CN102929070A - Structure for increasing secondary harmonic conversion efficiency of micro-fiber surface - Google Patents

Abstract

The invention provides a structure for increasing secondary harmonic conversion efficiency of a micro-fiber surface. The structure is a groove structure along the length direction of the tapering micro-fiber, and the groove structure is located on the waist of the tapering micro-fiber. The cross section of the groove structure takes the shape of a rectangle, V-shape, rounded rectangle and the like, and the micro-fiber is allowed to be provided with a dual-groove structure and a multi-groove structure in addition to a single-groove structure. The introduction of the groove structure can enlarge the micro-fiber surface area, and effectively enhance the optical field strength of the optical fiber surface, thereby enhancing the secondary nonlinear overlapping integration of the micro-fiber surface, and improving the conversion efficiency of the secondary harmonic conversion efficiency on the micro-fiber surface.

Description

A kind of structure that improves micro optical fiber Surface Second Harmonic conversion efficiency

Technical field

The present invention relates to a kind of microstructure, be specifically related to a kind of micro optical fiber trench structure.

Background technology

It is a kind of general non-linear phenomena that harmonic wave produces, can be for generation of the light of visible region and ultraviolet region.

Strictly speaking, second harmonic produces (SHG) and can not occur in silica fibre, because silicon dioxide (SiO ₂) glass structure shows as inversion symmetry, comes from second order susceptibility χ ⁽²⁾All nonlinear effects can not produce.Yet several early stage experiments show, when 1.06 μ m heavy pumping pulses of Nd:YAG laser instrument generation are transmitted in optical fiber, will produce second harmonic.With frequently be about 0.1% with the conversion efficiency of second harmonic process, so high conversion efficiency is that the surperficial non-linear of fibre core-covering interface and the non-linear institute that comes from electric quadrupole moment and magnetic dipole moment are unaccountable, and therefore certainly existing some other mechanism has caused the SHG phenomenon in the silica fibre.

The initial clue of this mechanism was found in 1986.If with some fiber-optic illuminated several hours, then second-harmonic power will increase substantially with pump light at that time discovery, maximum conversion efficiency can reach 3%.This experiment causes the broad research to SHG in the optical fiber.The result shows, after the intense radiation with some wavelength exposed to optical fiber, because the light sensitive characteristic of optical fiber, its optical characteristics will permanently be changed.The alloy that fiber core refractive index is increased such as germanium and phosphorus, can strengthen the light sensitive characteristic of light.The χ that this experimental phenomena can be formed by the multiphoton process of pump light and second harmonic ⁽²⁾Grating is explained.In this model, χ ⁽²⁾Grating produces by quantum Interference, and this effect has caused depending on the electronic injection process of pump light and second harmonic relative phase.This Charge Transfer Model is consistent with most of feature of observing on qualitative results.

And a nearest experiment is found, can observe significant second harmonic signal in action length only is the pure silicon dioxide nano wire of 100 μ m.Because the pure silicon dioxide light sensitive characteristic is very weak, the χ that therefore can not use pump light and second harmonic multiphoton process to form ⁽²⁾This phenomenon of grating model explanation.In fact because the nano wire yardstick compares ordinary optic fibre and greatly reduces, the interface surperficial non-linear with come from the non-linear to effective second order susceptibility χ of electric quadrupole moment and magnetic dipole moment ⁽²⁾Contribution need to rethink.Nanometer silica line provides stronger optical power density, therefore fibre core-air cladding layer interface surperficial non-linear and come from electric quadrupole moment and magnetic dipole moment non-linear become nanometer silica line (be micro optical fiber, the microfiber) main mechanism of middle SHG phenomenon.In addition, the contrast of refractive index that fibre core and air cladding layer are higher is so that the second harmonic phase matching between basic mode and second order mode becomes possibility.

The present invention utilizes this mechanism just, increases the surface area of micro optical fiber by grooved (slot) microstructure, strengthens the non-linear overlap integral of micro optical fiber surface second, thereby improves the conversion efficiency of micro optical fiber Surface Second Harmonic.

Summary of the invention

The object of the invention provides a kind of micro optical fiber trench structure, by increase the surface area of micro optical fiber along the groove of micro optical fiber length direction, and by the larger optical fiber surface distribution of light intensity of trench structure introducing, thereby strengthen the quadratic nonlinearity overlap integral on surface, improve the Surface Second Harmonic conversion efficiency.

The technical solution used in the present invention is as follows:

A kind of structure that improves micro optical fiber Surface Second Harmonic conversion efficiency is drawing the cone micro optical fiber to have groove structure along its length direction, and described groove is positioned at the waist that draws the cone micro optical fiber.

The cross section of described groove can be the shapes such as rectangle, V-shape, round rectangle, is not limited to single groove structure, can also be double flute and multi-groove structure.The groove width of groove is less than the micro optical fiber waist diameter, and groove depth is less than or equal to the micro optical fiber waist diameter, can obtain optimum translation efficient when groove depth is got the micro optical fiber waist diameter.

The micro optical fiber of high conversion efficiency of second harmonic has a wide range of applications in fields such as fiber laser, super continuum sources, and the application in nonlinear optics has very large facilitation to micro optical fiber to improve conversion efficiency of second harmonic.Microstructure of the present invention can effectively strengthen the non-linear overlap integral of micro optical fiber surface second, thereby improves the conversion efficiency of micro optical fiber Surface Second Harmonic, has certain practical significance.

Description of drawings

Fig. 1 is the schematic diagram of micro optical fiber trench structure of the present invention.

Fig. 2 is Fig. 1 list trench structure micro optical fiber schematic cross-section, wherein h _s, w _sBe respectively the high and groove width of groove, n _Air, n _sBe respectively the refractive index of air and silicon dioxide, d is the diameter of micro optical fiber.

Fig. 3 is micro optical fiber diameter d and second harmonic coupling wavelength X _SHGGraph of relation.

Fig. 4 is circular micro optical fiber and single trench structure micro optical fiber overlap integral absolute value | ρ ₂| with second harmonic coupling wavelength X _SHGGraph of relation.

Fig. 5 is a kind of micro optical fiber micro-nano structure that has proposed, wherein n _Air, n _sIt is respectively the refractive index of air and silicon dioxide.

Fig. 6 is fine schematic cross-section, wherein h of dual-trench type structure micro _S1, h _S2For groove is high, w _S1, w _S2Be groove width, d _sBe separation, n _Air, n _sIt is respectively the refractive index of air and silicon dioxide.

Fig. 7 is the fine overlap integral absolute value of circular micro optical fiber and dual-trench type structure micro | ρ ₂| with second harmonic coupling wavelength X _SHGGraph of relation.

Embodiment

The present invention will be further described in detail below in conjunction with the drawings and specific embodiments.

Take fibre core be silicon dioxide, covering be air draw the cone micro optical fiber as example, principle of the present invention is as follows:

Ignore pumping depletion, adopt the small-signal limit approximate, the SHG process can with under the statement that establishes an equation:

$\frac{{\mathrm{<figure-callout\; id="2"\; label="dA"\; filenames="HDA00002425271400022.png,HDA00002425271400041.png"\; state="\{\{state\}\}">dA</figure-callout>}}_2}{\mathrm{dz}}-i{\mathrm{\&rho;}}_2{A}_1^2\exp \left(\mathrm{i\&Delta;\&beta;z}\right)=0---\left(1\right)$

A wherein ₁, A ₂Respectively the amplitude of basic mode and second harmonic signal light field, Δ β=2 β ₁-β ₂The amount of mismatch (wave vector amount of mismatch) of basic mode and the effective propagation constant of second harmonic, ρ ₂Overlap integral,

${\mathrm{\&rho;}}_2=\frac{{\mathrm{\&omega;}}_2}{4N_1\sqrt{{\mathrm{<figure-callout\; id="2"\; label="N"\; filenames="HDA00002425271400022.png,HDA00002425271400041.png"\; state="\{\{state\}\}">N</figure-callout>}}_2}}{\&Integral;\mathrm{<figure-callout\; id="2"\; label="e"\; filenames="HDA00002425271400022.png,HDA00002425271400041.png"\; state="\{\{state\}\}">e</figure-callout>}}_2^{*}\&CenterDot;P^{\left(2\right)}\mathrm{dS}---\left(2\right)$

ω wherein ₂It is the angular frequency of second harmonic.Integration carries out in the optical fiber cross section, and dS is integral area unit.Field component is all passed through normalization, and normalized factor is:

$N_j=\frac{1}{2}\&Integral;|(e_j^{*}\&times;h_j)\&CenterDot;\hat{z}|\mathrm{dS},(j=\mathrm{1,2})---\left(3\right)$

The Electric and magnetic fields of guided mode can be expressed as respectively

E(r,ω _j)=A _j(ω _j)e _j(r,ω _j)exp(i(β _jz-ω _jt))（4）

H(r,ω _j)=A _j(ω _j)h _j(r,ω _j)exp(i(β _jz-ω _jt))（5）

P ⁽²⁾The second nonlinear polarization intensity, respectively from the contribution of optical fiber surface dipole and body multipole two aspects.By pertinent literature and calculate as can be known, the body multipole is to P ⁽²⁾Contribution can ignore compared to the contribution of optical fiber surface dipole, so the second nonlinear polarization intensity can be similar to surperficial second nonlinear polarization intensity and replace,

$P_{\&perp;}^{\left(2s\right)}={\mathrm{\&epsiv;}}_0{\mathrm{\&chi;}}_{\&perp;}^{\left(2s\right)}{e}_{1\&perp;}^2{\hat{r}}_{\&perp;}---\left(7\right)$

Wherein surperficial second nonlinear polarization intensity is broken down into three components, and S represents optical fiber surface, It is the unit normal vector of optical fiber surface.Surface second order susceptibility χ ^(2s)Three components measure by experiment, by the pertinent literature data as can be known: ${\mathrm{\&chi;}}_{\&perp;}^{\left(2s\right)}=6.3\&times;{10}^3{\mathrm{<figure-callout\; id="2"\; label="pm"\; filenames="HDA00002425271400022.png,HDA00002425271400041.png"\; state="\{\{state\}\}">pm</figure-callout>}}^2/V;$

For circular micro optical fiber, the analytic solution of guided mode electromagnetic field can be tried to achieve by the Maxwell equation group.In calculating, the incident pumping is at basic mode HE ₁₁Middle propagation (corresponding class HE in the microstructure micro optical fiber ₁₁Basic mode), second harmonic is at HE ₂₁Middle generation (corresponding class HE in the microstructure micro optical fiber ₂₁Mould).HE in the cylindrical coordinate _N1Mould (n=1,2 ... .) electric field can be expressed as:

$R_{\mathrm{nr}}\left(r\right)=\frac{1-s_n}{2}{J}_{n-1}\left(\frac{u_n}{a}r\right)-\frac{1+s_n}{2}{J}_{n+1}\left(\frac{u_n}{a}r\right)---\left(13\right)$

$R_{\mathrm{n\&theta;}}\left(r\right)=\frac{1-s_n}{2}{J}_{n-1}\left(\frac{u_n}{a}r\right)-\frac{1+s_n}{2}{J}_{n+1}\left(\frac{u_n}{a}r\right)---\left(14\right)$

$u_n={\mathrm{ak}}_n\sqrt{n_s^2-{\mathrm{<figure-callout\; id="2"\; label="n\; n"\; filenames="HDA00002425271400022.png,HDA00002425271400041.png"\; state="\{\{state\}\}">n</figure-callout>}}_n^{}}$

$w_n={\mathrm{ak}}_n\sqrt{n_n^2-1}---\left(15\right)$

$s_n=\frac{n(u_n^{-2}+w_n^{-2})}{\frac{J_n^{\&prime;}\left(u_n\right)}{u_n{J}_n\left(u_n\right)}+\frac{K_n^{\&prime;}\left(w_n\right)}{w_n{K}_n\left(w_n\right)}}$

J wherein _nBessel function of the first kind, K _nThe Equations of The Second Kind modified Bessel function, k _nThe wave vector of light in the vacuum, n _sThe refractive index (function of optical wavelength) of silicon dioxide, n _n=β _n/ k _nBe the effective refractive index of pattern, a is the micro optical fiber radius,

Be the phase place that determines polarization, two vertical polarization states can be by order Obtain.HE ₁₁Pump light in the mould can only have same phase

HE ₂₁Excite second harmonic signal in the mould.Propagation constant β _nCan obtain by separating eigenvalue equation: $[\frac{J_n^{\&prime;}\left(u_n\right)}{u_n{J}_n\left(u_n\right)}+\frac{K_n^{\&prime;}\left(w_n\right)}{w_n{K}_n\left(w_n\right)}][\frac{J_n^{\&prime;}\left(u_n\right)}{u_n{J}_n\left(u_n\right)}+\frac{1}{n_s^2}\frac{K_n^{\&prime;}\left(w_n\right)}{w_n{K}_n\left(w_n\right)}]$ （16）

$=n^2(\frac{1}{u_{\mathrm{<figure-callout\; id="2"\; label="n"\; filenames="HDA00002425271400022.png,HDA00002425271400041.png"\; state="\{\{state\}\}">n</figure-callout>}}^2}+\frac{1}{w_n^2})[\frac{1}{u_{\mathrm{<figure-callout\; id="2"\; label="n"\; filenames="HDA00002425271400022.png,HDA00002425271400041.png"\; state="\{\{state\}\}">n</figure-callout>}}^2}+\frac{1}{{\left(n_s{w}_n\right)}^2}]$

Can calculate circular micro optical fiber diameter d and second harmonic coupling wavelength X by above formula _SHGRelation curve (shown in Figure 3), and can try to achieve the absolute value of overlap integral | ρ ₂| with λ _SHGVariation relation (shown in Figure 4).

In circular micro optical fiber, introduce the overlap integral that micro-nano structure can change optical fiber.Once introducing micro-nano structure as shown in Figure 5 in circular micro optical fiber of researcher is arranged, this structure can increase the surface area of micro optical fiber, but the field distribution of micro optical fiber is disperseed, reduced the distribution of light intensity of optical fiber surface, these two factor actings in conjunction, the result so that the more circular micro optical fiber of the overlap integral of micro optical fiber slightly reduce.Contrast this structure, trench structure can not only increase the micro optical fiber surface area, and can effectively strengthen the optical fiber surface distribution of light intensity, therefore can effectively increase the overlap integral of micro optical fiber.The below illustrates the advantage of trench structure with numerical result.

The rectangle list groove structural representation of the present embodiment is to introduce groove 2 structures in the micro optical fiber 1 of sub-wavelength magnitude at waist diameter as shown in Figure 1, the xsect of groove 2 as shown in Figure 2, h _s, w _sBe respectively the high and groove width of groove.This structure can adopt the processing of the micro-processing technology means such as focused-ion-beam lithography (FIB) to obtain.Described example rectangle list groove structure is introduced micro optical fiber, make h _s=0.75d, w _s=0.05d in like manner can draw by the method for numerical evaluation above-mentioned curve (Fig. 3, Fig. 4).By relatively finding out of curve, the grooved micro optical fiber of equivalent diameter can be realized the more phase matching of small wavelength second harmonic, and for the co-wavelength second harmonic, the absolute value of its overlap integral is approximately 4 times of typical circular micro optical fiber overlap integral.Through similar calculating, the overlap integral of the trench structure of other shapes (comprising single groove and multiple-grooved) micro optical fiber all is significantly improved.Fine (the high h of groove of dual-trench type structure micro shown in Figure 6 for example _S1=h _S2=0.75d, groove width w _S1=w _S2=0.05d, separation d _s=0.075d), its overlap integral absolute value approximately reaches 5 times (shown in Figure 7) of typical circular micro optical fiber overlap integral.

Under the small-signal limit condition of (pumping depletion can be ignored), when wave vector mismatch Δ β=0, can derive second-harmonic power and satisfy:

P ₂(z)=[ρ ₂P ₁z] ² （17）

Be conversion efficiency of second harmonic:

$\frac{P_2}{P_1}{\left({\mathrm{\&rho;}}_{2}z\right)}^2{P}_1---\left(18\right)$

We can find out thus, conversion efficiency and overlap integral square be directly proportional the overlap integral absolute value | ρ ₂| larger then conversion efficiency of second harmonic is higher, so the present invention can improve conversion efficiency of second harmonic by introduce trench structure in micro optical fiber.

Claims (3)

1. a structure that improves micro optical fiber Surface Second Harmonic conversion efficiency is characterized in that, draws the cone micro optical fiber along having groove structure on its length direction, and described groove is positioned at the waist that draws the cone micro optical fiber.

2. a kind of structure that improves micro optical fiber Surface Second Harmonic conversion efficiency according to claim 1 is characterized in that, the cross section of described groove is rectangle, V font or round rectangle; The quantity of described groove is single or multiple.

3. a kind of structure that improves micro optical fiber Surface Second Harmonic conversion efficiency according to claim 1 and 2 is characterized in that, the groove width of described groove is less than the micro optical fiber waist diameter, and groove depth is less than or equal to the micro optical fiber waist diameter.

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