WO2010024264A1

WO2010024264A1 - Fiber ring laser and fiber ring laser gyro using the same

Info

Publication number: WO2010024264A1
Application number: PCT/JP2009/064816
Authority: WO
Inventors: 能徳久保田; 英之岡本; 健春日; 育成原
Original assignee: セントラル硝子株式会社
Priority date: 2008-09-01
Filing date: 2009-08-26
Publication date: 2010-03-04
Also published as: JP2010062224A

Abstract

Provided is a fiber ring laser which comprises an excitation light source exciting a gain medium, at least one resonator in the shape of a closed ring (ring resonator) and a rare earth-doped fiber serving as the gain medium in the ring laser resonator, characterized in that a fiber provided with a core doped with at least one rare earth selected from among Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er and Tm is used as the rare earth-doped fiber and the laser oscillation wavelength is a short wavelength that is less than 1 μm.

Description

Fiber ring laser and fiber ring laser gyro using the same

The present invention relates to a fiber ring laser used for angular velocity measurement, distance measurement light source, surface shape measurement light source, display light source, image projection light source and the like. The present invention also relates to a fiber ring laser gyro using the same.

Background of the Invention

Recently, a high-quality and high-efficiency light source for visible light has been demanded for reasons such as high definition of image projection display, large display area, 3D display technology advancement, and 3D content enhancement. In addition, short wavelength light sources such as a visible light source are expected to be realized in application fields such as precise surface shape inspection, internal processing of transparent bodies such as glass, and precision laser processing.

Among existing high-precision lasers, fiber lasers are widely used as lasers with high laser beam quality. In particular, the fiber ring laser (F-RLG) is known as one of the main forms of fiber lasers because of its low laser oscillation threshold and high design freedom.

For this reason, a fiber laser capable of directly oscillating a short wavelength of less than 1 μm such as visible light is required. However, in a fiber laser using a general rare earth-doped silica fiber, the influence of non-radiation relaxation due to the high phonon energy of the medium is large, and it is difficult to directly oscillate a short wavelength of less than 1 μm. Therefore, second-order harmonic generation technology that converts the wavelength of a fiber laser to half has attracted attention, and has been widely used particularly for green laser generation. LiNbO3 crystal (LN) and β-BaB2O4 crystal (β-BBO) are used for second harmonic generation, and a waveguide type and a periodically poled type are known in addition to the bulk type. In particular, LN with periodically poled is widely used as PPLN (Periodically Polly Lithium Niobate).

In addition to this, as a method for obtaining a short wavelength, there is a short wavelength laser by multistage excitation of an infrared laser, which is known as an upconversion laser. The lifetime of the excited intermediate level is very important for upconversion lasers, and low phonon glass materials such as fluoride glass doped with rare earths and low phonon crystal compositions such as fluoride crystals doped with rare earths are used. (Non-Patent Document 1).

Recently, in the wavelength bands of red and blue, the output of semiconductor lasers has rapidly increased, and the progress of 340-500 nm lasers using GaN-based semiconductor lasers has been remarkable. Therefore, attempts have been actively made on a visible laser using a GaN-based semiconductor laser as an excitation light source (Non-patent Document 2).

In addition, 630-670 nm band red semiconductor lasers are already being applied to laser television, and 405 nm band GaN semiconductor lasers are being used for high density DVD recording. The 460 nm band is expected as a blue light source for display applications.

As described above, a ring laser using an existing rare earth-doped fiber is invisible light having a laser oscillation wavelength of 1 μm or more and cannot be used for a display or projection application. In addition, in order to inspect a machined finished surface composed of irregularities having a surface shape of about several tens to 100 nm with high accuracy, a light source having a wavelength of at least a visible light region or less is required. Furthermore, a visible light wavelength region in which the material is transparent is desired for fine processing inside the transparent body.

As described above, as a technique for obtaining a laser having a laser oscillation wavelength of less than 1 μm, a combination with wavelength conversion using a nonlinear optical crystal such as LN or β-BBO is known. It is difficult to grow and has a problem of low wavelength conversion efficiency. Further, in order to improve the wavelength conversion efficiency, LN or the like that is periodically poled is used. However, since the optimum wavelength conversion wavelength is sensitive to temperature, precise temperature control is required. For this reason, there is a problem that the power consumption of the laser device increases and the device becomes large. Further, a Fabry-Perot laser using a GaN-based semiconductor laser as an excitation source is known. However, since it is necessary to use a spatial optical system, it is easily affected by temperature changes and vibrations, and the installation environment is limited. In particular, when a moving body such as a ring laser gyro is incorporated, the optical system is misaligned due to vibration, and accurate measurement becomes difficult.

Furthermore, as described above, an upconversion laser is known as a method for obtaining a short-wavelength laser beam directly from a fiber. However, an upconversion laser is a multi-step process, and laser power fluctuations due to temperature fluctuations are likely to occur. There is a problem of low conversion efficiency.

In addition, the application of angular velocity detection to fiber ring lasers has been studied, and very much like the He-Ne ring laser gyro (RLG) and interference type fiber gyro (I-FOG) that have been used mainly in aircraft until now. It is expected as a small high-precision optical gyro that can replace high-precision but expensive and large machines (Non-patent Documents 3 and 4, Patent Document 1). Further, since narrowing of the laser line width is effective for improving the sensitivity of F-RLG, a method in which a rare earth-doped fiber is disposed as a saturable absorber in a part of the ring has been proposed (Non-patent Document 5). ). In addition, a method for facilitating detection by using a fiber ring laser as a mode-locked laser has been proposed (Non-Patent Document 6).

Optical gyros, not limited to F-RLG, use the Sagnac effect, but in a ring laser with an amplifying medium in the ring, the following formula (1) is applied to the clockwise (CW) and counterclockwise (CCW) laser oscillation frequencies: ) Is generated.

However,
Δf: Laser oscillation frequency difference A: Ring area Ω: Angular velocity λ: Laser oscillation wavelength when there is no rotation P: Ring laser resonator length.
In the ring laser gyro, this frequency difference is detected by a method such as interference.

When a fiber is used for the ring laser, assuming that the fiber is wound in a circular shape, it can be further arranged and can be expressed as the following formula (2).

However,
R: represents the ring radius.

As can be readily understood from Equation (2), the conditions for the laser oscillation frequency difference to be large and easy detection are as follows: a) large angular velocity, b) large ring diameter, or c) short lasing wavelength. There are three conditions. As a laser material that can be used as a fiber ring laser, Nd-doped silica fiber, Yb-doped silica fiber, Er-doped silica fiber, and the like are known, and the laser oscillation wavelength is 1 μm or more.

In these fiber ring lasers and optical amplifiers, there is a case where there is no particular notice, but a quartz optical fiber containing 100 to 1000 ppm of Er, Nd, Yb, etc. is used as a rare earth, which is necessary for absorption of pumping light. Due to this limitation, a fiber of about 10 m to 100 m is used.

As described above, when a ring laser is applied to a ring laser gyro, there are three parameters [a), b), and c)] related to angular velocity detection conditions. However, in order to improve the accuracy of the ring laser gyro, only two conditions, that is, a large ring diameter [b)] and a short laser oscillation wavelength [c)] are effective. In the existing fiber laser, it is difficult to make λ of the above formula (2) to be a short wavelength of less than 1 μm. Therefore, there is no method for high resolution other than increasing the ring diameter, and an increase in size cannot be avoided. Furthermore, the conventional rare earth-doped silica-based fiber has a rare earth addition concentration as low as several hundred ppm or less, and a fiber having a ring laser resonator length of at least 10 m is required, so that miniaturization is difficult.

JP 2007-212247 A

As described above, it is difficult to obtain a fiber laser that can oscillate at a short wavelength of less than 1 μm, particularly a fiber laser that can directly oscillate visible light and that can stably oscillate laser light and has high power conversion efficiency. Met.

Also, as described above, when a ring laser is applied to a ring laser gyro, it is difficult to reduce the size for high accuracy.

Therefore, the present invention provides a fiber ring laser capable of directly oscillating a short wavelength, which is useful for angular velocity measurement, distance measurement, surface shape measurement, display, image projection, high-precision processing, etc. An object is to provide a fiber ring laser gyro.

As a result of intensive studies to achieve the above object, the present inventors have found that when a rare earth-doped fluoride fiber pumped with a semiconductor laser having a wavelength of 340 nm or more and 500 nm or less is used as a gain medium of a ring laser, the wavelength is less than 1 μm. The inventors have found that it is possible to directly oscillate a short wavelength, and have reached the present invention.

According to the present invention, an excitation light source for exciting a gain medium, at least one closed ring resonator (ring laser resonator), and a rare earth-doped fiber as the gain medium in the ring laser resonator. In a fiber ring laser, as the rare earth-doped fiber, a fiber having a core to which at least one kind of rare earth selected from Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, and Tm is added is used. There is provided a fiber ring laser (first laser) characterized in that the oscillation wavelength is a short wavelength of less than 1 μm.

The first laser is a fiber ring laser (second laser), wherein the rare earth-doped fiber is a fluoride glass fiber, and a semiconductor light source that emits a wavelength of 340 nm to 500 nm is used as the excitation light source. ).

The first or second laser is a fiber ring laser (third laser) characterized in that it comprises at least two or more of the rare earth-doped fibers and can individually control the pumping conditions of the rare earth-doped fibers. May be.

Any one of the first to third lasers includes a ring laser resonator including at least one saturable absorber that operates at a laser oscillation wavelength of the fiber ring laser. 4 lasers).

Any one of the first to fourth lasers is characterized in that at least one fluoride fiber having a core transparent to the excitation wavelength and the laser oscillation wavelength is provided in the ring laser resonator. It may be a ring laser (fifth laser).

According to the present invention, a beat obtained by extracting a clockwise (CW) laser signal and a counterclockwise (CCW) laser signal from a ring laser resonator and causing the extracted CW laser signal and CCW laser signal to interfere with each other. In a fiber ring laser gyro (F-RLG) using an angular velocity detection method for detecting a signal, a fiber ring laser gyro characterized by using any one of first to fifth lasers is provided.

It is a figure which shows the structure of the fiber ring laser part of Example 1,2. It is a figure which shows the oscillation spectrum of the fiber ring laser of Example 1. FIG. It is a figure which shows the ring laser light source part of Example 3. FIG. FIG. 6 is a configuration diagram of a ring laser gyro according to a third embodiment. It is a figure which shows the setup which measures the angular velocity detection characteristic of the ring laser gyro of Example 3. FIG. It is a figure which shows the measurement result of the relationship between the angular velocity of Example 3, and an interference frequency.

Detailed description

According to the present invention, a short-wavelength laser beam having a wavelength of less than 1 μm can be obtained directly and stably in a fiber. Surface shape measurement with visible light, display, projector, 3D television, high-precision surface shape measurement, high Application to precision processing, transparent body internal processing, high-performance fiber laser gyro, etc. becomes possible. In addition, for F-RLG applications, since a short-wavelength fiber laser can be used, a laser frequency shift equal to or higher than that of a conventional fiber ring laser using a quartz fiber can be obtained even when the ring diameter is small. Therefore, the ring laser gyro can be made smaller and more accurate.

Furthermore, by using the present invention, a laser having a short wavelength with a laser oscillation wavelength of less than 1 μm can be obtained directly without using a method such as wavelength conversion. Further, since the laser resonator is a ring laser resonator, no optical alignment shift occurs with respect to vibration or temperature fluctuation, so that stable laser oscillation can be obtained.

In the present invention, a ring laser can be constructed at low cost by using a semiconductor light source as an excitation light source.

Also, in the present invention, by controlling the pumping state independently for a plurality of rare earth doped fibers, the gain of CW light and CCW light is controlled to be the same, and the power difference due to the rotation direction is made very small. Therefore, in applications that use interference between CW light and CCW light, the contrast of interference fringes can be maximized. Further, such a laser can be used widely as a light source for interference measurement because it can simultaneously obtain a set of laser beams having the same power.

Also, in the present invention, at least one saturable absorber that operates at the laser oscillation wavelength of the fiber ring laser is provided in the ring laser resonator, thereby removing unnecessary noise components and controlling the laser oscillation state. Is possible. Reduction in laser noise improves processing accuracy and measurement accuracy. Further, by absorbing and removing the stimulated Brillouin scattering, the occurrence of rocking can be suppressed when applied to a laser gyro.

Here, since the gain medium of the ring laser is a rare earth doped fiber, scattered light or spontaneous emission light may circulate in the ring resonator in addition to the laser oscillation wavelength. In particular, the wavelength of spontaneous emission light generated by the transition between the same levels as laser oscillation is close to the laser oscillation wavelength. Therefore, the amplified spontaneous emission passes through and circulates through the optical components that make up the ring laser resonator with low loss. It becomes light (ASE light). Since the ASE light has a wide emission spectrum width, the integrated power is relatively large even when the peak power is low, which not only prevents laser oscillation but also causes laser noise. The saturable absorber has a characteristic of absorbing when the light intensity is low and transmitting when the light intensity is high. For this reason, if the saturable absorber is in the ring laser resonator, it can be absorbed and removed before the ASE light grows, and there is an advantage that the noise of the laser is greatly reduced. In addition, although scattered light having a laser wavelength is difficult to remove by a normal method, scattered light existing at a time other than when the laser pulse passes can be removed when pulsed laser oscillation is performed, so that noise can be reduced.

Further, in the present invention, by providing at least one fluoride fiber having a core transparent to the excitation wavelength and the laser oscillation wavelength in the ring laser resonator, the change in the optical length due to the temperature change can be suppressed. . As a result, it is possible to prevent laser oscillation wavelength changes and power fluctuations due to sudden temperature changes, and to ensure stable laser oscillation operation. Here, the optical length is represented by [ring laser resonator length] × [average refractive index of ring laser resonator]. Of the materials constituting the ring laser resonator, most of oxide-based materials and semiconductor-based materials have a positive linear expansion coefficient and a positive temperature dependency of the refractive index. For this reason, when the ambient temperature rises, the optical length increases monotonously. On the other hand, fluoride glass has a positive coefficient of linear expansion but negative temperature dependency of the refractive index. For this reason, an increase in the optical length due to the temperature dependence of other materials can be corrected by providing a combination of fluoride fibers having an appropriate length in the ring laser resonator. Furthermore, since the fluoride fiber to be used is a fiber that is transparent to the laser wavelength and the excitation wavelength, the temperature dependence can be suppressed without affecting the optical characteristics of the ring laser.

Also, in the present invention, the accuracy of detecting the angular velocity of the laser gyro is improved by applying the short wavelength ring laser of the present invention to the laser gyro. Further, since the ring laser of the present invention oscillates at a short wavelength, a highly accurate laser gyro can be realized even if the diameter of the ring laser resonator is reduced. For this reason, a laser gyro can be reduced in size.

The composition of the rare earth-doped fiber used for laser oscillation must be at least the core glass composition of low phonon energy, and low phonon energy laser base materials include fluoride glass, tungstate glass, and molybdic acid. Salt glass, germanate glass, tellurite glass, bismuthate glass, chloride glass, fluorophosphate glass, chalcogenide glass and the like can be used. Among these, fluoride glass is particularly preferable because of its ease of production and high emission efficiency of short wavelength light. In a glass fiber with a high phonon energy such as a silica-based glass fiber, laser oscillation at a short wavelength is hindered by multiphonon relaxation. On the other hand, when a rare earth-doped fiber or a transition metal-doped fiber is used for the saturable absorber, there is no problem with a rare-earth-doped silica fiber or a transition metal-doped silica fiber.

Structural parameters such as fiber core diameter and numerical aperture are determined taking into account the mode overlap and pumping light density determined by the pumping light wavelength and laser oscillation wavelength. In this case, it is preferable to set the cutoff wavelength to a wavelength shorter than the laser oscillation wavelength. At this time, it may be designed to be a single mode at the excitation light wavelength, or may be a multimode design in which a low-order mode can exist. This is because laser oscillation is possible without propagation over a long distance as in a transmission fiber for optical communication.

On the other hand, in the case of multimode laser oscillation used in a high-output light source or the like, it is necessary to design around a core diameter that can obtain the required power. Since it is necessary to adjust according to the required power, it cannot be specified unconditionally. However, when the output is about 1 W to 10 W, the core diameter is about 10 μm to 50 μm, and when the output is about 100 W, the core diameter is 100 μm to 500 μm. A range is preferred. As a limitation of power density when a fluoride fiber is used as a laser medium, a value obtained by dividing the total optical power of pumping light and ring laser light by the core area is preferably 100 MW / cm ² or less. Exceeding this power density significantly increases the risk of fiber breakage. On the other hand, when the power density is 50 kW / cm ² or less, efficient laser oscillation is difficult.

The concentration of rare earth added cannot be unconditionally defined because the optimum value varies depending on structural parameters such as the core diameter and numerical aperture of the fiber and the confinement efficiency of the resonator, but the concentration in the range of approximately 100 ppm to 10,000 ppm can be determined. It is suitable to add to. If it is less than 100 ppm, since the absorption coefficient at the excitation light wavelength is too small, the required fiber length becomes long, which is not preferable for practical use. A concentration exceeding 10,000 ppm is not preferable because the non-emissive transition probability increases due to energy transfer between rare earth ions.

It is preferable to use a semiconductor light source as the excitation light source in terms of cost and productivity. Semiconductor light sources with a light emitting part in a spot form include semiconductor lasers and LEDs (Light Emission Diodes), which are not only used as they are, but also one-dimensional light sources integrated in an array, two-dimensional integrated in a planar shape. The light source can be used as a three-dimensional light source that is three-dimensionally arranged. Moreover, since organic EL and inorganic EL can be originally produced as a two-dimensional light-emitting panel, they are suitable for uniformly exciting a large area. These excitation light sources can be used in appropriate combinations in order to obtain the necessary excitation power. Among these light sources, the use of a GaN-based semiconductor laser whose output has recently been greatly increased and the wavelength range has been greatly expanded is particularly preferable because of its good compatibility with the fiber system. In order to obtain a high output, a laser obtained by converting the wavelength of a solid laser, fiber laser or the like with a nonlinear optical element can be used as an excitation light source.

Regarding the relationship between the rare earth to be added and the excitation wavelength, for example, when Pr is added to fluoride glass, excitation is performed at 420 nm to 490 nm, and laser oscillation is possible in the 490 nm band, 520 nm band, 600 nm band, 710 nm band, and the like. Further, excitation is performed at 560 nm to 600 nm, and laser oscillation is possible in the 600 nm band, the 710 nm band, and the like. In particular, the 600 nm band has a wide laser oscillating band and can oscillate in a very wide wavelength range from 598 nm to 643 nm, and can be applied to a broadband wavelength tunable laser and an ultrashort pulse laser.

For example, when Nd is added to fluoride glass, excitation is possible in the band of 320 nm to 370 nm, 420 nm band, 440 to 480 nm, and laser oscillation is possible in the 490 nm band, 850 to 900 nm band, 1 μm band, and the like. Further, excitation is performed at 490 nm to 540 nm, 550 nm to 600 nm, and laser oscillation is possible in the 850 to 900 nm band, 1 μm band, and the like.

For example, when Sm is added to fluoride glass, excitation is possible particularly in the 400 nm band, and laser oscillation is possible in the 420 nm band, 450 nm band, 560 nm band, 595 nm band, 640 nm band, and 700 nm band.

For example, when Eu is added to fluoride glass, excitation is performed in the 340 to 410 nm band and the 465 nm band, and laser oscillation is possible in the 590 nm band, 613 nm band, and 696 nm band.

For example, when Tb is added to fluoride glass, it is excited in the 340 nm to 390 nm band and can oscillate in the 488 nm band and the 543 nm band. Further, it can be excited in the 470 to 500 nm band and can oscillate in the 543 nm band.

For example, when Dy is added to fluoride glass, excitation is performed in the 320 nm band, 350 nm band, and 364 nm band, and laser oscillation is possible in the 370 nm band, 480 nm band, 575 nm band, 660 nm band, 749 nm band, and the like. Further, excitation is possible in the 387 nm band and 450 nm band, and laser oscillation is possible in the 480 nm band, 575 nm band, 660 nm band, 749 nm band, and the like.

For example, when Ho is added to fluoride glass, excitation is performed in the 360 nm band, 415 nm band, and laser oscillation is possible in the 430 nm band, 545 nm band, 658 nm band, and 750 nm band. Further, it can be excited in the 450 nm to 500 nm band and can oscillate in the 545 nm band, 658 nm band, and 750 nm band. Further, it is excited in the 640 nm band and can oscillate in the 658 nm band and the 750 nm band.

For example, when Er is added to fluoride glass, excitation is possible in the 378 nm band, 405 nm band, 440 nm band, 485 nm band, 520 nm band, and 540 nm band, and laser oscillation occurs in the 543 nm band, 667 nm band, 850 nm band, and the like. Further, excitation is possible in the 650 nm band, and laser oscillation occurs in the 667 nm band, the 850 nm band, and the like.

For example, when Tm is added to fluoride glass, excitation is performed in the 355 nm band, 455 nm to 485 nm band, and laser oscillation is possible in the 480 nm band, 650 nm band, 795 nm band, and the like.

In addition, by dividing the rare earth doped fiber into at least two and making a fiber ring laser in which the pumping conditions can be individually controlled, it becomes possible to configure the optical configuration symmetrically, clockwise (CW) The same excitation condition and resonator loss can be realized counterclockwise (CCW).

In addition, by providing at least one saturable absorber operating in the laser oscillation wavelength of the fiber ring laser in the ring laser resonator, it is possible to absorb unnecessary scattered light and spontaneous emission light, but to pass the laser light. Can be adjusted. As the saturable absorber, a rare earth-doped fiber, a transition metal-doped fiber, a dye-added plastic, or the like having a slight absorption at a target laser oscillation wavelength is suitable. For example, for laser oscillation of a Pr-doped fluoride fiber, a fluoride fiber or quartz fiber doped with Nd, Er, Sm, Tm, or the like can be used. Thus, it may be effective to mix different types of rare earth-doped fibers. As the saturable absorber, a semiconductor thin film element, a rare earth-added crystal, a transition metal-added crystal, an element using a carbon nanotube, or the like can be used in addition to the above candidates.

Also, in order to adjust the absorption saturation power of the saturable absorber, it is possible to excite the saturable absorber and adjust the laser oscillation state. For example, when a Pr-doped fluoride fiber is excited at 440 nm and lasing at 520 nm, when an Er-doped fluoride fiber is used as a saturable absorber, the Er-doped fluoride fiber is excited at 440 nm and absorbed saturation power If it is configured to be lowered, it can be used for laser oscillation wavelength control and power control in the 520 nm band. It can also be used to select continuous laser oscillation or pulsed laser oscillation. In this way, a saturable absorber having an appropriate amount of absorption is obtained by setting at least one of the rare earth-doped fibers in a non-excited or low-pumped state and adjusting the resonance state and balance of the fiber ring laser. It becomes possible to operate.

Furthermore, in addition to selection of pulsed laser oscillation by saturable absorption control, if the dispersion amount of the ring laser resonator is controlled by a technique such as fiber Bragg grating, a short pulse with a pulse width of several tens of nanoseconds or less can be obtained. it can.

In addition, by providing at least one fluoride fiber having a core transparent to the excitation wavelength and the laser oscillation wavelength in the ring laser resonator, quartz-based fibers used in the ring laser resonator and others It is possible to compensate for the change in the optical length due to the temperature dependence of the optical component, and to perform an operation that apparently suppresses the temperature dependence. As a result, it is possible to suppress frequency fluctuations, power fluctuations, detection sensitivity fluctuations at low angular velocities, and the like caused by the change in optical length.

Here, the negative dn / dT value of the fluoride glass is almost the same as the positive dn / dT value of optical materials used for other optical components, but at least in the laser medium. It is not uncommon for the energy difference between the excitation light wavelength and the laser oscillation wavelength to be converted into heat and become about ten degrees higher than the ambient temperature. For this reason, the fluoride glass as the laser medium can increase the compensation amount of the optical length even when it is shorter than the length of other general optical materials. For example, the dn / dT value of fluoride glass is about −15 × 10 ⁻⁶ / K, and that of quartz glass is 11.9 × 10 ⁻⁶ / K. However, when the fluoride glass is a laser medium, the temperature fluctuation is larger than that of the quartz fiber, so that it can be compensated with a length of 50% or less with respect to the quartz fiber. The optimum compensation length varies depending on the excitation conditions, the heat dissipation state of the fiber, the Ge addition amount of the silica fiber used, etc., but it cannot be generally stated, but in general, about 5 to 20% of the length of the silica fiber is appropriate. is there. When the quartz fiber is a pure silica core fiber, the length of the fluoride fiber relative to the quartz fiber is suitably about 1 to 15%.

However, the length adjustment of rare-earth doped fibers is difficult to fine-tune because priority is given to the output characteristics and mode characteristics of the fiber ring laser. Therefore, by using an appropriate amount of a fluoride fiber transparent to the pumping light and laser light in the ring laser resonator, the temperature dependence of the optical length is compensated without affecting the optical characteristics of the fiber ring laser, It is possible to suppress power fluctuations due to the longitudinal mode hop of the fiber ring laser and detection sensitivity fluctuations at a low angular velocity.

The appropriate length of the fluoride fiber that is transparent at the laser light wavelength and the pump light wavelength varies depending on the resonator configuration and the excitation state of the rare earth-doped fiber. A range of about 5 to 50% of the total silica fiber length in the ring laser resonator is preferable. This compensation makes it possible to suppress changes in the resonator length to about several μm / ° C., and to easily suppress longitudinal mode hops due to temperature changes.

If gain hole burning occurs, it becomes difficult to oscillate the CW light and CCW light at close wavelengths. For this reason, if there is one rare earth doped fiber in the ring laser resonator, it will be at an arbitrary location in the ring laser resonator, and if there are multiple rare earth doped fibers in the ring laser resonator, its center or rare earth doped It is preferable to insert a Faraday rotator, optical rotator, polarization controller, polarization scrambler, etc. between the fibers to avoid oscillation in a specific polarization state. As the optical rotator, a transparent crystal having a wide wavelength range is suitable. The polarization controller is preferably of a type that utilizes the twist of a fiber or a type that adjusts the angle between the paddles that controls the winding diameter and the number of turns. Other polarization controllers include those using a birefringent crystal such as calcite and those using a polarization maintaining fiber, which can be used according to the purpose. For the polarization scrambler, a device in which a plurality of stages are fused by shifting the angle of the stress applying portion within the fiber cross section is commercially available and is preferable because it is easily connected to a ring laser resonator. In addition to these methods, it is preferable to limit the oscillation wavelength by inserting a narrow band filter or the like in the ring laser resonator.

In applications that use interference between CW light and CCW light, the remaining optical power that has been returned can be circulated many times by setting the TAP rate (the ratio of extracted power) of the fiber ring laser to 50% or less. Thus, the same effect can be obtained as when the interference fiber length is made longer and the sensitivity is increased. Here, when the TAP rate is lowered, the effective number of rotations increases, so the sensitivity is improved, but the power that can be extracted becomes small, so a low-noise detection method or an improvement in the optical power in the resonator is required. On the other hand, when the TAP rate is increased, the sensitivity decreases, but the power that can be extracted increases, so that detection becomes easy. The optimal TAP rate varies depending on the rare earth species, doping concentration, pumping wavelength, pumping power, fiber structural parameters, loss in the ring laser resonator, etc., but cannot be specified unconditionally, but generally from 0.1% It is in the range of 50%. In the case of a highly accurate interference light source ring laser, the TAP rate is preferably 0.1% or more and 20% or less. When the TAP rate is less than 0.1%, the optical power incident on the light receiving element becomes extremely low, and detection with high accuracy becomes difficult. On the other hand, if it exceeds 20%, it becomes difficult to obtain the effect of improving the sensitivity due to the multiple turns.

Further, using the fiber ring laser described so far, a beat signal obtained by extracting a clockwise (CW) laser signal and a counterclockwise (CCW) laser signal and causing the CW laser signal and the CCW laser signal to interfere with each other. A fiber ring laser gyro (F-RLG) can be constructed using an angular velocity detection method for detecting the above.

When applied as an F-RLG, the CW light and CCW light extracted from the ring laser resonator by the TAP are interfered by the 3 dB coupler through the optical isolator, and the interference output is incident on the photodiode and converted into an electric signal. By providing at least one optical isolator between the TAP coupler and the light receiving element, the reflected return light on the light receiving element side can be suppressed, and the operation of the fiber ring laser can be stabilized and the destruction can be prevented. Not only the fiber ring laser but also a functional component using a fiber as a gain medium is very susceptible to the return light. In particular, at the end of the fiber, there is a risk of the light receiving element approaching, the angle between the light receiving surface and the fiber end surface being parallel, and the return light re-entering the fiber. In the worst case, the fiber ring laser may be destroyed. There is. Such an accident can be prevented by inserting an optical isolator between the TAP coupler and the detection unit. As a light receiving element, not only a PIN photodiode or an avalanche photodiode can be used as a simple light receiving element formed of a single element, but also a line sensor or an image pickup element formed of a large number of elements can be used. In addition, a fiber-type polarization controller can be installed between the optical isolator and the 3 dB coupler to control the polarization. Since polarization control can be performed with respect to either the extracted clockwise laser signal or counterclockwise laser signal, a fiber-type polarization controller can be installed between the optical isolator and the 3 dB coupler. Good.

The fiber ring laser of the present invention can be equipped with generally attached polarization control means and temperature control means. Also, a modulator can be inserted to initiate the mode lock operation. As the polarization control means, a polarization controller using a combination of fiber rings and a polarization controller using a combination of anisotropic optical crystals such as calcite and quartz can be used. For temperature control, electronic cooling using a Peltier device or the like, or a method using refrigerant circulation can be used, but electronic cooling is preferable from the viewpoint of vibration. As the modulator, a vibration type modulator that vibrates the fiber ring or an optical waveguide device using a crystal exhibiting an electro-optic effect can be used. Among them, a LiNbO ₃ (LN) microwave-coupled modulator is particularly preferable because it has a high modulation frequency and operates outside the band used as a sensor.

The entire fiber ring laser is housed in an iron alloy casing having good thermal conductivity, and a copper heat channel is provided in the casing to intensively cool the excitation laser, and the temperature of the entire casing is The casing is preferably feedback-controlled by bringing the Peltier element into contact with the casing so as to be uniformly stable. Furthermore, it is preferable to seal the outside of the housing with a high-performance heat insulating material in order to prevent heat from entering. In this case, the temperature fluctuation in the housing can be suppressed to ± 0.01 ° C./hr or less when the outside air temperature fluctuation is about 5 ° C./hr.

Furthermore, when applied as a ring laser gyro, in order to detect rotation at a low angular velocity, a technique called dithering that raises the apparent angular velocity by vibrating the entire fiber system can be used. In general, periodic vibrations caused by piezoelectric elements are used for dithering. By applying vibrations with a frequency higher than the audible range to the housing part and bobbin part to which the fiber is fixed, the frequency at a low angular velocity is used. It is possible to prevent a dead zone (rocking) from occurring due to the pull-in. In order to effectively remove the heat generated by the vibrator, it is preferable to provide a cooling mechanism that can cool the vibrator by disposing the vibrator in a cooling material having good thermal conductivity such as copper. For this cooling mechanism, it is particularly preferable to use electronic cooling using the Peltier effect from the viewpoint of preventing vibrations that cause noise.

In addition, as a method for obtaining the same effect as the above-described mechanical dithering, locking is prevented by applying temporal phase modulation to the laser light circulating in the ring laser resonator. You can also. For example, a sine wave can be used as the phase modulation waveform. As a phase modulation method, a device such as an LN modulator may be used, or a periodic stress can be applied to the fiber in the ring laser resonator with a piezoelectric element or the like.
Examples are shown below. However, the present invention is not limited to these examples.

The fiber ring laser unit used in the experiment will be described with reference to FIG. The feature of this arrangement is that most parts are arranged symmetrically, and consideration is given so that no difference occurs in the clockwise (CW) and counterclockwise (CCW) laser characteristics. In particular, the gain medium (fluoride fiber 20a, fluoride fiber 20b) and the saturable absorber (fluoride fiber 23a, fluoride fiber 23b) and their excitation sources (excitation semiconductor laser 22a, excitation semiconductor laser 22b, excitation) The semiconductor laser 25a for excitation and the semiconductor laser 25b for excitation are completely symmetrical. Further, in order to prevent polarization hole burning of the circulating fiber ring laser, a crystal rotator (crystal rotator 27) is inserted into the resonator, and the laser beam is polarized by 90 degrees each time the laser beam goes around the ring. Rotate. As a result, the power ratio between the CW light and the CCW light approaches 1: 1, and the power stability is improved.

The gain media (fluoride fiber 20a and fluoride fiber 20b) are fluoride fibers having the same composition, fiber parameters, and fiber length. Its core composition is 53ZrF ₄ -18.5BaF ₂ -3AlF ₃ -18.5NaF-3.2YF ₃ -3.5LaF ₃ -0.3ErF ₃ (the number before each component is mol%) It is glass. The clad composition is fluoride glass represented by 38ZrF ₄ -15HfF ₄ -20BaF ₂ -1LaF ₃ -3AlF ₃ -3YF ₃ -20NaF (the number before each component is mol%). The NA of the

fluoride fibers

20a and 20b is 0.13, the cutoff wavelength is 0.52 μm, and the core diameter is 3 μm. The fiber length is 60 cm. Both ends of these fibers are fused and connected to a quartz fiber 34 which is a main component of the ring laser resonator. The quartz fiber 34 has the same fiber parameters as the gain medium (fluoride fiber 20a and fluoride fiber 20b), and is fused and connected to the gain medium (fluoride fiber 20a and fluoride fiber 20b).

The saturable absorbing media (fluoride fiber 23a and fluoride fiber 23b) are fluoride fibers having the same composition, fiber parameters, and fiber length. Its core composition is fluoride glass represented by 53ZrF ₄ -18.5BaF ₂ -3AlF ₃ -18.5NaF-3YF ₃ -3.5LaF ₃ -0.5ErF ₃ (the numbers before each component are mol%). is there. The clad composition is fluoride glass represented by 38ZrF ₄ -15HfF ₄ -20BaF ₂ -1LaF ₃ -3AlF ₃ -3YF ₃ -20NaF (the number before each component is mol%). The NA of the

fluoride fibers

23a and 23b is 0.13, the cutoff wavelength is 0.52 μm, and the fiber length is 10 cm. Both ends of these fibers are fused and connected to a quartz fiber 34 which is a main component of the ring laser resonator. The quartz fiber 34 has the same fiber parameters as the saturable absorption medium (fluoride fiber 23a and fluoride fiber 23b), and is fused and connected to the saturable absorption medium (fluoride fiber 23a and fluoride fiber 23b).

There are four multiplexing / demultiplexing elements in the excitation light wavelength band of 440 to 450 nm and the signal light wavelength band of 540 to 550 nm (the multiplexing / demultiplexing element 21a, the multiplexing / demultiplexing element 21b, the multiplexing / demultiplexing element 24a, the multiplexing / demultiplexing element). The wave element 24b) is inserted into the quartz fiber 34 in the ring laser resonator, and each is used to introduce an excitation laser. Further, in order to remove excess excitation laser light of the opposing excitation laser between the two gain media (fluoride fiber 20a and fluoride fiber 20b), light having a wavelength of 540 nm to 550 nm is transmitted and a wavelength of 440 nm to A multiplexing / demultiplexing element 26 for removing excess excitation light of 450 nm outside the ring laser resonator is inserted.

励起 There are 2 types of excitation lasers, 2 each for a total of 4 units. For the excitation of the gain medium (fluoride fiber 20a, fluoride fiber 20b), a semiconductor laser with a fiber pigtail (excitation semiconductor laser 22a, excitation semiconductor laser 22b) having a wavelength of 444 nm and an output of 140 mW was used. For controlling the amount of absorption of the saturable absorber, a semiconductor laser with a fiber pigtail (excitation semiconductor laser 25a, excitation semiconductor laser 25b) having a wavelength of 448 nm and an output of 50 mW was used.

For the purpose of adjusting the polarization state in the ring laser resonator, a fiber type polarization controller (polarization controller 29) was installed in the ring laser resonator.

In order to extract CW light and CCW light from the ring laser resonator, a TAP coupler 28 that can extract 5% of the optical power in the ring is installed, and each isolator 30 is set so that reflected light does not return to the fiber ring laser during measurement. Installed in the port. The optical spectrum was measured with an optical spectrum analyzer 31 and the optical power was measured with an optical power meter 32 attached to the target port.

The length of the ring laser resonator in the above configuration is 5 m, and the fiber in the ring laser resonator other than the optical components is wound to a diameter of 6.7 cm. Gain medium excitation condition = 60 mW, saturable absorber excitation condition = 15 mW FIG. 2 shows a spectrum of light emitted from the ring laser measured in (1). It can be seen that the laser oscillates at a wavelength of 544 nm, and a visible light laser is obtained directly from the fiber ring laser. Moreover, the CW output and the CCW output were 0.52 mW and 0.54 mW, respectively, and the total calculation power was 1.06 mW, which coincided within the measurement error range.

The configuration is the same as in Example 1, but the laser oscillation experiment similar to that in Example 1 was performed (Nos. 1 to 8) with various changes in the type of fiber used, the rare earth to be added, the excitation laser wavelength, and the like. Table 1 summarizes the results of the measured oscillation wavelength and the total calculation force. Two types of pumping light were used separately: a semiconductor laser and a wavelength conversion laser in which the wavelength of the semiconductor laser oscillation was converted into half using a PPLN which is a SHG (Second Harmonic Generation) element. As a result, laser oscillation with a wavelength of less than 1 μm was obtained from any of the fiber ring lasers.

The same ring laser as in Example 1 was used for the ring laser part, a ring laser gyro was constructed, and the angular velocity was detected. The experimental configuration of the light source part is shown in FIG. In order to maximize the interference fringe contrast of the CW light or CCW light extracted from the TAP coupler 28, a fiber type polarization controller 33 for controlling the polarization state is installed. The CW light and the CCW light were caused to interfere with each other by the 3 dB coupler 34, and after passing through the optical isolator 35, the light intensity modulation signal after the interference was received by the silicon PIN light receiving element 36.

FIG. 4 shows an outline when the fiber ring laser unit and the electrical wiring of FIG. A Kovar alloy casing (ring laser casing 40) having a small coefficient of thermal expansion and relatively good thermal conductivity is surrounded by a Peltier element 43, and waste heat is radiated from the radiation fins 44 to reduce the temperature inside the casing. It can be kept uniform. When the internal set temperature was 25 ° C, forced air cooling was required when the ambient temperature was 40 ° C or higher. The internal temperature could be controlled to a constant 25 ° C. up to 60 ° C. by forced air cooling at a wind speed of 2 m / sec. Further, when the internal set temperature is 25 ° C., the control is difficult when the ambient temperature is −5 ° C. or lower. A connector (electric connector 41) is attached to the housing for supplying power to the Peltier element 43 and the four excitation lasers, supplying a control signal to the polarization controller, taking out the received interference signal, and the like.

Fig. 5 shows the setup for angular velocity detection. A housing 50 in which the fiber ring laser shown in FIG. 4 is accommodated, a ring laser control board 52, and an oscilloscope 51 for detecting an interference signal are installed and fixed on a turntable 53, and a rotation control computer (turntable control computer). 54), the interference signal was measured while controlling the rotational angular velocity.

FIG. 6 shows the relationship between the angular velocity and the interference frequency measured under temperature control setting = 25 ° C., gain medium excitation condition = 60 mW, and saturable absorber excitation condition = 15 mW. Sufficient interference contrast was obtained. As a low-speed rotation, an angular velocity of 110 ° / h and a beat frequency of 3 KHz were measured, but no rocking was observed.

20a,

20b Fluoride fibers

21a, 21b, 24a, 24b, 26 Multiplexing /

demultiplexing elements

23a,

23b Fluoride fibers

22a, 22b, 25a, 25b Excitation semiconductor laser 27 Crystal rotator 28 TAP coupler 29 Polarization controller 30 Optical isolator 31 Optical spectrum analyzer 32 Optical power meter 33 Fiber type polarization controller 34 3 dB coupler 35 Optical isolator 36 Silicon PIN light receiving element 40 Ring laser housing 41 Electrical connector 43 Peltier device 44 Radiation fin 50 Housing 51 containing fiber ring laser Oscilloscope 52 Ring laser control board 53 Turntable 54 Turntable control computer

Claims

In a fiber ring laser having a pumping light source for exciting a gain medium, at least one closed ring resonator (ring laser resonator), and a rare earth-doped fiber as the gain medium in the ring laser resonator,
As the rare earth-doped fiber, a fiber having a core to which at least one kind of rare earth selected from Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, and Tm is added is used, and the laser oscillation wavelength is less than 1 μm. A fiber ring laser characterized by having a short wavelength.
2. The fiber ring laser according to claim 1, wherein the rare earth-doped fiber is a fluoride glass fiber, and a semiconductor light source that emits a wavelength of 340 nm to 500 nm is used as the excitation light source.
3. The fiber ring laser according to claim 1, wherein at least two of the rare earth-doped fibers are provided, and excitation conditions of the rare earth-doped fibers can be individually controlled.
The fiber ring laser according to any one of claims 1 to 3, wherein the ring laser resonator includes at least one saturable absorber that operates at a lasing wavelength of the fiber ring laser. .
5. The device according to claim 1, wherein at least one fluoride fiber having a core transparent to an excitation wavelength and a laser oscillation wavelength is provided in the ring laser resonator. Fiber ring laser.
Angular velocity detection method for extracting a clockwise (CW) laser signal and a counterclockwise (CCW) laser signal from a ring laser resonator and detecting a beat signal obtained by causing the extracted CW laser signal and the CCW laser signal to interfere with each other In a fiber ring laser gyro (F-RLG) using
A fiber ring laser gyro using the fiber ring laser according to any one of claims 1 to 5.