domingo, 27 de junio de 2010

Zigzag slabs for solid-state laser amplifiers: batch fabrication and parasitic oscillation suppression

Applications such as gravitational-wave detectionrequire high-average-power laser sources with ahigh degree of spectral and spatial coherence. Forexample, the proposed Advanced Laser InterferometerGravitational Wave Observatory (LIGO)requires a 200 W, single-longitudinal-mode, singletransverse-mode Nd:YAG laser as compared with the10 Wlaser used continually on site since 1997.1,2 Severalapproaches to meet this requirement are beinginvestigated. Traditional rod-based injection-lockedlasers operate at 114 W.3 Thermal lensing and stressinducedbirefringence present challenges to theirpower scaling. Large-core double-clad fiber amplifierbasedsources have reached 264 W of output power.4However, their phase noise, pointing stability, andlong-term reliability have to be characterized withrespect to the demanding LIGO requirements. Wehave investigated a master oscillator power amplifier(MOPA) approach based on end-pumped zigzag slabamplifiers to take advantage of the well-knownpower scaling of slab lasers and the reliability andcoherence-preserving properties of power amplification.In addition to scientific applications, commercialapplications also motivate several approaches forscaling solid-state lasers to high average powers.For example, active mirror slab lasers also knownas thin-disk lasers, first invented by Martin andChernoch5 and extensively developed by Giesen andcolleagues,6,7 have reached the 1 kW class in multipletransverse modes. Power scaling is difficult becauseof their one-sided cooling and the practicalaspects to operate at a thickness below 100 m. Thezigzag, rectilinear geometry, slab-based5 MOPA systemcan scale to higher average powers while maintaininga high beam quality. The slab lasers arecooled symmetrically on both sides and their averagepower output scales with the cooled area. The nearlyone-dimensional thermal gradients and the zigzagoptical path of slab laser gain media significantlyreduce thermal lensing and stress-induced birefringencecompared with traditional rod-based designs.8,9Early zigzag slab laser designs had low efficienciesdue to flashlamp pumping. Residual phase distortionsand a complex direct water-cooled laser headadded to the engineering challenges.10 Most of theseengineering problems have now been solved by laserdiode pumping through the end11 and edge12 ofconduction-cooled zigzag slabs.13 Nd:YAG zigzag slablasers now operate at multikilowatt output powerswith good beam quality.

Optical bistability in diode-laser amplifiers and injection-locked laser diodes

Optical bistability (OB) in semiconductor lasers andin resonant-type laser amplifiers has attracted muchattention recently because of its potential applicationin optical computing and optical communicationnetworks. When a semiconductor laser, biased justbelow threshold, is operated as an optical amplifier,dispersive OB has been demonstrated.'1-4 On theother hand, OB has also been found recently in opticallyinjection-locked semiconductor lasers.5 ' Thesephenomena have been theoretically explained eitherwhen a laser amplifier operates below threshold 3'4or when an injection-locked laser works abovethreshold.5',7 Quite different physical images havehitherto been considered for these two cases. Sincethere is no discontinuity at the threshold of asemiconductor laser, the OB should be continuousfrom below to above threshold. However, a unifiedtreatment has not yet been found. In this Letter wepresent our experimental measurement on the OBproperties of a distributed-feedback semiconductorlaser biased from below to above threshold.The experimental setup is described as follows.Two identical distributed-feedback laser diodes withan emission wavelength of 1554 nm are used. Thefirst laser (master), biased at three times its threshold,is used to generate the signal light, and its outputis injected into the second laser (slave) that worksas an OB element; both lasers have one facet antireflectioncoated and the other facet cleaved. Eachlaser used is isolated from external reflection with adouble-section Faraday optical isolator that providesmore than 70 dB of isolation. A monochromator anda scanning Fabry-Perot interferometer are used forrough and fine measurement of the optical spectrum.Frequency matching and detuning between the twolasers is accomplished by adjusting the heat-sinktemperature of the master laser. The injected opticalpower into the slave laser can be evaluated asfollows: first, measure the linewidth-enhancementfactor a of the slave laser by using the injectionlockingmethod8 ; with this ae value known, the injectedoptical power can then be obtained throughthe measured injection-locking bandwidth when theslave laser is biased above threshold.9 At each biaslevel of the slave laser and with a definite opticalpower injected from the master, a bistable outputfrom the slave laser can be obtained versus the signalfrequency detuning. As an example, the squares inFig. 1 give the measured bistable loop width versusthe relative bias level of the slave laser from belowto above threshold while the injected optical power iskept at approximately -23 dBm. Below threshold,the OB loop increases its width with the increaseof the bias level, while at above threshold, the loopwidth decreases with the bias level. The maximumOB loop width is obtained, in our case, at -1.03 timesthe free-running laser threshold, and this value is, ingeneral, dependent on the amount of injected opticalpower.In order to simulate the OB operation, a unifiedtreatment is obviously necessary, which allows oneto consider the laser biased from below to abovethreshold. Our theoretical analysis is based on therate-equation model of Lang

Fig. 1. Measured bistable loop width versus the normalizedinjection current with the optical injection levelat approximately -23 dBm (squares) together with thecalculated results with an injected optical power Pi. of-25 dBm (short-dashed curve), -23 dBm (solid curve),and -21 dBm (long-dashed curve). Ith is the free-runinglaser threshold.
Fig. 1. Measured bistable loop width versus the normalizedinjection current with the optical injection levelat approximately -23 dBm (squares) together with thecalculated results with an injected optical power Pi. of-25 dBm (short-dashed curve), -23 dBm (solid curve),and -21 dBm (long-dashed curve). Ith is the free-runinglaser threshold.

Semiconductor optical amplifier (SOA)

Semiconductor optical amplifiers are amplifiers which use a semiconductor to provide the gain medium.[3] These amplifiers have a similar structure to Fabry–Pérot laser diodes but with anti-reflection design elements at the endfaces. Recent designs include anti-reflective coatings and tilted waveguide and window regions which can reduce endface reflection to less than 0.001%. Since this creates a loss of power from the cavity which is greater than the gain it prevents the amplifier from acting as a laser.
Semiconductor optical amplifiers are typically made from group III-V compound semiconductors such as GaAs/AlGaAs, InP/InGaAs, InP/InGaAsP and InP/InAlGaAs, though any direct band gap semiconductors such as II-VI could conceivably be used. Such amplifiers are often used in telecommunication systems in the form of fibre-pigtailed components, operating at signal wavelengths between 0.85 µm and 1.6 µm and generating gains of up to 30 dB.
The semiconductor optical amplifier is of small size and electrically pumped. It can be potentially less expensive than the EDFA and can be integrated with semiconductor lasers, modulators, etc. However, the performance is still not comparable with the EDFA. The SOA has higher noise, lower gain, moderate polarization dependence and high nonlinearity with fast transient time. This originates from the short nanosecond or less upper state lifetime, so that the gain reacts rapidly to changes of pump or signal power and the changes of gain also cause phase changes which can distort the signals. This nonlinearity presents the most severe problem for optical communication applications. However it provides the possibility for gain in different wavelength regions from the EDFA. "Linear optical amplifiers" using gain-clamping techniques have been developed.
High optical nonlinearity makes semiconductor amplifiers attractive for all optical signal processing like all-optical switching and wavelength conversion. There has been much research on semiconductor optical amplifiers as elements for optical signal processing, wavelength conversion, clock recovery, signal demultiplexing, and pattern recognition.
[edit] Vertical-cavity SOAA recent addition to the SOA family is the vertical-cavity SOA (VCSOA). These devices are similar in structure to, and share many features with, vertical-cavity surface-emitting lasers (VCSELs). The major difference when comparing VCSOAs and VCSELs is the reduced mirror reflectivities used in the amplifier cavity. With VCSOAs, reduced feedback is necessary to prevent the device from reaching lasing threshold. Due to the extremely short cavity length, and correspondingly thin gain medium, these devices exhibit very low single-pass gain (typically on the order of a few percent) and also a very large free spectral range (FSR). The small single-pass gain requires relatively high mirror reflectivities to boost the total signal gain. In addition to boosting the total signal gain, the use of the resonant cavity structure results in a very narrow gain bandwidth; coupled with the large FSR of the optical cavity, this effectively limits operation of the VCSOA to single-channel amplification. Thus, VCSOAs can be seen as amplifying filters.
Given their vertical-cavity geometry, VCSOAs are resonant cavity optical amplifiers that operate with the input/output signal entering/exiting normal to the wafer surface. In addition to their small size, the surface normal operation of VCSOAs leads to a number of advantages, including low power consumption, low noise figure, polarization insensitive gain, and the ability to fabricate high fill factor two-dimensional arrays on a single semiconductor chip. These devices are still in the early stages of research, though promising preamplifier results have been demonstrated. Further extensions to VCSOA technology are the demonstration of wavelength tunable devices. These MEMS-tunable vertical-cavity SOAs utilize a microelectromechanical systems (MEMS) based tuning mechanism for wide and continuous tuning of the peak gain wavelength of the amplifier. SOAs has a more rapid gain response ,which is in the order of 1 to 100ps.

Erbium-doped fibre amplifiers

The erbium-doped fibre amplifier (EDFA) is the most deployed fibre amplifier as its amplification window coincides with the third transmission window of silica-based optical fibre.
Two bands have developed in the third transmission window – the Conventional, or C-band, from approximately 1525 nm – 1565 nm, and the Long, or L-band, from approximately 1570 nm to 1610 nm. Both of these bands can be amplified by EDFAs, but it is normal to use two different amplifiers, each optimized for one of the bands.
The principal difference between C- and L-band amplifiers is that a longer length of doped fibre is used in L-band amplifiers. The longer length of fibre allows a lower inversion level to be used, thereby giving at longer wavelengths (due to the band-structure of Erbium in silica) while still providing a useful amount of gain.
EDFAs have two commonly-used pumping bands – 980 nm and 1480 nm. The 980 nm band has a higher absorption cross-section and is generally used where low-noise performance is required. The absorption band is relatively narrow and so wavelength stabilised laser sources are typically needed. The 1480 nm band has a lower, but broader, absorption cross-section and is generally used for higher power amplifiers. A combination of 980 nm and 1480 nm pumping is generally utilised in amplifiers.
The optical fiber amplifier was invented by H. J. Shaw and Michel Digonnet at Stanford University, California, in the early 1980s. The EDFA was first demonstrated several years later [1] by a group including David N. Payne, R. Mears, and L. Reekie, from the University of Southampton and a group from AT&T Bell Laboratories, E. Desurvire, P. Becker, and J. Simpson.[2]
[edit] Doped fibre amplifiers for other wavelength rangesThulium doped fibre amplifiers have been used in the S-band (1450–1490 nm) and Praseodymium doped amplifiers in the 1300 nm region. However, those regions have not seen any significant commercial use so far and so those amplifiers have not been the subject of as much development as the EDFA. However, Ytterbium doped fiber lasers and amplifiers, operating near 1 micrometre wavelength, have many applications in industrial processing of materials, as these devices can be made with extremely high output power (tens of kilowatts).

Laser amplifiers

Almost any laser active gain medium can be pumped to produce gain for light at the wavelength of a laser made with the same material as its gain medium. Such amplifiers are commonly used to produce high power laser systems. Special types such as regenerative amplifiers and chirped-pulse amplifiers are used to amplify ultrashort pulses.Doped fibre amplifiers
Schematic diagram of a simple Doped Fibre AmplifierDoped fibre amplifiers (DFAs) are optical amplifiers that use a doped optical fibre as a gain medium to amplify an optical signal. They are related to fibre lasers. The signal to be amplified and a pump laser are multiplexed into the doped fibre, and the signal is amplified through interaction with the doping ions. The most common example is the Erbium Doped Fiber Amplifier (EDFA), where the core of a silica fiber is doped with trivalent Erbium ions and can be efficiently pumped with a laser at a wavelength of 980 nm or 1,480 nm, and exhibits gain in the 1,550 nm region.Amplification is achieved by stimulated emission of photons from dopant ions in the doped fibre. The pump laser excites ions into a higher energy from where they can decay via stimulated emission of a photon at the signal wavelength back to a lower energy level. The excited ions can also decay spontaneously (spontaneous emission) or even through nonradiative processes involving interactions with phonons of the glass matrix. These last two decay mechanisms compete with stimulated emission reducing the efficiency of light amplification.The amplification window of an optical amplifier is the range of optical wavelengths for which the amplifier yields a usable gain. The amplification window is determined by the spectroscopic properties of the dopant ions, the glass structure of the optical fibre, and the wavelength and power of the pump laser.Although the electronic transitions of an isolated ion are very well defined, broadening of the energy levels occurs when the ions are incorporated into the glass of the optical fibre and thus the amplification window is also broadened. This broadening is both homogeneous (all ions exhibit the same broadened spectrum) and inhomogeneous (different ions in different glass locations exhibit different spectra). Homogeneous broadening arises from the interactions with phonons of the glass, while inhomogeneous broadening is caused by differences in the glass sites where different ions are hosted. Different sites expose ions to different local electric fields, which shifts the energy levels via the Stark effect. In addition, the Stark effect also removes the degeneracy of energy states having the same total angular momentum (specified by the quantum number J). Thus, for example, the trivalent Erbium ion (Er+3) has a ground state with J = 15/2, and in the presence of an electric field splits into J + 1/2 = 8 sublevels with slightly different energies. The first excited state has J = 13/2 and therefore a Stark manifold with 7 sublevels. Transitions from the J = 13/2 excited state to the J= 15/2 ground state are responsible for the gain at 1.5 µm wavelength. The gain spectrum of the EDFA has several peaks that are smeared by the above broadening mechanisms. The net result is a very broad spectrum (30 nm in silica, typically). The broad gain-bandwidth of fibre amplifiers make them particularly useful in wavelength-division multiplexed communications systems as a single amplifier can be utilized to amplify all signals being carried on a fiber and whose wavelengths fall within the gain window.

Los amplificadores ópticos

Estos dispositivos generan una réplica de la señal de entrada pero con mayor nivel de potencia, operando completamente en el dominio óptico. Además pueden emplearse en otors procesos como la conmutación, la demultiplexación, o bien en la conversión de longitud de onda, aprovechando su comportamiento no lineal.Las ventajas de estos dispositivos frente a los regeneradores: Funcionamiento independiente del tipo de modulación de la señal. Tiene un amplio ancho de banda, por lo que amplifica varias longitudes de onda simultáneamente. Mayor simplicidad y por tanto menor probabilidad de fallos y menor coste que los regeneradores. Permiten emplear reflectómetros ópticos para el testeo y supervisión de las líneas de fibra óptica. Pueden ser integrados. Las limitaciones más importantes que supone su empleo son:Introducen un ruido adicional que es amplificado junto con la señal. Al no regenerar la señal se produce un efecto acumulativo de la dispersión. Su ancho de banda es finito por lo que limita el número de canales en los sistemas WDM. Su ganancia no es uniforme en todo el rango de amplificación, por lo que debe ser ecualizada. Tipos de amplificadores ópticos según su aplicación. La siguiente figura muestra las principales tipos de amplificadores según su aplicación: Como amplificador de línea en un enlace con fibra monomodo, como el que se muestra en la figura (a), se emplea para elevar el nivel de potencia de la señal y compensar así las pérdidas sufridas por la propagación de la señal. Frecuentemente se instalan varios amplificadores en cascada a lo largo de la línea. Como preamplificador front-end en un receptor, como muestra la figura (b), su misión es amplificar la señal antes de ser detectada por el fotodetector para mejora así la relación señal ruido. Como amplificador de potencia situándose a continuación de la fuente láser, se emplea para elevar el nivel de potencia de la señal e incrementar la distancia de transmisión. En la configuración de la figura (c) su objetivo es compensar las pérdidas debidas al modulador externo. En la configuración de la figura (d) busca compensar las pérdidas que sufre una señal al atravesar un divisor.


Cuando una señal se propaga por la fibra óptica se necesitan emplear regeneradores para amplificar la señal debido a los efectos de la atenuación y la dispersión, así como de la longitud máxima permitida para la fibra entre transmisor y receptor, que no alcanza para cubrir todo la distancia del enlace.Al principio se empleaban regeneradores o repetidores electrónicos. Estos realizan una conversión de la señal del dominio óptico al eléctrico, amplifican la señal eléctrica, la resincronización, recuperan su forma y realizan una conversión del dominio eléctrico al óptico. Atendiendo al procesado que se efectúa sobre una señal, los regeneradores se clasifican en tres tipos, como se muestra en la figura:1R , Regeneration . Amplificación de la señal. Son por tanto transparentes al formato de la modulación y se pueden aplicar a señales analógicas. Por contra, añaden ruido y no contrarrestan los efectos de la dispersión y de las no linealidades. 2R , Regeneration & Reshaping. Además de amplificar, se recupera de la forma de la señal. Por tanto sólo son aptos para señales digitales. 3R , Regeneration, Reshaping & Reclocking. Además de amplificar y regenerar la señal, la sincroniza. Este tipo de regeneradores cancela los efectos de las no linealidades y de la dispersión.

Estos regeneradores que actúan en el dominio eléctrico no son adecuados cuando se trabaja con sistemas con varias longitudes de onda y de alta velocidad, además de ser caros y complejos debido al uso de electrónica de alta frecuencia. Por ello surgen los amplificadores ópticos.

Amplificador óptico de semiconductor (Semiconductor optical amplifier, SOA)

Los amplificadores ópticos de semiconductor tienen una estructura similar a un láser Fabry-Perot salvo por la presencia de un antireflectante en los extremos. El antireflectante incluye un antireflection coating y una guía de onda cortada en ángulo para evitar que la estructura se comporte como un láser.
El amplificador óptico de semiconductor suele ser de pequeño tamaño y el bombeo se implementa de forma eléctrica. Podría ser menos caro que un EDFA y puede ser integrado con otros dispositivos (láseres, moduladores...).
Sin embargo, en la actualidad, las prestaciones no son tan buenas como las que presentan los EDFAs. Los SOAs presentan mayor factor de ruido, menos ganancia, son sensibles a la polarización, son muy no lineales cuando se operan a elevadas velocidades...
Su elevada no-linealidad hacen atractivos los SOAs para aplicaciones de procesado como la conmutación todo óptica o la conversión de longitud de onda. También se está estudiando su uso para implementar puertas lógicas.
[editar] Amplificadores RamanEstos dispositivos se basan en amplificar la señal óptica mediante el efecto Raman. A diferencia de los EDFAs y de los SOAs, los amplificadores Raman se basan en un una interacción no lineal entre la señal óptica y la señal de bombeo de alta potencia. De esta forma, la fibra convencional ya instalada puede ser usada como medio con ganancia para la amplificación Raman. Sin embargo, es mejor emplear fibras especialmente diseñadas (fibra altamente no lineal) en las que se introducen dopantes y se reduce el núcleo de la fibra para incrementar su no linealidad.
La señal de bombeo se puede acoplar a la fibra tanto en la misma dirección en la que se transmite la señal (bombeo codireccional) o en el sentido contrario (bombeo contradireccional). Es más habitual el bombeo contradireccional para evitar la amplificación de las componentes no lineales.
El máximo de ganancia se consigue 13 THz (unos 100 nm) por debajo de la longitud de onda de bombeo.
Para obtener una buena amplificación es necesario usar potencias de bombeo elevadas (de hasta 1 W y hasta 1,2 W para amplificación en banda L en fibra monomodo estándar). Normalmente se emplean más de dos diodos de bombeo. El nivel de ruido que se obtiene es bajo especialmente cuando se usa junto con EDFAs.

Amplificadores de fibra dopada

Amplificadores en fibra son amplificadores ópticos que usan fibra dopada, normalmente con tierras raras. Estos amplificadores necesitan de un bombeo externo con un láser de onda continua a una frecuencia óptica ligeramente superior a la que amplifican. Típicamente, las longitudes de onda de bombeo son 980 nm o 1480 nm y para obtener los mejores resultados en cuanto a ruido se refiere, debe realizarse en la misma dirección que la señal[1] .
Un amplificador óptico es capaz de amplificar un conjunto de longitudes de onda (WDM, wavelength division multiplexing).
[editar] Amplificador de fibra dopada con Erbio (EDFA) Diagrama esquemático de un amplificador de fibra dopada.El amplificador de fibra dopada más común es el EDFA (del inglés, Erbium Doped Fiber Amplifier) que se basa en el dopaje con Erbio de una fibra óptica.
Algunas características típicas de los EDFAs comerciales son:
Frecuencia de operación: bandas C y L (approx. de 1530 a 1605 nm). Para el funcionamiento en banda S (below 1480 nm) son necesarios otros dopantes. Baja figura de ruido (típicamente entre 3-6 dB). Ganancia entre (15-40 dB). Baja sensibilidad al estado de polarización de la luz de entrada. Máxima potencia de salida: 14-25 dBm. Ganancia interna: 25-50 dB. Variación de la ganancia: +/- 0,5 dB. Longitud de fibra dopada: 10-60 m para EDFAs de banda C y 50-300 m para los de banda L. Número de láseres de bombeo: 1-6. Longitud de onda de bombeo: 980 nm o 1480 nm2. Ruido predominante: ASE (Amplified Spontaneous Emission). El ruido ASE generado a la salida de un amplificador de este tipo se puede calcular como:

donde, nsp es el factor de emisión espontánea, G es la ganancia del amplificador y B0 es el ancho de banda óptico del amplificador.