3.1 volt 400 mA laser driver
You've misread the data sheet. Nominal operating current (according to the data sheet) is ~200 mA. In fact, the 400 mA limit is mentioned nowhere in the data sheet, and I'd guess you got it from the eBay listing. A word of advice - when it comes to believing an eBay listing or a data sheet - go with the data sheet.  With this in mind, your best bet (for consistent power levels) is







simulate this circuit &ndash; Schematic created using CircuitLab

I recommend that you find a copy of the LM317 data sheet and read it. The circuit is one of the application circuits. The biggest drawback of the circuit is that the LM317 will dissipate nearly 2 watts, so you need a decent heat sink on it.

Going with the 5 volts supply/resistor is workable, but the resistor should be 10 ohms, in order to limit the current appropriately.

And don't forget to heatsink the laser.