We investigate two schemes for wavelength conversion based on the cascading of two successive second order processes in a quasi-phase-matched lithium niobate channel waveguide. Efficient conversion over the full erbium amplifier bandwidth is possible with a single multi-hundred milliwatt pump laser operating around 1.55 mu m

Assanto, Gaetano

Gallo, Katia

Stegeman, George I.

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University of Central Florida STARS Faculty Bibliography 1990s Faculty Bibliography 1-1-1997 Efficient Wavelength Shifting Over The Erbium Amplifier Bandwidth Via Cascaded Second Order Processes In Lithium Niobate Waveguides Katia Gallo Gaetano Assanto George I. Stegeman University of Central Florida Find similar works at: https://stars.library.ucf.edu/facultybib1990 University of Central Florida Libraries http://library.ucf.edu This Article is brought to you for free and open access by the Faculty Bibliography at STARS. It has been accepted for inclusion in Faculty Bibliography 1990s by an authorized administrator of STARS. For more information, please contact STARS@ucf.edu. Recommended Citation Gallo, Katia; Assanto, Gaetano; and Stegeman, George I., "Efficient Wavelength Shifting Over The Erbium Amplifier Bandwidth Via Cascaded Second Order Processes In Lithium Niobate Waveguides" (1997). Faculty Bibliography 1990s. 1917. https://stars.library.ucf.edu/facultybib1990/1917 Appl. Phys. Lett. 71, 1020 (1997); https://doi.org/10.1063/1.119714 71, 1020© 1997 American Institute of Physics.Efficient wavelength shifting over theerbium amplifier bandwidth via cascadedsecond order processes in lithium niobatewaveguidesCite as: Appl. Phys. Lett. 71, 1020 (1997); https://doi.org/10.1063/1.119714Submitted: 05 March 1997 . Accepted: 23 June 1997 . Published Online: 04 June 1998Katia Gallo, Gaetano Assanto, and George I. StegemanARTICLES YOU MAY BE INTERESTED INFirst-order quasi-phase matched LiNbO3 waveguide periodically poled by applying anexternal field for efficient blue second-harmonic generationApplied Physics Letters 62, 435 (1993); https://doi.org/10.1063/1.1089251.5 μm band efficient broadband wavelength conversion by difference frequency generation ina periodically domain-inverted LiNbO3 channel waveguideApplied Physics Letters 63, 3559 (1993); https://doi.org/10.1063/1.110096Periodic domain inversion in x-cut single-crystal lithium niobate thin filmApplied Physics Letters 108, 152902 (2016); https://doi.org/10.1063/1.4946010Efficient wavelength shifting over the erbium amplifier bandwidth viacascaded second order processes in lithium niobate waveguidesKatia Gallo and Gaetano Assantoa)Department of Electronic Engineering, Terza University of Rome, Via della Vasca Navale 84, 00146 Rome,ItalyGeorge I. StegemanCREOL University of Central Florida, Orlando, Florida 32816-2700~Received 5 March 1997; accepted for publication 23 June 1997!We investigate two schemes for wavelength conversion based on the cascading of two successivesecond order processes in a quasi-phase-matched lithium niobate channel waveguide. Efficientconversion over the full erbium amplifier bandwidth is possible with a single multi-hundredmilliwatt pump laser operating around 1.55mm. © 1997 American Institute of Physics.@S0003-6951~97!01834-2#Efficient wavelength conversion of very high bit rate sig-nals is a key issue in wavelength division multiplexing~WDM! networks. The most widely investigated conversiontechniques have utilized semiconductor amplifiers either forfour-wave mixing or modulation of the saturated gain.1,2 Anumber of issues such as very limited 3 dB bandwidth, crosstalk, insertion loss, amplifier noise, etc. make this a nonidealsolution and there is a continuing search for other ap-proaches. Among them, those exploiting nonlinear opticalwave mixing via second order nonlinearities appear promis-ing because they also provide spectral inversion with theaddition of low excess noise.3 In fact, wavelength shifting ofsignals has been demonstrated via difference frequency gen-eration ~DFG! in quasi-phase-matched~QPM! lithium nio-bate (LN[LiNbO3) with a pump laser operating in thewavelength range of 750–800 nm.4,5 Although successful,this particular approach is problematic because thewaveguides are typically multimode for input in the near-infrared and require lasers outside of the communicationsbands. Here we describe an approach which also relies onDFG, but which is based on cascading of second harmonicgeneration~SHG! of a pump laser in the communicationsband and then DFG, both in the same channel waveguide.Cascading of second order processes has been shown toencompass new exciting possibilities for all-optical signalprocessing.6 From the early days of nonlinear optics it isknown that x (2):x (2) processes could mimic four-wave-mixing in a noncentrosymmetric material. In the 1980’sBaranova and Zel’dovich predicted large efficienciesfor SHG via x (2)(22v;v;v) followed by DFG viax (2)(2v3 ;2v,2v2) taking place in a noncentrosymmetricnonlinear medium.7 This phenomenon has been experimen-tally observed for wavelengths around 1064 nm in bulkb-barium borate by Tanet al.8 and has interesting implica-tions for wavelength shifting devices: the SHG from a stronginput wave atv interacting with a weak input signal atv25v2dv ~with dv!v! can generate an output signal atv352v2v25v1dv. The wavelength shifter proposedhere operates with inputs around 1.55mm, an appealing fea-ture for WDM networks where high quality lasers have beendeveloped specifically for communications. The chosenguided-wave geometry in QPM lithium niobate leads tomulti-hundred milliwatt operating powers. In this letter weeport on numerical investigations of two simple configura-tions, and evaluation of the device performance in terms ofignal attenuation/gain and bandwidth.Simulations were performed for the different field polar-izations allowed by the tensorial structure of the LN nonlin-ear susceptibility for SHG at wavelengths in the fiber com-munication spectral window atlp51.55mm ~input pump!,l25lp1dl ~input signal!, l351/(2/lp21/l2) ~frequencyshifted signal through DFG! with dlumax5650 nm. It wasfound that an efficient implementation of the cascaded inter-action requires the fields atl2 , l3 , andlp/2 to be collinearbecause this prevents the DFG wave vector mismatch fromvarying appreciably when tuning the input signal around1.55mm.The device proposed here is readily available, an an-nealed proton exchanged~APE! channel waveguide in aZ-cut LN crystal, supporting onlyX-propagating quasi-TM00modes at 1.55mm. The transverse refractive index profile istaken to be:n(y,z)5ne1Dne exp(2uzu/d)exp(2uyu/w), withne given by standard Sellmeier equations.9 The effective in-dex method with a finite difference algorithm was employedto calculate the transverse field profiles and their wave vec-tors along the propagation direction for each mode of thestructure. For typical parametersDne50.025, T5300 K,a!Electronic mail: assanto@ele.uniroma3.itFIG. 1. Sketch of the device in two simple configurations:~a! copropagatingpump and signal;~b! counterpropagating pump and signal, with a coatedendface, reflecting atlp/2 and transmitting atlp .1020 Appl. Phys. Lett. 71 (8), 25 August 1997 0003-6951/97/71(8)/1020/3/$10.00 © 1997 American Institute of Physicsd52 mm, w53 mm, the waveguide is single moded atlp l2 l3 , while several modes (TM00,TM01,TM10) aresupported at the second harmonic (lp/2). First order quasi-phase matching~QPM! through ferroelectric domain inver-sion with periodL516.7mm is then required for efficientSHG from lp . Under these conditions only the followinginteractions are significant inside the structure: TM00(lp)1TM00(lp)→TM00(lp/2) and TM00(lp/2)2TM00(l2)↔TM00(l3); the others being negligible because of theirlarge wave vector mismatch or small overlap integrals.While the first interaction indicates a pumping mechanism toupconvert the input frequency, in the second the double ar-row points out the presence of a slight mismatch which oc-curs as wavelength is scanned over the Er1 amplifier band-width.The modeling of the interactions utilizes the standardcoupled mode theory for waveguides. In the lossless case,this leads to the coupled mode equations in the slowly vary-ing envelope approximation:dA1dx52 iK 11BA1* exp~2 iDb1x!,dA2dx52 i ~lp /l2!K23BA3* exp~2 iDb2x!,dA3dx52 i ~lp /l3!K23BA2* exp~2 iDb2x!,dBdx52 iK 11A12 exp~ iDb1x!22iK 23A2A3 exp~ iDb2x!,where A1 , A2 , A3 , and B are the field amplitudes atlp ,l2 ,l3 ,lp/2, respectively; Db154p@(NSH2N1)/lp#22p/L and Db252p(2NSH/lp2N2 /l22N3 /l3)22p/L are the SHG and DFG wave vector mismatches~with N indicating the effective guided mode indices!, andthe two nonlinear coupling coefficients are:Ki , j52plpA2cm0Si jdeffANiNjNshwith ( i , j )5(1,1) or ~2,3!, if Kleinman symmetry holds. Forl2 ~signal! ranging from 1.5 to 1.6mm, we found effectivechannel waveguide cross sectionsS23>S11>49.5mm2, inagreement with values obtained through more accurate indexprofile modeling.10 For the nonlinear strength, the measuredvalue of d33527 pm/V in a uniform, depth-independentQPM grating obtained via electric-field poling was used.11 Ityields deff5(2/p)d33 and K23>K11>0.086@W1/2 mm21#,corresponding to a normalized SHG conversion efficiency ofhnor574% @W21 cm22#, which compares well with reportedexperimental values of 43%@W21 cm22# that include propa-gation losses.12Let us first consider the case@Fig. 1~a!# of both signaland pump inputs entering the crystal from the same side. Forlp51.55mm, l35l21dl and Db150 ~i.e., perfectlyphase matched SHG!, Fig. 2 shows thel2→l3 conversionfficiency with cw inputs versusl2 . Notice the broad con-version bandwidth, comparable with DFG processes in thenear IR with QPM-LN.13 Signal amplification, due to theFIG. 2. cw response of the single pass device@Fig. 1~a!# vs signal wave-length for various waveguide lengthsL. The vertical axes are the conversionefficiencies in dB. HerePsignal51 mW andPpump5300 mW.FIG. 3. Contour map response of the single pass device@Fig. 1~a!# vs wave-guide lengthL and pump powerPpump. The lines are marked with thecorresponding conversion efficiencies in dB.FIG. 4. Response of the two-pass device@Fig. 1~b!# vs signal wavelength forvariousL. The vertical axis is the conversion efficiency in dB, which clearlyshows an enhancement compared to the case in Fig. 2. HerePsignal51 mWandPpump5300 mW.1021Appl. Phys. Lett., Vol. 71, No. 8, 25 August 1997 Gallo, Assanto, and Stegemanparametric nature of the interaction, is obtained by increasingthe crystal length and/or the pump power atl51.55mm.Both parameters affect the conversion efficiency, as shownin Fig. 3 for input atl251.55mm120 nm and for a signalpower of 1 mW. The lines are marked by the signal gain indB. The simulations have confirmed that the output signalpower, due to the cascading of SHG and DFG, increaseslinearly with the signal power, quadratically with the pumppower and varies asLa ~with 3,a,4! for pump powersbelow 1 W.The simple geometry of Fig. 1~a! can be modified withthe addition of alp/2 reflective coating in order to maximizethe SHG prior to DFG, as sketched in Fig. 1~b!. The pump,injected through the uncoated endface, is partially convertedto the SH on the first pass through the crystal and then leavesit. The SH wave atlp/2 is reflected and interacts throughDFG with the signal launched from the opposite~ o the lplaunching! side. We assumed 100% reflectivity atlp/2.Highly transmitting coatings atlp are preferable to eliminatethe pump wavelength at the device output. As is intuitivelyclear, the more of the second harmonic generated in the firstpass, the greater the advantage in adopting this latter scheme.Figures 4 and 5 are the counterparts of Figs. 2 and 3 calcu-lated for the same input conditions. The device still exhibitsa wide conversion bandwidth typical of the DFG process,which compares favorably with other techniques for wave-length shifting and with the possibility of signal amplifica-tion at higher pump excitation.In conclusion, single- and double-pass configurations fora broadband, and integrated wavelength shifter for WDM ina lithium niobate quasi-phase-matched channel waveguidehave been analyzed numerically. The wavelength convertersoperate in the 1.55mm spectral window for optical fibercommunications and require reasonable pump powers andpropagation lengths to provide lossless, or even amplifiedwavelength conversion. The adoption of a contradirectionalgeometry for the signal and pump further improves the pre-dicted performance, and offers a simple method for remov-ing the pump beam. We are confident that this novel elementfor WDM communications, which is well within reach oftoday’s technological capabilities, might become one of thefirst practical devices based on nonlinear cascading.The work in Italy was supported by the National Re-search Council~Grants 96.02238.CT07 and 96.02514.CT07!and in the USA by DARPA.1J. Zhou, N. Park, K. J. Vahala, M. A. Newkirk, and B. I. Miller, Electron.Lett. 30, 859 ~1994!; W. Shieh, E. Park, and A. E. Willner, IEEE Photo-nics Technol. Lett.8, 524 ~1996!.2R. J. Manning and D. A. O. Davies, Opt. Lett.19, 889~1994!; T. Durhuus,B. Mikkelson, C. Joergensen, S. Lykke Danielson, and K. E. Stubkjaer, J.Lightwave Technol.14, 942 ~1996!.3S. J. B. Yoo, J. Lightwave Technol.14, 955 ~1996!.4C. Q. Xu, H. Okayama, K. Shinozaki, K. Watanabe, and M. Kawahara,Appl. Phys. Lett.63, 1170~1993!; C. Q. Xu, H. Okayama, and M. Kawa-hara, Appl. Phys. Lett.63, 3559~1993!.5M. L. Bortz, D. Serkland, and M. M. Fejer,Technical Digest of theCLEO’94 Meeting~Optical Society of America, Washington, DC, 1994!,pp. 288–289.6G. I. Stegeman, R. Schiek, L. Torner, W. Torruellas, Y. Baek, D. Baboiu,Z. Wang, E. Van Stryland, D. Hagan, and G. Assanto, inNovel OpticalMaterials and Applications, edited by I. C. Khoo, F. Simoni, and C. Ume-ton ~Wiley, New York, 1997!, pp. 49–76.7N. B. Baranova and B. Ya. Zel’dovitch, Sov. Phys. Dokl.27, 222 ~1982!;Dokl. Akad. Nauk SSSR263, 325 ~1982!.8H. Tan, G. P. Banfi, and A. Tomaselli, Appl. Phys. Lett.63, 2472~1993!.9D. F. Nelson and R. Mikulyak, J. Appl. Phys.45, 3698~1974!.10M. L. Bortz, Ph.D. dissertation, Stanford University, 1994.11K. Kintaka, M. Fujimura, T. Suhara, and H. Nishihara, J. Lightwave Tech-nol. 14, 462 ~1996!.12M. A. Arbore and M. M. Fejer, Opt. Lett.22, 151 ~1997!.13C. Q. Xu, H. Okayama, and T. Kamijoh, Jpn. J. Appl. Phys.34, 1543~1995!.FIG. 5. Contour map response of the two-pass device@Fig. 1~b!# vs L andpump power. The lines are marked with the resulting conversion efficienciesin dB, and in this case also show the enhancement afforded by the counter-propagating configuration.1022 Appl. Phys. Lett., Vol. 71, No. 8, 25 August 1997 Gallo, Assanto, and Stegeman

Efficient Wavelength Shifting Over The Erbium Amplifier Bandwidth Via Cascaded Second Order Processes In Lithium Niobate Waveguides

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Efficient Wavelength Shifting Over The Erbium Amplifier Bandwidth Via Cascaded Second Order Processes In Lithium Niobate Waveguides

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