CN114839790A - Laser decoherence method based on Brownian-like motion model - Google Patents
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Abstract
The invention relates to a laser decoherence method based on a Brownian-like motion model, which comprises the following steps of: preparing a cylindrical light guide rod; irradiating the side surface of the light guide rod by adopting corresponding laser with light-sensitive wavelength of the light guide rod material so as to generate a random refractive index distribution surface grating on the micro-nano-scale thickness of the side surface of the light guide rod and form a random refractive index distribution surface grating light guide rod; the random refractive index of the side surface of the random refractive index distribution surface grating light guide rod follows normal distribution; therefore, phase changes correspondingly generated by total reflection of p-polarized laser and s-polarized laser in the random refractive index distribution surface grating light guide rod are also subjected to normal distribution, so that the cumulative phase changes of multiple total reflections of emergent rays are also subjected to normal distribution, no fixed phase relation exists between the emergent rays, and decoherence is realized. The invention realizes laser decoherence by changing the phase relation of emergent rays of the random refractive index distribution surface grating light guide rod, and solves the problems of high cost, large energy loss and the like of the existing decoherence technology.
Description
Technical Field
The invention relates to the technical field of laser decoherence, in particular to a laser decoherence method based on a Brownian-like motion model.
Background
Laser is increasingly used as a light source in optical application scenes such as precision imaging and holographic projection based on high brightness, good monochromaticity and collimation characteristics. Due to high coherence of laser, when the laser passes through a scattering environment (such as dust particles) with fluctuating refractive index, wavelet coherent superposition is generated by randomly distributed surface element scattering, uneven spatial light intensity distribution is caused, and laser speckle is formed. Speckle limits the use of laser light in full-field imaging, including wide-field microscopes, projectors, holography, lithography systems, and laser imaging. The laser decoherence technology is one of the most effective methods for reducing laser speckles, greatly improves the image resolution in the application fields of laser imaging and the like, and the decohered laser keeps the excellent characteristics of the laser and has low coherence and is not easy to form the laser speckles.
The currently mainstream laser decoherence technology includes that from the perspective of a laser source, a stimulated radiation process is optimized, so that a plurality of modes are simultaneously excited and emitted, and the coherence of laser is reduced. Typical lasers include random lasers, chaotic microcavity lasers, and degenerative cavity lasers incorporating phase diffusers. Although some success has been achieved by changing the stimulated emission process to achieve the decoherence effect, it has limitations in the selection of the wavelength of the output laser. The system has complex structure and high cost, so the wide application of the system is limited.
In addition, a method for suppressing the coherence of laser light during the transmission of laser light has a higher practicability in the optical application field such as imaging, and is receiving much attention. The basic principle is that the phase, polarization or angle characteristics of the laser after being emitted are changed by using structures such as a cascade scattering sheet, a micro-electro-mechanical mirror, a diffraction optical element or a rotating light pipe, random components are introduced, and the coherence of the laser is reduced. While the use of scattering elements can cause some loss in the laser energy, actively moving optical elements can introduce vibrations into the optical path, increase the power consumption of the system, and reduce the reliability of the system.
Therefore, it is necessary to provide a laser decoherence method based on a brownian-like motion model to solve the problems of high cost, large energy loss and the like of the existing decoherence technology.
Disclosure of Invention
The invention aims to provide a laser decoherence method based on a Brownian-like motion model, which aims to solve the problems of high cost, large energy loss and the like of the existing decoherence technology.
In order to solve the problems in the prior art, the invention provides a laser decoherence method based on a Brownian-like motion model, which comprises the following steps:
preparing a cylindrical light guide rod;
irradiating the side surface of the light guide rod by adopting corresponding laser with light-sensitive wavelength of the light guide rod material so as to generate a random refractive index distribution surface grating on the micro-nano-scale thickness of the side surface of the light guide rod and form a random refractive index distribution surface grating light guide rod; the random refractive index of the side surface of the random refractive index distribution surface grating light guide rod follows normal distribution;
the phase changes correspondingly generated by the total reflection of the p-polarized laser and the s-polarized laser in the random refractive index distribution surface grating light guide rod are respectively as follows:
wherein n is 1 The refractive index of the interior of the random refractive index distribution surface grating light guide rod, n i The refractive index theta of the ith total reflection incident point of the laser on the side surface of the random refractive index distribution surface grating light guide rod i Is the incident angle of the ith totally reflected light, Δ φ i p And
are all subject to normal distribution;
after the light is transmitted by the multiple total reflection of the random refractive index distribution surface grating light guide rod, the cumulative phase of the multiple total reflection of the emergent light is changed into
Or
Wherein N is the number of total reflection,
and
also obey a normal distribution; between the emergent raysThere is no fixed phase relation, realizing the decoherence of the laser.
Preferably, in the laser decoherence method based on the brownian motion-like model, the material of the light guide rod includes lead silicate glass, quartz and photothermal conversion glass.
Preferably, in the laser decoherence method based on the brownian motion-like model, the side surface of the light guide rod is irradiated with the corresponding laser with the photosensitive wavelength of the light guide rod material:
when the material of the light guide rod is lead silicate glass and photothermal conversion glass, ultraviolet laser is adopted for irradiation; when the light guide rod is made of quartz, infrared laser is adopted for irradiation.
Preferably, in the laser decoherence method based on the Brownian-like motion model,
the light guide rod is cylindrical lead silicate glass.
Preferably, in the laser decoherence method based on the Brownian motion-like model, the diameter of the bottom surface of the light guide rod is 30mm, and the height of the side surface is 90 mm.
Preferably, in the laser decoherence method based on the Brownian motion-like model, the wavelength of the ultraviolet laser irradiating the side surface of the light guide rod to generate the random refractive index profile grating is 266 nm.
Preferably, in the laser decoherence method based on the Brownian-like motion model,
the wavelength of the incident laser meets the condition of total reflection in the random refractive index distribution surface grating light guide rod;
the incident laser is pulse laser or continuous laser;
the incident laser facula is smaller than the cross section of the random refractive index distribution surface grating light guide rod, and the shape is not limited.
Preferably, in the laser decoherence method based on the brownian motion-like model, the random refractive index of the side surface of the random refractive index distribution surface grating light guide rod follows a normal distribution:
wherein f (n) is a random refractive index probability density function of the side surface of the random refractive index distribution surface grating light guide rod, n is the refractive index of the side surface of the random refractive index distribution surface grating light guide rod, mu is a mean value of the refractive index distribution, and sigma is a variance of the refractive index distribution.
Preferably, in the laser decoherence method based on the Brownian-like motion model,
and
obey a normal distribution:
the mean value and the variance of the phase change accumulated by N times of total reflection generated by p-polarized laser in the random refractive index distribution surface grating light guide rod are as follows:
the mean value and the variance of the phase change accumulated by N times of total reflection generated by the s-polarized laser in the random refractive index distribution surface grating light guide rod are as follows:
wherein E (N) and Var (N) are the mean and variance of total reflection times, m p And m s Mean value of the phase change of the single total reflection of the p-polarized laser and the s-polarized laser, respectively p And σ s The variance of the phase change of the single total reflection of the p-polarized laser and the s-polarized laser respectively, and N is taken according to the actual total reflection times of the incident light.
Compared with the prior art, the invention has the following advantages:
(1) the laser is totally reflected and transmitted in the random refractive index distribution surface grating light guide rod, and the emergent rays have no fixed phase relation, so that the laser decoherence is realized;
(2) the wavelength, light intensity, area, shape and the like of incident laser are not required, and continuous laser or pulse laser can be adopted;
(3) the laser is reflected in the random refractive index distribution surface grating light guide rod, and the energy loss of the laser is extremely low;
(4) the laser decoherence is realized only by adopting the random refractive index distribution surface grating light guide rod without adopting a moving element, so that the stability of a decoherence light path is improved;
(5) the random refractive index distribution surface grating light guide rod has a simple structure and low cost.
Drawings
FIG. 1 is a schematic diagram showing the refractive index distribution of lead silicate glass after random irradiation with ultraviolet laser according to an embodiment of the present invention;
FIG. 2 is a graph illustrating the phase distribution of incident laser light and emergent light according to an embodiment of the present invention;
FIG. 3 is a graph of distribution of incident laser and emergent ray intensities according to an embodiment of the present invention;
fig. 4 is an experimental optical path diagram of the anti-coherent processing provided by the embodiment of the present invention.
Detailed Description
The following describes in more detail embodiments of the present invention with reference to the schematic drawings. The advantages and features of the present invention will become more apparent from the following description. It is to be noted that the drawings are in a very simplified form and are not to precise scale, which is merely for the purpose of facilitating and distinctly claiming the embodiments of the present invention.
Also, in the following, the terms "first", "second", and the like are used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances. Similarly, if a method described herein includes a series of steps, the order in which the steps are presented herein is not necessarily the only order in which the steps may be performed, and some of the described steps may be omitted and/or some other steps not described herein may be added to the method.
Although the mode of achieving the decoherence effect by changing the stimulated radiation process in the prior art achieves certain effect, the mode has limitation on the selection of the wavelength of the output laser. The system has complex structure and high cost, so the wide application of the system is limited. In addition, coherence of laser can be suppressed during laser transmission, but the use of a scattering element during laser transmission can cause a certain loss of laser energy, and an actively moving optical element can introduce vibration to an optical path, increase power consumption of a system and reduce reliability of the system.
Therefore, it is necessary to provide a laser decoherence method based on the brownian motion-like model, which includes the following steps:
preparing a cylindrical light guide rod;
irradiating the side surface of the light guide rod by adopting corresponding laser with light-sensitive wavelength of the light guide rod material so as to generate a random refractive index distribution surface grating on the micro-nano-scale thickness of the side surface of the light guide rod and form a random refractive index distribution surface grating light guide rod; the random refractive index of the side surface of the random refractive index distribution surface grating light guide rod follows normal distribution;
the phase changes correspondingly generated by the total reflection of the p-polarized laser and the s-polarized laser in the random refractive index distribution surface grating light guide rod are respectively as follows:
wherein n is 1 The refractive index of the interior of the random refractive index distribution surface grating light guide rod, n i Guiding light for laser at the random refractive index distribution surface gratingRefractive index of ith total reflection incident point of side surface of rod, theta i Is the incident angle of the ith totally reflected light, Δ φ i p And
are all subject to normal distribution;
after the light is transmitted by the multiple total reflection of the random refractive index distribution surface grating light guide rod, the cumulative phase of the multiple total reflection of the emergent light is changed into
Or
Wherein N is the number of total reflection,
and
also obey a normal distribution; there is no fixed phase relation between the emergent rays, thus realizing the decoherence of the laser.
Further, the random refractive index of the side surface of the random refractive index distribution surface grating light guide rod follows normal distribution:
wherein f (n) is a random refractive index probability density function of the side surface of the random refractive index distribution surface grating light guide rod, n is the refractive index of the side surface of the random refractive index distribution surface grating light guide rod, mu is a mean value of the refractive index distribution, and sigma is a variance of the refractive index distribution.
when light is transmitted in a random refractive index distribution surface grating light guide rod in a total reflection mode, the phase change process of s-polarized laser and p-polarized laser caused by total reflection simultaneously meets the following three conditions:
(1) when light is transmitted in the random refractive index distribution surface grating light guide rod, for different times of total reflection, the incident angle of the total reflection and the random refractive index at the incident point are mutually independent, and the corresponding total reflection phase variation is mutually independent.
(2) The accumulated phase change amount of the total reflection light from any j to k does not depend on the values of j and k, and is only related to the total reflection times k-j, and the accumulated phase change amount of the total reflection has a stable increment characteristic.
(3) The total reflection accumulated phase change amount is equal to a superposition of a plurality of mutually independent total reflection phase changes:
or
Therefore, the total reflection accumulates the phase change amount according to the central limit theorem
And
obeying a normal distribution.
Further, the mean and variance of the phase change accumulated by N total reflections generated by p-polarized laser in the random refractive index profile grating light guide rod are as follows:
the mean value and the variance of accumulated phase changes of N times of total reflection generated by the s-polarized laser in the random refractive index distribution surface grating light guide rod are as follows:
wherein E (N) and Var (N) are the mean and variance of total reflection times, m p And m s Mean value of the phase change of the single total reflection of the p-polarized laser and the s-polarized laser, respectively p And σ s The variance of the phase change of the single total reflection of the p-polarized laser and the s-polarized laser respectively, and N is taken according to the actual total reflection times of the incident light. When laser is transmitted in the random refractive index distribution surface grating light guide rod, along with the increase of the total reflection times, the variance of the change of the accumulated phase of the light is increased, namely after the transmission of the random refractive index distribution surface grating light guide rod, the dispersion degree of the phase distribution of the light on the emergent end surface is increased, no fixed phase relation exists between emergent lights, and the coherence is reduced.
Preferably, in the laser decoherence method based on the Brownian motion-like model, the material of the light guide rod comprises lead silicate glass and quartz (SiO) 2 ) And photo-thermal glass (photothermal glass). Of course, if there are other materials that generate random refractive index distribution by irradiation to realize laser decoherence, they can also be used as the material of the light guide rod of the present invention. Further, the side surface of the light guide rod is irradiated by adopting corresponding laser with the photosensitive wavelength of the light guide rod material: for example: when the material of the light guide rod is lead silicate glass and photo-thermal conversion glass, ultraviolet laser is adopted for irradiation; when the material of the light guide rod is quartz (SiO) 2 ) During the irradiation, infrared laser is adopted.
In one embodiment, the light guide rod is cylindrical lead silicate glass, the diameter of the bottom surface of the light guide rod is 30mm, the height of the side surface of the light guide rod is 90mm, and ultraviolet laser with the wavelength of 266nm is selected to irradiate the side surface of the light guide rod so as to generate a random refractive index distribution surface grating on the micro-nanometer level thickness of the side surface of the light guide rod, so that the random refractive index distribution surface grating light guide rod can be formed.
In the present invention, the shape of the light guide rod may be not only a cylindrical shape but also other triangular prisms, quadrangular prisms, polygonal prisms, and the like. The wavelength of the irradiated laser is not limited, as long as the random refractive index distribution surface grating can be generated on the micro-nano-scale thickness of the side surface of the light guide rod. The area, height and the like of the bottom surface are selected in various ways as long as the total reflection phase accumulated by the laser in the random refractive index distribution surface grating light guide rod meets normal distribution and the emergent light spot is ensured to be in a required shape. Therefore, in the above embodiment, when the light guide rod is made of cylindrical lead silicate glass, the wavelength of the irradiated ultraviolet laser, the diameter of the bottom surface and the height of the side surface have various choices; the light guide rod is cylindrical lead silicate glass, and when the wavelength of ultraviolet laser is 266nm, the diameter of the bottom surface and the height of the side surface are also selected in various ways.
Preferably, in the laser decoherence method based on the brownian motion-like model, the wavelength of the incident laser light only needs to satisfy the condition of total reflection in the random refractive index distribution surface grating light guide rod; the incident laser may be a pulsed laser or a continuous laser; the incident laser facula is smaller than the cross section of the random refractive index distribution surface grating light guide rod, and the shape is not limited; the light intensity of the incident laser is only lower than the material damage threshold of the random refractive index distribution surface grating light guide rod; therefore, the invention has no requirements on the wavelength, the light intensity, the area, the shape and the like of the incident laser, and can be used for both continuous laser and pulse laser.
In one embodiment, the refractive index of the lead silicate glass after ultraviolet laser irradiation is illustrated. As shown in fig. 1, when the lead silicate glass after 266nm ultraviolet laser is randomly irradiated is tested by using a fizeau interferometer, the lead silicate glass after irradiation has obvious refractive index change due to the photosensitivity of the lead silicate glass to the 266nm wavelength ultraviolet laser. The refractive index value of the lead silicate glass obtained by measurement after ultraviolet laser irradiation is subjected to statistical analysis to obtain a statistical result shown in figure 1, after ultraviolet laser random irradiation, the refractive index of the lead silicate glass approximately follows normal distribution, the mean value is 1.69, the standard deviation is 0.04, the minimum value is 1.59, and the maximum value is 1.80.
In one embodiment, the phase distribution of the emergent light after passing through the random refractive index profile grating light guide rod is illustrated. As shown in fig. 2, Zemax optical simulation software is used to analyze the laser decoherence effect of the random refractive index distribution surface grating light guide rod through simulation, the refractive index distribution of the central layer of the random refractive index distribution surface grating light guide rod is uniform and is 1.81, the refractive index of the outermost layer of the side surface in the thickness range of about 100nm follows normal distribution, the mean value is 1.69, the standard deviation is 0.04, the maximum value is 1.80, and the minimum value is 1.59. The incident Gaussian laser is transmitted through the random refractive index distribution surface grating light guide rod, the phase distribution of the incident end surface Gaussian laser and the phase distribution of the emergent end surface light of the random refractive index distribution surface grating light guide rod are shown in figures 2a and 2b, the phase distribution of the incident end surface Gaussian laser is uniform, the phase distribution of the emergent end surface light takes a value randomly in a range from-pi to pi, the standard deviation is 0.49 pi, and no definite relation exists between the phases. The random refractive index distribution surface grating light guide rod converts incident laser light distributed regularly in phase into emergent light distributed randomly in phase, and decoherence of the incident laser light is achieved. The intensity distribution of the incident Gaussian laser and the emergent light is represented by gray values respectively, as shown in figure 3a, the intensity of the incident laser is in uneven Gaussian distribution, and after the incident laser is transmitted by the random refractive index distribution surface grating light guide rod, as shown in figure 3b, the intensity distribution of the emergent light is even, and no obvious speckles are formed. The speckle contrast SC of the emergent light spot image obtained by calculation is 0.031 and is less than 0.05, the condition that the speckle pattern is invisible is met, and the incident laser is decohered by the random refractive index distribution surface grating light guide rod.
In one embodiment, as shown in fig. 4a, the experimental optical path of the cylindrical random refractive index distribution surface grating light guide rod for performing coherent elimination on incident laser light includes that the wavelength of a continuous laser is 532nm, L1 and L2 are collimating lenses, P1 is a conical lens, P2 is a random refractive index distribution surface grating light guide rod, the refractive index distribution of the central layer is uniform and is 1.81, the refractive index in the thickness range of about 100nm of the outermost layer of the side surface follows normal distribution, the average value is 1.69, the standard deviation is 0.04, the maximum value is 1.80, and the minimum value is 1.59. Incident laser is converted into conical Gaussian-Bessel light through a conical lens P1 after being collimated by L1 and L2 lenses, the conical Gaussian-Bessel light obliquely enters a cylindrical surface grating light guide rod P2 with random refractive index distribution, light spots are formed on a light screen after being emitted, the light spots on the light screen are shot by a CCD Camera, and gray level images of the intensity of the emitted light spots obtained by the CCD and gray level distribution curves of the gray level images are shown in FIGS. 4(b) and 4 (c). The speckle contrast SC of the emergent light spot image obtained by calculation is 0.044 and is less than 0.05, namely, the emergent light spot speckle pattern is invisible through the transmission of the random refractive index distribution surface grating light guide rod, and the coherence of incident laser is reduced.
In summary, in the laser decoherence method based on the Brownian motion model provided by the invention, the random refractive index distribution surface grating light guide rod is constructed based on the Brownian motion model, random phase distribution is introduced to the laser through total reflection to achieve the decoherence effect, and meanwhile, the optical loss is low. The simple, compact and low-cost light path design enables the decoherence system to be stable and reliable, and has strong robustness.
The above description is only a preferred embodiment of the present invention, and does not limit the present invention in any way. It will be understood by those skilled in the art that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.
Claims (9)
1. A laser decoherence method based on a Brownian motion model is characterized by comprising the following steps:
preparing a cylindrical light guide rod;
irradiating the side surface of the light guide rod by adopting corresponding laser with light-sensitive wavelength of the light guide rod material so as to generate a random refractive index distribution surface grating on the micro-nano-scale thickness of the side surface of the light guide rod and form a random refractive index distribution surface grating light guide rod; the random refractive index of the side surface of the random refractive index distribution surface grating light guide rod follows normal distribution;
the phase changes correspondingly generated by the total reflection of the p-polarized laser and the s-polarized laser in the random refractive index distribution surface grating light guide rod are respectively as follows:
wherein n is 1 The refractive index of the interior of the random refractive index distribution surface grating light guide rod, n i The refractive index theta of the ith total reflection incident point of the laser on the side surface of the random refractive index distribution surface grating light guide rod i Is the incident angle of the ith totally reflected light, Δ φ i p And
are all subject to normal distribution;
after the light is transmitted by the multiple total reflection of the random refractive index distribution surface grating light guide rod, the cumulative phase of the multiple total reflection of the emergent light is changed into
Or
Wherein N is the number of total reflection,
and
also obey a normal distribution; there is no fixed phase relation between the emergent rays, thus realizing the decoherence of the laser.
2. The laser decoherence method based on the Brownian-like motion model as claimed in claim 1, wherein the material of the light guide rod comprises lead silicate glass, quartz and photothermal conversion glass.
3. The laser decoherence method based on the Brownian-like motion model according to claim 2, wherein the side surface of the light guide rod is irradiated with corresponding laser light of a photosensitive wavelength of a material of the light guide rod:
when the material of the light guide rod is lead silicate glass and photothermal conversion glass, ultraviolet laser is adopted for irradiation; when the light guide rod is made of quartz, infrared laser is adopted for irradiation.
4. The method of claim 3, wherein the laser decoherence method based on the Brownian-like motion model,
the light guide rod is cylindrical lead silicate glass.
5. The method for decoherence of laser light based on the Brownian-like motion model according to claim 4, wherein the diameter of the bottom surface of the light guiding rod is 30mm, and the height of the side surface is 90 mm.
6. The laser decoherence method based on the Brownian-like motion model according to claim 5, wherein the wavelength of the ultraviolet laser irradiating the side surface of the light guide rod to generate the random-index-distribution surface grating is 266 nm.
7. The method of claim 1, wherein the laser decoherence method based on the Brownian-like motion model,
the wavelength of the incident laser meets the condition of total reflection in the random refractive index distribution surface grating light guide rod;
the incident laser is pulse laser or continuous laser;
the incident laser facula is smaller than the cross section of the random refractive index distribution surface grating light guide rod, and the shape is not limited.
8. The laser decoherence method based on the Brownian motion model as claimed in claim 1, wherein the random refractive index of the side surface of the random refractive index distribution surface grating light guide rod follows a normal distribution:
wherein f (n) is a random refractive index probability density function of the side surface of the random refractive index distribution surface grating light guide rod, n is the refractive index of the side surface of the random refractive index distribution surface grating light guide rod, mu is a mean value of the refractive index distribution, and sigma is a variance of the refractive index distribution.
9. The method of claim 1, wherein the laser decoherence method based on the Brownian-like motion model,
and
obey a normal distribution:
the mean value and the variance of the phase change accumulated by N times of total reflection generated by p-polarized laser in the random refractive index distribution surface grating light guide rod are as follows:
the mean value and the variance of the phase change accumulated by N times of total reflection generated by the s-polarized laser in the random refractive index distribution surface grating light guide rod are as follows:
wherein E (N) and Var (N) are the mean and variance of total reflection times, m p And m s Mean value of the phase change of the single total reflection of the p-polarized laser and the s-polarized laser, respectively p And σ s The variance of the phase change of the single total reflection of the p-polarized laser and the s-polarized laser respectively, and N is taken according to the actual total reflection times of the incident light.
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