Vibrational dephasing times for benzene and carbon disulfide are measured using a custom single-beam Coherent Anti-Stokes Raman Spectroscopy (CARS) setup. A femtosecond oscillator is used to pump a polarization maintaining all normal dispersion photonic crystal fibre (PM-ANDi-PCF) to generate a broad band supercontinuum, covering a spectral region from 680 to 900 nm. The dispersion properties of the PM-ANDi-PCF ensures the supercontinuum is stable and there exists a fixed phase relationship between the spectral components of the supercontinuum. This enables its temporal compression using i2PIE, implemented using a liquid crystal spatial light modulator (SLM) in a 4f geometry. This SLM is also used to shape the pulse spectrally and temporally. With this setup we could demonstrate time-resolved CARS, measuring the vibrational relaxation times of a carbon disulfide (CS2)/benzene mixture, and eliminate the non-resonant background completely. The main advantage of this setup is the fact that it is a single beam technique, eliminating the requirement for aligning the overlap of the pump and probe, both spatially and temporally, in the focal plane of the microscope. The strengths and limitations of the technique are highlighted and the route to time-resolved/background free vibrational microscopy is proposed.

AbstractUltrafast and low-noise supercontinuum (SC) sources based on all-normal dispersion (ANDi) fibers are emerging as key-enabling technology for new applications in spectroscopy, microscopy, and ultrafast photonics. In this chapter we review the fundamental physics, fiber designs, and applications of this unique white light source.

The low-noise and phase-coherent nonlinear transformation of a narrowband laser into a broadband supercontinuum (SC) in an optical fiber forms the basis of extremely precise applications ranging from optical frequency comb technology to ultrafast photonics and biomedical imaging. A major challenge of this process is the avoidance of incoherent nonlinear effects that amplify random quantum noise, requiring careful birefringence and dispersion engineering of the fiber. However, fundamental trade-offs exist between working in normal or anomalous dispersion regimes. Here, we combine the benefits of nonlinear dynamics in both regimes by cascading soliton compression and optical wave breaking in a hybrid fiber, formed by joining two widely available, commercial, polarization-maintaining step-index fibers exhibiting anomalous and all-normal dispersion, respectively. We experimentally demonstrate that this hybrid approach results in an ultra-low-noise fiber SC source covering the 930–2130 nm range with phase coherence near unity, spectrally resolved relative intensity noise (RIN) as low as 0.05%, and averaging 0.1% over a bandwidth of 750 nm, approaching the theoretical limits close to the pump laser noise. This corresponds to a doubling of the generated spectral bandwidth and a decrease of RIN by up to 1 order of magnitude compared to direct pumping of the individual fibers, where modulational polarization instabilities play a limiting role. Owing to its simplicity and its scalability to high repetition rates, our hybrid scheme is readily applicable to various laser platforms and could enhance the performance of applications such as hyperspectral nonlinear microscopy, coherent optical communications, and photonic signal processing.

By cascading soliton compression in an anomalous dispersion fiber and optical wave-breaking in a normal dispersion fiber, we demonstrate a coherent ultra-low noise fiber SC source covering the 930 - 2130 nm range.

A single-beam geometry implementation of a Coherent Anti-Stokes Raman Scattering (CARS) setup [1] , is capable of producing well resolved Raman spectra and can be used for CARS microscopy applications. Single-beam CARS (SB-CARS) is an affordable approach to performing CARS measurements, as laboratories with existing femtosecond laser infrastructure can be retrofitted for SB-CARS measurements.

Supercontinuum (SC) sources based on nonlinear spectral broadening of ultrashort pulses in specialty optical fibers have become an indispensable tool in a diverse range of application fields such as biomedical imaging, precision spectroscopy, and ultrafast photonics. After the initial race to maximize their spectral bandwidth and coverage, the noise properties of SC sources have recently shifted into focus. The large pulse-to-pulse fluctuations and correspondingly large relative intensity noise (RIN) present in commercial SC sources strongly limit the sensitivity, precision or resolution of many applications [1] . Recent work suggests that the dispersion engineering of the nonlinear fiber plays an important role for the resulting SC noise properties, as all-normal dispersion (ANDi) fibers suppress the noise-amplifying nonlinear processes that typically dominate the spectral broadening in their anomalously pumped counterparts [2] , [3] .

A new generation of ultrafast and low-noise supercontinuum (SC) sources is currently emerging, driven by the constantly increasing demands of spectroscopy, advanced microscopy, and ultrafast photonics applications for highly stable broadband coherent light sources. In this Perspective, we review recent progress enabled by advances in nonlinear optical fiber design, detail our view on the largely untapped potential for noise control in nonlinear fiber optics, and present the noise fingerprinting technique for measuring and visualizing the noise of SC sources with unprecedented detail. In our outlook, we highlight how these SC sources push the boundaries for many spectroscopy and imaging modalities and focus on their role in the development of ultrafast fiber lasers and frequency combs with ultra-low amplitude and phase noise operating in the 2 mu m spectral region and beyond in the mid-IR.

Many ultrafast optics applications require uniform and smooth spectral and temporal intensity profiles of ultrashort laser pulses. All normal dispersion (ANDi), coherent light pulses from fiber-based supercontinuum (SC) are often assumed to fulfill these requirements, backed up by many numerical simulations. However, detailed experimental data on the spectro-temporal characteristics of ANDi SC pulses is scarce.

We present a novel nonlinear microscopy modality using a time-domain ptychographic phase measurement, i2PIE, to compress 80 MHz supercontinuum pulses from an ANDi PCF used as excitation source, improving contrast at reduced average power.

Highly coherent and low noise supercontinuum (SC) sources based on nonlinear spectral broadening of femtosecond pulses in all-normal dispersion (ANDi) fibers are attractive for many applications in ultrafast photonics, such as nonlinear bio-photonic imaging, ultrafast spectroscopy, coherent X-ray generation, and lownoise ultrafast fiber laser development, amongst others [1].The noise properties of the SC source are of particular importance as fluctuations translate to intensity noise, pulse duration noise, or timing jitter, affecting sensitivity, resolution, or synchronisation of ultrafast experiments.While ANDi SC exhibit superior noise properties compared to their conventional counterparts pumped in the anomalous dispersion regime, recent theoretical studies suggest that polarization modulation instability (PMI) can severely degrade their stability [2].Here we present, for the first time, detailed experimental polarization-dependent relative intensity noise (RIN) measurements of ANDi SC sources based on both polarization-maintaining (PM) and non-PM fibers, and compare them to a conventional SC source (Fig. 1).All fibers were pumped with an ultrafast Er:fiber laser (80 fs, 40 MHz, 0.05% RIN), generating SC with comparable spectral bandwidths in the range 1.2 -2.2 µm.A rotating half-wave plate in front of the fiber and a synchronized analyzer at the fiber exit controlled the plane of pump pulse and detection polarization with respect the fiber's principal axes.Integrated RIN values from 0-18 MHz were measured using a photodiode and electronic spectrum analyzer with an angular resolution of approximately 0.2°.

Specialty optical fiber designs are presented that minimize quantum, technical, and polarization noise amplification during supercontinuum generation. Applications in ultrafast photonics are discussed.

We report a low noise, broadband, ultrafast Thulium/Holmium co-doped all-fiber chirped pulse amplifier, seeded by an Erbium-fiber system spectrally broadened via coherent supercontinuum generation in an all-normal dispersion photonic crystal fiber. The amplifier supports a − 20 dB bandwidth of more than 300 nm and delivers high quality 66 fs pulses with more than 70 kW peak power directly from the output fiber. The total relative intensity noise (RIN) integrated from 10 Hz to 20 MHz is 0.07%, which to our knowledge is the lowest reported RIN for wideband ultrafast amplifiers operating at 2 µm to date. This is achieved by eliminating noise-sensitive anomalous dispersion nonlinear dynamics from the spectral broadening stage. In addition, we identify the origin of the remaining excess RIN as polarization modulational instability (PMI), and propose a route towards complete elimination of this excess noise. Hence, our work paves the way for a next generation of ultra-low noise frequency combs and ultrashort pulse sources in the 2 µm spectral region that rival or even outperform the excellent noise characteristics of Erbium-fiber technology.

We report a low noise, broadband, ultrafast Thulium/Holmium co-doped all-fiber chirped pulse amplifier, seeded by an Erbium-fiber system spectrally broadened via coherent supercontinuum generation in an all-normal dispersion photonic crystal fiber. The amplifier supports a - 20 dB bandwidth of more than 300 nm and delivers high quality 66 fs pulses with more than 70 kW peak power directly from the output fiber. The total relative intensity noise (RIN) integrated from 10 Hz to 20 MHz is 0.07%, which to our knowledge is the lowest reported RIN for wideband ultrafast amplifiers operating at 2 mu m to date. This is achieved by eliminating noise-sensitive anomalous dispersion nonlinear dynamics from the spectral broadening stage. In addition, we identify the origin of the remaining excess RIN as polarization modulational instability (PMI), and propose a route towards complete elimination of this excess noise. Hence, our work paves the way for a next generation of ultra-low noise frequency combs and ultrashort pulse sources in the 2 mu m spectral region that rival or even outperform the excellent noise characteristics of Erbium-fiber technology.

Nonlinear microscopy has evolved over the last few decades to become a powerful tool for imaging and spectroscopic applications in biological sciences. In this study, i 2 P I E , a novel spectral phase control technique, was implemented in order to compress broad-bandwidth supercontinuum light pulses generated in an all-normal-dispersion (ANDi) photonic crystal fiber (PCF). The technique, based on time-domain ptychography, is demonstrated here in a nonlinear microscopy application for the first time, to the best of our knowledge. The first real-world application of this technique for second-harmonic generation and two-photon excitation fluorescence microscopies in biological samples is presented. We further show that in our implementation, i 2 P I E leads to improved contrast and signal-to-noise ratios in the generated images, compared to conventional compression techniques used in nonlinear microscopy.

We extend the time-domain ptychographic iterative engine to generalized spectral phase-only transfer functions. The modified algorithm, i$^2$PIE, is described and its robustness is demonstrated by different numeric simulations. The concept is experimentally verified by reconstruction of a complex supercontinuum pulse from an all normal dispersion fiber.

We present an improvement on the signal-to-background of single-beam coherent anti-Stokes Raman scattering (SB-CARS) spectroscopy measurements for systems employing ultrafast supercontinuum sources based on all-normal dispersion photonic crystal fibers. Improvements to the signal-to-background arise in the use of a new pulse-reconstruction algorithm based on temporal ptychography, i2PIE. A simple SB-CARS strategy is used to measure the spectrum of para-xylene, where the supercontinuum pulses used are compressed using multiphoton intrapulse interference phase scan (MIIPS) and, for the first time to the best of our knowledge, i2PIE using the same single-beam setup. With the i2PIE implementation, the signal-to-background is improved by nearly a factor of 4 in comparison with MIIPS. More notably, the integrated SB-CARS spectral intensity is increased by a factor of 6.5.

We experimentally investigate the spectro-temporal characteristics of coherent supercontinuum (SC) pulses generated in several implementations of silica and soft-glass all-normal dispersion (ANDi) photonic crystal fibers optimized for pumping with Erbium (Er):fiber femtosecond laser technology. We characterize the resulting SC using time-domain ptychography, which is especially suitable for the measurement of complex, spectrally broadband ultrashort pulses. The measurements of the ANDi SC pulses reveal intricate pulse shapes, considerable temporal fine structure, and oscillations on time scales of < 25 femtoseconds, which differ from the smoothness and simplicity of temporal profiles obtained in numerical simulations and observed in previous experiments. We link the measured complex features to temporal sub-structures of the pump pulse, such as pre- and post-pulses and low-level pedestals, which are common in high pulse energy ultrafast Er:fiber systems. We also observe spectro-temporal structures consistent with incoherent noise amplification in weakly birefringent fiber samples. Our results highlight the importance of the pump source and polarization-maintaining (PM) fibers for high-quality SC generation and have practical relevance for many ultrafast photonics applications employing ANDi fiber-based SC sources.

We report the amplification of an all-normal dispersion supercontinuum pulse in a Thulium / Holmium co-doped all-fiber chirped pulse amplification system. With a -20 dB bandwidth of more than 300 nm in the range 1800-2100 nm the system delivers high quality 66 fs pulses with more than 70 kW peak power directly from the output fiber. The coherent seeding of the entire emission bandwidth of the doped fiber and the stability of the supercontinuum generation dynamics in the silicate glass all-normal dispersion photonic crystal fiber result in excellent noise characteristics of the amplified ultrashort pulses.

Summary form only given. Ultrashort laser pulse characterization presents us with an interesting problem since these pulses are temporally too short to directly measure. Indirect measurement of pulses are possible through a variety of pulse reconstruction schemes such as FROG[1], SPIDER[2], MIIPS[3] and many more. Ptychography[4] a lens-less imaging technique in the spatial domain used to reconstruct images from sets of recorded diffraction patterns by application of the ptychographic iterative engine (P1E)[5] was recently migrated to the time-domain[6]. The ptychographic reconstruction algorithm has several advantages such as ease of implementation, quick convergence and being based in in the Fourier transform, the temporal resolution is determined by the spectral bandwidth and the temporal window is determined by the spectral resolution. Further the method does not require a square sample matrix nor a regularly spaced time delayed sample set. Pulse cross -correlation type schemes have been demonstrated using time-domain ptychography [6-11]. In this family of measurements an unknown pulse E(t) is mixed in a non-linear medium (NLM) with a time delayed probe pulse P(t) and an intensity spectrum I(co) is measured, where in some of the above references the probe is known, Fig. 1(a)i, and in some generated from the input pulse Fig. 1(a)ii by applying a filter H. In our latest work we show that one can generalize the PIE algorithm for application to single beam measurements, Fig. 1(a)iii, with excellent results. Here a known phase only transfer function from a set h n , is applied to the input pulse sequentially, in each case recording an intensity spectrum, i(o) = 1g. {[E(t) x h n (t)] 2 }1 2 , ( 1 ) which we refer to as i 2 P1E. Here Jr refers to the Fourier transform and x denotes the convolution operation. Once the pulse is characterized, the transfer function is effectively removed. In Fig. 1(b), (c) and (d) we show an example of a transfer function set, the resultant spectrogram and the reconstructed spectral amplitude and phase respectively. The i 2 P1E method is very versatile, broad in definition and performance is robust.

One of the most robust techniques solving the so-called phase problem in X-ray diffraction imaging is ptychography. It produces the correct real-space image if the illumination beam is known [1], but works even if it is unknown [2]. In 2015 we were the first to extend ptychography to the time domain and further to the reconstruction of temporal objects. In comparison to existing algorithms, ptychography minimizes the data to be recorded and processed, and thereby significantly reduces the computational time for reconstruction.