Coherent light generators – Particular resonant cavity – Folded cavity
Reexamination Certificate
2002-02-20
2003-09-09
Ip, Paul (Department: 2828)
Coherent light generators
Particular resonant cavity
Folded cavity
C372S018000
Reexamination Certificate
active
06618423
ABSTRACT:
The invention regards a passive mode-locked femtosecond laser having a ring resonator comprising a laser-active element, an optical output coupler and at least one mirroring element. Further the invention regards a femtosecond laser according the preamble part of claim
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and still further a femtosecond laser according to the preamble part of claim
1
. Further the invention regards a method for tuning a femtosecond laser. Also the invention regards a method of use of a femtosecond laser for generation of laser pulses with a duration below one picosecond.
With passive mode-locked femtosecond laser systems relatively high puls repetition rates in the range of several hundred MHz may be reached. These laser systems nevertheless cannot be referred to as being of high repetition rate as their repetition rates do not exceed 500 MHz. Due to extremely low pulse durations the pulses have a high peak intensity. Femtosecond lasers nowadays are successfully employed in the field of time-resolved spectroscopy, non-linear optics, multiple-photon-microscopy, micro-material engineering, optical frequency metrology and optical coherence-tomatography. Further in the future such lasers will also play a substantial role in the field of optical data communication.
Conventional passive mode-locked femtosecond laser systems rely on the use of titan doped sapphire crystals as laser-active elements. Upon optical excitation these develope a broad fluorescence spectrum in the range between 700 and 1000 nm. From this a gain profile of comparable range results, which means that Ti:Sapphire laser systems are suitable for generation of laser light in said range of wavelengths.
For generation of ultra-short laser pulses it must be observed that a laser pulse which is short in the time domain is correlated with a broad frequency spectrum. Because of this reason only laser elements with a broad gain profile are suitable for generation of ultra-short pulses.
All yet known passive mode-locked femtosecond laser systems with a solid state laser-active element (a CPM-dye laser is also passive mode-locked) rely on the concept of “Kerr-lens mode-locking”. This non-linear optical effect results, by self-focussing of an intensive light beam in a non-linear medium, in a temporarily gain of a single pulse in the laser-active element during its round trip in the resonator, compared to a continuous operation of the laser.
The repetition rate of such a femtosecond laser system is determined by the duration of a round trip of the pulse circulating in the resonator.
The duration of the pulse circulating in the resonator is nevertheless not able to reach the theoretical limit, which is determined by the width of the gain profile of the laser-active elements. This is caused by the phenomenon of pulse-broadening, which is experienced by the laser pulse in particular in the laser-active element during its roundtrip in the resonator. This effect is due to the so-called positive group velocity dispersion of the laser-active elements and further optical components in the resonator. The consequence is, that the various portions of wavelengths of the circulating pulse pass the laser-active element within varying time periods, whereby the laser pulse passing through the laser-active element is broadened in its time duration.
The basic approach to compensate pulse broadening which is caused by the positive group velocity dispersion of the laser-active elements and further optical components of the resonator is the use of an arrangement in the laser resonator comprising a negative group velocity dispersion which at least compensates the pulse broadening which has been caused by the laser-active element and the further optical components.
Known in prior art is for instance an arrangement of two dispersive elements, for instance prisms in a laser resonator, a so-called prism compensator.
Basics of femtosecond laser systems which rely on “Kerr-lens mode-locked” Ti:Sapphire lasers with prism compensators can for instance be drawn from the publication of D. E. Spence, P. N. Kean, W. Sibbet in
Optics Letters
16, page 42 and following pages (1991).
Recently as an alternative to said prisms or prism compensators dielectric mirrors have been developed which provide a negative group velocity dispersion GVD. This is achieved by a suitable sequence of dielectric layers on a substrate. The basic concept can be drawn from the publication of R. Szipöcs, K. Ferencz, Ch. Spielmann, F. Krausz in
Optics Letters
19, page 201 and following pages (1994).
The use of such mirrors with negative group velocity dispersion GVD in a laser resonator offers a substantial advantage, i.e. in contrast to the above mentioned prisms- or prism compensators only a non-significant prolongation of the optical path in the resonator occurs.
A femtosecond laser system, which is based on said mirrors, may for instance be drawn from the publication of H. Stingl, Ch. Spielmann, R. Szipöcs, F. Krausz in
Conference on Lasers and Electro-Optics
9, 1996 OSA Technical Digest Series (O.S.A., Washington D.C., 1996) page 66 and following.
Most of the Ti:Sapphire femtosecond laser systems relying on the phenomenon of “Kerr-lens mode-locking” comprise a Fabry-Perot-resonator, whose markable feature is a planar end mirror and which in particular has a folded configuration. In such kind of configuration elements belonging to a pulse compression such as prisms may be allocated simply in one arm of the resonator. The total length of such a resonator amounts typically in the range of 2 meters. Therefrom typical pulse repetition rates in the range of a few megahertz of normally below 100 MHz result. Such laser systems are not labelled as having a high-repetition rate.
From the above-mentioned publication of A. Stingl et. al. for instance a passive mode-locked Ti:Sapphire femtosecond laser system is known, which relies on a Fabry-Perot-resonator and which makes use of mirrors having a negative group velocity dispersion GVD.
Further from the U.S. Pat. No. 5,383,198 a self-starting passive mode-locked femtosecond laser system is known, having a prism compressor and a ring resonator and also from the U.S. Pat. No. 5,799,025 a self-starting passive mode-locked femtosecond laser system is known, having a prism compressor and a Fabry-Perot-resonator.
Due to the respective resonator geometries none of the mentioned laser systems allows to achieve pulse repetition rates of above 500 MHz and therefore these laser systems cannot be labelled as having high repetition rates.
An alternative approach may be drawn from the publication of M. Ramaswamy and J. G. Fujimoto in
Optics Letters
19, page 1756 and following pages (1994) (see also U.S. Pat. No. 5,553,093). The approach is based on a simplified resonator configuration with making use of a specific prism compressor. Instead of a conventional resonator-internal pair of prisms a prism-shaped laser crystal and a prism-shaped output coupler is used. A specific geometry of the laser resonator configurated as a Fabry-Perot-resonator and also the simplified prism compressor allows a shortening of the resonator length to about 30 cm so that a repetition rate of 1 GHz may be achieved.
Disadvantageous on this concept is, that in the outgoing beam the various in a laser pulse superimposed spectral components spread apart in a spatial direction perpendicular to the direction of the laser beam (“spatial chirp”), which at least complicates a practical use of such laser concept.
In fact the prism compressor being located inside the resonator enforces a minimal length of the resonator upon which it appears unlikely and even impossible that higher repetition rates than 1 GHz may be achieved.
A prism compressors causes, additionally to a negative group velocity dispersion, in some parts of the resonator a spatial split-up of the various spectral components of a laser pulse circulating in the resonator. A selection of wavelength may be established by use of a suitable, re-allocatable aperture in such area of a resonator, in which the spectral components are spatially
Bartels Albrecht
Dekorsy Thomas
Kurz Heinrich
Gigaoptics GmbH
Ip Paul
Shaw Pittman LLP
Vy Hung T
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