CN110896192A

CN110896192A - Non-quartz glass-based medium-infrared special fiber Raman DFB fiber laser

Info

Publication number: CN110896192A
Application number: CN201911105505.3A
Authority: CN
Inventors: 施进丹; 冯宪
Original assignee: Jiangsu Normal University
Current assignee: Jiangsu Normal University
Priority date: 2019-11-13
Filing date: 2019-11-13
Publication date: 2020-03-20

Abstract

The invention discloses a non-quartz glass-based medium-infrared special fiber Raman DFB fiber laser, which comprises a pumping light source, a dichroic mirror 1, a coupling lens, a medium-infrared special fiber pi phase shift DFB fiber grating, a medium-infrared lens, a dichroic mirror 2, a laser signal detection device and a temperature controller. The laser is based on the fiber nonlinear Raman effect, effectively solves the problems that the output wavelength of a rare earth doped intermediate infrared fiber laser is limited to a plurality of discrete wave bands within 4 micrometers, efficiency is low due to a thermal effect and the like, adopts pi phase shift DFB to form a single-frequency narrow-linewidth laser cavity, and generates tunable intermediate infrared laser with any wavelength above 2.5 micrometers of intermediate infrared and a single-frequency narrow linewidth.

Description

Non-quartz glass-based medium-infrared special fiber Raman DFB fiber laser

Technical Field

The invention belongs to the field of mid-infrared fiber lasers, and particularly relates to a non-quartz glass-based mid-infrared special fiber Raman DFB fiber laser.

Background

The mid-infrared band (2.5-25 μm, 4000-: 3-5 μm and 8-14 μm, and there are abundant, fingerprint-characterized, narrow line-width, strong absorption lines of fundamental frequency vibrations of gas molecules [ a. schliesser, n. picqu é, and T.W.

“Mid-infrared frequency combs,”Nat.Photonics 6(7),440–449(2012)]. Single root of the same gasTypical line widths of spectral lines are in the order of MHz, the spacing of the spectral lines is several nanometers, and overlapping cross interference exists among the spectral lines of different gases. The spectrum analysis technology based on the mid-infrared single-frequency narrow-linewidth laser light source can realize the high-sensitivity detection of the gas concentration part per billion (ppt) level, wherein the narrow linewidth characteristic of the light source is one of the basic premises for realizing the high-precision detection. In order to avoid cross interference of different gas molecules and accurately position a specific gas, the light source also needs to have certain wavelength tunable characteristics to realize high selectivity of spectral analysis. Therefore, the intermediate infrared single-frequency narrow-linewidth wavelength-tunable laser light source has important application requirements in the fields of national defense, medical treatment, atmospheric environment monitoring, medical treatment and health care and the like.

Currently, the Mid-infrared monochromatic laser light source mainly includes Distributed Feedback (DFB) semiconductor quantum cascade laser (DFB-QCL) [ c.s.kim, m.kim, j.abell, w.w.bewley, c.d.merritt, c.l.cancer, i.v. soot man, and j.r.meyer, "Mid-induced distributed-feedback and cassette Lasers with coherent-wave single mode emission to 80 ℃", "application.phys.le101, 061104(2012)," and interband cascade laser (DFB-ICL) [ w.zeller, l.naehle, p.fuels, f.geschuetz, l.hibanfeltdt, j.laser, "laser b" lat-laser, "laser and laser 1, 12 μ g", and "laser 1, 12", w.12, 12 μ g, 12 μ g, 20, 12 μ g, 12, w.r, 12, wo.12, w.r, 12, w.12, w.r, w.12, w.r, w.12, w.r, w.12, w..

The solid intermediate infrared OPO and OPA system has the advantages of wide output line width, complex optical path, high cost, large volume and poor portability.

The output wavelengths of the semiconductors DFB-ICL and DFB-QCL can cover all middle infrared bands above 3 μm; because the DFB resonant cavities of the QCL and the ICL are very short (millimeter magnitude) and are simultaneously influenced by spontaneous radiation, carrier noise and the like, the actual output line width is limited to MHz magnitude, and the output power of the QCL and the ICL is changed in the wavelength tuning process; in addition, the semiconductor laser is very sensitive to the influence of temperature, electrostatic shock and the like from the environment, and a temperature control unit and a circuit protection unit are needed during working, so that the complexity of the device is increased.

Compared with the prior art, the DFB fiber laser not only inherits the advantages of good output space mode, low-loss fusion with other fiber devices, compact structure, strong output power improving capability and the like, but also has the advantages that the resonant cavity length of the DFB fiber laser is in the range of several to dozens of centimeters and is 3-4 orders of magnitude longer than that of a semiconductor DFB laser resonant cavity, so that the laser output line width is narrower and is in the kHz order of magnitude; furthermore, the output power remains substantially constant during the wavelength tuning process. Therefore, the optical fiber DFB laser technology is one of effective ways for breaking through the technical bottleneck of kHz line width faced by the infrared semiconductor DFB-QCL and DFB-ICL narrow line width lasers.

DFB fiber lasers are mainly classified into two main categories according to gain type: rare earth doped DFB fiber lasers based on active optical fibers and nonlinear gain DFB fiber lasers based on passive fibers [ p.s.westbrook, k.s.abedin, j.w.nicholson, t.kremp, and j.port, "Raman fiber distributed feedback lasers," Optics Letters 36(15), 2895-. Fig. 1 summarizes the wavelength distribution and development history of the DFB fiber laser confirmed by experiments so far.

The rare earth doped fiber laser is limited by the energy level of doped rare earth ions, and the laser output wavelength can only cover a plurality of discrete wave bands (such as 1 μm, 1.3 μm, 1.5 μm, 1.7-2.1 μm and 2.8 μm) determined by the emission of the rare earth ions within 4 μm, as shown in figure 1; after the fiber enters the mid-infrared band, the non-radiative relaxation transition caused by the high phonon energy of the glass matrix becomes obvious, the laser emission efficiency is greatly reduced, non-oxide glass (such as fluoride glass) with low phonon energy is required to be adopted as a doping matrix material, and meanwhile, the thermal effect becomes a main factor influencing the efficiency and the performance of the rare earth doping mid-infrared fiber laser due to the enhancement of multi-phonon scattering.

Different from rare earth gain, the output wavelength of the Raman laser is only related to the wavelength of the pump laser and the Raman frequency shift of the optical fiber matrix, and the laser output can be realized at any wavelength theoretically; moreover, due to the characteristic of nonlinear stimulated Raman scattering, quantum defects are small, the thermal effect of the Raman DFB fiber laser is very low theoretically, and the Raman DFB fiber laser has lower noise. Therefore, the DFB (Raman-DFB) optical fiber laser based on the stimulated Raman gain of the optical fiber matrix material can effectively break through the problems of output wavelength and thermal effect of the rare earth doped DFB optical fiber laser, and is an effective way for generating mid-infrared single-frequency narrow linewidth laser. At present, the Raman DFB fiber laser only realizes near-infrared band single-frequency laser output in a quartz glass matrix fiber, and the experimental result of the infrared Raman DFB fiber laser in a special fiber based on non-quartz glass is not shown in the literature. In addition, since the transmission loss of the quartz fiber is rapidly increased above 2.4 μm, the laser output of the mid-infrared band [ h.jiang, l.zhang, and y.feng, 'Silica-based fiber Raman laser at >2.4 μm,' Optics Letters 40(14),3249 and 3252(2015) ] cannot be realized, so the realization of the mid-infrared Raman-DFB fiber laser requires a non-quartz glass based mid-infrared special fiber.

Disclosure of Invention

The invention aims to solve the technical problem of providing a medium infrared special fiber Raman DFB fiber laser based on non-quartz glass aiming at the defects in the background technology.

The invention adopts the following technical scheme to solve the technical problems

A middle infrared special optical fiber Raman DFB optical fiber laser based on non-quartz glass comprises a pumping light source, a dichroic mirror 1, a coupling lens, a middle infrared special optical fiber pi phase shift DFB optical fiber grating, a middle infrared lens, a dichroic mirror 2, a laser signal detection device and a temperature controller;

the pumping light source is used for providing a pumping light source;

the dichroic mirror 1 is used for highly transmitting a pump light source, highly reflecting a generated signal light source and separating a reverse Raman DFB laser signal;

the coupling lens is used for improving the coupling efficiency of the pump light entering the fiber core of the optical fiber;

the intermediate infrared special optical fiber pi phase shift DFB fiber grating is used for generating a core device of an intermediate infrared single-frequency narrow linewidth laser signal;

the intermediate infrared lens is used for collimating the output signal light so as to facilitate subsequent detection;

the dichroic mirror 2 is used for highly transmitting the generated signal light source and highly reflecting the pump light source to separate out residual pump light;

the laser signal detection device is used for detecting a laser signal;

and the temperature controller is used for ensuring that the DFB fiber grating is heated uniformly and avoiding the interference of the environmental temperature and noise on the DFB grating.

As a further preferable scheme of the non-quartz glass-based medium-infrared special fiber Raman DFB fiber laser, the numerical aperture and the focused light spot size of the coupling lens are respectively matched with the fiber NA and the fiber core diameter.

As a further preferable scheme of the non-quartz glass-based medium-infrared special fiber Raman DFB fiber laser, the laser signal detection device comprises a power meter, a spectrometer and a frequency spectrograph.

As a further preferable scheme of the non-quartz glass-based middle infrared special fiber Raman DFB fiber laser, the middle infrared special fiber pi phase shift DFB fiber grating comprises germanate glass, fluoride glass, tellurate glass, chalcogenide glass and a mixture glass fiber thereof.

As a further preferable scheme of the non-quartz glass-based medium infrared special fiber Raman DFB fiber laser, a pumping light source can adopt a fiber laser, a solid laser or a mixed laser system and is used for ensuring that the output power and the central wavelength of the pumping light source meet the production conditions of the medium infrared Raman fiber laser;

if a single-frequency narrow linewidth Raman laser with a 2.5-micron waveband is expected to be generated, a thulium-doped or holmium-doped high-power optical fiber laser can be used as a pumping source;

if it is expected to produce a single-frequency narrow linewidth Raman laser with a linewidth of 2.5 microns or more, the pumping light source can be 2.5-3 microns of chromium-doped zinc sulfide (Cr)²⁺ZnS) crystal laser, 2.7-2.9 micron erbium-doped sesquioxide ceramic laser, or rare earth doped fluoride glass fiber laser.

As a further preferable scheme of the non-quartz glass-based medium infrared special fiber Raman DFB fiber laser, the medium infrared band Raman DFB fiber laser with pi phase shift has the following principle:

step 1, uniformly distributing gratings in a fiber core of a non-quartz glass fiber, wherein the period is lambada, the length is L cm, the pi phase shift is positioned at the Z pi position of the grating, the function of the pi phase shift is to form a unique narrow linewidth transmission window in a DFB grating stop band, namely the central frequency of the DFB fiber grating, the loss of the frequency laser in a resonant cavity is minimum, and the competition of other longitudinal modes is inhibited, so that the single-frequency narrow linewidth laser output is selectively realized at the frequency;

step 2, the grating length for realizing pi phase shift is recorded as delta L-pi/(2 β)_s) Wherein, β_sThe grating is divided into two parts of L1 and L2 for the transmission constant of the signal light; the center wavelength design of the grating satisfies the following relationship:

m·λ_DFB＝2·n_eff·Λ

wherein m is a natural number representing the grating order, n_effIs the core effective refractive index; the central wavelength of the grating is the wavelength lambda s of the Raman DFB signal light, is positioned in the Raman gain bandwidth of the used optical fiber, and satisfies the following relation with the pump light lambda p:

where c is the speed of light, f_RFor first-order Stokes Raman frequency shift, the optical fiber is in single-mode transmission at the wavelengths of pumping and signal light, and the effective mode field area A_effApproximately the fiber core area; a. the_p、_b、_fThe electric field amplitudes of the pump light, the reverse signal light and the forward signal light respectively meet the requirements of respective power density and powerThe following relationships:

|A_p，f，b|²≡I_p，f，b≡P_p，f，b/A_eff

the dynamic relation among the pump light, the forward signal light and the reverse signal light is represented by a two-dimensional nonlinear coupled wave equation system of a time domain and a space domain, and a numerical solution is obtained through a Runge-Kutta high-order iterative algorithm proposed by de Sterke et al;

step 3, under the excitation of continuous pump light, after the system reaches a stable state, the signal change in the time domain can be approximate to zero, the influence of material dispersion on the pump light and the signal light can be ignored, and the nonlinear coupling wave equation system can be simplified into one dimension; wherein g is_p、_sRaman gain coefficients for the pump wavelength λ p and the signal wavelength λ s, respectively, and satisfy the inverse relationship with wavelength: g_p＝g_s·λs/λp；γ_p、_sKerr nonlinear coefficients for pump wavelength and signal wavelength, respectively, α_lp、_lsThe transmission loss of the optical fiber at the pump wavelength and the signal wavelength respectively; kappa is the DFB grating coupling coefficient, delta_βIs detuned from harmonic transmission constant by a magnitude of delta_β＝2πn_eff(1/λ_DFB- λ s), where λ_DFBDesigning a central wavelength for the DFB grating;

the simplified equation sets (4b) and (4c) are converted into partial differential coupling wave equation sets, a matrix conversion method of a coupling wave theory can be adopted, the method is widely used in the field of grating simulation, and a Matlab tool is combined, so that the operation efficiency is improved.

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

the non-doped intermediate infrared special optical fiber is adopted, the length of the optical fiber laser cavity is only the length of a single DFB fiber grating, the DFB fiber grating is 1-100 cm, the structure is compact, the size is small, and the cost of the optical fiber laser is greatly reduced; under the excitation of a pump, according to the relationship of fiber Raman gain frequency shift, a DFB fiber grating with a specific wavelength is designed and prepared, single-frequency laser output with any wavelength can be generated, the heat effect is small, the output laser efficiency is high, and the line width is narrow; by changing the temperature, the tensile force or the pressure of the DFB fiber grating, the continuous tuning of the output laser is realized.

Drawings

Fig. 1 is a wavelength distribution and development history of a DFB fiber laser realized so far, in which a solid line and a broken line are predicted development trends;

FIG. 2 is a diagram of an experimental apparatus of an infrared single-frequency narrow linewidth DFB fiber laser in the present invention;

FIG. 3 is a ZBLAN, Tellurite glass and As of the present invention₂S₃Fiber normalized raman spectra;

fig. 4(a) is a schematic diagram of a raman DFB fiber laser;

FIG. 4(b) illustrates a pump versus signal light wavelength diagram based on a tellurate glass fiber Raman DFB laser;

FIG. 5 is a theoretical calculation of the relationship between Raman DFB laser threshold power and DFB grating length based on fluoride glass (ZBLAN), Tellurite glass (Tellurite), and sulfide glass mid-IR single mode fibers of the present invention;

FIG. 6 is a theoretical calculation chart of the output wavelength range of a middle infrared single-frequency narrow linewidth Raman DFB laser based on fluoride glass (ZBLAN), Tellurite glass (Tellurite) and sulfide glass middle infrared special optical fiber;

FIG. 7 is a schematic diagram of the experimental principle (top) and apparatus (bottom) for tuning wavelength by DFB FBG compression method according to the present invention;

Detailed Description

The technical scheme of the invention is further explained in detail by combining the attached drawings:

the invention provides a non-quartz glass-based medium infrared special fiber Raman DFB fiber laser, which realizes a tunable single-frequency narrow linewidth laser source with any wavelength of more than 2.5 microns in medium infrared.

FIG. 2 is a diagram of an experimental apparatus for implementing the laser, wherein ① is a pump light source, the bandwidth of which is not required, the center wavelength of which is determined by the wavelength of a signal light generated by a target, ② is a dichroic mirror 1, which is highly transparent to the pump light source and highly reflective to the signal light source and used for separating a reverse Raman (Raman) DFB laser signal, ③ is a coupling lens, the Numerical Aperture (NA) and the size of a focusing spot of which are respectively matched with the diameters of an optical fiber NA and a fiber core and improve the coupling efficiency of the pump light entering the fiber core, ④ is a medium infrared special fiber pi phase shift DFB fiber grating which is a core device for generating a medium infrared single-frequency narrow-line width laser signal, ⑤ is a medium infrared lens and used for collimating output signal light and facilitating subsequent detection, ⑥ is a dichroic mirror 2, which is highly transparent to the generated signal light source and highly reflective to the generated signal light source and used for separating residual pump light, ⑦ is a laser signal detection device which comprises a power meter, a spectrometer, a spectrum instrument and the like, and ⑧ is a temperature controller and used for ensuring that the DFB fiber grating.

The non-quartz glass medium infrared special optical fiber comprises germanate glass (GeO)₂Group), fluoride glass (ZrF)₄Base, AlF₃Base, InF₃Basal, etc.), tellurate glass (TeO)₂A chalcogenide glass (an SSeTe base), and a mixture thereof.

The pump light source can be a fiber laser, a solid laser or a hybrid laser system, and the output power and the central wavelength of the pump light source need to be ensured to meet the generation condition of the mid-infrared Raman fiber laser. A single-frequency narrow linewidth raman laser intended to produce a 2.5 micron band may employ a thulium-doped or holmium-doped high power fiber laser as the pumping source. In anticipation of generating a single-frequency narrow linewidth Raman laser with a linewidth of 2.5 microns or more, the pumping light source can be 2.5-3 microns chromium-doped zinc sulfide (Cr)²⁺ZnS) crystal laser, 2.7-2.9 μm erbium-doped sesquioxide ceramic laser, or rare earth-doped fluoride glass fiber laserA device.

Fluoride glass (ZBLAN) commercially available from Fiberlabs of Japan and tellurate glass (TeO) manufactured by laboratories₂-ZnO-BaO) and sulfide glass (As)₂S₃) The single-mode fiber is taken as an example to explain the laser generation principle and the specific implementation of the 3-5 micron single-frequency narrow linewidth Raman fiber.

FIG. 3 shows ZBLAN, tellurite glass (tellurite) and sulfide glass (As)₂S₃) Fiber normalized raman spectrogram. Basic parameters such as peak raman gain coefficient, frequency shift, and bandwidth based on 2.78 μm pump wavelength 3 μm band signal light are shown in table 1. Wherein the ZBLAN fiber has the smallest Raman gain coefficient and narrower bandwidth; the peak Raman gain coefficient of the Tellurite glass fiber is between ZBLAN and As₂S₃The Raman gain frequency shift and the bandwidth are widest, and the wide-range tunable output is most favorably realized; as₂S₃The raman gain coefficient of the fiber is the largest, but the raman shift and bandwidth are smaller. Therefore, Raman-DFB lasers can be realized in the three non-quartz intermediate infrared glass fibers in thousands of years, and flexible selection is performed according to actual requirements.

The principle schematic diagram of the raman DFB fiber laser with pi phase shift in the mid-infrared band is shown in fig. 4(a), the gratings are uniformly distributed in the fiber core of the non-quartz glass fiber, the period is Λ, the length is L cm, the pi phase shift is located at the position of the grating Z pi, the grating output spectrum is shown in fig. 4(b), the pi phase shift has the effect of forming a unique narrow line width transmission window in the stop band of the DFB grating, the frequency has the minimum loss in the resonant cavity, and the competition of other longitudinal modes is inhibited, so that the single-frequency narrow line width laser output is selectively realized at the frequency.

The grating length to achieve pi phase shift is noted as Δ L pi/(2 β)_s) (wherein β_sWhich is a transmission constant of the signal light), the grating is divided into two parts, L1 and L2. The center wavelength design of the grating satisfies the following relationship:

m·λ_DFB＝2·n_eff·Λ (1)

wherein m is a natural number representing the grating order, n_effThe core effective index. The central wavelength of the grating is the Raman DFB signalThe optical wavelength (λ s) is within the raman gain bandwidth of the fiber used, as shown in fig. 4(b), and satisfies the following relationship with the pump light (λ p):

where c is the speed of light, f_RFor first-order Stokes raman shift (for example, tellurate glass, see table 1, summary of basic parameters of three mid-infrared fibers:

TABLE 1

First-order Stokes Raman peak gain frequency shift is 741cm-¹). When the input pump power is increased and reaches a threshold value, the raman DFB signal light is output from the grating in the forward direction (i.e. at the right end of the DFB grating, where z is equal to L) and in the reverse direction (i.e. at the left end of the DFB grating, where z is equal to 0), and the remaining unconverted pump light is output from the right end of the DFB grating. With a continuous or quasi-continuous pump light source, the effective mode field area (A) is provided assuming that the fiber is single mode transmission at both the pump and signal wavelengths_eff) Approximately the fiber core area. A. the_p、_b、_fThe electric field amplitudes of the pump light, the reverse signal light and the forward signal light respectively satisfy the following relations with the power density and the power of the pump light, the reverse signal light and the forward signal light respectively:

|A_p，f，b|²≡I_p，f，b≡P_p，f，b/A_eff(3)

the dynamic relationship between the pump light, the forward signal light and the backward signal light is represented by a two-dimensional nonlinear coupled wave equation system in time domain and space domain, and a numerical solution [ J.Shi, S.Alam, and M.Ibsen, "high efficiency Raman distributed feedback resonators," Optics Express 20(5) 5082-. Because the length of the DFB grating is long, the calculation grid array is very huge, the successive iteration greatly consumes the memory of a computer, and the simulation calculation time is long. Raman-DFB excitation of quartz fiber from earlier stageThe research of the optical instrument shows that a certain noise signal needs to be injected in the theoretical simulation, and the numerical system reaches a convergence stable state through iteration of a coupled wave equation set, so that a laser signal is generated. Under the excitation of continuous pump light, after the system reaches a stable state, the signal change in the time domain can be approximately zero, at this time, the influence of material dispersion on the pump light and the signal light can be ignored, and the nonlinear coupling wave equation system can be simplified into one dimension as shown in the formula (4 a-c). Wherein g is_p、_sRaman gain coefficients of a pump wavelength (λ p) and a signal wavelength (λ s), respectively, and satisfies an inverse relationship (g) with the wavelength_p＝g_s·λs/λp)；γ_p、_sKerr nonlinear coefficients for pump wavelength and signal wavelength, respectively, α_lp、_lsThe transmission loss of the optical fiber at the pump wavelength and the signal wavelength respectively; kappa is the DFB grating coupling coefficient, delta_βIs detuned from harmonic transmission constant by a magnitude of delta_β＝2πn_eff(1/λ_DFB- λ s), where λ_DFBThe center wavelength is designed for the DFB grating.

The simplified equation sets (4b) and (4c) are converted into partial differential coupling wave equation sets, a matrix conversion method of a coupling wave theory can be adopted, the method is widely used in the field of grating simulation, and the operational efficiency is greatly improved by combining a Matlab tool.

Based on the three non-quartz glass mid-infrared fibers in table 1, single-mode fibers with fiber core diameters of 4.5 μm and 7.5 μm are adopted, and the intensity constant (κ L) of the DFB grating with center pi phase shift is assumed to be 12 and remains unchanged, and the simulation result of the relationship between the pump threshold power of the raman DFB fiber laser and the length change of the DFB grating is shown in fig. 5. Obviously, the smaller the core diameter of the single-mode fiber is, the smaller the effective mode field area of the single-mode fiber is, the higher the laser power density and the nonlinear coefficient are, so that the pumping threshold power of the Raman DFB fiber laser can be reduced. As can be seen from fig. 5, the use of 10-20 cm long central pi-phase shifted DFB gratings illustrates that the pumping threshold of all three low-loss mid-infrared fiber raman DFB lasers is within 10W, especially for chalcogenide glass fibers with the highest raman gain coefficient, where the threshold power is expected to be less than 100 mw.

The length of the non-quartz glass DFB fiber grating is 1-100 cm, the grating strength constant (kL) is 4-50, and the pi phase shift is 0.2-0.8, wherein: and L is the total length of the grating.

The above is the principle of first order raman DFB laser signal generation. The first-order Raman DFB fiber laser source is used as a pumping light source of a high-order Raman DFB laser, so that a single-frequency narrow-linewidth Raman DFB laser signal with longer wavelength is generated. Fig. 6 shows the wavelength range that can be covered by the generation of 1-5 order raman DFB laser signals based on ZBLAN, tellurate and sulphide glass mid-infrared fibers, where the long wavelength boundary is limited by the transmission bandwidth of the fiber used in the mid-infrared band.

The tunability of the output wavelength of the Raman DFB laser signal is realized by adopting a method of temperature regulation, stretching and compressing a DFB fiber grating. In general, temperature control and stretched grating tuning wavelength ranges are small, a compressed grating can achieve a large range of wavelength tuning, and in quartz Fiber DFB Lasers, 22.5nm of large range wavelength tuning is achieved by compression [ m.ibsen, s.y.set, g.s.goh, and k.kikuchi, "Broad-Band continuous thin tube All-Fiber DFB Lasers," IEEE Photonics Technology Letters,14(1),21-23(2002). Fig. 7 is a schematic diagram of a principle and experimental apparatus for compressing the DFB fiber grating, in which the DFB fiber grating is encapsulated between elastic beams, the length of the central axis of the fiber distance is d, and the initial length of the elastic beam is L. Under the condition of applied pressure, the bending angle of the elastic beam is theta, the bending radius is R, and the relative compression amount of the DFB grating is epsilon, d/R, d, theta/L, so that the wavelength variation of the Raman-DFB laser can be calculated to be delta lambda (1-rho) approximately equal to (1-rho)_e)*ε*λ_DFBWhere ρ is_eIs the photoelastic coefficient of the material. Therefore, pm-50nm range can be achieved with temperature controlled and compressed DFB fiber gratings.

Claims

1. A middle infrared special fiber Raman DFB fiber laser based on non-quartz glass is characterized in that:

the system comprises a pumping light source, a dichroic mirror 1, a coupling lens, a middle infrared special fiber pi phase shift DFB fiber grating, a middle infrared lens, a dichroic mirror 2, a laser signal detection device and a temperature controller;

the pumping light source is used for providing a pumping light source;

the laser signal detection device is used for detecting a laser signal;

2. The non-quartz glass based mid-infrared special fiber Raman DFB fiber laser as claimed in claim 1, wherein: the numerical aperture and the size of the focusing light spot of the coupling lens are respectively matched with the fiber NA and the fiber core diameter.

3. The non-quartz glass based mid-infrared special fiber Raman DFB fiber laser as claimed in claim 1, wherein: the laser signal detection device comprises a power meter, a spectrometer and a frequency spectrograph.

4. The non-quartz glass based mid-infrared special fiber Raman DFB fiber laser as claimed in claim 1, wherein: the intermediate infrared special fiber pi phase shift DFB fiber grating comprises germanate glass, fluoride glass, tellurate glass, chalcogenide glass and their mixture glass fiber.

5. The non-quartz glass based mid-infrared special fiber Raman DFB fiber laser as claimed in claim 1, wherein: the pumping light source can adopt a fiber laser, a solid laser or a hybrid laser system and is used for ensuring that the output power and the central wavelength of the pumping light source meet the generation conditions of the middle infrared Raman fiber laser;

6. The non-quartz glass based mid-infrared special fiber Raman DFB fiber laser as claimed in claim 1, wherein: the principle of the Raman DFB fiber laser with pi phase shift in the middle infrared band is as follows:

step 2, realizing grating length record of pi phase shiftIs Δ L ═ pi/(2 β)_s) Wherein, β_sThe grating is divided into two parts of L1 and L2 for the transmission constant of the signal light; the center wavelength design of the grating satisfies the following relationship:

m·λ_DFB＝2·n_eff·Λ

where c is the speed of light, f_RFor first-order Stokes Raman frequency shift, the optical fiber is in single-mode transmission at the wavelengths of pumping and signal light, and the effective mode field area A_effApproximately the fiber core area; a. the_p、b、fThe electric field amplitudes of the pump light, the reverse signal light and the forward signal light respectively satisfy the following relations with the power density and the power of the pump light, the reverse signal light and the forward signal light respectively:

|A_p，f，b|²≡I_p，f，b≡P_p，f，b/A_eff

step 3, under the excitation of continuous pump light, after the system reaches a stable state, the signal change in the time domain can be approximate to zero, the influence of material dispersion on the pump light and the signal light can be ignored, and the nonlinear coupling wave equation system can be simplified into one dimension; wherein g is_p、sRaman gain coefficients for the pump wavelength λ p and the signal wavelength λ s, respectively, and satisfy the inverse relationship with wavelength: g_p＝g_s·λs/λp；γ_p、sKerr nonlinear coefficients for pump wavelength and signal wavelength, respectively, α_lp、lsThe transmission loss of the optical fiber at the pump wavelength and the signal wavelength respectively; kappa is the DFB grating coupling coefficient, delta_βIs detuned from harmonic transmission constant by a magnitude of delta_β＝2πn_eff(1/λ_DFB- λ s), where λ_DFBDesigning a central wavelength for the DFB grating;