Remote Subsurface Water Temperature Measuring Apparatus With Brillouin Scattering

The invention described herein relates generally to the following patent applications:

"APPARATUS FOR AND METHOD OF REMOTELY SENSING SUB-SURFACE WATER TEMPERATURES," Ser. No. 064,371, filed June 27, 1987 now U.S. Pat. No. 4,867,564;

"METHOD OF REMOTELY DETECTING SUBMARINES USING A LASER," Ser. No. 064,375, filed June 22, 1987 now U.S. Pat. No. 4,867,558; and,

"REMOTE SUBSURFACE WATER TEMPERATURE MEASURING APPARATUS AND METHOD," Ser. No. 386,383, filed July 28, 1989.

1. Field of the Invention

The present invention relates to the remote measurement of properties of transparent media, such as subsurface ocean temperature profiles, and in particular to improved apparatus for and a method of remotely measuring such temperature profiles from surface or subsurface vessels or aircraft.

2. Description of the Prior Art

There are several applications for remotely sensing or measuring the temperature of a bulk transparent medium such as water. One such application is the sounding of temperature profiles in the ocean which is useful for a variety of oceanographic purposes such as in measuring the depth of the thermocline, sensing internal waves, measuring heat content of the oceans for meteorological applications and mapping acoustical propagation paths sensitive to temperature gradients. Intrasensitive temperature sensors, such as thermistors, thermocouples, etc., have been used in the past for these purposes but, because they are not remote sensors, they are slow and awkward. A remote sensing technique in wide use is the monitoring of thermal radiation; this technique, however, is limited to measuring predominantly surface temperatures.

This invention is directed to an improved technique for remotely measuring temperatures within, i.e., below the surface of suitable transparent media or substances, for example sea water.

The foregoing problems are generally solved and a technical advance achieved in an embodiment of the invention which remotely and substantially instantly senses the subsurface temperature of a transparent substance, such as water.

An object of this invention is the provision of such apparatus that is relatively compact and highly portable.

Another object of this invention is the provision of apparatus that produces a high signal-to-noise (S/N) ratio and therefore generates highly accurate measurements.

A broad object of this invention is the provision of such apparatus in which the signal level in the received return signal from the target substance is relatively high providing an excellent S/N ratio.

These features and other objects of the invention are achieved with apparatus consisting of a pulsed laser having a highly intense output beam capable of producing stimulated Brillouin scattering (SBS), a beam splitter for splitting this beam into two sub-beams and for directing one sub-beam into the transparent medium of unknown temperature (the sample medium), thereby producing a phase-conjugate (PC) beam from the sample medium. The other sub-beam is directed into a transparent medium of known temperature (the reference medium) thereby producing a phase-conjugate beam from the reference medium. The return phase-conjugate beams from both mediums are mixed together to derive a difference frequency proportional to the difference between the temperature of the sample medium and the reference medium. The difference frequency value is then converted to a value indicative of the unknown temperature.

The invention also comprehends the use of a cw laser to generate a PC signal from the reference substance, and mixing the PC signals from both the pulsed laser and from the cw laser to produce a difference frequency proportional to the unknown temperature. The advantage of the later apparatus is that the range of the test medium need not be equal to the range of the reference cell, and indeed can be constantly changing.

For a better understanding of the invention reference is made to the following description which is to be read in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic drawing of apparatus embodying the invention and useful in probing ocean water for temperatures at a constant depth.

FIG. 2 is a schematic drawing of apparatus embodying the invention and useful in probing ocean water for temperatures at varying depths.

FIG. 3 is a schematic representation of a modified form of the invention of FIG. 2 showing a variable focus lens.

Referring now to the drawings, FIG. 1 illustrates temperature measuring apparatus 10 useful in illustrating the basic principles of the invention. Such apparatus comprises a pulsed laser 12 having an output beam 13 and a partially transmitting beam splitter 15 which further divides beam 13 into a first sub-beam 17 and a second sub-beam 18, the directions of propagation of which are indicated by arrows 17 and 18. Sub-beam 17 passes through focussing lens 19 which focusses the beam at focal point P.sub.s in a sample cell 20 containing a transparent substance or medium, such as sea water, having an unknown temperature T.sub.s at the focal point P.sub.s.

Sub-beam 18 passes to mirror 21 which directs beams 18 to focussing lens 22 which focusses beam 18 at focal point P.sub.r in a reference cell 23 containing a transparent material or substance preferably similar to that of cell 20 and having an known temperature T.sub.r at focal point P.sub.r.

When the intensity or irradiance of the pulses comprising output beam 13 of laser 12 exceeds a predetermined threshold level, sub-beams 17 and 18 produce stimulated Brillouin scattering (SBS) in cells 20 and 23 resulting in the generation of phase-conjugate or "time-reversed" beams whose rays run along the same trajectories, but in opposite directions, as sub-beams 17 and 18. the directions of the PC beams are designated by the arrows 17.sub.pc and 18.sub.pc. This phenomenon, called optical phase conjugation, is well known and is described in detail in articles entitled Optical Phase Conjugation by V. V. Shkunov et al., Scientific American, pages 54-59 (September 1985) and Applications of Optical Phase Conjugation by D. M. Pepper, Scientific American, pages 74-83 (January 1986).

The Brillouin backscatter optical wave from a spontaneously generated phonon interacts with the probe beam and the interaction produces a traveling electric field that travels exactly at the sound velocity in the particular substance. If the light intensity is great enough and the electrostrictive coefficient of the medium is sufficiently strong, this traveling electric field will create an acoustic field which aligns itself in a column with the probe beam and the backscattered beam at the expense of scattering in other directions. Hence the collection of light is more efficient in SBS than in spontaneous Brillouin scattering.

The presence of the induced electric field traveling at the sound velocity can be derived in the following manner. Assume that an electromagnetic wave (the probe laser) propagates in a substance in the +x direction magnetic wave (the probe laser) propagates in a substance in the +x direction with velocity c, then the electric field may be written as

where,

E.sub.O =magnitude of the wave

.omega.=frequency, and

t=time parameter,

and assume that a wave of different frequency (the Brillouin scattering) propagates in the -x direction, the electric field of which is written as,

where, .omega..sub.2 =frequency of the second wave. The total electric field in their common region is

which can be arranged by trigonometric identities to the form

which is of the form of a high frequency (.omega..sub.1 +.omega..sub.2) signal modulated with a low frequency (.omega..sub.1 -.omega..sub.2) envelope. The velocity of the envelope is found by making its argument constant, i.e.,

The envelope velocity is therefore, ##EQU1## In the case of Brillouin scattering f.sub.2 has been produced by Doppler shift by interacting with an acoustic wave at velocity v. The Doppler equation, namely

which when solved results in the following expression:

A substitution into the prior expression (6) for dx/dt yields dx/dt=v.

The envelope of the composite electric field wave therefore travels exactly at the acoustic velocity.

In order to achieve SBS, a predetermined threshold intensity level for the optical probe beam must be exceeded. This intensity level must be sufficient so that the following relationship exists:

or,

(since 10.sup.13 =exp[30]).

Or more simply, the intensity, I.gtoreq.30/GL, where,

G=a gain parameter which is a property of the medium, m/W

I=intensity of the optical probe beam (H/m.sup.2) and,

L=interaction length, m,

see Principles of Phase Conjugation by Zel'dovich et al., Springer-Verlag, Vol. 42, page 29, Springer Series on Optical Sciences (Springer Verlag Berlin Heidelberg, 1985). (For water G is typically 5.times.10.sup.-11 m/W.)

The sound velocity in water is related to the temperature and salinity and is given in the article entitled, Development of Simple Equations for Accurate and More realistic Calculation of the Speed of Sound in Sea Water, by C. C. Leroy, Journal of Acoustical Society of America, No. 216, page 216 (1969), as follows:

where,

T=temperature in .degree.C.,

S=salinity in parts per thousand,

Z=depth in meters.

Referring again to FIG. 1, the phase conjugate of sub-beam 17, designated 17.sub.pc, propagates from cell 20 precisely along the same path as sub-beam 17, except in the opposite direction, reflected off beam splitter 15 to photodetector 25 such as a photodiode. Similarly, the phase conjugate of sub-beam 18, designated 18.sub.pc, propagates from reference cell 23, through lens 22, off mirror 21, through splitter 15 and ultimately to photodetector 25.

A phase-conjugated beam derived from SBS has its optical frequency shifted by a frequency that produces an acoustical wavelength in the water equal to half the optical wavelength, i.e. ##EQU2## where, .DELTA.v=the optical frequency shift,

.sup.va =the acoustic velocity in water,

n=the index of refraction in water, and

.lambda.=the wavelength of the incident beam in a vacuum.

Since both phase-conjugate beams 17.sub.pc and 18.sub.pc are conjugate of the same incident beam, their respective wavefronts are identical. Photodetector 25 mixes beams 17.sub.pc and 18.sub.pc and because their wavefronts are identical, ideal optical heterodyning occurs with high mixing efficiency. The heterodyne frequency produced by photodector 25 is proportional to the difference of the shifted frequencies which is directly proportional to the difference in the acoustic velocities which in turn is predominantly proportional to the difference in temperatures T.sub.s and T.sub.r at the respective focal points P.sub.s and P.sub.r of cells 20 and 23, respectively. By assigning cell 23 as a reference cell and accurately controlling its temperature, the unknown temperature T.sub.s of cell 20 can then be measured. The output of photodetector 25 is received by a frequency measuring device 26, such as a frequency discriminator, which directly converts it into a value of temperature equal to .vertline.T.sub.s -T.sub.r .vertline.. From this .DELTA.T it is a simple matter to derive T.sub.s.

It is important that the pulses from phase-conjugate beams 17.sub.pc and 18.sub.pc arrive at the active mixing surface of photodetector 25 at the same time for effective heterodyning of these two signals. Since there is no optical storage mechanism, if the pulses from the two beams are not properly timed to arrive at photodetector 25 at substantially the same instant, the two pulses will not mix with each other. To this end, the optical spacing of the distance of focal points P.sub.r and P.sub.s from photodetector 25 need to be the same. This means that the optical path length between photodetector 25 and each of the cell focal points must be equal.

Certain variations of the basic technique may be used to achieve beneficial results. For example, referring again to FIG. 1, the substances or media of cells 20 and 23 need not be the same. Rather the material of reference cell 23 and its temperature may be selected to produce a convenient heterodyne difference frequency for the expected test sample of cell 20. Also, by controlling the temperature of reference cell 23 advantageously enables selection of a reference temperature T.sub.r close to the expected temperature of the test cell, T.sub.s. This results in less demand on the speed of the photodetector. For example, if the expected temperature, T.sub.s, of the test media is 20.degree. C., this produces a 7.45 GHz frequency shift from the frequency of the incident beam. By selecting the reference cell temperature T.sub.r to be 10.degree. C., which corresponds to a frequency shift of 7.20 GHz, the frequency difference between the two is only 250 MHz (i.e.. 7.45 GHz-7.20 GHz=250 MHz) as compared to 7.45 GHz without the reference cell. In addition, apparatus which utilizes two phase-conjugate beams provides optimum mixing performance.

As mentioned above, a precondition to successful utilization of the two-cell apparatus and technique is the necessity that phase-conjugate reference beam pulse 18.sub.pc be present at the photodetector cathode at essentially the same time as the return phase-conjugate sample beam pulse 17.sub.pc. In some applications, however, the probe range may be unknown or varying, as when the apparatus is mounted on an aircraft and thus time coincidence of pulse excitation of the photodetector is more difficult to control.

In order to overcome this limitation under such circumstances, a continuous-wave laser may be employed as is shown by apparatus 28 in FIG. 2. Apparatus 28 comprises a low-power spectrally-pure continuous wave (cw) laser 29, called a "seeder" laser, which feeds a low-level cw input to a pulse-amplifier laser 30, which is triggered by an optical pulse (flash) from a suitable source (not shown) on path 31. By way of example, laser 29 may be a Nd:YAG laser and laser 30 a flash-pumped Nd:YAG. The output beam 32 of pulse laser 30 comprises a low power level cw beam and periodic high power level pulses. Output beam 32 is incident on beam splitter 34 which divides laser beam 32 into sub-beams 35 and 36. Sub-beam 35 passes from splitter 34 to laser amplifier 37. Amplifier 37 operates near saturation to suppress the pulse components in sub-beam 35 and to amplify the cw component to an intensity level which exceeds that needed to produce SBS. The output beam 38 of amplifier 37 is incident on converging lens 39 which focusses the intense cw beam on reference cell 40 similar to cell 23 of FIG. 1. Cell 40 contains a transparent material such as water and acts as a temperature reference for the measuring apparatus. Lens 39 focusses sub-beam 35 at the point within reference cell 40 designated P.sub.r, the temperature of which, T.sub.r, is controlled and known. The other portion of beam 32 is partially reflected as sub-beam 36. Converging lens 41 focusses sub-beam 36 on focal point P.sub.s below the surface of a body of water 43, the temperature T.sub.s of which at point P.sub.s is unknown.

As discussed above, the pulse power of sub-beam 36 and the continuous power of sub-beam 38 exceed the power threshold needed to produce SBS in body 43 and cell 40. The phase-conjugates 35.sub.pc and 36.sub.pc of those beams travel through or reflect off of splitter 34 to photo-detector 44. Since the phase conjugate of the continuous wave from reference cell 40 is present at all times on the photocathode of detector 44, the heterodyning with pulse 36.sub.pc from body 43 is assured regardless of variations in the spacings of focal point P.sub.s within body 43 and focal point P.sub.r in cell 40 from photodetector 44. In this manner, the range of the test medium need not be equal in length to the range of the reference cell. Such an arrangement would be most advantageous when applied to an aircraft making temperature readings (profiles) of subsurface ocean waters. Also for sea water, optical wavelengths in the blue-green spectrum should be used since the penetration is greatest in this region.

Referring to FIG. 3 the embodiment depicted in FIG. 2 may be modified to measure temperatures at varying ranges or depths in test body 43, designated in FIG. 3 as P.sub.s1, P.sub.s2 and P.sub.s3, by substituting for transmitter lens 41, a transmitter lens 47 with an adjustable focus.

While the invention has been described with reference to its preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the true spirit and scope of the invention. In addition many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the scope thereof as defined in the appended claims.