Modulated pulse laser with pseudorandom coding capabilities for underwater ranging, detection, and...

Modulated pulse laser with pseudorandomcoding capabilities for underwater ranging,

detection, and imaging

Brandon Cochenour,1,2,* Linda Mullen,1 and John Muth2

1Naval Air Systems Command, 22347 Cedar Point Road, Building 2185, Suite 1100,Patuxent River, Maryland 20670, USA

2Electrical and Computer Engineering Department, North Carolina State University,Box 7911, Raleigh, North Carolina 27695, USA

*Corresponding author: [email protected]

Received 2 March 2011; revised 3 June 2011; accepted 28 August 2011;posted 7 September 2011 (Doc. ID 143445); published 15 November 2011

Optical detection, ranging, and imaging of targets in turbid water is complicated by absorption and scat-tering. It has been shown that using a pulsed laser source with a range-gated receiver or an intensitymodulated source with a coherent RF receiver can improve target contrast in turbid water. A blendedapproach using a modulated-pulse waveform has been previously suggested as a way to further improvetarget contrast. However only recently has a rugged and reliable laser source been developed that iscapable of synthesizing such a waveform so that the effect of the underwater environment on thepropagation of a modulated pulse can be studied. In this paper, we outline the motivation for themodulated-pulse (MP) concept, and experimentally evaluate different MP waveforms: single-tone MPand pseudorandom coded MP sequences.OCIS codes: 010.3640, 010.4450, 110.7050, 120.0280, 290.1350, 290.2558.

1. Introduction

Since the invention of the laser, optical methods fordetecting or imaging targets in seawater have beeninvestigated due to the potential for high temporaland spatial resolution [1]. High temporal (range) re-solution can be achieved with short-pulse or high-frequency intensity modulated sources. High spatialresolution is achieved due to the excellent collima-tion properties of laser light. However, the propaga-tion of light underwater is complicated by absorptionand scattering. Utilizing blue/green laser sourcesexploits an absorption minimum observed in sea-water. Scattering, however, in both the forward andbackward directions, is typically more difficult toovercome.

In the forward direction, imaging systems sufferfrom blurring due to a broadening of the beam, asscattered light is returned from adjacent pixels. Indetection and ranging applications, if the degree of

forward scattering is significant enough, multiplereturns from beyond the target of interest may beobserved which appear as “clutter” relative to theintended target. The impact of forward scatteringis dependent not only on the medium, but also on thesystem characteristics (for example, receiver field ofview). In addition to the spatial degradation and in-creased clutter, spatial broadening also results insmall temporal variations arising from path lengthdifferences between the scattered and ballistic(nonscattered) returns. These path length differ-ences deteriorate the temporal (range) resolutionof the system.

A more significant issue in underwater ranging,detection, and imaging occurs in the backward direc-tion. In imaging systems, backscatter reduces imagecontrast, a situation analogous to car headlights infog. In detection and ranging applications, backscat-tered light may appear as false targets. In all appli-cations, backscattered light can occupy a significant

6168 APPLIED OPTICS / Vol. 50, No. 33 / 20 November 2011

portion of the receiver dynamic range. This is oftenundesirable, as the backscattered light contains noinformation about the target of interest. As such, thegoal for system designers is to detect a weak targetreturn in the presence of a relatively large backscat-ter return.

Solutions to this problem have generally fallen un-der two broad categories. The first utilizes a pulsedsource and range-gated receiver. In this approach,only photons that arrive within the receiver gate ontime are collected [2–6]. The gate timing can be var-ied to coincide with the expected target location, andthe target range is determined by measuring thepulse time of flight. The backscatter component is at-tenuated while the receiver is gated off. Additionally,since most practical blue/green laser sources havebeen frequency doubled from the IR, the conversionefficiency may be higher for pulsed laser sources dueto the higher peak powers. The drawback of thepulsed range-gated technique is that it requires astable, high energy, short-pulse (<5ns) laser source,which can be difficult to synthesize at blue/greenwavelengths. Additionally, gating a large area, highgain receiver like a photomultiplier tube (PMT) is dif-ficult electronically due to the large voltages needed,as well as significant impedance mismatch betweendriver circuit and PMT dynodes. The use of shortpulses also requires a wideband receiver, which canintroduce more noise. However, the pulsed/range-gated technique in theory provides a relativelystraightforward method for suppressing backscatterwhile providing target range information.

The other approach aims to reduce backscatter byintensity modulating a CW laser source at RF fre-quencies (>0:01–1GHz) [7–9]. In this scenario, rangeis determined by calculating the phase of the de-tected RF envelope relative to the modulation drivesignal using a standard RF coherent receiver. Themajor motivation of the intensity modulated methodlies in its ability to separate target photons frombackscatter photons. Here, nonscattered and mini-mally forward-scattered target-reflected photonsretain their modulation. However, backscattered re-turns, distributed in range between the source andtarget, destructively interfere with each other, caus-ing the modulation of the backscattered light towash-out at high frequencies [10–12]. The target(AC) component and backscatter (DC) componentcan be separated at the receiver with a bandpassfilter centered at the modulation frequency.

The intensity modulation approach has beenshown to reduce the contribution from backscatterand improve target contrast in some scenarios. How-ever, in order to achieve high average powers, blue/green CW sources are often modulated externally bya bulk electro-optic modulator. Since these modula-tors typically have bandwidths below <100MHz,backscattered light may not return completely inco-herent and may appear as a target [7,8]. This signaldependent clutter (modulated backscatter) can inter-fere with the modulated return from the object, and

complicates target localization. Another drawback ofthis approach is the additional shot-noise component(relative to the pulsed/range-gated system) caused bythe continuous contribution of backscattered light. Asimilar conclusion was reached by Machol [13] whencomparing direct detection pulsed LIDAR to CW li-dars modulated by continuous pseudorandom codes,where the continuous pseudorandom coded lidarmay end up with lower signal-to-noise ratios due toan ever present shot noise caused by scattering fromthe atmosphere.

Mullen et al. have reported in the past on a modu-lated-pulse method that combines the attractivefeatures of both the pulsed and intensity modulatedtechniques [14,15]. Here, instead of a single shortpulse (<5ns), a longer macropulse of tm > 10ns isused. The macropulse is then modulated at high fre-quencies (f m > 0:1Ghz). The modulated pulses aregenerated at a repetition rate of 1=trep ¼ 0:1–1MHz.This technique makes use of the target/backscatterdiscrimination techniques of both the short-pulserange-gated technique (time discrimination/receivergating) and the intensity modulation technique (fre-quency discrimination). In addition, the use of awider macropulse leads to a reduced receiver band-width, and possibly relaxed PMT gating require-ments. The timing of the macropulse return providescoarse target range information, while the high-frequency modulation within the macropulse offersa high degree of range resolution. In this way, finerange resolution can be determined by the RF mod-ulation frequency rather than the pulse width.

Since the modulated-pulse technique was de-scribed in the mid-1990s, there has been little in-vestigation since then, owing to the difficulty of inimplementing both the laser and receiver. Only re-cently has a stable laser source that synthesizesmodulated pulses been developed. In this paper,we experimentally investigate the detection capabil-ities of two categories of modulated-pulse waveforms.The first type of modulated-pulse waveform is thesingle-tone modulated pulse (ST-MP), where themacropulse is intensity modulated up to 1GHz. Thisis the same type of modulated-pulse waveform usedin previous studies. While the high-frequency modu-lation within the macropulse is desirable for back-scatter/target discrimination, the ST-MP suffers inrange resolution due to its broad cross-correlationshape. As an alternative, we also examine in thiswork the pseudorandom coded modulated pulse(PN-MP), where the modulation within the macro-pulse has been coded according to a deterministicpseudorandom sequence. The PN-MP contains thehigh-frequency components needed for backscatter/target discrimination, yet achieves high range reso-lution through the effect of pulse compression. In thisway, the PN-MPmay help buy back the range resolu-tion lost due to use of the wider macropulse.

While there has been some similar investigationon the effect of atmospheric turbulence for codedwaveforms in atmospheric lidar systems [16], the

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propagation characteristics of modulated-pulsewaveforms (single-tone or pseudorandom coded) ina turbid underwater channel is one that is currentlyunderstudied. Presented here are preliminaryexperiments displaying the potential of themodulated-pulse waveform for turbid underwaterenvironments.

2. Mathematical Description

In this section, we develop a mathematical descrip-tion of the laser waveforms used in this study. A con-ceptual plot of the laser output for the PN-MPwaveform is shown in Fig. 1, though the followinganalysis holds for either the ST-MP or the PN-MP.For each macropulse, a waveform of N subpulsesis transmitted, where N is equal to the number ofchips in the waveform. Therefore, the macropulsewidth is given as tmacro ¼ N � tc, where tc is the chipwidth. In some optical ranging systems using pseu-dorandom coding, the bit width, tb, is made smallerthan the chip width in order to improve range reso-lution. However in this study, it is due to the imple-mentation of the laser source, tb ¼ tc. The bit pulseshape is defined as

pðtÞ ¼�1 ð0 ≤ t < tbÞ0 otherwise : ð1Þ

A single modulated macropulse is given by

xðtÞ ¼XN−1

i¼0

aipðt − itbÞ; ð2Þ

where ai is 1 or 0. We define the weight of a code asthe number of times ai ¼ 1 in a particular sequence.In a similar fashion, we define the macropulse envel-ope as

mðtÞ ¼�1 ð0 ≤ t < tmacroÞ0 otherwise : ð3Þ

Then, the complete description of the laser waveformis given by

sðtÞ ¼X∞j¼0

xðtÞmðt − jtrepÞ; ð4Þ

where trep is the macropulse repetition period. This isthe scenario shown in Fig. 1.

Considering a single burst period of sðtÞ [i.e., xðtÞ ofEq. (2)]. The demodulated waveform zðtÞ can be givenby the autocorrelation of xðtÞ:

zðtÞ ¼ RxxðτÞ ¼Z

trep

0xðtÞxðt − τÞdt; ð5Þ

where RxxðτÞ is the autocorrelation of either thesingle-tone or PN-MP, and τ is the shift variable.Note that due to the pulsed nature of modulated-pulse waveforms, this is an aperiodic correlation asthe integral is bounded by the laser repetitionperiod, trep.

For intensity modulated, direct detection imple-mentations, the coefficients ai ¼ 1 or 0. However,for most of the pseudorandom codes developed by theradar community (or coherent optical applications),the coefficients take on the values a0

i ¼ 1 or −1. Sincethis is physically impossible for incoherent laser de-tection, it is typical [17–20] to use the transformationa0i ¼ 2ai − 1 (a0

i ¼ 1 or −1), and write Eq. (5) as

Rxx0 ðτÞ ¼Z

trep

0xðtÞx0ðt − τÞdt; ð6Þ

with

x0ðtÞ ¼XN−1

i¼0

a0ipðt − itbÞ: ð7Þ

In other words, the unipolar laser signal, xðtÞ, is crosscorrelated with a reference, x0ðtÞ, which is a bipolarversion of itself. While this subtle point is often men-tioned in works dealing with pseudorandom codingusing incoherent optical signals, rarely do implica-tions arise because of it. However, we shall latersee that this transformation can have significantimpact on the shape of the cross correlation for thisapplication depending on the type of code used.

We describe the impulse response of the mediumas

hðtÞ ¼ hobjðtÞ þ henvðtÞ ¼ hobjðtÞ þ hbksðtÞ þ hfwdðtÞ;ð8Þ

where hobjðtÞ is the impulse response of the target,henvðtÞ is the response of the environment, withhbksðtÞ and hfwdðtÞ being the response of the backscat-tered and forward scattered light, respectively.

In clear waters, we are concerned only with the re-sponse of the target hobjðtÞ, which may be approxi-mated as a δ-like impulse located at the targetlocation such that hobjðtÞ ¼ δðt − ttgtÞ. In this case,the cross-correlation is given as

Fig. 1. Conceptual sketch of the modulated-pulse laser output,shown here as PN-MPs.


Rxx0 ðτÞclear ¼Z

trep

0xðt − ttgtÞx0ðt − τÞdt: ð9Þ

We note that the response in clear water is the sameas the waveform’s cross correlation as defined inEq. (6), only shifted to the target location τ ¼ ttgt

at the corresponding range d ¼ vτ=2n, where v isthe speed of light and n is the index of refraction(n ¼ 1:33 for water). In other words, Rxx0 ðτÞclear ¼Rxx0 ðτ − ttgtÞ.

In turbid media, we must account for all of theterms in Eq. (8). Here, it may be more convenientto express the demodulated signal as zðtÞ ¼ Rxx0 ðτÞ �hðtÞ or

zðtÞ ¼ Rxx0 ðτÞclear þ Rxx0 ðτÞ � hbksðtÞ þ Rxx0 ðτÞ � hfwdðtÞ:ð10Þ

It is noted that the impulse responses hbksðtÞ andhfwdðtÞ may vary in appearance from scenario to sce-nario depending on system geometry (e.g., transmit-ter/receiver orientation, field of view, etc.) or targetlocation (e.g., proximity to ocean bottom) in additionto water clarity.

Clearly, the accuracy at which we measure zðtÞ isdependent upon the shape of the cross correlation.However, even if one were to use a waveform witha narrow cross-correlation shape, the contribution

Fig. 2. Experimental setup. MPL: modulated-pulse laser(530nm), RD: reference detector.

Fig. 3. Results in clear water, c ¼ 0:05 =m, cd ¼ 0:35. On the left, figures (a), (b), and (c) show the PMT returns for the ST-MP, the M-sequence PN-MP, and theMPS PN-MP, respectively. On the right, figures (d), (e), and (f) show the cross correlations for each correspondingwaveform on the left.


of the forward and backward scatter terms stand toproduce a signal dependent clutter which may makedetection of the target difficult. As stated in the in-troduction, the goal of this work is to further under-stand the impact of these clutter terms relative to theshape of the clear water cross correlation.

3. Experimental Setup

The experimental setup is shown in Fig. 2. The lasersource was developed by Fibertek (Herndon, Virgi-nia) under a Navy SBIR Phase II program. Thetransmitter is a fiber optic MOPA system thatsynthesizes coded pulses at 1064nm and amplifiesthem via fiber amplifiers before frequency doublingto 532nm. The macropulse repetition rate is 1=trep ¼500kHz. A personal computer is used to configurethe high-speed modulation within the macropulse(i.e., modulation frequency, code type, length, bitwidth, etc.). Because of limitations of the custom la-ser source, only code lengths of N ¼ 15 and N ¼ 31were examined with a tb (and hence, tc) of 960ps(∼1GHz). The single-tone modulated-pulse wave-

form is synthesized as alternating 1s and 0s withweight ðN=2Þ þ 1. Two pseudorandom modulated-pulse waveforms are investigated: the M-sequence[17,18] and the minimum peak sidelobe (MPS) code[21,22]. The M-sequence code achieves near-zerosidelobes for a periodic cross correlation performedon a continuous, periodic transmitted waveform.However, because the modulated pulse does nottransmit continuously, the aperiodic correlationmust be used which leads to higher sidelobe levelsin the correlation function. Despite this nonideality,M-sequence codes are easy to generate, and it wasinitially unknown whether or not the sidelobe levelsor environmental clutter would be the limiting factorin target detection. On the other hand, the MPScodes are specifically designed to have minimumsidelobe levels (or equivalently, maximize the peak-to-sidelobe level) for aperiodic correlation.

The experimental setup, including the testing tank,is shown in Fig. 2. A simple target with a high reflec-tance and low reflectance portion is submerged in thetank. Data are presented for the low reflectance

Fig. 4. Results for c ¼ 0:35 =m, cd ¼ 2:45. On the left, figure parts (a), (b), and (c) show the PMT returns for the ST-MP, the M-sequencePN-MP, and the MPS PN-MP, respectively. On the right, figure parts (d), (e), and (f) show the cross correlations for each correspondingwaveform on the left.


portion of the target, as it most clearly illustrates thechallenge of target detection in a high backscatter en-vironment. A high-speed silicon photodiode is used tomonitor the transmitted waveform prior to enteringthe tank, and is also digitized as a reference. Ahigh-speed PMT from Photonis (Sturbridge, Massa-chusetts) is used that has a 25mm aperture and ameasured −3dB bandwidth of 1GHz. The collectionoptics consists of a 50mm, f =2 lens and a 532nm in-terference filter to suppress ambient room light. Aniris is placed at the focal point of the lens to restrictthe receiver field of view to 1:7°. The PMT output,as well as the transmitted reference from the high-speed silicon photodiode is digitized by a 10 giga-samples per second oscilloscope. The oscilloscopeaverages 100 macropulses, and the averaged wave-form is transferred to the CPU where the cross corre-lationandother processingareperformed off line.Thedynamic range is controlled by varying the laserpowerandusingneutral density filters at the receiver.An optical power meter is used to record the powerlevels, so that slow varying power fluctuations be-tween data sets can be calibrated out in postproces-sing. Other details can be found in the figure.Maalox antacid was used as the scattering agent

[23], and the total attenuation coefficient, c ð=mÞ,which includes both absorption and scattering losses,is measured in situ with a 532nm transmissometer.

4. Results

Figures 3–6 show on the left side (a)–(c) the returnsdigitized after the PMT and (d)–(f) the cross correla-tions for the ST-MP, the M-sequence PN-MP, andMPS PN-MP coded waveforms, respectively, forincreasing water turbidity. For each figure theattenuation coefficient, c ð=mÞ, and corresponding at-tenuation length, cd (the attenuation coefficient mul-tiplied by the physical range) is given. The crosscorrelations are normalized to the highest value(not necessarily the target location in turbid water).Below, we detail some observations in both clear andturbid waters.

A. Detected Returns in Clear Water

Figures 3(a)–3(c) shows the ST-MP, the M-sequencePN-MP, and the MPS PN-MP waveforms in clearwater (c ¼ 0:05=m, cd ¼ 0:35). The target return isclearly observed at t ¼ 84ns (relative to the referencedetector and including extra propagation delay fromthe experimental setup) for each waveform in the



absence of backscatter clutter. The correlations inclear water for each waveform are shown to the rightof the PMT returns in Figs. 3(d)–3(f). For the ST-MPin Fig. 3(d), the broad shape is expected as given theburst envelope width, while the coded waveforms[Figs. 3(e) and 3(f)] exhibit narrow correlation peakscharacteristic of pseudorandom sequences.

It is evident that in clear waters with no signal-dependent clutter, target localization with any ofthe investigated waveforms is possible so long asthere are enough received photons for sufficientsignal-to-noise. However, as with other pulse com-pression radar and atmospheric lidar systems, thenarrow cross correlation of the PN-MP waveformsshow that one may buy back range resolution despitethe larger width of the macropulse.

B. Effect of Backscatter

Turbid waters, however, provide a different chal-lenge, as previously illustrated by Eq. (10). Theresults for c ¼ 0:35 =m (cd ¼ 2:45), c ¼ 0:54 =m(cd ¼ 3:78), and c ¼ 0:66 =m (cd ¼ 4:62) are shownin Figs. 4–6, respectively. For the PMT return of eachwaveform, we note that as turbidity increases, the

backscatter clutter (a broad peak between approxi-mately 20–80ns) becomes more prominent. In gener-al, the response of the backscatter does not retain themodulation in the same way the target return does.This is due to the distributed backscatter returnscombining incoherently at the receiver, as discussedpreviously. The effect of backscatter can also be seenin the cross correlations. The ST-MP andM-sequencePN-MP cross correlations exhibit a broad, positivegoing feature between 20–80ns. The cross correla-tions of the MPS code [Figs. 4(f), 5(f), and 6(f)],however, exhibits a negatively going backscatter sig-nature, due to the code’s lower weight and referencetransformation explained in Eqs. (6) and (7). Whilenot investigated here, this suggests that careful de-sign of code weight/length/etc. may result in methodsto shape or even diminish the backscatter signaturein the cross-correlation shape, thereby improvingtarget detection reliability. Custom modulated-pulsewaveform/code design, given some knowledge of thebackscatter response [hbksðtÞ of Eq. (10)], is a topic offuture study.

Regardless, we see that in turbid waters, target lo-calization by envelope detection or thresholding for



either the ST-MP or PN-MP would prove unsuccess-ful since the backscatter signature can be greaterthan the target return. Also, depending on the loca-tion of the target relative to the backscatter clutter,the backscatter may also interfere with the targetsignature. Clearly, additional measures must be ta-ken. As discussed, receiver gating would help mini-mize unwanted backscatter. As we will see in a latersection, a simple filtering technique can be used totake advantage of the differences in frequency con-tent between the backscatter and target signatures.

C. Effect of Forward Scatter

Effects due to both single and multiple small-angleforward scattering are also observed in the digitizedsignal from the PMT. Single small-angle forwardscattering can be observed as a loss in modulationdepth for the ST-MP returns in Figs. 4(a), 5(a),and 6(a). Multiple small-angle forward scatteringis observed as well. Using the ST-MP return inFig. 5(a) (c ¼ 0:54 =m) as an example, observe thatin the area of the target return, three distinct regionsare evident. It is believed that this is actually due totwo separate returns: one from the target itself, andanother due to multiply forward-scattered light thathas scattered around the target, and is reflectedfrom the tank walls. In the first region, from

84 < t < 89ns, object-only light is reflected. In thesecond region, from 89 < t < 99ns, the average levelincreases, suggesting that returns are summed fromboth the target as well as the surrounding tank wall.In the third region, for t > 99ns, the average level isreduced, as the target return has ceased, and light isnow only returned from the tank walls. The time(∼5ns) between the start of the object return (region1), and the start of the object plus tank wall return(region 2), is consistent with the time of flight delaybetween the target and tank wall (∼0:6m). Also ob-serve that for the single-tone returns, themodulationdepth of the multiply forward-scattered light return-ing from the tank walls after the object return thathas ended in region 3 is clearly lower than the othertwo regions that do include a return from the object.This loss of modulation is reasonable to expect giventhe large amount of scattering events required toscatter around the target to reach the back of thetank given the relatively high modulation frequency(∼1GHz). This may also explain why the differencebetween zone 1 (object only) and zone 2 (object plustank walls) appears only to be a shift in DC level, andnot a change in the peak-to-peak value of the modu-lation. We point out a significant result where athigh enough frequencies (such as the 1GHz tonepresented here), the high-frequency content of the

Fig. 7. Results for c ¼ 0:66 =m (cd ¼ 4:62), but withN ¼ 31. On the left, figure parts (a), (b), and (c) show the PMT returns for the ST-MP,the M-sequence PN-MP, and the MPS PN-MP, respectively. On the right, figure parts (d), (e), and (f) show the cross correlations for eachcorresponding waveform on the left.


modulated pulse may also be used to discriminateagainst multiple forward scatter as well as backscat-ter. In other words, ballistic or minimally scatteredlight from the target will retain the high-speedmodulation better than multiple forward-scatteredlight will, resulting in a method of discrimination be-tween target signatures and surrounding clutter.

Forward scatter can also be observed on the shapeof the cross correlations. For the ST-MP, we notice aslight broadening of the correlation shape of the tar-get signature in more turbid waters (Fig. 6) due tothe path delays between ballistic and multiply scat-tered light. Unfortunately, the broad shape of thecross correlation makes resolving the separate targetand tank wall returns impossible, despite our abilityto discriminate between the two in the raw returns.In this situation, the PN-MP waveforms may be ofhelp due to their narrower correlation shapes. Thisis most easily observed with a longer code length,such as theN ¼ 31 code length shown in Fig. 7. Here,the cross correlations of the PN-MP waveforms[Fig. 7(e) and 7(f)] show both the target return (t ¼84ns) as well as the tank wall return (t ¼ 89ns). Itis also observed that the tank wall return (t ¼

89ns) is slightly broadened relative to the target re-turn, which is consistent with the effect that signifi-cant multiple forward scattering would have on the1GHz coding within the macropulse. Clearly, futureexperiments should include multiple closely spacedtargets, or three-dimensional targets in order tomore accurately quantify detection capabilities un-der these forward-scattering conditions.

D. Backscatter Suppression through Filtering

In previous work using intensity modulated cw andmodulated-pulse lasers [7,8,12,13,24], it was foundthat performing bandpass filtering on the receivedsignal helps suppress backscatter. By centering thebandpass filter at the modulation frequency, the lowfrequency backscatter clutter can be removed, asonly the modulated component from the object ispassed. In this subsection, we examine whether ornot a similar filtering technique may be employed forPN-MPs. Since the pseudorandom codes are inher-ently wideband, any filtering performed to removespectral content of backscatter clutter will simulta-neously remove spectral content of the desiredtarget-reflected waveform. Therefore, a tradeoff

Fig. 8. Unfiltered cross correlations forN ¼ 15 and c ¼ 0:66 =m (cd ¼ 4:62) of the (a) ST-MP, (b)M-sequence PN-MP, and (c) MPS PN-MPwaveforms. Also, the cross correlations of the high-pass filtered versions of the (d) ST-MP, (e) M-sequence PN-MP, and (f) MPS PN-MPwaveforms.


between backscatter rejection and detection sensitiv-ity (signal-to-noise) must be performed.

To test whether or not the benefits of filteringto suppress backscatter are outweighed by thesimultaneous removal of spectral content from thetarget, the digitized PMT return is passed througha digital high-pass filter with a cutoff at 150MHz.Figures 8(a)–8(c) shows the cross correlations of theST-MP, the M-sequence PN-MP, and MPS PN-MPcodes, respectively, as measured previously for theunfiltered case in Fig. 6 for c ¼ 0:66 =m (cd ¼ 4:62).Figures 8(d)–8(f) are the result of cross correlationafter the detected signal was high-pass filtered. Notethat for each of the three waveforms, high-pass filter-ing eliminates much of the backscatter signatureseen in the unfiltered cross correlations. In the caseof the M-sequence [Fig. 8(e)], high-pass filtering pre-serves both the target return as well as the forward-scattered return from the tank walls. However, theforward scatter feature is less evident in the MPScode [Fig. 8(f)], though the target signature is muchmore clearly recognized compared to the nonfilteredcase [Fig. 8(c)]. The differences in forward scatter dis-crimination when high-pass filtering for backscatterbetween the two coded waveforms is currently un-known and is a topic for future work.

The implication of filtering, as a method of back-scatter discrimination, should not be understated.For example, placing an analog high-pass filter be-fore the digitizer would eliminate the high amplitudebackscatter signal, thereby offering the opportunityto optimize the digitizer dynamic range for the smal-ler amplitude target signature. When combined withreceiver gating or optical shuttering, an optimizationof the photoreceiver dynamic range could also begained.

Regardless, it appears that while high-pass filter-ing the PN-MP waveforms does result in some degra-dation to the ideal cross-correlation shape (relative toclear water), these degradations may be acceptablein light of the benefits gained from removal of thebackscatter signature. Furthermore, it suggests thatan ideal waveform or code should not have frequencycomponents close to DC. In other words, the idealwaveform should be broadband enough to result insufficient pulse compression, yet not include anylow frequency components that will be removed byfiltering for backscatter suppression.

5. Summary

In this work, we have revisited the modulated-pulse concept for optical imaging and detectionunderwater. Both single-tone and pseudorandommodulated pulses were examined. For underwaterapplications, the modulated-pulse waveform blendsthe benefits from both the short-pulse range-gatedand intensity modulated techniques. We have shownthat in clear water, the PN-MP stands as an improve-ment over the ST-MP due to their excellent correla-tion properties of pseudorandom codes. The PN-MPwaveforms also appear to exhibit the ability to distin-

guish target returns frommultiply forward-scatteredreturns. In high backscatter environments, the high-pass filtering technique previously described for theST-MP can be used for the PN-MP as well, eventhough some spectral content of the pseudorandomwaveform is removed in the process. Future studieswill examine custom code design given a priori knowl-edge of the impulse response of the underwater chan-nel or a priori knowledge of the desire to use ahigh-pass filtering technique, both of which can beused to mitigate backscatter. When coupled withother techniques like photoreceiver gating, additionalclutter suppression may be realized. It is our hopethat this study provides an entry point for furtherstudy into waveform design for optical imaging, ran-ging, and detection for underwater applications.

This work is sponsored by the Naval InnovativeScience and Engineering (NISE/Section 219) pro-gram at NAVAIR, Patuxent River, Maryland.

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