JPH0118371B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for measuring group delay time differences for each mode group of a multimode optical fiber.

Conventionally, as a method for measuring the group delay time difference for each mode group of a multimode optical fiber, the deviation time of the pulse waveform obtained by passing the light emitted from the multimode optical fiber through a mode analyzer with an equal ring-shaped transmittance distribution is compared. method is known. However, several meters
At present, when trying to measure the group delay time difference for each mode group using a short optical fiber of
Due to the limited time resolution of the measuring device, for example, several p
It was completely impossible to measure group delay time differences on the order of seconds.

Therefore, the present invention aims to improve the above-mentioned drawbacks of the conventional technology, and its purpose is to provide a method for measuring the group delay time difference for each mode group using a short optical fiber with a length of about several meters. , its characteristics are that coherent light is input from a coherent light source into an optical fiber to be measured and a single mode optical fiber for reference, and the output light from each optical fiber is made into linearly polarized light by a polarizer, and each light is at least polarized. A group of modes of interest in the output light emitted from the multimode optical fiber to be measured whose optical path length maximizes the clarity of the interference fringes obtained on the observation surface by interfering using an optical system including an optical path length adjustment mechanism for one optical path. There is a measurement method that takes advantage of the fact that each mode is different, and obtains the group delay time difference for each mode group from these differences.

Examples will be described below with reference to the drawings.

FIG. 1 shows an embodiment of the present invention, in which 1 is a coherent light source, 2 is a coherent light beam, 3 is a semi-transparent mirror for amplitude-splitting the light, and 4 is a multimode optical fiber to be measured (hereinafter referred to as a multimode optical fiber to be measured). 5 is a fiber to be measured, 6 is a single mode fiber used as a reference fiber, and 7 is a lens for each of the above-mentioned fibers. 8 is a polarizer for making the light emitted from each fiber into linearly polarized light; 9 is an optical attenuator for adjusting the amount of light; 10 is a semi-transparent mirror; The output light of 9 and the output light of 12 are combined. 11 is a mirror, 12 is an optical path length coarse adjustment medium, and 13 is a light receiving and recording system for measuring the sharpness of interference fringes arranged on the observation surface. As this light-receiving recording system, an electrical signal recording system such as a silicon videocon or an image sensor, or a photosensitive material such as a photographic film or a photographic plate can be used.

To operate this, the lens 4 is arranged so that the coherent light 2 from the coherent light source 1 can be incident on the fiber 5 to be measured and the reference fiber 6 via the semi-transparent mirror 3. Next, a lens 7 is arranged so that the light emitted from the fiber 5 to be measured and the reference fiber 6 becomes parallel light, and after being divided by the semi-transparent mirror 3, the light is transmitted to the light receiving and recording system 13 arranged on the observation surface. The coarse optical path length adjustment medium 12 and the mirror 11 are arranged so that the optical paths of the images are observed to be approximately equal. Specifically, as the optical path length rough adjustment medium 12, an optical fiber of the same type as the reference fiber 6 of an appropriate length, or a glass tube filled with an appropriate gas at an appropriate pressure, etc. are used. It is possible. At this time, the optical attenuator 9 adjusts the intensities of these two light beams guided to the light receiving and recording system 13 to be equal. Continuing with such rough adjustment work, the mirror 11 is moved, and the relationship between the moving distance at this time and the clarity of the interference fringes observed in the light receiving recording system 13 is obtained, and the position of the mirror 11 with the maximum clarity is found. Next, similar measurements are made for any other mode group included in the light emitted from the fiber 5 to be measured, the position of the mirror 11 with maximum clarity is determined again, and the moving distance Δd of the mirror 11 from the reference position is determined. By measuring _0n ,
Find the group delay time difference for each mode group. Here, when the maximum intensity of the interference fringes is _Inax and the minimum intensity is _Inio , the visibility V is defined as V=( _Inax - _Inio )/ _Inax + _Inio ). Note that the exit angle from the optical fiber to be measured differs depending on the mode group, and the reference mode is located at the center,
Since the higher-order modes are emitted to the periphery, interference fringes for each mode group can be obtained.

Now, considering the center of the emission pattern from the fiber under test, which corresponds to the position of the lowest mode group, the relationship between the clarity V of the interference fringes and the group delay time difference τ is that the optical attenuator 9 has a uniform light intensity. If adjusted to V
It is expressed as = |γ(τ)|. Here, γ(τ) is the degree of complex coherence determined by the light source used. Next, when examining the clarity of the interference fringes in the radial direction from the center of the emission pattern, the relationship V=|γ(τ+Δr _0n )| holds true. Here, Δτ _0n is the group delay time difference between the lowest mode group and the m-th mode group, and if the moving distance of the mirror 11 that maximizes the visibility for each mode group is Δd _0n , then Δτ _0n = 2Δd _0n / It is represented by C. However, C is the speed of light in free space. Therefore, by measuring the moving distance Δd _0n of the mirror at the position where the sharpness obtained for each mode group is maximum, the group delay time difference Δτ _0n for each mode group can be determined.

An example of the measurement results is shown in FIG. Figure 2 is 6.4
The figure shows the visibility curves obtained for the lowest mode group and other arbitrary mode groups using a short multimode fiber of m length as a sample. From this result, the distance between the positions with maximum visibility is 1.4 cm, and Δτ _0n is 47 p sec (=
1.4cm/3×10 ² cm). This result agrees well with the results obtained by the pulse method of the group delay time difference between the lowest mode group and the highest mode group.

As explained above, this measurement method measures the clarity of interference fringes caused by the interference effect of the light emitted from the optical fiber to be measured and the light emitted from the reference optical fiber, and determines the maximum clarity obtained for each mode group. The group delay time difference is measured for each mode group based on the distance interval between the positions. For example, if this distance interval is equivalent to several millimeters, the group delay time difference for each mode group will be several millimeters.
ps, making it impossible to measure using existing pulse methods; however, by using the method of the present invention, it is possible to measure short optical fibers as samples. Furthermore, the measuring device is also simplified.

[Brief explanation of drawings]

FIG. 1 shows an example of the configuration of a measuring device according to the present invention, and FIG.
The figure shows the optical path difference 2d and sharpness V obtained by the present invention.
It is a graph showing the relationship between. 1... Coherent light source, 2... Coherent light, 3... Semi-transparent mirror, 4... Lens, 5... Fiber to be measured,
6...Single mode fiber, 7...Lens,
8... Polarizer, 9... Optical attenuator, 10... Semi-transparent mirror, 11... Mirror, 12... Optical path length rough adjustment medium, 1
3...Light reception recording system.

Claims (1)

[Claims] 1 Coherent light from a coherent light source is input into a multimode optical fiber to be measured and a reference single mode optical fiber, and output light of an arbitrary mode group emitted from the multimode optical fiber to be measured and the reference single mode optical fiber are input. Each output light emitted from a one-mode optical fiber is linearly polarized using a polarizer, and each light is made to interfere with each other using an optical system that includes an optical path length adjustment mechanism for at least one of the optical paths, resulting in clear interference fringes obtained on the observation surface. The optical path length that maximizes the degree of interference is determined, and then the interference pattern is obtained in the same manner for the output light of any other mode group that is output from the multimode optical fiber to be measured and the output light that is output from the reference single mode optical fiber. A measurement method characterized by determining the optical path length that maximizes the clarity of the image, and obtaining the group delay time difference for each mode group from the difference between these two optical path lengths.