Self-mixing sensor apparatus and method

A self-mixing sensor usable for remotely measuring speed, vibrations, range, and length is provided in a manner making the device practical for economic implementation while retaining accuracy. In one embodiment, the device is configured to avoid mode hopping, such as by providing for relatively high loss for all modes other than the desired mode. Preferably this is accomplished by utilizing laser types that have a high degree of side mode supression, such as DFB lasers or through active or passive control of the amount of light permitted to reenter the laser.

Cross reference is made to disclosure document No. 419765, received in the
 U.S. Patent and Trademark Office on May 19, 1997 and the disclosure
 document No. 432,032, received in the U.S. Patent and Trademark Office on
 Jan. 23, 1998, both titled "Improved Self-Mixing Sensor".
 FIELD OF THE INVENTION
 The present invention relates to apparatus for remotely measuring speed,
 vibrations, range, and displacements using laser light.
 BACKGROUND INFORMATION
 Measurement of range, vibration, or speed of an object can be done without
 physical contact between a sensor and the object. Optical non-contact
 measurements usually involves transmitting light from a laser to the
 object and measuring changes in light scattered from the object by
 suitable detection means. Self-mixing laser interferometry, also known as
 laser-feedback interferometry and backscatter modulation interferometry,
 has been known for some time, and some of the characteristics of such
 sensors have been described previously. In comparison with traditional
 laser radar and laser Doppler velocimeters, typical previous devices are
 very simple, the entire optical assembly typically consisting of a laser
 and a lens to focus light on or near an object whose properties are being
 measured. The apparent simplicity of the hardware offers potential to
 manufacture sensors at a relatively low cost compared with the alternative
 techniques, in particular coherent (heterodyne) laser radar. In its
 simplest form the self-mixing sensor relies on the fact that if light at a
 substantially fixed frequency is sent from a laser to an object, and if a
 fraction of the light scattered from the object is permitted to re-enter
 the laser, interference causes a modulation of the laser power. The
 amplitude of the modulation is dependent on the amount of scattered power
 coupled back into the laser. The modulation is at least partly dependent
 on the phase of the backscattered light relative to the phase of the light
 circulating in the laser cavity. If the relative phase varies in time, as
 is the case if the scattered light is shifted in frequency due, e.g., to
 the Doppler effect, the result is a periodic variation of the laser power
 with time. Detection of the periodicity through spectral analysis provides
 a direct measurement of the Doppler frequency. Since the Doppler frequency
 shift f.sub.Doppler is related to the speed of the object v as
 f.sub.Doppler =2v/.lambda., where .lambda. is the laser wavelength,
 determination of the Doppler frequency directly leads to a determination
 of the object speed v. By introducing a known time-varying laser frequency
 shift, for example, where the transmitted laser frequency increases or
 decreases linearly with time, the range to a stationary or moving object
 can also be determined. In this case the time t between transmission and
 reception of light is given by t=2L/c, where L is the optical distance
 from the sensor to the target and c is the speed of light. If the laser
 frequency is varied at a rate df/dt, the sensor measures a frequency shift
 (df/dt) t if the object is stationary. If the object is moving, the
 detected frequency shift is (df/dt) t.+-.f.sub.Doppler, the + or - sign
 depending on whether the object is moving toward or away from the sensor,
 respectively. If the transmitted frequency is alternately moved up and
 down in a linear fashion, a measurement of the resulting two frequencies
 can permit determination of the range, speed, and direction of the object
 motion. Vibrations can also be detected using self-mixing sensors, since a
 vibration is simply motion with a velocity varying periodically in time.
 Length can also be measured by integrating the measured speed over time.
 Early discussions of the self-mixing effect were given by D. M.Clunie and
 N. H.Rock (J. Sci. Instru., 41, pp. 489) in 1964 and M. J.Rudd (J. Sci.
 Instrum. Series 2, 1, pp.723) in 1968, where the researchers used a gas
 laser. Gas lasers were subsequently used also by Churnside (Appl. Optics
 23, pp.61 and pp. 2097, 1984), Bearden and O'Neill (U.S. Pat. No.
 5,260,562), and Hinz (U.S. Pat. No. 5,069,545). Such lasers are
 characterized by long laser cavities (typically on the order of 10 cm or
 more) and very high output coupler reflectivities (such as much greater
 than 90% in the case of HeNe lasers).
 Self-mixing using semiconductor lasers (henceforth also called diode
 lasers) has been studied by several researchers. Shimizu (Appl. Optics 26,
 pp. 4541, 1987), Shinohara et al. (Appl. Optics, 25, pp. 1417, 1986),
 Meinzer et al. (U.S. Pat. No. 5,267,016), and deGroot et al. (Appl.
 Optics, 27, pp. 4475, 1988), describe various basic sensor aspects.
 DeGroot and Gallatin (Opt. Lett., 14, pp. 165, 1989) subsequently
 discussed the use of a long external cavity laser to enable measurements
 to a range of 50 m. As implemented, it is believed the laser required an
 optical spectrum analyzer and a feedback control system to keep the laser
 stable.
 A previous patent to Gerardin (U.S. Pat. No. 4,928,152), as well as
 publications by Jentink et al. (Appl. Opt. 27, pp. 379, 1988), and Otsuka
 (Appl. Opt., 33, pp. 111, 1994) using a diode-pumped solid-state laser,
 improperly discuss the self-mixing effect as a conventional heterodyne
 mixing process. Wang et al. (J. Lightwave Techn., 12, pp. 1577, 1994) and
 Koelink et al. (Appl. Opt., 31, pp. 3401, 1992) note that at high feedback
 levels the laser can operate in multiple longitudinal modes.
 Although the self-mixing phenomenon has been known for a relatively long
 time, several assumptions are usually made which limit the practical use
 of these sensors. For example, previously it was often assumed , that if a
 semiconductor diode laser is substituted in place of a gas laser, the
 sensor performance will not be strongly affected, in terms of sensitivity
 and dynamic range. Dynamic range refers to the ratio of the strongest
 signal that can be detected to the weakest detectable signal. We find
 that, with good design, the lower limit can approach the quantum limit. In
 this limit, the noise level is given by the expression hvB, where hv
 denotes the photon energy and B denotes the detection bandwidth.
 Previously, it was frequently (implicitly) assumed that the upper limit is
 determined by how much light can be collected and allowed to re-enter the
 laser cavity. This assumption is, however, incorrect. The upper limit is
 more frequently determined by the onset of external-cavity mode hopping,
 hereinafter referred to as EMH. If a laser is operating substantially on a
 single longitudinal mode (a single frequency), feedback perturbs the laser
 frequency, in addition to perturbing the laser power. If the feedback
 level is high enough and the phase of the returned light is changing with
 time, the laser may jump between different frequencies separated
 approximately by an integer number times the quantity c/2L, where L
 denotes the distance from the sensor to the target. In the case of a
 Fabry-Perot laser cavity, for reflectivity values below a limiting value
 R.sub.max given by the expression
 ##EQU1##
 such EMH does not occur. Here L.sub.d denotes the optical length of the
 laser cavity, R.sub.oc denotes the laser output coupling mirror
 reflectivity, and .alpha. denotes the so-called linewidth enhancement
 parameter having a typical value in the range of 2-5. For external target
 reflectivity values in excess of R.sub.max, EMH becomes important and may
 have a significant influence on the accuracy of measurements. It is
 evident from the above equation 1, that as the distance L increases, the
 maximum allowable value of R.sub.max decreases. It is also evident that
 R.sub.max can be increased by increasing the laser length L.sub.d.
 For gas lasers used at a modest distance, the combination of a large
 R.sub.oc and a large L.sub.d cause the upper limit on reflected light to
 be so high that the upper limit given by equation 1 is almost never
 encountered. Consequently, the EMH effect typically has not been discussed
 in the context of gas lasers. Furthermore, the EMH concept is believed
 unique to self-mixing sensors. Consequently, it typically has not been
 recognized by those authors who discuss the sensors as conventional
 heterodyne devices.
 Among the prior authors who do note the existence of EMH, it is believed
 Wang makes no comments about whether the effect affects the sensor
 performance and shows no results to support any potential performance
 degradation. Koelink notes that under high feedback conditions, more noise
 results, and reducing the aperture can result in a larger signal without
 mention of attenuation effects or other possible rationales. It is
 believed no discussion is given of whether the issue is detrimental to
 unambiguous measurements, or how it can be circumvented.
 In the one case where a long cavity diode laser has been fabricated, the
 device was put together in order to improve the coherence length of the
 laser source and hence extend the range of the sensor. The authors do not
 mention EMH, or it potential importance. The authors state that they used
 the long cavity to ensure a long coherence length of the source laser, but
 make no mention of the fact that a long laser may be more immune to high
 feedback levels. Having a sufficient coherence length is necessary, but
 not a sufficient requirement for operation at extended distances. In other
 words, it is believed a very short laser will never be highly suitable for
 very long range measurements, regardless of its coherence length.
 It is believed the laser used by deGroot also does not enable low cost
 manufacturing. As discussed in the reference, the laser comprises a short
 diode laser together with an external mirror and control electronics to
 keep the laser properly aligned. Such lasers involve many precision
 components and are relatively expensive to fabricate.
 Of fundamental importance is also that previously used diode laser devices
 are intrinsically unsuitable for sensor manufacturing. This is
 particularly true when the objective is to manufacture sensors with high
 speed or range resolution, high accuracy, and high reliability over long
 time periods. The detailed commercial device testing and research efforts
 of the present inventors have demonstrated that so-called Fabry-Perot
 lasers employed by previous researchers using diode lasers, are inadequate
 for practical commercial self-mixing sensors that employ diode lasers.
 Accordingly, it would be advantageous to provide the means whereby lasers
 employing the self-mixing principle may be used to form the basis of
 practical, reliable, and commercially viable sensors,
 It would be further advantageous to improve the dynamic range of
 diode-laser based self- mixing sensors and to prevent EMH from causing
 ambiguous measurements or measurements with decreased accuracy.
 It would be further advantageous to achieve significantly higher
 sensitivity as well as variable attenuation, compared with the simplest
 form of the sensors discussed in the prior art previously.
 It would be further advantageous to increase the detectability of weakly
 scattering objects, such as objects with low reflectivity, or small
 particles.
 It would be further advantageous to enhance diode laser sensor operation at
 long ranges, without requiring the complications of optical spectral
 analysis discussed previously.
 It would be further advantageous to achieve highly accurate speed
 measurements without requiring that the sensor is accurately aligned in
 angle to the target.
 It would be further advantageous to obtain range and/or Doppler information
 from multiple spatial locations without the use of photo-detector arrays,
 as in the previous devices.
 It would be further advantageous to design sensors that are highly reliable
 and highly manufacturable.
 SUMMARY OF THE INVENTION
 A self-mixing sensor usable for remotely measuring speed, vibrations,
 range, and length is provided in a manner making the device practical for
 economic implementation while retaining accuracy. In one embodiment, the
 device is configured to avoid mode hopping, such as by providing for
 relatively high loss for all modes other than the desired mode. Preferably
 this is accomplished by utilizing laser types that have a high degree of
 side-mode suppression, such as DFB lasers or through active or passive
 control of the amount of light permitted to reenter the laser.

DETAILED DESCRIPTION
 In order to properly discuss the disclosed invention it is useful to also
 relate it directly to previous approaches. FIG. 1 shows a conventional
 heterodyne ("Michelson") interferometer configuration, where a light beam
 from a laser 10 is directed along a path 16a and 16b via optical isolator
 15 to a beam splitter 11, where part of the light is directed along a path
 17 to a mirror 12. Light reflected from the mirror along path 18 is
 partially transmitted through the beam splitter and impinges on
 photo-detector 13. Light is also transmitted in the forward pass from the
 laser along path 19, and is partially scattered from object 14 back along
 path 20. The beam splitter directs part of he light from path 18 along
 path 21 onto the detector 13. Although, for clarity, the light paths 18
 and 21 are shown as spatially separated in the figure, the beams in actual
 interferometers must be carefully aligned in position and direction to
 achieve optical interference. If the object 14 is moving, a Doppler
 frequency shift exists between the light reflected from mirror 12 and the
 light scattered from the object 14. As is well known in the art, this
 results in a temporal variation of the power detected on photo-detector
 13. The frequency of this power variation can be determined using
 conventional frequency spectrum analysis, and indicates the radial (along
 the direction of the laser beam) speed of the object toward or away from
 the sensor. The reflected light can be analyzed in a number of other
 fashions as well, such as detecting phase of said first light; detecting
 amplitude of said first light; detecting frequency content of said first
 light; detecting temporal evolution of the amplitude of said first light;
 detecting the width-of-signal of said first light in spectral space;
 detecting the signal to noise ratio; and/or detecting the number of peaks
 per unit time of said first light. To prevent the laser operation from
 being perturbed, it is common to place an optical isolator 15 between the
 laser and the beam splitter. This isolator 15 permits light to be
 transmitted from left to right in the figure, but substantially prevents
 light from passing through the isolator in the opposite direction. The
 heterodyne or Michelson interferometer scheme shown in FIG. 1 works well
 for speed detection and can be modified for range detection by varying the
 laser frequency in time, as is well-known in the art.
 The main drawbacks with such heterodyne techniques are the need for
 expensive optical isolation, such as with optical isolator 15, the need
 for multiple high quality optical elements, such as mirror 12 and beam
 splitter 11, and highly precise alignment of all beam paths to ensure
 optimal sensor efficiency. A key feature of this type of system is that
 the detected signal power increases as the laser output power increases.
 FIG. 2 shows a simplified graphical representation (in arbitrary unit) of
 signal power measured on a detector in relation to laser output power, at
 a given Doppler frequency. For laser power levels below a point denoted by
 P.sub.min, the measured power is limited by noise in the detection system,
 denoted by the detected power level N. For laser power levels above this
 threshold value P.sub.min, the detected power increases substantially
 linearly with increasing laser power, as indicated by plotted line 32.
 The typical self-mixing configuration shown in FIG. 3 reduces the alignment
 requirements as compared to a heterodyne sensor. Here a laser 40 transmits
 light to an object 42 along path 41 through beam splitter 45. Scattered
 light is reflected along the same path 41 back into the cavity of laser
 40, where it perturbs the laser oscillation in such a manner that the
 laser 25 exhibits output power fluctuations. If the phase of the light
 returned from object 42 into the laser 40 varies periodically at a Doppler
 frequency, the output power is also modulated at the same Doppler
 frequency. This power modulation can be detected by directing a portion of
 the light output along path 43, via beam splitter 45, to photo-detector
 44. This arrangement eliminates the need for highly accurate alignments,
 since, if he object 42 is at least partially reflecting, a fraction of the
 light scattered from the object 42 will be scattered back along path 41.
 In an alternative configuration, shown in FIG. 4, the photo-detector 54 is
 placed behind the laser 50, and light transmitted as well as received
 along path 51 and scattered from object 52 is directed through a partially
 transmitting rear mirror 55 along path 53 to a photo-detector 54. This
 arrangement eliminates the need for the beam splitter 45 shown in FIG. 3.
 Description of experiments by the Inventors
 It is also known in the prior art that a separate photo-detector is not
 needed to operate individual self-mixing sensors, particularly if the
 laser is a diode laser. A sensor, according to this invention, which
 eliminates the need for a separate photo-detector is shown in FIG. 5. In
 FIG. 5a current source 60 is used to drive the laser diode 61. Light from
 laser diode 61 is collected by lens 62 and focused along path 63 near the
 object 64. Scattered light returns to the laser diode and causes a
 temporally varying change in the diode laser junction impedance. Capacitor
 65 provides a return path for the fluctuating component of the voltage
 across the series combination of diode laser 61 and inductor 66. The
 junction impedance changes in diode laser 61 discussed above then causes
 voltage fluctuations to appear across inductor 66. These fluctuation can
 be amplified by amplifier 67, and periodic variations can be measured
 using, for example, spectrum analyzer 68.
 FIG. 6 illustrates the variation of signal power and noise power
 (Signal-to-Noise Ratio or SNR) as a function of diode laser drive current
 (proportional to diode laser power) observed by the present inventors for
 a sensor using the configuration shown in FIG. 5. In this case the sensor
 used a model 531 1 -G 1 diode laser from SDL, Inc. (San Jose, Calif.) and
 the target object consisted of a rotating wheel covered with white paper
 placed approximately 1 m from the sensor. The rotating wheel impressed a
 substantially constant Doppler frequency shift on the reflected light.
 Laser threshold occurs near point 72 where the noise level, indicated by
 curve 70, increases rapidly with small increases in laser current. Near
 the same current level, the signal, indicated by curve 71, also rises
 quickly. As the laser current increases above the laser threshold value,
 the noise decreases rapidly while the signal level flattens out to a
 substantially constant value. This behavior is fundamentally different
 from that illustrated in FIG. 2 and is characteristic of self-mixing
 sensors.
 FIG. 7 illustrates the behavior of the sensor in FIG. 5 under low and high
 feedback conditions. The amount of feedback was varied by inserting
 calibrated attenuation filters in the laser beam between the lens 62 and
 the object 64. The signal was again monitored using an RF spectrum
 analyzer 68. FIGS. 7a-d show the detected RF spectral signal peaks when 30
 dB, 20 dB, 6 dB, and 0 dB round-trip attenuation respectively was used. In
 each spectrum analyzer photograph in FIG. 7, the horizontal scale measures
 frequency. The frequency span of the display is 2 MHZ, with the center of
 the display corresponding to a frequency value of 10 approximately 25 MHZ.
 The vertical scale indicates signal power, with the scale marker showing
 the display size corresponding to a 10 dB change in signal strength. In
 going from FIGS. 7a to 7b, the round-trip attenuation decreased by 10 dB
 and one can see that the signal increased approximately 10 dB. In this
 case the sensor responds linearly with increasing feedback signal
 magnitude. However, in FIG. 7c the attenuation was reduced a further 14
 dB, but the signal does not show a significant increase in height. At the
 same time, the spectral width of the signal is clearly greater than in 7a
 or 7b. In FIG. 7d, finally, the attenuation was decreased a further 6 dB.
 The signal is now so wide that it is difficult to distinguish from the
 background noise, which is also increasing as the amount of attenuation is
 decreased. We estimate that the condition for the allowable reflectivity
 to prevent EMH according to equation 1 is exceeded between FIGS. 7b and
 7c. Clearly, as the maximum allowable reflectivity is exceeded, the
 measured bandwidth broadens and the noise floor increases. Speed
 measurement accuracy is dependent both upon SNR and spectral width through
 the approximate proportionality
EQU .DELTA.v.varies..DELTA..function./SNR,
 where .DELTA.v is the speed uncertainty and .DELTA.f is the measured
 frequency uncertainty (width of the spectral line). When a sensor is used
 to measure range as discussed earlier, a similar expression relates range
 uncertainty to the measured frequency uncertainty and SNR. The indicated
 spectral broadening and reduction of SNR can therefore be detrimental to
 achieving a high measurement accuracy.
 FIG. 8a shows RF spectra when the target is moved to a distance of
 approximately 10 m. With no laser beam attenuation, the RF spectrum shows
 a set of indicated peaks spaced approximately 14.5 MHZ apart, and
 indicated by vertical arrows in the figure. These spectral peaks
 correspond to the external cavity mode hopping (EMH) frequency for the
 approximately 10 m range used. Note that the peaks to the right and left
 of the indicated peaks are due to pickup of signal emanating from radio
 stations and other interfering sources and are not caused by the sensor.
 In the figure shown, there is no discernible trace of a Doppler peak which
 should have been present at the location of the vertical dashed arrow.
 However, by placing an attenuator in the laser path we can strongly reduce
 the magnitude of the EMH peaks, and in so doing the Doppler peak manifests
 itself as shown in FIG. 8b. The vertical scale is the same in FIGS. 8a and
 b, but the horizontal scale has been expanded in FIG. 8b.
 From the measurements it is clear that under these circumstances, if an
 attenuator is used which can change the attenuation from a high value to a
 low value, while spectral measurements are being made, the Doppler peak
 would manifest itself first as the attenuation is decreased. Only at lower
 attenuation would the EMH spectral features appear. It is equally clear
 that a signal processor which only detects spectral peaks, in particular
 the lowest frequency peak, cannot distinguish between spectral features
 corresponding to EMH and Doppler shifts, particularly if the spectral
 regions where those features may both occur, as in the example of FIG. 8.
 For example, if the laser wavelength is 800 nm, an object moving at a
 radial speed of 5.8 m/s would cause a 14.5 MHZ Doppler shift, which, to
 the signal processor, would look identical to an EMH spectral feature from
 a non-moving object placed approximately 10 m from the sensor. The
 previous approach, in particular the apparatus of Gerardin, could not, it
 is believed, distinguish between these two cases, which would consequently
 cause false Doppler estimates under many common circumstances.
 The following provides a description of an improved sensor using an optical
 attenuator. As demonstrated in the previous section, control of EMH is
 important to high performance self-mixing sensor operations. For fixed
 operations where little change in the operating conditions such as target
 range and reflectivity, can be expected, a simple fixed optical
 attenuator, such as a neutral density filter, may be selected for the
 application conditions, and fixed in place between the sensor and the
 target. However, for other common applications, the target range and/or
 reflectivity may be not be known. One method of providing automatic
 control of the target reflectivity to regulate self mixing sensor
 performance includes the following steps. First determine how much signal
 is acceptable without causing EMH. This is estimated using Equation 1 or
 through experimentation. Once this maximum signal level is determined, it
 is used as a reference level against which detected signals are compared.
 A second method measures the quiescent wideband noise of the laser diode
 and electronics, and uses fluctuations in that level to indicate and
 control the occurrence of EMH.
 One embodiment of the invention incorporating both of these EMH control
 techniques is shown in FIG. 9. Control electronics 109 in conjunction with
 a laser diode driver 100 provide a substantially constant drive current to
 laser diode 101. Radiation from the diode laser 101 is collected and
 transmitted to object 104 along path 103 using lens 102. Feedback control
 element 111 is inserted in the optical path between the diode laser and
 the object. Feedback control element 111 may be any of the available
 electrically operable optical attenuator devices, selected to function at
 the laser wavelength, and suitable drivers. Suitable electrically operable
 optical attenuators are well known in the art, and include, but are not
 limited to, liquid crystal attenuators, shutters, electro-absorption
 filters, electro-optic modulators, and mechanically operated crossed
 polarizers. Capacitor 105 and an inductor 106 are used, as in FIG. 5, to
 enable coupling of AC signals into amplifier 107. The output of the
 amplifier consists of the Doppler signal plus wideband electronic noise
 from the laser and electronics. The Doppler signal is demodulated by a
 signal analyzer 108 which comprises a narrowband filter used to determine
 the amplitude of the Doppler signal within a select bandwidth. Such
 techniques are well known in the art and suitable methods may include, but
 are not limited to, narrowband filter constructions such as one or
 combination of phase-locked loops, digital filters implemented on a
 digitized version of the amplifier 107 output, or passive or active LC or
 RC filters. Signal rectification and magnitude determinations may be
 performed by, but are not limited to, synchronous or asynchronous AM
 detection, peak picking, or other common analog or digital techniques. The
 wideband signal +noise power may be determined by rectification and low
 pass filtering of the wideband signal present at the output of the
 amplifier 107. A signal representing the wideband signal+noise is output
 to the control electronics 109 via wire 108a. The outputs of the signal
 analyzer 108 thus represent both the magnitude of the AC narrowband
 Doppler signal and the wideband signal noise induced by impedance
 variations in laser 101 and transmits said magnitudes to the control
 electronics 109.
 In the high feedback regime, as illustrated in FIGS. 7c and 7d, for
 example, EMH causes a broadband increase in the noise floor, which would
 manifest as an increase in the wideband signal+noise signal from the
 signal analyzer 108. If the wideband signal+noise magnitude exceeds a
 predetermined high limit value, a signal indicative of the high feedback
 condition is sent from control electronics 109 via wire 110 to the
 feedback control element 111 in order to increase the element 111 optical
 attenuation, thereby decreasing the amount of light reflected back into
 the laser. If the narrowband Doppler signal magnitude falls below a second
 predetermined value, a signal from control electronics 109 is sent via 110
 to the feedback control element 111 to decrease attenuation in the light
 path between the laser 101 and the object 104, in order to increase the
 SNR. Continuous proportional control of the feedback control element 111
 can be implemented using either or both narrowband and wideband signal
 magnitudes to provide as tightly controlled a s/n operating regime as
 desired for the particular application. If needed for stable operation of
 a particular laser 101, the control electronics 109 may also regulate the
 diode laser driver current to obtain improved signal to noise performance
 of the system.
 Once the appropriate SNR has been set by the operation of the above control
 feedback loop, the signal analyzer 108 also transmits the Doppler signal,
 wideband signal+noise, and other analyzed information (such as Doppler
 frequency, power spectrum if calculated, etc.) to a user or computer
 interface 112 via wire 108b, which formats and scales the data, and
 presents the analyzed information, which may include, but is not limited
 to, vibration frequency or spectra, velocity, distance, or position, to a
 display terminal for human interface, and/or to electronic interfaces to
 facilitate automatic industrial process control.
 A preferred operational flow for implementation of the control loop
 outlined in connection with FIG. 9 is shown in FIG. 10. Logical flow
 begins at 300 and proceeds to 305, where the wideband (WB) signal+noise is
 measured, and proceeds to 310 where the WB signal+noise is compared to an
 expected threshold value from a sensor free of EMH. If the threshold value
 is exceeded, flow proceeds to 320 where the optical attenuation in the
 path between the sensor and the target is increased by a predetermined
 amount appropriate to eliminate the measured excess noise signal, and flow
 returns to 300. If at 315, the signal+noise was found within acceptable
 limits, flow proceeds to 325 where the narrowband signal strength is
 determined, and flow proceeds to 330 where this signal is compared to an
 expected value and flow proceeds to 335. If at 335 the narrowband signal
 is not within expected limits, flow proceeds to 340 where the attenuator
 is adjusted by a predetermined amount to approximately bring the signal
 within limits. If at 335 the signal was within acceptable limits, flow
 returns to 300 and begins again. This control loop function maintains the
 sensor-detected target reflectivity within the range appropriate for
 reliable sensor performance.
 An alternative embodiment for implementing automatic optical feedback
 regulation incorporates digital processing of the sensor including the
 steps described below.
 An example of sensor attenuation measured using a liquid crystal attenuator
 placed between the lens and the target, relative to the liquid crystal
 applied voltage, is shown in FIG. 11. By varying the applied voltage
 between 0 V and 24 V, the attenuation could be varied by approximately 34
 dB, thereby providing a method for compensating for high feedback levels.
 A second embodiment implementing automatic EMH control using digitally
 processed feedback control is shown in FIG. 12. In FIG. 12 control
 electronics 124 outputs a control voltage 125, for example near 20 V, to a
 liquid crystal attenuator device 126, in order to provide a high degree of
 attenuation. The signal 120 from sensor 127, in the presence of this large
 attenuation, is converted to digital form by A/D converter 121a and
 injected into an FFT (Fast Fourier Transform) processor 121b. The FFT
 processor 121b can be effected in a specialized hardware processor, such
 as a DSP (Digital Signal Processor), or it may be implemented in software
 on a general purpose computer to perform an FFT calculation. The FFT
 processor calculates a power spectrum of the signal. The spectrum is then
 passed to a signal analyzer 122 which measures the presence of peaks in
 the spectrum above a predetermined noise level and below a predetermined
 maximum level. If a peak is detected, the corresponding frequency is
 output to a user interface 123. The user interface 123 can include a
 computer monitor or other local or remote readout and/or storage device,
 or it can be an electrical connection, such as an RS-232 or IEEE 488
 connection to a remote process control computer.
 If no peak is detected, the control signal 125 applied to the LC device 126
 is changed to reduce the attenuation by a suitable amount, for example, 5
 dB. A new signal then results, which is again processed by the FFT
 processor 121 and signal analyzer 122. This process is repeated until a
 spectral peak is found and output to the user interface 123. If no peak is
 detected at the lowest attenuation level, a signal indicating no detection
 may also be provided to the user interface 123. Numerous alternative
 implementations of the requisite electronics are readily apparent to those
 skilled in the art. One alternative is to use analog or digital circuitry
 to search for and lock onto a spectral peak. If that peak signal is
 varying in strength, a control loop can easily be implemented to increase
 or decrease the attenuation as required to keep the signal level within
 predetermined high and low limits. This technique is particularly well
 suited for tracking of signals which vary relatively slowly in frequency
 and strength.
 The following provides a description of an improved sensor using an optical
 amplifier. In an alternative embodiment the feedback control element
 consists of a laser amplifier whose gain or loss can be controlled by
 applying a signal to the amplifier. When the sensor laser is a diode
 laser, the amplifier can be, for example, a semiconductor diode amplifier,
 or a doped optical fiber amplifier. The preferred configuration when a
 semiconductor diode amplifier (SDA) is utilized is shown in FIG. 13. The
 configuration has many elements in common with FIG. 9 and corresponding
 parts have been given the same numbers in both figures. In this
 configuration, the light from laser 101 is coupled into SDA element 200
 through the use of one or more optical elements 201. In addition,
 secondary optical elements, such as a lens 202, are used to collect the
 light from the output of the SDA and focus it near the object. The control
 signal 204 is used to drive a current source 203 in order to produce a
 variable gain in the SDA. When no electrical current is applied to the SDA
 the amplifier is operating with a gain G&lt;1, and the incident light is
 attenuated in passing through the SDA 200. At a particular level of
 current, termed the transparency level, the incident light is passed
 through the SDA with no loss and no amplification, corresponding to a net
 SDA gain G=1. At a current higher than the transparency level, the light
 output from the SDA is greater than the input level and the SDA is
 operating with a gain G&gt;1. Gain is measured as the ratio of power coupled
 out from the SDA to the incident power. A great improvement in sensor
 dynamic range, as well as detection of weakly reflecting signals can be
 achieved with an SDA.
 FIG. 14 shows a plot of measured signal-to-noise ratio as a function of
 injection current for an SDA manufactured by SDL, Inc. In the absence of
 the SDA element, the measured SNR was approximately 20 dB. With reference
 to FIG. 14 it can be seen that 20 dB SNR was detected at an SDA current
 near 100 mA. That current consequently defines the transparency point of
 the SDA. It can be seen that reduction of the SDA current to approximately
 45 mA reduced the SNR by 20 dB, so the device can clearly be used as an
 attenuator. Similarly, by increasing the current to 280 mA, the signal
 increased by 28 dB with respect to the transparency point. This shows that
 the device can be used to increase signals, in addition to attenuating
 them. Since this device can amplify weak signals by 28 dB and also
 attenuate strong signal by 20 dB, the device can enable a 47 dB increase
 in dynamic range compared with a sensor not having an SDA present.
 When an optical amplifier is used it is important that the input and output
 facets produce low reflections of power, preferably lower than 10.sup.-4,
 back towards the laser. A low reflectivity is particularly important for
 the output facet, since light reflected from that facet will be amplified
 in the return pass before re-entering the laser. If the amplifier has a
 single-pass gain G, which may be 10-100, and the output facet has a
 reflectivity Rf of 10.sup.-4, the power fraction reflected towards the
 laser could be 0.01 to 1.0. If the distance between the laser and
 amplifier is large, for example several cm, this could cause EMH between
 the laser and the amplifier. For this reason, the amplifier should be
 placed as close to the laser as possible, the exact distance being
 determined by a number of practical factors.
 One solution to this potential problem is to place the amplifier on the
 same substrate as the laser, as shown in FIG. 15. In this case the laser
 section 151 and amplifier section 153 are manufactured at the same time
 and are separated by a grating 150. The grating is designed as the output
 coupler of the laser section and provides a relatively low, for example
 less than 15%, reflectivity at the laser wavelength. The left end 152 of
 the laser section preferably is provided with a high reflectivity coating,
 although a second grating could be used in place of the coating in order
 to improve the single-frequency characteristics of the laser. The output
 end facet 154 of the amplifier should be designed to provide a low
 reflectivity, for example 10.sup.-4 or less, in order to scatter little
 light back towards the laser section. The amplified light from the
 amplifier is collected with an optic 159, such as a lens, and the laser
 light is directed towards the measurement object along a path 160. The
 laser diode driver and other requisite electronics 157 can be connected to
 the laser section through a wire bonded to the semiconductor, a technique
 which is well known in the art. An amplifier current source 158 is
 connected in a similar manner to the amplifier section. We note that this
 arrangement does not preclude using same current source for the laser and
 the amplifier.
 In certain cases, for example if the target object is placed far from the
 sensor, it is desirable to place a beam expander between the sensor and
 the object. The advantage is that a larger transmitted beam can be used to
 focus the light to a smaller spot near the target and also that the amount
 of scattered light collected with the larger aperture increases. Both of
 these effects lead to an increased amount of power detected by the sensor.
 FIG. 16 shows one example of how such a configuration can be implemented.
 In the figure the self-mixing sensor 180 is shown as a single element. The
 laser light from the sensor is preferably, but not necessarily, collimated
 as it propagates along direction 181 to beam expander 182. The beam
 expander typically consists of several optical elements, for example two
 lenses 183a and 183b. At the output of the beam expander the transverse
 cross-section of the laser beam is larger than at the input, and this beam
 is transmitted to the object. Wavy lines 184 are used to indicate a break
 in the propagation path to the object.
 Optimal selection of a beam expander can lead to an additional advantage of
 using an SDA in the system, since it can be used to lessen potentially
 stringent requirements on focusing the transmitted light close to an
 object, whose distance from the sensor is often not known. Self-mixing
 sensors have the highest sensitivity to scattered light when the optical
 system focuses the transmitted light on the object. If the object is not
 at this focus, the amount of light coupled back into the laser is reduced.
 As a result, the sensitivity can be strongly dependent on where the object
 is with respect to the beam focus. An example of this is illustrated in
 FIG. 17, where the anticipated signal-to-noise ratio (SNR) is plotted as a
 function of object distance from the sensor. Since exact calculations of
 SNR as a function of object distance depends on a number of factors,
 including object reflectivity, the exact shape and SNR values indicated
 are not meant as absolute indicators of performance. What is of interest
 is the general shape of the curves as well as the relative vertical
 position of the curves. The curve indicated by numeral 130 indicates SNR
 when an optical system is transmitting a laser beam with transverse
 diameter of approximately 10 cm, and the beam is focused at 1500 meters.
 It is clearly seen that the SNR is highest near 1500 meters and decreases
 rapidly if the object is not near that range. If instead a 3.5 cm diameter
 beam is transmitted, the resulting curve is indicated by numeral 131. That
 curve shows less variation with range, but it also indicates a smaller
 maximum value of the SNR. If an amplifier with a single-pass gain of 10 dB
 is added to the self-mixing sensor, that same 3.5 cm transmitted beam
 would give rise to the curve 132. This curve shows that only a small
 variation of SNR with object range, but the peak SNR value is higher than
 either of the two previous curves. If the single-pass gain is increased to
 15 dB we obtain curve 133, which shows a further increase in SNR. Based
 upon these calculations it is clear that peak SNR can be traded off
 against insensitivity to object range variations. Such insensitivity is
 often desired to pre-empt the need for focusing mechanisms. As illustrated
 in FIG. 17, the use of an amplifier enables collateral use of a smaller
 beam expander than if no amplifier is used. This has the additional
 potential advantages of reducing the system size and cost.
 The following provides a description of an improved sensor using a long
 laser and a long laser with a gain element. Equation 1 implies that the
 amount of acceptable feedback can be increased if the length of the laser
 is increased. Semiconductor diode lasers normally have a very short
 length, such as less than 1 mm. As a result they are more prone to exhibit
 EMH than longer lasers, such as helium-neon or carbon dioxide lasers. By
 making use of an external cavity, the effective length of a semiconductor
 laser can be increased. One way of making an external cavity laser is
 shown in FIG. 18. A diode laser chip 200 has an active gain region
 indicated by hatched area 201. The laser chip 200 emits light which is
 collected and collimated with lens 202 and directed to a partially
 reflecting mirror 203. The mirror reflects a fraction, such as 5%-50%, of
 the light back into the laser diode chip 200. Provided that the laser chip
 output surface 204 has a lower reflectivity than the reflectivity of
 mirror 203, and that laser chip facet 205 has a sufficiently higher
 reflectivity, such as greater than 30%, a laser with a length equal to the
 distance L indicated in FIG. 18 will be formed. This simple configuration
 is highly likely to operate on several longitudinal modes. This is because
 the gain of a semiconductor laser is high over a very wide range of
 frequencies, such as more than 100 GHz. The frequency separation of
 different longitudinal modes is much smaller for a long laser, such as 1.5
 GHz for a laser cavity length of 10 cm. Many longitudinal modes
 (frequencies) then experience similar amounts of gain, and the device may
 lase at several frequencies. Such a device is not highly suitable for
 self-mixing sensors, unless its length is actively stabilized. Active
 stabilization adds considerable cost and complexity to the system. One
 approach disclosed here for incorporating a long cavity diode laser is
 shown in FIG. 19 and does not require such active stabilization
 techniques.
 The device shown in FIG. 19 contains a semiconductor laser chip 200 with a
 gain region 201. Light is coupled out of the laser through a partially
 reflecting surface 204. This surface 204 preferably has a relatively low
 reflectivity, such as less than 15%. At the other end of the laser chip
 the surface 209 has a very low reflectivity, such as less than 0.1%. Light
 is coupled from the gain region into an optical fiber 207 for example
 through the use of an optical element, such as a lens 206. The optical
 fiber contains near the far end a grating 208, which permits strong
 reflection of only a narrow frequency spectrum. The frequency selectivity
 of the grating permits the laser to operate on a single longitudinal mode
 without requiring active stabilization of the cavity length. The required
 length of the fiber, and the length Lg of the grating section, is
 dependent on the anticipated object reflectivity and distance from the
 sensor, and may, e.g., be in the range of about 1 cm to &gt;10 cm.
 It is also evident that a long cavity laser as described above can be
 combined with an attenuator or amplifier. A preferred way is to combine
 the long cavity laser with an amplifier placed on the same chip as the
 laser. A device of this type is shown in FIG. 20. In this figure the laser
 generally includes elements 150-151 and 153-159 as previously discussed in
 relation to FIG. 14. However, in place of reflector 152, the end facet is
 provided with a low reflectivity coating 209. Furthermore, the fiber optic
 assembly including elements 206-208 is also utilized to ensure a
 spectrally selective reflector for the laser. A long laser can also
 clearly be used in conjunction with liquid crystal and other attenuators.
 The following provides a description of monolithic sensor arrays.
 Semiconductor lasers can easily be fabricated with multiple laser elements
 placed in parallel on a laser chip. deGroot has previously discussed the
 use of a sensor array which used a matching photo-detector array for
 detection of signals from multiple sources. Using a photo-detector array
 has significant drawbacks. First, it is likely to require considerable
 design to ensure the photo-detector elements operate in or close to the
 shot-noise limited regime. Second, the detector array may need to be
 carefully aligned in space so that light from a single sensor element
 falls on exactly one corresponding detector element. Even if this can be
 accomplished, there is great potential for spurious scattered light to
 cause cross-talk between elements, thereby degrading the resulting sensor
 imaging performance.
 The arrangement shown in FIG. 21 eliminates the need for a photo-detector
 array, while still permitting an array of sensors to be operated from a
 single current source. A two-element sensor array is shown, although
 extensions to larger numbers of elements will be apparent to those of
 skill in the art after understanding the present disclosure. In the figure
 a current source 220 is used to drive two diode laser elements 227 and
 228. Inductors 221 and 222 are used to permit direct current to flow
 through the lasers, while blocking high-frequency components. In practical
 chip level implementations of self-mixing array sensors, this function may
 be performed by an active low pass filter to obviate the need for physical
 inductors. Capacitors 223 and 224 present a low impedance at high
 frequencies and permit voltage fluctuations to be transmitted to an
 amplifier 225 and signal processor 226 by connection to points A or B for
 the two channels. FIG. 22 shows spectrum analyzer traces for an
 experimental configuration where this arrangement was used. The light from
 a 2-element array was imaged onto a rotating disk 230 (FIG. 21), using a
 lens 229 (FIG. 21) such that each scattered signal experienced a slightly
 different Doppler frequency shift. The spectral trace in FIG. 22a
 corresponds to connecting the amplifier and spectrum analyzer to both
 points A and B, which results in two spectral peaks corresponding to the
 two different speeds. Spectra corresponding to connection of the amplifier
 to points A and B in FIG. 21 are shown in FIGS. 22b and c. It is clear
 that this arrangement permits separate detection of each array element. It
 is also clear that there is no discernible crosstalk, since neither of
 FIGS. 22b or 22c shows any evidence of a peak at the location
 corresponding to the peak of the other figure. Using this arrangement a
 large number of array elements can be implemented. Each element can be
 decoupled from the others using low-pass filtering and each resulting
 channel can have its own amplifier and signal processing channel.
 Alternatively, a number of channels can multiplexed into a single
 amplifier/signal processor and each channel demodulated sequentially.
 Although FIG. 21 illustrates a situation in which the sensed object was a
 spatially extended object, the present invention can also be used in
 connection with detecting motion, velocity or other characteristics of
 small objects such as small atmospheric particulates, dust or aerosol
 particles, and the like, e.g. particles with a longest dimension less than
 about 1-2 microns.
 Single particles scatter light during their passage through the laser beam.
 The motion of the particle induces a sensor signal similar, but
 time-varying, to the signal from the solid target. Unlike a solid target,
 the signal from a particle has a short duration determined by the width of
 the laser beam, the proximity of the particle to the laser beam focus, and
 the particle speed. A particle that crosses the laser beam near the focus
 produces a much stronger signal than a same-size particle crossing the
 beam far from the beam focus. The term far refers to distance in units of
 the confocal beam parameter of the focused beam. Particles passing a
 distance of several confocal beam parameters from the focus may not
 produce a detectable signal and are not considered to be signal inducing.
 This range dependence of the signal strength provides a means of
 determining the approximate range from where signals originate, provided
 that similar size particles are present in the beam on subsequent
 detection events.
 A small particle moving through a 0.2 mm wide beam at a speed of 10 m/s
 produces a signal for an approximate duration of 20 microseconds. Under
 many common circumstances, such as measuring the speed of a liquid or gas
 flow in which particles are entrained, the fraction of time particles are
 present relative to the time no signal-inducing particles are present in
 the beam is very small, for example less than 0.001. In order to be useful
 as a measurement instrument, the sensor should be equipped with a signal
 processing system that is capable of distinguishing the presence of
 particles. In the case of a high wideband SNR conditions this is possible
 by simply triggering a detection system (such as an oscilloscope) whenever
 the AC or DC signal level exceeds a predetermined threshold. When the
 wideband SNR is low but the narrowband SNR is modest to high, for example
 more than 3 dB, detection and processing system that substantially
 continuously measure the narrowband SNR as a function of frequency can be
 used to detect signals and calculate particle speeds. One example is a
 system that continuously calculates power spectra of the input signal and
 indicated the presence of a particle by detecting spectral features
 substantially above the background noise floor.
 An array of sensors can also be fabricated with amplifiers and with long
 cavities. One implementation of a general amplified sensor array is
 illustrated in FIG. 23. In this configuration multiple laser (600) and
 amplifier (601) sections are preferably fabricated on a single substrate
 (602), the two sets of sections being separated by, for example, a grating
 (603). To provide for a set of long laser cavities, a set of optical
 fibers (604) are aligned with the laser sections in such a manner that
 each fiber interacts with a single laser/amplifier channel. The individual
 fibers may also contain gratings (605) to ensure single longitudinal mode
 operation of each channel. We also note that optical waveguide devices can
 be used in place of the optical fibers. In this configuration all
 amplifier sections may be driven in parallel from a single current source
 (606). The laser sections are preferentially also driven from a single
 current source (607), but each channel is also isolated with respect to
 high frequency impedance fluctuations in a manner similar to FIG. 21. As
 illustrated in FIG. 23 an optical system, such as a lens (608), is used to
 collect the light from each channel and direct it towards the object
 (609). In place of a single lens (608), a lens array could be used, such
 that each channel has connected to it a separate lens. Binary optics or
 holographic elements could also be used to image the array output onto an
 object.
 The following provides a description of a method to achieve high
 reliability sensors. In order to be of practical use for many
 applications, a sensor should operate with high accuracy over a long
 period of time. For example, many industrial monitoring applications
 require speed accuracy of 0.1% with no measurable degradation over
 several-year time periods. Diode laser based sensors built as described in
 the prior art cannot produce such accuracy and reliability. All diode
 lasers used in the prior art have employed Fabry-Perot laser
 configurations wherein the laser cavity is formed by two parallel mirrors
 separated by a gain medium. If the cavity has an optical length L, the
 laser can potentially operate at frequencies given by {m*(c/2L)}, where m
 is an integer. The spacing between such potential frequencies is therefore
 c/2L, also known as the free spectral range. In a normal semiconductor
 laser the gain medium produces gain over a frequency range that is
 substantially larger than the free spectral range. As a result, such a
 Fabry-Perot laser may simultaneously operate at several different
 frequencies, and may also jump discontinuously between different
 frequencies. Since Doppler frequency is proportional to the wavelength or
 frequency of the light used, laser frequency uncertainties cause
 uncertainties in the measured Doppler frequency. In some instances such
 lasers may operate at a single frequency. This is sometimes due to
 unintentional perturbations, such as weak parasitic reflections from
 windows in the optical system, external to the laser cavity. Such
 parasitic reflections are usually not controllable or repeatable, and
 therefore there exists a fundamental uncertainty about the laser
 wavelength. Furthermore, even if a Fabry-Perot laser operates at a single
 frequency at a given time, there is no guarantee that it will operate at
 the same frequency after a significant period of time. Aging of the laser
 material can cause drifts of the laser frequency in an uncontrollable
 manner.
 The present invention permits some or all of these problems to be
 circumvented, to enable the manufacture of practical, reliable self-mixing
 sensors, e.g. by employing a novel combination of a mode-selective laser
 with a self-mixing sensor. An important feature is that the self-mixing
 laser cavity has at least one element that imposes a differential loss
 between the different potential laser frequencies. By permitting one
 frequency to have a lower loss than all other potential frequencies, that
 one frequency will oscillate at a much higher power level than any other
 frequencies, e.g., by a factor of 100-10,000. Frequency exclusive self
 mixing sensors can be fabricated in several different manners using known
 laser topologies such as distributed Feedback (DFB), distributed Bragg
 reflector (DBR), and fiber Bragg grating (FBG) lasers, in addition to
 several other types. Such lasers use corrugations or gratings to achieve a
 low loss at one frequency and higher loss at other frequencies. A further
 advantage of using such lasers is that the frequency is determined
 primarily by the grating, rather than the semiconductor material. As a
 result, aging of the material, and thermal mounting and packaging
 variations, do not cause the same problem with frequency drift in
 frequency exclusive self mixing sensors as with previously demonstrated
 sensors using Fabry-Perot topology lasers.
 Semiconductor lasers also change frequency with temperature. The tuning
 rate is normally of the order 0.05-0.3 nm/.degree. C. In order to ensure
 that the laser frequency is kept within a suitable tolerance band, such as
 0.01%, or 0.15 nm for a laser operating at 1500 nm, the temperature must
 be maintained to a specified set point within a corresponding temperature
 range. For the above example, the permissible temperature variation is
 preferably 0.3-3.degree. C., depending on the tuning rate of the laser.
 Such temperature control can be implemented by connecting the laser to a
 suitable temperature controlled mount. In a preferred case such a mount
 includes a thermoelectric cooler and temperature sensor contained within
 the laser diode package.
 A novel approach to laser temperature measurement uses the standard
 photodiode generally included on commercial laser substrates to facilitate
 laser power monitoring, as a temperature sensor. In this mode, a forward
 bias is applied across the photodiode, and the diode junction voltage drop
 is monitored to indicate diode laser substrate temperature. By alternately
 monitoring the forward bias voltage drop across the photodiode, and the
 reverse bias current through the photodiode, both the laser diode
 temperature and optical power may be monitored using the same device in
 intimate thermal contact with the laser diode.
 The following provides a description of a technique for accurate linear
 speed measurements. Doppler sensors of the type described herein measure
 the radial speed of moving objects. If an object, such as a sheet of
 paper, is moving at a linear speed vo along an axis, and if a Doppler
 speed sensor is pointed at the object at an angle 0, the speed sensor will
 measure a radial speed v=v.sub.0 cos(.theta.). If the angle .theta. is not
 accurately known or varies with time, this will introduce errors in the
 measurement of the speed v.sub.0. By using two sensors 235 and 236 to
 produce two laser beams 232 and 233, as shown in FIG. 24a, each one being
 incident at a different angle onto the surface 234, this problem can be
 eliminated, provided that the angle .phi. between the two beams is known.
 Since the incidence angles are .theta..sub.1 and .theta..sub.2, the two
 sensors measure speeds V.sub.1 =V.sub.0 cos(.theta..sub.1) and V.sub.2
 =V.sub.0 COS(.theta..sub.2)=V.sub.0 COS(.theta..sub.1 +.phi.). If we
 denote the ratio of the measured speeds as x=v.sub.2 /v.sub.1, one can
 show that v is given by
 ##EQU2##
 This expression is not dependent on the individual angles .theta..sub.1 and
 .theta..sub.2. By measuring the two radial speeds and independently
 measuring the separation angle, the linear speed v of the object can be
 inferred. In practice this can be done by measuring the speeds using
 analog or digital methods. In the latter case the two speeds are converted
 to digital numbers and a microprocessor is used to calculate expression
 (2). The angle .phi. is easily measured to a precision of, for example,
 less than 1 mrad. Each of the speeds can be measured to a precision of
 0.1% or better, which can then result in a linear speed measurement with,
 for example, 0.1% accuracy.
 The speed can also be extracted from the two measured speeds v.sub.1 and
 v.sub.2 using the
 ##EQU3##
 Likewise, orthogonal pairs of sensors operable to provide angle independent
 speed measurement as outlined above can be used to provide 2-D linear
 sheet speed independent of tilt and azimuthal orientation of the sensor
 planes to the sheet velocity.
 FIG. 24b illustrates a system and method for providing automatic correction
 for pointing angle using a combination of a single-beam self-mixing sensor
 and inclinometers. In many industrial applications, where sensor cost is
 an issue, but high precision measurement is still needed, a single beam
 self-mixing sensor may be combined with one or more accelerometers, or
 other inclinometers, to effect angle corrected speed or measurement, for
 example. The beam from a self-mixing sensor 400 impinges on a moving
 target 405 at an angle .alpha. with respect to the target motion (assumed
 in the plane of the figure). An inclinometer 403 providing electrical
 signals proportional to inclination angle is affixed to the sensor and
 previously aligned so that the angle between the sensor beam and the
 inclinometer axis is precisely known. A second inclinometer 410 is
 similarly affixed to the target, or target support, as might be found on
 an assembly line conveyor belt. Signals from inclinometer 410 are
 transmitted by hardwire, RF, or optical link to memory 415 where the
 inclination angle of the target may be stored once during setup, or on a
 periodic basis as required. The most recent inclinometer 410 reading is
 conducted to a differencing circuit 420 where it is subtracted from the
 current inclinometer 403 readings of the sensor 400 inclination angle to
 determine the angle .alpha.. The cosine of the angle signal is taken by
 arithmetic function generator 425 and output to the multiplier 430.
 Meanwhile, the Doppler signal from sensor 400 is reduced to a velocity
 signal by the signal processor 435 and output to the multiplier 430. The
 output 435 of multiplier 430 provides the desired speed signal
 automatically corrected for the angle .alpha.. Signal processing to effect
 the angle correction can be performed on analog or digital representations
 of the Doppler and inclinometer signals as appropriate, by choosing the
 appropriate devices well known in the art. In an alternative embodiment,
 the inclinometer 410 may be a portable test jig which may be affixed
 temporarily to the target or its support during a calibration period. The
 target inclination can be stored in memory 415. This is useful where the
 target inclination is substantially fixed, but where the sensor
 orientation may be moved during maintenance, for example. Another
 embodiment employing a movable link between the target inclinometer (such
 as a flexible cable, or RF or optical link) and the memory 415, allows
 constant changes in the target orientation while still providing fully
 corrected speed measurements. This is useful in robotics applications, for
 instance.
 The following provides a description of a technique for accurate linear
 speed measurements using a single laser source. A technical issue with
 using two lasers is the difficulty involved in maintaining the angle
 between two separate lasers to a sufficient level of accuracy, such as 0.1
 milliradians. Because the angular extent of the emission from diode lasers
 is normally large, such as 25-30 degrees, a lens having a short focal
 length F, typically in the range of 2-10 mm, is used for collimation and
 focusing onto a target. The short focal length leads to a high sensitivity
 to transverse misalignments between the laser emission point and the
 optical axis of the lens. If a transverse displacement y takes place
 between the laser and optical axis of the lens, the pointing angle of the
 laser beam following the lens shifts by an approximate amount y/F. In
 order to maintain an angular shift of less than 1 milliradians, y/F must
 be less than 0.001. If F is 5 mm, this means that y must be less than
 0.005 mm. This can be difficult to achieve in practice, particularly in
 the presence of temperature and other environmental variations within the
 system.
 In an alternative embodiment that circumvents this problem, accurate linear
 speed measurements can also be carried out using a single laser source
 according to this invention. A preferred implementation is shown in FIG.
 25. Here, the output beam from a laser 240 is split into two paths using a
 beam splitter 241 and a mirror 242. The two beams are directed to
 intersect a moving surface 234 at different angles. Light scattered from
 the two points on the object is scattered back into the laser, where it
 gives rise to several frequency spectrum components. Each Doppler
 frequency gives rise to its own individual frequency, f.sub.1 and f.sub.2.
 In addition, a frequency peak corresponding to the difference frequency
 f.sub.2 -f.sub.1 is present. Determination of the speed v can then be done
 by measuring each of the two individual frequencies. Alternatively, one of
 the individual frequencies in conjunction with the difference frequency
 can be used.
 Under relatively high feedback, each source at frequency f.sub.1 gives rise
 to sidebands at multiples of the fundamental frequency. As a result,
 additional peaks will appear in the spectrum, e.g, at (2f.sub.2 -f.sub.1)
 and (2f.sub.1 -f.sub.2). In order for unambiguous speed detection to be
 possible, a knowledge of which peak is which, is needed. To aid in
 automating speed measurements, in an alternative embodiment, a shutter
 243, for example, a mechanical device that may be flipped in and out of a
 laser beam path, or an electrically controlled liquid crystal attenuator,
 can be temporarily inserted into one of the laser beams. This eliminates
 the additional frequency peaks and permits a signal processor to lock onto
 the remaining lowest spectral peak, i.e., if beam 2 is blocked only
 frequency f, (and weaker harmonics thereof) is present. Once a servo has
 been used to lock to this peak, the second beam can be unblocked. A second
 processor can then be used to search for and lock onto either the second
 individual peak or to the difference frequency peak. The outputs from the
 two servos can then be used in conjunction with a microprocessor to
 calculate the linear speed using expression 2 above. Alternatively,
 sophisticated processing of the whole spectrum may be employed using DSP's
 or other hardware to decipher complex relationships between the spectral
 features.
 Sophisticated processing (including analog techniques) can also be used to
 determine the sign of the velocity. Normally, Doppler shifted frequencies
 give no information regarding the direction of motion towards or away from
 the sensor. When a self-mixing signal is large enough, harmonic sidebands
 of the fundamental Doppler frequency are present. The sign of the second
 harmonic relative to the fundamental signal is indicative of the direction
 of motion. With signal processors that operate in the amplitude domain,
 such phase differences can be derived. It is also noted that conventional
 and well known techniques for deriving the velocity sign, such as
 inclusion of a frequency shifter to move the zero Doppler away from zero
 frequency, can also easily be incorporated into the present invention.
 One advantage of this approach over using two lasers is that only a single
 angle between the two beams needs to be maintained with high accuracy.
 Motion between the lens and the laser as discussed above is no longer a
 critical issue, since both beam paths are affected by the same amount, and
 therefore the relative angle is not affected. A second advantage is that
 only a single laser is used, which reduces cost. A third advantage is that
 laser wavelength changes affect both frequency determinations to the same
 degree, reducing the measurement error in comparison with two lasers
 independently drifting in wavelength.
 In using this technique, it makes a significant difference whether or not
 the two beams are physically overlapped on the surface of the object. We
 show in FIG. 26 several RF spectrum analyzer traces taken under several
 conditions. FIG. 26a shows the spectrum analyzer trace when both beams are
 blocked. The peaks seen are due to spurious pickup in the sensor system
 and therefore represent noise. FIG. 26b shows the spectrum when both beams
 are incident on the surface, but when the two beams are separated by at
 least one beam diameter. The two big peaks (denoted f.sub.1 and f.sub.2)
 to the right in the figure correspond to the two individual Doppler
 frequencies, while the smaller peak (denoted .DELTA.f) represents the
 difference frequency. FIG. 26c shows the spectrum when the two beams are
 made to physically overlap on the moving surface.
 In addition to the peaks seen in FIG. 26b, an additional peak (denoted
 "sideband") can be seen between the individual Doppler peaks. A further
 peak (denoted 1/2.DELTA.f) can also be seen at low frequencies. These
 additional peaks are due to light transmitted through one beam to the
 target and received through the other beam back to the sensor. Analysis
 shows that the low frequency peak should be at 1/2 of the difference
 frequency peak, which can be seen from the figure. Analysis also predicts
 the peak centered between the two individual Doppler peaks. Because of the
 presence of these additional spectral features, it is often desirable to
 operate a two-beam sensor with non-overlapping beams. However, the extra
 information provided by the additional spectral peaks may also be useful,
 for example to indicate that a target is located in the beam intersection
 region.
 The features of invention herein described include improvements over
 previous self-mixing sensor technology, and the novel constructions of
 self-mixing sensors and of self-mixing sensors in combinations with other
 devices enables significant performance enhancements, including greatly
 improved dynamic range and sensitivity.
 While the above description contains many specifications, these should not
 be construed as limitations on the scope of the invention, but rather as
 examples of preferred embodiments thereof. Numerous variations and
 combinations of features are possible, including, but not limited to the
 following examples:
 Any signal processing technique capable of determining a signal magnitude
 can be used to provide a feedback signal to an attenuator or amplifier
 driver, including analog servos, fast Fourier transform (FFT), fast Walsh
 transform (FWT), fast Walsh-Hadamard transform (FHT) and auto-correlation
 processors (including those using pulse-pair and poly-pulse-pair
 algorithms).
 The technique of using an optical amplifier applies to any system where a
 laser and reasonably high gain medium is available, for example more than
 5 dB, amplifiers can be constructed, including: carbon dioxide lasers;
 numerous solid-state lasers (including neodymium, erbium, ytterbium, and
 holmium lasers); and lasers used in conjunction with high gain nonlinear
 optical amplifiers (including optical parametric amplifiers).
 Several alternative techniques exist to achieve a low reflectivity from the
 input and output ends of an amplifier, including the use of dielectric
 anti-reflection coatings, cleaving or polishing the end at an angle
 different from the normal to the light propagation path, and burying the
 facet or facets.
 The technique of using a variable attenuator applies to any laser used in a
 self-mixing configuration.
 Numerous ways of implementing a variable attenuator exist, including a
 variable iris (aperture), liquid crystal devices based on scattering light
 as opposed to altering the polarization state, electro-absorption
 modulators, rotating polarizers, and others.
 An indirect way of implementing a variable attenuator is to misfocus the
 transmitted beam on the target, which can be implemented using a variable
 focus lens.
 Signal detection techniques discussing detection of laser junction
 impedance variations can also be employed if detection of power
 fluctuations using an external detector is used.
 If the laser is a gas laser, laser power fluctuations can be detected by
 measuring fluctuations in the current through or voltage across the laser
 tube.
 Although single lenses are used in the figures to indicate means for
 collecting light from a laser and focusing the same on an object, more
 complex optics can be used for this purpose, including the use of separate
 optical elements for collimating light and subsequently expanding the beam
 and focusing it on or in the vicinity of the object.
 The sensors are also functional if the light is not focused on the object,
 although the amount of signal is generally reduced in comparison with the
 focused case.
 The technique of increasing the dynamic range using a long laser can also
 be implemented by using a laser with an external cavity including a
 grating, such as laser devices containing a grating mounted in the Littrow
 configuration.
 The techniques discussed in the description apply to the generic sensor,
 not to a particular sensor use, such as speed or vibration sensing, or
 displacement measurements.
 The apparatus shown in FIG. 25 is only one way of generating two beams from
 a single laser. Other methods include use of monolithic elements
 containing partially reflective mirror surfaces.
 Embodiments of the present invention include a number of features
 including:
 System and method for automatically controlling EMH in self-mixing sensor
 devices
 angle independence of radial speed sensors using 2 or more beams
 angle independence using inclinometer and self mixing laser device
 Control of self mixing laser device mode hopping using frequency selective
 laser cavity elements
 Laser amplifier with self mixing laser device to increase dynamic range
 Stable integral extended laser cavity construction of self mixing laser
 device sensor to control EMH and extend self-mixing laser device range
 Use of an optical amplifier as a variable attenuator as well as gain medium
 to increase self mixing laser device dynamic range
 Use of beam expander with Self mixing laser device to extend sensitivity
 range
 Elimination of cross-talk in sensor arrays by using laser diode as sources
 as sensors
 Temperature control using in-case photodiode or diode laser as a
 temperature sensing device, e.g. utilizing the temperature dependence of
 the voltage drop across the diode junction
 Length measurement using self mixing laser device
 Signal attenuation can also be effected through the use of variable beam
 distorters (for example liquid crystal devices or mechanical elements
 inserted into the laser beam) as well as through variable defocusing
 mechanisms (for example using a focusing lens attached to a voice coil,
 and defocusing the beam if the received signals are too high).
 In one embodiment, an optical sensor includes a semiconductor diode laser,
 configured to operate substantially in a single longitudinal mode; means
 or devices for providing light from said semiconductor diode laser to a
 first region and coupling scattered light from said first region back into
 said semiconductor diode laser, wherein self-mixing is provided; means or
 devices for detecting variations related to said semiconductor diode laser
 in response to changes in said scattered light; and means or devices for
 controllably changing the amount of said light coupled back into said
 semiconductor laser.
 In one embodiment, an optical sensing method includes transmitting light
 from a semiconductor diode laser to a first region, said semiconductor
 diode laser operating substantially in a single longitudinal mode;
 coupling scattered light from said first region back into said
 semiconductor diode laser, wherein self-mixing is provided; detecting
 variations related to said semiconductor diode laser in response to
 changes in said scattered light; and controllably changing the amount of
 said light coupled back into said semiconductor laser.
 In one embodiment, a sensor includes means or devices for outputting at
 least first and second light beams, wherein said means or devices for
 outputting includes at least a first self-mixing laser; and means or
 devices for obtaining a linear velocity of a target, using said means or
 devices for outputting, which is substantially insensitive to the angle
 between said beams.
 In one embodiment, a sensor apparatus includes a self-mixing laser; and
 stable integral extended laser cavity means or devices to control mode
 hopping of said self-mixing laser.
 In one embodiment, a sensor apparatus includes a self-mixing laser; and
 stable integral extended laser cavity means or devices to extend range of
 said self-mixing laser.
 In one embodiment, a sensor apparatus includes a self-mixing laser; and
 optical amplifier means or devices for providing variable attenuation.
 In one embodiment, a sensor apparatus includes a self-mixing laser; and
 optical amplifier means or devices for providing variable gain.
 In one embodiment, a sensor apparatus includes a plurality of laser diodes;
 and means or devices for reducing cross-talk among said plurality of laser
 diodes.
 In one embodiment, a sensing method includes providing a self-mixing laser
 having at least a first laser diode; and using a diode junction voltage
 drop to monitor laser substrate temperature. In one embodiment, such a
 sensing method further includes using said diode junction voltage drop to
 monitor laser power.
 In one embodiment, a self-mixing sensor includes a semiconductor diode
 laser and a lens, said laser having a laser cavity; and at least a first
 frequency selective means or device. In one embodiment, such a sensor is
 provided wherein said frequency selective means or device includes means
 or devices for imposing a differential loss between different potential
 laser frequencies. In one embodiment, such a sensor is provided further
 comprising a temperature monitor and cooling means or devices. In one
 embodiment, such a sensor further includes a frequency selective grating.
 In one embodiment, a sensing method, includes providing a self-mixing
 sensor comprising a laser diode defining a laser diode junction; using
 said laser diode junction as a temperature monitor; and selectively
 cooling said self-mixing sensor in response to temperature sensed by said
 temperature monitor. In one embodiment, said grating is provided in an
 optical fiber coupled to said laser.
 The present invention, in various embodiments, includes components,
 methods, processes, systems and/or apparatus substantially as depicted and
 described herein, including various embodiments, subcombinations, and
 subsets thereof. Those of skill in the art will understand how to make and
 use the present invention after understanding the present disclosure. The
 present invention, in various embodiments, includes providing devices and
 processes in the absence of items not depicted and/or described herein or
 in various embodiments hereof, including in the absence of such items as
 may have been used in previous devices or processes, e.g. for improving
 performance, achieving ease and.backslash.or reducing cost of
 implementation.
 The foregoing discussion of the invention has been presented for purposes
 of illustration and description. The foregoing is not intended to limit
 the invention to the form or forms disclosed herein. Although the
 description of the invention has included description of one or more
 embodiments and certain variations and modifications, other variations and
 modifications are within the scope of the invention, e.g. as may be within
 the skill and knowledge of those in the art, after understanding the
 present disclosure. It is intended to obtain rights which include
 alternative embodiments to the extent permitted, including alternate,
 interchangable and/or equivalent structures, functions, ranges or steps to
 those claimed, whether or not such alternate, interchangable and/or
 equivalent structures , functions, ranges or steps are disclosed herein,
 and without intending to publicly dedicate any patentable subject matter.