Jacob M. Hammer

Scattering in ZnO-Sapphire Optical Waveguides

Optical waveguide loss due to scattering in epitaxial ZnO-sapphire films was studied by measuring the dependence of attenuation on mode number and on optical wavelength. Analysis of the results in terms of wave-optical scattering theory enabled the bulk and surface scattering contributions to be determined. The bulk scattering showed the lambda(-4) dependence characteristic of Rayleigh scattering. A white noise spectrum of surface roughness was found to be in good agreement with surface scattering observed in most of the waveguides studied. The attenuation coefficients derived from the data were related to specific aspects of the waveguide fabrication process and allowed rational improvements resulting in a decrease in loss from 20 dB/cm to 1 dB/cm.

Phase‐locked operation of coupled pairs of grating‐surface‐emitting diode lasers

Grating‐surface emission from phase‐locked monolithic pairs of grating‐surface‐emitting diode lasers has been observed for the first time. Near‐diffraction‐limited far fields of 0.06°, spectra and power versus current plots of the coherent emission from the three second‐order distributed Bragg reflectors which form the laser cavity for two collinear gain sections are presented. The results suggest that linear arrays of grating‐surface emitters can be formed by connecting additional gain‐grating sections in series.

Optical grating coupling between low‐index fibers and high‐index film waveguides

Grating couplers between low‐refractive‐index (n ≈1.5) optical fibers and high‐refractive‐index (n ≈2.2) electro‐optic planar guides have been analyzed and demonstrated. Coupling efficiencies of 6% between single modes of a film and a fiber have been observed. Efficiencies of approximately 86% between single‐mode fibers and films using blazed backward‐wave gratings are predicted. In addition, low‐loss low‐optical‐aperture multimode fibers may be coupled to single‐mode films with efficiencies greater than 50% and angular spreads less than 0.5°.

Single‐wavelength operation of the hybrid‐external Bragg‐reflector‐waveguide laser under dynamic conditions

Dynamic wavelength stabilization of an AlGaAs diode laser has been demonstrated. The diode laser is butt coupled to a stripe‐loaded optical waveguide which contains a Bragg reflecting grating. The laser is pulsed with 50‐ns‐long, 25% duty cycle, 100% modulation depth pulses and the waveguide output is observed to remain at single wavelengths over a peak current range of 120–160 mA and 15 °C temperature variations.

High-speed electrooptic waveguide grating modulator using epitaxial ZnO

The results of a detailed study of electrooptic waveguide grating modulators are presented. Using low-loss epitaxial ZnO on sapphire waveguides with deposited interdigital electrodes diffraction percentages up to 80 percent at 80 V with rise times below 3 ns are observed. Capacitive power requirements on the order of 4 mW/MHz are predicted. A theoretical description which includes carrier and piezoelectric effects is given and is shown to be adequate for designing this type of modulator. The grating waveguide modulator spatially separates the diffracted from the main beam and thus can be used for switching. In addition, the spatial separation allows a number of these to be cascaded on a common waveguide. These features make it probable that grating modulators will find a variety of uses in integrated optics.

Observations and theory of high-power butt coupling to LiNbO 3 -type waveguides

We report the study and observation of high-efficiency (greater than 60 percent) and high-power (13 mW CW and 27 mW, 5 μs 5 percent duty cycle pulse) butt coupling of diode lasers to indiffused LiNbO 3 -type waveguides. We verified the predictions of the existing coupling theory at previously unreported power levels, and present a novel theoretical explanation of the effect of multiple reflections on laser output and waveguide coupling. The theory predicts our experimental observation that the amplitude of the periodic variation of laser output with laser-waveguide separation distance is a nonmonotonic function of laser drive current. Our measurements also lead us to infer that the onset of optical damage in Ti:LiNbO 3 occurs at a CW power density of 4 \times 10^{5} W/cm2in the 0.83 μm wavelength region.

High‐efficiency high‐power butt coupling of single‐mode diode lasers to indiffused LiNbO3 optical waveguides

We have butt coupled 13‐mW cw and 27‐mW (5‐μs‐long pulses, 5% duty cycle) optical power into a single mode Ti‐LiNbO3 optical waveguide by using single‐mode diode lasers at λ = 0.84 μm. Coupling efficiencies greater than 68% were observed. These are the highest diode‐laser powers coupled by any means into the technologically important class of LiNbO3 optical waveguides. Our observations support the theory of butt coupling between injection lasers and waveguides at hitherto unavailable power levels and efficiencies. The light coupled into the waveguide has a substantially reduced incoherent power level when compared to that emitted by the laser.

Closed form theory of multicavity reflectors and the output power of external cavity diode lasers

A closed form solution for the transmission and reflection coefficients of a double cavity Fabry-Perot resonator is given. The explicit expressions for these coefficients are used in a formula to give the output power of diode lasers coupled to multiple external cavities. Analysis of a cleaved or etched coupled-cavity laser shows that stable operation depends on proper choice of the phase length of both the gap and the control section. A study of a diode laser coupled to an external waveguide containing a Bragg reflector shows that for correct choice of grating reflectivity only modestly effective antireflection coatings (5 percent on laser facet and 2 percent on waveguide face) are required to allow the grating to dominate the operating wavelength of the laser diode. A single external cavity with loss coupled to a laser diode is also considered. In this case the theory indicates the necessity of proper control of loss or coupling fraction between diode and cavity for there to be effective control of wavelength.

Efficient modulation and coupling of CW junction laser light using electrooptic waveguides

The use of an electrooptical-waveguide modulator to achieve over 80-percent modulation of 7980-Å light from a room temperature laser diode operating at 10 mW CW is reported. The insertion loss of the modulator including couplers is 8.5 dB, and the output beam has high coherence.

Simple Acoustic Grating Modulators

Simple low-cost modulators based on the Raman-Nath regime of acoustooptic diffraction have been demonstrated. Experimental results on internal laser modulators constructed from some dense flint glasses and fused quartz are presented. 100% modulation of He-Ne lasers with rise times on the order of 1 musec are obtained with 10-MHz modulator drive powers of 100 mW. The theory necessary for the design of this class of modulators is reviewed and compared with the experiments.