The optical lines are 34 m long and must have two breakable
connections for insertion reasons. In
telecommunication, where signals have to travel over large distances, monomode
fibers with a core diameter of 7 
m are used. In a typical particle physics
experiment the distances are of order 10-50 m, and multimode fibers with
a transmission bandwidth of 20 MHz 
km are sufficient. Such cables
can have core
diameters of a few 100 m. Large fiber diameters are preferable
for efficiently coupling the light of the LED diodes into the fibers,
and for being insensitive to small misalignments at the
connections, offering cheaper solutions.
Mechanical stress when bending a multifiber bundle on the other hand
is less for
small diameter fibers, so we selected as a compromise a fiber core diameter of
200 m (230 m cladding diameter, total fiber diameter 500 m).
A halogen-free cable with nine fibers has been manufactured by Huber and
Suhner [15].
A miniature 9-fold optical multiconnector ( 
matrix) has been designed
(see Figure 10). For assembly, the coating at the fiber end
is removed, the fiber is inserted from behind into the connector body
(Aluminium) and glued
with Araldit S50 [16]. The fiber tips exceeding the front face of the
connector are cut and glued with
Epo-tek 302 [17]. Then the connector face is polished.
Light losses have been measured to be around 40% through two naked connections and 15% when optical grease is used. These numbers are reproducible for repeated connections within 10%. The connector has been patented in Switzerland [14].
At the fiber ends, diodes are attached which are plugged into transistor sockets. For assembly of these pigtails, the diodes are provided with housings as shown on the left side of figure 11. The fiber ends are glued into small tubes (see right side of figure 11) as described above for the multifiber connector. The tubes with polished faces are then glued into the diode housings. The relative orientation of the two parts is optimized for maximum light transmission.
Acknowledgement
This work was
supported by the Swiss National Science Foundation. We thank for the substantial
help of P. Dick and S. Streuli.