You will be presented with the full text of a science research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
modern and demanding application of radial velocities , like those associated with the search for extra - solar planets , have promoted the introduction of new methods of wavelength calibration .
the key to success has been recording of the reference wavelength grid for to the whole duration of the science exposure and through the same optical train and path .
two approaches have been followed : the _ parallel _ ( where thorium lines going through telescope optics are recoded on the ccd close and parallel to the science spectrum with an exposure lasting as long as the science observation ) , and the _ iodine - cell _ one ( the stellar spectrum passes through and is absorbed by a cell filled with iodine vapors , that mark the stellar spectrum with a huge number of sharp and equally intense absorption lines ) .
both approaches require delicate instrumentation , careful maintenance and dedicated spectrographs .
as such they are not easily manageable on small , all - purpose telescopes .
we propose here a third , independent way to mark with reference wavelengths the science spectrum as it is recorded , through exactly the same optical train and path .
it consists of using optical fibers with recorded in them bragg gratings that allows undisturbed passage of all wavelengths except a selection of few ones that will appear in the spectrum as sharp , deep absorption lines of constant and pre - selected wavelengths .
the latter are then used as a reference grid to accurately wavelength calibrate the science spectrum .
the system is easy and fast , allows to decouple the spectrograph from the telescope and operate it in a controlled environment , requires minimal mechanical work to be adapted to existing spectrographs , the marking absorption lines can be placed at any preferred wavelength , and the fibers can be mass - produced at low cost for distribution to interested observatories .
fbgs can be easily installed on spectrographs already designed to operate in multiple fiber mode .
the rave project ( steinmetz 2003 ) , with its fixed wavelength range , would represent an ideal case for the application of fbgs .
the fbgs ( fiber bragg grating ) appear of potential interest also to gaia .
the latter s spectrograph operates in tdi , slit - less mode with thousands of spectra simultaneously crossing the field of view as the satellite spins .
no conventional calibration lamp can be used , obviously .
the base - lined wavelength calibration procedure foresees using stars of recognized constant radial velocity as they travel across the spectrograph focal plane , by accurately linking their astrometric position on the sky to the wavelength of their absorption lines on the pixels of the ccds .    a bundle of fbgs properly introduced in the gaia spectrograph optical train would mark with a set of properly placed reference absorption lines the spectra of all stars entering the field of view .
the fbgs could become the prime wavelength calibration mean of gaia spectra ( particularly for preliminary solutions while the mission is still flying ) , or a backup for the method based on standard radial velocity stars in case this would not satisfactorily operate for whatever reason once the satellite will reach l@xmath0 .
the fbgs dissipate no power and have no movable parts .
they should also stand the low operating temperatures of the satellite in l@xmath0 and should do not degrade with time , and thus looking ideal for use in space .
no fbg has ever been used as proposed in this paper , and thus confirmatory tests are required . at the astronomical observatory of padova
we plan to manufacture and test prototype fibers for the gaia / rave wavelength range to check their performance with actual observations at the telescope .
fiber gratings are made by laterally exposing the core of the fiber to a periodic pattern of intense ultraviolet light .
the exposure produces a permanent increase in the refractive index of the fiber s core , creating a fixed index modulation according to the exposure pattern : this fixed pattern is called a grating . at each periodic change of refraction , a small amount of light is reflected ( bragg diffraction ) .
all the reflected light signals combine coherently to one large reflection at a particular wavelength when the grating period is approximately half the input light wavelength .
this is referred as the bragg condition and the wavelength at which this reflection occurs is called the bragg wavelength ( @xmath1 ) .
light at wavelengths different from @xmath1 , which are not phase matched , will pass unabsorbed through the fiber . thus , light propagates through the grating with negligible attenuation and only those wavelengths that satisfy the bragg condition are affected and strongly back - reflected .
application of this technology to astronomical spectra at gaia wavelengths is attractive for two basic reasons : ( @xmath2 ) the technology is already mature with the tele - communication industry that operate it in the near - ir , down to 1  @xmath3 m , and ( @xmath4 ) the limited @xmath5 range observed by gaia and rave requires only a limited number of reference lines to be properly calibrated ( see discussion in desidera and munari 2003 ) , which in turn means an easier fbgs production .
exposing a photosensitive fiber , usually germanium - doped ( yuen , 1982 ) , to an intensity pattern of ultraviolet radiation ( at wavelengths between 240 and 255 nm , where the germanium has an absorption band ) is used to produce ( i.e. write ) a fiber bragg grating . in its basic form
the grating selectively reflects light at the bragg wavelength @xmath6 , where @xmath7 is the grating spacing and @xmath8 is the effective index of refraction .
both @xmath7 and @xmath8 depend on fabrication parameters : @xmath8 depend on the modal dispersion characteristics of the fiber and lies , for each mode , between the core and cladding indices .
the functional form of the normalized frequency ( @xmath9 ) parameter is given by : @xmath10 where @xmath11 is the diameter of the fiber core , @xmath12 is the free space wavelength , @xmath13 the core index and @xmath14 the clad index .
note that the difference between the two squared index is also the numerical aperture of the fiber . for low enough @xmath9 values
the fiber is single mode and only one bragg reflection is observed at @xmath15 wavelength given above . at a certain value of @xmath9 the fiber become two moded with two separated bragg wavelengths , and for larger @xmath9 values many more modes begin to appear in the fiber grating spectrum .
the lowest order or fundamental mode has the longest @xmath1 , and the higher order modes have progressively shorter @xmath1 with the cladding index giving the limit on the shortest @xmath1 .
the peak reflectivity is an increasing function of the grating length and of the difference between @xmath13 and @xmath14 ( lam & garside , 1981 ) .    recording fbgs inside a fiber usually involves interference between two coherent uv laser beams that precisely define the spacing between the grating planes . for symmetric incidence and an angle @xmath16 between the uv beams ,
the pitch of the recorded grating is 0.5@xmath17 , where @xmath12 is the illumination wavelength .
the basic interferometric setup splits the laser beam to create an interference pattern on the fiber ( morey et al .
, 1989 ) as shown in figure 2 ( left panel ) .
because the beams travel different paths , the laser must have sufficient temporal coherence to account for path - length mismatch , and good spatial coherence .
the sensitivity to mechanical instabilities of this method of fbg production can make difficult to produce identical replicas .
the other way for writing fbgs is to use phase mask to generate multiple beams ( hill et al . , 1993 ) .
two of the beams carry approximately 40% of the total energy and closely overlap the phase mask surface to create an interference pattern at the desired bragg wavelength .
the fiber - phase mask assembly can be illuminated either by a large beam to cover the full fiber bragg grating length or by a small scanning beam . because the fiber is usually in proximity to the phase mask , the assembly is a very stable mechanical system , suitable for mass - production of identical fbg replicas .
the pattern recorded into the fiber is a copy of the phase mask ( pitch and chirp ) scaled by about 50% .
desidera , s. , munari , u. , 2003 , this volume , pag .
xxx hill k. , malo b. , bilodeau f. , johnson d. 1993 , appl.phys.lett .
62 ( 10 ) , 1035 lam d.k.w . ,
garside 1981 , appl .
optics , 20 ( 3 ) , 440 morey w.w . , meltz g. , glenn w.h .
1989 , proc .
spie , 1169 , 98 steinmetz m. , 2003 , this volume , pag .
xxx ( and http://www.aip.de./rave/ ) yuen m.j .
1982 , appl .
optics , 21 ( 1 ) , 136
we propose a new technique , the use of fbgs ( fiber bragg gratings ) , for accurate , easy and low cost wavelength calibration of gaia , rave and follow - ups spectra at local observatories . 
 fbgs mark the spectra with absorption lines , freely defined in number and position during the fibers manufacturing . 
 the process goes in parallel with the science exposure and through the same optical train and path , thus ensuring the maximum return in wavelength calibration accuracy . 
 plans to manufacture and test fbgs for the caii / paschen region are underway at the astronomical observatory of padova .    
 # 1_#1 _ # 1_#1 _ =    # 1 1.25 in .125 in .25 in