Characteristics of the resonant EOM with a passive circuit for TAMA300
Jan 30, 2001
Peter Beyersdorf
The characteristics of the new EOM which is packaged in a New Focus housing
have been investigated and are presented here. The response of the modulator was foundto have a center frequency of 15.225 MHz and a lorentzian linewidth of 384 kHz. The
modulation efficiency was measured and found to be 0.112 rad/vrms.
Figure 1 shows the experimental setup for measureing the charactersitics of themodulator. The frequency response of the modulator was measured by phase modulating
the attenuated laser beam at various frequencies and measureing the power in one of thefirst order sidebands, relative to the unmodulated carrier beam. The pmodulator wasdriven with a series of sine waves that had frequencies near 15.2 MHz and an rms
amplitude of 8.42 v. The power of the sidebands and carrier were by the opticalspectrum analyzer, which measures the power transmitted through a scanning FabryPerot cavity.
To measure the modulation efficiency of the modulator the attenuated laser beam
was phase modulated while the power in both first order sidebands and the carrier were
measured for various amplitude 15.225 MHz sine waves applied to the modulator.

Figure 1. The experimental setup for measuring the charateristics of the electrooptic modulator. The laserb e a m w a s a t t e n u a t e r b y a h a l f- w a v e p l a t e a n d p o l a r i z i n g b e a m s p l i t t e r . T h e a t t e n u a t e d b e a m w a s p h a s e
modulated by the EOM driven by various amplitude and frequency sine waves. The spectrum of themodulated light was measured by an opical spectrum analyzer.

The linewidth of the modulators response was found by measureing the responseover a broad frequency range and fitting the data to a lorentzian curve with only two freeparameters, the center freqeucny and linewidth. The lorenzian curve, shown in figure 2
with the data, isP(ω m )
P0=

∆f 2

( f − f0 ) + ∆ f 22

and was best fit with the parameters fo =15.174 MHz and ∆f=384 kHz. The measurement
covered a broad spectrum, and so measured far into the tails of the lorentzian curve
making it well suited for measuring the linewidth. With only 6 data points within one
linewidth of the center frequency, however, the precision of the measurement of thecenter frequency was poor.

normalized sideband power

1

0.8

0.6

0.4

0.2

012

12.5

1313.51414.5Frequency (MHz)

15

15.5

16

Figure 2. The response of the modulator over 4 MHz near the center frequency. A lorentzian fit to the datais also shown.

To increase the precision of the measurement of the center frequency, the response
was again measured with 100 data points inside of one linewidth. The center frequency
was found to be 15.225 MHz as shown in figure 3.

normalized sideband power

1

0.8

0.6

0.4

0.2

0
15

15.1

15.2
15.3
Frequency (MHz)'

15.4

15.5

Figure 3. The response of the modulator near the center frequency. Notice the discontinuity near 15.432
MHz.

The modulation efficiency was determined by measuring the power in the carrierand sidebands at different drive voltages to the EOM. The power levels, whennormalized to the unmoculated carrier power obey
P0 (m)
2= J 0 (m )P0 (0 )Pω (m) P−ω (m)2== J1 (m)P0 (0)P0 (0)
Figure 4 shows the data which fit to the these bessel functions when the modulation
efficiency is m=0.112 rad/vrms.

power normalized to unmodulated carrier power

1.2
1
0.8
0.6
0.4
0.2
00246
root-mean-square voltage

8

Figure 4. The power of the carrier and both first order sidebands normalized to the unmodulated carrier
power as a function of the root-mean-square drive voltage on the electrooptic modulator. The squaremarkers represnet the power in the carrier, the diamond and circular markers represent the power in each of
the first order sidebands, and the line at the top is the total power measured in the carrier and first order
sidebands.

These measurements characterize the electrooptic modulator. It has a linewidth of
384 kHz, a center frequency of 15.225 MHz and a modulation efficiency of 0.112
rad/vrms.