JP3107096B2 - Optical pulse tester - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pulse tester used for test monitoring of an optical fiber transmission line in the field of optical communication.

[0002]

2. Description of the Related Art The optical pulse tester detects a backscattered light intensity of an optical signal pulse incident on a test optical fiber in an optical fiber in a time series and obtains a backscattered light intensity distribution. It is widely used to measure the loss and break point of an optical fiber. Since this backscattered light is extremely weak, a detection method has been studied for the purpose of improving the measurement performance, and the effectiveness of coherent detection has been shown as compared with conventional direct detection. FIG. 1 shows a configuration example of a coherent optical pulse tester (hereinafter, referred to as a heterodyne optical pulse tester) that has been reported so far.

In FIG. 1, 1 is a narrow band semiconductor laser,
2 is an optical fiber, 3 is an isolator, 4 is a fiber coupler, 5 is an acousto-optic switch, 6 is a test optical fiber, 7
Is a light receiving element, 8 is a local oscillator, 9 is a mixer, 10 is a low frequency transmission filter, 11 is an A / D converter, 12 is a data processing system, 13 is a synchronization circuit, 14 is 7, 9, 10, 11
Is a heterodyne light receiver. The light emitted from the narrow-band semiconductor laser 1 is divided into main signal light and local light by a coupler 4, and the main signal light is pulse-modulated by an acousto-optic switch 5. The main signal light pulse is incident on the test optical fiber, and the backscattered light from this optical fiber is received by the heterodyne light receiver 14. Generally, an acousto-optic switch shifts the frequency of light by about 100 MHz, so that the frequency distribution of the electric signal of the light receiving element 7 after being multiplexed with local light is around the above-mentioned about 100 MHz (intermediate frequency signal: hereinafter referred to as IF signal). concentrate. For this reason, the 100 MHz signal from the local oscillator 8 is combined by the mixer 9 and heterodyne detected to demodulate the signal to the baseband frequency to obtain the backscattered light distribution.

[0004] Among the above-described configurations, the electric circuit for demodulating an IF signal into a baseband frequency signal is particularly complicated and complicated, so that it is difficult to reduce the size of the conventional coherent optical pulse tester. Had the disadvantage that In general, in a heterodyne detection system that demodulates an intermediate frequency (IF) signal into a baseband signal using a local oscillator and a mixer, a signal-to-noise ratio (hereinafter, referred to as an SN ratio) filters local optical shot noise. The bandwidth (hereinafter referred to as the noise bandwidth) is not the bandwidth 1 / 2T (where T is the pulse width) for the baseband signal,
Since the bandwidth is 1 / T for the IF signal, it is 3 dB worse than the ideal electric field amplitude detector. On the other hand, in the optical homodyne detection system, since the frequencies of the signal light and the local light are almost the same, the beat components of both lights appear directly as baseband signals, and the noise bandwidth becomes 1 / 2T. Therefore, an ideal electric field amplitude detector becomes possible, and the SN ratio becomes 3 dB theoretically better than that of the optical heterodyne detection method (see literature: supervised by Shimada; coherent optical communication, Chapter 4 modulation / demodulation theory; electronic information) Communication Society).

[0005]

As described above, the conventional
Heterodyne optical pulse tester is difficult to miniaturize
And the S / N ratio is not good .

An object of the present invention is to effectively solve the above-mentioned problem, and an object of the present invention is to provide an optical pulse tester which solves the complicated structure of an electric system which has been a problem as described above.

[0007]

In order to solve the above-mentioned problems, in the present invention, in addition to an acousto-optic switch used for pulse modulation, one or more frequency shifters (such as an acousto-optic switch) are inserted. The optical frequency of the acousto-optic switch is offset. A means for inserting an acousto-optic switch for pulse modulation and another acousto-optic switch with the opposite frequency shift polarity in series on the main signal light side, or an acousto-optic switch with the same frequency shift for the main signal light and the local light respectively There are means to both can be made substantially equal to the frequency of the main signal light and the local light and that Do.

[0008]

According to the present invention, from the F-heterodyne optical pulse tester, and dynamic range expansion, further electrical circuit for demodulating the IF signal into a baseband signal becomes unnecessary.

[0009]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing a first embodiment of the present invention; The embodiment of claim 1 .
An embodiment of the homodyne optical pulse tester according to claim 1 will be described below with reference to FIG. 2. In FIG. 5, reference numeral 5 (a) denotes an acousto-optic switch which can be modulated to the plus primary side and pulse-driven simultaneously. (B) can be modulated to the minus primary side,
It is an acousto-optic switch that can be pulsed at the same time. 5 (a) and 5 (b) can be used as optical frequency shifters when driven continuously. Reference numeral 15 denotes a homodyne light receiver.

The light emitted from the narrow-band semiconductor laser 1 is coupled to a coupler 4.
The main signal light is modulated by two acousto-optic switches 5 (a) and 5 (b), and is converted into a light pulse at the same time. At this time, since the two acousto-optic switches 5 (a) and 5 (b) are modulating at 120 MHz to the plus primary side and minus primary side, respectively, this main signal light pulse incident on the test optical fiber is used. After the backscattered light and the local light are combined, a signal received by the light receiving element 7 is a baseband signal. With this configuration, an optical pulse tester using optical homodyne detection was realized.

In this configuration, 5 (a) is an optical frequency shifter by driving continuously, and 5 (b) is an acousto-optical switch by pulse driving. Also, pulse drive may be performed for both 5 (a) and 5 (b). Hereinafter, the details of the acousto-optic switch 5 (b) used here will be described.

The acousto-optic switch used here receives a synchronization signal from a synchronization circuit and is driven by a TTL level electric signal of 120 MHz. At this time, the incident signal light is modulated at 120 MHz to the primary side and emitted. Therefore, when the primary port is connected to the main signal light side and the zero-order port is connected to the test optical fiber and the optical receiver, the synchronous circuit 1
The zero-order side and the primary side are connected for the time T (pulse width) during which the acousto-optic switch 5 (b) is driven in response to the synchronization signal from 3, and the main signal light is modulated and pulsed to enter the test optical fiber. Will do. When the synchronization signal is not received, the ports on the 0-th side are connected, and the test optical fiber 2 and the photodetector are connected.

The measurement results of the test of the test optical fiber with the homodyne optical pulse tester of the present invention will be described below. The signal light wavelength is 1.55 μm, the peak power of the incident light pulse on the main signal light side is 0 dBm, and the pulse width T is 1 μse.
c, the light intensity on the local light side is 5 dBm, the bandwidth of the low frequency transmission filter 10 is 1 MHz, and the dynamic range of the present pulse tester at this time is 20 dB, which is 1 dB larger than that of the heterodyne optical pulse tester. It is improved by 2 dB at the minimum light receiving level. In addition, this configuration eliminates the need for a local oscillator and a mixer, which was required in a heterodyne optical pulse tester.
The requirements for the electrical circuit on the receiver have been simplified.

An embodiment according to claim 2 . Figure 3 is serial to claim 2
In the configuration shown in the figure, an acousto-optic switch 5 (a) is inserted on the main signal light side, the main signal light is pulse-modulated, and an acousto-optic switch is provided on the local light side. Is inserted, and the local light is modulated at substantially the same frequency as the main signal light. Therefore, the above-mentioned optical homodyne detection was possible, and an optical pulse tester was realized using the homodyne light receiver 15 as in the first embodiment. Also in the present embodiment, the dynamic range is improved by 1 dB and the minimum light receiving level is improved by 2 dB as compared with the heterodyne optical pulse tester.

In the above embodiment, it is sometimes difficult to combine the main signal light and the local light using one optical frequency shifter. In this case, however, a plurality of light beams for coarse adjustment and fine adjustment are used. You may comprise using a frequency shifter.

[0016]

As described above, according to the present invention, a homodyne optical pulse tester which solves the complicated configuration of the electric system which has been a problem in the conventional heterodyne optical pulse tester has been realized. The improvement of the minimum light receiving level of the present invention, that is, the improvement of the dynamic range of the optical pulse tester is realized, the test monitoring of the optical transmission path can be made longer, and the optical transmission system of the present invention can be used for optical transmission. The reliability of the road is expected to improve, which can contribute to the improvement of service quality. Ma
According to the present invention, the frequencies of the main signal light and the local light are
Because they can be almost the same, the heterodyne detection method
Using a homodyne detection method with a higher SN ratio compared to the formula
A light pulse tester can be realized.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a conventional heterodyne optical pulse tester.

FIG. 2 is a configuration diagram of a homodyne optical pulse tester according to a first embodiment of the present invention.

FIG. 3 is a configuration diagram of a homodyne optical pulse tester according to a second embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Narrow band semiconductor laser 2 Optical fiber 3 Isolator 4 Optical fiber coupler 5 Acousto-optic switch 5 (a) Acousto-optic switch 5 (b) Acousto-optic switch 5 (c) Optical frequency shifter 6 Test optical fiber 7 Light receiving element 8 Local oscillator Reference Signs List 9 mixer 10 low frequency transmission filter 11 A / D converter 12 data processing system 13 synchronization circuit 14 heterodyne light receiver 15 homodyne light receiver

──────────────────────────────────────────────────続 き Continuation of the front page (72) Izumi Mikawa, Nippon Telegraph and Telephone Corporation, 1-6, Uchisaiwaicho, Chiyoda-ku, Tokyo (56) References JP-A-62-217135 (JP, A) JP-A Sho 62-266434 (JP, A) JP-A-64-46627 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01M 11/00-11/02 Practical file (PATOLIS) Patent file (PATOLIS)

Claims (2)

(57) [Claims] 1. A main signal light pulse-modulated by an acousto-optic switch is incident on a test optical fiber using light emitted from a laser as a main signal light and a local light. An optical pulse tester for multiplexing light and obtaining a backscattered light intensity distribution of a test optical fiber from a beat component of both lights, comprising one or more frequency shifters, and
Modulated by the wave number shifter to the opposite sign side from the acousto-optic switch
And the output of the frequency shifter is applied to the acousto-optic switch.
An optical pulse tester characterized by inputting . 2. The light emitted from a laser is divided into a main signal light and a local signal light.
Principal signal used for light and pulse modulated by acousto-optic switch
No. light is incident on the test optical fiber and its main signal light pulse
Of the backscattered light and the local light,
The backscattered light intensity distribution of the test optical fiber is calculated from the components.
Optical pulse tester Comprising one or more frequency shifters, wherein the local light
Almost the same frequency as the acousto-optic switch with the frequency shifter
An optical pulse tester characterized by modulating by: