Photonic-crystal Fiber Output Research Articles

Single‐beam dual‐color alternate‐pathway two‐photon spectroscopy: Toward an optical toolbox for redox biology

AbstractWe demonstrate a versatile single‐laser platform for single‐beam dual‐color two‐photon spectroscopy that combines a short‐pulse laser source with a tunable broadband wavelength converter based on a highly nonlinear photonic‐crystal fiber (PCF). We show that the short‐pulse PCF output can be tailored, via dispersion and nonlinearity management, to deliver a broadband optical driver whose spectral structure is ideally suited for a single‐beam two‐photon absorption (2PA) spectroscopy of the next‐generation genetically encodable fluorescent‐protein (FP) sensors of pH and redox‐responsive contrast agents. As a promising spectroscopic resource for redox biology, the short‐pulse PCF output can be finely sculpted to alternately drive an FP system via a one of its two 2PA‐allowed quantum pathways, yielding a high‐contrast fluorescence readout for a highly sensitive detection of redox reactions and signaling, pH sensing, and oxidative‐stress diagnosis.

Broadly wavelength- and pulse width-tunable high-repetition rate light pulses from soliton self-frequency shifting photonic crystal fiber integrated with a frequency doubling crystal

Soliton self-frequency shift (SSFS) in a photonic crystal fiber (PCF) pumped by a long-cavity mode-locked Cr:forsterite laser is integrated with second harmonic generation (SHG) in a nonlinear crystal to generate ultrashort light pulses tunable within the range of wavelengths from 680 to 1800 nm at a repetition rate of 20 MHz. The pulse width of the second harmonic output is tuned from 70 to 600 fs by varying the thickness of the nonlinear crystal, beam-focusing geometry, and the wavelength of the soliton PCF output. Wavelength-tunable pulses generated through a combination of SSFS and SHG are ideally suited for coherent Raman microspectroscopy at high repetition rates, as verified by experiments on synthetic diamond and polystyrene films.

Air-guided photonic-crystal-fiber pulse-compression delivery of multimegawatt femtosecond laser output for nonlinear-optical imaging and neurosurgery

Large-core hollow photonic-crystal fibers (PCFs) are shown to enable a fiber-format air-guided delivery of ultrashort infrared laser pulses for neurosurgery and nonlinear-optical imaging. With an appropriate dispersion precompensation, an anomalously dispersive 15-lm-core hollow PCF compresses 510-fs, 1070-nm light pulses to a pulse width of about 110 fs, providing a peak power in excess of 5MW. The compressed PCF output is employed to induce a local photodisruption of corpus callosum tissues in mouse brain and is used to generate the third harmonic in brain tissues, which is captured by the PCF and delivered to a detector through the PCF cladding. V C 2012 American Institute of Physics. [doi:10.1063/1.3681777]

Tailoring the soliton output of a photonic crystal fiber for enhanced two-photon excited luminescence response from fluorescent protein biomarkers and neuron activity reporters

Dual-cladding photonic crystal fibers (PCFs) with two zero-dispersion points are used to enhance the two-photon excited luminescence (TPL) response from fluorescent protein biomarkers and neuron activity reporters in dye-cell experiments and in in vivo work on transgenic mice and tadpoles. The soliton transmission of ultrashort pulses through a PCF suppresses dispersion-induced temporal pulse spreading, maintaining a high level of field intensity needed for efficient TPL excitation. The soliton self-frequency shift, stabilized against laser power fluctuations by a specific PCF dispersion design, is employed to accurately match the wavelength of the soliton PCF output with the two-photon absorption spectrum of dye or fluorescent protein biomarker molecules, enhancing their TPL response and allowing the laser damage of biotissues to be avoided.

Powerful wavelength-tunable ultrashort solitons in a solid-core photonic-crystal fiber

A solid-core photonic-crystal fiber (PCF) with an effective mode area of 20 microm2 is used to demonstrate the generation of sub-100-kW, 30-70 fs wavelength-tunable solitons within a wavelength range of 1300-1800 nm at a repetition rate of 18 MHz. An energy of 2.9 nJ per pulse is achieved for a 35 fs soliton PCF output centered at 1770 nm. Our numerical analysis supports experimental findings and suggests that frequency-shifted solitons in solid-core PCFs can be scaled up to a submegawatt level of peak powers.

Spectral interference of frequency-shifted solitons in a photonic-crystal fiber

Frequency-shifted solitons in a highly nonlinear photonic-crystal fiber (PCF) are shown to give rise to high-visibility interference fringes in PCF output spectra, indicating flat spectral phase profiles of individual solitons in the PCF output. This experimental finding, supported by numerical simulations, suggests a promising method of fiber-format pulse shaping and an attractive technology for few-cycle pulse synthesis through a coherent addition of frequency-shifted solitons generated in a highly nonlinear fiber.

Stabilized soliton self-frequency shift and 0.1- PHz sideband generation in a photonic-crystal fiber with an air-hole-modified core

Fiber dispersion and nonlinearity management strategy based on a modification of a photonic-crystal fiber (PCF) core with an air hole is shown to facilitate optimization of PCF components for a stable soliton frequency shift and subpetahertz sideband generation through four-wave mixing. Spectral recoil of an optical soliton by a red-shifted dispersive wave, generated through a soliton instability induced by high-order fiber dispersion, is shown to stabilize the soliton self-frequency shift in a highly nonlinear PCF with an air-hole-modified core relative to pump power variations. A fiber with a 2.3-microm-diameter core modified with a 0.9-microm-diameter air hole is used to demonstrate a robust soliton self-frequency shift of unamplified 50-fs Ti: sapphire laser pulses to a central wavelength of about 960 nm, which remains insensitive to variations in the pump pulse energy within the range from 60 to at least 100 pJ. In this regime of frequency shifting, intense high- and low-frequency branches of dispersive wave radiation are simultaneously observed in the spectrum of PCF output. An air-hole-modified-core PCF with appropriate dispersion and nonlinearity parameters is shown to provide efficient four-wave mixing, giving rise to Stokes and anti-Stokes sidebands whose frequency shift relative to the pump wavelength falls within the subpetahertz range, thus offering an attractive source for nonlinear Raman microspectroscopy.

Solitonic dynamics of ultrashort pulses in a highly nonlinear photonic-crystal fiber visualized by spectral interferometry

A linear technique of phase measurement based on spectral interferometry is employed to visualize the fine details in the spectral phase of a soliton produced in a highly nonlinear photonic crystal fiber (PCF) with a resolution better than 0.1 nm. Gigahertz features have been resolved in the spectral phase of the soliton PCF output, allowing the accuracy of time-domain soliton envelope reconstruction to be improved on the timescale of a few femtoseconds.

High peak power operation and harmonic generation of a single-polarization, Yb-doped photonic crystal fiber amplifier

We report on the use of a single-polarization, 41 μm core-diameter, intrinsically single-mode photonic crystal fiber (PCF) to obtain high peak power (up to ∼800 kW), 1 ns-duration pulses in a 100:1 linearly polarized, intrinsically single-mode ( M 2 ∼ 1.2) output. By transmitting the PCF output through nonlinear crystals, we also obtained efficient second, third, and fourth harmonic generation resulting in peak power >400 kW in the visible (green, 531 nm) and ∼200 kW in the UV (265.5 nm). To our knowledge these results represent the highest peak power obtained in a linearly polarized output from a fiber and the highest peak power in the visible and UV obtained through harmonic generation of the direct fiber output.

Multifrequency third-harmonic generation by red-shifting solitons in a multimode photonic-crystal fiber

While the standard scenario of third-harmonic generation (THG) by a dispersive-wave pump involves the emission of light with a frequency 3omega, thrice the frequency omega of the input pump field, solitons undergoing a continuous shift of their central frequency omega due to the Raman effect in a multimode optical fiber can generate the third harmonic in a different fashion. In the experiments reported here, we provide the first direct experimental evidence of THG by a continuously red-shifting soliton pump by studying the third-harmonic buildup in relation to the spectral evolution of the soliton pump field in a silica photonic-crystal fiber (PCF). We show that solitons excited in a PCF by unamplified femtosecond pulses of a Cr:forsterite laser sweep through the spectral range from 1.25 to 1.63 microm , scanning through a manifold of THG phase-matching resonances with 3omega dispersive waves in PCF modes. As a result, intense third-harmonic peaks build up in the range of wavelengths from 370 to 550 nm at the output of the fiber, making PCF a convenient fiber-format multifrequency source of short-wavelength radiation. Time-resolved fluorescence measurements with photoexcitation provided by the third-harmonic PCF output are presented, demonstrating the high potential of PCF sources for an ultrafast photoexcitation of fluorescent molecular systems in physics, chemistry, and biology.

Optical parametric amplification of a blueshifted output of a photonic-crystal fiber

Efficient optical parametric amplification of a frequency-upconverted output of a photonic-crystal fiber (PCF) is demonstrated using a double-pass parametric amplifier pumped by 65 mW 150 fs 100 kHz second-harmonic pulses of a Ti:sapphire laser. Such an amplifier provides gain factors in excess of 103 for the blueshifted PCF output, generating wavelength-tunable light pulses within the spectral range from 420 to 650 nm. Parametrically amplified pulses have a peak power up to 250 kW and a pulse width of about 200 fs at a repetition rate of 100 kHz, providing an average power up to 5 mW.

Frequency shifting and pulse shaping with photonic‐crystal fibers for coherent nonlinear spectroscopy

AbstractThe frequency‐shifted and pulse‐shaped output of photonic‐crystal fibers (PCFs) pumped by amplified femtosecond Cr:forsterite laser pulses is employed for frequency‐ and time‐resolved four‐wave mixing (FWM) spectroscopy. Cross‐correlation frequency‐resolved optically gated laser‐induced grating (LIG) technique is applied for the characterization of the pulse‐shaped blue‐shifted PCF output. The light pulses produced in the PCF are combined with the second‐harmonic output of the Cr:forsterite laser, to probe a doublet of Raman resonances in the nonlinear response of a test object by means of coherent anti‐Stokes Raman scattering (CARS). Copyright © 2006 John Wiley & Sons, Ltd.

Highly birefringent silicate glass photonic-crystal fiber with polarization-controlled frequency-shifted output: A promising fiber light source for nonlinear Raman microspectroscopy

A highly birefringent silicate glass photonic-crystal fiber (PCF) is employed for polarization-controlled nonlinear-optical frequency conversion of femtosecond Cr: forsterite laser pulses with a central wavelength of 1.24 mum to the 530--720-nm wavelength range through soliton dispersion-wave emission. The fiber exhibits a modal birefringence of 1.2.10(-3) at the wavelength of 1.24 mum due to a strong form anisotropy of its core, allowing polarization switching of the central wavelength of its blue-shifted output by 75 nm. Polarization properties and the beam quality of the blue-shifted PCF output are shown to be ideally suited for polarization-sensitive nonlinear Raman microspectroscopy.

A hollow beam from a holey fiber

A high-quality spectrally isolated hollow beam is produced through a nonlinear-optical transformation of Ti: sapphire laser pulses in a higher order mode of a photonic-crystal fiber (PCF). Instead of a doughnut shape, typical of hollow beams produced by other methods, the far-field image of the hollow-beam PCF output features perfect sixth-order rotation symmetry, dictated by the symmetry of the PCF structure. The frequency of the PCF-generated hollow beam can be tuned by varying the input beam parameters, making a few-mode PCF a convenient and flexible tool for the guiding and trapping of atoms and creation of all-fiber optical tweezers.

Mode-selective mapping and control of vectorial nonlinear-optical processes in multimode photonic-crystal fibers

We demonstrate an experimental technique that allows a mapping of vectorial nonlinear-optical processes in multimode photonic-crystal fibers (PCFs). Spatial and polarization modes of PCFs are selectively excited in this technique by varying the tilt angle of the input beam and rotating the polarization of the input field. Intensity spectra of the PCF output plotted as a function of the input field power and polarization then yield mode-resolved maps of nonlinear-optical interactions in multimode PCFs, facilitating the analysis and control of nonlinear-optical transformations of ultrashort laser pulses in such fibers.

Multistage Yb-doped fiber amplifier generating megawatt peak-power, subnanosecond pulses

A Q-switched microchip laser generating 1064 nm wavelength, subnanosecond pulses at a 13.4 kHz repetition rate was used to seed a dual-stage amplifier featuring a 40 microm core Yb-doped photonic-crystal fiber (PCF) as the power amplifier. From this source, we obtained diffraction-limited (M2 = 1.05), approximately 450 ps pulses of energy > 0.7 mJ, peak power in excess of 1.5 MW, and an average power of approximately 9.5 W. By further amplifying the PCF output in a multimode 140 microm core Yb-doped fiber, we generated a peak power in excess of 4.5 MW, the highest obtained in a fiber source to our knowledge.

Ultrafast photonic-crystal fiber light flash for streak-camera fluorescence measurements

Photonic-crystal fibers (PCFs) provide a high efficiency of frequency upconversion of femtosecond Cr: forsterite laser pulses through the emission of dispersive waves by solitons and third-harmonic generation. Dispersion management allows the central wavelength of the frequency-upconverted signal in PCF output to be tuned within the range of wavelengths from 400 to 900 nm. PCF frequency shifters are employed as excitation sources for time-resolved fluorescence streak-camera measurements on fluorescein solution.

Pulse Compression Using Direct Feedback of the Spectral Phase From Photonic Crystal Fiber Output Without the Need for the Taylor Expansion Method

Characterization and compensation of the complex spectral phase and the temporal profile of output pulses from a photonic crystal fiber (620-945-nm spectral broadening) were performed using a computer-controlled feedback system that combines a modified spectral-phase interferometry for direct electric-field reconstruction apparatus and only a 4-f chirp compensator having a spatial light modulator. These pulses were adaptively compressed from 12-fs input pulses to 6.8-fs. In addition, the compressed pulse profile showed excellent agreement with results measured independently with fringe-resolved autocorrelation.