Irfan Fazal

Optical Signal Processing Using Tunable Delay Elements Based on Slow Light

Tunable optical delay lines have many applications for high-performance optical switching and signal processing. Slow light has emerged as an enabling technology for achieving continuously tunable optical delays. Delay reconfigurability opens up a whole new field of nonlinear signal processing using slow light. In this paper, the authors review recent advances in slow-light-based optical signal processing, with a focus on the data fidelity after traversing the slow light elements. The concept of slow-light-induced data pattern dependence is introduced and is shown to be the main signal degrading effect. We then propose and experimentally demonstrate phase-preserving slow light by delaying 10 Gb/s differential phase-shift keying (DPSK) signals with reduced DPSK pattern dependence. Spectrally efficient slow light using advanced multilevel phase-modulated formats is further described. With this technique, doubled bit-rate signals can be transmitted through a bandwidth-limited slow light element. We finally show several novel slow-light-based signal processing modules. Unique features such as multichannel operation, variable bit-rate capability, and simultaneous multiple functions are highlighted.

Fiber structure to convert a Gaussian beam to higher-order optical orbital angular momentum modes

We propose a fiber structure of a square core and ring refractive index profile that converts an input circular polarized Gaussian mode into optical orbital angular momentum (OAM) modes. By breaking the circular symmetry of the waveguide, the input circularly polarized fundamental mode in the square core can be coupled into the ring region to generate higher-order OAM modes, corresponding to the transference of spin angular momentum and orbital angular momentum. We show, by using simulation, the generation of OAM modes with a topological charge l up to 9 using <10 mm long fiber. The mode purity is above 96.4% and the extinction ratio can be 30 dB.

Continuously-tunable, bit-rate variable OTDM using broadband SBS slow-light delay line

We conceptually compare the advantages of the proposed slow-light-based tunable OTDM to conventional fiber-based fixed OTDM multiplexer. We experimentally demonstrate continuously-controllable OTDM of two 2.5-Gb/s return-to-zero (RZ) signals using broadband SBS-based slow-light as the tunable optical delay line. We show that the time slot of one signal path can be manipulated relative to the other by as much as 75-ps. This continuous slow light tunability dramatically enhances the OTDM system performance which results in a power penalty reduction of 9-dB for the multiplexed data stream. We also demonstrate variable-bit-rate OTDM by dynamically adjusting the tunable slow-light delay according to the input bit-rates. We show efficient two-by-one optical time multiplexing of three different input data streams at 2.5-Gb/s, 2.67-Gb/s and 5-Gb/s.

Slow light on Gbit/s differential-phase-shift-keying signals

We demonstrate, via simulation and experiment, slowing down of a phase-modulated optical signal. A 10.7-Gb/s NRZ-DPSK signal can be delayed by as much as 42 ps while still achieving error free via broadband SBS-based slow light. We further analyze the impact of slow-light-induced data-pattern dependence on both constructive and destructive demodulated ports. By detuning the SBS gain profile, we achieve 3-dB Q-factor improvement by the reduction of pattern dependence. Performance comparison between NRZ-DPSK and RZ-DPSK shows that robustness to slow-light-induced pattern dependence is modulation format dependent.

Fiber coupler for generating orbital angular momentum modes

We propose a fiber coupler consisting of a central ring and four external cores to generate up to ten orbital angular momentum (OAM) modes. Four coherent input lights are launched into the external cores and then coupled into the central ring waveguide to generate OAM modes. By changing the size of the external cores, one can selectively excite a high-order OAM mode. The quality of the generated OAM modes can be enhanced by adjusting the polarization state and the phase of input lights. We show the generation of OAM modes with odd charge numbers of -9 to +9 (i.e., 10 modes totally) with mode purity of >99% using <2 mm long fiber. This fiber coupler design can be extended to enable all-fiber spatial-mode (de)multiplexing.

160 Gb/s Time-Domain Channel Extraction/Insertion and All-Optical Logic Operations Exploiting a Single PPLN Waveguide

160 Gb/s all-optical signal processing is demonstrated exploiting pump depletion in addition to sum and difference frequency generation (SFG/DFG) in a single periodically poled lithium-niobate (PPLN) waveguide. 160 Gb/s time-domain extraction and insertion operations of channels are obtained in an optical time division multiplexing (OTDM) system. Moreover, 160 Gb/s digital operations including half-adder, half-subtracter and AND/OR/XOR functions are carried out. The use of pump depletion effect allows to process ultrafast signals due to its high efficiency and ultrafast dynamics. 160 Gb/s bit error rate (BER) measurements confirm the effectiveness of all presented functionalities.

Photonic processing of 320 Gbits/s based on sum-/difference-frequency generation and pump depletion in a single PPLN waveguide

The possibilities and limitations of using nonlinearities in periodically poled lithium-niobate waveguides for ultrafast all-optical processing are experimentally investigated. A combination of the sum-/difference-frequency generation and pump depletion effect are exploited to obtain optical demultiplexing, add/drop multiplexing, and wavelength conversion of up to 320 Gbits/s.

Tunable 105 ns optical delay for 80 Gb/s RZ-DQPSK, 40 Gb/s RZ-DPSK, and 40 Gb/s RZ-OOK signals using wavelength conversion and chromatic dispersion

We demonstrate a variable, optical-delay element using tunable wavelength conversion in a periodically poled lithium niobate waveguide, dispersion-compensating fiber and intrachannel dispersion compensation. A delay of up to 105 ns is demonstrated using 80 Gb/s return-to-zero differential-quadrature phase-shift keying, 40 Gb/s return-to-zero differential phase-shift keying, and 40 Gb/s return-to-zero on-off keying modulation formats. Bit-error rates <10−9 are demonstrated for each waveform at various delay settings.

2 Tbit/s free-space data transmission on two orthogonal orbital-angular-momentum beams each carrying 25 WDM channels

We demonstrate a 2 Tbit/s free-space data link using two orthogonal orbital angular momentum beams each carrying 25 different wavelength-division-multiplexing channels. We measure the performance for different modulation formats, including directly detected 40 Gbit/s nonreturn-to-zero (NRZ) differential phase-shift keying, 40 Gbit/s NRZ on-off keying, and coherently-detected 10 Gbaud NRZ quadrature phase-shift keying, and achieve low bit error rates with penalties less than 5 dB.

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.