Electro Optic Phase Modulators
By making the electro-optic crystal part of a tuned circuit, relatively low drive voltages can be used to achieve up to p radians of phase modulation. For the range 50 MHz to 100 MHz the enhancement of the voltage which is able to be applied across the crystal as a result is typically around 10 times that of the 50 ohm input drive.
For the tuned circuit arrangements the drive power required from the source is usually less than one watt.
These phase modulators can currently only be supplied without a drive source.
|Outline Drawing of Phase
|Optical wavelength range||
600 - 1300 nm
Brewster cut or
ar/ar normal incidence rod
|RF Drive power||2 watt maximum for p radians|
|Tuning range of optical head||± 15% of centre frequency|
|Voltage gain from tuned circuit||» 10|
|Input impedance of head||» 50 W|
Note: These are typical specifications taken from a
phase modulator operating at 70 MHz.
Travelling Wave Broad-Band Phase Modulator: PM1000 (up to 1GHz available)
For frequencies higher than ~100Mhz, we have found that the enhancement in voltage available from using a resonant circuit is soon lessened to the extent that it is not so effective. We then find that a more conventional modulator is beneficial, however we use a pseudo-travelling wave arrangement to provide a 50 ohm characteristic impedance and also some matching of the optical and electrical phase velocity to maximize the bandwidth. Using this arrangment we have made phase modulators with useful response to ~1GHz. The reasonable availability of moderately high powered (50W) r.f. amplifiers up to this frequency makes it possible to provide a voltage swing of around 140V peak to peak across the crystal which is sufficient to provide a sufficient degree of modulation depth for most applications. The device is supplied without driver or load (the r.f. signal must be terminated in a suitably specified r.f. load)
|Optical Wavelength range||
600 - 1300 nm
200 - 600 nm (ADP)
|Characteristic design impedance||» 50W|
|Drive Power||Up to 50 watts|