Fiber for optical power amplification and/or optical power generation

Abstract

A fluid filled fiber for a quasi-phase matched generator and a laser incorporating such a fluid filled fiber. The liquid filled fiber has charge transfer molecules dissolved in a solvent. In another embodiment, the liquid of the LF fiber comprises or consists essentially of highly polar liquids and/or charge transfer molecules having relatively high molecular dipole values. The liquid filled fiber is usable with a laser for differential frequency generation.

Claims

1. An elongate liquid filled fiber for differential frequency generation in conjunction with a laser source and pump operably associated therewith, the liquid filled fiber comprising: an external cladding surrounding a polar liquid, the fiber having a ratio of the gain of QPM/DFG to the gain of the Raman scattering SRS greater than 1 when subjected to an applied voltage of 10 kV per mm to 100 kV/mm with a pump power from 25 W to 800 W.

2. A liquid filled fiber according to claim 1 wherein the ratio of the gain of QPM/DFG to the gain of the Raman scattering SRS is greater than 3.

3. A liquid filled fiber according to claim 1 wherein the ratio of the gain of QPM/DFG to the gain of the Raman scattering SRS is greater than 10.

4. A liquid filled fiber according to claim 1 wherein the liquid comprises a polar solvent suitable for optical power amplification.

5. A liquid filled fiber according to claim 4 wherein the liquid further comprises a charge transfer molecule solvent suitable for optical power amplification and optical power generation.

6. A liquid filled fiber according to claim 1 wherein n.sub.core>n.sub.cladding.

7. A liquid filled fiber according to claim 1 wherein the applied voltage of ranges from 10 KV per mm to 50 kV/mm.

8. An elongate liquid filled fiber for differential frequency generation in conjunction with a dual electrode laser source and pump operably associated therewith, the liquid filled fiber comprising: an external cladding surrounding a polar liquid, the fiber having a ratio of the gain of QPM/DFG to the gain of the Raman scattering SRS greater than 2 when subjected to an applied voltage of 10 kV per mm to 100 kV/mm with a pump power from 25 W to 1000 W.

9. A liquid filled fiber according to claim 8 wherein the laser has an electrode separation D and a wavelength and $2.3\frac{}{D}666.7.$

10. A liquid filled fiber according to claim 9 wherein $3\frac{}{D}650.$

11. A liquid filled fiber according to claim 8 having a ratio of the gain of QPM/DFG to the gain of the Raman scattering SRS greater than 8 when subjected to an applied voltage of 10 kV per mm to 50 kV/mm.

12. A liquid filled fiber according to claim 11 having a ratio of the gain of QPM/DFG to the gain of the Raman scattering SRS greater than 15 when subjected to an applied voltage of 10 kV per mm.

13. A liquid filled fiber according to claim 12 having a ratio of the gain of QPM/DFG to the gain of the Raman scattering SRS greater than 9 at a pump power of 150 W to 800 W.

14. A liquid filled fiber according to claim 13 having a ratio of the gain of QPM/DFG to the gain of the Raman scattering SRS greater than 12 at a pump power of 150 W to 500 W.

15. A liquid filled fiber according to claim 13 having a ratio of the gain of QPM/DFG to the gain of the Raman scattering SRS greater than 15 at a pump power of 150 W to 300 W.

16. An elongate liquid filled fiber for differential frequency generation in conjunction with a dual electrode laser source and pump operably associated therewith, the liquid filled fiber comprising: an external cladding surrounding a polar liquid, the fiber having a ratio of the gain of QPM/DFG to the gain of the Raman scattering SRS greater than 2 when subjected to an applied voltage of 10 kV per mm to 100 kV/mm with a pump power from 25 W to 1000 W, wherein the laser has an electrode separation D and a wavelength and $2.3\frac{}{D}666.7$ and n.sub.core>n.sub.cladding.

17. A liquid filled fiber according to claim 16 having an applied voltage of 50 kV/mm and a ratio of the gain of QPM/DFG to the gain of the Raman scattering SRS greater than 3 at a pump power of 25 W to 400 W.

18. A liquid filled fiber according to claim 16 having an applied voltage of 50 kV/mm and a ratio of the gain of QPM/DFG to the gain of the Raman scattering SRS greater than 3 at a pump power of 25 W to 400 W.

19. A liquid filled fiber according to claim 16 having an applied voltage of 10 kV per mm to 50 kV/mm and a ratio of the gain of QPM/DFG to the gain of the Raman scattering SRS greater than 8 at a pump power of 300 W to 900 W.

20. A liquid filled fiber according to claim 16 wherein the liquid further comprises a CTM.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a Feynman diagram of scattering between two electrons by emission of a virtual photon.

(2) FIG. 2 is a Jablonski energy level diagram schematically representing Rayleigh and Raman scattering processes for a hypothetical analyte.

(3) FIG. 3 is a perspective view, shown partially in cutaway, of a fluid filled fiber according to the present invention of indeterminate length and showing the fluid can be either a liquid or gas.

(4) FIG. 4 is a schematic side elevational view of a nonlimiting, exemplary laser according to the present invention.

(5) FIG. 5 is a graphical representation of the effect of electrode period to separation distance ratio on electrostatic field for an exemplary 93% bromotrichloromethane/7% perfluorohexane mixture.

(6) FIG. 6 is a graphical representation of the influence of pump power on the ratio of QPM OPG gain/SRS gain vs laser pump power for several cases.

(7) FIG. 7 is a graphical representation of the effect of electrode wattage on differential electric field gain for three different liquids

(8) FIG. 8 includes three tables of solvent to voltage ratios showing the approximate relationship between the ratio of the Gain of QPM/DFG to the Gain of the Raman scattering SRS.

DETAILED DESCRIPTION OF THE INVENTION

(9) Referring to FIG. 3, a longitudinally elongate liquid 12L filled optical fiber 10 according to the present invention comprises a hollow core fillable with a fluid 12. The fluid 12 may be a liquid 12L or a gas 12G, as described herein whereby the invention excludes solid core fibers 10. The core is encased by a cladding 13 which is preferably concentric with the core. The cladding 13 may be encased by a protective jacket, in known fashion and which is preferably concentric with the cladding 13. The index of refraction of the liquid 12L determines which type of waveguide is feasible. For example, total internal reflection fibers 10/waveguides require n.sub.core>nc.sub.ladding, whereas anti-resonant hollow-core fiber 10 and hollow dual bandgap photonic crystal fibers 10 require n.sub.core<nc.sub.ladding. The hollow fiber 10 may be filled with one of a liquid 12L which typically yields from 100 m to 1000 m, and a period error tolerance from 0.1 m to 10 m, or with a gas 12G with from 1000 m to 10000 m, and from 1.0 m to 10 m. Generally, as fiber interaction length increases, the allowed period error decreases.

(10) Referring to FIG. 4, the optical parametric amplifier 20 according to the present invention may be a laser 21. A laser 21 according to the present invention comprises a light source, one or more dichroic beam combiner and splitter 25 and lenses 24 as helpful for the particular configuration and a light transmitting fiber. The fiber 10 has a cladding 13 surrounding a core therein wherein n.sub.core>nc.sub.ladding. The light source may be a pump laser 21. The pump laser 21 may be augmented by a seed laser 22 for QPA. The seed laser 22 is not used for QPG. The pump laser 21 may have an associated pump power of at least 500 W with a 40 micron core fiber 10 to provide a ratio of ub/gamma to greater than 8*10{circumflex over ()}14 statvolts{circumflex over ()}2. The mu*beta/gamma term is independent of the pump power and fiber core. However, the resulting gain for QPM OPA and SRS is dependent on pump power and fiber core.

(11) The LFF 10 is located between two periodic electrodes 23 with a period . The periodic electrodes 23 produce a periodic electrostatic field along the LFF 10 length enabling QPM. QPM difference frequency generation (DFG) gain G. DFG occurs as the difference in frequency between two electric fields, producing a third electric field. DFG gain may be implemented through OPA where a weak signal is amplified to produce a third field. DFG may also be implemented through OPG where a single high power field is present. In OPG quantum noise produces random photons which mixes with the single high power field to provide amplification. The DFG gain may depend upon the effective nonlinearity .sup.(2) through the component of the electrode 23 electrostatic field (E.sub.DC,y) parallel to a pump laser 21 polarization. Accordingly the liquid 12L filled fiber 10, as optionally doped as described herein, of the present invention enables a /D ratio suitable to increase E.sub.DC,y.

(12) The fiber 10/waveguide may be a hollow channel, photonic crystal structure or slot waveguide. In one embodiment, the liquid 12L of the LF fiber 10 has a nonlinear molecule with a of at least 2000 esu 10.sup.48, preferably at least 3000 esu 10.sup.48, at least 5000 esu 10.sup.48, preferably at least 10000 esu 10.sup.48 but not more than 100000 esu 10.sup.48 in order to be adequately greater than the of bromotrichloromethane or other fluid 12 in the fiber 10. The molecule may have a negative bond length alternation (BLA), corresponding to a sufficiently large value for the molecular dipole, , k is Boltzmann constant, T is temperature and is the 2.sup.nd hyperpolarizability (.sup.(3)N*) as is normally associated with 3.sup.rd order nonlinear optics and N is molecular density.

(13) The averaged difference of bond lengths within the bridge is called bond length alternation (BLA) and ranges between approximately 0.12 to +0.12 . Bond length alternation is a measure of molecular geometric structure defined as the average of the difference in the length between adjacent carbon-carbon bonds in a polymethine ((CH)n) chain. The BLA can be determined by the dielectric constant of the liquid 12L in the core. The desired dielectric constant can be achieved by properly selecting a solvent for the liquid 12L in the core of the optical fiber 10.

(14) The molecule may exist in a state of superposition between neutral and zwitterionic. In such molecules having a negative BLA, the neutral structure is predominant in the ground electronic state while the charge transfer structure dominates an excited electronic state. The resulting difference in dipole moment () between the two electronic states may provide advantageously large values for , where .sub.ge is the transition dipole moment between the states and E is the energy difference between the ground and excited state. A BLA value of +/0.045 provides a sufficiently large value of and a sufficiently small value of , thereby increasing the ratio of /. CT molecules unexpectedly provide nonlinear gain orders of magnitude greater than periodically poled lithium niobate (PPLN), a domain-engineered lithium niobate crystal used for achieving quasi-phase-matching in nonlinear optics according to the prior art. CT Molecules can provide very large QPM DFG gain. Given the potential for long fiber interaction length, CT molecules may enable LCF to be competitive with workhorse bulk nonlinear crystals such as PPLN.

(15) Referring to FIG. 5, the liquid 12L of the LF fiber 10 may have a dielectric constant from 1 to 40, 1.5 to 7.5, 2 to 7 or 2.5 to 6.5 for control of hyperpolarizability . The liquid 12L may comprise one or more NIR transmissive solvents hydrogen free, polar solvents, and more particularly are charge transfer molecule (CTM) solvents. Polar liquids are suitable for use with OPA. Adding CTM to the polar liquid makes it suitable for OPA or OPG. Particularly, the solvents may be selected from the group consisting of bromotrichloromethane, perfluorohexane, trichloroacetonitrile, acetonitrile, trichloroacetontricle, hexachloroacetone and combinations thereof. The liquid 12L may comprise from 80 w/% to 95 w/% bromotrichloromethane and from 5 w % to 15 w/% perfluorohexane. The nonlinear molecule and solvent(s) may be combined to provide a solvent mixture having a dielectric constant of at least 1.5, 2 or 2.5 with a maximum in order to provide sufficiently large values for and provide that QPM DFG gain is greater than SRS gain.

(16) The present invention applies a spatially varying electric field across liquid 12L filled fibers 10 to simultaneously achieve quasi-phase matching (QPM) and an effective .sup.(2). QPM is a technique which employs an electrostatic field with the appropriate spatial modulation to meet the phase matching condition and enable efficient transfer of energy from the pump laser 21 to the desired signal and idler fields. The QPM Condition is

(17) $k-\frac{2}{}=k_p\left({}_p\right)-k_s\left({}_s\right)-k_i\left({}_i\right)-\frac{2}{}=0$
where the propagation constant is k.sub.x=k.sub.o,x*n.sub.eff(.sub.x), k.sub.o,x is the vacuum propagation constant at .sub.x, n.sub.eff (.sub.x) is the effective index of refraction due to the liquid 12L filled fiber 10 or waveguide and

(18) $\frac{2}{}$
is spatial frequency of the periodic electrode 23 with period and k is the phase mismatch due to the LF design that is correctable by application of the QPM. Generally, a generator 20 according to the present invention may satisfy deff, QPM DFG=k*(2)/2 where k is a geometric factor related to the QPM electrostatic field distribution. The ability to select different liquids or to mix different liquids for the liquid fill 12L allows for potential tunability of the polling period, which tunability is infeasible in a solid matrix according to the prior art.

(19) For appropriate transmission the LF fibers 10 and waveguides are chosen to have sufficiently high transmission at relevant wavelengths. For example, short-wave to mid-wave IR applications can benefit from hydrogen free molecules as H-X modes are a major source of absorption in that spectral range.

(20) Referring to FIG. 6, for the appropriate electrode 23 period, the liquid 12L solvent and waveguides or fibers 10 are chosen to enable conveniently manufacturable electrodes 23. The prior art silica fibers exhibit /D ratios<1.5. But according to the present invention /D>2.3 may provide efficient QPM difference frequency generation (DFG) and optical parametric amplification, where D is the separation distance between opposing electrodes 23 as described above. Fiber 10 or wave guide diameter is preferably </2.3. Since QPM period is preferably

(21) $=\frac{2}{k},$
the polling period is advantageously determined by the dispersion of the LF. Practical considerations of liquid 12L dispersion set the maximum 2000 um and fiber 10 diameter that set the minimum D3 um to yield a ratio in the range of

(22) $2.3\frac{}{D}666.7$
and preferably in the range of 3 to 650 and more preferably 5 to 600 for efficient operation. A maximum allowable fringing threshold can be ascertained by requiring a minimum conversion efficiency of 60% in the limit of no loss, un-depleted pump, and perfect phase matching. The electrode 23 period error is preferably maintained to 0.1-1.0%*. Since the polling period is defined by the combined LCF dispersion, different LCF designs yield different polling period. The range and allowed as follows: solid fiber (10-100 m, 0.01-1.0 m), liquid 12L HCF (100-1000 m, 0.1-10 m), and gas 12G HCF (>1000 m, >1.0 m). A liquid 12L filled fiber 10 is attractive for QPM DFG as and are more relaxed and high nonlinearity is still attainable.

(23) The breakdown strength of the LF fiber 10 and liquid 12L are sufficient to prevent breakdown of the electric field strength applied for QPM. Since higher electrostatic fields generally yield higher .sup.(2) values, LF with relatively high breakdown strengths are suitable. Similarly, since high power lasers 21 are typically used to pump the QPM LF, the LF needs a sufficiently high optical intensity breakdown threshold. If one of skill approximates the E.sub.DC,y (z) as sinusoidal, the QPM prefactor is approximately k0.5. For QPM DFG to dominate over SRS in the gain competition the ratio of gains is greater than one: G.sub.QPM,DFG/G.sub.SRS>1. The QPM gain is greater than the Raman gain.

(24) Referring to FIG. 7, the differential electric field gain ratios for a 40 um core liquid 12L filled fiber 10 with 106 um outer diameter, 320 um,

(25) $\frac{}{D}3,$
and a 1.06 um wavelength pump laser 21 converted to 2.12 um wavelength output laser 21 are shown. Bromotrichloromethane (BTM) with electrode 23 voltage yielding E.sub.DC,y10 kV/mm and 50 kV/mm are specifically examined in a non-limiting example. It is believed that the higher field case is best attained using short (<10 us voltage pulses). These results show that polar solvents benefit from high voltage operation and lower pump power. But low pump power may be unattractive as requiring inversely proportional longer fiber 10 length, thereby increasing sensitivity to liquid 12L loss and complicating electrode 23 design. The inclusion of charge transfer molecule (CTM) according to the present invention (and optional moderate field strength) advantageously allows the ratio to exceed 1 even for high pump power which tends to favor SRS gain over QPM DFG gain in conventional polar solvents. A further benefit of the invention is the inversely proportional relationship providing that greater the ratio of the differential electric field gain, the shorter the LF fiber 10 can be. A ratio of 25*10{circumflex over ()}52<<5*10{circumflex over ()}42 esu is believed to be advantageous for the claimed invention.

(26) FIG. 7 shows that as the /D ratio approaches 1, the needed fiber length increases. But if one of skill wants to operate at higher power levels, e.g. to image a target scene at a greater distance, the power must also increase in nonlinear fashion. One suitable execution without CTM is a 50:50 mixture of bromotrichloromethane (BTM) and trichloroacetonitrile (TCN). One suitable execution with CTM is a 2:3 mixture of bromotrichloromethane (BTM) and trichloroacetonitrile (TCN). The ratio of the gain of QPM/DFG to the gain of the Raman scattering SRS is advantageously greater than 1 for three different BTM and CTM filled fibers 40.

(27) The fiber 10 may have a predetermined length overlapping both electrodes 23. For DFG comprising OPA without the benefit of CTM, a fiber 10 having a length of 0.2 M to 1 M and preferably 0.3 M to 0.4 M to increase signal output. Prophetically, for DFG comprising OPA with the benefit of CTM, a fiber 10 having a length of 0.02 M to 0.2 M is believed suitable.

(28) For DFG comprising OPG with the benefit of CTM, a fiber 10 having a length of 0.08 M to 0.4 M is believed to be suitable. DFG comprising OPG without the benefit of CTM is believed to be infeasible for and outside of the present invention.

(29) Referring to FIG. 8, Table 1 shows an approximate relationship between the ratio of the Gain of QPM/DFG to the Gain of the Raman scattering SRS for a 40 micron fiber 40 BTM with a 10 kV/mm potential. It can be seen that for this execution gain ratios of about 3 and 2 advantageously occur with pump powers of 25 and 50 watts, respectively.

(30) Referring to FIG. 8, Table 2 shows an approximate relationship between the ratio of the Gain of QPM/DFG to the Gain of the Raman scattering SRS for a 40 micron fiber 40 BTM with a 50 kV/mm potential. It can be seen for this execution gain ratios of about 13, 8, 5, 4, 3, and 2 advantageously occur with pump powers of 25, 50, 100, 200, 400 and 800 watts, respectively.

(31) Referring to FIG. 8, Table 3 shows an approximate relationship between the ratio of the Gain of QPM/DFG to the Gain of the Raman scattering SRS for a 40 micron fiber 40 CTM with a 10 kV/mm potential. It can be seen that for this execution gain ratios of about 24, 23, 15, 13, 12, 11, 10, 9 and occur with pump powers of 150, 200, 300, 400, 500, 600, 700, 800 and 900 watts, respectively.

(32) FIG. 8 shows the maximum pump power can be 900 watts or 1000 watts and still provide a ratio of the Gain of QPM/DFG to the Gain of the Raman scattering SRS greater than 1, preferably greater than 5 and more preferably greater than 8. The applied voltage may be as high as 10 kV/mm, 50 kv/mm, 75 kV/m or 100 kV/mm.

(33) Using the present invention parasitic Raman scattering is suppressed-thereby overcoming a major challenge as amorphous media (glass, gas 12G, liquid 12L) typically and intrinsically have large Raman gain and weak second order nonlinearity. The present invention overcomes this problem through selection and engineering of molecules to yield large dipole and 1.sup.st hyperpolarizability (or .sup.(2)) while having small imaginary 2.sup.nd order hyperpolarizability (or Im[.sup.(3)]). Particularly, one suitable approach to suppressing Raman scattering uses polar liquids 12L yielding relatively large values of .sup.(2) and Raman output power less than 1 W and preferably less than 1E-3 W. Also, one may dissolve molecules in such solvents to increase .sup.(2) relative to .sup.(3) achieving a sufficiently large .sup.(2)/.sup.(3) ratio, or a large /.sup.(3) ratio.

(34) All values disclosed herein are not strictly limited to the exact numerical values recited. Unless otherwise specified, each such dimension is intended to mean both the recited value and a functionally equivalent range surrounding that value. For example, a dimension disclosed as 40 mm is intended to mean about 40 mm. Every document cited herein, including any cross referenced or related patent or application, is hereby incorporated herein by reference in its entirety unless expressly excluded or otherwise limited. The citation of any document or commercially available component is not an admission that such document or component is prior art with respect to any invention disclosed or claimed herein or that alone, or in any combination with any other document or component, teaches, suggests or discloses any such invention. Further, to the extent that any meaning or definition of a term in this document conflicts with any meaning or definition of the same term in a document incorporated by reference, the meaning or definition assigned to that term in this document shall govern. All limits shown herein as defining a range may be used with any other limit defining a range of that same parameter. That is the upper limit of one range may be used with the lower limit of another range, and vice versa. While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended the appended claims cover all such changes and modifications that are within the scope of this invention.