Optical position system

An apparatus and method are provided for determining the position of a reflective target surface using one or more laser cavities. A transmitting laser cavity produces a stable resonator output beam which is modulated with a compound beam pattern including a first wave of frequency f.sub.1 and a second wave of frequency f.sub.2. The reflected compound modulated beam enters a receiving laser cavity and fundamental and harmonic waves f.sub.1 and f.sub.2 are generated in the receiving laser cavity. The power of the compound modulated output beam of the receiving laser cavity is measured by a photodiode and a deconvolution system is connected to the photodiode for filtering the fundamental frequency of f.sub.1 and the second harmonic of wave f.sub.2. Time-dependent changes in the measured power of the modulated compound output beam are used to derive positional information of the target. In another form of the invention, two or more matched lasers form transmitting and receiving cavities and changes in the measured output power of the receiving cavity are used to derive positional information of the target. In another form of the invention, an amplitude modulated scheme is utilized wherein the coherence length of the output beam is reduced and time-dependent changes in the measured power of the modulated amplitude output beam in the receiving laser cavity are used to derive positional information of the target. [In another embodiment of the invention, a pair of matched diode lasers form the transmitting and receiving laser cavities, and wherein the output of the transmitting laser cavity is electronically modulated and wherein the impedance of the receiving diode laser is measured and time-dependent changes in the measured impedance are used to derive positional information of the target.]

BACKGROUND 
The present invention relates in general to laser technology. More 
particularly, the present invention provides an apparatus and method for 
determining the position, speed and direction of motion of reflective 
targets, as well as for detecting changes in refractive indices of light 
transmissive gases. According to the present invention, an improved 
optical disc reader is provided capable of reading at relatively high 
speeds compared to prior art optical disc readers and/or capable of 
operation with extremely small pit depths. The present invention also 
facilitates improved transmission and reception through fiberoptic cable 
networks. 
The prior art includes the Bearden et al U.S. Pat. No. 5,029,023 dated Jul. 
2, 1991 which teaches a laser motion detector utilizing a single laser 
cavity and laser feedback interferometry to measure displacements in a 
target surface. In contrast to the teachings of the Bearden '023 patent, 
the present invention utilizes in one embodiment a pair of matched, 
monolithic diode lasers, wherein the first laser transmits an output beam 
and the second laser receives the reflected beam from a target. The 
present invention differs from Bearden '023 in several significant 
respects. First, the present invention utilizes in several embodiments 
matched diode lasers, using two or more laser cavities, wherein the output 
beams are easily modulated electronically. The Bearden '023 patent teaches 
the use of a single laser cavity which must be modulated mechanically or 
electro-optically and which cannot be effectively modulated 
electronically. Secondly, the present invention incorporates dual 
frequency injection to detect a target whereas Bearden '023 teaches a 
single frequency injection to stabilize the device (see column 7, line 53 
thru column 8, line 19). Thirdly, the present invention in several 
embodiments includes the second harmonic as part of a compound modulated 
operating signal to detect the target, whereas Bearden '023 does not use 
harmonics in any fashion to detect the target. 
The prior art also includes the Bearden et al U.S. Pat. No. 5,235,587 dated 
Aug. 10, 1993 which discloses a method and apparatus for storing and 
retrieving data from an optical disc using multiple pit depths. The 
Bearden '587 patent differs from the present invention in that it utilizes 
a single laser cavity which requires optical modulation of the beam to 
maintain stability. The use of a single laser cavity as taught in Bearden 
'587 has certain inherent problems in operating at relatively high 
frequencies. The primary problem is that the feedback light momentarily 
decreases the output power of the single laser cavity, requiring that 
before the next bit is retrieved, the laser must be allowed to return to 
its original operating power. At higher frequencies, the laser does not 
have adequate time to return to its original output power and would have 
inherent difficulty in detecting a string of zeroes, for example. 
The prior art also includes Bearden et al U.S. Pat. No. 5,260,562 dated 
Nov. 9, 1993 which teaches a high resolution light microscope. This patent 
includes the same disadvantages of the other Bearden prior art summarized 
above. 
SUMMARY OF THE INVENTION 
A primary object of the present invention is to provide a system for 
detecting a reflective target wherein the output laser beam may be phase 
modulated electronically and the reflective beam is interpreted using a 
harmonic deconvolution scheme. 
Another object of the invention is to utilize a compound modulated laser 
output beam which utilizes a second harmonic as part of the operating 
signal to detect the target. 
Another object of the invention is to utilize a pair of matched lasers to 
detect a target, to detect motion in a target, to transmit bit streams 
through fiberoptic cables and to sense changes in the refractive index of 
gases. 
A further object of the present invention is to provide a phase modulated 
laser output beam which has the capability of phase quadrature detection 
wherein the position of the target can be located within the range of 
.lambda./4. 
A still further object of the invention is to provide an optical disc 
reader capable of reading at relatively high speeds and capable of reading 
shallow pit depths. 
Yet another object of the invention is to provide a fiberoptic cable system 
capable of transmitting greatly increased bit streams between two stations 
in both directions simultaneously. 
It is one of the purposes of the invention to measure surface motion and 
topography in the picometers or less and to provide a method and apparatus 
to simply accomplish the same. 
A related purpose of the invention is to provide a system for measuring the 
induced phase or amplitude variations caused by a reflective or 
translucent material using one or more lasers. 
The invention includes a method for measuring time-dependent phase or 
amplitude distortions which are induced for measurement reasons or data 
transmission and retrieval reasons. A coherent incident light beam from a 
stable-resonator is current modulated with a current offset. The current 
modulation pattern consists of one or more frequencies. In one embodiment 
of the device, the components of the current frequency are two sinusoidal 
waves f and 2f, 2f being twice the frequency of f with a phase shift. 
The modulated beam is then transmitted through or reflected off the 
material to be studied. The beam then enters a receiving or transceiving 
cavity. The receiving or transceiving cavity is then optically modulated 
by the re-entering light. This induced optical modulation produces a 
fundamental for each of the components of the transmitting frequency in 
the intensity of the receiving cavity. The amplitude of the intensity 
modulation in prior art alternates between maximum and minimum peaks every 
.lambda./2 total phase change. However a strong second harmonic signal is 
produced when the fundamental is at a minimum. Therefore, if two 
frequencies are introduced at 1 MHz and 2 MHz and, if the receiving cavity 
intensity is filtered for 2 MHz, each component of the introduced 
frequency produces a fundamental and harmonics. Then at every point along 
the beam path the reflected light will produce a 2 MHz signal. The 
filtered signal is either from the fundamental of the 2 MHz or when the 
fundamental is at a minimum then the second harmonic of the 1 MHz is at a 
maximum. The second harmonic signal has an extra degree of phase shift 
that is introduced in the cavity, this can be compensated for by phase 
shifting one component of the driving signal relative to the other so that 
the fundamental of one and the second harmonic of the other are phase 
matched on the induced intensity modulation pattern. This ability to have 
a strong signal at all points allows for the easy use of this technology 
in various applications. This particular frequency modulation scheme 
allows for monitoring the amplitude of the feedback light. 
The use of the alternating phase pattern also allows for the monitoring of 
the phase of the feedback signal. In another embodiment of the device, the 
modulating signal may be 1 MHz. The induced fundamental alternates 
180.degree. in phase every .lambda./2, the induced second harmonic also 
shifts 180.degree. every .lambda./2. The two alternating patterns are 
shifted relative to each other by .lambda./4. Therefore, every .lambda./4 
there is either a change in the phase of the fundamental or the harmonic, 
this allows for phase quadrature detection. For example, if each phase 
position were labeled relative to the driving signal or reference signal: 
1-in phase, 0-out of phase, then the signal would alternate between 
00,01,11, 10,00,01,11,10 . . . The amplitude of the signal could also be 
analyzed to further detect the exact pathlength change of the re-entering 
light. Higher order harmonics can be used in any of the methods described. 
The ability of the device to utilize resonant optical detection without the 
constraint of limited pathlength range also allows for data transmission 
by having carrier frequencies and then having a strong data modulation 
signal corresponding to the actual bit stream. 
One general object of the invention is to provide a high speed, high 
density digital data storage apparatus and storage disc in the apparatus. 
The optical data storage disc of the invention has a substrate which 
defines a plurality of data storage positions, i.e. two or more. In one 
embodiment of the device, the disc can have multi-bit information at each 
data location. The data storage apparatus also can detect smaller pit 
depths than conventional techniques, therefore allowing the archival CD 
recording with lower wattage recording lasers or higher speed recording. 
Normally this would have the detrimental effect of not creating pits deep 
enough to be read. This present invention is not limited by small pit 
depths. The invention can also be used in magneto-optic drives to simply 
enhance the signal strength, similar to any optical amplifier, with ideal 
polarization qualities. 
Also disclosed is a method of retrieving digital information. In the 
method, a focused laser beam is directed onto the surface of the optical 
data disc, of any number of formats, and a portion of the light is 
reflected back into the same laser or another laser using the modulation 
scheme described and which causes an intensity change in the laser that is 
detected and converted into a bit stream. Because of the stronger signal 
detection method, the disc can be read at a significantly higher speed. 
More than two lasers may also be used in this modulation scheme. For 
example, there may be more than one transmitting laser or more than one 
receiving laser, each modulated or filtered independently. 
This technique also has the advantage of having a specific depth of field. 
Therefore information can be stacked at different optical layers and the 
information can be read at these different layers by focusing through the 
top layers to retrieve information at the lower layers. This allows an 
enormous amount of information to be stored on a disc. The layers can also 
be frequency separated. As the retro-reflected light is only amplified if 
it is within the gain curve of the laser, multiple wavelengths can be 
used. For example, different layers may have photoabsorbant pits for 
different wavelengths, allowing for easy multi-layer scanning. Due to the 
increased sensitivity of the method, standard photoabsorptive material 
could easily be detected. The apparatus can operate with or without the 
optics that are used in prior art for optical disc detection. 
The invention can be used to examine surface features of a target. The 
position dependent variation can be either a phase or an amplitude 
variation. 
The present invention may be used for retrieving information from 
position-dependent surface displacements by moving the incident beam to 
selected positions on the target or for moving the target to place the 
beam on selected portions of the target. 
In still another embodiment, the method is used for data transmission and 
reception. A fiber optic is used to couple two lasers with matching gain 
curves or is used to couple a laser and an optical phase modulator. The 
harmonic deconvolution scheme is then used to generate a continuous 
operating region, i.e., using f and 2f. A bit stream is then superimposed 
upon the carrier modulation signal. In the two laser configuration, both 
the lasers can act as transmitters or receivers. In the one laser design, 
light is transmitted through a fiber into an optical path length 
modulator, the light is then reflected back into the fiber and 
subsequently into the laser for amplification and detection. In this 
particular configuration, the optical modulator is the transmitter of the 
bit stream and the laser is the detector. One of the current limitations 
in the resonant optical amplification configurations of the prior art is 
the existence of inoperable regions of the phase matching curve; this is 
not a limitation of this invention. One of the invention's primary 
advantages lies in its ability to convert phase modulation to intensity 
modulation without complex optical elements. This allows for the use of 
rapid phase modulation which has the potential for higher bit rates than 
intensity modulation. But because of the ability to also use intensity 
modulation, this technique has increased versatility. 
In still another embodiment, the method is used for fiberoptic transmission 
using wavelength division multiplexing and frequency division 
multiplexing. Due to the inherent frequency selectivity of the resonant 
cavity, the invention allows for the modulation and transmission of 
multiple wavelengths and the independent detection of each of the 
wavelengths and their respective bit streams. This has great potential for 
gigabit networks, where the primary limitation is the cost of the 
wavelength division due to complex optics and alignment. The invention in 
another aspect can be used to transmit bit streams with each temporal 
modulation or position corresponding to more than one bit. For example, 
instead of transmitting the bit streams based on a binary format, the 
signal can be transmitted with a base number of three. This is possible 
due to the multiple position phase detection abilities of the device. The 
device can also conveniently be used with non-monolithic lasers in spite 
of the intensity losses of the inserted optics due to the optical 
amplification of the invention. 
The invention can also be used to measure the frequency and amplitude of 
vibrations of a target. The amplitude of the vibration is determined from 
the time dependent variations in the power level measured by the light 
intensity detector and the frequency is determined from the frequency of 
the time-dependent variations and the power level. The invention may be 
used to provide a highly sensitive microphone. The invention has 
significant advantages over conventional techniques because no phase 
distortion is introduced in the audio spectrum by the transducer. Using an 
optical transducer also allows for large area detection. For example, the 
transmitting laser is coupled to a beam expander with a large beam waist, 
allowing motion detection over a large area. This allows for the ability 
to cancel Brownian motion noise due to the high sensitivity of the laser 
feedback technique. 
In still another embodiment, the invention is used to provide an instrument 
capable of measuring change in position or distance while keeping track of 
information bidirectionally to nanometer scale accuracy. This is done by 
quadrature detection of the intensity and phase signal of the laser. The 
phase logic states are divided into four distinct states, which can then 
be stored and the direction of the target surface can then be determined 
by the next relative state. For example, if the target surface is 
currently in a position corresponding to the phase state "00" and the next 
state is "01" or "10," the target surface can be said to have moved 
forward or backward, respectively. Accuracy can be improved by including 
additional optical reflections between the control unit and the target, as 
this would multiply the induced path length change caused by the motion of 
the target surface. The accuracy could also be increased by coupling the 
quadrature detection with the harmonic intensity analysis. 
The measurement of change in position of the target surface can also be 
analyzed for its rate of change, thereby being able to determine the speed 
of motion of a target. This embodiment of the invention, a velocimeter, is 
able to operate using scattered light and non-reflective targets, giving 
it great versatility. 
In still another embodiment, the method is used for the detection of vapor 
or liquid density. Due to the change in the effective refractive index of 
a medium, the invention can detect changes in density or composition. For 
example, the transmitted beam passes through a chemical vapor chamber, and 
the light re-enters the receiving or transceiving laser cavity. In this 
configuration, the vapor density in the chamber directly modulates the 
phase and intensity of the feedback light allowing for direct density 
measurement using the method. Accuracy can be increased by increasing the 
pathlength through the vapor chamber. Chemical composition can also be 
detected by use of specific wavelengths and appropriately extrapolating 
the induced phase modulation pattern for each of the appropriate 
wavelengths and determining a unique phase fingerprint. 
Further objects and advantages of the invention will become apparent from 
the following description and the drawings wherein:

DETAILED DESCRIPTION OF THE DRAWINGS 
FIGS. 1 and 2 show the fundamental envelope 10 known in the prior art. 
Envelope 10 is generated in a laser receiving cavity. The peaks of the 
envelope shown in FIGS. 1 and 2 represent absolute values, and the 
horizontal axis of FIGS. 1 and 2 represents the distance of a mirror or 
reflective surface from a laser output. 
Referring to FIG. 2 regions 11 are highlighted which represent regions of 
relatively low intensity feedback signal strength of a laser output beam 
reflected back from a target into a laser receiving cavity. The laser 
detection systems of the prior art are essentially inoperable within 
regions 11 because of the weakness of the feedback signal. The dotted 
horizontal line 12 of FIG. 2 represents the feedback signal threshold 
amplitude below which the feedback is insufficient to create a reliable 
and useful signal. 
FIGS. 3-5 represent schematically how one embodiment of the present 
invention provides a compound modulated beam which avoids the inoperable 
regions experienced by the prior art as shown in FIG. 2. FIG. 3A 
schematically represents the wave form of a typical laser output beam 
having a wavelength .lambda. of typically 780 nanometers. FIG. 3B 
represents the fundamental envelope 10 within which the output beam of 
FIG. 3A oscillates, showing absolute value along the vertical axis and 
along the horizontal axis distance from the laser output to a reflective 
target or mirror surface. 
FIG. 3C represents a second harmonic envelope 20 generated by the wave 9 
from FIG. 3A as the fundamental envelope 10 of FIG. 3B is created. 
A significant aspect of the present invention is shown schematically in 
FIG. 4 wherein the fundamental envelope 10 is shown together with the 
second harmonic envelope 20. According to the present invention, a laser 
output beam is modulated in such fashion that the fundamental envelope 10 
of a first modulating wave f.sub.1 is combined with the second harmonic 20 
of a second wave f.sub.2 and in the receiving laser cavity the feedback 
signal utilized includes portions of the fundamental envelope and portions 
from the second harmonic envelope shown in FIG. 4. This feedback signal 
remains at an intensity level greater than that represented by threshold 
12 in FIG. 4 irrespective of the path length between the output laser to 
the target and back into the receiving cavity. 
FIG. 5 is a schematic representation of the feedback signal utilized 
according to the present invention wherein the intensity level remains 
above the threshold level 12 irrespective of path length. 
The feedback signal represented in FIG. 5 can be generated using either a 
single wave f.sub.1 or by using the fundamental of a first wave f.sub.1 
and the second harmonic of a second wave f.sub.2. The amplitude of the 
second harmonic can be increased to flatten the feedback signal by simply 
increasing the amplitude of wave f.sub.2 to the point where its amplitude 
is twice that of the wave f.sub.1. In that case, the amplitude of the 
second harmonic 120, as shown in FIG. 7, would be as great as the 
fundamental 10 of f.sub.1 shown in FIG. 7. 
As used herein and in the claims, the phrase "beam modulation means" 
includes frequency and/or amplitude modulation. The phrase also includes 
direct current modulation or optical pathlength modulation. The preferred 
form is direct current modulation of a diode laser, as shown for example 
in FIG. 21. Although the preferred embodiment utilizes different 
frequencies for waves f.sub.1 and f.sub.2, a single frequency can be used 
wherein f.sub.1 =f.sub.2. 
In accordance with the present invention, deconvolution means are provided 
to filter the fundamental 10 of wave f.sub.1 and the second harmonic 20 of 
wave f.sub.2. Although other harmonics may be utilized, in the preferred 
embodiment the second harmonic is the preferred harmonic. The 
deconvolution means includes commercially available, standard filters for 
separating the fundamental of wave f.sub.1 and the second harmonic of wave 
f.sub.2. 
FIG. 6 is a schematic representation of the phase quadrature detection 
system according to the present invention. As shown in FIG. 6, the 
reference fundamental 10 is generated and a reference harmonic 20 is 
generated. The fundamental patterns inside the laser receiving cavity of 
the present invention are shown as 15a,15b,15c and 15d. The second 
harmonic being generated inside the receiving cavity is represented as 
25a,25b,25c and 25d. The upper graph of FIG. 6 represents on the 
horizontal axis 31 the four quadrants of a given wavelength and the 
vertical axis 32 represents the presence of a signal when the reference 
fundamental 10 is in phase with the fundamental inside the receiving 
cavity, i.e. 15a,15b,15c or 15d. In the first quadrant represented by 
reference numeral 36, the fundamental 10 is in phase with fundamental 15a 
in the receiving cavity and therefore a positive signal, shown by the 
solid line 33, is detected in quadrant 36. The reference harmonic 20 in 
first quadrant 36 is inverted or out of phase with the harmonic 25a inside 
the laser receiving cavity and therefore no signal is detected as 
indicated by 34b. Therefore, in first quadrant 36, the fundamental is in 
phase and can be represented by a "1," the second harmonic is out of 
phase, no signal is generated and that condition can be represented by a 
"0." In the second quadrant 37, the reference fundamental 10 is again in 
phase with the receiving cavity fundamental 15b, the signal as shown as 33 
in quadrant 37 is again positive and may be represented by a "1." In 
quadrant 37, the reference harmonic 20 is in phase with the harmonic 
inside the receiving cavity 25b and the signal 34a is positive which may 
be represented by a "1." In third quadrant 38, the fundamental 10 is out 
of phase with the fundamental inside the receiving cavity 15c, the signal 
33 shifts to the "0" position shown in FIG. 6 and may be represented by a 
"0." The reference harmonic 20 is in phase with the harmonic inside the 
receiving cavity, as shown by 25c, the signal 34a remains positive which 
may be represented by a "1." In the fourth quadrant 39, the fundamental is 
out of phase with the receiving cavity fundamental 15d and the reference 
harmonic 20 is out of phase with the harmonic inside the receiving cavity, 
as shown by 25d, both of which are conveniently represented by "0." These 
relative phase shifts of 180.degree. of either the fundamental or the 
second harmonic produce four distinct logic states as represented in FIG. 
6 and which may be represented in digital format as 1,0; 1,1; 0,1; and 00 
for each of the four quadrants. The presence of these four distinct logic 
states represented by the presence or absence of phase inversion between 
the fundamental and the second harmonic facilitates the use of "phase 
quadrature detection." 
FIGS. 7 and 8 are schematic representations which illustrate the phase 
quadrature detection system of the present invention. FIG. 7 illustrates 
an alternate embodiment wherein the fundamental envelope represented by 10 
and the second harmonic envelope is shown by 120. The second harmonic 
envelope 120 is of the same amplitude as that of the fundamental 10 which 
in effect produces a "flat" feedback signal which is strong in amplitude 
regardless of path length. This combination of feedback signals can be 
used in a second embodiment of the present invention wherein amplitude 
modulation is used for target detection. 
FIG. 8 is a schematic representation showing the phase quadrature detection 
scheme and four distinct logic states occurring in quadrants 36 thru 39. 
FIG. 8 shows the upper graph of FIG. 6 in greater detail. 
FIG. 9 represents a schematic diagram of a preferred embodiment of the 
present invention. This embodiment shows a pair of "matched" diode lasers 
40 and 41. These lasers may be obtained from SDL Inc. in San Jose, Calif. 
as Model Nos. SDL-5601-V1. These are dual beam separately addressable high 
power laser diodes. The lasers 40 and 41 are monolithic, in that they are 
made from the same substrate material and both are stable-resonator 
lasers. The first matched laser 40, as shown in FIG. 9, is a transmitting 
laser, the output of which is focused by lens 50 onto a reflective target 
surface 60. The reflected beam passes back through lens 50 into the second 
matched laser 41. The output of the receiving laser cavity 41 is 
transmitted to a photodiode 70 which measures the output power of the 
receiving laser cavity 41. 
According to one aspect of this invention, a light block 71 is mounted 
between the first matched laser 40 and photodiode 70 to prevent the output 
from transmitting laser 40 from entering photodiode 70. The presence of 
light block 71 in the overall arrangement shown in FIG. 9 adds 
considerably to the overall sensitivity of the system. 
As used herein, and in the claims, the phase "matched lasers" refers to two 
or more lasers having line width curves which overlap to a sufficient 
degree wherein each of the matched lasers is capable of operating at a 
wavelength at which the other matched laser or lasers are capable of 
operating. FIGS. 17A-17C show line width curves for three pairs of 
"matched lasers" 740 and 741, 840 and 841 and 940 and 941. The shaded 
areas represent common operating regions or overlap of each pair of 
matched lasers. In certain applications of the arrangements shown in FIG. 
9, the reflected beam 42 entering the receiving laser cavity 41 will cause 
resonant optical amplification in the receiving cavity and cause 
amplification of the feedback signal. The amplification allows the system 
of FIG. 9 to operate with higher speeds of motion of the reflective target 
60. 
The system represented in FIG. 9 may be used with or without the compound 
modulated beam illustrated in FIG. 5. In the preferred embodiment of the 
invention, the compound modulated beam would be utilized in the system of 
FIG. 9. The waves of frequency f.sub.1 and f.sub.2 are electronically 
modulated onto the output beam of laser 40 (also sometimes referred to 
herein as "frequency injection") by the use of well-known commercially 
available electronic signal generators. The system represented in FIG. 9 
may also be operated without compound modulated beam pattern as 
represented in FIG. 5. 
The photodiode 70 forms a means for measuring the power of the output beam 
of receiving laser cavity 41. Photodiode 70 may be an EE&G SGD 100-A 
silicon photodiode. 
Alternately, as shown in FIG. 23, instead of using a photodiode, the 
impedance of a receiving diode laser may be measured. The system of FIG. 
23 does not require a lens. 
Deconvolution means 80 is connected to the photodiode 70 and uses 
commercially available electronic filters for filtering the fundamental 
frequency of wave f.sub.1 and the second harmonic of wave 2 (or other 
particular harmonic being utilized). 
FIG. 10 represents cavities or pits which are typically utilized in optical 
and video discs, including CD ROMs. Disc 90 is shown having a plurality of 
pits 91 having a uniform depth d. FIG. 10B shows an alternate disc 
substrate 190 having shallow pits 191 of uniform depth d which may be 
utilized according to the present invention with a system such as 
illustrated in FIG. 9. The pits which may be used in conjunction with the 
optical reader illustrated in FIG. 9 can be less than 100 nanometers. 
In another aspect of the present invention, the alternate disc design, 
shown in FIG. 10C, utilizes a substrate 290 having pits 291 of varying 
depths capable of storing multiple bits of information. In one aspect of 
the present invention, the multiple pit depths, shown in FIG. 10C, are 
read by the matched laser configuration illustrated in FIG. 9. The 
preferred embodiment of the optical reader illustrated in FIG. 9 used with 
the multi-pit disc illustrated in FIG. 10C would utilize the "frequency 
injection" technique of introducing a first wave f1 and a second wave 
f.sub.2. An optical reader of such configuration would be capable of 
operating at much higher frequencies than the prior art Bearden optical 
reader because the present invention utilizes separate transmitting and 
receiving laser cavities, which configuration is inherently capable of 
operating at much greater speeds than the Bearden single cavity 
configuration. 
Referring to FIGS. 11A and 11B, first and second matched lasers 40 and 41 
are shown coupled by a fiberoptic cable 52. Photodiodes 70 and 72 measure 
the output beams of the laser cavities 40 and 41, respectively. In 
accordance with the present invention, the output beam of 1 or both lasers 
may be modulated by the "frequency injection" technique wherein a first 
wave frequency f.sub.1 and a second wave frequency f.sub.2 are used to 
modulate the output of either or both lasers 40 and 41. In the preferred 
embodiment, both lasers 40 and 41 would operate with the "frequency 
injection" modulated beam and would incorporate deconvolution means 
filtering the fundamental of f.sub.1 and the second harmonic of wave 
f.sub.2. Lasers 40 and 41 could transmit and receive bit streams 
simultaneously. 
A similar fiberoptic network system is represented in FIG. 11B wherein 
external electro-optic modulators 56 and 57 are positioned between laser 
cavity 40 and fiberoptic cable 52 and laser cavity 41 and the other end of 
fiberoptic cable 52. The purpose of the external electro-optic modulators 
53 and 54 is to allow modulation other than electronic modulation. 
FIG. 12 is a schematic representation of a microphone system utilizing the 
present invention. Sound waves 8 impact a diaphragm 9, causing the 
diaphragm to vibrate. The motion of the vibrating diaphragm 9 is detected 
by the use of matched lasers 40 and 41, wherein the output of transmitting 
laser 40 is directed at diaphragm 9 and the reflected laser output beam is 
received by laser cavity 41. The output of laser cavity 41 is measured by 
photodiode 70. The system shown schematically in FIG. 12 can be used with 
or without the "frequency injection" technique of the present invention. 
If the "frequency injection" technique is utilized, the necessary filters, 
which comprise the harmonic deconvolution means, would be connected to the 
photodiode 70. 
FIG. 13 comprises a schematic representation of a system utilizing the 
present invention to sense the change of the index of refraction of a 
light transmitting medium. The system includes chamber 6 into which a 
gaseous medium 7 may be introduced which would change the index of 
refraction of the ambient atmosphere in the chamber 6. A pair of matched 
lasers 40 and 41 are mounted adjacent the chamber 6 so that the output 
beam of transmitting laser 40 passes through the chamber, is reflected off 
a mirror 60, passes back through the chamber and into the second matched 
laser 41 comprising the receiving laser cavity in the design shown in FIG. 
13. Photodiode 70 is utilized to measure the output power of laser cavity 
41. As gas 7 enters the chamber 6, the index of refraction of the material 
through which the laser beam is passing will change, and the phase of the 
laser beam entering cavity 41 will accordingly change. The phase 
modulation caused by the presence of gas 7 will be detected in the 
receiving laser cavity 41. 
FIG. 14 shows a system for sensing the change in length of a fiberoptic 
cable 52 which is wound around a member 4. As the temperature of member 4 
increases, the length of fiberoptic cable 52 will be increased slightly 
and the change in path length will be detected by the receiving laser 
cavity 41. The design shown in FIG. 14 can be used with or without the 
"frequency injection" technique. The system shown in FIG. 14 is capable of 
measuring temperature and pressure changes in member 4 which could be a 
vessel, a pipe or any object which expands or contracts causing changes in 
the length of fiberoptic cable 52. In using the system shown in FIG. 14, 
it is ordinarily preferable to use acid-formed lenses 53 on both ends of 
fiberoptic cable 52. 
FIG. 15 is a schematic representation of another form of the invention 
wherein multiple matched lasers 40,41,42 are utilized. It is understood 
that the concept of the invention would work with larger combinations of 
matched lasers than the three shown in FIG. 15. The invention includes 
arrays of m matched transmitting lasers wherein the output beam of each 
laser may be modulated with a complex wave form and n receiving matched 
lasers. For example, eight matched lasers could be used wherein five of 
the lasers are transmitting lasers and the middle three lasers would be 
receiving lasers. The advantage of using multiple matched lasers is to be 
able to electronically extrapolate higher resolution data. 
FIG. 16 shows another embodiment of the invention wherein fiberoptic cables 
52 and 54 are utilized to transmit the output beam of transmitting laser 
40 to lens 50 and to transmit the reflected beam from lens 50 to receiving 
laser cavity 41. It is understood that the lens 50, shown in FIG. 16, 
could be deleted from the design if acid formed lenses were formed on the 
ends of fiberoptic cables 52 and 54 which are adjacent lens 50 in FIG. 16. 
FIGS. 17A, 17B and 17C show examples of line width curves of "matched 
lasers" to illustrate the meaning of that phase. 
FIG. 18 illustrates a beam amplitude modulation scheme for determining the 
position of a reflective target surface. The embodiment shown in FIG. 18 
utilizes matched lasers 40 and 41 which represents the preferred 
embodiment for amplitude modulation. As shown by the schematic 
representation of the beam pattern 49, the coherence length of the output 
beam is reduced intentionally by applying a rather strong RF signal to 
modulate laser 40 and to dissipate the coherence of the beam in order to 
"flatten" the phase response of the reflected beam entering the receiving 
laser cavity 41. As the amplitude of the output beam of laser 40 varies, 
the amplitude of the output beam of the receiving laser cavity 41 sensed 
by photodiode 70 will also rise and fall in time-dependent fashion. The 
change in amplitude is utilized to derive positional information regarding 
the target. The system, shown in FIG. 18 for amplitude modulation, could 
be operated with a single laser cavity, although that embodiment would not 
function as well as the matched laser system shown in FIG. 18. 
FIG. 19 shows a fiberoptic system in which multiplexing of the signal is 
facilitated by the use of multiple sets of matched lasers. Lasers 40 and 
41 are matched. Lasers 140 and 141 are matched but at a different 
wavelength. Lasers 140 and 141 will not resonate at a common frequency 
with either of lasers 40 or 41. A third set of matched lasers 240 and 241 
is also provided. Lasers 240 and 241 are matched with each other but, 
again, are not matched with any of lasers 40,41 or 140,141. The purpose of 
matching multiple lasers in this fashion is to allow for simultaneous 
transmission of bit streams bidirectionally through fiberoptic cable 52. A 
big stream transmitted from laser 40 will be received only by laser 41, 
whereas a bit stream transmitted by laser 140 through the same fiberoptic 
cable 52 will only be received by its matched laser 141. The output from 
the lasers may be easily introduced into fiberoptic cable 52 by beam 
splitters 98,99 and by mirrors 67 and 68. Photodiodes 70 measure the 
output power of each receiving cavity, the variation in the output power 
forming the transmitted bit stream. 
FIGS. 20A and 20B represent the use of multiple layer optical discs wherein 
each layer is read independently of the other. For example, upper layer 95 
and lower layer 96 may have pits formed therein. For clarity, the 
individual pits are not shown in FIG. 20. FIG. 20A shows a pair of laser 
beams 101 and 102 being simultaneously focused at different depths on disc 
90, so that laser output beam 101 is reading the pits along layer 96 and 
focused output beam 102 is simultaneously reading the pits at layer 95. 
Alternately, as shown in FIG. 20B, laser output beam 101 may be used 
individually to scan the pits in layer 96. 
FIG. 21 is a schematic representation of the drive and receive electronics 
that may be used in one embodiment of the invention. The specific 
relationship of modulating signal f.sub.1 to f.sub.2 is f.sub.1 =2f.sub.2. 
A frequency generator generates a 2f signal which passes through a 
bandpass filter, resulting in reference signal S.sub.1. Signal f may be 
generated by dividing the 2f signal, passing it through a bandpass filter 
resulting in input reference signal S.sub.2. Input reference signals 
S.sub.1 and S.sub.2 are amplified and added to a DC offset signal which is 
typically 1.5 volts. The reference signals are then applied to the laser. 
The lower portion of the schematic shown in FIG. 21 illustrates the 
deconvolution electronics of the present invention and includes a 
photodiode 70, the output of which is amplified and passed through a 
bandpass filter for the 2f signal and a separate bandpass filter for the f 
signal. The outputs are then entered into a phase comparator which 
compares the phase of input reference signal f.sub.1 with the phase of 
R.sub.1 +2R.sub.2. The output of the f bandpass filter is R.sub.2 which is 
entered into phase comparator to compare the phase of R.sub.2 with 
S.sub.2. These comparisons form the basis of the quadrature detection 
scheme according to the present invention. 
FIG. 22 is a schematic representation of yet another embodiment of the 
present invention wherein a single laser 40 is utilized as both the 
transmitting and receiving laser using the "frequency injection" technique 
and harmonic deconvolution means of the present invention. The schematic 
shown in FIG. 22 may be used in target motion detection. 
FIG. 23 is a schematic representation of matched diode lasers 40 and 41, 
wherein no photodiode and no focusing lens are required to determine the 
position of target 60. The output of diode laser 40 is electronically 
modulated. The output beam is scattered off the target or mirror 60 with a 
portion being reflected into receiving laser cavity 41. The impedance of 
receiving diode laser 41 is measured, and time-dependent changes in 
impedance are used to derive positional information of the target 60. 
The invention described above may be used in a variety of end uses. In 
addition to the end uses discussed above in the application, the invention 
may be used in a laser micrometer device. This would be a rather 
inexpensive device for measuring micromotion bidirectionally. 
The invention as described above could also readily be used for a variety 
of pressure and vacuum sensors. Such a device could utilize a diaphragm 
for sensing pressure or vacuum and the motion of the diaphragm could be 
readily detected by the embodiments disclosed above. 
Another end use of the invention described herein is a robotic sensing 
device which could be used in conjunction with a variety of robotic 
elements to assist in tracking the precise location of one or more robotic 
elements. 
The present invention could also be used as a thermometer by sensing the 
thermal-expansion of a variety of metals or ceramics. 
The invention can also be utilized to measure the thickness of 
electroplating material being applied to a substrate. The device could be 
mounted in the electroplating chamber. The invention can also be used to 
measure the thickness of molecular deposition such as used on optical 
lenses. 
The amplitude modulation technique of the present invention could also be 
used to read high speed optical tape. The amplitude modulation technique 
of the present invention could also be used as a bar code reader. 
The present invention can also be utilized in conjunction with scanners and 
facsimile machines.