Richard DeSalvo

Self-focusing and self-defocusing by cascaded second-order effects in KTP

We monitor the induced phase change produced by a cascaded chi((2)):chi((2)) process in KTP near the phase-matching angle on a picosecond 1.06-microm-wavelength beam using the Z-scan technique. This nonlinear refraction is observed to change sign as the crystal is rotated through the phase-match angle in accordance with theory. This theory predicts the maximum small-signal effective nonlinear refractive index of n(eff)(2) congruent with +/-2 x 10(-14) cm(2)/W (+/-1 x 10(-11) esu) for an angle detuning of +/-5 degrees from phase match for this 1-mm-thick crystal with a measured d(eff) of 3.1 pm/V. For a fixed phase mismatch, this n(eff)(2) scales linearly with length and as d(eff)(2) however, for the maximum n(eff)(2) the nonlinear phase distortion becomes sublinear with irradiance for phase shifts near pi/4.

Infrared to ultraviolet measurements of two-photon absorption and n/sub 2/ in wide bandgap solids

The bound electronic nonlinear refractive index, n/sub 2/, and two-photon absorption (2PA) coefficient, /spl beta/, are measured in a variety of inorganic dielectric solids at the four harmonics of the Nd:YAG laser using Z scan. The specific materials studied are: barium fluoride (BaF/sub 2/), calcite (CaCO/sub 3/), potassium bromide (KBr), lithium fluoride (LiF), magnesium fluoride (MgF/sub 2/), sapphire (Al/sub 2/O/sub 3/), a tellurite glass (75%TeO/sub 2/+20%ZnO+5%Na/sub 2/O) and fused silica (SiO/sub 2/). We also report n/sub 2/ and /spl beta/ in three second-order, /spl chi//sup (2)/, nonlinear crystals: potassium titanyl phosphate (KTiOPO/sub 4/ or KTP), lithium niobate (LiNbO/sub 3/), and /spl beta/-barium berate (/spl beta/-BaB/sub 2/O/sub 4/ or BBO). Nonlinear absorption or refraction can alter the wavelength conversion efficiency in these materials. The results of this study are compared to a simple two-parabolic band model originally developed to describe zincblende semiconductors. This model gives the bandgap energy (E/sub g/) scaling and spectrum of the change in absorption. The dispersion of nl as obtained from a Kramers-Kronig transformation of this absorption change scales as E/sub g//sup -1/. The agreement of this theory to data for semiconductors was excellent. However, as could be expected, the agreement for these wide bandgap materials is not as good, although general trends such as increasing nonlinearity with decreasing bandgap energy can be seen.

Measurement of nondegenerate nonlinearities using a two-color Z scan

A simple dual-wavelength (two-color) Z-scan geometry is demonstrated for measuring nonlinearities at frequency omega(p) owing to the presence of light at omega(e). This technique gives the nondegenerate two-photon absorption (2PA) coefficient beta(omega(p); omega(e)) and the nondegenerate nonlinear refractive index n(2)(omega(p); omega(e)), i.e., cross-phase modulation. We demonstrate this technique on CS(2) for n(2) and on ZnSe where 2PA and n(2) are present simultaneously.

Z-scan measurements of the anisotropy of nonlinear refraction and absorption in crystals.

We introduce a method for measuring the anisotropy of nonlinear absorption and nonlinear refraction in crystals by incorporating a wave plate into the Z-scan apparatus. We demonstrate this method by measuring the polarization dependence of the nonlinear refractive index or two-photon absorption coefficient in BaF2, KTP, and GaAs at wavelengths of 532 and 1064 nm.

A novel technique for double sideband suppressed carrier modulation of optical fields

We present a simple and efficient technique to produce double sideband suppressed carrier (DSBSC) modulation for coherent analog optical communications. When operating a Mach-Zehnder electro-optic modulator at the appropriate bias point the carrier exits one of the output ports while the information carrying sidebands exit the other output port. The absence of a strong carrier component in the DSBSC output allows the use of an optical amplifier immediately following the modulator where the light signals are still relatively strong hence preserving the high S/N. In our experiments using heterodyne detection at 1319 nm, we show a -28-dB carrier suppression using a commercial M-Z modulator.<<ETX>>

Thresholds for dielectric breakdown in laser‐irradiated diamond

We report on picosecond laser‐induced damage experiments that were carried out on a natural type‐IIa diamond and a thick specimen of high‐quality chemically vapor‐deposited (CVD) diamond. In conjunction with earlier measurements performed elsewhere on an ‘‘optically thick’’ single crystal, it is shown that for spot sizes (2ω) ranging from 3 to 60 μm, the breakdown field strength (EBD) at the damage threshold of diamond obeys a pattern best described as follows: EBD≂A/√2ω, where A=30.7 and 38.7 MVμ1/2/cm at 532 and 1064 nm, respectively. The case of CVD diamond demonstrates that if problems arising from localized high absorption at the deposition surface can be avoided, this material should be of much promise for contemplated high‐power free‐electron laser window applications.

High-performance and linear thin-film lithium niobate Mach-Zehnder modulators on silicon up to 50 GHz

Compact electro-optical modulators are demonstrated on thin films of lithium niobate on silicon operating up to 50 GHz. The half-wave voltage length product of the high-performance devices is 3.1 V.cm at DC and less than 6.5 V.cm up to 50 GHz. The 3 dB electrical bandwidth is 33 GHz, with an 18 dB extinction ratio. The third-order intermodulation distortion spurious free dynamic range is 97.3  dBHz2/3 at 1 GHz and 92.6  dBHz2/3 at 10 GHz. The performance demonstrated by the thin-film modulators is on par with conventional lithium niobate modulators but with lower drive voltages, smaller device footprints, and potential compatibility for integration with large-scale silicon photonics.

Extending frequency and bandwidth through the use of agile, high dynamic range photonic converters

RF photonic techniques can be used to extend operating frequencies and bandwidth of mission communications and sensing systems. Photonic links inherently cover wide frequency bandwidths and optical modulators and detectors have been demonstrated that operate up to 100 GHz. This paper presents a photonic method for tunable high frequency carrier generation and wideband tunable RF frequency conversion. Photonic down-conversion measurements are presented with +24 dBm OIP3 and positive gain across RF input signals from 5 to 20 GHz. Additionally, measurements of undesired higher order mixing spurious products were more than 30 dB lower than those of typical RF mixers. Coupling wide-band optical components with a reconfigurable RF payload provides unprecedented agility and multi-mission utility for remote sensing, communications and SAR radar missions.

Improved microwave photonic link performance through optical carrier suppression and balanced coherent heterodyne detection

We present a microwave photonic link architecture that enables high gain and dynamic range, low noise figure, and multi-octave bandwidth operation. Our method uses double sideband suppressed carrier modulation together with a balanced coherent heterodyne detection scheme. The modulation method increases link linearity by producing carriersuppressed amplitude modulation based on the optical field rather than intensity. The combination of carrier suppression, optical amplification, phase-locked local oscillator insertion, and balanced detection provide high signal-efficient gain, reduced intermodulation distortion, wide-band operation, frequency agile spectrum access, and low link noise.

High performance microwave photonic links using double sideband suppressed carrier modulation and balanced coherent heterodyne detection

Optical transmission of microwave signals offers many advantages such as increased bandwidth; immunity to electromagnetic interference; reduction of size, weight and power consumption; and low, frequency-independent loss over long distances. But microwave photonic links often lack the performance required to replace traditional microwave links. We present a microwave photonic link architecture that enables high gain and dynamic range, low noise figure, and multi-octave bandwidth operation. Our method uses double sideband suppressed carrier modulation together with a balanced coherent heterodyne detection scheme. The modulation method increases link linearity by producing amplitude modulation based on the optical field rather than intensity. The combination of carrier suppression, optical amplification, phase-locked local oscillator insertion, and balanced detection provide high signal-efficient gain, reduced intermodulation distortion, wide-band operation, and low link noise. The resulting link places this microwave photonic approach in the same performance realm as state-of-the-art microwave links.