Gain Linewidth Research Articles

Sodium zinc phosphate glasses doped with Tb3+ and Tb3+/Eu3+ as multicolor phosphor for green laser stimulated emission and WLED applications

Sodium zinc phosphate glasses doped with Tb3+ and Tb3+/Eu3+ were synthesized by the melt quenching method. Spectroscopic properties of the these glasses were analyzed to investigate their suitability as multicolor phosphor for green laser stimulated emission and WLED applications. In the 1.0 mol% Tb2O3 doped sodium zinc phosphate glass, the 5D4→7F5 green transition exhibits gain linewidth and optical gain values larger than those reported in other phosphate glasses doped with the same terbium content. The tonality of emitting light from the glass singly doped with terbium shifts from green to bluish-white and greenish-white as Tb2O3 content decreases from 2.0–2.5 mol% to 0.5–1.0 mol% under excitation at 350 nm. A 574 nm yellow tonality, close to warm-white one of 3564 K, is attained in the co-doped glass, wherein the europium emission is sensitized through the terbium excitation at 340 nm. The co-doped glass can also emit 591 nm orange light and 609 nm reddish-orange light, with color purities above 93 %, under Tb3+/Eu3+ (377 nm) and Eu3+ (393 nm) excitations, respectively.

Mid-infrared dual-wavelength power regulation and linewidth narrowing using an F-P etalon in a multi-optical parametric oscillator based on MgO: APLN

The multi-optical parametric oscillator based on MgO: APLN has the problems of unbalanced gain and wide linewidth. Because the F-P etalon can not only narrow the linewidth, but also adjust the transmittance of different wavelengths. The narrow linewidth multi-optical parametric oscillator based on MgO: APLN by using an F-P etalon in the cavity is proposed. For adding an F-P etalon, the linewidth of 3.30 μm and 3.84 μm is narrowed from 2.98 nm and 3.15 nm to 0.81 nm and 1.07 nm. By changing the tilt angle of the F-P etalon, the output power differential between 3.84 µm and 3.30 µm is reduced from 1.325 W to 0.226 W. When the tilt angle is 1.84°, the balanced output power with 1.326 W at 3.30 µm and 1.552 W at 3.84 µm is achieved. The corresponding conversion efficiency is 3.02% and 3.54%, and the pulse width is 12.4 ns and 11.8 ns, respectively. The experimental result shows that power regulation and linewidth narrowing can be realized by using an F-P etalon.

Intrinsic cascade-free intramode scattering Brillouin laser

The cascade operation of Brillouin lasers (BLs) is an identified obstacle to single-frequency power scaling and further compression of the fundamental linewidth. In this study, we reveal the relationship between the maximum cascade order and system parameters, starting from the phase-matching conditions of the Stokes cascade. The second Stokes is suppressed for modes that fall away the Brillouin gain linewidth (ΓB), which is heightened for Brillouin gain media with high sound velocity, large refractive index, and narrow linewidth. Diamond, with its extremely high product of speed of sound and refractive index, satisfies these requirements and is found to achieve cascade-free intramode scattering (TEM00) without manipulating cavity mode structures. This study elucidates a route to single-frequency, narrow-linewidth BLs via Brillouin material selection.

Green emission characteristics of Er3+ -doped TeO2-WO3-GeO2 glasses through up and down conversion processes

Different concentrations of Er3+ions doped TeO2-WO3-GeO2 transparent oxyfluoride glassy materials were fabricated by conventional melt quench method and analyzed through X-ray diffraction, Fourier Transform infrared, scanning electron microscope and energy dispersive X-ray, photoluminescence-excitation, visible emission and luminescence decay studies. Various physical, structural, spectroscopic and laser characteristic parameters were evaluated following the Judd-Ofelt theory. The studied glasses were capable of emitting intense green luminescence through (2H11/2,4S3/2) → 4I15/2 transitions. The emission of green luminescence was studied through down-conversion process at 377 nm excitation and up-conversion process at 980 nm excitation. The color coordinates were found to fall in the green region of chromaticity diagram and close to the European Broadcosting Union green illuminant. The evaluated branching ratio, stimulated emission cross-section, gain linewidth, figure of merit and the quantum efficiency corresponding to 4S3/2 → 4I15/2 transition suggests that the studied glasses find wide suitability to design green emitting solid state lighting devices.

Stokes linewidth narrowing by stimulated Brillouin scattering in liquid media

As an effective means to obtain a narrow-linewidth laser, stimulated Brillouin scattering (SBS) has not only the advantages of pulse compression but also controllable Stokes linewidth output. However, most research thus far has been focused on continuous-wave lasers, with little emphasis on short-pulse lasers. This work demonstrates that the Brillouin gain linewidth and pump power density are the primary factors affecting the linewidth of the Stokes pulse. As the pump power density increases, the Stokes linewidth tends to narrow and approaches the pump linewidth. This is the first study to reveal that the pump linewidth is the limiting factor in narrowing the Stokes linewidth. The Stokes linewidths of different liquid media were compared, and it was found that media with a wide Brillouin gain linewidth can be used to obtain lasers with a wider range of linewidths.

Compact 15 mJ Fiber–Solid Hybrid Hundred-Picosecond Laser Source for Laser Ablation on Copper

We report on a millijoule-level fiber–solid hybrid hundred-picosecond laser system with a stable performance and compact structure. The laser system is based on a master oscillator power amplifier structure containing an all-fiber master oscillator, a quasi-continuous-wave side-pumped Nd:YAG regenerative amplifier, and a double-pass amplifier. By using the filtering effect of fiber Bragg grating and the dispersion characteristics of single-mode fiber stretcher, the spectrum broadening caused by self-phase modulation effect is effectively suppressed. Thus, the gain linewidth of the Yb-doped fiber seed source and Nd:YAG laser amplifiers is accurately matched. The reason for thermally induced depolarization in the solid-state laser amplifier is theoretically analyzed, and a more flexible depolarization compensation structure is adopted in amplifier experiment. Furthermore, the pulse energy of 14.58 mJ and pulse width of 228 ps is achieved at 500 Hz repetition rate. The central wavelength is 1064.1 nm with a 3 dB bandwidth of 0.47 nm. The beam quality factors in the horizontal and vertical directions are 1.49 and 1.51, respectively. This laser system has a simple and compact structure and has a power stability of 1.9%. The high pulse energy and beam quality of this hundred-picosecond laser are confirmed by latter theoretical simulation of copper laser ablation. It is a very practical laser system for material processing and laser-induced damage.

Synthesis and characterization of Sm3+ doped BaO-ZnO-LiF-B2O3 glass system for reddish-orange light generation with high color purity

Sm3+ containing BaO-ZnO-LiF-B2O3 glass system by a melt-quenching procedure was synthesized and the structural, thermal, dielectric, optical, and radiative properties were studied. The creation of Non-Bridging Oxygens (NBOs) with Sm3+ doping was manifested from Fourier Transform Infrared (FTIR) spectroscopy by the decrement of the relative area of component peaks associated with the [BO4] units and simultaneous increment of the relative area linked with the [BO3] units. The glass transition temperature was identified by Differential Scanning Calorimetry (DSC). The dielectric properties were analyzed with varying frequency and temperature through dielectric constant, loss tangent, conductivity, and capacitance. The optical transition properties were studied in the framework of Judd-Ofelt (JO) theory. JO parameters were confirmed, and furthermore, the optical transition rates and fluorescence branching ratios were also derived. With 401 nm excitation, the emission spectra of the glass system exhibited emissions of the Sm3+ ion corresponding to transitions 4G5/2→6H5/2, 6H7/2,6H9/2,6H11/2 at 563, 601, 647, and 708 nm, respectively. The higher emission rate, branching ratio, stimulated emission cross-section, gain linewidth, and figure of merit for the transition 4G5/2→6H7/2 calculated by Fuchtbauer-Ladenburg theory indicated potential of the titled glass system for reddish-orange solid-state lasers. The investigated glass system also exhibited high quantum efficiency and color purity (99%) for emission device application.

Slope-Assisted Brillouin Optical Wavemeter

We introduce a slope-assisted Brillouin optical wavemeter. Existing Brillouin-based spectrum analyzers measure the Brillouin gain as the frequency of a pump laser is swept across the frequency band of interest. However, this process is slow since only one spectral channel is measured at a time and the spectral resolution is typically limited to the Brillouin gain linewidth. In this work, we introduce a technique that uses both the Brillouin gain and phase to construct a single-shot wavemeter with frequency resolution orders of magnitude below the Brillouin linewidth. We also show that combining information from the Stokes and anti-Stokes interactions allows us to reduce the sensitivity to environmental fluctuations such as temperature, enabling a robust system with frequency noise of 20 Hz/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\surd $ </tex-math></inline-formula> Hz. This noise level enables a measurement of a 200 Hz frequency shift with a 10 ms measurement.

Nanosecond pulsed single longitudinal mode diamond Raman laser in the 1.6 µm spectral region.

We demonstrate the first nanosecond pulsed single longitudinal mode (SLM) intracavity-pumped diamond Raman laser, to the best of our knowledge. The eye-safe coherent source at 1634 nm, which was converted from the actively Q-switched 1342 nm Nd:YVO4 laser, yielded 4.35 W of multimode average output power with a pulse duration of 6 ns and peak power of 29 kW. By exploiting the spatial hole burning free gain mechanism in the Raman media, stable SLM operation was observed at low output power (0.46 W) for the free-running case. Furthermore, by incorporating an etalon in the fundamental standing-wave cavity, the spectral linewidth of the fundamental field was suppressed substantially below the diamond Raman gain linewidth and slightly less than the free spectral range of the mm-scale Raman resonator. Thereby, a much higher SLM output power of 1.63 W was obtained with a pulse duration of ∼9 ns and a spectral linewidth of ∼77 MHz.

Analytical theory of frequency-modulated combs: generalized mean-field theory, complex cavities, and harmonic states.

Frequency-modulated (FM) combs with a linearly-chirped frequency and nearly constant intensity occur naturally in certain laser systems; they can be most succinctly described by a nonlinear Schrödinger equation with a phase potential. In this work, we perform a comprehensive analytical study of FM combs in order to calculate their salient properties. We develop a general procedure that allows mean-field theories to be constructed for arbitrary sets of master equations, and as an example consider the case of reflective defects. We derive an expression for the FM chirp of arbitrary Fabry-Perot cavities-important for most realistic lasers-and use perturbation theory to show how they are affected by finite gain bandwidth and linewidth enhancment in fast gain media. Lastly, we show that an eigenvalue formulation of the laser's dynamics can be useful for characterizing all of the stable states of the laser: the fundamental comb, the continuous-wave solution, and the harmonic states.

Coherent terahertz Smith–Purcell free-electron laser with possible quantum effects

A quantum approach is proposed to describe the Smith–Purcell free-electron laser (SP–FEL) where the interacting electron is described quantum mechanically as a wave packet with a finite width. A quantum regime for the SP–FEL operation is newly defined when the electron wave packet width is larger or comparable to the interaction length. This quantum regime can be realized in SP–FELs in periodic nanostructures enabled by current nanofabrication advances. In this quantum regime, the gain linewidth is determined by the longitudinal size of the electron wave packet. The latter result can be used in measuring the spreading length of a free-electron quantum wave packet. It is found that as the width of the electron wave packet increases, the coherence of the emitted radiation is enhanced. Finally, whether in the classical or quantum regime, an analytical expression for the gain in a SP–FEL is derived. Then we propose an efficient scheme for a SP–FEL amplifier to generate terahertz radiation.

Monolithic Integrated Semiconductor Optical Amplifier With Broad Spectrum, High Power, and Small Linewidth Expansion

Semiconductor optical amplifiers (SOAs) offer direct electrical injection, power consumption, integration, and anti-radiation advantages over optical fiber amplifiers. However, saturation output power and gain bandwidth have been limited in traditional structure SOAs. We demonstrate a monolithic integrated SOA with broad spectrum, high power, high gain, and small spectral linewidth expansion. The device adopts a two-stage amplified large optical cavity structure, and a lower optical field confinement factor is obtained by adjusting the thickness of the waveguide layer. The lower optical field confinement factor is conducive to improving the coupling efficiency and the maximum output power. Our device, fabricated only by standard i-line lithography with micron-scale precision, obtains excellent and stable performance. When the input power is set to 1 mW, the output power is 419 mW with a gain of 26.23 dB. When the input power is set to 25 mW at 25 °C, the output power increases to 600 mW with a gain of 13.8 dB. The corresponding gain bandwidth of 3 dB measures at least 70 nm. The spectral linewidth after the SOA is 1.15 times wider than that of the seed laser.

Interaction of cavity solitons on an unstable background.

The interaction of two cavity solitons in a driven semiconductor laser above lasing threshold is investigated. We focus on the case in which the background field of the solitons is turbulent because the laser is below the injection locking point. We show that the solitons move spontaneously and either reach some equilibrium distance or merge. Different behaviors are found depending on how far from the injection locking point the laser is. The laser is modeled by a set of effective Maxwell-Bloch equations which include an equation for the macroscopic polarization that mimics the complex susceptibility of the semiconductor. In that way we avoid the emergence of an unphysical behavior of the background which instead appears when the polarization is adiabatically eliminated, which amounts to assuming infinite gain linewidth. The simulations are slow because the time scales of the different dynamical variables differ by four orders of magnitude. Yet, we show that the results of the complete set of equations can be accurately reproduced with a reduced set of equations where the polarization is adiabatically eliminated but a diffusion term is included in Maxwell equation, which accounts for the finiteness of the gain linewidth.

Influence of Integrated Optical Feedback on Tunable Lasers

In this paper we explain how to use rate equations to describe a laser that includes integrated optical feedback. We find a relation between the threshold current, the voltage drop at the gain section, output power, linewidth, and side mode suppression ratio, and show experimental results.

Narrowing Spectral Linewidth in Brillouin Optical Time Domain Analyzers With Adjustable Spatial Resolution

In order to both adjust the spatial resolution (SR) and narrow the Brillouin gain spectrum (BGS) linewidth in Brillouin optical time domain analysis system, a novel scheme based on differential pulse pair technique is proposed. By designing the phase and amplitude of the pulse properly, the higher peak gain and narrower linewidth of BGS are achieved compared to the conventional single pulse scheme with same SR. The pulse parameters for different SR are analytically solved and the performances of the sensor are theoretically and experimentally studied. Experimental results show that the linewidth of BGS is narrowed to 19 and 22 MHz in the case of 2 m SR and 1 m SR, which indicates 2.3 times and 4.3 times frequency accuracy improvements are achieved, respectively. Furthermore, the advantages of our proposal for small Brillouin frequency shift change are also investigated.

Bandwidth-reconfigurable single-passband microwave photonic filter based on stimulated Brillouin scattering

A bandwidth-reconfigurable filter is necessary to meet the need of growing communication capacity and signal coverage frequency range. The stimulated Brillouin scattering (SBS) gain spectrum as a filter passband is used to extract desired microwave signals. Due to the fact that the passband is formed by mapping the Brillouin gain spectrum, bandwidth reconfigurability can be implemented by changing Brillouin gain linewidth. In this paper, a scheme using a 5-lines optical comb acting as a Brillouin pump is experimentally demonstrated. The spectrum of the 5-lines optical comb is extended by a binary phase shift keying (BPSK) pulse modulated signal. Experiments have shown that the 3-dB and 20-dB bandwidths of the filter are 196 MHz and 257 MHz, respectively, and its 20-dB shape factor is 1.31. The passband ripple is ~3.5 dBm with stop-band rejection of 20 dBm under 15 dBm optical pump power.

Optimization of Phase Modulation Formats for Suppression of Stimulated Brillouin Scattering in Optical Fibers

We theoretically investigate nonlinear optimization of periodic phase modulation for suppression of stimulated Brillouin scattering (SBS) in single-mode optical fibers. We use nonlinear multiparameter Pareto optimization to find modulations that represent the best tradeoff between SBS and optical linewidth, as measured by its rms value. The optimization uses a temporal-amplitude-domain finite-difference Brillouin solver with noise initiation to find the best phase modulation patterns in the presence of coherent so-called cross-interactions. These can be important in short fibers, when the period is large enough to make the frequency-domain separation of the modulated signal comparable to, or smaller than, the Brillouin gain linewidth. We calculate the SBS threshold for the optimized modulation patterns and find that smaller spectral line spacing improves the SBS threshold for the same linewidth. By contrast, whereas the maximum modulation depth and modulation frequency influence the range of accessible linewidths, they do not significantly alter the threshold for a given linewidth. We investigate the dependence on fiber length and find that while shorter fibers have a higher threshold, the increase is smaller than the often-assumed inverse dependence on length. Furthermore, we find that optimized formats are superior in terms of SBS threshold as well as in terms of linewidth control, compared to random modulation.

Highly nonlinear yttrium-aluminosilicate optical fiber with a high intrinsic stimulated Brillouin scattering threshold.

Highly nonlinear (high-NA small-mode-area) optical fibers also possessing an intrinsically high stimulated Brillouin scattering threshold are described. More specifically, silica clad, yttrium-aluminosilicate core fibers are shown to exhibit an intrinsically low Brillouin gain coefficient between 0.125 and 0.139×10-11 m/W and a Brillouin gain linewidth of up to 500MHz. Losses on the order of 0.7dB/m were measured, resulting from impurities in the precursor materials. Nonlinear refractive index values are determined to be similar to that of silica, but significant measurement uncertainty is attributed to the need to estimate dispersion curves since their direct measurement could not be made. The interest for highly nonlinear optical fibers with a low intrinsic Brillouin gain coefficient is expected to continue, especially with the growing developments of narrow-linewidth high-energy laser systems.

Fibre laser system providing generation of frequency-modulated pulses with a spectral width exceeding the gain linewidth

We propose an improved scheme of an amplifier similariton laser with a spectral width of the output significantly exceeding the gain linewidth. In the system, an additional dispersion element is inserted into the cavity to provide a local increase in the peak pulse power. The proposed scheme allows a reduction of pulse duration and an increase in peak power of the output pulse after compression.

Effect of gain compression above and below threshold on the chirp characteristics of 1.55 µm distributed feedback laser

We have observed and quantified the adiabatic and transient chirp in a directly modulated laser. The wavelength excursion for both chirp terms is well characterized by the phase rate equation model that describes the chirp behavior. In this study, the effect of gain compression and linewidth enhancement factor, below and above threshold, on the chirp characteristics is investigated by simulation. We have observed the trade-off between the two chirp terms i.e., transient part of the chirp is reduced by the strong damping introduced by gain compression, while as the adiabatic part increases with gain compression. We have observed that above threshold the α-factor increases with bias current, which is attributed to the enhancement of gain compression coefficient. It is shown that the higher the maximum gain, the lower the effects of gain compression and lower the α-factor. Finally, the effects of gain compression on the transmission characteristics are investigated.