laser warning receiver is a type of warning system used as a passive military defence. It detects, analyzes, and locates directions of laser emissions from  laser guidance systems and laser rangefinders. Then it alerts the crew and can start various countermeasures, like smoke screen, aerosol screen (e.g. Shtora), active laser self-defence weapon with laser dazzler (LSDW, used on the Chinese Type 99 main battle tank ), laser jammer, etc.
Detectors used in LWR are usually based on a semiconductor photodetector array, which is typically cryogenically or thermal-electric cooled. Sometimes avalanche photodiodes (APD), photoconductivity, photoelectromagnetic, or photodiffusion devices are used even without cooling.
Some devices detect only the main beam of foreign lasers while others detect even scattered rays.
Some models used by US are listed:
AN/AVR-2 and AN/AVR2A
Electronic Warfare in Operations: U. S. Army Field Manual FM 3-36. 25 February 2009. p. A-4. ISBN 9781437914245 . Retrieved . 7 February 2015 (PDF version 4.53 MB)
"Archived copy". Archived from the original on 2011-12-06 . Retrieved . 2012-01-31 CS1 maint: archived copy as title (link)
"Laser Warning Systems". BAE Systems. 2008-02-14. Archived from the original on 2009-11-06 . Retrieved . 2009-10-22
"Laser Warning Detector". Thales. 2008-02-14. Archived from the original on 2012-03-08 . Retrieved . 2009-10-22
^  PentaTec ARGUS
ASELSAN LİAS for Airborne Platforms
ASELSAN LİS for Naval Platforms
ASELSAN TLUS for Ground Platforms
AN/AVR-2 LWS at fas (airborne)
Goodrich AN/VVR-1 Laser Warning Receiver System (LWRS)
Goodrich AN/VVR-2 Laser Warning Receiver System (LWRS) (land laser warner)
High-speed FT Spectrometer for Laser Warning Receivers
Battlefield Laser Warning Receiver Homebrew laser warning receiver