Hello,
I'm trying to understand how one can input a laser light into a cavity, bow-tie for example, for mode cleaning or SHG/OPO light generating.
If all the mirrors are high reflectivity, that means that barely any of the laser light will be coupled. Even if I drop the reflectivity of the input mirror by a few percents, I'm still losing most of my power, as it seems to me.
Please suggest some material to read. I've read a ton of paper of SHG generation in resonant cavities, but nobody supplies any details.
This is a classic fundamental problem, and there isn't a magic way around it. How you deal with it depends on what you are trying to do.
You specifically mention SHG and OPO. In both those cases the answer is simple. The entrance face let's all of the pump light in. You then try and get as much conversion as you can before the remaining pump light escapes back out either the same face it entered, or the other end. Making the far end reflective and double passing the pump gives a longer conversion length to get as much of the pump converted as possible, but is often less convenient both for mechanical layout and complexity of optical coatings. Generally SHG is easy and there is no need to double pass. On the other hand OPO conversion is often less efficient and double passing the pump is typical. (In very high power laser amplifiers, the parametric conversion is efficient enough that you don't reflect anything. everything happens in one pass and then it is called an OPA)
The cavity of the OPO is then resonant (reflective) NOT for the pump light, but only for the signal light, or the idler, or "doubly resonant" for both. Because the intensity has to be very high for efficient conversion, laser damage thresholds of the optical coatings are the main design limit. Often we will only make the signal resonant to keep the coatings simple and the damage thresholds high. Many non-linear crystals have a high absorption for the idler anyway, and so you don't necessarily lose much. On the other hand coating technology has improved, and materials which are highly transparent to the idler are more readily available (RTA for example) and you can get much better efficiency with a doubly resonant cavity. However, you can't keep the pump light in the cavity forever. It either single passes, or with one reflective coating double passes, or by flipping the polarization you can even make it 4 pass. But you can't keep the pump in.
There is one way to keep the pump in the conversion cavity, and that is by making it there. You can make a laser cavity for the pump light that is essentially 100% reflective at both ends. You put the conversion crystal in the same cavity, and you can make the cavity reflective for the signal and idler as well. You leave one mirror partially reflective for the signal, and only the signal leaves the cavity. This approach is a little dangerous because when the conversion crystal is misaligned during construction the energy does not leave the cavity and it is very easy to damage the system. So you make one mirror only 90 or 95% reflective for the pump wavelength and during alignment you slowly ramp up the power only going to full power when you have it very well aligned.
Further Mike's comments, please see the following link of one thesis which could give more details
https://eprints.soton.ac.uk/210235/1/Kienle_Thesis.pdf&ved=2ahUKEwjt873Ru8vtAhUHT8AKHX1SAmQ4ChAWMAh6BAgFEAE&usg=AOvVaw33mpxezX62kgt5t2g8lj2s
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