Tunable Optical Delay Line Research Articles

High-rate data buffering in silicon nanophotonic devices

Buffers represent one of the most important components in a communications router. The device must be able to store data packets for a substantial period of time and release the data within an acceptable delay when the switch is clear. Optical buffers—devices that store optically transmitted data—are considered key elements to achieve synchronized and contention-free traffic in the design of all-light-based communication networks. Conventional buffers use optical fiber delay lines, but since the delay is for a fixed amount of time, they are unable to guarantee contention-free connections in the switch or through a network. Hence, research on tunable optical delay lines is generating significant interest, especially with optical communications now gearing up for large-scale integration. Advances in silicon technology are making this material a choice candidate for future integrated photonics. On-chip delay lines based on silicon-on-insulator (SOI) photonic wire waveguides were recently demonstrated, consisting of up to 100 microring resonators cascaded in either coupled-resonator or all-pass filter (APF) configurations with error-free operation up to 5Gbps.1 This demonstration was a major milestone in the development of all-optical buffers for interconnects. Tunable delays in an SOI planar waveguide based on slow light induced by stimulated Raman scattering (SRS) have also been reported,2 as well as intensity-tunable group delays in siliconmicroresonators enhanced by SRS.3 The functional mechanism of a ring resonator-based optical delay line can be described as follows: When a probe beam is injected into a ring with a frequency matching the resonance of the ring, light is forced to circle multiple times, lengthening the delay. If beam frequency deviates from resonance, it will bypass the ring and no delay will occur. When a pump light, whose frequency is of a different resonance, is injected into the microring resonator, the absorbed energy is eventually converted to therFigure 1. (a) Scanning electron microscope image of a microring resonator and (b) its resonance spectrum.

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.

Two-Pump Parametric Optical Delays

Continuously tunable optical delay lines based on parametric process in optical fibers are described theoretically and demonstrated experimentally. Performance limits are outlined and compared to alternative approaches capable of continually adjustable delay management. The current record of 105 ns tunable optical delay at 10 Gb/s is presented, with a delay-bandwidth product of 1055.

Tunable microwave photonic filter based on a fiber ring and erbium-doped fiber amplifier

The tunable microwave photonic filter based on a fiber ring and erbium-doped fiber amplifier (EDFA) was proposed. By introducing a section of erbium-doped fiber (EDF) into the fiber ring, the loss of the signal can be compensated by the EDFA gain through adjusting the pump power. This can largely increase the number of the effective sampling taps, and then improve the performance of the microwave photonic filter notably. When the pump power was set to be 42.7 mW, a microwave bandpass filter with the 3-dB bandwidth of 0.15 MHz, the Q factor up to 100 and extinction ratio up to 20 dB was achieved. By employing a tunable optical delay line in the above fiber ring, a tunable microwave photonic filter has been realized through tuning the length of the optical delay line. The proposed tunable microwave photonic filter can find great applications in microwave signal processing and ROF system.

All-Optical Clock Recovery Using Erbium-Doped Fiber Laser Incorporating an Electroabsorption Modulator and a Linear Optical Amplifier

We demonstrated a 10-GHz all-optical clock recovery system using an erbium-doped fiber laser that incorporates an electroabsorption modulator and a linear optical amplifier. Stable pulses with peak power of 200 mW and pulsewidth of 6 ps are obtained. The output power and the pulsewidth of the recovered clock pulses are independent of the input data pattern. Stable optical clock can still be observed when the input data rate varies by more than 60% of the fundamental frequency without any optical tunable delay line inside the laser cavity. The scheme is essentially wavelength transparent for the whole C-band which recovers clock pulses from input data wavelength between 1525 and 1565 nm

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.

Tunable polarization optical delay line based on a linear chirped fiber Bragg grating

AbstractA new technique for inducing relative group delays between the orthogonal transversal propagation modes in a single mode fiber is presented. It is based on a tunable linear chirp induced in a Bragg grating written in a highly birefringent fiber by thermal means. The method can induce a relative delay over a wide range, making it possible for use in a polarization mode dispersion compensator. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48:2357–2359, 2006; Published online in Wiley InterScience (www.interscience.wiley.com).DOI 10.1002/mop.21945

Optical tunable delay lines using fiber ring with acousto-optic frequency shifters and EDFAs: I. experimental demonstration

Optical buffers are key components in optical packet switching network systems that prevent packet collision along the same virtual path and wait for header processing time. Conventional fiber loops for optical buffers present the drawback of unexpected oscillation at gain peak wavelength of the fiber loop. In this paper, an optical tunable delay line using a fiber ring with an acousto-optic frequency shifter and an erbium-doped fiber amplifier (EDFA) are demonstrated experimentally. The frequency shifter plays the role of preventing unexpected oscillation. Clear eye heights are visible in data packets through the delay line.

Optical tunable delay lines using fiber ring with acousto-optic frequency shifters and EDFAs: II. SNR analysis

Optical tunable delay lines using fiber ring with acousto-optic frequency shifters and EDFAs: II. SNR analysis

Adiabatically tunable optical delay lines and their performance limitations

A simple tunable optical delay line based on adiabatic zone folding in coupled resonator lines is proposed, and its performance is analyzed. The results, valid for all adiabatically tunable optical buffers based on resonators, indicate that, while at low to medium bit rates the performance is improved substantially compared with that of the fixed delay lines, this improvement vanishes for high bit rates and large storage capacities.

Tunable optical group delay in an active waveguide semiconductor resonator

We investigate a novel optical filter with specific application to tunable optical delay lines. The structure consists of cascaded chirped grating and phase-shifter sections, providing two optical cavities which generate a wide range of potential filter responses. The active waveguide nature of the filter allows current-controlled tunability of both the spectral response and the corresponding group delay. The filter is studied experimentally and theoretically for both transmission and reflection schemes, highlighting the tradeoffs. We demonstrate that continuous group delay tunability of 43 ps can be achieved in reflection with small power variation by optimizing the injection currents.

An Active Semiconductor Tunable-Shape Optical Filter and Delay Line

We present a novel active semiconductor optical filter consisting of a curved-waveguide distributed-feedback (DFB) section and a phase-shifter section with a highly reflective coating. Separate electrical contacts to the DFB and phase-shifter sections allow independent tuning of the gain and the phase accumulated through the structure. The waveguide curvature forms an additional cavity, leading to interesting and controllable coupled-cavity effects. We show experimental and theoretical transmission properties of this filter operating with fixed gain in the DFB and current-controlled absorption in the phase-shifter and suggest potential applications to active filter control and tunable optical delay lines.

Control of Group Delay and Chromatic Dispersion in Tunable Hollow Waveguide with Highly Reflective Mirrors

We propose the control of group delay and chromatic dispersion in a hollow waveguide with a variable air core and highly reflective mirrors. Both group delay and chromatic dispersion in a hollow optical waveguide increase markedly with decreasing air core. A wide tunability of group velocity and chromatic dispersion is predicted by reducing the air core to nearly half of wavelength. We discuss the potential applications of the proposed hollow waveguide for compact photonic integrated circuits including tunable optical delay lines, interferometers, chromatic dispersion compensators and dispersion slope compensators.

Highly scalable serial–parallel optical delay line

We present a complete analysis of the operation of an ultra-fast tunable parallel optical delay line. Based on our analysis, we propose and experimentally demonstrate a generalized serial–parallel delay line structure, which provides better scalability with respect to the number of timeslots. The delay line uses one electro-optic modulator and an all-fiber delay lattice. We also present a discussion on two important performance measures of this delay line – the tuning speed and the optical power loss.

PMD-insensitive SCM optical receiver using polarization diversity

Subcarrier multiplexing (SCM) optical systems with high subcarrier frequencies are susceptible to power fading caused by fiber polarization-mode dispersion (PMD). In this letter, an SCM optical receiver free of carrier fading is proposed and demonstrated using polarization diversity. Unlike conventional PMD compensators, this setup does not require a tunable optical delay line.

Fast tunable parallel optical delay line

A new tunable optical delay line for OTDM applications can achieve sub-nanosecond tuning time across three time slots spaced 100ps apart. Using parallel fiber delays, the delay line requires only one modulator operating below the baseband data rate. A simple control algorithm based on a latency diagram can further reduce the average latency beyond that of the straightforward hardware implementation by 50%. Using the demonstrated delay line, 10 GHz electronics has the potential to access a throughput of 160Gb/s optical data.

Near-field optics with photonic crystals

Summary form only given. Much attention has been recently focused on the investigation of photonic band gap (PBG) structures. In particular, it was demonstrated that unusual dispersion properties of such structures allow compact tunable optical delay lines to be implemented, parameters of short laser pulses to be controlled, and nonlinear-optical interactions to be phase-matched. We employ the finite difference time-domain (FDTD) technique to analyze the propagation of a light beam in a two-dimensional (2D) PBG structure with a defect. We performed FDTD simulations for macroporous-silicon 2D PBG structures consisting of five to ten periods of cylindrical air holes arranged in a triangular lattice in silicon irradiated with a plane light wave.

Tunable optical delay line based on a fibre Bragggrating

A tunable fibre optic delay line is demonstrated using a uniform fibre Bragg grating. A segment of the grating is heated, increasing its Bragg wavelength so that a signal propagating outside the nominal stop-band is reflected from that point. The position of the heater along the length of the grating determines the time delay encountered by the signal. For a 10 cm apodised grating, a delay of up to 700 ps was obtained.