U.S. patent application number 10/153455 was filed with the patent office on 2002-09-26 for photonic band gap device and method using a periodicity defect region doped with a gain medium to increase photonic signal delay.
Invention is credited to Scalora, Michael.
Application Number | 20020135853 10/153455 |
Document ID | / |
Family ID | 26832420 |
Filed Date | 2002-09-26 |
United States Patent
Application |
20020135853 |
Kind Code |
A1 |
Scalora, Michael |
September 26, 2002 |
Photonic band gap device and method using a periodicity defect
region doped with a gain medium to increase photonic signal
delay
Abstract
A photonic band gap structure device and method for delaying
photonic signals of a predetermined frequency and a predetermined
bandwidth by a predetermined delay is provided. A Fabry-Perot delay
line device has several regions of periodically alternating
refractive material layers which exhibit a series of photonic band
gaps and a periodicity defect region, interposed between the
regions of periodically alternating refractive material layers. The
Fabry-Perot delay line device imparts a predetermined delay to
photonic signals that pass therethrough. The introduction of the
periodicity defect region into this photonic band gap structure
creates a sharp transmission resonance within the corresponding
photonic band gap of the structure and causes at least an order of
magnitude improvement in photonic signal delay for a band-edge
delay line device of similar size. Variable photonic delays to
multiple photonic signals are also generated by this Fabry-Perot
delay line device. In addition, a photonic signal delay device
based on an optical fiber grating structure is provided. A defect
layer doped with a gain medium increases the density of modes
thereby increasing the density of photons at a particular
frequency. The introduction of gain automatically causes an
increase in the photon density, and therefore, a narrowing of the
density of modes. This allows tuning of the group velocity, just as
if periods were being added. This also enables larger delays.
Inventors: |
Scalora, Michael;
(Huntsville, AL) |
Correspondence
Address: |
STERNE, KESSLER, GOLDSTEIN & FOX PLLC
1100 NEW YORK AVENUE, N.W., SUITE 600
WASHINGTON
DC
20005-3934
US
|
Family ID: |
26832420 |
Appl. No.: |
10/153455 |
Filed: |
May 24, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10153455 |
May 24, 2002 |
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09572290 |
May 17, 2000 |
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6396617 |
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60134536 |
May 17, 1999 |
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Current U.S.
Class: |
359/248 ;
359/245; 359/254 |
Current CPC
Class: |
B82Y 20/00 20130101;
H01S 5/183 20130101; G02B 6/1225 20130101; H01S 5/11 20210101 |
Class at
Publication: |
359/248 ;
359/254; 359/245 |
International
Class: |
G02F 001/03; G02F
001/07 |
Goverment Interests
[0002] This invention was made with Government support under
Contract DAAHO1-96-R234 awarded by the U.S. Army Missile Command.
The Government has certain rights in the invention.
Claims
What is claimed is:
1. A device for delaying a photonic signal by a predetermined
delay, the photonic signal having a predetermined frequency and a
predetermined bandwidth, comprising: a first region having
periodically alternating refractive material layers which exhibit a
photonic band gap structure, said first region comprising a first
refractive material layer having a first thickness and a first
index of refraction, and a second refractive material layer having
a second thickness and a second index of refraction; a second
region having periodically alternating refractive material layers
which exhibit a photonic band gap structure, said second region
comprising a third refractive material layer having a third
thickness and a third index of refraction, and a fourth refractive
material layer having a fourth thickness and a fourth index of
refraction; and a periodicity defect region, interposed between
said first and second regions of periodically alternating
refractive material layers, wherein said periodicity defect region
is doped with a gain medium; wherein the first predetermined delay
is imparted to the photonic signal during passage through the
device when said periodicity defect region is excited by an energy
source.
2. The device of claim 1, wherein said energy source is an
electrical current.
3. The device of claim 1, wherein said energy source is an optical
signal having the predetermined frequency.
4. A method of imparting a delay to a photonic signal of a
predetermined frequency and a predetermined bandwidth by a
predetermined temporal duration, wherein the photonic signal is
generated by an optical source, comprising the steps of: sending
the photonic signal along an optical path; passing the photonic
signal through a first region of periodically alternating
refractive sections in said optical path of the photonic signal;
passing the photonic signal through a periodicity defect region in
said optical path, wherein the periodicity defect region is doped
with a gain medium and excited by an energy source; and passing the
photonic signal through a second region of periodically alternating
refractive sections in said optical path.
5. The method of claim 4, wherein the energy source is an
electrical current; and further comprising the step of setting the
electrical current to impart the desired delay.
6. The method of claim 4, wherein the energy source is an optical
signal having the predetermined frequency; and further comprising
the step of setting the optical signal to impart the desired delay.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This patent application claims priority to and is entitled
to the benefit of U.S. Provisional Patent Appl. No. 60/134,536,
filed May 17, 1999, and U.S. Nonprovisional patent application Ser.
No. 09/572,290, filed May 17, 2000, now allowed, entitled "Photonic
Band Gap Device and Method Using a Periodicity Defect Region Doped
with a Gain Medium to Increase Photonic Signal Delay." These
applications are incorporated herein by reference in their
entireties.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention relates to optical communications and
optical signal control. In particular, the present invention
relates to an optical device and method for delaying photonic
signals.
[0005] 2. Related Art
[0006] The fields of communications and data processing are
currently transitioning from using electrical signals to using
optical signals. As a result, there is an increased need for
optical devices that perform various tasks in the control of these
optical signals. Such devices include delay blocks for
synchronizing optical pulses for communications. The
synchronization of optical pulses requires the use of controllable
delay devices which do not significantly distort the optical beam
as it passes through the device. In particular, robust, compact,
lightweight delay devices for use with a variety of predetermined
optical frequencies must be developed to be easily integrated into
existing optical systems.
[0007] One method of creating a low distortion, controllable
photonic delay is through the use of "uniform" photonic band gap
(PBG) structures, also called band-edge delay line devices. Uniform
PBG structures typically comprise a stack of alternating layers of
refractive materials of similar thicknesses, such as gallium
arsenide and aluminum arsenide, which exhibit photonic band gaps in
their transmission spectra. These alternating layers have different
indices of refraction and can be deposited by well known deposition
techniques onto a substrate.
[0008] By sending a photonic signal of a given frequency (.omega.))
through this type of delay device, the discontinuity of the indices
of refraction imparts a delay to the photonic signal. These devices
slow down the photonic signal as a result of scattering inside the
uniform PBG structure. Since the photonic delay is proportional to
the square of the number of periods contained in the uniform PBG
structure, a device can be constructed that imparts a predetermined
delay to a photonic signal. The physical processes involved in the
photonic signal delay imparted by a uniform PBG structure are
described in detail in Scalora, et al., "Ultrashort pulse
propagation at the photonic band edge: large tunable group delay
with minimal distortion and loss," Phys. Rev. E Rapid Comm. 54(2),
R1078-R1081 (August 1996), which is incorporated by reference
herein.
BRIEF SUMMARY OF THE INVENTION
[0009] The present invention generally relates to a device and
method of creating an optical signal delay using a compact and
readily manufacturable structure. In particular, the present
invention interposes a periodicity defect region into a uniform
photonic band gap (PBG) structure in order to generate a
transmission resonance spike of very narrow bandwidth at or near
the center of the photonic band gap of the structure. The
introduction of this periodicity defect causes at least an order of
magnitude greater photonic signal delay duration than for a uniform
PBG device of similar size.
[0010] According to one embodiment of the present invention, a
Fabry-Perot delay line device is provided for delaying photonic
signals of a predetermined frequency and a predetermined bandwidth
by a predetermined delay. The Fabry-Perot delay line device
includes a first region of periodically alternating layers of
refractive materials which exhibit a photonic band gap structure, a
second region of periodically alternating layers of refractive
materials which also exhibit a photonic band gap structure, and a
periodicity defect region interposed between the first and second
alternating layer regions. The first region of periodically
alternating layers of refractive materials comprises one or more
first refractive material layers having a first thickness and a
first index of refraction, and one or more second refractive
material layers having a second thickness and a second index of
refraction. The second region of periodically alternating layers of
refractive materials comprises one or more third refractive
material layers and one or more fourth refractive material layers.
The third refractive material layer has a thickness and an index of
refraction similar to or identical to the first refractive material
layer of the first alternating layer region. The fourth refractive
material layer has a thickness and an index of refraction similar
to or identical to the second refractive material layer of the
first alternating layer region. The interposed periodicity defect
region has a different thickness than either the first or second
thickness of the alternating layers of refractive materials in the
first and second alternating layer regions. Additionally, the
periodicity defect region can have a third index of refraction and
a photonic transmission resonance, which are predetermined along
with thickness to impart predetermined delay to the photonic
signals that pass therethrough.
[0011] According to one feature of the present invention, the
transmission resonance due to the interposition of this periodicity
defect region is located in the center of the photonic band gap.
Further, the transmission resonance corresponds to the
predetermined frequency of the photonic signal traveling through
the Fabry-Perot delay line device.
[0012] According to a preferred embodiment of the present
invention, the Fabry-Perot delay line device is constructed on a
semiconductor substrate using semiconductor materials, such as
aluminum arsenide (AlAs) and gallium arsenide (GaAs), as the first
and second refractive material layers, respectively. Additionally,
the periodicity defect region is also a semiconductor material.
[0013] According to a second embodiment of the present invention,
the Fabry-Perot delay line device includes two or more periodicity
defect regions interposed between the periodically alternating
layer regions. This embodiment provides for delaying photonic
signals of differing predetermined frequencies and predetermined
bandwidths by a predetermined range of photonic signal delays.
[0014] According to one feature of the second embodiment of the
present invention, the first and second photonic transmission
resonances are located at different frequency positions in the
corresponding photonic band gap of the Fabry-Perot delay line
device. Further, the first transmission resonance corresponds to a
first predetermined photonic signal frequency, and the second
transmission resonance corresponds to a second predetermined
photonic signal frequency. Thus, the device may impart different
predetermined delays to the first and second predetermined photonic
signals traveling therethrough.
[0015] According to a third embodiment of the present invention, a
tunable laser source generates photonic signals traveling through a
Fabry-Perot delay line device. The delay imparted by the device
varies as a function of the frequency of the photonic signals that
pass therethrough.
[0016] According to another embodiment of the present invention,
Fabry-Perot delay line device also includes electrical contacts
located on the periodicity defect regions. A power supply source is
coupled to the contact points to impart a variable voltage across
the device which varies the indices of refraction of the
periodicity defect regions, thereby varying the delay imparted to
photonic signals passing therethrough.
[0017] Further, according to another embodiment of the present
invention, both the band-edge or the Fabry-Perot delay line device
can also be constructed using an optical fiber grating. According
to this embodiment of the present invention, a fiber grating delay
device comprises regions of alternating first and second sections,
each section having a different index of refraction, which are
periodically spaced along the fiber or waveguide. In addition, a
periodicity defect region having a thickness from either the first
or second section can be interposed between the regions of
alternating sections. Further, piezo-electric or other suitable
means can be coupled to the periodicity defect region to alter the
optical path length of the periodicity defect region, thereby
varying the photonic delay imparted to a photonic signal passing
through the fiber grating delay device.
[0018] According to another embodiment of the present invention, a
defect layer is doped with a gain medium.
[0019] Further features and advantages of the present invention, as
well as the structure and operation of various embodiments of the
present invention, are described in detail below with reference to
the accompanying drawings.
BRIEF DESCRIPTION OF THE FIGURES
[0020] The present invention is described with reference to the
accompanying drawings. In the drawings, like reference numbers
indicate identical or functionally similar elements. Additionally,
the left-most digit(s) of a reference number identifies the drawing
in which the reference number first appears.
[0021] FIG. 1A is a schematic representation of a photonic signal
delay device with a uniform PBG structure. This device is also
referred to as a band-edge delay line device.
[0022] FIG. 1B is a diagram of the characteristic index of
refraction profile of the uniform PBG structure shown in FIG.
1A.
[0023] FIG. 2A is a characteristic transmission profile for the
band-edge delay line device shown in FIG. 1A.
[0024] FIG. 2B is a transmission profile for the band-edge delay
line device that also includes a group velocity curve for a
photonic signal traveling through the structure.
[0025] FIG. 3A is a schematic representation showing a photonic
delay imparted by the band-edge delay line device of FIG. 1A.
[0026] FIG. 3B is a plot of the photonic delay imparted by the same
band-edge delay line device.
[0027] FIG. 4A is a schematic representation of a PBG structure
photonic signal delay device according to one embodiment of the
present invention. This device is herein referred to as a
Fabry-Perot delay line device.
[0028] FIG. 4B is a characteristic transmission profile for the
Fabry-Perot delay line device of FIG. 4A.
[0029] FIG. 5 is a graph representing photonic signal delay as a
function of the number of periods that compares a band-edge delay
line device to a Fabry-Perot delay line device according to the
present invention.
[0030] FIG. 6A is a schematic representation of a Fabry-Perot delay
line device containing two periodicity defect regions according to
a second embodiment of the present invention.
[0031] FIG. 6B is the corresponding transmission profile for the
photonic signal delay device containing two periodicity defect
regions shown in FIG. 6A.
[0032] FIG. 7 is a schematic representation of a tunable laser
source, according to a third embodiment of the present
invention.
[0033] FIG. 8 is a schematic representation of the Fabry-Perot
delay line device of FIG. 7 having a voltage applied to the
device.
[0034] FIG. 9 is a schematic representation of a GaAs/AlAs PBG
structure.
[0035] FIG. 10 depicts delay versus gain coefficient for the
structure depicted in FIG. 9.
[0036] FIGS. 11 and 12 provide two views of a vertical cavity
surface emitting laser (VCSEL), according to the present
invention.
[0037] FIGS. 13-19 are plots illustrating VCSEL performance.
DETAILED DESCRIPTION OF THE INVENTION
Table of Contents
[0038] 1. Overview and Discussion of the Invention
[0039] 2. Terminology
[0040] 3. Example Environment
[0041] 4. Band-edge Delay Line Devices
[0042] 5. Fabry-Perot Delay Line Devices
[0043] a. Interposed Periodicity Defect in a Semiconductor
Structure
[0044] b. Multiple Periodicity Defects
[0045] c. Variable Photonic Signal Delay
[0046] d. Fiber Grating Photonic Delay Devices
[0047] e. Defect Layer Doped with a Gain Medium
[0048] 6. Conclusion
[0049] Overview and Discussion of the Invention
[0050] The present invention improves the current state of the art
of photonic delay devices by interposing a periodicity defect into
a uniform PBG structure in order to create a Fabry-Perot delay line
device having a longer duration optical signal delay and which is
compact and readily manufacturable. A Fabry-Perot delay line
device, according to the present invention, can have an order of
magnitude longer duration photonic signal delay compared to a
uniform PBG structure of similar size.
[0051] According to the present invention, the introduction of a
periodicity defect into a uniform PBG structure generates a
transmission resonance spike of very narrow bandwidth in the
photonic band gap of the device. This transmission resonance spike
has a very narrow resonance frequency bandwidth. Thus, a photonic
signal of a similar bandwidth to the resonance frequency bandwidth
and an optical frequency corresponding to the frequency location of
the transmission resonance spike is delayed by a predetermined
amount.
[0052] Terminology
[0053] For the purposes of this disclosure, the following terms are
used in describing the present invention:
[0054] "Photonic signals" refer to electromagnetic signals,
including, but not limited to, optical signals, x-ray signals, and
microwave signals. Photonic signals are described either in terms
of their in-air wavelength (.lambda.) or their photonic signal
frequency (.omega.), where .lambda.=c/(2.pi..omega.).
[0055] "Photonic signal delay device" refers to any device that
imparts a temporal delay on a photonic signal traveling
therethrough.
[0056] "Uniform photonic band gap (PBG) structure" refers to a
dielectric or semiconductor structure or fiber grating with
alternating layers of material or sections of similar thicknesses
having different indices of refraction periodically spaced on a
substrate or along a fiber or waveguide. This type of structure is
referred to, for example, as a "PBG structure" or a band-edge delay
line device (See Scalora, et al., "Ultrashort pulse propagation at
the photonic band edge: large tunable group delay with minimal
distortion and loss," Phys. Rev. E Rapid Comm. 54(2), R1078-R1081
(August 1996) (incorporated by reference herein in its entirety).
The word "uniform" describes the uniform periodical spacing of
alternating layers or sections of similar thicknesses on a
substrate or along a fiber. In addition, the term "uniform"
distinguishes the conventional PBG structure, such as the band-edge
delay line device described with respect to FIG. 1A, from the
Fabry-Perot delay line device of the present invention.
[0057] "Fabry-Perot delay line device" refers to a PBG structure
having at least one periodicity defect region interposed between
periodically spaced alternating layer regions or sections in a
semiconductor or fiber grating structure, according to the present
invention.
[0058] "Periodicity defect" refers to a break in the periodicity of
alternating refractive material layers or sections.
[0059] Example Environment
[0060] Before describing the present invention in further detail,
it is useful to describe an example environment in which the
invention can be implemented. The present invention can be
implemented in any system utilizing optical signals. The present
invention is particularly helpful in communications systems that
require the synchronization of optical signals generated from a
variety of sources and locations. The present invention allows a
degree of design flexibility with respect to the amount of delay
imparted on the signals and also with respect to the optical
frequency of the signal traveling through the device. This
flexibility in the design of the photonic delay structures will
allow a user to implement this invention in a variety of
settings.
[0061] The present invention is described in terms of this example
environment. Description in these terms is provided for convenience
only. It is not intended that the invention be limited to
application in this example environment. In fact, after reading the
following description, it will become apparent to a person skilled
in the relevant art how to implement the invention in alternative
environments.
[0062] Band-edge Delay Line Devices
[0063] FIG. 1A is a schematic representation of a uniform PBG
structure that can impart a predetermined photonic delay having a
relatively limited duration as compared to the present invention.
This type of device is also referred to as a band-edge delay line
device. Device 102 receives photonic signals 104 of a predetermined
frequency .omega. and a predetermined bandwidth d.omega. at the
entrance interface 106. Device 102 comprises a stack of alternating
layers 108 and 110 of refractive materials having predetermined
indices of refraction n.sub.1 and n.sub.2, and predetermined
thicknesses a and b, respectively. For example, first layer 108 can
be designed to have a thickness (a) corresponding to the
predetermined wavelength of the incoming photonic signal
(.lambda.), determined by the equation a=.lambda./4n.sub.1.
Similarly, second layer 110 can have an index of refraction
n.sub.2, and a thickness (b), where b=.lambda./4n.sub.2. This
pattern can be repeated for N periods 122, where a period is equal
to one set of alternating layers 112.
[0064] Photonic signal delay can be determined as a function of the
number of periods (N) 122 chosen and the characteristic photonic
band gap transmission profile seen in FIG. 2, which is described
below. This type of PBG structure is also known as a distributed
Bragg reflector (DBR) and is typically used as a highly reflecting
mirror in laser devices.
[0065] FIG. 1B is a diagram of a characteristic index of refraction
square-wave pattern profile of a uniform PBG structure for N
periods. Diagram 150 plots the index of refraction (n) 152 of a
uniform PBG structure as a function of distance (z) 154, which is
limited by the number of periods 156 in the device. Diagram 150
illustrates the periodic nature of the abrupt refractive index
changes occurring in the material. This abrupt change or
discontinuity in the index of refraction of the material can be
used to create photonic delay effects.
[0066] FIG. 2A displays a characteristic transmission profile 204
for a band-edge delay line device 202. Typically, the uniform PBG
structure associated with device 202 exhibits an infinite number of
photonic band gaps and band edges. Transmission profile 204 is a
plot of such a representative photonic band gap. Transmission
profile 204 is obtained by plotting the optical transmission 206 of
device 202 as a function of frequency (.omega.) 208. The maximum
possible transmission 210 of the device is 1, which corresponds to
all of the input photonic signal being transmitted through the
device.
[0067] Transmission curve 205 undergoes a series of relative maxima
and minima depending on the frequency .omega. of the input photonic
signal. The photonic band gap 212 corresponds to the low
transmission region of the profile. The maximum in transmission
occurs at the photonic band edge 213. The band-edge delay line
device can be designed so that the frequency corresponding to the
photonic band-edge 213 matches the center frequency .omega. of the
input photonic signal. In addition, the photonic band-edge
transmission resonance bandwidth (.delta..omega.) 214 can be
designed to be at least as wide as the bandwidth d.omega. of the
photonic signal in order to insure that the input photonic signal
is not significantly distorted by the photonic signal delay device,
whose pulsewidths are arbitrary in duration. Specifically, these
devices can be designed to impart photonic delays on signals whose
pulsewidths are on the order of picoseconds (ps) or less.
[0068] Band-edge delay line device 202 operates as a photonic delay
line because at every place in the band structure that there is a
maximum in the transmission function, there is a corresponding
minimum in the group velocity: hence, the smaller the group
velocity, the larger the photonic delay. This principle is
graphically illustrated in FIG. 2B. FIG. 2B shows a graph 250 where
both the transmission curve 258 (solid-line) and the group velocity
(v.sub.g) curve 262 (dashed-line) are plotted as a function of
frequency (.omega.) 254. The transmission 252 of the input photonic
signal is inversely related to the photonic signal's group velocity
v.sub.g 256 as it travels through the band-edge delay line device
202. Graph 250 demonstrates that when the signal undergoes a
maximum in transmission, such as at the photonic band-edge 261, the
signal's group velocity v.sub.g is at a corresponding minimum 262.
In practical terms, device 202 operates as a photonic signal delay
line in that not only is the input photonic signal delayed
substantially, but the light is maximally transmitted at the
predetermined frequency .omega..
[0069] FIGS. 3A and 3B illustrate a photonic signal delay generated
by band-edge delay line device 202. FIG. 3A shows a schematic
representation of a photonic signal delay experiment 302. First, a
picosecond (ps) pulse 304, having a center frequency .omega. and a
bandwidth d.omega. is directed towards a bulk material 306 that
does not contain a stack of alternating layers of refractive
materials having varying indices of refraction. Output signal 310
is then measured at some predetermined time (T) after being
transmitted through the bulk material 306. The position in distance
of output signal 310 after the time T is measured at a point in
space z.sub.0. Next, another ps pulse is delivered to band-edge
delay line device 308 described in detail above. After a time T,
this output signal 312 has a measured position in space
corresponding to z.sub.1. Thus, the photonic delay 314 imparted by
band-edge delay line device 308 is the measured difference
z.sub.0-z.sub.1.
[0070] FIG. 3B shows a graph 315 that illustrates the delay
imparted by band-edge delay line device 308 in terms of signal
intensity 316 as a function of distance traveled (z) 318 over a
time T. Graph 315 plots the photonic signal delay (.delta.) 324
between the control photonic signal 320 (dashed-line), which
traveled through bulk material 306, and the delayed photonic signal
322 (solid-line), which traveled through band-edge delay line
device 308, when both are measured at a time T. Additionally, graph
315 demonstrates that band-edge delay line device 308 sufficiently
preserves signal quality in terms of signal intensity 316 and pulse
shape when comparing delayed signal 322 to control photonic signal
320. Because input pulse 304 has a sufficiently narrow bandwidth,
all the frequency components of the pulse will have nearly the same
group velocity (v.sub.g), thereby avoiding any breakup or spreading
of the pulse as it travels through band-edge delay line device 308.
The net effect of the band-edge delay line device 308 on the
photonic pulse is solely the delay, since there are no significant
shaping effects taking place.
[0071] Fabry-Perot Delay Line Devices
[0072] In order to create a useful device for photonic delay in an
optical system, an important parameter to maximize is the ratio
(.delta./L), where .delta. is the delay imparted by the PBG
structure, and L is the length of the PBG structure. One known
method of increasing photonic signal delay is to increase the
number of periods (N) contained in a uniform photonic band gap
structure. However, by increasing N, one simultaneously increases
L, thereby failing to maximize the important .delta./L ratio.
[0073] Interposed Periodicity Defect in a Semiconductor
Structure
[0074] The present invention is directed towards maximizing the
.delta./L ratio by interposing a periodicity defect region into a
uniform PBG structure. The periodicity defect causes a transmission
resonance in the center of the photonic band gap, and thus
increases .delta. dramatically without increasing L. This
phenomenon is illustrated in FIGS. 4A and 4B.
[0075] FIG. 4A is a schematic representation of a Fabry-Perot delay
line device 406 according to one embodiment of the present
invention. Device 406 receives photonic signal 404 of a
predetermined frequency.omega. and a predetermined bandwidth
d.omega. at entrance port 405. Device 406 comprises at least two
stacks (or regions) 420 and 422 of alternating layers 408 and 410
of refractive materials having predetermined indices of refraction
n.sub.1 and n.sub.2, and predetermined thicknesses (a) 414 and (b)
416, respectively. In the center of the Fabry-Perot delay line
device 406, a periodicity defect region 412 is interposed between
two stacks 420 and 422, with each stack having an equal number of
alternating layers of refractive material. Periodicity defect
region 412 is also a refractive material having an index of
refraction (n) that is equivalent to either n.sub.1 or n.sub.2. As
mentioned previously, the term "defect", in this context, simply
means a break in the periodicity of the structure. The thickness
(c) 418 of periodicity defect region 412 is designed to be
different than thicknesses a and b. For example, if layer
thicknesses in the uniform stacks 420, 422 are taken to be one
quarter-wavelength long, then the thickness of periodicity defect
region 412 can be one half or one wavelength in thickness. However,
other thicknesses for periodicity defect region 412 can be
utilized, although only certain arrangements will maximize both
transmission and delay at the same time. As the signal travels
through the Fabry-Perot delay line device 406, a predetermined
delay .delta. is imparted on the photonic signal, resulting in a
delayed output photonic signal 426.
[0076] For example, in one embodiment of the present invention, a
photonic signal 404 of in-air wavelength .lambda. is delivered to
Fabry-Perot delay line device 406. Layer 408 is a layer of gallium
arsenide (GaAs), with an index of refraction of about 3.4 and a
thickness (a) given by a=.lambda./4n.sub.1. Layer 410 is a layer of
aluminum arsenide (AlAs), with an index of refraction of about 2.9
and a thickness (b) given by b=.lambda./4n.sub.2. Periodicity
defect region 412 in this embodiment is a layer of GaAs, with a
thickness (c) 418 of about .lambda.. It should be noted that this
type of structure is not limited to using only AlAs and GaAs
periodic layers: other commonly known semiconductor materials such
as indium arsenide (InAs), for example, can be also utilized in
this structure, depending on the delay requirements of the user.
Additionally, in one preferred example, thickness c is about two to
four times greater than thicknesses a or b, which results in the
photonic band gap profile 450 displayed in FIG. 4B. Of course, the
present invention is not intended to be so limited as the
thicknesses a, b, or c can be different relative distances.
Further, instead of a semiconductor structure, another possibility
includes the use of optical fiber or waveguide gratings to create
photonic delay. A photonic delay device based on an optical fiber
grating is described in detail below.
[0077] The interposition of periodicity defect region 412 creates a
characteristic photonic band gap which is displayed in FIG. 4B,
described in detail below. Fabry-Perot delay line device 406 is
designed to alter incident photonic signals 404 only in terms of
temporal delay: no shaping or pulsewidth characteristics are
significantly altered.
[0078] The present invention represents a significant improvement
over the state of the art in terms of photonic signal delay
(.delta.); for the same number of periods (N) in a Fabry-Perot
delay line device, the .delta. created can be at least an order of
magnitude greater in the invention than in a band-edge delay line
device (with no periodicity defect interposed). In practical terms,
the present invention allows for an improved device with a length
(L) equivalent to the length of band-edge delay line devices, but
with a much larger .delta..
[0079] The physical principle giving rise to this dramatic increase
in the photonic delay is illustrated in FIG. 4B. Transmission
profile 450 of Fabry-Perot delay line device 406 is plotted in
terms of photonic signal transmission 452 as a function of
frequency (.omega.) 454. Device 406 is referred to as a Fabry-Perot
delay line device because the corresponding transmission resonance
is usually found in Fabry-Perot resonators, well known to those
skilled in the art. Transmission curve 456 undergoes a similar
series of relative maxima and minima previously seen for the
band-edge delay line device illustrated in FIG. 2A. The
transmission curve 456 (solid-line) for device 406 also contains a
relative transmission maximum at the photonic band-edge 458 that
was also seen for the band-edge delay line device of FIG. 2A.
Unlike FIG. 2A, however, a new transmission resonance peak 462
(dashed-line) is created at the center of the photonic band gap 460
as a result of the interposition of periodicity defect region 412.
Fabry-Perot delay line device 406 can be designed so that the
frequency corresponding to transmission resonance peak 462 matches
the center frequency .omega. of photonic signal 402 (where,
.lambda.=c/(2.pi..omega.)) to impart the maximum predicted photonic
signal delay.
[0080] In general, the improvement in delay .delta. caused by the
Fabry-Perot delay line device of the present invention is based on
the narrower transmission resonance peak bandwidth
.delta..omega..sub.1; where
.delta..omega..sub.1.about..delta..omega./10 to .delta..omega./100,
and .delta..omega. is the frequency bandwidth of the photonic
band-edge 459. Since .delta..omega..sub.1 is narrower than
.delta..omega., the frequency bandwidth of the input photonic
signal (d.omega.) must also be narrower to avoid beam distortion.
This bandwidth requirement necessitates an increase in the photonic
signal pulsewidth delivered to the Fabry-Perot delay line device of
the present invention. For example, instead of delivering a
picosecond (ps) pulse to the Fabry-Perot delay line device, the
present invention is best utilized in a system where nanosecond
(ns) pulses are sought to be delayed.
[0081] It should be noted that the above periodicity defect
structure represents only one embodiment of the invention. One of
the important aspects of the present invention is the flexibility
to create a desired delay. For example, there is no particular
restriction to work around the one-quarter wavelength periodicity
of the alternating layers. In addition, the interposed periodicity
defect region does not have to be larger than the alternating layer
thicknesses nor is it required to be placed in the center of the
structure. While manufacturing considerations may make a one
wavelength or a one-half wavelength periodicity defect preferable
over another thickness, there are no physical reasons why other
thicknesses can not be attempted. Further, the placement of a
periodicity defect region is not restricted to only the center of
the delay device; a periodicity defect region can be placed at many
different locations in the delay device, depending on the type of
photonic delay desired. Fundamentally, as long as there is a break
in the periodicity of the structure, the transmission resonance
spike will appear at some location in the photonic band gap of the
structure.
[0082] FIG. 5 graphically illustrates the improvement in photonic
signal delay created by the present invention. Graph 502 plots
photonic signal delay 504 as a function of the number of periods
(N) 506 within a band-edge delay line device 508 (curve (a),
solid-line) and a Fabry-Perot delay line device 510 (curve (b),
dashed-line) such as that described in FIG. 4. While curve (a) 508
follows an N.sup.2 dependence, curve (b) 510 represents a steeper,
N.sup.2.multidot.e.sup.N dependence. As a result, according to the
present invention, a significantly greater photonic signal delay is
obtained as compared to a band-edge delay line device.
[0083] For example, a Fabry-Perot delay line device similar to
device 406 described above was fabricated and tested. The
Fabry-Perot delay line device comprised a total of 40 periods of
GaAs/AlAs alternating layers grown on a GaAs substrate. In order to
maximize the transmission from the structure to unity, a
one-wavelength defect layer was introduced 18 periods from the
surface, and 22 periods from the substrate. Actual measured delay
was approximately 10 ps. Thus, the delay-to-device-length ratio was
improved by approximately a factor of 20 over a band-edge delay
line device, such as band-edge delay line device 102 described
above.
[0084] Multiple Periodicity Defects
[0085] According to a second embodiment of the present invention, a
Fabry-Perot delay line device is designed to impart predetermined
delays on two photonic signals having different frequencies that
travel through the device. FIG. 6A is a schematic representation of
a second embodiment 603 of the present invention: a Fabry-Perot
delay line device with at least two periodicity defect regions.
Device 602 receives photonic signals 603 and 604 of different
signal frequencies .omega..sub.1 and .omega..sub.2, respectively,
and a predetermined bandwidth d.omega. at entrance port 605. Device
602 comprises at least three stacks (or regions) 616, 618 and 620
of alternating layers 606 and 608 of refractive materials having
predetermined indices of refraction n.sub.1 and n.sub.2, and
predetermined thicknesses (a) 626 and (b) 628, respectively. Two
separate periodicity defect regions 610 and 612 are interposed in
separate locations between the three stacks 616, 618 and 620, with
each stack having an equal number of alternating layers of
refractive material. Periodicity defect regions 610 and 612 are
also refractive materials respectively having indices of refraction
n.sub.3 and n.sub.4, that can be the same as either n.sub.1 or
n.sub.2. The thicknesses (c) 622 and (d) 624 of-the periodicity
defect layers 610 and 612 are designed to be greater than the
thicknesses a and b. In addition, periodicity defect layers 610 and
612 can be of equal thickness or of differing thickness, depending
on the desired transmission resonances. As signals 603 and 604
travel through the delay device 602, a predetermined delay
.delta..sub.1 can be imparted on the photonic signal 603, and a
second predetermined delay .delta..sub.2 can be imparted on the
photonic signal 604, resulting in delayed output photonic signals
614 and 615.
[0086] Transmission profile 650 is shown in FIG. 6B. This graph
represents the transmission profile of the second embodiment of the
present invention, Fabry-Perot delay line device 602. FIG. 6B plots
photonic signal transmission 652 as a function of frequency
(.omega.) 653. Transmission curve 656 undergoes a similar series of
relative maxima and minima as previously seen in FIG. 2B. While the
transmission curve 656 (solid-line) for Fabry-Perot delay line
device 602 contains a relative transmission maximum at the photonic
band-edge 659 that was also seen for the device of FIG. 4A, two
transmission resonance peaks 660 and 662 (dashed-lines) are now
created about the center of the photonic band gap 460 as a result
of the interposition of the two periodicity defect regions 610 and
612. These separate resonance peaks 660 and 662 occur at
frequencies .omega..sub.1 and .omega..sub.2, respectively. In order
to best utilize delay device 602, it can be designed so that the
frequencies corresponding to transmission resonance peaks 660 and
662 match the frequencies .omega..sub.1 and .omega..sub.2 of
photonic signals 603 and 604 in order to impart the maximum delay
on each optical signal.
[0087] As described above, the present invention provides the user
with the flexibility of controlling the delay imparted by the
Fabry-Perot delay line device by choosing an appropriate set of
delay parameters. The user can control the location of the
transmission resonance spikes and the input frequencies of the
photonic signals. For example, one periodicity defect region can
have a thickness on the order of .lambda./2, and a second
periodicity defect region can have a thickness on the order of
.lambda., where .lambda. corresponds to the optical in-air
wavelength of the photonic signal(s) delivered to a Fabry-Perot
delay line device, such as device 602. This arrangement generates
separate transmission resonance spikes in the photonic band gap
and, thus, can be used to pass two different frequencies of light.
In one embodiment, the two periodicity defect regions can be placed
in a PBG structure so that their positions are symmetric: the
corresponding resonance spikes will also be symmetrically
positioned about the center of the band gap. However, these
periodicity defect regions do not have to be placed symmetrically
within a PBG structure. In addition, the invention does not require
that there be an equal number of alternating layers on either side
of the periodicity defect regions, or that the number of
periodicity defect regions be limited to only two. Different
relative layer thickness and different numbers of layers will be
apparent to one skilled in the art given this description.
[0088] Variable Photonic Signal Delay
[0089] A third embodiment of the present invention is illustrated
in FIG. 7. A tunable laser source 704 delivers various photonic
signals via a fiber optic cable 706 to Fabry-Perot delay line
device 708. These delayed signals can then be launched into a
communications network 710 in synchronization with other photonic
signals. Since a tunable laser source is capable of generating
photonic signals that comprise a range of photonic frequencies,
using the Fabry-Perot delay line device will impart varying
photonic delays depending upon the frequency of the input photonic
signals.
[0090] The embodiments described above represent the invention
being used as a passive device, in which no voltage, bias current,
or electromagnetic field is applied to the Fabry-Perot delay line
device. However, a voltage can be applied to one or more defect
regions of a Fabry-Perot delay line device to vary the index of
refraction of the respective periodicity defect regions which
comprise the structure. By varying the index of refraction of the
periodicity defect regions, the delay imparted on the input
photonic signals can be varied. This embodiment of the present
invention is illustrated in FIG. 8, which displays a schematic of
Fabry-Perot delay line device 802 connected to a voltage source
816. The power supply can apply a positive 812 or negative 814
voltage to the periodicity defect region of the structure at
electrical contacts 804. By changing the voltage applied to the
periodicity defect region 810, the index of refraction of region
810 changes. As a result, device 802 can thereby impart a varying
photonic delay on the photonic signals passing therethrough.
[0091] Fiber Grating Photonic Delay Devices
[0092] According to a further embodiment of the present invention,
optical fiber or waveguide gratings are also used to create
photonic delay. Optical fiber gratings are similar to the types of
PBG structures described above in that they are also periodic
structures. The index of refraction for a fiber grating can achieve
an index modulation depth (i.e., a high and low value) similar to
that of high index contrast semiconductor structures. However,
fiber gratings are structures with a smaller index discontinuity
than that associated with a semiconductor PBG structure: for a
fiber grating an index modulation along its core is typically on
the order of .delta.n=10.sup.-3 to 10.sup.-4, as opposed to a
semiconductor PBG structure with an index modulation approaching
unity. Since the bandwidth of transmission resonances and band gaps
are proportional to .delta.n (the index modulation depth), fiber
grating delay devices are preferred for use with optical pulses of
longer (i.e., nanosecond) duration in order to preserve their
shape.
[0093] A fiber grating can be created on an optical fiber by
well-known fabrication techniques. For example, see the fiber
grating applications and fabrication techniques described in
"Continuously tunable single-mode erbium fiber laser," by G. Ball
and W. Morey, Optics Letters, Vol. 17, No. 6, p.420 (1992) and
"Spatially-multiplexed fiber-optic Bragg grating strain and
temperature-sensor system based on interferometric wavelength
shift," by Y. Rao, et al., Electronics Letters, Vol. 31, No. 12, p.
1009 (1995), which are both incorporated by reference in their
entirety herein.
[0094] For example, fiber grating fabrication can be accomplished
by placing an optical "mask" over a photo-sensitive fiber core and
then by illuminating the mask-fiber assembly with a high intensity
ultraviolet light beam, such as an Excimer laser. The resulting
grating, referred to as a fiber grating, displays much the same
properties of a high index contrast semiconductor PBG structure,
especially with respect to band gaps and transmission resonances.
In addition, a mask can be designed to create a grating which
imparts either a band-edge delay line effect or a Fabry-Perot delay
line effect.
[0095] According to one example embodiment of the present
invention, a fiber grating photonic delay device comprises
alternating sections having different indices of refraction
periodically spaced along the fiber or waveguide. According to
another example embodiment of the present invention, the fiber
grating photonic delay device can comprise a periodicity defect
region interposed between the regions of alternating sections.
Thus, the fiber grating photonic delay device operates according to
the same physical principles that apply to the semiconductor
band-edge and Fabry-Perot devices described above: the periodic
nature of the abrupt refractive index changes caused by the grating
occurring in the material can be used to create photonic delay
effects. When an electromagnetic field is applied to the fiber core
with the mask, the index of refraction is changed in the portion of
the fiber that is exposed to the illumination. In other words, once
the field enters the fiber core it permanently alters the
refractive index of that region. Thus, the fiber device consists of
sections where the index of refraction is altered and sections
where the refractive index is unchanged. The spacing of these
alternating refractive index sections can be based on one-quarter
wavelength section thicknesses such as those described for the
above band-edge delay line device. Alternatively, the periodic
nature of the spacings can be broken, with one or more periodicity
defect regions interposed, depending on the delay required, similar
to the Fabry-Perot delay line devices described above. For example,
two or more regions of the fiber grating can comprise sections that
periodically alternate proportional to the in-air wavelength
.lambda. of the incoming optical signal in order to maximize both
transmission and delay. A periodicity defect region of a different
optical path length can be interposed between these two regions of
alternating sections, resulting in a transmission resonance similar
to those described above.
[0096] As mentioned earlier, a PBG structure can also be used as a
variable photonic delay device. However, in the case of a fiber
grating, the location of a Fabry-Perot resonance can be changed in
the frequency domain, and hence tuned by piezo-electric means, as
opposed to electro-optic means. That is, a voltage can be applied
to the defect region, as shown in FIG. 8, in the fiber grating
embodiment of the present invention not to cause a change in index
of refraction, but rather to cause slight pressure to be applied in
the periodicity defect region. The pressure, in turn, causes a
slight elongation (or physical stretching) of the periodicity
defect region, and hence a change of its optical path. This method
is thus equivalent to modifying the refractive index by
electro-optic means, leading to a variable photonic delay.
[0097] Defect Layer Doped with a Gain Medium
[0098] An embodiment of the present invention includes a defect
layer doped with a gain medium. An increase in the density of modes
essentially means that the density of photons increases at a
particular frequency. This increase can be accomplished by
utilizing a defect layer that is doped with a gain medium. The
introduction of gain automatically causes an increase in the photon
density, and therefore, a narrowing of the density of modes. This
allows tuning of the group velocity, just as if periods were being
added.
[0099] There are several advantages to this scheme. First, once a
structure including a defect layer doped with a gain medium is
built, the maximum delay can simply be modified by changing the
structure's gain. Second, since this scheme does not alter delay by
tuning a field across a structure's resonance, there is no loss of
signal. On the contrary, the signal is amplified. Absorption (which
represents negative gain) can also be used to tune the group
velocity, although the transmitted signal may in this case be
reduced.
[0100] FIG. 9 depicts a structure 900 grown on a GaAs substrate.
Structure 900 includes at least two stacks (or regions) 902 and 906
of alternating layers of refractive materials. The alternating
layers in stack 902 have predetermined indices of refraction
n.sub.1 and n.sub.2. The alternating layers in stack 906 have
predetermined indices of refraction n.sub.3 and n.sub.4. The
alternating layers in stacks 902 and 906 have predetermined
thicknesses (a) 910, (b) 912, (c) 914, and (d) 916, respectively.
In the center of the structure 900, a periodicity defect region 904
is interposed between stacks 902 and 906. As illustrated in FIG. 9,
stack 902 comprises 18 periods of alternating GaAs and AlAs layers.
However, stack 906 comprises 22 periods of alternating GaAs and
AlAs layers. Periodicity defect region 904 is a GaAs cavity having
a one wavelength thickness 918. As mentioned previously, the term
"defect", in this context, simply means a break in the periodicity
of the structure. Structure 900 receives a photonic signal 920 of a
predetermined frequency .omega. and a predetermined bandwidth
d.omega. at an entrance port 922. As photonic signal 920 travels
through structure 900, a delay d is imparted on photonic signal
920, resulting in a delayed output photonic signal 924.
[0101] This configuration creates a sharp transmission resonance
with a full width at half maximum of .delta..lambda.=0.34 nm in the
middle of the photonic band gap, at .lambda.=1550 nm, and maximizes
the transmission to unity. The properties of this transmission
resonance inside the gap are similar to the properties of the
photonic band edge resonances, except for the location of the
transmission resonance, and the magnitude of the group velocity, or
group index, at the defect mode, which is about two orders of
magnitude larger than the group index of the band edge delay line
discussed above.
[0102] For structure 900, the maximum group index is about Ng=300.
Due to the limited bandwidth of structure 900, any pulse tuned at
the resonance maximum, and whose duration is greater than about 100
ps or so will propagate at the group velocity of C/300. The
physical length of structure 900 is approximately L=10.3 microns;
taking C=3.times.10.sup.8 m/sec., thereby producing a delay of
approximately d=8-10 picoseconds.
[0103] The exploration of ways to tune delay in photonic band gap
structures has led to some unexpected and interesting new results.
One such result occurs when a gain medium is introduced in
periodicity defect region 904. This means that electrical or
optical pumping of the Fabry-Perot (F-P) cavity (periodicity defect
region 904) is introduced. As photonic signal 920 makes its way
through structure 900, it experiences some gain, i.e., the density
of photons increases. This causes photonic signal 920 to slow down
further. This reduction in speed depends exponentially on the
amount of gain present, as described below with reference to FIG.
10.
[0104] According to the present invention, periodicity defect
region 904 is doped to change the absorptive and emissive
properties of the original semiconductor so that the electrons of
periodicity defect region 904 are excited to a higher state,
thereby placing them in a position to give up that energy to
photonic signal 920. Accordingly, photonic signal 920 is amplified
according to a gain. Such doping can be with beryllium atoms,
carbon atoms, and/or with any other substance, as would be apparent
to persons skilled in the relevant art(s).
[0105] In a doped semiconductor material, such as doped periodicity
defect region 904, there are many ways to put electrons in an
excited state. One such way involves optical pumping. Optical
pumping involves the injection of optical energy in a manner that
excites electrons from a lower level to an upper level. Such
electrons will remain at an upper level until photonic signal 920
absorbs photons from them.
[0106] Another way to put electrons in an excited state is through
the injection of an electrical current. The electrical current
causes the electrons to enter an excited state, until photonic
signal 920 absorbs photons from the excited electrons. This
approach differs from the approach described above with reference
to FIG. 8, where a voltage is applied to periodicity defect region
810 to change its index of refraction. In contrast, applying an
electrical current to doped periodicity defect region 904 produces
a gain. Thus, some of the electrical current is captured in doped
periodicity defect region 904 region to generate light, thereby
producing gain.
[0107] FIG. 10 is a plot 1000 of delay d as a function of the gain
coefficient for structure 900. As illustrated in FIG. 10, delay d
of photonic signal 920 increases with the gain coefficient. As
described herein, the gain coefficient increases as more energy is
injected in periodicity defect region 904. Thus, the present
invention provides a mechanism that enables the tuning of gain (or
loss). Accordingly, this tuning mechanism provides the ability to
modulate, or change, the delay of photonic signal 920 by changing
the gain of structure 900.
[0108] It is important to note that the generic term gain can also
be used to indicate absorption. That is, gain causes an increase in
the density of modes, a reduction of the group velocity, and an
increase in delay. On the other hand, absorption causes a reduction
of the density of modes, an increase of the group velocity, and
hence a decrease in delay.
[0109] This gain(or loss)-tuning mechanism for photonic signals
920, such as nanosecond pulses, constitutes a major achievement
because of the limitations involving the production of devices,
such as structure 900. One such limitation involves device
geometry, which is dictated by molecular beam epitaxy (MBE)
fabrication techniques. For instance, according to current MBE
techniques, it is difficult to regulate the thickness of layers in
structures having greater than 40 layers. Another limitation
involves economics. According to some estimates, the cost to
produce each layer of a structure is relatively expensive. Without
the introduction of a gain medium, these limitations place
constraints on the maximum delay obtainable at a reasonable
cost.
[0110] However, as FIG. 10 illustrates, the above limitations are
removed by devices, such as structure 900, or any similar or
equivalent structure as would be apparent to persons skilled in the
relevant art(s). For such devices, by simply introducing and
modulating the maximum gain, the maximum delay can be tuned well
beyond the maximum value dictated by the geometry of the
device.
[0111] According to one embodiment of the present invention,
ordinary vertical cavity surface emitting laser (VCSEL) diodes can
be implemented as a defect layer doped with a gain medium. These
devices include a gain doped defect layer as in a delay line
cavity. Furthermore, VCSELs are currently available at negligible
cost. The gain coefficients present in ordinary VCSELs or diode
lasers can be made to equal or approximately equal the gain
coefficients illustrated above with reference to FIG. 10.
[0112] FIGS. 11 and 12 provide two views of a VCSEL 1100 according
to the present invention. VCSEL 1100 includes two stacks 1102 and
1106 of alternating layers of refractive materials, each layer
having predetermined indices of refraction and predetermined
thicknesses. In an embodiment, each stack is a quarter-wave mirror
stack, where stack 1102 is 99.0% reflective and stack 1106 is 99.9%
reflective. Adjacent to stack 1106 is a GaAs substrate 1108. In the
center of the structure 1100, a periodicity defect region 1104 is
interposed between stacks 1102 and 1106. Periodicity defect region
1104 is doped with a gain medium. In an embodiment, this doping
comprises three quantum wells 1110a-1110c. This doping is performed
in such a way that when a current is injected in periodicity defect
region 1104, atoms go to an excited state, and give up a photon
when returning to the ground state. Such an injection of current
creates gain. Accordingly, when a photonic signal 1112 at a
predetermined frequency .omega. and bandwidth d.omega. is input to
an entrance interface 1116, a delayed photonic signal 1114 results.
The magnitude of this delay varies according to the amount of
electrical current injected in periodicity defect region 1104.
[0113] FIG. 12 illustrates a second view of VCSEL 1100. In addition
FIG. 12 illustrates the injection of an electrical current 1202
into VCSEL 1100. The gain of VCSEL 1100 is controlled by modulating
electrical current 1202 (as indicated by the arrow). The power
output of VCSEL 1100, and hence its gain, is proportional to
electrical current 1202. Thus, the velocity of photonic signal 1112
is controlled by modulating electrical current 1202. Accordingly,
as electrical current 1202 increases, the velocity decreases. A
decrease in velocity causes an increase in delay of the input
pulse.
[0114] According to the present invention, the injection of
electrical current in VCSEL 1100 causes lasing at a particular
wavelength. In an embodiment, the wavelength of photonic signal
1112 has the same wavelength.
[0115] A doped VCSEL structure whose initial resonance width is not
too thin that it can be easily fabricated. In this embodiment, an
injected external pulse laser source is input to the doped VCSEL
structure.
[0116] In one example a VCSEL structure as described by Bouche et
al. is used as a delay device according to the present invention.
See Bouche et al., Dynamics of Gain in Vertical Cavity Lasers and
Amplifiers at 1.53 um using femtosecond photoexcitation. 73 Applied
Physics Letters 2718 (Nov. 9, 1998) (hereinafter Bouche)
(incorporated herein by reference in its entirety). As described in
Bouche, this structure can be a 3.5.lambda. cavity with an active
medium composed of 2.times.9 InGaAsP strained multiple quantum
wells placed at antinodes. The active medium is sandwiched between
two and four pairs of Si/SiO2 mirrors with reflectivities of 96.2%
and 99.7%, respectively.
[0117] FIG. 13 is a plot 1300 of the reflectance 1302 and
transmittance 1304 for the passive structure described in
Bouche.
[0118] According to one example of the invention, gain in the
central part of the quantum well region is introduced in the
structure described in the Bouche et al. article. An increase was
observed in the transmission. FIG. 14 is a plot of the resonance
1402 of this structure located near 1528 nm when gain is
introduced. This gain causes significant narrowing of the line.
[0119] FIG. 15 is a plot illustrating the propagation of the
transmitted and reflected pulses 1502, for an input pulse whose
FWHM=120 fs, without gain or loss (i.e., the linear results of the
Bouche structure are displayed). For comparison, a pulse 1504 that
propagates in free space through a distance D=to the length of the
device is also plotted.
[0120] FIG. 16 is a plot illustrating the delay of a 1 ps pulse to
show that it couples better with the structure, and hence the delay
increases. This plot includes a reference free space pulse 1602, a
reflected pulse 1604, and a delayed transmitted pulse 1606.
[0121] FIG. 17 is a plot illustrating the delay of a 10 ps input
pulse. This plot includes a reference free space pulse 1702, a
reflected pulse 1704, and a delayed transmitted pulse 1706.
[0122] FIG. 18 is a plot illustrating the delay of a 5 ps input
pulse when the Bouche structure is doped with a gain material. This
plot includes a delayed transmitted pulse 1802 and a reflected
pulse 1804.
[0123] FIG. 19 is a plot illustrating the delay of a 5 ps input
pulse without the addition of a gain material to the Bouche
structure. This plot includes a transmitted pulse 1902 and a
reflected pulse 1904. As indicated in FIG. 19, the delay obtained
is less than in FIG. 18.
[0124] Accordingly, the present invention includes a method of
imparting a delay to an input photonic signal of a predetermined
frequency and a predetermined bandwidth. In one embodiment, the
semiconductor structure is a photonic band gap structure having a
defect layer doped with a gain medium, such as structure 900. In a
further embodiment, the semiconductor structure is a VCESL, as
illustrated is FIGS. 11 and 12. In other embodiments, the structure
can be a structure that exhibits a photonic band gap structure.
[0125] This method includes the steps of sending the input photonic
signal along an optical path; passing the photonic signal through a
first region of periodically alternating refractive sections in
said optical path of the photonic signal; passing the photonic
signal through a periodicity defect region in said optical path,
where the defect region is doped with a gain medium and excited by
an energy source; and passing the photonic signal through a second
region of periodically alternating refractive sections in said
optical path. The energy source can be electrical current, a second
photonic signal such as a continuous wave signal having the same
frequency as the input photonic signal, mechanical energy such as
force and/or pressure, and/or thermal energy.
[0126] Conclusion
[0127] Thus, the invention recognizes that the group velocity of
the probe pulse can be controlled by adjusting the gain or
absorption present in a photonic band gap structure. When gain is
dominant, such as when an input pulse is tuned to the low frequency
band edge resonance, the pulse group velocity decreases. When loss
is present, such as when an input pulse is tuned to the high
frequency band edge resonance, the group velocity of the pulse
increases.
[0128] Embodiments of the present invention can include any number
of periodic stacks and periodicity defect regions. In such
embodiments, periodic stacks can be doped with gain media. Also, in
embodiments including multiple periodicity defect regions, any
number of them can be doped with a gain medium in any
combination.
[0129] While various embodiments of the present invention have been
described above, it should be understood that they have been
presented by way of example only, and not limitation. It will be
understood by those skilled in the art that various changes in form
and details may be made therein without departing from the spirit
and scope of the invention as defined in the appended claims. Thus,
the breadth and scope of the present invention should not be
limited by any of the above-described exemplary embodiments, but
should be defined only in accordance with the following claims and
their equivalents.
* * * * *