Transient optical laser reflectivity measurement: finding FEL and OL time overlap¶

Transient optical laser reflectivity is a technique to determine the temporal overlap between the FEL and the optical laser (OL). The FEL is pumping a large band gap material (usually a 1 micrometer thick Si\(_3\)N\(_4\) membrane) and the OL, spatially overlaped with the FEL, is reflected off the sample. The incoming (\(I_0\)) and reflected (\(I_r\)) laser beams are monitored by photodiodes. The FEL pump alters the electronic properties of the material, which in turn modifies the reflectivity. By varying the delay between OL and FEL through the scanning of the optical delay line, the transient response of the material is measured and the exact time overlap between the two beams can be extracted.

To increase the signal to noise ratio, pumped and unpumped signals acquired closely in time are compared. The reflectivity is then defined as:

\(\Delta R [\%] = 100\times(\frac{R(pumped)}{R(unpumped)} - 1)\), with \(R = I_r / I_0\)

In the toolbox_scs, there is a convenience function reflectivity that allows the quick calculation of \(\Delta R\). It performs binning along the motor position axis and sorts the data according to the bunch pattern and the sequence of pumped, unpumped.

Below is an example, where \(I_0\) (\(I_r\)) is measured a by photodiode on the Fast ADC channel 5 (channel 3), respectively. The pump FEL is set at half the repetition rate of the OL, to have alternating pumped/unpumped/pumped/unpumped/… pulses within each train.

[1]:

import toolbox_scs as tb
proposal, runNB = 2769, 425
fields = ['FastADC5raw', 'FastADC3raw', 'PP800_DelayLine', 'BAM1932S']
run, ds = tb.load(proposal, runNB, fields)

refl = tb.reflectivity(ds, Iokey='FastADC5peaks', Irkey='FastADC3peaks',
                       delaykey='PP800_DelayLine',
                       binWidth=0.01, plot=True)
refl

_images/Transient_reflectivity_measurement_2_1.png

The output is an xarray.Dataset that contains the binned deltaR, its standard deviation and standard error, as well as the delay line position bins and the counts per bin.

One can also convert the motor position axis in mm into temporal axis in ps. For this, the argument positionToDelay can be used, in combination with origin which gives the motor position for time zero and invert, which gives the sign of time axis relative to the motor axis. In this case, binWidth is the width in picosecond, and the output has a new coordinate delay_ps.

[2]:

refl = tb.reflectivity(ds, Iokey='FastADC5peaks', Irkey='FastADC3peaks',
                       delaykey='PP800_DelayLine',
                       positionToDelay=True, origin=228.845, invert=False,
                       binWidth=0.01, plot=True)
refl

_images/Transient_reflectivity_measurement_5_1.png

Transient optical laser reflectivity measurement: finding FEL and OL time overlap¶

Correction by the BAM¶