Optical Low-pass Filter Research Articles

Study transmission characteristics of graded photonic crystal as low pass filter

Here, we investigate propagation characteristics of graded thickness one-dimensional photonic crystal structure made of silicon and air. The optical transmission is obtained by transfer matrix method (TMM) and studied impact of graded index and refractive index variation on the transmission spectra. The optical transmission of suggested structure drops as frequency increases and falls to zero more rapidly as graded index increases. The graded index variable can be used to tune the suggested structure’s cut off frequency, which corresponds to half of the transmission value. So, such device has useful application as tunable optical low-pass filter.

Flame detection by heat from the infrared spectrum: Optimization and sensitivity analysis

Accurate detection of unwanted fires at their early stage is crucial for efficient mitigation and loss prevention. Moreover, the detection strategy must avoid false alarms and the associated disruptions in workplaces. Thermal radiation-based flame detection is the fastest detection method and is commonly used in critical industrial spaces, such as air hangars and petroleum manufacturing and storage. The main challenge is distinguishing the radiation of flames from other sources, e.g., hot objects or the Sun. The principles of radiation-based flame detection have been known for a long time, but open data and worked-out feasibility studies are rare. This work takes advantage of the recent advances in experimental and numerical methods of characterizing the infrared spectra. Combining high-resolution spectra from flames and blackbody emitters with virtual low-pass filters allows us to simulate the response of a hypothetical sensor. To maximize the difference between flame and blackbody responses, we use a pattern search algorithm to find optimal filtering wavelengths for two different detection strategies based on three or four optical low-pass filters. The optimal wavelengths are reported along with the sensitivity of the detection signal to the filter non-ideality. Our results give guidelines for design of efficient and highly selective flame-radiation-based fire detection sensors.

System to eliminate the graininess of an integral imaging 3D display by using a transmissive mirror device

We propose a system to eliminate the graininess of an integral imaging 3D display by using a transmissive mirror device (TMD). The proposed system consists of a 2D display, a micro-lens array (MLA), and a TMD. The TMD comprises square apertures with mirror-reflective inner wall. The light rays pass through the square aperture to form a diffraction spot, and the diffraction light intensity has a Sinc-function distribution. Therefore, the TMD can be used as an optical low-pass filter. In a certain imaging range, the mainlobe of the Sinc-function distribution is almost unchanged. The TMD has the property of a volumetric optical low-pass filter. It can interpolate the interval between discrete 3D pixels. Therefore, the TMD can be used to eliminate the graininess. The resolution of the 3D image is improved by 2.12 times. The experimental results verify the feasibility of the proposed system.

Moire compensation in 2D colorimeter using adaptive spatial-frequency filter

The paper describes effective compensation of moire in a two-dimensional (2D) colorimeter using CMOS sensor without an Optical Low Pass Filter (OLPF) for display measurements. First, frequency characteristics of moire were modeled using a conventional system theory, and based on this moire model, a measured moire was characterized experimentally using the discrete Fourier transform (DFT). A detected moire in the frequency domain will be then compensated using spatial and frequency filters with minimizing loss of image details of displays, such as pixel-, line-defects and local variations in tristimulus values. Result images of the proposed method were compared with moire-free reference images. Experimental results have shown that the proposed method has sufficient performance to be used in high-precision imaging colorimeters.

Experimental Research on the Noise Characteristics of the Output Field of the Optical Filter Cavity

Precision measurement is an important direction of today's frontier scientific research. Using lasers to achieve high-precision target measurement has become an important way to improve measurement accuracy, which can be applied in various fields. However, for a certain application, the measurement accuracy will directly depend on the noise level of the laser source. Most applications require that the measurement frequency band is concentrated in the audio frequency band. In order to obtain a low-noise laser source with shot noise limited in the applied frequency band, active and/or passive noise reduction are the usual choice, i.e., active feedback control and filter cavity technique, and so on. Therefore, noise analysis and suppression techniques are the main concern of the precision measurement. The optical filter cavity acts as an optical low-pass filter, which can effectively suppress high-frequency noise beyond its linewidth. In this work, we found that the intensity noise of the output field of an optical filter cavity is higher than the noise floor of the laser. The main sources of noise are analyzed through experiments:(1) excess noise introduced by cavity length locking; (2) laser phase and pointing noises coupled to the intensity one by the cavity. To cancel the excess noise as much as possible, we optimize the feedback control loop by measuring the open-loop and closed-loop transfer functions of the MC, combined with the critical proportionality method. All the control loop are homemade, and the PID is designed with a FPGA board for expediently achieving a noise reduction up to 30 dB at the audio frequency. Then the control loop is optimized as the best condition without introducing the excess noise. Compared with the free-running laser, MC filters out the high-frequency noise, meanwhile converts the phase noise and pointing noise of input field into the intensity noise of the output field. Therefore, the power noise spectrum in the audio segment is still higher than that of the input optical field itself. In the future, an active control loop will be applied to suppress the noise power. The experimental results provide the basic means for applied research such as feedback control loop noise analysis, which will promote the development of precision measurement to higher measurement accuracy.

Experimental study on noise characteristics of audio frequency band in output field of optical filter cavity

Precision measurement is an important direction of today’s frontier scientific research. Using lasers to achieve high-precision target measurement has become an important way to improve measurement accuracy, which can be used in various fields. However, for a certain application, the measurement accuracy will directly depend on the noise level of the laser source. Most of applications require that the measurement frequency band is concentrated in the audio frequency band. In order to obtain a low-noise laser source with shot noise limited in the applied frequency band, active and/or passive noise reduction is usually an option, i.e. active feedback control or filter cavity technique, etc. Therefore, noise analysis and suppression techniques are the main concern of the precision measurement. The optical filter cavity acts as an optical low-pass filter, which can effectively suppress high-frequency noise beyond its linewidth. In this work, we find that the intensity noise of the output field of an optical filter cavity is higher than the noise floor of the laser. The main sources of noise are analyzed experimentally, showing that 1) excess noise is introduced by cavity length locking, and 2) laser phase and pointing noises are coupled to the intensity one by the cavity. To cancel the excess noise as much as possible, we optimize the feedback control loop by measuring the open-loop and closed-loop transfer functions of the mode cleaner (MC), combined with the critical proportionality method. All the control loops are homemade, and the proportional-integral-derivative (PID) is designed with a field programmable gate array board for expediently achieving a noise reduction up to 30 dB at the audio frequency. Then the control loop is optimized to the best condition without introducing the excess noise. Compared with the free-running laser, MC filters out the high-frequency noise, meanwhile converts the phase noise and pointing noise of input field into the intensity noise of the output field. Therefore, the power noise spectrum in the audio band is still higher than that of the input optical field itself. In the future, an active control loop will be used to suppress the noise power. The experimental results provide the basic means for application research such as feedback control loop noise analysis, which will promote the development of precision measurement toward higher measurement accuracy.

A practical approach on non-regular sampling and universal demosaicing of raw image sensor data

Non-regular sampling is a well-known method to avoid aliasing in digital images. However, the vast majority of single sensor cameras use regular organized color filter arrays (CFAs), that require an optical-lowpass filter (OLPF) and sophisticated demosaicing algorithms to suppress sampling errors. In this paper a variety of non-regular sampling patterns are evaluated, and a new universal demosaicing algorithm based on the frequency selective reconstruction is presented. By simulating such sensors it is shown that images acquired with non-regular CFAs and no OLPF can lead to a similar image quality compared to their filtered and regular sampled counterparts. The MATLAB source code and results are available at: http://github. com/PhilippBackes/dFSR

Composite filtering effect in a SESAM mode-locked fiber laser with a 3.2-GHz fundamental repetition rate: switchable states from single soliton to pulse bunch.

States that are switchable from single soliton to pulse bunch in a compact semiconductor saturable absorber mirror (SESAM) mode-locked fiber laser with a fundamental repetition rate of 3.2 GHz are experimentally investigated and further studied via simulations. A composite filtering effect comprising an intracavity low-finesse Fabry-Perot (FP) filter, an artificial optical low-pass filter, and a gain filter implements the state switching to pulse bunch. A numerical model is proposed to clarify the mechanism underlying the switching. It reveals that, for pulse interval ∆T > τA (relaxation time of the SESAM) in a pulse bunch, the laser operates in pulse-bound build up. In an inverse mechanism the state returns to single soliton, in which the ∆T is obtained from the free spectral range Ωc of the intracavity FP filter by mechanically controlling the distance between the SESAM and gain fiber. This pulse bunch regime of operation ought to be amenable to a quasi-steady-state treatment. It represents an alternative emergence trait in the temporal domain between a main soliton with strong sidelobes in both sides and a bound soliton pair with weak sub-sidelobes. Another profile of the pulse bunch state is that the side peak amplitude in the autocorrelation trace is more than 50%, which is distinct and larger than that in the conventional bound state regime in fiber lasers. The optical spectra, radio frequency spectra, and frequency chirp are further analyzed. These numerical results agree well with the experimental ones within the variation range of the crucial values of Ωc and enable the explicit understanding of such behavior in SESAM mode-locked high-repetition-rate fiber lasers.

Design and analysis of miniature high-resolution omnidirectional gaze optical imaging system

A miniature high-resolution omnidirectional gaze optical imaging system comprising one optical low-pass filter and 5 lens elements with just one aspheric surface was successfully designed. The total tracking length of the system is 14.7 mm and fit on a 5MPx 2/3in CCD with 2448 × 2050 pixel of 3.45 μm for the full 360° × 200° imaging. We introduced the difficulties in the design of ultrawide field of view system and proposed specific computations to overcome them by aberration theories, furthermore making a simulated analysis and performance evaluation using ZEMAX optical design software. The result shows that an admirable corrected field curvature along with a low F-theta distortion, and a perfect lateral and longitudinal color aberration correction were achieved; relative illumination of all field is over 68%; when the ambient temperature changes from −40 to 70 ℃, the modulation transfer function performs in a steady range at 75 lp/mm; all of these will promote the wide-field lens application in medical, automotive, and stereo vision.

多光谱干涉成像仪频谱传输特性研究

分析了多光谱干涉仪成像采样过程的频谱传输特性，针对频谱混叠现象采用以下措施进行消除：首先从光学器件参数上保证CCD上相邻两像元对应的最大光程差间隔不大于最大采样间隔，使其满足采样条件；其次通过引入抗光谱混叠滤波器——光学低通滤波器，利用石英晶体的双折射特性，将同一目标图像信息分成错开的o光像和e光像，分开的距离由石英晶体片厚度决定，以满足消除伪低频信号的条件。两种措施结合有效地消除了频谱混叠现象。 This paper analyzed the sampling frequency spectrum transmission characteristics of the inter-ference multi-spectral imaging interferometer imaging process, in view of the spectrum aliasing phenomenon using the following measures to improve: First choosing appropriate optical device parameters to guarantee the maximum optical path difference interval corresponding to any two adjacent pixels of the CCD is not greater than the maximum sampling interval, which can meet the sampling conditions; Secondly, by introducing anti-spectrum aliasing filter—optical low pass filter, which uses the quartz crystal’s birefringence properties to split the same target image information into stagger o image and e image; separate distance is determined by the thickness of quartz crystal slice. It can eliminate false low frequency signal. Combining the above two measures, it can eliminate the spectrum aliasing effectively.

Asymmetric transmission and optical low-pass filtering in a stack of random media with graded transport mean free path

Light transport and the physical phenomena related to light propagation in random media are very intriguing, they also provide scope for new paradigms of device functionality, most of which remain unexplored. Here we demonstrate, experimentally and by simulation, a novel kind of asymmetric light transmission (diffusion) in a stack of random media (SRM) with graded transport mean free path. The structure is studied in terms of transmission, of photons propagated through and photons generated within the SRM. It is observed that the SRM exhibits asymmetric transmission property with a transmission contrast of 0.25. In addition, it is shown that the SRM works as a perfect optical low-pass filter with a well-defined cutoff wavelength at 580nm. Further, the photons generated within the SRM found to exhibit functionality similar to an optical diode with a transmission contrast of 0.62. The basis of this functionality is explained in terms of wavelength dependent photon randomization and the graded transport mean free path of SRM.

Thulium-doped fiber laser utilizing a photonic crystal fiber-based optical low-pass filter with application in 1.7 μm and 1.8 μm band.

This paper describes a low pass filter based on photonics crystal fiber (PCF) partial ASE suppression, and its application within a 1.7 µm to 1.8 µm band thulium-doped fiber amplifier (TDFA) and a thulium-doped fiber laser (TDFL). The enlargement of air holes around the doped core region of the PCF resulted in a low-pass filter device that was able to attenuate wavelengths above the conventional long cut-off wavelength. These ensuing long cut-off wavelengths were 1.85 μm and 1.75 μm, and enabled a transmission mechanism that possessed a number of desirable characteristics. The proposed optical low-pass filter was applied within a TDFA and TDFL system. Peak spectrum was observed at around 1.9 μm for conventional TDF lasers, while the proposed TDF laser with PCF setup had fiber laser peak wavelengths measured at downshifted values of 1.74 μm and 1.81 μm.

Focus on lasers and imaging

Focus on lasers and imaging

Depth of field of diffraction-limited imaging system incorporating electronic devices

The depth of field is investigated for an imaging system in which optical imaging and electronic devices, such as an electronic sensor and a display, are combined. When the spatial frequency of pixels in the electronic devices is higher than the cut-off frequency of the optical system, it is shown that the depth of field is almost the same as that of the optical system itself. In the case where the spatial frequency is lower than the cut-off frequency of the optical system, the depth of field increases, and the features of the increase are shown in imaging systems both with and without an optical low-pass filter.

주파수 도메인의 변곡점을 이용한 디지털 카메라의 moire 제거 방법

Digital still camera generally uses optical low-pass filter(OLPF) to enhance its image quality because it removes high spatial frequencies causing aliasing. However, the use of OLPF causes some loss of detail. On the other hand, when image are captured by using no OLPF, the moir is generally existed in high spatial frequency region of an image. Therefore, in this paper, moir reduction method in case of using no OLPF is suggested. To detect the moir, spatial frequency response(SFR) of camera was firstly analyzed by using ISO 12233 resolution chart. Then, moir region is detected by using the patterns that are related to the SFR of camera. next, this region is analysed in the frequency domain. Then, the moir is reduced by removing its frequency component, which represents inflection point between high frequency and DC components. Through the experimental results, it is shown that the proposed method can achieve moir reduction with preserving the detail.

Echelette optical low-pass filter as a Bangerter filter for diagnosis and treatment of amblyopia and diplopia

This paper presents a new echelette optical low-pass filter that serves as a Bangerter filter (BF) for diagnosis and treatment of amblyopia and diplopia. Theoretical analysis and experimental measurements showed that echelette BFs have several advantages over the modern BFs used in medicine. Four echelette BFs with different grating periods and one BF were manufactured, and image blurring in an optical visual system with an echelette BF was experimentally investigated. The experimentally measured modulation transfer function and line spread function demonstrated definite image blurring (line expansion) that is inversely proportional to the width of the grating period. The results of measurements of the angular resolution of the optical system with an echelette BF coincide well with the results obtained by theory.

Adjustable active optical low-pass filter

This paper presents an active optical low-pass filter (AOLPF) capable of changing the resolution of an imaging system on demand in order to remove aliasing noise from a sampling image. This is advantageous over conventional optical low-pass filters, which are fixed image-blurring optical components that are built into the imaging system, in order to remove aliasing in the image. Furthermore, conventional filters smear images regardless of the presence or lack of high spatial frequency, which can exceed the Nyquist limit of the sensor. On the contrary, the proposed AOLPF can dynamically adjust the modulation transfer function of an imaging system to eliminate aliasing artifacts. In addition, this filter can be turned off in the absence of high spatial frequency to maximize resolution and prevent unnecessary blurring of the sampling image.

Theoretical Assessment of an All-Optical Temporal Low-Pass Filter for Dynamic Fiber Bragg Grating Signals

Fiber Bragg grating (FBG) sensors are used in numerous applications. Currently, the assessment of dynamic loads with FBGs is of increasing interest. Those applications include vibration analysis, acceleration, or dynamic strain estimation. The acquisition of dynamic signals usually incorporates the application of signal low-pass filtering before signal quantization. FBG interrogators based on spectrometers or scanning lasers inherently have a limited antialiasing capability for dynamic FBG signals. We present a theoretical analysis of an all optical low-pass filter for spectrometer-based interrogators. The low-pass filter is based on the application of a luminescent material onto the detector. Light absorption and continuous light reemission lead to an advanced attenuation of high FBG signal frequencies. An analytic derivation of the system transfer function and numerical signal simulations are presented.

Optical temporal frequency low-pass filtering and heterodyning with a microchannel plate

A microchannel plate is demonstrated as both an optical low-pass filter and a heterodyning modulator. The frequency response function of the microchannel plate indicates it behaves as a low-pass filter. Antialiasing with the microchannel plate is demonstrated using a two-tone amplitude modulated laser beam, with the higher tone above the cutoff frequency being successfully suppressed while the frequency within the bandwidth of the filter is passed through. Selection of different commercially available phosphors controls the filtering and frequency response characteristics of the optical filter, effectively moving the cutoff frequency. By gating the microchannel plate at different frequencies, the microchannel plate can also be used analogously to a heterodyning spectrum analyzer, supplying all of the required functions of a local oscillator, mixer, and filter.

Study of modular transfer function-based optieal low-pass filter evaluation model and experiment

To evaluate the performance of optical low pass filter (OLPF) accurately, a new 2D digital imaging system model including an OLPF was created for the first time using the modular transfer function (MTF). An evaluation function was also established based on this model. To manifest the model and its evaluation, three typical OLPFs were studied. The results show that the Δ value of each filter undergoes a slow and slight decrease and then rapidly increases when the ideal optical system cutoff frequency is increased. The optieal system embedded with the 2-chip-4-point type OLPF created a better image than those with the other two OLPF's when the F number is greater than 44; whereas the OLPF of 3-chip-8-point type worked best when the F number is lower than 44 When the F number of the optical system is 28, namely, with a spatial cutoff frequency of 649 lp/mm, the filter of 3-chip-8-point type performs better than that of the 2-chip-4-point and 3-chip-4-point types. The Δ value of the first filter is 283% and 375%, lower than that of the other two filters, respeatively. That is, the filter performance is 283% and 375% higher. Finally, spatial frequency test shows that the moiré fringe and false colors can be seen in the last two pictures, while in the first, these effects are well eliminated. The theoretical calculations agree with the experimental results.