I am sure most of us have seen those movies where red laser beams are used to guard a room against unwanted entry. They are based on a simple principle, someone crosses the beam, the beam no longer reaches its remote detector, the detector output changes state from "beam present" to "beam absent" and an alarm is raised.
This may seem all very well but what if
1. The intruder is a ghost, if such entities exist, and the beam is only slightly occluded, if at all?
or
2. The intruder has an "invisibility suit" that receives the beam, conducts it around the offending biology component and regenerates its path uninterrupted to the detector
In both cases we would have no way of knowing if some pesky intruder had transgressed our room?
Would it be preferable to monitor the phase of the received beam rather than just its amplitude? Ghosts are usually reported to be very ethereal things and probably would make little impact on the beam's amplitude but perhaps a slight difference in the speed of light would occur, much the same way as the speed of light "c" is usually explained to slow down when traveling through refractive materials such as glass. Equally, if not more so, this hypothetical "invisibility suit" would certainly introduce a longer path length; in both cases the phase of the received light beam would be altered even if its magnitude had minimal modification.
Since red light (TV movies always use red lasers for some reason) is about 850 nm it might be difficult to measure phase differences directly at this wavelength. However what if we modulated the light source's amplitude at 30 MHz for example. Since the speed of light in a vacuum isthen the "envelope} would have a much larger wavelength given by
This relatively short wavelength would allow fairly simple "optical envelope phase" measurements or more specifically, perturbations in "optical envelope phase"
i.e.- in radians
For example, let's say a fairly large intruder passed the beam with an invisibility suit on and this had an effective "additional path length"of 1 meter. Since 10 meters path loss would correspond to 360 degrees of optical envelope phase perturbation, 1 meter would create 36 degrees or perturbation whilst the beam was crossed. This would be easy to detect.
If a hypothetical ghost passed through the beam, perhaps its influence would be less. Perhaps it would causeth or
the optical envelope phase perturbation of an invisible man - would this be detectable?
Since we are detecting a change in phase we are looking for frequency modulation
Consequently, fast impatient intruders should create more FM than more lethargic ones. However the detection capability may now go well down into milli-degrees.
So it seems to me that if anyone is concerned about unwanted and invisible intruders sneaking into rooms where they really are not welcome, then a simple on/off laser beam detector isn't going to be much use. I can see the use of optical envelope phase promoted as a superior detection method however. Also, if the FM output was feed into an audio amplifier and speaker, I wonder if the ghosts, secretive aliens or invisible men could explain their intrusion by direct "voice over optical envelope phase modulation" of the beam?
(Also, would it need to be a laser - and old CD/DVD laser diode could be used, or perhaps a LED and lens with a photodiode and lens detector?)