GIANT-CHIRP ALL-NORMAL-DISPERSION SUB-NANOSECOND FIBER OSCILLATOR

Abstract

A single mode fiber pulsed oscillator includes an all normal dispersion ring cavity provided with a mode-locking fiber loop component and a giant chirp generating fiber component. The mode-locking fiber loop component is configured with a hybrid of NOLM and NALM configurations which is operative to induce a first phase acquisition of a spectrally narrow pulse due to SPM. The giant chirp generating fiber loop component is configured to induce the additional phase acquisition to the pulse broadened in the mode-locking fiber component so as to generate a pulse with a giant chirp. The fiber loop components each include a fiber amplifier and a coil of fiber. The amplifiers each are configured with an active fiber provided with a core which supports multiple transverse mode in a range of wavelength except for the desired wavelength at which the core is configured to support a single fundamental mode.

Claims

1. An all normal dispersion self-starting single mode (SM) pulsed fiber oscillator, comprising: an 8-shaped resonant ring cavity provided with: a mode-locking fiber loop component configured to provide a pulse with a first phase acquisition due to a Self-Phase Modulation (SPM) phenomenon, and a giant chirp generating fiber loop component receiving the pulse with the first phase acquisition and configured to provide the pulse with a second phase acquisition due to the SPM phenomenon, wherein the second phase acquisition is so greater than the first phase acquisition that the giant chirp generating fiber component outputs the pulse with a giant chirp, the mode-locking and giant chirp generating fiber components including respective first and second fiber amplifiers, the fiber amplifiers each being based on an active fiber with a multimode core (MM) which is configured to support propagation of a single fundamental mode at a desired wavelength in a 1 micron wavelength range.

2. The SM pulsed fiber oscillator of claim, wherein the mode locking fiber loop component is configured with an interferometric structure including a Polarization maintaining linearly polarized (LP) fused fiber coupler which provides optical communication between the mode-locking and giant chirp generating fiber components.

3. The SM pulsed fiber oscillator of claim 2, wherein the first fiber amplifier of the mode locking fiber loop component is located asymmetrically relative to the LP fiber coupler.

4. The SM pulsed fiber oscillator of claim 3, wherein the mode locking fiber loop component further includes a first coil of SM passive fiber located between an output of the first fiber amplifier and the fiber coupler.

5. The SM fiber oscillator of claim 4, wherein the output coupler has an asymmetric structure so that the mode locking fiber loop component is configured as a hybrid of NOLM and NALM architectures.

6. The SM pulsed fiber oscillator of claim 2, wherein the giant chirp generating fiber component further includes a second coil of SM passive fiber coupled to an output of the second fiber amplifier and a first linear polarized isolator coupled between the fused coupler and an input of the second fiber amplifier and preventing coupling of radiation backreflected from the second fiber amplifier into the first fiber amplifier.

7. The SM pulsed fiber oscillator of claim 6, wherein the giant chirp generating fiber component further includes a second linearly polarized isolator coupled between an output of the second fiber amplifier and the fused coupler and operative to prevent coupling of radiation reflected from the fused coupler into the output of the second fiber amplifier.

8. The SM pulsed fiber oscillator of claim 2, wherein the giant chirp generating fiber component further includes a filter providing periodic narrowing of spectral and temporal shapes of the pulse.

9. The SM pulsed fiber oscillator of claim 7, wherein the giant chirp generating fiber component further includes an output coupler between the output of the second fiber amplifier and second linearly polarized isolator, the output coupler being a beam splitter.

10. The SM pulsed fiber oscillator of claim 1, wherein the MM active fiber has a double bottleneck shaped cross-section.

11. The SM pulsed fiber oscillator of claim 1 further comprising a plurality of MM pumps each coupled to the active fiber to define a side pumping technique.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] The features and advantages of the disclosed oscillator will become more readily apparent from the following specific description accompanied by the drawings, in which:

[0056] FIGS. 1A and 1B illustrate broadening of a linear chirped pulse in spectral domain due SPM in material with normal dispersion;

[0057] FIGS. 2A and 2B illustrate broadening of a chirped pulse in time domain due SPM in material with normal dispersion;

[0058] FIG. 3 illustrates an exemplary schematic of passively mode-locked fiber laser through nonlinear polarization rotation;

[0059] FIG. 4A illustrates an exemplary schematic of the NOLM;

[0060] FIG. 4B illustrates an exemplary schematic of the NALM;

[0061] FIG. 5 illustrates an exemplary schematic of known passively mode-locked fiber laser through NOLM/NALM architecture;

[0062] FIG. 6 illustrates a schematic of a passively mode-locked laser configured in accordance with the disclosure;

[0063] FIG. 7 illustrates the operation of the fused coupler incorporated in the mode-locking component of the disclosed oscillator;

[0064] FIG. 8 illustrates a schematic of gain block incorporated in the disclosed laser of FIG. 6;

[0065] FIG. 9 illustrates a schematic of altered gain block in the laser of FIG. 6.

[0066] FIG. 10 illustrates an output spectrum obtained in one of the experimental devices configured in accordance with FIG. 6.

[0067] FIG. 11 illustrates another output spectrum in a different experimental device configured in accordance with FIG. 6. Destroy

[0068] Throughout the drawings, similar components are denoted by identical reference numerals.

SPECIFIC DESCRIPTION

[0069] By way of introduction, the disclosed passively mode-locked oscillator is configured with a novel all normal dispersion interferometric architecture enabling a stable mode-locked operation which results in picosecond, self-similar parabolic pulses emitted in a 1 micron wavelength range and having an output pulse energy of up to tens of nano joules (nJ).

[0070] FIG. 6 illustrates the all-normal dispersion fiber oscillator 100 configured with an 8-shaped all fiber or integrated components laser ring cavity void of free space. The ring cavity includes a giant chirped pulse forming fiber loop component, an interferometric mode-locking fiber loop component 104, and a fused coupler 106 providing light communication between the fiber loop components. The mode-locking fiber loop component 104 is operative to convert continuous radiation to pulsed radiation so that a pulse acquires a first phase acquisition due SPM, whereas the pulse forming fiber loop component 102 is operative to provide the pulse broadened in the mode-locking component 104 with a greater phase acquisition so as to output a giant-chirp pulse.

[0071] The oscillator 100 is self-starting and operates in the following manner. As fiber amplifiers 108 and 110 of respective components 102 and 104 are turned on, a random signal—white noise present in the fiber components is amplified. At a certain point of time, a first beat notch or spike with a relatively high amplitude builds up its intensity over multiple round-trips around the ring cavity while slightly spectrally broadening. The rest of the spectrum undergoes certain amplification, but compared to the amplification of the spike, it is insignificant. Every round trip the spike is further amplified and spectrally and temporally broadened. At a certain point of time, the intensity of the spike is amplified to the desired peak level capable of inducing SPM in mode-locking loop component 104 configured with a fused coupler 118, first fiber amplifier 118 and a fiber coil 114.

[0072] The operation of any oscillator is subject to a periodic boundary condition including the substantial uniformity of the pulse temporal and spectral shapes. To meet this condition, the broadened spike is processed in a pulse-forming dissipative component 116 once a spectral linewidth of the spike approaches that one of pulse forming component 116 of pulse forming fiber loop component 102 which may be configured as an inline filter or an off-line circulator with fiber Bragg gratings. The pulse-forming component 116 cuts out a segment out of the broadened spike to form a pulse with a narrow spectral line and also reduces the duration of the spike. The spectral and temporal shapes of thus formed pulse may mimic those of the initial spike.

[0073] This circulation around the ring cavity continues with the intensity of the pulse gradually increasing to the peak intensity which is sufficient to trigger nonlinear processes in first fiber coil 112, such as SPM, of mode-locking fiber loop component 104 providing the pulse with a phase acquisition. In other words, the pulse starts acquiring additional spectral components or modes in opposite increasing and decreasing wavelength directions in fiber coil 112 upon amplification in amplifier 110. This leads to a first spectral and temporal broadening of the pulse with consecutive longitudinal modes being delayed in time relative to one another in such a manner that a phase changes linearly across the pulse. The result of the above disclosed mechanism is the formation of the linear chirp shown in FIG. 1B having its peak intensity being now stabilized, i.e., while the pulse keeps propagating further through chirp generating fiber component 102 where it propagates through fiber amplifier 108 and second fiber coil 114. The second amplification and broadening provides the pulse with a phase acquisition greater than that acquired in mode-locking fiber component 104. The pulse further propagates along fiber component 102 through pulse-shaping element 116 where its spectral and temporal width are reduced before it is guided again through mode-locking component 104 where its peak intensity remains either the same or slightly lowers. From this point on, all the energy pumped in the pulse as it propagates through amplifiers 110 and 108 is redistributed among the longitudinal mode to broaden the pulse in both spectral and temporal domains. Thus, oscillator 100 moves from brief CW regime to a self-starting mode-locked regime.

[0074] Turning specifically to mode-locking component 104, upon coupling into coupler 118, the high intensity, spectrally and temporally shortened pulse is split in propagating and counter-propagating directions along the component 106. The latter has an interferometric architecture and may be configured as a NOLM, NALM or a hybrid of these, i.e., a combination of an asymmetric coupler and amplifier, with the latter being shown in FIG. 6

[0075] Referring to FIG. 7 in addition to FIG. 6, the configuration of mode-locking component 104 is operative to provide the pulses propagating in respective opposite directions with different amplitudes since the interferometric architecture requires that the signals propagate along respective different optical paths, which in turns requires different peak intensities. In light of this requirement, the physical path between asymmetric coupler 118 and amplifier 110 of the counterclockwise propagating signal Ice (FIG. 6) is longer than the path between the coupler and amplifier of the clockwise propagating signal Ic because of coil 112. Accordingly, the clockwise propagating pulse replica Ic is coupled into amplifier 110 first with the intensity thereof being increased in the amplifier so that, while it further propagates through coil 112, this pulse has a first phase acquisition.

[0076] On the other hand, counterclockwise propagating pulse replica Icc is guided through coil 112 with a relatively low intensity since it has not been yet amplified, and therefore its intensity is lower than that of the clockwise propagating replica Ic. Consequently, its phase acquisition within coil 112 is smaller than that of the clockwise pulse replica Ic because, as discussed above in detail, it is the intensity magnitude that defines the phase acquisition. After counterclockwise pulse replica Icc is amplified in amplifier 110, it is coupled into coupler 118 with the amplitude practically matching that of the clockwise propagating pulse, but its phase is different. The replicas Ic and Icc further propagate through fused coupler 118 where they have respective overlapping spectral zones 122 and 125 (FIG. 7) which constructively or distractively interfere with one another depending on the phase. As a result, high intensity light having a substantially parabolic spectrum is further transmitted through coupler 118 in a clockwise direction (circular directions are exemplary) along chirp generating fiber component 102. There the pulse replica undergoes a much greater broadening in both domains than that in mode-locking component 104 than in component 102 and is finally coupled out through an output coupler or beam splitter 124 with a giant chirp. The low intensity light replica Icc propagating along mode locking component 104 and coupler 118 in a counterclockwise propagating direction is reflected into component 102 in a direction opposite to that of high intensity pulse replica Ic, and its further propagation is prevented in isolator 120.

[0077] Referring to FIGS. 8 and 9, amplifiers 108 and 110 each include a combination of ytterbium (Yb) doped fiber 128 having its opposite ends which are spliced to respective input and output passive fibers 130. The Yb doped fiber has a core 132 capable of supporting multiple transverse modes (MM). However, at the wavelength of interest, for example 1.06 micron, core 132 is configured to support only one fundamental mode. This configuration is realized by doping MM core 132 with not light emitting dopants which provide this core with a mode field diameter (MFD) substantially matching that of single mode (SM) passive fibers 130. As a result, when SM light is coupled into MM core 132 of Y-doped fiber 128, it excites only a fundamental transverse mode which, as known to artisan, has close to Gaussian shape intensity profile similar to that of SM fibers. As a consequence, generated pulses emitted from oscillator 100 each carry a single mode radiation.

[0078] The MM fibers provide amplifiers with the opportunity to use a side pumping scheme which may have certain advantages over an end pumping scheme with necessarily in conjunction with the SM active fibers. First, the side-pumping scheme does not require the use of wavelength division multiplexer (WDM) that can tolerate only limited powers. As a consequence, the other advantage of the side pumping scheme is the possibility of generating pulses with powers higher than those of SM amplifiers.

[0079] FIG. 9 illustrates MM Yb-doped fiber 128 having a double bottleneck-shaped cross section. This modification provides a central enlarged core part 132 with a greater diameter than that of core ends 134. The core ends 134 are configured similar to the ends of FIG. 8 and each have an MFD matching that of SM passive fibers. Alternatively, as numerous experiments show, MM Yb-doped fiber 128 may have a uniformly dimensioned core.

[0080] Returning briefly to FIG. 6, oscillator 100 has an additional LP isolator 126 to prevent backreflection from amplifier 108 to amplifier 110. All of the components of oscillator 100 are operated with linearly polarized light. Due to its flexibility any of isotropic fibers can lose the desired polarization which inevitably would lead to unstable pulses. To prevent it, all of the components of the disclosed ring cavity, including both isolators 120 and 126, are polarization maintaining. All fibers except Yb-doped fibers are SM PM fibers.

[0081] Preliminary experiments using oscillator 100 of FIG. 6 brought encouraging results. For example, FIG. 10 illustrates a smooth output spectrum. However, the output pulses in this experiment were rather energy unstable. FIG. 11, in contrast, shows ripples in the output spectrum, but the pulse energy is stable. The drawbacks shown in respective FIGS. 10 and 11 can be easily fixed with better manufactured components. FIG. 12 illustrates a pulse train with a uniform interval between stable pulses. Finally, FIG. 12 illustrates a giant chirped pulse having a 12 ps duration at half the maximum peak which is about 2 kW. Further preliminary experiments with giant chirped pulses showed excellent compression to a pulse duration below 200 fs.

[0082] All the features disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. Any feature, whether preferred or not, may be combined with any other feature, whether preferred or not.