erratic output from green laser pointer

[Dave Martindale]

I have a green laser pointer, labelled "Orion Skyline". It's rated <= 5 mW
output power. I don't have a way to measure actual output, but I'll
assume that it's operating at approximately rated power when it's nice
and bright and the brightness is steady. Unfortunately, the brightness
is constant only when it has "warmed up".

If it sits in a cool room (say 15 degrees C) for a while before I use
it, the inital brightness of the beam appears quite low - perhaps 10%
or less of maximum output. If I keep it turned on, it will flicker -
becoming brighter then dimmer again multiple times, remaining at one
brightness for a fraction of a second to a couple of seconds before
abruptly changing brightness again. After about a minute of operation,
it seems to come to full brighness and remain there, without flickering.

It's not due to bad batteries or poor electrical contact on the
batteries - I've swapped in new alkalines, tested with a battery tester,
with just-cleaned terminals. But it *does* seem to be related to
temperature: if I hold the laser-containing end of the pointer in my
hand for a minute before use, warming it through skin contact while it's
still turned off, it comes on at full brightness (or close to it)
immediately.

So what is likely causing the brightness changes? I can think of 3
plausible explanations (there may be more).

1. A bad electrical contact somewhere in the circuit. When cold, it has
high resistance that prevents sufficient current from getting to the
power supply.

2. The constant-current supply for the IR laser diode is flakey and IR
output is changing, which results in changes in the amount of green
light generated.

3. The IR laser output is constant, but there's some sort of problem in
the 1064 nm laser or the frequency doubler crystal that makes its
efficiency very temperature-sensitive.

Anyone who has seen this problem care to guess what's happening in mine?

Also, how is it likely to be held together? The main body of the
pointer is a brass tube. Batteries are inserted by unscrewing an end
cap from one end. The laser assembly *looks* like it's screwed into
the other end of the tube, but it's not willing to budge with just hand
force attempting to unscrew it.

Dave

 

[Mike S.]

I have a green laser pointer, labelled "Orion Skyline". It's rated <= 5 mW
output power. I don't have a way to measure actual output, but I'll
assume that it's operating at approximately rated power when it's nice
and bright and the brightness is steady. Unfortunately, the brightness
is constant only when it has "warmed up".

The KTP frequency-doubling crystal may be more temperature-sensitive than
is normal. This is a known characteristic, but perhaps the crystal is
damaged or improperly aligned.

 

[Samuel M. Goldwasser]

The KTP frequency-doubling crystal may be more temperature-sensitive than
is normal. This is a known characteristic, but perhaps the crystal is
damaged or improperly aligned.

It may be normal. Green pointers have no temperature control and the
entire process is tempterature critical. So as various parts warm up,
the efficiency changes and this can be significant. Some are worse
than others but they all vary. At least yours is brightest when
it's warm.

Much more in the Laser FAQ.

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[GregS]

It may be normal. Green pointers have no temperature control and the
entire process is tempterature critical. So as various parts warm up,
the efficiency changes and this can be significant. Some are worse
than others but they all vary. At least yours is brightest when
it's warm.

Much more in the Laser FAQ.

Mine has a hang up when warm. There is a dim then bright effect.
I measured mine close to 5 mw as advertised. Many are a lot less.
I thought mine was a gonner one time. The dot had a strange display and was
very dim. After I removed the lint it was fine. I suppose there are a few variables that
are going to cause effects.

greg

 

[Dave Martindale]

[email protected] (Mike S.) writes:

It may be normal. Green pointers have no temperature control and the
entire process is tempterature critical. So as various parts warm up,
the efficiency changes and this can be significant. Some are worse
than others but they all vary. At least yours is brightest when
it's warm.

More info: I put the pointer into the refrigerator to cool for a few
hours. At that temperature, the green output is barely visible
projected onto a surface a half inch from the end of the pointer. I
measured the current drawn from the batteries (remove end cap, insert
ammeter into the circuit), and it was about 270 mA - almost the same as
when it is operating at its brightest. Current appeared to be steady
too.

I also had the beam hitting a surface at a shallow angle, and I could
see that as the output changed in intensity, the intensity profile
across the beam changed a great deal. Looks like one of the lasers (I
don't know whether the 800 nm or the 1060 nm one) was changing its mode
mixture quickly.

Once fully warmed up, it was back to normal.

From this, I'd conclude that the electrical side of the pointer is fine:
no intermittent connections, and the laser diode current regulator is
working. But at least one of the optical components is extremely
temperature sensitive. It seems that if this particular pointer is
going to be useful for pointing out things in the sky on cold clear
nights, it will have to be kept in an inside pocket between uses.

Thanks to everyone who commented.

Dave

 

[[email protected]]

"If it sits in a cool room (say 15 degrees C) for a while before I use
it, the inital brightness of the beam appears quite low - perhaps 10%
or less of maximum output. If I keep it turned on, it will flicker -
becoming brighter then dimmer again multiple times, remaining at one
brightness for a fraction of a second to a couple of seconds before
abruptly changing brightness again. After about a minute of operation,
it seems to come to full brightness and remain there, without flickering."

Laser pointer needs to be preheated before pointing if the room temperature is lower than 25 degrees C.Usually you can hold the laser in hands tightly about 30 seconds before pointing.
The low temperature might cause unstable and lower power pointing in operation.

 

[William Sommerwerck]

Laser pointer needs to be preheated before pointing if the room

temperature is lower than 25 degrees C. Usually you can hold the
laser in hands tightly about 30 seconds before pointing. The low
temperature might cause unstable and lower power pointing in
operation.

My condo is unheated, and is currently around 60 degrees F. (It has been as
low as the high 40s during this winter.) All the laser-optical devices in my
condo work properly -- including the DVD players in my computers, the DVD
player in my bedroom, the BD player in the living room, and even a 25-year-old
LV player.

The idea that a semiconductor laser has to be at a minimum temperature to work
strikes me as decidedly odd. I'd expect the pointer to work if it were dropped
in liquid nitrogen (at least until the battery froze).

 

[Mike S.]

My condo is unheated, and is currently around 60 degrees F. (It has been as
low as the high 40s during this winter.) All the laser-optical devices in my
condo work properly -- including the DVD players in my computers, the DVD
player in my bedroom, the BD player in the living room, and even a 25-year-old
LV player.

The idea that a semiconductor laser has to be at a minimum temperature to work
strikes me as decidedly odd. I'd expect the pointer to work if it were dropped
in liquid nitrogen (at least until the battery froze).

Hint: In a DPSS laser assembly, it isn't the semiconductor component that
is the heat-sensitive factor.

 

[Paul Drahn]

"5mw Green laser warmup - highter power trick"
<

>

The common green laser pointer starts with an 808 nm IR laser, which
excites a crystal to produce 1064 nm light, which then feeds a
frequency doubling crystal to produce green 532 nm.
<http://en.wikipedia.org/wiki/Diode-pumped_solid-state_laser>
Both crystals are temperature sensitive.

I have an early model green pointer which I use to boresight align
mountain top dish antennas. My personal best is 5 miles, although
others have gone much further with high power green lasers.

The instructions didn't say anything about warming up the laser, so my
first attempt on a 6400 ft mountain top in Smog Angeles failed due to
lack of light output. I eventually figured it out and now have a
small battery operated hand heater and temp controller to keep around
room temperature.

One of the printed circuit boards we build for a local customer is a
heater board for their green diode laser. The board is .090 thick, with
silver plated traces. 4 wire wound resistors surround a hole where the
laser diode goes.

These are for a outside, cold winter, application.

Paul

 

[William Sommerwerck]

Thanx to Jeff and Mike for the clarifications.

 

[gregz]

"If it sits in a cool room (say 15 degrees C) for a while before I use
it, the inital brightness of the beam appears quite low - perhaps 10%
or less of maximum output. If I keep it turned on, it will flicker -
becoming brighter then dimmer again multiple times, remaining at one
brightness for a fraction of a second to a couple of seconds before
abruptly changing brightness again. After about a minute of operation,
it seems to come to full brightness and remain there, without flickering."

Laser pointer needs to be preheated before pointing if the room
temperature is lower than 25 degrees C.Usually you can hold the laser in
hands tightly about 30 seconds before pointing.
The low temperature might cause unstable and lower power pointing in operation.

Mine goes dim when warm, after trying to use it or a while.

Greg