private link - MT
https://wp.optics.arizona.edu/opti511r/wp-content/uploads/sites/23/2016/01/HW9_2016.pdf
https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-974-fundamentals-of-photonics-quantum-electronics-spring-2006/lecture-notes/chapter6.pdf
http://home.thep.lu.se/~larsg/Site/SM2.pdf