The method I'd recommend would be to build a simple spectrograph. It'll give you 100% accurate readings. It's much much easier and quicker to build than you might think and requires no electronics or expensive parts. You only need a simple lens (the one that came with the diode is probably OK) and a grating. I haven't tested this myself, but a thin radial slice of a DVD will probably do as well, since it works as a grating with about 1200 lines/mm. You may have to keep it flat by glueing it to a glass plate. Use the lens to create a sharp image of the diode's output at a large distance. 10 meters will be ok. Of course you can use mirrors to fold the path if space is limited. Make sure the diode is oriented in such a way that its highest divergence before the lens is in the horizontal plane. This will give you the smallest spot size in the horizontal direction in the projected image. Now bounce the beam of the grating/DVD, placed just after the lens and take a look at the first order. Make sure the lines of the grating are in the vertical plane, so the wavelength seperation is in the horizontal. Because the mode spacing for this diode is around 0.1 nm, the spacing of the modes in the image will be around 0.26 mm per meter distance. So at a distance of 10 meters, it will be 2.6 mm, not that hard to see. Because the optical system is less than perfect, instead of clear lines, you'll probably see a couple of dots seperated in the horizontal plane. There may even be some overlap, but that's no problem. If the dots come and go with changing the current, you spectrograph is working OK. The laser will be working single mode when only one dot is visible. It requires some fine tuning with the current to get it like this and over time it may slowly drift to multimode. Adjust the current a tiny little bit to get it single mode again and keep on doing this until it settles. You'll find that just waving your hand around the diode+heatsink or current source will thermally influence it, so try to avoid this, especially during an exposure.
Joe Farina wrote:Hi Thieu, yes thermal compound is part of the plan, I will be sure to keep it off the facet or window. By the way, I was wondering what you think of noise-detection methods for the 445nm diode. Do you think sampling the light output with a photodiode (and having an audible indicator, like with Wolfgang's noise-detector) is good, or would it be better to use something which generates fringes? Or maybe both? I have the feeling that the key to success with this diode will be in knowing when it's single-mode or not.