U.S. patent application number 11/105850 was filed with the patent office on 2006-10-19 for optical fiber systems for delivering short high power pulses.
Invention is credited to Siddharth Ramachandran, Stephan Wielandy.
Application Number | 20060233554 11/105850 |
Document ID | / |
Family ID | 36564051 |
Filed Date | 2006-10-19 |
United States Patent
Application |
20060233554 |
Kind Code |
A1 |
Ramachandran; Siddharth ; et
al. |
October 19, 2006 |
Optical fiber systems for delivering short high power pulses
Abstract
Described is an optical fiber system for delivering ultrashort
pulses with minimal distortions due to nonlinearity. The system is
based on delivering the optical pulses in a higher order mode (HOM)
of a few-moded fiber. The fiber is designed so that the dispersion
for the HOM is very large. This results in a dispersion length
L.sub.D for the delivery fiber that is exceptionally small,
preferably less than the non-linear length L.sub.NL. Under these
conditions the system may be designed so the optical pulses
experience minimum non-linear impairment, and short pulse/high peak
power levels are reproduced at the output of the delivery
fiber.
Inventors: |
Ramachandran; Siddharth;
(Hoboken, NJ) ; Wielandy; Stephan; (Hillsborough,
NJ) |
Correspondence
Address: |
Law Firm of Peter V.D. Wilde
301 East Landing
Williamsburg
VA
23185
US
|
Family ID: |
36564051 |
Appl. No.: |
11/105850 |
Filed: |
April 14, 2005 |
Current U.S.
Class: |
398/141 |
Current CPC
Class: |
G02B 6/02023 20130101;
G02B 6/03644 20130101; G02B 6/02214 20130101; H04B 10/2507
20130101; H04B 10/2525 20130101; H04B 10/25 20130101 |
Class at
Publication: |
398/141 |
International
Class: |
H04B 10/12 20060101
H04B010/12 |
Claims
1. A method comprising: (a) generating optical pulses, the optical
pulses having a pulse width W, (b) propagating the optical pulses
through a pulse stretcher, (c) converting the propagating mode of
the optical pulses to a HOM, (d) propagating the optical pulses
along a length L of optical fiber to an output, where L is chosen
such that the pulse width W.sub.O at the output is approximately
equal to W.
2. The method of claim 1 wherein L is less than 20 meters.
3. The method of claim 2 wherein the optical pulses are less than
200 femtoseconds.
4. An optical device comprising: (a) a source of optical pulses,
(b) a pulse stretcher coupled to the source of optical pulses, (c)
a mode converter coupled to the pulse stretcher, (d) an optical
fiber coupled to the mode converter, the optical fiber supporting a
HOM.
5. The optical device of claim 4 wherein the optical pulses have a
wavelength in the range 700-900 nm.
6. The optical device of claim 5 wherein the optical fiber has a
dispersion value of less than -150 ps/nm-km.
7. The optical device of claim 4 wherein the optical fiber has a
dispersion length L.sub.D and a non-linear length L.sub.NL, where
L.sub.D is less than L.sub.NL.
8. The optical device of claim 4 wherein the optical fiber has a
dispersion length L.sub.D and a non-linear length L.sub.NL, where
L.sub.D is less than 0.5 L.sub.NL.
9. The optical device of claim 5 wherein the optical fiber has
A.sub.eff less than 50 microns.sup.2.
10. The optical device of claim 4 wherein the source of optical
pulses is a short pulse laser.
11. The optical device of claim 10 wherein the optical pulses are
less than 200 femtoseconds.
12. The optical device of claim 11 wherein the short pulse laser is
a titanium/sapphire laser.
13. The optical device of claim 4 wherein the mode converter
comprises a long period grating.
14. The optical device of claim 13 wherein the long period grating
has a TAP.
15. The optical device of claim 4 wherein the pulse stretcher
comprises a bulk-optics element.
16. The optical device of claim 4 wherein the optical fiber
terminates with a collimating element.
17. The optical device of claim 4 wherein the optical fiber has a
length of less than 20 meters.
18. An optical fiber supporting a HOM, and having a dispersion
length L.sub.D and a non-linear length L.sub.NL, where L.sub.D is
less than L.sub.NL.
19. The optical fiber of claim 16 where L.sub.D is less than 0.5
L.sub.NL.
20. The optical fiber of claim 16 wherein the optical fiber
supports LP02.
Description
FIELD OF THE INVENTION
[0001] This invention relates to optical fiber systems that produce
very short, high power, optical pulses.
BACKGROUND OF THE INVENTION
[0002] (Parts of the following section may not be prior art.)
[0003] Optical fiber lasers are available that produce optical
pulses with high pulse energy, good beam quality and excellent
optical characteristics. Several applications for these optical
pulse lasers exist, ranging from time-resolved near-field scanning
optical microscopy (NSOM) pump-probe experiments for understanding
ultrafast electronic processes in materials (see S. Smith, N. C. R.
Holme, B. Orr, R. Kopelman and T. B. Norris, "Ultrafast measurement
in GaAs thin films using NSOM," Ultramicroscopy, vol. 71, pp.
213-223, 1998); for two-photon fluorescence of dyes (see A. Lago,
A. T. Obeidat, A. E. Kaplan, J. B. Khurgin, P. L. Shkilnikov and M.
D. Stern, "Two-photon-induced fluorescence of biological markers
based on optical fiber," Optics Letters, vol. 20, pp. 2054-2056,
1995); for studying biological processed in living tissues (see G.
Alexandrakis, E. B. Brown, R. T. Tong, T. D. McKee, R. B. Campbell,
Y. Boucher, and R. K. Jain, "Two-photon fluorescence correlation
microscopy reveals the two-photon natures of transport in tumors,"
Nature Medicine, vol. 10, pp. 203-207, 2004). The last application
has potential impact on the prospects for non-invasive cancer
detection schemes where the delivery fiber is an endoscope (see E.
B. Brown, Y. Boucher, S. Nasser, R. K. Jain, "Measurement of
macromolecular diffusion coefficients in human tumors,"
Microvascular Research, vol. 67, pp. 231-236, 2004).
[0004] For the case of studying live tissues, the fs pulse acts as
the pump beam that excites fluorescence mediated by a 2-photon
process. Since multi-photo processes are by nature inefficient,
high peak powers are needed. However, this cannot be achieved by
increasing the average power of the source, because high average
power will cause tissue damage. Hence, such applications typically
require pulses of the duration of roughly 100 fs, with pulse
energies as high as 1 nJ, while the average power is maintained at
roughly 100 mW or lower. A commonly used laser source for such
schemes is a mode-locked Ti:Sapphire laser that can output very
high peak powers with repitition rates of .about.80 MHz.
[0005] The delivery fiber desirably propagates the high power,
short pulses through a (typically) 1-2 meter-long fiber, and
provides an output that is close in characteristics to the laser
output. However, there are two physical constraints that affect the
output from the delivery fiber. The dispersion of the fiber, due to
material as well as waveguide dispersion, leads to pulse broadening
that transforms the 100-fs pulse at the input of the fiber into
10-20 ps long pulse at the fiber-output. In addition, since the
peak power levels are so high, nonlinear phase shifts due
self-phase modulation (SPM) lead to a narrowing of the spectral
width of the pulse, further broadening the pulse. The dispersion
effect is linear, and thus may be compensated by a bulk linear
chirp element between the Ti:Sapphire output and fiber input. The
linear chirp element may be a bulk grating or prism pair used to
stretch or compress pulses. Such elements are capable of providing
arbitrary amounts of positive or negative dispersion. For this
application, they may be adjusted to provide dispersion that is
equal in magnitude, but opposite in sign to that of the specified
length of the fiber endoscope/delivery medium.
[0006] Accordingly, while the dispersion problem may be addressed
with some effectiveness, the nonlinear SPM effect is
non-recoverable. Hence, a majority of fiber delivery schemes work
with special fibers or complicated phase engineering of pulses to
counteract the SPM effect.
[0007] In a high performance system the delivery fiber should also
be a single mode fiber with low loss. Propagation in multiple modes
degrades the ability to tightly focus the output from the fiber. A
tightly focused output enables concentrating the high peak power
pulse on a small region, thus enabling efficient 2-photon
fluorescence, as well as ensuring high resolution for microscopy
applications. Propagation in multiple modes also spreads the pulses
due to modal dispersion, which lowers the peak power and reduces
the efficiency of nonlinear measurement techniques.
[0008] Low bend losses are desirable in applications, such as
endoscopes, where the fiber, even though short in overall length,
may still undergo multiple bends. Connection losses include the
concatenation of a collimating element at the fiber output. This
element will focus the output to a tight spot. Candidates for the
collimating element are fiber-GRIN lenses, or thin-film-based
diffractive optic elements such as mode transformers, or other beam
shaping elements. Normally, such miniature beam shaping elements
can be epoxy-bonded to the tip of the fiber, but considering that
very high peak powers will emanate from the fiber, it is desirable
that a mode transforming element such a long-period grating or GRIN
lens be used, since they are fiber-based, and can be easily fusion
spliced to the output of the fiber, with low loss and high power
handling capability.
[0009] The baseline candidate for the delivery fiber is a standard
fiber (doped core, and silica cladding) which is single-moded at
the desired wavelength of operation. A typical desired operating
wavelength is .about.800 nm (for Ti:Sapphire lasers), and the
effective area (A.sub.eff) of a standard single mode fiber (SMF) at
this wavelength is <25 .mu.m.sup.2. This fiber satisfies all the
above criteria, but at pulse energies >0.1 nJ, SPM severely
distorts the pulse. The pulse width rapidly expands past 250 fs
(desired pulse widths are <200 fs).
[0010] A variety of solutions to the SPM problem have been
proposed. Among these are using a multimoded fiber, but forcing
signal propagation in the fundamental mode to enable signal
propagation in a large Aeff, thus decreasing SPM. See F. Helmchen,
D. W. Tank and W. Denk, "Enhanced two-photon excitation through
optical fiber by single-mode propagation in a large core," Applied
Optics, vol. 41, pp. 2930-2934, 2002. However, pulse widths
obtained with this method are still undesirably large, especially
for two-photon applications.
[0011] A variation of the above solution is to use a large core,
multimoded microstructure fiber. See D. Ouzounov, K. Moll, M.
Foster, W. Zipfel, W. W. Webb and A. L. Gaeta, "Delivery of
nanojoule femtosecond pulses through large-core microstructured
fibers," Optics Letters, vol. 27, pp. 1513-1515, 2002.
Microstructured fibers are guided by a photonic crystal of air
holes running through the glass fiber, and this mitigates mode
coupling problems. However, it appears that coupling effectively
only into the fundamental mode in microstructured fibers is a
problem, and significant power is lost to higher order modes. This
causes unwanted modal noise in the system. In addition,
microstructured fibers have poor geometric control compared to
standard doped fibers, and a potential drawback is geometric
ovalities that would cause large polarization mode dispersion
(PMD), a source of additional noise.
[0012] Another option is the use of photonic bandgap fibers, where
the signal propagates in a central air core. In this case, most of
the signal energy resides in air, and hence undergoes negligible
amounts of SPM-based pulse broadening. However, photonic bandgap
fibers are difficult to manufacture in comparison to doped fibers,
and hence are not a cost-effective solution. Geometric regularity
problems are severely exacerbated, leading to the possibility of
high PMD and associated problems. They also suffer from the
inability to splice a mode-shaping element at the fiber output,
because the splice causes the photonic bandgap effect to disappear,
and will yield large losses.
[0013] Another proposal is to use pulse shaping schemes to combat
the nonlinear broadening in standard fibers. The pulse is
temporally chirped, and spectrally narrowed before launching into
the delivery fiber. While this produces short pulses, the power
levels are low and not desirable for two-photon applications.
[0014] New approaches that can minimize the deleterious effects of
nonlinear SPM, while maintaining the advantages of a standard
fiber, such as low propagation and bend losses, low PMD, ability to
splice to GRINs and other lenses, and high manufacturing yield and
control, would represent a significant advance in the
technology.
BRIEF STATEMENT OF THE INVENTION
[0015] We have developed an optical fiber system for delivering
ultrashort pulses with minimal distortions due to nonlinearity. The
system is based on delivering the optical pulses in a higher order
mode (HOM) of a few-moded fiber. The fiber is designed so that the
dispersion for the HOM is very large. This results in a dispersion
length L.sub.D for the delivery fiber that is exceptionally small,
preferably less than the non-linear length L.sub.NL. Under these
conditions the optical pulses experience minimum non-linear
impairment, and short pulse/high peak power levels are reproduced
at the output of the delivery fiber.
BRIEF DESCRIPTION OF THE DRAWING
[0016] FIG. 1 is a refractive index plot for a specially designed
few mode/HOM fiber optical fiber;
[0017] FIG. 2 is a plot showing dispersion for LP02 mode
propagation in the optical fiber of FIG. 1;
[0018] FIG. 3 is a plot of effective area (A.sub.eff) for the
optical fiber of FIG. 1;
[0019] FIG. 4 is a refractive index plot for another few mode/HOM
fiber optical fiber;
[0020] FIG. 5 is a plot showing LP02 dispersion for the optical
fiber of FIG. 4;
[0021] FIG. 6 is a plot of A.sub.eff for the optical fiber of FIG.
4;
[0022] FIG. 7 is a refractive index plot for three different
optical fibers A, B, and C, suitable for use in the invention;
[0023] FIG. 8 is a plot showing dispersion for the HOM LP02 in the
optical fibers A, B, and C of Fig. 7;
[0024] FIG. 9 is a schematic presentation of a short pulse delivery
system according to one embodiment of the invention;
[0025] FIGS. 10-13 are schematic presentations of the system of
FIG. 9 showing optional collimating elements.
DETAILED DESCRIPTION
[0026] The relative magnitudes of dispersive and nonlinear effects
in fibers used for short pulse propagation are succinctly described
by two characteristic lengths, the dispersion length L.sub.D, and
the nonlinear length L.sub.NL, given by: L D = ( .tau. 2 D ) ( - 2
.times. .pi. .times. .times. c .lamda. 2 ) .times. .times. L NL = c
.times. .times. A eff n 2 .times. .omega. peak ( 1 ) ##EQU1## where
.tau. is the undistorted pulsewidth, .beta..sub.2 is the dispersion
of the fiber waveguide, c is the speed of light, .omega. is the
central frequency of the pulse, n.sub.2 is the nonlinear response
of the fiber material, P.sub.peak is the peak power of the pulse in
the fiber, A.sub.eff is its effective area, and .lamda. is the
central wavelength of the pulse. These characteristic lengths
describe the maximum distance a pulse can travel before it becomes
significantly distorted by the corresponding impairment. Since
dispersion can be easily compensated but SPM cannot, it is
desirable to design a fiber such that L.sub.D is as small as
possible in comparison to L.sub.NL. In this case, a highly chirped
pulse can be launched into a fiber so that it is compressed as it
propagates through the fiber and reaches its shortest duration and
highest peak power (and hence becomes subject to significant SPM)
only near the output end of the fiber. In other words, for the
condition: L.sub.D<<L.sub.NL (2) the pulse will not travel a
large enough distance with high peak power to experience
significant nonlinear pulse distortion, hence facilitating high
energy pulse transmission.
[0027] In standard SMFs an 800-nm, 100-fs pulse with 1 nJ energy,
typical values for the characteristic lengths are L.sub.D.about.9
cm and L.sub.NL.about.1.3 cm (the corresponding dispersion of SMF
is -100 ps/nm-km). Thus L.sub.D>>L.sub.NL, and nonlinear
effects dominate, yielding undistorted pulses only for energies as
low 0.1 nJ (that is, only for pulse energies as low as 0.1 nJ, the
L.sub.D/L.sub.NL ratio is substantially smaller than unity).
Existing fiber designs to combat this problem, as mentioned
earlier, concentrate on satisfying condition (2) by increasing the
A.sub.eff for signal propagation. This serves to make L.sub.NL
significantly larger than L.sub.D (which is held nominally constant
and similar to SMF).
[0028] The novel class of fiber designs proposed here yield an
innovative means to satisfy condition (2). Instead of increasing
A.sub.eff (and thus L.sub.NL), the signal is propagated in a higher
order mode (HOM) of a fiber specially designed to yield very high
negative dispersions for one particular HOM. Hence, condition (2)
is satisfied by holding L.sub.NL nominally constant and similar to
that of SMF, but L.sub.D is significantly shortened by increasing
the magnitude of (negative) dispersion provided by the HOM of the
fiber.
[0029] HOMs of specially designed few moded fibers are especially
suited for this application, because HOMs can offer very high
dispersion values, while maintaining a large A.sub.eff, and very
low propagation and bend losses. It has been demonstrated that the
LP.sub.11 mode of a fiber can have dispersions as high as -700
ps/nm-km at the operation wavelength of 1550 nm. It has also been
shown that the LP.sub.02 mode at the operation wavelength of 1550
nm can have -210 ps/nm-km dispersion, and only 0.45 dB/km loss,
yielding very high figures of merit (FOM=dispersion/loss) of 466
ps/nm-dB. This enables up to 50% longer transmission distances for
communication pulses, because the large dispersion and A.sub.eff of
these fibers mitigate nonlinear distortions in comparison to a
communications system that uses single mode dispersion compensating
fibers.
[0030] Optical fibers suitable for use in the invention have low
ratios of L.sub.D/L.sub.NL, which enables high power pulse
propagation for fs laser pulse delivery systems, as described
earlier. In the preferred embodiments, this ratio is less than 1,
and preferably less than 0.5. The specific optical fibers described
here utilize the LP.sub.02 of the fiber for pulse propagation, but
similar designs can be achieved for any HOM. While such designs can
be applied for any wavelength of operation, illustrative designs
described below are optimized for fiber-delivery of Ti:Sapphire
laser pulses, which nominally operate in the 800-nm wavelength
range. That suggests that the wavelength range over which the
devices of the invention preferably operate is 700-900. However,
other wavelength regimes may also be found useful. As a reference,
the specific optical fiber designs used to illustrate the invention
can be compared to SMF, which has a L.sub.D/L.sub.NL ratio of
.about.6.92, which yields undistorted pulses for pulse-energies up
to 0.1 nJ (maximum undistorted pulse energy achievable with a fiber
is roughly proportional to the L.sub.D/L.sub.NL ratio-value of
L.sub.NL depends on pulse energy as well as undistorted width--this
has been calculated for 1-nJ pulses of 100 fs width, in all cases
illustrated here). The objective, in some preferred embodiments, is
to achieve L.sub.D/L.sub.NL ratios smaller than unity. Few mode
fibers supporting these specially designed HOMs can be
distinguished from standard multimode fibers in two respects.
Firstly, they are intentionally designed to be highly dispersive
for one particular, desired HOM, in contrast with multimode fibers,
where most of the modes experience negligible waveguide dispersion,
and the dispersion of all modes is similar to the material
dispersion of silica glass. Secondly, the fibers are designed such
that the propagation constant of the desired HOM of propagation is
sufficiently separated from other modes, so as to avoid intermodal
coupling at bends.
[0031] FIG. 1 shows an experimentally measured refractive index
profile of a fiber fabricated to yield low L.sub.D/L.sub.NL ratios
for 100-fs, 1-nJ pulses traveling in the LP.sub.02 mode. FIG. 2
shows the measured dispersion for this fiber. FIG. 3 shows the
A.sub.eff for the LP.sub.02 mode. As is clear from FIGS. 2 and 3,
the LP.sub.02 mode of this fiber has approximately 8 times larger
negative dispersion compared to SMF (corresponding to reduction of
dispersion length, L.sub.D by a factor of 8). At the same time, the
A.sub.eff is approximately 25 .mu.m.sup.2 at 840 nm, which is
similar to that of SMF (SMF A.sub.eff.about.20 .mu.m.sup.2).
Maintaining A.sub.eff similar to SMF has significant advantages
over conventional large A.sub.eff designs because mode coupling and
bend losses increase quadratically with A.sub.eff. A delivery fiber
with high bend losses or mode coupling would in most cases be
unsuitable, for example, in endoscope applications where the fiber
is expected to substantially bend during operation. To reduce
losses in a delivery fiber operating at approximately 800 nm,
A.sub.eff may be held to less than 50 microns.sup.2.
[0032] Given the properties of the novel HOM fiber illustrated in
FIG. 1, one can estimate that the L.sub.D/L.sub.NL ratio.about.0.69
for 100 fs, 1 nJ pulses, leading to an order of magnitude increase
in extractable undistorted pulse energies in comparison to SMF.
Thus, these fibers can support short (100 fs) pulse delivery of
energies up to 1 nJ.
[0033] The flexibility of this design space is further illustrated
with the theoretically designed fiber whose refractive profile is
illustrated in FIG. 4. FIGS. 5 and 6 show the dispersion and
A.sub.eff for the LP.sub.02 mode at 800 nm for this fiber,
respectively. As can be seen, dispersion values as high as -2300
ps/nm-km are easily achievable, with a similar A.sub.eff (.about.21
.mu.m.sup.2). The resultant L.sub.D/L.sub.NL ratio for this fiber
is 0.29 (a factor of 24 smaller than SMF), yielding undistorted 100
fs pulses with energies as high as 2.4 nJ. This value comfortably
surpasses the typical pulse energies required for several
applications such as 2-photon fluorescence imaging for in-vivo
cancer detection in live tissues.
[0034] HOMs also provide a greater degree of design-freedom to
achieve desired dispersion profiles, in addition to the large
dispersion magnitudes. Note that the dispersion profiles in FIGS. 2
and 5 have high, negative-dispersion-slopes, in addition to high
magnitudes of dispersion. All fibers, including the large A.sub.eff
microstructured fibers, and conventional SMF previously used for
this application, have similar high dispersion slope values. This
causes problems because most commonly available, bulk-optic-based
pulse chirping elements used before the light enters the fiber,
impart negligible dispersion slope to the pulse. As a result, even
in the absence of SPM nonlinearities, the recompressed pulse at the
fiber-output is often broadened compared to the original laser
pulse, due to uncompensated dispersion arising from the
dispersion-slope mismatch between the bulk optic pulse stretcher
and that of the fiber. Dispersion engineering flexibility with HOMs
enables achieving any desired dispersion slope. FIG. 7 shows three
different refractive index profiles, for fibers A, B, and C, each
yielding high negative dispersion for the LP.sub.02 mode. The
dispersion-curves for these three fibers are shown in FIG. 8. In
the 795 nm to 805 nm wavelength range (the shaded region
illustrates the bandwidth of operation), the profile for fiber A
yields a negative dispersion slope, as do standard SMF or large
area microstructured fibers, but profiles for fibers B and C yield
zero and positive dispersion slope values, respectively. This
enables efficient dispersion matching with any kind of bulk-optic
pulse stretcher design.
[0035] The inventive fiber designs illustrated here can be utilized
in a high power pulse delivery system. FIG. 9 shows an exemplary
device schematic. The fiber device is preceded by an assembly
comprising the short pulse laser followed by a bulk-optic pulse
stretcher (the pulse stretcher is illustrated as a pair of bulk
gratings, but the equivalent function can also be achieved with a
pair of prisms or a specialized dispersive element such as a
photonic bandgap fiber). Then, the signal enters the HOM fiber via
a mode converter, which converts the incoming beam with a nominally
Gaussian spatial profile, to match that of the HOM in the fiber.
Bulk diffractive-optic elements can be used to achieve broadband
mode conversion with more than 99% conversion efficiency. A
preferred schematic uses in-fiber long-period gratings that operate
at the so called turn-around-point (TAP), as disclosed in U.S. Pat.
No. 6,768,835. That patent is incorporated herein by reference for
more details on TAPs in optical fibers. It describes mode
converters that can be induced in fibers similar to the dispersive
HOM fiber disclosed here. These mode converters can be fabricated
to achieve up to 99.997% mode conversion efficiency, with losses
less than 0.2 dB.
[0036] For most applications, the high power pulse at the output of
the fiber is focused on to a small spot size to obtain large
2-photon fluorescence. As mentioned earlier, the HOM fiber should
not be confused with a multimode fiber. The signal exits the fiber
in a single well-defined mode. Hence it can be focused with lenses
in a manner identical to conventional Gaussian beams, to achieve
any desired spot size. FIGS. 10-13 illustrate this, and show the
modal image of the LP.sub.02 mode in the HOM fiber, as well as a
similar, but spatially contracted mode pattern after a collimating
device following the fiber. FIG. 10 shows the use of a standard
bulk-optic lens, while FIG. 11 shows the use of a fiber-based GRIN
lens. Alternatively, with reference to FIG. 12, if a Gaussian
output is desired a TAP grating similar to those described earlier
may be used. In FIG. 13, the collimating element represents
generically any one of a variety of beam shaping elements that can
convert a complex spatial pattern into a Gaussian pattern.
[0037] While the mode converters described in connection with FIG.
9 for converting the incoming, and optionally the outgoing, signals
between modes are long period gratings, the mode converters may be
of any suitable design. The mode converting functionality may be
achieved within the delivery fiber using in-fiber grating mode
converters. Alternatively, holographic free-space mode converters,
or tapered hollow-core fibers, may be employed.
[0038] While different types of mode converters may be used for the
invention, as indicated above, a preferred means to obtain the
mode-converting device functionality is with a broadband long
period fiber grating (LPG). The LPG may be induced in the HOM fiber
itself, enabling a low cost, low loss, mode-converting device.
Broadband mode converters are known that cover a wavelength range
as large as 500 nm. For more details see S. Ramachandran, M. Yan,
E. Monberg, F. Dimarcello, P. Wisk and S. Ghalmi, "Record bandwidth
microbend gratings for spectrally flat variable optical
attenuators," IEEE Photon. Tech. Lett., vol. 15, pp. 1561-1563,
2003; S. Ramachandran, U.S. Pat. No. 6,768,835, both of which are
incorporated by reference herein.
[0039] Whereas it is shown or may be inferred that the output from
the short pulse device of the invention is propagated in free
space, using standard collimating devices, it may also be coupled
to other forms of media.
[0040] Methods for making optical fibers with profiles like those
in FIG. 7 are well known and well developed. The core region
generally consists of silica doped with germanium at concentrations
less than 10 wt % at the position of maximum index, and graded with
radius to provide the shape desired. The center core is typically
has a radius of less than 20 microns. The inner cladding region may
be undoped, or lightly doped.
[0041] Optical fibers as described above that are specially
designed to support HOMs may be construed as meaning that a
substantial portion, typically a predominant portion, of the
optical energy propagating in the optical fiber is in a mode higher
than the fundamental mode LP.sub.01. Preferred HOMs are LP02
through LP0,10; and LP11 through LP1,10.
[0042] The element used to chirp the pulses, in the systems
described here, is referred to as a pulse stretcher, which is a
term familiar to those skilled in the art. For a another
description of these elements see http://www-
phys.llnl.qov/Orqanization/VDivision/Research/USP/USPFacilityVirtualTour/-
cpa.h tml incorporated herein by reference. The preferred choice of
pulse stretchers are those operating on bulk optics, i.e. the
optical pulses propagate through the stretching element.
High-quality gratings and prisms are in this category.
[0043] The operation of the devices described above relies in part
on having relatively high dispersion in the HOM fiber. While the
actual dispersion value will vary, the typical dispersion value
will be less than (more negative than) -150 ps/nm-km. The length of
the delivery fiber will in part be determined by the dispersion
value. In a qualitative sense, that length is where the dispersion
in the HOM fiber compensates for the nominal dispersion from the
pulse stretcher that appears at the input of the HOM fiber, but
before the optical pulses undergo significant non-linear
distortion. That length is typically from 1-20 meters. It should be
evident that this relatively short length distinguishes in the
usual sense this fiber from a transmission fiber.
[0044] While in principle the devices described here may function
over a wide band of pulse frequencies and pulse length, the
invention is preferably directed to devices where the pulses are
femtosecond pulses (i.e. less than 1 picosecond), or shorter. In
preferred embodiments the pulses are less than 200
femtoseconds.
[0045] Various other modifications of this invention will occur to
those skilled in the art. All deviations from the specific
teachings of this specification that basically rely on the
principles and their equivalents through which the art has been
advanced are properly considered within the scope of the invention
as described and claimed.
* * * * *
References