Method and system for generating optical pulses of light

Abstract

A laser system for generating optical pulses of ultra-short optical pulses comprises an oscillator emitting low-power and negatively chirped optical pulses having a spectral bandwidth W1, a dispersive connection section for maintaining the sign of the chirp of the pulses of the oscillator, an optical amplifier for amplifying the optical pulses, and a negative group-velocity dispersion section for compensating the phase contribution over the entire propagation. During the propagation from the output of the oscillator to the end of the optical amplifier, the chirp of the optical pulses will change once from negative to positive. After a last compression stage, an ultra-short optical pulse can be generated.

Description

[0001] Cross-references to related applications

[0002] This application claims the benefit and priority of Luxembourg Patent Application No. LU101629, filed on January 30, 2020. The entire disclosure of Luxembourg Patent Application No. LU101629 is incorporated herein by reference. Technical Field

[0003] This invention relates to methods and systems for generating optical pulses of light with spectral bandwidths significantly greater than the normal gain bandwidth of typical rare-earth-doped laser media or laser-active crystals. By compressing these generated ultra-wideband optical pulses, shorter optical pulses than those in conventional laser systems can be produced. Background Technology

[0004] Today, ultrashort optical pulses of laser light with pulse durations of less than one picosecond are widely used in many applications. Examples include the fabrication of micro and nanomaterials, medical applications (such as ophthalmology or nanosurgery), and scientific or biomedical applications (such as multiphoton microscopy). One of the well-known limitations of using these optical pulses is the so-called heat-affected zone (HAZ). The HAZ is caused by thermal heating of the environment surrounding the sample or, in biomedical applications, the heating of living cells. This can lead to thermal damage to the sample or living cells.

[0005] Generally, reducing the illumination time of the sample by the optical pulse can reduce the heat-affected zone or thermal damage. If the duration of the optical pulse (i.e., the illumination time) is shorter than the relaxation time of heat to the surrounding environment, more accurate results can be obtained. For example, this can significantly improve the accuracy of material processing or the excitation of individual cells.

[0006] Therefore, shorter optical pulses are clearly required. It is known that ultrashort optical pulses encompass very broad spectra due to the product of time and bandwidth. This means that ultrashort optical pulses consist of multiple optical frequencies (wavelengths) coupled through a mode-locking mechanism. Generally, the generation of ultrashort optical pulses is based on the interaction between the optical pulse and linear effects (affecting only the temporal envelope of the pulse) and nonlinear effects (affecting only the spectral envelope of the pulse) in the material. The linear effect is due to material dispersion effects. For example, group velocity dispersion (GVD; (β2, in ps) 2The group velocity (β²) (in nm*m) affects the optical frequency of an optical pulse passing through a material, thus influencing the time envelope of the optical pulse. GVD is mathematically the differential of the inverse of the group velocity with respect to the angular frequency. Normal dispersion or positive group velocity dispersion (β²>0) in the material causes the optical pulse to diverge in time. In this case, the red spectral component of the optical pulse shifts faster than the blue spectral component, resulting in a time-stretched optical pulse. If anomalous or negative group velocity dispersion (β²<0) exists, the red spectral component shifts slower than the blue spectral component, also leading to a time-stretched optical pulse.

[0007] On the time axis, the term "chirp" describes which spectral components propagate ahead of the optical pulse. In this case, positive chirp means that the red spectral component propagates ahead of the blue spectral component (faster). Similarly, negative chirp describes that the blue spectral component propagates ahead of the red spectral component. If the chirp is zero, then all spectral components arrive simultaneously. If all spectral components of an optical pulse propagate at the same speed, then there exists a shortest possible duration. This shortest possible pulse duration is called the "Fourier limit" of the optical pulse.

[0008] Positive chirp increases normal (positive) dispersion and results in longer stretched optical pulses (time domain). On the other hand, anomalous (negative) dispersion will reduce positive chirp, resulting in shorter optical pulses.

[0009] This effect means that the amount of negative chirp in an optical pulse will decrease in the normal dispersion region, and this decrease results in a pulse with a shorter duration. On the other hand, the amount of negative chirp will increase as the optical pulse passes through the negative dispersion region. The sign of the chirp only changes when the optical pulse reaches the Fourier limit within the dispersion region and continues to propagate further through the dispersion region.

[0010] The larger the spectral bandwidth of an optical pulse, the more effects of higher-order material dispersion must be considered. These higher-order effects are mathematically derivatives of GVD. For example, third-order dispersion (TOD) will cause temporal shape asymmetry in the optical pulse.

[0011] On the other hand, nonlinear effects, and therefore intensity-dependent effects, affect only the spectral domain of the optical pulse. For example, self-phase modulation (SPM) is an effect resulting from the change in refractive index in the material due to the optical Kerr effect. If the optical pulse is positively chirped and has positive group velocity dispersion, then SPM results in the generation of new spectral components. Conversely, if the optical pulse is negatively chirped and has positive group velocity dispersion, then SPM destroys the spectral components.

[0012] Depending on the peak power of the optical pulse, higher-order nonlinear effects may also appear in the material.

[0013] The most common method for generating strong ultrashort optical pulses is based on the "chirped pulse amplification (CPA)" method (described by D. Strickland and G. Mourou in "Compression of amplified chirped optical pulses," Opt. Commun. 56, 219, 1985). The basic idea described in the paper is to stretch the optical pulse generated by the oscillator in time through the positive dispersion segment, and then amplify the optical pulse in an optical amplifier. A typical stretching factor >> 100. Finally, the amplified optical pulse is recompressed in time in the negative group velocity dispersion segment. This is called linear amplification because the optical pulse is stretched in time, avoiding nonlinear effects during propagation. This also means that SPM cannot generate new spectral components. Therefore, the optical bandwidth of the optical pulse should ideally remain unchanged after amplification. Typically, the spectral bandwidth of the optical pulse decreases due to the gain narrowing effect during amplification. Therefore, it is impossible to generate a pulse duration shorter than that of the oscillator itself.

[0014] To generate new spectral components, a nonlinear process with self-phase modulation, as mentioned earlier, is necessary. Therefore, a new method must be chosen compared to the existing CPA method.

[0015] There are various methods for generating very short pulse durations in the range of a few femtoseconds to tens of femtoseconds. For example, after optical amplification, nonlinear effects can be used to propagate through a segment with positive group velocity dispersion. This is described, for example, in Optics Lett., 43, pp. 5877-5880 (2018) as a multi-pass concept, or in gas-filled hollow fibers (e.g., Opt. Lett. 40, 1238-1241 (2015)). However, for this purpose, very high pulse peak power must already be available in order to generate significant spectral broadening. Furthermore, the additional phase of the broadened optical pulse must be compressed again in another segment with negative group velocity dispersion.

[0016] Another method is described, for example, in U.S. Patent No. 9,362,702B2 (Delague et al., assigned to AmplitudeSystèmes). A segment with negative group velocity dispersion is used after the oscillator, before amplifying the optical pulse. Typically, a grating compressor, prism compressor, or other segment with negative group velocity dispersion is suitable for this compression. In this '702 patent, the positive chirp of the optical pulse from the oscillator is reduced within the negative group velocity dispersion segment. Furthermore, the method described in US'702 can achieve negative chirp. This results in a change in the sign of the chirp during propagation within the negative group velocity dispersion segment. By amplification and due to nonlinear effects within the positive group velocity dispersion amplifier, the negatively chirped optical pulse from the negative group velocity dispersion segment returns the chirp sign to a positive value. This double change in the chirp sign makes it necessary to perform time compression on the amplified optical pulse after amplification with a further negative group velocity dispersion segment.

[0017] In another U.S. patent No. 5,513,194A (Tamura et al., assigned to MIT), a negatively chirped optical pulse from a so-called stretched pulse oscillator is disclosed. Due to the evolution of the optical pulse within the "stretched pulse laser," the chirp changes sign twice in one round trip.

[0018] Wang Sija et al., “Intensity noise reduction of a high-power nonlinear femtosecond fiber amplifier based on spectral-breathing self-similar parabolic pulse evolution,” Proceedings of SPIE, Vol. 9893, April 17, 2016, pp. 98930J-98930J, DOI: 10.1117 / 12.2227743, discloses a laser system comprising a ytterbium-doped fiber oscillator that generates multiple positively chirped optical pulses with a first spectral width. A large-mode-area (LMA) photonic crystal fiber (PCF) amplifier receives the multiple optical pulses and amplifies the optical pulses to generate optical pulses with a second spectral width, wherein the second spectral width is greater than the first spectral width. A connection segment including a diffraction grating to a pre-chirped element is directly connected between the oscillator and the amplifier. To generate broadband optical pulses, the sign of the chirp must be changed twice during propagation.

[0019] Song Huanyu et al., “Femtosecond Laser Pulse Generation from Picosecond Laser Source with Self-Similar Amplification,” 2018 Conference on Lasers and Electro-Optics (CLEO), OSA, May 13, 2018, pp. 1-2, also disclose a laser system with an oscillator that generates multiple positively chirped optical pulses. An amplifier receives and amplifies the multiple optical pulses to generate an optical pulse with a second spectral width greater than a first spectral width. A pre-chirper is directly connected between the oscillator and the amplifier, wherein the pre-chirper has negative group velocity dispersion to reduce the positive chirp of the oscillator.

[0020] Song Huanyu's publication teaches a picosecond fiber laser used with a fiber Bragg grating (FBG). An FBG is a narrowband filter in a positively dispersive laser (β2 is positive for fibers with wavelengths below 1.3 μm). The laser system taught does not include a negatively dispersive section, therefore the optical pulses generated by the fiber oscillator are positively chirped. A grating compressor, used as a pre-chirper in front of the main amplifier, has negative dispersion to reduce the positive chirp of the oscillator.

[0021] Finally, Wang Sija et al., “On the efficiency of self-similar pulse evolution in fiber amplifiers with gain shaping,” 11th Conference on Lasers and Electro-Optics Pacific Rim, (CLEO-PR), IEEE, Vol. 4, August 24, 2015, pp. 1-2, XP032841059, DOI: 10.1109 / CLEOPR.2015.7376296. This publication by Wang Sija et al. explains the physical mechanism behind the laser amplification process and how this mechanism depends on the initial system parameters. In this case, a positively chirped optical pulse from an oscillator is also used, and pre-chirping is performed in a negatively dispersive pre-chirped section before the amplifier, resulting in the chirp sign changing twice throughout the setup. Summary of the Invention

[0022] This document discloses a laser system in which a stretched pulse oscillator that generates negatively chirped pulses can be directly connected to an amplifier via an optical fiber with positive group velocity dispersion, resulting in an alignment-free setup. The laser system of this document eliminates the need for a section with negative group velocity dispersion after the oscillator and before amplification. Negative dispersion elements (β2 < 0) are typically used because optical pulses usually have positive chirp when exiting the oscillator, and the optical pulses need to be compressed in a compressor to achieve so-called self-similar amplification. These compressors are typically free-space and / or bulky grating, prism, or GRISM compressors.

[0023] This document teaches a system that results in a cost-effective, robust, and very simple setup. In this system, compared to prior art systems where the chirp of the optical pulse changes twice, the chirp sign of the oscillator decreases and changes only once in the normal dispersion region after reaching the Fourier limit in the amplifier.

[0024] This document describes a laser system including an oscillator that generates a negatively chirped optical pulse having a first spectral width W1, an amplifier that receives the optical pulse and amplifies the light to generate an optical pulse having a second spectral width W2, the second spectral width being greater than the first spectral width, and a connection section directly connected between the oscillator and the amplifier, wherein the connection section has a positive dispersion and maintains the chirp of the oscillator.

[0025] In one aspect, the laser system also includes a second section with positive group velocity dispersion following the optical amplifier.

[0026] On the other hand, the laser system also includes a negative group velocity dispersion section, which is connected to the output of the amplifier and is adapted to compensate for the phase of the optical pulse.

[0027] Laser systems can be implemented as solid-state systems (i.e., integrated on a chip) or as fiber-optic based systems. In the latter case, the amplifier is a fiber-optic amplifier. A combination of solid-state and fiber-optic based systems can also be used.

[0028] On one hand, the spectral width of the optical pulse decreases from the input of the connecting section within the amplifier to a minimum value in the amplifier, and then increases to a larger amount at the output of the amplifier, which means that the chirp has changed. The connecting section in the laser system can be a waveguide and this can be implemented on a chip in a solid-state system, as a section of optical fiber in an optical fiber-based system, or as a positive group velocity dispersive material.

[0029] The laser system may also include at least one optical isolator or component that suppresses the propagation of optical signals toward the oscillator, which may be either free-space or fiber-optic coupled. This optical isolator or component may be located after the oscillator in the laser system or within a connecting section. At least one preamplifier or attenuator may also be incorporated within the connecting section.

[0030] The laser system effectively creates a virtual nonlinear optical bandpass filter through the interaction of pulse chirp, dispersion effects occurring within the amplifier, and nonlinear effects.

[0031] The laser system may optionally include at least one optical pulse selector to reduce the repetition rate of optical pulses or to add on-demand pulse functionality to the laser system.

[0032] On the other hand, the negative chirp of the oscillator can be increased by using an additional negative dispersion element within the connection section. The additional negative dispersion element only changes the amount of chirp, not the sign of the chirp within the connection section. The additional negative dispersion element will only shift the position of the Fourier-restricted pulse within the amplifier.

[0033] By adding a negative dispersion element to the connecting section, the location of the chirp-free point will move to the end of the amplifier if the amount of normal dispersion remains constant. During amplification, the SPM will reduce the spectral bandwidth in the presence of negative chirp, resulting in a narrow optical bandwidth W3.

[0034] If optional components, such as fiber-based pulse selectors, would increase the overall positive dispersion of the connection segment, then additional negative dispersion components can be used to fix the location of the chirp-free point in the amplifier.

[0035] Generally speaking, the amount of chirp will limit the spectral bandwidth of the amplifier when the overall dispersion of the system is taken into account.

[0036] On the other hand, due to the interaction of linear and nonlinear effects (SPM), the spectral width of the optical pulse decreases from the input of the connection section within the amplifier to a minimum value after the amplifier, which depends on the power level.

[0037] The oscillator includes negative group velocity dispersion sections and positive group velocity dispersion sections, and their arrangement ensures that the total net dispersion of the cavity is less than 0.1 ps. 2 (β2<0.1ps 2 ).

[0038] The oscillator and amplifier include a laser-active medium. The laser-active medium is selected from, but is not limited to, rare-earth dopants including, for example, ytterbium, neodymium, thulium, or erbium. It should be noted that the materials of the oscillator and amplifier do not need to be exactly the same.

[0039] In one aspect of a laser system, the oscillator includes a linear cavity having an absorber at one end and a grating compressor at the other end.

[0040] The amplifier can be pumped by at least a single-mode diode laser or a multimode diode laser.

[0041] The fiber optic portion of the oscillator and amplifier includes, but is not limited to, single-clad fiber, double-clad fiber, or photonic crystal fiber, as well as rod-shaped fiber. Attached Figure Description

[0042] Figure 1 A schematic general diagram of the first aspect of the present invention is shown.

[0043] Figure 2 A schematic diagram of another aspect of the invention is shown, which includes a second section with positive group velocity dispersion after the amplifier.

[0044] Figure 3 A third aspect of the invention is shown, which includes at least an optical isolator.

[0045] Figure 4 A fourth aspect of the invention is shown, which includes at least an optical isolator and an attenuator.

[0046] Figure 5 A fifth aspect of the invention is shown, which includes at least an optical isolator, an attenuator, or an optical preamplifier.

[0047] Figure 6 A sixth aspect of the invention is shown, which includes at least an optical isolator, an attenuator or an optical preamplifier and a pulse selector.

[0048] Figure 7 A schematic diagram of another aspect of the laser is shown, wherein an additional negative dispersion section is present within the connecting section.

[0049] Figure 8 A schematic diagram of pulse evolution within a stretched pulsed laser system is shown.

[0050] Figure 9 A schematic embodiment of a stretching pulse oscillator configured in a linear cavity is shown.

[0051] Figure 10 A schematic embodiment of a stretching pulse oscillator configured in a ring cavity is shown.

[0052] Figure 11 A schematic embodiment of a stretch pulse oscillator configured in a Sigma arm cavity is shown.

[0053] Figure 12A schematic embodiment of a stretched pulse oscillator configured in a linear cavity is shown, with dispersion compensation achieved by using a chirped fiber Bragg grating.

[0054] Figure 13 A schematic embodiment of a stretched pulse oscillator configured in a linear cavity is shown, with fiber-based dispersion compensation achieved using photonic crystal fiber or hollow core fiber.

[0055] Figure 14 The time-compressed optical pulse with a pulse duration of less than 50 fs following the compressor of the system is shown.

[0056] Figure 15 The corresponding spectrum of the optical pulse at the output of a laser system with a spectral bandwidth W2 is shown.

[0057] Figure 16 A typical spectrum of an optical pulse at the output of an oscillator with a spectral bandwidth W1 is shown.

[0058] Figure 17 The corresponding spectrum of the optical pulse at the output of a laser system with a spectral bandwidth of W3 is shown. Detailed Implementation

[0059] The invention will now be described based on the accompanying drawings. It will be understood that the embodiments and aspects of the invention described herein are merely examples and do not in any way limit the scope of the claims. The invention is defined by the claims and their equivalents. It will be understood that a feature of one aspect or embodiment of the invention may be combined with features of one or more different aspects and / or embodiments of the invention.

[0060] Figure 1 One aspect of a laser system 10 for generating ultrawideband optical pulses of light is shown, which is based on a stretched pulse oscillator 20 that emits multiple negatively chirped optical pulses 30. The input 41 of a positive group velocity dispersion connecting section 40 is connected to the output 22 of the stretched pulse oscillator 20. An optical amplifier 50 with positive group velocity dispersion is connected to the output 42 of the connecting section 40 and amplifies the optical pulses 30. IThen there is the negative group velocity dispersion section 60 (also called a compressor). The negative group velocity dispersion section 60 is used to compensate for the phase contributions of linear and nonlinear effects that have occurred during propagation through the connecting section 40 and the optical amplifier 50, including the phase of the output pulse 30 of the oscillator 20. The sign of the chirp between the outputs of the oscillator 20 and the optical amplifier 50 changes only once through the optical path. In some aspects of the invention, the laser system 10 does not include the negative dispersion section 60. In another aspect, the negative group velocity dispersion section 60 may be replaced or supplemented by a filter to reduce the width of the output spectrum, thus reducing the amount of amplitude noise in the output spectrum by removing light from the spectral edges.

[0061] In a first aspect of the invention, the stretch pulse oscillator 20 emits negatively chirped optical pulses as a plurality of optical pulses 30. In this first aspect, unlike the connection section 40 known in the prior art, the connection section 40 does not alter the sign of the chirp of the oscillator 20. The input terminal 41 of the connection section 40 is connected to the output terminal 22 of the stretch pulse oscillator 20 via an optical fiber connector or free-space coupling. The output terminal of the connection section 42 is connected to an amplifier 50.

[0062] At the input terminal 41 of the connection section 40, and at the output terminal 42 of the connection section 40, the optical pulse 30 I In comparison, optical pulse 30 has a larger negative chirp. In other words, optical pulse 30 I The negative chirp and therefore the optical pulse 30 at the output 42. I The duration of the optical pulse 30 decreases during propagation in the positive group velocity dispersive fiber forming the connection section 40. Depending on the optical power of the optical pulse 30, a nonlinear effect occurs in the connection section 40, which causes the optical pulse 30 at the output end 42 of the connection section 40 to... I The spectral bandwidth is reduced.

[0063] It will be recognized that it may be necessary to implement a short connecting segment 40' with positive group velocity dispersion after amplifier 50, such as Figure 2 As shown in the diagram. For example, it is necessary to remove the pump light (from the optical pump) from amplifier 50, and this is done by removing the residual pump light from amplifier 50 using a cladding stripper. In this case, optical pulse 30 II The positive chirp increases slightly at the output of the short connection segment 40'.

[0064] At least one optical isolator 43 or 43' may be implemented in one or more connecting segments 40 or 40' following the stretched pulse oscillator 20, and Figure 3The optical isolator 43 or 43' is schematically shown in the diagram. The position of the optical isolator 43 or 43' can be at the input end 41 or 41' of the connecting section 40 (40'), within the connecting section 40 (40'), or at the output end 42 (42') of the connecting section 40 (40'). In this case, due to the presence of the optical isolator 43 in the optical path... I Therefore, optical pulse 30 III It will have a slightly altered chirp.

[0065] Following the (first) connecting segment 40, the negative chirped optical pulse 30 I Propagation to increase optical pulse 30 I The optical amplifier 50 has a power level of [unspecified]. Due to the nonlinear effect of the SPM mentioned above within the optical amplifier 50, the optical pulse 30 [unspecified]. I The chirp is reduced to essentially zero at the "chirp-free" point 52 within the optical amplifier 50, such as... Figure 1-6 As shown, this is because the optical amplifier 50 has positive group velocity dispersion. At the output 54 of the optical amplifier 50, the optical pulse 30 II The positive chirp, and the chirp can be compensated by the negative group velocity dispersion segment 60 connected to the output 54 of the optical amplifier 50, for a pulse duration 30 close to the Fourier limit. IV .exist Figure 14 The image shows an example of the measured FROG trajectory of a compressed optical pulse. The corresponding spectrum is shown in... Figure 15 As shown in the diagram. The negative group velocity dispersion section 60 can be, but is not limited to, a grating compressor, a prism compressor, a GRISM compressor, a chirped mirror, or a hollow fiber section. For ideal compression, a pulse shaper can also be integrated.

[0066] The dispersion estimate of the negative group velocity dispersion segment 60 is less than three times the sum of the group velocity dispersions of the connecting segment 40 and the amplifier 50, i.e., (3*(|b 40 +b 50 |)>|b 60 |), where b 40 b represents the group velocity dispersion of the connected segment. 50 This represents the group velocity dispersion of the optical amplifier 50, and b 60 This represents the group velocity dispersion in the negative group velocity dispersion region 60. However, the value of the dispersion is not a limitation of the present invention.

[0067] In another aspect of the laser pulse system 10, the power level within the connection section 40 or 40' can be controlled by at least attenuator 44 (or 44') (in Figure 4-6 (as shown in the image) or preamplifier 45 (as shown in the image) Figure 5 and 6 (as shown) or a combination of both (such as) Figure 5 and6 The attenuator 44 or 44' or the preamplifier 45 can be adjusted (as shown in the diagram) to control nonlinear effects in the connection section 40 and amplifier 50. The position of the attenuator 44 or 44' or the preamplifier 45 can be at the input terminals 41 / 41' of the connection sections 40 and 40', within the connection sections 40 and 40', or at the output terminals 42 or 42' of the connection sections 40 or 40'. Higher power output can be achieved by increasing the diameter of the mode field in the fiber.

[0068] A preamplifier can be one of the following: a fiber-optic preamplifier, a (fiber-coupled) semiconductor optical amplifier, or a solid-state amplifier.

[0069] An optical isolator 43 can be used after the preamplifier 45 (e.g. Figure 5 and 6 (as shown in the image).

[0070] By combining the negative chirp of optical pulse 30 and the positive group velocity dispersion in the connecting section 40, the optical pulse 30 at the output end 42... I The spectral bandwidth will decrease, thus creating a nonlinear bandpass filter at the "chirp-free point" 52 within the optical amplifier 50, such as... Figure 1-6 As shown in the diagram, it is possible to couple the optical pulse 30 to the vicinity of this point by reducing the length of the amplifier 50 to provide an optical pulse 30 with a very narrow spectrum W3.

[0071] The sixth aspect of the laser system 10 is Figure 6 As shown, this enables the integration of an optical pulse selector 46 to reduce the repetition rate of the stretched pulse oscillator 20, thereby increasing the pulse energy after the optical amplifier 50. A second pulse selector 46' can be added before the compressor 60 within the connection section 40' to generate on-demand pulse functionality.

[0072] Figure 7 The seventh aspect of the laser system 10 is shown. An additional portion of the negative dispersion 40a is added to the positive dispersion 40b of the connecting section 40, while only changing the amount of chirp without changing the sign. This allows adjustment of the position of the chirp-free point 52 within the amplifier 50. The optical pulse 30 at the output 42 of the connecting section 40 is compared to the optical pulse 30 from the oscillator 20 that reaches the input 41 of the connecting section. V This will result in a larger negative chirp. The position of the chirp-free point 52 will shift towards the end of amplifier 50. This mechanism can be used to adjust the chirp-free point 52. Optical pulse 30 VI The chirp after amplifier 50 will be positive and propagates through optional sections 43', 44', and 46', and their dispersion results in optical pulse 30. VII Slightly increased.

[0073] If the chirp-free point 52 is moved to the end of amplifier 50, then an optical pulse 30 with a narrow spectrum W3 can be generated. VIII .

[0074] The stretch pulse oscillator 20, the connecting section 40, and the optical amplifier 50 are connected to the fiber optic connector. However, the transition between any of the stretch pulse oscillator 20, connecting sections 40 and 40'(one or more), and the optical amplifier 50 can also be achieved through free-space coupling. Therefore, free-space isolators 43 and 43', pulse selectors 46 and 46', attenuators 44 and 44', or preamplifier 45 can also be used.

[0075] The generation of ultrawideband optical pulses is based on the interaction of linear and nonlinear effects within the amplifier, and therefore the maximum energy or spectral bandwidth can be controlled by selecting different mode field diameters during propagation.

[0076] In a non-limiting example, the optical amplifier 50 may be made of a ytterbium-doped fiber amplifier. It is considered that the optical amplifier 50 can be applied to all laser materials, such as, but not limited to, neodymium (Nd), thulium (Tm), erbium (Er), and erbium-ytterbium (Er-Yb).

[0077] This principle is not limited to fiber laser technology, and in various aspects, it can also be applied to solid-state amplifiers, including, for example, disk amplifiers, planar amplifiers, crystal-based amplifiers, rod amplifiers, or other types. For a more general approach, the negatively chirped optical pulse 30 from the stretched pulse oscillator 20 or soliton oscillator must propagate through a medium in the positive group velocity dispersion section. This medium is not limited to optical fibers, but can also be waveguides (including those implemented as micro-optical devices on a wafer) or materials with positive group velocity dispersion.

[0078] The use of positive group velocity dispersion in the connecting section 40 requires the generation of negatively chirped optical pulses 30 in the stretching pulse oscillator 20. This is in Figure 8 The diagram illustrates this, and it can be achieved by using two distinct dispersion sections within the oscillator cavity. The oscillator cavity includes a positive group velocity dispersion section 21 and a negative group velocity dispersion section 23. The total net group velocity dispersion (GVD) must be less than 0.1 ps. 2 (β 2net <0.1ps 2 In this case, the optical pulse 30 undergoes a change in chirp sign in two dispersion segments (i.e., negative dispersion segment 21 and positive dispersion segment 23). Figure 7The evolution of the optical pulse chirp within the stretched pulse cavity during propagation (x-axis, in meters) is shown (normalized y-axis in arbitrary units (arb.unit)). Furthermore, the corresponding group velocity dispersion segment is shown (normalized y-axis in arbitrary units (arb.unit)). Figure 7 As seen in the diagram, during propagation, each round-trip pulse must have two chirp-free cycles. According to... Figure 7 The chirp-free point is located at the end of the linear cavity. The position of the chirp-free pulse can be changed based on the overall net dispersion.

[0079] Starting from the chirp-free point 31 within the positive group velocity dispersion section 21, positive chirp is generated by propagation through the positive dispersion section 21, which forms part of the stretched pulse oscillator 20. This positive chirp decreases within the negative group velocity dispersion section 23, which forms the second part of the oscillator cavity 20, thereby resulting in a chirp-free optical pulse at position 31' within section 23 and subsequently changing the sign of the chirp. The negative chirp increases until the end of the negative group velocity dispersion section 23. Finally, by entering the positive group velocity dispersion section 21, the negative chirp decreases and returns to the chirp-free starting point 31 after one round trip. For the apparatus of this document, the optical output pulse of the oscillator 30 must have a negative chirp at the output coupler 27 of the oscillator 20.

[0080] An example of a stretched pulse oscillator 20 is in Figure 9 As shown in the diagram, the stretch pulse oscillator 20 includes an optical pump 28 for generating pump light for the laser active fiber segment 20'. The optical pump 28 is coupled to the cavity using a pump coupler 29. The negative dispersion segment 23 is implemented by a grating compressor at one end of the linear cavity.

[0081] The fiber portion forms a positive dispersion section 21. The stretched pulse oscillator 20 is mode-locked by using a saturable absorber mirror 24 at the other end of the linear optical cavity. The output coupler 27 is placed after the negative dispersion section 23 (in the propagation direction).

[0082] Other different aspects of the laser optical cavity of the stretched pulse oscillator 20 Figure 10 and 11 As shown in the figure. When using optical isolator 25 and saturable absorber 24', the optical cavity can also be implemented as an annular cavity, or as a sigma cavity when the saturable absorber mirror 24 is implemented in the optical cavity using circulator 26.

[0083] To further reduce the free space portion inside the linear cavity oscillator, a chirped fiber Bragg grating 23' can be used to realize the negative dispersion segment 23. Figure 12 Alternatively, instead of a grating compressor, a negative dispersion photonic crystal fiber can be used in combination with a fiber-based mirror (23”). Figure 13 ).

[0084] In one respect, all optical fibers used are polarization-maintaining fibers to achieve an environmentally stable system. However, in general, the laser pulse system of this disclosure is not limited to polarization-maintaining fibers. Non-polarization-maintaining fibers can also be used. Furthermore, the laser pulse system 10 is not limited to single-clad fibers. In addition, other types of optical fibers can be used as double-clad fibers. Depending on the type of fiber, single-mode or multimode laser diodes can be used for pumping. Mode locking can also be achieved by using any kind of saturable absorber (24') or dummy saturable absorber, for example, as nonlinear pulse evolution.

[0085] As described above, the laser pulse system 10 of this document can be implemented in a bulk solid, provided that management of the dispersion of the optical pulses is provided. This will require at least one positive dispersion element and one negative dispersion section.

[0086] Figure Labels

[0087] 10 Laser Systems

[0088] 20 Stretching Pulse Oscillator

[0089] 20' Laser-Activated Fiber Segment

[0090] 21 Positive dispersion section

[0091] 22 Output terminal

[0092] 23 Negative dispersion section

[0093] 24 Absorption Mirror

[0094] 25 Optical isolators

[0095] 26 Circulators

[0096] 27 Output Coupler

[0097] 28 Optical Pumps

[0098] 29 Pump Coupler

[0099] 30 optical pulses

[0100] 31 No chirp point

[0101] 40 Connecting Section

[0102] Negative dispersion (β2<0) section within the 40a connecting segment

[0103] 40b Positive dispersion (β2>0) section within the connecting section

[0104] 41 Input Terminal

[0105] 42 Output terminal

[0106] 43 Isolators

[0107] 44 Attenuator

[0108] 45 Preamplifier

[0109] 46 Optical Pulse Selector

[0110] 50 Optical Amplifier

[0111] 52 No chirp point

[0112] 54 Output terminals

[0113] 60 negative dispersion section

Claims

1. A laser system (10), comprising: - Oscillator (20) generates multiple negatively chirped optical pulses (30) with a first spectral width W1; - An amplifier (50) for receiving a plurality of optical pulses (30) and amplifying the plurality of optical pulses to generate an optical pulse having a second spectral width W2 at the output of the amplifier (50); as well as - A connecting section (40) is directly connected between the input terminals of the oscillator (20) and the amplifier (50), wherein the connecting section (40) is adapted to maintain the sign of the chirp of the plurality of negative chirped optical pulses (30) and reduce the chirp and duration of the plurality of negative chirped optical pulses; as well as The sign of the chirp changes from negative to positive once between the outputs of the oscillator (20) and the amplifier (50); The connecting segment (40) has a positive group velocity dispersion (β2 > 0) without changing the sign of the chirp.

2. The laser system (10) as claimed in claim 1, wherein the second spectral width W2 at the output of the amplifier is greater than the first spectral width W1 of the oscillator.

3. The laser system (10) as claimed in claim 1 further includes a second segment (40') of the positive group velocity dispersion segment (β2) connected to the output of the amplifier (50).

4. The laser system (10) of claim 1 further includes a negative group velocity dispersion section (β2) (60) connected to the output of the amplifier (50) and adapted to compensate for the phase of the optical pulse (30).

5. The laser system (10) as claimed in claim 1, wherein the amplifier (50) is one of an optical fiber amplifier, a rod amplifier, a flat panel amplifier, a disk amplifier, or a solid-state amplifier.

6. The laser system (10) of claim 1, wherein the spectral width of the optical pulse (30) is reduced from the input (42) of the connecting section (40) through the amplifier (50) to a minimum value in the amplifier (50), and then increased to a larger amount at the output (54) of the amplifier (50).

7. The laser system (10) as claimed in claim 1 or 2, wherein the connecting section (40) is a combination of a positive group velocity dispersion section (β2) and a negative group velocity dispersion section (β2) without changing the sign of the chirp.

8. The laser system (10) of claim 1, wherein the spectral width of the optical pulse (30) is reduced from the input (42) of the connecting section (40) to a minimum value in the amplifier (50) through the amplifier (50), thereby resulting in a small amount W3 at the output (54) of the amplifier (50).

9. The laser system (10) of claim 1 further includes at least one optical isolator located after the oscillator (20).

10. The laser system (10) of claim 9, wherein the optical isolator is either a free space or fiber optic coupler.

11. The laser system (10) of claim 1 further includes at least one of a preamplifier or an attenuator in the connection section.

12. The laser system (10) of claim 11, wherein the preamplifier is one of an optical fiber amplifier, a semiconductor optical amplifier, or a solid-state amplifier.

13. The laser system (10) of claim 1 further includes at least one optical pulse selector to adjust the time separation between optical pulses.

14. The laser system (10) of claim 1, wherein the oscillator (20) includes at least a segment (23) of positive group velocity dispersion (β2) and a segment (21) of negative group velocity dispersion (β2).

15. The laser system (10) of claim 1, wherein the laser-active medium of at least one of the oscillator (20) or amplifier (50) is selected from the group consisting of dopants including ytterbium, neodymium, thulium or erbium.

16. The laser system (10) of claim 1, wherein the oscillator (20) comprises a linear cavity having a saturable absorber at one end and a grating compressor at the other end.

17. The laser system (10) of claim 1, wherein the amplifier (50) is pumped by at least one of a single-mode diode laser or a multimode laser.

18. The laser system (10) of claim 1, wherein the oscillator (20) and the amplifier (50) are one of a single-clad fiber or a double-clad fiber or a combination of a single-clad fiber and a double-clad fiber.

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