铌酸锂电光调制器应用于低频调制.doc

Use of LiNb03 modulators at low frequencies LiNb03 modulators are widely used for their high bandwidth performances that make them favored devices for high data rate optical communications (up to 40 Gb/s and high frequency (20 GHz analog transmission. They are less often used at low frequencies under 1 GHz. However LiNb03 phase modulators have also strong benefits at low frequencies (compactness, ease of use, low drive voltage compared to devices based on alternative technologies and are thus components to be considered even for kHz to MHz frequency range applications. Users searching to implement LiNb03 phase modulators with modulation signals showing low and high frequency components, and the typical case is the one of a pulse signal with sharp rising and fall edges and low repetition rate or long pulse duration, must be very cautious. A “high bandwidth” phase modulator, and here “high bandwidth” means 1 GHz typically, is not performing extremely well with such a modulation signal. The reason is that in order to get good high bandwidth performance, the impedance of the microwave line of the modulator is matched near to 50 ohms and a load resistance termination is connected at the end of the RF line to reduce or avoid electrical RF reflection. Thus, a significant level of current is traveling in the RF electrodes, leading to local temperature increases by Joule effect. Heating and thermal dissipation becomes a problem when the repetition period or the pulse duration becomes longer than the time constant of the thermal effects (in the range of 1kHz or below. Then the physical properties of the electrodes and waveguide are changing during the heat-on and cool-down periods, leading to unwanted phase drifts. Standard 5, 10 or 20 GHz phase modulators are not suitable for such applications involving very low repletion rate. To suppress that phenomenon, a solution is to use a modulator with a high input impedance load (typ 10 kΩ or directly an opened electrode line (MΩ