JPS5917771B2 - Optical fiber frequency characteristic measurement method - Google Patents

Description

[Detailed description of the invention] It is about law.

Optical fibers with low transmission loss have been developed, and optical fibers are being put into practical use as high-speed, large-capacity transmission lines.

When using this optical fiber as a communication transmission line, it is necessary that the optical transmission loss is small, and it is also desired that the frequency characteristics are good. In other words, if the frequency band is wide, even if an optical pulse with a steep rise is input into an optical fiber and transmitted, the spread of the pulse width will be small.
Optical 9 Pulse signals can be transmitted without error. Therefore, when using an optical fiber as a communication transmission line, it is necessary to understand the transmission loss and frequency characteristics of the optical fiber when designing a transmission system. For this reason, various methods for measuring the transmission characteristics of optical fibers have been proposed, but many of them require complicated and expensive equipment, and it is not easy to perform highly accurate measurements. An object of the present invention is to provide a method that allows highly accurate measurement with a relatively simple configuration.

Examples will be described in detail below. 1 and 2 are explanatory diagrams of an embodiment of the present invention, in which 1 is an oscillator that can sweep the frequency, 2 is a laser oscillator, and 3 is an output light of the laser oscillator 2 connected to one end of an optical fiber 4 to be measured. 5 is a light receiving part such as a lens for entering light emitted from the other end of the optical fiber 4 to be measured; 6 is a light receiving element such as a photodiode; T is a light receiving element 6VC. 8 is a detection processing circuit that amplifies the output of the light receiving element 6 and performs synchronous detection at the oscillation frequency a of the oscillator 1; 9 is an optical attenuator such as a filter; 10 is a detector for the laser oscillator 2; This is the output light. First, as shown in FIG. 1, the output light 10 of the laser oscillator 2, which has been amplitude-modulated by the output of the oscillator 1, is inputted from the negative end of the long optical fiber 4 to be measured via the excitation section 3, and the other end is The emitted light is made to enter the light receiving element 6 via the light receiving section 5, and its output is subjected to 8K detection processing circuits.

Since the detection processing circuit 8 synchronously detects the output of the light receiving element 6 with the oscillation frequency of the oscillator 1, the frequency band of the optical fiber 4 to be measured can be determined from the detection output. Next, as shown in FIG. 2, measurements are performed on a short optical fiber 4'' obtained by cutting the long optical fiber 4 to be measured along, for example, line A-A''.

In this case, a light attenuator 9 which provides a loss approximately equal to the transmission loss of the long optical fiber 4 to be measured is inserted in front of the light receiving element 6, and the measurement is carried out in the same manner as shown in FIG. In this case, a short optical fiber to be measured 4'' whose length is several hundred to several thousandths of the length of the long optical fiber to be measured 4''
However, by providing the optical attenuator 9, the amount of light incident on the light receiving element 6 is reduced, which is the same as in the case of a long optical fiber 4 to be measured. Therefore, there is no need to change the DC bias, the amplification degree of the detection processing circuit 8, the output light of the laser oscillator 2, etc. Also, by changing the output of the oscillator 1, the detection processing circuit 8
There is no need to keep the output the same. That is, long and short optical fibers to be measured can be measured under the same conditions. By comparing the measurement results for the long and short optical fibers mentioned above, it is possible to measure the transmission characteristics corresponding to the length of the optical fiber.

Also, by calibrating the attenuation amount of the optical attenuator 9 in advance,
This makes it possible to simultaneously measure the frequency characteristics and transmission loss characteristics of the optical fiber. As explained above, the present invention allows the output light 10 of the laser oscillator 2 that is amplitude-modulated by the output of the oscillator 1 to be incident on long and short optical fibers to be measured, and the output light that has propagated through the optical fiber is Since the detection output is detected by the light receiving element 6 and synchronized with the oscillation frequency of the oscillator 1, the bandwidth of the optical fiber to be measured can be easily measured by making the oscillator 1 capable of frequency sweeping. In addition, the frequency characteristics can be measured based on the difference between the measurement results of long and short optical fibers. Furthermore, by inserting an optical attenuator when measuring a short optical fiber to be measured, the characteristics of the light receiving element 6 and the detection processing circuit 8 can be measured under the same conditions as when measuring a long optical fiber to be measured. , errors due to nonlinear characteristics of the light receiving element 6 are eliminated, errors due to changes in the characteristics of the detection processing circuit 8 are also eliminated, and furthermore, errors due to changes in the characteristics of the laser oscillator 2 and the characteristics of the oscillator 1 are eliminated. ,
There are advantages such as Note that the output of the detection processing circuit 8 can be added to a recording device or a computer to automatically calculate recording or transmission characteristics.

[Brief explanation of drawings]

FIGS. 1 and 2 are explanatory diagrams of embodiments of the present invention. 1 is an oscillator, 2 is a laser oscillator, 3 is an excitation unit, 4 is a long optical fiber to be measured, 4″ is a short optical fiber to be measured,
5 is a light receiving part, 6 is a light receiving element, 7 is a DC bias power supply, 8
9 is a detection processing circuit, 9 is an optical attenuator, and 10 is an output light.

Claims (1)

[Claims] 1 Add the output of a frequency-sweepable oscillator to a laser oscillator, amplitude-modulate the output light of the laser oscillator, input the output light from one end of the optical fiber to be measured, and detect the output light from the other end with a light receiving element. In the method of measuring the frequency characteristics of an optical fiber by synchronously detecting the detection output with the sweep frequency of the oscillator, the detection output of the long optical fiber to be measured is measured with respect to the sweep frequency, and then the long optical fiber is Cut the short optical fiber to a length sufficiently short compared to the total length, adjust the detection output of the cut short optical fiber to be approximately equal to the detection output of the long optical fiber using an optical attenuator, and detect the short optical fiber again with respect to the sweep frequency. A method for measuring frequency characteristics of an optical fiber, characterized in that after measuring the output, a difference between a detection output of a long optical fiber and a detection output of a short optical fiber with respect to the sweep frequency is determined.