During mod-locked laser generation, it is necessary to create phase matched condition. But when we place the nonlinear crystal within the cavity lots of green lights are generated but mode-locking is not happening. From green lightning, II have understood that upconversion is happening but I am not sure about the phase matching. In all total, I could not understand for perfect phase matched mode-locked laser generation, which points we should follow.
Can you please describe your system in detail. Best companion in such case is the user manual. In order to generate upconverted light first it has to made sure that the focus is perfect. After that you need to rotate the non linear crystal to get the phase matched condition.
Dear Rumba,
It is a well-known fact that the process of SHG strongly favors a single longitudinal mode operation [1]. It is virtually the opposite of a saturable absorber.
This is beneficial to operate a laser in single frequency operation. For a mode locked laser you should place the crystal outside the cavity.
Greetings,
Karsten
[1] T. Baer, J. Opr. Soc. Am., B3, 1175 (1986).
Dear Tuhin Khan, thank you for your kind response. Actually, we are trying to build mode-locked laser by side-pumped Nd:YVO4 gain medium oscillating at 1064 nm from a z-cavity. However,t by tuning the NLC (KTP), whenever the maximum fluorescent of green light is generated, the output power increases and a small reflection part of that light is misplaced from the path of oscillation which does not follow the gain medium rod. Moreover, if we try to minimize that reflection for maximum phase matching, the output power automatically goes down. I could not understand which one should I follow for perfect mode-locked laser generation.
Dear Ruma,
I am very sorry I got your name wrong. However, you can believe me intracavity SHG prevents mode locked operation. This is simply caused by the fact that mode locking increases peak power. This increases losses for the peaks and suppresses them.
All the best,
Karsten
Dear Karsten Schuhmann, thank you for your kind reply. In our case, we are using a side-pumped configuration of a gain medium having several wavelength coating. And we are using mirrors having coating of1064 nm. In this case, should we place the crystal outside the z-cavity, please suggest? If we place the NLC outside of the cavity, then how the concept of "Nonlinear mirror mode-locked laser" will be fruitfull, kindly illaborate us.
Dear Karsten Schuhmann, It's ok for misspell. I really don't mind. However, there are so many works articles in different journals regarding "Nonlinear mirror mode-locked laser" where they have shown the NLC is placed inside the cavity. I am confused now and what should I do, kindly suggest. Please show me the way.
Dear Ruma,
in this cases, the nonlinear effect is used to reduce the OC-rate. Therefore, the nonlinear effect provides mode locking. In SHG, the nonlinear effect increases the out coupling rate. Therefore, the nonlinear effect prevents mode locking.
Normally single pass SHG is not an issue for the output beam of a mode locked laser.
All the best,
Karsten
Dear Ruma,
in case you are still interested in using SHG for mode locking there is a recent publication I just came across:
Modelocking of a thin-disk laser with the frequency-doubling nonlinear-mirror technique
They use the SHG for mode locking. However, as I pointed out they use the SHG to reduce cavity out coupling, as the green light is stored in the cavity and not extracted from the cavity and converted back to 1030nm.
All the best,
Karsten
Thank You Mr. Karsten Schuhmann. I will go through the work. Sorry for late respond.
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