Spectroscopy >Volume Phase Holographic Diffraction Gratings
Volume Phase Holographic (VPH) diffraction gratings enhance P&P Optica's ability to provide our customers with comprehensive spectroscopy and spectrum measurement solutions. When compared to etched or ruled varieties, VPH gratings provide much lower scattering, as well as higher polarization independent efficiency. This, together with their tough design, makes them ideal for use in high-grade spectroscopy or telecommunications.
What are VPH diffraction gratings?
The most commonly used gratings are generally ruled or etched using either a process involving the mechanical scratching of the surface of a substrate, or exposure of a photoresist layer deposited on a substrate.where the interference pattern produced by two coherent light beams is used to expose the photoresist. The diffraction gratings produced with this process are often called holographic. Both of these processes are used to create a fine, periodic relief pattern on the surface of a blank substrate. Such a relief structure can predictably modify the amplitude and/or phase of the incident light. The superposition of the effects of many such grooves allows for the angular separation of the spectral components of the incident light. Depending on whether the light is reflected from the grooved surface, or transmitted through it, the grating is called either a reflection or transmission diffraction grating.
Volume phase holographic gratings work on a somewhat different principle. A photosensitive gel or polymer is deposited on a substrate. An interference pattern is then shone onto the gel which changes its index of refraction accordingly. A chemical process is then used to enhance the refractive index modulation and to “save” the pattern on the grating. As light passes through the different areas of the gel, it bends according to its wavelength, resulting in an angular separation of spectral components of the incident light.
A typical VPH grating manufacture process is illustrated below. A layer of photosensitive gel is enclosed by two substrates. Usage of gel between two substrates gives many more degrees of freedom in the spectral response of these gratings. The width, shape and depth of the fringes all affect the grating performance.
What does this mean?
Grating properties can be influenced by several factors. The angular separation of spectral components of the incident light beam is determined by the spatial frequency of a grating. The diffraction efficiency, or the percentage of light which is angularly separated by the grating is controlled not only by the spatial frequency, but also by the angle of incidence of the incoming light and the optical properties of the grating material. The diffraction efficiency of reflecting relief gratings is also determined by the shape of the grooves and the reflectivity of the grating. In the case of transmission relief diffraction gratings, the diffraction efficiency depends on the shape of the grooves and the refractive index of the grating material. In both cases, diffraction efficiency strongly depends on the polarization of incident light and wavelength dependence of efficiency is quite different for the two polarizations.
The diffraction efficiency of VPH gratings depends on a greater number of factors than that of relief gratings. The gel thickness, its refractive index, the modulation depth of the refractive index and the distribution of refraction index across the gel layer all contribute to the diffraction efficiency. This gives more degrees of freedom to produce a grating with much higher efficiency and lower polarization dependency. Unlike relief gratings, VPH diffraction gratings can reach close to 100% efficiency, even for high spatial frequencies, if a sufficiently thick layer of gel is used. Furthermore, VPH gratings produce less scattered light due to the absence of groove edges of standard gratings. Overall efficiency over a given spectral range can therefore be greatly increased, as shown in the illustration below. Another advantage of VPH gratings is that more than one grating can be registered in the same layer of gel, giving an opportunity to develop completely new applications.
Benefits of P&P Optica VPH gratings.
The VPH gratings provided by P&P Optica have limited ghosting and scatter problems, which are often noticeable in other VPH gratings. For low noise, high efficiency systems, P&P Optica offers only the highest quality available. The image below shows ghosting and scatter effects in gratings by four different manufacturers. All are illuminated with HeNe laser at 2 mW, and oriented so that the first order is at a maximum. The first order images of all four gratings are shown below. Clearly, the P&P Optica grating has very little ghosting and low scatter as compared to other manufacturers' gratings.
Comparison of VPH and standard relief gratings (HRR)
Tested Parameter |
Performance |
|
|---|---|---|
| HRR |
VPH |
|
| Efficiency, narrow band (tens of nm) | ~ 90% | Close to 100% |
| Average efficiency, (one octave) | 20% - 30% | > 60% |
| Band and efficiency product | Hardly controlled | Well controlled |
| Efficiency, as a f(dispersion) | Decreasing | Increasing |
| Polarization dependency | Strong | Weak |
| Efficiency curve f(wavelength) | Irregular curve | Regular |
| Scattered light | At detectable limit | Below detectable limit |
| Focusing ability (no additional optics) | Over the entire spectral range | Limited capabilities |
| Imaging quality, focusing grating | Strongly astigmatic | Astigmatic |
| Imaging quality, plane grating | Diffraction limited | Diffraction limited |
| Work in high dispersion Littrow configuration without additional optics | Possible in whole spectral range | In limited part of visible and short NIR range |
| Work in high dispersion Littrow configuration with additional optics | Limited efficiency in whole spectral range | Highly efficient in whole spectral range |
| Multiple function in a single element | Impossible | Possible |
| Stacking of multiplicity of elements | Impossible | Possible |
| Surface protection | Impossible | Can be fully protected |
| Temperature range | Standard | Up to above 100°C |
| Humidity | Non-condensing | Boiling water (with surface protection) |
| Power resistant | Limited | High, depends on spectral range |
| Production technology | Mature, well established | Limited number of experienced companies |
| Pricing | Established price | Higher, low volume production |
| Price of imaging gratings | High | Competitive |
| Price of plane gratings (replica) | Low | Higher (if protected) |
