Pentagon-shaped 0 degrees -cut BaTiO3 as an efficient self-pumped phase conjugator

A pentagon-shaped 0(o)-cut undoped BaTiO(3) crystal was used to construct a high-quality self-pumped phase conjugator. The conjugator shows a steady phase-conjugate output with a high reflectivity (as high as ~68%), a fast rising time, and a large angular range of acceptance. It also shows a good fidelity in the phase-conjugate reconstructed image; the resolution reaches 17 mum.     

Enhanced stimulated photorefractive scattering self-pumped phase conjugator in Ce-doped BaTiO 3 with external mirrors

The characteristics of stimulated photorefractive scattering self-pumped phase conjugator (SPS-SPPC) in BaTiO3 :Ce crystal without and with an external mirror have been studied and compared. The experimental results show that the SPS-SPPC becomes unstable at a larger incident angle. An additional external mirror can create a stable optical path in the crystal and improve the stability of the SPS-SPPC effectively, and, at the same time, it can enhance the response rate and the phase conjugate reflectivity as well. We offer a reasonable explanation of the experimental results by using a semilinear cavity model.     

Coherence-based imaging through turbid media by use of degenerate four-wave mixing in thin liquid-crystal films and photorefractives

We describe a coherence-based imaging technique based on degenerate four-wave mixing in two materials. First, a 45 degrees -cut BaTiO(3) crystal was used as a self-pumped phase conjugator to obtain depth-resolved images through a 4 mean-free-path (mfp) scattering media. In addition, an HITC dye-doped K15 liquid-crystal layer was used to provide single-shot image acquisition through a 2 mfp scattering media. This technique can be used to provide instantaneous (single-pulse), depth-resolved, two-dimensional images of the internal structure of scattering materials. Potential applications of this technique include subsurface imaging of biomedical tissue and nondestructive evaluation of composite materials.     

Self-pumped phase-conjugation properties of cerium-doped BaTiO(3) crystals in the near infrared

Studies of a new Ce-doped BaTiO(3) crystal as an efficient self-pumped phase conjugator in the near infrared are presented. The internal beam production during the establishment of self-pumped phase conjugation (SPPC) is closely observed, and the corresponding mechanisms of SPPC are identified accordingly. When the incident beam is in an a-face-incidence geometric arrangement, it is observed for the first time that SPPC mechanism has a transition from stimulated photorefractive backscattering and four-wave mixing to total internal reflection when λ is varied from 706 to 733 nm. Variations of the SPPC reflectivity with the λ, incident angle, and position of the input beam have been investigated, and a high reflectivity of approximately 80% has been obtained. In addition, SPPC reflectivity of approximately 40% is obtained when the incident beam enters the crystal by the +c face, and a variation with λ of optical beam patterns in the crystal has also been observed. Qualitative explanations of the phenomenon of SPP mechanism transition with λ that will be very useful in practical applications are given.     

High Reflectivity Broad Band Self-pumped Phase Conjugator at Near Infrared Wavelengths Using SBN : Ce

Abstract

A high reflectivity broad-band SBN : Ce phase conjugator with an external ring cavity in the near infrared wavelength region, 706 to 830 nm, is reported. Phase conjugation reflectivity was measured to be greater than 60% in the wavelength region, 706-820 nm, and as high as 67% at 716 nm. To our knowledge, this is the highest phase conjugation reflectivity by a SBN : Ce self-pumped phase conjugator. Both c.w. and square-wave pump sources were used. No significant changes of the phase conjugation reflectivity were observed when the pump source was changed from c.w. to a square wave at each different wavelength. The wave form of the phase conjugate wave follows the pump square wave. The build-up time of the phase conjugate wave under a square-wave pump was found to be twice that under a c.w. pump when the peak power of the square wave was the same as the power of the c.w. beam.     

Compact reflection-grating-based phase conjugator in KNbO(3):Fe crystals

We present the design and the results of experimental investigations of a self-pumped phase conjugator through a reflection grating in a special-cut photorefractive KNbO(3):Fe crystal. High reflectivity, good phase-conjugate fidelity, and fast response time are obtained.     

Low-power fault-tolerant laser-to-fiber coupling with photorefractive BaTiO(3)

A novel device for low-power fault-tolerant laser-to-fiber coupling is described; it makes use of self-pumped phase conjugation in photorefractive BaTiO(3). We found the degree of fault tolerance to improve by 2 orders of magnitude to 0.02 dB per 100-mum displacement within the determined displacement range at intensities incident on the crystal as low as 2 mW, compared with the losses without the BaTiO(3) present.     

Influence of the internal photoinduced electric field on the formation of self-pumped phase conjugation in doped (K(0.5)Na(0.5))(0.2)(Sr(0.75)Ba(0.25))(0.9)Nb(2)O(6) crystals

The obvious influence of the internal photoinduced electric field on the formation of self-pumped phase conjugation in doped (K(0.5) Na(0.5))(0.2) (Sr(0.75) Ba(0.25))(0.9) Nb(2)O(6) crystals has been investigated with intermittent and continuous laser beams. An analysis is given that involves the migration of charge carriers driven by the photovoltaic (PV) and the pyroelectric (PY) electric field. Deviation from the optimal state of energy transfer owing to the PV effect and the self-adjusting of holograms owing to the PY field are the two important dominant processes in the self-pumped phase conjugator. The influence of the photoinduced electric field on the conjugation fidelity and the stability of the phase-conjugate output is also observed and analyzed. The bump feature of the two-wave mixing and the oscillating situation of the self-pumped phase conjugator are observed and are explained in view of the photoinduced internal field.     

Double phase conjugation in copper-doped potassium sodium strontium barium niobate crystals

We show a double phase conjugator with novel crystals of copper-doped potassium sodium strontium barium niobate. The phase conjugation reflectivity can reach 278% and depends on relative input positions of both incident beams. The response time with the input beam power ratio is measured and modeled as a function of input beam power ratio. The formation of the arch coupling loop is analyzed with theoretical simulation results and a positive feedback process is proposed. The influence of a self-pumped phase conjugation process on double phase conjugation fidelity is also discussed. It is necessary to avoid generating self-pumped phase conjugation to prevent the reduction of double phase conjugation fidelity.     

Spatial-Frequency Response of Photorefractive Phase Conjugators with Ce:BaTiO(3)

We propose and demonstrate an interferometric method to measure thespatial-frequency response of photorefractive phase conjugators withCe:BaTiO(3). Two coherent beams are incident on a crystaland form an interference pattern inside the crystal. The two beamsundergo stimulated photorefractive backscattering, which creates theircorresponding phase conjugations. Then the four waves interactwithin the crystal. The spatial-frequency resolution of the phaseconjugators is measured to be as high as 3750 line pairs/mm by use ofthe interferometric method. There are several factors that limitthe measured spatial resolution when using a U.S. Air Force ResolutionChart. The output modulation deviates from the input modulation forhigh spatial frequencies. In the presence of a strong additionalpump beam, the output modulation of the phase conjugators is almost thesame as the input modulation for a wide range of input spatialfrequencies. The phase conjugator exhibits a large dynamic range ofintensity. We analyze theoretically the modulation transferfunction of photorefractive phase conjugators withCe:BaTiO(3) for two mutually coherent beams. Thetheoretical analysis is in agreement with the experimental resultswithin a small incident-angle region.     

Broadly tunable femtosecond pulse generation in the near and mid-infrared

High-repetition-rate (80-MHz) femtosecond infrared pulses are generated by difference frequency mixing (DFM) a femtosecond Ti:sapphire laser with a phase-locked synchronized cw mode-locked Nd:YAG picosecond laser. This DFM scheme is of particular interest for generating ultrashort near-IR pulses (~10 fs) because group velocity mismatch with a pump pulse can be ignored. The simplicity and the broad wavelength tunability (from the near IR to the mid-IR) of this scheme is demonstrated. Short (125-fs FWHM) optical pulses in the near IR around 1.5 mum are obtained with noncritical type-I phase-matched LiB(3) O(5). We also used a similar scheme to generate mid-infrared pulses at 3.0 mum with type-II phase-matched KTiOPO(4).     

Clustered gases as a medium for efficient plasma waveguide generation

Clustered gas jets are shown to be an efficient means for plasma waveguide generation, for both femtosecond and picosecond generation pulses. These waveguides enable significantly lower on-axis plasma density (less than 10(18) cm(-3)) than in conventional hydrodynamic plasma waveguides generated in unclustered gases. Using femtosecond pump pulses, self-guided propagation and strong absorption (more than 70%) are used to produce long centimetre scale channels in an argon cluster jet, and a subsequent intense pulse is coupled into the guide with 50% efficiency and guided at above 10(17)W cm(-2) intensity over 40 Rayleigh lengths. We also demonstrate efficient generation of waveguides using 100 ps axicon-generated Bessel-beam pump pulses. Despite the expected sub-picosecond cluster disassembly time, we observe long pulse absorption efficiencies up to a maximum of 35%. Simulations show that in the far leading edge of the long laser pulse, the volume of heated clusters evolves to a locally uniform and cool plasma already near ionization saturation, which is then efficiently heated by the remainder of the pulse.     

Rhodium-Doped Barium Titanate Phase-Conjugate Mirror for an All-Solid-State, High-Repetition-Rate, Diode-Pumped Nd:YAG Master-Oscillator Power Amplifier Laser

Self-pumped phase conjugation in infrared-sensitive photorefractive rhodium-doped barium titanate (Rh:BaTiO(3)) is investigated with a kilohertz-repetition-rate, diode-pumped Nd:YAG laser. We also demonstrate the operation of a diode-pumped Nd:YAG master-oscillator power-amplifier system with such a compact and solid-state phase-conjugate mirror for wave-front restoration. We obtain near-diffraction-limited output pulses of 20-mJ energy at a repetition rate of 200 Hz and a 1-min response time of the phase-conjugate mirror. The performances of the Rh:BaTiO(3) crystal are compared with those obtained with a conventional Brillouin mirror.     

Nondestructive readout of a photorefractive hologram by phase-conjugate copying in a one-crystal configuration

We propose a novel phase-conjugate copying method for nondestructive readout of a volatile photorefractive hologram. In the one-crystal configuration, two holographic memories and a mutually pumped phase conjugator (MPPC) are formed within a single photorefractive crystal, instead of using multiple crystals. Two memories share the same hologram and complement each other in refreshing the hologram. A MPPC suppresses fanning noise and automatically aligns the wavefront of the reference and readout beams. We found the optimum configuration to achieve nondestructive readout from calculations and geometric consideration. In the experiments with a BaTiO(3) crystal, a continuous readout of 20 times longer than the recording time was achieved.     

Seeded femtosecond optical parametric amplification in the mid-infrared spectral region above 3 mum

Femtosecond optical parametric amplification that results in microjoule mid-infrared pulses at wavelengths exceeding 3 mum is demonstrated. Narrow-band quasi-cw seeding at the signal wavelength is applied to ensure the generation of nearly transform-limited femtosecond pulses at the idler wavelength. The broad bandwidth of the parametric amplification provided by pumping with femtosecond pulses from a Ti:sapphire regenerative amplifier at high intensity results in idler pulse durations shorter than the pump pulse length. The potentials of three nonlinear optical crystals that belong to the potassium titanyl phosphate family are comparatively studied. At 1-kHz repetition rate our all-solid-state system produces highly synchronized ~100-fs pulses in the spectral range between 3 and 4 mum.     

High-intensity Effects in Self-pumped Photorefractive Phase Conjugation Using Nanosecond Pulses

Abstract

This paper reports on self-pumped photorefractive phase conjugation using repetitively pulsed high intensity (MW cm^?2) pulses of nanosecond duration. An intensity-dependent phase conjugate reflectivity is measured experimentally. In the corner self-pumped geometry the direction of beam fanning and corner self-pumping is observed to reverse above a critical intensity. Formation of a higher order spatial mode is reported for the external loop self-pumped geometry.     

Automatic time delay optimization between the pump and seed pulses of a broadly tunable femtosecond optical parametric amplifier

We propose a scheme for optimizing the time delay between the pump and seed pulses of an optical parametric amplifier (OPA) over a large spectral range. The efficiency of this method is demonstrated for a femtosecond BBO parametric amplifier seeded with a white-light continuum pulse. The error signal used for intensity stabilization results from a modulation of the temporal delay between the pump and the continuum pulses and phase-sensitive detection of the amplified signal. It allows us to lock the delay to the position that maximizes the OPA gain.     

Femtosecond optical parametric generation of noncollinear phase matching for a biaxial crystal

In an optical parametric generation of a femtosecond pulse for a biaxial crystal, the interaction of three waves can be used as a model of noncollinear phase matching in which the group velocities of the interacting pulses are suitably linked to each other. For satisfaction of group-velocity matching, the tunable parametric generation of femtosecond pulses must use noncollinear phase matching. We consider three conditions of group-velocity matching for femtosecond pulses. Signal and idler pulses can be obtained when the coupled-wave equations, including the group-velocity mismatch and group-velocity dispersion effects, are solved. A Fourier method is an effective method for solving the equations, and from the solution of the equations the relation between duration of output pulses and wavelengths can be obtained. In a comparison of collinear and noncollinear matches, when the latter is group-velocity matched, the duration of its outpulses are smaller, and the outpulses can be continually tuned from the visible to the mid-infrared.     

Carbon Nanotubes as an Ultrafast Emitter with a Narrow Energy Spread at Optical Frequency

Ultrafast electron pulses, combined with laser‐pump and electron‐probe technologies, allow ultrafast dynamics to be characterized in materials. However, the pursuit of simultaneous ultimate spatial and temporal resolution of microscopy and spectroscopy is largely subdued by the low monochromaticity of the electron pulses and their poor phase synchronization to the optical excitation pulses. Field‐driven photoemission from metal tips provides high light‐phase synchronization, but suffers large electron energy spreads (3–100 eV) as driven by a long wavelength laser (>800 nm). Here, ultrafast electron emission from carbon nanotubes (≈1 nm radius) excited by a 410 nm femtosecond laser is realized in the field‐driven regime. In addition, the emitted electrons have great monochromaticity with energy spread as low as 0.25 eV. This great performance benefits from the extraordinarily high field enhancement and great stability of carbon nanotubes, superior to metal tips. The new nanotube‐based ultrafast electron source opens exciting prospects for extending current characterization to sub‐femtosecond temporal resolution as well as sub‐nanometer spatial resolution.     

Low-threshold self-starting femtosecond Ti:sapphire laser

A low-threshold self-starting Kerr-lens mode-locked Ti:sapphire laser is demonstrated that uses a tight-focusing cavity design in conjunction with a semiconductor saturable-absorber mirror (SESAM). With 3% and 12% output couplers, we achieve mode-locking thresholds as low as 390 and 600 mW, respectively. Stable femtosecond laser pulses with average power of 114 mW are generated at a pump power of 1.2 W, which corresponds to a typical duration of 17 fs and bandwidth of 47 nm. Mode-locking operation is achieved in a pump power range of 600 mW to 4.8 W at an output coupling of 12%; the advantages of using a SESAM for low-power mode-locking operation are demonstrated.