Few-optical-cycle laser pulses: from high peak power to frequencytunability

Recent advances in ultrafast laser technology have led to the generation of light pulses comprising few optical cycles employing different compression techniques. In particular, two techniques have been developed, which allow addressing the issues of high peak power or frequency tunability in a wide spectral range, namely: the hollow-fiber compression technique and the optical parametric amplification. The paper analyzes the general scheme of pulse compression and reports on the most interesting results obtained using the above-mentioned techniques. The combination of spectral broadening in a gas-filled hollow fiber with ultrabroad-band dispersion control, has led to the generation of pulses with duration of ~5 fs with peak powers up to 0.11 TW. Using optical parametric amplifiers with different configurations sub-15-fs laser pulses have been generated tunable in the visible and in the near-infrared     

Recent Advances in Optical Processing Techniques Using Highly Nonlinear Bismuth Oxide Fiber

We report our recent studies on nonlinear processing of optical signals using a 35-cm highly nonlinear bismuth oxide fiber (Bi-NLF). Our findings are based on self-phase modulation, cross-phase modulation, and four-wave mixing in the Bi-NLF. We demonstrate applications of the nonlinear techniques in optical signal regeneration, tunable optical delay, stabilization of multiwavelength laser source, tunable optical pulse generation, microwave photonic carrier frequency multiplication, and all-optical wavelength conversion.     

Intracavity spectral shaping in external cavity mode-lockedsemiconductor diode lasers

Intracavity spectral shaping techniques have been employed to artificially shape and broaden the optical spectrum of external cavity mode-locked semiconductor lasers. Using an intracavity spectrometer and intensity Fourier-plane filtering, active mode-locked output spectra with a multiplicity of independently tunable wavelengths have been generated, while an adjustable intracavity etalon has been employed to generate an 18-nm spectral width. Furthermore, hybrid mode-locking with a multiple-quantum-well (MQW) saturable absorber combined with the intracavity etalon, followed by dispersion compensation has led to the generation of optical pulses of 330 fs in duration     

Monochromatic-Tunable Terahertz-Wave Sources Based on Nonlinear Frequency Conversion Using Lithium Niobate Crystal

We review widely tunable terahertz (THz)-wave generation by optical parametric processes using lithium niobate crystal. Applying the parametric oscillation of or a MgO-doped crystal pumped by a nanosecond Q-switched Nd:YAG laser, we realized coherent THz-wave sources with a simple configuration widely tunable in the range of 0.7-3 THz. For efficient coupling of the THz wave, we used a monolithic grating coupler or a Si-prism array coupler. In addition, Fourier transform limited THz-wave spectrum narrowing was achieved by introducing the injection seeding method. A line width of about 110 MHz (0.003 cm^-1) was assured by measuring the absorption spectrum of low-pressure water vapor. Using the difference frequency generating method with a periodically poled crystal, we achieved higher conversion efficiency and realized continuous THz-wave generation. This room temperature operated, tabletop system promises to be a widely tunable THz-wave source suited to a variety of applications.     

Generation of mid-infrared pulses suitable for two-colorIR-spectroscopy with 300-fs time resolution, temporal analysis viatwo-photon absorption in germanium and first application

The generation of two independently tunable light pulses in the mid-infrared spectral region is reported here. Synchronously pumping two optical parametric oscillators in parallel with a flashlamp-pumped, pulsed Nd:YLF laser delivers tunable radiation in the range 1.2-1.7 μm with duration of 300 fs. Subsequent parametric amplification in AgGaS₂ crystals yields intensive light pulses in the wavelength range of 2.7-7 μm with duration of approximately 1 ps and energies up to 10 μJ. Pulse analysis determining also the zero delay time and the temporal resolution of the experimental system is performed via two-photon absorption in germanium, utilizing the direct band gap. The temporal resolution of the two-color IR-system is determined to be approximately 300 fs and first application to time-resolved spectroscopy in the condensed phase is demonstrated     

Relative carrier-envelope-offset phase control between independent femtosecond light sources

The authors studied the measurement and control of relative carrier-envelope-offset phases among femtosecond light pulses whose optical frequencies have a subharmonic relation. They report the fluctuation dynamics of the phases and their control of two types of passive pulse-timing synchronized systems: passively synchronized mode-locked Ti:sapphire and Cr:forsterite lasers and a femtosecond subharmonic optical parametric oscillator. These techniques will facilitate Fourier synthesizing among subharmonic pulses in order to realize subfemtosecond pulse-train generation in the visible and infrared regions.     

Widely tunable, near- to mid-infrared femtosecond and picosecondoptical parametric oscillators using periodically poledLiNbO3 and RbTiOAsO4

We describe the operation and characterization of Ti:sapphire laser-pumped femtosecond and picosecond optical parametric oscillators based the new quasi-phase-matched nonlinear materials of periodically poled LiNbO₃ and RbTiOAsO₄ with broad tunability in the near- to mid-infrared. We discuss the merits of the two materials for use in ultrafast optical parametric oscillators (OPOs) and compare and contrast their properties to the birefringent materials. We demonstrate an extended spectral coverage from <1 μm to >5 μm, pump power thresholds as low as 45 mW, average mid-infrared output powers in excess of 100 mW, and pulse durations of 100-200 fs and 1-2 ps at ~80 MHz repetition rate. We also report the efficient operation of Ti:sapphire-pumped femtosecond OPOs in all-solid-state configurations by utilizing diode-laser-based input pump sources     

Fiber-based optical parametric amplifiers and their applications

An applications-oriented review of optical parametric amplifiers in fiber communications is presented. The emphasis is on parametric amplifiers in general and single pumped parametric amplifiers in particular. While a theoretical framework based on highly efficient four-photon mixing is provided, the focus is on the intriguing applications enabled by the parametric gain, such as all-optical signal sampling, time-demultiplexing, pulse generation, and wavelength conversion. As these amplifiers offer high gain and low noise at arbitrary wavelengths with proper fiber design and pump wavelength allocation, they are also candidate enablers to increase overall wavelength-division-multiplexing system capacities similar to the more well-known Raman amplifiers. Similarities and distinctions between Raman and parametric amplifiers are also addressed. Since the first fiber-based parametric amplifier experiments providing net continuous-wave gain in the for the optical fiber communication applications interesting 1.5-μm region were only conducted about two years ago, there is reason to believe that substantial progress may be made in the future, perhaps involving "holey fibers" to further enhance the nonlinearity and thus the gain. This together with the emergence of practical and inexpensive high-power pump lasers may in many cases prove fiber-based parametric amplifiers to be a desired implementation in optical communication systems     

Terahertz Sources Based on Intracavity Parametric Down-Conversion in Quasi-Phase-Matched Gallium Arsenide

We have efficiently generated tunable terahertz (THz) radiation using intracavity parametric down-conversion in gallium arsenide (GaAs). We used three types of microstructured GaAs to quasi-phase-match the interaction: optically contacted, orientation-patterned, and diffusion-bonded GaAs. The GaAs was placed in an optical parametric oscillator (OPO) cavity, and the THz wave was generated by difference-frequency mixing between the OPO signal and idler waves. The OPO used type-II phase-matched periodically poled lithium niobate as a gain medium and was synchronously pumped by a mode-locked laser at 1064 nm (7 ps and 200 nJ at 50 MHz). With center frequencies spanning 0.4-3.5 THz, 250-GHz bandwidth radiation was generated. We measured two orders of optical cascading generated by the mixing of optical and THz waves. In a doubly resonant oscillator (DRO) configuration, the efficiency increased by 21times over the singly resonant oscillator performance with an optical-to-THz efficiency of 10^-4 and average THz power of 1 mW. The GaAs stabilized the DRO by a thermooptic feedback mechanism that created a quasi- continuous-wave train of THz pulses.     

Wavelength tunable 10-GHz 3-ps pulse source using a dispersion decreasing fiber-based nonlinear optical loop mirror

We experimentally demonstrate the use of a dispersion decreasing fiber (DDF)-based nonlinear optical loop mirror (NOLM) for the generation of wavelength tunable soliton-like pulses at a repetition rate of 10 GHz. We compress /spl sim/12-ps Gaussian pulses from an electro-absorption modulator (EAM) (followed by 125 m of DCF for preliminary linear dispersion compensation) into 3-ps pedestal-free pulses using both high-order soliton compression and nonlinear switching effects within an 8.5-km DDF-based loop mirror. The output pulses from the DDF-based NOLM show considerable pedestal reduction compared to those obtained by directly compressing the EAM seed pulses via a single passage through the DDF. Wavelength tuning of the compressed pulses over a /spl sim/15-nm bandwidth (from 1541 to 1556 nm) is demonstrated without a significant increase in pulse duration or degradation in pulse quality.     

Frequency tripling of a Q-switched Cr:LiSAF laser to the UV region

Frequency tripling of a Q-switched, tunable Cr:LiSAF laser to the UV wavelength region is accomplished by a mixing scheme involving second harmonic generation in lithium triborate (LBO), followed by sum frequency generation in β-barium borate (BBO). The generated UV output is tunable between 260 nm and 320 nm     

UV laser generation in GdYCOB crystal

We have developed a new rare‐earth calcium oxyborate crystal Gd_xY_1‐xCa₄O(BO₃)₃ (GdYCOB) in order to control birefringence in nonlinear optical crystals. This GdYCOB has been grown by the Czochralski technique. In addition, we have succeeded in generating noncritically phase‐matched (NCPM) second and third harmonics of Nd:YAG laser radiation. We observed two types of photoinduced damage in high‐power third‐harmonic generation (THG). We report the prevention of these kinds of damage by means of an elevated crystal temperature and describe THG performance in the GdYCOB crystal. © 2001 Scripta Technica, Electr Eng Jpn, 136(2): 26–30, 2001     

Laser-Induced Line Patterning of Nonlinear Optical Crystals in Glass

This paper reports the progress in the patterning of nonlinear optical crystal lines on a glass surface by laser irradiation techniques. Two techniques for the patterning of crystal lines have been developed, i.e., rare-earth atom heat processing and transition metal atom heat processing, in which continuous-wave lasers such as Nd:YAG laser (wavelength: lambda = 1064 nm) are irradiated onto the glasses containing rare-earth ions such as Sm³⁺ and Dy³⁺ or transition metal ions such as Ni²⁺ and Cu²⁺. The patterning of lines consisting of nonlinear optical crystals such as beta-BaB₂O₄, SmxBi_1- xBO₃, (Sr,Ba)Nb₂O₆, and LiNbO₃ has been achieved. It is clarified from the azimuthal dependence of second harmonic intensities and polarized micro-Raman scattering spectra that nonlinear optical crystals in the lines are highly oriented along the laser scanning direction, i.e., the patterning of single-like crystal lines. It is also possible to pattern two-dimensional crystal bending or curved lines by just changing the laser scanning direction, and such bending crystal lines have a potential for optical waveguides.     

In Vivo Cell Tracking With Video Rate Multimodality Laser Scanning Microscopy

Studies of biological processes, such as disease progression and response to therapy, call for live imaging methods that allow continuous observation without terminating the study subject for histological tissue processing. Among all current imaging modalities, optical microscopy is the only method capable of probing live tissue with cellular and subcellular resolution. We present a video-rate (30 frames/s), multimodality imaging system that is designed specifically for live animal imaging and cell tracking. In vivo depth-sectioned, high-resolution images are obtained using confocal and nonlinear optical techniques that extract structural, functional, and molecular information by combining multiple contrast mechanisms, including back scattering, fluorescence (from single- and two-photon excitation), second harmonic generation, and coherent anti-Stokes Raman scattering. Simultaneous use of up to three modalities is possible and eliminates the need for coregistration, especially on large-scale images. A real-time movement correction algorithm was developed to extend integration times in cases where the image needs to be stabilized against subject movement. Finally, imaging of fast moving leukocytes in blood vessels is made possible with a modification that permits operation at 120 frames/s over a smaller area. Sample imagery obtained in vivo with the microscope is presented to illustrate the capabilities.     

Electron beam poling of thin fluoropolymer layers

Electron beam poling by use of a scanning electron microscope was developed and applied to thin polymer layers. The electron beam charging process was studied by measurement of the surface potential, dependent on the acceleration voltage. The induced orientation of CF₂ dipole groups of the ferroelectric copolymer vinylidene fluoride/trifluoroethylene was detected by means of IR spectroscopy as well as by nonlinear optical methods. Using electron beam microlithography, microscopic polarization structures have been written into thin fluoropolymer layers by means of a direct, computer-controlled writing process; they were read out through potential contrast images as well as by means of SHG (second harmonic generation)     

Second-harmonic generation in potassium niobate waveguides

We report on our progress in the formation of waveguides in potassium niobate (KNbO₃) using techniques such as ion implantation and ion sputtering. Different methods for the structuring of channel waveguides are presented, and their advantages and disadvantages are discussed in terms of their optical and nonlinear optical properties. The excellent power-handling capability of KNbO₃ waveguides is compared to other waveguide materials, and we highlight the influence of postimplantation annealing and repoling on the waveguide attenuation and the nonlinear optical coefficient. We also review recent results on second-harmonic generation in KNbO₃ waveguides focusing on blue light generation     

Production of high average power UV by second-harmonic andsum-frequency generation from copper-vapor lasers

Progress in high average power UV generation by nonlinear frequency conversion of the output of copper-vapor lasers (CVL's) is reviewed. The specific parameters controlling the efficiency of nonlinear frequency conversion using CVL's are highlighted, with CVL beam quality and matching the optical beam delivery system to the characteristics of the nonlinear crystal being identified as the most significant issues. Recent experimental studies of second harmonic generation (SHG) with single-CVL oscillators and CVL oscillator-amplifier systems show that by careful optimization of the CVL pump laser and beam delivery systems, it is now possible to generate multiwatt average powers in the UV with high optical conversion efficiency (up to 35%) and overall electrical efficiency (approaching 0.1%)     

Broad-bandwidth parametric amplification in the visible:femtosecond experiments and simulations

A general geometric condition, involving group velocities, for large bandwidth parametric amplification with all types of phase-matching and in certain wavelength ranges is derived. This condition has been exploited to produce visible widely tunable sub-20-fs pulses from an optical parametric oscillator and an optical parametric amplifier based on β-barium borate (BaB₂O₄) pumped in the blue. These results are interpreted in terms of a modified soliton model for the optical parametric oscillator (OPO) and detailed numerical simulations for both devices     

Two-Pump Parametric Optical Delays

Continuously tunable optical delay lines based on parametric process in optical fibers are described theoretically and demonstrated experimentally. Performance limits are outlined and compared to alternative approaches capable of continually adjustable delay management. The current record of 105 ns tunable optical delay at 10 Gb/s is presented, with a delay-bandwidth product of 1055.     

Novel approach to RF photonic signal processing using an ultrafast laser comb modulated by traveling-wave tunable filters

The analogy between optical frequencies and RFs leads to a novel technique for RF photonic signal processing using a femtosecond laser comb modulated by a traveling-wave tunable filter, such as an acoustooptic tunable filter (AOTF) or a novel electrooptic tunable filter (EOTF). In this new scheme, the recent history of an applied RF waveform to-be-processed slides into the tunable filter and a femtosecond pulse train diffracts off the moving acoustooptically or electrooptically induced dielectric grating, producing a shaped optical pulse train with each shaped pulse as a compressed replica of the RF waveform contained within the device aperture that is sped up by the ratio between the center optical and RF frequency. For a CW RF tone input, only a narrowband group of the frequency comb lines in the incoming laser comb is spectrally filtered and modulated due to the phase-matching condition in the filter and is simultaneously Doppler shifted by that RF due to the traveling-wave grating obeying the conservation of energy. This allows us to use the traveling-wave tunable filter as a spectrally mapped Doppler-shifted modulator that encodes different RF components onto the corresponding optical frequency comb lines. RF signal processing can then be performed by using optical techniques to manipulate the spectrally modulated laser comb. To reconstruct the processed RF signal, the Doppler-shifted and optically processed pulse train is heterodyne detected by beating with a reference femtosecond pulse train from the same laser source. A high repetition rate femtosecond laser comb is modulated by an AOTF to experimentally demonstrate this novel RF photonic signal processing technique. We demonstrate an RF tunable bandpass/notch filter, an RF down-converter, and an RF jammer er as novel applications of ultrafast lasers.