JP2002344051A - Optical resonator, laser oscillator equipped therewith, and wavelength conversion device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical resonator that can optimize transmissivity (reflection coefficient) of an output mirror of a resonator in various lasers such as wavelength variable laser, solid-state laser, etc., of continuous oscillation and pulse oscillation, without having to change optical elements in the resonator in matching with oscillation forms, such as continuous oscillation, pulse oscillation, etc., in the laser device, and can be applicable as an optical resonator for a wavelength changing device. SOLUTION: A multilayer coating partially reflection mirror, used as an output mirror of the conventional optical resonator is replaced with a structure combined with a polarizer and a polarization rotating element, thus enabling change of the transmissivity from zero to 100%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transmittance (reflectance) of an output mirror of a resonator in various kinds of lasers such as a wavelength tunable laser and a solid-state laser of continuous oscillation and pulse oscillation.
The present invention relates to a resonator configuration that can be optimized without changing optical elements in the resonator in accordance with an oscillation mode such as continuous oscillation or pulse oscillation using the same laser device. This optical resonator can also be applied as an optical resonator of a wavelength conversion device.

[0002]

2. Description of the Related Art A conventional laser oscillator uses a multilayer-coated output mirror having an optimum transmittance depending on the gain, optical loss, and oscillation wavelength of a laser medium. In addition, each device is required depending on the oscillation mode of continuous oscillation and pulse oscillation, or it is necessary to replace the output mirror with an output mirror having an optimum transmittance (reflectance).

In the injection seeding, the seed light was input as p-polarized light from a polarizer in the resonator, but almost completely transmitted outside the resonator, and the seed light was not used effectively.

In a wavelength conversion optical resonator, the transmittance of a partially transmitting mirror is designed according to the output of an incident fundamental wave beam and the like, and a partially transmitting multilayer coating mirror is used.

[0005]

[0005] A multilayer coating having an optimum coupling ratio (transmittance) designed in consideration of the gain and optical loss of a laser medium as an output mirror in a resonator of a conventional laser device. Mirrors have been used. However, when the gain, the loss, and the oscillation wavelength change, the optimum coupling ratio also changes. Therefore, each time these parameters change, it is necessary to replace the output mirror with an optimum one.

Further, an output mirror having an optimum coupling ratio must be used for each of the continuous oscillation and the pulse oscillation. When both oscillation modes are used by one laser device, the output is adjusted according to each oscillation mode. The mirror needs to be replaced. Further, the cost of a broadband mirror required for a wavelength tunable laser or an ultrashort pulse laser is high, for example, in the case of multi-layer coating, 100 or more layers must be coated.

When a laser beam having a high beam quality is incident on an optical resonator as seed light, conventionally, the seed light is incident as p-polarized light from a polarizer installed in the optical resonator in order to align the polarization. Most of the energy of the seed light is transmitted to the outside of the optical resonator, and the energy of the seed light entering the optical resonator is so small that the seed light cannot be used efficiently.

As a method of extracting energy from a laser oscillator, a cavity dumping method has been conventionally used. This method uses an E-OQ switch to change the polarization of the laser light energy stored in a resonator, and a polarizer. And 100% transmitted or reflected, and is not used to obtain an arbitrary coupling ratio.

[0009] Even when an external resonator is used for wavelength conversion, the transmittance cannot be changed by a conventionally used partially transmitting multilayer film-coated mirror, and the optimum transmission is determined according to the output of the incident fundamental wave beam. A partially transmissive mirror having a refractive index was designed. Therefore, when the output of the fundamental wave beam changes, the optimum transmittance of the partially transmitting mirror also changes, and the partially transmitting mirror also needs to be replaced.

[0010]

According to the present invention, a multi-layer coating partially reflecting mirror used in an output mirror of a conventional optical resonator is replaced with an optical coupler having a combination of a polarizer and a polarization rotating element. Thus, it is possible to arbitrarily change the transmittance from 0 to 100%.

Utilizing that the polarizer transmits almost 100% of p-polarized light and reflects almost 100% of s-polarized light, λ / polarized light is incident before the polarizer with respect to the incident direction of the laser light polarization.
By arranging a polarization rotating element such as a two-wavelength plate, the laser light polarization can be continuously rotated from p-polarized light to s-polarized light, and only the p-polarized light component is output from the polarizer. By rotating the polarization rotator, the transmitted light from the polarizer can be freely and continuously changed from 0% to 100%, and the change in gain and optical loss, the oscillation wavelength, and whether pulse oscillation or continuous oscillation occurs. Regardless of the oscillation mode, an optimum coupling ratio of the output mirror can be obtained.

Further, by using the polarizer used for aligning the polarized light as an optical coupling element, 100% of the energy of the seed light can be input into the resonator as p-polarized light from the polarizer. The energy of the wasted seed light can be used efficiently.

This optical resonator can be applied as an optical resonator for wavelength conversion. A nonlinear crystal for wavelength conversion is installed in the optical resonator to control the polarization rotation element and the resonator length. By doing so, the energy of laser light can be stored in the resonator, and high wavelength conversion efficiency can be obtained.

[0014]

FIG. 1 shows a configuration example of a laser oscillator using an optical resonator of the present invention. Since all the mirrors of the resonator are replaced with polarizers, the oscillation wavelength range can be made wider than that of a conventional multilayer mirror, and it can be used for a wavelength tunable laser with a wide band.

The optical resonator according to the present invention comprises a polarizer 2,
In the ring resonator constituted by 7, 9, and 10, the oscillation wavelength range is narrowed, but the polarizers 7, 9, and 10 may be replaced with mirrors. The oscillation direction of the beam advances clockwise by the Faraday rotator 4 and is amplified by the laser head 5.

The oscillation oscillates with the s-polarized light 12 horizontal to the polarizer, but passes through the Faraday rotator 4 and becomes the inclined polarized light 13 rotated by the Faraday rotator 4, and then passes through the λ / 2 wave plate 3 and passes through the λ / 2 The light becomes the tilted polarized light 14 rotated by the wave plate, and further, only the p-polarized light component 15 of the output laser light perpendicular to the output polarizer 2 is output as the output beam 1.

By rotating the λ / 2 wavelength plate 3, the polarization can be continuously rotated, and the coupling ratio in the polarizer 2 can be changed from 0% to 100%.
Lenses 8 and 11 are used to adjust the mode of the beam. When the AO-Q switch 6 is not operating, continuous oscillation is performed. When the AO-Q switch 6 is operating, pulse oscillation is performed. Moreover, in each oscillation mode,
By rotating the λ / 2 wavelength plate 3, the coupling rate in the polarizer 2 can be set optimally. Further, the injection lock can be applied by the p-polarized light 17 which is incident on the polarizer 7 and transmitted by the seed light 16.

That is, the optical resonator of the present invention is characterized by having an optical coupler combining the polarizer 2 and the polarization rotating element of the λ / 2 wavelength plate 3. In this resonator, the polarizer transmits almost 100% of the p-polarized light and almost 10% of the s-polarized light.
By utilizing the 0% reflection and arranging a polarization rotation element in front of the polarizer 2 with respect to the incident direction of polarized light, the laser light polarization is continuously rotated from s-polarized light to p-polarized light, and only the p-polarized light component is Is output from the polarizer.

In the laser oscillator of the present invention, the polarizers 2, 7, 9 and 10 are arranged in a ring shape, and the polarizers 2 and 7 have a Faraday rotator 4 and a polarization rotating element of the wave plate 3, a laser head 5 and the like. And AO-Q
It is configured by installing the switch 6. In this oscillator, one of the oscillated s-polarized light 12 is rotationally polarized by the Faraday rotator 4 and the wave plate 3, and only the p-polarized light component 14 perpendicular to the polarizer 2 is output as the output beam 1.

FIG. 2A shows a configuration example of a wavelength converter using the optical resonator of the present invention. Since all of the mirrors of the resonator are replaced by polarizers, the oscillation wavelength range can be made wider than that of a conventional multilayer mirror, so that it can be used for a wide-band tunable laser or the like. The resonator is a ring-type resonator composed of polarizers 21, 26, 28 and 30.
It is also possible to replace 30 with a mirror.

The fundamental wave input beam 19 is a polarizer 21
Is input as p-polarized light 20, but becomes a tilted polarized light 22 in combination with the returning fundamental wave beam. FIG. 2B shows the polarization of the fundamental wave beam when the fundamental wave input beam and the returning fundamental wave beam are combined. When the polarization 37 of the fundamental wave input beam and the polarization 38 of the returning fundamental wave beam are combined by the polarizer 21, the polarizations of the beams are superposed vectorwise, and become linearly polarized light or elliptically polarized light 39 as shown in FIG. Whether the polarization of the combined beam is tilted linear or elliptically polarized depends on the relative positions of the polarization 37 of the fundamental input beam and the polarization 38 of the return fundamental beam. It is necessary to align the phases of the fundamental wave input beam and the returning fundamental wave beam so as to be inclined linearly polarized light. If the phases are not uniform and the polarization is circular or elliptically polarized, the polarizer 26 has a p-polarized component, and the p-polarized component is transmitted and detected by the photodetector 34. The position of the mirror or the polarizer 28 is controlled by a piezo mirror controller 35 or a piezo actuator 36 for finely adjusting the position of the polarizer so that the detection output of the photodetector indicating the transmitted light is minimized.

The polarization of the combined beam is then changed to λ / 2
The s-polarized light 2 is rotated by adjusting the rotation of the wave plate 23.
The light is reflected by the polarizers or mirrors 8 and 10 and guided to the nonlinear crystal 29. The fundamental wave beam is a lens 27
Is focused by the nonlinear crystal 29, and the wavelength is converted.
When a type I crystal is used as the nonlinear crystal 29, the polarization 32 of the wavelength-converted beam is p-polarized, and then output from the polarizer 30.

When a mirror is used instead of the polarizer 30, a dichroic mirror that reflects a fundamental wave beam and passes a wavelength-converted beam is used. The fundamental wave beam whose wavelength has not been converted is reflected by the polarizer 30 and is reflected by the lens 3.
The beam is returned to a parallel beam by 1 and is recombined with the fundamental incident beam 19 by the polarizer 21.

An embodiment of the present invention will be described below.

[0025]

FIG. 3 shows an oscillation characteristic when the laser oscillator shown in FIG. 1 is used. Nd: YAG crystal was used as a laser medium, and was excited by a continuous wave semiconductor laser. The horizontal axis shows the rotation angle of the polarized light, and the vertical axis shows the laser output.

In the present laser oscillator, the polarization of the oscillation laser beam 18 can be continuously rotated by rotating the λ / 2 wavelength plate 3, and only the p-polarized light passes through the polarizer 2. The transmittance changes depending on the inclination of the polarized light. If the polarization is completely p-polarized light, the transmittance is almost 100%. However, in this case, feedback is not performed and amplification does not actually occur. On the other hand, if the light is completely s-polarized, the beam is confined in the resonator and the beam energy is accumulated in the resonator, but the beam energy cannot be extracted out of the resonator. Therefore, there is an optimum transmittance for maximizing the laser output, and the element for adjusting the transmittance is a λ / 2 wavelength plate, and the component of the p-polarized light is adjusted by rotating the polarized light.

FIG. 3 shows a change in the output when the rotation angle of the polarized light is changed.
Is a state in which beam energy is confined in the resonator with almost perfect s-polarized light. However, since there is no perfect ideal polarizer, a beam is slightly output outside the resonator.

In continuous oscillation without operating the AO-Q switch, the output was maximum when the rotation angle of the polarization was 8 °, and 2% was obtained as the optimum transmittance of the output coupling element at this time.

On the other hand, when the AO-Q switch was operated and pulse oscillation was performed, the output became maximum at the polarization rotation angle of 6 °, and 1% was obtained as the optimum transmittance of the output coupling element. The peak positions of the continuous oscillation and the pulse oscillation are slightly different from each other because the gains of the amplification of the continuous oscillation and the pulse oscillation are different.

By using the optical resonator of the present invention, an optimum coupling ratio can be obtained even when the optical resonator is used in such different oscillation modes, gains, and optical losses.

[0031]

The following effects are produced by the present invention. (1) Optimal optical coupling conditions can be obtained with the output coupling element regardless of the gain, optical loss, oscillation wavelength, and mode of laser oscillation of the laser medium.

(2) Pulse oscillation by one laser device,
Both modes of continuous oscillation are possible. (3) An optimum output mirror transmittance can be obtained in a wide wavelength tunable region with a wavelength tunable laser.

(4) Injection seeding can be performed effectively. (5) Since it is a wide wavelength region, it can be applied to an ultrashort pulse laser.

(6) An optimum coupling ratio can be obtained with an optical resonator used for wavelength conversion, and high wavelength conversion efficiency can be obtained.

[Brief description of the drawings]

FIG. 1 shows an example of a laser oscillator using a variable coupling ratio optical resonator.

2A is a diagram showing a structure of a wavelength conversion device using a variable coupling ratio optical resonator, and FIG. 2B is a diagram showing a fundamental wave beam when a fundamental wave input beam and a returning fundamental wave beam are combined. It is a figure showing polarization.

FIG. 3 is a diagram illustrating a relationship between an optical coupling rate and a laser output performed using the system illustrated in FIG. 1; The horizontal axis indicates the polarization rotation angle of the beam with respect to the polarizer 2, and the vertical axis indicates the laser output.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Output beam 2 Polarizer for output 3 λ / 2 wavelength plate 4 Faraday rotator 5 Laser head 6 AO-Q switch 7 Polarizer 8 Lens 9 Polarizer 10 Polarizer 11 Lens 12 Polarization of laser light (s polarization) 13 Rotated by Faraday rotator 4 Polarization of laser light 14 Polarization of laser light rotated by λ / 2 wavelength plate 15 Polarization of output laser light 16 Seed light 17 Polarization of seed light (p-polarized) 18 Oscillated laser beam 19 Fundamental wave input beam 20 Fundamental wave input beam Polarized light (p-polarized light) 21 Polarizer 22 Polarized light when the fundamental wave input beam and return fundamental wave beam are combined 23 λ / 2 wavelength plate 24 Polarized light (s) obtained by rotating the tilted fundamental wave beam polarized light 22 by the λ / 2 wavelength plate (Polarized light) 25 Basic wave beam in resonator 26 Riser 27 lens 28 polarizer 29 nonlinear crystal 30 polarizer 31 lens 32 wavelength-converted output beam 33 polarization of wavelength-converted output beam 34 control photodetector 35 piezo mirror controller 36 piezo actuator 37 polarization of fundamental wave input beam 38 return basic Polarization of the wave beam 39 Polarization of the fundamental beam combined with the fundamental input beam and the returning fundamental beam

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Masaaki Kato 2-4 Shirane, Shirakata, Tokai-mura, Naka-gun, Ibaraki Prefecture F-term in the Japan Atomic Energy Research Institute Tokai Research Laboratory (reference) 2K002 AB12 BA03 HA20 5F072 AB02 HH05 HH06 HH07 JJ20 KK01 KK12 KK15 KK18 KK30 MM04 MM12 QQ02

Claims (6)

[Claims] 1. A ring type optical resonator comprising: a polarizer; and an optical coupler in which a polarization rotating element such as a λ / 2 wavelength plate is disposed in front of the polarizer with respect to the incident direction of laser light polarization. An optical coupler, wherein the polarizer transmits the p-polarized light almost 100% and the s-polarized light almost 1%.
Utilizing the reflection of 00%, the laser light polarization can be continuously rotated from s-polarized light to p-polarized light, and only the p-polarized light component can be transmitted by the polarizer.
Ring type optical resonator. 2. A laser oscillator, wherein a laser head and a Q switch are provided in the optical resonator according to claim 1, and both continuous oscillation and pulse oscillation are possible. 3. The optical resonator according to claim 1, wherein all of the resonator mirrors are replaced with polarizers. 4. The laser oscillator according to claim 2, wherein all of the resonator mirrors of the optical resonator are replaced with polarizers. 5. The laser oscillator according to claim 2, wherein the seed light is incident into the resonator as p-polarized light from a polarizer. 6. A wavelength converter, wherein a nonlinear crystal is provided in the optical resonator according to claim 1.