CN100360967C - Small mode-field fiber lens and manufacture thereof - Google Patents

Abstract

A fiber lens includes a graded-index lens (308), a single-mode fiber (310) disposed at a first end of the graded-index lens (308), and a refractive lens (306) having a hyperbolic or near-hyperbolic shape disposed at a second end of the graded-index lens (308) to focus a beam from the single-mode fiber (310) to a diffraction-limited spot.

Description

Optical fiber lens and manufacture method

The cross reference of related application

The application requires the U.S. Provisional Application No.60/437 that on Dec 31st, 2002 is that submit, be entitled as " little mould field optical fiber lens ", the right of priority that on October 31st, 328 and 2003 is that submit, be entitled as the U.S. Patent application 10/699,450 of " little mould field optical fiber lens ".

Background technology

The present invention relates generally to the optical devices that are used to be coupled in the optical signalling between the optical element.More particularly, the present invention relates to be used to be coupled in the optical fiber lens of the signal between the optical element.

In optical communication, adopted the whole bag of tricks to be coupled in optical signalling between the optical element, for example optical fiber, laser diode and semiconductor optical amplifier.A kind of method relates to the use of optical fiber lens, and it is the single unit system of the lens of an end with the pigtail fibers of being arranged on.Luminous energy scioptics or pigtail fibers enter or leave this optical fiber lens.This optical fiber lens can become to have the hot spot of desired size and brightness from the optical convergence of pigtail fibers on selected focal length.Yet prior art exists and to have that how little spot size could realize the limitation of desired Luminance Distribution and the limitation of attainable focal length when controlling spot size and Luminance Distribution.To some application, be desirably under the situation of focal length greater than 5 μ m places maintenance Gauss Luminance Distribution and obtain the same little mode field diameter with 2.5 to 3.0 μ m.The embodiment of these applications comprises from the semiconductor optical amplifier to optical fiber, the optical signalling coupling from high index of refraction (index) semiconductor or dielectric waveguide to optical fiber etc.

According to foregoing, expect a kind of optical fiber lens, it can produce has little mode field diameter and for the focused light spot of the desired Luminance Distribution of the focal length of wide region.

Summary of the invention

On the one hand, the present invention relates to a kind of optical fiber lens, it comprises gradual index lens, is arranged on the single-mode fiber and the refractor of this gradual index lens first end, this refractor is arranged on this gradual index lens the second end, have shape hyp or the approximate Double curve, be used for the beam convergence from this single-mode fiber is become the hot spot of diffraction-limited.

On the other hand, the present invention relates to a kind of optical fiber lens, the lens that it comprises single-mode fiber and is arranged on this single-mode fiber one end, wherein, the mode field diameter of the beam waist that presents at lens one end is less than 10 μ m, and from these lens should end to the ratio of distance with a tight waist and mode field diameter with a tight waist greater than 5.

Another aspect, the present invention relates to a kind of method of making optical fiber lens, it comprises that the joint single-mode fiber to graded index fiber, excises the length of this graded index fiber to expectation, and this graded index fiber one end rounding is become shape hyp or the approximate Double curve.

On the other hand, the present invention relates to a kind of method of making optical fiber lens, it comprises that the joint single-mode fiber is to graded index fiber, excise the length of this graded index fiber to expectation, engage hollow-core fiber to this graded index fiber, excise the length of this hollow-core fiber, and this hollow-core fiber one end rounding is become shape hyp or the approximate Double curve to expectation.

These and other feature and advantage of the present invention will be by to following detailed description of the present invention with discussed in more detail in conjunction with the accompanying drawings.

Description of drawings

The present invention passes through embodiment in conjunction with the accompanying drawings, but is not limited to these embodiment in the accompanying drawing, and reference marker identical in the accompanying drawing is represented components identical, wherein:

Figure 1A is optical fiber lens synoptic diagram according to an embodiment of the invention.

Figure 1B is the geometric representation of hyperbolic lens.

Fig. 1 C illustrates has the optical fiber lens that inserts the centreless spacer rod between grin lens and the refractor.

The curve map that Fig. 2 A changes with focal length for hyperbolic lens end radius-of-curvature under the situation that is arranged on single mode pigtail fibers one end when hyperbolic lens.

Fig. 2 B illustrates the variation of the MFD of hyperbolic lens end in Fig. 2 A illustrated embodiment with focal length.

Fig. 3 is the beam propagation synoptic diagram via optical fiber lens of the present invention.

Fig. 4 A is the plane and geometric representation light beam wave front that disperse.

The variation synoptic diagram with formation near-hyperbolic lens of Fig. 4 B for hyp shape is made.

Fig. 5 is the curve map that the far field Luminance Distribution of optical fiber lens according to an embodiment of the invention changes with far-field divergence angle.

Embodiment

Now, will and describe the present invention in conjunction with the accompanying drawings in detail by some preferred embodiments.In ensuing explanation, many details have been set forth so that complete understanding of the present invention to be provided.Yet, it is evident that those of ordinary skills can some or all these details realize the present invention.In other situation, operation of knowing and/or feature are not described in detail, to avoid unnecessarily obscuring the present invention.The features and advantages of the present invention can be by being better understood with ensuing argumentation with reference to the accompanying drawings.

The embodiment of the invention provides a kind of optical fiber lens, and it can become the hot spot that has the size and the Luminance Distribution of expection on the distance of the application's expectation with the optical convergence from optical fiber.This optical fiber lens produces convergent beam by engaging refractor and graded index (GRIN) lens.In one embodiment, this refractor has hyp shape, is used for collimated beam is become the hot spot of diffraction-limited.In another embodiment, this refractor has the shape of approximate Double curve, is used for uncollimated rays is converged to the hot spot of diffraction-limited.By the multimode parameter of control grin lens and the shape of refractor, realize for example little mode field diameter from 2 to 5 mu m ranges, that have reasonable Gauss's Luminance Distribution (MFDs).In addition, when keeping this little MFDs and Gauss's Luminance Distribution, obtain the same big long focal length with 25 to 40 μ m in 1550nm operating wave strong point.

Fig. 1 illustrates optical fiber lens 100 according to an embodiment of the invention.This optical fiber lens 100 comprises graded index (GRIN) lens 102, be arranged on the refractor 104 of these grin lens 102 1 ends and be arranged on the single mode pigtail fibers 106 of these GRIN optical fiber lens 102 other ends.This grin lens 102 has core 108, and it can be subjected to or not be subjected to the constraint of covering 110.The core 108 of this grin lens 102 preferably has the index distribution that radially increases along the optical axis of optical fiber lens 100, for example square type of law or parabolic type.In one embodiment, this refractor 104 is for having the hyperbolic lens of hyperbolic surface 112.This hyperbolic lens 104 has core 114, and it can be subjected to or not be subjected to the constraint of covering 116.In theory, this core 114 should have homogeneous refractive index, still, is polished to the perhaps easier formation refractor 104 of hyperbolic surface 112 by the end with grin lens 102, in this case, core 114 will have the index distribution that radially increases along the optical axis of optical fiber lens 100.

The shape of hyperbolic lens 104 provides as follows:

$\frac{u^2}{a^2}-\frac{v^2}{{\mathrm{<figure-callout\; id="2"\; label="b"\; filenames="C20038010799000171.png,C20038010799000211.png"\; state="\{\{state\}\}">b</figure-callout>}}^2}=1---\left(1a\right)$

Figure 1B is the diagrammatic representation of above-mentioned expression formula.In the figure, hyperbolic lens 104 is a hyp part in the u-v coordinate system, and (a, 0) that the summit of this hyperbolic curve part is positioned at the u axle is located.The focus of this hyperbolic curve part is positioned at (c, 0), and c provides as follows:

$c=\sqrt{a^2+b^2}---\left(1b\right)$

This hyperbolic curve partly comprises two asymptotic lines, is expressed as follows:

bu±av＝0 (1c)

Asymptotic slope is+b/a and-b/a.Asymptotic line locates to intersect to form the wedge shape with apex angle at initial point (0,0), and α provides as follows:

α＝2tan ^-1(b/a) (1d)

According to people's such as Edwards desirable hyperbolic shape, it just in time is transformed into plane wave with the incident spherical wave, and factor a and b in the equation (1a) to (1d) obtain by following formula:

$a^2={\left(\frac{n_2}{{\mathrm{<figure-callout\; id="1"\; label="n"\; filenames="C20038010799000171.png,C20038010799000211.png"\; state="\{\{state\}\}">n</figure-callout>}}_1+n_2}\right)}^2{\mathrm{<figure-callout\; id="2"\; label="r"\; filenames="C20038010799000171.png,C20038010799000211.png"\; state="\{\{state\}\}">r</figure-callout>}}_2^2---\left(2a\right)$

$b^2=\left(\frac{n_1-n_2}{{\mathrm{<figure-callout\; id="1"\; label="n"\; filenames="C20038010799000171.png,C20038010799000211.png"\; state="\{\{state\}\}">n</figure-callout>}}_1+n_2}\right)r_2^2---\left(2b\right)$

N wherein ₁Be hyperbolic lens core refractive index, n ₂Be the refractive index of hyperbolic lens core surrounding medium, r ₂Be hyperbolic lens end radius-of-curvature.(Edwards, ChristopherA., Presby, Herman M., and Dragone, Corrado. " is used for the desirable lenticule of laser instrument to optical fiber coupling " lightwave technology periodical, Vol11, No.2, (1993): 252.) have such hyperbolic shape, the spot size that being positioned at shown in Figure 1B located plane (1) and (2) equates, and the radius-of-curvature that (2) are located on the plane is for infinitely great, that is the light beam wave front that, (2) are located on the plane is a planar.

Get back to Figure 1A, this pigtail fibers 106 can be the single-mode fiber of any standard, for example CorningSMF-28 ^Optical fiber, or special single-mode fiber, for example polarization keeps (PM) optical fiber.When the end is seen, this pigtail fibers 106 can be circular symmetry or have other shape, for example, square or oval.This grin lens 102 preferably is fixed on this pigtail fibers 106.From reliability and long-time stability, this grin lens 102 can engage by welding and be fixed on this pigtail fibers 106.This hyperbolic lens 104 can directly form on the centreless spacer rod that forms or be attached to this grin lens 102 on this grin lens 102.Shown in Fig. 1 C, this hyperbolic lens 104 also can be attached to centreless spacer rod 120, and this centreless spacer rod 120 is attached to grin lens 102.Because the last size of refractor and spacer rod is very little usually, in a preferred method, at first will be attached to pigtail fibers than long GRIN optical fiber or spacer rod, then before forming refractor in the end with its excision or decompose to desired length (centreless spacer rod also can between this grin lens 102 and pigtail fibers 106).This hyperbolic lens 104 can be configured as by the optical fiber that makes a segment length to have a drift angle taper/wedge shape of (α among Figure 1B) forms.For example, this optical fiber laser capture microdissection machining that can handle (for symmetry) by taper-cut or have a polishing function forms taper/wedge shape.Then, form the hyperbolic shape of bending curvature in the taper that obtains/tapered end to obtain expecting.Though do not illustrate in the accompanying drawings, this grin lens and/or this single mode pigtail fibers can be tapers.The integral diameter of this pigtail fibers can be littler or equal substantially than the integral diameter of this grin lens.

This grin lens 102 and hyperbolic lens 104 produce convergent beams, and it has little mode field diameter (MFD), good wave front characteristic and the focal length of growing.In one embodiment, following feature is desired: place's mode field diameter with a tight waist (MFD) is less than 10 μ m, preferably in the scope of 2 to 5 μ m, has rational Gauss's Luminance Distribution, focal length is greater than 5 μ m, preferably in the scope of 20 to 60 μ m, from the lens end to the ratio of distance with a tight waist and MFD with a tight waist greater than 5, and for the coupling efficiency between the operation wavelength lens in 250 to 2000nm scopes greater than 65%.This hyperbolic lens 104 and grin lens 102 are for realizing that little MFDs and long focal length all are important.For example, if do not use this grin lens 102, the spot size of hyperbolic lens 104 ends will be limited in the MFD of single mode pigtail fibers 106, and it will limit attainable focal length to little numerical value.For example, the single-mode fiber of most of practical application has the MFD of 10-12 mu m range in the operating wave strong point of 1550nm.Because single-mode fiber has the angle of divergence of MFD and the 38 μ m of 10 μ m, its convergence MFD for 3 μ m is essential, if only use hyperbolic lens, the longest focal length will be limited in about 14 μ m.

In order to further specify the importance of using hyperbolic lens 104 and grin lens 102 simultaneously, with reference to the funtcional relationship of the focal length of the radius-of-curvature of the hyperbolic lens end shown in the figure 2A and optical fiber lens, this optical fiber lens is provided with hyperbolic lens at single mode pigtail fibers one end that does not insert grin lens.This illustrates optical fiber lens and has good wave front characteristic.Fig. 2 B illustrates the MFD of hyperbolic lens end of embodiment shown in Fig. 2 A with the function of focal length.This illustrates in order to obtain the convergence MFDs in the 2.0-3.5 mu m range, and the MFD of hyperbolic lens end must be greater than 10 μ m to realize that focal length is greater than 20 μ m.Unless between hyperbolic lens and single mode pigtail fibers, insert grin lens, otherwise, the MFD of hyperbolic lens end use the single-mode fiber that the most generally uses can not realize much bigger focal length, because will be limited in the MFD of single mode pigtail fibers than 20 μ m.

Fig. 3 illustrates the light beam 300 that passes plane (1), (2), (3) and (4) transmission.Plane (1) comprises an end face of optical devices 302.Plane (2) overlaps with the end of optical fiber lens 304.Plane (3) overlaps with surface of contact between hyperbolic lens 306 and grin lens 308.Plane (4) comprises an end face of single mode pigtail fibers 310.Suppose that these optical devices 302 have 2w ₀MFD, and the distance that is arranged on from this optical fiber lens 304 ends is the d place.In this case, expectation is design optical fiber lens 304 like this, when operation wavelength 8, the focused light spot size of this optical fiber lens 304 at d place, distance optical fiber lens 304 end as much as possible near 2w ₀The characteristic of the light beam of these grin lens 308 ends and the characteristic of hyperbolic lens 306 have determined the spot size feature of convergent beam.In one embodiment, the design procedure of optical fiber lens 304 comprises (1) calculating needed radius-of-curvature in optical fiber lens 304 ends and spot size, to produce the focused light spot size, (2) decide the shape of hyperbolic lens 306 by the radius-of-curvature that calculates, and (3) decide the parameter of grin lens 308 by the mould field of the mould field that calculates and pigtail fibers 310.It below is the argumentation of carrying out a kind of possible method of this step.

For step (1), be positioned at plane (2), that is, and the spot size (w of optical fiber lens 304 ends ₂) and radius-of-curvature (r ₂) can adopt the Gaussian beam propagation formula of knowing to decide.For example, people such as above-mentioned Edwards provide about w ₂And r ₂Following expression formula:

${\mathrm{<figure-callout\; id="2"\; label="w"\; filenames="C20038010799000171.png,C20038010799000211.png"\; state="\{\{state\}\}">w</figure-callout>}}_2=w_0\sqrt{1+{\left(\frac{\mathrm{\&lambda;d}}{\mathrm{\&pi;}{w}_0^2}\right)}^2}---\left(2a\right)$

${\mathrm{<figure-callout\; id="2"\; label="r"\; filenames="C20038010799000171.png,C20038010799000211.png"\; state="\{\{state\}\}">r</figure-callout>}}_2={\left(\frac{\mathrm{\&pi;}}{\mathrm{\&lambda;}}\right)}^2\frac{{\left(w_0{w}_2\right)}^2}{d}---\left(2b\right)$

For step (2), can be by from equation (2a) and the radius-of-curvature (r that (2b) obtains ₂) decide the shape of this hyperbolic lens 306 together with equation (1a)-(1d).

For step (3), this grin lens 308 will be positioned at plane (3) and locate, has spot size w ₃With radius-of-curvature r ₃Light beam be transformed into and be positioned at plane (4) and locate, have spot size w ₄With radius-of-curvature r ₄Light beam.For optimal design, w ₄Need as much as possible spot size w near single mode pigtail fibers 310 _pA kind of method that realizes this optimal design is to select this special single mode pigtail fibers, its w _pW with the grin lens 308 that can utilize easily ₄Equate.On the other hand, the parameter of this grin lens 308 can be selected like this, its w ₄As far as possible near particular value W _pIn this case, standard single-mode fiber, for example Corning SMF-28 ^Optical fiber can be used as pigtail fibers.This grin lens parameter comprises the dependent index of refraction difference between core diameter, outside (or covering) diameter, index distribution, fibre core and the covering and the length of grin lens.In one embodiment, the core diameter of this grin lens is in the scope of about 50 to 500 μ m, and its outer dia is in the scope of about 60 to 1000 μ m.With the dependent index of refraction difference of the compatible mutually high silicon constituent of the optical fiber that in optical communication system, uses preferably in about scope of 0.5 to 3%.

For r ₃The hyperbolic lens situation of=∞, that is, light beam on the plane (3) locate to be plane front and w ₃=w ₂, the length of this grin lens 308 is condensed into 1/4th spacings.In this simple scenario, according under establish an equation spot size w ₃And w ₄With this grin lens parameter correlation:

$w_3\&CenterDot;w_4=\frac{\mathrm{\&lambda;}}{\mathrm{\&pi;ng}}---\left(3a\right)$

Wherein

$g=\frac{{\left(2\mathrm{\&Delta;}\right)}^{1/2}}{a}---\left(3b\right)$

Wherein g is a focusing parameter, and a is the fiber core radius of grin lens, and Δ is the fibre core of this grin lens and the refractive index difference between the covering.The formula of this 1/4th spacing (L/4) provides as follows:

$\frac{\mathrm{<figure-callout\; id="4"\; label="L"\; filenames="C20038010799000211.png,C20038010799000231.png"\; state="\{\{state\}\}">L</figure-callout>}}{4}=\frac{\mathrm{\&pi;}\&CenterDot;a}{2\&CenterDot;\left({(2\&CenterDot;\mathrm{\&Delta;})}^{1/2}\right)}---\left(3c\right)$

Wherein

$\mathrm{\&Delta;}=({\mathrm{<figure-callout\; id="1"\; label="n"\; filenames="C20038010799000171.png,C20038010799000211.png"\; state="\{\{state\}\}">n</figure-callout>}}_1^2-{\mathrm{<figure-callout\; id="2"\; label="n"\; filenames="C20038010799000171.png,C20038010799000211.png"\; state="\{\{state\}\}">n</figure-callout>}}_2^2)/(2\&CenterDot;{\mathrm{<figure-callout\; id="1"\; label="n"\; filenames="C20038010799000171.png,C20038010799000211.png"\; state="\{\{state\}\}">n</figure-callout>}}_1^2)---\left(3d\right)$

Wherein L is a spacing, n ₁Be the fiber core refractive index of this grin lens, n ₂Cladding index for this grin lens.

For the grin lens that does not have 1/4th spacings, Gaussian beam changes and can calculate by the abcd matrix step that people such as Emkey propose.(Emkey, William L. and Jack, Curtis A., " calculating of graded index fiber lens and analysis " lightwave technology periodical, Vol.LT-5, No.9, (1987): 1156-1164) this method is used a composite light beam parameter " q ", and it is defined as follows:

$\frac{1}{q\left(z\right)}=\frac{1}{r\left(z\right)}-i\frac{\mathrm{\&lambda;}}{\mathrm{\&pi;}{w}^2\left(z\right)n}---\left(4a\right)$

Wherein r is the radius-of-curvature of Gaussian beam, and w is Gauss's spot size, and λ is a free space wavelength, and n is a refractive index.Q (z) from the plane (4) that comprises single mode pigtail fibers 310 end faces to comprising that optical fiber lens 304 final planes (1) with a tight waist change, and provide as follows:

${\mathrm{<figure-callout\; id="1"\; label="q"\; filenames="C20038010799000171.png,C20038010799000211.png"\; state="\{\{state\}\}">q</figure-callout>}}_1=\frac{A{q}_4+B}{C{q}_4+D}---\left(4b\right)$

Q wherein ₁And q ₄Be respectively and be positioned at the composite light beam parameter that plane (1) and (4) are located.

A, B, C, the D factor are and the relevant light beam entry of a matrix element of (4) to plane (1) light beam parameters from the plane, and are obtained by following formula:

$\left[\begin{array}{cc}A& B\\ C& D\end{array}\right]=M_1{M}_2{M}_3{M}_4---\left(5a\right)$

M wherein ₁For being positioned at the transformation of light beam parameters between plane (1) and plane (2), it is expressed as follows:

${\mathrm{<figure-callout\; id="1"\; label="M"\; filenames="C20038010799000171.png,C20038010799000211.png"\; state="\{\{state\}\}">M</figure-callout>}}_1=\left[\begin{array}{cc}1& \mathrm{<figure-callout\; id="0"\; label="z"\; filenames="C20038010799000171.png,C20038010799000231.png"\; state="\{\{state\}\}">z</figure-callout>}\\ 0& 1\end{array}\right]---\left(5b\right)$

Wherein z is the final beam waist position of relative hyperbolic lens end.M ₂Be the transformation of light beam parameters in the hyperbolic lens, it is expressed as follows:

${\mathrm{<figure-callout\; id="2"\; label="M"\; filenames="C20038010799000171.png,C20038010799000211.png"\; state="\{\{state\}\}">M</figure-callout>}}_2=\left[\begin{array}{cc}1& 0\\ -(n_2-n_1)/n_2{r}_2& {\mathrm{<figure-callout\; id="1"\; label="n"\; filenames="C20038010799000171.png,C20038010799000211.png"\; state="\{\{state\}\}">n</figure-callout>}}_1/n_2\end{array}\right]---\left(5c\right)$

M ₃Be the transformation of light beam parameters in the grin lens, it is expressed as follows:

${\mathrm{<figure-callout\; id="3"\; label="M"\; filenames="C20038010799000211.png"\; state="\{\{state\}\}">M</figure-callout>}}_3=\left[\begin{array}{cc}\cos \left(\mathrm{gL}\right)& \sin \left(\mathrm{gL}\right)/g\\ -g\sin \left(\mathrm{gL}\right)& \cos \left(\mathrm{gL}\right)\end{array}\right]---\left(5c\right)$

Wherein g is provided by the equation (3b) about grin lens, and this grin lens has length L and index distribution provides as follows:

n′(r)＝n(1-g ²r ²) ^1/2 (5d)

Wherein r is the radial position from the axis of lens.M ₄For plane (4) are located from refractive index from n ₁Transformation to the light beam parameters of the medium of n is expressed as follows:

${\mathrm{<figure-callout\; id="4"\; label="M"\; filenames="C20038010799000211.png,C20038010799000231.png"\; state="\{\{state\}\}">M</figure-callout>}}_4=\left[\begin{array}{cc}1& 0\\ 0& {\mathrm{<figure-callout\; id="1"\; label="n"\; filenames="C20038010799000171.png,C20038010799000211.png"\; state="\{\{state\}\}">n</figure-callout>}}_1/n\end{array}\right]---\left(5e\right)$

The length L of this grin lens and focusing parameter g scalable, so that after light beam passes this grin lens, the spot size w that plane (4) are located ₄Be transformed into as far as possible spot size w near the single mode pigtail fibers _p

Hyperbolic lens becomes the hot spot of diffraction-limited with collimated beam, but uncollimated rays can not be converged to the hot spot of diffraction-limited, because it does not make the path of all light equate at a hot spot place.For the grin lens with 1/4th spacings, the light beam of this grin lens output face place is collimated.Therefore, if hyperbolic lens one 1/4th spacing grin lens and then will be converged to the hot spot of diffraction-limited from the light beam of single mode pigtail fibers.For the grin lens that does not have 1/4th spacings, the output beam of this grin lens end will be discrete or convergent, and whether its length that depends on this grin lens is shorter or long than 1/4th spacings.Therefore, preferably this grin lens is designed to or near 1/4th spacings.But should be noted that has expectation to have the application of the grin lens that does not have 1/4th spacings.For these application, the inventor provides a near-hyperbolic lens, and it can be converged to uncollimated rays the hot spot of diffraction-limited.

1 funtcional relationship that has shown the MFD of output beam and radius-of-curvature (R) and grin lens length (Z) of tabulating down.The parameter of using in the calculating is as follows: fiber core radius a=50 μ m, relative index of refraction difference Δ=0.01, operation wavelength λ=1550nm, and the spot size w of single mode pigtail fibers _p=10.6 μ m.

Table 1

Z(mm)	MFD(μm)	R(mm)
			0.15	9.544436065	0.276522
0.16	9.880469672	0.302607
			0.17	10.19672103	0.333614
0.18	10.49111006	0.370933
			0.19	10.76181305	0.416623
0.2	11.00723608	0.473827
			0.21	11.22599466	0.547572
0.22	11.41689802	0.646393
			0.23	11.57893731	0.786001
0.24	11.7112765	0.99881
			0.25	11.81324573	1.364093
0.26	11.88433631	2.140502
			0.27	11.92419729	4.933849
0.2776	11.9334793	472.7168

0.28	11.93263319	-16.3388
			0.29	11.90960273	-3.07433
0.30	11.85521866	-1.69397
			0.31	11.76974837	-1.16665
0.32	11.65361566	-0.88767
			0.33	11.50740347	-0.71467
0.34	11.33185804	-0.59666
			0.35	11.12789463	-0.51087
0.36	10.89660521	-0.44559
			0.37	10.63926875	-0.39422
0.38	10.3573647	-0.35272
			0.39	10.05259072	-0.31854
0.4	9.726885718	-0.28996

For design shown in the table 1, the gap length of this GRIN is about 1110 μ m (or 1.11mm).Utilize equation (3c), this 1/4th spacing is about 277.6 μ m (or 0.2776mm).For the grin lens length near 1/4th spacings, R is very big.For the grin lens length less than 1/4th spacings, R disperses.For example, for the grin lens length of 260 μ m, R is about 2.14mm.For the grin lens length greater than 1/4th spacings, R is a convergent.For example, for the grin lens length of 290 μ m, R is about-3.07mm.For R is convergence or discrete grin lens length, needs a near-hyperbolic wire shaped to obtain the focused light spot of a diffraction-limited, and this near-hyperbolic wire shaped is to have a distortion hyperbolic shape that is used for the correction coefficient of light beam bending curvature compensation.

The near-hyperbolic lens shape can determine that with appropriate precision this change that hyperbolic shape is made is used for compensating the light beam bending curvature by the change of calculating optical path and physical path length.Fig. 4 A illustrates a planar beam wavefront 400, and it is positioned at or produces during near 1/4th spacings when this grin lens length, and a discrete light beams wave front 402, and it produces during less than 1/4th spacings when grin lens length.Compare with the optical path length of planar beam wavefront 400, the optical path length of discrete light beams wave front 402 reduces in the direction of leaving optical axis 404.This optical path length difference, L _Opt(r), can calculate by following formula with funtcional relationship from the radial distance of optical axis 404:

L _opt(r)＝R(1-cosφ) (6a)

Wherein

φ＝sin ^-1(r/R) (6b)

This physical path length difference, L _p(r), provide as follows

$L_p\left(r\right)=\frac{L_{\mathrm{opt}}\left(r\right)}{(n-1)}---\left(6c\right)$

Wherein n is the refractive index of lens material.In a similar fashion, longer than 1/4th spacings for grin lens length, that is, the converging beam wave front can calculate the near-hyperbolic wire shaped.In this case, optical path length difference need be function from optical axis distance and increase.Fig. 4 B has been depicted as the synoptic diagram that obtains near-hyperbolic wire shaped 408 and hyperbolic shape 406 is made change, and this near-hyperbolic wire shaped 408 can be converged to discrete light beam wave front the hot spot of diffraction-limited.

For the specific embodiments with 473.8 μ m radius-of-curvature (R), grin lens length 200 μ m, optical path length provides table 2 from the skew of hyperbolic shape and funtcional relationship from the radial position of optical axis.

Table 2

r(μm)	L opt
		2	0.004228
4	0.016913
		6	0.038054
8	0.067652
		10	0.105704
12	0.152212
		14	0.207173
16	0.270587
		18	0.342453
20	0.42277
		22	0.511536

The difference of physical path length calculates divided by (n-1) by optical path length, and wherein n is the refractive index of lens material.As can be seen from Table 2, for big radius-of-curvature, change very little in the hyperbolic shape of closely locating from optical axis.Yet this skew becomes big under optical axis and the little radius-of-curvature situation leaving.The above-mentioned calculating that illustrates is for the embodiment of a decision near-hyperbolic wire shaped step is provided.This near-hyperbolic wire shaped is more accurate to be determined to utilize suitable lens design model to make.

According to the present invention, each grin lens length of using in the optical fiber lens where necessary can be different with 1/4th spacings.As a result, according to the present invention, in various application, can adopt same blank to draw grin lens.Because do not need to change the index distribution of blank, the manufacturing process of blank and grin lens can be simplified.Thereby same blank can be used for different modular transformations and uses.For different application, this blank preferably is drawn into different outer dias again, and the grin lens that obtains can be cut or resolve into different length to satisfy the demand of different application.The parameter of grin lens, for example grin lens 1/4th spacings can be by the step decision of before having described.In the present invention, to have eliminated grin lens must be 1/4th spacings to this near-hyperbolic lens with the restrictions of the hot spot of realizing diffraction-limited.This near-hyperbolic wire shaped combines hyperbolic lens effectively and is used to proofread and correct the function of the spherical lens of residual bend curvature.

In one embodiment, adopt Corning SMF-28 ^Optical fiber is as the single mode pigtail fibers, but not with the described content of this embodiment as limitation of the present invention.This pigtail fibers is engaged to an end of grin lens, and the other end at this grin lens forms near-hyperbolic lens simultaneously.Distance between this joint and this hyperbolic lens end is about 275 μ m.This grin lens has the core diameter of 50 μ m and the outer dia of 125 μ m.The fibre core of this grin lens and the relative index of refraction difference between the covering are 1%.Fig. 5 illustrates the function relation curve figure of far field Luminance Distribution and far-field divergence angle among this embodiment.Obtain this characteristic at 1.31 μ m places, far field full width half maximal value (FWHM) angle of divergence of this optical fiber lens is about 20 °.It is complete Gaussian that this curve map illustrates this Luminance Distribution.

An application of optical fiber lens of the present invention is the optical signalling coupling between optical fiber and semiconductor optical amplifier (SOA) or other waveguide.The standard specification of these application comprises: MFD＜3.0 μ m, and from distance＞10 μ m with a tight waist, return loss＞45dB, and firm lens shape is to prevent the breakage in the element assembling process.For SOA and wave guide applications, this optical fiber lens must change the mould field of this pigtail fibers to mate this waveguide or SOA.Nowadays, this waveguide and the many SOA devices MFDs that generally has abundant circle.For SOAs, MFDs is 2.5-3.8 μ m in 1550nm place scope.These are the same with the 18-22 degree high with the corresponding value of far field full width half maximal value (FWHM) angle of divergence.At～13.5% (1/e ²) half far-field divergence angle (θ) at luminance level place provides as follows:

$\mathrm{\&theta;}=\frac{\mathrm{\&lambda;}}{\mathrm{\&pi;}{w}_0}---\left(7\right)$

Wherein λ is an optical wavelength, w ₀Be the light beam spot size.MFD is 2w ₀

Another attribute and the desired feature of optical fiber lens to the influential SOAs of SOA assembling process are the dip plane of SOA.In order to reduce echo reflection, the dip plane of SOAs tilts into about 15 °.Because the existence of dip plane, the gap rationally becomes important between edge, dip plane and the optical fiber lens end.Otherwise, aim at SOA when obtaining optimum coupling when optical fiber lens, optical fiber lens is very big with the probability that the SOA dip plane contacted and damaged it.The current available optical fiber lens that has a 2.5-3.8 mu m range MFDs in 1550nm operating wave strong point of the overwhelming majority has the same little focal length with 5-10 μ m.Thereby, help increasing its focal length to greater than 15-20 μ m improving this characteristic, and reduce the probability that damages SOA in the assembling process.Equally, SOAs is also very responsive to echo reflection.If focal length is big, will have than the echo reflection arrival SOA of small part from the optical fiber lens end.This has also improved performance and the stability of SOA.

Use the wave front characteristic that another useful feature is Luminance Distribution and the light assembled by optical fiber lens for SOA.This wave front characteristic should and the mould field brightness between SOA and pigtail fibers distribute and to be complementary, approaching as much as possible on size, Luminance Distribution and phase place.This shows that the convergent beam that passes optical fiber lens from pigtail fibers must have the size of 2.5-3.8 mu m range, and is Gaussian as far as possible.Current available optical fiber lens reaches this characteristic at big MFDs place, but so not good at little MFDs place.The performance that this just causes high coupling loss and reduces SOA.The present invention helps improving this performance.

Another characteristic is the stability of optical fiber lens.For example, in the various procedure of processings that comprise in the preparation in optical fiber lens being assembled into the SOA packing,, will exist the lens end to damage and reduce the possibility of performance if optical fiber lens is designed to have very little He frangible end.Physically firm non-friable optical fiber end will be useful characteristic.Design and processing that another useful characteristic is lens, this processing are stable and have for the more tolerance of machining deviation.For example, if the lens end has the radius-of-curvature of 10 μ m, the variation of the 1 μ m that radius-of-curvature is very little will be 10% variation, and can change focus characteristics significantly.The variation of 1 same μ m may not can reduce the performance of same degree in the design of 25 μ m radius-of-curvature.In the present invention, some in these problems are suggested and improve.

The Another application of optical fiber lens of the present invention is the optical signalling coupling between optical fiber and laser diode.This laser diode that is used to launch laser has the same high far-field divergence angle with 40 °, and its MFD with the about 0.8 μ m in 1550 mum wavelength places is consistent.This laser emission aspect ratio in the x and y direction is changed to 4 from 1.The MFD of lens matches more, and coupling efficiency is high more.In the content of the disclosure, optical fiber lens is typically connected on the device, and it has the aspect ratio near 1.In order to make lens in assembling, avoid damaging laser instrument, expectation be greater than 10 μ m from distance with a tight waist.The application's optical fiber lens is preferably as follows characteristic: in the MFD＜3.0 μ m of 1550nm operating wave strong point or the angle of divergence＞22 °, return loss＞45dB, from distance＞10 μ m with a tight waist, and between lens coupling efficiency greater than 90%.

The Another Application of optical fiber lens of the present invention is the optical signalling coupling of process between optical fiber and detector.With in the above-mentioned application not only spot size but also Luminance Distribution and phase front mistake also be critical different, this detector application need to spot size and only within the specific limits power quantity control.For the application, the spot size during focal length 50-60 μ m is less than 3-5 μ m, and it is low-cost to help assembling.

Optical fiber lens of the present invention has one or more advantages.This optical fiber lens allows the optical signalling coupling between the optical devices.The hyperbolic lens or the near-hyperbolic lens that are formed on this optical fiber lens end are mechanically firm, and compare with for example tapered lens, seldom might be damaged and reduce performance.The multimode parameter of this grin lens and the shape of hyperbolic or near-hyperbolic lens can be controlled to realize having the little MFDs and the long focal length of rational Gauss's pattern of brightness.This hyperbolic lens becomes collimated beam the hot spot of diffraction-limited.This near-hyperbolic lens is used for proofreading and correct the wave front bending curvature of uncollimated rays, becomes the hot spot of diffraction-limited to allow this beam convergence.

Though describe the present invention by several preferred embodiments, other variation that these preferred embodiments have been done, displacement, equivalence are replaced and are also all dropped in the scope of the invention.Therefore, accessory claim subsequently has been interpreted as comprising all and has dropped on practicalness of the present invention and interior this variation, displacement, the equivalence replacement of scope.

Claims (9)

1, optical fiber lens comprises:

Gradual index lens;

Single-mode fiber is arranged on this gradual index lens first end; And

Refractor, shape with approximate Double curve, be arranged on this gradual index lens the second end, be used for to become the hot spot of diffraction-limited from the beam convergence of this single-mode fiber, this near-hyperbolic wire shaped comprises a correction factor, be used for the compensation of light beam bending curvature, and allow uncollimated rays is converged to the hot spot of this diffraction-limited.

2, according to the optical fiber lens of claim 1, wherein between this refractor and gradual index lens, insert spacer rod.

3, according to the optical fiber lens of claim 1, wherein between this gradual index lens and this single-mode fiber, insert spacer rod.

4, according to the optical fiber lens of claim 1, wherein the mode field diameter of this hot spot is less than 10 μ m.

5, according to the optical fiber lens of claim 4, wherein the mode field diameter of this hot spot is in the scope of about 2-5 μ m.

6, according to the optical fiber lens of claim 4, wherein the focal length of this optical fiber lens is greater than about 5 μ m.

7, according to the optical fiber lens of claim 4, wherein the focal length of this optical fiber lens is in about 20-60 mu m range.

8, according to the optical fiber lens of claim 4, wherein the distance of the beam waist that presents of this refractor end to lens one end and ratio of this place with a tight waist mode field diameter are greater than about 5.

9, according to the optical fiber lens of claim 1, wherein the core diameter of this gradual index lens is in the scope of about 50-500 μ m.

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