Optical system eliminates displacement occurring when wavelengths are selected in multiple wavelength laser utilizing intracavity wavelength selection by first-order Littrow reflection of plane grating. Output coupling varies direction of output beam as different wavelengths are selected by grating rotation

Hard, T. M.

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Figure 1. Four Ray Paths 
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December 1970	 Brief 70-10507 
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NASA TECH BRIEF 
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nology Utilization Division, NASA, Code UT, Washington, D.C. 20546. 
Laser Wavelength Selector and Output Coupler 
An optical system eliminates the displacement that 
occurs when wavelengths are selected in a multiple 
wavelength laser which utilizes intracavity wave-
length selection by the first-order Littrow reflection 
of a plane grating. In such systems, some radia-
wavelength selection takes place about the line of 
intersection. The included angle need not be 90; 
in fact, the most useful configurations result for 
other angles. The ray path may form a delta, 
a Z, or a U. 
tion inevitably leaves the cavity through the zeroth 
order (specular) reflection of the grating. A simple, 
efficient output coupling can correct the problem 
by varying the direction of the output beam as dif-
ferent wavelengths are selected by grating rotation. 
A plane mirror, or a plane-mirror extension, inter-
sects the plane of the grating (Fig. I). Rotation for
In the Z configuration, one external plane mirror 
makes the output beam collinear with the laser 
axis. Two laser cavities can then be coupled at a 
selectable, varying wavelength. If another coupling 
scheme is already in use, an external plane mirror 
perpendicular to the output beam can return the 
zeroth
 order loss to the cavity.
(coillinued 0' crlcal) 
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Both input and output rays lie equidistant from 
the rotation axis. A given input ray and its two re-
flections lie tangent to the same circle, but dif-
ferent input rays lie tangent to different circles. 
Figure 2 shows one such construction. This system 
relies on mirror properties combined with the mir-
ror-like grating response of the zeroth order. 
Figure 2. 
The technique was used to select and couple 
out a range of single wavelengths in a CO-' laser. 
The system was pre-aligned with a visible laser. 
In a test using a helium-neon alignment laser, 
the carefully aligned CO ? selector was rotated 
through an angle exceeding 10. The output spot 
moved less than 1 mm at a distance of 7 m - 
2500 times lessthan the uncorrected motion of the 
grating's zero th order. 
This optical system is simple and inexpensive to 
construct, and is easy to align. The optics are 
either fixed in space or rigidly attached to the ro-
tating grating, and any optical wavelength can be 
used. 
Applications include higher dispersion of unwanted 
wavelengths, for communications; efficient output 
coupling of a wide range of single wavelengths in 
the CO-) laser; continuous wavelength tuning of or-
ganic dye lasers, while constant output beam position 
is maintained; and extracting the output of a variable 
angle-of-incidence reflectance spectrophotometer (sam-
ple replaces grating). 
References: 
I. Hard, Thomas M.: IEEE J. Quantum Electron., 
vol. QE-5, 1969, p. 321 (summary of paper pre-
sented at IEEE Conference on Laser Engineering 
Applications). 
2. Hard, Thomas M.: Laser Wavelength Selection and 
Output Coupling by a Grating. AppI. Optics, 
vol.9, Aug. 1970. 
Note: 
Requests for further information may be directed 
to:
Technology Utilization Officer 
Headquarters 
National Aeronautics 
and Space Administration 
Washington, D. C. 20546 
Reference: TSP70-10507 
Patent status: 
This invention is owned by NASA, and a patent 
application has been filed. Royalty-free, nonexclusive 
licenses for its commercial use will be granted by 
NASA. Inquiries concerning license rights should 
be made to: 
Patent Counsel 
Mail Code 200-11 A 
Ames Research Center 
Moffett Field, California 94035 
Source: T. M. Hard
Electronics Research Center
(ERC- 10248)
[1 
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Brief 70-10507	 Category 02


Laser wavelength selector and output coupler

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