How best to detect laser?

[Michael]

Hi - recently I found a cheap source for red laser diodes with built in
power supplies. So - I was wondering - what kind of sensor should I use to
detect whether the laser was focused on that sensor? Hopefully something
that wouldn't be bothered much by other light sources. Better yet -
something that could handle high frequencies. Anyone have any
reccomendations? Thanks!

-Michael

 

[Bill Sloman]

Hi - recently I found a cheap source for red laser diodes with built in
power supplies. So - I was wondering - what kind of sensor should I use to
detect whether the laser was focused on that sensor? Hopefully something
that wouldn't be bothered much by other light sources. Better yet -
something that could handle high frequencies. Anyone have any
reccomendations? Thanks!

By "red" I presume you mean 633nm or 670nm.

Your choice of sensor depends on the separation between the laser and
your sensor.

You can see the red spot produced by a 633nm or or a 670nm laser, so
for short distances and low frequencies, a bit of white paper is all
you need.

For many applications, a silicon PIN photodiode is the next step up -
the Siemens/Infineon/Osram SFH213 (end-viewing, 20 degree acceptance
half angle, 1mm squared receiving area) and the Siemens/Infineon/Osram
SFH206 (side viewing, 70 degree acceptance half angle 7mm squared
receiving area) are cheap and widely available.

The SFH213 can be fast (in the right circuit) with a data sheet
risetime of 5nsec. The larger-area SFH206 is rather slower at 20nsec -
the larger area means a greater parallel capacitance.

If you really want to detect the output of laser a long way away from
the laser, you might want to use a photomultiplier tube with a
red-sensitive photocathode, but these aren't cheap.

Silicon photodetectors react to any photon in the range 400nm to about
900nm, so they can be bothered by other light sources. You can use
multilayer narrow band interference filters to reject all normally
incident light falling outside a narrow band at around 633nm or 670nm
- Edmund Optics Ltd. stock such filters, but they aren't cheap either.

 

[colin]

if your modulating the laser then u probably not gona be bothered too much
with stray light just filter through the frequencie u want.

if its a single high frequency then an inductor as the load for the detector
can greatly increase the signal, otherwise the capacitance can rob all the
signal. i found a dual gate mosfet seems to be the best choice of amp.

an avalanche photodiode detector probably the ultimate, a bit expensive tho,
they can detect single photons, or just multiply by 150+, and theyr still
dam fast.

with such and a small lense ive managed to detect reflected 50mhz signal
from an object several meters away and still get a usable signal to noise
ratio.

Colin =^.^=

 

[Bill Sloman]

if your modulating the laser then u probably not gona be bothered too much
with stray light just filter through the frequencie u want.

if its a single high frequency then an inductor as the load for the detector
can greatly increase the signal, otherwise the capacitance can rob all the
signal. i found a dual gate mosfet seems to be the best choice of amp.

an avalanche photodiode detector probably the ultimate, a bit expensive tho,
they can detect single photons, or just multiply by 150+, and theyr still
dam fast.

with such and a small lense ive managed to detect reflected 50mhz signal
from an object several meters away and still get a usable signal to noise
ratio.

Colin =^.^=

Modulating your light source is an excellent way to distinguish it from
stray light, but enough stray light can always saturate your detector and
lower levels can still generate enough random noise to make it quite
difficult to detect your modulated source - narrow-band interference filters
can prevent most of the stray light from getting to the detector in the
first place.

Dual gate MOSFET can be attractive as front end amplifiers if you are
modulating at high frequencies, but when I last looked at them (which was a
long time ago) the 1/f noise corner tended to be close to 1MHz, and the DC
stability was rotten.

For most purposes a good FET-input op amp is better, and much easier to
use - the Burr-Brown (now Texas Instruments) OPA655 comes to mind.

If you are working with modulation frequencies of the order of 50MHz, an
inductor can be an attractive load, but you have to make sure that your
inductor's self-resonant frequency is higher than you modulation frequency -
the interwinding capacitance for a single-layer wound inductor is usually of
the order of 1pF, equivalent to a parallel impedance of only 3k at 50MHz.

Alavanche photo-diodes can detect single photons if operated in the Geiger
mode, and can provide a very prompt output, but the recovery time is
relatively long, which restricts them to looking at very low light levels.

Photomultiplier tubes are a lot more flexible, and can offer steady state
gains of up to about ten million, where the avalanche photo-diode is limited
to about 150 when not operating in the Geiger mode.

 

[colin]

Modulating your light source is an excellent way to distinguish it from
stray light, but enough stray light can always saturate your detector and
lower levels can still generate enough random noise to make it quite
difficult to detect your modulated source - narrow-band interference filters
can prevent most of the stray light from getting to the detector in the
first place.

Dual gate MOSFET can be attractive as front end amplifiers if you are
modulating at high frequencies, but when I last looked at them (which was a
long time ago) the 1/f noise corner tended to be close to 1MHz, and the DC
stability was rotten.

For most purposes a good FET-input op amp is better, and much easier to
use - the Burr-Brown (now Texas Instruments) OPA655 comes to mind.

If you are working with modulation frequencies of the order of 50MHz, an
inductor can be an attractive load, but you have to make sure that your
inductor's self-resonant frequency is higher than you modulation frequency -
the interwinding capacitance for a single-layer wound inductor is usually of
the order of 1pF, equivalent to a parallel impedance of only 3k at 50MHz.

Alavanche photo-diodes can detect single photons if operated in the Geiger
mode, and can provide a very prompt output, but the recovery time is
relatively long, which restricts them to looking at very low light levels.

Photomultiplier tubes are a lot more flexible, and can offer steady state
gains of up to about ten million, where the avalanche photo-diode is limited
to about 150 when not operating in the Geiger mode.

im using a visible light laser so i can see its pointing at the right
target, not sure how much a narow band filter would help me but snds
interesting, id only thought of infra red filters till now, main problem i
think is signal to noise/interfenernce at the detector amplifier stage, i
might look and see how much noise is being cuased by natural light,
obviously if i shine a torch on the sensor it swamps it as it has a high
resiatance in the hv supply, but dont expect it to have to cope with that.

initialy i was using a low resitance load to get good wide frequency
response and using a rf bipolar tranny as they best noise figure at low
source impedance, that wasnt good enough so i had to go for a single
frequency and tuned input stage to get the impedance up and the performance
diference i get now from a bf998 dual gate mosfet is quite remarkable. im
measuring very small delays so phase change with signal level is a problem
such as ive found are cuased by an op amp with negative feedback, but
otheriwse that would be easier.

im just using a 15 turn (found by trial and error) air cored inductor with a
tuning slug and no extra capacitance to resonate, so this is quite an easy
solution at this frequency. i dont have any tables to work out capacitance
of inductors, but as you say as long as the srf is higher than what u using,
but if you can use a single reasonably high frequency i think a tuned input
stage is an optimal solution. although i still have a 100k resistor to limit
the q as it was unstable but i think that was becuse i had a tuned drain
load too wich i since did away with.

a gain of 150 is still very significant to me, however im not sure if the
range inbetween this and geiger mode is usuable i think its just rather
unstable, but ive operated it what must be close to its vbr (250v ish) and
it seems to perform very well indeed, infact the latest problem was that a
small un noticable amount of light was being reflected of the
mirror/lense/case and finding its way back to the detector, so il have to re
orient them. (not to mention the interference from the 250v inverter)

APD are quite expensive tho, but i expect PM tubes are more so and bulky too
(this is hopefuly going to be a small hand held device), and do they still
take a while to recover from exposure to excesive light levels ? cant see
why APD are so expensive maybe cos they just not used in quantity yet. not
sure how noise level compares, im using the C30902E from RS wich specifies
gains greater than 150 or upto 250 or greater.

Colin =^.^=

 

[dave vanhorn]

There was a guy that used to hang out here, who could detect all sorts
of lasers and beam weapons. I guess "they" finally finished him off,
I haven't seen him around in a while.



First, keep the detector out of saturation.
If it saturates, you're done.
Interference filters, polarization, mechanical "blinders"...

Then, modulate the source, and if at all possible, use synchronus
detection.
AofE talked about a lock-in detector, that would give you pretty
amazing results.