Space-time coherence adjustable light source based on external cavity feedback and degenerate cavity

Abstract

The invention relates to the technical field of laser devices, and provides a space-time coherence adjustable light source based on external cavity feedback and a degenerate cavity, the space-time coherence adjustable light source comprises a degenerate cavity laser and an external feedback assembly which are coaxially arranged, and the degenerate cavity laser comprises a first reflector, a first lens, a variable aperture diaphragm, a second lens and an output coupling mirror which are sequentially arranged. According to the invention, the adjustable space-time low-coherence light source can be realized in a specific application scene, and the laser use effect is improved.

Description

Technical Field

[0001] The present application relates to the technical field of laser devices, and in particular to a spatiotemporal coherence tunable light source based on external cavity feedback and a degenerate cavity. Background Art

[0002] Traditional laser is a light source with high temporal and spatial coherence, which enables laser to produce focusing, interference and other precision optical phenomena, but high temporal and spatial coherence can have a negative impact on certain applications. For example, in space laser communication, high coherence light beams are greatly affected by atmospheric disturbances, making the light intensity distribution uneven, resulting in an increase in the bit error rate at the receiving end; in the field of optical imaging, high coherence causes severe speckle in the imaging pattern, reducing the imaging quality; in laser nuclear fusion, the high coherence of strong lasers will enhance the nonlinear local interaction between lasers and plasma, seriously restricting the beam-target coupling performance; in laser heat treatment, the interference caused by high coherence leads to a rough processing surface.

[0003] Therefore, in order to improve the above situation in application fields such as space laser communication, optical imaging, and laser heat treatment, there is an urgent need to develop low-coherence light sources.

[0004] Application Contents

[0005] The purpose of the present application is to provide a spatiotemporal coherence tunable light source based on external cavity feedback and a degenerate cavity to reduce the coherence of the light source and improve various aspects of the use of the laser in specific application scenarios.

[0006] The embodiments of the present application are implemented through the following technical solutions: a spatiotemporal coherence tunable light source based on external cavity feedback and a degenerate cavity, comprising a coaxially arranged degenerate cavity laser and an external feedback component, wherein the degenerate cavity laser comprises a first reflector, a first lens, a variable aperture diaphragm, a second lens and an output coupling mirror arranged in sequence.

[0007] Furthermore, the variable aperture diaphragm includes a cover ring, an arc-shaped piece and a mounting ring stacked in sequence, one end of the arc-shaped piece is equipped with a rotating shaft for rotationally connecting to the end face of the cover ring; a plurality of adjustment grooves are arranged around the mounting ring, and the other end of the arc-shaped piece is inserted into the adjustment groove to slide with it; and a light-transmitting hole is formed in the middle of a plurality of the arc-shaped pieces.

[0008] Furthermore, an adjustment handle is provided on the outer edge surface of the mounting ring.

[0009] Furthermore, the external feedback component includes a non-polarizing cubic beam splitter, an adjustable neutral density attenuation plate and a second reflector, and the non-polarizing cubic beam splitter is arranged close to one side of the output coupling mirror.

[0010] Furthermore, the degenerate cavity laser further includes a pump source and a gain medium, wherein the pump source and the gain medium are arranged between the first reflector and the first lens, and the pump source is arranged above the gain medium.

[0011] Furthermore, the first lens and the second lens are plano-convex lenses or bi-convex lenses.

[0012] Furthermore, the focal lengths of the first lens and the second lens are 20 cm or 30 cm.

[0013] The technical solution of the embodiments of the present application has at least the following advantages and beneficial effects: the present application starts with the regulation of spatial spectrum and temporal spectrum, adopts a degenerate cavity structure, and uses a variable aperture diaphragm to achieve the regulation of spatial coherence, and adopts an external cavity feedback structure and uses an adjustable neutral density attenuation plate to achieve the regulation of temporal coherence. The two together construct an adjustable spatiotemporal low-coherence light source, which is helpful for applying lasers in different fields to achieve better results. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present application and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without paying creative work.

[0015] Figure 1 A schematic diagram of the structure of a spatiotemporal coherence tunable light source based on external cavity feedback and a degenerate cavity provided in an embodiment of the present application;

[0016] Figure 2 Schematic diagram of the structure of the variable aperture diaphragm in this application;

[0017] Figure 3 An exploded view of the variable aperture diaphragm in this application; Figure 4 A schematic diagram of the near-field distribution and the number of modes of the light field generated as the aperture d of the pinhole stop increases in the present application; like Figure 5 Schematic diagram of the speckle image and speckle contrast C as the aperture d of the pinhole stop increases in the present application.

[0018] Icons: 1-degenerate cavity laser, 11-first reflector, 12-gain medium, 13-pump source, 14-first lens, 15-variable aperture diaphragm, 151-cover ring, 152-arc mosaic, 1521-rotating shaft, 1522-light-transmitting hole, 153-mounting ring, 1531-adjusting slide groove, 1532-adjusting handle, 16-second lens, 17-output coupling mirror, 2-external feedback component, 21-non-polarizing cubic beam splitter, 22-adjustable neutral density attenuator, 23-second reflector. DETAILED DESCRIPTION

[0019] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. The components of the embodiments of the present application described and shown in the drawings here can be arranged and designed in various different configurations.

[0020] Therefore, the following detailed description of the embodiments of the present application provided in the accompanying drawings is not intended to limit the scope of the present application for which protection is sought, but merely represents selected embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in the field without creative work are within the scope of protection of the present application.

[0021] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.

[0022] Example

[0023] The following is further described in conjunction with specific embodiments. Figure 1-Figure 3As shown, this embodiment is a spatiotemporal coherence tunable light source based on external cavity feedback and degenerate cavity, including a coaxially arranged degenerate cavity laser 1 and an external feedback component 2, the degenerate cavity laser 1 includes a first reflector 11, a first lens 14, a variable aperture diaphragm 15, a second lens 16 and an output coupling mirror 17 arranged in sequence, the degenerate cavity laser 1 also includes a pump source 13 and a gain medium 12, the pump source 13 and the gain medium 12 are arranged between the first reflector 11 and the first lens 14, and the pump source 13 is arranged above the gain medium 12; specifically, the first reflector 11 is installed on the front focal plane of the first lens 14, the first reflector 11 and the output coupling mirror 17 form a laser resonant cavity, and the gain amplification of the laser is achieved by reflecting the light back and forth, and the gain is increased. The beneficial medium 12 provides an energy level structure with inversion of the particle number. Under the stimulation of external photons, the electrons undergo stimulated radiation transitions between energy levels to generate lasers. The pump source 13 serves as an excitation source to pump the particles from the low energy level to the high energy level to achieve particle number inversion. The first lens 14 and the second lens 16 form a self-imaging structure, so that the resonant cavity is a self-imaging cavity. All transverse modes are intrinsic modes of the cavity, have the same Q factor, and can output spatially partially coherent light. The variable aperture 15 has a greater loss in the high-order transverse mode than the low-order transverse mode in the spectrum, and is used to perform transverse selection of the transverse mode, adjust the number of transverse modes, and regulate the spatial coherence dynamics of the degenerate cavity laser 1. The degenerate cavity structure is adopted, and the variable aperture 15 is used to adjust the spatial coherence, thereby reducing the coherence.

[0024] Spatial coherence describes the phase relationship between points on the wave surface perpendicular to the propagation direction of the light beam. It refers to the coherence of the light field at different spatial points in the light field at the same time, which can be described by the coherence width d. That is, λ is the laser wavelength, and θ is the plane divergence angle of the light beam. The better the directivity of the light beam, the better its spatial coherence. For a laser, all photons belonging to the same transverse mode are spatially coherent, and photons that do not belong to the same transverse mode are incoherent. Therefore, the spatial coherence of the laser is determined by its transverse mode structure. The laser with a single transverse mode is completely coherent, and the coherence of the multi-transverse mode light beam deteriorates. At the same time, the directionality of a single transverse mode light beam is the best, and the higher the transverse mode order, the worse the directionality. It can be seen that the spatial coherence of the light beam and its directionality (described by the beam divergence angle) are closely related. The resonant frequency ν of the eigenmode of the resonant cavity qmn for

[0025]

[0026] The subscript q represents the longitudinal mode number of the eigenmode, the subscripts m and n represent the two transverse mode numbers of the eigenmode, c is the speed of light in vacuum, and L is the optical length of the resonant cavity. In a degenerate cavity, since g1g2 = 1, This shows that there are countless transverse modes independently oscillating at the same time under the same longitudinal mode. The number of transverse modes can be adjusted by adjusting the aperture size of the pinhole aperture 15. The larger the aperture, the more transverse modes there are, the worse the directivity of the light beam, and the lower the spatial coherence. Therefore, the spatial coherence of the light beam can be controlled by changing the pinhole aperture.

[0027] In addition, the gain medium 12 can be made of Nd:YAG, Nd:YLF, Nd:YVO4 or other Nd 3+ The doped gain medium 12 has a concentration of 0.8 at%-1.2 at%, a thickness of 1-3 mm, and a cylindrical or square structure;

[0028] The gain medium 12 and the pump source 13 as a whole can be installed between the first lens 14 and the variable aperture diaphragm 15 , between the variable aperture diaphragm 15 and the second lens 16 , and between the second lens 16 and the output coupling mirror 17 .

[0029] Reference Figure 2 and Figure 3 As shown, the variable aperture diaphragm 15 in this embodiment includes a cover ring 151, an arc-shaped piece 152 and a mounting ring 153 which are stacked in sequence. A rotating shaft 1521 is installed at one end of the arc-shaped piece 152 for rotationally connecting with the end face of the cover ring 151. A plurality of adjusting grooves 1531 are arranged around the mounting ring 153. The other end of the arc-shaped piece 152 is inserted into the adjusting groove 1531 to slide with it. A light-transmitting hole 1522 is formed in the middle of the plurality of arc-shaped pieces 152.

[0030] In order to facilitate the aperture adjustment operation, an adjustment handle 1532 is provided on the outer edge surface of the mounting ring 153. By turning the adjustment handle 1532 and rotating it circumferentially by a certain angle, the size of the middle light-transmitting hole 1522 can be changed to meet the usage requirements.

[0031] like Figure 1As shown, the external feedback component 2 in this embodiment includes a non-polarizing cubic beam splitter 21, an adjustable neutral density attenuator 22 and a second reflector 23. The non-polarizing cubic beam splitter 21 is arranged close to the output coupling mirror 17. Specifically, in order to achieve the regulation of temporal coherence, the non-polarizing cubic beam splitter 21 divides the light beam into two beams, one for outputting a spatiotemporal coherence adjustable light beam, and the other for external feedback. The adjustable neutral density attenuator 22 is used to adjust the feedback intensity and regulate the temporal coherence dynamics of the degenerate cavity laser 1. The second reflector 23 is used to reflect the light beam back into the resonant cavity. Since temporal coherence refers to the coherence between light wave fields at different times in the same space. The coherence time describes the maximum time interval that can maintain the coherence of the light wave field at a certain point in the direction of light propagation. It represents the coherence of the wave train emitted by the light source within a limited duration. Since the narrow line width means that the frequency of the light wave is stable, the increase in the line width of the light source will extend the coherence time, thereby increasing the ability of the light wave field to maintain coherence. The external feedback component 2 generates an external cavity mode in the cavity through delayed self-feedback, which can be used to broaden the spectral linewidth of the laser. Under steady-state conditions, the characteristic equation of the external cavity mode is:

[0032] ω s -ω0-κsin(ω s τ)=0

[0033] Among them, ω s is the steady-state angular frequency of the laser output, ω0 is the isolated frequency of the degenerate cavity laser, which corresponds to the eigenfrequency of the laser output without external cavity feedback, κ is the feedback intensity, which can be adjusted by the adjustable neutral density attenuator 22 and the external cavity length, is the round trip delay of the laser in the external cavity, L is the cavity length, c is the speed of light. As the feedback intensity increases, the number of steady-state angular frequencies of the laser increases, the range expands, the bandwidth Δν of the spectrum increases, and the coherence time Therefore, the temporal coherence decreases with the increase of feedback strength.

[0034] The positions of the non-polarizing cubic beam splitter 21 and the adjustable neutral density attenuator 22 can be exchanged; the distances among the output coupling mirror 17, the non-polarizing cubic beam splitter 21, the adjustable neutral density attenuator 22 and the second reflector 23 can be changed between 1 and 10 cm.

[0035] It is worth noting that the first lens 14 and the second lens 16 are plano-convex lenses or bi-convex lenses; and the focal lengths of the first lens 14 and the second lens 16 are 20 cm or 30 cm, and their diameters are 25.4 cm and 50.8 cm, respectively, and they need to be coated with a high transmittance film for a wavelength of 1064 nm.

[0036] like Figure 4As shown in the figure, the near-field distribution and mode number N of the light field generated as the aperture d of the pinhole aperture increases in this embodiment, as the pinhole aperture increases, the number of high-order modes supported by the cavity increases, and the output spot area increases.

[0037] like Figure 5 As shown in FIG. 1 , the speckle image and the speckle contrast C increase with the increase of the pinhole aperture d in this embodiment. As the pinhole aperture increases, the number of independent modes increases, the spatial coherence decreases, and the speckle contrast decreases.

[0038] The above are only preferred embodiments of the present application and are not intended to limit the present application. For those skilled in the art, the present application may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A spatiotemporal coherence tunable light source based on external cavity feedback and degenerate cavity, characterized in that: The invention comprises a coaxially arranged degenerate cavity laser (1) and an external feedback component (2), wherein the degenerate cavity laser (1) comprises a first reflecting mirror (11), a first lens (14), a variable aperture diaphragm (15), a second lens (16) and an output coupling mirror (17) which are arranged in sequence.

2. The spatiotemporal coherence tunable light source based on external cavity feedback and degenerate cavity according to claim 1, characterized in that: The variable aperture diaphragm (15) comprises a cover ring (151), an arc-shaped patch (152) and a mounting ring (153) which are stacked in sequence, and a rotating shaft (1521) is mounted on one end of the arc-shaped patch (152) for rotationally connecting with the end surface of the cover ring (151); The mounting ring (153) is provided with a plurality of adjusting slots (1531) around it, and the other end of the arc-shaped piece (152) is inserted into the adjusting slot (1531) to slide with it; A light-transmitting hole (1522) is formed in the middle of a plurality of the arc-shaped pieces (152).

3. The spatiotemporal coherence tunable light source based on external cavity feedback and degenerate cavity according to claim 2, characterized in that: An adjustment handle (1532) is provided on the outer edge surface of the mounting ring (153).

4. The spatiotemporal coherence tunable light source based on external cavity feedback and degenerate cavity according to claim 1, characterized in that: The external feedback component (2) comprises a non-polarizing cubic beam splitter (21), an adjustable neutral density attenuation plate (22) and a second reflector (23); the non-polarizing cubic beam splitter (21) is arranged close to one side of the output coupling mirror (17).

5. The spatiotemporal coherence tunable light source based on external cavity feedback and degenerate cavity according to claim 1, characterized in that: The degenerate cavity laser (1) further comprises a pump source (13) and a gain medium (12); the pump source (13) and the gain medium (12) are arranged between the first reflector (11) and the first lens (14); and the pump source (13) is arranged above the gain medium (12).

6. The spatiotemporal coherence tunable light source based on external cavity feedback and degenerate cavity according to claim 1, characterized in that: The first lens (14) and the second lens (16) are plano-convex lenses or bi-convex lenses.

7. The spatiotemporal coherence tunable light source based on external cavity feedback and degenerate cavity according to claim 6, characterized in that: The focal lengths of the first lens (14) and the second lens (16) are 20 cm or 30 cm.

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