INSTRUMENTATION
SDectroscoDic ADDlications of Uolume HolograDhic ODtics James M. Tedesco and Harry Owen Kaiser Optical Systems, Inc. P.O. Box 983
Ann Arbor, MI 48106
David M. Pallister and Michael D. Morris Department of Chemistry University of Michigan Ann Arbor, MI 48109
Holographic optical elements (HOES) are finding new application in spectroscopic instruments, serving a broad range of optical functions previously provided by other optical technologies. In this INSTRUMENTATION we will review the operating principles and several new applications for a particular family of HOEs, called “volume” HOEs, in spectroscopic instrumentation. Unlike conventional surface -relief holographic gratings t h a t have been used i n spectroscopy for many years, volume HOEs function on the principle of volume or Bragg diffraction. Although volume holograms have been used for several y e a r s i n head-up displays (1,21,3D imagery, laser eye protection (3,41, and fiheroptic multiplexers, their widespread application in spectroscopy is relatively recent. Their unique combination of high efficiency, controllable spectral response, and low scattering is useful in high-performance components such as laser notch filters, laser bandpass filters, laser heamsplitters, and dispersive spectrograph “gratings” for spectroscopy ( 5 ) . 0003-270019310365-441A1$04.0010 0 1993 American Chemical Society
We will review me opricai principles of these applications and present experimental results comparing the performance of volume HOEs with the component technologies they replace. Fundamentals of HOEs We begin with a brief and qualitative introductory background to enable readers to understand the specific spectroscopic applications of HOEs that will be discussed in the following sections. I t is not our intent to provide a comprehensive tutorial on Bragg diffraction or HOEs; interested readers should consult the listed references. Transient holographic gratings and acousto-optic tunable filters, which have analytical applications that have been reviewed in the A pages of this JOURNAL (6, 7),are other systems that operate by Bragg diffraction.
Surface-relief holographic gratings. Since the late 1960s holographic gratings have displaced ruled gratings in monochromators and spectrographs. These conventional holographic gratings consist of a metallized surface-relief pattern that is holographically generated in photoresist by the interference of two mutually coherent laser beams. Normally, coarse pitch holographic gratings are manufactured as replicated or embossed versions of holographically produced masters. Similarly, conventionally ruled gratings normally are constructed as masters for replication. Holographic gratings, both masters and replicates, yield optical performance superior to that of mechanically ruled gratings, which consist of patterns machined into a surface by a precision ruling engine. The mechanical ruling process yields higher scattering and poorer periodicity than do holographic gratings, although somewhat higher peak efficiency can be achieved with mechanical ruling. In addition, holographic gratings can he fabricated on curved surfaces to yield spectrographic configurations that may be simpler and have lower aberrations than those based on plane gratings. In a surface-relief grating, the periodicity of the surface structure determines the dispersion of the device. The depth and shape of t h e surface relief pattern determine its diffraction efficiency a t any wavelength. The metallized surface relief reflects close to 90% of the incident light-some by specular reflection,
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INS7RUMENTATON some into the desired fxst diffracted order, and some into higher (usually undesirable) orders. Volume phase HOES. Volume holographic elements operate on very different physical principles. As the name suggests, the diffraction from such elements is a volume effect as opposed to a surface phase effect, although both are predicted by Maxwell’s equations of classical electromagnetic theory. Volume holograms physically consist of fringes formed by materials of periodically varying refractive index (or the correlated variable, hardness) throughout the volume of a n optically thick film, which is at least several wavelengths deep. The refractive index (RI) is modulated in a nominally sinusoidal profile over a range of as much as 0.2 units about a nominal film RI of -1.5. Volume holograms have been conveniently analyzed by using coupled wave theory (8). taking into account parameters such as film thickness, RI variation, absorption, fringe period, and fringe tilt. Volume diffraction can result in
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either transmission or reflection, depending on the fringe geometry. Important cases are shown schematically in the configuration matrix of Figure 1. Figure l a shows for reference a conventional surface-relief grating, which may be either holographic or ruled. Such a grating uormally i s reflective because i t i s formed on a metal surface. However, the same pattern formed on the surface of a transparent material produces a similarly dispersive transmission grating. Although it is rarely encountered in modern instruments, this transmission grating is often treated in introductory textbook discussions of diffraction. Conformal reflection holograms. Figure l b represents a “conformal’’ volume reflection hologram, so named because ita fringes are conformal or parallel to the surface. Perhaps the most widely applied volume HOE configuration, it is used for notch filters and beamsplitters. The magnitude of spectral dispersion in any surface or volume grating structure, holographic or other-
Conventionalblazed diffractiongrating (Reflectivesulface relief)
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Flgure ;on of conventional surface-relief grating used in spe (a) with various volume holographic element configurations (b-e). 442 A
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wise, is directly proportional to the spatial frequency or pitch of t h e grating as measured parallel to its surface. The conformal reflection hologram is the only type that is completely nondispersive, because the planes of equal RI are parallel to the surface; that is, the hologram has zero spatial frequency or pitch along its surface. The spectral content of the incident light is not angularly separated upon reflection from the hologram. All diffracted light is reflected about the surface normal, as from a conventional mirror. The conformal reflection hologram is the simplest to record in practice. It is essentially a coherent contact copy of a mirror surface. It is readily made in thick films (> 100 pm), with a very low and controlled RI modulation, yielding “notch” filters that have high reflectivity (> 99.99%, equivalent to an optical density of 4), narrow reflective spectral bandwidth (< 10 nm), and steep reflective band edges. These holographic notch filters, which will be discussed later, have superior performance characteristics as laser-blocking filters in Raman spectroscopy. Their combination of high reflectivity to the laser wavelength and steep band edges allows efficient rejection of Rayleigh scattering a t the laser wavelength as well as transmittal of Raman scattering a t very small wavelength shifts (< 50 em-’) for measurement at a detector. Furthermore, their narrow spectral bandwidths allow simultaneous acquisition of b o t h Stokes and anti-Stokes Raman data. Comparison to other notch filter technologies. Before holographic notch filters were available, dielectric stack notch filters (which also operate on diffractive principles) were the devices capable of the highest performance. Dielectric filters consist of quarter-wave stacks of alternating high- and low-RI evaporated layers. In fact, the conformal reflection hologram used as a notch filter is similar to a quarter-wave dielectric stack, except that the RI profile is continuous a n d sinusoidal rather than a series of discrete steps. The smooth variation leads to much lower “sidelohe” reflection bands in the holographic filters. As a practical matter, it is difficult t o evaporate dielectric stacks with the small RI differentials and tight layer thickness control achievable in holographic filters. The holographic filters therefore have narrower spectral notches and steeper band edges. The r u g a t e filter i s a form of
dielectric filter that mimics the sinusoidal RI variation of a conformal volume hologram. It is made by simultaneous vapor deposition of two dissimilar dielectric materials in continuously varying proportions. The resulting hologram-like structure has the same advantages of low sidelobe reflections and high laser damage threshold. The rugate process has a theoretical advantage over holography in some applications because materials t h a t have higher R I s t h a n holographic films (- 2.0 vs. 1.5) can be used. The higher RI produces a spectral response that shifts more slowly with incidence angle. However, rugate filters are subject to the same practical limitations a s dielectric stacks, because tight control of periodicity and RI mix is difficult. These fiters have not produced the combination of high optical density, steep band edges, and narrow bandwidth currently provided by holographic filters. Colloidal Bragg filters, which have also been available for several years (91, provide performance approaching that of holographic notch filters. However, because they are fragile and exDensive..~ thev have not been widely k e d . N o n o o n f o d reflection HOEs. Reflection holograms need not have fringes that are conformal to the surface. Figure l e shows the diffraction and fringe configuration of a nonconformal reflection hologram. This configuration exhibits some spectral dispersion because of the nonzero fringe frequency parallel t o the film surface. The direction of reflected light within the response bandwidth is also different from that of a simple mirror. The amount of spectral dispersion and deviation from the hehavior of a mirror are proportional to the amount of fringe tilt. Although not widely applied in spectroscopy, the nonconformal reflection hologram can, in principle, function as both a notch filter andlor a dispersive grating. It is used as a combining element for a head-up display in some high-performance aircraft. In these head-up displays, the tilt of the fringes varies across the surface of a concave spherical combiner to correct the inherent geometrical aberrations of a n off-axis spherical mirror. Other applications for nonconformal reflection holograms include certain laser eye protection filters in which there is a performance advantage to keeping the fringe surfaces optically perpendicular to the line of
sight from the eye to every point on the filter surface (3). This manipulation is accomplished by recording “curved” fringe surfaces that have different local tilt angles across the surface of a device. V o l u m e t r a n s m i s s i o n HOEs. Volume transmission holograms bear physical resemblance to nonconformal reflection holograms, but the fringe planes of constant RI are tilted to be more perpendicular to the surface. They a r e typically much more dispersive than nonconformal reflection holograms and have a much higher fringe frequency parallel to the surface. An untilted fringe transmission hologram, shown schematically in Figure Id, has fringe planes exactly perpendicular to the surface. This holographic configuration is often used when t h e incident and diffracted paths fall at equal angles to the surface normal. The diffracted path is a ”mirror image” of the incident path in the film surface. This behavior is analogous to the Littrow condition for a conventional surface relief reflection grating. Volume holographic gratings of this type currently are used for dispersive laser bandpass filtering. In this application, the primary emission line of, for example, an NdYAC, HeNe, or diode laser is physically separated by the hologram’s dispersive characteristic from the u n wanted plasma or sidelobe emissions, as shown in Figure 2. Proper tuning of the’spatial frequenq, film thickness, and RI modulation in such a transmission hologram can yield a combination of very high efliciency and very high disper-
sion. The efficiency of high-dispersion designs is sensitive to the polarization of t h e incident light; t h e s-polarized component is diffracted most efficiently (approaching 100%). Dispersion is maximized by embedding the holographic film on the diagonal of a glass cube and positioning the lasers such that the incident and diffracted paths propagate normal to the cube faces. The same type of volume transmission hologram can be used as a spectrograph grating. Thin holographic films typically are used to enable efficient operation over a broad spectral bandwidth. One advantage of transmission gratings over conventional reflective gratings is that they allow the use of on-axis imaging lenses, capable of highresolution imaging over a wide field of view, in compact configuration with very little vignetting. The tilted fringe transmission hologram, shown i n Figure le, h a s fringe planes that are closer to surface normal than surface parallel hut have some tilt to the surface normal. This class of volume HOES is similar in operation and potential application to the untilted fringe transmission hologram. However, it is typically used in optical geometries in which the incident and diffracted radiation do not form equal angles to the film. I n comparison to the reflective surface-relief grating technology, this device is analogous to operating “off Littrow.” Fabrication of volume phase HOEs. Volume phase holograms physically consist of fringes of differential RI throughout the volume of an optically thick film. These fringes
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Figure 2. Laser bandpass filter configuration using volume transmission hologri embedded in a cube for maximum dispersion. ANALYTICAL CHEMISTRY, VOL. 65, NO. 9, MAY 1.1993 * 443 A
INSTRUMENTATION are formed by the photographic process outlined schematically in Figure 3. Fringes of varying light intensity are formed by the interference of two mutually coherent laser beams, which is recorded in a photosensitive medium. The photosensitive medium is processed to convert the latent light intensity pattern into a corresponding RI pattern.
In the case of the conformal reflection hologram, the holographic film is placed in contact with (or parallel to) a mirror surface. That mirror reflects the first laser beam to form a second laser beam, creating interference fringes parallel to the mirror surface (Figure 3a). The fringes can be tilted with respect to the film surface by tilting the mirror.
F' (a) Reflectionhologram exposure. (b) Transmission hologram exposure. (c) Top-level volume hologram pmcess using dichromated gelatin (DCG) film.
The angle of the incident beam with respect to the surface normal of the mirror, in combination with the laser wavelength, determines the spatial frequency of the fringe pattern. Consequently, a reflection hologram can be designed to operate at a different wavelength than that of the recording laser by appropriately shifting the incidence angle of the recording laser. In the case of the untilted fringe transmission hologram, the two laser beams are incident from the same side of the fhat equal and opposite angles to the surface normal (Figure 3b). These beams are derived from the same laser via a beamsplitter arrangement. A tilted fringe transmission hologram is formed by using beams that are asymmetrical t o the surface normal. The fringes will optically bisect the propagation vectors of the two recording beams. Like the conformal reflection hologram, the angles of the exposure beams to the surface normal, combined with the wavelength of the laser, determine the spatial frequency of the transmission hologram fringe pattern. Similar anglelwavelength shifting techniques also can be used for operating the finished hologram at a wavelength other than that of the laser. As a n example of angle/wavelength shifting capabilities, volume holograms designed to operate at the fundamental NdYAG wavelength of 1.06 Fm are routinely exposed at a compensating angle with a n argon ion laser operating at 0.514 pm. This wavelength shift is implemented in fabricating both reflection holograms for notch filters and transmission holograms for bandpass fdters. Similar shifting techniques and shorter exposure wavelengths are used to manufacture components for hightransmission operation at wavelengths as short as 440 nm. Device transmission falls off rapidly at shorter operating wavelengths-to 20% at 320 nm-because of the inherent W absorption characteristic of holographic gelatin film. Dichromated gelatin (DCG) is typically used as t h e photosensitive holographic medium in applications requiring the highest optical performance in terms of clarity, low scattering, and high efficiency (IO).Less demanding applications-those requiring low cost andlor insensitivity to moisture-sometimes use a photopolymer material instead of DCG. However, DCG is the preferred medium in spectroscopic applications of volume holography.
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Flgure 4. Raman spectra of CCI, obtained with holographic edge filter, showing effect of angle tuning. From top to bonom, filter angle is 22", la", 14". and Oo. (Adapted with permission from Reference 13.) 444 A * ANALYTICAL CHEMISTRY, VOL. 65, NO. 9, MAY 1,1993
The postexposure DCG film process sequence is shown in simplified form in Figure 3c. The exposed film is immersed in a water bath, which washes out t h e residual sensitizer and swells t h e gelatin to several times its dry thickness. The exposed film is immersed in a n alcohol bath to drive out the water. It is then rapidly dried, collapsing t h e swollen film. The resulting density (hence RI) of the dried film varies through the surface, according to the differential cross-linking t h a t occurred during laser exposure. The dried DCG hologram is finally sealed with a n optical cement between two layers of glass to protect it from absorbing moisture from the environment. In practice, a large number of process variables affect t h e final key physical parameters of the hologram: RI differential (primarily affecting efficiency), spatial frequency (primarily affecting response wavelength), clarity, haze, and wavelength stability. Key process variables include film thickness, sensitizer concentration, pre-exposure film conditioning, exposure beam intensities and their ratios, exposure time, process bath temperature and purity, drying parameters, and environmental temp e r a t u r e a n d r e l a t i v e humiditv through all phases of t h e process (pre- and postexposure). Small impurities such a s dust or lint in or on the holographic film, on the glass substrate, or on the exposure optics yield larger cosmetic defects in t h e finished hologram because they produce scattering of the coherent laser exposure beam. HOES are typically manufactured in class 100 or better clean room facilities that control airborne particulates as well as temperature and humidity. T h i s environment is maintained from glass preparation for film coat ing through final sealing operations. Holographic optics in modern instruments The Raman revolution. The origins of the revolution lie in the development of colloidal band rejection filters by Asher and co-workers (9). Although t h e filters are not holograms, they were the first optical elements based on Bragg diffraction that were used in Raman instrumentation. This work showed that excellent performance could be obtained with optics based on volume diffraction effects. Volume HOES were first employed in Raman spectrographs by Xiong and co-workers (11, 12). They used transmission holograms as delivery and pickup lenses in a
UMENTS
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1 fiber-optic illumination system and demonstrated the advantages of holographic optics in off-axis operation. Because the available fabrication fa cilities were limited, their holograms suffered from poor efficiency and short working life. The Raman revolution began in earnest with the introduction of holographic band rejection filters by Carrabba and co-workers (13).Using a commercially available holographic f i l t e r , t h e y demonstrated h i g h quality Raman spectra as close as 200 cm-' from the 514.5-nm exciting line with a n instrument consisting of the holographic element, a singlestage spectrograph, and a n array detector (Figure 4). The emphasis on high transmission ( 7 5 8 0 % ) and smooth edges of the reflection band of holographic filters brought holography to the forefront of spectroscopic instrumentation. Although early filters did not allow convenient observation of very low frequency bands even with angle tuning, the first combination of holographic filter and single-stage spectrograph provided a 3.5- to 5-fold improvement in throughput over a triple spectrograph and was adequate for most purposes. The properties of first-generation
filters were further investigated by Pelletier and Reeder, who pointed out the optical density-incidence angle tradeoffs involved in angle tuning (14).They also demonstrated t h e slight ringing a t the edges of the rejection band that results from deviations from sinusoidal RI profiles. The holographic rejection filters first used in Raman spectrographs were edge filters, which have a high RI and are 30 pm thick. Notch filters, which have a lower index modulation but are about twice as thick, provide a narrower bandpass with steeper edges, as shown in Figure 5. Notch filters were introduced in Raman practice by our groups (15);we demonstrated the ability to obtain Raman spectra as close as 60 cm-' from the exciting line. Like edge filters, notch filters require angle tuning to optimize performance close to the exciting line. Although they have somewhat lower peak optical densities, holographic notch filters are the preferred devices for most Raman spectroscopic applications, largely because they can be used for very low frequency Stokeslanti-Stokes spectroscopy. Like the dielectric stack filter, the conformal holographic filter is not polarization sensitive if operated a t
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normal incidence. However, angle tuning introduces a small polarization dependence (15). Measured depolarization ratios can be 5-10% too large when the filter is operated 1020" off-normal. Similar errors would be expected for both edge filters and notch filters. Within two years of the first publication on these systems, single-stage dispersive Raman systems incorporating holographic band rejection filters became commercially available. Filter design has also advanced, and with commercial ultra-narrow-band notch filters, one can approach to as close as 50-60 cm-' of the exciting line with almost no angle tuning. For most dispersive Raman spectroscopy, the single-stage spectrograph with holographic prefilter is the preferred configuration. Compact systems can be built around these spectrographs, particularly if semiconductor diode lasers are used as the exciting sources (16). The common 0.25-m, fl4 Czerny-Turner spectrograph can be used, and the resulting system is small and has high etendue. Replacement of the filter stage of a triple spectrograph with a holographic filter is also a common and inexpensive way to upgrade instruments. A throughput increase of about fourfold has been achieved with a typical commercial Raman microprobe system (17). In the Raman microprobe, a holographic beamsplitter can replace the conventional dielectric element (18). The holographic beamsplitter is essentially a notch filter designed to operate at a 45O incidence angle. It delivers - 90% of the incident laser power to the microscope objective and transmits 75-80% of the Raman mattered light. The microprobe typically operates with low incident laser power (5-30 mW) to avoid heating or even destroying the sample. With a holographic element, this requirement can be satisfied with a lowpower laser, because losses at the beamsplitter are small. Microprobe spectra can be obtained in 10-20 s, even with a 10-mW He-Ne laser as the excitation source (Figure 6). Because it is not operated at normal incidence, the beamsplitter is polarization sensitive, particularly at the laser line and at low frequencies (< f 300 cm-'1. In this region reflection of p-polarized light is lesi efficient than reflection of s-polarized light. Correspondingly, transmission of p-polarized light is more efficient. Similar effects are observed with dielectric beamsplitters, of course. In both cases, the spectrosco446 A
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pist must make suitable corrections. HOES are rapidly displacing interference filters- in FT-Raman spectrometers. The Michelson interferometer does not reject the laser line as well as a grating instrument does; thus, the filter must attenuate the laser intensity by 10' or greater. Two filters must be used in series to achieve this attenuation, and the performance advantages of holographic filters are especially impor-
tant. The typical configuration is one
filter at the interferometer entrance aperture and one at the exit (19).Together these filters allow measurement of Raman bands within a remarkable 100 cm-' of the laser line, a n d they eliminate t h e artifacts caused by ripples in dielectric interference filter rejection bands. The superior throughput of the HOE also decreases measurement time, particularly for low-frequency bands.
Flgure 5. Schematic representationof film thickness. bulk RI. and RI modulation for dielectric, holographic edge, and holographic notch filters. (Adapted wilh permission from Reference 15.)
Figure 6. Raman microprobe spectrum of Bi,O,, obtained with holographic beamsplitter, holographic narrow-band notch filter, and a single-stage spectrograph. The Iwer CUNe is the raw d a m The upper curve is corrmed lor anenualion by the Lwamsplinerand the notch filter. (Adapled wilh permission from Reference 18.)
ANALYTICAL CHEMISTRY, VOL. 65, NO. 9, MAY l;i993
Holographic bandpass filters are also used to condition the Nd:YAG laser output. State-of-the-art instruments use a holographic transmission grating filter configuration, which has high transmission (> 90%) and a narrow bandpass. The usable laser power is about 50-75% greater than that obtainable with narrowband interference filters (50-60% transmission). The laser power requirement is reduced sufficiently so that a diode-pumped Nd:YAG laser can replace the larger, less stable, a n d less energy-efficient l a m p pumped laser in many, and possibly most, FT-Raman applications. Other instrumentation. Many of the filters developed for Raman spectroscopy are equally useful in laserexcited fluorescence spectroscopy. Ogasawara, Yang, and Bobbitt recently used an edge filterlmonochromatorlphotomultiplier tube configuration a s the detector system for a fiber-optic sensor for riboflavin 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 intercalated form of TOTO, the homodimer of thiazole orange, &el(abs) = 486 nm and &ax(fl) = 505 nm, a Stokes shift of 19 nm. The oxazole yellow homodimer (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 holographic rejection filters can improve t h e performance of argon laserbased fluorescence detection systems. Similarly, in laser fluorescence microscopy, whether confocal or wide field, the use of these stains or other materials with low Stokes shifts can benefit from substitution of holographic beamsplitters for conventional dichroic mirrors. Several laboratories a r e investigating such applications. Volume holographic transmission gratings have been fabricated on a n experimental basis for many years. The optical engineering literature contains many proposals for spectrographs and monochromators based on these gratings a s well as descriptions of actual experimental systems. In addition to high diffraction efficiency, t h e advantages typically sought are a simple design requiring
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few elements, good imaging quality at low flnumber operation because of on-axis design, or both. We have constructed a Raman spectrograph that exploits these advantages of HOES. It features a u fll.8 on-axis design that uses wellcorrected photographic lenses as the collimating and focusing elements, a volume holographic grating for dispersion, and a holographic notch filter for laser line rejection. The allholographic construction gives the instrument high throughput. The low f/number makes efficient use of the light from optical fiber Raman probes, which are -f/Z. This combination allows the instrument t o acquire spectra in 1s with very low laser power (Figure 7) and to serve as a real-time monitor with only moderate (10-50 mW) laser power. The on-axis design also minimizes aberrations and gives the system good imaging properties over its entire operating range. The design of a single-element imaging spectrograph in which the hologram performs both dispersion and focusing has intrigued many investigators. The design of a functioning instrument is not trivial, because the hologram itself introduces chromatic aberration. Chromatic aberration can be minimized with a second glass element, much as in the conventional achromatic doublet lens. Vila and co-workers have developed procedures for exact compensation at
several fixed wavelengths without a second element (22).Their instrument consists only of an entrance slit or aperture, a hologram, and an array detector o r photographic film plane. It is designed for monitoring atomic emissions from selected elements at fixed Wavelengths, however, and would not necessarily function well for general use. Recently de Castro Carranza and colleagues developed a dual spatial frequency plane holographic grating (23).Two gratings of different pitch are recorded successively in a single hologram. Consequently, light from two entirely different regions of the spectrum is diffracted efficiently and can be focused onto the same array detector. The grating is planar, so that a complete instrument requires a collimating lens and a focusing lens. Special-purpose high-resolution atomic (or molecular) spectrographs can use these interleaved gratings. With a suitable choice of wavelengths, the dual-pitch grating can make efficient use of array detector real estate and may well make array detectors economical even for many routine applications. Conclusions and future directions Until recently the power of volume holographic optics has gone unused because there has been little communication between the holography and analytical chemistry communities. However, this situation is changing.
Driven by t h e needs of Raman spectroscopy, manufacturers have developed high-performance filters and beamsplitters for use in fluorescence spectroscopy. We can expect to see them appear in such systems as optodes and capillary electrophoresis fluorescence detectors in the near future. The tradeoff between rejection band and peak reflectance will be optimized for important fluorescence applications such as protein and nucleic acid detection &s well as for optodes. The first spectrograph incorporating a volume holographic transmission grating was introduced by Kais e r Optical Systems a t t h e 1993 Pittsburgh Conference. It is a fast cfll.81, compact ( 8 5 - m m focal length), on-axis imaging system designed for Raman spectroscopy with green laser excitation. In the near future we will see spectrographs optimized for different wavelengths or for higher or lower resolution. Even these developments are only the early fruits of the holography/ spectroscopy marriage. In the future, HOES will combine multichannel focusing and dispersive or filtering functions in a single element. Sophisticated ultracompact singleelement spectroscopic systems are already on the horizon. References (1)Fisher, R. L.Proc. SPIE 1988,883,2835.
(2) Wood. R. B.: Havford. M. J. Pro
