CN111541140B - Yb-YAG ultrashort pulse laser amplifier based on brightness cascade pump - Google Patents

Yb-YAG ultrashort pulse laser amplifier based on brightness cascade pump Download PDF

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CN111541140B
CN111541140B CN202010253299.7A CN202010253299A CN111541140B CN 111541140 B CN111541140 B CN 111541140B CN 202010253299 A CN202010253299 A CN 202010253299A CN 111541140 B CN111541140 B CN 111541140B
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laser
yag
fiber
crystal
brightness
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CN111541140A (en
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赵智刚
王上
刘兆军
丛振华
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Shandong University
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/23—Arrangements of two or more lasers not provided for in groups H01S3/02 - H01S3/22, e.g. tandem arrangements of separate active media
    • H01S3/2375—Hybrid lasers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/0602—Crystal lasers or glass lasers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
    • H01S3/094042—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light of a fibre laser
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/106—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
    • H01S3/108—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using non-linear optical devices, e.g. exhibiting Brillouin or Raman scattering
    • H01S3/109—Frequency multiplication, e.g. harmonic generation

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Abstract

The invention relates to a Yb: YAG ultrashort pulse laser amplifier based on brightness cascade pumping, which comprises a 1938nm narrow-linewidth fiber laser system, a nonlinear laser frequency doubling system and a Yb: YAG crystal amplifier system which are sequentially arranged along a light path. According to the invention, a 793nm LD pumped thulium-doped fiber laser is used for obtaining a 1938nm laser light source with high brightness and narrow line width, and then the 1938nm laser is frequency-doubled to 969nm by a laser frequency doubling technology. And finally, using high-brightness 969nm laser as a pumping source of an Yb: YAG ultrashort pulse laser amplifier. The whole laser system realizes brightness cascade pumping conversion, the improvement of the brightness of the pumping source is beneficial to realizing good mode matching of pumping light and signal light in Yb: YAG crystal, the conversion efficiency of the laser is greatly improved, the heat effect of the laser is relieved, and the beam quality and the output power of the signal light are improved.

Description

Yb-YAG ultrashort pulse laser amplifier based on brightness cascade pump
Technical Field
The invention relates to a Yb-YAG ultrashort pulse laser amplifier based on brightness cascade pumping, belonging to the technical field of laser amplifiers.
Background
Ultrashort pulse lasers are typically referred to as lasers with pulse widths in picoseconds (10)-12s) and femtosecond (10)-15s) magnitude pulse laser light source, has characteristics such as extremely narrow pulse, extremely wide spectrum, extremely high peak power, is known as "brightest light", "most accurate chi", "fastest sword". At present, ultrashort pulse laser has been widely used in the fields of scientific research, industrial production, material processing, biomedical treatment, etc.
The mode-locked laser is an effective technical means for obtaining ultrashort pulse laser, and the currently commonly used mode-locked laser mainly comprises a solid mode-locked laser and an optical fiber mode-locked laser. However, the power and energy of the pulse laser directly obtained from the mode-locked resonator are generally low, and in order to further improve the characteristics of the pulse laser, the seed light needs to be amplified by an amplifier. Among them, the Yb: YAG crystal is widely used as a gain medium of an ultrashort pulse laser amplifier due to its excellent physical and optical characteristics. YAG crystal geometry is currently divided into a number of classes, and a typical amplifier structure mainly comprises: rod crystal amplifier, crystal fiber amplifier, slab amplifier, and chip amplifier. In the structural design of the amplifier, one of the key requirements is to improve the heat dissipation efficiency of the crystal as much as possible under the condition of ensuring the high gain characteristic of the laser crystal. This is because in high power laser amplifiers, thermal effects can severely affect the gain characteristics of the crystal and degrade the beam quality of the laser. The thermal effect of the crystal is mainly due to the quantum defect between the pump light and the signal light, so the choice of the pump laser wavelength has an important influence on the thermal effect of the amplifier.
YAG crystal has two main spectrum absorption peaks, one is near 940nm, and the other is near 969 nm. YAG crystal has a large absorption bandwidth around 940nm, so the requirement on the wavelength stability of the pump laser is relatively low. In contrast, the narrow absorption bandwidth of Yb: YAG crystals at 969nm typically requires wavelength locking of the pump laser, which adds to some degree cost and complexity to the system. However, since 969nm is within the zero phonon absorption linewidth of Yb: YAG crystals, the thermal effect due to quantum defect can be reduced by about 34% compared to 940nm pumping, which means that the pumping power density can be increased by about 48% with 969nm laser pumping at the same degree of thermal effect (Optics Letters,2012,37(15): 3045-3047.). YAG crystal fiber amplifiers with 969nm pumping are known to have a maximum output power of about 70W (Optics letters,2015,40(11): 2517-. While the chip amplifier benefits from the excellent heat dissipation structure, the maximum output power is close to 2kW under 969nm pumping (Laser consistency 2019(ASSL, LAC, LS & C), OSA Technical Digest (Optical Society of America,2019), paper ATh1A.8.).
Theoretically, the thermal effect due to quantum defect is certain at the same power density after the wavelength of the pump laser is determined. Therefore, in order to further increase the output power of the amplifier, it is necessary to reduce the thermal effect by optimizing the energy conversion efficiency of the amplifier. The degree of mode matching and the working distance of the signal light and the pump light in the gain medium have an important influence on the energy conversion efficiency of the amplifier. However, the 940nm and 969nm pump lasers commonly used at present are mainly fiber-coupled-out Laser Diodes (LDs), wherein one of the salient features of LDs is low brightness. Taking a typical 100W-level 940nm LD as an example, the numerical aperture NA of the coupled-out optical fiber is 0.22, the fiber core diameter is 105 μ M, and the corresponding beam quality factor M2Approximately 40 a. The output signal light of the conventional mode-locked laser is usually a near diffraction limit beam (beam quality factor M)2Close to 1). Therefore, the difference between the two beam qualities greatly limits the axial mode matching of the pump light and the signal light in the Yb: YAG crystal. Especially for Yb: YAG crystal amplifiers based on end-pumping, the mode mismatch can lead to a reduction in the energy conversion efficiency of the laser, which in turn leads to an increase in the thermal effect phenomenon. In addition, the gain of the laser signal generally increases with the increase of the effective length of the gain medium under the unsaturated amplification state, however, the limited rayleigh length of the LD pump light greatly limits the effective length available for the Yb: YAG crystal. Taking an end-pumped rod-shaped Yb: YAG crystal amplifier as an example, under the premise of not considering the influence of the doping concentration of rare earth ions, the typical crystal length values are 30mm and 40 mm. The insufficient effective length of the crystal, on the one hand, does not provide sufficient gain for the signal light at high power and, on the other hand, increases the thermal load on the crystal. The above problems all indicate that the LD pump light source with low brightness is an important factor for limiting the performance improvement of the Yb to YAG ultrashort pulse laser amplifier.
The fiber laser is widely applied due to the characteristics of compact structure, good beam quality, high output power and the like. However, it is difficult to obtain high power 940nm and 969nm laser light sources directly through a fiber laser in practical applications. In contrast, thulium (Tm) doped fibers have a broad emission spectrum in the 2.0 μm band, and it is known that 1.94 μm laser output on the order of hundred watts can be obtained with conventional 793nm LD pumping (Applied optics,2018,57(20): 5574-. Therefore, a 1938nm fiber laser light source with high brightness and narrow line width can be obtained from a 793nm LD light source with low brightness and wide spectrum by a Tm-doped fiber laser. By combining the mature laser frequency doubling technology at present, the frequency of 1938nm laser can be further doubled to 969nm, so that a 969nm laser pumping source with high brightness, narrow line width and stable wavelength is provided for the Yb: YAG ultrashort pulse laser amplifier.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a Yb: YAG ultrashort pulse laser amplifier based on brightness cascade pumping.
The technical scheme of the invention is as follows:
a Yb: YAG ultrashort pulse laser amplifier based on brightness cascade pumping comprises a 1938nm narrow-linewidth fiber laser system, a nonlinear laser frequency doubling system and a Yb: YAG crystal amplifier system which are sequentially arranged along a light path;
the 1938nm narrow linewidth fiber laser system comprises a first semiconductor laser, a first pump beam combiner, a high-reflectivity fiber grating, a first thulium-doped fiber, a low-reflectivity fiber grating, a fiber isolator, a second semiconductor laser, a second pump beam combiner, a second thulium-doped fiber, a fiber cladding light stripper and a fiber collimator which are sequentially arranged along a light path;
the nonlinear laser frequency doubling system comprises a first half-wave plate, a first spatial optical isolator, a second half-wave plate, a first optical lens, a nonlinear laser frequency doubling crystal and a first dichroic mirror which are sequentially arranged along a light path;
the Yb: YAG crystal amplifier system comprises a single-pass amplifier structure or a double-pass amplifier structure;
the single-pass amplifier structure comprises a seed light source, a third half-wave plate, a second spatial light isolator, a 45-degree reflector, a second optical lens, a second dichroic mirror, a Yb (Yb) YAG crystal, a third dichroic mirror, a third optical lens, a fourth dichroic mirror, a Yb YAG crystal, a third dichroic mirror and a light trap which are sequentially arranged along a seed light path, and a 969nm pump light path;
the double-pass amplifier structure comprises a seed light source, a third half-wave plate, a second spatial light isolator, a 45-degree reflector, a fourth half-wave plate, a polarization beam splitter, a second optical lens, a second dichroic mirror, a Yb: YAG crystal, a third dichroic mirror, an 1/4 wave plate and a 0-degree reflector which are sequentially arranged along a seed light path, and a third optical lens, a fourth optical lens, a second dichroic mirror, a Yb: YAG crystal, a third dichroic mirror and a light trap which are sequentially arranged along a 969nm pump light path.
Preferably, the center wavelengths of the first semiconductor laser and the second semiconductor laser are both 793 nm.
Preferably, the core diameters of the coupling-out optical fibers of the first semiconductor laser and the second semiconductor laser comprise 105/125 μm or 200/220 μm, the numerical aperture NA of the core is 0.22, and the output power of a single LD is 50W.
Preferably, the first pump combiner and the second pump combiner each have a plurality of pump ports. The advantage of this design is that multiple semiconductor lasers can be pumped simultaneously.
Further preferably, the first pump beam combiner has a (2+1) × 1 structure, the fiber core diameter at the pump input end is 105/125 μm, and the fiber core numerical aperture NA is 0.22.
Further preferably, the second pump beam combiner is of a (4+1) × 1 structure, the diameter of the optical fiber core at the pump input end is 105/125 μm, and the numerical aperture NA of the optical fiber core is 0.22; the diameter of the optical fiber core at the signal end is 25/400 μm, and the numerical aperture of the optical fiber core is 0.1.
Preferably, the first thulium-doped optical fiber and the second thulium-doped optical fiber are both polarization maintaining optical fibers.
Preferably, the optical fiber length of the first thulium-doped optical fiber is 3.0-4.0 m.
Further preferably, the optical fiber length of the first thulium-doped optical fiber is 3.0 m.
Preferably, the optical fiber length of the second thulium-doped optical fiber is 3.0-5.0 m.
Further preferably, the optical fiber length of the second thulium-doped optical fiber is 4.0 m.
Preferably, the reflection bandwidth of the high-reflectivity fiber grating is 1.0-3.0 nm, and the reflectivity is more than 99%.
Further preferably, the central wavelength of the high-reflectivity fiber grating is 1938.0nm, the reflection bandwidth is 2.0nm, and the reflectivity is more than 99%.
Preferably, the reflection bandwidth of the low-reflectivity fiber grating is 1.0-2.0 nm, and the reflectivity is 10-20%.
More preferably, the low-reflectivity fiber grating has a center wavelength of 1938.0nm, a reflection bandwidth of 1.0nm and a reflectivity of 10%.
Preferably, the nonlinear laser frequency doubling crystal comprises a magnesium oxide periodically poled lithium niobate crystal, a barium metaborate crystal, a lithium triborate crystal or a potassium titanyl phosphate crystal.
Preferably, the seed light source comprises a mode-locked fiber laser or a solid mode-locked laser.
Preferably, the pulse width of the seed light emitted by the seed light source is picoseconds (10)-12s) order or femtosecond (10)-15s) order of magnitude and pulse repetition frequency in the kHZ to MHz order of magnitude.
Preferably, the Yb: YAG crystal is a rod-shaped structure, the diameter of the crystal is 1.0-2.0 mm, the length of the crystal is 30-50 mm, and the ion doping concentration is 1 at% or 1.5 at% or 2.0 at%.
The invention has the beneficial effects that:
1. the ultrashort pulse laser amplifier firstly utilizes a Tm-doped fiber laser of a 793nm LD pump to obtain a 1938nm laser light source with high brightness and narrow line width, and then multiplies the frequency of the 1938nm laser to 969nm by a laser frequency multiplication technology. Finally, a high-brightness 969nm laser was used as a pump source for a Yb: YAG ultrashort pulse laser amplifier. The whole laser system realizes effective brightness cascade pumping conversion, namely realizes the output from a low-brightness 793nm LD light source to high-brightness 1938nm and 969nm laser light sources and finally to ultrashort pulse laser with higher brightness, high power and high energy. The improvement of the brightness of the pumping source is beneficial to realizing good mode matching of the pumping light and the signal light in the Yb-YAG crystal, thereby greatly improving the conversion efficiency of the laser, relieving the heat effect of the laser and improving the beam quality of the signal light. Meanwhile, the increase of the axial mode matching range creates favorable conditions for adopting a Yb: YAG crystal with a longer size, and the increase of the crystal length can provide higher signal gain and better heat management capability for the amplifier.
2. Compared with 940nm and 969nm LD lasers, the 969nm laser has the advantages of narrow line width, stable wavelength and no need of wavelength locking; compared with 940nm and 969nm LD lasers, the 969nm laser has high brightness and large Rayleigh length. Comparing the 969nm LD laser with the 969nm laser of the present invention, it can be found that the 969nm LD laser has low brightness and small Rayleigh length, so that the pumping light can only realize mode matching with the seed light within a very limited range in the Yb: YAG crystal, resulting in short effective length of the crystal, low signal gain, low laser conversion efficiency and serious thermal effect. In contrast, the 969nm laser of the invention has high brightness and large Rayleigh length, and the pump light and the signal light can realize good mode matching in a longer range in the crystal, thereby having high laser conversion efficiency and reduced thermal effect. The larger rayleigh length also allows the effective length of the Yb: YAG crystal to be significantly increased, thereby providing higher gain for the signal light.
Drawings
FIG. 1 is a schematic structural diagram of a 1938nm narrow linewidth fiber laser system according to the present invention.
Fig. 2 is a schematic structural diagram of the nonlinear laser frequency doubling system according to the present invention.
Fig. 3 is a schematic diagram of a single pass amplifier according to the present invention.
Fig. 4 is a schematic structural diagram of a double-pass amplifier according to the present invention.
FIGS. 5(a) and 5(b) are diagrams illustrating the different mode distributions of the 969nm LD laser and the 969nm laser of the present invention in the Yb: YAG crystal.
In the figure: 1. a first semiconductor laser; 2. a first pump combiner; 3. high-reflectivity fiber grating; 4. a first thulium (Tm) doped fiber; 5. a low-reflectivity fiber grating; 6. a fiber isolator; 7. a second semiconductor laser; 8. a second pump combiner; 9. a second thulium (Tm) doped fiber; 10. an optical fiber cladding stripper; 11. a fiber collimator; 12. a first half wave plate; 13. a first spatial light isolator; 14. a second half-wave plate; 15. a first optical lens; 16. a nonlinear laser frequency doubling crystal; 17. a first dichroic mirror; 18. a seed light source; 19. a third half-wave plate; 20. a second spatial light isolator; 21. a 45 degree mirror; 22. a second optical lens; 23. a second dichroic mirror; 24. yb is YAG crystal; 25. a third dichroic mirror; 26. a third optical lens; 27. a fourth optical lens; 28. a light trap; 29. a fourth half-wave plate; 30. a polarizing beam splitter; 31. 1/4 a wave plate; 32. a 0 degree mirror.
Detailed Description
The present invention will be further described by way of examples, but not limited thereto, with reference to the accompanying drawings.
Example 1:
as shown in fig. 1 to fig. 3, the present embodiment provides a Yb: YAG ultrashort pulse laser amplifier based on brightness cascade pumping, which includes a 1938nm narrow-linewidth fiber laser system, a nonlinear laser frequency doubling system, and a Yb: YAG crystal amplifier system sequentially arranged along an optical path;
the 1938nm narrow-linewidth fiber laser system comprises a first semiconductor laser 1, a first pump beam combiner 2, a high-reflectivity fiber grating 3, a first thulium-doped fiber 4, a low-reflectivity fiber grating 5, a fiber isolator 6, a second semiconductor laser 7, a second pump beam combiner 8, a second thulium-doped fiber 9, a fiber cladding light stripper 10 and a fiber collimator 11 which are sequentially arranged along a light path;
the nonlinear laser frequency doubling system comprises a first half-wave plate 12, a first spatial optical isolator 13, a second half-wave plate 14, a first optical lens 15, a nonlinear laser frequency doubling crystal 16 and a first dichroic mirror 17 which are sequentially arranged along a light path;
the Yb: YAG crystal amplifier system is of a single-pass amplifier structure; the single-pass amplifier structure comprises a seed light source 18, a third half-wave plate 19, a second spatial light isolator 20, a 45-degree reflecting mirror 21, a second optical lens 22, a second dichroic mirror 23, a Yb: YAG crystal 24 and a third dichroic mirror 25 which are sequentially arranged along a seed light optical path, and a third optical lens 26, a fourth optical lens 27, the second dichroic mirror 23, the Yb: YAG crystal 24, the third dichroic mirror 25 and a light trap 28 which are sequentially arranged along a 969nm pump light optical path.
As shown in fig. 1, a coupling output fiber of a first semiconductor laser 1 is connected to a pump input end of a first pump combiner 2, an output end of the first pump combiner 2 is connected to an input end of a first thulium (Tm) doped fiber 4, an output end of the first thulium (Tm) doped fiber 4 is connected to an input end of a fiber isolator 6, an output end of the fiber isolator 6 is connected to a signal input end of a second pump combiner 8, the coupling output optical fiber of the second semiconductor laser 7 is connected with the pumping input end of the second pumping beam combiner 8, the output end of the second pumping beam combiner 8 is connected with the input end of the second thulium (Tm) doped optical fiber 9, the output end of the second thulium (Tm) doped optical fiber 9 is connected with the input end of the optical fiber cladding light stripper 10, and the output end of the optical fiber cladding light stripper 10 is connected with the input end of the optical fiber collimator 11.
The first semiconductor laser 1 and the second semiconductor laser 7 each have a center wavelength of 793nm and do not require wavelength locking. The core diameter of the coupling-out optical fiber is 105/125 μm, the numerical aperture NA of the core is 0.22, and the output power of a single LD is 50W.
The first pump beam combiner 2 and the second pump beam combiner 8 are both provided with a plurality of pump ports, so that a plurality of semiconductor lasers can be pumped simultaneously.
In this embodiment, the first pump beam combiner 2 has a (2+1) × 1 structure, the fiber core diameter at the pump input end is 105/125 μm, and the fiber core numerical aperture NA is 0.22.
The second pump beam combiner 8 is of a (4+1) multiplied by 1 structure, the diameter of an optical fiber core at the pump input end is 105/125 mu m, and the numerical aperture NA of the optical fiber core is 0.22; the fiber core diameter of the signal end is 25/400 μm, and the numerical aperture of the fiber core is 0.1(Nufern, FUD-3716, PLMA-GDF-25/400-10 FA).
The central wavelength of the high-reflectivity fiber grating 3 is 1938.0nm, the reflection bandwidth is 2.0nm, and the reflectivity is more than 99%.
The central wavelength of the low-reflectivity fiber grating 5 is 1938.0nm, the reflection bandwidth is 1.0nm, and the reflectivity is 10%.
The first thulium (Tm) doped fiber 4 is a polarization maintaining double-clad fiber (Nufern, PM-TDF-10P-130-HE), the core diameter is 10/130 μm, the numerical aperture NA of the core is 0.15, the numerical aperture NA of the inner cladding is 0.46, the cladding absorption coefficient is 4.70dB/m @793nm, and the length of the fiber is 3.0 m.
The second thulium (Tm) doped fiber 9 is a polarization maintaining double clad fiber (Nurfern, PLMA-25P/400-HE), the core diameter is 25/400 μm, the numerical aperture NA of the core is 0.09, the numerical aperture NA of the inner cladding is 0.46, the cladding absorption coefficient is 2.40dB/m @793nm, and the length of the fiber is 4.0 m.
The optical fiber cladding stripper 10 and the optical fiber collimator 11 adopt the same optical fiber, the core diameter of the optical fiber is 25/400 μm, and the numerical aperture of the core is 0.1(FUD-3716, PLMA-GDF-25/400-10 FA). The optical fiber collimator 11 is used for shaping and collimating the 1938nm laser beam.
A 1938nm laser resonant cavity is formed by a first semiconductor laser 1, a first pump beam combiner 2, a high-reflectivity fiber grating 3, a first thulium (Tm) doped fiber 4 and a low-reflectivity fiber grating 5 and is used for generating low-power 1938nm seed light; the second semiconductor laser 7, the second pump beam combiner 8 and the second thulium (Tm) doped optical fiber 9 form an amplifier part for amplifying the 1938nm seed light.
As shown in fig. 2, the 1938nm laser sequentially passes through the first half-wave plate 12, the first spatial optical isolator 13, and the second half-wave plate 14, and then enters the nonlinear laser frequency doubling crystal 16, the 969nm laser obtained by frequency doubling is reflected by the first dichroic mirror 17 and output, and the rest of the 1938nm laser passes through the first dichroic mirror 17 and is output.
The first half-wave plate 12 is used to adjust the polarization direction of the 1938nm laser light to pass through the first spatial optical isolator 13.
The second half-wave plate 14 is used for adjusting the polarization direction of the 1938nm laser, so that the requirements of the nonlinear laser frequency doubling crystal 16 on the polarization direction of the laser are met.
The first optical lens 15 is used for adjusting the spot size of the 1938nm laser, so that the laser frequency doubling requirement is met.
The nonlinear laser frequency doubling crystal 16 is a magnesium oxide periodically poled lithium niobate crystal (MgO-PPLN), the length of the crystal is 40mm, the width is 10mm, the thickness is 0.5mm, the poling period is 29.0 μm, phase matching is carried out by temperature adjustment, and the temperature adjustment range is 50-150 ℃.
The first dichroic mirror 17 has an extremely high reflectance for 969nm laser light and an extremely high transmittance for 1938nm laser light.
As shown in fig. 3, the propagation path of the seed light: the seed light enters a 45-degree reflector 21 after passing through a third half-wave plate 19 and a second spatial light isolator 20, is reflected by the 45-degree reflector 21 and then is subjected to beam shaping through a second optical lens 22, the shaped seed light is reflected by a second dichroic mirror 23 and enters a Yb: YAG crystal 24 for amplification, and the amplified seed light is reflected and output by a third dichroic mirror 25.
Propagation path of 969nm pump laser: the 969nm pump laser sequentially passes through the third optical lens 26 and the fourth optical lens 27 for beam shaping, the shaped pump laser is coupled into the Yb: YAG crystal 24 through the second dichroic mirror 23 to provide energy for amplification of seed light, and the rest pump laser is collected by the light trap 28 after passing through the third dichroic mirror 25.
The seed light source 18 is a mode-locked fiber laser, the center wavelength is 1030nm, the spectral bandwidth is 15nm, the pulse width is 400fs, and the pulse repetition frequency is 69 MHz.
The forward transmittance of the second spatial light isolator 20 to the seed light is greater than 95%, and the reverse isolation is greater than 33 dB.
The 45-degree mirror 21 has an extremely high reflectance when the seed light is incident at an angle of 45 degrees.
The second optical lens 22, the third optical lens 26 and the fourth optical lens 27 are respectively used for beam shaping of the seed light and the 969nm pump light, so that mode matching is realized in the Yb: YAG crystal 24; the size of the focal length of the lens depends on the actual spot sizes of the seed light and the pump light and their spot sizes in the crystal.
The second dichroic mirror 23 and the third dichroic mirror 25 both have extremely high reflectivity for 1030nm seed light and extremely high transmittance for 969nm pump light.
The Yb: YAG crystal 24 is a rod-like structure, and the diameter and length of the crystal depend on the spot size and beam quality of the signal light and the pump light. In this example, the Yb: YAG crystal 24 had a diameter of 1mm, a length of 50mm and an ion doping concentration of 1 at%.
Example 2:
as shown in fig. 4, this embodiment provides a Yb: YAG ultrashort pulse laser amplifier based on brightness cascade pumping, the structure and connection relationship of which are as described in embodiment 1, and the differences are as follows: the Yb: YAG crystal amplifier system is a double-pass amplifier structure. The double-pass amplifier structure comprises a seed light source 18, a third half-wave plate 19, a second spatial light isolator 20, a 45-degree reflecting mirror 21, a fourth half-wave plate 29, a polarization beam splitter 30, a second optical lens 22, a second dichroic mirror 23, a Yb: YAG crystal 24, a third dichroic mirror 25, an 1/4 wave plate 31 and a 0-degree reflecting mirror 32 which are sequentially arranged along a seed light optical path, and a third optical lens 26, a fourth optical lens 27, the second dichroic mirror 23, the Yb: YAG crystal 24, the third dichroic mirror 25 and a light trap 28 which are sequentially arranged along a 969nm pump light optical path.
Propagation path of seed light: the seed light enters the 45-degree reflector 21 after passing through the third half-wave plate 19 and the second spatial optical isolator 20, is reflected by the 45-degree reflector 21, passes through the fourth half-wave plate 29 to adjust the polarization direction of the seed light to enable the seed light to horizontally pass through the polarization beam splitter 30, then passes through the second optical lens 22 to be subjected to beam shaping, the shaped seed light is reflected by the second dichroic mirror 23 to enter the Yb: YAG crystal 24 for amplification, the amplified seed light is reflected by the third dichroic mirror 25, passes through the 1/4 wave plate 31, is reflected by the 0-degree reflector 32, then passes through the 1/4 wave plate 31 again, the seed light enters the Yb: YAG crystal 24 again after being reflected by the third dichroic mirror 25 for amplification, and the second amplified seed light is reflected by the second dichroic mirror 23, passes through the second optical lens 22, and then is output from the polarization direction perpendicular to the polarization beam splitter 30.
Propagation path of 969nm pump light: the 969nm pump laser sequentially passes through the third optical lens 26 and the fourth optical lens 27 for beam shaping, the shaped pump laser is coupled into the Yb: YAG crystal 24 through the second dichroic mirror 23, and the remaining pump laser is collected by the light trap 28 after passing through the third dichroic mirror 25.
The fourth half-wave plate 29 is used to adjust the polarization direction of the seed light to pass horizontally through the polarization beam splitter 30.
The 0 degree mirror 32 has an extremely high reflectivity for the signal light, and the signal light can pass through the Yb: YAG crystal 24 for the second time to be amplified after being reflected by the 0 degree mirror.
1/4 wave plate 31 is used to adjust the polarization direction of the reflected signal light to make it perpendicular to the incident polarization direction. When the signal light passes through 1/4 wave plate 31 twice, the polarization direction is perpendicular to the original direction, so that the amplified signal light can be output from the perpendicular component of polarization beam splitter 30.
Example 3:
the Yb: YAG ultrashort pulse laser amplifier based on brightness cascade pumping has the structure and the connection relation as the embodiment 1, and the difference is that: the coupling-out fiber core diameters of the first semiconductor laser 1 and the second semiconductor laser 7 include 200/220 μm. The optical fiber length of the first thulium-doped optical fiber 4 is 4.0m, and the optical fiber length of the second thulium-doped optical fiber 9 is 5.0 m. The reflectivity of the low-reflectivity fiber grating 5 is 15%. The nonlinear laser frequency doubling crystal 16 is a barium metaborate crystal (BBO). YAG crystal 24 of Yb has an ion doping concentration of 1.5 at% and a length of 40 mm.
Example 4:
the Yb: YAG ultrashort pulse laser amplifier based on brightness cascade pumping has the structure and the connection relation as the embodiment 1, and the difference is that: the nonlinear laser frequency doubling crystal 16 is a lithium triborate crystal (LBO). The Yb: YAG crystal 24 had an ion doping concentration of 2.0 at% and a length of 30 mm.
Example 5:
the Yb: YAG ultrashort pulse laser amplifier based on brightness cascade pumping has the structure and the connection relation as the embodiment 1, and the difference is that: the nonlinear laser frequency doubling crystal 16 is a potassium titanyl phosphate crystal (KTP). The seed light source 18 is a solid mode-locked laser.

Claims (8)

1. A Yb: YAG ultrashort pulse laser amplifier based on brightness cascade pumping is characterized by comprising a 1938nm narrow-linewidth fiber laser system, a nonlinear laser frequency doubling system and a Yb: YAG crystal amplifier system which are sequentially arranged along a light path;
the 1938nm narrow linewidth fiber laser system comprises a first semiconductor laser, a first pump beam combiner, a high-reflectivity fiber grating, a first thulium-doped fiber, a low-reflectivity fiber grating, a fiber isolator, a second semiconductor laser, a second pump beam combiner, a second thulium-doped fiber, a fiber cladding light stripper and a fiber collimator which are sequentially arranged along a light path;
the nonlinear laser frequency doubling system comprises a first half-wave plate, a first spatial optical isolator, a second half-wave plate, a first optical lens, a nonlinear laser frequency doubling crystal and a first dichroic mirror which are sequentially arranged along a light path;
the Yb: YAG crystal amplifier system comprises a single-pass amplifier structure or a double-pass amplifier structure;
the single-pass amplifier structure comprises a seed light source, a third half-wave plate, a second spatial light isolator, a 45-degree reflector, a second optical lens, a second dichroic mirror, a Yb (Yb) YAG crystal, a third dichroic mirror, a third optical lens, a fourth dichroic mirror, a Yb YAG crystal, a third dichroic mirror and a light trap which are sequentially arranged along a seed light path, and a 969nm pump light path;
the double-pass amplifier structure comprises a seed light source, a third half-wave plate, a second spatial light isolator, a 45-degree reflector, a fourth half-wave plate, a polarization beam splitter, a second optical lens, a second dichroic mirror, a Yb (yttrium aluminum garnet) crystal, a third dichroic mirror, an 1/4 wave plate and a 0-degree reflector which are sequentially arranged along a seed light path, and a third optical lens, a fourth optical lens, a second dichroic mirror, a Yb (yttrium aluminum garnet) crystal, a third dichroic mirror and a light trap which are sequentially arranged along a 969nm pump light path;
the central wavelengths of the first semiconductor laser and the second semiconductor laser are 793 nm;
the diameters of the coupling output optical fiber cores of the first semiconductor laser and the second semiconductor laser comprise 105/125 micrometers or 200/220 micrometers, the numerical aperture NA of the fiber cores is 0.22, and the output power of a single LD is 50W;
the central wavelength of the high-reflectivity fiber grating is 1938.0nm, the reflection bandwidth is 2.0nm, and the reflectivity is more than 99%;
the central wavelength of the low-reflectivity fiber grating is 1938.0nm, the reflection bandwidth is 1.0nm, and the reflectivity is 10%.
2. The Yb: YAG ultrashort pulse laser amplifier based on brightness cascade pumping of claim 1, wherein the first pump beam combiner and the second pump beam combiner each have a plurality of pump ports.
3. The Yb: YAG ultrashort pulse laser amplifier based on brightness cascade pumping as claimed in claim 1, wherein the first thulium doped fiber and the second thulium doped fiber are both polarization maintaining fibers.
4. The Yb: YAG ultrashort pulse laser amplifier based on brightness cascade pumping as claimed in claim 1, wherein the optical fiber length of the first thulium doped optical fiber is 3.0-4.0 m.
5. The Yb: YAG ultrashort pulse laser amplifier based on brightness cascade pumping as claimed in claim 1, wherein the optical fiber length of the second thulium doped optical fiber is 3.0-5.0 m.
6. The Yb: YAG ultrashort pulse laser amplifier based on brightness cascade pumping of claim 1, wherein the nonlinear laser frequency doubling crystal comprises a magnesium oxide periodically poled lithium niobate crystal, a barium metaborate crystal, a lithium triborate crystal or a potassium titanyl phosphate crystal.
7. The Yb: YAG ultrashort pulse laser amplifier based on brightness cascade pumping of claim 1 wherein the seed light source comprises a mode-locked fiber laser or a solid-state mode-locked laser.
8. The Yb: YAG ultrashort pulse laser amplifier based on brightness cascade pumping as claimed in claim 1, wherein the Yb: YAG crystal is rod-shaped, the diameter of the crystal is 1.0-2.0 mm, the length of the crystal is 30-50 mm, and the ion doping concentration is 1 at% or 1.5 at% or 2.0 at%.
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