U.S. patent application number 13/112720 was filed with the patent office on 2011-09-15 for wavelength tuning source based on a rotatable reflector.
This patent application is currently assigned to The General Hospital Corporation. Invention is credited to Brett E. Bouma, Wang-Yuhl Oh, Guillermo J. Tearney, Benjamin J. Vakoc, Seok-Yun Yun.
Application Number | 20110222563 13/112720 |
Document ID | / |
Family ID | 39401084 |
Filed Date | 2011-09-15 |
United States Patent
Application |
20110222563 |
Kind Code |
A1 |
Bouma; Brett E. ; et
al. |
September 15, 2011 |
WAVELENGTH TUNING SOURCE BASED ON A ROTATABLE REFLECTOR
Abstract
An apparatus and source arrangement for filtering an
electromagnetic radiation can be provided which may include at
least one spectral separating arrangement configured to physically
separate one or more components of the electromagnetic radiation
based on a frequency of the electromagnetic radiation. The
apparatus and source arrangement may also have at least one
continuously rotating optical arrangement, e.g., polygonal scanning
mirror and spinning reflector disk scanner, which is configured to
receive at least one signal that is associated with the one or more
components. Further, the apparatus and source arrangement can
include at least one beam selecting arrangement configured to
receive the signal.
Inventors: |
Bouma; Brett E.; (Quincy,
MA) ; Yun; Seok-Yun; (Cambridge, MA) ; Oh;
Wang-Yuhl; (Cambridge, MA) ; Vakoc; Benjamin J.;
(Cambridge, MA) ; Tearney; Guillermo J.;
(Cambridge, MA) |
Assignee: |
The General Hospital
Corporation
Boston
MA
|
Family ID: |
39401084 |
Appl. No.: |
13/112720 |
Filed: |
May 20, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12015642 |
Jan 17, 2008 |
7949019 |
|
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13112720 |
|
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60885660 |
Jan 19, 2007 |
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Current U.S.
Class: |
372/20 |
Current CPC
Class: |
H01S 3/10046 20130101;
G01J 3/10 20130101; H01S 3/106 20130101; H01S 3/08009 20130101;
G02B 26/04 20130101; H01S 3/08 20130101; G02B 26/002 20130101; G01J
3/06 20130101; G01J 3/021 20130101; G01J 3/0229 20130101; G01J 3/02
20130101; H01S 3/08063 20130101; H01S 3/105 20130101; H01S 3/0805
20130101 |
Class at
Publication: |
372/20 |
International
Class: |
H01S 3/10 20060101
H01S003/10 |
Claims
1-21. (canceled)
17. An apparatus comprising: an arrangement including at least one
section thereon which is configured to receive a first
electro-magnetic radiation, wherein the at least one section is
configured to at least one of transmit or reflect a second
electro-magnetic radiation associated with the first
electro-magnetic radiation, and wherein the at least one section is
configured to modify the second electro-magnetic radiation to have:
a. a particular wave number which varies linearly in time, and b. a
mean frequency which changes over time at a rate that is greater
than 100 terahertz per millisecond.
18. The apparatus according to claim 17, wherein the mean frequency
changes repeatedly at a repetition rate that is greater than 5
kilohertz.
19. The apparatus according to claim 17, wherein the spectrum has
an instantaneous line width that is smaller than 100 gigahertz.
20. The apparatus according to claim 17, wherein the arrangement
includes a continuously rotating disk which at least one of
reflects or transmits through the at least one section the second
electro-magnetic radiation to a particular location
21. The apparatus according to claim 17, wherein the at least one
section has a curved shape.
22. The apparatus according to claim 17, wherein the arrangement is
provided in a laser cavity.
23. The apparatus according to claim 17, further comprising a
control arrangement which facilitates at least one of a processing
arrangement or a user to control the mean frequency.
24. An apparatus comprising a continuously rotating disk
arrangement including at least one section which is configured to
receive a first electro-magnetic radiation, wherein the at least
one section is configured to at least one of transmit or reflect a
second electro-magnetic radiation associated with the
electro-magnetic radiation, wherein the second electro-magnetic
radiation has a mean frequency that varies over time at a rate that
is greater than 100 Terra Hertz per millisecond, and wherein the
sections are provided on or in the arrangement, and defined by a
line whose points are provided at a radial distance which are
different from one another.
25. The apparatus according to claim 24, wherein a shape of the
line controls the rate.
26. The apparatus according to claim 25, wherein the shape of the
line controls the rate to be linear over time.
27. The apparatus according to claim 24, wherein the line is
curved.
28. The apparatus according to claim 24, wherein the line is
straight.
29. The apparatus according to claim 24, wherein the at least one
section has a curved shape.
30. The apparatus according to claim 24, wherein the arrangement is
provided in a laser cavity.
31. The apparatus according to claim 24, further comprising a
control arrangement which facilitates at least one of a processing
arrangement or a user to control the mean frequency.
32. An apparatus for providing an electromagnetic radiation,
comprising: a first continuously-rotating arrangement including at
least one section thereon which is configured to receive a first
electro-magnetic radiation, wherein the at least one section is
configured to at least one of transmit or reflect a second
electro-magnetic radiation associated with the first
electro-magnetic radiation, wherein the transmission or the
reflection by the at least one section is wavelength-independent,
and wherein a wavelength of the second electro-magnetic radiation
is scanned in a characteristic repetition time; and a second
arrangement including at least one laser cavity which is configured
to receive the second electro-magnetic radiation, wherein a
roundtrip travel time of the second electro-magnetic radiation in
the laser cavity is substantially equal to an integer multiple of
the characteristic repetition time.
33. The apparatus according to claim 32 further comprising a third
arrangement configured to control at least one of the roundtrip
travel time or the characteristic repetition time.
34. The apparatus according to claim 33, further comprising a
fourth arrangement configured to determine a relationship between
the roundtrip travel time and the characteristic repetition time,
and control the third arrangement based on the relationship.
35. The apparatus according to claim 32, further comprising a
further arrangement configured to control at least one of the
roundtrip travel time or the characteristic repetition time and
determine a relationship between the roundtrip travel time and the
characteristic repetition time, wherein the further arrangement is
controlled based on the relationship.
36. The apparatus according to claim 32, wherein each of the at
least one section transmits or reflects the second electro-magnetic
radiation for all wavelengths in the scan.
37. An apparatus for providing an electromagnetic radiation,
comprising: a first arrangement including at least one section
thereon which is configured to receive a first electro-magnetic
radiation, wherein the at least one section is configured to at
least one of transmit or reflect a second electro-magnetic
radiation associated with the first electro-magnetic radiation,
wherein the transmission or the reflection by the at least one
section is wavelength-independent, and wherein a wavelength of the
second electro-magnetic radiation is scanned in a characteristic
repetition time; a second arrangement including at least one laser
cavity which is configured to receive the second electro-magnetic
radiation, wherein a roundtrip travel time of the second
electro-magnetic radiation in the laser cavity is substantially
equal to an integer multiple of the characteristic repetition time;
and a third arrangement provided internally or externally with
respect to the second arrangement, and configured to amplify at
least one of the first electro-magnetic radiation or the second
electro-magnetic radiation based on at least one Raman
amplification characteristic.
38. The apparatus according to claim 37, further comprising a
fourth arrangement configured to control at least one of the
roundtrip travel time or the characteristic repetition time.
39. The apparatus according to claim 38, further comprising a fifth
arrangement configured to determine a relationship between the
roundtrip travel time and the characteristic repetition time, and
control the fourth arrangement based on the relationship.
40. The apparatus according to claim 37, further comprising a
further arrangement configured to control at least one of the
roundtrip travel time or the characteristic repetition time and
determine a relationship between the roundtrip travel time and the
characteristic repetition time, wherein the further arrangement is
controlled based on the relationship.
41. The apparatus according to claim 37, wherein each of the at
least one section transmits or reflects the second electro-magnetic
radiation for all wavelengths in the scan.
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] This application is based upon and claims the benefit of
priority from U.S. Patent Application Ser. No. 60/885,660, filed
Jan. 19, 2007, the entire disclosure of which is incorporated
herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates generally to optical systems,
and more particularly to an optical wavelength filter system for
wavelength tuning and a wavelength-swept laser using the optical
wavelength filter system.
BACKGROUND INFORMATION
[0003] Considerable effort has been devoted for developing rapidly
and widely tunable wavelength laser sources for optical
reflectometry, biomedical imaging, sensor interrogation, and tests
and measurements. A narrow line width, wide-range and rapid tuning
have been obtained by the use of an intra-cavity narrow band
wavelength scanning filter. Mode-hopping-free, single-frequency
operation has been demonstrated in an extended-cavity semiconductor
laser by using a diffraction grating filter design. Obtaining
single-frequency laser operation and ensuring mode-hop-free tuning,
however, may use a complicated mechanical apparatus and can limit
the maximum tuning speed. One of the fastest tuning speeds
demonstrated so far has been limited less than 100 nm/s. In certain
exemplary applications such as biomedical imaging,
multiple-longitudinal mode operation, corresponding to an
instantaneous line width as large or great than 10 GHz, may be
sufficient. Such width may provide a ranging depth of a few
millimeters in tissues in optical coherence tomography and a
micrometer-level transverse resolution in spectrally-encoded
confocal microscopy.
[0004] A line width on the order of 10 GHz can be achieved with the
use of an intra-cavity tuning element (such as an acousto-optic
filter, Fabry-Perot filter, and galvanometer-driven diffraction
grating filter). However, the sweep frequency previously
demonstrated has been less than 1 kHz limited by finite tuning
speeds of the filters. Higher-speed tuning with a repetition rate
greater than 15 kHz may be needed for video-rate (>30 frames/s),
high-resolution optical imaging in biomedical applications.
[0005] Recent implementation of a wavelength-swept laser using
polygon scanning filter has provided high-speed wavelength tuning
up to 10,000 nm/ms. While the high-speed polygon based
wavelength-swept light source enabled high-speed imaging as fast as
200 frames/s, wavelength tuning rate as fast as 10,000 nm/ms
keeping an instantaneous linewidth narrower than 0.15 nm has
already reached to the limit of the polygon based wavelength-swept
laser.
[0006] Accordingly, there may be a need for new wavelength scanning
filter and laser scheme for faster tuning and especially for wide
wavelength tuning range and narrow instantaneous linewidth at fast
tuning rate.
[0007] One of the objects of the present invention is to overcome
the above-described deficiencies.
SUMMARY OF EXEMPLARY EMBODIMENTS OF PRESENT INVENTION
[0008] An exemplary embodiment of the present invention can be
provided which may include apparatus and source arrangement for
lightwave filtering that provides high-speed wavelength-swept light
with broad spectral tuning range and narrow instantaneous
linewidth. In exemplary variant of the exemplary embodiment of the
present invention, the optical filter can include a diffraction
grating, a focusing lens and a spinning disk. The spinning disk can
have reflector patterns and/or transmission window patterns.
Certain optical components and arrangement and a proper design of
the disk enables high-speed wavelength sweeping over a broad tuning
range with narrow instantaneous linewidth.
[0009] In another exemplary embodiment of present invention, the
wavelength-swept filter is combined with a proper gain medium
implementing a wavelength tunable light source. The filter and gain
medium may further be incorporated into a laser cavity. For
example, a laser can emit a narrow band spectrum with its center
wavelength being rapidly swept over a broad wavelength range. The
exemplary laser resonator may include a unidirectional fiber-optic
ring and/or a full free space linear cavity with a specially
designed semiconductor optical gain medium to minimize the cavity
length of the laser.
[0010] According to one exemplary embodiment of the present
invention, an apparatus can be provided which may include an
arrangement that has at least one section thereon which is
configured to receive a first electro-magnetic radiation. The
section may be configured to transmit and/or reflect a second
electro-magnetic radiation associated with the first
electro-magnetic radiation. The section can be configured to modify
the second electro-magnetic radiation to have (i) a particular wave
number which varies linearly in time, and (ii) a mean frequency
which changes over time at a rate that is greater than 100
terahertz per millisecond. The mean frequency may change repeatedly
at a repetition rate that is greater than 5 kilohertz. The spectrum
can have an instantaneous line width that is smaller than 100
gigahertz.
[0011] According to yet another exemplary embodiment of the present
invention, the arrangement can include a continuously rotating disk
which reflects and/or transmits through the section the second
electro-magnetic radiation to a particular location. The rotating
disk can include at least one portion which reflects and/or
transmits the second electro-magnetic radiation and which has a
curved shape. The arrangement may be provided in a laser cavity. A
control arrangement can also be provided which may allow a
processing arrangement and/or a user to control the mean
frequency.
[0012] According to a further exemplary embodiment of the present
invention, an apparatus can provide an electromagnetic radiation,
and may include a first continuously-rotating arrangement that has
at least one section thereon which is configured to receive a first
electro-magnetic radiation. The section may be configured to
transmit and/or reflect a second electro-magnetic radiation
associated with the first electro-magnetic radiation. A wavelength
of the second electro-magnetic radiation may be scanned in a
characteristic repetition time. A second arrangement can be
provided which may include at least one laser cavity which may be
configured to receive the second electro-magnetic radiation. A
roundtrip travel time of the second electro-magnetic radiation in
the laser cavity can be substantially equal to an integer multiple
of the characteristic repetition time.
[0013] According a still further exemplary embodiment of the
present invention, a third arrangement can be provided which is
configured to control the roundtrip travel time and/or the
characteristic repetition time. A fourth arrangement may be
provided which is configured to determine a relationship between
the roundtrip travel time and the characteristic repetition time,
and control the third arrangement based on the relationship. In
addition, a further arrangement may be provided which is configured
to control the roundtrip travel time and/or the characteristic
repetition time and determine a relationship between the roundtrip
travel time and the characteristic repetition time. The further
arrangement may be controlled based on the relationship. In another
exemplary embodiment of the present invention, yet another
arrangement can be provided internally or externally with respect
to the second arrangement, and configured to amplify at least one
of the first electro-magnetic radiation or the second
electro-magnetic radiation based on at least one Raman
amplification characteristic.
[0014] These and other objects, features and advantages of the
present invention will become apparent upon reading the following
detailed description of embodiments of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] Further objects, features and advantages of the invention
will become apparent from the following detailed description taken
in conjunction with the accompanying figures showing illustrative
embodiments of the invention, in which:
[0016] FIG. 1A is a first exemplary embodiment of a spinning
disk-based wavelength tuning filter according to the present
invention;
[0017] FIG. 1B is a second exemplary embodiment of the spinning
disk-based wavelength tuning filter according to the present
invention;
[0018] FIG. 2 is a third exemplary embodiment of a short linear
cavity laser using a disk reflector-based wavelength tuning filter
according to the present invention;
[0019] FIG. 3 is an exemplary embodiment of a fiber ring laser
using the disk reflector-based wavelength tuning filter according
to the present invention;
[0020] FIG. 4 is an exemplary embodiment of a resonant cavity fiber
ring laser using the disk reflector-based wavelength tuning filter
according to the present invention;
[0021] FIG. 5 is an exemplary embodiment of a resonant cavity fiber
Raman ring laser using the disk reflector-based wavelength tuning
filter according to the present invention;
[0022] FIG. 6A is a first exemplary embodiment of a disk reflector
according to the present invention;
[0023] FIG. 6B is a second exemplary embodiment of the disk
reflector according to the present invention; and
[0024] FIG. 6C is a third exemplary embodiment of the disk
reflector according to the present invention.
[0025] Throughout the figures, the same reference numerals and
characters, unless otherwise stated, are used to denote like
features, elements, components or portions of the illustrated
embodiments. Moreover, while the subject invention will now be
described in detail with reference to the figures, it is done so in
connection with the illustrative embodiments. It is intended that
changes and modifications can be made to the described embodiments
without departing from the true scope and spirit of the subject
invention.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0026] FIG. 1A shows a schematic of a first exemplary embodiment of
the disk reflector-based wavelength tuning filter in accordance
with the present invention. For example, the optical wavelength
filter can comprises of a collimated input/output beam 100, a
diffraction grating 200, a focusing lens 400, and a spinning
reflection disk 500. Light input to the optical wavelength filter
is provided as a collimated input beam 100. Wavelength filtered
output is retro-reflected as a collimated light output 100. The
diffraction grating 200 is used as a wavelength dispersing element,
which may include but is not limited to, a reflection grating, a
transmission grating, a prism, a diffraction grating, an
acousto-optic diffraction cell or combinations of one or more of
these elements.
[0027] The diffraction grating 200 can have a concave curvature
that has a focal length and thereby eliminates the need for the
focusing lens 400. The focusing lens 400 is located approximately
at the distance of its focal length Fl from the diffraction
grating. The focusing lens 400 receives collimated wavelength
components diffracted from the grating 200 and focuses them onto an
image plane IP. At the image plane IP, a disk 500 with reflection
patterns 520 is placed. As the reflection disk 500 spins 560 around
its center 540, each wavelength component is selectively reflected
from one of the reflector patterns 520 one by one providing a
continuous wavelength sweep over time. After one reflector strip
passes through the desired spectrum of wavelengths 340-320, the
next reflector repeats the scan. Different types of materials can
be used to make the disk 500, including light weight metals, a
light weight plastic, and a substrate of different materials like
glass substrate or silicon substrate. The focusing lens 400 can be
also composed of different materials depending on applications, for
example, a plastic molded aspheric lens can be used for low cost
application.
[0028] The exemplary orientation of the incident beam 100 with
respect to the optic axis 210 and a rotation direction 560 of the
disk reflector 500 can be used to determine the direction of
wavelength tuning, e.g., a wavelength up (positive) scan or a
wavelength down (negative) scan. The spinning speed of the disk 500
may be monitored and controlled by using a feedback loop circuit. A
monitoring beam 110 can be used to provide a feedback. The
exemplary arrangement shown in FIG. 1A can generate a positive
wavelength sweep. It should be understood that although the disk
reflector arrangement 500 is shown in FIG. 1A as having, e.g.,
twenty reflector patterns 520, reflector pattern arrangements 520
which may have fewer than twenty reflector strips or greater than
twenty reflector strips can also be used. While generally not
considering practical mechanical limits, based upon conventional
manufacturing techniques, a particular number of reflector strips
520 of the disk reflector arrangement 500 to use in any application
may depend on a desired scanning rate and scanning range for a
particular application.
[0029] Furthermore, the size of the disk 500 may be selected based
on preferences of a particular application, and preferably taking
into account certain factors including, but not limited to,
manufacturability and weight of the disk 500.
[0030] In one exemplary embodiment according to the present
invention, a Gaussian beam 100 can be utilized with a broad optical
spectrum incident to the grating. A conventional grating equation
can be expressed as .lamda.=p(sin .alpha.+sin .beta.) where .lamda.
is the optical wavelength, p is the grating pitch, and .alpha. and
.beta. are the incident and diffracted angles of the beam with
respect to the normal axis of the grating, respectively. The center
wavelength of tuning range of the filter may be defined by
.lamda..sub.0=p(sin .alpha.+sin .beta..sub.0) where .beta..sub.0 is
the angle between the optic axis 210 and the grating normal axis.
FWHM bandwidth of the spectral resolution of the diffraction
grating arrangement is defined by
(.delta..lamda.).sub.FWHM/.lamda..sub.0=A(p/m)cos .alpha./W, where
A= {square root over (4ln2)}/.pi. for double pass, m is the
diffraction order, and W is l/e.sup.2-width of the Gaussian beam at
the fiber collimator.
[0031] The tuning range of the filter may be given by
.DELTA..lamda.=p cos .beta..sub.0(L/F1), where L=2 F1
tan(.DELTA..beta./2) denotes the distance between the stripes.
Since the beam spot size (measured at the l/e.sup.2 intensity
points) at the image plane can be given by
w.sub.s=4.lamda.F1/.pi.W', where W'=W(cos .beta./cos .alpha.) is
l/e.sup.2-width of the collimated beam of each wavelength
components at the focusing lens 400, the effective finesse of the
filter, which can be defined as (Tuning range)/(Linewidth) of the
filter, can be determined as
= L / w s = L .pi. W ' / 4 .lamda. F 1 = .pi. 2 .lamda. tan (
.DELTA..beta. / 2 ) W ' .apprxeq. .pi..DELTA..lamda. W 4 .lamda. p
cos .alpha. . ##EQU00001##
[0032] As can be determined from this relation, large groove
density of the grating and large beam incident angle are required
for high finesse of the filter, assuming that the spectral
resolution of the grating is sufficiently high. For example, with
W=0.5 mm, .lamda.=1.3 .mu.m, .DELTA..lamda.=120 nm, and p= 1/1200
mm, incident angle .alpha. should be 86.9.degree. (W=0.5 mm and
(.delta..lamda.).sub.FWHM=0.062 nm) to achieve a finesse of 800
(.DELTA..lamda.=120 nm and (.DELTA..lamda.).sub.filter=0.15 nm).
Since shorter focal length (higher NA) provides smaller spacing
between reflector strips (and smaller spot size), using short focal
length lens 400 is better for having larger number of reflector
strips, on the same size disk, therefore possibly higher wavelength
sweep repetition rate, as long as the clear aperture of the lens
400 is large enough to prevent beam clipping. For example, with
F1=10 mm and D=10 mm (NA.about.0.5), where D is the clear aperture
of the lens 400, L=1.74 mm and w.sub.s=2.16 .mu.m.
[0033] The width of the strip, w, can preferably be substantially
equal to the beam spot size, w.sub.s, at the surface of the disk.
For w>w.sub.s, the filter bandwidth may become greater, and for
w<w.sub.s, the filter bandwidth may become narrower but the
efficiency (reflectivity) of the filter can be decreased by beam
clipping.
[0034] A second exemplary embodiment of the optical wavelength
filter is shown in FIG. 1B. In this exemplary filter arrangement,
strips of transmission windows may be placed on the spinning disk.
Only wavelength components that pass through the transmission
window are relayed to the reflection mirror 600 via a telescope
arrangement 420 and 440 and then retro-reflected to the input port
100.
[0035] FIG. 2 shows an exemplary embodiment of the wavelength-swept
laser using a spinning reflector disk. Collimated light output 100
from a semiconductor optical amplifier (SOA) 700 is directly
coupled into the spinning disk wavelength filter. A small portion
of the light from the reflection facet side of the SOA 710 can be
coupled into the single mode fiber 720 providing output of the
laser 740.
[0036] A frequency downshift in the optical spectrum of the
intra-cavity laser light may arise as the light passes through the
SOA gain medium, as a result of an intraband four-wave mixing
phenomenon. In the presence of the frequency downshift, greater
output power can be generated by operating the wavelength scanning
filter in the positive wavelength sweep direction. Since the
combined action of self-frequency shift and positive tuning allows
higher output to be obtained and enables the laser to be operated
at higher tuning speed, the positive wavelength scan may be the
preferable operation. The output power can be decreased and the
instantaneous linewidth can be broadened with an increasing tuning
speed. A short cavity length may be desired to reduce the
sensitivity of the output power and instantaneous linewidth to the
tuning speed.
[0037] With a short length wavelength scanning filter based on the
disk reflector and direct free-space coupling between the gain
medium and the optical wavelength filter, the total cavity round
trip length can be shorter than 20 cm, which is advantageous for
reducing the sensitivity of the output power and instantaneous
linewidth to the tuning speed. Transmission type spinning disk
filter can also be used, but reflection type may be preferred due
to the shorter cavity length.
[0038] FIG. 3 shows another exemplary embodiment of the
wavelength-swept laser using spinning reflector disk. A fiber ring
cavity 702 can be coupled to the disk scanning filter via
collimating lens 750. For the applications where the high speed
tuning is not essential so that the relatively long cavity length
can be allowed, fiber ring cavity with a conventional dual port SOA
712 can be an optional exemplary configuration.
[0039] FIG. 4 shows an exemplary embodiment of the disk-based fiber
ring wavelength swept-laser with long cavity length. Increasing the
cavity length so that the laser light becomes resonant after a
round trip of the cavity is another way to reduce the sensitivity
of the output power and instantaneous linewidth to the tuning
speed. Additional length of fiber 760, whose length depends on the
tuning repetition rate, in the ring cavity 702 enables resonant
tuning. Cavity length variation of the laser cavity with disk
scanner may be smaller than that of the polygon scanner based
laser, therefore better resonant may be obtainable. A further
preferable cavity resonant may be obtained by using transmission
type disk scanning filter, because the cavity mirror position is
fixed and the cavity length for each wavelength is not changing as
the disk spins. The spinning speed of the disk 500 can be
maintained constant by using a feedback loop maybe with a
monitoring beam 110 for measuring the rotational speed. Active
phase tuning with an electro-optic phase modulator or a piezo
modulator can be also utilized to remove the phase variation due to
non-uniformities in disk thickness and flatness. The monitoring
beam 110 can also be used to provide a cavity length change
feedback to the phase modulator.
[0040] FIG. 5 shows an exemplary embodiment of the resonant cavity
fiber Raman ring laser using the disk scanning filter. Since long
length of optical fiber 760 is used for resonant wavelength tuning,
Raman gain can be induced in the long length of fiber 760 with
proper pump light 770 supplied through a WDM coupler 780. Special
type of fiber can be used as a long length fiber 760 in the cavity
to enhance the Raman gain efficiency. Since the Raman gain
wavelength band is determined by the wavelength band of the pump
light, wavelength swept-laser with arbitrary wavelength tuning band
may be obtained as far as the pump light with proper wavelength
band is available. Also, depending on the pump light power and the
Raman gain efficiency in the fiber, high power wavelength-swept
laser may be implemented. Pump light for the Raman gain can be also
provided in backward direction to the laser light and both forward
and backward pumps can be used simultaneously to obtain higher
gain. The pump light is not limited to the light with a single
wavelength component. To obtain a broad bandwidth Raman gain, a
multiple wavelength pump light can be preferably utilized. This
exemplary configuration can be further expanded to achieve a laser
tuning range beyond the filter free spectral range by using
multiple Raman pump light staggered in wavelength, whose gain
bandwidth is broader than the free spectral range of the filter,
that are progressively cycled on and off.
[0041] FIG. 6A shows an exemplary embodiment of the scanning disk
reflector (or transmission window) pattern 520 configuration. For
example, more than a hundred reflector strips 520 can be written on
the spinning disk 500. Spatially dispersed line of wavelength
components 580 is incident on the disk preferably with 90 degree
orientation to the reflector strip. The thickness and the spacing
between the reflector strips can be determined based on the
consideration explained above. The region where there is no
reflector (or transmission window) may be anti-reflection coated.
The bigger (larger diameter) disk may be preferred as far as it's
spinning speed is not significantly slower than that of the smaller
disk, because larger number of reflector strip elements can be
written on the disk providing faster tuning repetition rate with
the same spinning speed. If the disk is spun 560 at 1000
rotations/s, with more than a hundred reflector strips, faster than
100 kHz tuning repetition rate can be obtained.
[0042] FIG. 6B shows another exemplary embodiment of the scanning
disk reflector (or transmission window) pattern 520 configuration.
In this exemplary embodiment the reflector strip is not a straight
line but a curved line. The curvature of the reflector strip is
carefully designed so that any arbitrary desired wavelength tuning
slope can be obtained with the disk scanner spinning at constant
speed. The angle f between, the reflector strip element and the
line of wavelength components 580 should be accurately aligned to
the pre-designed value to have desired wavelength tuning slope. One
exemplary tuning slope may be desired for OFDI (optical frequency
domain imaging) is that the wavenumber of the filtered light is
linearly swept over time as the disk spins at constant speed.
[0043] FIG. 6C shows another exemplary embodiment of the scanning
disk reflector (or transmission window) pattern 520 configuration.
In this exemplary configuration, multiple rings of the reflector
strips 522 may be written on the disk. Each reflector strip ring
corresponds to specific wavelength filtering condition (e.g.,
tuning range, linewidth), and multiple rings can provide various
options for different wavelength sweep requirements.
[0044] The foregoing merely illustrates the principles of the
invention. Various modifications and alterations to the described
embodiments will be apparent to those skilled in the art in view of
the teachings herein. Indeed, the arrangements, systems and methods
according to the exemplary embodiments of the present invention can
be used with any OCT system, OFDI system, spectral domain OCT
(SD-OCT) system or other imaging systems, and for example with
those described in International Patent Application
PCT/US2004/029148, filed Sep. 8, 2004, U.S. patent application Ser.
No. 11/266,779, filed Nov. 2, 2005, and U.S. patent application
Ser. No. 10/501,276, filed Jul. 9, 2004, the disclosures of which
are incorporated by reference herein in their entireties. It will
thus be appreciated that those skilled in the art will be able to
devise numerous systems, arrangements and methods which, although
not explicitly shown or described herein, embody the principles of
the invention and are thus within the spirit and scope of the
present invention. In addition, to the extent that the prior art
knowledge has not been explicitly incorporated by reference herein
above, it is explicitly being incorporated herein in its entirety.
All publications referenced herein above are incorporated herein by
reference in their entireties.
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