Overview of High-Efficiency Transmission Gratings for Molecular Spectroscopy - - Spectroscopy
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Overview of High-Efficiency Transmission Gratings for Molecular Spectroscopy


Spectroscopy
Volume 29, Issue 4, pp. 32-39

Grating Example

To illustrate how effective transmission gratings can be, we will consider two common spectrometer applications: a Raman spectrometer with a laser wavelength of 785 nm and a UV spectrometer.

High-Dispersion Raman grating for 785 nm

The first example is a high-dispersion grating well suited for Raman spectroscopy at a laser wavelength of 785 nm. The groove density of the grating is 1765 lines/mm (with a 566-nm period) and the angle of incidence in air is 50. Ideally, the grating should provide polarization-independent high efficiency from 785 nm to 1000 nm. We used a finite element method (4) to solve Maxwell's equations combined with topological optimization (5) to find the best VPH and SRTG grating structures for this problem.


Figure 6: Diffraction efficiencies for SRTG and VPH gratings. The solid lines are theoretical curves and the colored squares are actual measurements on realized gratings.
Figure 6 shows the resulting theoretical efficiencies for the SRTG and the VPH. The efficiencies are very high for both gratings and both polarizations at the center wavelength. As expected, the VPH displays a smaller bandwidth than the SRTG. Also plotted on Figure 6 are real measurements on realized SRTG and VPH gratings, which confirms that an efficiency of >90% can be obtained at the peak wavelength and >80% efficiency is obtained over a 200-nm bandwidth around the peak.

UV Grating


Figure 7: Diffraction efficiencies for an SRTG UV grating in pure fused silica. The solid lines are theoretical curves and the marks with error bars are actual measurements on realized gratings.
The second example is a broadband UV grating for the 190–435 nm wavelength range. This grating has a groove density of 2420 lines/mm (period of 413 nm). Since this is a UV grating we have only considered a SRTG. Figure 7 shows the theoretical grating efficiency as well as the efficiency measured at three different wavelengths. The error bars indicate the total spread of efficiency over more than 60 individual gratings. As can be seen, the efficiency is high for both polarizatons and fairly flat across the complete range from 190 nm to 435 nm.

Summary

In summary, we have shown how near 100% efficiency, polarization-independent gratings can be designed by choosing the average refractive index, index modulation, and grating thickness appropriately. Furthermore, we have described two types of transmission gratings — the surface relief transmission grating (SRTG) and the volume phase hologram (VPH) — and shown that both types can provide the average index, index modulation, and grating thickness required for producing gratings with near 100% efficiency. VPHs generally have weak index modulation and SRTGs have strong index modulation. As a result, VPHs generally have thick gratings and a smaller bandwidth compared to SRTGs, which have thin gratings with a larger bandwidth. SRTGs based on fused silica can be made for any wavelength range from UV through NIR and VPH can be made for any range from ~350 nm through NIR.

References

(1) M.G. Moharam and T.K. Gaylord, J. Opt. Soc. Am. 71, 811–818 (1981).

(2) K. Baldry et al., Publ. Astron. Soc. Pac. 116, 403–414 (2004).

(3) H. Kogelnik, The Bell System Technical Journal 48, 2909–2947 (1969).

(4) J. Jin, The Finite Element Method in Electromagnetics (Wiley, Hoboken, New Jersey, 2002).

(5) J. Jensen and O. Sigmund, Laser Photonics Rev. 5, 308–321 (2011).

Thomas Rasmussen is VP Business Development for Sales and Marketing at Ibsen Photonics in Farum, Denmark. Direct correspondence to:


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