outreachspeedoflight-tspspi 0.0.1a21

Simple frontend to illustrate our speed of light measurements for Wiener Forschungsfest

Outreach project utility: Speed of light

Work in progress

This is the frontend to the speed of light outreach project. This project has been developed to measure the speed of light similar to the idea that Hippolyte Fizeau developed in 1848. In contrast to his experiment in this case a little cheat is used - a bicycle works as chopper to produce a sharp difference in light intensity of a laser source. The light travels through two arms of the experiment - one being only a few meters away from the chopper, the other being about hundred meters away. The signal of both pulses is recorded on photodiodes and sampled by a fast oscilloscope (this is where the cheating comes in - we have a definition of time and fast direct sampling).

Since this project has been developed to increase the interest in science and let people participate actively during exhibitions it's by no way a precision measurement - it's more of a (working) experiment that people can look at, play with and grasp the idea of light traveling at finite speeds.

This application:

• Gathers data from the oscilloscope
• Visualizes the samples
• Calculates a cross correlation of the symmetrized periodicalized recorded function
• Estimates the speed of light from this correlation function by doing a simple peak search
• Does averaging over the estimated speed of light and visualized this.

Experimental setup

The experimental setup consists of two basic parts:

• A chopper assembly - where a bicycle is used (or any other kind of chopper)
• A beam splitter and photodiode assembly as well as a long beam line

Example screenshots

Running the embedded DAQ simulation

Example run in the lab

Configuration files

daq.conf

The data acquisition module configures the communication with the MSO5000 oscilloscope.

Example configuration:

{
	"osci" : {
		"ip" : "10.0.0.196",
		"port" : 5555,
		"sperdiv" : 1e-6,
		"trigch" : 1,
		"triglvl" : 0.5,
		"ch1" : {
			"offset" : 0,
			"scale" : 0.5
		},
		"ch2" : {
			"offset" : 0,
			"scale" : 0.1
		},
		"maxqueryrate" : 2
	},
	"chopper" : {
		"diameter" : 68e-2
	},
	"path" : {
		"length" : 100,
		"n" : 1.4
	},
	"loglevel" : "debug",
	"mode" : "continuous"
}

The osci section configured the MSO5000 oscilloscope:

• Connectivity:
  • The ip field can be either the IP or hostname as string
  • port is optional and defaults to 5555
• Optional Horizontal axis (time) configuration:
  • sperdiv specifies the seconds per division on the horizontal axis. This has to be a value supported by the oscilloscope
• Optional Trigger configuration:
  • trigch selects a channel (1 or 2) for the trigger to act on
  • triglvl selects the trigger level in volts.
• Optional channel configuration ch1 and ch2:
  • One can specify an offset in volts
  • and a scale specified in volts per division that has to be a supported value by the oscilloscope
• A optional maximum query rate in Hz (i.e. queries per second) that can be used to limit the amount of queries to the oscilloscope since at some point it won't update it's own local display anymore due to prioritization of network queries. When not specified the application queries as fast as possible.

The chopper section configures the chopper that is used. For our experimental setup this can be a bicycle or a simple wooden wheel:

• diameter configures the diameter of the wheel in meters. This is only used for velocity calculation from trigger rate assuming that only one trigger is issued per cycle.

An important configuration is the path. This describes the free path the light pulse is traveling through. One can describe the length in meters and the refractive index n (1 for air or vacuum or about 1.4 for glass fibers)

At last one can select between two modes of operation using the mode parameter:

• triggered uses the scope in single trigger mode and re-arms when ready to gather more data. This ensures that both channels correspond to the same event but looks less real time on the oscilloscopes local display.
• continuous continuously samples on the scope and queries data as fast as possible.

gui.conf

The user interface can be configured using gui.conf. This allows one to configure some analysis parameters:

• The number of last measurements to be shown using lastsamples
• The number of measurements to include in rolling average averagecount
• A optional fit of a Gaussian function into the difference signal (that can also be used instead of the autocorrelation function to detect the time delay). This is done in the difffit dictionary:
  • enable is set to the strings "true" or "false". When set to true fitting is enabled. Keep in mind this is numerically more demanding than the other methods
  • primary can be either "true" or "false". If set to true the FWHM of the fit is used as measure for time delay of the signals and thus to calculate the speed of light. When set to "false" the fit is only plotted and the speed is still calculated using the cross correlation of both signals
  • dump is set to "true" to dump fitting result and parameters to the standard output (for debugging purposes)
• To handle noisy signals on the photodiodes movingaverage can be set to any integer value larger than 0 (or to 0 to disable the feature). It applies a moving average filter of the specified number of samples - and thus applies a low pass filter to the signal.

In addition one can directly configure some layout parameters:

• plotsize with x and y parameters directly scales all plots
• mainwindowsize with x and y parameters scales the main window as initially created.

Note that invalid setting of those parameters might clip some graphs or cause some strange behavior of the user interface.

highscore.conf

{
  "loglevel" : "debug",
  "highscorefile" : "/tmp/highscore.dat",
  "mainwindowsize" : {
    "x" : 1920,
    "y" : 1500
  },
  "tablesize" : {
    "x" : 1350,
    "y" : 250
  }
}