Holographic bandpass filters are also used to condition the Nd:YAG laser output. S t a t e - o f - t h e - a r t in struments use a holographic t r a n s mission grating filter configuration, which has high transmission (> 90%) and a narrow bandpass. The usable laser power is about 50-75% greater than t h a t obtainable with narrow band interference filters ( 5 0 - 6 0 % transmission). The laser power r e quirement is reduced sufficiently so that a diode-pumped Nd:YAG laser can replace the larger, less stable, and less e n e r g y - e f f i c i e n t l a m p pumped laser in many, and possibly most, FT-Raman applications. Other instrumentation. Many of the filters developed for Raman spec troscopy are equally useful in laserexcited fluorescence spectroscopy. Ogasawara, Yang, and Bobbitt r e cently used an edge filter/monochromator/photomultiplier tube configu ration as the detector system for a f i b e r - o p t i c s e n s o r for r i b o f l a v i n binding protein (20). In this case, as in much of fluorometry, the Stokes shift is great enough that the narrow rejection band of a notch filter is not really necessary. Although moderately wide notches often are adequate for fluorescence, there are important exceptions. The recently developed homodimer stains for nucleic acids have small Stokes shifts (21). In the case of the interca lated form of TOTO, the homodimer of thiazole orange, Xmax(ahB) = 486 nm and λ _ „ ί η ι = 505 nm, a Stokes shift of 19 nm. The oxazole yellow h o m o d i m e r (YOYO) h a s a 2 0 - n m Stokes shift (489 nm to > 509 nm). These stains are nicely matched to the argon ion 488-nm line, and holo graphic rejection filters can improve t h e p e r f o r m a n c e of a r g o n l a s e r based fluorescence detection sys tems. Similarly, in laser fluorescence mi croscopy, whether confocal or wide field, the use of these stains or other materials with low Stokes shifts can benefit from s u b s t i t u t i o n of holo graphic b e a m s p l i t t e r s for conven tional dichroic mirrors. Several labo ratories are investigating such applications. Volume holographic transmission gratings have been fabricated on an experimental basis for many years. The optical engineering l i t e r a t u r e contains many proposals for spectro graphs and monochromators based on these gratings as well as descrip tions of actual experimental systems. In addition to high diffraction effi ciency, t h e a d v a n t a g e s t y p i c a l l y sought are a simple design requiring
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