Synchronous pumping of harmonically mode-locked fiber lasers and mid-infrared picosecond optical parametric oscillators

Abstract

The present invention discloses synchronous pumping of a harmonically mode-locked fiber laser and a mid-infrared picosecond optical parametric oscillator. The mid-infrared picosecond optical parametric oscillator includes a pump source, a quarter-wave plate, a half-wave plate, and a polarization beam splitter for controlling the pump power, all arranged on the same horizontal line. The polarization beam splitter and a high-reflection mirror are arranged on the same vertical line. The high-reflection mirror, a first plano-convex lens, a first plano-concave mirror, an MgO:PPLN crystal, a second plano-concave mirror, a second plano-convex mirror, a reflector, and a beam splitter are also arranged on the same horizontal line. The synchronous pumping of a harmonically mode-locked fiber laser includes a pump source, which includes a harmonically mode-locked erbium-doped fiber laser and a MOPA amplifier. The present invention achieves low average dispersion and high average nonlinearity in the fiber laser, enabling high signal-to-noise ratio and high stability of harmonically mode-locked laser output, overcoming the technical biases of uneven pulse intensity and low signal-to-noise ratio in traditional mid-infrared tunable ultrafast laser output.

Description

Technical Field

[0001] The present invention relates to a pump and an oscillator, in particular to a synchronous pump of a harmonic mode-locked fiber laser and a mid-infrared picosecond optical parametric oscillator. Background Art

[0002] Ultrafast mid-infrared lasers are widely needed in fields such as spectroscopy, remote sensing, and biomedicine. In particular, high-repetition-rate ultrafast mid-infrared lasers are important tools in fields such as nonlinear optics, pump detection, and biophotonics. Optical parametric oscillators (OPOs) are the most effective way to generate ultrafast mid-infrared lasers. They have the advantages of extremely wide spectral coverage and tuning range, high power output, and good stability. Currently, the main methods for generating high-repetition-rate ultrafast mid-infrared lasers are synchronously pumped OPOs with titanium sapphire lasers, harmonic pumped OPOs, and Vernier effect pumped OPOs. Although these methods have achieved certain results, they all have their inevitable disadvantages. Titanium sapphire lasers usually require large water cooling systems and pumping systems, and are very expensive, which greatly limits their practical applications. With harmonic pumping and Vernier effect pumping, because the signal light circulates N times within the cavity before encountering the next pump light and thus being amplified, the pulse amplitude of the signal light output is very uneven, affecting the uniformity of the pulse energy and peak power of the generated mid-infrared laser and resulting in an extremely low signal-to-noise ratio. Furthermore, because only the signal light oscillates, the repetition rate of the output idler light is not increased. These shortcomings significantly limit their application and are urgently in need of resolution.

[0003] In summary, the problems and defects of the existing technology are as follows:

[0004] (1) The amplitude of the pulses output by existing high-repetition-rate mid-infrared ultrafast lasers is very uneven, and the peak power and pulse energy are also uneven, resulting in poor stability.

[0005] (2) The signal-to-noise ratio of existing high-repetition-rate mid-infrared ultrafast lasers is low;

[0006] (3) The repetition rate of the idler light output by the existing high-repetition-rate mid-infrared ultrafast laser has not been improved. Summary of the Invention

[0007] Purpose of the invention: In order to overcome the deficiencies in the prior art, the purpose of the present invention is to provide a harmonically mode-locked fiber laser synchronous pumping with good stability, high repetition rate, high output power, and easy operation. Another purpose of the present invention is to provide a high repetition rate, high signal-to-noise ratio, and tunable mid-infrared picosecond optical parametric oscillator.

[0008] Technical solution: The present invention discloses a synchronous pumping method for a harmonic mode-locked fiber laser, comprising a pump source, a harmonic mode-locked erbium-doped fiber laser and a MOPA amplifier, the harmonic mode-locked erbium-doped fiber laser comprising a first laser, a first wavelength division multiplexer, a first erbium-doped gain fiber, a mode matcher, a highly nonlinear fiber, a first polarization controller, a polarization-dependent optical isolator, a second polarization controller, an output coupler, a first polarization-independent optical isolator, the first wavelength division multiplexer, the first erbium-doped gain fiber, the mode matcher, the highly nonlinear fiber, the first polarization controller, the polarization-dependent optical isolator, the second polarization controller, and the output coupler forming a ring cavity, the wavelength division multiplexer being connected to the first laser, and the output coupler being close to the first laser. One end near the first wavelength division multiplexer is also connected to the first polarization-independent optical isolator; the MOPA amplifier includes a second erbium-doped gain fiber, a second wavelength division multiplexer, a second polarization-independent optical isolator, a second laser, a cladding light stripper, an erbium-ytterbium co-doped fiber, a beam combiner, a high-power optical isolator, a third laser, and a collimator. The first polarization-independent optical isolator, the second erbium-doped gain fiber, the second wavelength division multiplexer, the second polarization-independent optical isolator, the cladding light stripper, the erbium-ytterbium co-doped fiber, the beam combiner, the high-power optical isolator, and the collimator are connected in sequence. One end of the second wavelength division multiplexer near the second polarization-independent optical isolator is also connected to the second laser, and one end of the beam combiner near the high-power optical isolator is also connected to the third laser.

[0009] For the first time, a harmonically mode-locked fiber laser has been used to synchronously pump an optical parametric oscillator (OPO). The resulting mid-infrared ultrafast pulses not only maintain a high repetition rate while maintaining highly uniform pulse intensity and signal-to-noise ratio, but also boast a compact design and exceptionally stable operation. This addresses the challenges of achieving high-repetition-rate, high-SNR ultrafast tunable lasers in the mid-infrared band. By optimizing the OPO pump source structure and type, and using harmonic mode locking and a MOPA amplifier for synchronous pumping, the researchers offer unparalleled advantages, including low cost, excellent stability, all-fiber operation, and a high SNR.

[0010] Furthermore, the first laser and the second laser are both 976nm single-mode semiconductor lasers.

[0011] Furthermore, the third laser is a 976nm multi-mode semiconductor laser.

[0012] Furthermore, the ratio of the intracavity circulating oscillation to the power output of the output coupler is 95:5.

[0013] Furthermore, the first wavelength division multiplexer and the second wavelength division multiplexer are both 980 / 1550 nm wavelength division multiplexers.

[0014] Furthermore, the length of the first erbium-doped gain fiber and the second erbium-doped gain fiber is 2-3 m, and the dispersion is -20 ps / nm / km.

[0015] The mid-infrared picosecond optical parametric oscillator described in the present invention includes a pump source, a quarter-wave plate, a half-wave plate, and a polarization beam splitter for controlling the pump power, which are arranged on the same horizontal line. The polarization beam splitter and the high-reflection mirror are arranged on the same vertical line. The high-reflection mirror, a first plano-convex lens, a first plano-concave mirror, an MgO:PPLN crystal, a second plano-concave mirror, a second plano-convex mirror, a reflector, and a beam splitter are arranged on the same horizontal line.

[0016] Furthermore, the pump source is used to excite the optical parametric oscillation process, including a harmonically locked erbium-doped fiber laser and a MOPA amplifier. The harmonically locked erbium-doped fiber laser includes a first laser, a first wavelength division multiplexer, a first erbium-doped gain fiber, a mode matcher, a highly nonlinear fiber, a first polarization controller, a polarization-dependent optical isolator, a second polarization controller, an output coupler, and a first polarization-independent optical isolator. The first wavelength division multiplexer, the first erbium-doped gain fiber, the mode matcher, the highly nonlinear fiber, the first polarization controller, the polarization-dependent optical isolator, the second polarization controller, and the output coupler form a ring cavity. The wavelength division multiplexer is connected to the first laser, and the output coupler is close to the first wavelength division multiplexer. One end of the MOPA amplifier is also connected to the first polarization-independent optical isolator; the MOPA amplifier includes a second erbium-doped gain fiber, a second wavelength division multiplexer, a second polarization-independent optical isolator, a second laser, a cladding light stripper, an erbium-ytterbium co-doped fiber, a beam combiner, a high-power optical isolator, a third laser, and a collimator. The first polarization-independent optical isolator, the second erbium-doped gain fiber, the second wavelength division multiplexer, the second polarization-independent optical isolator, the cladding light stripper, the erbium-ytterbium co-doped fiber, the beam combiner, the high-power optical isolator, and the collimator are connected in sequence. The end of the second wavelength division multiplexer close to the second polarization-independent optical isolator is also connected to the second laser, and the end of the beam combiner close to the high-power optical isolator is also connected to the third laser.

[0017] Furthermore, the reflector can separate the pump light and the idler light.

[0018] Furthermore, the beam splitter can separate the signal light and the idler light.

[0019] Furthermore, the high-reflection mirror is a 1550nm high-reflection mirror.

[0020] Working Principle: Pump light is input from the input mirror and converted into signal light and idler light after passing through the crystal. The signal light is reflected by the plano-concave mirror and continues to oscillate in the cavity. The residual pump light and idler light are directly output. The structure for achieving harmonic mode-locking operation is to use nonlinear polarization rotation technology in a low-dispersion and high-nonlinear cavity to achieve 51st-order (800MHz) harmonic mode-locked output. By properly adjusting the PC, harmonic mode-locked laser output can be easily achieved. A polarization-independent optical isolator (PI-ISO 1) is used in the connection with the seed source to prevent backlight. The prepared MOPA amplifier section is connected to the harmonic mode-locked seed source to achieve high-power pulsed laser output, which can be used as the pump source of the optical parametric oscillator. A quarter-wave plate, a half-wave plate, and a polarization beam splitter are used to control the pump power. A 1550nm high-reflectivity mirror is used to change the propagation direction of the pump light. A plano-convex lens is used to focus the pump light to 100 microns to achieve cavity mode matching. An MgO:PPLN nonlinear crystal is used as a frequency conversion crystal. A plano-concave mirror provides high reflection for the signal light and enhances the transmittance of the pump and idler light, forming a resonant cavity together with the MgO:PPLN nonlinear crystal.

[0021] Beneficial effects: Compared with the prior art, the present invention has the following significant features:

[0022] 1. Low average dispersion and high average nonlinearity are achieved in fiber lasers, making it easy to achieve high signal-to-noise ratio and high stability harmonic mode-locked laser output, overcoming the technical bias of uneven pulse intensity and low signal-to-noise ratio in traditional mid-infrared tunable ultrafast laser outputs;

[0023] 2. The MOPA amplifier used is an all-fiber structure with reverse pumping, which has high amplification efficiency and output power, and has the advantages of good heat dissipation, high efficiency, high stability and easy operation;

[0024] 3. Using a harmonically mode-locked synchronously pumped optical parametric oscillator, we achieve high repetition rate and high signal-to-noise ratio picosecond tunable mid-infrared laser output. The output mid-infrared ultrafast laser has very uniform pulse intensity and high power stability.

[0025] 4. The mid-infrared picosecond optical parametric oscillator features a high repetition rate and signal-to-noise ratio, very uniform output pulse intensity, good power stability, all-fiber pumping, a very compact structure, and excellent tunability. It also has high long-term operating stability. The output signal and idler light both have a high repetition rate, maintaining a repetition rate of up to 800 MHz while maintaining a signal-to-noise ratio of up to 52 dB. The output pulse width is only 6.8 ps, and the RMS of the signal and idler light are only 0.8% and 1.1% when operating at maximum pump power.

[0026] 5. The pulse train has a very uniform intensity distribution, peak power and pulse energy. The tuning ranges of the signal light and idler light are 2.3 to 2.46 microns and 4.18 to 4.74 microns, respectively. It can provide 120mW of idler light power at 4.18 microns.

[0027] 6. It can be used as a mid-infrared ultrafast pulse and high-energy pulse laser light source in the fields of ultrafast laser and micro-nano material interaction, medical processing, space laser communication, ablation cooling material removal and optical frequency comb. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a schematic structural diagram of the pump source 1 of the present invention;

[0029] Figure 2 Schematic diagram of the structure of the mid-infrared picosecond optical parametric oscillator of the present invention;

[0030] Figure 3 is a pulse sequence diagram output by harmonic mode locking of the present invention;

[0031] Figure 4 is a spectrum diagram of the harmonic mode-locked output of the present invention;

[0032] Figure 5 This is a radio frequency spectrum diagram of the harmonic mode-locked output of the present invention;

[0033] Figure 6 is a graph of the autocorrelation pulse width of the harmonic mode-locked output of the present invention;

[0034] Figure 7 This is the power diagram of the harmonic mode locking of the present invention as a seed source after prevention;

[0035] Figure 8 It is the main amplified power diagram of the present invention;

[0036] Figure 9 It is the seed source, prevention and main amplification output spectrum of the present invention;

[0037] Figure 10 This is a pulse sequence diagram of the 2.3 micron signal light output of the present invention;

[0038] Figure 11 This is a radio frequency spectrum diagram of the 2.3 micron signal light output pulse of the present invention;

[0039] Figure 12 This is a graph showing power variations of the output signal light and idler light when the crystal planning period is 29.84 microns;

[0040] Figure 13 This is a pulse width measurement diagram of a 2.3 micron signal light output pulse of the present invention;

[0041] Figure 14 1. Spectra of signal light and idler light output at different periods of the present invention;

[0042] Figure 15 1 is a diagram of the maximum output power of signal light and idler light at different wavelengths of the present invention, a is the idler light, b is the signal light;

[0043] Figure 16 This is a graph showing the stability of the signal light and idler light output powers of the present invention. DETAILED DESCRIPTION

[0044] Example 1

[0045] like Figure 1Pump source 1 includes a harmonically mode-locked erbium-doped fiber laser and a MOPA amplifier. The pump source is seeded by the harmonically mode-locked erbium-doped fiber laser and amplified in two stages by the MOPA amplifier. The seed source achieves passive soliton harmonic mode locking through nonlinear polarization rotation technology. Harmonically mode-locked erbium-doped fiber lasers have the advantages of easy operation, all-fiber operation, high signal-to-noise ratio, and stability. The harmonically mode-locked erbium-doped fiber laser includes a first laser 101, a first wavelength division multiplexer 102, a first erbium-doped gain fiber 103, a mode matcher 104, a highly nonlinear fiber 105, a first polarization controller 106, a polarization-dependent optical isolator 107, a second polarization controller 108, an output coupler 109, and a first polarization-independent optical isolator 110. A first wavelength division multiplexer 102, a first erbium-doped gain fiber 103, a mode matcher 104, a highly nonlinear fiber 105, a first polarization controller 106, a polarization-dependent optical isolator 107, a second polarization controller 108, and an output coupler 109 are connected by fusion splicing single-mode optical fibers together using a fiber fusion splicer to form a ring cavity. The wavelength division multiplexer 102 is connected to the first laser 101, and the end of the output coupler 109 near the first wavelength division multiplexer 102 is also connected to the first polarization-independent optical isolator 110. The MOPA amplifier has the advantages of high amplification efficiency, stability, and good heat dissipation. The MOPA amplifier comprises a preamplifier and a main amplifier, both of which are reversely pumped. The pre-amplification part includes a second erbium-doped gain fiber 111, a second wavelength division multiplexer 112, a second polarization-independent optical isolator 113, and a second laser 114; the main amplification part includes a cladding light stripper 115, an erbium-ytterbium co-doped fiber 116, a combiner 117, a high-power optical isolator 118, a third laser 119, and a collimator 120. The first polarization-independent optical isolator 110, the second erbium-doped gain fiber 111, the second wavelength division multiplexer 112, the second polarization-independent optical isolator 113, the cladding light stripper 115, the erbium-ytterbium co-doped fiber 116, the combiner 117, the high-power optical isolator 118, and the collimator 120 are connected in sequence. The end of the second wavelength division multiplexer 112 close to the second polarization-independent optical isolator 113 is also connected to the second laser 114, and the end of the combiner 117 close to the high-power optical isolator 118 is also connected to the third laser 119.

[0046] The first laser 101 (Pump 1) and the second laser 114 are both 976nm single-mode semiconductor lasers, the third laser 119 is a 976nm multi-mode semiconductor laser, and the ratio of intracavity circulating oscillation to power output of the output coupler 109 is 95:5. The first wavelength division multiplexer 102 and the second wavelength division multiplexer 112 are both 980 / 1550nm wavelength division multiplexers. The length and dispersion of the first erbium-doped gain fiber 103 and the second erbium-doped gain fiber 111 are 2m and D = -18.5ps / nm / km, respectively. The length, dispersion and nonlinear coefficient of the highly nonlinear fiber 105 are 7m, 0±1ps / nm / km and 14W, respectively.-1 km -1 The length of the single-mode fiber (SMF-28e) is 4.1 m, and the dispersion is D = 17 ps / nm / km. The length and total dispersion of the ring cavity are 13.1 m and 0.044 ps, respectively. 2 , the average cavity dispersion and nonlinearity are -3.3 ps respectively. 2 km -1 and 7.5W -1 km -1 Such low dispersion and high nonlinearity are very beneficial for generating harmonic mode-locked operation. Erbium-ytterbium co-doped fiber 116 is 3 meters long. Both first and second wavelength division multiplexers 102 and 112 are 980 / 1550 nm wavelength division multiplexers, capable of combining 980 nm and 1550 nm light into a single beam for transmission through a single optical fiber.

[0047] The preparation method of synchronous pumping of harmonic mode-locked fiber laser comprises the following steps:

[0048] A. Preparation of harmonic mode-locked fiber laser:

[0049] A1. Prepare the nonlinear polarization rotation part: Take a polarization-dependent optical isolator 107 (PS-ISO), install the first polarization controller 106 (PC) and the second polarization controller 108 (PC) to the two ends of the polarization-dependent optical isolator's optical fiber pigtail, and finally fix them on an optical breadboard.

[0050] A2. This harmonically mode-locked fiber laser has low average dispersion and high average nonlinearity. The entire ring cavity consists of a first wavelength division multiplexer 102 (WDM), a first erbium-doped gain fiber 103 (DEF1), a mode matcher 104 (MFA), a highly nonlinear fiber 105 (HNLF), a nonlinear polarization rotation section (PS-ISO and PC), and a 95:5 output coupler 109 (OC). The ring cavity is formed by fusion-splicing these components in sequence using a fiber fusion splicer. By appropriately adjusting the first polarization controller 106 and the second polarization controller 108, harmonically mode-locked laser output can be easily achieved.

[0051] B. Preparation of MOPA amplifier part:

[0052] B1. Prepare the preamplifier: The preamplifier uses reverse pumping, with a second laser 114 (Pump 2) as the pump source. It consists of a second wavelength division multiplexer 112 and a second erbium-doped gain fiber 111. The connection to the seed source uses a first polarization-independent optical isolator 110 (PI-ISO 1) to prevent return light.

[0053] B2. Preparation of the main amplifier section: The main amplifier section also uses reverse pumping, with a third laser 119 (Pump 3) as the pump source. It consists of a cladding light stripper 115 (CPS), an erbium-ytterbium co-doped fiber 116 (EYDF), a combiner 117 (Combiner), a high-power optical isolator 118 (PI-ISO 3) and a collimator 120. A second polarization-independent optical isolator 113 (PI-ISO 2) is used to prevent return light in the connection part with the pre-amplifier.

[0054] C. Connect the prepared MOPA amplifier part to the harmonic mode-locked fiber laser seed source to achieve high-power pulsed laser output and obtain synchronous pumping of the harmonic mode-locked fiber laser.

[0055] Example 2

[0056] like Figure 2 The pump source 1 of Example 1 serves as the pump source 1 of a mid-infrared picosecond optical parametric oscillator, exciting the optical parametric oscillation process. The pump source 1, quarter-wave plate 2, half-wave plate 3, and polarization beam splitter 4 for controlling the pump power are arranged on the same horizontal line. The polarization beam splitter 4 and the high-reflection mirror 5 are arranged on the same vertical line. The high-reflection mirror 5, first plano-convex lens 6, first plano-concave mirror 7, MgO:PPLN crystal 8, second plano-concave mirror 9, second plano-convex mirror 10, reflector 11, and beam splitter 12 are arranged on the same horizontal line. The high-reflection mirror 5 is a 1550nm high-reflection mirror. A quarter-wave plate 2, a half-wave plate 3, and a polarization beam splitter 4 are used to control the pump power. A high-reflection mirror 5 is used to change the propagation direction of the pump light. A first plano-convex lens 6 and a second plano-convex mirror 10 are used to converge the pump light to 100 microns to achieve cavity mode matching. An MgO:PPLN crystal 8 serves as a frequency conversion crystal. A first plano-concave mirror 7 and a second plano-concave mirror 9 provide high reflection for the signal light and enhance the transmittance of the pump and idler light, forming a resonant cavity together with the MgO:PPLN crystal 8. A beam splitter 12 is used to separate the residual pump light, signal light, and idler light. The distance between the first plano-concave mirror 7 and the second plano-concave mirror 9 is 160 mm. This mid-infrared picosecond optical parametric oscillator simultaneously features a high repetition rate, a high signal-to-noise ratio, relatively uniform pulse intensity, a small size, and good stability.

[0057] Due to the lower dispersion and higher nonlinearity in the cavity, harmonic mode-locking operation can be easily achieved by properly adjusting the first polarization controller 106 and the second polarization controller 108 . Figure 3-6 The output characteristics of the 51st harmonic mode-locked operation are shown when the pump power is 800 MW by adjusting the first polarization controller 106 and the second polarization controller 108. The pulse sequence output by the harmonic mode-locked operation is as follows: Figure 3As shown, the pulse time interval is 1.25ns, corresponding to a repetition frequency of about 800MHz, and the pulse amplitude presents a relatively uniform pattern, indicating good stability.

[0058] Figure 4 The spectrum of harmonic mode locking is 1550.6nm in the center, and the Kelly sidebands on both sides also prove that it works in the soliton state. The output RF spectrum is measured for further evaluation, such as Figure 5 As shown in the figure, under the conditions of a span of 2 GHz and a resolution bandwidth of 10 kHz, a repetition rate of 800.7 MHz and an excellent signal-to-noise ratio of up to 60 dB are displayed, further demonstrating the good stability of the harmonically mode-locked laser. The laser cavity length of 13.1 m corresponds to a fundamental repetition rate of 15.7 MHz, and a repetition rate of 800.7 MHz corresponds to 51st-order harmonic mode-locked operation. Figure 6 The autocorrelation trace of the harmonic mode-locked pulse is shown using sech 2 The pulse width obtained after fitting the function is 6ps.

[0059] When the pump power is 800mW, the power output by the harmonic mode-locked seed source is 15mW. The maximum pump power that the pre-amplifier pump source (second laser 114) can provide is 414mW. At this time, the maximum output power of 120mW is obtained, and the corresponding optical conversion efficiency is 29%. Figure 7 The pump source of the main amplifier (third laser 119) can provide a maximum pump power of 27W. In order to minimize the nonlinear effect during the amplification process, the maximum pump power is set to 13.3W. At this time, the maximum output power obtained is 4W, and the corresponding optical conversion efficiency is 30%, as shown in FIG. Figure 8 The output spectrum at different powers is shown in Figure 9 As shown, it can be seen that after amplification, the spectrum does not undergo obvious deformation and broadening, indicating that the amplification effect is good.

[0060] Figure 10 The image shows the pulse sequence of the measured signal light. It can be seen that the amplitude of the output pulse is relatively uniform, and the pulse interval is 1.25ns, corresponding to a repetition frequency of 800MHz. The inset shows the output pulse sequence with a time span of 180ns. The pulses are also relatively uniform under large time spans, indicating its good working stability.

[0061] Figure 11 The corresponding RF spectrum was displayed, demonstrating a signal-to-noise ratio of 52dB at a span of 1.5GHz and a resolution bandwidth of 100Hz, further demonstrating the excellent stability of the output pulses. Even at a repetition rate of up to 800.7MHz, the signal maintained a high signal-to-noise ratio and good pulse amplitude uniformity, far exceeding previous reports.

[0062] Under the condition of crystal period of 29.84 microns, the threshold of optical parametric oscillator is 3.8W. Figure 12 The output power variations of the signal and idler light are shown for pump powers ranging from 3.8 to 4 W. At a pump power of 4 W, the maximum output power of the signal light (2302.8 nm) reaches 7 mW, and the maximum output power of the idler light (4746.9 nm) reaches 20 mW. Because the absorptivity of the MgO:PPLN crystal 8 increases above 4.5 μm, and the antireflection coating on both ends of the crystal only covers the wavelength range of 2.2 to 4.5 μm, the threshold of the optical parametric oscillator increases and the output power decreases when the idler light output wavelength exceeds 4.5 μm.

[0063] The pulse width of the signal light was measured using the cross-correlation method. The results are as follows: Figure 13 As shown in the figure, using an MgO:PPLN crystal to generate a sum frequency of pump light and signal light, the pulse width of the signal light can be calculated by observing the relationship between the optical path difference between the two beams and the sum frequency intensity. The measured results show that the half-maximum width of the sum frequency intensity is 2.9mm, and the calculated pulse width of the signal light (2302.8nm) is approximately 6.8ps.

[0064] To characterize the wavelength tunability of the optical parametric oscillator, the output performance of the signal and idler light in the six channels of the crystal was measured. The crystal temperature was fixed at 26.85°C, and the output spectra of the signal and idler light in the six channels with polarization periods of 29.87, 30.28, 30.72, 31.16, 31.60, and 32.14 μm were as follows: Figure 14 As shown in the figure, the corresponding signal light center wavelengths are 2302.8, 2330.3, 2357.9, 2385.4, 2418.4, and 2462.5 nm, respectively, and the idler light center wavelengths are 4746.9, 4634, 4528.9, 4430.7, 4321.1, and 4187.2 nm, respectively. The center wavelengths of the signal and idler lights are essentially consistent with those calculated using the Sellmeier equation. The signal light tuning range is 2.3-2.46 μm, with a span of 169 nm. The idler light tuning range is 4.18-4.74 μm, with a span of 560 nm.

[0065] In addition, under the six channels with polarization periods of 29.87-32.14 μm, the thresholds of SPOPO are 3.8, 3.6, 3.4, 3.2, 3.3 and 3 W, respectively, and the corresponding maximum output powers are as follows Figure 15As shown, the maximum signal and idler power outputs were achieved at a period of 32.14 microns. The maximum signal power reached 15 mW at 2462.5 nm, and the maximum idler power reached 120 mW at 4187.2 nm. The low signal output power is due to the optical parametric oscillator cavity's signal output coupling ratio being only 0.2%. The SPOPO's output power can be increased by increasing the output coupling ratio and boosting the pump power (further increasing the number of MOPA amplification stages).

[0066] Finally, we measured the stability of the maximum output power of the signal light and the idler light when the polarization period is 32.14 μm. The results are as follows: Figure 16 As shown in the figure, the corresponding RMS values ​​within 8 hours of operation are 1.1% (4187.2nm) and 0.8% (2462.5nm), respectively. SPOPO has good power stability.

[0067] Application Example 1

[0068] The high-performance mid-infrared picosecond optical parametric oscillator of Example 2 was used as a laser light source to successfully achieve the fine processing, repair, and treatment of high-end micro- and nanomaterials. The high-performance mid-infrared picosecond optical parametric oscillator of this invention can be used as an optical frequency comb to achieve precise laser ranging, and can also be used for coherent optical communications and optical atomic clocks. It can also be used as a practical laser light source in the medical and life science fields.

Claims

1. A harmonic mode-locked fiber laser synchronous pumping system, characterized by: The invention comprises a pump source (1), wherein the pump source (1) comprises a harmonically mode-locked erbium-doped fiber laser and a MOPA amplifier, wherein the harmonically mode-locked erbium-doped fiber laser comprises a first laser (101), a first wavelength division multiplexer (102), a first erbium-doped gain fiber (103), a mode matcher (104), a highly nonlinear fiber (105), a first polarization controller (106), a polarization-dependent optical isolator (107), a second polarization controller (108), an output coupler (109), a first polarization-independent optical isolator (108), and a second polarization controller (109). (110), the first wavelength division multiplexer (102), the first erbium-doped gain fiber (103), the mode matcher (104), the highly nonlinear fiber (105), the first polarization controller (106), the polarization-dependent optical isolator (107), the second polarization controller (108), and the output coupler (109) form a ring cavity, the wavelength division multiplexer (102) is connected to the first laser (101), and the end of the output coupler (109) close to the first wavelength division multiplexer (102) is also independent of the first polarization. The MOPA amplifier comprises a second erbium-doped gain fiber (111), a second wavelength division multiplexer (112), a second polarization-independent optical isolator (113), a second laser (114), a cladding light stripper (115), an erbium-ytterbium co-doped fiber (116), a beam combiner (117), a high-power optical isolator (118), a third laser (119), and a collimator (120). A wavelength division multiplexer (112), a second polarization-independent optical isolator (113), a cladding light stripper (115), an erbium-ytterbium co-doped optical fiber (116), a beam combiner (117), a high-power optical isolator (118), and a collimator (120) are connected in sequence; an end of the second wavelength division multiplexer (112) close to the second polarization-independent optical isolator (113) is also connected to a second laser (114); an end of the beam combiner (117) close to the high-power optical isolator (118) is also connected to a third laser (119).

2. The synchronous pumping system for a harmonic mode-locked fiber laser according to claim 1, characterized in that: The first laser (101) and the second laser (114) are both 976nm single-mode semiconductor lasers.

3. The synchronous pumping method of a harmonic mode-locked fiber laser according to claim 1, characterized in that: The third laser (119) is a 976nm multi-mode semiconductor laser.

4. The synchronous pumping system for a harmonic mode-locked fiber laser according to claim 1, characterized in that: The ratio of intracavity circulating oscillation to power output of the output coupler (109) is 95:

5.

5. The harmonic mode-locked fiber laser synchronous pumping system according to claim 1, characterized in that: The first wavelength division multiplexer (102) and the second wavelength division multiplexer (112) are both 980 / 1550 nm wavelength division multiplexers.

6. The harmonic mode-locked fiber laser synchronous pumping system according to claim 1, characterized in that: The length of the first erbium-doped gain optical fiber (103) and the second erbium-doped gain optical fiber (111) is 2-3 m, and the dispersion is -20 ps / nm / km.

7. A mid-infrared picosecond optical parametric oscillator, characterized by: The invention comprises a pump source (1), a quarter wave plate (2), a half wave plate (3), and a polarization beam splitter (4) for controlling the pump power, which are arranged on the same horizontal line. The polarization beam splitter (4) and the high-reflection mirror (5) are arranged on the same vertical straight line. The high-reflection mirror (5), a first plano-convex lens (6), a first plano-concave mirror (7), an MgO:PPLN crystal (8), a second plano-concave mirror (9), a second plano-convex mirror (10), a reflector (11), and a beam splitter (12) are arranged on the same horizontal line. The pump source (1) includes a harmonic mode-locked erbium-doped fiber laser and a MOPA amplifier. The harmonic mode-locked erbium-doped fiber laser includes a first laser (101), a first wavelength division multiplexer (102), a first erbium-doped gain fiber (103), a mode matcher (104), a highly nonlinear fiber (105), a first polarization controller (106), a polarization-dependent optical isolator (107), a second polarization controller (108), an output coupler (109), and a first polarization-independent optical isolator (110). The first wavelength division multiplexer (102), the first erbium-doped gain fiber (103), the mode matcher (104), the highly nonlinear fiber (105), the first polarization controller (106), the polarization-dependent optical isolator (107), the second polarization controller (108), and the output coupler (109) form a ring cavity. The wavelength division multiplexer (102) is connected to the first laser (101), and one end of the output coupler (109) close to the first wavelength division multiplexer (102) is also connected to the first polarization-independent optical isolator ( 110); the MOPA amplifier comprises a second erbium-doped gain fiber (111), a second wavelength division multiplexer (112), a second polarization-independent optical isolator (113), a second laser (114), a cladding light stripper (115), an erbium-ytterbium co-doped fiber (116), a beam combiner (117), a high-power optical isolator (118), a third laser (119), and a collimator (120); the first polarization-independent optical isolator (110), the second erbium-doped gain fiber (111), the second wavelength division multiplexer (112), a second polarization-independent optical isolator (113), a second laser (114), a cladding light stripper (115), an erbium-ytterbium co-doped fiber (116), a beam combiner (117), a high-power optical isolator (118), a third laser (119), and a collimator (120); The multiplexer (112), the second polarization-independent optical isolator (113), the cladding light stripper (115), the erbium-ytterbium co-doped optical fiber (116), the beam combiner (117), the high-power optical isolator (118), and the collimator (120) are connected in sequence; an end of the second wavelength division multiplexer (112) close to the second polarization-independent optical isolator (113) is also connected to the second laser (114); an end of the beam combiner (117) close to the high-power optical isolator (118) is also connected to the third laser (119).

8. The mid-infrared picosecond optical parametric oscillator according to claim 7, characterized in that: The reflector (11) is capable of separating pump light and idle light.

9. The mid-infrared picosecond optical parametric oscillator according to claim 7, characterized in that: The spectroscope (12) is capable of separating signal light and idler light.

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