JPH0322071B2 - - 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 a second harmonic generator (SHG), and This invention relates to a wavelength tunable laser device that obtains a broadband optical wavelength tunable laser by changing the angle between the optical axis of the crystal and the optical axis of the crystal to achieve phase matching.

"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 rotatably mounted nonlinear optical crystal 6,
In addition, SHG2 is a lens 7 that increases power density,
8 and an SHG crystal 9.

In such a configuration, the OPO crystal 6
is excited with a powerful laser beam to generate two lights with 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 p, signal light s, and idler light i are ωp, ωs, and ωi, and the wave number vectors are kp, ks, and ki, respectively, then a wavelength that satisfies the condition ωp=ωs+ωi kp=ks+ki oscillates. . Generally, only one of these two wavelengths is extracted.

In addition, in the SHG2, when the SHG 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)} ² = cos ² θm÷n ₀ ² +
The phase matching angle can be found from the relationship 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 If two lights of 870 nm and 870 nm are generated and the rotation angle (θ) is changed in the same way, 500
Coherent light in the range ~1230 nm is obtained.
The laser beam emitted from the OPO crystal 6 oscillates between the input and output mirrors 4 and 5 and is output. When the light emitted from the OPO 1 is incident on the SHG 2 and the SHG 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. 6, the optical axes 10 of crystals 6 and 9 are parallel to the plane of the paper, the pumping light is an ordinary light, and if one of the two generated lights (signal light, idler light) is an ordinary light, the other is an extraordinary light. becomes. 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.

“Problems 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),
Only the fundamental wave of a single polarization component, either the o component or the e component, can be used. For example, in Fig. 6, OPO crystal 6 is urea crystal, SHG crystal 9 is β-BaB ₂ O ₄ , pumping light is 355 nm ordinary light, OPO1 signal light is ordinary light (o component), OPO
If the idler light of No. 1 is extraordinary light (e component), the fundamental wave of SHG2 is ordinary light (o component), and the second harmonic of SHG2 is extraordinary light (e component), then the crystal 6 for OPO is centered around the rotation axis. When the angle (θ) is rotated by 50 to 90 degrees, as shown in Figure 9, the e component of OPO1 is 660 to 1230 nm, and the o component of OPO1 is 780 to 500 nm.
and changes. In SHG2, when only the o component of OPO1 is used as ordinary light and multiplied, a laser with a wavelength of 390 to 250 nm can be obtained. However, as shown in FIG. 9, a laser with a wavelength in the range x of 390 to 500 nm cannot be obtained. 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) extends to the long wavelength side, and the wavelength range multiplied by SHG 9 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, and if the optical axis 10 deviates, it will deviate from the incident plane of the SHG crystal 9.
The size of the SHG crystal 9 must be increased, the reflectance at the end face of the crystal increases, and even if an AR coating is applied to reduce the reflectance and the conversion efficiency is increased, the incident angle is widened. There were problems such as the difficulty of applying an AR coat.

"Means for Solving the Problems" The present invention has been made to solve the above-mentioned problems, and it generates signal light by exciting the nonlinear optical crystal of the first optical parametric oscillation section with pumping light. An idler light is emitted, and one of the lights emitted from the first optical parametric oscillation section is converted into a multiplied harmonic by an optical wavelength conversion section (second harmonic generation section) and emitted. In the wavelength tunable laser device, the output light of the first optical parametric oscillator is matched to the incident side of the optical wavelength converter (second harmonic generator) with respect to the optical wavelength converter. This device is equipped with a second optical parametric oscillator that can emit light with polarization planes that cannot be matched as light that can be matched.

"Operation" The nonlinear optical crystal of the first optical parametric oscillator (OPO) is excited with pumping light to emit signal light and idler light. Said first OPO
By changing the angle between the optical axis of the OPO crystal and the optical axis of the pumping light (for example, the rotation angle of the first OPO crystal), the wavelength of the output light can be varied and laser light in a predetermined wavelength range can be obtained. For example, one ordinary light among the output lights of the first optical parametric oscillator is made incident on the second harmonic generator (SHG), and the angle formed between the optical axis of this incident light and the optical axis of the SHG crystal is (for example, by rotating the SHG crystal in a plane parallel to the optical axis) to cause phase matching of 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 polarization direction of the output from the second optical parametric oscillator to the first optical parametric oscillator is
Emit light rotated 90 degrees. Then, the second
For the OPO crystal, the component that was the extraordinary light for the first OPO crystal becomes the ordinary light, so the phase matching of the laser beam, which is the extraordinary light and has become the ordinary light component, is caused and the wavelength is multiplied. Obtain the output light of the area.
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.

1a is a first optical parametric oscillator (hereinafter referred to as the first OPO), 1b is a second optical parametric oscillator (hereinafter referred to as the second OPO), and 2 is an optical wavelength conversion device, specifically, the first optical parametric oscillator. 2nd harmonic generation section (hereinafter
(called SHG). On the incident side of the first OPO 1a, a pulse laser with a wavelength of around 350 nm is required as excitation light, so an Nd-YAG laser device 11 that generates an infrared pulse of 1060 nm;
A module (SHG) 12 that generates a second harmonic of 532 nm and a module (THG) 13 that generates a third harmonic of 355 nm are arranged, and further a prism 14 and total reflection mirrors 15 and 16 are interposed. The first OPO 1a has a collimator section 3a, an input reflection mirror 4a, an output reflection mirror 5a, and a nonlinear optical crystal 6a, as in FIG. 6. This nonlinear optical crystal 6a is stored together with matching oil 20a in a container 19a with an entrance window 17a and an exit window 18a formed on both sides, and is rotated by a table 21a of a rotating device within the plane of the figure (in the same plane as the plane of the paper). It is rotatably provided.

As shown in FIG. 6, 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, and the SHG crystal 9 is parallel to the optical axis 10. It is attached to a rotating table 22 of a rotating device that rotates within a plane (in the same plane as the plane of the paper).

On the optical axis 10 between the first OPO 1a and (SHG) 2, a 355 nm pumping light is reflected,
Filter 2 that transmits laser light of 500 to 1230 nm
3 and a half mirror 24 are provided. At the reflection optical axis 25 of the filter 23 and between it and the half mirror 24, there is a mirror 26 interposed therebetween.
Wave plate 27, second OPO1b and mirror 28
is provided. The Θ wavelength plate 27 rotates the polarization direction of this pumping light by 90 degrees. The second OPO 1b is substantially the same as the first OPO 1a, and includes a collimator section 3b and an input/output reflection mirror 4.
b, 5b, OPO crystal 6b, rotary table 21
b, etc., but while the rotation direction of the first OPO crystal 6a is in the same plane as the plane of the paper, the second OPO coupling 6b is arranged in a plane perpendicular to the plane of the paper. It is being

The operation of the above configuration will be explained. As shown in Figure 1, the first layer consists of urea crystals.
When the OPO crystal 6a is excited with 355 nm pumping light, the angle (θ) of this first OPO crystal 6a is
When the angle is 90 degrees as shown in Figure 7, ordinary light of 500 nm (o component) and extraordinary light of 1230 nm (e component) are emitted, and when the angle (θ) is changed, the laser light of these wavelengths has the characteristics shown in Figure 4. It changes continuously like A and B.

Next, of the output light of the first OPO1a,
The 355 nm pumping light is reflected by the filter 23 and then totally reflected by the mirror 26 to be transmitted to the Θ wavelength plate 27.
Enter. Then, the polarization direction of this pumping light is rotated by 90 degrees. Here, the second OPO crystal 6b is in a positional relationship perpendicular to the first OPO crystal 6a. Therefore, the 355 nm pumping light whose polarization direction has been rotated by 90 degrees by the Θ wavelength plate 27 is the second
The signal becomes ordinary light for the OPO crystal 6b, is phase matched with the rotation direction in the vertical plane of the second OPO crystal 6b, and has a polarization direction perpendicular to the output light of the first OPO 1a. It will output light and idler light. In other words, the second OPO1
The output light of b is the first OPO for the SHG crystal 9.
The ordinary light of 1a becomes the extraordinary light, and the extraordinary light becomes the ordinary light,
This is totally reflected by the mirror 28 and the half mirror 24
Here, the output light from the first OPO 1a and the output light from the second OPO 1b are combined into SHG.
2. Then, ordinary light in the range of 500 to 1230 nm is obtained for the SHG crystal 9. This is
When the wavelength is converted by rotating the SHG crystal 9 by 50 to 90 degrees, characteristic lines C and D in Fig. 4 are obtained.
Output light of 250 to 500 nm can be obtained, and therefore all the wavelength tunable laser beams indicated by the bold lines in FIG. 4 can be obtained.

Next, FIG. 2 shows another embodiment of the present invention. The difference in Fig. 2 from Fig. 1 is that the optical axis 1 of the first OPO 1a and SHG 2 is
Two total reflection mirrors 29 and 30 are arranged on 0,
These total reflection mirrors 29 and 30 are movably attached to linear tables 31 and 32, respectively. Specifically, the linear table 31,
32 are placed on linear guides 33, 33, 34, 34 perpendicular to the optical axis 10, respectively, and are driven by linear pulse motors 35, 36.
6 is coupled to a control circuit 37. This control circuit 37 includes an output wavelength data output circuit 38 that outputs data corresponding to the output wavelength from the first OPO 1a.
and a memory 39 for commanding the polarization direction according to the wavelength.
, a comparator circuit 40 , an arithmetic circuit 41 , and a drive circuit 42 .

The SHG crystal 9 in SHG2 is the first and second
Of the signal light and idler light from OPOs 1a and 1b, only the ordinary light is phase matched.

In the above configuration, the linear pulse motors 35 and 36 are driven by a command from the control circuit 37, the linear tables 31 and 32 are moved to the right in the figure, and the total reflection mirrors 29 and 30 are aligned with the optical axis 10. Remove from. In this state, the ordinary light component of the first OPO 1a (characteristic A in FIG. 4) enters the SHG 2, undergoes wavelength conversion, and is converted into characteristic C in FIG. 4.

Next, when the total reflection mirrors 29 and 30 are moved to the left in the figure according to a command from the control circuit 37 to reach the state shown in the figure, the output light from the first OPO 1a passes through the total reflection mirrors 29 and 26 and is then transferred to the second OPO1a. Sent to OPO1b. In this second OPO1b, the first OPO1a
The extraordinary light component (Characteristic B in Figure 4) becomes the ordinary light.
Since it is sent to SHG2, the first OPO in SHG2
The wavelength of the extraordinary light 1a (characteristic B) is converted into the characteristic D shown in FIG. 4 as ordinary light.

In the example shown in Fig. 2, a total reflection mirror 2 is installed between the output side of the first OPO 1a and the input side of SHG2.
The second OPO 1b is placed with the mirrors 9 and 30 interposed between them, but as shown in FIG.
1a and the second OPO 1b may be installed in parallel. Rather, it is better to install it in parallel like this to directly transmit 355nm from the Nd-YAG laser device 11.
laser light can be introduced as pumping light, improving efficiency.

In the above embodiment, the present invention is applied to a wavelength tunable laser device that changes the wavelength of output light from each of the first and second OPOs and the optical wavelength conversion section by rotating the respective crystals. , the present invention is not limited to this, but the first, the first
Regarding each of 2OPO and optical wavelength conversion section,
It can be used in a wavelength tunable laser device that changes the wavelength of output light by changing the angle between the optical axis of the crystal and the optical axis of input light.

For example, a crystal is fixed without rotating, and a rotatable reflecting mirror for changing the traveling direction of incident light and a concave mirror for inputting the reflected light from this reflecting mirror to the crystal are added, and the rotatable It can also be used in a wavelength tunable laser device that changes the wavelength of output light by rotating a reflecting mirror to change the angle between the optical axis of the crystal and the optical axis of input light.

"Effects of the Invention" Since the present invention is configured as described above, the laser beams emitted from the first and second optical parametric oscillation sections both become ordinary light components even though their wavelengths are different, so the optical wavelength conversion section Both can perform optical wavelength conversion, and therefore, a broadband optical wavelength tunable laser can be obtained without making a large angle (for example, rotation angle) between the optical axis of the crystal for the optical wavelength conversion section and the optical axis of the input light. . In addition, since there is no need to make a large angle (for example, rotation angle) between the optical axis and the optical axis of the input light, there is no need to correct optical axis misalignment, or even if it is necessary, it is extremely easy, and it also reduces reflection. There is no need for an AR coating that supports wide angle incidence.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing a first embodiment of an optical wavelength tunable laser device according to the present invention, and FIG.
FIG. 3 is an explanatory diagram showing a third embodiment of the present invention; FIG. 4 is a characteristic diagram of the device of the present invention; FIG. 5 is an explanatory diagram of changes in wavelength in OPO and SHG; Figure 6 is an illustration of the conventional device, Figure 7 is an illustration of the output light when the rotation angle (θ) of the OPO crystal is 90 degrees, and Figure 8 is the rotation angle (θ) of the OPO crystal.
FIG. 9 is an explanatory diagram of output light when the angle is 60 degrees, and FIG. 9 is a characteristic diagram of a conventional device. 1, 1a, 1b... Optical parametric oscillation unit (OPO), 2... Optical wavelength conversion unit (second harmonic generation unit) (SHG), 3, 3a, 3b... Collimator unit,
6, 6a, 6b...OPO crystal, 9...SHG crystal,
10... Optical axis, 11... Nd-YAG laser device,
21a, 21b, 22... rotary table, 23...
...filter, 24...half mirror, 27...Θ
Wave plate, 29, 30... Total reflection mirror, 31, 3
2... Linear table, 35, 36... Linear pulse motor, 37... Control circuit.

Claims (1)

[Claims] 1. The nonlinear optical crystal of the first optical parametric oscillator is excited with pumping light to emit signal light and idler light, and of the light emitted from the first optical parametric oscillator, In a wavelength tunable laser device in which one of the lights is converted into a multiplied harmonic by an optical wavelength converter and then emitted, the optical wavelength converter has a plurality of wavelengths on the incident side of the optical wavelength converter. An optical wavelength tunable laser device comprising a second optical parametric oscillator that can emit light with a polarization plane that cannot be matched by the output light of the first optical parametric oscillator as matching light. . 2. The second optical parametric oscillation section is arranged along with a Θ wavelength plate on an optical axis branched by a filter and a half mirror provided on the optical axis between the first optical parametric oscillation section and the optical wavelength conversion section. An optical wavelength tunable laser device according to claim 1, wherein the optical wavelength tunable laser device is interposed. 3. The second optical parametric oscillator has a wavelength of The optical wavelength tunable laser device according to claim 1, which is interposed together with a plate. 4 The second optical parametric oscillator has a Θ
Claim 1 interposed with a wave plate
2. Optical wavelength tunable laser device as described in .