Lab 4: Lockin Amplifier Light Detection

PHYS 471, Spring 2023

In this lab, we will learn how to measure low light levels using a Lock-In Amplifier.

This lab consists of 1 Exercise and 1 Experiment

Exercise 1

This exercise is intended to make sure you can operate the SRS 830 Lock-In Amplifier. You will make a known signal using the function the generator and confirm that your measurement in the Lock-In is correct.

  1. Use the function generator to generate 400 Hz Sine wave.
  2. Connect the oscilloscope to monitor the output.
  3. Set the amplitude of the output to be 0.1 V RMS by measuring the wave on the oscilloscope.
  4. Connect the TTL output of the function generator to the REF IN of the Lockin.
  5. Using the T-Connector, also send the sine wave output to the A input channel on the lockin.
  6. Adjust the settings of the Lock-In until you can measure the 100 mV, 400 Hz signal.
  7. Use the Auto-Phase button to set the phase of the Lock-In so that the signal shown is predominately in the X channel.

Exercise 1 Set Up

Experiment 1

Now, we will use the Lock-In to measure the light output from a Laser diode. The Light sensor will be a Phototransistor (NTE3xxx). This element will create a voltage output proportional to the amount of light reaching the sensor.

Here is the general schematic for the experiment. A small circuit has been built to power the phototransistor and provide an output voltage to the Lock-In. The laser is powered by the function generator (which you can monitor using the oscilloscope). The laser will start emitting light at about 1.5 Volts RMS, the max Voltage should be kept under 5 V, but something in the order of 2 V RMS is appropriate for this experiment. The Reference signal to the Lock-In is provided by the TTL output of the Function Generator. The frequency you choose is up to you, but stay away from integer multiples of 60 Hz. Something between 400 Hz and 1 kHz should be good.

The experiment is to measure how the size of the aperture affects the light received by the sensor. There are 4 apertures available on the slides with diameters of 2mm, 1 mm, 0.75mm and 0.5 mm. Essentially, the goal is to show that as you restrict the amount of the light received by the sensor, the Lock-In input amplitudes follow the expected relationship. It's up to you to figure out what that expected relationship is (i.e. how does the amount of the light depend on the size of the aperture).

Experiment 1 Set Up

Procedure

  1. First, make sure you can illuminate the laser with the function generator. Please keep the voltage to the laser under 5 V or you might break it. Use the oscilloscope and a T connector to monitor the output of the function generator.

    2. The Function Generator and the Oscilloscope connections.

  2. Set up the optical axis so that the laser is pointed at the phototransistor and that the apertures can be placed in the beam line.
  3. Make sure you can simply measure the phototransistor signal with the Lock-In before adding the apertures. Keep the signals small. (i.e. around 4-5 volts from the Signal Generator is enough. You should be able just see the laser light, but it doesn't need to be very bright.

    4. The beam can be seen incident on the 2 mm aperture.

  4. Experiment with the frequency of the function generator to find a suitable value.
  5. If it all makes sense so far, then begin to take measurements with the 4 apertures.

There is no computer data acquisition available for this experiment currently, so you will have manually record the values from the Lock-In front panel display. Choose and appropriate time constant so that values are stable. Do the measurement for each of the 4 apertures and prepare a plot showing Light amplitude (intensity) vs. aperture size. Prepare an analytical curve to compare with your experimental values on the same plot.

Example Graph (not what it really looks like, but just to give an idea of what should be on the axes)

Report to submit:

Please type up a short report that shows your measurements.

Due May 8, 4 pm

Include:

  1. Brief Introduction
  2. Visual schematic of the experiment
  3. Plot of your data
  4. Comparison to theoretical prediction
  5. Links to your data online
  6. Links to your analysis code online
  7. References in proper format for any external sources you used.
  8. Scan of your handwritten lab notes.