Filter Cavity Research Articles

Nonlinear amplification of side-modes in frequency combs

We investigate how suppressed modes in frequency combs are modified upon frequency doubling and self-phase modulation. We find, both experimentally and by using a simplified model, that these side-modes are amplified relative to the principal comb modes. Whereas frequency doubling increases their relative strength by 6 dB, the growth due to self-phase modulation can be much stronger and generally increases with nonlinear propagation length. Upper limits for this effect are derived in this work. This behavior has implications for high-precision calibration of spectrographs with frequency combs used for example in astronomy. For this application, Fabry-Pérot filter cavities are used to increase the mode spacing to exceed the resolution of the spectrograph. Frequency conversion and/or spectral broadening after non-perfect filtering reamplify the suppressed modes, which can lead to calibration errors.

High resolution 3D NanoImprint technology: Template fabrication, application in Fabry–Pérot-filter-array-based optical nanospectrometers

We report a novel 3D NanoImprint methodology with very high vertical resolution well below 1nm and a minimum surface roughness down to 0.2nm in root mean square (rms) which is attractive for several applications. We introduce our 3D NanoImprint methodology in fabricating 3D filter cavities of Fabry-Perot filter arrays for optical nanospectrometers. A large number of different cavity heights can be fabricated in one single imprint step using this high resolution, low-cost 3D NanoImprint technology. The key issue of developing 3D NanoImprint technology is the fabrication of high resolution templates. The novel 3D templates contain arrays of negative or positive checkerboard-like mesa structures with currently up to 64 different heights. Different types of 3D templates were developed and adapted to cutting edge NanoImprint methods and imprint resists. The performance of different 3D NanoImprint recipes was investigated. Based on the novel 3D NanoImprint technology developed in this work, static Fabry-Perot filter arrays with NanoImprinted cavities indicated rather high filter transmission (best single filter >90%), small line widths (about 2nm in full width at half maximum) and broad stop bands.

A Novel Demultiplexing Photodetector with Integrated Vertical Conical Structure

A novel demultiplexing photodetector which consists of a filtering cavity and a conical absorption cavity is proposed. The filtering cavity is formed by four coupled （Fabry-Pérot）F-P cavities, which can realize flat-top and steep-edge spectral response. The top mirror of the absorption cavity is designed to be conical, which can increase the times of light reflection. Then the quantum efficiency is improved. Through the theoretical analysis and simulation, the peak quantum efficiency of the photodetector is 88.87% around 1550nm. In addition, this photodetector has good performance in the flat-top and steep-edge spectral response: the 0.5dB bandwidth is 0.4nm , the 3dB bandwidth is 0.56nm and the 20dB bandwidth is 0.98nm.

A ${\rm TM}_{11}$ Dual-Mode Dielectric Resonator Filter With Planar Coupling Configuration

This paper presents a novel dielectric resonator filter exploiting dual TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> degenerate modes. The dielectric rod resonators are short circuited on the top and bottom surfaces to the metallic cavity. The dual-mode cavities can be conveniently arranged in many practical coupling configurations. Through-holes in height direction are made in each of the dielectric rods for the frequency tuning and coupling screws. All the coupling elements, including inter-cavity coupling elements, are accessible from the top of the filter cavity. This planar coupling configuration is very attractive for composing a diplexer or a parallel multifilter assembly using the proposed filter structure. To demonstrate the new filter technology, two eight-pole filters with cross-couplings for UMTS band are prototyped and tested. It has been experimentally shown that as compared to a coaxial combline filter with a similar unloaded <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> , the proposed dual-mode filter can save filter volume by more than 50%. Moreover, a simple method that can effectively suppress the lower band spurious mode is also presented.

Filtration Performance of Particle Removal from Gas with Ceramic Candle Filter

A gas-particle separation system was established to study the filtration performance where twelve ceramic filters were divided into four groups. Filters in the same group will be cleaned by pulse back-flow at once. The pressure distribution and variation during filtration and pulse back-flow washing in four group filter and along filter length was measured. The main pressure drop occurs across the filter porous wall. The pressure at filter cavity decreases from the close end to open end of the candle filter. The gas velocity magnitude inside the filter system during filtration process is generally relatively low inside the vessel. The velocity increases as the gas passes across the filter walls and enters the outlet of the filter tube.

Flow Pattern in Ceramic Filter System for Particle Removal

A gas-particle separation system was established to study the filtration performance where twelve ceramic filters were divided into four groups. Filters in same group will be cleaned by pulse back-flow at one time. The pressure distribution and variation during filtration and pulse back-flow washing in filter cavity and along filter length was measured. The main pressure drop occurs across the filter porous wall. The pressure at filter cavity decreases from the close end to open end of the candle filter. The gas velocity inside the filter system is generally relatively low in the vessel during filtration process.

Cryogenic optomechanics with a Si3N4 membrane and classical laser noise

We demonstrate a cryogenic optomechanical system comprising a flexible Si3N4 membrane placed at the center of a free-space optical cavity in a 400 mK cryogenic environment. We observe a mechanical quality factor Q > 4 × 106 for the 261 kHz fundamental drum-head mode of the membrane, and a cavity resonance halfwidth of 60 kHz. The optomechanical system therefore operates in the resolved sideband limit. We monitor the membrane's thermal motion using a heterodyne optical circuit capable of simultaneously measuring both of the mechanical sidebands, and find that the observed optical spring and damping quantitatively agree with theory. The mechanical sidebands exhibit a Fano lineshape, and to explain this we develop a theory describing heterodyne measurements in the presence of correlated classical laser noise. Finally, we discuss the use of a passive filter cavity to remove classical laser noise, and consider the future requirements for laser cooling this relatively large and low-frequency mechanical element to very near its quantum mechanical ground state.

Optimization of filtering schemes for broadband astro-combs

To realize a broadband, large-line-spacing astro-comb, suitable for wavelength calibration of astrophysical spectrographs, from a narrowband, femtosecond laser frequency comb ("source-comb"), one must integrate the source-comb with three additional components: (1) one or more filter cavities to multiply the source-comb's repetition rate and thus line spacing; (2) power amplifiers to boost the power of pulses from the filtered comb; and (3) highly nonlinear optical fiber to spectrally broaden the filtered and amplified narrowband frequency comb. In this paper we analyze the interplay of Fabry-Perot (FP) filter cavities with power amplifiers and nonlinear broadening fiber in the design of astro-combs optimized for radial-velocity (RV) calibration accuracy. We present analytic and numeric models and use them to evaluate a variety of FP filtering schemes (labeled as identical, co-prime, fraction-prime, and conjugate cavities), coupled to chirped-pulse amplification (CPA). We find that even a small nonlinear phase can reduce suppression of filtered comb lines, and increase RV error for spectrograph calibration. In general, filtering with two cavities prior to the CPA fiber amplifier outperforms an amplifier placed between the two cavities. In particular, filtering with conjugate cavities is able to provide <1 cm/s RV calibration error with >300 nm wavelength coverage. Such superior performance will facilitate the search for and characterization of Earth-like exoplanets, which requires <10 cm/s RV calibration error.

Half mode substrate-integrated waveguide-loaded evanescent-mode bandpass filter

In this article, a filter size reduction of 46% is achieved by reducing a substrate-integrated waveguide (SIW)-loaded evanescent-mode bandpass filter to a half-mode SIW (HMSIW) structure. SIW and HMSIW filters with 1.7 GHz center frequency and 0.2 GHz bandwidth were designed and implemented. Simulation and measurements of the proposed filters utilizing combline resonators have served to prove the underlying principles. SIW and HMSIW filter cavity areas are 11.4 and 6.2 cm2, respectively. © 2012 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2013.

Large-angle scattered light measurements for quantum-noise filter cavity design studies

Optical loss from scattered light could limit the performance of quantum-noise filter cavities being considered for an upgrade to the Advanced Laser Interferometer Gravitational Wave Observatory (LIGO) gravitational-wave detectors. This paper describes imaging scatterometer measurements of the large-angle scattered light from two high-quality sample optics, a high reflector and a beamsplitter. These optics are each superpolished fused silica substrates with silica:tantala dielectric coatings. They represent the current state-of-the art optical technology for use in filter cavities. We present angle-resolved scatter values and integrate these to estimate the total scatter over the measured angles. We find that the total integrated light scattered into larger angles can be as small as 4 ppm.

Offline estimation of decay time for an optical cavity with a low pass filter cavity model

This Letter presents offline estimation results for the decay-time constant for an experimental Fabry-Perot optical cavity for cavity ring-down spectroscopy (CRDS). The cavity dynamics are modeled in terms of a low pass filter (LPF) with unity DC gain. This model is used by an extended Kalman filter (EKF) along with the recorded light intensity at the output of the cavity in order to estimate the decay-time constant. The estimation results using the LPF cavity model are compared to those obtained using the quadrature model for the cavity presented in previous work by Kallapur et al. The estimation process derived using the LPF model comprises two states as opposed to three states in the quadrature model. When considering the EKF, this means propagating two states and a (2×2) covariance matrix using the LPF model, as opposed to propagating three states and a (3×3) covariance matrix using the quadrature model. This gives the former model a computational advantage over the latter and leads to faster execution times for the corresponding EKF. It is shown in this Letter that the LPF model for the cavity with two filter states is computationally more efficient, converges faster, and is hence a more suitable method than the three-state quadrature model presented in previous work for real-time estimation of the decay-time constant for the cavity.

Note: A monolithic filter cavity for experiments in quantum optics

By applying a high-reflectivity dielectric coating on both sides of a commercial plano-convex lens, we produce a stable monolithic Fabry-Perot cavity suitable for use as a narrow band filter in quantum optics experiments. The resonant frequency is selected by means of thermal expansion. Owing to the long term mechanical stability, no optical locking techniques are required. We characterize the cavity performance as an optical filter, obtaining a 45dB suppression of unwanted modes while maintaining a transmission of 60%.

DC readout experiment in Enhanced LIGO

The two 4 km long gravitational wave detectors operated by the Laser Interferometer Gravitational-wave Observatory (LIGO) were modified in 2008 to read out the gravitational wave channel using the DC readout form of homodyne detection and to include an optical filter cavity at the output of the detector. As part of the upgrade to Enhanced LIGO, these modifications replaced the radio-frequency (RF) heterodyne system used previously. We describe the motivations for and the implementation of DC readout and the output mode cleaner in Enhanced LIGO. We present characterizations of the system, including measurements and models of the couplings of the noises from the laser source to the gravitational wave readout channel. We show that noise couplings using DC readout are improved over those for RF readout, and we find that the achieved shot-noise-limited sensitivity is consistent with modeled results.

Monolithically integrated tunable dual-wavelength photodetector with flat-top response

This paper elaborates the design and analysis of a novel monolithically integrated tunable dual-wavelength photodetector which can be formed by heteroepitaxial growth of an InP-based p-i-n absorption structure on a GaAs-based Fabry–Pérot filter. The photodetector presents two response peaks resulting from the introduction of 4-step Fabry–Pérot cavity. The wavelength spacing between the two peaks, as well as the peak shapes, can be accurately and easily tailored by adjusting the corresponding step height. This photodetector can be tuned via thermal-optic effect. With optimum design by using Transfer Matrix Method, the detectable wavelength range of the photodetector will be twice as that of its counterpart being with a uniform filter cavity; what is more, flat-top responses of the two peaks can also be obtained, simultaneously. We discuss the epitaxial growth and fabrication of the photodetector at the end, indicating the mature technique available for the device.

Numerical Simulation of Gas Purification by Filtration with Ceramic Filter

The numerical simulation method is used to analyze the process of gas purification with ceramic filters. The gas /solid two-phase flow field in the ceramic filter vessel was simulated using the Eulerian two-fluid model provided by FLUENT code. The variations of the pressure distribution in the filter cavity were analyzed. The distribution of the dust cake density along the under cleaning filter length was studied.

Optical Frequency Comb as a general-purpose and wide-band calibration source for astronomical high resolution infrared spectrographs

A spectrally filtered Optical Frequency Comb (OFC) laser is proposed as a versatile calibration source for astronomical spectrometers in the 1–2 μm spectral range. Such a source overcomes the limitations of current calibration lamps providing a uniform spectrum of equally spaced lines with similar intensity and extremely high long-term frequency stability. We present preliminary studies and results of a system which filters the OFC from a 100 MHz comb spacing to 16 GHz one, an adequate spacing for spectrometers with resolving power Δλ/λ >30000. The first approach employs two Fabry-Perot cavities in series, made of dielectric coated mirrors, followed by a non-linear optical broadening system. The limitations of such a filtering process are discussed. These can be overcome by the second approach, based on filtering cavities with metallic coated mirrors.

Modulation-free optical locking of an external-cavity diode laser to a filter cavity

Optical locking to a filter cavity is an effective method to eliminate the limitations of an external-cavity diode laser, such as broad spontaneous emission backgrounds and frequency jitters. Stable operation of the optical locking requires simultaneous control of the feedback phase and the diode-laser frequency. Frequency dither is usually used to extract the two error signals, but this causes extra frequency modulation in the output beam. A modulation-free method for deriving the error signals by modulating the laser-cavity coupling strength is demonstrated with a violet diode laser. A modulation-free linewidth upper limit of about 7 kHz for a 1 s measurement is realized by the method.

Filtration and Pulse Cleaning System for Particle Removal from Gas

The application of rigid ceramic filter for gas filtration on an industrial scale has shown unstable operation over periods of days. The behavior of the ceramic filtration in further application can be predict by measuring properties of a variety of dust and filtration operation condition. The aim of the experimental investigation is to analysis the pressure distribution inside and outside the ceramic filter along the filter length, and to analyze the pressure drop across the filter along the filter length. Ceramic filter filtration system composes multi-pipe filters for gas clean up is introduced. The operation parameters can be measured by this test system, such as the pressure in the filter cavity and outside the filter surface, velocity outside the filter, gas temperature and humidity, flow rate, dust particle size concentration before gad entering the filter tank and after the gas leaving the filer tank, the pulse cleaning pressure.

Optimal configurations of filter cavity in future gravitational-wave detectors

Sensitivity of future laser interferometric gravitational-wave detectors can be improved using squeezed light with frequency-dependent squeeze angle and/or amplitude, which can be created using additional so-called filter cavities. Here we compare performances of several variants of this scheme, proposed during last years, assuming the case of a single relatively short (tens of meters) filter cavity suitable for implementation already during the life cycle of the second generation detectors, like Advanced LIGO. Using numerical optimization, we show that the phase filtering scheme proposed by Kimble et al [Phys.Rev.D 65, 022002 (2001)] looks as the best candidate for this scenario.

Quantum Enhancement of the Zero-Area Sagnac Interferometer Topology for Gravitational Wave Detection

Only a few years ago, it was realized that the zero-area Sagnac interferometer topology is able to perform quantum nondemolition measurements of position changes of a mechanical oscillator. Here, we experimentally show that such an interferometer can also be efficiently enhanced by squeezed light. We achieved a nonclassical sensitivity improvement of up to 8.2dB, limited by optical loss inside our interferometer. Measurements performed directly on our squeezed-light laser output revealed squeezing of 12.7dB. We show that the sensitivity of a squeezed-light enhanced Sagnac interferometer can surpass the standard quantum limit for a broad spectrum of signal frequencies without the need for filter cavities as required for Michelson interferometers. The Sagnac topology is therefore a powerful option for future gravitational-wave detectors, such as the Einstein Telescope, whose design is currently being studied.