Self-optical parametric oscillation in periodically poled neodymium-doped lithium niobate.

Self-optical parametric oscillation is demonstrated for the first time to our knowledge in a periodically poled neodymium-doped lithium niobate (Nd:PPLN) crystal. The crystal is pumped by a cw Ti:sapphire laser at 813.5 nm. The Nd(3+) ions absorb the 813.5-nm radiation to generate 1084-nm laser oscillation. The internally Q switched 1084-nm radiation pumps the periodically poled lithium niobate host matrix to generate optical parametric oscillation at 1.55 and 3.6 microm . Up to 24% conversion efficiency from laser to signal is observed.

8-mJ TEM(00) diode-end-pumped frequency-quadrupled Nd:YAG laser.

A frequency-quadrupled Nd:YAG laser pumped by a commercially available high-brightness stack is described. A master oscillator power amplifier configuration was implanted, and the laser delivered energy of 8 mJ at 0.266microm . The laser was air cooled for easy use.

Self-pumped phase-conjugate diode-pumped Nd:YAG loop resonator.

We demonstrate a self-pumped phase-conjugate diode-pumped Nd:YAG loop resonator. In the injected configuration a maximum phase-conjugate reflectivity of 32 is obtained. The output beam has a pulse shape of ~30 ns , which shows the self- Q -switching ability of the resonator. By use of a low-refractive-output coupler, a self-starting version was produced. The resonator operated in a stable diffraction-limited TEM(00) mode, even with severe intracavity phase aberrations. A maximum energy of 5.5 mJ (2.4% of the energy of the high-brightness stacks that was incident upon the Nd:YAG crystals) in a 20-ns single-longitudinal-mode pulse was measured.

Demonstration of optically controlled data routing with the use of multiple-quantum-well bistable and electro-optical devices.

We present experimental results on a 1-to-64-channel free-space photonic switching demonstration system based on GaAs/GaAlAs multiple-quantum-well active device arrays. Two control schemes are demonstrated: data transparent optical self-routing usable in a packet-switching environment and direct optical control with potential signal amplification for circuit switching. The self-routing operation relies on the optical recognition of the binary destination address coded in each packet header. Address decoding is implemented with elementary optical bistable devices and modulator pixels as all-optical latches and electro-optical and gates, respectively. All 60 defect-free channels of the system could be operated one by one, but the simultaneous operation of only three channels could be achieved mainly because of the spatial nonhomogeneities of the devices. Direct-control operation is based on directly setting the bistable device reflectivity with a variable-control beam power. This working mode turned out to be much more tolerant of spatial noises: 37 channels of the system could be operated simultaneously. Further development of the system to a crossbar of N inputs and M outputs and system miniaturization are also considered.

Large-field-of-view, high-gain, compact diode-pumped Nd:YAG amplifier.

We present a compact diode-end-pumped Nd:YAG amplifier with a FWHM angular bandwidth of 100 degrees . Using a 1.064-microm probe beam, we measured a 2-pass gain of 23 (13.6dB) and a 4-pass gain of 420 (26.2dB). High-gain image amplification with such a laser amplifier is also demonstrated.

Widely tunable optical parametric oscillator with electrical wavelength control.

A LiNbO(3) optical parametric oscillator (OPO) pumped at 930nm shows a wide phase-matching curve. Each pulse produced by the OPO has a very broad natural linewidth, from 1480 to 1800 nm for the signal and from 1950 to 2550 nm for the idler. The emission wavelength is controlled thanks to an electrically tunable Fabry-Perot interferometer inserted into the OPO cavity. The signal wavelength is electrically tuned in the range 1450-1850nm without crystal rotation.

High-efficiency TEM(00) Nd:YVO(4) laser longitudinally pumped by a high-power array.

A new simple, compact, and efficient optical device is presented that allows any commercially available arrays or stacks to longitudinally pump laser materials. A Nd:YVO(4) laser was built and an optical-optical efficiency as great as 50% was demonstrated with a diffraction-limited beam.

Efficient degenerate four-wave mixing in a diode-pumped microchip Nd:YVO(4) amplifier.

The process of saturable-gain degenerate four-wave mixing in a diode-pumped microchip Nd:YVO(4) amplifier is investigated. To enhance the efficiency of the interaction, multipass geometries are employed in which the weak signal beam and therefore the conjugate beam experience several passes in the gain medium. Degenerate fourwave mixing reflectivities as high as R = 10% and R = 170% have been obtained experimentally for 130-W diode pumping with two-pass and four-pass geometries, respectively. Finally, the imaging capabilities of the volume population hologram written in the Nd:YVO(4) amplifier are demonstrated.

Tunable IR laser source with optical parametric oscillators in series.

A tunable IR laser source is described. A Q-switched Nd:YAG laser at 1.064 µm is used to pump two optical parametric oscillators (OPO's) in series. The first OPO uses a LiNbO(3) crystal and works in the vicinity of degeneracy. Its signal beam (adjusted to 1.82 µm) is used to pump the second OPO, which uses a AgGaSe(2) crystal and covers the range 2.6-6 µm. The conversion efficiency between the energy produced by this OPO in the signal and idler beams (λ(s) = 2.6 µm and λ(i) = 6 µm, respectively) and the input energy at 1.064 µm is 1.75%.

Nd:MgO:LiNbO(3) waveguide laser and amplifier.

We report efficient operation of a channel waveguide laser and a channel waveguide amplifier in Nd:MgO:LiNbO(3). For the laser a cw output power of 2.9 mW was obtained for 23.6 mW of absorbed pump power. The absorbed pump power at threshold was 1.5 mW, and a slope efficiency of 13% was achieved. For the amplifier a small-signal gain of 7.5 dB was achieved for 22 mW of coupled pump power.

[Use of specific photo-markers in experimental laser coronary angioplasty]. / Utilisation de photomarqueurs spécifiques pour l'angioplastie coronaire expérimentale par rayonnement laser.

Research into improving the efficacy of laser in the vaporisation of atheromatous plaques has led to the study of their modes of emission and to attempts at changing the natural absorption properties of the target lesion. This was achieved in vitro by incubation with solutions of derived haematoporphyrin and in vivo by oral administration of therapeutic doses of tetracycline. The selective nature of the coloration produced having been proved, the use of appropriate lasers allowed reduction of 50 to 100 p. 100 of the emitted energy; the exploitation of the fluorescent properties of tetracycline, investigated by laser spectrofluoroscopy in this study, has led to the concept of in situ photodiagnosis of the plaque. These results may open up a new field of research into unknown photobiological characteristics radically different to the thermic energy used at present.

[Experimental coronary angioplasty using hematoporphyrin coloration simultaneously with a pulsed laser beam]. / Angioplastie coronaire expérimentale utilisant simultanément la coloration par l'hématoporphyrine et un rayon laser pulsé.

The setting up of a reliable method for angioplasty by laser irradiation is based. On the one hand, on the simultaneous control of the observed thermal, mechanical and photochemical effects within the atheromatous material and, on the other, on the demonstration of the absence of effects damaging to the arterial tunica. Theoretical modelling and experimental use have together proved the unique importance of pulsed emission, which strongly reduces thermal diffusion. Labelling of the atheromatous plaque by a hematoporphyrin derivative (HPD), as exemplified in the carcinological process, cancels the superficial reflectivity and strongly reduces the diffusion coefficient in the plaque. It confers on it a spectral absorption characteristic which is the basis of the exact physical coupling between the frequency of the exciting radiation and the absorption which generates vaporization in the pathological material. The preparation of chemically more homogeneous HPD and the spectrometric study of other colouring agents in vivo point to a further development of this technique of angioplasty.

[Pulsed emission in laser coronary angioplasty. Theoretical bases and experimental application]. / Apport de l'émission pulsée dans l'angioplastie coronaire par laser. Bases théoriques et application expérimentale.

The aim of this study was to evaluate the thermal diffusion of a pulsed laser beam in atheroma and to obtain in vitro vaporisation of the plaque without causing arterial wall lesions. A computerised mathematical model integrated 4 parameters: reflectivity, thermal conduction, the absorption factor and coefficient of diffusion. The thermal diffusion was shown to be dependent on the time constant and the temperature of vaporisation may be best attained with a short burst (200 ns) with a high peak power (6000 w). The experimentation was performed on fresh debris and segments of epicardial coronary arteries which were exposed to a pulsed laser beam with a frequency of 1000 Hz in bursts of 200 ns at wave lengths of 1060 and 532 nm. The results were evaluated by microscopic examination of transverse sections perpendicular to the lumen of the artery. Effective vaporisation of atheroma was observed with weak mean dissipating powers (0.4 w) about 10 times weaker than with continuous node emission; examination of the underlying arterial wall showed no thermal or mechanical damage.

Selective mode excitation of graded index optical fibers.

A method is described permitting excitation of a small number of modes in graded index fibers, the order of the launched mode being easily varied and determined. Using this launching technique, it is possible to determine the index profile and index difference of graded core fibers; results are compared with those obtained by other techniques. Other important fiber parameters, such as the differential attenuation and the differential propagation delay time as a function of the mode parameter, are also obtained, giving an insight into the fundamental propagation characteristics of the fiber.

Propagation model for long step-index optical fibers.

This paper reports some theoretical and experimental investigations on the propagation of a pulse in long step-index optical fibers where mode conversion is present. We derive simple analytical expressions for the frequency response and the radiation pattern as a function of the fiber's length provided the launching conditions are known. A comparison with experimental observations made on a 3-km long Corning fiber shows reasonable agreement. We conclude that the approximations used to obtain the simple analytical solutions are satisfactory for predicting the transmission characteristics of the fiber.

Mode coupling in a multimode optical fiber with microbends.

Experimental observations of the radiation pattern of a short multimode fiber with different induced deformations are reported here. Theoretical calculations are also made, based on the assumption of strong coupling between two particular modes of the fiber, and are in good agreement with experiment. We obtain numerical evaluations of the coupling coefficients and excess loss as a function of the amplitude of the deformation, and indications on the requirements to be met in cabling the fibers are derived from the experimental results.