photoacoustic audio transmission without receiver

622

Letter

Vol. 44, No. 3 / 1 February 2019 / Optics Letters

Photoacoustic communications: delivering audible
signals via absorption of light by atmospheric H2O
RYAN M. SULLENBERGER,* SUMANTH KAUSHIK,

AND

CHARLES M. WYNN

Massachusetts Institute of Technology, Lincoln Laboratory, 244 Wood Street, Lexington, Massachusetts 02421, USA
*Corresponding author: ryan.sullenberger@ll.mit.edu
Received 25 September 2018; revised 29 November 2018; accepted 11 December 2018; posted 3 January 2019 (Doc. ID 346799);
published 25 January 2019

We describe a means of communication in which a user
with no external receiver hears an audible audio message
directed only at him/her. A laser transmits the message,
which is encoded upon a modulated laser beam and sent
directly to the receiver?s ear via the photoacoustic effect.
A 1.9 ?m thulium laser matched to an atmospheric water
vapor absorption line is chosen to maximize sound pressure
while maintaining eye-safe power densities. We examine the
photoacoustic transfer function describing this generation
of audible sound and the important operational parameters,
such as laser spot size, and their impact on a working
system. © 2019 Optical Society of America
https://doi.org/10.1364/OL.44.000622

The ability to communicate with a specific subject at a prescribed location who lacks any communication equipment
opens up many intriguing possibilities. Communication across
noisy rooms, hail and warn applications, and localized communication directed at only the intended recipient are a few possibilities. We demonstrate a method for localized acoustic
communication with a listener at long standoff distances using
a modulated laser transmitted toward the receiver?s ear. The optically encoded information is converted into acoustic messages
via the photoacoustic effect. The photoacoustic conversion of
the optical information into an audible signal occurs via the
absorption of light by ambient water vapor in the near area
of the receiver?s ear followed by airborne acoustic transmission
to the ear. The recipient requires no external communication
equipment in order to receive audible messages. We refer to this
means of communication as ?photoacoustic communications.?
Alexander Graham Bell previously described a ?photophone? means of using modulated light to create sound [1].
However, Bell?s invention never anticipated a means by which
the sound could be sent directly to the user without the need
for an intermediary material. Later, a photoacoustic speaker was
patented [2] in which modulated laser light was shined into ?a
gas absorption chamber.?
Again, this device failed to anticipate the possibility of using
open air as the absorbing medium. Recently, there has been
work investigating a photoacoustic means of communication
0146-9592/19/030622-04 Journal © 2019 Optical Society of America

that does not require a medium other than air. This technique,
known as laser-induced plasma effect (LIPE), uses a laser to
ionize the air, creating a plasma and ultimately a sound near
the end receiver [3]. Physical Optics Corporation is currently
developing this technique primarily for military use. The use of
ionizing radiation for producing sound, as well as the need for
very high-power lasers are safety concerns for the viability of
this approach.
Limited work has been performed examining the use of microwaves to stimulate sound directly in a user [4,5]. However,
the communication has been limited to barely audible clicks
(no complex messages) due to the inefficiencies in the transmission through bone and tissue. Furthermore, none of the microwave work has the ability to localize an individual in the
manner a laser-based photoacoustic communications system
does. Underwater photoacoustic communication has also been
explored [6].
Phased array acoustic systems (e.g., Audio Spotlight by
Holosonics) and nonlinear frequency conversion (e.g., Long
Range Acoustic Devices by LRAD Corp) have also been used
for projecting sound [7,8]. However, the acoustic spot size produced by linear acoustic arrays is much larger than what is possible with optical conversion due to diffraction (?sound ? mrad,
?opt ? ?rad). Parametric acoustic sources overcome diffraction
by transmitting higher frequency ultrasound and taking advantage of nonlinear mixing of two beams at a range. Haupt and
Rolt used such a system in a landmine detection scheme [9],
though in theory it could be used for communication. The
range of such a system is limited, however, by the lossiness
of high-frequency sound. Such systems have limits on the order
of 10 meters, much shorter than the photoacoustic communications system described here.
This Letter reports on two new approaches of efficiently
producing localized continuous-wave (CW) and pulsed sound
at >0 dB and distances > 2.5 m using photoacoustics in air.
A schematic illustrating the two different photoacoustic communication schemes is shown in Fig. 1. In the first method
[Fig. 1(a)], an acousto-optic modulator (AOM) provides an
amplitude modulation of the 1.9 ?m thulium laser, which produces CW audible signals near the receiver via the absorption of
light by ambient water vapor. In the second method [Fig. 1(b)],
a fast-steering mirror is used to sweep the laser beam such that

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Vol. 44, No. 3 / 1 February 2019 / Optics Letters

623

propagates to the receiver (lower absorption yields more optical
energy near the receiver), but it is also directly proportional to
the acoustic signal near the receiver (higher absorption yields
more local acoustic energy). For a given range, R, a balance
between these two constraints occurs when A 
 1?R, where
R is the distance from the transmitting laser to the receiver
(end user). We choose R by selecting for a particular absorption, A. A is in turn dictated by choice of laser wavelengths
commercially available. A highly attractive laser for a photoacoustic communications system is a 1.9 ?m thulium-based fiber laser we procured from IPG Photonics. Note that the gain
bandwidth of thulium is sufficiently wide enough that alternate
laser wavelengths can be obtained useful for alternate operating
ranges.
Of critical importance to a photoacoustic communications
system is the efficient conversion of optical energy into acoustic
energy using safe laser levels. Reference [13] describes a relationship between photoacoustically created sound pressure
and optical/physical parameters:
?I D1?2 Av2
???
Pr 
 psafe
,
2 2f L C P r 1?2

Fig. 1. Delivery of audible messages via photoacoustics. (a) Traditional
photoacoustic configuration: 1907.2 nm laser light is absorbed by
ambient water vapor. The laser beam is amplitude modulated via an
acousto-optic modulator. (b) Dynamic photoacoustic communication
amplifies the audible signal. (c) H2 O absorptivity near 1.9 ?m, with
an overlay of the laser emission from our thulium fiber laser.

the laser spot travels at the speed of sound over some arch
(?360°) adjacent to the receiver. The resulting coherent addition of acoustic waves results in an amplification of the acoustic
signal and produces pulsed acoustic emission without the need
for a resonant chamber. This method is similar to dynamic
photoacoustic spectroscopy, which has been used successfully
for standoff detection of trace explosives [10,11].
The laser wavelength was chosen to enable efficient long-range
communication as well as to satisfy requirements for laser eye
safety. Since acoustic pressure is directly proportional to optical
absorption, [see Eqs. (1) and (2) below], a laser wavelength for
which water is strongly absorbing is advantageous. Even in very
dry environments, there exist appreciable amounts of water in the
air. The upper bound for airborne water vapor is 100% relative
humidity (RH), for which at standard temperature (25°C), there
exist 4.4 · 104 ppm water molecules in the air. Water has several
particularly strongly absorbing features in the near infrared.
Because the near infrared is strongly absorbed by water, it poses
significantly less safety risk than wavelengths that can penetrate
through the eye to the retina. The primary safety risk at these
wavelengths is thermal damage with an eye and skin safety
threshold of 100 mW?cm2 [12]. Many commercial high-power
(typically fiber) lasers exist in this regime, including 1.4 ?m,
1.5 ?m, and 1.9 ?m varieties. For these three reasons, we find
the near infrared a very attractive regime for efficient operation.
Atmospheric optical absorption, A, affects the acoustic signal via two opposing roles. It attenuates the optical energy as it

(1)

where P is the pressure, ? is the expansion coefficient of the gas,
I safe is the laser intensity (assumed to be bounded by the safe
limit at the given wavelength), A is the optical absorption, v is
the speed of sound, C P is the specific heat of air, r is the distance from the photoacoustic absorption, f L is the laser modulation frequency, and D is the laser beam diameter. This
equation is valid in the large-beam limit in which the laser beam
diameter, D, is larger than the characteristic acoustic size
vT pulse , where T pulse describes the time duration that the laser
is on (for a 50% duty cycle waveform, this is half the period of
the acoustic waveform). Since typical acoustic frequencies range
between 20 Hz and 20,000 Hz, this period ranges between
50 ms and 50 us. When the D < vT pulse (small-beam limit),
the following equation derived from [13] describes the relevant
physics:
Pr 


?I safe D2 Av 2
:
8f L C P r 1?2 vT pulse 3?2

(2)

Equations (1) and (2) provide the guidelines for creating a useful photoacoustic communications system. We use a
1.9072 ?m thulium-based fiber laser (IPG Photonics) to assess
the relevant acoustic transfer functions describing the conversion of optical energy into acoustic energy and verify their relevance to our photoacoustic communications concept. The
output spectrum of this laser is overlaid on a water-vapor absorption spectrum in Fig. 1(c). At 50% RH and 1.9072 ?m
laser wavelength, we operate with an atmospheric optical absorption of A 
 0.04 m?1 . We use AOMs to modulate the laser (square wave, 50% duty cycle) over a range of audible and
ultrasonic frequencies [Fig. 1(a)]. Since safe levels are defined
by the laser energy per unit area, the laser spot size D is of particular importance. We systematically vary D (using a variety of
lenses, and maintaining optical intensity at the target) to examine its effect on the system. An Earthworks M30 microphone
(bandwidth 
 50 kHz) is placed ?1 cm away from the edge of
the laser beam. The resultant transfer functions describing the
conversion of eye-safe optical energy (100 mW?cm2 ) into
acoustic energy are shown in Fig. 2(a).

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We carried out experiments to demonstrate and characterize
traditional and dynamic photoacoustic communication configurations. Figures 2 and 3 show the measured sound pressure
levels produced by traditional and dynamic operation, respectively, and Fig. 4 plots the spatial distribution of the measured
photoacoustic spectra.
For the traditional photoacoustic experiment, several important trends emerge from the data in Fig. 2(a). As can be seen in
Eqs. (1) and (2) (dashed and solid lines, respectively) and our
data, each spot size has a corresponding cutoff frequency above
which the pressure decreases from its maximum value. The
maximum pressure occurs at the boundary between the small
and large spot limits, i.e., when D > vT pulse . In the large-beam
limit, contributions from different locations in the source
do not coherently add due to the long acoustic transit time
across the diameter [13]. The pressure [and corresponding
sound pressure level (SPL)] are in the audible regime
(SPL > 0 dB) for D > 1 cm. Higher SPLs are achieved by using larger beam diameters, at the sacrifice of higher frequency
content. Measurements of the photoacoustic signal strength
while varying the RH [Fig. 2(b)] show the expected linear relationship. An example photoacoustic waveform (sent and received; frequency sweep, from 20 kHz to 1 kHz) is shown in
Fig. 2(c). The agreement of the measured data over frequency
range is good, with deviations at higher frequencies that are
likely related to several simplifications in Eq. (1).
We obtained similar positive results for the dynamic photoacoustic concept shown in Fig. 1(b). Figure 3(a) shows an image plot of the dynamic photoacoustic time series data with
respect to laser beam sweep velocity. Individual waveforms
for Mach M  
 1.05, 1.00, and 0.95 are shown to the right
of the image plot. For M > 1, we see a time lag start to grow

Fig. 2. Results from our tests utilizing the traditional photoacoustic
configuration. (a) Transfer functions describing the conversion of
eye-safe optical energy at 50% RH into acoustic energy for various laser
spot sizes. Markers represent measured data, and lines represent theory
[solid = Eq. (2), dashed = Eq. (1)]. (b) Measured photoacoustic signal
(in mPa) versus RH. The result shows that signal strength is linear
with RH. (c) Demonstration of a photoacoustic communications waveform, 20 kHz to 1 kHz frequency sweep, sent (T) and received (R).

Letter

Fig. 3. Results from our tests utilizing the dynamic photoacoustic configuration (sweep length 
 50 cm, range 
 2.5 m). (a) Photoacoustic
signal heat map, sweep velocity (in Mach #) versus time, for a 5 mm
laser spot at target. Waveforms at M 
 1.05, M 
 1.00, and M 

0.95 are shown to the right of the heat map. Positive and negative
values represent compression and rarefaction, respectively. (b) Pressure
versus laser spot size. (c) Compression timescale (duration of the leading
compressive wave) of dynamic photoacoustic waveform versus spot size.
The compression timescale is indicative of the forcing function on the
water vapor molecules from the swept laser beam.

between the leading compression and trailing rarefaction of the
dynamic photoacoustic signal. This is caused by the swept laser
beam traveling faster than the speed of sound, giving additional
width (temporal length) to the signal. Measurements of the
photoacoustic signal strength and waveform compression timescale versus spot size (for constant laser power) are shown in
Figs. 3(b) and 3(c), respectively. We see that both parameters
vary linearly with spot size, with higher signal levels and shorter
timescales for smaller laser spots. Overlaid on Fig. 3(b) is the
signal level produced using the simple (static) photoacoustic
configuration. Our results show that dynamic photoacoustics
achieves an amplification proportional to L/D, where L is
the length over which the laser beam is swept, and D is the
spot size. We note that the signal produced via this method
is easily audible to the naked ear.
Another important feature of dynamic photoacoustics is its
ability to generate spatially localized sound. The feature has
been used recently to amplify faint photoacoustic signals from
gases as well as aerosols [10,11]. Dynamic photoacoustics
sweeps a laser beam at the speed of sound through an absorbing
medium (ambient water vapor in our case) [Fig. 1(b)]. The
acoustic waves add coherently along the sweep direction creating a local sound front similar to a shock wave that propagates
in the direction of the laser sweep. Both the amplification
and directionality of this process are highly advantageous to
photoacoustic communications, in that they increase the local
sound levels and provide a means of localizing the signal and
directing it toward a preferred receiver.

Letter
Measurements of the spatial extent of the dynamic photoacoustic signal at a range of 2.5 m are made by placing our
microphone on motorized translation stages arranged such that
we measure the plane perpendicular to the sweep direction
[Fig. 4(d)]. Results of these measurements are shown in
Figs. 4(a) and 4(b) for 50 cm and 25 cm propagation distances,
respectively. We define propagation distance as the distance between the microphone and the starting location of the laser
sweep [Fig. 4(d)]. A horizontal position of 0 mm corresponds
to a laser sweep speed equal to Mach 1. Horizontal positions >
0 mm correspond to sweep speeds > Mach 1, and horizontal
positions < 0 mm correspond to sweep speeds < Mach 1.
Analyzing both datasets, we see a vertical separation (?h) of
acoustic energy for sweep speeds > Mach 1. The separation distance ?h increases linearly with Mach number as well as propagation distance. Simple computer simulations modeling the
interference of spherically propagating wavefronts indicate that
the ?h separation is linear with Mach number, consistent with
our experimental results. The results of this simulation are
shown in Fig. 4(c). The horizontal spatial extent of the photoacoustic signal becomes larger at longer standoff ranges because
the relative Mach shifts occur at greater horizontal positions
due to simple geometry. We confirmed this behavior at
10 m standoff range for which we measured a photoacoustic
signal proportionally larger along the horizontal axis.
There is a tradeoff between sweep length (which directly corresponds to gain) and pulse repetition frequency (PRF) of the

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625

audible signal, where PRF of a dynamic photoacoustic communications system is v?L. This means that, for a dynamic photoacoustic communication system designed with a swept path
length of L 
 1 m, a single audible tone of frequency PRF 

343 m?s?1 m 
 343 Hz can be produced. To increase the
audible frequency, either the sweep length could be reduced (at
the cost of gain), or more laser beams could be added. The laser
spot size places an upper limit on the PRF, as the spot size dictates the lower bound on the waveform timescale. For likely operational parameters, e.g., D 
 3 cm, PRF 
 1 kHz, ANSI
constraints (for eye and skin safe operation) on average power
(100 mW?cm2 ) are more stressing than peak fluence
(100 mJ?cm2 ). Since average power is proportional to PRF, this
implies that low frequencies can be generated more loudly and
safely than higher frequencies (everything else being equal). A
trade study and systems engineering effort to design a dynamic
photoacoustic communications system with the bandwidth
necessary to encode more detailed messages (e.g., spoken words,
music, etc.) is reserved for a later study.
In summary, we have demonstrated the use of a 1.9 ?m thulium laser to produce photoacoustic signals from the ambient
water vapor in air (50% RH), with sound pressure levels well
into the audible regime (SPL > 0 dB) while using eye-safe laser
powers. We also demonstrated the use of dynamic photoacoustics to amplify the signal beyond what is possible with traditional photoacoustic techniques. The methods described
here provide new opportunities for development of photoacoustic communications systems capable of delivering audible
messages to subjects who lack any communication equipment.
Funding. Assistant Secretary of Defense for Research and
Engineering (Air Force Contract No. FA8702-15-D-0001).
Acknowledgment. Opinions, interpretations, conclusions, and recommendations are those of the author and are
not necessarily endorsed by the United States Government.

REFERENCES

Fig. 4. Measured spatial extent of the acoustic signal (mV p?p ) produced via the dynamic photoacoustic configuration at a range of
2.5 m, sweep length of 25 cm, and total propagation distance from
start of sweep to receiver of (a) 50 cm and (b) 25 cm. A horizontal
position of 0 mm corresponds to Mach 1. Horizontal positions > 0
correspond to sweep speeds > Mach 1, and horizontal positions <
0 correspond to sweep speeds < Mach 1. (c) Simulation of the ?separation distance? (?h) measured at supersonic sweep speeds growing
linearly with propagation distance, which agrees with our measured
data. (d) Schematic of the spatial measurement.

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