Patent Publication Number: US-6343167-B1

Title: Photonic band gap device and method using a periodicity defect region to increase photonic signal delay

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. application Ser. No. 08/956,112, filed on Oct. 24, 1997 now Pat. No. 5,907,421 
     This patent application is related to the following application. 
     “Photonic Bandgap Apparatus and Method for Delaying Photonic Signals,” Ser. No. 08/584,403, now U.S. Pat. No. 5,751,466, by J. Dowling, M. Scalora, M. Bloemer, M. Tocci, C. Bowden, R. Fork, S. Reinhardt, and R. Flynn, filed on January 11, 1996, now pending. 
    
    
     FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     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. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     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. 
     2. Related Art 
     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. 
     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. 
     By sending a photonic signal of a given frequency (ω) 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. 
     SUMMARY OF THE INVENTION 
     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 
     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. 
     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. 
     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 
     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. 
     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. 
     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. 
     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. 
     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. 
     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 
     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. 
     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. FIG. 1B is a diagram of the characteristic index of refraction profile of the uniform PBG structure shown in FIG.  1 A. 
     FIG. 2A is a characteristic transmission profile for the band-edge delay line device shown in FIG.  1 A. FIG. 2B is a transmission profile for the bandedge delay line device that also includes a group velocity curve for a photonic signal traveling through the structure. 
     FIG. 3A is a schematic representation showing a photonic delay imparted by the band-edge delay line device of FIG.  1 A. FIG. 3B is a plot of the photonic delay imparted by the same band-edge delay line device 
     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. FIG. 4B is a characteristic transmission profile for the Fabry-Perot delay line device of FIG.  4 A. 
     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. 
     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. FIG. 6B is the corresponding transmission profile for the photonic signal delay device containing two periodicity defect regions shown in FIG.  6 A. 
     FIG. 7 is a schematic representation of a tunable laser source, according to a third embodiment of the present invention. 
     FIG. 8 is a schematic representation of the Fabry-Perot delay line device of FIG. 7 having a voltage applied to the device. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Table of Contents 
     1. Overview and Discussion of the Invention 
     2. Terminology 
     3. Example Environment 
     4. Band-edge Delay Line Devices 
     5. Fabry-Perot Delay Line Devices 
     a. Interposed Periodicity Defect in a Semiconductor Structure 
     b. Multiple Periodicity Defects 
     c. Variable Photonic Signal Delay 
     d. Fiber Grating Photonic Delay Devices 
     6. Conclusion 
     1. Overview and Discussion of the Invention 
     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. 
     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. 
     2. Terminology 
     For the purposes of this disclosure, the following terms arc used in describing the present invention: 
     “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 (λ) or their photonic signal frequency (ω), where λ=c/(2πω) 
     “Photonic signal delay device” refers to any device that imparts a temporal delay on a photonic signal traveling therethrough. 
     “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 the Scalora, et al. article referenced herein). 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. 
     “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. 
     “Periodicity defect” refers to a break in the periodicity of alternating refractive material layers or sections. 
     3. Example Environment 
     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. 
     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. 
     4. Band-edge Delay Line Devices 
     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 ω and a predetermined bandwidth dω 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 1  and n 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 (λ), determined by the equation a=λ/4n 1 . Similarly, second layer  110  can have an index of refraction n 2 , and a thickness (b), where b=λ/4n 2 . This pattern can be repeated for N periods  122 , where a period is equal to one set of alternating layers  112 . 
     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. 
     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. 
     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 (ω)  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. 
     Transmission curve  205  undergoes a series of relative maxima and minima depending on the frequency ω 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 ω of the input photonic signal. In addition, the photonic band-edge transmission resonance bandwidth (δω)  214  can be designed to be at least as wide as the bandwidth dω 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. 
     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.  2 B. FIG. 2B shows a graph  250  where both the transmission curve  258  (solid-line) and the group velocity (vg) curve  262  (dashed-line) are plotted as a function of frequency (ω)  254 . The transmission  252  of the input photonic signal is inversely related to the photonic signal&#39;s group velocity v 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&#39;s group velocity v 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 ω. 
     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 ω and a bandwidth do 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 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 1 . Thus, the photonic delay  314  imparted by band-edge delay line device  308  is the measured difference z 0 −z 1 . 
     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 (δ)  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 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. 
     5. Fabry-Perot Delay Line Devices 
     In order to create a useful device for photonic delay in an optical system, an important parameter to maximize is the ratio (δ/L), where δ 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 δ/L ratio. 
     a. Interposed Periodicity Defect in a Semiconductor Structure 
     The present invention is directed towards maximizing the δ/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 δ dramatically without increasing L,. This phenomenon is illustrated in FIGS. 4A and 4B. 
     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 ω and a predetermined bandwidth dω 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 1  and n 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 1  or n 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 δ is imparted on the photonic signal, resulting in a delayed output photonic signal  426   
     For example, in one embodiment of the present invention, a photonic signal  404  of in-air wavelength λ 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=λ/4n 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=λ/4n 2 . Periodicity defect region  412  in this embodiment is a layer of GaAs, with a thickness (c)  418  of about λ. 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.  4 B. 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. 
     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. 
     The present invention represents a significant improvement over the state of the art in terms of photonic signal delay (δ); for the same number of periods (N) in a Fabry-Perot delay line device, the δ 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 δ. 
     The physical principle giving rise to this dramatic increase in the photonic delay is illustrated in FIG.  4 B. Transmission profile  450  of Fabry-Perot delay line device  406  is plotted in terms of photonic signal transmission  452  as a function of frequency (ω)  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.  2 A. 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.  2 A. 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 ω of photonic signal  402  (where, λ=c/(2πω) to impart the maximum predicted photonic signal delay. 
     In general, the improvement in delay δ caused by the Fabry-Perot delay line device of the present invention is based on the narrower transmission resonance peak bandwidth δω 1 ; where δω 1 ˜δω/10 to δω/100, and δω is the frequency bandwidth of the photonic band-edge  459 . Since δω 1  is narrower than δω, the frequency bandwidth of the input photonic signal (dω) 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. 
     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. 
     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 2  dependence, curve (b)  5   10  represents a steeper, N 2 ·e 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. 
     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. 
     b. Multiple Periodicity Defects 
     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 ω 1  and ω 2 , respectively, and a predetermined bandwidth dω 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 1  and n 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 3  and n 4 , that can be the same as either n 1 or n   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 δ 1  can be imparted on the photonic signal  603 , and a second predetermined delay δ 2  can be imparted on the photonic signal  604 , resulting in delayed output photonic signals  614  and  615 . 
     Transmission profile  650  is shown in FIG.  6 B. 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 (ω)  653 . Transmission curve  656  undergoes a similar series of relative maxima and minima as previously seen in FIG.  2 B. 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 ω 1  and ω 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 ω 1  and ω 2  of photonic signals  603  and  604  in order to impart the maximum delay on each optical signal. 
     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 λ/2, and a second periodicity defect region can have a thickness on the order of λ, where λ 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. 
     c. Variable Photonic Signal Delay 
     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. 
     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. 
     d. Fiber Grating Photonic Delay Devices 
     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 δn=10 −3  to 10 −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 δn (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. 
     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. 
     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. 
     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 inter posed 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 λ 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. 
     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. 
     6. Conclusion 
     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.