Julius Goldhar

Optimization of the frequency response of a semiconductor optical amplifier wavelength converter using a fiber Bragg grating

Cross-gain modulation in a semiconductor optical amplifier (SOA) is one of the simplest techniques for all-optical wavelength conversion. However, the finite gain recovery time of the semiconductor optical amplifier causes distortion and pattern dependence at high bit rates. Here we show that filtering the output of a semiconductor optical amplifier wavelength converter with the transmission edge of a fiber grating filter improves its frequency response. The grating sharpens the transition between the bits by converting the phase modulation at the edge to useful amplitude modulation. We determine the filtering condition that produces the optimum frequency response and reduces bit-pattern dependence for nonreturn-to-zero (NRZ) data. For small modulation, the apparent frequency response increases by the linewidth enhancement factor /spl alpha//sub H/ of the SOA. In this case, pattern dependence is eliminated completely by the fiber grating filter. For large modulation, pattern dependence can be substantially reduced, but not completely eliminated. We show that after spectral filtering, the residual pattern dependence of an SOA depends only on modulation depth. For a given SOA, we find the optimum grating for minimum conversion penalty at 12 Gb/s for a wide range of operating parameters. Using a fiber grating filter reduces the required optical power for conversion in a semiconductor optical amplifier.

A stable smoothly wavelength-tunable picosecond pulse generator

Smooth wavelength tuning over 10 nm has been realized in a dispersion-tuned harmonically mode-locked fiber ring laser. Supermode noise is suppressed by 13 dB by adding a semiconductor optical amplifier (SOA) into the cavity and the suppression improves as the SOA gain increases. 5.3-ps pulses are obtained at a repetition rate of 10 GHz. A simple numerical model demonstrates the effects of the intracavity dispersion and the SOA on the pulse characteristics.

Intensity averaging and four-wave mixing in Raman amplifiers

Intensity averaging of pump beams in a Raman amplifier can be treated using a four-wave mixing model. The model shows explicitly the consequences of finite pump beam correlation length in four-wave mixing processes. Phase conjugate effects in a Raman amplitier are observed and described. Measurements of four-wave mixing effects in a KrF laser-pumped methane Raman amplifier agree with the model.

Lightwave switching in semiconductor microring devices by free carrier injection

Using a conventional microwave photonic technique, we demonstrate lightwave switching in laterally coupled GaAs-AlGaAs microring resonators by free carrier injection. The ring waveguide is optically pumped just above its bandgap energy, which results in a temporal tuning of the microring resonant wavelengths by the refractive index change due to the induced free carriers. Both the transmission and the phase function of the resonators are investigated and used to demonstrate all-optical switching. The switching time, limited by surface recombination of carriers, is 20 ps. A switching energy of a few picojoules shifts the microring resonance by 1.2 nm.

Active harmonic modelocking of an erbium fiber laser with intracavity Fabry-Perot filters

We present both experimental and theoretical investigations of the operation of a harmonically modelocked erbium fiber ring laser stabilized by an intracavity bit-rate etalon. Our model analyzes the effects of cavity components and operating parameters on laser stability and output pulse characteristics. The model predicts the output pulsewidth variation with laser cavity parameters such as cavity length, dispersion, and finesse of intracavity Fabry-Perot etalons. If the laser cavity length is not optimized, a maximum 50% increase in pulsewidth can occur at 5 Gb/s pulse rate. A repetition rate etalon with a finesse of 50 is sufficient to provide a side-mode suppression ratio of over 50 dB in the laser output. We also discuss how detuning from the optimal modulation frequency increases the excess noise that affects the laser stability. The theory predicts a maximum detuning range of /spl plusmn/100 kHz, which agrees with the experimental observations. These theoretical results can guide the design of similar lasers over a wide range of operating parameters.

Optical clock recovery from a data stream of an arbitrary bit rate by use of stimulated Brillouin scattering

Using a fiber-optic stimulated-Brillouin-scattering amplifier as an active filter, we have demonstrated optical clock recovery from 5-Gbit/s return-to-zero-format optical data. Definite patterns and pseudorandom bit sequences were tested. This scheme requires no prior knowledge of the clock frequency and is well suited for operation at higher data rates.

Performance characterization of a harmonically mode-locked erbium fiber ring laser

We describe the performance of a harmonically mode-locked, erbium fiber ring laser. Transform-limited pulses with durations of 20-50 ps at 5.044 GHz repetition rate are obtained. The modulation frequency can be detuned by up to /spl plusmn/70 kHz and the lasing wavelength can be varied over the entire erbium gain bandwidth. Pulsewidths and pulse bandwidths are measured as a function of modulation power and frequency detuning. The laser is actively stabilized by locking intra-cavity Fabry-Perot etalon passbands to the laser modes.<<ETX>>

Subharmonic optical clock recovery from 160 Gb/s using time-dependent loss saturation inside a single electroabsorption modulator

A single electroabsorption modulator (EAM) is utilized to perform subharmonic all-optical timing extraction from 160 Gb/s. The extraction technique is based on fast time-dependent loss saturation inside the EAM. The extracted timing is used for optical clock recovery with /spl les/340 fs of timing jitter. The clock recovery technique is bit-rate flexible, and the bit-error-ratio measurements reveal an error-free operation with no receiver sensitivity penalty.

Technique for measurement of the Raman gain coefficient in optical fibers

We demonstrate a simple, novel technique for measurement of the Raman gain spectrum in optical fibers. We measure the stimulated Raman scattered power generated by a square pulse as a function of its pulse width. Because of fiber dispersion, increasing the pulse width increases the interaction length of the pump and Stokes pulses and leads to a higher Stokes power. The dependence of growth rate of Stokes power on pulse width gives the Raman gain coefficient. Our technique does not require calibration against a standard or absolute measurement of the Stokes signal. We show that gain values obtained with this technique agree well with direct pump-probe measurements.

Implementation of discrete unitary transformations by multimode waveguide holograms.

Integration of holograms into multimode waveguides allows the implementation of arbitrary unitary mode transformations and unitary matrix-vector multiplication. Theoretical analysis is used to justify a design approach to implement specific functions in these devices. Based on this approach, a compact mode-order converter, a Hadamard transformer, and a spatial pattern generator-correlator are proposed and analyzed. Beam propagation simulations are used to verify the theoretical calculations and to address bandwidth, scalability, and fabrication criteria. Optical pattern generators were successfully fabricated using standard photolithographic techniques to demonstrate the feasibility of the devices.