Narrow-band Wavelength Filters Research Articles

Grating-Position-Shifted Cavity-Resonator- Integrated Guided-Mode Resonance Filter

A cavity-resonator-integrated guided-mode resonance filter (CRIGF) is a type of narrowband wavelength filter for an incident free-space beam of small diameter. A CRIGF consists of a grating coupler (GC) and a pair of distributed Bragg reflectors used to construct a waveguide cavity resonator. This time, the dependence of the reflection spectrum on the relative position of the centers of the GC and the waveguide cavity resonator is investigated in detail. The position shift of 3/16 of the GC period reduces the bandwidth from 6 to 1 nm, and the position shift of a quarter of the GC period eliminates the guided-mode resonance effect.

Narrow-band wavelength tunable filter based on asymmetric double layer metallic grating.

In this paper, the optical properties of asymmetric double layer metallic gratings are presented theoretically. The asymmetric structure is achieved by two main factors: one corresponding to moving alternatively metal nanowires of the top layer metallic grating, the other corresponding to possessing different thickness of the top and down layer metallic gratings. Our proposed structure shows one remarkable narrow-band transmission dip at normal incidence, which is distinct different from that of symmetric structure. The results are further confirmed by using different numerical computation methods, and explained by the analytical model of Fano-like resonance. We find that, only when the thickness of the down layer metallic grating has certain fixed value, transmission dip can be transformed from two to only one dip even if the existence of symmetry breaking. However, the wavelength position of the dip can be easily controlled by adjusting the thickness of the top layer metallic grating without the need to modify the structure period, and the width of metal nanowire. Moreover, the influence of other structure parameters on the dip is also investigated. Surprisingly, in order to keep the appearance of one dip in the transmission spectrum of designed structure, there is a good linear approximation between the refractive index of waveguide layer and the thickness of down layer metallic grating, and the relation of waveguide layer thickness and the thickness of down layer metallic grating satisfy secondary polynomial fitting. This work can be used to develop subwavelength metallic-grating-based and narrow-band tunable wavelength filters.

Simultaneous dual-wavelength-band common-path swept-source optical coherence tomography with single polygon mirror scanner

We report a novel (to the best of our knowledge) simultaneous 1310/1550 two-wavelength band swept laser source and dual-band common-path swept-source optical coherence tomography (SS-OCT). Synchronized dual-wavelength tuning is performed by using two laser cavities and narrowband wavelength filters with a single dual-window polygonal scanner. Measured average output powers of 60 and 27 mW have been achieved for the 1310 and 1550 nm bands, respectively, while the two wavelengths were swept simultaneously from 1227 to 1387 nm for the 1310 nm band and from 1519 to 1581 nm for the 1550 nm band at an A-scan rate of 65 kHz. Broadband wavelength-division multiplexing is used for coupling two wavelengths into a common-path single-mode GRIN-lensed fiber probe to form dual-band common-path SS-OCT. Simultaneous OCT imaging at 1310 and 1550 nm is achieved. This technique allows for in vivo high-speed OCT imaging with potential application in functional (spectroscopic) investigations.

Optimization design and realization of Gaussian apodized sample gratings for narrowband wavelength selectivity filters

A sample fiber Bragg grating (SFBG) with Gaussian apodization profile is designed and optimized for the narrowband wavelength filter, and the relevant conclusions provide a guide for the further filter fabrication. With the grating length limited, a narrowband (5 GHz) grating filter used in the radio over fiber (ROF) system is fabricated, which has insertion loss of 3.463 dB, sideband suppression of 24.21dB, 3 dB bandwidth of 40 pm, and 20 dB bandwidth of 106.3 pm.

Fiber Fabry-Pe/spl acute/rot interferometer with precision glass-ceramic jacketing

A novel passive-alignment fiber Fabry-Pe/spl acute/rot interferometer (FFPI) was developed by direct jacketing of an optical fiber with a low-expansion glass-ceramic (GC). It was first demonstrated that a transmission spectrum with no peak splitting is obtained by the FFPI developed. The polarization-dependent loss of this FFPI was also improved to one-third of the conventional FFPI fabricated with a ferrule and an organic adhesive. The transmission peak wavelength of the FFPI shows a sensitive linear temperature dependency. The FFPI with a GC jacket is expected as a promising candidate for narrow-band optical wavelength filters or temperature sensors that can be easily embedded in standard optical fiber connectors.

Subwavelength Structured Optical Elements and Resonant Grating Filters

ABSTRACTWe developed several optical elements with subwavelength-structured surfaces. Antireflection surfaces were fabricated on a diffraction grating. A micro-retarder array realized by the form-birefringent effect has been made for an application to a polarization camera system. And we developed narrow-band reflection wavelength filters called “guided-mode resonant grating filters”. This filter bases on a coupling of guided mode and radiation mode. After describing some examples of the filters, we mention a grating structure for an optical switch with nonlinear optical material.

Gas analysis using an infrared source with temporally varyingtemperature

A novel technique of analysing mixtures of infrared active gaseswithout the need of narrow-band wavelength filters uses the dependence ofthe absorption on the temperature of a black body source. By pulsing thevoltage on a filament light source, the temperature-dependent absorptionspectrum is displayed as a function of time. Individual and simultaneousmonitoring of carbon dioxide and water vapour in the atmosphere isdemonstrated. The performance is considerably enhanced by using a sequenceof pulses of different lengths.

Low-crosstalk WDM by Bragg diffraction from thermally fixed reflection holograms in lithium niobate

Dense wavelength-division-multiplexing (WDM) systems require narrowband wavelength filters for simultaneous multiplexing/demultiplexing of several channels. As a proof of concept the authors report the demultiplexing of two channels (channel spacing 0.8 nm) by Bragg diffraction from two thermally fixed volume phase reflection holograms in photorefractive lithium niobate crystals doped with iron. More than 25 dB crosstalk suppression is achieved, and intensity losses are only between 2.3 and 5.2 dB.

Spectral characteristics of coupled-waveguide Fabry-Perot resonators and filters

Conventional Fabry-Perot resonators provide a narrow spectral width but lack the capability of longitudinal mode discrimination. A coupled-waveguide Fabry-Perot structure made of two parallel waveguides with reflecting mirrors at the ends is proposed for applications as mode selective resonator in single-mode diode lasers and as narrow-band wavelength filters. The interference of counter propagating waves from reflection by end mirrors and the coupling of waves between the two parallel waveguides contribute to the operation of this resonator structure. Thus, the device exhibits the attributes of both Fabry-Perot resonator and directional coupler. The coupled-mode theory of parallel waveguides is employed to analyze the structure. Both cases of identical and unidentical waveguides are examined. Resonance conditions and spectral characteristics are determined. It is shown that the coupled-waveguide Fabry-Perot resonator provides significant improvement in mode discrimination capability and longitudinal mode spacing over the conventional Fabry-Perot resonator.

Tunable wavelength filters based on nonlinear optical interactions in semiconductor amplifiers

A novel class of narrow-band tunable wavelength filters is proposed and evaluated. Wavelength selectivity of the proposed filters Is derived from the finite time response of an optical nonlinearity. The nonlinearity is gain saturation in semiconductor optical amplifier structures. The filters are shown to have very narrow passbands tunable over the entire semiconductor gain bandwidth. The key to filter implementation is a device configuration in which the wave-mixing products can be isolated from the amplified inputs. Three integrated optics compatible configurations are considered and shown to have high filter throughputs 34 to 180% and subangstrom bandwidths.

Narrow-band vertically stacked filters in InGaAlAs/InP at 1.5 μm

A narrow-band wavelength filter in InGaAlAs/InP has been modeled, fabricated, and tested. A highly asymmetrical, vertically coupled directional coupler operating near the band-edge is formed by a narrow ridge and a wide strip-loaded waveguide. The results of numerical simulation, performed by employing the spectral index method, effective-index method, and a modified coupled-mode theory, are used to fabricate a filter structure with a prescribed filter response. Operation at a center wavelength around 1.5 /spl mu/m with a bandwidth of 18 /spl Aring/ and transfer efficiency of /spl sim/46-68% is achieved. Excellent agreement between the. Designed and measured bandwidth is demonstrated. A multichannel filter device based on an array of individual filters that is capable of extending the usable spectral range is analyzed.

Rapidly tunable narrowband wavelength filter using LiNbO/sub 3/ unbalanced Mach-Zehnder interferometers

We demonstrate a rapidly tunable narrowband wavelength filter using unbalanced Mach-Zehnder interferometers in lithium niobate. It is designed to switch among eight channels spaced 50 GHz apart in optical frequency (approximately 0.4 nm in wavelength) in less than 50 ns. The insertion loss is 19 dB and crosstalk is less than -22 dB. The filter consists of a cascade of three electrooptically tunable unbalanced Mach-Zehnders and a feedback circuit to keep the filter centered on the desired wavelength.

Highly asymmetrical vertical coupler wavelength filter in InGaAlAs/InP

For the first time a narrowband wavelength filter in InGaAlAs/InP is reported. A highly asymmetrical, vertically-coupled directional coupler operating near the band edge is formed by a narrow ridge and a wide strip-loaded waveguide. Operation at a centre wavelength of ~1.5 µm with a bandwidth of 18 Å and transfer efficiency of ~50–70% has been achieved.

Grating assisted vertical coupler/filter for extended tuning range

Broadly tunable, narrowband wavelength filters that can be integrated with other photonic devices are important elements for WDM multi/demultiplexing systems. Recently, we have demonstrated a broadly tunable, narrowband wavelength selective grating assisted vertical directional coupler with a 215 Å tuning range. In the Letter, an improved version of the vertically stacked buried rib InGaAsP/InP integrable wavelength filter that has a tuning range greater than 370 Å is reported.

A coupled-waveguide grating resonator filter

A coupled-waveguide grating resonator is proposed and analyzed for implementing a tunable narrowband wavelength filter. Exchange Bragg reflection between two strongly coupled dissimilar waveguides is utilized in the filter design. The input and output (reflected) signals are spatially separated so that the device may be suitable as a signal dropping filter in a unidirectional ring photonic network. >

Proposed semiconductor narrow-band wavelength filter using directional coupler

A GalnAsP/InP waveguide narrow-band wavelength filter is proposed and analysed. The proposed wavelength filter is based on an asymmetric semiconductor directional coupler having a step-index waveguide and a double clad waveguide. It is numerically shown that a bandwidth as narrow as 5.4nm is feasible for 1.3 mm long devices at a 1.53 =m centre wavelength.

The Bragg reflection waveguide directional coupler

A directional coupler formed by two Bragg reflection waveguides (BRWs) with a common finite periodic region between them is analyzed. The coupling coefficient is a sensitive function of the index difference in the periodic coupling section. This feature can lead to a novel kind of electrooptic switch. In addition, the BRW directional coupler exhibits dispersion properties characteristic of the periodic bounding media, which can be used to implement a narrowband wavelength filter and a TE polarizer.

Polarization-independent electro-optically tunable narrow-band wavelength filter

We propose and demonstrate the first polarization-independent electro-optically tunable wavelengh filter with single-mode waveguides. The Ti:LiNbO3 filter utilizes narrow-band electro-optic TE ↔ TM conversion and employs TE/TM polarization splitters in the input and output waveguides. The filter operates at 1.52 μm with a bandwidth of only 12 Å and can be electro-optically tuned over at least 110 Å at a tuning rate of 0.55 Å/V.

Bandwidth of forward and backward coupling directional couplers

This paper deals with the filter properties of directional couplers. Formulas are derived for the bandwidth in wavelength units of couplers consisting of two waveguides that exchange their power either in the direction of the incident wave (forward couplers) or in the opposite direction (backward couplers). Backward couplers require the presence of a diffraction grating on one or both waveguides but they can achieve very much smaller bandwidth than forward couplers and hence are useful for constructing narrow-band optical wavelength filters.

Zero order anomaly of dielectric coated gratings

The zeroeth order diffraction efficiency of a three-layer dielectric grating is studied theoretically and experimentally. A sharp resonance anomaly due to the excitation of guided waves is observed, which increases significantly the reflectance of the system. The operation of the structure as a tunable narrow-band wavelength filter in a reflection regime is demonstrated.