Wide Free Spectral Range Research Articles

Analysis on characteristic and application of THz frequency comb and THz sub-comb

In this paper, we proposed a method for THz sub-comb generation based on spectral interference. The result of our calculation indicated that the THz pulse train, generated by surface-emitted optical rectification of femtosecond (fs) laser pulse in periodically poled lithium niobate (PPLN), has a comb-like spectrum. The characteristic of this THz sub-comb was analyzed both in frequency and time domain. Compared with the THz frequency comb emitted by a photoconductive antenna (PCA), THz sub-comb has a lower spectral resolution and wider free spectral range. Thus it could be an ideal source for wavelength division multiplexing (WDM) in THz wireless communication system.

Single Ring Interferometer Configuration With Doubled Free-Spectral Range

We proposed and experimentally demonstrated a single ring interferometer configuration that produces doubled free-spectral range (FSR) by in-phase combining light from the pass-through port and drop port of the resonator. It exhibits higher extinction ratio than conventional single ring resonators, brings no additional stress to fabrication, works for both polarizations, and does not require any particular design on the ring structure. By using this configuration, a wide FSR can be obtained while maintaining a high quality factor.

Microresonators in CMOS Compatible Substrate

We report our recent progress in designing and developing traveling-wave microresonators on CMOS compatible substrate, with a focus on ultra-compact Si-based microring and microdisk resonators in the visible and near infrared wavelengths with ultra-high Q and small mode volume, suitable for strong light-matter interaction. The performance of these resonators is discussed, and the design and fabrication challenges and solutions to achieve efficient coupling from the bus-waveguide to the resonator is mentioned. Coupled-resonator architectures for the design of filters are analyzed and theoretical and experimental results for the flat-band filters with wide bandwidth and large free spectral range are presented. Furthermore, by using a detailed thermal model we demonstrate optimized microresonator structures on semiconductor substrates with improved thermal conductivity that can sustain a high circulating optical intensity at the resonance wavelength, which is particularly useful for nonlinear optics and optical signal processing applications. Finally, we address some of the applications and future prospects of such microresonators in CMOS-compatible substrate technology, including portable multi-purpose bio/chemical sensing systems, reconfigurable optical signal processing modules and chip-scale optical amplifiers and lasers.

Aperture-coupled MIM plasmonic ring resonators with sub-diffraction modal volumes

We propose and investigate ultracompact aperture-coupled plasmonic ring resonators with submicron bending radii based on strongly-confined metal-insulator-metal plasmonic waveguides. Enhanced coupling can be obtained via diffraction by small apertures having typical widths between 50-100 nm in the metallic sidewall between the ring and bus waveguides. Both analytical model and rigorous FDTD simulations show that 500 nm-radius ring resonators can be obtained with low insertion loss, wide free spectral range and sub-diffraction cavity volume of less than 0.1(lambda(0)/n(eff))(3).

Ultracompact SOI Microring Add–Drop Filter With Wide Bandwidth and Wide FSR

We report the design and fabrication of an ultracompact silicon-on-insulator add-drop microring filter with a 1.5-mum radius for wavelength-division-multiplexing on-chip interconnect applications. The fabricated device was measured to have a wide free-spectral range of 52 nm and a wide channel bandwidth of 210 GHz, with a 2.9-dB insertion loss and a deep through-port extinction of nearly 25 dB.

A Hollow-Core Photonic Crystal Fiber Cavity Based Multiplexed Fabry–PÉrot Interferometric Strain Sensor System

A hollow-core photonic crystal fiber Fabry-Perot interferometric (HC-PCF-FPI) strain sensing system is proposed and demonstrated. The sensing system has an excellent multiplexing capability owing to the wide free-spectral range of the HC-PCF-FPI structure. Moreover, with a simple fast Fourier transform demodulation method, the proposed strain sensing system could be potentially applied in structural health monitoring.

Compact Thermally Tunable Silicon Wavelength Switch: Modeling and Characterization

A wavelength-selective photonic switch is developed based on a Si microring resonator using thermooptic effect. The 10-mum-diameter microring resonator uses single-mode strip Si waveguides of dimension 0.25 mum times 0.45 mum operating at 1.5 mum. Full-width at half-maximum are in the range 0.1-0.2 nm. The ultrawide tunable range (>6.4 nm) and wide free spectral range (~18 nm) of the switch element enables wavelength reconflgurable multichannel matrix switch by wavelength multiplexing/demultiplexing. Average rise delay time of 14 mus with low switching power of 3.15 mW has been achieved with 0.2-nm wavelength tuning and 78 mus, 104 mW for 6.4-nm tuning. Fall delay times are usually less than 10 mus. Thermal simulations show 10%-20% agreement with the measurements up to 3.2-nm tuning. The compact size of the switch shows its potential as an active element in photonic integrated circuits.

Tunable silicon microring resonator with wide free spectral range

The authors present a silicon-on-insulator single ring resonator with a free spectral range equal to 47nm, which is the widest known value for this type of resonators. The ring radius is 2μm and is the smallest ring resonator ever reported, achieving experimentally such a wide spectral range. For this ring resonator, the authors demonstrate the quality factor to be equal to 6730±60. They thermally tune the resonant wavelength with 0.11nm∕°C, thus showing the ring resonator as an attractive component for on-chip ultracompact photonic add/drop filters and switches.

Ladder-Type Tunable Filter Connected by High-Diffraction-Order Ladder Interferometer for Use in Widely Tunable Laser Diodes

We propose and demonstrate a novel ladder-type tunable filter cascade-connected by a high-diffraction-order ladder interferometer, and discuss its potential application to a high-performance laser source having high output power and a wide tuning range. We design a novel ladder interferometric tunable filter that satisfies such essential requirements as having a narrow bandwidth for stable single longitudinal oscillation mode control together with a low insertion loss, a low wavelength-sensitive filter spectrum, a wide free-spectral range and a wide tuning range. We successfully demonstrate all of these characteristics in the fabricated device.

Birefringent Fourier-transform imaging spectrometer

Fourier-transform imaging spectrometers offer important advantages over other spectral imaging modalities, such as, a wider free spectral range, higher spectral resolutions and, in low-photon-flux conditions, higher signal-to-noise ratios can be achieved. Unfortunately, for application in harsh environments, deployment of Fourier-transform instruments based on traditional moving-mirror interferometers is problematic due to their inherent sensitivity to vibration. We describe a new Fourier-transform imaging spectrometer, based on a scanning birefringent interferometer. This system retains the advantages of traditional Fourier transform instruments, but is inherently compact and insensitive to vibration. Furthermore, the precision requirements of the movement can be relaxed by typically two orders of magnitude in comparison to a traditional two-beam interferometer. The instrument promises to enable application of Fourier-transform imaging spectrometry to applications, such as airborne reconnaissance and industrial inspection, for the first time. Example spectral images are presented.

Parallel-cascaded semiconductor microring resonators for high-order and wide-FSR filters

We demonstrate an optical channel dropping filter (OCDF) using three parallel-cascaded vertically coupled microrings with improved rolloff, bandpass flattening, and wide free spectral range (FSR) compared to a single-ring OCDF using single-mode tightly confined waveguides in both GaAs-AlGaAs and GaInAsP-InP. We achieve FSRs of 30 nm for GaAs-AlGaAs and 40 nm for GaInAsP-InP devices, which are three and four times greater, respectively, than those for single rings. The rolloff is 2.8 times faster than that for a single ring.

Static Fourier transform spectroscopy with enhanced resolving power

Static Fourier transform (StFT) spectrometers, as well as multichannel dispersive spectrometers, normally have a small value for the product of resolving power and relative free spectral range. This value is limited by the number of pixels in a row of the detector array used by the spectrometer. A method of increasing the resolving power of static Fourier transform spectrometers, while preserving their wide free spectral range, is proposed. The technique introduces a stepped retardation into a double-beam interferometer with a two-dimensional detector array. Several configurations of StFT spectrometers with stepped retardation are presented and the theoretical model, performance limitations and experimental results are discussed. The possibility of achieving large values of resolving power is demonstrated.

Novel double-ring optical-guided-wave Vernier resonator

A theoretical study of a novel optical-guided-wave Vernier resonator is presented. The resonator is a compound configuration of two triple-coupler ring-based resonators (CTCRR) with different radii. Transmittance characteristics of the CTCRR are derived by using a combination of transfer-matrix and chain-matrix formulations. The dependence of the transmittance characteristics of the CTCRR on the amplitude-coupling ratios of the directional couplers are studied, and optimum design relationships are determined. An extensive comparison between the CTCRR and two double-ring-resonator (DRR) configurations reported in the literature is carried out. It is shown that the CTCRR features double free spectral range (FSR), higher transmission at resonance, and better crosstalk than DRRs with the same radii of curvature, waveguide propagation loss and finesse. These advantages make the CTCRR particularly attractive for those applications, such as frequency-selective filtering in high-density multichannel optical FDM systems, where conflicting requirements of wide FSR and low insertion loss have to be satisfied.

Integrated-optic double-ring resonators with a wide free spectral range of 100 GHz

Integrated-optic double-ring resonators with a wide free spectral range (FSR) of 100 GHz are fabricated using GeO/sub 2/-doped silica waveguides with a high relative refractive index difference (/spl Delta/) of 1.5%. The resonators are composed of two ring resonators comprising small ring waveguides with radii of 1.75 and 2.0 mm. The double-ring resonator module exhibited a wide FSR of 98.0 GHz, a finesse of higher than 138, a low crosstalk of less than -11.7 dB, and a low insertion loss of 6.1 dB. The measured FSR of 98.0 GHz is wider than any previously reported ring resonator composed of optical waveguides. >

Integrated-optic ring resonators with two stacked layers of silica waveguide on Si

A ring resonator with a radius of 1.5 mm is demonstrated using a stack structure consisting of two layers of GeO/sub 2/-doped silica waveguide with different refractive index differences ( Delta ). One is a high- Delta waveguide layer with a Delta of 2% for a small-radius ring and the other is a waveguide layer with a Delta of 0.75% for good fiber coupling. The waveguides and a flattened middle-cladding layer are stacked on a Si substrate. The ring resonator exhibited a wide free spectral range (FSR) of 21.6+or-0.1 GHz and a high finesse of 110+or-10.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Isogyration birefringent filter

A new narrow-band polarization spectral filter with an infinitely wide free spectral range is proposed. This filter utilizes the characteristics of propagation of light in a gyrotropic birefringent medium exhibiting reversal of the sign of gyration (isogyration medium). It is shown that the width of the spectral transmission maximum may reach a few angstroms and that the angle of view is limited only by the aperture of polarizing prisms.