High-efficiency femtosecond laser frequency doubling device based on combined lens and adjusting method thereof
阅读说明：本技术 基于组合透镜的高效率飞秒激光倍频装置及其调节方法 (High-efficiency femtosecond laser frequency doubling device based on combined lens and adjusting method thereof ) 是由 李峰 赵卫 杨小君 杨直 王娜 王屹山 吕志国 ***龙 于 2020-05-28 设计创作，主要内容包括：本发明提供一种基于组合透镜的高效率飞秒激光倍频装置及其调节方法,解决现有飞秒激光倍频过程中,晶体易损伤、倍频光束质量易受影响、倍频后脉冲宽度二次展宽严重,及成本增加的问题。该装置包括沿光源出射方向依次设的飞秒激光光源、衰减器、第二半波片、双透镜组、倍频晶体及双色镜；衰减器用对飞秒激光进行功率衰减并输出线偏振激光；第二半波片可实现转动调节,用于控制线偏振激光的偏振方向；双透镜组包括距离可调整的凸透镜和凹透镜,用于改变激光在倍频晶体上的聚焦光斑大小；倍频晶体为Ⅰ类相位匹配的BBO晶体,用于对激光实现倍频；倍频晶体可实现俯仰和偏摆调节；双色镜用于将倍频后的光束分为2路,2路光束出射方向上均设有功率计。(The invention provides a high-efficiency femtosecond laser frequency doubling device based on a combined lens and an adjusting method thereof, and solves the problems that crystals are easy to damage, the quality of frequency doubling light beams is easy to influence, the pulse width after frequency doubling is seriously widened twice and the cost is increased in the existing femtosecond laser frequency doubling process. The device comprises a femtosecond laser light source, an attenuator, a second half-wave plate, a double-lens group, a frequency doubling crystal and a dichroic mirror which are sequentially arranged along the emergent direction of the light source; the attenuator is used for carrying out power attenuation on the femtosecond laser and outputting linear polarized laser; the second half-wave plate can realize rotation adjustment and is used for controlling the polarization direction of the linear polarized laser; the double-lens group comprises a convex lens and a concave lens with adjustable distances and is used for changing the size of a focused light spot of the laser on the frequency doubling crystal; the frequency doubling crystal is a BBO crystal with I-type phase matching and is used for realizing frequency doubling on laser; the frequency doubling crystal can realize pitch and deflection adjustment; the dichroic mirror is used for dividing the frequency-doubled light beams into 2 paths, and power meters are arranged in the outgoing directions of the 2 paths of light beams.)
1. The utility model provides a high efficiency femto second laser frequency doubling device based on combination lens which characterized in that: the device comprises an attenuator, a second half-wave plate (6), a double-lens group, a frequency doubling crystal (9) and a dichroic mirror (10) which are sequentially arranged along the emergent direction of a femtosecond laser light source (1);
the attenuator comprises a first half wave plate (4) and a polarization beam splitting prism (5) which are coaxially arranged along the laser emergent direction in sequence, and is used for performing power attenuation on femtosecond laser and outputting linearly polarized laser;
the second half-wave plate (6) can realize rotation adjustment and is used for controlling the polarization direction of the linear polarized laser;
the double-lens group comprises a convex lens (7) and a concave lens (8) which are coaxially arranged in sequence along the laser emitting direction and have adjustable distances, and is used for changing the size of a focusing spot of laser on a frequency doubling crystal (9);
the frequency doubling crystal (9) is a BBO crystal with I-type phase matching and is used for realizing frequency doubling on laser; the frequency doubling crystal (9) can realize pitching and deflection adjustment and is used for adjusting the frequency doubling matching angle of the femtosecond laser and the frequency doubling crystal (9) and the peak power density on the frequency doubling crystal (9);
the thickness of the BBO crystal is 0.5 mm-1.5 mm;
the dichroic mirror (10) is used for dividing the frequency-doubled light beam into 2 paths of frequency-doubled light and fundamental-frequency light respectively;
a frequency doubling optical power meter (11) is arranged in the emergent direction of the frequency doubling light;
and a fundamental frequency light power meter (12) is arranged in the emergent direction of the fundamental frequency light.
2. The high-efficiency femtosecond laser frequency doubling device based on the combined lens according to claim 1, wherein: the femtosecond laser device is characterized by further comprising a near-infrared high-reflection mirror pair arranged between the femtosecond laser light source (1) and the first half-wave plate (4) and used for adjusting the femtosecond laser in the optical axis direction and the direction perpendicular to the optical axis direction to realize the adjustment of the optical path direction.
3. The high-efficiency femtosecond laser frequency doubling device based on the combined lens according to claim 2, wherein: the near-infrared high-reflection mirror pair comprises a first reflecting mirror (2) and a second reflecting mirror (3) which are arranged in parallel;
the parameters of the first reflector (2) and the second reflector (3) are HR @1030 nm;
the included angle between the reflecting surface of the first reflector (2) and the optical axis of the femtosecond laser light source (1) is 45 degrees;
and the included angle between the reflecting surface of the second reflecting mirror (3) and the optical axis of the first half-wave plate (4) is 45 degrees.
4. The high efficiency femtosecond laser frequency doubling device based on the combined lens according to the claim 1, the claim 2 or the claim 3, characterized in that: the convex lens (7) is a positive lens, and the focal length is 100 mm;
the concave lens (8) is a negative lens, and the focal length is-100 mm;
the distance between the convex lens (7) and the concave lens (8) is 13 mm.
5. The high-efficiency femtosecond laser frequency doubling device based on the combined lens according to claim 5, wherein:
the BBO crystal size parameter is 6 x 1.2mm, the cutting angle theta is 23.4 degrees,
6. The high-efficiency femtosecond laser frequency doubling device based on the combined lens according to claim 5, wherein: the frequency doubling crystal (9) is arranged on a two-dimensional adjusting frame, and the two-dimensional adjusting frame is used for adjusting the pitching and the deflection of the frequency doubling crystal (9).
7. A method for adjusting a high-efficiency femtosecond laser frequency doubling device based on a combined lens is characterized by comprising the following steps:
1) performing power attenuation on the femtosecond laser to enable the attenuated power to meet the debugging requirement of a frequency doubling light path;
wherein, the frequency doubling crystal (9) adopts a BBO crystal with I-type phase matching, and the thickness is 0.5 mm-1.5 mm; (ii) a
2) The angle of the second half-wave plate (6) is adjusted to be the optimal angle by monitoring the frequency doubling efficiency, so that the femtosecond laser polarization direction entering the frequency doubling crystal (9) is the optimal polarization direction required by the frequency doubling crystal (9);
the frequency doubling efficiency is obtained by dividing a light beam into reflected frequency doubling light and transmitted fundamental frequency light through a dichroic mirror (10) positioned on an emergent light path of a frequency doubling crystal (9), respectively measuring the light power of the frequency doubling light and the light power of the fundamental frequency light, and calculating according to the two measured power values;
3) the size of a focusing light spot of the femtosecond laser on the frequency doubling crystal (9) is changed by adjusting the distance between the double-lens-group convex lens (7) and the concave lens (8), so that the peak power density entering the frequency doubling crystal (9) is controlled;
4) the adjustment of the pitching and the deflection of the frequency doubling crystal (9) is realized to adjust the frequency doubling matching angle of the femtosecond laser and the frequency doubling crystal (9) and the peak power density on the frequency doubling crystal (9).
8. The method for adjusting the high-efficiency femtosecond laser frequency doubling device based on the combined lens as claimed in claim 7, wherein: in step 2), the frequency doubling efficiency calculation formula is as follows:
9. the method for adjusting the high-efficiency femtosecond laser frequency doubling device based on the combined lens as claimed in claim 8, wherein: in the step 1), before the power attenuation of the femtosecond laser, the near-infrared high-reflection mirror is adopted to adjust the optical axis direction and the direction vertical to the optical axis direction of the incident femtosecond laser, so as to realize the adjustment of the optical path direction;
the near-infrared high-reflection mirror pair comprises a first reflecting mirror (2) and a second reflecting mirror (3) which are arranged in parallel;
the parameters of the first reflector (2) and the second reflector (3) are HR @1030 nm;
the included angle between the reflecting surface of the first reflector (2) and the optical axis of the femtosecond laser light source (1) is 45 degrees;
the included angle between the reflecting surface of the second reflecting mirror (3) and the optical axis of the first half-wave plate (4) is 45 degrees;
in the step 4), the frequency doubling crystal (9) is arranged on the two-dimensional adjusting frame, so that the pitching and deflection adjustment of the frequency doubling crystal (9) is realized.
10. The method for adjusting the high-efficiency femtosecond laser frequency doubling device based on the combined lens as claimed in claim 9, wherein: in step 1), the BBO crystal size parameter is 6 × 1.2mm, the cutting angle theta is 23.4 degrees,
in the step 3), the convex lens (7) is a positive lens, and the focal length is 100 mm;
the concave lens (8) is a negative lens, and the focal length is-100 mm;
the distance between the convex lens (7) and the concave lens (8) is 13 mm.
Technical Field
The invention relates to the technical field of laser, in particular to a high-efficiency femtosecond laser frequency doubling device based on a combined lens and an adjusting method thereof.
Background
Ultrafast femtosecond laser source has been applied to the field of material processing, the heat effect in the material processing process is small, the material processing capability is wide, the processing precision is high, the processing surface quality is good, the attention is widely paid, and the application is realized in the fields of aerospace, semiconductor material welding and cutting, flexible circuit board cutting, glass cutting and punching, and the like.
Compared with fundamental frequency light, the femtosecond green light has smaller thermal effect and higher processing precision in most processing processes, and particularly in some copper alloy material processing projects, the material has very small infrared light absorption rate, but the absorption rate at the femtosecond green light position is obviously improved, so that the frequency-doubled femtosecond green light laser source has important requirements in the field of laser fine welding. However, unlike the conventional narrow linewidth frequency doubling laser process, the femtosecond frequency doubling process involves very high peak power density, the frequency doubling efficiency is also related to the peak power density, the crystal is very easy to damage in the high-efficiency conversion process, and the time domain walk-off effect caused by the group velocity dispersion problem limits the improvement of the frequency doubling efficiency. Therefore, the problems that crystals are easy to damage, the quality of frequency doubling beams is easy to influence, the frequency doubling efficiency is low, the secondary broadening of pulse width after frequency doubling is obvious and the like exist in the frequency doubling process of the existing femtosecond laser; in order to improve the frequency doubling efficiency, a harsh temperature control system is adopted, and the cost is increased due to the high precision requirement of the temperature control system.
Disclosure of Invention
The invention provides a high-efficiency femtosecond laser frequency doubling device based on a combined lens and an adjusting method thereof, aiming at solving the technical problems that crystals are easy to damage, the quality of frequency-doubled light beams is easy to influence, the secondary broadening of pulse widths after frequency doubling is serious, and the cost is increased because a harsh temperature control system is required to ensure the frequency doubling efficiency in the frequency doubling process of the conventional femtosecond laser.
In order to achieve the purpose, the technical scheme provided by the invention is as follows:
a high-efficiency femtosecond laser frequency doubling device based on a combined lens is characterized in that: the system comprises a femtosecond laser source, an attenuator, a second half-wave plate, a double-lens group, a frequency doubling crystal and a dichroic mirror which are sequentially arranged along the emergent direction of the femtosecond laser source;
the attenuator comprises a first half wave plate and a polarization beam splitting prism which are coaxially arranged along the laser emergent direction in sequence, and is used for performing power attenuation on femtosecond laser and outputting linearly polarized laser;
the second half-wave plate can realize rotation adjustment and is used for controlling the polarization direction of the linear polarized laser;
the double-lens group comprises a convex lens and a concave lens which are coaxially arranged in sequence along the laser emergent direction and have adjustable distances, and is used for changing the size of a focusing spot of the laser on the frequency doubling crystal;
the frequency doubling crystal is a BBO crystal with I-type phase matching and is used for realizing frequency doubling on laser; the frequency doubling crystal can realize pitching and deflection adjustment and is used for adjusting the frequency doubling matching angle of the femtosecond laser and the frequency doubling crystal and the peak power density on the frequency doubling crystal;
the thickness of the BBO crystal is 0.5 mm-1.5 mm;
the dichroic mirror is used for dividing the frequency-doubled light beam into 2 paths of frequency-doubled light and fundamental-frequency light respectively;
a frequency doubling optical power meter is arranged in the emergent direction of the frequency doubling light;
and a fundamental frequency light power meter is arranged in the emergent direction of the fundamental frequency light.
And the near-infrared high-reflection mirror pair is arranged between the femtosecond laser light source and the first half-wave plate and is used for adjusting the femtosecond laser in the optical axis direction and the direction perpendicular to the optical axis to realize the adjustment of the optical path direction.
Further, the near-infrared high-reflection mirror pair comprises a first mirror and a second mirror which are arranged in parallel;
the parameters of the first reflector and the second reflector are HR @1030 nm;
the included angle between the reflecting surface of the first reflector and the optical axis of the femtosecond laser light source is 45 degrees;
and the included angle between the reflecting surface of the second reflector and the optical axis of the first half-wave plate is 45 degrees.
Further, the convex lens is a positive lens, and the focal length is 100 mm;
the concave lens is a negative lens, and the focal length is-100 mm;
the distance between the convex lens and the concave lens is 13 mm.
Further, the BBO crystal size parameter was 6 x 1.2mm, the cut angle θ was 23.4 °,
Further, the frequency doubling crystal is mounted on a two-dimensional adjusting frame, and the two-dimensional adjusting frame is used for adjusting the pitching and the deflection of the frequency doubling crystal.
The invention also provides a method for adjusting the high-efficiency femtosecond laser frequency doubling device based on the combined lens, which is characterized by comprising the following steps:
1) performing power attenuation on the femtosecond laser to enable the attenuated power to meet the debugging requirement of a frequency doubling light path;
wherein, the frequency doubling crystal adopts a BBO crystal with I-type phase matching, and the thickness of the frequency doubling crystal is 0.5 mm-1.5 mm; (ii) a
2) Adjusting the angle of the second half-wave plate to be an optimal angle by monitoring the frequency doubling efficiency, so that the polarization direction of the femtosecond laser entering the frequency doubling crystal is the optimal polarization direction required by the frequency doubling crystal;
the frequency doubling efficiency is obtained by dividing a light beam into reflected frequency doubling light and transmitted fundamental frequency light through a dichroic mirror positioned on an emergent light path of a frequency doubling crystal, respectively measuring the light power of the frequency doubling light and the light power of the fundamental frequency light, and calculating according to the two measured power values;
3) the size of a focusing light spot of the femtosecond laser on the frequency doubling crystal is changed by adjusting the distance between the convex lens and the concave lens of the combined double-lens group, so that the peak power density entering the frequency doubling crystal is controlled;
4) the frequency doubling matching angle of the femtosecond laser and the frequency doubling crystal and the adjustment of the peak power density on the frequency doubling crystal are realized by adjusting the pitching and the deflection of the frequency doubling crystal.
Further, in step 2), the frequency doubling efficiency calculation formula is as follows:
further, in the step 1), before the power attenuation of the femtosecond laser, the near-infrared high-reflection mirror is adopted to adjust the optical axis direction and the direction vertical to the optical axis direction of the incident femtosecond laser, so as to realize the adjustment of the optical path direction;
the near-infrared high-reflection mirror pair comprises a first reflection mirror and a second reflection mirror which are arranged in parallel;
the parameters of the first reflector and the second reflector are HR @1030 nm;
the included angle between the reflecting surface of the first reflector and the optical axis of the femtosecond laser light source is 45 degrees;
and the included angle between the reflecting surface of the second reflector and the optical axis of the first half-wave plate is 45 degrees.
And 4), mounting the frequency doubling crystal on a two-dimensional adjusting frame to realize the pitch and yaw adjustment of the frequency doubling crystal.
Further, in step 1), the BBO crystal size parameter is 6 × 1.2mm, the cutting angle θ is 23.4 °,BBO crystal double-sided plating with 1030nm&An antireflection film of 515 nm;
in the step 3), the convex lens is a positive lens, and the focal length is 100 mm;
the concave lens is a negative lens, and the focal length is-100 mm;
the distance between the convex lens and the concave lens is 13 mm.
Compared with the prior art, the invention has the advantages that:
1. the frequency doubling device can dynamically adjust the peak power density incident on the frequency doubling crystal by adopting the double lens group, meets the requirement of femtosecond lasers with different injection parameters (different powers, different repetition frequencies and different pulse widths), realizes high-efficiency frequency doubling output, can avoid the damage problem of the frequency doubling crystal, and ensures long-time high-efficiency stable operation; the thin BBO crystal with high nonlinear coefficient is adopted, the quality of frequency doubling light beams is not easily influenced, the quality of femtosecond frequency doubling green light beams is free of distortion, the secondary pulse broadening influence is small, meanwhile, active precise temperature control is not needed, and a frequency doubling device has the characteristics of simple structure and low cost.
2. Before the frequency doubling device and the method of the invention attenuate the laser, the near infrared high reflection mirror is adopted to adjust the height and the left and the right of the incident femtosecond laser, thereby realizing the adjustment of the direction of the frequency doubling light path.
3. In the frequency doubling device and the frequency doubling method, the thickness of the BBO crystal is 0.5-1.5 mm, the efficiency of the frequency doubling crystal is high, the damage threshold of the crystal device and the influence on the beam quality are small, furthermore, the BBO crystal is preferably selected to be 6 x 1.2mm in size parameter, the cutting angle theta is 23.4 degrees,
4. The frequency doubling crystal is arranged on the two-dimensional adjusting frame, and the two-dimensional adjusting frame has pitching and deflection adjusting functions, so that the position of the frequency doubling crystal is finely adjusted, and the optimized frequency doubling matching angle is further realized.
5. The adjusting method of the invention adjusts the angle of the second half-wave plate by monitoring the frequency doubling efficiency, so that the polarization direction entering the frequency doubling crystal is the optimal polarization direction required by the crystal, the frequency doubling efficiency is improved, and the long-time high-efficiency stable operation is ensured.
6. The adjusting method of the invention changes the size of the laser focusing light spot on the frequency doubling crystal by adjusting the distance between the convex lens and the concave lens of the double lens group, realizes the control of the peak power density entering the frequency doubling crystal, and can avoid the easy damage of the crystal in the frequency doubling process.
Drawings
FIG. 1 is a schematic structural diagram of an embodiment of a high-efficiency femtosecond laser frequency doubling device based on a combined lens according to the invention;
FIG. 2 is a graph of the femtosecond light source power output in the embodiment of the high-efficiency femtosecond laser frequency doubling device based on the combined lens according to the invention;
FIG. 3 is a graph of the pulse autocorrelation of the femtosecond light source in the embodiment of the high-efficiency femtosecond laser frequency doubling device based on the combined lens according to the invention;
FIG. 4 is a femtosecond frequency doubling efficiency curve diagram in an embodiment of the high-efficiency femtosecond laser frequency doubling device based on the combined lens according to the invention;
FIG. 5 is a stable double-lens set BBO crystal formed stable double-frequency output power stability test chart in an embodiment of the high-efficiency femtosecond laser frequency doubling device based on the combined lens according to the invention;
FIG. 6 is a frequency doubling efficiency test chart of the combined lens-based high-efficiency femtosecond laser frequency doubling device under the condition of about 6W of maximum injection power;
wherein the reference numbers are as follows:
the device comprises a 1-femtosecond laser light source, a 2-first reflector, a 3-second reflector, a 4-first half-wave plate, a 5-polarization beam splitter prism, a 6-second half-wave plate, a 7-convex lens, an 8-concave lens, a 9-frequency doubling crystal, a 10-dichroic mirror, an 11-frequency doubling optical power meter and a 12-fundamental frequency optical power meter.
Detailed Description
The invention is described in further detail below with reference to the figures and specific embodiments.
The invention provides a method for realizing high-efficiency, nondestructive and low-cost stable femtosecond laser frequency doubling by adopting a light beam transformation method based on positive and negative lens combination and a temperature-control-free direct frequency doubling mode of a high-nonlinearity thin BBO crystal through monitoring frequency doubling efficiency and adjusting peak power, and the method comprises the following steps:
1) performing power attenuation on the femtosecond laser to enable the attenuated power to meet the power injected into the
2) the angle of the second half-
3) the size of a focused light spot of the femtosecond laser on the
4) the optimized frequency doubling matching angle of the femtosecond laser and the