CN101593925A - A kind of fiber coupling method for diode pumping solid-state laser - Google Patents

Abstract

The present invention relates to the direct fiber coupling technique of high efficiency laser output, particularly a kind of fiber coupling method for diode pumping solid-state laser, it is characterized in that: the laser outgoing mirror is composite mirrors (2) in the diode-pumped solid-state laser, described composite mirrors (2) is a planoconvex spotlight, its planar ends, as the outgoing mirror of resonant cavity, be coated with the reflectance coating planar ends plated film (7) of optical maser wavelength; The convex surface end is coated with the anti-reflection film convex surface end plated film (8) of optical maser wavelength, and transmitted light is converged, and the convex surface end light of composite mirrors (2) connects coupled fiber (3); Focal spot after the output beam of laser converges is less than fiber core, and the angle of divergence is less than the twice numerical aperture of coupled fiber (3).Owing to use composite mirrors to replace the outgoing mirror and the coupled lens of diode pumping solid laser, greatly simplify the structure, operation becomes simple and convenient.The use of composite mirrors solved coupled lens and outgoing mirror can't lean on a very near difficult problem, reached the zero distance of theoretical limit, reduced for numerical aperture and the effectively requirement of clear aperature, help obtaining higher optical coupling efficiency.

Description

A kind of fiber coupling method for diode pumping solid-state laser

Technical field

The present invention relates to the direct fiber coupling technique of high efficiency laser output, particularly a kind of fiber coupling method for diode pumping solid-state laser.

Technical background

The heavy-duty diode light-pumped solid state laser is widely used in fields such as military affairs, industry and medical science, if the output laser energy of DPL is coupled in the optical fiber, so, will become operation for such one operation with laser of certain volume and weight for coupled fiber with certain flexibility, greatly improved the use flexibility, realize the processing of flexibility three-dimensional laser, this technology can be applicable to fields such as industrial cutting, welding, surface treatment, has simplified the design difficulty and the cost of system of processing greatly.The fiber coupling technique of DPL output laser has important use value.

The method that the optical fiber coupling of diode pumping solid laser adopts usually is: in resonant cavity outgoing mirror rear end, assemble the condenser lens of a short focal length, focal spot after the output beam of laser converged is slightly less than fiber core, and the angle of divergence is less than the twice numerical aperture of coupled fiber; Optic fibre input end is handled: peel off protective layer and covering, perpendicular cuts end face, and polishing; Regulate the locus of coupled fiber, make its input be positioned at the waist spot place of converging beam just, fine adjustment optical fiber azimuth, with the method, optical coupling efficiency can surpass 90%.

But, there is certain defective in this kind optical fiber coupled modes: at first, the output beam of DPL is a Gaussian beam, if outgoing mirror is a level crossing, then girdling the waist is positioned on the outgoing mirror in theory, therefore, have only the as close as possible outgoing mirror of the coupled lens of allowing, just can make the light beam beamwidth and the angle of divergence that arrive coupled lens as far as possible little, reduce coupled lens numerical aperture and the effectively requirement of clear aperature, but because structural limitations, it is very near that coupled lens and outgoing mirror can't lean on; Secondly, in actual mechanical process, need regulate coupled lens, optical fiber locus and the angle to output beam simultaneously, operation easier is very big.The existence of coupled lens has increased the complexity of adjustment structure.Directly the optical fiber coupling can be avoided the problems referred to above, but is difficult to realize high coupling efficiency.

Summary of the invention

The purpose of this invention is to provide a kind of simple in structure, coupling is convenient, coupling efficiency is high fiber coupling method for diode pumping solid-state laser.

The object of the present invention is achieved like this, a kind of fiber coupling method for diode pumping solid-state laser, it is characterized in that: the laser outgoing mirror is a composite mirrors in the diode-pumped solid-state laser, described composite mirrors is a planoconvex spotlight, its planar ends, as the outgoing mirror of resonant cavity, be coated with the reflectance coating planar ends plated film of optical maser wavelength; The convex surface end is coated with the anti-reflection film convex surface end plated film of optical maser wavelength, and transmitted light is converged, and the convex surface end light of composite mirrors connects coupled fiber; Focal spot after the output beam of laser converges is less than fiber core, and the angle of divergence is less than the twice numerical aperture of coupled fiber.

Effective clear aperature of described composite mirrors is more than or equal to 1.5mm.

Described composite mirrors diameter is between 5mm～20mm, and focal length is between 5～25mm.

The pumping mode of described diode-pumped solid-state laser is end-pumping or side pumping.

The transmitance T of described reflectance coating planar ends plated film is 10%～35%.

Described composite mirrors focal length calculates according to formula (11),

$F=0.7f/\sqrt{{\left(\frac{4w_0}{d}\right)}^2-1}$

In the formula: f is equivalent confocal cavity parameter, and d is a core diameter, w ₀Be laser radius with a tight waist.

Described coupled fiber incident end is positioned at the position that Gaussian beam focuses on, and vertical incidence.

Operation principle of the present invention is: select suitable composite mirrors parameter, the focal spot after the output beam of laser is converged is slightly less than fiber core, and the angle of divergence is less than the twice numerical aperture of coupled fiber; The protective layer and the covering of optic fibre input end are peelled off; the end face perpendicular cuts; and do polishing; effectively reduce the end face reflection loss; regulate the locus of coupled fiber; make its input be positioned at the waist spot place of converging beam just, the spatial attitude of fine adjustment optical fiber makes that laser energy is as much as possible to be coupled in the optical fiber.The use of composite mirrors is integrated coupled lens and outgoing mirror, has solved the very near difficult problem that both can't lean on, and makes that in theory the distance between them equals zero, and has promptly leaned against together fully.Use this kind method can obtain 90% above optical coupling efficiency.

Advantage of the present invention is: owing to use composite mirrors to replace the outgoing mirror and the coupled lens of diode pumping solid laser, greatly simplify the structure, operation becomes simple and convenient.The use of composite mirrors solved coupled lens and outgoing mirror can't lean on a very near difficult problem, reached the zero distance of theoretical limit, reduced for numerical aperture and the effectively requirement of clear aperature, help obtaining higher optical coupling efficiency; The present invention is applicable to the diode-pumped solid-state laser of various kinds, and the pumping mode can be end-pumping, side pumping, and working method can be continuous, Q impulse, and laser medium material can be Nd:YAG, Nd:VYO ₄, NdGdVO ₄Deng, the laser medium shape can be pole, lath, disc, and is suitable equally for the diode-pumped solid-state laser of inner cavity frequency-doubling.

Description of drawings

Fig. 1 realizes direct optical fiber coupling schematic diagram for using composite mirrors in the end-face pump solid laser;

Fig. 2 is the composite mirrors schematic diagram;

Fig. 3 is the coupled fiber schematic diagram;

Fig. 4 realizes direct optical fiber coupling schematic diagram for using composite mirrors in the light-pumped solid state laser of side;

Fig. 5 uses composite mirrors to realize direct optical fiber coupling schematic diagram for transferring in the Q diode pumping solid laser;

Fig. 6 realizes direct optical fiber coupling schematic diagram for using composite mirrors in the diode pumping frequency double laser;

Among the figure, 1, laser crystal, 2, composite mirrors, 3, coupled fiber, 4, thermal lens; 5, pumping light, 6, planoconvex spotlight, 7, the planar ends plated film, 8, convex surface end plated film, 9, fiber cores; 10, fiber cladding, 11, the fiber optic protection layer, 12, Q switching, 13, frequency-doubling crystal, 14, harmonic reflection mirror.

Embodiment

Diode-pumped solid-state laser has the characteristics of compact conformation, reliability height, good beam quality, and for rhabdolith, common pumping mode is divided into two kinds of end-pumping and side pumping.

Fig. 1 provides the end-pumping mode, pumping light 5 to laser crystal 1 (as Nd:YAG, Nd:YVO ₄Deng) end face inject, laser crystal 1 end face is coated with usually that pump light is anti-reflection, oscillation light high-reflecting film layer (as AR@808nm/HR@1064nm), as the completely reflecting mirror of resonant cavity.Usually adopt level crossing as outgoing mirror, at this moment, resonant cavity is Ping-Ping chamber, belongs to metastable resonators.During laser works, laser crystal 1 end face can produce strong thermal effect, and it can just because of the existence of thermal lens 4, make resonant cavity become stable cavity with thermal lens 4 equivalences of end.The end-pumping mode can obtain better beam quality.But, because near the strong thermal effect pumping end surface has limited the increase of power output.

Figure 4 shows that side pumping mode, compare that pumping mode in side can obtain higher power output, but beam quality is relatively poor with the end-pumping mode.

The present invention has used composite mirrors 2, both as the outgoing mirror of laser, again output beam is played the effect of converging.

The structure of composite mirrors 2 as shown in Figure 2, for one side is of a size of the Φ D (planoconvex spotlight 6 of 5mm≤D≤20mm), focal length F is 5mm～25mm, plane one side is coated with the partial reflection membrane plane end plated film 7 (transmitance T is 10%～35%) of laser works wavelength, convex surface one side is coated with the anti-reflection film convex surface end plated film 8 of identical wavelength, and effective clear aperature DL of composite mirrors equals 1.5mm greatly.

Coupled fiber 3 is referring to Fig. 3, and coupled fiber 3 comprises fiber cores 9, fiber cladding 10 and fiber optic protection layer 11, for fear of when coupling composite mirrors focus on the fiber cladding 10 of coupled fiber 3 or the fiber optic protection layer 11 and burn optic fibre end; Simultaneously in order to obtain higher optical coupling efficiency, require the angle of divergence θ of incident beam to equal angle, the effective aperture α of optical fiber for a short time, and the focal spot size w after converging equal core diameter d for a short time.Angle, effective aperture α=2arcsin (NA) ≈ 2NA of optical fiber, wherein, NA is the numerical aperture of optical fiber, and is very little usually, for example, the numerical aperture span of CCITT suggestion multimode fiber is 0.18～0.23.

The concrete operations step is: at first, coupled fiber 3 input sections are peelled off fiber optic protection layer 11 and the fiber cladding 10 of about 6mm, and use the fiber cut cutter that input is cut about 1mm, and microscopically is observed fiber end face, if not cut surface out-of-flatness need be done polishing; Secondly, regulate the optical axis of composite mirrors 2 and the optical axis coincidence of laser crystal 1, and regulate the pitching and the scanning of composite mirrors 2, make laser export the laser power maximum; Once more, coupled fiber 3 is fixed on the device for precisely regulating, input end face is positioned near composite mirrors 2 focuses, and meets to the laser incident direction, and the output of optical fiber is aimed at laser power meter; At last, accurately regulate the position and the attitude of coupled fiber, the situation of change of Real Time Observation laser power is till the hot spot of output laser is single speck and power maximum.So far, just finished all operations.

Above-mentioned laser can use the end-pumping mode, sees Fig. 1, also can be used for side pumping mode, sees Fig. 4, and is suitable equally for Q-switched laser, sees Fig. 5, Q switching 12 can be acousto-optic, electric light with passive, mode of operation is the same.

For frequency double laser, the present invention is suitable equally, see that Fig. 6 is in conjunction with shown in Figure 2, the harmonic reflection mirror 14 of frequency double laser is high saturating for fundamental frequency light, high anti-for frequency doubled light, frequency-doubling crystal 13 can be selected KTP, LBO etc. for use, and the planar ends plated film 7 of composite mirrors 2 is high anti-, the frequency doubled light high transmittance film of fundamental frequency light, and convex surface end plated film 8 is the frequency doubled light high transmittance film.At this moment, before carrying out aforesaid operations, need regulate the locus and the attitude of laser crystal, harmonic reflection mirror, frequency-doubling crystal.

Being calculated as follows of composite mirrors focal length is described:

Thermal focal length calculates according to (1) formula,

$f_T=\frac{\mathrm{\π}{K}_c{\mathrm{\ω}}^2}{\mathrm{\δ}{P}_{\mathrm{in}}(\mathrm{dn}/\mathrm{dT})}\frac{1}{1-\exp (-\mathrm{\αl})}$ Formula (1)

In the formula, K _CBe thermal conductivity, ω is the radius of pumping light, and δ is the percentage of the shared absorbed power of hear rate, P _InBe draw power, dn/dT be crystal refractive index with the variation of temperature rate, α is an absorption coefficient, l is a crystal length.

For end-face pump solid laser, thermal lens is positioned at the pumping end, can equivalence be a concave mirror usually, and radius of curvature is R=f _TTherefore, when outgoing mirror was level crossing, resonant cavity was a flat-concave cavity.In order to guarantee the resonant cavity steady operation, require R 〉=L, L is long for the resonant cavity chamber.

According to Gaussian Beam Theory, can obtain the spatial distribution of the fundamental-mode gaussian beam that this resonant cavity produces, see formula (2), formula (3) and formula (4).

$f=\sqrt{L(R-L)}$ Formula (2)

$w\left(z\right)=\sqrt{\frac{\mathrm{f\λ}}{\mathrm{\π}}[1+{\left(\frac{z}{f}\right)}^2]}=w_0\sqrt{1+{\left(\frac{z}{f}\right)}^2}$ Formula (3)

$\mathrm{\θ}=2\sqrt{\frac{\mathrm{\λ}}{\mathrm{f\π}}}$ Formula (4)

Wherein, f is equivalent confocal cavity parameter, and w (z) is a beam radius, and the origin of coordinates of z is positioned on the level crossing, and θ is far-field divergence angle (full-shape).

The light beam of resonant cavity output can produce by composite mirrors the time and converge, by the transmission theory of Gaussian beam as can be known

${w_0}^{\′}=\frac{w_0}{\sqrt{1+{\left(\frac{f}{F}\right)}^2}}$ Formula (5)

${z_0}^{\&prime;}=\frac{\mathrm{<figure-callout\; id="1"\; label="F"\; filenames="A2009103040010009H1.png,A2009103040010010H2.png"\; state="\{\{state\}\}">F</figure-callout>}}{1+{\left(\frac{F}{f}\right)}^2}<F$ Formula (6)

$k=\frac{{w_0}^{\&prime;}}{w_0}=\frac{1}{\sqrt{1+{\left(\frac{f}{F}\right)}^2}}<1$ Formula (7)

Wherein, W ₀' and Z ₀' being respectively hot spot beam waist and position after converging, K is a magnification ratio.

Obviously, in order to obtain high optical coupling efficiency, the optical fiber input end should be positioned at the position that Gaussian beam focuses on, and vertical incidence, makes ${w_0}^{\&prime;}<\frac{d}{2}$ (d is a fibre core diameter).Can calculate W with a tight waist ₀The percentage that laser power in the ' scope accounts for gross power is

$\mathrm{\&eta;}=\frac{{\&Integral;}_0^{2\mathrm{\&pi;}}{\&Integral;}_0^{{w_0}^{\&prime;}}{I}_0\exp (-2\frac{r^2}{w_0^{\&prime;2}})}{{\&Integral;}_0^{2\mathrm{\&pi;}}{\&Integral;}_0^{\&infin;}{I}_0\exp (-2\frac{r^2}{w_0^{\&prime;2}})}=1-e^{-2}=86.47\%$ Formula (8)

As seen, if fibre diameter equals the beam waist size, the theoretical maximum of coupling efficiency is 86.47%; And the percentage that laser power accounts for gross power in the twice waist spot size is

$\mathrm{\&eta;}=\frac{{\&Integral;}_0^{2\mathrm{\&pi;}}{\&Integral;}_0^{2{w_0}^{\&prime;}}{I}_0\exp (-2\frac{r^2}{w_0^{\&prime;2}})}{{\&Integral;}_0^{2\mathrm{\&pi;}}{\&Integral;}_0^{\&infin;}{I}_0\exp (-2\frac{r^2}{w_0^{\&prime;2}})}=1-e^{-4}=98.17\%$ Formula (9)

Therefore, in order to obtain higher coupling efficiency, can get ${w_0}^{\&prime;}<\frac{\mathrm{<figure-callout\; id="4"\; label="d"\; filenames="A2009103040010010H2.png"\; state="\{\{state\}\}">d</figure-callout>}}{4},$ Like this, specific requirement has been proposed for the focal length F of composite mirrors, promptly

$F<\frac{f}{\sqrt{{\left(\frac{4w_0}{d}\right)}^2-1}}$ Formula (10)

Consider that high power DPL output beam is not the high-rder mode composition that includes significant proportion usually, practical laser waist spot is bigger than theoretical value, and therefore, focal length F should be slightly less than theoretical value; In addition, focal length F can not be too small, otherwise the influence of aberration is serious.Take all factors into consideration, desirable composite mirrors focal length is

$F=0.7f/\sqrt{{\left(\frac{4w_0}{d}\right)}^2-1}$ Formula (11)

For end-pumping Nd:YAGLaser, draw power P _In=20W, the long L=120mm in chamber, Optical Fiber Numerical Aperture NA=0.22, core diameter d=400 μ mIn addition, mix for 1.0% Nd:YAGCrystal, α=5cm ^-1, l=10mm, K _c=14W/ (mK), dn/dT=7.3 * 10 ^-6K ^-1, δ is taken as 0.3, and ω is taken as 0.4mm.

Calculate and to get by formula (1), formula (2) and formula (3): R=f _T=161.8mm, f=70.8mm, w ₀=0.15454mm; Can get by formula (8): F=42.1mm; Bringing formula (5), formula (6) and formula (7) into can get: w ₀'=0.079mm, z ₀'=31.1mm, k=0.5112.

Claims (7)

1. fiber coupling method for diode pumping solid-state laser, it is characterized in that: the laser outgoing mirror is composite mirrors (2) in the diode-pumped solid-state laser, described composite mirrors (2) is a planoconvex spotlight, its planar ends, as the outgoing mirror of resonant cavity, be coated with the reflectance coating planar ends plated film (7) of optical maser wavelength; The convex surface end is coated with the anti-reflection film convex surface end plated film (8) of optical maser wavelength, and transmitted light is converged, and the convex surface end light of composite mirrors (2) connects coupled fiber (3); Focal spot after the output beam of laser converges is less than fiber core, and the angle of divergence is less than the twice numerical aperture of coupled fiber (3).

2. a kind of fiber coupling method for diode pumping solid-state laser according to claim 1 is characterized in that: effective clear aperature of described composite mirrors (2) is more than or equal to 1.5mm.

3. a kind of fiber coupling method for diode pumping solid-state laser according to claim 1 is characterized in that: described composite mirrors (2) diameter is between 5mm～20mm, and focal length is between 5～25mm.

4. a kind of fiber coupling method for diode pumping solid-state laser according to claim 1 is characterized in that: the pumping mode of described diode-pumped solid-state laser is end-pumping or side pumping.

5. a kind of fiber coupling method for diode pumping solid-state laser according to claim 1 is characterized in that: the transmitance T of described reflectance coating planar ends plated film (7) is 10%～35%.

6. a kind of fiber coupling method for diode pumping solid-state laser according to claim 3 is characterized in that: described composite mirrors (2) focal length calculates according to formula (11),

$F=0.7f/\sqrt{{\left(\frac{4w_0}{d}\right)}^2-1}$

In the formula: f is equivalent confocal cavity parameter, and d is a core diameter, and w0 is a laser radius with a tight waist.

7. a kind of fiber coupling method for diode pumping solid-state laser according to claim 1 is characterized in that: described coupled fiber (3) incident end is positioned at the position that Gaussian beam focuses on, and vertical incidence.

Images