Optical Spectral Analyzer Research Articles

Solar spectrum measured by waveguide spectral lens

We demonstrate solar spectrum measurement in the visible-near-infrared region by integrated waveguide spectral lens (WSL). Light through the chip is dispersed and focused on a camera placed at the designed distance without any free-space lens. Commercial polymer materials with low propagation loss near visible region are chosen for wafer-scale fabrication. The solar radiation is coupled to a fiber via a home-made setup and the captured light is transmitted over ∼ 50 m to the lab, where the spectrum is examined by two WSL chips for broadband and fine analysis, respectively. The observed O2 (687.7 nm, 761.9 nm) and H2O (718.4 nm) absorption lines are in good agreement with the results from a commercial optical spectral analyzer and the literature. This experiment proves the feasibility of using planar WSL chips as low-cost and versatile components for the development of compact and portable spectrometer equipment.

High-precision fiber optic liquid level sensor based on fast Fourier amplitude demodulation in a specific range of spectrum

This study presents a fiber optic liquid level sensor (FOLLS) by acquiring information from the amplitude of spectral fast Fourier transform (FFT) in a specified narrow wavelength range. The sensing light path is simply formed by embedding a section of Panda-type polarization-maintaining fiber (PMF) into the sensing arm of a conventional Michelson interferometer (MI) structure. The output composite spectrum consists of two parts: the fine interference fringes due to MI optical path difference (OPD) and the envelope fringes stemming from the PMF’s birefringence. By applying liquid level variations on the sensing structure, shifts in envelope wavelength indicate a sensitivity of −0.193 nm/cm, and a liquid level resolution of 1 mm can be easily acquired. Furthermore, by demodulating the amplitude after spectral FFT in a specific narrow wavelength range, and intensity sensitivity of 1.906 dB/cm is achieved, leading to a greatly improved overall resolution of 0.03 mm in liquid level measurement. Besides, the sensing element is sealed and packaged by two sheets of silicone rubber, which can be thrown into liquid when in use, thus easing the sensor installation. Moreover, due to the proposed demodulation method, the sensor is insensitive to the light source power jitter, reference arm phase shift and optical spectral analyzer wavelength shift. Therefore, the merits of the proposed sensor include high precision, high stability, ease of installation, which are beneficial in pore water pressure and liquid level monitoring.

Optical Fiber Tip wth Integrated Mach-Zehnder Interferometer for Sensor Applications

In this paper, we present design and preparation of novel Mach-Zehnder Interferometer (MZI) based structure for sensing applications. We present integration of this structure at the end of a single-mode optical fiber. Direct laser writing technology was used to prepare MZI-based structure with a hoop for tight connection to optical fiber facet. Details of the design, preparation process and connectorizing process are described and finally, transmission spectral characteristic of the prepared structure was measured using optical spectral analyzer.

High Load-Induced Birefringence of PMMA Investigated in VIS/NIR Spectral Range

In the paper, we present the results of an investigation of polymethylmetacrylate’s birefringence induced due to applying high-load. The stress-induced birefringence of the polymethylmetacrylate samples was experimentally investigated by an apparatus combining an in-house made plane polariscope with a machine for fatigue testing of materials. A halogen lamp was used as the source of white light. The polariscope’s basis was formed by two sets of plane polarizers, one designed for visible (470-750) nm and another one for near-infrared (900-1800) nm spectral range, respectively. The investigated sample was placed between the polarizers and light that passed through the polariscope was collected by an optical fiber connected to an appropriate optical spectral analyzer. The spectra of transmitted light were measured as a function of load and their analysis provided information on the load-induced birefringence of the investigated polymethylmetacrylate samples.

Design and Testing of a 2D Optical Fiber Sensor for Building Tilt Monitoring Based on Fiber Bragg Gratings

In this paper, a new type of optical fiber tilt sensor based on fiber Bragg grating (FBG) is presented for 2D dual-axis tilt angle sensing. The tilt sensor is composed of two cylindrical floats suspended in water, connected with FBG. When the external environment causes the tilting of the sensor, the surface of the liquid within the container will form a new balanced liquid surface plane due to the gravity and change the height of the liquid at different locations in the container. So the buoyancy force of the cylindrical floats of the sensor will vary with the depth of liquid, thus the changed FBG strain will cause the FBG reflection central wavelength shift. According to the measured central wavelength shift by the optical spectral analyzer (OSA), we can obtain the two-axis tilt angle of the sensor. The proposed sensor can detect a tilt angle range of −5 degrees to +5 degrees and achieve a sensitivity of 0.1° with optical spectrum analyzer resolution of 0.01 nm. Due to its good sensing linearity, the proposed sensor can be applied in building tilt measurement.

Experimental Assessment of Filter Narrowing Effect for Low Bandwidth Connections in EON

In this letter, we experimentally analyze the filter narrowing effect for low bandwidth connections in the context of the flexi-grid paradigm for optical aggregation networks. First of all, we characterize the concatenation of up to 12 filters by means of a high resolution optical spectral analyzer. Then, we set up the programmable filters with the obtained characterization, in order to emulate the filter concatenation impairments. The studied filters have nominal bandwidths of 12.5 GHz, 25 GHz, and 50 GHz. The two potential candidates analyzed are intensity modulation/direct detection on-off keying and direct detection orthogonal frequency division multiplexing (OFDM) with uniform and bit loading. Results show a significant signal degradation for filter bandwidths of 12.5 GHz with respect to 25 GHz and 50 GHz, considering a net bit rate of 10 Gb/s. We also conclude that for future entire flexi-paradigm, OFDM is an appropriate candidate.

Wide Range and Accurate Measurement of Wafer Thickness Gauge Using Optical Spectral Analyzer

Applicability of a generic optical spectrum analyzer that is familiar with laser experiments is investigated for on-site wafer thickness measurements in the thinning process, to resolve the issues regarding mismatch in the thickness range of previous thickness measurement system. The optimizations in terms of the spectral range, the spectral resolution and the dynamic range are successfully conducted by use of the optical spectrum analyzer. Owing to both high spectral resolution and wide dynamic range in near infrared spectral measurements, full range thickness measurements for the initial thickness 775μm and the terminal thickness 1μm and nanometer order accuracy are implemented.

Wide Range and Accurate Measurement of Wafer Thickness Gauge Using Optical Spectral Analyzer

Applicability of a generic optical spectrum analyzer that is familiar with laser experiments is investigated for on-site wafer thickness measurements in the thinning process, to resolve the issues regarding mismatch in the thickness range of previous thickness measurement system. The optimizations in terms of the spectral range, the spectral resolution and the dynamic range are successfully conducted by use of the optical spectrum analyzer. Owing to both high spectral resolution and wide dynamic range in near infrared spectral measurements, full range thickness measurements for the initial thickness 775μm and the terminal thickness 1μm and nanometer order accuracy are implemented.

Fiber ring laser sensor based on hollow-core photonic crystal fiber

We report an erbium-doped fiber ring laser intra-cavity sensor. Hollow-core photonic crystal fiber (HC-PCF) used as the gas absorption chamber is introduced into the laser cavity. When the HC-PCF is filled with gas, its absorption attenuation changes the cavity loss and the laser output. We use a modular NI PXI platform equipped with a programmable voltage and current source and a LabVIEW program to generate driving voltage for the tunable optical filter (TOF) to ensure high precision. The relationship between the concentrations of acetylene, coupling ratios, pump power and output power is theoretically and experimentally investigated. Different output spectra are measured by the optical spectral analyzer (OSA). A minimum detectable acetylene concentration (MDAC) of 5.4ppm has been experimentally achieved.

Single‐mode–multimode–single‐mode fiber structures for simultaneous measurement of strain and temperature

AbstractThis article presents two related techniques for the simultaneous measurement of strain and temperature: (1) using two single‐mode–multimode–single‐mode (SMS) fiber structures accompanied with an optical spectral analyzer (OSA); (2) using three SMS fiber structures and a ratiometric power measurement system.Experimental results show that (1) the technique accompanied with an OSA has a resolution of 1.64 με for strain in the range from 0 to 1000 με and 0.11°C for temperature and (2) the technique based on ratiometric system offers a resolution of better than 2 με for strain measurements in the range from 0 to 1000 με and a resolution better than 0.84°C for temperature. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26205

Rapid and accurate chromatic dispersion measurement of fiber using asymmetric Sagnac interferometer

We propose and experimentally demonstrate an advanced method for chromatic dispersion measurement of fiber. This technique is based on spectral interferometry by using an asymmetric Sagnac loop and broadband optical source. The chromatic dispersion can be directly obtained from the spectral interferogram seen from an optical spectral analyzer. This method is rapid (<1 s), accurate, simple, low cost, and can provide a large dispersion measurement range.

Experimental demonstration of a simple displacement sensor based on a bent single-mode–multimode–single-mode fiber structure

A simple displacement sensor based on a bent single-mode–multimode–single-mode (SMS) fiber structure is proposed and experimentally investigated. The sensor offers a wider displacement range, not limited by the risk of fiber breakage, as well as a three-fold increase in displacement sensitivity by comparison with a straight SMS structure sensor. This sensor can be interrogated by either an optical spectral analyzer (OSA) or a ratiometric interrogation system: (1) if interrogated by an OSA assuming a resolution of 1 pm, it has a sensitivity of 28.2 nm for a displacement measurement range from 0 to 280 µm; (2) if interrogated by a ratiometric interrogation system, it has worst and best case resolutions of 556 and 38 nm, respectively, for a displacement measurement range from 0 to 520 µm.

Fiber loop ring-down optical fiber grating gas pressure sensor

A kind of bare fiber Bragg grating (FBG) gas pressure sensor based on fiber loop ring-down is presented, in which there are no sensitizing means or assistant structure on the bare FBG. The pressure sensitivity is analyzed theoretically via discussing the relationship between the ring-down time and the gas pressure. By detecting the ring-down time at different gas pressure levels, the gas pressure characteristics of the bare FBG are investigated in the time domain. The experimental results demonstrate that there exists a linear dependence of the pressure applied to the bare FBG on the ring-down time. A high sensitivity of −0.384 μs/MPa (being equivalent to −240 pm/MPa) could be achieved, which is 79 times larger than that of the optical spectral analyzer interrogated. This kind of pressure sensor could be used for gas or liquid pressure measurement.

Fiber Loop Ringdown — a Time-Domain Sensing Technique for Multi-Function Fiber Optic Sensor Platforms: Current Status and Design Perspectives

Fiber loop ringdown (FLRD) utilizes an inexpensive telecommunications light source, a photodiode, and a section of single-mode fiber to form a uniform fiber optic sensor platform for sensing various quantities, such as pressure, temperature, strain, refractive index, chemical species, biological cells, and small volume of fluids. In FLRD, optical losses of a light pulse in a fiber loop induced by changes in a quantity are measured by the light decay time constants. FLRD measures time to detect a quantity; thus, FLRD is referred to as a time-domain sensing technique. FLRD sensors have near real-time response, multi-pass enhanced high-sensitivity, and relatively low cost (i.e., without using an optical spectral analyzer). During the last eight years since the introduction of the original form of fiber ringdown spectroscopy, there has been increasing interest in the FLRD technique in fiber optic sensor developments, and new application potential is being explored. This paper first discusses the challenging issues in development of multi-function, fiber optic sensors or sensor networks using current fiber optic sensor sensing schemes, and then gives a review on current fiber optic sensor development using FLRD technique. Finally, design perspectives on new generation, multi-function, fiber optic sensor platforms using FLRD technique are particularly presented.

Comparing thermal and electrical methods for controlling the wavelength of semiconductor-laser radiation

This paper presents the results of a study of the spectral characteristics of commercial semiconductor lasers, using an optical spectral analyzer and a supply-and-control unit for the laser. Families of spectral characteristics are obtained in a wide range of injection currents and temperatures. The temperature and current coefficients for the wavelength of the laser radiation are determined.

Multiplexing multimode fiber and Fizeau etalon: a simultaneous measurement scheme of temperature and strain

We propose a sensing scheme to measure temperature and strain simultaneously by multiplexing a section of multimode fiber (MMF) and a Fizeau etalon to construct a novel fiber optic sensor unit. The multimode interference of MMF can monitor the environmental temperature separately without being affected by strain applied on the sensor unit. The Fizeau etalon can measure the strain applied on the sensor unit. The sensor unit is interrogated by a broadband source ASE and its demodulation is performed by an optical spectral analyzer (OSA) combined with fast Fourier transform (FFT) techniques in the spectral domain. A sensor unit is fabricated and tested to prove the feasibility of the sensing scheme.

Passive and temporally continuous optical spectral analyzer of RF signals via wavelength coding

A novel approach realizing an optical spectrum analyzer for photonic detection of an unknown RF carrier signals is presented. The described module may be part of an electronic warfare system in which detection of a narrow band RF signal is required. Moreover, The RF signal is characterized by an unknown time varying carrier frequency embedded in wide band noise. The system uses a passive, fiber based photonic configuration. It allows the spectrum mapping of an incoming electronic RF signal modulated on an optical carrier. The spectral analyzer configuration uses a finite impulse response (FIR) filter that is realized by two different optical paths of parallel fibers which generate a spectral notch filter. Hence, a wavelength coding is realized by chromatic dispersion such that each wavelength is filtered by a different FIR filter. Therefore, the energy at a WDM demux output channels is actually proportional to the spectrum of the input RF signal. This spectral mapping is obtained without lose of temporal RF information.

Single-mode vertical integration of p-i-n photodetectors with optical waveguides for monitoring in WDM transmission systems

A new InP-based waveguide photodetector utilizing single-mode vertical integration of a p-i-n structure atop of a low-loss optical waveguide is reported. Over all states of polarization, in a spectral range across the C-band, the device exhibits high on-chip responsivities of /spl sim/1.1 A/W, which remains nearly constant at a reverse bias above 5 V and optical powers up to 1 mW. If integrated within the output channels of a planar demultiplexer, it enables a very compact optical spectral analyzer to be realized.

Fluorescence guided excimer laser ablation of intervertebral discs in vitro

Laser induced fluorescence of intervertebral discs was investigated in this study for the first time in order to develop a fluorescence guided percutaneous excimer laser discectomy. For this purpose 35 human cadaveric intervetebral discs from level L1 to L5 were irradiated with a 308 nm XeCl excimer laser with a 60 nsec pulse width at 10, 20 and 30 mj/mm2. Laser light was transmitted over a 1000 microns core optical fiber. Fluorescence emitted from the irradiated tissue was transmitted back over the same fiber over a semireflective mirror to another optical fiber, coupled into a spectrograph and O-SMA optical simultaneous multichannel spectral analyzer system. The spectral lineshape of both the annulus fibrosus and the nucleus pulposus was characterized by a broadband emission between 320 nm and 616 nm with 3 peak emissions at 358, 423 and 457 nm. Relative intensities were calculated by forming two ratios of the peak intensities of the three peaks (R1 = I 423/I 358; R2 = I 423/I 457). Statistical analysis of both ratios revealed a highly significant discrimination between annulus fibrosus and nucleus pulposus (P less than 0.001). Under penetrating conditions multiple recorded spectra showed the spectral profile of subsequently ablated nucleus and annulus. The results of this study demonstrate that discrimination between annulus fibrosus and nucleus pulposus by excimer laser-induced fluorescence is feasible under laboratory conditions. Fluorescence spectroscopy could possibly be developed into a valuable guiding system for percutaneous laser discectomy.

Remote biomedical spectroscopic imaging of human artery wall.

We discuss a general technique, laser spectroscopic imaging (LSI), remote acquisition of spectroscopic images of biological tissues and tissue conditions. The technique employs laser-induced spectroscopic signals, collected and transmitted via an array of optical fibers, to produce discrete pixels of information from which a map or image of a desired tissue characteristic is constructed. We describe a prototype LSI catheter that produces spectral images of the interior of human arteries for diagnosis of atherosclerosis. The diagnostic is based on the fact that normal artery wall and atherosclerotic plaque exhibit distinct fluorescence spectra in the 500-650 nm range when excited by 476-nm laser light; the fluorescence from blood is minimal. The catheter is composed of 19 optical fibers enclosed in a transparent, protective shield. Argon ion laser radiation is used for excitation, and an optical multichannel spectral analyzer is used for detection. Sequential sampling is used to minimize crosstalk among fibers and reduce blurring of the image. Computer-processed 19-pixel spectroscopic images are produced of fresh cadaver artery in vitro. Regions of normal tissue, plaque, and blood are identified, and the diagnoses are confirmed histologically and by direct spatial correlation. The results demonstrate the concept of using this laser catheter system for real-time imaging.