A dispersive Raman spectrometer was used with three different excitation sources (Argon-ion, He-Ne, and Diode lasers operating at 514.5 nm, 633 nm, and 782 nm, resp.). The system was employed to a variety of Raman active compounds. Many of the compounds exhibit very strong fluorescence while being excited with a laser emitting at UV-VIS region, hereby imposing severe limitation to the detection efficiency of the particular Raman system. The Raman system with variable excitation laser sources provided us with a desired flexibility toward the suppression of unwanted fluorescence signal. With this Raman system, we could detect and specify the different vibrational modes of various hazardous organic compounds and some typical dyes (both fluorescent and nonfluorescent). We then compared those results with the ones reported in literature and found the deviation within the range of ±2 cm−1, which indicates reasonable accuracy and usability of the Raman system. Then, the surface enhancement technique of Raman spectrum was employed to the present system. To this end, we used chemically prepared colloidal suspension of silver nanoparticles as substrate and Rhodamine 6G as probe. We could observe significant enhancement of Raman signal from Rhodamine 6G using the colloidal solution of silver nanoparticles the average magnitude of which is estimated to be 103.

Ultrahigh-resolution fiber-optic sensing has found a wide range of potential applications. However, the techniques reported so far are all based on highly specialized fiber structures and interrogation lasers, which are not widely available. In this paper, we report the demonstration of ultrahigh strain resolutions using only off-the-shelf commercial components. Our method leverages the high wavelength discrimination of long, high-finesse fiber Fabry-Perot interferometers (FFPI), using two 1 m-long FFPIs, one as the sensor and the other as a frequency reference. By locking the interrogation laser to the reference interferometer, which is co-packaged with the sensor interferometer, large, environmentinduced sensing background is removed. This allows the laser to reliably probe the strains applied on the sensor with very high resolutions. A nominal, noise-limited strain resolution of 800 fε/ÝHz has been achieved within 1-100 Hz. Strain resolution further improves to 75 fε/ÝHz at 1 kHz, 60 fε/ÝHz at 2 kHz and 40 fε/ÝHz at 23 kHz, demonstrating better resolutions than proven techniques such as π-phase-shifted and slow-light fiber Bragg gratings. The work lays out a cost-effective scheme to achieve ultrahigh-resolution fiber-optic sensing.

High-resolution strain sensing based on long, high-finesse fiber Fabry–Perot interferometers (FFPIs) has been demonstrated with a special focus on the infrasonic frequency range. A novel dual-FFPI scheme allows the large environment-induced background at low frequencies to be suppressed, permitting high strain resolution limited only by excess electronic noise. Noise-equivalent strain resolution of 257 p ε / √ H z has been achieved at 6 mHz, and the resolution improves to ∼ 200 f ε / √ H z between 4–20 Hz. Without the use of any additional optical frequency references and with only off-the shelf commercial components, these resolutions are much better than most in the prior reports. Especially, an improvement of a factor of 1.8 is achieved in comparison with the highest resolution reported so far near 5 Hz. The limiting factors of the current scheme have been analyzed in detail, and the application prospects have been demonstrated using an acoustic transducer. The work lays out the potential of using long FFPIs with high finesse for high-resolution fiber-optic sensing in the infrasonic frequency range.

A new type of interferometric fiber sensor based on a Mach-Zehnder Fabry-Perot hybrid scheme has been experimentally demonstrated. The interferometer combines the benefits of both a double-path configuration and an optical resonator, leading to record-high strain and phase resolutions limited only by the intrinsic thermal noise in optical fibers across a broad frequency range. Using only off-the-shelf components, the sensor is able to achieve noise-limited strain resolutions of 40 f$$\varepsilon $$ ε /$$\sqrt{(}Hz)$$ ( H z ) at 10 Hz and 1 f$$\varepsilon $$ ε /$$\sqrt{(}Hz)$$ ( H z ) at 100 kHz. With a proper scale-up, atto-strain resolutions are believed to be within reach in the ultrasonic frequency range with such interferometers.

Lifetimes of the upper states of excited oxygen atoms corresponding to transitions to the common lower state in the case of the emission lines around 777 nm and 844 nm have been estimated by measuring the line intensities as a function of the delay time between the Q-switching of the laser and the opening of the window of the ICCD. In the case of the emission line around 777 nm, resulting from the transitions from the three very closely spaced upper energy levels, 2s2 2p3 (4S0) 3p5P3 (777.194 nm), 2s2 2p3 (4S0) 3p5P2 (777.417 nm) and 2s2 2p3 (4S0) 3p5P1 (777.539 nm) to the common lower energy level 2s2 2p3 (4S0) 3s5S2, the lifetime was measured to be 253 ns. Similarly, in the case of the emission-line around 844 nm, resulting from the transitions from the three very closely spaced upper energy levels 2s2 2p3 (4S0) 3p3P0 (844.625 nm), 2s2 2p3 (4S0) 3p3P2 (844.636 nm) and 2s2 2p3 (4S0) 3p3P1 (844.676 nm) to the common lower energy level 2p3 (4S0) 3s3S1, the lifetime was measured to be 278 ns by this technique. The measured lifetimes in our experiment, for both 777 nm and 844 nm atomic transition lines of oxygen (O), are almost nine times higher than the theoretical value. Self-absorption and radiation trapping are possible mechanisms responsible for the mismatch between measured and intrinsic lifetimes. Last but not least, utilizing the TR-LIBS technique is yet another incredible application to estimate the prolonged lifetime of closely spaced excited states of an atom in the presence of self-absorption and radiation trapping. The average plasma cooling temperature (excitation temperature) lifetime was found to be 1183 ns, which is compatible with the previously reported values. J. Bangladesh Acad. Sci. 46(2); 175-183: December 2022