Specifications

Features

Large Operating Frequency Range Coverage

We provide standard phase modulators with operating frequencies from DC up to 9.2 GHz.

Typical modulation depth of our resonant phase modulators plotted as a function of modulator frequency for three families of modulators.

Resonant Designs Offer Very Low Drive Voltages

We offer both broadband modulators that can be used at a wide range of frequencies and resonant modulators that operate at a single frequency with much lower drive voltage requirements. The resonant modulators feature a resonant tank circuit to maximize power transfer from driver to crystal, thereby maximizing the voltage across the crystal, lowering required drive voltages by a factor of nearly 10. Each resonant phase modulator is built to the your exact specified frequency.

Typical RF drive power as a function of modulation depth. Phase shifts of 1, 2, 3, and 4 radians are represented by the dark blue, magenta, green and cyan, respectively.

Phase Modulator Bessel Functions

This spectrum of a phase-modulated electric field is given by Bessel functions. The optical intensity of each sideband is proportional to the square of the electric field amplitude. The amplitude of the kth sideband is proportional to Jk(m), where Jk is the Bessel function of order k. The fraction of optical power transferred into each of the first-order sidebands is [J1(m)]2, and the fraction of optical power that remains in the carrier is [J0(m)]2.

Phase Modulator Operation

When a phase modulator is used, the laser beam should be well collimated and its polarization should be oriented vertically to within 1°. For an unpolarized laser, the polarizer should have an extinction ratio greater than 100:1. We recommend our Glan-Thompson polarizers or our low-cost sheet polarizers.

Mechanical Apertures for Easy Optical Alignment

The key to obtaining this pure phase modulation is good optical alignment of the beam to the crystal’s propagation axis and accurate orientation of the laser’s electric field with the crystal’s electro-optic axis. New Focus makes proper alignment easy, simply pass the beam through the mechanical apertures.

Move Your Optical Isolator Instead of the Beam

Our multi-axis stages are designed for applications where precise positioning and high stability are required. Shown here is a typical application of aligning an optical modulator to a laser beam with our four-axis kinematic device alignment stage.

Observing Phase Modulation

A typical setup for observing phase modulation: a HeNe laser beam is sent through a Model 4001 resonant phase modulator operating at 29 MHz, and the beam is focused into an optical spectrum analyzer. The laser’s phase-modulated spectrum, with its characteristic frequency sidebands, is observed on an oscilloscope.

Laser Frequency Stabilization Application

A Model 4001/3 or 4061/3 resonant phase modulator is the ideal component to use in a Pound-Drever-Hall laser frequency stabilization system. This optical FM frequency discriminator technique* is used to lock the optical frequency of a laser to a stable Fabry-Perot reference cavity. The system consists of a single-frequency laser beam that is sinusoidally phase modulated and coupled into an axial mode of the Fabry-Perot resonator cavity. The stabilization signal is fed back to a high-voltage amplifier that drives a piezoelectric transducer (PZT). The frequency-stabilized light transmitted by the cavity is clean spatially as well as spectrally. *R.W.P. Drever, et al. “Laser Phase and Frequency Stabilization Using an Optical Resonator,” Appl. Phys. B31, pp. 97–105 (1983).

Diagram for a Pound-Drever-Hall laser-frequency-stabilization system.

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