1. Milano Laser Pulse Shaping group
< 1 ps
(800 nm)
(266 nm)
0.5 mJ
This is the talk of the Milano laser system group.
Out gol is to obtain a rectangular pulse in third harmonic of 10 ps with rise and fall time less than 1 ps for sparc laser system.
Beforetoday I talk only about the shape of the first harmonic, today I will introduce the problem of the laser pulase shaping in connection with the third harmonic egenration.
S. Cialdi* , I. Boscolo , F. Castelli , A. Flacco , C. Borghi, A. Ostini

2. Pulse shaping and third harmonic
Outline
Milano Laser system
Pulse shaping and third harmonic
Mask calibration
This is the outline
First I will talk about the milano laser system. In milano we are building a laser system to develop the pulse shapering technology useful for SPARC.
Than I will talk about the problem of the laser pulse shaping in connection with the third harmonic generation, this is a very important point for SPARC.
And, at the end, I will talk about the measure that we made in out laboraory in Milano about the liquid cristal mask calibration for the pulse shaper.

3. Milano Laser System Project
This is the milano laser system sketch.
We have a Nd:YAG laser that generates a pulse of 100 ps. Than we have 500m of monomode optic fiber. With this opt. Fiber we are able to broaden the spectrum of the pulse. Than there is an optical compressor. At the output the compressor the pulse i only 1 ps. And with this short pulse we can test the technology for the laser pulse shaping. Here we have the 4-f apparatus with the liquid cristal mask for the pulse shaping and hire the crystals for the third harmonic generation.
This is our Nd:YAG laser, this is the opt. Fiber, the compressor, we develop also a program to simulate the pulse propagation through the system fiber/compressor. The liquid crystal masch for the pulse shaper. And the crystals for the third harmonic genration.

4. Laser pulse propagation in Optical Fiber
This is our program to simulate the propagation through the optic fiber.
These are the amplitude spectrum and the spectral fase funcion of the pulase at the uotput of the fiber.
Split-Step Fourier Method

5. Laser pulse compression
This is our program to simulate the optical compressor.
With this program we can simply define all the parameter of the compressor and also to test the sensitivity of the system about the disallaiment.
And this is very important.

6. Milano Laser System This is the milano laser system.
This is the laser, the fiber, the compressor, here we can see the autocorrelator for the measure of the pulse length, and here we have the spectrometer to measure the pulse spectrum at the output of the opt. Fiber.
Here we can see the RF apparatus to drive the laser mode-locker. This is the RF generator, this is the amplifier and the power on/off controll developed by Claudio Borghi.

7. Harmonics generation effects relative to the pulse shape1w 2w 1w 1w 2w
Now I will talk about the hermonics generation effects relative to the pulase shape.
At the moment we studied the effect relative to the second harmonic.
Here we can see the simulation about he second harmonic, we made these simulation with a program developed by Fabrizio Castelli and based on the method of the finite difference,
This is the first harmonic and here we can see the intensity rofile of the second harmonic for different crystal lengh, 200 micron, 400 micron and 600 micron.
We can see that for thin crystal the shape is good but the efficincy is low. Instead for thick cristall the efficincy is good but we have a deformation of the pulse.
So we can’t neglect this effect, and we have to compensate it with the pulse sheper.

8. Phase-matching Bandwidth
Here we can see the interpretation of this effect in second harmonic.
The point is that the power of the second harmonic increase with the crystal length, but the crystal is also a filter for the spectral components far from the
central sepctral component.
And also this filter effect increase with the crystal length.
Here we can see in wich way the crystal modulates the spectrum of the second harmonic.
For thin cristal (200 micron) we can neglect the moulation on the spectrum, but for exemple for a cristal of 1 mm the modulation is big.
The point is the to obtain the maximum efficiency for the second harmonic generation, we have to obtain the phase matchin condition. This means that the index of refraction of the second harmonic and that of the first harmonic must be equal. But the index of refracion change with the frequency, and so we can obtain the maximum efficiency only for one spectral compoentn, and so we obtain a modulation of the spectrum.
So we have to compensate this effect and we can do it in defferent ways.
Phase-matching

9. New Laser system proposal
Phase Mask
It is possible to compensate the effetc of the laser system, the effects relative to the third harmonic generation but also the effects of the amplifier system, with this apparatus based on the genetch alghoritm and the phasse mask, on the pulse shape.
In this case the genetic alghoritm compare the target funciont with the pulse shape in third harmonic. This is possibile if we build a cross correlator.
So in this way the genetic alghoritm automatically fainf the proper spectral phase function on order to have a good pulse in third harmonic.

10. 4f-2g / Dazzler-2g Dazzler
We can compensate the effect of the amplifier and the third harmonic also with this apparatus.
In this case we have the 4f-2g apparatus or dazzler with a streatcher instead the 4f apparatus with the phase mask.
In this case alse without the genetic alghoritm is very easy to compoensate the distortions. Infact in this the intensity profile is similar to the power soectrum profile. So if we know the intensity prfile we can introduce the proper corrections to the spectrum amplitude modulation to comensate the distotions.

11. Mask calibration
Now I talk about the measere that we made in our laboratory to make the calibration of the liquid crystal mask.
In this photo we can see the apparatus for the calibration based on a He:ne laser, this is the mask.
And here we can see the result.
Now we can convert the phase in a number beetween 0 and 4095 that we have to send to the mask.
With lab view is very easy to control the mask and thee aren’t problems about this.

12. Conclusions Next step: 1 ps laser pulse
Harmonics effects: third harmonic and working point (L1, L2, w1, w2)
Mask
About the milano laser system the next step is to abtined 1ps before this summer. And also a first pulse shaping measure with the masklocated near the prism of the compressor.
And about the harmonics effects we have to developed the program to obtain the effects in third harmonics. In this way we will able to fainf the proper length of the crystal and the proper beam waist in the crystal.