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Optical Design & Engineering

Novel uses for deep-etched, fused-silica diffraction gratings

Advances in methods of fabricating diffraction gratings enable or greatly simplify exotic applications such as high-efficiency polarizers and two- and three-port beam splitters.
2 June 2011, SPIE Newsroom. DOI: 10.1117/2.120114.003457

Diffraction gratings are among the most venerable tools of optical physics, with the first gratings produced in the mid-1700s. They are used in chemical analysis, astronomic spectroscopy, light-based communications, and other applications. Unlike a prism, diffraction gratings operate by interference effects as incident light impinges on rulings or grooves cut or etched onto the grating surface. Over the last 10 years, advances in fabrication techniques—many by our own group—have made possible gratings of unprecedented groove depth and density. These gratings offer new efficiencies, for example, in transmissivity, beam splitting, bandpass filtering, and polarization separation.

High density refers to the groove-to-groove distance or period. Groove depth is defined by the gratings' depth-to-period ratio. We reasoned that optimizing the groove depth to bring the grating period close to the wavelength of the incident light should enable novel functions. Our group faced two fundamental challenges concerning deep-etched, high-density gratings. The first was theoretical: How would such gratings perform? As a practical matter, only when we were fairly sure that the effort would produce useful new devices did we try to fabricate them. Fabrication was the second challenge. Which techniques and materials would be needed to make these gratings? We settled on fused silica, an excellent optical material with good thermal stability, high transmission at our wavelengths of interest, and a high laser-damage threshold. However, fused silica is hard and difficult to etch. Developing ways to work with it took considerable time and experimentation.

Table 1.Comparison of deep-etched fused silica grating and other methods for polarization separation.
Natural birefringence crystalRare, expensive, but with high isolation
Molecular polarizerCheap, low efficiency
Deep-etched gratingOptimized depth and period for polarization separation,
high isolation, high efficiency
Photonic crystalComplex 3D structure, difficult to fabricate

Figure 1. Scanning-electron microscopy image of a deep-etched fused-silica grating 50mm in diameter.

Figure 2. Applications of deep-etched, fused-silica gratings. TM: Transverse magnetic polarization. TE: Transverse electric polarization.

For the first challenge, it is necessary to understand exactly what is going on inside the grating. Rigorous coupled wave analysis3,4 is one of the most widely used numerical methods for predicting the performance of gratings, but it is computationally intensive and yields an approximation. If we had a better model of the diffraction mechanism, it would be much easier to design a practical deep-etched grating. The eigenfunction or modal method, first proposed by Botten,5 is a powerful method for revealing the diffraction mechanism by defining the modes within a grating design. We feel that Botten's method has not received enough attention in the 30 years since it was formulated. Tishchenko6 developed the modal method and later Clausnitzer7 presented a simplified version, which illustrated clearly that the diffraction can be simply described by a sine (or cosine) function for a subwavelength, low-contrast grating.

We further developed the simplified modal method to illustrate a series of novel and interesting optical functions of deep-etched, high-density, fused-silica gratings.8–13 We used the overlap integral and even and odd modes to reach simplified equations,11 including those for triangular10 and sinusoidal grooves.14,15 We obtained unified designs of these gratings16 by defining the ratios of period-to-wavelength and depth-to-wavelength, which can be shown clearly using the effective-mode indices of the method.

With that knowledge, we developed several applications for deep-etched, high-density, fused-silica gratings, including high-efficiency polarizers,8 a 1×2 beam splitter,9 a 1×3 beam splitter,17 and other designs. The gratings also offer highly efficient diffraction at the minus-one diffraction order with polarization-independence over a wide spectrum.18,19 In fabricating devices like these, it is a real technical challenge to achieve narrow (smaller than several hundred nanometers) and deep (>1μm) grooves on practically sized substrates with diameters in the tens of centimeters. Eventually, we employed several procedures, including fringe recording of holographic interference, lithographic developing, and dry etching using an inductively coupled plasma (ICP) facility. Over 10 years, we successfully made a series of deep-etched fused-silica gratings one by one.1,2,8–25 We found that ICP for etching the grating is a crucial step.

Figure 3. Double-pulse generation using two deep-etched gratings (a) and single-pulse generation by shifting one grating vertically by a quarter period.25
Table 2.Compression techniques for femtosecond laser pulses.
Prism pairWidely used
Chirped mirrorExpensive
Equal-density reflective gratingsChirped pulse amplification
Double-line density gratingsReflective compressor1
Deep-etched gratingsTransmittive compressor2

Figure 1 shows a photomicrograph of a section of a sample grating. These devices present some remarkable technical opportunities. Table 1 offers a comparison of fused-silica gratings with other methods for polarization separation. Several applications for deep-etched-gratings are shown in Figure 2. We note several new or unusual uses for them. A subwavelength, fused-silica grating can function as a polarizer to separate two polarized waves into two directions of the zeroth order and the minus-one order,8,14 respectively. It can also work as a high-efficiency, polarization-independent grating for two polarizations over a 100nm-wide band.19 With two such gratings, a miniaturized pulse compressor2 can be built that is much smaller than the traditional pulse compressor that uses prism pairs. (Table 2 compares current pulse compressors.) Double pulses can be generated using two deep-etched gratings25 (see Figure 3). This approach is much less complex than the usual Michelson structure for generating double pulses.

In conclusion, deep-etched, fused silica gratings have shown a series of novel functions that include high-efficiency polarization-independent diffraction, polarization beam splitting, and two- and three-port beam splitting. These functions will enable us to develop new optical devices as well as other practical applications in optical-fiber communications, spectrometry, femtosecond laser-pulse compression, and carrier-envelope phase-stabilized laser systems.26

The authors acknowledge research funding from the National Science Foundation of China (60878035, 61078050).

Changhe Zhou, Wei Jia, Hongchao Cao, Shaoqing Wang, Jianyong Ma
Shanghai Institute of Optics and Fine Mechanics
Shanghai, China

Changhe Zhou is a professor. He is the executive editor-in-chief of Chinese Optics Letters. Along with two other professors, he is the guest editor of Applied Optics for a special issue of Applications of Nano-optics in 2011.

Wei Jia, Hongchao Cao, Shaoqing Wang, and Jianyong Ma are assistant researchers.

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10. J. Zheng, C. Zhou, J. Feng, B. Wang, Polarizing beam splitter of deep-etched triangular-groove fused silica gratings, Opt. Lett. 33, pp. 1554-1556, 2008.
11. J. Zheng, C. Zhou, B. Wang, J. Feng, Beam splitting of low-contrast binary gratings under the second Bragg angle incidence, JOSA A 25, pp. 1075, 2008.
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14. J. Feng, C. Zhou, H. Cao, P. Lv, Deep-etched sinusoidal polarizing beam splitter grating, Appl. Opt. 49, pp. 1739-1743, 2010.
15. J. Feng, C. Zhou, H. Cao, P. Lu, Unified design of sinusoidal-groove, fused-silica grating, Appl. Opt. 49, pp. 5697-5704, 2010.
16. P. Lv, C. Zhou, J. Feng, H. Cao, Unified design of wavelength-independent deep-etched fused-silica gratings, Opt. Commun. 283, pp. 4135-4140, 2010.
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18. S. Wang, C. Zhou, Y. Zhang, H. Ru, Deep etched high-density fused silica transmission gratings with high efficiency at wavelength of 1550nm, Appl. Opt. 45, pp. 2567-2571, 2006.
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20. S. Wang, C. Zhou, H. Ru, Y. Zhang, Optimized condition for etching fused silica phase gratings with inductively coupled plasma technology, Appl. Opt. 44, pp. 4429-4434, 2005.
21. J. Feng, C. Zhou, J. Zheng, H. Cao, P. Lv, Dual-function beam splitter of a subwavelength fused silica grating, Appl. Opt. 48, pp. 2697-2701, 2009.
22. J. Feng, C. Zhou, J. Zheng, B. Wang, Modal analysis of deep-etched low-contrast two-port beam splitter grating, Opt. Commun. 281, pp. 5298-5301, 2008.
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24. H. Cao, C. Zhou, J. Feng, J. Ma, Polarization-independent triangular-groove fused-silica gratings with high efficiency at a wavelength of 1550nm, Opt. Commun. 283, pp. 4271-4273, 2010.
25. T. Wu, C. Zhou, J. Zheng, J. Feng, H. Cao, L. Zhu, W. Jia, Generation of double femtosecond pulses by using two transmissive gratings, Appl. Opt. 49, pp. 4506-4513, 2010.
26. C. Zhou, Deep-etched fused silica grating and applications, Proc. SPIE 7848, pp. 78480R, 2010. doi:10.1117/12.869033