Determining Q-switch operating frequency

[macona]

With the two other lightwave lasers I have I also acquired a Spectra-Physics YHP70-106QM head unit that is an engineering sample. I plan on running it off a couple Coherent FAP modules I have. Real basic head. End pumped Vandanate q-switched head. Nothing special here. The problem is the Q-switch. I have no idea what frequency it ran at. I know SP used 41Mhz and possibly 80 Mhz drivers. I have no idea which though for this one. The q-switch in the head is different than the ones I have seen in similar heads.

Any ideas on how to figure out what this thing ran at? Try a driver and watch for reflected energy on the rf?

[Laserman532]

Dont know for sure...but 41 mhz sounds high to me...41 mhz was the frequency that we mode locked the ion lasers for sync pumped dye laser applications. For q switching (at least for green yags) we used about 10 k to 25 k hz. Steve will probably know more but i think for q switching applications you will need to give the cavity more time to build up...you need to balance the cavity gain, recovery time with q switching speed. Sorry im not more help but the 41mhz thing jogged my memory (which rarely happens)

[planters]

The 41Mhz may be OK as this is for the RF signal which is modulated at the 10's of Khz range. The big 'ol LS rods 10-25kHz, the several W ( average ) Coherent markers up to about 100Khz. The crystal will have to be driven at a RF that it was designed for and I'm not knowledgeable enough to know how out of spec is OK. I am curious though, the Q-switch module might have a performance plate that identifies the required input power/frequency/wavelength etc?

[macona]

Laserman, the q-switch has a carrier frequency that activates it. Most modern ones are between 27MHz and 80MHz. Apparently the higher carrier frequencies are better for shorter beam paths. The head I have is actually pretty long, over 2 feet. This RF signal is modulated and thats what turns the q-switch on and off. No RF and the beam passes though, RF and it is deflected.

The Q-switch is inside the head which is pretty much sealed. I did peek and there is nothing marked on it, nor on the one that I took apart after they started scrapping. I have a NEOS Q-swiitch driver that came out of a SP power supply that was similar to the ones that pumped these heads. Just don't know for sure.

[Laserman532]

Oh yes, I should have considered that you might be talking about the carrier frequency, which for the devices i use is 27.12mhz. I had the PRF in my mind, sorry. We did however did use 41mhz to create a standing wave in a 171 ion laser for modelocking. We would adjust the frequency while watching output power (dip) to match the resonance of the prism assembly (highest depth of modulation) then adjust the cavity length to create a train of short pulses (increasing power) (approx 200 picoseconds) at approx 41 mhz...(some cobwebs are clearing)

[planters]

Laserman532,

You are the LS guru right? What about simply replacing the entire Q-switch assembly in the lightwave with a LS Q-switch/driver assembly as this would also include the LF standoff function (prevents cavity dumping). How much would this cost, hundreds?

[mixedgas]

Here is the rub, you need to know the carrier frequency or the crystal blows its piezo transducer to smithereens.
If you have a modern driver, the driver might live, dispite the bad HIGH VSWR reflection. Being 5 Mhz off in some cases is enough to cause problems.

Common license free freqs used are 27.125 (Aka cb channel 14, the Cbers love this, I'm sure! )

Qswitch frequencies I've seen:

27.125
40.66 ( some pump lasers for dye and modelocking ion/yag for used for a cavity resonance reason)
48.1
50.00
68.xx
76.0 (pump lasers for dye and picosecond systems, used for a cavity resonance reason)
80.0
110.00
200.00



13.553 MHz to 13.567 MHz , 26.957 MHz to 27.283 MHz, and 40.660 MHz to 40.700 MHz are popular because they are "ISM" frequencies dedicated to be designated electronic wasteland for industrial use. You wont see a 13 Mhz AO though, too slow, so the crystal would have to be huge.
Not all AOs are ISM, some piggyback on frequencies used for other services. Xerox AOs from printers are right in the heart of the Air Navigation bands.
So I go out of my way never to run a xerox driver without a load to adsorb the RF.

80.0 is popular because it used to be ISM, makes for a good small spot size out of the AO cell, and the use carried over to modern use.

I have the frequency sweeper and calibrated wideband bridge needed to test the AO cell. I can test it with a few milliwatts, which will not blow the cell. Takes 10 minutes on the bench. I can sweep 0-250 Mhz in a few minutes. I can also sweep 2 Ghz to 22 Ghz, but that is outside the scope of this descussion.

These cells need anywhere from 0.25 watts to 50 watts drive, depending on the Q-switch.

Otherwise you have to send it to Neos-Gooch and Housego, or ISOMET for a sweep. And that is not going to be cheap.

Does the Q-Switch have water cooling? If water cooled, it is usually 27 to 68 Mhz, because RF power amplifier transistors for this band are cheap.

If you shutter the intracavity light at a N/2 frequency of the cavity longitudinal mode spacing in Megahertz, you can modelock and make short pulses with some lasers. That is what Laserman532 is talking about. It is becoming the bain of my existance in this lab, so much so that my output coupler is on a micrometer "Z" stage to adjust the mode length of the laser to the AO.

76 Mhz is popular with Coherent, because of their larger ring dye lasers, the Antares-76 Doubled YLF laser, and some of their ti-saphs are all length adjusted for 76 Mhz for picosecond or femtosecond pulses.

TOO LOW A PRF and YOU GIANT PULSE, which may damage the laser. You may also need "first pulse supression" to avoid the stored energy in the laser crystal to not be released in one Q-Switched BIG first pulse. FPS ramps the RF power up or down depending on the model, bleeds the stored energy, then goes pulsed. Every time you blank/unblank, you may need FPS. This is why laser show AO blanking some Laserscopes(or Industrial yags) using the wrong AO driver can result in expensive damage to the KTP or Optics. This is not just a YAG thing, on Ruby can blow off parts of the rod.

Steve

[macona]

Thanks for the info.

Spectra Physics just sent me the manual for it and it specs it at 40MHZ at 10 watts. So it looks like I have the right driver after all. It does have the first pulse suppression feature.

-Jerry

[Laserman532]

I cant really add anything to what Steve said except...yeah what he said...

isnt it cool steve when you are watching the scope and tuning the cavity length then all of a sudden...BAM...a nice short pulse (200ps)

THEN...you are watching the autocorrelator scope signal and adjusting the length of a dye laser and then all of a sudden....BAM....2 pico seconds

THEN....if you are totally OLD SCHOOL and you are matching the length with a fiber grating pulse compressor then BAM....50 fempto seconds

THEN...if you want to get totally retarded....you can pulse pick and amplify!!!! MUAHAAAAAAAAAA

all of that train of pulses at 41 mhz....good times

Well that is some really old school stuff right there...it is probably all done in a laser pointer now.

[planters]

It does have the first pulse suppression feature.

That was my biggest fear with alt. sourcing a driver. Good.

Now, you have these neat lightwave lasers and you don't have a plan for them. OK, but what do you think is interesting? Machining? Pumping? Display? (I know maybe not with the 355)