14-2012-2806 | Adaptive Spectral Filtering for Nyquist-WDM Generation with an LCoS-based Photonic Spectral Processor
, HUJI, School of Computer Science and Engineering, Applied Physics
Multi-Channel All-Optical Signal Shaping
Nyquist WDM, Fine-resolution optical filter, spectral shaping
In order to maximize capacity carried in fiber-optic communication systems, spectrally efficient modulation formats are being intensely investigated. One such modulation format is called Nyquist-WDM (Wavelength Division Multiplexing) which results in a rectangular-like spectrum and concurrently a sinc-shaped time domain response.
Nyquist-WDM shaping can be performed at the transmitter using digital signal processing on the data symbols. This requires up-sampling (typically to two samples per symbol) and shaping in a computationally intensive manner per channel, increasing cost and power consumption.
Optical spectral shaping to date has not been performed at sufficient fidelity to achieve rectangular spectrum, limiting the spectral density of multiplexing.
We have designed a photonic spectral processor (PSP) having unprecedented filtering ability. We address the limitation of limited performance achieved to date with optical spectral shaping by designing a custom grating based upon an arrayed waveguide grating (AWG). The custom AWG can be designed to achieve very fine spectral resolution, which allows spectral shaping for telecommunication signals with high fidelity. The AWG provides the spectral resolution, and we introduce a spatial light modulator for the dynamic spectral shaping element. Jointly, these two elements form a photonic spectral processor (PSP) having unprecedented filtering ability.
Another favorable attribute of the fine resolution AWG is its multi-channel support. The AWG has a spectrally periodic response, or free-spectral range (FSR), resulting in identical filtering performed for channels spaced at the FSR. Hence one common filtering apparatus, the PSP, simultaneously processes all wavelength channels spaced by the FSR.
The PSP can be configured to shape a directly modulated signal, formed by using data symbols to directly drive the modulator (at one data sample per symbol) without any need for digital signal processing, by prescribing a spectral transfer function that limits the spectral extent (truncating higher frequencies) to the baud rate, and also shaping the pass bandwidth components roughly in an inverse-Gaussian shape which results in a rectangular spectrum (since the directly modulated signal has a particular signature of roll-off about its center frequency).
Since it is desired to pack the Nyquist-WDM channels contiguously, two PSPs can perform the shaping, one for the even channels and one for the odd channels. The two interleaved WDM channels can then be passively combined, forming a nearly continuous power spectrum filled with modulated data.
Top-right: Photonic spectral processor combining custom fine resolution AWG and LCoS spatial light modulator for synthesizing arbitrary optical transfer functions with high fidelity.
Top-left: PSP transfer function set to rectangular shape (red) or raised cosine filter with 0.5 roll-off factor (blue)
Bottom-left: Experimental result of Nyquist-WDM generation at 20 Gbaud and 16-QAM modulation. Black: modulator output when driven by one sample per symbol; Red & Blue: filtered signal with PSP.
Bottom-right: Time domain response exhibiting sinc behavior (top) and Nyquist-WDM error vector magnitude as a function of filter tightness, as prescribed by raised cosine filter roll-off factor (tightest square filter at ‘0’ setting).
The PSP can prescribe any spectral transfer function, subject to the designed resolution constraint, by encoding the LCoS (Liquid Crystal on Silicon) spatial light modulator. For example, not only square spectrum carving is possible, but also raised cosine filters (see above), sinc-shaped spectral filters for OFDM signal generation of matched filtering, and more.
Photonics spectral processor having fine resolution by custom arrayed waveguide gratings
Can be used to optically synthesize and modify signal waveforms by passive filtering techniques
Multi-channel support in one device making implementation simple and inexpensive
Low power consumption (only to drive spatial light modulator)
Optical losses can be compensated by EDFA. Overall power budget and cost extremely favorable
Can be packaged in a very small assembly for 1U line card installation
Demonstrated single and multi-channel Nyquist WDM channel shaping over single polarization.
Polarization diversity now being introduced.
Demonstrated operation with 3.5 GHz resolution and 0.8 GHz AWG. Can be designed to match system requirements, balancing complexity and performance.
Applications in all areas where fine resolution control over optical signals is required, such as optical communications, radar signals, and defense.
US patent filed