JP2010101669A - Device and method for measuring band of multimode optical fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for measuring an effective mode band of a multimode optical fiber inexpensively and easily. <P>SOLUTION: The band measuring device for a multimode optical fiber includes a laser light source for generating an optical signal in response to an input signal, an exciter connected to the laser light source to excite the input optical signal, and a detector connected to the exciter through the multimode optical fiber being a measuring object. The core diameter of an optical fiber constituting the exciter is 0.36-0.9 times the core diameter of the multimode optical fiber being the measuring object, and the numerical aperture of the optical fiber constituting the exciter is nearly the same of the numerical aperture of the multimode optical fiber being the measuring object. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a band measuring apparatus and a band measuring method for a multimode optical fiber used in the fields of optical communication and optical interconnection.

  Since optical communication is capable of long-distance and large-capacity transmission, long-distance communication has been more widely used than before. In recent years, with the development of computers and networks, high-speed communication has been required even at relatively short distances such as between computers and in data centers. A transmission system using a multimode optical fiber is suitable for such short-range communication. A multimode optical fiber is a communication method for propagating a plurality of modes in a fiber, and is not suitable for long-distance transmission. On the other hand, since cost reduction such as easy mounting of optical components can be expected, it is suitable for short distance communication requiring an inexpensive transmission system.

  In such a communication system using a multi-mode optical fiber, a light emitting diode (LED) is initially used as a light source. However, light-emitting diodes are limited in speeding up, and semiconductor lasers such as surface emitting lasers (VCSELs) have come to be used with the recent increase in data communication speed. In particular, it has recently been used for high-speed communications of 8 to 10 gigabit class.

Mode dispersion is dominant as a factor limiting the transmission rate in multimode optical fiber transmission. Mode dispersion is dispersion that occurs because the propagation time differs between different modes. Therefore, a high-speed multimode optical fiber usually needs to have a graded index structure. This graded index fiber has a high refractive index at the center of the core through which light propagates, and the refractive index gradually decreases toward the outside. Since light propagates at an effective refractive index speed, the mode passing through the center has a slow propagation speed instead of a short propagation distance, and light propagating outside has a propagation speed instead of a long propagation distance. To be faster. That is, the refractive index distribution is such that the light propagating anywhere in the core has substantially the same propagation time. Here, it is known that there is no difference in propagation time theoretically by making the refractive index distribution parabolic (square function). However, because the refractive index of SiO ₂ and GeO ₂ used for multimode optical fiber has wavelength dependence, and to reduce the mode dispersion at the target wavelength, the refractive index distribution is adjusted according to the characteristics of the fiber. There are many cases to do.

  As a method for evaluating the transmission characteristics of the manufactured fiber, there is a method for evaluating all-mode excited fibers described in Non-Patent Document 1. In this method, the characteristics are measured in a state where the mode distribution of light is spread over the entire fiber using an all-mode exciter. The effect of fiber mode dispersion uses the mode bandwidth as an index. This is a product of a band in which the length × frequency characteristic is reduced by 3 dB. In general, such band measurement by all-mode excitation is used for characteristic evaluation in which a measurement result is 300 MHz-km or more. Fibers with such evaluation results are used for applications below the gigabit class or for applications where the distance is shorter than that.

  On the other hand, for a high-speed transmission system such as 10 gigabit and used for applications such as 100 m or more, a broadband fiber with smaller mode dispersion is required. Further, in such a high-speed transmission system, it is necessary to couple the fibers so that the fields are distributed concentrically except for the pole center portion of the core instead of the entire region of the multimode core. That is, it is necessary to use in a limited mode excitation (also called effective mode) state. The reason for this is that the refractive index at the pole center of the multimode optical fiber is likely to fluctuate due to manufacturing reasons, and that the outer edge of the outer side tends to generate modal noise and the mode dispersion increases. It is done.

  As a method for obtaining a band in such a limited mode excitation state, there is a measurement method described in Non-Patent Document 2. This method is called DMD (Differential Mode Delay) measurement. In this method, the response of the pulse train is measured while focusing the beam to a single mode and shifting the coupling position of the light to the multimode optical fiber. Then, the response of the pulse train is Fourier transformed to obtain a band. The effective mode band required by this method is used for a broadband fiber of 2000 MHz-km or more. That is, this method is suitable for selecting a fiber for high-speed multimode transmission. For this reason, it is generally used as a method for evaluating a broadband fiber that is used at a transmission speed of 10 gigabits or more and a distance of 100 m or more.

  For transmission of multimode optical fibers, the 1310 nm band and the 850 nm band have been mainly used so far. An edge-emitting laser is mainly used in the 1310 nm band, and a surface-emitting laser is exclusively used in the 850 nm band. Particularly suitable for high-speed applications is the 850 nm band. Surface emitting lasers have features such as low power consumption and low power consumption, and because they are easy to produce in the vicinity of this wavelength, they are widely used for datacom applications regardless of high speed or low speed. ing.

  On the other hand, in a large-scale data center or a network switch system in which the amount of processing is remarkably increased, a further increase in data speed has been demanded. In view of this, there is known a technique for virtually speeding up a plurality of transmission lines by using a plurality of ports such as a link aggregation technique. In addition, transmission systems that increase the bandwidth by using a fiber in which a plurality of fibers are formed in a ribbon shape to have one input / output port and sending signals in parallel have come to be used.

  However, it is desirable to increase the transmission rate per channel in order to meet future needs for higher speeds and lower costs. As a method for speeding up, wavelength multiplexing and speeding up of the signal itself can be considered. In order to increase the speed of the signal itself, it is more suitable to use a longer wavelength than 850 nm. Here, if the wavelength is up to about 1100 to 1200 nm, it can be improved by extending the existing surface emitting laser. That is, a relatively low difficulty change such as changing the resonance wavelength of the active layer and the reflecting mirror is sufficient. A high differential gain (dg / dn) can be expected by using a strain compensation active layer containing In as the active layer, and as a result, a high speed can be achieved.

  Further, as a characteristic of the optical fiber, there is a phenomenon in which the propagation speed also changes for each wavelength because the refractive index changes for each wavelength. As a feature of the operation of the surface emitting laser, it is in a transverse multimode during operation. At this time, the oscillation wavelength is not a complete single mode, but has a broad spectrum corresponding to each transverse mode. Therefore, the propagation speed changes for each transverse mode, and transmission characteristics deteriorate due to this chromatic dispersion. In order to suppress these phenomena, it is necessary to suppress the spectrum spread, and severe restrictions are imposed on the spectrum spread in accordance with the transmission speed. In general, in order to increase the speed of the surface emitting laser, it is necessary to increase the optical confinement and the spectrum tends to be broadened, so that it is technically difficult to increase the speed in the 850 nm band. However, this amount of chromatic dispersion is inversely proportional to the fourth power of the wavelength, and is the smallest in the 1300 nm band. That is, in the 1000-1200 nm band, the amount of chromatic dispersion can be greatly reduced as compared with 850 nm. For this reason, it is considered that the use of this nearby wavelength band is more suitable for optical transmission.

  On the other hand, in the case of wavelength multiplexing, even if the center is 850 nm, it is necessary to arrange the wavelengths separately. In consideration of wavelength variation and ease of wavelength separation, when each channel is separated by 20 nm and four waves are multiplexed, it is used over 60 nm.

  Patent Documents 1 and 2 disclose techniques related to the present invention.

JP 59-111103 A Japanese Patent Laid-Open No. 11-038255 Japanese Industrial Standard JIS-C6824 Japanese Industrial Standard JIS-C6864

  Thus, in speeding up multimode optical fiber transmission, it is effective to use not only 850 nm but also various wavelength bands up to 1300 nm. However, the mode band of a multimode optical fiber greatly depends on the wavelength. Also. Because of manufacturing variations, it is desirable to measure the effective mode bandwidth for each wavelength used. Furthermore, even if wavelengths that are relatively adjacent to each other are used for wavelength multiplexing or the like, it is desirable to measure at each wavelength. Here, as a matter of course, measurement of a mode band suitable for high-speed applications is required, and thus it is necessary to perform measurement by the method described in Non-Patent Document 2.

  However, in the method described in Non-Patent Document 2, it is necessary to change the mode diameter of the single mode according to the wavelength, and a precise optical system is required. And it is difficult to prepare such a system for each wavelength to be used.

  An object of the present invention is to provide an inexpensive and simple method for measuring the effective mode bandwidth of a multimode optical fiber.

In the present invention, a multimode optical fiber bandwidth measuring device
A laser light source that generates an optical signal according to an input signal;
An exciter connected to the laser light source and exciting the input optical signal;
A detector connected to the exciter through a multimode optical fiber to be measured,
The core diameter of the optical fiber constituting the exciter is 0.36 to 0.9 times the core diameter of the multimode optical fiber to be measured, and the aperture of the optical fiber constituting the exciter The number is substantially the same as the numerical aperture of the multimode optical fiber to be measured.

  According to the present invention, it is possible to provide a high-speed transmission system that is bidirectional communication and has visibility.

  The core diameter of a standard multimode optical fiber is 50 μm or 62.5 μm. Among these, what is used for high-speed transmission is a GI50 having a core diameter of 50 μm. In general, as a feature of a multimode optical fiber, a propagating mode greatly changes depending on an incident mode and an optical coupling state between a light source and a fiber. In order to evaluate the characteristics of fibers, it is necessary to use a stable mode that does not depend on them. In order to achieve a stable mode, it is known to use a fiber in which a step index fiber (S) and a graded index fiber (G) are connected in order of SGS (or GSG) by 2 m. In particular, when measuring the bandwidth, many are connected in the order of SGS.

  The inventors have found that a stable mode with a small mode diameter can be obtained by thinning the core of the SGS fiber. Thereby, the mode shape necessary for measuring the effective mode can be adjusted. The size of this mode diameter is determined by the core diameter of the thin core diameter. In high-speed multimode transmission, an index called Encircled Flux (EF) plays an important role. The encircled flux value is obtained by integrating the light intensity distribution from the center toward the outer periphery (r), and the light mode distribution has a proportion of the total intensity from the center to the radius (r). It is an index indicating whether or not.

  As high-speed data communication, Ethernet (registered trademark) standardized by IEEE (American Institute of Electrical and Electronics Engineers) 802.3ae is generally used. In this Ethernet, there is 10G-Base-SR standardized for multimode optical fiber transmission. Among them, the encircled flux is specified to be 0.3 or less within a radius of 4.5 μm and 0.86 or more when the radius is 19 μm or more.

  This is because, in view of the characteristics of the multimode optical fiber, the characteristics such as mode dispersion are stabilized because the radius is 4.5 to 19 μm. In particular, at the central portion of the radius of 4.5 μm or less, the doping during the fiber manufacturing process cannot be sufficiently controlled, and the refractive index is often smaller than the design value. On the other hand, speckle noise, which is called modal noise, due to interference between modes is more likely to occur at the outer periphery than 19 μm. The outer peripheral portion corresponds to a higher order mode even in the fiber mode. Higher-order modes close to the cut-off have a large mode dispersion, so that it is necessary to transmit them away from the outer periphery. Therefore, it is necessary to measure the effective mode band in a mode in which this encircled flux value is included in this regulation.

  Therefore, in the present invention, the exciter portion described in Non-Patent Document 1 is an exciter composed of an SGS fiber having a fine core diameter of 0.36 to 0.9 times the core diameter of the multimode optical fiber to be measured. Used for. As a result, the effective mode band can be easily measured with the same device configuration as the all-mode excitation. Further, in this method, if only the light source is replaced, it is possible to easily measure at a plurality of wavelengths. Moreover, if a thin core diameter fiber is measured in multiple ways within the range of 18-45 micrometers, accuracy will also improve.

  Hereinafter, specific embodiments to which the present invention is applied will be described in detail with reference to the drawings. However, the present invention is not limited to the following embodiment. In addition, for clarity of explanation, the following description and drawings are simplified as appropriate.

Embodiment 1
The effective mode band device according to the first embodiment of the present invention will be described below with reference to FIG. The effective mode band measuring apparatus 100 includes a signal generator 101, a semiconductor laser light source 102, a fine core diameter SGS exciter 103, a measured fiber 104, a detector 105, a signal receiver 106, and a signal processor 107.

  The signal generator 101 uses a tracking generator, a sweep generator, a synthesizer, or the like. As this high-frequency signal generator 101, a high-frequency signal generator that can output a sufficiently high-frequency signal with respect to the band of the fiber 104 to be measured is used.

  As the semiconductor laser light source 102, a laser beam having a sufficiently narrow spectrum width and single axis mode laser oscillation is used. Further, the semiconductor laser light source 102 has a mechanism capable of modulating the optical output signal based on the signal from the signal generator 101, and can sufficiently respond to the band of the measured fiber 104.

  As shown in FIG. 2, the narrow core diameter SGS exciter 103 uses step index fibers 201 and 203 having a core diameter of 18 to 45 μm and a graded index fiber 202 to shape the incident single mode into an effective mode. . The length of each of the fibers 201 to 203 is 2 m. Details of FIG. 2 will be described later.

  A detector 105 that can receive all of the light emitted from the optical fiber 104 is used. The detector 105 is linear with respect to the incident intensity level of light and can respond up to a high frequency.

The signal receiver 106 is for measuring the amplitude of the high-frequency signal from the detector 105 in synchronization with signal generation.
The signal processor 107 controls the signal generator 101 and the signal receiver 106 and observes the frequency characteristics of the signal level.
Here, when a network analyzer in which the signal generator 101, the signal receiver 106, and the signal processor 107 are integrated is used, measurement can be performed more easily.

  Next, an effective mode band measurement method using the fine core diameter SGS exciter 103 will be described in detail with reference to FIGS. As shown in FIG. 2A, the fine core diameter SGS exciter 103 includes an SGS fiber 200 including a step index fiber 201, a graded index fiber 202, and a step index fiber 203 connected in order. The step index fibers 201 and 203 are the same, and each fiber is joined by a fiber fusion method.

  FIG. 2B shows the cross section and refractive index distribution of the step index fiber. For the step index fibers 201 and 203, those having a step structure with a refractive index of the core and the clad are used. The numerical apertures of the step index fibers 201 and 203 (coefficient determined by the relative refractive index difference between the core and the clad) are set to 0.2, which is the same as that of the measured fiber 104.

  FIG. 2C shows the cross section and refractive index distribution of the graded index fiber. The graded index fiber 202 has a graded index structure in the refractive index profile of the core. The numerical aperture of the graded index fiber 202 is also set to 0.2.

  If the numerical aperture of the SGS fiber 200 is too small, a sufficient difference in refractive index between the core and the clad cannot be obtained, making it difficult to set the mode to the effective mode state. On the other hand, if the numerical aperture is too large, the mode can be shaped, but there will be a difference in the equivalent refractive index of the light propagation mode with the fiber 104 to be measured. May be excited. Therefore, it is preferable that the numerical aperture of the SGS fiber 200 is the same as the aperture ratio of the measured fiber 104. Specifically, the deviation between the two is preferably within ± 10%. In the present embodiment, since the numerical aperture of the measured fiber 104 is 0.2, the numerical aperture of the SGS fiber 200 is preferably 0.2 ± 0.02.

  The length of each of the fibers 201 to 203 constituting the SGS fiber 200 is 2 m as in the Japanese Industrial Standard (see Non-Patent Document 1). Specifically, 2 m ± 0.3 m is preferable. If it is shorter than this, the mode does not spread sufficiently, and depends on the mode of input to the SGS exciter. On the other hand, if the length is longer than this, the mode shape hardly changes, and the measurable bandwidth is limited by the bandwidth of the exciter itself.

  Next, the core diameter and mode diameter of the thin core diameter fiber will be described with reference to FIG. The figure shows encircled flux (EF) after passing through an SGS exciter composed of 18 μm, 32 μm and 45 μm fine core diameters. Areas with a radius of 4.5 μm or less, EF 0.3 or more, and a radius of 19 μm or more and EF 0.86 or more, which are non-specified regions of TIA / EIA455-203, are shown as hatched portions. This data was obtained by a simulation of multimode propagation based on a light source that oscillated in a single mode. The curve that does not pass through the two shaded portions is the effective mode.

  As is clear from this data, when the core diameter is 18 μm, the inner hatched area is barely reached, and when the core diameter is 45 μm, the outer hatched area is barely reached. From these facts, the effective range of the fine core diameter is 18 to 45 μm. That is, the core diameter of the SGS fiber 200 is set to 0.36 to 0.9 times the core diameter of the multimode optical fiber to be measured. Further, by measuring not only in the vicinity of 32 μm in the center but also in both cases of 18 μm and 45 μm, it is possible to measure an effective mode that is more realistic.

  In addition, although the example of SGS which is easy to shape to the most stable mode diameter was given as this exciter structure, not only the order of SGS but the combination in which mode shaping is possible can be selected suitably. On the other hand, although only a single mode wavelength is described here as a measurement wavelength, a generally suitable wavelength corresponds to the most used range of 850 nm to 1310 nm. All multimode optical fibers that have been manufactured and installed so far have been optimized for one or both of these wavelengths. For this reason, at least a wavelength of about several hundred MHz-km can be expected for existing wavelengths, and a good one can obtain a bandwidth of 3000 MHz-km or more. Therefore, it can be said that 850 nm to 1310 nm is suitable for the transmission band of the multimode optical fiber.

Next, the measurement method will be described. First, in FIG. 1, the frequency response of the measurement system is measured without the fiber to be measured. This corresponds to calibration of the measurement system. At this time, it is necessary to select a frequency range corresponding to the band and length of the fiber. Let A (f) be the frequency characteristic obtained without the fiber to be measured. If the frequency characteristic measured with the measured fiber connected is B (f), the frequency response characteristic C (f) of the measured fiber is expressed by the following equation.
C (f) = − 20 × log (B (f) / A (f)) + 20 log (B (0) / A (0))
Finally, in order to eliminate the influence of the DC response characteristics (the influence of the conversion efficiency of the measurement system and the loss of the fiber), the DC component is added.

When this frequency response characteristic is reduced by 6 dB, f0 is 3 dB and f1 is 3dB, and L is the measured fiber length. The following Fa and Fb are compared, and the smaller one per unit length of fiber. Is assumed to be a band F.
Fa = f0 × L ^γ
Fb = 1.512 × f1 × L ^γ
Here, γ is a distance conversion factor between 0.5 and 1. For a uniform fiber with little loss, γ≈1.

  The reason for this is that the response characteristic may be raised after the frequency response function of the fiber is once deteriorated, and the fiber band may be erroneously determined. Therefore, it is closer to the effective mode band that is originally required if the fiber band is obtained by extrapolating the lowered shape. In general, it has been found that the fiber band is close to the response of a Gaussian function. In this case, 3dB is multiplied by 1.412 and the value at 6dB is the same. In this way, the mode band of the fiber can be measured.

  The difference between the evaluation method according to the present embodiment and the evaluation method for all mode bands defined by JIS is whether the configuration of the exciter is the same as that of the core of the fiber to be measured or is thin. Therefore, if the exciter has a thin core diameter, the effective mode band can be easily measured in this way.

Embodiment 2
Next, an effective mode bandwidth measuring apparatus 300 according to the second embodiment will be described with reference to FIG. The effective mode band measuring apparatus 300 includes a pulse signal generator 301, a semiconductor laser light source 302, a fine core diameter SGS exciter 303, a fiber to be measured 304, a detector 305, a signal amplifier 306, an oscilloscope 307, and a signal processor 308.

  For the pulse signal generator 301, a pulse generator or a pulse pattern generator is used. The pulse signal generator 301 generates a short pulse at a high speed such that a sufficiently high frequency response is obtained for the band of the fiber 304 to be measured.

  As the semiconductor laser light source 302, a laser light source having a sufficiently narrow spectrum width and having a single axial mode laser oscillation is used. Further, the semiconductor laser light source 302 has a mechanism capable of modulating the optical output signal based on the signal from the signal generator 301, and can sufficiently respond to the band of the measured fiber 304.

  The thin core diameter SGS exciter 303 shapes the incident single mode into an effective mode using a step index fiber and a graded index fiber having a core diameter of 18 to 45 μm. The SGS small diameter core fiber is the same as that in the first embodiment.

  A detector that can receive all of the outgoing pulsed light from the optical fiber 304 is used as the detector 305. The response of the detector 305 is linear with respect to the incident intensity level of light and responds up to a high frequency.

The signal amplifier 306 amplifies the high frequency signal from the detector 305.
The oscilloscope 307 observes the output pulse of the signal amplifier 306 in synchronization with the clock signal obtained from the pulse signal generator 301.
The signal processor 308 performs data processing on the pulse observed with the oscilloscope.

  Next, a measurement method will be described. First, in FIG. 4, the pulse response of the output of the measurement system is measured without the fiber to be measured. This corresponds to calibration of the measurement system. At this time, it is necessary to make a pulse shape according to the band and length of the fiber. Let A (t) be the pulse waveform obtained in this way without the measured fiber. A pulse waveform measured with the measured fiber connected is defined as B (t). Also, let D (t) be the output pulse waveform of the pulse signal generator that has not passed through the measurement system.

The frequency response A (f) of the measurement system is expressed by the following expression when the fast Fourier transform processing is FFT.
A (f) = FFT (A (t)) / FFT (D (t))
Further, the frequency response B (f) in a state where the measured fibers are connected is expressed by the following equation.
B (f) = FFT (B (t)) / FFT (D (t))
From these results, the frequency response characteristic of the fiber is expressed by the following equation as in the first embodiment.
C (f) = − 20 × log (B (f) / A (f)) + 20 log (B (0) / A (0))
Finally, in order to eliminate the influence of the DC response characteristics (the influence of the conversion efficiency of the measurement system and the loss of the fiber), the DC component is finally added.

As in the first embodiment, when the frequency response characteristic is reduced by 6 dB, f0 is reduced by 3 dB, f1 is decreased by 3 dB, and the measured fiber length is L. This is the band F per unit length of fiber.
Fa = f0 × L ^γ
Fb = 1.512 × f1 × L ^γ
Here, γ is a distance conversion factor between 0.5 and 1. For a uniform fiber with little loss, γ≈1.

  In the second embodiment, what is measured in the time domain instead of the frequency domain is Fourier-transformed to obtain the fiber characteristics. As a result, the same result as in the first embodiment is obtained.

  As a first embodiment, an effective mode band measuring method and apparatus using a frequency domain method of a wavelength of 1000 nm band using a thin core diameter fiber will be described with reference to FIG.

  The signal generator 101 is a synthesizer that can generate signals up to 5 GHz.

  As the semiconductor laser light source 102, a laser beam that has a modulation bandwidth of 7 GHz, an oscillation wavelength of 1000 nm, a sufficiently narrow spectrum width, and a single-mode laser oscillation is used. In addition, the signal generator 101 is connected using a bias so that the optical output signal of the semiconductor laser 102 can be modulated. At this time, 3 mA is used for the DC bias so as not to generate a plurality of modes.

  As the fine core diameter SGS exciter 403, a step index fiber and a graded index fiber each having a length of 2 m and a NA of 0.2 are used for a core diameter of up to 32 μm.

  The detector 105 is an InGaAs photodiode having a large light receiving diameter of 50 μm and a 3 dB band of 8 GHz so that all of the light emitted from the optical fiber 104 can be received. The response of the detector 105 is terminated with a resistor in order to make it linear with respect to the incident intensity level of light, and then a baseband amplifier is connected so that a high S / N ratio can be obtained.

The signal receiver 406 uses a spectrum spectrum analyzer or a power meter that measures the amplitude of the high-frequency signal from the detector 405 in synchronization with signal generation.
The signal processor 407, the signal generator 401, and the signal receiver 406 are controlled by GPIB, and the frequency characteristics of the signal level are observed. The fine core diameter SGS exciter 403 is measured for core diameters of 18 μm, 32 μm, and 45 μm.

Next, the measurement method will be described. First, in FIG. 1, the frequency response of the measurement system is measured without the fiber to be measured. This corresponds to calibration of the measurement system. At this time, it is necessary to select a frequency range corresponding to the band and length of the fiber. Let A (f) be the frequency characteristic obtained without the fiber to be measured. If the frequency characteristic measured with the measured fiber connected is B (f), the frequency response characteristic C (f) of the measured fiber is expressed by the following equation.
C (f) = − 20 × log (B (f) / A (f)) + 20 log (B (0) / A (0))
Finally, a DC component is added in order to eliminate the influence of the DC response characteristic (conversion efficiency of the measurement system and the influence of fiber loss).

When this frequency response characteristic is reduced by 6 dB, f0 is 3 dB and f1 is 3dB, and L is the measured fiber length. The following Fa and Fb are compared, and the smaller one per unit length of fiber. Is assumed to be a band F.
Fa = f0 × L ^γ
Fb = 1.512 × f1 × L ^γ
Here, γ is a distance conversion factor between 0.5 and 1. For a uniform fiber with little loss, γ≈1.

  The reason for doing this is that the response characteristic may be raised after the frequency response function of the fiber once deteriorates, and the band of the fiber may be erroneously determined. Therefore, it is closer to the effective mode band that is originally required if the fiber band is obtained by extrapolating the lowered shape. In general, it has been found that the fiber band is close to the response of a Gaussian function. In this case, 3dB is multiplied by 1.412 and the value at 6dB is the same. As a result, if Fa = 1.2 GHz is obtained, the bandwidth is 6000 MHz-km when the distance conversion coefficient γ = 1 is calculated.

  In this way, measurement is performed for three core diameters in the 1000 nm band, and the smallest of the three measurement results is the effective mode band. The 1000 nm band is a wavelength band that is not used for general multimode optical fiber communication. However, the characteristics can be easily obtained with only a single mode light source and the fine core diameter SGS. By using the fiber selected by this method, high-speed long-distance transmission can be stably performed in the 1000 nm band.

  As described above, in the embodiment of the present invention, the effective mode band can be measured easily. In addition, measurement at a plurality of wavelengths can be easily performed, and all mode excitation can be measured by exchanging only the excitation system, so that the apparatus can be shared and the cost can be reduced.

It is a figure which shows the effective mode band measuring apparatus which concerns on 1st Embodiment and Example 1. FIG. It is a figure which shows the structure of the exciter which concerns on 1st Embodiment. It is a figure which shows the encircled flux at the time of using the mode exciter which concerns on 1st Embodiment. It is a figure which shows the effective mode band measuring apparatus which concerns on 2nd implementation.

Explanation of symbols

100 Effective Mode Bandwidth Measurement Device 101 Signal Generator 102 Semiconductor Laser Light Source 103 Thin Core Diameter SGS Exciter 104 Fiber to be Measured 105 Detector 106 Signal Receiver 107 Signal Processor 200 SGS Fiber 201 Fine Core Diameter Step Index Fiber 202 Fine Core Diameter Graded index fiber 203 Fine core diameter step index fiber 300 Band measurement device 301 of multimode optical fiber according to the second embodiment 301 Pulse signal generator 302 Semiconductor laser light source 303 Fine core diameter SGS exciter 304 Fiber to be measured 305 Detection 306 Signal amplifier 307 Oscilloscope 308 Signal processor

Claims (8)

A laser light source that generates an optical signal according to an input signal;
An exciter connected to the laser light source and exciting the input optical signal;
A detector connected to the exciter through a multimode optical fiber to be measured,
The core diameter of the optical fiber constituting the exciter is 0.36 to 0.9 times the core diameter of the multimode optical fiber to be measured, and the aperture of the optical fiber constituting the exciter A multimode optical fiber bandwidth measuring apparatus, wherein the number is substantially the same as the numerical aperture of the multimode optical fiber to be measured.   2. The multimode optical fiber according to claim 1, wherein a deviation between the numerical aperture of the optical fiber constituting the exciter and the numerical aperture of the multimode optical fiber to be measured is within 10%. Bandwidth measuring device.   The multimode optical fiber band measuring device according to claim 1 or 2, wherein a core diameter of the multimode optical fiber to be measured is 50 m.   The multimode optical fiber band measuring device according to any one of claims 1 to 3, wherein a numerical aperture of the multimode optical fiber to be measured is 0.2.   The multiwavelength optical fiber band measuring device according to any one of claims 1 to 4, wherein a measurement wavelength is 850 to 1310 nm.   The band measurement of a multimode optical fiber according to any one of claims 1 to 5, wherein the optical fiber constituting the exciter is a connection of a step index fiber and a graded index fiber. apparatus.   The optical fiber constituting the exciter is composed of three fibers of two step index fibers and one graded index fiber connected therebetween, and the length of the three fibers. The multi-mode optical fiber band measuring device according to claim 6, wherein each of the two is 2 m ± 0.3 m.   A band measuring method for a multimode optical fiber using the band measuring apparatus according to claim 1.

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