Tunable spatial multiplexing multi-wavelength laser based on Ho: LLF and wavelength tuning method

Abstract

The invention discloses a tunable spatial multiplexing multi-wavelength laser based on Ho: LLF and a wavelength tuning method, and belongs to the technical field of optoelectronic device manufacturing and optical engineering laser. In order to realize the output of a plurality of selectable wavelengths of the tunable spatial multiplexing multi-wavelength laser, a 1940nm pumping source is connected with an optical fiber coupling lens group through an optical fiber; a micro lens array, a polaroid group, an input plane mirror, a c-axis cut Ho: LLF crystal, an output plane mirror, a 1900nm narrow-band filter and a group of polarization beam splitting prisms are sequentially arranged on the right side of the optical fiber coupling lens group along a light path; and the rotation angle of the polaroid group is set to be in gradient change of 0-90 degrees. The surface of the input plane mirror is plated with a 2050nm narrow-band reflecting film, the surface of the output plane mirror is plated with a 2050nm 5% transmission film, and the c-axis cut Ho: LLF crystal is in a cuboid shape. According to the invention, the design of a miniaturized multi-wavelength laser is realized.

Description

Technical Field

[0001] The present invention belongs to the technical fields of optoelectronic device manufacturing and optical engineering laser technology, and particularly relates to a tunable spatial multiplexing multi-wavelength laser based on Ho:LLF and a wavelength tuning method. Background Art

[0002] In modern communication and computing systems, lasers play an indispensable role. However, traditional lasers rely on complex tuning mechanisms or multiple independent laser sources when generating multi-wavelength light, which not only increases the complexity and cost of the system but also makes it difficult to ensure long-term stable performance, especially in the face of environmental changes. In addition, with the rapid expansion of wavelength division multiplexing (WDM) technology in frontier fields such as optical fiber communication and optical computing, the information industry's demand for efficient and highly integrated laser output is growing. However, due to the inherent absorption and emission spectral limitations of laser crystals, most traditional lasers can only operate at a single wavelength. The ability to generate multi-wavelength signals is limited by the need to deploy multiple independent laser sources, restricting their use in compact applications, and there are problems of high energy loss in wavelength tuning, which not only increases the complexity and cost of the system but also limits the flexibility and integration of the system.

[0003] The "orthogonal polarization dual-wavelength laser with adjustable proportion" with the authorization announcement number CN216648854U uses a single-wavelength laser as the fundamental frequency light and performs frequency conversion using a nonlinear crystal inside / outside the resonant cavity to obtain the simultaneous output of dual-wavelength lasers. It can also achieve the output of dual-wavelength lasers by selecting the emission spectra of different laser working media through a frequency selection device. However, the operation is difficult, and the method of beam splitting inside the cavity will increase the loss inside the cavity, resulting in a significant decrease in the output power.

[0004] The "compact multi-wavelength laser" with the authorization announcement number CN116885540A uses a single crystal to achieve the co-aperture coaxial output of four wavelengths in a highly integrated manner and realizes the output of four wavelengths under room temperature conditions without harsh environmental conditions. However, it only provides a method for the output of four wavelengths and cannot switch wavelengths freely. External devices are required for filtering or beam splitting to extract a specific wavelength.

[0005] The "dual-crystal multi-wavelength laser" with the authorization announcement number CN213071699U uses the structural design of a focusing device, a fixing block, and a laser to solve the problem that the existing dual-crystal multi-wavelength laser cannot collect data on the numerical value of the laser and the distance of the beam refraction point during actual use. However, it only modulates the beam using components such as optical lenses without changing the premise of "dual crystals", and the number of wavelengths that can be output is small. Summary of the Invention

[0006] The problem to be solved by the present invention is to achieve the output of multiple selectable wavelengths of a tunable spatial multiplexing multi-wavelength laser, and a tunable spatial multiplexing multi-wavelength laser based on Ho:LLF and a wavelength tuning method are proposed.

[0007] To achieve the above object, the present invention is realized through the following technical solutions:

[0008] A tunable spatial multiplexing multi-wavelength laser based on Ho:LLF includes a 1940nm pump source, an optical fiber, an optical fiber coupling lens group, a microlens array, a polarizer group, an input flat mirror, a c-axis cut Ho:LLF crystal, an output flat mirror, a 1900nm narrowband filter, and a group of polarization beam split prisms;

[0009] The 1940nm pump source is connected to the optical fiber coupling lens group through an optical fiber. On the right side of the optical fiber coupling lens group, a microlens array, a polarizer group, an input flat mirror, a c-axis cut Ho:LLF crystal, an output flat mirror, a 1900nm narrowband filter, and a group of polarization beam split prisms are arranged in sequence along the optical path;

[0010] The rotation angle of the polarizer group is set to vary in a gradient from 0 to 90°.

[0011] Further, the surface of the input flat mirror is coated with a 2050nm narrowband reflection film, the surface of the output flat mirror is coated with a 5% transmission film of 2050nm, and the c-axis cut Ho:LLF crystal is in the shape of a cuboid.

[0012] Further, the microlens array is composed of microlenses, and the focal length of the convex lens corresponds to the position of the front end face of the c-axis cut Ho:LLF crystal.

[0013] Further, the microlens array is replaced by a group of beam splitters and an optical path compensation plate.

[0014] Further, in a group of beam splitters, the n-way beam splitting optical paths are numbered from bottom to top. The transmittance of the first beam splitter is 1 / n and the reflectance is (n - 1) / n. The transmittance of the second beam splitter is (n - 2) / (n - 1) and the reflectance is 1 / (n - 1). The transmittance of the third beam splitter is (n - 3) / (n - 2) and the reflectance is 1 / (n - 2)... The transmittance of the i-th beam splitter is (n - i) / (n - i + 1) and the reflectance is 1 / (n - i + 1).

[0015] Furthermore, the optical path compensation plate is set as a glass sheet inclined at 45° to the horizontal plane. The thickness of the glass sheet is determined by the spacing s between the beam splitters. The thickness difference of the glass sheets between every two adjacent paths is 0.58×s. The optical path compensation plate is not placed in the uppermost optical path. In the order from top to bottom, the thickness of the first optical path compensation plate is 0.58×s, the thickness of the second optical path compensation plate is 0.58×2s, and so on. The thickness of the nth optical path compensation plate is 0.58×ns.

[0016] A wavelength tuning method for a tunable spatial multiplexing multi-wavelength laser based on Ho:LLF is realized relying on the tunable spatial multiplexing multi-wavelength laser based on Ho:LLF as described above, and includes the following steps:

[0017] The pump light output by the 1940nm pump source is transmitted through the optical fiber and coupled by the fiber coupling lens group to output parallel light. After the parallel light is split by the microlens array, the polarization direction of the pump light is adjusted by the polarization beam splitter group, and then input into the plane mirror, so that the light in the 1940nm band is focused on the front end face of the Ho:LLF crystal cut along the c-axis;

[0018] After the Ho:LLF crystal cut along the c-axis absorbs the input light in the 1940nm band, it experiences stimulated absorption and stimulated transition in the resonant cavity and continuously gains, and then outputs laser light through the output plane mirror and the 1900nm narrowband filter;

[0019] Then, a group of polarization beam split prisms is used to separate the σ polarization and π polarization of the orthogonally polarized dual-wavelength lasers with different wavelengths generated by each branch, and multi-wavelength laser outputs of 2052 - 2056.3nm and 2063.1 - 2065.7nm are respectively realized in the horizontal direction and the vertical direction.

[0020] Furthermore, when the Ho:LLF absorbs the 1940nm laser, 5 I8 and 5 the energy level transition between I7 occurs. The multi-wavelength laser output by this laser system is actually 5 I8 and 5 the result of the transition from the upper energy level to the lower energy level between a large number of fine energy levels generated by the energy level splitting of I7. Here, the rate equation during the transition is simplified for analysis, and the obtained expression is:

[0021]

[0022] Among them, N1 is 5 the number of particles in the I8 energy level, N2 is 5 the number of particles in the I7 energy level, W 12 is the 1940nm laser pumping rate, W 21 is the output laser rate, σ 12 and σ21 is the emission or absorption cross-sectional area, and W has different values when the output laser wavelength is different 21 and σ 12 have different values, corresponding to different fine energy levels; P is the total power absorbed by the energy level transition, h is Planck's constant, A is the cross-sectional area of the light beam, υ P is the central frequency of the pump light, υ L is the central 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 non-radiative transition time;

[0023] Equations (1) and (2) are based on 5 the I8 energy level, 5 the rate equations for the increase and decrease of the number of particles in the I7 energy level. (3) and (4) respectively represent the expressions for the 1940 nm laser pumping rate and the output laser rate, and (5) represents the relationship between the relaxation time, spontaneous emission time, and non-radiative transition time of the particles in the energy level.

[0024] Furthermore, when there is an angular change α between the polarization direction of the incident light and the crystal principal axis, it causes wavelength changes in the σ polarization state and π polarization state output by the Ho:LLF crystal, and the obtained expression is:

[0025] λ σ (θ1) = λ σ (θ0) + Δλ σ (α) (6)

[0026] λ π (θ1) = λ π (θ0) + Δλ π (α) (7)

[0027] where θ0 is defined as the initial angle between the polarization direction of the input light and the crystal principal axis, and after rotating α, the angle becomes θ1. λ σ (θ0) and λ σ (θ1) are respectively defined as the central wavelengths output in the σ polarization state when the angles are θ0 and θ1. λ π (θ0) and λ π (θ1) are respectively defined as the central wavelengths output in the π polarization state when the angles are θ0 and θ1. Δλ σ (α) and Δλ π (α) represent the central wavelength change amounts of the σ polarization state and the π polarization state after rotating α;

[0028] The expression for the refractive index relationship in the plane of the a-axis and c-axis of the Ho:LLF crystal is:

[0029]

[0030] where n a and n c are the refractive indices of the a-axis and c-axis respectively, and n c (θ) is the refractive index when the polarization direction of the input light makes an angle θ with the crystal principal axis;

[0031] Based on the fact that the output light is restricted by the standing-wave condition of the resonator between the input mirror and the output mirror, the expression is obtained as:

[0032] 2kL' = 2mπ (9)

[0033] L' = n0L1 + n c (θ)L2 (10)

[0034] where k is the modulus of the wave vector, L’ is the total optical path, n0L1 and n c (θ)L2 represent the optical paths outside and inside the crystal in the resonator respectively, and m is the first positive integer; L1 and L2 represent the propagation distances of light outside and inside the crystal during oscillation in the resonator respectively;

[0035] Combining Equation (8), Equation (9) and Equation (10), we get:

[0036]

[0037] When the angle θ changes by α, Equation (6) changes to:

[0038]

[0039] where k' is the modulus of the wave vector after the change, and m' is the second positive integer;

[0040] Introducing the wavelength λ into Equation (12), the expression is obtained as:

[0041]

[0042] where m1' and m2' are the third positive integer and the fourth positive integer respectively, and θ π0 and θ σ0 represent the angles between the output σ polarization and π polarization and the crystal principal axis respectively; Based on Equation (8) and Equation (9), it is obtained that after introducing α, the variable Δλ π (α) / Δλ σ (α) and m1' / m2' will also change synergistically; When the polarizer is rotated, as θ changes, the refractive index n c (θ) changes, thus changing the optical path L’, and in order to satisfy the standing-wave condition, the modulus k of the wave vector will change accordingly, that is, the wavelength λ changes.

[0043] An adjustable spatial multiplexing multi-wavelength laser based on Ho:LLF, comprising a 795 nm LD pump source, an optical fiber, an optical fiber coupling lens group, a second input plane mirror, a Tm:YAP crystal cut along the b-axis, a second output plane mirror, a set of beam splitters, an optical path compensation plate, a polarizer group, an input plane mirror, a Ho:LLF crystal cut along the c-axis, an output plane mirror, a 1000 nm narrowband filter, a 1900 nm narrowband filter, and a set of polarization beam split prisms;

[0044] The 795 nm LD pump source is connected to the optical fiber coupling lens group through an optical fiber. On the right side of the optical fiber coupling lens group, the second input plane mirror, the Tm:YAP crystal cut along the b-axis, the second output plane mirror, a set of beam splitters, the optical path compensation plate, the polarizer group, the input plane mirror, the Ho:LLF crystal cut along the c-axis, the output plane mirror, the 1000 nm narrowband filter, the 1900 nm narrowband filter, and a set of polarization beam split prisms are arranged in sequence along the optical path;

[0045] The surface of the second input plane mirror is coated with a 1950 nm narrowband reflection film, and the surface of the second output plane mirror is coated with a 1950 nm 5% transmission film; the Ho:LLF crystal cut along the c-axis is in the shape of a cuboid;

[0046] In a set of beam splitters, the n-way beam splitting optical paths are numbered from bottom to top. The transmittance of the first beam splitter is 1 / n and the reflectance is (n - 1) / n. The transmittance of the second beam splitter is (n - 2) / (n - 1) and the reflectance is 1 / (n - 1). The transmittance of the third beam splitter is (n - 3) / (n - 2) and the reflectance is 1 / (n - 2)… The transmittance of the i-th beam splitter is (n - i) / (n - i + 1) and the reflectance is 1 / (n - i + 1);

[0047] The optical path compensation plate is set as a glass sheet inclined at 45° to the horizontal plane. The thickness of the glass sheet is determined by the distance s between the beam splitters. The thickness difference of the glass sheet between every two adjacent paths is 0.58×s. The optical path compensation plate is not placed in the uppermost optical path. In the order from top to bottom, the thickness of the first optical path compensation plate is 0.58×s, the thickness of the second optical path compensation plate is 0.58×2s, and so on. The thickness of the n-th optical path compensation plate is 0.58×ns.

[0048] Advantages of the present invention:

[0049] For the adjustable spatial multiplexing multi-wavelength laser based on Ho:LLF of the present invention, the multi-wavelength output in two bands in the horizontal and vertical directions is realized by using the rotation angle gradient design of multiple polarizers, and all laser wavelength components can be extracted separately. In addition, the multi-wavelength tuning function can be realized by adjusting the polarizer.

[0050] A tunable spatially multiplexed multi-wavelength laser based on Ho:LLF according to the present invention uses a Ho:LLF crystal and a pump source to generate multi-wavelength laser outputs in two bands of 2052 - 2056.3 nm and 2063.1 - 2065.7 nm in the horizontal and vertical directions respectively.

[0051] For the tunable spatially multiplexed multi-wavelength laser based on Ho:LLF according to the present invention, any wavelength component of the multi-wavelength laser can be split and independently output.

[0052] For the tunable spatially multiplexed multi-wavelength laser based on Ho:LLF according to the present invention, by simply rotating the angle of a polarizer, the output multi-wavelength laser can be tuned, and tunable multi-wavelength laser outputs in different bands can be achieved in the horizontal and vertical directions. Utilizing the characteristics of spatial multiplexing and the output characteristics of the Ho:LLF crystal, the space required for multi-wavelength output is optimized, realizing the design of a miniaturized multi-wavelength laser. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 FIG. 1 is a schematic diagram of the first structure of a tunable spatially multiplexed multi-wavelength laser based on Ho:LLF according to the present invention;

[0054] Figure 2 FIG. 2 is a schematic diagram of the structure of a c-axis cut Ho:LLF crystal according to the present invention;

[0055] Figure 3 FIG. 3 is a schematic diagram of the angle between the polarization direction of the input light and the crystal axis according to the present invention;

[0056] Figure 4 FIG. 4 is a schematic diagram of the refractive index ellipse of a c-axis cut Ho:LLF crystal according to the present invention;

[0057] Figure 5 FIG. 5 is a schematic diagram of the second structure of a tunable spatially multiplexed multi-wavelength laser based on Ho:LLF according to the present invention;

[0058] Figure 6 FIG. 6 is a schematic diagram of the third structure of a tunable spatially multiplexed multi-wavelength laser based on Ho:LLF according to the present invention;

[0059] Figure 7 FIG. 7 shows the wavelength change under angle tuning of a c-axis cut Ho:LLF crystal according to the present invention, where a is π polarization and b is σ polarization. DETAILED DESCRIPTION OF THE INVENTION

[0060] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention, that is, the specific embodiments described are only a part of the embodiments of the present invention, rather than all of the specific embodiments. The components of the specific embodiments of the present invention usually described and shown in the drawings here can be arranged and designed in various different configurations, and the present invention can also have other embodiments.

[0061] Therefore, the following detailed description of the specific embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but merely represents the selected specific embodiments of the present invention. Based on the specific embodiments of the present invention, all other specific embodiments obtained by those skilled in the art without creative efforts fall within the scope of protection of the present invention.

[0062] To further understand the content, features and effects of the present invention, the following specific embodiments are exemplified and described in detail in conjunction with the attached Figure 1 - Attached Figure 7 as follows:

[0063] Example 1:

[0064] A tunable spatial multiplexing multi-wavelength laser based on Ho:LLF includes a 1940 nm pump source 1, an optical fiber 2, an optical fiber coupling lens group 3, a microlens array 4, a polarizer group 5, an input plane mirror 6, a c-axis cut Ho:LLF crystal 7, an output plane mirror 8, a 1900 nm narrowband filter 9, and a group of polarization beam split prisms 10;

[0065] The 1940 nm pump source 1 is connected to the optical fiber coupling lens group 3 through the optical fiber 2, and on the right side of the optical fiber coupling lens group 3, the microlens array 4, the polarizer group 5, the input plane mirror 6, the c-axis cut Ho:LLF crystal 7, the output plane mirror 8, the 1900 nm narrowband filter 9, and a group of polarization beam split prisms 10 are arranged in sequence along the optical path;

[0066] The rotation angle of the polarizer group is set to vary in a gradient of 0 - 90°.

[0067] Furthermore, the surface of the input plane mirror 6 is coated with a 2050 nm narrowband reflection film, the surface of the output plane mirror 8 is coated with a 5% transmission film of 2050 nm, and the c-axis cut Ho:LLF crystal 7 is in the shape of a cuboid.

[0068] Furthermore, the microlens array 4 is composed of microlenses, and the focal length of the convex lens corresponds to the position of the c-axis cut Ho:LLF crystal 7.

[0069] Furthermore, ensure that each beam of the split laser is focused on the crystal end face. During operation, the placement position of the Ho:LLF crystal should be adjusted according to the focal length of the microlens array; the 1940 nm pump light realizes multi-channel output through the microlens array, and linearly polarized light with different polarization directions is output through a series of polarizers with different rotation angles. The polarizer is an extremely important device in this design. The reason for the output of different wavelengths according to different rotation angles of the polarizer is as follows: When the laser with different polarization directions output by different polarizers is input to the end face of the c-axis cut Ho:LLF, according to different rotation angles, the refractive index corresponding to the polarization direction of the light wave also changes. Considering the special energy level structure of the Ho:LLF crystal, it has the ability to output dual-wavelength lasers under the pumping condition of 1940 nm. When the polarization direction of the incident light generates an angular change α with the main axis of the crystal, this leads to wavelength changes in the σ polarization state and π polarization state output by the Ho:LLF crystal.

[0070] Furthermore, the function of the 1900 nm narrowband filter is to filter out the 1940 nm pump light, so that only the output component of the Ho:LLF crystal remains in the laser.

[0071] Furthermore, due to the ingenious design of the structure and the innovative application of the polarizer, according to the gradient rotation of the polarizer, multi-wavelength laser outputs in two bands are realized in the vertical and horizontal directions respectively, and each wavelength component of the output can be separately separated.

[0072] Embodiment 2:

[0073] A wavelength tuning method for a tunable spatially multiplexed multi-wavelength laser based on Ho:LLF is realized relying on the tunable spatially multiplexed multi-wavelength laser based on Ho:LLF described in Embodiment 1, and includes the following steps:

[0074] The 1940 nm pump source outputs pump light. The pump light is transmitted through an optical fiber and coupled by an optical fiber coupling lens group to output parallel light. After the parallel light is split by the microlens array, the polarization direction of the pump light is adjusted by the polarizer group, and then input to the plane mirror, and the light in the 1940 nm band is focused on the front end face of the c-axis cut Ho:LLF crystal;

[0075] After the c-axis cut Ho:LLF crystal absorbs the input light in the 1940 nm band, it experiences stimulated absorption and stimulated transition in the resonant cavity and continuously gains, and then outputs laser through the output plane mirror and the 1900 nm narrowband filter;

[0076] Then, a group of polarization beam splitters are used to separate the σ polarization and π polarization of the orthogonally polarized dual-wavelength lasers with different wavelengths generated by each branch, and multi-wavelength laser outputs at 2052 - 2056.3 nm and 2063.1 - 2065.7 nm are realized in the horizontal and vertical directions respectively.

[0077] Furthermore, when the Ho:LLF absorbs the 1940 nm laser, the 5 energy level transition between I8 and 5 I7 occurs. The multi-wavelength laser output by this laser system is actually 5 the result of the transition from the upper energy level to the lower energy level between a large number of fine energy levels generated by the energy level splitting of I8 and 5 I7. Here, the rate equation during the transition is simplified for analysis, and the obtained expression is:

[0078]

[0079] Among them, N1 is the 5 population of the I8 energy level, N2 is the 5 population of the I7 energy level, W 12 is the pumping rate of the 1940 nm laser, W 21 is the output laser rate, σ 12 and σ 21 are the emission or absorption cross-sectional areas. When the output laser wavelength is different, the values of W 21 and σ 12 are different, corresponding to different fine energy levels; P is the total power absorbed by the energy level transition, h is Planck's constant, A is the cross-sectional area of the light beam, υ P is the central frequency of the pump light, υ L is the central 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 non-radiative transition time;

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

[0081] Furthermore, when there is an angular change amount α between the polarization direction of the incident light and the crystal principal axis, it causes wavelength changes in the σ polarization state and π polarization state output by the Ho:LLF crystal, and the obtained expression is:

[0082] λσ λ(θ1) = σ λ(θ0) + Δλ σ (α)(6)

[0083] λ π λ(θ1) = π λ(θ0) + Δλ π (α)(7)

[0084] Where θ0 is defined as the initial angle between the polarization direction of the input light and the principal axis of the crystal. After rotating by α, the angle becomes θ1. λ(θ0) and λ(θ1) are respectively defined as the central wavelengths of the output in the σ polarization state when the angles are θ0 and θ1. λ(θ0) and λ(θ1) are respectively defined as the central wavelengths of the output in the π polarization state when the angles are θ0 and θ1. Δλ(α) and Δλ(α) represent the change in the central wavelength of the σ polarization state and the change in the central wavelength of the π polarization state after rotating by α; σ λ(θ0) and λ σ (θ1) π λ(θ0) and λ π (θ1) σ Δλ(α) and Δλ π (α) represent the change in the central wavelength of the σ polarization state and the change in the central wavelength of the π polarization state after rotating by α;

[0085] The expression for the refractive index relationship in the plane of the a-axis and c-axis of the Ho:LLF crystal is:

[0086]

[0087] Where n a and n c are the refractive indices of the a-axis and c-axis respectively. n(θ) is the refractive index when the angle between the polarization direction of the input light and the principal axis of the crystal is θ; c (θ) is the refractive index when the angle between the polarization direction of the input light and the principal axis of the crystal is θ;

[0088] Based on the fact that the output light is restricted by the standing wave condition of the resonator between the input mirror and the output mirror, the expression is obtained as:

[0089] 2kL' = 2mπ (9)

[0090] L' = n0L1 + n c (θ)L2 (10)

[0091] Where k is the modulus of the wave vector, L’ is the total optical path, n0L1 and n(θ)L2 respectively represent the optical paths outside and inside the crystal in the resonator, and m is the first positive integer; L1 and L2 respectively represent the propagation distances of the light outside and inside the crystal during oscillation in the resonator; c (θ)L2 respectively represent the optical paths outside and inside the crystal in the resonator, and m is the first positive integer; L1 and L2 respectively represent the propagation distances of the light outside and inside the crystal during oscillation in the resonator;

[0092] Combining equations (8), (9) and (10), we get:

[0093]

[0094] When the angle θ changes by α, Equation (6) changes to:

[0095]

[0096] where k' is the magnitude of the wave vector after the change, and m' is the second positive integer;

[0097] Introducing the wavelength λ into Equation (12), the resulting expression is:

[0098]

[0099] where m1' and m2' are the third positive integer and the fourth positive integer respectively, and θ π0 and θ σ0 respectively represent the angles between the output σ polarization and π polarization and the crystal principal axis; Based on Equations (8) and (9), it is obtained that after introducing α, the variable Δλ π (α) / Δλ σ (α) and m1' / m2' will also change synergistically; When rotating the polarizer, as θ changes, the refractive index n c (θ) changes, thus changing the optical path L', and in order to satisfy the standing wave condition, the magnitude of the wave vector k will change accordingly, that is, the wavelength λ changes.

[0100] Furthermore, the variables Δλπ(α) / Δλσ(α) and m' / n' will also change synergistically. Under the condition of satisfying the equation, a wide range of frequencies can be output. According to Walsh.B.M et al. in "Branching ratios, cross sections, and radiative lifetimes of rare earth ions in solids::Application to Tm3+ and Ho3+ ions in LiYF4", the absorption and emission spectra of the σ / π polarization states of Ho:LLF crystals are presented. The Ho:LLF crystal exhibits a broad emission wavelength range at approximately 2050 nm, which is consistent with the theoretical derivation.

[0101] Furthermore, according to the above derivation, it can be seen that when the polarizer rotates, as θ changes, the refractive index nc(θ) changes, thus changing the optical path L'. In order to satisfy the standing wave condition, the magnitude of the wave vector k will change accordingly, that is, the wavelength λ changes. This angular tuning refers to changing the angle between the polarization state of the 1940 nm laser (the pump light of the Ho:LLF crystal) and the crystal axis direction. That is to say, each part of the Ho:LLF crystal outputs lasers with different wavelengths due to the change in the polarization direction, and due to the characteristics of the Ho:LLF crystal, the lasers in each path are orthogonally polarized dual-wavelength lasers. Through the design of the polarizer angle gradient, an orthogonally polarized dual-wavelength laser group with a monotonically changing output wavelength is realized for each optical path, achieving angularly controlled wavelength tuning.

[0102] It can be deduced that the change in the angle between the polarization direction of the pump light and the crystal principal axis of the Ho:LLF crystal affects the wavelength of the output laser. Therefore, by rotating the angle of the polarizer, the wavelength tuning of each output laser can be achieved. After the orthogonally polarized dual-wavelength lasers with different wavelengths of each output pass through the polarization beam splitting prism, the dual-wavelength components are separated from each other, and each wavelength component of the multi-wavelength laser is output separately. Figure 7 It shows the wavelength change of the output laser under the angle tuning of 13° - 80°, which is the same as the result of theoretical derivation. Among them, the wavelength change range of the π polarization output in the horizontal direction is 2053 - 2056.3 nm, and the wavelength change range of the σ polarization output in the vertical direction is 2063.1 - 2065.7 nm. The test results verify the feasibility of the device.

[0103] Example 3:

[0104] The difference between this example and Example 1 is that the micro-lens array 4 is replaced by a group of beam splitters 11 and an optical path compensation plate 12.

[0105] Furthermore, the n beam splitting optical paths in a group of beam splitters 11 are designed with numbers from bottom to top. The transmittance of the first beam splitter is 1 / n and the reflectance is (n - 1) / n. The transmittance of the second beam splitter is (n - 2) / (n - 1) and the reflectance is 1 / (n - 1). The transmittance of the third beam splitter is (n - 3) / (n - 2) and the reflectance is 1 / (n - 2)… The transmittance of the i-th beam splitter is (n - i) / (n - i + 1) and the reflectance is 1 / (n - i + 1);

[0106] For example, if 5 beam splitters are used to split 5 optical paths, the transmittance of the first beam splitter in the 2μm band is 20% and the reflectance is 80%. The transmittance of the second beam splitter is 75% and the reflectance is 25%. The transmittance of the third beam splitter is 66.7% and the reflectance is 33.3%. The transmittance of the fourth beam splitter is 50% and the reflectance is 50%. The transmittance of the fifth beam splitter is 0 and the reflectance is 100%.

[0107] Furthermore, the optical path compensation plate 12 is set as a glass sheet inclined at 45° to the horizontal plane. The thickness of the glass sheet is determined by the spacing s between the beam splitters. The thickness difference of the glass sheet between every two adjacent optical paths is 0.58×s. The optical path compensation plate is not placed in the uppermost optical path. In the order from top to bottom, the thickness of the first optical path compensation plate is 0.58×s, the thickness of the second compensation plate is 0.58×2s, and so on. The thickness of the n-th optical path compensation plate is 0.58×ns.

[0108] Example 4:

[0109] The difference between this embodiment and Embodiment 3 is that the 1940 nm pump source 1 is replaced by a 795 nm LD pump source 13, and the tunable space-division multiplexing multi-wavelength laser based on Ho:LLF is arranged such that the 795 nm LD pump source 13 is connected to a fiber coupling lens group 3 through an optical fiber 2. On the right side of the fiber coupling lens group 3 along the optical path, a second input plane mirror 14, a b-axis cut Tm:YAP crystal 15, a second output plane mirror 16, a set of beam splitters 11, an optical path compensation plate 12, a polarizer group 5, an input plane mirror 6, a c-axis cut Ho:LLF crystal 7, an output plane mirror 8, a 1000 nm narrowband filter 17, a 1900 nm narrowband filter 9, and a set of polarization beam split prisms 10 are arranged in sequence.

[0110] The surface of the second input plane mirror 14 is coated with a 1950 nm narrowband reflection film, and the surface of the second output plane mirror 16 is coated with a 1950 nm narrowband reflection film.

[0111] Furthermore, considering that the 795 nm LD pump source is more common and the system is more mature in practical applications, the Tm:YAP crystal is pumped by the 795 nm LD pump source to output 1940 nm laser, and the Ho:LLF crystal is pumped by this laser. In this scheme, an additional 1000 nm narrowband filter (17) is required to filter out the 795 nm pump light.

[0112] It should be noted that relational terms such as "first" and "second" are only used 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 "include", "comprise" or any other variation thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not expressly listed, or also includes elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "including a..." does not exclude the existence of additional identical elements in the process, method, article or device including the element.

[0113] Although the present application has been described above with reference to specific embodiments, various improvements can be made thereto and components thereof can be replaced with equivalents without departing from the scope of the present application. In particular, as long as there is no structural conflict, the features in the specific embodiments disclosed in the present application can be combined with each other in any way, and the exhaustive description of these combinations is not given in this specification only for the consideration of saving space and resources. Therefore, the present application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A tunable spatial multiplexing multi-wavelength laser based on Ho:LLF, characterized in that: It includes a 1940nm pump source (1), an optical fiber (2), a fiber-coupled lens group (3), a microlens array (4), a polarizer group (5), an input plane mirror (6), a c-axis cut Ho:LLF crystal (7), an output plane mirror (8), a 1900nm narrowband filter (9), and a group of polarization beam splitting prisms (10); The 1940 nm pump source (1) is connected to a fiber coupling lens group (3) via an optical fiber (2); a microlens array (4), a polarizing plate group (5), an input plane mirror (6), a c-axis cut Ho:LLF crystal (7), an output plane mirror (8), a 1900 nm narrowband filter (9), and a group of polarization beam splitting prisms (10) are sequentially arranged on the right side of the fiber coupling lens group (3) along the optical path; The rotation angle of the polarizer group is set to change gradually from 0 to 90 degrees.

2. The tunable spatial multiplexing multi-wavelength laser based on Ho:LLF according to claim 1, characterized in that: The surface of the input plane mirror (6) is coated with a 2050nm narrow-band reflection film, the surface of the output plane mirror (8) is coated with a 2050nm 5% transmission film, and the c-axis cut Ho:LLF crystal (7) is in a rectangular parallelepiped shape.

3. A tunable spatial multiplexing multi-wavelength laser based on Ho:LLF according to claim 1 or 2, characterized in that: The microlens array (4) is composed of micro-convex lenses, and the focal length of the convex lenses corresponds to the position of the front end surface of the c-axis cut Ho:LLF crystal (7).

4. The tunable spatial multiplexing multi-wavelength laser based on Ho:LLF according to claim 3, characterized in that: The microlens array (4) is replaced by a group of beam splitters (11) and an optical path compensation plate (12).

5. The tunable spatial multiplexing multi-wavelength laser based on Ho:LLF according to claim 4, characterized in that: The n splitting optical paths in a group of splitters (11) are numbered from bottom to top, the transmittance of the first splitter is 1 / n and the reflectance is (n-1) / n, the transmittance of the second splitter is (n-2) / (n-1) and the reflectance is 1 / (n-1), the transmittance of the third splitter is (n-3) / (n-2) and the reflectance is 1 / (n-2)... the transmittance of the i-th splitter is (ni) / (n-i+1) and the reflectance is 1 / (n-i+1).

6. The tunable spatial multiplexing multi-wavelength laser based on Ho:LLF according to claim 5, characterized in that: The optical path compensation plate (12) is set as a glass sheet placed at an angle of 45 degrees to the horizontal plane. The thickness of the glass sheet is determined by the spacing s between the beam splitters. The thickness difference of the glass sheet between each two adjacent paths is 0.58×s. No optical path compensation plate is placed on the top optical path. In order from top to bottom, the thickness of the first optical path compensation plate is 0.58×s, the thickness of the second optical path compensation plate is 0.58×2s, and so on. The thickness of the nth optical path compensation plate is 0.58×ns.

7. A wavelength tuning method for a tunable spatial multiplexing multi-wavelength laser based on Ho:LLF, implemented by a tunable spatial multiplexing multi-wavelength laser based on Ho:LLF according to any one of claims 1 to 6, characterized in that: The steps include: The 1940nm pump source outputs pump light, which is transmitted through the optical fiber and then coupled into the optical fiber coupling lens group to output parallel light. After the parallel light is split by the microlens array, the polarization direction of the pump light is adjusted by the polarizer group, and then input into the plane mirror to obtain the light in the 1940nm band, which is focused on the front end face of the c-axis cut Ho:LLF crystal. After the c-axis cut Ho:LLF crystal absorbs the input 1940nm band light, it undergoes stimulated absorption and stimulated transition in the resonant cavity and continuously gains gain, and then outputs multi-wavelength laser through the output plane mirror and 1900nm narrow-band filter. Then, a set of polarization beam splitting prisms is used to separate the σ polarization and π polarization of the orthogonally polarized dual-wavelength lasers of different wavelengths generated by each branch, and multi-wavelength laser output of 2052-2056.3nm and 2063.1-2065.7nm is achieved in the horizontal direction and vertical direction respectively.

8. The wavelength tuning method of a tunable spatial multiplexing multi-wavelength laser based on Ho:LLF according to claim 7, characterized in that Ho :When 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 that the rate equation of the transition is simplified here to obtain the expression: 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 21 is 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 Planck's 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; Formulas (1) and (2) are based on 5 I8 energy level, 5 The rate equation for the increase and decrease of the number of particles at the I7 energy level, (3) and (4) represent the expressions of the 1940nm laser pumping rate and the output laser rate, respectively, and (5) represents the relationship between the relaxation time of particles at the energy level and the spontaneous radiation time and the radiationless transition time.

9. The wavelength tuning method of a tunable spatial multiplexing multi-wavelength laser based on Ho:LLF according to claim 8, characterized in that: When the angle between the polarization direction of the incident light and the principal axis of the crystal changes by an amount α, the wavelength of the σ polarization state and the π polarization state output by the Ho:LLF crystal changes, and the expression is: l σ (θ1)=λ σ (θ0)+Δλ σ (a)(6) l π (θ1)=λ π (θ0)+Δλ π (a)(7) Among them, θ0 is defined as the initial angle between the polarization direction of the input light and the main axis of the crystal. After rotating α, the angle becomes θ1, λ σ (θ0) and λ σ (θ1) are defined as the central wavelength of the σ polarization state when the angle is θ0 and θ1, λ π (θ0) and λ π (θ1) are defined as the central wavelength of the π polarization state when the angle is θ0 and θ1, 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 Ho:LLF 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 and the crystal principal axis include an angle θ; Based on the fact that the output light is subject to the resonant cavity standing wave condition between the input mirror and the output mirror, the expression is obtained: 2kL'=2mπ(9) <h2 style=";text-align:left;direction:ltr">L'=n0L1+n<h2 style=";text-align:left;direction:ltr"> c <h2 style=";text-align:left;direction:ltr"> (θ)L2(10) 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 path of light outside and inside the crystal when oscillating in the resonant cavity, respectively; Combining equation (8), equation (9) and equation (10), 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 (12), we get the following expression: Where m1' and m2' are the third positive integer and the fourth positive integer respectively, θ π0 and θ σ0 Respectively represent the angles between the output σ polarization and π polarization and the crystal axis; Based on equations (8) and (9), after introducing α, the variable Δλ π (α) / Δλ σ (α) and m1' / m2' also change in tandem; when the polarizer is rotated, the refractive index n changes with the change of θ. c (θ) changes, thus changing the optical path L', and the modulus k of the wave vector will change accordingly, that is, the wavelength λ changes.

10. A tunable spatial multiplexing multi-wavelength laser based on Ho:LLF, characterized in that: It comprises a 795 nm LD pump source (13), an optical fiber (2), an optical fiber coupling lens group (3), a second input plane mirror (14), a b-axis cut Tm:YAP crystal (15), a second output plane mirror (16), a group of splitting mirrors (11), an optical path compensation plate (12), a polarizing plate group (5), an input plane mirror (6), a c-axis cut Ho:LLF crystal (7), an output plane mirror (8), a 1000 nm narrowband filter (17), a 1900 nm narrowband filter (9), and a group of polarization beam splitting prisms (10); The 795 nm LD pump source (13) is connected to a fiber coupling lens group (3) via an optical fiber (2); a second input plane mirror (14), a b-axis cut Tm:YAP crystal (15), a second output plane mirror (16), a group of splitting mirrors (11), an optical path compensation plate (12), a polarizer group (5), an input plane mirror (6), a c-axis cut Ho:LLF crystal (7), an output plane mirror (8), a 1000 nm narrowband filter (17), a 1900 nm narrowband filter (9), and a group of polarization beam splitting prisms (10) are arranged in sequence on the right side of the fiber coupling lens group (3) along the optical path; The surface of the second input plane mirror (14) is coated with a 1950nm narrow-band reflection film, and the surface of the second output plane mirror (16) is coated with a 1950nm narrow-band reflection film; the c-axis cut Ho:LLF crystal (7) is in a rectangular parallelepiped shape; The n-way splitting optical paths in a group of splitting mirrors (11) are numbered from bottom to top, the transmittance of the first splitting mirror is 1 / n, and the reflectance is (n-1) / n, the transmittance of the second splitting mirror is (n-2) / (n-1), and the reflectance is 1 / (n-1), the transmittance of the third splitting mirror is (n-3) / (n-2), and the reflectance is 1 / (n-2)... The transmittance of the i-th splitting mirror is (ni) / (n-i+1), and the reflectance is 1 / (n-i+1); The optical path compensation plate (12) is set as a glass sheet placed at an angle of 45 degrees to the horizontal plane. The thickness of the glass sheet is determined by the spacing s between the beam splitters. The thickness difference of the glass sheet between each two adjacent paths is 0.58×s. No optical path compensation plate is placed on the top optical path. In order from top to bottom, the thickness of the first optical path compensation plate is 0.58×s, the thickness of the second optical path compensation plate is 0.58×2s, and so on. The thickness of the nth optical path compensation plate is 0.58×ns.

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