NASA Technical Reports Server (NTRS) 20110024096: Laser Ranging Simulation Program

2 Pages·0.06 MB·English

by  NASA Technical Reports Server (NTRS)

centrations of CO . Because at least one
2
CO calibration line is present in every
2
1.0000 spectral scan of the laser during absorp-
tion measurements, the atmospheric
CO serves continuously as a calibration
2
standard for every measurement point.
Figure 1 depicts simulated spectral
transmission measurements in a wave-
0.9990
on H2O number range that contains two absorp-
ssi tion lines of N O and one of CO . The
mi 2 2
s simulations were performed for two dif-
Tran 1-m SPpaethc,t r7a lm Sbimaru, l2a0ti0o nK f,o Cr OM2a =rs 9:6%, ferent upper-atmospheric pressures for
HO = 20 Parts per Billion an airborne instrument that has a path
2
0.9980 By Volume length of 80 m. The relative abundance
CO2 of CO2 in air was assumed to be 360
parts per million by volume (approxi-
mately its natural level in terrestrial air). In
applying the present method to mea-
5327.0 5327.2 5327.4 5327.6
surements like these, one could average
Laser Wavenumber, cm–1 the signals from the two N O absorption
2
lines and normalize their magnitudes to
that of the CO absorption line. Other
Figure 1. These Simulated Spectral Transmission Measurementsillustrate the present method 2
as applied to a wave-number range that contains absorption lines of N O and CO . gases with which this calibration method
2 2
can be used include H O, CH , CO, NO,
2 4
NO , HOCl, C H , NH , O , and HCN.
2 2 2 3 3
One can also take advantage of this
1.0
method to eliminate an atmospheric-
pressure gauge and thereby reduce the
mass of the instrument: The atmospheric
0.9
pressure can be calculated from the tem-
perature, the known relative abundance
n 0.8 N2O N2O of CO2, and the concentration of CO2as
sio Pressure = 400 mbar, measured by spectral absorption.
nsmis 200 PaNrt2sO P Aerb uBnildlioann cbey =Volume 13CO2 ideNaal tucraall iCbrOa2tiolenv elsst aonnd Maardrs. pFroigvuidree a2n
Tra 1.0 shows a second example of the applica-
tion of this method to Mars atmospheric
0.9 N2O N2O gas measurements. For sticky gases like
H O, the method is particularly powerful,
Pressure = 50 mbar, 2
NO Abundance = since water is notoriously difficult to han-
2
0.8 100 Parts Per Billion by Volume 13CO dle at low concentrations in pre-flight
2 calibration procedures.
This work was done by Chris
2232.5 2233.0 2233.5
Webster of Caltech for NASA’s Jet
Laser Wave Number, cm–1
Propulsion Laboratory. For further
information, access the Technical
Figure 2. The Spectral Region Near 1.88 µmwould be appropriate for measurements of water on Support Package (TSP) free on-line
both Mars and the Earth where suitable near-infrared laser devices would exist. at www.nasatech.com.
NPO-30401
known and varies slowly and by a small Hence, absorption-spectral measure-
enough amount to be considered constant ments of the concentrations of gases of
for calibration in the present context. interest can be normalized to the con-
Laser Ranging Simulation Program
Laser Ranging Simulation Program map terrain from a distance of several kilo- interface that includes a pop-up menu that
(LRSP) is a computer program that pre- meters. Designed to run in a more recent prompts the user for the input of data that
dicts selected aspects of the performances version (5 or higher) of the MATLAB pro- determine the performance of a laser rang-
of a laser altimeter or other laser ranging or gramming language, LRSP exploits the ing system. Examples of input data include
remote-sensing systems and is especially numerical and graphical capabilities of duration and energy of the laser pulse, the
applicable to a laser-based system used to MATLAB. LRSP generates a graphical user laser wavelength, the width of the laser
26 NASA Tech Briefs, January 2003
beam, and several parameters that charac- and noise currents, and ranging and Package (TSP) free on-line at www.
terize the transmitting and receiving optics, pointing errors. nasatech.com.
the receiving electronic circuitry, and the This program was written by Sabino This software is available for commer-
optical properties of the atmosphere and Piazzolla of USC and Hamid Hemmati and cial licensing. Please contact Don Hart of
the terrain. When the input data have been David Tratt of Caltech for NASA’s Jet the California Institute of Technology at
entered, LRSP computes the signal-to- Propulsion Laboratory. For further infor- [phone]. Refer to NPO-30549.
noise ratio as a function of range, signal mation, access the Technical Support
Micro-Ball-Lens Optical Switch Driven by SMA Actuator
This could be a prototype of low-loss, NASA’s Jet Propulsion Laboratory,
mass-producible optical switches. Pasadena, California
The figure is a simplified cross section
of a microscopic optical switch that was SMA Actuators
partially developed at the time of report-
Glass
ing the information for this article. In a
fully developed version, light would be Input Optical Fiber
coupled from an input optical fiber to
one of two side-by-side output optical
Ball Lens
fibers. The optical connection between
the input and the selected output fiber
would be made via a microscopic ball Output Optical Fibers
lens. Switching of the optical connection
from one output fiber to another would
be effected by using a pair of thin-film
SIMPLIFIED CROSS SECTION OF SWITCH
shape-memory-alloy (SMA) actuators to
toggle the lens between two resting
Ball Lens in Upper Position
switch positions.
There are many optical switches —
some made of macroscopic parts by
conventional fabrication techniques and
some that are microfabricated and,
hence, belong to the class of micro-
electromechanical systems (MEMS).
Conventionally fabricated optical switch-
es tend to be expensive. MEMS switch-
es can be mass-produced at relatively
low cost, but their attractiveness has
been diminished by the fact that, hereto-
Ball Lens in Lower Position
fore, MEMS switches have usually been
OPTICAL SWITCHING GEOMETRY
found to exhibit high insertion losses.
The present switch is intended to serve
as a prototype of low-loss MEMS The SMA Actuatorswould move the ball lens between its two resting positions. Light would be
switches. In addition, this is the first coupled to one or the other output optical fiber, depending on which position was selected.
reported SMA-based optical switch.
The optical fibers would be held in V
assembled into a sandwich structure to and a switching contrast of 70 dB. One
grooves in a silicon frame. The lens
complete the fabrication of the switch. the basis of prior research on SMA actu-
would have a diameter of 1 µm; it would
To effect switching, an electric current ators and assuming a lens displacement
be held by, and positioned between, the
would be passed through one of the of 125 µm between extreme positions, it
SMA actuators, which would be made
SMA actuators to heat it above its tran- has been estimated that the fully devel-
of thin films of TiNi alloy. Although the
sition temperature, thereby causing it to oped switch would be capable of operat-
SMA actuators are depicted here as
deform to a different “remembered” ing at a frequency as high as 10 Hz.
having simple shapes for the sake of
shape. The two SMA actuators would This work was done by Eui-Hyeok
clarity of illustration, the real actuators
be stiff enough that once switching had Yang of Caltech for NASA’s Jet
would have complex, partly netlike
taken place and the electrical current Propulsion Laboratory. For further
shapes. With the exception of the lens
was turned off, the lens would remain information, access the Technical
and the optical fibers, the SMA actua-
latched in the most recently selected Support Package (TSP) free on-line at
tors and other components of the switch
position. www.nasatech.com.
would be made by microfabrication
In a test, the partially developed switch NPO-30434
techniques. The components would be
exhibited an insertion loss of only –1.9 dB
NASA Tech Briefs, January 2003 27