Hello --

If you just want to check spatial and temporal overlap of two beams, all you need is a crystal supporting sum frequency generation (SFG) of the two input beams. You can search online for the correct crystal and phase matching angle for these wavelengths, or you could calculate them yourself very easily using some free software like SNLO's Qmix function. In any case, oftentimes the required crystal cut isn't too different than one you might use for SHG (like in your FROG), so you can often use a crystal you already have (e.g. BBO or KDP) and just tilt/rotate it a little bit to get the desired angle. (Be sure to pay attention to your desired polarization to know if you should be targeting type-I or type-II phase matching! Also, if you reuse a crystal, be aware that some crystals might be coated specifically for use with certain wavelengths, so be aware you might incur some extra losses by using unintended wavelengths.)

Once you've got the right crystal set at the desired angle, align both beams to the crystal using your favorite IR card/viewer in order to ensure spatial overlap. For the beampaths themselves, measure out as carefully as possible the propagation distance of your 800nm and 1040nm beams to get their lengths starting as close as possible to each other. (It would also be wise to use a photodiode and an oscilloscope to directly view the arrival time of your two different pulses, which should easily let you get them overlapped to well within a foot/nanosecond of each other if you've got a diode and scope with halfway decent bandwidth.) In one beam, incorporate a roof mirror on a delay stage so you can easily add and subtract propagation distance without changing the pointing (and therefore the spatial overlap); this will allow you the fine tuning needed to achieve temporal overlap at the fs level (imagine that, for a 200 fs pulse, your path length needs to be accurate to within a few tens of microns (200fs * c (0.3um/fs)) at the very most). To find the temporal overlap at first, adjust the micrometer of the delay stage to the end of its range, and then manually push the stage back and forth, quickly scanning multiple times across the full range while looking at the output for flashes of blue/violet (the sum frequency should generate ~450nm light). If you see the flash, eyeball the position of the stage where you get this temporal overlap, and adjust the micrometer to the position until you can fine tune the temporal overlap with your delay stage, and you're done!

(If you don't see the blue flashes, any number of things could've gone wrong: Do you still have spatial overlap? Do you have the right crystal and polarization? Are the path lengths still too mismatched? Are the intensities of your beams too low to generate the nonlinearity? Is there an underlying synchronization problem? Is the individual jitter in timing/pointing/path length/etc. too large to overcome?)

(All of this assumes that your two beams share a common source/are mutually coherent; if not, none of this will work under normal circumstances.)

(An SHG-FROG is usually designed to accept a single input of a pre-designed wavelength and pulse duration, so the purpose of the device is not specifically to look at overlap of two different beams.)

Good luck!

~Eric

See the interference pattern.

Interference does not work since you have two different wavelengths. But the method, Eric explained, works fine.

Dear Eric,

Thank you so much for your complete guidance. The current crystal I have is coated specifically for 800 nm laser pulse, so I don't think It could be used for the other beam. I am familiar with SNLO's software but it didn't come to my mind that it could help me for this problem. I am trying to generate a white light with the 1040 (with around 400-500 nm bandwidth ) so I'm trying to find a crystal that would show me where the white light and the other beam overlap with each other.

I have also measured the path length of the beam which showed about a meter offset, now that I have corrected that delay line it is difficult to separate the two beams with the oscilloscope ( by separation I mean the peaks are not being showed as they were before, it is showing a pulse train with on peak in each pulse, which I guess its showing that the pulses are overlapping now in temporal domain), I believe that as you mentioned I have to experimentally find the overlap with the adjustment of the delay stage, is that correct?

Again I really appreciate your kind and complete responce.

Thanks,

Shahriar

Omid and Nikolay,

thank you for your responses as well.