JP2021100040A - Optical fiber output light source device - Google Patents

Abstract

To solve a problem in which a fiber laser that has been conventionally proposed as a white light source which is an important device for measuring the wavelength-dependent characteristics of an optical fiber and an optical component does not have sufficient characteristics for a high-precision measuring instrument in terms of high stability and high brightness.SOLUTION: An optical fiber output light source device includes an optical path in which an amplification optical fiber and a dispersion compensation optical fiber are connected via a polarization controller and a combiner, a 90° polarization rotation reflector connected to one end of the optical path, a single polarization reflection type polarization beam splitter connected to the other end of the optical path, a reflector arranged to face a reflected light plane of the single polarization reflection type polarization beam splitter, and an excitation light source connected to the combiner, and has unprecedented intensity stability and wavelength control performance, and can demonstrate sufficient performance as a white light source.SELECTED DRAWING: Figure 1

Description

The present invention relates to an optical fiber output light source device.

A white light source is an important device for measuring wavelength-dependent characteristics of optical fibers and optical components. Conventionally, in order to obtain a white light source, light from a halogen lamp or the like has been focused on a fiber to produce a fiber output. However, the core diameter of the fiber is about 10 μm, and high light output cannot be obtained due to the limitation of the brightness of the filament surface used in the halogen lamp.

In order to obtain high light output, there is a fiber laser in which a rare earth element or the like is doped in a fiber and excited by a semiconductor laser, but the fiber laser cannot be used as a substitute for a white light source because the spectrum width is narrow.

On the other hand, there is an ASE (Amplified Spontaneous Emission) light source that extracts a fluorescent component that is excited until just before laser oscillation and causes some induced amplification from the fiber.

Patent Document 1 discloses a technique for reducing a noise component (ASE optical component) generated by an optical amplifier when a short pulse light source is made to have multiple wavelengths by using an optical amplifier and a nonlinear optical medium. Specifically, the ASE optical component is interposed between the optical amplifier and the nonlinear optical medium.

Further, as a recent technique, a method (SC (Super Continuum) light source) of widening a band by incident a pulse generated in a fiber into a photonic band gap fiber or the like having a high nonlinear optical effect has been put into practical use.

Patent Document 2 discloses a method in which a fundamental wave pulse emitted from an ultrashort pulse laser light generator of picoseconds or less is widened by utilizing a self-phase modulation effect through a nonlinear optical material.

Japanese Unexamined Patent Publication No. 2007-178681 Japanese Unexamined Patent Publication No. 2001-083558

M. Martinelli, "Aniversal compression for polarization changes indeced by birefringence on a birefringence beam," Opt. Commun. , Vol. 72, no. 6, pp. 341-344, 1989

However, the ASE light source shown in Patent Document 1 and the like cannot be said to be sufficiently high to be used as a white light source when the fiber output is used. Further, although the SC light source disclosed in Patent Document 2 and the like can obtain a spectral width exceeding an octave, it is difficult to make it a stable light source because the nonlinear optical effect fluctuates due to the fluctuation of the incident pulse. In other words, it was difficult to perform highly accurate evaluation because the light source became unstable when the bandwidth was widened or the brightness was increased.

An object of the present invention is to provide a high-stable and high-luminance wideband light source in order to solve the above problems.

More specifically, the optical fiber output light source device according to the present invention is
An optical path in which an amplification optical fiber and a dispersion compensation optical fiber are connected via a combiner,
A 90 ° polarization rotation reflector connected to one end of the optical path,
A single polarization reflection type polarization beam splitter connected to the other end of the optical path,
A reflector arranged to face the reflected light plane of the single polarization reflection type polarizing beam splitter, and
With the combiner inserted in the optical path,
It is characterized by having an excitation light source connected to the combiner.

Since the optical fiber output light source device according to the present invention is configured to enable laser oscillation only when a nonlinear optical effect occurs in an optical system in which laser oscillation becomes difficult, a pulse is generated only when strong laser oscillation occurs. appear. Since this pulse is a bunch (group) of extremely short pulses of about 100 fs, a wide-band spectral characteristic can be obtained due to the non-linearity generated by a large group of short pulses. As a result, a stable wideband high-intensity light source can be constructed.

It is a figure which shows the structure of the optical fiber output light source apparatus which concerns on this invention. It is a figure which shows the internal structure of the single polarization reflection type polarization beam splitter. It is a figure explaining the basic operation of the optical fiber output light source apparatus which concerns on this invention. It is a figure explaining the operation when the optical fiber output light source device which concerns on this invention oscillates. It is a figure which shows the internal structure of the single polarization reflection type polarization beam splitter of another structure. It is a graph which shows the output spectrum characteristic of an optical fiber output light source apparatus. It is a graph which shows the waveform characteristic of the output of an optical fiber output light source apparatus. It is a graph which shows the waveform characteristic of the output when a highly nonlinear fiber is connected to a resonator. It is a graph which shows the time stability of the spectrum of SC light obtained by using a highly nonlinear fiber. It is a graph which shows the output spectrum of SC light and a fiber laser when the polarization state of the light in an optical path is changed by a polarization controller. It is a graph which shows the output spectrum of SC light and a fiber laser when the polarization state of the light in an optical path is changed by a polarization controller. It is a graph which shows the output spectrum of SC light and a fiber laser when the polarization state of the light in an optical path is changed by a polarization controller.

The optical fiber output light source device according to the present invention will be described below with reference to drawings and examples. The following description exemplifies one embodiment and one embodiment of the present invention, and the present invention is not limited to the following description. The following description can be modified without departing from the spirit of the present invention.

FIG. 1 shows the configuration of the optical fiber output light source device 1 according to the present invention. The optical path 10 is configured by connecting a dispersion compensating optical fiber (DCF: Dispersion Compensating Fiber) 12, an amplification optical fiber 14, and a polarization controller 70. Then, a combiner 16 is inserted in the optical path 10. The dispersion compensation optical fiber 12 may be located anywhere in the optical path 10. Further, the dispersion compensation optical fibers 12 may be located at a plurality of locations in the optical path 10. There may be a dispersion compensation optical fiber between the combiner 16 and the amplification optical fiber 14.

As the amplification optical fiber 14, an optical fiber to which rare earths such as Er (erbium), Pr (praseodymium), and Tm (thulium) are added can be preferably used. An erbium-containing optical fiber (EDF) can be preferably used.

The combiner 16 is a coupler that inputs excitation light into the optical path 10. An excitation light source 18 is connected to the combiner 16. A laser diode can be preferably used as the excitation light source 18.

The polarization controller 70 controls the polarization of the laser in the optical path 10. The polarization controller 70 is known to be of a bulk type, a fiber type, or the like, but is not particularly limited. Further, it is desirable that the polarization controller 70 can adjust the polarization state by a signal from the outside.

The dispersion compensating optical fiber 12 and the amplification optical fiber 14 constituting the optical path 10 are composed of only a horizontal single-mode fiber. Since these fibers do not use multimode fiber, they operate in a complete horizontal single mode. The optical fiber output light source device 1 does not use the plane of polarization preservation fiber. The plane of polarization preservation fiber is a fiber having a function of providing a propagation constant difference between modes and fixing the plane of polarization in the fiber within the cross section of the fiber in the modes of two orthogonal planes of polarization existing in the single mode fiber. Is. Therefore, if the plane-preserving fiber is used for pulse oscillation utilizing the rotation of the plane of polarization due to the nonlinear optical effect, which is the object of the present invention, laser oscillation does not occur.

Further, the present invention has a configuration capable of self-compensating for fluctuations (unstable fluctuations) in the plane of polarization due to reciprocal birefringence caused by bending methods, lateral pressures, temperature changes, etc. depending on the laying (installation) state of the fiber. ing. Therefore, it is not necessary to use the plane of polarization preservation fiber in the first place. This is because the self-compensation ability of the optical fiber output light source device 1 is due to the effect of the 90 ° polarization rotation reflector 22.

A 90 ° polarization rotation reflector 22 is provided at one end of the optical path 10. The 90 ° polarization rotation reflector 22 is a reflector in which the polarization planes of the incident light and the reflected light are rotated by 90 °. Alternatively, the 90 ° polarization rotation reflector 22 may be a plurality of components including means capable of obtaining the same effect as the reflector. For example, the Faraday rotating mirror can preferably be used as the 90 ° polarization rotating reflector 22.

Further, the 90 ° polarization rotation reflector 22 is not limited to the Faraday rotation mirror. For example, it is caused by the double refraction generated by the fiber used between the incident port 20a of the single polarization reflection type polarizing beam splitter 20 described later and the reflector placed at the position of the 90 ° polarization rotation reflector 22. A configuration that obtains the same effect by using an optical system that adjusts the polarization of the light can be used as the 90 ° polarization rotation reflector 22.

More specifically, first, the 90 ° polarization rotation reflector 22 is replaced with a normal dielectric multilayer mirror. Then, the birefringence generated by the fiber corresponding to the incident end of the dielectric multilayer mirror is compensated from the incident port 20a, and the polarized light is in a linear state when the reflected light returns to the single polarization reflection type polarizing beam splitter 20. Set to rotate 90 ° while maintaining. The dielectric multilayer mirror may be a metal vapor-deposited film mirror or the like.

Further, the light reflected from the dielectric multilayer mirror is transmitted and emitted without being reflected by the reflecting surface 44r inside the polarization beam splitter 44 (see FIG. 2) inside the single polarization reflection type polarizing beam splitter 20. The polarization state is adjusted using a wavelength plate or the like so that the light is output to the port 20b at a high rate. Even if such a configuration is used as the 90 ° polarization rotation reflector 22, it is possible to obtain the same operation as when the Faraday rotation mirror is used, and pulse oscillation becomes possible.

A single polarization reflection type polarization beam splitter 20 is provided at the other end of the optical path 10. The single polarization reflection type polarization beam splitter 20 reflects only the light having one of the two planes of polarization out of two orthogonal planes of polarization, and transmits the light having the other plane of polarization. Therefore, the single polarization reflection type polarization beam splitter 20 has an entrance port 20a and an exit port 20b. The optical path 10 is connected to the incident port 20a.

The combiner 16 is connected to the amplification optical fiber 14, and the excitation light is on the side to which the single polarization reflection type polarizing beam splitter 20 is connected or the side to which the 90 ° polarization rotation reflector 22 is connected. It may be sent from either of. However, as shown in FIG. 1, the excitation light is most preferably sent toward the amplification optical fiber 14 connected to the 90 ° polarization rotation reflector 22.

The resonator 32 is formed by a single polarization reflection type polarization beam splitter 20, an optical path 10, and a 90 ° polarization rotation reflector 22 that rotates the polarization plane by 90 °. The resonator 32 is the minimum configuration of the optical fiber output light source device 1 according to the present invention.

An optical fiber 24 is connected to the exit port 20b of the single polarization reflection type polarization beam splitter 20. The optical fiber 24 is provided with an antireflection device 26. The optical fiber 24 allows light to pass from the single polarization reflection type polarization beam splitter 20 to the subsequent stage in only one direction. Here, the "second stage (same below)" refers to the configuration in the output direction from the constituent elements.

The antireflection device 26 is a means for preventing reflection from returning the reflected light from the outside of the resonator 32. The antireflection device 26 is not limited to a device such as an isolator, but may be a means such as obliquely polishing the connector end face of the exit end of the light emitting port 28 or applying a non-reflective coating to the connector end face of the light emitting end of the light emitting port 28. Good.

That is, if such a measure is applied to the light emitting port 28, it can be said that the antireflection device 26 is provided. Further, it can be said that the antireflection device 26 is provided at the outlet 20b of the single polarization reflection type polarization beam splitter 20. Further, the single polarization reflection type polarization beam splitter 20 may have the same function as the antireflection device 26 inside.

A light emitting port 28 is formed at the end of the antireflection device 26. On the exit side of the single polarization reflection type polarization beam splitter 20, the optical pulse intended by the optical fiber output light source device 1 according to the present invention has already been completed. Therefore, as the optical fiber 24, a desired optical fiber may be used depending on the purpose of use of the optical fiber output light source device 1. For example, a fiber such as a plane of polarization preservation fiber may be used.

A highly non-linear fiber 30 such as a photonics crystal fiber may be connected to the subsequent stage of the resonator 32. Further, the highly non-linear fiber 30 may be connected to the subsequent stage of the antireflection device 26. As shown in Examples described later, the optical fiber output light source device 1 according to the present invention exhibits a stable output spectrum over a wide wavelength range. Therefore, if the output light of the optical fiber output light source device 1 is passed through the highly nonlinear fiber 30, a light source capable of stably outputting light having a wider spectrum band can be obtained. As the optical fiber 24, an optical fiber other than the horizontal single mode fiber may be used.

In particular, by connecting a highly non-linear fiber having a characteristic of zero dispersion with small dispersion to the highly non-linear fiber 30, a wide-band spectral light capable of generating the resonator 32 can be converted into a wider-band spectrum light (SC (Super Continuum) light). Can be taken out as.

More specifically, the "highly non-linear fiber (30)" in the present invention means that the range of the dispersion value is from -1 to 3 at the maximum in the effective spectrum of the laser light output from the resonator 32, centering on zero dispersion. There is a region in the range of [ps / (nm · km)]. Further, more preferably, the "highly non-linear fiber (30)" is a fiber having a region in the range of 0 to 1 [ps / (nm · km)].

Further, it is desirable that the "highly nonlinear fiber (30)" has a flat dispersion in the effective spectrum. Preferably, the spectrum can be expanded if it is ± 0.1 [ps / (nm ^{2 · km)] or less.}

Here, the effective spectrum refers to the wavelength width of −30 dB from the maximum value of the envelope obtained by obtaining the envelope of the spectrum of the laser light output from the resonator 32. In addition, there may be a wavelength region without light in the effective spectrum. Needless to say, the region having the above dispersion value range is a region (wavelength region) in which light exists in the effective spectrum.

Further, in the single polarization reflection type polarizing beam splitter 20, the combiner 16, the antireflection device 26, etc., which are optical fiber input / output, fibers are attached in advance to these parts, and the optical fiber 12 for dispersion compensation is attached to the fiber. And the amplification optical fiber 14 may be connected by means such as fusion splicing or connector connection.

Further, a spectrum monitor 72 can be arranged after the highly nonlinear fiber 30. Further, a control device 80 for receiving the output of the spectrum monitor 72 may be provided. The control device 80 is connected to the polarization controller 70. As in the embodiment described later, the state of the output spectrum of the highly non-linear fiber 30 can be controlled by changing the polarization state in the optical path 10 with the polarization controller 70. That is, the control device 80 that adjusts the polarization controller 70 according to the output of the spectrum monitor 72 can control the adjustment of the output light band.

FIG. 2 shows the internal configuration of the single polarization reflection type polarization beam splitter 20. The single polarization reflection type polarization beam splitter 20 includes an incident lens 40, an exit lens 42, a polarization beam splitter 44, and a reflecting mirror 46. The incident lens 40 is arranged immediately after the incident port 20a. In other words, the incident lens 40 is arranged in the single polarization reflection type polarizing beam splitter 20 at a position optically opposed to the dispersion compensation optical fiber 12 which is the other end of the optical path 10.

The exit lens 42 is arranged in the single polarization reflection type polarization beam splitter 20 immediately before the exit port 20b. The optical fiber end face corresponding to the entrance port 20a and the exit port 20b inside the single polarization reflection type polarizing beam splitter 20 is subjected to non-reflection treatment to prevent Fresnel reflection generated at the interface with air on the end face. Has been done. This is to prevent an unexpected laser cavity from being accidentally configured.

The polarization beam splitter 44 is arranged between the incident lens 40 and the exit lens 42. The polarization beam splitter 44 has a reflection surface 44r inside which only a specific polarization surface passes through. Further, the polarization beam splitter 44 includes an incident light surface 44a for incident light, a transmitted light surface 44b for emitting only light from a specific polarization surface, and light in which the transmitted light and the polarization surface are rotated by 90 °. It also has a reflected light surface 44c that emits light.

The polarization beam splitter 44 is arranged so that the incident light surface 44a faces the incident lens 40 and the transmitted light surface 44b faces the emitting lens 42. Further, the reflecting mirror 46 is arranged so as to face the reflected light surface 44c. As the reflecting mirror 46, a high-reflection mirror can be preferably used.

The operation of the optical fiber output light source device 1 having the above configuration will be described. In the following operation description, the operation will be described when there is no polarization controller 70 in the optical path 10. First, the operation of the single polarization reflection type polarization beam splitter 20 will be described with reference to FIG. Light enters the incident port 20a from the optical path 10. This optical LB1 has planes of polarization that are orthogonal to each other. Let this be an S wave and a P wave. The S wave is a polarized wave that oscillates in the incident surface including the normal of the reflecting surface 44r of the polarized beam splitter 44 and the light traveling direction, and the P wave is a polarized wave that oscillates in the plane perpendicular to the incident surface. is there. In FIG. 2, it is described as LB1 (S, P).

The light is spread afoally by the incident lens 40 and becomes parallel rays. Then, it is incident on the polarization beam splitter 44 from the incident light surface 44a. The light on one of the polarization planes of the light incident on the polarization beam splitter 44 passes through the reflection surface 44r of the polarization beam splitter 44 and is emitted from the transmitted light plane 44b. It is assumed that the polarized light that can be transmitted now is a P wave. The light LB3 (P) is focused by the exit lens 42 and emitted from the exit port 20b.

On the other hand, the S wave is reflected by the reflecting surface 44r and emitted from the reflected light surface 44c. This was described as optical LB2 (S). Since the reflecting mirror 46 is arranged to face the reflected light surface 44c, the S wave is reflected and incident on the polarization beam splitter 44 again from the reflected light surface 44c. The light LB4 (S) is reflected again by the reflecting surface 44r, emitted from the incident light surface 44a, and returns to the optical path 10 (light LB5 (S)). The reflecting mirror 46 may be formed on the surface of the reflected light surface 44c by a method such as thin film deposition.

As described above, the single polarization reflection type polarization beam splitter 20 transmits one of the orthogonal polarization planes, reflects the other, and returns it to the optical path 10.

FIG. 3 shows the basic operation of the optical fiber output light source device 1. As the excitation light L1 from the excitation light source 18, a semiconductor laser having a single polarization or the like can be usually used. Since the excitation light L1 excites the amplification optical fiber 14 and is converted into fluorescence L2 (P, S) having other wavelengths, the polarization state of the excitation light L1 is described below in the generation of fluorescence and pulsing. Does not affect the mechanism of. The plane of polarization of fluorescence L2 (P, S) does not have a specific polarization and is non-polarized. However, here, for the sake of explanation, the P wave and the S wave with respect to the reflecting surface 44r are described.

The fluorescence L2 (P, S) is reflected by the 90 ° polarization rotation reflector 22 (fluorescence L3 (S, P)). At this time, the plane of polarization is rotated by 90 °. The fluorescence L3 (S, P) is incident on the single polarization reflection type polarization beam splitter 20 via the amplification optical fiber 14 and the dispersion compensation optical fiber 12 (fluorescence L4 (S, P)).

At this time, the fluorescence L4 (S, P) was initially generated by the amplification optical fiber 14, and the fluorescence component reflected by the 90 ° polarization rotation reflector 22 was amplified by stimulated emission by the amplification optical fiber 14. Includes components and newly added fluorescence in the amplification optical fiber 14. Therefore, the light after fluorescence L4 (S, P) is referred to as induced light.

In the single polarization reflection type polarization beam splitter 20, as described above, the P wave is transmitted (guided light L6 (P)), the S wave is reflected (guided light L5 (S)), and the optical path is again. It is returned to 10 (guided light L7 (S)).

The guided light L7 (S) returns to the optical path 10 again and is amplified by the amplification optical fiber 14, and then input to the 90 ° polarization rotation reflector 22 (guided light L8 (S)). The polarization plane of the induced light L8 (S) is rotated by 90 ° again by the 90 ° polarization rotation reflector 22, and becomes a P wave (guided light L9 (P)).

The induced light L9 (P) passes through the optical path 10 and is amplified again by the amplification optical fiber 14, and then incident on the single polarization reflection type polarizing beam splitter 20 (guided light L10 (P)). Since the guided light L10 (P) is a P wave, it is transmitted through the single polarization reflection type polarizing beam splitter 20 (guided light L11 (P)) and is emitted from the light emitting port 28.

As described above, in the optical fiber output light source device 1 according to the present invention, since all the induced light is transmitted through the single polarization reflection type polarizing beam splitter 20 and emitted from the light emitting port 28, laser oscillation is basically performed. It is a difficult structure.

Next, refer to FIG. As shown in FIG. 3, the optical fiber output light source device 1 according to the present invention basically has a structure in which laser oscillation is difficult. However, since the optical path 10 is composed of an optical fiber, there is light having a polarization plane deviated from the polarization state of the S wave or the P wave in a state where the nonlinear optical effect is generated and the nonlinear optical effect is not generated. To do. This deviation of the plane of polarization occurs more strongly as the optical power increases. As a result, laser oscillation occurs even with the configuration shown in FIG.

The excitation light L1 from the excitation light source 18 is introduced into the optical path 10 from the combiner 16. The excitation light L1 may be of a single polarization that is usually used by using a semiconductor laser or the like. The excitation light L1 excites the amplification optical fiber 14 and is converted into fluorescence (spontaneous emission light) L2 (P, S) having another wavelength. The plane of polarization of fluorescence L2 (P, S) is in a non-polarized state, and the degree of polarization is zero (zero).

The fluorescence L2 (P, S) is reflected by the 90 ° polarization rotation reflector 22 (fluorescence L3 (S, P)). At this time, the plane of polarization is rotated by 90 °. The fluorescent L3 (S, P) is incident on the single polarization reflection type polarizing beam splitter 20 via the amplification optical fiber 14 and the dispersion compensation optical fiber 12 (guided light L4 (S, P)).

At this time, the fluorescence L4 (S, P) was initially generated by the amplification optical fiber 14, and the fluorescence component reflected by the 90 ° polarization rotation reflector 22 was amplified by stimulated emission by the amplification optical fiber 14. Includes components and newly added fluorescence in the amplification optical fiber 14. Therefore, the light after fluorescence L4 (S, P) is referred to as induced light.

In the single polarization reflection type polarization beam splitter 20, the P wave (guided light L6 (P)) is transmitted. However, the S wave contained in the guided light L4 (S, P) is reflected by the reflecting surface 44r (guided light L5 (S)), reflected by the reflecting mirror 46, and reflected again by the reflecting surface 44r to the optical path 10. It is returned (guided light L7 (S)). Here, the P wave is not included in the light that is reflected and returns to the optical path 10.

The guided light L7 (S) returns to the optical path 10 again, is amplified by the amplification optical fiber 14, and then is input to the 90 ° polarization rotation reflector 22 (guided light L8 (S, (P))). At this time, the induced light L8 (S, (P)) is affected by the non-linear optical effect generated in the optical path 10, so that the component of strong light causes the rotation of polarized light and has the (P) wave component. Become. In the induced light L8 (S, (P)), the plane of polarization is rotated by 90 ° again by the 90 ° polarization rotation reflector 22, the S wave becomes a P wave, and the reflected (P) wave becomes an (S) wave. (Induction light L9 (P, (S)).

The induced light L9 (P, (S)) passes through the optical path 10 and is amplified again by the amplification optical fiber 14 and incident on the single polarization reflection type polarizing beam splitter 20 (guided light L10 (P, (S))). ). Since the induced light L10 (P, (S)) is amplified by the amplification optical fiber 14, the component having a strong light intensity receives a nonlinear optical effect stronger than that of L8 (S, (P)). The polarization rotates more greatly, and the (S) polarization component is generated.

Of the amplified (guided light L10 (P, (S)), the P wave passes through the single polarization reflection type polarizing beam splitter 20 (guided light L12 (P)) and is emitted from the light emitting port 28. On the other hand, the (S) wave, which is a component orthogonal to the (P) wave, is reflected again and returns to the optical path 10 (guided light L13 (S)).

The induced light L11 (S) is amplified more than the induced light L5 (S), and as a result, the induced light L13 (S) is stronger than the induced light L7 (S) and the time width is narrowed.

Through the above steps, the fluorescent component having the (P) wave and the (S) wave strengthens the intensity fluctuation while repeating the reciprocation of the optical path 10.

By the way, the induced light L13 (S) is amplified by the amplification optical fiber 14 until it reaches the 90 ° polarization rotation reflector 22, and is further strongly affected by the non-linear optical effect.

That is, the strong component receives a stronger nonlinear optical effect, and the induced light L13 (S) generates the (P) component more strongly due to the rotation of the polarized light. On the other hand, the weak component does not change significantly from the (S) polarization. On the reflecting surface 44r, a component having a weak intensity is not easily reflected. Therefore, the low-intensity component is not reflected by the reflecting surface 44r and the reflecting mirror 46 of the polarization beam splitter 44, and the component re-entered into the optical path 10 is relatively lower than the high-intensity component.

Due to this effect, the intense induced light component becomes stronger. When the intensity of the incident light to be amplified, that is, the induced light referred to here increases, the gain of the amplification optical fiber 14 is saturated, and the loss and the gain in the resonator 32 (see FIG. 1) are balanced. Therefore, the low-intensity induced light component that cannot sufficiently obtain the effect of reflection by the single polarization reflection type polarizing beam splitter 20 has a high loss and is relatively attenuated because the loss is higher than the gain. As a result, only the high-intensity component of the induced light component is selectively and repeatedly amplified to grow into a short pulse.

It should be noted that such an effect is obtained by returning the component whose plane of polarization is rotated by the nonlinear optical effect in the optical path 10 to the optical path 10 again. In the above description, the (P) wave component in L8 (S, (P)) generated from L7 (S) is the (S) wave component of L9 (P, (S)) generated by the Faraday rotating mirror 22. is there. The (S) wave component is further subjected to non-linear polarization rotation by the dispersion compensating optical fiber 12 and the amplification optical fiber 14, reflected by the reflecting mirror 46 in the beam splitter 20, and returned to the optical path 10 as L13 (S). Has been done.

By repeating this action, the change in light intensity is emphasized, and finally the strongest light that can be generated by the entire optical system remains in the resonator 32, and the reflector 46 and the 90 ° polarization rotation reflector. Laser resonance is established between 22 and pulse oscillation is continuously generated.

The light reflected by the 90 ° polarization rotation reflector 22 plays an important role in emphasizing the intensity fluctuation. However, not only that, the 90 ° polarization rotation reflector 22 exchanges the (S) wave and the (P) wave on the outward path and the return path at the time of reflection. Therefore, the (P) wave receives the linear polarization fluctuation caused by the birefringence of the fiber caused by the temperature change and deformation of the fiber received by the (S) wave on the outward path. Similarly, the (S) wave receives the polarization fluctuation of the outward path received by the (P) wave on the return path.

As a result, the resonator 32 is compensated for fluctuations in linear polarization. In other words, in the resonator 32, actions such as reciprocal polarization fluctuations are canceled (Non-Patent Document 1). That is, once oscillation occurs in the resonator 32, it can be said that the oscillation is extremely robust against linear polarization fluctuations caused by birefringence of the fiber caused by temperature change or deformation of the fiber.

Due to this effect, the resonator 32 device of the optical fiber output light source device 1 according to the present invention has high resistance to environmental changes and vibrations, and enables highly robust laser oscillation that can obtain stable oscillation without adjustment.

Further, when the pulse is amplified by stimulated emission, the energy of the pulse becomes high, and the amplification is repeated while reciprocating in the resonator 32 and narrowing the pulse width. As the pulse width becomes narrower, the spectral width of the pulse related to the Fourier transform widens. Then, when the energy of the pulse is saturated and stabilized, the bunch composed of the pulse having a narrow pulse width and a wide spectrum width is stably output.

The rotation of the polarized wave due to the non-linear optical effect shifts the polarization rotation effect by the 90 ° polarization rotation reflector 22 from 90 °. Therefore, the (S) wave component contained in the pulse component that is not a perfect (P) wave on the reflecting surface 44r is reflected by the reflecting mirror 46 and returned to the optical path 10 again. At the same time, the remaining (P) wave component that has not been returned to the optical path 10 passes through the single polarization reflection type polarizing beam splitter 20 and is emitted from the light emitting port 28. As a result, a short pulse laser beam having a pulse length of femtoseconds can be obtained from the emission port 28.

The induced light oscillates at a single wavelength, but the frequency band is widened by shortening the pulse. In the optical fiber output light source device 1 according to the present invention, as shown in Examples described later, pulse light emission in a band of 100 nm or more can be obtained.

FIG. 5 shows the internal configuration of a single polarization reflection type polarization beam splitter having another configuration. This is referred to as a single polarization reflection type polarization beam splitter 50. The single polarization reflection type polarization beam splitter 50 includes an incident lens 60, an emitting lens 62, a birefringence prism 64, and a reflecting mirror 66. The incident lens 60 is arranged immediately after the incident port 60a. In other words, the incident lens 60 is arranged in the single polarization reflection type polarizing beam splitter 50 at a position optically opposed to the dispersion compensation optical fiber 12 which is the other end of the optical path 10.

The exit lens 62 is arranged in the single polarization reflection type polarization beam splitter 50 and immediately before the exit port 60b.

The birefringence prism 64 is arranged between the incident lens 60 and the exit lens 62. The birefringent prism 64 has an incident light surface 64a for incident light and a transmitted light surface 64b for emitting transmitted light.

The birefringence prism 64 is arranged so that the incident light surface 64a faces the incident lens 60 and the transmitted light surface 64b faces the emitting lens 62. On the transmitted light surface 64b, the reflecting mirror 66 is arranged so as to block a portion through which normal light rays pass from the optical axis of the birefringence prism 64. As the reflecting mirror 66, a high-reflection mirror can be preferably used.

The single polarization reflection type polarization beam splitter 50 configured in this way also has the same function as the single polarization reflection type polarization beam splitter 20 described with reference to FIG. That is, the normal light ray Lnr and the abnormal light ray Lab of the birefringence prism 64 are generated by different polarization directions.

Therefore, the abnormal light ray Lab is emitted from the exit port 60b, and the normal light ray Lnr is reflected and returns to the optical path 10. Therefore, even if the single polarization reflection type polarization beam splitter 20 is replaced with the single polarization reflection type polarization beam splitter 50, the optical fiber output light source device 1 according to the present invention can be obtained.

Further, as the single polarization reflection type polarization beam splitter 20, a single polarization reflection type polarization beam splitter 20 may be used which is composed of a melt-stretched optical fiber coupler in which two fibers are melt-stretched to exhibit polarization separation characteristics.

An example of the optical fiber output light source device according to the present invention is shown below. In the following description of the examples up to FIG. 9, examples will be described from FIG. 1 in a configuration in which the polarization controller 70 is not inserted. Specifically, it is the configuration of FIG. In the configuration of FIG. 4, a laser diode having a wavelength of 1480 nm was used as the excitation light source 18. As the amplification optical fiber 14, an erbium-containing optical fiber (EDF) was used at 15 m. The main specifications are a specific refractive index difference of about 1.5%, a blocking wavelength of 850 nm, a zero dispersion wavelength of about 1700 nm, and normal dispersion (normal dispersion) in the wavelength 1560 nm band.

As the dispersion compensating optical fiber 12, a single mode fiber was used at 30 m. The main specifications are a specific refractive index difference of 0.4%, a blocking wavelength of 1250 nm, a zero dispersion wavelength of about 1315 nm, and anomalous dispersion in the wavelength 1560 nm band.

With the above configuration, the resonator 32 can be oscillated at room temperature, and a short pulse laser beam is obtained from the outlet 20b of the single polarization reflection type polarizing beam splitter 20. FIG. 6 shows the results of the obtained laser light spectrum analyzer. With reference to FIG. 6, the horizontal axis is the wavelength (nm), the vertical axis is the light intensity (dBm / nm), and the spectral resolution is 1 nm. The solid line and the chain line are spectra obtained when the optical path 10 is stably obtained due to a difference in the conditions (difference in installation state). If the two spectra are called by their peak values, the solid line can be called the 1570 nm band and the chain line can be called the 1670 nm band.

The spectrum in the 1570 nm band had a band of about 130 nm, and the spectrum in the 1670 nm band showed a wide band of about 150 nm or more. Particularly characteristic is that in the band where the spectrum was obtained, at a light intensity of -40 dB / nm or more, there was no part such as drop or chip of the spectrum, and the intensity stability was less than 0.1 dB. From this, it was considered that the oscillating short pulse laser oscillated stably over the above band.

In the spectrum of the 1570 nm band, the maximum value was -19.3 dBm / nm, and the effective spectrum was 171.1 nm of 1524.9 nm to 1696.0 nm. In the spectrum of the 1670 nm band, the maximum value was −23.7 dBm / nm, and the effective spectrum was 204.3 nm of 1519.7 nm to 1724.0 nm.

FIG. 7 shows an example of a time waveform. With reference to FIG. 7, the horizontal axis is the time axis (5 ns / div) and the vertical axis is the intensity (500 mV / div). Further, in the small window, the horizontal axis is the time axis (1 μs / div), and the vertical axis is the intensity (arbitrary). The unit of the horizontal axis is different between the small window and the large window, and the large window is an enlarged view of a part of the small window.

With reference to the small window, it can be seen that there are about three waveforms in 1 μs, and that they oscillate stably with a repetition period of about 3 MHz. Also, referring to the large window, there was no drop or chip in the waveform when the oscilloscope was swept. Moreover, since a waveform having a substantially symmetrical shape is obtained, it can be seen that stable oscillation is performed.

Next, at the exit 20b (output end of the resonator 32) of the single polarization reflection type polarization beam splitter 20, a dispersion having a value close to zero dispersion near the center wavelength of the laser-output optical spectrum (see FIG. 6). A highly non-linear fiber having characteristics was connected as a high non-linear fiber 30. The specifications of the highly non-linear fiber 30 have a mode field diameter of 5 μm or less and a non-linear coefficient of 10 [1 / (Wm)] or more. The dispersion was 0.3 [ps / (nm · km)] at 1550 nm, and the fiber length was 400 m. Therefore, the highly nonlinear fiber 30 has a region having a dispersion value in the range of -1 to 3 [ps / (nm · km)] in the range of the effective spectrum.

FIG. 8 shows a graph of the output result. With reference to the graph of FIG. 8, the horizontal axis is the wavelength (nm) and the vertical axis is the intensity (dBm / nm). The dotted line shows the output of the resonator 32 (the output of the outlet 20b), and the solid line is the output of the highly non-linear fiber (highly non-linear fiber 30). As can be seen from FIG. 8, stable oscillation could be obtained in the band of 1100 nm to 1700 nm or more and 600 nm or more.

FIG. 9 shows the test results of stability. The horizontal axis is wavelength (nm) and the vertical axis is intensity (dBm / nm). The dotted line is the spectrum immediately after the start, and the solid line is the spectrum after 50 minutes. As can be seen from FIG. 9, there was no change in the spectrum so that the dotted line overlapped the solid line and could not be discriminated. That is, it was possible to obtain an SC (Super Continuum) light source with extremely high stability.

From the above, the optical fiber output light source device according to the present invention can be suitably used for a light source that requires highly accurate measurement. In addition, although it is basically a configuration that does not easily oscillate, once it oscillates, it has an optical configuration that stabilizes the oscillation conditions, so there is little change in characteristics due to long-term operation, and robustness against temperature fluctuations and vibrations. Is also expensive. Further, by connecting a highly nonlinear fiber having a value close to zero dispersion near the center wavelength of the oscillated spectrum, a very wide band SC light source can be obtained.

Next, an embodiment of the optical fiber output light source device 1 having the configuration shown in FIG. 1 will be described. The polarization controller 70 used was a paddle type. The control device 80 was omitted, and the experimenter controlled the polarization controller 70 instead. The fiber was branched into two by a tap coupler at the subsequent stage of the antireflection device 26, a highly non-linear fiber 30 was connected to one end, and the output was connected to the spectrum monitor 72. The other end was directly connected to the spectrum monitor 72.

Then, the resonator 32 was oscillated, and the output signal was examined while changing the polarization state of the laser with the polarization controller 70. The output of the highly non-linear fiber 30 is referred to as an SC spectrum, and the side to which the highly nonlinear fiber 30 is not connected (the "other end" above) is referred to as an FL spectrum.

The results are shown in FIGS. 10 to 12. Each figure represents a spectrum obtained as a result of changing the polarization state in the optical path 10. In each figure, the horizontal axis is the wavelength (nm) and the vertical axis is the intensity (dBm / nm).

First, when paying close attention to the FL spectrum (the output to which the highly nonlinear fiber 30 is not connected), the monomodal shape was maintained between the wavelengths of about 1520 nm and 1680 nm regardless of the polarization state. The intensity varied from -25 dBm / nm to -20 dBm / nm.

On the other hand, when paying close attention to the SC spectrum (output to which the high nonlinear fiber 30 is not connected), from FIG. 10 to FIG. 12, the intensity of -40 dBm or more is 1500 nm (FIG. 10), 1200 nm (FIG. 11), and 1100 nm (FIG. 12). ), And the intensity increased as it spread.

As described above, by adjusting the polarization controller 70 in the optical path 10, the wavelength spread and intensity could be controlled.

The optical fiber output light source device according to the present invention can be suitably used as a light source for various measurements.

1 Optical fiber output light source device 10 Optical path 12 Optical path for dispersion compensation 14 Optical fiber for amplification 16 Combiner 18 Excitation light source 20 Single polarization reflection type polarized beam splitter 20a Inlet port 20b Outlet port 22 90 ° polarization rotation reflection Instrument 24 Optical fiber 26 Anti-reflection device 28 Emitting port 30 Highly non-linear fiber 32 Resonator 40 Incident lens 42 Emission lens 44 Polarizing beam splitter 44a Incident light surface 44b Transmitted light surface 44c Reflected light surface 44r Reflecting surface 46 Reflector 50 Single Polarization Reflective Polarized Beam Splitter 60 Incident Lens 60a Incident Port 60b Exit Port 62 Exit Lens 64 Double Refractive Prism 64a Incident Light Surface 64b Transmitted Light Surface 66 Reflector 70 Polarization Controller 72 Spectrum Monitor 80 Control Device

Claims (11)

An optical path connecting an optical fiber for amplification and an optical fiber for dispersion compensation,
A 90 ° polarization rotation reflector connected to one end of the optical path,
A single polarization reflection type polarization beam splitter connected to the other end of the optical path,
A reflector arranged to face the reflected light plane of the single polarization reflection type polarizing beam splitter, and
With the combiner inserted in the optical path,
An optical fiber output light source device having an excitation light source connected to the combiner. The optical fiber output light source device according to claim 1, further comprising a polarization controller inserted in the optical path. The optical fiber output light source device according to claim 1 or 2, wherein the 90 ° polarization rotating reflector is a Faraday rotating mirror. The optical fiber output according to any one of claims 1 to 3, wherein all the optical fibers used in the optical path are composed of only fibers having no plane of polarization preservation ability. Light source device. The optical fiber output light source device according to any one of claims 1 to 4, wherein an antireflection device is provided at an outlet of the single polarization reflection type polarization beam splitter. The optical fiber output light source device according to any one of claims 1 to 5, wherein a highly non-linear fiber is further connected to a subsequent stage of the single polarization reflection type polarization beam splitter. A spectrum monitor arranged after the highly non-linear fiber and
The optical fiber output light source device according to claim 6, further comprising a control device that receives the output of the spectrum monitor and controls the polarization controller. In the highly non-linear fiber, the range of the dispersion value in the effective spectrum of the laser light emitted from the exit port of the single polarization reflection type polarization beam splitter is -1 to 3 [ps / (, centering on zero dispersion. The optical fiber output light source device according to claim 6 or 7, wherein the optical fiber output light source device has a region in the range of nm · km)]. The single polarization reflection type polarization beam splitter
Polarization beam splitter and
A lens arranged on the incident surface side of the polarization beam splitter and
A reflector arranged on the reflecting surface side of the polarization beam splitter and
The optical fiber output light source device according to any one of claims 1 to 8, further comprising a lens arranged on the transmission surface side of the polarization beam splitter. The single polarization reflection type polarization beam splitter
Birefringence prism and
A lens arranged on the incident surface side of the birefringence prism and
The optical fiber output light source device according to any one of claims 1 to 8, further comprising a reflecting mirror arranged on a part of the transmission surface of the birefringent prism. The single polarization reflection type polarization beam splitter
The invention according to any one of claims 1 to 8, wherein the two fibers are melt-stretched and composed of a melt-stretched optical fiber coupler exhibiting polarization separation characteristics. Optical fiber output light source device.

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