I need a CW laser diode, with an integrated photodiode, with wavelength around 800nm and power 1-1.5mW. Although I didn't find anything that exactly matching my specifications, I did find this from Thorlabs and this from Roithner Lasertechnik. Unlike some of the other laser diodes I browsed through, these only specify a maximum power (of 10mW and 5mW, respectively), rather than a typical power, so this seems to imply that I can adjust the output power (in this case, to 1-1.5mW). Am I correct here? If so, then what are the consequences of doing such a thing (is any aspect of the laser diode and/or beam quality degraded in any way by this)?
Laser diodes have what is called a
threshold current which is the current through the diode at which it starts to lase.
With your thorlabs diode, the typical threshold is
25mA (The catalogue uses this term rather than the datasheet. https://www.thorlabs.com/catalogpages/V21/1234.PDF)
With your diode running at
2V 25mA you'd be delivering
50mW to the diode and it would start to lase, after that a parameter called
slope efficiency comes into effect which in your sheet is typically 0.5mW/mA. To get 1.5mW out you'd need to drive around 28mA into the diode.
Thorlabs don't seem to indicate what level of optical power output they define as lasing though so you might be well over 1.5mW by the time you reach that point.
Since you've got a bit of spread with regards to threshold current and slope efficiency you might have a hard time locking to 1.5mW without an external power meter and calibration of the diode driver.
Edit: It looks like the diode has an integrated photodiode which you could use as part of a closed control loop for controlling power. Which was probably your plan.
This isn't the best datasheet, to be honest, I would expect to see some LIV curves here that describe the diode a bit better.
I'm not an optical engineer but I don't expect an impact on beam quality based on how much power you supply the diode.
Additonally, note that this diode can put you into Class 3B territory which is why you need to make sure that if you use this diode or a diode capable of driving >5mW that you build your design so it is not capable of driving enough current to exceed the safe limit, attenuate the laser, or use a laser that is impossible to drive beyond the limit.
I suspect you're going about this the wrong way.
As VBwhatnow has answered, for the Thorlabs part, trying to run at 1.5 mW will probably be right down near the threshold current. Also note that you really can't tell exactly how you'll need to drive your laser: at the very least, for the nominal test power the photodiode current can vary over a 20:1 range and still be in spec. This doesn't mean that it will vary in operation, it means that from unit to unit the sensitivity can vary wildly.
I'd suggest that what you need to do is run the laser somewhere near maximum power. Note that, unless you have a power meter you will run the risk of overdriving the laser, so be careful. Once you have the laser running at a stable power, put an attenuator in the output. Thorlabs will provide you exactly what you need (for a price). Consider running the laser near 5 mW, and putting a 3x attenuator to get it near 1.5 mW.
I've mentioned a power meter. You don't say what your tolerance is for your desired power, but be aware that you'll need something to measure the power. The specs on the diode are simply not tight enough to closely predict what you'll get. Notice, for instance, that at 10 mW the Thorlabs laser will typically require 50 mA, but no more than 70 - and maybe as little as 25. So, if you drive 25 mA through it and it lases (which will be difficult to tell, since you can't see the output) you might be at the maximum 10 mW. Or you might be down around 1-2 mW. If your target is 10 mW, so you increase the current to 50 or 70, a sensitive laser will be grossly overdriven and may not survive.
You could roll your own using a commercially-available photodiode with a large(ish) active area and a well-defined sensitivity, and get decent results. Or you can try eBay, as long as you keep caveat emptor firmly in view. As a start, a lot of lab-grade laser power meters being offered do not include the (required) sensor head. Or you can get a new one from someone like Thorlabs. They sell good stuff, and charge accordingly.