If you have a pulsed laser then, by decreasing the spot size you could increase the peak laser fluence and peak power but the pulse energy would remain same.

Since energy conservation holds also for lasers you have no chance to increase laser power, however reducing the spot beam would actually increase irradiance (or radiant exposure) on the target. Any optical component will introduce some loss but you can choose the one with minimum absorbtion at your wavelength.

The only issue I see is that you will have also a defocusing of the beam beyond the "waist" that is beyond the minimum spot distance but this could be not relevant for your experiment

If it is a pulsed laser with a regular repetition rate you may be able to increase the energy per pulse using a reflective tuned cavity via coherent pulse stacking. For an example, see 

https://www.osapublishing.org/DirectPDFAccess/249E8F8E-9A28-05C1-DEC1520784BA9A1E_312940/oe-23-6-7442.pdf?da=1&id=312940&seq=0&mobile=no

Use of any optical device on the laser light path would definitely reduce the power due to intrinsic material absorption of the used device. It depends on the nature of your experiment and type of laser, whether using semiconductor laser or solid state types.

One way is to replace your laser with a higher output power. If you need high pointing power, you may use optical fiber probes, which can concentrate the power on few um2. 

Besides my previous suggestions above, If the laser output is accessible, you may couple the laser output to a fiber optic ring resonator to increase the power at least 20 times, which can be tapped out from its loop power. To this aim, usually, FORR is used for developing a high power to create non-linear medium, which requires high  laser power, as you know. 

Wish you success. 

First of all, thank you so much for all your answers. I am a beginner in this field and I have learnt a lot with all your comments. Thank you for sharing your knowledge with other people.

My laser is a CW laser, not of a great quality, as it does not have a defined peak, so I just wanted to know if there was anything 'magical' that could increase the power of it without manipulating itself, as my knowledge in electronics is quite reduced.

Dear YUrema,

It's OK, you will learn through time. Actually, there may be some "scientific trick", but not any "magic" in this regard. Had there been, there would not have been a researcher like you to try. Anyway, Let's see, something you should try, then we shall discuss if you like.

Good success.

As previous responders have stated, any passive optical element in the beam path will actually reduce the power delivered through the system (hopefully by not much if you have quality optics).  And increasing the intensity at a spot by reducing the beam size at that point (as previously suggested) is also a simple and inexpensive approach.  A more complicated/expensive alternative is to take an active approach whereby you construct an optical amplifier and pass your laser output through this device. Several good texts are available in the literature that will show you how to do this and estimate the power gain you can expect to achieve.

Hi Yurema,

Could you tell me a bit more about your laser - what wavelength, power, is it an external cavity diode laser, etc. It sounds like you have a 'broad' or multimode emission which I am guessing is a either a simple fabry-perot laser diode, orperhaps even a DBR laser?

For CW lasers such as external cavity diode lasers, it is possible to increase laser output power without changing the laser itself, e.g. you could use a tapered amplifier gain chip or a slave laser gain chip. These are separate devices that you couple your existing laser power into and they amplify the 'seed' CW laser power.

A tapered amplifier can increase a 10mW CW seed laser to 500mW or 3000mW depending on the laser wavelength. There is quite a cost (several thousand USD) for these items as you require several stages of optical isolation, lenses, mounts, the TA chip itself (at least USD$2000 itself) as well as a current/temperature controller.

A slave laser setup is cheaper but not as much amplification, typically 10mW seed will get you about 100mW output, again very dependent on wavelength. For this you will need simply an appropriate laser diode, a collimation lens, a current/temperature controller and an PBS type optical isolator with at least 30db of isolation, preferably closer to 60dB, depending on your needs for the output beam stability and quality.

Cheers,

Sebastian

I think this thread is suffering from a lack of definition.  

First, a motherhood statement:  in principle the mean delivered power cannot be increased beyond the output power of the laser: all you can change using passive componenets is where and when the power is delivered.  The added passive components will in practice absorb some of the power, meaning that the power available to your load will always be less than the output power of the laser. 

What can (generally) be increased are 'power density', 'circulating power', and 'peak power'; any practical manipulation of these types can only reduce the mean delivered power below the total output of the laser, but of ourse this can be beneficial if it means that more of the delivered power goes where you want it.

Power density: subject to diffraction limits, changing the beam dimensions can ensure that a higher proportion of the laser's power is delivered to the reqired load; this change in power density can be achieved using a single lens or focussing mirror; but beam expanders (essentially two or more more focussing elements) may be more convenient if you want to route the beam through narrow gaps.

Circulating power: you can increase circulating power using a cavity (look up Fabri-Perot resonators as an example). 

In principle the cavity could allow you to increase the peak delivered power: as an intuitive example you might shoot a bullet through the circulating beam.  Alternatively you might use a non-linear element to release the power out of the cavity when the circulating power reaches a high enough level.  Another example of increasing peak power is to us non-linear elements within the cavity to cause the circulating power to "bunch up".  This is more commonly done to shorten existing pulses, but it is still possible with a continuous beam.

The above are only examples of the methods that may be used; however, regardless of being boringly repetitive, no passive method will increase the total mean power beyond the power output of the laser itself.

N.B. that there is a situation in which you mcan increase the delivered power beyond the output of the as-delivered laser: some lasers are not designed to deliver the maximum possible external power; in this case it may be possible to use external optical components to increase the efficiency of the laser. This is clearly not an exception to the first paragraph - these components become part of the laser.