CN111525379A - A Broadband Topological Charge Tunable Laguerre Gaussian Optical Parametric Oscillator - Google Patents

Abstract

The invention discloses a broadband topological charge tunable Laguerre Gaussian optical parametric oscillator which comprises a nanosecond pulse laser, a light beam shaping device, a coated cavity mirror, a periodically polarized lithium niobate crystal, a temperature control device, a polarization element and a vector vortex wave plate. The film coating cavity mirror comprises a film coating front cavity mirror and a film coating rear cavity mirror which form a resonant cavity. The invention enables the relative position of the vector vortex wave plate and the film-coating front cavity mirror to meet the requirement of the concave mirror 1: 1, thereby forming a self-reproduction cavity mode smoothly transiting from a solid beam to a hollow Laguerre Gaussian beam, improving the coupling efficiency of the cavity mode and pump light, reducing the loss in the cavity, and finally realizing the Laguerre Gaussian beam output with tunable high efficiency, high purity, wavelength and topological load.

Description

A Broadband Topological Charge Tunable Laguerre Gaussian Optical Parametric Oscillator

technical field

The invention relates to an optical parametric oscillator, in particular to an optical parametric oscillator outputting a wide-band, topologically charge-tunable Laguerre Gaussian beam.

Background technique

表示。当l,p＝0时，拉盖尔高斯光束退化为基模高斯光束；当l≠0时，拉盖尔高斯光束中每个光子都携带轨道角动量，大小为

角向指数l又称为拓扑荷数。携带轨道角动量的拉盖尔高斯光束在光学操控、光学精密测量、光通信、超分辨成像、量子信息处理等领域有着广阔应用。高纯度的拉盖尔高斯光束能够有效的提高光学精密测量的信噪比、提高超分辨成像的分辨率、提高和轨道角动量光学芯片的耦合效率等。因此针对上述领域通用，功能集成的高质量的拉盖尔高斯光束光源是非常必要的。Laguerre Gaussian (LG) modes are a set of eigensolutions of the paraxial approximate wave equation in cylindrical coordinates. The different Laguerre Gaussian modes are denoted by angular exponent l and radial exponent p, denoted by

express. When l,p=0, the Laguerre Gaussian beam degenerates into a fundamental mode Gaussian beam; when l≠0, each photon in the Laguerre Gaussian beam carries orbital angular momentum, the magnitude of which is

The angular index l is also called the topological charge. Laguerre Gaussian beams carrying orbital angular momentum have broad applications in optical manipulation, optical precision measurement, optical communication, super-resolution imaging, quantum information processing and other fields. The high-purity Laguerre Gaussian beam can effectively improve the signal-to-noise ratio of optical precision measurement, improve the resolution of super-resolution imaging, and improve the coupling efficiency of orbital angular momentum optical chips. Therefore, a high-quality Laguerre Gaussian beam light source with integrated functions is very necessary for the above-mentioned fields.

Optical parametric oscillators rely on a quasi-phase matching mechanism to convert a pump photon of frequency _ω1 into a signal photon _of frequency _ω2 and an idler photon of frequency ω3 through a parametric downconversion process, where _ω1 =ω ₂ +ω ₃ , and changing the phase matching conditions can adjust the wavelengths of the output signal light and idler light, so it is widely used in the generation of broadband light sources. Combining the optical parametric oscillation system with the generation of Laguerre Gaussian beams, a broadband Laguerre Gaussian beam source can be obtained.

At present, the generation technologies for broadband Laguerre Gaussian beams can be divided into two categories: one is to use ordinary optical parametric oscillators to output broadband fundamental mode Gaussian beams, and then use mode conversion devices outside the resonator to convert the Gaussian beams into Laguerre Gaussian beams, these mode conversion devices generally include vector vortex waveplates, fork gratings, helical phase plates and spatial light modulators. The modal purity of the Laguerre Gaussian beams produced by such methods is generally low and the system is not sufficiently integrated. The second type of method can directly output the Laguerre Gaussian beam from the resonator, and use the mode selection effect of the resonator to improve the mode purity of the output beam. For example, a beam carrying orbital angular momentum is used as the pump light [Controlled switching of orbital angular momentum in an optical parametric oscillator], due to the conservation of orbital angular momentum in the process of parametric down-conversion, the generated parametric light carries orbital angular momentum, and the interaction with the resonator can directly output Laguerre Gaussian mode, but the ring pump beam light carrying orbital angular momentum The power density is low, resulting in low frequency conversion efficiency; in addition, Gaussian beam pumping is also used, and the output of Laguerre Gaussian beams is realized by adding mode conversion devices in the resonator cavity, for example [a tunable Laguerre output at 1064nm Gaussian beam laser], but in this scheme only single-wavelength Laguerre Gaussian beam output is achieved, and the use of pinhole mode selection improves the purity of the output mode while increasing the loss in the resonator cavity, which needs to be improved in terms of efficiency improvement.

SUMMARY OF THE INVENTION

Object of the invention: The present invention provides a Laguerre Gaussian optical parametric oscillator with tunable broadband topological charge in view of the problems existing in the prior art, with high efficiency and high purity.

Technical scheme: The broadband topological charge tunable Laguerre Gaussian optical parametric oscillator according to the present invention includes a nanosecond pulse laser, a beam shaping device and a coated cavity mirror placed in sequence along the light propagation direction, and the coated cavity mirror has an inner edge along the light propagation direction. Periodically polarized lithium niobate crystal, polarizing element and vector vortex wave plate are placed in order in the light propagation direction, and a temperature control device is arranged under the periodically polarized lithium niobate crystal, wherein:

Nanosecond pulsed laser, used as a pump light source to provide a pump beam with a specified wavelength;

The beam shaping device is used to shape and shrink the pump beam, and adjust the size and position of the beam waist to obtain the pump Gaussian beam that matches the size and position of the beam waist of the intrinsic cavity mode of the resonator;

Coated cavity mirrors, including pre-coated cavity mirrors and post-coated cavity mirrors, which form a resonant cavity;

Periodically polarized lithium niobate crystal, used to interact with the pump Gaussian beam, provide parametric gain, amplify the parametric down-converted light field, and generate signal light;

The temperature control device is used for stabilizing and regulating the temperature of the periodically polarized lithium niobate crystal, so as to change the quasi-phase matching condition and control the wavelength of the signal light;

The polarization element is used to adjust the polarization state of the signal light at each position in the resonant cavity, so that the polarization can be reversibly converted to ensure that the resonant cavity satisfies the polarization self-reproduction condition;

The vector vortex wave plate is used to provide a helical phase to generate Laguerre Gaussian beams with different angular exponents to achieve topological charge tunability of the output Laguerre Gaussian beam.

Further, the coated cavity mirrors are all coated with multi-layer films, the pre-coated cavity mirror is a concave mirror, and the concave surface is coated with a film that is anti-reflection to the pump light and highly reflective to the signal light; the post-coated cavity mirror is Concave mirror, the concave surface is coated with signal light reflection film.

Further, the effective distance between the cavity mirror before coating and the cavity mirror after coating is L, and the following conditions are met:

R ₁ and R ₂ represent the curvature of the mirror before and after coating.

Further, the polarizing element includes a broadband Faraday rotator and a broadband quarter-wave plate placed in sequence along the light propagation direction.

Further, the vector vortex wave plate is a broadband vector vortex wave plate, which is placed at the center of curvature of the cavity mirror before coating.

Further, the effective distance between the vector vortex wave plate and the front cavity mirror is L ₁ , and the effective distance from the back cavity mirror is L ₂ , and the following relationships are satisfied: L ₁ =R ₁ and L ₁ +L ₂ =L, where R ₁ is the curvature of the cavity mirror before coating, and L is the effective distance between the cavity mirror before coating and the cavity mirror after coating.

Further, the periodically polarized lithium niobate crystal is placed at the beam waist of the pump beam, and the front and rear surfaces are coated with a broadband antireflection film.

Further, the temperature control device includes a temperature control power supply and a temperature control furnace connected with the temperature control power supply wire, the temperature control furnace is located under the crystal, and a groove is provided on the surface to fix the periodically polarized lithium niobate crystal. , the temperature control device keeps the temperature of the periodically polarized lithium niobate crystal stable.

Beneficial effects: The present invention provides an optical parametric oscillator capable of outputting high-purity, wavelength and topological charge tunable Laguerre Gaussian beams with high efficiency:

(1) The present invention combines the design of an optical parametric oscillation system with the generation of the Laguerre Gaussian beam, so that the optical parametric oscillation resonator satisfies the stability conditions and the wavefront and polarization self-reproduction conditions, and the Laguerre Gaussian beam can be directly generated by the resonator cavity. The output has a wide wavelength tuning range and a large topological charge tuning range. Taking a specific implementation example as an example, the output wavelength tuning range can reach 100 nm, and the topological charge range of the output Laguerre Gaussian beam can reach -4 to 4.

(2) The positional relationship between the vector vortex wave plate and the front cavity mirror satisfies the 1:1 concave mirror imaging condition, and the cavity mode smoothly evolves from a solid beam to a hollow Laguerre Gaussian beam during the propagation process, and no aperture is required in the resonator cavity. Due to the mode selection, the purity is improved, lower intracavity loss is achieved, and at the same time, it is matched with the pump Gaussian beam to obtain higher gain, so that the optical parametric oscillator has higher output efficiency. Taking a specific implementation example as an example, the output Laguerre Gaussian beam has a maximum efficiency of more than 15% in the wavelength range of 1500nm-1600nm.

(3) Due to the setting of the imaging relationship between the vector vortex wave plate and the concave mirror of the front cavity mirror, the parametric light gains gain at the front cavity mirror and smoothly evolves into a hollow annular distribution before passing through the vector vortex wave plate, which effectively suppresses This ensures the generation of higher-order radial modes, thus guaranteeing a high-purity Laguerre-Gaussian mode output. Taking a specific implementation example as an example, the output Laguerre Gaussian mode is in the topological charge tuning range of -4 to 4, and the mode purity is up to more than 97%.

(4) The Laguerre Gaussian optical parametric oscillator designed by the present invention can meet the requirements of high-efficiency output of high-purity Laguerre Gaussian beams in different wavelength ranges by replacing components with different working wavelengths and pump lasers of corresponding wavelengths.

Description of drawings

Fig. 1 is the light path diagram of the broadband topological charge tunable Laguerre Gaussian optical parametric oscillator provided by the present invention;

Fig. 2 is a simulation diagram of the intensity distribution of the optical field profile during the propagation of the cavity mode of the resonator;

光束的转换效率散点图,(b)为输出1550nm的

光束光强分布图；Figure 3(a) is the output of the optical parametric oscillator in the operating bandwidth of 1500nm-1600nm

Scatter diagram of the conversion efficiency of the beam, (b) is the output of 1550nm

beam intensity distribution map;

光束的模式成分权重分布图，(b)为调节光学参量振荡器输出波长为1525nm和1575nm时，输出

光束的模式成分权重分布图。Figure 4(a) is the output of the optical parametric oscillator at 1550nm

Mode component weight distribution diagram of the beam, (b) is the output wavelength of the adjusted optical parametric oscillator when the output wavelength is 1525nm and 1575nm.

A map of the mode component weight distribution of the beam.

Detailed ways

1 is a schematic diagram of the optical path structure of the broadband topological charge tunable Laguerre Gaussian optical parametric oscillator provided by the present invention, the laser is a solid-state laser, as shown in FIG. 1 , including a nanosecond pulse laser 1, a beam shaping device 2, Coated cavity mirror 3 , periodically polarized lithium niobate crystal 4 , temperature control device 5 , polarizing element 6 and vector vortex wave plate 7 . The nanosecond pulse laser 1, the beam shaping device 2, and the coated cavity mirror 3 are placed in sequence along the light propagation direction, and the periodically polarized lithium niobate crystal 4, the polarizing element 6 and the vector vortex wave plate 7 are placed in the coated cavity in sequence along the light propagation direction. Inside the mirror 3 , the temperature control device 5 is arranged under the periodically polarized lithium niobate crystal 4 . The nanosecond pulsed laser 1 serves as a pump light source to provide a pump beam with a specified wavelength, which is a 1064 nm pump beam in this embodiment. The beam shaping device 2 is composed of a lens group, which is used to adjust the beam waist and position of the 1064nm beam to generate a 1064nm Gaussian beam, and the pump power can reach up to 5w. The coating cavity mirror 3 includes a pre-coated cavity mirror 31 and a post-coated cavity mirror 32, both of which form a resonant cavity. Finally, both the pre-coating cavity mirror 31 and the post-coating cavity mirror 32 are coated with multi-layer films, the pre-coating cavity mirror 31 is a concave mirror with a curvature R ₁ , and the concave surface is coated with a 1064nm antireflection film and a 1450nm-1650nm broadband high-reflection film. The rear cavity mirror 32 is a concave mirror, the curvature is R ₂ , and the concave surface is coated with a 1450nm-1650nm broadband reflective film. Periodically polarized lithium niobate crystal 4 measures 25mm×12.3mm×1mm, and is placed at the beam waist of the 1064nm pumping Gaussian beam to interact with the 1064nm pumping Gaussian beam to provide parametric gain and amplify the parametric down-converted light field. Both the pump light and the parametric light are vertically polarized. The front and rear surfaces of the periodically polarized lithium niobate crystal 4 are coated with a 1380nm-1800nm broadband antireflection coating. There are ten channels with different polarization periods. The signal light with wavelength of 1480nm-1650nm is generated in the range. The temperature control device 5 is used for stabilizing and regulating the temperature of the periodically polarized lithium niobate crystal to change the quasi-phase matching condition and control the wavelength of the signal light. The temperature control device 5 includes a temperature control furnace and a temperature control power supply, and the surface of the temperature control furnace is A groove is provided for fixing the crystal; the temperature control accuracy of the temperature control device is 0.1°C, which can keep the temperature of the periodically polarized lithium niobate crystal constant and ensure the stable output of the optical parametric oscillator. The polarizing element 6 includes a broadband Faraday rotator 61 and a broadband quarter-wave plate 62 placed in sequence along the beam propagation direction. The phase distribution and polarization state are reversible, and the light incident on the vector vortex wave plate 7 is circularly polarized. The vector vortex wave plate 7 is a broadband vector vortex wave plate with a working band of 1500nm-1600nm. It is used to add a helical phase to the circularly polarized incident light to generate pull caps with angular indices l=±1, ±2, and ±4 respectively. Laguerre Gaussian beam to obtain topological charge tunable Laguerre Gaussian beam output.

以保证谐振腔能够稳定起振。为了使圆偏振光入射矢量涡旋波片以获得螺旋相位，谐振腔内引入四分之一波片用于偏振转换，同时为了满足谐振腔的偏振可逆条件，需引入旋光角为45度的法拉第旋光器，置于周期极化铌酸锂晶体和四分之一波片之间，使得光束在谐振腔内往返一周后偏振态得以复原。For the design of the resonator of the optical parametric oscillator, the stability condition of the resonator and the polarization reversibility condition should be satisfied first. For the curvatures R ₁ and R ₂ of the cavity mirror before and after coating, and the effective cavity length L of the resonator, the stability conditions of the resonator should be satisfied:

In order to ensure that the resonator can stably start to vibrate. In order to make the circularly polarized light incident on the vector vortex wave plate to obtain the helical phase, a quarter wave plate is introduced into the resonator for polarization conversion. At the same time, in order to meet the polarization reversibility condition of the resonator, a Faraday with an optical rotation angle of 45 degrees needs to be introduced. The optical rotator is placed between the periodically polarized lithium niobate crystal and the quarter-wave plate, so that the polarization state of the beam can be restored after one round trip in the resonator.

In order to reduce the loss in the resonator cavity and obtain high-purity Laguerre Gaussian beam output, the cavity mode of the resonator needs to be designed. On the one hand, the cavity mode should have a solid optical field distribution at the front cavity mirror to match the pump Gaussian beam. The beam ensures high conversion efficiency; on the other hand, the cavity mode located at the rear cavity mirror should be a standard Laguerre Gaussian mode to ensure high-purity output, so the cavity mode propagated from the front cavity mirror to the rear cavity mirror should evolve smoothly from a solid beam A standard Laguerre Gaussian beam is generated after passing through a vector vortex waveplate as a hollow beam.

In the present invention, the vector vortex wave plate is set at the imaging position of a 1:1 concave mirror formed with the front cavity mirror, that is: L ₁ =R ₁ , and L ₁ is the effective distance between the vector vortex wave plate and the front cavity mirror. Many optical elements with refractive index greater than 1 are placed between the mirror and the vector vortex wave plate, and the actual distance between the vector vortex wave plate and the front cavity mirror should be slightly larger than R ₁ .

According to the resonant cavity configuration of the optical parametric oscillator provided by the present invention, using scalar diffraction theory analysis, the standard Laguerre Gaussian mode starts from the rear cavity mirror and returns to the starting position after one round trip of the resonant cavity to realize the self-reproduction of amplitude and phase distribution. Through the fox-li iterative algorithm simulation, the light intensity distribution of the cavity mode of the resonator cavity during the propagation process is obtained, which verifies the theoretical analysis. As shown in Figure 2, the cavity mode beam propagates from the front cavity mirror to the vector vortex wave plate and is smoothed by the solid beam. Evolved into a hollow Laguerre Gaussian beam intensity distribution, and then added a helical phase through a vector vortex wave plate to generate a standard Laguerre Gaussian beam and output. The optical parametric oscillator resonant cavity cavity mode provided by the invention has different light field distributions at the front and rear cavity mirrors, the light field evolution process is smooth, and the high-purity Laguerre Gaussian beam output is guaranteed. Diffraction loss is reduced, and the cavity mode and the pump beam are well matched to ensure high output efficiency.

光束的转换效率散点图，在1550nm输出时，

光束转换效率分别高达15.2％、15.8％和15.6％，在超过80nm的工作带宽内可以达到10％以上的输出效率；图3(b)为输出1550nm的

光束光强分布图；实验结果表明本发明提供的光学参量振荡器能够在宽带工作范围内高效率输出多种拓扑荷数的拉盖尔高斯光束。Figure 3(a) shows the optical parametric oscillator using this example, when the pump power is 4.2w, the output is in the range of 1500nm to 1600nm

Scatter plot of the conversion efficiency of the beam, when output at 1550nm,

The beam conversion efficiency is as high as 15.2%, 15.8% and 15.6%, respectively, and the output efficiency can reach more than 10% in the working bandwidth of more than 80nm; Figure 3(b) shows the output efficiency of 1550nm.

The light intensity distribution diagram of the beam; the experimental results show that the optical parametric oscillator provided by the invention can output Laguerre Gaussian beams with various topological charges with high efficiency in a broadband working range.

当标准模式与被测模式一致时，会得到中心光强极大的光场分布，根据测量结果中心光强的占比，可获得被测模式的成分组成和权重。图4(a)为光学参量振荡器在1550nm输出

光束的模式成分权重分布图，可以看到光学参量振荡器输出

光束模式纯度高达97％和96.6％，当输出较高阶模式

光束时，模式纯度仍然能够达到95.2％和93.7％；图4(b)为光学参量振荡器在1525nm和1575nm输出

光束的模式成分权重分布图，输出

光束的模式纯度分别为97.1％和95.9％；实验结果表明本发明提供的光学参量振荡器在宽带工作范围内能够保证不同拓扑荷的拉盖尔高斯光束的高纯度输出。Figure 4 is the test result of the mode purity of the output beam of the optical oscillator in this example. The feature identification method is used for the mode purity analysis of the output Laguerre Gaussian beam, and the spatial light modulator is used to generate standard pulls with different angular and radial indices. The Gal-Gaussian mode is cross-correlated with the beam to be measured, and the measurement result can be expressed as:

When the standard mode is consistent with the mode under test, a light field distribution with extremely large central light intensity will be obtained. According to the proportion of the central light intensity in the measurement result, the composition and weight of the mode under test can be obtained. Figure 4(a) shows the optical parametric oscillator output at 1550nm

Mode component weight distribution of the beam, you can see the optical parametric oscillator output

Beam mode purity up to 97% and 96.6% when outputting higher order modes

When the light beam is used, the mode purity can still reach 95.2% and 93.7%; Figure 4(b) shows the output of the optical parametric oscillator at 1525nm and 1575nm

Mode component weight distribution map of the beam, output

The mode purities of the beams are 97.1% and 95.9% respectively; the experimental results show that the optical parametric oscillator provided by the invention can ensure the high-purity output of Laguerre Gaussian beams with different topological charges in the broadband working range.

What is disclosed above is only a preferred embodiment of the present invention, which cannot limit the scope of the rights of the present invention. Therefore, equivalent changes made according to the claims of the present invention are still within the scope of the present invention.

Claims (8)

1. A broadband topology load tunable Laguerre Gaussian optical parametric oscillator is characterized in that: including nanosecond pulse laser, beam shaping device and the coating chamber mirror of placing in proper order along light propagation direction, periodic polarization lithium niobate crystal, polarization component and vector vortex wave plate have been placed in proper order along light propagation direction in the coating chamber mirror, be provided with temperature control device under the periodic polarization lithium niobate crystal, wherein:

the nanosecond pulse laser is used as a pumping light source to provide pumping light beams with specified wavelengths;

the beam shaping device is used for shaping and shrinking the pumping beam and adjusting the beam waist size and position of the pumping beam so as to obtain a pumping Gaussian beam matched with the beam waist size and position of the intrinsic cavity of the resonant cavity;

the coating cavity mirror comprises a coating front cavity mirror and a coating rear cavity mirror which form a resonant cavity;

the periodically polarized lithium niobate crystal is used for interacting with the pumping Gaussian beam, providing parametric gain, amplifying the parametric down-conversion light field and generating signal light;

the temperature control device is used for stabilizing and regulating the temperature of the periodically polarized lithium niobate crystal so as to change the quasi-phase matching condition and control the wavelength of the generated signal light;

the polarization element is used for adjusting the polarization state of the signal light at each position in the resonant cavity, so that the polarization can be reversely converted, and the resonant cavity is ensured to meet the polarization self-reproduction condition;

and the vector vortex wave plate is used for providing a spiral phase and generating Laguerre Gaussian beams with different angular indexes so as to realize tunable topological charge of the output Laguerre Gaussian beam.

2. The broadband topologically tunable laguerre gaussian optical parametric oscillator of claim 1, wherein: the coating cavity mirrors are all plated with multilayer films, the coating front cavity mirror is a concave mirror, and the concave surface is plated with a film which is anti-reflection to pumping light and highly reflective to signal light; the film-coated rear cavity mirror is a concave mirror, and the concave surface of the film-coated rear cavity mirror is plated with a signal light reflection film.

3. The broadband topologically tunable laguerre gaussian optical parametric oscillator of claim 1, wherein: the effective distance between the front film-coating cavity mirror and the rear film-coating cavity mirror is L, and the following conditions are met:

R₁and R₂Showing the curvature of the pre-coated cavity mirror and the coated back cavity mirror.

4. The broadband topologically tunable laguerre gaussian optical parametric oscillator of claim 1, wherein: the polarization element comprises a broadband Faraday rotator and a broadband quarter-wave plate which are sequentially arranged along the light propagation direction.

5. The broadband topologically tunable laguerre gaussian optical parametric oscillator of claim 1, wherein: the vector vortex wave plate is a broadband vector vortex wave plate.

6. The broadband topologically tunable laguerre gaussian optical parametric oscillator of claim 1, wherein: the effective distance between the vector vortex wave plate and the front cavity mirror is L₁An effective distance L from the rear cavity mirror₂And satisfies the following relationship: l is₁＝R₁And L₁+L₂L, wherein R₁The curvature of the film coating front cavity mirror is shown, and L is the effective distance between the film coating front cavity mirror and the film coating rear cavity mirror.

7. The broadband topologically tunable laguerre gaussian optical parametric oscillator of claim 1, wherein: the periodically polarized lithium niobate crystal is arranged at the beam waist of the pumping beam, and the front end face and the rear end face are plated with broadband antireflection films.

8. The broadband topologically tunable laguerre gaussian optical parametric oscillator of claim 1, wherein: the temperature control device comprises a temperature control power supply and a temperature control furnace connected with a temperature control power supply lead, the temperature control furnace is positioned below the crystal, a groove is formed in the surface of the temperature control furnace and used for fixing the periodically polarized lithium niobate crystal, and the temperature control device enables the temperature of the periodically polarized lithium niobate crystal to be kept stable.

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