Ultrafast Laser Laboratory

Electro-Optical Modulators

In systems with very many thousand signal channels, such as detectors for Large Hadron Colliders, identifying the signal information at the trigger level with a high degree of parallelism in signal transfer and processing, the readout electronics using the traditional electronic technique becomes rather complex and is almost a formidable task. Conventional detector signal transfer schemes relying on copper cables and electronic drive circuits for signal transmission result in a limited bandwidth and/or cable length, they are also vulnerable to signal cross-talk and electromagnetic interference. Adding, and ultimately replaced by, optoelectronics and fiber optic cables for the readout of analog signals in high energy particle detectors is an attractive alternative.

One early approach utilized a system of integrated Mach-Zehnder optical intensity modulators, fabricated on a single substrate, and powered by a diode-pumped Nd:YAG at the wavelength of 1.3 micron. In this scheme, signal charge from a detector may be applied directly to the electrodes of the modulator. Alternatively, charge is fed first to a preamplifer and the output signals are then applied to the electrodes of an optical modulator. Laser light is brought in from a remote source on a single-mode polarization preserving fiber, and the optical output signals are carried on ribbon single-mode fibers. This appoach can dramatically reduce the mass-volume in detectors with many signal channels by eliminating copper cables, increase the signal bandwidth, improve the noise immunity, and provide a high radiation resistance. Furthermore, the capability of operating the optical modulators in a cryogenic detector such as a liquid ionization calorimeter also simplifies the cold feedthroughs and lowers the heat loss due to copper cables. An advantage of optical modulators is that they are passive devices that do not generate heat. But a small amount of optical power is lost in the substrate due to branching and coupling, yet the heat dissipation of the modulators is still significantly less than that of the conventional electronic devices. The capability of summing all detector signals optically rather than electronically to deduce the total energy collected by a detector prior to any electronic signal processing may also be attractive in some applications.

The principle of operation is based on the linear electro-optic effect, or the Pockels effect: a change of optical birefringence (refractive index) in the waveguide due to application of an electric field. A polarized light beam traveling in a single transverse mode optical waveguide is split by refraction into two beams, carried a suitable distance and then recombined, giving rise to interference. The interference is constructive when the two beams are in phase, and the full power exits to the output waveguide. If the optical lengths of the two arms differ by half of a wavelength (optical phase of ${\rm\pi}$), the interference is destructive. The output is then splitted by a 3 dB coupler producing two complementary optical outputs with no power loss to the substrate, hence no thermal loading which would significantly relax the thermal budget in a particle detector. A push-pull type rf-signal electrode and a dc-bias electrode are separately defined on the waveguide structure. The modulator is pigtailed with a single-mode polarization maintaining fiber at the input end and a standard single-mode fiber at the output end. Both fiber ends are terminated with ST (AT\&T registered trademark) fiber connectors. A change of the refractive index of the waveguide resulting from the application of an electrical signal to the electrode causes an optical phase shift, hence intensities change in the outputs of the interferometer.

A schematic of an integrated optical modulator and its tranfer characterestic are shown here. The V-pi voltage or equivalently the switching charges, the charges required to switch the device from zero to its maximum intensity, are 4.5 Volt and 63 pC, respectively.

Schematic of the charge measurements from a detector element of a multiwire proportional chamber, the signal output after the shaping amplifier, and the integral linearity of the optical modulator system are shown here.

For more information or preprint request contact Thomas Y. F. Tsang

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