JP3369888B2 - Optical fiber laser - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber laser using a rare earth doped fiber mainly used as a light source for optical communication. 2. Description of the Related Art In recent years, the development of optical fiber communication has been remarkable, and the amount of communication has been remarkably increased. Therefore, there is a demand for a high density in which a large amount of optical information is loaded on one optical transmission line. For example,
Attempts have been made to transmit signal light of many wavelengths at narrow wavelength intervals in a wavelength division multiplexing communication system in the wavelength band of 1.55 μm. For this reason, a light source with a narrow wavelength width that causes the oscillation wavelength to differ only by about several nm is required. As a light source satisfying such conditions, an optical fiber laser using an optical fiber doped with Er as a laser medium has been actively studied. The optical fiber laser can obtain a large finesse inside the resonator due to the high gain of the amplifying medium, so that the line width of the oscillation spectrum can be reduced to about several KHz. An optical fiber laser configured for wavelength division multiplex communication is required to have a single longitudinal mode (single frequency) oscillation from the viewpoint of wavelength and output intensity stability. In order to realize single longitudinal mode oscillation, it is necessary to insert a mode selection element in the resonator. Fiber gratings have attracted particular attention as mode selection elements. A fiber grating is a wavelength selection element that reflects light of an arbitrary wavelength by giving a refractive index change to the core of an optical fiber. As an example of a resonator configuration using a fiber grating, there is a reflection type (Fabry-Perot type) resonator using a fiber grating at a reflection end. FIG. 9 shows a configuration example of a reflection type resonator. In FIG. 9, 31 is an excitation light source, 32 is an optical isolator, 33 is a fiber grating, 34 is an optical fiber doped with a rare earth metal element, 35 is a fiber grating, and 36 is an optical isolator. In general, as a method for selecting an oscillation mode of a laser, a composite resonator configuration having two or more resonance conditions has been used. An example of an optical fiber laser using a Fox-Smith resonator with such a configuration (Peter Urquhartetal., Opt. Soc .. Am .. A Vo
l.5, No. 8, p. 1339, 1988), there is a resonator as shown in FIG. In FIG. 10, 31 is an excitation light source, 32 is an optical isolator, 34 is an optical fiber doped with a rare earth metal element, 37 is a lens, 38 is an input / output terminal, 39 is an optical coupler, 40 is an input / output terminal, and 41 is a Bragg The grating mirror 42 is a reflection end. [0005] In the case of using an optical fiber doped with Er for a laser medium for the purpose of laser oscillation in the 1.55 μm band, the amplification band is spread to around 1.53 μm to 1.56 μm. In a resonator in which a wavelength selection element is not introduced, many longitudinal modes oscillate simultaneously. Therefore, a device for realizing single mode oscillation is required. For the reflection type resonator length,
The interval Δf between the oscillating longitudinal modes is as shown in Expression (1). Δf = c / 2nL (1) L: length of reflection type resonator, Δf: interval of longitudinal mode, c: light velocity, n: refractive index From among these many longitudinal modes In order to select one mode, a mode selection element having a wavelength width equivalent to the longitudinal mode interval Δf represented by Expression (1) is required. The selection wavelength width of the fiber grating is narrow, and the full width at half maximum is 0.1 nm
It is about. Therefore, the length of the resonator must be shortened so that one longitudinal mode can be selected with a wavelength width of about 0.1 nm. In the reflection type resonator, the longitudinal mode interval is set to be equal to 0.
The cavity length to be 1 nm is about 0.85 cm. A configuration example of such a short resonator length is disclosed in the following document. (S.
V. Chernikov, and JRTalor, Opt. Lett. 18, No. 23, p2023
(1993)) However, with the shortening of the cavity length, the length of the Er-doped optical fiber, which is the laser medium, also becomes shorter, and the output intensity may decrease, or the amplification required for laser oscillation may not be obtained. . For this reason, the use of optical fibers with a high concentration of Er has been considered, but the problem is that Er ions form clusters due to the high concentration, and the absorption efficiency of excitation light decreases due to ion-ion interactions. is there. In addition, in order to prevent a decrease in the absorption efficiency of the excitation light and to achieve high-concentration addition of Er, a new device such as co-addition of Yb is required. As another method of realizing the single mode oscillation, there is a method of configuring the composite resonator shown in FIG. 10, but there is a problem that the number of components used is large and the configuration becomes complicated. An object of the present invention is to solve the above-mentioned problems and to provide an optical fiber laser having an oscillation wavelength width and an output intensity required as a light source for wavelength division multiplex communication. The present invention has the following means for solving the above-mentioned problems. An optical fiber laser according to the present invention includes an optical coupler having four terminals, two of the four terminals of the optical coupler being connected by optical fibers to form two loops, An optical amplifier having a figure 8 resonator formed by providing a fiber grating in one loop, and an excitation light source, a wavelength multiplexing directional coupler, and an optical fiber doped with rare earth metal ions in the other loop. And in the other loop, an optical component that restricts light passing through the loop in one direction, and an optical component that outputs predetermined light passing through the loop outside the figure-eight resonator, and the inside of the loop. It is characterized by having an optical filter for removing unnecessary wavelength components of light passing therethrough. According to the optical fiber laser of the present invention, an eight-shaped resonator is formed by using an optical coupler having four terminals, and an optical amplifier is provided in one loop of the eight-shaped resonator. And an optical component that restricts light passing through the loop in one direction, an optical component that outputs predetermined light passing through the loop outside the figure-eight resonator, and an unnecessary wavelength component of light that passes through the loop. Since the optical filter to be removed is provided and the fiber grating is provided in the other loop, the following operation is performed. The light circulating in the resonator selects a longitudinal mode that satisfies the laser oscillation condition by limiting the traveling direction to one direction, such as a directional coupler or an optical circulator, and further selects a longitudinal mode that satisfies the laser oscillation medium. Spatial hole burning can be suppressed. Also, since an optical filter and a fiber grating which remove unnecessary wavelength components, which are wavelength selecting elements, are provided, the optically amplified oscillation light has improved selectivity of the oscillation wavelength and excellent wavelength stability. It will be. Further, the optically amplified oscillation light is split into two optical paths by an optical coupler having four terminals to form a composite resonator,
Since a single mode is performed by selecting a wavelength using a fiber grating, stable mode selection is achieved. That is, according to the optical fiber laser of the present invention, it is not necessary to shorten the optical fiber to which the rare earth metal ions are added in the optical amplifier for mode selection. The required light intensity can be easily achieved. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an optical fiber laser according to the present invention will be described in more detail with reference to FIGS. FIG.
An optical fiber laser 10 shown in FIG. 1 includes an excitation light source 11, a wavelength multiplexing coupler 12, and an optical fiber 1 doped with a rare earth metal element.
3, a ring-type optical fiber resonator including an optical circulator 14, a wavelength selection filter 15, an optical coupler 16, and a fiber grating 17. In FIG. 1, reference numeral 18 denotes an optical fiber connecting each optical component. The laser light generated from the optical fiber 13 excited by the excitation light from the excitation light source 11 is limited to one direction by the action of the optical circulator 14 as shown in FIG. The signal is input to the optical coupler 16.
The optical coupler 16 has four terminals 16A, 16B, 16C, 1
6D, the laser light from the wavelength selection filter 15 is input to the terminal 16D of the optical coupler 16. Terminal 1
The laser light input to 6D is split by the optical coupler 16 and output from the terminals 16A and 16B. The laser beams output from the terminals 16A and 16B are input to the terminals 17A and 17B of the fiber grating 17 as shown in FIG. Each of the laser beams input to the terminals 17A and 17B reflects only a predetermined wavelength selected when traveling through the fiber grating 17, and the original terminals 17A and 1B are reflected.
7B, and is input to the original terminals 16A and 16B of the optical coupler 16. The reflected laser light selected to have a predetermined wavelength by the fiber grating 17 input to the terminals 16A and 16B is coupled in the optical coupler 16 and then distributed by the optical coupler 16 to be output from the terminals 16C and 16D. You. Here, the reflected laser light output from the terminal 16D is output to the outside from the output terminal 14A of the optical circulator 14 via the wavelength selection filter 15. On the other hand, the reflected laser light output from the terminal 16C passes through the optical fiber 13, the optical circulator 14, and the wavelength selection filter 15 as shown in FIG.
Is input to the terminal 16D. Hereinafter, the same route as described with reference to FIG. 2 will be followed. By the way, of the light generated from the optical fiber 13 excited by the excitation light from the excitation light source 11, there is light directly input to the terminal 16 </ b> C of the optical coupler 16 without passing through the optical circulator 14. This light is the spontaneous emission light of the optical fiber 13 to which the rare earth metal element is added, and the residual excitation light from the excitation light source 11 that is not absorbed by the optical fiber 13. The spontaneous emission light and the residual excitation light are input to the terminal 16C of the optical coupler 16, distributed by the optical coupler 16, and output from the terminals 16A and 16B. The spontaneous emission light and the residual excitation light output from the terminals 16A and 16B are input to the terminals 17A and 17B of the fiber grating 17 as shown in FIG. The other terminal 16B of the optical coupler 16 passes through the terminal 1
6A. The light input to the optical coupler 16 and passing through the fiber grating 17 is coupled by the optical coupler 16,
It is distributed and output from the terminals 16C and 16D. Here, since the light output from the terminal 16D is absorbed by the wavelength selection filter 15, it is not output to the outside from the output terminal 14A of the optical circulator 14. Terminal 1
The light output from 6C is the optical fiber 1 as shown in FIG.
3. The signal is again input to the terminal 16D of the optical coupler 16 via the optical circulator 14 and the wavelength selection filter 15. Hereinafter, the same route as described with reference to FIG. 2 will be followed. As described above, in the optical fiber laser 10 composed of a ring type optical fiber resonator, the wavelength that circulates in the resonator and satisfies the laser oscillation condition is selected by the wavelength selection filter 15 and the fiber grating 17. Area. Further, two optical paths 18 formed between the optical coupler 16 and the fiber grating 17 are provided.
A and 18B form a composite resonator, and mode selection is performed. The resonance conditions of the composite resonator between the optical coupler 16 and the fiber grating 17 are the optical fiber length L1 between the terminals 16A and 17A, the terminals 16B and 17B.
B is determined by the difference in the optical fiber length L2. Assuming that the single mode oscillation is possible under the condition that the half width of the reflected wavelength of the fiber grating 17 as the wavelength selection element is substantially the same as the free spectrum region of the longitudinal mode of the composite resonator, When the reflection wavelength half width of the grating 17 is 0.1 nm,
The difference between optical fiber length L1 and optical fiber length L2 is about 0.85cm
It would be good if it was. FIG. 6 shows two optical paths 18A, 18 formed between an optical coupler 16 and a fiber grating 17 using an optical fiber 13 doped with erbium as a rare earth metal used as a laser active medium.
The difference between the optical fiber length L1 and the optical fiber length L2 of B is about 0.85
FIG. 3 shows an output diagram of laser oscillation of the optical fiber laser 10 which oscillates an optical laser of 1.55 μm in cm. Here, in the case of the optical fiber laser 10 which oscillates an optical laser of 2.0 μm, an optical fiber 13 doped with holonium as a rare earth metal is used.
In the case of the optical fiber laser 10 that oscillates an optical laser of m, this can be achieved by using an optical fiber 13 doped with ytterbium as a rare earth metal. In the above-described embodiment, as shown in FIG. 7A, the traveling directions of the excitation light and the laser light are different from those of the excitation light of the excitation light source 11 and the laser light of the optical fiber 13 doped with the rare-earth metal element. Although different backward pumping is performed, forward pumping may be performed in which the traveling directions of the pump light and the laser light are the same as shown in FIG. Alternatively, as shown in FIG.
The two pumps 1 may be used for bidirectional pumping in which both pumping light and laser light travel in different directions, and pumping light and laser light travel in the same direction. Furthermore,
In the above embodiment, an optical circulator is used to output the laser light to the outside. However, as shown in FIGS. 8A and 8B, the laser light is output to the outside by combining the optical isolator 19 and the branch coupler 20. You may make it. As described above, according to the optical fiber laser of the first aspect of the present invention, an eight-shaped resonator is formed by using an optical coupler having four terminals. An optical component that restricts light passing through the optical amplifying section and the loop in one direction and outputs predetermined light passing through the loop outside the figure-shaped resonator in one loop of the figure-shaped resonator. And an optical filter for removing unnecessary wavelength components of light passing through the loop, and a fiber grating provided in the other loop, the following effects are obtained. The light circulating in the resonator selects a longitudinal mode that satisfies the laser oscillation condition by limiting the traveling direction to one direction, such as a directional coupler or an optical circulator, and further selects a longitudinal mode that satisfies the laser oscillation medium. Spatial hole burning can be suppressed. Further, since an optical filter and a fiber grating for removing unnecessary wavelength components, which are wavelength selecting elements, are provided, the optically amplified oscillation light has improved selectivity of the oscillation wavelength and excellent wavelength stability. It will be. Further, the optically amplified oscillation light is split into two optical paths by an optical coupler having four terminals to form a composite resonator,
Since a single mode is performed by selecting a wavelength using a fiber grating, stable mode selection is achieved. That is, according to the optical fiber laser of the present invention, it is not necessary to shorten the optical fiber to which the rare earth metal ions are added in the optical amplifier for mode selection. The required light intensity can be easily achieved.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a configuration diagram showing an embodiment of an optical fiber laser according to the present invention. FIG. 2 is an explanatory diagram showing a traveling direction of light in the optical fiber laser of FIG. FIG. 3 is an explanatory diagram showing a traveling direction of light in the optical fiber laser of FIG. FIG. 4 is an explanatory diagram showing a traveling direction of light in the optical fiber laser of FIG. 5 is an explanatory diagram showing a traveling direction of light in the optical fiber laser of FIG. FIG. 6 is an output diagram of laser oscillation of the optical fiber laser of the present invention. FIG. 7 is a configuration diagram of an optical amplifier used in the optical fiber laser of the present invention. FIG. 8 is a configuration diagram of an optical component that outputs laser light used for the optical fiber laser of the present invention to the outside. FIG. 9 is a configuration diagram illustrating an example of a conventional optical fiber laser. FIG. 10 is a configuration diagram showing another example of a conventional optical fiber laser. DESCRIPTION OF SYMBOLS 10 Optical fiber laser 11 Excitation light source 12 Wavelength multiplexing coupler 13 Optical fiber 14 doped with rare earth metal element 14 Optical circulator 15 Wavelength selection filter 16 Optical couplers 16A, 16B, 16C, 16D Terminal 17 of optical coupler Fiber grating 17A, 17B Fiber grating terminal 18 Optical fiber

Claims (1)

(57) [Claim 1] An optical coupler having four terminals is provided, and two terminals of the four terminals of the optical coupler are connected by optical fibers to form two loops. A fiber grating was provided in one loop to form a figure-eight resonator, and the other loop was provided with an excitation light source, a wavelength multiplexing directional coupler, and an optical fiber doped with rare earth metal ions. An optical component for providing an optical amplifying unit, and in the other loop, an optical component for limiting light passing through the loop to one direction, and an optical component for outputting predetermined light passing through the loop to outside the figure-eight resonator; An optical fiber laser comprising an optical filter for removing unnecessary wavelength components of light passing through a loop.