Section 4.1
Photodiode Basics
Photodiodes are widely used to measure the relative power in a beam. In order to understand the signal coming out of the photodiode, however, you have to understand how it operates, because its output depends not only on its input, but also on how it interfaces with your electronics.
Photodiodes are often packaged in a photodetector with a reverse bias for high speed operation. The photodiode produces an output current, and the detected voltage will then depend on the input impedance of the device the photodiode is connected to. The bias voltage will limit the power level at which the detector saturates, because when the power level reaches a maximum value the output voltage rises to the bias voltage and can rise no further.
The response time of the photodiode is also dependent on the input impedance of the device it is connected to, because the intrinsic capacitance of the diode junction combines with this impedance to produce a low pass filter.
Section 4.2
Oscilloscope Basics
Oscilloscopes are useful for measuring amplitude and temporal properties of signals from photodetectors in optics experiments. Since they are so widely used, it is helpful to understand how they work and how to use some of the basic features. At a minimum you should be able to set a trigger to get a stable display, zoom into the region of interest, and use the cursors to measure values such as amplitudes, periods of oscillation and full width at half maximum values for pulses.
Section 4.3
Power Measurement
Whether considering the power out of your laser, or looking at how the power transmitted through your system is affected by some parameter, it is important to be able to measure laser power. Thermal photometers and photodiodes are the most common tools for the job. Thermal meters provide excellent accuracy over a large spectral region, but are slower than photodiodes and less able to respond to rapid modulation.
Section 4.4
Characterizing Polarization
Control of polarization is often used as a means to manipulate the path that light takes through an experiment and the phase it acquires while doing so. With a few of the same tools used to manipulate polarization - a polarizer, a quarter-wave plate and a half-wave plate - as well as a photodetector we can measure the polarization state of light: its degree of polarization, its polarization state and the angle and ellipticity of the general elliptical polarization state.
Lasers will tend to have a specific polarization state if there are polarization sensitive elements in the laser cavity. As an example, Helium-Neon lasers with intracavity gas tubes that have a window at Brewster’s angle to reduce reflection losses will produce linearly polarized light, while HeNe lasers that have the cavity mirrors bonded onto the gas tube at normal incidence will produce unpolarized light.
Section 4.5
Measuring Gaussian Beam Parameters
Laser beams can be described as Gaussian beams, which are narrowest at their waist and then diverge at an angle inversely proportional to the radius of the beam waist. The size of the beam and the radius of curvature of its wavefronts can be calculated at any position along the beam’s path once the location and size of the waist are determined (assuming the wavelength is known). Therefore it is important to know the location and size of the waist exiting your laser. This is typically determined by measuring the Gaussian radius of the beam at three or more positions along the beam path, and curve fitting to the data to determine the free parameters. The Gaussian beam radius may be measured by an imaging sensor, by a commercial
Section 4.6
Balanced Detection
When an experiment seeks to measure small signals introduced to the light by the optical system, it can be difficult to distinguish these signals from any laser amplitude noise that is present. A powerful technique to isolate these signals is “balanced detection”.
For balanced detection, the signal to be measured is compared to a reference beam - a portion of the laser that doesn’t contain the signal. In some incarnations the signal and reference are measured separately. In others the reference beam is optically combined with the signal using a beamsplitter so that the sum and difference of the reference and signal are measured. The value of combining the signal and the reference optically, is that if the signal amplitude is very small, combining it with a strong reference of the same, or difference frequency (called homodyne detection and heterodyne detection respectively) allows small changes in the signal amplitude to produce a much larger change in the detected power than would otherwise be the case.
Section 4.7
Modulation-Demodulation
When an experiment seeks to measure small signals introduced to the light by the optical system, it can be difficult to distinguish these signals from any other detected light. For instance if one wishes to measure a weak fluorescence signal from a sample pumped by a laser, but the ambient room light is much greater, the fluorescence signal will not be distinguishable from the background. One way around this is to modulate the signal that you wish to detect at high frequency, for example, amplitude modulate the beam illuminating the fluorescing target using a chopper. The signal being measured is then embedded on the high frequency modulation which is distinguishable from the background light in the system due to its higher frequency. To recover the original signal, the output of the photodetector is demodulated using a mixer that combines the photodetectors signal with the “local oscillator” - a copy of the modulation waveform (that could be from a photodetector measuring the laser power after the chopper for instance). This returns the intended signal to its original frequency, but shifts the background to the higher frequency. The output of the mixer is low pass filtered to remove the background, recovering the original signal. This technique can be combined with balanced detection by modulating the reference beam. More on modulation theory is available here.