JPH0322070B2 - - Google Patents

Description

[Detailed Description of the Invention] "Industrial Application Field" The present invention inputs light emitted from an optical parametric oscillator (OPO) into a crystal of an optical wavelength converter (for example, a second harmonic generator/SHG). The present invention relates to a wavelength tunable laser device which obtains a broadband optical wavelength tunable laser by rotating and phase matching the crystal.

"Prior Art" In general, a wavelength tunable laser device includes an optical parametric oscillator (OPO) 1 and a second harmonic generator (SHG) as an optical wavelength converter, as shown in FIG.
2, of which (OPO) 1 includes a collimator section 3 for increasing the power density of the incident laser beam, and an input reflection mirror 4 and an output reflection mirror 5 having a predetermined reflectance in a predetermined bandwidth. and a rotatable nonlinear optical crystal 6, and an SHG
2 are lenses 7 and 8 that increase power density and crystal 9
It consists of.

In such a configuration, the OPO crystal 6 is excited with a powerful laser beam to generate two lights having different wavelengths from the excitation light. The excitation light is called pumping light, one of the two output lights is called signal light, and the other is called idler light. These signal lights and idler lights are amplified and oscillated within a cavity composed of input and output mirrors 4 and 5 provided with an OPO crystal 6 in between.

If the angular frequencies of the pumping light p, signal light s, and idler light i are respectively ωp, ωs, and ωi, and the wave number vectors are kp, ks, and ki, respectively, then the wavelength that satisfies the condition ωp=ωs+ωi kp=ks+ki oscillates. do. Generally, only one of these two wavelengths is extracted.

In addition, in the SHG2, when the crystal 9 is a uniaxial crystal and the phase matching condition called TYPE-1 is used, n ₀ (λ) = ne (θm, λ/2) {1/ne (θm, λ/ 2)} The phase matching angle can be found from the relationship: ² = cos ² θm/n ₀ ² + sin ² θm/ne ² . Here, n ₀ is the refractive index of ordinary light, and ne is the refractive index of extraordinary light.

For example, at 355 nm of Nd-YAG laser, urea crystal 6
When excited, in TYPE-2 matching, crystal 6
When the rotation angle (θ) of It generates two types of light. In this way, by changing the rotation angle (θ),
Coherent light in the range 500-1230 nm is obtained.
The laser beam emitted from the crystal 6 is sent to the input/output mirror 4,
It oscillates and outputs within 5 seconds. When the light emitted from the OPO 1 is incident on the SHG 2 and the crystal 9 is rotated for phase matching, the frequency can be multiplied and a broadband oscillation light of 250 to 1230 nm can be obtained. In FIG. 4, optical axis 1 of crystals 6 and 9
0 is parallel to the paper surface, the pumping light is ordinary light, and if one of the two generated lights (signal light, idler light) is ordinary light, the other becomes extraordinary light.
Since the SHG crystal 9 has a TYPE-1 phase matching condition, it is phase matched to the ordinary ray by crystal rotation and outputs multiplied harmonics.

“Problem to be solved by the invention” In OPO1 with TYPE-2 matching, the output light is
As shown in the figure, one side is ordinary light (o component) and the other side is extraordinary light (e component). Here, to multiply by SHG2 according to the first type matching (TYPE1), o
Only the fundamental wave of a single polarization component, either the e-component or the e-component, can be used. For example, in Figure 4, OPO crystal 6 is a urea crystal, SHG crystal 9 is a β
−BaB ₂ O ₄ , pumping light is 355 nm ordinary light, OPO
The signal light of OPO1 is the ordinary light (o component), the idler light of OPO1 is the extraordinary light (e component), the fundamental wave of SHG2 is the ordinary light (o component), and the second harmonic of SHG2 is the extraordinary light (e component).
component), when the OPO crystal 6 is rotated by an angle (θ) of 50 to 90 degrees around the optical axis 10,
As shown in characteristic lines A and B in Figure 7, the e component of OPO1 is from 660 to 1230 nm, and the o component of OPO1 is from 780 to
It changes to 500nm. In SHG2, when only the o component of OPO1 is used as ordinary light and multiplied, a laser having a wavelength of 390 to 250 nm is obtained, as shown by characteristic line C in FIG. 7. However, in Figure 7, 390~
A laser with a wavelength in the range x of 500 nm is not available.
If the rotation angle of the OPO crystal 6 is greatly rotated to 50 degrees or less, the wavelength range of ordinary light (o component) will be extended to the long wavelength side.
The wavelength range multiplied by SHG9 also extends to the long wavelength side.
However, when the rotation angle (θ) is increased, the following problems occur. The deviation of the optical axis 10 due to the refractive index of the crystal increases.If the optical axis 10 shifts, SHG
Since the SHG crystal 9 is out of the incident plane, it is necessary to increase the size of the SHG crystal 9, the reflectance at the end face of the crystal increases, and in order to reduce the reflectance, an AR coating is applied to increase the conversion efficiency. However, there were also problems such as the difficulty of AR coating when the incident angle was wide.

"Means for Solving the Problems" The present invention has been made to solve the above-mentioned problems, and it generates signal light and idler light by exciting the nonlinear optical crystal of the optical parametric oscillation section with pumping light. In the wavelength tunable laser device, the wavelength tunable laser device is configured to emit a harmonic that is multiplied by rotating the crystal of an optical wavelength converter (for example, a second harmonic generator) from the light emitted from the optical parametric oscillator. Rotation consisting of a first rotating section that rotates the crystal for optical wavelength conversion section (second harmonic generation) in a plane parallel to the optical axis, and a second rotating section that rotates in a plane perpendicular to the optical axis. It is equipped with a device.

"Function" The nonlinear optical crystal of the optical parametric oscillator is
It is excited with pumping light and emits signal light and idler light. When the rotation angle of the optical crystal is changed,
The wavelength of the output light is varied to obtain laser light in a predetermined wavelength range. Among the output lights of this optical parametric oscillation section, for example, one ordinary light is made incident on the second harmonic generation section, and the second harmonic crystal is rotated in a plane parallel to the optical axis by the first rotation section. Then, phase matching is caused for the ordinary light to obtain output light in a multiplied wavelength range. If only the ordinary light is multiplied, the matching condition is not satisfied for the extraordinary light, and a broadband output cannot be obtained.

Therefore, the second harmonic crystal is rotated by 90 degrees in a plane perpendicular to the optical axis by the second rotating section. Then, the component that was previously extraordinary light for that crystal becomes ordinary light, and is further rotated in a plane parallel to the optical axis by the first rotating section to cause phase matching, and the component in the multiplied wavelength range is Obtain output light. In this way, a broadband optical wavelength tunable laser can be obtained.

“Example” An example of the present invention will be described below with reference to FIG.

1 is an optical parametric oscillator (hereinafter referred to as OPO), and 2 is an optical wavelength converter, specifically, a second harmonic generator (hereinafter referred to as SHG). Said OPO1
4, it has a collimator section 3, an input reflection mirror 4, an output reflection mirror 5, and a nonlinear optical crystal 6. This nonlinear optical crystal 6 has entrance windows 11 on both sides.
It is housed together with matching oil 14 in a container 13 having an exit window 12 formed therein, and is rotatably provided by a table 15 of a rotating device.

As shown in FIG. 4, the SHG 2 is composed of an SHG crystal 9 made of β-BaB ₂ O ₄ and lenses 7 and 8 provided on both sides of the SHG crystal 9.
is attached to a rotation device 18 consisting of a first rotation section 16 that rotates in a plane parallel to the optical axis 10 and a second rotation section 17 that rotates in a plane perpendicular to the optical axis 10 . That is, the first rotating section 16 is
A rotary table 19 on which the SHG crystal 9 is placed and fixed;
It is comprised of a stepping motor 20 that rotates the rotary table 19 about its central axis, and rotates the SHG crystal 9 in a plane parallel to the optical axis 10 to set a rotation angle (θ). In addition, the second
The rotating section 17 is provided with an index table 23 that is supported by bearings 22, 22 on a support frame 21 and rotatable around the optical axis 10, and the first rotating section 16 is attached to this index table 23. Parallel index scum 24 divided into four parts
This parallel index scum 2
The rotation shaft 25 of No. 4 is connected to a motor 27 by a coupling 26. And motor 27 is 1
When rotated, the index table 23 rotates 90 degrees.

The operation of the above configuration will be explained. As shown in Figure 1, when the OPO crystal 6 is excited with 355 nm pumping light, the angle (θ) of the OPO crystal 6 is
When is 90 degrees, 500nm ordinary light (o component) and 1230nm
The extraordinary light (e component) is emitted, and when the angle (θ) of the laser light of these wavelengths is changed, the characteristics A,
It changes continuously like B.

Next, the ordinary light (o component) of the output light of OPO1
The first of rotation device 18 enters SHG2 about
The rotating unit 16 rotates the SHG crystal 9 in a plane parallel to the optical axis 10. This rotation angle (θ) is 90 to 50
When it is varied within a range of degrees, it changes as shown by characteristic line C in FIG.

Next, when the motor 27 of the second rotating section 17 of the rotating device 18 is rotated once, the index table 23 is rotated by 90 degrees via the parallel index cam 24.
degree about the optical axis 10. Then,
The e component, which was an extraordinary light for the SHG crystal 9, becomes an ordinary light. In this state, the SHG crystal 9 is rotated by the motor 20 of the first rotating section 16 to cause phase matching, and the wavelength is converted into the ordinary light component of the SHG output as shown by the characteristic line D in FIG. As a result, a wavelength tunable laser can be obtained over the entire range of 250 to 1230 nm even in a rotation range of 90 to 50 degrees.

In the embodiment described above, the index cam 24 of the second rotating section 17 is divided into four parts of the index table 23, so that phase matching occurs only with either the ordinary light or the extraordinary light. However, by using a stepping motor as the motor 27, using a normal rotary table 23 for the index table 23 and index cam 24, and driving the rotary table rotationally with the stepping motor, it is possible to perform arbitrary positioning. Make it. With this configuration, the SHG of the ordinary light and extraordinary light of the OPO output can be adjusted at any position.
It can be phase matched with the ordinary light component for the crystal 9. Therefore, by selecting the rotation angle, the second harmonics of ordinary light and extraordinary light can be obtained simultaneously at an arbitrary output ratio.

"Effects of the Invention" Since the present invention is configured as described above, the e-component and o-component of the laser emitted from the optical parametric oscillation section are
It is possible to convert the optical wavelength of any of the polarized light components, and therefore, it is possible to obtain a broadband optical wavelength tunable laser without increasing the rotation angle of the crystal of the optical parametric oscillation section. Also, since there is no need to take a large rotation angle, there is no need to correct optical axis misalignment, or even if it is necessary, it is extremely easy, and there is no need for an AR coating that supports wide angle incidence to reduce reflections. .

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram showing the first embodiment of the optical wavelength tunable laser device according to the present invention, and Fig. 2 shows OPO and SHG.
Fig. 3 is a characteristic diagram of the device of the present invention, Fig. 4 is an explanatory diagram of the conventional device, and Fig. 5 is the output when the rotation angle (θ) of the OPO crystal is 90 degrees. An explanatory diagram of light, Figure 6 is an explanatory diagram of output light when the rotation angle (θ) of the OPO crystal is 60 degrees, Figure 7 is an explanatory diagram of the output light when the rotation angle (θ) of the OPO crystal is 60 degrees
The figure is a characteristic diagram of a conventional device. 1...Optical parametric oscillator (OPO), 2...
...Optical wavelength conversion section (second harmonic generation section) (SHG),
3... Collimator section, 4, 5... Reflection mirror, 6
...OPO crystal, 7, 8...lens, 9...SHG
Crystal, 10... Optical axis, 11, 12... Window, 13...
... Container, 14 ... Matching oil, 15 ... Rotating table, 16 ... First rotating section, 17 ... Second rotating section, 18 ... Rotating device, 19 ... Rotating table, 20 ... Steth Ping motor, 21...
Support frame, 22... Bearing, 23... Index table, 24... Parallel index cam, 25
...rotating shaft, 26...coupling, 27...motor.

Claims (1)

[Claims] 1. The nonlinear optical crystal of the optical parametric oscillation section is excited with pumping light to emit signal light and idler light, and the emitted light from the optical parametric oscillation section is transferred to the crystal of the optical wavelength conversion section. In a wavelength tunable laser device configured to emit multiplied harmonics through rotation, a first rotating section rotates the crystal for the optical wavelength conversion section in a plane parallel to the optical axis, and a first rotating section rotates the crystal for the optical wavelength conversion section in a plane parallel to the optical axis; 1. An optical wavelength tunable laser device comprising: a rotating device comprising a second rotating section that rotates at a oscillation speed. 2. The optical wavelength tunable laser device according to claim 1, wherein the optical wavelength conversion section comprises a second harmonic generation section. 3. The first rotating section comprises a rotating table on which a crystal for an optical wavelength conversion section is installed, and a stepping motor that rotates this rotating table.
2. Optical wavelength tunable laser device as described in . 4. The optical wavelength tunable laser device according to claim 1, wherein the second rotating section comprises an index table on which the first rotating section is installed and a motor for rotating the index table. 5. The optical wavelength tunable laser device according to claim 1, wherein the second rotating section comprises a rotating table on which the first rotating section is installed and a stepping motor that rotates the rotating table.