How to supply power to a diode for a laser and be able to vary it?

Hello! I am a student working on a portable laser project. The laser is going to be powered by two lithium-ion batteries at 3.4A capacity and 3.7 Nominal voltage (4.2 charging voltage cut-off and 2.5 discharging voltage cut-off). I'm using voltage regulators to step up or step down voltages for the components. I was informed by the optics student that he want to be able to vary the voltage to obtain different output power values. Unfortunately we've had problems with designing the laser since the 1064nm diode that we were going to use for the laser gave very poor results and we have to change that part. Now I am going to wait for my partner to decide on a laser pointer since this will be the next step. This information might not be useful right now but we were looking into powering the diode (1064nm) up to 1.8V max at a current of 640.104 mA. I wanted suggestions on how to power a diode or laser pointer and being able to lower and vary this voltage below that max voltage threshold. Should I used a voltage regulator and something after to vary the voltage or take a different approach?

You power a laser diode from a current source for best performance, and if it has a monitor diode you use that in a feedback loop to set the output power, since the monitor diode current is proportional to light intensity. There is a lot of information on the internet, including schematics, that should help you to design an appropriate circuit. Laser diodes, as I have found out, are quite sensitive to overcurrent, which may occur as a result of spikes on the power supply, it is customary to power a diode up slowly, and have good transient protection on the supply, they turn into very expensive LEDs otherwise.
I have used controllers from https://www.ichaus.de/ to good effect in the past, they have application notes, and used to sell through a web shop, contact them to get more details.
Cheers,
Richard

That's a mean laser. 1.8V at 640mA is over a watt! Unless you're cutting stuff I would not expect that much current??

I'm no expert with lasers but they work like LEDs. They are not voltage driven but current driven, so you want to regulate the CURRENT through them rather than the voltage across them. You can use the good old LM317 with a resistor between OUT and ADJ. for 640mA you'll need a 1.875 ohm resistor - you'd probably get away with a 1.8 ohm unless 640mA is the absolute limit (if so use 2 x 1 ohm in series). Put a 1 to 5 ohm pot in series with it to adjust the current down - 1 5 ohm pot would give you a current of 176mA when its at max resistance.

You could also use Pulse Width Modulation - arrange it so when pulse width is maximum you get your 640mA, then as you decrease the pulse duty cycle the average current (and the Laser brightness) will go down/ You could do this with a 555 and a transistor and a resistor in series with the laser.

I'd post diagrams but I've found it impossible to do so lately, if you really need them I can get Maurizio to put them in.

Further to your comments, laser pointer diodes are low power, no where near what you apparently need. They are also operate in the visible spectrum, unlike the infrared diode you initially mentioned.
What are you trying to achieve?
It seems to me that the optics person you mention needs to supply a specification, then a laser diode chosen and the control electronics designed.
At the power level you mention, permanent eye damage is possible, you need safety goggles for the wavelength you decide to use. I worked in a research laser lab for some years, and am only too aware of the dangers of these things.
More detail on your end application would help!
Cheers,
Richard