Hiva Shahoei

Tunable microwave photonic phase shifter based on slow and fast light effects in a tilted fiber Bragg grating

A continuously tunable microwave phase shifter based on slow and fast light effects in a tilted fiber Bragg grating (TFBG) written in an erbium/ytterbium (Er/Yb) co-doped fiber is proposed and experimentally demonstrated. By optically pumping the TFBG, the magnitude and phase responses of the cladding mode resonances are changed, which is used to introduce a tunable phase shift to the optical carrier of a single-sideband modulated signal. The beating between the phase-shifted optical carrier and the sideband will generate a microwave signal with the phase shift from the optical carrier directly translated to the generated microwave signal. A tunable phase shifter with a tunable phase shift of 280° at a microwave frequency tunable from 24 to 36 GHz is experimentally demonstrated.

Continuously Tunable Time Delay Using an Optically Pumped Linear Chirped Fiber Bragg Grating

A simple method to achieve a large and tunable time delay of an optical signal by using a linearly chirped fiber Bragg grating (LCFBG) written in an erbium-ytterbium (Er/Yb) codoped fiber is proposed and demonstrated. The group delay response of the LCFBG can be tuned by optically pumping the LCFBG with different pumping powers, which leads to the tuning of the time delay. An LCFBG written in an Er/Yb codoped fiber is fabricated. A continuously tunable time delay up to 200 ps for a Gaussian pulse with a full-width at half-maximum of 7.6 GHz is experimentally demonstrated. The influence of the dispersion, the magnitude and group delay ripples of the LCFBG on the time delay performance, and also the stability of operation are investigated.

Photonic Fractional-Order Differentiator Using an SOI Microring Resonator With an MMI Coupler

An optically tunable fractional order temporal differentiator implemented using a silicon-on-isolator microring resonator with a multimode interference (MMI) coupler is proposed and experimentally demonstrated. Through changing the input polarization state, the self coupling coefficient and the loss factor of the designed ring resonator with the MM! coupler are changed. Correspondingly, the coupling regime is changed. Through changing the coupling regime from over-coupled to under-coupled regime, the phase shift in the resonance wavelength is changed from <;π to >π. This tunable phase shift is used to implement a tunable fractional order photonic differentiator with an order tunable from <;1 to 1. The proposed fractional order differentiator is demonstrated experimentally. A Gaussian pulse with a bandwidth of 45 GHz is temporally differentiated with a tunable order of 0.37, 0.67, 1, 1.2, and 1.3.

Tunable Fractional Order Temporal Differentiator by Optically Pumping a Tilted Fiber Bragg Grating

We propose and demonstrate an optically tunable photonic fractional temporal differentiator using a tilted fiber Bragg grating written in an erbium/ytterbium (Er-Yb) co-doped fiber. Thanks to the high absorption of the Er-Yb co-doped fiber, when it is pumped the refractive index is changed, and thus the phase of a cladding mode resonant wavelength is changed continuously by continuous tuning of the pumping power. By locating the wavelength of the input light wave at the location of a cladding mode resonant wavelength, a temporal differentiator with a tunable fractional order is achieved. The proposed technique is experimentally evaluated. A temporal differentiator with a tunable fractional order is demonstrated. The use of the fractional differentiator to implement temporal differentiation of a Gaussian pulse with a bandwidth of 28 and 75 GHz is also demonstrated.

Continuously tunable photonic fractional Hilbert transformer using a high-contrast germanium-doped silica-on-silicon microring resonator

We propose and experimentally demonstrate a continuously tunable fractional Hilbert transformer (FHT) based on a high-contrast germanium-doped silica-on-silicon (SOS) microring resonator (MRR). The propagation loss of a high-contrast germanium-doped SOS waveguide can be very small (0.02 dB/cm) while the lossless bend radius can be less than 1 mm. These characteristics lead to the fabrication of an MRR with a high Q-factor and a large free-spectral range (FSR), which is needed to implement a Hilbert transformer (HT). The SOS MRR is strongly polarization dependent. By changing the polarization direction of the input signal, the phase shift introduced at the center of the resonance spectrum is changed. The tunable phase shift at the resonance wavelength can be used to implement a tunable FHT. A germanium-doped SOS MRR with a high-index contrast of 3.8% is fabricated. The use of the fabricated MRR for the implementation of a tunable FHT with tunable orders at 1, 0.85, 0.95, 1.05, and 1.13 for a Gaussian pulse with the temporal full width at half-maximum of 80 ps is experimentally demonstrated.

Continuously Tunable Microwave Frequency Multiplication by Optically Pumping Linearly Chirped Fiber Bragg Gratings in an Unbalanced Temporal Pulse Shaping System

We propose and demonstrate a new and simple method for achieving continuously tunable microwave frequency multiplication using an unbalanced temporal pulse shaping (TPS) system incorporating two linearly chirped fiber Bragg gratings (LCFBGs) written in erbium/ytterbium co-doped fibers. By optically pumping the LCFBGs with different pumping power, the dispersion of the LCFBGs is tuned. The incorporation of the LCFBGs in an unbalanced TPS system would enable the generation of a frequency tunable microwave waveform. The operation of the system is discussed which is then verified by an experiment. Continuously tunable microwave frequency multiplication with a multiplication factor from 5.14 to 11.9 is experimentally demonstrated. The impact of the ripples in the magnitude and group delay responses of the LCFBGs on the performance of the microwave generation is also studied.

Continuously Tunable Chirped Microwave Waveform Generation Using a Tilted Fiber Bragg Grating Written in an Erbium/Ytterbium Codoped Fiber

An optical approach to generating continuously tunable chirped microwave waveforms using a tilted fiber Bragg grating (TFBG) written in an erbium/ytterbium (Er/Yb) codoped fiber is proposed. By pumping the TFBG, the magnitude and group delay responses of the cladding mode resonances are changed, which can be used to implement a photonic microwave delay-line filter with increasing or decreasing tap spacing. If an ultranarrow pulse is sent to the photonic microwave delay-line filter, a pulse burst with increasing or decreasing pulse spacing is generated. The photodetection of the pulse burst would lead to the generation of a chirped microwave waveform. The proposed technique is demonstrated by an experiment in which a chirped microwave waveform with a tunable chirp rate from 1.8 to 7 GHz/ns is generated.

Continuous Slow and Fast Light Generation Using a Silicon-on-Insulator Microring Resonator Incorporating a Multimode Interference Coupler

Continuously tunable slow and fast light generation using a silicon-on-insulator microring resonator (MRR) incorporating a multimode interference (MMI) coupler is proposed and experimentally demonstrated. The MMI coupler is optimized for the transverse-magnetic mode. By changing the input polarization state, the self-coupling coefficient and the loss factor of the MRR are changed. The depth and the bandwidth of the MRR are tunable by tuning the self-coupling coefficient and the loss factor; thus, a tunable phase shift can be achieved at the resonance wavelength, which leads to the generation of a tunable slow and fast light. The proposed scheme is experimentally evaluated. A tunable slow light with a maximum time delay of 35 ps and a slow-to-fast light with a continuously tunable range of 102 ps are achieved for a 13.5-GHz Gaussian optical pulse by using a double-MMI coupler MRR and a single-MMI coupler MRR, respectively.

Continuously Tunable Slow and Fast Light by Using an Optically Pumped Tilted Fiber Bragg Grating Written in an Erbium/Ytterbium Co-Doped Fiber

Continuously tunable slow and fast light is achieved by using a tilted fiber Bragg grating (TFBG) written in an erbium/ytterbium co-doped fiber. By pumping the TFBG, the magnitude and group delay responses of the cladding mode resonances are changed, which can be used to achieve a tunable time delay or time advance. The proposed method is demonstrated by an experiment in which a tunable time delay from to 18 ps corresponding to a fast to a slow light is achieved for a 13.5-GHz Gaussian pulse.

A continuously tunable multi-tap complex-coefficient microwave photonic filter based on a tilted fiber Bragg grating.

The coupling coefficients of the cladding-mode resonances of a tilted fiber Bragg grating (TFBG) are linearly increasing or decreasing in different wavelength regions. Based on the Kramers-Kronig relations, when the coupling coefficients are linearly increasing, the phase shifts are linearly increasing correspondingly. This feature is employed, for the first time, for the implementation of a multi-tap continuously tunable microwave photonic filter with complex coefficients by using a TFBG. By locating the optical carriers of single-sideband-modulated signals at the cladding-mode resonances of the TFBG which has linearly increasing depths, linearly increasing phase shifts are introduced to the optical carriers. By beating the optical carriers with the single sidebands, the phase shifts are translated to the microwave signals, and thus complex coefficients with the required linearly increasing phase shifts are generated. The tunability of the complex coefficients is realized by optically pumping the TFBG which is written in an erbium/ytterbium (Er/Yb) co-doped fiber. A proof-of-concept experiment is performed; a three- and four-tap filter with a frequency tunable range of 150 and 120 MHz, respectively, are demonstrated.