Influence of phase coherence on seeded supercontinuum generation

The noise properties of supercontinuum (SC) sources are of importance in many applications. This intensity noise of the SC arises from nonlinear amplification of the input-pulse shot noise and the spontaneous Raman scattering down the fiber [1]. Low noise femtosecond SC generation (SCG) has previously been demonstrated [2]. However, in commercial SC sources, the SC is generated by pumping with picosecond to nanosecond pulses and the SCG is thus initiated by modulation instability (MI). Therefore, the SC is characterized by low coherence and high shot-to-shot fluctuations, in particular at the spectral edges [3-5]. However, the influence of the noise properties when pumping in the normal dispersion regime has so far not been investigated for long pulse pumping. In this work we have measured and compared the relative intensity noise (RIN) properties of long-pulse MI initiated SCG in three different regimes: (i) all-normal SCG, (ii) normal-pumped SCG where higher-order Raman lines occur in the anomalous dispersion regime, and (iii) anomalous-pumped SCG. We show that the noise properties for the three regimes are similar: when increasing the input pump power, the intensity noise will continuously decrease for a given wavelength, and for a given input power the intensity noise will be lowest at the pump wavelength and increase when moving towards the spectral edge [6].

We experimentally investigate continuous-wave (CW) laser-pumped cascaded random Raman fiber-laser (CRRFL) architectures based on standard telecom fiber for low intensity noise supercontinuum (SC) generation. Our objective is to identify conventional pump laser designs that minimize wavelength and radio frequency (RF) resolved relative intensity noise (RIN) - a key determinant of sensitivity of low coherence interferometry application like ultrahigh-resolution optical coherence tomography (UHR-OCT). Three CW Yb-fiber pump configurations were evaluated: a single-stage fiber Bragg grating (FBG)-based bidirectional linear cavity, an FBG and a unidirectional ring-cavity seeded two-stage master-oscillator power-amplifier (MOPA) systems. Each laser delivered ∼75 W at 1064 nm into 2 km of SMF-28 Ultra, generating up to ∼34 W of SC spanning 1000-1700nm (measurement-limited) with power spectral densities (PSDs) exceeding 50 mW/nm. Spectrally resolved RIN was measured from 1200-1700nm across two RF bands: the quantum-noise-dominated regime (1-40 MHz) and the UHR-OCT relevant regime (10-160 kHz). Ring-based CRRFL SC systems consistently exhibited the lowest integrated rms RIN (∼2.5% in the 1-40 MHz band and ∼0.35% in the 10-160 kHz band), outperforming FBG-based implementations (5% and 0.4%, respectively). Importantly, this improvement persists even when the ring-laser free spectral range (FSR) is matched to that of the FBG laser, confirming that the RIN reduction arises from the intrinsic unidirectional lasing dynamics of the ring cavity rather than cavity FSR differences. To assess application-level relevance, we benchmark our best CRRFL-SMF SC sources against a commercial picosecond photonic crystal fiber (PCF)-based SC system. Across 1200-1700nm, the ring-MOPA-pumped SC exhibits at least a four-fold reduction in spectrally resolved RIN while providing comparable spectral flatness and nearly an order-of-magnitude higher PSD (>50 mW/nm vs ∼5 mW/nm). To our knowledge, this is the first systematic wavelength and RF frequency resolved noise characterization, including an explicit assessment of cavity FSR effects of conventional CW pumped CRRFL based SC sources, establishing their strong potential as low-cost, low-noise alternatives to commercial PCF-SCs for UHR-OCT and other low-coherence interferometry applications.

Optical fiber based supercontinuum light sources combine the brightness of lasers with the broad bandwidth of incandescent lamps and thus are promising candidates for sources in spectroscopic applications requiring high brightness and broad bandwidth. Herein, near-infrared (IR) Fourier transform (FT) spectrometry with a supercontinuum (SC) light source is investigated. The efficient, collimated propagation of broad bandwidth SC light through an 18 m path length multipass cell is demonstrated. A normalized spectral difference is calculated for the SC spectrum on consecutive FT mirror scans and is found to vary by less than 0.5%, indicating excellent spectral stability. The rms noise on zero absorbance lines is obtained as a function of the number of mirror scans at 0.125, 2, and 16 cm(-1) resolution for both the SC and conventional tungsten lamp source. The SC source has approximately a factor of ten times more noise than the lamp under comparable conditions for each resolution and data acquisition time. This clearly indicates that spectral acquisition with the SC source is not detector noise limited. NIR-FT spectra of methane and methyl salicylate, acquired with both the SC and lamp source, are reported. These spectra illustrate the advantage the SC source has over the incandescent source in that it can efficiently traverse long path lengths, thus providing a sensitivity advantage. The spectra also demonstrate the disadvantage of the SC source with respect to the lamp in the increased level of amplitude noise. Prospects for the future use of SC sources in absorption spectroscopy, including possible noise mitigation strategies, are briefly discussed.

A promising supercontinuum (SC) source made of dispersion varying 2-mm silicon-rich silicon nitride tapered waveguide is numerically proposed for highly coherent broad-band SC generation from 825 nm to 5660 nm employing pump at 1.55 $\mu$m with a pulse width of 50-fs duration. Simulation result shows that the SC bandwidth obtained at the waveguide output cross the material transparency limit with a very low peak power of 25 W. Our proposed design exhibits a significant potentiality in the mid-infrared SC generation in terms of spectral flatness and long wavelength expansion compared to the traditional planar waveguide design. Such a highly coherent SC source can be used in the field of optical coherent tomography, biomedical imaging and variety of industrial optical sensing applications.

Ultra-broadband high-power supercontinuum (SC) light sources are currently being applied to a wide range of biooptical applications, in which noise is a severely limiting factor. With picosecond pumping, as we consider here, noise is inherent in the SC generation process, because the generation is initiated by modulation instability (MI), which amplifies random fluctuations in the pump pulse. Above a certain threshold power the original pulse breaks up into a train of solitons, which evolves into a supercontinuum [1]. Recently it has been shown that by seeding the four-wave mixing (FWM) gain wavelengths, SC generation close to the threshold can be accelerated and the noise can be greatly reduced [2,3]. The noise is reduced because the seed initiates the MI in a controlled manner, thereby removing a great deal of the randomness in the SC generation process [3]. The theory behind it builds on the recent discovery of Rogue waves in SC [4,5].

The emergence of new laser-based mid-infrared (MIR) sources, such as quantum cascade lasers (QCL), led to substantial developments in the field of MIR spectroscopy in the last decade. Recently, also MIR supercontinuum (SC) sources became available. They combine broadband spectral emission known from thermal sources emission with coherent properties known from laser sources like QCLs. Nevertheless, while the latter already find practical application in the field of optical sensing, SC sources have yet to prove their applicability. In this contribution we present the development, characterization and application of a measurement concept consisting of a fiber-coupled broadband MIR SC source (1.75 μm-4.2 μm, 75 mW optical power) and a fully-integrated MOEMS-based Fabry-Perot microspectrometer (FPMS) for MIR spectroscopy. The main hindrance for the use of SC sources in spectroscopy so far, are the significant pulse-to-pulse fluctuations arising from the non-linear nature of the SC generation process. We show to what extent spectral averaging makes sense and evaluate the noise performance. By combining a SC source and a FPMS it was possible to significantly reduce noise in spectral, time and polarization domain, resulting in a set-up suitable for MIR spectroscopy. The performance of the set-up was characterized both in transmission and reflection geometry. Low-noise absorption spectra of oils, polymers and aqueous solutions of acetic acid were acquired . Furthermore, time-resolved measurements of the curing process of ethyl-2-cyanoacrylate and results of the chemical mapping of a painted metal surface are reported. The obtained results prove the concept of SC-FPMS promising for MIR spectroscopy, characterized by its simplicity and versatility.

Summary form only given. In the long pulse regime, supercontinuum (SC) generation is very sensitive to the input noise of the pump pulses resulting in poor coherence properties and stability [1]. Recently, the susceptibility of the dynamics to the input noise was shown to induce large shot-to-shot fluctuations when the initial stage of the SC mechanism is driven by modulation instability, leading to the generation of rare pulses with abnormally high intensity [2]. Recently, the noise amplification characteristics of modulation instability and subsequent stages of SC generation were observed in real time using a time-stretching technique which allows to capture single-shot SC spectra [2,3]. In this way, significant insight can be obtained into the SC dynamics that would be otherwise impossible to observe from average quantities, and indeed wavelength correlations in the SC have been shown to exhibit a rich structure that can be connected to the energy exchange dynamics along propagation [3].Here, we report on the experimental investigation of the spectral correlations in the SC regime of cascaded stimulated Raman scattering (SRS) where a large number of Stokes orders are generated leading to the formation of a broadband SC spectrum. We launch 355 nm nanosecond pulses from a 10 kHz repetition rate Q-switched laser deep into the normal dispersion region of highly nonlinear fiber single-mode at the pump wavelength and above. These pulses lead to the generation of a SC whose dynamics are mainly dominated by cascaded SRS [1]. Using and a direct single-shot detection technique where the output end of the fiber is connected to the entrance aperture of a spectrograph with an ICCD camera mounted on the spectrograph exit aperture, we can capture the spectra produced by individual pulses. Significant variations are observed and the captured single-shot spectra allow us to study in detail the SC spectral correlation map at the fiber output as shown in Fig 1.The correlation map analysis reveal how the pump and all the Stokes orders are generally highly correlated except at the long wavelength edge of the mean spectrum, beyond 400 nm, where the Stokes lines appears to be somewhat anti-correlated with the lower Stokes orders. Such correlations are indicative of the energy exchange dynamics between the different cascaded Raman orders and allow to interpret the noise dependence between the different Stokes components. Specifically, the region of high correlations are indicative of the saturated regime of cascaded SRS with little wavelength jitter from noise effects whilst the unsaturated regime of SRS at the edge of the SC is associated with a region of anti-correlations and correspond to large shot-to-shot fluctuations. Similar correlation/anti-correlation features were recently observed in modulation instability dynamics using [4]. We anticipate the new type of correlation analysis reported here to provide new insights into the study of broadband nonlinear propagation dynamics and noise amplification processes.

We demonstrate a 143.4 W high power combined white supercontinuum (SC) source using a (7 × 1) SC fiber combiner. Based on the criteria of adiabatic tapering and brightness conservation, a (7 × 1) SC fiber combiner is designed and theoretically investigated, and the simulated results of the transmission efficiencies of laser at different wavelengths of the combiner verify that the combiner has the ability of combining white SC sources efficiently. Then, the (7 × 1) SC fiber combiner is fabricated, and three white SC fiber laser sources with average power of about 50 W are set up for beam combining experiment. Finally, a 143.4 W high power combined white SC source is achieved with high combining efficiency of 97.4%. and the corresponding combined SC spectrum ranges from 450 nm to 1700 nm, which remains consistent with the overall spectral range of the three SC sources. The experiment result indicates that the (7 × 1) SC fiber combiner is greatly capable of combining high power white SC sources efficiently, thereby increasing the average power of the white SC source significantly. To the best of our knowledge, this is the first demonstration of a fiber combiner for high power white SC sources with such a high combining efficiency.

Supercontinuum (SC) sources based on optical fibers exhibiting flat and normal group velocity dispersion (GVD) in the entire wavelength region of interest, so-called all-normal dispersion (ANDi) fibers, have become a well-accepted alternative to the more conventional approach of pumping fibers in the anomalous GVD region. The conservation of a single ultrashort pulse and the suppression of noise-amplifying nonlinear effects during SC generation make ANDi fibers particularly attractive for ultrafast and noise-sensitive applications. Here we discuss current possibilities and future prospects, but also the limitations of ANDi fiber SC sources, focusing particularly on coherence and noise performance for various pump pulse regimes.

Temperature effects on supercontinuum generation using a continuous-wave Raman fiber laser

Diabetes mellitus 1 requires tight control of the blood glucose levels to avoid harmful effects of either too high (hyperglycemia) or too low (hypoglycemia) blood sugar. Due to the availability of low-cost components, fiber- coupled near infrared (NIR) absorption spectroscopy could be a feasible measurement method. From the molar absorptivity of glucose, it is shown that to achieve high accuracy using near infrared spectroscopy for glucose sensing, relative noise levels should not exceed 0:003 %. Two supercontinuum (SC) sources and one broadband lamp were investigated with a low-cost portable spectrometer. The SNR of the two SC sources was limited by amplitude fluctuations and could be improved by averaging. The SNR of the broadband source was found to be largely limited by the detector noise due to the weak intensity. 16 aqueous glucose samples ranging from 0 to 500mm were measured with the broadband source and an SC laser. A partial least squares regression (PLSR) model was built for both measurement sets, yielding root mean square errors of 49 and 54mm, illustrating how the limit of detection is restrained by the high relative intensity noise. A reference arm setup was built and could account for much of the variability of the SC source. A glucose measurement series using this setup and five samples (100 to 500mm) yielded a root mean square error of 10:6mm. The results indicate that an SC source can be feasible for absorption spectroscopy in a reference arm setup.

With the rapid developments and widespread applications of supercontinuum (SC) sources, ocular damage induced by this new light source becomes possible and receives our concern. To explore the ocular damage effect of an SC source, a series of experiments were conducted in a chinchilla grey rabbit model to determine the in-vivo retinal damage thresholds induced by a 420-750 nm SC source and a 532 nm laser. For the SC source, the beam divergence and the corneal 1/e2 beam diameter were 3.8 mrad and 2.45 mm, respectively. The determined ED50 values given in terms of total intraocular energy (TIE) for exposure durations of 0.1, 1.0, and 10.0 s were 1.57, 12.1, and 86.0 mJ, respectively. For the 532 nm laser, the beam divergence and the corneal 1/e2 beam diameter were 0.9 mrad and 2.25 mm, respectively. The determined ED50 value for an exposure duration of 0.1 s was 1.39 mJ. By employing the retinal thermal action spectrum in the ICNIRP guidelines, the damage thresholds for SC sources could be compared with the exposure limits for incoherent and laser radiation. Between the 420-750 nm SC source and the 532 nm laser, no significant difference could be found for the damage effects including damage threshold, retinal lesion size, and histological damage characteristics.

We have determined the cause of performance variation as a function of system parameters in supercontinuum (SC) sources in a normal-dispersion fiber by numerically analyzing the noise evolution of the SC. High-order nonlinearity was considered in such an analysis for the first time to our knowledge. We found that the evolution of noise along the SC fiber has two stages: First, the noise is governed by dispersion, which is different from that of the signal, whereas self-steeping leads to an asymmetric noise distribution across the spectrum. Second, four-wave mixing generates new noise components, whereas Raman scattering augments the asymmetry. We evaluated the qualities of the spectrally sliced pulses at several stages to verify the analysis and found an asymmetric profile of the sliced-pulses' quality versus frequency.

Supercontinuum (SC) generation in all-normal dispersion (ANDi) photonic crystal fibers (PCFs) has recently been investigated due to its ability to generate a low-noise broadband coherent spectrum, with a noise level much lower than typical SC sources based on anomalous dispersion fibers [1] , [2] . Several recent works have however reported some limitations on the input pulse duration due to both stimulated Raman scattering (SRS) and polarization modulation instability (PMI) [2] . Here, using a polarization-maintaining (PM) ANDi PCF pumped by a compact ytterbium-doped femtosecond laser at 1049 nm, we overcome these limitations and demonstrate an ultra-flat octave-spanning (670 nm-1390 nm) coherent SC with excellent low noise (RIN<0.54 % from 700 nm to 1100 nm) and polarization (PER>17 dB) properties. This highly-stable broadband PM-fiber based source could find strong potential applications in biomedical imaging and microscopy where improved signal to noise ratio is essential.

We have theoretically investigated supercontinuum (SC) generation with dual-wavelength pumping. The dual-wavelength-pumped source at 1064/686nm generated through four-wave mixing and SC generation have been realized in an all-fiber device. A good agreement between our simulation and experimental results has proved the conjugate action of cross-phase modulation and soliton self-frequency shift can obviously improve the power at the visible wavelengths with group-velocity matching between the visible wave at 686nm and the infrared wave at 1064nm.