Patent Application: US-2412701-A

Abstract:
in the basic diode - pumped alkali laser device , excitation to the n 2 p 3 / 2 electronic level by a single diode laser pump source leads to a population inversion between the first excited electronic 2 p 1 / 2 level and the ground 2 s 1 / 2 level , permitting the construction of efficient , high - power , compact dpal laser oscillators in the near infrared spectral region . the present invention extends the single - step excitation dpal to a two - step excitation , or up - conversion dpal to produce efficient , powerful laser operation in the visible blue and near uv spectral regions . the present invention describes an apparatus and method that efficiently sums the energy of two , near - infrared diode pump photons in alkali vapor atoms , followed by stimulated emission to their electronic ground levels .

Description:
the energy level scheme for the up - conversion dpal ( or uc - dpal ) is shown in fig1 . in the basic infrared dpal [ 10 ], only the ground and first two excited energy levels are involved in laser action . in the uc - dpal device , additional higher lying electronic levels are involved in generating visible laser emission . in fig1 n is the principal quantum number of the alkali atoms ( n = 2 , 3 , 4 , 5 , 6 for lithium , sodium , potassium , rubidium , and cesium , respectively ). in the uc - dpal device , two diode pump sources are utilized . the first pump , p 1 , is set to the wavelength of either of the first resonance ( so - called ) d 1 or d 2 transition wavelengths ( d 1 : n 2 s 1 / 2 − n 2 p 1 / 2 , or d 2 : n 2 s 1 / 2 − n 2 p 3 / 2 ). the second pump , p 2 , is set to a wavelength that equals the wavelength of a transition between either the n 2 p 1 / 2 level or the n 2 p 3 / 2 level , and the n 2 d 3 / 2 level ( or possibly another 2 d j level , not shown ). with both pump excitation sources present , alkali atoms are successively excited from the ground n 2 s 1 / 2 electronic level , into either the n 2 p 1 / 2 or n 2 p 3 / 2 levels , and subsequently into the n 2 d 3 / 2 level . in the presence of an appropriate buffer gas mixture ( see below ), the alkali atom populations excited to the n 2 p 1 / 2 and n 2 p 3 / 2 levels come into thermal equilibrium with each other , characterized by a temperature equal to that of the buffer gas , due to rapid collisional mixing ( exchange of energy ) between these levels by the buffer gas . similarly , due to collisional mixing among the n 2 d 3 / 2 , n + 1 2 p 1 / 2 and n + 1 2 p 3 / 2 levels due to presence of an appropriate buffer gas , the alkali atom population excited by the second step pump rapidly comes to thermal equilibrium with the latter two levels , characterized by the temperature of the buffer gas . with the appropriate excitation fluxes from the first and second diode pump sources , a population inversion is generated between the n + 1 2 p 1 / 2 and n + 1 2 p 3 / 2 levels and the ground n 2 s 1 / 2 level . when the doubly - excited alkali / buffer - gas mixture is contained with an appropriate laser cavity , laser action is generated in either of the two “ second series d ′- transitions ” of the alkali atoms : d 1 ′ : n + 1 2 p 1 / 2 − n 2 s 1 / 2 ; d 2 : n + 1 2 p 3 / 2 − n 2 s 1 / 2 . the required pump fluxes for efficient laser action in the d 1 or d 2 , transitions of an alkali atom depends directly on the collision - broadened spectral widths , effective transition cross - sections , and the saturation fluxes of the pump and laser transitions , which in turn depend on the type and partial pressures of the buffer gases utilized . the spectroscopic properties of the first and second series d - transitions of the alkali vapor atoms have been extensively studied [ 11 ], first as model systems of atomic structure , and more recently as preferred species for producing bose - einstein condensates . likewise , also extensively studied have been the collisional effects of all of the rare - gases and selected molecular gases on the spectroscopic and population kinetics of excited alkali atoms , including spectral broadening of the d - line transitions [ 11 - 15 ], collisional mixing rates of excited 2 p 1 / 2 , 3 / 2 alkali atoms [ 16 - 23 ], and inelastic quenching rates of excited alkali atoms [ 24 ]. table 1 gives a summary of the electronic level energies and corresponding transition wavelengths relevant to uc dpal devices , for each of the five alkali atoms . from table 1 , it is noted ( in italics ) that the demand pump wavelengths for the cesium and rubidium uc - dpals lie in the 762 - 921 nm spectral range , for which powerful and efficient high power laser diode and diode arrays are commercially available . therefore , these particular alkali atoms are preferred alkali atoms for practical uc - dpal devices . the basic functional elements of an uc - dpal device are shown in fig2 . the uc - dpal laser gain cell 8 contains the laser active alkali vapor and an appropriate buffer gas ( e . g ., a mixture of a rare gas such as helium and a selected molecular gas a such as ethane ). generally , the gain cell 8 will have a length “ l ” and transverse cross - section that is generally circularly symmetric with radius “ r ”, and having an aspect ratio , l / r , of typically & gt ; 10 . however , the cross - sectional shape of the gain cell may take many forms ( circular , square , rectangular , or higher polygonal form ) and may also be designated as a tube , capillary , hollow - waveguide , etc . the gain cell is fitted with flat optical windows 9 and 10 at either end so as to contain the alkali atomic vapor . the cell windows 9 and 10 are coated on their exterior surfaces with a multilayer dielectric stack to form an anti - reflection coating at both of the pumping wavelengths , and at the operating laser wavelength ( either put lambda here l 03 or l 04 ) of the uc - dpal . the diode pump sources 1 and 2 are collimated with lenses 3 and 4 , respectively , spatially combined by the thin - film polarizer or dichroic beam combiner optic 11 , and focused into the laser gain cell by lens 5 , through laser cavity mirror 6 . the laser cavity mirror 6 is coated with a dielectric stack that produces high reflectivity at the uc - dpal laser wavelength , and high transmission in the near infrared wavelengths of the two pumps . the laser cavity output mirror 7 is coated with a dielectric stack that highly reflects in the near infrared at the two pump wavelengths , and partially transmits at the uc - dpal laser wavelength ( with a reflectivity that optimizes the conversion efficiency of diode pump light to blue laser output ). the main purpose of the buffer gas is two - fold : 1 ) the buffer gas collisionally broadens the optical transitions , renders the transitions spectrally homogeneous with predominantly lorentzian lineshapes , and facilitates increased spectral coupling between the pump radiation and alkali atom absorption ; and 2 ), the buffer gas collisionally relaxes doubly - excited alkali atoms from the 2 d 3 / 2 level to the n + 1 2 p 1 / 2 and 2 p 3 / 2 levels ( the upper laser levels for the two uc - dpal laser transitions , n + 1 2 p 3 / 2 − n 2 s 1 / 2 or the n + 1 2 p 1 / 2 n 2 s 1 / 2 . the buffer gas composition and density is chosen so that this relaxation rate substantially exceeds the radiative relaxation rate of the 2 d 3 / 2 level . for example , using atomic collision data from the literature [ 11 - 24 ], a suitable buffer gas mixture for a cesium uc - dpal is ˜ 1 - 2 atm of helium and ˜ 0 . 1 atm of ethane . such a buffer mixture can sufficiently relax the excited populations to the desired levels before significant undesired radiative emission takes place . up - conversion laser action can be efficient in the uc - dpal because the ground level population can be readily bleached using commercially available laser diodes , and substantial population inversions ( and small signal gain ) can be produced on the n + 1 2 p 3 / 2 − n 2 s 1 / 2 and the n + 1 2 p 1 / 2 − n 2 s 1 / 2 transitions . to gain insight into the laser performance of uc - dpal devices , it is necessary to construct a computer code capable of tracking alkali level populations in all of the relevant energy levels ( those strongly coupled together by the radiation fields of the two spatially - overlapping pump beams , and by the simultaneous oscillation and saturation of level populations due to laser action within the laser cavity [ 25 ]). it is because the magnitudes of the collisionally - broadened alkali atom pump transition peak cross - sections are large (& gt ; 10 − 13 cm 2 ), and that the corresponding population saturation fluxes are small (˜ 30 watts / cm 2 , relative to pump fluxes available with pump laser diodes ) that substantial and practical amounts of pump radiation absorption can be achieved within the lorentzian wings of the transitions , thus enabling practical uc - dpal designs . 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[ 24 ] e . speller , b . staudenmayer , and v . kempter , “ quenching cross sections for alkali - inert gas collisions ”, z . phys ., a291 , 311 ( 1979 ). [ 25 ] r . j . beach , “ cw theory of quasi - three - level end - pumped laser oscillators ”, opt commun ., 123 , 385 ( 1995 ). the foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed . many modifications and variations are possible in light of the above teaching . the embodiments were chosen and described to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best use the invention in various embodiments and with various modifications suited to the particular use contemplated . the scope of the invention is to be defined by the following claims .