CN119787068B - A femtosecond disk laser generating an eye-safe wavelength - Google Patents

Abstract

The invention discloses a femtosecond disc laser generating a human eye safety wave band, belonging to the technical field of femtosecond laser. The femtosecond disc laser for generating the human eye safety wave band comprises a pump source, a disc crystal, a pump cavity and a resonant cavity, wherein the pump cavity enables pump light emitted by the pump source to pass through the disc laser crystal for multiple times and then excite to generate oscillation laser, a first branch in the resonant cavity comprises a high-dispersion mirror and an output mirror which are sequentially arranged along a light path, the oscillation laser is transmitted to the output mirror along the first branch, returns to the disc laser crystal along the original path and is reflected to a second branch, the second branch comprises a first concave mirror, a second concave mirror, a Cr ⁴⁺:YAG crystal and a high-reflection mirror which are sequentially arranged along the light path, and the Cr ⁴⁺:YAG crystal is used as a passive mode locking device and is used as a gain medium to generate a femtosecond laser pulse sequence of the human eye safety wave band. The femtosecond disk laser has a simple structure and can generate stable femtosecond laser output with a specific wave band.

Description

A femtosecond disk laser generating an eye-safe wavelength

Technical Field

The present invention belongs to the field of disk lasers, and more particularly, relates to a femtosecond disk laser that generates an eye-safe wavelength band.

Background Art

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.

Currently, high-power eye-safe femtosecond lasers are primarily generated using bulk crystals and nonlinear frequency conversion. Key approaches include using Cr ⁴⁺ :YAG crystals in combination with passive mode-locking mechanisms, femtosecond Raman frequency shifting, and optical parametric oscillators (OPOs). These methods can achieve femtosecond laser output in specific wavelength bands. However, existing lasers present several challenges. For example, the use of bulk crystals is limited by thermal effects and thermal distortion of the crystal under high-power pumping conditions. Without subsequent amplification, the average power of femtosecond pulses in all-solid-state femtosecond lasers is only in the milliwatt range. This low average power significantly reduces the signal-to-noise ratio of femtosecond pulses. Furthermore, the use of femtosecond Raman frequency shifting and optical parametric oscillation techniques is limited by self-steepening, stimulated Raman scattering, and nonlinear walk-off, resulting in low femtosecond laser conversion efficiency. Therefore, finding a method that can directly output high-power eye-safe femtosecond pulses has become a critical challenge.

Summary of the Invention

In response to the defects of the existing technology and the need for improvement, the present invention provides a femtosecond laser that generates a waveband that is safe for the human eye, aiming to solve the problem that the existing laser cannot directly generate a high-power femtosecond laser pulse sequence in the waveband that is safe for the human eye.

To achieve the above-mentioned object, the present invention provides a femtosecond disk laser that generates an eye-safe wavelength band, characterized in that it comprises: a pump source, a disk laser crystal, a pump cavity and a resonant cavity;

The pump cavity is arranged on the output light path of the pump source; the disk laser crystal is arranged in the pump cavity; the pump cavity allows the pump laser emitted by the pump source to pass through the disk laser crystal multiple times and then excite the disk laser crystal to generate oscillating laser light;

The resonant cavity is arranged on one side of the output of the oscillating laser describing the disk laser crystal, and the resonant cavity includes a first branch and a second branch;

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

The second branch includes a first concave mirror, a second concave mirror, a Cr ⁴⁺ :YAG crystal, and a high-reflection mirror sequentially arranged along the optical path; the focal points of the first concave mirror and the second concave mirror coincide with each other, the Cr ⁴⁺ :YAG crystal is located at the focal points, and 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 Cr ⁴⁺ :YAG crystal serves as a saturable absorber for passive mode locking. The Cr ⁴⁺ :YAG crystal also serves as a gain medium for generating a femtosecond laser pulse sequence in an eye-safe band under the action of an oscillating laser. The femtosecond laser pulse sequence in the eye-safe band is reflected by the high-reflection mirror, returns along the original path to the disk laser crystal, and then enters the first branch again after being reflected by the disk laser crystal. The output mirror outputs the femtosecond laser pulse sequence in the eye-safe band.

Optionally, the disk laser crystal is a Yb:YAG crystal, wherein the Yb ³⁺ doping concentration is 7%;

The Cr ⁴⁺ doping concentration of the Cr ⁴⁺ :YAG crystal is in the range of 2%-7%, and the length of the Cr ⁴⁺ :YAG crystal is in the range of 10 mm-30 mm;

The central wavelength of the femtosecond laser pulse sequence in the human eye-safe band is 1500 nm.

Optionally, both sides of the surface of the Cr ⁴⁺ :YAG crystal are coated with broadband antireflection films for wavelengths of 1000nm-1070nm and 1300-1600nm, with a transmittance greater than 99.9%.

Optionally, the light-facing surfaces of the dispersion mirror group, the first concave mirror, the second concave mirror and the high-reflection mirror are coated with broadband high-reflection films for the 1000nm-1070nm band and the 1300-1600nm band, with a reflectivity greater than 99.9%.

Optionally, the dispersion mirror group includes a first high dispersion mirror and a second high dispersion mirror arranged in sequence along the optical path.

Optionally, the light-facing surface of the output mirror is coated with a film layer that fully reflects the oscillating laser in the 1000nm-1070nm band and partially transmits the 1300-1600nm band, with a transmittance range of 1% to 20%;

The light-emitting surface of the output mirror is coated with an anti-reflection film for oscillating lasers in the 1300-1600nm band.

Optionally, the side of the disk laser crystal facing the resonant cavity is coated with an anti-reflection film for the pump laser and the oscillator laser, and the side facing away from the resonant cavity is coated with a high-reflection film for the pump laser and the oscillator laser.

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

Optionally, the femtosecond disk laser further includes a crystal fixture;

The crystal fixture clamps the Cr ⁴⁺ :YAG crystal, and external circulating cooling water is passed through the crystal fixture to dissipate heat from the Cr ⁴⁺ :YAG crystal.

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

1. The present invention provides a femtosecond disk laser that generates a specific wavelength band. It uses a disk laser crystal with a shape similar to a one-dimensional thin sheet as a gain medium. Under high-power pumping, it can effectively dissipate heat, minimize the generation of thermal effects, and the small amount of heat generated can be efficiently removed by a stroke-type water-cooling structure. Therefore, the disk crystal is more compatible with high-power pumping conditions, thereby outputting high-power femtosecond lasers, while also having a high average power within the resonant cavity. A passive saturable absorber (Cr ⁴⁺ :YAG crystal) whose absorption spectrum range includes the central wavelength of the oscillating laser generated by the disk laser crystal is used for passive mode locking. At the same time, it also serves as a gain medium for generating a femtosecond laser pulse sequence in the target wavelength band (eye-safe band) under the action of the oscillating laser. This achieves the direct generation of high-power, eye-safe femtosecond laser pulses, broadening the application scenarios of femtosecond disk lasers.

2. The present invention provides a femtosecond disk laser that generates light in a specific wavelength band. This device uses a thin, 10mm-diameter, 130μm-thick Yb:YAG disk as a gain medium. A Cr ⁴⁺ :YAG crystal serves as both a passive mode-locking device and the gain medium for generating laser light in the eye-safe wavelength band. This device directly outputs high-power femtosecond laser light in the eye-safe wavelength band within the resonant cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic structural diagram of a femtosecond disk laser generating a specific wavelength band according to an embodiment of the present invention;

The reference numerals in the accompanying drawings are as follows:

1. Pump source, 2. Disk laser crystal, 3. Pump cavity, 4. Resonant cavity, 5. Cr ⁴⁺ :YAG crystal, 6. First high-dispersion mirror, 7. Second high-dispersion mirror, 8. Hard aperture, 9. Output mirror, 10. First concave mirror, 11. Second concave mirror, 12. High-reflection mirror.

DETAILED DESCRIPTION

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.

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

As shown in FIG1 , a femtosecond disk laser generating an eye-safe wavelength band includes: a pump source 1, a disk laser crystal 2, a pump cavity 3, and a resonant cavity 4;

The pump cavity 3 is arranged on the output light path of the pump source 1; the disk laser crystal 2 is arranged in the pump cavity 3; the pump cavity 3 allows the pump laser emitted by the pump source 1 to pass through the disk laser crystal 2 multiple times and then excite the disk laser crystal 2 to generate oscillating laser light;

The resonant cavity 4 is arranged on one side of the output of the oscillating laser describing the disk laser crystal, and the resonant cavity 4 includes a first branch and a second branch;

The first branch includes a dispersion mirror group, a hard aperture 8, and an output mirror 9 arranged in sequence along the optical path; the hard aperture 8 is used to assist in passive mode locking; after the oscillating laser is transmitted along the first branch to the output mirror 9, a portion of the laser light is output through the output mirror 9, and the remaining laser light returns to the disk laser crystal 2 along the original path and is reflected to the second branch;

The second branch includes a first concave mirror 10, a second concave mirror 11, a Cr ⁴⁺ :YAG crystal 5, and a high-reflection mirror 12, which are sequentially arranged along the optical path. The focal points of the first concave mirror 11 and the second concave mirror 11 coincide with each other, the Cr ⁴⁺ :YAG crystal 5 is located at the focal points, 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.

The Cr ⁴⁺ :YAG crystal 5 serves as a saturable absorber for passive mode locking. The Cr ⁴⁺ :YAG crystal also serves as a gain medium for generating a femtosecond laser pulse sequence in the eye-safe band under the action of the oscillating laser. The femtosecond laser pulse sequence in the eye-safe band is reflected by the high-reflection mirror 12 and returns along the original path to the disk laser crystal 2. After being reflected by the disk laser crystal 2, it enters the first branch again. The output mirror 9 outputs the femtosecond laser pulse sequence in the eye-safe band.

The central wavelength of the oscillating laser generated by the disk laser crystal 2 is within the absorption spectrum of the Cr ⁴⁺ :YAG crystal 5 . The oscillating laser serves as the pump light source of the Cr ⁴⁺ :YAG crystal 5 to generate a femtosecond laser pulse sequence in the human eye-safe band.

To ensure stable and sustainable femtosecond pulse output from a disk laser without increasing structural complexity or energy loss, the present invention provides a femtosecond disk laser that generates eye-safe wavelengths. This device uses a Cr ⁴⁺ :YAG crystal 5 as a saturable absorber for passive mode locking in the 1030nm band. Furthermore, because the central wavelength of the oscillating laser light generated by the disk laser crystal 2 falls within the selected fluorescence absorption spectrum of the Cr ⁴⁺ :YAG crystal 5, the Cr ⁴⁺ :YAG crystal 5 also serves as a gain medium, generating an eye-safe femtosecond laser pulse train under the influence of the oscillating laser light generated by the disk laser crystal 2. The femtosecond laser pulse train output by this femtosecond disk laser has a femtosecond-scale width, and the femtosecond laser pulses are both high power and highly stable, significantly simplifying the design and operation of the device.

The disk laser crystal 2 serves as a gain medium, absorbing the energy from the pump laser to generate oscillating laser light. It also acts 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: an anti-reflection coating for the pump and oscillating laser light on the side facing the resonant cavity, and a high-reflection coating for both the pump and oscillating laser light on the side facing away from the resonant cavity. 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.

Optionally, according to the mode distribution in the cavity, hard apertures 8 of different sizes are set at different positions in the cavity to increase the diffraction loss in the cavity to assist in mode locking.

As shown in Figure 1, a pump source 1 is used to generate pump laser light. Semiconductor lasers, fiber lasers, and solid-state lasers can be used. In this embodiment, a fiber-coupled semiconductor laser is used as the pump source, with a pump laser wavelength of 940 nm or 969 nm. In this embodiment, a pump cavity 3 comprises a parabolic mirror and a series of refraction prisms, which are positioned in the optical path between the pump source 1 and a disk laser crystal 2 according to a predetermined pattern. The disk laser crystal 2 is positioned at the focal point of the parabolic mirror. The input pump light is focused onto the disk laser crystal 2 after multiple reflections from the parabolic mirror and the refraction prisms. The disk laser crystal 2 is a disc-shaped Yb:YAG (doping concentration 7%) laser crystal with a diameter of 10 mm and a thickness of 130 μm. Made from the well-established Yb:YAG material, it provides the gain medium for 1030 nm laser generation and is located within the pump cavity 3. The pump cavity 3 allows the pump laser emitted by the pump source 1 to pass through the disk laser crystal 2 multiple times and then excite the disk laser crystal 2 to generate oscillating laser light.

Furthermore, this embodiment employs a 48-pass pumping structure, meaning the pump laser passes through the disk laser crystal 2 48 times, significantly improving the disk crystal's absorption efficiency of the pump light. In other embodiments, a 24-pass pumping structure, a 36-pass pumping structure, or even a 72-pass pumping structure may be employed, depending on actual needs.

In this embodiment, the resonant cavity 4 is used to output femtosecond pulses in a wide wavelength band of 1300-1600 nm. It is located on one side of the oscillating laser output from the disk laser crystal and includes a first branch and a second branch.

In this embodiment, the dispersion mirror specifically includes a first high-dispersion mirror 6 and a second high-dispersion mirror 7 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 enters the first high-dispersion mirror 6, and then is reflected on the second high-dispersion mirror 7, and then is reflected on the output mirror 9. Part of the laser light is output through the output mirror 9, 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.

In this femtosecond disk laser, the Cr ⁴⁺ :YAG crystal 5 serves as the modulation device for passive mode locking in the 1030nm band. Furthermore, the Cr ⁴⁺ :YAG crystal 5 exhibits strong fluorescence absorption lines between 970nm and 1100nm; therefore, the 1030nm femtosecond laser within the resonant cavity serves as the pump source for the Cr ⁴⁺ :YAG crystal's eye-safe femtosecond laser.

The disk laser crystal 2 of the femtosecond disk laser is a Yb:YAG crystal, which generates an oscillating laser with a central wavelength of 1030nm. The oscillating laser oscillates and strengthens back and forth between the first branch, the disk laser crystal, and the second branch, and is continuously modulated by the Cr ⁴⁺ :YAG crystal, thereby forming a high-power stable femtosecond laser pulse sequence with a central wavelength of 1030nm.

Furthermore, the Cr ⁴⁺ :YAG crystal 5 continuously absorbs the 1030nm band femtosecond laser in the resonant cavity. When the gain of the laser in the 1300-1600nm band is greater than the loss, a broadband eye-safe band femtosecond laser will be generated in the resonant cavity. The laser is continuously amplified by the reflector in the resonant cavity to form a stable pulse sequence. Part of the laser 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.

Furthermore, the length of the Cr ⁴⁺ :YAG crystal 5 is selected within the range of 10 mm to 30 mm, and the doping concentration of Cr ⁴⁺ ions is selectable within the range of 2% to 7%. That is, different crystal lengths and doping concentrations are combined and adjusted accordingly based on experimental phenomena.

To further reduce energy loss in the resonant cavity, the light-transmitting surface of the Cr ⁴⁺ :YAG crystal 5 was optimized. Specifically, both sides of the Cr ⁴⁺ :YAG crystal surface were coated with broadband antireflection coatings for the 1000nm-1070nm and 1300-1600nm bands, with a transmittance greater than 99.9%.

Optionally, the light-facing surfaces of the dispersion mirror group, the first concave mirror 10, the second concave mirror 11 and the high-reflection mirror 12 are coated with broadband high-reflection films for the 1000nm-1070nm band and the 1300-1600nm band, with a reflectivity greater than 99.9%.

Optionally, the light-facing surface of the output mirror 9 is coated with a film layer that fully reflects the oscillating laser in the 1000nm-1070nm band and partially transmits the 1300-1600nm band, with a transmittance range of 1% to 20%;

The light-emitting surface of the output mirror 9 is coated with an anti-reflection film for oscillating lasers in the 1300-1600 nm band.

The reflective surface of the output mirror 9 is also coated with a high-reflective film for 1000nm-1070nm and oscillating lasers, with a reflectivity greater than 99.9%.

The front surfaces of the first concave mirror 10, the second concave mirror 11 and the high-reflection mirror 12 in the resonant cavity 4 are coated with a 1300-1600nm high-reflection film, and the front surface of the output mirror 9 is coated with a 1300-1600nm partially transmissive film, thereby ensuring that the 1300-1600nm femtosecond laser oscillates and strengthens back and forth between the first branch, the disk laser crystal and the second branch, and ultimately outputs high-power femtosecond laser pulses in the human eye-safe band.

Optionally, the femtosecond disk laser further includes a crystal fixture;

The crystal fixture holds the Cr ⁴⁺ :YAG crystal 5 , and external circulating cooling water is passed through the crystal fixture for dissipating heat from the Cr ⁴⁺ :YAG crystal 5 .

Furthermore, the Cr ⁴⁺ :YAG crystal 5 is wrapped with silver foil and held in a copper fixture. The fixture has external cooling water and adjusts its temperature based on the average power within the resonant cavity. The silver foil ensures a tighter fit between the Cr ⁴⁺ :YAG crystal 5 and the copper, improving cooling efficiency.

By employing the aforementioned femtosecond disk laser, the Cr ⁴⁺ :YAG crystal serves both as a passive mode-locking component in the 1μm band and as a gain medium for eye-safe femtosecond laser generation. This allows for stable, high-power femtosecond laser pulses in the eye-safe band (the target band), significantly simplifying the design and operation of the device. The femtosecond disk laser generating a specific wavelength, provided by the present invention, possesses a simple and compact structure and is widely applicable in fields such as lidar, industrial ranging, and ophthalmic surgery, with promising development and application prospects.

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 (9)

1. A femtosecond disk laser generating an eye-safe wavelength band, characterized by comprising: a pump source (1), a disk laser crystal (2), a pump cavity (3) and a resonant cavity (4); The pump cavity (3) is arranged on the output light path of the pump source (1); the disk laser crystal (2) is arranged in the pump cavity (3); the pump cavity (3) allows the pump laser emitted by the pump source (1) to pass through the disk laser crystal (2) multiple times and then excite the disk laser crystal (2) to generate oscillating laser light; The resonant cavity (4) is arranged on one side of the output of the oscillating laser describing the disk laser crystal, and the resonant cavity (4) comprises a first branch and a second branch; The first branch includes a dispersion mirror group, a hard aperture (8), and an output mirror (9) arranged in sequence along the optical path; the hard aperture (8) is used to assist passive mode locking; after the oscillating laser is transmitted along the first branch to the output mirror (9), a portion of the laser is output through the output mirror (9), and the remaining laser returns to the disk laser crystal (2) along the original path and is reflected to the second branch; The second branch comprises a first concave mirror (10), a second concave mirror (11), a Cr ⁴⁺ :YAG crystal (5) and a high reflective mirror (12) arranged in sequence along the optical path; the focal points of the first concave mirror (11) and the second concave mirror (11) coincide with each other, the Cr ⁴⁺ :YAG crystal (5) is located at the focal point, 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; The Cr ⁴⁺ :YAG crystal (5) serves as a saturable absorber for passive mode locking, and the Cr ⁴⁺ :YAG crystal also serves as a gain medium for generating a femtosecond laser pulse sequence in the human eye-safe band under the action of an oscillating laser; the femtosecond laser pulse sequence in the human eye-safe band is reflected by the high-reflection mirror (12), returns along the original path to the disk laser crystal (2), and enters the first branch again after being reflected by the disk laser crystal (2); the output mirror (9) outputs the femtosecond laser pulse sequence in the human eye-safe band. 2. The femtosecond disk laser according to claim 1, wherein the disk laser crystal (2) is a Yb:YAG crystal, wherein the Yb ³⁺ doping concentration is 7%; The Cr ⁴⁺ doping concentration of the Cr ⁴⁺ :YAG crystal (5) is in the range of 2%-7%, and the length of the Cr ⁴⁺ :YAG crystal (5) is in the range of 10 mm-30 mm; The central wavelength of the femtosecond laser pulse sequence in the eye-safe band is 1500 nm. 3. The femtosecond disk laser according to claim 2, characterized in that both sides of the surface of the Cr ⁴⁺ :YAG crystal (5) are coated with broadband antireflection coatings for the 1000nm-1070nm and 1300-1600nm bands, and the transmittance is greater than 99.9%. 4. The femtosecond disk laser according to claim 2, characterized in that the light-facing surfaces of the dispersion mirror assembly, the first concave mirror (10), the second concave mirror (11), and the high-reflection mirror (12) are coated with a broadband high-reflection film for the 1000nm-1070nm band and the 1300-1600nm band, with a reflectivity greater than 99.9%. 5. The femtosecond disk laser according to claim 4, wherein the dispersion mirror assembly comprises a first high-dispersion mirror (6) and a second high-dispersion mirror (7) sequentially arranged along the optical path. 6. The femtosecond disk laser according to claim 2, characterized in that the light-facing surface of the output mirror (9) is coated with a film layer that fully reflects the oscillating laser in the 1000nm-1070nm band and a film layer that partially transmits the 1300-1600nm band, with a transmittance range of 1% to 20%; The light-emitting surface of the output mirror (9) is coated with an anti-reflection film for oscillating laser light in the 1300-1600 nm band. 7. The femtosecond disk laser according to any one of claims 1 to 6, characterized in that the disk laser crystal (2) is coated with an anti-reflection film for pump laser and oscillator laser on the side facing the resonant cavity (4), and is coated with a high-reflection film for pump laser and oscillator laser on the side facing away from the resonant cavity (4). 8. The femtosecond disk laser according to claim 7, characterized in that the surface of the disk laser crystal (2) coated with the high-reflection film is fixed on a water-cooled heat sink. 9. The femtosecond disk laser according to claim 1, further comprising a crystal holder; The crystal fixture clamps the Cr ⁴⁺ :YAG crystal, and external circulating cooling water is passed through the crystal fixture to dissipate heat from the Cr ⁴⁺ :YAG crystal.