1
Opto-Mechanical Transduction in a
Microelectromechanical Systems-Based Optical
Pressure Transducer
Chase Coffman, Jessica Sockwell, Benjamin A. Griffin, and Mark Sheplak

Abstract—MEMS-based fiber optic lever sensors have
previously shown promise for measurement ability in high
thermal energy and hypersonic flows. In the design of these
sensors, an understanding of the intensity modulation mechanism
is critical to optimizing their performance. Prior work has
attempted to construct a suitable mathematical model for this
mechanism with some success.
This paper presents an
experimental
characterization
of
the
opto-mechanical
transduction mechanism of a fiber optic lever using an industrystandard multimode telecommunications fiber optic. The power
coupling distribution, sensitivity, and linearity characteristics are
reported.
Index Terms—fiber optic lever, MEMS, optical pressure
transducer, opto-mechanical transduction
I. INTRODUCTION
T
HE development of hypersonic transports and
improvement of existing high speed re-entry vehicles and
gas turbine engines has been hindered in recent years by an
inability to accurately measure pressure fields in the high
temperature flows surrounding or permeating these bodies.
Critical to the continued advancement of these platforms is an
understanding of their interactions with high thermal energy
flow phenomena, resulting in a need for more robust sensors
capable of withstanding adverse environmental conditions.
Previous work has demonstrated the promise of a MEMSbased, intensity-modulated fiber optic lever for providing
direct and accurate measurement of surface pressure
fluctuations within these hostile flow regimes [1]-[3].
Manuscript received May 6, 2010. This work was supported in part by the
Air Force Office of Scientific Research (AFOSR).
Chase Coffman was an undergraduate student with the Department of
Mechanical and Aerospace Engineering, University of Florida, Gainesville,
FL 32611 USA. Beginning Fall 2010 he will be a graduate student with the
Department of Aeronautics and Astronautics, Massachusetts Institute of
Technology, Cambridge, MA 02139 USA.
Jessica Sockwell is a graduate student with the Department of Mechanical
and Aerospace Engineering, University of Florida, Gainesville, FL 32611
USA.
Benjamin A. Griffin is a post-doctoral associate with the Department of
Mechanical and Aerospace Engineering, University of Florida, Gainesville,
FL 32611 USA.
Mark Sheplak is a Professor with the Department of Mechanical and
Aerospace Engineering, University of Florida, Gainesville, FL 32611 USA.
Fig. 1. Potential package illustrating the manner in which beam
spreading along the optical path serves to modulate the intensity of
the received optical signal.
The challenges that high temperature, high Reynolds
number flows pose to current pressure sensors have rendered
them largely ineffective. Near the surface of a body in such
flows, viscous forces or combustion kinetics generate
excessive amounts of thermal energy that result in
temperatures that approach or eclipse the melting point of
traditional sensor materials, causing thermally induced
measurement error [4]. The presence of electromagnetic
interference (EMI) may compound performance degradation
by disrupting the proper function of electrical-based devices.
Further challenges stem from flow instability and shock
wave/boundary layer interaction, phenomena which dictate
device operation at high frequencies and acoustic intensities
[2]. In the interest of improving measurement performance
under these stringent demands, advanced instrumentation is
shifting toward the union of micro-machined components,
high temperature materials, and fiber optics [4].
An intensity-modulated fiber optic lever is comprised of a
fiber optic orthogonally oriented with respect to a reflective
surface. An optical signal is transmitted through the fiber
toward the reflective surface where it is modulated by the
optical path length, or physical distance between the surface
and fiber end-face. It is then re-transmitted to the proximal
end of the fiber and collected for analysis. In pressure
transduction applications, the reflective surface used is a
diaphragm which deflects in response to pressure waves,
thereby altering the optical path length and modulating the
intensity of the optical signal. A MEMS-based pressure
2
transducer has the potential to meet the demands of high
temperature flow measurement through the use of high
melting temperature components, such as sapphire and
platinum, and fiber optics that facilitate the placement of
necessary electronics remotely with respect to the physical
testing location [5]. MEMS-based diaphragms, due to their
small length scale and high stiffness, also provide the
bandwidth and sensitivity characteristics suitable for the
measurement of flow instability and shock wave/boundary
layer interaction, with the ancillary benefit of enhanced spatial
resolution.
In the design of such fiber optic sensors, the nominal optical
path length is a critical parameter. The sensitivity and
linearity of the sensor are a strong function of the parameter,
and so an understanding of the underlying mechanism
governing intensity modulation is necessary to determine the
path length that will optimize device performance. Previous
work has attempted to pose a suitable mathematical model for
this mechanism, with the particular application of fiber optic
sensor design in mind [6]-[7]. The model is formulated only
for fiber optics of the step-index type, and so it is shown here
that the predictive capability is sufficiently inaccurate for
modern, graded-index fibers that an experimental
characterization may be preferential.
The purpose of this paper is to delineate an experimental
investigation of the so-called ‘opto-mechanical’ transduction
mechanism in support of the development of a lowtemperature, intensity-modulated fiber optic lever pressure
sensor of the single transmit/receive fiber variation. The
experience gained from this experimental investigation, and
subsequent sensor construction, will guide the development of
a high temperature counterpart suitable for measurement at
elevated temperatures.
II. EXPERIMENTAL PROCEDURE
A. Test Platform
The opto-mechanical transduction mechanism is
characterized experimentally by aligning the fiber of interest
with a reflective surface and monitoring the re-transmitted
optical power as the optical path length is altered
systematically. A test platform has been devised by the
Fig. 2. Traverse schematic depicting the means by which the optical
path length is controlled.
Interdisciplinary Microsystems Group (IMG) at the University
of Florida to realize a systematic path length variation by way
of a nano-positioning device. The fiber of interest, an
industry-standard 62.5/125 μm graded-index Silica fiber, is
clasped by a chuck that is housed within a Newport FPR-2
fiber mount and directed at a small mirror stationed on an
optical table. The fiber mount is positioned atop a Newport
441 Series linear stage controlled by a Newport 850G
Actuator attachment as illustrated in Fig. 2.
The proximal end of the mounted fiber optic is connected to
one pigtail of a Newport F-CPL-M22855 2x2 multimode
coupler that facilitates monitoring of both the transmitted and
received optical signals (Fig. 3). The remaining three pigtails
connect the Doric Lenses high brightness LED source, a
reference signal photodiode, and an output signal photodiode.
A New Focus 2001 reference photodiode monitors the input
signal to provide power fluctuation information that is used in
data post-processing. The output photodiode is a ThorLabs
DET110 device that monitors the optical signal received from
the optical lever.
The current output of the ThorLabs sensor is converted to a
voltage signal by way of a Stanford Research Systems Model
SR570 low-noise transimpedance amplifier prior to sampling
by the HP34970A integrating voltmeter. The New Focus
sensor incorporates a built-in transimpedance amplifier which
allows it to connect directly. Remote, automatic operation of
the nano-positioning device and voltmeter is realized through
a LabView control and data collection routine.
B. Data Collection Procedure
Prior to data collection the fiber distal end is always
checked for sediment and imperfections; if necessary it is
cleaved transverse to the fiber. The fiber mount affords two
degrees of rotational freedom controlled independently by
two, 100 threads-per-inch screws. Manual operation of the
positioner is used to place the mounted fiber and mirror in
close proximity for monitoring and alignment. Vertically and
horizontally mounted CCD cameras with Olympus LMPlanFl
20x objective attachments are employed to view the fiber and
its mirror reflection from roughly orthogonal observation
planes. The mount is used to adjust the fiber until it is parallel
Fig. 3. Layout of coupler connections.
3
with its reflection, as observed from both planes, ensuring an
orthogonal alignment of the fiber with respect to the reflective
surface.
Manual positioner control is used thereafter to orient the
fiber and mirror such that the fiber distal end and mirror are in
nearly direct contact. With the LED source powered and
connected appropriately to the coupler, the LabView routine
that simultaneously controls the optical path length and
collects data from the photodiodes can be executed.
Data has been collected at 20 μm intervals along the optical
path (i.e. the positioner steps at intervals of 10 μm; the
distance from fiber distal end to mirror is one half the total
path length). The positioner steps from the initial point out to
1 mm (2mm optical path length) where negligible optical
power is re-transmitted via the transduction mechanism under
investigation. The data is used to elucidate the optical power
coupling sensitivity as a function of the optical path and,
thereafter, investigate the linearity characteristics.
In collecting all data sets, 30 averages of both the reference
and output signals have been taken at each point along the
optical path, and uncertainties estimated. A black shroud was
also draped over the test platform to mitigate the transmission
of ambient optical power through the fiber network.
C. Data Post-Processing
The reference photodiode collects input optical power data
that is used to normalize the output signal. The input power is
an important parameter to monitor as the source LED output
tends to drift slightly as a function of time. This warrants a
need to sample the input power each time the output signal is
sampled, at each step in the traverse of the total optical path
considered. Doing so allows the output signal to be
appropriately normalized by the input power corresponding to
the time at which it was sampled.
Data processing has addressed two key issues, back
reflections and data scaling for normalization purposes. Back
reflections of the optical input are phenomena internal to the
fiber network, resulting in a quantifiable bias of the output
signal. This bias is a function of the input optical power,
which may vary across experiments, and therefore must be
uniquely quantified each time data is collected. At path
lengths approaching 2 mm, negligible power is re-transmitted
via the transduction mechanism, meaning the signal level for
large paths must equate to the bias offset. Subtracting this
offset from the data appropriately corrects for the bias.
After back reflections have been accounted for, the
reference and output powers must be corrected to yield
normalized data of physical significance. The uncorrected
reference and output photodiode responses are of disparate
scales for two distinct reasons: (1) the devices are of varying
spectral responsivity, and (2) the reference and output signals
navigate separate paths through the fiber network, incurring
separate and distinct power losses along the way. Using a
known response ratio between the two devices and known
power losses through each pigtail of the coupler permits each
signal to be appropriately scaled to the total input power. In
other words, the reported output, or re-transmitted optical
power is given as a fraction of the total input power, where the
fraction indicates only how much power was lost via the
intensity modulation mechanism. Reporting the data in such
fashion also facilitates comparison with the model posed in
[6].
The ultimate objective of investigating the re-transmitted
power as a function of the optical path is to elucidate the
sensitivity characteristics of the modulation mechanism. To
delineate the sensitivity of the mechanism as a function of the
optical path, the intensity modulation data is differentiated
using a numeric central differencing scheme. Both the
intensity modulation and differentiated sensitivity data are
reported for graphical clarity.
III. RESULTS
A. Sensitivity
Experiment indicates a maximum sensitivity of -0.078
normalized power units per normalized optical path length
unit (W-m/W-m), or -2.51E-3 normalized power units per
micrometer (W/W-μm). The maximum sensitivity occurs at
an optical path length of 140 μm (i.e. the distance from fiber
distal end to mirror is 70 μm). The re-transmitted power as a
function of path length (Fig. 4) and sensitivity (Fig. 5) are
delineated alongside the model posed in [6] for comparative
purposes.
Investigating Fig. 4 reveals the discrepancy between model
and experiment, particularly for the small optical paths at
which a physical sensor might operate. The model suggests
that all input optical power should be coupled back into fiber
for a diminishingly small path length, whereas experiment
indicates otherwise. At this path length, approximately 80%
of the input power was re-transmitted in practice. Though
model and theory have focused on discrete fiber variations, the
fundamental theory suggests that both should couple all input
Fig. 4. Normalized power re-transmission as a function of normalized
optical path. The re-transmitted power is scaled by the total input
power, whereas the path length is scaled by the core radius of the fiber
optic, 31.25 μm.
4
Fig. 5. Mechanism sensitivity as a function of the normalized optical
path. The reported sensitivity is in units of normalized optical power per
normalized path length. The figure indicates that model and experiment
are in disagreement as to both the magnitude and location of maximum
sensitivity.
power under this condition. The observed deviation from
theory is most likely due to imperfections in the actual fiber
optic and mirror used, which facilitated power loss via an
optical beam that deviated from the ideal. Furthermore, slight
alignment error may have resulted in optical power leakage.
From both figures it is evident that model and experiment
disagree as to the mechanism sensitivity distribution. The
model fails to accurately predict either the magnitude of
maximum sensitivity or corresponding optical path. Likely,
the most significant factor contributing to the discrepancy is
the behavioral variance between the graded-index fiber optic
employed in this characterization and a step-index fiber for
which the model was originally formulated. Non-idealities in
fiber and beam may also have contributed, as indicated by the
power coupling discrepancy for a diminishingly small optical
path noted above.
In the design of a fiber optic lever sensor, the nominal
optical path length is a critical parameter of which the device
performance and package layout are a strong function.
Consequently, accurate information regarding the sensitivity
distribution of the transduction mechanism is paramount to a
design that effectively realizes the full operational potential of
the sensor. The reported data suggests that the model posed in
[6] may insufficiently predict the sensitivity distribution for a
graded-index fiber optic, necessitating an experimental
characterization of the mechanism, for design purposes, when
this fiber genre is employed.
B. Linearity
The reported results indicate that the nonlinear transduction
mechanism exhibits approximately linear behavior within a
small region encompassing relatively short optical paths (Fig.
6). The point of maximum sensitivity lies near the center of
this region, meaning an accurate linear approximation can be
made for operation about this location. For the sensors being
Fig. 6. Region of approximated linearity. The region encompasses
small optical path lengths and is roughly centered about the point of
maximum sensitivity.
developed by IMG at the University of Florida, full-scale
deflections of the mechanical diaphragm on the order of 1-2
μm are expected about the operating point (i.e. the maximum
change in the optical path is 4 μm). This paper suffices to
show that the linearity error does not exceed 0.06% over a 40
μm range of optical path lengths centered about the point of
maximum sensitivity (Fig. 7). Consequently, a physical
sensor operating within this envelope can be expected to
perform in highly-linear fashion.
IV. CONCLUSION
A maximum sensitivity of -2.51E-3 W/W-μm (normalized
power units per micrometer) is reported for the optomechanical transduction mechanism of a fiber optic lever of
the single transmit and receive fiber variation, where an
Fig. 7. Linear approximation about the point of maximum sensitivity.
The surrounding points bound an optical path length region of 40 μm in
which the linearity error does not exceed 0.06%.
5
industry-standard multimode telecommunications fiber
(62.5/125 μm Silica) has been used. The optical path length
corresponding to this sensitivity is shown to lie in a region of
high linearity, where the error does not exceed 0.06%. A
model posed in previous work is referenced and delineated
alongside experimental data for comparative means. The
comparison indicates that the model may be insufficiently
accurate for the purposes of designing a fiber optic lever
sensor as it fails to accurately predict the mechanism
sensitivity distribution.
The experimental characterization presented in this paper
will ultimately be used to guide the design of a MEMS-based
optical pressure transducer for high temperature measurement
applications. Further work is needed to explore the behavior
of the opto-mechanical transduction mechanism as a function
of the transmit/receive fiber properties. Sapphire fiber optics
have been proposed for use in high temperature sensors due to
their favorable melting point, with respect to traditional fiber
optic materials such as Silica. The performance parameters of
these fibers, such as the numerical aperture, differ
significantly from those of the fiber reported in this paper, and
necessitate a need for future experiments to employ them in a
similar characterization. Doing so will provide a more
accurate stream of information that designers may use to
develop higher performance devices.
ACKNOWLEDGMENT
Author Chase Coffman would like to heartily thank John
Griffin of IMG for his assistance in the development of a
LabView code for control of the nano-positioner and
voltmeter.
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
A. J. Zuckerwar and F. W. Cuomo. Fiber optic sensor for measurement
of pressure fluctuations at high temperatures. IEEE CH2762-3/89, 1989,
pp. 503-509.
A. J. Zuckerwar, F. W. Cuomo, T. D. Nguyen, S. A. Rizzi, and S. A.
Clevenson. High-temperature fiber-optic lever microphone. J. Acous.
Soc. Am., vol. 97 (6), 1995, pp. 3605-3616.
N. Bilaniuk. Optical Microphone Transduction Techniques. Applied
Acoustics, vol. 50, 1997, pp. 35-63.
W. Pulliam and J. Schetz. Development of Fiber Optic Aerodynamic
Sensors for High Reynolds Number Supersonic Flows. AIAA Paper
2001-0245, 2001.
K. Kadirvel, R. Taylor, S. Horowitz, L. Hunt, M. Sheplak, and T.
Nishida. Design and Characterization of MEMS Optical Microphone
for Aeroacoustic Measurement. AIAA Paper 2004-1030, 2004.
G. He and F. W. Cuomo. A Light Intensity Function Suitable for
Multimode Fiber-Optic Sensors. Journal of Lightwave Technology, vol.
9 (4), 1991, pp. 545-551.
G. He and F. W. Cuomo. Displacement Response, Detection Limit, and
Dynamic Range of Fiber-Optic Lever Sensors. Journal of Lightwave
Technology, vol. 9 (11), 1991, pp. 1618-1625.