JPS59125037A - Reflection loss measuring method - Google Patents

Abstract

PURPOSE:To measure a reflection loss by a difference of peak values of both pulses by using an optical pulse generator as a light source, and separating an optical pulse which leaks at the near end of an optical directional coupler, and an optical pulse which is reflected by an optical boundary surface of an optical fiber, etc. and goes and returns. CONSTITUTION:A pulse signal outputted from a pulse generator 5 is converted to an optical pulse having the same pulse width and period as an input by an optical transmitter 7. In an optical directional coupler 2, the optical pulse signal leaks into a point C by a near-end leakage to the point C from a point A, and it is inputted to an optical receiver 6. On the other hand, the optical pulse signal which passes through a point B from the point A passes through an optical fiber 4 for a delay, is reflected by an optical boundary surface 8, passes through the optical fiber 4 again, passes through the point C from the point B, and is inputted to the optical receiver 6. As for said two pulses which are photoelectric converted and amplified, a level ratio of its peak values is read by an oscilloscope 9.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a return loss measuring method for measuring the amount of light reflected at an optical boundary surface such as an end face of an optical fiber.

Conventional configuration and its problems Conventionally, the end face of optical fiber (including the joint of optical connector)
The measurement system shown in FIG. 1 was used to measure the amount of light reflected in the .

The output light (stationary light) from the light source 1 directly enters the optical power meter 3 from point A to point B due to near-end leakage of the optical directional coupler 2 . - The light that passes from point A to point C and the optical directional coupler 2 reaches the end face of the optical fiber 4 to be measured, is reflected there, and passes from point C to point B and the optical directional coupler 2 again. and enters the optical power meter 3. The return loss of the end face of the optical fiber 4 to be measured can be calculated from the difference between these two incident powers.

Now, the light output at point A of the light source is P. dBm, the optical power Pr1 of the reflected light from the end face of the optical fiber at point B is expressed by equation (1).

P, 1-Po (L (+L b+2 ・L,
) + R(dBrr+) ・-・(1) However, L is the optical directional coupler connection loss, Lbll': i is the optical directional coupler connection loss, L8 is the optical fiber loss, Ri
・：With end face reflection attenuation. It's the neck, see Figure 1.
Ll is the near-end leakage amount of the optical directional coupler.

On the other hand, the optical power Pr2 leaking into point B due to near-end leakage of the optical directional coupler 2 is expressed by equation (2).

pr2=p0+I,, (dBm)−・−・(2)
The measurement limit is Pr1 and Pr2, and the connection loss of the current optical directional coupler 2 is about 2 dB, and the near-end leakage is about -
Since it is 30 dB, the measurement limit for return loss H is approximately -2
It becomes 4dB. However, although the fiber loss is expressed in odB, if there is fiber loss, the value will be greater than -24 dB.

In the case of analog transmission using a semiconductor laser, it is necessary to determine the return loss at the end face of the optical fiber to -40 dB or less, and measurement of return loss of -40 dB or less is impossible with conventional methods for the reasons mentioned above. Ta.

Purpose of the Invention The present invention is intended to eliminate the above-mentioned conventional drawbacks, and to reduce the return loss (-4 odB or less) of (I1), which was previously impossible.
1 constant'f: It is possible.

Structure of the Invention In order to achieve the above object, the present invention uses an optical pulse generator as a light source, and combines the optical pulse leaking at the near end of an optical directional coupler with the optical pulse reflected at an optical boundary surface of an optical fiber or the like. This method separates the reciprocating optical pulse and measures the return loss from the difference in the peak values of both pulses.

DESCRIPTION OF EMBODIMENTS FIG. 2 shows the basic configuration of the measurement system of the present invention. Second
In the figure, 5 is an optical pulse generator, 2 is an optical directional coupler, 4 is an optical fiber to be measured, and 6 is an optical receiver.

The optical pulse from the optical pulse generator 5 is transmitted to the optical directional coupler 2
directly leaks into the optical receiver 6 due to proximity leakage. The received waveform P1 in FIG. 3 is the pulse waveform generated by this. On the other hand, the optical fiber 4 to be measured passes through the optical directional coupler 2.
The optical pulse guided by the optical fiber 4 is reflected at the end face of the optical fiber 4 and guided to the optical receiver 6 through the optical directional coupler 2 again. The received waveform PR in FIG. 3 is the pulse waveform generated by this. The received waveform PR is as shown in FIG. It is separated with a delay of time τd from the received waveform Pl. τd is expressed as in equation (3), and the length L
The longer (m) is, the thicker τd can be.

τd=2XLXn/3X10'' (seconds) (3) where n is the refractive index of the optical fiber core.

Here, the peak power of the optical pulse at point A is P. (dBm), the wave height of each of the received waveforms Pl and PR is 1. iR is represented by formulas (4) and (5), respectively.

11ocP1=Po-t-Ll (dBm) ---
(4) i6cx: Ps=Po-(L(+Lb-+-2-
Ls)+R(dBm)・・・・・・・・・・・・・・・
...(5) If we now consider il and iRO ratio, )(=101og(i 1./iH)=P1-Ps(d
Bm)-=-(6) Therefore, the return loss R can be obtained as in equation (7).] R= (Lf+Lb+ 2 L 6) + L 1-1
o log (i8/iH) (dB)...
......(7) As mentioned above, the connection loss of the optical directional coupler 2 is approximately 2 dB,
If the near end leakage amount is -3 odB and the fiber loss is odB, then R=-24-101og (i 1/iH) (dB)-
− axis). Considering the S/N of the optical receiver 6, (8
) Since the second term of the equation can be ensured to be 2 odB or more, the measurement method according to the present invention makes it possible to measure return loss of -40 dB or less.

Next, an embodiment of the present invention will be described with reference to FIG.

A pulse signal having a pulse width eonS and a period of about 480 nS outputted from the pulse generator 5 is converted by the optical transmitter 7 into light/curse having the same pulse width and period as the input. 2 is an optical directional coupler, and the passing loss from point A to point B is about 2 dB, similarly the throughput loss from point B to point C is about 2 dB, and the near-end leakage from point A to point C is Approximately 3o
The optical pulse signal C having a performance of dB leaks from point A to point C by near-end leakage and enters the optical receiver 6. - The optical pulse signal that has passed from point A to point B passes through the delay optical fiber 4, is reflected at the optical interface 8, passes through the optical fiber 4 again, and passes from point B to point C to the optical receiver 6.
to go into. The optical receiver 6 has a frequency bandwidth of 30 MHz and an input equivalent noise current of about 1 P A/%. The two pulses optically/electrically converted and amplified by the optical receiver 6 are read by an oscilloscope 9 for the level ratio of sonopeak values.

Here, a semiconductor laser is used as the light source of the optical transmitter 7 which performs an electrical/optical converter, and the peak power of the output optical pulse is OdBm or more. Therefore, the received peak power of the optical pulse leaking into point C due to near-end leakage of the optical directional coupler 2 at the optical receiver 6 is less than 130 dBm, and considering the performance of the optical receiver described above, , S/ at the output of the optical receiver
N can be secured at 40dB L, I, and above. Therefore (8
) It is clear that the second term in the equation can be maintained at 20 dB or more.

On the other hand, the W9L optical fiber has a length of 6 m or more.

The delay time resulting from this is approximately 60 nS or more from equation (3), and the two received optical pulses can be separated in time.

Effects of the Invention The present invention has the above configuration, and according to the present invention, the amount of reflection at an optical boundary surface such as a fiber end face (this is not limited to the reflection at the fiber end face, but also the reflection due to scattering inside the fiber). The ratio of the incident light to the incident light on the fiber end face can be measured by -40 dB or more by reading the peak value of the pulse on an oscilloscope.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a conventional measurement system for measuring the optical return loss at the end face of an optical fiber, and FIG. 2 is a block diagram showing the basic configuration of the measurement system of the return loss measurement method of the present invention.
Figure 3 is a received waveform diagram of the optical receiver in the same measurement system;
The figure is a block diagram of a measurement system in an embodiment of the present invention. 2... Optical directional coupler, 4... Optical fiber, 5... Optical pulse generator, 6... ... optical receiver, 7 ... optical transmitter, 8 ... optical interface, 9 ... oscilloscope. Name of agent: Patent attorney Toshio Nakao and 1 other person
l F't

Claims (2)

[Claims] (1) The optical pulse generated by the optical pulse generating means, incident on the optical directional coupler, and leaking at the near end, and the light reflected at the optical boundary surface of the optical fiber, etc. on the output side of the optical directional coupler. A method for measuring return attenuation, characterized in that the return attenuation of the optical interface is measured from the difference in the peak values of both optical pulses. (2) An optical transmitter that generates optical pulses with a pulse width of 60 nS or less is used as the optical transmitter constituting the optical pulse generation means, and the optical pulse leaking at the near end of the optical directional coupler and the optical fiber etc. 2. The return loss measuring method according to claim 1, wherein the bandwidth of the optical receiver that receives the optical pulse reflected from the optical boundary surface is 30 MHz or more.