JP2004020571A - Wavelength transformation device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wavelength transformation device with a new connecting structure with which any trouble of connecting a light source with an element in the wavelength transformation device is improved. <P>SOLUTION: In constructing the wavelength transformation device with a laser light source 1 and a wavelength transformation element 2 (an element to transform a wavelength of laser light L1 emitted from the light source 1), the light source 1 and the element 2 are connected via a photonic crystal fiber 3 so as to make the laser light L1 emitted from the light source 1 incident on the element 2. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a wavelength conversion device that combines a laser light source and a wavelength conversion element.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, a wavelength conversion device in which a laser light source and a wavelength conversion element are combined has been known. The “wavelength conversion device” here includes not only an independent device as one product but also a so-called module, unit or the like, a wavelength conversion component constituting a part of the product.
[0003]
FIG. 5A shows an example of the configuration of a conventional wavelength converter. In the example of FIG. 5A, a semiconductor laser (LD) 50 and an optical waveguide type wavelength conversion element 70 are used. The wavelength conversion element 70 of the optical waveguide type has a band-shaped optical waveguide 72 formed on the surface layer of a ferroelectric crystal substrate 71 and a periodic domain-inverted structure (hereinafter also referred to as domain-inverted structure) overlapping with the optical waveguide. Is formed. Illustration of the domain-inverted structure is omitted. The LD 50 is coupled to the optical waveguide 72 of the wavelength conversion element 70 via a lens system (optical system) 60, and the laser light L10 passing through the optical waveguide is subjected to a nonlinear optical effect (the phase matching is performed by a pseudo-polarization structure. The wavelength is converted by phase matching, and the converted light is output as wavelength converted light L20. The LD 50 has an oscillation wavelength variable so that the wavelength of the laser light L10 can be matched with the phase matching wavelength of the wavelength conversion element 70. A wavelength plate 62 is arranged in the lens system 60. The wavelength plate 62 is for adjusting the relationship between the polarization direction of the laser light emitted from the LD and the polarization direction of the optical waveguide, but may be omitted by changing the polarization direction of the wavelength conversion element.
[0004]
FIG. 5B is a modification of the configuration in FIG. 5A, in which the lens system is omitted by directly coupling the LD 50 and the optical waveguide of the wavelength conversion element 70 on the submount substrate 80. Structure (submount structure).
[0005]
FIG. 5C shows an example of a configuration using a solid-state laser (laser medium 55, resonators 56a and 56b, etc.) and a bulk-type wavelength conversion element 75. The bulk type wavelength conversion element 75 is formed by forming a domain-inverted structure (structure indicated by hatching) in the entire ferroelectric crystal substrate, and does not have an optical waveguide. In the example shown in the figure, a wavelength conversion element 75 is inserted inside the solid-state laser resonator (56a, 56b), and the oscillation light L11 of the solid-state laser is wavelength-converted by the element 75 and output as wavelength-converted light L21. (Internal resonator type). In the example shown in the figure, an LD 51 is used as an excitation light source for a solid-state laser. An etalon plate 57 is inserted inside the resonators (56a, 56b) in order to unify the longitudinal mode of the solid-state laser, reduce mode competition, and stabilize the output.
[0006]
In any of the above embodiments, a temperature control mechanism for stabilizing the temperature of the light source, the wavelength conversion element, and the like is provided, and further, a signal processing unit (a monitor circuit for temperature and light output and a feedback circuit) as necessary. Is added.
[0007]
[Problems to be solved by the invention]
However, the present inventors have found the following problems to be improved in the configuration of the conventional wavelength converter as described above.
[0008]
First, in a configuration using a lens system for coupling as shown in FIGS. 5A and 5C, there is a problem in mechanical stability of the lens system, and a highly reliable product cannot be obtained. Also, the cost is high due to the large number of optical components. Further, in the embodiment of FIG. 5A, the numerical aperture of the LD (Numerical Aperture) is usually large (0.3 or more), and there is a large gap from the numerical aperture of the optical waveguide (0.2 or less). Efficiency does not increase.
[0009]
In the submount structure as shown in FIG. 5B, the polarization direction of the laser light emitted from the LD and the polarization direction of the optical waveguide (= the polarization direction of the crystal related to wavelength conversion) coincide with each other. It is necessary to assemble with the polarization directions matched, and the crystal orientation when producing the wavelength conversion element is limited. That is, in FIG. 5B, since the polarization direction of the laser beam emitted from the LD is perpendicular to the plane of the drawing, the polarization direction of the wavelength conversion element is set to be perpendicular to the plane of the drawing to match the direction. Then, the ferroelectric crystal substrate, which is the material of the wavelength conversion element, is limited to an X-axis cut substrate (the Z axis to be polarization-inverted is parallel to the substrate surface). With such an X-axis cut substrate, it is difficult to invert the polarization in the Z-axis from the substrate surface to a deep region, and it is not possible to secure an optical path having a sufficiently wide cross section in the depth direction, and thus the conversion efficiency is low. In addition, since both the cross-sectional shapes of the optical waveguide and the LD in the propagation mode are as small as several μm, very high positioning accuracy is required at the time of positioning, and production is difficult.
[0010]
An object of the present invention is to provide a wavelength converter having a novel connection structure that can improve the above-described problem.
[0011]
[Means for Solving the Problems]
The present invention has the following features.
(1) a laser light source, and a wavelength conversion element capable of wavelength-converting the laser light emitted from the light source, and the light source and the element so that the laser light emitted from the light source is incident on the element. Are connected via a photonic crystal fiber.
[0012]
(2) The wavelength conversion element is an optical waveguide type element, and the photonic crystal fiber has a numerical aperture equal to or larger than the numerical aperture of the light source at the end face on the laser light source side, and the end face on the wavelength conversion element side. The wavelength conversion device according to (1), wherein the device has a structure in which the cross-sectional shape of the propagation mode changes along the longitudinal direction so as to have a numerical aperture equal to or less than the numerical aperture of the optical waveguide of the element.
[0013]
(3) The photonic crystal fiber is twisted so that the polarization direction of the laser light emitted from the laser light source and the polarization direction related to wavelength conversion of the wavelength conversion element match. Or the wavelength converter of (2).
[0014]
(4) A reflection structure is further provided on any one of the following optical waveguides (A) to (U), and the reflection structure is provided in a predetermined portion of the oscillation spectrum of the laser light emitted from the laser light source. The wavelength converter according to the above (1), which can reflect light having a wavelength so as to return to the laser light source.
(A) On an optical waveguide of a photonic crystal fiber.
(I) On the optical waveguide of the wavelength conversion element.
(C) On another optical waveguide that is further added downstream of the emission surface of the wavelength conversion element.
[0015]
(5) The wavelength converter according to (4), wherein the reflection structure is a Bragg grating formed on an optical waveguide.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
The wavelength conversion device according to the present invention includes a laser light source (hereinafter also referred to as a light source) 1 and a wavelength conversion element (hereinafter also referred to as an element) 2 as schematically shown in FIG. The element 2 is an element that transmits a laser beam L1 emitted from the light source 1, converts the wavelength of the laser beam L1 by a nonlinear optical effect, and emits it as output light (wavelength-converted light) L2. Then, a photonic crystal fiber (hereinafter, also referred to as PCF) 3 is emitted from the light source 1 so that the laser light L1 emitted from the light source 1 is incident on an optical path of the element 2 that can convert the wavelength. The light source 1 and the element 2 are optically connected to each other between the surface and the incident surface of the element 2.
[0017]
As will be described in more detail later, PCF is a type of optical fiber that has been reported in recent years. If single-mode propagation is possible even in a short wavelength range, PCF is mainly used in the field of energy transmission at short wavelengths and sensor applications. It is used.
The present inventors have focused on the problems unique to the wavelength conversion device (the instability of the lens system, the high cost due to the use of optical components, and the difference in numerical aperture) existing in the connection portion between the light source and the element. Unlike a normal fiber (in which a refractive index distribution is formed inside), the PCF can maintain single mode propagation even when the core diameter is changed into a tapered shape. We focus on the fact that the wavefront can be kept constant over the entire length. Then, as in the configuration shown in FIG. 1, the present inventors have conceived of applying a PCF as a member for connecting a light source and an element, and the above-mentioned features of the PCF have solved the problem of the connection portion. A specific mode of the PCF for solving the problem will be described later.
[0018]
The PCF in the present invention includes not only a photonic crystal fiber in a narrow sense (to be described later) but also a so-called holey fiber (microstructured fiber). In any of the embodiments, the hole extends in the fiber along the longitudinal direction, and the hole ensures the polarization plane maintaining property and / or the variability / maintaining property of the cross-sectional shape of the propagation mode. Whether to use the PCF having the property of maintaining the polarization plane and / or the property of changing or maintaining the cross-sectional shape of the propagation mode may be appropriately selected according to the embodiment of the present invention.
[0019]
A PCF in a narrow sense has a periodic array of holes h1 and an array that breaks the periodicity of the array (a hole h2 at the center) as illustrated in FIG. 2 showing a cross section perpendicular to the optical axis. It is. These arrangements create a photonic bandgap, and light is localized and propagates in the hole core, which breaks the periodicity. A PCF based on this principle is also called a photonic bandgap fiber.
The holey fiber does not necessarily have a periodic arrangement of holes, and includes a waveguide guided by total reflection. However, depending on the properties required by the present invention, the polarization maintaining property and / or the propagation mode may be different. What is necessary is just to give the variability of the cross-sectional shape.
For PCF, reference may be made to JP-A-10-95628.
[0020]
There is no limitation on the laser light source and the wavelength conversion element used in the device, and a conventionally known device may be combined. As the laser light source, any laser light source such as an LD, a solid-state laser, a fiber laser, a gas laser, and a dye laser can be used. The wavelength conversion element includes a bulk type and an optical waveguide type.
As described in the description of the related art, the combination of the semiconductor laser and the optical waveguide type wavelength conversion element is difficult to match with each other in terms of the numerical aperture and the direction of the polarization plane, and there are many problems that can be improved by the present invention. It is a combination that makes the usefulness of the present invention more remarkable.
[0021]
There is no limitation on the semiconductor laser that can be used as the light source, and a conventionally known semiconductor laser may be used. Preferable examples include, for example, GaAs, InGaAs, and GaN as material systems, and 800 nm to 1000 nm, 1550 nm to 1700 nm, and 400 nm to 600 nm as oscillation wavelengths.
[0022]
A well-known wavelength conversion element may be used. As a basic structure of the element, there is a structure in which a domain-inverted structure is formed on a substrate made of a ferroelectric crystal. The optical waveguide type is further provided with an optical waveguide so as to cross the domain-inverted structure.
[0023]
As shown in FIGS. 3A and 3B, the domain-inverted structure is a structure in which the polarization direction in the ferroelectric crystal substrate 20 is locally inverted. In this structure, the regions R20 and the non-inversion regions N20 in the original polarization direction of the crystal substrate are alternately arranged in a stripe pattern at a predetermined period.
FIG. 3A shows an example of an optical waveguide type. An optical waveguide 21 is formed on a surface layer of a crystal substrate so as to cross a domain-inverted structure. FIG. 3B shows an example of a bulk type, in which a domain-inverted structure is formed over the entire substrate surface and the entire thickness of the crystal substrate, and the optical path is not limited. Input light L1 to be wavelength-converted is incident on these crystal substrates by the PCF, and the light L1 is wavelength-converted according to the quasi-phase matching method by passing through the non-inversion region and the inversion region alternately, and the output light L2. The wavelength conversion includes second harmonic generation (SHG), optical parametric oscillation / amplification / generation (OPO, OPA, OPG), difference frequency generation (DFG), sum frequency generation (SFG), and the like.
[0024]
Ferroelectric crystal may be a known, for _{example, LiNbO 3, LiTaO 3, X} A TiOX B O 4 (X A = K, Rb, Tl, Cs, X B = P, As) representative of such And those doped with various elements such as Mg.
[0025]
The structure of the optical waveguide type wavelength conversion element may refer to a known technique, but for the optical waveguide structure, for example, an embedded waveguide structure in which the refractive index of the optical waveguide portion is higher than that of the surroundings by an ion exchange method, A ridge waveguide structure in which only the optical waveguide portion is left in a ridge shape (ridge shape) and its periphery is removed, a loaded waveguide structure in which a dielectric or metal is loaded on the surface, and the like are given.
[0026]
Unlike ordinary fibers (in which a refractive index distribution is formed inside), PCF can maintain single mode propagation as it is even when the core diameter is changed. It can be connected with high efficiency. For example, in a combination of an LD and an optical waveguide type wavelength conversion element, the diameter of the cross-sectional shape of the propagation mode of the PCF is changed along the longitudinal direction, the diameter on the LD side is increased, and the coupling efficiency with the LD is improved. On the optical waveguide side, the size can be reduced in accordance with the optical waveguide mode shape.
More specifically, as shown in FIG. 1, at the end face on the LD light source side, the numerical aperture is equal to or larger than the numerical aperture of the light source, and at the end face on the element side, the numerical aperture is equal to or smaller than the numerical aperture of the optical waveguide of the element. In such a case, a tapered shape that gradually becomes thinner from the light source 1 side toward the element 2 side can be given. Thereby, the coupling efficiency between the light source 1 and the PCF 3 and the coupling efficiency between the element 2 and the PCF 3 are sufficiently increased, and the coupling efficiency between the LD light source and the optical waveguide type wavelength conversion element can be improved.
Furthermore, the cross-sectional shape of the propagation mode of the optical waveguide, for example, the cross-sectional shape of the propagation mode can be controlled in accordance with the ellipse, so that the coupling efficiency with the optical waveguide can be significantly improved.
[0027]
Here, the cross-sectional shape of the propagation mode is the shape of the distribution when the electric field intensity distribution generated as the propagation mode by the propagating light is viewed in a cross section perpendicular to the optical path (optical axis) of the optical waveguide.
[0028]
As a specific example of the case where the PCF is tapered, the total length is about several mm to several hundreds mm, the diameter of the propagation mode on the end face on the LD light source side is 5 to 50 μm, and the propagation mode on the end face on the element side is An embodiment in which the diameter is 2 to 15 μm is given. The change between the end faces may be linear or at an arbitrary rate. The change in the cross-sectional shape of the propagation mode of the PCF may be determined as appropriate according to the light source and the element, such as the change from the circular cross section to the circular cross section, as well as the change from the circular cross section to the elliptical cross section.
[0029]
The connection between the end face of the PCF and the emission face of the light source, and the end face of the PCF and the incidence face of the element may be a connection in which the faces are in close contact with each other or a connection with a gap. Each of these end faces is characterized in that the propagation mode size, the numerical aperture, etc. can be controlled according to the object of the light source and the element, so that the degree of freedom in optical design can be increased. It can be designed as one similar optical element.
[0030]
In the present invention, the polarization plane of the laser beam from the laser light source (particularly a problematic LD) is hardly coincident with the polarization direction of the element. By applying a twist to the PCF, the polarization direction of the laser beam incident from the LD into the PCF is made to coincide with the useful polarization direction of the wavelength conversion element (direction in which the nonlinear optical constant is large). It is configured to enter the wave path. The specific torsional angle is, for example, 90 degrees when both polarization directions are orthogonal. Thus, for example, it is not necessary to use an X-cut substrate having a horizontal polarization direction (instead, the depth of the domain-inverted region cannot be sufficiently secured), and a Z-cut substrate having a sufficiently deep domain-inverted structure is used. As a result, the alignment becomes easier and the wavelength conversion efficiency is improved.
[0031]
In addition, the present invention focuses on a further problem that the oscillation wavelength of the LD varies depending on conditions such as a temperature change. Since the wavelength conversion element is designed to convert only a specific wavelength, if the oscillation wavelength fluctuates, the wavelength conversion itself becomes impossible. On the other hand, in the present invention, a reflection structure is provided in any part of the optical path of the device, and about 5% to 50% of light of a predetermined wavelength in the oscillation spectrum of the laser light emitted from the LD. Is returned to the LD, whereby the oscillation wavelength of the LD is fixed at the predetermined value, and the fluctuation of the oscillation wavelength is suppressed.
The principle that the oscillation wavelength is fixed is based on the fact that the LD is oscillated in such a manner that the LD is oscillated by this wavelength by stably injecting a spectrum of a specific wavelength into the LD.
[0032]
The reflection structure may be any as long as it can selectively reflect only light of a predetermined wavelength, and even a reflection layer provided with a coating on the element end face, or an optical member may be used. An embodiment in which a Bragg grating is provided is preferable in that a narrow and stable oscillation wavelength suitable for wavelength conversion is obtained. For the method of forming the Bragg grating on the optical waveguide, a known technique may be referred to.For example, a dielectric thin film formed on the optical waveguide is irregularly processed periodically along the optical waveguide according to the grating period. Things.
[0033]
The portion where the Bragg grating is provided is provided on the optical waveguide of the PCF 3 (BG1 in FIG. 4), on the optical waveguide of the wavelength conversion element 2 (BG2 in FIG. 4), and at a further stage after the emission surface of the wavelength conversion element. Any portion on the optical waveguide 4 (BG3 in FIG. 4) and the like can be mentioned. Among these, the mode provided on the optical waveguide of the PCF 3 is a preferable mode in which the number of components can be reduced, and the number of connection points is small, so that the return light to the laser can be returned stably.
[0034]
When a Bragg grating is provided in a PCF or a subsequent optical waveguide, the temperature of the optical waveguide in that portion is adjusted by heating, cooling, voltage application, etc., and local expansion (heating) or contraction (cooling) is performed in the Bragg grating portion. Alternatively, a configuration may be adopted in which a change in the refractive index is caused and the period (interval) of the Bragg grating can be adjusted to be longer or shorter.
As a result, the wavelength returned to the LD can be stabilized, and the oscillation wavelength of the LD can be finely adjusted according to the characteristics of the wavelength conversion element. Examples of the heating means include various types of heaters and light irradiation, and examples of the cooling means include an electronic cooling element and water cooling. Voltage application includes appropriately providing electrodes. The control circuit may refer to a known technique.
[0035]
【Example】
Example 1
In this embodiment, as shown in FIG. 1, an InGaAs-based LD (edge emitting type) light source having an oscillation wavelength of 980 nm and an optical waveguide type wavelength conversion element (SHG element) are connected by a PCF, and an output of a wavelength of 490 nm is output. An apparatus capable of emitting light was configured.
[0036]
The wavelength conversion element has a domain-inverted structure (inversion period of 5.3 μm) formed on a Z plate (thickness: 0.5 mm, width: 3 mm, length in the optical path direction: 10 mm) made of Mg-doped LiNbO ₃ crystal, and has a mode size on a surface layer. An optical waveguide having a width of 8 μm and a thickness of 5 μm is formed.
[0037]
The PCF is a holey fiber having a periodic hole array and a hole array breaking the periodicity. The cross-sectional shape of the propagation mode on the light source side is elliptical with a long side of 15 μm × short side of 10 μm. It is formed in a tapered shape so that the cross-sectional shape of the propagation mode is an ellipse having a long side of 7 μm × a short side of 5 μm.
The extinction ratio for the preserved vertical and horizontal polarized light is 30 dB or more.
When connecting the LD light source and the wavelength conversion element by the PCF, the PCF was twisted so that the polarization direction of the laser light coincided with the polarization inversion direction of the element.
[0038]
Further, in this embodiment, as shown by reference numeral BG1 in FIG. 4, a Bragg grating which reflects 10% of light having a center wavelength of 980 nm in the oscillation spectrum of the laser light emitted from the LD is formed at an intermediate position of the PCF. The light is returned to the LD to stabilize the oscillation wavelength.
[0039]
According to the present embodiment, it was found that the optical components can be omitted and a wavelength converter can be obtained with a simple configuration. In addition, it was found that the matching of the numerical aperture of the PCF and the LD light source and the matching of the numerical aperture of the PCF and the optical waveguide of the element improved the coupling efficiency and were useful for improving the conversion efficiency.
In addition, when the second harmonic was actually generated using this device, it was found that the oscillation from the LD light source was stable at a wavelength of 980 nm, and that SHG light of 490 nm was always output stably with high efficiency. Was.
[0040]
Example 2
In this embodiment, as shown in FIG. 4, an optical fiber (PCF) 4 is further connected to the emission surface of the wavelength conversion element, and the Bragg grating of the first embodiment is moved to this optical fiber (reference numeral in FIG. 4). A wavelength conversion device was configured in the same manner as in Example 1 except for the Bragg grating shown by BG3). As a result, similar to the first embodiment, stable SHG light was obtained with a simple configuration.
[0041]
Example 3
In the present embodiment, a wavelength conversion device was configured in the same manner as in the first embodiment except that the wavelength conversion element 2 in the device configuration of FIG. 1 was replaced with a bulk type device shown in FIG. As a result, similar to the first embodiment, stable SHG light was obtained with a simple configuration.
[0042]
【The invention's effect】
As described above, in the present invention, since the PCF is applied to the coupling between the laser light source and the wavelength conversion element, the following effects can be obtained.
{Circle around (1)} It is possible to make the fundamental wave light incident from the light source (especially LD) to the wavelength conversion element (especially optical waveguide type) with high efficiency.
{Circle around (2)} The polarization plane can be adjusted only by adding a twist to the PCF, so that a wavelength conversion element formed of a Z plate can be easily used. The wavelength conversion element made of a Z plate has good reproducibility in producing a domain-inverted crystal, and also has good reproducibility of a wavelength conversion device.
{Circle around (3)} There are few optical components, and the mechanical stability is good.
{Circle around (4)} The optical axis alignment accuracy is eased, and mass production becomes easy.
(5) Since the oscillation wavelength of the LD is stabilized by providing the reflection structure and returning the light of the predetermined wavelength to the LD, the LD having a special control structure is unnecessary.
[Brief description of the drawings]
FIG. 1 is a sectional view schematically showing a configuration of a wavelength converter according to the present invention. In the figure, the apparatus is viewed from the side, and the thickness of the crystal substrate 20 and the thickness of the LD appear in the figure.
FIG. 2 is a diagram schematically illustrating an example of a cross section (a cross section perpendicular to a waveguide direction) of a PCF.
FIG. 3 is a perspective view schematically showing a structure of a wavelength conversion element.
FIG. 4 is a diagram schematically illustrating an example of an arrangement position when a Bragg grating is provided on each optical waveguide in the device of the present invention. In the figure, the crystal substrate surface of the wavelength conversion element 2 is viewed, and an optical waveguide appears at the center on the substrate surface.
FIG. 5 is a diagram schematically illustrating an example of an arrangement configuration of a light source to a wavelength conversion element in a conventional wavelength conversion device.
[Explanation of symbols]
1 laser light source 2 wavelength conversion element 3 photonic crystal fiber

Claims (5)

Laser light source, having a wavelength conversion element that can convert the wavelength of the laser light emitted from the light source,
A wavelength converter, wherein the light source and the element are connected via a photonic crystal fiber such that the laser light emitted from the light source is incident on the element. The wavelength conversion element is an optical waveguide type element, and the photonic crystal fiber has a numerical aperture equal to or greater than the numerical aperture of the light source on the end face on the side of the laser light source, and the photonic crystal fiber on the end face on the wavelength conversion element side. The wavelength conversion device according to claim 1, wherein the wavelength conversion device has a structure in which a cross-sectional shape of a propagation mode changes along a longitudinal direction so as to have a numerical aperture equal to or smaller than a numerical aperture of an optical waveguide of the element. 3. The photonic crystal fiber according to claim 1, wherein a twist is applied to the photonic crystal fiber such that a polarization direction of the laser light emitted from the laser light source and a polarization direction regarding wavelength conversion of the wavelength conversion element match. Wavelength converter. A reflection structure is further provided on any one of the following optical waveguides (a) to (u), and the reflection structure is a light having a predetermined wavelength in the oscillation spectrum of the laser light emitted from the laser light source. 2. The wavelength conversion device according to claim 1, wherein the wavelength conversion device can reflect the light to return to the laser light source.
(A) On an optical waveguide of a photonic crystal fiber.
(I) On the optical waveguide of the wavelength conversion element.
(C) On another optical waveguide that is further added downstream of the emission surface of the wavelength conversion element. 5. The wavelength conversion device according to claim 4, wherein said reflection structure is a Bragg grating formed on an optical waveguide.

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