FREE-SPACE RESONANTLY ENHANCED PHASE MODULATORS
Electro-optic phase modulators (PM) are devices that change the phase of an optical signal by applying an electric field to an electro-optic material. PMs can operate over a wide spectral range from ultraviolet (UV) to infrared (LWIR) with low optical loss, high optical power and modulation frequencies ranging from 50 kHz up to 20 GHz.
Phase Modulators can achieve high modulation depths with low driving voltages, making them attractive for various applications, including laser cooling, spectroscopy and spectral broadening. QUBIG offers Phase Modulators as bulk/free-space (PM), fiber-coupled (PM.FC) and surface-mount (PM.SMD) devices.
Altered laser property: Phase
An important and easy to manipulate property of laser light that is commonly used for signal modulation is the phase. In general, optical radiation such as laser light is associated with electromagnetic waves which can be characterised with an amplitude and a phase. The phase determines in which part of an oscillation cycle the electric field is. Light where the optical phase evolves systematically and predictably in time possesses a high temporal coherence.
QUBIG’s resonant modulators consist of an electro-optic crystal that is coupled via a resonance circuitry to an RF input connector. This high-Q tank circuit is used to boost the input signal which eventually reduces the required RF power needed to achieve a desired modulation depth. An impedance matching network transforms the reactive crystal load to a 50Ohms input to allow for easy matching to standard RF drivers and function generators.
Effect on the laser light: Sideband generation
Resonant phase modulators are used to vary the phase of an optical laser beam. The induced sinusoidal phase variation f(t)=β*sin(Ω*t) at the modulation frequency Ω and peak phase change generates frequency sidebands at multiples of Ω about the central cw optical frequency, ω. The spectrum of a sinusoidally phase-modulated electric field after passing through the modulator is given by Bessel functions:
The amplitude in the m-th sideband at ω+m*Ω is proportional to Jm(β), where Jm is the m-th order Bessel function of the first kind. The amount of energy transferred from the fundamental J0(β) to the m-th sideband is proportional to the square of the electric field amplitude |Jm(β)|2. The modulation index β which describes how much energy is transferred from the carrier to the sidebands depends on many parameters such as the crystal material and its geometry, the laser wavelength as well as the applied RF power. The required value itself, which can be easily adjusted by tuning the RF level, varies a lot among the applications.