Why Coherent uses low gain

[atomd]

Recently, I looked at construction of countless CW DPSS lasers (Adlas DPY, Coherent Verdi to name a few) and they all work on a very wide beam in crystal with very low gain. I'm curious why is it so, as I see many downsides of such construction:

• lower efficiency do to lower losses to output coupler transmittance ratio
• require very expensive coatings to get very good or very low reflectivity
• high threshold
• intracavity doubling requires higher focusing that brings resonator closer to stability edge

I guess the big upside is lower alignment sensitivity and lower thermal lensing, but is it really that important? This lasers have crazy sturdy construction anyway.

[mixedgas]

In DPSS, low gain = lower RMS noise and easier mode control.

Google "The Green Problem" for DPSS lasers

chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://novantaphotonics.com/wp-content/uploads/2021/12/NOVT_Whitepaper_Noise_Measurement.pdf

low mode noise = important for things like Ti:Saph pumping and Quantum Physics.

In frequency doubling for example, a very poor design can limit the green significantly, It took 20 years for 532 nm lasers to become efficient.

Steve

[atomd]

Single mode is important for many things, but not for pumping other DPSS lasers (OPSLs pumping TI:sapphire are often multimode) so single mode control isn't crucial everywhere.

Can you elaborate on the green problem in this context (I read the article about someone trying to build DPSS but can't find it now)?

I'm trying to build intracavity doubled Q-switched laser so by design it'll be largely multimodal.

[mixedgas]

Be very careful that you do not confuse longitudinal and transverse modes.

Yes, in a highly tunable femto ring ti:saph, amplitude and both transvers and longnitudal mode noise in the pump can cause Issues when Kerr Lens Modelocking. I spent a good three semesters with Mira and Chameleon at one university and a lot of time with Mira [years] at another.

Published Works by
Robert Byer, Vittoria Magni, and Tony Siegman's book are what you seek.


The green problem manifests itself as high losses in the doubling crystal. Chaotic time domain effects in the crystal tied to polarization, pump adsorption, and changing local electric field.

Cures involve crystal cut axis, wave plates, and careful design of longitudinal and transverse modes.

ST

[mixedgas]

[QUOTE=mixedgas;365223]Be very careful that you do not confuse longitudinal and transverse modes.

Yes, in a highly tunable femto ring ti:saph, amplitude and both transvers and longnitudal mode noise in the pump can cause Issues when Kerr Lens Modelocking. I spent a good three semesters with Mira and Chameleon at one university and a lot of time with Mira [years] at another.

Published Works by
Robert Byer, Vittoria Magni, and Tony Siegman's book are what you seek.


The green problem manifests itself as high losses in the doubling crystal. Chaotic time domain effects in the crystal tied to polarization, pump adsorption, and changing local electric field.

Cures involve crystal cut axis, wave plates, and careful design of longitudinal and transverse modes.

"Super moding" Where huge amounts of longnitudal modes are lasing in the cavity, ie Spectra Physics Millenia and Magni et All's design were a later way out.

Steve

[atomd]

Right, now that I think about it Kerr lens modelocking should be sensitive to amplitude and traverse mode fluctuations as they change gain distribution, but how longnitudal mode noise influences it?

Just to clarify what sources you are referring to:

1. Tony Siegman's book is "Lasers"? I read it and it's really nice introduction, but it's missing many parts required to fully understand lasers
2. By published works you're referring to these people: https://ntt-research.com/phi-people/byer-profile/ and whom? I was unable to find anything that looked relevant

Also, where can I find anything more about green problem? Surely there's another way around it, as there are countless intracavity doubled Q-switched lasers (so by design transverse multimodal)?

[mixedgas]

ScienceDirect.com
https://www.sciencedirect.com › vitt...
Vittorio C. Magni

Not all his papers are listed there.

THE one you want is actually a lamp pumped YAG paper.

The methodology of Spectra Physics old model Millennia V is what you want. First of the tight 532s with good mode quality and small diameter beams.

Steve

[mixedgas]

The longitudinal modes do frequency mixing / beating against each other. The mixing products cause problems in the doubler, especially if there is excess nonlinearity.

I'm going from memory as these were all 1990s problems for me. I had a 150 watt lamp pumped laser that I wanted to convert to green. Problem was all existing designs produced fat, q-switched ,highly divergent , and excessively noisy beams that would not be useful for projected graphics. A team at Spectra Physics and Magni's team solved the problem nearly simultaneously.

Magni probably published first, but Spectra shipped the product first... There was a bit of a controversy at the time.

In the past few years on PL we've had a few talented. Grad students and one Engineer come to PL asking for help on building high power green. Most of them were hitting only 30 percent of design goals.

When you phrased your question I had a "senior moment" and realized I had forgotten the nature of the problem they had.

One other thing. If using multiple fiber coupled pump diodes on a ring cavity, make sure you have an optical isolator on the pump system. One of the brightest men I know found out the hard way that diode one would burn the output fawcett of
diode two when diode two was started. Conjugate focus at the diode face due to cavity symmetry.

He killed two 700$ butterfly packages before he realized what was going on.

The publications make it look easy, but there is an art to making a good Dpss Green.

STeve

[mixedgas]

HERE:

I used to know him personally. I had a bit of hand in helping him make some design choices on this beast.

https://youtu.be/Pp9hliLJjg4?si=vuP17YP9ZEV0Rf7H

He later switched to a "capped" crystal with undoped ends. That reduces the lensing even more.

STEVE




Steve

[atomd]

The longitudinal modes do frequency mixing / beating against each other. The mixing products cause problems in the doubler, especially if there is excess nonlinearity.

I'm going from memory as these were all 1990s problems for me. I had a 150 watt lamp pumped laser that I wanted to convert to green. Problem was all existing designs produced fat, q-switched ,highly divergent , and excessively noisy beams that would not be useful for projected graphics. A team at Spectra Physics and Magni's team solved the problem nearly simultaneously.

Magni probably published first, but Spectra shipped the product first... There was a bit of a controversy at the time.

In the past few years on PL we've had a few talented. Grad students and one Engineer come to PL asking for help on building high power green. Most of them were hitting only 30 percent of design goals.

When you phrased your question I had a "senior moment" and realized I had forgotten the nature of the problem they had.

One other thing. If using multiple fiber coupled pump diodes on a ring cavity, make sure you have an optical isolator on the pump system. One of the brightest men I know found out the hard way that diode one would burn the output fawcett of
diode two when diode two was started. Conjugate focus at the diode face due to cavity symmetry.

He killed two 700$ butterfly packages before he realized what was going on.

The publications make it look easy, but there is an art to making a good Dpss Green.

STeve

My current plan is to use a set of 808nm MM diodes, knife edged along fast axis to get beam with roughly similar M^2 in both axis (I convinietly aquired FAC diodes with M^2 of 1.3 x 15 with divergence in similar ratios) and possibly PBS combined if I need more power. I know that pumping from both sides is complex (and isolators are surprisingly hard to get), though I can't see where the isolator in Verdi is.