JPH06283793A - Light wavelength converting device - Google Patents

Abstract

PURPOSE:To stabilize the oscillation wavelength of a semiconductor laser and provide a stable power wavelength converted wave by using a device which converts the wavelength of a laser beam generated by the semiconductor laser or a laser beam generated from solid-laser crystal pumped by the semiconductor laser. CONSTITUTION:A semiconductor laser 20 is arranged near the beam incident face 10a of a light wavelength converting element 10 with a slight space L in between so as to constitute an external resonator between the front face 20a of the semiconductor laser 20 and the beam incident face 10a. The light intensity of a rear outgoing beam 15R, which corresponds to the light intensity of a laser beam 15 as a fundamental wave, is detected by a photodetector 21. Based on the output S of the photodetector 21, the driving current I of the semiconductor laser 20 is controlled by a feedback control circuit 22 so as to match the oscillation frequency of the semiconductor laser 20 with the resonance frequency of the external resonator.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical wavelength converter for converting a fundamental wave into a second harmonic wave or the like, and more particularly to a wavelength conversion of a laser beam emitted from a semiconductor laser by an optical wavelength conversion element. The present invention relates to an optical wavelength conversion device and an optical wavelength conversion device that pumps a solid-state laser crystal with a laser beam emitted from a semiconductor laser and converts the wavelength of the solid-state laser beam emitted from the solid laser crystal with an optical wavelength conversion element.

[0002]

2. Description of the Related Art Conventionally, nonlinear optical materials have been used to
Various attempts have been made to convert the wavelength of laser light into a second harmonic wave (shorter wavelength). As a light wavelength conversion element that performs wavelength conversion in this manner, specifically, a bulk crystal type, an optical waveguide type, and the like are known. Further, this type of optical wavelength conversion element is often used in combination with a semiconductor laser.

On the other hand, as disclosed in, for example, Japanese Patent Application Laid-Open No. 62-189783, a solid-state laser crystal doped with a rare earth element such as neodymium is used as a semiconductor laser (laser diode).
Laser diode pumped solid-state lasers are known which are pumped by. Also in this type of laser diode pumped solid-state laser, as shown in, for example, Japanese Patent Application Laid-Open No. 2-77181, in order to obtain a laser beam having a shorter wavelength, the solid-state laser beam is passed through a nonlinear optical material to generate a second harmonic And so on.

[0004]

As described above, in the optical wavelength converter using the semiconductor laser as the fundamental wave light source or the pumping light source, the output wavelength of the wavelength converted wave is changed because the oscillation wavelength of the semiconductor laser changes with time. May become unstable. In such an optical wavelength conversion device, the semiconductor laser often causes mode hopping under the influence of light emitted from the semiconductor laser and then reflected by the end surface of the optical wavelength conversion element or the solid laser crystal and returned to the semiconductor laser. . When this mode hopping occurs, the output of the solid-state laser beam becomes unstable, which in turn makes the output of the wavelength converted wave unstable.

The present invention has been made in view of the above circumstances, and an object thereof is to provide an optical wavelength conversion device capable of stabilizing the oscillation wavelength of a semiconductor laser and obtaining a wavelength-converted wave with stable output. It is what

[0006]

A first optical wavelength conversion device according to the present invention is an optical device including a semiconductor laser and an optical wavelength conversion element for wavelength-converting a laser beam emitted from the semiconductor laser as described above. In the wavelength conversion device, the semiconductor laser is arranged in close proximity to the light incident end face with a minute gap so as to form an external resonator between the front end face and the light incident end face of the light wavelength conversion element. At the same time, a photodetector for detecting the light intensity of the laser beam as a fundamental wave, and a drive current for the semiconductor laser based on the output of the photodetector, and an oscillation frequency (wavelength) of the semiconductor laser for the external resonator are provided. Resonance frequency (wavelength)
And a control circuit for controlling so as to match with.

The second optical wavelength converter according to the present invention wavelength-converts the semiconductor laser, the solid-state laser crystal pumped by the semiconductor laser, and the laser beam emitted from the solid-state laser crystal as described above. In a light wavelength conversion device including a light wavelength conversion element, a semiconductor laser is configured to have an external resonator between a front end face of the semiconductor laser and the light incidence end face of the solid-state laser crystal so as to be minute with respect to the light incidence end face. Along with being spaced apart, a photodetector that detects the light intensity of the laser beam as pumping light emitted from this semiconductor laser, and the drive current of the semiconductor laser based on the output of this photodetector, And a control circuit for controlling the oscillation frequency of the semiconductor laser to match the resonance frequency of the external resonator. It is characterized in that it was.

[0008]

When the drive current of the semiconductor laser is controlled as described above, its oscillation frequency (that is, wavelength) is locked to the resonance frequency (wavelength) of the external resonator. In this case, when the resonance frequency of the external resonator changes due to changes in ambient temperature, the oscillation frequency of the semiconductor laser changes so as to follow it, but this change is slight and not enough to jump the mode. , Mode hopping is suppressed.

If the mode hopping of the semiconductor laser is prevented in this way, the wavelength conversion efficiency of the optical wavelength conversion element in the first device becomes stable, and the output of the solid laser beam in the second device becomes stable. Stable, and as a result, the wavelength converted wave output is stabilized in both devices.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below based on the embodiments shown in the drawings. 1 and 2 are respectively the first of the present invention.
3A and 3B are a side view and a perspective view of an optical wavelength conversion device according to an example. This optical wavelength conversion device is, for example, an optical waveguide type optical wavelength conversion element 10, a semiconductor laser (laser diode) 20 that emits a fundamental wave whose wavelength is shortened by the optical wavelength conversion element 10, and the semiconductor laser 20. Photodetector 21 such as a photodiode for detecting the rearward emitted light 15R
And a feedback control circuit 22 to which the output S of the photodetector 21 is input.

In the optical wavelength conversion element 10, a periodic domain inversion structure 13 is provided on a substrate 11 made of a crystal of LiNbO ₃ (hereinafter, referred to as LN) which is a ferroelectric non-linear optical material, and the domain inversion portion is provided. A channel type optical waveguide 12 extending in the arranging direction is formed. The periodic domain inversion structure 13 is a structure in which the spontaneous polarization (domain) of LN is periodically inverted. In this structure, the period Λ of the domain inversion part is Λc = 2π / {β (2ω) -2β (ω)} (1) where β (2ω) is the propagation constant of the second harmonic 2β (ω) Is set to be an integral multiple of the coherent length Λc given by the propagation constant of the fundamental wave, whereby phase matching (pseudo phase matching) can be achieved between the fundamental wave and the second harmonic.

The above-mentioned periodic domain inversion structure 13 is formed by, for example, irradiating an electron beam with an electron beam drawing device to draw a predetermined periodic pattern on the LN substrate 11 that has been subjected to a single polarization process. You can In this example, the LN substrate 11 has a length of 7 mm.

On the other hand, as the semiconductor laser 20, a single longitudinal / transverse mode laser emitting a laser beam 15 having a fundamental wavelength λ ₁ = 860 nm is used. This semiconductor laser 20 is fixed to the light wavelength conversion element 10 via a mounting member 23, and its front end face 20a and the light incident end face of the light wavelength conversion device 10 are fixed.
10a (more specifically, the end face of the optical waveguide 12) is arranged in close proximity to the light incident end face 10a with a minute gap L so as to form an external resonator. In this example, the distance L is 9 μm.

The laser beam 15 emitted from the semiconductor laser 20 enters the optical waveguide 12 through the light incident end face 10a. The laser beam 15 traveling in the waveguide mode in the optical waveguide 12 has a wavelength λ ₂ =
It is wavelength-converted into the second harmonic 16 of λ _1/2 = 430nm. At this time, in the periodic domain inversion structure 13, the above-mentioned quasi phase matching is achieved between the laser beam 15 and the second harmonic wave 16.

A laser beam having a wavelength of 860 nm is provided on the light incident end face 10a and the light emitting end face 10b of the light wavelength conversion element 10.
A coating having the following characteristics is applied to 15 and the second harmonic 16 having a wavelength of 430 nm. HR is highly reflective (reflectance 99.9% or more), AR is non-reflective (transmittance 99%).
Above).

860 nm 430 nm Light incident end face 10a-HR Light emitting end face 10b-AR Therefore, the second harmonic 16 is emitted from the light emitting end face 10b to the outside of the light wavelength conversion element 10. The light emitted from the light emitting end face 10b also includes the laser beam 15 that is the fundamental wave, but the laser beam 15 and the second harmonic wave 16 can be separated by a dichroic mirror or the like.

Next, the point of stabilizing the output of the second harmonic wave 16 will be described. The photodetector 21 described above is arranged at a position facing the rear end face 20b of the semiconductor laser 20, and detects the light intensity P of the rear emission light 15R of the semiconductor laser 20. The light intensity P of the backward emitted light 15R corresponds to the light intensity of the laser beam 15 incident on the light wavelength conversion element 10. The output S of the photodetector 21 is the feedback control circuit 22.
Entered in.

Here, the relationship between the drive current I supplied to the semiconductor laser 20 and the light intensity P of the rear emission light 15R detected by the photodetector 21 is approximately as shown in FIG.
That is, in the region of the laser oscillation threshold value I ₀ or more, the light intensity P basically increases as the drive current I increases. The oscillation frequency (wavelength) of the semiconductor laser 20 changes as the drive current I changes,
When it coincides with the resonance frequency of the external resonator composed of the both end faces 10a and 20a, the light intensity P is lowered due to the confinement of light in the external resonator. That is, FIG.
When the light intensity P is partially reduced as indicated by the arrow D, the oscillation frequency of the semiconductor laser 20 matches the resonance frequency of the external resonator, so that the semiconductor laser drive current I is always kept in this arrow D portion. If so, the oscillation frequency of the semiconductor laser 20 is locked to the resonance frequency of the external resonator.

The feedback control circuit 22 is shown in FIG.
In the drive current range indicated by I _{1 to} I ₂ in FIG.
The semiconductor laser drive current I is controlled so that the output S of the above-mentioned S has a minimum value. By performing such feedback control, the oscillation frequency of the semiconductor laser 20 is locked to the resonance frequency of the external resonator, so that the wavelength of the laser beam 15, which is the fundamental wave, becomes stable, and the wavelength conversion efficiency becomes constant. , The second harmonic 16 with stable output can be obtained. In this embodiment, the output of the semiconductor laser 20 is 100 mW
At that time, the second harmonic wave 16 having an output of 5 mW was obtained.

Next, a second embodiment of the present invention will be described with reference to FIG. In FIG. 4, the same elements as those in FIG. 1 are designated by the same reference numerals, and duplicate description thereof will be omitted (the same applies hereinafter).

In the device of the second embodiment, the dichroic mirror 30 that reflects the laser beam 15 having a wavelength of 860 nm and transmits the second harmonic wave 16 having a wavelength of 430 nm in the optical path of the light emitted from the optical wavelength conversion element 10. Are arranged, and the light intensity of the laser beam 15 reflected there is detected by the photodetector 21. The control of the semiconductor laser drive current I based on the output S of the photodetector 21 and the positional relationship between the optical wavelength conversion element 10 and the semiconductor laser 20 are the same as those in the first embodiment. Second
The output of harmonic 16 is stabilized.

Further, the coating applied to both end faces 10a and 10b of the light wavelength conversion element 10 in the second embodiment is the laser beam 15 having a wavelength of 860 nm and the wavelength 43.
It has the following characteristics for the second harmonic 16 of 0 nm.

860 nm 430 nm light incident end face 10a 70% reflection 99% reflection light emission end face 10b 99.9% reflection 95% transmission Therefore, in the second embodiment, the laser beam 15 is reflected by the light incident end face 10a and returns to the semiconductor laser 20. Since the amount of light is larger than that in the first embodiment and optical feedback is easily applied, the frequency range (locking range) in which the oscillation frequency of the semiconductor laser 20 is locked to the resonance frequency of the external resonator is further expanded. Become. Further, in this configuration, the laser beam 15 which is the fundamental wave is confined in the optical waveguide 12 between the both end faces 10a and 10b of the optical wavelength conversion element 10 and resonates, and the power thereof is increased, so that the second output of higher output Harmonics 16 can be obtained. In this embodiment, when the output of the semiconductor laser 20 is 100 mW, the output
A second harmonic 16 of 30 mW was obtained.

Next, a third embodiment of the present invention will be described with reference to FIG. The optical wavelength conversion device of the third embodiment comprises an optical wavelength conversion element 40 arranged in the resonator of a laser diode pumped solid-state laser.

That is, with respect to the YAG crystal 41 which is a solid-state laser crystal doped with Nd (neodymium), the semiconductor laser 50 as a pumping light source has a front end face 50a which is slightly spaced from the light incident end face 41a of the YAG crystal 41. They are placed close to each other with L placed. And YAG crystal 41
Is disposed on the front side (right side in the figure) of the solid-state laser together with the light incident end face 41a, and the wavelength conversion is performed between the resonator mirror 42 and the YAG crystal 41. An element 40 is provided.

The light wavelength conversion element 40 is made of the above-mentioned LN bulk crystal, and the periodic domain inversion structure 13 is formed therein. On the other hand, as the semiconductor laser 50, one having a single longitudinal / transverse mode which emits a laser beam 51 having a fundamental wavelength of 809 nm is used. Elements 40, 41, 4 mentioned above
2, 50 and the photodetector 21 for detecting the rear emission light 51R of the semiconductor laser 50 are fixed on a common mount 52,
The temperature is maintained at a predetermined temperature by a temperature adjusting means (not shown).

The YAG crystal 41 is irradiated with N by a laser beam 51 having a wavelength of 809 nm emitted from a semiconductor laser 50.
The d atoms are excited and emit a laser beam 53 having a wavelength λ ₁ = 946 nm. The laser beam 53 is incident to the optical wavelength conversion element 40 is wavelength-converted into the wavelength λ ₂ = λ _1/2 = second harmonic 54 of 473 nm. At this time, in the periodic domain inversion structure 13, the above-mentioned quasi-phase matching is established between the laser beam 53 and the second harmonic wave 54.

A laser beam 5 having a wavelength of 809 nm is provided on both end surfaces 41a and 41b of the YAG crystal 41, both end surfaces 40a and 40b of the optical wavelength conversion element 40, and the mirror surface 42a of the resonator mirror 42.
1, laser beam 53 with wavelength 946 nm, and wavelength 4
A coating having the following characteristics is applied to the second harmonic wave 54 of 73 nm.

809 nm 946 nm 473 nm End face 41a AR HR-End face 41b-AR HR End face 40a-AR AR End face 40b-AR AR Mirror face 42a-HR AR Therefore, the laser beam 53 which is the fundamental wave is the end face 41a.
Between the mirror surface 42a and the mirror surface 42a to resonate and enter the optical wavelength conversion element 40 in a state where the power is increased, so that the second harmonic wave 54 of high output can be obtained. This second harmonic 54
The light is emitted from the resonator mirror 42.

In the above structure, the semiconductor laser 50 is
The light incident end face 41a is formed so as to form an external resonator between the front end face 50a and the light incident end face 41a of the YAG crystal 41.
Are closely arranged with a minute gap L therebetween. In this example, the distance L is 50 μm. Further, the control of the semiconductor laser drive current I based on the output S of the photodetector 21 is also the first.
In the same way as in the embodiment, the output of the second harmonic 54 is again stabilized.

The non-linear optical material used in the present invention is not limited to the above-mentioned LN, but other materials such as LiTaO ₃ (LT), MgO-LT and MgO are also used.
_{-LN, KNbO 3, KTP, BBO} , can also be used LBO like.

Further, in the embodiment described above, the quasi phase matching is performed between the fundamental wave and the wavelength converted wave, but the phase matching method is not limited to this, and the angular phase and The temperature phase matching may be performed.

[Brief description of drawings]

FIG. 1 is a schematic side view showing an optical wavelength conversion device according to a first embodiment of the present invention.

FIG. 2 is a perspective view of the device of the first embodiment.

FIG. 3 is a graph showing the relationship between the semiconductor laser drive current and the fundamental wave light intensity in the device of the first embodiment.

FIG. 4 is a schematic side view showing an optical wavelength conversion device according to a second embodiment of the present invention.

FIG. 5 is a schematic side view showing an optical wavelength conversion device according to a third embodiment of the present invention.

[Explanation of symbols]

 10 Waveguide type optical wavelength conversion element 10a, 10b, 40a, 40b End facet of optical wavelength conversion element 11 LN substrate 12 Channel type optical waveguide 13 Periodic domain inversion structure 15 Laser beam (fundamental wave) 16, 54 Second harmonic 20, 50 semiconductor laser 20a, 50a front end face of semiconductor laser 21 photodetector 22 feedback control circuit 30 dichroic mirror 40 bulk crystal type optical wavelength conversion element 41 YAG crystal 41a, 41b end face of YAG crystal 42 resonator mirror 51 laser beam (pumping light) ) 53 Solid-state laser beam (fundamental wave)

Claims (2)

[Claims] 1. An optical wavelength conversion device comprising a semiconductor laser and an optical wavelength conversion element for converting the wavelength of a laser beam emitted from the semiconductor laser, wherein the semiconductor laser includes a front end face of the semiconductor laser and the optical wavelength conversion element. A photodetector that is arranged close to the light-incident end face with a minute gap so as to form an external resonator with the light-incident end face, and that detects the light intensity of the laser beam as the fundamental wave. And a control circuit for controlling the drive current of the semiconductor laser based on the output of the photodetector so that the oscillation frequency of the semiconductor laser matches the resonance frequency of the external resonator. Optical wavelength converter. 2. An optical wavelength conversion device comprising a semiconductor laser, a solid-state laser crystal pumped by the semiconductor laser, and an optical wavelength conversion element for wavelength-converting a laser beam emitted from the solid-state laser crystal, wherein the semiconductor comprises: The laser is arranged in close proximity to the light-incident end face with a small distance so as to form an external resonator between the front end face and the light-incident end face of the solid-state laser crystal. A photodetector for detecting the light intensity of the laser beam as the emitted pumping light, and a drive current of the semiconductor laser based on the output of the photodetector, the oscillation frequency of the semiconductor laser being the resonance frequency of the external resonator. An optical wavelength conversion device, comprising: a control circuit for controlling so as to match the frequency.