CN103820858A - A Class of Erbium or Holmium Activated ABC3O7 Mid-infrared Ultrafast Laser Crystals - Google Patents

Abstract

The purpose of the present invention is to disclose a kind of mid-infrared ultrafast laser crystal with ABC ₃ O ₇ mellow feldspar structure activated by erbium and holmium. Aiming at the problems of complex devices and low beam quality in the prior art, the present invention proposes to use LD to pump a type of Er ³⁺ and Ho ³⁺ activated crystal of ABC ₃ O ₇ mellow feldspar structure to directly realize the mid-infrared in the 2.5-3 μm band Mode-locked ultrafast laser, the chemical formula of the crystal is Er _x AB _1-x C ₃ O ₇ or Ho _y AB _1-y C ₃ O ₇ (A is at least one of Ca, Sr, Ba; B is Y, La , at least one of Gd; C is Ga, at least one of Al), wherein Er ³⁺ doping concentration x=0.01~0.5, Ho ³⁺ doping concentration y=0.01~0.3, Er ³⁺ and Ho ³⁺ all replace B ³⁺ in the crystal (namely Y ³⁺ , La ³⁺ , Gd ³⁺ ).

Description

A Class of Erbium or Holmium Activated ABC3O7 Mid-infrared Ultrafast Laser Crystals

technical field

The invention relates to the field of laser crystal materials.

Background technique

The 2.5-3.0μm band is located in the "molecular fingerprint area", which is a transmission window of the atmosphere. The laser in this band has strong penetrating power to smog, smoke, etc., and the scattering of gas molecules on sea level transmission is small. The detectors of infrared guided missiles and infrared early warning systems are sensitive to this band. Ultrafast mid-infrared lasers can instantly provide high peak power enough to damage these detection devices. Therefore, ultrafast mid-infrared lasers with high peak power are used in military confrontation There are many potential applications in . In civilian use, since water has a strong absorption of light with a wavelength of 2.5 to 3.0 μm, the absorption coefficient of water (α=13000cm ^-1 ) is 2.1μm (α=50cm ^-1 ) or 1.064μm (α=l cm -1 ) respectively ^{. 1} ), and the ablation threshold and the thermal diffusion of the surrounding tissue are inversely proportional to the absorption coefficient. Fast lasers are very effective in ophthalmic, dental and endoscopic microsurgery. In addition, 2.5-3.0μm ultrafast lasers also have important applications in remote sensing, environmental detection, optical soliton communication, fine machining, etc., and are also used to study the transient optical transition process between narrow-gap semiconductors and superlattice multi-quantum well bands. , Photoexcitation dynamics in semiconductors, as well as intra-molecular and intermolecular energy transfer and phase-dissolution phenomena and other important means of kinetic problems.

At present, there are four main methods for realizing ultrashort mid-infrared laser pulses in the world:

(1) Parametric Oscillation Method (OPO). The conversion efficiency of the parametric oscillation method is high, but the single pulse energy is low; the quasi-phase-matched optical parametric oscillation has a high nonlinear conversion efficiency, which can realize the phase matching of crystals and light-passing bands that cannot be phase-matched under normal conditions. Frequency transformation, suitable for frequency transformation under continuous and miniaturization.

(2) Difference frequency generation method. This method is often used to generate tunable infrared radiation with high spectral resolution, especially the main method for generating far-infrared light. Commonly used crystals are mainly semiconductor crystals such as AgGaS ₂ , AgGaSe ₂ , GaAs, etc. These crystals have a wide effective light transmission range, large nonlinear coefficient, and a wide light transmission range. The disadvantage is that the damage threshold is low. At present, the mid-infrared laser with an output power of 100 milliwatts and a pulse width of tens of fs can be realized by using the difference frequency method.

(3) Parametric amplification (OPA). Using the high gain of parametric amplification to amplify at the gain saturation point can obtain high-power femtosecond mid-infrared output. At present, optical parametric amplification of supercontinuum injection is the main method to generate high-power femtosecond mid-infrared laser pulses. Commonly used crystals are mainly MgO:LiNbO ₃ and KTP and its isotype crystals with very high damage threshold. Domestically, Professor Qian Liejia from Shanghai Jiaotong University successfully tunable the 100fs mid-infrared femtosecond laser in the range of 2.0-4.5μm.

(4) Direct output. Utilize the emission of Er ³⁺ , Ho ³⁺ or Cr ²⁺ doped fiber or crystal near 2.5-3.0 μm, adopt appropriate pump source, mode-locking element and mode-locking technology, etc., to directly realize ultrafast laser in this band . This method has simple and compact devices, high beam quality, and can generate diffraction-limited and Fourier-transform-limited ultrashort pulse lasers, which can overcome the high complexity, high cost, poor beam quality and Disadvantages such as low signal-to-noise ratio are another preferred technical solution to achieve ultrafast laser output in the 2.5-3.0 μm band, and one of the most effective technical approaches. It is currently a research hotspot in the field of all-solid-state lasers in the world. For a long time, people have focused on the first three methods of ultrafast lasers, while ignoring the broader research space of direct output.

The transition of Er ³⁺ ions ⁴ I _11/2 → ⁴ I _13/2 and Ho ³⁺ ions ⁵ I ₆ → ⁵ I ₇ can produce ultrafast laser in the mid-infrared band of 2.5-3.0 μm. Er ³⁺ and Ho ³⁺ doped crystals are the core gain medium of all-solid-state ultrafast mode-locked lasers in the 2.5-3.0μm band, and one of the important factors affecting the output of ultrafast lasers. Generally speaking, in order to achieve 2.5-3.0 μm ultrashort pulse laser, the host material should at least meet the following conditions: ① The phonon energy is low, so the probability of multi-phonon relaxation and non-radiative transition is reduced, which helps to improve the laser efficiency, Therefore, the host crystals commonly used in the near-infrared band, such as borate, tungstate, vanadate, etc., have too high phonon energy, and it is difficult to achieve high-power, ultra-short pulse width mid-infrared lasers. Lower phonon energy can be selected. Gallates, chalcogenides, fluoride halides, etc. ②Choose a crystal with a partially disordered structure. After they are doped with rare earth luminescent ions, the absorption and fluorescence emission spectra are obviously broadened. The larger the width of the laser crystal fluorescence emission spectrum, the narrower the width of the laser pulse it can achieve. Therefore, crystals with disordered structure characteristics are excellent ultrashort pulses. Laser Gain Materials. ③ After the active ions Er ³⁺ and Ho ³⁺ are doped, they have excellent spectral properties: for example, the fluorescence emission cross-section is large in the 2.5-3.0 μm band, the fluorescence branch ratio is high, and the laser upper energy level ⁴ I _11/2 has a long lifetime to achieve Population inversion etc. ④ It has good physical and chemical properties, easy to grow and obtain, and good crystal optical quality. Cr:ZnS and Cr:ZnSe crystals that have recently achieved mid-infrared femtosecond laser output have problems such as poor mechanical strength, harsh growth conditions, and uneven doping concentration, while chalcogenides, fluorine halides, etc. also face the problem of difficult growth. Seriously hinder the possibility of further development and practical application.

Therefore, we will choose Er ³⁺ , Ho ³⁺ activated ABC ₃ O ₇ (A is at least one of Ca, Sr, Ba; B is at least one of Y, La, Gd; C is Ga, Al At least one of) series crystals are used as ultrafast laser gain media in the mid-infrared band, mainly for the following reasons:

First, ABC ₃ O ₇ series crystals, as an excellent laser host material, are one of the preferred materials for generating high-power, short-pulse mid-infrared lasers.

具有黄长石结构。该体系具有物化性能良好(不吸潮、不溶于酸碱)，热导率高（SrGdGa₃O₇:11W·m^-1·K^-1）、机械强度大、声子能量低（SrLaGa₃O₇:560cm^-1）等优点，熔点1500～1650℃左右、同成分熔化，可采用提拉法较容易获得大尺寸优质单晶。这些特点使得晶体不仅满足2.5～3.0μm波段超快激光的一般要求，而且具有高的光损伤阈值和激光量子效率。而且，ABC₃O₇系列晶体具有一定的无序结构特征，这是商业化的YAG、YSGG、YAP等晶体所不具备的。以SrGdGa₃O₇晶体为例，该分子由层状GaO₄ ^5-四面体构成，层与层之间Sr²⁺、Gd³⁺离子以1:1比例、镜面对称分布在相应晶格点位置上。由于其价态、粒子半径和结晶性能的差异，造成晶体内部的无序结构，使得掺杂离子Nd³⁺取代Gd³⁺离子后基质晶体中形成许多结构各异的激活中心，导致它的吸收和发射光谱获得非均匀展宽，同样地在Er:SrLaGa₃O₇晶体上也发现了～2.7μm波段荧光光谱展宽的现象，这样，一方面将有利于锁模激光的产生，另一方面也将引起Er³⁺准三能级系统激光下能级的劈裂增大，从而克服由Er³⁺系统中存在的上转换和反向能量传递等能量损耗所引起的高激光阈值和低激光功效等缺点，可以大大地降低激光阈值和提高激光输出功效。The ABC ₃ O ₇ crystal family belongs to the tetragonal crystal system, the space group

With yellow feldspar structure. The system has good physical and chemical properties (no moisture absorption, insoluble in acid and alkali), high thermal conductivity (SrGdGa ₃ O ₇ : 11W·m ^-1 ·K ^-1 ), high mechanical strength, and low phonon energy (SrLaGa ₃ O ₇ :560cm ^-1 ), the melting point is about 1500～1650℃, and the same composition melts, and it is easier to obtain large-sized high-quality single crystals by using the pulling method. These characteristics make the crystal not only meet the general requirements of ultrafast lasers in the 2.5-3.0 μm band, but also have high optical damage threshold and laser quantum efficiency. Moreover, the ABC ₃ O ₇ series crystals have certain disordered structure characteristics, which are not available in commercial YAG, YSGG, YAP and other crystals. Taking SrGdGa ₃ O ₇ crystal as an example, the molecule is composed of layered GaO ₄ ^5- tetrahedron, and the Sr ²⁺ and Gd ³⁺ ions between the layers are distributed at the corresponding lattice points in a mirror-symmetrical ratio of 1:1. superior. Due to the difference in valence state, particle radius and crystallization properties, the disordered structure inside the crystal is caused, so that after the dopant ion Nd ³⁺ replaces the Gd ³⁺ ion, many activation centers with different structures are formed in the host crystal, resulting in its absorption. And the emission spectrum is broadened non-uniformly. Similarly, the fluorescence spectrum broadening in the ~2.7μm band has also been found on Er:SrLaGa ₃ O ₇ crystals. Cause the splitting of the lower energy level of the Er ³⁺ quasi-three-level system laser to increase, thereby overcoming the high laser threshold and low laser efficacy caused by energy losses such as up-conversion and reverse energy transfer in the Er ³⁺ system Disadvantages, can greatly reduce the laser threshold and improve laser output efficacy.

Second, Er ³⁺ activated ABC ₃ O ₇ series crystals can be excellent gain media for 2.5-3.0 μm ultrafast lasers.

ATu3A.32,2013】，同时在Nd:SrLaGa₃O₇晶体实现飞秒激光输出，即在1061nm时脉宽为378fs，在1071nm时脉宽为534fs【Advanced Solid-StateLasers Congress Technical Digest

AM1A.7,2013】。以上研究结果表明：具有无序结构特征的ABC₃O₇晶体家族是综合性能非常突出的超快激光晶体,与现有的YAG\YAP\YSGG\GGG晶体相比，它们的熔点比较低，较容易采用提拉法获得大尺寸优质晶体，而与其它镓酸盐例如YSGG和GGG晶体相比，它还有一个突出的优点——晶体结构无序，从而导致Ho³⁺，Er³⁺激活的ABC₃O₇晶体的光谱显著展宽，非常有利于实现中红外波段超快激光。该系列晶体中Nd³⁺掺杂的研究比较多，Er³⁺、Ho³⁺掺杂报道极少，而且目前尚无Er³⁺、Ho³⁺激活的ABC₃O₇晶体作为2.5～3.0μm波段超快激光晶体的研究报道。Nd ³⁺ doped ABC ₃ O ₇ crystals have attracted extensive research interest as gain media for femtosecond lasers. For example, Nd:SrGdGa ₃ O ₇ crystals have achieved self-mode-locked pulsed laser output with output power of 400mW and repetition power 80GHz, the pulse width is 616fs, which is the shortest pulse width achieved by Nd ³⁺ doped crystal self-mode-locked laser [Optics Letters, 37(4), pp461-463, 2012]. In 2013, A.Agnesi et al. adopted passive mode-locking Technology realizes broadband tunable femtosecond pulse laser on Nd:BaLaGa ₃ O ₇ crystal, when the output wavelength is 1060nm, the pulse width is 316fs, and when the pulse width is ps level, the output wavelength is tunable from 1072 to 1090nm【Advanced Solid-State Lasers Congress Technical Digest

ATu3A.32, 2013], and achieved femtosecond laser output in Nd:SrLaGa ₃ O ₇ crystal, that is, the pulse width is 378fs at 1061nm, and the pulse width is 534fs at 1071nm【Advanced Solid-StateLasers Congress Technical Digest

AM1A.7, 2013]. The above research results show that the ABC ₃ O ₇ crystal family with disordered structure characteristics is an ultrafast laser crystal with outstanding comprehensive performance. Compared with the existing YAG\YAP\YSGG\GGG crystals, their melting point is relatively low, It is easy to use the pulling method to obtain large-sized high-quality crystals, and compared with other gallates such as YSGG and GGG crystals, it has another outstanding advantage-the crystal structure is disordered, resulting in Ho ³⁺ , Er ³⁺ activated The spectrum of ABC ₃ O ₇ crystals is significantly broadened, which is very conducive to the realization of ultrafast lasers in the mid-infrared band. There are many studies on Nd ³⁺ doping in this series of crystals, but there are very few reports on Er ³⁺ and Ho ³⁺ doping, and there is no Er ³⁺ , Ho ³⁺ activated ABC ₃ O ₇ crystals as 2.5～3.0μm Research reports on ultrafast laser crystals with high frequency bands.

Contents of the invention

The purpose of the present invention is to disclose a new class of mid-infrared ultrafast laser crystals with ABC ₃ O ₇ mellow feldspar structure activated by erbium and holmium. Aiming at the problems of complex devices and low beam quality in the prior art, the present invention provides a kind of Er ³⁺ , Ho ³⁺ activated crystal of ABC ₃ O ₇ mellow feldspar structure by LD pumping to directly realize ultra-fast mid-infrared mode-locking Laser, the chemical formula of the crystal is Er _x AB _1-x C ₃ O ₇ or Ho _y AB _1-y C ₃ O ₇ (A is at least one of Ca, Sr, Ba; B is Y, La, Gd at least one; C is at least one of Ga and Al), where Er ³⁺ doping concentration x=0.01~0.5, Ho ³⁺ doping concentration y=0.01~0.3, both Er ³⁺ and Ho ³⁺ Substitute B ³⁺ in the crystal (namely Y ³⁺ , La ³⁺ , Gd ³⁺ ).

Detailed ways:

Growth Preparation and Spectral Properties of Er _x AB _1-x C ₃ O ₇ or Ho _y AB _1-y C ₃ O ₇ Crystals

The instrument used for crystal pulling method growth is DJL-400 intermediate frequency pulling furnace, and the model of intermediate frequency power supply is KGPF25-0.3-2.5. Use Pt/Pt-Rh thermocouple and type 815EPC ohmmeter for temperature control. The crucible used is a Ф55mm×30mm iridium crucible, and the raw materials used are 4N grade SrCO ₃ , Gd ₂ O ₃ , Ga ₂ O ₃ , Er ₂ O ₃ , Ho ₂ O ₃ and so on. Prepare raw materials according to the following chemical reaction formula:

2ACO ₃ +(1-x)B ₂ O ₃ +xEr ₂ O ₃ +3C ₂ O ₃ →2AB _(1-x) Er _x C ₃ O ₇ +CO ₂ ↑

2ACO ₃ +(1-y)B ₂ O ₃ +yHo ₂ O ₃ +3C ₂ O ₃ →2AB _(1-y) Ho _y C ₃ O ₇ +CO ₂ ↑Among them, A is Ca, Sr, Ba At least one; B is at least one of Y, La, Gd; C is at least one of Ga, Al, Er ³⁺ doping concentration x=0.01～0.5, Ho ³⁺ doping concentration y=0.01 ~0.3, both Er ³⁺ and Ho ³⁺ replace B ³⁺ in the crystal (namely Y ³⁺ , La ³⁺ , Gd ³⁺ ). Mix the raw materials evenly, press them into flakes, put them into a platinum crucible, put them into a common sintering furnace, slowly raise the temperature to 1080°C at 150°C/h, keep for 48h, repeat this process, and then put them into a high-temperature sintering furnace at 1200°C Sinter at a constant temperature for 72 hours, take out the polycrystalline material until the X-ray powder diffraction is completely consistent with the standard card.

Put the raw materials into the iridium crucible of Ф55mm×30mm. In order to avoid the oxidation of the iridium crucible, firstly extract the air in the furnace to make the air pressure in the furnace reach minus 0.01MPa, then fill in high-purity nitrogen to make the air pressure reach 0.04MPa, and then raise the temperature To a temperature 50°C higher than the melting point, keep the temperature constant for 1 hour, so that the raw materials are completely melted. During the growth process, the pulling rate of the seed rod is 1.3-1.5mm/h, the cooling rate is 2-10°C/h, the rotation rate of the seed rod is 12-20r.pm, and the crystal is lifted out of the liquid after the growth is completed. The surface is lowered to room temperature at a rate of 10-30°C/h, and the size is larger than of transparent crystals.

Claims (2)

1. a class Er ³⁺or Ho ³⁺the ABC activating ₃o ₇the crystal of configuration, is characterized in that: described crystal belongs to melilite structure, and molecular formula is Er _xaB _1-xc ₃o ₇or Ho _yaB _1-yc ₃o ₇, wherein, A is Ca, Sr, and at least one in Ba, B is Y, La, at least one in Gd, C is Ga, at least one in Al; Er ³⁺doped parameterx=0.01～0.5, Ho ³⁺doping content y=0.01～0.3, Er ³⁺and Ho ³⁺all replace the B in crystal.

2. the purposes of crystal described in claim 1, directly realizes the infrared locked mode ultrafast laser output of 2.5～3 mu m waveband for semiconductor laser pumping.