CN101476978B - A Method for Measuring Geometric Parameters of Single-Mode Optical Fiber - Google Patents

Abstract

A method for measuring the geometric parameters of a single-mode fiber, using a common laser as a light source, coupling light into the fiber through a precision five-dimensional adjustment frame with a lens and a fiber clamp, using a CCD or other imaging system for data collection, and using the obtained data with Computer analysis, Mathematica mathematical software is used to process and calculate the measured light intensity distribution under visible light, and the geometric parameters of the optical fiber are calculated by nonlinear least square fitting method, including the core refractive index n ₁ and the cladding refractive index Rate n ₂ , core radius a. Using ordinary semiconductor lasers as light sources is easy to use, requires less equipment, is low in cost, and has no strict requirements on the environment. The visible light band is used for measurement, avoiding the use of expensive infrared detectors and infrared laser light sources, and at the same time it is easy to adjust with the naked eye, and it is also convenient for the collection of light intensity information. Using CCD to detect the light intensity distribution at the output end of the optical fiber can be used for full-field measurement, and all data can be acquired at one time, avoiding the use of mechanically moving scanning equipment.

Description

A Method for Measuring Geometric Parameters of Single-Mode Optical Fiber

technical field

The invention relates to a method for measuring geometric parameters of a single-mode optical fiber, which belongs to the field of information technology.

Background technique

The geometric parameters of the single-mode fiber include the core refractive index n ₁ , the cladding refractive index n ₂ , and the core radius a. They are the most basic parameters of optical fiber, and other characteristic parameters of optical fiber, such as optical characteristic parameters and transmission characteristic parameters, all depend on geometric parameters. Therefore, it is of great significance to measure the geometric parameters of single-mode fiber. The traditional method of measuring the refractive index of optical fiber is the near-field scanning method, which requires highly delicate optical equipment, expensive equipment, complicated measurement process and long measurement time. The traditional method of measuring the radius of the fiber core is the backscattering method, which is only suitable for the measurement during the fiber manufacturing process.

Contents of the invention

The invention provides a method for measuring the geometric parameters (including core refractive index n ₁ , cladding layer refractive index n ₂ and core radius a) of single-mode optical fiber. The method comprises a semiconductor laser light source, which is irradiated to the uncoated input end face of the optical fiber in such a way that it is most suitable to be incident parallel to the longitudinal axis of the optical fiber, so that the output light intensity distribution image meets the experimental measurement requirements as much as possible, and then according to the measured The geometric parameters of the single-mode fiber are calculated from the obtained light intensity distribution at the output end of the fiber.

The purpose of the present invention is achieved in that a method for measuring the geometric parameters of a single-mode optical fiber comprises the following steps;

Use ordinary semiconductor lasers as the light source, and the wavelength of the light source is in the visible light band;

Use CCD to detect the light intensity distribution at the output end of the optical fiber, and perform full-field measurement;

A high-order mode filtering device is used to filter out the high-order modes transmitted in the optical fiber, and only the fundamental mode reaches the output end of the optical fiber;

Process the obtained image with a computer, and calculate the geometric parameters of the single-mode fiber by fitting the light intensity distribution, including the core refractive index n ₁ , the cladding refractive index n ₂ and the core radius a;

The nonlinear least squares method is used for fitting, and the parameters to be fitted are the core refractive index n ₁ , the cladding refractive index n ₂ and the fiber core radius a; according to the optical waveguide theory, the optical intensity distribution of the fundamental mode at the output end of the fiber is calculated The formula I(n ₁ , n ₂ , a) extracts the light intensity distribution data points from the measured image, and uses the formula I(n1, n2, a) to fit the data points to obtain the best n ₁ , n ₂ , a value.

Using a common laser as a light source, the light is coupled into the optical fiber through a precision five-dimensional adjustment frame with a lens and a fiber clamp, and the data is collected using a CCD or other imaging system, and the obtained data is analyzed by a computer to obtain the intensity distribution of the image. Use the Mathematica mathematical software of Wolfram Corporation of the United States to process and calculate the measured light intensity distribution under visible light,

The calculation method is as follows:

According to the optical waveguide theory, the ordinary single-mode fiber with a working wavelength of 1310nm will work in a multi-mode state when visible light is injected. The conditions for optical fiber single-mode transmission are:

V<2.405;

Where V is the normalized frequency, which is determined by the fiber structure parameters:

$\mathrm{<span\; class="notranslate">\; V</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}\sqrt{{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}\mathrm{<span\; class="notranslate">\; a</span>}<span\; class="notranslate">\; ;</span>$

where n ₁ is the core refractive index, n ₂ is the cladding refractive index, and a is the core radius. Assuming that visible light with a wavelength of 635nm is injected, the condition that V satisfies when an ordinary single-mode fiber with a working wavelength of 1310nm works at a wavelength of 635nm can be calculated from the above two formulas:

V<4.9615;

Since the cut-off condition of the LP ₃₁ mode is V=5.316, the LP ₃₁ mode and the modes after LP ₃₁ are all cut-off, and four modes may be transmitted in the optical fiber: LP ₀₁ , LP ₁₁ , LP ₀₂ , LP ₂₁ , and the cut-off frequency of each mode They are 0, 2.405, 3.832, 3.832 respectively.

Assuming that n ₁ , n ₂ , and a are known, then the field distribution of each mode can be calculated, and the total field distribution in the fiber is the coherent superposition of the light fields of each mode.

Perform Hankel transformation on the near-field distribution of the fundamental mode to obtain the far-field light intensity distribution, which is related to the fiber parameters n ₁ , n ₂ , a, the wavelength λ of the light source, and the included angle θ of the light deviation from the fiber axis, record its expression The formula is P(n ₁ , n ₂ , a; λ; θ). If the far-field distribution diagram of the fundamental mode of the fiber is measured experimentally, then the best n ₁ can be obtained by fitting the measured data points with the expression P(n ₁ , n ₂ , a; λ; θ) , n ₂ , a.

The geometrical parameters of the optical fiber are calculated by nonlinear least square fitting method, including the core refractive index n ₁ , the cladding refractive index n ₂ , and the core radius a.

The light source is an ordinary semiconductor laser light source, the power is greater than 10mW, and the wavelength is in the visible light band.

The imaging system is a CCD or other equipment that can be used to collect light intensity information, and the spectral response characteristics should be consistent with the semiconductor laser light source.

The beneficial effects produced by the present invention are:

1. The present invention uses a common semiconductor laser as a light source, which is easy to use, requires less equipment, is low in cost, and has no strict requirements on the environment.

2. The present invention uses the visible light band for measurement, avoids the use of expensive infrared detectors and infrared laser light sources, and at the same time facilitates adjustment with the naked eye and facilitates the collection of light intensity information.

3. The present invention uses CCD to detect the light intensity distribution at the output end of the optical fiber, and can perform full-field measurement, and obtain all data at one time, avoiding the use of mechanically moving scanning equipment.

Description of drawings

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

The present invention will be further described below in conjunction with drawings and embodiments.

Detailed ways

Embodiment 1: As shown in Figure 1, in this embodiment, the patented measurement device of the present invention includes a light source, an optical fiber coupling unit, a high-order mode filtering unit, an image acquisition unit, and an image processing unit. In this embodiment, the light source is a common semiconductor laser.

In this embodiment, the fiber coupling unit includes a precision five-dimensional adjustment frame with a lens and a fiber clamp. The model of the precision five-dimensional adjustment frame is OM-TZ-112, and the model of the fiber clamp is F1-109, both of which are manufactured by Shanghai Lianyi Optical Fiber Laser Instrument Factory. Production.

In this embodiment, the high-order mode filtering unit is a cylinder with a certain radius. The radius range is 3mm-5mm, and the optical fiber winds more than one circle on the cylinder.

In this embodiment, the image acquisition unit utilizes a commercially available common CCD and image acquisition card to transmit data to a computer.

In this embodiment, the image processing unit is an ordinary computer equipped with relevant software. Including supporting software for CCD image acquisition (this software is matched with the image acquisition card, and the software is obtained when purchasing the image acquisition card), and Mathematica mathematical software.

Working principle and operation steps of the present invention are:

The first step is to connect the optical fiber according to Figure 1.

The second step is to turn on the light source and adjust the precision five-dimensional adjustment frame to maximize the light intensity coupled into the optical fiber.

The third step is to adjust the position of the optical fiber output end and the image receiving unit so that the optical fiber output end is vertically aligned with the center of the photosensitive surface of the image receiving unit, and the distance between the optical fiber output end and the photosensitive surface is within the range of 5mm-20mm.

The fourth step is to adjust the background light intensity. The image receiving unit is irradiated with a red light-emitting diode with adjustable brightness, and the brightness is adjusted so that the image received by the image receiving unit has uniform background light, and the maximum light intensity is less than the saturation threshold of the image receiving unit.

The fifth step is to record the distance between the output end of the optical fiber and the image acquisition system. Collect images, and import the collected images into the computer through the image acquisition card, and process them with supporting programs (supporting software for CCD image acquisition and Mathematica mathematical software), and then you can get the geometric parameters of the optical fiber. The program is written on the basis of the Mathematica mathematical software platform of Wolfram Corporation in the United States. The geometric parameters of the optical fiber are calculated according to the light intensity distribution diagram by using the nonlinear least squares fitting algorithm. The specific algorithm is as described in the calculation method above.

While the invention has been shown and described herein in what is contemplated to be the most practical and preferred embodiment; it is to be recognized that changes may be made thereby within the scope of the invention and the invention should not be limited to what is disclosed herein. details, but the full scope of the claims should be followed so as to encompass any equivalents.

Claims (4)

1. the method for a photo measure geometric parameter of single mode fiber is characterised in that may further comprise the steps:

Do light source with the general semiconductor laser instrument, optical source wavelength is at visible light wave range;

With the light distribution of CCD detection optical fiber output terminal, carry out measurement of full field;

Use a kind of high-order mode filtering device, make the high-order mode of transmitting in the optical fiber by filtering, only basic mode arrives fiber-optic output;

The image that obtains is handled with computing machine,, calculated the geometric parameter of single-mode fiber, comprise fiber core refractive index n by match is carried out in light distribution ₁, cladding index n ₂With fiber core radius a;

Carry out match with nonlinear least square method, treat that fitting parameter is fiber core refractive index n ₁, cladding index n ₂With fiber core radius a; According to Light Wave Guide Theory, calculate the formula I (n of fiber-optic output basic mode light distribution ₁, n ₂, a), from being extracted the light distribution data point the altimetric image, with formula I (n ₁, n ₂, a) data point is carried out match, obtain best n ₁, n ₂, a value.

2. method according to claim 1 is characterised in that: used light detection device can carry out measurement of full field, the light intensity value of disposable measuring optical fiber output terminal each point, thus can obtain the space two-dimensional surface of intensity distribution.

3. method according to claim 1 is characterised in that: used high-order mode filtering device is the certain right cylinder of radius, optical fiber on this right cylinder around a fixing turn.

4. method according to claim 1, be characterised in that: CCD directly links to each other with computing machine by image pick-up card, can show the light distribution image that measures on computers in real time, and can be fast, in real time image is handled, calculate the geometric parameter of optical fiber.

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