Salman Khaleghi

All-Optical Signal Processing

Optical signal processing brings together various fields of optics and signal processing - namely, nonlinear devices and processes, analog and digital signals, and advanced data modulation formats - to achieve high-speed signal processing functions that can potentially operate at the line rate of fiber optic communications. Information can be encoded in amplitude, phase, wavelength, polarization and spatial features of an optical wave to achieve high-capacity transmission. We revisit advances in the key enabling technologies that led to recent research in optical signal processing for digital signals that are encoded in one or more of these dimensions. Various optical nonlinearities and chromatic dispersion have been shown to enable key sub-system applications such as wavelength conversion, multicasting, multiplexing, demultiplexing, and tunable optical delays. We review recent advances in high-speed optical signal processing applications in the areas of equalization, regeneration, flexible signal generation, and optical control information (optical logic and correlation).

High-Speed Correlation and Equalization Using a Continuously Tunable All-Optical Tapped Delay Line

We demonstrate a reconfigurable high-speed optical tapped delay line (TDL), enabling several fundamental real-time signal processing functions such as correlation (for pattern search) and equalization. Weighted taps are created and added using optical multicasting and multiplexing schemes that utilize the nonlinear wave mixings in the periodically poled lithium niobate (PPLN) waveguides. Tunable tap delays are realized using the conversion-dispersion technique. In the demonstrated TDL, the amplitude and phase of tap coefficients can be varied, enabling signal processing on amplitude- and phase-encoded optical signals. We experimentally demonstrate the tunability of the TDL in time, amplitude, and phase. We analyze the TDLs theory of operation and present experimental results on reconfigurable pattern search (correlation) on on-off-keyed and phase-shift-keyed signals at data rates of up to 80 Gb/s, as well as equalization for chromatic dispersion.

Optical Nyquist channel generation using a comb-based tunable optical tapped-delay-line

We demonstrate optical Nyquist channel generation based on a comb-based optical tapped-delay-line. The frequency lines of an optical frequency comb are used as the taps of the optical tapped-delay-line to perform a finite-impulse response (FIR) filter function. A single optical nonlinear element is utilized to multiplex the taps and form the Nyquist signal. The tunablity of the approach over the baud rate and modulation format is shown. Optical signal-to-noise ratio penalty of 2.8 dB is measured for the 11-tap Nyquist filtering of 32-Gbaud QPSK signal.

Demonstration of reconfigurable optical generation of higher-order modulation formats up to 64 QAM using optical nonlinearity

We demonstrate a reconfigurable optical transmitter of higher-order modulation formats including pulse-amplitude-modulation (PAM) signals and quadrature-amplitude-modulation (QAM) signals. We generated six different modulation formats by multiplexing 10 Gbit/s on-off-keying (OOK) signals (10 Gbaud binary phase-shift keying, 4-PAM, 8-PAM quadrature phase-shift keying (QPSK), 16-QAM and 16-star-QAM with error-vector magnitudes (EVMs) of 8.1%, 7.5%, 7.8%, 8.2%, 7.2%, and 6.9%, respectively) and 80 Gbit/s 16-QAM with an EVM of 8.5%, as well as 120 Gbit/s 64-QAM with an EVM of 7.1%, using two or three 40 Gbit/s QPSK signals, respectively. We also successfully transmitted the generated 16-QAM signals through a 100 km transmission line with negligible power penalty.

Tunable 503 ns optical delay of 40 Gbit/s RZ-OOK and RZ-DPSK using a wavelength scheme for phase conjugation to reduce residual dispersion and increase delay

We demonstrate a method for a tunable optical delay element that allows for minimal residual dispersion and double the relative delay. A tunable delay of 503 ns for 40 Gbit/s return-to-zero on-off keying and return-to-zero differential phase-shift keying is shown with a reduction in residual dispersion of approximately 95%.

Demonstration of all-optical phase noise suppression scheme using optical nonlinearity and conversion/dispersion delay.

We propose and demonstrate an all-optical phase noise reduction scheme that uses optical nonlinear mixing and tunable optical delays to suppress the low-speed phase noise induced by laser linewidth. By utilizing the phase conjugate copy of the original signal and two narrow-linewidth optical pumps, the phase noise induced by laser linewidth can be reduced by a factor of ∼5 for a laser with 500-MHz phase noise bandwidth. The error-vector-magnitude can be improved from ∼30% to ∼14% for the same laser linewidth for 40-Gbit/s quadrature phase shift keying signal.

True time delays using conversion/dispersion with flat magnitude response for wideband analog RF signals

We demonstrate optical true time delays using wavelength conversion coupled with chromatic dispersion. Using offset pumps, < 1 dB magnitude response fluctuations over ∼40 GHz of bandwidth is achieved. Calculated linear frequency-modulated impulse response maintains a <-32 dB peak-to-sidelobe-ratio.

Coherent correlator and equalizer using a reconfigurable all-optical tapped delay line.

We experimentally demonstrate a reconfigurable optical tapped delay line in conjunction with coherent detection to search multiple patterns among quadrature phase shift keying (QPSK) symbols in 20 Gbaud data channel and also to equalize 20 and 31 Gbaud QPSK, 20 Gbaud 8 phase shift keying (PSK), and 16 QAM signals. Multiple patterns are searched successfully on QPSK signals, and correlation peaks are obtained at the matched patterns. QPSK, 8 PSK, and 16 QAM signals are also successfully recovered after 25 km of SMF-28 with average EVMs of 8.3%, 8.9%, and 7.8%. A penalty of <1 dB optical signal to noise penalty is achieved for a 20 Gbaud QPSK signal distorted by up to 400  ps/nm dispersion.

Tunable radio frequency photonics filter using a comb-based optical tapped delay line with an optical nonlinear multiplexer

A radio frequency (RF) photonic filter is experimentally demonstrated using an optical tapped delay line (TDL) based on an optical frequency comb and a periodically poled lithium niobate (PPLN) waveguide as multiplexer. The approach is used to implement RF filters with variable bandwidth, shape, and center-frequency.

Tunable optical correlator using an optical frequency comb and a nonlinear multiplexer

We experimentally demonstrate a tunable optical correlator to search for multiple patterns among QPSK symbols. We utilize an optical frequency comb to generate the coherent signals and multiplex them coherently in a single PPLN waveguide. Multiple patterns with different lengths are successfully searched within QPSK symbols in a 40-Gb/s signal.