Conventional Optical Filters Research Articles

Optical notch filters with tunable central wavelength and reconfigurable free spectral range on SOI platform

Conventional optical filter is typically easy to realize adjustable central wavelength and bandwidth, but it's quite challenging to obtain reconfigurable free spectral range (FSR), thus limiting its application in a variety of fields. In this paper, we propose novel schemes and implement two kinds of tunable and reconfigurable optical notch filters (TRNF) on Silicon-on-Insulator (SOI) platform for the first time. The experimental results indicate that the FSR reconfigurable range is 20 GHz–200 GHz for a five-stage TRNF based on microring resonators (MRR), and the central wavelength can be tuned over the whole free spectral range. Moreover, a five-stage TRNF based on Mach-Zehnder interferometers (MZI) offers a broader FSR reconfigurable range of 7.5 GHz–232.5 GHz. Our results will improve the spectrum usage efficiency of programmable photonic integrated circuits and pave the way for the development of next generation elastic optical network (EON).

Wavelength Conversion of 25-Gbit/s PAM-4 Signals Using a Quantum-Dot SOA

We demonstrate all-optical wavelength conversion of a 4-level pulse-amplitude modulation (PAM-4) signal by cross gain modulation (XGM) in a quantum-dot semiconductor optical amplifier (QD-SOA). In this scheme, the center passband of a conventional optical filter located at the QD-SOA output is slightly shifted to the shorter wavelength side [blue-shifted filtering (BSF)], which enables us to accelerate the gain recovery time of the QD-SOA and reduce the bandwidth limitation in the process of the wavelength conversion. By using the combination of XGM and BSF, we successfully achieve 25-Gbit/s PAM-4 wavelength conversion, with good conversion performance in terms of error-vector magnitude. We also investigate the conversion performance in a wide wavelength-hopping range and the high tolerance to the shift in input signal wavelength for use in data center applications.

Direct Selection and Amplification of Individual Narrowly Spaced Optical Comb Modes Via Injection Locking: Design and Characterization

Many applications of optical frequency combs (OFCs) require manipulation and amplification of individual comb modes, e.g., arbitrary waveform generation, terahertz generation and telecommunications. Extracting individual comb modes can be a challenging task for OFCs with narrow comb mode spacings (100 MHz to 10 GHz) due to the limitations of conventional optical filters. Optical injection locking can address this problem, but-due to the relatively large bandwidth (1 to 10 GHz) required for simple (i.e., without the need for additional feedback loops) and stable locking-can struggle when processing OFCs with sub-GHz comb mode spacings. Here, we present an approach to optical injection locking which incorporates a dither-free phase locked loop that allowed for long-term locking to OFCs with comb spacings below the high power injection locking bandwidth. As a result, we achieved robust injection locking directly to a sub-GHz OFC (250 MHz in our experiments). Optimization of the optical injection power is carried out using detailed phase noise characterization. We achieved an Allan deviation for the frequency variation of the slave laser with respect to the injected comb mode (1 s gate time) down to 9.7 × 10-17 and 4.4 × 10-19 at 1 s and 1000 s averaging times respectively, and a phase error variance of 0.02 rad2 (integration bandwidth of 100 Hz to 500 MHz).

Feasibility of nanofluid-based optical filters

In this article we report recent modeling and design work indicating that mixtures of nanoparticles in liquids can be used as an alternative to conventional optical filters. The major motivation for creating liquid optical filters is that they can be pumped in and out of a system to meet transient needs in an application. To demonstrate the versatility of this new class of filters, we present the design of nanofluids for use as long-pass, short-pass, and bandpass optical filters using a simple Monte Carlo optimization procedure. With relatively simple mixtures, we achieve filters with <15% mean-squared deviation in transmittance from conventional filters. We also discuss the current commercial feasibility of nanofluid-based optical filters by including an estimation of today's off-the-shelf cost of the materials. While the limited availability of quality commercial nanoparticles makes it hard to compete with conventional filters, new synthesis methods and economies of scale could enable nanofluid-based optical filters in the near future. As such, this study lays the groundwork for creating a new class of selective optical filters for a wide range of applications, namely communications, electronics, optical sensors, lighting, photography, medicine, and many more.

Nanofluid-based optical filter optimization for PV/T systems

Optical filters are essential in a wide range of applications, including optical communications, electronics, lighting, optical sensors and photography. This article presents recent work which indicates that optical filters can be created from specialized nanoparticle suspensions. Specifically, this article describes a theoretical optimization process for designing nanofluid-based filters for hybrid solar photovoltaic/thermal (PV/T) applications. This particular application is suitable because nanofluids can be utilized as both volumetric solar absorbers and flowing heat transfer mediums. The nanofluid filters described in this work compare favorably with conventional optical filters for five photovoltaic (PV) cell alternatives: InGaP, CdTe, InGaAs, Si, and Ge. This study demonstrates that nanofluids make efficient, compact and potentially low-cost, spectrally selective optical filters.

Changes in contrast sensitivity perception due to protector screens for welding tasks: conventional protective optical filter vs novel protective optical filter of selective absorbance

AbstractPurpose To evaluate differences in contrast sensitivity perception in central visual field using protective optic filters for welding tasks. Two interposed filters are compared: a conventional protective optical filter and a new protective optical filter with selective absorption designed by University Complutense of Madrid (UCM).Methods 36 people of working‐age were included in a cross‐sectional prospective observational study. Contrast sensitivity threshold in central visual field was evaluated using FDT Perimeter, C‐20 procedure (Humphrey systems, USA). Three conditions were set: a) without optical filters; b) with an interposition of conventional protective optical filter for welding (shade 2.5) and c) with an interposition of a novel protective optical filter of selective absorbance that fully blocks the short wavelength light and minimally attenuates the remaining bands of the visible spectrum.Results Contrast sensitivity in all visual field areas evaluated diminished 91‐98% with the conventional protective optical filters. UCM optical filters provoked a low‐level decrease in contrast sensitivity (9‐19%). Mean contrast sensitivity thresholds were 29.5±5dB, 25.5±4dB, 1.8±2dB in conditions a, b and c respectivelyConclusion Conventional optical filters resulted in drastically diminished contrast sensitivity. UCM optical filter minimally diminish the visual function, allowing a suitable visibility and also maintaining an appropriate level of protection against phototoxic damage.

Design and Simulation of a Narrow Passband Electro-Optical Tunable Filter with Band-pass and Band-rejection Output

An electro-optical tunable filter comprised of two paralleled identical long-period waveguide gratings is proposed in this paper. The filter has one input port and two separate output ports, outputting band-pass and band-rejection light respectively. Using poled electro-optical polymer material to fabricate its waveguide cores enable the filter’s tuning speed to approach nanosecond order. With the aid of long-period waveguide gratings, the tuning range of the filter is Λ times larger than the conventional electro-optical filters. We built up a model to design and simulate the proposed tunable optical filter. We investigate the relationship between the parameters of the long-period waveguide grating and the characteristics of the tunable filter, such as tuning range and FWHM. The simulation results show that the tunable optical filter can realize high-speed tuning in the wavelength range of 1530 nm to 1560 nm, and the FWHM width can reach 0.8 nm. Compared with the conventional optical filters, the results show that a high-speed widely tunable optical filter with narrow pass band can be achieved.

Mach-Zehnder interferometer with a uniform wavelength period.

We have realized a first optical filter composed of optical delay lines and couplers that has a periodic response with respect to wavelength, whereas a conventional optical filter is known to have a periodic response to relative optical frequency. This new filter can be constructed by using a phase-generating coupler (PGC) that supplies the optical delay lines with a wavelength-dependent nonlinear phase. The PGC, which is also composed of optical delay lines and couplers, can simultaneously provide a desired phase and an arbitrary amplitude coupling ratio. We applied our transformation method to a conventional Mach-Zehnder interferometer (MZI) as an example of an optical frequency filter and converted it to an optical wavelength filter. We fabricated the designed MZI on a silica-based planar lightwave circuit and successfully realized what is believed to be the first MZI with a uniform wavelength period.

Double exposure speckle technique for imaging system analysis and use in rod lens arrays

New double exposure speckle techniques for imaging system analysis are developed and their feasibility is experimentally examined. Using a single convex lens as the test lens, the MTF of the lens is obtained from the measured visibility of Young's fringes which have been produced by Fourier transforming a double exposure specklegram recorded through the test lens. By using conventional optical filtering techniques the spatial variations in the imaging quality are visualized for GRIN rod lens arrays.

The Optical Convolution of Time Functions

The ease with which an optical system can achieve an extremely large time-bandwidth product makes optical filters and cross-correlation function generators attractive for signal detection and parameter estimation when signal-to-noise ratios are very low. The principles of a new optical filter that overcomes the sometimes severe positioning requirements of conventional optical filters are described. The relationship between the signal amplitude and matched-filter impulse response amplitude is discussed, and a method for reducing a noise component that is common to all optical filters is given. A matched filter was constructed and used to detect a radar-like pulse when the signal-to-noise ratio was -40 dB. A digital computer may be used with an optical system to filter time signals with an accuracy that potentially is as good as the accuracy of an all-digital system, but with a significant reduction in computation. The theory of an optical digital filter is presented.