JP2988114B2 - Optical fiber amplifier - 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 fiber amplifier for directly amplifying light.

[0002]

2. Description of the Related Art In recent years, with the advent of optical fiber amplifiers, systems constituting a light distribution network have been actively studied.
Optical fiber amplifiers used as post-amplifiers or booster amplifiers adversely affect the human body when directly illuminated with eyes to amplify input light to an output of 10 to 50 mW. As a countermeasure, various detection methods have been considered to block the excitation light when an abnormality occurs in the transmission line and stop the amplification operation. Hereinafter, a conventional detection method will be described.

FIG. 3 shows an example of a conventional detection system. In the drawing, reference numerals 1 and 2 denote polarization-independent optical isolators for suppressing laser oscillation due to reflected return light, 3 denotes an Er-doped fiber for amplifying incident light, and 4 denotes a signal light for amplifying signal light. Reference numeral 5 denotes an excitation light source, 6 denotes an optical splitter that splits reflected return light, and 8 denotes a light receiving element that detects reflected return light. Reference numeral 9 denotes a control circuit for shutting off the excitation light source 5 according to the level of the light receiving element.

The operation of the optical fiber amplifier configured as described above for detecting a transmission line abnormality will be described. Er
If the optical signal amplified by the dope fiber is disconnected from the optical connector or the optical fiber is disconnected in the transmission line from the optical branching device 6 to the optical fiber amplifier of the next stage,
Fresnel reflected light from the end face of the optical fiber is generated and propagates in the opposite direction to the signal light. This reflected return light is split by the optical splitter 6 and enters the light receiving element 8. In accordance with the level of light incident on the light receiving element 8, the control light source 5
Cut off the output from Here, even when there is no abnormality in the transmission line, the reflected return light from the transmission line is incident on the light receiving element 8, and a threshold value is provided to distinguish the reflected return light from the abnormal transmission line. ing. The control circuit 9 shuts off the excitation light source 5 when receiving light receiving power greater than the threshold.

[0005]

In the method of detecting the Fresnel reflected light from the end face of the optical fiber, there are two methods of detecting the reflected light returned from the end of the optical fiber due to an abnormal transmission line (detachment of the optical connector, disconnection of the optical fiber, etc.) and a normal transmission state. It must be distinguished from the reflected light returning from the transmission path at the time. In particular, in the case of an optical fiber disconnection, etc., the amount of reflected return light differs depending on the broken state of the end face (in many cases, it is smaller than normal mirror polishing), so the light reception level that is the limit of abnormality detection is set to be lower than in the normal mirror surface Must be set lower.

In the above conventional configuration, when the next-stage optical fiber amplifier is of the backward pumping type, pump light not absorbed by the Er-doped optical fiber passes through the optical isolator and returns to the preceding-stage optical amplifier. Will go on. Therefore, even when the transmission path abnormality does not occur, the light receiving element receives the sum of the reflected return light quantity from the transmission path during normal transmission and the leakage light quantity of the excitation light from the next-stage optical fiber amplifier. As a result, the light reception level when the transmission path is normal becomes large, and the difference from the light reception level of the reflected return light generated at the time of the abnormality is reversed depending on the location where the abnormality occurs, and the detection becomes impossible.

In order to prevent this, a method of improving the isolation by using a double isolator for the optical amplifier in the next stage is considered. However, when the pump light becomes large, the absolute power after passing through the isolator is considered. And the difference from the power of the Fresnel reflection return light caused by the transmission path abnormality becomes too close to detect.

SUMMARY OF THE INVENTION The present invention solves such a conventional problem, and prevents leakage of pumping light from an optical fiber amplifier at the next stage when a transmission line abnormality does not occur. Is provided.

[0009]

SUMMARY OF THE INVENTION An optical splitter for detecting reflected return light from an end face of an optical fiber caused by a transmission line abnormality is provided in a part of a transmission line after amplification, and after the optical branching, a signal light wavelength is reduced. It comprises an optical filter that passes light and cuts off the wavelength of the excitation light, a light receiving element that receives the passed light, and a control circuit that controls the output of the excitation light source.

[0010]

With this configuration, the pumping light from the next-stage optical fiber amplifier using the backward pumping system is passed through the wavelength multiplexer to Er.
The light that contributes to the amplification of the signal light with the dope fiber and partially cuts the wavelength of the pump light after being branched by the optical splitter provided in the optical fiber amplifier in the preceding stage following the path in the opposite direction to the signal light. The light is blocked by the filter and does not enter the light receiving element.

If the transmission path is interrupted between the optical amplifier at the preceding stage and the optical amplifier at the next stage, a part of the signal light returns due to Fresnel reflection, and the optical splitter and optical filter provided in the optical amplifier at the preceding stage. Passes through the light receiving element. If this light is at or above a level caused by a transmission line abnormality, it is determined that the light is abnormal, and the excitation light is automatically cut off by the excitation light source control circuit.

[0012]

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a configuration diagram of an optical fiber amplifier according to the present invention. Reference numerals 1 and 2 denote polarization-independent optical isolators for suppressing laser oscillation due to reflected return light, and 3 denotes a wavelength of 1.55.
An Er-doped fiber 4 for amplifying a signal light of μm, a wavelength multiplexer 4 for injecting pump light into the Er-doped fiber 3, and a pump light source 5 for a wavelength of 1.48 μm. Reference numeral 6 denotes an optical splitter for splitting the reflected return light of the amplified signal light. 7
Is an optical filter which cuts off the pumping light having a wavelength of 1.48 μm from an optical fiber amplifier (not shown) at the next stage and transmits signal light having a wavelength of 1.55 μm. Reference numeral 8 denotes a light receiving element that receives the amount of reflected return light from the transmission path. Reference numeral 9 denotes a control circuit for cutting off the output of the excitation light source 5 according to the light receiving level of the light receiving element 8.

FIG. 2 shows the relationship between the amount of reflected light returning from the transmission line at the light receiving section and the position from the branch end of the branching device 6 to the point of occurrence of the transmission line abnormality. Here, the transmission path abnormality is indicated by the fact that the Fresnel reflected light amount due to a perfect mirror surface returns.If a passive component such as an optical switch or an optical splitter is inserted during this time, the reflected return light amount is reduced by the insertion of the passive component. It will be reduced by the round trip loss.

The operation of detecting a transmission line abnormality will be described with reference to FIGS. When the transmission path is normal, the pump light of the next-stage backward pumping optical fiber amplifier is Er-doped fiber,
It contributes about 70% as an amplifying function to the signal light, and a part of the remaining light passes through the optical isolator (in the optical isolator, the transmittance is reduced by about 20 to 30 dB), and returns to the optical fiber amplifier in the preceding stage. To go. This light enters from the output end of the optical fiber amplifier in the preceding stage, and is pumped by the optical branching device 6 into the pump light 1.4.
8 μm light will be split. This light is cut by the optical filter 7 and does not enter the light receiving element 8.
On the other hand, the weak reflected return light amount of the signal light reflected from the connection end of the transmission path or the optical passive component passes through the optical filter 7 and enters the light receiving element 8. When light larger than the weak reflected return light amount is detected, an abnormality in the transmission path is detected.

Next, when an abnormality occurs in the transmission line (such as disconnection of the optical connector or disconnection of the optical fiber), the Fresnel reflected light from the end face of the optical fiber propagates in the opposite direction to the signal light, and is branched by the optical branching device 6. Pass through the optical filter 7 and enter the light receiving element 8. At this time, the incident light level is higher than when the transmission path is normal, and it is determined that the transmission path is abnormal, and
The control circuit 9 cuts off the excitation light output.

In the state where the optical filter 7 is not inserted, as shown in FIG. 2, depending on the position of the transmission path abnormal point, the transmission line becomes smaller than the normal state and the abnormality cannot be detected.

[0018]

As described above, by using the transmission line abnormality detection structure of the present invention, a large difference between the reflected return light amount when the transmission line is abnormal and the reflected return light amount when the transmission line is normal can be obtained. Can be eliminated.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of an optical amplifier of the present invention.

FIG. 2 is a diagram showing the amount of reflected return light when the transmission path is abnormal and the amount of reflected return light when normal using the optical amplifier of the present invention.

FIG. 3 is a diagram showing a configuration of an optical amplifier using a conventional transmission path abnormality detection method.

[Explanation of symbols]

 1, 2 Optical isolator 3 Er-doped fiber 4 Wavelength multiplexer 5 Excitation light source 6 Optical splitter 7 Optical filter 8 Light receiving element 9 Control circuit section

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazuhiko Fujikawa 1006 Kazuma Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) References JP-A-5-291667 (JP, A) JP-A-3- 214936 (JP, A) JP-A-5-292036 (JP, A) JP-A-5-83201 (JP, A) JP-A-4-324335 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H01S 3/10 H01S 3/06-3/07 G02F 1/35 H04B 10/00-10/28

Claims (1)

(57) [Claims] An optical splitter for branching reflected return light from a transmission line to a part of an output transmission line after amplification of the optical fiber, wherein the optical branching device is a system using an optical fiber amplifier connected in multiple stages by backward pumping. An optical filter that blocks light having a wavelength of an excitation light source in an optical path; a light receiving element that receives reflected return light of an amplified signal that has passed through the optical filter; and a reception level of detection light of the light receiving element that indicates a transmission path abnormal level. An optical fiber amplifier comprising: a control circuit for shutting off the output of the pump light at the preceding stage when the output power exceeds the limit.