Rapidly tunable ultra-broadband femtosecond optical parametric oscillator

Abstract

The application discloses a kind of ultra-wideband femtosecond optical parametric oscillators that can be quickly tuned, and relates to the technical field of optical parametric oscillator.The oscillator includes frequency doubling device, optical parametric oscillator resonant cavity device and dispersion compensation chirped mirror pair module.By using the non-collinear phase matching mode of the transmission angle between pump light and signal light as 2.5°, the original phase matching bandwidth is further broken through, wavelength fast tuning can be realized by only fine tuning cavity length without adjusting the angle of nonlinear crystal, the bottleneck of wavelength tuning range, wavelength tuning speed and other aspects of existing near-infrared optical parametric oscillator can be broken through, and a new type of light source with larger wavelength tuning range and faster tuning speed is provided for the field of spectroscopy, micro-nano processing, optical frequency comb and other fields requiring wideband tunable light source.

Description

Technical Field

[0001] This invention relates to the field of optical parametric oscillator technology, and more particularly to a fast-tunable ultrawideband femtosecond optical parametric oscillator. Background Technology

[0002] The principle of an optical parametric oscillator (OPO) is based on frequency conversion of the second-order nonlinear optical effect in a nonlinear medium. After the pump light enters the nonlinear crystal, it is converted into two laser beams, a signal beam and an idler beam, under phase-matching conditions. The signal beam oscillates repeatedly within the resonant cavity to achieve a higher energy output. OPOs can significantly extend the output wavelength of lasers and offer the advantage of wavelength tunability. Ultrafast lasers in the 620–1050 nm wavelength range are widely used in biophotonics, micro / nano fabrication, and optical frequency combs. OPOs capable of outputting in this wavelength range are indispensable light sources in these research fields.

[0003] However, existing femtosecond optical parametric oscillators (OPOs) still face significant limitations in generating tunable femtosecond lasers in the visible to near-infrared band. Currently, the tuning bandwidth of femtosecond OPOs in this band is relatively narrow. The main reason for this limitation is the limited bandwidth of the nonlinear crystal phase matching under traditional collinear phase matching. Simultaneously, since tuning traditional femtosecond OPOs requires adjusting the crystal angle, temperature, or period, and then changing the cavity length to achieve wavelength tuning, the output wavelength tuning speed of femtosecond OPOs is also slow. The tuning speed for a 100nm spectral coverage range can only reach the order of seconds, and no femtosecond OPOs with faster tuning speeds have been reported. Summary of the Invention

[0004] The technical problem to be solved by the present invention is how to provide an ultrawideband femtosecond optical parametric oscillator that can greatly improve the wavelength tuning range, while making wavelength tuning more convenient and faster, requiring only fine adjustment of the cavity length without changing the crystal angle for rapid tuning.

[0005] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: an ultra-wideband femtosecond optical parametric oscillator, comprising a 1030nm near-infrared femtosecond laser source. The 1030nm near-infrared light emitted from the 1030nm near-infrared femtosecond laser source passes through a first half-wave plate and is incident on a frequency doubling crystal to generate 515nm green light. After passing through a dichroic mirror, the 1030nm near-infrared light is reflected and leaves the oscillator, while the 515nm green light passes through the dichroic mirror. The 515nm green light passes through a second half-wave plate, is focused by a focusing lens, and enters the resonant cavity of the optical parametric oscillator, and is incident on a nonlinear crystal to generate signal light and idler light. The idler light is transmitted at a second concave focusing cavity mirror and leaves the resonant cavity. The signal light is reflected and collimated at the second concave focusing cavity mirror. After being reflected sequentially by the first, second, and third reflecting mirrors, the light is incident on the output coupling mirror. Part of the energy is transmitted out of the resonant cavity, while the rest is reflected and continues to oscillate within the resonant cavity. The reflected signal light is reflected by the fourth and fifth reflecting mirrors and then incident on the dispersion-compensating chirped mirror module, which includes a first dispersion-compensating reflector and a second dispersion-compensating reflector. Finally, after being reflected sequentially by the first and second dispersion-compensating reflectors, the light is reflected to the first concave focusing cavity mirror, completing one cycle within the cavity. Then, the signal light is focused and reflected by the first concave focusing cavity mirror, and synchronously meets the newly incident 515nm green light, which is then injected into the nonlinear crystal again, thus oscillating back and forth within the cavity.

[0006] The beneficial effects of adopting the above technical solution are as follows: This application employs a multicolor phase matching method, that is, using a BBO crystal as the nonlinear crystal for the optical parametric process. By using a nonlinear phase matching method, the propagation direction of the pump light within the crystal forms a 2.5° angle with the signal light. A single pass of the pump light can excite ultra-wideband parametric fluorescence, thereby significantly improving the wavelength tuning range without changing the crystal angle. Simultaneously, piezoelectric ceramics are used to fine-tune the cavity length, achieving an output wavelength tuning speed on the order of milliseconds per 100nm. Attached Figure Description

[0007] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0008] Figure 1 This is a schematic block diagram of the oscillator described in an embodiment of the present invention;

[0009] Figure 2 This is a schematic diagram of the optical path of the oscillator described in an embodiment of the present invention;

[0010] The components are: 1. 1030nm near-infrared femtosecond laser source; 2. First half-wave plate; 3. Frequency doubling crystal; 4. Dichroic mirror; 5. Second half-wave plate; 6. Focusing lens; 7. First concave focusing cavity mirror; 8. Nonlinear crystal; 9. Second concave focusing cavity mirror; 10. First reflecting mirror; 11. Second reflecting mirror; 12. Third reflecting mirror; 13. Output coupling mirror; 14. Fourth reflecting mirror; 15. Fifth reflecting mirror; 16. First dispersion-compensating reflecting mirror; 17. Second dispersion-compensating reflecting mirror; 18. 1030nm near-infrared light; 19. 515nm green light; 20. Signal light; 21. Idle light; 22. Piezoelectric ceramic. Detailed Implementation

[0011] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0012] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0013] like Figure 1 As shown, this invention discloses a fast-tuned ultrawideband femtosecond optical parametric oscillator, comprising: a frequency doubling device, an optical parametric oscillator resonant cavity device, and a dispersion-compensating chirped mirror pair module. The frequency doubling device includes a 1030nm near-infrared femtosecond laser source 1, a first half-wave plate 2, and a frequency doubling crystal 3. The 1030nm near-infrared femtosecond laser source 1 is used to generate pump light. Preferably, the 1030nm near-infrared femtosecond laser source 1 is a femtosecond pulse laser oscillator or femtosecond pulse amplifier with ytterbium-doped fiber as the gain medium, and a repetition frequency of 49.22MHz. The 1030nm near-infrared light 18 emitted by the 1030nm near-infrared femtosecond laser source 1 passes through the first half-wave plate 2 and is incident on the frequency doubling crystal 3 to generate 515nm green light 19. After passing through a dichroic mirror 4, the 1030nm near-infrared light 18 is reflected and leaves the system, while the 515nm green light 19 passes through the dichroic mirror 4.

[0014] The first half-wave plate 2 is a 1030nm half-wave plate coated with a 1030nm anti-reflection film, and the frequency doubling crystal 3 is an LBO crystal, cut vertically, with a size of approximately 5×5×5mm. 3With θ = 90°, it can perform frequency-second conversion on 1030nm near-infrared light 18. Both its front and rear light-transmitting surfaces are coated with 1030nm and 515nm anti-reflection films, with a transmittance greater than 95%. The coating characteristics of the dichroic mirror 4 are high reflectivity for 1030nm laser and high transmittance for 515nm laser. The 1030nm near-infrared light 18 is in an s-polarized state relative to the frequency-doubling crystal 3 after passing through the first half-wave plate 2, and the green light generated by the frequency-doubling crystal 3 is in a p-polarized state.

[0015] The optical parametric oscillator resonant cavity device includes a first concave focusing cavity mirror 7, a nonlinear crystal 8, a second concave focusing cavity mirror 9, a first reflecting mirror 10, a second reflecting mirror 11, a third reflecting mirror 12, an output coupling mirror 13, a fourth reflecting mirror 14, and a fifth reflecting mirror 15. The first concave focusing cavity mirror 7 and the output coupling mirror 13 are the two end mirrors of the device. 515nm green light 19 passes through the second half-wave plate 5, is focused by the focusing lens 6, and then passes through the first concave focusing cavity mirror 7 into the optical parametric oscillator resonant cavity. It then enters the nonlinear crystal 8 to generate signal light 20 and idler light 21.

[0016] Furthermore, the second half-wave plate 5 is a 515nm half-wave plate coated with a 515nm anti-reflection film; the focusing lens 6 is coated with a 515nm anti-reflection film; the nonlinear crystal 8 is a BBO crystal, cut using Brewster angle cutting, θ B =59.2°, approximately 5×5×15mm 3 With θ = 24.5°, it can perform optical parametric conversion on the 515nm laser 19, generating signal light 20 of 620–1050nm. Since the broadband antireflection coating can only withstand a low optical power density, no coating is applied to either end of the crystal. The signal light 20 and the idler light 21 have the same polarization state, and are s-polarized relative to the nonlinear crystal 5.

[0017] Furthermore, the nonlinear crystal 8 is placed on a one-dimensional angular displacement platform, and the tilt angle of the displacement platform is adjusted to adjust the angle between the crystal optical axis and the 515nm green light 19, so as to adjust the efficiency of phase matching and achieve the best output efficiency at different output wavelengths.

[0018] The idler light 21 and the residual 515nm green light 19 are transmitted through the second concave focusing mirror 9 and leave the resonant cavity. The signal light 20 is reflected and collimated at the second concave focusing mirror 9, and continues to be reflected by the first reflecting mirror 10, the second reflecting mirror 11, and the third reflecting mirror 12 before entering the output coupling mirror 13. At the output coupling mirror 13, a portion of the energy is transmitted out of the resonant cavity, while the remaining energy is reflected and continues to pass through the fourth reflecting mirror 14 and the fifth reflecting mirror 15. The first reflecting mirror 10, the second reflecting mirror 11, the third reflecting mirror 12, the fourth reflecting mirror 14, the fifth reflecting mirror 15, the first concave focusing mirror 8, and the second concave focusing mirror 9 are coated with a high-reflection film in the 600–1100nm wavelength range. The output coupling mirror 13 has the characteristic of 15% transmission and 85% reflection for light in the 600–1100nm wavelength range.

[0019] Furthermore, the output coupling mirror 13 is mounted on a displacement platform to adjust the cavity length of the resonant cavity, ensuring that the time for the signal pulse to travel one revolution within the cavity is equal to the interval between two pump pulses, thus achieving synchronous pumping. The piezoelectric ceramic 22 is in close contact with the rear surface of the output coupling mirror 13, with a range of 95μm and an accuracy of 1μm. It is used to finely adjust the position of the output coupling mirror 13 in the horizontal direction to finely adjust the cavity length of the resonant cavity, thereby achieving the screening of the signal light wavelength oscillating within the resonant cavity and thus realizing rapid tuning of the output wavelength.

[0020] Furthermore, the dispersion-compensating chirped mirror module includes a first dispersion-compensating reflector 16 and a second dispersion-compensating reflector 17. Signal light 20 is compensated for by the first dispersion-compensating reflector 16 and the second dispersion-compensating reflector 17, which compensate for the dispersion accumulated in the preceding optical path components. Signal light 20 is reflected by the second dispersion-compensating reflector 17 and incident on the first concave focusing cavity mirror 7, thus completing one cycle within the optical parametric oscillator resonant cavity. Subsequently, the signal light 22 is focused and reflected by the first concave focusing cavity mirror 7, and together with the newly incident 515nm green light 21, it re-enters the nonlinear crystal 8, oscillating cyclically within the cavity. The group delay dispersion compensation of the first dispersion-compensating reflector 16 and the second dispersion-compensating reflector 17 for lasers in the 600–1100nm wavelength band is 70 fs. 2 .

[0021] In this application, the BBO crystal employs a non-collinear phase matching method. By adjusting the incident angle of the pump light, the propagation angle of the pump light within the BBO crystal is made 2.5° to the signal light. At this angle, the BBO exhibits a maximum phase matching bandwidth of 600–1200 nm, and within this bandwidth, the propagation direction of the signal light is collinear; only the angle between the idler light and the crystal's optical axis changes. The schematic diagram of this non-collinear phase matching is shown below. Figure 2 As shown, where, Let be the wave vector of the pump light. The wave vector of the broadband signal light. The wave vector of the idler light corresponding to the signal light with the subscript represents the wavelength of the signal light. Signal lights of different wavelengths differ only in the length of their wave vectors, but their directions are the same. Therefore, wavelength tuning of this optical parametric oscillator only requires controlling the piezoelectric ceramic 22 to adjust the cavity length of the optical parametric oscillator, thereby selecting wavelengths that meet the resonance conditions from the broadband phase-matching bandwidth, achieving wavelength tuning from 620 to 1050 nm without adjusting the crystal angle. The tuning speed can reach 12.5 ms / 10⁹ nm.

[0022] This invention further breaks through the original phase matching bandwidth by adopting a non-collinear phase matching method with a transmission angle of 2.5° between the pump light and the signal light. It can achieve rapid wavelength tuning by only fine-tuning the cavity length without adjusting the nonlinear crystal angle. It can overcome the bottlenecks of existing near-infrared optical parametric oscillators in terms of wavelength tuning range and wavelength tuning speed, and provide a new type of light source with a larger wavelength tuning range and faster tuning speed for fields such as spectroscopy, micro-nano fabrication, and optical frequency combs that require broadband tunable light sources.

Claims

1. A rapidly tunable ultra-broadband femtosecond optical parametric oscillator, characterized by: The 1030nm near-infrared femtosecond laser source (1) is used for generating 515nm green light (19) by a first half-wave plate (2) and a frequency doubling crystal (3), and the 1030nm near-infrared light (18) is reflected out of the oscillator through a dichroic mirror (4), and the 515nm green light (19) is transmitted through the dichroic mirror (4); the 515nm green light (19) is transmitted through a second half-wave plate (5) and focused by a focusing lens (6) to enter an optical parametric oscillator resonant cavity and be incident on a nonlinear crystal (8) to generate signal light (20) and idler light (21); the idler light (21) is transmitted at a second concave focusing cavity mirror (9) and exits the resonant cavity; the signal light (20) is reflected and collimated at the second concave focusing cavity mirror (9), and is reflected in turn by a first mirror (10), a second mirror (11) and a third mirror (12) to be incident on an output coupling mirror (13), part of the energy is transmitted out of the resonant cavity, and the remaining energy is reflected to continue oscillating in the resonant cavity; the reflected signal light (20) is reflected by a fourth mirror (14) and a fifth mirror (15), and then is incident on a dispersion compensation chirped mirror pair module, the dispersion compensation chirped mirror pair module includes a first dispersion compensation mirror (16) and a second dispersion compensation mirror (17), and finally is reflected in turn by the first dispersion compensation mirror (16) and the second dispersion compensation mirror (17) to be reflected to a first concave focusing cavity mirror (7) to complete one cavity circulation; then the signal light (20) is focused and reflected by the first concave focusing cavity mirror (7) to meet the newly incident 515nm green light (19) synchronously, and is incident on the nonlinear crystal (8) again to reciprocally oscillate in the cavity. 2.The fast-tunable ultra-wideband femtosecond optical parametric oscillator of claim 1, wherein: the 1030nm near-infrared femtosecond laser source (1) is a femtosecond pulse laser oscillator or a femtosecond pulse amplifier using a ytterbium-doped optical fiber as a gain medium, and the repetition frequency is 49.22MHz. 3.The fast-tunable ultra-wideband femtosecond optical parametric oscillator of claim 1, wherein: the first half-wave plate (2) is a 1030nm half-wave plate coated with a 1030nm antireflection film, and the second half-wave plate (5) is a 515nm half-wave plate coated with a 515nm antireflection film; the focusing lens (6) is coated with a 515nm antireflection film; and the dichroic mirror (4) has a film coating property of high reflection to 1030nm laser and high transmission to 515nm laser. 4.The fast-tunable ultra-wideband femtosecond optical parametric oscillator of claim 1, wherein: ​ ​ The frequency doubling crystal (3) is LBO crystal, which is used for doubling frequency conversion of 1030nm near infrared light (18), and the front and rear light transmission surfaces are coated with 1030nm and 515nm anti-reflection films; the nonlinear crystal (8) is BBO crystal, which is used for optical parametric conversion of 515nm green light (19) to generate 620-1050nm signal light (20).

5. The rapidly tunable ultra-broadband femtosecond optical parametric oscillator of claim 1, wherein: The 1030nm near infrared light (18) is s-polarized relative to the frequency doubling crystal (3) after passing through the first half-wave plate (2), and the green light generated by the frequency doubling crystal (3) is p-polarized; the signal light (20) and the idler light (21) have the same polarization state and are s-polarized relative to the nonlinear crystal (8).

6. The rapidly tunable ultra-broadband femtosecond optical parametric oscillator of claim 1, wherein: The first mirror (10), the second mirror (11), the third mirror (12), the fourth mirror (14), the fifth mirror (15), the first concave focusing cavity mirror (7) and the second concave focusing cavity mirror (9) are coated with high reflection films in the 600nm-1100nm waveband.

7. The rapidly tunable ultra-broadband femtosecond optical parametric oscillator of claim 1, wherein: The output coupling mirror (13) has 15% transmission and 85% reflection characteristics for light in the 600nm-1100nm waveband.

8. The rapidly tunable ultra-broadband femtosecond optical parametric oscillator of claim 1, wherein: The output coupling mirror (13) is mounted on a displacement platform for adjusting the resonant cavity length to ensure that the time for the signal pulse to travel around the cavity is equal to the interval time of two pulses of the pump pulse, achieving synchronous pumping.

9. The rapidly tunable ultra-broadband femtosecond optical parametric oscillator of claim 1, wherein: The piezoelectric ceramic (22) is tightly attached to the rear surface of the output coupling mirror (13), with a range of 95μm and an accuracy of 1μm, for fine-tuning the position of the output coupling mirror (13) in the horizontal direction to fine-tune the resonant cavity length, thereby achieving rapid tuning of the output wavelength.

10. The rapidly tunable ultra-broadband femtosecond optical parametric oscillator of claim 1, wherein: The first dispersion compensation mirror (16) and the second dispersion compensation mirror (17) compensate the group delay dispersion of 70 fs for the laser in the wave band of 600-1100 nm 2 .

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