Single mode and multi mode

[gozert]

Could someone explain me how I can tell whether a diode is single mode or multi mode? I've been looking at datasheets of several high and lower wattage/wavelength diodes but I can't seem to figure it out. I searched the forum before making my thread, but couldn't find what I was looking for.

 

[ChaosLord]

Hmmm.. I've seen diode data sheets list it. If I recall correctly, it may be under the transverse mode.

 

[USAbro]

Can't one tell from the beam characteristics? I could be wrong though.

 

[The Lightning Stalker]

It isn't always easy to tell. The
multimode 5mW 638s don't look any different
from single mode. Usually it's obvious,
though. There will be visible mode lines
and artifacts. Just search for pictures of
M140 and P73 dots. That will give you a
good idea. Unless you're talking
longitudinal mode which is completely
different and can only be seen with a high
res spectrometer.

 

[gozert]

It isn't always easy to tell. The
multimode 5mW 638s don't look any different
from single mode. Usually it's obvious,
though. There will be visible mode lines
and artifacts. Just search for pictures of
M140 and P73 dots. That will give you a
good idea. Unless you're talking
longitudinal mode which is completely
different and can only be seen with a high
res spectrometer.

The problem with measuring it by myself is that I can't tell if a diode is single or multi-mode when I order it online. I know that pretty much all 405nm diodes are single mode, but when it comes to 638 and 445nm I get lost.

 

[rhd]

Datasheets will all indicate the info you're looking for.

Give us an example of a datasheet you've looked at that doesn't seem to indicate mm vs sm, and we'll tell you where it's listed.

 

[gozert]

Datasheets will all indicate the info you're looking for.

Give us an example of a datasheet you've looked at that doesn't seem to indicate mm vs sm, and we'll tell you where it's listed.

http://www.lillyelectronics.com/download/ICPDF-SLD3232VF-GD.PDF

http://www.princetel.com/datasheets/635 nm DL-3148-025.pdf

I found these two sheets, hope they're good enough. Thanks for taking the time to explain it.

 

[The Lightning Stalker]

The Sony is designed for Blu-Ray tech. and
is almost certainly single mode.

http://www.sony.net/Products/SC-HP/cx_news_archives/img/pdf/vol_40/sld3233vf.pdf

The Sanyo on the other hand is more
difficult. Just based on the perpendicular
divergence angle being 30° and there being
relatively few single mode in 638nm, it is
most likely multimode.

 

[lazeristasUVISIR]

405 - it is an earlier version of violet diodes. They can have an elliptical beam with a small side effect. Like clipping on the cap exit hole or on an edge of the waveguide.

635 - I have those. It produces a nice beam. Focuses to a dot. If you know what parameters are important to you I could look in more details.

Multimode vs single mode => you shall say why you care about them. To get a pure single mode lasing is not that simple as it means a very narrow spectra (a single longitudinal mode) and a single spatial mode (though it can be not only a perfect TEM00)

An older thread on the topic http://laserpointerforums.com/f54/single-mode-multi-mode-laser-diodes-53695.html

 

[The Lightning Stalker]

Can you please enlighten us a bit more?

What effect do longitudinal modes have on
spatial modes?

This page seems to state otherwise.

http://www.photonics.com/EDU/Handbook.aspx?AID=25099

The Fabry-Perot resonator in a single-spatial-mode laser diode can support multiple longitudinal modes — wavelengths separated by about 1 Å.

 

[lazeristasUVISIR]

Can you please enlighten us a bit more?

What effect do longitudinal modes have on spatial modes?

This page seems to state otherwise. Semiconductor Lasers | Photonics Handbook®

I stated that a single mode operation is when a laser emits a single transverse (spatial) mode and a single longitudinal mode at the same time. :na:

What effect do longitudinal modes have on spatial modes?

Spatial modes (even if they all are TEM00) will have different wavelengths (different longitudinal modes). In general a single spatial mode operation is achieved by inserting an aperture (easy), and a single longitudinal mode inserting etalons (difficult). With laser diodes things are more complicated :can: