This section provides some insight into the manufacturing methods for SRTG and VPH gratings and presents some of the fine-tuning
that can be done to design practical polarization-independent, high-efficiency gratings.
Surface Relief Transmission Gratings
SRTGs can be manufactured by etching into the surface of a glass substrate using a mask layer to define the periodic pattern.
If fused silica is used as the substrate, gratings for wavelengths ranging from UV through NIR can be made. The index modulation
goes from ~1.5 in glass to 1.0 in air — that is, an index modulation of ±0.25. The periodic pattern can be created by either
a holographic technique or by using a conventional photo-mask. By controlling the pattern exposure and etching process the
following parameters can be varied (within certain limits):
- The shape of the grating "teeth" — both sinusoidal and rectangular is possible
- The grating thickness, that is, the etching depth
- The duty cycle of the index modulation, that is, the width of the high index portion relative to the total period
- The sidewall angle of the grating structures — even asymmetric triangular (blazed) grating structures can be produced
Furthermore, dielectric materials with refractive indices different than the glass substrate can be added to the structure.
Figure 4 shows the various parameters that can be used to optimize the grating's diffraction efficiency.
As discussed in the previous section, a high-efficiency, polarization-independent design requires that the average index,
the index modulation, and the grating thickness are controlled and — for the SRTG — this is done by choosing the physical
grating parameters appropriately. For instance, for a pure rectangular SiO2 grating profile the average index can be controlled between 1.0 (air) and ~1.5 (SiO2) by choosing the duty cycle appropriately. A 50% duty cycle would yield an average index of ~1.25 and an index modulation
of ~0.25. By adding dielectric layers with an index larger than glass, the average index and index modulation can be made
even larger than 1.5 and 0.25, respectively.
Figure 4: SRTG structural parameters.
When light propagates through the complete SRTG in Figure 4 it passes through three transitions coming from left to right:
the air–glass interface; the glass–grating interface; and the grating–air interface. Special care must be taken in the grating
design to avoid Fresnel reflections at these interfaces. The first air–glass interface simply requires an antireflection (AR)
coating, but the glass-grating and grating-air interfaces must be designed such that the structure itself acts as AR transitions.
Volume Phase Holograms
VPHs are typically manufactured in a dichromated gelatin (DCG) that is sandwiched between two glass substrates as shown in
Figure 5. The DCG is transparent from ~350 nm through NIR, so VPHs generally cannot be made for UV spectroscopy. The refractive
index of DCG can be changed permanently by illumination with laser light. The permanent index modulation is created in the
DCG by illuminating the material with an interference pattern (holographic technique). The thickness of the grating is simply
the thickness of the DCG layer. The index modulation and average index can be controlled by the exposure intensity or exposure
time of the illumination. Unexposed DCG has a refractive index of 1.54 and strongly exposed DCGs yield a minimum average index
of ~1.25 and a maximum index modulation of ~0.15.
Figure 5: VPH grating structural parameters.
A benefit of the VPH type of gratings is that the grating structure is fully protected between two glass plates, which makes
handling and cleaning less of an issue compared to SRTGs.
Similar to the SRTG, the glass–air interfaces in the VPH must be AR-coated. The VPH process does not provide a way of controlling
the glass–grating interfaces.
Comparison of SRTG and VPH
With the parameter space available by the manufacturing techniques described above, both SRTGs and VPHs can provide high-efficiency,
polarization-independent diffraction gratings. In general, SRTGs provide high index modulation (>0.25) and VPHs provide lower
index modulation (<0.15). This means that VPHs typically require thick gratings (t > 5λ) and SRTGs require thinner gratings (t < 2λ).
The bandwidth of the grating is inversely proportional to the thickness of the grating, which means that SRTGs will provide
a wider bandwidth than VPHs.
Table I provides a general overview of the two transmission grating types.
Table I: Comparison of SRTG and VPH capabilities