Patent Publication Number: US-11658450-B2

Title: Wavelength flexibility through variable-period poling of a compact cylindrical optical fiber assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority under 35 U.S.C. § 119(e) to U.S. Provisional Application Ser. No. 63/150,614 entitled “High Power Laser Using Quasi-Phase-Matched Electric Field Induced Optical Parametric Amplification in Hollow Core Photonic Crystal Fibers”, filed 18 Feb. 2021. 
     This application claims the benefit of priority under 35 U.S.C. § 119(e) to U.S. Provisional Application Ser. No. 63/150,698 entitled “Quantum entanglement system using quasi-phase-matched electric field induced optical parametric amplification in hollow core photonic crystal fibers”, filed 18 Feb. 2021. 
     This application is a continuation-in-part under 35 U.S.C. § 120 to U.S. patent application Ser. No. 16/986,408 entitled “Wavelength Flexibility through Variable-Period Poling of a Compact Cylindrical Optical Fiber Assembly,” filed 6 Aug. 2020, which in turn claimed the benefit under 35 U.S.C. § 120 to U.S. patent application Seri. Ser. No. 16/920,994 entitled “Wavelength Flexibility Through Variable-Period Poling of Fluid-Filled Hollow-Core Photonic Crystal Fiber,” filed 6 Jul. 2020, which in turn claimed the benefit of priority under 35 U.S.C. § 119(e) to U.S. Provisional Application Ser. No. 62/872,316 entitled “Wavelength Flexibility Through Variable-Period Poling of Fluid-Filled Hollow-Core Photonic Crystal Fiber,” filed 10 Jul. 2019, the contents of all of which are incorporated herein by reference in their entirety. 
    
    
     ORIGIN OF THE INVENTION 
     The invention described herein was made by employees of the United States Government and may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefore. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure generally relates to fiber laser systems, and more particularly to wavelength adjustable fiber laser systems. 
     2. Description of the Related Art 
     Applications exist for laser communication, sensing, and designating that benefit from being operable in more than one frequency. However, providing more than one laser each operable in respective frequencies can be prohibitive in cost, size, weight and, power (CSWAP). Alternatively, it is generally known that wavelength conversion of a single laser can be achieved with nonlinear crystals by nonlinear optical processes such as sum frequency and difference frequency generation or more specifically with optical parametric generation (OPG), optical parametric amplification (OPA), and optical parametric oscillation (OPO). However, efficient operation of typical processes are believed to require housing of the nonlinear crystal in an optical cavity external to the pump laser, which seem to function near optical damage threshold and are opto-mechanically sensitive. Thus, this alternative is also CSWAP prohibitive. Dynamic wavelength agility can be achieved by pump power modulation above and below the nonlinear process operational threshold or by using a phase modulator to change the pump polarization state such that the requisite phase matching conditions are inhibited. 
     In optical parametric generation (OPG), a high power pump laser is shined through a nonlinear medium with a second order nonlinearity (χ 2 ). OPG is also referred to as difference frequency generation. Optical parametric amplification (OPA) is similar to OPG. OPA uses a high power pump that is shined in along with a weak seed laser. The seed can be either the corresponding signal or idler wavelength. The seed is generated while the complimentary signal or idler is generated. If the phase matching condition (photon conservation of momentum) is met and sufficient pump power is used, the OPA process can be used to generate two longer wavelength photons called the signal (shorter or desired wavelength) and idler (longer wavelength). The phase matching condition is often met by sending the pump laser through a χ 2  nonlinear crystal in a very specific direction and with a specific polarization. This technique is called birefringence phase matching. 
     Although this technique enables OPA operation the efficiency is sometimes lower than ideal because the conditions for phase matching usually come at the cost of directing the laser beam through the crystal in a direction for which the nonlinearity is low, the crystals are short, and the crystals can only be exposed to a limited pump power before damage. 
     To remedy this problem another technique called quasi-phase matching (QPM) is used to simultaneously achieve phase matching and high nonlinearity. The crystal nonlinearity is modified by applying a periodic high voltage to the crystal. At high enough voltage, this process produces a permanent periodic inversion of the crystal polarization. The poling period is chosen to enable phase matching for a specific signal wavelength. The process is permanent and the crystal can only be used to produce a specific wavelength. Although an effective solution to low nonlinearity, the periodically poled crystals are relatively short and sensitive to optical damage since they require very high pump intensity. Also, this technique requires the addition of free space optics to couple the pump laser to the crystal. If reduced pump power is required then the crystal has to be placed in an optical cavity. 
     A fiber based solution is based on gas or liquid filled hollow-core photonic crystal fiber (HCPCF) and solid core fibers. χ 2  is typically negligible for glass, gases and liquids due to the random molecular orientations; the media is centrosymmetric and lacks a significant chi-2 value. However, application of an electric field to a glass, gas, or liquid can induce an effective χ 2 . So if the gas-filled HCPCF is sandwiched between two electrodes with a voltage applied to the electrodes, a χ 2  will be induced but the phase matching condition will likely not be met. Pressure tuning the gas along with the fiber properties enables some level of control over the phase matching conditions and may enable higher order modes to be phase matched. This is unattractive because the higher modes will yield lower conversion efficiency and yield a poor beam quality laser emission. 
     An alternative is to enable quasi-phase matching by applying a periodic electric field along the length of the gas filled HCPCF similar to periodically poled niobate. The field will induce a χ 2  and proper poling period will enable phase matching of the fundamental modes. This was proposed in 2016 (Broadband electric-field-induced LP01 and LP02.second harmonic generation in Xe-filled hollow-core PCF) by JEAN-MICHEL MÉNARD, but an enabling method to implement was not contemplated or disclosed. 
     SUMMARY 
     The present innovation overcomes the foregoing problems and other shortcomings, drawbacks, and challenges of wavelength adjustable fiber laser systems by providing a compact, method of quasi-phase matching in optical fibers. While the present innovation will be described in connection with certain embodiments, it will be understood that the invention is not limited to these embodiments. To the contrary, this invention includes all alternatives, modifications, and equivalents as may be included within the spirit and scope of the present invention. 
     According to one aspect of the present innovation, a cylindrical electrode module of a fiber optic laser system includes an inner cylinder having an inner repeating pattern of longitudinally-aligned positive and negative electrodes on an outer surface of the inner cylinder. The cylindrical electrode mode includes an outer cylinder that encloses the inner cylinder. The outer cylinder that has an outer repeating pattern of longitudinally-aligned negative and positive electrodes on an inner surface of the inner cylinder that are in corresponding and complementary, parallel alignment with the positive and negative electrodes of the inner repeating pattern on the outer surface of the inner cylinder. The cylindrical electrode module includes an optical fiber having an input end configured to align with and be optically coupled to a high power pump laser. The optical fiber is wrapped around the inner cylinder within the outer cylinder to form a cylindrical fiber assembly. The electrodes are activated to achieve quasi-phase matching. In one or more embodiments, at least one of the pattern of electrodes is fabricated using additive manufacturing. 
     According to another aspect of the present innovation, a fiber laser system includes a pump laser, a cylindrical electrode module, and a controller. The cylindrical electrode module includes an inner cylinder having an inner repeating pattern of longitudinally-aligned positive and negative electrodes on an outer surface of the inner cylinder. An outer cylinder encloses the inner cylinder and that has an outer repeating pattern of longitudinally-aligned negative and positive electrodes on an inner surface of the inner cylinder that are in corresponding and complementary, parallel alignment with the positive and negative electrodes of the inner repeating pattern on the outer surface of the inner cylinder. An optical fiber has: (i) an output end; and (ii) an input end aligned with and optically coupled to the high power pump laser. The optical fiber is wrapped around the inner cylinder within the outer cylinder to form a cylindrical fiber assembly, the output end extending out of the outer cylinder. The controller is communicatively coupled to, and activates, the inner repeating pattern of positive and negative electrodes and the outer repeating pattern of negative and positive electrodes to perform adjustable period poling to achieve QPM with wavelength agility in a compact form factor. In one or more embodiments, at least one of the pattern of electrodes is fabricated using additive manufacturing. 
     Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The description of the illustrative embodiments can be read in conjunction with the accompanying figures. It will be appreciated that for simplicity and clarity of illustration, elements illustrated in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to other elements. Embodiments incorporating teachings of the present disclosure are shown and described with respect to the figures presented herein, in which: 
         FIG.  1    depicts a fiber laser system that dynamically changes signal wavelength of, according to one or more embodiments; 
         FIG.  2 A  is an isometric diagram illustrating a gas or fluid-filled hollow-core photonic crystal fiber (HCPCF) with orthogonally opposed, periodic electrode structure, according to one or more embodiments; 
         FIG.  2 B  is an isometric diagram illustrating the gas or fluid-filled HCPCF of  FIG.  1    with a second pair of orthogonally opposed, periodic electrode structure that is orthogonal to the first pair, according to one or more embodiments; 
         FIG.  3 A  depicts a diagrammatic side view of a linear fiber assembly with a single pair of electrode sets on diametrically opposed sides of an optical fiber, according to one or more embodiments; 
         FIG.  3 B  depicts a diagrammatic side view of a linear fiber assembly with periodic groupings of three (3) positive electrodes and periodic grouping of three (3) negative electrodes, according to one or more embodiments; 
         FIG.  3 C  depicts a cross sectional diagrammatic side view of a linear fiber assembly with four selectable pairs of electrode sets, according to one or more embodiments; 
         FIG.  3 D  depicts a side diagrammatic side view of a linear fiber assembly with a single pair of electrode sets that is spiraled around the optical fiber with complementary corresponding positive and negative electrode being diametrically opposed, according to one or more embodiments; 
         FIG.  4    depicts a three-dimensional, disassembled view of the cylindrical fiber assembly of  FIG.  1   , according to one or more embodiments; 
         FIG.  5    is a graphical plot of electrode poling period versus signal wavelength for different xenon (Xe) pressures and HCPCF diameters, according to one or more embodiments; 
         FIG.  6    is a graphical plot for a liquid-filled HCPCF that uses carbon disulfide (CS 2 ), according to one or more embodiments; 
         FIG.  7    is a three-dimensional view of an example multiple-period cylindrical fiber assembly, according to one or more embodiments; 
         FIG.  8    is a diagram of multiple electrode patterns with complementary top-bottom electrode pairs across fiber, according to one or more embodiments; 
         FIG.  9    is a three-dimensional view of creating concentric cylindrical shells with fiber sandwiched there between, according to one or more embodiments; 
         FIG.  10    is a three dimensional view of fabricating another example cylindrical electrode, according to one or more embodiments; 
         FIG.  11    is a three-dimensional view of assembling an example electrode assembly using additive manufacturing, according to one or more embodiments; 
         FIG.  12    is a depiction of an electrode structure with interdigitated opposite electrode patterns printed in multiple strips per sheet, according to one or more embodiments; 
         FIG.  13    is a depiction of another electrode structure with mirrored opposite electrode patterns with varied periodicity printed on respective strips, according to one or more embodiments; 
         FIG.  14    is a three dimensional view of assembling the electrode of  FIG.  13   , according to one or more embodiments; 
         FIG.  15    is a depiction of an accordion-shaped electrode structure, according to one or more embodiments: 
         FIG.  16    is a depiction of a spiral electrode structure, according to one or more embodiments; and 
         FIG.  17    is a depiction of the spiral electrode structure assembled with a fiber, according to one or more embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     According to aspects of the present invention, wavelength flexibility through variable-period poling, or variable period quasi-phase matching (QPM), of fibers, including liquid or gas-filled hollow-core photonic crystal fiber (HCPCF) and solid core fibers, enables wavelength flexibility in a fiber based laser system for applications such as communications, three-dimensional laser scanning (“LADAR”), military illuminators, beacons, and designators. In particular, applying different poling periods around the fiber enables rapid wavelength agility. In one or more embodiments, the fiber based laser system is one or more of: (i) a simple, small, low cost and efficient; (ii) opto-mechanically insensitive to misalignment; and (iii) exhibit rapid wavelength switching; and (iv) provide considerable flexibility in wavelength selection. 
     In one or more embodiments, the present innovation uses solid core fibers. In one or more embodiments, the present innovation uses gas or liquid filled fibers. In one or more embodiments, the present innovation uses temporarily gas or liquid filled fibers. In one or more embodiments, aspects of the present innovation provide periodic poling of a gas/liquid filled hollow core fiber or solid core fiber with dynamically adjustable poling period to enable wavelength flexibility. In contrast to the crystal, the effect of the poling will not be permanent; once the field is turned off the molecules will relax and there will be no optical axis. Thus, an optical parametric amplification (OPA) process can be turned on and off by turning the electrode voltage on and off. If the electrode period can be dynamically changed, the OPO process can enable the phase matching condition to be met for different signal and idler wavelengths. 
     In one aspect, the present disclosure provides dynamic wavelength agility. The ability to dynamically change wavelength is achieved by energizing different electrodes where different electrodes have different periods. Each period is designed to produce a particular wavelength. In one or more embodiments, a second technique uses a single electrode poling period with modulating power to the electrodes. So when there is no power you get the pump wavelength out and when there is a voltage applied to the electrode you get the design wavelength. 
     The present innovation has a number of applications that include optical parametric generation (OPO), optical parametric amplification (OPA), and optical parametric oscillation (OPO). In OPG, the input is one light beam of frequency ω p , and the output is two light beams of lower frequencies ω s  and ω i , with the requirement ω p =ω s +ω i . These two lower-frequency beams are called the “signal” and “idler”, respectively. This light emission is based on the nonlinear optical principle. The photon of an incident laser pulse (pump) is divided into two lower-energy photons. The wavelengths of the signal and the idler are determined by the phase matching condition. The wavelengths of the signal and the idler photons can be tuned by changing the phase matching condition. 
     In one aspect, the present innovation provides wavelength flexibility—reconfigurable electrodes to produce different periods or multiple period electrodes around the fiber. Generally-known approaches are limited to solid core fibers and use a planar ground electrode and not a periodic electrode of oppose polarity on either side of the fiber. The generally-known approach only achieved a conversion efficiency of about 0.1% and only focused on second harmonic generation. By using periodic electrodes of opposite polarity opposed to one another, our OPA is anticipated to have a conversion efficiency of approximately 30% based on modeling. The SHG and OPA process are similar enough that one can directly compare the efficiencies. Also, we propose using additive manufacturing or lithography techniques for electrode production to enable very short periods—as short at microns for lithography or ˜10&#39;s of microns for additive manufacturing. The generally-known approaches use printed circuit board with electrode period of ˜1.5 mm. This short period forces them to work at low pressure where nonlinearity chi-2 is low. Highest chi-2 occurs at highest pressure which requires shortest electrode period. Although some generally-known approaches use QPM, there was no suggestion of dynamic wavelength flexibility. 
       FIG.  1    depicts a diagram of a fiber laser system  100  that provides dynamic wavelength agility by configuring paired periodic electrodes  102   a - 102   b  positioned on opposite sides of an optical fiber  104  provide a fiber assembly  106  for adjusting quasi-phase matching. In one or more embodiments, the paired periodic electrodes  102   a - 102   b  and optical fiber  104  are arranged in a linear fiber assembly  106   a . In addition, the electrode period at different azimuthal angles along the cylinder are different—the phase matched wavelengths will vary depending on which electrode is powered. The poling period necessary to achieve phase matching for a particular signal wavelength is a function of the pump wavelength, desired signal/idler wavelengths, fiber design and effective modal index of refraction. For a gas-filled fiber, pressure can be used to tune the modal index of refraction. In one or more embodiments, the paired periodic electrodes  102   a - 102   b  and optical fiber  104  are arranged in a cylindrical fiber assembly  106   b . In one or more embodiments, the fiber  104  is 250 μm in diameter and is wrapped within a cylindrical electrode  107  is approximately 300 μm and encases the fiber  104  to form a cylindrical fiber assembly  106   b.    
     In one or more embodiments, the fiber laser system  100  is configured as an OPA with a pump output  108  from a high power pump laser  110  being combined by a dichroic mirror or fiber combiner (“combiner”)  112  with a seed output  114  from a seed laser  116 . The seed output  114  can have a signal wavelength or an idler wavelength. The combiner  112  directs the pump output  108  and the seed output  114  in a single direction for coupling to the fiber assembly  106  via a first lens  118  at one end of an optical cavity  120  that contains the fiber assembly  106 . In one or more embodiments, the optical fiber  104  of the fiber assembly  106  has a solid core. In one or more embodiments, the optical fiber  104  of the fiber assembly  106  is has a hollow core that is filled with a fluid comprising one or more of a gas and a liquid. In one or more embodiments, the hollow core of the optical fiber  104  is encapsulated between an input gas cell  124  and an output gas cell  126  that include a window respectively to receive combined input  128  to and direct output  130  from the fiber assembly  106 . A collimation lens  132  collimates the output  130 . A first output dichroic mirror  134  separates residual pump laser  136  from beam path  138 . A second output dichroic mirror  140  separates residual idler laser  142  from beam path  138 . In one or more embodiments, the seed laser  116  receives residual pump laser or  136  residual idler laser  142 . What remains in beam path  138  is laser output signal beam  144 . In an exemplary embodiments, the pump laser  110  produces pump output  108  having a wavelength of 1.06 μm, which results in residual idler laser  142  of wavelength 3.1 μm and laser output signal beam  144  of wavelength 1.6 μm. 
     In one or more alternative embodiments, the fiber laser system  100  includes two mirrors  146   a - 146   b  that define a mirror cavity  148  to operate as an optical parametric oscillator (OPO). The OPO employs the same fundamental nonlinear process, difference frequency generation, as is used for the OPA. The difference is that the nonlinear medium (here the quasi-phase matched) of fiber assembly  106  is part of the optical cavity  120  that serves to reduce pump power threshold and increase conversion efficiency. Mirrors  1146   a - 146   b  reflect the signal wavelength with high reflectivity so that the signal wavelength oscillates in the mirror cavity  148 . This reduces threshold pump power and increases efficiency. Neither the pump nor the idler oscillate but leak out instead. 
       FIG.  2 A  depicts a linear fiber assembly  206   a  that includes gas/liquid-filled HCPCF  204  with periodic electrode structure or set  202   a - 202   b  enabling QPM OPA laser generation. Reconfigurable electrodes could be constructed by making the electrodes much smaller than the necessary period for phase matching and dynamically grouping electrodes to produce the desired periods. Integrated circuit techniques could be employed to dynamically group electrodes. The electrode structure could be produced through lithography or additive manufacturing techniques depending on the required periods. 
       FIG.  2 B  depicts an alternative design of a linear fiber assembly  206   a ′ that includes gas/liquid-filled HCPCF  204  gas/liquid-filled HCPCF  200   b  that employs two orthogonal electrode sets or patterns  202   a - 202   b ,  204   a    204   b . In one or more embodiments, electrode sets or patterns can be used that are at angles that are not orthogonal to each other. The poling period of the top and bottom electrode pattern  202   a - 202   b  respectively has a different poling period than the orthogonal right and left electrode pattern  204   a - 204   b . By switching voltage between top and bottom, electrode pattern  202   a - 202   b  and right and left electrode pattern  204   a - 204   b  quasi-phase matching is achieved at two different signal wavelengths. In addition, one could rotate this electrode structure around the fiber such that at different azimuth angles there are different electrode periods which phase match different wavelengths. 
       FIG.  3 A  depicts a diagrammatic side view of a linear fiber assembly  306   a  with a single pair of electrode sets  302   a - 302   b  on diametrically opposed sides of optical fiber  304  and that alternate positive and negative electrode  307   a ,  307   b . Wavelength agility can be achieved by activating and deactivating all of the positive and negative electrode  307   a ,  307   b . In one or more embodiments, a periodic subset of the positive and negative electrode  307   a ,  307   b  is activated and deactivated to perform wavelength agility. 
       FIG.  3 B  depicts a diagrammatic side view of a linear fiber assembly  306   b  with a single pair of electrode sets  303   a - 303   b  on diametrically opposed sides of optical fiber  304  and that repeatedly alternate in a grouping of three (3) positive electrodes  307   a  and a grouping of three (3) negative electrode  307307   b . Wavelength agility can be achieved by activating and deactivating all of the grouped positive and negative electrode  307   a ,  307   b . In one or more embodiments, the positive and negative electrode  307   a ,  307   b  are dynamically reconfigurable into a different number (e.g., 1, 2, 3, 4, etc.) of repeated groupings of positive and negative electrode  307   a ,  307   b  to perform wavelength agility. 
       FIG.  3 C  depicts a cross sectional diagrammatic side view of a linear fiber assembly  306   c  with four selectable pairs of electrode sets  311   a - 311   b ,  312   a - 312   b ,  313   a - 313   b , and  314   a - 314   b , that alternate positive and negative electrode  307   a ,  307   b . Each electrode set  311   a - 311   b ,  312   a - 312   b ,  313   a - 313   b , and  314   a - 314   b  are diametrically opposed to the corresponding other electrode set  311   a - 311   b ,  312   a - 312   b ,  313   a - 313   b , and  314   a - 314   b  of the pair. Each electrode set  311   a - 311   b ,  312   a - 312   b ,  313   a - 313   b , and  314   a - 314   b  is at different radial positions to other electrode set  311   a - 311   b ,  312   a - 312   b ,  313   a - 313   b , and  314   a - 314   b . Wavelength agility can be achieved by activating one of the four pairs of electrode sets  311   a - 311   b ,  312   a - 312   b ,  313   a - 313   b , and  314   a - 314   b  that has respectively a different period of positive and negative electrode  307   a ,  307   b  from other pairs. The first pair of electrode sets  311   a - 311   b  (“A1”, “B1”) has a period that is shorter than the second pair of electrode sets  312   a - 312   b  (“A2”, “B2”). The third pair of electrode sets  313   a - 313   b  (“A3”, “B3”) has period that is shorter than the first pair of electrode sets  311   a - 311   b  (“A1”, “B1”). The fourth pair of electrode sets  314   a - 314   b  (“A4”, “B4”) has period that is longer than the first pair of electrode sets  311   a - 311   b  (“A1”, “B1”) and that is shorter than the second pair of electrode sets  312   a - 312   b  (“A2”, “B2”). 
       FIG.  3 D  depicts a side diagrammatic side view of a linear fiber assembly  306   d  with a single pair of electrode sets  321   a - 321   b  that spiral around the optical fiber  304  with complementary corresponding positive and negative electrode  307   a ,  307   b  diametrically opposed. The spiral shape can provide a structural benefit and can enable a higher density of electrodes per longitudinal length of fiber. 
     In one or more embodiments,  FIG.  4    depicts cylindrical electrode module  401  that encases a long length of wrapped optical fiber  404  that performs wavelength agility as a cylindrical fiber assembly  406 . The cylindrical electrode module  401  includes an inner cylinder  405   a  that is nested within an outer cylinder  405   b . The optical fiber  404  is run through the cylindrical electrode module  401  between the inner and outer cylinders  405   a    405   b  to form the cylindrical fiber assembly  406 . An outer surface of the inner cylinder  405   a  includes an inner periodic electrode set  402   a  having a repeating pattern of positive and negative electrodes  407   a    407   b . An inner surface of the outer cylinder  405   b  includes an outer periodic electrode set  402   b  having a complementary repeating pattern of positive and negative electrodes  407   a    407   b  to the inner periodic electrode set  402   a . The inner and outer periodic electrode set  402   a    402   b  that interact with the long length of the optical fiber  404  to achieve quasi-phase matching. 
     In one or more embodiments, the optical fiber  404  is gas or liquid filled and thus has a longer poling periods than a solid filled fiber. A longer poling period makes the corresponding larger dimensions of the inner and outer periodic electrode sets  402   a - 402   b ,  404   a    404   b  easier to manufacture. For instance Xe at 50 atmospheres has a period of approximately 1 mm whereas fused silica (glass) has a period of approximately 100 μm. 
     Liquid could be used instead of gas.  FIG.  5    depicts a graphical plot  500  of electrode poling period versus signal wavelength for different xenon (Xe) pressures and HCPCF diameters.  FIG.  6    depicts a graphical plot  600  for a liquid-filled HCPCF that uses carbon disulfide (CS 2 ). 
     If gas/liquid nonlinearity is too low then conversion efficiency will be low. This could be remedied by placing the periodically poled gas/liquid filled hollow fiber inside of an optical cavity to create an optical parametric oscillator (OPO). OPOs are a common technology for wavelength conversion but are usually constructed from a nonlinear crystal and cavity mirrors. 
     A significant problem with operation in a material outside of Noble gases (like Xe) is that parasitic processes like Raman scattering can beat out the OPO/OPA channeling the pump energy into unwanted Raman output wavelengths. A solution to this is to introduce significant loss at the Raman wavelength so that the OPA/OPO process can dominate. 
     The following four (4) references are included in the priority document to the present application and are hereby incorporated by reference in their entirety: (i) https://www.rp-photonics.com/optical_parametric_generators.html; (ii) https://en.wikipedia.org/wiki/Optical_parametric_amplifier; (iii) Quasi-phase matching. David S. Hum*, Martin M. Fejer, C. R. Physique 8 (2007), pp. 180-198; and (iv) Phase-matched electric-field-induced second harmonic generation in Xe-filled hollow-core photonic crystal fiber, JEAN-MICHEL MÉNARD AND PHILIP ST. J. RUSSELL, Vol. 40, No. 15/Aug. 1, 2015/Optics Letters 3679. 0146-9592/; and (v) JEAN-MICHEL MÉNARD FELIX KÖTTIG, AND PHILIP ST. J. RUSSELL, Vol. 41, No. 16/Aug. 15, 2016/Optics Letters 3795. 
     In one or more embodiments, the present disclosure provides wavelength flexibility through application of multiple periodic electrode structures. The specific wavelength that is produced depends on which electrode is powered. This process is useful for any nonlinear process that requires phase matching and can be used in gas or liquid filled fibers and fused silica solid core fibers. The latter includes fibers such as telecom fibers. 
     In one or more embodiments, the present disclosure provides a cylindrical electrode that enables use quasi-phase matching on any type of fibers. This can be for the purpose of creating a specific wavelength or to achieve wavelength agility. The key is that cylindrical electrode enables long interaction length in a practical electrode structure. One aspect of using the term “cylindrical” is associated with producing a number of different periodic electrodes (different periods) and wrapping them around the fiber. By choosing which one is powered, the wavelength produced can be chosen. The second usage of “cylindrical” to implementing single electrode period within cylindrical packaging for reduced length that is more compact and practical. In one or more embodiments, the cylinder electrode could have different periods at different heights to enable use of multiple electrode periods. 
       FIG.  7    is a three-dimensional view of example multiple-period cylindrical fiber assembly  700 . The process begins as did the concentric cylinder approach. A cylinder will have periodic electrodes printed in the azimuthal angle around its surface. An optical fiber is wrapped around the electrode. Now instead of sliding a slightly larger diameter cylinder, with periodic electrodes printed on its inner surface, over the fiber wrapped inner cylinder, we print the outer electrodes directly on the optical fiber. This has a couple of advantages. First, it minimizes the distance between the inner and outer electrodes increasing field strength and reducing field fringing. Both of these factors increase nonlinearity and therefore conversion efficiency. Secondly, it removes concern for mechanical tolerances between the cylinders. Sine we desire the two cylinders to have as close to the same diameters as possible without mechanical interference, any mechanical tolerances required to accommodate uncertainties in the mechanical stack up will reduce the conversion efficiency. Since we are printing directly on the fiber there is reducing concern over tolerances. 
     The additive manufacturing printing process in principle is summarized as: 1) a sticky substance is sprayed on the cylinder prior to wrapping with fiber; this ensures the fiber is secured once the material is cured and it also removes air from the stack increasing the breakdown voltage. After the fiber is wrapped another material is sprayed/printed in the spaces between the fibers. This serves to remove air and increase electrical breakdown but it also helps to smooth out the outer surface on which the outer electrodes will be sprayed. A certain level of smoothness is probably necessary to ensure the nonlinear optical process occurs as predicted for QPM. These materials are cured so that they harden. Lastly, the outer electrodes is sprayed on. As indicated this is a rough sketch of the process and this will be tested/finalized in the coming months. 
     Unfortunately, using a single polarity does require longer fiber, which reduces conversion efficiency due to fiber loss. The advantage is that it is much easier to print this electrode configuration. For certain configurations that may be a reasonable trade. For example, if a fiber has high nonlinearity and/or low loss then the reduced field modulation can be made up for using longer fiber. If the fiber has low nonlinearity and high/moderate loss then conversion efficiency will definitely suffer. This is the case for Xe filled fiber at low to moderately high pressure. We haven&#39;t examined other materials but there may be cases where this is an effective design. 
     Initial work is underway as to appropriate applications and details of implementation for quantum tech. The same nonlinear processes previously discussed are relevant but the focus tends to be on spontaneous parametric down conversion (SPDC, difference frequency generation) and spontaneous four wave mixing, SFWM). One big difference here is that the pump powers have to be kept low so that there is no amplification of signal/idler; ideally we produce one signal/idler photon per pump pulse. Similar to the current work, a phase, matching condition must be met. 
     Traditionally, SPDC was accomplished in nonlinear crystals to produce entangled (strong correlation between signal and idler properties) or unentangled (weak/no correlation between signal/idler) photon pairs. Although this works, it is difficult to efficiently couple these photons into fibers which form the basis of quantum networks. Also, these crystals offer little flexibility in emission wavelengths. 
     Traditional fibers do not have an inherent 2nd order nonlinearity and cannot produce SPDC. Since fibers have a 3rd order nonlinearity they can be used for SFWM. The practical difference between SPDC and SFWM is the location of the signal and idler relative to the pump wavelength. For SPDC, the signal and idler are both on the long wavelength side of the pump whereas for SFWM the signal on the short wavelength side of the pump and the idler is on the long wavelength side. There has been some work producing photon pairs using traditional fibers and Xe filled hollow fibers. 
     The selection of entangled vs unentangled photons is accomplished through a quantity called joint spectral amplitude (JSA), JSA˜Phase matching function*energy conservation function. To produce unentangled photons, the appropriate JSA is accomplished by certain constraints on group velocity of the pump, signal, and idler fields, An example is that that group velocity of pump=signal V, or pump V=idler V. So now a phase matching condition and group velocity matching condition is required. There has been one recent paper on SFWM in XE hollow fiber where they showed the ability to meet these conditions through a combination of fiber design nd pressure tuning. 
     There are at least two novel aspects of our quasi-phase matching technique: 1) we can accomplish 2nd order nonlinear optics such as SPDC and 2) we can, potentially, accomplish 3rd order processes such as SFWM in the same, single fiber device. We may need a different pump for each process though. For SPDC, the ability to independently meet group velocity (GV) and phase matching conditions gives much greater control and flexibility in design. For example, pressure tuning and fiber design can be used to meet GV where as QPM can meet phase matching conditions. Our preliminary analysis also shows that we can meet both conditions over a broad spectral range by pressure tuning. This could be important as it might be valuable to have biphoton source that can work throughout the common fiber telecom bands that span 1460 nm-1625 nm; this might be valuable in quantum networks for communication or quantum computers. 
     It may be possible that SFWM could occur in the same fiber. SFWM has a spectral distribution of signal/idler that is more favorable for certain unentangled photon applications requiring heralded photons. Here SFWM is used to produce a near IR signal (˜800 nm) and a telecom idler (˜1550 nm) and this allows a super-efficient silicon single photon detector to sense the presence of the signal photon and then let the system using the photons know, herald, the idler photon is therefore present. The idler photon would then be used for communication or computation work. Heralding is necessary because these sources are probabilistic and, given the need to avoid stimulated emission, the source produces ˜0.1 photon pairs/pump pulse. 
     QUASI-PHASE MATCHING OPTICAL FIBERS USING FLEXIBLE PRINTED CIRCUIT TECHNOLOGY: We advance the application of flexible printed circuit (FPC) tech. to quasi-phase matched (QPM) nonlinear optical processes in optical fiber for development of wavelength flexible fiber lasers and sources of quantum states of light. The FPC maybe packaged in a cylindrical, linear, or other geometry to enable desired electrode interaction length in compact volume. The nonlinear optical processes may include difference frequency generation, optical parametric amplifiers and oscillators, second harmonic generation, etc. The application of FPC to quasi-phase matching in fiber is new and potentially disruptive. The disruptive element arises from the potential, for the first time, to QPM long lengths of optical fiber to enable efficient nonlinear optical processes in any time of fiber at any high transmission wavelength in the fiber. The FPC technology has recently been extended to circuits as long as 70 m. Electrical elements as small as 20 μmm (periods ˜80 μm) can be produced and future development may further extend printed lengths and minimal size elements. 
     The traditional methods of quasi-phase matching (QPM) in fiber are based on external application of electrostatic fields (we also use static E fields) and thermal poling. Application of external fields has been investigated by many groups on gas filed hollow fiber and traditional fused silica solid core fiber. The research has demonstrated the potential of QPM in fiber but efficiency and broader adoption of the technology has been hampered by the need for long fiber lengths (1-100 m) for efficient operation and the difficulty in making long electrodes with small electrode period (˜10&#39;s-100&#39;s micron). Thermal poling occurs by raising the temperature of an appropriately doped fiber while simultaneously applying an external field for some duration. Proper technique causes a built in periodic electric field so that after completion of the external electrode is no longer needed. Although attractive, this technique is limited by weak built in fields, short fiber lengths, and the decay of the built in fields over time. 
     Our approach also employs externally applied fields but does so using flexible printed circuits (FPC). The FPC should enable high efficiency interaction along with greatly improved wavelength flexibility. The efficiency is increased by long interaction length; current FPC can be made as long as 70 m. Electric field QPM in the literature has limited efficiency due to electrode lengths limited to ˜10-30 cm. Current FPC technology allows for periodic, alternating polarity electrodes with electrode period &gt;80 um; implementations in literature generally have much longer periods which limited wavelength flexibility. The combination of long FPC with ˜80 um electrode period allows efficient interaction and considerable wavelength flexibility in gas filled hollow fiber, and a number of different liquid filled hollow fiber and solid fiber geometries. Continued development of FPC tech will yield even small electrode periods and longer lengths which will allow even greater flexibility of fiber geometry and performance. 
     Another attractive element of the FPC is the way it can be used and packaged. For instance the FPC technology may be used to construct the inner and outer electrode shells of a cylindrical electrode, which would enable an ultra compact laser/quantum source geometry. Although very attractive this approach has tight mechanical tolerances; we continue to work this approach. A more straight forward implementation is to encase the fiber in the long FPC and then compactly the package the long FPC 
       FIG.  8    is a diagram of multiple electrode patterns with complementary top-bottom electrode pairs across fiber. Create concentric cylindrical shells with fiber sandwiched in between—each shell has complementary electrodes that are aligned. We introduce multiple approaches based on additive manufacturing printing and flexible printed circuits. 
     I. Two rigid, cylinders:  FIG.  9    is a three-dimensional view of creating concentric cylindrical shells with fiber sandwiched there between. Using additive manufacturing techniques (or possibly lithography), print periodic electrodes on both the outer surface of the inner cylinder and the inside surface of the outer (hollow) cylinder. Wrap the fiber around the inner cylinder, and slide the outer cylinder over the wrapped fiber. Region between shells filled with high dielectric strength material. Critical aspects are cylinder diameter size for close fit and rotational alignment such that opposite polarity electrodes oppose one another. 
     II. One rigid, inner cylinder:  FIG.  10    is a three dimensional view of fabricating another example cylindrical electrode. Using additive manufacturing techniques (or possibly lithography), print periodic electrodes on the outer surface of the inner cylinder. Wrap the fiber around the inner cylinder, and fill and smooth the empty space around fiber. Directly print the electrodes on the fiber/dielectric fill medium surface. This method removes size tolerance concerns of multiple cylinders but introduces additive manufacturing challenges. 
     III. Flexible shell approach (additive manufacturing):  FIG.  11    is a three-dimensional view of assembling an example electrode assembly using additive manufacturing. Using additive manufacturing techniques, print periodic electrodes on two flexible rectangular sheets, which can be wrapped and joined at a seam into a cylindrical shape. Wrap the first sheet on an inner cylinder, followed by the fiber and the second electrode sheet. This method enables initial planar electrode characterization and removes size tolerance concerns of multiple cylinders but requires fine rotational alignment. 
     IV. Flexible shell approach (flexible printed circuits):  FIG.  12    is a depiction of an electrode structure with interdigitated opposite electrode patterns printed in multiple strips per sheet. This approach follows geometry of previous flexible shell approach #3, with the advantage of more proven method to create Case 2 electrodes. Interdigitated opposite polarity electrode patterns are printed in multiple strips per sheet. These strips are separated; inner sheet is wrapped and secured on cylinder, followed by fiber and out sheet wrapping. Top bus strip is separated from bottom bus strip at a separation line. Each bus strip includes a positive electrode array interdigitated with a negative electrode array. In one embodiment, the strips are 22.5 inches long. 
       FIG.  13    is a depiction of another electrode structure with mirrored opposite electrode patterns with varied periodicity printed on respective strips. 
       FIG.  14    is a three dimensional view of assembling the electrode of  FIG.  13   , according to one or more embodiments; 
     Advanced FPC technology can now enable printed circuits as long as 70 m; this is a recent world record set by TrackwiseDesigns. Incidentally, our QPM optical parametric amplifier in Xe filled fiber requires a fiber length of ˜70 m. May also be possible to stitch multiple FPCs together to make longer length. Electrode widths as small as ˜20 um can be produced which enable electrode periods of ˜80 μm. These short periods would be necessary for many applications using solid core fiber and liquid filled hollow fiber. These type fibers would likely require ˜10 m of fiber for single pass optical amplifiers and generators. FPC technology also applicable for optical parametric oscillators which would require electrode length of ˜cm&#39;s. 
       FIG.  15    is a depiction of an accordion-shaped electrode structure. Although 10s of meters in length, the FPC technology would allow the electrode-fiber structure to be shaped to fill a small volume thus making the technology practical. Long electrode could be wrapped on a cylinder or spiral structure etc., including an accordion shape that could be compressed. 
       FIG.  16    is a depiction of a spiral electrode structure.  FIG.  17    is a depiction of the spiral electrode structure assembled with a fiber. Spiral design has advantages of cylindrical electrode but are in a 2-D plane making fabrication easier. Could be produced through: (i) flexible printed circuits; (ii) Electroplated flat substrate followed by laser removal of regions intended to be insulating between periodic electrodes; (iii) Could be produced through lithographic techniques. Spiral electrodes fabricated in the plane of the flexible electrode or other substrate with fiber wrapped on the spiral electrode structure. Following fiber wrap, either a second spiral electrode is aligned on top of the fiber or a common voltage plane is place on top. The common V plane is simply a planar metallic surface. 
     FIG&gt; While the disclosure has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the disclosure. In addition, many modifications may be made to adapt a particular system, device or component thereof to the teachings of the disclosure without departing from the essential scope thereof. Therefore, it is intended that the disclosure not be limited to the particular embodiments disclosed for carrying out this disclosure, but that the disclosure will include all embodiments falling within the scope of the appended claims. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. 
     In the preceding detailed description of exemplary embodiments of the disclosure, specific exemplary embodiments in which the disclosure may be practiced are described in sufficient detail to enable those skilled in the art to practice the disclosed embodiments. For example, specific details such as specific method orders, structures, elements, and connections have been presented herein. However, it is to be understood that the specific details presented need not be utilized to practice embodiments of the present disclosure. It is also to be understood that other embodiments may be utilized and that logical, architectural, programmatic, mechanical, electrical and other changes may be made without departing from general scope of the disclosure. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present disclosure is defined by the appended claims and equivalents thereof. 
     References within the specification to “one embodiment,” “an embodiment,” “embodiments”, or “one or more embodiments” are intended to indicate that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present disclosure. The appearance of such phrases in various places within the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Further, various features are described which may be exhibited by some embodiments and not by others. Similarly, various requirements are described which may be requirements for some embodiments but not other embodiments. 
     It is understood that the use of specific component, device and/or parameter names and/or corresponding acronyms thereof, such as those of the executing utility, logic, and/or firmware described herein, are for example only and not meant to imply any limitations on the described embodiments. The embodiments may thus be described with different nomenclature and/or terminology utilized to describe the components, devices, parameters, methods and/or functions herein, without limitation. References to any specific protocol or proprietary name in describing one or more elements, features or concepts of the embodiments are provided solely as examples of one implementation, and such references do not limit the extension of the claimed embodiments to embodiments in which different element, feature, protocol, or concept names are utilized. Thus, each term utilized herein is to be given its broadest interpretation given the context in which that terms is utilized. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     The description of the present disclosure has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the disclosure in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope of the disclosure. The described embodiments were chosen and described in order to best explain the principles of the disclosure and the practical application, and to enable others of ordinary skill in the art to understand the disclosure for various embodiments with various modifications as are suited to the particular use contemplated.