Chapter 1: Description
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Chapter 1 Description
1.1. Overview
The electro-optic phase modulator (EO-PM) consists of an electro-optic crystal,
lithium niobate, packaged in a compact housing with an RF input connector. By
applying an appropriate voltage to the RF input, a corresponding change in the
extraordinary refractive index will occur. If the optical input is both linearly
polarized and aligned with the extraordinary axis of the modulator crystal, the
output will undergo a pure phase shift with no change in the state of polarization.
This effect is utilized in applications where only a phase shift is required (i.e.,
generating RF sidebands and Mach Zhender interferometers).
The extraordinary axis (the crystal Z-axis or optic axis) is oriented vertically with
respect to the modulator package. With linearly polarized input light, the angle of
the input polarization with respect to the optic axis determines how the output will
be affected: When the input polarization is vertically aligned, a pure phase shift
will occur. If the input polarization is horizontal, a pure phase shift will also occur
but with approximately 30% less efficiency than for vertical polarization (r13/r33 =
30% for Lithium Niobate). If the input polarization is at 45
, the output polarization
will undergo a change from linear to elliptical. In this mode, the modulator can be
considered an electronically tunable waveplate.
1.2. EO Phase Modulator Versions
Thorlabs offers the EO phase modulators in two basic versions:
1.
DC-coupled Broadband Version
: The electro-optic crystal is directly
coupled to the RF modulation input connector, allowing the full
bandwidth of the crystal to be utilized. The broadband version can be
driven from DC up to the frequency limit of the RF driver (
Note
: The
external driver must be capable of driving the capacitive load of the
crystal).
2.
High Q Resonant Mode Modulator
: A high Q tank circuit is used to
boost the RF input voltage up to the high voltage needed by the EO
crystal. An impedance matching network transforms the reactive crystal
load to a 50
input to allow for easy matching to standard RF drivers
and function generators.
Note
: By definition, high Q circuits have a narrow operating bandwidth. The
resonant EO modulators have a typical operating bandwidth that is 5% of the
center frequency. Therefore, the operating frequency must be specified at time
of purchase.
