JPH0561079A - Optical filter - Google Patents

Abstract

PURPOSE:To obtain an amplifying optical filter for enhancing an optical multioplication factor in the band of 1.3mum wavelength and the other bands by removing unnecessary light disturbing the amplifying of light. CONSTITUTION:The prescribed part of an optical fiber 20 provided with a core made of an optical functional glass that Pr is added as an active material, keeps a constant curvature by a holding means 22. Therefore, the light of the longer wavelength side than specific wavelength, out of propagation light transmitting the optical fiber 20, is attenuated. When the necessary light such as excitating light, and light, enter the one end of the optical fiber 20, the excitating light excites the Pr. excited Pr emits the unnecessary light together with necessary emission light having the same wavelength as that of the signal light, etc., but the unnecessary light is attenuated on the prescribed part of the optical fiber 20. Therefore, the emission of necessary induced emission light having the same wavelength as that of the signal light caused by the emission of the unnecessary light can be prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an amplification type optical filter, an optical active device, a fiber amplifier, a fiber laser and the like used for optical amplification in the 1.3 .mu.m band and other wavelength bands.

[0002]

2. Description of the Related Art An optical functional glass doped with a rare earth element is considered to be applied to a fiber amplifier used for optical communication in a wavelength band of 1.3 μm, which is generally performed in a range of 1.310 ± 0.025 μm. ing. For example, wavelength 1.3μ
Optical fibers doped with praseodymium ion (Pr ³⁺ ) as an active material for achieving optical amplification near m have been reported (OFC '90 Post Deadline Papers (PD2-
1)). In this report, regarding a fluoride glass doped with Pr ³⁺ , a wavelength of 1.3 which enables optical enhancement in this band.
In addition to the transition ¹ G ₄ → ³ H ₅ corresponding to the fluorescence in the μm band, there is a transition ¹ G ₄ → ³ H ₆ corresponding to the fluorescence in the wavelength 1.8 μm band. Transition ¹ G
A value of 60% of ₄ → ³ H ₅ is reported.

[0003]

Therefore, when an optical amplifier is constructed by using the Pr ^{3+ -doped} fluoride glass shown in the above report, the transition ¹ G ₄ → ³ increases with an increase in the intensity of the pumping light. The fluorescent emission in the wavelength band of 1.8 μm due to H ₆ also starts to increase exponentially due to the stimulated emission, and there is a problem that the decrease in the amplification efficiency in the wavelength band of 1.3 μm due to this is not negligible. Further, there has been a problem that the above-mentioned propagating light other than the signal light is deteriorated as noise by deteriorating the signal transmission characteristic itself unless it is removed by an optical filter, a demultiplexing coupler or the like. Furthermore, when an optical amplifier is constructed using Pr ^{3+ -doped} fluoride glass, the amplification factor is small, so the loss of pumping light and signal light due to the insertion of optical filters, demultiplexing couplers, etc. into the optical path is also ignored. There was a problem that it became impossible.

Therefore, according to the present invention, unnecessary light that interferes with light emission and optical amplification in the wavelength band of 1.3 μm and other bands is removed from the inside of the optical fiber so that light in the wavelength band of 1.3 μm and other bands is removed. It is an object of the present invention to provide an amplification type optical filter which enables amplification or can increase its amplification factor.

Another object of the present invention is to provide an optical active device having the above amplification type optical filter.

It is another object of the present invention to provide a fiber amplifier, a fiber laser, etc., which is composed of the above-mentioned optical active device.

Another object of the present invention is to provide an optical filter that removes propagating light of a specific wavelength from the inside of an optical fiber.

[0008]

In order to solve the above-mentioned problems, the amplification type optical filter according to the present invention comprises (a)
An optical fiber having a core formed from an optical functional glass in which a rare earth element is added as an active substance to a host glass,
(B) Holding means for holding a predetermined portion of the optical fiber at a constant curvature is provided. In this case, Ytterbium (Y
b), Dysprosium (Dy), Samarium (S
m), promethium (Pm), erbium (Er), and various rare earth elements can be used.

In the above-mentioned amplification type optical filter, since the predetermined portion of the optical fiber is kept at a constant curvature, the light on the longer wavelength side than the specific wavelength of the propagation light transmitted through this optical fiber is attenuated. This specific wavelength can be appropriately adjusted according to the bending radius of the optical fiber, fiber parameters, and the like. When necessary light such as pumping light and signal light is incident on one end of an optical fiber provided in this amplification type optical filter, this pumping light pumps the rare earth element to a constant state.
The rare earth element excited in a certain state can generate unnecessary light of a specific wavelength or more together with necessary emitted light equal to the wavelength of signal light or the like, but this unnecessary light is attenuated at a predetermined portion of the optical fiber. Therefore, it is possible to prevent the generation of necessary stimulated emission light equal to the wavelength of signal light or the like due to the generation of unnecessary light. Such optically active devices include optical amplifiers, lasers,
It can be applied to various devices such as optical switches.

An optical active device of the present invention comprises: (a) the above-mentioned amplification type optical filter; (b) an excitation light source for generating excitation light for exciting a rare earth element; and (c) excitation light emitted from the excitation light source. And a pumping light coupling means for guiding the fiber.

In the above-mentioned photoactive device, the rare earth element is excited by the excitation light introduced into the optical fiber by the excitation light coupling means. Unwanted light from the excited rare earth element having a specific wavelength or longer is attenuated at a predetermined portion of the optical fiber. As a result, it is possible to prevent unnecessary light from interfering with the generation of necessary emitted light. In other words, it is possible to prevent the generation of emitted light having a wavelength equal to this induced by the unnecessary light,
It is possible to suppress a decrease in the probability of generation of necessary emitted light.
The required emission light generated by the excited rare earth element or the signal light incident on the optical fiber further induces the rare earth element to generate stimulated emission light with high efficiency, and the optical amplification function in this band, the optical Make it easy to perform various functions such as switch function and optical sensor function.

The fiber amplifier of the present invention comprises (a) the above-mentioned photoactive device using Pr as an active substance and light having a wavelength of about 1 μm or less as pumping light, and (b) a wavelength of 1.3 μm.
Signal light coupling means for guiding the m-band signal light to the optical fiber is provided. In the above fiber amplifier, Pr in the optical fiber is pumped by the pumping light having a wavelength of about 1 μm or less introduced into the optical fiber by the pumping light coupling means. Unwanted light having a wavelength of 1.8 μm from the excited Pr is absorbed by a predetermined part of the optical fiber. Therefore, it is possible to prevent the unnecessary light from interfering with the generation of the emitted light in the required 1.3 μm wavelength band. The signal light that has entered the optical fiber in this state induces Nd to generate stimulated emission light with high efficiency, which enables optical amplification in the wavelength band of 1.3 μm.

The fiber laser of the present invention comprises (a) the above optical active device using Pr as an active substance and 0.8 μm wavelength band light as pumping light, and (b) a wavelength of 1. And a resonator structure for feeding back light in the 3 μm band or its vicinity to an optical fiber. In the above fiber laser, Pr in the optical fiber is excited by the excitation light having a wavelength of about 1 μm or less introduced into the optical fiber by the excitation light coupling means. Unwanted light having a wavelength of 1.8 μm from the excited Pr is absorbed by a predetermined part of the optical fiber. Therefore, the wavelength 1.3 μm required by this unnecessary light
Not only is it possible to prevent generation of emitted light in the m band from being disturbed, but the emitted light in the 1.3 μm wavelength band generated by the optical fiber induces Nd to generate stimulated emission light with high efficiency. Laser oscillation in the 3 μm band becomes possible.

The optical filter of the present invention comprises an optical fiber and a holding means for keeping a predetermined portion of the optical fiber at a constant curvature.

In the above optical filter, since the predetermined portion of the optical fiber is kept to have a constant curvature, the light on the longer wavelength side than the specific wavelength of the propagation light transmitted through the optical fiber is attenuated. As described above, this specific wavelength can be appropriately adjusted according to the bending radius of the optical fiber, fiber parameters, and the like.

[0016]

The principle of the present invention will be briefly described below before the description of the embodiments.

It has been found that the bending loss α _B per unit table of a single mode fiber is given by the following equation.

Α _B = 4.34 × (πaw) ^−1/2 R ⁻¹ (w / v) ² × exp [−4w ³ × (3v ² ) ⁻¹ × Δ × R × a ⁻¹ ] where , W, k, v, Δ are given by the following equations.

W = | βt ₂ | a = β ² −k ² n ₂ ² ) ^1/2 a k = 2π / λ v = (k ² n ₁ ² a ² 2Δ) ^1/2 Δ = (n ₁ ² −n ₂ ² ) / 2n ₁ ² where R is the bending radius, a is the core radius, n ₁ is the core refractive index, and n ₂ is the cladding refractive index.

According to this equation, the longer the wavelength of light or the smaller the bending radius, the greater the loss 2α. Therefore, by setting an appropriate bending radius according to the fiber parameter and forming a coil, an optical filter that attenuates light on the longer wavelength side than the specific wavelength can be manufactured. With this optical filter, unnecessary light on the long wavelength side generated by a rare earth element such as Pr which is an active substance can be removed.

FIG. 1 is a diagram showing the structure of an optical fiber used in the amplification type optical filter of the embodiment. This optical fiber has a W-shaped structure and has a core 2 with a diameter of 5 μm and a diameter of 8 μm.
m inner clad 4 and 125 μm diameter outer clad 6
With. The relative refractive index difference Δn ₁ between the core 2 and the inner cladding 4 was set to 1%, and the relative refractive index difference Δn ₂ between the inner cladding 4 and the outer cladding 6 was set to 0.3%. The core 2 is formed of optical functional glass in which a rare earth element is added to a fluoride as an active substance.

FIG. 2 is a diagram showing loss characteristics when the optical fiber of FIG. 1 is coiled. The optical fiber wound around a coil with a diameter of 50 mm has a wavelength of about 1.8 as shown by the solid line.
Attenuates light above μm. Further, in the optical fiber wound around a coil having a diameter of 30 mm, the wavelength is about 1.
Attenuate light of 5 μm or more. As the active substance, for example, P
When an optical fiber doped with r is wound around a coil having a diameter of 50 mm, Pr in the optical fiber is excited by the introduced excitation light having a wavelength of about 1 μm or less. Unwanted light with a wavelength of 1.8 μm from the excited Pr is absorbed by the coil portion of the optical fiber. Therefore, it is possible to prevent the unnecessary light from interfering with the generation of the required emitted light in the 1.3 μm wavelength band, and it is possible to improve the generation efficiency of the stimulated emission light in this band, and it is possible to reduce the light emission in the 1.3 μm wavelength band. The efficiency of amplification and laser oscillation can be improved.

FIG. 3 shows an example of the configuration of a fiber amplifier of the wavelength 1.3 μm band using the optical fiber of FIG. The illustrated fiber amplifier includes an optical fiber 20 including a core doped with Pr ³⁺ , a bobbin 22 that holds the optical fiber 20 in a coil shape, and a Ti-sapphire laser that generates pumping light in a wavelength band of 1.017 μm. The pump light source 24 configured and the pumping light from the pump light source 24 to the optical fiber 2
And a coupling coupler 26 for making the light incident inside 0.

One of the input ports provided in the coupler 26 is connected to the signal light source 10 composed of a laser diode or the like for generating a signal light of wavelength 1.30 μm, and the other input port thereof has a wavelength 1.017 μm. Pump light source 4 is connected. The output port provided on the coupler 26 is
The input side of the optical fiber 20 is coupled, and the signal light and the excitation light are made incident on the inside of the optical fiber 20. The amplified signal light from the output side of the optical fiber 20 is guided to the photodetector 32 via the filter 30 that cuts the excitation light.
The photo detector 32 detects the intensity of the amplified light of wavelength 1.30 μm and the like.

The operation of the fiber amplifier shown in FIG. 3 will be briefly described. The signal light of wavelength 1.30 μm from the signal light source 28 enters the optical fiber 20 through the coupler 26. At the same time, the wavelength from the pump light source 10 is 1.017 μm
The excitation light of (3) also enters the optical fiber 20 through the coupler 26 and excites the active substance Pr ³⁺ . Excited Pr
³⁺ is induced by this signal light and generates radiation light having a wavelength of 1.30 μm corresponding to the transition ¹ G ₄ → ³ H ₅ . When the excitation light exceeds a predetermined intensity, the signal light will be amplified. In this case, the wavelength of 1.8 at which the excited Pr ³⁺ is generated
Since unnecessary spontaneous emission light and stimulated emission light in the μm band are absorbed by the coiled portion of the optical fiber, the wavelength of 1.
The stimulated emission in the 8 μm band can be suppressed to a low level, and the optical amplification factor at the wavelength of 1.30 m can be increased.

Specifically, an optical fiber 20 having 500 ppm of Pr ³⁺ added to a core is wound around a coil having a diameter of 50 mm and a length of 8 m, and the optical fiber 20 has a wavelength of 1.017 μm and an input intensity of 300 mW. As a result of entering the pumping light of
A gain of dB was obtained. Under the same conditions, when the optical fiber 20 was not wound into a coil, a gain of 12 dB was obtained.

The present invention is not limited to the above embodiment. A fiber laser can also be constructed using the optical fiber 20 of FIG. Specifically, both end surfaces of the optical fiber 20 are mirror-finished, and pumping light is introduced into the optical fiber 20 by using a light source similar to the pump light source 42 shown in FIG.

[0028]

As described above, in the amplification type optical fiber according to the present invention, unnecessary light of a specific wavelength from the rare earth element excited by the excitation light propagating in the optical fiber is attenuated at a predetermined portion of the optical fiber. It Therefore, it is possible to prevent the generation of necessary stimulated emission light equal to the wavelength of the signal light or the like from being caused by the unnecessary light, and to enable optical amplification in the wavelength 1.3 μm band or other bands, or Can be increased.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of an optical fiber used for an amplification type optical fiber of the present invention.

FIG. 2 is a diagram showing attenuation characteristics of the optical fiber of FIG.

FIG. 3 is a diagram showing an embodiment of an amplification type optical fiber of the present invention.

[Explanation of symbols]

 20 ... Optical fiber 22 ... Holding means 24 ... Excitation light source 26 ... Coupler which is excitation light coupling means

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁵ Identification number Internal reference number FI Technical indication location H01S 3/0915 (72) Inventor Takashi Mugo 1 Taya-cho, Sakae-ku, Yokohama-shi, Kanagawa Sumitomo Electric Industries Co., Ltd. Company Yokohama Works (72) Inventor Yoshiaki Miyajima 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (6)

[Claims] 1. An amplification type device comprising: an optical fiber having a core formed of an optical functional glass obtained by adding a rare earth element as an active substance to a host glass; and a holding means for holding a predetermined portion of the optical fiber at a constant curvature. Optical filter. 2. The amplification type optical filter according to claim 1,
An optical active device comprising: an excitation light source that generates excitation light for exciting a rare earth element; and excitation light coupling means that guides the excitation light from the excitation light source to the optical fiber. 3. The photoactive device according to claim 2, wherein Pr is used as the active substance, and light having a wavelength of about 1 μm or less is used as the excitation light. 4. A fiber amplifier comprising: the optical active device according to claim 3; and a signal light coupling means for guiding a signal light having a wavelength band of 1.3 μm to the optical fiber. 5. A fiber laser comprising the optical active device according to claim 3, and a resonator structure for feeding back light in the 1.3 μm wavelength band or in the vicinity thereof from the inside of the optical fiber to the optical fiber. 6. An optical filter comprising an optical fiber and a holding means for holding a predetermined portion of the optical fiber at a constant curvature.