JPH053356A - Optical fiber amplifier - Google Patents

Abstract

PURPOSE:To provide an optical fiber amplifier having a high signal gain, high saturated output, and low-noise characteristics. CONSTITUTION:Signal light is made incident to an Er-added single mode optical fiber 1 after the signal light is synthesized with excited-light emitted from an exciting light source 4 by means of an optical multiplexer/demultiplexer 21 so that the signal light and excited light can be propagated through an optical fiber in the same direction and passed through the optical isolator 11. The signal light amplified in the optical fiber 1 and the excited light which is not absorbed into the fiber 1 have signal light wavelength band and excited light wavelength band passing characteristics and the center wavelength of the two kinds of light coincide with the wavelength of the signal light in the signal light wavelength band. Therefore, the two kinds of light pass through another optical isolator 12 and made incident to and amplified in an Er-added optical fiber 12 after passing through an optical filter 3 using an interference fringe filter having a band width of 3nm. The output of the fiber 2 is outputted after the output is separated from the excited light by means of an optical multiplexer/demultiplexer 22.

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 optically amplifying signal light in a rare earth-doped optical fiber.

[0002]

2. Description of the Related Art Recently, as an amplifier for optical fiber communication,
Researches on optical amplifiers that directly amplify signal light without photoelectrically converting the signal light are being actively conducted. As an optical amplifier, an optical amplifier in which Er, which is a rare earth element, is added to an optical fiber (Er-doped optical fiber amplifier) has a wavelength of 1.55 μm, which is the lowest loss wavelength region of the optical fiber, and is 30
Features such as high gain of B or higher, almost no polarization dependence of gain, high output of +10 dBm or higher, excellent noise characteristics, and wide amplification bandwidth of 40 nm. Recently, active research and development have been conducted (see, for example, Journal of Applied Physics, Volume 59 (1990), pp. 1175-1192).

Since a high gain is obtained as the pumping wavelength of this Er-doped optical fiber amplifier and a semiconductor laser has already been developed as a pumping light source, the 0.98 μm band and 1.
The 48 μm band is considered to be practical. 0.98 μm
In the band-pumped Er-doped optical fiber amplifier, since the pumping light wavelength and the amplified light wavelength are distant from each other, an ideal three-level system can be formed and a noise limit of 3 dB can be reached. In the 1.48 μm band pumping, since the pumping light wavelength and the amplified light wavelength are close to each other, the noise figure is 4 to 5 dB, which is inferior in low noise property as compared with 0.98 μm pumping, but from the high pumping light power of 70% or more. The conversion efficiency to the amplified light power can be obtained.

From the above characteristics, as an application example of the Er-doped optical fiber amplifier to the optical fiber communication system, the optical booster amplifier positively utilizing the high output characteristic utilizes the high gain and low noise characteristic. There is an optical repeater in which an optical preamplifier arranged in front of an optical receiver has characteristics of both an optical booster amplifier and an optical preamplifier. In the optical booster amplifier,
Output +20 dBm or more (JF Massicottet a
l., "Efficient, High Power, High Gain, Er ³ + Doped Sil
ica Fiber Amplifier, "Electronics Letters, 1990,26,
pp.605-607), and an optical preamplifier reports high-sensitivity reception of 150 photons / bit or less by an intensity modulation-direct detection method (T. Saito et al., "High Receiver Sensi".
tivity at 10 Gb / s Using an Er-Doped Fiber Preampli
fier Pumped by A 0.98 μm Laser-Diode, "1991 Optic
al Fiber Communication Conference, Post-Deadline Pa
perPD-14) is available.

[0005]

When the rare earth-doped optical fiber amplifier is applied to optical communication or the like, it is desirable that the optical amplifier has a large gain, a large saturation output and a low noise. However, the rare earth-doped optical fiber amplifier has a drawback in that the spontaneous emission light itself generated in the amplification process is amplified, which hinders the amplification of the signal light and makes it difficult to obtain high gain and high output.
Further, there is a drawback that the noise characteristic is deteriorated when the output is saturated by the spontaneous emission light. Further, this becomes more remarkable as the signal light power incident on the optical fiber amplifier becomes smaller.

An object of the present invention is to provide an optical fiber amplifier having a large signal gain and saturated output and low noise.

[0007]

An optical fiber amplifier according to the first aspect of the present invention comprises two or more rare earth-doped optical fibers which are optically cascaded, and a wavelength corresponding to the absorption wavelength of the rare earth-doped optical fibers. A pumping light source that emits light, an optical coupling unit that pumps pumping light output from the pumping light source into the rare earth-doped optical fiber, an optical signal input unit that causes the signal light to be amplified to enter the rare earth-doped optical fiber, and is amplified. In an optical fiber amplifier including an optical output unit that extracts the signal light from the rare earth-doped optical fiber, the signal light wavelength and the excitation light wavelength are passed, and spontaneous emission light output from the rare earth-doped optical fiber is removed. The optical filter is inserted between the rare earth-doped optical fibers.

The optical fiber amplifier according to the second invention comprises the first
In the optical fiber amplifying device of the present invention, an optical isolator is inserted between the respective rare earth-doped optical fibers.

An optical fiber amplifier according to a third aspect of the invention comprises two optically connected rare earth-doped optical fibers, an excitation light source that emits light at a wavelength corresponding to the absorption wavelength of the rare earth-doped optical fibers, and a signal. First optical multiplexing / demultiplexing means for multiplexing light and pumping light output from the pumping light source to enter the rare earth-doped optical fiber from the same direction, and the pumping light for signal light emission side of the rare earth-doped optical fiber In the optical fiber amplifying device including a second optical multiplexing / demultiplexing means for making the amplified signal light incident on the optical fiber and separating the amplified signal light from the pumping light, the pumping light that passes the signal light wavelength and is incident from both sides An optical filter that totally reflects the light and makes it enter the rare earth-doped optical fiber again, and that removes spontaneous emission light is inserted between the rare earth-doped optical fibers.

[0010]

[Operation] During the amplification process of the signal light, it occurs until then,
By removing the amplified spontaneous emission light with the optical filter and amplifying it again, the limitation of the saturation output due to the presence of the spontaneous emission light can be relaxed and a high output can be obtained.

The present invention pays attention to this fact, and in the first invention, two or more rare earth-doped optical fibers that are optically cascaded are used, and at least one portion between the rare earth-doped optical fibers is used. In addition, a band-pass type optical filter that passes signal light and removes spontaneous emission light is provided. However, if the optical filter blocks the pumping light as well, the pumping efficiency is reduced. Therefore, the present invention is characterized in that the pass characteristics are provided at two points of the signal light wavelength band and the pumping light wavelength band. In this case, a plurality of rare earth-doped optical fibers are required, but the excitation light may be incident from one place or from a plurality of places. The pumping method can be either forward pumping or backward pumping, and the pumping wavelength can be freely selected to match the absorption wavelength of the rare earth optical fiber.

In the second invention, in the first invention, when the gain in each optical amplification section is increased, an interaction may occur between the amplification sections, which is caused by the Er-doped optical fiber. The feature of the present invention is that it can be avoided by inserting an optical isolator between them.

According to a third aspect of the present invention, two rare earth-doped optical fibers that are optically cascade-connected are used, and a signal light is passed between the rare earth-doped optical fibers and a spontaneous emission light is removed. It is equipped with an optical filter. However, if the optical filter also blocks the pumping light, the pumping efficiency is lowered, so that the signal light wavelength band passes and the pumping light wavelength band has a characteristic of reflecting with high efficiency. There are features.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the optical fiber amplifying device of the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of the first invention. Optical amplification is performed by the following procedure. First, the signal light is an optical fiber type so as to propagate in the same direction as the excitation light emitted from the excitation light source 4 which is an InGaAs / InP Fabry-Perot type semiconductor laser oscillating at a wavelength of 1.48 μm. After being passed through the polarization independent optical isolator 11 with a loss of 1 dB after being multiplexed by the optical multiplexer / demultiplexer 21 of the above, a core diameter of 5 μm produced by the internal chemical vapor deposition method (MCVD method).
m, Er ion concentration 1000ppm, length 50m Er
It is incident on the doped single mode optical fiber 1. The signal light amplified in the Er-doped optical fiber 1 and the pumping light not absorbed by the Er-doped optical fiber 1 have the characteristics of passing the signal light wavelength band and the pumping wavelength band, and the central wavelength thereof in the signal light wavelength band. Corresponds to the wavelength of the signal light and has a bandwidth of 3 nm
After passing through the optical filter 3 using a 50-layer interference film type filter made of io ₂ / SiO ₂ , it again passes through the polarization-independent optical isolator 12 with a loss of 1 dB, and is the same kind as the Er-doped optical fiber 1 and has a length of 50 m. The signal light incident on the Er-doped optical fiber 2 is amplified. The output of the Er-doped optical fiber 2 is demultiplexed with the excitation light by the optical multiplexer / demultiplexer 22 and output.

First, in order to obtain a characteristic similar to that of ordinary amplification, the optical filter 3 is removed, and a signal light having a wavelength of 1.55 μm and an optical power of −40 dBm is made incident on the optical multiplexer / demultiplexer 21, and the excitation light power is set. Was set to 100 mW, a signal gain of 40 dB, a signal output of about 0 dBm and a noise figure of 10 dB were obtained. Therefore, the optical filter 3 is inserted between the Er-doped optical fibers. As a result, a high gain and a high output of a signal gain of 60 dB and a signal output of about 10 dBm were obtained, and a low noise of a noise figure of 5 dB was obtained.

FIG. 2 is a diagram showing the pass characteristic of the optical filter used in this embodiment. It has a characteristic of passing a signal light wavelength band of 1.55 μm and an excitation wavelength band of 1.48 μm, and has a bandwidth of 3 nm in the signal light wavelength band.

FIG. 3 is a block diagram showing an embodiment of the third invention. Optical amplification is performed by the following procedure. First,
The signal light is InGaAs / oscillating at a wavelength of 1.48 μm.
The pumping light emitted from the pumping light source 4 which is an InP Fabry-Perot type semiconductor laser is multiplexed by the optical fiber type optical multiplexer / demultiplexer 21 so as to propagate in the same direction in the optical fiber, and the internal chemical Vapor deposition (MCVD
Method), core diameter 5μm, Er ion concentration 1
The light enters the Er-doped single mode optical fiber 1 having a length of 000 ppm and a length of 50 m. On the other hand, the pumping light emitted from the pumping light source 5, which is an InGaAs / InP Fabry-Perot type semiconductor laser oscillating at a wavelength of 1.48 μm, is an optical fiber type optical coupling / decoupling so as to propagate in the optical fiber in the opposite direction to the signal light. The Er-doped optical fiber 1 is combined by the wave filter 22.
It is incident on the Er-doped optical fiber 2 of the same type as that of 50 m long. The signal light amplified in the Er-doped optical fiber 1 passes through an optical filter 3 using an interference film type optical filter having a center wavelength that coincides with the signal light wavelength in the signal light wavelength band and has a bandwidth of 3 nm, and then Er The signal light incident on the doped optical fiber 2 is amplified. The excitation light that has not been absorbed by the Er-doped optical fiber 1 is reflected by the optical filter 3 that also has a characteristic of reflecting the wavelength of the excitation light, and is incident on the Er-doped optical fiber 1 again. Similarly, the excitation light that is not absorbed by the Er-doped optical fiber 2 is reflected by the optical filter 3 and again Er
It is incident on the doped optical fiber 2. Er-doped optical fiber 2
The output of is separated from the excitation light by the optical multiplexer / demultiplexer 22 and output.

First, in order to obtain characteristics similar to ordinary amplification, the optical filter 3 is removed, and a signal light having a wavelength of 1.55 μm and an optical power of −40 dBm is made incident on the optical multiplexer / demultiplexer 21 and two pumps are used. When the optical power was set to 100 mW, a signal gain of 55 dB, a signal output of about 5 dBm and a noise figure of 11 dB were obtained. Therefore, the optical filter 3 is inserted between the Er-doped optical fibers. As a result, the signal gain 65
A high gain and high output of dB and a signal output of about 15 dBm were obtained, and a low noise of a noise figure of 5 dB was obtained.

FIG. 4 is a diagram showing the structure of the optical filter used in this embodiment. The signal light 101 and the excitation light 102 emitted from the optical fiber 6 are collimated by the lens 31. The signal light 101 is transmitted through the optical filter 41 having the characteristic of transmitting the signal light wavelength and reflecting the excitation light wavelength. The signal light 101 is transmitted by the optical filter 41, but the excitation light 102 is reflected by the lens 31. The light is condensed by and is incident on the optical fiber 6. Signal light 10 transmitted through the optical filter 41
In No. 1, the spontaneous emission light is removed by the optical filter 42 having a bandwidth of 3 nm whose center wavelength coincides with the signal light wavelength.
The light is transmitted through the optical filter 43 having the same characteristics as No. 1, is condensed by the lens 32, and is incident on the optical fiber 7. on the other hand,
The excitation light 103 emitted from the optical fiber 7 is reflected by the lens 3
The light is collimated by 1 and totally reflected by the optical filter 43, condensed by the lens 32, and incident on the optical fiber 7. The optical filters 41, 42, and 43 are respectively made of TiO ₂ /
This is an interference film type filter in which oxide thin films of SiO ₂ are multilayered.

The present invention has various modifications other than the above. As the signal light, the signal light wavelength is not limited to 1.55 μm, and may be 1.53 μm or any other wavelength within the amplification band of the Er-doped optical fiber amplifier.

The wavelength of the pumping light source is not limited to 1.48 μm, but may be any laser that matches the absorption wavelength of the Er-doped optical fiber such as 0.8 μm or 0.98 μm, and any laser may be used. . The optical multiplexing means for the signal light and the pumping light is not limited to the optical fiber type, but a dichroic mirror or the like may be used, and any element or element may be used as long as it has the performance.

The optical filter is not limited to the interference film type,
A diffraction grating or a Fabry-Perot interferometer may be used, or another one may be used. Further, the bandwidth of the signal light pass band of the optical filter is not limited to 3 nm, and may be 4 nm above this or 1 nm below this. The Er concentration and size of the Er-doped optical fiber, the number of cascade-connected fibers, and the like are not limited to those in this embodiment.

[0023]

As described above, according to the present invention, it is possible to obtain a rare earth-doped optical fiber amplifying device with high gain, high output and low noise.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a first invention.

FIG. 2 is a diagram showing a pass characteristic of the optical filter in the embodiment of the first invention.

FIG. 3 is a block diagram showing an embodiment of a third invention.

FIG. 4 is a structural diagram of an optical filter in an embodiment of the third invention.

[Explanation of symbols]

1,2 Er-doped optical fiber 3,41,42,43 Optical filter 4,5 excitation light source 6,7 optical fiber 11,12 Optical Isolator 21,22 Optical multiplexer / demultiplexer 31,32 lens 101 signal light 102,103 Excitation light

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location H01S 3/091

Claims (3)

[Claims] 1. Optically cascaded two or more rare earth-doped optical fibers, a pumping light source emitting light at a wavelength corresponding to an absorption wavelength of the rare earth-doped optical fibers, and pumping light output from the pumping light source. An optical coupling means for making the rare earth-doped optical fiber incident, an optical signal input section for making the amplified signal light incident on the rare earth-doped optical fiber, and an optical output section for taking out the amplified signal light from the rare earth-doped optical fiber. In an optical fiber amplifier including, an optical filter that passes the signal light wavelength and the pumping light wavelength and removes spontaneous emission light output from the rare earth-doped optical fiber is inserted between the rare earth-doped optical fibers. An optical fiber amplifier characterized by the above. 2. The optical fiber amplifier according to claim 1, wherein an optical isolator is inserted between each of the rare earth-doped optical fibers. 3. Optically cascaded two rare-earth-doped optical fibers, a pumping light source that emits light at a wavelength corresponding to the absorption wavelength of the rare-earth-doped optical fibers, signal light, and output from the pumping light source. First optical multiplexing / demultiplexing means for multiplexing pumping light to enter the rare-earth-doped optical fiber from the same direction, and pumping light to be incident from the signal light emitting side of the rare-earth-doped optical fiber and amplified signal In an optical fiber amplifier including a second optical multiplexer / demultiplexer that separates and extracts light from the pumping light, while allowing the signal light wavelength to pass therethrough, the pumping light incident from both sides is totally reflected and the rare earth-doped light is again reflected. An optical fiber amplifying device, characterized in that an optical filter for making a fiber incident and removing spontaneous emission light is inserted between the rare earth-doped optical fibers.