U.S. patent application number 09/854154 was filed with the patent office on 2001-09-27 for fiber delivery system for ultra-short pulses.
Invention is credited to Kafka, James D., Spence, David E..
Application Number | 20010024546 09/854154 |
Document ID | / |
Family ID | 23234665 |
Filed Date | 2001-09-27 |
United States Patent
Application |
20010024546 |
Kind Code |
A1 |
Kafka, James D. ; et
al. |
September 27, 2001 |
Fiber delivery system for ultra-short pulses
Abstract
Fiber delivery systems are desirable to provide convenient
delivery of an output beam from a laser system to a target
distanced from the source. For ultra-short pulse lasers, a limiting
factor in fiber delivery is the dispersion of the optical fiber. A
fiber delivery system for ultra-short pulses that uses a photonic
crystal fiber to Fprovide the appropriate dispersion is
described.
Inventors: |
Kafka, James D.; (Mountain
View, CA) ; Spence, David E.; (Sunnyvale,
CA) |
Correspondence
Address: |
WILSON SONSINI GOODRICH & ROSATI
650 PAGE MILL ROAD
PALO ALTO
CA
943041050
|
Family ID: |
23234665 |
Appl. No.: |
09/854154 |
Filed: |
May 11, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09854154 |
May 11, 2001 |
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09317652 |
May 24, 1999 |
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6236779 |
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Current U.S.
Class: |
385/31 ;
385/141 |
Current CPC
Class: |
G02B 6/02214 20130101;
G02B 6/02295 20130101; B82Y 20/00 20130101; G02B 6/4204
20130101 |
Class at
Publication: |
385/31 ;
385/141 |
International
Class: |
G02B 006/26; G02B
006/16 |
Claims
What is claimed is:
1. A system that delivers sub-picosecond pulses, comprising: a
source that produces an output beam of sub-picosecond pulses at a
wavelength no greater than 1.27 microns; and a photonic crystal
fiber coupled to the source to receive the output beam.
2. The system of claim 1, further comprising: a first optical
device positioned between the source and the fiber, the first
optical element coupling the output beam into an input end of the
fiber.
3. The system of claim 2, further comprising: a second optical
device positioned at an output end of the fiber that delivers the
output beam to a selected target.
4. The system of claim 2, wherein the first optical device is
selected from a lens, a waveplate, an attenuator, a filter, a
polarizer and combinations thereof.
5. The system of claim 3, wherein the second optical element is
selected from a lens, a waveplate, an attenuator, a filter, a
polarizer, an acousto-optic modulator, an electro-optic modulator,
a scanner, a microscope and combinations thereof.
6. The system of claim 1, wherein the source is a mode-locked
Ti:sapphire laser.
7. The system of claim 1, wherein the source is a synchronously
pumped OPO.
8. The system of claim 1, wherein the source is a mode-locked
Cr-doped colquiriite laser.
9. The system of claim 1, wherein the source is a mode-locked fiber
laser.
10. The system of claim 1, wherein the source is a mode-locked
Forsterite laser.
11. The system of claim 1, wherein the source is a mode-locked
Nd-doped glass laser.
12. The system of claim 1, wherein the source is a mode-locked
Yb-doped glass laser.
13. The system of claim 1, wherein the fiber provides delivery of
pulses at the target that have a pulsewidth less than 2 times the
pulsewidth of the source.
14. The system of claim 1, wherein the fiber provides delivery of
pulses at the target that have a bandwidth less than 2 times the
bandwidth of the source.
15. The system of claim 1, wherein the photonic crystal fiber is a
large core photonic crystal fiber.
16. The system of claim 1, wherein the photonic crystal fiber is a
dispersion flattened photonic crystal fiber.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates generally to fiber delivery of
ultra-short pulses, and more particularly to the use of photonic
crystal fibers in an ultra-short pulse delivery system.
[0003] 2. Description of Related Art
[0004] Fiber delivery systems are desirable for laser systems to
provide convenient delivery of an output beam to a target distanced
from the source. In particular, for ultra-short pulse lasers, a
limiting factor in fiber delivery is the dispersion of the optical
fiber.
[0005] At wavelengths of less than 1.27 microns, all step-index
fibers have normal dispersion. In this regime, the ultra-short
pulses broaden substantially while propagating in a fiber of
lengths as short as a few meters. Prism or grating pairs, which
provide anomalous dispersion, have been used to compensate the
dispersion of the fiber. However, this increases complexity and
cost and in the case of grating pairs, is inefficient.
Additionally, with a tunable laser, the prism or grating pair
requires adjustment as the wavelength is tuned.
[0006] There have been suggestions to use photonic crystal fibers
to shift the zero dispersion wavelength to shorter values. In
"Group-velocity dispersion in photonic crystal fibers", by D.
Mogilevtsev, T. A. Birks and P. St. J. Russell, in Optics Letters
23, 1662 (1998) it is suggested that this may be useful in
telecommunication systems. In "Efficient visible continuum
generation in air-silica microstructure optical fibers with
anomalous dispersion at 800 nm", by J. K. Ranka, R. S. Windeler and
A. J. Stentz, Postdeadline paper at CLEO 1999 (Optical Society of
America), it is shown that in combination with a Ti:sapphire laser,
novel non-linear effects are possible.
[0007] There is a need for a fiber delivery system for delivering
ultra-short laser pulses. As a result, there is a need for a fiber
that has an appropriate value of dispersion at wavelengths where
common ultra-short pulse lasers operate.
SUMMARY OF THE INVENTION
[0008] Accordingly, an object of the present invention is to
provide a fiber delivery system for delivering ultra-short
pulses.
[0009] Another object of the present invention is to provide a
fiber delivery system with a fiber that has an appropriate value of
dispersion at wavelengths where common ultra-short lasers
operate.
[0010] These and other objects of the invention are achieved in a
system that delivers sub-picosecond pulses. Included is a source
that produces an output beam of sub-picosecond pulses at a
wavelength no greater than 1.27 microns. A photonic crystal fiber
is coupled to the source to receive the output beam.
BRIEF DESCRIPTION OF THE FIGURES
[0011] FIG. 1 is a schematic diagram of one embodiment of the
present invention illustrating a laser and a photonic crystal
fiber.
DETAILED DESCRIPTION
[0012] The present invention can utilize several sources of
ultra-short pulses in the wavelength range between 700 and 1270 nm.
The most popular is the Ti:sapphire laser, however other sources
include optical parametric oscillators and the Cr doped
colquiriites such as LiSAF, LiCAF, LiSCAF and LiSGAF. Also included
are longer wavelength ultra-short pulse sources, which are then
frequency doubled. Examples include frequency doubled Erbium doped
fiber lasers, frequency doubled optical parametric oscillators and
frequency doubled Forsterite lasers. Finally, there are sources of
sub-picosecond pulses at wavelengths between 1000 and 1100 nm such
as Nd or Yb doped glass.
[0013] Additionally, to deliver low power sub-picosecond pulses
without substantial pulse broadening, the present invention
utilizes a photonic crystal fiber designed to have nearly zero
dispersion at the wavelength that the laser operates. For example,
a Ti:sapphire laser operating at a wavelength of 800 nm with
transform limited pulses with duration of 100 fs, will have a
bandwidth of 7 nm. Typical step-index fibers have a normal
dispersion D of -120 ps/nm-km at 800 nm. The pulse will broaden by
an amount D times the bandwidth, or 840 fs, for each meter of fiber
it passes through. To prevent significant broadening the dispersion
of the fiber should be kept between -20 and +20 ps/nm-km. Clearly
the lower the absolute value of the dispersion, the longer the
fiber that can be used without broadening the pulse.
[0014] As the power of the pulse is increased, nonlinear effects
will become important. Nonlinear effects, such as self phase
modulation (SPM), Raman generation or continuum generation will
broaden the bandwidth of the pulse. This distortion of the pulse is
clearly undesirable for a sub-picosecond pulse delivery system. A
small amount of SPM can be compensated, however, by choosing a
fiber with a small amount of anomalous dispersion. When the correct
balance is chosen, the pulse becomes a soliton and can propagate
long distances in the fiber without changing pulse duration. This
is clearly a desirable situation for a sub-picosecond pulse
delivery system. To obtain a soliton with a given pulse duration
and energy, the dispersion and the core size of the fiber must be
chosen appropriately. Consider the Ti:sapphire laser operating at a
wavelength of 800 nm with transform limited pulses with duration of
100 fs. For a photonic crystal fiber with a dispersion D of +100
ps/nm-km and a core size of 10 microns, the N=1 soliton will have a
peak power of 13 kW. At a repetition rate of 80 MHz this
corresponds to 100 mW of average power.
[0015] Referring now to FIG. 1, one embodiment of the present
invention is a system 10 that delivers sub-picosecond pulses.
System 10 includes a source 12 of sub-picosecond pulses as
described above and a photonic crystal fiber 14 coupled to source
12. Suitable sources 12 include but are not limited to a
mode-locked Ti:sapphire laser, a synchronously pumped OPO, a
mode-locked Cr-doped colquiriite laser, a mode-locked fiber laser,
a mode-locked Forsterite laser, a mode-locked Nd-doped glass laser,
a mode-locked Yb-doped glass laser and the like. A first optical
device 16 is positioned between source 12 and fiber 14. First
optical element 16 couples an output beam from source 12 into an
input end of fiber 14. Suitable first optical elements 16 include
but are not limited to a lens, a waveplate, an attenuator, a
filter, a polarizer and combinations thereof.
[0016] A second optical device 18 is positioned at an output end of
fiber 14 to reduce the divergence of the output beam from 14 fiber
and deliver the output beam to a selected target 20. Suitable
second optical elements 18 include but are not limited to a lens, a
waveplate, an attenuator, a filter, a polarizer, an acousto-optic
modulator, an electro-optic modulator, a scanner, a microscope and
combinations thereof.
[0017] Photonic crystal fibers typically preserve the polarization
of a linearly polarized input beam that is oriented correctly with
respect to fiber 14. To orient the polarization, a half waveplate
can be used. Further, an attenuator, which may consist of a
polarizer and waveplate, can be used to adjust the power delivered
to fiber 14. The output of photonic crystal fiber 14 may be
directed to an attenuator, or an acousto-optic or electro-optic
modulator to modulate the intensity of the output beam. A scanning
system may be used to deflect the direction of the beam. Further,
the output of fiber 14 may be directed to an optical instrument
including a microscope.
[0018] In a second embodiment, fiber 14 is a large core photonic
crystal fiber. Typical fibers have core sizes of 1-2 microns in
radius. As the power of the ultra-short pulses in fiber 14 is
increased, nonlinear effects begin to broaden the bandwidth and
distort the pulse. A fiber 14 with a larger core size can deliver
higher peak power pulses without pulse distortion. For a given
length of fiber 14 with twice the core size, four times the power
can be delivered with a comparable amount of pulse distortion. A
practical upper limit is placed on the core size, since the bending
losses also increase with larger core size.
[0019] In a third embodiment, fiber 14 consists of a dispersion
flattened photonic crystal fiber. In a typical fiber, the
dispersion will remain between -20 and +20 ps/nm-km for less than
100 nm. In dispersion flattened fiber 14, the dispersion remains
small over a larger range of wavelengths. When used in conjunction
with a tunable source of sub-picosecond pulses, such as a
Ti:sapphire laser, fiber 14 allows the delivery of sub-picosecond
pulses over a large range of wavelengths.
[0020] The foregoing description of a preferred embodiment of the
invention has been presented for purposes of illustration and
description. It is not intended to be exhaustive or to limit the
invention to the precise forms disclosed. Obviously, many
modifications and variations will be apparent to practitioners
skilled in this art. It is intended that the scope of the invention
be defined by the following claims and their equivalents.
* * * * *