Comb-based radio-frequency photonic filtering with 20 ns bandwidth reconfiguration

Optics LettersThe article of record as published may be found at http://dx.doi.org/10.1364/ol.38.002735We 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.Funded by Naval Postgraduate SchoolThis project was supported in part by the Naval Postgraduate School under grant N00244-09-1-0068 under the National Security Science and Engineering Faculty Fellowship progra

Microwave photonic filter design via optical frequency comb shaping

The field of microwave photonics (MWP), where the wideband and low-loss capability in optics is utilized to enhance the performance of radio frequency (RF) systems, has been significantly explored over the last decades. This perspective offers benefits that are unattainable with conventional electronics solutions, such as ultra-broad bandwidth, insensitivity to electromagnetic interference, transport through optical fiber networks, easy tuning control, or programmability. One important application of microwave photonics is the implementation of microwave filters for high carrier frequency and wide bandwidth RF waveform. In this thesis, MWP filters based on an optical frequency combs (OFCs) and a dispersive medium are presented. First, noise evaluation of MWP filters based on OFCs is explored to show the potential of optical frequency comb technology to operate over large distances in MWP filter links. Then, amplitude and phase control complex coefficient taps MWP filters are presented. We demonstrate reconfigurable and tunable flat-top MWP filters by applying positive and negative weights across the comb lines and adding a phase ramp onto the tap weights. Furthermore the application of this technique to phase filtering operation over an ultra-wide bandwidth will be demonstrated through high-speed real-time measurement. We present the implementation of matched filter to compress the chirped pulses to their bandwidth limited duration. We also explore the group delay ripple (GDR) compensation of chirped fiber Bragg grating (CFBG) which would reduce the delay of MWP filter links

Multitap microwave photonic filters with programmable phase response via optical frequency comb shaping

Optics LettersThe article of record as published may be found at http://dx.doi.org/10.1364/ol.37.000845We 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.Funded by Naval Postgraduate SchoolThis project was supported in part by the Naval Postgraduate School under grant N00244-09-1-0068 under the National Security Science and Engineering Faculty Fellowship program

Noise Comparison of RF Photonic Filters Based on Coherent and Incoherent Multiwavelength Sources

A filtered microwave photonic (MWP) link implemented with an optical frequency comb as a multitap optical source offers a significant improvement in noise characteristics when compared to a spectrally sliced broadband incoherent source with the same number of taps and identical tap delay. Our results show that frequency combs also enable a better use of the optical bandwidth for filtering with minimum noise-induced fluctuations. These results highlight the potential of optical frequency comb technology to operate over large distances in MWP filter links

Multitap microwave photonic filters with programmable phase response via optical frequency comb shaping

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

Compression of ultra-long microwave pulses using programmable microwave photonic phase filtering with > 100 complex-coefficient taps

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

Anomalous Photocurrent Reversal Due to Hole Traps in AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes

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

Compression of ultra-long microwave pulses using programmable microwave photonic phase filtering with > 100 complex-coefficient taps

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