Device including a transversely anisotropic optical fiber, and a method of manufacturing it

The side surface of a ferrule (16) carrying a polarization-maintaining option fiber (10) has a projecting substantially dihedral-angled portion (17, 19) engaged in a complementary re-entrant substantially dihedral angled portion (22, 24) formed in a ferrule receiver (20). The invention is particularly applicable to making a laser-emitting head for an optical fiber telecommunications network.

The present invention relates to the use of optical fibers that are 
anisotropic in a transverse plane. A typical purpose of such anisotropy is 
for the light injected into such fibers to be guided and/or absorbed to a 
greater or lesser extent in a manner that depends on the polarization of 
said light. 
Such fibers comprise in particular polarizing fibers and fibers for 
maintaining polarization. When such a fiber is installed during the 
manufacture of such a device, there is a particular need to ensure correct 
angular positioning of said fiber about its own longitudinal axis. The 
purpose of such positioning is typically to cause a privileged plane of 
the fiber, which plane is defined relative to the fiber and includes said 
axis, to coincide with a plane of polarization for light that is to travel 
along the fiber, which plane is defined relative to the device to be 
manufactured. 
In addition, when such proper positioning has been obtained, there is a 
need for it to be conserved unchanged throughout the service life of the 
device. 
A known device for this purpose comprises the following elements: 
a support structure; 
optical components fixed to said structure to interact with light having a 
plane of polarization, such that said plane of polarization is defined 
relative to said structure; 
a transversely anisotropic optical fiber for guiding said light, said fiber 
having a longitudinal axis and defining a privileged plane including said 
axis and which is to coincide with said plane of polarization; 
a ferrule securely carrying and surrounding said fiber, a side surface of 
said ferrule being parallel to said longitudinal axis, a cross-section of 
said side surface having a positioning projection which is angularly 
located so that the privileged plane of said fiber is defined by the 
angular position of said projection, said side surface having bearing 
surfaces for enabling said ferrule to be carried and for defining its 
position in transverse translation; and a ferrule receiver carried by said 
support structure for receiving said ferrule, said receiver having 
reception surfaces for receiving the bearing surfaces of said ferrule and 
having a positioning recess which is angularly positioned to receive said 
positioning projection in such a manner that when said bearing surfaces 
bear against said reception surfaces and said projection is inserted in 
said recess, said plane of polarization of the light is caused to coincide 
with the privileged plane of the fiber. 
This known device is described in the article "Enhanced rotary mechanical 
splice for rectangular polarization-maintaining fibers" by Frederick M. 
Sears, Calvin M. Miller, W. A. Vicory, and D. N Ridgway given at the 
OFC'89 "Conference on optical fiber communication" that took place in 
Houston, Feb. 6-9, 1989. 
The ferrule of that known device has a cross-section that is circular and 
it includes a positioning projection that is rectangular in section. 
To obtain accurate angular positioning of said ferrule relative to its 
receiver requires firstly difficult machining during manufacture of the 
ferrule and its receiver, and then greater care during assembly thereof, 
such that the accuracy finally obtained is sometimes poor. Finally, it is 
difficult to obtain final fixing of the ferrule in its receiver in such a 
manner as to avoid any risk of subsequent relative displacement. 
Particular objects of the present invention include easily providing a 
ferrule for receiving a fiber as defined above, easily and reliably 
obtaining accurate angular position of said ferrule in its receiver, and 
easily obtaining final mechanically-strong fixing of said ferrule in said 
receiver. More particularly, an object of the invention is to facilitate 
bonding said ferrule to said receiver. 
These objects are achieved, in particular, by a transversely anisotropic 
optical fiber device characterized by the fact that the side surface of a 
ferrule carrying said fiber presents a projecting substantially 
dihedral-angled portion engaged in a complementary re-entrant 
substantially dihedral-angled portion formed in a ferrule receiver. 
An implementation of the present invention is described in greater detail 
below by way of non-limiting example and with reference to the 
accompanying diagrammatic figures. When the same item is shown in a 
plurality of figures, it is designated in all of them by the same 
reference symbol. It should be understood that the items described may be 
replaced by other items providing the same technical functions.

The device comprises a laser-emitting head for use, for example, in an 
optical fiber telecommunications network. It includes a support structure 
constituted by a metal base plate 2, and by optical components fixed on 
said base plate by means of elements 3. 
These optical components comprise a semiconductor laser chip 4, a crystal 
plate 6 surrounded by a permanent magnet 7 to constitute a Faraday 
rotator, and a focusing lens 8. The laser chip emits light polarized in a 
plane perpendicular to the plane of the figure and the Faraday rotator 
rotates the plane of polarization through 45.degree.. 
At the outlet from these components, the plane of polarization of the light 
is thus defined relative to the structure 2. 
Thereafter, the device includes a transversely anisotropic optical fiber 10 
which receives the light focused by the lens 8. This fiber guides the 
light. It has a longitudinal axis 12 and it defines a privileged plane 14 
including said axis and which must be caused to coincide with said plane 
of polarization. 
The fiber is more particularly a polarizing fiber which passes light 
vibration in a privileged plane only. 
This provides an optical isolator. The function of the isolator is to 
prevent interfering light returned towards the laser-emitting head by the 
fiber 10 reaching the laser chip 4 in a plane of polarization parallel to 
that of the light emitted by the chip. This function is useful since such 
light in said plane of polarization disturbs the operation of the laser 
chip. 
At a distance from the laser emitting head, the polarizing fiber 10 is 
connected to an ordinary optical fiber, i.e. to a fiber that is 
transversely isotropic. 
To secure the fiber 10, the device includes a ferrule 16 surrounding said 
fiber and carrying it securely. The outside surface of said ferrule is 
parallel to the longitudinal axis 12 and it has bearing surfaces 17 and 
19. The cross-section of said outside surface has a positioning projection 
18. 
A ferrule receiver 20 is carried by the support structure 2 for the purpose 
of receiving the ferrule. It has receiving surfaces 22 and 24 for 
receiving the bearing surfaces of said ferrule, and it has a positioning 
recess 26 for receiving the positioning projection 18. 
In accordance with the present invention, the two bearing surfaces 17 and 
19 are plane and constitute a projecting substantially dihedral-angled 
portion, while the ferrule receiver 20 has two plane reception surfaces 22 
and 24 form a re-entrant substantially dihedral-angled portion 
complementary to said projecting dihedral-angled portion 18, 19. This 
simple disposition ensures that said two bearing surfaces press 
simultaneously against respective ones of the two reception surfaces, said 
projecting and re-entrant dihedral-angled portions also constituting the 
positioning projection 18 and the position recess 26, respectively. 
Preferred additional dispositions also included in the device given by way 
of example are now described. 
Each bearing surface such as 17 remains in contact with the complementary 
reception surface 22 up to a longitudinal line of contact 28 between the 
ferrule 16 and its receiver 20. A side face 30 of the ferrule then slopes 
away from a disengaged face 32 of the ferrule receiver constituting a 
re-entrant substantially dihedral-angled portion 30, 32 which defines a 
dihedral angle of more than 90.degree.. This dihedral angle may be 
referred to as an angle "of accessibility" in the sense that the larger 
the angle the easier the access to said line of contact for a beam of 
welding energy. In order to fix the ferrule on its receiver by means of 
such a beam, the materials of the ferrule and of the receiver should be 
selected so as to be suitable for being welded together. 
More particularly, the material selected for the ferrule and for its 
receiver should be a hard metal having a low coefficient of thermal 
expansion, and the dihedral angle of accessibility should be about 
135.degree.. 
The device is then made by means of the following operations: 
the support structure 2 carrying the optical components 4, 6, and 8, and 
the ferrule receiver 20 are made; 
the ferrule 16 is made; 
the transversely anisotropic fiber 10 is positioned and secured in said 
ferrule 16; 
the bearing surfaces 17 and 19 of said ferrule are pressed against the 
reception surfaces 22 and 24 of the receiver; and 
the ferrule is welded to the receiver along their longitudinal join lines 
28. 
More particularly, the ferrule 16 is fixed to the receiver 20 by spot 
welding, e.g. two spots along each of the two join lines such as 28. These 
weld spots are made by means of a YAG laser whose beam 40 (represented by 
an arrow in FIG. 3) maintains an unchanging direction which is common to 
both of the dihedral angles of accessibility defined at each of the two 
join lines such as 28. 
The cross-section of the ferrule 16 is preferably substantially a 
quadrilateral having two adjacent sides defining the two bearing surfaces 
17 and 19, two opposite vertices 36 and 38 of said quadrilateral defining 
the two join lines 28, with the two disengaged faces 32 of the ferrule 
receiver extending in the same plane that includes said two join lines. 
More particularly, as shown, a square shape is chosen because it is easy 
to make and to handle. 
Advantageously, in particular when a ferrule is used whose cross-section is 
square, means are provided for identifying the two bearing surfaces: said 
means may be a line or a point formed on the ridge of the dihedral angle 
formed substantially by said two surfaces. 
Conventional techniques are used for positioning and fixing the fiber 10 
inside the ferrule 16 and for positioning and fixing the ferrule receiver 
20 on the support 2, the receiver being previously fitted with the ferrule 
having the fiber secured therein.