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WHITE PAPER

Programmable narrow-band filtering using the WaveShaper

1000S and WaveShaper 4000S

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 50- and 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

retraced upon reflection from the LCoS, with the beam-

steering 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

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Figure 6: Overlay of flat-top filters referenced to the ITU Grid (a) shows

80 x 50 GHz channels overlaid (b) shows 45 x 100 GHz channels

overlaid

4.2 Arbitrary Channel Bandwidth The WaveShaper is not limited to 50- and 100 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 10 GHz steps between 20 GHz and 100 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


WaveShaper 4000S

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.

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 4000S, any arbitrary channel shape can

be used be used as the shape for channel switching.

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 10) or can have a

controlled amplitude using the attenuation control function.

The filter shaping functions (linear and quadratic) can also

be applied to band-stop filter shapes created using the

attenuation control function.

Figure 10: Example of Amplitude control. Grey trace shows six

unattenuated 100 GHz channels, separated by blocked channels

whilst blue trace shows the same channels attenuated by 0-10 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 11.

Figure 11: Arbitrary user-generated filter functions created using the

‘File Import’ function of the WaveShaper [3]

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WaveShaper 4000S

5. Filter Centre Frequency and Alignment

to ITU Grid The default setting for the WaveShaper is to align the

channels with the ITU 50- or 100 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 50 GHz offset for a 100 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

4000S) 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 0 and 15 dB with 0.1 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 10, 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.

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

1000S and 4000S.

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 50- or 100 GHz grid if

required.

9. References 1. http://electronics.howstuffworks.com/LCoS3.htm

2. 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.,2006, 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.

10152-10157

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1389 Moffett Park Drive

Sunnyvale, CA 94089

Tel.: +1-408-548-1000

Fax: +1-408-541-6138

[email protected]

http://www.finisar.com/optical-instrumentation

©2012 Finisar Corporation. All rights reserved.

Finisar is a registered trademark. WSPR 03/12

white paper - finisar · white paper: programmable narrow-band filtering using the waveshaper 1000s...

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