DPSS questions

Can anybody answer these questions (or any of them)?

1. Is the beam quality of DPSS lasers better than that of single diode lasers? If so, why?

2. Why did Nichia bother to invent the 405nm diode? Why not just frequency double the output from an 808 LD?

3. What is the size of the laser light emitting area in a laser LD? How does this compare to the laser light emitting area in the lasing crystal of a DPSS?

Just wondering....

David

1. No, because DPSS lasers are unstable, unless it's property made with for example TEC to stay at constant temperature of crystals and diode (like CNI does with their labb lasers - I think so).
2. It's not worth it, doubling 808nm is quite hard, very low effective, there is no point to pump 1W into crystal just to made 10mW of violet.
3. It depends on the power of diode, diodes in DPSS lasers are normal diodes, propably or most likely just using extra FAC lens (Fast Axis Collimation) to correct multi mode diode beam.
For example here it shows emiter size in um:

http://www.rpmclasers.com/Multimode%20Single%20Emitter%20Diode%20Lasers.htm

Just an example, it of course may be different.

2. - imagine how would you put an DPSS module into bluray drive:na:

davidgdg said:

1. Is the beam quality of DPSS lasers better than that of single diode lasers? If so, why?

Click to expand...

Well, yes and no. Stable DPSS setups have incredible beam qualities, which is why, despite it being inefficient, 473nm is used in holography. The Coherent Compass 315m is *I think* the best long coherence length laser in the world. But, unstable DPSS will skew this, obviously.

Prototype said:

Well, yes and no. Stable DPSS setups have incredible beam qualities, which is why, despite it being inefficient, 473nm is used in holography. The Coherent Compass 315m is *I think* the best long coherence length laser in the world. But, unstable DPSS will skew this, obviously.

Click to expand...

Yes! He's right. The effiency of DPSS is quite bad and they're really hard to control. But you'll never ever reach the DPSS beam quality with a "bulk" single mode laser diode.

e.g. the coherence length of laser is more or less depending of the overall cavity length - a HeNe has a cavity length of multiple cm - a small laser diode has a cavity length of a few um.

mo

davidgdg said:

Can anybody answer these questions (or any of them)?
2. Why did Nichia bother to invent the 405nm diode? Why not just frequency double the output from an 808 LD?

Click to expand...

Mainly because it's hard to fit a DPSS unit into a blu-ray drive!

808nm direct doubling has been done before, and I believe such systems can be bought, but the main issue is that the beam quality going to a KTP, LBO, or BiBo crystal MUST be exceptional, or you get no frequency doubling.

Since the most abundant source of 808nm light is a laser diode, the cost to correct the beam into something usable for a doubling crystal outweighs the benefits. 808nm diodes often come in multi-mode setups, or even arrays of multimode outputs.. quite the mess if you ask me.

They CAN use 808nm diodes to pump neodymium doped lasing mediums because such mediums aren't so picky about the quality of light they are pumped with. Thus, for a DPSS green to work, you start with crappy 808nm light, which excites higher quality 1064nm light into the KTP, which then does it's job since the output of the solid state lasing medium is way better than the output of any laser diode.

Hope this helps...

Many thanks for the responses. What's confusing me is that if the entire front of the KTP crystal emits a lasing output from its face, why is the DPSS beam width at the aperture less than the beam width from an LD which has a far smaller emitting area? Surely the smaller the emitting area the smaller the potential beam width? What am I missing?

It isn't the entire face of the KTP that lases, the portion that lases isn't all that big, the rest of the crystal is for heat, otherwise the crystal would crack from the heat.


Edit: If memory serves, I'm quoting the FAQ from memory, which may or may not be 100% accurate (my memory, not the FAQ)

The active portion of the lasing medium and non linear optic is a function of the beam waist of the pump diode as well as the cavity configuration. Flat mirrors for example will output a much wider beam whereas a hemispherical OC will output a narrower beam because the light is refocused into a smaller waist with each reflection. Cavity length makes a big difference but for small DPSS they are quite short to begin with. Single mode linear cavity DPSS lasers are created using extremely short lasing media that inherently limits the oscillating longitudinal modes. This limits the maximum output power. High power single frequency lasers use ring or other cavity designs so as to allow components such as etalons to be inserted. The cavity design can be used to control the longitudinal modes because you basically can use interferometer configurations.

Light emitting areas on diodes are measured in microns whereas in DPSS they are in mm.

FC -- It's good to see your knowledge back here again.
HMike