U.S. patent application number 09/983999 was filed with the patent office on 2003-05-01 for system and method for measuring physical, chemical and biological stimuli using vertical cavity surface emitting lasers with integrated tuner.
Invention is credited to Kochergin, Vladimir, Swinehart, Philip.
Application Number | 20030081875 09/983999 |
Document ID | / |
Family ID | 25530228 |
Filed Date | 2003-05-01 |
United States Patent
Application |
20030081875 |
Kind Code |
A1 |
Kochergin, Vladimir ; et
al. |
May 1, 2003 |
SYSTEM AND METHOD FOR MEASURING PHYSICAL, CHEMICAL AND BIOLOGICAL
STIMULI USING VERTICAL CAVITY SURFACE EMITTING LASERS WITH
INTEGRATED TUNER
Abstract
An optical sensor diagnostic system utilizing a tunable Vertical
Cavity Surface Emitting Laser (VCSEL) that incorporates an
integrated MEMS tuning mechanism provides variable wavelength light
into an optical fiber with improved wavelength scanning speed and
greater simplicity of construction. Sensors, such as Bragg
gratings, are disposed along the fiber in the light path. Each
sensor reflects or transmits light exhibiting a characteristic
amplitude feature with respect to wavelength, the wavelength
position of which is affected by an environmental stimulus imposed
thereon. The power of the reflected light is converted to an
electrical signal by a simple detector and monitored by circuitry
that detects changes in reflected power and provides output signals
indicative of the environmental stimulus for each sensor.
Inventors: |
Kochergin, Vladimir;
(Westerville, OH) ; Swinehart, Philip; (Columbus,
OH) |
Correspondence
Address: |
NIXON & VANDERHYE P.C.
8th Floor
1100 North Glebe Road
Arlington
VA
22201
US
|
Family ID: |
25530228 |
Appl. No.: |
09/983999 |
Filed: |
October 26, 2001 |
Current U.S.
Class: |
385/12 ;
250/227.14; 385/15 |
Current CPC
Class: |
G01N 21/7703 20130101;
G02B 6/29358 20130101; G02B 26/001 20130101; G01N 21/64 20130101;
G01N 21/39 20130101; G02B 6/42 20130101; H01S 5/18366 20130101;
G02B 6/4202 20130101; G01N 21/553 20130101; G01N 21/45 20130101;
G01N 2021/399 20130101; G02B 6/4206 20130101 |
Class at
Publication: |
385/12 ; 385/15;
250/227.14 |
International
Class: |
G02B 006/26 |
Claims
We claim:
1. An optical sensor diagnostic system, comprising: a tunable VCSEL
incorporating an integrated MEMS wavelength tuner for providing
wavelength-tunable light in response to a tuning control signal,
said tunable light being launched into an optical waveguide, said
tunable VCSEL including a movable tuning mirror and a capacitive or
optical detector for detecting the position of the movable tuning
mirror and providing feedback; at least one optical sensor,
disposed in the path of said tunable light, said at least one
sensor providing a transmitted light having at least one associated
characteristic amplitude feature selected from the group consisting
of a minimum, a maximum or a slope located at a particular
wavelength within the transmitted wavelength range, said wavelength
at each minimum, maximum or sloped transmission amplitude being
responsive to an environmental stimulus imposed upon said at least
one sensor; said tunable VCSEL individually illuminating said at
least one sensor in a wavelength range spanning said wavelength
location of said associated characteristic transmission amplitude
feature; an optical isolator, disposed in the path of said tunable
light between said tunable VCSEL and said at least one sensor, for
isolating said tunable light source from light reflected from said
at least one sensor; an optical detector, disposed in the path of
said transmitted light, for detecting said transmitted light from
said at least one sensor and for providing an electrical detection
signal indicative of the power of said transmitted light throughout
a predetermined wavelength range; a controller for providing a
variable tuning control signal to said tunable VCSEL indicative of
the desired wavelength of said tunable light; at least one
wavelength reference independent of said tuning control signal and
moveable mirror position detector disposed in the path of the
light; and a signal processor responsive to said electrical
detection signal, for detecting a wavelength defined on the
characteristic transmission amplitude feature in order to
quantitatively detect the effect on said at least one sensor due to
said environmental stimulus, changes in said wavelength at the
characteristic transmission amplitude feature caused by changes in
said environmental stimulus, and for providing a signal indicative
of said stimulus or change therein.
2. The optical sensor diagnostic system of claim 1 wherein said at
least one sensor comprises plural wavelength division multiplexed
optical sensors.
3. The optical sensor diagnostic system of claim 1 wherein said at
least one sensor comprises plural time division multiplexed optical
sensors.
4. The optical sensor diagnostic system of claim 1 wherein said at
least one sensor comprises an environment reference or compensation
sensor.
5. The optical sensor diagnostic system of claim 1 wherein said
detection and signal processor comprises a tracker, responsive to
said electrical detection signal, for adjusting a voltage or other
control signal to allow said tunable light to track static and
dynamic values of said characteristic transmission amplitude
feature for said at least one sensor, thereby providing utilization
of the control signal as the output characteristic of the physical
stimulus, making unnecessary scanning of the complete wavelength
range and greatly increasing the speed of data acquisition.
6. The optical sensor diagnostic system of claim 1 wherein said
controller comprises a modulator for modulating said voltage
control signal at a predetermined modulation frequency.
7. The optical sensor diagnostic system of claim 1 wherein said
signal processor comprises a demodulator operating at said
modulation frequency, for demodulating said electrical detection
signal and for providing a demodulated signal indicative
thereof.
8. The optical sensor diagnostic system of claim 1 wherein said
signal processor incorporates a computational element for
increasing the accuracy and precision of determining the wavelength
position of said characteristic transmission amplitude feature and
changes therein for each of said sensor.
9. The optical sensor diagnostic system of claim 1 wherein: said at
least one sensor comprises plural sensors; said controller
comprises a scanner that scans said control signal for the purpose
of causing said tunable VCSEL to scan its wavelengths across said
characteristic transmission amplitude features of said plural
sensors; and said signal processor determines, in response to said
voltage or other control signal, the wavelength of said tunable
light from the magnitude of said voltage or other control signal
and/or mirror position feedback signal and for determining which of
said plural sensors is being illuminated, thereby determining the
value of the environmental stimulus at the position of said
illuminated sensor.
10. The optical sensor diagnostic system of claim 1 wherein: said
at least one sensor comprises plural sensors; said controller
comprises a scanner that scans said control signal so as to cause
said tunable VCSEL to scan across the characteristic transmission
amplitude features of said plural sensors and for providing a
synchronization signal indicative of when said voltage control
signal begins and ends said scanning; and said signal processor
determines, in response to said synchronization signal, which of
said plural sensors is being illuminated, thereby determining
changes in said wavelength at said characteristic transmission
amplitude feature.
11. The optical sensor diagnostic system of claim 1 wherein said at
least one sensor comprises at least one fiber or planar Bragg
grating.
12. The optical sensor diagnostic system of claim 1 wherein said at
least one sensor includes at least one Bragg grating that
incorporates phase shift in its structure, said phase shift
producing a sharper maximum within said transmitted wavelength band
minimum.
13. The optical sensor diagnostic system of claim 1 wherein said at
least one sensor comprises at least one Fabry-Perot etalon.
14. The optical sensor diagnostic system of claim 1 wherein said at
least one sensor comprises at least one Surface Plasmon Resonance
structure.
15. The optical sensor diagnostic system of claim 1 wherein said at
least one sensor comprises at least one thin film or bulk material
characteristic absorber material.
16. The system of claim 15 wherein characteristic absorber material
comprises one or more vibronic, excitonic or fluorescent
materials.
17. The optical sensor diagnostic system of claim 1 wherein said
environmental stimulus comprises any combination of mechanical
stress, temperature, pressure, electrical current, electrical
field, magnetic field or chemical or biological material on said
sensor.
18. The optical sensor diagnostic system of claim 1 wherein at
least one wavelength reference, not affected by any environmental
stimulus, comprising at least one of the group of a Bragg grating,
a phase shift Bragg grating, a Fabry-Perot etalon or a
gas-containing chamber, is disposed in the optical path.
19. The optical sensor diagnostic system of claim 1 wherein the
wavelength reference comprises at least one gas-containing chamber
containing acetylene gas.
20. An optical sensor diagnostic system, comprising: a VCSEL
incorporating integrated MEMS wavelength tuner for providing
wavelength-tunable light in response to a tuning control signal,
said tunable light being launched into an optical waveguide,
wherein is provided at least one optical sensor, disposed in the
path of said tunable light, each providing a reflected light having
at least one associated characteristic amplitude feature from but,
not limited to, the group, a minimum, a maximum or a slope located
at a particular wavelength within the reflected wavelength range,
said wavelength at each minimum, maximum or sloped reflection
amplitude being responsive to an environmental stimulus imposed
upon a corresponding sensor; said tunable VCSEL for individually
illuminating each of said sensor in a wavelength range spanning
said wavelength location of said associated characteristic
reflection amplitude feature; optical detector, disposed in the
path of said reflected light, for detecting said reflected light
from each of said sensor and for providing an electrical detection
signal indicative of the power of said reflected light throughout
the appropriate wavelength range; optical circulator, disposed in
the path of said tunable light between said tunable VCSEL and said
sensor, for isolating said tunable light source from light
reflected from said sensor and directing the light to said
detector; voltage or other controller for providing a variable
tuning control signal to said tunable VCSEL indicative of the
desired wavelength of said tunable light; capacitive or optical
detector that detects the position of the movable tuning mirror and
providing feedback; and at least one wavelength reference
independent of said tuning control signal and moveable mirror
position detector disposed in the path of the light, and signal
processor responsive to said electrical detection signal, for
detecting a wavelength defined on the characteristic reflection
amplitude feature in order to quantitatively detect the effect on
said sensor due to said environmental stimulus, changes in said
wavelength at the characteristic reflection amplitude feature
caused by changes in said environmental stimulus, and for providing
a signal indicative of said stimulus or change therein for each of
said sensor.
21. The optical sensor diagnostic system of claim 20 wherein said
optical sensors are wavelength division multiplexed.
22. The optical sensor diagnostic system of claim 20 wherein said
optical sensors are time division multiplexed.
23. The optical sensor diagnostic system of claim 20 wherein at
least one of said sensors serves as an environment reference or
compensation sensor.
24. The optical sensor diagnostic system of claim 20 wherein said
detection and signal processor comprises tracker, responsive to
said electrical detection signal, for adjusting said voltage or
other control signal to allow said tunable light to track static
and dynamic values of said characteristic reflection amplitude
feature for each of said sensor, thereby providing utilization of
the control signal as the output characteristic of the physical
stimulus, making unnecessary scanning of the complete wavelength
range and greatly increasing the speed of data acquisition.
25. The optical sensor diagnostic system of claim 20 wherein said
voltage controller comprises a modulator for modulating said
voltage control signal at a predetermined modulation frequency.
26. The optical sensor diagnostic system of claim 20 wherein said
signal processor comprises a demodulator operating at said
modulation frequency, for demodulating said electrical detection
signal and for providing a demodulated signal indicative
thereof.
27. The optical sensor diagnostic system of claim 20 wherein said
signal processor performs computations that increase the accuracy
and precision of determining the wavelength position of said
characteristic reflection amplitude feature and changes therein for
each of said sensor.
28. The optical sensor diagnostic system of claim 20 wherein: said
voltage or other controller comprises a scanner that scans said
voltage control signal so as to cause said tunable VCSEL to scan
its wavelength across said characteristic reflection amplitude
feature of any or all of said sensor; and said signal processor
determines, in response to said voltage or other control signal,
the wavelength of said tunable light from the magnitude of said
voltage or other control signal and/or mirror position feedback
signal and for determining which of said sensor is being
illuminated, thereby determining the value of the environmental
stimulus at the position of said individual sensor.
29. The optical sensor diagnostic system of claim 20 wherein: said
voltage or other controller comprises a scanner that scans said
voltage control signal so as to cause said tunable VCSEL to scan
across the wavelengths of the characteristic reflection features of
all of said sensor; and for providing a synchronization signal
indicative of when said voltage control signal begins said
scanning; and said signal processor determines, in response to said
synchronization signal, which of said sensor is being illuminated,
thereby determining changes in said wavelength at said
characteristic reflection amplitude feature.
30. The optical sensor diagnostic system of claim 20 wherein said
at least one sensor comprises at least one fiber or planar Bragg
grating.
31. The optical sensor diagnostic system of claim 30 wherein at
least one Bragg grating of at least one sensor comprises at least
one incorporated phase shift in its structure, said phase shift
producing a sharper minimum within said reflected wavelength band
maximum.
32. The optical sensor diagnostic system of claim 20 wherein said
at least one sensor comprises at least one Fabry-Perot etalon.
33. The optical sensor diagnostic system of claim 20 wherein said
at least one sensor comprises at least one Surface Plasmon
Resonance structure.
34. The optical sensor diagnostic system of claim 20 wherein at
least one sensor is disposed in a branch waveguide or optical fiber
coupled to the main trunk waveguide by a coupler.
35. The optical sensor diagnostic system of claim 34 wherein said
at least one sensor comprises at least one thin film or bulk
material characteristic absorber material.
36. The characteristic absorber material of claim 35 comprising at
least one semiconductor.
37. The characteristic absorber material of claim 36 chosen from
the full possible range of alloys and compounds of zinc, cadmium,
mercury, silicon, germanium, tin, lead, aluminum, gallium, indium,
bismuth, nitrogen, oxygen, phosphorus, arsenic, antimony, sulfur,
selenium and tellurium.
38. The characteristic absorber material of claim 35 comprises one
or more vibronic, excitonic or fluorescent materials.
39. The characteristic absorber material of claim 35 wherein said
sensor comprised of said characteristic absorber material
incorporates a mirror at the distal end, providing signal
reflection by double-pass transmission.
40. The optical sensor diagnostic system of claim 20 wherein at
least one sensor produces a characteristic absorption feature in
the form of a slope, wherein: the said wavelength indicative of the
characteristic absorption slope is determined by taking the first
derivative of the light amplitude with respect to the wavelength,
and by analytically extracting the wavelength position of resulting
said first wavelength derivative extremum, or, alternatively, the
said wavelength indicative of the characteristic absorption slope
is determined by taking the second derivative of the light
amplitude with respect to the wavelength and by analytically
extracting the wavelength positions of said second wavelength
derivative zeros.
41. The optical sensor diagnostic system of claim 20 wherein said
environmental stimulus is any combination of mechanical stress,
temperature, pressure, electrical current, electrical field,
magnetic field or chemical or biological material on said
sensor
42. The optical sensor diagnostic system of claim 20 wherein at
least one wavelength reference, not affected by any environmental
stimulus, comprising at least one of the group of a Bragg grating,
a phase shift Bragg grating, a Fabry-Perot etalon or a
gas-containing chamber, is disposed in the optical path.
43. The optical sensor diagnostic system of claim 42 wherein the
gas-containing chamber contains acetylene gas.
44. The optical sensor diagnostic system of claim 20 wherein a said
sensor comprising an optical fiber having a core waveguide and a
cladding or cladding/buffer layer surrounding the core waveguide,
in addition incorporating an input/output end and a terminal
reflection end, wherein the terminal reflection end is defined by
an end face of the core waveguide in contact with a mirrored layer
such that the light is caused to reverse its direction of
propagation and exits the input/output end. In this embodiment, the
majority of the optical fiber length does not support surface
plasmon resonance, but instead the optical fiber incorporates a
sensing area located between the input/output end and terminal
reflection end or at the terminal reflection end. Said sensing area
is defined by a surface plasmon resonance-supporting metal in
contact with at least a portion of the surface of the optical fiber
core waveguide free from the surrounding cladding or
cladding/buffer layer.
45. The optical sensor diagnostic system of claim 44 wherein the
said sensing area further contains at least one additional
functional layer adhered to the surface plasmon
resonance-supporting metal layer.
46. The optical sensor diagnostic system of claim 45 wherein the at
least one additional layer comprises a chemically reactive
layer.
47. The optical sensor diagnostic system of claim 45 wherein the at
least one additional layer comprises a biologically reactive
layer.
48. The optical sensor diagnostic system of claim 44 wherein the
said plasmon resonance supporting metal is one or more layers of
elements or alloys chosen from the group silver, gold, copper or
aluminum.
49. The optical fiber sensor according to claim 44 further
incorporating a polarizer positioned anywhere between said tunable
VCSEL and said sensor, said polarizer selecting light with
polarization P.
50. The optical fiber sensor according to claim 45 further
comprising a first polarizer positioned between the said tunable
VCSEL and said circulator, said polarizer selecting light with
polarization state between S and P polarizations.
51. The optical fiber sensor according to claim 50 further
comprising a second polarizer positioned between the said sensor
and said detector, said second polarizer being oriented with
respect to the said first polarizer such that a phasepolarization
enhancement is obtained of the ratio of the power amplitudes at
wavelengths outside the said minimum of reflection feature to the
power amplitude at the exact minimum of reflection.
52. In a sensing system of the type including an optical fiber
sensor or optical waveguide having a radiation reflectance or
transmissivity characteristic that varies in response to a
stimulus, an optical path being defined between a coupler and said
optical fiber sensor or optical waveguide, a sensing method
comprising: (1) operating a Vertical Cavity Surface Emitting Laser
to generate radiation; (2) tuning the Vertical Cavity Surface
Emitting Laser to vary the wavelength of said Vertical Cavity
Surface Emitting Laser-generated radiation; (3) coupling at least
some of the radiation emitted by said Vertical Cavity Surface
Emitting Laser to said coupler; and (4) analyzing radiation
transmitted or reflected by said sensor or waveguide for variations
of said characteristic caused by said stimulus.
53. A sensing system including an optical fiber sensor or optical
waveguide, said system comprising: a coupler being coupled to the
optical fiber sensor or optical waveguide having a
wavelength-selective radiation transrnissivity characteristic that
varies in response to a stimulus; a Vertical Cavity Surface
Emitting Laser coupled to said coupler, said Laser operated to
generate radiation and supply said radiation to said coupler; a
tuning device that tunes the Vertical Cavity Surface Emitting Laser
to vary the wavelength of said Vertical Cavity Surface Emitting
Laser-generated radiation; a detector that detects radiation
transmitted or reflected by the optical fiber sensor or waveguide;
and an analyzer that analyzes said detected radiation for at least
one variation caused by said stimulus.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not applicable.
FIELD OF THE INVENTION
[0003] This invention relates to systems using vertical cavity,
surface emitting lasers (VCSELs) having integrated MEMS
(micro-electromechanical) wavelength tuner to interrogate optical
sensors, including fiber and planar Bragg gratings, etalons,
characteristic absorption or reflection sensors such as bandgap
semiconductors and surface plasmon resonance sensors sensitive to
physical, chemical and biological stimuli and, more particularly,
to specific system configurations for use with such Bragg grating,
etalon, absorption/reflection and surface plasmon resonance sensing
devices.
BACKGROUND OF THE INVENTION
[0004] Fiber optic sensors employing measurements of the shift of
wavelength position of a sensor spectral peculiarity (maximum,
minimum, slope or some other function) under the influence of a
physical stimulus are well known to those skilled in the art. The
examples of such sensors include Bragg grating-based strain,
pressure, temperature and current (via the associated magnetic
fields) sensors, surface plasmon resonance (SPR) biological and
chemical sensors, semiconductor absorption bandedge based
fiber-optic sensors and Fabry-Perot (FP) etalon pressure,
temperature and sensors. The utilization of such sensors has been
retarded in the marketplace because of many well known problems,
including the susceptibility of simple, inexpensive sensing systems
to optical noise and the great expense of most of the solutions
found to overcome said susceptibility. It will be revealed that
combining a new type of laser, a vertical cavity, surface emitting
laser (VCSEL) with an integrated microelectromechanical (MEMS)
tuning mechanism, as an interrogating instrument with sensors of
many different types will enable new, less expensive and more
reliable class of optical sensor systems.
[0005] A Bragg grating is a series of optical elements that create
a periodic pattern of differing indices of refraction in the
direction of propagation of a light beam. A Bragg grating is formed
in an optical fiber by means of exposing ultraviolet sensitive
glass (usually germanium doped fiber) with an ultraviolet (UV) beam
that varies periodically in intensity, usually accomplished by
means of an interference pattern created by a phase mask or split
beam, such as with a Lloyd's mirror apparatus. Planar Bragg
gratings are created by exposing a "photoresist" of any of a number
of types through a phase shift or other type of mask, or they can
be written directly with an electron beam. Light reflections caused
by the periodic index of refraction pattern in the resulting
grating interfere constructively and destructively. Since the
refractive index contrast between UV-exposed and unexposed sections
of fiber is small but the number of sections is very large, the
reflected beam narrows its spectrum to a very sharp peak, as narrow
as a fraction of a nanometer in spectral width. It can also be
arranged by means of a phase shift design that the reflected peak
can contain within it an even narrower "valley" of absorption, as
narrow as a few picometers in spectral width. Conversely, the
transmitted portion of the light beam exhibits complimentary
spectral power characteristics, i.e., a broader valley with a
narrower peak within it.
[0006] It is known that Bragg gratings patterned into optical
fibers or other waveguides may be used to detect physical stimuli
caused by various physical parameters, such as, for example,
strain, pressure, temperature, and current (via the associated
magnetic fields) at the location of the gratings,-such as is
described in U.S. Pat. Nos. 4,806,012 and 4,761,073 both to Meltz,
et al; U.S. Pat. No. 5,380,995 issued to E. Udd; U.S. Pat. No.
6,024,488 issued to J. Wu; and the publication authored by Kersey,
A. D., et. al. [10.sup.th Optical Fiber Sensors Conference,
Glasgow, Oct 1994, pp.53-56]. Generally, in such a sensor, the core
and/or cladding of the optical fiber (or planar waveguide) is
written with periodic grating patterns effective for selectively
reflecting a narrow wavelength band of light from a broader
wavelength band launched into the core (waveguide layer in the
waveguide). The spectral positions of sharp maxima or minima in the
transmitted and reflected light intensity spectra indicate the
intensity of strain, temperature, pressure, electrical current, or
magnetic field variations at the location of the grating. The
mechanism of the spectral position changes lies in changes either
the in grating period or the indices of refraction, or both, which
can be affected by various environmental physical stimuli, such as
temperature and pressure. Frequently, more than one stimulus or
physical parameter affects the sensors at the same time, and
compensation must be designed into the sensor or the measurement
technique for all the variables but one, which can be accomplished
by many physical, optical and electronic techniques known in the
art. The typical sensitivity limits of fiber grating sensors in the
current art are about 0.1.degree. C. and/or 1 microstrain,
respectively. Advantages of a spectral shift method of sensor
interrogations include the high accuracy of wavelength
determination (akin to the advantages of measuring electrical
frequency instead of magnitude) and immunity to "optical noise" due
to fluctuations in fiber transmission amplitude (microbending
losses, etc.). It also allows the multiplexing of many sensors on
the same fiber via wavelength dependent multiplexing techniques
(WDM), e.g., dividing the total wavelength band into sections
dedicated to individual sensors.
[0007] The precision, dynamic range and multiplexing capabilities
of the all optical sensor interrogation techniques are defined in
part by the spectral power of the light source, especially in cases
in which a broadband source is used. The LEDs, SLDs
(superluminescent diodes) and various lamps usually used provide
spectral power that can be too little when divided into
nanometer-sized segments. This limits critical parameters such as
the magnitude of the reflected peak available to the optical
sensor, causing lower than desirable signal to noise ratios.
Another technique, the use of a conventional laser diode tuned with
motorized external cavity, electrical current or temperature
mechanisms is more effective because all the power of the laser is
contained in a narrow beam as it is tuned across the spectrum.
Several techniques have been proposed: see for example Froggatt,
(U.S. Pat. No. 5,798,521); the use of a conventional laser diode
tuned with electrical current has been proposed by Dunphy et. al.
(U.S. Pat. No. 5,401,956); and the use of a tunable fiber laser has
been proposed by G. A. Ball et. al. [J. of Lightwave Technology,
vol. 12, no. 4, April 1994 p 700]. When using a scanning laser
technique, an inexpensive detector and electronics system simply
determines the wavelength at the peak (or null) of the reflected
(or transmitted) light intensity against a known wavelength
reference. However, past art approaches are generally too
expensive, too slow, too unstable or too inaccurate to have a wide
range of practical applications. Laser diodes tuned with current,
while inexpensive and faster than thermal methods, suffer from
narrow tuning wavelength spans, which limits practical applications
to only time division-multiplexed (TDM) Bragg sensors. Such lasers
are completely unusable in surface plasmon or semiconductor
absorption edge shift sensors. The broadband light source method
utilizes an inexpensive light source, but requires a spectrometer
to read the signals (an optical spectrum analyzer may cost as much
as $35,000). It is most practical when many sensors are multiplexed
on the same fiber. Still, spectrometers are temperamental and not
well suited to field use. The lasers tuned with external cavities
that are now in use, on the other hand, typically are more
expensive than spectrometers, but have the advantage of using an
inexpensive detector. In addition, such lasers are typically slow
to tune, such as 100 nm/sec, and may be even more delicate than
spectrometers. Scanning (or tuning) speed is especially important
in applications in which absorption and polarization related noise
are significant because of negative effects on the signal to noise
ration (SNR). Mass produced MEMS-tunable VCSELs, configured as
sensing instruments, are expected to cost at least an order less
than prior art lasers and be at least two orders of magnitude
faster than prior art lasers.
[0008] Surface plasmon resonance-based sensors for biological
and/or chemical monitoring are well known to those skilled in the
art. Surface plasmon waves are electromagnetic waves that may exist
at the boundary between a metal and a dielectric (hereinafter
referred to as the "sample"). Such waves can be excited by light
that has its electric field polarized parallel to the incident
plane (i.e., transverse magnetic (TM) polarized). When the parallel
component of the propagation constant of the incident light equals
the real part of the surface plasmon wave propagation constant, the
incident light resonantly excites the surface plasmon waves, and a
fraction of the incident light energy is transferred or dispersed
to surface plasmon resonance (SPR). This dispersion of energy
depends on both the dielectric constant of the metal and that of
the sample in contact with the metal. By monitoring the resonance
wavevector of the metal/sample interface, the dielectric constant
of the sample (gas or solution) may be obtained. Alternatively, if
the sample is contaminated by a chemical species, dielectric
constant measurements may provide the concentration of the chemical
species in the sample. The typical SPR spectral minimum is at least
two orders of magnitude wider than the typical Bragg grating
minimum or maximum.
[0009] Traditionally, SPR has been measured using the Kretschmann
configuration (Kretschmann and Raether, Z. Naturforsch. Teil A
23:2135-2136, 1968). In this configuration, a thin layer of highly
reflective metal (such as gold or silver) is deposited on the base
of a prism. The metal surface is then contacted with the sample,
and the SPR reflection spectra of the sample is measured by
coupling TM polarized, monochromatic light into the prism and
measuring the reflected light intensity as a function of the angle
of incidence. The angle of minimum reflective intensity is the
resonance angle at which maximum coupling occurs between the
incident light and the surface plasmon waves. This angle, as well
as the half-width of the resonance spectrum and the intensity at
the angle of minimum reflective intensity, may be used to
characterize or sense the sample that is in contact with the metal
surface (Fontana et al., Applied Optics 27:3334-3339, 1988).
[0010] Optical sensing systems have been constructed based on the
Kretschmann configuration described above. Such systems utilize the
sensitivity of SPR to changes in the refractive indices of both
bulk and thin film samples, as well as to changes in the thickness
of thin films. These systems, in conjunction with appropriate
chemical sensing layers, have led to the development of a variety
of SPRbased chemical sensors, including immunoassay sensors (e.g.,
Liedberg et al., Sensors and Actuators 4:299-304, 1983; Daniels et
al. [Sensors and Actuators 15:11-17, 1988]; Jorgenson et al.,
[IEEE/Engineering Medicine and Biology Society. Proceedings
12:440-442, 1990]), gas sensors (e.g., Liedberg et al., ibid, Gent
et al., [Applied Optics 29:2843-2849, 1990]), and liquid sensors
(e.g., Matsubaru et al., [Applied Optics 27:1160-1163, 1988]). An
SPR sensor usually utilizes the wavelength of minimum amplitude as
a function or angle of reflection. However, the shape of the minima
can be modified if an additional polarizer and phase plate (or
retarder) are introduced between the sensor and detector at some
predetermined angle with respect to the polarization of light
illuminating the SPR sensor (Homola J., et al, Sensors and
Actuators B, B51 (1-3), August 1998, p.331, Kabashin A. V et al,
Sensors and Actuators B, B54 (1-2), January 1999, p.51). This
modification is due to phase and polarization peculiarities near
the surface plasmon resonance excitation conditions. Moreover,
minima can be transformed into maxima (Homola, ibid), which has the
potential for increasing the resolution of SPR sensors
[0011] While the Kretschmann configuration for SPR-based chemical
sensors offers significant sensitivity, their relatively large size
has severely restricted their application. An optical fiber sensor
that utilizes SPR to detect a material in contact with the sensor
and utilizes incident light having multiple wavelengths as the
excitation energy is described by Jorgenson, et al. (U.S. Pat. No.
5,359,681). While being small and considerably less expensive than
the non-waveguide optical sensor, it is at least an order of
magnitude less sensitive. The reason for this drop in sensitivity
is obvious--in a non-waveguide optical scheme with 5000 pixels, the
SPR minimum is read by at least 2000 pixels. However, in the fiber
optic scheme with a spectrometer as a readout instrument
(wavelength resolution not better than 0.1 nanometer and SPR
minimum spectral width around 60 nm) the SPR minimum will be
characterized by 600 pixels at most, leading to less precise
interpolations to locate the minimum, and hence changes in the
wavelength position of the minimum. Conversely, the use of
inexpensive tunable lasers with sub-nm wavelength resolution as
light sources in fiber systems will eliminate the expensive
spectrometer and yield precision at least comparable to that of the
non-waveguide optical system. The illustrative problem that arises
in the broadband light source/spectrometer configuration,
specifically a lack of optical intensity per measurement point,
has, in the case of SPR sensors, the additional undesirable
attribute of overheating of the sensor. If the total light
intensity of the broadband light source is increased to compensate
for the small intensity available to each pixel in the spectrometer
charge coupled device (CCD) array, overheating of the SPR sensor
will occur because approximately half the incident light is
dissipated in heat in the metal layer whether it contributes to the
usable signal or not. Heating is not only harmful to the biological
and/or chemical sample under test, but also can induce refractive
index changes in the fiber and/or sample, causing much larger
variations in the SPR wavevector than the perturbation to be
detected. Very fast tuning lasers, having very narrow emission
spectra, are ideal to address this problem, since the total
intensity illuminating the sensor at any given time will be almost
exactly equal to the intensity of the reflected light available to
detect the change in stimulus.
[0012] An illustrative example of a characteristic
absorber/reflector material is a semiconductor. A semiconductor
means that comprises an optical sensor with
optical-wavelength-dependent characteristics that may vary as a
function of a physical parameter such as temperature is well known
to those skilled in the art (see, for example patents, issued to
Christenson (U.S. Pat. No. 4,136,566) or Quick et al. (U.S. Pat.
No. 4,355,910). The optical-wavelength-dependent characteristic
(semiconductor absorption band edge) is usually monitored by
present art methods in one wavelength band, in which case
measurements are intensity-dependent, or in two wavelength bands,
after which a ratio of intensities is taken. In both cases, the
sensitivity and accuracy of such sensor systems are low and
more-or-less sensitive to optical noise (microbending, etc.).
Scanning very rapidly through the whole semiconductor transmission
intensity slope related to the forbidden band edge using a tunable
VCSEL with high wavelength resolution will provide the opportunity
for mathematical enhancement of the sensitivity of such sensors by
at least in order of magnitude. Broadband light source/spectrometer
configurations are not suitable for reasons similar to those
described in [0011]. The high total illumination intensity will
cause self-heating of the sensor, which is crucial especially for
temperature sensors. Reducing the illumination intensity, on the
other hand, will cause uncertainty due to photodetector dark noise
and other sources of optical noise.
[0013] Fiber etalon-based sensors are well known to those skilled
in the art (see, for example, U.S. Pat. No. 5,646,401 issued to E.
Udd). Etalons consist of two mirrored surfaces that may be internal
or external to the optical fiber. The reflectivity of an etalon is
defined by interference between light waves reflected from first
and second mirrors. The advantages of etalon-based pressure,
temperature and/or stain sensors include the low cost of etalons
and very high sensitivity. However, with broadband light sources
used for interrogation, measurements that are intensity based or
count interference fringes are very susceptible to optical noise or
other technical problems (e.g., losing count of the fringes), to
the point of being impractical. The sole practical,
self-calibrating system uses an optical cross-correlating
interferometer as a detector, also an expensive technique (see, for
example, U.S. Pat. Nos. 5,202,939 and 5,392,117 both issued to
Belleville, et al.).
[0014] A new kind of laser, a vertical cavity surface emitting
laser (VCSEL), has recently been invented. Generally, VCSELs are
made completely with waferlevel processing and the chips emit from
the direction of the broad surface of the wafer, rather than having
to be cleaved out of the wafer in order to have an exposed pn
junction edge from which to emit, as in older art. This enables
another benefit to be designed into the wafer
structure--tunability. This is done with micromachining (MEMS)
technology by placing a stack of optical layers, forming a mirror,
in front of the emitting surface in such a way that the stack can
be varied in its distance from the emitting surface by
piezoelectric, magnetic, electrostatic or some other microactuating
means. The groups of C. J. Chang-Hasnain (US Patent, [IEEE J. on
Selected Topics in Quantum Electronics, V 6, N 6, November 2000, p.
978]), J. S. Harris Jr. (U.S. Pat. No. 5,291,502, [Appl. Phys.
Lett. 68 (7), February 1996 p. 891]), and Vakhshoori [Electronics
Letters, May 1999, V. 35, N.11 p. 900] have shown the potential for
making tunable VCSELs with MEMS tuning mechanisms with wide tuning
ranges and fast tuning speeds. Tunable VCSELs are relatively simple
to manufacture, exhibit continuous mode-hop-free tunability over a
wide spectrum, and potentially offer orders of magnitude lower cost
as compared to prior art tunable lasers or optical spectrometers.
Integrated, MEMS-tunable VCSELS make possible truly affordable and
accurate optical sensor systems by combining low cost detectors and
low cost excitation sources, one or the other of which is very
expensive in the prior art systems with the accuracy and resolution
to be viable commercially. In addition to the orders of magnitude
lower cost of source/detector combinations, lower cost sensor will
become available because of the orders of magnitude greater tuning
speed.
BRIEF SUMMARY OF THE INVENTION
[0015] The present invention provides a means of optical wavelength
scanning Bragg grating, characteristic absorber/reflector, etalon
and surface plasmon resonance sensors of all types with integrated,
MEMS-tunable VCSELs in order to measure various physical parameters
at several orders of magnitude lower cost than prior art, with the
added benefits of enhanced accuracy, ruggedness and
reliability.
[0016] In more detail, the present invention provides, as an
illustrative embodiment, a diagnostic system which interfaces with
optical fibers or optical waveguides having Bragg grating or other
types of sensors as described herein, embedded therein for the
determination of static and dynamic values of various physical,
chemical or biological parameters, and, further, to provide means
of guaranteeing wavelength accuracy during the scanning cycle.
[0017] In accordance with one aspect of a preferred illustrative
embodiment of the invention, an optical sensor diagnostic system
includes an integrated MEMStunable VCSEL for providing a
wavelength-tunable light in response to a voltage or other control
signal, the tunable light being launched into an optical waveguide.
At least one optical sensor, disposed in the path of the tunable
light, provides a reflected light having an associated local
amplitude minimum, maximum or slope. The said local amplitude
maximum could contain one or more local amplitude minimums inside
said local amplitude maximum, while said local amplitude minimum
could contain one or more local amplitude maximums inside said
local amplitude minimum. The wavelength at said minimum, maximum or
slope of amplitude varies in response to an environmental stimulus
imposed upon the corresponding sensor. The tunable VCSEL
individually illuminates each of the sensors throughout its
associated wavelength band of an amplitude minimum, maximum or
slope. An optical circulation device, disposed in the path of the
tunable light between the tunable VCSEL and the sensors, isolates
the tunable VCSEL from the reflected light and directs the
reflected light from each of the sensors to the optical detector
means, disposed for detecting the reflected light and for providing
an electrical detection signal indicative of the power of the
reflected light. A tuning controller provides a variable voltage or
other signal to the tunable VCSEL indicative of the desired
wavelength of the tunable light. A signal processor responsive to
the electrical detection signal interprets a shift in the
wavelength of the magnitude minimum, maximum or slope due to the
environmental stimulus, and provides a signal indicative of said
stimulus.
[0018] According to another aspect provided by an illustrative
embodiment of the present invention, an optical sensor diagnostic
system includes an integrated MEMS-tunable VCSEL for providing a
wavelength-tunable light in response to a voltage or other control
signal, the tunable light being launched into an optical waveguide.
At least one optical sensor, disposed in the path of the tunable
light, provides a transmitted light having an associated local
amplitude minimum, maximum or slope. The said local amplitude
maximum could contain one or more local amplitude minimums inside
said local amplitude maximum, while said local amplitude minimum
could contain one or more local amplitude maximums inside said
local amplitude minimum. The wavelength at said minimum, maximum or
slope of amplitude varies in response to an environmental stimulus
imposed upon the corresponding sensor. The tunable VCSEL
individually illuminates each of the sensors throughout its
associated wavelength band of an amplitude minimum, maximum or
slope. An optical isolation device, disposed in the path of the
tunable light between the tunable VCSEL and the sensors, isolates
the tunable VCSEL from the reflected light. The light transmitted
through the said at least one optical sensor is directed by an
out-going fiber to the optical detector means, disposed for
detecting the transmitted light and for providing an electrical
detection signal indicative of the power of the transmitted light.
A tuning controller provides a variable voltage or other signal to
the tunable VCSEL indicative of the desired wavelength of the
tunable light. A signal processor responsive to the electrical
detection signal interprets a shift in the wavelength of the
magnitude minimum, maximum or slope due to the environmental
stimulus, and provides a signal indicative of said stimulus.
[0019] In accordance with one aspect of a preferred illustrative
embodiment of the invention, the said optical sensors are of
reflective Bragg grating type. The sensors reflect light, having
maxima or minima inside the maxima at a different reflection
wavelength for each sensor, which vary their spectral positions due
to an environmental stimulus, such as strain, pressure,
temperature, electrical current or magnetic field imposed
thereon.
[0020] In accordance with another aspect of a preferred
illustrative embodiment of the invention, the said optical sensors
are of transmittive Bragg grating type. The sensors transmit light,
having minima or maxima inside the minima at a different
transmission wavelength for each sensor, which vary their spectral
positions due to an environmental stimulus, such as strain,
pressure, temperature, electrical current or magnetic field imposed
thereon.
[0021] In accordance with further aspect of a preferred
illustrative embodiment of the invention, the said optical sensors
are of reflective etalon type. The sensors reflect light, having
maxima, minima or maxima and minima at a different reflection
wavelength for each sensor, which vary their spectral positions due
to an environmental stimulus, such as strain, pressure,
temperature, electrical current or magnetic field imposed
thereon.
[0022] In accordance with further aspect of a preferred
illustrative embodiment of the invention, the said optical sensors
are of transmittive etalon type. The sensors transmit light, having
maxima, minima or maxima and minima at a different transmission
wavelength for each sensor, which vary their spectral positions due
to an environmental stimulus, such as strain, pressure,
temperature, current or magnetic field imposed thereon.
[0023] In accordance with further aspect of a preferred
illustrative embodiment of the invention, the said optical sensors
are of reflective Surface Plasmon Resonance type. The sensors
reflect light, having maxima or minima at a different reflection
wavelength for each sensor, which vary in their spectral positions
due to an environmental stimulus, such as temperature, biological
or chemical stimuli imposed thereon.
[0024] In accordance with further aspect of a preferred
illustrative embodiment of the invention, the said optical sensors
are of transmittive Surface Plasmon Resonance type. The sensors
transmit light, having maxima or minima at a different reflection
wavelength for each sensor, which vary in their spectral positions
due to an environmental stimulus, such as temperature, or
biological and chemical stimuli imposed thereon.
[0025] In accordance with further aspect of a preferred
illustrative embodiment of the invention, the said optical sensors
are of a characteristic absorber/reflector type. Said
characteristic absorber/reflector sensors, disposed in the path of
the tunable light, provide a transmitted light having an associated
local amplitude slope or local amplitude minimum, the wavelengths
of which vary their spectral position due to an environmental
stimulus, such as temperature, imposed thereon. This embodiment
could be also realized in reflection mode, with multiple sensors
coupled off the main fiber, if the reflective means is disposed in
the light path such that the light is double-passed through the
each of the sensors by means of a mirror and time domain
multiplexing is utilized. In the case of an absorber/reflector
exhibiting a minimum, wavelength division multiplexing can be
utilized to the degree the width of the minimum allows as a
fraction of the available tuning spectrum. The isolator in this
realization of the present embodiment must be replaced by a
circulator means.
[0026] The illustrative embodiments of the invention provide low
cost, workable, practical diagnostic systems which function in
cooperation with remote optical fiber sensor systems to measure
static and dynamic strain, pressure, temperature, electrical
currents and magnetic fields as well as acoustic or vibratory
perturbations of items or structures and chemical and biological
parameters. The remote sensors may be disposed on structures made
of metal, plastic, composite, or any other materials that expand,
contract, or vibrate, or the sensors may be embedded within such
structures or immersed in liquids or gasses. The embodiments also
provide a wavelength-tunable VCSEL, tunable smoothly and
monotonically, and in particular, linearly or sinusoidally tunable
with time. The embodiments further provide individual illumination
of each sensor, thereby allowing all the tunable VCSEL power to be
resident in a single narrow wavelength band at any instant in time.
As a result, the reflected or transmitted light from each optical
sensor has a high intensity, thereby providing a signal-to-noise
ratio of such reflected or transmitted light that is much greater
than systems that illuminate all sensors at the same time using a
broadband source. Ultra-fine tuning of tunable VCSELs to a few
parts per million will allow another order of magnitude increase in
precision due to higher resolution and improved computational
methods and statistical processing. The very low mass of the MEMS
tuning mechanisms allow very high tuning speeds with very low
hysteresis, providing the ability to average out optical noise in
the sensor systems with many data points and allowing very close
spacing of data in wavelength.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] The foregoing and other objects, features and advantages of
the present invention will become more apparent in light of the
following brief description of exemplary embodiments thereof as
illustrated in the accompanying drawings, of which:
[0028] FIG. 1 is a schematic drawing of an illustrative VCSEL
incorporating one example of an integrated MEMS
(micro-electromechanical machined system) tuning mechanism in the
form of a cantilevered mirror and optional lens;
[0029] FIG. 2A is a schematic block diagram of a first state of an
illustrative, exemplary non-limiting sensor diagnostic system
capable of determining the value of static and dynamic physical,
chemical or biological physical stimuli in a reflection mode
employing a tunable VCSEL as an optical excitation source and a
photodiode or similar simple device as a detector;
[0030] FIG. 2B is a schematic block diagram of a first state of an
illustrative, exemplary non-limiting sensor diagnostic system
capable of determining the value of static and dynamic physical,
chemical or biological stimuli in a transmission mode, employing a
tunable VCSEL as an optical excitation source and a photodiode or
similar simple device as a detector;
[0031] FIGS. 3A-3J are a series of illustrative, exemplary graphs
showing time-varying tuning control signals, Vt, applied to a
tunable VCSEL and the resulting spectral power transmission and/or
reflection signals produced by the sensor;
[0032] FIG. 4 is a graph of an illustrative, exemplary transmission
optical power profile of a sensor employing Surface Plasmon
Resonance to produce a shift in the wavelength position of the
spectral power minimum in response to a physical, chemical or
biological stimulus;
[0033] FIG. 5A is an illustrative, exemplary diagram of the
spectral power transmission of a characteristic absorber/reflector
in the form of a bandgap semiconductor showing a shift in
wavelength due to an increase in temperature; and
[0034] FIGS. 5B and 5C are, respectively, illustrations of the
increases in the accuracy of wavelength shift determination with
the knowledge of the first and second derivatives of the power
transmission spectra.
BEST MODES FOR CARRYING OUT THE INVENTION
[0035] FIG. 1 is an illustrative schematic drawing of a VCSEL
incorporating one example of an integrated MEMS
(micro-electromechanical machined system) tuning mechanism in the
form of a cantilevered mirror and optional lens. Substrate chip 30
has fabricated upon it, when in wafer form, a multilayer stack of
materials forming the light emitting VCSEL 31 and the tuning
components consisting of mirror stack 32, actuator and structural
means 33 that change the tuning cavity length 38, diffractive
optical lens 34 (optional), capacitive cantilever position monitor
35 (optional) and light beams emitted in directions 36 and/or 37.
If either mirror 32 or bottom of stack 31 is opaque, light is
emitted only in one direction 37 or 36 respectively. Emission in
both directions is possible if both 32 and 31 are partially
transparent. This arrangement provides the simplest means for
optical power monitoring with a photodiode 38 for the purpose of
spectral power uniformity control. Electrical connections are not
shown for simplicity.
[0036] FIG. 2A shows an example illustrative embodiment of a
diagnostic system 40 provided in accordance with one aspect of this
invention. In more detail, FIG. 2A is a schematic block diagram of
a first state of an illustrative, exemplary sensor diagnostic
system capable of determining the value of static and dynamic
physical, chemical or biological physical stimuli in a reflection
mode employing a tunable VCSEL as an optical excitation source and
a photodiode or similar simple device as a detector. Preferred
embodiment diagnostic system 40 includes a MEMStunable VCSEL 30, a
fiber light coupling means 41, an optical circulator 47, a
wavelength reference 43, exterior fiber 44 and coupling means to
the sensor or sensor array 45, a photodetector 48, and a control
block 49. In said illustrative example, a tunable VCSEL 30 is
assembled with necessary means to fiber couple 41 the emitted
light, provide an accurate wavelength reference 43, and couple the
VCSEL assembly 40 to an external fiber 44 to convey the laser light
to an optical sensor or sensor array 45 in a reflection mode. A
coupler or circulator 47 must be provided to divert the optical
signal reflected from the sensor 45 to the photodetector 48, the
electrical signal from which is relayed to the control block
circuitry 49 and external electronic circuitry as required. A
circulator also provides the function of isolating the VCSEL from
back-reflected light. If a coupler is used to divert the light to
the detector, a separate isolator must be incorporated between it
and the laser. The control block 49 may or may not control the
laser temperature via a thermoelectric element or other means and
may or may not adjust the laser power output according to a signal
from a monitor photodiode 38, as required.
[0037] In this embodiment, the tunable VCSEL 30 provides a
wavelength-tunable light in response to a tuner control signal
provided by control block 49. This tunable light provided by
tunable VCSEL 30 is launched into an optical waveguide 44 such as
an optical fiber. A sensor or sensor array 45, in this embodiment a
Bragg grating sensor array, providing at least one optical sensor,
is disposed in the path of the tunable light. The sensor array 45
includes individual Bragg gratings that each reflects light having
different, non-overlapping, associated amplitude reflection maxima
at individual reflection wavelengths, spectrally distinguishable
one from the other. In the exemplary embodiment, the wavelength
position of the amplitude maximum reflected by each of the Bragg
gratings in the array 45 varies in response to a physical stimulus
or perturbation imposed on the corresponding sensor.
[0038] The tunable VCSEL 30, by continuously scanning its output
spectrum, individually illuminates each of the sensors in turn
within the sensor array 45 in a wavelength band including the
wavelength of maximum or minimum reflection associated with each
sensor. An optical isolation and directing device such as optical
circulator 47 is disposed in the path of the tunable light between
the tunable VCSEL 30 and the sensor array 45. The circulator
efficiently isolates the tunable VCSEL from light reflected by the
sensor array 45 and diverts the reflected signals to a simple and
inexpensive optical detector 48, such as a photodiode, disposed in
the path of the light. The detector 48 provides an electrical
detection signal indicative of the power of the reflected light
that is directly related to the wavelength through the tuning
control signal and the wavelength reference 43, if utilized. The
series of optical signals obtained during a scanning cycle can
contain one or more absorption or reflection bands from one or more
wavelength reference devices 43 that can be incorporated for
additional wavelength accuracy. Said reference signals do not
change in wavelength position with any of the external stimuli
measured by the sensors, and can be related in time to the tuning
control signal.
[0039] Control block 49 responds to the electrical detection signal
from the photodetector 48 in the example embodiment by calibrating
a variable voltage or other tuning signal for the tunable VCSEL 30
to the wavelengths of the wavelength references, and providing said
tuning signal to said VCSEL. Control block 49 may also include a
signal processor responsive to the electrical detection signal for
detecting a shift in the wavelength of maximum reflection due to a
physical, chemical or biological stimulus on each of the sensors,
and/or may cooperate with external circuitry to provide a signal
indicative of the stimulus for each of the sensors. Control block
49 may also control the laser temperature by any of several known
means and adjust the laser power to provide a constant power output
with respect to wavelength using an independent monitor detector 38
(FIG. 1).
[0040] In more detail, referring to FIG. 2A, diagnostic system 40
includes a tunable VCSEL 30 which in this embodiment (FIG. 1) has a
rear reflector stack of alternating quarter-wave layers of two
different materials 31, the Fabry-Perot cavity region that contains
the active material 31 (here a solid optical cavity), and an upper
reflector 32, made as movable, suspended mirror layers with
different indices of refraction of transparent material on a
cantilever as illustrated, or, alternately, as a reflective or
partially reflective single layer, such as aluminum. The relative
position of the movable mirror structure with respect to the rest
of the structure is changeable by the application of an
electrostatic field or other control force, forming a variable
optical cavity 38 (here an air or vacuum optical cavity). The
mirror structure could be made in a form of a diaphragm suspended
by other means by selective etching and release techniques, the
relative position of which with respect to the rest of the
structure is also changeable by the application of an electrostatic
force, magnetic force or other force. The result of this is that
the effective optical distance between the two reflectors making up
the cavity 38 is adjustable. Since the resonant wavelength depends
on this distance, the characteristic wavelength of the tunable
VCSEL is continually tuned, for example, by varying the applied
voltage and thereby the electrostatic field between the upper
reflector and the remainder of the device.
[0041] It is desirable to provide energy within the tunable VCSEL
30 to achieve lasing. It should be noted that the energy could be
provided by optical pumping means or by electrical pumping means
(p-n or p-i-n junction). Although both methods are suitable for a
sensor system, the electrically pumped embodiment is preferred from
the point of view of lowest cost and greatest simplicity.
[0042] The operating wavelengths of the tunable VCSEL can be in the
communication wavelength band (Chang-Hasnain [IEEE J. on Select.
Topics in Quantum Electronics, V 6, N 6, November 2000, p. 978]
Vkhshoori [Electronics Letters, may 1999, V. 35, N.11 p. 900]) or
around 960 nm (J. S. Harris, [Appl. Phys. Lett. 68 (7), February
1996 p. 891]) or in any other desired band in which VCSELs are
produced. When the distance between the tunable VCSEL 30 and a
Bragg grating or other type of sensor 45 does not exceed about 1
km, many wavelength bands are usable. When this distance exceeds
about 1 km, the losses may become too high at wavelengths not in
the communications bands and tunable VCSELs 30 emitting within the
communication wavelength bands may be more suitable.
[0043] A current control circuit within control block 49 (FIG. 2A)
provides an electrical current to the tunable VCSEL 30, which
controls the intensity of the output light. Adjusting the current
through the diode (VCSEL active area 31) also causes slight changes
in wavelength. However, this effect is not significant for this
application. A pulsed current can be used to cause pulsed light,
which would be required for Time Division Multiplexing (TDM)
(although it should be noted that TDM could be realized by placing
an electro-optical modulator anywhere between tunable VCSEL 30 and
sensor array 45). In addition, a temperature control circuit could
be used in the illustrative embodiment to provide a current drive
to a thermoelectric (TE) cooler to stabilize the temperature of the
tunable VCSEL 30 if needed. Other devices may be used to control
the temperature if desired. A voltage control circuit can be used
to control the electrostatic force between the movable reflector 32
and the active layers of the tunable VCSEL 31 and, by such means,
can control the wavelength emitted by the tunable VCSEL in the
illustrative embodiment. It should be noted that other control
mechanisms than electrostatic can be used to position the VCSEL
tuning mirror, and the tuning signal may or many not be a
voltage.
[0044] In all exemplary embodiments, the tunable VCSEL 30 can
provide a divergent output light beam to either the end plane of
fiber 41, placed in close vicinity to the tunable VCSEL and
perpendicular to the direction of emitted light propagation
(butt-coupling method) or to a focusing lens, also represented by
element 41, that provides focused light to optical fiber component
isolator 42 or circulator 47. The lens may instead be a lens system
that provides this function. The lens also could be realized as a
diffractive element 34 written photolithographically on the surface
of the VCSEL mirror, adjacent to the fiber 41 or on the backside of
the chip in the path of the light beam 36.
[0045] It should be noted that the optical circulator 47 may be
replaced by an optical isolator 42 and a wavelength-independent
two-way splitter, placed in line. This approach is less costly,
although at least half of the optical power will be lost going each
way. In addition, an optical isolator could be placed between the
tunable VCSEL 30 and the optical circulator 47 if very high
suppression of back-reflected light is needed. In this embodiment,
the sensor can be a fiber Bragg grating, a planar Bragg grating, a
surface plasmon resonance sensor, a Fabry-Perot etalon sensor or a
characteristic absorber/reflector sensor.
[0046] Additionally in the FIG. 2A embodiment, the light from the
tunable VCSEL 30 propagates toward the sensor 45 that is composed
of an array of sensors disposed at intervals along the optical
fiber 44. Each Bragg grating sensor within sensor array 45 reflects
a predetermined narrow wavelength band of light and passes the
remaining wavelengths on toward the next sensor. The transmitted
beam therefore contains a narrow absorption band corresponding to
the reflected band, but the remainder of the light is available to
use with the other sensors. The sensors in array 45, in the
illustrative embodiment, can be placed in parallel or in series and
multiplexed by Wavelength Division Multiplexing (WDM) and/or TDM.
WDM is realized by sensors 45 having different central reflection
wavelengths, while TDM is realized by intentionally introduced time
delays, by any technique known to those skilled in the art, between
sensors that may or may not have the same central reflection
wavelengths. In this embodiment, the sensors can be fiber Bragg
gratings, planar Bragg gratings, surface plasmon resonance sensors,
characteristic absorption/reflection sensors or Fabry-Perot etalon
sensors.
[0047] FIG. 2B shows an example illustrative embodiment of a
diagnostic system 40 provided in accordance with a second aspect of
this invention. FIG. 2B is a schematic block diagram of a first
state of an illustrative, exemplary sensor diagnostic system
capable of determining the value of static and dynamic physical,
chemical or biological stimuli in a transmission mode; necessarily
employing a tunable VCSEL as an optical excitation source and a
photodiode or similar simple device as a detector. Preferred
embodiment diagnostic system 40 includes a MEMS-tunable VCSEL 30, a
fiber light coupling means 41, an optical isolator 42, a wavelength
reference 43, exterior fiber 44 and coupling means to the sensor or
sensor array 45, exit fiber 46, a photodetector 48, and a control
block 49. In said illustrative example, a tunable VCSEL 30 is
assembled with necessary means to fiber couple 41 the emitted
light, isolate 42 the VCSEL from back-reflections, provide an
accurate wavelength reference 43 and couple the VCSEL assembly 40
to an external fiber 44 to convey the laser light to an optical
sensor or sensor array 45 in a transmission mode. An exit optical
fiber 46 is provided to couple the optical output transmitted
through the sensor or sensor array 45 to the photodetector 48, the
electrical signal from which is relayed to the control block
circuitry 49 and external electronic circuitry as required. The
control block may or may not control the laser temperature via a
thermoelectric element or other means and may or may not adjust the
laser power output according to a signal from a monitor photodiode
38, as required.
[0048] In this embodiment, the sensor array 45 includes individual
Bragg gratings that each transmits light having different,
non-overlapping, associated amplitude transmission minima at
individual transmission wavelengths, spectrally distinguishable one
from the other. In the exemplary embodiment, the wavelength
position of the amplitude minimum transmitted by each of the Bragg
gratings in the array 45 varies in response to a physical stimulus
or perturbation imposed on the corresponding sensor. The sensors in
array 45, in the illustrative embodiment, can be placed in parallel
or in series and multiplexed by Wavelength Division Multiplexing
(WDM). TDM is not applicable in this embodiment. Other aspects of
this embodiment are the same as in the first embodiment, drawn in
FIG. 2A. In this embodiment, the sensors can be fiber Bragg
gratings, planar Bragg gratings, surface plasmon resonance sensors,
characteristic absorption/reflection sensors or Fabry-Perot etalon
sensors.
[0049] In preferred embodiments illustrated in FIGS. 2A and 2B, the
fiber 44 and the sensor array 45 may be bonded to or embedded in a
structure which is being monitored for a perturbation change, such
as dynamic or static strain and/or temperature and/or pressure
and/or electrical current/or magnetic field. The structure may be
made of metal, plastic, composite, or any other materials and the
sensors may be disposed on or within the structure.
[0050] Signal processing circuits (FIGS. 2A, 2B) analyze the
electrical signals and provide a plurality of output electrical
perturbation signals, indicative of the perturbation being measured
by the sensors within the structure. It should be understood that a
single line that is time multiplexed or that provides serial
digital data for each sensor might also be used.
[0051] In the embodiments illustrated in FIGS. 2A and 2B, the
wavelength tuning control circuitry in control block 49 may include
a function generator in order to produce the control signal
waveforms in illustrated in FIGS. 3A-3J. FIGS. 3A-3J are a series
of graphs showing exemplary, illustrative time-varying tuning
control signals, represented by V.sub.t, applied to a tunable VCSEL
30. Output wavelengths, .lambda., and the resulting optical power
spectrum from the sensors 45 as a function of both time and
wavelength in reflection and transmission modes are shown as well.
The waveforms shown are a sawtooth waveform (3A-3D), sinusoidal
waveform (3E-3H) and triangular waveform (3I, 3J), but many others
could be used. It is important to the sensor system operation that
the wavelength versus time should be known accurately, and the
linear triangle wave (3I, 3J) would be superior from that point of
view. The triangle waveform also allows reading all sensors 45
twice per cycle. In the exemplary embodiment, the control signal
V.sub.t relates directly to the expansion or contraction of the
cavity 38 in the VCSEL 30, thereby causing the wavelength .lambda.
of the output light to vary in proportion to the applied control
signal V.sub.t. Thus, the wavelength .lambda. of the light varies
linearly from .lambda..sub.1 to .lambda..sub.2, which range
includes at least one peak reflection or transmission minimum
wavelength .lambda..sub.b from a sensor and optionally at least one
peak or minimum from a wavelength reference, which is shown as WR.
For the sake of simplicity of the figures, reflection or
transmission from just one sensor is shown. Also for simplicity of
illustration, the optical power graphs for the triangular wave are
not shown, as they are similar to 3C and 3D with the sensor signals
occurring twice per cycle.
[0052] The triangle waveform (FIGS. 3I, 3J), although providing
linear dependence of the wavelength vs. time, has the disadvantage
of having a discontinuity in the waveform that will by its nature
induce higher frequencies, or ringing, into the system. The ringing
can be filtered out by various means known in the art, but a
penalty is paid in time and efficiency. The sinusoidal control
signal will provide frequency stability and power-conserving
scanning with much faster scanning rate due to the elimination of
the stabilizing time required of a mechanical structure when a
discontinuous forcing function is applied, such as the triangle
wave. With the sinusoidal waveform, the entire scan can occur in a
few microseconds or shorter time. This is two orders of magnitude
speed advantage over conventional lasers, allowing better
statistical averaging techniques to be used and allowing
non-spectral shift sensors (e.g., bandgap semiconductor or other
characteristic absorber/reflectors) to be scanned fast enough to
minimize optical noise interference. The optical power sensor
signals for the sinusoidal waveforms are illustrated schematically
in FIGS. 3G and 3H, showing the non-linear nature of the signals in
time.
[0053] As a result of the scan through the wavelength range, the
optical signal at the input to the optical detector 48 as well as
the electrical signal from the optical detector will appear as
indicated in illustration FIG. 3C or 3G for the reflection mode and
FIG. 3D or 3H for the transmission mode. In particular, the
electrical signal from the optical detector will experience a sharp
increase or decrease respectively (as dictated by sensor design) in
power centered at the central wavelength .lambda..sub.a of each
sensor. If TDM is also used, the output is somewhat more complex,
but know to those skilled in the art. For the sinusoidal drive, the
results of a scan are given in FIGS. 3G, 3H.
[0054] In the illustrative embodiment, the control block 49, in
cooperation with additional signal processing circuitry, determines
the static or dynamic value of the sensor stimulus by determining
at what wavelengths the maxima or minima in signal level occur and
determining the amount of change from the wavelength maximal or
minima of the unperturbed sensors. Calibration defines the
relationship between a change in the stimulating parameter and a
corresponding change in wavelength. The wavelength value is
determined by monitoring the wavelength control signal and
comparing it to the wavelength reference 43 or mirror 32 position
feedback 35 (capacitive), as required. Because this signal is
directly related to the wavelength of the tunable VCSEL 30, it
provides a directly proportional value of the instantaneous
wavelength. Many computation algorithms known to those skilled in
the art can perform the determination of wavelength position of the
minima or maxima. For the illustrative purposes only, one of the
possible algorithms is described below. The ability to calculate
the position of an extremum from relatively few data points enables
enhanced accuracy with lower computational overhead.
[0055] During the each full tunable VCSEL wavelength scan N
intensity measurement points are taken. Since photodetector 48 and
the signal processor in the control block 49 can be made to operate
by known art at GHz frequencies, the number N could be adequately
large even if the tunable VCSEL could be operated at a maximum
tuning speed of tens of kHz. Let us assume that the tunable VCSEL
operates at a frequency of only 1 kHz (each scan takes 1
millisecond), which is still two orders of magnitude faster than
commonly available from non-MEMS tunable lasers. In this case, at
least 100,000 intensity measurements could be taken during each
scan. Further assuming the maximum wavelength span of the tunable
VCSEL does not exceed 50 nm, intensity measurements could be made
every 0.5 picometers. Since communications art has advanced into
the tens of GHz and slower scanning speeds can be tolerated in
practical situations, greater wavelength resolution could be
obtained. Further, since typical Bragg reflection peaks (or
transmission minima) wavelength widths are on the order of hundreds
of picometers, the embodiments illustrated in FIGS. 2A and 2B are
capable of extremely high wavelength resolution. In the embodiment
of Bragg gratings employing phase shifts, in which a much narrower
peak or valley (20 picometers or narrower) is incorporated inside
the primary valley or peak (respectively), the sensor data rate
obtainable, as illustrated, will enable several data points to be
taken within the phase shift band. This in turn will allow the
interpolation of the spectral data from the sensor array by a
mathematically smooth, continuous function of time, F(t). F(t) can
then be transformed to a function of wavelength, F(X), according to
the type of VCSEL tuning drive used, or directly into a function of
the parameter being measured. Many applicable mathematical
techniques and their electronic implementations are known in the
art. When used, wavelength references can be analyzed in the same
manner. Further simple algorithms are used to compute wavelength
changes for each sensor by comparison to the previous F(.lambda.)
and the wavelength reference.
[0056] Instead of relying on the tuning control signal or feedback
from a cantilever or diaphragm position monitoring means, such as
capacitance, to calibrate the VCSEL wavelength against time, an
additional unstrained or unperturbed reference means in the form of
at least one Bragg grating, Fabry-Perot etalon or absorption cell
may be inserted into the optical path at 43. Said reference grating
or cell must cause at least one reflection peak or absorption
valley within the tuning range and not interfering with any sensor
wavelength band, and may provide multiple extrema at
.lambda..sub.ref 1, .lambda..sub.ref 2, .lambda..sub.ref 3, . . .
.lambda..sub.ref n that are always located at the same wavelength
positions. Knowledge of the predetermined cycle rate, or waveform,
of the voltage or other tuning signal, together with such reference
wavelengths, provides the signal processing circuit with sufficient
information to synchronize the beginning of each new tuning cycle
with the laser wavelength. The number of wavelength reference
points is determined by the accuracy and linearity of the laser
tuning mechanism and the required accuracy of the physical
parameter measurement. The fewest reference points will provide the
most economical system. In place of a reference Bragg grating or
gratings, a number of high finesse Fabry-Perot cavity filters could
be used. Another applicable method of maintaining wavelength
accuracy would be to place an acetylene cell in the optical path.
Acetylene exhibits a number of very sharp absorption peaks in the
communications wavelength bands that can be used to calibrate the
system on every cycle or every half cycle. Other techniques may
also be employed to maintain calibration accuracy to needed levels
by those skilled in the art.
[0057] Even though the embodiments have been most frequently
described as using Bragg gratings as the sensors that detect the
environmental stimulus, any reflective or transmittive device
having a narrow reflection or transmission wavelength band, or
transition slope (e.g., bandgap semiconductors) or any other
reflection or transmission spectral peculiarity that shifts with
applied perturbation may be used. Some examples of such sensors
include Fabry-Perot cavity pressure, temperature and/or
displacement sensors, waveguide and surface plasmon resonance-based
biological and/or chemical sensors, semiconductor bandgap strain,
temperature or pressure sensors and fluorescent and vibronic
materials. In the latter two types of materials, the absorption
bands are much narrower than that of semiconductors, and they do
not have to exhibit fluorescent light output in the spectral range
of use.
[0058] An example of a surface plasmon resonance-based sensor
transmission spectrum is given in FIG. 4. The valley in the curve,
caused by a surface plasmon, is shifted in wavelength by, for
example, a biomass specimen to be detected as it is adsorbed onto
the sensor surface. The absolute value of said shift provides very
precise information about the concentration of, for example, a
reagent in the solution. Additionally, the temporal behavior of
said shift can provide information about the kinetics of associated
chemical interactions. The high tuning rate of the MEMStunable
VCSEL enables faster and higher resolution kinetics studies
possible, widening the range of applications. The employment of
MEMS-tunable VCSEL 30 as a light source will make the
afore-described types of sensor systems considerably less expensive
and more functional than those employing prior art lasers and/or
optical spectrometers and will provide at least an order of
magnitude increase in the system resolution through computational
and statistical means. Another feature of surface plasmon resonance
sensors is that the resonance transmission valley can be located at
any predetermined wavelength within the near infrared or infrared
spectrum. Thus, tunable VCSELs 30 operated at 950-980 nm, for
example, will be equally as useful as VCSELs operating in the
communications bands in the 1310 nm and 1550 nm ranges, with little
change in cost. This further advantage of tunable VCSEL sensor
systems allows a wide variety of sensor types and materials to be
matched with a suitable, inexpensive tunable laser. The
mathematical algorithm for extracting the position of the
wavelength at the minimum of transmission will be similar to the
one described for the reflective or tranmissive Bragg grating
sensor above. Wavelength multiplexing in this case will be limited
by the greater width (typically tens on nanometers) of SPR
reflectivity minima and the maximum tuning wavelength span of the
tunable VCSEL, possibly as low as two sensors. Time division
multiplexing is applicable in this case, as well.
[0059] An illustrative example of the transmission spectrum of a
semiconductor bandgap absorption-edge temperature sensor is shown
in FIG. 5. The bandgap edge wavelength position is a function of
the semiconductor temperature according to well-known mathematical
and physical formulations. Such an absorption edge is blue-shifted
as a whole when the temperature decreases and red-shifted when the
temperature increases, as illustrated in FIG. 5A. In this case,
which is illustrative of its type, the scanned absorption edge will
only yield accurate temperature or pressure data if the scanning
process is completed in a much faster time than either the thermal
response time of the semiconductor mass or the speed and/or the
frequency of absorption-dependent noise (e.g., microbending noise)
in the remainder of the fiber circuit. This is because the
absorption of the semiconductor cannot be distinguished from
absorption noise unless the shape of the edge can be traced out
very rapidly. For the greatest accuracy, the spectral shape,
undistorted by optical noise, of the absorption curve is required,
not the absolute value of the absorption. Thus an advantage of the
exceptional tuning speed of the MEMs-tuned VCSEL 30 in this
arrangement will make possible inexpensive fiber optic temperature
sensors using chips of many semiconductor materials, with or
without a mirrored surface, with good sensitivity and adequate
accuracy for applications such as microwave ovens. Further, such
sensors can be selected to match the desired temperature range and
VCSEL properties by using alloy compositions available with
continuously varying bandgaps, such as alloys and compounds of
indium, aluminum, gallium and arsenic or silicon and germanium.
[0060] FIGS. 5B and 5C are illustrative examples of improved means
of detecting precisely the spectral position of an absorption band
with a very wide maximum, such as the "long pass" band of a
semiconductor. The "S"-shaped absorption curve can be converted to
a peaked curve by taking the first derivative, as shown in FIG. 5B.
The computational algorithm for this case is then similar to the
cases of Bragg grating sensors and SPR-based sensors described
heretofore. This is equally applicable to a single pass of the
light through the absorber in transmission, or a double pass
through the material if a mirror means is utilized on the output
side of the sensor. A second means of accurately determining the
amount of wavelength change from the unperturbed wavelength is
provided by taking the second derivative of the transmission
spectrum, as shown in FIG. 5C, which provides the opportunity to
use a "zero cross-over" point to define the spectral shift. In the
case of a pure semiconductor, the sensors will be self-calibrating
since the wavelength dependence of the absorption edge is very well
known. In the case of alloys, calibration may be performed. Many
computation algorithms known to those skilled in the art can
perform the determination of wavelength position utilizing the peak
of the first derivative or the zero crossing of the second
derivative. The spectra can be interpolated, smoothed or subjected
to any other mathematical analysis known to those skilled in the
art.
[0061] Referring to Bragg grating sensors, the sensors 45 need not
be written into the same type of fiber 44 as the fiber that feeds
the sensors, e.g., the sensors can be spliced into the fiber 44 or
they can be separate planar chips, optically coupled to the fiber
by means commonly known in the art.
[0062] Further, the embodiments have been described as employing an
optical fiber 44, but any other form of optical waveguide may be
used if desired.
[0063] Also, it should be understood that the tuning control
circuit 49 and subsequent signal processing can be done with any
degree of combination of software and hardware by many methods
known in the art.
[0064] Although the invention has been described and illustrated
with respect to the exemplary embodiments thereof, it should be
understood by those skilled in the art that the foregoing and
various other changes, omissions and additions may be made without
departing from the scope of the invention.
* * * * *