Signal carrier laser beam is optically modulated by a second laser beam of different frequency acting on a free carrier source to which the signal carrier laser is directed. The second laser beam affects the transmission characteristics of the free carrier source to light from the signal carrier laser, thus modulating it

Gruber, C. L.

Richards, W. E.

English

NASA Technical Reports Server

April 1969
	 Brief 69-10114 
NASA TECH BRIEF	
AL 
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. 
Optically Induced Free Carrier Light Modulator 
Signal Generator 
Free Carrier Source 
Beam Combiner 
Signal 
Carrier
-
—/Combined Beam Laser
Magnetic Field 
I
Modulating 1
Laser 
A system has been devised that optically modulates 
a signal carrier laser beam by a second laser beam of 
different frequency acting on a free carrier source to 
which the signal carrier laser beam is directed. The 
second or modulating laser beam affects the trans-
mission characteristics of the free carrier source to 
light from the signal carrier laser, thus modulating it. 
Since development of the laser, efforts have been to 
modulate its beam in order to develop a laser com-
munications system. Such devices as electro-optical 
and magneto-optical modulators, optical waveguides, 
electro-acoustic modulators, band gap modulators, 
and electrically depleted free carrier modulators have 
successfully modulated laser beams but have inherent 
disadvantages. For example, electro-optical and mag-
neto-optical modulators require large amounts of drive 
power per unit bandwidth. Optical waveguides and 
electrically depleted frçe carrier modulators suffer 
from low power handling capabilities and low modula-
tion indices. Electrp-acoustic modulators are limited 
by narrow information bandwidths, while band gap 
modulators have low modulation indices. As a result, 
modulators of the foregoing types have found only 
limited use, usually in laboratory applications.
The solution is a laser beam modulating system with 
low drive power requirements per unit bandwidth. This 
invention provides such a system and makes possible 
a laser beam communications system. Referring to the 
illustration, the signal carrier laser generates a signal 
to which the light beam combiner is transparent, while 
the modulating laser generates a beam to which the 
light beam combiner is reflective. By positioning the 
combiner as shown, the two laser beams join on 
a mutual axis to form a combined beam that impinges 
on the free carrier source. The free carrier source is 
transparent to the signal portion of the combined beam 
but relatively opaque to the modulating portion. This 
is due to the difference in wavelengths of the two 
beams plus the constituent makeup of the free carrier 
source (a situation similar to the beams-to-combiner 
relationship). 
Radiation from the modulating laser creates free 
carriers (relative to its wavelength) in the free carrier 
- source to vary its degree of transparency to the signal 
carrier laser beam, thus modulating the latter. Control 
of the spatial distribution of the signal carrier beam is 
achieved by application of a d.c. magnetic field along 
(continued overleai( 
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the direction of propagation of the modulating light 
signal. This restricts free carrier diffusion to a path 
parallel to the direction of propagation of the modu-
lated light. 
Notes: 
1. Drive power requirements for this system are cal-
culated to be 3 watts for a 100 MHz bandwidth, 
while prior art optical modulators require approxi-
mately 20 kilowatts for that bandwidth. Addition-
ally, the efficiency of this system is 7.5 % whereas 
other optical modulators have efficiencies of about 
6 x 10 % making this system's theoretical effi-
ciency almost 1000 times greater.
No further documentation is available. Inquiries 
may be directed to: 
Technology Utilization Officer 
Goddard Space Flight Center 
Greenbelt, Maryland 20771 
Reference: B69-10114 
Patent status: 
This invention is owned by NASA, and a patent ap-
plication has been filed. Royalty-free, non-exclusive 
licenses for its commercial use will be granted by 
NASA. Inquiries concerning license rights should be 
made to NASA, Code GP, Washington, D.C. 20546. 
Source: William E. Richards and Carl L. Gruber

Goddard Space Flight Center

(GSC- 10216)
El 
is 
Brief 69-10114	 .	 Category Ol


Optically induced free carrier light modulator

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