Amplitude Modulators

FREE-SPACE RESONANTLY ENHANCED AMPLITUDE MODULATORS

QUBIG provides resonantly enhanced, very efficient polarisation shifters that can be converted with appropriate polarisation optics into high-frequency amplitude/intensity modulators. Large spectral bandwidth, very low insertion loss, high optical damage thresholds and extinction ratios up to 30dB enable a wide range of applications.

The spectrum ranges from mode locking, optical chopping in industry to Lock-In amplification, multiplexing of single photon trains as well as rapid polarisation modulation for state manipulations in novel Quantum technology systems. Together with matching drivers that include cutting edge features a simple and quick implementation in experimental setups is provided.

General product overview

AM6 0.1 - 1 MHz Hz 10 ⁶	AM7 2 - 30 MHz 10 ⁷	AM8 31 - 160 MHz 10 ⁸
AM6-UV See details	AM7-UV See details	AM8-UV See details	UV 200 - 400 nm
AM6-VIS See details	AM7-VIS See details	AM8-VIS See details	VIS 360 - 650 nm
AM6-NIR See details	AM7-NIR See details	AM8-NIR See details	NIR 0.6 - 1.1μm
AM6-SWIR See details	AM7-SWIR See details	AM8-SWIR See details	SWIR 1.0 - 1.7μm

Background Information

**Fig. 1** | *Symbolisation of a polarisation modulation.*

**Fig. 2** | *Characteristic features (Smith chart with Q-circle, S21) of a resonant system visualised with a vector network analyser (VNA).*

**Fig. 3** | A typical setup for generating fast a) polarisation or b) amplitude modulation in a laser beam. c) Optical transfer function of a Sénarmont-type setup b) as a function of the analyser (P2) angle and the applied voltage U.

Altered Laser Property: Polarization/Amplitude

A transverse electromagnetic wave such as light consists of a coupled electric and magnetic field which oscillate perpendicularly to the direction of propagation. By convention, the “polarisation” of electromagnetic waves refers to the direction of the electric field. In case the direction of this electric field is well defined, the light is called polarised. Depending on how the electric field is oriented, one distinguishes between

Linear polarisation: the electric field of light oscillates in a single plane along the direction of propagation
Circular polarisation: the electric field rotates in a circle around the direction of propagation. Depending on the rotation direction, it is called left- or right-hand circularly polarised light.
Elliptical polarisation: the electric field of light describes an ellipse.

Mathematically polarisation is treated either by the Jones or the Mueller calculus, depending on whether the light is fully polarised or not.

Coupling: Resonant

QUBIG’s resonant modulators consist of electro-optic crystals that are 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.

Effect on the Laser Light: Polarisation / Amplitude Modulation

Electro-optic amplitude modulators (AM) are essentially Pockels cells consisting of a single or multiple electro-optic crystals with zero or compensated natural birefringence. Applying an electric field to the crystal induces a change in both the ordinary and extraordinary indices of refraction giving rise to an electric field dependent birefringence which leads to a change in the polarisation state of the transmitted optical beam. In this way the electro-optic crystal acts as a variable waveplate with retardance linearly dependent on the applied electric field. By placing a linear polariser after the modulator, the beam intensity through the polariser varies quasi sinusoidally with linear change in applied voltage. The voltage required to produce a retardance of π radians is called the halfwave voltage or simply Vπ.

Typical Applications

QUBIG provides resonantly enhanced, very efficient polarisation modulators that can be converted with appropriate polarisation optics into high-frequency amplitude/intensity modulators. Large spectral bandwidth, very low insertion loss, high optical damage thresholds and extinction ratios up to 30dB enable a wide range of applications.

**Fig. 1** | *Typical Lock-In detection scheme for measuring the phase and amplitude of very weak optical signals relative to a reference.*

**Fig. 2** | *Basic LiDAR operating principle based on the Time Of Flight (TOF) method.*

**Fig. 3** | *Schematic of a basic single photon multiplexing arrangement.*

**Fig. 4** | *Simplified schematic illustration of the Bell-Bloom magnetometer principle.*

Lock-In Detection

Amplitude modulators in combination with Lock-In amplifiers are suited for ultra-precision spectroscopy applications like optical gas detectors which are commonly used to measure air pollution or highly toxic gases / materials. Furthermore, in science and research fast intensity modulation is a common and established method for driving quantum state transitions in e.g. Rydberg atoms or ultra-cold quantum gases made of neutral atoms, ions or molecules.

LiDAR

(Light Detection And Ranging) is a surveying method that measures distance to an object by illuminating the target with pulsed laser light and measuring the reflected pulses with a sensor. Differences in laser return times and wavelengths can then be used to make digital 3-D representations of the target. LiDAR is the analogy to Radar, using laser instead of radio wave. Amplitude modulators from QUBIG are useful tools to generate ultra-fast laser intensity modulations.

Single Photon Multiplexing

Parametric single-photon sources are promising candidates for large-scale quantum networks due to their potential for photonic integration. Optical switches are used for active multiplexing of photons in order to overcome the intrinsically probabilistic nature of these sources, resulting in near-deterministic operation. The switching itself can be achieved by fast polarisation modulators that possess a high optical transmission and extinction ratio.

Magnetometry

The atomic magnetometer detects an external magnetic field by exciting atomic magnetic dipole transitions via modulated light (Bell–Bloom magnetometer) and measuring the coherent precession frequency of atomic spins about the external magnetic field . Besides SQUIDs, atomic magnetometer are the most sensitive magnetometers which are used in many significant fields, such as medicine, tests of fundamental symmetries, space exploration, and detection of nuclear magnetic resonance signals.