JP2005221782A - Wavelength tunable visible light source - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wavelength tunable visible light source which has practical light intensity and is tunable. <P>SOLUTION: The tunable visible light source includes a first laser 63 emitting first laser beam with stabilized wavelength, a second laser 65 outputting tunable second laser beam, and a wavelength conversion element 61 which receives the first laser beam and the second laser beam to output coherent light having a wavelength different from those of the laser beams. The wavelength conversion element comprises a nonlinear optical crystal having a periodical polarization reversal structure. The light source has a light amplifier 64 connected between the second laser 65 and the wavelength conversion element 61. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a visible wavelength tunable light source, and more particularly to a visible wavelength tunable light source that changes the wavelength of visible light using second harmonic generation, difference frequency generation, and sum frequency generation effects generated in a nonlinear optical medium. .

  Conventionally, as a wavelength conversion element for converting the wavelength of light, an element using a semiconductor optical amplifier, an element using four-wave mixing, and the like are known. On the other hand, application of a wavelength conversion element using second harmonic generation, sum frequency generation, and difference frequency generation by pseudo phase matching, which is a kind of second-order nonlinear optical effect, is expected (for example, see Patent Document 1).

  FIG. 1 shows a configuration of a conventional quasi phase matching type wavelength conversion element. The wavelength conversion element includes a multiplexer 11 that combines the signal light A having a relatively small light intensity and the control light B having a relatively large light intensity, and a waveguide made of a nonlinear optical crystal having a polarization inversion structure. 12 and a demultiplexer 13 that separates the difference frequency light C and the control light B. The signal light A is converted into the difference frequency light C having another wavelength in the waveguide 12 and emitted together with the control light B. For example, when the excitation light B has a wavelength λ1 = 1.06 μm and the excitation light A having a wavelength λ2 = 1.55 μm is input, the mid-infrared light C having a wavelength λ3 = 3.35 μm can be obtained by difference frequency generation. Can do.

  When such a wavelength conversion element is applied as a mid-infrared laser light source, a highly sensitive gas sensor using mid-infrared light can be realized.

  Further, in another embodiment of FIG. 1, relatively large excitation light A and excitation light B are combined by a multiplexer 11 and are incident on a waveguide 12 made of a nonlinear optical crystal having a polarization inversion structure. In the waveguide 12, the sum frequency light of the excitation light A and the excitation light B is generated and emitted from the waveguide 12. For example, when the excitation light A has a wavelength λ1 = 1.06 μm and the excitation light B has a wavelength λ2 = 1.32 μm, yellow visible light C having a wavelength λ3 = 0.58 μm can be obtained by sum frequency generation. it can.

  When such a wavelength conversion element is used as a visible light source, it can be applied to a light source for refractive index measurement as an alternative to the D line of a conventional Na lamp, and high sensitivity of optical equipment using visible light such as a fluorescence microscope. There is a significant effect on the conversion.

  The yellow visible light source includes an external resonator type wavelength stabilized 1.06 μm semiconductor laser using a fiber grating (hereinafter referred to as FBG) as an excitation light source, a DFB semiconductor laser having an oscillation wavelength of 1.3 μm, and a WDM coupler. Etc., and a modularized wavelength conversion element. Here, the excitation light source is preferably a light source that oscillates in a single mode, such as a DBR or a DFB laser, and when it does not oscillate in a single mode, the wavelength is stabilized by the addition of an external resonator using an FBG. It is desirable to use a light source.

  Wavelength stabilization is an essential requirement because the phase matching bandwidth of the wavelength conversion element used to obtain visible light is narrow. FIG. 2 shows phase matching conditions for obtaining green light having a wavelength of 0.53 μm by second harmonic generation. It is the figure which calculated the phase matching curve when using lithium niobate as a nonlinear optical material, the period of the polarization inversion structure was 6.76 μm, and using the wavelength conversion element with a length of 10 mm. The vertical axis represents the light intensity of the second harmonic obtained. As can be seen from FIG. 2, the phase matching band is 0.2 nm or less. Therefore, the oscillation wavelength of the 1.06 μm semiconductor laser needs to be stabilized within the spectral width of 0.2 nm.

In the above-described visible light source, when changing the wavelength λ _{3 of the} visible light to be output, it is necessary to change the wavelength of the excitation light so that the relationship of the following equation is satisfied.
n ₃ / λ ₃ −n ₂ / λ ₂ −n ₁ / λ ₁ −1 / Λ = 0 (Formula 1)
Here, n is the refractive index, λ is the wavelength, and Λ is the period of the domain-inverted structure. Subscript 3 is visible light, 1 and 2 are excitation light,
1 / λ ₃ = 1 / λ ₁ + 1 / λ ₂ (Formula 2)
Meet the relationship. When the period Λ of the domain-inverted structure is constant, in order to change the wavelength of the visible light λ ₃ , it is necessary to change the wavelength of the excitation light of λ ₂ or λ ₁ .

JP 2003-140214 A

However, when a 1.06 μm semiconductor laser that has been wavelength-stabilized using an FBG as described above as an excitation light source is used, the oscillation wavelength is uniquely determined by the wavelength reflected by the FBG. There is a problem that it is difficult to change the wavelength of the 1.06 μm semiconductor laser in accordance with the change of λ ₃ .

  On the other hand, the oscillation wavelength of the 1.3 μm band DFB laser can be varied by changing the temperature, but the practical variable range is about 4 nm, and it is difficult to vary the wavelength over a wide range. There was also a problem.

  As another embodiment, it is also conceivable to use a 1.06 μm semiconductor laser whose wavelength is stabilized by FBG and a 1.3 μm band tunable laser which is put into practical use as an excitation light source. However, the 1.3 μm band wavelength tunable laser has a problem that the intensity of output light is weak and it is difficult to construct a yellow visible wavelength tunable light source having practical light intensity.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a visible wavelength variable light source having practical light intensity and capable of changing the wavelength.

  In order to achieve the above object, according to the present invention, a first laser that outputs a first laser beam having a stabilized wavelength and a second laser that outputs the first laser beam are provided. Wavelength conversion for inputting a second laser capable of changing the wavelength of light, the first laser light and the second laser light, and outputting coherent light having a wavelength different from the wavelength of the laser light In the visible wavelength tunable light source including the element, the wavelength conversion element includes a nonlinear optical crystal having a periodic polarization inversion structure, and an optical amplifier connected between the second laser and the wavelength conversion element. It is characterized by having.

  According to a second aspect of the present invention, the wavelength of the second laser light according to the first aspect is a 1.3 μm band, and the amplification band of the optical amplifier is a 1.3 μm band.

  The invention described in claim 3 is characterized in that the optical amplifier according to claim 1 or 2 is either a semiconductor optical amplifier or a rare earth-doped optical fiber amplifier.

  The invention according to claim 4 is characterized in that the rare earth ion of the rare earth doped optical fiber amplifier according to claim 3 is Pr.

According to a fifth aspect of the present invention, the nonlinear optical crystal according to any one of the first to fourth aspects of the present invention includes LiNbO ₃ , LiTaO ₃ , LiNb _(x) Ta _(1-x) O ₃ (0 ≦ x ≦ 1 Or at least one selected from the group consisting of Mg and Zn as an additive.

  The invention described in claim 6 is characterized in that the nonlinear optical crystal according to any one of claims 1 to 5 has a waveguide structure.

  The invention described in claim 7 includes a light source that emits laser light and a detection unit that detects fluorescence by the laser light from the object to be measured, and is a fluorescence microscope apparatus that analyzes fluorescent proteins in living cells. The light source includes a first laser that outputs a first laser beam having a stabilized wavelength, a second laser that can change the wavelength of the output second laser beam, and periodic polarization inversion A wavelength conversion element that is formed of a nonlinear optical crystal having a structure, inputs the first laser light and the second laser light, and outputs coherent light having a wavelength different from the wavelength of the laser light; And an optical amplifier connected between the wavelength conversion element and the wavelength conversion element.

  As described above, according to the present invention, the optical amplifier is provided between the second laser capable of changing the wavelength of the output second laser light and the wavelength conversion element. It is possible to provide a visible wavelength tunable light source having a high light intensity and capable of changing the wavelength.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The visible wavelength tunable light source according to the present embodiment includes a 1.06 μm semiconductor laser stabilized by an FBG as an excitation light source, a 1.3 μm band wavelength tunable laser, a wavelength conversion element module, and a 1.3 μm band wavelength tunable laser. And an optical amplifier connected between the wavelength conversion element module. According to the present invention, a practical yellow visible wavelength variable light source can be configured by amplifying the output of a 1.3 μm band wavelength variable laser with an optical amplifier.

Here, the wavelength conversion element module includes a nonlinear optical crystal having a periodic polarization inversion structure, and any of LiNbO ₃ , LiTaO ₃ , LiNb _(x) Ta _(1-x) O ₃ (0 ≦ x ≦ 1). Alternatively, a nonlinear optical crystal containing at least one selected from the group consisting of Mg and Zn as an additive can be used. Further, as the optical amplifier, a semiconductor optical amplifier or a rare earth doped optical fiber amplifier may be used. The semiconductor optical amplifier has an advantage that the device size is reduced, but has a disadvantage that the light output of the light source cannot be sufficiently obtained because the output light intensity is relatively weak. On the other hand, the optical fiber amplifier has an advantage that a light intensity close to 100 mW can be obtained, but has a disadvantage that the apparatus becomes large.

FIG. 3 shows the wavelength variable range of yellow light of the visible wavelength variable light source according to the embodiment of the present invention. In a wavelength conversion element using LiNbO ₃ as a nonlinear optical material, the wavelength variable range of yellow light obtained when the period of the domain-inverted structure is 9.1 μm and a 1.06 μm semiconductor laser is used as an excitation light source FIG. The temperature of the wavelength conversion element is shown on the horizontal axis, the wavelength of the 1.3 μm band wavelength variable laser is shown on the left vertical axis, and the wavelength of yellow light that is the output of the visible wavelength variable light source is shown on the right vertical axis. By changing the temperature of the wavelength conversion element from 10 ° C. to 80 ° C., which is a practical range, the wavelength of yellow light can be changed in the range of 582 nm to 585 nm.

  At this time, the output wavelength of the 1.3 μm band wavelength tunable laser needs to be changed in the range of 1290 nm to 1305 nm. Since this wavelength range is included in the amplification band (1280 nm to 1330 nm) of the rare earth-doped optical fiber amplifier containing Pr (praseodymium) as the rare earth ion, a yellow visible wavelength variable light source having practical light intensity Can be configured.

  EXAMPLES Hereinafter, although demonstrated using the Example of this invention, this invention is not limited to these Examples at all.

The visible wavelength tunable light source of Example 1 includes a wavelength conversion element module using LiNbO ₃ as a nonlinear optical material, a 1.064 μm semiconductor laser stabilized in wavelength by FBG, and a 1.3 μm band tunable laser, A Pr-doped 1.3 μm band optical fiber amplifier is connected between the wavelength conversion element module and the 1.3 μm band tunable laser to constitute a yellow visible wavelength tunable light source.

The wavelength conversion element uses a ridge-type waveguide using a directly bonded substrate. FIG. 4 shows a method for producing a single-mode ridge waveguide according to the first embodiment. The first substrate 41 is a Z-cut Zn-added LiNbO ₃ substrate in which a periodic domain-inverted structure is previously prepared, and the second substrate 42 is a Z-cut Mg-added LiNbO ₃ substrate. The substrates 41 and 42 are both 3-inch wafers whose surfaces are optically polished, and the thickness of the substrate is 300 μm. After the surfaces of the first substrate 41 and the second substrate 42 are rendered hydrophilic by normal acid cleaning or alkali cleaning, the substrates 41 and 42 are superposed in a clean atmosphere. The superposed substrates 41 and 42 are placed in an electric furnace and subjected to diffusion bonding by heat treatment at 500 ° C. for 3 hours (first step). The bonded substrates 41 and 42 were void-free, and no cracks or the like occurred when returned to room temperature.

  Next, using a polishing apparatus in which the flatness of the polishing surface plate is controlled, polishing is performed until the thickness of the first substrate 41 of the bonded substrates 41 and 42 becomes 6 μm. After the polishing process, a mirror-polished polishing surface is obtained by performing a polishing process (second step). The polished thin film substrate is set on a dicing saw, and a ridge-type waveguide having a waveguide width of 10 μm is manufactured using a diamond blade having a particle diameter of 4 μm or less (third step). The manufactured ridge-type waveguide is cut into a strip shape from the substrate, and the end face of the waveguide is optically polished to prepare a wavelength conversion element having a length of 60 mm.

Note that a LiTaO ₃ substrate may be used as the second substrate 42. Further, when the non-doped LiNbO ₃ substrate is used as the first substrate 41 and the LiTaO ₃ substrate is used as the second substrate 42, a similar wavelength conversion element can be manufactured. Furthermore, the thickness of the substrate can be 200 μm or more and 1 mm or less in addition to 500 μm.

  FIG. 5 shows the configuration of the wavelength conversion element module according to the first example. The wavelength conversion element 51 manufactured as described above is fixed on the carrier 52. A polarization maintaining single mode optical fiber 54 is connected to the input side of the wavelength conversion element 51. The output side of the wavelength conversion element 51 is optically coupled to the lens module 53 and emitted through the filter 55. A Peltier element 57 thermally coupled to the carrier 52 is built in and enclosed in a package 56 to produce a wavelength conversion module.

  FIG. 6 shows a configuration of a visible wavelength variable light source according to the first embodiment. The optical fiber on the input side of the wavelength conversion element module 61 shown in FIG. 5 and the output optical fiber of the WDM coupler 62 that multiplexes the input lights having wavelengths of 1.06 μm and 1.3 μm are fusion-connected. A pigtail of a 1.064 μm semiconductor laser 63 is fused and connected to the 1.06 μm input optical fiber of the WDM coupler 62. The pigtail is provided with an FBG 66 for wavelength stabilization.

  Further, the output fiber of the 1.3 μm band optical fiber amplifier 64 is fusion-connected to the 1.3 μm input optical fiber of the WDM coupler 62. An output fiber of the 1.3 μm band wavelength tunable laser 65 is fused and connected to the input fiber of the 1.3 μm band optical fiber amplifier 64. In the visible wavelength tunable light source according to the example 1, when the wavelength of the 1.3 μm band wavelength tunable laser 65 is changed from 1.29 μm to 1.33 μm, the wavelength of yellow visible light that is the output of the wavelength conversion element module 61 is changed. However, it changes from 583 nm to 591 nm, and an optical output of 10 mW on average can be obtained.

  The second embodiment includes a wavelength conversion element module similar to that of the first embodiment, a 0.98 μm semiconductor laser stabilized by FBG, and a 1.3 μm band wavelength tunable laser. A Pr-doped 1.3 μm band optical fiber amplifier is connected to the 3 μm band tunable laser to constitute a visible wavelength tunable light source. For wavelength synthesis of the excitation light, a WDM coupler that multiplexes input light having wavelengths of 0.98 μm and 1.3 μm is used.

  In the visible wavelength tunable light source according to Example 2, when the wavelength of the 1.3 μm band wavelength tunable laser was changed from 1.29 μm to 1.33 μm, the wavelength of visible light as the output of the wavelength conversion element module was changed from 557 nm. It changes up to 564 nm, and an average optical output of 10 mW can be obtained. In addition, when the wavelength-stabilized 0.94 μm semiconductor laser is used in Example 2, the wavelength of visible light can be changed from 543 m to 550 nm.

  FIG. 7 shows a configuration of a fluorescence microscope apparatus to which the visible wavelength variable light source according to the second embodiment is applied. The fluorescence microscope apparatus incorporates the visible wavelength variable light source according to the second embodiment as a light source, and irradiates the object to be measured with the visible light from the light source via the optical system including the lens 75, the dichroic mirror 73, and the objective lens 74. . The fluorescence output from the object to be measured is observed by the high sensitivity camera 71 through the objective lens 74, the dichroic mirror 73 and the prism 72. When used for observation of a fluorescent protein (GFP) fused to a gene of another protein and introduced into a cell, a specific structure or functional molecule can be fluorescently labeled with high sensitivity in a living cell.

  The visible wavelength variable light source according to the present invention has practical light intensity and the wavelength of output light can be varied. Therefore, the sensitivity of refractive index measurement using visible light is increased, and fluorescence by a fluorescence microscope is used. It can be applied to increase the sensitivity of protein observation.

It is a perspective view which shows the structure of the conventional quasi phase matching type wavelength conversion element. It is a figure which shows the phase matching curve for obtaining the green light of wavelength 0.53 micrometer by a 2nd harmonic generation. It is a figure which shows the wavelength variable range of the yellow light of the visible wavelength variable light source concerning one Embodiment of this invention. 6 is a diagram illustrating a method for producing a single-mode ridge waveguide according to Example 1. FIG. It is a figure which shows the structure of the wavelength conversion element module concerning Example 1. FIG. It is a figure which shows the structure of the visible wavelength variable light source concerning Example 1. FIG. It is a figure which shows the structure of the fluorescence microscope apparatus to which the visible wavelength variable light source concerning Example 2 is applied.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Multiplexer 12 Non-linear waveguide 13 Demultiplexer 41 1st board | substrate 42 2nd board | substrate 51 Wavelength conversion element 52 Carrier 53 Lens module 54 Polarization-maintaining single mode optical fiber 55 Filter 56 Package 57 Peltier element 61 Wavelength Conversion element module 62 WDM coupler 63 1.064 μm semiconductor laser 64 1.3 μm band optical fiber amplifier 65 1.3 μm band wavelength tunable laser 66 FBG
71 High-sensitivity camera 72 Prism 73 Dichroic mirror 74 Objective lens 75 Lens

Claims (7)

A first laser that outputs a first laser beam having a stabilized wavelength; a second laser that can vary the wavelength of the second laser beam that is output; the first laser beam; A visible wavelength tunable light source including a second laser beam and a wavelength conversion element that outputs coherent light having a wavelength different from the wavelength of the laser beam.
The wavelength conversion element is composed of a nonlinear optical crystal having a periodic polarization inversion structure,
A visible wavelength variable light source comprising an optical amplifier connected between the second laser and the wavelength conversion element.   2. The visible wavelength variable light source according to claim 1, wherein the wavelength of the second laser light is in a 1.3 [mu] m band, and the amplification band of the optical amplifier is in a 1.3 [mu] m band.   3. The visible wavelength tunable light source according to claim 1, wherein the optical amplifier is either a semiconductor optical amplifier or a rare earth-doped optical fiber amplifier.   4. The visible wavelength variable light source according to claim 3, wherein the rare earth ion of the rare earth doped optical fiber amplifier is Pr. The nonlinear optical crystal is one of LiNbO ₃ , LiTaO ₃ , LiNb _(x) Ta _(1-x) O ₃ (0 ≦ x ≦ 1), or selected from the group consisting of Mg and Zn. The visible wavelength tunable light source according to any one of claims 1 to 4, wherein at least one kind is contained as an additive.   The visible wavelength variable light source according to claim 1, wherein the nonlinear optical crystal has a waveguide structure. A fluorescence microscope apparatus for analyzing a fluorescent protein in a living cell, comprising: a light source that emits laser light; and a detection unit that detects fluorescence by the laser light from an object to be measured.
A first laser that outputs a first laser beam having a stabilized wavelength;
A second laser capable of varying the wavelength of the output second laser beam;
A wavelength conversion element made of a nonlinear optical crystal having a periodic polarization inversion structure, which inputs the first laser light and the second laser light and outputs coherent light having a wavelength different from the wavelength of the laser light When,
A fluorescence microscope apparatus comprising: an optical amplifier connected between the second laser and the wavelength conversion element.

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