JP3731977B2 - Optical wavelength converter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical waveguide type optical wavelength conversion element that converts a fundamental wave into a second harmonic, and more specifically, a ferroelectric crystal substrate is used as an optical waveguide substrate, and a periodic domain inversion structure is formed in the optical waveguide. The present invention relates to an apparatus for converting the wavelength of a laser beam emitted from a semiconductor laser using an optical wavelength conversion element.
[0002]
[Prior art]
A method for converting the wavelength of a fundamental wave into a second harmonic using an optical wavelength conversion element provided with a region in which spontaneous polarization (domain) of a ferroelectric material having a nonlinear optical effect is periodically inverted has already been proposed by Bleombergen et al. (See Phys. Rev., vol. 127, No. 6, 1918 (1962)). In this method, the period Λ of the domain inversion part is
Λc = 2π / {β (2ω) -2β (ω)}
Where β (2ω) is the propagation constant of the second harmonic
β (ω) is the fundamental wave propagation constant
By setting so as to be an integral multiple of the coherent length Λc given by (1), phase matching (so-called pseudo phase matching) between the fundamental wave and the second harmonic can be achieved.
[0003]
And, for example, as disclosed in Japanese Patent Laid-Open No. 7-152555, an optical waveguide type optical wavelength conversion element that has an optical waveguide made of a nonlinear optical material and converts the wavelength of the fundamental wave guided therethrough is as described above. Attempts have been made to form such a periodic domain inversion structure and achieve phase matching efficiently.
[0004]
By the way, the optical waveguide type optical wavelength conversion element having the above-mentioned periodic domain inversion structure is often used for wavelength conversion of a laser beam emitted from a semiconductor laser. In that case, if the oscillation wavelength of the semiconductor laser does not match the wavelength that is phase-matched with the period Λ of the domain inversion part, the wavelength conversion efficiency will be extremely low, and it will be difficult to obtain a practical short wavelength light source. Become.
[0005]
In view of such circumstances, conventionally, as shown in, for example, the above-mentioned Japanese Patent Application Laid-Open No. 7-152555, a narrow-band bandpass filter (in the laser beam optical path between the semiconductor laser and the optical wavelength conversion element) In the following, it has been proposed to adjust and lock the oscillation wavelength of the semiconductor laser to a desired value.
[0006]
An example of such a configuration is shown in FIG. In the figure, 1 is a semiconductor laser that emits a laser beam 2 as a fundamental wave, 3 is an incident optical system, and 4 is an optical waveguide type optical wavelength conversion element having a channel optical waveguide 4a and a periodic domain inversion structure 4b. The incident optical system 3 includes a collimator lens 5 that collimates the laser beam 2 emitted from the semiconductor laser 1 in a divergent light state, a condensing lens 6 that converges the collimated laser beam 2, and these A λ / 2 plate 7 for polarization control disposed between the lenses 5 and 6 and a narrow band BPF 8 made of a dielectric multilayer filter, for example, are provided.
[0007]
The narrow band BPF 8 as described above generally has a spectral transmittance characteristic as shown in FIG. The maximum transmission wavelength λ in this characteristic ₀ Generally changes depending on the light incident angle θ to the BPF 8 as shown in FIG. The laser beam 2 transmitted through the BPF 8 having such characteristics is focused by the condenser lens 6 so as to converge at the incident end face of the optical waveguide 4a, and the component input to the optical waveguide 4a in the TM mode and the incident end face. It is divided into the component to reflect.
[0008]
The laser beam 2 input to the optical waveguide 4 a passes through the periodic domain inversion structure 4 b and is converted to the second harmonic 9. On the other hand, the laser beam 2 reflected by the incident end face of the optical waveguide 4a follows an optical path opposite to the optical path so far and is fed back to the semiconductor laser 1 and resonates between the incident end face and the rear end face of the semiconductor laser 1. , So that the semiconductor laser 1 has a wavelength λ ₀ It will oscillate at.
[0009]
And the maximum transmission wavelength λ of the BPF 8 ₀ 6 depends on the light incident angle θ to the BPF 8 as shown in FIG. 6, and by rotating the BPF 8 as shown by the arrow A in FIG. 4, the oscillation wavelength of the semiconductor laser 1 is changed to the periodic domain inversion structure. It can be adjusted and locked to a value that is phase matched to the period Λ of 4b.
[0010]
[Problems to be solved by the invention]
In this conventional apparatus, as schematically shown in FIG. 7 (1), the BPF 8 is rotated to adjust the oscillation wavelength even if the laser beam 2 is correctly applied to the incident end face of the optical waveguide 4a before adjustment. Then, the optical path of the laser beam 2 may be tilted as indicated by a broken line in FIG. This is because the BPF 8 is not a perfect parallel plate and the laser beam 2 is not a perfect parallel light.
[0011]
When the optical path of the laser beam is inclined as described above, the convergence position on the end face of the optical waveguide is shifted. Although this deviation is slight, the diameter of the optical waveguide is generally very small, such as about 2 to 3 μm, so that the optical coupling efficiency of the laser beam to the optical waveguide is reduced, and the output of the wavelength conversion wave is reduced. In the worst case, the amount of light input to the optical waveguide may be extremely reduced, making it impossible to adjust the oscillation wavelength to match the phase matching wavelength.
[0012]
Further, in the above-described conventional apparatus, since it is difficult to produce a BPF having a high transmittance, the amount of light input to the optical wavelength conversion element tends to be low, and thus it is difficult to obtain a high-output wavelength converted wave. The problem is also recognized.
[0013]
More specifically, in order to oscillate the semiconductor laser in a single longitudinal mode, it is generally necessary that the transmission wavelength half-width of the BPF be 0.5 nm or less. In producing a BPF by a multilayer thin film technique, a very advanced film forming technique is required in order to achieve such a transmission wavelength half width and to obtain a high transmittance. For example, if it is attempted to achieve a transmission wavelength half width of 0.5 nm and a transmittance of 80% or more with the current film forming technology, the yield of the film forming process becomes very low, and as a result, the BPF becomes extremely expensive. . If it is attempted to realize a transmission wavelength half width of 0.5 nm with the current normal film forming technology, the transmittance is only about 30%, and the output of the wavelength-converted wave is very low.
[0014]
The present invention has been made in view of the above circumstances, and in an apparatus for converting the wavelength of a laser beam emitted from a semiconductor laser by an optical waveguide type optical wavelength conversion element having a periodic domain inversion structure, the oscillation of the semiconductor laser An object of the present invention is to accurately lock the wavelength to a wavelength that is phase-matched with the period of the domain inversion unit and to obtain a high wavelength converted wave output.
[0015]
Another object of the present invention is to achieve the above object by using an inexpensive BPF.
[0016]
[Means for Solving the Problems]
The first optical wavelength conversion device according to the present invention comprises:
As described above, an optical waveguide type optical wavelength conversion element having a periodic domain inversion structure and a laser beam incident on the optical wavelength conversion element as a fundamental wave are emitted. Oscillate on its own In an optical wavelength conversion device comprising a semiconductor laser,
The collimator lens collimates the laser beam emitted from the semiconductor laser,
The laser beam before entering the optical wavelength conversion element is partly branched by the light branching means,
This branched laser beam is converged on a mirror by a condenser lens,
The laser beam is reflected by this mirror and fed back to the semiconductor laser,
A narrow band BPF is rotatably arranged between the mirror and the light branching means so that the pass wavelength is changed, and the narrow band BPF is fixed when the pass wavelength reaches a predetermined wavelength. .
[0017]
In addition, as said light branching means, what makes 50 to 90% of the laser beam which injected there into inject into an optical wavelength conversion element, and makes the remainder inject into the said mirror is used suitably. Specifically, a beam splitter is preferably used as this light branching means.
[0018]
On the other hand, the second optical wavelength conversion device according to the present invention is an optical wavelength conversion device comprising an optical wavelength conversion element and a semiconductor laser similar to those used in the first optical wavelength conversion device.
From this semiconductor laser, after collimating the laser beam as the backward emission light emitted to the opposite side of the laser beam to be wavelength-converted from the collimator lens, it is converged on the mirror by the condenser lens,
The laser beam is reflected by this mirror and fed back to the semiconductor laser,
Then, a narrow band BPF is provided in the optical path of the laser beam as the backward emission light. The narrow band BPF is fixed when the passing wavelength reaches a predetermined wavelength. It is characterized by that.
[0019]
【The invention's effect】
In the optical wavelength conversion device of the present invention, the adjustment and locking of the oscillation wavelength of the semiconductor laser can be performed in the same manner as in the conventional device described above by rotating the BPF.
[0020]
In the first optical wavelength conversion device of the present invention, the laser beam that has passed through the BPF is not incident on the optical wavelength conversion element, but the laser beam that has passed through the optical branching unit is incident on the optical wavelength conversion element. As this optical branching means, for example, a beam splitter having a transmittance of 70 to 90% can be used, so that a high input light quantity to the optical wavelength conversion element is secured to obtain a high-output wavelength converted wave. It becomes possible.
[0021]
As described above, if the laser beam that has passed through the BPF does not enter the optical wavelength conversion element, the output of the wavelength-converted wave is reduced even if an inexpensive BPF having a transmittance of about 30% is used. There is no. If an inexpensive BPF can be used in this way, an optical wavelength conversion device can be manufactured at a relatively low cost.
[0022]
In the second optical wavelength conversion device of the present invention, only the laser beam as the backward emission light passes through the BPF, and the laser beam to be wavelength-converted (front emission light) basically has the entire optical wavelength. Since the light is guided to the conversion element, in this case as well, it is possible to secure a high input light amount to the optical wavelength conversion element and obtain a high-output wavelength conversion wave.
[0023]
In the second optical wavelength conversion device of the present invention, when the reflectance of the rear end face of the semiconductor laser is as low as 0%, the semiconductor laser does not oscillate by itself, and the mirror and the semiconductor laser that reflect the backward emitted light Laser oscillation is performed using the front end face as an external resonator. At that time, if a BPF having a low transmittance is used as the BPF disposed between the mirror and the semiconductor laser, the internal optical power of the resonator is lowered, and the oscillation efficiency is lowered. If this is the case, the amount of light emitted from the front of the semiconductor laser is reduced, and as a result the output of the wavelength-converted wave is reduced.
[0024]
On the other hand, in the second optical wavelength converter, when the reflectance of the rear end face of the semiconductor laser is about 2 to 3%, the semiconductor laser oscillates by itself. Even if configured, the amount of light emitted from the front of the semiconductor laser is almost the same as when there is no external resonator. That is, in this case, the light from the external resonator acts weakly and perturbatively on the semiconductor laser. Therefore, the transmittance of the BPF does not affect the amount of light emitted from the front of the semiconductor laser and the output of the wavelength conversion wave.
[0025]
Further, in the optical wavelength conversion device of the present invention, only the laser beam for adjusting the oscillation wavelength passes through the BPF, and the laser beam to be converted is incident on the optical wavelength conversion element separately from the laser beam for adjusting the oscillation wavelength. Even if the optical path of the laser beam is tilted when the BPF is rotated, the amount of light input to the optical wavelength conversion element is not reduced thereby, and the output of the wavelength converted wave is not reduced.
[0026]
In particular, in the second optical wavelength conversion device of the present invention, since the BPF is disposed outside the semiconductor laser and the optical wavelength conversion element, the BPF is rotated for adjusting and locking the oscillation wavelength of the semiconductor laser. However, the input coupling efficiency of the fundamental wave to the optical waveguide of the optical wavelength conversion element is not affected. For this reason, the fundamental wave wavelength can be easily adjusted to the phase matching wavelength while maintaining the input coupling efficiency of the fundamental wave high, and thus extremely high wavelength conversion efficiency can be realized.
[0027]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an optical wavelength converter according to a first embodiment of the present invention. As shown in the figure, this optical wavelength conversion device is converted into parallel light by a semiconductor laser (laser diode) 10, a collimator lens 12 for collimating a laser beam 11 emitted from the semiconductor laser 10 in a divergent light state. A converging lens 13 for converging the laser beam 11, a λ / 2 plate 14 for polarization control disposed between the lenses 12 and 13, and a space between the λ / 2 plate 14 and the condensing lens 13. A beam splitter 20 as an optical branching unit, and an optical wavelength conversion element 15.
[0028]
Further, a mirror 21 is disposed at a position where the laser beam 11 reflected by the beam splitter 20 is incident. On the other hand, a narrow band BPF 22, a condensing lens 23, and the like so that the laser beam 11 reflected by the mirror 21 is sequentially incident. And a mirror 24 is provided. As the BPF 22, for example, a relatively inexpensive dielectric multilayer filter having a transmittance of 30% is used.
[0029]
As shown in the perspective view of FIG. 2, the optical wavelength conversion element 15 is made of MgO—LN (MgO-doped LiNbO) which is a ferroelectric material having a nonlinear optical effect. _Three ) A periodic domain inversion structure 17 in which a domain inversion portion in which the direction of spontaneous polarization parallel to the z-axis is inverted is periodically formed on a crystal substrate 16, and a channel extending along the periodic domain inversion structure 17 An optical waveguide 18 is formed.
[0030]
The MgO-LN crystal substrate 16 is, for example, doped with 5 mol% of MgO. The periodic domain inversion structure 17 is formed so that domain inversion portions are arranged in the x-axis direction of the substrate 16, and the period Λ is in consideration of the wavelength dispersion of the refractive index of MgO-LN with respect to wavelengths near 980 nm. Therefore, it is set to 5.3 μm so as to have a primary cycle. In this example, the length of the periodic domain inversion structure 17 (a dimension in FIG. 2) is 10 mm. Such a periodic domain inversion structure 17 can be formed by various methods disclosed in, for example, JP-A-6-242478.
[0031]
On the other hand, in the channel optical waveguide 18, after forming the periodic domain inversion structure 17, a metal mask pattern is formed on the + z surface of the substrate 16 by known photolithography and dry etching, and this substrate 16 is immersed in pyrophosphoric acid. It can be manufactured by a method such as performing a proton exchange process, removing the mask, and then performing an annealing process. Thereafter, the end faces 18a and 18b of the channel optical waveguide 18 are edge-polished, whereby the optical wavelength conversion element 15 is completed.
[0032]
The proton exchange treatment is performed, for example, at a temperature of 170 ° C. and a proton exchange time of 68 minutes. On the other hand, the annealing treatment is performed, for example, at an annealing temperature of 350 to 370 ° C. and an annealing time of 1 to 2 hours. The width of the channel optical waveguide 18 (dimension b in FIG. 2) is, for example, 6 to 9 μm.
[0033]
As an example of the semiconductor laser 10, a laser that emits a laser beam 11 having a wavelength near 980 nm is used. The laser beam 11 is collimated by the collimator lens 12, and then the polarization direction is adjusted in the z-axis direction of the channel optical waveguide 18 by the λ / 2 plate 14 and enters the beam splitter 20. A beam splitter 20 having a transmittance of 70% is used, and 70% of the incident laser beam 11 passes through the beam splitter 20.
[0034]
The laser beam 11 transmitted through the beam splitter 20 is condensed by the condenser lens 13 and converges on the end face 18 a of the channel optical waveguide 18. As a result, the laser beam 11 enters the channel optical waveguide 18 and is guided there.
[0035]
The laser beam 11 as the fundamental wave traveling in the waveguide mode is phase-matched (so-called quasi-phase matching) in the periodic domain inversion region in the channel optical waveguide 18 and converted into the second harmonic 19 having a wavelength of 490 nm. Is done. This second harmonic wave 19 also propagates through the channel optical waveguide 18 in the waveguide mode and exits from the optical waveguide end face 18b.
[0036]
On the other hand, the laser beam 11 reflected by the beam splitter 20 is reflected by the mirror 21, passes through the narrow band BPF 22, is condensed by the condensing lens 23, converges, and is reflected by the mirror 24 arranged at this convergence position. . The laser beam 11 reflected by the mirror 24 follows the optical path opposite to the optical path up to that point and is fed back to the semiconductor laser 10. The condensing lens 23 is not always necessary, and the laser beam 11 may be incident and reflected on the mirror 24 as a parallel beam. However, in that case, the mirror 24 and the rear end face of the semiconductor laser 10 do not form a confocal optical system, so the angle of the mirror 24 can be adjusted with high accuracy so that the amount of feedback to the semiconductor laser 10 is maximized. Necessary.
[0037]
Here, by rotating the BPF 22 in the direction of arrow A in FIG. 1, only the laser beam 11 having a predetermined wavelength can be fed back to the semiconductor laser 10. The detailed reason is basically the same as that in the conventional apparatus shown in FIG. However, in this case, the rear end face of the semiconductor laser 10 and the mirror 24 constitute an external resonator of the semiconductor laser 10.
[0038]
Thus, if only the laser beam 11 having a predetermined wavelength is fed back to the semiconductor laser 10, the semiconductor laser 10 oscillates at this wavelength. Therefore, by appropriately rotating the BPF 22, the oscillation wavelength of the semiconductor laser 10 can be selected and locked to a desired wavelength that is phase-matched with the period Λ of the periodic domain inversion structure 17.
[0039]
In this apparatus, the laser beam 11 transmitted through the BPF 22 having a low transmittance is not incident on the light wavelength conversion element 15, and the laser beam 11 transmitted through the beam splitter 20 having a high transmittance of 70% is used as the light wavelength conversion element. Since the light is incident on the light 15, it is possible to obtain a high-output second harmonic 19 while ensuring a high amount of light input to the optical wavelength conversion element 15.
[0040]
As described above, if the laser beam 11 transmitted through the BPF 22 does not enter the optical wavelength conversion element 15, even if a relatively inexpensive one having a transmittance of 30% is used as the BPF 22, the second harmonic is used. The output of 19 never drops. If an inexpensive BPF 22 can be used in this way, an optical wavelength conversion device can be manufactured at a relatively low cost.
[0041]
In this apparatus, only the laser beam 11 for adjusting the oscillation wavelength passes through the BPF 22, and the laser beam 11 to be wavelength-converted is incident on the optical wavelength conversion element 15 separately. Even if the optical path of the laser beam 11 is tilted when the BPF 22 is rotated for adjustment, the amount of light input to the optical wavelength conversion element 15 is not reduced thereby, and the output of the second harmonic 19 is not reduced.
[0042]
Hereinafter, the output of the second harmonic 19 and the like will be described with specific numerical values. In this first embodiment, first, the oscillation wavelength of the semiconductor laser 10 with an output of 100 mW is not selected as a wavelength (980 nm) that is phase-matched with the period Λ of the periodic domain inversion structure 17 in particular. Locked to 983 nm.
[0043]
At this time, the output of the laser beam 11 collimated by the collimator lens 12 was 90 mW due to the loss at the collimator lens 12. Further, the output of the laser beam 11 after passing through the λ / 2 plate 14 and the beam splitter 20 was 63 mW because the transmittance of the beam splitter 20 was 70%. In this case, since the output of the laser beam 11 from the optical wavelength conversion element 15 is 41 mW, the optical coupling efficiency of the laser beam 11 with respect to the optical waveguide 18 is estimated to be about 65%.
[0044]
Next, the BPF 22 was rotated in order to select the oscillation wavelength of the semiconductor laser 10 to a wavelength that is phase-matched with the period Λ of the periodic domain inversion structure 17. With this rotation, the output of the second harmonic 19 gradually increased, and a maximum output of about 1 mW was obtained. The oscillation wavelength of the semiconductor laser 10 at this time was 980 nm, which is phase-matched with the period Λ of the periodic domain inversion structure 17. The output of the laser beam 11 that is the fundamental wave from the optical wavelength conversion element 15 was about 40 mW.
[0045]
As described above, in this embodiment, it is confirmed that even if the BPF 22 is rotated to adjust the oscillation wavelength of the semiconductor laser 10, the optical coupling efficiency of the laser beam 11 with respect to the optical waveguide 18 does not decrease as in the conventional apparatus. It was done.
[0046]
In the embodiment described above, the semiconductor laser 10 is DC driven. In this driving method, several to several tens of percent of light intensity noise in the low frequency region (MHz level or lower) due to the semiconductor laser 10 itself, that is, light amount level fluctuations were observed. This is due to the fact that the light amount level is not stabilized (of course, the wavelength is locked to the phase matching wavelength according to the present invention, so that the level fluctuation due to the wavelength fluctuation does not occur). Conventionally, in order to reduce such noise, it is known that a semiconductor laser should be driven at a high frequency. Therefore, when the semiconductor laser 10 was driven at a high frequency with a frequency of 0.1 to 1 GHz, the fluctuation of the light amount level became 1% or less, and a more desirable result was obtained.
[0047]
The oscillation wavelength lock of the semiconductor laser 10 operates stably when the transmittance of the beam splitter 20 is in the range of 50 to 90%. When the transmittance of the beam splitter 20 exceeds 90%, the oscillation wavelength lock becomes slightly unstable.
[0048]
Next, a second embodiment of the present invention will be described with reference to FIG. In FIG. 3, the same elements as those in FIGS. 1 and 2 are denoted by the same reference numerals, and redundant description thereof will be omitted unless necessary (the same applies hereinafter).
[0049]
In the optical wavelength converter of FIG. 3, the laser beam 11 reflected by the beam splitter 20 is directly bent by the mirror (see the mirror 21 of FIG. 1), and the narrow band BPF 22, the condenser lens 23, and It is configured to enter the mirror 24.
[0050]
Even when the optical path of the laser beam 11 reflected by the beam splitter 20 is bent by the mirror as described above, the direction of the bending is not limited to the direction in FIG. 1, for example, it is bent to the right in FIG. It may be.
[0051]
In the above embodiment, a z-cut substrate (z plate) is used as the MgO-LN crystal substrate 16, and therefore the direction of spontaneous polarization is perpendicular to the substrate surface. However, the present invention is not limited thereto, and as shown in the specification of Japanese Patent Application No. 8-47591 filed by the present applicant, a substrate cut so that the direction of spontaneous polarization is not perpendicular to the substrate surface is used. It is also possible to form a periodic domain inversion structure by directly applying an electric field to.
[0052]
Next, a third embodiment of the present invention will be described with reference to FIG. In the optical wavelength conversion device of FIG. 8, the laser beam 11 emitted from the semiconductor laser 10 to the front side, that is, the right side in the figure, is guided to the optical wavelength conversion element 15 without being branched. The laser beam 11 incident on the light wavelength conversion element 15 is converted into the second harmonic 19 in the same manner as in the apparatus of FIG.
[0053]
Here, the laser beam 11R is emitted from the semiconductor laser 10 to the rear side, that is, the left side in the drawing. This laser beam 11R, generally referred to as backward emission light, is emitted from the semiconductor laser 10 as diverging light, converted into parallel light by a collimator lens 30, and then transmitted through a narrow band BPF 22 and condensed by a condenser lens 23. Has been converged. The laser beam 11R reflected by the mirror 24 arranged at the convergence position follows an optical path opposite to the optical path up to that point, and is almost entirely fed back to the semiconductor laser 10.
[0054]
Also in this case, only the laser beam 11R having a predetermined wavelength can be fed back to the semiconductor laser 10 by rotating the BPF 22 in the direction of arrow A in the figure. In this case, however, the front end face 10b of the semiconductor laser 10 and the mirror 24 constitute an external resonator of the semiconductor laser 10. Therefore, the reflectance of the rear end face 10a of the semiconductor laser 10 is set to, for example, about 0 to 3%, and the reflectance of the front end face 10b is set to about 10 to 20%, which is higher than that.
[0055]
Thus, if only the laser beam 11R having a predetermined wavelength is fed back to the semiconductor laser 10, the semiconductor laser 10 oscillates at this wavelength. Therefore, by appropriately rotating the BPF 22, the oscillation wavelength of the semiconductor laser 10 can be selected and locked to a desired wavelength that is phase-matched with the period Λ of the periodic domain inversion structure 17.
[0056]
Also in this apparatus, only the laser beam 11R for adjusting the oscillation wavelength passes through the BPF 22, and the laser beam 11 subjected to wavelength conversion is incident on the optical wavelength conversion element 15 completely separately. Therefore, even if the optical path of the laser beam 11R is tilted when the BPF 22 is rotated to adjust the oscillation wavelength, the amount of light input to the optical wavelength conversion element 15 is thereby reduced, and the output of the second harmonic 19 is reduced. There is no such thing.
[0057]
Hereinafter, the output of the second harmonic 19 in the optical wavelength converter of the third embodiment will be described with specific numerical values. In this apparatus, first, the oscillation wavelength of the semiconductor laser 10 was locked to 983 nm by the BPF 22 without selecting the phase matching wavelength (980 nm). At this time, the output from the optical wavelength conversion element 15 of the laser beam 11 which is the fundamental wave was 40 mW.
[0058]
Next, in order to select the oscillation wavelength of the semiconductor laser 10 as the phase matching wavelength, the BPF 22 was rotated. With this rotation, the output of the second harmonic 19 gradually increased, and a second harmonic output of about 1 mW at maximum was obtained. At this time, the oscillation wavelength of the semiconductor laser 10 was 980 nm, which coincided with the phase matching wavelength. The output of the laser beam 11 that is the fundamental wave from the optical wavelength conversion element 15 was about 39 mW.
[0059]
From the above, it was confirmed that the optical coupling efficiency of the laser beam 11 to the optical waveguide 18 does not decrease as in the conventional apparatus even if the BPF 22 is rotated to adjust the oscillation wavelength of the semiconductor laser 10 in this apparatus. . That is, according to the present apparatus, high wavelength conversion efficiency can be obtained without changing the input coupling efficiency of the fundamental wave with respect to the optical waveguide of the optical wavelength conversion element.
[0060]
Next, a fourth embodiment of the present invention will be described with reference to FIG. The optical wavelength conversion device of FIG. 9 is different from that of FIG. 8 in that the semiconductor laser 10 is directly coupled to the light incident end face of the optical wavelength conversion element 41. The optical wavelength conversion element having such a configuration can be formed to be small and light because there are few optical components, and has high optical stability because there is little axial displacement of the optical components.
[0061]
When the semiconductor laser 10 is directly coupled to the light incident end face of the light wavelength conversion element 41 as described above, a polarization control element such as the λ / 2 plate 14 in FIG. 1 cannot be provided between them. Therefore, if the optical wavelength conversion element is configured using a z-cut substrate as in the apparatus of FIG. 1 and the TM mode waveguide is adopted, the polarization direction of the laser beam 11 is changed to the z-axis of the channel optical waveguide 18. In order to match the direction, for example, the semiconductor laser 10 needs to be rotated 90 ° from the state shown in FIG. However, if so, the laser beam patterns in the semiconductor laser 10 and the channel optical waveguide 18 will be different, and the optical coupling efficiency between them will deteriorate.
[0062]
Because of these circumstances, when the semiconductor laser 10 is directly coupled to the light incident end face of the light wavelength conversion element, it is not necessary to rotate the polarization direction of the laser beam 11 by 90 °. The use of an element is advantageous in keeping the optical coupling efficiency between the semiconductor laser 10 and the channel optical waveguide 18 high. Therefore, in the present embodiment, the TE mode waveguide type optical wavelength conversion element 41 is configured using an x-cut MgO-LN crystal substrate 40 whose z-axis is horizontal with respect to the substrate surface.
[0063]
In place of the x-cut MgO-LN crystal substrate 40, a TE-mode waveguide type optical wavelength conversion element can be obtained by using a y-cut substrate in which the z-axis direction is horizontal to the substrate surface. Obtainable. Further, as shown in the specification of Japanese Patent Application No. 8-47591 mentioned above, a substrate cut so that the direction of spontaneous polarization is not perpendicular to the substrate surface is used, and an electric field is directly applied to the substrate. By forming a periodic domain inversion structure, a TE mode waveguide type optical wavelength conversion element can be obtained.
[Brief description of the drawings]
FIG. 1 is a schematic side view showing an optical wavelength converter according to a first embodiment of the present invention.
2 is a perspective view of an optical wavelength conversion element used in the optical wavelength conversion device of FIG.
FIG. 3 is a schematic side view showing an optical wavelength converter according to a second embodiment of the present invention.
FIG. 4 is a schematic side view showing a conventional optical wavelength converter.
FIG. 5 is a schematic diagram showing the spectral transmittance characteristics of a narrow-band bandpass filter.
FIG. 6 is a schematic diagram showing the light incident angle versus the maximum transmission wavelength characteristic of a narrow-band bandpass filter.
FIG. 7 is an explanatory diagram for explaining a decrease in optical coupling efficiency of a fundamental wave with respect to an optical wavelength conversion element in a conventional optical wavelength conversion device.
FIG. 8 is a schematic side view showing an optical wavelength converter according to a third embodiment of the present invention.
FIG. 9 is a schematic side view showing an optical wavelength converter according to a fourth embodiment of the present invention.
[Explanation of symbols]
10 Semiconductor laser
11 Laser beam (fundamental wave)
12 Collimator lens
13 Condensing lens
14 λ / 2 plate
15 Optical wavelength conversion element
16 MgO-LN crystal substrate (z plate)
17 Periodic domain inversion structure
18-channel optical waveguide
19 Second harmonic
20 Beam splitter
21 mirror
22 Narrow bandpass filter
23 Condensing lens
24 mirror
30 Collimator lens
40 MgO-LN crystal substrate (x plate)
41 Optical wavelength conversion element

Claims (4)

An optical waveguide is formed on a ferroelectric crystal substrate having a nonlinear optical effect, and a domain inversion portion in which the direction of spontaneous polarization of the substrate is inverted is periodically formed on the optical waveguide. An optical wavelength conversion element that converts the wavelength of the fundamental wave guided in the direction of arrangement of the inversion parts;
A semiconductor laser that emits a laser beam incident on the optical wavelength conversion element as the fundamental wave and oscillates by itself ;
A collimator lens that collimates the laser beam emitted from the semiconductor laser;
A light branching means for branching a part of the laser beam before entering the light wavelength conversion element;
A mirror for reflecting the branched laser beam and feeding it back to the semiconductor laser;
A condenser lens for converging the branched laser beam on the mirror;
A narrow band-pass filter that is rotatably arranged in the optical path of the laser beam between the mirror and the optical branching unit so as to change a passing wavelength, and is fixed when the passing wavelength reaches a predetermined wavelength. An optical wavelength conversion device.   2. The optical wavelength according to claim 1, wherein the optical branching unit causes 50 to 90% of the laser beam incident thereon to enter the optical wavelength conversion element and the remainder to enter the mirror. Conversion device.   3. The optical wavelength converter according to claim 1, wherein the optical branching unit is a beam splitter. An optical waveguide is formed on a ferroelectric crystal substrate having a nonlinear optical effect, and a domain inversion portion in which the direction of spontaneous polarization of the substrate is inverted is periodically formed on the optical waveguide. An optical wavelength conversion element that converts the wavelength of the fundamental wave guided in the direction of arrangement of the inversion parts;
A semiconductor laser that emits a laser beam incident on the optical wavelength conversion element as the fundamental wave and oscillates by itself ;
From this semiconductor laser, a collimator lens that collimates a laser beam as backward emission light emitted to the opposite side of the laser beam;
A mirror that reflects the laser beam and feeds it back to the semiconductor laser;
A condensing lens that converges a laser beam as the backward emission light on the mirror;
An optical wavelength conversion device comprising a narrow-band bandpass filter that is rotatably arranged in the optical path of the laser beam as the backward emission light so that the passing wavelength changes, and is fixed when the passing wavelength reaches a predetermined wavelength.

Images