WHITE PAPER. Programmable narrow-band filtering using the WaveShaper 1000S and WaveShaper 4000S. Abstract. 2. WaveShaper Optical Design

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1 WHITE PAPER Programmable narrow-band filtering using the WaveShaper 1S and WaveShaper 4S Abstract 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 5- and 1 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 4S, the input fibre array uses 5 fibers (1 x common, 4 x input/output) whilst the WaveShaper 1S 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 Page 1

2 WHITE PAPER: Programmable narrow-band filtering using the WaveShaper 1S and WaveShaper 4S 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. (for the WaveShaper 4S) low cross talk between ports. Figure 5 shows the C-band optical response of a WaveShaper 4S 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. Crosstalk - Switching to Port 3, db, C Port 1-1 Port 2 Port 3 Figure 3: LCOS Image showing different channel bandwidths (horizontal axis) for 2, 1 and 5 GHz channels and different grating patterns (vertical axis) for switching to 2 different ports. Example Mixed Channel Plan: 5, 1, 2GHz Insertion Loss (db) - Port 1 Crosstalk (db) - Ports Port Frequency (THz) Figure 4: The resulting optical spectra for the two ports showing a programmable interleaver with 2, 1 and 5 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 2, 1 and 5 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 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 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 and 1 GHz Flat-top Filter Function Typical flat-top filtering functions for the WaveShaper 1S, configured for 5 GHz and 1 GHz (3dB Bandwidth) channels, are shown in Figure 6 (a) and (b) respectively. In this, 8 x 5 GHz channels (a) and 45 x 1 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. Page 2

3 WHITE PAPER: Programmable narrow-band filtering using the WaveShaper 1S and WaveShaper 4S Figure 6: Overlay of flat-top filters referenced to the ITU Grid (a) shows 8 x 5 GHz channels overlaid (b) shows 45 x 1 GHz channels overlaid 4.2 Arbitrary Channel Bandwidth The WaveShaper is not limited to 5- and 1 GHz bandwidth filters. Since the channel bandwidth is set by the number of columns grouped together on the LCoS backplane, the filter bandwidth can be controlled by grouping together the appropriate number of columns. The WaveShaper software provides 1 GHz resolution for setting the channel bandwidth. Examples of variable bandwidth filters are shown in Figure 7 where the channel bandwidth is varied in 1 GHz steps between 2 GHz and 1 GHz. Figure 7: Variable Bandwidth, Narrow-Band, Flat-top Filters 4.3 Non Flat-top Filter Functions Whilst for many applications, a flat-top filter response provides the optimum performance, other applications may require different filter shapes. By varying the amplitude control function within the channel, the LCoS transfer function can be shaped to control the filter shape [2]. The WaveShaper has three controls for channel shaping built into the software. Each channel can be individual shaped or a common shape can be applied to all channels using the <All Channels> function. Linear adds a linear amplitude ramp across the selected channel, with either positive or negative slope. Examples of this are shown in Figure 8. Figure 8: Sloped filter shapes generated by using the Linear Channel Shaping function of the WaveShaper Page 3

4 WHITE PAPER: Programmable narrow-band filtering using the WaveShaper 1S and WaveShaper 4S Quadratic applies a quadratic function to the shape of the channel. The sign of the quadratic can be either positive or negative, which gives the spectra shown in Figure 9. In both cases, the graph shows and overlay of multiple filters generated by varying the amplitude of the quadratic term. The Linear and Quadratic functions can be combined to any channel to produce more complex filter shapes. The filter shaping functions (linear and quadratic) can also be applied to band-stop filter shapes created using the attenuation control function. Offset Both the Linear and Quadratic functions are, by default, applied symmetrically across the filter bandwidth. The WaveShaper also allows the applied function to be offset from the central position if required. For the WaveShaper 4S, any arbitrary channel shape can be used be used as the shape for channel switching. -1 Attenuation (db) Frequency Offset (GHz) Figure 1: Example of Amplitude control. Grey trace shows six unattenuated 1 GHz channels, separated by blocked channels whilst blue trace shows the same channels attenuated by -1 db in 2 db steps. All other channels are blocked. 4.5 Arbitrary Filter Transfer Functions For the majority of applications, the front-panel controls provided by the WaveShaper software will provide sufficient control of the filter transfer functions. However, more complex filter functions can be created by using the WaveShaper s ability to import a user-generated filter profile (amplitude and phase). The details of this are the subject of a separate Application Note, but examples of the complexity of filtering that can be obtained are shown in Figure Port P 16 Port P 27-2 Figure 9: Non-uniform filter shapes generated by applying the quadratic filter shaping functionality of the WaveShaper. 4.4 Band-stop Filters Whilst for the majority of applications, the WaveShaper will be used as a band-pass filter, the WaveShaper can also create band stop filters. These can either be fully-blocked (as in the blocked channels in Figure 1) or can have a controlled amplitude using the attenuation control function. Power (db) W avelength (nm) Figure 11: Arbitrary user-generated filter functions created using the File Import function of the WaveShaper [3] Page 4

5 WHITE PAPER: Programmable narrow-band filtering using the WaveShaper 1S and WaveShaper 4S 5. Filter Centre Frequency and Alignment to ITU Grid The default setting for the WaveShaper is to align the channels with the ITU 5- or 1 GHz channel spacing grid. This simplifies its use in DWDM testing and simulation applications. However, the WaveShaper also allows each channel to be offset from the ITU grid by up half of the channel spacing (e.g. up to 5 GHz offset for a 1 GHz channel spacing) This frequency offset can be set with a resolution of 1 GHz and, as with all WaveShaper settings, can be applied to each channel individually or to all channels together. This capability allows the user to investigate, for example, the effects of filter drift on receiver performance, or to work with frequencies which are off the ITU grid. The WaveShaper also has the ability to control transmitted optical power on a per-channel and per-port (WaveShaper 4S) basis. This per-channel attenuation control is achieved by setting the grating pattern on the LCoS to one in which splits the light into two paths and directs part of the light to the output fibre, with the remaining, unwanted, power directed to a `dump location within the WaveShaper. The attenuation is therefore controlled by varying the relative powers in each of these two beams. This attenuation control mechanism does not rely on displacement of the image on the output fibre and so does not require feedback mechanisms to stabilize the attenuation. 6. Optical Power Control Attenuation control between and 15 db with.1 db resolution is provided. The WaveShaper also provides the ability to block light a channel where an attenuation of greater than 15 db is required. An example of both channel blocking and amplitude control is shown in Figure 1, which also shows how the WaveShaper can be used to generate arbitrary comb filters with programmable amplitude response. Figure 12 shows how the amplitude control function of the WaveShaper can be used to generate a broadband saw-tooth filter profile. Attenuation (db) Figure 12: Broadband saw-tooth filter function generated using the WaveShaper amplitude control function. 7. Dispersion Control The WaveShaper also provides control of group delay within the channel. For details of how this is achieved, please see the companion White Paper Dispersion Trimming using the Programmable Group Delay capability of the WaveShaper 1S and 4S. 8. Conclusions This white paper has outlined the basic operating principles and narrow-band filter amplitude control capabilities of the WaveShaper series of Optical Processors. The ability to control the channel bandwidth, shape, centre frequency and attenuation have been described, along with the ability to align the channels with the ITU 5- or 1 GHz grid if required. 9. References Frequency (THz) G. Baxter, S. Frisken, D. Abakoumov, H. Zhou, I. Clarke, A. Bartos and S. Poole, Highly programmable wavelength selective switch based on liquid crystal on silicon switching elements in Proc. Optical Fiber Communication Conf., Anaheim, CA.,26, OTuF2. 3. Michaël A. Roelens, Jeremy A. Bolger, David Williams, and Benjamin J. Eggleton, Multi-wavelength synchronous pulse burst generation with a wavelength selective switch in Optics Express, Vol. 16, Issue 14, pp Moffett Park Drive Sunnyvale, CA 9489 Tel.: Fax: waveshaper@finisar.com Finisar Corporation. All rights reserved. Page 5 Finisar is a registered trademark. WSPR 3/12