JPH10206662A - Optical fiber laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical fiber laser as a light source which transmits plural stable spectrum whose line width and interval are short, without requiring multiple number of laser light sources which have various wavelength. SOLUTION: The optical fiber laser consists of an optical fiber 11 to which rare earth metal element is added as a laser active medium, an optical fiber grating 12 having prescribed reflection wavelength at one end 11a of the optical fiber 11, connected optical fiber gratings 131-13n in series which have various prescribed reflection wavelength, which is within wavelength range of the optical fiber grating 12, at the other end 11b of the optical fiber 11, and an excitation light source 14 for entering excitation light which excites natural radiation into the optical fiber 11.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical fiber laser mainly used as a light source for optical communication, and more particularly to an optical fiber laser using an optical fiber doped with a rare earth metal element.

[0002]

2. Description of the Related Art In recent years, with the expansion of an optical communication network, an optical communication system has been demanded to have a higher speed and a larger capacity. As a result,
2. Description of the Related Art In a wavelength division multiplexing (WDM) communication system, for example, an attempt has been made to transmit light of a large number of wavelengths (four to 32 waves) in a wavelength band of 1.55 μm at narrow wavelength intervals.

For this reason, a light source having a narrow oscillation wavelength width having oscillation wavelengths different by several nm is required. As this light source, an optical fiber laser using Er as a laser medium has been actively studied.
Since the optical fiber laser can obtain a large finesse inside the resonator due to the high gain of the amplification medium, the oscillation spectral line width can be narrowed to several kHz. There are two types of resonators in an optical fiber laser in which a resonator is formed by optical fibers. One is a Fabry-Perot type (reflection type), and the other is a ring type. In the laser resonator, many longitudinal modes corresponding to the period of the resonator length oscillate within the natural width of the gain spectrum of the laser medium. Therefore, in order to obtain a laser output of a single wavelength, it is necessary to insert a mode selection (wavelength selection) element into the resonator.

In the literature reported so far, (1) Fabry-Perot etalon (JLZyskind, JWSulhoff, J. Stone, DJ Digiovanni,
LWStulz, HMP Presby, A. Piccirilli and PEPramayon
Electron Lett., 27, No. 21, 1950 (1991)) (2) Fiber grating (for example, SVChernikov, and JRTaylor, Opt.
t. 18, No. 23, 2023 (1993)) (3) Mach-Zehnder interferometer (EWDianov, TRMartirosian, OGOkhotnikov, and V.
M. Paramonov, OFC / IOOC'93 Technical Digest WG6 p10
3) are used as mode selection elements.

FIGS. 13 and 14 show examples of the configuration of an optical fiber laser using a conventional optical fiber grating.
In the optical fiber laser 100 shown in FIG.
Reference numerals 2 and 104 denote fiber gratings, 103 denotes an optical fiber doped with a laser active medium, 105 denotes a wavelength division multiplexing directional coupler, 106 denotes an optical isolator, and 107 denotes an excitation light source. Optical fiber laser 110 shown in FIG.
Is an application of the optical fiber laser 100 shown in FIG. In FIG. 14, 111, 112 and 11
Reference numerals 3 and 114 are optical fiber gratings, and 115 and 116 are optical fibers to which a laser active medium is added.
In the laser resonator shown in FIG. 14, two resonators 115A and 116A exist for one pumping light source 117, and have different wavelength outputs. Accordingly, the optical fiber laser 110 shown in FIG. 14 has been studied as a wavelength division multiplexing communication light source as a wavelength division multiplex simultaneous oscillation laser.

[0006]

The characteristics that are important for the light source to be used in a wavelength division multiplexing communication system include:
Examples include the wavelength of the oscillation spectrum of the light source, the spectral line width, the stability of the wavelength, and the wavelength multiplexing. It is important that the width of the oscillation spectrum of the light source be as small as possible to obtain high-density multiplexing. In the case of wavelength multiplexing, the spectral line width and its shape have a direct effect on the level of crosstalk between adjacent channels. A set of light sources having a narrow oscillation spectrum with a spectral line width of several kHz and non-overlapping spectra is effective for wavelength multiplexing within a certain spectral range.

[0007] An ideal stable continuous light source is a single-wavelength (single longitudinal mode) laser having a narrow oscillation line width without fluctuation in oscillation wavelength and intensity. For example, when an Er-doped optical fiber is used as a laser medium, its amplification band is 15
It has a width of around 30 nm around 50 nm, and when oscillating a laser, many longitudinal modes oscillate simultaneously and do not become a single mode. Here, in order to stabilize the wavelength, it is necessary to insert a mode selection element in the resonator to select one longitudinal mode. Mode selection elements often used in optical fiber lasers are dielectric filters, etalon filters, and the like. These filters have a narrow band width (0.1n).
m) is difficult to select, and the large insertion loss also affects the laser output.

Further, for use in wavelength division multiplexing communication, a large number of light sources having oscillation wavelengths different by several nm are required. The light source conventionally used in the wavelength division multiplexing communication is mainly a semiconductor laser. In this case, it is necessary to prepare a number of laser light sources having different wavelengths used for multiplexing.

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems, and does not require the use of laser light sources having different numbers of wavelengths used for multiplexing. It is an object of the present invention to provide an optical fiber laser.

[0010]

The present invention has the following means to solve the above problems.

In the optical fiber laser according to the present invention, an optical fiber doped with a rare earth metal element is used as a laser active medium, and an optical fiber grating having a predetermined reflection wavelength width is provided at one end of the optical fiber. A plurality of optical fiber gratings having different predetermined reflection wavelength widths included in the wavelength selection region of the optical fiber grating are connected in series to the other end of the optical fiber, and the rare earth metal element is added. An excitation light source for inputting excitation light for exciting spontaneous emission light to the optical fiber is provided.

According to a second aspect of the present invention, in the optical fiber laser according to the second aspect, the directional couplers are provided inside the optical fiber gratings provided at both ends of the optical fiber doped with the rare earth metal element. An optical fiber is arranged in parallel with the optical fiber to which the rare earth metal element is added via the optical fiber.

In the optical fiber laser according to a third aspect of the present invention, a directional coupler is provided in an optical fiber arranged in parallel with the optical fiber doped with a rare earth metal element, and output light is output from the directional coupler. It is characterized by being emitted.

In the optical fiber laser according to a fourth aspect of the present invention, the excitation light from the excitation light source input to the optical fiber doped with a rare earth metal element is input via a wavelength division type directional coupler. I do.

According to the optical fiber laser of the first aspect of the present invention, an optical fiber doped with a rare earth metal element is used as a laser active medium, and an optical fiber grating provided at both ends thereof acts as a resonator. With respect to the light amplified by the laser active medium, only the wavelengths that meet the reflection conditions are selected and reflected by the optical fiber gratings provided at both ends constituting the reflection end of the resonator. Here, one reflection end is provided with a plurality of optical fiber gratings having different reflection wavelengths in series, and the other reflection end is provided with all of the plurality of optical fiber gratings having different reflection wavelengths provided on one side. Since the optical fiber grating includes the reflection wavelength selection region, the laser active medium is 1
In spite of this, simultaneous independent multi-wavelength oscillation corresponding to a plurality of optical fiber grating reflection wavelengths having different resonance conditions is achieved. That is, by providing the optical fiber grating at both ends of the resonator, it plays a role of both the reflection end and the wavelength selection element.

According to the optical fiber laser of the second aspect of the present invention, since the optical fiber is arranged in parallel with the optical fiber doped with the rare-earth metal element, the operation of the directional couplers provided at both ends of the two. Thereby, the traveling direction of the laser light satisfying the laser resonance condition propagating in the optical fiber doped with the rare earth metal element can be defined in one direction. Here, by propagating the traveling wave in the optical fiber to which the rare earth metal element is added, the spatial hole burning effect which is a problem with the standing wave can be suppressed.

According to the optical fiber laser of the third aspect of the present invention, the output light is extracted from the directional coupler provided in the optical fiber arranged in parallel with the optical fiber doped with the rare earth metal element. By adjusting the output ratio of the sexual coupler, it is easy to improve the laser efficiency.

According to the optical fiber laser of the fourth aspect of the present invention, since the pumping light from the pumping light source is input via the wavelength division multiplex type directional coupler, it is independent of the reflection wavelength region of the optical fiber grating. It is possible to use a plurality of types of wavelengths of the excitation light source. As a result, the wavelength band of the excitation light can be widened.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the optical fiber laser according to the present invention will be described below in more detail with reference to FIGS. (Embodiment 1) An optical fiber laser 10 shown in FIG. 1 has an optical fiber 11 doped with a rare earth metal element as a laser active medium, and an optical fiber having a predetermined reflection wavelength width at one end 11a of the optical fiber 11. Grating 12,
The other end 11b of the optical fiber 11 is provided with n optical fiber gratings 131 to 13n. The n optical fiber gratings 131 to 13n have different predetermined reflection wavelength widths within the reflection wavelength width of the optical fiber grating 12, respectively. FIG.
In the figure, 14 is an excitation light source, and 15 is an optical isolator.

The rare earth metal element added to the optical fiber 11 as a laser active medium is, for example, erbium when outputting a wavelength of 1.55 μm, holonium when outputting a wavelength of 2.0 μm, and 1.06 μm. Is output by doping with ytterbium. In the optical fiber laser 10 shown in FIG. 1, an optical fiber grating 12 and n optical fiber gratings 131 to 13n form n resonators having different resonance conditions.

That is, the n optical fiber gratings 131 to 13n have a role as both a reflection end and a wavelength selection element. Since the optical fiber grating 12 includes all the reflection wavelength regions of the optical fiber gratings 131 to 13n, it is necessary that the optical fiber grating has a wide reflection wavelength region. For example, a chirped fiber grating is used as an optical fiber grating having a wide reflection wavelength region. A chirped fiber grating is one in which the period of the change in the refractive index formed in the core in the optical fiber is not uniform, but the period is reduced along the longitudinal direction of the optical fiber.

FIG. 2 shows how the grating period of the chirped grating changes. When light having a wide wavelength width is incident, the light is reflected by a grating having a period corresponding to the wavelength. That is, an optical path difference is generated due to a difference in a reflection position depending on a wavelength, and this property is used for chromatic dispersion compensation in an optical fiber communication system. FIG. 3 shows an example of the wavelength characteristic of the chirped Bragg grating. FIG. 4 shows an example of the laser output characteristics of the optical fiber laser 10 shown in FIG.

As described above, the feature of the optical fiber laser 10 of the present invention is that the resonator is formed by using a plurality of optical fiber gratings in spite of the fact that only one laser active medium constitutes the resonator. That is, light of a plurality of wavelengths having different conditions oscillates simultaneously from one laser resonator. For example, a conventional optical fiber laser 110 shown in FIG. 14 for multi-wavelength simultaneous output has optical fiber gratings 111, 112 and 113,
14 and a plurality of resonators 115A and 116A having laser active media 115 and 116 between them (2 in FIG. 14).
The laser excitation medium is excited by the same excitation light source 117 to achieve a laser oscillation state in each of the resonators 115A and 116A.

However, the optical fiber laser 10 of the present invention
Since multiple wavelengths oscillate simultaneously from one resonator and the number of required optical fiber gratings is reduced, the structure of the optical fiber laser is simplified, and the cost is reduced. A composite resonator 120 shown in FIG. 15 is also known as a wavelength selection method when configuring a laser resonator.

In the composite resonator 120 shown in FIG.
1 is an optical fiber doped with a rare earth metal element as a laser active medium, and 122, 123, and 124 are reflection ends constituting a reflection type resonator. Reflecting ends 122, 123
The reflection end 124 is a mirror having a predetermined reflectance. The light output from the reflection type composite resonator 120 satisfies both the resonance conditions of the respective resonators including the reflection end 122 and the reflection end 123 and the reflection end 122 and the reflection end 124. That is, the composite resonator 120 having this configuration
In (2), the wavelength selection is such that only those having two resonance conditions and satisfying the conditions oscillate, and there is only one resonance condition.

On the other hand, the optical fiber laser 10 of the present invention has one reflecting end composed of an optical fiber grating provided at one end of an optical fiber doped with a rare earth metal element as a laser active medium, and the other end. Since the plurality of reflection ends made of optical fiber gratings connected to each other are in an independent resonance state, simultaneous oscillation of multiple wavelengths is possible.

(Embodiment 2) Another embodiment of the optical fiber laser of the present invention is shown in FIG. The optical fiber laser 20 shown in FIG. 5 is characterized in that each end 11a, 1a of the optical fiber 11 doped with a rare earth metal element as a laser active medium.
1b is provided with directional couplers 21A and 21B. An optical fiber grating 12 having a predetermined reflection wavelength width via directional couplers 21A and 21B;
Optical fiber gratings 131 to 13n are connected. Further, an optical fiber 22 is arranged in parallel with the optical fiber 11 doped with the rare earth metal element via the directional couplers 21A and 21B. The other parts are the same as in the first embodiment, and the same parts are denoted by the same reference numerals and detailed description thereof will be omitted.

As described above, the optical fiber 22 is arranged in parallel with the optical fiber 11 to which the rare earth metal element is added, and the rare earth metal element is added through the directional couplers 21A and 21B provided at both ends. If the traveling direction of the laser light satisfying the laser resonance condition propagating in the optical fiber 11 is defined in one direction, and the traveling wave propagates through the optical fiber 11 doped with the rare-earth metal element, a space which becomes a problem due to the standing wave The target hole burning effect can be suppressed.

(Embodiment 3) FIG. 6 shows another embodiment of the optical fiber laser of the present invention. The characteristics of the optical fiber laser 30 shown in FIG.
The directional coupler 31 is inserted into n resonators formed by the optical fiber gratings 131 to 13n having different resonance conditions, and output light is output through the directional coupler 31. is there. Other configurations are the same as those in the first embodiment, and the same portions are denoted by the same reference numerals and detailed description thereof will be omitted.

(Embodiment 4) FIG. 7 shows another embodiment of the optical fiber laser of the present invention. The feature of the optical fiber laser 40 shown in FIG. 7 is that the non-reflection end 41 is connected to the outside of the optical fiber grating 12, and the excitation light of the excitation light source 14 is formed by the optical fiber grating 12 and n optical fiber gratings 131 to 13n. Is input via the directional coupler 42 inserted inside the n resonators having different resonance conditions. Other configurations are the same as those in the first embodiment, and the same portions are denoted by the same reference numerals and detailed description thereof will be omitted.

(Embodiment 5) FIG. 8 shows another embodiment of the optical fiber laser of the present invention. The feature of the optical fiber laser 50 shown in FIG. 8 is that the non-reflection end 51 is connected to the outside of the optical fiber grating 12, and the excitation light of the excitation light source 14 is formed by the optical fiber grating 12 and n optical fiber gratings 131 to 13n. That is, the signal is input via a directional coupler 52 provided outside the n resonators having different resonance conditions. Other configurations are the same as those in the first embodiment, and the same portions are denoted by the same reference numerals and detailed description thereof will be omitted.

(Embodiment 6) FIG. 9 shows another embodiment of the optical fiber laser of the present invention. The optical fiber laser 60 shown in FIG. 9 is characterized in that the non-reflection end 61 is connected to the outside of the optical fiber grating 12, and the excitation light of the excitation light source 14 is formed by the optical fiber grating 12 and n optical fiber gratings 131 to 13n. Is input via the directional coupler 62 inserted inside the n resonators having different resonance conditions. Other configurations are the same as those in the second embodiment, and the same portions are denoted by the same reference numerals and detailed description thereof will be omitted.

(Embodiment 7) FIG. 10 shows another embodiment of the optical fiber laser of the present invention. The feature of the optical fiber laser 70 shown in FIG.
A non-reflection end 71 is connected to the outside, and the pumping light of the pumping light source 14 is provided outside a resonator formed by the optical fiber grating 12 and the n optical fiber gratings 131 to 13n having different resonance conditions. Is input via the directional coupler 72. Other configurations are the same as those in the second embodiment, and the same portions are denoted by the same reference numerals and detailed description thereof will be omitted.

Embodiment 8 FIG. 11 shows another embodiment of the optical fiber laser of the present invention. The characteristic of the optical fiber laser 80 shown in FIG.
n is connected to the non-reflective end 81 outside the directional coupler 21.
A directional coupler 82 is inserted into the optical fiber 22 disposed in parallel with the optical fiber 11 doped with the rare-earth metal element through A and 21B, and output light is output through the directional coupler 82. It is. Other configurations are the same as those in the second embodiment, and the same portions are denoted by the same reference numerals and detailed description thereof will be omitted.

(Embodiment 9) FIG. 12 shows another embodiment of the optical fiber laser of the present invention. The feature of the optical fiber laser 90 shown in FIG.
The non-reflection ends 91A and 91B are connected to the outside of the optical fiber grating 13n and to the outside of the optical fiber grating 13n. The signal is input via a directional coupler 92 inserted inside a resonator having different conditions, and is arranged in parallel with the optical fiber 11 doped with a rare earth metal element via the directional couplers 21A and 21B. Directional coupler 93 is inserted into the existing optical fiber 22, and the output light is
Is output via Other configurations are the same as those in the second embodiment, and the same portions are denoted by the same reference numerals and detailed description thereof will be omitted.

[0036]

As described above, according to the optical fiber laser of the first aspect of the present invention, the optical fiber doped with the rare earth metal element is used as the laser active medium, and the optical fiber gratings provided at both ends of the optical fiber grating resonate. Acts as a vessel. With respect to the light amplified by the laser active medium, only the wavelengths that meet the reflection conditions are selected and reflected by the optical fiber gratings provided at both ends constituting the reflection end of the resonator. Here, one reflection end is provided with a plurality of optical fiber gratings having different reflection wavelengths in series, and the other reflection end is provided with all of the plurality of optical fiber gratings having different reflection wavelengths provided on one side. Since it is an optical fiber grating that includes a reflection wavelength selection region, independent multi-wavelength simultaneous oscillations corresponding to a plurality of optical fiber grating reflection wavelengths with different resonance conditions despite having one laser active medium are provided. Achieved. That is, by providing the optical fiber grating at both ends of the resonator, it plays a role of both the reflection end and the wavelength selection element.

According to the optical fiber laser of the second aspect of the present invention, since the optical fiber is arranged in parallel with the optical fiber doped with the rare earth metal element, the action of the directional couplers provided at both ends of the two. Thereby, the traveling direction of the laser light satisfying the laser resonance condition propagating in the optical fiber doped with the rare earth metal element can be defined in one direction. Here, by propagating the traveling wave in the optical fiber to which the rare earth metal element is added, the spatial hole burning effect which is a problem with the standing wave can be suppressed.

According to the optical fiber laser of the third aspect of the present invention, the output light is extracted from the directional coupler provided in the optical fiber arranged in parallel with the optical fiber doped with the rare earth metal element. By adjusting the output ratio of the sexual coupler, it is easy to improve the laser efficiency.

According to the optical fiber laser of the fourth aspect of the present invention, since the pumping light from the pumping light source is input via the wavelength division multiplex type directional coupler, it is independent of the reflection wavelength region of the optical fiber grating. It is possible to use a plurality of types of wavelengths of the excitation light source. As a result, the wavelength band of the excitation light can be widened.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an embodiment of an optical fiber laser of the present invention.

FIG. 2 is an explanatory diagram showing how the grating period of an optical fiber grating changes.

FIG. 3 is an explanatory diagram illustrating an example of a wavelength characteristic of an optical fiber grating.

FIG. 4 is an explanatory diagram showing an example of a laser output characteristic of the optical fiber laser of FIG.

FIG. 5 is a configuration diagram showing another embodiment of the optical fiber laser of the present invention.

FIG. 6 is a configuration diagram showing another embodiment of the optical fiber laser of the present invention.

FIG. 7 is a configuration diagram showing another embodiment of the optical fiber laser of the present invention.

FIG. 8 is a configuration diagram showing another embodiment of the optical fiber laser of the present invention.

FIG. 9 is a configuration diagram showing another embodiment of the optical fiber laser of the present invention.

FIG. 10 is a configuration diagram showing another embodiment of the optical fiber laser of the present invention.

FIG. 11 is a configuration diagram showing another embodiment of the optical fiber laser of the present invention.

FIG. 12 is a configuration diagram showing another embodiment of the optical fiber laser of the present invention.

FIG. 13 is a configuration diagram illustrating an example of a conventional optical fiber laser.

FIG. 14 is a configuration diagram showing another example of a conventional optical fiber laser.

FIG. 15 is a configuration diagram showing another example of a conventional optical fiber laser.

[Explanation of symbols]

Reference Signs List 10 optical fiber laser 11 optical fiber doped with rare earth metal element as laser active medium 11a end of optical fiber 11b end of optical fiber 12 optical fiber gratings 131 to 13n optical fiber grating 14 excitation light source 15 optical isolator

Claims (4)

[Claims] An optical fiber doped with a rare earth metal element is used as a laser active medium, an optical fiber grating having a predetermined reflection wavelength width is provided at one end of the optical fiber, and the other end of the optical fiber is provided. A plurality of optical fiber gratings having different predetermined reflection wavelength widths included in the wavelength selection region of the optical fiber grating are connected in series, and pumping to excite spontaneous emission light to the optical fiber doped with the rare earth metal element is performed. An optical fiber laser comprising an excitation light source for inputting light. 2. A directional coupler is provided inside each of optical fiber gratings provided at both ends of an optical fiber doped with a rare-earth metal element, and the light doped with the rare-earth metal element via the directional coupler. The optical fiber laser according to claim 1, wherein an optical fiber is arranged in parallel with the fiber. 3. The directional coupler according to claim 2, wherein a directional coupler is provided on the optical fiber arranged in parallel with the optical fiber doped with the rare earth metal element, and output light is emitted from the directional coupler. An optical fiber laser as described. 4. The pump light according to claim 1, wherein the pump light from the pump light source input to the optical fiber doped with the rare earth metal element is input via a wavelength division multiplex type directional coupler. Optical fiber laser.