Special cut-angle broadband frequency-doubling device and its application

Abstract

The present invention relates to a broadband frequency doubling device with a special cutting angle and its application, and belongs to the field of laser and nonlinear optics. The special cutting angle is (113°, 25.9°) for YCOB crystals and (114.1°, 32°) for GdCOB crystals. When the center wavelength of the fundamental frequency light is within the range of 1550-1700nm, the YCOB and GdCOB crystals processed according to the above-mentioned cutting angle can achieve high-efficiency, wide-band frequency doubling output. The present invention has the advantages of low production cost, mature crystal growth process, high temperature stability, high anti-light damage threshold, high conversion efficiency, and large spectral bandwidth. Combined with erbium-doped solid-state laser technology, it can achieve high-efficiency, wide spectrum, narrow pulse width, and high peak power ultrafast pulse laser output with a wavelength of near 800nm.

Description

Technical Field

[0001] The invention relates to YCOB and GdCOB crystals with special cutting angles and their high-efficiency and broadband frequency doubling applications, belonging to the technical field of laser and nonlinear optics. Background Art

[0002] Currently, nonlinear optical crystals are the primary medium for optical frequency upconversion. Commonly used crystals include KDP, ADP, BBO, LBO, YCOB, and GdCOB. Using the Czochralski method, large, high-optical-quality YCOB and GdCOB single crystals can be obtained in a short period of time. These two crystals offer advantages such as large nonlinear optical coefficients, high laser damage thresholds, and stable physicochemical properties. Consequently, they have found widespread application in various nonlinear frequency conversion applications, such as frequency doubling, frequency tripling, optical parametric chirped pulse amplification, and self-frequency doubling.

[0003] For birefringent nonlinear optical crystals, noncritical phase matching is known as the best phase matching due to its high tolerance to operating conditions. Based on the factors affecting phase mismatch, noncritical phase matching can be divided into angular noncritical phase matching, temperature noncritical phase matching, and spectral noncritical phase matching. By doping with rare earth elements of different types and proportions, YCOB and GdCOB series crystals can achieve frequency doubling of angular noncritical phase matching over a wide band, with a tunable range of 750 to 1250 nm and an angular acceptance bandwidth of up to 43-84 mrad·cm. 1 / 2 (Opt. Express 25, 11867-11893, 2017). By studying the full-space temperature bandwidth, the temperature non-critical phase matching directions of YCOB and GdCOB crystals for 1064nm frequency doubling were determined to be (θ=149.2°, φ=0°) and (θ=135°, φ=47.3°), respectively. The corresponding measured temperature acceptance bandwidths were 490℃·cm and 430℃·cm, respectively (Opt. Lett. 44, 1742-1745, 2019; Opt. Express 28, 33274-33284, 2020). So far, there have been no reports on the spectral non-critical phase matching performance of YCOB and GdCOB crystals.

[0004] Ultrashort pulse lasers are finding increasingly widespread applications in fields such as biomedicine, spectroscopy, high-speed communications, environmental sensing, and the study of ultrafast nonlinear optical phenomena. Broadband spectrum, a fundamental characteristic of ultrashort pulse lasers, places special demands on nonlinear optical crystals when considering frequency conversion. To improve conversion efficiency and avoid harmonic pulse broadening, the group velocity mismatch between the fundamental wave and the newly generated pulse in the crystal must be sufficiently small compared to the fundamental pulse duration. In other words, in the frequency domain, the spectral acceptance bandwidth of the nonlinear optical crystal must be sufficiently large compared to the spectral bandwidth of the fundamental wave. When the group velocity mismatch is zero, the first-order wavelength derivative vanishes. Frequency conversion then depends solely on the second and higher-order wavelength derivatives, which are inversely proportional to the square root of the crystal thickness. This frequency conversion method, which simultaneously achieves both phase matching and group velocity matching, is known as spectrally noncritical phase matching. Due to its simplicity, reliability, and high efficiency, it has become the preferred method for ultrafast laser frequency conversion. To date, several nonlinear crystals have been used for broadband frequency doubling at various wavelengths, such as BBO at 1550nm, BiBO at 1600nm, LBO at 1300nm, MgO-doped PPLN at 1550nm, DKDP at 1034-1179nm, and DADP at 1027-1161nm. However, these nonlinear crystal materials have drawbacks such as difficulty in growth, small size, high cost, low light damage threshold, low conversion efficiency, low output energy, and a narrow operating band.

[0005] Currently, ultrashort pulse lasers mainly use femtosecond Ti:sapphire lasers. However, these lasers are expensive, bulky, and complex. Therefore, there is an urgent need to develop an ultrashort pulse laser with a simple structure and low cost. Summary of the Invention

[0006] In response to the shortcomings of the existing technology, the present invention provides a broadband frequency-doubling device with a special cut angle and its application. The present invention utilizes the low symmetry and refractive index characteristics of YCOB and GdCOB crystals to provide a broadband frequency-doubling device with a special cut angle, YCOB and GdCOB. The device can be applied to the frequency doubling of 1550-1700nm broadband laser light sources (such as erbium-doped ultrafast fiber lasers), achieving high-efficiency, high-power broadband laser output near 800nm. The device has the advantages of easy growth, low cost, high light damage threshold, high conversion efficiency, large output energy, wide operating band, small device size, easy processing, easy installation, and long life. It solves the problems of the currently commonly used titanium sapphire laser, such as high price and bulky size. SUMMARY OF THE INVENTION

[0008] The present invention uses YCOB and GdCOB crystals cut in a specially oriented direction to achieve high-efficiency broadband frequency-doubled output. When frequency-doubled in the near-infrared band (1550-1700nm), these YCOB and GdCOB crystal devices can generate a frequency-doubled spectrum with a bandwidth of 14±1nm for a fundamental spectrum with a bandwidth of 22±1nm. The spectrum-bandwidth conversion efficiency is as high as 64%, and the maximum energy conversion efficiency can reach 58%.

[0009] Terminology Notes:

[0010] YCOB: abbreviation of the chemical formula YCa4O(BO3)3.

[0011] GdCOB: abbreviation of the chemical formula GdCa4O(BO3)3.

[0012] The technical solution of the present invention is:

[0013] A broadband frequency-doubling device with a special cutting angle, whose chemical composition is YCOB crystal or GdCOB crystal. The spatial direction of the YCOB crystal or GdCOB crystal is described by polar coordinates (θ, φ), where θ is the angle between the direction and the optical axis Z, and φ is the angle between the projection of the direction vector in the XY principal plane and the X-axis, i.e., the azimuth angle. The device is obtained by cutting the YCOB crystal at an angle of (113°, 25.9°) or the GdCOB crystal at an angle of (114.1°, 32°).

[0014] According to the present invention, preferably, the broadband frequency doubling device with a special cut angle can achieve broadband frequency doubling of a fundamental frequency laser with a central wavelength within the range of 1550-1700 nm.

[0015] According to the present invention, the broadband frequency-doubling device with a special cut angle, i.e., the YCOB crystal with a cut angle of (113°, 25.9°) or the GdCOB crystal with a cut angle of (114.1°, 32°), has a turning point in its phase matching curve within the 1.55-1.7 μm band, thereby achieving efficient, broadband frequency-doubling output.

[0016] According to the present invention, preferably, when a broadband frequency doubling device with a special cut angle is used for broadband frequency doubling, as the wavelength of the fundamental light increases from 1550nm to 1700nm, the phase matching angle of the YCOB crystal with a cut angle of (113°, 25.9°) changes by ≤0.3°, and the phase matching angle of the GdCOB crystal with a cut angle of (114.1°, 32°) changes by ≤0.1°. That is, when the wavelength of the fundamental light varies within the range of 1550-1700nm, the corresponding phase matching angle φ of the YCOB crystal with a cut angle of (113°, 25.9°) changes by 25.9°-26.2°, while θ remains unchanged; and the corresponding phase matching angle φ of the GdCOB crystal with a cut angle of (114.1°, 32°) changes by 32°-32.1°, while θ remains unchanged.

[0017] According to the present invention, preferably, the size of the YCOB crystal or the GdCOB crystal is 6×6×10 mm 3 , that is, the light-transmitting cross section of the YCOB crystal and the GdCOB crystal is 6×6mm 2 , the length in the light-transmitting direction is 10 mm.

[0018] According to the present invention, a broadband frequency-doubling device with a special cutting angle is used in an ultrashort pulse laser.

[0019] An ultrashort pulse laser comprises a light source, a focusing lens, a broadband frequency doubling device with a special cutting angle and a color filter which are sequentially placed along the propagation direction of the light path.

[0020] According to the present invention, preferably, the focusing lens is coated with a broadband anti-reflection film of 1400-1800 nm.

[0021] According to the present invention, preferably, the color filter is coated with a broadband high-reflection film of 1400-1800nm ​​and a broadband anti-reflection film of 700-900nm.

[0022] According to the present invention, when fundamental frequency light with a central wavelength range of 1550-1700nm is focused by the focusing lens, the broadband frequency doubling device with a special cut angle converts the fundamental frequency light into frequency doubling light, and the remaining fundamental frequency light and frequency doubling light pass through the color filter at the same time, wherein the fundamental frequency light is reflected and the frequency doubling light is output through the color filter.

[0023] According to the present invention, preferably, when the fundamental frequency light spectrum bandwidth is 22±1 nm, the frequency doubled light spectrum bandwidth of the broadband frequency doubler device with a special cut angle is 14±1 nm, and the spectrum bandwidth conversion rate reaches 64%.

[0024] Anything not described in detail in the present invention is based on the existing technology in this field.

[0025] The beneficial effects of the present invention are:

[0026] To date, all spectral non-critical frequency doubling angles have been cut within the principal plane of the nonlinear optical crystal. This invention proposes for the first time an optimal cut angle outside the principal plane, significantly improving conversion efficiency while ensuring that the frequency doubling bandwidth is not attenuated, with an overall effect significantly superior to the principal plane cut angle. Specifically, when the fundamental frequency light wavelength varies within the range of 1550-1700nm, the broadband frequency doubling conversion efficiency of the YCOB crystal with the special cut angle (113°, 25.9°) described in this invention is far greater than that of the conventional principal plane cut angle (139.7°, 0°). The broadband frequency doubling conversion efficiency of the GdCOB crystal with the special cut angle (114.1°, 32°) described in this invention is far greater than that of the conventional principal plane cut angle (146.4°, 0°).

[0027] Taking a fundamental wavelength of 1650nm as an example, with an average power of 100mW, a repetition rate of 100kHz, and a pulse width of 160fs, the conversion efficiency of a YCOB crystal at the tangent direction (113°, 25.9°) is 51.4%, and at the tangent direction (139.7°, 0°) is 39.2%. The conversion efficiency of a GdCOB crystal at the tangent direction (114.1°, 32°) is 39.8%, and at the tangent direction (146.4°, 0°) is 31.5%. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 Schematic diagram of the main structure of the ultrashort pulse laser in Example 5 of the present invention.

[0029] Figure 2 This is a type of frequency-doubled phase matching curve within the main plane of the YCOB crystal and the GdCOB crystal in Example 1 of the present invention.

[0030] Figure 3 The phase matching curves of the 1.64 μm type I frequency-harmonic generation of the YCOB crystal and the GdCOB crystal in Example 1 of the present invention and the corresponding d eff curve.

[0031] Figure 4 Graph showing the variation of the phase matching angles of the four tangential crystals with the fundamental frequency wavelength in Example 2 of the present invention.

[0032] Figure 5 Spectra of the fundamental frequency and the doubled frequency of YCOB and KDP crystals in Example 3 of the present invention.

[0033] Among them: 1. light source, 2. fundamental frequency light, 3. focusing lens, 4. broadband frequency doubling device with special cutting angle, 5. color filter, 6. frequency doubling light. DETAILED DESCRIPTION

[0034] The present invention will be further described below with reference to the accompanying drawings and examples, but is not limited thereto.

[0035] Example 1

[0036] A broadband frequency-doubler device with a special cut angle is constructed from YCOB or GdCOB crystals. Polar coordinates (θ, φ) are used to describe the spatial orientation of the YCOB or GdCOB crystals, where θ is the angle between the orientation and the optical axis Z, and φ is the angle between the projection of the orientation vector in the XY principal plane and the X-axis, i.e., the azimuth angle. The device is obtained by cutting the YCOB crystal at an angle of (113°, 25.9°) or the GdCOB crystal at an angle of (114.1°, 32°). The frequency-doubler phase-matching curve exhibits a turning point in the 1.55-1.7μm range, enabling efficient, broadband frequency-doubler output.

[0037] The device crystal size is 6×6×10mm 3 , meaning the length of the YCOB and GdCOB crystals in the light-transmitting direction is 10 mm. When the fundamental wavelength increases from 1550 nm to 1700 nm, the phase-matching angle φ of the YCOB crystal with a tangential orientation of (113°, 25.9°) changes by only 0.3°, while θ remains unchanged. The phase-matching angle φ of the GdCOB crystal with a tangential orientation of (114.1°, 32°) changes by only 0.1°, while θ remains unchanged.

[0038] YCOB and GdCOB are monoclinic biaxial crystals, and the phase matching conditions are determined by the parameters (θ, φ). Based on the dispersion equations of YCOB and GdCOB crystals, for the frequency doubling process (ω+ω→2ω), the variation of the phase matching angle in the main plane with the wavelength of the fundamental frequency light is calculated, as shown in the following example: Figure 2 As shown in the figure, the solid line is the GdCOB crystal, and the dotted line is the YCOB crystal. From this, it can be determined that within the principal plane, the spectral non-critical SHG matching angle of the YCOB crystal is (139.7°, 0°), and the spectral non-critical SHG matching angle of the GdCOB crystal is (146.4°, 0°).

[0039] Effective nonlinear coefficient d eff Related to the phase matching angle (θ, φ), the phase matching angle and d of YCOB crystal and GdCOB crystal at 1640nm eff Curves such as Figure 3 As shown, Figure a is a YCOB crystal and Figure b is a GdCOB crystal. eff The maximum is 1.55pm / V, d in the tangential direction of the main plane (139.7°, 0°) eff The magnitude is only 1.23pm / V; the d of the GdCOB crystal at (114.1°, 32°) is eff The maximum is 1.52pm / V, d in the tangential direction of the main plane (146.4°, 0°) eff The size is only 1.1pm / V.

[0040] Therefore, the special cutting angles selected in the present invention are (113°, 25.9°) for the YCOB crystal and (114.1°, 32°) for the GdCOB crystal.

[0041] Example 2

[0042] When the wavelength of the fundamental frequency light changes in the range of 1550-1700nm, the phase matching angle φ of the (113°, 25.9°) tangential YCOB crystal changes from 25.9° to 26.2°, and θ remains unchanged. Figure 4As shown in (a); the phase matching angle θ of the YCOB crystal with a tangential direction of (139.7°, 0°) changes from 139.7° to 139.9°, while φ remains unchanged. Figure 4 As shown in (b); the phase matching angle φ of the GdCOB crystal with a tangential direction of (114.1°, 32°) changes from 32° to 32.1°, while θ remains unchanged. Figure 4 As shown in (c); the phase matching angle θ of the GdCOB crystal tangent at (146.4°, 0°) changes from 146.4° to 146.9°, and φ remains unchanged, as shown in Figure 4 (d) shown.

[0043] When the wavelength of the fundamental frequency light varies in the range of 1550-1700nm, the phase matching angles of the YCOB crystal and the GdCOB crystal in four different planes vary within 0.5°, showing excellent broadband frequency doubling characteristics. Figure 4 The theoretical calculation results and measured results of the phase matching angle in the 1500-1800nm ​​band are shown, and the two are basically consistent. The errors may come from multiple aspects, such as the measurement and fitting accuracy of the dispersion equation, the crystal orientation and processing accuracy, and the degree of collimation of the experimental optical path.

[0044] Example 3

[0045] The ORPHEUS-HP tunable femtosecond laser from Light Conversion was used as the fundamental frequency light source, and the fundamental frequency spectra of different wavelengths were recorded using a spectrometer, such as Figure 5 As shown in (a), the half-peak width of the 1064nm spectrum is 15nm, and the half-peak widths of the 1550nm, 1600nm, 1650nm, and 1700nm spectra are 22±1nm; the doubled frequency spectrum is as shown in Figure 5 As shown in (b), a YCOB crystal with a (113°, 37.4°) cut was used to perform ordinary frequency doubling of the 1064nm fundamental frequency light, and the half-peak width of the obtained 532nm spectrum was 3nm. A YCOB crystal with a (113°, 25.9°) cut was used to perform near-spectral noncritical or spectral noncritical frequency doubling of 1550nm, 1600nm, 1650nm, and 1700nm, and the half-peak width of the obtained 775nm, 800nm, 825nm, and 850nm spectra was 14±1nm. Figure 5 (b) The inset shows the 775 nm spectrum obtained by using a (52.6°, 45°)-cut KDP crystal to perform ordinary frequency doubling of the 1550 nm fundamental frequency light, with a half-peak width of 4 nm.

[0046] From the above data, it can be seen that the spectral width conversion rate of the ordinary frequency doubling of YCOB crystal ((113°, 37.4°) tangent, 1064nm frequency doubling) is 20%, the spectral width conversion rate of the ordinary frequency doubling of KDP crystal ((52.6°, 45°) tangent, 1550nm frequency doubling) is 18%, and the spectral width conversion rate of the non-critical frequency doubling of the YCOB crystal spectrum of the present invention ((113°, 25.9°) tangent, 1550-1700nm frequency doubling) is as high as 64%, showing a significant advantage in broadband frequency doubling.

[0047] Example 4

[0048] The frequency-doubled conversion efficiency of the YCOB crystal at the (113°, 25.9°) tangent direction is greater than that at the (139.7°, 0°) tangent direction, and the frequency-doubled conversion efficiency of the GdCOB crystal at the (114.1°, 32°) tangent direction is greater than that at the (146.4°, 0°) tangent direction, as shown in Table 1.

[0049] Table 1

[0050] Tangential crystal Input=40mW Input=60mW Input=80mW Input=100mW YCOB (113°, 25.9°) 28.3％ 36.3％ 43.9％ 51.4％ YCOB (139.7°, 0°) 19.4％ 25.5％ 31％ 39.2％ GdCOB (114.1°, 32°) 21.8％ 28.9％ 35.6％ 39.8％ GdCOB (146.4°, 0°) 13.2％ 19.7％ 25.9％ 31.5％

[0051] The data in Table 1 show that compared with the traditional main plane tangential spectral non-critical phase matching method, the special tangential spectral non-critical phase matching method of the present invention has higher efficiency and is more advantageous for application.

[0052] When the fundamental wavelength is 1650nm, the average power is 40mW, the repetition rate is 100kHz, and the pulse width is 160fs, the YCOB crystal achieves a GSH conversion efficiency of 28.3% at the (113°, 25.9°) tangent and 19.4% at the (139.7°, 0°) tangent. The GdCOB crystal achieves a GSH conversion efficiency of 21.8% at the (114.1°, 32°) tangent and 13.2% at the (146.4°, 0°) tangent.

[0053] When the fundamental wavelength is 1650nm, the average power is 60mW, the repetition rate is 100kHz, and the pulse width is 160fs, the YCOB crystal has a GSH conversion efficiency of 36.3% at the (113°, 25.9°) tangent and 25.5% at the (139.7°, 0°) tangent. The GdCOB crystal has a GSH conversion efficiency of 28.9% at the (114.1°, 32°) tangent and 19.7% at the (146.4°, 0°) tangent.

[0054] When the fundamental wavelength is 1650nm, the average power is 80mW, the repetition rate is 100kHz, and the pulse width is 160fs, the YCOB crystal has a GSH conversion efficiency of 43.9% at the (113°, 25.9°) tangent and 31% at the (139.7°, 0°) tangent. The GdCOB crystal has a GSH conversion efficiency of 35.6% at the (114.1°, 32°) tangent and 25.9% at the (146.4°, 0°) tangent.

[0055] When the fundamental wavelength is 1650nm, the average power is 100mW, the repetition rate is 100kHz, and the pulse width is 160fs, the YCOB crystal has a GSH conversion efficiency of 51.4% at the (113°, 25.9°) tangent and 39.2% at the (139.7°, 0°) tangent. The GdCOB crystal has a GSH conversion efficiency of 39.8% at the (114.1°, 32°) tangent and 31.5% at the (146.4°, 0°) tangent.

[0056] It can be seen that the frequency doubling conversion efficiency of the broadband frequency doubling device with a special cut angle of the present invention is significantly higher than that of the crystal whose cut direction is within the main plane of the nonlinear optical crystal.

[0057] Example 5

[0058] like Figure 1 As shown, an ultrashort pulse laser includes a light source 1, a focusing lens 3, a broadband frequency doubling device 4 with a special cut angle, and a color filter 5, which are sequentially placed along the propagation direction of the optical path;

[0059] The broadband frequency multiplying device 4 with a special cut angle is the device described in Example 1;

[0060] The focusing lens is coated with a broadband anti-reflection film of 1400-1800nm;

[0061] The color filter is coated with a 1400-1800nm ​​broadband high-reflection film and a 700-900nm broadband anti-reflection film.

[0062] When the fundamental frequency light 2 with a central wavelength range of 1550-1700nm is focused by the focusing lens 3, the broadband frequency doubling device 4 with a special cut angle converts the fundamental frequency light 2 into the frequency doubling light 6. The remaining fundamental frequency light 2 and the frequency doubling light 6 pass through the color filter 5 at the same time, wherein the fundamental frequency light 2 is reflected and the frequency doubling light 6 is output through the color filter 5.

Claims

1. A broadband frequency doubling device with a special cut angle, characterized in that: The chemical composition of the device is YCOB crystal or GdCOB crystal, and the spatial direction of the YCOB crystal or GdCOB crystal is described by polar coordinates (θ, φ), θ is the angle between the direction and the optical axis Z, and φ is the angle between the projection of the direction vector in the XY principal plane and the X-axis, that is, the azimuth angle; the device is obtained by cutting the YCOB crystal at an angle of (113°, 25.9°) or the GdCOB crystal at an angle of (114.1°, 32°).

2. The broadband frequency doubling device with a special cut angle according to claim 1, characterized in that: The broadband frequency doubling device with a special cut angle can realize broadband frequency doubling of a fundamental frequency laser with a central wavelength within the range of 1550-1700 nm.

3. The broadband frequency doubling device with a special cut angle according to claim 1, characterized in that: The broadband frequency doubling device with a special cut angle, i.e., a YCOB crystal with a cut angle of (113°, 25.9°) or a GdCOB crystal with a cut angle of (114.1°, 32°), has a turning point in its phase matching curve within the 1.55-1.7μm band, thereby achieving efficient and broadband frequency doubling output.

4. The broadband frequency doubling device with a special cut angle according to claim 1, characterized in that: When using a broadband frequency doubling device with a special cut angle for broadband frequency doubling, as the fundamental frequency light wavelength increases from 1550nm to 1700nm, the phase matching angle of the YCOB crystal cut at (113°, 25.9°) changes by ≤0.3°, and the phase matching angle of the GdCOB crystal cut at (114.1°, 32°) changes by ≤0.1°.

5. The broadband frequency doubling device with a special cut angle according to claim 1, characterized in that: The size of the YCOB crystal or GdCOB crystal is 6×6×10mm 3 .

6. Use of the broadband frequency-doubling device with a special cutting angle as claimed in claim 1 in an ultrashort pulse laser.

7. An ultrashort pulse laser, characterized in that: The invention comprises a light source, a focusing lens, a broadband frequency doubling device with a special cutting angle as claimed in claim 1 and a color filter, which are sequentially placed along the propagation direction of the light path.

8. The ultrashort pulse laser according to claim 7, characterized in that The focusing lens is coated with a broadband anti-reflection film of 1400-1800nm.

9. The ultrashort pulse laser according to claim 7, characterized in that The color filter is coated with a 1400-1800nm ​​broadband high-reflection film and a 700-900nm broadband anti-reflection film.

10. The ultrashort pulse laser according to claim 7, characterized in that: When the fundamental frequency light spectrum bandwidth is 22±1nm, the frequency doubled light spectrum bandwidth of the special-angle-cut broadband frequency doubler device is 14±1nm.

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