NASA/CR--2003-212100

Micro-Optical Distributed Sensors

Aero Propulsion Applications

S. Arnold and V. Otugen

Polytechnic University, Brooklyn, New York

for

January 2003
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NASA/CR--2003-212100

Micro-Optical Distributed Sensors

Aero Propulsion Applications

for

S. Arnold and V. Otugen

Polytechnic University, Brooklyn, New York

Prepared under Grant NAG3-2679

National Aeronautics and

Space Administration

Glenn Research Center

January 2003
The Propulsion and Power Program at

NASA Glenn Research Center sponsored this work.

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

Available from

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Available electronically at http://gltrs.grc.nasa.gov
Micro-Optical Distributed Sensors for Aero Propulsion Applications

S. Arnold and V. Otugen

Polytechnic University

Brooklyn, New York 11201

Summary

The objective of this research is to develop micro-opto-mechanical system (MOMS)-based

sensors for time- and space-resolved measurements of flow properties in aerodynamics

applications. The measurement technique we propose uses optical resonances in dielectric microspheres that can be excited by radiation tunneling from optical fibers. It exploits the tunnelinginduced and morphology-dependent shifts in the resonant frequencies. The shift in the resonant

frequency is dependent on the size, shape and index of refraction of the micro-sphere. A physical

change in the environment surrounding a micro-bead can change one or more of these properties

of the sphere thereby causing a shift in frequency of resonance. The change of the resonance

frequency can be detected with high resolution by scanning a frequency-tunable laser that is

coupled into the fiber and observing the transmission spectrum at the output of the fiber. It is

expected that, in the future, the measurement concept will lead to a system of distributed microsensors providing spatial data resolved in time and space. The present project focuses on the

development and demonstration of temperature sensors using the morphology-dependent optical

resonances although in the latter part of the work, we will also develop a pressure sensor. During

the period covered in this report, the optical and electronic equipment necessary for the

experimental work was assembled and the experimental setup was designed for the single sensor

temperature measurements. Software was developed for real-time tracking of the optical

resonance shifts. Some preliminary experiments were also carried out to detect temperature using

a single bead in a water bath.

Introduction

We are interested in using resonances or, the so-called Whispering Gallery Modes (WGM),

of a microsphere as a sensor for properties of the local environment. The resonances are of

extremely high Q and are caused by the entrapment of photons due to total internal reflection.

The light orbits the microsphere as illustrated in Fig. 1 (i.e. wave point of view). Each orbit has a

constant number of wavelengths 1, similar to orbits within a Bohr atom. Consequently the

circumference will be equal to a whole number of internal wavelengths,

2rca _ 1 k/n (1)

where n is the refractive index of the sphere and a is its radius. As a consequence of Eq. (1), any

change in the size of the particle or its refractive index leads to a certain resonance shift. In the

case of temperature, the sensitivity can be extremely large. In quartz microspheres of submillimeter size Q values of 101° have been reported. The thermal expansion coefficient of quartz

is _10 6 K1, which translates into a sensitivity for temperature change of 10 5 K. Considering

that fused silica has a particularly small thermal expansion coefficient, the extreme sensitivity to

temperature is surprising. It is clearly a consequence of the large quality factor associated with

the resonances. To drive a WGM, we utilize a particular discovery made at the Polytechnic.

NASA/CR--2003-212100 1
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Figure 1. Stimulation of a Microsphere Resonance by a naked optical fiber.

A fiber etched to its core with its evanescent field overlapping the evanescent field of the

spherical mode can stimulate a mode (as depicted in Fig. 1). Therefore, it is possible to couple

light into an optical microbead from a bare fiber that is touching it if the frequency of the light

traveling through the fiber matches one of the resonances of the bead. Thus, one can interrogate

the resonances of the bead by scanning a tunable laser that is coupled into the fiber. The location

of the resonances can be monitored by observing the dips in the transmitted light through the

fiber by means of a photo detector.

Theory

The electrical field within a dielectric particle satisfies the vector Helmholtz equation of the

form, 1,2

V2/_ + k2/_ = 0 (2)

where k = nk o is the wave vector, where ko is the vacuum wave vector and n is the refractive

index. In the spherical coordinates, the equation becomes

C_2 /_2 _

--r/_ +k2/_ 0 (3)

FC_F2

- r--TE =

where /_ is the angular momentum operator/_ = if x V. We note the commutation relationship

between/_ and £2. The solution of the field therefore can be written as

_p_ L_ (4)

r

NASA/CR--2003-212100 2
where q_ is a scalar function, so that one only needs to solve following one-dimensional equation.

C_2q/ £2

c_r2 r2 _+nZko2_ = 0 (5)

For a sphere, the refractive index can be described as:

(6)

where a is the radius of the spherical bead, n is the absolute refractive index of the bead material,

n is the refractive index of surrounding material. For example, the approximate refraction index

for silica, air, and water are 1.53, 1, and 1.33, respectively.

The solutions of Eq. (5) with boundary conditions (6) are the Ricatti-Bessel functions. These

functions correspond to the electrical mode inside the sphere. The resonance frequencies are

related to the complex eigenvalues {ko}. The complex number indicates the light leakage from

the sphere. For a sphere with a radius around several hundred microns, a geometric picture of the

internal reflection can lead to an approximation resonance solution 3 given earlier by

Eq. (1) for wavelength

12 = 2 _cna

where I is an integral number and is the same as the quantum number of angular momentum, /_2.

The leaking rate is very small for an isolated sphere. However, the tunneling loss increases

significantly when the sphere is put very close to a fiber. Conversely, light in fiber can also

tunnel into the sphere. The tunneling effect becomes much stronger when the light matches the

electromagnetic mode of the sphere. Therefore we expect to see resonant absorption by the

sphere at the wavelengths given by Eq. (1).

When a temperature change induces a change in the refractive index and the radius of the

bead, the resonant wavelength also changes. From Eq. (1)

AZ A(n0a) Aa An 0

- - + : + )Ar

Z noa a n o

(7)

1 A%

AT - (8)

where, ct is the thermal expansion coefficient, 13is the thermal coefficient of refractive index, and

T is temperature of the bead. The thermal expansion coefficient for silica is ct = 5.5E-7 °C 1

while the thermal coefficient of refractive index is 13= 8.53 E-6 °C _. Most other glass materials

have smaller thermal coefficient of refractive index than silica.

NASA/CR--2003-212100 3
We note that if the micro bead shape is not spherical, its resonance wavelengths cannot be

described by the simple relation in Eq. (1). For a bead with a quadmpole shape 4 it is defined as

Yl=Yl

Yl = Yl S

(9)

2 rmax -- rmin

where, e is the small deviation coefficient, e=--

3a/-5 a

deviates from the corresponding sphere resonant position by

• The new resonant position

z_ = z o 1+ 1 -- (10)

l(l + 1)

where m = -/, .... l, and )_0is the quantum number/-related resonant wavelength of sphere with

radius a, determined by Eq. (1).

Progress

During the initial period of February 1 to September 30, 2002, an experimental set up was

designed and manufactured and software was developed for automatic data acquisition and

processing to track optical resonance frequency shifts in micro-beads in response to changes

their surrounding. A post-doctoral fellow, Dr. Jason Guan has come aboard and begun to

assemble a highly sensitive precision spectrometer with which to measure the physical properties

of the microsphere resonances under temperature perturbations.

Experimental Setup

We designed an experiment to measure the resonant frequency shift and infer the temperature

from that shift. A schematic of the experimental setup is shown in Fig. 2. The set up consists of a

laser source, operating around 1306 nm wavelength and associated optical, electronic and

mechanical elements including mounts, controller, etc. The laser controller can be driven by an

external voltage signal. The laser beam is coupled into a single mode fiber (Coming SMF-28

single mode fiber) using a Newport single mode coupler F-1015LD. Light in the fiber travels

only inside the core. The core diameter of SMF-28 single mode fiber is 8 microns. The details of

the test section is shown in Fig. 3. In the test section, a certain length of the fiber is stripped of its

cladding and the core fiber is etched to a diameter of about 4 to 5 _tm. A silica bead is made by

heating fiber in a micro-torch. The diameter of the spheroid-shaped bead ranges from 150 to

400 _tm. The bead is placed such that it touches the bare fiber around the bead's maximum

diameter. A photograph of this sensor arrangement is shown in Fig. 4. The optical sensor is

placed in a small water bath along with a thermistor as shown in Fig. 3. The Thermometrics

micro-thermistor MC65F103A is used to monitor the temperature change. We use operational

amplifier to make a constant current source. The voltage between two legs of the thermistor is

proportional to the resistance, which is a strong function of temperature.

NASA/CR--2003-212100 4
An InGaAs photodiode detector measures the transmission laser light intensity at the end of

the fiber. The voltage output of the photodiode is fed into a 16-bit analog-to-digital converter

(National Instruments PCI 6036E DAQ card) that is connected to a personal computer that

controls the data acquisition and performs the real time analysis. We developed a Labview-based

software to acquire and analyze the data.

A function generator (HP 3325A) provides input to the laser diode controller and the DAQ

card during the experiment. It provides a ramp function to the laser diode controller and square

signal to trigger input of the DAQ card in order to synchronize the data acquisition with the

scanning of the laser. The ramp signal allows for a linear increase of the laser current with time.

A transmission spectrum is obtained for each ramp cycle. The Labview software is designed to

follow the absorption resonance, which indicates the sphere property and thus contains the

environmental change information. In our preliminary measurements, the sensor is immersed in

water in order to avoid rapid changes in temperature of the environment of the bead sensor.

The thermal relaxation time of the small water bath is in the order of several seconds whereas

this time is in the order of a millisecond for the sensor bead itself. In the initial development of

the measurement technique, we would like to first test the system and the associated software in

a slow time-varying environment. In the future, we will progressively increase the frequency of

temperature variations and track the resonances in order to make fast and accurate temperature

measurements.

3

ii

5

:::::::::::

Figure 2. Experimental setup: 1-Function generator; 2-Laser diode controller; 3-DFB laser

diode; 4-Single mode fiber coupler; 5-Test section; 6-Photodiode; 7-Resistance-Voltage

converter; 8-National Instrument DAQ card; 9-Personal computer.

NASA/CR--2003-212100 5
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Figure 3. Top view of test section; 1-Fiber; 2-Fiber holder; 3-Etched section of fiber; 4-Silica

bead; 5-Thermistor; 6-InGaAs Photodiode; 7-Glass cell; 8-Water bath.

Figure 4. Photo of MOMS sensor: Fiber diameter -5 _tm and sphere diameter -350 _tm.

NASA/CR--2003-212100 6
Choice of Laser Source

It is conventional in high-resolution spectroscopy to use a semiconductor laser having an

electro-mechanically tuned external cavity. Since we wanted our system to be both miniature and

fast, we substituted this by an electronically-tuned Distributed Feedback Laser (DFB). This

choice not only allows for much more rapid tuning, but also anticipates an integrated optics

solution. DFB lasers are of a single heterojunction type with a grating imprinted within their

structure. This constrains the laser to operate at a single frequency with a linewidth of-30 MHz.

The frequency of the laser can be rapidly modulated (>MHz) by perturbing electronic state

populations using the drive current. We use an ILX Lightwave LDC-3714B laser diode

controller and Mitsubishi ML725B11F DFB laser diode in our setup. The laser controller can be

driven by an external voltage signal. A Newport F-1015LD single-mode fiber coupler allows us

to focus energy into a single-mode fiber.

The laser operates around 1306 nm wavelength. This is a wavelength widely used in the

telecommunications industry. Thus we can find suitable and economic laser diode and associated

optical elements fairly easily.

Microsphere and Eroded Fiber Fabrication

We use bare single mode fiber (Coming SMF-28) as our sphere material. The microsphere is

formed by using a micro-torch to melt the fiber end. As a result, a spherical droplet forms at the

end of the fiber. The sphere size is controlled from the amount of melted glass (i.e. heating time).

A typical sphere is shown above in Fig. 4. In the figure, the thin line is the image of the 4_m

diameter etched fiber that is used to excite WGM's of the sphere. The bare fiber is fabricated

through chemical etching with concentrated hydrofluoric acid over a 12 mm length. The fiber is

etched into the core. Although the un-etched core is 8 _tm in diameter, the diameter of the etched

core is - 4 to 5 _tm.

Alternate Measurement Cell Design

The preliminary measurements that will be described later in the report were carried out

using the test cell arrangement shown in Fig. 3. However, an alternate cell is also developed and

is currently being tested in connection with measuring the effect of temperature, stress and other

environmental influences on the microsphere resonances. Figure 5 shows this cell design.

3mm _ -_x - Open at top

Eroded Inside ' '_Y / To Computer

Optical _

I" -- Flow Out T

TE Cooler Flow In [___l i x Photodetector

10 mm (Inside) Telemici°sc°pe'_)-_

Figure 5. Cell for controlling the environment of the microsphere.

NASA/CR--2003-212100 7
In this design, the enclosure entraps air (rather than water) with inlet and outlet ports. The cell is

cooled at the bottom. We expect that the temperature of the air will be tightly controlled with this

arrangement.

Frequency Tracking Software

The 16-bit DAQ card is installed in the personal computer as the interface for analog/digital

converting. An external square signal is provided to the DAQ card for triggering. The voltage

signal from photodiode is sampled by the DAQ card. The voltage signal from the thermistor is

also sampled by the DAQ card. The square signal from the function generator (synchronized

with the ramp signal provided to the laser controller) is also fed to the DAQ card as trigger.

We chose to use National Instruments' Labview as our measurement platform. A Labview

program is designed to acquire the data and perform the analysis. The program front panel is

shown in Fig. 6. The function generator generates a ramp signal at a specified magnitude with a

scan frequency of 30 Hz. This scan frequency, current offset and modulation magnitude are

provided to the Labview program to calculate the scan range. The DAQ card samples 500 points

in each scan (or ramp cycle). Sampling starts whenever an external trigger signal is received.

Therefore, the actual data-record rate is smaller than the scan rate. We have about 15 Hz data

record rate when scan rate is at 30 Hz and sampling rate is 500 points per scan. A real time

spectral diagram of light intensity against laser current is plotted in the center of the display

window (Fig. 6). The Labview program also gives the option to display all results in light

Figure 6. Labview program front panel.

NASA/CR--2003-212100 8
wavelength since the laser wavelength is uniquely determined by the driving laser current. There

are several resonant absorptions in our scan range, each of which corresponds to an

electromagnetic resonant mode of the bead. We use an algorithm that fits a quadratic polynomial

to sequential groups of data points representing a dip (for example 10 points in Fig. 6) to

determine the resonance positions in the spectrum. The software locks into a simple resonance

by finding the minimum shift between two consecutive scans. The scan is fast enough so that the

resonance shift between two consecutive scans does not exceed the free spectral range

(overtaking of one resonance dip the next one by a large frequency shift). Therefore, in this

mode, as long as the temperature driven resonance shifts are slower than the data acquisition and

analysis speed, resonance shifts larger than the free spectral range can be easily measured.

A diagram of resonance positions versus time is plotted at the right of the spectral diagram.

The program also samples the resistance of thermistor after each current scan. The temperature is

calculated from the resistance. Temperatures versus time are plotted in the low right corner. The

spectrum, the resonance and the temperature can be saved at any time.

Calibration of the Laser Wavelength

We calibrate the DFB laser wavelength by using Burleigh Instruments wavemeter WA-1150.

Figure 7 shows the relation between the wavelength of the DFB laser and the driving current.

The black symbols are the measured wavelength at each driving current. The solid red line is a fit

function using a second order polynomial as follows

)_ = 1306.30205 + 0.00465 1 + 0.0000566 12 (11)

where, the wavelength, )_, is in nanometers and the current, I, is in mA. The accuracy of

wavelength by fitting is better than 0.001 nm, which for silica sphere represents 0.08 °C error in

temperature measurement.

A

E

ee130 6.55 -

130 6.50 -

130 6.45 -

130 6.40 -

1306.35-

1'0 l'S =.0 ='s 3'0 3's 40

C urrent (mA)

Figure 7. Laser wavelength versus current.

NASA/CR--2003-212100 9
Results'

We carried out measurements in water using the test section of Fig. 3. The glass cavity filled

with water. The thermistor is placed near the sphere to monitor the temperature of water as

shown in Fig 3. The bead, the fiber, and the thermistor are all immerse in the water. The bead

and fiber touch each other as shown. The function generator generates ramp signal at 30 Hz

repetition rate. 500 points of the photodiode are sampled at each ramp cycle. Sampling starts

whenever an external trigger signal is received. As discussed before, the actual measurement rate

is 15 Hz.

A typical scan spectrum is shown in Fig. 8. There are usually several resonances in one

spectrum. The Labview program locks the cursor to one resonance position as indicated by the

arrow. The resonance position is related to the bead size, shape and index of refraction. A slight

change in bead temperature can induce significant change in the position of the particular

resonance. Thus, when we measure the shift in the resonance position, we can infer the change in

temperature provided that there are no additional changes in the environmental conditions other

than temperature. During the experiment, we heat the air around the test section with a heat gun

thereby causing a change in the temperature of water surrounding the sensor bead. We

independently measure the temperature of the water using the thermistor during the experiments.

The results of a typical experiment are shown in Fig. 9. Here, the temperature measured by the

optical sensor is compared to that form the thermistor. The optical measurement is made by

tracking the shift of the resonance dip identified in Fig. 8. The shift is converted to temperature

using Eq. (8).

2.0

1.5

d

V

1.0

ee'-

0.5

0.0 1's 2'o 2's 3'o 3's

Current (mA)

Figure 8. Transmission spectrum in a typical run.

NASA/CR--2003-212100 10
0 10 20 30

34 34

A

03

"10

O.

E

32 32

30 30

28 28

A

03

"10

8.

E

26 26 o 1'o 2'o 3'o

t (s)

Figure 9. Measured temperature against time: Black dot by bead and red line by thermistor.

Discussion

The temperature increases by a total amount of - 5.5 °C during heating according to the

thermistor measurement. The resonance shifts by a total amount of - 10 mA at the end of the

heating. The corresponding temperature change calculated from Eq. (8) is about 6.7 °C. Figure 9

shows that the two temperatures match each other reasonably well although there are slight

mismatches between them as time progresses. However, we note that there is no significant

relative bias between the two types of measurements. The mismatch is believed to be primary

due to non-homogeneous heating of the water in the enclosure by the heat gun, particularly

considering that the thermal relaxation time of the water in the enclosure is several tens of

seconds. It is noted that the contribution to resonant shift comes primary from change in

refractive index for silica material (rather than the diameter change of the bead) since silica has

very small thermal expansion coefficient. It turns out that silica is the best material to sense the

temperature due to large thermal coefficient of refractive index.

We call the "free spectral range" (FSR) the wavelength jump between two consecutive

resonances. Typically, FSR determines the upper bound of the dynamic range of the

measurement unless the measurement is fast enough to lock onto a specific resonance and track

it past a the FSR. An approximate calculation of the free spectral range can be accomplished

using Eq. (10) for the quadrupole shape particle as follows

L m - Lm_ 1 (2m - 1)e L2

- = (2m - 1)e 2 2 (12)

L 0 1(1 + 1) 4_z a

We can also obtain the FSR from the measured spectrum. For example, in Fig. 8 the two adjacent

resonances are separated by about 3 mA, which corresponds to 0.024 nm in wavelength and

about 2 °C of temperature change for the silica bead. To ensure that the software successfully

NASA/CR--2003-212100 11
locks on one resonance and tracks its shift, the measurement rate should be high enough so that

resonance shift is less than half of the free spectrum or (1 °C in the present setup). At 15 Hz

measurement rate, we can successfully measure temperature change rates up to about 10 °C/s.

Again, these numbers are for the present setup and software which is only a preliminary effort.

We expect to improve these numbers significantly in the near future by both hardware and

software. Also, we will carry out temperature measurements with the improved test section (cell)

that has provides a much more uniform temperature distribution within it.

References

1. J.D. Jackson, "Classical Electrodynaxnics," John Wiley & Sons, Inc, 3rd edition, p. 380 (1998).

2. B.E. Little, J.P. Laine, and H.A. Haus, J. Lightwave Technology, vol. 17, no. 4, p. 704 (1999).

3. R.K. Chang and A.J. Caxnpilla, "Optical processes in microcavities," World Scientific Publishing Co.

Pte. Ltd., p. 10 (1996).

4. H.M. Lai, P.T. Leung, and K. Young, Physical Review A, 41, no.9, p. 5187 (1990).

NASA/CR--2003-212100 12

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Micro-Optical Distributed Sensors for Aero Propulsion Applications

6. AUTHOR(S)

S. Arnold and V. Otugen

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

Polytechnic University

Six Metro Tech Center

Brooklyn, New York 11201

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

WBS-22-708-87-23

NAG3-2679

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AGENCY REPORT NUMBER

NASA CR--2003- 212100

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organization code 5520, 216-433-3754.

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13. ABSTRACT (Maximum 200 words)

The objective of this research is to develop micro-opto-mechanical system (MOMS)-based sensors for time- and space-resolved

measurements of flow properties in aerodynamics applications. The measurement technique we propose uses optical resonances in

dielectric micro-spheres that can be excited by radiation tunneling from optical fibers. It exploits the tunneling-induced and morphology-dependent shifts in the resonant frequencies. The shift in the resonant frequency is dependent on the size, shape, and index of

refraction of the micro-sphere. A physical change in the environment surrounding a micro-bead can change one or more of these

properties of the sphere thereby causing a shift in frequency of resonance. The change of the resonance frequency can be detected

with high resolution by scanning a frequency-tunable laser that is coupled into the fiber and observing the transmission spectrum at

the output of the fiber. It is expected that, in the future, the measurement concept will lead to a system of distributed micro-sensors

providing spatial data resolved in time and space. The present project focuses on the development and demonstration of temperature

sensors using the morphology-dependent optical resonances although in the latter part of the work, we will also develop a pressure

sensor. During the period covered in this report, the optical and electronic equipment necessary for the experimental work was

assembled and the experimental setup was designed for the single sensor temperature measurements. Software was developed for

real-time tracking of the optical resonance shifts. Some preliminary experiments were also carried out to detect temperature using a

single bead in a water bath.

14. SUBJECT TERMS

Temperature measurements; MOMS sensors

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