Tunable Optical Delay Research Articles

Tunable photonic elements at the surface of an optical fiber with piezoelectric core.

Tunable photonic elements at the surface of an optical fiber with piezoelectric core are proposed and analyzed theoretically. These elements are based on whispering gallery modes whose propagation along the fiber is fully controlled by nanoscale variation of the effective fiber radius, which can be tuned by means of a piezoelectric actuator embedded into the core. The developed theory allows one to express the introduced effective radius variation through the shape of the actuator and the voltage applied to it. In particular, the designs of a miniature tunable optical delay line and a miniature tunable dispersion compensator are presented. The potential application of the suggested model to the design of a miniature optical buffer is also discussed.

Resonant Graphene-Based Tunable Optical Delay Line

The design of a new graphene-based continuously tunable optical delay line formed by two vertically stacked microring resonators coupled to a straight waveguide is proposed. High values of delay time $(\tau_{g}=360 \mbox{ps})$ and a wide tuning range $(\mathit{\Delta}\tau_{g}=230 \mbox{ps} ) $ have been calculated, due to the graphene sandwiched between the stacked ring resonators, which also provides an electrooptical tuning of the delay with low energy consumption $(E_{\mathrm{switch}}=3.4 \mbox{pJ})$ and fast switching time $(t_{\mathrm{switch}} . The ratio $\mathit{\Delta}\tau_{g} /A$ represents an important figure of merit (FOM) for optical delay lines. A value $\mbox{FOM}=1.4\times 10^{-1} \mbox{ps}/\mu\mbox{m}^{2} $ has been calculated, which corresponds to an enhancement of about a factor 4 compared with the state-of-the-art of the integrated optical delay lines, also providing a switching time several times faster. Such performance, together with a small device footprint $( , gives a significant contribution to the state-of-the-art of optical delay lines, confirming the suitability of the graphene-based resonant cavity as a high-efficient optical delay line for applications in which fast tuning and wide range of tunability are required, e.g., phased array antennas.

Tunable optoelectronic oscillator incorporating a carrier phase-shifted double sideband modulation system

A tunable optoelectronic oscillator (OEO) implemented by using a carrier phase-shifted double sideband modulation (CPS-DSB) system consisting of an optical coupler (OC), a Mach–Zehnder modulator (MZM) biased at the minimum transmission point, a polarization beam splitter (PBS), and a tunable optical delay line (TODL) is proposed and experimentally demonstrated. The key device in the system is the CPS-DSB system, which functions in conjunction with a chirped fiber Bragg grating (CFBG) in the loop form a high-Q microwave photonic filter (MPF). Through simply adjusting the TODL, the central frequency of the MPF is shifted and the frequency tunability of the OEO can be realized. A detailed theoretical analysis is provided and the results are confirmed by an experiment. A microwave signal with a frequency-tuning range from 7.24 to 14.05GHz is generated. The phase noise, the long-term stability and the side-mode suppression performance of the generated microwave signal are also investigated.

Integrated remotely tunable optical delay line for millimeter-wave beam steering fabricated in an InP generic foundry.

A compact and fabrication-tolerant integrated remotely tunable optical delay line is proposed for millimeter-wave beam steering and is fabricated in an InP generic foundry. The proposed delay line is based on a spectrally cyclic-arrayed waveguide grating feedback loop. Its major features include the tolerant architecture with reduced chip size, and bi-directional operation with simplified remote tuning. Moreover, its cyclic feature guarantees further cascaded operations either for 2D radio beam steering or for high-resolution delay generation. The experimental results show less than 6.5-dB insertion loss of the integrated delay line. Five different delays from 0 to 71.6 ps are generated with less than 0.67-ps delay errors.

Improved Phase Stabilization of a Radio-Over-Fiber Link Under Large and Fine Delay Tuning

In this paper, we propose and demonstrate a phase-stabilized radio-frequency (RF) reference transmission scheme by actively stabilizing the optical path delay of the fiber link. The reference RF tone is round-trip transferred between the central station and the remote end to obtain the delay variation. The fast and small part of the delay fluctuation is cancelled by a piezoelectric fiber stretcher, benefitting from its short response time and fine adjusting granularity. The slow and large part of the delay variation is used to alter a wavelength tunable laser, which results in a dispersion-induced tunable optical delay line with very large compensation range. Experimentally, a 2.48-GHz RF signal has been transferred through a 60-km optical fiber link with suppressed noise. The fractional frequency stability achieved is 6.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-14</sup> at 1 sand 2.1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-17</sup> at 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> s averaging time, respectively.

Pulsed laser deposited barium titanate thin film for tunable optical delay application

The thin film of barium titanate was fabricated by pulsed laser deposition (PLD) technique. Tetragonal phase in the film has been confirmed by XRD and laser Raman spectra. The film thickness estimated from the analysis of the fringes observed in UV–visible spectra was ∼3.3μm. Barium titanate thin film was subjected to Optical delay studies via degenerate two-wave mixing at room temperature, using 200ns pulse of 2nd harmonic of Nd:YAG laser. Sub as well as superluminal pulse propagation has been observed by tuning the film orientation. The optical delay has been compared for BTO crystal and thin film.

Temporal cloak based on tunable optical delay and advance.

A temporal cloak scheme based on tunable optical delay and advance is theoretically proposed. An input optical signal is divided into two parts, and the front part is advanced while the rear part is delayed. A temporal gap which contains negligible optical energy is opened to hide events from observers, which can be closed by doing the inverse process. Different from the previous schemes that use continuous wave light, we use an optical data stream as the probe beam. Simulations have been done to investigate performance of the temporal cloak. The developed scheme can be applied in fiber-optical signal processing and secure communications.

A Simultaneous Variable Optical Weight and Delay in a Semiconductor Optical Amplifier for Microwave Photonics

In this paper, we demonstrate how a single semiconductor optical amplifier can serve as a simultaneous variable optical weight and tunable optical delay for microwave photonics. The device weight, or power transmission, and delay can be controlled simultaneously and independently from each other by varying the input optical power and the semiconductor bias current. The dual functionality is achieved by combining the effects of slow and fast light with cross-gain modulation in the semiconductor. We experimentally demonstrate a tunable delay range of 100 ps and RF gain range of $-$ 6 to +3 dB for a 600-MHz microwave signal and show how the weight and delay of the device can be separately tuned. The delay range and bandwidth of the device are limited by the semiconductor carrier lifetime and characteristic of slow and fast light. As a simultaneous optical weight and delay, as well as a wavelength converter, a semiconductor optical amplifier operating in this manner can be a compact and versatile element in microwave photonics, radio-over-fiber, and general analog optical signal processing.

A multi-VCR system for dispersion compensation and tunable group delay

The vertically coupled resonator (VCR) provides a new insight into the optical delay lines. However, the group velocity dispersion has remained a significant challenge. Here, we describe a multi-VCR system to solve the problem and achieve a tunable optical delay line. The dispersion and the group delay properties of the system are analyzed both theoretically and numerically. Some practical considerations are also discussed. Its flexibility, robustness, compactness, and zero GVD properties make it advantageous over single vertically coupled resonator delay lines and provide a new way to slow light, filtering and optical communication.

BER Analysis Using Beat Probability Method of 3D Optical CDMA Networks with Double Balanced Detection

This study proposes novel three-dimensional (3D) matrices of wavelength/time/spatial code for code-division multiple-access (OCDMA) networks, with a double balanced detection mechanism. We construct 3D carrier-hopping prime/modified prime (CHP/MP) codes by extending a two-dimensional (2D) CHP code integrated with a one-dimensional (1D) MP code. The corresponding coder/decoder pairs were based on fiber Bragg gratings (FBGs) and tunable optical delay lines integrated with splitters/combiners. System performance was enhanced by the low cross correlation properties of the 3D code designed to avoid the beat noise phenomenon. The CHP/MP code cardinality increased significantly compared to the CHP code under the same bit error rate (BER). The results indicate that the 3D code method can enhance system performance because both the beating terms and multiple-access interference (MAI) were reduced by the double balanced detection mechanism. Additionally, the optical component can also be relaxed for high transmission scenery.

The manipulation of light pulse from subluminal to superluminal propagation in a degenerate two-level Cs atomic system

We experimentally demonstrate the propagation of light pulse from subluminal to superluminal light based on quantum coherence in a degenerate two-level atomic system in a Cs vapor cell. It is shown that the group velocity of light pulse can be switched from subluminal to superluminal propagation via changing the coupling field from a traveling wave to a standing wave, while can also be continuously manipulated by varying the intensity of two waves superposed to form a standing wave. The observed maximum delay and advance times are about 0.45 and 0.54 μs, corresponding to the group velocity of Vs = 168 km/s and vg =−138 km/s, respectively. This investigation may have the practical applications of devices for optical tunable delay lines, optical switching and optical buffering.

Phase stabilized downlink transmission for wideband radio frequency signal via optical fiber link.

In this paper, we propose and demonstrate a phase stabilized wideband downlink transmission scheme, which directly transmits the received radio frequency (RF) signals from remote antennas to central station. A reference RF tone is round-trip transferred between the central station and remote end to obtain the delay variation caused by the fiber link. The delay variation is then used to alter a tunable laser. Since optical carriers with different wavelengths propagate at different velocities in fiber, a tunable optical delay line is realized to cancel the delay variation of the fiber link. The tunable delay range is in proportion to the length of the fiber link, which means a very long delivery distance can be expected. Experimentally, a RF signal at frequency of 2.50 GHz has been downlink transferred through a 45 km fiber link, with stability of 3.3 x 10⁻¹³ at 1 s and 7.5 x 10⁻¹⁷ at 10⁴s.

Brillouin slow light: substantial optical delay in the second-order Brillouin gain spectrum.

We experimentally demonstrate optical delay in the second-order Brillouin gain spectrum by incorporating a double Brillouin-frequency shifter into the system. By coinciding the seed signal with the second-order Brillouin gain spectrum, it was found that the seed signal experienced significantly larger delay as compared to the Brillouin slow light generated from the first-order Brillouin spectrum. At a Brillouin gain of 17 dB, the delay was found to be at maximum of 60 ns. This widens the window of promising opportunities into the deployment of all optical tunable delay into the existing optical signal processing.

Stable radio-frequency phase distribution over optical fiber by phase-drift auto-cancellation.

We report a new radio-frequency (RF) phase stabilization approach for long-haul optical fiber distribution. The phase drift of an RF signal induced by fiber-length variations can be canceled out automatically via RF mixing without using active phase discrimination and dynamic phase tracking. A key significance of our approach is that no assistant local oscillator (LO) signal is needed. Consequently, frequency estimation of the received RF signal, as well as frequency locking between the LO and the received RF signal, is no longer required, which simplifies the system. A proof-of-concept experiment shows that the phase drift of the received RF signal at 9.6GHz is significantly reduced using the proposed method. The root mean square (RMS) timing jitter is 0.76ps when a tunable optical delay line (TODL) inserted between the remote antenna unit (RAU) and local station is changed from 0 to 300ps.

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.

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).

Tunable nanophotonic delay lines using linearly chirped contradirectional couplers with uniform Bragg gratings

We demonstrate an integrated tunable optical delay line in grating-assisted contradirectional couplers using a CMOS-compatible photonic technology. The input signal is delayed through dispersive Bragg gratings and distributedly coupled to the drop port of the coupler without backreflections. This add-drop design enables monolithic integration of grating-based delay lines without using optical circulators. The gratings are formed by slab perturbations in rib waveguides, with the index chirping realized by linearly tapering the rib widths. Both the pitch and size of the gratings are constant through the entire coupler, for a higher tolerance to fabrication errors. Continuous tuning of the optical group delay of up to 96 ps has been obtained, with a low insertion loss of less than 2 dB and a negative chromatic dispersion of -11 ps/nm that allows for bit rates of up to almost 100 Gb/s at the maximal delay. The device has a small footprint of 0.015 mm2, and can be used for on-chip optical buffering, dispersion compensation, and pulse compression.

Extended tunable optical delay using gain-transparent stimulated Brillouin scattering control in four-wave-mixing wavelength conversion

We propose and experimentally demonstrate an extended range of tunable optical delay obtained from four-wave mixing wavelength conversion and dispersion. The conversion bandwidth and the maximum delay are enlarged through dynamic control of the optical phase by gain-transparent stimulated Brillouin scattering. The delay range is increased by 37%. Bit-error-rate measurements show a maximum power penalty of 2.0 dB with reference to back-to-back performance. The technique can be applied to different configurations of tunable delay lines constructed with a fiber parametric process and dispersion.

Continuously tunable reflective-type optical delay lines using microring resonators

We present a reflective-type optical delay line using waveguide side-coupled 13 microring resonators terminated with a sagnac loop reflector. Light passes through the microring resonator sequence twice, doubling the delay-bandwidth product. Group delay is tuned by p-i-p type microheaters integrated directly in the microring waveguides. Experiment demonstrates that the delay line can potentially buffer 18 bits and the delay can be continuously tuned for 100 ps with a power tuning efficiency of 0.34 ps/mW. Eye diagrams of a 20-Gbps PRBS signal after 10 and 110 ps delays are also examined.

A promising optoelectronic clock solution for time-interleaved ADC

Abstract A promising optoelectronic clock solution with low jitter, high bandwidth and accurate delay for a TIADC is proposed by exploiting optical pulses and tunable optical delay lines. It is feasible to construct the optoelectronic clock system with optical pulse source, dispersive medium, tunable delay line in optical domain and conditioning circuits in electronic domain while photodiodes as the bridge. The testing result of the designed conditioning circuit shows that using electronic method to adjust the signal will induce jitter degradation of the optoelectronic clock system.