JP2018096880A - Mode field diameter measurement method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a mode field diameter measurement method capable of establishing measurement method of MFD with respect to a higher mode of multi-mode optical fiber while clarifying a relation between MFD and connection loss.SOLUTION: Disclosed mode field diameter measurement method is a VA method capable of precisely measuring MFD in high mode, in which, defining an angle which is formed by a center axis of the optical fiber and a variable opening as angle θ, by adopting the maximum opening angle θmax to satisfy the following formula 1.SELECTED DRAWING: Figure 1

Description

  The present disclosure relates to a method for measuring a mode field diameter in an optical fiber capable of propagating a plurality of propagation modes.

  A multi-core optical fiber having a plurality of core regions and a number mode optical fiber capable of propagating a plurality of propagation modes have been actively studied for a dramatic increase in transmission capacity by using a spatial multiplexing technique. In particular, mode multiplex transmission using a plurality of propagation modes has been attracting attention as a new large-capacity transmission system because the transmission capacity can be improved by the number of modes that can be propagated.

  Here, in an optical communication system, the mode field diameter (MFD) of an optical fiber indicates the spread of a light wave that is being propagated. Particularly, since it can be used to estimate connection loss, it is an important parameter directly related to communication system design. It is. In the conventional single mode optical fiber (SMF), three methods of MFD measurement are defined by international standards: NFP (Near Field Pattern) method, FFP (Far Field Pattern) method, and VA (Variable Aperture) method. .

  Even in a few mode optical fiber, the fundamental mode exhibits the same behavior as the SMF, so that the existing measurement method can be used. However, for the higher-order mode MFD in the several-mode optical fiber, since the electric field distribution and FFP are significantly different from the fundamental mode, the relationship between the MFD and the connection loss in the higher-order mode is not clear, and the measurement method has not been established. .

  Non-Patent Document 1 reports that MFD in a higher-order mode of a several-mode optical fiber is numerically calculated to show a high correlation with connection loss by using the square root of the effective area. Non-Patent Document 2 shows that the beam width of the fundamental mode and the higher-order mode can be measured by applying an image processing technique in the NFP method.

K. Ozaki et al. "Effective Mode Field Diameter Definition and Splice Loss Estimation of LP11 Mode in Few Mode Fibers", Proc. ACP2014, ATh3A-98 (2014). Y. Wakayama et al. “MFD Measurement of a Six-Mode Fiber with Low-Coherence Digital Holography”, Proc. OECC / PS2016, ThC2-3 (2016).

  However, the relationship between the measured value of the MFD for the higher order mode and the connection loss is not clear, and the applicability of the MFD measurement method to the higher order mode is not clear, so how to measure the MFD for the higher order mode There was a problem that it was unclear whether it was good.

  Therefore, in order to solve the above-mentioned problems, the present invention provides a mode field diameter measurement method that establishes an MFD measurement method for higher-order modes of a few-mode optical fiber and clarifies the relationship between MFD and connection loss. With the goal.

  In order to achieve the above object, the mode field diameter measuring method according to the present invention sets the maximum opening angle with respect to the emission angle of the optical fiber to be measured based on the expected MFD in the VA method.

ただし、ＭＦＤ^*は予想される前記被測定光ファイバのモードフィールド径である。

Specifically, the mode field diameter measuring method according to the present invention is a mode field diameter in which the effective mode field diameter of the fundamental mode and the higher order mode in an optical fiber capable of propagating a plurality of modes is measured by the VA (Variable Aperture) method. A measuring method,
The light selectively excited in any propagation mode is incident on the optical fiber to be measured,
When the angle between the central axis of the optical fiber to be measured and the opening of the variable opening is θ,
When the light emitted from the optical fiber to be measured passes through the variable aperture, the θ is changed from zero to a maximum aperture angle θmax that satisfies Equation 1,
A mode field diameter measuring method characterized in that the mode field diameter MFD is calculated from the light intensity P (θ) of the emitted light that has passed through the variable aperture at the time of θ and the θ, using Equation (2).

Where MFD ^* is the expected mode field diameter of the optical fiber to be measured.

  In this mode field diameter measurement method, when measuring the MFD for each mode of the several-mode optical fiber by the VA method, an accurate MFD is calculated in Equation 2 for calculating the MFD (for example, when sufficiently large θmax is set). Therefore, the minimum value of θmax is obtained from the equation (1). The connection loss based on the MFD of the present mode field diameter measurement method obtained by measuring the light intensity from θ to zero to θmax substantially coincides with the connection loss measured in the experiment. Therefore, the present invention can establish a method for measuring MFD for higher-order modes of a few-mode optical fiber and provide a mode field diameter measuring method for clarifying the relationship between MFD and splice loss.

  The present invention can provide a mode field diameter measurement method that establishes an MFD measurement method for higher-order modes of a few-mode optical fiber and clarifies the relationship between MFD and splice loss.

It is the schematic of the apparatus which performs the mode field measuring method which concerns on this invention. It is the example which measured the light intensity of LP01 mode and LP11 mode radiate | emitted from a to-be-measured optical fiber with the apparatus which performs the mode field measuring method which concerns on this invention. It is an example which shows the relationship between the MFD precision and numerical aperture range of LP01 mode and LP11 mode which were measured with the mode field measuring method concerning the present invention. It is an example which shows the relationship between the connection loss estimated from MFD measured with the mode field measuring method which concerns on this invention, and the measured connection loss. It is a figure explaining the maximum numerical aperture required for the MFD measurement of LP01 mode and LP11 mode in the mode field measurement method according to the present invention.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

ただし、ＭＦＤ^*は予想される前記被測定光ファイバのモードフィールド径である。

The mode field measurement method of the present embodiment is a mode field diameter measurement method that measures the effective mode field diameter of the fundamental mode and the higher-order mode in an optical fiber capable of propagating a plurality of modes by a VA (Variable Aperture) method.
The light selectively excited in any propagation mode is incident on the optical fiber to be measured,
When the angle between the central axis of the optical fiber to be measured and the opening of the variable opening is θ,
When the light emitted from the optical fiber to be measured passes through the variable aperture, the θ is changed from zero to a maximum aperture angle θmax that satisfies Equation 1,
A mode field diameter measuring method characterized in that the mode field diameter MFD is calculated from the light intensity P (θ) of the emitted light that has passed through the variable aperture at the time of θ and the θ, using Equation (2).

Where MFD ^* is the expected mode field diameter of the optical fiber to be measured.

  FIG. 1 is a schematic view of a measuring machine that performs this mode field measurement method. The measuring instrument has a light source 11, a mode exciter 12, a variable aperture 13, and a light receiver 14. The light wave is incident on the measured optical fiber 50 from the light source 11 through the mode exciter 12 and selectively excites an arbitrary propagation mode. A light wave propagating through the optical fiber 50 to be measured and emitted from the emission end passes through the variable opening 13 and is received by the light receiving unit 14, and its light intensity is measured. At this time, the light intensity is measured by the light receiving unit 14 while changing the numerical aperture in the variable opening 13.

  If the received light intensity with respect to the angle θ formed by the central axis of the optical fiber and the opening is P (θ), the MFD is obtained by Equation 2. Note that θmax is θ at the end of measurement when increasing from zero.

  In order to align the center of the opening of the variable opening 13 with the center axis of the optical fiber 50 to be measured, it is preferable to install the aligning table 15 at the emission part of the optical fiber 50 to be measured. In addition, it is preferable to install an optical lens (collimator) 16 between the variable opening 13 and the light receiving unit 14 because the light receiving efficiency in the light receiving unit 14 is improved.

  FIG. 2 is a diagram for explaining the light intensity measured by the light receiving unit 14 when the aperture (sin θ) of the variable aperture 13 is changed in the measuring machine of FIG. Here, the measured optical fiber 50 is a step type number mode optical fiber, and the core diameter and the relative refractive index difference are 14 μm and 0.4%, respectively. The optical fiber can propagate two LP modes at a wavelength of 1550 nm. Here, the mode exciter 12 selectively excited the fundamental mode (LP01 mode) and the first higher-order mode (LP11 mode) to obtain a change in light intensity with respect to the numerical aperture. When MFD was calculated using Equation 2, they were 12.5 μm and 8.1 μm for the LP01 mode and the LP11 mode, respectively.

  FIG. 3 illustrates the relationship between the error between the MFD calculated when the light intensity is measured by the light receiving unit 14 by changing the maximum numerical aperture (sin θmax) in the variable aperture 13 and the MFD estimated from the measured connection loss. It is a figure to do. It can be seen that the MFD error is larger as the numerical aperture range (sin θmax) is smaller for any mode. Here, it can be seen that the maximum numerical apertures of 0.12 and 0.18 are necessary for the LP01 mode and the LP11 mode to have a measurement error of 1% or less, respectively.

ここでは軸ずれ量ｄを３μｍとした。
黒丸は本モードフィールド径測定方法のＶＡ法によって得られたＬＰ０１モードの接続損失、
白丸は本モードフィールド径測定方法のＶＡ法によって得られたＬＰ１１モードの接続損失、
黒三角はＮＦＰ法によって得られたＬＰ０１モードの接続損失、
白三角はＮＦＰ法によって得られたＬＰ１１モードの接続損失、
を表す。 FIG. 4 is a diagram for explaining the relationship between the measured connection loss and the connection loss value estimated from the MFD of Equation 2. The horizontal axis represents connection loss obtained by experiments for the LP01 mode and the LP11 mode. The vertical axis represents the connection loss estimated from the MFD measurement value of Equation 2 using the following equation.

Here, the axis deviation d was set to 3 μm.
The black circle is the connection loss of the LP01 mode obtained by the VA method of this mode field diameter measurement method,
The white circle is the connection loss of the LP11 mode obtained by the VA method of this mode field diameter measurement method,
The black triangle is the LP01 mode splice loss obtained by the NFP method.
The white triangle is the LP11 mode splice loss obtained by the NFP method.
Represents.

  From FIG. 4, it can be seen that in the NFP method, the estimated values from the experiment and the MFD are generally the same for the LP01 mode, but greatly deviated from the LP11 mode. Although the LP11 mode has a larger connection loss than the LP01 mode in the experiment, the NFP method is not suitable from the viewpoint of connection loss evaluation because the higher order mode has a larger MFD. On the other hand, the value obtained by the VA method of this mode field diameter measurement method is close to the experimental value for both the LP01 and LP11 modes. From this result, it can be seen that the MFD obtained by the VA method of the mode field diameter measurement method is effective in estimating the connection loss for both the fundamental mode and the higher order mode.

  FIG. 5 shows the relationship between the minimum required numerical aperture (sin θmax) for the MFD to be measured in the VA method. The horizontal axis is the MFD (true value) obtained with a sufficiently large maximum numerical aperture (for example, the maximum numerical aperture that can be set by the variable aperture portion 13 in FIG. 1), and the vertical axis is the error calculated by the VA method. This is the maximum numerical aperture (sin θmax) that is minimum required for measurement at 1% or less.

The black circles in the figure are the results of the LP01 mode, and the white circles are the results of the LP11 mode. From this comparison result, the maximum numerical aperture required in this mode field diameter measurement method can be approximated by the exponential function of Formula 1 with respect to the MFD value to be measured.
That is, by setting the maximum numerical aperture (sin θmax) satisfying Equation 1 in the measuring device of FIG. 1, it is possible to measure the MFD with sufficient accuracy in both the basic mode and the higher order mode.

(Effect of the present invention)
In the mode field measurement method of the present invention, it is possible to perform the MFD measurement for the higher-order mode and to estimate the connection loss for the higher-order mode based on the obtained MFD value.

  The present invention can be used for optical characteristic test for several mode optical fibers and system design of a space division multiplexing transmission system.

11: light source 12: mode exciter 13: variable aperture 14: light receiving unit 15: alignment table 16: optical lens 50: optical fiber to be measured

Claims (1)

A mode field diameter measurement method for measuring an effective mode field diameter of a fundamental mode and a higher-order mode in an optical fiber capable of propagating a plurality of modes by a VA (Variable Aperture) method,
The light selectively excited in any propagation mode is incident on the optical fiber to be measured,
When the angle between the central axis of the optical fiber to be measured and the opening of the variable opening is θ,
When the light emitted from the optical fiber to be measured passes through the variable aperture, the θ is changed from zero to a maximum aperture angle θmax that satisfies Equation 1,
A mode field diameter measuring method characterized in that the mode field diameter MFD is calculated from the light intensity P (θ) of the emitted light that has passed through the variable aperture at the time of θ and the θ, using Equation (2).

Where MFD ^* is the expected mode field diameter of the optical fiber to be measured.

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