You can use 980 nm laser light

You can use a LED or a laser source. Try in Newport, Edmund optics, etc

The simplest  and cheapest way is to use tungsten filament-lamp (about 200 W in the case of using monochromator-spectrgraph for the spectral dependence of the photoresponse or 100 W or less in the case of the direct incident geometry). All that remains is to callibrate lamp with the power-meter if it is necessary. Inspite of weak efficiency of filament-lamp as a source of the light, in my current expirements with NIR photodetectors characterizing, I was abble to achive several mW of incident light at 1550 nm after the lens, monochromator and filter. If you have some doubts you are welcome to see my last publication. I wish you success in experiments.

Thanks all friends, specially Alexander.

Your answers helped me by providing sufficient information.