Moshe Horowitz

Broadband second‐harmonic generation in SrxBa1−xNb2O6 by spread spectrum phase matching with controllable domain gratings

We have demonstrated, for the first time to our knowledge, second‐harmonic generation for a broad input wavelength range of 750–1064 nm in SrxBa1−xNb2O6 crystals, with a controllable spread spectrum of quasiphase matching. Second‐harmonic conversion efficiencies of up to ∼1% were observed. This was done without any temperature or angular tuning of the crystal. The phase matching was obtained by inducing alternating ferroelectric domains in the crystal in real time, using a novel fixing process which is based on screening. The broadband capability of the conversion is allowed by a spread in the range of the domain widths.

Linewidth-narrowing mechanism in lasers by nonlinear wave mixing

We propose a linewidth-narrowing mechanism in lasers by nonlinear absorptive wave mixing of the counterpropagating beams in the cavity. We give a theoretical analysis and report on a demonstration of the effect with an erbium-doped fiber laser. The system also exhibits bistability in the dependence of the oscillation intensity versus pump power.

Interrogation of fiber gratings by use of low-coherence spectral interferometry of noiselike pulses

We demonstrate an innovative method for a real-time interrogation of fiber Bragg gratings based on low-coherence spectral interferometry of noiselike pulses. By analyzing the spectral interference at the output of a Michelson interferometer we obtained the impulse response of the grating with a time resolution of ~350 fs . Using the Gabor transformation, we could directly detect nonuniform regions inside the grating and could measure the spatial dependence of the resonance wavelength along the grating.

Theoretical and experimental study of harmonically modelocked fiber lasers for optical communication systems

We study theoretically and experimentally actively modelocked fiber lasers that are used in high repetition rate optical communication systems. Using an innovative numerical technique and a reduced model, we have found that the laser can operate in four different operating regimes when the laser intensity was changed; three of the regimes were experimentally observed in a laser with a sigma configuration. An excellent quantitative agreement between the theoretical and the experimental results was obtained. The use of dispersion management in the sigma laser was found to significantly improve the laser performance.

Control of noiselike pulse generation in erbium-doped fiber lasers

Noiselike generation in lasers can be controlled by changing the cavity in order to obtain pulses with unique properties. Intense noiselike pulses, as narrow as a few picoseconds, were obtained. These are two orders of magnitude narrower than pulses obtained in previous work. In long cavities, coherent and incoherent two-color noiselike generation were demonstrated. The wavelength difference between the generated pulses could be tuned in a wide wavelength range, which is much broader than the amplifier bandwidth. High-energy /spl ap/16-nJ noiselike pulses with a broad-band spectrum and narrow intensity autocorrelation trace were also demonstrated.

Spurious mode reduction in dual injection-locked optoelectronic oscillators

Optoelectronic oscillators (OEOs) are promising sources of low phase noise radio frequency (RF) signals. However, at X-band frequencies, the long optical fiber delay line required for a high oscillator Q also leads to spurious modes (spurs) spaced too narrowly to be filtered by RF filters. The dual injection-locked OEO (DIL-OEO) has been proposed as a solution to this problem. In this work, we describe in detail the construction of a DIL-OEO. We also present experimental data from our systematic study of injection-locking in DIL-OEOs. With this data, we optimize the DIL-OEO, achieving both low phase noise and low spurs. Finally, we present data demonstrating a 60 dB suppression of the nearest-neighbor spur without increasing the phase noise within 1 kHz of the 10 GHz central oscillating mode.

Modeling optoelectronic oscillators

We have developed a comprehensive simulation model for accurately studying the dynamics in optoelectronic oscillators (OEOs). Although the OEO is characterized by three widely separated time scales, our model requires neither long run times nor a large amount of memory storage. The model generalizes the Yao-Maleki model and includes all of the physical effects in the Yao-Maleki model as well as other physical effects that are needed to calculate important features of the OEO dynamics, such as the impact of the fast response time of the modulator on the phase noise power spectral density, the fluctuations of the OEO output due to the input noise, the cavity mode competition during the OEO start-up, and temporal amplitude oscillations in steady state. We show that the absolute value of the phase noise is 2-3 dB lower than predicted by the Yao-Maleki model. The Yao-Maleki model does not take into account amplitude noise suppression due to the fast time response of the modulator, which accounts for this difference. We show that a single cavity mode oscillates in the OEO at steady state, and this mode is determined by the noise that is present when the OEO is turned on. When the small-signal open-loop gain is higher than 2.31, we show that the OEO amplitude oscillates in steady state. This temporal amplitude oscillation can be suppressed by using a narrow filter. Our simulation model, once extended to include flicker (1/f) noise and different amplifier and modulator designs, will enable its users to accurately design OEOs.

Time-dependent behavior of photorefractive two- and four-wave mixing

A theoretical and experimental study of the temporal response of photorefractive two- and four-wave mixing processes is given. We examine the buildup and the decay of the output signal and the gratings when the input signal beam is turned on and off. For two-wave mixing we have performed an analysis that includes the depleted-pump regime. The experiment was done with a BaTiO3 crystal. The buildup-and-decay behavior is strongly dependent on the coupling constant and the ratio of the signal and pump intensities. From these measurements we obtain the crystal time constant, which was found to have an intensity dependence with a power of −0.7. The coupling constant is extracted from the data of the whole dynamic process of buildup and erasure and not only from the steady-state point, as is usually done. This is the first analytic study, to the best of our knowledge, for the four-wave mixing decay and buildup. We find that the solution for time-dependent four-wave mixing processes (the phase-conjugate mirror and the double phase-conjugate mirror) with undepleted pumps is identical to that for two-wave mixing with unidirectional optical feedback circuits. This similarity provides a direct way to study the stability properties of the devices. We find, for example, that in the unstable regime of four-wave mixing a self-pulsation of the output corresponds to the frequency detuning of the unidirectional two-wave mixing oscillator.

Measuring the structure of highly reflecting fiber Bragg gratings

We demonstrate a new technique that enables us to measure the structure of highly reflecting fiber Bragg gratings. The impulse response function is measured from both sides of the grating using a low-coherence spectral interferometry technique. An inverse scattering algorithm is used to extract the refractive-index profiles from the measured impulse responses. The reconstruction of the grating is performed by combining the refractive-index profiles, measured from both sides of the grating. The transfer function of the optical spectrum analyzer is measured and used to correct the measured results. The interrogation of an apodized grating with a reflectivity of 99.91% is demonstrated.

Comprehensive computational model of single- and dual-loop optoelectronic oscillators with experimental verification

We describe a comprehensive computational model for singleloop and dual-loop optoelectronic oscillators (OEOs). The model takes into account the dynamical effects and noise sources that are required to accurately model OEOs. By comparing the computational and experimental results in a single-loop OEO, we determined the amplitudes of the white noise and flicker noise sources. We found that the flicker noise source contains a strong component that linearly depends on the loop length. Therefore, the flicker noise limits the performance of long-cavity OEOs (≧5 km) at low frequencies (f<500 Hz). The model for a single-loop OEO was extended to model the dual-loop injection-locked OEO (DIL-OEO). The model gives the phase-noise, the spur level, and the locking range of each of the coupled loops in the OEO. An excellent agreement between theory and experiment is obtained for the DIL-OEO. Due to its generality and accuracy, the model is important for both designing OEOs and studying the physical effects that limit their performance. We demonstrate theoretically that it is possible to reduce the first spur in the DIL-OEO by more than 20 dB relative to its original performance by changing its parameters. This theoretical result has been experimentally verified.