JPH04271327A - Optical amplifier - Google Patents

Abstract

PURPOSE:To allow the synthesis of signal light and stimulating light and the shut off of the stimulating light after the amplification of the signal light without using a special wavelength filter of high isolation. CONSTITUTION:An LD light source 13 for excitation outputs the stimulating light 14 which is 1st linearly polaized light for excitation. A polarized light synthesizer 15 polarizes and synthesizes the 2nd linearly polarized light of the signal light 12 and the 1st linearly polarized light and outputs the synthesized light. A rare earth-doped polarization maintaining optical fiber 16 is inputted with the output light from the polarized light synthesizer 15. An optical isolator 17 is connected to the output end of the optical fiber 16. A polarizer 18 is optically connected to the output end of the optical isolator 17 and allows the passage of only the signal light.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplifier for directly amplifying signal light, and more particularly to an optical amplifier using a rare earth doped optical fiber.

0002

2. Description of the Related Art Regarding conventional optical amplifiers, an Er ion-doped fiber optical amplifier using Er (erbium) ions as rare earth ions will be explained with reference to FIG.

A laser diode (hereinafter referred to as LD) module 9 having an oscillation center wavelength of 1.55 μm is used as a light source for signal light. An optical signal 2 with an intensity of 0 dBm is connected from the LD module 9 via the optical fiber 1 to the Er ion-doped fiber optical amplifier 101. The Er ion-doped fiber optical amplifier 101 includes a multiplexer 5, a pumping L
It includes a D light source 3, an Er ion-doped optical fiber 6, an optical isolator 7, and an optical filter 8. The center wavelength of the optical output from the excitation LD light source 3 is 1.48μ
m and the optical output is +20 dBm. The output end of the Er ion-doped fiber optical amplifier 101 is an optical star coupler 103.
The optical fibers are branched into six optical fibers 104-109. Normally, when optical output is split into six optical fibers and output, the optical intensity input to each fiber is approximately 1/10th.
(-10dBm). Here, the optical output of the LD module 9 can be amplified by about 10 dB by the rare earth doped fiber optical amplifier 101, so even if the light is split by the optical star coupler 103, the intensity of the light transmitted to each optical fiber 104-109 is sufficiently high. Therefore, the Er-doped fiber optical amplifier 101 has a feature that it can compensate for the branching loss of the optical star coupler 103 and extend the transmission distance.

Signal light 2 with center wavelength 1.55 μm and light intensity 0 dBm and center wavelength 1.48 μm and light intensity +20 dBm
The excitation lights 4 are combined by a multiplexer 5. A wavelength combining film 21 is built into the multiplexer 5 . Wavelength synthesis film 2
1 has the function of reflecting light with a wavelength of 1.48 μm and passing light with a wavelength of 1.55 μm. 1.55 for the wavelength synthesis film 21
A material with a characteristic that the reflectance ratio differs by 20 dB at two wavelengths of μm is used. The light input into the Er iodoped optical fiber 6 is for blocking the long wavelength and short wavelength components of the pumping light having a wavelength of 1.48 μm and the pumped signal light.

[0005] In order to reduce noise, the optical filter 8 is
It is necessary to select a very special wavelength separation film 22 that has a narrow passband width of 0.01 μm or less centered around a wavelength of 1.55 μm and a high isolation of 30 dB or more. Further, the optical isolator 7 is used to prevent oscillation within the Er ion-doped optical fiber due to reflected return light. In order to be unaffected by the polarization of the signal light and return light, it is necessary that the characteristics be polarization-independent.

[0006]

The conventional optical amplifier described above requires a wavelength combining film and a wavelength separating film for combining the pumping light and the signal light and for separating the amplification control signal light and the pumping light. The wavelength separation film removes leakage of excitation light and noise caused by long wavelength and short wavelength components of the excited signal light.
It has the disadvantage that it has to use a very special configuration with a narrow passband width of 0.01 μm or less centered around 55 μm and high isolation of 30 dB or more.

[0007]

[Means for Solving the Problems] An optical amplifier of the present invention includes a laser diode that outputs first linearly polarized light for excitation;
a polarization combiner that combines and outputs the second linearly polarized light of the signal light and the first linearly polarized light; a polarization-preserving rare earth doped optical fiber that inputs the output light from the polarization combiner; and the optical fiber. and a polarizer that is optically connected to the output end of the optical isolator and allows only the signal light to pass through.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be explained with reference to the drawings. FIG. 1 is a block diagram of a first embodiment of the present invention.

FIG. 1 shows a rare earth-doped fiber optical amplifier 1.
02 into optical fibers 104 to 109.
1 with polarization maintaining optical fiber as a light source for 55μm signal light
．． A 55 μm Fabry-Perot type LD module 19 was used, and the optical output was set to 0 dBm. Six optical fibers 104 to 109 are connected to the output end of the rare earth-doped fiber optical amplifier 102 via an optical star coupler 103 to branch optical signals with sufficiently strong intensity.

A wavelength of 1.48μ is emitted from the excitation LD light source 13.
Pumping light of +20 dBm is output. The output of the excitation LD light source 13 is output from a polarization maintaining fiber. The excitation light 14 and signal light 12 from the excitation LD light source 13 are combined by a polarization combiner 15. A polarization synthesis film 31 is included inside the polarization synthesizer 15 . The polarization synthesis film 31 contains Ti.
Using a multilayer film of two types of dielectrics, O2 and SiO2,
Light having a polarization component parallel to the film surface of the polarization synthesis film 31 is reflected, and light having a polarization component perpendicular to the film surface is transmitted. Therefore, the respective input directions are adjusted so that the excitation light 14 becomes linearly polarized light in a direction parallel to the film surface, and the signal light 12 becomes linearly polarized light in a direction perpendicular to the film surface. The combined light is input into a polarization preserving rare earth doped optical fiber 16. An optical isolator 17, a polarizer 18, and an optical filter 33 are connected to the output end of the polarization-preserving rare earth doped optical fiber 16.

The polarization-maintaining rare earth-doped optical fiber 16 is an elliptical clad type polarization-maintaining optical fiber whose core is doped with Er ions. A fiber cord with a length of 200 m and a degree of polarization conservation (polarization extinction ratio) of 35 dB is used. The polarization combiner 15 and the polarization preserving rare earth doped optical fiber 16 are fusion-spliced so that the principal axes of the linearly polarized light coincide, that is, the polarization direction of the signal light 12 coincides with the direction perpendicular thereto.

Further, the optical isolator 17 uses a polarization maintaining optical fiber at the input and output ends, and preserves the linear polarization of input and output. A polarization extinction ratio of 25 dB is used. A polarizer 18 that transmits polarized waves in only one direction is connected to the output end of the optical isolator 17. Polarization-maintaining optical fibers for input and output of each component are connected with their principal axes of polarization aligned, and a signal light with a wavelength of 1.55 μm is transmitted in the direction of the principal axis of polarization of the input fiber of the polarizer 18. Excitation light having a length of 1.48 μm is transmitted in the vertical direction. The polarizer 18 adjusts the internal polarization separation film 32 so that the linearly polarized signal light is transmitted and the linearly polarized excitation light is blocked. The polarization separation film 32 also has the same structure as the polarization synthesis film 31.

[0013] Normally, a polarization separation film made of a dielectric multilayer film has three
Polarization separation degree of about 0 dB (ratio of intensity of transmitted light and blocked light)
can be obtained. In the case of this embodiment, the polarization synthesis film 31
The degree of polarization separation of the polarizing film 32 is 27 dB or more in both cases. At each connection point of the optical components, a normal level polarization maintaining optical fiber was fusion spliced. As a result, the ratio of the intensity of the signal light in the direction of the main polarization axis input to the polarizer 18 and the excitation light was 19 dB, and the excitation light could be blocked by the polarizer 18. An optical filter 33 is connected to the output of the polarizer 18, and further removes the excitation light component and the long wavelength and short wavelength adjustment components of the excited signal light. Therefore, the optical filter 33 can reduce the noise caused by the excitation light by setting the ratio (isolation) of the light transmittance at the wavelength of 1.55 μm at the center and the pass band with a width of 0.01 μm to the other wavelengths to about 15 dB. It can be suppressed. If the isolation is about 15 dB, a commonly available optical filter can be used.

FIG. 2 is a block diagram of a second embodiment of the invention. In FIG. 2, in the first embodiment, an LD module with a polarization-maintaining optical fiber is used as a light source in order to input signal light as linearly polarized light, but in the second embodiment, input signal light of arbitrary polarization is used. is converted into linearly polarized light by a phase compensator 40 attached to the input section of the polarization combiner 25. In this case, as shown in FIG. 3, this rare earth-doped fiber optical amplifier 110 can be connected in series using an optical fiber cable 111 to be applied as an optical repeater. Further, in the second embodiment, the excitation LD light source 23 was directly attached to the polarization combiner 25.

Furthermore, in the embodiment shown in FIGS. 1 and 2, the polarizer was constructed as a separate component from the optical isolator, but in order to construct the rare earth-doped fiber optical amplifier of this embodiment, the polarizer is It is also effective to use an optical isolator that has multiple functions.

[0016]

[Effects of the Invention] As explained above, the optical amplifier of the present invention inputs an input signal light and a pump light into a polarization-maintaining rare earth-doped optical fiber by polarization synthesis, and inputs the input signal light and pump light into the output end of the polarization rare-earth doped optical fiber. By providing a polarizer that allows only the signal light to pass through, it becomes possible to combine the signal light and the pumping light and to block the pumping light after the signal light has been amplified, without using a wavelength filter with special isolation.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a first embodiment of the invention.

FIG. 2 is a block diagram of a first embodiment of the invention.

FIG. 3 is a block diagram for explaining application of this embodiment.

FIG. 4 is a block diagram of an example of a conventional optical amplifier.

[Explanation of symbols]

1 Optical fiber 2, 12 Signal light 3, 13 Pumping laser diode (LD) light source 4, 14 Pumping light 5 Multiplexer 6 Er-doped optical fiber 7, 17 Optical isolator 8 Optical filter 9, 19 Laser diode (LD) module 11 Polarization maintaining optical fiber 15 Polarization combiner 16 Polarization maintaining rare earth doped optical fiber 18
Polarizer 21 Wavelength synthesis film 22 Wavelength separation film 23 Excitation LD light source 25 Polarization combiner 31 Polarization synthesis film 32 Polarization separation film 33 Optical filter 34 Polarization synthesis film 40 Phase compensator

Claims (2)

[Claims] 1. A laser diode that outputs a first linearly polarized light for excitation, a second linearly polarized light of signal light, and a laser diode that outputs a first linearly polarized light for excitation;
a polarization-preserving rare-earth-doped optical fiber that inputs the output light from this polarization combiner, an optical isolator that connects to the output end of this optical fiber, and An optical amplifier comprising: a polarizer optically connected to an output end of an isolator and passing only the signal light. 2. The optical amplifier according to claim 1, wherein the polarization combiner includes a phase compensator that converts any input signal light into the second linearly polarized light.