A femtosecond disk laser combining active and passive mode locking

Abstract

The present invention discloses a femtosecond disk laser combining active and passive mode locking, belonging to the field of disk lasers. The femtosecond disk laser comprises: a pump source, a pump cavity, a disk laser crystal, and a resonant cavity. The pump source emits pump laser light, which is reflected multiple times in the pump cavity and passes through the disk laser crystal multiple times to generate oscillating laser light. The resonant cavity comprises a first branch and a second branch. The first branch comprises a dispersion mirror group, a hard aperture, and an output mirror arranged in sequence along the optical path. 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 arranged in sequence along the optical path. The acousto-optic modulator is located at the focus of the third concave mirror and modulates periodic loss during active mode locking and initiates passive mode locking. The oscillating laser light generates a femtosecond laser pulse sequence after passing through the acousto-optic modulator. The pulse sequence is then reflected back to the disk laser crystal along the original path, enters the first branch, and 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. The heat on the disk is efficiently removed by an impact water cooling method, so that the crystal has only a 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, achieving ultrafast laser output in the femtosecond range requires combining 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 precise control of the characteristics of the output pulse 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 applications that require long-term stable operation. It 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 to active mode-locking, passive mode-locking primarily exploits the saturable absorption characteristics of nonlinear devices. The absorption loss of these devices varies gradually with the light intensity within the resonant cavity, resulting in a faster response time for the resulting femtosecond laser. This makes it an important technical approach for achieving high-power femtosecond pulsed lasers. However, limited by the damage threshold of passive mode-locking devices, mode-locked lasers cannot operate stably for extended periods, and laser regulation is also complex. To address these issues, reports have combined the two mode-locking techniques, achieving higher average power and single-pulse energy. However, both mode-locking methods utilize their own components, making laser regulation difficult and the output laser pulse widths mostly in the picosecond range. Therefore, finding a simple and reliable way to combine active and passive mode-locking techniques to simultaneously achieve high-power femtosecond ultrafast laser output remains a pressing issue. Summary of the Invention

[0005] To address the shortcomings of related technologies, the present invention aims to provide a femtosecond disk laser that combines active and passive mode locking. This approach aims to address the limitations of existing lasers, including the inability to integrate active and passive mode locking technologies and the resulting wide output pulse width. Furthermore, the invention combines disk technology to achieve high-power femtosecond-scale ultrafast laser output.

[0006] To achieve the above objectives, 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 provided 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 light;

[0010] A resonant cavity is provided on the output side 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 light is dispersion-compensated by the dispersion mirror group and then transmitted to the output mirror through the hard aperture; the hard aperture is used to assist in passive mode locking; a portion of the laser light is output through the output mirror, and the remaining laser light is reflected by the output mirror and returns to the disk laser crystal along the original path, and then 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 arranged in sequence 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 also to serve 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, the transmittance of which ranges from 1% to 20%.

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

[0021] 1. The present invention provides a femtosecond disk laser that combines active and passive mode locking. An acousto-optic modulator (AOM) is placed at the focal point of a concave mirror. The AOM serves as both a modulator for active mode locking and an AO crystal for passive mode locking, effectively integrating the two mode locking methods into a single laser. This eliminates the need for an additional Kerr medium and results in a simple and reliable structure. The AOM is used to modulate periodic loss during active mode locking and also serves as a Kerr medium for passive mode locking, forming a stable femtosecond laser pulse train. This solution utilizes only the AOM to achieve both active and passive mode locking, thereby enabling high-power femtosecond-level ultrafast laser output.

[0022] 2. This invention provides a femtosecond disk laser that combines active and passive mode locking. The acousto-optic crystal in the acousto-optic modulator is a quartz crystal with a thickness of 10mm-30mm. This thicker Kerr medium enhances the self-phase modulation effect within the resonant cavity. By properly adjusting the dispersion within the resonant cavity, the output femtosecond pulse width can be effectively shortened, resulting in ultrafast laser light with high peak power. This laser can be used for precise cutting at the micron scale, facilitating the manufacture of precision parts and products with complex geometries.

[0023] 3. This invention provides a femtosecond disk laser that combines active and passive mode locking. The quartz crystal in the acousto-optic modulator (AOM) is cut to the Brewster angle, effectively reducing the etalon effect within the crystal and optimizing the pulse width of the mode-locked laser. Simultaneously, the transducer on the AOM applies the modulated signal to the acousto-optic crystal, creating the conditions for active mode locking and achieving high-power, stable ultrafast laser output.

[0024] 4. This invention provides a femtosecond disk laser that combines active and passive mode locking. Using a quartz crystal as the Kerr medium in an acousto-optic modulator (AOM), the surface of the quartz crystal is coated with a broadband antireflection coating covering the wavelength range of 1000nm to 1070nm. This effectively reduces photon energy loss within the resonant cavity while ensuring the desired function. Experimental results demonstrate that adjusting the angle and position of the AOM in this scheme does not affect the output laser quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 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 objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely for the purpose of explaining 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 may 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] Pump source 1, used for emitting pump laser;

[0032] A pump cavity 3 is provided on the outgoing 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 light;

[0034] The resonant cavity 4 is provided on the output side 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. After dispersion compensation by the dispersion mirror group, the oscillating laser light is transmitted through the hard aperture 7 to the output mirror 6. The hard aperture 7 is used to assist in 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 and returns to the disk laser crystal 2 along the original path. After being reflected by the disk laser crystal 2, it enters the second branch.

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

[0038] The transducer on the acousto-optic modulator applies a modulation frequency signal to the acousto-optic crystal, and the modulation frequency is adjusted to match the longitudinal mode spacing of the laser. The laser mode-locking status is observed using an oscilloscope or other monitoring equipment. While ensuring stable pulse operation within the resonant cavity, the Kerr lens mode-locking technology achieves a high modulation depth. The position of the acousto-optic device and related parameters within the cavity are continuously adjusted to achieve a femtosecond-scale output mode-locked pulse width. Combining the advantages of the two mode-locking technologies, high-power, stable femtosecond pulse laser output is generated, significantly simplifying 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 lasers, fiber lasers, solid lasers, etc. can be selected. In this embodiment, a semiconductor laser with fiber-coupled output is specifically used as the pump source, and the pump laser wavelength is 940nm or 969nm.

[0040] The pump cavity 3 is positioned in the optical path of the pump source 1. The disk laser crystal 2, a disc-shaped Yb:YAG (doping concentration 7%) laser crystal with a diameter of 10 mm and a thickness of 130 μm, is located within the pump cavity 3. The pump cavity 3 allows the pump laser light emitted by the pump source 1 to pass through the disk laser crystal 2 multiple times, thereby exciting the disk laser crystal 2 to generate oscillating laser light. In this embodiment, the pump cavity 3 comprises a parabolic mirror and a series of refraction prisms, which are positioned in a predetermined pattern in the optical path between the pump source 1 and the disk laser crystal 2. The disk laser crystal 2 is positioned at the focus of the parabolic mirror. The input pump light is focused onto the disk laser crystal 2 after multiple reflections from the parabolic mirror and refraction prism. Furthermore, this embodiment employs a 48-pass pumping configuration, meaning that the pump laser light passes through the disk laser crystal 48 times, significantly improving the disk laser crystal's absorption efficiency of 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 adopted according to actual needs.

[0041] In this embodiment, the resonant cavity 4 is used to output femtosecond pulses in the 1030nm band. It is arranged on the side where the oscillating laser is output from the disk laser crystal, and includes a first branch and a second branch. In this 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 first is incident on the second high-dispersion mirror 9, and then is reflected on the first high-dispersion mirror 8, and then is reflected on the output mirror 6. 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 is reflected to the second branch.

[0042] 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 AOM 5, and a second high-reflection mirror 14, arranged sequentially along the optical path. The focal points of the first and second concave mirrors 10 and 11 coincide, and the AOM 5 is placed near the second high-reflection mirror 14. Simultaneously, the AOM 5 is located at the focal point of the third concave mirror 13, creating conditions for the passive mode-locking mechanism. The focal point of the third concave mirror 13 provides a higher power density in the Kerr medium, increasing the modulation depth of the disk mode-locked laser. Furthermore, an electrical signal of a carrier frequency is applied to the acousto-optic modulator 5 through the modulation power supply 15. The electrical signal is converted into an ultrasonic signal after being converted by the transducer. The ultrasonic signal is then transmitted to the acousto-optic crystal, causing a refractive index change 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 precisely equal to the longitudinal mode frequency interval of the resonant cavity 4. After the laser is 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. 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 serves as the Kerr medium, constructing a combined active and passive mode-locking mechanism. To further reduce energy loss within the resonant cavity, the light-passing surface of the acousto-optic crystal was optimized while ensuring the laser is controlled by a combination of active and passive mode-locking. Specifically, the surface of the acousto-optic crystal is coated with a broadband anti-reflection coating for the 1000nm-1070nm band, with a transmittance greater than 99.9%. The crystal is also cut to the Brewster angle, meaning that the angle between the quartz crystal in the acousto-optic modulator 5 and the horizontal plane is the Brewster angle. This reduces the etalon effect in the crystal and improves the stability of the disk laser's mode-locked pulse.

[0044] Combine Figure 1 As can be seen, the gain medium and Kerr medium are separated and can be optimized and adjusted independently. The acousto-optic crystal length can be selected within the range of 10 mm to 30 mm. This enhances the self-phase modulation effect within the resonant cavity, making continuous light spikes or double pulses more likely to occur during mode locking, seriously affecting the stability of the output pulses. 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 within the resonant cavity. Initially, the acousto-optic modulator 5 is placed at the focus of the third concave mirror 13, and the spectral profile and pulse sequence of the laser output are observed. If continuous peaks or multiple pulses are present, the position of the acousto-optic crystal is adjusted within a preset range near the focus, or the dispersion compensation amount within the resonant cavity is adjusted until the continuous light spikes or multiple pulses disappear, ultimately obtaining high-quality femtosecond laser pulses in the 1030 nm band.

[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. Furthermore, 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 oscillating laser light. Simultaneously, it serves as a reflective mirror within the resonant cavity, reflecting the oscillating laser light. Therefore, different dielectric coatings are applied to both sides of the disk laser crystal 2: the side facing the resonant cavity is coated with an anti-reflection coating for the pump laser and oscillating laser light, while the side facing away from the resonant cavity is coated with a high-reflection coating for both the pump laser and oscillating laser light. The surface of the disk laser crystal 2 coated with the high-reflection coating is fixed to a water-cooled heat sink with a stroke structure.

[0049] Stable, high-power femtosecond lasers in the 1μm wavelength band have important applications in both scientific research and industry. In scientific research, they can be used to generate broadband femtosecond mid-infrared lasers, terahertz lasers, and high-harmonics, thanks to the availability of mature, high-damage-threshold nonlinear frequency-converting crystals in the 1μm wavelength range. In industry, stable 1μm femtosecond lasers enable precise cutting at the micron scale, enabling the manufacture of precision parts and products with complex geometries. This high-precision processing capability is particularly important in aerospace, semiconductor microelectronics, biomedicine, and other fields, significantly improving product performance and reliability. During processing, the extremely short pulse duration of femtosecond lasers prevents heat from being transferred to the surrounding material, resulting in a minimal heat-affected zone. This is crucial for applications that are temperature-sensitive or require preserving the original properties of the material, such as processing polymers, glass, and biomaterials. Furthermore, frequency doubling a 1μm femtosecond laser can produce a 500nm wavelength laser, which can be used in information storage, laser printing, and submarine communications. Further frequency doubling or focusing of the 1μm femtosecond laser can also produce 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 this paper combines active and passive mode-locking technologies, leveraging the advantages of both technologies to achieve highly stable femtosecond-scale mode-locked laser pulse output. The overall system offers the advantages of simplicity, ease of operation, and high efficiency. It holds the potential to serve as an important femtosecond laser source for driving and processing applications in future fundamental research and industrial processing, with broad application prospects and significant value.

[0051] It will be easily understood by those skilled in the art that the above description is merely 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 scope of protection of the present invention.

Claims

1. A femtosecond disk laser combining active and passive mode locking, characterized in that: include: A pump source (1), configured to emit 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 one 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) 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, wherein: 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, wherein: 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, wherein: Both sides of the surface of the acousto-optic modulator (5) are plated with a broadband anti-reflection film for the 1000nm-1070nm wave band.

5. The femtosecond disk laser according to claim 1, wherein: 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, wherein: 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, wherein: 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, wherein: 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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