Seppo Honkanen

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Photonic Sensors

• Seppo Honkanen
• OPTI 507 - Guest Lecture
• October 6, 2005

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Overview

• Optical Fiber Sensors
• - Fiber grating based sensors
• Fiber grating fabrication
• Sensing applications
• - Fiber gyroscope
• Basic principles
• Integrated gyro chip
• Surface Plasmon Resonance (SPR)
• Integrated Optic Biosensor

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Single-Mode Fiber
k0ncore gt ß (k0neff) gt k0nclad
Typical parameters of a single-mode fiber Core
radius a 4 µm ? (ncore - nclad)/ ncore
0.005 (ncore 1.45)
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Fiber Based Devices

• Inherently low insertion loss (splice loss)
• Economical (SMF-28 / meter)
• Mature fabrication
• Mechanically rugged
• Immunity to EM interference
• Tunable in wavelength

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Giber Grating Theory(first order)
Conservation of momentum
neff
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Fiber Grating Fabrication

• Hill, et al (1978) self-induced gratings
• Absorption of UV light introduces spectral
  artifacts (a, n) at telecom wavelengths through
  various physical mechanisms
• Common UV sources
• excimer lasers (pulsed, ArF, 193 nm or KrF, 248
  nm)...high intensity, poor spatial coherence
• frequency-doubled argon ion lasers (cw, 244-248
  nm)...lower intensity, very high spatial coherence

K.O. Hill et al., Photosensitivity in optical
fiber waveguides Application to reflection
filter fabrication, Appl. Phys. Lett. 32,
647-649 (1978).
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Fiber Grating Fabrication

• Interferometric photowriting setup

Amplitude Splitting
Lloyd Mirror

• Requires very high coherence length (amplitude
  splitting) or spatial coherence across beam
  width (Lloyd mirror)
• Single frequency gratings only (no chirp)

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Fiber Grating Fabrication

• Phase mask photowriting setup

• Low - coherence (excimer) UV sources OK
• Arbitrary ?(z) profiles possible, depends only
  on the mask

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Grating Response
Linear
Logarithmic
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Environmental Sensitivity

• Good for sensors!
• Bad for telecom devices (stability, packaging)
• Bragg wavelength is dependent on strain and
  temperature

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Pressure and Temperature Sensitivity
Pressure
Temperature
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Measurement Scheme Example
Single Bragg grating Strain sensor
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Fiber Optic Gyroscope (Gyro)

• Introduction
• Operation principle
• Fiber Gyro types
• Interferometric fiber Gyro
• Ring resonator Gyro
• Integrated optic gyro

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Applications Navigation and Robotics
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Applications requirements
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Optical gyroscope principle
System at rest
M
Light pass point M every
System rotating
Rotation cause a delay in co-propagating light
Demonstrated by Sagnac in 1913
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Optical gyroscope principle
System rotating
Rotation cause a delay in co-propagating light
So the phase difference between clockwise (CW)
and counter-clockwise (CCW) light is
With N loops
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Types of Optical gyros
Interferometric
Non-Interferometric
Resonant Fiber Optic Gyros (R FOG)
Ring Laser Gyros (RLG)
Fiber Optic Gyros (FOG)
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Fiber Optic Gyros (FOG)
The rotation causes interference between the CW
and CCW waves. With a perfect coupler the
intensity at the detector is
Where
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FOG Open loop configuration
- Moving the point of operation to the maximum
sensitivity. - However, the sensitivity decrease
with increasing rotation rates
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FOG Closed loop configuration
In a closed-loop configuration a feedback
mechanism maintains the open-loop signal at zero
by compensating for the Sagnac phase shift by
introducing an equal and opposite phase shift
within the sensing loop.
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Ring Resonator Gyroscope

• Rotating a Ring Resonator

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Fiber Ring Resonator(Radius 1 m, Loss 0.2db/km)
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Ring Resonator Gyroscope
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Integrated Optical Active Ring Resonator ?
Passive Ring Resonator
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Ion Exchanged Glass Waveguides

• Waveguide geometry is defined by openings in an
  oxidized metal mask
• The sample is placed in a melt containing silver
  ions
• Ag is driven into the substrate by a chemical
  potential gradient, and Na is released into the
  melt to preserve charge neutrality
• Ag is distributed within the substrate by
  thermal diffusion

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Surface Plasmon Resonance (SPR)

• Surface plasmons are excited (TM-polarized light)
  by the evanescent wave of the incoming light.
  The wavenumber along the surface must match with
  that of the surface plasmon.
• At resonance a sharp dip in reflection is
  observed.
• In SPR sensors thin metal films are used to give
  access to the other side and change the resonance
  condition (i.e. angle).

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SPR Sensor
 
L light source D photodiode array P prism S
sensor surface F flow cell
The two dark lines in the reflected beam
projected on to the detector symbolise the light
intensity drop following the resonance
phenomenon t1 The situation before binding of
antigens to the antibodies on the surface t2 The
situation after binding.
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Integrated Optical Biosensor

• A guided-wave optics approach to develop
  integrated optical chemical and biological
  sensors.
• The waveguide configuration combines an
  ion-exchanged glass channel structure and a
  sol-gel cladding film the waveguide is exposed
  to analytes through openings in the sol-gel
  cladding.

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Results

• With a 5 µM of horse heart cytochrome-c dissolved
  in phosphate buffer solution at the waveguide
  superstrate, a half monolayer is expect to form
  on a hydrophylic silica surface (data from
  literature).
• By using a wavelength tuned to cyt c absorption
  band (532 nm), we measured a decrease of 30 in
  the device transmission.
• For a control signal (633 nm) outside the cyt c
  absorption band, the transmission remained
  unchanged when propagating through the same
  channel.
• Demonstrated ultra-sensitive detection for a
  fraction of protein monolayer adsorbed to the
  waveguide surface.

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• Ultimate optical sensitivity for probing
  biomolecular films is obtained when the
  analytical optical beam is confined into a
  single-mode structure as it provides for an
  enhanced optical interaction with
  surface-adsorbed analytes.
• Limit of detection, which is a critical
  characteristic of sensor devices, benefits
  directly from an enhanced sensitivity.
• In addition to sensitivity, limit of detection
  is also dictated by the noise present in the
  device signal a low background noise translates
  into a much improved limit of detection.
• For this purpose, we have designed, fabricated,
  and successfully tested integrated optical
  waveguides with two optical channels, where one
  channel is exposed to the analytes to be probed
  and the other channel is used for eliminating any
  noise present in the device (e.g. due to source
  and/or coupling fluctuations).
• Top on the right, we schematically present the
  developed dual-channel integrated optical
  waveguide sensor, and on the bottom we show
  experimental results demonstrating the much
  improved signal-to-noise ratio when operating
  under the dual-channel waveguide configuration.
• A detection limit of less than 0.1 pmol/cm2 was
  measured for a protein film composed of horse
  heart cytochrome-c molecules that were
  electrostatically adsorbed onto the waveguide
  surface from a phosphate buffer solution. The
  wavelength of the probing laser beam was chosen
  at 532 nm to match the absorption band of
  oxidized cyt c.

Dual-Channel Integrated Optical Waveguide Sensor
Improvements in Limit of Detection with the Novel
Dual-Channel Integrated Optical Waveguide Sensor