Hello,

Q-switch typically associate with generation of ns pulses. Therefore there is no direct link of so long switching time to the laser radiation. Everything is finished long before the trailing pulse of the thing which produces Q-switching operation.

Here, I believe, you are talking about something else. For example about semiconductor laser (which pumps Nd:... laser) current on time. If so, you need to keep conditions "as designed" because semiconductor pump laser emitted energy is linked to energy dissipated in the "final" laser Nd:... Dissipated energy (most often average power) is proportional to strength of thermal lens which defines cavity parameters and as result divergence and beam diameter of the laser radiation.

On the other hand if the Nd:... laser is passively Q-switched there may be not enough accumulated inversion and (as result) light amplification gain to overcome attenuation of the unsaturated saturable absorber which performs Q-switching. The laser would simply do not generate. So, pump time is proportional to accumulated energy which is proportional to inversion which is proportional to gain of laser medium which must be grater compared to loses produced mainly by saturable absorber (if laser is passively Q-switched).

Different story is for the laser which is actively (electro-optically or acusto-oprically Q-switched). There is some freedom to change pump conditions in order to modify output energy, but not so much also.

Anyway, thermal balance is quite important to obtain certain thermal lensing which is criteria for optimization when designing the laser (except if the laser is flashlamp pumped running at very low frequency).

Ok, preset answer is quite messy, but it is correct. Keep the laser running at specified parameters (you can modify a little bit if it is actively Q-switched) and use dielectric coating beam splitters (mirrors) to attenuate. It may happen that power dissipated in the glass filter forms thermal lens which distort beam.

Gintaras

Your question is not clear. What repetition are you mentioning? 200us is a little bit long q-switch opening time for Nd:YAG lasers. ND Filter - is it inside laser resonator or outside laser resonator? Changing q-switch opening time can change many things such as Pulse Width, Beam mode, Pulse stability, power stability, giant pulse, and others,     

Please provide more information about your laser...

You use a active Q-switch laser and the 200us is the time delay?

in that case, i assume, that the recommendation of using highest pulse energy is just to avoid situation of unstable population inversion.

R

Nam and Ran,

It is a passively q-switched system shown here (The Surelite I-20): http://www.continuumlasers.com/files/products/surelite/SureliteI_II_III.pdf

Hello,

surprisingly no information about the laser is on the web. Indirectly and by you words this should be flashlamp pumped passively Q-switched laser. In that case better keep it running as specified. Passive mode locker is quite sensitive to variations of pump energy. You can still have Q-switching with lower pump energy, but pulse duration will vary as well as delay (referencing to electric trigger).

Energy is massive - half joule. There are definitely some problems handling such a beam. Talking about result you want to obtain - variable output energy - I would recommend waveplate-polarizer attenuator (WPMH10M-1064 and PBSW-1550 from Thorlabs) (461-4205 and 420-1258E from eksmaoptics.com).  There are assemblies with waveplate and polarizer at eksmaoptics.com also. As prescaler - beamsplitting mirror is recommended which should be mounted before attenuator which is less robust to damage and environment (dust and fingerprints :)  ).  BST11 from Thorlabs or 042-7170A from eksmaoptics or similar can be used. Choose one taking care of double reflection. Often wedged mirror is the best.

Do not use metalic coated variable transmittance mirrors - layer will evaporate.

Gintaras

Gintaras

Pulse duration will vary? I thought laser pulse duration had more to do with cavity length. How does it change with q-switching?

It seems that your laser should be a flash pumped Nd:YAG laser with  an active Q-switching.

Passive Q-switch will not support the population inversion for such a huge laser pulse. 

The 200us seems to be the delay time after the flash lamp and you should keep it at 200 for Nd:YAG, whose excited state has a comparable life time.

Changing the optimized Q-switch delay will affect the pulse Energy and the pulse width of your output.

From my experience with such a Laser (i dont remember the exact model), you can definitely control the output power by increasing or decreasing the time delay. the pulse temporal length is independent of the time delay. 

Anyway, note that the pulse is not very stable in terms of energy (comparing one pulse to another), and can turn to be 'spiky' (very short and strong spikes my appear ontop of the pulse, it will be unnoticed unless your detector is fast enough).

Hello,

sorry for not following this discussion. I thought it was finished...

Pulse duration of Q-switched laser depends on: cavity length, losses of the cavity, modulator absorbance. I am not sure does it "mathematically" depend on pump energy. Formally it should be invariant: depends when gain is reached to overcome losses and generation in tact with Q-switching appears. That moment is assigned for the same inversion value in the laser medium. However in reality things are bit more complicated. Delay which vary depending on pump causes different thermal lensing and change of cavity parameters. There is also certain delay (which becomes more significant for longer cavity lasers) while initial generation builds up. In case of high pump some additional energy deposition (not a lot) will be fast enough to take place over few microseconds. This just slightly increases gain of the laser but causes measurable shortening of the pulse. Anyway, the result will not be like 10 times. We are talking about ~10%. This is a lot if you are interested not only in energy deposition but in effect depending on intensity.

And do not forget comment of Ran - stability decreases a lot closer to threshold.

Gintaras

Of course, Q-Switch delay will affect the pulse duration, shape and characteristics.

When the Q-Switch is off, You will not have any laser oscillation, and the population inversion will be affected with this delay.

The shape and width of the pulse depend on the initial population inversion when your Q-Switch is on.

Gintaras, basically you are right.

But, in these lasers you can expect more than 10% pulse-to-pulse variations also in the optimal delay (i.e. max power). Greater instability will appear by changing the rep-rate, due to thermal stability issues you mentioned.

Apparently, the thermal lensing variation are negligible once you reached a certain population inversion. If thermal lensing is so crucial you will not be able to vary any laser output power.

Also, i do not see how all the above mentioned influence the pulse temporal length.

Hello,

OK I might be wrong. My experience is with laser with relatively long cavities. It may be that for cavities

The flash duration in flash pumped Nd:YAG lasers is around 220us. So, the Q-switch delay is crucial to obtain an optimal output.

you can control the output power by increasing or decreasing the time delay, but  the pulse characteristics will vary also.

It's recommended for spectroscopic applications that the pump power and Q-switch delay remain constants and to change the power by a beam splitter or a special filter.

Hi

Gintaras - i dont have this laser anymore. 

Benjamin - please measure and share the results.

Is the Q switching method active( EO, mechanically or acoustically) or passive method by dye?

One thing to consider here is the difference between trying to create a giant pulse with active q-switching has a much longer delay than passive. It takes time for the active switched method to build up gain from several resonator passes, AND that from a steady state CW pumped crystal and the need for first pulse suppression else the giant first pulse can destroy crystals. Also if the timing is off then you can get relaxation oscillations as someone else described above, multi-pulsing and other non-stable phenomenon.

Truth is that, as you change the q-switch delay, the energy also changes. However, from 200 us down to 180 us, the laser ionisation is high and therefore favours more of ionic and atomic signals that overshadow greatly the molecular signals. So depending on ur sample and what exactly u wish to analyse, if u are interested in molecular fingerprints like in the case of polymeric materials, I will suggest you set ur q-swish flashlamp delay above 200 us.