The availability of low cost but relatively high power laser pointers (hundreds of mW) has led to misuse with potentially dangerous consequences, such as dazzling aircraft which has raised concerns about aircraft safety. A low cost laser detection system based on coherence detection has been developed and is able to detect weak, continuous laser sources even against bright background light. In this paper, we introduce the use of a cone mirror to extend the horizontal field of view of the detector (originally at 3°) to 360° to detect incoming beams from different directions. With the additional use of a camera in the system, we also determine the direction of the incoming laser beam. Finally, the sensitivity between the original system and the cone mirror system are compared: the new system showed promising results with a sensitivity below 100nW

Benton, David M.

Grasso, Robert J.

Richardson, Mark A.

Sugden, Kate

Titterton, David H.

Zandi, Marie

English

Aston Publications Explorer

PROCEEDINGS OF SPIESPIEDigitalLibrary.org/conference-proceedings-of-spieDevelopments in low-cost laserdetection: wide field of viewimplementation and directiondeterminationMarie A. Zandi, David M. Benton, Kate SugdenMarie A. Zandi, David M. Benton, Kate Sugden, "Developments in low-costlaser detection: wide field of view implementation and directiondetermination," Proc. SPIE 11161, Technologies for Optical CountermeasuresXVI, 111610H (7 October 2019); doi: 10.1117/12.2532560Event: SPIE Security + Defence, 2019, Strasbourg, FranceDownloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 14 Oct 2019  Terms of Use: https://www.spiedigitallibrary.org/terms-of-useDevelopments in low-cost laser detection: wide field of viewimplementation and direction determinationMarie A. Zandia, David M. Bentona, and Kate SugdenaaAston Institute of Photonics Technologies, Aston University, Aston Triangle, Birmingham B47ET, UKABSTRACTThe availability of low cost but relatively high power laser pointers (hundreds of mW) has led to misuse withpotentially dangerous consequences, such as dazzling aircraft which has raised concerns about aircraft safety. Alow cost laser detection system based on coherence detection has been developed and is able to detect weak,continuous laser sources even against bright background light.1 In this paper, we introduce the use of a conemirror to extend the horizontal field of view of the detector (originally at 3◦) to 360◦ to detect incoming beamsfrom different directions. With the additional use of a camera in the system, we also determine the direction ofthe incoming laser beam. Finally, the sensitivity between the original system and the cone mirror system arecompared: the new system showed promising results with a sensitivity below 100nW.Keywords: Laser warning systems, coherence discrimination, cone mirror, laser direction, field of view1. INTRODUCTIONWith the creation and the development of lasers in the early 1960s, the question of how to detect this technologyhas naturally followed. In the military, this question has raised even more interest since lasers have been usedfor decades for targeting, range finding, designation, and missile control.2 Laser Warning Receivers (LWRs)have been developed to identify the nature of the laser threats as well as the direction of irradiation. Once thisinformation is known, the appropriate countermeasure such as smoke screens can be taken. The LWR systemsare meant to be used on land, sea or vehicles but can also be used on buildings and air platforms. It is importantto get a wide field of view from those devices as it will allow a better coverage on the battlefield.Most often, LWRs are composed of three units: an optical unit, a detection unit, and a processing unit.3 Thefield of view of a LWR depends on the composition of the optical and/or detection unit but can be extended withadditional optical components. For example, imaging laser detection systems (ILDS) are usually composed of afish-eye lens and CCD or CMOS camera devices4 and can reach 180◦ of field of view. Oftentimes several ILDSunits needs to be combined together to reach a 360◦ full azimuthal coverage.5 However, this is not a cost-effectivesystem. Interferometer based detection systems are another type of LWRs and are based on coherence detection,their field of view will be restricted by the size of the interferometer aperture. A Fabry-Prot based LWR hasalready achieved a horizontal field of view of 160◦6 while a static Michelson interferometer associated with anoptical antenna has demonstrated a 45◦ horizontal coverage.7 Sometimes, the field of view needs to be reducedto 120◦ in order to increase sensitivity in the system.8 Diffraction gratings can also be used as part of detectionsystems, and typically relies on the use of a CCD camera. In 2014 , a fish eye lens was combined with a sinusoidalamplitude grating and the system is able to achieve a 160 ◦ field of view.9The laser detection system used for this work is composed of a Mach-Zehnder interferometer, combined witha balanced detector composed of two photodiodes at the outputs of the interferometer and a data acquisitionunit (USB-6341 Multifunction I/O Device) used to digitize and send the data to a computer.1 The field of viewof this system has been measured and is equal to ±3◦, this value being due to the detector size (3mm long).The aim of this work was to expand the field of view thereby making the device more practical. In addition tothe field of view, the incoming beam direction has been studied as it is essential to determine the origin of thethreats.Further author information:David M. Benton: E-mail: d.benton@aston.ac.ukMarie A. Zandi: E-mail: zandima@aston.ac.ukTechnologies for Optical Countermeasures XVI, edited by David H. Titterton, Robert J. Grasso, Mark A. Richardson, Proc. of SPIE Vol. 11161, 111610H · © 2019 SPIE · CCC code: 0277-786X/19/$21 · doi: 10.1117/12.2532560Proc. of SPIE Vol. 11161  111610H-1Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 14 Oct 2019Terms of Use: https://www.spiedigitallibrary.org/terms-of-use2. CONFIGURATION OF THE LASER DETECTION SYSTEM - COHERENCEMODULATION2.1 Principle of the Original SystemThe laser detection system works by detecting light sources that have a significant coherence length ratherthan the conventional detection of enhanced brightness. For this to work, the path difference between the twoMach Zehnder arms are set to be longer than the coherence length of the background light, which is the casewith distances over few micrometers long. A piezo-mounted mirror replaces one of the two reflective mirrorsof the original Mach-Zehnder interferometer to modulate the path length in one arm. Thus the path differencecombined with the modulating element cause the output signal to be modulated at a known frequency only ifthe input light has a longer coherence length than the path difference. In other words, detecting a modulatingsignal will indicate the presence of laser light in the system. A schematic representation of the detection systemis shown in Fig. 1.Figure 1: Configuration of the Mach-Zehnder laser detection systemThe plano-convex lens (focal length f=7.5cm) helps to focus the laser beam on the two photodetectors.The beam splitters and the mirrors are 1cm long. The total optical path length from the focusing lens to thephotodetectors is equal to 7.5 cm and this limits the field of view. The photodiodes are Si type and have awavelength detection range from 350nm to 1100nm. For this experiment, we used a 635nm laser diode and thepiezo mirror is modulated at a frequency of 600Hz and an amplitude of 2.7V.To detect modulating signals at the output, the output voltage was sampled and a Fourier transform analysisis made: peaks in the Fourier transform at the modulation frequency (and its harmonics) are evidence of coherentinput. A rolling average spectrum was generated by summing consecutive frequency spectra. The graph in Fig.2 gives an example of a signal modulated at 1200Hz detected by the two photodiodes and the Fourier transformapplied to the signal where the first and second harmonics are visible.2.2 Implementation of the Wide Field of ViewA Mach-Zehnder interferometer has the advantage of two distinct outputs, which implies that the two outputscan be used for two different applications at the same time. In this work, one of the outputs will be usedto measure the intensity of the modulating signal via a photodiode while the second output will be used todetermine the direction of the beam. A wide field of view will be achieved by using a cone mirror. A cone mirrorProc. of SPIE Vol. 11161  111610H-2Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 14 Oct 2019Terms of Use: https://www.spiedigitallibrary.org/terms-of-use-1.14-1.13-1.12-1.11-1.1-1.090 200 400 600 800 1000Amplitude (a.u.)Time (ms)1.2E-04 1.0E-04 8.0E-05 6.0E-05 4.0E-05 2.0E-05 0.0E+000 500 1000 1500 2000 2500 3000 3500 4000Amplitude (a.u.)Frequency (Hz)Figure 2: Example of an output signal modulated at 1200 Hz received by the photodiodes and its Fouriertransformis composed of a cylindrical base and a cone top. The top has a coating allowing reflection of a good percentageof the light towards the direction perpendicular to the input light as shown in Fig. 3. The cone mirror used inthe experiment has a diameter of 1cm and a total length of 1.5cm and reflects at least 80% of the light.Figure 3: Cone mirror principleThis element is positioned to direct light into the interferometer. A camera looking through the interferometerat one of the output ports is focused on the cone mirror. Capturing an image of the cone mirror allows thedirection of the laser input to be observed. A laser was shone into the system at various angles separated by 45◦angles. The amplitude of the first harmonic and a camera image were recorded in each case. The set-up of ourexperiment with the cone mirror is shown in Fig. 4 and the 0◦ angle was set as illustrated.Proc. of SPIE Vol. 11161  111610H-3Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 14 Oct 2019Terms of Use: https://www.spiedigitallibrary.org/terms-of-use635nmCone mirrorCameraModulating signalSignal processingPhotodetectorDetector 0° 180°90°Amplifier Signal generatorFigure 4: Configuration of the laser detection system with the cone mirror3. RESULTS3.1 Determination of the Direction of IrradiationCaptured images of the cone mirror when the laser was shone from 0◦ and 45◦ are shown in Fig. 5. The reflectedbeam is visible on the cone mirror; from these captures the direction is easily determined.Figure 5: Captured images of the cone mirror when the laser is at 0◦ and 45◦.All the data of the first harmonic amplitude and of captures of the cone mirror at different angles are combinedin the graph of Fig. 6. These results show that the laser is detected as a modulating input at every given angle.The measured amplitudes of the FFTs are very good and the direction of the laser beam can be determined fromthe camera.3.2 Comparison of Input and Output Power for Original and Cone Mirror SystemsIt is then interesting to look at the difference between the laser power before the laser beam entered the detector(measuring the power of the laser beam itself) and the laser power after it left through the last beam splitter(measuring the power at one of the two outputs) to know the attenuation of the light through the two differentdetection systems. In Fig. 7, the input/output power ratio for the original system and for the cone mirror systemare shown.For the original system, the transmission efficiency is 37% while for the wide field of view system, thetransmission efficiency is 2.5%. For the original system, 50% efficiency was to be expected as only one of the twoProc. of SPIE Vol. 11161  111610H-4Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 14 Oct 2019Terms of Use: https://www.spiedigitallibrary.org/terms-of-useFigure 6: Combined data of experiment results on cone mirroroutput ports is used in the measurement. The efficiency loss is due to the optics. In the cone mirror system,the additional losses comes from the reflected beam, which was widely distributed by the cone mirror and onlya part of the laser light would reach the detector input.3.3 Comparison of Sensitivity for Original and Cone Mirror SystemsIn this part, the sensitivity of the system has been measured. It corresponds at the minimum laser power weneed to distinguish the signal from the noise level. The sensitivity measurements have been done by measuringthe Signal to Noise ratio (S/N) for different laser powers. The intensity of the laser diode is attenuated by usingNeutral-Density filters and a power meter is used to measure the new laser input power level (values from 100µWto 10nW). For each laser power level, the signal to noise ratio is calculated by measuring the amplitude of thefirst harmonic of the signal FFT and by comparing it to the noise level. The results are plotted in Fig. 8. If theultimate sensitivity is defined to be equal at the power when S/N=1, then the system is able to detect a signaldown to around 70nW for the cone mirror system. These results are quite interesting, knowing that without acone mirror (original system) the ultimate sensitivity was around 2nW.3.4 Verification of Laser Detection SystemTo demonstrate that the cone mirror system detects only coherent sources, an LED and a laser were both directedtowards the cone mirror. Both were at similar wavelength (635 nm) and can be seen on the camera image inFig. 9. The LED appears significantly brighter on the camera image; however, no change in the amplitude ofthe first harmonic was seen when the LED was turned on and off. As expected the LED contributes nothing tothe modulation signal.Proc. of SPIE Vol. 11161  111610H-5Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 14 Oct 2019Terms of Use: https://www.spiedigitallibrary.org/terms-of-use0.11101001000100000.1 1 100 1000Input power (μW)10Output power (μW)Cone mirror systemOriginal systemLinear (Cone mirror system) y = 39.28x - 5.34Linear (Original system)y = 2.70x + 0.24Figure 7: Comparison of input and output power for the cone mirror and the original systemsR² = 0.9988R² = 0.99651.E+001.E+011.E+021.E+031.E+041.E+051.E+061.E+070.001 0.01 0.1 10 100 1000S/N1Input power (μW)Cone mirror systemOriginal systemFigure 8: Comparison of sensitivity for the cone mirror and the original systems4. CONCLUSIONWith a cone mirror input, a CW laser is identified as a modulating input for a range of input angles whichdemonstrates a 360◦ field of azimuthal detection. Images captured by the camera show the position of the lasersource on the cone mirror from which the source direction can be inferred. Using only one output of the Mach-Zehnder interferometer with a photodiode is enough to detect a laser signal with a large amplitude. Moreover,an LED is not detected by our detector showing the ability to discriminate lasers from non laser sources. Sothis detection system based on interferometry has been shown to be reliable in detecting the presence of a CWlaser and to find the direction of the beam. This system has a high sensitivity and a wide spectral range (in thiscase the photodiode has a spectral range from 350nm to 1100nm) and the spectral range can easily be extendedto infrared with the adapted optical sensors. The acquisition time of the detector is less than one second and itdepends on the source intensity. Futhermore, the determination of the wavelength is possible if we use the piezomirror modulation.1Proc. of SPIE Vol. 11161  111610H-6Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 14 Oct 2019Terms of Use: https://www.spiedigitallibrary.org/terms-of-useFigure 9: Captures of the LED and the laser lights on the cone mirror.ACKNOWLEDGMENTSI would like to thank the Defence Science and Technology Laboratory (Dstl) for the funding of my PhD as partof the Future Sensing and Situational Awareness programme and to thank my supervisors for their regular helpon this research.REFERENCES[1] Benton, D., “Low-cost detection of lasers,” Opt. Eng. 56(11), 114104 (2017).[2] Dubois, J. and Reid, F., “Detecting laser sources on the battlefield,” Proc. SPIE 6796, 67962F (2007).[3] Mohammad Nejad, S., Arab, H., and Ronagh Sheshkelani, N., “Analysis of new laser warning technologiesto propose a new optical subsystem,” IJEEE 14(3), 213–221 (2018).[4] Chen, B. and Zhang, W., “Study on warning radius of diffuse reflection laser warning based on fish-eye lens,”Proc. of SPIE 8907, 89072Z (2013).[5] Dabrowski, M., Mlodzianko, A., Pietrzak, J., Zygmunt, M., and Niedzielski, R., “Laser warning receiverlwr-h,” Proc. of SPIE 6598, 65980S (2007).[6] Siebert, E., “Analyzer for coherent radiation,” Pat. 4,309,108 (1982).[7] Li, X., Zhang, J., Tian, E., Zhang, Y., and Wang, Z., “A new design for laser warning system,” SSIP’07Proceedings of the 7th WSEAS International Conference on Signal, Speech and Image Processing, 187–189(2007).[8] Russell, J., “Coherent laser warning system,” Pat. 6,151,114 (2000).[9] Wang, L., Dong, H., Zhang, W., and Shen, X., “Optimum design of wide angle laser detecting system basedon fish eye lens and sinusoidal amplitude grating,” Opt. Commun. 310, 173–178 (2014).Proc. of SPIE Vol. 11161  111610H-7Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 14 Oct 2019Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

Developments in low-cost laser detection: wide field of view implementation and direction determination

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Developments in low-cost laser detection: wide field of view implementation and direction determination

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