A multi-wavelength selection laser based on ho-doped crystal and a wavelength tuning method thereof

Abstract

A kind of multi-wavelength selection laser based on Ho doped crystal and its wavelength tuning method belong to the field of optoelectronic device manufacturing and optical engineering laser technology.To solve the function of multi-wavelength output of multi-wavelength selection laser, the pump source of 1940nm is connected with fiber coupler through fiber, the fiber coupler is arranged with fiber coupling lens group, polaroid, input plane mirror, Ho doped mixed crystal, output plane mirror and 1900nm narrow band filter in sequence along the light path;The Ho doped mixed crystal is composed of first square doped crystal, second square doped crystal, third square doped crystal and fourth square doped crystal, and the combination includes Ho: LLF crystal, Ho: YAP crystal and Ho: YLF crystal cut in different axial directions.The present application can efficiently realize tunable multi-wavelength output.By combining Ho doped crystals with different substrates, a laser is used to realize multi-wavelength output in a wide wavelength range.

Description

Technical Field

[0001] The present invention belongs to the field of optoelectronic device manufacturing and optical engineering laser technology, and particularly relates to a multi-wavelength selective laser based on Ho-doped crystal and a wavelength tuning method thereof. Background Art

[0002] Wavelength division multiplexing (WDM) technology, as a core technology in cutting-edge fields such as fiber-optic communications and optical computing, is rapidly expanding to multiple sectors of the information industry. Its demand for efficient and highly integrated laser output poses a major challenge to modern laser technology. However, due to the inherent absorption and emission spectrum limitations of laser crystals, traditional lasers are mostly limited to single-wavelength operation, and the ability to generate multi-wavelength signals is limited by the number of lasers. Traditional lasers usually rely on complex tuning mechanisms or multiple independent laser sources when generating multiple different wavelengths, which increases the complexity and cost of the system and makes it difficult to ensure long-term stable performance, especially in the face of environmental changes. In addition, existing designs are difficult to meet the requirements of high integration with micro-nano optics and microelectronics, limiting their use in compact applications. At the same time, wavelength tuning also has the problem of high energy loss.

[0003] The "A Compact Multi-Wavelength Laser" with the authorization announcement number CN116885540A uses a single crystal to achieve four-wavelength co-aperture coaxial output in a highly integrated manner. However, the patent only provides four-wavelength output and cannot switch wavelengths freely. External devices are required for filtering or beam splitting to extract a specific wavelength. The "A Dual-Crystal Multi-Wavelength Laser" with the authorization announcement number CN213071699U uses a focuser, a fixed block, and a laser structural design to solve the problem that existing dual-crystal multi-wavelength lasers cannot collect data on the laser's value and the distance of the beam refraction point during actual use. However, this method only uses components such as optical lenses to modulate the light beam, and does not change the premise of the "dual crystal". The "Multi-Wavelength Laser" with the authorization announcement number CN203674555U uses a green laser module, a red laser module, and a blue laser module placed in parallel. Different colors of laser light are obtained by lighting up the red, blue, and green laser modules in different combinations. However, this patent lacks wavelength diversity output. "A multi-wavelength laser" with authorization announcement number CN112928588B uses the resonant cavity of the imaging structure and the cavity mirror coating process to enable the laser to simultaneously output lasers of different wavelengths. However, its actual application will face a series of complex problems, such as the coating process and the need for different pump sources for different wavelength outputs. Summary of the Invention

[0004] The problem to be solved by the present invention is to realize the multi-wavelength output and arbitrary wavelength switching function of a multi-wavelength selective laser, and propose a multi-wavelength selective laser based on Ho-doped crystal and a wavelength tuning method thereof.

[0005] To achieve the above object, the present invention is implemented through the following technical solutions:

[0006] A multi-wavelength selective laser based on Ho-doped crystal, comprising a 1940nm pump source, an optical fiber, an optical fiber beam splitter, an optical fiber coupling lens group, a polarizer, an input plane mirror, a Ho-doped mixed crystal, an output plane mirror, and a 1900nm narrowband filter;

[0007] The 1940nm pump source is connected to a fiber optic beam splitter via an optical fiber. A fiber coupling lens group, a polarizer, an input plane mirror, a Ho-doped mixed crystal, an output plane mirror, and a 1900nm narrowband filter are sequentially arranged on the right side of the fiber optic beam splitter along the optical path.

[0008] The Ho-doped mixed crystal is composed of a first square-doped crystal, a second square-doped crystal, a third square-doped crystal, and a fourth square-doped crystal. The first square-doped crystal, the second square-doped crystal, the third square-doped crystal, and the fourth square-doped crystal are a combination of several types of Ho:LLF crystals, Ho:YAP crystals, and Ho:YLF crystals cut in different axes.

[0009] Furthermore, the surface of the input plane mirror is coated with a 2050nm narrow-band reflection film, and the surface of the output plane mirror is coated with a 2050nm 5% transmission film.

[0010] Furthermore, when the 1940 nm pump source is directly connected to the fiber-coupled lens group through an optical fiber, a microlens array is provided between the polarizer and the input plane mirror.

[0011] Furthermore, in the Ho-doped mixed crystal, the first square doped crystal is a b-axis cut Ho:YAP crystal, the second square doped crystal is a c-axis cut Ho:LLF crystal, the third square doped crystal is an a-axis cut Ho:YLF crystal, and the fourth square doped crystal is a c-axis cut Ho:YLF crystal.

[0012] Furthermore, in the Ho-doped mixed crystal, the first square doped crystal is an a-axis cut Ho:YAP crystal, the second square doped crystal is a c-axis cut Ho:YAP crystal, the third square doped crystal is a c-axis cut Ho:LLF crystal, and the fourth square doped crystal is a b-axis cut Ho:YLF crystal.

[0013] Furthermore, in the Ho-doped mixed crystal, the first square doped crystal is an a-axis cut Ho:YAP crystal, the second square doped crystal is a b-axis cut Ho:YAP crystal, the third square doped crystal is a c-axis cut Ho:LLF crystal, and the fourth square doped crystal is a c-axis cut Ho:YLF crystal.

[0014] Furthermore, the first square doped crystal, the second square doped crystal, the third square doped crystal, and the fourth square doped crystal are combined together via a heat sink.

[0015] A wavelength tuning method for a multi-wavelength selective laser based on a Ho-doped crystal is implemented based on the multi-wavelength selective laser based on a Ho-doped crystal, and includes the following steps:

[0016] The 1940nm pump source outputs pump light, which is transmitted through the optical fiber and the optical fiber beam splitter into the fiber coupling lens group for coupling and focusing. Then, it passes through the polarizer and the input plane mirror to obtain the 1940nm band light that converges on the front face of the Ho-doped hybrid crystal.

[0017] The four different doped crystals in the Ho-doped mixed crystal absorb the input 1940nm band light and output four different bands of light;

[0018] Light of four different wavelength bands passes through the output plane mirror and then is filtered through a 1900nm narrowband filter to obtain multi-wavelength laser.

[0019] Furthermore, when the angle α between the polarization direction of the input light and the principal axis of the crystal changes, the wavelength of the σ polarization state and the π polarization state output by the doped crystal changes, and the expression is obtained as follows:

[0020] λ σ (θ1)=λ σ (θ0)+Δλ σ (α) (1)

[0021] λ π (θ1)=λ π (θ0)+Δλ π (α) (2)

[0022] Where θ0 is defined as the initial angle between the input light polarization direction and the crystal axis. After the rotation α, the angle becomes θ1, λ σ (θ0) and λ σ (θ1) are defined as the central wavelength of the output of the σ polarization state when the angle is θ0 and θ1, respectively, and λ π (θ0) and λ π (θ1) are defined as the central wavelength of the π polarization state when the angles are θ0 and θ1, respectively, and Δλ σ (α) and Δλ π (α) represents the change in the central wavelength of the σ polarization state and the central wavelength of the π polarization state after a rotation of α;

[0023] The expression for the refractive index relationship between the a-axis and c-axis planes of the doped crystal is:

[0024]

[0025] Among them, n a and n c are the refractive indices of the a-axis and c-axis, n c (θ) is the refractive index when the input light polarization direction is at an angle θ with the principal axis of the crystal;

[0026] Based on the fact that the output light is subject to the standing wave condition of the resonant cavity between the input mirror and the output mirror, the expression is obtained as follows:

[0027] 2kL'=2mπ (4)

[0028] L'=n0L1+n c (θ)L2 (5)

[0029] Where k is the modulus of the wave vector, L' is the total optical path, n0L1 and n c (θ)L2 represent the optical path outside and inside the crystal in the resonant cavity, respectively, and m is a first positive integer; L1 and L2 represent the propagation distance of light outside and inside the crystal during oscillation in the resonant cavity, respectively;

[0030] Combining equations (3), (4) and (5), we get:

[0031]

[0032] When the angle θ changes by α, equation (6) changes to:

[0033]

[0034] Among them, k' is the modulus of the wave vector after the change, and m' is the second positive integer;

[0035] Introducing wavelength λ into equation (7), we get the expression:

[0036]

[0037] Among them, m1' and m2' are the third positive integer and the fourth positive integer respectively, θ π0 and θ σ0 represent the angles between the output σ polarization and π polarization and the principal axis of the crystal respectively;

[0038] Based on equations (8) and (9), we can get the variable Δλ after introducing α π (α) / Δλ σ (α) and m1' / m2' will also change in tandem. When the polarizer is rotated, the refractive index n c(θ) changes, thereby changing the optical path L', and the mode k of the wave vector will change accordingly, that is, the wavelength λ changes; under the conditions of satisfying equations (8) and (9), the multi-wavelength solid-state laser based on Ho-doped crystal outputs 2μm band lasers in different frequency ranges.

[0039] The above derivation is based on c-axis cut Ho:LLF crystal. The theoretical model is also applicable to Ho:YLF and Ho:YAP. The only difference is that they can output single-wavelength laser or dual-wavelength laser.

[0040] Furthermore, assuming that the laser intensity before passing through the polarizer is I0, the energy E∝I0, and the angle between the incident light polarization direction and the polarizer is θ', according to Malus's law, the laser intensity after passing through the polarizer is:

[0041]

[0042] Where I is the intensity of light passing through the polarizer;

[0043] After setting the polarizer to rotate β, we get:

[0044]

[0045] Where I' is the intensity of light passing through the polarizer after rotating the polarizer;

[0046] The expression for calculating the attenuation of light intensity is:

[0047] ΔI=I'-I; (12)

[0048] When the polarizer rotates, the change in laser gain affects the output. The expression of laser gain is as follows:

[0049]

[0050] Where G is the net laser gain, g(θ') is the gain contributed by the gain medium associated with θ', α' is the absorption loss, L represents the optical path length of one cycle of laser oscillation, and R1 and R2 are the reflectivities of the laser cavity mirrors.

[0051] Ignoring the change in absorption loss, when the polarizer is rotated by β, we can obtain:

[0052]

[0053] Among them, σ e is the emission cross section, σ α is the absorption cross section, N1 is the number of particles in the lower energy level, N2 is the number of particles in the upper energy level, h is the Planck constant, and υ is the central wavelength of the emitted laser.

[0054] Further analysis shows that the main attenuation source is the light intensity loss caused by the angle between the input light and the polarizer. This solution greatly alleviates the laser attenuation problem caused by the frequency tuning device.

[0055] Beneficial effects of the present invention:

[0056] The multi-wavelength selective laser based on Ho-doped crystals described in the present invention can efficiently output multi-wavelength lasers by utilizing the strong absorption characteristics of 1940nm laser by three Ho-doped crystals, Ho:LLF, Ho:YAP, and Ho:YLF. In addition, the simultaneous adjustment of multi-wavelength lasers is achieved by innovatively changing the angle of the polarizer. In addition, when only any one of the four crystals is used, the function of selecting a single wavelength can also be achieved, that is, by adjusting the angle of the polarizer, lasers of different wavelengths can be selectively output.

[0057] The multi-wavelength selective laser based on Ho-doped crystal described in the present invention can efficiently realize wavelength-tunable multi-wavelength laser output through only a single pump and a laser system.

[0058] The multi-wavelength selective laser based on a Ho-doped crystal described in this invention achieves multi-wavelength laser output using a single pump source by designing a Ho-doped mixed crystal. Wavelength selection is achieved by varying the angle of the polarizer, and rotating the polarizer can output different multi-wavelength laser combinations. By designing a special Ho-doped mixed crystal and utilizing different tangential directions and polarizer rotation angles, different multi-wavelength outputs and multi-wavelength selection can be achieved.

[0059] The wavelength tuning method of a multi-wavelength selective laser based on Ho-doped crystal described in the present invention innovatively uses polarizers to achieve wavelength tuning by deducing and analyzing the angle change of polarizers, thereby designing the overall structure of the laser. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Figure 1 This is a schematic diagram of the first structure of a multi-wavelength selective laser based on Ho-doped crystal according to the present invention;

[0061] Figure 2 Schematic diagram of the Ho-doped mixed crystal structure of the present invention;

[0062] Figure 3 Schematic diagram of the angle between the polarization direction of the pump light and the crystal axis in the present invention;

[0063] Figure 4 The refractive index ellipse of the c-axis cut Ho:LLF crystal of the present invention;

[0064] Figure 5This is a schematic diagram of a second structure of a multi-wavelength selective laser based on Ho-doped crystal according to the present invention;

[0065] Figure 6 This is a wavelength variation diagram of the c-axis cut Ho:LLF crystal under angle tuning of the present invention, where (a) is π polarization and (b) is σ polarization. DETAILED DESCRIPTION

[0066] 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 specific embodiments. It should be understood that the specific embodiments described herein are only intended to explain the present invention and are not intended to limit the present invention. That is, the specific embodiments described herein are only some embodiments of the present invention, not all embodiments. Generally, the components of the specific embodiments of the present invention described and illustrated in the drawings herein can be arranged and designed in various different configurations, and the present invention can also have other embodiments.

[0067] Therefore, the following detailed description of the specific embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention as claimed, but is merely representative of selected specific embodiments of the present invention. All other specific embodiments obtained by those skilled in the art based on the specific embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0068] In order to further understand the content, features and effects of the present invention, the following specific embodiments are given as examples, and the attached Figure 1 -Attached Figure 6 The detailed instructions are as follows:

[0069] Example 1:

[0070] A multi-wavelength selective laser based on Ho-doped crystal, comprising a 1940 nm pump source 1, an optical fiber 2, an optical fiber beam splitter 3, an optical fiber coupling lens group 4, a polarizer 5, an input plane mirror 6, a Ho-doped mixed crystal 7, an output plane mirror 8, and a 1900 nm narrowband filter 9;

[0071] The 1940nm pump source 1 is connected to the fiber optic beam splitter 3 through the optical fiber 2. The right side of the fiber optic beam splitter 3 is provided with a fiber coupling lens group 4, a polarizer 5, an input plane mirror 6, a Ho-doped mixed crystal 7, an output plane mirror 8, and a 1900nm narrowband filter 9 arranged in sequence along the optical path;

[0072] The Ho-doped mixed crystal 7 is composed of a first square-doped crystal 11, a second square-doped crystal 12, a third square-doped crystal 13, and a fourth square-doped crystal 14. The first square-doped crystal 11, the second square-doped crystal 12, the third square-doped crystal 13, and the fourth square-doped crystal 14 are a combination of several types of Ho:LLF crystals, Ho:YAP crystals, and Ho:YLF crystals cut in different axes.

[0073] Furthermore, the surface of the input plane mirror 6 is coated with a 2050nm narrow-band reflection film, and the surface of the output plane mirror 8 is coated with a 2050nm 5% transmission film.

[0074] Furthermore, in the Ho-doped mixed crystal 7, the first square doped crystal 11 is a b-axis cut Ho:YAP crystal, the second square doped crystal 12 is a c-axis cut Ho:LLF crystal, the third square doped crystal 13 is an a-axis cut Ho:YLF crystal, and the fourth square doped crystal 14 is a c-axis cut Ho:YLF crystal.

[0075] Furthermore, the first square doped crystal 11 , the second square doped crystal 12 , the third square doped crystal 13 , and the fourth square doped crystal 14 are combined together via a heat sink.

[0076] Furthermore, the function of the coupling lens group is to couple the pump light in the optical fiber and focus it;

[0077] Furthermore, each component of the Ho-doped mixed crystal absorbs 1940nm light and outputs lasers of different wavelengths based on their respective energy level structures. Taking Ho:LLF crystal as an example, Ho:LLF crystal absorbs light in the 1940nm band based on its energy level structure. When Ho:LLF absorbs 1940nm laser, 5 I8 and 5 The energy level transition between I7, the multi-wavelength laser output by the laser system is actually 5 I8 and 5 The result of the transition between the upper energy level and the lower energy level between the large number of fine energy levels generated by the energy level splitting of I7 is a simplified analysis of the rate equation during the transition, and the expression is:

[0078]

[0079] Among them, N1 is 5 The number of particles at the I8 energy level, N2 is 5 The number of particles at the I7 energy level, W 12 is the 1940nm laser pumping rate, W 21 is the output laser rate, σ 12 and σ 21is the emission or absorption cross-sectional area, when the output laser wavelength is different, W 21 and σ 12 Different values ​​of correspond to different fine energy levels; P is the total power absorbed by the energy level transition, h is the Planck constant, A is the cross-sectional area of ​​the beam, and υ P is the center frequency of the pump light, υ L is the center frequency of the emitted laser, τ ij is the relaxation time between energy levels, τ ij sp is the spontaneous emission time, τ ij nr is the radiationless transition time;

[0080] Formulas (1-1) and (1-2) are based on 5 I8 energy level, 5 The rate equation for the increase or decrease of the number of particles at the I7 energy level, (1-3) and (1-4) represent the expressions of the 1940nm laser pumping rate and the output laser rate, respectively, and (1-5) represents the relationship between the relaxation time of particles at the energy level, the spontaneous radiation time, and the non-radiative transition time.

[0081] Furthermore, the wavelength and wavelength selection functions are implemented as follows: All three Ho-doped crystals absorb light strongly in the 1940nm band and output lasers of different wavelengths (the Ho:LLF dual-wavelength output band is around 2060nm, the Ho:YAP output band is around 2130nm, and the Ho:YLF output band is around 2050nm). Therefore, due to the different substrate materials, the four crystals output lasers of four different wavelengths. Secondly, rotating the polarizer angle can change the output wavelength of the laser. Figure 6 The wavelength change of the output laser from the c-axis cut Ho:LLF crystal under angle tuning of 13°-80° is shown, which is consistent with the results of theoretical derivation. The wavelength change range of the π polarization output in the horizontal direction is 2053-2056.3nm, and the wavelength change range of the σ polarization output in the vertical direction is 2063.1-2065.7nm. The test results verify the feasibility of the device.

[0082] Example 2:

[0083] A wavelength tuning method for a multi-wavelength selective laser based on a Ho-doped crystal is implemented based on the multi-wavelength selective laser based on a Ho-doped crystal described in Example 1, and is characterized by comprising the following steps:

[0084] The 1940nm pump source outputs pump light, which is transmitted through the optical fiber and the optical fiber beam splitter into the fiber coupling lens group for coupling and focusing. Then, it passes through the polarizer and the input plane mirror to obtain the 1940nm band light that converges on the front face of the Ho-doped hybrid crystal.

[0085] The four different doped crystals in the Ho-doped mixed crystal absorb the input 1940nm band light and output four different bands of light;

[0086] Light of four different wavelength bands passes through the output plane mirror and then is filtered through a 1900nm narrowband filter to obtain multi-wavelength laser.

[0087] Furthermore, when the angle α between the input light polarization direction and the main axis of the crystal changes, the wavelength of the σ polarization state and the π polarization state output by the doped crystal changes, and the expression is obtained as follows:

[0088] λ σ (θ1)=λ σ (θ0)+Δλ σ (α) (1)

[0089] λ π (θ1)=λ π (θ0)+Δλ π (α) (2)

[0090] Where θ0 is defined as the initial angle between the input light polarization direction and the crystal axis. After the rotation α, the angle becomes θ1, λ σ (θ0) and λ σ (θ1) are defined as the central wavelength of the output of the σ polarization state when the angle is θ0 and θ1, respectively, and λ π (θ0) and λ π (θ1) are defined as the central wavelength of the π polarization state when the angles are θ0 and θ1, respectively, and Δλ σ (α) and Δλ π (α) represents the change in the central wavelength of the σ polarization state and the central wavelength of the π polarization state after a rotation of α;

[0091] The expression for the refractive index relationship between the a-axis and c-axis planes of the doped crystal is:

[0092]

[0093] Among them, n a and n c are the refractive indices of the a-axis and c-axis, n c (θ) is the refractive index when the input light polarization direction is at an angle θ with the principal axis of the crystal;

[0094] Based on the fact that the output light is subject to the standing wave condition of the resonant cavity between the input mirror and the output mirror, the expression is obtained as follows:

[0095] 2kL'=2mπ (4)

[0096] L'=n0L1+n c (θ)L2 (5)

[0097] Where k is the modulus of the wave vector, L' is the total optical path, n0L1 and n c (θ)L2 represent the optical path outside and inside the crystal in the resonant cavity, respectively, and m is a first positive integer; L1 and L2 represent the propagation distance of light outside and inside the crystal during oscillation in the resonant cavity, respectively;

[0098] Combining equations (3), (4) and (5), we get:

[0099]

[0100] When the angle θ changes by α, equation (6) changes to:

[0101]

[0102] Among them, k' is the modulus of the wave vector after the change, and m' is the second positive integer;

[0103] Introducing wavelength λ into equation (7), we get the expression:

[0104]

[0105] Among them, m1' and m2' are the third positive integer and the fourth positive integer respectively, θ π0 and θ σ0 represent the angles between the output σ polarization and π polarization and the principal axis of the crystal respectively;

[0106] According to the derivation, the variable Δλ π (α) / Δλ σ(α) and m1' / m2' also change in synergy, enabling a wide range of frequencies to be output under the conditions that satisfy the equation. According to the absorption and emission spectra of the σ / π polarization state of Ho:LLF crystals published by Walsh BM et al. in "Branching ratios, cross sections, and radiative lifetimes of rare earth ions in solids::Application to Tm3+ and Ho3+ ions in LiYF4," Ho:LLF crystals exhibit a broad emission wavelength range at approximately 2050 nm, which is consistent with theoretical derivation. From the above derivation, it can be seen that when the polarizer is rotated, the refractive index nc(θ) changes with the change in θ, thereby changing the optical path L'. To meet the standing wave condition, the modulus k of the wave vector also changes, that is, the wavelength λ changes. The above derivation is based on Ho:LLF crystals, and this theoretical model is also applicable to Ho:YLF and Ho:YAP, with the only difference being the output of single-wavelength lasers and dual-wavelength lasers. In other words, each part of the Ho-doped mixed crystal outputs lasers of different wavelengths due to the change in polarization direction.

[0107] Furthermore, assuming that the laser intensity before passing through the polarizer is I0, the energy E∝I0, and the angle between the incident light polarization direction and the polarizer is θ', according to Malus's law, the laser intensity after passing through the polarizer is:

[0108]

[0109] Where I is the intensity of light passing through the polarizer;

[0110] After setting the polarizer to rotate β, we get:

[0111]

[0112] Where I' is the intensity of light passing through the polarizer after rotating the polarizer;

[0113] The expression for calculating the attenuation of light intensity is:

[0114] ΔI=I'-I; (12)

[0115] Compared with the mainstream solid-state laser wavelength tuning method (inserting a frequency modulation device), the laser attenuation in this embodiment is smaller, and the attenuation is very small when the rotation angle is small.

[0116] When the polarizer rotates, the change in laser gain affects the output. The expression of laser gain is as follows:

[0117]

[0118] Where G is the net laser gain, g(θ') is the gain contributed by the gain medium associated with θ', α' is the absorption loss, L represents the optical path length per one cycle of laser oscillation, and R1 and R2 are the reflectivities of the laser cavity mirrors. In this expression, the three terms on the right correspond to the gain contributed by the gain medium, the absorption loss, and the energy loss caused by laser oscillation, respectively.

[0119] Ignoring the loss change, when the polarizer is rotated by β, we can get:

[0120]

[0121] The net gain of a laser is closely related to the absorption and emission cross-sections. Combining the absorption and emission spectra of various components within the Ho-doped crystal allows analysis of the specific power enhancement and attenuation. While these variations are clearly minimal, the primary attenuation is due to intensity loss caused by the angle between the input light and the polarizer. This approach significantly mitigates the problem of laser attenuation caused by the frequency tuning device.

[0122] Example 3:

[0123] The difference between this embodiment and embodiment 1 is that the first square doped crystal 11 in the Ho-doped mixed crystal 7 is an a-axis cut Ho:YAP crystal, the second square doped crystal 12 is a c-axis cut Ho:YAP crystal, the third square doped crystal 13 is a c-axis cut Ho:LLF crystal, and the fourth square doped crystal 14 is a b-axis cut Ho:YLF crystal.

[0124] In this embodiment, a polarizer is placed after the coupling lens group to adjust the polarization direction of the pump light. The polarization angle of the pump light can be arbitrarily selected to achieve output of different wavelengths. The laser of the present invention is a multi-wavelength laser, which ultimately outputs frequency-adjustable multi-wavelength laser light. The wavelength group selection is achieved by rotating the polarizer angle.

[0125] Example 4:

[0126] The difference between this embodiment and Example 1 is that the first square doped crystal 11 in the Ho-doped mixed crystal 7 is an a-axis cut Ho:YAP crystal, the second square doped crystal 12 is a b-axis cut Ho:YAP crystal, the third square doped crystal 13 is a c-axis cut Ho:LLF crystal, and the fourth square doped crystal 14 is a c-axis cut Ho:YLF crystal.

[0127] Example 5:

[0128] The difference between this embodiment and embodiment 1, embodiment 4 or embodiment 5 is that when the 1940 nm pump source 1 is directly connected to the fiber coupling lens group 4 through the optical fiber 2, a microlens array 10 is provided between the polarizer 5 and the input plane mirror 6.

[0129] It should be noted that relational terms such as "first" and "second" are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus comprising a series of elements includes not only those elements, but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus. In the absence of further limitations, an element defined by the phrase "comprising a ..." does not exclude the presence of additional identical elements in the process, method, article, or apparatus comprising the element.

[0130] Although the present application has been described above with reference to specific embodiments, various modifications may be made thereto and components may be substituted with equivalents without departing from the scope of the present application. In particular, as long as there are no structural conflicts, the various features of the embodiments disclosed herein may be combined with each other in any manner, and the omission of an exhaustive description of these combinations in this specification is solely for the sake of space and resource conservation. Therefore, the present application is not limited to the specific embodiments disclosed herein, but includes all technical solutions within the scope of the claims.

Claims

1. A multi-wavelength selective laser based on Ho-doped crystal, characterized in that: It includes a 1940 nm pump source (1), an optical fiber (2), an optical fiber beam splitter (3), an optical fiber coupling lens group (4), a polarizer (5), an input plane mirror (6), a Ho-doped mixed crystal (7), an output plane mirror (8), and a 1900 nm narrowband filter (9); The 1940 nm pump source (1) is connected to a fiber beam splitter (3) via an optical fiber (2); a fiber coupling lens group (4), a polarizer (5), an input plane mirror (6), a Ho-doped mixed crystal (7), an output plane mirror (8), and a 1900 nm narrowband filter (9) are sequentially arranged on the right side of the fiber beam splitter (3) along the optical path; The Ho-doped mixed crystal (7) is composed of a first square-shaped doped crystal (11), a second square-shaped doped crystal (12), a third square-shaped doped crystal (13), and a fourth square-shaped doped crystal (14); the first square-shaped doped crystal (11), the second square-shaped doped crystal (12), the third square-shaped doped crystal (13), and the fourth square-shaped doped crystal (14) are a combination of several types of Ho:LLF crystals, Ho:YAP crystals, and Ho:YLF crystals cut in different axes.

2. The multi-wavelength selective laser based on Ho-doped crystal according to claim 1, characterized in that: The surface of the input plane mirror (6) is coated with a 2050nm narrow-band reflection film, and the surface of the output plane mirror (8) is coated with a 2050nm 5% transmission film.

3. A multi-wavelength selective laser based on Ho-doped crystal according to claim 1 or 2, characterized in that: When the 1940nm pump source (1) is directly connected to the optical fiber coupling lens group (4) through the optical fiber (2), a microlens array (10) is arranged between the polarizer (5) and the input plane mirror (6).

4. The multi-wavelength selective laser based on Ho-doped crystal according to claim 3, characterized in that: In the Ho-doped mixed crystal (7), the first square-doped crystal (11) is a b-axis-cut Ho:YAP crystal, the second square-doped crystal (12) is a c-axis-cut Ho:LLF crystal, the third square-doped crystal (13) is an a-axis-cut Ho:YLF crystal, and the fourth square-doped crystal (14) is a c-axis-cut Ho:YLF crystal.

5. The multi-wavelength selective laser based on Ho-doped crystal according to claim 4, characterized in that: In the Ho-doped mixed crystal (7), the first square-doped crystal (11) is an a-axis-cut Ho:YAP crystal, the second square-doped crystal (12) is a c-axis-cut Ho:YAP crystal, the third square-doped crystal (13) is a c-axis-cut Ho:LLF crystal, and the fourth square-doped crystal (14) is a b-axis-cut Ho:YLF crystal.

6. The multi-wavelength selective laser based on Ho-doped crystal according to claim 5, characterized in that: In the Ho-doped mixed crystal (7), the first square-doped crystal (11) is an a-axis-cut Ho:YAP crystal, the second square-doped crystal (12) is a b-axis-cut Ho:YAP crystal, the third square-doped crystal (13) is a c-axis-cut Ho:LLF crystal, and the fourth square-doped crystal (14) is a c-axis-cut Ho:YLF crystal.

7. The multi-wavelength selective laser based on Ho-doped crystal according to claim 6, characterized in that: The first square doped crystal (11), the second square doped crystal (12), the third square doped crystal (13), and the fourth square doped crystal (14) are combined together via a heat sink.

8. A wavelength tuning method for a multi-wavelength selective laser based on a Ho-doped crystal, implemented by the multi-wavelength selective laser based on a Ho-doped crystal according to any one of claims 1 to 7, characterized in that: The steps include: The 1940nm pump source outputs pump light, which is transmitted through the optical fiber and the optical fiber beam splitter into the fiber coupling lens group for coupling and focusing. Then, it passes through the polarizer and the input plane mirror to obtain the 1940nm band light that converges on the front face of the Ho-doped hybrid crystal. The four different doped crystals in the Ho-doped mixed crystal absorb the input 1940nm band light and output four different bands of light; Light of four different wavelength bands passes through the output plane mirror and then is filtered through a 1900nm narrowband filter to obtain multi-wavelength laser.

9. The wavelength tuning method of a multi-wavelength selective laser based on Ho-doped crystal according to claim 8, characterized in that: When the angle α between the polarization direction of the input light and the principal axis of the crystal changes, the wavelength of the σ polarization state and the π polarization state output by the doped crystal changes, and the expression is: l σ (θ1)=λ σ (θ0)+Δλ σ (a)(1) l π (θ1)=λ π (θ0)+Δλ π (a)(2) Where θ0 is defined as the initial angle between the input light polarization direction and the crystal axis. After the rotation α, the angle becomes θ1, λ σ (θ0) and λ σ (θ1) are defined as the central wavelength of the output of the σ polarization state when the angle is θ0 and θ1, respectively, and λ π (θ0) and λ π (θ1) are defined as the central wavelength of the π polarization state when the angles are θ0 and θ1, respectively, and Δλ σ (α) and Δλ π (α) represents the change in the central wavelength of the σ polarization state and the central wavelength of the π polarization state after a rotation of α; The expression for the refractive index relationship between the a-axis and c-axis planes of the doped crystal is: Among them, n a and n c are the refractive indices of the a-axis and c-axis, n c (θ) is the refractive index when the input light polarization direction is at an angle θ with the principal axis of the crystal; Based on the fact that the output light is subject to the standing wave condition of the resonant cavity between the input mirror and the output mirror, the expression is obtained as follows: 2kL'=2mπ(4) L'=n0L1+n c (θ)L2(5) Where k is the modulus of the wave vector, L' is the total optical path, n0L1 and n c (θ)L2 represent the optical path outside and inside the crystal in the resonant cavity, respectively, and m is a first positive integer; L1 and L2 represent the propagation distance of light outside and inside the crystal during oscillation in the resonant cavity, respectively; Combining equations (3), (4) and (5), we get: When the angle θ changes by α, equation (6) changes to: Among them, k' is the modulus of the wave vector after the change, and m' is the second positive integer; Introducing wavelength λ into equation (7), we get the expression: Among them, m1' and m2' are the third positive integer and the fourth positive integer respectively, θ π0 and θ σ0 represent the angles between the output σ polarization and π polarization and the principal axis of the crystal respectively; Based on equations (8) and (9), we can get the variable Δλ after introducing α π (α) / Δλ σ (α) and m1' / m2' will also change in tandem. When the polarizer is rotated, the refractive index n c (θ) changes, thereby changing the optical path L', and the mode k of the wave vector will change accordingly, that is, the wavelength λ changes; under the conditions of satisfying equations (8) and (9), the multi-wavelength solid-state laser based on Ho-doped crystal outputs 2μm band lasers in different frequency ranges.

10. The wavelength tuning method of a multi-wavelength selective laser based on Ho-doped crystal according to claim 9, characterized in that The laser intensity before passing through the polarizer is I0, the energy E∝I0, the angle between the incident light polarization direction and the polarizer is θ', and according to Malus's law, the laser intensity after passing through the polarizer is: Where I is the intensity of light passing through the polarizer; After setting the polarizer to rotate β, we get: Where I' is the intensity of light passing through the polarizer after rotating the polarizer; The expression for calculating the attenuation of light intensity is: ΔI=I'-I;(12) When the polarizer rotates, the change in laser gain affects the output. The expression of laser gain is as follows: Where G is the net laser gain, g(θ') is the gain contributed by the gain medium associated with θ', α' is the absorption loss, L represents the optical path length of one cycle of laser oscillation, and R1 and R2 are the reflectivities of the laser cavity mirrors. Ignoring the change in absorption loss, when the polarizer is rotated by β, we can obtain: Among them, σ e is the emission cross section, σ α is the absorption cross section, N1 is the number of particles in the lower energy level, N2 is the number of particles in the upper energy level, h is the Planck constant, and υ is the central wavelength of the emitted laser.

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