JP2842481B2 - Optical amplifier - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical amplifier configured using a rare earth doped fiber or a rare earth doped optical waveguide doped with a rare earth element.

BACKGROUND ART In an optical fiber communication system currently in practical use, repeaters are inserted at regular intervals in order to compensate for attenuation of an optical signal due to loss of an optical fiber. The repeater converts an optical signal into an electric signal using a photodiode, amplifies the signal using an electronic amplifier, converts the signal into an optical signal using a semiconductor laser or the like, and sends the signal again to an optical fiber transmission line. If this optical signal can be directly amplified with low noise as it is, the optical repeater can be reduced in size and economical.

Therefore, research on optical amplifiers capable of directly amplifying optical signals is being actively pursued, and the optical amplifiers to be studied are roughly classified into (a) optical amplifiers doped with rare earth elements (Er, Nb, Yb, etc.). A combination of fiber and pumping light,
There are (b) a semiconductor laser doped with a rare earth element, and (c) a non-linear effect in an optical fiber.

Among them, the optical amplifier combining the rare-earth-doped fiber and the pumping light of (a) has no polarization dependency,
It has excellent features such as low noise and small coupling loss with a transmission line, and is expected to enable a dramatic increase in transmission repeater distance and distribution of optical signals to a large number in an optical fiber transmission system.

FIG. 1 shows the principle of optical amplification by a rare earth doped fiber. Reference numeral 2 denotes an optical fiber composed of a core 2a and a clad 2b, and the core 2a is doped with erbium (Er). When pumping light is incident on such Er-doped fiber 2, Er atoms are excited to a high energy level. When the signal light enters the Er atoms in the optical fiber 2 pumped to the high energy level, the Er atoms transition to the low energy level. At this time, stimulated emission of light occurs, and the power of the signal light is increased. Gradually increases along the optical fiber, and signal light is amplified.

The Er doping concentration in the core 2a is generally uniform in the longitudinal direction and the radial direction of the Er-doped fiber 2.

According to the principle of optical amplification described above, the pumping light consumes energy when exciting the rare earth atoms in the rare earth doped fiber to a high energy level, so that as the pumping light propagates through the rare earth doped fiber, the power of the pumping light is reduced. Absorption will occur. On the other hand, it is known that pumping light power smaller than a certain threshold intensity does not generate enough excitation of rare earth atoms for optical amplification. Therefore, in an optical amplifier using a rare-earth-doped fiber in which the doping concentration of the rare-earth element in the core is uniform, the power loss of the signal light and the pump light is caused by the doping of the rare-earth element. May be caused. Therefore, the optical amplifier having the conventional configuration as described above cannot be said to be suitable for increasing the amplification efficiency (the amplification degree of the signal light with respect to the constant input pump light).

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an optical amplifier suitable for overcoming the above-mentioned problems of the prior art and increasing amplification efficiency.

DISCLOSURE OF THE INVENTION According to one aspect of the present invention, in an optical amplifier configured to propagate signal light and pumping light to a rare earth-doped fiber doped with a rare earth element to amplify the signal light,
There is provided an optical amplifier in which the diameter of a portion of the rare-earth-doped fiber doped with a rare-earth element is gradually reduced in the propagation direction of the pumping light. The propagation direction of the pumping light may be either the same direction as the signal light or the opposite direction.

As means for gradually reducing the diameter of the portion doped with the rare earth element in the propagation direction of the pumping light, the diameter of the portion doped with the rare earth element is continuously increased by heating and stretching the rare earth doped fiber. Let it change. As another means, a plurality of rare-earth-doped fibers having different diameters at portions where the rare-earth element is doped may be connected in series.

According to another aspect of the present invention, in an optical amplifier configured to propagate signal light and pumping light to a rare-earth-doped optical waveguide doped with a rare-earth element to amplify the signal light, the rare-earth element in the rare-earth-doped optical waveguide is An optical amplifier is provided in which the width of the doped portion is gradually reduced in the propagation direction of the pumping light.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram showing the principle of optical amplification by a rare earth doped fiber, FIGS. 2A and 2B are schematic diagrams illustrating the principle of the present invention, and FIG. 3 is each point in FIG. 2A. FIG. 4 is a configuration diagram of an optical amplifier according to an embodiment of the present invention, FIG. 5A and FIG. 5B are cross-sectional views of an upstream fiber and a downstream fiber, and FIG. FIG. 7A and FIG. 7B are explanatory diagrams of a method of manufacturing a stretched fiber, FIG. 8 is a structural diagram of an optical amplifier according to still another embodiment of the present invention, FIG. 9 is an enlarged view of the optical waveguide shown in FIG. 8, and FIG. 10 is a configuration diagram of an optical amplifier according to still another embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION First, the principle of the present invention will be described with reference to FIGS. 2A and 2B. FIG. 2A shows a case where the signal light and the pumping light propagate in the same direction in the rare-earth doped fiber 2, and FIG. 2B shows a case where the signal light and the pumping light propagate in the rare earth-doped fiber 2 in the opposite directions.

In the optical amplifier of the present invention, the signal light and the pumping light are propagated through the rare-earth doped fiber 2 to amplify the signal light, and the diameter of the portion of the rare-earth doped fiber 2 where the rare-earth element is doped is adjusted. , As shown by the broken line, gradually reduced in the propagation direction of the pumping light.

Whether the signal light and the pumping light propagate through the rare-earth doped fiber 2 in the same direction, or whether the signal light and the pumping light propagate through the rare earth-doped fiber 2 in the opposite direction are determined by the optical amplifier of the present invention. The selection can be made according to the configuration of the applied optical communication system or the like.

In FIG. 2A, point A indicates a position on the upstream side in the propagation direction of the signal light and pumping light of the rare earth doped fiber 2, point C indicates a position on the downstream side, and point B indicates a position between points A and C. (A), (B) and (C) of FIG. 3 are A, B, and
It is a graph showing the intensity distribution of the pumping light at point C, where the vertical axis represents the electric field amplitude of the pumping light and the horizontal axis represents the position of the rare-earth doped fiber 2 in the radial direction.

The pumping light has a so-called Gaussian distribution in which the electric field amplitude at the center of the fiber increases, and the amplitude gradually decreases along the propagation direction due to the pumping light exciting the rare earth atoms. The level indicated by _Pth in FIG. 3 is a threshold level at which light amplification is performed at a level higher than the threshold and at which no light amplification is performed. R _a , R _b , and R _c indicate the radii of the portions giving the electric field amplitude larger than the threshold value P _{th at the} points A, B, and C, respectively, and the relation between them is as follows: R _c <R _b <R _a It is. For example, regarding the point B, even if the rare earth element is doped, it does not contribute to optical amplification at any part outside the part of the radius _Rb , but the pumping light is attenuated by the presence of the rare earth element, Efficient optical amplification becomes difficult. In the configuration of the present invention, the diameter of the portion of the rare-earth doped fiber 2 where the rare-earth element is doped is gradually reduced in the direction of propagation of the pumping light. There is provided an optical amplifier suitable for increasing the amplification effect by eliminating or reducing a harmful rare earth element doped region which does not contribute to amplification and attenuates pumping light.

In the case of so-called backward pumping in which the signal light and pumping light shown in FIG. 2B are propagated in opposite directions, the operation is similar to that of the forward pumping shown in FIG. 2A.

 Next, specific examples of the present invention will be described.

FIG. 4 is a block diagram of an optical fiber amplifier showing an embodiment of the present invention. A plurality (two in this embodiment) of rare earth-doped fibers 21 and 22 having different diameters of a portion doped with a rare earth element are connected in series. The diameter of the portion of the rare-earth-doped fiber where the rare-earth element is doped is gradually reduced in the propagation direction of the pumping light. The connection of the rare earth doped fibers 21 and 22 is made by, for example, fusion (splicing). At both ends of the rare earth doped fibers 21 and 22 connected in series,
Similarly, the input side optical fiber 4 for transmitting the signal light to be amplified and the output side optical fiber 6 for transmitting the amplified light are connected, for example, by fusion.

A fiber fusion type optical coupler 8 is formed in the middle part of the input side optical fiber 4 by fusing another optical fiber to the side and extending the fusion portion. In the fiber fusion type optical coupler 8, the portion corresponding to the input side optical fiber 4 is the first input port 8a and the first input port 8a.
A portion corresponding to the other optical fiber forms an output port 8c, and forms a second input port 8b and a second output port 8d. A semiconductor laser 10 serving as a pumping light source is connected to the second input port 8b.

In the case where the doped rare earth element is erbium (Er), when amplifying the signal light having a wavelength of 1.55 μm, the wavelength of the pumping light is 0.80 μm, 0.98 μm.
A μm band, a 1.48 μm band, or the like is selected. Almost 100% of the signal light input to the first input port 8a is guided to the first output port 8c so that the pump light and the signal light selected in this way are incident on the rare-earth doped fiber with high efficiency.
Almost 100% of the excitation input to input port 8b is also the first
The structural parameters of the fiber-fused optical coupler 8 are set so as to be guided to the output port 8c.

The rare-earth-doped fiber 21 located on the upstream side in the propagation direction of the signal light and the pumping light is referred to as an upstream fiber, and the rare-earth-doped fiber 22 located on the downstream side is referred to as a downstream fiber. And 5B
Shown in the figure. The upstream fiber 21 includes a cladding 21a and a core 21b having a higher refractive index than the cladding 21a, and the core 21b is doped with Er at a uniform concentration. The downstream fiber 22 includes a clad 22a, a first core 22b, and a second core 22c, and the refractive index distribution in the first core 22b and the second core 22c is the same as the refractive index distribution of the core 21b of the upstream fiber, Clad 22a
Is the same as the refractive index of the cladding 21a of the upstream fiber.

In the first core 22b and the second core 22c of the downstream fiber, only the second core 22c corresponding to the central portion is doped with Er at a uniform concentration. In order to dope a rare earth element only in a specific portion of the core like the downstream fiber 22, for example, when manufacturing a preform by the MCVD method, a core glass in which the inner wall of the quartz reaction tube is not doped with the rare earth element is used. May be formed, and a core glass doped with a rare earth element may be formed thereon.

In this embodiment, the upstream fiber 21 and the downstream fiber
Although two rare earth-doped fibers consisting of 22 are used, a plurality of rare earth-doped fibers having different diameters of the portion doped with the rare earth element are manufactured according to the method of manufacturing the downstream fiber 22, and the rare earth doped fiber from this is manufactured. The fibers may be connected in series such that the diameter decreases in the direction of propagation of the pumping light. Further, in this embodiment, the concentration of the doped Er is made uniform in the radial direction, but the concentration distribution of the doped rare earth element becomes higher in the center as in the intensity distribution of the propagating light. As an alternative, efficient optical amplification may be achieved.

In the optical fiber amplifier shown in FIG. 4, when the input signal light and the pumping light from the semiconductor laser 10 are combined in the fiber fusion type optical coupler 8 and incident on the upstream fiber 21, there is not enough absorption yet. The signal is amplified by the pumping light having intensity. When the pumping light and the amplified signal light having a relatively small intensity distribution due to the optical amplification enter the downstream fiber 22, the diameter of the Er-doped second core 22c of the downstream fiber becomes upstream. Since the diameter is smaller than the diameter of the core 21b of the side fiber, efficient optical amplification is performed without causing unwanted absorption of pumping light. Applying this principle to an optical waveguide, the same optical amplification can be performed even when multiple optical waveguide substrates having different widths of Er-doped optical waveguides are connected in multiple stages.

FIG. 6 is a diagram showing the configuration of an optical fiber amplifier according to another embodiment of the present invention, in which the same reference numerals are given to the same portions as those in the above-described embodiment. In this example,
By using the rare earth-doped fiber 23 in which the diameter of the portion doped with the rare earth element is continuously changed instead of the upstream fiber 21 and the downstream fiber 22, the rare earth element in the rare earth doped fiber is reduced. The diameter of the doped portion is gradually reduced in the direction of propagation of the excitation light. In order to continuously reduce the diameter of the portion doped with Er as described above, for example, the following is performed. First, as shown in FIG. 7A, the rare-earth-doped fiber 2 is heated so that the heating temperature at the approximate center of the rare-earth-doped fiber 2 of a predetermined length becomes high.
Is heated by the burner 12 and stretched in the direction of the arrow in the figure to form a rare-earth doped fiber 2 'having a diameter at the center smaller than that at the end as shown in FIG. 7B. Then, by cutting the rare earth doped fiber 2 ′ at the central portion, a rare earth doped fiber 23 (drawn fiber) is obtained in which the diameter of the portion doped with Er is continuously reduced. The broken lines in FIGS. 7A and 7B show portions of the drawn fiber or the like where Er is doped.

According to this embodiment, since the diameter of the portion of the rare-earth-doped fiber where the rare-earth element is doped changes continuously, compared to the previous embodiment in which the above-mentioned diameter changes stepwise, more amplification is achieved. An optical fiber amplifier with high efficiency can be provided.

In the embodiments described above, the signal light and the pumping light propagate in the same direction through the rare-earth-doped fiber. However, the signal light and the pumping light may propagate in the opposite direction through the rare-earth-doped fiber. .

Next, another embodiment of the present invention using a rare earth doped optical waveguide doped with a rare earth element such as Er will be described with reference to FIG. The signal light input from the input side optical fiber 4 and the pumping light emitted from the semiconductor laser (LD) 10 for pumping light are converted into a fiber fusion type optical coupler 8.
Are multiplexed. The thus combined signal light and pumping light are coupled by a pair of lenses 25, 26 to an optical waveguide 28 formed on a waveguide substrate 27. The ninth optical waveguide 28
As shown in the drawing, the Er-doped portion 29 doped with Er is formed so that the width thereof is gradually reduced along the propagation direction of the pumping light. The Er-doped portion 29 can be formed in the optical waveguide 28 while controlling the width thereof, for example, by a thermal diffusion method. In the present embodiment, the pumping light incident on the waveguide 28 loses energy when exciting Er in the waveguide 28 to a high energy level, so that the pumping light power is attenuated as it propagates through the waveguide 28. As in this embodiment, if the Er doping range in the waveguide 28 is formed so as to be continuously narrowed in accordance with the attenuation of the pumping light power,
Pump light below the threshold can be prevented from being absorbed by Er. The signal light amplified by the Er-doped optical waveguide 28 is coupled to the output side optical fiber 6 by a pair of lenses 30 and 31.

Referring to FIG. 10, there is shown still another embodiment of the present invention. This embodiment uses a waveguide type multiplexer 35,
The optical amplifier has a structure that can be integrated. That is, an optical waveguide 33 on which signal light is incident and an optical waveguide 34 on which pumping light from the pumping LD 10 is incident are formed on the waveguide substrate 32, and the signal light propagating through these optical waveguides 33, 34 is formed. And the pumping light are multiplexed by the multiplexer 35,
The light enters the Er-doped optical waveguide formed on the substrate 27.
The waveguide substrates 27 and 32 are bonded at their waveguide portions with an optical adhesive or the like.

The embodiment shown in FIGS. 8 and 10 is an embodiment of the forward pumping in which the incident directions of the pumping light and the signal light are the same, but the backward pumping in which the pumping light and the signal light are incident from different directions. In this case, the same effect can be obtained.

INDUSTRIAL APPLICABILITY As described above, according to the present invention, the diameter of the portion of the rare-earth-doped fiber where the rare-earth element is doped is gradually reduced in the propagation direction of the pumping light. It is possible to provide an optical amplifier suitable for increasing the optical amplifier. This is the same in the case of an optical waveguide doped with a rare earth element.

When the amplification effect can be increased, a relatively low-power semiconductor laser can be used as a pumping light source. Further, when the amplification efficiency can be increased, the length of the rare-earth-doped fiber used can be shortened when the output of the semiconductor laser used as the pumping light source is equivalent, and a compact optical amplifier can be provided. Become.

Continued on the front page (51) Int.Cl. ⁶ Identification symbol FI H01S 3/17 G02B 6/12

Claims (9)

(57) [Claims] An optical amplifier in which signal light and pumping light are propagated through a rare-earth-doped fiber doped with a rare-earth element to amplify the signal light, wherein the portion of the rare-earth-doped fiber doped with the rare-earth element is An optical amplifier having a diameter gradually reduced in a propagation direction of the pumping light. 2. The optical amplifier according to claim 1, wherein the signal light and the pumping light propagate in the rare-earth doped fiber in the same direction. 3. The optical amplifier according to claim 1, wherein the signal light and the pumping light propagate in the rare-earth doped fiber in opposite directions. 4. The optical amplifier according to claim 1, wherein a plurality of rare-earth-doped fibers having different diameters at portions where the rare-earth element is doped are connected in series. 5. The light according to claim 1, wherein the rare-earth-doped fiber is heated and stretched so that the diameter of the rare-earth-doped portion of the rare-earth-doped fiber is continuously changed. amplifier. 6. An optical amplifier in which signal light and pumping light are propagated through a rare earth-doped optical waveguide doped with a rare earth element to amplify the signal, wherein the rare earth element of the rare earth doped optical waveguide is doped. An optical amplifier characterized in that the width of the portion is gradually narrowed in the propagation direction of the pumping light. 7. The optical amplifier according to claim 6, wherein the signal light and the pumping light propagate in the rare-earth-doped optical waveguide in the same direction. 8. The optical amplifier according to claim 6, wherein the signal light and the pumping light propagate in the rare earth doped optical waveguide in opposite directions. 9. The optical amplifier according to claim 6, wherein a plurality of rare-earth-doped optical waveguides having different widths in a portion doped with a rare-earth element are connected in series.