EP1516218A2

EP1516218A2 - Device for automatic centering of a laser beam and method for making same

Info

Publication number: EP1516218A2
Application number: EP03761641A
Authority: EP
Inventors: Pascal Leclerc; José Garcia
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2002-06-27
Filing date: 2003-06-25
Publication date: 2005-03-23
Also published as: CA2490345A1; FR2841657B1; WO2004003616A2; CN1666127A; JP2005531037A; FR2841657A1; WO2004003616A3; RU2005101890A; KR20050013645A; US20050213881A1; AU2003260627A1

Abstract

The invention concerns a device for automatic centering of a laser beam and method for making same. Said device comprises a volume diffuser (2) for diffusing a laser beam and automatically centering it in an optical waveguide (32), for example a monomode or multimode optical fiber. The method for making said device consists in producing a tubular optical waveguide (6) followed by the diffuser, from a diffusing material, using the guide as hollow punch.

Description

DEVICE FOR AUTOMATIC CENTERING OF A LASER BEAM AND METHOD FOR MANUFACTURING THE DEVICE

DESCRIPTION

TECHNICAL AREA

The present invention relates to a device for automatically centering a laser beam, in particular in a single-mode optical fiber or in a multimode optical fiber, after a defocusing or a decentering of said beam.

This device applies more particularly to laser beams whose deflections or offsets are greater than the transverse dimensions of the optical fibers or close to these dimensions.

The invention also relates to a method of manufacturing this device.

STATE OF THE PRIOR ART Known centering devices can be classified into two categories:

- static devices, tolerating variations in pointing and centering for injecting the laser beam into a fiber, and - dynamic devices, tolerating variations in pointing and centering and provided with a system for refocusing the laser beam by relative to the entry of the fiber, either by deflecting this laser beam, or by orienting the fiber. Static devices mainly use surface light scatterers, more simply called "surface diffusers", that is to say means whose surface is capable of diffusing ("scatter") the light from the incident laser beam, but do not provide sufficient uniformity for injections into the fibers, because

- On the one hand of the initial no-uniformity of the laser beam, which is only partially corrected, and on the other hand of the coherence of this laser beam.

Indeed, when an object diffusing on the surface is illuminated by a laser, the points which compose this object diffuse a coherent light and produce a granularity ("speckle") of Fresnel type in all the space which surrounds them.

Dynamic devices have the major drawback of requiring a priori knowledge of pointing and off-center variations to correct the positioning of the optical fiber relative to the laser beam.

'They are therefore generally only applicable to recurrent lasers since they require several laser pulses to converge to the optimal coupling position.

Such devices use electronic means which are controlled by a CCD type sensor or a four-quadrant sensor, this sensor being placed on a position which is the image of the core ("core") of the optical fiber.

They control mobile optics which must compensate for variations in the pointing of the laser beam in order to optimize coupling in the fiber.

The advantage of such devices is that they can achieve high coupling rates (of the order of 50%). They are however very expensive due to their complexity and require very fine alignments, sensitive to temperature variations and vibrations.

This constraint results from the small size of the fiber core and its small angular opening, which require an optic of relatively high focal length - typically of the order of 20 cm - the positioning of which must be of the order of 1 μm.

PRESENTATION OF THE INVENTION The aim of the present invention is to remedy the above drawbacks.

To do this, a static centering device is used, comprising a volume light diffuser, more simply called a "volume diffuser", that is to say a means whose volume - and no longer the surface - is suitable to scatter the light of the incident laser beam that we want to center.

Specifically, the present invention relates to a device for automatically centering a laser beam in a light guide, this device being characterized in that it comprises a volume diffuser, comprising an entry face of the laser beam and provided for diffusing this laser beam and automatically centering it in the light guide. This light guide can be a single mode optical fiber or a multimode optical fiber.

According to a preferred embodiment of the device which is the subject of the invention, the thickness of the volume diffuser is at least equal to 100 times the wavelength of the laser beam.

The volume diffuser can be made of polytetrafluoroethylene.

According to a particular embodiment of the device which is the subject of the invention, the volume diffuser has a cylindrical shape.

Preferably, the volume diffuser has a side face and the device further comprises a light reflector which surrounds this side face. According to a first preferred embodiment of the device which is the subject of the invention, this device further comprises a lens which is placed on the entry face of the volume diffuser and provided for defocusing the laser beam on this entry face. _.

According to a second preferred embodiment, the volume diffuser has a side face and the device further comprises a light reflector which surrounds this side face, extends beyond the entry face and guides the laser beam to this entry face. According to a third preferred embodiment, the device which is the subject of the invention further comprises an auxiliary optical fiber which is optically coupled to the entry face of the volume diffuser and guides the laser beam to this entry face.

The present invention also relates to a method of manufacturing the device which is the subject of the invention, in which a tubular light guide is produced and the volume diffuser is manufactured from a material capable of diffusing light, using the tubular light guide as a cookie cutter.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be better understood on reading the description of exemplary embodiments given below, by way of purely indicative and in no way limitative, with reference to the appended drawings in which: - Figure 1 schematically illustrates a example of a volume diffuser that can be used in the present invention,

FIG. 2 is a schematic sectional view of a first particular embodiment of the device which is the subject of the invention,

- ^' Figure 3 is a schematic sectional view of a second particular embodiment of the device object of the invention,

FIG. 4 is a schematic sectional view of a third particular embodiment of the device which is the subject of the invention, FIG. 5 is a schematic sectional view of a fourth particular embodiment of the device which is the subject of the invention,

- Figure 6A schematically illustrates a step of manufacturing a device according to

The invention,

- Figure 6B is a schematic sectional view of a device according to the invention.

FIG. 7 schematically illustrates the scattering of light by an elementary volume of diffusing material, and

- Figure 8 shows variation curves of 1 scattered illumination and 1 incident reduced illumination as a function of the distance.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

As seen above, the device which is the subject of the invention overcomes the drawbacks of the prior art, on the one hand, because it is static and, on the other hand, because it uses a volume diffuser. In this case, it is possible to reduce the coherence of the laser beam and therefore the resulting granularity.

By using media whose inhomogeneities are small compared to the size of the beam, multiple diffusions introduce random phase relationships between the different points of the beam and degrade spatial coherence.

The volume diffuser is made of a suitable material in order to obtain correct uniformities. The choice of this material is made in function of the optical scattering coefficient, which must be as large as possible, and the absorption coefficient which must be as low as possible.

On this subject, one will refer to the end of the description where one gives a theory of the radiative transfer.

A material such as polytetrafluoroethylene or Teflon (registered trademark) is well suited to laser beams from the visible and near infrared spectra.

It has also been found that a device according to the invention does not degrade the temporal shape of a pulsed laser beam, as long as the duration of the pulses is not less than 10 ¹¹ s, and that the coherence of the beam does not harm not to the uniformity of this beam at the outlet of the diffuser, due to the superposition of decorrelated granularity figures.

In addition, a volume diffuser is used; this means that this diffuser has a length L, or thickness, which is significant with respect to the wavelength of the incident laser beam F which it is desired to center (FIG. 1). Preferably, the thickness of this diffuser is at least equal to 100 times this wavelength.

This volume diffuser advantageously has the shape of a cylinder whose length is a function of the uniformity and of the desired overall transmission.

This is schematically illustrated in Figure 1 where we see a device according to the invention, comprising a volume diffuser 2, of cylindrical shape, in Teflon (registered trademark), length L.

A laser beam F is focused on one end 4 of the diffuser 2 forming an entry face. The laser light is diffused in the form of spherical waves S at the outlet of the diffuser, on the side opposite to the inlet face 4.

In addition, the increase in uniformity at the outlet of the diffuser 2 as well as the increase in the overall transmission are obtained by placing the diffuser in volume in a reflective waveguide.

This is schematically illustrated by FIG. 2 where we see the diffuser 2 inserted in a metallic tubular reflector 6 which thus surrounds the lateral face 8 of the diffuser 2.

This reflector 6, or guide, reflects the laser light which reaches this lateral face 8 and thus guides this light in the diffuser 2.

An empirical formula, which is verified experimentally, makes it possible to simply calculate the overall transmission and to size the centering device with respect to the deflection to be corrected.

This formula, which gives the transmission T of the device provided with a metal guide, is as follows:

₇ , a sin α

T. e ^' " ^z .

4A

In this formula:

- A is the section of the metal guide (in m ² ), a is the section (in m ² ) of the optical fiber which is coupled to the diffuser and in which we want to center the laser beam,

- α is the digital opening angle of the fiber, - z is the length of the guide (in m),

- p is the density (number per m ³ ) of the particles which scatter light, and

- σ (in m ² ) is the cross section of diffusion

("scattering cross section"). Auxiliary means can advantageously be added to the reflecting guide in order to increase the flow resistance of the automatic centering device. Indeed, if the laser beam is focused on the entrance face of the diffuser, it risks damaging it.

According to a first possibility, to reduce the risks of degradation, a micro-lens is added in front of the diffuser to defocus the laser beam on the entrance face of the diffuser, that is to say so that the laser beam is not focused on this entry face.

This is schematically illustrated in FIG. 3 where we see a micro-lens 10 placed against the entry face 4 of the diffuser 2. This micro-lens 10 is able to defocus the incident laser beam 12 on the face 4 of the diffuser , the latter and the microlens 10 being coaxial.

In the example of FIG. 3, the diameter of the microlens is equal to the diameter of the diffuser 2. According to a second possibility, by extending the waveguide towards the front of the diffuser, the laser beam is guided to this diffuser and the geometric extent of the beam is increased by increasing its surface at the level of the diffuser, which therefore reduces the risk of degradation of this diffuser.

This is schematically illustrated in Figure 4 where we see a tubular reflector 14 which surrounds the cylindrical diffuser 2 and protrudes from the inlet face 4 of this diffuser.

In the description of FIG. 6A, a method of manufacturing the diffuser 2 of FIG. 2 will be explained in a tubular reflector of the same length.

The diffuser of Figure 4 can be obtained in the same way, in a longer tubular reflector and then pushing the diffuser to the side of the reflector opposite to that by which the diffusing material was introduced.

According to a third possibility, a large diameter optical fiber is added in front of the volume diffuser to increase the flow resistance of the automatic centering device. This is schematically illustrated in FIG. 5. In this example, a section of optical fiber 16 is added to the device in FIG. 4, the core ("core") and the cladding ("cladding") of which have the references 18 and 20 respectively. The core 18 and the diffuser 2 are coaxial. The section 16, the diameter of which is substantially equal to that of the diffuser 2, is housed in the part of the guide 14 which projects beyond the inlet face

4. The latter is in contact with the fiber section 16.

The fiber section 16 thus receives the laser beam 12 before the diffuser, which makes it possible to avoid hot spots in the latter.

The reflective guide 6 can advantageously serve as a cookie cutter for the development of the diffuser in a flexible diffusing material (if this guide is made of a material of sufficient hardness).

This is schematically illustrated by the example of FIG. 6A where we see the tubular guide 6, for example made of steel, which is made rigidly integral with a plate 22 of steel and thus forms a projection of this plate 22.

As can be seen in FIG. 6B, this plate 22 is fitted, by means of this projection, into a support 24 and made integral with this support by screws symbolized by dashed lines

26.

The support 24 has a threaded portion 28 onto which an optical fiber connector 30 can be screwed. It is thus able to optically connect the diffuser 2 to the optical fiber 32 with which this connector 30 is provided, the plate 22 and the support 24 being suitably drilled for this purpose.

In particular, as can be seen in FIG. 6B, the drilling of the plate 22 causes the diffuser 2 to be located in a reflector of the kind 03616

12

that of figure 4, rather than in a guide of the kind of that of figure 2.

The device in FIG. 6B allows the laser beam 12 to be centered on the optical fiber 32 thanks to the volume diffuser 2.

To manufacture this device, a plate 34 of the flexible diffusing material is used, for example a Teflon plate (registered trademark), and the steel plate 22 is applied against this plate 34 (FIG. 6A).

The projection formed by the tubular guide 6 of FIG. 6A sinks into the material and part of the latter penetrates into the tubular guide to form the diffuser 2. By means of an appropriate cutting tool 36, the diffuser is then separated. thus formed from the rest of the material.

As a purely indicative and in no way limitative, to center a laser beam whose wavelength is 1064 nm, a Teflon diffuser (registered trademark) is used whose length (thickness) is 750 μm, or nearly 700 times the length d wave of the laser beam, and a metal waveguide of polished steel, which protrudes from the diffuser by 0.3mm on the side from which the laser beam arrives.

The present invention is not limited to centering a laser beam in an optical fiber (single mode or multimode).

It also applies to centering a laser beam in other light guides, for example planar guides. 3616

13

We explain in what follows the theory of radiative transfer, that is to say the transfer of light by the diffuser

In the case of a straight propagation, the variation dL of the luminance L (in / m ² / sr) through a thickness dz of an elementary volume is such that dL

-. - (..>) L dz where α is the absorption coefficient (in m ^"1 ) and β the diffusion coefficient (in m ^{" 1} ).

In the case of diffusing particles, for which the cross sections of diffusion σ _s , of absorption σ _a and of extinction σ _t = σ _a + σ _s are defined

(in m ² ), we also express the incident luminance I (r, s) at point r in the direction s, on an elementary cylindrical volume of length ds (see Figure 7) as follows: dl (r, s ) _ = pσ _t I (r, s) ds where p is the density of the particles. At the end of absorption and diffusion in the direction s, we must add all the diffusions and absorptions coming from all the directions s'. They are expressed from the phase diffusion function of the particles p (s, s') which is defined by:

03616

14

where Wgest is the albedo of a single particle and dω the elementary solid angle.

It is also necessary to add a term (in / m ³ / sr) which corresponds to the emission of the elementary volume of length ds in the direction s and which is denoted ε (r, s).

By integrating all of these contributions, we obtain the transfer equation: dl (r, s)

= -pσ _t I (r, s) + ^ - I p (s, s') I (r, s') dω '+ ε (r, s). ds 4π J

4π The total luminance I in the direction s at point r is dissociated in two terms corresponding to the reduced incident luminance I _r i and to the diffused luminance la. We obtain the following two equations: dl ^L r _τ ι (r, s) = -pσ _t I _r i (r, s) ds

s') I _d (r, s') dω '+ ε (r, s) + ε _ri (r, s)

4π

We draw the illumination U _d and the flow vector F _d which are scattered at point r:

^u _d ( ^r ) = - and F _d (r, s) = - jl (r,) sdω

In the case of a collimated or Gaussian beam arriving on a plane sample, one can calculate the scattered illumination U _d (r) at any point. For this, it is necessary to introduce the Green G functions (r, r ') which satisfy the propagation equation and with boundary conditions for a plane sample of length d:

VG (r, r ') - KG (r, r') = -δ (r, r ') dd G (r, r') - h - G (r, r ') = 0 z = 0 dz d G (r, r ') + h - G (r, r') = 0 z = d dz

In these equations, with (1-μ) where μ is the cosine of the mean scattering angle. The illumination diffused at a point rs' then expresses by:

G (r, r ') Ql (r')

U _d (r) = FG (r, r ') Q (r') dV '+ F dS'

2πh

V

Q (r) = Q (r, θ, z) = exp 3pσ _tr -2- (-pσ _t z) exp (-), where Qα πw w (r) is zero for a ^• isotropic scattering is dV the volume of the sample, P ₀ is the incident power of the laser beam and W is the radius at 1 / e ² of the laser beam.

It is possible to express analytically

1 diffused illumination U _d by means of the modified Bessel functions and to calculate it for different values of p, of σ _t , and of the thickness of

1 sample.

Various simulations have been carried out which give the variations of U _d and U _r i (reduced incident illumination) as a function of the density of particles and the cross-section for three thicknesses (0.5mm, 1mm and 2mm) of the sample.

The laser used had a power of lmW and a numerical aperture of 0.11. Figure 8 shows the variation curves of U _d and U _r i as a function of z.

The reduced incident illumination U _r i decreases as a function of exp (-pσ _t z) and of the dimension of the laser beam, while the scattered illumination U _d increases firstly as a function of z and then decreases thereafter.

With the chosen configuration, which is linked to the input laser beam, the product pσ _t z must be of the order of 10 for U _{d to} be of the order of

Uri- We can find the order of magnitude of this value from simple considerations. The reduced incident illumination decreases in the form:

where Kl is a constant of proportionality and θ is the aperture angle at l / e ² of the laser beam in the material, while we can write for 1 scattered illumination, due to the conservation of energy and considering that this illumination is constant on a sphere of radius z:

4πzU _d (z) = K2x (l-exp (-pσ _t z)) where K2 is a constant of proportionality. When U _d is equal to U _ri , exp (-pσ _t z) is little θ ² different from - so pσ _t z is little different from 7. 4π

We find the order of magnitude indicated above.

Claims

1. Device for automatically centering a laser beam in a light guide (32), this device being characterized in that it comprises a volume diffuser (2), comprising an entry face of the laser beam and provided for scatter this laser beam and automatically center it in the light guide.

2. Device for automatically centering a laser beam in a single-mode or multimode optical fiber (32), this device being characterized in that it comprises a volume diffuser (2), comprising an entry face of the laser beam and intended to diffuse this laser beam and automatically center it in the optical fiber.

3. Device according to any one of claims 1 and 2, wherein the thickness (L) of the volume diffuser (2) is at least equal to 100 times the wavelength of the laser beam.

4. Device according to any one of claims 1 to 3, wherein the volume diffuser (2) is made of polytetrafluoroethylene.

5. Device according to any one of claims 1 to 4, wherein the volume diffuser (2) has a cylindrical shape.

6. Device according to any one of claims 1 to 5, wherein the volume diffuser (2) has a side face and the device further comprises a light reflector (6, 14) which surrounds this side face.

7. Device according to any one of claims 1 to 6, further comprising a lens (10) which is placed on the entry face of the volume diffuser (2) and intended to defocus the laser beam on this face of Entrance.

8. Device according to any one of claims 1 to 5, wherein the volume diffuser (2) has a side face and the device further comprises a light reflector (14) which surrounds this side face, extends au- beyond the entry face and guides the laser beam to this entry face.

9. Device according to any one of claims 1 to 6 and 8, further comprising an auxiliary optical fiber (16) which is optically coupled to the input face of the volume diffuser (2) and guides the laser beam up to to this entrance face.

10. A method of manufacturing the device according to any one of claims 1 to 5, in which a tubular light guide (6) is manufactured and the volume diffuser (2) is manufactured from a material (34 ) suitable for diffusing light, using the tubular light guide as a cookie cutter.