JPS61228327A - Measuring method by optical fiber - Google Patents

Abstract

PURPOSE:To attain to enhance the measuring accuracy of transmission characteristics by preventing the formation of modal noise, by cutting the terminal surface of an optical fiber receiving the incidence of the optical signal from a LD light source so as to be made oblique by a specific angle or more to the surface vertical to the axis of the optical fiber or forming the same into a curved surface. CONSTITUTION:In order to substantially reduce the irregularity in a measured value, the incident terminal surface of an optical surface is inclined by 1 deg. or more from the surface vertical to the fiber axis. In order to reduce the effect of the modal noise of reflected light, the incidence terminal surface is formed into a smoothly curved surface shape such as a spherical surface in place of oblique cutting and the same effect can be obtained. It is unnecessary to necessarily apply the above mentioned special cutting or processing of the incidence terminal surface to a fiber itself to be measured and said cutting or processing may be preliminarily applied to an exciting fiber to connect the output terminal of the exciting fiber to a measuring fiber. By this method, the measuring accuracy of the characteristics of the optical fiber is enhanced.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method that can improve the deterioration of measurement accuracy due to the influence of modal noise in measuring the transmission characteristics of an optical fiber.

(Prior art and its problems) Conventionally, when measuring transmission characteristics by inputting a frequency-modulated optical signal from an LD light source into an optical fiber, the oscillation spectrum of modal noise (LI) is not pure, and some The problem was that the measurement accuracy deteriorated due to the coexistence of several modes and fluctuations in the abundance ratio of these modes. For example, FIG. 2 shows the results of repeated measurements of the baseband frequency characteristics of a multimode fiber using the frequency sweep method. In the figure, the shaded area indicates the variation in measured values for each measurement.

If we define the transmission bandwidth as 6dB lower frequency.

In the illustrated example, the frequency varies in the range of 490 MHz to 560 MHz, and this variation is a value that cannot be ignored in practice. One of the causes of modal noise that causes such variations in measured values is the influence of reflected light that occurs when laser light is incident on an optical fiber. In other words, when signal light is incident on the input end face perpendicular to the fiber axis, a resonance phenomenon occurs due to repeated reflections between the optical fiber end face and the output end face of the LD, resulting in an undesirable oscillation wavelength other than the original LD oscillation wavelength. Oscillation mode occurs. This oscillation is unstable and causes unstable output at the fiber exit point, that is, variations in measured values.

(Means for Solving the Problems) The present invention has been made to eliminate the above-mentioned conventional drawbacks, and therefore, the present invention provides a method for measuring the transmission characteristics of an optical fiber using an LD light source. The optical fiber is characterized in that the end face of the optical fiber, into which the optical signal is incident from the light source, is cut at an angle of one degree or more from a plane perpendicular to the optical fiber axis, or is formed into a curved surface to allow the optical signal to enter.

(Function) By cutting the optical fiber end face diagonally or forming it into a curved surface as described above, the reflected light from the optical fiber end face can be
The beam is deflected so as not to collide with the output end face of D, thereby preventing the generation of oscillation modes other than the LD's original oscillation wavelength formed by repeated reflections.

(Example) Examples of the present invention will be described below.

Figure 1 shows the same measurement of the optical border as in Figure 2.

The measurement results are shown when the input end face was cut obliquely by 7 degrees from the plane perpendicular to the optical fiber axis, and the measurement was performed under exactly the same conditions as the other conditions. The variation in measured values in this example is 530~
550 MHz, which is about 1/6 of the variation in FIG. 2, which confirms the effect of the method of the present invention. It has been found that in order to substantially reduce the variation in measured values, it is sufficient to incline the input end face of the optical fiber by one degree or more from a plane perpendicular to the fiber axis. In addition, to reduce the influence of modal noise caused by such reflected light, the same effect can be obtained by using a curved surface such as a spherical surface for the hole that cuts the incident end face diagonally. . Furthermore, it is not necessary to perform such special cutting or processing of the input end face on the fiber to be measured itself, and the excitation fiber VC may be cut or processed in advance. The output end may be connected to the fiber under test.

Although the above explanation has been made specifically regarding band measurement of a multimode fiber, the present invention is not limited to this, and the present invention is not limited to this. It is also applicable to various measurements such as failure point measurement and single-dispersion measurement, and measurements of single mode fibers.

(Effects of the Invention) As described above, according to the present invention, when measuring the transmission characteristics of an optical fiber using an LD light source, it is possible to
It is possible to prevent the generation of modal noise due to repeated reflections with the D output end face, thereby improving the measurement accuracy of optical fiber transmission characteristics.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the results of band measurement of a multimode fiber using the method of the present invention, and FIG. 2 is a graph similar to FIG. 1 using the conventional method. Patent applicant Sumitomo Electric Industries, Ltd. (5 others) Circumferential liquid number CMt-br) Circumferential liquid number CMHX)

Claims (2)

[Claims] (1) When measuring the transmission characteristics of an optical fiber using an LD light source, the end face of the optical fiber into which the optical signal is input from the light source is cut at an angle of at least one degree from the plane perpendicular to the optical fiber axis, or is formed into a curved surface. An optical fiber measurement method characterized by inputting an optical signal into an optical fiber. (2) The measurement method according to claim 1, wherein the optical fiber is a fiber to be measured itself or an excitation optical fiber connected to the fiber to be measured.