Product Features

● Annealing Proton Exchange
● High Temperature Proton Exchange (Soft Proton Exchange)
● Fiber pigtailing
● High resolution Poling
● X, Y and Z cuts of LiNbO3, 
● Dielectric and metal deposition
● Photolithography

Definition of parameters

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The definition of Vπ is the same as the Vπ in an intensity modulator which interferometric configuration adopts the same push-pull electrodes (and optical waveguides) used in the gyro chip.

A test amplitude modulator is included in the wafer. Then we measured the Vπ for this device. The Vπ of an interferometric integrated optical amplitude modulator is defined as the voltage change requested to move the modulator from the minimum to the maximum optical transmission. It is a function of frequency f of the modulating signal V(f), and is defined in the modulator optical transmission function:




1.1.1 Phase Modulators

The electro-optic effect is used to modulate the optical carrier fase. In this device a couple of electrodes are located on the sides of a single waveguide (figure 1) .

The modulator transfer function is simply given by:




Where Δф is the relative phase delay, V is the voltage applied to the electrodes and Vπ is the necessary voltage required to obtain a phase delay equal to π.


1.1.2 Coupling Electrodes

When the signal is time varying, the simple capacitive electrode configuration, in which they can be considered a capacitor C in parallel with the terminating resistor R, is frequency limited by the RC cut. The value of this cut is usually in the range of 1-2 GHz. In order to obtain modulators efficient enough for the microwave frequencies (20 GHz), coplanar transmission line configurations are adopted for the electrodes, allowing Travelling Wave (TW) coupling between the modulating signal and the optical carrier.
The higher frequency limit of the TW electrode configuration is possible due to:
the difference between the propagation speeds of the modulating signal (c/n RF ) and the optical carrier (c/n opt );
the resistive/dielectrical losses experimented by the modulating in the coupling electrodes. The difference between the propagation speeds is the major limitation to the bandwidth in the case of conventional TW electrodes. The -3dB bandwidth (BW)is given in this case by the following expression:




where L is the length of the coupling between the TW electrodes and the optical waveguide Δn=nRF-nopt is the difference between the two propagation indexes (typically Δn=2). Because the efficiency of the modulator (Vπ) decreases with L, wider bandwidths imply lower electro-optic efficiencies.

Alenia Marconi Systems has developed a new TW electrode configuration enabling us to overcome this limitation. In this new configuration, called “Velocity Matched” (VM) the propagation speed of the modulating signal is adapted to that of the optical carrier. The bandwidth of the modulator is now limited by resistive/dielectric losses only, which increase with frequency. Moreover, the electro-optic efficiency is practically bandwidth independent (figure 2) . This makes the VM increasingly advantageous, in terms of efficiency, over the conventional TW electrodes as the bandwidth increases. The electro-optic efficiency of the conventional TW still remains higher than that of the VM configuration for a bandwidth lower than a threshold value, typically in the range of 3¸5 GHz. A further advantage of the VM configuration is the value of the characteristic impedance close to 50 Ohms, perfectly matched with the external signal line, in spite of the 25-35 Ohms of the conventional TW electrodes.




Figure 2: Electro-optic efficiency vs bandwidth