Active and passive mode locking combined femtosecond disc laser

Abstract

The invention discloses an active and passive mode locking combined femtosecond disc laser, and belongs to the field of disc laser. The femtosecond disc laser comprises a pumping source, a pumping cavity, a disc laser crystal and a resonant cavity, pumping laser emitted by the pumping source is reflected for multiple times in the pumping cavity and passes through the disc laser crystal for multiple times to generate oscillation laser; the resonant cavity comprises a first branch and a second branch, the first branch comprises a dispersion mirror group, a hard diaphragm and an output mirror which are sequentially arranged along a light path, and the second branch comprises a first concave mirror, a second concave mirror, a first high-reflection mirror, a third concave mirror, an acousto-optic modulator and a second high-reflection mirror which are sequentially arranged along the light path; the acousto-optic modulator is located at the focal point of the third concave mirror, and modulates periodic loss and starts passive mode locking in the active mode locking process; oscillation laser generates a femtosecond laser pulse sequence after passing through the acousto-optic modulator, the femtosecond laser pulse sequence is reflected to return to the disc laser crystal along the original path and enters the first branch, and the femtosecond laser pulse sequence is output through the output mirror.

Description

Technical Field

[0001] The present invention belongs to the field of disk lasers, and more specifically, relates to a femtosecond disk laser combining active and passive mode locking. Background Art

[0002] The disk technology is to make the gain medium into a thin sheet structure with a diameter of more than 10mm and a thickness of 100-300μm. At the same time, the disk is welded to a tungsten copper heat sink or glued to a diamond heat sink with better thermal conductivity, and the heat on the disk is efficiently taken away by impact water cooling, so that the crystal has only one-dimensional thermal gradient distribution, effectively avoiding problems such as thermal lensing, thermal distortion and nonlinear excitation of the gain medium, and providing a guarantee for the generation of femtosecond lasers with high peak power, high average power and high beam quality.

[0003] For disk lasers, if you want to achieve ultrafast laser output in the femtosecond range, you need to combine mode-locking technology. Commonly used mode-locking technologies are divided into two types: active and passive mode-locking. Active mode-locking technology has high flexibility and stability, allowing the characteristics of the output pulse to be precisely controlled by adjusting the frequency and amplitude of the modulation signal. At the same time, by optimizing the design of the modulation signal and the laser cavity, the stability of the pulse output can be improved, which is especially important for application scenarios that require long-term stable operation, and has broad application prospects and market space in scientific research and industrial fields. However, these modulators may introduce noise during operation. These noise signals oscillate with the optical signal in the resonant cavity and continue to accumulate, causing the laser pulse to be distorted, which is manifested macroscopically as pulse broadening.

[0004] Compared with active mode-locking technology, passive mode-locking technology mainly utilizes the saturable absorption characteristics of nonlinear devices. The absorption loss of the device gradually changes with the light intensity in the resonant cavity. The constructed femtosecond laser has a fast response time and is an important technical means for the laser to achieve high-power femtosecond pulse lasers. However, due to the limitation of the damage threshold of the passive mode-locking device, the mode-locked laser cannot operate stably for a long time, and the laser adjustment is also more complicated. To solve the above problems, there have been related reports combining the two mode-locking technologies and achieving higher average power and single pulse energy. However, both mode-locking methods use related devices, the laser adjustment is difficult, and the output laser pulse width is mostly in the picosecond level. Therefore, how to find a simple and reliable way to combine active and passive mode-locking technologies and achieve high-power femtosecond ultrafast laser output at the same time is a problem that needs to be solved at present. Summary of the invention

[0005] In view of the defects of the related art, the purpose of the present invention is to provide a femtosecond disk laser combining active and passive mode locking, aiming to solve the problems of the existing lasers that cannot combine active mode locking and passive mode locking technologies and cannot be integrated and the output pulse width is relatively wide. At the same time, the disk technology is combined to achieve high-power femtosecond-level ultrafast laser output.

[0006] To achieve the above object, the present invention provides a femtosecond disk laser combining active and passive mode locking, comprising:

[0007] A pump source, used to emit pump laser;

[0008] A pump cavity is arranged on the output light path of the pump source;

[0009] The disk laser crystal is arranged in the pump cavity; the pump laser is reflected multiple times in the pump cavity and passes through the disk laser crystal multiple times, thereby exciting the disk laser crystal to generate oscillating laser;

[0010] A resonant cavity is arranged on one side of the output of the disk laser crystal, and the resonant cavity includes a first branch and a second branch;

[0011] The first branch includes a dispersion mirror group, a hard aperture and an output mirror arranged in sequence along the optical path; the oscillating laser is transmitted to the output mirror through the hard aperture after dispersion compensation by the dispersion mirror group; the hard aperture is used to assist passive mode locking; part of the laser is output through the output mirror, and the remaining laser is reflected by the output mirror and returns to the disk laser crystal along the original path, and enters the second branch after being reflected by the disk laser crystal;

[0012] The second branch includes a first concave mirror, a second concave mirror, a first high-reflection mirror, a third concave mirror, an acousto-optic modulator and a second high-reflection mirror, which are sequentially arranged along the optical path; the distance between the first concave mirror and the second concave mirror is located at the center of the stable region of the resonant cavity; the acousto-optic modulator is close to the high-reflection mirror and is located at the focus of the third concave mirror, and is used to modulate periodic loss during active mode locking, and is also used as a Kerr medium to start passive mode locking; the oscillating laser generates a femtosecond laser pulse sequence after passing through the acousto-optic modulator, and is reflected by the second high-reflection mirror, returns to the disk laser crystal along the original path, and enters the first branch again after being reflected by the disk laser crystal, and the output mirror outputs the femtosecond laser pulse sequence.

[0013] Optionally, the second concave mirror is placed on a precision translation stage to adjust the stable region of the laser.

[0014] Optionally, the angle between the quartz crystal in the acousto-optic modulator and the horizontal plane is the Brewster angle.

[0015] Optionally, both sides of the surface of the acousto-optic modulator are coated with a broadband anti-reflection film for the 1000nm-1070nm band.

[0016] Optionally, the disk laser crystal is coated with an anti-reflection film on a side facing the resonant cavity, and is coated with a high-reflection film on a side facing away from the resonant cavity.

[0017] Optionally, the surface of the disk laser crystal coated with a high-reflection film is fixed on a water-cooled heat sink.

[0018] Optionally, the light-facing surfaces of the first high-reflection mirror and the second high-reflection mirror are coated with a high-reflection film for the oscillating laser, and the light-emitting surfaces are coated with an anti-reflection film for the oscillating laser.

[0019] Optionally, the light-facing surface of the output mirror is coated with a film layer, and the transmittance range of the film layer is 1% to 20%.

[0020] Compared with the prior art, the above technical solution conceived by the present invention can achieve the following beneficial effects:

[0021] 1. The present invention provides a femtosecond disk laser combining active and passive mode locking. An acousto-optic modulator is placed at the focal position of a concave mirror. The acousto-optic modulator can be used as a modulation device for active mode locking, and its acousto-optic crystal can also serve as a Kerr medium required in the passive mode locking process. The two mode locking methods are effectively integrated into one laser, and no additional Kerr medium is required. The structure is simple and reliable. The acousto-optic modulator is used to modulate the periodic loss in the active mode locking process, and is also used as a Kerr medium for passive mode locking to form a stable femtosecond laser pulse sequence. The present scheme can realize active mode locking and passive mode locking only through the acousto-optic modulator, thereby realizing high-power femtosecond-level ultrafast laser output.

[0022] 2. The present invention provides a femtosecond disk laser combining active and passive mode locking. The acousto-optic crystal in the acousto-optic modulator is a quartz crystal with a thickness of 10mm-30mm. As a Kerr medium, the thicker Kerr medium increases the self-phase modulation effect in the resonant cavity. After reasonably adjusting the dispersion in the resonant cavity, the output femtosecond pulse width can be effectively shortened to obtain ultrafast laser with high peak power. It can be used for precise cutting on a micrometer scale, which makes it possible to manufacture precision parts and products with complex geometric shapes.

[0023] 3. The present invention provides a femtosecond disk laser that combines active and passive mode locking. The quartz crystal in the acousto-optic modulator needs to be cut to the Brewster angle, which can effectively reduce the etalon effect in the crystal, thereby optimizing the pulse width of the mode-locked laser. At the same time, the transducer on the acousto-optic modulator is used to load the modulation signal on the acousto-optic crystal to construct the conditions for the generation of the active mode locking mechanism to achieve high-power stable ultrafast laser output.

[0024] 4. The present invention provides a femtosecond disk laser combining active and passive mode locking. When the quartz crystal in the acousto-optic modulator is used as the Kerr medium, the surface of the quartz crystal is coated with a broadband anti-reflection film in the 1000nm-1070nm band, which can effectively reduce the loss of photon energy in the resonant cavity while ensuring the realization of the corresponding functions. The experimental results show that the adjustment angle and position of the acousto-optic modulator in this scheme does not affect the quality of the output laser. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is a schematic diagram of a femtosecond disk laser combining active and passive mode locking provided by an embodiment of the present invention;

[0026] The reference numerals in the accompanying drawings are as follows:

[0027] 1. Pump source, 2. Disk laser crystal, 3. Pump cavity, 4. Resonant cavity, 5. Acousto-optic modulator, 6. Output mirror, 7. Hard aperture, 8. First dispersion mirror, 9. Second dispersion mirror, 10. First concave mirror, 11. Second concave mirror, 12. First high-reflection mirror, 13. Third concave mirror, 14. Second high-reflection mirror, 15. Modulation power supply. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0029] The contents involved in the above embodiment are described below in conjunction with a preferred embodiment.

[0030] like Figure 1 As shown, a femtosecond disk laser combining active and passive mode locking comprises:

[0031] A pump source 1, used for emitting a pump laser;

[0032] A pump cavity 3, arranged on the output light path of the pump source 1;

[0033] The disk laser crystal 2 is disposed in the pump cavity 3; the pump laser is reflected multiple times in the pump cavity 3 and passes through the disk laser crystal 2 multiple times, thereby exciting the disk laser crystal 2 to generate oscillating laser;

[0034] A resonant cavity 4 is arranged at one side of the output of the disk laser crystal, and the resonant cavity 4 includes a first branch and a second branch;

[0035] The first branch includes a dispersion mirror group, a hard aperture 7 and an output mirror 6 arranged in sequence along the optical path; the oscillating laser is transmitted to the output mirror 6 through the hard aperture 7 after dispersion compensation by the dispersion mirror group; the hard aperture 7 is used to assist passive mode locking; part of the laser is output through the output mirror 6, and the remaining laser is reflected by the output mirror 6 and returns to the disk laser crystal 2 along the original path, and enters the second branch after being reflected by the disk laser crystal 2;

[0036] The second branch includes a first concave mirror 10, a second concave mirror 11, a first high-reflection mirror 12, a third concave mirror 13, an acousto-optic modulator 5, and a second high-reflection mirror 14, which are arranged in sequence along the optical path; the distance between the first concave mirror 10 and the second concave mirror 11 is located at the center of the stable region of the resonant cavity 4; the acousto-optic modulator is close to the high-reflection mirror 14 and is located at the focus of the third concave mirror 13, and is used to modulate the periodic loss during the active mode locking process, and is also used as a Kerr medium to start the passive mode locking; the oscillating laser generates a femtosecond laser pulse sequence after passing through the acousto-optic modulator 5, and is reflected by the second high-reflection mirror 14, returns to the disk laser crystal 2 along the original path, and enters the first branch again after being reflected by the disk laser crystal 2, and the output mirror 6 outputs the femtosecond laser pulse sequence.

[0037] In order to enable the mode-locked laser to produce stable and sustainable femtosecond pulse output without increasing the structural complexity and energy loss, the present invention provides an active-passive combined femtosecond disk laser, which uses an acousto-optic modulator as an active mode-locking device, and its acousto-optic crystal can also be used as a Kerr medium in passive Kerr lens mode-locking.

[0038] The transducer on the acousto-optic modulator applies a modulation frequency signal to the acousto-optic crystal, and adjusts the modulation frequency to match the longitudinal mode interval of the laser. Use an oscilloscope or other monitoring equipment to observe the mode locking of the laser. While ensuring the stable operation of the pulse in the resonant cavity, the Kerr lens mode locking technology has a high modulation depth, and the position of the acousto-optic device and related parameters in the cavity are continuously adjusted to make the width of the output mode-locked pulse reach the femtosecond level. The advantages of the two mode-locking technologies are combined to produce high-power stable femtosecond pulse laser output, which significantly simplifies the design and operation of the device.

[0039] like Figure 1 As shown, the pump source 1 is used to generate pump laser, and semiconductor laser, fiber laser and solid laser etc. can be selected. In this embodiment, a semiconductor laser with fiber coupled output is specifically used as the pump source, and the wavelength of the pump laser is 940nm or 969nm.

[0040] The pump cavity 3 is arranged on the outgoing optical path of the pump source 1; the disk laser crystal 2 is a disk-shaped Yb:YAG (doping concentration is 7%) laser crystal with a diameter of 10 mm and a thickness of 130 μm, which is arranged in the pump cavity 3. The pump cavity 3 makes the pump laser emitted by the pump source 1 pass through the disk laser crystal 2 for many times and then excite the disk laser crystal 2 to generate oscillating laser. In this embodiment, the pump cavity 3 includes a parabolic mirror and a series of folding prisms, which are placed in the optical path between the pump source 1 and the disk laser crystal 2 according to a set rule. The disk laser crystal 2 is placed at the focus of the parabolic mirror, and the input pump light is focused on the disk crystal 2 after multiple reflections of the parabolic mirror and the folding prism. Further, in this embodiment, a 48-pass pumping structure is adopted, that is, the pump laser passes through the disk crystal 48 times, which greatly improves the absorption efficiency of the disk crystal for the pump light. In some other embodiments, a 24-channel pumping structure, a 36-channel pumping structure, or a higher 72-channel pumping structure may be used according to actual needs.

[0041] In the present embodiment, the resonant cavity 4 is used to output femtosecond pulses in the 1030nm band, and is arranged on the side where the oscillating laser is output from the disk laser crystal, including a first branch and a second branch; in the present embodiment, the dispersion mirror group specifically includes a first high-dispersion mirror 8 and a second high-dispersion mirror 9 arranged in sequence along the optical path; after the oscillating laser is incident on the first branch from the disk laser crystal 2, it is first incident on the second high-dispersion mirror 9, and then reflected on the first high-dispersion mirror 8, and then reflected on the output mirror 6, part of the laser is output through the output mirror 6, and the remaining laser is reflected by the output mirror 6, returns to the disk laser crystal 2 along the original path, and is reflected to the second branch.

[0042] The second branch includes a first concave mirror 10, a second concave mirror 11, a first high reflector 12, a third concave mirror 13, an acousto-optic modulator 5 and a second high reflector 14 arranged in sequence along the optical path; the focal points of the first concave mirror 10 and the second concave mirror 11 coincide; the acousto-optic modulator 5 is placed close to the second high reflector 14. At the same time, the acousto-optic modulator 5 is located at the focal position of the third concave mirror 13, providing conditions for the generation of a passive mode-locking mechanism. The focus of the third concave mirror 13 provides a higher power density for the Kerr medium, increasing the modulation depth of the disk mode-locked laser. Furthermore, an electric signal of a carrier frequency is applied to the acousto-optic modulator 5 through the modulation power supply 15, and the electric signal is converted into an ultrasonic signal after being converted by the transducer; the ultrasonic signal is transmitted to the acousto-optic crystal, and a refractive index change is formed in the crystal; the angle and position of the acousto-optic modulator 5 are adjusted to ensure that the laser can pass smoothly and be effectively modulated. At the same time, the modulation frequency of the acousto-optic modulator 5 should be accurately equal to the longitudinal mode frequency interval of the resonant cavity 4; after the laser and are continuously modulated, the phases between the longitudinal modes gradually tend to be consistent, forming a coherent superposition, thereby forming a stable pulse sequence in the resonant cavity 4, and part of it is output through the output mirror 6, and the remaining part returns to the disk laser crystal 2 along the original path, and is reflected to the second branch, realizing the round-trip oscillation of the oscillating laser in the resonant cavity.

[0043] At the same time, the quartz crystal in the acousto-optic modulator 5 is used as a Kerr medium to construct a combined active and passive mode-locking mechanism. In order to further reduce the energy loss in the resonant cavity, the light-passing surface of the acousto-optic crystal is optimized while ensuring that the laser is regulated by a combination of active and passive mode-locking. Specifically, the surface of the acousto-optic crystal is coated with a broadband anti-reflection film for the 1000nm-1070nm band, with a transmittance greater than 99.9%, and the crystal is cut to the Brewster angle, that is, the angle between the quartz crystal in the acousto-optic modulator 5 and the horizontal plane is the Brewster angle, which weakens the etalon effect in the crystal and improves the stability of the disk laser mode-locked pulse.

[0044] Combination Figure 1 It can be seen that the gain medium and the Kerr medium are separated from each other and can be optimized and adjusted separately. The length of the acousto-optic crystal can be selected within the range of 10mm-30mm, that is, the self-phase modulation effect in the resonant cavity is enhanced, and continuous light peaks or double pulses are prone to occur during the mode locking process, which seriously affects the stability of the output pulse. In practical applications, the spectral profile and pulse sequence of the laser mode locking can be optimized by adjusting the position of the acousto-optic crystal and the dispersion amount in the resonant cavity. At the initial moment, the acousto-optic modulator 5 is placed at the focus of the third concave mirror 13, and then the spectral profile and pulse sequence output by the laser are observed. If there are continuous peaks and multiple pulses, the position of the acousto-optic crystal is adjusted within a preset range near the focus, or the dispersion compensation amount in the resonant cavity is adjusted until the continuous light peak or multiple pulse phenomenon disappears, and finally a high-quality 1030nm band femtosecond laser pulse is obtained.

[0045] Furthermore, the reflective surface of the end mirror (output mirror 6) is also coated with a high-reflective film for the oscillating laser, with a reflectivity greater than 99.9%; the light-facing surface of the output mirror 6 is coated with a film layer that partially reflects and partially transmits the oscillating laser, with a transmittance range of 1% to 20%; the light-emitting surface of the output mirror 6 is coated with an anti-reflection film for the oscillating laser.

[0046] Optionally, the focal points of the first concave mirror 10 and the second concave mirror 11 coincide, and the distance between the first concave mirror 10 and the second concave mirror 11 is located at the center of the stable region of the resonant cavity 4. Further, the second concave mirror 11 is placed on a precision translation stage for adjusting the stable region of the laser.

[0047] Optionally, the light-facing surfaces of the first high-reflection mirror 12 and the second high-reflection mirror 14 are coated with a high-reflection film for the oscillating laser, and the light-emitting surfaces are coated with an anti-reflection film for the oscillating laser.

[0048] Furthermore, the disk laser crystal 2 acts as a gain medium, absorbing the energy generated by the pump laser to generate an oscillating laser. At the same time, the disk laser crystal 2 acts as a return mirror of the resonant cavity to reflect the oscillating laser. Therefore, different dielectric films are coated on both sides of the disk laser crystal 2. The side facing the resonant cavity is coated with an anti-reflection film for the pump laser and the oscillating laser, and the side facing away from the resonant cavity is coated with a high-reflection film for the pump laser and the oscillating laser. The surface of the disk laser crystal 2 coated with the high-reflection film is fixed on a water-cooled heat sink with a stroke structure.

[0049] Stable high-power femtosecond lasers in the 1μm band have important applications in both scientific research and industry. In the field of scientific research, broadband femtosecond mid-infrared lasers, terahertz and high-order harmonics are driven to generate, because there are many mature nonlinear frequency conversion crystals with high damage threshold in the 1μm band; in the industrial field, stable 1μm femtosecond lasers can be precisely cut at the micron scale, which makes it possible to manufacture precision parts and products with complex geometries. This high-precision processing capability is particularly important in aerospace, semiconductor microelectronics, biomedicine and other fields, and can significantly improve product performance and reliability. During the processing of femtosecond lasers, due to the extremely short pulse duration, the heat does not have time to be transferred to the surrounding of the material, thus achieving a very small heat-affected zone. This is crucial for application scenarios that are sensitive to temperature or need to maintain the original properties of the material, such as the processing of polymers, glass and biomaterials. At the same time, doubling the frequency of a 1μm femtosecond laser can obtain a 500nm wavelength laser, which can be used in fields such as information storage, laser printing and submarine communications. By further doubling the frequency or focusing the 1μm femtosecond laser, it is also possible to generate lasers in the ultraviolet and extreme ultraviolet bands, which can be used in technical fields such as industrial processing and lithography machines.

[0050] The femtosecond disk laser proposed in the present invention combines the advantages of the two mode-locking technologies to achieve highly stable femtosecond-level mode-locked laser pulse output. The overall system has the advantages of being simple, easy to operate and highly efficient. It can be used as an important driving and processing femtosecond laser light source for future basic scientific research and industrial processing, and has broad application prospects and value benefits.

[0051] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A femtosecond disk laser combining active and passive mode locking, characterized in that: include: A pump source (1) for emitting pump laser light; A pump cavity (3) is arranged on the output light path of the pump source (1); The disk laser crystal (2) is arranged in a pump cavity (3); the pump laser is reflected multiple times in the pump cavity (3) and passes through the disk laser crystal (2) multiple times, thereby exciting the disk laser crystal (2) to generate oscillating laser light; A resonant cavity (4) is arranged on a side of the disk laser crystal output, and the resonant cavity (4) comprises a first branch and a second branch; The first branch comprises a dispersion mirror group, a hard aperture (7) and an output mirror (6) arranged in sequence along the optical path; the oscillating laser passes through the hard aperture (7) to the output mirror (6) after dispersion compensation by the dispersion mirror group; the hard aperture (7) is used to assist passive mode locking; part of the laser light is output through the output mirror (6), and the remaining laser light is reflected by the output mirror (6), returns to the disk laser crystal (2) along the original path, and enters the second branch after being reflected by the disk laser crystal (2); The second branch comprises a first concave mirror (10), a second concave mirror (11), a first high-reflection mirror (12), a third concave mirror (13), an acousto-optic modulator (5) and a second high-reflection mirror (14) which are arranged in sequence along the optical path; the distance between the first concave mirror (10) and the second concave mirror (11) is located at the center of the stable region of the resonant cavity (4); the acousto-optic modulator is close to the high-reflection mirror (14) and is located at the focus of the third concave mirror (13), and is used to modulate periodic loss during active mode locking, and is also used as a Kerr medium to start passive mode locking; the oscillating laser generates a femtosecond laser pulse sequence after passing through the acousto-optic modulator (5), and is reflected by the second high-reflection mirror (14), returns to the disk laser crystal (2) along the original path, and enters the first branch again after being reflected by the disk laser crystal (2), and the output mirror (6) outputs the femtosecond laser pulse sequence.

2. The femtosecond disk laser according to claim 1, characterized in that: The second concave mirror (11) is placed on a precision translation stage and is used to adjust the stable region of the laser.

3. The femtosecond disk laser according to claim 1, characterized in that: The angle between the quartz crystal in the acousto-optic modulator (5) and the horizontal plane is the Brewster angle.

4. The femtosecond disk laser according to claim 1, characterized in that: Both sides of the surface of the acousto-optic modulator (5) are plated with a broadband anti-reflection film for the 1000nm-1070nm band.

5. The femtosecond disk laser according to claim 1, characterized in that: The side of the disk laser crystal (2) facing the resonant cavity (4) is coated with an anti-reflection film, and the side facing away from the resonant cavity (4) is coated with a high-reflection film.

6. The femtosecond disk laser according to claim 5, characterized in that: The surface of the disk laser crystal (2) coated with a high-reflection film is fixed on a water-cooled heat sink.

7. The femtosecond disk laser according to claim 1, characterized in that: The light-facing surfaces of the first high-reflection mirror (12) and the second high-reflection mirror (14) are coated with a high-reflection film for the oscillating laser, and the light-emitting surfaces are coated with an anti-reflection film for the oscillating laser.

8. The femtosecond disk laser according to claim 1, characterized in that: The light-facing surface of the output mirror (6) is coated with a film layer, the transmittance of which ranges from 1% to 20%.

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