laser pointer - coherence length
surfer
really laser but rather LED as I uderstand.
TIA
Mike
Sam Goldwasser
> What is the coherence length of a regular red laser pointer? It isnot
> really laser but rather LED as I uderstand.
No, it is a real laser. But without testing, there is no way to know.
Some can be many meters while others only fractions of a mm. Depends on
the type of laser diode, temperature, phase of the moon, etc. :)
--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html
Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.
Rene Tschaggelar
> What is the coherence length of a regular red laser pointer? It isnot
> really laser but rather LED as I uderstand.
Due to the short cavity length of 1-2mm, there
are many competing modes. These modes also interfer
for the coherence length. Without special treatment,
I'd assume the coherence length to be somewhat
below a millimeter.
They are sufficient to read hologram. A LED is also
sufficient to read a hologram at a somewhat lower
resolution though.
Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net
Helpful person
Rene Tschaggelar wrote:
> Due to the short cavity length of 1-2mm, there
> are many competing modes. These modes also interfer
> for the coherence length. Without special treatment,
> I'd assume the coherence length to be somewhat
> below a millimeter.
>
Completely untrue.
Please visit my web site at www.richardfisher.com
Rene Tschaggelar
Nice - and you claim to achieve centimeters ?
decimeters ? meters ? of coherence length without
external cavity ?
Rene
Sam Goldwasser
Fleetie
> decimeters ? meters ? of coherence length without
> external cavity ?
Well I myself have made holograms with a 658nm nominal
laser diode. Ok, it wasn't mounted inside a laser pointer,
but that's irrelevant. The coherence length was certainly
better than "millimetres".
Martin
--
M.A.Poyser Tel.: 07967 110890
Manchester, U.K. http://www.livejournal.com/userinfo.bml?user=fleetie
Helpful person
>
> Nice - and you claim to achieve centimeters ?
> decimeters ? meters ? of coherence length without
> external cavity ?
>
> Rene
Yes. In the applications that I have used, a short coherence length
has been desirable. However, for the 850nm laser diodes I used the
typical coherence length was about 2cm. The lasers I was using had a
single transverse mode and mode hopped about several longitudinal
modes. However, laser diodes without an external cavity can be made to
maintain close to single longitudinal mode operation without external
feedback.
Using external feedback is certainly necessary for very stable single
mode opperation. However, it is not necessary for obtaining coherence
lengths greater than 20mm.
surfer
Thanks to all who responded. As there were different opinions in the
responses and also contradicting references in our online searches, we
went ahead and measured the coherence length of two red pointers in an
Michelson interferometer setup. The first one was a no brand from an
all-for-a-dollar store. Coherence length was in the ~100 micron range.
The other one was a 200-yard pointer powered by 3 AAA batteries from
Radio Shack. Its coherence length was very long. Our translation
stage only went an inch or so; there was no change in the fringe
pattern.
Fleetie
~658nm) from Frank DeFreitas, which just used a regular laser
diode, but with some fancy-ass PSU he and his mate had designed,
and he _claimed_ >14 FEET coherence length for it.
I dunno about that but, it made holograms ok.
Now I have a ~650nm diode from a DVD recorder running at 300mA (!!!) on
my living room table. It must be doing between 100 and 200mW, but
I have no way of measuring it. It lights a match easily. I wonder
whether it has a decent coherence length. If so, it would be wicked
for holography, with all that power.