JP2008003422A - Optical fiber light guiding method and device for wavelength conversion laser output - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical fiber light guiding method which makes optical adjustments easy and simple even when the wavelength of output light is varied as a method of individually separating and guiding a difference frequency light output to a small light guide-in portion of an optical fiber, the method being applied to a wavelength conversion laser device which makes nonlinear optical crystal outputs the difference frequency light by making pump light and signal light incident thereupon. <P>SOLUTION: Angles of incidence of the pump light ω1 and signal light ω2 made incident on the nonlinear optical crystal 1 are given a slight angle difference θin, and the crystal axis of the nonlinear optical crystal 1 is adjusted according to a phase matching condition. At this time, a common passage point 2 that an optical path always passes even when the projected difference frequency generation (DFG) ω3 is different in wavelength is regarded as a virtual spot light source, and the DFG having passed the common passage point 2 is guided to a light guide-in port 31 of the optical fiber 3 by an optical system such as a dispersing element 4, an image relay lens system 6, and a rotary reflector 7. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a light guide method and apparatus for guiding a difference frequency light output to an optical fiber in a wavelength conversion laser device that makes two laser beams having different frequencies incident on a nonlinear optical crystal and outputs a laser light having a difference frequency between the incident laser beams. About.

Conventionally, when two excitation lights having different wavelengths are mixed in a nonlinear optical crystal, a long wavelength coherent light corresponding to the difference in the frequency of the excitation light is generated. 2. Description of the Related Art A wavelength conversion laser device that generates laser light is known.
In this wavelength conversion laser device, the first pumping light (pump light) having a short wavelength interacts with the second pumping light (signal light) in the nonlinear optical crystal and is wavelength-converted to convert the long-wavelength difference frequency light (DFG). Since it is generated, the wavelength of the DFG can be adjusted by changing the wavelength of the second excitation light, so that the wavelength-tunable long-wavelength laser generator is obtained.

For example, in Patent Document 1, a neodymium YAG (Nd: YAG) laser is used as the pump light on the short wavelength side, and a chromium forsterite (Cr: forsterite) laser is used as the signal light on the longer wavelength side. A high-power, compact, and wavelength-tunable infrared light generator is disclosed.
In this infrared light generator, an Nd: YAG laser is incident on a nonlinear optical crystal using a pulse laser having a wavelength of 1.064 μm as pump light, and a Cr: forsterite laser synchronized therewith is in a range of 1.15 to 1.35 μm. The light is incident on the nonlinear optical crystal as signal light of a tunable solid-state laser whose wavelength can be selected, and infrared light in a wavelength range of 5 to 14 μm according to the difference frequency between the pump light and the signal light is selectively generated.

The nonlinear optical crystal emits pump light and signal light in addition to the difference frequency light (DFG). Therefore, the DFG is selected and output by a filter with high infrared selection characteristics, and is input to an optical fiber. Transmit to location.
Since germanium (Ge) has high infrared selectivity, it is expected that if a Ge filter is used, high-purity infrared light will be obtained by well blocking YAG laser light and Cr: forsterite laser light having a wavelength of around 1 μm. Is done.
However, the infrared light output in the difference frequency light generator actually constructed by the inventors of the present invention is about half the design value, which is a result that deviated from the initial expectation.

As a result of researches on this problem, the inventors have grasped a phenomenon in which the transmittance of DFG decreases when a Ge filter is irradiated with Nd: YAG laser light.
When the separated DFG is irradiated onto the Ge filter, the portion through which the DFG passes is irradiated with an Nd: YAG laser, and the intensity of the Nd: YAG laser is changed to measure the transmittance of the DFG with respect to the Ge filter. It has been confirmed that the transmittance of DFG decreases as the value of A increases. Although the mechanism by which the transmittance is reduced by the YAG laser is not clear, it is certain that this phenomenon causes the DFG output of the difference frequency light generator using the Ge filter not to be as expected.
Therefore, when a Ge filter is used, it is preferable that the YAG laser light does not overlap with the DFG irradiation position.

  Patent Document 2 describes a method of taking out pure DFG while avoiding overlap of DFG and YAG laser. In Patent Document 2 that discloses a wavelength tunable terahertz wave generation device, a configuration in which the output light is purified by making the emission direction of the DFG different from the emission direction of the pump light or signal light instead of providing a Ge filter in the output portion. Is described.

Assuming that the angular frequency of the pump light incident on the nonlinear optical crystal is ω1, the angular frequency of the signal light is ω2, and the angular frequency of the DFG is ω3, the angular frequency ω3 is represented by ω3 = ω1−ω2, where ω is a wave number vector. Have a relationship.
Therefore, as shown in FIG. 9, when the optical axis of the pump light and the signal light has a slight angle difference θin, the DFG wave vector ω3, which is the difference between the wave vectors ω1 and ω2, sandwiches the pump light. And is refracted on the crystal surface and emitted in a direction deviating by the angle difference θout. Therefore, DFG can be separated from pump light and signal light at a position sufficiently away from the nonlinear optical crystal.
Therefore, by giving a difference θin of only a few minutes between the incident angles of the pump light and the signal light, the exit angle θout of the DFG from the nonlinear optical crystal is made several tens of degrees, and the DFG is obtained from the pump light and the signal light. To separate.

However, in the configuration described in Patent Document 2, since the emission angle of the DFG changes every time the wavelength of the DFG is changed, if the outgoing light is made to enter the optical fiber, the optical axis of the light guide optical system is adjusted. It is inconvenient to do.
For example, if the incident angle difference θin between the pump light and the signal light is 0.3 degrees, changing the DFG wavelength from 5.5 μm to 10 μm changes the DFG emission direction by about 1.3 degrees. Optical adjustment during use becomes extremely complicated.

Reference 3 discloses a wavelength tunable light source device that tracks the selected wavelength of the variable bandpass filter in accordance with the wavelength change of the wavelength tunable light source.
In the disclosed variable bandpass filter system, the input light to the variable bandpass filter is controlled to a predetermined value using an APC control circuit, and a part of the selection light of the variable bandpass filter is branched by a non-polarizing beam splitter. The variable wavelength filter is driven by the wavelength tracking circuit, and the selected wavelength is adjusted so that the output light becomes constant. A variable bandpass filter suitable for the disclosed apparatus is required to have a special output characteristic, but can be formed of a dielectric multilayer film.

The output wavelength selection method disclosed in the cited document 3 does not require optical position adjustment because the optical axis of the output light is constant, but a variable bandpass filter capable of tracking the transmission wavelength, and a light source stabilization control circuit In addition, it is necessary to provide a transmission wavelength tracking circuit for the filter, and the complexity of the system is inevitable.
Therefore, in a wavelength tunable laser device using a nonlinear optical crystal for generating a difference frequency, the background light is effectively removed and only the desired difference frequency is injected into the core of the optical fiber, which is an extremely small target, at a low cost. The provision of a new method is awaited.
JP 2005-331599 A JP 2004-318028 A JP 2004-039677 A

  Therefore, the problem to be solved by the present invention is to provide a simpler optical fiber light guiding method that separates the difference frequency light output from the pump light and signal light and guides it to a small light introducing portion of the optical fiber in the wavelength conversion laser device. It is to provide an optical fiber light guiding method that is easy to optically adjust even when the wavelength of output light is changed.

  In order to solve the above problems, the optical fiber light guiding method of the wavelength conversion laser output of the present invention gives a slight angle difference between the incident angles of the pump light and the signal light incident on the nonlinear optical crystal, and the crystal axis of the nonlinear optical crystal. Is adjusted according to the phase matching condition, and even when the difference frequency light (DFG) to be emitted has a different wavelength, a common passing point through which the optical path always passes is found, and this common passing point is used as an object point. An image relay is arranged so that the incident point is an image point, and DFG generated in the nonlinear optical crystal is guided to an optical fiber.

When the pump light and the signal light are incident on the nonlinear optical crystal, the vector difference between the pump light and the signal light wave vector is obtained when the optical axis of the pump light and the signal light and the optical axis of the nonlinear optical crystal satisfy the phase matching condition. A difference frequency light having a wave vector is output.
In the wavelength conversion laser output device capable of adjusting the wavelength, even if the incident angles of the pump light and the signal light are not changed, the output direction from the nonlinear optical crystal is changed by changing the wavelength of the output DFG. As the wavelength increases, the output axis of the DFG tilts and the emission angle increases.

However, the angle between the optical axis of the pump light in the nonlinear optical crystal and the crystal optical axis becomes larger as the wavelength of the DFG becomes longer based on the phase matching condition.
Therefore, if the optical path of the pump light and the signal light before entering the nonlinear optical crystal is fixed and the direction of the nonlinear optical crystal axis is adjusted according to the phase matching condition, the longer the DFG wavelength, the more the pump light etc. The position where the crystal is ejected moves backward in the DFG ejection direction.

  Therefore, the optical paths of DFGs having different wavelengths have points that intersect after emission. Actually, when optical paths are drawn for DFGs of various wavelengths, it can be seen that all DFGs in a certain wavelength range pass through a relatively narrow region having a width of about 1 to 2 mm. The position in the space where the area through which all the DFG passes is the narrowest is obtained. Since this region is sufficiently small, it can be regarded as a pseudo point, and any wavelength DFG can be treated as if there is a light source at this point. This point is called a common passing point.

  In the optical fiber light guiding method of the present invention, the image relay is arranged so that the common passing point is an object point and the incident point guided to the core of the optical fiber is the image point. Therefore, the DFG at the common passing point is smaller than that of the optical fiber. Projected to the core position. Therefore, even DFGs of different wavelengths pass through the common passing point, and can be accurately introduced into the optical fiber.

According to the second optical fiber light guiding method of the wavelength conversion laser output of the present invention, a dispersive element is arranged at a common passing point so that the emitted light is diffracted in the same direction regardless of the wavelength by a dispersive element such as a prism. The DFG is guided to an optical fiber by an optical lens.
Since the incident angles of the DFGs gathered at the common passing point change according to the wavelength, the dispersion elements such as prisms are appropriately selected to cancel the difference in the incident angle, and the DFGs having different wavelengths transmitted through the dispersing elements have almost the same direction. Try to face.

For example, since the refractive index of an optical crystal is larger for a light beam having a longer wavelength than for a light beam having a shorter wavelength, the prism of the optical crystal is used as a dispersion element, and the incident angle of DFG to the dispersion element is shorter for a longer wavelength. If it arrange | positions so that it may become larger than a thing, an incident angle difference can be canceled.
Therefore, if a condensing lens is arranged in front of the optical fiber so that the optical axis is parallel to the optical path of the DFG and the core of the optical fiber is located at the focal position, all DFGs having different wavelengths can be introduced into the optical fiber. Will be able to.

In the third optical fiber light guiding method of the wavelength conversion laser output of the present invention, a rotatable reflecting mirror is arranged at the common passing point, and the reflection direction is adjusted so that the reflected light is directed in the same direction regardless of the wavelength. The DFG is guided to an optical fiber by a condensing lens having an optical axis in this direction.
Since the DFG that passes through the common passage point has a different direction of incidence on the common passage point depending on the wavelength, the optical path coincides with the optical axis of the condenser lens that adjusts the direction of the reflecting mirror and concentrates light on the core of the optical fiber, Alternatively, if an optical path parallel to the optical axis is formed, DFGs having different wavelengths can be reliably introduced into the optical fiber.

  In order to solve the above problems, an optical fiber light guiding device with a wavelength-converted laser output according to the present invention includes a pump light generator that supplies pump light, a signal light generator that supplies signal light, a pump light and a signal A nonlinear optical crystal that outputs difference frequency light when light is incident, a control device that adjusts the inclination of the nonlinear optical crystal, and an image relay that transmits an image of an object point to the image point are provided.

In addition, the optical path is such that the pump light and the signal light are incident on the nonlinear optical crystal with a slight angle difference, and the control unit adjusts the crystal axis of the nonlinear optical crystal in accordance with the phase matching condition. Even if the wavelength of the difference frequency light (DFG) output from the optical fiber is different, an image relay is arranged so that the common passing point through which the optical path always passes is the object point and the incident point of the optical fiber is the image point. Lead to fiber.
Instead of the image relay, a combination of a dispersion element and a condensing lens or a combination of a reflecting mirror and a condensing lens may be provided on the optical path.

  In the method and apparatus of the present invention, a neodymium YAG (Nd: YAG) laser with a wavelength of 1.064 μm is used as pumping light, and signal light is tunable in the range of 1.15 to 1.35 μm. It is preferable to apply it to the output section of a wavelength conversion laser device that obtains a variable DFG with a wavelength range of 5 to 14 μm using laser light as signal light.

  The present invention relates to a wavelength-tunable wavelength conversion laser output device that outputs a difference frequency light (DFG) by making pump light and signal light incident on a nonlinear optical crystal. By utilizing the existence of a common passing point that passes therethrough, the DFG is reliably guided to the light entrance of a narrow optical fiber even if the wavelength changes.

When the direction of the nonlinear optical crystal axis is adjusted in accordance with the phase matching condition for the pump light and the signal light, the longer the DFG wavelength, the larger the output axis of the DFG and the larger the emission angle, while the pump light and DFG are nonlinear. The position at which the optical crystal is ejected moves backward in the DFG ejection direction. For this reason, the optical paths of DFGs having different wavelengths intersect with each other at a common passing point which is a narrow area.
Therefore, even for DFGs of different wavelengths, if the optical system is designed as if there is a point light source at the common passing point, the DFG generated in the wavelength conversion laser output device can be reliably introduced into the optical fiber. it can.

Hereinafter, based on an Example, this invention is demonstrated in detail using drawing.
FIG. 1 is a vector diagram of pump light, signal light, and difference frequency light in a nonlinear optical crystal, FIG. 2 is a relationship diagram showing an example of a change in phase matching angle with respect to the wavelength of the signal light, and FIG. Drawing which shows the optical path when the wavelength of frequency light is changed, FIG. 4 is a graph showing the relationship between the optical path of difference frequency light, and a common passing point.

When the difference frequency light (DFG) is obtained by inputting the pump light and the signal light to the nonlinear optical crystal, between the wave number vectors k1, k2, and k3 of the pump light, the signal light, and the DFG,
k1 = k2 + k3
The relationship is established. Also, if each wavelength is λ1, λ2, λ3,
1 / λ1 = 1 / λ2 + 1 / λ3
The relationship is established.

  As shown in the vector diagram of FIG. 1, even when the angle α between the pump light k1 and the signal light k2 is constant in the crystal, the signal light k2 changes to k2 ′, K2 ″, and the vector of the pump light and the signal light When the difference (k1−k2) becomes smaller, the difference frequency light vector k3 becomes smaller as k3 ′, k3 ″, so that the wavelength λ3 becomes longer, and the angle β between the difference frequency light and the pump light becomes β ′, β ″. The angle β is larger as the wavelength λ3 of the difference frequency light is longer.

  Actually, if the angle α between the pump light and the signal light is slightly shifted, for example, by about 0.1 to 0.5 °, the angle β formed by the difference frequency light and the pump light becomes 4 to 8 times α. Therefore, the difference frequency light is spatially separated from the pump light and signal light at a position sufficiently away from the nonlinear optical crystal, and the difference frequency light can be handled independently without using a special separation mechanism. .

However, in order to generate the difference frequency light, it is necessary to satisfy a phase matching condition depending on the angle between the optical path and the crystal optical axis.
The phase matching condition is a condition for the phases of the three lights to coincide when they exit from the crystal.
n _1e (θ) / λ1 = n _2o / λ2cos α + n _3e (θ−β) / λ3 cos β
n _2o / λ2sinα = n _3e (θ−β) / λ3sinβ
The relationship expressed by must be satisfied.

The wave number vectors of pump light, signal light, and difference frequency light are
k1 = 2πn _1e (θ) / λ1
k2 = 2πn _2o / λ2
k3 = 2πn _3e (θ−β) / λ3
It is expressed.
Here, θ is the angle formed by the optical path of the pump light and the crystal axis, n _1e (θ) is the refractive index of the nonlinear optical crystal with respect to the pump light anomalous wave having the angle of the crystal axis and θ, and n _2o is the normal signal light of the crystal N _3e (θ−β) is a refractive index for a difference frequency optical anomalous wave having an angle of (θ−β) with respect to the crystal axis.

FIG. 2 is a graph showing an example of changes in the phase matching angle. The horizontal axis of the graph indicates the wavelength of a chromium forsterite (Cr: forsterite) laser used as signal light, and the vertical axis indicates the phase matching angle θ.
The graph shown in FIG. 2 shows that when Nd: YAG laser light is used as pump light, Cr: forsterite laser is used as signal light, and the pump light and signal light are incident on a non-linear optical crystal having a length of 24 mm from the same direction, That is, it is the result of calculating the angle θ formed by the optical path of the pump light that is phase-matched and the crystal axis when the angle α is zero.

As can be seen in FIG. 2, the phase matching angle θ increases monotonically from 38 ° to 50 ° while the wavelength of the Cr: forsterite laser changes from 1.17 μm to 1.35 μm.
When an Nd: YAG laser having a wavelength of 1.06 μm is used as pump light, the wavelength λ3 of the difference frequency light generated in this range varies in the range of 11.3 μm to 4.9 μm.
Note that when the angle α is in the range of 0.1 ° to 0.5 °, the difference between the phase matching angle and the value in FIG. 2 is slight.

FIG. 3 is a diagram illustrating an optical path when the wavelength of the difference frequency light is changed in the nonlinear optical crystal and explaining the common passing point.
A Nd: YAG laser with a wavelength of 1.06 μm is used as pump light ω1, and a tunable Cr: forsterite laser is used as signal light ω2 to enter the nonlinear optical crystal 1 having a length of 24 mm. In order to avoid complicated laser light path adjustment, the incident directions and the incident positions 11 of the pump light ω1 and the signal light ω2 are determined and fixed in advance, and are not changed even when the wavelength λ3 of the difference frequency light ω3 is changed.

The inclination δ of the nonlinear optical crystal surface 1 is adjusted so that the angle between the optical path in the crystal of the pump light ω1 incident on the nonlinear optical crystal 1 and the crystal optical axis 12 becomes the phase matching angle θ.
Since the angle θin between the incident directions of the pump light ω1 and the signal light ω2 to the nonlinear optical crystal 1 is fixed and the incident angle to the nonlinear optical crystal 1 is different, the inclination δ of the nonlinear optical crystal surface changes. Then, the angle α between the optical paths in the crystal changes according to Snell's law of refraction. Therefore, strictly speaking, the phase matching angle θ slightly changes.

The test shows that if there is an angle shift of 0.04 ° with respect to the correct phase matching angle, the conversion efficiency to the difference frequency light is reduced by about 50%. Thus, it is preferable to calculate the phase matching angle θ with respect to this and adjust the inclination δ of the crystal plane in accordance with the newly calculated phase matching angle.
When the incident direction of the signal light ω2 can be accurately adjusted, the angle difference θin in the incident direction may be adjusted to keep the angle difference α in the crystal constant.

When the pump light ω1 and the signal light ω2 travel in the nonlinear optical crystal 1 at an angle α between the optical paths, a difference frequency light ω3 having an angle β with respect to the pump light ω1 is generated, and from the end face of the nonlinear optical crystal 1 Exit. The difference frequency light ω3 is refracted on the exit surface in accordance with Snell's law, and the optical path after emission has an angle θout with respect to the pump light ω1 after emission. When the incident end face and the exit end face of the nonlinear optical crystal 1 are parallel, the direction in which the pump light ω1 is emitted from the end face is the same as the incident direction.
As the wavelength λ3 of the difference frequency light is longer, the angle β is larger, so the angle θout of the optical path after emission with respect to the pump light is also larger.

On the other hand, the position 13 where the pump light or the like exits from the nonlinear optical crystal 1 transitions in a direction in which the exit surface tilts and the original exit position shifts when the nonlinear optical crystal 1 tilts. When the difference frequency light having a long wavelength λ3 is generated, the nonlinear optical crystal 1 is tilted to the right side in the drawing so as to match the small phase matching angle θ. For example, the difference frequency light ω3 having a wavelength of 10 μm indicated by a thick line in the drawing is generated. As in the configuration to be generated, the emission point 13 of the pump light ω1 transitions downward in the figure.
On the other hand, when the wavelength λ3 of the difference frequency light is short, the phase matching angle θ is large, so that the crystal optical axis is lifted upward in the drawing and adjusted, for example, when the wavelength is expressed as 5.5 μm. Therefore, the emission point 13 of the pump light ω1 transitions upward in the drawing.

  As described above, when the wavelength λ3 of the difference frequency light is long, the angle θout with respect to the pump light is large and the emission position 13 is shifted downward in the figure, and when the wavelength λ3 is short, the opposite is obtained. The short difference frequency light paths intersect at one point 2 as seen in the figure.

  FIG. 4 is a view showing an optical path when the wavelength λ3 of the difference frequency light is variously changed in the apparatus constructed by the inventors. FIG. 4 shows the distance after emission from the nonlinear optical crystal 1 on the horizontal axis, the distance from an appropriate reference line parallel to the pump light on the vertical axis, and the lower limit of the difference frequency light that can be generated by this apparatus. In this wavelength range from 5.5 μm to the upper limit of 10 μm, the locus of the difference frequency light after exiting the nonlinear optical crystal 1 is drawn.

It can be seen from FIG. 4 that the region enclosing all the trajectories has a very small region 2 having a width of about 2 mm. This region can be regarded as a point, and here this point is called a common passing point. That is, the difference frequency light generated by this apparatus passes through the common passing point 2 even when the wavelengths are different.
When the common passing point 2 is regarded as a pseudo point light source and an optical system is configured to guide light emitted from the point light source to the core of the optical fiber, the difference frequency light having different wavelengths emitted from the wavelength conversion laser is formed. Can be introduced into the optical fiber.

FIG. 5 is a conceptual diagram illustrating an optical fiber light guiding method and apparatus for outputting a wavelength-converted laser according to Embodiment 1 of the present invention.
The optical fiber light guiding method of the first embodiment uses a dispersive element.
In the optical fiber light guide device of the present embodiment that implements this method, the dispersive element 4 and the converging optical system 5 are disposed between the difference frequency light emission point 13 of the nonlinear optical crystal 1 and the optical entrance 31 of the optical fiber 3. To do. Although not shown, the optical fiber light guide device of this embodiment includes a control device that precisely adjusts the inclination δ of the nonlinear optical crystal 1.

By disposing the dispersive element 4 at the common passing point 2, when the difference frequency light is emitted from the dispersive element 4 corresponding to different incident angles as the wavelength changes, the difference frequency light of any wavelength has a traveling direction. Try to be the same. For example, the refractive index of the material, the inclination of the incident surface and the inclination of the exit surface, and the like can be adjusted using a prism that has been appropriately selected.
A converging optical system 5 for converging parallel rays to the focal point is arranged behind the dispersive element 4 so that the traveling direction of the difference frequency light ω3 and the optical axis are parallel, and the optical entrance 31 of the optical fiber 3 is located at the focal position. Place.

Then, even when the wavelength λ3 of the difference frequency light ω3 changes, the difference frequency light emitted from the emission point 13 of the nonlinear optical crystal 1 passes through the common passing point 2. The difference frequency light ω3 incident on the dispersive element 4 placed at the common passing point 2 has a different incident angle depending on the wavelength, but when exiting from the dispersive element 4, the difference frequency light of any wavelength travels in the same direction. become. That is, the difference frequency lights having different wavelengths travel on optical paths parallel to each other.
Since the optical axis of the converging optical system is parallel to this parallel optical path, the difference frequency light of any wavelength passes through the same focal point. That is, the difference frequency light is reliably introduced into the light entrance provided at the focal point.

FIG. 6 is a drawing showing an example of an optical path of each wavelength when the traveling direction is corrected in substantially the same direction using a calcium fluoride (CaF ₂ ) prism and converged on an optical fiber by a condenser lens. The horizontal axis is a distance in the direction perpendicular to the crystal exit surface, and the vertical axis is an arbitrary scale representing the distance in the direction parallel to the exit surface. The vertical axis is greatly enlarged compared to the horizontal axis.

As shown in the drawing, the difference frequency light beams having wavelengths of 5.5 μm, 7.5 μm, and 9.5 μm are all collected at the common passing point 2. The refractive index of CaF ₂ increases as the wavelength of incident light increases, and is 1.424 when the wavelength is 2 μm and 1.327 when the wavelength is 9 μm. Therefore, in consideration of the optical path difference depending on the wavelength, the prism 4 having a triangular section with a wedge angle of 8.5 ° is placed at the common passing point 2 so that the incident surface is inclined by 8.5 ° with respect to the optical path of the pump light. To place.

Then, when the difference frequency light beam enters and exits the prism 4 provided at the common passing point 2, the difference in the incident angle is canceled by performing refraction slightly different for each wavelength, and as a result, the difference frequency light beam is almost independent of the wavelength. Drive in the same direction.
Thus, if the optical axis of the condensing lens 5 is arranged so as to be parallel to the traveling direction of the difference frequency light beam, the beam of any wavelength is converged to the focal point by the condensing lens 5 and placed at the focal position. It is introduced into the optical fiber via the optical fiber input section.

FIG. 7 is a conceptual diagram illustrating an optical fiber light guiding method and apparatus for outputting a wavelength-converted laser according to Embodiment 2 of the present invention.
The optical fiber light guiding method of the second embodiment uses an image relay.
In this method, the image relay lens system 6 is disposed between the difference frequency light exit surface 13 of the nonlinear optical crystal 1 and a light entrance 31 of the optical fiber 3 to which a control device for precisely adjusting the tilt of the crystal is attached. The image relay lens system 6 is composed of two or more focusing lenses 61 and 62, and the object point side focal point of the image relay lens system 6 is located at a common passing point, and the optical fiber 3 is located at the image point side focal point. The light inlet 31 is disposed.

Note that the first lens 61 of the image relay optical system 6 receives all the trajectories of the difference frequency light ω3 within the target wavelength range so that the image relay can be performed.
The difference frequency light ω 3 emitted from the nonlinear optical crystal 1 enters the image relay lens system 6, expands and contracts, and converges to the optical entrance 31 of the optical fiber. At this time, although the locus of the difference frequency light ω3 is shifted depending on the wavelength, an optical system that forms an object image at the common passing point 2 at the position of the light entrance 31 as a real image having a size of 1/25, for example, is assembled. Therefore, the difference frequency light ω3 can be injected into the optical entrance 31 of the optical fiber regardless of the wavelength.

In order to more accurately guide the difference frequency light ω3 to the optical fiber 3, a fine adjustment mechanism for finely adjusting the position of the image relay lens system 6 may be provided so that the fine adjustment can be performed for each wavelength. Needless to say.
With the optical fiber guiding method of this embodiment, the difference frequency light of each wavelength passing through the common passing point 2 can be introduced into the optical fiber 3.

FIG. 8 is a conceptual diagram illustrating an optical fiber light guiding method and apparatus for outputting a wavelength-converted laser according to Example 3 of the present invention.
The optical fiber light guiding method of the third embodiment uses a rotatable reflecting mirror.
In this method, a rotatable reflecting mirror 7 is disposed between the difference frequency light emitting surface 13 of the nonlinear optical crystal 1 capable of adjusting the tilt of the crystal and the light entrance 31 of the optical fiber 3, and the reflected light is transmitted through the optical fiber. 3 is incident on the light entrance 31 of the light source.
The rotating shaft 71 of the reflecting mirror 7 is arranged on the reflecting mirror surface 72 so that the common passing point 2 comes to the position of the rotating shaft 71.

  The difference frequency light ω <b> 3 emitted from the nonlinear optical crystal 1 is reflected by the reflecting surface 72 of the reflecting mirror 7 and is condensed by the focusing lens 5 on the light entrance 31 of the optical fiber 3. When the wavelength λ3 of the difference frequency light 3 changes, the incident angle with respect to the reflecting mirror 7 changes. Therefore, the direction of the reflecting surface 72 is adjusted so that the light is always focused on the light entrance 31 of the optical fiber 3.

The difference frequency light ω3 always passes through the common passing point 2 even if the wavelength λ3 changes, and only the incident direction changes. Therefore, the rotation axis 71 of the reflecting mirror 7 is on the reflecting surface 72 and the common passing point 2 is changed. Is on the rotation axis 71, even if the wavelength of the difference frequency light ω3 changes and the incident direction changes, it is only necessary to adjust the direction by rotating the reflecting surface 72 while fixing the other optical arrangement. The difference frequency light ω <b> 3 reflected by the reflecting surface 72 can be condensed on the optical entrance 31 of the optical fiber 3.
In this embodiment, when the wavelength of the difference frequency light is changed, the difference frequency light can be continuously guided to the optical fiber without any interruption simply by adjusting the angle of the reflecting mirror surface.

  According to the present invention, it is possible to provide a simpler method and apparatus for guiding a difference frequency light to an optical fiber in a wavelength tunable wavelength conversion laser device that generates a difference frequency light using a nonlinear optical crystal.

It is a vector diagram of pump light, signal light, and difference frequency light in a nonlinear optical crystal in the optical fiber light guiding method of wavelength conversion laser output of the present invention. It is a related figure which shows one example of the change of the phase matching angle with respect to the wavelength of the signal light in the optical fiber light guide method of this invention. It is drawing which shows the optical path when the wavelength of difference frequency light is changed in the nonlinear optical crystal in the optical fiber light guide method of this invention. It is a graph showing the relationship between the optical path of the difference frequency light and the common passing point in the optical fiber light guiding method of the present invention. It is a conceptual diagram explaining the optical fiber light guide method and apparatus of the wavelength conversion laser output concerning Example 1 of this invention. 2 is a conceptual diagram illustrating an example of an optical path of each wavelength in Example 1. FIG. It is a conceptual diagram explaining the optical fiber light guide method and apparatus of the wavelength conversion laser output which concern on Example 2 of this invention. It is a conceptual diagram explaining the optical fiber light guide method and apparatus of the wavelength conversion laser output which concerns on Example 3 of this invention. It is drawing explaining the mechanism which generate | occur | produces difference frequency light with a nonlinear optical crystal.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Nonlinear optical crystal 11 Incident position 12 Crystal optical axis 13 Output position 2 Common passing point 3 Optical fiber 31 Optical entrance 4 Dispersive element, prism 5 Converging optical system, Condensing lens 6 Image relay lens system 61, 62 Converging lens 7 Reflection Mirror 71 Rotating shaft 72 Reflecting surface

Claims (8)

  When a slight angle difference is given to the incident angles of the pump light and signal light incident on the nonlinear optical crystal, and the crystal axis of the nonlinear optical crystal is adjusted according to the phase matching condition, the difference frequency light (DFG) emitted is the wavelength. Even if the optical path is different, the position of the common passing point through which the optical path always passes is obtained, and the image relay lens system is arranged so that the common passing point is an object point and the optical fiber entrance is an image point. An optical fiber light guide method for outputting a wavelength-converted laser, characterized in that DFG generated in an optical crystal is guided to an optical fiber.   When a slight angle difference is given to the incident angles of the pump light and the signal light incident on the nonlinear optical crystal, and the crystal axis of the nonlinear optical crystal is adjusted according to the phase matching condition, the difference frequency light (DFG) emitted is the wavelength. Even when the optical path is different, the position of the common passing point through which the optical path always passes is obtained, a dispersive element is arranged at the common pass point, and the dispersive element refracts the emitted light in the same direction regardless of the wavelength. A method for guiding an optical fiber having a wavelength-converted laser output, wherein the DFG is guided to the optical fiber by an optical lens.   When a slight angle difference is given to the incident angles of the pump light and signal light incident on the nonlinear optical crystal, and the crystal axis of the nonlinear optical crystal is adjusted according to the phase matching condition, the difference frequency light (DFG) emitted is the wavelength. Even when the optical path is different, the position of the common passing point through which the optical path always passes is obtained, a rotatable reflecting mirror is arranged at the common passing point, the reflection direction is adjusted, and the reflected light is directed in the same direction regardless of the wavelength. Thus, a method for guiding an optical fiber with a wavelength-converted laser output, wherein the DFG is guided to the optical fiber by a condenser lens.   4. The wavelength according to claim 1, wherein the pump light is a neodymium YAG (Nd: YAG) laser having a wavelength of 1.064 μm, and the DFG has a variable range of a wavelength range of 5 to 14 μm. Optical fiber light guide method for conversion laser output.   A pump light generating device that supplies pump light, a signal light generating device that supplies signal light, a nonlinear optical crystal that outputs difference frequency light when pump light and signal light are incident, and a tilt of the nonlinear optical crystal is adjusted A control device and an image relay lens system for transmitting an image of an object point to the image point, and adjusting the optical path of the pump light and the signal light so that the pump light and the signal light have a slight angle difference and the nonlinear optical When the control device adjusts the crystal axis of the nonlinear optical crystal in accordance with a phase matching condition so that the wavelength of the difference frequency light (DFG) output from the nonlinear optical crystal is different. In addition, the wavelength change is characterized by being arranged such that a common passing point through which the optical path always passes is an object point and an optical entrance of the optical fiber is an image point, and DFG is guided to the optical fiber. Optical fiber light guide device of the laser output.   A pump light generating device that supplies pump light, a signal light generating device that supplies signal light, a nonlinear optical crystal that outputs difference frequency light when pump light and signal light are incident, and a tilt of the nonlinear optical crystal is adjusted A control device and a dispersive element having a refraction angle different depending on the wavelength are provided, and the optical path of the pump light and the signal light is adjusted so that the pump light and the signal light are incident on the nonlinear optical crystal with a slight angle difference. The control device adjusts the crystal axis of the nonlinear optical crystal according to the phase matching condition, and the optical path always passes even when the difference frequency light (DFG) output from the nonlinear optical crystal by the dispersion element has a different wavelength. The optical path of the DFG transmitted through the dispersive element is arranged in the same direction even if the wavelength is different, and is transmitted through the dispersive element by a condenser lens. Directing the by've DFG the optical fiber an optical fiber light guide device of the wavelength conversion laser output, wherein.   A pump light generator for supplying pump light, a signal light generator for supplying signal light, a non-linear optical crystal that outputs difference frequency light (DFG) when pump light and signal light are incident, and a tilt of the non-linear optical crystal And a non-linear optical device having a slight angle difference between the pump light and the signal light by adjusting the optical path of the pump light and the signal light. The optical path is always present even when the control device adjusts the crystal axis of the nonlinear optical crystal in accordance with the phase matching condition and the DFG output from the nonlinear optical crystal has a different wavelength. An optical fiber light guide device for outputting a wavelength-converted laser, wherein the DFG arranged at a common passing point and passing through the reflecting mirror is guided to an optical fiber by a condenser lens. 8. The wavelength according to claim 5, wherein the pump light is a neodymium YAG (Nd: YAG) laser having a wavelength of 1.064 μm, and the DFG has a variable range of a wavelength range of 5 to 14 μm. Optical fiber light guide device with converted laser output.

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