CN102680096A - Low resolution optical fiber monochromator - Google Patents

Abstract

The invention relates to the field of optical fiber devices, in particular to a low resolution optical fiber monochromator. The basic structure of the low resolution optical fiber monochromator is that a Y-shaped multimode optical fiber beam splitter is adopted. Two branches on the same side are used for transmitting exciting light and signal light respectively, an optical fiber end face of an signal light emitting optical fiber and the axial direction of the optical fiber forms an inclined angle gamma =90 degree-arcsin {lambda /[d (n1-1)]}, parallel and equidistant grating indentations are fully distributed in a fiber core range of a light emitting end face, and the indentations are perpendicular to the end face inclined direction and the axial direction of the optical fiber. The signal light emitted by the signal light emitting optical fiber is dispersed into monochromatic light by end face grating, the monochromatic light is received by a photoelectric conversion element which is coaxially installed with the signal light emitting optical fiber, and an end face inclined angle enables the first-level diffraction direction of central wavelength of the signal light to be along the axial direction of the optical fiber. The low resolution optical fiber monochromator is easy to assemble and can be applied to temperature sensing, chemical sensing, absorption rate measurement analysis, colorimetry measurement and the like.

Description

A low-resolution fiber optic monochromator

technical field

The invention relates to the field of optical fiber devices, in particular to a low-resolution optical fiber monochromator.

Background technique

In fiber optic spectrum measurement and sensing technology methods based on fiber optic spectroscopy, the dispersive elements include fiber gratings, AWG wavelength division multiplexers, multilayer film filters, Mach-Zehnder interference, Fabry-Perot micro-optical elements, etc. Type; when the wavelength range of the signal light is wide and the resolution accuracy of the wavelength is not high, a small fiber optic spectrometer is often used to analyze the low-resolution spectrum. In contrast, the optical fiber monochromator of the present invention can provide low-resolution dispersion, and has a small volume, stable and reliable manufacturing process, and convenient use, and has great advantages in terms of product cost and application range.

Contents of the invention

The object of the present invention is to provide a low-resolution optical fiber monochromator that can be used for low-resolution spectral measurement and optical sensing, and its specific applications include temperature sensing, chemical sensing, absorbance measurement and analysis, colorimetric measurement, and the like.

The present invention is realized in such a way that two same-side fiber branches of a Y-shaped multimode fiber beam splitter are respectively used for the transmission of excitation light and signal light, wherein the end face of the signal light exit fiber is in a specific direction relative to the fiber axis. The angle is inclined, the transmissive grating is made on the end face of the signal light output fiber, and the photoelectric conversion element is installed coaxially with the signal light output fiber.

The inclination angle of the end face of the signal light output fiber of the Y-shaped multimode fiber beam splitter relative to the fiber axis is γ = 90° – arcsin{ λ /[ d ( n ₁ -1)]}.

Each score of the transmissive grating is perpendicular to the axis of the optical fiber and the inclination direction of the end face of the optical fiber from which the signal light exits, and is distributed in parallel and equidistant.

The branching ratio of the Y-shaped multimode fiber beam splitter is determined according to the intensity of the excitation light and the intensity of the signal light, usually 50/50.

The inclination angle γ = 90° – arcsin{ λ /[ d ( n ₁ –1 )]} of the end face of the signal light output fiber is polished or cut so that the first-order diffraction direction of the central wavelength of the signal light is along the fiber axis.

The transmissive grating is directly photoengraved, or pasted in parallel, or embossed and replicated on the end face of the signal light exiting optical fiber.

The transmission grating according to the present invention is characterized in that the grating is directly photoengraved, or pasted in parallel, or embossed and copied on the end face of the signal light output optical fiber. The fabrication method depends on the accuracy requirements and the fiber material.

The invention has the advantages of convenient assembly, extremely small device size, convenient use and low cost.

Description of drawings

Fig. 1 is a structural schematic diagram of the present invention.

Fig. 2 is a partial block diagram of the present invention A.

In the figure, 1. Y-shaped multimode fiber beam splitter 11, excitation light incident fiber 12, signal light exit fiber 13, fiber pigtail 2, transmissive grating 3, photoelectric conversion element

The figure shows the first-order diffraction of the central wavelength λ of the signal light propagates along the fiber axis and enters the photoelectric conversion element. n ₁ is the refractive index of the fiber core; n ₂ is the air refractive index; θ is the angle between the signal beam and the normal line of the signal light exiting fiber end face; Ф is the diffraction angle; γ is the inclination angle of the signal light exiting fiber end face.

Detailed ways

As shown in Fig. 1 and Fig. 2, the present invention is realized in this way, and it comprises a Y-shaped multimode fiber beam splitter 1, and two branches of the same side of the beam splitter are respectively used for the transmission of excitation light and signal light , the excitation light is injected into the sensor probe through the excitation light incident optical fiber 11, the signal light is exported through the signal light output optical fiber 12, the excitation light is partially absorbed and partially reflected, or the wavelength of the excitation light is modulated and occurs in the Y-shaped multimode fiber waveguide beam splitter On the other side of the fiber pigtail 13. The end face of the signal light output fiber 12 is inclined at a specific angle relative to the fiber axis, and the transmissive grating 2 is fabricated on the end face of the signal light output fiber 12 . The signal light is dispersed into monochromatic light after passing through the transmissive grating 2 . The central wavelength component of the monochromatic light belonging to the first-order diffraction is output along the axial direction of the signal light output fiber 12 . The photoelectric conversion element 3 coaxially installed at the end of the signal light output fiber 12 receives the dispersed signal light, and outputs the relative values of light intensities of different wavelength components in the signal light, that is, the spectrum of the signal light.

The inclination angle of the end face of the signal light exit fiber 12 of the Y-shaped multimode fiber beam splitter 1 according to the present invention relative to the fiber axis is γ , and γ = 90° - arcsin{ λ /[ d ( n ₁ -1 )]}. Where λ is the central wavelength of the signal light, d is the grating constant of the transmission grating 2, _{and n1} is the refractive index or effective refractive index of the fiber core.

Each score of the transmissive grating 2 in the present invention is perpendicular to the axial direction of the optical fiber and the inclination direction of the end face of the optical fiber from which the signal light exits, and is distributed in parallel and equidistant.

According to the grating equation of oblique incidence: d ( n ₁ sin θ + n ₂ sin Ф ) = ± kλ , the signal light is received by the photoelectric conversion element 3 after it exits the air, and the refractive index n ₂ of the air is 1. The signal light is transmitted along the fiber axis, and the incident angle at the end face of the signal light exiting the optical fiber 12 is θ , and the inclination angle of the end face relative to the fiber axis is γ =90°- θ . For the first-order diffraction k = 1, the specific tilt angle γ described in the present invention makes the first-order diffraction direction of the center wavelength of the signal light along the fiber axis, that is, the first-order diffraction angle Ф = – θ .

The end face of the signal light output optical fiber 12 can be processed by grinding or cutting. Grinding or cutting can use conventional fiber grinders and fiber cutters.

The branching ratio of the Y-shaped multimode optical fiber beam splitter 1 according to the present invention is determined according to the excitation light intensity and the signal light intensity, usually 50/50. However, the selection of other branching ratios does not affect the color resolution capability of the fiber optic monochromator of the present invention.

The transmissive grating 2 described in the present invention is directly photoengraved, or pasted in parallel, or embossed and replicated on the end face of the signal light output optical fiber 12 . The fabrication method depends on the fiber material. Plastic optical fiber is suitable for duplicating the grating on the end face of the fiber by molding; quartz or sapphire optical fiber is suitable for direct photolithography; different types of optical fiber can be pasted with prefabricated grating.

Controlling the blaze angle of the transmission grating 2 can concentrate most of the light energy of the signal light to a predetermined spectral order, that is, the first-order diffraction order set in the present invention.

The first-order diffracted monochromatic light is then received by the photoelectric conversion element 3 . The photoelectric conversion element 3 may use an array detector such as a CCD or a CMOS camera, or may use a discrete photodetector such as a phototransistor or a photoresistor.

As a specific example, take a low-resolution fiber optic monochromator connected to an Er ³⁺ ion fluorescence temperature sensing probe with the fiber pigtail 13 as an example:

The wavelength range of Er ³⁺ ion fluorescence used for temperature sensing by fluorescence specific intensity method is 515nm~570nm. Taking 545nm as the central wavelength of the signal fluorescence, the resolution of the fluorescence spectrum can be practically applied when it reaches 15nm.

A commercially available Y-shaped multimode quartz fiber beam splitter is used, the refractive index of the fiber core is n ₁ =1.468, and the core diameter is 62.5 microns. The excitation light is introduced by the excitation light incident optical fiber 11, and the end face of the signal light exit optical fiber 12 is ground to a 39-degree inclination, and then the parallel and equidistant grating notches are photocut on the end face inclined surface, the grating constant is 1.5 microns, and the grating notches are uniform It is perpendicular to the fiber axis and the inclination direction of the signal light exiting fiber end face. Calculated from the grating equation, the first-order diffraction of light with a wavelength of λ = 545nm (that is, k = 1) is along the fiber axis. The number of scored lines in the core range of the optical fiber is N = 62.5 / ( 1.5 * sin39°) and counts 66 lines. According to the calculation formula R = kN –1 = λ /Δ λ from the color resolution of the grating, the resolution of the first-order diffraction spectrum of the end grating at a wavelength of 545nm is 8.4nm.

In the second specific example, the optical fiber pigtail 13 is directly facing the surface of an object to be measured to measure the reflection spectrum. The light reflected from the surface of the object returns to the optical fiber pigtail 13 , and is finally received by the photoelectric conversion element 3 after being diffracted at the end face of the signal light output optical fiber 12 . With a grating constant of 1.2 microns and an end-face inclination angle of 27 degrees, the first-order diffraction of 500nm wavelength light can exit along the fiber axis and achieve a resolution of about 4nm.

The present invention is not limited to the above-mentioned embodiments.

Claims (6)

1. A low-resolution optical fiber monochromator is characterized in that: two same-side optical fiber branches of a Y-shaped multimode fiber beam splitter are used for the transmission of excitation light and signal light respectively, wherein the end of signal light exit optical fiber The surface is inclined at a specific angle relative to the axis of the optical fiber. The transmission grating is fabricated on the end face of the signal light output optical fiber, and the photoelectric conversion element is installed coaxially with the signal light output optical fiber. 2. A low-resolution optical fiber monochromator according to claim 1, characterized in that the end face of the signal light exit optical fiber of the Y-shaped multimode optical fiber beam splitter is γ = 90° with respect to the axial direction of the optical fiber - arcsin{ λ /[ d ( n ₁ –1)]}. 3. A low-resolution fiber monochromator according to claim 1, characterized in that each notch of the transmission grating is perpendicular to the axial direction of the optical fiber and the inclination direction of the end face of the optical fiber from which the signal light exits, and is parallel to equidistant distribution. 4. a kind of low-resolution optical fiber monochromator according to claim 1, is characterized in that the branching ratio of described Y-shaped multimode fiber beam splitter is determined according to excitation light intensity, signal light intensity, usually gets 50/ 50. 5. A kind of low-resolution optical fiber monochromator according to claim 2, characterized in that the end face of the signal light output optical fiber is ground or cut inclination angle γ = 90 ° - arcsin{ λ / [ d ( n ₁ -1 )]} Make the first-order diffraction direction of the central wavelength of the signal light along the fiber axis. 6. A low-resolution fiber optic monochromator according to claim 3, characterized in that the transmission grating is directly photo-engraved, or pasted in parallel, or molded and copied on the end face of the signal light exiting optical fiber.

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