George Rakuljic

Optical data storage by using orthogonal wavelength-multiplexed volume holograms

We propose a volume holographic data storage scheme that employs counterpropagating reference and image beams and wavelength multiplexing for page differentation. This method is compared with that based on angular multiplexing. A reduction in holographic cross talk is predicted. Further cross-talk reduction that is due to sidelobe suppression is observed in experiments by using photorefractive crystals and the proposed orthogonal data storage.

Volume holographic narrow-band optical filter

An 0.0125-nm (FWHM) bandwidth optical filter with >10% throughput, clean sidebands, f/12 speed, and a response roll-off of -40 dB/decade has been developed by using volume holography in photorefractive materials. The performance of the Accuwave holographic filter is characterized and compared with that of a Lyot filter, which is the standard for solar astronomy today. Moreover, this device represents the first commercial application of volume holography in photorefractive crystals.

Precise control of broadband frequency chirps using optoelectronic feedback

We demonstrate the generation of wideband frequency sweeps using a semiconductor laser in an optoelectronic feedback loop. The rate and shape of the optical frequency sweep is locked to and determined by the frequency of a reference electronic signal, leading to an agile, high coherence swept-frequency source for laser ranging and 3-D imaging applications. Using a reference signal of constant frequency, a transform-limited linear sweep of 100 GHz in 1 ms is achieved, and real-time ranging with a spatial resolution of 1.5 mm is demonstrated. Further, arbitrary frequency sweeps can be achieved by tuning the frequency of the input electronic signal. Broadband quadratic and exponential optical frequency sweeps are demonstrated using this technique.

All-fiber zero-insertion-loss add-drop filter for wavelength-division multiplexing

We developed and fabricated an all-fiber add-drop filter by recording a Bragg grating in the waist of an asymmetric mode converter-coupler formed by adiabatic tapering and fusing of two locally dissimilar, single-mode optical fibers. The insertion loss of the device was ~0.1 dB .A narrow spectral bandwidth (<1 nm) and a large add-drop efficiency (>90%) were also demonstrated. In addition, the filter was polarization independent.

Narrow bandwidth volume holographic optical filter operating at the Kr transition at 1547.82 nm

We describe and characterize a narrow bandwidth volume holographic optical filter operating at the Krypton transition line (1s2‐2p8) at 1547.82 nm, which corresponds to the center wavelength of the proposed International Telecommunications Union wavelength standard. A reflectivity of 98% and a bandwidth full width at half‐maximum of 0.18 nm are measured. The filter exhibits clean sideholes with a −20 dB optical response 0.5 nm away from the peak. The filter can be fabricated and operated with an absolute wavelength precision better than 0.005 nm.

Holographic storage dynamics in lithium niobate: theory and experiment

We present a theoretical model that describes holographic ionic fixing and storage dynamics in photorefractive crystals. Holographic gratings that are based on charge redistribution inevitably decay because of ionic and electronic conduction. Relevant decay rates and transient hologram field expressions are derived. Ionic gratings are partially screened by trapped electrons on readout. The lifetimes of fixed ionic holograms are limited by the finite ionic conductivity at low (i.e., room) temperatures. Only under certain and restricted conditions can these decay times be acceptably long. A significant increase in fixed ionic hologram lifetime is realized in lithium niobate with a low hydrogen-impurity content. The residual ionic conductivity (decay-time constant) in these samples exhibits ~1.4-eV activation energy and is not due to protonic conduction. Fixed hologram lifetimes of ~2 years at room temperature in dehydrated lithium niobate crystals are projected.

Real time image subtraction and "exclusive or" operation using a self-pumped phase conjugate mirror

Real time ‘‘exclusive or’’ operation with an interferometer using a self‐pumped phase conjugate mirror is reported. Also, results of image subtraction and intensity inversion are shown.

Holographic fixing, readout, and storage dynamics in photorefractive materials

Holographic gratings in photorefractive crystals that are based on charge redistribution inevitably decay as a result of ionic and electronic conduction. Under certain and restricted conditions these decay times can be acceptably long. Relevant decay rates and transient hologram field expressions are derived with special reference to LiNbO(3). Some experimental data are presented.

Coherent combining of the output of two semiconductor lasers using optical phase-lock loops

We have experimentally demonstrated current-injection optical phase-lock loops (OPLLs) based on commercial single-section semiconductor distributed-feedback (DFB) lasers. Using two parallel OPLLs, we have obtained 87% efficient coherent power combining of the two DFB lasers. The rms differential phase error between the two lasers is about 30 degrees .

Photorefractive Properties Of Undoped, Cerium-Doped, And Iron-Doped Single-Crystal SrO.6BaO.4Nb2O6

We present the results of our theoretical and experimental studies of the photorefractive effect in single-crystal SBN:60, SBN:Ce, and SBN:Fe. The two-beam coupling coefficients, response times, and absorption coefficients of these materials are given.