Patent Description:
The invention relates to a method and system for generating optical pulses of light with a spectral bandwidth much larger than the normal gain bandwidth of a typical rare earth doped laser medium or laser active crystal. Shorter optical pulses can be generated compared to a conventional laser system by compressing these generated ultrabroadband optical pulses.

Today ultrashort optical pulses of laser light with pulse durations less than a picosecond are widely used for many applications. Examples include the use in processing of micromaterials and nanomaterials, medical applications, like ophthalmology or nanosurgery, as well as in scientific applications or biomedical applications, like multiphoton microscopy. One of the well-known limitations of using these optical pulses is the so-called heat affected zone (HAZ). The heat affected zone is due to a thermal heating of the environment about a sample or a heating of living cells in biomedical applications. This can lead to thermal damage of the sample or the living cells.

The heat affected zone or thermal damage in general can be reduced by reducing the illumination time of the sample by the optical pulse. If the duration of the optical pulse (i.e., illumination time) is shorter than a relaxation time of heat to the surrounding environment, then more precise results can be obtained. This can significantly improve the accuracy of the material processing or the excitation of individual cells, for example.

There is therefore clearly a need for shorter optical pulses. It is known that ultrashort optical pulses comprise very broad optical spectra due to the product of time and bandwidth. This means that the ultrashort optical pulses are constructed from a plurality of optical frequencies (wavelengths) which are coupled by a mode locking mechanism. In general, the generation of ultrashort optical light pulses is based on an interaction of optical pulses with linear effects (affecting only the temporal profile of a pulse) and nonlinear effects (affecting only the spectral profile of a pulse) in materials. The linear effects are due to material dispersion effects. For example, the group velocity dispersion (GVD; (β<NUM> in ps<NUM>/(nm*m)) affects the optical frequencies of the optical pulse travelling through the material and thus influences the temporal profile of the optical pulse. The GVD is mathematically the derivation of the inverse group velocity with respect to the angular frequency. Normal dispersion or positive group velocity dispersion (ß<NUM>><NUM>) in the material causes the optical pulses to diverge in time. In this case, red spectral components of the optical pulse move faster than blue spectral components of the optical pulse and lead to a temporal stretched optical pulse. If anomalous or negative group velocity dispersion (ß<NUM><<NUM>) is present, then the red spectral components move slower than the blue spectral components leading also to a temporal stretched optical pulse.

On a temporal axis, the term "chirp" describes which ones of the spectral components propagates at the front of the optical pulse. A positive chirp in this case means that the red spectral components propagate in front (faster) of the blue spectral components. Similarly, a negative chirp describes that the blue spectral components propagate in front of the red spectral components. If the chirp is zero, then all of the spectral components arrive at the same time. The shortest possible time duration is present if all spectral components of the optical pulse propagate with the same velocity. This shortest possible pulse duration represents the so called "Fourier Limit" of the optical pulses.

A positive chirp results in an increase in normal (positive) dispersion and leads to a longer stretched optical pulse (time domain). On the other hand, a positive chirp will be reduced due to an anomalous (negative) dispersion which leads to a shortened optical pulse.

This effect means that the amount of the negative chirp of the optical pulse will be reduced in a normal dispersion segment and this reduction leads to a time shortened pulse. On the other hand, the amount of the negative chirp will increase when the optical pulse passes through a negative dispersion segment. The sign of the chirp can only change if the optical pulse reaches the Fourier Limit within the dispersion segment and continues propagating further through the dispersion segment.

The larger the spectral bandwidth of the optical pulse, the more the effects of higher order material dispersion must be taken into account. These higher order effects are mathematically a derivation of the GVD. For example, third order dispersion (TOD) will lead to an asymmetry of the temporal shape of the optical pulses.

On the other hand, nonlinear effects and therefore intensity dependance effects only affecting the spectral domain of the optical pulses. For example, self-phase modulation (SPM) is an effect due to a varying refractive index in the material due to the optical Kerr effect. The SPM leads to the generation of new spectral components if the optical pulse is positively chirped, and positive group velocity dispersion is present. Conversely, the SPM can destroy spectral components if the optical pulse is negatively chirped and positive group velocity dispersion is present.

Higher order nonlinear effects can also occur in the material depending on the peak power of the optical pulses.

The most common approach to generate intense ultrashort optical pulses is based on the "Chirped pulse amplification (CPA)" method (described by <NPL>). The basic idea described in the paper is to stretch the optical pulse in time generated from an oscillator by a positive dispersive segment and subsequently amplify the optical light pulse in an optical amplifier. Typical stretching factors are >> <NUM>. Finally, the amplified optical light pulses will be recompressed in time in a negative group velocity dispersive segment. This is a so-called linear amplification, as the optical pulses are stretched in time avoiding nonlinear effects during propagation. This also means that no new spectral components can be generated by SPM. Therefore, the optical bandwidth of the optical pulses remains at the best the same after amplification. Typically, the spectral bandwidth of the optical pulse decreases due to gain narrowing effects during amplification. Thus, it is not possible to generate shorter pulse durations than would be possible with the oscillator itself.

To generate new spectral components, a nonlinear process like the afore-mentioned self-phase modulation is necessary. Therefore, a new approach must be chosen compared to the prior art CPA method.

There are different approaches to generate very short pulse durations in the range of a few femtoseconds to a few ten femtoseconds. For example, after optical amplification it is possible to use nonlinear effects for propagation through a segment with positive group velocity dispersion. This is described, for example, as the multi pass concept in <NPL>) or are gas filled hollow core fibers (for example <NPL>.

For this purpose, however, a very high pulse peak power must already be available in order to generate a significant spectral broadening. Furthermore, the additional phase of the broadened optical pulses must be compressed again in a further segment of negative group velocity dispersion.

Another approach is described for example in <CIT>, assigned to Amplitude Systèmes). A segment with negative group velocity dispersion is used after an oscillator prior to amplification of the optical pulses. Typically grating compressors, prism compressors or other segments with negative group velocity dispersion are suitable for this compression. In this '<NUM> patent, the positive chirp of the optical pulses from the oscillator will be reduced within the segment of negative group velocity dispersion. Furthermore, a negative chirp can be achieved by this method set out in US'<NUM>. This leads to a change in the sign of the chirp during the propagation within the segment of negative group velocity dispersion. By amplifying and due to nonlinear effects within the positive group velocity dispersion amplifier, the negative chirped optical pulses from the segment of negative group velocity dispersion will change the chirp sign again to a positive value. This two times change of the chirp sign makes it necessary to time-compress the amplified optical light pulses after amplification with a further negative group velocity dispersion segment.

In another <CIT>), negatively chirped optical pulses from a so-called Stretched Pulse Oscillator are disclosed. Due to the evolution of the optical light pulse inside a "stretched pulse laser" the chirp changes its sign twice during one roundtrip.

<NPL> discloses a laser system comprising an Yb-doped fiber oscillator producing a plurality of positively chirped optical pulses having a first spectral width. A large mode area (LMA) photonic crystal fiber (PCF) amplifier receives the plurality of optical pulses and amplifies the optical pulse of light to produce an optical light pulse having a second spectral width, wherein the second spectral width being greater than the first spectral width. A connecting segment comprising a diffraction grating-pair pre-chirper is connected directly between the oscillator and the amplifier. To generate the broadband optical pulses the sign of the chirp has to change twice within the propagation.

<NPL>, also discloses a laser system with an oscillator producing a plurality of positively chirped optical pulses. An amplifier receives the plurality of optical pulses and amplifies the optical pulse of light to produce an optical light pulse having a second spectral width, the second spectral width being greater than the first spectral width. A prechirper is connected directly between the oscillator and the amplifier, wherein the prechirper has a negative group velocity dispersion to reduce the positive chirp of the oscillator.

The Song Huanyu publication teaches a picosecond fiber laser used with a fiber Bragg grating (FBG). The FBG is a narrow band filter in a positive dispersion laser (β2 is positive for optical fibers below <NUM> wavelength). There is no negative dispersion segment includes in the taught laser system and so the optical pulses produced by the fiber oscillator are positively chirped. The grating compressor used as the prechirper in front of the main amplifier has a negative dispersion to reduce the positive chirp of the oscillator.

Finally, <NPL>. This Wang Sija et al publication explains the physical mechanism behind the laser amplification process and how this mechanism depends on the initial system parameters. In this case positive chirped optical pulses of an oscillator were also used and a prechirping is done in a negative dispersion pre chirper segment in front of the amplifier leading to a change twice of the sign of the chirp within the whole setup.

This document discloses a laser system in which a stretched pulse oscillator producing a negatively chirped pulse can be connected directly to the amplifier via an optical fiber with positive group velocity dispersion leading to an alignment free setup. The laser system of this document eliminates the need for a segment with negative group velocity dispersion after the oscillator and before amplification. A negative dispersion element (β2<<NUM>) is typically used as the optical pulses are typically chirped positively when coming out of the oscillator and the optical pulses need to be compressed in a compressor to achieve so called self-similar amplification. These compressors are typically a free space and/or bulky grating, prism or GRISM compressor.

This document teaches a system which leads to a cost effective, robust, and very simple setup. In this case the sign of the chirp of the oscillator will be reduced in a normal dispersion segment and changes a single time, after reaching the Fourier Limit in the amplifier, compared to the prior art systems in which the chirp of the optical pulses changes twice.

The scope of the invention is defined by independent claim <NUM>. This document describes a laser system comprises an oscillator producing a negatively chirped optical pulse having a first spectral width W1, an amplifier having a positive group velocity dispersion for receiving the plurality of negatively chirped optical pulse and amplifying the plurality of negatively chirped optical pulses of light to produce at an output of the amplifier a plurality of positively chirped optical light pulses having a second spectral width W2, the second spectral width being greater than the first spectral width, and a positive group velocity dispersion connecting segment connected directly between the output of the oscillator and the input of the amplifier, wherein the connecting segment has a positive dispersion and maintains the sign of the chirp of the plurality of negatively chirped optical pulses and wherein the sign of the chirp is changed no more than once from a negative sign to a positive sign between the oscillator and the output of the amplifier.

In one aspect, the second spectral width W2 at the output of the amplifier is greater than the first spectral width W1 of the oscillator.

In another aspect, the connecting segment is adapted to change the amount of the chirp.

In one aspect, the laser system further comprises a second segment of positive group velocity dispersion after the optical amplifier.

In another aspect, the laser system further comprises a negative group velocity dispersion segment connected to an output of the amplifier and which is adapted to compensate the phase contributions of linear and nonlinear effects that have occurred during propagation through the connecting segment and the optical amplifier including the phase of the optical pulse.

The laser system can be implemented as a solid-state system (i.e., integrated on a chip) or as a fiber-based system. In the latter case, the amplifier is a fiber amplifier. It is also possible to use a combination of a solid-state system and a fiber-based system.

In one aspect, the amplifier is one of a fiber amplifier, a rod-type amplifier, a slab amplifier, thin disk amplifier or a solid-state amplifier.

The spectral width of the optical pulse decreases from an input of the connecting segment within the amplifier to a minimum in the amplifier and then increases to a larger amount at an output of the amplifier, meaning that the chirp has changed. The connecting segment in the laser system can be a waveguide and this can be implemented on a chip in a solid-state system, as a length of fiber in a fiber-based system or as a positive group velocity dispersion material.

In another aspect, the optical amplifier has a positive group velocity dispersion and the optical amplifier comprises a "chirp-free" point at which, in use, the chirp of the optical pulses is substantially reduced to zero by nonlinear effects resulting from self-phase modulation inside the optical amplifier, wherein at the output of the optical amplifier, the optical pulses are positively chirped.

In one aspect, the connecting segment is a combination of positive group velocity dispersion and negative group velocity dispersion segments without changing the sign of the chirp.

At an output of the laser system a spectral width may be generated by shifting the "chirp-free" point to the end of the amplifier.

The laser system may further comprise at least one optical isolator or a component which suppresses an optical signal propagating towards the oscillator, could be one of a free space or fiber coupled. This one optical isolator or a component can be located in the laser system after the oscillator or within the connecting segment. At least a pre-amplifier or an attenuator can also be incorporated within the connecting segment. The optical isolator may be one of a free space or fiber coupled.

The laser system creates in effect a virtual nonlinear optical bandpass filter through the interaction of the chirp of the pulses, the dispersive effects and nonlinear effects occurred within the amplifier.

The laser system may include at least one of a pre-amplifier or an attenuator within the connecting segment. The preamplifier may be one of a fiber-based amplifier, a semiconductor optical amplifier or a solid-state amplifier.

The laser system includes optionally at least one optical pulse picker to reduce the repetition rate of the optical pulses or adding a pulse on demand functionality to the laser system.

In one aspect, the oscillator may comprise at least a segment of positive group velocity dispersion and a segment of negative group velocity dispersion.

In another aspect, the negative chirp of the oscillator can be increased by using an additional negative dispersion element within the connecting segment. The additional negative dispersion element will only change the amount of chirp but not the sign of the chirp within the connecting segment. The additional negative dispersion element will shift only the position of the Fourier limited pulse within the amplifier.

By adding the negative dispersion element to the connection segment, the position of the chirp free point will shift to the end of the amplifier if the amount of normal dispersion is kept constant. During the amplification process, the SPM will reduce the spectral bandwidth in the presence of a negative chirp leading to a narrow optical bandwidth W3.

If optional elements like a fiber-based pulse picker will increase the overall positive dispersion of the connection segment the additional negative dispersion element can be used to fix the position of the chirp free point in the amplifier.

In general, the amount of the chirp will define the spectral bandwidth of the amplifier taking into account the overall dispersion of the system.

In another aspect the spectral width of the optical pulse decreases due to the interplay of linear and nonlinear effects (SPM) from an input of the connecting segment within the amplifier to a minimum after the amplifier depending on the power level.

The oscillator may comprise a negative group velocity dispersion segment and a positive group velocity dispersion segment arranged in a way that the overall net dispersion of the cavity is less than <NUM>. 1ps<NUM> (β<NUM><<NUM> ps<NUM>).

The oscillator and the amplifier may comprise a laser active medium. The laser active medium is selected from, for example, but not limited to the group of rare earth dopants comprising Yb, Nd, Tm, or Er. It should be noted that the materials need not be identical in both the oscillator and the amplifier.

In one aspect of the laser system, the oscillator comprises a linear cavity with an absorber at one end and a grating compressor at the other end.

The amplifier can be pumped by at least a single mode diode laser or a multi-mode diode laser.

In one aspect, the amplifier is pumped by at least one of a single mode diode laser or multi-mode laser.

The fiber sections of the oscillator and the amplifier may comprise, but not limited to, single clad fibers, double clad fibers or photonic crystal fibers, also including rod-type fibers.

The invention will now be described on the basis of the drawings. It will be understood that the embodiments and aspects of the invention described herein are only examples and do not limit the protective scope of the claims in any way. It will be understood that features of one aspect or embodiment of the invention can be combined with a feature of a different aspect or aspects and/or embodiments of the invention as long as the resulting combination of features does fall under the scope claimed.

<FIG> shows an aspect of a laser system <NUM> for generating ultrabroadband optical pulses of light is based on a stretched pulse oscillator <NUM> emitting a plurality of negatively chirped optical pulses <NUM>. An input <NUM> of a positive group velocity dispersion connecting segment <NUM> is connected to the output <NUM> of the stretched pulse oscillator <NUM>. An optical amplifier <NUM> having positive group velocity dispersion is connected to an output <NUM> of the connecting segment <NUM> and amplifies the optical pulses <NUM>' and is followed by a negative group velocity dispersion segment <NUM> (also called compressor). The negative group velocity dispersion segment <NUM> is used to compensate the phase contributions of linear and nonlinear effects that have occurred during propagation through the connecting segment <NUM> and the optical amplifier <NUM> including the phase of output pulses <NUM> of the oscillator <NUM>. The sign of the chirp between the oscillator <NUM> and the output of the optical amplifier <NUM> only changes once through the optical path. In some aspects of the invention, the laser system <NUM> does not include the negative dispersion segment <NUM>. In another aspect, the negative group velocity dispersion segment <NUM> could be replaced or complemented by an optical filter to reduce the width of the output spectrum and thus decrease the amount of amplitude noise in the output spectrum by removing light from the edges of the spectrum.

In a first aspect of the invention, the stretched pulse oscillator <NUM> emits the negatively chirped optical pulses as the plurality of optical pulses <NUM>. In this first aspect, the connecting segment <NUM> does not change the sign of the chirp of the oscillator <NUM> unlike the connecting segments <NUM> known in the art. The input <NUM> of the connecting segment <NUM> is connected to the output <NUM> of the stretched pulse oscillator <NUM> by a splice of optical fibers or a free space coupling. The output of the connecting segment <NUM> is connected to the amplifier <NUM>.

At the input <NUM> of the connecting segment <NUM>, the optical pulses <NUM> have a larger negative chirp compared to the optical pulses <NUM>I at the output <NUM> of the connecting segment <NUM>. In other words, the negative chirp of the optical pulses <NUM>I and therefore the pulse duration of the optical pulses <NUM>I at the output <NUM> is reduced during propagating in a positive group velocity dispersion fiber which forms the connecting segment <NUM>. Depending on the optical power of the optical pulses <NUM>, nonlinear effects can occur in the connecting segment <NUM>, which lead to a reduced spectral bandwidth of the optical pulses <NUM>I at the output <NUM> of the connecting segment <NUM>.

It will be appreciated that it may be necessary to implement a short connecting segment <NUM>' with positive group velocity dispersion after the amplifier <NUM>, shown in <FIG>. For example, there is a need to remove the pump light (from the optical pump) from the amplifier <NUM> and this is done by using a cladding light stripper to remove residual pump light from the amplifier <NUM>. In this case the positive chirp of the optical pulse <NUM>II slightly increases at the output of the short connecting segment <NUM>'.

At least one optical isolator <NUM> or <NUM>' can be implemented in the connecting segment(s) <NUM> or <NUM>' after the stretched pulse oscillator <NUM> and is shown schematically in <FIG>. The position of the optical isolator <NUM> or <NUM>' could be at the input <NUM> or <NUM>' of the connecting segment <NUM> (<NUM>'), within the connecting segment <NUM> (<NUM>'), or at the output <NUM> (<NUM>') of the connecting segment <NUM> (<NUM>'). In this case the optical pulse <NUM>III will have a slightly changed chirp due to the presence of the optical isolator <NUM>I in the optical path.

After the (first) connecting segment <NUM>, the negative chirped optical pulses <NUM>I propagate into the optical amplifier <NUM> which is used to increase the power level of the optical pulses <NUM>I. Due to the nonlinear effects of the afore-mentioned SPM inside the optical amplifier <NUM> the chirp of the optical pulses <NUM>I is substantially reduced to zero at a "chirp-free" point <NUM> within the optical amplifier <NUM>, as shown in <FIG>, as the optical amplifier <NUM> has a positive group velocity dispersion. At the output <NUM> of the optical amplifier <NUM>, the optical pulses <NUM>II are positively chirped and the chirp can be compensated by a negative group velocity dispersion segment <NUM> connected to the output <NUM> of the optical amplifier <NUM> to a near Fourier limited pulse duration <NUM>IV. In <FIG> an example of a measured FROG trace of the compressed optical light pulse is displayed. The corresponding optical spectrum is show in <FIG>. The negative group velocity dispersion segment <NUM> can be, but are not limited to, a grating compressor, a prism compressor, a GRISM compressor, chirped mirrors, or a hollow core fiber segment. For an ideal compression, a pulse shaper can also be integrated.

The dispersion of the negative group velocity dispersion segment <NUM> is estimated to be smaller than three times the sum of the group velocity dispersion of the connecting segment <NUM> and the amplifier <NUM>, i.e. (<NUM>*(b<NUM>+b<NUM>|)>|b<NUM>|) in which b<NUM> represents the group velocity dispersion of the connecting segment, b<NUM> represents the group velocity dispersion of the optical amplifier <NUM> and b<NUM> represents the group velocity dispersion of the negative group velocity dispersion segment <NUM>. The value of the dispersion is not, however limiting of the invention.

In a further aspect of the laser pulse system <NUM>, the power level inside the connecting segment <NUM> or <NUM>' can be adjusted by at least an attenuator <NUM> (or <NUM>') (shown in <FIG>) or a preamplifier <NUM> (shown in <FIG>), or a combination of both (as in <FIG>) for controlling the non-linear effects in the connecting segment <NUM> and the amplifier <NUM>. The position of the attenuators <NUM> or <NUM>' or the preamplifier <NUM> could be at the input <NUM>/<NUM>' of the connecting segments <NUM> and <NUM>', within the connecting segments <NUM> and <NUM>', or at the output <NUM> or <NUM>' of the connecting segments <NUM> or <NUM>'. A higher power output can be achieved by increasing the diameter of the mode field in the optical fiber.

The preamplifier could be one of a fiber-based preamplifier, a (fiber-coupled) semiconductor optical amplifier or a solid-state amplifier.

An optical isolator <NUM> can be used after the preamplifier <NUM> (as shown in <FIG>).

By combining a negative chirp of the optical pulses <NUM> and a positive group velocity dispersion in the connecting segment <NUM>, the spectral bandwidth of the optical pulses <NUM>I at the output <NUM> will be reduced, thus creating a "nonlinear band pass filter" at a "chirp free point" <NUM> inside the optical amplifier <NUM>, as shown in <FIG>. It would be possible to couple the optical pulse <NUM> out near to this point to provide an optical pulse <NUM> with a very narrow spectrum W3 by reducing the length of the amplifier <NUM>.

A sixth aspect of the laser system <NUM> is shown in <FIG> and enables the integration of an optical pulse picker <NUM> to reduce the repetition rate of the stretched pulse oscillator <NUM> to increase the pulse energy after the optical amplifier <NUM>. A second pulse picker <NUM>' can be added in front of the compressor <NUM> within the connecting segment <NUM>' to generate a pulse on demand functionality.

A seventh aspect of the laser system <NUM> is shown in <FIG>. An additional part of negative dispersion 40a is added to the positive dispersion 40b of the connecting segment <NUM> while changing only the amount of the chirp without changing the sign. This allows for an adaption of the position of the chirp free point <NUM> inside the amplifier <NUM>. The optical pulse <NUM>V at the output <NUM> of the connecting segment <NUM> will have a larger negative chirp compared to the optical pulse <NUM> coming from the oscillator <NUM> and arriving at the input <NUM> of the connecting segment. The position of the chirp free point <NUM> will shift towards the end of the amplifier <NUM>. This mechanism can be used to adapt the chirp free point <NUM>. The chirp of the optical pulse <NUM>VI will be positive after the amplifier <NUM> and slightly increased by propagating through the optional parts <NUM>', <NUM>' and <NUM>' and their dispersion leading to an optical pulse <NUM>VII.

If the chirp free point <NUM> is shifted to the end of the amplifier <NUM>, an optical pulse <NUM> VIII with a narrow spectrum W3 can be generated.

The stretched pulse oscillator <NUM>, the connecting segment <NUM> and the optical amplifier <NUM> are connected with fiber splices. Nevertheless, transitions between any of the stretched pulse oscillator <NUM>, the connecting segment(s) <NUM> and <NUM>' and the optical amplifier <NUM> can also be implemented by free space coupling. Therefore, free space isolators <NUM> and <NUM>' pulse pickers <NUM> and <NUM>', attenuators <NUM> and <NUM>' or preamplifiers <NUM> can also be used.

The ultrabroadband generation of the optical pulses is based on the interaction of linear effects and nonlinear effects within the amplifier and so the maximum energy, or the spectral bandwidth, can be controlled by choosing different mode field diameters during the propagation.

The optical amplifier <NUM> can be made in a non-limiting example of a fiber amplifier doped with Ytterbium. It is thought that the optical amplifier <NUM> can be adapted to all lasing materials, such as but not limited to, Nd, Tm, Er, Er-Yb.

The principle is not limited to fiber laser technology and in different aspects, the principle can also be adapted to solid state amplifiers, including for example thin disk amplifiers, slab amplifiers, crystal-based amplifiers, rod amplifiers or other types. For a more general approach the negative chirped optical pulses <NUM> from the stretched pulse oscillator <NUM> or a soliton oscillator have to propagate through a medium of positive group velocity dispersion segment, which is not limited to optical fibers, but can also be waveguides (included those implemented as micro-optics on a wafer) or materials with positive group velocity dispersion.

The use of a positive group velocity dispersion for the connecting segment <NUM> requires the production of the negatively chirped optical pulses <NUM> in the stretched pulse oscillator <NUM>. This is illustrated in <FIG> and can be achieved by using two different dispersion segments inside an oscillator cavity. The oscillator cavity comprises a positive group velocity dispersion segment <NUM> and a negative group velocity dispersion segment <NUM>. The overall net group velocity dispersion (GVD) has to be less than <NUM> ps<NUM> (β2net <<NUM> ps<NUM>). In this case the optical pulses <NUM> undergo a change of the chirp sign within the two dispersion segments, i.e., a negative dispersion segment <NUM> and a positive dispersion segment <NUM>. <FIG> shows the evolution of the chirp (normalized y-axis in arbitrary units (arb. unit)) of an optical light pulse inside a stretched pulse cavity during propagation (x-axis in meter). Furthermore, the corresponding group velocity dispersion segments (normalized y-axis in arbitrary unit (arb. unit)) are displayed. During the propagation, the pulse has to be free of chirp twice per round trip, as can be seen in <FIG>. According to <FIG> the chirp free points are located at the ends of the linear cavity. Depending in the overall net dispersion, the position of the chirp free pulses can be changed.

Starting from a chirp free point <NUM> within the positive group velocity dispersion segment <NUM> a positive chirp is generated by propagating through the positive dispersion segment <NUM> forming one part of the stretched pulse oscillator <NUM>. This positive chirp will be reduced within the negative group velocity dispersion segment <NUM> forming a second part of the oscillator cavity <NUM>, leading to a chirp free optical pulse at a position <NUM>' within the segment <NUM>, and changing the sign of the chirp afterwards. This negative chirp increases up to the end of the negative group velocity dispersion segment <NUM>. Finally, the negative chirp will be reduced by entering the positive group velocity dispersion segment <NUM> and after one roundtrip will reach the chirp free starting point <NUM> again. For the apparatus of this document the optical output pulses of the oscillator <NUM> have to have a negative chirp at the output coupler <NUM> of the oscillator <NUM>.

One example of the stretched pulse oscillator <NUM> is shown in <FIG>. The stretched pulse oscillator <NUM> comprises an optical pump <NUM> for generating the pump light for the laser active fiber segment <NUM>'. The optical pump <NUM> is coupled to the cavity by using a pump coupler <NUM>. The negative dispersion segment <NUM> is implemented by a grating compressor at one end of a linear cavity.

The optical fiber part forms the positive dispersion segment <NUM>. The stretched pulse oscillator <NUM> is mode-locked by using a saturable absorber mirror <NUM> at the other end of the linear optical cavity. An output coupler <NUM> is placed behind the negative dispersion segment <NUM> (in propagation direction).

Other different aspects of the laser optical cavity of the stretched pulse oscillator <NUM> are shown in <FIG> and <FIG>. The optical cavity can also be implemented as a ring cavity when using an optical isolator <NUM> and a saturable absorber <NUM>' or as a sigma cavity when using a circulator <NUM> to implement a saturable absorber mirror <NUM> in the optical cavity.

To further reduce free space parts inside the linear cavity oscillator the negative dispersion segment <NUM> can be implemented using a chirped fiber Bragg grating <NUM>' (<FIG>) or a negative dispersion photonic crystal fiber combined with a fiber-based mirror (<NUM>") can be used (<FIG>) instead of a grating compressor.

In one aspect all of the optical fibers used are polarization maintaining fibers to achieve an environmental stable system. In general, however, the laser pulse system of this disclosure is not limited to polarization maintaining fibers. Non-polarization maintaining fibers could also be used. Furthermore, the laser pulse system <NUM> is not limited to single clad fibers. In addition, other types of fibers as double clad fibers can be used. Depending on the type of fibers single or multimode laser diodes can be used for pumping. Mode-locking can also be achieved by using any kind of saturable absorber (<NUM>') or virtual saturable absorber as for example nonlinear pulse evolution.

Claim 1:
A laser system (<NUM>) comprising:
an oscillator (<NUM>) configured to produce a plurality of negatively chirped optical pulses (<NUM>) having a first spectral width W1;
a positive group velocity dispersion connecting segment (<NUM>) connected directly between an output of the oscillator (<NUM>) and an input of an amplifier (<NUM>), wherein the connecting segment (<NUM>) is adapted to maintain the sign of the chirp of the plurality of negatively chirped optical pulses (<NUM>) and to reduce a duration of the plurality of negatively chirped optical pulses (<NUM>);
the amplifier (<NUM>) having a positive group velocity dispersion and is configured to receive at the input of the amplifier the plurality of negatively chirped optical pulses (<NUM>) and to amplify the plurality of negatively chirped optical pulses to produce, at an output of the amplifier (<NUM>), a plurality of positively chirped optical light pulses having a second spectral width W2;
wherein the sign of the chirp of the optical pulses is changed no more than once from a negative sign to a positive sign between the output of the oscillator (<NUM>) and the output of the amplifier (<NUM>).