Electro-Optic Amplitude Modulator
Page 3
15956-D02 Rev.B, 1/26/2012
The halfwave voltage of lithium niobate is dependent on the optical wavelength and is given by:
L
d
r
n
r
n
V
o
e
13
3
33
3
−
=
λ
π
,
Where
λ
= optical wavelength
d
= electrode spacing
L
= optical path length
r
ij
= electro-optic coefficients
n
oe
= ordinary and extraordinary indices of refraction
The indices of refraction themselves are also functions of wavelength and temperature. While the above formula for the
halfwave voltage is relatively complicated, a reasonably close approximation for this modulator is obtained using a linear fit
over the typical operating range,
844
.
23
361
.
0
−
=
λ
π
V
where
λ is the optical wavelength in nm.
1.2. EO Amplitude Modulator Versions
Thorlabs offers the EO amplitude 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 have sufficient slew
rate, and output current level for driving the capacitive load of the crystal. Driver limitations and crystal
capacitance typically determine the limit for maximum drive frequency)
2.
High Q Resonant Mode Modulator
: A high Q resonant circuit is used to boost the RF input voltage up to
the high voltage needed by the EO crystal. The required peak to peak input voltage (as given in the
formula above) is then reduced by the Q-factor of the tank circuit, when driven sinusoidally at the
resonant frequency. An impedance matching network transforms the reactive crystal load to a 50
Ω
input
to allow for easy matching to standard RF drivers and function generations.
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.
1.3. Standard Features
•
Broadband DC coupled and High Q Resonant models
•
MgO doped Lithium Nioabate crystal
•
Standard Broadband AR coatings cover 400 to 1650 nm range
•
2 mm diameter clear aperture
•
SMA RF modulation input connector
•
DC to 100 MHz
•
Flange with thru holes on housing for secure mounting
•
#1/4-20 and #8-32 tapped mounting holes on bottom
