D. H. Reitze

In vivo OCT imaging of hard and soft tissue of the oral cavity

We use optical coherence tomography (OCT) to perform a comprehensive program of in vivo and in vitro structural imaging of hard and soft tissues within the oral cavity. We have imaged the different types of healthy oral mucosa as well as normal and abnormal tooth structure. OCT is able to differentiate between the various types of keratinized and non-keratinized mucosa with high resolution. OCT is also able to provide detailed structural information on clinical abnormalities (caries and non-caries lesions) in teeth and provide guidance in dental restorative procedures. Our investigations demonstrate the utility of OCT as a diagnostic imaging modality in clinical and research dentistry.

Femtosecond spectral holography

Storage, recall, and processing of shaped femtosecond waveforms are achieved by performing spectral holography within a femtosecond pulse shaping apparatus. Time reversal, as well as correlation and convolution, of femtosecond temporal signals is demonstrated. Applications of this technique to matched filtering, dispersion compensation, encryption and decoding and femtosecond waveform synthesis are also discussed. The work extends the powerful principles of holographic signal processing, which have been used extensively for pattern recognition and filtering of two-dimensional spatial signals, to the femtosecond time domain. >

Evaluating the effect of transmissive optic thermal lensing on laser beam quality with a Shack–Hartmann wave-front sensor

We examine wave-front distortion caused by high-power lasers on transmissive optics using a Shack-Hartmann wave-front sensor. The coupling coefficient for a thermally aberrated Gaussian beam to the TEM(00) mode of a cavity was determined as a function of magnitude of the thermally induced aberration. One wave of thermally induced phase aberration between the Gaussian intensity peak and the 1/e(2) radius of the intensity profile reduces the power-coupling coefficient to the TEM(00) mode of the cavity to 4.5% with no compensation. With optimal focus compensation the power coupling is increased to 79%. The theoretical shape of the thermally induced optical phase aberration is compared with measurements made in a neutral-density filter glass, Faraday glass, and lithium niobate. The agreement between the theoretical and the measured thermal aberration profiles is within the rms wave-front measurement sensitivity of the Shack-Hartmann wave-front sensor, which is a few nanometers.

Effect of terbium gallium garnet crystal orientation on the isolation ratio of a Faraday isolator at high average power

We present a comprehensive and systematic investigation of the fundamental physical limitations of Faraday isolation performance at high average powers that are due to thermally induced birefringence. First, the operation of various Faraday isolator designs by use of arbitrary orientation of cubic magneto-optic crystals is studied theoretically. It is shown that, for different Faraday isolator designs, different crystal orientations can optimize the isolation ratio. Second, thermo-optic and photoelastic constants for terbium gallium garnet crystals grown by different manufacturers were measured. Measurements of self-induced depolarization are made for various orientations of crystallographic axes. The measurements are in good agreement with our theoretical predictions. Based on our results, it is possible to select a crystal orientation that optimizes isolation performance at high average powers, resulting in a 5-dB enhancement over nonoptimized orientations.

Investigation of self-induced depolarization of laser radiation in terbium gallium garnet

Absorption of laser radiation in magnetooptical materials results in a temperature gradient which induces depolarization due to both the temperature dependence of the Verdet constant and the photoelastic effect of thermal strains. This results in a limitation of the isolation ratio of Faraday isolators in high average power lasers. Here, we derive expressions for the isolation ratio as a function of beam power, beam radius, angle between incident polarization and crystal axis, and characteristics of the magnetooptical material. The theoretical results are compared with experiments for a terbium gallium garnet crystal. Our results allow us to choose the optimal parameters to maximize the isolation ratio and to compare different materials from this point of view.

Spectral holography of shaped femtosecond pulses.

Storage and subsequent recall of shaped femtosecond waveforms is achieved by performing spectral holography within a femtosecond pulse-shaping apparatus. Readout of the spectral hologram by using a short reference pulse yields both real and time-reversed reconstructions of the original signal. Convolution and correlation operations are achieved by using shaped pulses for readout. These experiments demonstrate the possibility of nonlinear spectral filtering and signal processing of femtosecond optical waveforms.

Programmable shaping of ultrabroad-bandwidth pulses from a Ti:sapphire laser

We have used a commercially available liquid-crystal spatial light modulator within a reflective optics pulse-shaping apparatus to shape ultrashort pulses with temporal resolution approaching 10 fs. Using the spatial light modulator as a phase modulator, we produce a variety of complex ultrafast waveforms, including odd pulses, high repetition rate (>23 THz) pulse trains, and asymmetric pulse trains. We also show that it is possible to compensate for large amounts of high-order phase dispersion (in excess of 60π) by appropriate cubic- and quartic-phase modulations of the pulse. Finally, we examine the limitations of shaping ultrabroad-bandwidth pulses. We find that, for specific classes of waveforms, Fourier-transform pulse-shaping techniques can be used for pulses with 5-fs durations, which exceed the current state of the art in ultrashort pulse generation. However, synthesis of general waveforms with 5-fs resolution will require compensating for nonlinear spatial dispersion of frequency in the masking plane.

Compensation of thermally induced modal distortions in Faraday isolators

Two methods of compensation of thermal lensing in high-power terbium gallium garnet (TGG) Faraday isolators have been investigated in detail: compensation by means of an ordinary negative lens and compensation using FK51 Schott glass possessing a negative dn/dT. Key thermooptic constants for TGG crystals and FK51 glass were measured. We find that the contribution of the photoelastic effect to the total thermal lens cannot be neglected for either TGG or FK51. We define a figure of merit for compensating glass and show that for FK51, an ordinary negative lens with an optimal focus is more efficient, but requires physical repositioning of the lens for different laser powers. In contrast, the use of FK51 as a compensating element is passive and works at any laser power, but is less effective than simple telescopic compensation. The efficiency of adaptive compensation can be considerably enhanced by using a compensating glass with figure of merit more than 50, a crystal with natural birefringence or gel.

Two-photon absorption in diamond and its application to ultraviolet femtosecond pulse-width measurement.

Intensity-dependent transmission measurements of 310-nm femtosecond pulses show that diamond has a twophoton absorption coefficient of 0.75 +/- 0.15 cm/GW, in approximate agreement with universal scaling formulas for two-photon absorption in diamond-structure materials. We then demonstrate that two-photon absorption is strong enough to permit simple measurements of ultraviolet femtosecond pulse widths in single-crystal diamond plates that are thin enough (250 microm) to be both inexpensive and dispersion free. Autocorrelation measurements of 10-50-nJ, 0.18-1.4-ps pulses are presented. The method requires no phase matching and can be applied to pulses in the wavelength range of 220-550 nm.

Shaping of wide bandwidth 20 femtosecond optical pulses

We investigate the propagation of 20 fs pulses through a generalized pulse shaper/compressor and synthesize shaped waveforms with 20 fs features by linear spectral filtering using a generalized pulse shaper consisting of gratings and reflective optics. The use of reflective optics in the pulse shaper avoids cubic phase dispersion associated with lenses which significantly broaden short 20 fs pulses. As an example of our pulse‐shaping capabilities, we generate pulse trains with repetition rates in excess of 12 THz using phase‐only filtering.