source (opto-acoustic)

https://www.google.com.br/search?q=laser+light+source+(opto-acoustic)&source=lnms&tbm=isch&sa=X&ved=0ahUKEwirkfWXwYHWAhUDRyYKHWTMABAQ_AUIDCgD&biw=1440&bih=770#imgrc=EPhj1Gvz1MzNEM:

https://www.google.com.br/search?tbm=isch&q=using+optical+fibre+and+a+laser+light+source+measure+low+frequency&spell=1&sa=X&ved=0ahUKEwiuoIqTwIHWAhXD4SYKHc91DUIQBQgjKAA&biw=1440&bih=770&dpr=1#imgrc=kxFgSuKBMckhEM: