JP2008182072A - Fiber amplifier - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fiber amplifier efficiently taking out an energy excited by the fiber. <P>SOLUTION: The fiber amplifier comprises the fiber with a rare earth element added to its core, a first excitation light source for generating a first excitation light entered into one end part of the fiber, and a second exciation light source for generating a second excitation light entered into the other end part of the fiber. The absorption coefficient of the rare earth element in a wavelength of the second exciation light is larger than that in a wavelength of the first excitaion light, and a signal light entering into the one end part is amplified by the rare earth element excited by the first and the second excitation light, and emitted from the other end part. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a fiber amplifier.

In order to generate or amplify infrared light having a wavelength in the vicinity of 2 μm, a fiber added with Tm ³⁺ (thulium ion) which is a rare earth element is used. Since Tm ³⁺ has an absorption characteristic in the vicinity of 800 nm where 793 nm is a maximum, a semiconductor laser that emits light of this wavelength can be used as an excitation light source. Pumped by this excitation light source, for example, a laser beam having a wavelength of about 2 μm is amplified by a transition between energy levels from ³ H ₄ to ³ H ₆ .

There is a technology disclosure example of an optical amplifier that amplifies light in the 1.47 μm band using an optical fiber in which Tm and Nd (neodymium) are added to the core as a gain medium (Patent Document 1). In this example of technical disclosure, spontaneous emission (ASE: Amplified Spontaneous Emission) light having a wavelength of 800 nm generated during light amplification by Tm is absorbed by Nd, and light in the 1470 nm band is efficiently amplified.
JP 2002-246673 A

  However, when pumping from both ends of the fiber to perform high-power amplification, the input light of the signal light is small at the incident end, and the excited energy cannot be extracted effectively as amplified light, and the surplus energy is regarded as a noise component. ASE light or parasitic oscillation becomes lower and energy efficiency is lowered. The present invention provides a fiber amplifier that efficiently extracts energy excited in a fiber.

  According to one aspect of the present invention, a fiber in which a rare earth element is added to a core, a first excitation light source that generates a first excitation light incident on one end of the fiber, and the other end of the fiber A second excitation light source that generates second excitation light incident on the end portion, and the rare earth element has an absorption coefficient that is greater than that at the wavelength of the first excitation light at the wavelength of the second excitation light. The signal light incident on the one end is amplified by the rare earth element excited by the first and second excitation light and emitted from the other end. An amplifier is provided.

  The present invention provides a fiber amplifier that efficiently extracts energy excited in a fiber.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a configuration diagram of a fiber amplifier according to an embodiment of the present invention. The fiber 20 including Tm ³⁺ added to the core and containing silica has an incident end 22 of the signal light 30 and an output end 24 of the amplified light 32. The signal light 30 that is infrared light in the vicinity of a wavelength of 2 μm passes through the dichroic prism 40 and enters the incident end 22.

  The first excitation light 34 (wavelength λ1) from the first excitation light source 50 is bent by 90 degrees by the dichroic prism 40 and enters the fiber 20 from the incident end 22. On the other hand, the second excitation light 36 (wavelength λ2) from the second excitation light source 52 is bent by 90 degrees by the dichroic prism 42 and enters the fiber 20 from the emission end 24. As described above, the dichroic prisms 40 and 42 have wavelength selectivity that allows the signal light 30 and the amplified light 32 to pass therethrough and reflects the excitation lights 34 and 36.

As the excitation light sources 50 and 52, for example, a semiconductor laser having a wavelength near 800 nm can be used. Here, the energy level of Tm ³⁺ will be described. FIG. 2 is an example of an energy level diagram of Tm ³⁺ . ^{Tm 3+} it pumped by the pumping light source 50 and 52 having a wavelength of 800nm near ^the laser beam to induced emission having a wavelength of 2μm near the transitions between energy levels of from 3 _{H 4} to ³ H _6.

For this purpose, the signal light 30 is amplified and becomes amplified light 32 having the same wavelength as that of the signal light 30 and is extracted from the output end 24 of the fiber 20 to the outside through the dichroic prism 42. Incidentally, the energy level diagram is not limited thereto, it may induce emit laser light of 2μm near the transition from ³ F ₄ to ³ H _6.

FIG. 3 is a graph showing the gain or loss distribution of the fiber 20 in the present embodiment. The vertical axis represents gain or loss, the horizontal axis represents the position in the fiber length direction, P1 represents the entrance end 22, and P2 represents the exit end 24. The forward pumping gain represented by the dot line is generated when Tm ³⁺ is excited by the pumping light 34 incident from the incident end 22.

The backward pumping gain represented by the broken line is generated by exciting Tm ³⁺ by the pumping light 36 incident from the emission end 24. The sum of the forward excitation gain and the backward excitation gain is the double-side excitation gain represented by the solid line. The gain decreases from the incident end 22 toward the intermediate portion of the fiber 20, increases from the intermediate portion of the fiber 20 toward the output end 24, and becomes maximum at the output end 24.

  In the present embodiment, the absorption coefficient of the rare earth element is made larger at the wavelength λ2 of the backward pumping excitation light 52 than at the wavelength λ1 of the forward pumping excitation light 50 (λ1 ≠ λ2). That is, two-wavelength hybrid excitation in which forward excitation and backward excitation are performed at different wavelengths.

FIG. 4 is a graph showing the absorption spectrum of a Tm ³⁺ doped silica fiber at a wavelength near 800 nm, where the vertical axis represents the cross-sectional area (10 ⁻²⁵ m ² ) and the horizontal axis represents the wavelength (nm). This absorption causes a transition from ³ H ₆ to ³ H ₄ , ³ F ₄ , ³ F ₂ , ^3, etc., and the absorption cross section becomes a maximum of about 10 × 10 ⁻²⁵ m ^{2 at a} wavelength of 793 nm. That is, the absorption coefficient is maximized and the excited energy is also maximized. Within the excitation wavelength range of FIG. 4, λ1 and λ2 are set.

Next, the absorption loss represented by the alternate long and short dash line in FIG. 3 will be described. Tm ³⁺ can emit light in the vicinity of a wavelength of 2 μm, but there is also absorption. FIG. 5 is a graph showing light emission and absorption by Tm ³⁺ . The vertical axis represents the cross-sectional area (10 ⁻²⁵ m ² ), and the horizontal axis represents the wavelength (nm). At wavelengths between 1900 and 1982 nm, the emission and absorption cross sections decrease with increasing wavelength. For example, at 1920 nm, the emission cross-sectional area is about 4.8 × 10 ⁻²⁵ m ² , whereas the absorption also occurs in the vicinity of 2 μm so that the absorption is about 0.7 × 10 ⁻²⁵ m ² .

That is, even at the wavelength of the signal light 30, there is an absorption loss corresponding to this absorption cross-sectional area, which is distributed substantially flat over the length direction of the fiber 20. For this reason, the net gain obtained by subtracting the absorption loss from the gain due to Tm ³⁺ contributes to the amplification. In the present embodiment, a region where the net gain is positive is set wide. In particular, it is more preferable that the net gain is positive over the entire area of the fiber 20 because the absorption of the signal light 30 is reduced.

  The fiber amplifier of the present embodiment that is the two-wavelength hybrid pumping shown in FIG. 3 has a gain distribution accompanying an increase in the signal light 30. For this reason, it is possible to obtain an appropriate excitation intensity for the signal light 30 incident on the incident end 22 side, and to suppress a decrease in energy efficiency due to ASE light or parasitic oscillation.

  Further, if the absorption coefficient is too small, excitation becomes insufficient and the signal light 30 is absorbed and the amplification factor is lowered. However, in this embodiment, since the range that is a plus of the net gain is wide, the reduction of the amplification factor can be suppressed. If the net gain is set to be positive in the entire area of the fiber 20, a decrease in the amplification factor can be further suppressed, and the length of the fiber 20 can be shortened.

  Further, since the absorption coefficient at the wavelength λ2 of the backward pumping is larger than that at the wavelength λ1 of the forward pumping, the excitation of the fiber 20 is made stronger at the emission end 24 side than the incident end 22 side, and the excited energy is efficiently amplified. As high fiber output.

  Next, two-wavelength hybrid excitation combining 808 nm forward excitation and 793 nm backward excitation will be described. FIG. 6 is a graph showing the dependence of fiber output on pumping input. The vertical axis represents fiber output (W), and the horizontal axis represents pumping output (W). In FIG. 6, the case of the two-wavelength hybrid excitation according to the present embodiment is indicated by ●, the forward excitation at 793 nm is indicated by ◆, and the backward excitation at 793 nm is indicated by ■. Further, double-sided excitation at 793 nm is represented by ▲.

  Here, the vicinity of 793 nm where the absorption spectrum is maximized is set as the wavelength λ2 of the excitation light 36 for backward excitation, and the cross-sectional area of absorption is set to 70% or less of the maximum value as the wavelength λ1 of the excitation light 34 for forward excitation. If the cross-sectional area of absorption at λ1 is larger than 70%, excited energy may still remain even if the signal light 30 is amplified. In this embodiment (marked with ●) in FIG. 6, λ1 is 808 nm.

  FIG. 7 is a graph schematically showing the distribution of gain and absorption loss in the comparative example, which is excitation by the excitation light sources 50 and 52 having a wavelength of 793 nm at which the absorption coefficient is maximum at a wavelength near 800 nm. Forward excitation is represented by a dot line, backward excitation is represented by a broken line, bilateral excitation is represented by a solid line, and absorption loss is represented by an alternate long and short dash line. The absorption loss is substantially constant without depending on the position in the length direction of the fiber 20.

  First, in FIG. 6, the fiber output (♦) of 793 nm forward pumping is as low as about 2.5 W when the pumping power is 85 W. In this case, the gain becomes maximum at the incident end 22 and decreases with the position in the length direction from the incident end 22, as represented by the dot line in FIG. In the region from the vicinity of the middle portion of the fiber 20 to the emission end 24, the net gain becomes negative and the signal light 30 is not amplified. Further, since the signal level of the signal light 30 at the incident end 22 is small, the energy strongly excited at 793 nm cannot be sufficiently converted into amplified light, resulting in low energy efficiency. Further, since the excited energy is small on the emission end 24 side, a high fiber output cannot be obtained.

  Further, the fiber output (■) of the backward pumping at 793 nm is about 7 W when the pumping input is 85 W, which is higher than the forward pumping. However, as indicated by the broken line in FIG. 7, there is no forward pumping from the incident end 22, so there is no net gain in the vicinity of the intermediate portion from the incident end 22, and the signal light 30 is not amplified in this region. The net gain turns positive in the vicinity of the middle of the fiber, increases toward the exit end 24, and reaches its maximum at the exit end 24. Since the wavelength of the excitation light 36 is 793 nm at which the absorption coefficient is maximized, energy that is strongly excited in the vicinity of the emission end 24 can be extracted to the outside, and a fiber output 7 W higher than that of forward excitation can be obtained.

  Further, the fiber output (両 側) of both-side excitation by 793 nm is about 8.5 W when the excitation input is 165 W, and becomes saturated when the excitation input exceeds 130 W. In this case, since the excitation at the incident end 22 is strong, the fiber output can be increased in a region where the excitation input is 70 W or less. However, excitation at the incident end 22 side is too strong, and energy still remains even when the signal light 30 is amplified. The remaining energy causes ASE light and parasitic stopping, reducing energy efficiency, and saturating the fiber output in the vicinity of about 8.5 W as shown in FIG.

On the other hand, the fiber output of this embodiment (●), which is a two-wavelength hybrid pump that combines forward pumping with a wavelength of 793 nm and backward pumping of 808 nm, is approximately 13.5 W with a pump input of 175 W. In this case, at 808 nm, the cross-sectional area of absorption is 2.3 × 10 ⁻²⁵ m ² , which is about 23% of the maximum value, and excitation can be suppressed at the incident end 22 as shown in FIG. For this reason, ASE light and parasitic oscillation can be suppressed and high energy efficiency can be achieved.

Further, the closer to the emission end 24 of the fiber 20, the larger the excitation light 36 is, and Tm ³⁺ is strongly excited, and excitation is performed at a wavelength of 793 nm at which the absorption coefficient is maximized. Therefore, in the pumping wavelength range of FIG. 4, the energy pumped into the fiber 20 is maximized, the pumping can be maximized at the output end 24, and the energy pumped into the fiber 20 can be efficiently converted into the amplified light 32, and about A high fiber output of 13.5 W can be obtained.

  When the signal light 30 is pulse-modulated with a repetition frequency of 15 to 30 kHz, the pulse-modulated amplified light 32 having a high peak value can be obtained.

Here, a supplementary description will be given of the excitation light sources 50 and 52 for obtaining a high fiber output as shown in FIG. Laser light from a plurality of semiconductor lasers is collected and incident on a transmission fiber. Alternatively, a transmission fiber into which a plurality of semiconductor lasers are incident may be bundled and fused using a double clad fiber. When a plurality of semiconductor lasers arranged in parallel as described above are pulse-operated, a higher peak pumping input can be obtained, and the Tm ³⁺ doped fiber 20 can be pumped more strongly.

  The embodiments of the present invention have been described above with reference to the drawings. However, the present invention is not limited to this. Even if those skilled in the art have made various design changes regarding the material, size, shape, and arrangement of rare earth elements, fibers, excitation light sources, etc. constituting the fiber amplifier, they are included in the scope of the present invention without departing from the gist of the present invention. Is done.

It is a block diagram of the fiber amplifier concerning embodiment of this invention. It is an example of the energy level diagram of Tm ³⁺ . It is a graph showing distribution of gain and absorption loss in a Tm ³⁺ doped fiber. It is a graph showing the absorption spectrum of Tm3 ⁺ addition silica fiber in the 800 nm vicinity. It is a graph showing the light emission and absorption cross-sectional area in a Tm3 ⁺ addition silica fiber. It is a graph showing the excitation input dependence of fiber output. It is a graph showing the gain and absorption loss distribution by 793 nm excitation light which is a comparative example.

Explanation of symbols

20 fiber, 22 entrance end, 24 exit end, 30 signal light, 32 amplified light, 34 excitation light, 36 excitation light, 50 excitation light source, 52 excitation light source

Claims (5)

A fiber with a rare earth element added to the core;
A first excitation light source that generates first excitation light that is incident on one end of the fiber;
A second excitation light source for generating second excitation light to be incident on the other end of the fiber;
With
An absorption coefficient of the rare earth element is larger at a wavelength of the second excitation light than at a wavelength of the first excitation light;
The fiber amplifier, wherein the signal light incident on the one end is amplified by the rare earth element excited by the first and second excitation lights and is emitted from the other end.   The gain of amplification of the signal light by the rare earth element decreases from the first end toward the intermediate portion of the fiber, increases from the intermediate portion toward the second end, and the second 2. The fiber amplifier according to claim 1, wherein the maximum value is at the end of the fiber amplifier.   3. The fiber amplifier according to claim 2, wherein a net gain obtained by subtracting the absorption loss of the rare earth element from the gain is positive throughout the entire length of the fiber.   The wavelength of the second pumping light is in the vicinity of a wavelength that maximizes the absorption coefficient in a pumping wavelength range in which the signal light can be amplified. The fiber amplifier described. The fiber amplifier according to claim 1, wherein the rare earth element is thulium.

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