Liquid-crystal-based tunable filter for WDM (lambda=1550 nm)

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LIQUID-CRYSTAL-BASED

TUNABLE FILTER FOR WDM

(

λ=1550 nm)

Ci-Ling Pan a , Minjay Huang b & Ru-Pin Pan c a

Institute of Electro-Optical Engineering, National Chiao Tung University , Hsinchu, Taiwan 30010, R.O.C.

b

The Lee-MTI Center, National Chiao Tung University , Hsinchu, Taiwan 30010, R.O.C. c

Department of Electrophysics , National Chiao Tung University , Hsinchu, Taiwan 30010, R.O.C.

Published online: 07 Jan 2010.

To cite this article: Ci-Ling Pan , Minjay Huang & Ru-Pin Pan (2004)

LIQUID-CRYSTAL-BASED TUNABLE FILTER FOR WDM (λ=1550 nm), Molecular Crystals and Liquid Crystals, 413:1, 561-568, DOI: 10.1080/15421400490439301

To link to this article: http://dx.doi.org/10.1080/15421400490439301

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LIQUID-CRYSTAL-BASED TUNABLE FILTER

FOR WDM (

k=1550 nm)

Ci-Ling Pan

Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan 30010, R.O.C.

Minjay Huang

The Lee-MTI Center, National Chiao Tung University, Hsinchu, Taiwan 30010, R.O.C.

Ru-Pin Pan

Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan 30010, R.O.C.

We report a new configuration of tunable optical filter for DWDM applications. In this design, first-order diffracted signal light by a grating is directed to a lens and focused on to a transmission-type liquid crystal spatial light modu-lator (LC SLM). Wavelength channels are selected by opening the appropriate pixels of the LC-SLM for transmission. Filtering into 15 channels of the 1550 nm band is demonstrated in the initial experiment. The channels are designed according to the International Telecommunication Union (ITU) grid with channel spacing of 100 GHz. Channel crosstalk is less than 30 dB. The average 1 dB, 3 dB, and 30 dB passbands of the filter are 0.07 nm, 0.13 nm, and 0.91 nm, respectively. The extinction ratio can be as high as 24.7 dB.

Keywords: DWDM; liquid crystal; optical filter; router; spatial light modulator; switch

1. INTRODUCTION

High-performance and cost-effective tunable optical filters are essential for the next generation of dynamic WDM systems and networks [1]. A number

This work was supported by the National Science Council of the ROC under various grants, the Lee-MTI Center of NCTU, and the Radiantech Corporation. Ci-Ling Pan was also supported by the Pursuit of Academic Excellence Program of the Ministry of Education of ROC.

Address correspondence to Ci-Ling Pan, Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan 30010 R.O.C.

561=[2697]

Mol. Cryst. Liq. Cryst., Vol. 413, pp. 561=[2697]–568=[2704], 2004 Copyright # Taylor & Francis Inc.

ISSN: 1542-1406 print=1563-5287 online DOI: 10.1080=15421400490439301

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of designs have been implemented in the past, e.g., Fabry–Perot and Mach– Zehnder interferometers, Fiber Bragg Gratings, Acousto-optic as well as electro-optical tunable filters (AOTF and EOTF), and arrayed waveguide grating devices. Active filters based on laser structures operating below threshold are also attractive candidates.

In this work, we demonstrate a new configuration of tunable optical filter for applications in optical communication–systems of the 1550 nm band. A transmission-type liquid crystal spatial light modulator (LC SLM) is used for channel selection. Wavelength channels are selected by opening the appropriate pixels of the LC-SLM for transmission. This device is also expected to be useful for other WDM applications, e.g., switching and routing.

2. BASIC PRINCIPLES AND EXPERIMENTAL METHODS

The basic principle of the present device is similar to that of many grating-based spectral filtering devices. A schematic of the device and the testing setup is shown in Figure 1. This is a modification of our previous external-cavity design for multi-frequency tunable lasers [2]. Signal light is either the broadband emission from an Erbium Doped Fiber Amplifier (C-band power booster, New Elite technology) or a tunable laser diode. It is collimated by a lens, sent through an optical isolator before incidence on a grating

FIGURE 1A schematic diagram of the experimental setup. LC-SLM: liquid crystal spatial light modulator; EDFA: Erbium-doped fiber amplifier.

562=[2698] Ci-Ling Pan et al.

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(1100 lines=mm) at the grazing-incidence angle hi¼ 82). The first-order

diffracted light is directed to a lens and focused onto a liquid crystal spatial light modulator (LC-SLM). The LC-SLM is basically a normally off-state twisted nematic liquid crystal (NLC) cell (See Fig. 2). The cell was constructed with a 6-mm-thick NLC (E7 manufactured by Merck) layer sandwiched between indium-tin-oxide (ITO) glass plates. One of the ITO-electrodes was patterned. The pattern consisted of fifty 100mm 2 cm stripes with 5mm spacing. Using the grating equation, one can readily show that the dispersion of the filter is given by

Dx¼

dk

dx¼ a cos hm 1

f ; ð1Þ

where a is the grating period (1=1100 mm),hm(¼ 44fork¼ 1532 nm) is the

diffraction angle and f(¼ 257 mm) is the focal length of the lens. For pixels separated by Dx in mm, the wavelength separation Dk (for the present experiment) is given byDk=Dx¼ 2.547 nm=mm.

Output of the filter is collected and analyzed by using an optical spec-trum analyzer (Advantest Q8384 with a resolution of 0.01 nm). The authors presented demonstration of the design concept at 830 nm previously [3].

3. RESULTS AND DISCUSSIONS

In the first experiment, the channel spacing is designed to be 0.26 nm and the filtering bandwidth is 0.12 nm. Figure 3(a) is the output of the filter by turning on 10 neighboring channels in turn. For a channel with center wavelength of 1533.4 nm, we show the output spectrum with the pixel turned on and off, respectively (See Fig. 3(b)). The extinction ratio was 17.2 dB. Averaging over the 10 channels, the 1 dB, 3 dB and 20 dB pass-bands of the filter were 0.07, 0.13, and 0.52 nm, respectively. The channel cross-talk was less than 14.7 dB.

Using a tunable laser diode as the signal light, we demonstrate electro-nically tunable filtering operation of the device with 15 channels. This is shown in Figure 4(a). The nominal output wavelengths of the channels are precisely set according to the ITU (International Communication Union) grid with channel spacing of 100 GHz. It can be seen that a chan-nel spacing of 0.79 nm, a 3 dB pass band of 0.12 nm and a 30 dB passband of 0.91 nm have been realized. The channel isolation is thus better than 30 dB. The average extinction ratio of the channels is 18.2 dB and can be as high as 24.7 dB. This is illustrated in Figure 4(b). The predicted wavelengths are also in good agreements with theoretical predictions (See Fig. 5).

Liquid-Crystal-Based Tunable Filter for WDM 563=[2699]

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FIGURE 2Configuration of the LC-SLM in the off-state (a) and on-state. (See COLOR PLATE XXXVII)

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FIGURE 3(a) Output of the filter for a succession of 10 channels. The input signal was a broad band EDFA. Extinction of a given channel can be observed in (b). (See COLOR PLATE XXXVIII)

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The liquid crystal element allows additional functionalities for the present device: The channels can be switched on and off at a rate limited by the LC SLM. To demonstrate, a 1 kHz biasing signal was applied to one of the pixels. Results are shown in Figure 6. The upper trace is the switching waveform. The lower trace is the filter output. The switch-on

FIGURE 4(a) Performance of the tunable filter channels according to the ITU grid with a channel spacing of 100 GHz. (b) Extinction of a particular channel. (See COLOR PLATE XXXIX)

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time, the time it takes for a pixel to change from an off state to an on state, is about 16 ms. The switch-off time is about 242 ms. This is primarily determined by the dynamic characteristics of the twisted NLC cell.

FIGURE 5A comparison of the output wavelength of the channels and the theoretical prediction according to Eq. (1).

FIGURE 6The channels of the filter can be switched on and off electronically. The upper trace is the switching waveform. The lower trace is the filter output.

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4. CONCLUSIONS

We report a new configuration of tunable optical filter for DWDM applications. In this design, first-order diffracted signal light by a grating is directed to a lens and focused on to a transmission-type liquid crystal spatial light modulator (LC SLM). Wavelength channels are selected by opening the appropriate pixels of the LC-SLM for transmission. The device is demonstrated by using EDFA and tunable laser diode as the signal light. With 100 micron-wide pixels separated by 5 microns in the LC-SLM and an 1100 lines=mm grating, we show selection of 15 channels with channel spacing of 0.79 nm (or 100 GHz) according to the ITU grid. The 3dB band-width is 0.13 nm. The channel isolation is better than 30 dB. This device is also expected to be useful for other DWDM applications, e.g., switching, demultiplexing, routing, and gain equalization.

REFERENCES

[1] Sadot, D. & Boimovich, E. (1998). IEEE Communications Mag., December, 50. [2] Ci-Ling Pan, Shang-Huang Tsai, Ru-Pin Pan, Chia-Reng Sheu, & Wang, S. C. (1999).

Electron. Lett., 35, 1472.

[3] Pan, R.-P., Tung, X.-X., Chen, J.-Y., Huang, M.-J., & Pan, C.-L. (2001). In Active and Passive Optical Components for WDM Communication, Achyut K., Dutta, Abdul Ahad, S., Awwal, Niloy K., Dutta, Katsunari Okamoto, (Eds.), Proceedings of SPIE, Vol. 4532, pp. 244–248.