Dental optical coherence tomography: a comparison of two in vitro systems.

OBJECTIVE: To compare the imaging results obtained with two different in vitro prototype dental optical coherence tomography (OCT) systems. METHODS: Two prototypes were evaluated: an 850 nm wavelength, 700 microW OCT system with a relatively low numerical aperture (0.03) and a 1310 nm wavelength, 140 microW system with a higher numerical aperture (0.20). RESULTS: Using the 850 nm system a characteristic scattering signal was observed that correlated with the depth of a periodontal probe. There was, however, insufficient light penetration to create images with adequate resolution. Improved image quality was achieved with the 1310 nm OCT system; these images had sufficient resolution to allow identification of anatomical structures important for the diagnostic assessment of oral structures. CONCLUSIONS: These results illustrate the improvement in imaging dental structures that can be obtained with a prototype 1310 nm OCT system. The feasibility of OCT as a dental imaging technique is verified.

Imaging of hard- and soft-tissue structure in the oral cavity by optical coherence tomography.

We have developed a prototype optical coherence tomography (OCT) system for the imaging of hard and soft tissue in the oral cavity. High-resolution images of in vitro porcine periodontal tissues have been obtained with this system. The images clearly show the enamel-cementum and the gingiva-tooth interfaces, indicating OCT is a potentially useful technique for diagnosis of periodontal diseases. To our knowledge, this is the first application of OCT for imaging biologic hard tissue.

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.

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.

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.

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.