U.S. patent application number 12/027710 was filed with the patent office on 2009-03-05 for linearized swept laser source for optical coherence analysis system.
This patent application is currently assigned to AXSUN TECHNOLOGIES, INC.. Invention is credited to Walid A. Atia, Dale C. Flanders, Mark E. Kuznetsov.
Application Number | 20090059971 12/027710 |
Document ID | / |
Family ID | 40407389 |
Filed Date | 2009-03-05 |
United States Patent
Application |
20090059971 |
Kind Code |
A1 |
Atia; Walid A. ; et
al. |
March 5, 2009 |
Linearized Swept Laser Source for Optical Coherence Analysis
System
Abstract
A frequency swept laser source that generates an optical signal
that is tuned over a spectral scan band at single discrete
wavelengths associated with longitudinal modes of the swept laser
source. Laser hopping over discrete single cavity modes allows long
laser coherence length even under dynamic very high speed tuning
conditions. A ramp drive to the laser is used to linearize laser
frequency tuning. A beam splitter is used to divide the optical
signal between a reference arm leading to a reference reflector and
a sample arm leading to a sample. A detector system detects the
optical signal from the reference arm and the sample arm for
generating depth profiles and images of the sample.
Inventors: |
Atia; Walid A.; (Lexington,
MA) ; Kuznetsov; Mark E.; (Lexington, MA) ;
Flanders; Dale C.; (Lexington, MA) |
Correspondence
Address: |
HOUSTON ELISEEVA
4 MILITIA DRIVE, SUITE 4
LEXINGTON
MA
02421
US
|
Assignee: |
AXSUN TECHNOLOGIES, INC.
Billerica
MA
|
Family ID: |
40407389 |
Appl. No.: |
12/027710 |
Filed: |
February 7, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60968185 |
Aug 27, 2007 |
|
|
|
Current U.S.
Class: |
372/20 |
Current CPC
Class: |
H01S 5/02251 20210101;
G01N 21/4795 20130101; H01S 5/0687 20130101; H01S 5/141 20130101;
H01S 5/0683 20130101 |
Class at
Publication: |
372/20 |
International
Class: |
H01S 3/10 20060101
H01S003/10 |
Claims
1. A swept laser source for an optical coherence analysis system
comprising: a laser cavity; a semiconductor gain medium in the
laser cavity; and a tuning element in the laser cavity that is
tuned over a spectral scan band; a ramp generator for driving the
tuning element with a ramp function substantially to linearize
frequency tuning with time of an optical signal over the spectral
scan band.
2. An optical coherence analysis system as claimed in claim 1,
wherein a duty cycle of the linearized output is greater than
50%.
3. An optical coherence analysis system as claimed in claim 1,
wherein a duty cycle of the linearized output is greater than
80%.
4. An optical coherence analysis system as claimed in claim 1,
wherein the ramp generator drives the tuning element at greater
than 10 kHz.
5. An optical coherence analysis system as claimed in claim 1,
wherein the ramp generator drives the tuning element at greater
than 30 kHz.
6. An optical coherence analysis system as claimed in claim 1,
wherein the laser cavity is characterized by longitudinal modes and
the swept source only generates an optical signal at single
discrete wavelengths associated with the longitudinal modes.
7. An optical coherence analysis system as claimed in claim 1,
wherein the tuning element comprises a tunable pass band that
restricts the swept laser source to lasing at the single discrete
wavelengths of the longitudinal modes.
8. An optical coherence analysis system as claimed in claim 1,
wherein the tuning element comprises a Fabry-Perot tunable
filter.
9. An optical coherence analysis system as claimed in claim 1,
wherein the tuning element comprises a MEMS Fabry-Perot tunable
filter.
10. An optical coherence analysis system as claimed in claim 1,
wherein the laser cavity is defined by at least two reflectors.
11. An optical coherence analysis system as claimed in claim 10,
wherein one of the reflectors is integral with the semiconductor
gain medium.
12. An optical coherence analysis system as claimed in claim 10,
wherein one of the reflectors is integral with tuning element.
13. An optical coherence analysis system as claimed in claim 1,
wherein a bandwidth of the tuning element and spacing between
longitudinal modes enables only individual ones of the longitudinal
modes to lase as the optical signal is tuned over the spectral
band.
14. An optical coherence analysis system as claimed in claim 1,
wherein the optical signal is taken from the laser cavity through
the tuning element.
15. An optical coherence analysis system as claimed in claim 1,
wherein the optical signal is taken from the laser cavity through
the semiconductor gain medium.
16. An optical coherence analysis method, comprising: tuning an
optical signal over a scan band by a tuning element in a laser
cavity with a ramp function to linearize frequency tuning of the
optical signal over the spectral scan band; dividing the optical
signal between a reference arm leading to a reference reflector and
a sample arm leading to a sample; and detecting the optical signal
from the reference arm and the sample arm.
17. An optical coherence analysis method as claimed in claim 16,
wherein a duty cycle of the linearized output is greater than
50%.
18. An optical coherence analysis method as claimed in claim 16,
wherein a duty cycle of a linearized output is greater than
80%.
19. An optical coherence analysis method as claimed in claim 16,
further comprising driving the tuning element at greater than 10
kHz.
20. An optical coherence analysis method as claimed in claim 16,
further comprising driving the tuning element at greater than 30
kHz.
21. An optical coherence analysis method as claimed in claim 16,
wherein the laser cavity is characterized by longitudinal modes,
and further comprising only generating the optical signal at single
discrete wavelengths associated with the longitudinal modes.
22. An optical coherence analysis method as claimed in claim 16,
further comprising selecting a bandwidth of the tuning elements and
spacing between longitudinal modes to enable only individual ones
of the longitudinal modes to lase as the optical signal is tuned
over the spectral scan band.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit under 35 USC 119(e) of
U.S. Provisional Application No. 60/968,185, filed on Aug. 27,
2007.
[0002] This application is related to U.S. application No. ______
filed on an even date herewith, entitled Mode Hopping Swept
Frequency Laser for FD OCT and Method of Operation, now U.S. Patent
Publication No.: ______, which is incorporated herein by reference
in its entirety.
BACKGROUND OF THE INVENTION
[0003] Optical coherence analysis relies on the use of the
interference phenomena between a reference wave and an experimental
wave or between two parts of an experimental wave to measure
distances and thicknesses, and calculate indices of refraction of a
sample. Optical Coherence Tomography (OCT) is one example
technology that is used to perform usually high-resolution cross
sectional imaging that can provide images of biological tissue
structure, for example, on the microscopic scales in real time.
Optical waves are sent through an object and a computer produces
images of cross sections of the object by using information on how
the waves are changed.
[0004] The original OCT imaging technique, the time-domain OCT
(TD-OCT) uses a movable reference mirror in a Michelson
interferometer arrangement. Another type of optical coherence
analysis is termed Fourier domain OCT (FD-OCT). Other terms are
time encoded Frequency Domain OCT and swept source OCT. These
techniques use either a wavelength swept source and a single
detector, sometimes referred to as time-encoded FD-OCT or TEFD-OCT,
or a broadband source and spectrally resolving detector system,
sometimes referred to spectrum-encoded FD-OCT or SEFD-OCT. FD-OCT
has advantages over time domain OCT (TD-OCT) in speed and
signal-to-noise ratio (SNR).
[0005] TEFD-OCT has advantages over SEFD-OCT in some respects. The
spectral components are not encoded by spatial separation, but they
are encoded in time. The spectrum is either filtered or generated
in successive frequency steps and reconstructed before
Fourier-transformation. Using the frequency scanning light source
(i.e. wavelength tuned laser) the optical configuration becomes
less complex but the critical performance characteristics now
reside in the wavelength tuned laser.
SUMMARY OF THE INVENTION
[0006] Frequency swept laser source for TEFD-OCT imaging requires
tuning at very high repetition rates, in the tens of kilohertz, for
fast real-time image frame acquisition with a sufficiently large
image pixel count. At the same time, long coherence length of the
source is required for large imaging depth range. Simultaneous high
speed tuning while maintaining long coherence length is difficult
to achieve in conventional tunable lasers, such as conventional
external cavity semiconductor tunable lasers.
[0007] Long laser coherence length requires narrow spectral
linewidth operation and therefore demands a narrow bandwidth
tunable filter for the laser. Fast tuning of a narrow filter
implies laser light makes very few roundtrips inside the laser
cavity as the filter shifts its center frequency by its width. Few
roundtrips imply detrimental decrease of coherence length of the
laser. We define dwell time as the number of roundtrips light makes
inside the laser cavity while intracavity tunable filter tunes by
its full width at half maximum. As the laser tuning speed is
increased and the dwell time decreases to less than about 2, the
laser coherence length degrades very rapidly and eventually the
laser power turns off completely. One solution to this problem is
the frequency domain modelocked laser (FDML), where light in a
kilometer long fiber cavity is repetitively filtered by the tunable
filter on multiple round trips in the cavity, such that long
coherence length is maintained even at high tuning speeds. Such a
laser, however, is bulky, has complications related to using very
long fiber, and has a number of other limitations.
[0008] Dwell time of a tunable laser at high tuning speed can be
increased by using a short laser cavity length with a
correspondingly small cavity roundtrip time. Such a short required
cavity length, e.g. 10 to 30 millimeters (mm), is typically
difficult to achieve with conventional external cavity
semiconductor lasers. Using micro-electro-mechanical system (MEMS)
tunable filters and optical micro-bench packaging allows such
compact tunable lasers, however.
[0009] Furthermore, as the cavity length gets smaller, cavity mode
spacing gets larger, so that fewer modes remain lasing under the
narrow tunable filter spectral envelope, eventually leaving just a
single lasing mode. Conventional thinking has assumed that a
continuously tuned laser mode or a group of modes is required for
OCT imaging. However, since OCT data is read at discrete time
intervals anyway, a laser tuned over a discrete set of individual
wavelengths should be also suitable for OCT imaging. Such a
discretely tuned single mode laser can have a short laser cavity
and thus capable of operating at very high speeds with dwell times
sufficiently high for maintaining the long required dynamic
coherence length.
[0010] The present invention concerns a wavelength tuned laser for
an optical coherence analysis system. The laser hops between single
discrete modes of the laser cavity. The laser can be tuned at very
high scanning speeds while maintaining long coherence length.
Further, laser frequency tuning with time can be linearized with a
high utilization duty cycle.
[0011] In general, according to one aspect, the invention features
a swept laser source for an optical coherence analysis system. This
source comprises a laser cavity, a semiconductor gain medium in the
laser cavity, and a tuning element in the laser cavity that is
tuned over a spectral scan band. According to the invention, a ramp
generator is provided for driving the tuning element with a ramp
function substantially to linearize frequency tuning with time of
an optical signal over the spectral scan band.
[0012] The use of the ramp generator provides for linearized scan
utilization duty cycle of greater than 50%, usually better than
80%.
[0013] In general, according to one aspect, the invention features
an optical coherence analysis method, comprising tuning an optical
signal over a scan band by a tuning element in a laser cavity with
a ramp function to linearize frequency tuning of the optical signal
over the spectral scan band, dividing the optical signal between a
reference arm leading to a reference reflector and a sample arm
leading to a sample, and detecting the optical signal from the
reference arm and the sample arm.
[0014] The above and other features of the invention including
various novel details of construction and combinations of parts,
and other advantages, will now be more particularly described with
reference to the accompanying drawings and pointed out in the
claims. It will be understood that the particular method and device
embodying the invention are shown by way of illustration and not as
a limitation of the invention. The principles and features of this
invention may be employed in various and numerous embodiments
without departing from the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] In the accompanying drawings, reference characters refer to
the same parts throughout the different views. The drawings are not
necessarily to scale; emphasis has instead been placed upon
illustrating the principles of the invention. Of the drawings:
[0016] FIG. 1 is a schematic block diagram of an optical coherence
analysis system to which the invention is applicable;
[0017] FIG. 2 is a schematic diagram of a single longitudinal mode
hopping tunable laser for the optical coherence analysis system,
according to a first embodiment of the present invention;
[0018] FIG. 3 is a spectral plot illustrating the relationship
between the tunable signal emission from the mode hopping laser,
the laser's cavity modes, scan band, and intracavity filter
passband;
[0019] FIG. 4 is a schematic diagram of a single longitudinal mode
hopping tunable laser for the optical coherence analysis system
with phase compensation, according to a second embodiment of the
present invention;
[0020] FIG. 5 is a schematic diagram of a single longitudinal mode
hopping tunable laser for the optical coherence analysis system
using an single angle facet (SAF) semiconductor optical amplifier
(SOA) chip, according to a third embodiment of the present
invention;
[0021] FIG. 6 is a schematic diagram of a single longitudinal mode
hopping tunable laser for the optical coherence analysis system
taking the tunable optical signal directly from the SOA, according
to a fourth embodiment of the present invention;
[0022] FIG. 7 is a plot of optical frequency as a function of scan
period showing the inventive saw-toothed tuning ramp compared with
a convention sinusoid tuning drive; and
[0023] FIG. 8 is a flow diagram illustrating process for optical
coherence analysis.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] FIG. 1 shows an optical coherence analysis system to which
the present invention is applicable.
[0025] In more detail, the illustrated system is a time encoded
Fourier domain optical coherence tomography system (TEFD-OCT). As
such it comprises a tunable narrowband light source. Specifically,
a tunable semiconductor diode laser system 100 is used. The light
from the tunable laser 100 is output on a fiber 125 to a tap 50. A
tap 50 takes a portion of the tunable signal from the tunable laser
100 and directs it to a wavelength or frequency reference system.
Specifically, the tap 50 is coupled to a reference filter 52 that
applies a known spectral function against the tunable signal. A
reference detector 54 detects the tunable signal as it is filtered
by the reference filter 52. In one example, the reference filter 52
comprises a fixed etalon providing a series of transmission peaks
spaced at known wavelengths.
[0026] The light that is not diverted by the tap 50 is provided to
a circulator 56. This directs the light from the tunable laser 100
to a coupler 60. The tunable signal is divided by the coupler 60
into a reference arm and a sample arm of the system. Specifically,
the optical fiber of the reference arm terminates at the fiber
endface 62. The light exiting from the reference arm fiber endface
62 is collimated by a lens 64 and then reflected on a mirror 66 to
return back to the coupler 60.
[0027] The external mirror 66 has an adjustable fiber to mirror
distance. This distance determines the depth range being imaged,
i.e. the position in the sample of the zero path length difference
between the reference and the sample arms. The distance is adjusted
for different sampling catheters and/or imaged samples.
[0028] The fiber on the sample arm terminates at the sample arm
fiber endface 70. The light exiting from the fiber endface 70 is
focused by a lens 72 onto the sample 75. The fiber endface 70 and
the lens 72 are controlled by an x-y-z scanner 68. Specifically,
the x-y-z scanner 68 scans the focused beam from the fiber endface
70 relative to the sample 75 while collecting the spectral response
from the sample 75. The light reflected back into the fiber endface
70 is returned to the coupler 60 which then combines the signals
from the reference and sample arms. Light then returns to a first
detector 58 via circulator 56 and to a second detector 62.
[0029] In examples, the x-y-z scanning is implemented by moving the
lens 72 and endface 70 using a three dimensional positioner. In
other examples, the x-y-z scanning is implemented by moving the
sample 75 relative to the lens 72 and endface 70. In still other
examples, the scanning is implemented by rotating and laterally
moving the lens 72/endface 70.
[0030] The first detector 58 and the second detector 62 function in
a balanced detector scheme. Specifically, their electrical
responses are combined in a differential amplifier 86 and then
sampled in a current embodiment.
[0031] In one implementation, an analog to digital converter system
82 of a digital signal processor (DSP) 80 is used to sample the
output from the differential amplifier 86.
[0032] The analog to digital detection system 82, and usually a
second converter, is used to sample the output from the reference
detector 54. As a result, the DSP 80 is able to determine the
instantaneous wavelength or frequency of the tunable laser 100 as
it scans over the scan band.
[0033] In one embodiment, the output of the tunable laser is also
provided to a direct trigger circuit 75 to provide the sampling
trigger to the analog to digital converter system 82. Specifically,
the trigger 75 detects the instantaneous amplitude of the tunable
laser 100. When the amplitude jumps high as the tunable laser scans
and discretely hops between the modes of the laser, the trigger
circuit 75 generates a trigger signal that the analog to digital
converter system 82 of the digital signal processor 80 uses to
trigger and thus sample the output of the differential amplifier
86. In this case such direct laser trigger signal 75 is used
instead of the reference detector 54 trigger signal.
[0034] Once a complete data set has been collected from the surface
of the sample 75 by the operation of the scanner 68 and the
spectral response at each one of these points is generated from the
tuning of the tunable laser 100, the digital signal processor 80
performs a Fourier transform on the data in order to reconstruct
the image and perform a 2D or 3D tomographic reconstruction of the
sample 75. This information is generated by the digital signal
processor 80 is then displayed on a video monitor 84.
[0035] FIG. 2 shows the tunable semiconductor laser 100 according
to a first embodiment of the invention.
[0036] In more detail, the tunable laser 100 comprises a
semiconductor gain chip 110 that is paired with a
microelectromechanical (MEMS) angled reflective Fabry-Perot tunable
filter 112 to create external cavity tunable laser (ECL).
[0037] The semiconductor optical amplifier (SOA) chip 110 is
located within a laser cavity 105. In the current embodiment, both
facets of the SOA chip are angled and anti-reflection (AR) coated,
providing parallel beams from the two facets.
[0038] Specifically, each end facet of the SOA 110 has associated
lenses 114, 116 that are used to couple the light exiting from
either facet of the SOA 110. The first lens 114 couples the light
between the back facet of the SOA 110 and a mirror 118. Light
exiting out the front facet of the SOA 110 is coupled by a second
lens 116 to the reflective Fabry-Perot tunable filter 112.
[0039] The angled reflective Fabry-Perot filter is a
multi-spatial-mode tunable filter that provides angular dependent
reflective spectral response back into the laser cavity 105. This
phenomenon is discussed in more detail in incorporated
US20060215713A1.
[0040] In one specific example, the filter 112 is a curved-flat
Fabry-Perot tunable filter that supports multiple spatial
modes.
[0041] The light transmitted by the tunable filter 112 is coupled
through to a third lens 122. This focuses the light down into the
endface 124 of an output optical fiber 125, such as single mode
fiber, which can also be polarization controlling, such as
polarization maintaining (PM), fiber.
[0042] The single mode fiber 125 ultimately transports the tunable
signal to the coupler 60 and then to each of the reference and
sample arms of the optical coherence analysis system 100, see FIG.
1.
[0043] In one embodiment, the drive current to the semiconductor
optical amplifier 110 is controlled by the digital signal processor
80. Specifically, in one implementation the drive current is
controlled to stabilize the power output of the laser 100. Further,
in the preferred embodiment, the signal processor 80 either is
implemented to function as a ramp generator or a separate ramp
generator circuit 120 is provided, which is triggered by the DSP
80. The ramp generator 120 provides a saw-toothed tuning voltage
curve to the tunable filter 112 to thereby provide substantially
linear frequency tuning with time over the scan band, which also
maximizes the tuning performance of the tunable laser 100.
[0044] In the preferred embodiment the tunable laser 100 is
implemented on an optical bench 107, which is installed in a
hermetic package 106. Specifically, this is a micro-optical bench
to provide a relatively short laser cavity 105.
[0045] FIG. 3 is an exemplary spectral plot illustrating of the
relationship between the scan band 310, longitudinal cavity modes
152, the passband 154 for the tunable filter 112, and the laser
emission/tunable signal 150.
[0046] The scan band 310 for the tunable laser 100 extends over a
wavelength range. The passband 154 of the tunable filter 112 is
then scanned over the band 310. Its passband is such that it only
selects no more than one of the cavity modes 152 of the laser
cavity 105. Thus, at points when the passband is directly between
cavity modes, spectrally, it is possible that no cavity modes are
lasing. However, when the passband 154 aligns with a given cavity
mode then the output tunable signal 150 is produced. In this way,
the laser system 100 functions as a mode-hopping laser system that
only produces an optical signal at the discrete wavelengths of the
cavity modes 152.
[0047] FIG. 4 shows a second embodiment of the mode hopping tunable
laser 100. This includes a second semiconductor chip section 111
that functions as an intra cavity phase or dispersion compensator,
which is integral with the semiconductor gain medium in a preferred
embodiment. Specifically, there is a certain amount of dispersion
during scanning, which implies variation of cavity mode frequency
spacing. The cavity 105 is not all air; there is refractive index
dispersion in the laser chip, lenses, optical coatings. Also, laser
threshold changes over the tuning range. Thus, there will typically
be higher carrier density inside the gain chip 110, when it is
tuned to the edges of the spectrum. This increased carrier density
decreases the chip's refractive index and thereby causes a
dispersion-like effect.
[0048] The phase compensator 111 controls its refractive index and
thus the optical length of the cavity as compensation.
[0049] In other embodiments, the compensator includes an
electro-optic medium.
[0050] In other embodiments, signal processing by the DSP 80 is
used to compensate for dispersion in the case of the mode-hopping
laser. This involves measuring the laser's tuning dispersion. This
dispersion relates to the variation in the optical frequency
spacing between modes across the scan band. The collected data is
then resampled onto an equally spaced frequency grid before doing
the FFT to make the image.
[0051] FIG. 5 shows a third embodiment of the mode hopping tunable
laser 100. This uses a reflective SOA or single angled facet (SAF)
chip 110'. Specifically, the back normal facet 512 of the
semiconductor amplifier 110' is reflective to define one end of the
laser cavity 105. On the front angled non-reflective facet, a
series of three lenses 116-1, 116-2, 116-3 is used to collimate the
output of the semiconductor amplifier and relay it to the tunable
filter 112, thereby yielding the laser cavity with the desired
length. Also in one implementation, light output through fiber 510
is taken, alternatively or in addition to output fiber 125, through
the back facet 512 of the reflective optical amplifier SAF 110'.
This is used to provide an alternative output of the tunable laser
or a second laser output to serve as a possible input to the
trigger circuit 75, see FIG. 1.
[0052] In a variant design for longer laser cavities, the
reflective SOA 110' is replaced with an SOA having antireflection
(AR) coated front and rear facets. Further, the facet 540 of the
optical fiber 510 is AR coated. This extends the laser cavity 105
into the fiber 510. Thus, with a reflective coated end 530 of the
optical fiber, the length of the cavity is made longer to include a
portion of the cavity in the fiber 510.
[0053] FIG. 6 shows a fourth embodiment in which the tunable filter
112 forms the one end of the laser cavity 105. The other end of the
laser cavity is formed by a mirror 124' formed or deposited on the
endface of output optical fiber 125. In preferred embodiment, the
mirror is a dielectric multi-layer stack mirror. Further, the fiber
125 preferably has a mode expanded core or an integral mode
expander/collimator, such as a fused graded index lens, at the
endface/mirror 124' so that the mode size of the fiber output
matches the beam size produced by lens 114 from the output of SOA
110.
[0054] This embodiment provides higher levels of output power since
the output light does not need to pass through the tunable filter
112 and suffer corresponding insertion loss. Also, the beam profile
is less distorted, yielding good power coupling. On the other hand,
broadband amplified spontaneous emissions from the semiconductor
amplifier 110 are not filtered out, as they are when output is
taken through the filter 112.
Mode Hopping Laser Specifications
[0055] The laser cavity 105 is preferably only about 10-30
millimeters (mm) long, in air. This provides roundtrip times that
are on the order 0.07-0.20 nanoseconds (nsec). This short cavity
provides for good dynamic laser coherence length upon fast sweep
tuning: relatively long coherence length is largely preserved upon
fast tuning because of the relatively short cavity.
[0056] The passband of the tunable filter 112 is selected to be
relatively spectrally narrow in relation to the spectral
longitudinal cavity mode spacing of the cavity 105. Specifically,
the filter is narrow to ensure only a few, and preferably never
more than one, longitudinal mode of the cavity can lase during the
scanning of the filter 112. In one example, the passband of the
filter 112 is less than 10 giga-Hertz (GHz) wide and is preferably
about 5.0 GHz wide, tuning over 100 nanometer scan band near 1300
nm wavelength at 45 kilo-Hertz (kHz) repetition rate. Thus, the
filter 112 tunes by one bandwidth in 2.0 nanosecond (nsec). This
yields dwell times of 10-57 for 10-30 mm cavities, where dwell time
is the number of roundtrips light makes inside laser cavity while
the filter tunes by its full width at half maximum. Large dwell
times imply static laser coherence length is well preserved under
dynamic tuning conditions; for dwell times less than about 2,
dynamic laser coherence length decreases rapidly.
[0057] First Single-Mode Laser:
[0058] Filter 112, bandwidth 6-8 GHz.
[0059] Length of laser cavity 105: .about.17.5 mm, Mode
Spacing.about.8.6 GHz.
[0060] Lorentzian filter transmission at side-mode: 0.11-0.20. Good
single mode operation with low, >30 dB, sidemodes.
[0061] Second Single-Mode Laser:
[0062] Filter 112 bandwidth: 9-12 GHz.
[0063] Length of laser cavity 105: 21 mm, Mode Spacing.about.7.0
GHz.
[0064] Lorentzian filter transmission at side-mode: 0.29-0.42.
[0065] side-mode suppression ration (SMSR) only .about.10-15
dB.
[0066] FIG. 7 shows the conventional sinusoidal filter tuning
drive. As a result, only a small portion of the total scan period
has a near linear tuning curve (see sinusoid scan utilization time
period 710). This sinusoidal drive signal is necessitated by the
relative high mass spectral tuning elements used in convention ECL
devices, which requires driving such tunable filters sinusoidally
in time near their resonant frequency. MEMS tunable filters,
preferably used in the inventive lasers, have high resonance
frequencies and their frequency tuning with time can be linearized
for repetition frequencies of 50-100 kHz and higher.
[0067] According to the present invention, the ramp generator 120
produces a saw-toothed drive signal 714 to provide a linearized
sweep that extends over 50% and preferably greater than 80% of the
scan period 310, providing much higher duty cycles of 50-80% and
greater.
[0068] FIG. 8 illustrates the process performed by the optical
coherence analysis system. Specifically, in step 1010 the laser is
scanned through the discrete wavelengths of the spectral scan band.
In step 1012, the detector system response is stored as a function
of wavelength by the digital signal processor 80 in one example.
The data are then reassembled on to an equally spaced frequency
grid in step 114. This is required if the discrete laser scan
frequencies do not have the same spectral separation.
[0069] Finally, in step 1016 an inverse Fourier transform is
applied to the data in order to reconstruct the image of the
sample.
[0070] While this invention has been particularly shown and
described with references to preferred embodiments thereof, it will
be understood by those skilled in the art that various changes in
form and details may be made therein without departing from the
scope of the invention encompassed by the appended claims.
* * * * *