Introduction to Volume Phase Holographic (VPH) Transmission Gratings and Applications
Volume Phase Holographic (VPH) transmission gratings work much like conventional surface relief reflection gratings, except in transmission. They are periodic phase structures, whose fundamental purpose is to diffract different wavelengths of light from a common input path into different angular output paths.
A VPH grating is formed in a layer of transmissive material, usually dichromated gelatin, which is sealed between two layers of clear glass or fused silica. The phase of incident light is modulated as it passes through the optically thick film that has a periodic differential hardness or refractive index. Hence the term “Volume Phase”. This is in contrast to a conventional grating, in which the depth of a surface relief pattern modulates the phase of the incident light. This key distinction is shown schematically in Figure 1.
Figure 1. Conventional reflection gratings vs. VPH gratings.
As with conventional reflection gratings, the spatial frequency of the periodic structure determines the spectral dispersion, or angular separation of wavelength components, in the diffracted light. The average refractive index, the refractive index differential or modulation, and the thickness of the film determine the efficiency, polarization, and bandwidth performance characteristics of the grating.
VPH Gratings are manufactured using a complex proprietary process whose basic steps are as follows:
- A photosensitive film of dichromated gelatin is coated onto a glass or fused silica substrate.
- The coated substrate is exposed to a holographic fringe pattern formed by interfering two collimated beams of coherent light from a common laser source, as shown in Figure 2.
- The exposed gelatin coating is processed in a series of liquid baths to achieve the desired level of refractive index modulation.
- A second plate of glass or fused silica is laminated over the processed grating film to protect it from the environment, using optical adhesive.
Figure 2. Holographic exposure of a VPH grating
The resulting structure is rugged, cleanable, stable, and has excellent optical characteristics. Figure 3 shows schematically the resulting physical structure of a VPH grating. Note that the aperture of the gelatin grating film is normally smaller than that of the glass substrate and coverplate. This provides an outer border of optical adhesive that protects the edge of the gelatin film from encroachment of moisture, which can be harmful to the grating.This configuration provides an indefinite operating life under realistic environments, including MIL specifications. However, some of the more extreme environmental qualification levels of Telcordia GR-1221 (telecommunications industry) necessitate the addition of a fully hermetic metal-to-glass seal around the edge.
Figure 3. Structure of a VPH Grating.
Volume Phase Holographic Gratings are readily customized to your application, and offer premium optical performance, environmental ruggedness, and system packaging advantages.
Every VPH grating is a genuine holographic master, not a mechanical replication. As such, quoted production costs for VPH gratings may be higher than for similarly sized surface-relief reflection gratings. However, the advantages of VPH gratings in performance, durability, and system packaging actually result in a lower-cost, higher-performance system for many applications. Furthermore, because of our flexible VPH manufacturing facility, non-recurring costs for a custom or prototype VPH grating are substantially lower than those for tooling a custom conventional surface relief grating.
VPH gratings may be made with line frequencies from under 300 lines/mm to over 6000 lines/mm. They may be combined with prisms to enable higher dispersion than is possible in a flat grating. Their volume refractive index structure scatters less light than the typical surface relief structure of a conventional grating. The many degrees of freedom available to the design of a VPH grating enable a wide range of dispersion, efficiency, bandwidth, and polarization characteristics -- far surpassing anything you will find in a conventional grating catalog.
Conventional diffraction gratings are normally considered to be highly fragile components because of their microscopic surface relief structure. If anything comes into physical contact with the surface of a conventional grating, including something as benign as a human fingertip, the grating is generally rendered scrap. Not so with a VPH grating component, which is as physically rugged as its external glass or AR coating surface. They may be cleaned if touched or contaminated, and they can survive demanding environmental requirements of military and telecom industry specifications.
The transmission geometry of VPH gratings enables compact, high performance system geometries that cannot be realized using reflection gratings. Reflection gratings normally work in the Littrow geometry, in which the diffracted/dispersed light retroreflects back toward the source of incident light. In some applications this can be useful, but in many it is a nuisance that limits system performance. In contrast, the transmission geometry of a VPH grating places the input and output/dispersed regions on opposite sides of the grating plane. This distinction can be seen by referring back to Figure 1.
The transmission grating allows input and output to be operated on by independent optics and components such as detector, fiber, or MEMS arrays. Imaging optics may be placed very close to the grating, which minimizes lens size and vignetting losses. In some applications this further enables the use of commercial off-the-shelf lenses. These concepts are evident in the axial transmissive spectrograph layout of Figure 4. In addition, the use of a polarization-insensitive VPH grating design may eliminate the need for polarization splitting system configurations, further reducing component count and package size.
Figure 4. Axial transmissive spectrograph layout using VPH grating.
In any case, the net result is that a system designed around a VPH grating is likely to be smaller, lighter and less expensive than one designed around a conventional reflection grating, in addition to offering better optical performance.
Kaiser Optical Systems (Kaiser) has been making VPH products for nearly 25 years, utilizing manufacturing technology originally developed for primary flight displays on the world’s best tactical aircraft. We have delivered thousands of holographic head-up display (HUD) components for aircraft currently in production, such as the F-15 and F-18, and over 2 million cm2 of VPH components over all product lines.
Our facility, with its 6000 ft2 VPH manufacturing clean room complex, was designed and built for the specific purpose of manufacturing VPH components. It has been in operation since 1985. Its 6000 square foot clean room complex houses several 8 x 16 foot holographic exposure tables on isolated foundations, high power water-cooled ion lasers, and facilities for gelatin coating, hologram processing, optical testing, and glass lamination. The same manufacturing facility, expertise, and exacting mil-spec standards are applied to our commercial VPH transmission grating products. Indeed, Kaiser is widely recognized as the world’s leading manufacturer of VPH components
Because of the inherent flexibility of this product line, we do not have a fixed catalog of VPH grating products. This frees your engineers to create the best possible dispersive optical systems for your application. Our engineering staff is available to work with yours to assure the best possible result.
Our technical staff offers over 250 person-years of experience in the design and manufacture of VPH products. We are intimately familiar with the tradeoffs in the VPH grating design space. We have optical engineers who speak your technical language. We’ve engineered world-class industrial Raman analyzers recognized as the best available for on-line process applications in chemical and pharmaceutical industries. Areas of expertise include:
- Optical system design
- Mechanical system design
- Fiber optics
- Spectrograph design and manufacture
- Wavefront analysis
- External cavity stabilized diode lasers
- CCD cameras
- Optical component laminations
- Complex, precision glass cutting
- Military and Telecom (Telcordia) specs
Our intensive up-front technical/design support has led to successful, high-volume OEM applications in ultrafast laser pulse compression, telecom/DWDM, and spectrographic instruments, as well as one-of-a-kind applications in astronomical telescopes and research instrumentation.
VPH gratings manufactured at Kaiser are deployed in a variety of OEM and custom applications, including optical telecom/DWDM, industrial and laboratory spectrometers, ultrafast laser pulse compression, laser bandpass filtering, and astronomical telescopes.
Kaiser’s UltraSpec™-HT product line for telecommunications offers an unprecedented combination of dispersion, efficiency, and polarization insensitivity over the full telecom C-Band. The UltraSpec™-HT940 is a 940 line/mm grating in a flat package. The UltraSpec™ HT1350 is a higher-dispersion 1350 line/mm grating embedded between two prisms (sometimes referred to as a “grism”) that enable entrance and exit of otherwise non-propagating paths. There is some degree of flexibility to tailor the beam path and dispersion of the UltraSpec™-HT1350 by varying the prism angles. The UltraSpec™-HT940 amd one prism configuration of the UltraSpec™-HT1350 are pictured in Figure 5. These grating products are deployed in wavelength routing devices in which low insertion loss and low PDL are paramount.
Figure 5. UltraSpec™-HT products for telecommunications.
The UltraSpec-C160 is an even higher dispersion grism, consisting of a 1790 line/mm grating, also embedded between two prisms. This product was designed for a compact DWDM channel monitor application, shown schematically in Figure 6, in which high dispersion and low PDL are paramount, but insertion loss relatively unimportant.
Figure 6. Channel monitor layout using UltraSpec™-C160.
Kaiser-manufactured VPH gratings are widely deployed in industrial Raman spectrometers used to monitor reactions in the chemical and pharmaceutical manufacturing industries. The axial transmissive spectrograph geometry enabled by the VPH grating, shown previously in Figure 4, provides a fast, high-resolution acquisition of very weak Raman spectra for real-time process control.
Similar axial transmissive spectrograph products are deployed in research laboratories and universities performing Raman, fluorescence, and general-purpose spectrometry.
Kaiser-manufactured VPH gratings are deployed in commercial ultrafast laser products used in medical/surgical and materials processing applications. Matched grating pairs are used in ultrafast lasers in order to stretch and compress the temporal width of laser pulses. In their stretched or “chirped” state the pulses have substantially lower peak power, and can be amplified without damaging the amplifier components. After amplification, a double-pass trip through an appropriately configured grating pair temporally compresses the chirped pulse, achieving peak powers on the terawatt level. The concept is illustrated in Figure 7.
VPH transmission gratings enable compact, efficient ultrafast laser layouts compared to those using reflection gratings. As such, they are used by multiple laser OEMs in their “cutting edge” products.
Laser Bandpass Filters Kaiser’s VPH gratings are deployed in laser bandpass filter applications, in which the function is to eliminate low level, broadband emissions from sources such as diode lasers. The transmission geometry enables a convenient system layout in which the grating or grism is positioned, for example, in the collimated space between a laser source and a fiber-coupling lens. Here the fiber core itself acts as the spatial filter, accepting only the peak mode of the dispersed laser spectrum. This type of filtering is essential for high performance Raman spectroscopy, in which the wavelength-shifted Raman scatter is roughly 8 orders of magnitude (80 dB) weaker than the laser excitation. Without such filtering, the Rayleigh scatter of broadband diode excitation laser emissions would swamp the weak Raman scatter spectrum close to the laser line. Astronomical Telescopes
Leading astronomers throughout the world have recognized the benefit of using VPH transmission gratings in their telescopes that study the universe. They have also recognized Kaiser as the quality supplier of these gratings. In response to demand for ever-increasing apertures from this community, we have procured 12-inch aperture, 1/20th-wave off-axis parabolic collimators for the purpose of fabricating large aperture gratings.