The infrared digital Fourier transform spectrometer has several advantages, including small size, light weight, high stability, increased throughput, and enhanced spectral resolution, making it a valuable tool in the biomedical field. The data acquired by this instrument directly is interference data, and the required spectral data is obtained through a spectral recovery algorithm. The reconstruct spectral is determined by the acquired data quality and spectral recovery algorithm. However, the type of silicon photonics-based Fourier transform infrared spectrometers often encounter non-uniform optical path differences in collected interference data, due to the limitations in hardware design and manufacturing processes, leading to the spectral obtained by commonly used spectrum reconstruction methods inaccuracies. In this paper, a spectral recovery algorithm based on deep neural networks is proposed for the reconstruction of spectra from non-uniformly sampled interference data, Compared to other spectral recovery methods, the proposed method achieves better spectral angle (SA) and relative quality error (RQE) between the reconstructed spectra and ideal spectra.
In order to obtain a high-resolution image and realize the all-day imaging simultaneously, a visible and long-wave infrared dual band imaging detector is proposed and designed, by a meta surface structure through which the infrared light has been absorbed and the visible light passed. With a monolithic chip, the visible light of 400 nm to 900 nm and the long-wave infrared light of 8 μm to 14 μm have been detected, which have the apparent size and weight advantages. This validates the improved integration of structure and results in considerable reduction of the complexity of the imaging system, which is advantageous to target recognition in complex environment.
Polarization detection in the Medium Wave Infrared (MWIR) region presents broad applications in environmental monitoring, aerial reconnaissance and military target spying. Here, we have fabricated aluminum gratings with 1000 nm and 400 nm pitch by Electron Beam Lithography (EBL) on Si substrate with double sides antireflection coating. The polarization property is characterized by the transmittance and extinction ratio in medium wave infrared measured by the FTIR-based test system. Both Transverse Electric (TE) wave transmittances of Al wire-grid polarizers remain at low value indicating that the TE wave is effectively blocked by the designed grating. The Transverse Magnetic (TM) wave transmittance of Al grating with 400 nm pitch in medium wave infrared augments significantly leading to the increase of extinction ratio and the better polarization performance compared with that of the Al grating with 1000 nm pitch. The average TM transmittance of Al polarization grating with 400 nm pitch exceeds 80% in medium wave infrared which can meet the requirements of polarization detection integrated into medium wave infrared detectors. As the polarization angle of incident light rotates, the transmittances of both Al gratings are of remarkably periodic, which follows Malus’ law. The extinction ratios of Al grating with 400 nm pitch and 1000 nm pitch realize 219:1 and 90:1 at 4.5 μm, respectively. Al wire-grid polarizers manufactured in this paper can be a promising candidate for optical system applications, especially in the field of medium wave infrared detectors.
The tunability of optical transmittance spectra can be available by mounting one of the mirrors of the Fabry-Perrot cavity on a movable structure. The F-P filter prepared by adopting MEMS process can realize the advantages of miniaturization, array, and high output. The size of the MEMS F-P filter can be reduced to a few hundred micrometers. This feature introduces a new problem for the characterization of optical performance, that is, the incident light needs to be focused onto the mirror with a size of a few hundred micrometers. However, in the actual test, the incident light with a hundred-micron spot is usually a convergent beam with a certain cone angle. It is found that through theoretical analysis, compared to parallel incident light, the convergent light beam passed through the F-P cavity leads to the decrease at peak transmittance and the broadening of full width at half maximum. The reason for that was the converging light with a cone angle passing through the F-P cavity had different incident angles and caused diverse optical path difference. As a result, the light emitting from the cavity with various wavelength would appear in the transmission spectra. In summary, the test results under the converging light could not truly reflect the performance of the F-P cavity and the influence of the cone angle of incident light beam on the performance characterization of MEMS F-P filter was analyzed by theoretical arithmetic and simulation.
The graphene-based silicon modulator can benefit from the outstanding optical and electrical properties of graphene which makes this kind of modulator a potential candidate for applications such as optical communication and optical interconnect which have strict performance requirements, like broadband operation, low optical loss, low-cost and high speed. In this work, a graphene-based high speed electro-absorption modulator is constructed. Firstly, a typical graphene-oxide-silicon (GOS) structure modulator with one-layer graphene is designed and simulated. The mode field distribution and single mode loss is simulated. Secondly, in order to improve the performance of the graphene-based modulator, the structure of the modulator is optimized. Instead of one-layer graphene, two layers are used. By adjusting the position of graphene in the structure of modulator, a graphene-oxide-graphene (GOG) structure modulator is designed. The structures of graphene layers on top of the silicon-based waveguide and graphene layers sandwiched between two silicon-based waveguides are designed, and the latter one is chosen because it has the advantage of stronger interaction between graphene and light field. By simulation, the extinction ratio of this proposed modulator is as high as 15 dB, and the modulation depth is about 93.3%. The operation bandwidth is calculated to be 52 GHz with low insertion loss of 1.55 dB.
A new on-chip Fourier transform spectrometer has been developed for spectrum analysis application. This spectrometer, based on a thermally tuned Mach-Zehnder Interferometer(MZI) with silicon photonics technology, is small size, light weight and low power consumption. Experimental data have acquired in the O-band domain and the processing chain to convert raw data to spectral data is described. These experimental results show that the spectral resolution is close to the one expected, but also that the signal to noise ratio is limited by various factors. We discuss the origin of those limitations and suggest solutions to circumvent them.
The optical on-chip integration technology can be used to realize spectrum analysis. Compared with the traditional bulky high performance optical spectrum analyzer which is always used in laboratory, on-chip spectrometer has the advantages of chip-scaled, low-cost and suitable for detection in complicated environment. For example, it can be utilized for detection of toxic gases such as carbon monoxide and hydrogen fluoride. In the past decade, several kinds of on-chip spectrometer, dispersive spectrometer and Fourier transform spectrometers, have been demonstrated as promising candidates for wide range of spectral application. In this paper, a digital Fourier transform spectrometer based on an interferometer whose arms consist of several optical switches which are connected by Mach-Zechnder Interferomter (MZI) with different arm lengths is demonstrated. The optical switch based on thermo-optic effect is used to select the different arms of an MZI, so the combination of optical switches and MZIs can lead to a series of different optical path difference. The proposed on-chip spectrometer is designed for Original-band (O-band) and the prospective spectral resolution is 0.3 nm. The future effort will focus on the test of the proposed spectrometer and the followed data processing including dispersive compensation and spectral reconstruction.
In the silicon photonics field, coupling occupies an important position of propagating the light from the space to the waveguide. There are two normal coupling way. The one is end-face coupling and the other one is surface coupling. And the more popular way is to use the surface coupling, which can be put on anywhere of the chip and is much easier to measure. The specific surface coupling format is grating coupler. Grating coupler can be both input and output coupler and match the fiber to propagate the light from and to the space. However, the one-dimensional grating coupler, used in the most of silicon photonic chips, has polarization selectivity and can only transfer one single mode (TE mode) in the waveguide. That means the half of the light would be wasted during coupling. In order to improve the efficiency of the coupler, two-dimensional grating coupler is a better solution. It has two orthogonal waveguides and propagate the transverse-electric (TE) mode with opposite directions. And the transverse-magnetic (TM) mode is transferred to the TE mode when the light changes the propagating direction. In this paper, the two-dimensional grating coupler is designed to match the light whose wavelength is from 1260 nm to 1290 nm. The calculation and simulation method is finitedifference time domain (FDTD). After modeling and optimizing the structure, the coupling efficiency is 26.8%.
Transmission characteristics of Fabry-Perot (F-P) filter based on silicon substrate with different transmissivity of high reflection (HR) coatings, incident angle, and interference orders are investigated. The results show that the transmissivity of HR coatings has great effect on full width at half maximum (FWHM) of transmission spectrum, the FWHM of F-P filter reduced from 209 nm to 3.4 nm with the reflectivity of HR coatings increased from 84.7% to 99.6%. The peak wavelength shifts from 1546.3 nm at 0° to 1542.6 nm at 5°, indicating that the FWHM of transmission spectrum broadens as the incident angle increases. The 1st, 2nd, 3rd, and 4th order interference are 3.4 nm, 2.3 nm, 1.8 nm, and 1.5 nm, respectively. Thus, in the applications tuning in a narrow wavelength range, F-P filter can be designed to operate in high-order mode to achieve a narrow transmission spectrum.
Optical spectrum analysis has been widely used in numerous areas such as optical network performance monitoring, materials analysis and medical research. Although there are many kinds of spectrometers, on-chip spectrometer could be a promising alternative with apparent size and weight advantages,. Silicon-on-Insulator(SOI) waveguide technology offers means to miniaturize the different parts of the spectrometer, even if often at the cost of performance and scalability. In this work, a cascaded waveguide structure was proposed for a spectrometer, with a spectral range from 1150nm to 1550nm, which corresponds to the second overtone region of the NIR absorption, and a resolution of 2 nm for performing spectrum derivation. The spectrometer is realized by a SOI cascaded Mach-Zehnder Interferometer and four SOI arrayed waveguide gratings. The cascaded MZI based coarse wavelength division de-multiplexers was employed for the first stage of the spectrometer and was used to disperse the signal into four channels. The output signals of the four channels are further dispersed into eight channels by the second stage AWG structures. We further implemented the thermo-optic modulation to achieve a higher spectral resolution. The output channel wavelengths of the spectrometer are modulated (with a wavelength shift 2 nm) by the embedded heater to obtain the first order derivative spectra of the input optical signal. We present the theory, modeling, and experimental demonstration of the thermally tuned spectrometer. With respect to the computer simulation and device characterization results, the 400nm spectral range and the 2nm resolution have been demonstrated.
A mesa-type normal incidence separate-absorption-charge-multiplication (SACM) Ge0.95Sn0.05/Si avalanche photodiode (APD) was fabricated. The 60-μm-diameter avalanche photodiode achieved a responsivity of ~5A/W (gain=24) and ~3.1A/W (gain=20) at 98% breakdown voltage (-14.2V) under 1310nm and 1550nm illumination respectively with a low dark current of 10μA. The −3 dB bandwidth for a 60-μm-diameter APD is about 1-1.25GHz for gains from 5 to 20, resulting in a gain-bandwidth product of 25GHz for a C-band communication wavelength of 1550nm.
The photonic crystal structure can be utilized for improving the transmission within a broadband, and suppressing the dark current of detector efficiently as well. Considering such an advantage, the study on the multi-level profile photontrapping structure is performed; meanwhile, the enhancement of HgCdTe mid-wavelength infrared detector based on such a structure is analyzed. With the help of FDTD model and FEM model, via optimizing the structure, a multi-level profile photon-trapping detector scheme with a quantum efficiency enhancement of 20% is established. The proposed simulation results and structure are crucial for further acquiring HgCdTe detector with enhanced SNR.
The reflected or radiated electromagnetic wave from natural objects exhibits different polarization characteristics. By detecting the polarization properties of light waves, more information from the target can be obtained, as well as the target recognition capability can be strengthened. In this paper, the research progress of the image processing method for infrared polarization detection is introduced. The traditional extraction methods of polarization components and the theory of polarization information processing are initially demonstrated. The fabricated 128×128 integrated polarization infrared detector, whose extinction ratio is 10:1, is also proposed. By using of the image reconstruction method, polarization images captured from this detector, which can acquire four polarization components simultaneously, are processed. Further, the degree and angle of polarization of natural scene images are obtained as well.
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