A photonic approach for frequency-sextupled microwave signal generation without filter or precise phase control is demonstrated by computer simulations and experiments. Without any filter, a frequency-sextupled microwave signal is generated by adjusting bias voltages of the cascade modulators. This structure largely reduces the dependence of particular phase relation that is built between the different modulated signals. The approach is verified by simulations and experiments, and stable 18- and 24-GHz frequency-sextupled signals are generated by 3 and 4 GHz local signals without filter or precise phase control.
Tunable optoelectronic oscillator (OEO) with an embedded delay-line oscillator (EDO) for fine steps is presented. A delay-line oscillator consisting of an amplifier, phase shifter, and electrical delay line is embedded in the general single loop OEO. The oscillation frequency is controlled by the EDO loop phase, which is adjusted by the phase shifter. Without any narrow radio frequency bandpass filter, about 340 MHz tunable range with a fine step of ∼100 kHz is realized by directly tuning the direct current voltage of the phase shifter for 8 GHz oscillation. We also experimentally demonstrate κ-band tunable microwave signal generation within 22.18 to 23.32 GHz. The tunable accuracy and tunable range have a better performance compared to other OEO schemes with a fine frequency tunable step. The phase noise of the generated 22.25 GHz microwave signal is −108.7 dBc/Hz at 10 kHz offset.
A novel kind of terahertz (THz) photonic crystal fibers (PCFs) based on the material-filled structure is proposed in this paper. Different materials can be selectively filled into parts of air-holes in the designed THz PCFs, and then perfect features such as single-mode transmission and ultra-flattened dispersion are obtained easily in large frequency ranges near 1THz. Employing full-vectorial finite element method (FEM) and plane wave expansion method (PWE), confinement losses and modal dispersion with different structural parameters and diverse refractive indices of the filled materials are investigated respectively. Numerical results show that the proposed PCFs have acceptable confinement losses, low and flattened dispersions whose absolute values are lower than 1ps/nm*km. Moreover, its structure is simple and its feature is insensitive to variations of parameters. It is helpful for PCFs design and real fabrication in the potential THz applications.
A novel highly birefringent photonic crystal fiber with selectively liquid-filled structure in cladding is proposed. The birefringence is numerically investigated with variant structural parameters and refractive index of the infiltrating liquid. The results show that high birefringence can be achieved over a wide wavelength range, and the birefringence is increased because of the symmetry destroyed further in the proposed structure. However, the confinement loss keeps the same order of magnitude even though more materials with high refractive index are filled in the cladding. By varying the refractive index of the infiltrating liquid, the birefringence is shown to be well tuned. It has the potential to be used as optical devices such as all-fiber polarization controllers and high-sensitive sensors.
A novel highly birefringent photonic crystal fiber (PCF) with hybrid cladding is proposed. In this hybrid structural PCF,
some air holes of the cladding are selectively filled with high refractive index material. The increased birefringence
mainly results from that the symmetry is destroyed further in the designed PCF. The birefringence is theoretically
investigated with variant structural parameters and refractive index of the filled material. The plane wave expansion
method (PWE) and full-vector finite-element method (FEM) are respectively employed to investigate the optical
properties of the proposed PCF. The numerical results show that the designed PCF can provide high birefringence and be
well tuned by the structural parameters and refractive index of the filled material. It can be used as all-fiber polarization
controllers and highly sensitive sensors.
The fiber Bragg grating has been widely used in sensors. We have studied the spectral properties of uniform Bragg gratings in photonic crystal fibers under transverse pressure. By the finite element method, the relation between the birefringence and the pressure was simulated in bare photonic crystal fibers. The results show that the birefringence is changed even under weak pressure. A new method based on polarization-dependent loss is presented for measuring pressure. The maximal amplitude of the polarization-dependent loss varies linearly in the pressure range of 0 to 4 MPa with a slope of 0.75 MPa−1.
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