Electro-optical modulator realizes voltage transfer function in synthesizing birefringent networks. Choice of the voltage transfer function is important, the most satisfactory optimizes the modulator property

Ammann, E. O.

Yarborough, J. M.

English

NASA Technical Reports Server

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FIGURE 2 
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Light 
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Input	 FIGURE 1 
August 1968	 Brief 68-10275 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Synthesis of Electro-Optic Modulators for Amplitude Modulation of Light 
Output Polarizer
A procedure has been developed for synthesizing 
electro-optical amplitude modulators having arbitrary 
modulation characteristics. The technique is an adap-
tation of the procedure of Ammann and Yarborough 
for synthesizing naturally birefringent networks. With 
this procedure, a voltage transfer function K(v) ex-
pressed in terms of an exponential series can be 
realized by an electro-optic network of the form 
shown in Figure 1. The resulting modulator consists 
of a number of stages between an input and an output 
polarizer, with each stage consisting of an electro-
optic element (Pockels or Kerr cell) and an optical 
compensator. 
The manner in which K(v) is chosen is very impor-
tant. If sufficient care is not taken in this choice, 
the performance of the synthesized modulator can
easily be inferior to that obtained from the simple, 
conventional amplitude modulator of Figure 2. Sev-
eral techniques for choosing K(v) were tried with 
varying degrees of success. The most satisfactory re-
sults were obtained when the Ci coefficients of the 
K(v) expression were chosen to directly optimize the 
modulator property (or properties) of greatest interest. 
This was done for two cases of interest: the design of 
a modulator for use with (a) an envelope detector 
and (b) a square-law detector. 
The modulator properties which were chosen (arbi-
trarily) for optimization were the following. The 
modulating signal v was assumed to be of the form, 
v = Vcoswmt. The demodulated signal from the de-
tector will in general contain a dc term, a fundamen-
tal, and harmonics. It was deemed desirable for the 
-	 -	 (continued overleaf( 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
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fundamental to be linearly proportional to V. and for 
the harmonics to be minimized. Hence modulator per-
formance was measured by calculating the deviation 
from linearity of the fundamental and the amplitudes 
of the harmonics. 
Best results were obtained for both the envelope 
and square-law detector cases by writing the funda-
mental and harmonic amplitudes as power series in V. 
The C 1 coefficients were then chosen to eliminate 
as many nonlinear terms from the fundamental ex-
pression and as many low-order terms from the 
second harmonic expression as possible. The K(v) 
transfer functions so derived do indeed give im-
proved modulator performance: the modulator de-
signs corresponding to these functions are pre-
sented in tables. However the improvement is, in some
respects, less then might be hoped for. It is likely that 
still other approximation techniques will eventually 
be found which yield further improvement. 
Note: 
Complete details may be obtained from: 
Technology Utilization Officer 
Marshall Space Flight Center 
Huntsville, Alabama 35812 
Reference: B68-10275 
Patent status: 
No patent action is contemplated by NASA. 
Source: E. 0. Ammann and J. M. Yarborough 
of Sylvania Electric Products Inc. 
under contract to 
Marshall Space Flight Center 
(M FS- 14268) 
Brief 68-10275	 Category 02


Synthesis of electro-optic modulators for amplitude modulation of light

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