JP3415916B2 - 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 an optical fiber amplifying element for amplifying light by a stimulated emission effect as a laser medium. 2. Description of the Related Art In general, as a solid-state laser, a glass rod doped with a rare earth element or the like is used as a laser medium, and a pair of reflecting mirrors are arranged with the laser medium interposed therebetween to form a Fabry-Perot resonator. There are those that have been configured. However, in this type of apparatus, it is necessary to arrange the laser medium and the reflecting mirror spatially with their optical axes coincident with each other, so that assembly is troublesome, and the apparatus becomes large and expensive. Therefore, in the prior art, an optical fiber laser having a configuration as shown in FIG. 9 is provided. This optical fiber laser uses, as a laser medium, an optical fiber amplifier AE in which a core of an optical fiber is doped with a rare earth element such as Er. An optical resonator is formed by sequentially connecting the wave optical fiber coupler Ca, the branching optical fiber coupler Cb, and the polarization independent isolator I in a ring shape via a single mode optical fiber F.
Here, P is an excitation light source composed of a laser diode or the like. In this optical fiber laser, the optical fiber amplifying element AE is excited by pumping light introduced from a pumping light source P via an optical fiber coupler Ca, and light stimulated and emitted by the optical fiber amplifier AE passes through the optical resonator. The laser oscillates and oscillates, and is gradually amplified. Then, a part of the amplified laser light is taken out of the system via the optical branching optical fiber coupler Cb. The optical fiber laser having the structure shown in FIG. 9 has the features that the structure is relatively simple, the output is higher than that of an ordinary semiconductor laser or the like, and the wavelength band of laser oscillation is wide. For this reason, use of a signal source or the like for wavelength division multiplexing communication has been considered. On the other hand, laser light emitted from a normal solid-state laser has polarization characteristics and coherence. On the other hand, the polarization state of the laser light emitted from the optical fiber laser having the above configuration cannot be specified because the laser light propagates through the optical fiber in the ring-shaped optical resonator. Therefore, the laser light emitted from the optical fiber laser having the structure shown in FIG.
(Approximately そ の ま ま of linear polarization), this optical fiber laser cannot be applied to various devices using the coherence of the conventional laser light. For example, length measuring instruments, holography, heterodyne detectors, and the like use the coherence of laser light, but it is difficult to apply them to these devices. Therefore, in the prior art, in order to impart polarization characteristics to laser light emitted from an optical fiber laser,
For example, in the middle of the ring-shaped optical resonator shown in FIG.
A configuration provided with a polarizer is provided (for example, see Japanese Patent Application Laid-Open No. Hei 5-327096). [0011] However, even if a polarizer is arranged in the middle of a ring-shaped optical resonator, an optical fiber cannot be directly connected to the polarizer. It is necessary to perform optical coupling by inserting a module combining optical elements such as a polarizing plate into an optical resonance circuit. That is, since the optical axes are still required for spatially arranging optical elements such as lenses and polarizing plates, not only is it troublesome to assemble, but also drift due to temperature fluctuations and the like. , And lacks long-term stability. Further, in the configuration in which the polarizer is provided, although the laser beam has a polarization characteristic, simply rotating the laser beam in the optical resonator makes a round after passing through the polarizer. Before passing through the polarizer again, the polarization angle may be shifted. When the polarization angle is shifted, the laser beam does not pass through the polarizer, so that laser oscillation may not be continued and may be stopped. Therefore, in the case where a polarizer is provided, a polarization control element for matching the polarization angles after circulating is further required, which complicates the configuration. SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has a relatively simple structure in an optical fiber laser using an optical fiber amplifying element for amplifying light by a stimulated emission effect as a laser medium. Accordingly, it is an object of the present invention to stably generate linearly polarized laser light without performing polarization control. An optical fiber coupler formed by fusion splicing a pair of optical fibers has a wavelength dependence or a polarization dependence by adjusting the amount of stretching at the time of fusion splicing. (Eg, Applied Optic)
s, Vol.22 No.3 p484〜491,1983, JOURNAL OF LIGHTW
AVE TECHNOLOGY VOL9, NO.11 NOVEMBER 1991 p1503〜15
06). In the case where the optical fiber coupler has a 3 dB characteristic of splitting incident light into two optical fibers and emitting the same, if both ends of each optical fiber on the same side are connected to each other in a loop shape. It is known that, due to phase matching, light incident from one optical fiber acts as a reflector that exits from the optical fiber at the same incident end as it is (for example, JOURNAL OF LIGHTWAVE TECHNO
LOGY VOL6, NO.7 JULY1988 p1217-1224). The present invention has been made in view of such an event, and employs the following structure in order to solve the above-mentioned problems. That is, the optical fiber laser according to the present invention comprises a first optical fiber mirror section and a second optical fiber section before and after an optical fiber amplifying element for amplifying light by the stimulated emission effect.
An optical resonator is formed by individually connecting the optical fiber mirror sections to each other, and an excitation light source for generating excitation light is connected to the first optical fiber mirror section. A wavelength-dependent optical fiber coupler is provided that allows the light of the wavelength of the pump light to pass therethrough without branching, but the light of the wavelength of the laser light is equally branched by half and emitted. connecting one end portion of the same side of the pair of optical fibers constituting each other result
In both cases, one of the opposite ends is connected to the light
Connect to the fiber amplifier and connect the other to the excitation light source
Is configured, the second optical fiber mirror unit is provided with a polarization-dependent optical fiber coupler emitted by equally branches only light of a particular polarization angle of the laser light by 1/2, this polarization dependence One end of the same side of a pair of optical fibers constituting the optical fiber coupler is connected to each other , and
Is one of the opposite ends, and
It is configured to connect with a child . In the above configuration, the excitation light from the excitation light source is
The light is introduced into the first optical fiber mirror part constituting the optical resonator. Since the wavelength-dependent optical fiber coupler constituting the first optical fiber mirror portion passes the light having the wavelength of the excitation light as it is, the excitation light is introduced into the optical fiber amplifier, and the optical fiber amplifier is thereby excited. You. Then, the laser light generated by the stimulated emission of the optical fiber amplifier is introduced into the first and second optical fiber mirrors, respectively. The laser light introduced into the first optical fiber mirror section is equally branched by に よ っ て by the wavelength dependent optical fiber coupler, and each branched laser light is introduced again into the wavelength dependent optical fiber coupler. Is done. At this time, due to the phase matching, the split laser beams are multiplexed and output from the optical fiber on the same incident side. In other words, the first optical fiber mirror acts as a reflector for the laser light. Then, the laser light reflected by the first optical fiber mirror section is again introduced into the optical fiber amplifier and amplified. On the other hand, in the laser light introduced into the second optical fiber mirror section, only the light of the specific polarization angle is equally branched by に よ っ て by the polarization dependent optical fiber coupler, and each of the branched lights is split. The laser light is again introduced into the polarization dependent optical fiber coupler. At this time, due to the phase matching, the split laser beams are multiplexed and output from the optical fiber on the same incident side. That is, the second optical fiber mirror section acts as a reflector only for laser light having a specific polarization angle. Then, the laser light reflected by the second optical fiber mirror section is again introduced into the optical fiber amplifier and amplified. In this way, the laser light having a specific polarization angle reflected by each of the first and second optical fiber mirrors is repeatedly amplified by the optical fiber amplifying element, and laser oscillation is continued. Is taken out. In this laser oscillation, the laser light is reciprocated between the first and second optical fiber mirrors, instead of circulating the laser light in a conventional loop-shaped optical resonator. Even if the control is not performed, the polarization angles at the respective positions coincide by going back and forth on the same optical path of the forward path and the return path having the same optical path length. Therefore, the linearly polarized laser light is stably emitted. FIG. 1 is a block diagram of an optical fiber laser according to an embodiment of the present invention. The optical fiber laser of this embodiment includes an optical fiber amplifying element AE for amplifying light by the stimulated emission effect, and a first optical fiber mirror section FM ₁ is provided before and after the optical fiber amplifying element AE. is configured Fabry-Perot type optical resonator second optical fiber mirror FM ₂ and respectively connected individually, also, the first optical fiber mirror FM ₁ is the excitation light source P for generating excitation light It is connected. The above-described optical fiber amplifier AE amplifies light by the stimulated emission effect.
It is formed by doping a rare earth element such as r or Nd in the core of the optical fiber or in the outer peripheral portion of the core. Then, for example, when Er is doped, laser light of a constant wavelength λ _L (= 1.55 μm band) is emitted by stimulated emission. The pumping light source P is composed of a semiconductor laser such as a laser diode in this embodiment, and generates pumping light having a constant wavelength λp (1.48 μm in this embodiment). The first optical fiber mirror section FM ₁ has a wavelength-dependent optical fiber coupler C ₁ . As shown in FIG. 2, the wavelength-dependent optical fiber coupler C ₁ is formed by fusion splicing the side surfaces of a pair of optical fibers F ₁₁ and F ₁₂ , and adjusts the amount of stretching during fusion splicing. By doing so, as shown in FIG. 3, the light having the wavelength λ _P of the excitation light is allowed to pass as it is without branching, but the light having the wavelength λ _L of the laser light is equally branched by 、 and emitted. It has wavelength dependency. That is, in FIG. 2, a pair of optical fibers F ₁₁ , F ₁₁ constituting the wavelength-dependent optical fiber coupler C ₁ is shown.
When the symbols ~ are given to each of the ₁₂ ends,
The pumping light having the wavelength λ _P passes through without any branching between the-or-ends, but does not pass through between the-or-ends. On the other hand, when the laser light having the wavelength λ _L is incident from the end of the laser beam, the laser beam is equally branched into the terminals of the laser beam and emitted. That is, it has a 3 dB characteristic with respect to the incident light having the wavelength λ _L. In this embodiment, a pair of optical fibers F ₁₁ and F ₁₂ constituting the wavelength-dependent optical fiber coupler C ₁ are used.
One end portion of the same side (left side in FIG. 2), but are connected in a loop to each other via an optical fiber F ₁ of the single mode. Furthermore, the other end of the optical fiber F ₁₁ is the optical fiber amplifying element AE, the other end of the optical fiber F ₁₂ is connected to the pumping light source P. It is to be noted that the optical fibers F ₁₁ and F ₁₂ constituting the wavelength-dependent optical fiber coupler C ₁ may be directly joined to each other and connected in a loop without using the single mode optical fiber F _1. It is possible. On the other hand, the second optical fiber mirror unit FM ₂
It has an optical fiber coupler C ₂ polarization dependent. As shown in FIG. 5, the polarization-dependent optical fiber coupler C ₂ is formed by fusion-splicing the side surfaces of a pair of optical fibers F ₂₁ and F _22. Fig. 6
As shown in FIG. 3, only one linearly polarized light having specific polarization angles θ ₁ and θ ₂ (where θ ₁ −θ ₂ = 180 °) is _one of the laser light having the wavelength λ _L. The polarization dependence is given so that the light beam is split and output at equal intervals of / 2. That is, in FIG. 5, a pair of optical fibers F ₂₁ , F ₂₁ constituting the polarization dependent optical fiber coupler C ₂ is shown.
_When each of the ends of ₂₂ is denoted by the symbol ~,
In the light having the wavelength λ _{L of the} laser light, a specific polarization angle θ ₁ ,
For linearly polarized light having θ ₂ , or when incident from the end of
The light is split into two and emitted. That is, it is set to have a 3 dB characteristic for light having specific polarization angles θ ₁ and θ ₂ among the incident light having the wavelength λ _L. In this embodiment, a pair of optical fibers F ₂₁ and F ₂₂ constituting the polarization-dependent optical fiber coupler C ₂ is used.
One end portion of the same side (right side in FIG. 5), but are connected in a loop to each other via an optical fiber F ₂ single mode. Furthermore, the other end of the optical fiber F ₂₁ is connected to the optical fiber amplifying element AE, the other end of the optical fiber F ₂₂ is an exit end out of the laser beam. It is to be noted that the optical fibers F ₂₁ and F ₂₂ constituting the polarization dependent optical fiber coupler C ₂ are directly abutted with each other and connected in a loop without using the single mode optical fiber F _2. Is also possible. Next, the operation of laser oscillation of the optical fiber laser having the above configuration will be described. The pumping light of a constant wavelength λ _P (1.48 μm in this example) generated from the pumping light source P is introduced into the first optical fiber mirror section FM ₁ constituting the optical resonator. [0041] In this case, in the present example, the excitation light is inputted from one end of the optical fiber F ₁₂ constituting a wavelength independent optical fiber coupler C _1, the excitation light is branched by the optical fiber coupler C ₁ without Rukoto, of the optical fiber F _12,
Is Kaee folded in the optical fiber F ₁ through the respective end portions of the further, passes through the optical fiber F _11, the ends of are introduced into the optical fiber amplifying element AE. The laser light generated by the stimulated emission of the optical fiber amplifier AE excited by the excitation light is:
The light is introduced into the first and second optical fiber mirror units FM ₁ and FM ₂ , respectively. In this case, the first optical fiber mirror unit FM ₁
For, although will be laser light is input from one end of the optical fiber F ₁₂ constituting a wavelength independent optical fiber coupler C _1, the laser beam, the optical fiber F _11, F
_{At 12} , the light is equally branched by そ れ ぞ れ, and the light is emitted from each end. Then, the laser light emitted from one end, via the optical fiber F ₁ to the other end, also, the laser beam emitted from the other end, via the same optical fiber F ₁ whereas Are respectively input to the ends of. [0044] At this time, laser light is branched into two, in relation to the phase matching, are multiplexed by and emitted from the end of the same optical fiber F ₁₁ as the incident laser light first.
That is, by light having a multi-wavelength from the terminal of the optical fiber F _11, examining the intensity of light emitted from the same end, as shown in FIG. 4, the light of the wavelength of the laser beam lambda _L is preferentially Taken out. That is, the first optical fiber mirror unit FM
₁ will act as a reflector for laser light of wavelength λ _L. Thus, the first optical fiber mirror section FM ₁
Is reflected again by the optical fiber amplifier AE.
And amplified. On the other hand, the second optical fiber mirror FM ₂ receives laser light from one end of the optical fiber F ₂₁ constituting the polarization dependent optical fiber coupler C _2. Specific polarization angles θ ₁ , θ
_The linearly polarized light having ₂ (where θ ₁ −θ ₂ = 180 °) is preferentially split by the optical fibers F ₂₁ and F ₂₂ and emitted from the respective ends. Then, the laser light emitted from one end, via the optical fiber F ₂ at the other end, also, the laser beam emitted from the other end, via the same optical fiber F ₂ Meanwhile Are respectively input to the ends of. At this time, the two branched laser lights are multiplexed and emitted from the same end of the optical fiber F ₂₁ as the one into which the laser light was first incident due to phase matching.
That is, the light enters the circularly polarized wave from the end of the optical fiber F _21, examining the intensity of light emitted from the same end, as shown in FIG. 7, the polarization angle phase differs 180 ° theta ₁ , Θ ₂
Is preferentially extracted. That is,
The second optical fiber mirror section FM ₂ acts as a reflector for linearly polarized light having the polarization angles θ ₁ and θ ₂ . Thus, the second optical fiber mirror section FM ₂
The linearly polarized laser light reflected by the optical fiber is again introduced into the optical fiber amplifier AE and amplified. Then, the linearly polarized laser beams respectively reflected by the first and second optical fiber mirror units FM ₁ and FM ₂ are repeatedly amplified by the optical fiber amplifier AE, and the laser oscillation is continued. In this laser oscillation, the laser light does not circulate in the conventional ring-shaped optical resonator shown in FIG. 9, but the first and second optical fiber mirrors FM ₁ , FM ₁
Since the laser light is reflected back and forth between M ₂ , the linearly polarized laser light can be stably extracted without any particular polarization control. For example, as shown in FIG. 8, when the amplitude of the one-way component of the laser beam is a _x and the amplitude of the component in the direction perpendicular thereto is a _Y , the second optical fiber mirror unit FM ₂ performs linear polarization. When the wave laser light is reflected, for example, the polarization state is indicated by a solid line in FIG. Then, the laser beam is in the forward path leading to the first optical fiber mirror FM _1, +
When the deviation of the polarization angle of Δθ occurs (transition to the broken line position in the drawing), if the laser beam is reflected by the first optical fiber mirror FM _1, second optical fiber mirror F
_On the return path to M ₂ , a deviation of the polarization angle of −Δθ occurs. Therefore, the deviation of the polarization angle is canceled out by reciprocating reflection on the same optical path (return to the position indicated by the solid line in the figure). Therefore,
Only linearly polarized laser light having specific polarization angles θ ₁ and θ ₂ (where θ ₁ −θ ₂ = 180 °) repeats laser oscillation. Here, the second optical fiber mirror unit FM
_2, as described above, but reflects light of the linearly polarized wave, the reflectance is about 70%, the remaining approximately 30%, without being reflected by the polarization dependent optical fiber coupler C ₂ , it is taken out via the output terminal out from the terminal of the optical fiber F _22. [0052] Incidentally, the excitation light (wavelength lambda _P) also, but is output through the output terminal out from the terminal of the optical fiber F ₂₂ constituting the second optical fiber mirror FM _2, eliminating the need for excitation light In this case, for example, the second optical fiber mirror unit F
A filter that cuts the wavelength λ _{P of the} excitation light may be provided between M ₂ and the emission end out. Also, of the laser light (wavelength λ _L ) other than those having the specific polarization angles θ ₁ and θ ₂ , the second optical fiber mirror unit FM ₂ is formed at the beginning of the laser oscillation. although the terminal of the optical fiber F ₂₂ of the output through the output terminal out, according to the laser oscillation is continued, their light is attenuated gradually, specific polarization angle theta _1, linearly polarized with theta ₂ Only wave light is selectively amplified and emitted. [0054] In the above embodiment, derived is a laser beam from the second optical fiber emission ends out of the mirror unit FM _2, as shown by a broken line in FIG. 1, separately, an optical fiber coupler C ₃ the provided, it is possible to from the optical fiber coupler C ₃ configured as taking out the laser beam. According to the present invention, a first and second optical fiber mirror sections are provided in an optical fiber laser using an optical fiber amplifying element for amplifying light by stimulated emission effect as a laser medium. Since laser oscillation is performed by reciprocating reflection between the mirror portions, linearly polarized laser light can be stably generated. Further, the optical fiber laser having this configuration does not use a spatial optical system such as a lens and uses only an optical fiber as an optical element, so that it is extremely easy to assemble and has excellent stability. I have. Further, since the conventional polarization control element is not required, the configuration is simpler and the price is lower.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a configuration diagram of an optical fiber laser according to an embodiment of the present invention. FIG. 2 is a configuration diagram of a wavelength-dependent optical fiber coupler included in a first optical fiber mirror section constituting the optical fiber laser according to the embodiment of the present invention. FIG. 3 is a characteristic diagram showing a wavelength-dependent characteristic of a branching ratio of the wavelength-dependent optical fiber coupler of FIG. 2; FIG. 4 shows the wavelength-dependent optical fiber coupler of FIG.
FIG. 4 is a characteristic diagram showing the wavelength dependence of the intensity of light entering and exiting from one end. FIG. 5 is a configuration diagram of a polarization-dependent optical fiber coupler included in a second optical fiber mirror section constituting the optical fiber laser according to the embodiment of the present invention. FIG. 6 is a characteristic diagram showing polarization dependent characteristics of a branching ratio of the polarization dependent optical fiber coupler of FIG. 5; FIG. 7 shows a polarization-dependent optical fiber coupler shown in FIG.
FIG. 9 is a characteristic diagram showing polarization dependence of the intensity of light entering and exiting from one end. FIG. 8 is a diagram for explaining that linearly polarized laser light having a specific polarization angle can be extracted from the optical fiber laser of FIG. 1; FIG. 9 is a configuration diagram of a conventional optical fiber laser having a loop-type optical resonator. [Description of Signs] AE: Optical fiber amplifier, FM ₁ : First optical fiber mirror, FM ₂ : Second optical fiber mirror, C ₁ : Wavelength dependent optical fiber coupler, C ₂ : Polarization dependent light Fiber couplers, P: pumping light sources, F ₁₁ , F ₁₂ : optical fibers constituting wavelength-dependent optical fiber couplers, F ₂₁ , F ₂₂ : optical fibers constituting polarization-dependent optical fiber couplers.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hiroyuki Tanaka 4-3 Ikejiri, Itami-shi, Hyogo Mitsubishi Electric Wire Industry Co., Ltd. Itami Works (72) Inventor Yoshiyuki Imada 4-3-3 Ikejiri, Itami-shi, Hyogo Mitsubishi Electric Katsumi Morishita, Inventor Katsumi Morishita 958-4, Shirogane-cho, Fushimi-ku, Kyoto-shi, Kyoto (56) References Special Table 4-501037 (JP, A) Takashi Okuyama et al., 1994 Proceedings of the Academic Conference Spring Meeting, 4th Edition, 398 pages (issued March 10, 1994) Minoru Yoshida et al., 1992 IEICE Autumn Conference Proceedings, 4th Edition, 296 pages (58) (Int.Cl. ⁷ , DB name) H01S 3/00-3/30 G02B 6/00-6/293 JICST file (JOIS) WPI (DIALOG)

Claims (1)

(57) [Claim 1] A first optical fiber mirror section and a second optical fiber mirror section are separately provided before and after an optical fiber amplifying element for amplifying light by a stimulated emission effect. To form an optical resonator, and an excitation light source for generating excitation light is connected to the first optical fiber mirror section. The first optical fiber mirror section splits light having a wavelength of excitation light. Without passing through, but the light of the wavelength of the laser light is 1 /
A wavelength-dependent optical fiber coupler that splits and emits two light beams at equal intervals is provided, and one end on the same side of a pair of optical fibers constituting the wavelength-dependent optical fiber coupler is connected to each other , and on the opposite side. One end of the
Is connected to the optical fiber amplifier, and the other is an excitation light source.
Connect is configured, the second optical fiber mirror unit is provided with a polarization-dependent optical fiber coupler emitted by equally branches only light of a particular polarization angle of the laser beam halves, One end of the same side of a pair of optical fibers constituting the polarization dependent optical fiber coupler is connected to each other , and the opposite side is connected to the other end .
An optical fiber laser , wherein one of the one ends is connected to the optical fiber amplifying element .