A four-mirror ring cavity optical parametric oscillator based on non-collinear phase matching principle and a manufacturing method thereof

Abstract

The application relates to a four-mirror annular cavity optical parametric oscillator based on a non-collinear phase matching principle and realizing wavelength wide tuning and a manufacturing method thereof, which comprises a first 45-degree OPO input mirror, a KTA crystal, a second 45-degree OPO input mirror, a third 45-degree OPO input mirror and a 45-degree OPO output coupling mirror which are sequentially arranged along an optical path. According to the advantages of non-collinear phase matching in wavelength tuning of the optical parametric oscillator, the application innovatively proposes a method for realizing non-collinear parametric light oscillation paths and pump light paths under a four-mirror annular cavity structure. By only adding a one-dimensional displacement platform to a cavity mirror of the annular cavity and performing a translation operation, tunable parametric light output under different non-collinear angle phase matching can be realized. In addition, forward and backward displacement in the direction parallel to the mirror normal can also obtain the optical parametric oscillator with wide tuning output, and the displacement direction of the mirror can be selected according to actual experimental conditions and environmental requirements.

Description

Technical Field

[0001] This invention relates to the field of optical parametric oscillator (OPO) technology, and in particular to a four-mirror ring cavity optical parametric oscillator with wide wavelength tuning based on the principle of non-collinear phase matching and its fabrication method. Background Technology

[0002] Eye-safe wavelengths (wavelengths exceeding 1.4 μm) are widely used in infrared remote sensing, special night vision, coherent lidar, biomedicine, and military weaponry. Common methods for generating eye-safe wavelengths include rare-earth ion-doped crystals, Raman lasers, and optical parametric oscillators (OPOs). Compared to in-cavity laser oscillation and stimulated Raman scattering, wavelength tuning is one of the most significant advantages of OPOs. To broaden the output wavelength range of OPOs, wavelength tuning is typically employed to achieve wide-band mid-to-short-wave infrared laser output, fully utilizing the characteristics of each laser wavelength within its tuning range and expanding the application scenarios of OPOs.

[0003] Currently, the most common operating mode of OPO (Optical Probe Oscillator) is collinear phase-matched, meaning the pump light and parametric light oscillate along the same path. The wavelength tuning methods used primarily include temperature tuning and angle tuning. Temperature tuning is mainly applied to periodically polarized crystals, such as periodically polarized lithium niobate (MgO:PPLN), periodically polarized potassium titanyl phosphate (PPKTP), and periodically polarized potassium titanyl arsenate (PPKTA). By changing the crystal's temperature, the phase-matching state is altered, resulting in parametric light output at different wavelengths, achieving wavelength-tunable laser output. Angle tuning involves rotating the crystal to change its placement angle within the resonant cavity, thereby altering the angle between the pump light and the crystal's optical axis, achieving parametric light output at different phase-matching angles. For large-size nonlinear crystals suitable for high-energy pumping, such as potassium titanyl arsenate (KTiOAsO4, KTA) and potassium titanyl phosphate (KTiOPO4, KTP), the crystal is typically cut at the critical phase-matching angle, and angle tuning is used to achieve tunability of the parametric light output wavelength. However, nonlinear crystals cut at noncritical phase-matching angles have advantages such as large effective nonlinear coefficients, small walk-off effects, and large acceptable angles, enabling higher parametric light energy output. However, the output parametric light wavelength of these crystals is insensitive to changes in the crystal rotation angle, making tunable output impossible. To overcome this limitation, a non-collinear phase-matching method was proposed and used to achieve wide-range tunable OPO parametric light output.

[0004] Based on the principle of non-collinear phase matching, there are currently two methods to achieve this. One is to change the transmission path of the pump light, making it at a certain angle with the oscillation path (optical axis) of the parametric light, so as to achieve non-collinear phase matching with the parametric light. For details, see the literature: F.Li, K.Zhong, H.Qiao, X.Zhang, D.Xu, and J.Yao, “A Wavelength-Agile Eye-Safe Optical Parametric Oscillator Based on an X-Cut KTP Crystal,” IEEE PHOTONICS J13, 1-4 (2021). Using a 4f imaging system, the pump light transmission path is changed by rotating the angle of the mirrors in the optical path. This structure is not suitable for high-energy pumping, as the pump light will focus and ionize in the 4f system, making the experimental conditions relatively dangerous. Non-collinear phase matching output of the KTP crystal can be achieved in a three-mirror ring cavity structure, but the operation is complicated, requiring simultaneous translation and rotation of the mirrors. During the operation, it is easy to cause the resonant cavity to stop oscillating, making it difficult to implement. Another approach involves adjusting the cavity mirror to alter the oscillation path of the parametric light, creating an angle between it and the pump light's transmission path (optical axis). This achieves noncollinear phase matching between the parametric light and the pump light. See the reference: K. Zhong, J. Mei, Y. Liu, H. Qiao, K. Liu, D. Xu, and J. Yao, “Widely tunable eye-safe optical parametric oscillator with noncollinear phase-matching in a ring cavity,” OPT EXPRESS 27, 10449-10455 (2019). This method is more suitable for high-energy pumped noncollinear phase-matched optical parametric oscillators. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a four-mirror ring cavity optical parametric oscillator that achieves wide wavelength tuning based on the principle of non-collinear phase matching;

[0006] The present invention also provides a method for fabricating the above-mentioned four-mirror ring cavity wavelength-tunable optical parametric oscillator;

[0007] The tuning method of this invention is simple, easy to operate, and has a wide wavelength tuning range. This structure is also applicable to other nonlinear crystals to achieve tunable output wavelength.

[0008] The technical solution of this invention is as follows:

[0009] A wavelength-tunable four-mirror ring cavity optical parametric oscillator based on the principle of non-collinear phase matching includes a first 45-degree OPO input mirror, a KTA crystal, a second 45-degree OPO input mirror, a third 45-degree OPO input mirror, and a 45-degree OPO output coupling mirror placed sequentially along the optical path.

[0010] After the pump light is collimated, it is incident on the surface of the KTA crystal, generating a nonlinear frequency conversion. The angles of the first 45-degree OPO input mirror, the second 45-degree OPO input mirror, the third 45-degree OPO input mirror, and the 45-degree OPO output coupling mirror are adjusted to achieve parametric light output.

[0011] According to a preferred embodiment of the present invention, the first 45-degree OPO input mirror, the second 45-degree OPO input mirror, the third 45-degree OPO input mirror, and the 45-degree OPO output mirror are all plane mirrors, and are all placed at a 45-degree angle to the optical path to form a four-mirror annular cavity, and the center line of all the cavity mirrors forms a rectangle.

[0012] According to a preferred embodiment of the present invention, the KTA crystal is wrapped in indium foil and placed on a fixture for clamping and fixing.

[0013] According to a preferred embodiment of the present invention, the first 45-degree OPO input mirror, the second 45-degree OPO input mirror, and the third 45-degree OPO input mirror are all coated with a high transmittance film covering a wavelength of 1064nm on one side, and coated with an antireflection film covering a wavelength of 1064nm and a high reflectivity film covering a wavelength of 1.5μm on the other side.

[0014] According to a preferred embodiment of the present invention, one side of the 45-degree OPO output coupling mirror is coated with a 1064nm high transmittance film and a 1.5μm partial transmittance film, and the other side is coated with an antireflection film covering the 1064nm wavelength and a high transmittance film covering the 1.5μm band.

[0015] According to a preferred embodiment of the present invention, the KTA crystal satisfies a non-critical phase-matching state, and the KTA crystal is cut along the X direction at a cutting angle of θ = 90°. θ is the angle between the wave vector k and the optical axis of the KTA crystal. The angle of incidence of the pump light onto the KTA crystal is denoted as θ.

[0016] According to a preferred embodiment of the present invention, the KTA crystal is cuboid.

[0017] According to a preferred embodiment of the present invention, the third 45-degree OPO input mirror is mounted on a one-dimensional displacement stage for translation operation.

[0018] The fabrication method of the aforementioned wavelength-tunable four-mirror ring cavity optical parametric oscillator includes:

[0019] (1) Install the first 45-degree OPO input mirror, the second 45-degree OPO input mirror, the third 45-degree OPO input mirror, and the 45-degree OPO output coupling mirror on the mirror frame;

[0020] (2) Wrap the KTA crystal with indium foil and then place it on a fixture for clamping and fixing;

[0021] (3) The first 45-degree OPO input mirror, KTA crystal, second 45-degree OPO input mirror, third 45-degree OPO input mirror and 45-degree OPO output coupling mirror are placed and fixed in sequence so that the center line of all devices coincides with the optical axis.

[0022] (4) A 1064nm laser is incident on the end face of the KTA crystal, resulting in a nonlinear frequency conversion. The angles of the first 45-degree OPO input mirror, the second 45-degree OPO input mirror, the third 45-degree OPO input mirror, and the 45-degree OPO output coupling mirror are adjusted to output 1.5μm parametric light.

[0023] (5) Rotate the one-dimensional displacement stage on which the third 45-degree OPO input mirror is mounted, so that the third 45-degree OPO input mirror is translated along a direction that is at a 45-degree angle to the optical axis and parallel to the normal of the mirror, thereby changing the oscillation path of the signal light, achieving non-collinear phase matching, and changing the output wavelength of the four-mirror ring cavity optical parametric oscillator.

[0024] Compared with existing technical solutions, the significant features of this invention are:

[0025] 1. Based on the principle of non-collinear phase matching, this invention enables the output wavelength of an optical parametric oscillator to be tunable by simply shifting one cavity mirror in a four-mirror ring cavity structure, with simple operation steps;

[0026] 2. This invention provides a method to achieve wavelength tuning of the optical parametric oscillator output in both forward and backward displacement directions parallel to the lens normal, allowing the cavity mirror to be moved in any direction according to actual needs and experimental conditions. Attached Figure Description

[0027] Figure 1 This is a schematic diagram showing the detailed structure and implementation of the wavelength-tunable four-mirror ring cavity optical parametric oscillator based on the principle of non-collinear phase matching of the present invention.

[0028] Figure 2 This is a schematic diagram illustrating the principle of non-collinear phase matching in the XZ principal plane of a KTA crystal.

[0029] Figure 3(a) is a schematic diagram of the theoretical signal light wavelength tuning curve for the angle tuning method;

[0030] Figure 3(b) is a schematic diagram of the theoretical signal light wavelength tuning curve for non-collinear phase matching tuning.

[0031] Among them, 1. First 45-degree OPO input mirror, 2. KTA crystal, 3. Second 45-degree OPO input mirror, 4. Third 45-degree OPO input mirror, and 5. 45-degree OPO output coupling mirror. Detailed Implementation

[0032] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments, but is not limited thereto.

[0033] Example 1

[0034] A wavelength-tunable four-mirror ring cavity optical parametric oscillator based on the principle of non-collinear phase matching, such as Figure 1 As shown, it includes a first 45-degree OPO input mirror 1, a KTA crystal 2, a second 45-degree OPO input mirror 3, a third 45-degree OPO input mirror 4, and a 45-degree OPO output coupling mirror 5, which are placed sequentially along the optical path; the first 45-degree OPO input mirror 1, the KTA crystal 2, the second 45-degree OPO input mirror 3, the third 45-degree OPO input mirror 4, and the 45-degree OPO output coupling mirror 5 constitute a four-mirror ring cavity optical parametric oscillator;

[0035] After assembling and fixing the positions of all the components, the 1064nm pump light is collimated and incident on the surface of KTA crystal 2, generating a nonlinear frequency conversion. By adjusting the adjustment knobs on the lens holder, the horizontal and vertical deflection angles of the lenses are finely adjusted. The angles of the first 45-degree OPO input mirror 1, the second 45-degree OPO input mirror 3, the third 45-degree OPO input mirror 4, and the 45-degree OPO output coupling mirror 5 are finely adjusted to achieve 1.5μm parametric light output.

[0036] Example 2

[0037] The wavelength-tunable four-mirror ring cavity optical parametric oscillator based on the non-collinear phase matching principle described in Example 1 differs in that:

[0038] The first 45-degree OPO input mirror 1, the second 45-degree OPO input mirror 3, the third 45-degree OPO input mirror 4, and the 45-degree OPO output mirror are all plane mirrors, and are all placed at a 45-degree angle to the optical path, forming a four-mirror annular cavity. The center line of all the cavity mirrors forms a rectangle.

[0039] The KTA crystal 2 is wrapped in indium foil and placed on a fixture for clamping and fixation.

[0040] The first 45-degree OPO input mirror 1, the second 45-degree OPO input mirror 3, and the third 45-degree OPO input mirror 4 are all coated with a high transmittance film covering a wavelength of 1064nm on one side, and an antireflection film covering a wavelength of 1064nm and a high reflectivity film covering a wavelength of 1.5μm on the other side.

[0041] The 45-degree OPO output coupling mirror 5 has a 1064nm high transmittance film and a 1.5μm partial transmittance film on one side, and a 1064nm antireflection film and a 1.5μm high transmittance film on the other side.

[0042] KTA crystal 2 satisfies a non-critical phase-matching state. KTA crystal 2 is cut along the X direction at a cutting angle of θ = 90°. θ is the angle between the wave vector k and the 2-axis of the KTA crystal. θ is the angle of incidence of the pump light onto KTA crystal 2.

[0043] For nonlinear crystals, the direction of the crystal's optical axis is usually defined as the Z direction, and the X and Y coordinate axes of the crystal are determined according to the right-hand screw rule of the coordinate system.

[0044] θ is the angle between the wave vector k and the optical axis of the crystal. The angle of incidence of the pump light onto the crystal. That is, the pump light is perpendicular to the crystal end face. When cutting along the X direction, the cutting angle is θ = 90°. At this point, the non-critical phase condition is met, and the nonlinear coefficient is at its maximum.

[0045] KTA crystal 2 is a cuboid.

[0046] The third 45-degree OPO input mirror 4 is installed on a one-dimensional displacement stage for translation operation.

[0047] The principle of non-collinear phase matching is as follows: Figure 2 As shown, at this cutting angle, nonlinear phase matching occurs in the XZ principal plane of KTA crystal 2. According to the laws of energy conservation and momentum conservation, the phase matching of the three-wave coupling of pump light, signal light, and idler light can be expressed as: Where kp, ks, and ki are the wave vectors of the pump light, signal light, and idler light, respectively. Combining the refractive index curve equation of KTA crystal 2 and the Semir dispersion equation, the phase-matching angle can be obtained.

[0048] The theoretical tuning curves of the signal light wavelengths for non-collinear phase matching and angle tuning were calculated. Figure 3(a) shows a schematic diagram of the theoretical signal light wavelength tuning curve for angle tuning; Figure 3(b) shows a schematic diagram of the theoretical signal light wavelength tuning curve for non-collinear phase matching tuning. By comparing the theoretical tuning curves of the signal light wavelengths for angle tuning, it can be found that under the same angle change, the tuning range of non-collinear phase matching is much larger than that of angle tuning.

[0049] Example 3

[0050] The fabrication method of the wavelength-tunable four-mirror ring cavity optical parametric oscillator described in Example 1 or 2 includes:

[0051] (1) Install the first 45-degree OPO input mirror 1, the second 45-degree OPO input mirror 3, the third 45-degree OPO input mirror 4, and the 45-degree OPO output coupling mirror 5 on the mirror frame;

[0052] (2) Wrap the KTA crystal 2 with indium foil, and then place it on the fixture for clamping and fixing;

[0053] (3) Place and fix the first 45-degree OPO input mirror 1, KTA crystal 2, second 45-degree OPO input mirror 3, third 45-degree OPO input mirror 4, and 45-degree OPO output coupling mirror 5 in sequence so that the center line of all devices coincides with the optical axis.

[0054] (4) A 1064nm laser is incident on the end face of KTA crystal 2, and a nonlinear frequency conversion occurs. The angles of the first 45-degree OPO input mirror 1, the second 45-degree OPO input mirror 3, the third 45-degree OPO input mirror 4, and the 45-degree OPO output coupling mirror 5 are adjusted to output 1.5μm parametric light.

[0055] (5) Rotate the one-dimensional displacement stage on which the third 45-degree OPO input mirror 4 is mounted, so that the third 45-degree OPO input mirror 4 is translated along a direction that is at a 45-degree angle to the optical axis and parallel to the normal of the mirror, thereby changing the oscillation path of the signal light, achieving non-collinear phase matching, and changing the output wavelength of the four-mirror ring cavity optical parametric oscillator.

Claims

1. A wavelength-tunable four-mirror ring cavity optical parametric oscillator based on the principle of non-collinear phase matching, characterized in that, It includes a first 45-degree OPO input mirror, a KTA crystal, a second 45-degree OPO input mirror, a third 45-degree OPO input mirror, and a 45-degree OPO output coupling mirror, which are placed sequentially along the optical path. After the pump light is collimated, it is incident on the surface of the KTA crystal, generating a nonlinear frequency conversion. The angles of the first 45-degree OPO input mirror, the second 45-degree OPO input mirror, the third 45-degree OPO input mirror, and the 45-degree OPO output coupling mirror are adjusted to achieve parametric light output. The first 45-degree OPO input mirror, the second 45-degree OPO input mirror, the third 45-degree OPO input mirror, and the 45-degree OPO output mirror are all plane mirrors, and they are all placed at a 45-degree angle to the optical path to form a four-mirror annular cavity. The center line of all the cavity mirrors forms a rectangle. The third 45-degree OPO input mirror is mounted on a one-dimensional displacement stage for translation. Rotating the one-dimensional displacement stage on which the third 45-degree OPO input mirror is mounted causes the third 45-degree OPO input mirror to translate along a direction that forms a 45-degree angle with the optical axis and is parallel to the normal of the mirror, thereby changing the oscillation path of the signal light, achieving non-collinear phase matching, and changing the output wavelength of the four-mirror ring cavity optical parametric oscillator.

2. The wavelength-tunable four-mirror ring cavity optical parametric oscillator based on the non-collinear phase matching principle according to claim 1, characterized in that, The KTA crystal is wrapped in indium foil and placed on a fixture for clamping and fixation.

3. A wavelength-tunable four-mirror ring cavity optical parametric oscillator based on the principle of non-collinear phase matching according to claim 1, characterized in that, The first 45-degree OPO input mirror, the second 45-degree OPO input mirror, and the third 45-degree OPO input mirror are all coated with a high transmittance film covering a wavelength of 1064 nm on one side, and an antireflection film covering a wavelength of 1064 nm and a high reflectivity film covering a wavelength of 1.5 μm on the other side.

4. A wavelength-tunable four-mirror ring cavity optical parametric oscillator based on the principle of non-collinear phase matching according to claim 1, characterized in that, The 45-degree OPO output coupling mirror has a 1064 nm high transmittance film and a 1.5 μm partial transmittance film on one side, and a 1064 nm wavelength antireflection film and a 1.5 μm wavelength high transmittance film on the other side.

5. A wavelength-tunable four-mirror ring cavity optical parametric oscillator based on the principle of non-collinear phase matching according to claim 1, characterized in that, The KTA crystal satisfies a non-critical phase-matched state, and the KTA crystal along... X Directional cutting, cutting angle is θ = 90°, φ = 0°, θ wave vector k The angle relative to the optical axis of the KTA crystal, φ The angle of incidence of the pump light onto the KTA crystal is denoted as θ.

6. A wavelength-tunable four-mirror ring cavity optical parametric oscillator based on the principle of non-collinear phase matching according to claim 1, characterized in that, The KTA crystal is a cuboid.

7. A method for fabricating the wavelength-tunable four-mirror ring cavity optical parametric oscillator according to any one of claims 1-6, characterized in that, include: (1) Install the first 45-degree OPO input mirror, the second 45-degree OPO input mirror, the third 45-degree OPO input mirror, and the 45-degree OPO output coupling mirror on the mirror frame; (2) Wrap the KTA crystal with indium foil and then place it on a fixture for clamping and fixing; (3) Place and fix the first 45-degree OPO input mirror, KTA crystal, second 45-degree OPO input mirror, third 45-degree OPO input mirror and 45-degree OPO output coupling mirror in sequence, so that the center line of all devices coincides with the optical axis. (4) A 1064 nm laser is incident on the end face of the KTA crystal, resulting in a nonlinear frequency conversion. The angles of the first 45-degree OPO input mirror, the second 45-degree OPO input mirror, the third 45-degree OPO input mirror, and the 45-degree OPO output coupling mirror are adjusted to output 1.5 μm parametric light. (5) Rotate the one-dimensional displacement stage on which the third 45-degree OPO input mirror is mounted, so that the third 45-degree OPO input mirror is translated along a direction that is at a 45-degree angle to the optical axis and parallel to the normal of the mirror, thereby changing the oscillation path of the signal light, achieving non-collinear phase matching, and changing the output wavelength of the four-mirror ring cavity optical parametric oscillator.

Images