Theory and experiment of transient two-wave mixing in Yb-doped single-frequency fiber amplifiers induced by frequency modulation.

This paper presents a theoretical and experimental characterization of an instability phenomenon observed in single-frequency fiber amplifiers when the frequency of the seed laser is modulated. The instability manifests itself as fluctuating elastic back-reflections that occur only when the frequency is decreasing with time. The theory is a generalization of a coupled-mode model developed for a single-frequency fiber amplifier back-seeded with a constant frequency shift relative to the main signal. It can explain most observed features of the experiments in a qualitative and semi-quantitative way. Open questions and directions for further developments are also discussed.

Dispersive wave generation in single higher-order modes of a large-core silica step-index fiber with pulse energies up to 12 nJ.

The generation of light in a laser system is constrained by the gain medium, limiting the available wavelengths. We demonstrate in-fiber generation of ultrafast pulses between ∼550 and 800 nm via dispersive wave generation (DWG), in higher-order modes (HOMs). Using higher-order modes enables power scaling, due to their large effective area compared to the fundamental modes of single-mode fibers and dispersion engineering, even in simple step-index fibers. The process occurs in a single higher-order mode, which we excite using passive glass components (an axicon and two telescopes). The output pulses have energies up to 12 nJ at the biologically relevant wavelength of 705 nm.

Power scaling of normal-dispersion continuum generation using higher-order modes in microstructured optical fibers.

The use of a higher-order HE12-like mode to produce weak normal dispersion over a substantial wavelength range in a microstructured optical fiber is investigated numerically. It is shown that the effective area, and thereby the pulse energy, can in this way be scaled by an order of magnitude compared to using the fundamental mode in a single-mode fiber. Multimode nonlinear simulations indicate that nonlinear mode coupling will not disturb single-mode operation in the HE12 mode at least up to the threshold where polarization modulation instability sets in.

Observation of two-wave mixing in a single-frequency fiber amplifier induced by frequency modulation.

We report on the observation of unstable two-wave mixing in a Yb-doped optical fiber amplifier induced by frequency modulation of a single-frequency laser. What is believed to be a reflection of the main signal experiences a gain much higher than that provided by the optical pumping and potentially limits power scaling under frequency modulation. We propose an explanation for the effect based on the dynamic population and refractive index gratings formed by the interference between the main signal and its slightly frequency-detuned reflection.

Observation of dynamical eigenmodes induced by the moving refractive index grating of TMI in a rod amplifier.

We report a novel, to the best of our knowledge, analysis of high power rod fiber amplifiers by monitoring the cross-polarization of the output. Spatially and temporally resolved imaging of co- and cross-polarizations at high power amplification reveals dynamic eigenmode behavior of the rod fiber. The dynamic of the eigenmodes is caused by the moving refractive index grating written by the modal interference pattern of transverse mode instability and is the first direct observation of this refractive index grating, to our knowledge.

Experimental investigations of seeding mechanisms of TMI in rod fiber amplifier using spatially and temporally resolved imaging.

In this work we investigate transverse mode instability (TMI) in the presence of pump intensity noise and a controlled perturbation of the input coupling for a rod-type fiber amplifier using spatially and temporally resolved imaging (ST). We show that inherent pump intensity noise from the power supply can define significant peaks in the resulting TMI spectrum. ST measurements show that the TMI in the transition region consists of different orientations of LP11. This finding indicates that the simple picture of TMI being seeded by the combination of a static initial fraction of LP11 and pump or signal intensity noise is not valid for our measurements. Furthermore we present seeding of TMI by perturbing the input coupling dynamically. ST measurements of the resulting TMI as a function of perturbation frequency provides quantitative information regarding the frequency response of the non-linear coupling coefficient. Finally, ST measurements of the resulting TMI as a function of signal power shows that the TMI experiences an exponential gain long before visible beam fluctuations appear.

Multimode nonlinear simulation technique having near-linear scaling with mode number in circular symmetric waveguides: publisher's note.

This publisher's note contains a corrections to Opt. Lett.45, 4160 (2020).OPLEDP0146-959210.1364/OL.398412.

Static and dynamic mode coupling in a double-pass rod-type fiber amplifier.

This Letter describes an experimental realization of a double-pass amplifier using rod-type fiber. In this device, the gain reaches 26 dB amplifying a 300 mW, 20 ps, 20 MHz seed up to 120 W, with an optical-to-optical efficiency of 50% and excellent beam quality. In addition, by design the output of the amplifier has a polarization extinction ratio of 33 dB. Besides these good performances, we report a marginal degradation of mode quality and degree of polarization followed by the so-called transverse mode instability which occurs at 120 W signal power. The first degradation is static, and by analyzing its two polarizations, we conclude it is caused by a coupling between modes due to the formation of a static thermal long-period grating, which in turn initiates the dynamic instability.

Theory of thermo-optic instabilities in dual-core fiber amplifiers.

A coupled-mode theory for nonlinear mode coupling by the thermo-optic effect, originally developed for single-core fiber amplifiers, is applied to the case of dual-core amplifiers. It is shown that a non-phase-matched coupling term, which is usually irrelevant in single-core amplifiers, can strongly affect the mode stability when the coupling length between supermodes exceeds a few centimeters. The phase-mismatched coupling can lead to a strongly reduced instability threshold and static deformation effects for a range of intermediate coupling lengths.

Spatial beam cleanup by pure Kerr processes in multimode fibers.

Recent experiments with pulse propagation in multimode graded-index fibers have shown a nonlinear improvement in beam quality, even in situations where dissipative processes such as Raman scattering play no significant role. In this Letter, numerical simulations of beam cleanup by third-order Kerr nonlinearities in a multimode fiber are used to demonstrate that in the absence of dissipative processes beam cleanup is crucially dependent on spectral/temporal disorder and does not occur in a continuous-wave model. This finding is in accordance with fundamental considerations on entropy.

Cherenkov radiation from 1550 nm pumping in tapered photonic crystal fibers.

The generation of Cherenkov radiation from soliton compression of 1550 nm pulses in tapered photonic crystal fibers is analyzed numerically, with a view to generating short-wavelength-tunable output pulses in the visible range. It is shown that low-noise femtosecond light sources with spectral power densities approaching those of existing supercontinuum sources are feasible with existing fiber laser and tapering technology.

Flexible cross-correlated (C2) imaging method for the modal content characterization in a broad range of wavelengths.

We demonstrate a flexible cross-correlated (C2) imaging method in the time domain by application of a tunable and highly flexible light source. An advantage of the flexible C2 method is shown by characterization of the step-index fiber (SMF28) over a broad range of wavelengths from 870nm to 1090nm and by the modal analysis of the distributed modal filtering (DMF) rod fiber within a wavelength range from 1050nm to 1090nm. Also, the influence of the spectral shape and bandwidth on the imaging trace is investigated by deliberately adjusting the input spectrum of the light source. The modal intensity as well as the phase distribution are extracted by the alternative method of 2D FT filtering. Being exceptionally tunable the flexible C2 method gives an ability to adapt the system's parameters in a desired manner satisfying even measurements of very specific fiber designs opening up new possibilities for advanced modal characterization of fibers over broad range of wavelengths.

Nonlinearity-tailored fiber laser technology for low-noise, ultra-wideband tunable femtosecond light generation.

The emission wavelength of a laser is physically predetermined by the gain medium used. Consequently, arbitrary wavelength generation is a fundamental challenge in the science of light. Present solutions include optical parametric generation, requiring complex optical setups and spectrally sliced supercontinuum, taking advantage of a simpler fiber technology: a fixed-wavelength pump laser pulse is converted into a spectrally very broadband output, from which the required resulting wavelength is then optically filtered. Unfortunately, this process is associated with an inherently poor noise figure, which often precludes many realistic applications of such super-continuum sources. Here, we show that by adding only one passive optical element-a tapered photonic crystal fiber-to a fixed-wavelength femtosecond laser, one can in a very simple manner resonantly convert the laser emission wavelength into an ultra-wide and continuous range of desired wavelengths, with very low inherent noise, and without mechanical realignment of the laser. This is achieved by exploiting the double interplay of nonlinearity and chirp in the laser source and chirp and phase matching in the tapered fiber. As a first demonstration of this simple and inexpensive technology, we present a femtosecond fiber laser continuously tunable across the entire red-green-blue spectral range.

Stain-free histopathology by programmable supercontinuum pulses.

The preparation, staining, visualization, and interpretation of histological images of tissue is well-accepted as the gold standard process for the diagnosis of disease. These methods were developed historically, and are used ubiquitously in pathology, despite being highly time and labor intensive. Here we introduce a unique optical imaging platform and methodology for label-free multimodal multiphoton microscopy that uses a novel photonic crystal fiber source to generate tailored chemical contrast based on programmable supercontinuum pulses. We demonstrate collection of optical signatures of the tumor microenvironment, including evidence of mesoscopic biological organization, tumor cell migration, and (lymph-)angiogenesis collected directly from fresh ex vivo mammary tissue. Acquisition of these optical signatures and other cellular or extracellular features, which are largely absent from histologically processed and stained tissue, combined with an adaptable platform for optical alignment-free programmable-contrast imaging, offers the potential to translate stain-free molecular histopathology into routine clinical use.

Static thermo-optic instability in double-pass fiber amplifiers.

A coupled-mode formalism, earlier used to describe transverse mode instabilities in single-pass optical fiber amplifiers is extended to the case of double-pass amplifiers. Contrary to the single-pass case, it is shown that the thermo-optic nonlinearity can couple light at the same frequency between the LP01 and LP11 modes, leading to a static deformation of the output beam profile. This novel phenomenon is caused by the interaction of light propagating in either direction with thermo-optic index perturbations caused by light propagating in the opposite direction. The threshold power for the static deformation is found to be several times lower than what is typically found for the dynamic modal instabilities observed in single-pass amplifiers.

Hollow-core fibers for high power pulse delivery.

We investigate hollow-core fibers for fiber delivery of high power ultrashort laser pulses. We use numerical techniques to design an anti-resonant hollow-core fiber having one layer of non-touching tubes to determine which structures offer the best optical properties for the delivery of high power picosecond pulses. A novel fiber with 7 tubes and a core of 30µm was fabricated and it is here described and characterized, showing remarkable low loss, low bend loss, and good mode quality. Its optical properties are compared to both a 10µm and a 18µm core diameter photonic band gap hollow-core fiber. The three fibers are characterized experimentally for the delivery of 22 picosecond pulses at 1032nm. We demonstrate flexible, diffraction limited beam delivery with output average powers in excess of 70W.

Progress in Cherenkov femtosecond fiber lasers.

We review the recent developments in the field of ultrafast Cherenkov fiber lasers. Two essential properties of such laser systems - broad wavelength tunability and high efficiency of Cherenkov radiation wavelength conversion are discussed. The exceptional performance of the Cherenkov fiber laser systems are highlighted - dependent on the realization scheme, the Cherenkov lasers can generate the femtosecond output tunable across the entire visible and even the UV range, and for certain designs more than 40 % conversion efficiency from the pump to Cherenkov signal can be achieved. The femtosecond Cherenkov laser with all-fiber architecture is presented and discussed. Operating in the visible range, it delivers 100-200 fs wavelength-tunable pulses with multimilliwatt output power and exceptionally low noise figure an order of magnitude lower than the traditional wavelength tunable supercontinuum-based femtosecond sources. The applications for Cherenkov laser systems in practical biophotonics and biomedical applications, such as bio-imaging and microscopy, are discussed.

Optimizing Yb concentration of fiber amplifiers in the presence of transverse modal instabilities and photodarkening.

The Yb concentration of double-clad optical fiber amplifiers is numerically optimized with respect to maximizing the transverse modal instability threshold in the presence of absorption arising from photodarkening. The pump cladding area is scaled with the Yb concentration to approximately maintain the pump absorption in operation. It is found that approximate analytical expressions can predict the optimized concentration levels found in numerical simulations with sufficient accuracy to be useful in fiber design.

Optical frequency standard using acetylene-filled hollow-core photonic crystal fibers.

Gas-filled hollow-core photonic crystal fibers are used to stabilize a fiber laser to the 13C2H2 P(16) (ν1+ν3) transition at 1542 nm using saturated absorption. Four hollow-core fibers with different crystal structure are compared in terms of long term lock-point repeatability and fractional frequency instability. The locked fiber laser shows a fractional frequency instability below 4 × 10(-12) for averaging time up to 10(4) s. The lock-point repeatability over more than 1 year is 1.3 × 10(-11), corresponding to a standard deviation of 2.5 kHz. A complete experimental investigation of the light-matter interaction between the spatial modes excited in the fibers and the frequency of the locked laser is presented. A simple theoretical model that explains the interaction is also developed.

Intermodal and cross-polarization four-wave mixing in large-core hybrid photonic crystal fibers.

Degenerate four-wave mixing is considered in large mode area hybrid photonic crystal fibers, combining photonic bandgap guidance and index guidance. Co- and orthogonally polarized pump, signal and idler fields are considered numerically by calculating the parametric gain and experimentally by spontaneous degenerate four-wave mixing. Intermodal and birefringence assisted intramodal phase matching is observed. Good agreement between calculations and experimental observations is obtained. Intermodal four-wave mixing is achieved experimentally with a conversion efficiency of 17%.