WO2006013890A1 - コヒーレント光源 - Google Patents
コヒーレント光源 Download PDFInfo
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- WO2006013890A1 WO2006013890A1 PCT/JP2005/014203 JP2005014203W WO2006013890A1 WO 2006013890 A1 WO2006013890 A1 WO 2006013890A1 JP 2005014203 W JP2005014203 W JP 2005014203W WO 2006013890 A1 WO2006013890 A1 WO 2006013890A1
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- Prior art keywords
- wavelength
- light source
- fundamental wave
- wavelength conversion
- coherent light
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- 230000001427 coherent effect Effects 0.000 title claims abstract description 84
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Classifications
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- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/35—Non-linear optics
- G02F1/37—Non-linear optics for second-harmonic generation
- G02F1/377—Non-linear optics for second-harmonic generation in an optical waveguide structure
- G02F1/3775—Non-linear optics for second-harmonic generation in an optical waveguide structure with a periodic structure, e.g. domain inversion, for quasi-phase-matching [QPM]
Definitions
- the present invention relates to a coherent light source, and more particularly, to a coherent light source including a wavelength conversion element that receives light and emits light having a wavelength different from the wavelength of the received light by converting the wavelength.
- wavelength conversion technology for light has been continuously developed in connection with coherent light sources, and the efficiency and output of coherent light sources have been increasing.
- a method for realizing high efficiency of a coherent light source a method of improving wavelength conversion efficiency by increasing the power density of the fundamental wave using an internal resonator, and a high spire value by using a Q switch pulse
- Both methods achieve high-efficiency conversion with an efficiency of about 50%.
- SHG the second harmonic
- the second harmonic can be generated with high efficiency. For this reason, in order to realize highly efficient second harmonic generation, further improvement in the conversion efficiency of the nonlinear material responsible for wavelength conversion is desired.
- a wavelength conversion unit In addition to high efficiency, in order to generate visible light with high output, that is, to generate second harmonic having a wavelength in the visible light region with high output, a wavelength conversion unit is configured. In addition to high conversion efficiency, it is desirable to have excellent resistance in the wavelength region near the wavelength of the generated second harmonic. This is because if the nonlinear material is damaged by light such as the second harmonic wave that propagates inside the wavelength conversion unit and has high power, it may be difficult to obtain a desired output stably. Because there is.
- Mg-doped LiNb03 having a periodic domain-inverted structure in a crystal
- MgO LiNb03
- MgLN Mg-doped LiNb03
- MgLN has high efficiency for generating visible light. It is attracting attention as a nonlinear material.
- MgLN is known to be an inorganic material having the highest nonlinearity with respect to light having a wavelength in the visible light region, and to have excellent light damage resistance. For this reason, it is suitable for increasing the efficiency and output of light sources. More advantageously, MgLN can be produced at low cost because of its easy crystal growth.
- Patent Document 1 discloses a blue light coherent light source that uses MgLN (hereinafter also referred to as “PPMgLN (Periodically Poled MgO: LiNb03)”) having a periodic domain-inverted structure as an internal resonator.
- PPMgLN Periodically Poled MgO: LiNb03
- Non-Patent Document 1 reports a phenomenon in which a green induced infrared absorption (GRIIRA) force increases in MgLN with an Mg doping amount of 4.8 mol% or less. .
- GRIIRA green induced infrared absorption
- wavelength conversion By performing wavelength conversion using a nonlinear optical crystal produced by crystal growth of the above-described nonlinear material as a wavelength conversion element, from blue light having a wavelength of approximately 450 nm to green light having a wavelength of approximately 530 nm, Can be generated.
- High-efficiency wavelength conversion is realized by using high-power emitted light that can also generate light source as the fundamental wave, and converting it to the second harmonic with a nonlinear optical crystal with high conversion efficiency. Is realized.
- a wavelength converter that is a wavelength converter uses a material that is sufficiently stable in the fundamental and second harmonic (SHG) wavelength regions. Hope to do. If a material having the factor that optical characteristics become unstable due to light in the wavelength region including the SHG wavelength is used as the wavelength conversion element, stable SHG cannot be generated. Such a material can be said to be unsuitable for a wavelength conversion element.
- SHG fundamental and second harmonic
- LiNb03 (hereinafter also referred to as “LN”) and LiTa03 (hereinafter also referred to as “LT”), which are nonlinear materials
- LN LiNb03
- LT LiTa03
- photodamage (1) photodamage and (2) GRIIRA (red light of green light excitation) (Outside absorption), (3) No light Have been reported.
- Photodamage is a phenomenon of refractive index change caused by photoexcitation.
- the refractive index of an LN crystal fluctuates when irradiated with the above-mentioned short wavelength light.
- the phase matching condition is not satisfied in the part where the light damage occurs, and the conversion efficiency of the element is reduced. This phenomenon is a reversible phenomenon, and when the light irradiation is stopped, the changed refractive index is restored.
- Photodamage is a force that depends on the wavelength and intensity of light. It is observed in LN crystals with Mg added at 5 mol% or more.
- Non-patent document 1 reports that force absorption of infrared light by green light is observed and reported for MgLN with a Mg doping amount of 4.8 mol% or less.
- Light damage is a phenomenon that exists in all crystals.
- the crystal's resistance to light damage is determined by the minimum value of the light power density that causes crystal breakage.
- optical damage and GRIIRA are caused by LN, LT, etc. with relatively low power light. To be born. Therefore, it is difficult to construct a light source that generates high-power visible light using LN, LT, etc. In order to realize such a light source, for example, in order to obtain an output exceeding 1 W, it is necessary to heat the crystal temperature to 100 ° C or higher. In other words, if a configuration that stably converts high-power visible light using LN, LT, etc. as a wavelength conversion element is provided, there will be the problem of stability of the light source due to phenomena such as light damage at the same time. Become.
- KTiOP04 (hereinafter also referred to as "KTP") is known to have a phenomenon called "gray track" in which a color center is generated in a crystal by irradiation with visible light having a short wavelength. This phenomenon is a factor that limits the power of the converted light when using ⁇ as a wavelength conversion element.
- MgLN and MgLT are materials that are attracting attention as highly nonlinear materials having excellent resistance to visible light.
- This periodic domain-inverted structure (PPMgLN) is a nonlinear material with high conversion efficiency and excellent damage damage to light, and can be used for various applications including internal resonator structures.
- PPMgLN does not cause a GRIIRA phenomenon that causes practical problems if the Mg doping amount is 5 mol% or more for GRIIRA. In fact, as shown above, it is used as an internal resonator type wavelength conversion element at an output of 1 W or less.
- Patent Document 1 Japanese Patent Laid-Open No. 06-242478
- Non-Patent Document 1 Y. Furuka ⁇ , ⁇ . Kitamura, ⁇ . Alexandrovski, R. ⁇ . Laute, ⁇ . Fuezienore, G. Fullon (Y. FurukaWa, K. Kitamura, A. Alexandrovski, RK Route, and MM Fejer, G. Foulon), “Green-induced Infrared absorptio in MgO doped LiNb03”, Applied “Physics” Letters ( Applied Physics Lette rs), (USA), American 'Institut' Ob 'Physics (American Institute of Physics), April 2, 2001, vol. 78, p. 1970—1972
- MgO 2 LiNb03 (MgLN) and MgO 2: LiTa03 (MgLT) are highly nonlinear materials having excellent light damage resistance.
- PPMgLN wavelengths near room temperature Stable high output wavelength conversion is also possible in conversion.
- the inventors of the present application irradiate a fundamental wave having a high peak power to a crystal or the like having a periodic domain-inverted structure (PPMgLN, etc.) or generate a high output of visible light. In this case, we found a phenomenon that the output is unstable.
- an output instability phenomenon that seems to be caused by causes other than optical damage at high power conversion of 1 W or more.
- Such an unstable phenomenon is a factor that raises questions about the stability of the light source when a high-power coherent light source is constructed using PPMgLN or the like as a wavelength conversion element. If this phenomenon is neglected, the reliability of the light source is significantly impaired.
- An object of the present invention is to elucidate the cause of this output instability phenomenon and to show a method for avoiding this phenomenon, and to provide a coherent light source capable of stably outputting at a high output. Means for solving the problem
- a light source unit that emits a fundamental wave having a first wavelength of 1070 nm or more and a fundamental wave that emits a second harmonic of the fundamental wave at a predetermined average output or more. It is a coherent light source having a wavelength converter.
- the wavelength conversion section has Mg dopant LiNb03 having a periodic domain-inverted structure! /.
- the wavelength conversion unit has Sc dope LiNb03 having a periodic domain-inverted structure! /.
- the wavelength conversion unit preferably includes In-dope LiNb03 having a periodic domain-inverted structure! /.
- the wavelength converter has a Zn doped LiNb03 having a periodic domain-inverted structure! /.
- the present invention provides a light source unit that emits a fundamental wave having a first wavelength of 1027 nm or more, and emits a second harmonic of the fundamental wave at a predetermined average output or more upon receiving the fundamental wave.
- the present invention provides a light source unit that emits a fundamental wave having a first wavelength of 1018 nm or more, and emits a second harmonic of the fundamental wave at a predetermined average output or more upon receiving the fundamental wave.
- the present invention provides a light source unit that emits a fundamental wave having a first wavelength of 850 nm or more, and emits a second harmonic of the fundamental wave at a predetermined average output or more upon receiving the fundamental wave.
- the coherent light source has a wavelength conversion unit including KTiOP04 having a periodic domain-inverted structure.
- each aspect of the present invention it is preferable to further include an ultraviolet light shielding unit that covers at least a part of the wavelength conversion unit and protects the wavelength conversion unit even when the optical power is 400 nm or less incident from the outside. .
- the present invention provides a light source unit that emits a fundamental wave having a first wavelength of 800 nm or more, and light having a second wavelength that is a half wavelength of the first wavelength after receiving the fundamental wave.
- This is a coherent light source having a wavelength conversion unit that emits light at a predetermined average output or more and an ultraviolet light shielding unit that covers at least a part of the wavelength conversion unit and protects the wavelength conversion unit from light incident at a wavelength of 400 nm or less.
- the wavelength converter is operated at 100 degrees Celsius or less.
- the polarization reversal angle which is an angle formed by the normal line of the stripe indicated by the periodic polarization reversal structure of the wavelength converter and the traveling direction of the fundamental wave, is 3 degrees or more. Good.
- the wavelength converter has a crystal structure, and an angle formed between a stripe indicated by the periodic domain-inverted structure and a direction perpendicular to the a-axis and the c-axis of the crystal structure is 0. It is preferred to be greater than 1 degree and less than 1 degree.
- a light source unit that emits a fundamental wave having a predetermined first wavelength and a periodic polarization reversal structure are provided, and the second harmonic of the fundamental wave is predetermined by receiving the fundamental wave.
- the polarization reversal angle which is the angle formed by the normal of the stripe indicated by the periodic polarization reversal structure provided in the wavelength conversion portion and the traveling direction of the fundamental wave, is 3 degrees. This is the coherent light source characterized by the above.
- the wavelength conversion unit has a crystal structure and has periodic polarization reversal.
- the angle between the stripe indicated by the structure and the direction perpendicular to the a-axis and c-axis of the crystal structure is preferably greater than 0 degree and less than 1 degree.
- the electrode unit further arranged to be energized to the wavelength conversion unit;
- the light source section preferably has a fiber laser.
- the light source unit is Q-switch pulse driven and the repetition frequency thereof is 1 kHz or more.
- the predetermined average output of the second harmonic of the wavelength converter is 1
- the predetermined average output of the second harmonic of the wavelength converter is 2
- the predetermined average output of the second harmonic of the wavelength converter is 2
- the power is 5 W or more.
- the predetermined average output of the second harmonic of the wavelength converter is 3
- it is W or more.
- the present invention provides a high-power coherent light source using a wavelength conversion element in the visible light region.
- the coherent light source according to the present invention has an output instability at high output and a stable output characteristic free from reliability problems.
- FIG. 11 Graph showing the relationship between polarization reversal angle, SHG conversion efficiency, and wavelength conversion element tolerance
- the coherent light source according to the present invention uses high-output light emitted from a high-output light source unit as a fundamental wave, and converts the fundamental wave into a second harmonic by a wavelength conversion unit and emits it.
- the coherent light source useful in the present invention is a high-power coherent light source having a light source unit that emits a high-output fundamental wave and a wavelength conversion element that realizes highly efficient wavelength conversion.
- a high-power coherent light source having a light source unit that emits a high-output fundamental wave and a wavelength conversion element that realizes highly efficient wavelength conversion.
- SHG fundamental wave and second harmonic
- the present invention discloses a coherent light source that is capable of more stable and high output based on knowledge found in the phenomenon discovered by the present inventors.
- the inventor of the present application discovered an unstable phenomenon that has not been observed in the past when PPMgLN was used to generate high-power green light (wavelength: 532 ⁇ m) of 1 W or higher. Further, the inventor of the present application has found that there is a phenomenon that the output deteriorates even when the light source is used for a long time. The inventor of the present application elucidates the cause of the phenomenon that has been discovered, and discloses a configuration of a high-power coherent light source that does not cause such instability and deterioration of output with time.
- the inventor of the present application conducted a wavelength conversion experiment on PPMgLN doped with Mg 5 mol% using the optical system 100 shown in FIG.
- the optical system 100 includes a light source 101, a wavelength conversion element 102, and a condensing optical system 103.
- the light source 101 is a laser light source using Nd: YV04 as a solid-state laser, and generates laser light having a wavelength of 1064 nm by semiconductor laser excitation.
- the light source 101 is configured to insert an AO switch in the resonator of the solid-state laser and generate a pulse train having a high spire value by the Q switch.
- the wavelength conversion element 102 includes PPMgL N doped with Mg 5 mol%, has a polarization inversion structure with a period of 6.95 / zm, and has an element length of 10 mm.
- Light having a wavelength of 1064 nm emitted from the light source 101 is used as the fundamental wave 104.
- the fundamental wave 104 enters the wavelength conversion element 102 and is converted to SHG 105 having a wavelength of 532 nm.
- the fundamental wave 104 is generated as a pulse train as described above, and the average power can be set to several watts.
- the fundamental wave 104 may be condensed by the condensing lens constituting the condensing optical system 103 and converted in wavelength by the wavelength conversion element 102. The conversion efficiency for this wavelength conversion is around 50%.
- the inventor of the present application in addition to SHG105 with a wavelength of 532nm, emits ultraviolet light with a wavelength of 355nm (not shown) from wavelength conversion element 102 (PP MgLN). I found out that The generation of ultraviolet light was observed in the fundamental wave intensity region where the conversion efficiency of SHG105 was reduced. In addition, the propagation direction of the observed SHG105 and ultraviolet light (not shown), that is, the direction indicated by the pointing vector, was slightly shifted, and it was revealed that they occurred at different emission angles.
- FIG. 2A shows the optical system 200 used in this study.
- the optical system 200 includes two types of light sources, a light source 201 and an ultraviolet light source 202, a dichroic mirror 203, and a wavelength conversion element 102 ( PPMgLN), a filter 204, and a photo detector (PD) 205.
- the light source 201 is a light source that emits light having a predetermined wavelength in the infrared region (infrared light 210), and the ultraviolet light source 202 is light having a predetermined wavelength (for example, 355 nm) in the ultraviolet region (ultraviolet light). ).
- the light emitted from both the light sources 201 and 202 is combined by the dichroic mirror 203 and enters the wavelength conversion element 102 (PPMgLN).
- the light emitted from the wavelength conversion element 102 is transmitted through the filter 204 in a wavelength-selective manner, wavelength-separated, and detected by the infrared light 210 force SPD 205 that passes through the filter 204.
- the light source 201 continuously emits infrared light 210, and the ultraviolet light source 202 emits ultraviolet light while intensity-modulating it.
- FIG. 2B is a graph plotting the intensity of the ultraviolet light emitted from the ultraviolet light source 202 and the intensity of the infrared light 210 detected by the PD 205.
- the horizontal axis represents time, and the vertical axis represents light intensity.
- the ratio of the intensity 251 of infrared light 210 to the intensity 253 of ultraviolet light is not particularly important. Prioritizing the clarity of the graph, the plot is ignored.
- the important point here is the correlation between the time zone in which the intensity 253 of the ultraviolet light shows a non-zero value and the time zone in which the intensity 251 of the infrared light 210 is relatively low.
- the power of the ultraviolet light actually used in the experiment was about several mW, but it can be seen that the intensity of the infrared light 210 emitted from the wavelength conversion element 102 decreases with the irradiation of the ultraviolet light.
- the generated output instability phenomenon of SHG105 occurs when, for example, the fundamental wave 104 having a wavelength of 1064 nm is converted to SHG105 having a wavelength of 532 nm in PPMgLN (wavelength conversion element 102).
- SFG (not shown) in the ultraviolet region having a wavelength of 355 ⁇ m, which is the sum frequency of the wavelength of 1064 nm and the wavelength of 532 nm, is generated.
- the generation of SFG in the ultraviolet region increases the absorption of visible light, resulting in a thermal lens effect in which the temperature in the crystal (PPMgLN) partially rises, leading to unstable phase matching.
- the wavelength conversion element 102 generating visible light for example, SHG105
- ultraviolet light having a wavelength of 400 nm or less
- the thermal lens effect occurs, which causes the output of the wavelength conversion element 102 to fluctuate.
- the harmonic output is small, even if absorption occurs, the temperature rise in the wavelength conversion element 102 due to absorption is small, and the thermal lens effect does not occur.
- the output of harmonics for example, SHG105
- exceeds about 1W the range of temperature rise due to the absorption of harmonics increases, the thermal lens effect occurs, and output instability occurs.
- Such harmonic output instability is caused not only by ultraviolet light generated inside the wavelength conversion element 102 but also by ultraviolet light irradiated from the outside of the generated wavelength conversion element 102. Even when the power of ultraviolet light generated inside the wavelength conversion element 102 or irradiated from the outside is relatively small, the absorption of visible light increases. Therefore, it is preferable that the wavelength conversion element 102 is protected by the ultraviolet light shielding portion so that the external force is not incident on the ultraviolet light.
- This ultraviolet light shielding part desirably shields light having a wavelength of 40 Onm or less to protect the element 102. It is desired to have a high shielding ability (non-transmission) for light having a wavelength of at least 320 nm and not more than 400 nm.
- the ultraviolet light shielding part By providing the ultraviolet light shielding part, even if the ultraviolet light incident on the wavelength conversion element 102 from the outside is almost completely shielded, if the output of harmonics (for example, SHG) is increased, the output will be reduced. Stability phenomenon was observed. The cause is the presence of ultraviolet light generated inside the wavelength conversion element 120 substrate. However, in order to generate SFG with a degree that cannot be ignored, that is, in order to perform wavelength conversion to SFG with high efficiency, a predetermined phase matching condition must be satisfied. In the same device, it is unlikely that high-efficiency wavelength conversion of SFG that uses a wavelength different from the intended purpose from the element design stage will easily occur.
- harmonics for example, SHG
- the tolerance is high. Even if the average output of SHG is several W (power density: several MWZcm 2 or more), unstable phenomenon does not occur o
- PPMgLN The phase matching characteristics of were analyzed.
- PPMgLN In order for SFG to occur with high efficiency, PPMgLN must satisfy non-critical quasi-phase matching conditions. Even if the non-critical pseudo phase matching condition is not completely satisfied, the pseudo phase matching condition is satisfied by the fundamental wave and SFG propagating in different directions with a non-zero walk-off angle. There are things to do. As this walk-off angle decreases, SFG increases rapidly. If the walk-off angle becomes zero and the fundamental wave and the SFG travel in the same direction, this corresponds to the case where the non-talical quasi-phase matching condition is met!
- Fig. 3A shows the walk-off angle of the generated SFG.
- the arrow indicates the light propagation direction.
- the walk-off angle indicates the angle formed by the arrows indicating the propagation directions of SFG and SHG. In other words, this walk-off angle is the angle between the SFG and SHG pointing vectors.
- Periodic polarization with PPMgLN force period of about 6.95 m is required to satisfy the phase matching condition that generates SHG of 532 nm wavelength from light of 1064 nm wavelength (fundamental wave) by wavelength conversion with PPMgLN It is necessary to provide an inversion structure. This value is calculated from the refractive index dispersion of MgLN.
- light with a wavelength of 355 nm is generated by the sum frequency (SFG) of a fundamental wave with a wavelength of 1064 nm and SHG light with a wavelength of 532 nm.
- SFG sum frequency
- a polarization inversion period of 6.95 m suitable for generating SHG from the fundamental wave does not satisfy the polarization inversion period of 1. 95 suitable for generating the sum frequency of the fundamental wave and SHG.
- the possibility of phase matching in higher order periodic structures remains. If the periodic polarization inversion structure is an integer multiple (m times) of 1.79 / zm, phase matching is possible and sum frequency (SFG) can be generated with high efficiency. (However, in this case, the conversion efficiency decreases in proportion to lZ (m 2 ).)
- FIG. 3A shows that the fundamental, SHG, and force non-critical phase matching are performed.
- SHG in PPMgLN Generation can be highly efficient by performing non-critical phase matching in which the fundamental wave and SHG propagate in the same traveling direction.
- the fundamental wavelength is 1000 ⁇ !
- SFG wavelength ( ⁇ ⁇ 3)) of fundamental wave (wavelength ⁇ ) and SHG (wavelength ( ⁇ / 2)) is 4th and 5th order quasi phase matching (QPM) ( These are described as 4thQP M and 5thQPM, respectively).
- QPM quasi phase matching
- SFG due to fifth-order quasi-phase matching has a large walk-off angle of 30 degrees or more, so the output of SFG (sum frequency) has no effect on the tolerance.
- the walk-off angle with SFG which is the vertical axis in Fig. 3B, refers to the walk-off angle between SFG and the fundamental wave. If the walk-off angle between the fundamental wave and SHG is zero, the force matches the walk-off angle shown in Fig. 3A.
- the walk-off angle shown in Fig. 3A and the walk-off angle on the vertical axis in Fig. 3B Note the differences.
- SFG due to fourth-order quasi-phase matching occurs when the fundamental wavelength is 1030 nm or more, as shown in Fig. 3B.
- the walk-off angle is small, and the output of the SFG greatly increases near the fundamental wavelength of 1030 nm.
- Fig. 3C the SFG output greatly increases even in the region where the fundamental wave wavelength is about 1050 ⁇ m or less when the walk-off angle is 10 degrees or less, and the region near the fundamental wavelength of 1030 nm.
- the wavelength conversion element has a fundamental wavelength dependency.
- the fundamental wavelength is 1030 nm
- the SFG light is greatly increased, and at the same time, the tolerance of the wavelength conversion element is greatly reduced because the non-critical phase matching condition is established in which SFG is output in the same direction as the fundamental wave. To do.
- SFG generation by lower-order quasi-phase matching also exists in the vicinity of the fundamental wavelength of 1370 nm.
- the wavelength of SFG is 450 nm or more, and there is no effect on the output stability of PPMgLN.
- higher-order (sixth or higher) phase matching conditions may be satisfied, as described above, conversion efficiency decreases in inverse proportion to the square of the order, so higher-order ( Phase matching (sixth or higher) is negligible in measuring the tolerance of wavelength conversion elements.
- the generation power of SFG increases the absorption of infrared light, which is the fundamental wave, and visible light, which is SHG, in the phase matching in PPMgLN, causing a partial temperature rise in PPMgLN and causing phase matching to occur. It became clear that the output was unstable and the output was unstable.
- the walk-off angle between the SFG and the fundamental generated by the 4th and 5th quasi-phase matching of the PPMgLN is 15 degrees or more when the fundamental wavelength is 1070 nm or more. Become. If the walk-off angle is such a large angle, the SHG output will not be affected. Generally, the fundamental wavelength used for SHG generation is 1064nm. The walk-off angle at this time is 13 degrees. However, it is desirable for the inventors of the present application to use a fundamental wavelength of 1070 nm or more in order to stabilize the SHG output. In other words, the walk-off angle should be 15 degrees or more.
- FIG. 3D is a graph showing the walk-off angle between the SFG and the fundamental wave generated by the fourth-order and fifth-order quasi-phase matching of PPMgSLN.
- the horizontal axis is the fundamental wavelength. From this figure, in PPMgSLN, when the fundamental wavelength is 1027nm or more, the walk-off angle is 15 degrees or more.
- FIG. 3E is a graph showing the walk-off angle between the SFG and the fundamental wave generated by the fourth-order and fifth-order pseudo phase matching of PPLT (LiTa03 having a periodic domain-inverted structure).
- the horizontal axis is the fundamental wavelength. From this figure, in PPLT, the walk-off angle is 15 degrees or more when the fundamental wavelength is 1018 nm or more.
- FIG. 3F is a graph showing the walk-off angle between the SFG and the fundamental wave generated by the third-order and fourth-order quasi-phase matching of PPKTP (KTP having a periodic domain-inverted structure).
- the horizontal axis is the fundamental wavelength.
- the walk-off angle is 15 degrees or more when the fundamental wavelength is 850 nm or more.
- UV SFG is generated by fundamental wave and SHG.
- Thermal lens effect is generated in the wavelength conversion element due to partial temperature rise due to absorption.
- the wavelength of SFG is longer than the absorption edge of the crystal (which constitutes the wavelength conversion element). (SFG generation is suppressed by absorption of crystals below the absorption edge.)
- SFG is ultraviolet light with a wavelength of 400nm or less.
- the absorption coefficient of the nonlinear optical crystal is increased by ultraviolet light irradiation.
- the output instability phenomenon due to the thermal lens effect described above is a phenomenon in which the generation of ultraviolet rays causes the absorption of the fundamental wave and its (second) harmonics, resulting in the thermal lens effect and the output becoming unstable. It is. It has been clarified that the generation of the thermal lens effect due to the absorption of light is greatly influenced by the peak power and average power of the absorbed light, depending on the wavelength of the absorbed light. Each case will be described below.
- the above-described fundamental wave absorption corresponds to the case (i).
- the absorption coefficient is small, the power density at which the thermal lens effect is generated is relatively high.
- the thermal lens effect due to absorption of the fundamental wave appears remarkably in the case of wavelength conversion of pulsed light with a large peak power. This is because the heat lens effect is remarkably generated by the spire value of pulsed light with high power.
- FIG. 4A is a configuration diagram of the coherent light source 400 according to the first embodiment of the present invention.
- the coherent light source 400 includes a light source 401 that constitutes a light source unit, and a wavelength conversion element 402 that is a wavelength conversion unit. Further, the coherent light source 400 may include a condensing optical system 403 that is a condensing unit so that the fundamental wave 404 emitted from the light source 401 is condensed on the wavelength conversion element 402.
- the light source 401 can perform Q-switch pulse driving in order to improve the efficiency of wavelength conversion by the wavelength conversion element 402.
- the wavelength conversion element 402 includes PPMgLN which is a nonlinear optical material.
- the fundamental wave 404 emitted from the light source 401 is condensed in the wavelength conversion element 402 (PPMgLN) by the condensing optical system 403.
- the wavelength conversion element 402 converts the fundamental wave 404 into SHG 405 therein, and the fundamental waves 404 and SHG 405 are converted into SFG 406 inside the element 402.
- the wavelength of the fundamental wave 404 is obtained
- the wavelength of SHG405 is ( ⁇ / 2)
- the wavelength of SFG406 is ( ⁇ / 3).
- FIG. 4B is a configuration diagram of a coherent light source 450 according to a modification of the first embodiment.
- the coherent light source 450 is based on the configuration of the coherent light source 400 (see FIG. 4), and further includes an ultraviolet light shielding unit 451.
- the ultraviolet light shielding unit 451 shields light having a wavelength of 400 nm or less and protects the wavelength conversion element 402.
- the ultraviolet light shielding part 451 preferably has a high shielding performance (non-transparency) for light having a wavelength of at least 320 nm and at most 400 nm.
- the ultraviolet light shielding part 451 can protect the wavelength conversion element 402 by ultraviolet light force caused by ultraviolet light generation factors such as fluorescent lamps and sunlight.
- the wavelength conversion element 402 is free from the incidence of ultraviolet light from the outside.
- Coherent light source 451 shields ultraviolet light from the outside Therefore, green light (SHG405) with an output of about 1W can be generated stably.
- the SHG wavelength is 400 nm or more.
- the fundamental wave power is ⁇ OOnm
- the SFG wavelength of the fundamental wave and SHG is approximately 267nm (Z3). Therefore, no ultraviolet light having a wavelength in the range of 320 nm to 400 nm is generated in the wavelength conversion element 402, and only the ultraviolet light incident from the outside affects the increase in visible light absorption. For this reason, by adding the ultraviolet light shielding part 451, the stability of the coherent light source at the time of high output of SHG is dramatically improved.
- the ultraviolet light shielding part 451 can be constituted not only by the wavelength converting element 402 but also by an ultraviolet light impervious thin film formed on the surface of the wavelength converting element 402. .
- FIG. 5 is a graph showing the relationship between the wavelength conversion element 402 and the high output tolerance with respect to the fundamental wavelength ⁇ .
- the light source 401 is pulse-driven at a repetition frequency of 60 kHz, the pulse width of each pulse is about 20 ns, and the output characteristics are observed by changing the fundamental wavelength.
- the temperature of the observation environment is room temperature.
- the high output tolerance gradually increases depending on the wavelength (region 501).
- the walk-off angle of SFG406 is kept above 30 degrees, so the strength of SFG406 is very small (see Fig. 3B).
- SFG due to the 4th-order QPM does not occur because phase matching is not possible (see Figure 3B). Therefore, the output of the SFG 406 is very small, and the wavelength conversion element 402 exhibits high high output resistance.
- the wavelength of the fundamental wave emitted from the light source 401 is preferably 1030 nm or less.
- the walk-off angle of SFG406 and fundamental 404 due to the 4th and 5th order quasi-phase matching is both 10 degrees or more (see Fig. 3B), so the output of SFG406 is Low (area 505). Therefore, the wavelength conversion element 402 exhibits high high output resistance.
- the wavelength of the fundamental wave emitted from the light source 401 is also preferably 1050 nm or more. Furthermore, it is desirable that the walk-off angle is 15 degrees or more, and the fundamental wavelength is 1070 nm or more.
- Fig. 6 is a graph showing the relationship between the fundamental wavelength and the tolerance strength in the vicinity of the region where SFG406 is perfectly phase-matched, with the phase-matching temperature varied. As shown in region 5 01 in Fig. 5, the generation of SFG406 was suppressed at wavelengths below 1030 nm, indicating strong resistance. However, the inventor of the present application has found that there is a limit to the temperature of the crystal in order to achieve strong resistance in this fundamental wavelength region. In general, the wavelength conversion element 402 is often used at a crystal temperature of about 100 ° C. or higher in order to reduce the effects of optical damage and GRIIRA.
- the region where the tolerance is low shifts to the short wavelength side.
- the shortest fundamental wave wavelength end in the fundamental wave wavelength region where the tolerance becomes low becomes shorter as the temperature of the wavelength conversion element increases.
- the fundamental wavelength is! ⁇ !
- the phase matching temperature it is preferable to keep the phase matching temperature at 50 ° C. or lower in order to keep the high output resistance high.
- the fundamental wavelength when the fundamental wavelength is included in the range of 1020 nm to 1030 nm, it is desirable to use the crystal temperature with careful attention. In this wavelength region, the perfect phase matching condition is met with a slight temperature rise, the strength of SFG406 increases rapidly, and the high output tolerance is greatly degraded.
- a Yb: YAG solid-state laser or a Yb-doped fiber laser can be used as the light source 401. These light sources have high efficiency and high output.
- the combination of these light sources and the wavelength conversion element 4022 can realize a high-output coherent light source.
- the temperature of the wavelength conversion element 402 low, it has high output tolerance and high efficiency.
- a high-power coherent light source 400 is realized.
- the fundamental wave wavelength is included in the range of 1030nm to 1050nm, as shown in Fig. 3B, the fundamental wave 404 and SFG406 walk angle angle force is 10 degrees / J, and SFG406 is generated with force and strength. To do. For this reason, the resistance is lOMWZcm 2 or less.
- PPMgLN wavelength conversion element 4002
- the power density of the fundamental wave is about LMWZcm 2 is desirable.
- a high-power pulse is used as the fundamental wave 404 or the wavelength conversion element 402 is used as an internal resonator.
- the wavelength conversion element 402 when the fundamental wave wavelength is included in the range of 1060 nm to LlOOnm will be described.
- a high-power light source using a solid light source doped with Nd or Yb can be used as the fundamental light source 401.
- S FG406 there is S FG406 in the ultraviolet region generated by the fourth-order QPM.
- the walk-off angle between the fundamental wave 404 and SFG406 is 10 degrees or more (15 degrees or more at 1070 nm or more), the intensity of SFG406 generation is kept low and shows relatively high resistance.
- the resistance to the power density of the applied fundamental wave is about 50MWZcm 2 or more. This is reported as a laser damage metamorphosis in normal LN !, which is a low value compared to a value of about 100-200 MWZcm 2, and a high non-linear constant PPMgLN has a high efficiency of over 50% Since wavelength conversion can be carried out by this, it is resistant to practically no problem. Low non-linearity, high in material to increase efficiency! Power that requires optical power density and high resistance required for that.
- PPMgLN is used in the wavelength range from 1060 to L lOOnm, it is fundamental
- the wave 404 and SFG406 have a walk-off angle of 10 degrees or more (15 degrees or more at 1070 nm or more), so a practical and highly efficient coherent light source can be realized.
- this wavelength region (1060 nm to LlOOnm) it is preferable from the viewpoint of stable output that the power density of the fundamental wave is about 50 MWZcm 2 or less. Furthermore, it is more preferable to use at about 1 to 40 M WZcm 2 . In this region, conversion efficiency is as high as about 50%. Furthermore, even when used for a long period of time, no deterioration of the crystal is observed, and the life can be extended.
- the problem of crystal lifetime is closely related to the increase of absorption by infrared light (eg fundamental wave 404) and visible light (eg SHG405) crystal (wavelength conversion element 402) due to generation of ultraviolet light (eg SFG406).
- fundamental wave 404 infrared light
- visible light eg SHG405
- crystal wavelength conversion element 402
- ultraviolet light eg SFG406
- the walk-off angle of the fundamental wave 404 and the SFG 406 it is preferable to set to 10 degrees or more in order to ensure a long lifetime of the crystal. More preferably, it is more preferably 15 degrees or more, and further 20 degrees or more, because the resistance can be increased to almost the same level as the light damage resistance.
- control the crystal temperature It is preferable to set the phase matching wavelength to a desired value. In particular, when the fundamental wavelength is included in the range of 1060 nm to: LlOOnm, SFG406 is slightly emitted. For this reason, it is desirable to use the SHG405 with an output of 5 W or less.
- the average output of SHG405 is preferably 1 W or more, 2 W or more, 2.5 W or more, or 3. OW or more and 5.0 W or less. If it is used within this SHG output range, high-efficiency conversion, output stability, and long life can be achieved. In addition, when using at a higher power density, the crystal temperature may be increased.
- the light source 401 is preferably a force Q-switch pulse light source that can be a CW light source. This is because even if the average power of the fundamental wave 401 is low, high peak power can be used and high-efficiency conversion is possible.
- the repetition frequency is preferably 1 kHz or more. At repetition frequencies below this, the peak power may be too high. Since the coherent light source according to the present invention is used while being suppressed to a power density of about 50 MWZ cm 2 that is effective for stabilization, it may be necessary to increase the light beam spot and lower the average power. is there.
- the repetition frequency is preferably 1 kHz or more, more preferably 10 kHz or more.
- the light source 401 includes Nd: YV04, Nd: YAG, Nd: Nd material such as glass, or Yb:
- YAG, Yb Yb-doped material such as glass.
- the light source 401 preferably includes a Yb-doped fiber laser.
- the fiber laser is easy to increase the output, has excellent beam condensing characteristics with high beam quality, and can be converted with high efficiency. For example, if a 100W light source is condensed to about 20 / ⁇ ⁇ ⁇ , the power density is 30 MWZcm 2 , and depending on the fundamental wavelength, a value that may affect the durability of the wavelength conversion element 402 (P PMgLN) Obtainable.
- the light source 401 is configured so as to amplify light from a pulse-driven light source using a Yb-doped fiber laser as an amplifier, output with a high peak value is possible. Such a light source 401 is suitable for realizing a coherent light source with high efficiency and high output.
- the resonator When the wavelength conversion element 402 is used with an internal resonator structure, the resonator The power of the internal fundamental wave 404 easily reaches several tens or hundreds of times the external pump power. Therefore, it is normal for the internal power to exceed 100W. If the configuration of the coherent light source 400 according to the present invention is applied, a stable visible light coherent light source having high efficiency and high output can be realized.
- PPMgLN doped with Mg 5 mol% is used as an example for the purpose of PPMgLN, but the Mg doped amount of PPMgLN is preferably 4.9 mol% to 6 mol%. This is because the light damage resistance is excellent.
- PPMgLN having a stoichiometric composition can also be used because it is a highly nonlinear material with excellent light damage resistance.
- the Mg doping amount is preferably 1.5 mol% or more.
- Mg-doped LiTa03 Mg-doped stoichiometric LiTa03, KTP, and the like can also be used for the wavelength conversion element 402 of the coherent light source 400 according to the present invention.
- stable output characteristics can be realized by setting the walk-off angle between the fundamental wave 404 and SFG 406 to 10 degrees or more. More preferably, the walk-off angle is set to 15 degrees or more.
- the coherent light source according to the present invention is a coherent light source capable of receiving a fundamental wave having a light source power and emitting the second harmonic of the fundamental wave with high output.
- the output of the second harmonic can be more than 1W on average. It is also possible to obtain an average output of 2W or higher, 2.5W or higher, or 3W or higher.
- Output instability due to the thermal lens effect described above is caused by the generation of ultraviolet rays and absorption by the wavelength conversion element 402 of the fundamental wave and the harmonic wave, and the thermal lens effect is generated by the absorbed energy, resulting in unstable output. It is a phenomenon to hesitate. In the case of absorption of infrared light, the power density required to generate the thermal lens effect is high because the absorption coefficient is relatively small. For this reason, the thermal lens effect occurred due to the high peak power of the peak value. On the other hand, for visible light with a short wavelength, the occurrence of a thermal lens effect with a large absorption coefficient becomes more prominent.
- the SFG706 in the phase-mismatch state is also shown in FIG. However, it propagates in the same direction as the fundamental wave 404.
- the output 703 of the phase mismatched SFG 706 increases little with respect to the propagation distance.
- absorption occurs even with minute ultraviolet light (SFG706) generated in a phase mismatch state.
- FIG. 8 is a configuration diagram of the wavelength conversion unit 800 of the coherent light source according to the second embodiment of the present invention. Since other components may be the same as those in the first embodiment, description thereof is omitted.
- the fundamental wave 804 is wavelength-converted to SHG (second harmonic) 805 by the periodic domain-inverted structure 8 03 formed on the substrate (wavelength conversion element) 802.
- the light source unit (not shown) includes a fiber laser, and the wavelength of the emitted light is 1084 nm.
- the fundamental wave 803 having a wavelength of 1084 ⁇ m is wavelength-converted into green light having a wavelength of 542 nm by the PPMgLN (wavelength conversion element 802) having the wavelength conversion element 802 and the periodically poled structure 803.
- the period of the periodically poled structure 803 is about 7 m, and the phase matching condition is controlled by the temperature control of the element 802.
- the temperature control may be provided with a temperature control unit (not shown).
- the fundamental wave 804 and SHG805 propagate in the same direction.
- the temperature of the wavelength conversion element 802 is set to the state of complete phase matching.
- the phase mismatch is maintained by shifting the force.
- the SHG805 emits at an angle (walk-off angle) with respect to the fundamental wave 804. In this case, the conversion efficiency to SHG805 is reduced. Since the overlap between the fundamental wave 804 and the SHG805 beam is reduced, the sum frequency output generated by the fundamental wave 804 and SHG805 is greatly reduced. As a result, the output instability phenomenon due to SHG805 absorption is greatly reduced.
- the non-zero walk-off angle is generated to suppress the sum frequency.
- FIG. 9 is a configuration diagram of a wavelength conversion unit 900 according to a modification of the second embodiment according to the present invention.
- the normal direction of the stripe of the polarization inversion structure 903 is formed so as to be inclined by an angle ⁇ with respect to the optical axis of the fundamental wave 904.
- a walk-off angle ⁇ W is formed in SHG 905 and fundamental wave 904 by chromatic dispersion of SHG 905 and fundamental wave 904.
- the angle between the polarization inversion optical axis and the fundamental wave 904 is defined as the polarization inversion angle ⁇ .
- the output SHG805 is divided into two directions, the reduction in conversion efficiency due to the generation of a non-zero walk-off angle is relatively large.
- the angle at which the SHG 905 is generated that is, the walk-off angle
- the propagation direction of SHG905 is limited to one direction, a decrease in conversion efficiency can be reduced.
- the overlap between the fundamental wave 904 and SHG905 is reduced, so the sum frequency (SFG) generated by the overlap can be greatly reduced, and stability at high output is achieved. Will improve.
- Fig. 10 is a graph showing the relationship between the polarization inversion angle ⁇ (the deviation angle of the polarization inversion structure 903 from the state perpendicular to the fundamental wave beam) and the walk-off angle formed by the fundamental wave 904 and SHG905. It is. This graph was obtained by calculation with a fundamental wavelength of 1080 nm. Referring to FIG. 10, it can be seen that the walk-off angle ⁇ W is about 1Z30 of the polarization inversion angle ⁇ of the polarization inversion structure 903. To suppress the generation of harmonics, the walk-off angle needs to be 0.1 degree or more. Therefore, the polarization reversal angle ⁇ is 3 degrees or more It is preferable.
- FIG. 11 is a graph showing the relationship between the polarization inversion angle ⁇ , the conversion efficiency 1101 of the fundamental wave 904 into the SHG 905, and the high output tolerance 1103 of the wavelength conversion element 902.
- the polarization reversal angle is 2 degrees or more, the resistance is remarkably improved. If the polarization reversal angle is 5 degrees, the high output resistance is 1.4 times that of the case where the polarization reversal angle is zero degrees, and the conversion efficiency decreases to about 1Z2.
- the domain-inverted angle is preferably 3 degrees or more and 20 degrees or less. More preferably, it is 5 degrees or more and 10 degrees or less.
- the direction of the stripe of the domain-inverted structure 903 is the wavelength conversion element.
- the polarization inversion structure 903 of the Balta crystal is preferably formed on the Z substrate.
- An electrode is formed on the + Z surface of the Z substrate, and a voltage is applied to it.
- the stripe direction of the electrode needs to be formed so as to substantially coincide with the Y-axis direction of the crystal constituting the element 902.
- the polarization inversion structure 903 when the polarization inversion structure 903 is formed to be inclined with respect to the optical axis, it is preferable to form the polarization inverted structure 903 with the optical axis and the Y axis of the crystal being inclined. If the force in the stripe direction of the domain-inverted structure 903 deviates from the Y-axis of the crystal, the uniformity of the domain-inverted structure 903 deteriorates and the conversion efficiency is greatly reduced. Therefore, it is desirable to match the stripes of the domain-inverted structure 903 in the Y-axis direction. It is desirable to suppress the angle between the domain-inverted structure 903 and the Y axis within ⁇ 1 degree.
- the efficiency is reduced to 80% or less as compared with the case where the domain-inverted structure 903 is formed in an ideal direction due to the non-uniformity of the domain-inverted structure 903.
- the deviation between the stripe of the 903 and Y axis exceeds 5 degrees, the conversion efficiency decreases to less than half of the ideal state.
- the two fundamental waves have the same optical axis and emit a 450 nm sum frequency at different angles.
- a configuration in which the fundamental wave with a wavelength of 1080 nm and the optical axis of the sum frequency with a wavelength of 450 nm are matched, and the optical axis of the fundamental wave with a wavelength of 770 nm is slightly angled with the two previous lights is preferable.
- the generation or influence of ultraviolet light can be reduced, and high output tolerance can be improved.
- high power tolerance can be improved by the power ratio of the two fundamental waves.
- the output of the sum frequency is proportional to the product of the power of the two fundamental waves.
- SHG from fundamental waves with a short wavelength is a problem for fundamental wave power that causes absorption of the sum frequency. Therefore, in the case of sum frequency generation, if the power of the first fundamental wave with wavelength ⁇ 1 is Pl and the power of the second fundamental wave with wavelength ⁇ 2 is ⁇ 2, then if ⁇ 1> ⁇ 2, then ⁇ 1 > ⁇ 2 is preferred.
- ⁇ 1> ⁇ 2 is preferred.
- the light source unit constituting the coherent light source according to the present embodiment can be used as long as it is a light source indicated by V in other embodiments.
- FIG. 12 is a diagram illustrating a configuration of a coherent light source 1200 according to the third embodiment.
- the coherent light source 1200 of this embodiment has the same configuration as the coherent light source shown in the previous embodiment, but has the electrode 1211 arranged in the current source 1210 and the wavelength conversion element 1202 in the previous embodiment. Different from coherent light source.
- the fundamental wave 1204 emitted from the light source 1201 is converted into SHG 1205 by the wavelength conversion element 1202 (PPMgLN). Furthermore, SHG1205 and fundamental wave 1204 generate sum frequency, which may generate SFG1206.
- SFG1206 force When the light is in the ultraviolet region with a wavelength of 400 nm or less, free electrons increase inside the PPMgLN due to the generated ultraviolet light.
- the voltage applied to the electrode 1211 is AC It is preferable to change, and it is desirable to apply an AC voltage with a frequency of 100 Hz or higher.
- the inventor of the present invention has clarified the mechanism of the output instability phenomenon in the powerful SHG wavelength conversion that has not been clarified so far, thereby suppressing the generation of the sum frequency.
- a coherent light source capable of obtaining a stable second harmonic output is provided.
- the coherent light source according to the present invention suppresses the generation of the sum frequency by increasing the walk-off angle between the sum frequency light generated by the fundamental wave and the second harmonic and the fundamental wave, and the stable second high frequency. Harmonic output can be obtained.
- the coherent light source according to the present invention has a great practical effect as a coherent light source for high-power applications.
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