WO2022164781A1

WO2022164781A1 - Single-cavity all-fiber laser system with multiple synchronized polychromatic outputs

Info

Publication number: WO2022164781A1
Application number: PCT/US2022/013638
Authority: WO
Inventors: Farhad Akhoundi; Nasser Peyghambarian
Original assignee: Arizona Board Of Regents On Behalf Of The University Of Arizona
Priority date: 2021-01-28
Filing date: 2022-01-25
Publication date: 2022-08-04

Abstract

An all-fiber femtosecond laser system configured to generate multiple light outputs at different central/reference wavelengths from a single laser cavity. One implementation of the system is based on Er- doped fiber and is mode-locked by being fiber-coupled to a saturable-absorber reflector, thereby lending all multiple light outputs of the system to be synchronized and, aggregately, defining an optical supercontinuum. Generation of multiple outputs turns on taking advantage of fiber-optics nonlinearity, while at least one light output is maintained to be spectrally-tunable within a spectral range of at least 300 nm.

Description

SINGLE-CAVITY ALL-FIBER LASER SYSTEM

WITH MULTIPLE SYNCHRONIZED POLYCHROMATIC OUTPUTS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This patent application claims priority from and benefit of the US Provisional Patent

Application no. 63/142,642 filed on January 28, 2021, the disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

[0002] The present invention relates to laser systems configured as supercontinuum sources of light and, more particularly, to all-fiber laser systems generating multiple spatially-distinct outputs each of which possesses a corresponding broadened bandwidth (which bandwidth, in combination, forms the required spectral supercontinuum) and which are synchronized with one another.

RELATED ART

[0003] Solid-state femtosecond lasers have attracted a great interest in a variety of medical, commercial, and scientific applications including multiphoton microscopy (which provides resolution better than confocal systems and also provides deeper tissue imaging due to less scattering of the employed near infrared, NIR, wavelength), refractive surgery, spectroscopy, microablation, and as a pump for nonlinear frequency conversion. More recently, fiber-based mode-locked lasers have been considered as promising alternative sources of femtosecond pulses due to their compactness, environmental stability, lightweight, and a substantially maintenance-free nature. In particular, compact mode-locked fiber lasers at 1 tmi wavelengths have been employed in two-photon microscopy, microstructuring, and second harmonic generation, as well as in ultra-wide supercontinuum generation (which, in some cases, covers visible and NIR spectral windows). Furthermore, due to their small footprint, the fiber-based mode-locked lasers have been used in commercial two-photon polymerization system (e.g., for printing miniaturized 3D-structures). The use of mode-locked lasers at 1.55 rm was discussed in the context of three- photon microscopy, nonlinear characterization of materials, precision optical comb and spectroscopy, and fiber optics communications. Ultrashort pulsed sources operating at wavelengths near 2 um are becoming popular in LIDARs and the field-deployable environmental sensing applications due to abundance of unique molecular signatures in this spectral window). Due to the high absorption of light by liquid water at wavelengths near 1.9 um, these lasers also find applications in tissue ablation and microsurgery. Mid-infrared supercontinuum and THz generations are other popular applications of the femtosecond sources near 2 um wavelength. Near this spectral range, femtosecond lasers at 1.7 um has attracted a great interest in deep tissue 3D imaging.

[0004] Substantially each fiber laser cavity of related art remains specifically designed to target each desired wavelength in particular, with appropriately-compatible optical including a gain fiber, an output coupler, a wavelength division multiplexer (WDM), an appropriate mode-locking mechanism, a pump diode laser, an optical isolator, and so on. For instance, in IEEE J. of Selected Topics in Quantum Electronics, vol. 10, no. 1, pp. 129-136, 2004, Gomes et al. discussed a mode-locked laser cavity proposed to generate picosecond pulses at 1 um, while Cabasse et al. disclosed a system producing the output at 1.55 imi (Optics Express, vol. 17, no. 12, pp. 9537- 9542, 2009), Kivisto et al. (IEEE Photonics Technology Letters, vol. 19, no. 12, pp. 934-936, 2007) devised a system targeting the design wavelength near 2 um.

[0005] From this short overview, it becomes clear that development of fiber-lasers configured to generate light in the spectral window covering at least the range from about 1 micron to about 2 microns and with greater spectral flexibility has not been solved, and remains of great interest.

SUMMARY

[0006] Embodiments of the invention provide an all-fiber laser system that includes a single optical-fiber laser cavity (which contains a gain medium and which is configured to generate femtosecond cavity pulses of light having an optical spectrum when the gain medium is pumped with pump light at a predetermined wavelength). Here, the optical spectrum includes a target central wavelength and a target spectral bandwidth. The system additionally includes an optical-fiber amplifier device operably cooperated with the single laser cavity to receive the cavity pulses of light and configured to produce, at an output of the optical-fiber amplifier device, a first laser output at a first wavelength. (Such first laser output has a first spectral bandwidth.) The system further includes at least one optical-fiber wavelength-conversion device operably connected to the single laser cavity to receive the cavity pulses of light. Such wavelengthconversion device is configured to produce at least one corresponding auxiliary laser output having a corresponding auxiliary wavelength and a corresponding auxiliary spectral bandwidth. The system is characterized by having the auxiliary wavelength and the auxiliary spectral bandwidth of the at least one auxiliary laser output be different from the first wavelength and the first spectral bandwidth, respectively, an by having each of the first laser output and the at least one auxiliary laser output contain respective trains of pulses of light that are synchronized with the cavity pulses of light while having equal repetition rates.

[0007] In at least one implementation of the system, each of a repetition rate of a train of pulses in the first laser output and a repetition rate of a train of pulses of the at least one auxiliary laser output may be substantially equal to a repetition rate of the femtosecond cavity pulses. Alternatively or in addition, such implementation is structured to satisfy any combination of the following conditions: (i) the first wavelength is substantially equal to the target central wavelength; (ii) at least one auxiliary laser output includes first and second auxiliary laser outputs, the first auxiliary laser output having a first auxiliary wavelength and a first auxiliary spectral bandwidth, the second auxiliary laser output having a second auxiliary wavelength and a second spectral bandwidth. (Here, the first auxiliary wavelength may be longer than the first wavelength while the second auxiliary wavelength may be shorter than the first wavelength); (iii) the optical-fiber amplifier device and each of optical-fiber wavelength-conversion devices present in the all-fiber laser system are connected to one another in parallel; and (iv) when the at least one optical fiber wavelength-conversion device includes multiple optical-fiber wavelength conversion devices, corresponding auxiliary wavelengths and corresponding auxiliary spectral bandwidths at corresponding multiple auxiliary laser outputs are different from one another, respectively. In at least one embodiment of the system in which multiple wavelength-conversion devices are present, the system satisfies one or more of the following requirements: (a) each of the multiple wavelength-conversion devices includes a respective optical-fiber amplifier component that is spatially-distinct from any other optical-fiber amplifier component of another of the multiple wavelength-conversion devices; (b) each of the optical -fiber amplifier device and the multiple wavelength-conversion devices is connected to the single laser cavity via an optical-fiber splitter; (c) each of the optical-fiber amplifier device and the multiple wavelength-conversion devices includes a corresponding optical pulse-compressor element configured to reduce durations of pulses of light passing therethrough; and (d) the all-fiber laser system includes an optical pulse-compressor element disposed to be separated from the single laser cavity by a corresponding optical-fiber splitter and an optical-fiber amplifier device to prevent breaking of a pulse of light of the pulses of light that propagate through the optical pulse-compressor element. Additionally or in the alternative, at least one embodiment of the system that includes multiple optical-fiber wavelength-conversion devices is structured to satisfy one or more of the following conditions: - a first wavelength-conversion device of the multiple wavelength-conversion devices is configured to generate a first auxiliary laser output by red-shifting the first wavelength to a first auxiliary wavelength as a result of intrapulse stimulated Raman scattering (here, the first auxiliary laser output includes the first auxiliary wavelength and a first auxiliary spectral bandwidth); and - a second wavelength-conversion device of the multiple wavelength- conversion devices is configured to generate a second auxiliary laser output by forming an optical supercontinuum (here, the second auxiliary laser output includes a second auxiliary wavelength and a second auxiliary spectral bandwidth). Alternatively or in addition - and in at least one embodiment of the system that includes multiple wavelength-conversion devices - the system is structured to meet one or more of the following requirements: (i) each of the multiple wavelength-conversion devices includes a respective optical-fiber amplifier component that is spatially-distinct from other optical -fiber amplifier components; (ii) each of the multiple wavelength-conversion devices is connected to the single laser cavity via an optical-fiber splitter; (iii) at least one of the multiple wavelength-conversion devices includes a corresponding optical pulse-compressor element configured to reduce durations of pulses of light passing therethrough; and (iv) the all-fiber laser system includes an optical pulse-compressor element disposed to be optically separated from the single laser cavity by a corresponding optical-fiber splitter and an optical-fiber amplifier component to prevent breaking of a pulse of light of the pulses of light that propagate through the optical compressor element.

[0008] Embodiments of the invention additionally provide a method configured to effectuate, in an all-fiber laser system, the processes of (a) reproducing a seed train of femtosecond pulses of light formed in a laser cavity to create multiple trains of femtosecond pulses of light (the seed train of femtosecond pulses having a repetition rate); (b) generating a first train of femtosecond pulses of light at a first wavelength by amplifying a first chosen train of the multiple trains of femtosecond pulses of light while propagating said first chosen train along a first optical path; and (c) generating a second train of femtosecond pulses of light at a second wavelength that is different from the first wavelength by transforming a second chosen train of the multiple trains of the femtosecond pulses along a second optical path that is spatially-different from the first optical path. Here, each of the first and second trains of femtosecond pulses is not only synchronized with the seed train of femtosecond pulses as defined by spatial coordination between the first and second optical paths with the laser cavity, but also possesses the same repetition rate as tat of the seed train. In at least one embodiment of the method, the second wavelength may be longer than the first wavelength, while the method additionally includes a process of generating a third train of femtosecond pulses of light at a third wavelength that is shorter than the first wavelength by transforming a third chosen train of the multiple trains of femtosecond pulses while propagating the third chosen train along a third optical path. (Here, the third optical path is spatially-different from both the first optical path and the second optical path and/or the third train of femtosecond pulses has the repetition rate of and is synchronized with the seed train of femtosecond pulses as defined by spatial coordination between the third optical path and the laser cavity.) In at least this former implementation, the method satisfies one or more of the following requirements: (i) the method includes a process of amplifying pulses of light in at least one of the multiple trains of femtosecond pulses after the process of replicating, and further includes a process of temporally compressing pulses of light in at least one of the multiple trains of femtosecond pulses upon propagation thereof through the laser system outside the laser cavity after the process of amplifying to prevent breaking of the pulses of light upon propagation thereof along at least one of the first, second, and third optical paths; (ii) the process generating the second train of femtosecond pulses includes red-shifting a wavelength of light propagating along the second optical path by intra-pulse Raman-scattering of said light in an optical fiber; and (iii) the process of generating the third train of femtosecond pulses is followed by generating light with optical supercontinuum bandwidth including the third wavelength in the third optical path. In at least one implementation of the method, the process of generating the second train of femtosecond pulses includes at least one of the following steps: - spectrally tuning a central wavelength of a spectral bandwidth of pulses in the second train by adjusting pump power delivered to an optical-fiber amplifier disposed in the second optical path; - defining an initial value of said central wavelength of the spectral bandwidth of the pulses in the second train by pre-determining at least one of a length, a core size, a dispersion characteristic, a V-number of at least one of a pulse-compression optical-fiber element, and a soliton self-frequency shift optical-fiber element disposed at the second optical path; and - amplifying pulses of light in at least one of the multiple trains of femtosecond pulses of light. At least one embodiment of the method includes defining or setting the repetition rate to be substantially the same for each of the first, second, and third trains of pulses and temporally synchronizing the first, second, and third trains of pulses with the seed train of pulses by directing respective portions of light formed in the laser cavity along the first, second, and third optical paths without a use of free-space propagation of said light.

[0009] Embodiments of the invention additionally provide a method that effectuates, in an all-fiber laser system, a process of forming a laser light output containing multiple output trains of pulses with an aggregate output spectral bandwidth defining a supercontinuum optical bandwidth (such that each output train has the same repetition rate as any other output train of the multiple trains) by first replicating an initial train of femtosecond pulses generated in a laser cavity of the all-fiber laser system to form multiple intermediate trains of femtosecond pulses each of which is temporally synchronized with the initial train of femtosecond pulses; and then by spatially-distinctly transforming each of the multiple intermediate trains of femtosecond pulses to the multiple output trains of pulses having different respective output spectral bandwidths and different output central wavelengths. In at least one case, such method may additionally include a step of temporally compressing a pulse of light propagating through the all -fiber laser system after amplifying such pulse to prevent breaking of such pulse of light; and/or a step of delivering at least first and second of the multiple output trains of pulses from different output ends of the all -fiber laser system; and/or scattering light in pulses of a first intermediate train of the multiple intermediate trains of femtosecond pulses by intra-pulse Raman scattering process to red-shift a first central wavelength of a first spectral bandwidth of the first intermediate train of pulses. (At least in one such specific implementation, the method may additionally include a step of temporally compressing a pulse of light propagating through the all-fiber laser system after amplifying the pulse to prevent breaking of the pulse of light, while such temporal compression - in at least one implementation - may include propagating a frequency-chirped pulse of light through a hollow-core photonic-crystal optical fiber.) Additionally or in the alternative, and in at least one implementation of the method, the process of spatially-distinctly transforming may include modifying optical characteristics of pulses in the multiple intermediate trains by passing such pulses through optical-fiber-based devices disposed in spatially-distinct from one another optical-fiber branches of the all-fiber laser system to define a first output wavelength to be approximately 1,550 nm, a second output wavelength to be approximately 2,000 nm, and a third output wavelength to be approximately 1,000 nm while delivering pump laser light to at least two locations of at least one of the optical-fiber branches and/or the method may include a step of amplifying pulses of light of at least one of the multiple intermediate trains after the step of replicating.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The invention will be more fully understood by referring to the following Detailed

Description of Specific Embodiments in conjunction with the Drawings, of which:

[0011] FIG. 1 is a schematic diagram of an embodiment of the all-fiber laser cavity system of the present invention.

[0012] FIG. 2A and 2B provide, respectively, the spectrum and the temporal structure of the laser light generated in the single cavity of an embodiment of FIG. 1.

[0013] FIG. 2C is a plot representing the spectrum of light at the output of one of the branches of the all-fiber laser system of FIG. 1. [0014] FIG. 2D is the autocorrelation trace representing the light output of FIG. 2A;

[0015] FIG. 3A is a plot representing the spectrum of light at the output of another one of the branches of the all-fiber laser system of FIG. 1;

[0016] FIG. 3B is the autocorrelation trace representing the light output of FIG. 3 A;

[0017] FIG. 4A is a plot representing the supercontinuum bandwidth of light at the output of the

HNLF element of the remaining one of the branches of the all-fiber laser system of FIG. 1 ;

[0018] FIG. 4B is a plot depicting the spectrum of light at the output of the branch corresponding to

FIG. 4A;

[0019] FIG. 4C is the autocorrelation trace representing the light output of FIG. 4B.

[0020] Generally, like elements or components in different Drawings may be referenced by like numerals or labels and/or the sizes and relative scales of elements in Drawings may be set to be different from actual ones to appropriately facilitate simplicity, clarity, and understanding of the Drawings. For the same reason, not all elements present in one Drawing may necessarily be shown in another.

DETAILED DESCRIPTION

[0021] Embodiments of the present invention address industrial and scientific need in a fiber-based laser system that not only offers generation of laser light at multiple wavelengths, but provides for such generation in an optical supercontinuum fashion - that is, as a result of employing nonlinear processes that cause severe spectral broadening of light generated in the laser cavity of the discussed embodiments of an all- optical-fiber laser system.

[0022] As per implementation of the idea of the invention, shortcomings of the laser systems of related art preventing these systems from generating laser outputs with operationally-adequate substantially uniform supercontinuum spectral bandwidths are overcome with the use of an all-optical-fiber (or, all-fiber, for short) laser system by: generating femtosecond pulses turned into multiple trains of pulses, which have the same repetition rate, from a single a laser cavity with the use of spatially-different optical-fiber components acquiring light from such single laser cavity while providing each of these optical-fiber components to operate independently from one another by independently controlling pump light delivered to optical amplifiers of these optical-fiber components. Since light at the multiple wavelengths are generated from a single laser and they all have the same repetition rate, the pulses of light can be mutually synchronized so that at each given pulse duration, the entire supercontinuum of the available wavelengths can be used.

[0023] According to an idea of the invention, each of such temporally-synchronized trains of pulses

- or multiple of these trains of pulses - can be further configured as a corresponding source of laser light spectrally-targeting a specific application (such as a coherent anti-Stoke scattering, CARS, application or simultaneous imaging of various fluorophores and/or components of a biological tissue and/or various fluorophores associated with such tissue (with the use of, for example, multiphoton microscopy). Such sources of light will also find application in precision optical comb and spectroscopy as well as LiDARs and field-deployable environmental sensing. The all-fiber design of the laser system ensures compactness, reliability, and stability of operation of the system.

[0024] As commonly understood and defined in related art, a fiber laser is a laser in which the active gain medium is an optical fiber (doped with some rare-earth elements such as, for example, erbium, ytterbium, neodymium, dysprosium, praseodymium, thulium and/or holmium). For the purposes of this disclosure and appended claims, the all-fiber laser (or, all-fiber laser system) is defined as a laser system that is fully, completely identified by and disposed within an optical-fiber medium - such that not only all generation and amplification of light within the laser system cavity but also all transformations of the so- generated light required to be performed prior to forming the useful laser-light output are carried out fully and completely within the optical fiber medium/media and without any involvement of free-space propagation of laser light and/or utilization of non-optical-fiber-based optical devices. In other words, the only time laser light of an all-fiber laser system exits the optical -fiber medium into a free space is at the very terminus of the all-fiber laser system - that is, when delivering laser light with desired characteristics to a user.

[0025] A reference to a laser cavity being "a single" laser cavity implies that a considered laser system contains only one, the only, unique optical cavity (resonating cavity, optical resonator - whether configured in a ring-like or a linear format). The terms "to synchronize", "synchronized" and similar terms refer to events that are caused to agree in time of occurrence, occur at the same time, or coincide, or agree in time.

Examples of Non-Limiting Embodiments of the Invention

[0026] Oscillator Design and a Light Output at a Target Wavelength.

[0027] To this end - and in general reference to FIG. 1, as discussed below - an embodiment of allfiber laser system 100 includes a fiber-laser cavity 110 configured to generate, at the output of the cavity 110, light 110A substantially at a chosen communication (central, target) wavelength such as, for example, a wavelength of 1.55 um (where the fiber components are low cost and widely available). The fiber-laser cavity 110 is structured to operate in a mode-locked regime by being fiber-coupled to a commercial saturable-absorber mirror 112. Externally to the cavity 110, a light-transforming unit or sub-system 114 of the embodiment 100 is optically cooperated with the cavity 110. The light-transforming sub-system manifests as an array of constituent all-fiber optical pulse transformers that are arranged in parallel with one another and in sequence with the single cavity 110. The configuration of the light-transforming sub-system is judiciously defined to utilize a single light output formed by the cavity 110 and to transform it - whether substantially simultaneously or in temporally-sequential fashion - to the desired number of generally different light outputs at least one of which may be characterized as optical supercontinuum.

[0028] In the specific example of FIG. 1 , shown are only three all-fiber optical pulse transformers that are identified as optically-parallel fiber-based branches or devices 114A, 114B, and 114C of the subsystem 114. It is understood, however, that a sub-system 114 with a different number of all-fiber optical fiber transformers or branches can be employed in a related embodiment. The unit/sub-system 114 is optically- interruptingly connected to the laser cavity 110 to transfer light 110A through operationally-independent light-transformers 114A, 114B, 114C to deliver, at the multiple output ends or outputs or terminuses (in the specific example shown - output ends 120A, 120B, 120C) of the system 100 corresponding output trains of optical pulses at respective multiple wavelengths. In one specific example, as discussed below, such trains of optical pulses are delivered, at corresponding output ends of the system 100, at approximately 1000 nm, 1550 nm, and 2000 nm. All output trains or sequences of pulses are generated, therefore, based on the laser processes occurring in and from the very same single, only, unique cavity 110 and therefore have the same pulse repetition rate by the very nature of the system 100. [0029] Non-Limitin Example of Oscillator Design and Transformation of a Primary Light

Output at a Target Wavelength.

[0030] Referring again to block-diagram of an example 100 of the all-fiber laser system of the invention schematically shown in FIG. 1, in one case the cavity 100 was configured as the Er-doped fiber laser cavity that is mode-locked using a commercial saturable-absorber mirror 112 (SAM-1550-40-2ps, BATOP GmbH, Germany) that provides a modulation depth of about 24%, saturation fluence of about 40 j/cm², and a relaxation time of about 2 ps. In one specific implementation, the oscillator defined by the cavity 110 was configured to operate in a single pulse regime with a transform- limited pulse width of about 350 fs and a pulse repetition rate of about 37 MHz (see FIG. 2B). The cavity output power was 3 mW, and the light output 110A at a central wavelength located in the vicinity of 1550-1560 nm (see FIG. 2A) was split among the branches of the light-transforming unit 114 (in the specific example as shown in FIG. 1 - in three ways)to form individual inputs to the constituent devices ofthe unit 114thatare defined by fiber branches 114A, 114B, 114C. These individual portion of light 110A delivered into constituent all-fiber branches 114A, 114B, 114C were amplified separately from one another (due to the fact that the branches 114A, 114B, 114C are spatially discrete and not-overlapping) using respective independently-operated Er-doped fiber amplifiers installed in these branches.

[0031] The light-transforming device of the branch 114A of the light-transforming arrayed subsystem 114 was - at least in one implementation - judiciously configured as an optical-fiber amplifier device compressing, in operation, the duration of pulses of light 110A generated by the single cavity 110, optionally without any substantial modification of the optical spectrum of such light. (In other words, and depending on the specifics of particular implementation of the branch 114A, the reference wavelength of the optical spectrum of the light output at the terminus 120 A may or may not be substantially different from the reference wavelength of the optical spectrum of light 110A.)

[0032] Here - and referring to the structure of the branch 114A of FIG. 1, due to the normal dispersion of the used gain fiber, the light at the output from the fiber-amplifier 122 of this branch was highly chirped. To compress the pulses at the output from the amplifier 122, about 1.4 m of a standard single-mode fiber, SMF, 124 with anomalous dispersion at 1550 nm was additionally employed. The pulses were then further compressed down to about 160 fs with the use of a cut back method for an average output power, at the terminus 120A, of 75 mW. The spectrum and autocorrelation trace ofthe 1550 nm light generated in one specific example at the output end / terminus 120A are shown in FIGs. 2C, 2D.

[0033] Non-Limiting Example: Generation of Additional Light Output at an Auxiliary Pre-defined

Wavelength.

[0034] The upper (as shown in FIG. 1) arm or branch 114B of the light-transformer unit 114 of the all-fiber laser system 100 was employed to generate femtosecond pulses at an auxiliary wavelength different from that of light 110A (in this example - at a wavelength of about 2 rm). Accordingly, the device 114B of the arrayed sub-system 114 was configured as an all-optical-fiber wavelength-conversion device.

[0035] To achieve this goal, as schematically illustrated in FIG. 1, the Er-doped amplifier 132 ofthe device 114B is indirectly, through the pulse-compression optical fiber element 134, spliced to the SSFS fiber element 136 (which was chosen to be a 60 m long fiber with a 6 rm core diameter and a numerical aperture NA of 0.23). Here, soliton-self-frequency-shift, SSFS, process (which was employing a silica fiber element 136 to effectuate an intra-pulse stimulated Raman scattering) was used to red-shift (that is, to shift towards the red part of the wavelength spectrum) the 1550 nm wavelength of the light input 110 A; here, towards the 2 rm spectral window. The design of the device 114B was tailored to form a soliton pulse with maximized a red-wavelength shift.

[0036] The empirically demonstrated soliton self-frequency shift resulted in tuning the wavelength at the laser system output/terminus 120B from about 1700 nm to about 2000 nm by adjusting various structural and/or operational parameters of the system 100 (such as power of pump light with which the Er- doped fiber amplifier 132 was fed, the length of the compression fiber 134, and the length of the SSFS fiber element 136. Here, the parameters were optimized for an approximately 1900 nm output wavelength. The average power at about 1900 nm was approximately 20 mW, and in order to scale it up, a pulse chirp amplification (CPA) as additionally employed. In particular, a 6 m fiber element 138 with normal dispersion at 1900 nm was used to pre-chirp the pulses of light propagating therethrough towards the output 120B, and a Tm-doped fiber amplifier 139 was further used to amplify and temporally compress the pulse train thereafter delivered to the output 120B, where the output pulse train had an average power of about 300 mW and a pulse width of about 200 fs. (The output spectrum and autocorrelation trace are shown in FIGs. 3A, 3B.) Having the advantage of this disclosure, the skilled artisan will now readily appreciate that the wide gain bandwidth (of about 300 nm) of the Tm-doped fiber element 139 of the branch 114B easily supported the option to amplify the soliton self-frequency shifted light as spectrally-tunable, in this example, within a spectral range of about 300 nm (as was alluded to above) .

[0037] Non-Limiting Example: Additional Generation of Light Output at Yet Another Auxiliary

Pre-Defined Wavelength.

[0038] To transform the light (that is, to purposely modify characteristics of light) generated by the cavity 110 of the system 100 in the device defined by the branch 114C of the arrayed light-transforming subsystem 114 to produce, at the terminus 120C of the sub-system 114 light with yet another frequency spectrum that is different not only from the spectrum of light 110A generated by the cavity 110, but also from the spectrum of light produced by a parallel branch 114B. In one specific example - in which the device 114C was configured to generate optical supercontinuum extending all the way down to the vicinity of 1000 nm - the branch 114C was structured as follows. Here, a highly nonlinear fiber element, HNLF, 146 was employed by indirectly splicing the Er-doped fiber amplifier 142 of the device/branch 114C to about 4 cm of a highly-nonlinear fiber (HNLF-ST, OFS, USA). The splice was optimized to reduce the loss to less than 20%. A supercontinuum was generated at the output of the HNLF 146, which extended/expanded down to a 1 um spectral window (FIG. 4A). Light in that part of the spectrum was further amplified by a reverse- pumped Yb-doped fiber amplifier 148, positioned to receive light at the output end of the fiber element 146. The output from the amplifier 148, in turn, was highly chirped and the follow-up compression of the light pulses was accomplished with the use of a hollow core photonic-crystal fiber element 150 (HC-1060, NKT Photonics, Denmark). A fiber isolator 152 was used to reduce the Fresnel reflection from the glass/air interface of the hollow-core fiber splice. This splice was optimized to reduce the loss to 1.5 dB. The output spectrum and the autocorrelation trace of the approximate 1 um pulsed light at the output 120C is shown in FIGs. 4B, 4C.

[0039] In further reference to the embodiment of FIG. 1, a skilled person will readily appreciate that in the single cavity 110 and/or in at least one branch of the branches 114A, 114B, 114C of the lighttransforming arrayed sub-system 114 of the system 100, wavelength-division-multiplexer(s), WDM(s), can be used as appropriate to effectuate the delivery of pump radiation for light amplification in a given portion of the all-optical-fiber system 100. These structural operational arrangements are well-recognized in related art, and for that reason are not discussed here in any detail. In FIG. 1, the delivery of pump radiation to a corresponding WDM (configured for example as an optical -fiber-based splitter) portion of the system 100 is clearly indicated.

[0040] The skilled artisan will readily appreciate that - although the drawings used for illustration of the embodiments o the invention discussed here may not necessarily expressly display the microprocessor / electronic circuitry used in such embodiments - at least a part of the process of operation of an embodiment of the all-fiber laser system that has been described may and is likely include an appropriate processor (microprocessor, electronic circuitry; optionally programmable) that is controlled by instructions stored in a memory operably cooperated with the processor. Such electronic circuitry may be operably cooperated with at least some of the SESAM mirror 112 and sources of pump light (indicated throughout the schematic of Fig. 1 as delivering the radiation at 980 nm and/or 210 nm and/or 975 nm to corresponding optical fiber sections of the system 100). The memory may be random access memory (RAM), read-only memory (ROM), flash memory or any other memory, or combination thereof, suitable for storing control software or other instructions and data. Those skilled in the art should also readily appreciate that instructions or programs defining the functions of the present invention may be delivered to a processor in many forms, including, but not limited to, information permanently stored on non-writable storage media (e.g. read-only memory devices within a computer, such as ROM, or devices readable by a computer RO attachment, such as CD-ROM or DVD disks), information alterably stored on writable storage media (e.g. floppy disks, removable flash memory and hard drives) or information conveyed to a computer through communication media, including wired or wireless computer networks. In addition, while the invention may be embodied in software, the functions necessary to implement the invention may optionally or alternatively be embodied in part or in whole using firmware and/or hardware components, such as for example combinatorial logic, Application Specific Integrated Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs) or other hardware or some combination of hardware, software and/or firmware components.

[0041] For the purposes of this disclosure and the appended claims, the use of the terms

"substantially", "approximately", "about" and similar terms in reference to a descriptor of a value, element, property or characteristic at hand is intended to emphasize that the value, element, property, or characteristic referred to, while not necessarily being exactly as stated, would nevertheless be considered, for practical purposes, as stated by a person of skill in the art. These terms, as applied to a specified characteristic or quality descriptor means "mostly", "mainly", "considerably", "by and large", "essentially", "to great or significant extent", "largely but not necessarily wholly the same" such as to reasonably denote language of approximation and describe the specified characteristic or descriptor so that its scope would be understood by a person of ordinary skill in the art. In one specific case, the terms "approximately", "substantially", and "about", when used in reference to a numerical value, represent a range of plus or minus 20% with respect to the specified value, more preferably plus or minus 10%, even more preferably plus or minus 5%, most preferably plus or minus 2% with respect to the specified value.

[0042] The use of these terms in describing a chosen characteristic or concept neither implies nor provides any basis for indefiniteness and for adding a numerical limitation to the specified characteristic or descriptor. As understood by a skilled artisan, the practical deviation of the exact value or characteristic of such value, element, or property from that stated falls and may vary within a numerical range defined by an experimental measurement error that is typical when using a measurement method accepted in the art for such purposes. Other specific examples of the meaning of the terms "substantially", "about", and/or "approximately" as applied to different practical situations may have been provided elsewhere in this disclosure.

[0043] References throughout this specification to "one embodiment," "an embodiment," "a related embodiment," or similar language mean that a particular feature, structure, or characteristic described in connection with the referred to "embodiment" is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment," "in an embodiment," and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. It is to be understood that no portion of disclosure, taken on its own and in possible connection with a figure, is intended to provide a complete description of all features of the invention.

[0044] While the invention is described through the above-described specific non-limiting embodiments, it will be understood by those of ordinary skill in the art that modifications to, and variations of, the illustrated embodiments may be made without departing from the inventive concepts disclosed herein. For example - and referring again to Fig. 1 - in at least one case the light output at a terminus of one of the present branches 114A, 114B, 114C may be produced by generating an optical supercontinuum as understood in related art, while neither of the other light outputs from the remaining branches is formed with the use for formation on an optical supercontinuum and while, at the same time, each of the constituent light outputs is characterized by the same pulse repetition rate as any of these light outputs. Alternatively or in addition, the aggregate output spectral bandwidth of such multiple light outputs generated by the light- transforming sub-system of the embodiment of the overall all-fiber laser system of the invention may define a supercontinuum optical bandwidth. The disclosed aspects may be combined in ways not listed above. Accordingly, the invention should not be viewed as being limited to the disclosed embodiment(s).

Claims

CLAIMS What is claimed is:

1. An all-fiber laser system comprising: a single optical-fiber laser cavity containing a gain medium and configured to generate femtosecond cavity pulses of light having an optical spectrum when the gain medium is pumped with pump light at a predetermined wavelength, wherein the optical spectrum includes a target central wavelength and a target spectral bandwidth, an optical-fiber amplifier device operably cooperated with the single laser cavity to receive the cavity pulses of light and configured to produce, at an output of the optical-fiber amplifier device, a first laser output at a first wavelength, said first laser output having a first spectral bandwidth; at least one optical-fiber wavelength-conversion device operably connected to the single laser cavity to receive the cavity pulses of light, the at least one wavelength-conversion device configured to produce at least one corresponding auxiliary laser output at a corresponding auxiliary wavelength, the at least one auxiliary laser output having a corresponding auxiliary spectral bandwidth, wherein, the auxiliary wavelength and the auxiliary spectral bandwidth of the at least one auxiliary laser output are different from the first wavelength and the first spectral bandwidth, respectively, and wherein each of the first laser output and the at least one auxiliary laser output contain respective trains of pulses of light that are synchronized with the cavity pulses of light while having equal repetition rates.

2. An all-fiber laser system according to claim 1, wherein each of a repetition rate of a train of pulses in the first laser output and a repetition rate of a train of pulses of the at least one auxiliary laser output is substantially equal to a repetition rate of the femtosecond cavity pulses.

3. An all -fiber laser system according to claim 1, wherein at least one of the following conditions is satisfied:

(3a) the first wavelength is substantially equal to the target central wavelength;

(3b) wherein the at least one auxiliary laser output includes first and second auxiliary laser outputs, the first auxiliary laser output having a first auxiliary wavelength and a first auxiliary spectral bandwidth, the second auxiliary laser output having a second auxiliary wavelength and a second spectral bandwidth, and wherein the first auxiliary wavelength is longer than the first wavelength while the second auxiliary wavelength is shorter than the first wavelength;

(3c) wherein the optical-fiber amplifier device and each of optical-fiber wavelength-conversion devices present in the all-fiber laser system are connected to one another in parallel; and

(3d) wherein, when the at least one optical fiber wavelength -conversion device includes multiple optical-fiber wavelength conversion devices, corresponding auxiliary wavelengths and corresponding auxiliary spectral bandwidths at corresponding multiple auxiliary laser outputs are different from one another, respectively.

4. An all-fiber laser system according to claim 3, wherein the at least one optical-fiber wavelengthconversion device includes multiple wavelength-conversion devices and at least one of the following conditions is satisfied:

(4a) each of said multiple wavelength-conversion devices includes a respective optical-fiber amplifier component, said optical-fiber amplifier component being spatially-distinct from any other optical- fiber amplifier component of another of said multiple wavelength-conversion devices;

(4b) each of the optical-fiber amplifier device and the multiple wavelength-conversion devices is connected to the single laser cavity via an optical-fiber splitter;

(4c) each of the optical-fiber amplifier device and the multiple wavelength-conversion devices includes a corresponding optical pulse-compressor element configured to reduce durations of pulses of light passing therethrough; and

(4d) the all-fiber laser system includes an optical pulse-compressor element disposed to be separated from the single laser cavity by a corresponding optical-fiber splitter and an optical-fiber amplifier device to prevent breaking of a pulse of light of the pulses of light that propagate through the optical pulsecompressor element.

5. An all-fiber laser system according to claim 3, wherein the at least one optical-fiber wavelength-conversion device includes multiple optical-fiber wavelength-conversion devices, and wherein at least one of the following conditions is satisfied: (5a) a first wavelength-conversion device of said multiple wavelength-conversion devices is configured to generate a first auxiliary laser output by red-shifting the first wavelength to a first auxiliary wavelength as a result of intra-pulse stimulated Raman scattering, wherein the first auxiliary laser output includes said first auxiliary wavelength and a first auxiliary spectral bandwidth; and

(5b) a second wavelength-conversion device of said multiple wavelengthconversion devices is configured to generate a second auxiliary laser output by forming an optical supercontinuum, wherein the second auxiliary laser output includes a second auxiliary wavelength and a second auxiliary spectral bandwidth.

6. An all-fiber laser system according to claim 1, wherein the at least one optical-fiber wavelength-conversion device includes multiple optical-fiber wavelength-conversion devices, and wherein at least one of the following conditions is satisfied:

(6a) a first wavelength-conversion device of said multiple wavelength-conversion devices is configured to generate a first auxiliary laser output by red-shifting the first wavelength to a first auxiliary wavelength as a result of intra-pulse stimulated Raman scattering, wherein the first auxiliary laser output includes said first auxiliary wavelength and a first auxiliary spectral bandwidth; and

(6b) a second wavelength-conversion device of said multiple wavelengthconversion devices is configured to generate a second auxiliary laser output by forming an optical supercontinuum, wherein the second auxiliary laser output includes a second auxiliary wavelength and a second auxiliary spectral bandwidth.

7. An all-fiber laser system according to claim 1, wherein the at least one optical-fiber wavelength-conversion device includes multiple wavelengthconversion devices and at least one of the following conditions is satisfied:

(7a) each of said multiple wavelength-conversion devices includes a respective optical-fiber amplifier component, said optical-fiber amplifier components being spatially- distinct from one another;

(7b) each of the optical -fiber amplifier device and the multiple wavelengthconversion devices is connected to the single laser cavity via an optical-fiber splitter;

18 (7c) each of the optical-fiber amplifier device and the multiple wavelengthconversion devices includes a corresponding optical pulse-compressor element configured to reduce durations of pulses of light passing therethrough; and

(7d) the all-fiber laser system includes an optical pulse-compressor element disposed to be optically separated from the single laser cavity by a corresponding optical- fiber splitter and an optical-fiber amplifier device to prevent breaking of a pulse of light of the pulses of light that propagate through the optical compressor element.

8. A method comprising: in an all-fiber laser system, reproducing a seed train of femtosecond pulses of light formed in a laser cavity to create multiple trains of femtosecond pulses of light, the seed train of femtosecond pulses having a repetition rate; generating a first train of femtosecond pulses of light at a first wavelength by amplifying a first chosen train of the multiple trains of femtosecond pulses of light while propagating said first chosen train along a first optical path; generating a second train of femtosecond pulses of light at a second wavelength that is different from the first wavelength by transforming a second chosen train of the multiple trains of the femtosecond pulses along a second optical path that is spatially-different from the first optical path; and wherein each of the first and second trains of femtosecond pulses is a) synchronized with the seed train of femtosecond pulses as defined by spatial coordination between the first and second optical paths with the laser cavity, and b) has the repetition rate.

9. A method according to claim 8, wherein the second wavelength is longer than the first wavelength, and further comprising: generating a third train of femtosecond pulses of light at a third wavelength that is shorter than the first wavelength by transforming a third chosen train of the multiple trains of femtosecond pulses while propagating the third chosen train along a third optical path, the third optical path being spatially-different from both the first optical path and the second optical path;

19 wherein the third train of femtosecond pulses has the repetition rate and is synchronized with the seed train of femtosecond pulses as defined by spatial coordination between the third optical path and the laser cavity.

10. A method according to claim 9, wherein at least one of the following conditions is satisfied:

(10a) amplifying pulses of light in at least one of the multiple trains of femtosecond pulses after said replicating, and temporally compressing pulses of light in at least one of the multiple trains of femtosecond pulses upon propagation thereof through the laser system outside the laser cavity, after said amplifying to prevent breaking of the pulses of light upon propagation thereof along at least one of the first, second, and third optical paths;

(10b) wherein the generating the second train of femtosecond pulses includes red-shifting a wavelength of light propagating along the second optical path by intra-pulse Raman-scattering of said light in an optical fiber; and

(10c) wherein the generating the third train of femtosecond pulses is followed by generating light with optical supercontinuum bandwidth including the third wavelength in the third optical path.

11. A method according to claim 8, wherein said generating the second train of femtosecond pulses includes at least one of the following:

(l la) spectrally tuning a central wavelength of a spectral bandwidth of pulses in the second train by adjusting pump power delivered to an optical-fiber amplifier disposed in the second optical path;

(l lb) defining an initial value of said central wavelength of the spectral bandwidth of the pulses in the second train by pre-determining at least one of a length, a core size, a dispersion characteristic, a V- number of at least one of a pulse-compression optical-fiber element, and a soliton self-frequency shift optical- fiber element disposed at the second optical path; and

(l lc) amplifying pulses of light in at least one of the multiple trains of femtosecond pulses of light.

12. A method according to claim 8, comprising: defining said repetition rate to be substantially the same for each of the first, second, and third trains of pulses and temporally synchronizing said first, second, and

20 third trains of pulses with the seed train of pulses by directing respective portions of light formed in the laser cavity along the first, second, and third optical paths without a use of free-space propagation of said light.

13. A method comprising: in an all-fiber laser system, forming a laser light output containing multiple output trains of pulses with an aggregate output spectral bandwidth defining a supercontinuum optical bandwidth, each output train having the same repetition rate as any other output train from said multiple trains, by replicating an initial train of femtosecond pulses generated in a laser cavity of said all-fiber laser system to form multiple intermediate trains of femtosecond pulses each of which is temporally synchronized with the initial train of femtosecond pulses; and spatially-distinctly transforming each of said multiple intermediate trains of femtosecond pulses to said multiple output trains of pulses having different respective output spectral bandwidths and different output central wavelengths.

14. A method according to claim 13, comprising: temporally compressing a pulse of light propagating through said all-fiber laser system after amplifying said pulse to prevent breaking of said pulse of light.

15. A method according to claim 13, further comprising delivering and least first and second of said multiple output trains of pulses from different output ends of the all -fiber laser system.

16. A method according to claim 13, comprising: intra-pulse Raman scattering light in pulses of a first intermediate train of said multiple intermediate trains of femtosecond pulses to red-shift a first central wavelength of a first spectral bandwidth of said first intermediate train.

17. A method according to claim 16, comprising: temporally compressing a pulse of light propagating through said all-fiber laser system after amplifying said pulse to prevent breaking of said pulse of light.

18. The method according to claim 17, wherein said temporally compressing includes propagating a frequency-chirped pulse of light through a hollow-core photonic-crystal optical fiber.

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19. A method according to claim 13, wherein said spatially-distinctly transforming includes modifying optical characteristics of pulses in the multiple intermediate trains by passing said pulses through optical- fiber-based devices disposed in spatially-distinct from one another optical -fiber branches of the all-fiber laser system to define a first output wavelength to be approximately 1,550 nm, a second output wavelength to be approximately 2,000 nm, and a third output wavelength to be approximately 1,000 nm while delivering pump laser light to at least two locations of at least one of said optical-fiber branches.

20. A method according to claim 13, comprising amplifying pulses of light of at least one of the multiple intermediate trains after said replicating.

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