Intracavity-pumped continuous-wave optical parametric oscillator based on a ring cavity structure

Abstract

The application discloses a kind of intra-cavity pumping continuous wave light parametric oscillators based on annular cavity structure, it includes multimode semiconductor pump laser, multimode semiconductor pump laser is connected with signal pump combiner, polarization maintaining double-clad Yb-doped fiber, first fiber collimator in proper order, free space optical isolator, birefringent filter, first dichroic mirror, first concave mirror, MgO:PPLN crystal, second concave mirror, plane mirror, second dichroic mirror, second fiber collimator are sequentially arranged after first fiber collimator, and second fiber collimator is coupled into signal pump combiner again from the light of second dichroic mirror, and constitutes an annular cavity loop.The application can realize the single-end output of intra-cavity pumping OPO idler light, reduce the idler light linewidth of final output, and reduce the thermal effect of crystal, improve the stability of the whole system.

Description

Technical Field

[0001] This invention belongs to the field of lasers, specifically relating to an intracavity-pumped continuous-wave optical parametric oscillator based on a ring cavity structure. Background Technology

[0002] Continuous-wave optical parametric oscillators (OPOs) are currently considered a highly effective technique for generating wide-tunable, narrow-linewidth mid-infrared lasers, playing a crucial role in high-resolution molecular spectroscopy measurements and trace gas sensing. Based on the pumping method, OPOs can be classified into externally pumped OPOs and intracavity pumped OPOs. Intracavity pumped OPOs, by utilizing the high cyclic pump power within the laser resonant cavity, can significantly reduce the OPO threshold and offer advantages such as compact structure and high conversion efficiency, thus attracting wider research attention.

[0003] Currently, intracavity-pumped continuous-wave (OCP) optical oscillators (OPOs) commonly employ a standing-wave cavity resonant structure, utilizing periodically polarized lithium niobate crystal (MgO:PPLN) doped with magnesium oxide as the nonlinear medium. Mid-infrared idler light output is generated through frequency down-conversion, resulting in a simple and compact structure. However, the following problems exist: 1. The standing-wave cavity structure of the OPO pump light causes idler light to be generated in two directions of the nonlinear crystal, with the output in one direction often unused; 2. The nested resonant cavity structure of the OPO pump light and signal light increases the cavity length and loss of the pump light, making it difficult to achieve narrow linewidth operation, thus increasing the width of the final idler light output; 3. Due to the incomplete transparency of the nonlinear crystal MgO:PPLN, the pump light and signal light undergo two absorptions during their round trip within the cavity, leading to severe thermal effects and affecting the stability of the entire system. Summary of the Invention

[0004] The purpose of this invention is to provide an intracavity-pumped continuous wave optical parametric oscillator based on a ring cavity structure. This oscillator can achieve single-ended output of intracavity-pumped OPO idler light, reduce the width of the final output idler light, reduce the thermal effect of the crystal, and improve the stability of the entire system.

[0005] The technical solution adopted in this invention is:

[0006] An intracavity-pumped continuous-wave optical parametric oscillator based on a ring cavity structure includes a multimode semiconductor-pumped laser, a signal-pumped beam combiner, a polarization-maintaining double-clad ytterbium-doped fiber, a first fiber collimator, a free-space optical isolator, a birefringent filter, a first dichroic mirror, a first concave mirror, an MgO:PPLN crystal, a second concave mirror, a plane mirror, a second dichroic mirror, and a second fiber collimator.

[0007] A multimode semiconductor pump laser, a signal pump combiner, a polarization-maintaining double-clad ytterbium-doped fiber, and a first fiber collimator are connected in sequence. After the first fiber collimator, a free-space optical isolator, a birefringent filter, a first dichroic mirror, a first concave mirror, an MgO:PPLN crystal, a second concave mirror, a plane mirror, a second dichroic mirror, and a second fiber collimator are arranged in sequence. The second fiber collimator couples the light from the second dichroic mirror into the signal pump combiner to form a ring cavity circuit.

[0008] The signal pump combiner, polarization-maintaining double-clad ytterbium-doped fiber, first fiber collimator, and second fiber collimator constitute the fiber optic path; the first dichroic mirror, first concave mirror, MgO:PPLN crystal, second concave mirror, plane mirror, and second dichroic mirror constitute the OPO signal optical resonant cavity, which is an X-shaped four-mirror ring cavity structure; the free-space optical isolator, birefringent filter, and the cavities of the first dichroic mirror, first concave mirror, MgO:PPLN crystal, second concave mirror, plane mirror, and second dichroic mirror shared with the OPO signal optical resonant cavity constitute the free-space optical path; the fiber optic path and the free-space optical path constitute the OPO pump optical resonant cavity, which is a unidirectional ring cavity structure;

[0009] The OPO pump light operates in a unidirectional manner, and the idler light is generated only in the same direction as the pump light.

[0010] A further approach involves using 1064nm polarization-maintaining devices for the signal pump combiner, polarization-maintaining double-clad ytterbium-doped fiber, first fiber collimator, and second fiber collimator, connected via fiber fusion splicing.

[0011] A further approach is to have the outgoing light of the free-space optical isolator polarized horizontally, while its incident polarization direction is aligned with the slow axis of the first fiber collimator, meaning the slow axis of the first fiber collimator is consistent with the incident polarization direction of the free-space optical isolator.

[0012] A further proposed solution is that the parameters of the second fiber collimator are completely identical to those of the first fiber collimator, with its slow axis direction along the horizontal direction; and the parameters of the first concave mirror are completely identical to those of the second concave mirror.

[0013] A further approach is to use an off-axis quartz crystal plate as the birefringent filter, which is inserted into the pump light resonator of the OPO at a Brewster angle.

[0014] A further proposed solution is that the first dichroic mirror is coated with a high-reflection film for the OPO pump light and an anti-reflection film for the signal light, thereby combining the pump light and the signal light; the second dichroic mirror is coated with an anti-reflection film for the OPO pump light and a high-reflection film for the signal light, thereby separating the OPO pump light and the signal light.

[0015] A further improvement is that the first and second concave mirrors are coated with high-reflectivity films for OPO pump light and signal light, and with anti-reflection films for idler light.

[0016] A further approach is to coat the plane mirror with a high-reflectivity coating for both OPO pump light and signal light.

[0017] A further approach is to use an MgO:PPLN crystal composed of multiple different polarization periods, which allows for wavelength tuning of the OPO idler light by changing the polarization periods; both its front and back surfaces are coated with antireflective films that protect against OPO pump light, signal light, and idler light.

[0018] The first fiber collimator collimates the laser light in the fiber into free space, facilitating subsequent nonlinear frequency conversion; the second fiber collimator recouples the free space light back into the fiber, thus forming a ring cavity circuit.

[0019] The beneficial effects of this invention are as follows:

[0020] Free-space optical isolators are used to enable unidirectional operation of the OPO pump light;

[0021] Birefringent filters are used to limit the width of the OPO pump beam and tune the wavelength of the OPO pump beam;

[0022] MgO:PPLN crystal (nonlinear crystal MgO:PPLN) is used to realize optical parametric frequency conversion;

[0023] The OPO pump light has a ring cavity structure, which eliminates the spatial hole burning effect compared to the standing wave cavity structure, reduces the linewidth of the pump light, and thus reduces the width of the final output idler light.

[0024] The OPO pump light operates in a unidirectional state. According to the phase matching relationship, the idler light is generated only in the same direction as the pump light, so that the single-end output of the idler light can be achieved in the cavity pumped OPO.

[0025] The polarization direction of the outgoing light from the free-space optical isolator is horizontal in order to correspond to the quasi-phase matching of the crystal type 0 (e→e+e); its incident polarization direction is aligned with the slow axis direction of the first fiber collimator, which can improve the power stability of the OPO pump light.

[0026] Birefringent filters are used to limit the pump beam width of the OPO and tune the pump beam wavelength, thus enabling continuous tuning of the OPO idler wavelength by changing the pump beam wavelength.

[0027] The OPO signal optical resonator adopts a ring cavity structure, which improves the Q value of the signal optical resonator and enhances the wavelength selectivity of the resonator for the oscillating signal light inside the cavity, thereby reducing the bandwidth of the final output idler light.

[0028] The OPO employs a ring cavity structure for both the pump and signal light, which greatly reduces the absorption of the pump and signal light by the nonlinear crystal MgO:PPLN. This lowers the oscillation threshold of the OPO, improves the conversion efficiency of idler power, reduces the thermal effect of the crystal, and enhances the stability of the entire system. Attached Figure Description

[0029] The present invention will be further described below with reference to the accompanying drawings and embodiments. In the accompanying drawings:

[0030] Figure 1 This is a schematic diagram of a cavity-pumped continuous wave optical parametric oscillator based on a ring cavity structure.

[0031] In the figure: 1-Multimode semiconductor-pumped laser; 2-Signal pump combiner; 3-Polarization-maintaining double-clad ytterbium-doped fiber; 4-First fiber collimator; 5-Free-space optical isolator; 6-Birefringent filter; 7-First dichroic mirror; 8-First concave mirror; 9-MgO:PPLN crystal; 10-Second concave mirror; 11-Plane mirror; 12-Second dichroic mirror; 13-Second fiber collimator. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0033] like Figure 1 As shown, an intracavity-pumped continuous-wave optical parametric oscillator based on a ring cavity structure includes a multimode semiconductor-pumped laser 1, a signal pump combiner 2, a polarization-maintaining double-clad ytterbium-doped fiber 3, a first fiber collimator 4, a free-space optical isolator 5, a birefringent filter 6, a first dichroic mirror 7, a first concave mirror 8, an MgO:PPLN crystal 9, a second concave mirror 10, a plane mirror 11, a second dichroic mirror 12, and a second fiber collimator 13.

[0034] A multimode semiconductor pumped laser 1, a signal pump combiner 2, a polarization-maintaining double-clad ytterbium-doped fiber 3, and a first fiber collimator 4 are connected in sequence. After the first fiber collimator 4, a free-space optical isolator 5, a birefringent filter 6, a first dichroic mirror 7, a first concave mirror 8, an MgO:PPLN crystal 9, a second concave mirror 10, a plane mirror 11, a second dichroic mirror 12, and a second fiber collimator 13 are arranged in sequence. The second fiber collimator 13 couples the light transmitted from the second dichroic mirror 12 back into the signal pump combiner 2 to form a ring cavity circuit.

[0035] The signal pump combiner 2, polarization-maintaining double-clad ytterbium-doped fiber 3, first fiber collimator 4, and second fiber collimator 13 constitute the optical fiber path. All of these components are 1064nm polarization-maintaining devices and are connected via fiber fusion splicing. The first dichroic mirror 7, first concave mirror 8, MgO:PPLN crystal 9, second concave mirror 10, plane mirror 11, and second dichroic mirror 12 constitute the OPO signal optical resonant cavity, which is an X-shaped four-mirror ring cavity structure. The free-space optical isolator 5, the birefringent filter 6, and the cavity of the first dichroic mirror 7, the first concave mirror 8, the MgO:PPLN crystal 9, the second concave mirror 10, the plane mirror 11, and the second dichroic mirror 12 shared with the OPO signal optical resonator form the free-space optical path; the optical fiber path and the free-space optical path form the pump optical resonator of the OPO, and the pump optical resonator of the OPO is a ring cavity structure that operates in one direction.

[0036] The OPO pump light operates in a unidirectional manner, and the idler light is generated only in the same direction as the pump light.

[0037] In this embodiment, the multimode semiconductor pump laser 1 has a center wavelength of 976nm and a maximum output power of 30W. It serves as the pump light for the entire system and its output is connected to the signal pump combiner. The incident pump power is changed by controlling the current. The signal pump combiner 2 is a (2+1)×1 type. Its signal and common end pigtails are both passive double-clad optical fibers with a core diameter of 10μm and an inner cladding diameter of 125μm. Its pump end pigtail is a multimode optical fiber with a core diameter of 105μm, a cladding diameter of 125μm, and a core numerical aperture of 0.22. The polarization-maintaining double-clad ytterbium-doped fiber 3 is an active double-clad ytterbium-doped fiber with a core diameter of 10μm, an inner cladding diameter of 125μm, a core numerical aperture of 0.08, and a fiber length of 3m. Its two ends are connected to the common end of the signal pump combiner 2 and the pigtail of the first fiber collimator 4, respectively.

[0038] In this embodiment, the polarization direction of the emitted light from the free-space optical isolator 5 is horizontal. Its rotation angle is determined by a linearly polarized light source and an analyzer, and its incident polarization direction is aligned with the slow axis direction of the first fiber collimator 4. That is, the slow axis direction of the first fiber collimator 4 is consistent with the incident polarization direction of the free-space optical isolator 5. The collimated spot size of the first fiber collimator 4 is determined by the radius of curvature of the first concave mirror 8. The slow axis direction of the first fiber collimator 4 is determined by the power stability and output power of the OPO pump light. The optical path distance from the first fiber collimator 4 to the first concave mirror 8 is optimized by the idler frequency power output by the OPO. The parameters of the second fiber collimator 13 are completely identical to those of the first fiber collimator 4, and its slow axis direction is horizontal. The distance from the second fiber collimator 13 to the second dichroic mirror 12 is optimized by the conversion efficiency of the OPO pump light.

[0039] The birefringent filter 6 is an off-axis quartz crystal plate that is inserted into the pump light resonant cavity of the OPO at a Brewster angle. The Brewster angle of the birefringent filter 6 is determined by the output power and conversion efficiency of the OPO pump light.

[0040] The first dichroic mirror 7 is coated with a high-reflection film for the OPO pump light and an anti-reflection film for the signal light, which combines the pump light and the signal light; the second dichroic mirror 12 is coated with an anti-reflection film for the OPO pump light and a high-reflection film for the signal light, which separates the OPO pump light and the signal light.

[0041] As a nonlinear crystal, the position of the MgO:PPLN crystal 9 within the resonant cavity is optimized by the power of the idler light ultimately transmitted from behind the second concave reflector 10. The MgO:PPLN crystal 9 consists of multiple different polarization periods, and wavelength tuning of the OPO idler light is achieved by changing different polarization periods; both its front and back surfaces are coated with antireflective films for the OPO pump light, signal light, and idler light.

[0042] The parameters of the first concave mirror 8 and the second concave mirror 10 are completely identical. Their radius of curvature is determined by the length of the MgO:PPLN crystal 9, mainly based on the fact that the confocal parameter of the signal light in the crystal is greater than the length of the crystal. The distance between them is optimized by the output idler light power. The first concave mirror 8 and the second concave mirror 10 are coated with high-reflection films for the OPO pump light and the signal light, and with anti-reflection films for the idler light.

[0043] The plane mirror is coated with a high-reflectivity film for both OPO pump light and signal light. The distances of the plane mirror 11 and the second dichroic mirror 12 to the two concave mirrors affect the size of the focused spot of the signal light within the MgO:PPLN crystal 9, which is optimized by the output idler light power.

[0044] It should be understood that those skilled in the art can make improvements or modifications based on the above description, and all such improvements and modifications should fall within the protection scope of the appended claims.

Claims

1. A cavity-pumped continuous-wave optical parametric oscillator based on a ring cavity structure, characterized in that: It includes a multimode semiconductor-pumped laser, a signal-pumped beam combiner, a polarization-maintaining double-clad ytterbium-doped fiber, a first fiber collimator, a free-space optical isolator, a birefringent filter, a first dichroic mirror, a first concave mirror, an MgO:PPLN crystal, a second concave mirror, a plane mirror, a second dichroic mirror, and a second fiber collimator. A multimode semiconductor pump laser, a signal pump combiner, a polarization-maintaining double-clad ytterbium-doped fiber, and a first fiber collimator are connected in sequence. After the first fiber collimator, a free-space optical isolator, a birefringent filter, a first dichroic mirror, a first concave mirror, an MgO:PPLN crystal, a second concave mirror, a plane mirror, a second dichroic mirror, and a second fiber collimator are arranged in sequence. The second fiber collimator couples the light from the second dichroic mirror into the signal pump combiner to form a ring cavity circuit. The signal pump combiner, polarization-maintaining double-clad ytterbium-doped fiber, first fiber collimator, and second fiber collimator constitute the fiber optic path; the first dichroic mirror, first concave mirror, MgO:PPLN crystal, second concave mirror, plane mirror, and second dichroic mirror constitute the OPO signal optical resonant cavity, which is an X-shaped four-mirror ring cavity structure; the free-space optical isolator, birefringent filter, and the cavities of the first dichroic mirror, first concave mirror, MgO:PPLN crystal, second concave mirror, plane mirror, and second dichroic mirror shared with the OPO signal optical resonant cavity constitute the free-space optical path; the fiber optic path and the free-space optical path constitute the OPO pump optical resonant cavity, which is a unidirectional ring cavity structure; The OPO pump light operates in a unidirectional manner, and the idler light is generated only in the same direction as the pump light.

2. The cavity-pumped continuous wave optical parametric oscillator based on a ring cavity structure according to claim 1, characterized in that: The signal pump combiner, polarization-maintaining double-clad ytterbium-doped fiber, first fiber collimator, and second fiber collimator are all 1064nm polarization-maintaining devices, connected by fiber fusion splicing.

3. The cavity-pumped continuous wave optical parametric oscillator based on a ring cavity structure according to claim 1, characterized in that: The outgoing light of the free-space optical isolator is polarized horizontally, and its incident polarization direction is aligned with the slow axis of the first fiber collimator. That is, the slow axis of the first fiber collimator is consistent with the incident polarization direction of the free-space optical isolator.

4. The cavity-pumped continuous-wave optical parametric oscillator based on a ring cavity structure according to claim 1 or 3, characterized in that: The parameters of the second fiber collimator are exactly the same as those of the first fiber collimator, and its slow axis is in the horizontal direction; the parameters of the first concave mirror are exactly the same as those of the second concave mirror.

5. The cavity-pumped continuous wave optical parametric oscillator based on a ring cavity structure according to claim 1, characterized in that: The birefringent filter is an off-axis quartz crystal plate inserted into the pump light resonator of the OPO at Brewster angle.

6. The cavity-pumped continuous wave optical parametric oscillator based on a ring cavity structure according to claim 1, characterized in that: The first dichroic mirror is coated with a high-reflection film for the OPO pump light and an anti-reflection film for the signal light, which combines the pump light and the signal light; the second dichroic mirror is coated with an anti-reflection film for the OPO pump light and a high-reflection film for the signal light, which separates the OPO pump light and the signal light.

7. The cavity-pumped continuous wave optical parametric oscillator based on a ring cavity structure according to claim 1, characterized in that: The first and second concave mirrors are coated with high-reflectivity films for OPO pump light and signal light, and with anti-reflection films for idler light.

8. The cavity-pumped continuous wave optical parametric oscillator based on a ring cavity structure according to claim 1, characterized in that: The plane mirror is coated with a high-reflectivity film for OPO pump light and signal light.

9. The cavity-pumped continuous wave optical parametric oscillator based on a ring cavity structure according to claim 1, characterized in that: MgO:PPLN crystals consist of multiple different polarization periods. By changing different polarization periods, the wavelength of OPO idler light can be tuned. Both its front and back surfaces are coated with anti-reflection films that protect against OPO pump light, signal light, and idler light.

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