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Several questions about laser diodes...

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Externet

Well-Known Member
Several questions about laser diodes...

-When a laser diode is said to be a 30 mW, is that the optical power output or the power fed to it ?

-If the voltage across anode and cathode is 2.5Vf; a 100mW laser diode is to have 0.1 / 2.5 = 40mA current ?

-There was awell written tutorial article on the web about a clever simple method to determine the maximum safe operating current for an unknown/unmarked laser diode which I believe was based in perhaps observing a subtle 'knee' when plotting observed V I curves. I cannot find it now. Would you know about it ?

-A laser diode can focus to a few micrometers dot, but as seen on typical lasers, they project a spot of about a couple of millimetres diameter. What am I missing ? How to reduce the diameter of the beam ?

-Several same power laser diodes of different visible wavelenghts; which one is more dangerous, as to produce burns or retinal damage ? Red, green, blue... ?

-Laser diode dies do not use a mirror in one end as gas lasers? (https://assets.newport.com/web600w-EN/images/1225583.gif)

-Is there any passive goggles that allow seeing an infrared laser beam ? (not directly)

-Would a laser beam that can burn your skin do the same 500 yards away ?
 

richardb

New Member
-When a laser diode is said to be a 30 mW, is that the optical power output or the power fed to it ?

that's what a reputable seller would say

-If the voltage across anode and cathode is 2.5Vf; a 100mW laser diode is to have 0.1 / 2.5 = 40mA current
?

only if the laser was 100% efficient.

-
There was awell written tutorial article on the web about a clever simple method to determine the maximum safe operating current for an unknown/unmarked laser diode which I believe was based in perhaps observing a subtle 'knee' when plotting observed V I curves. I cannot find it now. Would you know about it ?

I don't know
-A laser diode can focus to a few micrometers dot, but as seen on typical lasers, they project a spot of about a couple of millimetres diameter. What am I missing ? How to reduce the diameter of the beam ?

what you see coming out of most lasers is just a collimated beam you will need a lens to focus down to a small spot.

-Several same power laser diodes of different visible wavelenghts; which one is more dangerous, as to produce burns or retinal damage ? Red, green, blue... ?

they are pretty much all the same danger, green will probably look brighter but its the power/pulse energy that will damage the cells
-Laser diode dies do not use a mirror in one end as gas lasers? (https://assets.newport.com/web600w-EN/images/1225583.gif)

depends on your definition of mirror. all oscillator lasers as far as I know have some kind of mirror at both ends. one will be as close to 100% reflective as possible the other one depends on the gain of the laser medium, sometimes 1% reflect sometimes 99%, don't forget uncoated glass is around 4% per surface.
-Is there any passive goggles that allow seeing an infrared laser beam ? (not directly)

no

-Would a laser beam that can burn your skin do the same 500 yards away ?

no
 

ronsimpson

Well-Known Member
Most Helpful Member
-When a laser diode is said to be a 30 mW, is that the optical power output or the power fed to it ?
Looking at some data sheets: "3mW output power". Most LEDs are measured in input power.
I noticed that some LED lasers are 1 to 3% efficient.
-A laser diode can focus to a few micrometers dot, but as seen on typical lasers, they project a spot of about a couple of millimetres diameter. What am I missing ? How to reduce the diameter of the beam ?
Lenses will focus the light to a small spot (at some point). This will also cause the light to spread out beyond that point.
-Would a laser beam that can burn your skin do the same 500 yards away ?
That depends on how much the light diverges. If the bean is twice as wide then the area is 4x and the energy/area is 1/4.
It also depends on how much energy is blocked by air. (that depends on the wave length of the light)
 

Joe G

Member
-Is there any passive goggles that allow seeing an infrared laser beam ? (not directly)

use your digital phone camera
 

crutschow

Well-Known Member
Most Helpful Member
-Is there any passive goggles that allow seeing an infrared laser beam ?

No goggles. But there are companies that make laser viewing cards such as this which fluoresce when placed in the light beam.
 
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steveB

Well-Known Member
Most Helpful Member
I'll run through and give my take on answering these. I apologize that many of my answers might be the same as above, but it is just easier to answer directly without trying to figure out where I'm duplicating and agreeing with answers above.

-When a laser diode is said to be a 30 mW, is that the optical power output or the power fed to it ?
Any of the good quality and expensive laser diodes I used to work with, were specified very precisely in terms of output optical power. If one wanted to know the electrical input power, the current curve and voltage specification could be used to figure that out quite easily.

-If the voltage across anode and cathode is 2.5Vf; a 100mW laser diode is to have 0.1 / 2.5 = 40mA current ?
Generally more current is needed. There is a threshold current needed to get lasing going, then there is a quantum efficiency issue which does not allow all power to be converted over.

-There was awell written tutorial article on the web about a clever simple method to determine the maximum safe operating current for an unknown/unmarked laser diode which I believe was based in perhaps observing a subtle 'knee' when plotting observed V I curves. I cannot find it now. Would you know about it ?
I never heard about this and I would be skeptical that this method can be used reliably in all cases. Obey manufacturers specifications and guidelines, or risk damage to the laser.

-A laser diode can focus to a few micrometers dot, but as seen on typical lasers, they project a spot of about a couple of millimetres diameter. What am I missing ? How to reduce the diameter of the beam ?
The spot size of a single spacial mode laser diode would indeed be at the micron level, not the mm level. But, lenses are used to create a collimated laser beam of the order of millimeters to make a beam that can travel long distance. The beam from a laser will not travel far at small size. This is just a diffraction limitation of light. You can reduce the size back down to micron level using very well design lenses suited to that purpose, but again, the focus point will be limited over a short distance.

-Several same power laser diodes of different visible wavelenghts; which one is more dangerous, as to produce burns or retinal damage ? Red, green, blue... ?
The visible wavelengths and near infrared are all pretty much equally dangerous. The so-called 'eye-safe' wavelengths are longer wavelengths (e.g. 1550 nm communications lasers, or 1480 nm pump lasers etc.) beyond 1400 nm which get efficiently absorbed by water. Hence, it is more difficult to damage the inside of the eye, because the wavelength is blocked and can not reach the retina. However, this is a bit of a misnomer, because high power lasers can still damage the eye. They really should be called "eye-safer"!

One benefit of visible lasers is that you can see them and can avert your eyes more easily. This is by no means a "safe" thing to do, but it can help minimize damage. Infrared lasers are particularly bad because you can't see it.

-Laser diode dies do not use a mirror in one end as gas lasers? (https://assets.newport.com/web600w-EN/images/1225583.gif)
The semiconductor/air interface is effectively a mirror, but it is a low reflection percentage. This is fine for the output end where you want to get light through anyway, but the other end wastes power, so often reflection coatings are added to that end, and sometimes also to the output end to improve the laser performance.

-Is there any passive goggles that allow seeing an infrared laser beam ? (not directly)
Not passive googles that I know of, but there are active googles made for different wavelength ranges. A passive method is the infrared cards mentioned above.

-Would a laser beam that can burn your skin do the same 500 yards away ?
If it is a collimated laser beam then yes the damage would be similar. It really depends on the quality of the beam. A high quality beam can go that distance and spread at the diffraction limit, which is determined by wavelength and spot size. Poor quality beams would diverge much faster and might cause less damage to skin, but would still not be safe for the eyes.
 

Externet

Well-Known Member
THANKS to all for elaborated responses on a bothersome multiple questions post; I know it may be discouraging to answer.

Externet said:
-There was a well written tutorial article on the web about a clever simple method to determine the maximum safe operating current for an unknown/unmarked laser diode which I believe was based in perhaps observing a subtle 'knee' when plotting observed V I curves. I cannot find it now. Would you know about it ?
...I never heard about this and I would be skeptical that this method can be used reliably in all cases. Obey manufacturers specifications and guidelines, or risk damage to the laser...

The situation implies unmarked/unknown data for the diode.
It is something like this:
Screenshot from 2014-02-02 11:36:30.png

Where reaching the first kink should be considered as a level to not surpass, the maximum safe operation point to be kept below that level.
 

steveB

Well-Known Member
Most Helpful Member
Ah, OK, this rings a bell now. I think what you are seeing is the appearance of additional spacial modes. So, if you want to operate a single mode laser, which is the most effective for coupling to fiber or for creating a diffraction limited free space beam, you want to operate below the knee.

I would still be cautious that perhaps not all lasers should be tested in this way, but many that I've dealt with can be operated at the higher power levels for short periods without degrading the laser. Usually it is the manufacturer that does this test and then qualifies the laser with an appropriate data sheet. However, if you are stuck and don't have the data, you might try to do these tests yourself.
 

ericgibbs

Well-Known Member
Most Helpful Member
hi Externet,
Do you have a specific application for this laser project.?
E
 

Externet

Well-Known Member
Hi Eric.
Am mostly learning about voids I have that did not see clear before or discerned from tutorials.
 
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