That was the PXIe-4844, an optical sensor interrogator. It wasn't just for temperature -- the gratings (microscopic cuts) can measure strain too. It's useful for taking lots of measurements across a long distance with just a single cable (although it was more like ~15 sensors per fiber, not 100), in an intrinsically safe environment, and/or an electrically noisy environment (since the fiber is not affected by noise).
$20k was the price of a typical interrogator ~10 years ago. PXIe-4844 was obsoleted because NI exited the market. Other manufacturers are still in it; performance has gone up and price has gone down since then.