A laser frequency conversion system and method based on thermal lens effect enhancement

Abstract

This invention discloses a laser frequency conversion system and method based on thermal lensing effect enhancement, belonging to the field of laser technology. The system includes a first pump source, a nonlinear frequency conversion crystal, and a second pump source. The crystal is a rare-earth-doped calcium borate crystal, which absorbs the first pump light to generate a controllable thermal lensing effect. The fundamental wave light emitted from the second pump source is coaxially incident on the crystal with the first pump light. The thermal lens focuses the fundamental wave light, significantly increasing its power density within the crystal. The crystal then utilizes a nonlinear optical effect to efficiently convert the high-power-density fundamental wave light into harmonic light output. This invention solves the problem of low efficiency in traditional extracavity frequency doubling. Through an active focusing mechanism, high-efficiency and high-stability visible light laser output can be achieved with only a lower fundamental wave power, making it particularly suitable for biological detection and medical aesthetics.

Description

Technical Field

[0001] This invention belongs to the field of laser technology, specifically relating to a laser frequency conversion system and method based on thermal lensing effect enhancement. Background Technology

[0002] With its unique advantages of high monochromaticity, high directionality, high brightness, and high coherence, laser technology has become an indispensable core tool in the field of modern biological detection. Its application background stems from the limitations of traditional detection technologies and the need for precision in biomedical research.

[0003] Before the widespread adoption of laser technology, biological detection relied heavily on broadband light sources such as mercury lamps and xenon lamps. These sources suffer from low spectral resolution, energy dispersion, and poor excitation specificity, making it difficult to simultaneously detect multiple targets and prone to low signal-to-noise ratios due to stray light interference, thus failing to meet the requirements for precise quantitative analysis at the cellular level. However, with the development of molecular biology, cell biology, and clinical diagnostics, the demand for technologies such as single-molecule tracking, live-cell dynamic imaging, multicolor flow cytometry sorting, and non-invasive deep tissue detection in research and medical settings is increasingly urgent. The high monochromaticity of lasers can precisely match the characteristic absorption peaks of fluorescent dyes or proteins, maximizing excitation efficiency and reducing background interference; high brightness enables efficient acquisition of weak biological signals, meeting the needs of single-molecule detection; and high directionality allows for focusing into micron-sized spots for targeted excitation and imaging within cells. These characteristics precisely compensate for the shortcomings of traditional light sources.

[0004] Meanwhile, the iteration of laser technology (such as the miniaturization and cost reduction of semiconductor lasers and DPSS lasers) and the integration of optical detection technology (such as the development of confocal microscopy, flow cytometers, and laser-induced fluorescence spectrometers) have further promoted the large-scale application of lasers in biological detection, enabling them to gradually move from laboratory research tools to industrial applications such as clinical diagnosis, biopharmaceuticals, and food safety testing.

[0005] For example, 488nm, as a classic wavelength for biofluorescence detection, is a common choice for flow cytometry and confocal microscopy. It can efficiently excite mainstream fluorophores such as FITC, Alexa Fluor 488, and GFP. With its mature multicolor matching schemes and broad instrument compatibility, it has become a benchmark tool for routine experiments such as protein localization, cell sorting, and immunoblotting. The 510nm wavelength fills the spectral gap between 488nm and 532nm, and is specifically adapted to next-generation yellow fluorescent proteins such as Venus and mCitrine. In live cell dynamic tracking and multi-target co-localization experiments, it can effectively reduce spectral crosstalk with the GFP channel and improve the imaging signal-to-noise ratio. 561nm is the optimal excitation wavelength for tandem dyes such as PE and PE-Cy5 / Cy7, as well as red fluorescent proteins such as mCherry and DsRed. Its longer excitation wavelength results in extremely low interference from autofluorescence in biological samples, making it particularly suitable for scenarios with stringent background noise requirements, such as deep tissue imaging and high-sensitivity flow cytometry quantitative analysis.

[0006] However, currently used lasers all have certain problems. 488nm lasers are mostly based on argon-ion or semiconductor pumping technology. Argon-ion lasers are large, consume high power, and have short lifespans (lamp lifespan is typically only a few thousand hours), resulting in high maintenance costs. Semiconductor-pumped lasers, while miniaturized, suffer from significant wavelength temperature drift (wavelength drift can reach over 0.1nm for every 1°C change in temperature), making it difficult to stably match the narrow absorption peak of the phosphor. Furthermore, mode switching is prone to occur at high power, leading to a decrease in beam quality. In addition, 488nm falls into the short-wavelength blue light category, making the laser cavity mirror coating susceptible to aging from high-energy photon bombardment. Long-term use can lead to rapid output power decay and increased noise (RIN), affecting the stability of quantitative detection. 510nm is a niche wavelength band, lacking mature direct-emission semiconductor chips. It often relies on nonlinear frequency conversion technologies (such as frequency doubling and mixing), resulting in complex overall structures, large size, and low frequency conversion efficiency. It requires a high-power pump source, causing high power consumption and heat dissipation pressure. Limited by the angle tuning accuracy of the frequency conversion crystal, this wavelength laser... The wavelength control of 510nm lasers is difficult, with center wavelength deviations easily exceeding ±1nm. Furthermore, the frequency conversion process introduces additional noise, resulting in a lower signal-to-noise ratio than direct-emission lasers. Additionally, the limited availability and high cost of 510nm-related optical components (such as cavity mirrors and coupling lenses) further restricts its commercialization and performance optimization. The mainstream 561nm laser architecture is DPSS (diode-pumped solid-state) laser, whose core challenge lies in power enhancement. High-power models require large-size gain media, leading to significant thermal lensing effects and beam divergence drift with power changes, making it difficult to maintain a stable Gaussian beam output. Moreover, DPSS lasers have a narrow repetition rate adjustment range in pulsed mode, making it difficult to meet the low duty cycle and high repetition rate pulse requirements of live-cell imaging. Furthermore, the coupling efficiency between the pump source and gain medium of this wavelength laser is easily affected by vibration and temperature, resulting in poor power stability during long-term use (power drift can reach ±2% / hour). Additionally, core components (such as frequency doubling crystals) are susceptible to optical damage due to temperature stress, shortening the overall lifespan of the laser. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a laser frequency conversion system and method based on thermal lensing effect enhancement. This system innovatively utilizes a nonlinear optical crystal doped with rare-earth ions. By introducing an auxiliary pump beam, a controllable thermal lensing effect is generated within the crystal, thereby actively focusing another fundamental wave beam, significantly increasing its energy density within the crystal, and ultimately achieving high-efficiency extracavity nonlinear frequency conversion.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: A laser frequency conversion system enhanced by thermal lensing effect, comprising: The first pump source is configured to emit the first pump light; A nonlinear frequency conversion crystal, wherein the crystal is made of calcium borate salt crystal doped with rare earth ions, with the molecular formula ReCa4O(BO3)3, where Re is a rare earth ion; the crystal is configured to receive a first pump light, wherein the rare earth ions in the crystal absorb the first pump light and generate a thermal lensing effect. The second pump source is configured to emit fundamental light; In this process, the fundamental light emitted by the second pump source is guided into a nonlinear frequency conversion crystal, and the optical axis of the fundamental light is highly coaxial with the optical axis of the first pump light within the crystal. The thermal lensing effect is used to focus the fundamental light within the crystal to increase its power density, and then, through the nonlinear optical effect of the crystal, at least a portion of the fundamental light is converted into harmonic light output.

[0009] Preferably, the rare earth ion Re is ytterbium ion Yb³⁺ or neodymium ion Nd³⁺.

[0010] Preferably, when the rare earth ion is Yb³⁺, the center wavelength of the first pump light is around 976 nm; When the rare earth ion is Nd³⁺, the center wavelength of the first pump light is around 808 nm.

[0011] Preferably, the second pump source is a single-mode fiber laser.

[0012] Preferably, the nonlinear optical effect is a frequency doubling effect; The wavelength of the fundamental light is selected from 1020 nm or 976 nm; The wavelengths of the harmonic light are 510 nm or 488 nm respectively.

[0013] Preferably, the nonlinear frequency conversion crystal is cut along the critical phase-matching direction of its nonlinear optical effect; The light-transmitting surface of the crystal is rectangular, with a thickness of 4mm, a width of 4mm, and a light-transmitting length of 5-50mm; The light-transmitting surface of the crystal is polished and coated with an optical film layer. The optical film layer is an anti-reflection film for the fundamental wave light and the harmonic light, and is an anti-reflection film for the first pump light, or its reflection characteristics are set according to the optical path requirements.

[0014] Preferably, it further includes at least one dichroic mirror; the dichroic mirror is disposed in the optical path for coaxially combining the first pump light and the fundamental light and guiding them into the nonlinear frequency conversion crystal, and / or for separating and outputting the generated harmonic light from the optical path.

[0015] Preferably, the nonlinear optical effect is a sum-frequency effect, a difference-frequency effect, a third harmonic effect, or a fourth harmonic effect.

[0016] Preferably, the second pump source is a wavelength-tunable fiber laser, which utilizes the characteristic of a small dispersion coefficient in a nonlinear frequency conversion crystal to achieve wavelength-tunable output of harmonic light.

[0017] Furthermore, this invention also mentions a method for enhancing the efficiency of extracavity optical frequency conversion using the thermal lensing effect, wherein the method employs a laser frequency conversion system based on the thermal lensing effect as described in any of the preceding claims; and includes the following steps: Step 1: Turn on the first pump source and let the emitted first pump light be incident on the nonlinear frequency conversion crystal, and form a thermal lens in the crystal through rare earth ion absorption; Step 2: Turn on the second pump source so that the emitted fundamental light and the first pump light are coaxially incident on the nonlinear frequency conversion crystal; Step 3: Use a thermal lens to focus the fundamental wave light, thereby increasing its energy density inside the crystal; Step 4: Through the nonlinear optical effect of the crystal, the high-energy-density fundamental light is efficiently converted into harmonic light and output.

[0018] The beneficial technical effects of this invention are as follows: 1. High Efficiency: This invention creatively transforms the traditionally harmful "thermal lensing effect" into a beneficial tool by actively focusing the fundamental wave light, thus multiplying its power density within the crystal. Since the nonlinear conversion efficiency is proportional to the fundamental wave power density, extremely high harmonic conversion efficiency can be achieved with relatively low fundamental wave input power, breaking through the efficiency bottleneck of traditional external cavity frequency doubling.

[0019] 2. High stability: The system is a single-pass structure without a complex resonant cavity, avoiding instability factors such as cavity length drift and mode competition. The fundamental source uses a highly stable fiber laser, resulting in excellent power and noise characteristics of the output laser.

[0020] 3. High controllability: By adjusting the power of the first pump light, the focal length and intensity of the thermal lens can be dynamically controlled, thereby adjusting the focusing degree of the fundamental light and the final harmonic output power, realizing the electrical adjustability of the output power.

[0021] 4. Flexible Applications: By selecting different rare-earth ions, fundamental wavelengths, and phase-matching methods, lasers in multiple commonly used biomedical wavelengths, ranging from blue to yellow-green, can be generated. Combining the characteristics of tunable fiber lasers and the wide phase-matching bandwidth of calcium borate crystals, tunable harmonic wavelength output can also be achieved.

[0022] 5. Relatively simple structure: Compared with complex intracavity frequency doubling lasers, this system has a simpler structure, higher reliability, and is easier to integrate and miniaturize. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the laser frequency conversion system structure of Embodiment 1 of the present invention.

[0024] Figure 2 A schematic diagram of the intracavity mode when a rare-earth-doped crystal is used as an independent laser cavity.

[0025] Figure 3 This is a schematic diagram of the propagation mode of the fundamental wave light in a crystal using a conventional external frequency doubling (without a thermal lens).

[0026] Figure 4 This is a schematic diagram of the propagation mode of the fundamental wave light in the crystal of the present invention (with thermal lens).

[0027] The labels in the diagram are as follows: 1-First pump source (semiconductor laser); 2-Nonlinear frequency conversion crystal (rare earth-doped calcium borate crystal); 3-First dichroic mirror; 4-Second dichroic mirror; 5-Coupled mirror; 6-Second pump source (fiber laser). Detailed Implementation

[0028] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments: Example 1: like Figure 1 As shown, this embodiment provides a laser frequency conversion system for generating 510nm green light.

[0029] First pump source 1: A high-power semiconductor laser with a center wavelength of 976nm is used to generate the first pump light.

[0030] Nonlinear frequency conversion crystal 2: A ytterbium-doped (Yb³⁺) calcium borate crystal (Yb:YCOB). The crystal is cut into a cuboid with dimensions of 4mm × 4mm × 20mm along the type I critical phase matching angle (θ = 115.3°, φ = 39.2°). The light-transmitting surface is polished. A film with high transmittance to 976nm and 1020nm and high reflectance to 510nm is deposited at the end near the first pump source 1; a film with high transmittance to 510nm, 976nm, and 1020nm is deposited at the end near the second pump source 6.

[0031] Second pump source 6: A single-mode polarization-maintaining fiber laser is used, with an output fundamental wavelength of 1020nm and a beam quality M²<1.1.

[0032] Optical path components: The first dichroic mirror 3 transmits 976nm light and reflects 1020nm light to form the optical path; the second dichroic mirror 4 transmits 1020nm fundamental light and reflects 510nm harmonic light to form the optical path. The coupling lens 5 is used to properly focus the collimated light output from the second pump source 6.

[0033] Working principle: The 976nm pump light is reflected by the first dichroic mirror 3 and then incident perpendicularly into the nonlinear frequency conversion crystal 2, where it is strongly absorbed by the Yb³⁺ ions. The absorbed energy is converted into heat, forming a thermally induced refractive index change region within the nonlinear frequency conversion crystal 2, which is approximately a positive lens, i.e., a "thermal lens".

[0034] The 1020nm fundamental light is emitted from the second pump source 6, passes through the coupling lens 5, and is reflected by the second dichroic mirror 4, then enters the nonlinear frequency conversion crystal 2 coaxially with the 976nm optical path. Upon entering the nonlinear frequency conversion crystal 2, the fundamental light is immediately focused by a thermal lens (e.g., ...). Figure 4 As shown in the figure, the beam diameter is compressed inside the crystal, and its power density increases sharply.

[0035] High-power-density 1020nm fundamental light undergoes nonlinear interaction in a Yb:YCOB crystal that satisfies phase-matching conditions, efficiently generating its frequency-doubled light, i.e., 510nm green laser. The 510nm light is output through the other end of the crystal and a second dichroic mirror 4.

[0036] By adjusting the driving current of the 976nm semiconductor laser 1, its output power can be changed, thereby changing the intensity of the thermal lens inside the nonlinear frequency conversion crystal 2, and thus continuously adjusting the output power of the 510nm harmonic light.

[0037] Example 2: This embodiment provides a laser frequency conversion system for generating 488nm blue light. Its structure is similar to that of Embodiment 1, except that: Replace the nonlinear frequency conversion crystal 2 with a neodymium-doped (Nd³⁺) calcium borate crystal (such as Nd:GdCOB).

[0038] Replace the first pump source 1 with a semiconductor laser with a center wavelength of 808nm.

[0039] Replace the second pump source 6 with a single-mode fiber laser that outputs 976nm fundamental light.

[0040] Accordingly, the phase matching angle of the crystal was adjusted to achieve frequency doubling from 976nm to 488nm, and the film systems of all optical elements were adjusted to match the three wavelengths of 808nm, 976nm, and 488nm.

[0041] The system works on the same principle as in Example 1, and ultimately outputs a 488nm blue laser.

[0042] Figure 2 The study demonstrates the mode distribution of the laser beam within an independent laser resonator when a rare-earth-doped crystal is used as the gain medium, and the beam propagates approximately parallel within the crystal.

[0043] Figure 3 This demonstrates the case of conventional external frequency doubling (such as using an LBO crystal). The fundamental wave diverges naturally in free space or within the crystal (such as a Gaussian beam), resulting in a short effective interaction region (sharp length) within the crystal and a low average power density.

[0044] Figure 4 The mechanism of the technical solution of this invention is demonstrated. Due to the focusing effect of the thermal lens, the fundamental wave light undergoes a "focusing-diverging" process within the crystal, its beam waist is "pulled" into the crystal interior, and the beam diameter remains small over a longer crystal length. This is equivalent to significantly extending the effective interaction length and increasing the average power density, which is the core advantage of this invention. This effect is not present in traditional nonlinear crystals (such as LBO and KTP).

[0045] This invention is not limited to the embodiments described above. For example, by using a tunable fiber laser as a second pump source and utilizing the small dispersion characteristics of calcium borate crystals, continuously tunable harmonic wavelengths can be achieved within a certain temperature tuning range. Furthermore, by designing different phase matching schemes and input fundamental wave combinations, various nonlinear frequency conversion processes such as sum-frequency (e.g., 1064nm + 1030nm → 523nm) and third harmonics can also be achieved.

[0046] Of course, the above description is not intended to limit the present invention, and the present invention is not limited to the examples given above. Any changes, modifications, additions or substitutions made by those skilled in the art within the scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A laser frequency conversion system based on the thermal lensing effect, characterized in that, include: The first pump source is configured to emit the first pump light; A nonlinear frequency conversion crystal, wherein the crystal is made of calcium borate salt crystal doped with rare earth ions, with the molecular formula ReCa4O(BO3)3, where Re is a rare earth ion; the crystal is configured to receive a first pump light, wherein the rare earth ions in the crystal absorb the first pump light and generate a thermal lensing effect. The second pump source is configured to emit fundamental light; In this process, the fundamental light emitted by the second pump source is guided into a nonlinear frequency conversion crystal, and the optical axis of the fundamental light is highly coaxial with the optical axis of the first pump light within the crystal. The thermal lensing effect is used to focus the fundamental light within the crystal to increase its power density, and then, through the nonlinear optical effect of the crystal, at least a portion of the fundamental light is converted into harmonic light output.

2. The laser frequency conversion system based on thermal lensing effect enhancement according to claim 1, characterized in that, The rare earth ion Re is either ytterbium ion (Yb³⁺) or neodymium ion (Nd³⁺).

3. The laser frequency conversion system based on thermal lensing effect enhancement according to claim 2, characterized in that: When the rare earth ion is Yb³⁺, the center wavelength of the first pump light is around 976 nm; When the rare earth ion is Nd³⁺, the center wavelength of the first pump light is around 808 nm.

4. The laser frequency conversion system based on thermal lensing effect enhancement according to claim 1, characterized in that, The second pump source is a single-mode fiber laser.

5. The laser frequency conversion system based on thermal lensing effect enhancement according to claim 1, characterized in that, The nonlinear optical effect is a frequency doubling effect; The wavelength of the fundamental light is selected from 1020 nm or 976 nm; The wavelengths of the harmonic light are 510 nm or 488 nm respectively.

6. The laser frequency conversion system based on thermal lensing effect enhancement according to claim 1, characterized in that, The nonlinear frequency conversion crystal is cut along the critical phase-matching direction of its nonlinear optical effect; The light-transmitting surface of the crystal is rectangular, with a thickness of 4mm, a width of 4mm, and a light-transmitting length of 5-50mm; The light-transmitting surface of the crystal is polished and coated with an optical film layer. The optical film layer is an anti-reflection film for the fundamental wave light and the harmonic light, and is an anti-reflection film for the first pump light, or its reflection characteristics are set according to the optical path requirements.

7. The laser frequency conversion system based on thermal lensing effect enhancement according to claim 1, characterized in that, It also includes at least one dichroic mirror; the dichroic mirror is disposed in the optical path for coaxially combining the first pump light and the fundamental light and guiding them into the nonlinear frequency conversion crystal, and / or for separating and outputting the generated harmonic light from the optical path.

8. The laser frequency conversion system based on thermal lensing effect enhancement according to claim 1, characterized in that, Nonlinear optical effects include sum-frequency effects, difference-frequency effects, third-harmonic effects, or fourth-harmonic effects.

9. The laser frequency conversion system based on thermal lensing effect enhancement according to claim 1, characterized in that, The second pump source is a wavelength-tunable fiber laser, which utilizes the characteristic of a small dispersion coefficient in a nonlinear frequency conversion crystal to achieve wavelength-tunable output of harmonic light.

10. A laser frequency conversion method based on thermal lensing effect enhancement, characterized in that, A laser frequency conversion system based on thermal lensing effect enhancement as described in any one of claims 1-9 is adopted, comprising the following steps: Step 1: Turn on the first pump source and let the emitted first pump light be incident on the nonlinear frequency conversion crystal, and form a thermal lens in the crystal through rare earth ion absorption; Step 2: Turn on the second pump source so that the emitted fundamental light and the first pump light are coaxially incident on the nonlinear frequency conversion crystal; Step 3: Use a thermal lens to focus the fundamental wave light, thereby increasing its energy density inside the crystal; Step 4: Through the nonlinear optical effect of the crystal, the high-energy-density fundamental light is efficiently converted into harmonic light and output.

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