Deep-etched high-density fused-silica transmission gratings with high efficiency at a wavelength of 1550 nm

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1 Deep-etched high-density fused-silica transmission gratings with high efficiency at a wavelength of 1550 nm Shunquan Wang, Changhe Zhou, Yanyan Zhang, and Huayi Ru We describe the design, fabrication, and excellent performance of an optimized deep-etched high-density fused-silica transmission grating for use in dense wavelength division multiplexing (DWDM) systems. The fabricated optimized transmission grating exhibits an efficiency of 87.1% at a wavelength of 1550 nm. Inductively coupled plasma-etching technology was used to fabricate the grating. The deepetched high-density fused-silica transmission grating is suitable for use in a DWDM system because of its high efficiency, low polarization-dependent loss, parallel demultiplexing, and stable optical performance. The fabricated deep-etched high-density fused-silica transmission gratings should play an important role in DWDM systems Optical Society of America OCIS codes: , , , , Introduction According to dense wavelength division multiplexing (DWDM) technology, different wavelengths carrying optical signals are transmitted simultaneously with very small frequency spacing, typically 100 GHz, or less than 0.8 nm at wavelength of 1550 nm. The DWDM technology greatly reduces the channel spacing and considerably enhances the number of communication channels. 1 Therefore the bandwidth of a normal fiber can be enlarged enormously. DWDM technology can be realized through thin-film filters, arrayed waveguide gratings, fiber Bragg gratings (FBGs), and free-space diffraction gratings. 2 Thinfilm filters can be virtually temperature insensitive, but they have higher losses and complexity with an increasing number of channels. Arrayed waveguide gratings suffer from large losses, low free-spectral range, relatively high cross talk, and polarizationdependent loss, which limit the practical number of channels. In addition, they cannot perform stably in The authors are with the Shanghai Institute of Optics and Fine Mechanics, Academia Sinica, P.O. Box , Shanghai , China, and the Graduate School of the Chinese Academy of Science, Beijing , China. C. Zhou s address is chazhou@ mail.shcnc.cn.cn. S. Wang s address is shunquanwang@ gmail.com. Received 22 August 2005; accepted 6 October /06/ $15.00/ Optical Society of America an environment with fluctuating temperatures. The FBG performs well when just a few channels are used. The main problem is that the FBG must be used with circulators or Mach Zehnder couplers that increase the losses and costs. Comparatively, free-space diffraction gratings have the obvious advantages of parallel demultiplexing, low polarizationdependent loss, and the lowest cross talk for the highest numbers of channels. They give a relatively stable performance in an environment with fluctuating temperatures. 3,4 Furthermore, the fabrication of free-space diffraction gratings can take advantage of microelectronic technology, which decreases the fabrication cost. All these advantages are important for a DWDM system. Figure 1 shows the demultiplexing principle with a free-space transmission grating used in a DWDM system. When a wideband incident light source impinges on a diffraction grating after a collimating lens (see Fig. 1), each wavelength is diffracted by the grating at a different angle and therefore to a different point in space. By use of a coupling lens, these wavelengths can be focused into individual single-mode fibers. Among different types of free-space diffraction gratings, metallic reflection gratings can achieve a diffraction efficiency of 85% through the high reflectivity of a metal film. However, the power absorption of a metal film is an impediment to further efficiency improvement. A dielectric film can be an alternative to the metal film because of its lower power-absorption rate. However, it is fairly difficult 20 April 2006 Vol. 45, No. 12 APPLIED OPTICS 2567

2 Fig. 2. Schematic diagram of the efficiency measurement setup for a high-density surface-relief rectangular grating. Fig. 1. Schematic diagram of the demultiplexing principle with a free-space transmission grating used in a DWDM system. SMF, single-mode fiber. to fabricate a highly reflecting dielectric stack below a surface-relief grating to realize high diffraction efficiency. 5 Fused silica is an excellent optical material, with a wide transmitting spectrum ranging from deep ultraviolet to far infrared, high optical quality, and stable performance in an environment with fluctuating temperatures. These properties of fused silica make the material suitable for use in an optical communication system. Furthermore, deep-etched fusedsilica transmission gratings can exhibit a theoretical efficiency of 95% when the parameters of the grating reach an optimized condition. 6 8 Nguyen et al. 6 fabricated a fused-silica grating with period of 350 nm, a height of 580 nm, and a duty cycle (DC) of 0.5 for use in high-power UV lasers. By use of the grating, high efficiency and low damage (induced by two-photon absorption and self-focusing) can be achieved at the same time. Néauport et al. 7 also designed and fabricated high-aspect-ratio transmission gratings with high efficiency for use in high-power laser facilities. However, to our knowledge, no one has presented the optimized design and fabrication process of the deepetched binary-phase fused-silica transmission grating with high efficiency at a wavelength of 1550 nm, which is used in optical-fiber communication. An optimized fused-silica transmission grating with high diffraction efficiency should be very useful in DWDM systems. the period is no larger than wavelength, wecan permit only two orders of m: m 1and m 0. Figure 2 shows the schematic diffraction diagram of a diffraction grating with a high density (period ). There are only two transmission diffraction orders, T 1 and T 0 (in Fig. 2). The next step is to design optimized parameters for the above highdensity diffraction grating to obtain the maximum high diffraction efficiency on the m 1 diffraction order. Parameters of a grating include grating period, groove depth, index of the fused silica, and DC: DC a, (2) where a is the width of a grating line. Other important parameters include incidence wavelength, polarization state (TE, TM) of the incident beam, and the absorption rate of the fused silica. By using the rigorous coupled-wave theory of binary gratings, 9 we found that the optimized profile of a grating will have the necessary minimal groove depth for DC 0.5. Nguyen et al. 6 and J Néauport et al. 7 also proved this conclusion. We use a rectangular profile for the grating with a DC value of 0.5, an index of at a wavelength of 1550 nm; TEpolarized light with a wavelength of 1550 nm is used 2. Optimized Design of the Grating To use a diffraction grating in a DWDM system, we must find an optimized structure for the grating to acquire a high diffraction efficiency. The basic grating equation can be expressed as sin d m sin i m, (1) where i and d m are the incidence angle and diffraction angle, respectively, is the wavelength, and is the period of the grating. To achieve high diffraction efficiency in a specific order, it is necessary to minimize the number of all diffraction orders (number of m). If Fig. 3. Theoretical efficiency of a fused-silica grating with 1550 nm, m 1order, and TE polarization as a function of groove depth and density for a rectangular profile with a DC of APPLIED OPTICS Vol. 45, No April 2006

3 as the incident beam. With the above parameters, we obtain the relation between the diffraction efficiency of T 1 order with the grating line density and the etched groove depth (Fig. 3). 10 From Fig. 3, we can see that fused-silica transmission gratings with a DC of 0.5, a density of lines mm, and a groove depth of m can reach a diffraction efficiency of more than 95% at a wavelength of 1550 nm. In this region, a TM-polarized light with a wavelength of 1550 nm can also reach an efficiency above 80%. The low polarization-dependent loss and the high efficiency indicate that a grating with parameters in this region is suitable for use in a DWDM system. 3. Fabrication Setup and Results A. Fabrication Procedure The fabrication of deep-etched high-density fusedsilica transmission gratings mainly includes a holographic recording process and an etching step. The holographic recording technology is an effective way of producing high-density gratings. The etching step is crucial here because an etched depth of mis very large compared with a linewidth of 800 nm. Generally speaking, there are two types of etching method: wet chemical etching 11 (WCE) and dryetching methods. WCE is cheap and fast. However, the undercut effect induced by the isotropic etching property of WCE makes it impossible to obtain a rectangular grating profile with an aspect ratio as high as Dry etching is realized in a plasma environment through chemical reactions and physical reactions between ionized erosive gases and substrate. Dry-etching methods include reactive-ion etching, 12 ion milling, electron cyclotron resonance, and inductively coupled plasma 13,14 (ICP), etc. An ICP is a plasma system with a high density of plasma at low pressure, which could achieve a high etch rate and anisotropic etching. ICP-etching technology is now used to fabricate micro-optical elements with high aspect ratios. 14 The detailed process flow for fabricating a deepetched high-density fused-silica transmission grating with ICP-etching technology, shown in Fig. 4, is as follows from the figure: 1. A chromium layer is evaporated on the surface of a clean and dry fused-silica wafer. 2. A positive photoresist (PR) film (Shipley, Model s1805, USA) is coated on the chromium layer. 3. Laser interference holographic lithography technology is used to form the desired grating pattern on the photoresist. The substrate with the PR on its surface is illuminated by two plane waves, and the interferogram is then recorded on the photoresist (a He Cd laser with a single wavelength of 441 nm is used in the recording system). 4. The developing process takes place, and the grating pattern forms on the chromium mask by chemical solution. 5. The PR layer is removed by chemical solution, and the fabricated sample with the chromium grating mask is placed into the ICP equipment for etching. 6. The remaining chromium mask is removed by chemical solution. An ICP-etching system and an optimized etching condition used in a previous work 14 were used in the etching process of this experiment. However, the fused-silica wafer temperature during the etching process was cooled down properly by cooling ice water of 4 C 5 C, and the metal mask film was used as the protective layer for deep etching. B. Fabrication Result Figure 5 shows two cross-sectional images of a fabricated optimized high-density fused-silica grating observed with a scanning electron microscope. Figure 5(a) is the scanning electron microscope image of a wide range of grating lines, which shows that the uniformity of the grating lines in a large area is good. Figure 5(b) shows a magnified part of Fig. 5(a). From Fig. 5(b), we can easily observe that the sidewall of the grating is vertical. From the two figures, we know that the etched groove depth of the grating is 2.5 m, the density of the grating lines is 674 lines mm, and the DC is 0.5. The aspect ratio of the fabricated grating is 3.37, which reaches a high level. The surface of the fabricated grating is clean, and no polymer deposition can be observed. 4. Analysis and Discussions A. Analysis of the Fabrication Result We measured the diffraction efficiency of the fabricated grating with TE-polarized light and TMpolarized light at a wavelength of 1550 nm. The instrument setup for the measurement of the diffraction efficiency is shown in Fig. 2, in which a laser beam with a wavelength of 1550 nm from an optical fiber impinges on the surface of the grating through a collimator. The angle between the normal direction of the grating surface and the direction of the laser beam can be changed freely. Theoretically, when the incident-beam angle equals the Bragg angle, which can be expressed by arcsin 2, (3) Fig. 4. Process flow for fabricating a deep-etched high-density fused-silica transmission grating with ICP-etching technology. PR, photoresist. the diffraction efficiency of the grating reaches its highest value. In Eq. (3), is the wavelength of the incident laser beam and is the grating period. In the experiment, we measured the m 1 order diffraction efficiency T 1 I 0 of the TE-polarized light and TM-polarized light at incident angles ranging from 20 April 2006 Vol. 45, No. 12 APPLIED OPTICS 2569

4 Fig. 5. Scanning electron micrograph cross-sectional images of (a) a large area and (b) a magnified local part of a fabricated fusedsilica grating with a density of 674 lines mm, a groove depth of 2.5 m, and a DC of to 45. The measured results are shown in Fig. 6. From Fig. 6(a), we can see that the measured diffraction efficiency of the TE-polarized incident light has the highest value of 87.1% at an incidence angle of 31.5 (Bragg angle). There is a difference between the experimental result, 87.1%, and the theoretical calculation result, 95%, which can be explained by several reasons. First, the back surface of the grating wafer reflects 4% of the whole incident power; the surface of the grating wafer is not so smooth, which will induce little scattering and absorption of the incident laser; also, deviations of the DC, the profile of the grating groove, and grating lines from their ideal conditions will also affect the diffraction efficiency. When all the above factors are taken into consideration, the experimental diffraction efficiency, 87.1%, of the grating is reasonable. The same analysis can be used to explain the deviation of the experimental efficiency of TMpolarized light from the relevant theoretical value [see Fig. 6(b)]. Fig. 6. Theoretical efficiencies and experimental efficiencies of the fabricated grating at different incidence angles. The incident beam is (a) TE polarized or (b) TM polarized with a wavelength of 1550 nm. B. Discussion Measures can be taken to decrease the power losses induced by the above factors, such as polishing the fused-silica wafer carefully before the fabrication process, depositing a suitable dielectric film on the back surface of the grating after the etching step for antireflection, and controlling the holographic lithographic process to get a precise 0.5 DC, etc. If all the above measures can be taken properly, an efficiency of greater than 90% can be achieved with TEpolarized incident light. At the same time, the diffraction efficiency of the TM-polarized light will also be increased. The fabricated optimized high-density fused-silica transmission grating has the highest theoretical efficiency of about 95% at a wavelength of 1550 nm. Theoretical calculations shown in Fig. 7 indicate that when TE-polarized light is used as the incident beam, the m 1 order diffraction efficiency of the grating keeps above 90% for the C L band with a wavelength from 1460 to 1620 nm. When TM-polarized light is used as the incident beam, the m 1 order 2570 APPLIED OPTICS Vol. 45, No April 2006

5 5. Conclusion DWDM technology can be realized through a freespace fused-silica diffraction grating, which has the obvious advantages of parallel demultiplexing, low polarization-dependent loss, and stable performance in an environment with fluctuating temperatures. The property of parallel demultiplexing makes it easy to demultiplex DWDM signals into multichannels. By using the rigorous coupled-wave theory of binary gratings, we successfully designed an optimized deepetched high-density fused-silica transmission grating with high efficiency at a wavelength of 1550 nm for use in a DWDM system. ICP-etching technology was used to fabricate the deep-etched fused-silica grating. The fabricated grating has a density of 674 lines mm, a DC of 0.5, and a groove depth of 2.5 m. The measured transmission efficiency of the fabricated grating can reach its highest value of 87.1% at a wavelength of 1550 nm. In addition, measurements and calculations show that the fabricated grating has a relatively low polarization-dependent loss. The fabricated optimized deep-etched high-density fusedsilica transmission gratings should be very useful in DWDM systems in optical communication. The authors acknowledge support from the National Outstanding Youth Foundation of China ( ) and the Shanghai Science and Technology Committee (05DZ22004) under the Program of Shanghai Subject Chief Scientist (03XD14005). Fig. 7. Theoretical efficiency of m 1 order of the fabricated grating with a density 674 lines mm, a groove depth of 2.5 m, and a DC of 0.5, at a wavelength from 1460 to 1620 nm. diffraction efficiency can also keep above 70% in the whole range. The relatively low polarizationdependent loss makes the element suitable to be used in a DWDM system. This result indicates that the fabricated deep-etched fused-silica high-density grating can be very useful in DWDM systems. References 1. J.-P. Laude, DWDM Fundamentals, Components, and Applications (Artech House Optoelectronics Library, 2002). 2. J. Qiao, F. Zhao, R. T. Chen, J. W. Horwitz, and W. W. Morey, Athermal low-loss echelle-grating-based multimode dense wavelength division demultiplexer, Appl. Opt. 41, (2002). 3. A. Sappey, Not all multiplexing technologies are on the same wavelength, Photonics Spectra 36, (2002). 4. A. Sappey and P. Huang, Free-space diffraction gratings allow denser channel spacing, Wavelength Division Multiplexing Solutions Res. Dev. 3, (2001). 5. L. Li and J. Hirsh, All-dielectric high-efficiency reflection gratings made with multilayer thin-film coatings, Opt. Lett. 20, (1995). 6. H. T. Nguyen, B. W. Shore, S. J. Bryan, J. A. Britten, R. D. Boyd, and M. D. Perry, High-efficiency fused-silica transmission gratings, Opt. Lett. 22, (1997). 7. J. Néauport, E. Journot, G. Gaborit, and P. Bouchut, Design, optical characterization, and operation of large transmission gratings for the laser integration line and laser megajoule facilities Appl. Opt. 44, (2005). 8. T. Clausnitzer, J. Limpert, K. Zöllner, H. Zellmer, H. J. Fuchs, E. B. Kley, A. Tünnermann, M. Jup, and D. Ristau, Highly efficient transmission gratings in fused silica for chirped-pulse amplification systems, Appl. Opt. 42, (2003). 9. M. G. Moharam, E. B. Grann, D. A. Pommet, and T. K. Gaylord, Formulation for stable and efficient implementation of the rigorous coupled-wave analysis of binary gratings, J. Opt. Soc. Am. A 12, (1995). 10. Y. Zhang and C. Zhou, High-efficiency reflective diffraction gratings in fused silica as (de)multiplexers at 1.55 mm for dense wavelength division multiplexing application, J. Opt. Soc. Am. A 22, (2005). 11. C. Zhou and L. Liu, Numerical study of Dammann array illuminator, Appl. Opt. 34, (1995). 12. J. N. Mait, A. Scherer, O. Dial, D. W. Prather, and X. Gao, Diffractive lens fabricated with binary features less than 60 nm, Opt. Lett. 25, (2000). 13. E. Gogolides and P. Vauvert, Etching of SiO 2 and Si in fluorocarbon plasma: A detailed surface model accounting for etching and deposition, J. Appl. Phys. 88, (2000). 14. S. Wang and C. Zhou, Optimized condition for etching fusedsilica phase gratings with inductively coupled plasma technology, Appl. Opt. 44, (2005). 20 April 2006 Vol. 45, No. 12 APPLIED OPTICS 2571