White Supercontinuum Research Articles

143.4 W high power combined white supercontinuum source using a (7 × 1) supercontinuum fiber combiner

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.

Investigation of the filament spatial structure by aberration focusing in air of a femtosecond radiation pulse

The appearance conditions of two filaments propagating at an angle to each other by aberration focusing in air of femtosecond radiation pulse with a 940 nm wavelength are studied. Interest to these beams is due to the formation of highly directed radiation of the white supercontinuum in them with diffraction divergence. It is shown that the pump beam diffraction on plasma and Kerr nonlinearity in filamentation area are responsible for the formation of two additional filaments. The diffraction maxima are beginnings of high-intensity pump beams due to Kerr self-focusing of surrounding energy reservoir of pump radiation. The dynamics of these beams from foci zone till far zone is demonstrated.

Broadband optical ultrafast reflectivity of Si, Ge and GaAs

Ultrafast optical reflectivity measurements of silicon, germanium, and gallium arsenide have been carried out using an advanced set-up providing intense subpicosecond pulses (35 fs FWHM, lambda = 400 nm) as a pump and broadband 340–780 nm ultrafast pulses as a white supercontinuum probe. Measurements have been performed for selected pump fluence conditions below the damage thresholds, that were carefully characterized. The obtained fluence damage thresholds are 30, 20.8, 9.6 mJ/hbox {cm}^2 for Si, Ge and GaAs respectively. Ultrafast reflectivity patterns show clear differences in the Si, Ge, and GaAs trends both for the wavelength and time dependences. Important changes were observed near the wavelength regions corresponding to the E_1, E_1+Delta singularities in the joint density of states, so related to the peculiar band structure of the three systems. For Ge, ultrafast reflectivity spectra were also collected at low temperature (down to 80 K) showing a shift of the characteristic doublet peak around 2.23 eV and a reduction of the recovery times.

Kerr nonlinearity effect on femtosecond pulse radiation filamentation in air

The conditions for filamentation of femtosecond pulse laser radiation when focusing in air are studied experimentally and theoretically. A good agreement between experimental and calculated results is shown if neglecting the filament plasma. It is shown that the Kerr nonlinearity plays a fundamental role in the generation, existence, and cessation of a filament at a small numerical aperture (NA ≤ 2.15 × 10–3). The Kerr effect first leads to the beam self-focusing and generation of a filament, and at the final stage, to radiation defocusing and a sharp decrease in its axial intensity due to the beam wavefront distortions. In the case of aberration focusing, a spatial quasi-soliton is formed after a visible filament due to the balance between Kerr self-focusing and diffraction spreading. The quasi-soliton is a source of the directional white supercontinuum.

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67.9 W high-power white supercontinuum all-fiber laser source.

We present a high-power white supercontinuum (SC) all-fiber laser source with average power of 67.9 W, spectrum ranging from 500 to 1700 nm, and spectral width exceeding 1000 nm for spectrum with flatness below 10 dB (except pump wavelength). Also, the visible waveband power (below 850 nm) occupies about 21% of the total SC power. A 145 W high-power picosecond pulse fiber laser is specially designed with high repetition frequency of 656 MHz to reduce nonlinear effects. Meanwhile, a homemade high-power mode field adaptor that can operate stably at hundreds of watts of pulse power has high coupling efficiency of 82%. To our knowledge, the 67.9 W white SC fiber laser source we achieved is the highest reported with such a wide and flat optical spectrum.

Modulated CMOS camera for fluorescence lifetime microscopy.

Widefield frequency-domain fluorescence lifetime imaging microscopy (FD-FLIM) is a fast and accurate method to measure the fluorescence lifetime of entire images. However, the complexity and high costs involved in construction of such a system limit the extensive use of this technique. PCO AG recently released the first luminescence lifetime imaging camera based on a high frequency modulated CMOS image sensor, QMFLIM2. Here we tested and provide operational procedures to calibrate the camera and to improve the accuracy using corrections necessary for image analysis. With its flexible input/output options, we are able to use a modulated laser diode or a 20 MHz pulsed white supercontinuum laser as the light source. The output of the camera consists of a stack of modulated images that can be analyzed by the SimFCS software using the phasor approach. The nonuniform system response across the image sensor must be calibrated at the pixel level. This pixel calibration is crucial and needed for every camera settings, e.g. modulation frequency and exposure time. A significant dependency of the modulation signal on the intensity was also observed and hence an additional calibration is needed for each pixel depending on the pixel intensity level. These corrections are important not only for the fundamental frequency, but also for the higher harmonics when using the pulsed supercontinuum laser. With these post data acquisition corrections, the PCO CMOS-FLIM camera can be used for various biomedical applications requiring a large frame and high speed acquisition.

Visibly “white” light generation in uniform photonic crystal fiber using a microchip laser

We report on visibly white supercontinuum generation in photonic crystal fibers using a sub ns pump source at 1064 nm. The spectra extend from below 400 nm to 2450 nm, some 50 nm further into the blue than previously reported spectra. The extra bandwidth which is achieved by a simple modification to the fiber structure gives a higher apparent color temperature and a truly "white" visual appearance. The mechanism for the generation of the deeper blue to ultraviolet frequencies is outlined and our modified fiber is compared with fibers which have been conventionally used for supercontinuum generation.

Supercontinuum assisted trapped electron accumulation in titanium oxide gel by femtosecond laser pulses

Long-lived trapped electrons are generated in titanium oxide gels in a convergent part of a loosely focused beam from a millijoule, femtosecond Ti:Sapphire laser. The appearance of trapped electrons is accompanied by the generation of the white supercontinuum. Considering the relevant physical mechanisms of trapped electron accumulation, we show that two-photon interband transitions with participation of photons at the fundamental frequency and from the blue part of the supercontinuum explain the experimental findings.

Fresnel diffraction supercontinuum generation

White supercontinuum (SC) produced from Fresnel diffraction was demonstrated in BK7 glasses using femtosecond laser pulses. For input pulse energy below the threshold for the onset of self-focusing and SC generation, an aperture was placed into the beam before the BK7 glass to redistribute the intensity by diffraction along the propagation axis to produce a stable SC. The SC occurred at various distances from the aperture containing odd number of Fresnel zones, mainly the first. This result plays an important role in studying the properties of SC sources as well as the diffraction field distribution. A distinct and stable interference ring pattern inside the conical four-photon emission is observed and attributed to the spatial arrangement of small-scale filaments at different distances in the glass.