Suppression of forward-scattered lightusing high-frequency intensitymodulation

Brandon CochenourShawn O’ConnorLinda Mullen

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Suppression of forward-scattered light usinghigh-frequency intensity modulation

Brandon CochenourShawn O’ConnorLinda MullenNaval Air Warfare CenterPatuxent River, Maryland 20670E-mail: brandon.cochenour@navy.mil

Abstract. Laser imaging through a turbid medium is complicated by scat-tering. Backscattered photons reduce image contrast as weak targetreturns compete against a large background of backscattered light.Forward scattering broadens the interrogating laser beam, thereby reduc-ing the spatial resolution of the target. Prior research has shown that inten-sity modulation (<100 MHz) can be used to “wash-out” the backscatter,resulting in better discrimination of the target and higher contrast. Weshow that the higher modulation frequencies (>100 MHz) can be alsoused to suppress forward scattered light, thereby increasing spatial res-olution. © 2014 Society of Photo-Optical Instrumentation Engineers (SPIE) [DOI: 10.1117/1.OE.53.5.051406]

Subject terms: scattering; underwater; laser; imaging; modulation.

Paper 131414SS received Sep. 13, 2013; revised manuscript received Nov. 6,2013; accepted for publication Nov. 7, 2013; published online Dec. 16, 2013.

1 IntroductionLasers are used for detection, ranging, and imaging in turbidmedia such as seawater. High spatial resolution can beachieved due to the excellent collimation properties oflaser light. High temporal (i.e., range) resolution can beachieved with short pulse or high-frequency intensity modu-lated sources. However, the propagation of light underwateris complicated by absorption and scattering. Utilizing blue/green laser sources exploits an absorption minimumobserved in seawater.1,2 However, scattering in both the for-ward and backward directions is typically more difficult toovercome. Collection of scattered light in the backwarddirection reduces image contrast as the backscattered lightfrom the environment adds a constant background compo-nent to a weak target return. Scattering in the forward direc-tion reduces image resolution as light intended for a certainpixel is scattered into adjacent pixels as the once collimatedbeam spreads in space.

A synchro-scan configuration, which uses a collimatedlaser source and a single pixel/narrow field-of-view (FOV)receiver, can be used to reduce the impact of scatteredlight.3 Still, beyond two to three attenuation lengths, addi-tional techniques are required to provide immunity to scat-tered light. One technique uses intensity modulated beams totemporally discriminate the object from the backscatter.4–8

The intensity modulated technique presumes that modulatedreturns from the backscatter, distributed in range and havingexperienced many multiple scattering events, will sum inco-herently at the receiver. This results in the backscattered lighthaving a direct current (DC) component only. However, lightreaching the target will retain the modulation. A bandpassfilter after the photodetector provides a means of separatingthe modulated light from the object and the nonmodulatedbackscattered light from the environment.

This technique effectively provides a method ofdiscriminating between object reflected light (which retainsmodulation) and backscattered light (whose modulation is

“washed-out”). The extent to which backscattered light issuppressed depends on the relationship between the modu-lation wavelength and the backscattered photon path lengthdifferences. Studies have shown that due to the large pathlength differences accumulated between backscatteredevents, a sufficient amount of backscatter reduction canbe obtained with relatively low modulation frequenciesof <100 MHz (i.e., long modulation wavelengths).4,5,9–13

As the contrast (backscatter) limit is mitigated, the for-ward scatter limit will become a serious limitation. In thiswork, we show that the intensity modulation can also beused to reduce forward scatter. Similar to the way that thepath length differences between modulated backscatteredphotons result in a loss of modulation when summed coher-ently at the receiver, modulated forward scattered light willalso accumulate phase differences. We posit, however, thatthe path length differences between forward scattered com-ponents will be much smaller than the backscattered lightdue to a peaked scattering phase function which tends tokeep scattering events close to the beam axis. In order toachieve the desired reduction in modulation of forward scat-tered photons, the modulation frequencies used will need tobe much higher than those determined in the previous back-scatter studies.

The task is then to determine what modulation frequen-cies are necessary to achieve a sufficient amount of forwardscattering reduction as described above. Unfortunately,measuring the temporal response in the forward directionhas been a complicated task. It requires a high power(>1 W) laser source in order to overcome the exponentialattenuation of light in water. This high power source mustalso be modulated higher than 100 MHz (ideally, up to1 GHz) in order to observe the effects associated with highermodulation frequencies (i.e., shorter modulation wave-lengths). Similarly, a high bandwidth photoreceiver isrequired to recover the high frequency modulation. It isdesired that this photoreceiver also have high gain inorder to combat the significant attenuation exhibited in a dis-persive channel. Until now, these technological hurdles have0091-3286/2014/$25.00 © 2014 SPIE

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prevented a clear experimental understanding into thetemporal behavior of forward scattered light.

The remainder of this paper is organized in the followingmanner. First, we describe an experimental technique to mea-sure the frequency response (from 0.1 to 1 GHz) of forwardscattered light. The results of laboratory experiments willshow support for our hypothesis that forward scatteredlight can be suppressed via high frequency modulation. Asimple imaging experiment is then performed to illustratethe resolution improvement when using the high frequencymodulation compared to an unmodulated system.

2 Experimental Measurements of Spatial andTemporal Dispersions of Forward Scattered Light

2.1 Experimental Technique

Recently, there have been several works aimed at experimen-tally measuring the impulse response of the underwater opti-cal channel for underwater laser communications andimagings.10,14–19 These works borrow a technique fromGloge et al.20 and Helkey et al.,21 where the measurementis made in the frequency domain instead of the time domain.Instead of a single measurement of the channel that requiresa wide bandwidth, one can make several measurementsacross many frequencies, but with a smaller receiver band-width for each instance. The trade-off for lower noise band-width is a longer overall measurement time to construct thefrequency response. This is an acceptable trade-off assumingthat there are no time constraints on obtaining a measurementof the channel. Furthermore, it is assumed that the channelresponse is not time varying. The other benefit of thefrequency domain measurement is that it does not requirea high speed digitizer. High frequency components can bedownconverted to an intermediate frequency where low sam-ple rate, high resolution (>12 bits) digitizers can be used. Infact, this is precisely how radio frequency (RF) spectrumanalyzers or software defined radios operate. Thus, the fre-quency domain measurement allows for a reduced measure-ment bandwidth and higher dynamic range and sensitivity.

The question then becomes how to implement a lasersource that is rich in frequency content that would allowthe channel response to be obtained. One method may beto intensity modulate a continuous wave (CW) laser source.While this method may be practical for optical fibers in theinfrared using telecom Mach–Zender modulators (which canachieve tens of GHz modulation), it is more difficult at blue/green wavelengths. This is particularly true for the opticalpowers (several Watts) needed for realistic underwater mea-surements where the optical extinction is so significant.Currently, intensity modulation at high powers in theblue/green are only available via bulk external electro-optic modulators, which are bandlimited to ∼100 MHz.

Instead, a wideband RF spectrum is generated using amode-locked laser (MLL). The laser outputs a Gaussianpulse train given by

PoptðtÞ ¼P̄optffiffiffiffiffi2π

p Tt

X∞n¼−∞

exp½−ðtþ nTÞ2∕2τ2�; (1)

where P̄opt is the average transmitted optical power, T is thepulse repetition period, and τ is the Gaussian pulse time con-stant. Recall that the Fourier transform of a train of Gaussian

pulses in the time domain yields a train of pulses in the fre-quency domain, whose harmonics are spaced at f ¼ 1∕T.Photodetection of this optical signal immediately at thelaser output, before entrance to the channel, is given as

PacðfÞ ¼ Pdc × exp½−ð2πτfÞ2�; (2)

where PacðfÞ is the electrical power at frequency, f. Theaverage, or DC component, is Pdc ¼ RLðRP̄optÞ2, whereRL is the load resistance and R is the detector responsivity.Instead of characterizing the temporal response of the chan-nel by trying to temporally resolve the individual short pulsesof Eq. (1), we choose to measure changes to the frequencyharmonics using the form of Eq. (2). The generated signal isclearly rich in frequency content, yet does not require a wide-band receiver to measure all spectral content simultaneously.Rather, multiple measurements can be made by centering anarrow bandwidth receiver around each frequency harmonic.This yields a measurement technique that has exceptionalsensitivity despite the broad range of frequencies requiredfor characterization. Assuming a narrow pulse width, τ(on the order of ps), the exponential envelope of the alternat-ing current (AC) harmonics is approximately unity forf < 1 GHz, and the AC and DC components are directlyproportional.

The laser signal is transmitted through the underwaterchannel. At the output of the channel, the received electricalpower at each harmonic after photodetection is given by

Pacðf; czÞ ¼ mðf; czÞ × PdcðczÞ; (3)

where c is the total attenuation coefficient (in m−1) and z isthe physical propagation distance (in m). The product cz isreferred to as the attenuation length. Note that while thepower in each harmonic is still proportional to the averagereceived electrical power, it is now also a function of theattenuation length. This is to reflect the attenuation of theaverage signal level due to the absorption and scattering.Note that Eq. (3) also introduces the modulation depth,mðf; czÞ, which represents an additional fractional amountof power lost due to the temporal dispersion caused by for-ward scattering, and therefore takes on the values0 < mðf; czÞ ≤ 1. The modulation depth is unity in clearwaters where no scattering occurs. In summary, Eq. (3) illus-trates that the received frequency components experienceattenuation due to two mechanisms; a frequency independentloss due to the absorption and scattering, and a frequencydependent loss arising from the temporal distortions causedby multiple scattering.

It may be more convenient to express Eq. (3) in terms ofvoltages, which yields

Vacðf; czÞ ¼ffiffiffi2

pmðf; czÞ × VdcðczÞ: (4)

Rearranging terms, we can express the modulation depthas

mðf; czÞ ¼ Vacðf; czÞffiffiffi2

pVdcðczÞ

: (5)

We see then that an experimental measurement ofthe modulation depth of the received signal provides a

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straightforward way to characterize the frequency responseof forward scattered light.

2.2 Experimental Setup

A diagram of the experimental setup is shown in Fig. 1. Thetest tank is z ¼ 7.62 m in diameter and 3.65 m in height. Atransmissometer is used to measure the beam attenuationcoefficient, c m−1, at 532 nm. The dimensionless attenuationlength is, therefore, defined as the product of the attenuationcoefficient and the physical range, or cz. Magnesium hydrox-ide [MgðOHÞ2] is used as an artificial scattering agent. Thescattering albedo, or ratio between the scattering coefficientand overall attenuation, is ω0 ≃ 0.95. The scattering phasefunction (i.e., the probability distribution of scattering eventswith an angle) of MgðOHÞ2 was measured with the laser in-situ scattering transmissometer and volume scattering func-tion (LISST-VSF) instrument from Sequoia Scientific(Bellvue, Washington), and is shown in Fig. 2. This particu-lar artificial scattering agent was chosen because of its sim-ilarity to scattering functions in real ocean waters. Petzold’s“Turbid Harbor,” measured in the San Diego Harbor, is alsoshown for comparison.22

The laser source is a diode pumped solid-state MLL fre-quency doubled to 532 nm with an average power of 4 W at532 nm. The pulse width is ≃ 10 ps with a repetition periodof 10 ns. This results in frequency harmonics spaced evenlyat 100 MHz. A small mirror placed on a motorized rotationstage is situated at the tank input window, which allows forthe beam to be scanned θ ¼ 30 deg (in water) off of align-ment with the photorecever. The photodetector, a photon is37,303 five-stage, high speed, photomultiplier tube (PMT) isplaced behind a window on the opposite side of the tank. ThePMT has a 25-mm aperture, but has been specificallydesigned to achieve −3 dB bandwidths out to 1 GHz. Anoptical front-end consisting of a 50 mm f∕2 lens and irisis used to control the FOV between 1 and 7 deg (measuredin air). In this section, results are shown for the widest FOVof 7 deg. A 532-nm optical interference filter is used to rejectambient light. Dynamic range is controlled by adjusting

transmit power, or by using neutral density filters at thereceiver.

A bias-tee placed after the PMT separates the AC and DCcomponents of the received signal. The DC component ismeasured with a mulitmeter, while a microwave spectrumanalyzer is used to measure the AC component at the indi-vidual frequency harmonics of the mode-locked pulse train.The resolution bandwidth (RBW) of the spectrum analyzer isnormally set at 5 kHz, but can be lowered to 1 kHz in turbidwaters where the signal level is low. The ability to narrow themeasurement bandwidth while still making measurements athigh frequencies is a benefit to a frequency domain measure-ment such as this. Capturing the AC and DC componentssimultaneously allows for easy calculation of the modulationdepth as a function turbidity, FOV, transmitter/receiver align-ment, and scattering agent.

The AC response on-axis in clear water (cz ¼ 0.684) isused as a system calibration, and accounts for any slight var-iations in the frequency response of the PMT, microwavecomponents, cables, etc. Since the on-axis clear waterresponse is unaffected by any scattering, this serves as anappropriate way to subtract out the system response fromthe rest of the data. It also means that any subsequentchanges in mðf; czÞ are solely due to the environment.

All AC and DC voltages are also normalized relative tothis clear water response according to

Axðθ; czÞ ¼ 10 log10

�Vxðθ; czÞ

Vxðθ ¼ 0; cz ¼ 0.684Þ�

− 10 log10½expð−0.684Þ�; (6)

where x ¼ AC or DC. The first term on the right-hand side ofEq. (6) normalizes all the amplitudes to the clearest waterinvestigated in this study (cz ¼ 0.684). The second termsimply accounts for Beer’s law, since the transmittedsignal experiences a small amount of loss even in clearwaters (cz ¼ 0.684).

Fig. 1 Experimental setup. The transmitter and receiver are placedbehind opposite facing windows. Tank geometry allows for thebeam to be deflected up to θ ¼ 30 deg.

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Fig. 2 The scattering phase function of MgOH2. The volume scatter-ing function was measured via the LISST-VSF instrument fromSequoia Scientific (Bellevue, WA). Petzold’s “Turbid Harbor” isshown for comparison (Ref. 22).

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2.3 Results

The results are presented in Fig. 3 which shows the relativeamplitude (intensity) of the DC component, as well as that of100 MHz, 500 MHz, and 1 GHz. The corresponding modu-lation depths are also shown for the AC components.Measurements were made from 0 deg < θ < 10 deg andare presented reflected about the origin for illustrative pur-poses. This is valid given the axial symmetry of the phasefunction.

The plots of the relative amplitude can be interpreted asthe light field intensity at the given attenuation length, whilethe modulation depth describes the fraction of light still

modulated as a function of position for each correspondingattenuation lengths. In clear waters of Fig. 3(a) (cz ¼ 0.68),the interrogating beam remains well collimated as evidencedby a ∼55 dB drop in intensity for all frequency componentsaway from the beam axis (i.e., θ > 0 deg). Despite a largevariation in intensity distribution, there is no loss of modu-lation depth [Fig. 3(b)] as a function of position since thereare few scattering events.

Recalling the loss due to spatial dispersion in Eq. (3) andnoting the change in the vertical axis scale, the relative ampli-tudes in more turbid waters of cz ¼ 11.25 [Fig. 3(c)] andcz ¼ 15.88 [Fig. 3(e)] show that the intensity distribution

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Fig. 3 The relative amplitude and modulation depth as a function of pointing angle for (a and b) cz ¼ 0.68, (c and d) cz ¼ 11.25, and (e and f)cz ¼ 15.88.

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is both reduced and more diffuse with increased scattering.Also note that the AC components have a lower amplituderelative to the DC component with increasing turbidity anddistance from the beam axis. This is evidence of temporaldispersion and a direct result of forward scattering, as the sum-mation of forward scattered light with different receivedphases result in a lower received amplitude. Naturally, theshorter modulation wavelengths of higher frequencies aremore susceptible to the path length differences due to themultiple scattering. This temporal dispersion can also beobserved in the associated modulation depth plots ofFigs. 3(d) and 3(f). The figures show that the modulationdepth is best maintained along the beam axis, where nonscat-tered light is most likely to dominate. Away from the beamaxis, modulation depth is lost more rapidly at highfrequencies.

The relative amplitudes of the DC components in Fig. 3clearly illustrate the forward scatter problem in imaging aspreviously described. In clear water [Fig. 3(a)], the beamremains well collimated at the target plane, so high spatialresolution can be expected. With increased scattering[Figs. 3(c) and 3(e)], the average intensity distribution atthe target plane is more broadened and forward scatteredlight is more likely to illuminate nearby pixels. In contrast,as the modulation frequency increases, the intensitydistribution at each frequency component undergoes aneffective narrowing. This is a result of an increased amountof destructive interference between forward scatteredcomponents with an increasing modulation frequency(i.e., decreasing modulation wavelength). The phenomenonis also seen in the modulation depth plots, which show thatthe modulation depth is highest at the beam axis and lowestat forward angles. As expected, this difference is moredramatic at higher frequencies. It is worthwhile to notethat this is essentially the first experimental confirmationof a theoretical prediction made over four decades agoby Luchinin and Savel’ev,23 who noted a similar “narrow-ing” of the intensity distribution for light modulated at highfrequencies.

There are some caveats regarding the above experiment.First, due to the larger photoreceiver aperture (50 mm), thereis some spatial averaging that is exhibited in the above data.In other words, some details may be lost in the immediatearea of the beam axis. Furthermore, the above experimentonly describes the scenario from the transmitter to the object.A diffuse reflection off of the object is likely, meaning thatthe return path has a completely different distribution thanthe transmitted one. This means that the small amount of col-limated and modulated light that does reach the target mustmaintain its modulation upon diffuse reflection off the objectand subsequent travel back to the receiver. Prior research hasprovided some insight into this behavior, and it is expectedthat diffuse light will maintain its modulation over 3 to 6attenuation lengths.14,15

Therefore the current experiment, while useful in provid-ing insight into the behavior of the temporal properties offorward scattered light, does not sufficiently represent theimaging geometry. In order to determine whether or notthe phenomena described in this section can be exploitedto improve spatial resolution when imaging in turbid envi-ronments, a modified experiment must be performed. Thisis the focus of the next section.

3 Forward Scatter Rejection in Imaging Geometries

3.1 Experimental Setup

The modulation transfer function (MTF) is a measure of thespatial frequency content in an image, and is therefore anappropriate metric to determine the amount of improvementhigh frequency modulation may provide. The question thenbecomes how to measure the MTF experimentally. Ourapproach is shown in Fig. 4, which uses a 1m × 1m targetwith step function reflectivity placed 6.2 m from the inputwindow. The target is translated in the x-direction, whichis equivalent to scanning the beam and receiver FOV acrossthe step-function edge. Doing so yields the edge spread func-tion (ESF), which is simply the received intensity as a func-tion of beam position on the target. The derivative of the ESFis the line spread function (LSF), which gives the rate ofchange in intensity as a function of position over the target.A Fourier transform of the LSF then yields the MTF.

To test the hypothesis that high frequency modulation canbe used to mitigate the effects of forward scattered light, thesame MLL source is used to interrogate the target. Recall thatthe mode-locked pulse train generates frequency compo-nents at 100 MHz intervals from DC-1 GHz simultaneously.The electrical power at each frequency, indicative of theamount of modulated light returning from the object, isrecorded as a function of position across the target. Thisyields an ESF at each frequency (and DC), by which indi-vidual LSF’s and MTF’s can then be calculated. The MTF’sat each frequency can be compared for increasing turbiditiesto determine if there is any resolution improvement in usinghigh frequency modulated light.

The receiver is the same high speed PMT also used pre-viously. A bias-T separates the RF and DC components. TheRF component is measured via spectrum analyzer, with a 1kHZ RBW. A 100 Hz RBW is used for cz > 5. A single datasample using such a small RBW (i.e., long integration time)is chosen instead of obtaining many samples with a wideRBW (i.e., a short integration time). The DC componentis measured via multimeter. To assure that this configurationwould not quickly become backscatter limited with increas-ing turbidity, a pseudomonostatic geometry was used witha transmitter/receiver separation distance of 39 cm. The

Fig. 4 Experimental setup used to evaluate forward scatter suppres-sion for underwater laser imaging.

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receiver FOV is 2.86 deg, resulting in a 15.5 cm “spot” onthe target. The transmit beam and receiver FOVare aligned inwater such that the beam is in the middle of the receiver FOVon the target at the given range. The beam was collimatedprior to the input window and the spot size, as estimatedby eye via ruler at the target, was ∼0.5 cm.

Water clarity is varied usingMgOH2 and the beam attenu-ation coefficient is measured in situ with a transmissometer.The target is translated �30 cm on either side of the reflec-tivity edge, and due to the control of the experimental setup,was limited to a scan resolution of 0.5 cm steps. Because thescan resolution is approximately equal to the beam diameter,the ESF in clear waters may not exactly represent the “best”capabilities of the system under test with regard to resolutionsince it does not scan many points across the reflectivity tran-sition. As a compromise, in postprocessing the data isresampled by a factor of 5 (to increase dx in the ESF,and fmax in the MTF). While this may be viewed as a difficultcompromise in clear water, it is not expected to have much ofan influence in turbid waters since forward scattering willsignificantly increase the spot size on the target, making itmuch larger than the lateral resolution of the scan.

The data was then fit to a sigmoid function of the formfðxÞ ¼ x∕ð1þ jxjÞ for de-noising purposes as well as tofacilitate easier computation in transforming to the MTF.The function bounds are extended to �40 cm to furtherreduce noise and increase df in the MTF. Each ESF is con-trast stretched in order to eliminate any effects of backscatter,so that any changes observed in MTF should be due to for-ward scatter only. The derivative of the fitted and contraststretched ESF fit is determined numerically to arrive at

the LSF, and the discrete Fourier transform is used to arriveat the MTF. In the results below, the fitted data for ESF, LSF,and MTF are shown.

3.2 Results

Figure 5 shows the results at low turbidity, cz ¼ 0.8. Asanticipated from the intensity and modulation depth distribu-tions in Fig. 3, the ESF strongly resembles the ideal step-function intensity of the target. It is also clear from theMTF that spatial resolution is not lost in this scenario.Both the results are obvious given the lack of scatteringin clearer waters. The results in Fig. 6 show degradationdue to forward scattering, evidenced by the notably lesssharp ESF Fig. 6(a). Careful examination, however, revealsthat the shape of this transition is frequency dependent. Thisis shown more clearly in a zoomed-in version of the white-to-black transition in Fig. 7. The trends show that as thefrequency increases from DC-1 GHz, the transition of theESF gets sharper. The corresponding LSF’s in Fig. 6(b)show that higher modulation frequencies result in a steeperand narrower LSF, and the MTF’s in Fig. 6(c) also show thatfor a given spatial frequency, the contrast is improved bymoving to high modulation frequencies.

In order to gain a qualitative appreciation for the contrastimprovement using high modulation frequencies, Fig. 8shows a simulated object with increasingly fine features[Fig. 8(a)] which is digitally filtered using the experimentallyobtained MTF’s in Fig. 6(c). A cross section across the simu-lated result is shown in Fig. 8(b), which shows the relativecontrast of the simulated target had it actually been

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interrogated with a nonmodulated and 1 GHz modulatedbeam. Since the data in Fig. 6 has already been contraststretched to remove the effects of backscatter on theexperimentally obtained data, the contrast difference seenin Fig. 8(b) is due to forward scatter suppression only.

4 Summary and DiscussionSection 2 presented an experiment that showed the temporalbehavior of forward scattered light. The results suggestedthat high frequency modulation could be used to effectively

“narrow” an interrogating beam that had been spatiallybroadened by forward scattering. In Sec. 3, a second experi-ment, made in an imaging geometry, showed that there is infact a spatial resolution improvement associated with therejection of forward scattered light using this form of tem-poral discrimination.

While the results show quantitative improvement in MTF,they do not make any assertions on what kind of qualitativeimpact this would have on an actual underwater image.Additionally, because of the narrow RBW used on the spec-trum analyzer (100 Hz for cz > 5) a number of practicalissues are largely ignored. For example, a practical syn-chro-scan system is likely to have integration times muchshorter that the integration time in this experiment inorder to maintain a reasonable scan and search rate. Thesepractical receiver bandwidths would also introduce moreshot noise relative to this experiment, which may or maynot be a more dominant source of image degradation.

While not explicitly investigated here, receiver FOV willhave an impact on the system’s susceptibility to forwardscatter and hence the shape of the MTF. It is expected atlarge FOV’s that the efficacy of high frequency modulationwould be increased, since more forward scattered lightwould be collected. Data (not shown) were taken at a6.8 deg FOV to verify this, however, a rapidly increasingbackscatter signal prevented the investigation to be carriedout much further than a few attenuation lengths before thedynamic range was compromised. Should the FOV bemade narrower than the 2.86 deg used for the presentedresults, it is expected that the MTF magnitude would behigher at a given spatial frequency, since forward scatteredlight would be rejected spatially due to the smaller FOV. It isunclear with this additional spatial rejection, whether or notthere would be additional temporal rejection with increasingmodulation frequency.

Finally, the scattering albedo of the artificial scatteringagents used in our experiments (ω0 ≃ 0.95) is known tobe higher than typically observed in real turbid ocean waters(ω0 > 0.75). This means that we will likely observe moreforward scattering in the laboratory as the increased absorp-tion in real ocean waters will also act to suppress forwardscatter. As such, the results presented here represent a“worst case scenario” in terms of the magnitude of forwardscattering observed. Future laboratory experiments shouldinclude an absorbing dye such that the ratio of scatteringand absorption can be better matched to real ocean condi-tions. Additionally, it is unknown exactly how the shapeof the forward peak of the phase function influences tempo-ral dispersion as a function of frequency in the context offorward scatter suppression for imaging. Future studieswill include this dependence.

In short, there are several practical issues that must beconsidered in the use of high frequency modulation for for-ward scatter suppression in the context of the operation ofactual imaging systems. However, initial results usingstate-of-the-art modulated pulse laser sources and receiverssuggest that high frequency modulation does exhibitimproved contrast compared to nonmodulated techniques.24

References

1. N. Jerlov, Marine Optics, Elsevier Science, Amsterdam (1976).2. C. Mobley, Light and Water, Academic Press/Elsevier Science, San

Diego, California (1994).

25 20 15 10 5 0 5 10

0.6

0.7

0.8

0.9

1

1.1

Distance (cm)

Con

tras

t St

retc

hed

ESF

DC100MHz300MHz500MHz700MHz1GHz

Lower Frequency

HigherFrequency

Fig. 7 A zoomed-in view of the contrast stretched ESF transition forcz ¼ 5.5.

(a)

(b)

0 10 20 30 40 50 60 70 80 90 1000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

cm

Rel

ativ

e C

on

tras

t

DC1 GHz

Fig. 8 (a) A simulated object of increasing resolution and (b) the rel-ative contrast improvement at 1 GHz is due to forward scattersuppression. The contrast is obtained by digitally filtering a cross sec-tion of the ideal object by the experimentally obtained MTF’s in Fig. 6.

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Brandon Cochenour received the BS degree from LafayetteCollege, Easton, Pennsylvania, in 2003, the MS degree fromJohns Hopkins University, Baltimore, MD, in 2008, and the PhDdegree from North Carolina State University, Raleigh, NC, in 2012,all in electrical and computer engineering. His research at theNaval Air Warfare Center in Patuxent River, MD, focuses on under-water laser communications, underwater laser imaging, and STEMoutreach. He is a 2006 and 2012 recipient of the US Navy'sDelores M. Etter, "Top Scientist and Engineers of the Year," awardand was named "Outstanding Young Engineer of the Year" by theMaryland Academy of Sciences in 2009.

ShawnO’Connor received the masters of science degree (1999) andhis undergraduate degree (1997) from the University of SouthernIllinois. He has served 13 years as a research physicist at theNaval Research Laboratory working in the area of lasers and materi-als development. His tenure was spent developing novel laser sys-tems, most notably, the radiation balanced laser, the first bulk-solid-state visible Dysprosium laser, a low-phonon infrared countermeasure laser, and a high power Holmium doped fiber laser. Herecently moved to the EO and Special Mission Sensors Divisionwhere he is now working on optical imaging techniques in turbidunderwater environments.

Linda Mullen received the BS degree in electrical engineeringfrom Trenton State College, Ewing, NJ, in 1992, and the MS andPhD degrees in electrical engineering from Drexel University,Philadelphia, PA, in 1993 and 1996, respectively. Mullen hasbeen a researcher at the Naval Air Warfare Center since 1996, lead-ing research efforts in underwater laser detection, ranging, andimaging.

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