Publications

Fiber Bragg gratings (FBGs) have found numerous applications in fiber lasers, sensors, telecommunication, and many other fields. Traditionally, they are fabricated using UV laser sources and a phase mask or other interferometric techniques. In the past two decades, FBGs have been fabricated with femtosecond lasers in either the point-by-point method or by using a phase mask, in a similar configuration as with UV laser sources. In the following, we briefly review the advantages of femtosecond fabrication of fiber Bragg gratings. We then focus on transient FBGs; these are FBGs that exist for a short duration only, for the purpose of all-optical, in-fiber switching and modulation and the possible mechanism to implement them with a high-power femtosecond laser. The theory behind transient grating switching is outlined, and we discuss related experimental results achieved by our group on both permanent grating inscription and the generation of transient (dynamic) fiber Braggs gratings.

We investigate configurations for upscaling the number of laser distributions that can be coherently added by means of intra-cavity
interferometric combiners. Experimental demonstrations of coherent addition of 5 and 25 laser distributions are presented. Calculated
results indicate that with our configurations upscaling to a large number of coherently added distributions is possible but strongly
depends on the tolerances of the interferometric combiners.

Compelling electromagnetic (EM) properties, such as tight EM-confinement and fabrication-compatible architectures, propel the importance of metal-based nanophotonics in small-footprint devices, such as highly efficient optical absorbers. Here, we focus on boosting the absorption of optically thick bulk Aluminum (Al), the most abundant metal, solely by corrugating its surface periodically. Application-oriented, we utilize the simplest one-dimensional (1D) rectangular-grating pattern that can be remotely engraved on a large scale, e.g., using femtosecond pulsed lasers. We devise four designs for various utilizations optimized for perfect absorption around the application-abundant  wavelength under transverse electric (TE), transverse magnetic (TM), and unpolarized (denoted TEM) irradiations. Lacking attractive plasmonic confinement properties, the s-polarization is considered electromagnetically “degenerate” to the p-polarization case, making it rarely explored in the literature of 1D-grating architectures. Nevertheless, we reveal that perfect s-polarized absorption is also attainable in such designs without adding extra layers. The designs’ performance, properties, limitations, and the propelling electromagnetic physics are theoretically and numerically discussed in-depth. A large-area sample of the TEM-optimized design was fabricated, facilitating uniquely extensive optical and radiative heating experimental investigation with realistic near-to-normal Gaussian-beam incidence. The absorption enhancement derived from wideband and single-frequency optical and radiative heating measurements agree well, also with the numerical simulations. Despite fabrication-derived thickness deviation, the design experimentally showed a 9.6-(TE), 6-(TM), and 7.8-(TEM) fold absorption enhancement over bulk Al. Considering the one-dimensional-patterning simplicity and its large-area fabrication compatibility, it is noteworthy and pioneering, particularly for the challenging s-polarization case. We also study a polarization-sensitive diffused reflection scattering pattern, evidently postulated to result from resonated surface roughness, highlighting the importance of radiative heating measurements. The elaborated original findings and methods are highly applicable alongside the prevalent material and spectrum.

A five fiber Bragg grating (FBG) array is inscribed at the same spot with a single uniform phase-mask (PM). The inscription setup consists of a near-infrared femtosecond laser, a PM, a defocusing spherical lens and a cylindrical focusing lens. The tunability of the center Bragg wavelength is achieved by a defocusing lens, and by translating the PM, which results in a different magnification of the PM. A first FBG is inscribed, followed by four cascading FBGs, which are inscribed exactly at the same spot only after the translation of the PM. The transmission and reflection spectra of this array are measured, showing a second-order Bragg wavelength at ~1.56 µm with a transmission dip of ~−8 dB. The spectral wavelength shift between each consecutive FBG is ~2.9 nm, and the total wavelength shift is ~11.7 nm. The reflection spectrum of the third-order Bragg wavelength is measured at ~1.04 µm, showing a wavelength separation of ~1.97 nm between neighboring FBGs, and the total spectral span between the first FBG and the last one is ~8 nm. Finally, the wavelength sensitivity to strain and temperature is measured.

Fiber Bragg gratings (FBGs) have found numerous applications in fiber lasers, sensors, telecommunication, and many other fields. Traditionally, they are fabricated using UV laser sources and a phase mask or other interferometric techniques. In the past two decades, FBGs have been fabricated with femtosecond lasers in either the point-by-point method or by using a phase mask, in a similar configuration as with UV laser sources. In the following, we briefly review the advantages of femtosecond fabrication of fiber Bragg gratings. We then focus on transient FBGs; these are FBGs that exist for a short duration only, for the purpose of all-optical, in-fiber switching and modulation and the possible mechanism to implement them with a high-power femtosecond laser. The theory behind transient grating switching is outlined, and we discuss related experimental results achieved by our group on both permanent grating inscription and the generation of transient (dynamic) fiber Braggs gratings.

Plasmonic designs for mid-infrared extraordinary optical transmission (EOT), a direct route to tailored filtering with broad-band out-of-band rejection, have long been constrained by a fundamental trade-off between high transmission efficiency and narrow linewidths, a challenge rooted in the material properties of noble metals. Here, we theoretically propose and numerically demonstrate a versatile design paradigm that resolves this challenge by functionally decoupling the tasks of light coupling and resonant filtering. Our approach uses a dual-stacked noble metal-dielectric grating architecture to surpass the intrinsic limitations of single-layer structures. This paradigm provides the flexibility to engineer devices for ultra-high spectral selectivity and transmission efficiency. We demonstrate this with distinct designs: one at 10 µm with a quality factor (Q-factor) > 2000 and > 91% transmission; a high-Q design at 4 µm and > 80% transmission; and a high-efficiency design at 4 µm with > 92% transmission over a uniquely broad spectral-angular range. These generic designs produce solitary, narrow EOT peaks originating from a “triple-coupling” mechanism that mitigates reflection and absorption losses, with symmetry-broken configurations capable of exceeding Q-factors of 16,000 while maintaining a peak transmission efficiency > 60%. Crucially, these compact two-layer designs exhibit exceptional robustness against fabrication variations, offering a broadly applicable route to ultra-compact, low-cost infrared components, enabling advanced architectures such as angular sensing, spectro-polarimetric imaging, and isotope-resolved gas diagnostics.

Compelling electromagnetic (EM) properties, such as tight EM-confinement and fabrication-compatible architectures, propel the importance of metal-based nanophotonics in small-footprint devices, such as highly efficient optical absorbers. Here, we focus on boosting the absorption of optically thick bulk Aluminum (Al), the most abundant metal, solely by corrugating its surface periodically. Application-oriented, we utilize the simplest one-dimensional (1D) rectangular-grating pattern that can be remotely engraved on a large scale, e.g., using femtosecond pulsed lasers. We devise four designs for various utilizations optimized for perfect absorption around the application-abundant  wavelength under transverse electric (TE), transverse magnetic (TM), and unpolarized (denoted TEM) irradiations. Lacking attractive plasmonic confinement properties, the s-polarization is considered electromagnetically “degenerate” to the p-polarization case, making it rarely explored in the literature of 1D-grating architectures. Nevertheless, we reveal that perfect s-polarized absorption is also attainable in such designs without adding extra layers. The designs’ performance, properties, limitations, and the propelling electromagnetic physics are theoretically and numerically discussed in-depth. A large-area sample of the TEM-optimized design was fabricated, facilitating uniquely extensive optical and radiative heating experimental investigation with realistic near-to-normal Gaussian-beam incidence. The absorption enhancement derived from wideband and single-frequency optical and radiative heating measurements agree well, also with the numerical simulations. Despite fabrication-derived thickness deviation, the design experimentally showed a 9.6-(TE), 6-(TM), and 7.8-(TEM) fold absorption enhancement over bulk Al. Considering the one-dimensional-patterning simplicity and its large-area fabrication compatibility, it is noteworthy and pioneering, particularly for the challenging s-polarization case. We also study a polarization-sensitive diffused reflection scattering pattern, evidently postulated to result from resonated surface roughness, highlighting the importance of radiative heating measurements. The elaborated original findings and methods are highly applicable alongside the prevalent material and spectrum.

Stimulated Brillouin scattering (SBS) typically limits power transmission in optical fibers and the power scalability of fiber lasers and amplifiers, particularly in applications requiring narrow-linewidth operation. In this study, we experimentally characterize SBS suppression through time modulation and multiwavelength operation when transmitting laser beams in 1.5-µm range through optical fibers. A continuous-wave (CW) narrow line-width input seeder for a Master Oscillator Power Amplifier (MOPA) system is replaced by several seeders with different wavelengths. These seeders are then modulated synchronously at high-repetition rates (∼10 MHz; ∼50 ns pulses), in a complementary manner – when one is active, the other is inactive. This hybrid method effectively mitigates SBS gain while maintaining a pseudo-continuous-wave (pseudo-CW) operation with multiple relatively narrow wavelengths. Experimental results indicate the conditions under which this mitigation occurs. For pulse widths exceeding 10 ns and inter-pulse delays greater than 50 ns we demonstrate an increase of at least two orders of magnitude in the SBS threshold compared to the single CW seed source operation with a band width of 1.7 MHz. To the best of our knowledge, this is the first experimental observation of such a significant SBS suppression under these conditions. Our findings present a viable pathway for SBS-free, high-power transmission in optical fibers, with potential applications in high-power fiber laser systems, fiber remote charging, sources for free space telecommunications, material processing, and defense.

Plasmonic designs for mid-infrared extraordinary optical transmission (EOT), a direct route to tailored filtering with broad-band out-of-band rejection, have long been constrained by a fundamental trade-off between high transmission efficiency and narrow linewidths, a challenge rooted in the material properties of noble metals. Here, we theoretically propose and numerically demonstrate a versatile design paradigm that resolves this challenge by functionally decoupling the tasks of light coupling and resonant filtering. Our approach uses a dual-stacked noble metal-dielectric grating architecture to surpass the intrinsic limitations of single-layer structures. This paradigm provides the flexibility to engineer devices for ultra-high spectral selectivity and transmission efficiency. We demonstrate this with distinct designs: one at 10 µm with a quality factor (Q-factor) > 2000 and > 91% transmission; a high-Q design at 4 µm and > 80% transmission; and a high-efficiency design at 4 µm with > 92% transmission over a uniquely broad spectral-angular range. These generic designs produce solitary, narrow EOT peaks originating from a “triple-coupling” mechanism that mitigates reflection and absorption losses, with symmetry-broken configurations capable of exceeding Q-factors of 16,000 while maintaining a peak transmission efficiency > 60%. Crucially, these compact two-layer designs exhibit exceptional robustness against fabrication variations, offering a broadly applicable route to ultra-compact, low-cost infrared components, enabling advanced architectures such as angular sensing, spectro-polarimetric imaging, and isotope-resolved gas diagnostics.

We investigate configurations for upscaling the number of laser distributions that can be coherently added by means of intra-cavity
interferometric combiners. Experimental demonstrations of coherent addition of 5 and 25 laser distributions are presented. Calculated
results indicate that with our configurations upscaling to a large number of coherently added distributions is possible but strongly
depends on the tolerances of the interferometric combiners.

Stimulated Brillouin scattering (SBS) typically limits power transmission in optical fibers and the power scalability of fiber lasers and amplifiers, particularly in applications requiring narrow-linewidth operation. In this study, we experimentally characterize SBS suppression through time modulation and multiwavelength operation when transmitting laser beams in 1.5-µm range through optical fibers. A continuous-wave (CW) narrow line-width input seeder for a Master Oscillator Power Amplifier (MOPA) system is replaced by several seeders with different wavelengths. These seeders are then modulated synchronously at high-repetition rates (∼10 MHz; ∼50 ns pulses), in a complementary manner – when one is active, the other is inactive. This hybrid method effectively mitigates SBS gain while maintaining a pseudo-continuous-wave (pseudo-CW) operation with multiple relatively narrow wavelengths. Experimental results indicate the conditions under which this mitigation occurs. For pulse widths exceeding 10 ns and inter-pulse delays greater than 50 ns we demonstrate an increase of at least two orders of magnitude in the SBS threshold compared to the single CW seed source operation with a band width of 1.7 MHz. To the best of our knowledge, this is the first experimental observation of such a significant SBS suppression under these conditions. Our findings present a viable pathway for SBS-free, high-power transmission in optical fibers, with potential applications in high-power fiber laser systems, fiber remote charging, sources for free space telecommunications, material processing, and defense.

Chirped fiber Bragg gratings (CFBGs) and arrays of fiber Bragg gratings (FBGs) are inscribed with femtosecond pulses and a single uniform phase-mask (PM). The inscription setup includes an 800 nm NIR femtosecond laser, a PM, a defocusing spherical lens and a cylindrical focusing lens. The wavelength tunability is achieved by a defocusing lens, and movement of the PM. A first FBG is inscribed, followed by nine cascading FBGs, where each one is inscribed only after the movement of the PM and a movement of the fiber, thus allowing inscription on a fresh fiber section without overlapping. The transmission and reflection spectra of these ten cascaded FBGs are measured showing a spectral shift of ∼0.9 nm between each one, and a total shift of ∼8 nm. The transmission dip of each FBG is roughly −10 dB. The CFBG is inscribed with the same single uniform PM and with the same setup, but with different movement increments of the PM and fiber. The transmission and reflection spectra of the resulting CFBG are measured with a strength of ∼20 dB and a continuous bandwidth of ∼4.7 nm at FWHM. This yields a chirp rate of ∼1 nm/cm, while the full length of the CFBG is ∼45 mm.