Programmable narrow-band filtering using the WaveShaper 1000 S and WaveShaper 4000 S

The WaveShaper family of Programmable Optical Processors provide unique capabilities for the manipulation and transformation of optical signals. They are based on Finisar’s field-proven Dynamic Wavelength Processor, at the heart of which is a Liquid Crystal on Silicon (LCoS) switching element. This White Paper provides an overview of the optical design of the WaveShaper family and provides insights into how the narrow-band amplitude filtering, power control and switching characteristics of the WaveShaper family are obtained. The application of these capabilities to the generation of arbitrary channel and filter transfer characteristics is discussed. 1. What is Liquid Crystal on Silicon? The key to the operation of the WaveShaper family of Programmable Optical Processors is the Liquid Crystal on Silicon (LCoS) optical switching element. LCoS is a display technology which combines Liquid Crystal and semiconductor technologies, to create a high resolution, solid-state display engine [1]. Figure 1 shows the structure of an LCoS display with the Liquid Crystal (LC) layer sandwiched between the Active Matrix silicon backplane and the ITO-coated top glass. Figure 1: Schematic of LCOS Structure In the WaveShaper, the LCoS is used to control the phase of light at each pixel to produce an electrically-programmable grating. This can control the beam deflection in a vertical direction by varying either the pitch or blaze of the grating whilst the width of the channel is determined by the number of pixel columns selected in the horizontal direction. (Figure 2). Figure 2: WaveShaper Optical Schematic 2. WaveShaper Optical Design The WaveShaper optical design is shown schematically in Figure 2. It incorporates polarisation diversity, control of mode size and a 4-f wavelength optical imaging in the dispersive axis of the LCoS providing integrated switching and optical power control. In this note we refer to each filtered set of wavelengths as a channel, as in a DWDM channel, although, as will be seen later, the channels do not have to correspond to the standard ITU 50and 100 GHz channel grids. In operation, the light passes from a fibre array through the polarization diversity optics which both separates and aligns the orthogonal polarization states to be in the high efficiency s-polarization state of the diffraction grating. (For the WaveShaper 4000S, the input fibre array uses 5 fibers (1 x common, 4 x input/output) whilst the WaveShaper 1000S uses 2 fibers (1 x input, 1 x output). ) The light from the input fibre is reflected from the imaging mirror and then angularly dispersed by the grating, reflecting the light back to the cylindrical mirror which directs each optical frequency (wavelength) to a different portion of the LCoS. The path for each wavelength is then WHITE PAPER: Programmable narrow-band filtering using the WaveShaper 1000S and WaveShaper 4000S Page 2 retraced upon reflection from the LCoS, with the beamsteering image applied on the LCoS directing the light to a particular port of the fibre array. Figure 3: LCOS Image showing different channel bandwidths (horizontal axis) for 200, 100 and 50 GHz channels and different grating patterns (vertical axis) for switching to 2 different ports. Example Mixed Channel Plan: 50, 100, 200GHz Figure 4: The resulting optical spectra for the two ports showing a programmable interleaver with 200, 100 and 50 GHz channel spacing. The phase image is, effectively, a portion of video image and the WaveShaper uses internal video-processing circuitry to generate and maintain the required image. A portion of an LCoS switching image is shown in Figure 3. For this example, the LCoS is programmed as an interleaver, with a common input port and two output ports. The channel bandwidth is varied by selecting different numbers of pixel columns as described above, with channels of 200, 100 and 50 GHz shown. In this example, alternating channels are switched to either output port 1 or output port 2 depending on the periodicity of the grating applied to that channel. The resulting spectral response measured on an Optical Spectrum Analyser with a broadband ASE input source is shown in Figure 4. 3. Insertion Loss and Crosstalk The WaveShaper has a low insertion loss, typically around 4.5dB), a very flat spectral response across the C band and (for the WaveShaper 4000S) low cross talk between ports. Figure 5 shows the C-band optical response of a WaveShaper 4000S configured as 1 x 4 drop Wavelength Selective Switch with a common input port and 4 output drop ports. The measurement was taken with a broadband light input which was directed by the WaveShaper to output port 1 and shows the flat response across the whole C-band, low insertion loss (4.65dB incl connectors) and low cross-talk from this port into the other drop ports. Figure 5: Wavelength Dependence of Insertion Loss and Crosstalk. 4. Filter Functions As the wavelength channels are separated on the LCoS the control of each wavelength is independent of all others and can be switched or filtered without interfering with the light on other channels. 4.1 50 and 100 GHz Flat-top Filter Function Typical flat-top filtering functions for the WaveShaper 1000S, configured for 50 GHz and 100 GHz (3dB Bandwidth) channels, are shown in Figure 6 (a) and (b) respectively. In this, 80 x 50 GHz channels (a) and 45 x 100 GHz channels (b) with adjacent channels blocked, are superimposed to show the uniformity of channel shape across the C-band and the alignment of the channels with the ITU grid. The flat-top channel shape and high extinction to adjacent channels is clearly visible. Crosstalk Switching to Port 3, 0dB, 0°C