CN115149382A - Narrow linewidth continuously tuned mid-infrared optical parametric oscillator and its working method - Google Patents

Abstract

The invention provides a narrow-linewidth continuous tuning mid-infrared optical parametric oscillator and a working method thereof, wherein the mid-infrared optical parametric oscillator comprises: the pump source, the convex lens, the plane reflector, the MgO PPLN crystal, the concave mirror, the first etalon, the second etalon and the plane output mirror are arranged in the resonant cavity in sequence along the light path; the PPLN crystal is placed in a temperature control furnace, a heating sheet is arranged on the first etalon, and the thickness of the first etalon is larger than that of the second etalon; the invention realizes continuous wavelength tuning output by combining the thermal tuning etalon on the basis of crystal temperature tuning.

Description

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

technical field

The invention relates to the technical field of mid-infrared optical parametric oscillators, in particular to a narrow linewidth continuous tuning mid-infrared optical parametric oscillator and a working method.

Background technique

The statements in this section merely provide background related to the present disclosure and do not necessarily constitute prior art.

The mid-infrared laser in the 3μm-5μm band has important application value in many fields such as medical diagnosis, atmospheric environment monitoring, optoelectronic countermeasures, and high-precision spectral analysis. The military infrared guidance system mainly conducts reconnaissance on the light source in the 3μm ~ 5μm band. If the missile seeker emits a strong laser in this band, the seeker can be blinded and the real target can be covered. In practical applications, in order to obtain a sufficiently strong echo signal and high resolution in the long-distance laser detection, and at the same time minimize the attenuation of the laser transmission caused by the complex atmospheric environment, there are not only strict requirements on the laser power, but also on the laser spectral width. And the tunability of the laser output wavelength also puts forward certain requirements. Based on the MgO:PPLN mid-infrared optical parametric oscillator, the near-infrared pump light is converted to the mid-infrared parametric optical band through frequency conversion, which has the advantages of high conversion efficiency and wide tuning range, and is currently the main means to achieve mid-infrared laser output. Unfortunately, the parametric light in the 3μm-5μm band obtained by using the MgO:PPLN optical parametric oscillator has a wide laser linewidth in the free-running state, up to ten or even tens of nanometers, which is difficult to meet the practical application requirements.

In order to obtain a laser light source with a narrow linewidth of 3μm to 5μm, there are usually two methods: one is to inject seed light to obtain a narrow linewidth mid-infrared laser output, and the other is to use a frequency selection element in the optical parametric oscillator system. For example, etalons and diffraction gratings, etc. The former system is complex and difficult to implement; the latter has the advantages of compact structure, flexibility and convenience.

The inventors found that, based on the etalon linewidth compression technology, wide-tuning narrow-linewidth laser output can be achieved by tuning the temperature of MgO:PPLN crystal; however, because the etalon has a certain free spectral range, this method alone cannot achieve continuous wavelength tuning. .

SUMMARY OF THE INVENTION

In order to solve the deficiencies of the prior art, the present invention provides a narrow linewidth continuous tuning mid-infrared optical parametric oscillator and a working method, which realizes continuous wavelength tuning output based on crystal temperature tuning combined with etalon thermal tuning.

In order to achieve the above object, the present invention adopts the following technical solutions:

A first aspect of the present invention provides a narrow linewidth continuously tuned mid-infrared optical parametric oscillator.

A narrow linewidth continuous tuning mid-infrared optical parametric oscillator, comprising: a pump source, a convex lens, a plane mirror, a MgO:PPLN crystal, a concave mirror, and a first etalon, which are arranged in a resonant cavity and are arranged in sequence along an optical path. , the second etalon and the plane output mirror;

The MgO:PPLN crystal is placed in a temperature-controlled furnace, the first etalon is provided with a heating plate, and the thickness of the first etalon is greater than that of the second etalon.

As an optional implementation manner, the pump source is an acousto-optic QNd:YAG pump source for outputting linearly polarized laser light with a wavelength of 1064 nm.

As an optional implementation manner, the plane mirror is used as the input cavity mirror, which has high transmittance at 1064 nm and high reflectivity in the wavelength band of 1.4 μm to 1.6 μm.

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

As an optional implementation, the MgO:PPLN crystal includes 7 different periodic channels ranging from 28.5 μm to 31.5 μm with an interval of 0.5 μm.

As an optional implementation, the concave mirror has a radius of curvature of 500mm, high transmittance in the 1064nm and 3.4μm～4.3μm bands, and high reflectivity in the 1.4μm～1.6μm band. The frequency parametric light is output from the concave mirror.

As an optional implementation, 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.

As an optional implementation manner, the focal length of the convex lens is 500mm, and the focused spot size is 0.59mm*0.62mm.

As an optional implementation manner, the resonant cavity is a V-shaped resonant cavity, and the concave mirror is arranged at a corner position of the V-shaped resonant cavity.

A second aspect of the present invention provides a working method of a narrow linewidth continuous tuning mid-infrared optical parametric oscillator.

A working method of a narrow linewidth continuous tuning mid-infrared optical parametric oscillator, using the narrow linewidth continuous tuning mid-infrared optical parametric oscillator according to the first aspect of the present invention, comprising:

1064nm laser-pumped MgO:PPLN crystal to generate signal light and idler light;

The signal light oscillates continuously in the resonant cavity, and the first etalon in the cavity compresses the spectral width of the oscillating signal light, indirectly constraining the spectral line width of the idler light;

Using the larger free spectral range of the second etalon, the parametric light width is further compressed, and finally a narrow linewidth and continuously adjustable mid-infrared laser output is obtained;

Wherein, wavelength tuning is performed by changing the temperature of the MgO:PPLN crystal; or continuous wavelength tuning is performed by changing the temperature of the first etalon.

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

The narrow linewidth continuous tuning mid-infrared optical parametric oscillator and the working method of the present invention, on the basis of crystal temperature tuning, combined with etalon thermal tuning, realizes high-power narrow linewidth continuous tuning mid-infrared laser output, and the output is in the range of 3 μm～ The output power of idler light in the 5μm band range is ~2W; by adding two etalons with different thicknesses in the cavity, a narrower linewidth laser output is achieved.

Advantages of additional aspects of the invention will be set forth in part in the description which follows, and in part will become apparent from the description which follows, or may be learned by practice of the invention.

Description of drawings

The accompanying drawings forming a part of the present invention are used to provide further understanding of the present invention, and the exemplary embodiments of the present invention and their descriptions are used to explain the present invention, and do not constitute an improper limitation of the present invention.

FIG. 1 is a schematic structural diagram of a narrow linewidth continuously tunable mid-infrared optical parametric oscillator based on a dual etalon provided by an embodiment of the present invention.

FIG. 2 is an output spectrum diagram before and after insertion of an etalon provided by an embodiment of the present invention.

3 is a MgO:PPLN narrow linewidth temperature tuning output spectrogram provided by an embodiment of the present invention.

FIG. 4 is an etalon transmission peak curve provided by an embodiment of the present invention.

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 sheet; 9-second etalon; 10-plane output mirror.

Detailed ways

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

It should be noted that the following detailed description is exemplary and intended to provide further explanation of the invention. Unless otherwise defined, 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 belongs.

It should be noted that the terminology used herein is for the purpose of describing specific embodiments only, and is not intended to limit the exemplary embodiments according to the present invention. As used herein, unless the context clearly dictates otherwise, the singular is intended to include the plural as well, furthermore, it is to be understood that when the terms "comprising" and/or "including" are used in this specification, it indicates that There are features, steps, operations, devices, components and/or combinations thereof.

Embodiments of the invention and features of the embodiments may be combined with each other without conflict.

Example:

The embodiment of the present invention provides a narrow linewidth continuously tuned mid-infrared optical parametric oscillator, comprising: a Nd:YAG pump source 1, a convex lens 2, a plane mirror 3, and a Nd:YAG pump source 1, a convex lens 2, a plane mirror 3, arranged in the resonator cavity and sequentially arranged along the optical path. MgO:PPLN crystal 5 , concave mirror 6 , first etalon 7 , second etalon 9 and flat output mirror 10 .

Specifically, as shown in FIG. 1 , an optical parametric oscillator with a V-shaped resonant cavity is built in this embodiment. 1 is a Nd:YAG pump source, and the output wavelength is a linearly polarized laser of 1064 nm; Focusing, the spot size after focusing is about 0.59mm*0.62mm; 3 is a flat mirror with high transmittance at 1064nm and high reflectivity in the 1.4μm-1.6μm band, as the input cavity mirror.

A PPLN crystal with a size of 50mm ³ × 8.6mm ³ × 1mm ³ , doped with 5 mol.% MgO, was placed in a temperature-controlled furnace 4. The temperature accuracy of the temperature-controlled furnace was 0.1 °C. By setting different temperature control The temperature of the furnace enables temperature tuning of the MgO:PPLN crystal. The MgO:PPLN crystal includes 7 different periodic channels, ranging from 28.5 μm to 31.5 μm, with an interval of 0.5 μm, and a 29.5 μm periodic channel is selected in this embodiment.

6 is a concave mirror with a radius of curvature of 500mm, high transmittance in the 1064nm and 3.4μm～4.3μm bands, and high reflectivity in the 1.4μm～1.6μm band, and the final mid-infrared idler parametric light is output from here. 7 is the first etalon, 9 is the second etalon, but the thicknesses of the two are different (the thickness of the first etalon is greater than the thickness of the second etalon), 8 is the heating piece, which is attached to the surface of the etalon 7 and passes through the heating piece The output wavelength can be continuously adjusted by changing the etalon temperature; 10 is a flat output mirror with high reflectivity at 1064nm, high transmittance in the 3.4μm-4.3μm band, and 10% transmittance in the 1.4μm-1.6μm band.

This embodiment is an etalon-based narrow linewidth, continuously tunable MgO:PPLN mid-infrared laser, 1064nm laser-pumped MgO:PPLN crystal, due to nonlinear effects, will generate signal light and idler light, wherein the signal light Continue to oscillate in the resonant cavity, and the etalon in the cavity compresses the spectral width of the oscillating signal light, thereby indirectly constraining the spectral width of the idler light, and obtaining a mid-infrared laser output with a narrow linewidth. Etalon transmission peak half-width formula:

where λ is the incident wavelength, n is the etalon refractive index, d is the etalon thickness, and R is the etalon inner surface light intensity reflectance. It can be seen from the above formula that the thickness of the etalon will affect the spectral line width of the transmission peak. The greater the thickness of the etalon, the smaller the full width at half maximum of the transmission peak of the etalon, and the more obvious the effect of narrowing the line width. 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, which is specifically expressed as follows:

where d is the thickness of the etalon. It can be seen that a thicker etalon has a small free spectral range, and for a laser with a relatively wide fluorescence linewidth, only a single thicker etalon cannot obtain a single spectral peak, and the linewidth cannot be well compressed. The method of this embodiment to solve this problem is to add a thin etalon, and use its larger free spectral range to obtain a single peak and further compress the line width.

There are two ways of wavelength tuning in this embodiment, namely, changing the temperature of the MgO:PPLN crystal and thermal tuning of the etalon. The OPO parametric optical tuning needs to satisfy the energy and momentum conservation equations:

In the formula, Λ is the polarization period of the MgO:PPLN crystal, n _p , _ns and _ni are the refractive indices of the pump light, signal light and idler light, respectively. These quantities are all related to the crystal temperature. Wavelength tuning was achieved by changing the temperature of MgO:PPLN crystal.

In addition, wavelength tuning can also be achieved by thermal tuning of the etalon. For the etalon, different wavelengths have different transmittances, which can be expressed as:

In the formula, F is the fineness of the etalon, and d is the thickness of the etalon. By heating the etalon, the thickness of the etalon changes due to heating, resulting in a change in transmittance, thereby achieving continuous wavelength tuning. Therefore, in this embodiment, there are two methods for obtaining a narrow linewidth, continuously tunable mid-infrared laser output. The first is to use a thin etalon to obtain narrow linewidth output, and to achieve output wavelength tuning by changing the temperature of the MgO:PPLN crystal. Another method is to add a thicker etalon based on the first method to further compress the line width, and to achieve continuous wavelength tuning by changing the temperature of the thicker etalon.

The output spectrum of the signal light is shown in Figure 2. When the etalon is not inserted, that is, when the OPO operates in a free working state, the output spectrum is wide, and the spectral width is about 1.8 nm. After inserting the etalon, the spectral width was compressed to 0.039 nm. It can be seen that the etalon shows an excellent ability in compressing the spectral width. According to theoretical calculation, the corresponding idler spectral width is compressed from the original 12.17nm to 0.995nm. The output wavelength tuning curve is shown in Figure 3. It can be seen that the etalon transmission peak is well matched with the signal light output spectrum.

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 (ie, the etalon transmission peak half-height width) is about 1.5 nm; for a 0.35 mm thick etalon, Δλ is about 2.1 nm.

Figure 4 shows the transmission peak curves of the 0.5mm and 0.35mm etalons. It can be seen that for the thicker 0.5mm etalon, its free spectral range, that is, the distance between two adjacent transmission peaks is small, and the output spectral lines in the OPO free state There are two transmission peaks under the wide, and a double peak output may be obtained when compressing the spectral linewidth; if a 0.35mm thick etalon is used, due to its large free spectral range, there is only one transmission peak in the OPO free state output spectrum . The actual output spectrum is the overlapping part of the two etalon transmission peaks, which not only ensures a narrow line width, but also obtains a single peak output, and changes the temperature of the thick 0.5mm etalon, which can move the transmission peak to the left and right, and realize the continuous tuning of the output wavelength. .

The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims (10)

1. A narrow linewidth continuous tuning mid-infrared parametric oscillator is characterized in that:

the method comprises the following steps: the pump source, the convex lens, the plane reflector, the MgO, the PPLN crystal, the concave mirror, the first etalon, the second etalon and the plane output mirror are arranged in the resonant cavity along the optical path in sequence;

the PPLN crystal is arranged in a temperature control furnace, a heating sheet is arranged on the first etalon, and the thickness of the first etalon is larger than that of the second etalon.

2. The narrow linewidth continuous-tuned mid-ir parametric oscillator of claim 1, wherein:

the pumping source is an acousto-optic modulation QNd-YAG pumping source and is used for outputting linear polarization laser with the wavelength of 1064 nm.

3. The narrow linewidth continuously tuned mid-infrared parametric oscillator of claim 1, wherein:

the plane reflector is used as an input cavity mirror, has high transmittance at 1064nm and high reflectivity at a wave band of 1.4-1.6 μm.

4. The narrow linewidth continuously tuned mid-infrared parametric oscillator of claim 1, wherein:

the size of MgO PPLN crystal is 50mm ³ ×8.6mm ³ ×1mm ³ 5mol% of MgO is doped.

5. The narrow linewidth continuous-tuned mid-infrared parametric oscillator of any of claims 1-4, wherein:

PPLN crystal comprises 7 different periodic channels ranging from 28.5 μm to 31.5 μm with 0.5 μm spacing.

6. The narrow linewidth continuously tuned mid-infrared parametric oscillator of claim 1, wherein:

the curvature radius of the concave mirror is 500mm, the concave mirror has high transmittance in the wave bands of 1064nm and 3.4-4.3 mu m, and has high reflectivity in the wave bands of 1.4-1.6 mu m, and finally generated mid-infrared idler parameter light is output from the concave mirror.

7. The narrow linewidth continuous-tuned mid-ir parametric oscillator of claim 1, wherein:

the plane output mirror has high reflectivity at 1064nm, high transmissivity at the wave band of 3.4-4.3 μm and transmissivity of 10% at the wave band of 1.4-1.6 μm.

8. The narrow linewidth continuous-tuned mid-ir parametric oscillator of claim 1, wherein:

the focal length of the convex lens is 500mm, and the focused spot size is 0.59mm x 0.62mm.

9. The narrow linewidth continuous-tuned mid-ir parametric oscillator of claim 1, wherein:

the resonant cavity is a V-shaped resonant cavity, and the concave mirror is arranged at the corner of the V-shaped resonant cavity.

10. A method of operating a narrow linewidth continuous tuned mid-ir parametric oscillator according to any one of claims 1 to 9, comprising:

PPLN crystal, generating signal light and idler frequency light, the signal light continuously oscillating in the resonant cavity, compressing the spectral width of the oscillating signal light through the first etalon in the cavity, indirectly constraining the spectral line width of the idler frequency light, obtaining a single peak by utilizing the larger free spectral range of the second etalon, further compressing the line width, and finally obtaining the narrow-line-width and continuously adjustable mid-infrared laser output;

wherein, the wavelength is tuned by changing the temperature of the MgO: PPLN crystal; or wavelength continuous tuning by changing the temperature of the first etalon.

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