (we have both in the
lab, so check the model number to know which one you are
using). It is important to realize that data sheets (also
called spec sheets or specification sheets) provide a model for
the realistic behavior of the device. This model can be tested
and improved, a process more commonly called “calibration.”
Note that there are two power switches, one on
the power supply and one on the photodetector. The
photodetector will respond to light with the power off but it
won’t work well and changing the gain will have little
effect.
5.1 Basic function of the
amplified photodetector
Spend a few minutes (no more than 10) to write an
explanation using words and diagrams to explain the physical
mechanism for how the photodetector converts light into
voltage. You may use the manufacturer’s specifications sheet
(linked to in the section above), trustworthy online resources,
a book, etc.
Use the data sheet to estimate the conversion of watts of
light into amps of current for Helium-Neon red wavelength
(632.8 nm) and for the frequency doubled Nd:YAG laser (Green
laser pointer wavelength, 532 nm).
How would you convert “amps per watt” into “electrons per
photon”?
What is the electron/photon conversion efficiency for the
red HeNe laser and green doubled Nd:YAG lasers?
Is this number less than, equal to, or greater than one?
What does this number tell you about how the photodiode
works?
5.2 Calibrating the
photodetector offset and gain
Calibrating the photodetector is especially important when
you take a data set that uses multiple gain settings. Having an
accurate calibration of the gain and offset will let you stitch
the data together accurately.
Here you will encounter gain values that are presented on a
logarithmic \(dB\) (decibel)
scale, which is obtained by taking \(20×log(V_{out}/V_{in})\). For
example, \(20\ dB\) of gain
corresponds to electronic voltage amplification by a factor of
10. A \(dB\) scale could also
be defined as \(10×log(P_{out}/P_{in})\), where
\(P\) is the power. Explain
the conversion between these two scales and why this makes
sense.
Calibrating the offset voltage (the output of the
photodetector when no light is incident upon the device).
Calibrate the offset of the photodetector as a function of
gain setting.
Quantitatively compare it to the specifications given in
the table. Is your measured value within the specified range
given on the PDA36A or PDA36A2 photodetector data sheet?
What measures did you take to eliminate stray light? Were
your measures sufficient for an accurate calibration?
Calibrating the gain.
Is it possible to measure the \(V/A\) gain for each setting, or
can you only measure the change in gain as you switch the
settings? Why? Note that this lab only requires relative
gain.
Make a measurement of the gain or relative gain for most of
the gain settings. If you need to adjust the laser power, try
blocking part of the beam. Note, you will need to make two
measurements at one gain setting when you block the beam. What
systematic error sources are of most concern?
Quantitatively compare your results with the range of
values given on the data sheet. Do you believe your results
provide a more accurate estimate of the photodetector gain than
the data sheet? Why or why not?
Using the appropriate spec sheet and your measurements,
what is the power of your laser? Does this agree with the laser
power shown on the laser?
Hypothetically, how would you measure the absolute
gain?
5.3 Follow up
Write mathematical expressions that converts the incident
power (the light) \(P_{in}\)
to the photodetector voltage \(V\) and the photodetector voltage
\(V\) to input power \(P_{in}\). Take into account all
relevant parameters such as the photodetector gain setting (in
\(dB\)) and offsets.
6 Review of Measurement
Uncertainty
When attempting to establish the validity of our
experimental results it is always important to quantify the
uncertainty. Measurement uncertainty wasn’t invented to make
lab classes tedious, rather it is a core part of any
experimental work that gives us a way to quantify how much we
trust our results.
A simple and rigorous way to make a measurement and estimate
its uncertainty is to take \(N\) measurements \(\{y_1,y_2,\ ...\ ,y_N\}\) and
estimate the value by the mean:
Make sure that your laser has been turned on for at
least 5 minutes so it has had the opportunity to warm
up. You will use the photodetector to measure the DC
optical power.
6.2 Measurement and
uncertainty using the multimeter
Using your digital multimeter, make a table of estimated DC
voltages from your photodiode as it is illuminated by your
laser and the corresponding uncertainties using the following
methods:
“Eyeball” the mean. “Eyeball” the amplitude of the random
fluctuations.
Set the multimeter on max/min mode to record the \(V_{max}\) and \(V_{min}\) fluctuations over a
certain time period. You can estimate the mean by \((V_{max}+V_{min})/2\) and the
uncertainty by \((V_{max}-V_{min})/2\).
Record the instantaneous voltage reading on the multimeter
\(N\) times and calculate the
estimated uncertainty from the standard deviation.
What is the resolution intrinsic to the multimeter
according to the spec
sheet (no measurement required)? How does this compare to
the observed uncertainty in parts 1-3?
6.3 Measurement and
uncertainty using the oscilloscope
Now connect the photodiode to an oscilloscope. Continue the
previous table of estimated DC voltages from your photodiode as
it is illuminated by your laser and the corresponding
uncertainties using the following methods. For each method
comment on if and how it depends on the setting for the time
scale or voltage scale on the oscilloscope.
“Eyeball” the mean. “Eyeball” the amplitude of the random
fluctuations (no cursors or measurement tools).
Use the measurement function on the scope to record the
mean and RMS fluctuations.
Use the cursors to measure the mean and size of
fluctuations.
Record the voltage from the oscilloscope \(N\) times and calculate the
estimated uncertainty from the standard deviation.
A comparison with the data sheet is difficult because so
many factors affect the observed noise in the oscilloscope. You
can find some information here
(there is information about the resolution and the DC
measurement accuracy) .
6.4 Summary of
methods
Make sure to support your answers for each question
below.
Did any methods overestimate the uncertainty?
Did any methods underestimate the uncertainty?
How reliable was “eyeballing”?
Did the time scale or voltage scale affect any of the
oscilloscope measurements? If yes, how? Does this tell you
anything about how to use the scope?
Suppose the light into the photodetector was not constant
during the measurements (due to variations of the laser, room
lights, etc.). In which estimates of the uncertainty will this
be included?
Which method(s) should give a true estimate for the
uncertainty?
6.5 Writing numbers and
their uncertainty
The convention used in this course is that we:
only display one significant digit of the uncertainty
(two are allowed if the first significant digit is a
1).
display the measurement to the same digit as the
uncertainty.
The numbers \(154\pm 3\),
\(576.33\pm 0.04\), and \(245.1\pm 1.4\) follow theis
convention. However, numbers copied from the computer are often
displayed as “machine precision” with no regard for significant
digits.
Mathematica generated the following fit parameters and
corresponding uncertainties:
\[a=-0.6699999999999988 \pm
0.6751049301158053\]
\[b=2.2700000000000005 \pm
0.2035517952102936\]
How should the two Mathematica fit parameters above be
rewritten such that they correspond with the convention
described above?
7 Measuring the Beam
Width
The goal of this section is to develop a measurement
technique and analysis scheme to measure the width of a laser
beam. The scheme will let you measure the width in one
dimension. The technique is most useful for beams that have an
approximately Gaussian intensity profile. You will improve and
refine this technique in the upcoming weeks of this lab.
Note: You may or may not find that completing this section
during your lab time this week is challenging due to time
constraints. This is okay - get as far as you can now. You’ll
have an opportunity to revisit this section during week 3.
However, don’t just skip it now as you’ll find the outcomes to
be useful in the upcoming weeks.
The basic scheme involves measuring the power in the laser
beam as the beam is gradually blocked by a knife edge (razor
blade) using a setup similar to Figure 3.
Suppose a laser beam has a Gaussian intensity profile \(I(x,y) =
I_{max}e^{-2(x^2+y^2)/w^2}\), and is incident upon a
photodiode. What is the expression for the power hitting the
photodiode when a portion of the beam is blocked by a razor
blade?
Draw a diagram showing the beam and the razor.
Using the above expression for \(I(x,y)\), write the mathematical
expression for the power incident on the photodiode as a
function of razor position. Note, to address this question, you
will need to become familiar with the Error Function, \(erf(x)\). What assumptions, if
any, did you need to make in evaluating the integral? Hint: if
you are moving in the \(x\)
direction, what is going on in the \(y\) direction?
7.1 Before you take data:
create an analysis function to fit a test set of data
Note: Nonlinear least squares fitting is covered in next
week’s prelab. There is also a YouTube video available on least
squares fitting in Mathematica.
What is the functional form for your fit function?
Is it a linear or nonlinear fit function? Why?
What are the fit parameters? Why do you need this
many?
How do the fit parameters relate to the beam width?
Check that the fit looks good and you get a beam width of
\(w=4.52 \times 10^{-4}\ m\).
If you get a different value, check with your instructor to
understand the problem. What is the uncertainty on your
measurement?
7.2 Build your setup for
measuring the beam width of your laser
Draw a detailed schematic of the setup (from the laser all
the way to the photodetector).
After assembling your experiment, but prior to taking a lot
of data, how can you quickly determine if the measurement is
working?
Is it preferable to use a digital multimeter or
oscilloscope? Why?
Use the measurement scheme to take data of incident power
on the photodiode vs. position of the razor. Pay attention to
the units of the translation stage. Pick a position where your
beam has a measurable width and measure it. Justify your
choice.
7.3 Analysis of the random
uncertainty sources
What are possible sources of random uncertainty in the
photodetector voltage?
How would you estimate the uncertainty in the photodetector
voltage measurement?
What is the largest source of uncertainty? Why?
7.4 Analysis of the real
data
Use the analysis procedures verified in section 7.1 to find the beam width for your
data. Be sure to include the uncertainty.
Plot your fit together with your data to make sure it is
good.
8 Postlab
Please choose either Mathematica, Matlab, or Python for this
assignment. Both Mathematica and Matlab licenses
are provided by CU and Python is free. You must submit both
code and results. Note that the following assignment was
created based on Mathematica and Matlab.
Evaluate the following math expressions:
\(e^{1.6\pi j}\)
\(4i\pi+e^{7\pi /4}\)
\(sin^2(\frac{\pi}{5})\)
\(log(3+\sqrt{3})\)
\(|3+4i|^{2/3}\)
Plot the following functions at the given ranges. Make
sure to add appropriate \(x\)
and \(y\)-axis ticks and
numeric labels at the ticks locations.
Plot \(sin^2~\theta\)
vs. \(\theta\) in the range
\(0\le\theta\le 6\pi\). Add
the legend that indicate the name of the function used.
Plot \(sin~2t\) and \(cos~5t\) together (using different
colored lines) on the sample for \(t\) in the range of \(0\le t \le 10\).
Plot the following data set (wavelength, \(\lambda\), and the corresponding
index of refraction, \(n\), of
a particular type of glass):
Make a plot of \(n\)
vs. \(\lambda\). Which
variable would you choose for \(x\)-axis? Please make sure to add
the axes names and appropriate ticks for each axis.
Add lines to connect each points. What can you tell about
the relation between the wavelength and the index of refraction
from this plot?
Provide your code to make your plot from 8.3.1 including
the following:
Title of “Refraction index as a function of \(\lambda\)”.
Add the linear fitting line $ y= ax +b$, where \(y\) corresponds to \(n\) and \(x\) to \(\lambda\) on the data.
Include the fitting linear equation on the plot.
If you would like to display the \(x\)-axis only below \(0.6\mu m\) (i.e. \(\lambda \le 0.6~ \mu m\)), how can
you do that?
Familiarize yourself with the HELP
functions of your chosen software package and perform the
following tasks and answer the questions.
Make a contour plot of any function of \(z=f(y,x)\) of your choice.
We would like to evaluate a function, \(f(x)\), at \(x= \frac{\pi}{10}, \frac{2\pi}{10},
~\cdots~,\frac{9\pi}{10}\frac{10\pi}{10}\).
How can we represent/create these \(x\) points in your chosen software
package? What should we modify in order to make the increment
of \(x\) be \(\frac{\pi}{100}\), instead of
\(\frac{\pi}{10}\)?
Now, define a function that represents \(y= f(x) = \frac{\sin x}{x}\) and
evaluate \(y\) at the given
\(x\) with increment of \(\frac{\pi}{10}\).
Export your \((x,y)\) data
created in 8.5.2 in .csv format.
Download this
data set (in .txt format), where the temperature of Boulder
was recorded at approximately every hour (\(\approx 0.04\) days) since January
1st. Using the HELP function of your chosen
software package, export this data and plot in an appropriate
way (make sure to include labels, ticks, titles etc).