LCOS-Based Wavelength Blocker Array With Channel-by-Channel Variable Center Wavelength And Bandwidth

LCOS-Based Wavelength Blocker Array With Channel-by-Channel Variable Center Wavelength And Bandwidth


概要

We developed a new class of compact wavelength blocker (WB) array based on liquid crystal on silicon (LCOS), which enables per-channel control of not only optical transmission power level but also channel center wavelength and channel bandwidth independently. We demonstrate an integrated device with WB array having 12 input/output ports for 50-GHz spaced ITU-T channels over the entire C-band. The center wavelength and bandwidth can be adjusted independently with minimum setting resolution of 3 GHz and 6 GHz, respectively.


Keywords

Liquid crystal devices, Optical components, Tunable filters, Wavelength division multiplexing, Optical switches, Optical fiber communications

著者

Yasuki Sakurai, Masahiro Kawasugi, Yuji Hotta, MD. Saad Khan, Hisashi Oguri, Katsuyoshi Takeuchi, Sachiko Michihata, Noboru Uehara

発表日

2011/6/22

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I. INTRODUCTION

INCREASING network capacity, efficiency and flexibility are driving the demand for optical signals with higher data rates, new modulation formats and channel management technologies for wavelength division multiplexing (WDM) optical communication networks. In such next generation optical networks, network architectures and optical devices should be unrestricted by transmitted WDM signal conditions such as data rates, modulation formats and channel spacing [1]. On the other hand, reconfigurable optical add-drop multiplexers (ROADMs) are the key network elements and have been already implemented in many flexible and efficient networks. In general, ROADMs consist of passive optical splitters, combiners and per-channel controllable devices such as wavelength selective switches (WSS) or wavelength blockers (WB), which can adjust optical power level and select add/drop channels with port switching or channel blocking functions.

Various technical approaches based on micro-electro-mechanical systems (MEMS) [2]-[4], liquid crystal (LC) technologies [5], [6], and planar lightwave circuit (PLC) technologies [7] have been reported. However, it is difficult for these devices to control spectral parameters like center wavelength and bandwidth because they have generally been limited by having inflexible configurations with one pixel or one switch element per WDM channel. In this regard, liquid crystal on silicon (LCOS) based WSSs and WBs are very attractive solutions for next generation optical networks because of their ability to adjust these spectral parameters arbitrarily by taking advantage of fine LCOS pixel resolution with individual electrical control [8]-[10].

In this paper, we introduce a new class of compact LCOS-based WB array with channel-by-channel variable center wavelength, bandwidth and optical power control. We demonstrate an integrated device with WB array having 12 input/output ports for 50-GHz spaced ITU-T channels over the entire C-band. The center wavelength and bandwidth can be adjusted independently with minimum setting resolution of 3 GHz and 6 GHz, respectively. Integrated WB array enables the handling of WDM signals from multiple optical fibers

independently at the same time, which will also contribute to cost reduction and downsizing of ROADM systems. To our knowledge, this is the fist demonstration of an integrated spectrum controllable device array in compact footprint.


II. DEVICE STRUCTURE

Figure 1 shows the schematic diagram of the developed 12 port WB array. The filter geometry is a free-space littman monochromator. The design is divided into three functional blocks; an input/output (I/O) block, a spectrometer block and a LCOS block. The I/O block containing optical circulators and polarization converters handles 12 input/output optical beams separated by 250 μm pitch collimating to the spectrometer block

with the pair of a microlens array and an anamorphic condenser lens. The input light is split spatially into two optical beams with orthogonal linear polarizations each other and the polarization axis of one beam is rotated by 90 degree for converting to the polarization state with higher diffraction efficiency of a spectrometer using a polarization splitter and a half-wave plate.

The spectrometer block, consisting of transform lens and dispersive element, disperses the WDM channels spatially and project onto channel-by-channel different LCOS pixels. Unlike the other WSS devices using 2-f imaging geometry [2], [3], we adopt a 4-f imaging system consisting of a pair of condenser and transform lens, which disperses wavelength in X-axis and split different ports along Y-axis as shown in Fig. 1 (b), thus enabling array function.

Fig. 1, 2, 3

We chose GRISM consisting of a 1800-lines/nm reflective diffraction grating and a fused silica prism as dispersive element to achieve higher dispersion, which contributes to higher wavelength and bandwidth resolutions. The focal length of

transform lens is 45 mm and focusing beam diameter on LCOS plane is 50 μm in X-axis and 100 μm in Y-axis. Dispersion of the GRISM was chosen so that the reciprocal linear dispersion along wavelength dispersion axis on the LCOS plane is 0.37GHz/μm. Our LCOS processor has 2-dimensional 1920 x 1080 pixels separated by 8.5 μm pitch and can address pixel-by-pixel.

The pixel width of 8.0 μm corresponds to 3 GHz on wavelength dispersion axis, and thus the minimum resolution of channel center wavelength and spectral bandwidth are 3 GHz and 6 GHz, respectively. Optical power attenuation and blocking functions can be achieved with a polarizer by changing polarization state at each pixel. The entire package size is 200 mm (width) x 120 mm (length) x 16 mm (Height) including electrical circuit board as shown in Fig. 2.


III. EXPERIMENTAL RESULT

Figure 3 shows typical filter transmission profile of 12 port counts WB array on the 50-GHz spaced ITU-T channels. Four out of 96 channels (5, 33, 61 and 89 channel) over the entire C-band (ITU-T channels of 191.3 GHz to 196.05 GHz) are set to 0 dB attenuation and all the other channels are set to blocking state. The 0.5 dB and 3 dB passband widths were greater than standard window of 30 GHz and 40 GHz for 50-GHz ITU-T channels, respectively. The total insertion loss is less than 6.0 dB across the entire C-band and extinction ratio of better than 35 dB was obtained. The low reflectivity of LCOS segmented aluminum mirror accounts for excess loss of 1.8 dB within the total loss, which can be improved by coating a high reflective dielectric mirror on the segmented aluminum. Crosstalk between ports of lower than -40 dB was also observed.

Fig. 4, 5, 6


Figure 4 shows the passband variable operation on ITU-T channel of 193.7 THz. The spectral 3dB-passband was set from 10 GHz to 100 GHz by 10 GHz increment through firmware.The bandwidth can be digitally tuned with the setting accuracy

of 6 GHz without significant filter shape distortion and degradation of insertion loss.

The optical power attenuation can be set from 0 dB to 15 dB with 0.1 dB minimum setting size as shown in Fig. 5. The optical passband ripple was about 0.4 dB at 15 dB attenuation setting. This ripple is caused by the optical interference due to refractive index mismatching between liquid crystal and LCOS indium tin oxide (ITO) glass, which should be reduced with optimum anti-reflection coating. Figure 6 shows the spectrum mixed with both 50-GHz and 100-GHz spacing channels with per-channel optical power equalization and bandwidth adjusting with LCOS pixel-by pixel controls.



IV. CONCLUSIONS

We introduced a compact LCOS-based WB array with independently channel-by-channel variable bandwidth and center wavelength, and demonstrated one-package integrated 12 input/output port counts WB array designed for 50-GHz channel spacing over the entire C-band.

Proposed array concept is cost effective and good downsizing solutions for ROADM systems, which opens up a new class of various ROADM architectures including colorless channel drop, N x M wavelength cross connect and truly transparent channel-by-channel path routing independent of transmitted signal conditions such as data rates, modulation format and channel spacing.