Underwater laser range finder

Underwater laser range finder Alan Laux* a, Linda Mullen a, Paul Perez b, Eleonora Zege c, a Naval Air Systems Command, NAVAIR, Electro-Optics and Special Mission Sensors Division, 22347 Cedar Point Road, Patuxent River, MD 267; b Clark University, Wallace H. Coulter School of Engineering, Potsdam, New York 369972; c Institute of Physics, Belarus Academy of Sciences, ScarynaAve. 68, 72, Minsk, Belarus ABSTRACT The conventional method used to detect the range to an underwater object is by sending and receiving some form of acoustic energy. However, acoustic systems have limitations in the range resolution and accuracy they can provide under certain conditions. The potential benefits of a laser-based range finder include high-directionality and covertness, speed of response, and the potential for high-precision, range accuracy. These benefits have been exploited in the above-water environment where kilometer propagation ranges are achieved with sub-meter range precision. The challenge in using optical techniques in the underwater environment is overcoming the exponential loss due to scattering and absorption. While absorption extinguishes photons, scattering redistributes the light and produces a clutter signal component from the surrounding water environment. Optical modulation techniques using compact laser diode sources are being investigated to help suppress this clutter and provide accurate target range information in a wide range of underwater environments. To complement the experimental efforts, a theoretical model has been developed to help optimize the system parameters and test the performance of various configurations as a function of different water optical properties. Results from laboratory water tank experiments will be discussed and compared with model predictions. Keywords: underwater, laser, modulation, range. INTRODUCTION Measuring the range to an object is accomplished by transmitting a time-dependent signal to the object of interest and measuring the time difference between the outgoing and object-reflected signals. Underwater range finders have historically used acoustic radiation due to their long range propagation characteristics. Optical techniques are currently being investigated due to the potential advantages that lasers provide, including high-directionality and covertness, speed of response, and high-precision, range accuracy. Laser-based range finders typically operate in either the time of flight or phase shift mode []. The time of flight method uses a pulsed laser and the receiver measures the time delay between the transmitted and detected laser pulse to determine range. For this approach, the range ambiguity is controlled by the pulse repetition rate while the range precision and accuracy is determined by the laser pulse width. The disadvantage is that to obtain high range precision and accuracy, a short pulse must be used, which then dictates a wide receiver noise bandwidth. The other approach is to use an amplitude-modulated continuous wave source and measure the phase shift between the transmitted and received signal to determine range. In this approach, both the unambiguous range, d UNAMB, and range precision, δd, are determined by the modulation frequency, f m : d δ d UNAMB v 4π v = () 2 f f m m =, (2) where v is the speed of light in the medium and δϕ is the precision of the phase shift measurement. Thus, there is a tradeoff between precision and unambiguous range when selecting the modulation frequency. The advantage of the δ ϕ Ocean Sensing and Monitoring IV, edited by Weilin Will Hou, Robert Arnone, Proc. of SPIE Vol. 8372, 8372B 22 SPIE CCC code: 277-786X/2/$8 doi: 7/2.9928 Proc. of SPIE Vol. 8372 8372B- Downloaded From: http://proceedings.spiedigitallibrary.org/ on 2/8/26 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx

phase shift approach is that it can be realized with inexpensive laser diodes and, depending on the modulation scheme, it can be accomplished using a relatively narrow bandwidth receiver. Implementing either the time of flight or phase shift approach in the underwater environment faces additional challenges due to scattering and absorption. Absorption of light in water is minimized by restricting the laser wavelength to the blue-green region of the optical spectrum. Optical scattering causes the laser beam to lose its initially collimated distribution and reduces the achievable spatial resolution. Furthermore, light scattered from the environment produces a clutter term that must be distinguished from the object-reflected light. Together, absorption and scattering exponentially reduce the amount of light that is detected and place demanding requirements on the dynamic range of the optical receiver. Fortunately, several techniques have been developed to help mitigate the effects of the underwater environment. For the pulsed time of flight approach, range gated receivers have been developed that are timed to open only when the objectreflected light reaches the receiver [2-4]. Therefore, the clutter or backscattered light from the environment, which arrives earlier, is not detected. This approach inherently requires a priori knowledge of the object s range and is therefore not suitable for some applications. Furthermore, gating a high speed receiver at short time intervals is difficult, especially when highly sensitive photomultiplier tubes are used. For the phase shift technique, experimental and theoretical studies have shown that selecting the proper modulation frequency helps to discriminate against light that is backscattered from the environment [-9]. While the backscattering process washes out the modulation, the objectreflected light remains modulated. A receiver centered at the modulation frequency can then be used to reject the nonmodulated clutter term. Results have shown that the efficiency of this approach is optimized at modulation frequencies greater than MHz []. This in turn places restrictions on the unambiguous range that is achieved. The focus of this paper is to describe a technique based on the phase shift approach that uses multiple modulation frequencies to both suppress backscattered light and increase the unambiguous range. The following sections detail this approach and describe the experimental setup that was developed to test the concept in a controlled laboratory environment. Results from the laboratory experiments will be discussed and compared with theoretical predictions. 2. MULTIPLE FREQUENCY PHASE SHIFT APPROACH As discussed previously, there is a tradeoff between unambiguous range and range precision when selecting a modulation frequency to be used in the phase shift approach. One compromise is to use two modulation frequencies a lower modulation frequency to meet the unambiguous range requirement and a higher modulation frequency to optimize range precision. One such approach switches between two frequencies to obtain both a coarse range measurement (at the lower frequency) and then a higher precision measurement (at the higher frequency) []. The problem with implementing such a technique in the underwater environment is that range measurements performed at the lower modulation frequency (less than MHz) will be affected by backscatter. An alternate approach is to amplitude modulate the higher frequency signal and use the modulation envelope to provide the coarse range measurement. This modulation scheme is shown in Figure where a direct digital synthesis (DDS) source provides a high frequency signal, f 2 >MHz, that is amplitude modulated at a lower frequency, f <MHz. This composite signal then amplitude modulates the intensity of a laser diode which is transmitted to the underwater object. At the receiver, the output of the optical detector is first filtered by a bandpass filter (BPF) centered at f 2 thereby eliminating any low frequency signal components. The low frequency envelope ( f ) is then recovered with an envelope detector and the phase shift is measured relative to the drive signal via the in-phase ( I ) and quadrature-phase ( Q ) samples generated by a software defined radio (SDR) receiver. In this configuration, the high frequency modulation is used only to suppress the contribution from backscattered light. To obtain the higher precision range measurement, the source can be reconfigured in software to output only the higher frequency ( f 2 ) to modulate the laser diode. At the receiver, the envelope detector is bypassed and the SDR is programmed to change the center frequency to f c = f 2. The disadvantage of the approach shown in Figure is that the dynamic range for measuring the phase shift at 2MHz is limited by the envelope detector. Through testing it was found that the phase delay through the envelope detector itself was not constant with changes in signal amplitude. This is a challenge since in the underwater environment, the signal level changes exponentially with target range. A different approach is to modulate the laser at two high frequencies (>MHz) that are separated in frequency by some Δf that meets the unambiguous range requirement (Figure 2). At the receiver, the phase at each modulation frequency is measured via separate, phase-locked SDR receivers. Here, the range ambiguity and the measured range become: Proc. of SPIE Vol. 8372 8372B-2 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 2/8/26 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx

Output modulated at f and f 2 Laser diode bias cos ( 2π t) [ + cos( π f t) ] f2 2 DDS source Optical filter PMT f = f 2 BPF Envelope Detector SDR receiver fc = f I/Q samples Figure. Block diagram of a two-frequency modulation scheme where a higher frequency signal, f 2, is amplitude modulated by a lower frequency signal, f. The higher frequency modulation is used to suppress backscatter and the lower frequency envelope is selected to meet the unambiguous range requirement. d UNAMB v = (3) 2 Δf v d meas = ( ϕ2 ϕ), (4) 4π Δf where ϕ 2 is the phase shift measured at the higher frequency ( f 2 ) and ϕ is the phase shift measured at the lower frequency ( f ). The advantage of this approach is that the higher modulation frequencies help suppress the contribution from backscatter while the unambiguous range is determined by the frequency difference. Furthermore, the dynamic range is now controlled by the SDR receiver, which is approximately 3dB for a given gain setting. A disadvantage of this approach is that the range precision is now lower due to the fact that the difference frequency, Δf, is used to compute the range, which is significantly less than either modulation frequency. In addition, the total noise, as given by the standard deviation of the range measurement, is also larger when using two phases since it is determined from the quadrature sum of the standard deviations of the individual phase measurements. However, once the course range is determined by using equation (4) above, the phase measured using either high frequency signal can be used to enhance the overall range precision. In the next section, the experimental setup developed to test this two frequency approach in a controlled laboratory environment is described in detail. Output modulated at f and f 2 Laser diode bias cos ( 2π t) + cos ( π f t) f2 2 DDS source PMT BPF SDR receiver Optical SDR receiver filter fc = f Figure 2. Block diagram of a two-frequency modulation scheme where a laser diode is modulated at two high frequencies > MHz. The frequency difference, Δf, is selected to meet the unambiguous range requirement. Each frequency is processed by a SDR receiver. fc = f 2 I/Q samples Proc. of SPIE Vol. 8372 8372B-3 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 2/8/26 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx

3. EXPERIMENTAL SETUP The experimental setup developed to test the two frequency phase shift approach is shown in Figure 3. The output from a single output DDS source is combined with a source to modulate the current of a 488nm laser diode. Three different modulation frequencies were used in the measurements f m = 2MHz, 6MHz, and MHz. The modulated light was transmitted through a window in a 3.6m long tank to a gray target mounted on a translation stage. The target was moved in cm increments from a range of.m to 3.m from the input window. The optical receiver collected light scattered from the submerged target through the same window. The transmitter-receiver separation was 2.cm, and the angle between the transmitter and receiver was fixed so that they intersected at a target depth of 2.m. The front-end optics on the photomultiplier tube optical receiver consisted of a 2, 488nm interference filter followed by a 2, f/2 lens. The 8mm input window of the Hamamatsu 783 PMT was placed approximately cm behind the focal point of the lens giving a receiver field of view of approximately 4 degrees (in water). A bias-tee at the output of the photomultiplier tube separated the and AC components of the photocurrent. The -coupled signal was monitored on a multimeter to ensure that the photomultiplier tube remained within its linear operating regime. The AC-coupled signal was band-pass filtered (either at f c = 2MHz or MHz, depending on the DDS setting) and inputted to the SDR receiver (ComBlock COM-3). The center frequency of the SDR was selected to match the modulation frequency (the transmitter and receiver were phase locked via a 2MHz external reference). Translation stage bias f = f m 488 nm Laser diode DDS source object Water tank Optical filter PMT Multimeter BPF f c = f m SDR receiver f c = f m I/Q samples Figure 3. Block diagram of the two-frequency phase shift scheme where a laser diode is modulated at one of three modulation frequencies f m = 2MHz, 6MHz or MHz. The modulated light is transmitted to an underwater object, and the object-reflected light plus backscatter is collected by a photomultiplier tube (PMT). The phase delay between the transmitted and received signals is computed by the I and Q samples generated by a SDR receiver. The optical clarity of the water in the test tank was varied through the addition of Maalox antacid. The water optical properties (scattering and absorption) at 488nm were measured with a WetLabs AC-9 instrument. For each Maalox concentration, measurements were made at each of the three modulation frequencies mentioned previously. The in-phase (I) and quadrature-phase (Q) signals generated by the SDR were used to measure the phase shift at each modulation frequency, f m, and target position, d: ( f ) m, d ( f d ) m, Q ϕ ( f d ) = m, tan. () I The range to the underwater object, d meas, was then calculated at each modulation frequency: d v 4π f ( f, d ) ϕ( f d ) =. (6) meas m m, m The range was also computed by using the phase measured at the two higher modulation frequencies ( f m = 6MHz or MHz): Proc. of SPIE Vol. 8372 8372B-4 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 2/8/26 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx

v d meas, 2 ϕ, 4π Δf ( f d ) = [ ϕ( f, d ) ( f d )] Δ, (7) where f 2 is the higher and f is the lower of the two modulation frequencies, respectively. The experimental parameters were used as inputs to a performance prediction model based on the theoretical approach described in a previous publication [6]. The one main difference was that in the version of the model used for this project, the transmitter/receiver angle remains fixed instead of changing to intersect at each target depth. Another feature of this new model is that the phase delay due to multiple forward scattering is included. The technique is based on the method of moments and the solution of the radiative transfer equation in the Small-Angle Diffusion Approximation (SADA) [2]. 4. RESULTS Data showing the underwater object range computed using the experimental phase data and the model phase data at f m =2MHz are plotted in Figures 4a and 4b, respectively, as a function of target position for a total attenuation of c =.4m -. Also shown are the amplitudes of the and components of the detected signal (right vertical axis). For both the model and experimental data, the measured position correlates well with the actual position and the amplitudes of the and signals decay at a similar rate with increasing range. However, as the water clarity decreases, both the experimental (Figure ) and model (Figure 6) data show an increase in error between the measured and actual target positions. This is due to the fact that as the backscatter grows, it eventually dominates the return signal. Since the backscatter is still modulated at f m =2MHz, the phase measurement is biased towards shallower depths where the backscatter signal is the largest. Evidence that the backscatter is still modulated is also seen in the amplitude data where the and signals have approximately the same decay rate with increasing object range. These results show that although a modulation frequency of f m =2MHz provides an unambiguous range of.64m, the accuracy of the measurement is affected by the collection of backscattered light that is still modulated. a. b. Figure 4. Graphs showing the underwater object position computed using the phase shift data from the experiment (a) and model (b) at f m =2MHz as a function of the actual object position for c=.4m -. Also plotted (on the right vertical axis) are the and signal levels normalized relative to their values at d=cm. Figure. Graphs showing the experimentally measured object position (left vertical axis) and the and signal amplitudes relative to their values at d=cm (right vertical axis) as a function of the actual object position at f m =2MHz for three different water clarities: (a) c=.8m -, (b) c=.2m -, and (c) c=.6m -. Proc. of SPIE Vol. 8372 8372B Downloaded From: http://proceedings.spiedigitallibrary.org/ on 2/8/26 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx

Figure 6. Graphs generated from model data showing the object position (left vertical axis) and the and signal amplitudes relative to their values at d=cm (right vertical axis) as a function of the actual object position at f m =2MHz for three different water clarities: (a) c=.8m -, (b) c=.2m -, and (c) c=.6m -. Experimental and model data for a modulation frequency of f m =MHz are shown in Figures 7-9. The data for the cleanest water at c =.4m - in Figure 7 correlates well with the data in Figure 4 for f m =2MHz. However, the data for more turbid water in Figures 8 (experimental data) and 9 (model data) show that the range computed using the phase shift data at f m =MHz is much closer to the actual object range than the corresponding f m =2MHz data in Figures and 6. This is due to the fact that at f m =MHz, the modulation becomes more washed out in the backscattered light. Evidence of this is shown by the increase in the signal decay rate relative to the signal decay rate with increasing object range. The small percentage of backscatter that is still modulated does interfere with the object-reflected light. These constructive and destructive interference effects were observed in previous measurements [6-8] are indicated by the fluctuations in the amplitude and range data at longer (d >cm) object ranges. a. b. Figure 7. Graphs showing the underwater object position computed using the phase shift data from the experiment (a) and model (b) at f m =MHz as a function of the actual object position for c=.4m -. Also plotted (on the right vertical axis) are the and signal levels normalized relative to their values at d=cm. Figure 8. Graphs showing the experimentally measured object position (left vertical axis) and the and signal amplitudes relative to their values at d=cm (right vertical axis) as a function of the actual object position at f m =MHz for three different water clarities: (a) c=.8m -, (b) c=.2m -, and (c) c=.6m -. Proc. of SPIE Vol. 8372 8372B-6 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 2/8/26 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx

Figure 9. Graphs generated from model data showing the object position (left vertical axis) and the and signal amplitudes relative to their values at d=cm (right vertical axis) as a function of the actual object position at f m =MHz for three different water clarities: (a) c=.8m -, (b) c=.2m -, and (c) c=.6m -. Experimental and model data corresponding to a modulation frequency of f m =6MHz are shown in Figures -2. The cleanest water data (Figure ) is nearly identical to the data shown in Figures 4 and 7 for f m =2MHz and f m =MHz, respectively. The data in Figures (experiment) and 2 (model) show similar trends to the f m =MHz data. Specifically, the measured range correlates well with the actual object position and the signal decay rate is higher than signal decay rate and increases with decreasing water clarity. Constructive and destructive interference effects between the backscatter and object-reflected signals are also observed in the amplitude and range data, especially at object ranges d >2cm. a. b. Figure. Graphs showing the underwater object position computed using the phase shift data from the experiment (a) and model (b) at f m =6MHz as a function of the actual object position for c=.4m -. Also plotted (on the right vertical axis) are the and signal levels normalized relative to their values at d=cm. Figure. Graphs showing the experimentally measured object position (left vertical axis) and the and signal amplitudes relative to their values at d=cm (right vertical axis) as a function of the actual object position at f m =6MHz for three different water clarities: (a) c=.8m -, (b) c=.2m -, and (c) c=.6m -. Proc. of SPIE Vol. 8372 8372B-7 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 2/8/26 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx

Figure 2. Graphs generated from model data showing the object position (left vertical axis) and the and signal amplitudes relative to their values at d=cm (right vertical axis) as a function of the actual object position at f m =6MHz for three different water clarities: (a) c=.8m -, (b) c=.2m -, and (c) c=.6m -. The data presented above show how using modulation frequencies >MHz help suppress backscatter and improve the accuracy of range measurements. However, since the backscattered light is still detected, it contributes to the overall noise level at the receiver. Furthermore, when the object-reflected light is weak and is close in amplitude to the small portion of the backscatter that is still modulated, fluctuations in the range measurement are observed due to constructive and destructive interference between the backscatter and target-reflected signals. The effects that these two parameters have on the experimental range measurement error for f m =MHz and f m =6MHz are shown in Figures 3 and 4, respectively. Here the range error (plus one and minus one standard deviation from the ) is plotted against the object position for three different integration times: 4μsec, 4μsec, and 4μsec. As expected, the standard deviation increases with object range and with increasing water turbidity. While increasing the integration time reduces the standard deviation due to shot noise, it does not help suppress the fluctuations caused by constructive and destructive interference. These interference effects are the most significant source of range error at object ranges greater than 2cm. There also appears to be a positive offset in the range error for c =.2m - and.6m - for both modulation frequencies. This could possibly be due to multiple small angle scattering that adds an additional phase delay to the modulated light on its path to and from the underwater object. - std. dev. sample (4μsec) std. dev. samples (4μsec) std. dev. samples (4μsec) - std. dev. sample (4μsec) std. dev. samples (4μsec) std. dev. samples (4μsec) - std. dev. sample (4μsec) std. dev. samples (4μsec) std. dev. samples (4μsec) Figure 3. Graphs showing range error (plus one and minus one standard deviation from the ) as a function of object position at f m =MHz for three different water clarities: (a) c=.8m -, (b) c=.2m -, and (c) c=.6m -. Proc. of SPIE Vol. 8372 8372B-8 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 2/8/26 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx

- std. dev. sample (4μsec) std. dev. samples (4μsec) std. dev. samples (4μsec) - std. dev. sample (4μsec) std. dev. samples (4μsec) std. dev. samples (4μsec) - std. dev. sample (4μsec) std. dev. samples (4μsec) std. dev. samples (4μsec) Figure 4. Graphs showing range error (plus one and minus one standard deviation from the ) as a function of object position at f m =6MHz for three different water clarities: (a) c=.8m -, (b) c=.2m -, and (c) c=.6m -. As mentioned previously, the main disadvantage of using a single high frequency modulated optical signal to measure object range is the small unambiguous range that it provides. For example, the unambiguous ranges corresponding to f m =MHz and f m =6MHz are d UNAMB = 62.2cm and d UNAMB = 7.cm, respectively. The ambiguity was removed in the data presented above since the object range was known a priori. An alternate way of reducing the ambiguity is to compute the range using the difference between the phase measurements at f m =MHz and f m =6MHz (Equation 7). In this case the unambiguous range becomes d UNAMB = 97cm. The data obtained by using this method to compute the object position is shown in Figures (experiment) and 6 (model). The most notable difference between the data in Figures and 6 and the data presented previously for a single modulation frequency is the increased deviation between the measured and actual position, especially at the ranges greater than 2cm and at the highest water turbidity (c=.6m -, Figures c and 6c). This is more evident in the data shown in Figure 7 where the range error is plotted for the same scenarios as those presented previously for the single frequency data. This increase in range error was anticipated due to the fact that when subtracting the phases at f m =MHz and f m =6MHz, the resulting standard deviation is the quadrature sum of the standard deviations of the data at the individual modulation frequencies. Furthermore, the variations due to constructive and destructive interference are amplified due to the fact that range is computed by dividing the resulting phase by the difference frequency. However, it is worth noting that the range error data in Figure 7 does not appear to contain the positive offset that was observed in the data in Figures 3 and 4 for the individual modulation frequencies. This may be due to the fact that when subtracting the phase shift measured at two closely spaced frequencies, the additional phase delay due to multiple small angle scattering is removed. Figure. Graphs showing the experimentally measured object position (left vertical axis) as a function of the actual object position. The data was computed by using Equation 7 with f 2 =MHz and f =6MHz for three different water clarities: (a) c=.8m -, (b) c=.2m -, and (c) c=.6m -. Proc. of SPIE Vol. 8372 8372B-9 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 2/8/26 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx

Figure 6. Graphs showing the model-generated object position (left vertical axis) as a function of the actual object position. The data was computed by using Equation 7 with f 2 =MHz and f =6MHz for three different water clarities: (a) c=.8m -, (b) c=.2m -, and (c) c=.6m -. 8 6 4 2-2 -4-6 -8 std. dev. sample (4μsec) std. dev. samples (4μsec) std. dev. samples (4μsec) - 8 6 4 2-2 -4-6 -8 std. dev. sample (4μsec) std. dev. samples (4μsec) std. dev. samples (4μsec) - 8 6 4 2-2 -4-6 -8 std. dev. sample (4μsec) std. dev. samples (4μsec) std. dev. samples (4μsec) - Figure 7. Graphs showing range error (plus one and minus one standard deviation from the ) as a function of object position for the data in Figure for three different water clarities: (a) c=.8m -, (b) c=.2m -, and (c) c=.6m -.. SUMMARY AND FUTURE WORK Various methods for measuring the range to an underwater object using modulated light were reviewed. While using a modulation frequency of 2MHz provided an unambiguous range of.97m, the collection of backscattered light that is still modulated at this low frequency produced a range measurement that was significantly biased towards shallow ranges where the backscatter signal is the largest. Increasing the modulation frequency to 6MHz or MHz provided a way to suppress the contribution from backscattered light, but the unambiguous range was reduced to <m. Furthermore, constructive and destructive interference between the object-reflected and backscattered light produced fluctuations in the measured range that increased the range error. A two-modulation frequency approach was evaluated where the phase difference between the MHz and 6MHz data was used to compute the range to the underwater object. The advantage was that the unambiguous range was increased by dividing phase difference by Δf = 2MHz. However, the range error also increased beyond that which was observed for the data taken with the individual modulation frequencies. This was due to the fact that the standard deviation increased as a result of the quadrature sum of the Gaussian noises. Furthermore, the fluctuations due to constructive and destructive interference increased because the range is computed by dividing the phase difference by a significantly lower frequency. However, it appeared that the additional phase delay due to multiple small angle scattering on the way to and from the underwater object can be eliminated by subtracting the phase shift at two closely spaced frequencies. The focus of future work will be to determine how to minimize the effects of constructive and destructive interference on the range measured using two modulation frequencies. One approach is to use techniques similar to those developed for underwater optical imaging where the phase of the local oscillator reference signal is varied in a controlled manner depending on the phase difference between the backscattered and object-reflected light [6]. Methods for enhancing the accuracy of the two frequency approach by using the phase measured at the individual modulation frequencies are also Proc. of SPIE Vol. 8372 8372B- Downloaded From: http://proceedings.spiedigitallibrary.org/ on 2/8/26 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx

under investigation. Here the range measured using the phase difference between the two frequencies is used to get an initial estimate of the object range and reduce the ambiguity of the range measured using each modulation frequency independently. An experimental setup is also being developed where two phase-locked SDR receivers are used to measure the phase at two different frequencies simultaneously or at alternating time intervals. Finally, the observation that the effects of multiple small angle scattering appear to be minimized by subtracting the phases at two closely spaced frequencies is also under investigation. REFERENCES [] Amann, M., Bosch, T., Lescure, M., Myllylä, R., and Rioux, M., "Laser ranging: a critical review of usual techniques for distance measurement", Opt. Eng. 4, (2). [2] McLean, E. A., Burris, H. R. and Strand, M. P., Short-pulse range-gated optical imaging in turbid water, Applied Optics 34, 4343-43 (99). [3] Dalgleish, F. R., Caimi, F. M., Britton, W. B. and Andren, C. F., Improved LLS imaging performance in scattering-dominant waters, Proc. SPIE 737 (29). [4] Weidemann, A., Fournier, G. R., Forand, L. and Mathieu, P., In harbor underwater threat detection/identification using active imaging, Proc. SPIE 78, 9-7 (2). [] Mullen, L., Laux, A., Concannon, B., Zege, E.P., Katsev, I. and Prikach, A., Amplitude-Modulated Laser Imager, Applied Optics, 43, 3874-3892 (24). [6] Mullen, L., Laux, A., Cochenour, B., Zege, E.P., Katsev, I. and Prikach, A., "Demodulation techniques for the amplitude modulated laser imager," Applied Optics, 46, 7374-7383 (27). [7] Ricci, R., Francucci, M., De Dominics, L., Ferri de Collibus, M., Fornetti, G., Guarneri, M., Nuvoli, M., Paglia, E. and Bartonlini, L., Techniques for effective optical noise rejection in amplitude-modulated laser optical radars for underwater three-dimensional imaging, EURASIP Journal on Advances in Signal Processing 2 (2). [8] De Dominicis, L., Ferri de Collibus, M., Fornetti, G., Guarneri, M., Nuvoli, M., Ricci, R. and Francucci, M., Improving underwater imaging in an amplitude-modulated laser system with radio frequency control technique, J. Europ. Opt. Soc. Rap. Public. 4 Vol (2). [9] Pellen, F., Olivard, P., Guern, Y., Cariou, J. and Lotrian, J., Radio frequency modulation on an optical carrier for target detection enhancement in sea-water, J. Phys. D Appl. Phys. 34, 22-3 (2). [] Pellen, F., Intes, X., Olivard, P., Guern, Y., Cariou, J. and Lotrian, J., "Determination of sea-water cut-off frequency by backscattering transfer function measurement, J. Phys. D Appl. Phys. 33, 349-34 (2). [] Poujouly, S. and Journet, B., A twofold modulation frequency laser range finder, J. Opt. A: Pure Appl. Opt. 4 (22). [2] Katsev, I. L., Zege, E. P., Prikhach, A. S., Kargin, B. A. and Kargin, A. B., Pulse stretching of the propagated laser beam, International Workshop on Multiple Scattering Lidar Experiments (MUSCLE-3), Russia, St. Petersburg (24). Proc. of SPIE Vol. 8372 8372B- Downloaded From: http://proceedings.spiedigitallibrary.org/ on 2/8/26 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx