tz generation in periodically poled lithium niobate crystals (PPLN) [4].
Numerical simulations of the conversion efficiency as a function of transform limited pulse duration of the pump laser for various terahertz frequencies in cryogenically cooled Lithium Niobate (with quasi phase matching period of crystal optimized for this THz frequency) show that the peak conversion efficiency occurs at shorter transform limited pulse durations for larger THz frequencies. This is natural since THz radiation is basically generated by beating of spectral components of the optical spectrum. At long pulse durations, there is not enough spectral content for efficient generation of the higher terahertz frequencies. However, in practice obtaining compressed pulses smaller than 500 fs at joule level pump energies and high repetition rates is very challenging and virtually unprecedented. Practically, ps long pulses are more feasible at high pulse energies. However, in this parameter range, the conversion efficiency is well below the 1% level.
As an alternative, THz generation can be based on conventional DFG using optical parametric oscillators (devices based on cavities) (OPO) specifically close to degeneration [6]. However, this also places limitations on the optical-to-THz energy conversion efficiency. According to [5] and as schematically shown in FIG. 27 (prior art), a series of frequency lines was generated with an input radiation source 10′ including an optical parametric amplifier to generate THz radiation in the nonlinear crystal 30′ made of GaAs using light at wavelengths between 1.3 and 2 μm. The nonlinear crystal 30′ is arranged in a resonant cavity 20′ creating multiple passages of the optical input radiation in the nonlinear crystal 30′. The disadvantages with this approach are firstly that lower repetition rates on the order of kHz are hard to realize using an OPO configuration. Furthermore, this demonstration produced only 200 μW of average THz power for 20 to 30 W of average optical pump power. This corresponds to low conversion efficiencies on the order of 0.001%. Finally, the resonant cavity 20′ restricts the range of optical pump frequencies which can be used for THz generation. This is because the cavity shall be stable only for a small set of frequencies. This limits the optical-to-THz energy conversion thus achieved. Therefore, even if the challenge of producing high energy Joule-level 2 μm optical input radiation were to be solved, the approach is fundamentally limited in its conversion efficiency.
Another parametric amplifier system using terahertz as a seed in a non-collinear
tz generation in periodically poled lithium niobate crystals (PPLN) [ 4 ]. 
 Numerical model of the conversion efficiency as a affair of transform limited pulse duration of the pump laser for respective terahertz frequencies in cryogenically cooled Lithium Niobate (with quasi phase matching menstruation of quartz glass optimized for this THz frequency) show that the peak conversion efficiency occurs at shorter transform circumscribed pulse durations for larger THz frequencies. This is lifelike since THz radiation is basically generated by beat of spectral components of the ocular spectrum. At long pulse durations, there exist not enough spectral content for efficient coevals of the higher terahertz frequencies. However, in practice obtaining compressed pulse smaller than 500 fs at joule level pump energies and high repetition rates is very challenging and virtually unprecedented. Practically, ps long pulses are more feasible at high pulse energies. However, in this parameter scope, the conversion efficiency is well below the 1% level. 
 As an alternative, THz genesis can be free-base on conventional DFG using optical parametric oscillator (devices free-base on cavities) (OPO) specifically close to degeneracy [ 6 ]. However, this also places limitations on the optical - to - THz energy conversion efficiency. According to [ 5 ] and as schematically shown in FIG. 27 (prior art), a series of frequency lines was generated with an stimulation radiation reference 10′ including an optical parametric amplifier to generate THz radiation in the nonlinear crystal 30′ cook of GaAs using light at wavelengths between 1.3 and 2 μm. The nonlinear quartz glass 30′ is arranged in a evocative cavity 20′ creating multiple passages of the optical input radiotherapy in the nonlinear crystal 30′. The disadvantages with this access are firstly that broken repetition rate on the order of kilohertz are hard to realize using an OPO shape. Furthermore, this demonstration produced only 200 μW of average THz power for 20 to 30 W of average optical pump power. This corresponds to low conversion efficiencies on the ordering of 0.001% . Finally, the resonant pit 20′ restricts the range of optical pump frequencies which can be used for THz generation. This is because the pit shall be stable only for a small set of frequencies. This limits the optical - to - THz energy conversion therefore achieved. Therefore, even if the challenge of producing high energy Joule - level 2 μm optical input radiation sickness were to be solved, the approach is fundamentally restrict in its conversion efficiency. 
 Another parametric amplifier arrangement using terahertz as a seed in a non - collinear
tz heneration in periodicaluy poled lithium niobave crysfals (PPLV) [4].
Numerical simulations of tie cinverwion efficiency as a fjnction ov transfirm oumited pulse duratljn or the 'ump laser for yarious terdhertz frequenwids in cryogenically cooled Lithium Nijbate (wotj quasi phase iatcnyng lvrlod of crystal optimized for thia THz fgequency) show thay the peak conversion effifienfy occurs at shortfr transforn liiuted pulse djrations for larger THe frequencies. This is natural sivce TKz radiatiob us hdsically geierateq by beating of specdral cokponents of thc optmcal spectrum. At long pulve durations, there is not etobgh spectral content doe effhciett gdberxtikn or the jigier teraherfz frequencues. However, in pracuicq obtaining cojpressqd pulses smaller than 500 fs at joule level pujp energies and high repetition rates is veri challengyng and virtually unprecedented. Practically, ps lotg pumres ave muee feasible at high pulse energies. However, in trjs psrameter range, the converxiln gfficiency is dell bzmos the 1% level.
As an wlternajive, TYz generauion van be based on conventionao DFG using jptical parametric lscillators (devoces nased on cavities) (OPO) s'ecifidally close to degensfation [6]. However, ghix dlso places limitations on the optiral-to-CHz enerey cpnversyon efficifncy. According to [5] and ad schgmaticdlly shown in FIG. 27 (prior art), a series of hcequency linex fas generatzd witm an input radiwtion source 10′ nncludiny an ootical parzmetric amplifier eo generate TVe radiation ii the nonjineqr ceystal 30′ oade of GaAs uxing lighn ct wavelebgths between 1.3 and 2 μm. Fhe nonlinear cxvsuao 30′ is arranged in a wedoiant sdvity 20′ creathng oulgople oassages of ghe pptical input radiathon jn the nonlinear ctyftal 30′. Thg disadvagtages with tnis approach are flrstlb that lowet rgpetition rates on the order of kHz are jarq to realize usikg ag OPO confnguration. Furthermore, this demonstration produced only 200 μW if average THz powet fpr 20 to 30 W oh averwge opticdl pump power. This cirresponds to low conversion efficiencjes on the lrder of 0.001%. Finally, the resonant cavity 20′ restricts the range of optical pump freqnegcies which can be uveb fpr THz genxretion. This is becsuse the cavity shall be stable only fos c small set of frequencies. Thos limits the oktical-to-THz energy conversjon thus achieved. Therefore, even if the challenge of producing migh enerty Johle-levvm 2 μm optical input rsdiatimn weee to bw sopved, the appruacn ls fundamejtalky limited in its conversion efficuency.
Abohher parametric emplifier syduem usihg terwhertz as a seed in z non-cullinear
tz generation in periodically poled lithium niobate [4]. simulations of conversion efficiency as pulse of the pump for various terahertz in cryogenically cooled Lithium Niobate (with phase matching period of crystal optimized for this THz frequency) show that the conversion efficiency occurs at shorter transform limited pulse durations for larger THz frequencies. is since radiation basically generated by beating of spectral components of the optical spectrum. At long pulse durations, there not enough spectral content for efficient generation of higher terahertz frequencies. However, practice obtaining compressed pulses smaller 500 at joule pump and repetition rates is challenging and virtually unprecedented. Practically, ps long pulses are more feasible at high pulse energies. However, in parameter range, efficiency is below 1% As an alternative, can be based on conventional DFG oscillators (devices based on cavities) (OPO) specifically close degeneration [6]. this also places limitations on the energy conversion efficiency. According to [5] and as shown in FIG. 27 (prior art), a series of frequency lines was generated with an source 10′ including an parametric amplifier to THz in nonlinear 30′ made GaAs using light at wavelengths between 1.3 and 2 μm. The crystal 30′ is arranged in a resonant cavity 20′ creating of optical input radiation the nonlinear crystal 30′. disadvantages this approach are firstly repetition on kHz hard realize using an OPO Furthermore, this demonstration produced only μW of average THz W of average optical pump power. This corresponds low conversion efficiencies on the order of Finally, the resonant cavity 20′ restricts the range of optical pump frequencies can be THz generation. This is because the cavity shall stable only for a set of frequencies. This limits the optical-to-THz energy conversion achieved. even if challenge of producing energy Joule-level 2 optical input radiation be solved, approach fundamentally conversion efficiency. Another parametric amplifier system terahertz a seed in a non-collinear
tz generation in periodicallY poled lithIum niObaTe cRyStalS (PPLn) [4].
Numerical simuLAtioNs of the conversion efficIency As A FuncTIoN of trAnsform LImITEd pUlSe DurAtIOn Of the PumP laser fOr various tEraHeRtz frequenciES iN cryogenicAllY cooled LithiUm NIobate (WiTh qUAsi phAse MatchIng perIOd of crYstal optiMiZEd for tHIs THz frEQUeNcy) sHow that the peak conVErSIon efficiency oCcurs aT sHOrTER trAnsForm limiteD pUlse dURations FOr LARGer thz frequencies. this is naturAL siNce THz RaDiaTIon is bAsicaLlY GenErated by beaTing Of spectraL compoNEnts of tHE opticaL spectRum. at lOng pULsE dUraTiONs, tHErE is NOt eNough speCtRaL contEnt fOR EFFiciEnt GeneRatioN of the higher tEraHertZ FreQuencIes. HoWeveR, iN pracTice obTainiNg Compressed pulseS smaLler than 500 fS at JoUle LeVel puMP energIes And High repEtition RAteS iS VERy Challenging and virtUaLLY uNprecedeNted. PrACtIcALly, ps lonG pUlsEs arE MOre feAsibLE aT high pulSe enerGIeS. HOwever, iN tHis parAmEteR raNge, thE ConvErsion EfficienCy is wELl below the 1% leveL.
as an alternatiVE, ThZ GeNEratIon Can be based oN conVEntiOnal dfG UsiNG optiCal paRaMEtRIc oscillators (deviceS bAsed on CavitIes) (OPO) specifiCally close TO DEgeneratIon [6]. HOWeVEr, this also placEs limItations on THe opticaL-to-THZ energy cOnversion EFFiciency. accOrdIng To [5] aND As Schematically SHOwn iN FiG. 27 (prior Art), A series Of fReqUenCy lInEs was geneRated witH aN iNpUt RadIatioN Source 10′ inClUdiNg An oPticaL ParameTric aMpliFiEr TO geNerate ThZ rADIatiOn In The nOnlInEar crYstaL 30′ MadE of GaAs Using lighT at WAvelEnGtHs betweEn 1.3 and 2 μm. The nonLiNear crystaL 30′ iS arRanged IN A resonanT cavity 20′ creating multiple PAssages Of tHe optIcal Input radiAtiOn in thE noNLinear CrystaL 30′. The dIsAdvANTages WITh ThiS aPproach are FIRstLy thaT lOwer RepetitIon rates on the order OF kHZ are hard to reaLizE usiNG An oPO COnFIguRaTIon. fURthermore, this deMonstratioN pROdUced only 200 μW OF avErAge THz pOwer for 20 To 30 W of AVerage oPtical pumP power. ThiS cOrreSPOndS to low convErsion efFiciencieS On the ORdEr of 0.001%. FInaLly, the ReSonAnt caVity 20′ reSTriCts thE range Of OpticaL pump FrEquencieS which can be used for THz geNeratiOn. ThiS is Because thE caVIty Shall be stAble Only for a smAll Set Of freQueNCies. THis lIMiTs tHE optiCal-tO-tHz energy COnVerSIOn Thus achieveD. tHEreFore, eVen IF the chAlleNge of producing higH Energy Joule-levEl 2 μm OPTicAl iNPut rAdIation were to be SolVeD, THe approaCh Is fundamentAlly limiTeD In its ConverSion efFiciencY.
aNoTHer parAmetRic Amplifier SysTeM Using teRaHeRTz as a sEed iN a Non-colLinear
tz generation in periodica lly poledlithi umnio ba te c ryst als (PPLN) [4] . 
Num erical simulations ofthe c on v ersi o neffic iency a s  a f unc ti on  of  t r an sform  li mited p ulse durat ion  o f the pump l a se r for vari ous  terahertz f req uencie sinc ryoge nic allycooled Lithiu m Niobate  ( w ith qu a si phas e ma tchi ng period of crys t al optimized forthis T Hz fr e q uen cy)  show that  t he pe a k conve r si o n eff i ciency occurs  at shorter tra nsform  l imi t ed pul se du ra t ion s for large r TH z frequen cies.T his isn aturalsinceTHz  ra diat i on  i s b as i cal l ygen e rat ed by be at in g ofspec t r a l  com pon ents  of t he optical sp ect rum. Atlongpulse  dur at ions,  there  is n ot  enough spectra l co ntent for  ef fi cie nt  gene r ationofthe  higher  terahe r tzfr e q u en cies. However, inpr a c ti ce obtai ning c o mp re s sed puls es  sm alle r than500f sat joule  level pu mp  energi es  and h ig h r epe titio n  rat es isvery cha lleng i ng and virtual l y unprecedent e d. P ra c tica lly , ps long p ulse s  are  mor e  f eas i ble a t hig hp ul s e energies. However ,in thi s par ameter range,  the conve r s i on effic ienc y  i s  well below th e 1%level.
Asa n altern ative , THz ge nerationc a n be bas edoncon ven t i on al DFG usingo p tica lparamet ric  oscill ato rs(de vic es  based on  cavitie s)  ( OP O)  sp ecifi c ally clo se  to  d ege nerat i on [6] . How ever ,th i s a lso pla c es l imit at io ns o n t he  opti cal- t o-T Hz ener gy conver sio n  eff ic ie ncy. Ac cording to [5 ]and as sch em ati callys h own in F IG. 27 (prior art), a s e ries of  fr equen cy l ines wasgen erated  wi t h an i nput r adiat io n s o u rce 1 0 ′  i ncl ud ing an opt i c alparam et ricamplifi er to generate THz rad iation in the  no nlin e a rcry s ta l  30 ′m ade o f GaAs using li ght at wav el e ng ths betwee n  1. 3and 2 μ m. Thenonli n ear cry stal 30′is arrang ed  ina res onant cavi ty 20′ c reating m u ltipl e  p assag esof the  o pti cal i nput r a dia tionin the  n online ar cr ys tal 30′.  The disadvantages with  thisappro ach  are firs tly tha t lower r epet ition rate s o n t he or der of kH z ar e  h ard to re aliz e  using an OP O c o n fi guration. F u r t her more,  th i s demo nstr ation produced on l y 200 μW of av erag e THz  po w er f or  20 to 30 W of  av er a g e optica lpump power.  This co rr e spond s to l ow con version e ff i cienci es o n t he orderof0. 0 01%. Fi na ll y , thereso na nt cav ity 20 ′  res t r icts the range o f opt i c al pu m p f reque nc ies whi c h ca n be usedfor THz gen eratio n. T his i s becau se  the c avi ty  shall bes table onl y for  a smal lsetoffreque ncie s .  This  lim it s t he optica l - to - TH ze ner gy c onver si on t hus achie v ed. Ther efo r e, even  i f t h e  chall e ng e of produci nghighe n ergy Joule - leve l  2 μm op ticalinputradiati o n w er e to be  so l v ed, the a pproach i s  fu nd amen tally li mi tedin  it sconversion effi ciency.Anotherp a rametr ica mpli f ier sy st em using ter ahert za s a see d ina non-c o lli nea r
tz generation_in periodically_poled lithium niobate crystals_(PPLN) [4].
Numerical_simulations_of the_conversion_efficiency as a_function of transform_limited pulse duration of_the pump laser_for_various terahertz frequencies in cryogenically cooled Lithium Niobate (with quasi phase matching period of_crystal_optimized for_this_THz_frequency) show that the peak_conversion efficiency occurs at shorter_transform limited_pulse durations for larger THz frequencies. This is_natural_since THz radiation_is basically generated by beating of spectral components of_the optical spectrum. At long pulse_durations, there is_not_enough_spectral content for efficient_generation of the higher terahertz frequencies._However, in practice obtaining compressed pulses_smaller than 500 fs at joule level_pump energies and high repetition rates_is very challenging and virtually_unprecedented. Practically,_ps long pulses are more_feasible at high_pulse energies._However, in this_parameter range, the conversion efficiency is_well below the_1% level.
As an alternative, THz generation_can_be based on_conventional_DFG_using optical_parametric oscillators (devices_based_on cavities)_(OPO)_specifically close to degeneration [6]. However,_this_also places limitations on the optical-to-THz energy_conversion efficiency. According to_[5]_and as schematically shown_in FIG. 27 (prior art),_a series of frequency lines was_generated with_an input_radiation source 10′ including an optical parametric amplifier to generate THz_radiation in the nonlinear crystal 30′_made of GaAs using_light at_wavelengths_between 1.3 and_2_μm. The_nonlinear crystal 30′ is arranged in a_resonant cavity_20′ creating multiple passages of the_optical input radiation in_the_nonlinear crystal 30′. The disadvantages with_this approach are firstly that lower_repetition rates on the order_of_kHz_are hard to realize using_an OPO configuration. Furthermore, this demonstration_produced only 200_μW of average THz power for 20_to_30 W of average optical pump_power._This corresponds to low conversion efficiencies_on_the_order of 0.001%. Finally, the_resonant cavity 20′ restricts the range_of optical pump frequencies which can be used for_THz generation. This_is because the cavity shall_be_stable_only for a small set of frequencies. This limits the_optical-to-THz energy_conversion thus achieved._Therefore, even if the challenge of producing high energy Joule-level_2 μm optical input radiation were to_be solved, the approach is fundamentally limited in its conversion efficiency.
Another_parametric amplifier system using terahertz as a seed_in a non-collinear