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I am interested in whether the combination of a spectrometer and a tunable notch filter with notch frequencies at the visible red, green and blue wavelengths which have very fast response times could be used to deflect laser beams aimed at airplane windows. The purpose of the spectrometer is to detect the wavelength(lambda) of the incoming laser beam and then tune the optical notch filter to the notch wavelength lambda.

I am currently reading a paper,

Independently tunable optical notch filter based on double ring resonator structure, J. Zhang, S. Guo, X. Li. Optik 124 no. 12 (2013), pp.1307-1310.

This paper describes a tunable optical notch filter based on the Mach-Zehnder interferometer (MZI) with cascaded double ring resonator structures. Does anyone know whether the optical notch filter described in this paper could be used to deflect red, green or blue laser beams aimed at airplane windows?

I am also reading a paper,

Demonstration of a Tunable Microwave-Photonic Notch Filter Using Low-Loss Silicon Ring Resonators, M. Rasras, et al. J. Lightwave Technol. 27 no. 12 (2009), p. 2105.

This paper discusses how to use thermo-optic phase shifters to set the coupling ratios, all pass filter phases and the Mach-Zehnder input-output couplers. While thermo-optic phase shifters can be tuned in milliseconds, subnanosecond tuning speeds should be achievable using technology currently being explored for the high-speed modulators such as carrier injection or carrier depletion in a reversed biased junction. The tuning speed must be fast as possible for the laser beam deflector to react as fast as possible to laser beams aimed at airplane windows

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    $\begingroup$ I disagree with the close vote: whether it's possible for such a device to exist is a good physics question. $\endgroup$
    – rob ♦
    Commented Aug 1, 2014 at 19:59
  • $\begingroup$ In theory, Gallium Arsenide and Indium Phoshide electro-optic modulators offer picosecond tuning speeds . $\endgroup$
    – Frank
    Commented Aug 2, 2014 at 8:58
  • $\begingroup$ @rob, Thank you for commenting on this post. May I ask you to briefly describe what factors I should take into account to determine whether it's possible for such a device to exist. $\endgroup$
    – Frank
    Commented Aug 3, 2014 at 7:30
  • $\begingroup$ Hi @Frank, this is out side of my area of expertise. My suspicion is that it'd be possible to construct such a filter on a laboratory bench, but a serious engineering challenge to make such a device as a window. $\endgroup$
    – rob ♦
    Commented Aug 3, 2014 at 12:50
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    $\begingroup$ @rob, This is the link , opticsinfobase.org/ol/upcoming_pdf.cfm?id=212486, for the paper, "Tunable Electro-Optic Filter based on Metal-Ferroelectric Nanocomposite for VLC(Visible Light Communications)" published by S. Arnon and Etai Rosenkranz. Thanks. $\endgroup$
    – Frank
    Commented Aug 5, 2014 at 7:45

2 Answers 2

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Generally speaking, tunable filters are not appropriate for airplane windows.

Optical notch rejection filters are based upon phase matching conditions. These phase matching conditions created by quarter-wave layers (e.g. see Thin-Film Optical Filters, by H.A. Macleod, for information on how to design thin film stacks). It is common for these filters to be sensitive to both wavelength and angle. So, a notch filter typically rejects a specific wavelength and a specific angle.

So, from a conventional thin film stack point of view, coatings can only reject a specific wavelength at a specific incident angle.

Next, consider the agile optical filter. This is typically created by changing the optical path length between two reflective layers. For example, a geometry that is similar to an airplane window would be a liquid crystal tunable filter or an electro-optic cell.

The LCTF (liquid crystal tunable filter) operates as a Fabry-Perot cavity (see this device for an example). This configuration transmits a series of narrow bands rather than rejecting a series of narrow bands.

Let's assume you get over this hurdle with the appropriate coatings. The geometry looks right (plane-parallel window) and you can reject the wavelengths you want. Let's also assume that your spectrometer is fast enough (pretty easy to do assuming the wavelength ranges are well known). Now you need to contend with speed.

In a plane-parallel configuration, any electro-optic type of filter will appear as a capacitor. The capacitance will determine the speed. Capacitance increases linearly with area and decreases with thickness. So, you need to generally divide up the area of the window into pixels so you have an array of capacitors rather than one big capacitor the size of the windshield.

To put this in perspective, 25 mm diameter LCTF's can operate in the kHz range. If you need to cover a windshield that is 250 mm in diameter, the operating frequency will drop by 10X.

Bear in mind that a more efficient solution may be a visor on a helmet rather than a windshield.

As you consider the possibilities, look at devices and projects like the following:

  1. Gentex Laser Visor
  2. Adaptive Agile Multi-Spectral Laser Protection Devices by Kent Optronics
  3. Kent Optronics Laser Protection

This is not a new problem and many have tried to address it with a variety of approaches.

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  • $\begingroup$ Please tell me how you determined that: "To put this in perspective, 25 mm diameter LCTF's can operate in the kHz range. If you need to cover a windshield that is 250 mm in diameter, the operating frequency will drop by 10X." Please tell me what technical papers to read. Thanks. $\endgroup$
    – Frank
    Commented Aug 16, 2014 at 23:42
  • $\begingroup$ When you discuss an array of capacitors are you referring to capacitors in series or parallel? Thank you $\endgroup$
    – Frank
    Commented Aug 19, 2014 at 20:22
  • $\begingroup$ By array of capacitors, what I mean is that a single, continuous windshield is a larger area. Dividing that up into a matrix of smaller areas is what I mean by an array of capacitors. Connecting them electrically is a different issue entirely that relies upon the driver electronics. Each capacitor must be driven separately if you want them to operate separately. In this case, you may want to operate the window as a single "device" (sort of like turning your computer screen totally white or totally black. $\endgroup$
    – user3533030
    Commented Aug 21, 2014 at 5:01
  • $\begingroup$ One might have to add a lot of VLSI wiring to control the individual capacitors for a 250 mm in diameter windshield. VLSI wires have a capacitance of 1 picofarad per centimeter. Is there a method to reduce the amount of VLSI wiring to control an array of capacitors? Thank you. $\endgroup$
    – Frank
    Commented Aug 21, 2014 at 19:33
  • $\begingroup$ LCD's operate this way. In the "old" days, a liquid crystal display had drivers on the edges (passive matrix). The modern LCD is active matrix where there is a transistor at every pixel (transparent!). Each pixel can be considered a capacitor that is charged with its own "local" transistor. $\endgroup$
    – user3533030
    Commented Aug 23, 2014 at 15:41
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I think people typically use conducting electrodes for window-sized areas, such as Indium Tin Oxide. If you want to tune the whole window at once, then maybe you can get away with just one large pair of electrodes with the nanosphere material sandwiched in-between.

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  • $\begingroup$ Indium Tin Oxide is best suited for near infrared. Please tell me what material is best suited for visible light Pockels effect. Thank you. Frank $\endgroup$
    – Frank
    Commented Sep 18, 2014 at 18:54
  • $\begingroup$ Lithium Niobate is not well suited for tunable optical notch filters operating in the visible light wavelengths because of absorption phenomena complications. Frank $\endgroup$
    – Frank
    Commented Sep 19, 2014 at 21:54
  • $\begingroup$ The M.I*.T Physics Ph.D candidate was correct when he told me that Lithium Niobate is not well suited for tunable optical notch filters operating in the visible light wavelengths because of absorption phenomena complications in the near infrared spectrum.Thank you. Frank $\endgroup$
    – Frank
    Commented Sep 24, 2014 at 8:12
  • $\begingroup$ Has anyone heard of MoS2(molybenium sulfide) tunable optical notch filters in the visible light range. Thank you. $\endgroup$
    – Frank
    Commented Oct 1, 2014 at 1:10

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