Automatic machine for welding two optical fibers end-to-end

An automatic machine for welding optical fibers end-to-end comprises a first plate (1) for supporting a first or "reference" fiber (2) and a second plate (5) for supporting a second or "moving" fiber (6). The second plate can be translated along Ox and Oy axes in a plane perpendicular to the axes of the fibers. The ends of the fibers are illuminated by light sources (30, 31), optical systems make images thereof, and optical pick-ups (12, 13, 18, 19) located at the image points provide optical signals representative of the positions of the optical fibers relative to each of said Ox and Oy axes. These signals are converted into electrical signals and are compared by electronic comparators (15, 21) which deliver corresponding error signals to control means (17, 23, 24) for moving the second plate until the moving fiber is in alignment with the reference fiber.

The present invention relates to an automatic machine for welding optical 
fibers end-to-end, the machine being of the type comprising: 
first and second optical fiber support plates respectively for supporting a 
first, or "reference" optical fiber, and a second or "moving" optical 
fiber; 
automatic translation means connected to said second support plate for 
moving said second support plate in two orthogonal directions which are 
perpendicular to the axes of the fibers; 
automatic translation means connected to one of said support plates for 
moving said support plate parallel to the axes of the fibers; 
manual translation means connected to the other one of sadi support plates 
for moving said other support plate parallel to the axes of the fibers; 
position detecting means comprising light source means for illuminating the 
ends of the optical fibers; optical lens means for forming magnified 
images thereof; and optical pick-up means responsive to the positions of 
said images; 
control means responsive to signals derived from said optical pick-up means 
and connected to control said automatic translation means; and 
fiber welding means. 
BACKGROUND OF THE INVENTION 
U.S. Pat. No. 3,960,531 describes a machine comprising a first plate for 
supporting a first optical fiber and equipped with means for translation 
parallel to the axis of the fiber, a second plate for supporting a second 
optical fiber and equipped with means for horizontal and vertical 
translation perpendicular to the axis of the fibers, a microscope for 
observing the ends of the optical fibers while their axes are being 
aligned by manual displacement of the plates, a moving wedge for marking a 
welding plane by the wedge abutting against the end of one of the fibers, 
and two electrodes for welding the ends of the fibers in the welding 
plane. 
Such a machine is only applicable to low accuracy end-to-end welding of 
optical fibers, eg. for multimode fibers having a core diameter of about 
100 microns. 
The machine is not suitable for the kind of accuracy required to minimize 
light losses due welding nor is it suitable under any circumstances for 
monomode fibers which have a core diameter of a few microns only. 
It is for such reasons that Research Disclosure No. 21643, April 1982 
describes an automatic machine for positioning optical fibers, in 
particular for welding, along three orthogonal axes by means of a TV 
camera controlling a microprocessor. However, such a machine is very 
complex and expensive. 
Preferred embodiments of the present invention provide an automatic machine 
for end-to-end welding of optical fibers with the axes of the fibers being 
aligned with great accuracy, eg. about 1/10-th of a micron. The machine 
can also reduce welding defects. Furthermore, alignment is performed 
rapidly without the need for operator dexterity, the heating of the fiber 
ends during welding can be closely controlled, and various welding cycles 
can be reproduced at will. 
SUMMARY OF THE INVENTION 
According to the present invention, a machine of the above-defined type 
includes the improvement wherein said position detecting means includes 
pick-up means for detecting the position of each fiber end in two 
orthogonal directions corresponding to the said orthogonal directions 
perpendicular to the axes of the fibers, each fiber end thus being 
associated with at least two optical pick-ups, one for each of said 
orthogonal directions, the signals from said optical pick-ups being 
applied to respective tansducers to be converted into respective 
electrical signals, which electrical signals are so connected to 
electronic comparators that, for each of said orthogonal directions, the 
electrical signals derived from pick-ups associated with different optical 
fiber ends are electronically compared, and the electrical signals 
resulting from said comparisons are connected to said control means to 
enable said control means to control said automatic translation means to 
bring said fiber ends into alignment, and into end-to-end contact ready to 
be welded together. 
A machine in accordance with the invention preferably includes at least one 
of the following features. 
For each of said orthogonal directions, said position detecting means 
comprises said light source means located on one side of the fiber ends, a 
common optical lens means located on the other side thereof for forming 
magnified real images of the optical fiber ends, and respective optical 
pick-ups located at said images. 
Said control means include a microprocessor. 
Said comparators are connected to operate respective light-emitting diodes. 
The idea is to provide a matrix display covering a range of positions and 
to indicate the position of each fiber on the matrix display in a unique 
manner (eg. a steady light for the reference fiber and a flashing light 
for the moving fiber). When placing the fibers in the machine, the 
operator then attempts to obtain as good an alignment as possible by 
superposing the two indications. The superposed flashing and steady lights 
may be anywhere in the matrix display, since absolute position is 
immaterial. 
Said automatic translation means for movement in a direction parallel to 
the fiber axes is connected to said second support plate for supporting 
the moving fiber. 
Said automatic translation means for movement in a direction parallel to 
the fiber axes is constituted by a high-resolution stepper motor. 
The machine further comprises means for backing off the moving fiber by a 
few tenths of a micron after end-to-end contact is established between the 
fibers. 
Said means for backing off the moving fiber are under the control of said 
control means. 
Second support plate for supporting the moving fiber is fixed to the free 
end of a cantilever bar having an intermediate point engaged in a bead of 
very hard material, said bead being in contact with wedges of 
piezoelectric material constituting said automatic translation means for 
moving said second support plate in two orthogonal directions 
perpendicular to the fiber axes. 
Said means for welding the ends of the fibers comprises electric arc 
welding electrodes made of platinum plated tungsten. 
Said machine includes means for pre-heating the ends of the aligned fibers 
while not in end-to-end contact, to polish their end faces prior to 
welding. 
Each image of a fiber end is formed by a respective optical lens means. 
Said light source means is constituted by a single light source for each of 
said orthogonal directions and common to both fiber ends. 
Each optical pick-up means associated with an image of a fiber end is 
constituted by a pair of optical pick-ups connected to a corresponding 
pair of respective transducers, the electrical signals from each pair of 
transducers being connected to a corresponding electronic comparator to 
provide an electrical signal representative of the position of the 
corresponding fiber end image. 
For each of said orthogonal directions, said electrical signals 
representative of the positions of the fiber end images are themselves 
compared by an electronic comparator.

MORE DETAILED DESCRIPTION 
In FIG. 1, a first plate 1 serves to support a first or "fixed" or 
"reference" optical fiber 2 which rests in the groove between two adjacent 
cylindrical metal rods 3 and 4 which are themselves crimped into grooves 
in the plate 1. A second plate 5 serves as a support for a second or 
"moving" optical fiber 6 which rests in the groove between the similar 
cylindrical metal rods 7 and 8 likewise crimped into grooves in the second 
plate. The optical fibers 2 and 6 are held in place on their support 
plates by clips (not shown). 
The end face 9 of the first fiber 2 is moved by adjusting the position of 
the first plate 1 by means of knurled adjustment knob (not shown) until it 
lies in the plane of welding electrodes 10A and 10B. The welding 
electrodes are preferably made of tungsten and thorium and include a 
platinum coating to prevent corrosion. 
Once the fibers have been initially loaded into the clips, the end face 11 
of the moving fiber is generally offset both vertically and horizontally 
from the end face of the fixed fiber, as indicated by arrow 11A. 
Throughout this description offsets are shown exaggerated, this is because 
monomode optical fibers need to be aligned very accurately, and commonly 
encountered degrees of offset in need of correction would hardly show up 
in the figures. 
The positions of the reference fiber 2 and the moving fiber 6 are detected 
by means of two optical pick-ups 12 and 13 which are optimized for 
subsequent use in adjusting the horizontal position of the fibers, and by 
means of two optical pick-ups 18 and 19 which are optimized for use in 
adjusting the vertical position of the fibers as explained below with 
reference to FIG. 2. 
The signals from the optical pick-ups 12 and 13 on receiving light beams 
(not shown in FIG. 1) are transformed into respective electrical signals 
by a first photo-receiver or transducer block 14. These electrical signals 
are compared by a first comparator 15 which generates an error signal 
which is amplified by a first amplifier 16 and is then applied to a 
microprocessor 17. 
Likewise, the signals from the optical pickups 18 and 19 are transformed 
into respective electrical signals by a second photo-receiver or 
transducer block 20 and these electrical signals are compared by a second 
comparator 21. The resulting error signal is applied to the microprocessor 
17 via a second error amplifier 22. 
On the basis of the signals derived from the optical pick-ups, the 
microprocessor delivers instructions for correcting the position of the 
second plate 5 which serves to move the "moving" fiber 6. A first 
correction instruction is applied to a first motor unit 23 which moves the 
plate in a vertical plane, and a second correction instruction is applied 
to a second motor unit 24 to move the moving plate 5 in a horizontal 
plane. These motor units may be constituted, for example, by piezoelectric 
blades whose operation is described in greater detail below with reference 
to FIG. 3. 
In addition, once the moving fiber has been correctly positioned, the 
microprocessor 17 uses a third motor unit 25 to control an approach 
sequence bringing the end 11 of the moving fiber 6 fixed to the plate 5 
into contact with the end 9 of the reference fiber. The pick-ups then 
sense the initial bending of the fibers due to their coming into contact 
and defining their contact plane. The microprocessor 17 then controls a 
slight backing off of the moving fiber, and welding can then take place. 
FIG. 2 shows in greater detail how two pick-ups, such as 12 and 13 are used 
for adjusting position. Light sources 30 and 31 direct respective light 
beams 32 and 33 towards the ends of the reference fiber 2 and the moving 
fiber 5 respectively. Downstream from each fiber, there is a zone in each 
beam which includes a projected shadow of the fiber. The shadowcontaining 
beams are then enlarged by a common optical lens system 34 to object image 
points at which the pick-ups 12 and 13 are located. Each pick-up has two 
adjacent rectangular light-receiving areas. The shadow zone cast the fiber 
overlaps partially onto both of these areas, which are generally not 
equally affected, thereby giving rise to electrical signals representative 
of the position of the shadow, and hence of the optical fiber that cast 
the shadow. When the moving optical fiber 6 is not in the same plane as 
the reference optical fiber 2, the offset between the fibers is 
represented by the offset between the positions of their respective 
shadows on the optical pick-ups 12 and 13. 
An entirely similar arrangement using the pick-ups 18 and 19 detects the 
vertical offset between the fibers 2 and 6 and transmits information 
representative thereof to the microprocessor 17. The microprocessor sends 
instructions to the motor units 23 and 24 as a function of the offset 
information it receives, thereby reducing the offsets. 
The motor units which respond to the microprocessor to reduce the Ox and Oy 
offsets (see FIG. 2) to substantially nil and to move the end faces of the 
fibers into mutual contact (along the Oz axis) are shown in greater detail 
in FIG. 3. 
The second support plate 5 is fixed to the free end of a cantilevered beam 
41 whose other end is fixed in a block 40. The beam passes through a 
spherical bead 42 for applying adjustment forces thereto. The bead 42 is 
made of hard material such as synthetic ruby and is in contact with two 
pairs of piezoelectric wedges which are shown diagrammatically by arrows 
43 & 44 and 45 & 46. The wedges 43 & 44 serve to displace the moving fiber 
in the horizontal plane, while the wedges 45 & 46 serve to displace the 
moving fiber in the vertical plane. The piezoelectric wedges could be 
replaced by micro-motors. 
Although the fiber displacements obtained by applying horizontal and 
vertical forces to the bar 41 are not themselves strictly horizontal and 
vertical, they can be treated as such in practice, given the length of the 
lever arm and the small size of the displacements involved. 
Axial displacements are obtained by moving the block 40 which is mounted 
via ball bearings on rails 36. When the block 40 moves, the arm 41 and the 
moving fiber at its end also move. The block 40 is moved by a high 
resolution micro-motor represented in FIG. 3 by arrows 37 and 38. 
When the end of the moving fiber 6 abuts against the end of the reference 
fiber 2, the pressure which it exerts thereon causes the ends of the 
touching fibers to deflect in the OX-OY plane. This deflection is detected 
by the optical system and is signalled to the microprocessor 17 which has 
stored the size of the advance step taken immediately prior to such 
deflection. The microprocessor responds to the deflection by instructing 
the moving fiber 6 to back off a little, (eg a few tenths of a micron), 
and then starts a welding cycle. 
The welding cycle includes a first operation of heating to below the 
melting point of the optical fiber material, in order to round the edges 
of their ends and thus avoid including any air bubbles at the interface 
during welding proper. This preheating may be performed by means of a 
carbon dioxide laser. 
The preheating is followed by a short pause, and then by welding proper in 
which the ends of the fibers are raised above their melting points. This 
cycle is likewise under the control of the microprocessor, but in a 
variant it could be under manual control. 
In FIG. 4 a single light source 30 directs two pencil beams (shown 
diagrammatically at 32 and 33) to the ends of the fixed and the moving 
fibers 2 and 5. The axis of the moving fiber 5 is shown offset from the 
axis of the fixed fiber 2 in a horizontal plane by an amount x. The pencil 
beams are directed by respective optical lens systems 34A and 34B towards 
the optical pick-ups 12 and 13. Since the reference fiber 2 is correctly 
placed, the beam 32 falls on the surface of the optical pick-up 12, 
together with a dark portion caused by the shadow cast by the fiber. Since 
the fiber 5 is off axis, the light beam 33 forms a spot of light 35 off 
the pick-up 13 (clearly if the moving fiber 5 were nearer to its correct 
position, then the spot would fall at least in part on the pick-up 13). 
The signals from the pick-ups 12 and 13 are converted into respective 
electrical signals by the transducers 14, and are compared by a comparator 
15 which generates an error signal for application to the microprocessor 
17 after amplification. The microprocessor then causes the moving fiber 5 
to be moved in a horizontal plane until it is in alignment with the fixed 
fiber 2, as described above. 
FIG. 5 shows a similar optical sensor arrangement to FIGS. 2 and 4, using a 
single light source 30 as in FIG. 4 and a single optical lens system 34 as 
in FIG. 2. 
FIG. 6 shows an improvement over the FIG. 5 arrangement, and constitutes 
the preferred embodiment of the invention. As in FIG. 5 there is a single 
light source 30 and a single optical lens system 34. However, each optical 
pick-up 12, 13 is now constituted by a pair of pick-ups 12A, 12B or 13A, 
13B. Each individual pick-up is connected to a corresponding transducer in 
a box marked 14A or 14B as the case may be. The electrical signals from 
each pair of pick-ups are then compared by respective comparators 15A and 
15B, and the output signals from these comparators are themselves compared 
in a comparator 15C which provides an error signal for feeding to the 
microprocessor. 
If a moving fiber 5 is off axis relative to the fixed fiber 2, its shadow 
does not fall equally on the two portions of the corresponding pick-up 13. 
The unequal electrical signals that result therefrom are a direct function 
of the position of the fiber 5. The same is true for the shadow of the 
reference fiber on the pair of pick-ups 12. The position of the moving 
fiber 5 can therefore be adjusted to ensure that it is exactly the same 
degree off its nominal position (as determined by the optical system from 
the source 30 via the lens system 34 and on to the pick-ups 12 and 13) as 
is the reference fiber 2. 
One of the advantages of using pairs of pick-ups is that information is 
available to the microprocessor concerning the sign of the offset, so it 
begins its corrective action in the right direction. Another advantage is 
that the differential system constituted by each pair of pick-ups is 
relatively insensitive to possible variations in the ratio of the image 
area to the active area of the pick-up. 
It will readily be understood that the various options described above (one 
or two light sources, one or two lens systems, single pick-ups or pairs of 
pick-ups) can be mixed in combinations other than the four specifically 
shown in the drawings. In particular, pairs of pick-ups can be used with 
two optical lens systems, and/or with two light sources.