Variable Optical Attenuators

Laser light emerged from the TDL at a power level of approximately 20 mW and was amplified to approximately 300 mW. A beam splitter directed 99% of the amplified laser power via a Variable Optical Attenuator (VOA) and a 350-400 foot long fiber optic cable to the launch module. The beam was transmitted across the... 

The main rack contains all components apart from the laser source and MCP-PMT cooler unit. Most of the electronics and the control PC are housed on the front of the 19-inch rack. Four variable optical attenuator boxes containing eight... 

Ru -Ru species that display intense intervalence charge-transfer bands at 1550 nm, close to the telecommunication wavelength of 1.5 pm (Fig. 8.6). Cross-linked films can be used in variable optical attenuator devices, and stability over 18,000 switching cycles was demonstrated with a response time of ca. 2 s (see Fig. 8.7) [99, 100]. 

Fig. 8.7 Variation of current (top) and transmittance (bottom) as a function of time for a variable optical attenuator device that uses crosslinked films of dendrimer 8.44 for 5 s A/itches (step time 2 s). (Adapted from [100])...

A variety of devices have been designed by using transparent electro-optical ceramic materials, including variable optical attenuators (VOA), polarization controllers (PC), sinusoidal filters, dynamic gain flattening filters, tunable optical filters, and (2-switches, which have been described in detail in Ref. [129]. A brief description is given for some devices as follows. 

Zhu L, Huang Y, Yariv A (2005) Integrated microfluidic variable optical attenuator. Opt Express 13(24) 9916-9921... 

Y.-Q. Lu, P. Du, Y.-H. Lin, and S.-T. Wu, Variable optical attenuator based on polymer stabiUzed twisted nematic liquid crystal. Optics Express, 12, 1222 (2004). 

In addition to displays, liquid crystals have also been used extensively in tunable photonic devices, such as optical phased array for laser beam steering, variable optical attenuator (VOA) for telecommunications, tunable-focus lens for camera zoom lens, LC-infiltrated photonic crystal fibers [1,2], diode laser-pumped dye-doped LC laser, just to mention a few. 

In this chapter, we only select four topics to illustrate the potential appUcations of liquid crystals in photonics and their technical challenges. The four representative subjects selected are (1) laser beam steering, (2) variable optical attenuator, (3) tunable-focus lens, and (4) polarization-independent LC devices. 

Figure 12.7 Schematic diagram of DFLC-based variable optical attenuator (1 = polarization beam displacer, 2 = half-wave plate, 3 = master LC cell, and 4 = compensation ceU).

X. Liang, Y. Q. Lu, Y. H. Wu, et al.. Dual-frequency addressed variable optical attenuator with submillisecond response time, Jpn. J. Appl. Phys. 44, 1292 (2005). 

The SOIMUMPS process is based on bulk micromachining of a silicon on insulator (SOI) wafer using four mask levels. It was originally developed for the fabrication of MEMS variable optical attenuators (VOAs) based on the use of a thermal actuator to control an optical shutter [18]. A cross-sectional diagram of an SOI wafer is shown in Figure 1.12. 

R. Wood, V. Dhuler, and E. Hill, A MEMS variable optical attenuator, Proc. lEEE/LEOS Int l Conf. Opt. MEMS, Kauai, HI, Aug. 2000, pp. 121-222 (2000). 

A special case are ion storage layers with complementary electrochromic activity, which may be assembled with a PEDOT layer. In a complementary electrochromic cell, both layers—dye and bleach—simultaneously increase to the overall optical contrast. Tungsten trioxide as well as Prussian Blue, iron(III) hexacyanoferrate(II/III) bleach upon reduction and were used in combination with PEDOT. Cells made from PEDOT in combination with Prussian Blue exhibited a deep blue-violet color at a potential of -2.1 V and become light blue at 0.6 V. Combination cells of alkylenedioxypyrrols or -thiophenes with ferrocene have been proposed as variable optical attenuator devices due to a large dynamic range of optical attenuation at the telecommunication wavelength of 1550 nm. ... 

J. D. Zhang, H. A. Yu, X. G. Wu, and Z. Y. Wang. 2004. Towards the improvement of attenuation range and response time of electrochromic polymer-based variable optical attenuators. Opt Mater 27(2) 265-268. 

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