JP2012242545A - Light wavelength conversion device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a high-output wavelength-converted wave in a light wavelength conversion device with a structure of looping back a fundamental wave within a light wavelength conversion element.SOLUTION: A light wavelength conversion device comprises: a light wavelength conversion element 15 that includes an incident end face 15a for allowing a fundamental wave 10 to enter and an emission end face 15b facing it and that converts the fundamental wave 10 into a wavelength-converted wave 14; a first reflection member 17 that reflects the fundamental wave 10 emitted from the emission end face 15b and transmits the wavelength-converted wave 14 emitted therefrom; a second reflection member 18 that transmits the fundamental wave 10 which is reflected by the first reflection member 17, is propagated through the light wavelength conversion element 15 and is emitted from the incident end face 15a, and that reflects the wavelength-converted wave 14 which is similarly emitted from the incident end face 15a; and phasing means 20 that mutually matches a phase of the wavelength-converted wave 14 which is propagated through the element 15 before reaching the first reflection member 17 and a phase of the wavelength-converted wave 14 which is reflected by and looped back at the second reflection member 18 and is propagated within the element 15.

Description

  The present invention relates to an optical wavelength conversion device using an optical wavelength conversion device, and more particularly to an optical wavelength conversion device having a structure in which a fundamental wave is folded in the optical wavelength conversion device.

  Conventionally, in order to obtain laser light in a short wavelength region, laser light as a fundamental wave emitted from a laser light source is passed through an optical wavelength conversion element, and the fundamental wave in the element is converted into a wavelength-converted wave having a shorter wavelength, for example, a wavelength. Various optical wavelength converters for converting to 1/2 second harmonics and the like are provided. In such an optical wavelength conversion device, it is desired to achieve higher wavelength conversion efficiency and obtain a high-power laser beam.

  As one optical wavelength converter for meeting such a demand, for example, as shown in Patent Document 1, wavelength conversion converted from a fundamental wave incident from the incident end face of the optical wavelength conversion element and propagating through the element By sequentially reflecting the wave and the remaining fundamental wave on the emission end face side and the incident end face side of the optical wavelength conversion element, the fundamental wave passes through the optical wavelength conversion element three times in total, and finally the wavelength conversion wave Is known that has a configuration for emitting light from the emission end face.

  In this configuration, since the action length of the fundamental wave and the optical wavelength conversion element is approximately three times the element length of the optical wavelength conversion element, it is expected to improve the wavelength conversion efficiency.

JP 2006-208629 A

  However, in the conventional optical wavelength conversion device shown in Patent Document 1, a high wavelength conversion efficiency may be obtained as expected, but on the other hand, compared to a normal case in which the fundamental wave is passed through the optical wavelength conversion element only once, On the other hand, there is a problem that the wavelength conversion efficiency is lowered.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a stable and high-output wavelength conversion wave in an optical wavelength conversion device having a structure in which a fundamental wave is folded in an optical wavelength conversion element. To do.

The optical wavelength converter of the present invention is
Basically, it has an incident end face and an exit end face facing it, and is incident from the incident end face and propagates to the exit end face side (hereinafter this direction is called “forward direction” and the opposite direction is called “reverse direction”) An optical wavelength conversion element that converts a wave into a wavelength-converted wave such as a second harmonic and emits the wave from the emission end face;
A first reflection member that reflects the fundamental wave and wavelength-converted wave emitted from the emission end face while allowing the former to be reflected and the latter to be transmitted;
The fundamental wave emitted from the incident end face after being reflected by the first reflecting member and propagating in the folded light wavelength conversion element, and the wavelength converted wave emitted from the incident end face after being converted from the fundamental wave, A second reflecting member that transmits while the latter reflects;
Before reaching the first reflecting member, the phase of the wavelength converted wave propagating in the optical wavelength conversion element (the wavelength converted wave propagating for the first time in the positive direction) and the reflected light reflected by the second reflecting member and turned back It is characterized by comprising phase adjusting means for aligning the phase of the wavelength converted wave propagating in the optical wavelength conversion element (the wavelength converted wave propagating second time in the positive direction) with each other.

  The first and second reflecting members can be composed of a coating or the like applied to the exit end face and the entrance end face of the light wavelength conversion element, respectively, but are formed separately from the light wavelength conversion element. It may be configured by a coating or the like applied to the optical member.

  Further, as the phase adjusting means, means for applying a variable voltage (voltage whose value can be adjusted) to at least a part of the optical wavelength conversion element, for example, the vicinity of the incident end face, or an optical wavelength conversion element Means for adjusting the temperature can be applied alone or in combination.

  Such a phase adjusting means is preferably capable of shifting the phase of the wavelength converted wave propagating first time in the positive direction and the phase of the wavelength converted wave propagating second time in the same direction from each other by one cycle or more. Used.

  In the optical wavelength conversion device of the present invention, it is particularly desirable to use an optical wavelength conversion element having a periodic polarization inversion structure in which the period of the domain inversion part is a chirp period.

  According to the inventor's research, it has been found that the output of the wavelength-converted wave becomes unstable in the conventional apparatus disclosed in Patent Document 1 described above due to the following causes. When the fundamental wave and the wavelength converted wave are reflected and folded at the output end face and the incident end face of the optical wavelength conversion element, a phase shift occurs due to interface reflection when each of the fundamental wave and the wavelength converted wave is reflected, and the shift value is Since the fundamental wave and the wavelength converted wave are different from each other, and because a phase difference occurs between the fundamental wave and the wavelength converted wave due to wavelength dispersion, the wavelength converted wave generated before the return and the wavelength conversion generated after the return There is a relative phase shift with the wave. This is mainly related to the optical path length of the wavelength conversion element when light propagates, that is, the element length, and high wavelength conversion efficiency can be obtained when the phases of the wavelength converted waves before and after the folding are coincidentally strengthened. However, if they are weakened, the wavelength conversion efficiency is lowered. In the conventional apparatus, no consideration has been given to this phase shift, and it has been impossible to stably obtain a high wavelength conversion wave output while maintaining a high wavelength conversion efficiency as designed.

  Based on the above knowledge, the optical wavelength conversion device according to the present invention includes the phase of the wavelength conversion wave propagating in the optical wavelength conversion element for the first time in the positive direction and the phase of the wavelength conversion wave propagating for the second time in the same direction. Are provided with phase adjusting means for aligning the two. In this way, if the two wavelength conversion waves traveling in the same positive direction are aligned, they are strengthened and combined, so that a high-power wavelength conversion wave is always generated from the emission end face of the optical wavelength conversion element. It will be taken out.

In particular, when the phases of the two wavelength conversion waves are completely matched, the amplitude of the wavelength conversion wave is doubled compared to the case where the fundamental wave is passed through the optical wavelength conversion element only once. Theoretically reaches 2 ² = 4 times. On the other hand, when the means for adjusting the phase of the two wavelength conversion waves is not provided, the output of the wavelength conversion wave after being combined is compared with the case where the fundamental wave is passed through the optical wavelength conversion element only once. And fluctuate between 0 and 4 times.

  The “phase adjustment” in the present invention means to shift the phase between the two wavelength-converted waves as described above, and the phase due to the difference in wavelength between the fundamental wave and the wavelength-converted wave. This is different from so-called “phase matching” that eliminates the phase shift caused by the difference in speed.

  As the phase adjusting means that performs the above-described operation, the phase between the wavelength-converted wave that propagates the first time in the positive direction and the wavelength-converted wave that propagates the second time in the positive direction can be relatively shifted by one period or more. If phase adjustment is not performed, it is possible to match the phases of the two wavelength-converted waves regardless of how they are shifted.

  Note that the wavelength converted wave that propagates the second time in the forward direction as described above is generated when the fundamental wave reflected and turned back by the first reflecting member propagates in the reverse direction in the optical wavelength conversion element, and thereafter The light is reflected by the second reflecting member and folded. That is, in the optical wavelength converter of the present invention, unlike the optical wavelength converter disclosed in Patent Document 1, the fundamental wave is not reflected by the second reflecting member. The maximum working length of the element is approximately twice the element length.

  The second reflecting member is configured to transmit the fundamental wave satisfactorily as described above so that the fundamental wave is first incident on the optical wavelength conversion element. On the other hand, since the first reflecting member takes out the wavelength conversion wave and uses it, the light wavelength conversion element is supposed to transmit well.

  In the optical wavelength conversion device of the present invention, when means for applying a variable voltage (voltage whose value is adjustable) is used as the phase adjustment means in at least a part of the optical wavelength conversion element, It is desirable that a voltage be applied to the vicinity of the incident end face of the wavelength conversion element. The reason is as follows.

  This arrangement is effective for the wavelength-converted wave generated when the fundamental wave travels the first time in the positive direction and the wavelength-converted wave generated when the fundamental wave travels the second time in the positive direction. This is to obtain a high wavelength conversion efficiency by adjusting the phase shift amount with the voltage. First, unlike this arrangement, for example, in the case where an electrode is arranged in the vicinity of the emission end face of the optical wavelength conversion element, the applied voltage acts on both the fundamental wave and the wavelength converted wave in this case, so the fundamental wave The phase difference between the wavelength conversion wave and the wavelength conversion wave cannot be made large. As a result, the phase difference between the wavelength conversion wave generated the first time and the wavelength conversion wave generated the second time cannot be made large. Therefore, a large voltage is required to set this phase difference to 2π or more, or adjustment of 2π or more cannot be performed and high wavelength conversion efficiency cannot be obtained.

  On the other hand, when the electrode is arranged in the vicinity of the emission end face of the optical wavelength conversion element, the wavelength conversion wave is hardly generated when the fundamental wave propagates in the positive direction of the voltage application portion, and the voltage is substantially It only affects the fundamental wave. On the other hand, a voltage acts simultaneously on the fundamental wave and the wavelength converted wave propagating in the opposite directions, but thereafter, the fundamental wave is emitted from the wavelength conversion element and is not used. As a result, the voltage substantially acts only on the second wavelength converted wave generated. Further, the action of the voltage on the fundamental wave propagating in the positive direction is equivalent to the action on the wavelength converted wave since the wavelength converted wave is generated from the fundamental wave thereafter. From the above, according to this arrangement, no voltage is applied to the fundamental wave, and the voltage is substantially applied only to the wavelength conversion waves generated when the fundamental wave propagates in the first and second times in the positive direction. It becomes possible to act. From this, it is possible to easily align the phase difference between the wavelength-converted waves generated at the first time and the second time at a practically low voltage.

  In the optical wavelength conversion device of the present invention, in particular, when an optical wavelength conversion element having a periodically poled structure in which the period of the polarization inversion part is a chirp period is used, the optical path of the fundamental wave is the polarization inversion part. It is possible to change the phase matching condition by changing the relative position of the optical wavelength conversion element and the fundamental wave so as to change in a direction crossing the arrangement direction of. If so, for example, by performing phase adjustment by adjusting the temperature of the optical wavelength conversion element, even if the element temperature has deviated from the phase matching temperature, the relative relationship between the optical wavelength conversion element and the fundamental wave is as described above. By changing the phase matching condition by changing the position, the temperature set by the temperature adjustment can be matched with the new phase matching temperature.

1 is a schematic side view showing an optical wavelength conversion device according to a first embodiment of the present invention. The schematic side view which shows the optical wavelength converter by 2nd Embodiment of this invention The graph which shows the relationship between the optical wavelength conversion element temperature and 2nd harmonic output in the apparatus of FIG. The schematic side view which shows the optical wavelength conversion element in the optical wavelength converter of 3rd Embodiment of this invention The figure explaining the effect by the optical wavelength converter of the said 3rd Embodiment. The schematic side view which shows the optical wavelength converter by 4th Embodiment of this invention Schematic side view showing an optical wavelength converter according to a fifth embodiment of the present invention. Schematic side view showing an optical wavelength converter according to a sixth embodiment of the present invention. Schematic side view showing an optical wavelength converter according to a seventh embodiment of the present invention. The figure explaining the path | route of the fundamental wave and the 2nd harmonic in the optical wavelength converter of this invention The graph which shows the 2nd harmonic output profile in case the phase adjustment is made by temperature adjustment in the optical wavelength converter of this invention

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

<< First Embodiment >>
FIG. 1 is a schematic side view of an optical wavelength conversion device 100 according to a first embodiment of the present invention. The optical wavelength converter 100 collects a fiber laser 11 that emits a fundamental wave 10 having a wavelength λ 1, a collimator lens 12 that collimates the fundamental wave 10, and a fundamental wave 10 that is collimated by the collimator lens 12. A light condensing lens 13 and an optical wavelength conversion element 15 arranged at a position where the fundamental wave 10 thus collected is incident and which converts the incident fundamental wave 10 into a second harmonic wave 14 having a wavelength λ2 = λ1 / 2. And an output mirror 16 composed of a concave mirror disposed at a position where the fundamental wave 10 and the second harmonic wave 14 emitted from the optical wavelength conversion element 15 are incident, and a second generated in two ways as will be described later. Phase adjustment means 20 for aligning the phases of the harmonics 14 with each other is provided.

  The fiber laser 11 has a polarization-preserving single mode fiber. As an example, the fiber laser 11 emits a linearly polarized laser beam (fundamental wave) 10 having a wavelength λ1 = 1120 nm. The output of the fiber laser 11 is variable between 1 and 10W.

The optical wavelength conversion element 15 is made of, for example, an MgO-doped LiNbO ₃ crystal having a periodically poled structure, an incident end face 15a on which the fundamental wave 10 from the fiber laser 11 is incident, and a face to the incident end face 15a. And an outgoing end face 15b. In this example, the optical wavelength conversion element 15 has a width of 2 mm and a thickness of 0.5 mm, and the element length, that is, the size in the propagation direction of the fundamental wave and the second harmonic wave is 20 mm. The light wavelength conversion element 15 is formed by cutting so that the c-axis direction of the crystal is the thickness direction.

  The inner surface of the output mirror 16, that is, the concave surface on the side of the light wavelength conversion element 15, is coated with a coat 17 as a first reflection member, and the incident end face 15 a of the light wavelength conversion element 15 is a second reflection member. A coat 18 is applied. The coat 17 is an HR coat (reflectance of 99.9% or more) that reflects well with respect to the fundamental wave 10 with a wavelength λ1 = 1120 nm, and transmits well with respect to the second harmonic 14 with a wavelength λ2 = 560 nm. AR coating (reflectance 1% or less) to be applied. On the other hand, the coat 18 is an AR coat (reflectance of 1% or less) that allows the fundamental wave 10 to pass well, and an HR coat (reflectance 99.9%) that reflects the second harmonic 14 well. That is the above.

  The phase adjusting means 20 includes a pair of EO electrodes 21 fixed to the element 15 so as to face each other in the vicinity of the incident end face 15 a of the optical wavelength conversion element 15, and the optical wavelength via these EO electrodes 21. The voltage application circuit 22 applies a variable voltage to the above portion of the conversion element 15. Each EO electrode 21 is made of, for example, an Au / Cr vapor deposition film. The thickness of the Au film is 1 μm, the thickness of the Cr film is 0.1 μm, and the propagation direction of the fundamental wave and the second harmonic is long. And a square having a width of 1 mm.

  Next, the operation of the optical wavelength conversion device 100 will be described. The fundamental wave 10 emitted from the fiber laser 11 is condensed by the collimator lens 12 and the condenser lens 13 and enters the light wavelength conversion element 15 from the incident end face 15a. In this way, the fundamental wave 10 having the wavelength λ 1 = 1120 nm propagating in the optical wavelength conversion element 15 in the positive direction, that is, in the left direction in the figure, has a wavelength while being so-called quasi-phase-matched by the periodic polarization inversion structure in the optical wavelength conversion element 15. It is converted into the second harmonic wave 14 of λ2 = λ1 / 2/2 = 560 nm.

In this case, the fundamental wave 10 is converted into an optical wavelength in a state where the linear polarization direction coincides with the c-axis direction of the MgO-doped LiNbO ₃ crystal so that the largest nonlinear optical constant d _{33 of} the MgO-doped LiNbO ₃ crystal is used. Incident on the element 15. The fundamental wave 10 is incident on the light wavelength conversion element 15 with its beam waist position on the incident end face 15a of the light wavelength conversion element 15 and with a beam waist radius of 80 μm.

  The second harmonic wave 14 generated and propagated in the positive direction as described above is emitted from the emission end face 15b of the light wavelength conversion element 15, and is satisfactorily transmitted through the coat 17 formed there, for a predetermined use. Used for On the other hand, the fundamental wave 10 that has propagated in the optical wavelength conversion element 15 in the positive direction and has not been converted into the second harmonic wave 14 is emitted from the emission end face 15 b of the optical wavelength conversion element 15 and is reflected well by the coat 17. The light propagates in the reverse direction in the optical wavelength conversion element 15. The fundamental wave 10 propagating in the opposite direction is also converted into the second harmonic wave 14 by the optical wavelength conversion element 15. The second harmonic wave 14 is emitted from the incident end face 15 a of the light wavelength conversion element 15, is favorably reflected on the coat 18, is folded, and propagates in the light wavelength conversion element 15 in the positive direction. At this time, the fundamental wave 10 that has not been converted to the second harmonic wave 14 passes through the coat 18 satisfactorily.

  The second harmonic wave 14 that is folded as described above (that is, the second harmonic wave that propagates the second time in the positive direction) is the second harmonic wave 14 (that is, the second harmonic wave that propagates the first time in the positive direction). If the phase is shifted with respect to (wave), in the worst case, the two may interfere with each other so that the output of the second harmonic wave emitted from the output mirror 16 may become zero.

  Moreover, even if the worst state where the output of the second harmonic is almost zero is not reached, depending on the sum of the optical path lengths in the forward direction and the reverse direction, the phase of the second harmonic propagating the first time in the forward direction Depending on the relationship with the phase of the second harmonic that propagates the second time in the positive direction, the second harmonic output may be lower than that of a general optical wavelength conversion device that does not return the fundamental wave.

  In the present embodiment, the above-described phase adjusting means 20 is provided so as to avoid the above-described problems. The voltage application circuit 22 constituting the phase adjusting unit 20 applies a variable voltage in the range of 0 to 1 kV as an example to the vicinity of the incident end face of the optical wavelength conversion element 15 via the EO electrode 21. In this voltage application portion, the refractive index of the optical wavelength conversion element 15 changes, and therefore, the phase of the fundamental wave 10 that passes through this portion and propagates in the positive direction for the first time and the second harmonic wave 14 converted therefrom. Respectively change in accordance with the value of the applied voltage. Then, the fundamental wave 10 propagating in the reverse direction through the voltage application portion and the phase of the second harmonic wave 14 converted therefrom also change in accordance with the value of the applied voltage. Further, the phase relationship between the fundamental wave 10 that propagates the first time in the forward direction and the fundamental wave 10 that reflects and propagates in the opposite direction also changes according to the element length of the optical wavelength conversion element 15.

  Therefore, if the value of the applied voltage is adjusted, the phase of the second harmonic wave 14 reflected by the coat 18 and propagated the second time in the positive direction is changed, and the phase of the second harmonic wave 14 propagated the first time in the positive direction. Can be matched. In this state, the output of the second harmonic wave 14 extracted from the optical wavelength conversion element 15 is maximized.

  When the output of the fundamental wave 10 is set to 1W, 3W, and 10W, and the applied voltage to the optical wavelength conversion element 15 is adjusted within the above range, the second as shown in Table 1 below in each case. Harmonic maximum output was obtained.

For comparison, the embodiment described above except that the phase adjusting means 20 is not provided and that an AR coat that satisfactorily transmits the fundamental wave 10 and the second harmonic wave 14 is formed instead of the coats 17 and 18. An optical wavelength conversion device of a comparative example having the same configuration as the optical wavelength conversion device 100 was produced, and the generation of the second harmonic wave 14 was also examined. In this case, the fundamental wave 10 propagates in the positive direction only once through the optical wavelength conversion element. Further, in this case, the fundamental wave 10 is incident on the optical wavelength conversion element 15 in a state where the beam waist position is at the central position in the length direction of the optical wavelength conversion element 15 and the beam waist radius is 60 μm. The measurement results in this comparative example are also shown in Table 1.

  As described above, according to the optical wavelength conversion device 100 of the present embodiment, the wavelength conversion efficiency is 1. as compared to the optical wavelength conversion device configured to pass the fundamental wave 10 only once in the optical wavelength conversion element 15. It can be increased by 7 to 2.4 times.

  In the present embodiment, the coat 17 is formed on the output mirror 16, but the coat 17 may be formed on the emission end face 15 b of the light wavelength conversion element 15. Conversely, the coat 18 formed on the incident end face 15 a of the light wavelength conversion element 15 may be formed on an optical member formed separately from the light wavelength conversion element 15.

<< Second Embodiment >>
Next, an optical wavelength conversion device 200 according to the second embodiment of the present invention will be described with reference to FIG. In FIG. 2, the same elements as those in FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted unless necessary (the same applies hereinafter). Compared with the optical wavelength conversion device 100 shown in FIG. 1, this optical wavelength conversion device 200 basically has a phase comprising means for adjusting the temperature of the optical wavelength conversion element 15 instead of the phase adjustment means 20 by applying a variable voltage. The difference is that the adjusting means 30 is provided.

  The phase adjusting means 30 includes a Peltier element 31 capable of heating / cooling the optical wavelength conversion element 15 as a whole, a temperature adjustment circuit 32 for supplying a current to the Peltier element 31, and the temperature of the optical wavelength conversion element 15. And a temperature sensor 33 that feeds back a temperature detection signal to the temperature adjustment circuit 32. In this phase adjustment means 30, the current supplied to the Peltier element 31 by the temperature adjustment circuit 32 is controlled based on the difference between the detected temperature indicated by the temperature detection signal of the temperature sensor 33 and the set temperature (target temperature). The optical wavelength conversion element 15 can be controlled to a set temperature.

  The above-described optical wavelength conversion device of the comparative example, that is, the optical wavelength conversion device in which the fundamental wave 10 propagates through the optical wavelength conversion element only once in the positive direction is provided with the same phase adjusting means 30 as described above. The temperature of the optical wavelength conversion element that provides the maximum second harmonic output was examined. According to this, it was found that the output of the second harmonic has temperature dependence as shown in FIG. 3 with respect to the set temperature of the optical wavelength conversion element. As shown in the figure, the maximum second harmonic output can be obtained when the temperature of the optical wavelength conversion element is 56.7 ° C. (this is the phase matching temperature).

On the other hand, in the optical wavelength conversion device 200 of this embodiment, when the set temperature of the optical wavelength conversion element 15 was continuously changed, the second harmonic output changed periodically at a temperature interval of 0.47 ° C. As in the first embodiment, when the output of the fundamental wave 10 is 1 W, 3 W, and 10 W, the maximum value of the second harmonic output that fluctuates as described above was measured. As shown in Table 2 below, Results were obtained. Table 2 also shows the measurement results of the comparative example.

  As described above, also in the present embodiment, higher conversion efficiency than in the comparative example is obtained. However, when the results of the first embodiment shown in Table 1 are compared with the results of the present embodiment, the improvement in conversion efficiency relative to the comparative example is 2.4, 2.0, and 1.7 (times) in the first embodiment. On the other hand, in this embodiment, it is 1.4, 1.5 and 1.3 (times), and the degree of improvement is low. As described above, the temperature at which the second harmonic output that periodically changes at a temperature interval of 0.47 ° C. takes a peak value is approximately 0 with respect to the phase matching temperature 56.7 ° C. of the second harmonic generation. This is because it is about 56.5 ° C. shifted by 2 ° C.

<< Third Embodiment >>
Next, an optical wavelength conversion device according to a third embodiment of the present invention will be described. The optical wavelength conversion device of the present embodiment has a configuration in which the optical wavelength conversion element 215 shown in FIG. 4 is applied in place of the optical wavelength conversion element 15 in the optical wavelength conversion element 200 of the second embodiment shown in FIG. Is. In the optical wavelength conversion element 215 shown in FIG. 4, the periodically poled structure formed in the MgO-doped LiNbO ₃ crystal has a chirp period instead of a constant period, as shown by hatching to indicate the polarization inversion part 215c. It is different from the above-described optical wavelength conversion element 15 in that it is made.

  When the optical wavelength conversion element 215 having the above-described structure is used, the fundamental wave 10 is moved relative to the fundamental wave 10 in a direction transverse to the traveling direction thereof, that is, in the vertical direction in FIG. It becomes possible to change the period of the periodically poled structure in the propagation part.

  In this embodiment, by utilizing this fact, first, the fundamental wave 10 is propagated to a portion where the period of the periodically poled structure of the optical wavelength conversion element 15 is medium, that is, near the central portion in the vertical direction in FIG. The set temperature at which the output of the second harmonic wave 14 was maximized was found by continuously changing the set temperature of the optical wavelength conversion element 215 below, and the optical wavelength conversion element 215 was maintained at that temperature. Under this state, the phase of the second harmonic 14 that propagates the first time in the positive direction and the phase of the second harmonic 14 that propagates the second time in the positive direction coincide with each other. Next, the optical wavelength conversion element 215 was moved in the vertical direction in FIG. 4, and the optical wavelength conversion element 215 was fixed at a position where the output of the second harmonic wave 14 was maximum within the movement range. At this time, the period of the periodically poled structure where the fundamental wave 10 passes is a value that satisfies the phase matching condition for generating the second harmonic at the maximum.

In the above state, the output of the fundamental wave 10 was changed to 1 W, 3 W, and 10 W as in the first and second embodiments, and the second harmonic output in each case was measured. The result as shown in FIG. Table 3 also shows the measurement results of the comparative example.

  When the results of the second embodiment shown in Table 2 are compared with the results of this embodiment, the improvement in conversion efficiency relative to the comparative example is 1.4, 1.5 and 1.3 (times) in the second embodiment. On the other hand, in this embodiment, it is 2.2, 1.8, and 1.5 (times), and it can be seen that the degree of improvement is higher. That is, in the present embodiment, the temperature of the optical wavelength conversion element 215 set to match the phase of each second harmonic wave 14 propagating in the first direction and the second time in the positive direction is the phase matching temperature of the second harmonic generation. The deviation from the above can be compensated by changing the period of the periodically poled structure.

  FIG. 5 conceptually shows the above. In the figure, a solid line a indicates a second harmonic output profile based on the phase relationship between the two second harmonics 14. When the temperature of the optical wavelength conversion element 215 is about 56.5 ° C., The phase of the second harmonic 14 matches and the maximum second harmonic output is obtained. On the other hand, a broken line indicates a second harmonic output profile based on phase matching in second harmonic generation, a broken line b indicates a profile before the position adjustment of the optical wavelength conversion element 215, and a broken line c indicates an optical wavelength. The profile after the position adjustment of the conversion element 215 is shown.

  More specifically, the period of the periodically poled structure of the optical wavelength conversion element 15 in the second embodiment is 8.48 μm, whereas in this embodiment, the portion of the portion of the optical wavelength conversion element 215 where the fundamental wave 10 propagates is transmitted. The period of the periodically poled structure can be changed in the range of 8.46 to 8.51 μm.

  The optical wavelength conversion element 215 shown in FIG. 4 is a bulk type. However, when a waveguide type optical wavelength conversion element is used, if a plurality of optical waveguides are formed apart in the vertical direction in FIG. Good. Then, if the optical waveguide through which the fundamental wave 10 is guided is appropriately selected by moving the optical wavelength conversion element thus formed in the vertical direction in FIG. It becomes possible to change the period of the periodically poled structure stepwise.

<< 4th Embodiment >>
Next, with reference to FIG. 6, the optical wavelength converter 400 by 4th Embodiment of this invention is demonstrated. In FIG. 6 and FIGS. 7 to 9 to be described later, the phase adjusting means is omitted, but the phase adjusting means 20 shown in FIG. 1 or the phase adjusting means shown in FIG. 30 may be used as appropriate.

  In the optical wavelength conversion device 400 of the fourth embodiment, the fiber laser 11 and the optical wavelength conversion element 15 are directly coupled by a GRIN (refractive index distribution type) lens 410. The laser beam as the fundamental wave 10 emitted from the core 11a of the fiber laser 11 generally has a large divergence angle. Therefore, if the spread of the fundamental wave 10 is suppressed by the GRIN lens 410, the optical wavelength conversion element 15 having a longer element length can be applied, and therefore the interaction length between the element 15 and the fundamental wave 10 is increased. Therefore, the optical wavelength conversion device 400 with high wavelength conversion efficiency can be realized.

<< 5th Embodiment >>
Next, with reference to FIG. 7, the optical wavelength converter 500 by 5th Embodiment of this invention is demonstrated. This optical wavelength conversion device 500 differs from the optical wavelength conversion device 400 of the fourth embodiment shown in FIG. 6 in that a GI (graded index) fiber 510 is applied instead of the GRIN lens 410. It is. The GI fiber 510 is formed so that the refractive index of the core 510a gradually decreases from the center toward the periphery, and has a function of converging the fundamental wave 10 emitted after propagating through the core 510a. Even when such a GI fiber 510 is used, the spread of the fundamental wave 10 can be suppressed, and thereby the same effect as that of the fourth embodiment shown in FIG. 6 can be obtained.

<< 6th Embodiment >>
Next, with reference to FIG. 8, the optical wavelength converter 600 by 6th Embodiment of this invention is demonstrated. This optical wavelength converter 600 is different from the optical wavelength converter 400 of the fourth embodiment shown in FIG. 6 in that a tapered fiber 610 is applied instead of the GRIN lens 410. The tapered fiber 610 has a shape in which the core 610a gradually spreads from the incident end face toward the outgoing end face (from the right to the left in the figure). It is possible to suppress the spread. Thereby, also in this embodiment, the same effect as that of the fourth embodiment shown in FIG. 6 can be obtained.

<< 7th Embodiment >>
Next, an optical wavelength conversion device 700 according to the seventh embodiment of the present invention will be described with reference to FIG. In this optical wavelength conversion device 700, the output mirror 16 used in each of the embodiments described above is omitted, and instead, the emission end face 15b of the optical wavelength conversion element 15 has a convex lens shape. The coat 17 that performs the above-described action is formed on the emission end face 15b. Considering the reflection on the coat 17, the output end face 15 b having the above shape acts as a concave mirror like the output mirror 16. The fiber laser 11 is directly coupled to the optical wavelength conversion element 15. The optical wavelength conversion device 700 having such a configuration can be small and light overall.

  In the present embodiment, the fundamental wave returns directly to the fiber laser 11 side. Therefore, it is desirable to provide an optical isolator in the middle of the optical path on the fiber laser 11 side to prevent the fiber laser 11 from being damaged by the return light. In particular, when the fiber laser 11 is of high power, destruction by return light is more likely to occur, so it is desirable to use an optical isolator that can more effectively suppress the return light.

  In the optical wavelength conversion device of the present invention, the phase of the wavelength converted wave propagating the first time in the positive direction and the phase of the wavelength converted wave propagating the second time in the positive direction are mutually greater than one cycle by the phase adjusting means. It is desirable to be able to adjust. If so, it becomes possible to make the phases of the two wavelength converted waves coincide with each other. Hereinafter, specific conditions for satisfying this requirement when the second harmonic is generated will be considered when the variable voltage applying unit is used as the phase adjusting unit and when the temperature adjusting unit is used.

<< When voltage application means is used >>
When the applied voltage to the optical wavelength conversion element is V, the wavelength of light is λ, and the refractive index of the optical wavelength conversion element for this wavelength λ is ne, the electric field E acting on the voltage application region of the optical wavelength conversion element, and the voltage application The refractive index change amount Δne of the optical wavelength conversion element and the second harmonic phase change amount Δφ due to voltage application are as follows.

E = V / d (1)
Δne = (1/2) γ ₃₃ · ne ³ · E (2)
Δφ = (2π / λ) L · Δne (3)
∵φ = (2π / λ) L · ne
Further, specific numerical values of elements related to optical wavelength conversion are considered as the following example.

Wavelength of fundamental wave λ _FM = 1000 nm
Second harmonic wavelength λ _SH = 500 nm
EO coefficient of optical wavelength conversion element γ ₃₃ = 30.8 (pm / V)
Refractive index ne _FM = 2.16 with respect to the fundamental wave of the optical wavelength conversion element
Refractive index ne _SH = 2.25 for the second harmonic of the optical wavelength conversion element
Light wavelength conversion element thickness d = 0.5 mm
EO electrode length L = 1mm
The refractive indexes ne _FM and ne _SH are values when the optical wavelength conversion element is not subjected to voltage application. The thickness d of the optical wavelength conversion element is the distance between the pair of EO electrodes, and the length L of the EO electrode is the length of the region where the second harmonic is subjected to phase adjustment in the optical wavelength conversion element.

  Here, FIG. 10 schematically shows the optical paths of the fundamental wave 10 and the second harmonic wave 14 that pass through the optical wavelength conversion element 15. The fundamental wave 10-1 first propagated in the forward direction is reflected by the coat 17 and folded back in the reverse direction. This folded fundamental wave is shown as 10-2. When the fundamental wave 10-1 propagates in the positive direction, a part of the fundamental wave 10-1 is converted into the second harmonic 14-1. Further, when the folded fundamental wave 10-2 propagates in the opposite direction, a part thereof is converted to the second harmonic 14-2, and this second harmonic 14-2 is reflected by the coat 18 and reflected to the second harmonic. It propagates in the positive direction as wave 14-3.

The phases of the fundamental wave and the second harmonic wave change as they pass through the voltage application region of the optical wavelength conversion element 15. Here, the phase difference Δφ _T between the second harmonic 14-1 propagating first in the positive direction and the second harmonic 14-3 propagating second in the positive direction is considered. The wave 14-1 inherits the phase of the fundamental wave 10-1. The second harmonic wave 14-3 propagating second in the positive direction passes through the voltage application region a total of two times together with the second harmonic wave 14-2 before being reflected by the coat 18, and changes its phase. . Therefore, if the phase change of the fundamental wave due to passing through the voltage application region is Δφ _FM and the phase change of the second harmonic is Δφ _SH , the condition that the phase difference Δφ _{T is} exactly one cycle is as follows:
Δφ _T = Δφ _SH + Δφ _SH −Δφ _FM = 2π (4)
It becomes.

Δφ _SH and Δφ _FM are obtained from the above-described equation (3).
Δφ _SH = (2π / λ _SH ) L · Δne _SH
Δφ _FM = (2π / λ _FM ) L · Δne _FM
Therefore, from equation (4),
2 (2π / λ _SH ) L · Δne _SH − (2π / λ _FM ) L · Δne _FM = 2π
Π2πL {(2Δne _SH / λ _SH ) −Δne _FM / λ _FM } = 2π (5)
It becomes.

From the equations (1) and (2),
Δne _SH = (1/2) γ ₃₃ · ne ³ _SH · E = (1/2) γ ₃₃ · ne ³ _SH V / d
Δne _FM = (1/2) γ ₃₃ · ne ³ _FM · E = (1/2) γ ₃₃ · ne ³ _FM V / d
Therefore, equation (5) is
πL {(2γ ₃₃ · ne ³ _SH V / d · λ _SH ) −γ ₃₃ · ne ³ _FM V / d · λ _FM } = 2π
And substituting λ _FM = 2λ _SH into
πL {(2γ ₃₃ · ne ³ _SH V / d · λ _SH ) −γ ₃₃ · ne ³ _FM V / d · 2λ _SH } = 2π
∴ (Lγ ₃₃ V / 2dλ _SH ) (4ne ³ _SH −ne ³ _FM ) = 2
It becomes.

  If the value of V is calculated by substituting the numerical values of the specific examples described above into the above equation, V = 915 volts. In other words, when each element has the above numerical value, the phase of the wavelength converted wave propagating for the first time in the positive direction can be obtained by applying a variable voltage in the range of 0 to 915 volts to the optical wavelength conversion element. And the phase of the wavelength-converted wave propagating the second time in the positive direction can always be matched.

<When temperature control means is used>
When the temperature of the optical wavelength conversion element is uniformly adjusted, the second harmonic undergoes a phase change of Δφ with respect to the temperature change ΔT of the element when propagating in the forward direction or in the reverse direction. . Therefore, when the second harmonic propagates the first time in the forward direction, when it propagates back and propagates in the reverse direction, the phase change when it propagates from there and propagates the second time in the forward direction is each Δφ (three The phase difference Δφ _T between the second harmonic propagating the first time in the positive direction and the second harmonic propagating the second time in the positive direction is
Δφ _T = Δφ− (Δφ + Δφ) = Δφ
Eventually, the second harmonic wave has the same value as the phase change amount Δφ caused by the temperature change ΔT when passing through the optical wavelength conversion element once.

Then, the phase φ is expressed as follows: n is the refractive index of the optical wavelength conversion element, and Le is its length.
φ = (2π / λ) n · Le
So, from this,
Δφ _T = Δφ = (2π / λ) Δn · Le
It becomes. In addition, the refractive index change amount Δn of the optical wavelength conversion element when the optical wavelength conversion element is subjected to a temperature change of ΔT is expressed as follows, where the temperature coefficient of the optical wavelength conversion element is δn / δT:
Δn = ΔT (δn / δT)
Therefore, if this is substituted into the above equation,
Δφ _T = ΔT (2π / λ) (δn / δT) · Le
It becomes.

Consider specific values from this equation. Temperature coefficient of optical wavelength conversion element δn / δT = 5.3 × 10 ⁻⁵ , n = 2.2, λ = 500 nm,
ΔT (2π / λ) (δn / δT) · Le = 2π
When ΔT satisfying the above is obtained, for example, ΔT = 0.94 ° C. when Le = 10 mm, and ΔT = 0.47 ° C. when Le = 20 mm. In other words, when the length of the optical wavelength conversion element is 10 mm and 20 mm, if the element temperature can be adjusted over the range of 0.94 ° C. and 0.47 ° C., respectively, the first time in the positive direction. It is possible to make the phase of the wavelength converted wave propagating the same as the phase of the wavelength converted wave propagating the second time in the positive direction.

  FIG. 11 shows the relationship between phase adjustment and phase matching when the temperature of the optical wavelength conversion element is adjusted over the range of 0.47 ° C. as described above. That is, as indicated by a solid line, if the element temperature is adjusted over a range of 0.47 ° C., the phase of the wavelength-converted wave that propagates the first time in the positive direction when the temperature reaches the center of the range. Thus, the phase of the wavelength converted wave propagating the second time in the positive direction matches. In addition, if the phase matching profile (two examples when the element length is relatively long and short) shown in the broken line is as shown in the figure, the profile matches the temperature at the center. The phase matching profile may be changed by applying the above-described optical wavelength conversion element 215 of FIG. 4 so as to take a peak, that is, so that the temperature at the center becomes the phase matching temperature.

100, 200, 400, 500, 600, 700 Optical wavelength converter 10 Fundamental wave 11 Fiber laser (fundamental wave light source)
DESCRIPTION OF SYMBOLS 12 Collimator lens 13 Condensing lens 14 2nd harmonic 15, 215 Optical wavelength conversion element 15a Incident end surface of optical wavelength conversion element 15b Output end surface of optical wavelength conversion element 16 Output mirror 17 Coat (1st reflection member)
18 Coat (second reflecting member)
20, 30 Phase adjustment means 21 EO electrode 22 Voltage application circuit 31 Peltier element 32 Temperature adjustment circuit 33 Temperature sensor 410 GRIN lens 510 GI fiber 610 Taper fiber

Claims (6)

An optical wavelength conversion element having an incident end face and an exit end face facing the incident end face, converting the fundamental wave incident from the incident end face and propagating to the exit end face side into a wavelength converted wave, and emitting from the exit end face;
A first reflection member that reflects the fundamental wave and wavelength-converted wave emitted from the emission end face while allowing the former to be reflected and the latter to be transmitted;
The fundamental wave emitted from the incident end face after being reflected by the first reflecting member and propagating in the folded light wavelength conversion element, and the wavelength converted wave emitted from the incident end face after being converted from the fundamental wave, A second reflecting member that transmits while the latter reflects;
The phase of the wavelength conversion wave propagating in the optical wavelength conversion element before reaching the first reflection member, and the phase of the wavelength conversion wave propagating in the return optical wavelength conversion element after being reflected by the second reflection member And a phase adjusting means for aligning each other.   2. The optical wavelength conversion device according to claim 1, wherein means for applying a variable voltage to at least a part of the optical wavelength conversion element is provided as the phase adjusting means.   3. The optical wavelength conversion device according to claim 2, wherein the means for applying the variable voltage applies the variable voltage to the vicinity of the incident end face of the optical wavelength conversion element.   4. The optical wavelength conversion device according to claim 1, wherein means for adjusting the temperature of the optical wavelength conversion element is provided as the phase adjusting means.   As the phase adjusting means, the phase of the wavelength conversion wave propagating in the optical wavelength conversion element before reaching the first reflection member, and the reflection in the second reflection member and propagating in the folded light wavelength conversion element 5. The optical wavelength conversion device according to claim 1, wherein the wavelength-converted waves to be shifted can be shifted in phase by one period or more.   The optical wavelength conversion device according to any one of claims 1 to 5, wherein an element having a periodic polarization inversion structure in which a period of a polarization inversion portion is a chirp period is used as the optical wavelength conversion element.