Curly R. Hoaglan

Precise diffraction efficiency measurements of large-area greater-than-99%-efficient dielectric gratings at the Littrow angle

We have developed improved cavity-finesse methods for characterizing the diffraction efficiencies of large gratings at the Littrow angle. These methods include measuring cavity length with optical techniques, using a Michelson interferometer to calibrate piezoelectric transducer nonlinearities and angle-tuning procedures to confirm optimal alignment. We used these methods to characterize two 20 cm scale dielectric gratings. The values taken from across their surfaces collectively had means and standard deviations of micro=99.293% and sigma=0.164% and micro=99.084% and sigma=0.079%. The greatest efficiency observed at a single point on a grating was (99.577+/-0.002)%, which is also the most accurate measurement of the diffraction efficiency in the literature of which we are aware. These results prove that a high diffraction efficiency with low variation is achievable across large apertures for gratings.

Gratings for High-Energy Petawatt Lasers

To enable high-energy petawatt laser operation we have developed the processing methods and tooling that produced both the worlds largest multilayer dielectric reflection grating and the worlds highest laser damage resistant gratings. We have successfully delivered the first ever 80 cm aperture multilayer dielectric grating to LLNLs Titan Intense Short Pulse Laser Facility. We report on the design, fabrication and characterization of multilayer dielectric diffraction gratings.

Wet-etch figuring for precision optical contouring

Wet-etch figuring utilizes free surface flows driven by surface tension gradients (the Marangoni effect) to confine and stabilize the size and shape of an etchant droplet attached to the underside of a glass surface. This droplet, or wetted zone, is translated on the surface, etching where it contacts and leaving behind no residue, to facilitate an etching-based small-tool figuring process that is free of mechanical and thermal stresses. The optic needs no backing plate, and its back side is free for inspection by optical means. When transmissive optics is figured, the optical thickness between the front and the rear surfaces of the optic is measured interferometrically and used in real time to control the local dwell time of the etchant zone. This truly closed-loop figuring process is robust, environmentally insensitive, and fully automated. It is particularly suited for figuring patterns such as phase plates, corrective elements, and optical flats on very thin (<< 1-mm) substrates that are difficult to figure with traditional abrasive polishing methods.

Manufacture, optical performance, and laser damage characteristics of diffractive optics for the National Ignition Facility

We have fabricated demonstration diffractive optic plates at full scale for the NIF laser. These include an off-axis focusing beam sampling grating, a color separation grating, and a kinoform phase plate for spatial beam smoothing. Fabrication methods and optical performance of these DOPs are discussed. IT was discovered that the sol-gel antireflective coating normally applied to high-power transmissive optics partially planarizes the diffractive structures, particularly on the color separation grating used for color management at target, to the extent that optical performance and laser damage threshold are negatively impacted. The effect of sol-gel coatings on grating performance, the feasibility of placing all diffractive structures on a single surface, and future work in this area are discussed.

Advanced dielectric grating technology for high-energy petawatt lasers

We describe the design, fabrication, and test of large-area multilayer dielectric gratings for high-energy petawatt lasers. 80/spl times/40-cm/sup 2/, 1780-mm/sup -1/ gratings have been fabricated, exhibiting high diffraction efficiency and >4 J/cm/sup 2/, 10-ps damage thresholds on witness samples.

Characterization of Polarization Sensitive, High Efficiency Dielectric Gratings for Formation Flight Interferometry

Reflective diffraction gratings enable novel optical configurations that simplify and improve laser interferometers. We have proposed an all-reflective grating interferometer that can be used in LISA type interferometers for space gravitational wave detection [1]. One configuration requires a highly polarization sensitive grating. We report on characterizations of a grating made atop high reflective dielectric layers. Using a direct measurement method, the diffraction efficiency at the Littrow angle for s-polarization is measured as 97.3% and for p-polarization 4.2%, leading to a s/p polarization diffraction ratio of 23.2. The depolarization from s- to p-polarization is measured to be ~1.7×10-4, and from p- to s-polarization 1.8×10-4. We derived a transfer matrix based on these measurements. Furthermore, we have developed a more accurate method for diffraction efficiency measurement using a grating cavity. These measurements are encouraging steps taken towards the requirements of an ideal grating interferometer.

Design and Installation of a static wavefront correction optic on the HELEN laser

This report discusses the design and installation of a static wavefront correction optic on the HELEN laser at AWE. The element is designed to compensate for static phase errors and prompt thermally induced aberrations on the backlighter beam of the laser. Partial compensation of cooling effects is also included in the design. A phase element has been fabricated using a recently developed novel wet etch figuring tool at LLNL. Performance evaluation through comparison of the focal spot pre- and post-installation is provided. The element has been tested on the laser to produce a 2x reduction in focal spot size.

Development of static phase control elements for high power solid state lasers

We describe development of passive phase correction elements to compensate for static phase errors and prompt thermally induced aberrations in the HELEN laser at AWE. Partial compensation of cooling effects is also included in the design. Phase elements have been fabricated through two processes, an indirect write lithographic process using amplitude masks generated from measured laser wavefronts and a direct write method using a novel wet etch figuring tool.

Fabrication and applications of large aperture diffractive optics

Large aperture diffractive optics are needed in high power laser applications to protect against laser damage during operation and in space applications to increase the light gathering power and consequently the signal to noise. We describe the facilities we have built for fabricating meter scale diffractive optics and discuss several examples of these.

Wet-etch figuring: optical surfacing by controlled application of etchant solution using the Marangoni effect

Wet-etch figuring (WEF), a computer-controlled method for generating arbitrarily shaped optical surfaces using wet chemical etching, has been developed. This method uses applicator geometry and surface tension gradients (the Marangoni Effect) to define and confine the footprint of a wetted etchant zone on the surface. Capillary forces attach the flowing etchant solution to the underside of the optic being figured. No mechanical or thermal stresses or residues are applied to the optic by this process. This enables interferometric measurement of the glass thickness while surfacing, which then controls the placement and dwell time of the wetted zone. The result is a truly deterministic, closed-loop figuring process with a high degree of optical precision. This process can figure submillimeter thickness, large-aperture plates or sheets that are very difficult to finish by conventional methods. Automated linear and circular spot etching tools were used to demonstrate surfacing on 380 micron-thick glass sheets, to Strehl better than 0.8, as specified by data array or Zernike polynomials.