The following lab manual was written by  Dr J Olof Johansson  and Kyle Barlow from The University of Edinburgh

For more information about Dr J Olog Johansson and his lab please visit the Johansson Group page 

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1. Background

In the optics workshop, you would normally get practical experience using lasers and placing and aligning optical components on a breadboard. Due to current circumstances, this initial workshop will take place online. However, do not worry; you will get practical optics experience in S2 during the investigation.

You will use a free online software called 3DOPTIX . This program was chosen because it allows you to choose the optical components you need, place them in a kinematic mount and finally place this on an optics table. You can then perform ray tracing calculations to understand what happens when you move the optics on the laser table or change the wavelength of the light.

2. Resources

3. Tasks

Don’t forget to document your progress in the electronic lab notebook!

First of all, you need to go to the website https://3doptix.com/ and press the “Register to the Beta” button on the website header to get started. Please also watch some of the tutorial videos that are available on the website. Apparently, the programme does not work well using the Firefox web browser. If you have problems, try a different browser (e.g. Chrome or Edge).

Task 1: Telescopes and beam expanders

You first task will be to place two lenses in a mount and build a telescope. Use a laser as a light source.

Telescope

 



  1. Adjust the distance between the two lenses to collimate the beam after the two lenses. A collimated laser beam does not change beam diameter as a function of propagation distance. 
  2. Study how the distance between the two lenses change as you change the lens material (refractive index of lenses) and wavelength of the laser. Do this in a systematic manner and compare to the dispersion of the material.

  3. Build both a Galilean and Keplerian telescope to triple the beam diameter. What is the difference between these two types of telescopes?

Task 2: Gratings and prisms

Gratings and prisms are used in spectrographs to spatially disperse a white-light beam onto an array of detectors to measure light intensity as a function of wavelength.

  1. Use a white-light source and explore how the light is dispersed as it is diffracted by a grating. Study how the dispersion changes as a function of the number lines and angle of incidence. Use a “screen” to project the dispersed beam onto a plane.
  2. Compare the difference between a prism and a grating. What is the advantage and disadvantage of using either of these components to disperse the light in a spectrometer?
prism

Task 3: Aligning a laser beam on a line

Quite often you will have to align a laser beam along a straight line, perhaps towards a detector or a sample. To align a beam onto a line, you need four degrees of freedom. This can be achieved using two mirror mounts that can both be adjusted in two dimensions (horizontal and vertical).

  1. Use the pre-built setup (Alignment Irises) to test how to do this practically using two mirrors and two irises. This was setup was made using a third party mount and you can use the setting under "optical element delta" to fine tune the angles by a few degrees. The setting for “delta theta” changes the left and right angle and “delta psi” changes the up and down angle. Hint: use mirror 1 (M1) to align the beam through iris 1 (I1) and M2 to align the beam through I2. Iterate until the beam passes through both I1 and I2.

Aligning a laser beam on a line
How to Align a Laser from Thorlabs 
Task 4: Pump-probe experiment

The absorption of light by molecules and the subsequent redistribution of this energy takes place on the femtosecond (fs) timescale. It is therefore important to use a tool that is fast enough to capture these initial fast events. It is possible to generate laser pulses as short as a few fs. However, there are no detectors that are fast enough to measure the short duration of the light pulses. Therefore, the laser pulse is split into two beams: a pump and a probe beam. The pump beam is used to excite the sample and the probe detects any changes to the absorption of the excited sample due to the pump beam. The optical path travelled by the probe beam can be adjusted so that it arrives at a well-defined delay relative to the pump beam. It is easy to adjust this path difference with sub-μm precision. 

The measurement is repeated for a range of different path length differences and a new pump beam excites the sample for delay point.

  1. Calculate how much later the probe beam arrives if the path length difference is 2 μm.
  2. Use the pre-made pump-probe setup (“Inline single-colour pump-probe”) in 3DOPTIX to see what a pump-probe experiment can look like. What are the different components and what are they used for?

4. Lab report

Include the answers to the questions above in your lab report. Explain how you solved the problems and plot the results (where possible). For example, you could plot the spatial position of the wavelength as a function of angle of incidence on the grating. Plan your “experiments” so that you vary a parameter systematically. Get into the habit of analysing your results and compare to model calculations. Often it is convenient to linearize the data based on the model. Make error estimates if possible, but of course that is difficult when using the simulation software.