A 4f system based partially coherent laser

Abstract

The utility model discloses a kind of partially coherent laser based on 4f system, including light source generation module and coherence control module, the light source generation module includes high-power total reflection mirror, gain medium and output coupling mirror, the high-power total reflection mirror is coupled with output mirror and constitutes short FP resonator with high F number characteristic, for through the back and forth reflection of light to prolong the working length of gain medium.The coherence control module includes first thin lens, adjustable diaphragm and second thin lens, the first thin lens is coupled with second thin lens and constitutes 4f system, for through adjustable diaphragm on fourier plane, selective adjustment is carried out to light field spectrum component;The adjustable diaphragm is used as band-pass filter.The utility model combines the high F number characteristic of short FP resonator, the spatial fourier transform characteristic of 4f system and the spatial band-pass filtering characteristic of adjustable diaphragm, realizes coherence regulation and control from spatial dimension, with good practicability.

Description

Technical Field

[0001] This utility model belongs to the field of optoelectronic technology, specifically relating to a partially coherent laser based on a 4f system. Background Technology

[0002] Maiman built the world's first ruby ​​laser in 1960. Currently, lasers are widely used in industrial processing, information communication, medical and health care, scientific research, and national defense. Traditional lasers possess high collimation, high coherence, high intensity, and high monochromaticity, making them suitable for laser communication, distance measurement, spectral analysis, and material cutting. However, because high coherence can more easily cause speckle and wavefront distortion, severely affecting beam quality, high spatial coherence can actually have negative consequences in some applications.

[0003] For example, in laser displays, the high coherence of the laser and the strong interference between the surface morphology of objects such as screens create "speckle noise," leading to a decrease in imaging performance (Journal of Display Technology, 2015, 11(4): 330-335.). In laser communication links, atmospheric turbulence disrupts the wavefront of the laser beam, causing degradation phenomena such as beam broadening, beam drift, and intensity jitter at the receiver, resulting in a decrease in the receiver signal-to-noise ratio and an increase in the bit error rate, severely restricting the system performance of free-space optical communication (IEEE Communications Surveys & Tutorials, 2017, 19(1):57-96). In high-power driven inertial confinement fusion, interference fringes caused by high coherence lead to non-uniformity in intensity distribution, severely restricting beam-target coupling performance. In laser thermal processing, interference fringes caused by high coherence result in non-uniformity of the material processing surface. Therefore, for applications such as space laser communication, optical imaging, and laser thermal processing, there is an urgent need to develop partially coherent light sources.

[0004] Regarding external beam manipulation, the research team at the Polytechnic University of Rome used a mechanical vibration method to add a non-uniform phase increment to the random perturbation of the optical field phase to achieve low coherence light. However, due to the speed of mechanical movement, it is impossible to achieve low spatial coherence for a short time (Journal of Applied Physics 42.1(1971):399-403.).

[0005] The research team at the Technical University of Helsinki used a photoelectric method to reduce spatial coherence by using acousto-optic devices to modulate the phase of the light field, but the cost was too high (Journal of applied physics 67.1(1990):49-59.).

[0006] The research team at Yale University used the multiple scattering properties of particles in the gain medium to achieve optical gain. The resulting random laser has low coherence, but the randomness of scattering leads to poor output directionality (Physical Review Letters, 2000, 84(24): 5584-5587; Nature Photonics, 2012, 6(6): 355-359.).

[0007] The research team at the Weizmann Institute of Science in Israel used a degenerate cavity laser to control the coherence of the laser. They achieved coherence modulation by adjusting the size of the spectral surface. They pointed out that increasing the number of transverse modes can effectively reduce speckle contrast in imaging. However, since the device is a combination of a resonant cavity and a 4f system, it is difficult to start up and the power density of the spectral surface inside the cavity is high, which may damage the optical components (Optics Letters, 2013, 38(19): 3858-3861; Optics Express, 2015, 23(10): 12989-12997; Physical Review A, 2018, 98(2): 023812.).

[0008] In 2015, a research team at Yale University used deformable microcavities to achieve low-coherence lasers and multimode lasing at relatively low pump values. However, this type of laser has low power, poor directionality, and complex fabrication process. (Proceedings of the National Academy of Sciences, 2015, 112(5): 1304-1309.)

[0009] In 2019, a research team at Yale University achieved multimode lasing using a near-concentric cavity, but the resulting low-coherence beam had an excessively large divergence angle. (Applied Physics Letters, 2019, 115(7): 071101.)

[0010] In 2020, a research team at the Institute of Semiconductors, Chinese Academy of Sciences, achieved multimode lasing using a dumbbell-shaped cavity. However, the low-coherence laser obtained could not be lased under continuous pumping, resulting in low practical value. (Optics Letters, 2020, 45(18): 5097-5100.) Utility Model Content

[0011] The purpose of this invention is to provide a partially coherent laser based on a 4f system, which aims to combine the high Fresnel number characteristics of a short FP resonator, the spatial Fourier transform characteristics of a 4f system, and the spatial bandpass filtering characteristics of an adjustable aperture to achieve coherence control in the spatial dimension.

[0012] This utility model is mainly achieved through the following technical solutions: A partially coherent laser based on a 4f system includes a light source generation module and a coherence control module arranged sequentially. The light source generation module includes a high-power total reflection mirror, a gain medium, an output coupling mirror, a pump source, and a DC power supply arranged sequentially from front to back. A pump source is disposed on one side of the gain medium, and the pump source is connected to a negative electrode and a DC power supply through a positive electrode wire. The high-power total reflection mirror and the output coupling mirror form a short FP resonant cavity with high Fresnel number characteristics, which is used to extend the working length of the gain medium by reflecting light back and forth. The coherence control module includes a first thin lens, an adjustable aperture, and a second thin lens arranged sequentially from front to back. The adjustable aperture is located on the common focal plane of the first and second thin lenses. The first and second thin lenses form a 4f system, which is used to selectively adjust the spectral components of the light field by placing the adjustable aperture on the Fourier plane. The adjustable aperture is used as a bandpass filter to selectively pass spatial frequency components, so as to adjust the spatial coherence length of the light field by adjusting the spatial spectral width. Among them, spatial coherence length for: ; Δ v f This represents the optical field spectral width after filtering by the bandpass filter.

[0013] To better realize this utility model, the outer layer of the high-power total reflection mirror is coated with a high-reflectivity film layer, the reflectivity of the high-reflectivity film layer is >99.9%, and the diameter of the high-power total reflection mirror is 25.4 mm or 50.8 mm.

[0014] To better realize this utility model, the outer side of the output coupling mirror is further coated with a reflective and transmissive film layer for a wavelength of 1064 nm; the outer sides of the first thin lens and the second thin lens are respectively coated with a high transmittance film layer.

[0015] To better realize this utility model, the thickness of the gain medium 2 is 1~3 mm and the diameter is 5~10 mm.

[0016] To better realize this invention, the cavity of the short FP resonant cavity can further support the number of modes. N =( πN f ) 2 ;in N f It is a Fresnel number.

[0017] To better realize this utility model, the diameter D of the adjustable aperture 5 is further 0~10 mm.

[0018] To better realize this utility model, the first thin lens and the second thin lens are respectively plano-convex lenses or biconvex lenses, and the focal length is 20cm or 30cm; the reflection and transmission splitting ratio of the output coupling mirror is 99:1 or 90:10, and the diameter is 25.4mm or 50.8mm.

[0019] To better realize this utility model, the output current of the DC power supply is further specified as 0~20A.

[0020] The beneficial effects of this utility model are as follows: This invention combines the high Fresnel number characteristics of a short FP resonant cavity, the spatial Fourier transform characteristics of a 4f system, and the spatial bandpass filtering characteristics of an aperture to perform filtering in the spatial frequency domain, selectively extracting the spatial spectral components of the light field, and achieving coherence control from the spatial dimension. This tunable light source can achieve spatial coherence adjustment and can be used in fields such as space laser communication, optical imaging, and laser processing.

[0021] This invention abandons the cumbersome and complex architecture of degenerate cavity tunable coherence, directly utilizing the filtering characteristics of the spatial spectral surface to achieve spatial coherence modulation while maintaining the energy of the light field. Simultaneously, it eliminates the need for a scattering device, reducing light loss and improving energy utilization. Compared to previous solutions, this invention starts with spatial spectral modulation, employing a 4f structure and utilizing a short FP cavity to achieve a high Fresnel number cavity, realizing the generation of a low-coherence multimode lasing light source with good collimation. A variable pinhole aperture is used for spatial spectral filtering to extract coherent components, thereby adjusting part of the light source's coherence, demonstrating good practicality. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the structure of the partially coherent laser based on the 4f system of this utility model.

[0023] Wherein: 1-High-power total reflection mirror; 2-Gain medium; 3-Output coupling mirror; 4-First thin lens; 5-Adjustable aperture; 6-Second thin lens; 7-Pump source; 8-Positive electrode wire; 9-Negative electrode wire; 10-DC power supply. Detailed Implementation

[0024] Example 1: A partially coherent laser based on a 4f system, such as Figure 1 As shown, the optical system includes, from front to back, a high-power total reflection mirror 1, a gain medium 2, an output coupling mirror 3, a first thin lens 4, an adjustable aperture 5, and a second thin lens 6, all of which are coaxial at the same height, as well as a pump source 7, a positive electrode wire 8, a negative electrode wire 9, and a DC power supply 10.

[0025] The high-power total reflection mirror 1 and the output coupling mirror 3 are respectively placed on both sides of the gain medium 2. The planes of the high-power total reflection mirror 1 and the output coupling mirror 3 are perpendicular to the optical axis, and the center point is located on the optical axis. The central axis of the gain medium 2 coincides with the optical axis. The first thin lens 4 and the second thin lens 6 form a confocal telescope system and are placed outside the cavity. The plane where the focal points of the first thin lens 4 and the second thin lens 6 are located is called the common focal plane. An adjustable pinhole aperture 5 is placed on the common focal plane. The distance between the second thin lens 6 and the partial reflector is the focal length of the second thin lens 6. The pump source 7 is placed at the same height on the side of the gain medium 2. The pump source 7 is connected to the DC current 10 through the positive electrode wire 8 and the negative electrode wire 9.

[0026] Preferably, the partially coherent laser based on the 4f system can be divided into two parts: a light source generation module and a coherence control module.

[0027] The light source generation module includes a high-power total internal reflection mirror 1, a gain medium 2, an output coupling mirror 3, a pump source 7, a positive electrode wire 8, a negative electrode wire 9, and a DC power supply 10. The high-power total internal reflection mirror 1 and the output coupling mirror 3 form an FP resonant cavity. By reflecting light back and forth, the working length of the gain medium 2 is extended, achieving gain amplification and suppressing spontaneous emission. The gain medium 2 provides a population inversion energy level structure, where electrons undergo stimulated emission transitions between energy levels to generate laser light. The pump source 7 acts as an excitation source, pumping electrons from lower energy levels to higher energy levels, achieving population inversion. The DC power supply 10 forms a current loop through the positive electrode wire 8 and the negative electrode wire 9 to power the pump source 7. Due to the parallel structure of the FP cavity, the emitted beam has good directionality. For the coherence of the FP resonant cavity, the Fresnel number is introduced. N f Quantitative description of it: ; in: a The dimensions of the FP resonant cavity; λ is the wavelength of light; L is the length of the FP resonant cavity.

[0028] For a laser of a certain wavelength, a short FP cavity can be used to achieve a small... L This increases the Fresnel number, and by using a large-aperture gain medium, high gain can be achieved. a This results in a high Fresnel number. The higher the Fresnel number, the more modes N the cavity can support. Therefore, the laser is a multimode laser, with each mode independent of the others, making it a low-coherence source. The number of modes N supported by the cavity is calculated as follows: πN f ) 2 .

[0029] The coherence control module includes a first thin lens 4, an adjustable aperture 5, and a second thin lens 6, which together form a 4f system. By placing the adjustable aperture 5 on the Fourier plane, selective adjustment of the spectral components of the light field can be achieved, thereby effectively controlling the coherence of the light field. The influence of the adjustable aperture on the spatial coherence length of the light field can be analyzed using Fourier transform. Specifically, the adjustable aperture can be considered as a bandpass filter, which selectively passes certain spatial frequency components, thus affecting the spatial frequency distribution and coherence properties of the light field. Spatial coherence length L s This refers to the length scale by which the light field maintains coherence in space, and it is related to the spectral width of the light field. For spatial frequencies... v The coherence length can be expressed as: ; Where, Δ v It refers to the spectral width. In a 4f system, it refers to the aperture size of the spectral plane. D This determines the bandpass filter width of the system. Specifically, the diameter of the aperture... D With spectral width Δ v The relationship between them is as follows: ; Where λ is the wavelength of light. f It is the focal length of the lens.

[0030] Assuming the bandpass filter selects a spatial frequency that passes through... v 1 to v The components between 2 and 3, then the filtered optical field spectral width Δ v f It can be represented as: Δ v f = v 1- v 2.

[0031] The Fourier transform effect of the first thin lens 4 can convert the real space distribution of the light field into a Fourier space spectral distribution, while bandpass filtering selectively preserves certain frequency components. For a given light field... I ( x , y ), its Fourier transform I ( k x , k y ) represents its spatial spectrum, where k x and k y It is the spatial frequency. The optical field spectrum after bandpass filtering is: ; in, H ( k x , k y ) is the bandpass filter function: ; Spatial coherence length after bandpass filtering The filtered spectral width Δ v f Decide: ; In summary, before filtering, the spatial spectral width Δ of the original light field is... v Determines its spatial coherence length L s After bandpass filtering, the selected frequency range of the bandpass filter is... v 1 to v 2 spectral width Δ v f The spatial coherence length of the filtered optical field is determined. Therefore, bandpass filtering selectively passes certain spatial frequency components, thereby changing the spatial spectral width Δ of the optical field. v f This, in turn, affects its spatial coherence length. L s Specifically, by reducing the spectral width Δ v f This can increase the spatial coherence length of the light field. L ' s Based on the above analysis, the spatial coherence length of the optical field can be adjusted by regulating the spatial spectral width through bandpass filtering. Therefore, by selecting an appropriate aperture size, precise control over the spatial coherence properties of the optical field can be achieved. Thus, spatial coherence can be manipulated through spatial Fourier filtering.

[0032] Preferably, the gain medium 2 can be Nd:YAG or other Nd... 3+ The doped gain medium has a thickness of 1~3 mm and a diameter of 5~10 mm; Pump source 7 can be a high-intensity radiation device such as a semiconductor laser, arc lamp, or diffuser; The high-power total internal reflection mirror 1 needs to be coated with a high reflectivity film for lasing wavelength, with a reflectivity >99.9% and a diameter of 25.4 mm or 50.8 mm; The output coupling mirror 6 can be a reflective or a transmissive beam splitter with a ratio of 99:1 or 90:10. It needs to be coated with reflective and transmissive films for a wavelength of 1064 nm, with a diameter of 25.4 mm or 50.8 mm. The first thin lens 4 and the second thin lens 6 can be plano-convex lenses or biconvex lenses with a focal length of 20cm or 30cm, and need to be coated with a high transmittance film for the lasing wavelength. The diameter of the adjustable aperture 5 can be continuously adjusted between 0 and 10 mm; The output current of the DC power supply is continuously adjustable from 0 to 20A.

[0033] The above description is merely a preferred embodiment of the present utility model and is not intended to limit the present utility model in any way. Any simple modifications or equivalent changes made to the above embodiments based on the technical essence of the present utility model shall fall within the protection scope of the present utility model.

Claims

1. A partially coherent laser based on a 4f system, characterized in that, The system includes a light source generation module and a coherence control module arranged in sequence. The light source generation module includes a high-power total reflection mirror (1), a gain medium (2), and an output coupling mirror (3) arranged in sequence from front to back, as well as a pump source (7) and a DC power supply (10). A pump source (7) is provided on one side of the gain medium (2), and the pump source (7) is connected to the negative electrode wire (9) and the DC power supply (10) through a positive electrode wire (8). The high-power total reflection mirror (1) and the output coupling mirror (3) form a short FP resonant cavity with high Fresnel number characteristics, which is used to extend the working length of the gain medium (2) by reflecting light back and forth. The coherence control module includes a first thin lens (4), an adjustable aperture (5), and a second thin lens (6) arranged sequentially from front to back. The adjustable aperture (5) is located on the common focal plane of the first thin lens (4) and the second thin lens (6). The first thin lens (4) and the second thin lens (6) constitute a 4f system, which is used to selectively adjust the spectral components of the light field by placing the adjustable aperture (5) on the Fourier plane. The adjustable aperture (5) is used as a bandpass filter to selectively pass spatial frequency components so as to adjust the spatial coherence length of the light field by adjusting the spatial spectral width. Among them, spatial coherence length for: ； Δ v f This represents the optical field spectral width after filtering by the bandpass filter.

2. A partially coherent laser based on a 4f system according to claim 1, characterized in that, The outer layer of the high-power total reflection mirror (1) is coated with a high-reflectivity film layer, the reflectivity of which is >99.9%, and the diameter of the high-power total reflection mirror (1) is 25.4 mm or 50.8 mm.

3. A partially coherent laser based on a 4f system according to claim 1, characterized in that, The outer side of the output coupling mirror (3) is coated with a reflective and transmissive film for a wavelength of 1064 nm; the outer sides of the first thin lens (4) and the second thin lens (6) are respectively coated with a high transmittance film.

4. A partially coherent laser based on a 4f system according to claim 1, characterized in that, The thickness of the gain medium (2) 2 is 1~3 mm and the diameter is 5~10 mm.

5. A partially coherent laser based on a 4f system according to any one of claims 1-4, characterized in that, The number of modes supported by the cavity of the short FP resonator N = ( πN f ) 2 ;in N f It is a Fresnel number.

6. A partially coherent laser based on a 4f system according to claim 1, characterized in that, The diameter D of the adjustable aperture (5) 5 is 0~10 mm.

7. A partially coherent laser based on a 4f system according to claim 1 or 6, characterized in that, The first thin lens (4) and the second thin lens (6) are respectively plano-convex lenses or biconvex lenses, and the focal length is 20cm or 30cm; the output coupling mirror (3) has a reflection and transmission splitting ratio of 99:1 or 90:10, and a diameter of 25.4mm or 50.8mm.

8. A partially coherent laser based on a 4f system according to claim 1, characterized in that, The DC power supply (10) has an output current of 0~20A.

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