Is there a tunable visible wavelength optical filter for exclusively filtering intense coherent laser light?

Question

emphasized text> Is it possible to design an tunable visible wavelength optical filter

for exclusively filtering intense coherent laser light while leaving the incoherent light background untouched?

Mr. Han-Kwang Nienhuys , the experimental physicist states that "If you want to protect a pilot against the most common high-power laser pointers (532 nm wavelength), you should be using dielectric mirrors as filters. Such mirrors have a coating that is a stack of thin layers, tuned to reflect light of a certain wavelength. The dielectric mirror can be designed to reflect only over a very narrow wavelength range, in which case they are called "notch filter". You can buy these off the shelf, for example Edmunds OD4 notch filter. A disadvantage for pilot protection is that the reflecting wavelength will depend on the angle of incidence αα. The only way to make them reflect 532 nm wavelength over a wide range of angles is by making them reflect over a large range of wavelengths. If you do that for green light, you will probably end up with something resembling very dark sunglasses, which only transmit deep red and deep blue light and block orange-yellow-green-cyan."

So another design criterion for the hypothetical tunable filter is insensitivity to large angles of incidence. It has been observed that large angles of incidence map to significant shifts in the center wavelength of the filter. What is the reason for this phenomena?

If so, how might one design such a thing?

There may be too much hoopala made today about the difference between coherent and incoherent light. If the source is fairly small (like sun, as it is very large but also at very large angle hence it is very small) , it will act as spatially coherent source. If you select a very small bandwidth it is also temporally coherent. Hence, one could stop worrying about coherent/incoherent thing.

The incoherent light form a background and the coherent light is a delta function riding over the background. If we have a narrow band reject filter(say 10 nanometers) could I exclusively remove the intense laser light without losing incoherent light visibility appreciably?

Please feel free to use formulas or drawings if you would like.

I spoke with the Edmund Optics application engineers in the United Kingdom this morning and asked how to exclusively remove red laser pointer light while letting red airport night runway light through untouched. The Edmund Optics application engineer stated that the spectral linewidth of incoherent red night runway lights is about 100 nanometers wide (50 nm FWHM) while the spectral linewidth of a red coherent laser pointer is about 2 to 3 nanometer. So , the Edmund Optics engineer recommended using either a custom notch matched filter with 2.5 nanometer FWHM similar to part number OD-86121 or an off-the-shelf notch filter with 25 nanometer FWHM. The off-the-shelf notch filter is less costly than the custom notch filter and is capable of blocking the variety of red laser pointers with slightly different center wavelengths.

The spectral linewidth is the first derivative of the optical bandwidth defined as 2 * pi * speed of light divided by center wavelength in nanometers.

Here is a proposed way to fix the weakness of Airbus and Nova Scotia's Metamaterials Inc.s Lambda Guard product where the only way to make them reflect 532 nm wavelength over a wide range of incidence angles is by making them reflect over a large range of center wavelengths because of the optical path length differences. One could design an array of 5 interferometers spanning 0 degrees of angle of incidence to 45 degrees of angle of incidence in 9 degree wide increments.

[EDIT 8/27/2016 11:46 AM Frank] The only way such an array would fix the weakness of Metamaterials Inc.s Lambda Guard product with regard to large incidence angles is to identify probable narrower angle ranges as I did in my volume of vulnerability analysis for the Boeing 767 which I will show here by Sunday evening August 28 2016, If I made the array wide band(0 degrees to 45 degrees) we would lose the transmission over all wavelengths hence defying our purpose.

Also , Metamaterials Inc.s Lambda Guard product may not be tunable with respect to wavelength causing it to be unresponsive to future frequency agile laser pointer threats.

I hope to show a drawing for this by tonight.

Answer 1

The problem is the dazzle effect overloading the retina. This has long been a problem for pilots in a (theoretical) nuclear war scenario, where the visor needs to blank within microseconds.

However, the modern problem, apart from laser "toys" is laser dazzle weapons. Once such approach to protection is this. Not perfect, but vastly cheaper than protecting from a nuclear weapon flash.

Answer 2

Last night, I read a new 2016 book called Seven Concurrency Models in Seven Weeks which has a chapter named Data Parallelism by Paul Butcher which I believe can be harnesssed to deconvolute the incoherent real image , in essence the landmarks , such as the Smithsonian Museum , where are obscured by a coherent RGB laser pointer shone by miscreants at a 767 cockpit window.

By calculating the correlation between the Fourier transform of two fields, Jeong-A Lee, In-Kyu Moon, Hailing Liu, and Faliu Yi wrote a paper called 3D Holographic Image Recognition by Using Graphic Processing Unit in the Journal of the Optical Society of Korea Vol. 15, Issue 3, pp. 264-271 (2011)

In this paper we examine and compare the computational speeds of three-dimensional (3D) object recognition by use of digital holography based on central unit processing (CPU) and graphic processing unit (GPU) computing. The holographic fringe pattern of a 3D object is obtained using an in-line interferometry setup. The Fourier matched filters are applied to the complex image reconstructed from the holographic fringe pattern using a GPU chip for real-time 3D object recognition. It is shown that the computational speed of the 3D object recognition using GPU computing is significantly faster than that of the CPU computing. To the best of our knowledge, this is the first report on comparisons of the calculation time of the 3D object recognition based on the digital holography with CPU vs GPU computing

On Jan 11 '15 at 21:05, Zaaikor wrote that an essential property of laser light is its coherence. This property makes it extremely useful, almost indispensable, for producing holograms.

But when Dennis Gabor invented holography in 1947, lasers didn't exist yet! He managed to squeeze some more-or-less coherent light out of ordinairy light from a mercury arc lamp by heavy filtering. Of course it was very far from perfect. Nevertheless, the first ever hologram was a fact.

Laser beams are often cleaned up with a so called spatial filter, basically a pinhole. Such a device was also part of Gabor's "heavy filtering".

http://light.ece.illinois.edu/ECE460/PDF/Holography.pdf

shareciteimprove this answer answered Jan 11 '15 at 21:05

I was pondering using Zaaikor's idea to clean up and filter tunable coherent laser pointer light which obscures an incoherent light image of a geographical landmark.