20100401 RGA Autocorrelations

An autocorrelator was setup with components brought from Oxford as shown in Fig 1.

Figure 1 : Photograph of autocorrelator setup.

The output of the regenerative amplifier (RGA) was taken before entering the spatial filter (link to laser schematic). Given the estimated length of ~400ps, the spatial filter and linear amplifiers will have little effect on the pulse length or temporal structure. Furthermore, the energy output of the RGA is more than sufficient for an autocorrelation.

An interferometric autocorrelator was built first which produces a noisy signal but allows easier alignment which can then be adjusted to make an intensity autocorrelator. See autocorrelator measurement guide.

A LabView VI was written to move the stage at a controllable constant speed and record the peak height from an oscilloscope connected to a photodiode. There were several version and data was recorded under different versions

Autocorrelator V1 VI

Using autocorrelator version 1 VI the following data was recorded

Stage Speed mm/s	Data Filename	Type
	3003100854ACDATA.txt	Interferometric
	3003100839ACDATA.txt	Interferometric
	3003100912ACDATA.txt	Interferometric

Autocorrelator V4 (Final) VI

Using version 4 of the autocorrelator VI the following data was recorded

Version 4 of the VI records a measurement every 750ms from the oscilloscope and stops recording data when the stage has reported back that it has finished. The scope could make measurements of the maximum value in the 5000 sample trace at 2.4Hz maximum for a measurement on the screen and 4.3Hz for a measurement off the screen (only one available). This was still too slow to run the laser at 6Hz and record data faster, so the original 1.56Hz was used. The time of 750ms was therefore set to ensure a new measurement each time the oscilloscope was queried.

Analysis

Interferometric Autocorrelations

Figure 2 : Graph of 3003100854ACDATA.

Figure 3 : Graph of 3003100839ACDATA.

Figures 2 and 3 show two interferometric autocorrelations representing 3003100854ACDATA and 3003100839ACDATA respectively. These have been manually fitted due to the difficulty in fitting the envelope of the data. Assuming a comletely stable experimental setup and a higher number of samples, it would be possible to fit the interference fringes, however the experiment is not that stablea and with a fixed sampling rate of 1.56Hz this is impractical. The interferotmetic setup serves only as an alignment tool for the intenstiy autocorrelations. Here, the data has been fitted to a Gaussian. Whilst the exact pulse shape is not known, the envelope matches reasonably well giving a of 200 and 210ps respectively. This must be deconvolved by dividing by , the deconvolution factor for two Gaussians, giving 141.4 and 148.5ps respectively.

Intensity Autocorrelations

Figure 4 : Intensity autocorrelation with stage moving at 5.

Figure 5 : Intensity autocorrelation with stage moving at 4.

Figure 6 : Intensity autocorrelation with stage moving at 3.

Figure 7 : Intensity autocorrelation with stage moving at 2.

Figures 4, 5, 6 and 7 show intensity autocorrelations recorded at different stage speeds. Again, these have been fitted to Gaussians but by Origin as with the intensity autocorrelator only the envelope is produced. The deconvolved results are summarised below.

Stage Speed (mm/s)	Number of Samples	Sigma (ps)	FWHM (ps)	4 Sigma (ps)
5	133	108.1	254.5	432.4
4	165	111.2	261.8	444.8
3	220	109.1	256.8	436.4
2	329	110.2	259.4	440.8
Average		109.7 1.2	258.1 2.7	438.6 4.7

Taking the average sigma and assuming a Gaussian profile (deconvolution factor of sqrt(2)), the deconvoluted pulse length is 77.6 $\pm$ 0.9 ps.

Graphs in emf (windows metafile format - vector based) are available below

• intensity_ac_graph_5mms.emf
• intensity_ac_graph_4mms.emf
• intensity_ac_graph_3mms.emf
• intensity_ac_graph_2mms.emf

Specific Equipment Used

• 500mm Sigma Koki Stage
• Tektronix TDS-684C Oscilloscope