Altered Light Property: Phase
Light in free space can be described by an electromagnetic wave which propagates transversely to the propagation direction with the speed of light. When passing through material the light velocity is reduced by the refractive index. The phase retardation caused by the element is obtained by measuring the optical path difference (OPD) in setups based on the interference principle (Michelson, Mach-Zehnder, Young).
Coupling: DC / Broadband
The electro-optic crystal is directly coupled to the RF modulation input connector, allowing the full bandwidth of the crystal to be utilised. The bandwidth of a DC-coupled phase shifter is limited by the output impedance of the driver that forms with the capacitance of the crystal a low-pass filter. For typical retarders with 50Ohms coupling (via e.g. SMA) the limit is about 150MHz. Applications with nano-second response times require much higher bandwidths. In such cases the high voltage is coupled directly to the eo-crystal via very short, flying leads.
Effect on the laser light: Optical retardation / Phase shifts
QUBIG’s phase shifters provide a voltage dependent phase retardation on a linearly polarised input beam by inducing a change in the crystal’s extraordinary index of refraction via the electro-optic effect.
The control signal can be a DC/HV signal for interferometry applications. In case the control voltage is a time varying RF signal, the optical beam undergoes a phase modulation whereby the energy at the carrier frequency is converted into sidebands separated from the fundamental frequency by the integer multiples of the modulating frequency. Note: As the electro-optic effect is generally weak and no resonant enhancement is implemented several hundred volts might be required to achieve a significant effect.
QUBIG provides DC-coupled phase shifters with large spectral bandwidth, very low optical insertion loss, high optical quality and damage thresholds that enable a wide range of applications such as:
Interferometry
Broadband DC-coupled electro-optic phase shifters are often used in one arm of an interferometer to precisely control the relative optical path length between the arms. Interferometers are widely used in science and industry for extremely accurate and complex measurements of small displacements, refractive index changes and surface irregularities. A recent example is the detection of extremely weak gravitational waves with a Michelson interferometer at LIGO (Nobel price 2017) which confirms yet again Einstein’s general relativity theory.
Optical lattices
Electro-optic phase shifters find versatile use in many areas of science and research. They have been successfully deployed for electro-optic bandwidth compression of optical wavepackets (eo time lens), optical serrodyne frequency shifting of laser light or the manipulation of neutral atoms in optical dipole traps and lattices. Potential applications can be found in ultrafast spectroscopy, high-speed communications and even optical quantum science and technology.
Intra-Cavity
Intracavity electro-optical modulators are commonly used in external cavity diode lasers (ECDL) for stabilising their frequency to an ultra-stable reference. Due to their fast response times frequency noise up to the high MHz regime can be efficiently suppressed. Only a few volts (i.e. small phase shifts) are usually required for significant noise reduction at high frequencies, as large amplitude, low frequency noise on the laser light is typically pre-compensated by piezo actuators.
