Optical Links

In the CMS tracker, optical fibers are advantageous compared to copper cable in several aspects: First of all, they place much less material within the tracker volume, minimizing unwanted multiple scattering and interaction probability. There are no problems with shielding and ground loops, since no electric connection is needed between transmitter and receiver. However, optical transmission is much more expensive than simple cabling, and optical connectors are not as robust as their electrical counterparts. Nevertheless, CMS has decided to use optical links for both digital and analog signal transmission.

Common specifications have been defined for the optical links within CMS, which include:

• $1310\,\rm nm$ wavelength
• Edge-emitting semiconductor lasers
• Single-mode fibers
• $40\,\rm MHz$ clocked transmission

The choice of the wavelength and the limited space imply the use of edge-emitting semiconductors. Multi-Quantum-Well (MQW) InGaAsP edge-emitting laser diodes have been selected because of their good linearity, low threshold current (below $10\,\rm mA$) and proven reliability. Photodiodes are epitaxially grown, planar InGaAs devices of small active volume. Both digital and analog signals are transmitted at $40\,\rm MHz$, which is quite slow compared to the capabilities of modern optoelectronics.

Figure: Optical links for readout and control in the CMS tracker.

Fig.  gives an overview of the optical links used in the CMS tracker. Approximately 50000 links are required for the analog readout, while the digital control needs only about 1000 lines. Most of the distance between detector and control room will be covered by 96-way multi-ribbon optical fibers. On a patch panel outside of the CMS experiment, the fibers are split into groups of twelve, leading to the edges of barrel and disk layers. Pigtails, which are attached to each laser and receiver, connect to the 12-way fibers there.