DPSS vs. diode laser advantages

[ixfd64]

I've seen a few threads comparing DPSS and diode lasers. From what I gather, these are the advantages for each type:

DPSS lasers

• better beam specs
• can reach very high powers while maintaining a low divergence and small beam diameter

Diode lasers

• lower cost
• more energy efficient
• more stable (DPSS lasers have a tendency to fluctuate or mode-hop)
• no residual IR
• less complex construction

Both types of lasers can produce exclusive wavelengths that the other type (yet) cannot.

Am I missing anything?

 

[Jaseth]

Diode lasers can be switched on/off almost instantly, resulting in better blanking capabilities compared to DPSS lasers which usually sit around a maximum blanking rate of 20kHz. This makes diode lasers superior for both laser shows and data transferral.

You could also add to the point about stability that DPSS lasers are far more temperature dependent.

Seb

 

[dnar]

DPSS laser are temperature sensitive and operate optimally over a relatively small temperature range.

 

[goninanbl00d]

As dnar said, DPSS lasers are very temperature sensitive.

At the higher powers, the doubling crystals (KTP or LBO) need to be stabilised within 0.1C of their operating temperatures. It's certainly not easy, and combined with the effects of modulation, it can be very difficult.

The gain media also require cooling. Although not as temperature-dependent as the nonlinear-optic crystal, it still requires to be held in a specific temperature range for maximum performance.

Some gain media (such as Nd:YLF) suffer from thermal expansion, and require strict cooling in order to prevent thermal stress and eventual cracking.

The pump diode also needs to be held at a consistent temperature in order to hold the wavelength steady. Although not as important for a (relatively) broadband absorber such as Nd:YVO4, other gain media such as Nd:YAG (used in 473nm and higher-end 532nm lasers) have absorption lines centered on a specific wavelength, with changes of 1nm having a major effect on efficiency, leading to a 20% or more performance drop for every nm shifted.

Efficiency in DPSS lasers also leaves much to be desired- 25% efficiency for 532nm is considered good. It requires a 2.5W pump diode to achieve 500mW of output, while the rest is simply dissipated as heat/electrical losses.

473nm is even less efficient, with 10% considered average. Not only is Nd:YAG very wavelength-selective, in order to create 473nm, the weak 946nm line of the YAG is used. You're competing with several other lasing lines, and only a tiny amount of pump energy turning into 473nm photons.

Diode lasers aren't without their problems either.

Although weaker diode lasers may seem to have good beam specs, once you start getting higher powers, beam specs go downhill.

These diodes often use multiple emitters, and the output is horribly hard to collimate, often requiring all sorts of correctional optics. The Casio 445nm diode is one of the better-looking multimode diodes.

Not only that, they also have terrible divergence. On higher-power multi-emitter reds, 4 to 5mRad is considered average. You get to a point where even after correctional optics, you just can't collimate the beam any further.

 

[Benm]

I think you should consider the difference between multimode and singlemode diodes too... its quite important when beam specs are concerned.

The difference is realy striking at a bit of distance. If you were to point a dpss green, diode red and 445 diode laser at a building a few hundred meters away, both the red and green would appear to project a circle, while the 445 projects a line.

Considering that, the loss of beam quality going from single to multimode is much larger than the difference between dpss and single mode diodes.

 

[532 with Envy]

I'm sorry for not having the knowledge of ya'll here, but have had interest in DPSS from when I started here.
I guess I would ask....aren't both really diode lasers when it comes down to it(DPSS and Diode), until a C-mount is used for higher output?
Is the difference coming from open can/closed can..5.6/9mm diodes when compared to a C-mount?:thinking:

 

[BShanahan14rulz]

Friend found me a laser that puts out a perfect, round, even, smooth red dot, brighter in the center. It makes the best beam of any of my diode lasers so far.

It was a cheap <1mW on a promotional free pen. There's definitely something to be said for single-mode diodes too.

 

[Gryphon]

I guess I would ask....aren't both really diode lasers when it comes down to it(DPSS and Diode), until a C-mount is used for higher output?
Is the difference coming from open can/closed can..5.6/9mm diodes when compared to a C-mount?:thinking:

Yes both lasers start out as diodes, but a DPSS uses crystals to achieve a desired wavelength, and thats when their differences start to appear, those like in the op.

Not really, both are just diodes. C-mounts offer more power because they are usually made to be multimode, pumping with that or a simple 5.6mm diode is basicaaly the same.

 

[532 with Envy]

So its the Crystals threshold(wavelength and power) that really shows the difference between DPSS and Diode?

Aside from Sam's laser does anyone have links to mounts for C-mount diodes....I'v looked into DPSS a bit and found 9mm 808nm cans that might be easier to start with as far as DIY, but would still like more information on how to mount and use C-mount diodes.

 

[RA_pierce]

So its the Crystals threshold(wavelength and power) that really shows the difference between DPSS and Diode?

Aside from Sam's laser does anyone have links to mounts for C-mount diodes....I'v looked into DPSS a bit and found 9mm 808nm cans that might be easier to start with as far as DIY, but would still like more information on how to mount and use C-mount diodes.

This topic is concerned more with the output characteristics of the two different types.

I'm not sure exactly what you are trying to say here...
If you are referring to differences in the beam when a DPSS laser is pumped by a single-mode diode or a multi-mode diode, there essentially is not one.

When the crystal is pumped, it absorbs the light and in a process similar to fluorescence (stimulated emission) it emits it's own light.
This light is then converted in the SHG crystal. In the case of green it is frequency doubled.
Because the crystal produces it's own light, the diode's beam profile does not translate to the final output.

C-mount diodes are just a diode type that uses several emitters to attain higher output. The construction of C-mount diodes is ideal for higher powers up to a few Watts. 5.6mm and 9mm diodes can also be multi-emitter diodes but not in as high powers as C-mounts, TO-3, or diode arrays.
Also, note that the Nichia 445nm 1W diodes are multi-mode but have only ONE emitter. This is one of the reasons collimation is better than with multi-emitter diodes.

 

[532 with Envy]

Thanks for the answer:wave:....didn't mean to stray off subject.

 

[Bluefan]

As far as I know, c-mount diodes are still single emitters, only diode bars and other fancy constructions have multiple emittters.

On the stability: diodes are worse than DPSS lasers. Diodes shift in wavelength with temperature. This is usually used to correct for the small wavelength error they have to start with.
Diode lasers have a very short cavity, and thus have a lot of modes. Because the gain curve is quite wide, the spectrum is also wide. A DPSS has a much and much larger cavity. The linewidth is much smaller and because the gain curve is smaller the spectrum is much better. A diode has many longitudinal modes, a DPSS only a few. But lasers like the Coherent Verdi or Compass have only a single longitudinal mode.

On transversal modes: transversal modehopping in a DPSS is a broken or bad designed laser. Multiple transversal modes cannot be avoided at higher powers for a diode lasers, but not for a DPSS. DPSS lasers are usually singe mode, only instable resonators are different. They usually have a tophead beam profile to extract the most energy out of the gain medium.

Because the gain of diode lasers per roundtrip is very high there's also higher spontaneous emission. The laser mode also has fewer roundtrips and the beam profile is lower because of this. A DPSS has a larger cavity, more roundtrips and a better beam profile. Only single mode diodes can get close, but those are limited in power, DPSS laser can go very high with a very good beam quality.
It's true that higher power DPSS lasers need more complicated cooling, they use a diode to begin with plus the crystals need cooling/heating. But the colling isn't very precise, the laser crystal just needs to be cooled, KTP is slightly more efficient at a higher temperature and that's it. Only LBO is more precise and needs to be regulated within 0.1deg to keep the phase matching, but it's not used much compared to KTP.

Take the Coherent Verdi for example. Unidirectional ring laser, 532nm, 18W and single mode, both transversal and longitudinal.

 

[drummerdimitri]

Sorry for going slightly off topic here, but what exactly is the difference between a multimode and singlemode diode? From my knowledge, the first has multiple emitters on the die and the later has only one. But how can I understand what that's supposed to mean? Maybe an explanation of what goes on inside a laser diode or some close up pics would help.

 

[Bluefan]

Short explanation: A single mode diode laser has a single transversal mode, a multimode diode laser has a large enough emitter that multiple transversal modes fit, which will mess up the beam quality. Diode bars have multiple multimode emitters.
Long answers will probably follow, I'm in a bit of a hurry

 

[ixfd64]

I have another question.

Does the beam quality of a DPSS laser depend on the number of crystals the light has to pass through? For example, suppose someone invents a crystal that converts 808 nm directly to 532 nm. Will that laser have better/worse specs than a "traditional" DPSS laser?

 

[goninanbl00d]

In short, no.

Beam quality is dependent on the process in which the light is produced. For example, diode lasers with a short cavity length often have very poor beam specifications while a gas laser with a significantly longer cavity will have much better beam specifications.

As a result, the beam specs are often determined by the crystals used, the properties of the crystals, and the size of these crystals. A longer cavity often results in much more stable emission.

However, the more components, the more room there is for instability. All the cavity components (such as the gain medium and the non-linear optics) will need to be stabilized and any instability in any element in the cavity can cause the beam quality (mode, generally) to go downhill.