Given your specifications of ~1ps 5mW (cw averaged) and 76MHz rep rate you have quite little pulse energy / intensity.*

For a common beam diameter of about 800µm you will need a f=+100 to f=+200mm lens and a ~5-10mm long BBO crystal to get a few µW of SHG. (Adjust the rayleigh length to the length of your BBO).

If you focus harder you will gain in efficiency by the higher intensity but loose effective length by spatial walkoff. If you havent you could take a day or two to download the free program SNLO (I am not the author) and simulate your process in 2D SP MIX to optimize the relevant quantities.

There are also a lot of good books on nonlinear optics. Given your plan to set up an autocorrelator I would recommend Rick Trebinos's book on Frequency Resolved Optical Gating wich also has a nice primer on nonlinear optics and also is a very good basement for interpreting autocorrelation traces.

Please be aware that even if you know the cut, the error margins on the cut angle are big enough to prevent phase matching for BBOs that are longer than a few hundred µm. So you WILL have to optimize the angle. The polarization sould of course be adjusted first.

If you do not have any clue about the cut of your BBO it might be helpful to request about 100mW of the Oscillator for a day and caracterize the BBO first. The cyan spot of ~1µW SHG will be barely visible when you adjust the SHG for the first time. It also helps to prealign the device with more power first and then reduce it to the desired SNR.

Please apologize the lack of quotations for my statements, I had only very limited time for this answer.

PS: It is always helpful to state all relevant physical quantities as for example beam diameter, focal length etc.

* It is also possible to get high efficiency SHG with ps-pulses but in my experience only if you start with 10+µJ or use a resonator. Keep in mind that you anyway do NOT want to drive the efficiency in saturation for a pulse measurement device.

It is advisable to look at the scheme of installation and think.

Best Regards

Thanks Nikolay. I followed the interferometric setup in the link below. We are not sure about how the non-linear crystal was cut (currently we assume it was cut in the c axis, but it won't work). The manual of the non-linear company gave us so little information.

https://www.rp-photonics.com/autocorrelators.html

You might want to check if the polarisation of the two beams are correct. You can use a waveplate before the BBO to rotate the polarization and maximize the signal.

with the output being quite in the center of the human visible range, the human eye in a completely dark room is likely more sensitive than most detectors. Further as has been mentioned the polarizations of the beams are important ( i am assuming you are trying oo-e) and the phase-matching condition can be quite critical, depending on the width of your laser spectrum, so some slight angle tuning of the crystal can help. If you have a laser at similar wavelength available with higher intensity you could also first try to set the crystal up with that.

If you think that focusing, overlap, phase matching is achieved can you try increasing the input power?

We have a Ti:sapphire laser (100 fs, 80 MHz) and we use pretty high power (700 mW) to generate SHG (BBO) power (~120 mW). This is 20% efficiency. Since SHG intensity is square of fundamental intensity I have no experience how much power is required to generate SHG from a ps laser.

Another point, are you sure that the path lengths (delay) of the two arms are exactly same? Is there any way you can try with some other laser sources. You can also try to bring the two arms of the input closer to each other (if that is possible) to increase the chance of spatial overlap.

You need to state the energy of the beam going into the BBO rod. If the beam is highly focused going in, you may have problems! Check the efficiency of the conversion so the output intensity is within your measurement instrument sensitivity. The 20% efficiency quoted above (by Tuhin) is quite high, you may get a few percents. It is assumed of course that you have a good quality BBO and setup.

You can also attempt it in a different way. Block the beam path with weaker intensity and try to get SHG from the other arm itself. Here you do not have to be worry about spatial overlap. But remember that the phase matching angle will  be different. By this way you can also adjust the focus. If do not get any output try adjusting the power. After you get the SHG you have to just (after allowing the other light beam to pass) do the spatial overlap (which you can check with a telescope) and look for the phase matching angle.

Given your specifications of ~1ps 5mW (cw averaged) and 76MHz rep rate you have quite little pulse energy / intensity.*

For a common beam diameter of about 800µm you will need a f=+100 to f=+200mm lens and a ~5-10mm long BBO crystal to get a few µW of SHG. (Adjust the rayleigh length to the length of your BBO).

If you focus harder you will gain in efficiency by the higher intensity but loose effective length by spatial walkoff. If you havent you could take a day or two to download the free program SNLO (I am not the author) and simulate your process in 2D SP MIX to optimize the relevant quantities.

There are also a lot of good books on nonlinear optics. Given your plan to set up an autocorrelator I would recommend Rick Trebinos's book on Frequency Resolved Optical Gating wich also has a nice primer on nonlinear optics and also is a very good basement for interpreting autocorrelation traces.

Please be aware that even if you know the cut, the error margins on the cut angle are big enough to prevent phase matching for BBOs that are longer than a few hundred µm. So you WILL have to optimize the angle. The polarization sould of course be adjusted first.

If you do not have any clue about the cut of your BBO it might be helpful to request about 100mW of the Oscillator for a day and caracterize the BBO first. The cyan spot of ~1µW SHG will be barely visible when you adjust the SHG for the first time. It also helps to prealign the device with more power first and then reduce it to the desired SNR.

Please apologize the lack of quotations for my statements, I had only very limited time for this answer.

PS: It is always helpful to state all relevant physical quantities as for example beam diameter, focal length etc.

* It is also possible to get high efficiency SHG with ps-pulses but in my experience only if you start with 10+µJ or use a resonator. Keep in mind that you anyway do NOT want to drive the efficiency in saturation for a pulse measurement device.

 Hi Tuhin Khan,

Why do I need two arms to have same intensity, I thought to have SHG, you only need 2 photons, one does not require same intensity on both arms to have SHG? Nevertheless, if both beams are equally strong, the SHG efficiency might be higher than uneven distribution of intensities. Because the final photon is recombining photons from both beams.

Dear James,

I do not recall or see mentioning that the intensity of the two arms have to be same. In first case I said "are you sure that the path lengths (delay) of the two arms are exactly same?" and later I said " Block the beam path with weaker intensity and try to get SHG from the other arm itself." which clearly means the intensity of the two arms are different. You are right, "one does not require same intensity on both arms to have SHG".

Hi Tuhin,

We got some SHG today, with just one beam, I am not sure why you said the phase matching angles would change in an autocorrelation setup with collinear beam. Could you tell me why it would change?

Good to know that you got some signal. The phase matching angle depends on the beam crossing angle  (for collinear arrangement it is zero).

For our setup (femtosecond fluorescence upconversion / femtosecond optical gating, BBO type I) for a 800/800 nm interaction the phase matching angle is 32.25 degree for 10 degree beam crossing angle.  For a 0 degree beam crossing angle (collinear) the corresponding phase matching angle is 29.02 degree. The phase matching angle has no practical significance and has to found experimentally for each setup. Also remember that the SHG direction for collinear and non-collinear geometry are different.

https://goo.gl/K9WTQ9

You can also think it this way, if the phase matching angle for both processes are same then the processes will compete with each other as a result it will severely undermine the purpose of autocorrelator (temporal overlap, non-collinear geometry).

Since you have got the SHG it means you have the right parameters in hand and you need to do the spatial overlap and phase matching angle. Don't forget that the delays have to be exactly same.

@Tuhin Khan Do you have any idea why the second harmonic generation in my experiment looks like an elliptical and has fringes in the middle? The crystal we use is a BBO crystal and a ultrafast picosecond Ti:S laser. We tried both fibre cleaning the mode before focusing onto the non-linear crystal, and we tried focusing onto crystal with different focal length, it doesn't help. This very weird spatial shape makes it very hard to couple into a single mode fibre. 

Are you sure that the laser is TEM00. If significant amount of different modes are present then you may see fringes. You will get poor 2nd harmonic because of the presence of such modes (because it has node at the centre). https://www.rp-photonics.com/resonator_modes.html

@Tuhin Khan, Yes. We mode filter the laser with a single mode fiber before that. Anyway now we are doing quasi-phase matching the result is much nicer.