Multi mode vs. Single mode

[Bradfo69]

Can someone please give me a definition of what is meant by multi-mode and single mode as it relates to these various red diodes. And, the pros and cons of each. I tried to do a search to try and figure it out and am pulling up threads from 2005.

It sounds like single mode diodes are better than multi-modes but, the powers are lower with single mode. What is the difference and why should I care? If I knew more about electronics maybe this wouldn't be a question but, a lot of times I have to just nod my head like I understand.

As always, thanks in advance!

[Stiffler]

I have pondered this as well.

[planters]

This sounds sophisticated, but it's only because of the jargon. When the laser oscillates the photons/waves bounce back and forth between the mirrors. In a diode laser the space where this occurs is the very small plane at the junction of the P doped and the N doped half's of the crystal. In a single mode laser there is one favored path that passes through this plane where gain occurs and which strikes the cavity mirrors for a return trip. By adjusting the mirrors for loss and spacing as well as for the lateral dimensions the laser can be prevented from oscillating along any other path. Typically, this single mode is TEM00 for a nice compact low divergence beam although it doesn't have to be. The major problem with single mode operation is that it can be difficult to extract a high percentage of the energy from the gain region. In a diode laser the gain plane is thin, wide and short and so in order for the laser to pass through much of this region, multi-mode operation with a few, to many simultaneous and approximately parallel "lasers" or modes run next to each other, filling this plane. Multi-mode produces complex beams with poor coherence, but more power. Some lasers (probably most) such as the 445nm diodes will operate single mode at a power level just above threshold because the gain is still a little better for this mode, but as the input power increases then progressively less favorable paths begin to lase and the output shows more complexity.

[pschlosser]

A nice treatise by the previous poster. In a gas laser, TEM00 produces a nice clean beam and dot. As one tries to squeeze more output power from a laser, other modes can occur, such as a split beam (TEM01) that produces two dots. Laser diodes are subject to this same problem, perhaps more so.

http://en.wikipedia.org/wiki/Transverse_mode

[Bradfo69]

Thanks Eric,

I'm reasonably certain I understand the difference between the two now. However... next question is in terms of preference. Seems therre are about three I regularly see listed for sale - a single mode 170mW, and then multimode at 300mW and 500mW. (I'm talking Mitsu's.) So, in the grand scheme of things, what's better - a set up with two single mode 170' s combined in some fashion (whether it's with a pbs or kife edging or something else) or 1 of the 300mW?

I'm considering actually trying to mess around with these a little more in what little spare time I have. I read the threads intently but, I think until you physically do it, some things just don't all make sense.

[helium]

Go for 2 x single mode instead ot the multi mode option. More expensive but much better beam quality.

[jeejeedr]

sorry to hijack, but what if you need eg 1W?
With 2 mitsu's and a pbs you're there, but with the single mode ones, you need about 6...

[Bradfo69]

I really don't have a specific project in mind or a power that I'm looking for. I've got some quad mounts and some dual mounts and other assorted machined hardware floating around, in the event I actually put on the big boy pants and really attempt to mess with these things someday.

[planters]

This is sort of a caramel vs fudge question. With no project in mind, it depends on what you want to play with. I have worked with both the P73 (500mw) and the G71 (300mw) and they require at least one pair and better two pairs of corrective lenses to produce the best possible beams. The lenses require either a collection of mounts and spacers or the ability to do some basic machining/fabrication in order to fine tune the alignment and spacing. But when you go through the effort the result is a PBS quad that can put at least 3W on a 7mm wide scanner mirror with well under 1mrad divergence. I like the beam quality of the G71's a little better. At this performance you will need to telescope a typical DPSS green to reduce its divergence down to the red. On the other hand with the appearance of the 100-200mw green diodes and their low divergence then a low power (170mw) red might make a good match for a low power /high speed/ low divergence graphics satellite projector.

[edison]

I agree with Helium , single mode is much better in beamquality then multimode. Even for beamshows single mode is more visible then multimode because of the thighter beam. However the mitsubishi multimode is a nice solution when you can,t afford single mode or you don,t have the skills and tools to build a multistacker. A multimode setup is cheaper because you need less diodes but is only when you build it your self. I can do both but the multimode is not much cheaper because of the time you need to setup all the optics. I prefer the single modes because i can do 0.6mrd full angle with a 3x2mm beam for a quad. You can,t do that with multimode. Multimode is more like 4-5mm at aperture and 1 mrd. Still allot better then a 635 chinese flashlight though and without tec and it is the cheaper solution for beamshows. Its less money compared to a single mode setup but not cheap unless you do it yourself but isn,t that with all things in life?:rolleyes

The marketprice for a red singlemode setup is a little less the 3000.(endusers price) So when looking at a multimode setup with less beamquallity its should be cheaper because you have less diodes but because of all the optics and time you spend it will not be half. Multimode diodes is mainly for the hobbyist who has all the time of the world to build it. There for the bigger module companies don,t build them because when the time is the same to build it why sacrifice the beamquality? Besides they buy single modes in 10000 pieces so single mode will be also the same costs.

If you are planning to build someting try a multimode setup then at least you have enough power and you can save 3000 euro