Spectral Operation Research Articles

Growth, structural, spectral and high-power continuous-wave laser operation of Yb0.11Gd0.89COB crystal

A Yb0.11Gd0.89Ca4O(BO3)3 crystal with new composition was grown by the Czochralski method. The crystal structure was measured and analyzed. The unit-cell parameters of the Yb0.11Gd0.89COB were calculated to be a=0.8089(7) nm, b=1.5987(6) nm, c=0.3545(8) nm, β=101.22°. The absorption and fluorescence spectra were measured. The maximum absorption cross-section of Yb0.11Gd0.89COB crystal was 0.79×10−20 cm2, which occurred at 976 nm with Y polarization. The emission cross-section at 1027 nm was calculated to be 0.33×10−20 cm2. The radiative lifetime τrad was calculated to be 2.74 ms. The Stark energy-level diagram of Yb3+ in the Yb0.11Gd0.89COB crystal field at room temperature was determined. The ground-state energy level 2F7/2 splitting was calculated to be as large as 1004 cm−1 and the zero-line energy was 10246 cm−1. A maximum output power of 9.35 W was achieved in continuous-wave (CW) mode, with the slope efficiency being 42.1%. Chemical etching experiment revealed that the dominating imperfections in the studied Yb0.11Gd0.89COB crystal were dislocations and sub-grain boundaries. The existence of crystal defects could cause light scattering, and degrade laser output efficiency. The influence of crystal defects on laser properties was discussed.

PAPR Reduction in OFDM using Combination of Statistical Transformation Distribution and Selected Mapping Method

Orthogonal Frequency Division Multiplexing (OFDM) offers a high spectral efficiency, smooth operation and effectiveness against multipath fading as its modulation technique. This modulation technique advancement is to increase the number of subcarriers to enhance efficiency. Despite of the number of subcarriers increases, certain time domain OFDM coefficients are likely to acquire unwanted high magnitudes of PAPR. This high PAPR cause the limit of the transmitter power efficiency. Therefore, cause spectral to spread and reduce the bit-error-rate (BER) performance. In order to allay these potential performance problems, a computationally efficient and low cost PAPR reduction method were proposed. The new method presents a combination of Statistical Redistribution Method (SRM) with Selected Mapping Method (SLM). The proposed scheme takes advantage of Selected Mapping (SLM) followed by the companding SRM technique to amplify the reduction of the PAPR of the OFDM signal. Simulation results indicate that about 5 dB reduction in PAPR is achieved compared with the conventional SLM algorithm.

All-optical background subtraction readout method for bimaterial cantilever array sensing.

Optical readout method plays a critical role in bimaterial cantilever array sensing system. The common optical readout methods are based on spectral plane filtering. In the paper an all-optical background subtraction readout approach inspired by total reflection and optical lever principle is presented for the bimaterial cantilever array sensing. Comparing with the spectral plane filtering methods the proposed approach eliminates digital subtraction operation by using optical total reflection instead of digital subtraction and avoids spectral filtering operation. An all-optical background subtraction directly-view infrared sensing system was developed to evaluate the approach. The infrared target can be directly acquired by the visible light CCD. The experimental results and analysis show its unique advantages.

Switchable beam steering with zenithal bistable liquid-crystal blazed gratings.

Switchable beam steerers based on zenithal bistable liquid crystal (LC) gratings are designed and theoretically investigated. The nematic orientation profiles and the optical transmittance properties of the gratings are rigorously calculated, respectively, via a tensorial formulation of the Landau-de Gennes theory and the full-wave finite-element-method. By proper design of the grating geometry, beam steering with high diffraction efficiency is demonstrated between the two stable LC states. The tolerance of the device performance with respect to material parameters is assessed, evidencing spectral operation windows of more than 50 nm in the visible for a beam steering efficiency higher than 90%.

High temperature, single mode, long infrared (λ = 17.8 μm) InAs-based quantum cascade lasers

We demonstrate quantum cascade lasers in the InAs/AlSb material system which operate up to 333 K (in pulsed regime) at λ = 17.8 μm. They employ metal-metal optical waveguides and the threshold current density is 1.6 kA/cm2 at 78 K. We also report distributed-feedback devices obtained using the same laser material via a 1st-order Bragg grating inscribed in the sole top metallic contact. Spectral single mode operation with more than 20 dB side mode suppression ratio is achieved at a temperature of 300 K. Large wavelength tuning rates, of the order of 1.5 nm/K, are demonstrated. A wavelength coverage of 0.38 μm is achieved in single-mode regime over a temperature range of 255 K.

Double antiresonant hollow core fiber--guidance in the deep ultraviolet by modified tunneling leaky modes.

Guiding light inside the hollow cores of microstructured optical fibers is a major research field within fiber optics. However, most of current fibers reveal limited spectral operation ranges between the mid-visible and the infrared and rely on complicated microstructures. Here we report on a new type of hollow-core fiber, showing for the first time distinct transmission windows between the deep ultraviolet and the near infrared. The fiber, guiding in a single mode, operates by the central core mode being anti-resonant to adjacent modes, leading to a novel modified tunneling leaky mode. The fiber design is straightforward to implement and reveals beneficial features such as preselecting the lowest loss mode (Gaussian-like or donut-shaped mode). Fibers with such a unique combination of attributes allow accessing the extremely important deep-UV range with Gaussian-like mode quality and may pave the way for new discoveries in biophotonics, multispectral spectroscopy, photo-initiated chemistry or ultrashort pulse delivery.

Modelling of surface plasmon resonance sensor for detection of mass concentration of ethanol and methanol in a binary mixture

Modelling of a surface plasmon resonance sensor for detection of ethanol or methanol content in water is presented. The reflectivities of these sensors are derived using transfer matrix method and hence the corresponding sensitivity and detection accuracy are obtained in the presence of high index overlayer, like silicon and chalcogenide. It is observed that the resonant wavelength of surface plasmon resonance sensor is shifted toward higher wavelength with increase of refractive index and thickness of overlayer. Hence, by adjusting their values, the desired spectral operation range of this sensor is obtained. The sensitivity of this sensor with silicon overlayer is larger than the chalcogenide overlayer but at the same time detection accuracy is smaller. This sensor can be used to detect the mass concentration of ethanol, methanol and other similar materials. A method to obtain the percentage mass concentration of ethanol is discussed.

Investigation of Bending Sensitivity in Partially Doped Core Fiber for Sensing Applications

A fiber based high sensitive bend sensor is proposed and demonstrated using a uniquely designed partially doped core fiber (PDCF). The fabrication method of PDCF with two core regions, namely an undoped outer region with a diameter of ~ 9.5 μm encompassing a doped, inner core region with a diameter of 4.00 μm is explained. The mechanism of bending effect in proposed PDCF and the experimental setup for amplified spontaneous emission (ASE) based sensor and fiber laser based sensor is illustrated. For ASE sensor, the higher ASE power level loss as the spooling radius is reduced from 20 to 3 cm is measured. The gain peak shift to shorter wavelength with respect to the decrease of the spooling radius from 20 to 3 cm due to higher bending loss at smaller bending radius is observed. The results are in agreement with overlap factor variation of PDCF. As expected from ASE peaks variation, the fiber laser sensor spectral operation is changed from 1539 to 1530 nm range. This phenomenon is due to higher mode field diameter of longer wavelength and result of optical filtering at longer wavelengths. The experimental results showed the output of the ASE is also highly stable, with no observable variation in the power output over a measurement period of 1 h. The PDCF is also temperature insensitive.

GaiaRadial Velocity Spectrometer Performance

We present the initial performance of the Gaia Radial Velocity Spectrometer, providing an overview of its performance, which is essentially nominal in terms of spectral resolution, throughput and operation, except for the presence of unexpectedly high levels of scattered background. This is mainly Solar in origin, and reduces the limiting magnitude for radial velocity measurements by ∼1 magnitude to V ∼ 16. Radial velocity calibration accuracies are compliant with requirements.

Coupled tandem cavities based electro-absorption modulator with asymmetric tandem quantum well for high modulation performance at low driving voltage

We propose and demonstrate a new electro-absorption modulator (EAM) based on coupled tandem cavities (CTC) having asymmetric tandem quantum well (ATQW) structure with separated electrode configuration to achieve large transmittance change over a broad spectral range at low driving voltage for high definition (HD) 3D imaging applications. Our theoretical calculations show that CTC with ATQW structure can provide large transmittance change over a wide spectral range at low driving voltage. By introducing separated electrode configuration, the fabricated EAM having CTC with ATQW structure shows a large transmittance change over 50%, almost three times larger spectral bandwidth compared to that of EAM having single cavity with a single thickness quantum well without significantly increasing the applied voltage. In addition, the CTC with ATQW structure also shows high speed modulation up to 28 MHz for the device having a large area of 2 mm x 0.5 mm. This high transmittance change, large spectral bandwidth and low voltage operation over a large device area for the EAM having CTC with ATQW demonstrates their huge potential as an optical image modulator for HD 3D imaging applications.

Photon management in two-dimensional disordered media

Elaborating reliable and versatile strategies for efficient light coupling between free space and thin films is of crucial importance for new technologies in energy efficiency. Nanostructured materials have opened unprecedented opportunities for light management, notably in thin-film solar cells. Efficient coherent light trapping has been accomplished through the careful design of plasmonic nanoparticles and gratings, resonant dielectric particles and photonic crystals. Alternative approaches have used randomly textured surfaces as strong light diffusers to benefit from their broadband and wide-angle properties. Here, we propose a new strategy for photon management in thin films that combines both advantages of an efficient trapping due to coherent optical effects and broadband/wide-angle properties due to disorder. Our approach consists of the excitation of electromagnetic modes formed by multiple light scattering and wave interference in two-dimensional random media. We show, by numerical calculations, that the spectral and angular responses of thin films containing disordered photonic patterns are intimately related to the in-plane light transport process and can be tuned through structural correlations. Our findings, which are applicable to all waves, are particularly suited for improving the absorption efficiency of thin-film solar cells and can provide a new approach for high-extraction-efficiency light-emitting diodes.

Design, modeling, and performance evaluation of a novel dye cell for a high repetition rate dye laser

In this paper, a new dye cell for transverse pumping was designed, modeled, and its performance in a narrow spectral width dispersive resonator, pumped by a high repetition rate copper vapor laser, was investigated. The scheme essentially involves the profiling of the cubical glass and stainless steel cylindrical surface such that convex-plano contour be present near the optical pumping region. The design is an amalgamation of straight and curved periphery to enhance the dye solution flow stabilities near the dye laser axis. A computational fluid dynamics analysis of the liquid flow through this dye cell has been carried out. The dye laser outputs such as optical average power, spectral width and wavelength stability, tuning range, pulse shape, through this new dye cell was evaluated. The dye laser average power about 30 mW was fairly steady over the observation period of more than an hour. The dye laser short-term (1 min) spectral width was within 0.824 ± 0.075 GHz, while, in a long-term, more than an hour, drifted by about 180 MHz. The dye laser wavelength in short-term fluctuates within ±0.0065 nm whereas in a long-term, more than an hour, drifted by about 0.0105 nm. The dye laser tuning range was 10 nm with a sub GHz spectral width operation. The pulse shape of the dye laser follows the pump laser pulse profile. Thus, the dye laser has demonstrated fairly long-term stability, without the use of either low expansion material or close loop control on the output.

Wide spectral bandwidth electro-absorption modulator using coupled micro-cavity with asymmetric tandem quantum well

For reliable three dimensional (3D) imaging system, it is necessary for the optical shutter to have a wide spectral bandwidth operation and enhanced modulation depth. We propose an electro-absorption modulator (EAM) based on coupled Fabry-Perot cavities with micro-cavity (CCMC) which uses asymmetric tandem quantum wells (ATQWs) to obtain improved spectral bandwidth and enhanced modulation depth. Several modulator designs are investigated to obtain improved modulation performance such as wider spectral bandwidth and enhanced modulation depth. It was found that among all the studied modulator geometries, CCMC structure with ATQWs provides the widest spectral bandwidth of 9.6nm and high modulation depth in excess of 50% at -24V, which is good agreement with theoretical calculations. These results suggest that EAM has excellent potential as optical shutter for 3D imaging application.

Spectral linewidth narrowing and tunable two-color laser operation of two diode laser arrays

We propose and implement a common external cavity to narrow spectral linewidth of two broad-area laser diode arrays (LDAs) and align their center wavelengths. The locked center wavelength of two LDAs can be tuned in the range of ~10 nm by tuning the tilted angle of the diffraction grating. The output beams of two LDAs are spatially overlapped through the polarization beam splitter of the common external cavity, and the total output power equals the power of two LDAs. The center wavelength of each LDA can be independently tuned by shifting the corresponding fast-axis collimation lens. As a result, the high-power two-color LDA operation is demonstrated with the tunable wavelength difference of up to 2 nm (~1 THz).

Broad spectral pulse operation of 2 μm Tm:YAP laser based on reflection-type carbon nanotube absorber

We demonstrated the first use of reflection-type single-walled carbon nanotube (RSWCNT) as a saturable absorber in the Q-switched mode-locking (QML) of a diode pumped Tm:YAP operating at 2 μm. The spectrum of the QML laser is centered at 1.97 μm with a broad spectral region of 36 nm. At the incident pump power of 10.33 W, as high as 432 mW average output power was produced in QML laser. The repetition rate of the mode-locked pulse inside the Q-switched envelope was 158 MHz. The dependence of the operational parameters on the pump power was also investigated experimentally.

Plutonium isotopic composition of high burnup spent fuel discharged from light water reactors

The isotopic composition and amount of plutonium (Pu) in spent fuel from a high burnup boiling water reactor (HB-BWR) and a high burnup pressurized water reactor (HB-PWR), each with an average discharge burnup of 70 GWd/t, were estimated, in order to evaluate fast breeder reactor (FBR) fuel composition in the transition period from LWRs to FBRs. The HB-BWR employs spectral shift rods and the neutron spectrum is shifted through the operation cycle. The weight fraction of fissile plutonium (Puf) isotopes to the total plutonium in HB-BWR spent fuel after 5 years cooling is 62%, which is larger than that of conventional BWRs with average burnup of 45 GWd/t, because of the spectral shift operation. The amount of Pu produced in the HB-BWR is also larger than that produced in a conventional BWR. The HB-PWR uses a wider pitch 17 × 17 fuel rod assembly to optimize neutron slowing down. The Puf fraction of HB-PWR spent fuel after 5 years cooling is 56%, which is smaller than that of conventional PWRs with average burnup of 49 GWd/t, mainly because of the wider pitch. The amount of Pu produced in the HB-PWR is also smaller than that in conventional PWRs.

A Compact and Highly Efficient Silicon-Based Asymmetric Mach—Zehnder Modulator with Broadband Spectral Operation

An asymmetric Mach—Zehnder electro-optic modulator is demonstrated by using a silicon-based p-i-n diode embedded in compact 200 μm long phase shifters. The measured figure of merit VπL = 0.23 V ·mm shows highly efficient modulation by the device, and an open eye-diagram at 3.2 Gbit/s confirmed its fast electro-optic response. Integrated with the grating coupler, the device exhibits a broad operational wavelength range of 70 nm with a uniform 18 dB extinction ratio covering the C-band and part L-band of optical communication.

Inline Conversion Between Transmission and Reflection Spectra of Fiber Bragg Grating Using Polarization-Diversity Loop Structure

By incorporating a polarization-diversity loop structure, we have demonstrated an inline conversion between transmission and reflection spectra of a fiber Bragg grating (FBG) without reconfiguration of the filter structure containing the FBG. The proposed apparatus consists of a polarization beam splitter, an FBG, and two quarter-wave plates (QWPs). Without requiring rearrangement of input/output ports or additional optical switches, the spectral switching operation of the FBG was carried out through proper adjustment of the QWPs contained in the proposed filter.

Polarized spectral properties and laser operation of Nd^3+-doped La_2(WO_4)_3 crystal

A Nd3+:La2(WO4)3 single crystal was grown by the Czochralski method. The orientation relationship between the optical indicatrix axes and the crystallographic axes was determined. Polarized spectral properties were studied in detail. Laser oscillations of this crystal under Ti:sapphire laser pumping at 802 nm were demonstrated. The achieved highest slope efficiencies were 51.3% and 12.1% for 1058 and 1332 nm, respectively, and the corresponding thresholds were 30 and 93 mW, respectively.

Microstructured and Photonic Bandgap Fibers for Applications in the Resonant Bio‐ and Chemical Sensors

We review application of microstructured and photonic bandgap fibers for designing resonant optical sensors of changes in the value of analyte refractive index. This research subject has recently invoked much attention due to development of novel fiber types, as well as due to development of techniques for the activation of fiber microstructure with functional materials. Particularly, we consider two sensors types. The first sensor type employs hollow core photonic bandgap fibers where core guided mode is confined in the analyte filled core through resonant effect in the surrounding periodic reflector. The second sensor type employs metalized microstructured or photonic bandgap waveguides and fibers, where core guided mode is phase matched with a plasmon propagating at the fiber/analyte interface. In resonant sensors one typically employs fibers with strongly nonuniform spectral transmission characteristics that are sensitive to changes in the real part of the analyte refractive index. Moreover, if narrow absorption lines are present in the analyte transmission spectrum, due to Kramers‐Kronig relation this will also result in strong variation in the real part of the refractive index in the vicinity of an absorption line. Therefore, resonant sensors allow detection of minute changes both in the real part of the analyte refractive index (10−6–10−4 RIU), as well as in the imaginary part of the analyte refractive index in the vicinity of absorption lines. In the following we detail various resonant sensor implementations, modes of operation, as well as analysis of sensitivities for some of the common transduction mechanisms for bio‐ and chemical sensing applications. Sensor designs considered in this review span spectral operation regions from the visible to terahertz.