Kim A. Winick

Fabrication and characterization of photonic devices directly written in glass using femtosecond laser pulses

Both straight and curved waveguides are written in a variety of silicate glasses using near-IR femtosecond laser pulses. Writing parameters are identified that produce waveguides that support only a single mode and yield smooth-mode profiles. The laser pulse-induced refractive index change is reconstructed from near-field mode profile data using the scalar wave equation and by refractive near-field profiling. Waveguide propagation losses are determined by throughput and Fabry-Perot resonator measurements. Both coarse and fine period gratings are written and characterized, and the thermal stability of these gratings is investigated. The utility of the femtosecond writing technique is demonstrated by fabricating an optical interleaver.

Cramér–Rao lower bounds on the performance of charge-coupled-device optical position estimators

The problem of optically estimating an object’s position by using a charge-coupled device (CCD) array composed of square pixels Δx on a side is analyzed. The object’s image spot at the CCD is assumed to have a Gaussian intensity profile with a 1/e point at a radial distance of 2σs from the peak, and the CCD noise is modeled as Poisson-distributed, dark-current shot noise. A two-dimensional Cramer–Rao bound is developed and used to determine a lower limit for the mean-squared error of any unbiased position estimator, and the maximum-likelihood estimator is also derived. For the one-dimensional position-estimation problem the lower bound is shown to be minimum for a pixel-to-image size ratio Δx/σs of between 1 and 2 over a wide range of signal-to-noise ratios. Similarly for the two-dimensional problem, the optimum ratio is shown to lie between 1.5 and 2.5. As is customary in direct detection systems, it is also observed that the lower bound is a function of both the signal power and noise power separately and not just of their ratio. Finally, the maximum-likelihood estimator is shown to be independent of the signal and noise powers at high signal-to-noise ratios.

Planar glass waveguide ring resonators with gain

The frequency resolution of an active waveguide ring resonator spectrometer is fundamentally limited by spontaneous emission noise produced by the gain medium. A closed-form expression for this resolution is derived, and the result is used to determine the minimum, rms, angular rotation rate, random walk error achievable by an active ring resonator gyroscope. An active waveguide ring resonator is demonstrated in a neodymium-doped glass, and a finesse of 250 at a signal wavelength of 1060 nm is achieved for the 1.6 cm diameter ring under laser diode pumping. This finesse corresponds to an effective propagation loss on the order of 0.013 dB/cm, which is the lowest value reported to date for rings of this size.

Effective-index method and coupled-mode theory for almost-periodic waveguide gratings: A comparison

Contradirectional propagation through active, first-order, almost-periodic, corrugated waveguide gratings is analyzed by using both coupled-mode theory and a combined effective-index/impedance-matching matrix technique. For TE-mode operation, which is near the first-order Bragg wavelength, the equivalence of the two techniques is analytically demonstrated for shallow surface corrugations.

Neodymium‐doped glass channel waveguide laser containing an integrated distributed Bragg reflector

An integrated, distributed Bragg reflector laser in a Nd‐doped, glass, channel waveguide is reported for the first time. The waveguide is fabricated using Ag+ thermal ion exchange in a soda‐lime‐silicate‐glass containing 2% Nd2O3 by weight. The distributed Bragg reflector grating is produced holographically in photoresist and then etched into the waveguide using argon ion milling. The device lases in a single longitudinal mode with a pump threshold of 50 mW and a slope efficiency of 1%.

Optimum holographic elements recorded with nonspherical wave fronts

The problem of designing a flat, aspheric, holographic optical element that images a finite set of input wave fronts into a finite set of output wave fronts is rigorously analyzed. The optimum phase transfer function of the holographic optical element is analytically determined. The optimum phase transfer function is defined as the one for which the element has minimum mean-squared wave-front error averaged over the set of input wave fronts. It is also shown that in general it may not be possible to obtain a low value for this average mean-squared wave-front error by using a single holographic element. Furthermore, the performance trade-off of spherical aberration, coma, and astigmatism versus geometric distortion is clearly indicated by example.

Designing efficient aberration-free holographic lenses in the presence of a construction–reconstruction wavelength shift

We have derived an analytic procedure for recording a flat, volume-phase-transmission holographic optical element at a wavelength different from that at which it is to be used. The procedure guarantees that the resulting element will have diffraction-limited aberration performance. Furthermore it guarantees, to a first order, that the Bragg condition for high diffraction efficiency will be satisfied. The technique gives simple analytic expressions for the required object and reference construction-beam phases at the element. In general, the object and reference construction beams must be realized by using computer-generated holograms in conjunction with conventional refractive or reflective optics.

Atmospheric turbulence-induced signal fades on optical heterodyne communication links.

The three basic atmospheric propagation effects, absorption, scattering, and turbulence, are reviewed. A simulation approach is then developed to determine signal fade probability distributions on heterodyne-detected satellite links which operate through naturally occurring atmospheric turbulence. The calculations are performed on both angle-tracked and nonangle-tracked downlinks, and on uplinks, with and without adaptive optics. Turbulence-induced degradations in communication performance are determined using signal fade probability distributions, and it is shown that the average signal fade can be a poor measure of the performance degradation.

An infrared integrated optic astronomical beam combiner for stellar interferometry at 3-4 μm

Integrated-optic, astronomical, two-beam and three-beam, interferometric combiners have been designed and fabricated for operation in the L band (3 microm--4 microm) for the first time. The devices have been realized in titanium-indiffused, x-cut lithium niobate substrates, and on-chip electro-optic fringe scanning has been demonstrated. White light fringes were produced in the laboratory using the two-beam combiner integrated with an on-chip Y-splitter.

Waveguide electro-optic modulator in fused silica fabricated by femtosecond laser direct writing and thermal poling

An integrated electro-optic waveguide modulator is demonstrated in bulk fused silica. A Mach-Zehnder interferometer waveguide structure is fabricated by direct writing with a femtosecond laser followed by thermal poling. A 20 degrees electro-optic phase shift is achieved at an operating wavelength of 1.55 microm with an applied voltage of 400 V and an interaction length of 25.6 mm, which correspond to an estimated effective electro-optic coefficient of 0.17 pm/V for the TE-polarized mode.