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Ultrafast Laser Pulse Measurement

OBJECTIVES: The accurate knowledge of the pulse width of the ultrafast laser pulse is of paramount importance during any experiment in an ultrafast lab. While modern day electronics can not measure any event faster than few Pico-seconds, there are some other state of the art techniques available for characterizing the laser pulses among which Autocorrelation is one of the most efficient techniques which we practice regularly.

THEORY: As the name suggests, the autocorrelation is the measurement of the pulse using the original pulse itself. Experimentally the original pulse is split in two equal parts and they are again made to recombine both specially and temporally using a variable time delay in one arm. A detector placed at the recombination point detects the interference between two pulses which in turn is converted to the autocorrelation spectrum. Depending on the measurement process this can be of two types- Intensity Autocorrelation and Field Autocorrelation.

Mathematically, autocorrelation function is defined as INTENSITY AUTOCORRELATION:

This is done using a non-collinear geometry where the two arms are focused to a point using a lens and a BBO is kept at the focal point. At this point the second harmonic generated is proportional to which is the sum of three components. The third component proportional to is our desired autocorrelation signal that is detected by the detector as  FIELD AUTOCORRELATION:

This is done in a collinear geometry where two pulses are recombined collinearly and the signal detected by the detector is given by Where is the first order autocorrelation function. If we place a BBO crystal in front of the detector and collect only the SHG signal we get the interferrometric (second order) autocorrelation signal as before(look at intensity autocorrelation for mathematical expression). INSTRUMENTS UTILIZED:

1.      LASER, Mirrors, mirror holders.

2.      Lenses, lens holders, irises, retro mirror.

3.      Beam splitter,

4.      Non Linear Crystal (BBO).

5.      Photo Diode.

6.      Oscilloscope, GPIB Card

7.      Motorized Linear Stage, Motorized Stage Controller

8.      BNC cable, GPIB connector.

9.      Laser glasses for eye safety.

SOFTWARE USED:

1. LabVIEW software for data acquisition.
2. Origin pro for data plotting and analysis.

Note: For user operation and usage no specific software needed.

EXPERIMENT PROCEDURE:

INTENSITY AUTOCORRELATION:

1.      Turn ON the key Switch (from OFF to ON position).

2.      Wait for 10-15 minutes.

3.      Open the Shutter by pressing the shutter open switch and then press the power level 2 switch.

4.      Switch on the Chiller.

5.      Wait for 40-45 minutes for stabilization of the laser; put a power meter in the optical path to measure the average power of the laser and then remove it.

6.      Make the laser from CW to Mode Locked Condition.

7.      Put one ultrafast thin Beam Splitter (BS), which divides the total optical path into two parts. One path is call pump path and another is called probe path.

8.      Put the Retro on a motorized stage in one path (Probe path) such that the beam height at the input and at the output paths are same & the beam path distances of the input and output paths are same up to 4-5 meter distances. If the beam paths are not parallel then recheck the alignment.

9.      Align the laser path according to schematic diagram (Fig: 1).

10.  Measure the Optical Path & make sure that the pump and probe path lengths are same, and make the output beam paths parallel (both Pump and probe), If both are not parallel then make it parallel and recheck the alignment properly.

11.  Put one Plano convex lens (15 mm) in to the parallel path such that Foci of the pump & probe beams are same.

12.  Put one thin BBO (SHG crystal) in to the foci and rotate the BBO such that in the middle of the two beam one extra signal is generated , which is SHG signal, if this signal is absent then move the motorized stage such that the SHG signal is generated, if not recheck the alignment .

13.  After SHG generation keep a Photo Diode (PD) into the SHG path and collect the SHG signal through digital Oscilloscope interfaced with GPIB to the computer.

14.  Move the Motorized stage and collect the SHG signal, after collecting data fit it into Gaussian and measure the Full width at Half Maxima (FWHM) which is the pulse width of the laser pulse.

FIELD AUTOCORRELATION:

1.      Turn ON the key Switch (from OFF to ON position).

2.      Wait for 10-15 minutes.

3.      Open the Shutter by pressing the shutter open switch and then press the power level 2 switch.

4.      Switch on the Chiller.

5.      Wait for 40-45 minutes for stabilization of the laser; put a power meter in the optical path to measure the average power of the laser and then remove it.

6.      Make the laser from CW to Mode Locked Condition.

7.      Put one ultrafast thin Beam Splitter (BS), which divides the total optical path into two parts. One path is called pump path and another is called probe path.

8.      Put the Retro on a motorized stage in one path (Probe path) such that the beam height at the input and at the output paths are same & the beam path distances of the input and output paths are same up to 4-5 meter distances. If the beam paths are not parallel then recheck the alignment.

9.      Put another ultrafast thin beam splitter in the crossing point of the pump and probe beam such that they are fully overlapped in the transmitted and reflected path, if they are not properly overlapped then recheck the alignment properly.

10.  Align the laser path according to schematic diagram (Fig: 3).

11.  Measure the Optical Path & make sure that the pump and probe path lengths are same. When both the arm are of same lengths then we can see the fringes by putting a white paper after the second beam splitter, if fringes are not visible, recheck the alignment properly.

12.  Put one Plano convex lens (15 mm) in to the transmitted path.

13.  After checking the fringes pattern keep a Photo Diode (PD) into the optical path and collect the signal through digital Oscilloscope interfaced with GPIB to the computer.

14.  Move the Motorized stage and collect the signal, after collecting data fit it into Gaussian and measure the Full width at Half Maxima (FWHM) which is the pulse width of the laser pulse.