Free Spectral Region Research Articles

Tunable laser frequency lock based on a temperature-dependent Fabry-Perot etalon.

A simple method for laser frequency stabilizing by a temperature-tuned Fabry-Perot etalon is reported. The etalon is a plano-convex lens that permits tuning the length and refractive index via controlling the temperature for shifting wavelengths in the region of 852 nm, with a transmission spectral linewidth of ∼72.5MHz and free spectral region of ∼16GHz. Using this scheme, arbitrary frequency locking of a laser with an adjustable frequency resolution of 2.34GHz/∘C is realized, and MHz-level long-term stability is demonstrated.

Compact Inner-Wall Grating Slot Microring Resonator for Label-Free Sensing.

In this paper, we present and analyze a compact inner-wall grating slot microring resonator (IG-SMRR) with the footprint of less than 13 μm × 13 μm on the silicon-on-insulator (SOI) platform for label-free sensing, which comprises a slot microring resonator (SMRR) and inner-wall grating (IG). Its detection range is significantly enhanced without the limitation of the free spectral region (FSR) owing to the combination of SMRR and IG. The IG-SMRR has an ultra-large quasi-FSR of 84.5 nm as the detection range, and enlarged factor is up to over 3 compared with the conventional SMRR. The concentration sensitivities of sodium chloride solutions and D-glucose solutions are 996.91 pm/% and 968.05 pm/%, respectively, and the corresponding refractive index (RI) sensitivities are 559.5 nm/RIU (refractive index unit) and 558.3 nm/RIU, respectively. The investigation on the combination of SMRR and IG is a valuable exploration of label-free sensing application for ultra-large detection range and ultra-high sensitivity in future.

All-optical demultiplexing of Nyquist OTDM signal using a biorthogonal Nyquist gate

We present an all-optical method for demultiplexing Nyquist optical time division multiplexed (OTDM) signals using a biorthogonal Nyquist gate, which is a Nyquist gate of reduced pulse width. The biorthogonality between the Nyquist symbol and Nyquist gate creates an intersymbol interference (ISI) free spectral region in the mixed product that theoretically allows for complete elimination of ISI in the demultiplexer. Furthermore, this format transparent all-optical method produces a Nyquist signal on the output suitable for further optical processing and multiplexing. We analyze the performance of this approach through simulation in the presence of nonidealities of imperfect pulse shape, timing jitter, and dispersion. Finally, we experimentally demonstrate the performance of this approach operating on an 80-GBd Nyquist OTDM signal.

Investigation of the absolute detection method of atmospheric temperature based on solid cavity scanning Fabry-Perot interferometer

measurement methods based on Rayleigh scattering are employed to relatively detect atmospheric temperature profiles. That is to say, the definition of response functions and calibration procedures is required for temperature retrieval. Because the thermal motion rate of gas molecule complies with Maxwell distribution, and gas molecule is always in motion state, the frequency of scattering return signal generates Doppler spectral broadening. There is a positive correlation between the full width at half maximum of widened Doppler spectrum and T1/2, atmospheric absolute temperature can be obtained by measuring the Doppler spectrum shape. In this paper, the fine detection method of the spectrum shape of Rayleigh scattering and residuary Mie-scattering correction method based on solid cavity scanning Fabry-Perot (F-P) interferometer are investigated. According to the characteristics of Rayleigh scattering spectrum, the free spectral range, the geometric length of solid cavity, the type of cavity media, the full width at half maximum, the reflectivity of cavity, and the scanning step are designed. When the electro-optical crystal of KD*P with the length of 8.5 mm acts as solid cavity medium of scanning F-P interferometer, the designed free spectral region and 3 dB bandwidth are 11.5 GHz and 60 MHz at the central wavelength of 354.7 nm, respectively. The energy datum of 185 discrete points at Rayleigh scattering spectrum are obtained by using an optimized solid cavity scanning F-P interferometer with the scanning voltage of 23.5 V. A fitting spectrum is generated by employing polynomial interpolation method at the atmospheric temperature of 300 K. The maximum absolute error and full width at half maximum error of Rayleigh scattering spectrum are 22 MHz and 337 kHz, respectively. In order to verify the results, a numerical simulation of Rayleigh scattering spectrum based on standard atmosphere model and S6 model is performed. The detection uncertainty of atmospheric temperature is up to 0.8 K. As SNR (signal to noise ratio) is 10, the detection distance is 4.5 and 7.9 km at day-time and night-time, respectively. The research provides a new solution of filter system for the achievement of all-time, high-precision, and absolute detection of atmospheric temperature in the future. In meteorology, in order to investigate the temporal and spatial characteristics, the change rules and physical mechanism of weather processes, the temperature in the boundary layer of urban atmosphere is absolutely detected, where human activities are frequent and the changes of weather elements are obviously at day and night. In addition, the absolute detection method of atmospheric temperature can provide the valid means to research urban heat island, weather forecast for urban environment, and high temperature alert. In environmental studies, the absolute detection of atmospheric temperature can provide the big amount of scientific data for establishment of numerical model and research on air pollution diffusion. There is reference significance for the investigation of filter system of similar lidar. Simultaneously, the scanning filter method provides a feasible solution for the filter system with the characteristics of miniaturization, high anti-interference and high stability in the space-based platform.

Research on Terahertz Filters Employing the Effect of Frustrated Total Internal Reflection

The optical filter under development employing frustrated total internal reflection is a micromechanical device containing two silicon rectangular prisms. There is flat silicon plane among hypotenuse edges of the prisms. The silicon plane is a Fabry-Perot resonator, with clearances h1 among the plate and prism edges being its mirrors. Theoretical resolution of the optical filter would be R ≈ 1,5∙103 if h1 = 50 µm and R ≈ 3∙105 if h1 = 100 µm with the thickness of the silicon plane being h = 65 µm; wavelength λ = 100 µm; free spectral region Δλ/λ ≈ 0,3…0,4, transmission in the maximum of spectral characteristics 0,6 (provided input and output silicon prism legs bloom).

Finite-difference time-domain analysis of deformed square cavity filters with a traveling-wave-like filtering response by mode coupling

An add-drop filter based on a perfect square resonator can realize a maximum of only 25% power dropping because the confined modes are standing-wave modes. By means of mode coupling between two modes with inverse symmetry properties, a traveling-wave-like filtering response is obtained in a two-dimensional single square cavity filter with cut or circular corners by finite-difference time-domain simulation. The optimized deformation parameters for an add-drop filter can be accurately predicted as the overlapping point of the two coupling modes in an isolated deformed square cavity. More than 80% power dropping can be obtained in a deformed square cavity filter with a side length of 3.01 microm. The free spectral region is decided by the mode spacing between modes, with the sum of the mode indices differing by 1.

Plasmonic waveguide ring resonator at terahertz frequencies

The authors theoretically demonstrate a plasmonic waveguide ring resonator (WRR) working at terahertz frequencies considering that surface plasmon polaritons (SPPs) can effectively propagate on bended plasmonic waveguides with dielectric claddings possessing high refractive index. A free spectral region of 110GHz has been achieved by decreasing ring radius of the WRR, which could be much smaller than the operating wavelength. By introducing dielectric and plasmonic waveguides at both sides of the resonant ring, respectively, SPPs can be efficiently excited at the output of the WRR through mode coupling. The plasmonic WRRs may find great applications in the integration of terahertz generation and control platform.