postęp optyczna

WSTĘP
Dominik Dorosz
Źródła koherentnego promieniowania optycznego, konstruowane z wykorzystaniem optycznie pobudzanych światłowodów aktywnych, charakteryzują się szeregiem unikalnych zalet, przez co są chętnie stosowane w wielu rozwiązaniach. Wyróżnić tu można: a) układy budowane na potrzeby telekomunikacji światłowodowej, generujące promieniowanie optyczne w zakresie widmowym II i III okna
transmisyjnego, b) lasery włóknowe z konwersją w górę (ang. up-conversion),
charakteryzujące się tym, że długość fali promieniowania pompującego jest większa niż długość fali generowanego promieniowania, promieniujące w zakresie VIS
i UV, c) układy emitujące promieniowanie spójne przestrzennie, wykorzystujące
zjawisko wzmocnionej emisji spontanicznej ASE (ang. Amplified Spontaneous
Emission). Bezpośredni wpływ na rozwój tego rodzaju źródeł ma niewątpliwie
dynamiczny postęp badań nad nowymi technologiami szkieł oraz światłowodów
aktywnych. Skupiają się one na opracowaniu matryc szklistych i odpowiadających
im technologii wytwarzania światłowodów tak, aby parametry fizyczne i optyczne
powstałych z nich światłowodów pozwalały na ich praktyczne wykorzystanie.
Te poszukiwania wymagają niekonwencjonalnego podejścia do problemów
(zagadnień) związanych z kształtowaniem więźby szkła o założonym rozkładzie
energii wiązań, w taki sposób, aby osiągnąć emisję promieniowania w wyniku
przejść optycznych w obrębie struktury energetycznej lantanowców. Jest to nowe
spojrzenie, w konfrontacji z powszechnie przyjętym uszeregowaniem osnowy
szklistej względem maksymalnej energii drgań fononów.
Powyższa tematyka od kilkunastu lat jest przedmiotem badań autorów tego
opracowania. W monografii przedstawiono wyniki prac autorów związanych
z poszukiwaniem szkieł (rozdział 4) i światłowodów (rozdział 5), w których luminescencja w zakresie widzialnym jest efektem procesu apkonwersji. W szczególności omówiono właściwości i strukturę szkieł o mieszanej więźbie nisko- i wysokofononowej, pozwalającej na kontrolę rozkładu energii fononów w matrycy szklistej, a przez to wzrost prawdopodobieństwa przejść promienistych, które warunkują uzyskanie (wywołanie zjawiska) luminescencji. Określone i praktycznie zweryfikowane zostały reguły umożliwiające optymalizację składu szkieł antymonowych
i tellurowych, pozwalające na uzyskanie silnej luminescencji. Scharakteryzowano
światłowody aktywne wytworzone z tych szkieł. Ponadto określono wpływ geometrii tych światłowodów na ich właściwości luminescencyjne. Porównano też zarejestrowane ich widma z widmami odpowiadających im szkieł.
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Szkła i światłowody domieszkowane pierwiastkami ziem rzadkich
Monografia może być przydatna doktorantom i pracownikom naukowym zajmującym się technologią szkieł i światłowodów (nie tylko aktywnych) oraz konstruktorom wzmacniaczy i laserów światłowodowych.
SUMMARY
Glasses, through their physicochemical properties, are excellent material for the
construction of optical fibers. Changes of their chemical composition and the use
of various methods of synthesis make it possible to design optical properties of
glasses in a very wide range allowing for the fabrication of advanced photonic
structures. In particular, glasses and optical fibers co-doped with rare earths (RE)
elements, offer new possibilities of constructing the radiation sources characterised
by unique features. Sources of optical radiation, constructed with the use of
optically pumped active fibers enable to obtain: 1) the generation of laser radiation,
2) the emission via up-conversion process, characterized by the higher wavelength
of the pumping radiation than the wavelength of the generated radiation, which
enables to obtain the radiation in UV – VIS range, 3) emission of radiation with
high spatial coherence and a short coherence length (it is possible through the use
of the phenomenon of Amplified Spontaneous Emission – ASE).
Undoubtedly, a direct impact on the development of optical fibers radiation
sources, has the progress of research on new technologies of glass and active
fibers. It focuses on developing new glassy matrices and technologies of
fabrication fiber optics corresponding to them, so that physical and optical
parameters of optical fibers made of them allow for their use in practical solutions.
These investigations require an unconventional approach to shaping the framework
of glass with the complex phonon distribution, in such a way that the radiation
emission can be achieved as a result of optical transitions within the energy
structure of lanthanides. It is a new look at the construction of glass in a
confrontation with generally accepted classification of glassy matrix in regard to
the maximum of phonon energy. The above investigations have been the subject of
research for the authors of this study for several years.
In this monograph, against other research, the results of the author's studies
connected with the search of new types of glasses and optic fibers, in which the
luminescence in the visible – infrared range have been presented. In particular,
properties and the technology of glasses with the mixed, low- and high-phonon
framework, enabling to control the energy distribution of phonons in a glass
matrix, have been discussed. The authors has determined and practically verified
the rules that allow for the optimization of the antimony and tellurium glass
composition, enabling to achieve strong luminescence. The active optical fibers
made of these glasses have been characterised. In particular, their luminescence
spectra have been compared to the glasses corresponding to them.
Summary
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Fabricated by the authors of this monograph, glasses and optical fibers co-doped
with lanthanides, which results in an improvement in pumping efficiency and the
enhanced luminescence, are also presented. It has been achieved by using the
processes of the energy transfer between RE ions placed within the same matrix.
The advantage of co-doping of active fibers is also the possibility of broadening
the spectra range of radiation by combining the emission transitions of particular
rare earth elements. Moreover, the choice of the type of the glass forming the core
of the active fiber and the its length have a significant impact on spectral
parameters of the the Amplified Spontaneous Emission (ASE).
It has been shown in the monograph, that not only concentrations, but also
relative proportions of lanthanides embedded in the glass are essential for efficient
emission. The optimisation of these values determine the energy transfer between
the donor and the acceptor, enabling to obtain the amplified spontaneous emission
in optical fiber. Furthermore, in fabricated optical fibers co-doped with several
lanthanides, the radiation emission is carried out inside the optical fiber. It
simplifies the construction of the emitter and allows efficient pumping by
semiconductor laser diodes.
The author's research on glassy materials for co-doped optical fibers
fabrication, has been concentrated not only on achieving the optimum conditions
for absorption and emission, but also on the efficient energy transfer between
active dopants. It has been shown that by combining suitable rare earth elements it
is possible to take advantage of their absorption and luminescent properties in such
a way that the radiation emission is a superposition of optical transitions of both
active ions. As a result of controlled changes in RE co-dopants and concentration
ratio of lanthanide ions, the identification of quantum processes occurring between
energy levels has been made. It has been found that in terms of the efficient energy
transfer, the best material for the construction of active optical fibers (along with
tellurium glass) is fabricated antimony glass, doped with Tm3+ and Ho3+ ions.
Closely located laser levels of both activators ΔE = 400 cm –1 significantly increase
the probability of excitation energy transfer between these ions. Measurement
results of luminescence of antimony glass, co-doped with Yb3+ and Ho3+ and Yb3+
and Tm3+ ions, confirmed that in the case of optical pumping by the radiation of the
laser diode, with a wavelength of 976 nm, there is an efficient energy transfer
resulting in luminescence at 2 μm and 1,8 μm wavelength, corresponding to 5I7 →
5
I8 (Ho3+) and 3F4 → 3H6 (Tm3+) transitions respectively. Moreover, the results
enable to explain processes occurring in glass doped with two lanthanides
simultaneously. This knowledge is essential to use energy transfer phenomena in
processes of designing materials for the construction of optical fibres sources.
On the basis of carried out research on tellurium glass, doped with selected
pairs of lanthanides of various concentrations, and results contained in the
monograph it can be concluded that the high intense emission has been obtained as
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Szkła i światłowody domieszkowane pierwiastkami ziem rzadkich
a result of the upconversion process. Tellurium glasses designed as the matrix of
rare earth is a compromise between required thermal stability, good optical
parameters and in particular low energy of phonon vibration. At the same time,
apart from the ability to accept rare earth ions without effect of clustering (up to
1,5%mol), an effective non-radiative energy transfer between donor and acceptor
ions is possible, and in a consequence luminescence in the visible range as a result
of upconversion process. In addition, determined luminescence spectra of glasses
doped with Yb3+/Ho3+, Yb3+/Tm3+, Yb3+/Er3+ ions, indicate the possibility of
developing active optical fibers emitting radiation in the visible range.
This monograph also contains the results of research of the authors on active optic
fibers. Their emission parameters enable to use them to construct laser sources and
amplified spontaneous emission (ASE), operating in the VIS-NIR range. The
novelty presented in this work is the development of the technology of fabrication
of the original structure of optical fibers co-doped with lanthanides, in which in the
active core the luminescence is obtained as a result of donor-acceptor energy
transfer processes. The amplified spontaneous emission in the range of 0,3 μm –
0,78 μm and 1,7 μm – 2,2 μm has been achieved in optical fibers. The advantage of
presented constructions is the possibility to increase the bandwidth of the radiation
emission as a result of piecing luminescence bands of lanthanide ions, which are
embedded in the fiber core. It has been shown that the intensity and the full width
at half maximum of emission band strongly depends not only on applied glass but
also on the structure and the length of the fiber.
The research results presented in this monograph open up new possibilities for
constructing optical fiber sources of radiation and can be the basis for further
research on active materials.