The fiber itself cannot emit light, but the fiber can transmit light for illumination. The fiber selected for fiber optic illumination can be divided into quartz fiber, multi-component glass fiber and plastic fiber POF according to different fiber materials. This paper mainly introduces POF. The principle of light transmission, other principles of fiber transmission are consistent with the principle of light transmission of POF.
It has long been observed that light travels forward through multiple total reflections in a transparent cylinder. They are ancient glass blowing artists. The first scientific explanation of this phenomenon was that John Dingdal of the Royal Society of the United Kingdom demonstrated a famous experiment to the Royal Society. He used a container filled with water to let water flow out of the side hole of the vessel. At this time, the light projected in the water is also transmitted along with the water flow.
In 1880, William Wheeler proposed the idea of ​​"pipe lighting" and obtained the US patent. This is the earliest "remote control lighting" device. The basic principle is: the inner wall is coated with a reflective layer. The tube directs the light from the central source to a number of locations that require illumination, which is actually the prototype of the fiber used for illumination. The fiber-optic illumination system can simply be seen as similar to the "pipe system" described above. A system in which the medium being transmitted is light, and the "pipe" used to transmit light is the fiber, which transmits light from the source to a specific area that needs to be illuminated. In 1954, Nature published an article in which Hopkin's and Kapany successfully used a bundle of 10,000 to 20,000 fibers to transmit images. Van Heel discovered the role of a low-refractive-index fiber cladding, the successful implementation of fiber image transmission, and fiber optic packages. The two advances of the layer marked the birth of optical fiber as a new discipline. In 1966, the British Standard Scientist of the British Standard Telecommunications Institute, Dr. KCKao and GAHockham, studied the transmission loss of glass in detail. The article "Mechanical Fiber Surface Waveguides for Optical Frequency" was published in the Journal of the Institute of Electrical Engineers (IEE) in London. They theoretically pointed out that if the harmful impurities such as transition metal ions in the optical fiber are reduced or eliminated, the fiber can be greatly reduced. Transmission loss, improve the light transmission capability of the fiber, and promote the research of optical fiber manufacturing process. DuPont of DuPont also introduced the world's first POF [1] to the market this year. POF is a kind of fiber, and the basic principle of fiber for fiber illumination is to use light at the interface of different refractive index media. Total reflection, the efficient transmission of light in the fiber and the full coupling of the fiber and the light source, and the combination of various optical components to achieve the desired lighting effect, in order to understand the transmission mode of light in the fiber, the introduction of meridian light Transmission characteristics in POF.
2. Basic knowledge of light
Light is radiation that is generated by the vibration of a large number of molecules or atoms in a light source. In 1894, Maxwell theoretically pointed out that light is an electromagnetic wave. In 1905, Einstein proposed that light is a particle flow of one grain, and each particle can be called a photon. That is to say, light has both particle and volatility, and light appears to be volatility when it is transmitted, and it is also particle-like when it interacts with matter. Generally speaking, light is a kind of electromagnetic wave. It is usually composed of ultraviolet light, visible light and near-infrared light. The light in the 1-390nm band is ultraviolet UV, and the wavelength in the 280-300nm band is UV-B. Strong light can kill or seriously damage the Earth's creatures; the 200-280um band is UV-C, and its glare can kill all living things on Earth, including humans, and X-rays and gamma are higher than ultraviolet light. Ray, etc.; light in the 390-760nm band is visible light; near-infrared light at 760-1500nm, 1.5-25μm in the mid-infrared band, 25-300μm in the far-infrared spectrum, smaller or wavelength than far-infrared Longer millimeter waves, microwaves, short waves, medium waves, and long waves. The visible light is composed of seven colors of light, that is, the visible light contains red light, orange light, yellow light, green light, blue light, and indigo light [2]:?
Purple/nm indigo/nm blue/nm green/nm yellow/nm orange/nm red/nm
390-430 430-450 450-500 500-570 570-600 600-630 630-760
The standard set by the International Commission on Illumination is: red light with a wavelength of 700 nm in the mercury spectrum is red-based light, green light with a wavelength of 546.1 nm is green-based light, and blue light with a wavelength of 435.8 nm is blue-based light. Conventional POF generally does not have good light transmission in the ultraviolet light band, while quartz fiber and special liquid core fiber have good light transmittance in this region, and POF has good light transmittance in the visible light region. The POF core material is made of fluorinated and deuterated polymer material, and the POF has good light transmittance in the near-infrared light region.
The propagation velocity C of light in vacuum is 3 × 108 m / s, the transmission wavelength of light λ, the relationship between frequency f and speed of light C is as follows:
C=fλ........................ (1)
Where f is in Hertz Hz or 1/second (s) and the unit of wavelength is meters (m).
Only the refractive index n of the vacuum is 1.0, so the propagation velocity V of light in any transmission medium is the speed of light divided by the refractive index of the medium, ie:
The speed of light propagation in vacuum is the fastest, the transmission medium is different, its refractive index is different, and the light transmission speed is also different. Relatively speaking, the transmission medium with a large refractive index is a light-tight medium, and the transmission medium with a small refractive index is a light-dissipating medium. For POF, the POF core material is a light-tight medium, and the POF leather material is a light-draining medium, since the light is The light-transmitting medium-propagation speed in the core material is reduced, so the transmission speed of light in the core material is slower than the transmission speed in the leather material; in air, due to n≈1, the propagation speed of light waves is close to the propagation in vacuum. Speed ​​C; pure PMMA has a refractive index of 1.49, so the transmission speed of light therein is about 2.01 × 108 m / s.
When light is transmitted in a uniform medium or an inhomogeneous medium, the Fermat principle is satisfied, that is, light travels from space to point at a distance along the path of time, that is, the light is minimized along the optical path or Maximum or constant path propagation.
3. Geometric optics theory
To understand the principle of POF light transmission, you must understand some knowledge of geometric optics.
First, optics are divided into geometric optics and physical optics. Geometric optics is the study of the propagation characteristics of light in a homogeneous medium. It is usually described by a straight line. It is the basic optical theory for studying the propagation of light in a medium. Physical optics is divided into wave optics and quantum optics. Wave optics considers light to be an electromagnetic wave, but it cannot explain the microscopic phenomenon of light. Quantum theory believes that the energy of light is not continuously distributed, and light is composed of photons moving by a grain. Each photon has a certain energy. The four basic laws of geometric optics theory are:
3.1 The Law of Straight Line Propagation of Light: In an isotropic homogeneous medium, light travels in a straight line.
3.2 The law of independent propagation of light: When light from different sources passes through a certain point, it does not affect each other, and the propagation of each light is not affected by other light.
3.3 The law of reflection of light: When a beam of light is projected onto a smooth surface of a medium, a part of the light is reflected back to the original medium. This light is called reflected light, and the reflected light, incident light and normal are in the same plane. The angle formed by the incoming ray and the normal is called the incident angle. The angle formed by the reflected ray and the normal is called the reflection angle. The reflection angle is equal to the incident angle, that is, θ1 = θ3, and the absolute value is equal. This is the law of reflection.
3.4 The law of refraction of light: When a beam of light is projected onto a smooth surface of a medium, in addition to a part of the light is reflected, a part of the light is incident into the second transmission medium through the interface of the medium. This part of the light is called refracted light. The refracted ray and the incident ray are respectively located on both sides of the normal, and the refracted ray is located in a plane determined by the incident ray and the normal. The angle formed by the refracted ray and the normal is called the angle of refraction. The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant value. This is the law of refraction. It should be pointed out that when using geometric optics to analyze the transmission characteristics of light in a research object, the geometry of the research object must be much larger than the wavelength of the transmitted light, so that the length of the wavelength can be ignored. Otherwise, physical optics must be used. Analyze the transmission characteristics of light in the subject. That is, when the fiber core diameter is several tens or hundreds of times the wavelength of the propagating light, the propagation phenomenon can be studied by geometric optics without wave optics.
4. Transmission of Meridian Light in Stepped POF
? Step type POF is an optical fiber with a core structure.
The meridian plane refers to the plane containing the fiber axis. The so-called meridian, that is, the propagation path of the light is always in the same plane, the meridional light always intersects the fiber axis, and the light propagates linearly in a uniform medium. : When light passes from one medium to another, reflection and refraction generally occur simultaneously; if light is incident from a light-reducing medium having a small refractive index into a dense medium having a large refractive index, the angle of refraction is smaller than the angle of incidence; When the light is incident from the optically dense medium into the light-diffusing medium, the angle of refraction will be greater than the angle of incidence. Therefore, when light is incident from the optically dense medium into the light-dissipating medium, there may be a phenomenon of only reflection without refraction, which is total reflection. Total reflection is a boundary effect of light refraction, that is, the phenomenon that light enters from one transparent medium into another medium and bends. POF is optical transmission through the principle of total reflection.
? From the law of refraction law:
N1sinθ1=n2sinθ2 (4)
Here, n1 and n2 are divided into core skin refractive indexes, θ1 and θ2 are divided into incident angle and refraction angle, and the critical angle at which total reflection occurs is θm, and θ2=90° at this time, therefore
When the incident angle θ1>θm, the light is totally reflected at the core skin interface, and when the incident angle θ1 < θm, the light is refracted on the surface of the core skin, and a part of the light leaks from the core material to the outside of the skin layer. From the critical angle of total reflection, the critical incident fiber angle θ0 of the fiber section can also be derived. At the interface between the air and the fiber section, the same is true:
N0sinθ0 = n1sin (90°—θm)
= n1cosθm
Where n0 is the refractive index of the air, and the value is set to be the same as the vacuum refractive index value of 1.0, that is, n0=1.0, thus
When the ambient light incident angle θ is smaller than θ0, the light can propagate forward in the form of total reflection in the fiber, from one end of the fiber to the other end of the fiber. Therefore, the critical acceptance angle of the fiber is:
Therefore, the transmission mode of light in the SI POF fiber is a total reflection type sawtooth type.
The numerical aperture of optical fiber is one of the important indexes of optical fiber. The larger the NA value is, the larger the θ0 is, and the larger the critical incident angle of the fiber is, the larger the angle of receiving or emitting light of the fiber end face is, the stronger the light collecting ability of the fiber is, the easier it is. The fiber is connected to the fiber or coupled to the source. The numerical aperture of the fiber of a conventional POF is shown in the following table.
? Table of conventional POF fiber numerical aperture parameters
POF PS core POF PMMA core POF PC core POF (ESK-PH) side illuminating POF
Core material refractive index 1.59 1.495 ? 1.59 ?1.475
Leather material refractive index 1.49 1.402 1.31 1.34
? Numerical aperture NA 0.55 0.5 0.9 ? 0.65
Maximum incident angle or angle of emission / degree 67 60 ? 128 75
5. Geometric travel and number of reflections of the meridian in the step fiber
Since the meridional rays are not incident at the same angle in the fiber, their geometrical travel in the fiber is also different. Whether it is the stroke calculation formula of the meridian in the light or the formula for calculating the number of reflections, it is assumed that the fiber is in a very ideal state: the fiber is very straight, the fiber diameter is uniform, the fiber is free of defects and the fiber entrance end is straight, etc., if the fiber is not Under this ideal condition, the state of total reflection of the incident meridian changes, and if some will be reflected from the optical fiber, some reflection angles will change, and thus the transmission loss of the optical fiber will also increase.
6. Transmission of oblique rays in step-type refractive index POF
The so-called oblique light, when the light is transmitted in the optical fiber, is not guaranteed to be in the same plane like the meridional light. When it is transmitted in the optical fiber, its orbit is usually a spatial spiral curve, and its maximum incident angle is larger than the meridian. However, it is usually the meridian transmission to characterize the transmission characteristics of the fiber, which is naturally the most ideal condition.
7. Transmission of light in a graded refractive index distribution POF
For the graded refractive index GI POF, there are also meridian and oblique fibers. The refractive index of this fiber is not a constant constant, but the refractive index decreases as the off-axis distance increases. The gradient refractive index is shown below; parabola The type of refractive index distribution fiber has the characteristics of small modal dispersion. The gradient refractive index has various forms. When the refractive index distribution is distributed by the quadratic parabola, the propagation path of the meridian in the fiber is sinusoidal. See the following figure. The propagation path of the oblique fiber is a spiral curve, and the gradient type refractive index POF is mostly used for short-distance data transmission, and is used for less fiber illumination.
The laser energy distribution of this fiber transmission is close to the Gauss distribution, that is, the higher the optical energy density near the fiber axis, that is to say, the laser energy is more concentrated, and the laser power density (or laser intensity) I transmitted can be considered It is proportional to the square of the core diameter α. If the laser power transmitted by the fiber is kept constant, reducing the core diameter of the fiber, that is, reducing the cross-sectional area of ​​the fiber core that transmits the laser energy, the laser power density of the fiber transmission will increase [5], when the light is in this GI. When transmitting POF, it can be said that it is a very low-energy transmission and also satisfies the formula as described above.
8. The principle of light transmission of side-emitting POF
The side-emitting POF means that the light transmits not only the transmitted light from the incident end face of the optical fiber to the exit end face, but also a part of the light is transmitted from the optical fiber cladding layer to form a side-emitting phenomenon of the optical fiber. It is called a side-emitting POF, and its light transmission diagram is as follows. The essence is that a part of the transmitted light leaks from the side of the fiber, which is a result of light scattering. For a single-core side-emitting POF, it is mostly caused by non-inherent loss. For multi-core side-emitting POFs, it is due to bending loss.
The most prominent feature of the side-emitting POF is the side-emitting. According to Janis Spigulis et al. [5], the side-emitting intensity of the side-emitting POF decreases exponentially with the increase of its length, which is the same as that of ordinary optical fiber. The luminous intensity is exponentially decreasing as the length of its transmission increases, and the conclusion is drawn after making the following assumptions:
8.1 The principle of side illumination is only considered to be due to the transmission of radiation from the fiber core.
8.2 All of the initial side-scattered light penetrates the circular surface of the fiber without loss, resulting in uniform transmission to the outer surface of the fiber.
The luminous intensity Is(x) of the side-emitting POF at a length of X meters can be expressed by the following formula:
Is(x)=Aexp(-kx) (24)
Where K is the side illuminance coefficient, the unit is m-1, and the constant A can be expressed by the following formula:
A = (4Ï€) - 1I. (expk-1) (25)
Where I. It is the side-emitting POF light input intensity.
Therefore, in actual use, in order to ensure the uniformity of the side illumination intensity of the side-emitting POF, the length of use of the side-emitting POF is usually limited, and a light source of the same power is disposed at both ends of the side-emitting POF or a total reflection mirror or reflection is disposed at one end. The film, of course, the former ensures the uniformity of the side illumination of the fiber over a longer length of use. The illumination intensity IS2(x) of the side-emitting POF using the dual source can be calculated by the following formula (26).
IS2(x)=A{exp(-kx)+exp[-k(Lx)]} (26)
Where L is the total length of the side-emitting POF.
The side-emitting POF intensity calculated by using the total reflection mirror can be calculated by the following formula. The relationship between the luminous intensity and the distance of the side-emitting POF is shown in the following figure.
ISR(x)=A{exp(-kx)+Rexp[-k(2L-x)]} .........(26)
Where R is the specular reflectance.
Due to the optical transmission loss, the brightness of the side illumination will decrease as the distance from the light source increases. In order to make the brightness of the fiber unit nearly uniform, the fiber of the single-ended source can be scored according to the length, with the fiber. The length is increased and the spacing of the scores is decremented. In the actual use process, when the length of the side-emitting POF is less than 30m, a 150W metal halide lamp source is used, and the other end is equipped with a mirror or a reflective film; when the side-emitting POF is used, the length is 30-60m. When using, two 150W metal halide light sources are used to ensure the uniformity of the side illumination of the side-emitting POF. The following figure shows the illuminance of three side-emitting POFs with a diameter of 14mm. It can be seen that when using one When the light source of 150W metal halide lamp is used, the illuminance of the POF side at 1.5m is about 800lx, while the illuminance at 60m is less than 20lx, and the distance from the surface of the fiber when the illuminometer is tested is 2.5cm.
9. Principle of light transmission of fluorescent POF
Fluorescent POF is a POF prepared by incorporating a certain amount of fluorescent agent into a POF core material. After being irradiated with light of a specific wavelength, the POF emits light of a specific wavelength, and the principle is relatively complicated, and can be simply considered to be in the ground state molecule. The bonding electrons absorb light and then excite, and then the singlet molecules return to the ground state, which fluoresces. Fluorescent POFs are classified by refractive index distribution structure and can be classified into fluorescent SI POF and fluorescent GI POF. The most important characteristic of POFA doped with organic dyes is to provide high power output over a wide wavelength range. The principle of the light transmission of the fluorescent POF is as follows. It satisfies the light transmission characteristics of a general SI fiber, but the wavelength of the incident light is different from the wavelength of the emitted light.
Fluorescent POF has another way of transmitting light, that is, the incident light can illuminate the fluorescent POF from the side, and the outgoing light is emitted from both end faces of the optical fiber. Of course, the wavelength of the incident light is different from the transmission wavelength of the outgoing light.
The optical properties of the fluorescent material are mainly dependent on the matrix material, and the fluorescence POF gain amplification characteristics are related to the pump wavelength, the length of the fluorescent POF, and the dopant and concentration used. The so-called gain G is a ratio between the POF output signal optical power Pout and the input optical power Pin.
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