A high efficiency kyw second order stokes raman laser

Abstract

The application provides a high-efficiency KYW second-order Stokes Raman laser, a second half-wave plate is used for rotation adjustment to align pump light with different optical axes of a KYW crystal, and the different optical axes of the KYW crystal are used for Raman frequency shift to obtain rich Raman wavelengths, such as second-order Raman wavelengths of 1270nm, 1285nm, 1294nm and 1318nm, and the KYW crystal can be used for realizing high-energy Raman laser pulse output.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lasers, and particularly relates to a high-efficiency KYW second-order Stokes Raman laser. BACKGROUND

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

[0003] 1.3 mu m waveband laser is in the atmospheric transmission window and the optical fiber transmission window area, and has wide application prospects in the fields of laser radar, laser communication, gas detection and sodium laser guide star. A traditional technical means is to directly obtain a laser by using a laser crystal doped with Nd ions, but this type of laser has problems of small emission cross section, single laser spectrum line and poor beam quality. In recent years, Raman laser technology has unique advantages in special wavelength expansion, beam purification, pulse compression and the like, and has become a research hotspot in the field of lasers. The working principle of a Raman laser is based on stimulated Raman scattering effect. When high-intensity pump light is incident into a Raman crystal, non-elastic scattering occurs, and a specific frequency shift of the pump light wavelength is generated, thereby generating first-order Stokes laser. The size of the frequency shift is the frequency shift of the Raman crystal. When the energy of the first-order Stokes light is strong enough, the second-order Stokes light is excited, and higher-order Raman lasers are generated in turn. However, the existing Raman crystal frequency shift peak mostly has only one, and multiple Raman frequency shift wavelengths cannot be generated in one Raman laser, resulting in limited Raman wavelengths and inability to generate rich Raman wavelengths. SUMMARY

[0004] In order to overcome the shortcomings of the prior art, the application provides a high-efficiency KYW second-order Stokes Raman laser, which utilizes the characteristic of multiple Raman frequency shift peaks of a KYW crystal, is based on cascade and cross-cascade mechanisms, and rotates the polarization direction of pump waves to obtain rich second-order Raman wavelengths.

[0005] To achieve the above object, the application provides a high-efficiency KYW second-order Stokes Raman laser, which comprises, in sequence along the beam direction, a pump source, a first half-wave plate, an optical isolator, a second half-wave plate, a focusing lens, an input mirror, a KYW crystal and an output mirror.

[0006] The pump source is configured to emit pump light.

[0007] The first half-wave plate and the optical isolator are configured to adjust the power of the pump light emitted by the pump source.

[0008] The second half-wave plate is adjusted in rotation according to the direction of the optical axis of the KYW crystal, so that the optical axis of the KYW crystal is aligned with the direction of the polarization of the pump light.

[0009] The adjusted polarized pump light passes through different crystal optical axes of the KYW crystal, and corresponding Raman wavelengths are outputted;

[0010] The input mirror and the output mirror constitute a Raman resonant cavity.

[0011] The above one or more technical solutions have the following beneficial effects:

[0012] In the present application, rotating the second half-wave plate makes the polarization direction of the pump light align with different optical axes of the KYW crystal, and different Raman wavelengths, such as 1270 nm, 1285 nm, 1294 nm, 1318 nm, etc., are obtained by using the Raman frequency shift characteristics of different optical axes of the KYW crystal, and high-energy Raman laser pulse output can be realized by using the high damage threshold characteristics (10 GW / cm 2 ) of the KYW crystal.

[0013] The advantages of the additional aspects of the present application will be partially given in the following description, partially become obvious from the following description, or be understood by the practice of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0014] The drawings accompanying the specification of the present application form a part thereof, serve to provide further understanding of the present application, and together with the description of the exemplary embodiments of the present application and the explanation thereof serve to explain the present application, and do not constitute improper limitations on the present application.

[0015] Figure 1 It is a structure diagram of the KYW second-order Stokes Raman laser in the embodiment one of the present application;

[0016] Figure 2 It is a schematic diagram of the optical axis of the KYW crystal in the embodiment one of the present application;

[0017] Figure 3 It is a spectrum of the KYW Raman laser with the pump polarization direction aligned with the N m axis at the highest output power in the embodiment one of the present application;

[0018] Figure 4 It is a spectrum of the KYW Raman laser with the pump polarization direction aligned with the N g axis in the embodiment one of the present application;

[0019] Figure 5 It is a spectrum of the KYW Raman laser with the pump polarization direction between the N g axis and the N m axis in the embodiment one of the present application;

[0020] Figure 6 It is a comparison diagram of the pump light and the Raman light pulse shape in the embodiment one of the present application;

[0021] Figure 7 Figure 1 shows the output mirror transmittance graph of embodiment one of the present application.

[0022] Legend of reference signs

[0023] 1, pump source, 2, first half-wave plate, 3, optical isolator, 4, second half-wave plate, 5, focusing lens, 6, input mirror, 7, KYW crystal, 8, output mirror. DETAILED DESCRIPTION

[0024] It should be noted that the following detailed description is exemplary in nature and is intended to provide further description of the application. 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 application belongs.

[0025] It is to be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments according to the present application.

[0026] In the case of no conflict, the embodiments in the present application and the features in the embodiments can be combined with each other.

[0027] Embodiment one

[0028] The embodiment discloses a high-efficiency KYW second-order Stokes Raman laser, comprising a pump source, a first half-wave plate, an optical isolator, a second half-wave plate, a focusing lens, an input mirror, a KYW crystal and an output mirror which are sequentially arranged along a light beam direction.

[0029] The pump source is used for emitting pump light.

[0030] The first half-wave plate and the optical isolator are used for adjusting the power of the pump light emitted by the pump source.

[0031] The second half-wave plate is adjusted in rotation according to the KYW crystal optical axis direction, so that the KYW crystal optical axis is aligned with the polarization direction of the pump light.

[0032] The adjusted polarized pump light passes through different crystal optical axes of the KYW crystal, and corresponding Raman wavelengths are output.

[0033] The input mirror and the output mirror constitute a Raman resonant cavity.

[0034] The Raman laser can be divided into two types of external cavity and internal cavity. The internal cavity design can fully utilize the power density in the cavity, reduce the light output threshold, and is currently used more frequently. However, the pump cavity and the Raman cavity are combined together and influence each other, and it is not easy to produce high-power output. The external cavity Raman laser design is independent of the pump cavity and the Raman cavity, and it is easy to optimize the pump beam and the Raman beam spot quality separately, and different Stokes light outputs can be obtained by changing the film system structure, so the embodiment designs an external cavity Raman cavity structure.

[0035] The experimental setup of the second-order Stokes KYW external cavity Raman laser is shown in Figure 1 Figure 1. The 1064 nm pump source 1 is a diode side-pumped Nd:YAG laser, which generates more than 30 W linearly polarized laser output at a repetition rate of 10 kHz.

[0036] The first half-wave plate 2 is placed before the optical isolator 3, and the pump power is adjusted by rotating the first half-wave plate 2 placed in front of the optical isolator 3. The optical isolator 3 can prevent the reflected light from entering the pump source.

[0037] Specifically, the first half-wave plate 2 is a 1064 nm half-wave plate.

[0038] The pump beam is focused to the center of the KYW crystal by the focusing lens 5, and the spot size is 280 μm. The second half-wave plate 4 placed in front of the coupling lens is used to rotate the pump polarization direction to align the N m or N g crystal optical axis direction.

[0039] Specifically, the second half-wave plate 4 is a 1064 nm half-wave plate.

[0040] The Raman medium is an undoped and uncoated KYW crystal with a size of 5 × 5 × 45 mm 3 . The KYW crystal has a moderate Raman gain coefficient (3.6 cm / GW@1064 nm) and thermal conductivity (3.3 W / m / K).

[0041] Potassium yttrium tungstate (KY(WO4)2, KYW) as a typical double-metal tungstate Raman medium has a very high optical damage threshold and good thermal conductivity. In addition, they have two large Raman shifts (765 cm -1 and 905 cm -1 ) and one small Raman shift (87 cm -1 ), which can produce a rich Raman wavelength by changing the polarization orientation of the pump wave.

[0042] As shown in Figure 2 , the KYW crystal has three crystal optical axes N p , N m and N g . The pump light propagates along the N p axis of the KYW crystal, thereby providing high gain for the 765 cm -1 and 905 cm -1 modes. When the pump polarization direction is aligned with the N g axis, the Raman laser enters the 765 cm -1 phonon mode. When the pump polarization direction is aligned with the N mWhen the axis is aligned, the Raman laser enters 905 cm. -1 Phonon mode.

[0043] The crystal is wrapped in indium foil, installed in a copper heat sink, and cooled to 20°C by circulating water.

[0044] The length of the plano-concave resonant cavity formed by the input mirror 6 and the output mirror 8 of the Raman laser is 50mm-60mm, preferably 55mm.

[0045] Input mirror 6 is a plane mirror. Its inner cavity surface is coated with a high-reflectivity film (>99.5%). Stokes light with a wavelength of 1150-1350 nm incident on this film undergoes total internal reflection. Both the inner and outer cavity surfaces of input mirror 6 are also coated with an anti-reflectivity film (<1%). Pump light with a wavelength of 1064 nm incident on this anti-reflectivity film exhibits high transmittance.

[0046] It is understood that in this embodiment, the "inner cavity surface" is the side of the input mirror or output mirror that is close to the KYW crystal, and the "outer cavity surface" is the side of the input mirror or output mirror that is far away from the KYW crystal.

[0047] Figure 7 The output mirror transmittance diagram shows that the output mirror 8 is a plano-concave mirror with a radius of curvature of 100mm-500mm, preferably 200mm.

[0048] The inner cavity surface of output mirror 8 is coated with a partially reflective film with a reflectivity of 95%. When first-order Stokes light with wavelengths in the 1064nm-1240nm wavelength range is incident on this film, 5% of the light is transmitted. The inner cavity surface of output mirror 8 is also coated with a partially reflective film with a reflectivity of 80%. When second-order Stokes light with a wavelength of 1318nm is incident on this film, 20% of the light is transmitted. This special coating design allows the first-order Stokes light energy within the cavity to be sufficiently strong to excite second-order Stokes light. To filter out residual pump light, a filter is placed behind the output mirror.

[0049] like Figure 3 As shown, the pump polarization direction is aligned with N. m Axis alignment, using 905cm -1 The phonon mode produces second-order Stokes light with a center wavelength of 1318 nm and a spectral bandwidth of 0.33 nm. A comparison of the pump light and Raman light pulse shapes is shown below. Figure 6 As shown, it exhibits a strong pulse width compression effect.

[0050] like Figure 4 As shown, the pump polarization direction is aligned with N. g Axis alignment, using 765cm -1The phonon mode produces second-order Stokes light with a central wavelength of 1270 nm, simultaneously exciting an 87 cm⁻¹ prism. -1 The phonon mode produces a wavelength of 1285nm.

[0051] like Figure 5 As shown, the pump polarization direction is in N g Axis and N m Between axes, a first-order Stokes beam at 1158 nm (765 cm⁻¹) will be excited. -1 ) and 1178nm (905cm) -1 Meanwhile, through cross-cascading, second-order Stokes light with wavelengths of 1271nm and 1294nm is generated.

[0052] This embodiment presents a pulsed KYW second-order Stokes Raman laser with an output wavelength of 1318 nm, a maximum output power of 1.06 W, and a pulse width of 23 ns. By changing the polarization of the pump light, a rich variety of Raman wavelengths, such as 1270 nm, 1285 nm, and 1294 nm, can also be achieved. This is the first demonstration of an external cavity pulsed KYW Raman laser with watt-level output power.

[0053] While the specific embodiments of the present invention have been described above in conjunction with the accompanying drawings, this is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art without creative effort based on the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims

1. A high-efficiency KYW second-order Stokes Raman laser, characterized in that, include: The pump source, first half-wave plate, optical isolator, second half-wave plate, focusing lens, input mirror, KYW crystal and output mirror are arranged sequentially along the beam direction; The pump source is used to emit pump light; the first half-wave plate and the optical isolator are used to adjust the power of the pump light emitted by the pump source. The second half-wave plate is rotated and adjusted according to the direction of the KYW crystal optical axis to align the KYW crystal optical axis with the direction of pump light polarization; The adjusted polarization pump light is passed through different crystal optical axes of the KYW crystal to output the corresponding Raman wavelength; The input mirror and the output mirror constitute a Raman resonant cavity; This will excite a first-order Stokes beam, and at the same time, through cross-cascade, generate a second-order Stokes beam; The inner cavity surface of the output mirror is coated with a partially reflective film with a reflectivity of 95% and a partially reflective film with a reflectivity of 80%. When first-order Stokes light with a wavelength of 1064nm-1240nm is incident on the partially reflective film with a reflectivity of 95%, 5% of the light is transmitted. When second-order Stokes light with a wavelength of 1318nm is incident on the partially reflective film with a reflectivity of 80%, 20% of the light is transmitted. The Raman laser has watt-level output power. The pump polarization direction is related to the N-axis of the KYW crystal. m Axis alignment, utilizing the 905cm of the KYW crystal. -1 The phonon mode produces second-order Stokes light with a center wavelength of 1318 nm; The pump polarization direction is related to the N-axis of the KYW crystal. g Axis alignment, using 765cm -1 The phonon mode generates second-order Stokes light with a central wavelength of 1270 nm, while simultaneously exciting an 87 cm⁻¹ ionosphere. -1 The phonon mode produces Stokes light with a wavelength of 1285 nm; The pump polarization direction is located at the N-axis of the KYW crystal. m Axis and N g Between axes, first-order Stokes light with wavelengths of 1158 nm and 1178 nm is excited, while second-order Stokes light with wavelengths of 1271 nm and 1294 nm is generated through cross-cascading.

2. The high-efficiency KYW second-order Stokes Raman laser as described in claim 1, characterized in that, The input mirror is a plane mirror, and the inner cavity surface of the input mirror is coated with a high-reflectivity film with a reflectivity >99.5%. Both the inner and outer cavity surfaces of the input mirror are coated with an anti-reflectivity film with a reflectivity <1%.

3. The high-efficiency KYW second-order Stokes Raman laser according to claim 2, characterized in that, Stokes light with wavelengths of 1150nm-1350nm undergoes total internal reflection when incident on a high-reflectivity film with a reflectivity >99.5%, while pump light with a wavelength of 1064nm undergoes high transmission when incident on an anti-reflectivity film with a reflectivity <1%.

4. A high-efficiency KYW second-order Stokes Raman laser as described in claim 1, characterized in that, The output mirror is a plano-concave mirror with a radius of curvature of 100mm-500mm.

5. A high-efficiency KYW second-order Stokes Raman laser as described in claim 1, characterized in that, It also includes a filter, through which the Stokes light output by the output mirror is filtered.

6. A high-efficiency KYW second-order Stokes Raman laser as described in claim 1, characterized in that, The input mirror and the output mirror constitute a Raman resonant cavity, which is an external cavity type.

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