Material characteristics measuring methods and devices

Radiation from coupled-cavity lasers is used to measure certain characteristics of materials, e.g., absorption, reflectance and other complex dielectric constants of solids, liquids and gases. Novel laser systems with electronic feedback loops are disclosed which provide compensation for laser changes with moderate temperature variations resulting in improved measurement accuracy without adversely affecting system power efficiency. In a preferred embodiment, the invention is used in the measurement of optical attenuance in submarine water over long path-lengths and at relatively specific wavelengths, e.g., about 800 nm.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the measurement of physical values, such as 
absorption, reflectance, attentuance, scatterance, etc., of materials by 
imposing laser radiations on the material and detecting radiations 
emitting from such radiated material. More particularly, it concerns use 
of coupled-cavity lasers combined with electronic feedback circuits to 
provide wavelength and radiance control in the lasers as the radiation 
source in performing such laser radiation measurements. 
2. Description of the Prior Art 
Optical techniques for the measurement of a wide variety of material 
characteristics are extensively used in industry, research, or elsewhere 
to determine the physical values of materials under test. Spectrometry 
methods and apparatus for determination of optical absorption, attenuance, 
scatterance and other values of gases, liquids and solids is a typical 
example of such optical techniques. 
Typical methods of measurement for optical absorption in transparent 
material are by conventional transmission, interferometry, laser 
intercavity absorption, photothermal detection, photoacoustic calorimetry 
and thermal lens calorimetry. The relative merits of these different 
methods have been adequately discussed in the literature (T.D. Harris, 
etal., Proceeding of SPIE, Vol. 426, pg. 110 T.D. Harris, Anal. Chem. 54, 
1982). The present invention relates to transmission-sensitive type 
methods for among the full spectrum of methods available. 
Such measurement operations often require highly collimated (parallel) 
radiation beams to attain accuracy in measurement values and propagation 
over long test paths. Lasers of various wavelengths and types have been 
used in the past in performing such measurements, e.g., study of trace 
materials (pollutants) in the atmosphere and elsewhere. Semiconductor 
(diode) lasers offer advantages in such procedures due to their small size 
and high conversion efficiency. However, such lasers suffer from several 
distinct problems, i.e., (a) emitted wavelength varies with temperature 
and excitation current and (b) emitted radiant flux varies substantially 
with emitter temperature. Methods have been proposed for correction of 
these defects including (1) wavelength stabilization by temperature 
control [L. W. Chaney, etal., Appl. Opt. 18, Sept., 17 1979], (2) 
wavelength stabilization by combined temperature and current control [R. 
A. Keller, Proc.SPIE, Vol. 426, 1983], (3) radianco stabilization by 
optical feedback [Amada, J. of Q.E. QE 19, Sept., 9 1983], (4) radiance 
stablization by synchronously modulation of beams by electro-optical 
feedback [Caimi, etal. Proc. SPIE Ocean Optics VII: 489, 1985], etc. 
In addition to the above listed problems, well known characteristics of 
solid-state laser diodes predicate use of complicated bias and modulation 
methods to avoid facet damage and operation below threshold over 
temperature extremes. Although temperature control of the laser emitter is 
possible to eliminate these problems and provide mode stabilization, 
system power efficiency is compromised. 
Sources of inaccuracy in measurements using prior art laser methods and 
devices include: 
A. The laser threshold current and differential efficiency decrease with 
inceasing temperature and age. 
B. Diode lasers can vary in wavelength while maintaining a single 
longitudinal mode at a bias somewhat above threshold. As temperatue 
increases, each longitudinal mode shifts to longer wavelengths as a result 
of refractive index changes. 
C. Asymetric aging of front and rear facets can cause long term output 
radiance changes in systems deriving radiance feedback from the alternate 
facet. 
D. The near field radiation pattern can become spotty with age. Angular 
changes in the far-field may result. 
E. Transverse/lateral mode changes can result depending upon device 
structure, temperature and current. 
F. Bandgap temperature dependence in any photodetector results in 
responsivity changes to the detected energy. 
The present invention makes possible the mitigation of these problems in 
the optical measurement operations to which the invention is directed. 
The recent development of coupled-cavity or distribuited lasers [Tsang, et 
al. "Semiconductors and Semimetals", Ch. 4, Vol. 22, Academic Press, 1985[ 
presents some advantage over the previous work cited above since very fine 
wavelength tuning is possible. Such coupled-cavity lasers were developed 
for communication systems, but in accordance with the present invention 
are applied with added improvements to spectrometry and comparable optical 
measurements. In addition, a utility of this invention is the application 
of cavity-tuned lasers, e.q., coupled-cavity lasers, to spectrometry of 
either broad or narrow absorbing test species. 
OBJECTS 
A principal object of the invention is provision of new optical methods and 
devices for the measurement of physical values such as absorption, 
reflectance, attentuance, to scatterance, etc. of gases, liquids and 
solids. 
Further objects include the provision of: 
1. Improvements in optical measurement techniques by imposing controlled 
laser radiations on test materials and detecting radiations emitting from 
such radiated material. 
2. Wavelength control in laser radiations in such optical measurements 
through electrical feedback to an element of the laser cavity while 
maintaining simultaneous electrical feedback for stabilization of the 
output radiance by control of the laser current. 
3. Auto-zero circuitry in the laser radiation devices to null the detector 
output for variation in reference and sensing optical paths. 
4. New laser radiation methods and devices having the ability to make 
spectral measurement at very precise wavelengths without need for precise 
temperature control of the laser. 
5. Such methods and devices that are adaptable to optical fiber measurement 
operations, e.g., operations in which optical fibers are used for most or 
all optical paths. 
6. Such methods and devices useable in environments where ambient lighting 
or other optical noise sources are mitigated as possible contaminates to 
the measurement results. 
7. Such measurement devices that do not require mechanically moving parts, 
e.g., mechanical choppers, motors, etc. 
8. Compensations in such optical measurements to mitigate changes in 
far-field radiance distribution or wavelength division operations or 
optical paths due to temperature changes or element aging. 
9. Reduction of temperature coefficient effects from optical detectors by 
time division multiplexing reference and signal beam originating from the 
same source thereby eliminating calbration problems relating to source 
aging and/or use of moving mechanical parts. 
10. Maintenance in such optical measurement operations of wavelength 
stability over a broader temperature range than has been possible 
heretofore with other laser based measurement systems. 
11. Such improved optical measurement methods that can be used with both 
single and double path measurement procedures. 
12. New laser methods and devices for measurement of optical attenuance, 
absorption or scattering function through a medium having large absorption 
profile width, e.g., sea water, compensated for temperature-induced 
longitudinal mode shifts of the laser. 
Other objects and further scope of applicability of the present invention 
will become apparent from the detailed description given hereinafter; it 
should be understood, however, that the detailed description, while 
indicating preferred embodiments of the invention, is given by way of 
illustration only, since various changes and modifications within the 
spirit and scope of the invention will become apparent to those skilled in 
the art from this detailed description. 
SUMMARY OF THE INVENTION 
The objects are accomplished, in part, in accordance with the invention by 
utilization of the inherent advantages of coupled-cavity laser structures, 
i.e., tunability and wavelength/longitudinal mode stability, in 
combination with additional temperature and radiance stabilization 
improvements to achieve new optical measurement methods and devices of 
unique accuracy and simplicity. 
Basically the invention involves the use of three electronic feedback 
control loops in coupled-cavity lasers, i.e., a wavelength loop, an 
intensity loop and a null (auto-zero) loop, to achieve wavelength and 
radiance stabilization over a range of temperature and other ambient 
condition variations. 
In the wavelength control loop, two control inputs are applied 
simultaneously. One input is derived from a high frequency oscillation 
(.omega..sub.1) and is applied to a modulator diode of the laser. Thus, 
the output wavelength of the coupled-cavity laser is modulated at an 
.omega..sub.1 rate and at small amplitude. Because the output radiance of 
the laser is non-linearly related to the modulator current, these small 
amplitude-induced current variation produce an intensity derived signal 
from the synchronous detection circuit in proportion to the deviation from 
optimal current for a given longitudinal mode. 
The second input to the wavelength control loop is a square wave or 
equivalent signal at a much lower oscillation frequency (.omega..sub.o). 
Thus, different longitudinal modes (different wavelengths) are selected 
during each half-wave of the square wave signal. 
The success of the invention is due, in part, on the use in combination, as 
desirable features, of collimated emission of the test material radiation 
and ambient light rejection by synchronous detection. A steady state 
average (bias) radiant output to the test material is controlled by 
feedback from a monitoring detector, while modulation about that bias is 
maintained and stabilized by separate feedback from a synchronous detector 
.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring in detail to the drawings, in which identical parts are 
identically marked, several laser feedback schemes used in the prior art 
are shown in FIGS. 1a 1b and 2. Thus, FIG. 1a showns a typical zero type 
control loop having K.sub.b as an input and any parameter which is a 
function of dc bias as the control parameter. In the loop 2, the d.c. 
input K.sub.B is fed to the algebraic summer 4 and passes via the gain 
amplifier 6 to the laser 8 causing it to emit the light flux L.sub.o to 
the photodetector 10. With the detector diode 10 connected to zero or 
positive voltage 11 via connection 12, its output loops via conductor 14 
and gain amplifier 16 back to summer 4. The resulting operation may be 
expressed by the formula: L.sub.o .dbd.f(I.sub.O), I.sub.D .dbd.g[L, 
.lambda.(I.sub.O),T], where .lambda. is wavelength as a mathematical 
function dependent upon I.sub.o, I.sub.o is the bias current to the laser, 
L is radiant flux, .omega. is frequency, g is a mathematical function 
characteristic of the particular semi-conductor laser construction and 
detection geometry and T is temperature. 
In FIG. 1b, the loop 22 uses a time varying signal as input K.sub.S. The 
a.c. input K.sub.S is fed to the algebraic summer 24 and passes via the 
gain amplifier 26 to the laser 28 causing it to emit the light flux 1 to 
the photodetector 30. With the detector diode 30 connected to zero or 
positive voltage 11 via connection 32, its output loops via conductor 34 
and gain amplifier 36 back to summer 24. The resulting operation may be 
expressed by the formula: l=f(i.sub.o), i.sub.d =g[l,.lambda.(I.sub.o),T], 
where .lambda. is wavelength, l is the ac component of radiant flux, f is 
frequency, g is a mathematical function characteristic of the particular 
semi-conductor laser construction and detection geometry and T is 
temperature. 
If K.sub.S is of nearly zero mean amplitude and spectrally removed from the 
response band of loop 22, then a degree of independence can be observed 
between the loops 2 and 22. Typically, the control parameter in loop 2 
would be a laser diode's average radiant output, while the control 
parameter in loop 22 would be a synchronously detected a.c. component of 
the radiant output. In this way, variation of the L and l values (due to 
changes in i.sub.th) are compensated to avert facet damage at low 
temperature and avert spectral broadening at high temperature where the 
threshold condition may not be met. Similar schemes have been disclosed to 
achieve radiance stabilization of.+-.several percent over a 50.degree. C. 
range and over the lifetime of the laser. 
Another method can be used to effect wavelength stabilization (over perhaps 
10.degree. C.) while also maintaining constant a.c. modulated output 
radiance. For this, loop 2 is given a control signal derived from a 
wavelength sensing device, e.g., grating. Loop 22 is used for detection of 
optical absorption. In any event, when optical transitions are narrow, 
frequency/amplitude stabilization techniques are common and result in 
acceptable accuracy of measurement, but the accuracy in such prior art 
techniques have been unacceptable where optical transitions are wide. 
FIG. 2 represents a known semiconductor transmittance measurement system 
combining two loops as illustrated in FIGS. 1a and 1b as a feedback system 
providing a double loop bias/small signal radiance compensator. 
In FIG. 2, the optical measurement system 40 comprises laser 42 connected 
via light path 44 to optical system 46, e.g., tandem lens arrangement, and 
via light path 48 to feedback control detector 50. The radiation output 
from system 46 passes via light path 52 to the material 54 under test and 
the radiation emitted (transmitted or reflected) from material 54 passes 
via light path 56 to photodetector 58 which inputs a signal via lead 60 to 
lock-in-amplifier (LIA) 62 which, in turn, passes a signal via lead 64 to 
a measurement unit (not shown), e.g., a calibrated meter. Typically, 
detector 58 will be identical to detector 50. 
The radiance compensator portion 66 of system 40 comprises the bias loop 68 
and the signal loop 70. The loop 68 includes lead 72 from feedback control 
detector 50 to summer 74, lead 76 to bias amplifier 78 and lead 80. 
The loop 70 includes lead 82 to the LIA 84, lead 86 to the summer 88, 
junction 90, signal amplifier 92 and lead 80. 
The dc bias K.sub.B is applied to summer 74 via input 94, dc signal K.sub.S 
controlling the ac radiance component is applied to summer 88 via input 96 
and the ac signal at frequency .sub.1 is applied to multipler junction 90 
via the input 98 to provide ac modulation of the laser current via 
amplifier 92 in proportion to the error signal from summer 66. 
Typical known single mode lasers exhibit mode hopping both with bias 
current and temperature change on the order of 0.2-0.5 nm/.degree. C. and 
roughly 0.2 nm/mA. Residual temperature coefficients of about 0.1-0.2 
nm/.degree. C. can be compensated by adding a carrier injection tuning 
mechanism to the laser structure. Typically, a 4 .ANG. shift can be 
induced with an injection current of 4 mA. Greater tuning range has been 
demonstrated for cleaved-coupled-cavity (C.sup.3) lasers. As a result of 
the present invention, it has been discovered that the greater tuning 
control available in C.sup.3 lasers allows a two loop feedback method to 
be used with this class of laser to create uniquely improved material 
characteristics measurement instrumentation. 
An embodiment of a three control loop laser optical measurement system of 
the present invention is shown in FIG. 3. In this system, a coupled-cavity 
laser is used in the wavelength tunable mode (one diode biased above and 
one biased below threshold). 
The laser system 100 of the invention comprises a wavelength control loop 
102, an intensity control loop 104, an auto-zero loop 106 and the optical 
measurement unit 108. 
The basic elements of unit 108 include the coupled-cavity laser 110, light 
path 112, control detector 114, light path 116, optical system 118, light 
path 120, wavelength-division-multiplexor (WDM) 122, light path 124 having 
a beam-splitter semi-mirror 126, test material holding section 128, light 
path 130, photodector 132, lead 134, LIA 136, measurement signal output 
138 and light path 140 with its beam-splitter semi-mirror 142. Some of the 
light paths, e.g., 112, may be integral transparent junctions between 
elements. More typically, they may be a series of lenses, fiber optics, 
etc. 
The wavelength control loop 102 comprises summer 144, LIA 146, leads 148, 
150, and 152 and inputs 154 and 156. 
The intensity control loop 104 comprises summer 158, leads 160 and 162 and 
input 164. 
The auto-zero loop comprises photodector 166, lead 168, LIA 170, lead 172, 
electronic switch 174, lead 176, summer 178, lead 180, input 182 and light 
paths 184 & 186. 
In operation of the device 100, wavelength control of the radiation from 
laser 110 to light path 112 is achieved via control loop 102 using an ac 
signal .omega..sub.1 from a high frequency oscillator (not shown) applied 
to input 154 and a square-wave signal .omega..sub.o applied to input 156. 
The control signal generated by the summer 144 is applied to the modulator 
diode M via lead 152 which exhibits non-linearity of output power versus 
modulator current. 
An oscillator signal is also applied to summer 144 via lead 148 from LIA 
controlled by an intensity derived signal from the synchronous detector 
114/146 which responds in proportion to the deviation from optimal current 
for a given longitudinal mode of the laser 110. By control of the dc input 
to the loop via 156, different longitudinal modes may be selected. In 
addition, since laser amplitude modulation is allowed under these 
conditions use of the auto-zero capability 106 is made possible. The bias 
input .omega..sub.o to input 156 is driven at a much lower frequency 
compared to .omega..sub.1 input 154 thereby alternatively switching the 
output wavelength of the laser 110 at an .omega..sub.o rate. The WDM 122 
switches the laser output via 112 . . . 120 between the light paths 124 
and 140 at the .omega..sub.o rate. 
The path 124 involves the material 128 under test and the path 140 serves 
as a reference. 
Advantageously, detectors 132 and 166 are a pair of matched, separate 
detectors at the same temperature, one to receive the test path signal and 
the other the reference path signal. Detector 132 responds to differences 
in signal proportional to the absorption of the test material, while the 
other 166 responds to unwanted absorption changes between the reference 
and test paths arising from (1) lateral/transverse mode changes of the 
laser affecting radiant flux coupling between the laser 110 and the 
detectors 132, 166, (2) optical radiant flux variations in the WDM 122 of 
paths 124, 130, 140 due to temperature changes or (3) any wavelength 
dependent mechanism affecting radiant flux transfer between the reference 
140, 184 and signal paths 124, 130. 
FIGS. 4-11 serve to illustrate improvements in wavelength and radiance 
stabilization in cavity-coupled lasers versus ambient temperature changes 
and in laser radiation material measurement operations attained by way of 
the present invention. In the following description of the gathering of 
the data, the number in () following a system element, e.g., a laser, 
relate that element corresponding numbered elements in FIG. 3. 
A series of experiments were conducted to obtain data on the short term 
stability of radiance output of a C.sup.3 laser versus temperature change 
using a TJS GaAlAs type laser (110) [commercially available as a 
Mitsubishi ML3101 diode] operating integral with a UDT silicon detector 
(114) at 800 nm. These elements were mounted in an insulated, 
temperature-controlled (0.1.degree. C.) silicon oil bath which was stirred 
constantly. To eliminate interference effects and temperature dependent 
back-reflections (phase changes) which might couple to the laser cavity, 
the laser window was removed. The optical path was kept clear in the 
mechanical design. 
Two-dimensional scans of far-field intensity patterns were conducted over 
most of the major lobe at temperatures of 23.7.degree. C. and 34.5.degree. 
C. Anomolies in the far-field intensity patterns were observed as a 
function of average injection current and/or temperature by lock-in 
detection of a 1 kHz modulating signal at 40% modulation. Feedback 
stabilization was implemented, as it would be in conventional transmission 
spectrometry and feedback parameters were monitored. 
Data recorded by these operations are shown isometrically in FIGS. 4 and 5. 
Contour plots and cross-sectional plots of the data are shown in FIGS. 
6-9. 
During the above described operations, the total optical power output from 
the rear facet of the laser was kept constant within about .+-.0.2%. 
Therefore, injection current and temperature increased simultaneously. In 
an effort to independently determine the effects of temperature and 
current, additional far-field data were gathered at slightly reduced, but 
constant ambient temperature while the bias-current was altered to the 
value the intensity loop (104) required at a higher temperature 
(30.4.degree. C.). The ambient temperature was reduced slightly to account 
for increased power dissipation with resulting temperature rise in the 
laser due to thermal resistance. Cross-sectional plots of the resulting 
data are shown in FIG. 10. 
It was discovered that mode changes are more significant with temperature 
change than with current change alone. Changes in current appear to be 
correlated with low-amplitude, higher order, mode competition. However, 
changes in temperature in these experiments produce angular emission 
changes (1.degree.) over a 10.degree. C. temperature change. Elevated 
current at a constant temperature and elevated temperature tend to reduce 
the far-field width at levels above 50% of peak intensity. In long-path 
measurement, variation of the wavefront curvature can result in 
significant error due to tracking misalignment or detector response 
changes over the active area. 
Another experiment was performed to further study the thermal behavior of 
the far-field. The integrally mounted detector diode (114) was again used 
to provide the feedback signals from the laser (110) rear facet. An 
externally mounted 1 cm diameter detector, operating within its linearlity 
range, was used to synchronously detect a portion (.+-.5 acceptance cone) 
of the flux from the front facet of the laser while temperature was 
varied. Plots of the resulting bias (d.c. error), a.c. error and detected 
output data are presented in FIG. 11. The output shows periodic 
oscillation (6.7.degree. C.) about an average negative slope of about 
0.15%/.degree. C. corresponding to the temperature coefficient of response 
(R.sub.T) of the internally mounted detector (114). The magnitude of the 
oscillation (3%) limits the measurement accuracy obtainable when 
temperature is not known. 
Additional points derived from the 3-dimensional plots of FIGS. 4 and 5 
were added to FIG. 11 at the three temperatures shown. These data were 
computed by integrating the far-field data over a .+-.5.degree. cone and 
normalizing these values to integrated data at angles subtended by the 
internally mounted detector. The data agreement show that the tracking 
between front and rear facets is good. This was substantiated by using an 
alternate feedback detector/beam-splitter arrangement about the front 
facet. The periodic variation remained, however, being indicative of 
periodic changes in shape of the far-field with temperature. This 
indicates the need in using identical f-number optics, e.g., in optical 
systems 108, 112, 116, 120, 124, 130, 140, 184 and 186 in the detection 
and feedback control optical paths in instrumentations of the invention. 
In specific embodiments of the invention, cleaved-cavity-coupled (C.sup.3) 
lasers are advantageously used as the cavity-coupled lasers (110). 
By way of example of components useable in specific embodiments, lock-in 
amplifiers (146, 136, 170) may be commercially available units from the 
Princeton Applied Research division of EG&G, Analog Devices AD 630 or two 
inverting amplifiers, a MOSFET SPDT switch and a filter amplifier. A 
coupled-cavity semiconductor laser structure (110) using a GaAlAs 
heterostructure device, e.g., ML3101, coupled with an external cavity may 
be used. Such external cavity can consist of two duplicate lasers in close 
proximity, e.g., 1 mm., to each other and aligned with parallel facets. 
Alternate Fabry-Perot or other interferometric cavities can also be used 
in place of one laser. In these latter configurations, it is necessary to 
utilize electrooptic modulators to vary the cavity tuning via the input in 
FIG. 3. 
Detector (114) should be sensitive to the wavelength emitted by the laser. 
Silicon PN or PIN detectors available from EG&G or United Detector will 
serve in the spectral region from the visible wavelengths to near 1 .mu.m. 
The optical system (108) can consist of a graded index optical element 
(Galileo Electrooptics) for connection to the fiberoptic cables (Corning 
Glass) (112, 116, 120, 124, 140, 130, 184, 186). Wavelength Division 
Multiplexor (122) may be a grating, internal reflection element, or filter 
type depending upon the degree of wavelength shift induced by the signal 
.omega..sub.o. These WDMs are commercially available from Microcoatings, 
Inc. 
Dectors 132 and 136 may be pyroelectric types (Eltec, Inc.) silicon, 
germanium or a variety of known infrared types (Infrared Industries). 
Switch (174) may be constructed from a MOSFET or other electric switch such 
as the RCA CD 4053. Summers (144, 158, 178) may be constructed from 
operational amplifiers (National LF 411) and others. Resistor network 
summing junctions may also be used. 
The clock signals may be generated using laboratory-type signal generators 
or CMOS crystal controlled oscillator/dividers. Typical valuse for 
.omega..sub.o are 2.times.100 Hz or less and for .omega..sub.1, much 
greater than 2.times.10,000 Hz. 
In an application of the invention near 800 nm in air, a 0.5 cm diameter 
collimated laser output beam was separated into two separate paths by 
reflection from a Bausch & Lomb diffraction grating (WDM). The beams were 
recombined onto a 1 cm. diameter silicon detector (UDT Pin10DP) and 
detected by a laboratory constructed LIA operating at a frequency of 1 
KHz. Optical absorption in the test path was induced by inserting a water 
sample into it and the resulting change in values from the detector was 
recorded. Signals derived from a beam splitter in both paths were used to 
compensate for variations of the transmitted light between both paths and 
constituted the auto-zero function. The auto-zero loop used a similar 
detection system to the one just described. The temperature of the laser 
diode was altered by heating an electical resistance in the laser 
collimating mount assembly over a 10.degree. C. temperature range and 
resulting changes in values from the detector were recorded. 
In another test, a microscopic cover glass was used as a test material 
sample. The Fresnel loss due to reflection was adequately measured as a 
loss in the test beam. 
The data acquired as a result of the above procedures showed that the 
detection output variations due to the temperature change were reduced by 
less than 1/10th by the use of the auto-zero loop as compared to detection 
output without such loop in operation.