Howard Nathel

Efficient continuous-wave chromium-doped YAG laser

Continuous-wave power performance of a chromium-doped YAG laser pumped by a Nd:YAG laser is investigated as a function of crystal temperature, output coupling transmission, and emission wavelength. Using a 2%-transmitting output coupler, we obtained as much as 1.9 W of cw output power at 1.45 μm with an absorbed power slope efficiency of 42% when the gain medium was cooled at 3°C. Using the laser efficiency data, we estimated the emission and excited-state absorption cross sections at 1.45 μm and compared them with the previously reported values.

17-fs pulses from a self-mode-locked Ti:sapphire laser

We have generated sub-17-fs-duration pulses directly from a self-mode-locked Ti:sapphire laser. These pulses are near transform limited, with a wavelength centered at 817 nm, a pulse repetition rate of 80 MHz, and an average power of 500 mW. By minimizing the amount of material inside the laser cavity and choosing the correct glass for the intracavity prism pair, third-order dispersion in the laser can be significantly reduced compared with that in previous designs. Extracavity compensation for group-velocity dispersion in the output coupler and autocorrelator optics is necessary to measure this pulse width. To our knowledge this laser generates pulses substantially shorter than any other laser to date.

Continuous-wave self-mode-locked operation of a femtosecond Cr 4+ :YAG laser

Continuous-wave self-mode-locked operation of a chromium-doped YAG laser pumped by a continuous-wave Nd:YAG laser at 20 degrees C is described. We used both regenerative initiation and continuous-wave self-mode-locking techniques to generate nearly transform-limited pulses of 120-fs (FWHM) duration at 1.52 microm. The TEM(00) output power was as high as 360 mW. The output of this femtosecond source was tunable from 1.51 to 1.53 microm.

Generation of tunable femtosecond pulses in the 1.21-1.27 /spl mu/m and 605-635 nm wavelength region by using a regeneratively initiated self-mode-locked Cr:forsterite laser

A detailed description of the design and operational characteristics of a regeneratively initiated, self-mode-locked Cr:forsterite laser pumped by a continuous-wave Nd:YAG laser is given. Without compensating for the intracavity positive group velocity dispersion, regenerative acoustooptic modulation produced pulses of between 41 and 6.5 ps (FWHM) at 1.23 /spl mu/m with average output powers of between 280 and 380 mW, respectively. Using intracavity negative group-velocity-dispersion compensation, nearly transform-limited femtosecond pulses of 48 fs (FWHM) duration were generated with average TEM/sub 00/ output powers of 380 mW at 1.23 /spl mu/m. By tuning the output of the mode-locked laser from 1.211 to 1.264 /spl mu/m, the dispersion (second and third order) of the Cr:forsterite gain medium has been measured. The operational wavelength range of the laser was extended to the visible region from 605 to 635 nm by external frequency doubling in a LiIO/sub 3/ nonlinear crystal. With approximately 250 mW of fundamental pump power at 1.23 /spl mu/m, red pulses of 116 fs (FWHM) duration at 615 nm were obtained with conversion efficiencies approaching 10 percent. >

Generation of 48-fs pulses and measurement of crystal dispersion by using a regeneratively initiated self-mode-locked chromium-doped forsterite laser

A regeneratively initiated self-mode-locked chromium-doped forsterite laser operated at 3.5 degrees C is described. By employing intracavity negative-group-velocity dispersion compensation, nearly transform-limited femtosecond pulses of 48-fs (FWHM) duration were generated with average TEM(00) output powers of 380 mW at 1.23 microm. Regenerative initiation provides improvement in the output stability and ease of operation compared with fixed-frequency acousto-optic modulators. By tuning the mode-locked laser in the range 1.21-1.26 microm, estimated values for forsterite dispersion constants have also been obtained for the first time to our knowledge. The demonstrated power and stability open the door to applications such as efficient second-harmonic generation.

Propagation of intense, ultrashort laser pulses in plasmas.

We have investigated the propagation of terawatt-power laser pulses in gases. The spatial distribution of focused radiation is modified by refraction that results from the spatially inhomogeneous refractive index of the plasma generated by high field ionization. We observe Thomson scattering, stimulated Raman scattering, and large wavelength shifting of the laser light.

Prepulse suppression for high-energy ultrashort pulses using self-induced plasma shuttering from a fluid target

The technique of self-induced plasma shuttering can be used to suppress prepulse energy from an ultrashort pulse. If a femtosecond pulse is incident upon a transparent target, the leading edge passes through while the peak reflects owing to ionization breakdown at the surface. We describe a fluid jet, enclosed in a vacuum chamber, that allows this technique to be used at high repetition rates. The jet has excellent stability and a fast (~500 micros) recovery time. At normal incidence, we demonstrate a reflection efficiency of 70% with a prepulse-to-main-pulse energy suppression ratio of >10, while at Brewster incidence we measure a reflection efficiency of 38% with a suppression ratio of nearly 400.

Optical coherence tomography for diagnosing periodontal disease

We have, in this preliminary study, investigated the use of optical coherence tomography for diagnosis of periodontal disease. We took in vitro OCT images of the dental and periodontal tissues from a young pig and compared them to histological sections. These images distinguish tooth and soft tissue relationships that are important in diagnosing and assessing periodontal disease. We have imaged the attachment of gingiva to the tooth surface and located the cemento-enamel junction. This junction is an important reference point for defining attachment level in the diagnosis of periodontal disease. the boundary between enamel and dentin is also visible for most of the length of the anatomical crown, allowing quantitation of enamel thickness and character.

Prepulse suppression using a self-induced ultrashort pulse plasma mirror

The plasma mirror is a self-induced, plasma-based optical element which can be inserted into existing experiments to reduce prepulse energy without significant degradation of ultrashort pulse laser light. We have directly observed the nonlinear reflectivity of the plasma mirror as well as the spatial and temporal characteristics of the reflected pulse. The initial measurements indicate that the incident pulse reflects specularly from a high density, highly reflective plasma. The reflected pulse has a smoothed spatial profile and reduced pulsewidth. We outline future work to characterize both the plasma mirror technique of prepulse suppression and its reflected pulse.

Sub-50-femtosecond high-peak-power pulses from a regeneratively initiated self-sustained continuous-wave mode-locked chromium-doped forsterite laser

Regeneratively-initiated, self-sustained, continuous-wave mode-locked operation of a chromium-doped forsterite laser operated at 3.5 degree(s)C is described. Without compensating for the positive group velocity dispersion of the cavity, regenerative, acousto-optic modulation produced pulses of between 41 and 6.5 psec (FWHM) at 1.23 micrometers with average output powers of between 280 and 380 mW, respectively. By employing intracavity negative group velocity dispersion compensation, nearly transform-limited femtosecond pulses of 48 fsec (FWHM) duration were generated with average TEM00 output powers of 380 mW at 1.23 micrometers . These represent the shortest and highest peak power pulses directly generated from this laser system to date.