Single-frequency continuous output of laser is converted into series of high-power laser pulses at high repetition rates. Applications include pollutant detection by absorption, laser gain measurements at discrete wavelengths, laser propagation measurement, and laser plasma diagnostics

Allario, F.

Lucht, R. A.

English

NASA Technical Reports Server

NASA TECH BRIEF 
Langley Research Center El fl September 1974	 874-10134 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
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be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Wavelength-Selective, Sequential Q-Switching Laser Cavity 
. 
.
A new device converts the single-frequency con-
tinuous output of a laser into a series of high-power 
laser pulses at high repetition rates, with each succeeding 
pulse in the series corresponding to a different emission 
wavelength of the particular laser. The process is known 
as- line-selective, sequential Q.switching, and it has a 
variety of applications, including pollutant detection by 
absorption, laser gain measurements at discrete wave-
lengths, laser propagation measurements, and laser 
plasma diagnostics. 
In the sequential Q-switching technique, an oscillating 
mirror is used as an optical folding element in a cavity 
consisting of a partially reflecting mirror and a stationary 
diffraction grating. The oscillating mirror sequentially 
aligns the various CO2 laser wavelengths in first-order 
Littrow reflection, converting the continuous wave 
(CW) output of the laser into a series of high-power 
pulses with multiwavelength spectral output. This tech-
nique has been used with a conventional CO 2 laser 
to Q-switch at least 62 rotational transitions between 
92 and 10.8 pm. The technique should be applicable 
to Q-switching other molecular lasers (e.g., CO, HF) by 
using appropriate diffraction gratings. 
Figure 1 shows the apparatus used to Q-switch 
sequentially a high-flow CO 2 laser. The inverted medium, 
a mixture of CO2 , N2 , and He, is excited by an 
electrical discharge and made to flow at high velocities 
in a tube 23 cm (1 in.) in diameter and 0.5 in (20 in.) 
long. Brewster angle windows of KC1 are used. The 
laser beam is coupled out with a 90-percent-reflecting 
germanium mirror having a 10-rn radius of curvature. A 
standard Bausch and Lomb grating, or equivalent, with 
150 grooves per millimeter is used. The scanning mirror 
is a Bulova model NAS.1.44 scanner, or equivalent, 
sinusoidally driven at a frequency of 200 Hz (for high-
repetition rates, scanning frequencies of 1000 Hz are 
available). The mirror on the scanner is optically flat 
to one-twentieth of a wavelength at the sodium D line.
Partial Reflecting	 Inverted	 Scanning 
Mirror	 Medium	 Mirror 
Figure 1. Sequential Q-Switching Apparatus 
To understand the device, consider a laser with three 
possible transitions and with the folding mirror to be 
stationary at an angular position where only one wave-
length incident on the grating is reflected back through 
the optical cavity (A1 ). The other wavelengths are 
reflected out of the optical cavity by the grating 
(A2 , A3). Thus, the laser output is single frequency and 
continuous. Then consider that the scanner rotates 
clockwise so that A1 is no longer reflected back through 
the cavity. Lasing stops until the angular orientation of 
the grating is such as to reflect wavelength A 2
 back into 
the optical cavity. The process continues so that each of 
the three wavelengths is Q-switched. 
Using the optical scanner, the continuous output is 
converted into three pulses, each pulse corresponding to 
a different possible lasing transition. For the CO 2
 laser, 
more than 62 transitions (wavelengths) have been 
obtained with the scanner. Using a scanning element 
with a 200-Hz vibrational frequency, the individual 
pulses are separated by approximately 25 in with output 
peak power in excess of 0.5 kW. 
Figure 2 shows the results of an experiment per-
formed to demonstrate the capability of this technique 
(continued overleaf) 
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. 
Figure 2. Oscilloscope Traces of Sequentially 0-Switched Laser Pulses of 
10.4-pm Band of CO  (50-ps/cm Sweep Rate) Showing 
Absorption by C 2 H 4 of R(22) Transition 
to measure small concentrations of atmospheric pol-
lutants over long path lengths. The top oscilloscope 
trace shows the results of transmitting a sequentially 
Q-switched laser beam through a 20-cm-long cell con-
taining no gas. A series of sequentially Q-switched 
laser pulses are displayed. The bottom trace shows the 
results of transmitting the beam through the same cell 
containing 10 tori (1.3x 103 N/rn 2 ) of C2 H4 (ethylene). 
Ethylene selectively absorbs the R(22) wavelength and 
none of the adjacent transitions. These results indicate 
that, assuming a capability of detecting a 5-percent 
signal change over a 1-kW path length, a concentration 
smaller than 20 ppm of C 2 H4 can be detected with a 
sequentially Q-switched laser. 
An important feature of this technique is the ability 
to select a limited number of wavelengths in either the 
P or R branch of the spectral lines in one sweep of the 
oscillating mirror. With a given grating, this can be 
accomplished by limiting the sweep of the laser beam 
across the grating with field stops or by limiting the 
scan angle of the oscillating mirror. By varying the scan 
angle of the oscillating mirror, the number of pulses for 
the configuration shown in Figure 1 was varied from a 
minimum of 3 to a maximum of 22 pulses. Another 
interesting feature of this technique is the ability to 
vary , the Q-switching time of the cavity either by 
changing the scan frequency of the oscillating mirror or 
by varying the dispersion of the grating. A grating with 
reduced dispersion will shorten the Q-switching time of 
the individual pulses and decrease the time interval 
between successive pulses.
Because of its unique characteristics of high power 
and rapid switching of laser wavelengths, the sequentially 
Q-switched laser represents an ideal radiation source for 
long-path absorption spectroscopy, since it provides the 
ability to discriminate rapidly between absorption by 
selectively absorbing molecules and absorption by the 
atmosphere. The use of CO 2 lasers in long-path ab-
sorption spectroscopy for the sensitive detection of 
NH 3 , C 2 H4 , and 0 3 has previously been suggested. 
Note: 
Requests for further information may be directed to: 
Technology Utilization Officer 
Langley Research Center 
Mail Stop 139-A 
Hampton, Virginia 23665 
Reference: B74-10134 
Patent status: 
Inquiries concerning rights for the commercial use of 
this invention should be addressed to: 
Patent Counsel 
Langley Research Center 
Mail Stop 456 
Hampton, Virginia 23665 
Source: Frank Allario and Roy A. Lucht 
Langley Research Center 
(LAR-1 1045)
. 
B74-10134	 Category 03,04


Wavelength-selective, sequential Q-switching laser cavity

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