Tunable narrow band difference frequency THz wave generation in DAST via dual seed PPLN OPG.

We report a widely tunable narrowband terahertz (THz) source via difference frequency generation (DFG). A narrowband THz source uses the output of dual seeded periodically poled lithium niobate (PPLN) optical parametric generators (OPG) combined in the nonlinear crystal 4-dimthylamino-N-methyl-4-stilbazolium-tosylate (DAST). We demonstrate a seamlessly tunable THZ output that tunes from 1.5 THz to 27 THz with a minimum bandwidth of 3.1 GHz. The effects of dispersive phase matching, two-photon absorption, and polarization were examined and compared to a power emission model that consisted of the current accepted parameters of DAST.

Experimental verification of sparse frequency linearly frequency modulated ladar signals modeling.

We present the results of an experiment designed to verify the results of a previously published theoretical model that predicts the range resolution and peak-to-side lobe ratio of sparse frequency linearly frequency modulated (SF-LFM) ladar signals. We use two ultra stable diode lasers which are frequency locked and can be current tuned in order to adjust the difference frequency between the two lasers. The results of the experiment verify the previously developed model proving that SF-LFM ladar signals have the ability to increase the range resolution of a ladar system without the need for larger bandwidth modulators. Finally we simulate a target at a range of approximately 150 meters through the use of a fiber optic delay line, and demonstrate the ability of SF-LFM ladar signals to detect a target at range.

Vectorial fiber laser using intracavity axial birefringence.

In this paper we investigate the polarization properties of a fiber laser with an intracavity c-cut calcite crystal that is capable of producing reconfigurable vectorial output modes. Vectorial modes with radial, azimuthal and generalized cylindrical vector polarizations can be generated by translating one lens within the laser cavity. Detailed studies of the mode polarization evolution show that the modes inside the laser cavity can be spatially homogeneously polarized in one section of the cavity while being spatially inhomogeneously polarized in another section of the cavity, which opens the opportunities for many potential new fiber laser design possibilities and applications. Furthermore, more complicated vectorial vortex output modes are also observed by purposefully introducing angular misalignments.

Sparse frequency LFM ladar signals.

Through modeling we explored the possibility of utilizing a sparse frequency linear frequency modulation (LFM) signal for laser radar (ladar) applications. We propose a potential transmit and receive experiment utilizing the superposition of two LFM laser sources with a known difference frequency to provide the necessary segmented bandwidth. Finally we analyzed the signal performance of the proposed system showing that the range resolution of the signal can be improved by two to three times while utilizing the same modulator bandwidth as that of a continuous LFM signal.

Optical characterization of wiregrid micropolarizers designed for infrared imaging polarimetry.

We report the optical characterization of a metal wiregrid micropolarizer array for IR imaging polarimetry. The micropolarizers are designed for operation in the 1.5-5.0 microm band with a specially designed thin SiO(2) layer between the silicon substrate and the wiregrids to improve the performance at the shorter wavelengths. Deep-UV projection lithography is used to fabricate 140-nm-deep wiregrids with a 400 nm period. The extinction ratio and the transmission coefficient are measured with a tunable IR laser. A TM transmission coefficient greater than 70% with an extinction ratio greater than 10(4) is achieved for the midwave-IR region while maintaining an extinction ratio better than 10(2) for the near-IR region above 1.5 microm.

Femtosecond micromachining in transparent bulk materials using an anamorphic lens.

A unique anamorphic lens design was applied to a circular 780nm femtosecond laser pulse to transform it into an elliptically shaped beam at focus. This lens was developed to give an alternative method of micromachining bulk transparent materials. The challenge for femtosecond laser processing is to control the nonlinear affect of self-focusing, which can occur when using a fast f-number lens. Once the focused spot is dominated by self-focusing the predicted focused beam becomes a filament inside the bulk, which is an undesirable effect. The anamorphic lens resolves this self-focusing by increasing the numerical aperture (NA) and employing an elliptical beam shape. The anamorphic lens was designed to furnish a 2.5mum by 190mum line at focus. Provided the pulse energy is high enough, transparent bulk material will be damaged with a single femtosecond laser pulse. Damage in this text refers to visual change in the index of refraction as observed under an optical microscope. Using this elliptical shape (or line), grating structures were micro-machined on the surface of SiC bulk transparent substrate. SiC was chosen because it is known for its micromachining difficulty and its crystalline structure. From the lack of self-focusing and using energy that is just above the damage threshold the focused line beam generated from the anamorphic lens grating structures produced a line shape nearly identical to the geometrical approximation. In this paper we discuss a new method of writing gratings (or other types of structures) in bulk transparent materials using a single femtosecond laser pulse. We will investigate the grating structures visually (inspected under an optical microscope) and also by use of an atomic force microscopy (AFM). In addition, we test the grating diffraction efficiency (DE) as a function of grating spacing, d.

Enhanced signal coupling into periodically poled lithium niobate with microlens arrays.

The return signal frequency of an eye-safe ladar system is upconverted from the infrared to the visible through sum-frequency generation by incorporation of periodically poled LiNbO3 into the receiver. A quantitative analysis of the angular acceptance and the quantum efficiency is then presented for a single macroscopic receiver optic and a multiaperture microlens array. Comparing both results, a 6x increase in the receiver field of regard and an 18% increase in beam coupling were realized for the microlens design over the macroscopic system.