Slow-light Element Research Articles

Numerical study on slow-light effects in an ultra-compact grating-based Fabry–Perot cavity

In this paper, we propose a slow-light element based on a grating-based Fabry–Perot cavity (GBFPC). Using three-dimensional finite-difference time-domain simulation, the slow-light properties that depend on its structural parameters are systematically investigated. A slow-light effect with the maximum group index about 14.6 and the slow-light bandwidth about 874 GHz at near 1550 nm wavelength is achieved. Moreover, we find that changing of the GBFPC’s structural parameters, such as bus waveguide width W, grating period p, and period number N, has greater impact on its slow-light properties than its other parameters. This study helps lay a solid foundation for designing excellent slow-light elements based on a Fabry–Perot cavity.

Wideband slow-light propagation with no distortion in a nanofiber-plane-grating composite waveguide

A nanofiber-plane-grating composite slow-light waveguide to achieve wideband slowlight propagation with no distortion is proposed. The waveguide is formed by embedding a tapered nanofiber into a V-groove on a plane-grating surface. By optimizing the waveguide structural parameters, a slow-light effect with bandwidth of about 1453 GHz is obtained. Based on finite-difference time-domain (FDTD) method, we analyze the waveguide’s optical properties and slow-light characteristics. Simulation results show that a picosecond optical pulse propagating in the slow-light waveguide can be delayed for about 980 fs and without distortion. The group velocity of the optical pulse can be reduced to about 0.3c (c is the speed of light in vacuum). This study will provide important theoretical basis and innovative ideas for the development of new-type slow-light elements.

Theoretical and experimental study of structural slow light in a microfiber coil resonator

In this paper, a compact slow-light microfiber coil resonator (MCR) is fabricated and the slow-light properties of it are analyzed and tested. Based on coupled-wave theory, a theoretical model for describing the slow-light propagation in the MCR is established. Experimentally, the MCR slow-light element is fabricated and its relative slow-light time delay is measured. The group velocity of the light pulse in the MCR slow-light element can be reduced to about 0.47c (c is the speed of light in vacuum) and the shape of the light pulse passing through the MCR is well preserved.

Stability-improved slow light in polarization-maintaining fiber based on polarization-managed stimulated Brillouin scattering

With the idea of controlling the polarizations of the pump and Stokes beams in an optical fiber, a simple scheme is presented for enhancing the stability of the stimulated Brillouin scattering (SBS) interaction and thus that of the SBS-based slow light. For this purpose, a special slow-light element is constructed by fusing a polarization-maintaining circulator (its fast axis being blocked) to each terminal of the polarization-maintaining fiber (PMF). This configuration ensures that the pump and Stokes beams always have identical polarization in the whole fiber during the SBS interaction. An experimental setup is established accordingly. The SBS gain feature and the slow-light performance are studied. A tunable time delay with a slope of 0.82 ns dB−1 is demonstrated for a Gaussian pulse with a width of 88.9 ns. The experimental results with and without polarization management are compared. It is found that such a polarization-managed scheme can improve both the stability of the Brillouin gain and that of the time delay of the Stokes pulse. Moreover, for the same pump power, the Brillouin gain is also enhanced.

Slow-light element for tunable time delay based on optical microcoil resonator

We propose a simple and compact slow-light element by use of an optical microcoil resonator (OMR) constituted by two microfiber coils. Based on the matrix exponential method, we solve the coupled-wave equations of the OMR with n turns of microfiber coils and obtain a general solution. Simulations indicate that a tunable slow-light propagation can be obtained by controlling the coupling coefficient between the two adjacent microfiber coils by means of regulating the voltage applied to the ferroelectric crystal. A slow-light time delay up to 62 ps with a bandwidth of 0.4 nm is performed at the wavelength around 1.5 μm.

Balancing interferometers with slow-light elements

In this Letter we show that interferometers with unbalanced arm lengths can be balanced using optical elements with appropriate group delays. For matched group delays of the arms, the balanced interferometer becomes insensitive to the frequency noise of the source. For experimental illustration, a ring resonator is employed as a slow-light element to compensate the arm-length mismatch of a Mach-Zehnder interferometer. An arm-length mismatch of 9.4 m is compensated by a ring resonator with a finesse of 70 and a perimeter of 42 cm.

The role of optical filtering in microwave phase shifting

We highlight the importance of the delay arising from optical filters in slow-light-based microwave phase shifting systems. We calculate the filter delay numerically from the measured amplitude response by using the well-known Kramers-Kronig relations. The complex filter transmission response is then incorporated within a numerical model with which we explain phase shifting results obtained from experiments employing semiconductor optical amplifiers as slow light elements.

Complete compensation of pulse broadening in an amplifier-based slow light system using a nonlinear regeneration element

We experimentally demonstrate complete compensation of pulse broadening in an amplifier-based slow light system. The configuration of the delay line basically consists of two stages: a conventional Brillouin slow light system and a nonlinear regeneration element. Signal pulses experienced both time delay and temporal broadening through the Brillouin delay line and then the delayed pulses were delivered into a nonlinear optical loop mirror. Due to the nonlinear response of the transmission of the fiber loop, the inevitably broadened pulses were moderately compressed in the output of the loop, without loss in the capacity to delay the pulses. The overall result is that, for the maximum delay, the width of the pulse could be kept below the input width while the time delays introduced by the slow light element were preserved. Using this delay line, a signal pulse with duration of 27 ns at full width at half maximum was delayed up to 1.3-bits without suffering from signal distortion.

Optoelectronic Oscillator Tunable by an SOA Based Slow Light Element

We present a continuously tunable optoelectronic oscillator with an intra cavity slow light element based on coherent population oscillations in a semiconductor optical amplifier. The performance of the oscillator is characterized in terms of its frequency tuning range and phase noise. These two characteristics can be traded for each other by choosing the operating conditions of the slow light element and the oscillator fiber length.

Discretely tunable optical packet delays using channelized slow light

We describe a procedure for increasing the fractional delay (or delay-bandwidth product) of a slow-light system. A broadband input signal is sliced into several frequency bands. The light in each band propagates through a separate channel which possesses a highly reduced group velocity over a narrow frequency band. The output of each channel is then combined to form a single output field. For certain discretely distributed values of the group velocity of each channel, the output can replicate the input waveform without the need to adjust the output phase of each channel. Because this scheme makes use of many parallel channels, it can overcome a fundamental limit [D. A. B. Miller, Phys. Rev. Lett. 99, 203903 (2007)] to the delay-bandwidth product for single-channel devices. A practical design is proposed using spectral slicers and stimulated Brillouin scattering as the narrow-band slow-light process. Numerical simulation shows that such a channelized slow-light element can have nearly uniform gain and group index over very large bandwidths.

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.

Spectrally efficient slow light using multilevel phase-modulated formats

A phase-preserving and spectrally efficient slow-light scheme has been proposed and demonstrated by utilizing advanced multilevel phase-modulated formats. A 60 ps symbol delay with error-free demodulation of both I and Q channels for 10 Gbit/s return-to-zero differential-quadrature-phase-shift-keyed (DQPSK) signals via a broadband stimulated Brillouin scattering-based slow-light medium is achieved experimentally. Simulation results on 20 Gbit/s DQPSK and 30 Gbit/s D8PSK propose to transmit very high spectrally efficient multilevel formats through a bandwidth-limited slow-light element.

Pattern Dependence of Data Distortion in Slow-Light Elements

Slow light has been proposed as a potential solution to all-optically tunable delay line. However, a slow-light element may degrade data quality in an optical communication system while decreasing group velocity of optical pulses. In this paper, pattern dependence of signal distortion is identified as a main reason for data degradation, which is caused by narrow-band amplitude and phase responses of the slow-light elements. We define figure of merit involving pulse delay and data degradation to optimize slow-light devices. It is shown that the pattern dependence can be reduced by detuning slow-light devices away from the signal carrier frequency, which allows using narrow-band slow-light techniques to increase normalized delay up to 0.8, with Q improvement of 2 dB.

A Single Slow-Light Element for Independent Delay Control and Synchronization on Multiple Gb/s Data Channels

Independent delay control and bit-level synchronization of multiple data channels are demonstrated through a single stimulated Brillouin scattering (SBS)-based slow-light element. Multiple, independently tunable SBS gain resonances are generated in a single piece of highly nonlinear fiber from multiple SBS pumps. Each pump is broadened by noise modulating the laser current so that the bandwidth is compatible with high-speed gigabit-per-second data signals. Bit-level synchronization on three 2.5-Gb/s nonreturn-to-zero on-off keying channels shows independent and continuous delay control of up to 112ps. Bit-error-rate measurement on the synchronized channels shows 3.5-dB power penalty. Effect of one channel on the other channel's delay is also investigated and the reason is attributed to the nonlinear crosstalk.