U.S. patent application number 10/928428 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, Charlton, Martin, Lincoln, John, Netti, Maria Caterina, Parker, Greg, Perney, Nicolas, Wilkinson, James, Zoorob, Majd.
Application Number | 20050047702 10/928428 |
Document ID | / |
Family ID | 34116789 |
Filed Date | 2005-03-03 |
United States Patent
Application |
20050047702 |
Kind Code |
A1 |
Parker, Greg ; 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. The
planar waveguide geometry permits easy integration in more complex
photonic integrated circuits such as a Michelson interferometer for
low coherence interferometry based optical coherence
tomography.
Inventors: |
Parker, Greg; (Brockenhurst,
GB) ; Baumberg, Jeremy; (Winchester, GB) ;
Wilkinson, James; (Southampton, GB) ; Charlton,
Martin; (Southampton, GB) ; Zoorob, Majd;
(Southampton, GB) ; Netti, Maria Caterina;
(Southampton, GB) ; Perney, Nicolas; (Southampton,
GB) ; Lincoln, John; (Wiltshire, GB) |
Correspondence
Address: |
MCDONNELL BOEHNEN HULBERT & BERGHOFF LLP
300 S. WACKER DRIVE
32ND FLOOR
CHICAGO
IL
60606
US
|
Assignee: |
Mesophotonics Limited
|
Family ID: |
34116789 |
Appl. No.: |
10/928428 |
Filed: |
August 27, 2004 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10928428 |
Aug 27, 2004 |
|
|
|
10648797 |
Aug 27, 2003 |
|
|
|
Current U.S.
Class: |
385/1 ;
385/129 |
Current CPC
Class: |
G02F 1/353 20130101;
G02F 1/365 20130101; G02F 1/3528 20210101 |
Class at
Publication: |
385/001 ;
385/129 |
International
Class: |
G02F 001/01; G02B
006/10 |
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 non-linear 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. A non-linear optical device 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
accessible bandwidth is at least 200 nm.
8. A non-linear 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 the
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
non-linear 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 crystal structure.
16. A non-linear optical device according to claim 14, wherein the
structure is operative to filter the optical input and/or optical
output.
17. A non-linear optical device according to claim 14, wherein the
structure is operative to compress temporally the optical input
and/or optical output.
18. A non-linear optical device according to claim 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 system including a non-linear optical device
according to claim 1.
21. 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 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.
22. An optical continuum source according to claim 21, wherein the
degree of non-linear broadening is by at least a factor of 4.
23. An optical continuum source according to claim 21, wherein the
output bandwidth of the optical continuum is at least 200 nm.
24. An optical continuum source according to claim 21, wherein the
non-linear optical process is seeded with an optical seed
input.
25. An optical continuum source according to claim 21, 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.
26. An optical continuum source according to claim 25, wherein the
structure comprises a photonic structure.
27. An optical continuum source according to claim 25, wherein the
optical dispersion characteristics of the planar optical waveguide
are modified to achieve zero dispersion at points along the
waveguide.
28. An optical continuum source according to claim 25, wherein the
optical dispersion characteristics of the planar optical waveguide
are modified to achieve normal dispersion at a predetermined
wavelength.
29. An interferometer comprising: 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 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;
a reference arm; a signal arm; a detection arm; a beam splitter
optically coupled to the optical continuum source, in use, the beam
splitter splitting the optical continuum output between the
reference arm and the signal arm; and, a beam combiner optically
coupled to the detection arm, in use, the beam combiner combining
optical beams reflected by part of the reference arm and part of
the signal arm.
30. An interferometer according to claim 29, wherein a single
optical component acts as the beam splitter and the beam
combiner.
31. An interferometer according to claim 29, wherein the
interferometer comprises a photonic integrated circuit.
32. An interferometer according to claim 29, wherein an arm of the
interferometer includes a photonic crystal structure.
33. A system for performing optical coherence tomography on a
sample, the system comprising: an interferometer according to claim
29, in use, the sample located in the signal arm; means for varying
an optical path length of the reference arm; and, a detector
located in the detection arm for detecting the recombined optical
beam comprising a reflected optical beam from the reference arm and
a beam back scattered by the sample in the signal arm.
34. 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.
35. An optical parametric oscillator according to claim 34, wherein
the accessible bandwidth is at least 200 nm.
36. An optical parametric oscillator according to claim 34, wherein
the optical feedback means is provided at least in part by a
photonic structure.
37. An optical parametric oscillator according to claim 34, 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.
38. An optical parametric oscillator according to claim 37, wherein
the structure comprises a photonic structure.
39. An optical parametric oscillator according to claim 37, 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, 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.
41. An optical parametric amplifier according to claim 40, wherein
the accessible 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 42, wherein
the structure comprises a photonic structure.
44. An optical parametric amplifier according to claim 42, 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 the 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 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.
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] One example is optical continuum generation (CG), whereby a
cascade of (generally) third order processes enables the generation
of an optical signal with a continuous, or near continuous,
spectrum over a very broad bandwidth. The continuum may share a
number of properties with laser light, including spatial coherence.
However, the broad bandwidth means the continuum is of lower
temporal coherence, which make it an attractive source for some
applications, such as low coherence interferometry. The continuum
generated can be used in its entirety or optically filtered or
sliced as the application requires.
[0006] Another example are the optical parametric processes, 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.
[0007] A variety of techniques and materials have been investigated
for enhancing the bandwidth that can be accessed by the nonlinear
processes described above, including CG and optical parametric
devices. However, as will now be described, all of these have their
attendant drawbacks.
[0008] 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. 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.
[0009] 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. A further problem is maintaining the polarization
(electric field orientation) of the light generated, which affects
its usefulness in applications.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] Another 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 the input pulse, the degree
of spectral broadening was not sufficient to generate an optical
continuum.
[0014] 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
[0015] 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.
[0016] However, it is preferred that the ratio of the accessible
bandwidth to the input bandwidth is at least 10.
[0017] 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.
[0018] Strong optical confinement in the planar waveguide is
obtained when the 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.
[0019] A variety of materials exhibit both the high linear and
nonlinear refractive index preferred for the core layer in the
present invention.
[0020] 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).
[0021] The performance and wavelength range of the nonlinear device
can be extended 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).
[0022] Although the performance of the nonlinear device may be
characterized in terms of the enhancement of the accessible
bandwidth relative to the bandwidth of the 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.
[0023] Preferably, the ratio of the accessible bandwidth to the
input bandwidth is non-linearly dependent on the peak intensity of
the optical input. Alternatively, it may be linearly dependent
intensity.
[0024] 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 one or more processes selected from a group which
includes self-phase modulation, self-focussing, four-wave mixing,
Raman scattering and soliton formation.
[0025] 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.
[0026] 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.
[0027] Preferably, a portion of the planar waveguide is tapered.
Preferably, the 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, the taper may
be characterized by a gradually decreasing waveguide (core) width
Preferably, the taper is symmetrical.
[0028] 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.
[0029] 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.
[0030] Preferably, the structure is operative to filter the 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.
[0031] 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.
[0032] 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.
[0033] 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.
[0034] Of course, there are many other applications of the present
invention, including integration into a more complex optical
system.
[0035] Preferably, an optical system includes a non-linear optical
device according to the first aspect of the present invention.
[0036] 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.
[0037] 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.
[0038] Preferably, the degree of non-linear broadening is by at
least a factor of 4.
[0039] Preferably, the output bandwidth of the optical continuum is
at least 200 nm.
[0040] Preferably, the degree of broadening is non-linearly
dependent on the peak intensity of the optical input.
Alternatively, the degree of broadening may be linearly dependent
on intensity.
[0041] In order to enhance the continuum generation process it is
preferred that the non-linear optical process is seeded with an
optical seed input.
[0042] 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.
[0043] Preferably, the structure comprises a photonic
structure.
[0044] 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.
[0045] The waveguide devices according to the first and second
aspects may be incorporated into more complex optical or photonic
devices.
[0046] According to a third aspect of the present invention, an
interferometer comprises:
[0047] 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 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;
[0048] a reference arm; a signal arm; a detection arm;
[0049] a beam splitter optically coupled to the optical continuum
source, in use, the beam splitter splitting the optical continuum
output between the reference arm and the signal arm; and,
[0050] a beam combiner optically coupled to the detection arm, in
use, the beam combiner combining optical beams reflected by part of
the reference arm and part of the signal arm.
[0051] Preferably, the interferometer comprises a photonic
integrated circuit, allowing easy integration of all the components
on a single chip.
[0052] Various arrangements of interferometer are possible.
However, it is preferred that a single optical component acts as
the beam splitter and the beam combiner, in a Michelson type
arrangement.
[0053] Preferably, an arm of the interferometer includes a photonic
crystal structure. Such a structure may be used to introduce
dispersion, dispersion compensation or time delay into the
interferometer arm.
[0054] This type of interferometer with broad band source is
particularly useful for applications involving low coherence
interferometry, such as optical coherence tomography.
[0055] Preferably, a system for performing optical coherence
tomography on a sample comprises:
[0056] an interferometer according to the third aspect, in use, the
sample located in the signal arm;
[0057] means for varying an optical path length of the reference
arm; and, a detector located in the detection arm for detecting the
recombined optical beam comprising a reflected optical beam from
the reference arm and a beam back scattered by the sample in the
signal arm.
[0058] 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.
[0059] According to a fourth aspect of the present invention, an
optical parametric oscillator comprises:
[0060] 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,
[0061] means for providing optical feedback at a wavelength within
the accessible bandwidth.
[0062] Preferably, the accessible bandwidth of the optical
parametric oscillator is at least 200 nm.
[0063] Preferably, the optical feedback means is provided at least
in part by a photonic structure.
[0064] 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.
[0065] Preferably, the structure comprises a photonic
structure.
[0066] Preferably, the optical dispersion characteristics of the
planar optical waveguide are modified to achieve negative
(anomalous) dispersion at a predetermined wavelength.
[0067] According to a fifth 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.
[0068] Preferably, the accessible bandwidth of the optical
parametric amplifier is at least 200 nm.
[0069] 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.
[0070] Preferably, the structure comprises a photonic structure
[0071] Preferably, the optical dispersion characteristics of the
planar optical waveguide are modified to achieve negative
(anomalous) dispersion at a predetermined wavelength.
[0072] According to a sixth aspect of the present invention, a
method for enhancing the bandwidth accessible in the generation of
an optical output, comprises the 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.
[0073] According to a seventh aspect of the present invention, a
method for generating an optical signal comprises the steps of:
[0074] receiving an optical input signal having an input bandwidth
at an optical input to a planar optical waveguide;
[0075] guiding the optical input signal along the planar optical
waveguide; and,
[0076] 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.
[0077] 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 the device can be greatly improved. The planar
waveguide device may be incorporated in more complex photonic
integrated circuits, such as a Michelson interferometer for low
coherence interferometry based optical coherence tomography.
BRIEF DESCRIPTION OF THE DRAWINGS
[0078] Examples of the present invention will now be described in
detail with reference to the accompanying drawings, in which:
[0079] FIG. 1A illustrates a system for non-linear bandwidth
generation according to the present invention;
[0080] FIGS. 1B and 1C show planar optical waveguides according to
the present invention;
[0081] 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;
[0082] 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;
[0083] FIGS. 4A to 4D illustrate, on a macroscopic and microscopic
level, a nonlinear optical interaction between light and
matter;
[0084] FIGS. 5A, 5B and 5C illustrate the nonlinear process of
self-focussing;
[0085] FIGS. 6A and 6B illustrate temporally and spectrally,
respectively, the nonlinear process of self-phase modulation of an
ultrashort pulse;
[0086] FIG. 7 illustrates an arrangement for optical CG in a
Ta.sub.2O.sub.5 planar waveguide with a pulsed input;
[0087] FIG. 8 shows the optical input and output spectra for CG in
the arrangement of FIG. 7;
[0088] FIG. 9A 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;
[0089] FIG. 9B shows the output spectra for CG with two different
wavelengths of pump laser source;
[0090] FIG. 10 shows a planar optical waveguide according to the
present invention;
[0091] FIG. 11 illustrates the concept of on-chip pre- and
post-processing in the device of FIG. 10;
[0092] FIG. 12 shows a planar waveguide with vertically tapered
input region;
[0093] FIG. 13 shows a planar waveguide with laterally tapered
input regions;
[0094] FIG. 14 shows a planar waveguide with a 2-D photonic
structure for on-chip filtering;
[0095] FIG. 15 shows a planar waveguide with a 1-D photonic
structure for on-chip filtering;
[0096] FIG. 16 shows a planar waveguide with laterally tapered
input regions and a photonic structure for on-chip filtering;
[0097] FIG. 17 shows a ridge type embodiment of a planar
waveguide;
[0098] FIG. 18 shows a rib type embodiment of a planar
waveguide;
[0099] FIG. 19 shows a ridge type planar waveguide with 1-D
photonic structure for on-chip filtering;
[0100] FIG. 20 shows a rib type planar waveguide with 1-D photonic
structure for on-chip filtering;
[0101] FIG. 21 shows a ridge type planar waveguide with laterally
tapered input region;
[0102] FIG. 22 shows a rib type planar waveguide with laterally
tapered input region;
[0103] FIG. 23 shows ridge type planar waveguides having laterally
up- or down-tapered input/output regions.
[0104] FIG. 24A shows a ridge type planar waveguide with laterally
tapered input region and photonic structure for on-chip
filtering;
[0105] FIG. 24B shows a rib type planar waveguide with laterally
tapered input region and photonic structure for on-chip
filtering;
[0106] FIG. 25A shows a three-channel ridge type planar waveguide
with laterally tapered input regions and photonic crystal
structures for on-chip filtering;
[0107] FIG. 25B shows a three-channel rib type planar waveguide
with laterally tapered input regions and photonic crystal
structures for on-chip filtering;
[0108] FIG. 26 shows a planar waveguide with a 1-D cladding
photonic structure for waveguide dispersion control;
[0109] FIG. 27 shows a planar waveguide with a 1-D core/cladding
photonic structure for waveguide dispersion control;
[0110] FIG. 28 shows a planar waveguide with a 2-D
buffer/core/cladding photonic crystal structure for waveguide
dispersion control;
[0111] FIG. 29 shows a planar waveguide with a multilayer structure
for waveguide dispersion control;
[0112] FIG. 30 shows a planar waveguide with 2-D photonic structure
for on-chip pulse compression;
[0113] FIG. 31 shows a planar waveguide with 1-D photonic structure
for on-chip pulse compression;
[0114] FIG. 32 illustrates the basic arrangement for Optical
Coherence Tomography (OCT) using low coherence interferometry;
[0115] FIG. 33 shows a block diagram of an interferometer
arrangement for OCT using CG as the optical source;
[0116] FIG. 34 shows a perspective view of an on-chip integrated
Michelson Interferometer with CG arm using planar ridge
waveguides;
[0117] FIG. 35 show a plan view of FIG. 34;
[0118] FIG. 36 shows on-chip polarization or wavelength splitting
for providing inputs to two OCT Michelson Interferometers;
[0119] FIG. 37 shows off-chip optical path length scanning of the
reference arm;
[0120] FIG. 38 shows on-chip optical path length scanning of the
reference arm;
[0121] FIG. 39 shows off-chip single-channel OCT detection;
[0122] FIG. 40 shows off-chip multi-channel OCT detection;
[0123] FIG. 41 shows on-chip integrated single-channel OCT
detection;
[0124] FIG. 42 shows on-chip integrated multi-channel OCT
detection;
[0125] FIG. 43 shows integrated on-chip modulation of the CG arm
shown in FIG. 35;
[0126] FIG. 44 shows integrated on-chip modulation of the
post-splitting two input arms of the device shown in FIG. 35;
[0127] FIG. 45 shows a known PPLN optical parametric generator;
[0128] FIG. 46 shows a known PPLN optical parametric
oscillator;
[0129] FIG. 47 illustrates the concept of an OPO based on a
nonlinear device according to the present invention;
[0130] FIG. 48 illustrates the concept of an OPO based on the
nonlinear device with pre- and post-processing;
[0131] FIG. 49 shows an OPO with feedback via on-chip discrete
photonic structures; and,
[0132] FIG. 50 shows an OPO with feedback via an on-chip waveguide
and waveguide beamsplitters.
DETAILED DESCRIPTION
[0133] The present invention is directed to a device for non-linear
bandwidth generation. FIG. 1A illustrates the overall system 1,
which comprises an optical source 2 for generating an optical input
3 that is coupled into an optical device 5 having a non-linear
portion 4 for generating an optical output 6 from at least a
portion of the optical input 3. The optical input 3 acts as a
"pump" for the non-linear process occurring in the non-linear
portion 4, and the optical output 6 generated has a wavelength
within an accessible bandwidth, which is substantially enhanced by
the operation of the non-linear portion. In the present invention,
the non-linear portion 4 is provided by a planar waveguide and the
optical source 2 is typically a mode-locked laser, which provides
an ultra-short pulsed optical input 3 to the non-linear waveguide
4.
[0134] 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.
[0135] 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. 1B and 1C 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. 1C. 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.
[0136] 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.sub.c 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.
[0137] 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. 2(a) 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.
[0138] 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
mode 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.
[0139] FIG. 2(b) shows a dispersion curve for the effective index
of a fundamental mode propagating in a planar optical waveguide of
the type shown in FIG. 1C. 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. 2(c). 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.).
[0140] Another useful way to express dispersion is to look at its
temporal effects on light propagation within a material. The phase
velocity, V.sub.100 of a wave is inversely proportional
(V.sub.100=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
[0141] 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 )
[0142] The D parameter accounts for the propagation delay per unit
wavelength 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.
[0143] 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. 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.
[0144] 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. 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.
[0145] 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.)I(r,t)
[0146] 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 I(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.
[0147] 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(I(r,t)), with
the spatially-varying contribution, I(r), giving rise to
self-focussing and the temporally-varying contribution, I(t),
giving rise to self-phase modulation.
[0148] 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. 5B, 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 the pulse than at the edges. Thus, in effect, a weak
positive lens is induced within the medium and the 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.
[0149] 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
[0150] 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, .DELTA..omega., across the pulse,
given by 4 = - ( ) t
[0151] 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. 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.
[0152] 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.
[0153] FIG. 7 shows in more detail a particular arrangement of the
system shown in FIG. 1A. The system 70 shown is suitable 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 substrate. A laser source 71, comprising a modelocked
titanium sapphire (TiS) laser oscillator 72 with regenerative
amplifier 73, is used to provide 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.sub.r=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.apprxeq.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 rate 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.
[0154] 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. The 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. The most commonly used
logarithmic scale is the decibel scale, on which relative intensity
is defined by 5 Relative Intensity in dB = - 10 log 10 I ( ) I max
( )
[0155] On the logarithmic decibel scale, the half-maximum intensity
point, at which FWHM is conventionally measured, corresponds to the
-3 dB, However, here the point at which the spectral (radiant)
intensity, I(.lambda.), falls permanently below 10.sup.-3 (0.001)
of the peak (maximum) value, I.sub.max is chosen for the measure of
bandwidth. This equates to a relative intensity of -30 dB on the
decibel scale. 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)
[0156] As can be seen from FIG. 8, based on this definition, the
input pump bandwidth of 50 nm is non-linearly broadened to 200 nm
and 600 nm, respectively, at an average pump power of 20 mW and 200
mW, corresponding to a peak power of approximately 0.5 and 5.5 mW.
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".
[0157] 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.
[0158] FIG. 9A 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 by self-phase
modulation, for the purposes of optical switching, has previously
been reported in a chalcogenide glass planar waveguide. However,
although the spectrum of the output was nonlinearly broadened, the
degree is much less than that obtained in a waveguide according to
the present invention.
[0159] FIG. 9B shows the spectra for continua generated with two
different infra-red input pump wavelengths, namely 800 nm, 1040 nm,
which are of particular interest for a variety of applications.
These continua were generated in a ridge waveguide embodiment of
the present invention, which will be described later. The results
clearly demonstrate the broad applicability of planar waveguides
according to the present invention for generating signals across a
wide range from visible wavelengths to wavelengths in the
near-infra-red. The continuum spectra shown in FIG. 9B each span a
bandwidth which includes the wavelength of the optical pump input
used to generate it. The spectral intensity of the two spectra have
been normalized, allowing easy comparison of the -3 dB and -30 dB
spectral widths. The -3 dB and -30 dB points are denoted by
intersection with the upper and lower horizontal dot-dash lines,
respectively. An optical continuum was also generated with a 1550
nm pump. The spectrum is not shown in FIG. 9B, but spanned from at
least 950 nm to 1750 nm, with measurement at shorter and longer
wavelengths limited by the detection system used.
[0160] Using a planar waveguide with a 500 nm Ta.sub.2O.sub.5 core
layer, continuum generation is achieved for an average pump power
as low as 10 mW (pulse energy 40 nJ, peak power 0.25 mW).
Alternative dimensions and materials should reduce the 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 which can be deposited in a thin
film to form the core of a planar waveguide. This is another
advantageous feature of the planar waveguide device according to
the present invention. Doping the core material 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. Further
advantages of the planar waveguide geometry for continuum
generation are preservation of polarization state and very low
noise. Other techniques for CG do not preserve the polarization
state of the continuum generated, leading to an optical signal with
a polarization state that is spatially-varying or possibly
depolarized. Such beams have a reduced range of applications.
Similarly, a low level of unwanted background noise is also
desirable.
[0161] 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
non-linear signal generation in the core layer 103. As a result of
the interaction a non-linearly 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.
[0162] As indicated in FIG. 1, the overall device 5 may have parts
other than the non-linear portion 4. FIG. 11 illustrates the broad
concept whereby a planar waveguide chip 110 comprises not only a
portion 113 fulfilling the role of enhancing the accessible
nonlinearly-generated bandwidth 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. The planar
waveguide geometry of the non-linear device according to the
present inventions facilitates integration, including monolithic,
with other components.
[0163] 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 non-linearity. 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 10 of FIG. 10,
suitably designed external optics are required to beam shape and
launch light efficiently into the waveguide core 103. However, an
alternative approach is to build beam shaping or mode converting
capability into the chip. An effective way to do this is 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.
[0164] FIG. 12 shows an embodiment of a planar waveguide 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. The waveguide 120 itself may be single-mode,
only supporting a single spatial mode, or may be multi-mode. As
shown, the waveguide is broad area, only providing significant
confinement in the vertical dimension.
[0165] 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.
[0166] 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 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
optical propagation constant.
[0167] 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. 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.
[0168] 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
associated optical transfer function is described in more detail in
co-pending U.S. patent application Ser. Nos. 10/196727, 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 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 155
filters out a band of wavelengths to produce the spectrum 159.
[0169] The on-chip processing functions described so far can, of
course, be combined. 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, 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.
[0170] 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. 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 the broad area device illustrated in FIG. 10. Input pulse
175 is converted in a broad continuum represented by spectrum 176.
FIG. 18 shows a device comprising a buffer 182 and a core rib 183
layer formed on a substrate. Again, the device operates in the same
manner as the broad area device illustrated in FIG. 10, with the
input pulse 184 non-linearly converted to a broad continuum
represented by spectrum 185.
[0171] 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. The precise dimensions of the structures
will determine whether the waveguide is single-mode or
multi-mode.
[0172] Of course, many of the pre- and post-processing structures
applied to the basic broad area 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 with narrow
spectrum 196 is therefore broadened into a continuum and filtered
to produce the spectrum 197, with a central missing waveband.
[0173] 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.
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.
[0174] 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 the input region 214. 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.
[0175] FIG. 23 illustrates other types of taper structure that may
be implemented in a ridge (or rib) waveguide. Again the basic
structure 230 comprises a buffer layer 231, a core layer 232 and a
core ridge layer 233. Both of the two core ridges shown provide
lateral confinement and have input waveguide regions 234 with the
same width w, taper region 235 and central region 236. However, in
one waveguide the ridge tapers down laterally to provide a narrower
central region, whilst in the other waveguide the ridge tapers up
laterally to provide a broader central region, width w.sub.r.
Optical signals propagating in the two waveguides will experience
differing degrees of dispersion in dependence on the width of the
ridge in the central region 236. In both cases, the taper is
reversed at the output region to return the waveguide to the same
width as the input. These types of structures are particularly
useful for incorporating the non-linear waveguide within an
integrated planar device.
[0176] In a manner analogous to that shown in FIG. 16, combinations
of tapers and photonic crystal structures can be fabricated in both
ridge and rib waveguides. FIG. 24A illustrates the combination of
tapers and photonic crystal structures applied to a ridge waveguide
embodiment of the present invention. FIG. 24A shows a device 240
including a buffer layer 242, a core layer 241 and a core ridge
layer 244. The core ridge layer includes 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
246 is incident on the tapered section 243 and an output is
generated with spectrum 247.
[0177] FIG. 24B illustrates a rib waveguide embodiment equivalent
to the ridge waveguide embodiment of FIG. 24A. The structure
includes a buffer layer 242 and a core rib layer 244. 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 246 is incident on the tapered
section 243 and an output is generated with spectrum 247.
[0178] 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. 25A illustrates the
concept with a three channel 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. 25A comprises three distinct waveguiding channels 254 on a
buffer layer 252, each with a separate input taper 253 and photonic
crystal structure 255. 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. FIG. 25B
illustrates the same three channel device implemented with rib type
waveguides.
[0179] 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.
The input 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. FIG. 27
illustrates a similar structure 270 to that shown FIG. 26, which
includes a buffer layer 272, a core layer 273 and a cladding layer
274. Here, the photonic crystal 275 is formed from slabs 276
located in both the cladding layer and the core layer.
[0180] 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.
[0181] 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 and low refractive index
(n.sub.1, n.sub.2 . . . n.sub.n) on a buffer layer 292. The
dimensions and materials used for each layer are calculated
according to the desired dispersion characteristics. In this
manner, as shown, three different spectral outputs can be obtained
from a single input.
[0182] 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 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.
[0183] 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 the input beam spectrum 307, in
order to give rise to a compression in the time domain. 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 the core 304 and a continuum 308 is
generated.
[0184] FIG. 31 shows an example of a broad area planar waveguide
310 with a 1-D photonic crystal 314 extending through a core layer
313. As with the photonic crystal shown 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.
[0185] The pulse compression described above may simply be for the
purposes of increasing peak power in the pulse in order to induce a
stronger nonlinear effect in the 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 the embodiments described previously.
[0186] We now turn our attention to an application of the optical
continuum generated in a planar waveguide according to the present
invention. In particular the integration of a CG waveguide in a
device for application to Optical Coherence Tomography (OCT). OCT
is a relatively recent technique for performing optical ranging in
biological tissue to determine local variations in the structure
and is similar to ultrasound imaging, but uses near-infrared
optical radiation. Optical ranging has been widely used in
telecommunications for locating faults or defects in optical fibers
by sending optical pulses through the fiber and measuring the time
delay between the original pulse and a reflected pulse. However,
due to the velocity of light, the time delay between original and
reflected pulses cannot be measured directly, and so an
interferometric technique is used.
[0187] One suitable method is low coherence interferometry, or
coherence domain reflectometry, performed using a Michelson type
interferometer. FIG. 32 shows a schematic diagram of an optical
fibre based Micheson Interferometer (MI) system 320 suitable for
OCT. The main interferometer region 321 includes a low coherence
source 322 and a beam splitter 323, which is ideally a 50:50 (3 dB)
intensity splitter. Low coherence radiation from the source 322 is
coupled into an optical fibre and is split by the splitter 323 to
provide signals for each of a reference arm 325 and a signal arm
327. The reference arm 325 includes a reflector and an instrument
for varying (scanning) the optical length of the reference arm,
either by varying the actual or effective physical path length or
refractive index. The signal arm 327 includes an instrument for
delivering the beam to the sample specimen 328 under test. The
optical signals reflected or back scattered from the reference arm
325 and signal arm 327 are recombined at the splitter 232, where
they interfere, and the combined signal propagates along a
detection arm to a photodetector 329 and any associated diagnostic
equipment, such as filter, analogue-to-digital converter and
computer.
[0188] Axial scanning of the reference arm optical length permits
measurement of interference fringes, and associated coherence
envelope, formed by interference between the reference and signal
beams. From this the time-of-flight and optical ranging information
can be deduced. By scanning the beam in the transverse directions,
horizontal and vertical, a two dimensional image may be built up.
Axial scanning gives depth information and the detected beam
intensity gives further information about the composition of the
sample. In this way, a data array of up to four-dimensions may be
built that represents the optical backscattering from the specimen
sample.
[0189] A key feature of this technique is that the axial (depth)
and transverse (lateral) resolution are independent. The axial
resolution is determined by the coherence length of the source of
the source and is inversely proportional to the spectral bandwidth.
For an optical source with a Gaussian-shaped spectral distribution,
the axial resolution is given by 6 z = 2 ln 2 2
[0190] where .lambda. is the centre wavelength and .DELTA..lambda.
is the spectral bandwidth. Thus, for high axial resolution, a wide
bandwidth and hence low coherence source is required, which makes
optical continua an attractive option. For application to
biological samples another consideration is the degree of scatter,
which is stronger at shorter wavelengths, and absorption, which is
stronger at longer wavelengths. Thus, an optimum "biological
window" exists in the near infrared between 800 nm and 1500 nm
which, as shown in FIG. 9A, makes optical continua generated in
planar waveguides of the present invention particularly well
suited. Finally, transverse resolution .DELTA.x is determined by
focused spot size and is given by 7 x 4 f d
[0191] where d is the diameter of the beam size incident on a lens
of focal length f. As .lambda. is largely fixed, .DELTA.x can be
varied by choosing suitable values of f and d, although a trade-off
exists between transverse resolution and depth of focus
(.varies..DELTA.x.sup.2/.lambda.- ).
[0192] FIG. 33 illustrates the broad concept of an MI based OCT
system 330, which comprises an optical continuum generation arm
331, a reference arm 332, signal arm 333, detection arm 334 and
splitter 335. As shown, the CG arm 331 generates an optical
continuum from a narrower band input. Although, an optical
continuum generator according to the present invention can be used
as a stand alone source for a variety of implementations of the
system shown in FIG. 33, the geometry of the planar waveguide lends
itself to simple monolithic integration of many of the components
of the system. In particular, the main interferometer region 321
shown in FIG. 32, can be formed as a planar photonic integrated
circuit on a single chip.
[0193] FIG. 34 shows one embodiment of such a device 340, which
implement each of the CG arm 341, reference arm 342, signal arm 343
and detection arm 344 as ridge waveguides and the splitter 335 as a
back-to-back Y-type ridge waveguide splitter/combiner. FIG. 35
shows a schematic plan view of the ridge geometry. As with previous
embodiments of the planar waveguide continuum generator, the CG arm
341,351 is pumped with short intense pulses of relatively narrow
bandwidth, which drive the nonlinear process that generates an
optical continuum that is confined and waveguided towards the
splitter 345,355. Of course, very many variations and functionality
may be built into the device. For example, the arrangement shown in
FIG. 23 may be employed to provide two arms of different width.
[0194] FIG. 36 shows a more complex variant 360 in which a
continuum is generated in a waveguide 361 and routed by the two
arms of a Y-branch splitter 362 to provide a CG beam as input to
two separate MI arrangements, each having a reference arm 363,
signal arm 364 and detection arm 365. The nature of the splitting,
especially intensity ratio, may be controlled by the geometry of
the Y-branch 362 or any other structure incorporated in it. FIG. 36
shows Y-branch structure 362 which incorporates a photonic crystal
structure 366 located between the CG region 361 and the Y-branch.
The two insets to FIG. 36 illustrate that the photonic crystal 366
of the Y-branch structure may be designed to split the CG signal
according to polarization state or wavelength to provide
independent and synchronized inputs to the two interferometers.
Polarization splitting facilitates simultaneous OCT probing of
samples using orthogonally polarized light of the same waveband. On
the other hand, wavelength splitting facilitates simultaneous OCT
probing of samples using light in different wavebands. For example,
a continuum generated in a ridge waveguide with a 1550 nm pump
contains two Gaussian-like bands at 1040 nm and 1300 nm, which are
the best wavelengths for OCT, as they exhibit minimum absorption by
water, melanin and haemoglobin and hence have a bigger penetration
depth. The two wavebands can be carved out of the continuum for
splitting at the Y-branch by suitable design of the photonic
crystal structure 366.
[0195] As described previously with reference to FIG. 32, generally
only the main interferometer region 321 is integrated on a single
chip. In this case the photonic integrated circuit (PIC) must be
optically coupled to the other necessary components. FIG. 37 shows
the PIC 370, with reference arm 371, signal arm 372, detection arm
373 and splitter/combiner 374 and also examples of the off-chip
components 375 for the reference arm, including fibre 376,
dispersion compensator 377 and scanning mirror 378. An optical
fibre 376 is employed for collection, optical delay and onward
transmission of the reference beam and a mirror 378 is mechanically
scanned to vary the optical path length of the reference arm. A
dispersion compensator 377 is employed to compensate for dispersion
experienced by the various wavelength components of the reference
signal while propagating along the reference arm.
[0196] FIG. 38 shows a variation 380 of the OCT device shown in
FIG. 37, in which the necessary components 385 for the reference
arm 381 are integrated with the rest of the structure. Here, the
reference arm 381 and signal arm 382 waveguides are extended, and
the reference arm includes two photonic crystal structures. One
photonic crystal 386 acts as both an optical delay line and a
dispersion compensator, while the other photonic crystal 388 acts
as the back-reflecting mirror. Dispersion compensation and optical
delay are tuned by tuning the refractive index of the filled
photonic crystals. Dynamic tuning permits axial scanning of the
optical path length of the reference arm.
[0197] FIG. 39 again shows the basic PIC structure 390, with
reference arm 391, signal arm 392, detection arm 393 and
splitter/combiner 394. Also shown here is a stand-alone
photodetector 395 for off-chip detection of the optical signal
propagating in the detection arm 393. In general, the photodetector
will be connected to suitable electronic circuitry for processing
of the electrical signal produced by the photodetector. This will
typically include bandpass filter, envelope detector and
analogue-to-digital converter. The resulting signal is fed to a
computer for final data analysis and display, which may include a
grey-scale or false-colour 2-D image, or tomogram, of the optical
backscatter signal obtained from the specimen. However, by
analysing the full interferometric data in more detail, other types
of tomogram may be displayed, which show spectrographic information
or information about the motion of scatter centers due to liquid
flow or tissue movement.
[0198] Such data analysis can be simplified by including further
functionality in the detection arm, such as a dispersive element.
FIG. 40 shows the basic PIC structure 400, in which the detection
arm 403 has an integral photonic crystal structure designed to
disperse the signal into three wavelength channels. The three
optical signals so derived are then detected by a suitable off-chip
photodector 405 and the resulting electrical signal(s) processed.
Of course, the photodetecting element may be integrated with the
rest of the PIC on a single chip to form the final part of the
detection arm. FIG. 40 shows a similar arrangement 410 to FIG. 39,
but with the photodetector 415 located on-chip and forming the end
of the integrated detection arm. FIG. 42 shows the arrangement 420
of FIG. 40, but again with an integrated on-chip photodetector 425.
It is noted that the location of the dispersive photonic crystal
structure 426 within the detection arm 423 is adjusted for optimum
coupling of the three channels to the photodetector 425.
[0199] A final addition to the MI-based OCT systems described above
is the use of modulation, which may be implemented by an integrated
on-chip modulator. FIG. 43 shows the familiar PIC interferometer
with a modulator 436 integrated in the CG source arm 435. Intensity
modulators can easily be realized in such a PIC, by employing
phenomena such as electroabsorption, for example. The modulator
region is typically driven by a suitable voltage signal applied to
an electrode located on the modulator region. In the device shown
in FIG. 43, the continuum is largely generated in input region 435
and then modulated by the modulator 435. This provides greater
functionality for OCT by allowing more control over the form of the
optical signal with which the sample is probed. Furthermore, the
modulation may provide a signature element to the optical signal
which aids in its detection. The functionality of the OCT device
shown in FIG. 36 may also be enhanced by the use of modulation.
FIG. 44 shows a similar arrangement 440, but in which a modulator
442 is built into each of the arms of the Y-branch leading to the
two interferometers. The modulators 442 can provide either phase or
amplitude modulation and may be designed according to the operation
of the photonic crystal structure 446 in the continuum arm 441.
Independent modulation of the continuum signals entering the two
interferometers significantly enhances the functionality of OCT
detection. In particular, the use of different modulation
frequencies permits different types of simultaneous scanning.
[0200] The majority of the foregoing discussion has centered on the
ability of the present invention to generate large bandwidth,
particularly in the context of optical continuum generation, and
its manipulation and application, such as in OCT. 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. The total photon energy is conserved in
the process such that the pump, .omega..sub.p, signal,
.omega..sub.s, and idler, .omega..sub.i, frequencies are related
by
.omega..sub.p=.omega..sub.s+.omega..sub.i
[0201] 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. 45 shows a known waveguide
device 450 of this type based on periodically poled Lithium Niobate
(Li.sub.2O.sub.3), also known as PPLN. A single frequency input
beam 453 is converted into a signal wavelength 454 and an idler
wavelength 455 by the waveguide 451. It is not shown in FIG. 45 but
the periodically poled regions 452 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.
[0202] 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. 46 shows a known PPLN
waveguide-based OPO 460, which utilizes distributed Bragg gratings
462 at each end of the waveguide 461 to provide optical feedback in
a linear cavity.
[0203] FIG. 47 shows the basic concept of an OPO 470 based on a
planar waveguide according to the present invention. The core
component is the waveguide region 471, 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. 47, also comprising
discrete or distributed reflectors. FIG. 47 shows an input optical
beam 472 incident on a partial reflector 473. Reflector 473 passes
the signal 472 to the waveguide region 471. The output from the
waveguide region 471 is incident on partial reflector 474. The
reflector 474 is designed to be partially transmitting at the
resonating wavelength to provide an output 475. A portion of the
output is reflected by reflector 474 around a loop 477 including
reflectors 476 and 478 back to the reflector 473. The reflected
signal is then fed back into the waveguide region 471. The signal
fed back into the waveguide region 471 is at the wavelength of the
input signal. The reflectors 476, 478 themselves may provide the
wavelength selectivity or a specific filter may be inserted in the
loop 477 to perform this function.
[0204] FIG. 48 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 481 and post-processing such as
filtering and output tapers 485. The waveguide region is indicated
at 482 and a partial/selective reflector or splitter is shown at
483. The feedback loop is shown with a specific filter 484.
[0205] FIG. 49 shows an embodiment of an OPO 490 according to the
present invention comprising a compact planar waveguide 492 with
feedback provided by four discrete 1-D or 2-D photonic crystals
491, 493, 494. One of the four crystals 491 is designed to transmit
the input 495 and another of the four crystals 493 is designed to
be partially transmitting at the resonating wavelength in order to
provide an output 496. FIG. 50 shows another embodiment 500
comprising an extended waveguide 502 with input 501 and output 504
tapers, and on-chip beam splitters 503 to couple light out into the
feedback loop. Wavelength selectivity and tuning is provided by
photonic crystal 505 located within the feedback loop.
[0206] 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 the 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.
[0207] 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
device may be pumped by a variety of standalone sources. Typically
these are ultrashort pulse laser sources. 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. Furthermore, the planar waveguide, as optical continuum
generator, may be integrated in a variety of larger photonic
devices. A particular example that has been described is a PIC
implemented Michelson interferometer with a continuum broadband
source, which finds great application in optical coherence
tomography by low coherence interferometry.
* * * * *