阅读说明：本技术 一种中红外光纤光栅稳频的量子级联激光器及其实现方法 (Quantum cascade laser for frequency stabilization of intermediate infrared fiber bragg grating and implementation method thereof ) 是由 施进丹 冯宪 于 2020-05-26 设计创作，主要内容包括：本发明公开了一种中红外光纤光栅稳频的量子级联激光器及其实现方法,该中红外激光器包括中红外量子级联激光器、第一中红外透镜、第二中红外透镜、中红外单模光纤、光纤布拉格光栅和温度控制单元；空间输出模式差、线宽稳频能力低的中红外量子级联激光器激光输出耦合进入一段中红外单模光纤,在中红外单模光纤上的窄线宽布拉格光栅作为外腔稳频单元,实现波长稳定、窄线宽光纤输出的单横模激光。本发明可以实现结构紧凑、输出模式质量高、中红外4微米以上的窄线宽光纤输出激光光源。(The invention discloses a quantum cascade laser for frequency stabilization of an intermediate infrared fiber bragg grating and an implementation method thereof, wherein the intermediate infrared laser comprises an intermediate infrared quantum cascade laser, a first intermediate infrared lens, a second intermediate infrared lens, an intermediate infrared single-mode fiber, a fiber bragg grating and a temperature control unit; the laser output of the intermediate infrared quantum cascade laser with poor space output mode and low line width frequency stabilization capability is coupled into a section of intermediate infrared single-mode fiber, and a narrow line width Bragg grating on the intermediate infrared single-mode fiber is used as an external cavity frequency stabilization unit, so that single transverse mode laser with stable wavelength and narrow line width fiber output is realized. The invention can realize the narrow linewidth optical fiber output laser light source with compact structure, high output mode quality and over 4 microns of mid-infrared.)

1. A quantum cascade laser for frequency stabilization of mid-infrared fiber gratings is characterized in that: the method comprises the following steps:

the intermediate infrared quantum cascade laser is used as an output gain substrate of the intermediate infrared QCL laser;

the first intermediate infrared lens is positioned at the front end of the intermediate infrared quantum cascade laser and used for parallel collimation QCL output;

the second intermediate infrared lens is positioned at the front end of the first intermediate infrared lens and used for focusing and coupling the QCL output to enter an intermediate infrared single-mode fiber core;

the intermediate infrared single-mode fiber is used for outputting the etched narrow-linewidth Bragg grating and the final narrow-linewidth QCL laser;

the fiber Bragg grating is engraved on the intermediate infrared single-mode fiber, the central wavelength of the fiber Bragg grating is positioned in the gain bandwidth of the intermediate infrared quantum cascade laser and is used as a narrow-linewidth wavelength selective unit, the QCL output falling in a Bragg grating window is used as seed light to be reflected back to the QCL chip, an external cavity is formed by the QCL chip and the seed light, and the seed light is allowed to repeatedly oscillate back and forth in the external cavity;

and the temperature control unit is arranged below the fiber Bragg grating and is used for tuning the central wavelength of the fiber Bragg grating.

2. The mid-infrared fiber grating frequency stabilized quantum cascade laser of claim 1, characterized in that: the central wavelength of the intermediate infrared quantum cascade laser is 4-14 microns.

3. The mid-infrared fiber grating frequency stabilized quantum cascade laser of claim 1, characterized in that: the first intermediate infrared lens is a spherical lens, is made of one of calcium fluoride, magnesium fluoride, germanium and zinc selenide, and has a focal length of 5-200 mm and a diameter of 1/4-1 inch.

4. The mid-infrared fiber grating frequency stabilized quantum cascade laser of claim 1, characterized in that: the second intermediate infrared lens is a spherical lens, is made of one of calcium fluoride, magnesium fluoride, germanium and zinc selenide, and has a focal length of 5-200 mm and a diameter of 1/4-1 inch.

5. The mid-infrared fiber grating frequency stabilized quantum cascade laser of claim 1, characterized in that: the intermediate infrared single-mode fiber adopts one of an intermediate infrared single-mode fluoride glass fiber, an intermediate infrared single-mode sulfide glass fiber, an intermediate infrared single-mode selenide glass fiber and an intermediate infrared single-mode telluride glass fiber.

6. The mid-infrared fiber grating frequency stabilized quantum cascade laser of claim 1, characterized in that: and (3) adopting a femtosecond laser direct writing technology to write the fiber Bragg grating.

7. The mid-infrared fiber grating frequency stabilized quantum cascade laser of claim 1, characterized in that: the temperature control unit is used for controlling the temperature of the liquid crystal display panel to be in a range of-5 ℃ to 200 ℃.

8. The method for realizing the frequency-stabilized quantum cascade laser based on the mid-infrared fiber grating of any one of claims 1 to 7 is characterized in that: the method comprises the following steps:

s1, enabling the laser output of the intermediate infrared quantum cascade laser to be parallel and collimated through the first intermediate infrared lens;

s2, enabling laser output of the intermediate infrared quantum cascade laser to enter a fiber core of the intermediate infrared single-mode fiber through focusing and coupling of the second intermediate infrared lens after parallel collimation;

s3, Bragg gratings with low reflectivity and narrow line width are engraved on the intermediate infrared single-mode fiber, the gratings play a role of a weak reflection cavity mirror and selectively reflect the intermediate infrared quantum cascade laser signals falling in a fiber Bragg grating reflection waveband window back to the quantum cascade laser; thus, the quantum cascade laser and the narrow-linewidth intermediate infrared fiber grating form an external cavity resonant cavity, a narrow-linewidth weak signal injected by reflection is used as a seed signal, and the seed signal light is amplified by the gain of the quantum cascade laser and finally realizes the narrow-linewidth laser output through repeated oscillation; the laser with narrow line width is finally output through the intermediate infrared single-mode fiber, so that the intermediate infrared narrow line width and single transverse mode output is realized;

and S4, adjusting the central wavelength of the intermediate infrared single-mode fiber grating through the temperature control unit, repeating the step S3, and outputting the laser with the tuned wavelength and the narrow line width through the intermediate infrared single-mode fiber to realize the output of the wavelength-tunable intermediate infrared narrow line width and the single transverse mode.

Technical Field

The invention relates to a semiconductor laser with a narrow line width in an intermediate infrared band, in particular to a single-mode fiber output and wavelength tunable intermediate infrared fiber grating frequency-stabilized quantum cascade laser and an implementation method thereof.

Background

The mid-infrared 2-20 micron wave band has important practical value, and fingerprint fundamental frequency vibration absorption spectral lines of gas molecules containing carbon, hydrogen, oxygen and nitrogen and volatile organic compounds related to life fall within the wave band; meanwhile, the 3-5 micrometers and the 8-12 micrometers are two transparent windows of the atmosphere, and laser signals can be transmitted in the atmosphere with low loss ranging from kilometers to dozens of kilometers, so that the intermediate infrared laser spectrum technology has important requirements in the fields of atmosphere remote sensing, satellite communication, national defense safety and the like, and can be used for performing rapid, accurate and synchronous online high-precision qualitative and quantitative analysis on multi-component trace gases in various application scenes such as environment monitoring, industrial production process monitoring, toxic and explosive gas detection, disease diagnosis and the like. The vibration absorption spectrum of the detected gas molecular group is represented as a comb-shaped spectral line with a MHz-level line width in the middle infrared, and the wavelength position of the spectral line is represented as a fingerprint type characteristic, so that a high-performance middle infrared laser with tunable wavelength and a line width below hundred kHz is urgently needed.

The intermediate infrared quantum cascade laser chip is constructed by periodic layered semiconductor materials, and electronic transition luminescence is realized between sub-energy levels of a superlattice semiconductor structure under the condition of electric excitation, so that intermediate infrared band laser is generated. The quantum cascade laser realizes laser output through electric-optical conversion, and has high electric-optical conversion efficiency; meanwhile, the laser output wavelength can cover all the mid-infrared wavelengths above 4 microns, so the quantum cascade laser is a compact mid-infrared laser light source with the most application prospect.

Currently, there are three main types of mid-infrared quantum cascade lasers: (1) fabry-perot cavity quantum cascade laser (FP-QCL): the end surfaces of natural reflection or the end surfaces plated with reflection media are used as laser feedback mirrors at two ends of a laser cavity to form a Fabry-Perot laser resonant cavity (FP cavity); the output power of the FP-QCL is large, but the output spectral bandwidth is wide (in the range of nanometers to dozens of nanometers), and the coherence of a light source is poor. (2) Distributed feedback quantum cascade laser (DFB-QCL): etching a distributed feedback grating (DFB) directly on a semiconductor quantum cascade chip; the laser output linewidth of the DFB-QCL is narrow, but the output power is much lower than that of the FP-QCL. (3) External cavity modulated quantum cascade laser (EC-QCL): combining the quantum cascade chip with a Volume Bragg Grating (VBG) to form QCL laser output of external cavity VBG frequency selection; such QCLs can provide both narrow spectral output and an overall gain bandwidth (hundreds of cm) for quantum cascade chips^-1) High speed tuning capability on the order of milliseconds is provided. However, the quantum cascade laser with the above three structures adopts a spatial light output mode, and the divergence angle of laser output is large, which is not beneficial to the problems of laser coupling, system integration and the like; on the other hand, due to the thermal problem in the middle infrared, the semiconductor material has larger carrier fluctuation under the condition of temperature fluctuation, so that the instability (jitter) of the laser output wavelength (namely, frequency) causes the actual line width to be widened due to the influence of the temperature; therefore, even for the narrow linewidth DFB-QCL and EC-QCL, the output linewidth is above MHz, and the narrower linewidth output must be realized by other complicated frequency stabilization technology.

The external cavity fiber bragg grating frequency stabilization technique is widely used in near-infrared semiconductor narrow linewidth lasers, and has realized hundreds of kHz magnitude narrow linewidth output. In the middle infrared region above 4 microns, because of the immature fiber Bragg grating preparation technology, the work of quantum cascade laser output by adopting the middle infrared fiber Bragg grating as an external cavity frequency stabilizing device is not seen so far.

Disclosure of Invention

Aiming at the technical problems of three core performances of line width, output mode and tunable wavelength of the quantum cascade laser with the intermediate infrared wavelength of more than 4 microns, the invention provides a quantum cascade laser with intermediate infrared fiber grating frequency stabilization and an implementation method thereof.

In order to realize the purpose of the invention, the technical scheme adopted by the invention is as follows: a frequency stabilized quantum cascade laser of mid-infrared fiber grating comprises:

the intermediate infrared quantum cascade laser is used as an output gain substrate of the intermediate infrared QCL laser;

the first intermediate infrared lens is positioned at the front end of the intermediate infrared quantum cascade laser and used for parallel collimation QCL output;

the second intermediate infrared lens is positioned at the front end of the first intermediate infrared lens and used for focusing and coupling the QCL output to enter an intermediate infrared single-mode fiber core;

the intermediate infrared single-mode fiber is used for outputting the etched narrow-linewidth Bragg grating and the final narrow-linewidth QCL laser;

the fiber Bragg grating is engraved on the intermediate infrared single-mode fiber, the central wavelength of the fiber Bragg grating is positioned in the gain bandwidth of the intermediate infrared quantum cascade laser and is used as a narrow-linewidth wavelength selective unit, the QCL output falling in a Bragg grating window is used as seed light to be reflected back to the QCL chip, an external cavity is formed by the QCL chip and the seed light, and the seed light is allowed to repeatedly oscillate back and forth in the external cavity;

and the temperature control unit is arranged below the fiber Bragg grating and is used for tuning the central wavelength of the fiber Bragg grating.

Preferably, the central wavelength of the intermediate infrared quantum cascade laser is 4-14 microns.

Preferably, the first intermediate infrared lens is a spherical lens, the material of the first intermediate infrared lens is one of calcium fluoride, magnesium fluoride, germanium and zinc selenide, the focal length is 5-200 mm, and the diameter is 1/4-1 inch.

Preferably, the second intermediate infrared lens is a spherical lens, the material of the second intermediate infrared lens is one of calcium fluoride, magnesium fluoride, germanium and zinc selenide, the focal length is 5-200 mm, and the diameter is 1/4-1 inch.

Preferably, the intermediate infrared single-mode fiber is one of an intermediate infrared single-mode fluoride glass fiber, an intermediate infrared single-mode sulfide glass fiber, an intermediate infrared single-mode selenide glass fiber and an intermediate infrared single-mode telluride glass fiber.

Preferably, the fiber Bragg grating is etched by a femtosecond laser direct writing technology.

Preferably, the temperature control unit has a temperature adjustment range of-5 ℃ to 200 ℃.

The method for realizing the frequency-stabilized quantum cascade laser of the mid-infrared fiber grating is characterized in that: the method comprises the following steps:

s1, enabling laser output of the intermediate infrared quantum cascade laser to be parallel and collimated through the first intermediate infrared lens;

s2, enabling laser output of the intermediate infrared quantum cascade laser to enter a fiber core of the intermediate infrared single-mode fiber through focusing and coupling of a second intermediate infrared lens after parallel collimation;

s3, a Bragg grating with low reflectivity and narrow line width is engraved on the intermediate infrared single-mode fiber, the grating plays a role of a weak reflection cavity mirror, and selectively reflects the intermediate infrared quantum cascade laser signal falling in a reflection waveband window of the fiber grating back to the quantum cascade laser; thus, the quantum cascade laser and the narrow-linewidth intermediate infrared fiber grating form an external cavity resonant cavity, a narrow-linewidth weak signal injected by reflection is used as a seed signal, and the seed signal light is amplified by the gain of the quantum cascade laser and finally realizes the narrow-linewidth laser output through repeated oscillation; the laser with narrow line width is finally output through the intermediate infrared single-mode fiber, so that the intermediate infrared narrow line width and single transverse mode output is realized;

and S4, adjusting the central wavelength of the intermediate infrared single-mode fiber grating through the temperature control unit, repeating the step S3, and outputting the laser with the tuned wavelength and the narrow line width through the intermediate infrared single-mode fiber to realize the output of the wavelength-tunable intermediate infrared narrow line width and the single transverse mode.

The working principle of the laser is as follows:

the intermediate infrared quantum cascade laser forms an F-P resonant cavity through two end faces of the intermediate infrared quantum cascade laser, the laser output linewidth is wide, output light is collimated in parallel through a first intermediate infrared lens and then enters a fiber core of an intermediate infrared single-mode fiber through a second intermediate infrared lens in a focusing and coupling mode, a narrow linewidth and weak reflection Bragg grating is engraved on the intermediate infrared single-mode fiber, the narrow linewidth weak reflection Bragg grating selectively reflects an output signal of the quantum cascade laser falling in a Bragg grating window back to a quantum cascade laser chip, the weak signal becomes seed light injected into the quantum cascade laser chip in the reverse direction, the quantum cascade laser and the fiber Bragg grating form an outer cavity, and the seed light signal injected in the reverse direction is subjected to multiple round-trip oscillation in the cavity to finally realize narrow linewidth output; the fiber Bragg grating is arranged on the miniature temperature control unit, and the central wavelength position of the fiber Bragg grating is tuned through temperature adjustment, so that the output of the narrow-linewidth intermediate infrared quantum cascade laser with tunable wavelength is realized.

Compared with the prior art, the technical scheme of the invention has the following beneficial technical effects:

a) the fiber Bragg grating can realize that the 3dB bandwidth is less than 1 nanometer due to the long length of the grating region, so that the output of the mid-infrared laser with the wavelength of more than 4 micrometers and narrow line width can be realized;

b) the intermediate infrared single-mode fiber directly ensures the laser output of single transverse mode with high beam quality without other transverse mode selection elements;

c) the high-precision tunable wavelength is realized by controlling the central wavelength of the fiber Bragg grating through temperature,

d) the bending resistance of the optical fiber enables the output of the whole laser to be more flexible, and is beneficial to realizing a laser device with a compact structure.

Drawings

FIG. 1 is a schematic structural diagram of a frequency stabilized quantum cascade laser of a mid-infrared fiber grating according to the present invention; wherein, 1-intermediate infrared quantum cascade laser; 2-a first mid-infrared lens; 3-a second mid-infrared lens; 4-mid-infrared single-mode fiber; 4.1-fiber core; 4.2-cladding; 5-fiber Bragg grating; 6-a temperature control unit;

FIG. 2 is a mid-infrared high power QCL output spectrum;

FIG. 3 is a schematic diagram of a medium infrared fiber Bragg grating structure;

FIG. 4 is an external cavity fiber Bragg grating transmission spectrum;

FIG. 5 is a narrow linewidth QCL laser output spectrum based on external cavity fiber Bragg grating frequency stabilization;

fig. 6 is a plot of narrow linewidth QCL laser output center wavelength versus temperature.

Detailed Description

The technical solution of the present invention is further described below with reference to the accompanying drawings and examples.

As shown in fig. 1, the frequency-stabilized quantum cascade laser of mid-infrared fiber grating according to the present invention includes:

a 4-micron intermediate infrared Fabry-Perot cavity quantum cascade laser 1 is used as a light source, the output spectrum is shown in figure 2, the central wavelength of the laser is 4.045 microns, and the 3dB bandwidth of the spectrum is 19 nanometers;

the first intermediate infrared lens 2 is CaF₂A crystal plano-convex spherical lens with a diameter ofInches, focal length 30 millimeters;

the second intermediate infrared lens 3 is CaF₂A crystal plano-convex spherical lens with a diameter ofInches, focal length 10 millimeters;

a section of mid-infrared single-mode fiber 4 is used as a carrier for inscribing fiber Bragg gratings and a final laser output tail fiber; in the embodiment, the intermediate infrared single-mode fiber is a medium infrared single-mode indium fluoride glass fiber, and comprises a fiber core 4.1 and a cladding 4.2 coated outside the fiber core 4.1, wherein the diameter of the fiber core is 9 microns, the numerical aperture is 0.26, and the total length of the fiber is 1.0 meter;

as shown in fig. 3, at the input end of the intermediate infrared single mode fiber 4, that is, at the side close to the quantum cascade laser 1 and the first and second intermediate infrared lenses 2 and 3, a fiber bragg grating 5 is engraved on the fiber core 4.1 of the single mode fiber by the 800 nm femtosecond laser direct writing technology;

in the present embodiment, the transmission spectrum of the measured fiber bragg grating 5 is shown in fig. 4, the central wavelength of the grating is located at 4.05 microns, the 3dB bandwidth is 0.46 nm, and compared with the baseline level, the transmittance at the peak wavelength is reduced to-80%, and the reflectance at the peak wavelength of 4.05 microns is calculated to be about 20%.

The laser output of the intermediate infrared quantum cascade laser 1 is parallel and collimated through a first intermediate infrared lens 2; then the fiber core 4.1 enters the intermediate infrared single-mode fiber 4 through the focusing coupling of the second intermediate infrared lens 3; the narrow-linewidth weak reflection optical fiber Bragg grating 5 selectively reflects an output signal of the quantum cascade laser 1 falling in a Bragg grating window back to the quantum cascade laser 1, the weak signal becomes seed light which is reversely injected into the quantum cascade laser 1, the quantum cascade laser 1 and the optical fiber Bragg grating 5 form an outer cavity, and the reversely injected seed light signal is subjected to multiple times of reciprocating oscillation amplification in the cavity to finally realize narrow-linewidth laser output; the laser with narrow line width is finally output through the intermediate infrared single-mode fiber 4, so that the intermediate infrared narrow line width and single transverse mode output is realized; fig. 5 shows the laser output of a mid-infrared frequency stabilized narrow linewidth quantum cascade laser tested by a spectrometer, the center wavelength of which is 4.05 microns and is limited by the spectral resolution of the spectrometer, and the actual line width of which is about 0.1 nm.

The fiber Bragg grating 5 is arranged on the miniature temperature control unit 6, the temperature control unit 6 is a semiconductor refrigerating sheet material, the temperature adjusting range is-5 ℃ to 200 ℃, and the central wavelength position of the fiber Bragg grating is tuned by adjusting the temperature, so that the output of the narrow-line-width intermediate infrared quantum cascade laser with tunable wavelength is realized. Fig. 6 shows the range of the center wavelength of the laser output from a narrow linewidth QCL by temperature tuning.

The foregoing is a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, several modifications and variations can be made without departing from the technical principle of the present invention, and these modifications and variations should also be regarded as the protection scope of the present invention.