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Optics Express (10944087) 33(10)pp. 21393-21412
We introduce what we believe to be a novel machine learning (ML)-based ResNet algorithm for predicting gas pressure from spectral imagery, eliminating the need for traditional peak fitting. Evaluated using simulated and experimental carbon monoxide (CO) spectra, the model accurately predicts pressures across a wide range (1 mbar - 2 bar), even with noisy data, outperforming conventional methods like PeakFit. The ResNet model demonstrates minimal discrepancies between predicted and actual pressures, achieving a mean absolute error (MAE) of 0.095 and mean squared error (MSE) of 0.009 in simulations, and maximum MAE of 1.2×10−2 and MSE of 1.46×10−4 experimentally below 94 mbar. This approach significantly enhances quantitative spectroscopy by focusing on line shape imagery, showing promising applications in atmospheric science, industrial monitoring, and environmental research. This work is a substantial improvement over our previous models. © 2025 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement.
Journal of Optics (United Kingdom) (20408986) 27(8)
This study presents a temperature-tunable liquid lens utilizing silicon oil (SO) to achieve precise focal length modulation without requiring mechanical displacement of the sample, typically required in conventional z-scan methodologies. The lens was fabricated by injecting SO into a cavity formed between two convex glass substrates, with temperature varied from 20 °C to 130 °C to enable a focal length tuning range from 69.30 mm to 77.78 mm, corresponding to an overall change of 8.48 mm. Key performance metrics include a focal length resolution of 7.2 × 10−2 mm, temperature sensitivity of 1 °C, and demonstrated long-term stability under repeated thermal cycling. The lens’s functionality was validated within a z-scan setup, where it replaced fixed-focal-length optics to eliminate sample translation along the z-axis. Nonlinear refractive index measurements of silver nanoparticles dispersed in distilled water at a concentration of 13 mM yielded a value of (−1.03 ± 0.08) × 10−7 cm2 W−1 using the proposed tunable lens, closely matching (−0.97 ± 0.06) × 10−7 cm2 W−1 obtained via the conventional z-scan method. The proposed approach provides a practical modification to the classical Z-scan technique by allowing real-time focal control, which can simplify experimental procedures and potentially improve measurement accuracy. © 2025 IOP Publishing Ltd. All rights, including for text and data mining, AI training, and similar technologies, are reserved.
Journal of Optics (United Kingdom) (20408986) 26(4)
A water-injected liquid lens is fabricated to tune its focal length using the change in water salinity. It is found that when the salinity of water is changed from zero to 34.25%, the focal length can be changed by about 12.6 mm from 73.7 mm to 86.3 mm. A focal length resolution of approximately 0.75 × 10−2 mm and high temporal stability over a long period have been achieved for the lens foci. This lens is then used to modify the z-scan technique where the lens and the sample both remain fixed without displacement. The performance of the fabricated lens is evaluated by nonlinear refractive index measurement of a sample containing 10.82-pH-synthesized Silver nanoparticles suspended in water with 15 mM of concentration. For verification of the results, a nonlinear refractive index of (−10.6 ± 1.0) × 10−7 cm2 W−1 is firstly measured for the sample using a classical z-scan benefiting from a conventional focal-fixed lens. Interestingly, we found out that when the fabricated lens is replaced in the modified z-scan, the nonlinear refractive index of about (−8.1 ± 0.2) × 10−7 cm2 W−1 can be measured, indicating a similarity in the order and small difference in the coefficient compared to the classical z-scan. This outcome highlights the potential capability and simplicity of the fabricated lens in the modification of the classical z-scan technique. © 2024 IOP Publishing Ltd.
Optical Materials (09253467) 148
We report on the nonlinear properties of the series of deep eutectic solvents (DESs) which are prepared by a certain ratio of choline chloride as the base and lactic acid as additive materials. The z-scan technique is used to characterize the nonlinear index and absorption of the solvents. Obtained results indicated a negative nonlinear index of the order of about 10−9 cm2/W for the DES solvents at 808 nm of a ∼60-mW CW laser source. It is further found that while the different ratios of lactic acid are increasingly added to the choline chloride, the nonlinear index of solvents are accordingly changed by a factor of 1.2 in magnitude. The results of the open-aperture z-scan confirmed that neither the solvents nor the lactic acid responded to the nonlinear absorption. The represented experiment suggested that with a novel look at these series of DES liquids, it is possible to improve their nonlinear properties by engineering the ratio of choline chloride and lactic acid. © 2024 Elsevier B.V.
Applied Optics (21553165) 63(11)pp. 2843-2853
We report on the new application, to the best of our knowledge, of a time-domain optical coherence tomography (TD-OCT) device used to measure the ordinary no and extraordinary ne indices of calcite birefringence crystal at room temperature. A 1.25±0.05mm thick slab of calcite crystal is cut, polished, and used as a sample in the OCT arm. While the calcite slab is axially scanned, the raw carrier ordinary signals that came from its front and rear facets are received and denoised with a set of digital filters. The extraordinary signals are generated by the change of beam polarization using a 90°-rotating polarizer plate. It is found that the wavelet transformis capable of reaching the highest signal-to-noise ratio (SNR) of about 24.50 and 23.91 for denoising the ordinary and extraordinary signals, respectively. Quantitative measurement of no and ne is carried out by extracting a desired envelope from the denoised signals using standard methods. Average values of 1.660 and 1.444 are obtained for no and ne , respectively, using the wavelet-denoised signals. The weights of the results are finally searched with ones obtained from two sets of dispersion equations.We found a very good agreement between the wavelet-denoised OCT- and dispersion equation-based values with a very lowrelative differences of 0.04% and 2.8% for no and ne , respectively, when theGhosh equation is used and averaged ones of 1.3% and 4.2% for no and ne , respectively, when the Zhao et al. equation is applied. © 2024 Optica Publishing Group.
IEEE Photonics Technology Letters (10411135) 36(18)pp. 1157-1160
We present the efficacy of deep learning (DL) in identifying and recovering the CO2 absorption line from a noisy spectrum. Following a simulation-based assessment of the DL method's capabilities, it was applied in an experiment utilizing a lock-in amplifier and mechanical chopper as a phase-sensitive detection unit. The results demonstrate that the DL method is fully qualified to replace traditional noise-reduction systems, simplifying spectroscopy while ensuring reliability and intelligence. © 1989-2012 IEEE.
IEEE Sensors Journal (1530437X) 23(14)pp. 15570-15577
With strong evanescent waves, optical microfibers (MFs) provide guided lights the ability to directly interact with surrounding environments, whereby several fiber optics chemical sensors have been realized. In this study, based on MFs external refractive index (RI) sensitivity, a multimode optical fiber (MMF) specklegram RI sensor with MMF-MF-MMF configuration is presented. As the MF Section is exposed to the liquids of different RIs (in the range of 1.333-1.368), the interaction between the liquids and evanescent waves modulates the guidance status of the MF, thereby changes the excited modes within the end MMF Section and affects the output speckle pattern accordingly. The evaluation of the functionalities of the MFs with different waist diameters (11, 18, 33, and 42 μ m ) shows that the MF with 33- μ m waist diameter results in the highest output specklegram RI sensitivity, which has been quantified by the zero-mean normalized cross correlation coefficient (ZNCC). Moreover, the response time and sensitivity of the proposed fiber specklegram sensor (FSS) have been simultaneously improved by applying spacial filter on the captured speckles. The RI sensor has also been studied for the temperature detection and showed 0.013°C-1 linear sensitivity within the range of 25 °C-65 °C. Finally, the theoretical analysis of the supported modes by the MFs of the specified waist diameters verifies that 33- μ m sample with high number of guided modes and strong total evanescent waves is the optimum case for the MMF-MF-MMF specklegram RI sensor. © 2001-2012 IEEE.
Optical and Quantum Electronics (discontinued) (03068919) 55(6)
A theoretical model is discussed to investigate the impact of a gas-filled hollow-core photonic crystal fiber (HC-PCF) on the temporal shape and width of a picosecond input pulse. For this purpose the nonlinear Schrödinger equation is numerically solved to simulate the evolution of a slowly varying approximated soliton propagating along the waveguide. To obtain a highly desirable pulse shape with the shortest width at the output, four fiber schemes proposed as case-I to case-IV HC-PCF are modelled in which the pressure style of the filler gas and the chirp of the input pulse are purposefully engineered. The well-known distance parameter and the Q factor are used to evaluate the temporal feature of the ultimate pulse. The final study on the compressed pulse characteristics is performed using a standard merit function (SMF) that is introduced to obtain the most optimum output pulse shape and width. It is found that the application of a positive gradient for the gas pressure and an adequate chirp value for the input makes it possible to reach the lowest width and the highest temporal quality. The final results suggest that the cascaded case-IV fiber is capable of generating the most optimal pulse width of 18.1 fs with the highest SMF value of 12, indicating the highest temporal quality and the lowest distance parameter of 46.3% and 194, respectively. The most interesting feature of the results is the fact that by the proper use of a gradient from for the gas pressure the effect of chirp can be directed in a way that the highest temporal quality and the most desired width of compressed pulse obtains while the input power is kept low. © 2023, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.
Optics Continuum (27700208) 2(8)pp. 1782-1802
This study examined the impact of chirping the input pulses on the amplification of a wide bandwidth THz wave emitted from a plasma slab within a range of input wavelengths from 800 nm to 3.9 µm. It was found that in a normal case of interaction when the injected pulses are highly chirped, flowerthorn-like fluctuations raised in the transverse current density of the plasma, and a THz emission with a higher amplitude within a wide band of 50 THz could be reached. Employing the cross-focusing scheme, the amplitude of the THz wave at 3.9 µm is further enhanced to about 43 times the ones obtained at 800 nm when the two-color pulses are weakly chirped down to 3 × 10−4 rad/s2. A significant increase of 233 times received in THz radiation when a weakly-chirped regime of delayed cross-focusing interaction is established and the intensity of the delayed pulse is increased to 1015 W/cm2, beyond the ionization threshold of Argon gas. The profile change of the fundamental and its second-harmonic waves indicated that a flat-top form is capable of enhancing the THz amplitude approximately by 8 times compared to the case when an ordinary Gaussian profile is used. The obtained results have confirmed that a combination of the type of interaction, chirp value, and the profile of inputs is crucial for enlarging the THz magnitude and domain. © 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement.
Applied Physics B: Lasers and Optics (09462171) 128(12)
The present study discussed a theoretical model to simulate the interaction of an ultra-short femtosecond laser pulse with a slab of neutral Ar gas. The calculation was based on using the Particle in Cell-Monte Carlo Collision (PIC-MCC) method to investigate the possibility of infrared emission. It was found that besides several emissions that are inevitably obtained from a high-density Ar-based plasma medium, resonance radiation can be achieved at about 165 THz whenever the stimulated Raman scattering (SRS) based half-harmonic and rippled density wavenumber are involved in phase match condition with the fundamental laser frequency. Among several parameters examined to amplify the resonance emission, the symmetry breaking of electric field (SBEF) and turning up the input intensity to about 1018 W/cm2 proved more efficient infrared emission. It turned out that compared to the SBEF effect, an almost three times stronger infrared intensity can be obtained using more intensive laser pulses. The transverse profile for half-harmonic emission and resonant radiation is presented. It is observed that, the infrared radiation has an off-axis profile. At an intensity of 1018 W/cm2, the effects of hot electrons and the nonlinear Kerr refractive index have been presented. It was found that these two parameters limit the generation mechanism at the beginning of the plasma and at the end of the interaction medium, respectively. © 2022, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.
Optical and Quantum Electronics (discontinued) (03068919) 54(2)
We report on the nonlinear properties of size-variable Silver nanoparticles (AgNPs) which are synthesized based on the complexing agent method using different pH of the synthesizer. The z-scan technique is used to characterize nonlinear optical properties of AgNPs products. The required z-scan sample is provided by suspending the AgNPs in distilled water and dispersing them in Potassium Bromide (KBr) as host materials. Obtained results indicated an average nonlinear index of − 0.79 × 10–7 cm2/W in water and of − 2.09 × 10–7 cm2/W in KBr at 1064 nm. Furthermore, an average nonlinear absorption of 2.23 × 10–3 cm/W in water and 5.09 × 10–3 cm/W in KBr is measured for the synthesized AgPNs. The results affirm that as the AgNPs sizes are increased, their nonlinear properties are significantly improved in both KBr and water. © 2022, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.
Journal of Optics (United Kingdom) (20408986) 23(12)
In this work, a theoretical model is discussed to investigate the performance of a core-engineered gas-filled hollow-core photonic crystal fiber (HC-PCF). To gain the shortest pulse width and the best beam quality at the output, the core geometry of the fiber is modified within four specific types referred to as type I to type IV fibers. It is found that, by using type III and type IV HC-PCF devices, a 5 ps laser pulse in the input can be respectively compressed to 18.5 fs and 13.7 fs at the output. It is found that, a 5-ps laser pulse in the input can be reduced to 18.5 fs and 13.7 fs if type III- and type IV of modified HC-PCF device are respectively used for compression. The structural similarity (SSIM) index is used to evaluate the quality of the beam cross-section that ultimately emerges from the end of the fiber. The results suggest that the highest SSIM value of 0.76 can be obtained if type III HC-PCF is employed for pulse compression. © 2021 IOP Publishing Ltd.
Applied Physics B: Lasers and Optics (09462171) 127(9)
A multi-segment oven scheme is proposed to control the temperature distribution inside a MgO:PPLN crystal to generate a second-harmonic generation (SHG) pulse possessing optimum characteristics. It is found by numerical solving of SHG coupled equations and steady-state heat equation that when the oven is divided into four segments, the most optimal pulse features can be reached. While the temperature of each segment is independently set at a certain value, and the input pump is raised to 103 GW/m2, the lowest distance parameter of about 10 and the highest efficiency of around 85% is entirely expectable, indicating a desirable case whereby keeping the crystal at the phase-matching condition could not be reached. © 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.
Journal of the Optical Society of America A: Optics and Image Science, and Vision (15208532) 38(1)pp. 1-9
We report on the quality assessment of an optical coherence tomography (OCT) image. A set of recent digital filters are used for denoising the interferometric signals. It is found that when a combination of continuous wavelet transform (WT) decomposition and the WT denoising techniques is imposed on raw signals, the highest signal-to-noise ratio of 17.8 can be reached. The structural similarity (SSIM) index is eventually employed to evaluate the modality of the reconstructed OCT image. Further, we found out that a SSIM value of about 0.95 can be reached, independent of the method used for envelope extraction. © 2020 Optical Society of America
Journal Of Theoretical And Applied Physics (22517227) 14(4)pp. 399-409
The performance of an apodized gas sensor is demonstrated through simultaneous detection of CO and CO2 absorption lines around 1.57 µm in the recuperator channel of a gas-fired industrial furnace at Shahid Montazeri power plant (SMPP) industry. This led to the concentration measurement of targeted molecules as less than ~ 1% and 9.5%, respectively, at atmospheric pressure and 350 °C, indicating close consistency with the reference data reported by SMPP. A minimum detectable absorption of ~ 0.4 × 10−3, corresponding to a detection sensitivity of ~ 4.8 × 10−9 cm−1 Hz−1/2 is measured in this application. © 2020, Islamic Azad University.
Journal of the Optical Society of America B: Optical Physics (07403224) 37(1)pp. 9-18
The performance of a double-pass scheme is theoretically investigated for the efficiency enhancement of a second-harmonic (SH) beam generated by using a high-power fundamental laser beam. Based on a modified version of coupled equations that include the possible effects leading to the thermal de-phasing, the Collins integral is used for optimizing the focusing optics to obtain maximum efficiency of SH conversion. We found through simulation that at a fundamental power of 40 W, when the focusing regime is set for the loose condition, a conversion efficiency of 74% can be reached. It is possible if the focusing points of the first and second passes are designed to be located at 19 mm and 6 mm away from the input face of the crystal, respectively. © 2019 Optical Society of America
Applied Physics B: Lasers and Optics (09462171) 126(5)
We introduced a theoretical model to preserve the quality and the efficiency of a picosecond second-harmonic generation (SHG) in a 30-mm-long LBO crystal which is surrounded by a thermal oven. It is proposed to arrange a temperature gradient throughout the crystal length for mitigating the undesirable consequences of dispersion and group velocity mismatch. To optimize the pulse shape, a distance parameter is introduced and in addition, the SHG efficiency and pulsewidth are included in a merit function. By setting the temperature gradient as the variable the merit function is optimized. The results show that for temperature gradient of 60 °C, an SHG pulse with mostly Gaussian shape with 80.7% efficiency and with 4.47 ps pulsewidth is attainable. Furthermore, the effect of the chirping on the SHG pulse is investigated. Final results have turned the proposed model into an excellent candidate for obtaining an efficient SHG conversion with an appreciable pulse quality. © 2020, Springer-Verlag GmbH Germany, part of Springer Nature.
Journal of Optics (United Kingdom) (20408986) 21(11)
This work provides a comprehensive study around the effect of a quality-deteriorated fundamental beam on the quality of a 355 nm wavelength beam experimentally generated using sum-frequency mixing of a second-harmonic generation (SHG) beam and its un-depleted fundamental inside a walk-off (WO) affected BBO nonlinear crystal. A slit-variable aperture is used inside the resonator of a utilized Nd:YAG resonator to generate a wide range of Hermite-Gaussian modes for the fundamental and subsequent SHG and sum-frequency generation (SFG) beams. The M2-factor of interacting beams is simulated based on the reconstruction of the recorded intensities using a recently reported technique known as few-frame decomposition method. It is found that in the worst case, due to the WO occurring in the X-direction, the M2-factor of the SFG beam is converged to 2.5 after 2 mm of crystal length and remained constant in the remaining length when the fundamental M2-factor is set for 4.5. Based on the simulation results, it is shown that the beam quality of SFG beam along the WO direction is independent of the fundamental M2-factor. It is further realized that by changing the focusing factor up to 2.84, again M2-factor of the UV-SFG beam is not exceeded from 2.5. © 2019 IOP Publishing Ltd.
Optical and Quantum Electronics (discontinued) (03068919) 50(10)
We report on the capability of adaptive neuro fuzzy inference system (ANFIS) coupled with discrete wavelet transform (DWT) for the de-noising of absorption signals. The proposed method is examined for the de-noising of a CO2 spectrum which is experimentally traced in the near-infrared around 1.57 μm. This is performed by contaminating the spectrum with a various types of complicated noise figures produced by several approaches. It is found that DWT-assisted ANFIS is quite sufficient for CO2 signal recovery that is obscured by a huge amount of unknown noises without need of using a prior estimation of the noise feature. Eventually, enhanced signal-to-noise ratio of about 11.7 is obtained. © 2018, Springer Science+Business Media, LLC, part of Springer Nature.
Iranian Journal of Physics Research (16826957) 18(2)pp. 207-220
An annular cell with elliptical cross section and small physical size is introduced and simulated for increasing the absorption length in the spectroscopy. In this investigation by changing of the geometrical radius of the ellipse an absorption length of 15.50 m is obtained. By the optimization of the cell and including the sagittal and tangential radius of the ellipse that reflect the beam, it is shown that at the obtained absorption length, it is possible to reduce the divergence of the beam down to 54 µm. Then, signal-to-noise ratio (SNR) is calculated for R(16) CO2 absorption line by assuming that the cell is filled by the gas. The results of this calculation indicated that under optimum condition the SNR can be increased up to 310. Finally, the cell is optimized using Genetic algorithm by including all the effective parameters which affect the efficiency of the cell. We found that in the best case an absorption length of 8.24 m with a SNR of 107 can be achieved when the geometrical radius of the cell being fixed at 5.95 and 4.59 cm. © 2018, Isfahan University of Technology. All rights reserved.
Journal of Optics (United Kingdom) (20408986) 20(4)
We present a theoretical model in order to study the effect of a thermally loaded crystal on the quality of a second-harmonic (SH) beam generated in a high-power pumping regime. The model is provided based on using a particular structure of oven considered for MgO:PPsLT nonlinear crystal to compensate for the thermal de-phasing effect that as the pumping power reaches up to 50 W degrades the conversion efficiency and beam quality of the interacting beams. Hereupon, the quality of fundamental beam is involved in the modeling to investigate the final effect on the beam quality of generated SH beam. Beam quality evaluation is subsequently simulated using Hermite-Gaussian modal decomposition approach for a range of fundamental beam qualities varied from 1 to 3 and for different levels of input powers. To provide a meaningful comparison numerical simulation is correlated with real data deduced from a high-power SH generation (SHG) experimental device. It is found that when using the open-top oven scheme and fixing the fundamental M 2-factor at nearly 1, for a range of input powers changing from 15 to 30 W, the M 2-factor of SHG beam is degraded from 9% to 24%, respectively, confirming very good consistency with the reported experimental results. © 2018 IOP Publishing Ltd.
Applied Optics (21553165) 57(30)pp. 9140-9147
We report on the performance of an enhanced-cavity (EC) designed for obtaining high-power and efficiency second-harmonic generation (SHG). This is performed by numerical simulation of SHG coupled equations in the presence of a thermal dephasing effect that is effectively intensified through embedding the periodically poled LiNbO3 crystal by an oven-surrounded scheme. It is found that by setting the PPLN temperature at an optimum value, adjusting the mirror reflectively, and pumping power at certain values, gaining SHG efficiency of more than 90% is possible. We further realized that by an ECSHG device SHG efficiency can be improved by about 15%-50%. Moreover, compared to a single-pass SHG scheme, the EC-based SHG device is shown to be a very promising candidate to reduce and suppress the effect of thermal dephasing on the stability and efficiency of SHG radiation. © 2018 Optical Society of America.
Iranian Conference on Machine Vision and Image Processing, MVIP (21666776) 2017pp. 114-118
In fiber optic sensors, laser source produces a coherent light which is transmitted through a fiber cable. The output of this laser forms a speckle. This speckle is captured by a CCD and then analyzed by a PC. The intensity and shape of the speckle change when external force is applied on the cable. In this paper, Laplacian of Gaussian (LoG) filter is used to improve the dynamic range and accuracy of the fiber optic sensor. Capturing images is done in a dark room and the simulation is done by Matlab. The difference between the intensities of a normal and abnormal state image was 4.0556 when using 2D Discrete Wavelet transform (DWT) filter. It is increased into 14.6191 when LoG filter is used, thus increasing the dynamic range and accuracy of the sensor. In addition, the time is decreased by about 24.5 %. The number of changed blocks is increased by about 22 % leading to double increase in the accuracy and dynamic range. © 2017 IEEE.
Applied Physics B: Lasers and Optics (09462171) 123(9)
An innovative optical method is introduced for the beam quality measurement of any arbitrary transverse mode based on the reconstruction of the mode from a few-frame image of the beam cross-section. This is performed by the decomposition of a mode to its basic Hermite–Gaussian modal coefficients. The performance of the proposed method is examined through M2-factor measurement of the beam of a Nd:YAG laser which was forced to oscillate in a certain mode using a crossed rectangular intracavity aperture. Obtained results have shown that this method can be alternatively replaced for the hologram- and ISO-based techniques recently exploiting for beam quality measurement regardless of the mode type and the position of utilized CCD camera along the beam direction. © 2017, Springer-Verlag GmbH Germany.
Optical and Quantum Electronics (discontinued) (03068919) 49(11)
In the present work, the effects of an intracavity etalon inside a singly resonant optical parametric oscillator (SRO-OPO) resonator on the power and spectrum stability and tuning characteristics of the signal and idler waves has been theoretically modeled. Real experimental parameters associated with a laboratory SRO-OPO are used to compute the spectral bandwidth of oscillating signal through calculation of the nonlinear coupled equations while the input pump power is assumed to be adjusted at several multiples of oscillation thresholds for sets of thickness-variable slab glass etalon. Simulation results show by using thicker etalons a significant reduction of ~ 60 times in the signal bandwidth can be obtained at higher levels of the SRO-OPO threshold which resulted in the more stability of the outcoupled waves. The simulated results have been realized by using a slab glass of etalon inside the cavity of a PPLN-base SRO-OPO setup constructed in the laboratory. It is found that the peak-to-peak fluctuations of both signal and idler waves reduces to less than ~ 0.05% whereas the thickness of the internal etalon is increased to ~ 1.1 mm and the device is adjusted to operate around the threshold. This reflects good consistency between experimental measurements and theoretical model. Finally, more than ~ ± 15 nm tuning of the SRO-OPO output is also performed by rotating the intracavity over ~ ± 5°. © 2017, Springer Science+Business Media, LLC.
Optical and Quantum Electronics (discontinued) (03068919) 48(4)
The potential and drawbacks of the apodized 2f/1f wavelength modulation spectroscopy are theoretically studied and experimentally characterized. We apply a near-infrared DFB-based laser spectrometer, tunable around the R(32) CO2 absorption line centered at 6369.408 cm−1. The performance of the apodized method is shown by minimizing the pressure deviation between the gauge and experimental pressures by using the beneficial effect of the scaling (Formula presented.) -factor. This factor equalizes the experimental and simulated peak heights of the CO2 absorption trace. We found that when (Formula presented.) -factor is varied up to its optimum value of ~200, a pressure deviation of nearly zero is obtained at a case pressure of 19 ± 0.5 mbar. Under such optimum condition a minimum uncertainty of ±1 mbar is also obtained for the pressure deviation. However, we further acquired that far from this optimum condition, compared to the common method, the apodized approach is also capable of reducing the pressure deviation by ~13.5 % at 20 ± 0.5 mbar of CO2 pressure, indicating the performance of the proposed method for precise pressure measurement of a gas sample, regardless of the optical limits. © 2016, Springer Science+Business Media New York.
Optica Applicata (00785466) 46(1)pp. 103-115
We report on a multichannel sensor for multipoint corrosion monitoring of an aluminum film, which was corroded by nitric acid as a corrosive solution. The sensor head was a commercial 1×4 optical fiber coupler combined with the optical time domain reflectometry technique. The effects of the concentration and pH of acid on the corrosion rate of the aluminum film are initially studied and characterized. Then, about 100 nm of pure aluminum is coated on the end facets of each fiber channel and reduction of the back reflected signals is simultaneously monitored as a measure of corrosion rate. It is found that the fabricated sensor is very sensitive to the variation of pH as by increasing of the pH up to ~13, the corrosion rate of aluminum is raised to ~75 millimeter per year. Our experimental results have been verified by the standard immersion test, indicating very good consistency and reliability. Eventually, the sensor is used to trace the corrosion of sea water provided from the Persian Gulf, confirming the validity of laboratory results.
Optica Applicata (00785466) 46(4)pp. 639-650
In this work, the second-harmonic component of wavelength modulation spectroscopy is simulated for R(22) CO2 absorption line to investigate the effect of gas temperature and pressure on the modulation index. We found that the optimum value of modulation index, that is 2.2, is not affected by temperature but gas pressure will change the optimized modulation index. Specifically, when the gas pressure decreased to lower pressures of less than 100 mbar, the modulation index is also decreased and tended exponentially to about two. Accordingly, the optimum value of modulation index is recalculated for a range of CO2 gas pressures to establish a nearly zero pressure deviation in the spectroscopy of very low pressure samples.
Optical and Quantum Electronics (discontinued) (03068919) 48(12)
We applied discrete wavelet transform (DWT) technique for de-noising of carbon monoxide (CO) spectrum which is experimentally obtained in the mid-infrared region using a difference-frequency-based spectrometer. The performance of DWT is firstly examined by de-noising of a simulated P(16) CO absorption line based on the real data associated with the experimental setup we arranged in laboratory. It is found that when the db20 function from Daubechie wavelet family is used at level 7 of composition, highest signal-to-noise ratio (SNR) ~23 can be achieved. Similar results are obtained in the experiment in which a noisy CO trace is de-noised using the same procedure as described in the simulation. Subsequently, highest SNR of ~9.2 and lowest residuals is experimentally obtained using the db20 function which confirms the merit of DWT technique in detection of low-level absorption signals. © 2016, Springer Science+Business Media New York.
Optica Applicata (00785466) 46(2)pp. 277-279
In the present work, the inner cladding geometries of a typical double-clad fiber laser are studied and numerically simulated for different cladding shapes to obtain the maximum absorption efficiency for the pump beam. This is performed by using the ray tracing approach and dislocating the fiber core from the center to impose the asymmetry on the investigated geometries. The high absorption efficiency of ∼94.5% was obtained for the optimized offset D-shaped double-clad fiber. The hexagonal shape is proposed as a new geometry for the inner cladding to attain higher absorption efficiency. It was found that the absorption efficiency of ∼ 68% for a symmetrical hexagonal can be improved to ∼95% for an asymmetrized hexagonal-shaped double-clad fiber laser. Eventually the genetic algorithm was used to enhance the performance of the investigated geometries. This resulted in the further increasing of pump beam absorption efficiency of 99.5% in the genetic algorithm optimized asymmetrized hexagonal shape.
Journal of the Optical Society of America B: Optical Physics (07403224) 33(12)pp. 1640-1648
In this paper, the effect of thermal dephasing on the efficiency of a high-power and high-repetition-rate singlepass second-harmonic generation (SHG) in the nanosecond regime is studied. We found that inside a thermal loaded crystal, in addition to the optimization of the focusing parameter and oven temperature, spillover loss can also play a controlling role in retaining the SHG efficiency. It is shown that with a proper choice of the focusing parameter and oven temperature, at about 20 W of averaged fundamental power, at least 60% SHG efficiency is achievable. Moreover, exceeding a certain limit of input power up to 80 W, inducing a spillover loss of about 10%, will help to maintain the SHG efficiency at a maximum value of about 50%. © 2016 Optical Society of America.
IEEE Journal of Quantum Electronics (00189197) 51(8)
In this paper, we present a theoretical model that describes the performance of a specific oven configuration used to actively control the effect of thermal dephasing on the output characteristics of a high-power single-pass second-harmonic generation (SHG) in a MgO:sPPLT nonlinear crystal. The provided model is based on using step and slope oven configurations and making a quantitative comparison with the recent results obtained by using open-top oven scheme. It is found that in the slope oven scheme the SHG power is enhanced by 190% at a fundamental power of 50 W, indicating the significance of the represented model. Eventually, the merit of the proposed slope oven indicated through saturation control in the generation of high-power SH radiation. © 1965-2012 IEEE.
Optical and Quantum Electronics (discontinued) (03068919) 47(10)pp. 3349-3363
We present a theoretical model to investigate the performance of an intracavity sum-frequency generation (SFG) inside the ring resonator of a singly-resonant optical parametric oscillator (SR-OPO) scheme. The effect of the resonator specifications on the efficiency of such SFG–OPO configuration is modeled and simulated by solving the coupled equations in a normalized from which describe simultaneous SFG and OPO interactions inside two individual phase-matched nonlinear crystals. Numerical simulation is carried out using real practical values associated with experimental results that are recently reported by Devi et al. (Opt Express 21:24829–24836, 2013). We found very good agreement and consistent with the used experimental data, indicating the validity of our simulation. The influence of the beam waist of interacting beams on the conversion efficiency of resultant SFG–OPO radiation is investigated through introducing a magnifying factor that relates optical and physical characteristics of the designed SFG–OPO configuration. It is found that when this factor is taken equal to 3, the maximum of normalized SFG power reaches to ~0.0034 for an OPO loss of ~10 %. This is very consistent with the measured value of ~0.0030, confirming that our represented model is valid. The SFG–OPO arrangement is then optimized by extracting a mathematical formula that relates the optimum magnifying factor to the gain parameters of OPO and SFG interactions. © 2015, Springer Science+Business Media New York.
IEEE Journal of Selected Topics in Quantum Electronics (1077260X) 21(1)pp. 185-192
We study thermal effects in high-power single-pass second-harmonic generation (SP-SHG) of continuous-wave (cw) green radiation in MgO:sPPLT and describe optimization of critical factors including the fundamental beam waist, position, and focusing parameter, for attainment of maximum SP-SHG efficiency. By developing a numerical model combining split-step Fourier and finite-element methods, we compute the relevant parameters for optimization of SP-SHG of a cw Yb-fiber laser at 1064 nm in a 30-mm-long crystal. Real experimental parameters and boundary conditions are used to simulate nonuniform heat distribution in the crystal under high-power operation and study its effects on SP-SHG optimization. The results indicate that for input powers exceeding a certain limit, the Boyd and Kleinman (BK) criteria for optimum focusing are no longer valid. By increasing the fundamental power to 50 W, we find that the optimum focusing parameter decreases by a factor of 4, from "\xi =2.84$ to 0.7. We also find that the maximum SP-SHG output power is always obtained for fundamental waist at the center of the crystal, and we present a general strategy for the choice of ξ at different fundamental powers. Using a top-sinked oven design, the BK focusing condition can be established even at high fundamental powers. © 1995-2012 IEEE.
Optica Applicata (00785466) 45(3)pp. 355-367
We introduce the genetic algorithm for the optimization of an Yb3+-doped double-clad fiber laser based on a multi-variable scheme. The output characteristic of the laser is numerically simulated using real practical values. This is performed through solving the associated steady-state rate equation and investigating the effects of input variables such as pump and signal wavelengths and length of the fiber on the laser output. It is found that pumping of the medium around 975 nm is conducted to attain the maximum output power of ∼34.8 W, while the stability of the outcoupled power is significantly improved when pumping at 920 nm, confirming good agreement with the reported experimental results. We have also found that by using genetic algorithm base multi -variable optimization, the output power can be significantly increased by about three orders of magnitude and reaches to ∼28.5 W with optimum and shorter fiber length of ∼57.5 m. Obtained results show that based on the genetic algorithm multi-variable discipline, fiber characteristics can be optimized according to the gaining of maximum output power.
Applied Optics (21553165) 54(29)pp. 8580-8586
We present a theoretical model that describes a multisegment oven configuration to compensate for the Gouy phase shift in the tight focusing regime. The model is provided to obtain maximum efficiency of the second-harmonic wave. Numerical simulations are performed based on the experimental setup employing an MgO:sPPLT nonlinear crystal. It is shown by the simulation that the three-segment oven is potentially capable of reaching the highest efficiency by canceling the effect of Gouy phase when the temperature of each segment is so optimized that the phase-matching condition is re-established. It is found that by using a multisegment oven scheme with optimized temperature, the highest attainable second-harmonic generation (SHG) efficiency exceeds about 4.4% at a confocal parameter of 3.32. Moreover, it is shown that for long crystals with large confocal parameter, the only way to attain the maximum efficiency is by using multisegment ovens. The results indicate that when the number of segments is changed from one to nine, the confocal parameter can be varied from 3.90 to 5.85. In the candidate three-segment oven, by applying active control on the temperature of each segment, for a certain combination of segment lengths, ∼4% improvement in the SHG efficiency is achieved. This results in a relatively large increase in the acceptance temperature bandwidth of the crystal of up to 2 times, reflecting a comparatively large enhancement in the second-harmonic power stability and efficiency. © 2015 Optical Society of America.
Applied Physics B: Lasers and Optics (09462171) 118(2)pp. 219-229
The performance of an optical sensor that employs an unbuffered polydimethylsiloxane (PDMS)-cladding fber optic is demonstrated for the sensitive detection of CO2 gas in the near-infrared region for around 1.57 µm using the apodized 2f/1f wavelength modulation spectroscopy method. The permeability and diffusion characteristics of the PDMS fber have been theoretically examined and numerically simulated. The results of the simulation are verifed by an experimental setup containing a DFB laser source and 5-m-coiled unbuffered PDMS fber placed in a pre-vacuumed cell flled with about 980 ± 10 Torr of pure CO2 gas. A minimum detectable absorption of ~0.9 × 10-4 is measured, corresponding to a detection sensitivity of ~4.5 × 10-11 cm_1/Hz1/2. The effect of the scaling k-factor on the apodized signal is subsequently studied, showing close agreement between the simulation and experimental results. © Springer-Verlag Berlin Heidelberg 2014
Chinese Physics B (16741056) 24(2)
To reduce the walk-off angle of the extraordinary third-harmonic ultraviolet wave at 355nm generated by type II KTiOPO4 and type I β-BaB2O4 optical crystals, and the Gaussian output beam of a Q-switched Nd:YAG laser, a simple theoretical model was developed based on a rotatable BK7 plate of variable thickness. By rotating the plate up to 35° along the beam direction, we reduced the walk-off angle up to ∼ 13%. The same phenomenon is predicted by the model, confirming the performance of the model. It is found that, due to the walk-off effect, the intensity profile of the third-harmonic generation beam is slightly degraded. To compensate for the observed phenomena and further reduce the walk-off, we used a combination of a convex lens and an axicon to transform the beam profile of the interacting fundamental and second-harmonic generation waves to the zero-order Bessel-Gaussian form. As a result, the walk-off is decreased to ∼48.81mrad, providing ∼30% relative reduction. By using the same BK7 plate rotated up to 35° along the third-harmonic beam direction, the walk-off angle is further reduced to 38.9mrad. Moreover, it is observed that the beam profile of the emerged Bessel-Gaussian third-harmonic generation beam remains unchanged with no degradation. © 2015 Chinese Physical Society and IOP Publishing Ltd.
Applied Physics B: Lasers and Optics (09462171) 116(3)pp. 521-531
We introduce the basics of an apodized 2f/1f wavelength modulation method for the spectroscopy of the R(9) transition line in the first overtone band of carbon monoxide (12C16O) in near-infrared (NIR) region around 2.33 μm. Performance of the method is investigated for high gas concentrations beyond the optically thin limit to generalize common 2f/1f wavelength modulation spectroscopy (WMS) reported by Rieker et al. (Appl Opt 48:5546, [28]). Numerical simulations are performed based on real experimental parameters associated with a NIR spectrometer designed in our laboratory. The results primarily show a more linear response and less error than occurred in the common WMS-2f/1f method for an optically thick sample. It is also theoretically shown that the apodized method enables sharpening the spectrum without peak displacement compared to the common WMS-2f/1f method. The validity of the method is verified experimentally by the trace detection of an air-broadened R(9) CO absorption line centered at 4,294.637 cm-1 at atmospheric pressure and room temperature. The effect of a so-called scaling k-factor on the sharpening of WMS-2f/1f signal is investigated through trace simulation and detection of CO and methane (CH4) lines in the scanning range of a distributed feedback laser. The obtained results show very good agreement between simulation and experiment. © 2013 Springer-Verlag Berlin Heidelberg.
Optical and Quantum Electronics (discontinued) (03068919) 46(12)pp. 1547-1559
We report a new method to measure amplitude small signal gain of a chopper base Q-switched Nd:YAG laser using saturation pulse width effect of nanosecond relaxation oscillation (RO) spikes. Formation of such RO pulses is theoretically modeled and simulated by attributing a loss function to the internal Q-switch chopper and solving the standard rate equations using real practical values associated with a laboratory Q-switched cw Nd:YAG high-power laser. It is found that at a certain chopper frequency, pulse width of the simulated RO spikes is saturated and tends to a minimum value. The performance of the represented model is verified by experimental measurements through generating nanosecond RO spiked using a frequency-variable chopper inside the cavity of the laboratory Nd:YAG laser. We observed that at a certain input pump power of 180 and 290 W, when the frequency of Q-switch chopper is respectively raised up to 5.8 and 12 kHz, oscillation of single RO pulse is established. We also found that by further increasing of the chopper frequency up to 18 kHz, FWHM pulse width of the RO fundamental pulse is saturated toward ~163 ns. Moreover, at this frequency of chopper and input pump powers of 270 and 180 W, saturated FWHM RO pulse widths of 170 and 330 ns are measured, respectively. Obtained results show good consistency between theoretical model and experimental measurements. This saturation effect, is used to obtain amplitude small signal gain of the utilized Q-switched Nd:YAG laser as 3 and 0.8 at the above input pumping levels, respectively. At the same time by using Findlay–Clay approach the small signal gain is measured to provide a quantitative comparison with the above results. From the final results we found excellent agreement between the presented method and Findlay–Clay approach. © 2014, Springer Science+Business Media New York.
IEEE Journal of Selected Topics in Quantum Electronics (1077260X) 20(5)
We present a theoretical model which describes the effects of thermal load distribution on single-pass second harmonic generation (SP-SHG) of high-power continuous-wave (cw) radiation in MgO:sPPLT nonlinear crystal to provide green output at 532 nm. Numerical simulations are performed based on real practical values and actual operating conditions associated with a recent SP-SHG experiment, generating 10 W of cw green radiation using a Yb-fiber laser. The model is used for four oven configurations to simulate the implications of thermal effects on SH power. The observed asymmetric feature of the phase-matching curves, particularly at higher fundamental powers up to 50 W, are characterized and explained by considering the generation of heat due to crystal absorption. The concept of optical path difference (OPD) is introduced to study the formation of thermal lens and its effects on the displacement of focal point inside the thermally loaded crystal. We further study the dependence of the SH power on the different oven schemes by increasing the input fundamental power up to 50 W. It is found that a top-sinked oven design is the optimum configuration for achieving maximum SHG efficiency without saturation. Comparison of the simulation results with experimental data confirms the validity of the theoretical model. © 1995-2012 IEEE.
Optics InfoBase Conference Papers (21622701) 2014
We report a systematic study of thermal effects in high-power single-pass SHG in the presence of absorption, and propose an optimum heating configuration for the crystal to minimize thermal lensing at various fundamental power levels. © 2014 Optical Society of America.
Optics and Laser Technology (00303992) 56pp. 436-442
We report the generation of a mid-infrared (MIR) difference-frequency radiation between 4.76 μm and 4.86 μm rang inside AgGaS2 nonlinear crystal using a V-shaped external cavity configuration fabricated around pump source. The effect of using such V-shaped scheme on tuning characteristics of a commercial diode laser have been studied and experimentally presented. More than ~11 nm continuous tuning is obtained for the emerging original beam through rotating a blazed grating over a certain angular range with a measured accuracy of 1.4 μm/degree. Such characterized V-shaped external cavity is used as pump source in a DFG experimental setup. An appreciable extension in generated MIR-DFG bandwidth is observed when the intracavity grating is rotated over ~600 μm in opposite directions. The performance of such extended V-shaped external cavity base MIR-DFG radiation is demonstrated as trace detection of P(16) and P(18) rovibrational lines of 1-0 fundamental band of carbon monoxide (12C16O) molecule with and without using V-shaped cavity. © 2013 Elsevier Ltd.
Applied Physics B: Lasers and Optics (09462171) 110(3)pp. 425-431
We report the generation of mid-infrared pulsed radiation between 2.2 and 3 μm range using a singly-resonant optical parametric oscillator (SR-OPO) based on a 40-mm-long crystal of periodically-poled LiNbO3 (PPLN) pumped by mechanically Q-switched pulses from a Nd:YAG laser, obtained by chopping the beam inside the laser resonator over a 1-10 kHz duty cycle. An appreciable reduction in pulse width as well as the number of relaxation oscillation pulses of the Nd:YAG pump laser is observed when the frequency of the Q-switch chopper is increased up to 10 kHz. Sub-nanosecond relaxation oscillation pulses of about 170-210 ns duration are generated under the width of the idler envelope varying from 4.6 to 8.55 μs. The same behavior is observed for the signal wave. A maximum extraction efficiency of 22 % is obtained for the idler, corresponding to 785 mW of output power at 10 kHz. The tuning of the signal and idler beams were performed by temperature variation of the PPLN crystal within 100-200 °C range. © 2013 Springer-Verlag Berlin Heidelberg.
Chinese Physics B (16741056) 21(6)
A tunable continuous wave (cw) mid-infrared (MIR) laser based on difference-frequency generation (DFG) in a 1.5-cm long AgGaS 2 nonlinear crystal for trace gas detection is reported. Two visible and near-infrared diode lasers were used as pump and signal sources. The MIR-DFG laser was tunable in a wavelength range of 4.75 μm-4.88 μm. The phase-matching (PM) condition was non-critically achieved by adjusting the temperature of the crystal for fixed pairs of input pump and signal wavelengths. The required PM temperatures of the generated MIR-DFG wavelengths have been calculated by using three sets of recent Sellmeier equations and the temperature-dispersion equations of AgGaS 2 given by Willer U, et al. (Willer U, Blanke T and Schade W 2001 Appl. Opt. 40 5439). Then the calculated PM temperatures are compared with the experimental values. The performance of the MIR-DFG laser is shown by the trace detection of the P(16) carbon monoxide ( 12C 16O) absorption line in a laboratory-fabricated absorption cell. The enhanced sensitivity of about 0.6×10 -4 was obtained through the long path absorption provided by consecutive reflections between coated cylindrical mirrors of a constructed cell. © 2012 Chinese Physical Society and IOP Publishing Ltd.
Optics Express (10944087) 20(25)pp. 27442-27455
We present a theoretical model on the effects of mechanical perturbations on the output power instability of singly-resonant optical parametric oscillators (SR-OPOs). Numerical simulations are performed based on real experimental parameters associated with a SR-OPO designed in our laboratory, which uses periodically-poled LiNbO3 (PPLN) as the nonlinear crystal, where the results of the theoretical model are compared with the measurements. The out-coupled power instability is simulated for a wide range of input pump powers the SR-OPO oscillation threshold. From the results, maximum instability is found to occur at an input pump power of ~1.5 times above the OPO threshold. It is also shown theoretically that the idler instability is susceptible to variations in the cavity length caused by vibrations, with longer cavities capable of generating more stable output power. The validity of the theoretical model is verified experimentally by using a mechanical vibrator in order to vary the SR-OPO resonator length over one cavity mode spacing. It is found that at 1.62 times threshold, the out-coupled idler suffers maximum instability. The results of experimental measurements confirm good agreement with the theoretical model. An intracavity etalon is finally used to improve the idler output power by a factor of ∼2.2 at an input pump power of 1.79 times oscillation threshold. © 2012 Optical Society of America.
Optics and Laser Technology (00303992) 44(5)pp. 1564-1569
In the present paper, a laser-coupled optical fiber is introduced for pH sensing of Methyl red solution in the Ethanol solvent. Then it is modified for corrosion detection when it was placed inside a corrosive solution. Second-harmonic (SH) radiation of a microchip Q-switched pulsed Nd:YAG laser operating at λ=532 nm is generated via KTP nonlinear crystal, and it is launched into the fabricated fiber sensor. The provided evanescent field is absorbed by the surrounding environment in the sensing region, and the output intensity of the absorbed laser beam is monitored and recorded in the presence of the different kind of solvents and corrosive solutions. To increase the sensitivity of the pH sensor the fiber-optic probe is coiled and fixed on a Poly Propylene (PP) mount with 6 cm diameter and 10 cm long. The fabricated sensor is then calibrated for pH measurement of unknown media. For corrosion detection, a spin motor is used to uniformly coat a small portion of the fiber designed as U-shaped after its clad was removed by a simple chemical method. It is then electroplated by a very thin FeC film to form a corrosion sensor. It is observed that while the concentration of the NH 4Cl solution is changed from 0.068 to 0.125 mol/l and its pH from zero to 14, the output intensity of the launched laser is increased due to the FeC film corrosion. © 2011 Elsevier Ltd. All rights reserved.
Chinese Optics Letters (16717694) 10(6)
An antiresonant ring (ARR) interferometer configuration is introduced for the characterization of a continuous wave (CW) Nd:YAG laser output. The output of the ARR device is precisely characterized to determine the gain and loss of a laboratory CW Nd:YAG laser by using the Findlay-Clay approach. The ARR arm is then experimentally arranged inside the cavity of an arranged high power side-pumped CW Nd:YAG laser. A coated beam splitter with 50-50% reflectivity at normal incidence is placed inside the cavity to provide a wide range of reflectivity from 0 to 100%. This is performed by a rotatable stage and tilting the beam splitter by 10° with the steps of 0.05. By changing the input electrical power of the laser pump the variation of the output laser power is monitored for 20 individual reflectivity of ARR arm. Average pump threshold power of about 180 W is obtained. With the help of the derived equations and obtained threshold power, small signal gain and loss associated with the emerging beam is estimated. It is verified that the former is very dependent to the input parameters. Laser efficiency is also measures 5.6% which is quite comparable with the reported values. © 2012 Chinese Optics Letters.
Applied Physics B: Lasers and Optics (09462171) 108(2)pp. 261-268
In the present article, the thermal distribution inside the gain medium of a passively Q-switched microchip laser is modeled and simulated for actual practical values associated with an available microchip laser constructed in our laboratory. The effects of the non-uniform heat distribution on the spectral properties of the output laser beam have been investigated and simulated with the variation of diode-pump power and pulse repetition rate. It is observed that the gain bandwidth as well as Optical Path Difference (OPD) values of the propagating pulses are significantly decreased, while the Nd:YAG chip is cooled down to a certain value. The validity of the utilized model is checked by setting and characterizing the spectral properties of a fabricated laboratory microchip laser under different heating conditions. It is verified that when the temperature of the gain material is changed by an electronically controlled Peltier device, the spectrum of the output laser beam can be switched between single- and dual-mode situations. This physical character has shown good agreement between the presented model and obtained experimental results. ©Springer-Verlag 2012.
Iranian Journal of Physics Research (16826957) 11(3)
A novel compact fiber-coupled NIR system based on a DFB diode laser source is employed as a portable and sensitive gas sensor for trace detection of combustion pollutant molecules. We demonstrate the performance of such an NIR gas sensor by tracing the absorption lines of CO and CH4 using 2f-WMS technique at moderate temperature of T ∼ 600°C in the recuperator channel of an industrial furnace provided by Mobarakeh steel company. This measurement shows the excellent sensitivity of the applied NIR gas sensor to the on-line and in-situ monitoring of such molecular species.
Chinese Optics Letters (16717694) 9(7)
In this letter, a thin slab of glass is used as Fabry-Perot etalon inside a cavity of a continuous wave (CW) Nd:YAG laser to change the behavior of its output longitudinal modes. The slab etalon is used as tuning element when it turns around the laser cavity axis. Two simultaneous longitudinal modes with a relatively wide tuning range from 5.83 MHz to 0.02 THz are obtained when the Nd:YAG laser is operated at moderate output power of about 120 mW. The mode structure of this configuration is modeled and simulated. Computer-generated diagrams are also presented schematically and compared with the experimental results. © 2011 Chinese Optics Letters.
Optics and Laser Technology (00303992) 43(5)pp. 956-959
We have reported a modified V-shaped external cavity, which is constructed around a commercial diode laser operating at a center wavelength of λ=785 nm by adding a new coated glass plate with about 50% reflectivity to the cavity. This allows simultaneous dual-wavelengths operation in the vicinity of Δνmin=0.18 THz to Δνmax=0.22 THz, which can be used as laser source for terahertz generation either for semiconductor devices or nonlinear schemes. © 2010 Elsevier Ltd. All rights reserved.
Chinese Physics B (16741056) 20(5)
We have generated a second-harmonic generation (SHG) of a Q-switched microchip Nd:YAG laser on the surface of a periodically poled LiNbO3 (PPLN) nonlinear crystal near the grazing incidence angle. Three individual SHG waves as transmitted homogeneous, inhomogeneous and reflected radiations have been generated and their intensities are measured and characterized within a desirable range of about 10 different incidence angles of the Nd:YAG laser as pump source on the PPLN surface. The basic of surface nonlinear radiation is also investigated and similar results are calculated and extracted from the theory. Comparison between calculated and measured data shows that they are in good agreement with each other. © 2011 Chinese Physical Society and IOP Publishing Ltd.
Optics and Lasers in Engineering (01438166) 44(7)pp. 699-710
The advantages of infrared laser monitoring in terms of sensitivity, selectivity and the ability of non-intrusive detection of gases are reviewed. Emphasis is laid on direct absorption spectroscopy and evanescent-field spectroscopy. The performance of the latter for gas detection in the near-infrared is demonstrated for the analysis of volcanic gases. For industrial process control, direct mid-infrared absorption spectroscopy is used to detect CO in the high-temperature atmosphere of a glass melting furnace. For both applications portable, stable, rugged and easy-to-handle laser systems are needed. Mid-infrared absorption spectroscopy is also applied to detect different explosives. Material evaporation is achieved by plasma generation with a pulsed laser at high repetition rate. Energetic materials contain high concentrations of nitrogen; therefore NO is present in the generated plasma. However, the rate at which NO is produced varies in a highly characteristic manner for different energetic materials. This enables the distinction between different types of explosives. © 2005 Elsevier Ltd. All rights reserved.
Optics InfoBase Conference Papers (21622701)
Mid-infrared laser radiation at 7.9 μm is generated by DFG in AgGaS2 for the detection of explosives TNT, RDX, PETN, and TATP. Fiber coupled absorption and evanescent field sensors are developed for remote detection. ©2004 Optical Society of America.
Applied Optics (21553165) 43(35)pp. 6481-6486
A compact mid-infrared (MIR) laser spectrometer based on difference-frequency generation (DFG) is applied as a portable and sensitive gas sensor for industrial process control and pollutant monitoring. We demonstrate the performance of such a MIR DFG gas sensor by recording the absorption spectra of the carbon monoxide (CO) P(28) absorption line in the atmosphere of a gas-fired glass melting furnace. For a gas temperature of approximately 1100°C, the CO concentration in the recuperator channel is measured to be 400 parts per million. © 2004 Optical Society of America.
Wondraczek, L. ,
Heide, G. ,
Frischat, G.H. ,
Khorsandi, A.R. ,
Willer, U. ,
Schade w., W. Glass Science and Technology (09467475) 77(2)pp. 68-76
Laser spectroscopic diagnostics of combustion species at elevated temperature has shown considerable progress during the last years and decades. Particularly, techniques of mid-infrared absorption spectroscopy using fundamental rotational/vibrational transitions offer potentially great sensitivity and selectivity for combustion control. However, applications are still limited to mostly laboratoryscale investigations, which is mainly due to drawbacks of light sources and beam guidance materials. On the other hand, many efforts are made to facilitate industrial application. In this context, the relevance of mid-infrared absorption spectroscopy for in situ monitoring of minor species in glass melting furnaces is obvious, too. The present paper gives a general review of available techniques and their impact on emission diagnostics in the glass industry, considering conventional as well as emerging light sources, detectors, peripheral devices and spectroscopic techniques.
Wondraczek, L. ,
Heide, G. ,
Frischat, G.H. ,
Khorsandi, A.R. ,
Willer, U. ,
Schade w., W. Glass Science and Technology (09467475) 77(3)pp. 131-136
Emerging techniques of mid-infrared absorption spectroscopy offer potentially great sensitivity and selectivity for combustion control and emission monitoring. Because of that, a difference frequency based mid-infrared absorption spectrometer has been considered for application in the glass industry. Based on preliminary tests within laboratory conditions, a spectrometer which operates at wavelengths around 5 μm was applied to online monitoring of the atmosphere of a gas fired glass melting furnace. The CO concentration was measured in order to demonstrate the feasibility of a mid-infrared absorption spectrometer for process control in the glass industry. A series of measurements was performed in situ as well as crossing the recuperator entry, resulting in general advice on the construction of a prototype device.
Wondraczek, L. ,
Khorsandi, A.R. ,
Willer, U. ,
Heide, G. ,
Schade w., W. ,
Frischat, G.H. Combustion and Flame (00102180) 138(1-2)pp. 30-39
Three-dimensional imaging of minor combustion species is of great interest for combustion engineering, process control and environmental analysis. On the other hand, difference frequency generation (DFG) based continuous wave (cw) laser sources offer a compact and low-cost alternative light source for high resolution spectroscopy. In this work, difference frequency generation (DFG) laser light sources are used in the mid infrared (MIR) for the optical tomography of combustion products, such as carbon monoxide, in laminar CH 4/air flames on a flat flame burner. The combination of DFG MIR spectroscopy and computerized tomography is shown to offer great potential for laminar flames, as well as environmental monitoring. © 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
Optics InfoBase Conference Papers (21622701)
The P(28) line is used to detect carbon monoxide directly in the atmosphere of a glass furnace. A concentration of 300 ppm is found, in good agreement with conventional measurements in the exhaust gas stream. © 2003 Optical Society of America.
Khorsandi, A.R. ,
Kostjucenko, I. ,
Geiser, P. ,
Willer, U. ,
Schade w., W. ,
Wondraczek, L. pp. 510-510
The P(26) rotational line of carbon monoxide is traced with a midinfrared laser in a methane/air combustion. 3-dimensional tomographic images of the relative carbon monoxide concentrations in the flame are computerized from the absorption data. © 2003 IEEE.
Applied Physics B: Lasers and Optics (09462171) 77(5)pp. 509-513
A compact mid-infrared (MIR) laser system is described applying two single mode diode-lasers as pump and signal sources to generate difference-frequency radiation in AgGaS2. A spectral tuning range of 71.5 cm-1 between 4.9-5.1 μm with an output power of 100 nW and linewidth of 1.2 × 10-2 cm-1 (3.6 GHz) for the DFG laser system is obtained by mode behavior improvement of the pump source using an external short-cavity. The performance of such an external short-cavity MIR-DFG laser spectrometer is demonstrated by recording the fundamental absorption spectra of carbon monoxide (12C16O) and nitric oxide (14N16O) in a 10 cm long cell in order to estimate line-broadening coefficients of CO and NO molecules, which are of general interest in combustion diagnostics.
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