Principles of Fluorescence Spectroscopy

PRINCIPLES OF FLUORESCENCE SPECTROSCOPY 165
Figure 5.8. Optical arrangement for an electrooptic modulator. P, polarizer; SB, Soliel-Babinet compensator; RF, radio frequency signal. The SB compensator
is replaced by an electrical DC voltage in some commercial FD instruments.
The cross-correlation method is surprisingly robust.
The harmonic content (frequency components) of almost
any excitation profile can be used if it contains frequency
components that are synchronized with the detector. Both
pulsed lasers and synchrotron radiation have been used as
modulated light sources. Pulse lasers provide harmonic
content to many gigahertz, so the bandwidth of the frequency-domain
instruments is then limited by the detector and
not the light modulator (Section 5.6).
5.2.3. Light Modulators
Adjustment of the frequency-domain instruments is aided
by understanding the operating principles of light modulators.
Light can be modulated with high efficiency using
acoustooptic modulators, which diffract light based on a
periodic density gradient caused by sound waves. Acoustooptic
(AO) modulators are typically resonant devices that
operate at only certain frequencies. While broadband or
variable frequency AO modulators are known, the active
area is usually small, limiting their use to focused laser
sources.
A general procedure to modulate light is provided by
the use of electrooptic (EO) modulators (Figure 5.8). EO
modulators are constructed of materials that rotate polarized
light when the material is exposed to an electrical
field. 54–56 The modulator is placed between crossed polarizers.
In the absence of any voltage there is no effect on the
incident light, and no light is transmitted. If a voltage is
applied to the modulator, the electric vector of the light is
rotated and some light passes through the second polarizer.
A voltage is applied to the modulator at the desired modulation
frequency.
When an EO modulator is used as just described, without
a bias, it provides modulated light at twice the frequency
of the electric field applied to the modulator. This occurs
because the optical system becomes transmissive whether
the voltage is positive or negative (Figure 5.9). However,
the amount of light transmitted is rather small. Hence, EO
modulators are usually operated with an optical or electri-
Figure 5.9. Modulated intensity from an electrooptic modulator. The
upper drawing shows the modulated intensity without an electrical or
optical bias, the lower drawing includes a λ/4 bias.