Compact ultrafast (ps or fs) pulsed sources, particular in the visible and the ultraviolet (UV) portion of the electromagnetic spectrum, allow both static and time-resolved studies in photobiology and photochemistry, and are therefore in high demand for applications such as fluorescence spectroscopy, short-wavelength multiphoton microscopy, and fluorescence lifetime imaging microscopy (FLIM). As an example, many fluorescent molecules of biomedical importance allow one-photon excitation within the 350-600 nm spectral range and two-photon excitation within the 500-700 nm spectral range [1
]. Presently, the unamplified tunable UV-visible ps-fs pulses are mainly generated from Ti:sapphire laser-pumped optical parametric oscillators or argon laser-pumped dye lasers, neither of which is compact, cost-effective, or easy to maintain. Since the desirable pulse properties, such as those of pulse duration, energy per pulse, and repetition rate, are more readily available in infrared sources, it is useful to frequency up-convert infrared pulses into the visible (or UV) while retaining, if not enhancing, requisite spectral and temporal characteristics. Following this guidance, researchers have actively pursued either the supercontinuum or the non-supercontinuum UV-visible sources based on nonlinear fiber optics.
Numerous efforts have been devoted to extend the blue edge of the supercontinuum toward the UV-visible region. The reported techniques include controlling four-wave mixing (FWM) [2
], pumping at multiple wavelengths [3
], using photonic crystal fiber (PCF) with two zero-dispersion wavelengths (ZDW) [4
], irradiating germanosilicate fiber by UV light [5
], cascading multiple fibers [6
], tapering the fiber [7
], selecting PCF with a high air-fill fraction [8
], and modifying the fiber glass composition [9
]. Supercontinuum sources with high power spectral density across 420-2400 nm have become commercially available. Selective spectral filtering of the supercontinuum sources in the visible region have allowed continuous tuning of the excitation wavelength in laser-scanning confocal microscopy [11
] and FLIM [13
]. Despite all these successes, it is challenging for the supercontinuum sources to cover the UV region (< 400 nm) with practically useful power spectral density. Also, the spectral filtering often results in elongated pulse duration and decreased pulse energy to forbid the applications of the supercontinuum sources to ultrafast fluorescence spectroscopy and short-wavelength multiphoton microscopy.
The non-supercontinuum sources involve specific routing of the pulse energy from the infrared pump to a narrow (< 30 nm 3-dB bandwidth) signal band in the UV-visible region. This approach is attractive because it minimizes the pulse energy loss to the nonspecific wavelength conversion of supercontinuum generation. Such specific wavelength conversion has been enabled by the phase-matching condition of either FWM or Cherenkov radiation (CR) (also known as dispersive wave generation or non-solitonic radiation). The FWM has generated tunable fundamental-mode signal across the visible spectrum, but with a small average power (< 1 mW) [14
], or multimilliwatt visible signal, but in an undesired higher-order fiber mode [15
]. To date, the pulse walk-off effect and/or the supercontinuum onset have largely limited the pump-to-signal conversion efficiency of the FWM, preventing the generation of a multimilliwatt-level fundamental-mode signal. Such signal can be produced through the CR, but at the cost of additional complexities introduced to the simple fiber-pumping procedure of the supercontinuum generation and FWM. These include accurately controlled fiber tapering [16
] and complicated combinations of fiber dispersion engineering and bulk optics frequency doubling [17
]. Another study has directly produced multimilliwatt visible CR signal through the simple fiber-pumping procedure [18
]. However, the tuning of the signal wavelength over 20 nm has not been demonstrated.
Our recent study has generated multimilliwatt fundamental-mode CR signal tunable across 485-690 nm from two dispersion-engineered PCFs, which are intrinsically poor candidates for supercontinuum generation due to their unique group-index profiles [19
]. The signal wavelength is tuned by varying the pump wavelength of a relatively expensive Ti:sapphire laser which affords a wide tuning range of 690-1020 nm. In this work, we show that neither the special dispersion engineering nor the tuning of the pump wavelength is necessary for generating such CR signal. By fixing the pump wavelength at 1020 nm, we obtain similar CR signal across 347-680 nm from one series of PCFs whose dispersion properties approximate those of single circular silica strands in air.