Narrow-linewidth continuously tunable mid-infrared optical parametric oscillator and working method

Abstract

The application provides a narrow linewidth continuous tuning middle infrared optical parametric oscillator and a working method thereof. The middle infrared optical parametric oscillator comprises a pump source, a convex lens, a plane mirror, a MgO:PPLN crystal, a concave mirror, a first etalon, a second etalon and a plane output mirror which are arranged in sequence along an optical path in a resonant cavity. The MgO:PPLN crystal is placed in a temperature control furnace, the first etalon is provided with a heating sheet, and the thickness of the first etalon is greater than that of the second etalon. On the basis of crystal temperature tuning, the application combines a thermal tuning etalon to realize continuous wavelength tuning output.

Description

Technical Field

[0001] This invention relates to the field of mid-infrared optical parametric oscillators, and particularly to a narrow-linewidth continuously tuned mid-infrared optical parametric oscillator and its operating method. Background Technology

[0002] The statements in this section are merely background information related to the present invention and do not necessarily constitute prior art.

[0003] Mid-infrared lasers in the 3μm–5μm band have significant applications in medical diagnostics, atmospheric environmental monitoring, optoelectronic countermeasures, and high-precision spectral analysis. Military infrared guidance systems primarily detect light sources in the 3μm–5μm band. Firing a strong laser in this band at a missile seeker head can blind it, effectively masking the real target. In practical applications, to obtain sufficiently strong echo signals and high resolution for long-range laser detection, while minimizing the attenuation of laser transmission in complex atmospheric environments, strict requirements are placed on not only laser power but also on the tunability of the laser spectral width and output wavelength. Based on a MgO:PPLN mid-infrared parametric oscillator, frequency conversion from near-infrared pump light to the mid-infrared parametric light band offers advantages such as high conversion efficiency and a wide tuning range, making it the primary method for achieving mid-infrared laser output. Unfortunately, the parametric light in the 3μm to 5μm band obtained by using the MgO:PPLN optical parametric oscillator has a wide laser linewidth in the free-running state, reaching tens of nanometers, which is difficult to meet the needs of practical applications.

[0004] To obtain a narrow-linewidth laser source in the 3μm–5μm wavelength range, there are generally two methods: one is to obtain a narrow-linewidth mid-infrared laser output by seed light injection, and the other is to use frequency-selective elements, such as etalons and diffraction gratings, in an optical parametric oscillator system. The former is complex and difficult to implement; the latter has advantages such as compact structure and flexibility.

[0005] The inventors discovered that, based on the standard etalon linewidth compression technology, wide-tunable narrow-linewidth laser output can be achieved by tuning the temperature of the MgO:PPLN crystal; however, since the standard has a certain free spectral range, continuous wavelength tuning cannot be achieved by relying solely on this method. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a narrow-linewidth continuously tuned mid-infrared parametric oscillator and its operating method. Based on crystal temperature tuning, it combines etalon thermal tuning to achieve continuous wavelength tuning output.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] The first aspect of the present invention provides a narrow-linewidth continuously tuned mid-infrared parametric oscillator.

[0009] A narrow-linewidth continuous-tuning mid-infrared optical parametric oscillator includes: a pump source, a convex lens, a plane mirror, an MgO:PPLN crystal, a concave mirror, a first etalon, a second etalon, and a plane output mirror arranged sequentially along the optical path within a resonant cavity;

[0010] The MgO:PPLN crystal is placed in a temperature-controlled furnace. A heating element is provided on the first standard, and the thickness of the first standard is greater than the thickness of the second standard.

[0011] As an optional implementation, the pump source is an acousto-optic Q-switched Nd:YAG pump source, used to output linearly polarized laser with a wavelength of 1064nm.

[0012] As an optional implementation, a plane mirror, used as an input cavity mirror, has high transmittance at 1064 nm and high reflectivity in the 1.4 μm to 1.6 μm band.

[0013] As an optional implementation, the MgO:PPLN crystal has a size of 50 mm. 3 ×8.6mm 3 ×1mm 3 It is doped with 5 mol% MgO.

[0014] As an alternative implementation, the MgO:PPLN crystal includes seven different periodic channels ranging from 28.5 μm to 31.5 μm, spaced 0.5 μm apart.

[0015] As an alternative implementation, the concave mirror has a radius of curvature of 500 mm, high transmittance in the 1064 nm and 3.4 μm to 4.3 μm bands, and high reflectivity in the 1.4 μm to 1.6 μm band. The resulting mid-infrared idler parametric light is output from the concave mirror.

[0016] As an alternative implementation, the planar output mirror has high reflectivity at 1064nm, high transmittance in the 3.4μm–4.3μm band, and 10% transmittance in the 1.4μm–1.6μm band.

[0017] As an optional implementation, the convex lens has a focal length of 500mm, and the focused spot size is 0.59mm*0.62mm.

[0018] As an optional implementation, the resonant cavity is a V-shaped resonant cavity, with the concave mirror positioned at the corner of the V-shaped resonant cavity.

[0019] The second aspect of the present invention provides a method for operating a narrow-linewidth continuously tuned mid-infrared optical parametric oscillator.

[0020] A method for operating a narrow-linewidth continuously tuned mid-infrared optical parametric oscillator, utilizing the narrow-linewidth continuously tuned mid-infrared optical parametric oscillator described in the first aspect of the present invention, includes:

[0021] A 1064nm laser pumps an MgO:PPLN crystal to generate signal light and idler light.

[0022] The signal light oscillates continuously in the resonant cavity. The first etalon inside the cavity compresses the spectral width of the oscillating signal light, thereby indirectly constraining the spectral linewidth of the idler light.

[0023] By utilizing the large free spectral range of the second standard etalon, the parametric beamwidth is further compressed, ultimately resulting in a narrow linewidth and continuously tunable mid-infrared laser output.

[0024] Wavelength tuning is achieved by changing the temperature of the MgO:PPLN crystal; or continuous wavelength tuning is achieved by changing the temperature of the first etalon.

[0025] Compared with the prior art, the beneficial effects of the present invention are:

[0026] The narrow-linewidth continuously tuned mid-infrared parametric oscillator and its operating method described in this invention, based on crystal temperature tuning and combined with etalon thermal tuning, achieve high-power narrow-linewidth continuously tuned mid-infrared laser output, with idler light output power of ~2W in the 3μm to 5μm band; by adding two etalons of different thicknesses in the cavity, even narrower linewidth laser output is achieved.

[0027] Advantages of additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0028] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0029] Figure 1 This is a schematic diagram of a narrow-linewidth continuously adjustable mid-infrared optical parametric oscillator based on dual etalons, provided as an embodiment of the present invention.

[0030] Figure 2 The output spectra before and after insertion of the etalon are provided in an embodiment of the present invention.

[0031] Figure 3 The output spectrum of MgO:PPLN with narrow linewidth temperature tuning provided in an embodiment of the present invention.

[0032] Figure 4 The standard etalon transmission peak curve is provided for embodiments of the present invention.

[0033] Among them, 1-Nd:YAG pump source; 2-convex lens; 3-plane mirror; 4-temperature control furnace; 5-MgO:PPLN crystal; 6-concave mirror; 7-first etalon; 8-heating plate; 9-second etalon; 10-plane output mirror. Detailed Implementation

[0034] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0035] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0036] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0037] Where there is no conflict, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0038] Example:

[0039] This invention provides a narrow-linewidth continuous-tuned mid-infrared parametric oscillator, comprising: an Nd:YAG pump source 1, a convex lens 2, a plane mirror 3, an MgO:PPLN crystal 5, a concave mirror 6, a first etalon 7, a second etalon 9, and a plane output mirror 10 arranged sequentially along the optical path within a resonant cavity.

[0040] Specifically, such as Figure 1 As shown, this embodiment constructs a V-shaped resonant cavity optical parametric oscillator. 1 is an Nd:YAG pump source that outputs linearly polarized laser light with a wavelength of 1064nm. The light is focused by a convex lens 2 with a focal length of 500mm, and the size of the focused spot is approximately 0.59mm*0.62mm. 3 is a plane mirror with high transmittance at 1064nm and high reflectivity in the 1.4μm~1.6μm band, serving as the input cavity mirror.

[0041] The size is 50mm 3 ×8.6mm 3×1mm 3 The PPLN crystal, doped with 5 mol.% MgO, was placed in a temperature-controlled furnace 4 with a temperature accuracy of 0.1℃. Temperature tuning of the MgO:PPLN crystal was achieved by setting different furnace temperatures. The MgO:PPLN crystal contained seven different periodic channels, ranging from 28.5 μm to 31.5 μm, with an interval of 0.5 μm. In this embodiment, a 29.5 μm periodic channel was selected.

[0042] 6 is a concave mirror with a radius of curvature of 500 mm. It has high transmittance in the 1064 nm and 3.4 μm–4.3 μm wavelength range and high reflectance in the 1.4 μm–1.6 μm wavelength range. The mid-infrared idler parametric light is output from here. 7 is the first etalon, and 9 is the second etalon, but they have different thicknesses (the first etalon is thicker than the second etalon). 8 is a heating element attached to the surface of etalon 7. The output wavelength is continuously adjustable by changing the temperature of the etalon through the heating element. 10 is a planar output mirror with high reflectance at 1064 nm, high transmittance in the 3.4 μm–4.3 μm wavelength range, and 10% transmittance in the 1.4 μm–1.6 μm wavelength range.

[0043] This embodiment describes a narrow-linewidth, continuously tunable MgO:PPLN mid-infrared laser based on an etalon. A 1064nm laser pumps the MgO:PPLN crystal. Due to nonlinear effects, signal light and idler light are generated. The signal light oscillates continuously in the resonant cavity. The etalon within the cavity compresses the spectral width of the oscillating signal light, thereby indirectly constraining the spectral width of the idler light, resulting in a narrow-linewidth mid-infrared laser output. The formula for the full width at half maximum (FWHM) of the etalon transmission peak is:

[0044]

[0045] In the formula, λ is the incident wavelength, n is the refractive index of the etalon, d is the thickness of the etalon, and R is the reflectivity of the inner surface of the etalon. As shown in the formula, the thickness of the etalon affects the linewidth of the transmission peak. The greater the etalon thickness, the smaller the full width at half maximum (FWHM) of the transmission peak, and the more significant the linewidth narrowing effect. However, the etalon also has a free spectral range. The free spectral range of the etalon refers to the interval between two adjacent transmittance maxima, specifically expressed as follows:

[0046]

[0047] In the formula, d represents the etalon thickness. It is evident that a thicker etalon has a smaller free spectral range. For lasers with relatively wide fluorescence linewidths, using only a thicker etalon cannot obtain a single spectral peak, thus failing to effectively compress the linewidth. This embodiment addresses this problem by adding a thinner etalon, utilizing its larger free spectral range to obtain a single peak and further compress the linewidth.

[0048] In this embodiment, wavelength tuning can be achieved through two methods: changing the temperature of the MgO:PPLN crystal and thermal tuning of the etalon. OPO parametric optical tuning must satisfy the energy and momentum conservation equations.

[0049]

[0050]

[0051] In the formula, Λ is the polarization period of the MgO:PPLN crystal, and n p n s and n i These are the refractive indices of the pump light, signal light, and idler light, respectively. These quantities are all related to the crystal temperature. Therefore, wavelength tuning can be achieved by changing the temperature of the MgO:PPLN crystal.

[0052] Furthermore, wavelength tuning can also be achieved through thermal tuning of the etalon. For the etalon, different wavelengths have different transmittances, which can be expressed as:

[0053]

[0054] In the formula, F represents the fineness of the etalon, and d represents the thickness of the etalon. Heating the etalon changes its thickness, thus altering its transmittance and enabling continuous wavelength tuning. Therefore, this embodiment provides two methods for obtaining narrow-linewidth, continuously tunable mid-infrared laser output. The first method uses a thinner etalon to achieve narrow-linewidth output, and wavelength tuning is achieved by changing the temperature of the MgO:PPLN crystal. The second method, based on the first, adds a thicker etalon to further compress the linewidth, and continuous wavelength tuning is achieved by changing the temperature of the thicker etalon.

[0055] Signal light output spectrum such as Figure 2 As shown, without the etalon inserted, i.e., when the OPO is operating in free mode, the output spectrum is relatively broad, with a width of approximately 1.8 nm. After inserting the etalon, the width is compressed to 0.039 nm. This demonstrates the excellent ability of the etalon to compress the spectral width. According to theoretical calculations, the corresponding idler spectrum width is compressed from 12.17 nm to 0.995 nm. The output wavelength tuning curve is shown below. Figure 3 As shown, the transmission peak of the etalon and the output spectrum of the signal light are well matched.

[0056] In this embodiment, the thickness of the first etalon is greater than that of the second etalon. For example, the thickness of the first etalon is 0.5 mm, and the thickness of the second etalon is 0.35 mm. For a 0.5 mm thick etalon, Δλ (i.e., the full width at half maximum (FWHM) of the etalon transmission peak is approximately 1.5 nm; for a 0.35 mm thick etalon, Δλ is approximately 2.1 nm.

[0057] Figure 4 The transmission peak curves of 0.5mm and 0.35mm etalons are shown. It can be seen that for the thicker 0.5mm etalon, its free spectral range (i.e., the spacing between adjacent transmission peaks) is smaller, resulting in two transmission peaks at the OPO free-state output spectral linewidth. This may lead to a double-peak output when compressing the spectral linewidth. If a 0.35mm thick etalon is used, due to its larger free spectral range, only one transmission peak is observed at the OPO free-state output spectrum. The actual output spectrum is the overlapping portion of the two etalon transmission peaks, ensuring both a narrow linewidth and a single-peak output. Furthermore, changing the temperature of the thicker 0.5mm etalon allows the transmission peak to shift left and right, achieving continuous tuning of the output wavelength.

[0058] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A narrow-linewidth continuously tuned mid-infrared optical parametric oscillator, characterized in that... : The system includes: a pump source, a convex lens, a plane mirror, an MgO:PPLN crystal, a concave mirror, a first etalon, a second etalon, and a plane output mirror arranged sequentially along the optical path within the resonant cavity. The plane output mirror has high reflectivity at 1064 nm, high transmittance in the 3.4 μm~4.3 μm band, and 10% transmittance in the 1.4 μm~1.6 μm band. The concave mirror has a radius of curvature of 500 mm, high transmittance in the 1064 nm and 3.4 μm~4.3 μm bands, and high reflectivity in the 1.4 μm~1.6 μm band. The resulting mid-infrared idler parametric light is output from the concave mirror. The MgO:PPLN crystal is placed in a temperature-controlled furnace. A heating element is provided on the first standard, and the thickness of the first standard is greater than the thickness of the second standard. The pump source is an acousto-optic Q-switched Nd:YAG pump source, used to output linearly polarized laser with a wavelength of 1064nm; Wavelength tuning can be achieved by changing the temperature of the MgO:PPLN crystal; or wavelength tuning can be achieved by changing the temperature of the first etalon; the spectral width can be compressed by the etalon, so that the transmission peak of the etalon and the output spectrum of the signal light can be better matched. The resonant cavity is a V-shaped resonant cavity, and the concave mirror is placed at the corner of the V-shaped resonant cavity; A 1064nm laser pumps a MgO:PPLN crystal to generate signal light and idler light. The signal light oscillates continuously in the resonant cavity. The first etalon in the cavity compresses the spectral width of the oscillating signal light and indirectly constrains the spectral linewidth of the idler light. The second etalon, with its large free spectral range, is used to obtain a single peak, further compressing the linewidth, and finally obtaining a narrow-linewidth and continuously tunable mid-infrared laser output. Based on crystal temperature tuning, combined with etalon thermal tuning, high-power, narrow-linewidth, continuously tuned mid-infrared laser output is achieved. The actual output spectrum is the overlapping part of the transmission peaks of the first and second etalons, which ensures both narrow linewidth and single-peak output. By changing the temperature of the first etalon, the transmission peak is shifted left and right, thus achieving continuous tuning of the output wavelength.

2. The narrow-linewidth continuously tuned mid-infrared optical parametric oscillator as described in claim 1, characterized in that... : As an input cavity mirror, the plane mirror has high transmittance at 1064nm and high reflectivity in the 1.4μm~1.6μm band.

3. The narrow-linewidth continuously tuned mid-infrared optical parametric oscillator as described in claim 1, characterized in that... : The MgO:PPLN crystals are 50 mm in size. 3 × 8.6mm 3 ×1 mm 3 It is doped with 5 mol% MgO.

4. The narrow-linewidth continuously tuned mid-infrared optical parametric oscillator as described in any one of claims 1-3, characterized in that... : The MgO:PPLN crystal comprises seven distinct periodic channels, ranging from 28.5 μm to 31.5 μm in size, spaced 0.5 μm apart.

5. The narrow-linewidth continuously tuned mid-infrared parametric oscillator as described in claim 1, characterized in that... : The convex lens has a focal length of 500mm, and the focused spot size is 0.59mm*0.62mm.

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