Narrow-linewidth 2μm single-frequency laser based on composite cavity structure

Abstract

The application discloses a kind of based on composite cavity structure narrow line width 2 μm single frequency laser, belong to laser technical field, to solve the existing 2 μm laser line width compression effect is limited, complex structure, poor stability, it is difficult to realize the problem of high-stable narrow line width single frequency output.The laser is composed of dichroic mirror, Ho:YAG crystal, polaroid, quarter-wave plate, first full reflection mirror, second full reflection mirror, double Fabry-Perot standard and 2 μm output coupling mirror composite cavity structure, adopts the scheme of double Fabry-Perot standard and composite cavity to narrow single longitudinal mode transmission peak, the length of resonant cavity is adjusted by piezoelectric ceramic, and the precise regulation and control of laser wavelength and spectral line width are realized.The application has simple structure, stable operation, can output pure narrow line width single longitudinal mode laser, can be used as high-precision laser light source in laser radar, Doppler wind measurement, target identification and tracking and the like.

Description

Technical Field

[0001] This invention belongs to the field of laser technology, and in particular relates to a narrow-linewidth 2μm single-frequency laser based on a composite cavity structure. Background Technology

[0002] 2μm wavelength lasers are considered ideal light sources for lidar due to their eye safety, high atmospheric transmittance, and coverage of absorption peaks for various gas molecules. In lidar systems, narrow-linewidth single-longitudinal-mode lasers can achieve higher range resolution, velocity resolution, and detection accuracy, thereby improving target detection accuracy. Their advantages are particularly evident in applications such as Doppler wind measurement, target identification, and tracking.

[0003] Currently, 2μm lasers still have many shortcomings in terms of narrow linewidth and single-frequency output. 2μm lasers using a single control structure have limited linewidth compression effect, making it difficult to meet the requirements of high-precision applications. 2μm lasers using a ring cavity structure rely on non-reciprocal devices such as Faraday rotators for mode control, which has problems such as low damage threshold, poor stability, and complex adjustment, making it impossible to achieve high-power, highly stable single-frequency pulse output. In addition, some solutions have complex structures and high costs, which are not conducive to large-scale application and promotion.

[0004] Therefore, developing a simple, stable, and reliable narrow-linewidth 2μm single-frequency laser to overcome existing technological bottlenecks and meet the demand for high-precision and high-stability laser sources has become a key issue that urgently needs to be addressed in this field, and it has significant research value and broad application prospects. Summary of the Invention

[0005] In view of this, the present invention aims to propose a narrow-linewidth 2μm single-frequency laser based on a composite cavity structure, in order to solve the problems of limited linewidth compression effect, complex structure, poor stability, and difficulty in achieving high-stability narrow-linewidth single-frequency laser output of existing 2μm lasers.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: a narrow-linewidth 2μm single-frequency laser based on a composite cavity structure, comprising a pump source, a dichroic mirror, a Ho:YAG crystal, a polarizer, a quarter-wave plate, a first total reflection mirror, a second total reflection mirror, a first Fabry-Perot etalon, a second Fabry-Perot etalon, and a 2μm output coupling mirror. The dichroic mirror, Ho:YAG crystal, polarizer, quarter-wave plate, first total reflection mirror, second total reflection mirror, first Fabry-Perot etalon, second Fabry-Perot etalon, and 2μm output coupling mirror together constitute a composite cavity Ho:YAG laser. The output end of the pump source faces the dichroic mirror. At the incident surface, the pump light is transmitted through a dichroic mirror and then incident on a Ho:YAG crystal. A polarizer and a quarter-wave plate are sequentially arranged along the laser transmission direction emitted from the Ho:YAG crystal. The first total reflection mirror is located on the emission side of the quarter-wave plate, with its reflecting surface facing the quarter-wave plate. The second total reflection mirror is located on the reflection side of the polarizer, with its reflecting surface facing the polarizer. The 2μm output coupling mirror is located on the reflection side of the dichroic mirror, with its reflecting surface facing the dichroic mirror. The first Fabry-Perot etalon and the second Fabry-Perot etalon are located between the dichroic mirror and the 2μm output coupling mirror.

[0007] Furthermore, the side of the dichroic mirror facing the pump source is the front side, and the positive and negative films of the dichroic mirror transmit the 1.9μm pump laser and reflect the 2μm laser.

[0008] Furthermore, the Ho:YAG crystal has a length of 50 mm and a cross-sectional diameter of 5 mm. 3+ The doping concentration is 0.5%, and the two ends of the crystal are coated with 1.9μm and 2μm laser antireflection films.

[0009] Furthermore, the polarizer's positive and negative film systems reflect 2μm vertically polarized light and transmit 2μm horizontally polarized light.

[0010] Furthermore, both sides of the quarter-wave plate are coated with a 2μm laser antireflection film, and the optical axis of the quarter-wave plate is placed at an angle that is not an integer multiple of 45°.

[0011] Furthermore, both the first and second total reflection mirrors have their optical path facing the front side, and the front film systems of both mirrors are totally reflective of 2μm lasers.

[0012] Furthermore, both the first and second Fabry-Perot etalons are uncoated etalons, and the thicknesses of the first and second Fabry-Perot etalons are different.

[0013] Furthermore, the laser also includes an optical path length adjustment module, which includes a piezoelectric ceramic and a piezoelectric ceramic driver. The piezoelectric ceramic is bonded to the first total reflection mirror, and the piezoelectric ceramic driver is electrically connected to the piezoelectric ceramic. When the piezoelectric ceramic driver is energized, it causes the piezoelectric ceramic to extend or retract, thereby causing the first total reflection mirror to move synchronously to adjust the cavity length of the composite cavity.

[0014] Furthermore, the base cavity length of the composite cavity Ho:YAG laser is the sum of the distances from the 2μm output coupling mirror, through the dichroic mirror, the Ho:YAG crystal, the polarizer, the quarter-wave plate, to the first total reflection mirror.

[0015] Furthermore, the change in the cavity length of the composite cavity Ho:YAG laser is the sum of the distances from the first total reflection mirror through the polarizer to the second total reflection mirror.

[0016] Compared with existing technologies, the advantages of this invention are as follows: This invention adopts a design combining a dual Fabry-Perot etalon and a composite cavity, which can narrow the transmission peak of a single longitudinal mode. The composite cavity can effectively extend the cavity length, and only light that simultaneously meets the resonance condition can oscillate within the cavity, ensuring stable operation of the single longitudinal mode and achieving narrow linewidth output. Combined with the dual Fabry-Perot etalon within the cavity, the longitudinal mode selection capability is further enhanced, which is beneficial for obtaining pure, narrow-linewidth single longitudinal mode laser output, meeting the requirements of high-precision applications.

[0017] This invention employs a composite cavity structure to achieve single-longitudinal-mode narrow-linewidth laser output, eliminating the need for non-reciprocal devices for mode control and avoiding the use of expensive components, thus reducing costs. Furthermore, the composite cavity design does not increase the system volume when increasing the cavity length, simplifying the overall system structure and allowing for more flexible layout. The laser's overall structure is simple, with no redundant components, reducing laser transmission losses within the cavity, and ensuring stable and reliable operation. It effectively improves upon the shortcomings of existing ring cavity structures, such as low damage threshold, poor stability, and complex adjustment, and also solves the problems of complex structures, high costs, and limitations on large-scale application and promotion found in some existing solutions.

[0018] By adjusting the phase delay and resonant cavity length within the cavity, the laser wavelength and spectral linewidth can be precisely controlled through the combined action of piezoelectric ceramics and the composite cavity, achieving simultaneous tuning of wavelength and linewidth and further optimizing the output characteristics of the laser. Attached Figure Description

[0019] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1This is a schematic diagram of a narrow-linewidth 2μm single-frequency laser structure based on a composite cavity structure according to the present invention. Figure 2 This is a schematic diagram of the cavity structure of the narrow linewidth single-longitudinal-mode laser described in this invention; Figure 3 This is a schematic diagram of a single-frequency laser with a narrow linewidth of 2μm based on a composite cavity structure, according to the present invention.

[0020] In the picture: 1-Dichroic mirror, 2-Ho:YAG crystal, 3-Polarizer, 4-Quarter-wave plate, 5-First total reflection mirror, 6-Second total reflection mirror, 7-2μm output coupling mirror, 8-First Fabry-Perot etalon, 9-Second Fabry-Perot etalon, 10-Piezoelectric ceramic, 11-Pump source, 12-Piezoelectric ceramic driver Detailed Implementation The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of the present invention can be combined with each other, and the described embodiments are only some embodiments of the present invention, not all embodiments.

[0021] See Figure 1-3 For a description of this implementation method, please refer to [link / reference]. Figure 1 This embodiment provides a narrow-linewidth 2μm single-frequency laser based on a composite cavity structure, including a pump source 11, a dichroic mirror 1, a Ho:YAG crystal 2, a polarizer 3, a quarter-wave plate 4, a first total reflection mirror 5, a second total reflection mirror 6, a 2μm output coupling mirror 7, a first Fabry-Perot etalon 8, a second Fabry-Perot etalon 9, a piezoelectric ceramic 10, and a piezoelectric ceramic driver 12. The dichroic mirror 1, Ho:YAG crystal 2, polarizer 3, quarter-wave plate 4, first total reflection mirror 5, second total reflection mirror 6, first Fabry-Perot etalon 8, second Fabry-Perot etalon 9, and 2μm output coupling mirror 7 together constitute the composite cavity Ho:YAG laser. The output end of the pump source 11 faces the incident surface of the dichroic mirror 1, and the 1.9μm pump laser emitted by the pump source 11 is transmitted from left to right through the dichroic mirror 1 to the center of the Ho:YAG crystal 2.

[0022] Ho:YAG crystal 2 has a length of 50mm and a cross-sectional diameter of 5mm. 3+The doping concentration is 0.5%, and both ends are coated with 1.9μm and 2μm laser antireflection films. The dichroic mirror 1, facing the optical path, is the front side. Both the front and reverse coatings exhibit high transmittance for the 1.9μm pump laser and high reflectivity for the 2μm laser, and can be used to reflect first, second, and third horizontally polarized light. The polarizer 3, facing the Ho:YAG crystal 2, is the front side. The front coating exhibits high reflectivity for the 1.9μm pump laser, and both the front and reverse coatings exhibit high reflectivity for 2μm vertically polarized laser light and high transmittance for 2μm horizontally polarized laser light. Polarizer 3 can transmit first, second, and third horizontally polarized light, and reflect first and second vertically polarized light.

[0023] A polarizer 3 and a quarter-wave plate 4 are sequentially arranged along the transmission direction of the 2μm laser emitted from the Ho:YAG crystal 2. A first total reflection mirror 5 is positioned on the emission side of the quarter-wave plate 4, with its reflecting surface facing the quarter-wave plate 4. Both sides of the quarter-wave plate 4 are coated with 2μm laser antireflection films, and the optical axis is positioned at angles not multiples of 45°. The angle of the quarter-wave plate can be adjusted as needed. Both the first total reflection mirror 5 and the second total reflection mirror 6 are front-facing, and their front-facing films provide total internal reflection of the 2μm laser light, altering the propagation direction and polarization state of the light to cause oscillation within the composite cavity Ho:YAG laser.

[0024] When the laser begins to oscillate, horizontally linearly polarized light is transmitted from the 2μm output coupling mirror 7 to the dichroic mirror 1 and reflected by it to the Ho:YAG crystal 2. The horizontally linearly polarized light is then transmitted through the Ho:YAG crystal 2 to the polarizer 3. The polarizer 3 has high transmission for the horizontally linearly polarized light, transmitting the first horizontally linearly polarized light to the quarter-wave plate 4. The quarter-wave plate 4 modulates the first horizontally linearly polarized light into first elliptically polarized light, which can be decomposed into horizontally linearly polarized light and vertically linearly polarized light. The first elliptically polarized light is transmitted to the first total reflection mirror 5, reflected by the first total reflection mirror 5, and then incident again on the quarter-wave plate 4. The quarter-wave plate 4 modulates the returning first elliptically polarized light into first linearly polarized light, which is neither horizontally nor vertically polarized.

[0025] The first linearly polarized light, after being incident on polarizer 3, is decomposed by polarizer 3 into second horizontally polarized light and first vertically polarized light. The second horizontally polarized light is transmitted through polarizer 3 to dichroic mirror 1, and reflected by dichroic mirror 1 to 2μm output coupling mirror 7. Part of the second horizontally polarized light is transmitted and output at 2μm output coupling mirror 7, and the remaining part is reflected back to dichroic mirror 1 by 2μm output coupling mirror 7, completing the first cycle. The second total reflection mirror 6 is disposed on the reflecting side of polarizer 3, with the reflecting surface of the second total reflection mirror 6 facing polarizer 3. The first vertically polarized light is reflected by polarizer 3 to the second total reflection mirror 6, and reflected back to polarizer 3. Polarizer 3 reflects the first vertically polarized light to quarter-wave plate 4.

[0026] After receiving the first vertically linearly polarized light, the quarter-wave plate 4 modulates it into a second elliptically polarized light and transmits it to the first total reflection mirror 5. The second elliptically polarized light can be decomposed into horizontally linearly polarized light and vertically linearly polarized light. After being reflected by the first total reflection mirror 5, the second elliptically polarized light is incident on the quarter-wave plate 4 again. The quarter-wave plate 4 modulates the returned second elliptically polarized light into a second linearly polarized light, which is neither horizontal nor vertical. After the second linearly polarized light is incident on the polarizer 3, it is decomposed into a third horizontally linearly polarized light and a second vertically linearly polarized light. The third horizontally linearly polarized light is transmitted through the polarizer 3 to the dichroic mirror 1, and reflected by the dichroic mirror 1 to the 2μm output coupling mirror 7. Part of the third horizontally linearly polarized light is transmitted at the 2μm output coupling mirror 7 as laser output, and the remaining part is reflected back to the dichroic mirror 1 by the 2μm output coupling mirror 7, completing the second cycle. The second vertically linearly polarized light is reflected by the polarizer 3 to the second total reflection mirror 6, and reflected back to the polarizer 3, for the next cycle of decomposition.

[0027] During transmission, the light beam undergoes multiple first and second cycles, creating an optical path difference. This optical path difference, based on each subsequent first and second cycle, is one more cycle than the previous beam's difference, resulting from one more cycle. For example... Figure 2 As shown in the figure, the cavity structure of a narrow linewidth single-longitudinal-mode laser is illustrated. Under the action of polarizer 3, only horizontally polarized light oscillates within the cavity.

[0028] The first Fabry-Perot etalon 8 and the second Fabry-Perot etalon 9 are positioned within the resonant optical path between the dichroic mirror 1 and the 2μm output coupling mirror 7 to screen the longitudinal modes and oscillation wavelength of the composite cavity Ho:YAG laser. Both the first Fabry-Perot etalon 8 and the second Fabry-Perot etalon 9 are uncoated etalons with different thicknesses. A thicker etalon results in a narrower free spectral range, and since the etalon thickness is much smaller than the laser's resonant cavity length, the free spectral range of the etalon is larger than the single longitudinal mode spacing of the solid-state laser. By employing a dual-etalon structure, inserting two etalons of different thicknesses into the resonant cavity of the solid-state laser, the thinner etalon allows for a wider selection range, while the thicker etalon further selects the longitudinal modes. The combination of the two etalons provides a narrower transmission peak for the 2μm laser, used to screen for narrow-linewidth single longitudinal mode laser oscillation, ultimately achieving single longitudinal mode laser output and resulting in a purer output longitudinal mode.

[0029] The piezoelectric ceramic 10 and the piezoelectric ceramic driver 12 constitute the optical path length adjustment module of this laser, used to adjust the length of the composite cavity to tune the oscillation frequency of the laser and adjust the laser linewidth. The piezoelectric ceramic 10 is bonded and fixed to the first total reflection mirror 5, and the piezoelectric ceramic driver 12 is electrically connected to the piezoelectric ceramic 10. The voltage applied to the piezoelectric ceramic by the piezoelectric ceramic driver can be adjusted as needed. When the piezoelectric ceramic driver 12 is energized, it drives the piezoelectric ceramic 10 to extend and retract, thereby causing the first total reflection mirror 5 bonded to the piezoelectric ceramic 10 to extend and retract synchronously with it, causing the first total reflection mirror 5 to make a slight extension and retraction movement, changing the cavity length of the composite cavity, so as to further adjust the laser frequency.

[0030] The base cavity length of the composite cavity Ho:YAG laser is the sum of the distances from the 2μm output coupler 7, through the dichroic mirror 1, Ho:YAG crystal 2, polarizer 3, quarter-wave plate 4, to the first total reflection mirror 5. The change in resonant cavity length is the sum of the distances from the first total reflection mirror 5, through the polarizer 3, to the second total reflection mirror 6. This sum of distances also represents the cavity length change for each of the first and second cycles. The cavity length difference between the second horizontally linearly polarized light output in the first cycle and the third horizontally linearly polarized light output in the second cycle corresponds precisely to the aforementioned cavity length change. The third horizontally linearly polarized light, based on the path taken by the second horizontally linearly polarized light, has this additional cavity length change. Different cavity lengths result in different laser oscillation frequencies, thus achieving laser frequency selection.

[0031] like Figure 3 As shown, according to the basic resonance condition of a Fabry-Perot resonator, the optical path length for one round trip in the Fabry-Perot cavity must be an integer multiple of the wavelength of light to form a standing wave resonance. The longitudinal mode spacing that satisfies the resonance condition is... When the cavity length of a solid-state laser increases, the longitudinal mode spacing of the laser narrows, thereby enabling fine selection of longitudinal modes and narrowing of the longitudinal mode linewidth. Within the composite cavity, only lasers that simultaneously meet the resonant cavity oscillation conditions can oscillate, i.e., the phase change caused by the change in cavity length is an integer multiple of 2π, further enabling the selection of narrow-linewidth single longitudinal modes.

[0032] The circuit consisting of polarizer 3, first total reflection mirror 5, and second total reflection mirror 6 is essentially equivalent to a reflective Fabry-Perot etalon. It forms a composite cavity with dichroic mirror 1 and 2μm output coupling mirror 7, and a multi-level mode selection structure with the dual Fabry-Perot etalon. The equivalent thickness of this reflective Fabry-Perot etalon is the distance from the first total reflection mirror 5, through polarizer 3, to the second total reflection mirror 6, enabling it to possess the core function of multi-beam interference frequency selection. The composite cavity effectively extends the cavity length, allowing light that simultaneously meets the resonance condition to oscillate, ensuring stable operation of the single longitudinal mode and achieving narrow linewidth output. Simultaneously, the Fabry-Perot etalon within the composite cavity enhances the longitudinal mode selection capability, facilitating the output of a narrow linewidth single longitudinal mode.

[0033] By adjusting the phase delay and resonant cavity length within the cavity, the laser wavelength and spectral linewidth can be precisely controlled through the combined action of the piezoelectric ceramic 10 and the composite cavity, achieving simultaneous tuning of wavelength and linewidth and optimizing the laser's output characteristics. This laser employs a composite cavity scheme to increase the cavity length, eliminating the need for redundant components and making the overall system structure more flexible. The absence of redundant components reduces laser transmission losses within the cavity; simultaneously, it avoids the use of expensive non-reciprocal components, thus reducing costs. Furthermore, the composite cavity scheme does not increase the system's volume when increasing the cavity length, simplifying the system structure and enabling stable and reliable narrow-linewidth single-frequency laser output.

[0034] The specific embodiments of the present invention disclosed above are merely illustrative of the invention. These embodiments do not exhaustively describe all details, nor do they limit the invention to the specific embodiments described. Many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention.

Claims

1. A narrow-linewidth 2μm single-frequency laser based on a composite cavity structure, characterized in that: The system includes a pump source (11), a dichroic mirror (1), a Ho:YAG crystal (2), a polarizer (3), a quarter-wave plate (4), a first total reflection mirror (5), a second total reflection mirror (6), a first Fabry-Perot etalon (8), a second Fabry-Perot etalon (9), and a 2μm output coupling mirror (7). The dichroic mirror (1), Ho:YAG crystal (2), polarizer (3), quarter-wave plate (4), first total reflection mirror (5), second total reflection mirror (6), first Fabry-Perot etalon (8), second Fabry-Perot etalon (9), and 2μm output coupling mirror (7) together constitute a composite cavity Ho:YAG laser. The output end of the pump source (11) faces the incident surface of the dichroic mirror (1). The pump light is transmitted through the dichroic mirror (1) and then incident on the laser. A Ho:YAG crystal (2) is provided with a polarizer (3) and a quarter-wave plate (4) arranged sequentially along the laser transmission direction emitted from the Ho:YAG crystal (2). A first total reflection mirror (5) is arranged on the emission side of the quarter-wave plate (4), with the reflecting surface of the first total reflection mirror (5) facing the quarter-wave plate (4). A second total reflection mirror (6) is arranged on the reflection side of the polarizer (3), with the reflecting surface of the second total reflection mirror (6) facing the polarizer (3). A 2μm output coupling mirror (7) is arranged on the reflection side of the dichroic mirror (1), with the reflecting surface of the 2μm output coupling mirror (7) facing the dichroic mirror (1). A first Fabry-Perot etalon (8) and a second Fabry-Perot etalon (9) are arranged between the dichroic mirror (1) and the 2μm output coupling mirror (7).

2. A narrow-linewidth 2μm single-frequency laser based on a composite cavity structure according to claim 1, characterized in that: The side of the dichroic mirror (1) facing the pump source (11) is the front side. The front and back films of the dichroic mirror (1) transmit the 1.9μm pump laser and reflect the 2μm laser.

3. A narrow-linewidth 2μm single-frequency laser based on a composite cavity structure according to claim 1, characterized in that: The Ho:YAG crystal (2) has a length of 50 mm and a cross-sectional diameter of 5 mm. 3+ The doping concentration is 0.5%, and the two ends of the crystal are coated with 1.9μm and 2μm laser antireflection films.

4. A narrow-linewidth 2μm single-frequency laser based on a composite cavity structure according to claim 1, characterized in that: The polarizer (3) has a positive and negative film system that reflects 2μm vertically polarized light and transmits 2μm horizontally polarized light.

5. A narrow-linewidth 2μm single-frequency laser based on a composite cavity structure according to claim 1, characterized in that: The quarter-wave plate (4) is coated with a 2μm laser antireflection film on both sides, and the optical axis of the quarter-wave plate (4) is placed at an angle that is not an integer multiple of 45°.

6. A narrow-linewidth 2μm single-frequency laser based on a composite cavity structure according to claim 1, characterized in that: Both the first total reflection mirror (5) and the second total reflection mirror (6) have a front side facing the optical path, and the front film system reflects the 2μm laser light completely.

7. A narrow-linewidth 2μm single-frequency laser based on a composite cavity structure according to claim 1, characterized in that: Both the first Fabry-Perot tetrometer (8) and the second Fabry-Perot tetrometer (9) are uncoated tetrometers, and the thicknesses of the first Fabry-Perot tetrometer (8) and the second Fabry-Perot tetrometer (9) are different.

8. A narrow-linewidth 2μm single-frequency laser based on a composite cavity structure according to claim 1, characterized in that: The laser also includes an optical path length adjustment module, which includes a piezoelectric ceramic (10) and a piezoelectric ceramic driver (12). The piezoelectric ceramic (10) is bonded to the first total reflection mirror (5), and the piezoelectric ceramic driver (12) is electrically connected to the piezoelectric ceramic (10). When the piezoelectric ceramic driver (12) is energized, it drives the piezoelectric ceramic (10) to extend and retract, thereby driving the first total reflection mirror (5) to move synchronously to adjust the cavity length of the composite cavity.

9. A narrow-linewidth 2μm single-frequency laser based on a composite cavity structure according to claim 1, characterized in that: The base cavity length of the composite cavity Ho:YAG laser is the sum of the distances from the 2μm output coupling mirror (7) through the dichroic mirror (1), Ho:YAG crystal (2), polarizer (3), quarter-wave plate (4) to the first total reflection mirror (5).

10. A narrow-linewidth 2μm single-frequency laser based on a composite cavity structure according to claim 1, characterized in that: The change in the cavity length of the composite cavity Ho:YAG laser is the sum of the distances from the first total reflection mirror (5) through the polarizer (3) to the second total reflection mirror (6).

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