Why are diode lasers different?

[lasergal]

Sorry if this has been discussed elsewhere, but I searched and Googled and still can't find a satisfactory answer, and this question has nagged me for years:

If one of the chief characteristics of laser light is its coherence, why then are the beams from diode lasers any less coherent than from other types of lasers? I've read that diode coherence is inferior to, say, a HeNe so many times I stopped counting. But never have I found a really good explanation for this.

Diode lasers are still real lasers, right? So why do they need special lenses to focus the beam into a tight line, and other lasers don't? :thinking:

Could someone explain it to me so even my "typical female" mind can understand? I'd appreciate it!

And I do apologize if I'm :horse:

 

[Sigurthr]

In a gas laser the bore shape yields the round spot and only light which is directly parallel to the bore gets amplified and emitted.

In a diode laser the active emitter area is super tiny, and can be considered a point source for many things, but it is not really a point source when you get down small enough. This is analogous to an isotropic antenna (it doesn't exist in the real world, nothing is infinitely small). Also, IIRC the photons don't pass through the gain medium as many times in diode lasers, and with each pass through a gain medium (and thus each reflection by the mirrors) the non parallel photons get weeded out. So, in a diode you have a definite emitter area where photons are emitted not just parallel to the axis of the mirrors but also in a cone, and there is less filtration of non-parallel photons.

It is because the diode's photons are emitted in a cone shape that we need a lens to focus it in to a beam.

 

[lasergal]

Ah, okay. So it has to do with the shape of the diode then?

 

[Sigurthr]

Not of the diode but of the part of the diode inside that actually produces the light. More so size than shape, to be accurate.

 

[Hiemal]

Basically, there's a lot more area "emitting" laser light in a HeNe tube, therefore it doesn't need optics to produce a dot...however, as you can probably tell, tubes aren't exactly high power.

Because of the size of a laser diode the photons tend to scatter out in a oval/circular shape and thus require a lens to collimate them together nicely.

 

[lasergal]

Thanks, that helps me understand it.

 

[MadEye]

Reading your question I think you dont really know what coherence really means. Coherence means the waves are of the same wavelength and are in phase with each other. So an output directly of a laser diode is exactly as coherent as the beam after it got focused to infinity.

But your main question was already answered

 

[Benm]

It doesnt depend on how you focus the light using lenses, but the coherence length of light from a laser diode is inferior to that of, say, a HeNe for the reasons descibed above.

 

[Oceansoul]

Firstly, divergence at aperture =/= coherence length.

Both the coherence length of the beam and the divergence at aperture are proportional to the length of the gain medium

A longer gain medium will produce beams with both better divergence (we're talking <0.9mRad at 1-2mm beam diameter here from a large-frame ArIon), as well as beams with a longer coherence length.

The length of the gain medium on a diode laser is measured in micrometers. It's tiny. The beam diameter upon leaving the gain medium is also tiny, and because beam diameter is proportional to divergence, the divergence is huge.

The collimating lens just keeps the expansion in check.

That's how beam expanders work, by the way. They trade off beam diameter for divergence.

Fact: y'know your average DPSS module? It's no better. The beam that leaves the front-side of the KTP is around the same diameter as that leaving a diode laser's gain medium (often smaller). It gets blown up a bit more by an expanding lens and then gets collimated again. That's why DPSS laser modules have a focusing lens as well.

The fact that taking the collimating lens off a diode laser or a DPSS laser module turns it into a flashlight doesn't make it any less of a 'real' laser. It's still coherent light, it's just that you're dealing with a tiny beam diameter and massive resultant divergence.

Both the HeNe and the diode laser are coherent; the distinction comes from the fact that they're coherent to different degrees.

Secondly, the books are talking about temporal coherence, not spatial coherence.

Put in really simple terms: that's the degree to which the beam is coherent if you travel down the beam longitudinally.

Diode lasers don't exactly have spectacular spatial coherence either; they're incredibly 'noisy' (the output is not always stable). This comes from the fact that they're driven by a power supply that may experience fluctuations in current (which leads to a change in output power).

 

[lasergal]

"Coherence means the waves are of the same wavelength..."

I thought that was what 'monochromatic' means- the same wavelength (color). Not coherence.

Coherence is the "and are in phase with each other" part.

 

[Oceansoul]

"Coherence means the waves are of the same wavelength..."

I thought that was what 'monochromatic' means- the same wavelength (color). Not coherence.

Coherence is the "and are in phase with each other" part.

Monochromatic: the waves are the same wavelength.

Coherence: the waves are in phase with each other.

Spatial coherence: (coherence in space) the wavefronts are in phase with each other.

Temporal coherence: (coherence in time) the power and wavelength of the beam remains consistent throughout the length of the beam.

 

[Videogamer555]

In a gas laser the bore shape yields the round So, in a diode you have a definite emitter area where photons are emitted not just parallel to the axis of the mirrors but also in a cone, and there is less filtration of non-parallel photons.

It is because the diode's photons are emitted in a cone shape that we need a lens to focus it in to a beam.

Still not quite true. In a laser several factors must be present simultaneously.

1: 2 paralel mirrors (doesn't work if not paralell and a lasing medium
2: distance between mirrors must be exactly equal to wavelength of light to amplify (much like "tuning" the length of an antanna to the right RF frequency).


The distance between the center of one mirror and the center of the other mirror is EXACTLY a multiple of the wavelength to amplify. However ANY photons bouncing at even a SLIGHT angle will travel a longer distance between mirrors, and thus will NOT cause laser amplification. So even in a DIODE laser, it should be a PERFECTLY PARALLEL SIDED beam emerging from the device. The fact that it does NOT do so, has baffeled scientists for a long time.

 

[BShanahan14rulz]

I think the trend is the longer your cavity, the lower your base divergence.

 

[ElektroFreak]

^yep. A tiny cavity like that which is found in a laser diode will always produce a highly diverging raw beam before correction. The same is true in most of our hobbyist-class DPSS lasers too, due to the fact that the cavity is usually between a few mm (in the case of bonded crystals) and a couple inches (discrete crystals) long. In contrast, most gas lasers have long cavities, between 6 inches and several feet. They produce well collimated beams naturally, without any further lenses or optics.

 

[Videogamer555]

Why is the beam more divergent in one axis than in the other (and thus forming an elliptical rather than circular beam)?

 

[ElektroFreak]

That has to do with the rectangular cross-section of the cavity. Due to the fact that one dimension (axis) is longer than the other, some of the light traveling within the cavity travels further in one dimension than it does in the other. Because of the effect of cavity dimensions described in my last post, the light that travels in the longer dimension will have lower divergence than light traveling in the shorter dimension. This results in an astigmatic beam, where one axis diverges faster than the other. With most other types of lasers, the cavity cross-section is symmetrical, often perfectly square or circular.

Here's a diagram that might help you visualize what's being described:

You can see that the beam is oval, like single-mode diodes. The cavity is the small rectangle with the V shaped dotted line in it, the length of which is marked by "D". The output facet of the diode is in the front and center of the area called "active layer". You can see that the thickness of the active layer is less than the width of the rectangle that marks the cavity. This is the difference in dimension that causes astigmatism.