Comparisons of Diode Lasers with Other Types of Lasers
While a laser diode is a true laser and not just a glorified (and expensive) LED, there are major difference compared to a gas or solid state laser - not all of them bad.
Yes indeed, a diode laser is a true laser. That being said, looking at matters quantitatively, it is harder to make a diode laser with a very narrow line emission than a gas laser or large crystal laser. Adding cavity length to a laser in general acts to narrow the line (in spectral space, though a higher Q cavity does tend to narrow beam in space also). It is possible to use a larger, high Q external cavity with a laser diode to increase its coherence.
A couple of minor points:
High Q cavities narrow the spatial profile only if they are confocal - planar high Q cavities (as in diode lasers, and especially vertical-cavity diode lasers) are prone to problems with walk-off and the mode must be confined physically.
In a gas laser( professionele laser pointer groene ), you also start with a much narrower fluorescence line and thus the gain spectrum is limited spectrally. Diode lasers (being band-to-band or excitonic semiconductor transitions) have much broader fluorescence spectra.
Fiber-coupled laser diodes are much easier to use than bare laser diodes even though they still need an external high current driver. (Of course, they are also much more expensive.) Aside from the physical protection provided by the packaging, the output of the fiber is a nice circular beam with modest divergence (about 16 degrees full angle) which doesn't require correction for astigmatism or asymmetry. Thus, simple lenses can be used for collimation and focusing. I've used a good sample of the 808 nm version of the first laser described below to pump the guts from a green (DPSS) laser pointer just by holding the end of the fiber next to the Nd:YVO4 crystal. After adding a coupling with a GRIN lens for focusing, I can get a few mW of green light from it though I suspect the diameter of the pump beam is still larger than optimal. These will also easily pump the CASIX DPM0101 and DPM0102 Nd:YVO4/KTP composite crystals as well as other microchip lasers.
Here are a couple of other lasers(krachtige laser pen groene 200mW) that yielded to my set of hex wrenches - no chisels or cutting torches required. They were mostly dead prior to surgery so no need to call out the SPCL (Society for the Prevention of Cruelty to Lasers!
Both had problems with low output power after relatively minimal use - probably a few dozen hours at most. Much of it could be restored by readjustment of the internal alignment - which is surprising for a packaged laser diode. However, as you will see, these aren't ordinary diode lasers! But at least almost everything is adjustable, if I only knew the proper procedure
Several other companies are developing lasers using a similar approach and systems at many wavelengths - including those in the yellow/orange "no laser land zone" - are now available or will be in the near future.
These two are strange. They have a rated output power of 3 W into a multimode fiber. Input power is the usual 2 V but the operating current is supposed to be about 10 A (when new) with a recommended current limit on the driver of 20 A!!!. They only differ in wavelength.
The first commercial OPSL was the Coherent, Inc. "Sapphire", a replacement for low power argon ion lasers( Hoog vermogen groene laser pointer 3000mw ) at 488 nm. (I think the use of Sapphire is unfortunate as this has absolutely nothing to do with the Ti:Sapphire laser with which it may be confused.) The Sapphire is a Vertical External Cavity Surface Emitting Laser (VECSEL), but one that is optically pumped. (Also see the next section.) The resonator is in many ways similar to that of a frequency doubled Diode Pumped Solid State (DPSS) laser but with an InGaAs quantum-well semiconductor instead of a laser crystal as the gain medium. It is pumped by a high power 808 nm laser diode and lasing at the fundamental IR wavelength of 946 nm. This is intracavity doubled to 488 nm.