Ehsan Hamidi

Comb-based radiofrequency photonic filters with rapid tunability and high selectivity

Using electro-optically generated frequency combs, scientists demonstrate radiofrequency photonic filters that can potentially provide simultaneous high stopband attenuation, fast tunability and bandwidth reconfiguration.

Tunable Programmable Microwave Photonic Filters Based on an Optical Frequency Comb

We demonstrate the application of optical combs to implement tunable programmable microwave photonic filters. We design well-known multi-tap microwave photonic filters; however, the utilization of an optical comb with a dispersive medium enables scaling of these filters to a large number of taps. We use optical line-by-line pulse shaping to program tap weights, which allows us to shape the filters bandpass. Our scheme is simple and easily implementable, which provides filters with arbitrary tap weights. As an example, we implement filters with Gaussian apodized tap weights, which achieve more than 35-dB sidelobe suppression. Our experiments provide usable bandwidth, free of sampling spurs, over a Nyquist zone of 5 GHz, equal to half of our 10-GHz comb repetition frequency. Furthermore, we introduce a simple new technique, based on a programmable optical delay line, to uniformly tune the passband center frequency across the free spectral range (FSR) of the filter, ideally without changing the bandpass shape. We demonstrate this scheme by tuning the filter over a full FSR, equal to 10.4 GHz in our experiments.

Optical coherence tomography – near infrared spectroscopy system and catheter for intravascular imaging

Owing to its superior resolution, intravascular optical coherence tomography (IVOCT) is a promising tool for imaging the microstructure of coronary artery walls. However, IVOCT does not identify chemicals and molecules in the tissue, which is required for a more complete understanding and accurate diagnosis of coronary disease. Here we present a dual-modality imaging system and catheter that uniquely combines IVOCT with diffuse near-infrared spectroscopy (NIRS) in a single dual-modality imaging device for simultaneous acquisition of microstructural and compositional information. As a proof-of-concept demonstration, the device has been used to visualize co-incident microstructural and spectroscopic information obtained from a diseased cadaver human coronary artery.

Phase-Only Matched Filtering of Ultrawideband Arbitrary Microwave Waveforms via Optical Pulse Shaping

We demonstrate compression of ultrawideband (UWB) microwave arbitrary waveforms via phase-only matched filtering implemented in a programmable hyperfine resolution optical pulse shaper. We synthesize spread-time UWB electrical waveforms and utilize programmable microwave photonic phase filters to impose the opposite of a waveforms spectral phase on its spectrum. This enables us to compress an UWB microwave waveform to its corresponding bandwidth-limited pulse duration via phase filtering. As an example, we present compression of a linear frequency-modulated electrical waveform with > 15 GHz frequency content with almost 200% fractional bandwidth with ~ 733 ps temporal window to a 40-ps duration pulse with more than 14-dB gain in peak power. Our technique is programmable and we believe it is applicable to a wide range of arbitrary spectral phase modulated UWB radio frequency (RF) waveforms.

Ex vivo catheter-based imaging of coronary atherosclerosis using multimodality OCT and NIRAF excited at 633 nm

While optical coherence tomography (OCT) has been shown to be capable of imaging coronary plaque microstructure, additional chemical/molecular information may be needed in order to determine which lesions are at risk of causing an acute coronary event. In this study, we used a recently developed imaging system and double-clad fiber (DCF) catheter capable of simultaneously acquiring both OCT and red excited near-infrared autofluorescence (NIRAF) images (excitation: 633 nm, emission: 680nm to 900nm). We found that NIRAF is elevated in lesions that contain necrotic core - a feature that is critical for vulnerable plaque diagnosis and that is not readily discriminated by OCT alone. We first utilized a DCF ball lens probe and a bench top setup to acquire en face NIRAF images of aortic plaques ex vivo (n = 20). In addition, we used the OCT-NIRAF system and fully assembled catheters to acquire multimodality images from human coronary arteries (n = 15) prosected from human cadaver hearts (n = 5). Comparison of these images with corresponding histology demonstrated that necrotic core plaques exhibited significantly higher NIRAF intensity than other plaque types. These results suggest that multimodality intracoronary OCT-NIRAF imaging technology may be used in the future to provide improved characterization of coronary artery disease in human patients.

Post-Compensation of Ultra-Wideband Antenna Dispersion Using Microwave Photonic Phase Filters and Its Applications to UWB Systems

We demonstrate experimental post-compensation of ultra-wideband (UWB) antenna dispersion at a receiver front-end by using programmable microwave photonic phase filtering. After the received RF signal is modulated onto an optical carrier, we utilize a hyperfine resolution optical pulse shaper to apply the conjugate of its spectral phase in the optical domain. After optical-to-electronic conversion, this yields an electrical waveform, which is compressed to bandwidth-limited duration. Further we use this technique in two schemes: a radar configuration in which we resolve two close echoes from different paths, which initially interfere and mask each other due to the dispersed response of the antenna link, and a spread-time UWB transmission configuration in which we retrieve and compress a distorted signal received in line-of-sight. To our knowledge, this is the first experimental demonstration of dispersion post-compensation of UWB RF waveforms to approach the ultimate bandwidth-limited resolution, as well as identification of such signals by matched filtering and compression. Our technique is programmable and offers potential to enhance performance in UWB radar and communications.

New aspects of temporal dispersion in high-resolution Fourier pulse shaping: a quantitative description with virtually imaged phased array pulse shapers

We report new aspects of temporal dispersion in Fourier pulse shapers that contain spectral dispersers with a nonlinear frequency to space mapping. These effects are particularly important in high-resolution operation, since high-resolution dispersers typically exhibit pronounced nonlinear angular dispersion over relatively small bandwidths. In this paper we present a general discussion of the new effects followed by quantitative analysis and experimental verification for pulse shapers, which utilize a virtually imaged phased array (VIPA) as the spectral disperser. Compared to the well-known 4-F configuration, our results demonstrate a substantial modification to the placement of the optical components required to obtain zero temporal dispersion. Furthermore, spectral phase variations associated with nonzero dispersion coupled with contributions from multiple diffraction orders are shown to give rise to a dramatic new spectral interference effect, which can be used to monitor temporal dispersion purely in the spectral domain. We expect the effects we present in this paper to become prominent even for more conventional diffraction-grating-based pulse shapers for bandwidths sufficiently large that nonlinear spectral mapping becomes strong.

Programmable multi-tap microwave photonic phase filtering via optical frequency comb shaping

We present a programmable multi-tap microwave photonic phase filter operating over an ultra-wide bandwidth. Complex programmability of >30 taps is achieved by optical line-by-line pulse shaping on a 10 GHz Gaussian optical frequency comb using a novel interferometric scheme. Through high-speed real-time measurements, we show programmable chirp control in the range ±0.6GHz ns.

Achieving arbitrary passband profiles and high stopband attenuation in microwave photonic filters

Microwave photonic filters utilizing optical frequency combs have been shown to achieve narrow bandwidths, high stopband attenuation as well as easy tunability of center frequency and bandwidth. The ability to achieve smooth apodization of optical frequency comb spectrum to specific spectral profiles is important to obtain high stopband attenuations. Previously, by utilizing an optical pulse shaper to crease a Gaussian profile spectrum, filters with high stopband attenuation and Gaussian passbands were demonstrated. However this technique is flexible and can be used to easily program the filter passband profile. We demonstrate this by creating filters with a flat top passband. We also demonstrate that the stopband attenuation can be significantly enhanced by utilizing frequency combs which achieve the necessary spectral shapes directly from the source. By generating an optical comb with a smooth quasi-Gaussian profile, we demonstrate filters with a record >60dB stopband attenuation.

Microwave photonic filters with > 65 dB sidelobe suppression using directly generated broadband, quasi — Gaussian shaped optical frequency combs

We present microwave photonic filters with record > 65dB sidelobe suppression using directly generated, broadband, quasi-Gaussian combs using cascaded-FWM based spectral broadening of combs generated from tailored E-O modulation of CW lasers.