Mei-ting Wang,Yan Zhu,Yi-ning Mu

School of Science,Changchun University of Science and Technology,Changchun 130022,Jilin,China

Keywords:Laser eavesdropping Waveguidefiber taper 4QD

A B S T R A C T In order to improve ability of amplification about optical arrival angle in laser eavesdropping system,and enhance receiving field of view(FOV)of laser eavesdropping system,this paper presents a new multistage amplifier laser eavesdropping model based on waveguidefiber taper.First of all,taking about four-quadrant(4QD)laser eavesdropping model as an example,from the theoretical view,analyses factors that,currently,laser eavesdropping system face many restrictive factors during practical applications.Besides,from the receiving FOV and ability of amplification about arrival angle of incoming optical axis angles,systematically analyses advantages of multi-stage amplifier model.What's more,through constructing laser eavesdropping test platform,we demonstrate Signal to Noise Ratio(SNR)of two stage amplification model is eventually promoted to 6-10 dB compared with one amplifying model,especially in low and high audio level,the advantages is more obvious.

1.Introduction

Since the end of last century,laser monitor become a new conception investigative techniques has received attention of domestic and foreign scholars in recent year.Literature[1,2]some laser monitor basic concepts,literature[3-7]presents an effective way in military field,the author proposes from literature[8,9].In term of the principle of laser monitor,this paper presents an quadrant difference provide an efficient of laser detection.After thirty years development,we have made great progress and development in laser monitor.

However,as a new conception in military field,there are many factors for restraining where the system performance is optimized further;Firstly,in term of the principle of laser monitor,the nature of laser monitor is the sound waves vibrate the surface of the object.The surface vibrations of the measured objects cause slight changes in the laser axis.Then the optical axis angle of the photoelectric detection terminal is affected.The reduction of acoustic wave is derived from real-time monitoring of the arrival angle of the optical axis.

By geometrical optics we know that,The detection precision of the optical system incident optical axis is directly related to the focal length of the optical system,the more bigger focal lengths,the better of capacity optical axis arrival angle fluctuation.No matter which instrumentation mode you select.Long focal refracting antenna is an inevitable choice for laser monitor system.But analysis from the perspective of application,the longer focal lengths,the less field bias optical system.It's difficult to realize enables rapid deployment.

To sum up,With the development of the investigative techniques,these problem have been overcome by achieving the laser monitor function.But how to solve the focal length and the optical axis alignment difficulty,how to solve the problem of the rapid deployment of the arrival.Angle and the rapid deployment of the monitoring system has become a key problem in the field of laser monitoring equipment.Therefore,this paper proposes a taper connected to waveguide achieve the second amplification.The angle optical amplification capability and the incident light axis are difficult to solve this problem radically.The possibility that enables rapid deployment of laser monitor will be turned into reality soon.

2.The principle of four-quadrant(4QD)laser eavesdropping model

The photosensitive diode can be arranged in image plane,performance parameters are approximately the same,which can be divided into four quadrants,the same area.Fig.1[10-12]are exemplary views the detective power of photoelectric imaging system that a laser facula into four.

Fig.1.Four-quadrant photo-detector.

Let IA、IB、IC、IDare output currents of the probe;let EA、EB、EC、EDare the amount of light incident upon the locator in the array of optical power;SA、SB、SC、SDare photosurface area for four photosensors.SXis the Airy disc area,SABCDis the overall area of probes,dxis a trench isolation width.When the Airy disc enter the field of view,each of quadrants are proportional to photoelectric current.

Meanwhile,because of the light current generated from different quadrants are proportional to light power distributed on the surface.The segment centred expression would look like this:

Where Δσx,Δσyare final 4QD optical axis of offset amount,the Detection accuracy of Four-Quadrant detector consists of two parts:self-segmentation accuracy and optical resolution accuracy.

In the system of whole detecting,the image plane of optoelectronic detector impact the whole system-inherent resolution.the minimal photoelectric imaging unit has a direct impact on the inherent resolution of the detection system.therefore the adjustment of the focal length f and the minimum imaging diameter dAhave direct in fluence on the intrinsic accuracy detection systemΔ:

Then in the ideal case,the whole detection system accuracy consist of two parts:inherent resolution accuracyΔand detection segmentation accuracy.Ideally,the final expression to optical axis detection precision is:

With respect to detection precision,to guarantee the range of the field.The photosensitive area size is of the order of millimeter.Although,you can improve the inherent of the system by reducing the size of Airy disc.But its segmentation accuracy is only related to atmospheric environment,without committing to other conditions.so the 4QD resolution systemθ4QDis:

In the laser listening system,the laser beam is assembled at the focal plane after being modulated by the vibration surface.By formula 6,the focal length f of the optical system has a linear amplification effect on the detection of angle-of-arrival fluctuation.When the speech caused by the optical axis angle-of-arrival fluctuation,the range of the optical system has been enlarged to match the size of the Airy disc(In general,the laser photosensitive surface size can be much larger than that of the Airy disc).The speech signal has the least distortion degree.And it can be seen from formula 2-3,the detection mode belong to differential mode.It can be extracted from its arrival Angle,compared to the singlequadrant photoelectric detection structure(e.g.traditional PIN or silicon photocell etc.).

3.Optical cone waveguide two-stage amplification principle

The last section 4QD laser monitoring principle can be seen,for laser monitoring,its view aperture is not the photosensitive surface of the photodetector in the traditional sense.It's the final focus of the Airy disc diameter.In addition,in order to achieve the high angle-of-arrival fluctuation magnification ability,the focal length of the optical system is the most effective and direct method.But that way,the receiving view of the entire monitoring system will be significantly lower.

In this paper,based on light cone waveguide structure,it propose a two-stage optical axis arrival angle amplification model.A waveguide optical alignment is placed at the focal plane of the optical system.Airy disc by the small end of the light cone of the waveguide,the Airy disc is emitted by the large end of the waveguide cone after the amplification of the light cone by the waveguide.At this point,a 4QD detector is abutted on a large end of a waveguide cone.The two stages of the arrival Angle of the incident light axis are amplified.The focal length of the optical antenna is effectively amplified by the first stage,and the second stage is amplified by the waveguide cone.The principle is shown in Fig.2.

It can be seen that the two-stage light cone is similar to a eyepiece,which increases the swing range of the incident to the Angle-of-arrival.And because the size of 4QD photosensitive surface can be made very large.Therefore,when the Airy disc size is amplified by the waveguide cone,its equivalent for the receiving field of laser monitoring is equally amplified.(The receiving field of laser monitoring is the opposite of the calculation method of the traditional telescope system.)

4.Desktop verified experiment

In order to further verify the superiority of the two-stage magnification model mentioned in this paper,we conducted the following verification experiments.

First,a standard C focal length with focal length of 50 mm is selected,the light source is 658 nm infrared laser light source,the beam divergence Angle is 2 mrad,the diameter of the laser beam waist is 30 mm,the taper ratio is 3.1:1.0,large end fiber diameter of 6μm.The concrete effect is shown in Fig.3.

Fig.2.Two-stage enlarged perspective view.

Fig.3.Appearance effect of waveguide light cone.

We use the external function of the smart phone to vibrate the air and trigger a glass sheet that is 2 mm thick as the voice trigger source.And by turning the volume of the phone to change the volume of the voice trigger.

By turning the volume of the mobile phone,the voice listening effect of the first-stage amplification mode and the two-stage amplification mode is compared.Compared with the results of the speech test of the two detection models,the two-stage amplification mode increased by 6-10 dB compared to the firststage amplification mode detection.Especially at low volume and high volume,the waveguide cone has the advantage of signal-tonoise ratio.

5.Conclusion

This paper presents the constraints of the current laser monitoring system in practical applications.This paper presents a laser monitoring mode based on multi-stage amplification of waveguide light cone.The advantages of the waveguide light cone in the field of detecting field are demonstrated by theoretical analysis.Meanwhile,the advantages of the waveguide cone in the Angle of arrival of the amplifying incident beam are verified by the verification experiment.

Foundation item

Jilin province of Science and technology development plan key scientific and technological breakthrough project.(2015 0204083GX,20150204049GX);The 13th five-year science and technology research project of Jilin education department.(Jilin Provincial Education and technology system 2016357);Jilin provincial science and technology smes innovation fund.(20170308029HJ).