Re: Submarines and mines truly undetectable to active sonar might become a reality so
What is the effective range of LIDAR? Fighter borne IRST struggle to get near the 100km mark for detecting things, and visibility in water is far worse than in the air.
Depends pretty much on the water conditions. Murky waters means practically useless detection ranges for LIDAR, crystal clear waters means really good detection ranges.
Also, LIDAR is usually used by aircraft looking down into the water and not so much by ships or subs looking for other subs. This means that LIDAR can cover an area quite fast and is pretty good at picking up subs in shallow depths. Not sure how much the technology has advanced though.
Also, regarding the article you posted, it talks about acoustic invisibility in the ultrasound frequencies against incoming soundwaves. That's fairly selective wording as it does not mention what frequencies it won't work against (non-ultrasonic frequencies?) and how it masks soundwaves that are not external in source (such as internal machinery noise, water-flow noise and propeller noise).
Nonetheless, this technology is quite ground-breaking as it will make a stationary object with no internal noise source (sub or mine) invisible to sonar. In the case of mines, it may be a double-edged sword though since it is also invisible to whoever planted it.