Dispersion and loss limitations on the performance of optical delay lines based on coupled resonant structures

The relative importance of group-velocity dispersion and loss-induced distortion in limiting the performance of optical delay lines based on coupled resonator structures is investigated. It is shown that for the current state of the quality of fabrication both factors play roughly comparable roles for bit rates of 2.5-40 Gbits/s and that as the storage capacity grows, the relative weight of loss-imposed limitation increases.     

Widely tunable optical delay generator

We propose and demonstrate a method for quasi storage of light based on periodic spectral filtering realized in the time domain by amplitude modulation using frequency-to-time conversion. The delay can be tuned in a wide range by changing the frequency of an electrical modulation signal. In our experiments, the delay of single 2.5 ps pulses varied by 66 pulse widths. The technique works equally well for more complex optical data packets. Contrary to known approaches, the method has a very large spectral bandwidth and can be implemented by either fiber or integrated solutions using existing technologies. Because of the large bandwidth, fractional delays up to several tens of thousands of pulse widths can be achieved potentially for subpicosecond pulses, which is a tremendous value regarding the implementation simplicity.     

Photodetectors,their performance and their limitations

With the human eye as a reference, a short survey of man-made photodetectors is given. The smallest number of detectable photons and the “detectivity” of present-day detectors, both as a function of the wavelength, are discussed in more detail. Finally, some special comments on the performance of photoconductive detectors are made since they are at present the most sensitive detectors for middle and far infrared radiation.     

Continuously tunable 1 byte delay in coupled-resonator optical waveguides

A reconfigurable coupled-resonator optical waveguide made of a few directly coupled ring resonators is employed to control the delay of data streams modulated at tens of gigabits per second. A delay of 8 bit lengths (1 optical byte) with a small pulse broadening and 1 dB/bit fractional loss is achieved by using only eight rings. The limiting role of waveguide loss and spurious backreflections is experimentally investigated. The high storage efficiency (1 bit/ring) of the device enables an easy, reliable, hitless, and relatively low-power-consuming management of the delay. A higher storage efficiency is demonstrated to be associated to an unavoidable higher pulse distortion.     

Tunable photonic delay lines in optical fibers

A comprehensive overview is presented about optical fiber‐based tunable photonic delay lines, which have been steadily developed over the last decade for the realization of all‐optically controlled timing functions. The most widely used techniques, such as those based on slow & fast light and wavelength conversion associated to dispersion, are described and their physical limitations are discussed in terms of the maximal achievable delay, the associated signal distortion and signal bandwidth. Besides, an entirely different approach for all‐optical signal delaying is introduced. This technique is based on movable grating reflectors dynamically generated in highly birefringent optical fibers. This type of delay line has experimentally demonstrated large tunable delaying with a moderate signal distortion for high capacity optical data streams and even for wideband analog signals.     

Full-range tunable optical delay line based on coupled and side-coupled resonators

An optical delay line based on a new kind of structure comprising coupled resonators and side-coupled resonators has been proposed and demonstrated. The structure has unique advantages in realizing tunable optical delay lines. The tuning range has covered almost the full range of the optical delay line, from minimum delay at which only one ring of the light pulse structure requires transmission, to maximum delay at which all rings are required. In addition, the input signal delivered from the input to output port travels the structure twice, thereby providing buffer delay that is twice longer than general coupled resonators.     

Reliability of multicore optical fibers in fiber-optic delay lines

Analytical expressions for estimating the minimum acceptable bending diameter are derived as a function of the expected service life, probability of premature fracture, and optical fiber diameter, which can be used to estimate the multicore optical fiber functionality in the particular applications.     

Performance limits of delay lines based on optical amplifiers

The performance of tunable buffers based on optical amplifiers and its improvement by use of gain flattening are analyzed. A simple analytical expression is obtained indicating that the gain required to attain tunable storage of N bits increases nonlinearly with N and reaches unrealistically high values for N>5. A comparison with other types of optical delay line is made.     

"Ideal" optical delay lines based on tailored-coupling and reflecting, coupled-resonator optical waveguides

We present a design of "ideal" optical delay lines (i.e., constant amplitude and constant group delay over the desired bandwidth). They are based on reflection from coupled-resonator optical waveguides (CROWs). The inter-resonator coupling coefficients are tailored and decrease monotonically with the distance from the input to realize all-pass Bessel filters. The tailored coupling coefficients result in a frequency-dependent propagating distance which compensates for the group velocity dispersion of CROWs. We present a simple formalism for deriving the time-domain coupling coefficients and convert these coefficients to field coupling coefficients of ring resonators. The reflecting CROWs possess a delay-bandwidth product of 0.5 per resonator, larger than that of any kind of transmitting CROW. In the presence of uniform gain, the gain enhanced by slow light propagation and the constant group delay result in efficient and dispersion-free amplifiers.     

Data pulse distortion induced by a slow-light tunable delay line in optical fiber

We experimentally demonstrate an all-optical tunable delay line based on slow light induced by stimulated Brillouin scattering in a highly nonlinear fiber (HNLF) and study the distortion of data pulses with different pulse widths and the induced pattern-dependent distortion on non-return-to-zero data in slow-light systems.     

Rapidly tunable all-optical wavelength converter based on single semiconductor optical amplifier delay interferometer

We demonstrate a rapidly tunable 10 Gb/s all-optical wavelength converter based on a semiconductor optical amplifier delay interferometer and a tunable laser. It uses a 16-channel 100 GHz-spacing digitally tunable multifrequency laser based on a novel external-cavity laser design. The bit sequence on the incoming wavelength is converted alternatively to different wavelengths. Power penalties are 1.1 dB for 30 ns guard-time spacing between wavelength packets.     

Integrated optical delay lines:a review and perspective[Invited]

Optical delay lines(ODLs) are one of the key enabling components in photonic integrated circuits and systems.They are widely used in time-division multiplexing, optical signal synchronization and buffering, microwave signal processing, beam forming and steering, etc. The development of integrated photonics pushes forward the miniaturization of ODLs, offering improved performances in terms of stability, tuning speed, and power consumption. The integrated ODLs can be implemented using various structures, such as single or coupled resonators, gratings, photonic crystals, multi-path switchable structures, and recirculating loop structures.The delay tuning in ODLs is enabled by either changing the group refractive index of the waveguide or changing the length of the optical path. This paper reviews the recent development of integrated ODLs with a focus on their abundant applications and flexible implementations. The challenges and potentials of each type of ODLs are pointed out.     

Slowing down the light for delay lines implementation: Design and performance

Optical delay lines are key building blocks for all-optical signal processing. Photonic crystal structures can demonstrate efficient group velocity reduction, together with a wide-bandwidth and reduced high-order group velocity dispersion. Theses structures also offer the ability for 2D integration within photonic integrated circuits. This paper presents the performances of photonic crystal structures engineered for slowing down the light, and discuss the actual limitation encountered due to fabrication imperfections. To cite this article: A. Talneau, C. R. Physique 10 (2009).     

A high performance tunable optical filter based on cascaded polarization interference filter

A new kind of tunable optical filter is proposed for DWDM optical communication application. It is based on cascaded polarization interference filter (PIF). The period and bandpass width of each PIF are decided by its optical path difference between o-ray and e-ray (OPDOE). When their OPDOEs are proportionately designed, the tuning range and bandpass width depend on OPDOE in the first and the last PIF, respectively. The tuning range, bandpass width and crosstalk are independent each other. The crosstalk is related to the OPDOE ratios among PIFs and can be suppressed by designing the PIF's 15 × 2N-7 × l1, 10 × 2N-6 × l1 and 2N-2 × l1 from the first to the last. This suggested OPDOEs can yield -50-dB crosstalk for any tuning range and bandpass width. The insert loss is less than i dB. As its loose alignment requirement, there is no limitation on cascaded PIF number. When 11 PIFs are cascaded, it can achieve 170-nm tuning range, -50-dB crosstalk, bandpass width applicable to 25-GHz channel spacing and 1 dB insert loss.     

All-optical tunable delay line based on wavelength conversion in semiconductor optical amplifiers and dispersion in dispersion-compensating fiber

We demonstrate an all-optical continuously tunable delay line system based on wavelength conversion in semiconductor optical amplifiers (SOAs), and group-velocity dispersion (GVD) in a dispersion-compensating fiber (DCF). The system operates, near 1550 nm, with a non-return-to-zero (NRZ) pattern at 10 Gb/s. A maximal optical delay up to 2700 ps is observed. The scheme achieves continuous control of a wide range of optical delays, wide signal bandwidth, nearly no pulse broadening, and very little spectral distortion.     

Design of a continuously tunable delay line using vectorial modulational instability and chromatic dispersion in optical fibers

We design an all-optical tunable delay line based on both dispersive and wavelength conversion stages involving modulational instability of a two-frequency pump field propagating in a highly birefringent fiber. More precisely, we numerically show that, by varying the frequency separation between the two orthogonally polarized pump waves, we achieve a controllable and continuous delay of hundreds of picoseconds for signal pulse durations from picoseconds to nanoseconds, without pulse distortion and with only small peak power fluctuations. The proposed method does not require any tunable bandpass filter and can be applied to delay digital data streams at tens of Gbit/s depending on the pump power level.     

A novel method for measuring optical fiber delay lines based on the optical fiber low-coherence Young's interferometer

According to the principle of double-beam interference imaging using an infrared broadband light source, a novel method based on the optical fiber low-coherence Young's interferometer for the measurement of programmable optical fiber delay line is proposed. By measuring the small length increments of delay fiber through the evaluation of the central positions of zero-order interference fringes, we successfully construct a system with a variety of important functions, such as real-time controlling, measurement and displaying. This system offers the benefit that it can be regulated precisely to generate interference stripes once again after the length of fiber segment changes. When the locations of central stripes are moved to coincide with the original markers, the determination of the corresponding fiber optical delay time becomes really simplified, as just by reading out the tinny displacement of the reference arm. Multiple groups of experimental results demonstrate the outstanding performance of the proposed system since it bears a measuring accuracy of μm and a measuring range from 1 mm to 20 cm.     

基于磁光开关的高精度光实时延时线

 给出了基于高速磁光开关的光实时延时技术的方案。研究了拓扑结构和磁光开关对光延时性能的影响。研制了5 bit,32位的光实时延时系统,并建立了光延时的测试平台,经测试,延时线延时精度优于3 ps。对光延时单元插损进行测试,结果显示:32态单元插损为0.12~0.88 dB,各态变化呈随机性。