Asher A. Friesem

The formation of laser beams with pure azimuthal or radial polarization

Laser resonator configurations for obtaining pure azimuthal and radial polarized beams are presented. They involve the coherent summation, inside the laser resonator, of two orthogonally polarized TEM01 modes. Basic principles and experimental results with a Nd:YAG laser are presented. The results include a full space variant polarization measurement and show efficient formation of high-quality azimuthal and radial polarized beams.

Two Photon Absorption and Coherent Control with Broadband Down-Converted Light

We experimentally demonstrate two-photon absorption with broadband down-converted light (squeezed vacuum). Although incoherent and exhibiting the statistics of a thermal noise, broadband down-converted light can induce two-photon absorption with the same sharp temporal behavior as femtosecond pulses, while exhibiting the high spectral resolution of the narrow band pump laser. Using pulse-shaping methods, we coherently control two-photon absorption in rubidium, demonstrating spectral and temporal resolutions that are 3-5 orders of magnitude below the actual bandwidth and temporal duration of the light itself. Such properties can be exploited in various applications such as spread-spectrum optical communications, tomography, and nonlinear microscopy.

Temporal shaping of entangled photons

We experimentally demonstrate shaping of the two-photon wave function of entangled-photon pairs, utilizing coherent pulse-shaping techniques. By performing spectral-phase manipulations we tailor the second-order correlation function of the photons exactly like a coherent ultrashort pulse. To observe the shaping we perform sum-frequency generation with an ultrahigh flux of entangled photons. At the appropriate conditions, sum-frequency generation performs as a coincidence detector with an ultrashort response time (approximately 100 fs), enabling a direct observation of the two-photon wave function. This property also enables us to demonstrate background-free, high-visibility two-photon interference oscillations.

Nonlinear interactions with an ultrahigh flux of broadband entangled photons.

We experimentally demonstrate sum-frequency generation with entangled photon pairs, generating as many as 40,000 photons per second, visible even to the naked eye. The nonclassical nature of the interaction is exhibited by a linear intensity dependence of the nonlinear process. The key element in our scheme is the generation of an ultrahigh flux of entangled photons while maintaining their nonclassical properties. This is made possible by generating the down-converted photons as broadband as possible, orders of magnitude wider than the pump. This approach can be applied to other nonlinear interactions, and may become useful for various quantum-measurement tasks.

Compact beam expander with linear gratings

Novel compact beam expanders that could be useful for applications such as providing light to flat panel displays are presented. They are based on a planar configuration in which three spatially linear gratings are recorded on one transparent substrate, so as to expand a narrow incoming beam in two dimensions. We present the design and recording procedures along with results, showing a relatively uniform intensity of the wide output beam. Such expanders can serve for illuminating flat panel displays.

Discontinuous phase elements for transverse mode selection in laser resonators

Discontinuous phase elements can be inserted into laser resonators so that the lasers will operate with only one desired high order transverse mode. These elements introduce sharp discontinuous phase changes so as to result in minimal losses for a desired transverse mode but high losses to others. The basic principles, along with experimental results with Nd:yttrium–aluminum–garnet and CO2 lasers, illustrating improved output powers with a high beam quality of low divergence, are presented.

Continuous-phase elements can improve laser beam quality

Siegman [Opt. Lett. 18, 675 (1993)] showed that binary-phase plates cannot improve laser beam quality. We demonstrate that continuous spiral phase elements can improve the quality of beams that originate from a laser operating with a pure high-order transverse mode. A theoretical analysis is presented, along with experimental results obtained with a CO(2) laser. The results reveal that a nearly optimal Gaussian output beam can be obtained with only a small decrease in the output power.

Very narrow spectral filters with multilayered grating-waveguide structures

Promising configurations for grating-waveguide structures are presented. In these structures, the grating layer, which is normally adjacent to the waveguide layer, is displaced by means of an intermediate layer, resulting in significant reduction of losses and weaker coupling. This leads to very narrow spectral bandwidths and high contrast ratios. Experimental results reveal that the spectral bandwidths can be as low as 0.1 nm with contrast ratios greater than 1000, suggesting that these grating-waveguide structures could be useful for optical communication networks.

Light modulation with electro-optic polymer-based resonant grating waveguide structures.

A novel modulator design incorporating an E-O polymer into a resonant grating waveguide structure is presented. Using purely polymeric material we developed a resonant grating waveguide structure having low loss and high finesse, with approximately 2nm spectral line width at 1.55 mum. An externally applied voltage modulates the refractive index of the E-O waveguide, thereby shifting the resonance wavelength and modulating the incident light at MHz rates. Such modulator operates in free space and does not involve waveguide patterning nor the need for facet conditioning and coupling common to operation in the Mach-Zehnder type configuration.

Efficient selection of high-order Laguerre-Gaussian modes in a Q-switched Nd:YAG laser

Intra-cavity binary phase elements are incorporated into a Q-switched Nd:YAG laser resonator to obtain efficient high-order transverse mode selection. The resonator configuration is analyzed using the propagation-matrix diagonalization method and the Fox-Li algorithm, and a simple model for predicting the relative output powers of the selected modes is developed. The predicted results are verified experimentally with binary phase elements for selecting the TEM/sub 01/, TEM/sub 02/ and TEM/sub 03/, degenerate Laguerre-Gaussian modes. The output energy per pulse was 15 mJ for the TEM/sub 01/, 16.5 mJ for TEM/sub 02/ and 18.3 mJ for TEM/sub 03/, all higher than the 10 mJ for the TEM/sub 00/. The performance in Q-switched operation was found to be similar to that in free-running operation. The numerical calculations, experimental procedure and experimental results are presented.