RESUMEN
In this study, we presented flat-topped coherent supercontinuum lasers with tunable repetition rates and programmable spectral bandwidths. Supercontinuum sources with ultra-broadband and high-repetition-rate coverage can be achieved by merging nonlinearly broadened electro-optic optical frequency combs with optical line-by-line spectrum shaping. Spectral bandwidth programming is implemented by iterative spectrum shaping and input power control of highly nonlinear stages, whereas repetition rate tuning is performed by modulation speed control in optical frequency combs. Herein, we implemented a programmable and tunable flat-topped supercontinuum with a maximum bandwidth and repetition rate of 55 nm at 10 dB and 50 GHz, respectively. To clarify the coherence of the supercontinuum during tuning and programming, we performed a phase-noise analysis. We proposed a remarkably modified self-heterodyne method to measure the phase noise of each mode precisely by filtering specific supercontinuum taps in a Mach-Zehnder interferometer. With this method, it has been proved that the single-sideband spectra in each mode are almost similar to that of the RF clock, indicating that our programmable and tunable supercontinuum generation process added minimal degradation to the phase noise properties. This study shows possibilities for generating hundreds of programmable and tunable flat-topped optical carriers with robustness and coherence.
RESUMEN
The trap states and defects near the active region in deep-ultraviolet (DUV) light-emitting diodes (LED) were investigated through wavelength-dependent photocurrent spectroscopy. We observed anomalous photocurrent reversal and its temporal recovery in AlGaN-based DUV LEDs as the wavelength of illuminating light varied from DUV to visible. The wavelength-dependent photocurrent measurements were performed on 265 nm-emitting DUV LEDs under zero-bias conditions. Sharp near-band-edge (~265 nm) absorption was observed in addition to broad (300-800 nm) visible-range absorption peaks in the photocurrent spectrum, while the current direction of these two peaks were opposite to each other. In addition, the current direction of the photocurrent in the visible wavelength range was reversed when a certain forward bias was applied. This bias-induced current reversal displayed a slow recovery time (~6 h) when the applied forward voltage was removed. Furthermore, the recovery time showed strong temperature dependency and was faster as the sample temperature increased. This result can be consistently explained by the presence of hole traps at the electron-blocking layer and the band bending caused by piezoelectric polarization fields. The activation energy of the defect state was calculated to be 279 meV using the temperature dependency of the recovery time.
RESUMEN
We present the flat-top supercontinuum source with high repetition rate over a broad bandwidth. The flatness and high repetition rate are achieved by iterative optical line-by-line spectrum shaping on electro-optic optical frequency combs. By applying Gaussian apodized pulse train to a highly nonlinear medium with optimized Gaussian coefficient and nonlinear polarization rotation techniques, we implemented here a flat-top supercontinuum with a 47.7â nm bandwidth at 3â dB and 30â GHz repetition rate. The generation of high repetition rate supercontinuum sources with smooth and coherent spectrum is the critical challenging task for many applications such as optical communications and the optical arbitrary waveform generation. This work leads us to new possibilities for generating hundreds or thousands of flattened coherent optical carriers with a simple configuration.
RESUMEN
Microwave photonic filters with arbitrary phase response can be achieved by merging high-repetition-rate electro-optic frequency comb technology with line-by-line pulse shaping. When arranged in an interferometric configuration, the filter features a number of programmable complex-coefficient taps equal to the number of available comb lines. In this work, we use an ultrabroadband comb generator resulting in a microwave photonic phase filter with >100 complex-coefficient taps. We demonstrate the potential of this filter by performing programmable chirp control of ultrawideband waveforms that extend over long (>10 ns) temporal apertures. This work opens new possibilities for compensating realistic linear distortion impairments on ultrabroadband wireless signals spanning over dozens of nanosecond temporal apertures.
RESUMEN
We present a scheme to generate a 10 GHz optical frequency comb that is bandwidth reconfigurable on a time scale of tens of nanoseconds via electronic control of the drive signal to a phase modulator. When such a comb is used as the source for a radio-frequency (RF) photonic filter employing dispersive propagation, the RF filter bandwidth varies in inverse proportion to the optical bandwidth. As a result we are able to demonstrate, for the first time to our knowledge, bandwidth-reconfigurable RF filtering with transition times under 20 ns. The reconfiguration speed is determined by the response time of a programmable RF variable attenuator.
RESUMEN
We present a programmable multitap microwave photonic filter with an arbitrary phase response operating over a broad bandwidth. Complex coefficient taps are achieved by optical line-by-line pulse shaping on a 10 GHz flat optical frequency comb using a novel interferometric scheme. Through high-speed real-time measurements, we demonstrate programmable chirp control of a waveform via phase filtering. This achievement enables us to compress broadband microwave signals to their corresponding bandwidth-limited pulse duration.
