Gain-flattening Filter Research Articles

SiON high-refractive-index waveguide and planar lightwave circuits

The rapidly growing optical communication market requires photonic components with ever-increasing functionality and complexity that can be fabricated reliably at low cost. Of the various approaches used to fabricate photonic components, those based on planar waveguides have achieved high performance and represent a promising path toward compact integration of optical functions. We present an overview of an approach used to produce an optical single-mode waveguide. Through its strong mode confinement, the approach makes it possible to integrate optical filter functions with higher functionality, as required for high-data-rate communication networks. The waveguide is based on the use of a silicon oxynitride (SiON) core and silicon oxide cladding layers, and can be fabricated using conventional chip fabrication techniques. Using the new approach, conventional passive optical components such as arrayed waveguide gratings for wavelength-division-multiplexed transmission systems can be fabricated in a more compact way than using standard silica-on-silicon waveguide methods. Moreover, the realization of more enhanced, adaptive optical functions such as finite-impulse-response as well as infinite-impulse-response filters is possible. Reconfiguration is achieved through the thermo-optic effect. A reconfigurable gain-flattening filter and an adaptive dispersion compensator are presented as examples.

Application of Nonlinear Fitting for Etalon-Type Gain-Flattening Filter Design

We describe a method of designing an etalon-type gain-flattening filter. The method is based on nonlinear fitting of transmission spectra of combined etalons, and it enables us to design gain-flattening filters that exhibit better compensation of any spectra than conventional methods. By manufacturing a typical filter module, we demonstrate excellent gain-flattening behavior applicable to the C-band optical amplifier for metro transmission systems.

Comparison of EDFA Setups using either Adaptive Equalization Filters or Fixed gain-flattening Filters with Respect to Noise Figure

Residual gain variations of erbium-doped fiber amplifiers (EDFAs) based on passive filtering techniques can accumulate over the entire transmission length and impose limitations on the link length. To overcome this problem, adaptive equalization filters have been developed that can rectify gain imbalances within each amplifier or at specific locations within the link. It is shown that such filters can also help to improve the noise figures provided by standard amplifier setups significantly. However, the investigation also reveals that amplifiers using a reconfiguration stage that can be inserted into or dropped out of the optical path can provide similar noise performance, although their realization is much easier and less cost intensive.

Dünnschichtfilter in der Telekommunikation

Market forces are pushing the performance of optics to their limits. Optical components must be developed to provide the best possible combination of manufacturability, performance and price. One vital step to success in creating WDM optics lies in a discipline that is often overlooked or misunderstood – coating engineering. A key technology for controlling light in WDM systems is the optical filter, which performs functions from simple filtering in multiplexers and demultiplexers, to more sophisticated functions in optical amplifiers, modulators and test equipment. The basic tool of multiplexing and demultiplexing devices, thin film filters offer accurate center wavelength, broad flattop passband and high isolation from adjacent and nonadjacent channels. Thin film filters are widely used for gain flattening, band splitting, C and L band separation and combining amplifier-pump beams. Choosing the right thin-film-deposition process is essential for the efficiency and productivity of vacuum coating systems. LEYBOLD OPTICS has developed and optimized a comprehensive range of vacuum coating processes and process tools. LEYBOLD OPTICS has been proven the ability of producing shift-free coatings on large substrate areas by means of PIAD (Plasma Ion Assisted Deposition) in many applications over the past ten years. The low loss and stress values achieved, especially with silica and tantala films, allows besides the production of narrow band pass filters (DWDM, CWDM) also the production of gain flattening filters (GFF). PIAD is considerably improving the properties of evaporated thin films by high energy ion bombardment during the growing of the film. PIAD allows to produce dense shift free thin films with high refractive index, good adhesion and extremely low absorption. With the Advanced Plasma Source (APS) LEYBOLD OPTICS has developed a high power plasma source for PIAD. The APS provides high ion current densities over a large surface area in a neutral plasma to produce high quality layers at a high productivity.

비대칭 구조의 커플러를 이용한 EDFA 이득평탄필터 설계

비대칭 구조의 커플러에 대한 이론적 고찰과 함께 EDFA 이득평탄용 필터로서의 응용과 그에 대한 설계를 하였다. 이득평탄필터를 구현하기 위한 비대칭 커플러의 광섬유 파라미터를 제시하였고 커플러의 광 결합 길이와 코어간의 거리에 따른 이득평탄필터의 투과 스펙트럼의 특성을 시뮬레이션을 통해 알아보았다. 그리고 시뮬레이션 결과를 바탕으로 하여 약 30nm의 대역폭에 걸쳐서 EDFA의 이득 스펙트럼을 <TEX>$\pm$</TEX>0.75 ㏈이하로 평탄화할 수 있는 이득평탄필터를 설계하였다. An asymmetrical directional coupler with two nonidentical fibers has, for the first time, been proposed and analyzed for an EDFA gain flattening filter (GFF). The characteristics of the transmission spectra of the GFFs have been theoretically investigated for the core spacings, the coupling lengths and the fiber parameters of the asymmetrical directional coupler. The analytical results show that an EDFA gain spectrum with flatness of ~7 ㏈ can be flattened to within <TEX>$\pm$</TEX>0.75 ㏈ over a bandwidth of 30 nm by using the asymmetrical directional coupler-based GFF.

Tunable gain-flattening filter using microbending long-period fiber gratings

A new type of gain-flattening filter using microbending long-period fiber gratings is presented. The fabrication and characterization of the device are described and its use in an erbium-doped fiber amplifier to flatten the amplified spontaneous emission (ASE) spectrum is demonstrated. The result shows that the ASE variation is less than 1 dB over 33 nm at two different pump powers of 45 and 68 mW.

Design of a robust thin-film interference filter for erbium-doped fiber amplifier gain equalization.

Gain-flattening filters (GFFs) are key wavelength division multiplexing components in fiber-optics telecommunications. Challenging issues in the design of thin-film GFFs were recently the subject of a contest organized at the 2001 Conference on Optical Interference Coatings. The interest and main difficulty of the proposed problem was to minimize the sensitivity of a GFF to simulated fabrication errors. A high-yield solution and its design philosophy are described. The approach used to control the filter robustness is explained and illustrated by numerical results.

Gain-flattening filter using long-period fiber gratings

We propose a novel structure for a gain-flattening filter, in which a conventional long-period fiber grating (LPFG) and a phase-shifted LPFG are written closely. The transmission characteristics are investigated in the case of a closely arranged configuration. For the fabrication of a phase-shifted LPFG, a UV-trimming method to control the amount of the phase shift is examined theoretically and experimentally. Finally, a 67-mm-long LPFG gain-flattening filter designed for an erbium-doped fiber amplifier is fabricated and the measured spectrum is shown.

Topical meeting on optical interference coatings (OIC'2001): design contest results.

A gain-flattening filter (GFF) for minimum manufacturing errors (12 designs submitted) and dense wavelength-division multiplex (DWDM) filters for low group-delay (GD) variation (9 designs submitted) was the subject of a design contest held in conjunction with the Optical Interference Coatings 2001 topical meeting of the Optical Society of America. Results of the contest are given and evaluated. It turned out that the parameter space for GFFs with optimum performance when manufacturing errors are not considered is much different from that when manufacturing errors are considered. DWDM filter solutions with low GD variation are possible.

Theoretical study of amplitude and phase filtering of guided waves.

The design of integrated optics filters by use of refinement software based on the Abelès thin-film computation method and the film mode matching method is studied. The results obtained with the two computation methods are compared. Good agreement is obtained provided that the fill factor of the guided mode in the component is high and that modal losses between waveguide sections are simulated by absorption with the Abelès computation method. Integrated optics devices that manage either the amplitude of guided waves such as a dense wavelength division multiplexing narrow-bandpass filter and a gain-flattening filter or the phase of guided waves such as a broadband dispersion compensator are

Wideband multiwavelength erbium-doped fiber ring laser with frequency shifted feedback

Wideband multiwavelength erbium-doped fiber ring lasers with frequency shifted feedback are described. The use of an intra-cavity gain flattening filter (GFF) was proposed in order to increase the lasing spectral bandwidth, leading to a demonstration of 34 lasing wavelengths in 28 nm bandwidth in C-band. The GFF induced spectral output power fluctuation is discussed. Multiwavelength operation was also demonstrated for the first time in L-band, where wideband laser operation was obtained without a GFF. Optical bistability and Kerr effect induced pulsation were determined to be limiting factors to stable operation range in this kind of multiwavelength lasers.

Custom design of long chirped Bragg gratings: application to gain-flattening filter with incorporated dispersion compensation

We present and experimentally demonstrate simple relationships between the refractive index modulation, the chirp-rate or dispersion and the transmission loss through, and reflection of, chirped Bragg gratings, and apply them to the design of a new type of gain flattening filter with incorporated dispersion compensation.

Actively gain-flattened erbium-doped fiber amplifier over 35 nm by using all-fiber acoustooptic tunable filters

We demonstrate an actively gain-flattened erbium-doped fiber amplifier (EDFA) using an all-fiber gain-flattening filter with electronically controllable spectral profiles. A good gain flatness (<0.7 dB) over a broad wavelength span (>35 nm) is achieved for a wide range of operational gain levels as well as input signal and pump powers.

Optical fibers and amplifiers for WDM systems

The development of optical-fiber amplifiers allowed a dramatic increase in the capacity of optical transmission systems while reducing system costs. Capacity increases are possible because the high output powers afforded by optical-fiber amplifiers support higher bit rates, while their broad bandwidth and slow gain dynamics allow multichannel operation. This benefit comes at the expense of having to manage signal-to-noise ratio degradations due to the accumulation of amplifier noise and dispersion distortions accumulated over the total system link. Furthermore, nonlinear optical effects become significant with the use of high power signals over long lengths of fiber, causing cross talk among the different channels and increasing signal distortions. To fully exploit the potential capacity of wavelength division multiplexing systems, the optical characteristics of the fibers and optical-fiber amplifiers must be optimized. The optical amplifiers should have low noise and flat gain, which can be achieved by using 980-nm pump lasers, optimized fiber glass composition, and gain-flattening filters. The optical fibers should have a small nonzero dispersion and large effective area. Both features can be achieved by optimizing the fiber index profile. This paper summarizes the state of the art in these components and points to directions for future exploration.

Erbium doped fibre amplifier with dynamic gain flatnessfor WDM

The authors have designed and numerically analysed an erbium doped fibre amplifier for wavelength division multiplexing with a wavelength-independent gain, irrespective of operating conditions. Furthermore, under some assumptions, they show that this property cannot be attained without a gain-flattening filter. A gain flat to within 1% over a bandwidth of 6 nm is demonstrated. >