U.S. patent application number 10/648797 was filed with the patent office on 2005-03-03 for nonlinear optical device.
This patent application is currently assigned to MESOPHOTONICS LIMITED. Invention is credited to Baumberg, Jeremy John, Charlton, Martin David Brian, Lincoln, John, Netti, Maria Caterina, Parker, Greg Jason, Perney, Nicolas, Wilkinson, James, Zoorob, Majd.
Application Number | 20050047739 10/648797 |
Document ID | / |
Family ID | 34116789 |
Filed Date | 2005-03-03 |
United States Patent
Application |
20050047739 |
Kind Code |
A1 |
Parker, Greg Jason ; et
al. |
March 3, 2005 |
NONLINEAR OPTICAL DEVICE
Abstract
There is provided a non-linear optical device for enhancing the
bandwidth accessible in the nonlinear generation of an optical
signal. The device comprises a planar optical waveguide, the planar
optical waveguide being operative to generate an optical output
from an optical input having an input bandwidth by means of a
non-linear optical process, the optical output having a wavelength
within an accessible bandwidth, wherein the planar optical
waveguide is operative to enhance the accessible bandwidth such
that the ratio of the accessible bandwidth to the input bandwidth
is at least 4. The device is particularly applicable to broad
optical continuum generation, but may also be used in a parametric
oscillator or amplifier arrangement with broad tuning range.
Inventors: |
Parker, Greg Jason;
(Brockenhurst, GB) ; Baumberg, Jeremy John;
(Winchester, GB) ; Wilkinson, James; (Southampton,
GB) ; Charlton, Martin David Brian; (Southampton,
GB) ; Zoorob, Majd; (Southampton, GB) ; Netti,
Maria Caterina; (Southampton, GB) ; Perney,
Nicolas; (Southampton, GB) ; Lincoln, John;
(Wiltshire, GB) |
Correspondence
Address: |
NIXON & VANDERHYE, PC
1100 N GLEBE ROAD
8TH FLOOR
ARLINGTON
VA
22201-4714
US
|
Assignee: |
MESOPHOTONICS LIMITED
Southampton
GB
|
Family ID: |
34116789 |
Appl. No.: |
10/648797 |
Filed: |
August 27, 2003 |
Current U.S.
Class: |
385/122 |
Current CPC
Class: |
G02F 1/365 20130101;
G02F 1/3528 20210101; G02F 1/353 20130101 |
Class at
Publication: |
385/122 |
International
Class: |
G02B 006/00 |
Claims
1. A non-linear optical device comprising a planar optical
waveguide, at least a section of the planar optical waveguide being
operative to generate an optical output from at least a portion of
an optical input having an input bandwidth by means of a non-linear
optical process, the optical output having a wavelength within an
accessible bandwidth, wherein the planar optical waveguide is
operative to enhance the accessible bandwidth such that the ratio
of the accessible bandwidth to the input bandwidth is at least 4,
the term "bandwidth" being defined here as the wavelength interval
beyond which the spectral radiant Intensity remains below a level
of -30 decibels (0.001) of the maximum value.
2. A non-linear optical device according to claim 1, wherein the
ratio of the accessible bandwidth to the input bandwidth is at
least 10.
3. A non-linear optical device according to claim 1, wherein the
planar optical waveguide has a core layer with a refractive index
of at least 1.7.
4. A non-linear optical device according to claim 1, wherein the
planar optical waveguide has a core layer which comprises a
material selected from a group including the oxides of tantalum,
hafnium, zirconium, titanium and aluminium.
5. A nonlinear optical device according to claim 1, wherein the
planar optical waveguide has a core layer which comprises a
material doped with a rare earth element.
6. An optical wavelength converter according to claim 1, wherein
the planar optical waveguide has a core layer which comprises
silicon nitride (SiN).
7. A non-linear optical device according to claim 1, wherein the
accessibl bandwidth is at least 200 nm.
8. A nonlinear optical device according to claim 1, wherein the
accessible bandwidth is at least 500 nm.
9. A non-linear optical device according to claim 1, wherein th
ratio of the accessible bandwidth to the input bandwidth is
non-linearly dependent on the intensity of the optical input.
10. A non-linear optical device according to claim 1, wherein the
nonlinear optical process comprises one or more processes selected
from a group which includes self-phase modulation, self-focussing,
four-wave mixing, Raman scattering and soliton formation.
11. A non-linear optical device according to claim 1, wherein the
planar waveguide comprises a ridge.
12. A non-linear optical device according to claim 1, wherein the
planar waveguide comprises a rib.
13. A non-linear optical device according to claim 1, wherein a
portion of the planar waveguide is tapered.
14. A non-linear optical device according to claim 1, wherein a
portion of the planar waveguide includes a structure, the structure
being operative to modify the optical input and/or optical
output.
15. A non-linear optical device according to claim 14, wherein the
structure comprises a photonic structure.
16. A non-linear optical device according to claims 14, wherein the
structure is operative to filter the optical input and/or optical
output.
17. A non-linear optical device according to claims 14, wherein the
structure is operative to compress temporally the optical input
and/or optical output.
18. A non-linear optical device according to claims 14, wherein the
structure is operative to modify the optical dispersion
characteristics of the planar optical waveguide.
19. A non-linear optical device according to claim 1, wherein the
planar optical waveguide comprises a further planar layer which is
operative to modify the optical dispersion characteristics of the
planar optical waveguide.
20. An optical continuum source comprising a non-linear optical
device according to claim 1, wherein the optical output has an
optical spectrum comprising an optical continuum as a result of
non-linear broadening of the optical input.
21. An optical continuum source according to claim 20, wherein the
degree of non-linear broadening is by at least a factor of 4.
22. An optical continuum source according to claim 20, wherein the
optical continuum has a bandwidth of at least 200 nm.
23. An optical continuum source according to claim 20, wherein the
degree of broadening is non-linearly dependent on the peak
intensity of the optical input.
24. An optical continuum source according to claim 20, wherein the
nonlinear optical process is seeded with an optical seed input.
25. An optical parametric oscillator comprising a non-linear
optical device according to claim 1 and means for providing optical
feedback at a wavelength within the accessible bandwidth.
26. An optical parametric oscillator according to claim 25, wherein
the optical feedback means is provided at least in part by a
photonic structure.
27. An optical parametric amplifier comprising a non-linear optical
device according to claim 1 adapted to receive a further optical
input to be amplified at a wavelength within the accessible
bandwidth.
28. An optical system including a non-linear optical device
according to claim 1.
29. An optical continuum source comprising a planar optical
waveguide, at least a section of the planar optical waveguide being
operative to generate an optical output having an output bandwidth
from at least a portion of an optical input having an input
bandwidth by means of a non-linear optical process, wherein the
optical output has an optical spectrum comprising an optical
continuum as a result of non-linear broadening of the optical
input, the planar optical waveguide being operative to nhance the
ratio of the output bandwidth to the input bandwidth to at least 4,
the term "bandwidth" being defined here as the wavelength interval
beyond which the spectral radiant intensity remains below a level
of -30 decibels (0.001) of the maximum value.
30. An optical continuum source according to claim 29, wherein the
output bandwidth of the optical continuum is at least 200 nm.
31. An optical continuum source according to claim 29, wherein a
portion of the planar waveguide includes a structure, the structure
being operative to modify the optical dispersion characteristics of
the planar optical waveguide.
32. An optical continuum source according to claim 31, wherein the
structure comprises a photonic structure.
33. An optical continuum source according to claim 31, wherein the
optical dispersion characteristics of the planar optical waveguide
are modified to achieve zero dispersion at points along the
waveguide.
34. An optical continuum source according to claim 31, wherein the
optical dispersion characteristics of the planar optical waveguide
are modified to achieve normal dispersion at a predetermined
wavelength.
35. An optical parametric oscillator comprising: a planar optical
waveguide, at least a section of the planar optical waveguide being
operative to generate an optical output from at least a portion of
an optical input having an input bandwidth by means of a non-linear
optical process, the optical output having a wavelength within an
accessible bandwidth, wherein the planar optical waveguide is
operative to enhance the accessible bandwidth such that the ratio
of the accessible bandwidth to the input bandwidth is at least 4,
the term "bandwidth" being defined here as the wavelength interval
beyond which the spectral radiant intensity remains below a level
of -30 decibels (0.001) of the maximum value; and, means for
providing optical feedback at a wavelength within the accessible
bandwidth.
36. An optical parametric oscillator according to claim 35, wherein
the accessible bandwidth is at least 200 nm.
37. An optical parametric oscillator according to claim 35, wherein
apportion of the planar waveguide includes a structure, the
structure being operative to modify the optical dispersion
characteristics of the planar optical waveguide.
38. An optical parametric oscillator according to claim 35, wherein
the structure comprises a photonic structure.
39. An optical parametric oscillator according to claim 35, wherein
the optical dispersion characteristics of the planar optical
waveguide are modified to achieve negative (anomalous) dispersion
at a predetermined wavelength.
40. An optical parametric amplifier comprising a planar optical
waveguide for receiving a first optical input having a first input
bandwidth and a second optical input having a second input
bandwidth, at least a section of the planar optical waveguide being
operative to amplify the second optical input by generating an
optical output from at least a portion of the first optical input
by means of a non-linear optical process, th optical output and the
second optical input having a wavelength within an accessibl
bandwidth, wherein the planar optical waveguide is operative to
enhance th accessible bandwidth such that the ratio of the
accessible bandwidth to the first input bandwidth is at least 4,
the term "bandwidth" being defined here as the wavelength interval
beyond which the spectral radiant intensity remains below a level
of -30 decibels (0.001) of the maximum value.
41. An optical parametric amplifier according to claim 40, wherein
the accessibl bandwidth is at least 200 nm.
42. An optical parametric amplifier according to claim 40, wherein
a portion of the planar waveguide includes a structure, the
structure being operative to modify the optical dispersion
characteristics of the planar optical waveguide.
43. An optical parametric amplifier according to claim 40, wherein
the structure comprises a photonic structure.
44. An optical parametric amplifier according to claim 40, wherein
the optical dispersion characteristics of the planar optical
waveguide are modified to achieve negative (anomalous) dispersion
at a predetermined wavelength.
45. A method for enhancing the bandwidth accessible in the
generation of an optical output, comprising th step of providing a
planar optical waveguide for receiving an optical input having an
input bandwidth, wherein at least a section of the planar optical
waveguide is operative to generate an optical output from at least
a portion of the optical input by means of a non-linear optical
process, the optical output having a wavelength within an
accessible bandwidth, wherein the planar optical waveguide is
operative to enhance the accessible bandwidth such that the ratio
of the accessible bandwidth to the input bandwidth is at least 4,
the term "bandwidth" being defined here as the wavelength interval
beyond which the spectral radiant intensity remains below a level
of -30 decibels (0.001) of the maximum value.
46. A method for generating an optical signal comprising the steps
of: receiving an optical input signal having an input bandwidth at
an optical input to a planar optical waveguide; guiding the optical
input signal along the planar optical waveguide; and, generating an
optical output signal from at least a portion of the optical input
signal by means of a non-linear optical process in at least a
section of the planar optical waveguide, the optical output signal
having a wavelength within an accessible bandwidth, wherein the
planar optical waveguide is operative to enhance th accessible
bandwidth such that the ratio of the accessible bandwidth to the
input bandwidth is at least 4, the term "bandwidth" being defined
here as the wavelength interval beyond which the spectral radiant
intensity remains below a level of -30 decibels (0:001) of the
maximum value.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to nonlinear optical devices
and in particular to a planar waveguide device for nonlinear
optical signal generation with large accessible bandwidth,
including optical continuum generation.
BACKGROUND TO THE INVENTION
[0002] Optical sources which can generate radiation over a wide
wavelength range currently have many applications in scientific
research, engineering and medicine. Often the application requires
the coherence properties associated with laser radiation and so
various laser materials have been developed which exhibit a broad
gain bandwidth when suitably pumped. A good example is titanium
doped sapphire, which is characterized by a gain bandwidth covering
the range 650-1100 nm. The coherence properties of lasers based on
broadband material systems may be further enhanced in a number of
ways. The technique of modelocking permits the utilization of much
of the available laser bandwidth to obtain a repetitive train of
short (or ultra short) pulses. Alternatively, the laser cavity may
contain frequency selective elements to ensure that the laser only
emits radiation with a relatively narrow spectrum centered around a
particular wavelength. By adjusting a frequency selective element
this wavelength may be tuned across the available gain
bandwidth.
[0003] However, as many laser sources only operate over narrower,
well-defined wavelength ranges, nonlinear optical processes have
been employed to generate other wavelengths using the output from
available laser sources. A wide range of nonlinear optical
processes are known, with the common factor being a nonlinear
dependence of the electric polarization that is induced in the
nonlinear material on the electric field (or intensity) of an
optical input, resulting in the generated optical output. The order
of nonlinearity relates to the specific integer power of the
electric field on which the induced polarization depends. Second
order effects include second harmonic generation, which is commonly
used to "frequency double" the output from a laser source, and also
two-wave mixing. Third order effects are more numerous and include
third harmonic generation, four-wave mixing, self-phase modulation,
self-focussing and Raman scattering.
[0004] As indicated, non-linear optical devices have the advantage
that they can be "bolted on" to the output of existing laser
sources in order to extend the available wavelength range or simply
to generate nonlinearly another well-defined wavelength from the
laser radiation. The strength of the nonlinear effect is generally
determined by the relevant non-linear coefficient of the material
and the peak intensity of the input (pump) beam inside the
material. However, other factors such as interaction length and
accurate phase-matching can be very important in maximizing the
efficiency of conversion in the non-linear interaction. Nonlinear
processes and materials capable of generating radiation over a wide
wavelength range from a relatively narrow band optical input are of
particular interest.
[0005] Optical continuum generation (CG) is an example whereby a
cascade of (generally) third order processes enables the generation
of a coherent optical signal with a continuous, or near continuous,
spectrum over a very broad bandwidth. The continuum generated can
be used in its entirety or optically filtered or sliced as the
application requires. Optical parametric processes are another
example where one or more optical signals that are tunable over a
wide bandwidth are generated from an input pump at a fixed
wavelength. In particular, the optical parametric amplifier (OPA)
provides parametric amplification at two tunable wavelengths (the
signal and idler), whereas the optical parametric oscillator (OPO)
employs (tunable) optical feedback at one, or both, of these
wavelengths to achieve self-oscillation. A variety of techniques
have been investigated for enhancing the bandwidth that can be
accessed by th nonlinear processes described above.
[0006] Optical CG has typically been performed in bulk materials,
both liquid and gas, due to the simplicity of implementation and
the relatively small sample size required.
[0007] However, due the low nonlinear coefficient associated with
many materials, the characteristic threshold intensity is high and
so a high peak intensity laser source is required. This usually
takes the form of a modelocked laser system generating very short
(or ultrashort) pulses which are then amplified and the radiation
focussed tightly onto the target material. Consequently, there is a
high attendant risk of surface or bulk damage to the sample unless
it is a material exhibiting a high damage threshold such as
sapphire, which also exhibits good stability of CG.
[0008] One approach to reducing the threshold pulse energy required
has been the utilization of optical fibers for CG. Despite a
relatively low nonlinear coefficient for the fiber material, the
lateral optical confinement ensures that an adequate optical
intensity can be maintained throughout a long interaction length of
fiber for efficient CG. Nevertheless, the associated pulse energy
damage threshold is also reduced and so end facet damage may occur,
requiring the cleaving of a new facet or provision of a new fiber
entirely. In addition, the stability of CG in optical fibers is
typically low, the overall size may limit the compactness of the
source and the optical mode properties are not easily compatible
with the planar waveguide devices used in photonic integrated
circuits.
[0009] Another issue associated with CG is the characteristic
optical dispersion (variation of refractive index with wavelength)
of the device in which the continuum is generated. It is known that
the threshold for CG is lowered when the pump is at a wavelength
where the dispersion of the device is near zero. Furthermore, due
to the proximity of the anomalous dispersion region, more nonlinear
processes may be accessed and bandwidth may more easily be
generated beyond the zero dispersion wavelength, extending the
spectrum further into the infrared. In the case of bulk materials,
the characteristic dispersion is simply the material dispersion,
which usually lies within the normal dispersion region at the
optical wavelengths of interest. But for waveguides, the situation
is more complex, with the total dispersion also depending on
waveguide and modal dispersion. This provides scope for controlling
the total dispersion via the waveguide design parameters. To this
end, zero dispersion fibers and tapered fibers have been
manufactured.
[0010] A further development in the control of fiber dispersion has
been the fabrication of the so-called microstructured fiber (MF) or
photonic crystal fiber (PCF). These fibers consist of a solid
silica core surrounded by an array of air holes running along the
fiber, which provides a wavelength-dependent effective index for
the cladding and can allow single-mode guidance throughout the
visible and near infra-red. By suitable choice of arrangement and
size of holes, the dispersion properties of the fiber can be
tailored, as can the effective area of the propagating mode. Such
dispersion engineered fibers have been used to enhance the
continuum bandwidth that can be generated from a short pulse pump,
resulting in so-called supercontinuum generation. Here, bandwidth
in excess of 800 nm has been generated as a result of a cascade of
processes, including self-phase modulation (SPM), four-wave mixing
(FWM), Raman scattering (RS), soliton formation and decay, soliton
self-frequency shifting (SSFS) and self-steepening (SS). However,
despite the improvement in CG bandwidth, the microstructured fibers
still suffer from the drawbacks associated with more conventional
fibers, as outlined above. In addition, there are many materials
that can not be fabricated in bulk or fiber form and are therefore
unavailable for CG in these configurations.
[0011] Another waveguide based approach for enhancing a nonlinear
phenomenon has been the employment of a planar chalcogenide glass
(ChG) waveguide. Here, the intention was to enhance the level of
nonlinear phase shift that could be obtained via SPM for a given
pump pulse energy, a key application being optical switching for
optical communication systems. A thin film of high refractive index
GeSe-based glass material formed the core of a planar waveguide
that was subjected to pump pulses from an amplified mode-locked
fiber ring laser. A maximum peak phase shift of 1.6.pi. was
recorded for an input pulse energy of 461 pJ. However, although the
process was accompanied by nonlinear spectral broadening, which
resulted in an optical output having a bandwidth broader than that
of th input pulse, th degree of spectral broadening was not
sufficient to generate an optical continuum.
[0012] Planar waveguides have also been employed for enhanced
performance in parametric devices, such as the optical parametric
oscillator (OPO) and optical parametric amplifier (OPA). Typical
devices comprise a layer of periodically poled material such as
Lithium Niobate (Li.sub.2O.sub.3), also known as PPLN. Although
improved performance is obtained in terms of threshold power and
conversion efficiency, the available tuning range is still
limited.
SUMMARY OF THE INVENTION
[0013] According to a first aspect of the present invention there
is provided a non-linear optical device comprising a planar optical
waveguide, at least a section of the planar optical waveguide being
operative to generate an optical output from at least a portion of
an optical input having an input bandwidth by means of a non-linear
optical process, the optical output having a wavelength within an
accessible bandwidth, wherein the planar optical waveguide is
operative to enhance the accessible bandwidth such that the ratio
of the accessible bandwidth to the input bandwidth is at least
4.
[0014] However, it is preferred that the ratio of the accessible
bandwidth to the input bandwidth is at least 10.
[0015] Here, the term "bandwidth" is defined as the wavelength
interval beyond which the spectral radiant intensity remains below
a level of -30 decibels (0.001) of the maximum value. The
logarithmic definition is appropriate due to the widely differing
spectral intensity of the different wavelengths that may be
generated by the nonlinear optical process.
[0016] Strong optical confinement in the planar waveguide is
obtained when th refractive index of the core layer of the
waveguide is high. Preferably, the planar waveguide has a core
layer with a refractive index of at least 1.7.
[0017] A variety of materials exhibit both the high linear and
nonlinear refractive index preferred for the core layer in the
present invention.
[0018] Preferably, the planar waveguide has a core layer comprising
a metal oxide material. More preferably, the planar waveguide has a
core layer comprising a material selected from a group including
the oxides of tantalum, hafnium, zirconium, titanium and aluminium.
Alternatively, the planar optical waveguide may have a core layer
which comprises silicon nitride (SiN).
[0019] The performance and wavelength range of the nonlinear device
can be extend d by suitable doping of the core material.
Preferably, the planar optical waveguide has a core layer which
comprises a material doped with a rare earth element. An example is
Neodymium, (Nd).
[0020] Although the performance of the nonlinear device may be
characterized in terms of the enhancement of the accessible
bandwidth relative to the bandwidth of th optical input, it is also
desirable that the device is characterized by a large absolute
accessible bandwidth. Preferably, the accessible bandwidth is at
least 200 nm. More preferably, the accessible bandwidth is at least
500 nm.
[0021] Preferably, the ratio of the accessible bandwidth to the
input bandwidth is non-linearly dependent on the peak intensity of
the optical input.
[0022] A non-linear optical device according to the present
invention will typically operate by means of a nonlinear
interaction, which comprises one or more third order nonlinear
optical processes. Preferably, the non-linear optical process
comprises on or more processes selected from a group which includes
self-phase modulation, self-focussing, four-wave mixing, Raman
scattering and soliton formation.
[0023] In addition to the simple broad area broad area waveguide
configuration, a nonlinear device according to the present
invention may comprise other forms of planar waveguide structure,
which may provide enhanced optical confinement. Preferably, the
planar waveguide comprises a ridge waveguide. Alternatively, the
planar waveguide may comprise a rib waveguide.
[0024] One of the problems associated with planar structures, is
the coupling in of light from other devices or sources having a
different geometry, such as optical fibre. This problem can be
mitigated by employing beam shaping or spot-size converting
structures, which can be integrated on the same chip.
[0025] Preferably, a portion of the planar waveguide is tapered.
Preferably, th tapered region is proximate the input of the planar
waveguide. It is preferred that the taper is characterized by a
gradually increasing waveguide (core) width. However, th taper may
be characterized by a gradually decreasing waveguide (core) width
Preferably, the taper is symmetrical.
[0026] The non-linear optical device may also comprise other
structures for pre-processing of the optical input or
post-processing of the optical output. Preferably, a portion of the
planar waveguide includes a structure, the structure being
operative to modify the optical input and/or optical output.
[0027] Preferably, the structure comprises a photonic structure.
Such structures may perform many functions and can be tailored by
appropriate design. Many examples of the photonic structure
(crystal) and applications of photonic structures (crystals) can be
found in the Applicant's co-pending U.S. patent application Ser.
Nos. 09/910,014, 10/147,328, 10/185,727, 10/196,727, 10/240,928,
10/287,792, 10/287,825 and 10/421,949, the discussion of which is
included herein by reference.
[0028] Preferably, th structure is operative to filter th optical
input and/or optical output. The optical transfer function of the
filter may result in changes to both the phase and amplitude of the
different spectral components of the optical signal spectrum. For
example, particular wavelengths or ranges of wavelengths may be
transmitted, whilst others are blocked or reflected.
[0029] Preferably, the non-linear optical device has a structure
which is operative to compress temporally the optical input and/or
optical output. Pulse compression can serve to increase pulse peak
power, leading to a stronger induced nonlinear effect, and can also
pre-compensate for pulse broadening during subsequent propagation
due to refractive index dispersion.
[0030] Preferably, the non-linear optical device has a structure
which is operative to modify the optical dispersion characteristics
of the planar optical waveguide. The structure will typically be
disposed either proximate or in the region of nonlinear signal
generation, and can serve to tailor the waveguide dispersion
characteristics to optimize device performance and accessible
bandwidth enhancement.
[0031] Preferably, the planar optical waveguide comprises a
photonic structure which is operative to modify the optical
dispersion characteristics of the planar optical waveguide.
Alternatively, the planar optical waveguide comprises at least a
further planar layer which is operative to modify the optical
dispersion characteristics of the planar optical waveguide.
[0032] A particular application of the present invention is in
optical continuum generation. The nonlinear device can give rise to
optical continua and supercontinua characterized by particularly
large bandwidth.
[0033] Preferably, an optical continuum source comprises a
non-linear optical device according to the first aspect of the
present invention, wherein the optical output has an optical
spectrum comprising an optical continuum as a result of non-linear
broadening of the optical input.
[0034] Preferably, the degree of non-linear broadening is by at
least a factor of 4.
[0035] Preferably, the optical continuum has a bandwidth of at
least 200 nm.
[0036] Preferably, the degree of broadening is non-linearly
dependent on the peak intensity of the optical input.
[0037] In order to enhance the continuum generation process it is
preferred that the non-linear optical process is seeded with an
optical seed input.
[0038] A further application of the present invention is in optical
parametric devices for the tunable generation and amplification of
an optical output over a wide wavelength range.
[0039] It is therefore preferred that an optical parametric
oscillator comprises a non-linear optical device according to th
first aspect of the present invention and means for providing
optical feedback at a wavelength within the accessible
bandwidth.
[0040] Preferably, the optical feedback means is provided at least
in part by a photonic structure.
[0041] In another application it is preferred that an optical
parametric amplifier comprises a non-linear optical device
according to the first aspect of the present invention adapted to
receive a further optical input to be amplified at a wavelength
within the accessible bandwidth.
[0042] Of course, there are many other applications of the present
invention. Preferably, an optical system includes a non-linear
optical device according to the first aspect of the present
invention
[0043] According to a second aspect of the present invention, an
optical continuum source comprises a planar optical waveguide; at
least a section of the planar optical waveguide being operative to
generate an optical output having an output bandwidth from at least
a portion of an optical input having an input bandwidth by means of
a non-linear optical process, wherein the optical output has an
optical spectrum comprising an optical continuum as a result of
non-linear broadening of the optical input, the planar optical
waveguide being operative to enhance the ratio of the output
bandwidth to the input bandwidth to at least 4, the term
"bandwidth" being defined here as the wavelength interval beyond
which the spectral radiant intensity remains below a level of -30
decibels (0.001) of the maximum value.
[0044] Preferably, the output bandwidth of the optical continuum is
at least 200 nm. Preferably, a portion of the planar waveguide in
the optical continuum source includes a structure, the structure
being operative to modify the optical dispersion characteristics of
the planar optical waveguide.
[0045] Preferably, the structure comprises a photonic
structure.
[0046] Preferably, the optical dispersion characteristics of the
planar optical waveguide are modified by the structure to achieve
zero dispersion at points along the waveguide. Alternatively, the
optical dispersion characteristics of the planar optical waveguide
are modified to achieve normal dispersion at a predetermined
wavelength.
[0047] According to a third aspect of the present invention, an
optical parametric oscillator comprises:
[0048] a planar optical waveguide, at least a section of the planar
optical waveguide being operative to generate an optical output
from at least a portion of an optical input having an input
bandwidth by means of a non-linear optical process, the optical
output having a wavelength within an accessible bandwidth, wherein
the planar optical waveguide is operative to enhance the accessible
bandwidth such that the ratio of th accessible bandwidth to the
input bandwidth is at least 4, the term "bandwidth" being defined
here as the wavelength interval beyond which the, spectral radiant
intensity remains below a level of -30 decibels (0.001) of the
maximum value; and,
[0049] means for providing optical feedback at a wavelength within
the accessible bandwidth.
[0050] Preferably, the accessible bandwidth of the optical
parametric oscillator is at least 200 nm.
[0051] Preferably, a portion of the planar waveguide in the optical
parametric oscillator includes a structure, the structure being
operative to modify the optical dispersion characteristics of the
planar optical waveguide.
[0052] Preferably, the structure comprises a photonic
structure.
[0053] Preferably, the optical dispersion characteristics of the
planar optical waveguide are modified to achieve negative
(anomalous) dispersion at a predetermined wavelength.
[0054] According to a fourth aspect of the present invention, an
optical parametric amplifier comprises a planar optical waveguide
for receiving a first optical input having a first input bandwidth
and a second optical input having a second input bandwidth, at
least a section of the planar optical waveguide being operative to
amplify the second optical input by generating an optical output
from at least a portion of the first optical input by means of a
non-linear optical process, the optical output and the second
optical input having a wavelength within an accessible bandwidth,
wherein the planar optical waveguide is operative to enhance the
accessible bandwidth such that the ratio of the accessible
bandwidth to the first input bandwidth is at least 4, the term
"bandwidth" being defined here as the wavelength interval beyond
which the spectral radiant intensity remains below a level of -30
decibels (0.001) of the maximum value.
[0055] Preferably, the accessible bandwidth of the optical
parametric amplifier is at least 200 nm.
[0056] Preferably, a portion of the planar waveguide in the optical
parametric amplifier includes a structure, the structure being
operative to modify the optical dispersion characteristics of the
planar optical waveguide.
[0057] Preferably, the structure comprises a photonic structure
[0058] Preferably, the optical dispersion characteristics of the
planar optical waveguide are modified to achieve negative
(anomalous) dispersion at a predetermined wavelength.
[0059] According to a fifth aspect of the present invention, a
method for enhancing the bandwidth accessible in the generation of
an optical output, comprises th step of providing a planar optical
waveguide for receiving an optical input having an input bandwidth,
wherein at least a section of the planar optical waveguide is
operative to generate an optical output from at least a portion of
the optical input by means of a non-linear optical process, the
optical output having a wavelength within an accessible bandwidth,
wherein the planar optical waveguide is operative to enhance the
accessible bandwidth such that the ratio of the accessible
bandwidth to the input bandwidth is at least 4, the term
"bandwidth" being defined here as the wavelength interval beyond
which the spectral radiant intensity remains below a level of -30
decibels (0.001) of the maximum value.
[0060] According to a sixth aspect of the present invention, a
method for generating an optical signal comprises the steps of:
[0061] receiving an optical input signal having an input bandwidth
at an optical input to a planar optical waveguide;
[0062] guiding the optical input signal along the planar optical
waveguide; and, generating an optical output signal from at least a
portion of the optical input signal by means of a non-linear
optical process in at least a section of the planar optical
waveguide, the optical output signal having a wavelength within an
accessible bandwidth, wherein the planar optical waveguide is
operative to enhance the accessible bandwidth such that the ratio
of the accessible bandwidth to the input bandwidth is at least 4,
the term "bandwidth" being defined here as the wavelength interval
beyond which the spectral radiant intensity remains below a level
of -30 decibels (0.001) of the maximum value.
[0063] Thus the present invention provides an extremely flexible
nonlinear device, which substantially enhances the bandwidth
accessible in a nonlinear optical interaction. The key element of
the device is a planar waveguide formed from material having both
high linear and nonlinear refractive index, which combines the
advantages of strong optical confinement and high intensity over an
extended interaction region with those of a highly nonlinear
material. The net result is an extremely efficient nonlinear
interaction with a considerably enhanced accessible bandwidth, as
compared to that achievable in prior art planar devices. The device
has particular application in optical continuum and supercontinuum
generation and also in broadly tunable parametric devices. The
geometry of the planar device makes it particularly amenable to the
integration of other functionality on the same chip and also
compatible with modern photonic integrated circuits. By using
tapers, ridge and rib waveguides, and also pulse compression,
dispersion modifying and filtering structures (particularly
photonic crystal structures) the performance and range of
applications of th d vice can be greatly improved.
BRIEF DESCRIPTION OF THE DRAWINGS
[0064] Examples of the present invention will now be described in
detail with reference to the accompanying drawings, in which:
[0065] FIGS. 1A an 1B show planar optical waveguides according to
the present invention;
[0066] FIG. 2 shows the wavelength dependence of (a) the material
refractive index, (b) the effective refractive index and (c) the
total refractive index for a waveguide of the type shown in FIG. 1B
with a Ta.sub.2O.sub.5 core;
[0067] FIG. 3 shows the wavelength dependence of both the group
velocity and the dispersion parameter (D) in a Ta.sub.2O.sub.5
planar waveguide;
[0068] FIGS. 4A to 4D illustrate, on a macroscopic and microscopic
level, a nonlinear optical interaction between light and
matter;
[0069] FIGS. 5A, 5B and 5C illustrate the nonlinear process of
self-focussing;
[0070] FIGS. 6A and 6B illustrate temporally and spectrally,
respectively, the nonlinear process of self-phase modulation of an
ultrashort pulse;
[0071] FIG. 7 illustrates an arrangement for optical CG in a
Ta.sub.2O.sub.5 planar waveguide with a pulsed input;
[0072] FIG. 8 shows the optical input and output spectra for CG in
the arrangement of FIG. 7;
[0073] FIG. 9 compares the optical input and output spectra
obtained for CG in a Ta.sub.2O.sub.5 planar waveguide with CG in
known bulk sapphire material;
[0074] FIG. 10 shows a planar optical waveguide according to the
present invention;
[0075] FIG. 11 illustrates the concept of on-chip pre- and
post-processing in the device of FIG. 10;
[0076] FIG. 12 shows a planar waveguide with vertically tapered
input region;
[0077] FIG. 13 shows a planar waveguide with laterally tapered
input regions; FIG. 14 shows a planar waveguide with a 2-D photonic
structure for on-chip filtering;
[0078] FIG. 15 shows a planar waveguide with a 1-D photonic
structure for on-chip filtering;
[0079] FIG. 16 shows a planar waveguide with laterally tapered
input regions and a photonic structure for on-chip filtering;
[0080] FIG. 17 shows a ridge type embodim nt of a planar
waveguide;
[0081] FIG. 18 shows a rib type embodiment of a planar
waveguide;
[0082] FIG. 19 shows a ridge type planar waveguide with 1-D
photonic structure for on-chip filtering;
[0083] FIG. 20 shows a rib type planar waveguide with 1-D photonic
structure for on-chip filtering;
[0084] FIG. 21 shows a ridge type planar waveguide with laterally
tapered input region;
[0085] FIG. 22 shows a rib type planar waveguide with laterally
tapered input region;
[0086] FIG. 23 shows a ridge type planar waveguide with laterally
tapered input region and photonic structure for on-chip
filtering;
[0087] FIG. 24 shows a rib type planar waveguide with laterally
tapered input region and photonic structure for on-chip
filtering;
[0088] FIG. 25 shows a three-channel rib type planar waveguide with
laterally tapered input regions and photonic crystal structures for
on-chip filtering;
[0089] FIG. 26 shows a planar waveguide with a 1-D cladding
photonic structure for 15, waveguide dispersion control;
[0090] FIG. 27 shows a planar waveguide with a 1-D core/cladding
photonic structure for waveguide dispersion control;
[0091] FIG. 28 shows a planar waveguide with a 2-D
buffer/core/cladding photonic crystal structure for waveguide
dispersion control;
[0092] FIG. 29 shows a planar waveguide with a multilayer structure
for waveguide dispersion control;
[0093] FIG. 30 shows a planar waveguide with 2-D photonic structure
for on-chip pulse compression;
[0094] FIG. 31 shows a planar waveguide with 1-D photonic structure
for on-chip pulse compression;
[0095] FIG. 32 shows a known PPLN optical parametric generator;
[0096] FIG. 33 shows a known PPLN optical parametric
oscillator;
[0097] FIG. 34 illustrates the concept of an OPO based on a
nonlinear device according to the present invention;
[0098] FIG. 35 illustrates the concept of an OPO based on the
nonlinear device with pre- and post-processing;
[0099] FIG. 36 shows an OPO with feedback via on-chip discrete
photonic structures; and, FIG. 37 shows an OPO with feedback via an
on-chip waveguide and waveguide beamsplitters.
DETAILED DESCRIPTION
[0100] The propagation of ultra-short, intense pulses in a high
index planar waveguide according to the present invention is
accompanied by an extremely large nonlinear spectral broadening,
which may be exploited in a number of ways. A particularly useful
application is in the generation of optical continua. Although the
phenomenon of continuum generation is well known in bulk material
and optical fiber, the spectral broadening achieved in planar
waveguides according to the present invention exhibits unique
characteristics. Unlike optical fiber, the high index planar
waveguide enables the generation of a broad continuum of
wavelengths by exploiting only a smaller number of nonlinear
effects.
[0101] Planar optical waveguides are key devices in the
construction of integrated optical circuits and lasers. The
potential of those waveguides lies in the way the electric fields
can distribute and propagate in the planar platform, providing a
unique way to implement functionality on chip. FIGS. 1A and 1B show
the typical geometry of two planar optical waveguides 10 according
to the present invention. The waveguide 10 shown in FIG. 1A
comprises buffer 12, core 13 and cladding 14 layers formed on a
substrate 11. Frequently, the cladding layer 14 is not necessary,
as shown in FIG. 1B. The main degrees of freedom in engineering
such a device are the dimensions of these waveguide layers and the
materials used to form them. These two parameters determine how the
light (signal) can propagate in the guide. The key material
property is the refractive index n, with the condition
n.sub.c>n.sub.s required for optical confinement within the
waveguide, where n is the refractive index of the core layer and
n.sub.s is the refractive index of the neighboring buffer and
cladding layers. The refractive index is generally a wavelength
dependent function, n(.lambda.), a property that is known in optics
as dispersion.
[0102] The wavelength dependence of the linear refractive index,
n.sub.0(.lambda.), of a material is the property that accounts for
the difference in propagation speed (phase velocity) experienced by
different wavelengths (colours) of light, when travelling through
the material. The graph of FIG. 2a shows the variation in linear
refractive index with wavelength for tantalum pentoxide
(Ta.sub.2O.sub.5), a preferred material for the core layer of a
planar optical waveguide according to the present invention. The
linear index is characterized by a steep fall with increasing
wavelength in the ultra-violet (UV) region, followed by a much more
gradual decline throughout the visible and near-infrared.
[0103] However, dispersion in optical waveguides is more complex
phenomena, being given by the combined effect of two contributions:
material dispersion and waveguide dispersion. Waveguide dispersion
may also be characterised in terms of index by introducing an
effective refractive index n.sub.e(.lambda.), which corresponds to
the guiding condition. This effective index takes into account the
speed at which a particular optical mode (described by both
polarization state and order) propagates in the waveguide. The
index is proportional to the propagation constant for the specific
mod and its value is always in the range
n.sub.s<n.sub.e<n.sub.c. Thus, for planar waveguides,
n.sub.e(.lambda.) depends on both the geometry of the waveguide
(core and cladding thickness) and on the value of the refractive
indices of the constituent materials.
[0104] FIG. 2b shows a dispersion curve for the effective index of
a fundamental mode propagating in a planar optical waveguide of the
type shown in FIG. 1B. The waveguide comprises a 500 nm thick layer
of Ta.sub.2O.sub.5 deposited on a 2 .mu.m thick layer of thermal
silicon dioxide (SiO.sub.2) located on a silicon wafer substrate.
The linear refractive indices of the buffer and core materials are
n.sub.s=1.46 and n.sub.c=2.1, respectively. The effective index
shows a much more uniform decrease with increasing wavelength than
the linear material index. Finally, the total dispersion of the
planar optical waveguide, combining the contributions from material
dispersion and waveguide dispersion, is shown in FIG. 2c. The
variation in total refractive index largely mirrors the material
contribution, but exhibits a slightly steeper fall at visible and
near-infrared wavelengths. Thus, although material dispersion is a
fixed intrinsic property of the waveguide material used, the
dispersion characteristics of the total index, n(.lambda.), can be
tailored by varying the material and layer thicknesses to vary the
contribution from waveguide dispersion, n.sub.e(.lambda.).
[0105] Another useful way to express dispersion is to look at its
temporal effects on light propagation within a material. The phase
velocity, v.sub..phi., of a wave is inversely proportional
(v.sub..phi.=c/n) to the linear refractive index of the material,
n. The phase velocity is the velocity at which the phase of any one
frequency component of the wave will propagate. This is not the
same as the group velocity of the wave, which is the rate at which
changes in amplitude (known as the envelope of the wave) will
propagate. The group velocity, v.sub.g, is often thought of as the
velocity at which energy or information is conveyed along the wave
and is given by 1 v g = c [ n - n ] - 1
[0106] The group velocity is also generally a function of
wavelength. This dependence results in group velocity dispersion
(GVD), which causes a short pulse of light to spread in time as a
result of different frequency components of the pulse travelling at
different velocities. This effect provides one of the limitations
on achieving and maintaining short pulse duration and high data
rate in optical communication systems. GVD is often quantified by
the group delay dispersion parameter. 2 D = - c ( 2 n 2 )
[0107] The D parameter accounts for the propagation delay per unit
wave length introduced by a unit length of material, and is usual
quoted in units of ps/nm/Km). If D is less than zero, the medium is
said to have normal, or positive dispersion. If D is greater than
zero, the medium has anomalous, or negative, dispersion. When a
light pulse is propagating through a normally dispersive medium,
the higher frequency components travel slower than the lower
frequency components. Conversely, when a light pulse travels
through an anomalously dispersive medium, high frequency components
travel faster than the lower frequency components. FIG. 3 shows a
graph of the GVD and corresponding D parameter for Ta.sub.2O.sub.5,
as used in a planar waveguide according to the present invention.
The group velocity increases to a maximum at a wavelength a little
over 1.6 .mu.m before gradually declining. At the same time the D
parameter increases, crossing the zero point at around 1.6 .mu.m,
in the transition from the normal to anomalous dispersion
region.
[0108] From the above discussion, it is clear that tailoring the
dispersion is a fundamental ingredient for controlling or
engineering the propagation of light and, as such, is currently
exploited in optical fibre communication. It is noted though that,
thus far, only linear processes have been considered. This,
however, is not a good approximation when dealing with the
propagation of intense, ultra-short laser pulses.
[0109] High index materials of the type used in the present
invention can exhibit nonlinear optical effects, which can be
particularly strong when induced by intense, ultra-short laser
pulses. Although the presence of these nonlinearities in bulk
material has been known since the introduction of ultrafast lasers,
the more recent diffusion of planar waveguides and use of optical
fibres in optical circuits has boosted investigation of nonlinear
optical effects in such systems.
[0110] One of the most important characteristics of ultra-short
laser pulse interaction with matter is the delivery of high energy
in a very short time (.about.100 femtoseconds=10.sup.-13 s) without
permanently damaging the material. The light pulse itself can
induce a whole range of physical phenomena as it propagates through
the material. When the electric field, E.sub.light, of a laser
pulse is comparable with the internal field, E.sub.at, of the atoms
in the material, the laser light can "drive" the atoms and, in
turn, be modified by this interaction. A simple representation of
such an interaction is illustrated in FIGS. 4A to 4D. FIGS. 4A and
4B illustrate the situation before the interaction on a macroscopic
and microscopic scale, respectively. One result of the interaction
can be the generation of a new range of wavelengths around the
input wavelength. This is illustrated on a macroscopic and
microscopic scale in FIGS. 4C and 4D, respectively.
[0111] This may take the form of pulse broadening, to the extent of
continuum generation, or a particular combination of discrete input
and output wavelengths, known as parametric effects. The strength
of the interaction is determined not only by the laser pulse
characteristics (spot size, pulse energy and pulse duration) but
also by the nature of the material.
[0112] A key parameter in quantifying these particular phenomena is
the strength of the nonlinearity which can be characterised by an
intensity-dependent higher order contribution to the index of
refraction, n, as follows
n(.lambda.,r,t)=n.sub.0(.lambda.)+n.sub.2(.lambda.)l(r,t)
[0113] where n.sub.0(.lambda.) is the ordinary linear index,
n.sub.2(.lambda.) is the nonlinear refractive index (in units of
m.sup.2W.sup.-1) and l(r,t) is the temporally and spatially varying
intensity of the laser pulse. The nature of the linear term has
been described previously and gives rise to optical phenomena such
as refraction and reflection, in which light is merely deflected or
delayed but remains unchanged in terms of its frequency
(wavelength). The nonlinear term is rather different and depends on
both the characteristic nonlinear coefficient of the material at
the laser wavelength and on the spatial-temporal characteristics of
the laser pulse. The higher the nonlinear index of refraction
n.sub.2 and/or the higher the intensity of the laser pulse, the
stronger the nonlinear effect and the greater the nonlinear
contribution to the total refractive index. This third order
nonlinear effect is commonly known as the Optical Kerr effect.
[0114] One practical use of the Optical Kerr effect is the
generation of a range of new wavelengths around the input
wavelength. If the spread of wavelengths in broad and continuous,
the result is termed an Optical Continuum. When there is both a
spatial and temporal variation in the local intensity of the
optical input field, the Optical Kerr effect can be resolved into
two contributions, known as Self-Focussing (SF) and Self-Phase
Modulation (SPM). Both effects have their origin in the
spatio-temporal dependence of the refractive index n(l(r,t)), with
the spatially-varying contribution, l(r), giving rise to
self-focussing and the temporally-varying contribution, l(t),
giving rise to self-phase modulation.
[0115] FIG. 5A illustrates the first process of self-focussing in
relation to a pulsed laser beam propagating through a nonlinear
material. The beam will typically have a Gaussian spatial profile,
as shown in FIG. 5A, with the intensity highest at the center of
the beam and falling radially so towards the edge of the beam. In
the presence of the Kerr effect, the center of the beam will be
give rise to a greater nonlinear refractive index within the Kerr
medium than at the edges, as shown in FIG. 5C. At the same time,
the beam will experience this self-induced refractive index
variation, with the phase velocity of the wavefront being less at
the center of th pulse than at the dg s.
[0116] Thus, in effect, a weak positive lens is induced within the
medium and th beam is gradually focused as it propagates. This can
lead to the propagating beam collapsing into a filament, which
concentrates the energy into a reduced volume inside the material.
The result of such an intense trapped beam within the material
gives rise to an even more intense interaction between the light
and the material, leading to a whole range of nonlinear
effects.
[0117] The intensity of an ultrashort pulse also changes rapidly
with time and so, due to the near-instantaneous response of the
material, different parts of the pulse will induce different
magnitudes of nonlinear refractive index. This time-varying
refractive index leads to a phase change, .DELTA..phi.(t), across
the temporal profile of the pulse, which is dependent on the
instantaneous intensity in the following manner 3 ( t ) = 2 n 2 I (
t ) L
[0118] where L is the length of the material. Since a time-varying
phase corresponds to frequency (frequency is the time derivative of
phase, .omega.=-d.phi./dt), the phase delay, .DELTA..phi.(t),
results in a frequency chirp, .phi..omega., across the pulse, given
by 4 = - ( ) t
[0119] As a result, the time-varying nonlinear refractive index of
the material leads to what is termed self-phase modulation of the
pulse. The frequency shift introduces new spectral components to
the pulse, leading to a broadening of the spectral bandwidth.
[0120] The phase shift and corresponding frequency shift induced
across a Gaussian pulse by self-phase modulation is illustrated in
FIG. 6A. The resulting impact on the spectrum of the propagating
pulse is shown in FIG. 6B. In this example, the initially Gaussian
spectrum is broadened, with two lobes either side of a suppressed
central wavelength region.
[0121] Thus, a planar waveguide with a high associated nonlinear
index of refraction has great potential, as it combines the
properties of light confinement and guiding with those of a highly
nonlinear material. Furthermore, by seeding the interaction with a
pulse having a wavelength in the vicinity of zero group velocity
dispersion or in the anomalous dispersion region, a wide range of
nonlinear processes can occur in the medium. Self-phase modulation,
self-focussing, four-wave mixing, Raman scattering, harmonic
generation, soliton formation are among the nonlinear effects that
may be initiated.
[0122] FIG. 7 shows a schematic of a suitable arrangement 70 for
optical continuum generation 76 in a 13 mm long Ta.sub.2O.sub.5
planar waveguide 75 according to the present invention. The
waveguide comprises a 800 nm thick core layer of undoped
Ta.sub.2O.sub.5 deposited on a 2 .mu.m thick buffer layer of
thermal silicon dioxide (SiO.sub.2), which is located on a silicon
wafer substrat. A laser system 71 comprising a modelocked titanium
sapphire (TiS) laser oscillator 72 with regenerative amplifier 73
is used to provid ultrashort laser pulses which act as the optical
pump source 74 in the interaction. The laser oscillator 72 alone
can provide pulses of duration t.sub.p=110-150 fs at a repetition
rate of f, =76 MHz and a center wavelength of 800 nm. A typical
pulse energy of E.sub.p=13 nJ corresponds to an average laser power
(P.sub.av=f.sub.rE.sub.p) of approximately 1W and a peak power
(P.sub.pk.about.E.sub.p/t.sub.p) of approximately 90 kW. With
amplification provided by the regenerative amplifier, the overall
laser system generates 150 fs pulses at a repetition ate of 250 kHz
and a center wavelength of 800 nm. Available pulse energy ranges
between 40 nJ and 2.4 .mu.J, corresponding to an average laser
power of between 10 mW and 600 mW and peak power of between 0.25 MW
and 16 MW.
[0123] FIG. 8 shows the spectrum of the output signal obtained at
two pump powers together with that of the input pump beam for
comparison. The results were obtained with amplified input pulse
energies of 80 nJ and 800 nJ, corresponding to an average input
power of 20 mW and 200 mW, respectively, at a repetition rate of
250 KHz. Th input pump pulses have a bandwidth of 10 nm, measured
by the full-width at half-maximum (FWHM) intensity. However, as can
be seen from the output spectrum, a measure of bandwidth on a
logarithmic scale is more appropriate. Therefore, the point at
which the spectral (radiant) intensity, l(.lambda.), falls
permanently below 10.sup.-3 (0.001) of the peak (maximum) value,
l.sub.max(.lambda.), is chosen for the measure of bandwidth. On the
decibel scale 5 Relative Intensity in dB = - 10 log 10 I ( ) I max
( )
[0124] the relative intensity equates to the -30 dB point. Thus,
the working definition of bandwidth is the wavelength interval
(.DELTA..lambda.) between the maximum (.lambda..sub.max) and
minimum (.lambda..sub.min) wavelengths beyond which the relative
intensity is less than -30 dB, as follows:
.DELTA..lambda.=.lambda..sub.max(-30 dB)-.lambda..sub.min(-30
dB)
[0125] As can be seen from FIG. 8, based on this definition, the
input pump bandwidth of 50 nm is nonlinearly broadened to 200 nm
and 600 nm at an average pump power of 20 mW and 200 mW and peak
power of approximately 0.5 and 5.5 MW, respectively. This equates
to a degree of nonlinear broadening
(.DELTA..lambda..sub.out/.DELTA..lambda..sub.in) of between 4 and
12 times. Despite the large variation in spectral intensity over
the nonlinearly generated spectrum, the small amount of radiation
present at wavelengths towards the blue (and UV) and red (and
infrared) is sufficient to make the output beam appear "white".
[0126] At .lambda..sub.p=800 nm, the pump is tuned to a wavelength
within the normal dispersion regime of the planar waveguide.
Furthermore, the dispersion present in the planar waveguide is less
structured than that in present in the microstructured fibres used
in prior art continuum generation. As a consequence the nonlinear
interaction is dominated by the twin Optical Kerr effects of
self-focussing and self-phase modulation. Nevertheless, it is clear
that unusually broad continuum generation is occurring, with the
spectrum extending into the anomalous dispersion region.
[0127] FIG. 9 shows a comparison between the output spectrum
obtained from continuum generation in bulk sapphire, as used in
commercial systems, and that obtained in a Ta.sub.2O.sub.5 planar
waveguide according to the present invention. The optical continuum
generated in the planar waveguide extends much further into the
near infrared spectral region. Non-linear broadening has also been
reported in a chalcogenide glass planar waveguide by self-phase
modulation, for the purposes of optical switching. However,
although the spectrum of the output is nonlinearly broadened, the
degree is much less than that obtained in a waveguide according to
th present invention.
[0128] Using a planar waveguide with a 500 nm Ta.sub.2O.sub.5 core
layer, continuum generation is achieved for average pump power as
low as 10 mW (pulse energy 40 nJ, 20: peak power 0.25 MW).
Alternative dimensions and materials should reduce th threshold
sufficiently for continuum generation with unamplified pump pulses
from the TiS laser oscillator alone. Suitable core materials
include oxides such as hafnium oxide, zirconium oxide, titania,
aluminium oxide and also silicon nitride. However, there are many
other possible high index candidate materials, some of which can
not be fabricated in the bulk but can be deposited in a thin film
to form the core of a planar waveguide. Doping these materials with
rare earth metals will modify the properties still further.
Tantalum pentoxide films doped with Neodymium (Nd) have been
investigated and demonstrate that such doping permits continuum
generation to be extended into the ultraviolet (UV) spectral
region.
[0129] The basic waveguide structure for a planar device 100
according to the present invention is reproduced in FIG. 10. As
shown, the device comprises a buffer 102, core 103 and cladding 104
layer formed on a substrate 101, although the cladding layer 104
may be neglected. A high intensity optical input with a
comparatively narrow spectrum 105 is used as the pump for nonlinear
signal generation in the core layer 103. As a result of the
interaction a nonlinearly generated output is obtained with a
spectrum, which may contain wavelength components over a very broad
accessible range. Nevertheless, despite the impressive performance
associated with the basic embodiment shown in FIG. 10, there are
many other possible embodiments which can further improve
performance and build additional functionality into the device.
[0130] FIG. 11 illustrates the broad concept whereby the planar
waveguide chip 110 comprises not only a portion fulfilling the role
of enhancing the accessible nonlinearly-generated bandwidth 113,
but also a portion 114 that provides on chip post-processing of the
generated signal prior to output 115. Indeed, the chip may also
comprise elements 112 to provide pre-processing of the optical
input 111.
[0131] Efficient coupling of the pump beam into the waveguide core
is one of the most important issues for generating continua, as
power density is critical for strong nonlinearity. Thus, by
launching most of the power into a high index thin film waveguide
for optical confinement, high power density can be maintained over
an interaction length. In the basic arrangement of FIG. 10,
suitably designed external optics are required to beam shape and
launch light efficiently into the waveguide. However, an
alternative approach is to build beam shaping or mode converting
capability into the chip. An effective way to do this by tapering
an input region of the waveguide vertically or horizontally, so
that there is no longer an abrupt facet extending along a plane
perpendicular to the propagation direction. This has the added
benefit of reducing the likelihood of surface or bulk damage to the
waveguide material in the region of the facet. FIG. 12 shows an
embodiment 120 with a vertically tapered input region 123 prior to
the main interaction region 124. The tapered, region 123 is
characterized by a gradual increase in the thickness of the
waveguide core located on the buffer layer 122. This can be
achieved during fabrication by growth or etching techniques. The
result of the taper is a gradual coupling of light into the
waveguide accompanied by a concentration of the input beam 125 as
the beam size reduces 126. The tapered input region allows the use
of a pump beam with a modal area much larger than the size of the
thin film itself, by acting as spot-size converter. FIG. 13 shows
an embodiment 130 with two adjacent horizontally (laterally)
tapering input regions 131 prior to the main interaction region
132. This embodiment allows for the independent coupling of two
separate beams into the waveguide or, alternatively, a more complex
coupling of a single large beam into the waveguide. Again, the
input beam 133 reduces in size 134 as it propagates. With a slow
smooth taper, input light is adiabatically coupled into a mode
supported by the waveguide and able to propagate therein. Provision
of an on-chip spot-size converter increases the range of light
delivery systems that can be used with the waveguide device,
including optical fiber.
[0132] Adding on-chip functionality is one of the great advantages
of planar waveguide devices. On-chip structures for spatial
profiling and beam shaping have already been described in the
context of waveguide tapers. However, other types of functionality
can be included that modify the phase or amplitude of a beam
propagating in the device. In particular, structures for filtering,
shaping or slicing the spectra of the optical input or output may
be integrated. Such filtering may be performed by more conventional
structures with wavelength-dependent absorption, transmission or
diffraction properties. However, all the above operations can be
performed by photonic crystal structures, which can control
propagation of light, both spatially and spectrally, by virtue of
their detailed structure which results in forbidden bands of
propagation constant.
[0133] FIG. 14 illustrates an embodiment of a planar waveguide 140
according to the present invention, which includes a 2-D photonic
crystal structure 145 for on-chip post-processing of the optical
output generated by the enhanced nonlinear process. In this
example, the photonic crystal structure comprises rod-like holes
extending through the cladding 144 and core 143 layers, but not
into the buffer layer 142. However the rods may extend into the
buffer layer or exist in any one or more of the cladding, core and
buffer layers. Furthermore, the holes may be filled with any
material. This concept is explained in more detail with reference
to FIG. 15. The input pulse 146 is converted into an optical
continuum within the planar waveguide and the photonic crystal
structure is designed with a complex optical transfer function to
filter the spectrum in a particular manner. As shown, the photonic
crystal structure 145 filters out a band of wavelengths to produce
the spectrum 147. The optical transfer function may include
spectral flattening to obtain a more uniform spectral intensity
across a range of wavelengths within the continuum. Alternatively,
discrete wavelengths or bands of wavelengths can be passed with
intervening wavelength ranges blocked. Both forms of spectral
processing may be employed in optical comb generation, which has
applications in optical communications, including dense wavelength
division multiplexing (DWDM). The photonic crystal structures may
also perform a beam shaping or optical routing function as
well.
[0134] FIG. 15 shows another embodiment of a planar waveguide 150
according to the present invention, which includes a 1-D photonic
crystal structure 155 for on-chip post-processing of the optical
output generated by the enhanced nonlinear process. In this
example, the photonic crystal structure comprises slabs extending
through the cladding 154 and core 153 layers, but not into the
buffer layer 152. As shown the slabs comprise a first material 156
in the core layer and a second material in the cladding layer 157.
The refractive index of the materials 156 and 157 affects the
optical confinement of signals within the waveguide and the optical
transfer function as is described in more detail in co-pending U.S.
patent application Ser. Nos. 10/196,727, 10/287,825 and 10/421,949,
also in the name of Mesophotonics Limited. The input pulse 158 is
converted into an optical continuum within the planar waveguide and
th photonic crystal structure is designed with a complex optical
transfer function to filter the spectrum in a particular manner. As
shown, the photonic crystal structure 155 filters out a band of
wavelengths to produce the spectrum 159. The on-chip processing
functions described so far can, of course, be combined.
[0135] FIG. 16 shows an embodiment of a planar waveguide 160
according to the present invention that combines a double lateral
taper input region 164 with a 1D/2D photonic crystal
post-processing region 165. The structure includes a buffer layer
162 and a core layer 163 as previously described. The input pulse
166 is converted and output as spectrum 167. Here, both efficient
coupling of the optical input is combined with spectral processing
of the nonlinearly generated optical output. Careful design of
these two features permits optimisation over specific wavelength
ranges.
[0136] Thus far, only conventional broad area planar waveguides
have been considered, which predominantly provide optical
confinement in only one dimension, the vertical. Lateral
confinement may be provided by employing ridge-or rib type-planar
waveguide structures, as illustrated in FIGS. 17 and 18.
[0137] FIG. 17 shows a device comprising a buffer 172, core 173 and
a core ridge 174 layer formed on a substrate. The device operates
in the same manner as th planar device illustrated in FIG. 10.
Input pulse 175 is converted in a broad continuum represented by
spectrum 176.
[0138] FIG. 18 shows a device comprising a buffer 182 a core rib
183 layer formed on a substrate. Again, the device operates in the
same manner as the planar device illustrated in FIG. 10. Input
pulse 184 is converted in a broad continuum represented by spectrum
185.
[0139] Such structures give rise to higher peak intensity within
the guided mode and a correspondingly larger degree of nonlinearity
induced in the nonlinear material. This in turn leads to a lower
threshold power (pulse energy) requirement for initiating the
nonlinear process. Furthermore, a more symmetrical optical mode may
be promoted within the waveguide, which is more easily mode matched
to other devices, such as optical fiber, when coupling light into
and out of the waveguide.
[0140] Of course, many of the pre- and post-processing structures
applied to the basic planar waveguide structure may also be applied
the ridge and rib waveguide embodiments. FIG. 19 shows the
application of 1-D photonic crystal structures to the ridge
waveguide of FIG. 17. The structure has the same basic features as
the structure of FIG. 17, including a buffer layer 192, core layer
193 and core ridge layer 194. The core ridge layer includes a
photonic crystal post-processing section 195 formed from a periodic
modulation of the core ridge layer. The periodic modulation of the
waveguide structure provides the means for filtering of the signal
(optical continuum) that is non-linearly generated in a region of
the waveguide proximate to the input. The input pulse 196 is
therefore broadened into a continuum and filtered to produce
spectrum 197.
[0141] FIG. 20 illustrates the same design applied to a rib
waveguide structure 200. The structure includes a buffer layer 202
and a core rib layer 203. The core rib layer includes a photonic
crystal section 204 for post processing so that after input pulse
205 has been broadened it is filtered to produce a spectrum
206.
[0142] The photonic crystal of FIG. 19 or 20 may be implemented in
rod or slab form and need not be periodic, as indicated, but have a
varying mark-space ratio according to particular design parameters.
The intervening "spaces" may remain air-filled or could be filled
with other materials having suitable refractive index
properties.
[0143] FIG. 21 illustrates a ridge embodiment 210 having a
horizontally tapered input region 214 for spot size conversion and
improved optical coupling efficiency. A vertical taper may also be
employed. The structure 210 includes a buffer layer 212 and a core
layer 213. On the core layer 213 is a core ridge layer 215 and it
is the ridge layer that is tapered at input region 214.
[0144] FIG. 22 illustrates the same design applied to a rib
structure 220. The structure includes a buffer layer 222 and a core
rib layer 224. An input region 223 of the rib layer 224 is
horizontally tapered for spot size conversion and improved optical
coupling efficiency. A vertical taper may also be employed.
[0145] In a manner analogous to FIG. 16, FIG. 23 illustrates the
combination of tapers and photonic crystal structures applied to a
ridge waveguide embodiment of th present invention. FIG. 23 shows a
device 230 including a buffer layer 232, a core layer 233 and a
core ridge layer. The core ridge layer includes a tapered input
section 234, a continuum generation section 235 and a
post-processing photonic crystal section 236. As with the basic
waveguide embodiment, the tapers provide for improved optical
coupling efficiency and pre-processing of the input beam, and the
photonic crystal structures provide for post-processing operations
on the nonlinearly generated output beam, including filtering. The
input pulse 237 is incident on the tapered section 234 and is
output as a spectrum 238.
[0146] FIG. 24 illustrates a rib waveguide embodiment equivalent to
the ridge waveguide embodiment of FIG. 23. The structure includes a
buffer layer 242 and a core rib layer. The core rib layer comprises
a tapered input section 243, a continuum generation section 244 and
a post-processing photonic crystal section 245. As with the basic
waveguide embodiment, the tapers provide for improved optical
coupling efficiency and pre-processing of the input beam, and the
photonic crystal structures provide for post-processing operations
on the nonlinearly generated output beam, including filtering. The
input pulse 246 is incident on the tapered section 243 and is
output as a spectrum 247.
[0147] The geometry of the ridge and rib waveguide structures
naturally lend themselves to the construction of a single chip
device with multiple waveguide channels. FIG. 25 illustrates the
concept with a three channel rib/ridge waveguide device 250, which
also incorporates tapered input regions 253 and photonic crystal
structured output regions 255. Unlike the broad area waveguide
device of FIG. 16, which has two tapered input regions, the device
of FIG. 25 comprises three distinct waveguiding channels 254 on a
buffer layer 252, each with a separate input taper and photonic
crystal structure. Typically, the device would be used with three
separate input beams 256, possibly derived from three separate
sources. If the three beams are derived from a common source, the
three photonic crystals 255 can be designed with different transfer
functions so as to sample different wavelength ranges from the
continua generated in each of the channels 254. In this way three
distinct output spectra 257 are produced. By adjusting the spacing
between the ridges/ribs, a degree of cross-talk can be introduced,
if desired. Alternatively, the optical input to the three channels
can be a single beam in a mode that extends across all three input
taper regions.
[0148] A further embodiment of the present invention incorporates a
structure within the region of the planar waveguide where the
signal is nonlinearly generated, the structure being designed to
modify the local dispersion characteristics of the waveguide in a
predetermined way. FIG. 26 shows an example of this embodiment, in
which the structure 260 comprises a 1-D photonic crystal 265
located in the cladding layer 264. The structure includes a buffer
layer 262 and a core layer 263 on which the cladding layer lies.
Input pulse 266 is converted into a continuum 267 in the core layer
263 whilst the photonic crystal structure may be designed to modify
the dispersion of the whole waveguide so as to lie in the normal,
zero or anomalous dispersion regimes at the central wavelength of
the input pump beam. Alternatively, the structure may be designed
to yield a specific dispersion, such as zero dispersion, at regular
points along the waveguide. This latter example is particularly
useful for minimizing dispersion induced increase in pulse
duration, which would otherwise lead to reduced peak power.
[0149] FIG. 27 illustrates a similar structure to FIG. 26. The
structure 270 of FIG. 27 includes a buffer layer 272, a core layer
273 and a cladding layer 274. The photonic crystal 275 is formed
from slabs 276 located in both the cladding layer and the core
layer.
[0150] Furthermore, operating near the zero dispersion point can
lead to broader continuum generation. FIG. 28 illustrates an
example of this embodiment 280 having a 2-D photonic crystal
structure 284 that extends from the cladding layer 285, through the
core layer 283 and into the buffer layer 282. The 2-D structure
permits additional functionality such as beam shaping within the
generating region.
[0151] An alternative device structure for modifying the dispersion
characteristics of the waveguide is a multilayered structure 290,
as illustrated in FIG. 29. Here, the "core" 296 comprises
alternating layers 293, 294, 295 of high (n.sub.1, n.sub.2 . . .
n.sub.n) refractive index and low refractive index on a buffer
layer 292. The dimensions and materials used for each layer are
calculated according to the desired dispersion characteristics. In
this manner three different spectral outputs can be obtained from a
single input.
[0152] As has been described previously, in relation to tapers, the
planar waveguide device may comprise integrated structures for the
pre-processing of the optical input signal prior to nonlinear
generation of the output signal. In the context of a short pump
pulse input, one particularly useful function provided by such
structure is an on-chip pulse compression. FIGS. 30 and 31
illustrate two examples of this embodiment based on a broad area
planar waveguide according to the present invention. Pulse
compression is performed by a region of the waveguide having a
photonic crystal structure.
[0153] FIG. 30 shows an example 300 with a 2-D photonic crystal 303
extending through a core layer 302. Here, the photonic crystal is
designed to impart the appropriate phase advance or delay to the
different wavelength components of th input beam spectrum 307, in
order to give rise to a compression in the time domain.
[0154] FIG. 30 shows the input pulse 305 delayed by the photonic
crystal 303 to produce compressed pulse 306. The compressed pulse
then passes through the rest of th core 304 and a continuum 308 is
generated.
[0155] FIG. 31 shows an example 310 with a 1-D photonic crystal 314
extending through a core layer 313. As with the photonic crystal of
FIG. 15, different materials are used in the core layer and the
cladding layer. The photonic crystal is designed to impart the
appropriate phase advance or delay to the different wavelength
components of the input beam spectrum 317, in order to give rise to
a compression in the time domain. FIG. 31 shows the input pulse 315
advanced by the photonic crystal section 314 to produce compressed
pulse 316. The compressed pulse then passes through the rest of the
core and a continuum 318 is generated.
[0156] The pulse compression may simply be for the purposes of
increasing peak power in the pulse in order to induce a stronger
nonlinear effect in th continuum generation section of the core.
Alternatively, or in addition, the pulse compression may be a
pre-processing of the pulse to compensate for dispersion effects on
propagation through the remainder of the waveguide device. Of
course, this pre-processing function may be applied to any of th
embodiments described previously.
[0157] Much of the above discussion has centered on the ability of
the present invention to generate large bandwidth, particularly in
the context of optical continuum generation. However, another
application of the planar waveguide according to the present
invention is in parametric devices such as the optical parametric
oscillator (OPO) and optical parametric amplifier (OPA). Such
devices are used to generate or amplify one or more signals at a
discrete wavelength from a input pump beam at a different
wavelength. Typically, in a third order parametric process, two
output beams are generated, the signal (s) and idler (i), from the
single input pump (p) beam. Th total photon energy is conserved in
the process such that the pump, .omega..sub.p, signal,
.omega..sub.s, and idler, .omega..sub.l, frequencies are related
by
.omega..sub.p=.omega..sub.s+.omega..sub.i
[0158] Each signal typically has a relatively narrow bandwidth
centered around these frequencies. However, the wavelengths of the
signal and idler can be tuned over a broad range by satisfying the
appropriate phase matching condition. In the case of bulk nonlinear
crystals, this is often achieved by rotation of the crystal. In the
case of modern planar devices, quasi phase-matching can be achieved
by periodic poling of the material. FIG. 32 shows a known waveguide
device 320 of this type based on 20 periodically poled Lithium
Niobate (Li.sub.2O.sub.3), also known as PPLN. A single frequency
input beam 323 is converted into a signal wavelength 324 and an
idler wavelength 325 by the waveguide 321. It is not shown in FIG.
32 but the periodically poled regions 322 may be angled and
tapered. This permits tuning of the signal and idler wavelengths by
translating the waveguide. In addition to spontaneous parametric
generation, the device can act as an OPA, whereby a second input
signal at either the signal or idler wavelength is amplified.
[0159] Another common parametric device is the OPO, which comprises
a suitable nonlinear material (parametric generator) with optical
feedback provided at either the signal or idler wavelength, or
both. A certain degree of feedback may also be provided at the pump
wavelength. In an OPO, the presence of optical feedback, together
with the parametric gain provided by the pump beam, gives rise to a
laser-like growth in the signal being fed back. The feedback may be
in the form of a linear cavity or a ring (loop) cavity. As with a
laser, by making one of the feedback elements partially
transmitting at the resonating wavelength, an optical output can be
obtained, which can be tuned by employing tunable wavelength
selective feedback element. FIG. 33 shows a known PPLN
waveguide-based OPO 330, which utilizes distributed Bragg gratings
322 at each end of the waveguide 331 to provide optical feedback in
a linear cavity.
[0160] FIG. 34 shows the basic concept of an OPO 340 based on a
planar waveguide according to the present invention. The core
component is the waveguide region 341, in which an optical output
is nonlinearly generated, whilst simultaneously the accessible
nonlinearly-generated bandwidth is enhanced. This region provides
the parametric generation of the signal and idler, which are
tunable over much of the broad accessible bandwidth. In order to
achieve optical parametric oscillation, the key addition to the
basic device is the provision of optical feedback to form a cavity.
This may be a linear cavity with discrete or distributed end
reflectors, or a ring cavity as shown in FIG. 34, also comprising
discrete or distributed reflectors. FIG. 34 shows an input optical
beam 342 incident on a partial reflector 343. Reflector 343 passes
th signal 342 to the waveguide region 341. The output from the
waveguide region 341 is incident on partial reflector 344. The
reflector 344 is designed to be partially transmitting at the
resonating wavelength to provide an output 345. A portion of the
output is reflected by reflector 344 around a loop 347 including
reflectors 346 and 348 back to the reflector 343. The reflected
signal is then fed back into the waveguide region 341. The signal
fed back into the waveguide region 341 is at the wavelength of the
input signal. The reflectors 346, 348 themselves may provide the
wavelength selectivity or a specific filter may be inserted in the
loop 347 to perform this function.
[0161] FIG. 35 shows a schematic of the basic components for an OPO
based on a planar waveguide according to the present invention. The
planar waveguide device may incorporate any of the on-chip features
described previously, input including tapers, ribs/ridges, pulse
compression, dispersion control 351 and post-processing such as
filtering and output tapers 355. The waveguide region is indicated
at 352 and a partial/selective reflector or splitter is shown at
353. The feedback loop is shown with a specific filter 354.
[0162] FIG. 36 shows an embodiment of an OPO 360 according to the
present invention comprising a compact planar waveguide 362 with
feedback provided by four discrete 1-D or 2-D photonic crystals
361, 363, 364. One of the four crystals 361 is designed to transmit
the input 365 and another of the four crystals 363 is designed to
be partially transmitting at the resonating wavelength in order to
provide an output 366.
[0163] FIG. 37 shows another embodiment 370 comprising an extended
waveguide 372 with input 371 and output 374 tapers, and on-chip
beam splitters 373 to couple light out into the feedback loop.
Wavelength selectivity and tuning is provided by photonic crystal
375 located within the feedback loop.
[0164] Optical parametric amplification of a second input beam at
the signal or idler wavelength is also possible using a planar
waveguide according to the present invention. The arrangement is
much the same as the OPO above but without any feedback mechanism.
In all parametric device embodiments it is preferred that the
nonlinearly generating region comprises a structure to modify the
dispersion of th planar waveguide. This may be provided by photonic
crystal structures or multilayer structures as described above, or
any other suitable structure. In order for accurate phase-matching
and efficient operation to be achieved, it is preferred that the
total dispersion is in the anomalous regime for the wavelengths of
interest.
[0165] In summary, the present invention provides an extremely
flexible nonlinear device, which substantially enhances the
bandwidth accessible in the nonlinear optical interaction. The use
of a planar waveguide formed from material having a high linear and
nonlinear refractive index combines the benefit of strong optical
confinement and high intensity with high material nonlinearity. The
net result is an extremely efficient nonlinear interaction with a
considerably enhanced accessible bandwidth. The device has
particular application in optical continuum and supercontinuum
generation, but also in broadly tunable parametric devices. The
geometry of the planar device makes it particularly amenable to the
integration of other functionality on the same chip and also
compatible with modern photonic integrated circuits. As has been
described, there are very many embellishments that can be made to
the basic device to incorporate added functionality. In particular,
the use of tapers, ridge and rib type waveguides, and other
modifying structures for pulse compression, dispersion control and
filtering (particularly photonic crystal structures) has been shown
to improve greatly the performance and range of applications of the
device.
* * * * *