red laser diode multimode

[harryd]

hello everyone,

i am doing an experiment to measure the beam profile. i am scanning an optical fiber across the beam and then using a self built detector.

my laser source is a red laser pointer and i would like to know if the red laser diode is a multimode or a single mode laser. also, i want to know whether to choose between the single mode fiber or a multi mode fiber as a tradeoff between resolution and intensity.

[krazer]

The diodes in a laser pointer are usually single 'latitudinal' mode (ie-they give a good Gaussian TEM 00 beam) however most of them have several longitudinal modes (ie, they lase at several very closely spaced wavelengths, all well within a 1nm range). Note however- because the emitter is not round on a diode laser, it is a rectangle roughly 1um tall by a few um long, the beam will have an elliptical profile which diverges faster in the vertical axis than the horizontal 'slow' axis. This roundness is not to be confused with a non-gaussian beam (such as a tem01 beam, or the type of beam that you get out of the new 445nm very multi mode diodes) because all you need to make it 'perfect' again is a beam stretcher that expands the fast axis to have the same divergence as the slow axis--after you get it corrected it will stay more or less round at any distance from the laser.

I personally would recommend using a piece of singlemode fiber because it is smaller (roughly 10um diameter, as opposed to 50um for the smallest multimode fiber) so you will get a higher resolution. Photo diodes are very sensitive, so even with a 1mw 1mm diameter beam you should get about .1uw through the fiber, which should be more than enough to detect on a decent photodiode. Just make sure you bias it properly and shield it from outside light.

Sorry if any of that was unclear, I am in the middle of finals and a bit out of it

[harryd]

well i have a question. if i a the laser pointer and scan a multimode fiber across it in the experiment. what will happen in terms of intermodal dispersion.

i think that since there is only one mode it wont make any difference right?