Phase Shifters - QUBIG GmbH

DC-COUPLED BROADBAND PHASE SHIFTERS

QUBIG provides DC-coupled Phase Shifters with large spectral bandwidth, very low insertion loss, high optical quality and damage thresholds that enable a wide range of applications such as

Laser frequency stabilisation (intra-cavity), interferometry, broadband phase modulation, optical dipole traps for the manipulation of novel quantum technology systems.

General product overview

UV 200 - 400nm	VIS 370 - 650nm	NIR 630 - 1100nm	SWIR 1.0 - 3.0um	MWIR 3 - 7um	LWIR 7 - 12um
PS - UV See details	PS - VIS See details	PS - NIR See details	PS - SWIR See details	PS - MWIR See details	PS - LWIR See details	PS HV short pulses / unshielded
PSA - UV See details	PSA - VIS See details	PSA - NIR See details	PSA - SWIR See details	PSA - MWIR See details	PSA - LWIR See details	PSA analog operation / shielded

Background Information

**Fig. 1** | *Illustration of a linear phase sweep representing arbitrary control of light retardation.*

**Fig. 2** | *Typical frequency response (depicted as Bode plot) of a DC coupled phase shifter.*

**Fig. 3** | *Mach-Zehnder interferometer with an electro-optic phase*

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.

Typical Applications

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:

**Fig. 1** | *Phase shifter placed in one arm of a Michelson interferometer.*

**Fig. 2** | *Illustration of single atoms in an optical lattice manipulated by a phase shifter.*

**Fig. 3** | *Intra-cavity phase shifter placed in a Fabry-Pèrot interferometer.*

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.