CN115528528A - Broadband Frequency Doubling Device with Special Cut Angle and Its Application - Google Patents

Abstract

The invention relates to a broadband frequency multiplier with a special cutting angle and application thereof, belonging to the field of laser and nonlinear optics. The specific cut angles were (113 °,25.9 °) for YCOB crystals and (114.1 °,32 °) for GdCOB crystals. When the central wavelength of the fundamental light is in the range of 1550-1700nm, the YCOB and GdCOB crystals processed according to the cutting angle can realize high-efficiency broadband frequency-doubled output. The invention has the advantages of low manufacturing cost, mature crystal growth process, high temperature stability, high light damage resistance threshold, high conversion efficiency, large spectral bandwidth and the like, and can realize the output of ultrafast pulse laser with high efficiency, wide spectrum, narrow pulse width and high peak power with the wavelength near 800nm by combining with the erbium-doped solid laser technology.

Description

Broadband frequency doubling device with special cut angle and its application

technical field

The invention relates to special angle-cut YCOB and GdCOB crystals and their high-efficiency, wide-band frequency doubling applications, belonging to the technical field of laser and nonlinear optics.

Background technique

At present, nonlinear optical crystals are the main medium for optical frequency up-conversion, and the commonly used crystals mainly include KDP, ADP, BBO, LBO, YCOB, GdCOB, etc. Using the pulling method, large-sized, high-optical-quality YCOB and GdCOB single crystals can be obtained in a short time. These two crystals have the advantages of large nonlinear optical coefficient, high laser damage threshold, and stable physical and chemical properties. Therefore, they have been widely used in various nonlinear frequency conversions, such as frequency doubling, frequency tripling, optical parametric chirped pulse amplification, and self-doubling.

For birefringent nonlinear optical crystals, noncritical phase matching is known as optimal phase matching due to its high tolerance to operating conditions. According to the influencing factors of phase mismatch, non-critical phase matching can be divided into angle non-critical phase matching, temperature non-critical phase matching, and spectral non-critical phase matching. By doping different types and different proportions of rare earth elements, YCOB and GdCOB series crystals can achieve angular non-critical phase-matched frequency doubling in a wide band, with a tunable range of 750-1250nm and an angular acceptance bandwidth of up to 43-84mrad cm ^{1 /2} (Opt. Express 25, 11867-11893, 2017). By studying the temperature bandwidth of the whole space, the temperature non-critical phase matching directions of YCOB and GdCOB crystals for 1064nm frequency doubling are determined, which are (θ=149.2°, φ=0°), (θ=135°, φ=47.3°) , and the corresponding measured temperature acceptance bandwidths are 490 °C cm and 430 °C cm (Opt. Lett. 44, 1742-1745, 2019; Opt. Express 28, 33274-33284, 2020). So far, there is no report about the spectral non-critical phase matching performance of YCOB and GdCOB crystals.

Ultrashort pulse laser has been more and more widely used in the fields of biomedicine, spectroscopy, high-speed communication, environmental detection, and research on ultrafast nonlinear optical phenomena. As a fundamental property of ultrashort pulse lasers, broadband spectroscopy places special demands on nonlinear optical crystals when frequency conversion is considered. To improve conversion efficiency and avoid harmonic pulse broadening, the group velocity mismatch between the fundamental and newly generated pulses in the crystal should 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 should be sufficiently large compared with the spectral bandwidth of the fundamental wave. When the group velocity mismatch is zero, the first wavelength derivative will disappear. In this case the frequency conversion is only related to the second and higher order wavelength derivatives and is inversely proportional to the square root of the crystal thickness. This frequency conversion method that satisfies phase matching and group velocity matching at the same time is spectral non-critical phase matching, which has become the first choice for ultrafast laser frequency conversion due to its simplicity, reliability and high efficiency. So far, some nonlinear crystals have been used for broadband frequency doubling of different wavelengths, such as BBO crystals at 1550nm, BiBO crystals at 1600nm, LBO crystals at 1300nm, MgO-doped PPLN crystals at 1550nm, DKDP crystals at 1034-1179nm, DADP crystals are at 1027-1161nm, etc. However, the above-mentioned nonlinear crystal materials have disadvantages such as difficult growth, small size, high cost, low light damage resistance threshold, low conversion efficiency, low output energy, and narrow working band.

At present, femtosecond Ti:sapphire lasers are mainly used as ultrashort pulse lasers, but femtosecond Ti:sapphire lasers are expensive, large in size, and complex in structure. Therefore, it is urgent to develop an ultrashort pulse laser with simple structure and low cost.

Contents of the invention

Aiming at the deficiencies of the prior art, the invention provides a broadband frequency doubling device with a special angle cut and its application. The present invention utilizes the low symmetry and refractive index characteristics of YCOB and GdCOB crystals to provide YCOB and GdCOB broadband frequency doubling devices with special cut angles, which can be applied to 1550-1700nm broadband laser sources (such as erbium-doped ultrafast fiber lasers). High-efficiency, high-power broadband laser output near 800nm, easy to grow, 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 long-term advantage. The problems of high price and bulky volume of the currently commonly used Ti:Sapphire laser are solved.

Summary of the invention

The invention uses YCOB and GdCOB crystals cut in special directions to realize high-efficiency broadband frequency multiplication output. When the YCOB and GdCOB crystal devices are frequency-multiplied in the near-infrared band (1550-1700nm), for a fundamental frequency spectrum with a bandwidth of 22±1nm, a frequency-doubled spectrum with a bandwidth of 14±1nm can be generated, and the spectral bandwidth conversion efficiency is as high as 64%. , the highest energy conversion efficiency can reach 58%.

Terminology Explanation:

YCOB: It is the abbreviation of chemical formula YCa ₄ O(BO ₃ ) ₃ .

GdCOB: the abbreviation of the chemical formula GdCa ₄ O(BO ₃ ) ₃ .

Technical scheme of the present invention is:

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

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

According to the present invention, the broadband frequency doubling device with special cut angle, that is, (113°, 25.9°) tangential YCOB crystal or (114.1°, 32°) tangential GdCOB crystal has a phase in the 1.55-1.7 μm wave band There is a turn-back point in the matching curve, which can realize high-efficiency, broadband multiplier output.

According to the present invention, preferably, when a broadband frequency doubling device with a special cut angle is used for broadband frequency doubling, when the wavelength of the fundamental frequency light increases from 1550nm to 1700nm, the (113°, 25.9°) tangential YCOB crystal phase matches The change of the angle is ≤0.3°, and the change of the phase matching angle of (114.1°, 32°) tangential GdCOB crystal is ≤0.1°. That is: when the wavelength of the fundamental frequency light changes within the range of 1550-1700nm, the (113°, 25.9°) tangential YCOB crystal corresponds to a change in the phase matching angle φ of 25.9°-26.2°, and θ remains unchanged; (114.1°, 32°) tangential GdCOB crystal corresponds to a change of phase matching angle φ of 32°-32.1°, and θ remains unchanged.

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

According to the invention, the application of the broadband frequency doubling device with special cut angle in the ultrashort pulse laser.

An ultrashort pulse laser, including a light source, a focusing lens, a broadband frequency doubling device with a special cut angle, and a color filter placed in sequence along the propagation direction of the optical path.

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

According to the present invention, preferably, the color filter is coated with a 1400-1800nm broadband high reflection coating and a 700-900nm broadband anti-reflection coating.

According to the present invention, when the 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 the multiplier The frequency light passes through the color filter at the same time, wherein the fundamental frequency light is reflected, and the double frequency light passes through the color filter and is output.

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

Everything that is not described in detail in the present invention is based on the prior art in this field.

The beneficial effects of the present invention are:

So far, the cutting angles of all spectral non-critical frequency doubling are in the main plane of the nonlinear optical crystal. The present invention proposes the best cutting angle outside the main plane for the first time, and the conversion efficiency is remarkable while ensuring that the frequency doubling bandwidth is not attenuated. Improvement, the overall effect is obviously better than the main plane chamfer. Specifically, when the wavelength of the fundamental frequency light changes within the range of 1550-1700nm, the broadband frequency doubling conversion efficiency of the special tangential direction (113°, 25.9°) of the YCOB crystal described in the present invention is far greater than that of the traditionally used principal plane tangential direction (139.7°, 0°), the GdCOB crystal in the present invention has a special tangential direction at (114.1°, 32°) and the broadband frequency multiplication conversion efficiency is far greater than the traditionally used main plane tangential direction (146.4°, 0°).

Taking the fundamental wavelength of 1650nm as an example, when the average power is 100mW, the repetition frequency is 100kHz, and the pulse width is 160fs, the conversion efficiency of YCOB crystal in the tangential direction of (113°, 25.9°) is 51.4%, and at (139.7°, 0°) tangential conversion efficiency is 39.2%. The conversion efficiency of GdCOB crystal in the (114.1°, 32°) tangential direction is 39.8%, and the conversion efficiency in the (146.4°, 0°) tangential direction is 31.5%.

Description of drawings

Fig. 1 is a schematic diagram of the main structure of an ultrashort pulse laser in Embodiment 5 of the present invention.

Fig. 2 is a kind of double frequency phase matching curve in the main plane of YCOB crystal and GdCOB crystal in embodiment 1 of the present invention.

Fig. 3 shows the phase matching curves of YCOB crystals and GdCOB crystals with a thickness of 1.64 μm and the corresponding d _eff curves in Example 1 of the present invention.

Fig. 4 is a graph showing the variation of the phase matching angles of the four tangential crystals with the wavelength of the fundamental frequency in Example 2 of the present invention.

Fig. 5 is a spectrum diagram of fundamental frequency and doubled frequency spectrum of YCOB and KDP crystals in Example 3 of the present invention.

Among them: 1. Light source, 2. Fundamental frequency light, 3. Focusing lens, 4. Broadband frequency doubling device with special cut angle, 5. Color filter, 6. Frequency doubling light.

detailed description

The present invention will be further described below in conjunction with the accompanying drawings and embodiments, but is not limited thereto.

Example 1

A broadband frequency doubling device with a special cut angle, the chemical composition is YCOB crystal or GdCOB crystal, the spatial direction of YCOB crystal or GdCOB crystal is described by polar coordinates (θ, φ), θ is the angle between the direction and the optical axis Z, φ is the angle between the projection of the direction vector in the XY main plane and the X axis, that is, the azimuth angle; the device is obtained from a YCOB crystal with a cutting angle of (113°, 25.9°), or a GdCOB crystal with a cutting angle of (114.1 °,32°) obtained. In the 1.55-1.7μm range, there is a turn-back point in the frequency-doubling phase matching curve, which can realize high-efficiency, broadband frequency-doubling output.

The crystal size of the device is 6×6×10mm ³ , that is, the length of the YCOB crystal and the GdCOB crystal in the direction of light transmission is 10mm. When the fundamental frequency light wavelength increases from 1550nm to 1700nm, (113°, 25.9°) tangential YCOB crystal corresponds to a change of phase matching angle φ of only 0.3°, and θ remains unchanged; (114.1°, 32°) tangential GdCOB The change of the corresponding phase matching angle φ of the crystal is only 0.1°, and θ remains unchanged.

YCOB crystals and GdCOB crystals are monoclinic biaxial crystals, and the phase matching conditions are jointly determined by parameters (θ, φ). Based on the dispersion equations of YCOB and GdCOB crystals, for the frequency doubling (ω+ω→2ω) process, the phase matching angle in the main plane varies with the wavelength of the fundamental frequency light, as shown in Figure 2, the solid line is the GdCOB crystal, and the dot Scribes are YCOB crystals. It can be determined that, in the main plane, the spectral non-critical frequency doubling matching angle of YCOB crystal is (139.7°, 0°), and the spectral non-critical frequency doubling matching angle of GdCOB crystal is (146.4°, 0°).

The effective nonlinear coefficient d _eff is related to the phase matching angle (θ, φ). The phase matching angle and d _eff curves of YCOB crystal and GdCOB crystal at 1640nm are shown in Figure 3. Figure a shows YCOB crystal, and picture b shows GdCOB crystal. The d _eff of the YCOB crystal in the (113°, 25.9°) tangential direction is the largest, which is 1.55pm/V, and the d _eff in the main plane tangential direction (139.7°, 0°) is only 1.23pm/V; the GdCOB crystal is in ( 114.1°, 32°) has the largest tangential d _eff of 1.52pm/V, and the tangential d _eff of the main plane (146.4°, 0°) is only 1.1pm/V.

Therefore, the special cutting angle selected in the present invention is (113°, 25.9°) for the YCOB crystal, and (114.1°, 32°) for the GdCOB crystal.

Example 2

When the wavelength of the fundamental frequency light changes in the range of 1550-1700nm, the (113°, 25.9°) tangential YCOB crystal corresponds to a change in the phase matching angle φ of 25.9°-26.2°, and θ remains unchanged, as shown in Figure 4(a) As shown; (139.7°, 0°) tangential YCOB crystal corresponds to a change of phase matching angle θ of 139.7°-139.9°, φ remains unchanged, as shown in Figure 4(b); (114.1°, 32°) tangential The change of the phase matching angle φ corresponding to the GdCOB crystal is 32°-32.1°, and θ remains unchanged, as shown in Figure 4(c); (146.4°, 0°) the tangential GdCOB crystal corresponds to the change of the phase matching angle θ is 146.4°-146.9°, and φ remains unchanged, as shown in Figure 4(d).

When the wavelength of the fundamental frequency light changes in the range of 1550-1700nm, the phase matching angles of YCOB crystal and GdCOB crystal in four different planes all change within 0.5°, showing excellent broadband frequency doubling characteristics. Figure 4 shows the theoretical calculation results and the actual measurement results of the phase matching angle in the 1500-1800nm band. The degree of collimation of the optical path, etc.

Example 3

Using the ORPHEUS-HP tunable femtosecond laser from Light Conversion Company as the fundamental frequency light source, the fundamental frequency spectra of different wavelengths were recorded with a spectrometer. The half peak width of 1600nm, 1650nm, 1700nm spectrum is 22±1nm; frequency doubling spectrum is shown in Figure 5(b), using (113°, 37.4°) tangential YCOB crystal to do ordinary frequency doubling of 1064nm fundamental frequency light , the obtained 532nm spectrum has a half-height width of 3nm, using (113°, 25.9°) tangential YCOB crystals to do 1550nm, 1600nm, 1650nm, 1700nm near-spectral non-critical or spectral non-critical frequency doubling, the obtained 775nm, 800nm, 825nm, and 850nm spectra have half-height widths of 14±1nm; the illustration in Figure 5(b) shows the 775nm spectrum obtained when the 1550nm fundamental frequency light is doubled by a (52.6°, 45°) tangential KDP crystal , with a half-maximum width of 4 nm.

From the above data, it can be seen that the spectral width conversion rate of YCOB crystal common frequency doubling ((113°, 37.4°) tangential, 1064nm frequency doubling) is 20%, and KDP crystal general frequency doubling ((52.6°, 45°) tangential, The spectral width conversion rate of 1550nm frequency doubling) is 18%, while the spectral width conversion rate of YCOB crystal spectrum non-critical frequency doubling ((113°, 25.9°) tangential, 1550-1700nm frequency doubling) of the present invention is as high as 64%. The obvious advantage of broadband frequency multiplication.

Example 4

The frequency doubling conversion efficiency of the YCOB crystal in the (113°, 25.9°) tangential direction is greater than that of the (139.7°, 0°) tangential direction, and the frequency doubling conversion efficiency of the GdCOB crystal in the (114.1°, 32°) tangential direction is greater than that of (146.4°, 0°) tangential direction, as shown in Table 1.

Table 1

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%

The data in Table 1 shows that compared with the traditional main plane tangential spectral non-critical phase matching method, the special tangential spectral non-critical phase matching method described in the present invention has higher efficiency and is more beneficial for applications.

When the wavelength of the fundamental frequency light is 1650nm, the average power is 40mW, the repetition frequency is 100kHz, and the pulse width is 160fs, the frequency doubling conversion efficiency of the YCOB crystal in the tangential direction of (113°, 25.9°) is 28.3%, and at (139.7°, 0°) tangential frequency doubling conversion efficiency is 19.4%. The frequency doubling conversion efficiency of the GdCOB crystal in the (114.1°, 32°) tangential direction is 21.8%, and the frequency doubling conversion efficiency in the (146.4°, 0°) tangential direction is 13.2%.

When the wavelength of the fundamental frequency light is 1650nm, the average power is 60mW, the repetition frequency is 100kHz, and the pulse width is 160fs, the frequency doubling conversion efficiency of the YCOB crystal in the tangential direction of (113°, 25.9°) is 36.3%, and at (139.7°, 0°) tangential frequency doubling conversion efficiency is 25.5%. The frequency doubling conversion efficiency of the GdCOB crystal in the (114.1°, 32°) tangential direction is 28.9%, and the frequency doubling conversion efficiency in the (146.4°, 0°) tangential direction is 19.7%.

When the wavelength of the fundamental frequency light is 1650nm, the average power is 80mW, the repetition frequency is 100kHz, and the pulse width is 160fs, the frequency doubling conversion efficiency of the YCOB crystal in the tangential direction of (113°, 25.9°) is 43.9%, and at (139.7°, 0°) tangential frequency doubling conversion efficiency is 31%. The frequency doubling conversion efficiency of the GdCOB crystal in the (114.1°, 32°) tangential direction is 35.6%, and the frequency doubling conversion efficiency in the (146.4°, 0°) tangential direction is 25.9%.

When the wavelength of the fundamental frequency light is 1650nm, the average power is 100mW, the repetition frequency is 100kHz, and the pulse width is 160fs, the frequency doubling conversion efficiency of the YCOB crystal in the tangential direction of (113°, 25.9°) is 51.4%, and at (139.7°, 0°) tangential frequency doubling conversion efficiency is 39.2%. The frequency doubling conversion efficiency of the GdCOB crystal in the (114.1°, 32°) tangential direction is 39.8%, and the frequency doubling conversion efficiency in the (146.4°, 0°) tangential direction is 31.5%.

It can be seen that the frequency doubling conversion efficiency of the broadband frequency doubling device with special cut angle of the present invention is obviously higher than that of the crystal whose tangential direction is in the main plane of the nonlinear optical crystal.

Example 5

As shown in Figure 1, 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 placed sequentially along the propagation direction of the optical path;

The broadband frequency doubling device 4 of the special cut angle is the device described in embodiment 1;

The focusing lens is coated with a broadband anti-reflection coating of 1400-1800nm;

The color filter is coated with a 1400-1800nm broadband high reflection coating and a 700-900nm broadband anti-reflection coating.

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 a frequency doubling light 6, leaving the remaining fundamental frequency light 2 The frequency-doubled light 6 passes through the color filter 5 at the same time, wherein the fundamental frequency light 2 is reflected, and the frequency-doubled light 6 passes through the color filter 5 and is output.

Claims (10)

1. A broadband frequency multiplier with special cut angles is characterized in that the chemical composition of the broadband frequency multiplier is YCOB crystals or GdCOB crystals, the spatial direction of the YCOB crystals or the GdCOB crystals is described by polar coordinates (theta, phi), theta is an included angle between the direction and an optical principal axis Z, and phi is an included angle between a projection of a direction vector in an XY principal plane and an X axis, namely an azimuth angle; the device is obtained for a YCOB crystal at a cut angle of (113 °,25.9 °), or a gdcoob crystal at a cut angle of (114.1 °,32 °).

2. The special-chamfer broadband frequency multiplier device according to claim 1, wherein said special-chamfer broadband frequency multiplier device is capable of realizing broadband frequency multiplication of fundamental frequency laser light having a center wavelength in the range of 1550-1700 nm.

3. The special-cut-angle broadband frequency multiplier according to claim 1, wherein the special-cut-angle broadband frequency multiplier has a break-back point in a phase matching curve of a (113 °,25.9 °) tangential YCOB crystal or a (114.1 °,32 °) tangential gdcoob crystal in a 1.55-1.7 μm band, and can realize high-efficiency and broadband frequency multiplication output.

4. The special-cut-angle broadband frequency multiplier of claim 1, wherein the phase matching angle of the (113 °,25.9 °) tangential YCOB crystal varies by 0.3 ° and (114.1 °,32 °) tangential gdcoob crystal varies by 0.1 ° during the wavelength of the fundamental light increases from 1550nm to 1700nm during broadband frequency multiplication using the special-cut-angle broadband frequency multiplier.

5. The special-cut wide-band frequency doubler device according to claim 1, characterized in that the size of the YCOB crystal or gdcoob crystal is 6 x 10mm ³ 。

6. Use of the special cut-angle broadband frequency doubling device of claim 1 in ultrashort pulse lasers.

7. An ultrashort pulse laser, comprising a light source, a focusing lens, the special corner-cut broadband frequency multiplier device of claim 1, and a color filter, which are sequentially disposed along a propagation direction of an optical path.

8. An ultrashort pulse laser as claimed in claim 7, wherein the focusing lens is coated with a broadband antireflection film of 1400-1800 nm.

9. The ultrashort pulse laser as claimed in claim 7, wherein the color filter is plated with a 1400-1800nm broadband high reflection film and a 700-900nm broadband antireflection film.

10. The ultrashort pulse laser as claimed in claim 7, wherein the spectral bandwidth of the frequency doubled light of the broadband frequency doubling device with special cut angle is 14 ± 1nm when the spectral bandwidth of the fundamental light is 22 ± 1nm.

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