Narrow Spectral Linewidth Research Articles

MEMS-based filter integrating tunable Fabry–Perot cavity and grating

We present a novel Micro-Electro-Mechanical-Systems (MEMS) filter integrating a tunable Fabry–Perot cavity and a grating (FP-G). It fuses the functions of the Fabry–Perot (FP) cavity and grating into a single device so as to realize both narrow spectral linewidth given by FP cavity and wide spectral range given by grating. A MEMS FP-G filter with 200μm cavity length was designed and fabricated. It is based on a W-shaped-section micro-bridge structure by which the electrostatic actuator is independent on the FP cavity and a long cavity length can be configured. A grating etched in the back of a cavity mirror serves to spatially separate the transmitted spectral lines corresponding to different interference orders of FP cavity. Experimental results indicated that the MEMS FP-G filter obtained a broader operation spectral range compared with the traditional MEMS FP filter, and achieved 1.2 nm spectral linewidth, which could be narrower by improving the effective finesse of the FP cavity in the fabrication process. Compared with the simulation results, there was considerable room for improvement in device performance by the optimal technique.

Tunable high-power blue external cavity semiconductor laser

A commercially available high-power GaN-based blue laser diode has been operated in a simple Littrow-type external cavity (EC). Two kinds of EC configurations with the grating lines perpendicular (A configuration) and parallel (B configuration) to the p-n junction are evaluated. Good performance has been demonstrated for the EC laser with B configuration due to the better mode selection effect induced by the narrow feedback wavelength range from the grating. Under an injection current of 1100mA, the spectral linewidth is narrowed significantly down to ∼0.1nm from ∼1nm (the free-running width), with a good wavelength-locking behavior and a higher than 35dB-amplified spontaneous emission suppression ratio. Moreover, a tuning bandwidth of 3.6nm from 443.9nm to 447.5nm is realized with output power of 1.24W and EC coupling efficiency of 80% at the central wavelength. The grating-coupled blue EC laser with narrow spectral linewidth, flexible wavelength tunability, and high output power shows potential applications in atom cooling and trapping, high-resolution spectroscopy, second harmonic generation, and high-capacity holographic data storage.

785 nm grating-coupled external-cavity laser for shifted-excitation Raman difference spectroscopy

A 785 nm grating-coupled external-cavity (EC) laser was fabricated and applied as a light source for shifted-excitation Raman difference spectroscopy (SERDS) to extract Raman signal from the fluorescence background. A simple Littrow-type EC configuration with the grating lines parallel to the p–n junction was applied to narrow the linewidth of a commercially available high-power broad-area laser diode. The characteristics of the EC laser, including wavelength tuning range, output power, and emission spectra, were investigated in detail. A wavelength tuning range of more than 6 nm was realized at an injection current of 900 mA by simply changing the grating rotation angle. The output power after Raman probe coupling exceeded 40 mW and the spectral linewidth was narrowed significantly down to less than 0.1 nm from 0.9 nm (the free-running width). Then, five shifted emission lines were used for the Raman measurement with sesame oil as the target, whose Raman signal was overwhelmed in the fluorescence background. Due to the constraint relationship between the signal intensity and the spectral resolution, the best reproducibility of the Raman signal without fluorescence background was obtained when the excitation wavelength interval was fixed at around the linewidth of the Raman peak. In conclusion, the grating-coupled EC laser with flexible wavelength tunability, high output power, and narrow spectral linewidth was demonstrated to be an ideal light source for SERDS application.

High-resolution spectroscopy of laser ablation plumes using laser-induced fluorescence

We report laser-induced fluorescence spectroscopy (LIF) of laser-produced plasmas under varying nitrogen pressure levels up to atmospheric pressure. The plasmas were generated on a glass target containing minor amounts of U and Al using 1064 nm, 6 ns pulses from a Nd:YAG laser. A frequency-doubled continuous-wave Ti:Sapphire laser was used as an ultra-narrowband tunable LIF excitation source to increase the magnitude and persistence of emission from selected U and Al atomic transitions in a laser-produced plasma. 2D-fluorescence spectroscopy (2D-FS) absorption/emission images were recorded at various nitrogen pressure levels, showing both excitation and emission spectral features. At lower pressure levels (⪝100 Torr), fluorescence emission was found to be well separated in time from thermally-excited emission. However, as the ambient pressure increased, the thermally-excited emission persisted for longer times along with a reduction of LIF emission persistence and intensity. The excitation spectral features showed the inherent linewidths of various transitions in the plasma, which have significantly narrower spectral linewidths than observed in emission spectra. We evaluated two nearby transitions separated by only 18 pm to demonstrate the effectiveness of fluorescence spectra over thermally-excited spectra for high-resolution studies. The present results highlight the importance of LIF as a diagnostic tool employing continuous-wave laser re-excitation, addressing some of the limitations of traditional emission and absorption spectroscopic methods.

High Speed VCSEL Transmission at 1310 nm and 1550 nm Transmission Wavelengths

Vertical cavity surface emitting lasers (VCSELs) operating at 1310 nm and 1550 nm are very promising sources for access and interconnections in telecommunication systems due to their technologically attractive properties such as low threshold currents and narrow spectral linewidth due to single mode operation. VCSELs are also being rapidly commercialized for single-mode fibre metropolitan area and wide area network applications. All these advantages leads to cost-effective wavelength-tunable lasers, which are essential for the future intelligent, all-optical networks. Direct modulation (DM) of VCSEL with separate optical and current apertures enables high modulation bandwidth operating at single mode at low current density. However, dispersion and attenuation is a major hurdle to VCSELs transmission at bit rate of 10 Gb/s and above. In this study, a 1310 and 1550 nm VCSELs were directly modulated with 10 Gb/s NRZ PRBS 2⁷-1 and transmitted over 25 km ITU. T G.652 and ITU. T G.655 fibres and optimized for metro-access distances. A low dispersion penalty was realized when a 1550 nm VCSEL was used on a G.655 fibre and when a 1310 nm source was transmitted over a G.652 fibre. The transmission system has been analyzed on the basis of different parameters, which are (Bit error rate) BER, Quality (Q) factor and Output power.

140 W high power all-fiber laser at 1940 nm with narrow spectral line-width by MOPA configuration

We report a diode-pumped Tm3+-doped double-clad all-fiber laser operating at 1940 nm with a master oscillator power amplifier configuration; 50 W of seed was generated in master oscillator with 144 W pump power, corresponding to a slope efficiency of 40.1 %. With 212 W pump power, the seed was amplified to 140.9 W in power amplifier, corresponding to a slope efficiency of 47.1 %. The peak wavelength was 1939.57 nm with a narrow spectral line-width of 0.09 nm. The beam quality factor of M 2 was 1.29. Neither amplified spontaneous emission nor parasitic lasing was observed during the amplification process. The output power was only limited by the pump power.

Two-dimensional fluorescence spectroscopy of laser-produced plasmas.

We use a two-dimensional laser-induced fluorescence spectroscopy technique to measure the coupled absorption and emission properties of atomic species in plasmas produced via laser ablation of a solid aluminum target at atmospheric pressure. Emission spectra from the Al I 394.4nm and Al I 396.15nm transitions are measured while a frequency-doubled, continuous wave (cw) Ti:sapphire laser is tuned across the Al I 396.15nm transition. The resulting two-dimensional spectra show the energy coupling between the two transitions via increased emission intensity for both transitions during resonant absorption of the cw laser at one transition. Time-delayed, gated detection of the emission spectrum is used to isolate resonantly excited fluorescence emission from thermally excited emission from the plasma. In addition, the tunable cw laser measures the absorption spectrum of the Al transition with ultrahigh resolution after the plasma has cooled, resulting in narrower spectral linewidths than observed in emission spectra. Our results highlight that fluorescence spectroscopy employing cw laser re-excitation after pulsed laser ablation combines benefits of both traditional emission and absorption spectroscopic methods.

Sub-kilohertz laser linewidth narrowing using polarization spectroscopy.

We identify several beneficial characteristics of polarization spectroscopy as an absolute atomic reference for frequency stabilization of lasers, and demonstrate sub-kilohertz laser spectral linewidth narrowing using polarization spectroscopy with high-bandwidth feedback. Polarization spectroscopy provides a highly dispersive velocity-selective absolute atomic reference based on frequency-dependent birefringence in an optically pumped atomic gas. The pumping process leads to dominance of the primary closed transition, suppressing closely-spaced subsidiary resonances which reduce the effective capture range for conventional atomic references. The locking signal is based on subtraction of two orthogonal polarization signals, reducing the effect of laser intensity noise to the shot noise limit. We measure noise-limited servo bandwidth comparable to that of a high-finesse optical cavity without the frequency limit or complexity imposed by optical modulation normally associated with high bandwidth laser frequency stabilization. We demonstrate narrowing to 600±100 Hz laser linewidth using the beatnote between two similarly locked external cavity diode lasers.

Application of CRAFT in two-dimensional NMR data processing.

Two-dimensional (2D) data are typically truncated in both dimensions, but invariably and severely so in the indirect dimension. These truncated FIDs and/or interferograms are extensively zero filled, and Fourier transformation of such zero-filled data is always preceded by a rapidly decaying apodization function. Hence, the frequency line width in the spectrum (at least parallel to the evolution dimension) is almost always dominated by the apodization function. Such apodization-driven line broadening in the indirect (t1 ) dimension leads to the lack of clear resolution of cross peaks in the 2D spectrum. Time-domain analysis (i.e. extraction of frequency, amplitudes, line width, and phase parameters directly from the FID, in this case via Bayesian modeling into a tabular format) of NMR data is another approach for spectral resonance characterization and quantification. The recently published complete reduction to amplitude frequency table (CRAFT) technique converts the raw FID data (i.e. time-domain data) into a table of frequencies, amplitudes, decay rate constants, and phases. CRAFT analyses of time-domain data require minimal or no apodization prior to extraction of the four parameters. We used the CRAFT processing approach for the decimation of the interferograms and compared the results from a variety of 2D spectra against conventional processing with and without linear prediction. The results show that use of the CRAFT technique to decimate the t1 interferograms yields much narrower spectral line width of the resonances, circumventing the loss of resolution due to apodization. Copyright © 2016 John Wiley & Sons, Ltd.

Low-nonlinearity spin-torque oscillations driven by ferromagnetic nanocontacts

Spin-torque oscillators are strong candidates as nano-scale microwave generators and detectors. However, because of large amplitude-phase coupling (non-linearity), phase noise is enhanced over other linear auto-oscillators. One way to reduce nonlinearity is to use ferromagnetic layers as a resonator and excite them at localized spots, making a resonator-excitor pair. We investigated the excitation of oscillations in dipole-coupled ferromagnetic layers, driven by localized current at ferromagnetic nano-contacts. Oscillations possessed properties of optical-mode spin-waves and at low field ($\approx$200 Oe) had high frequency (15 GHz), a moderate precession amplitude (2--3$^\circ$), and a narrow spectral linewidth ($<$3 MHz) due to localized excitation at nano-contacts. Micromagnetic simulation showed emission of resonator's characteristic optical-mode spin-waves from disturbances generated by domain-wall oscillations at nano-contacts.

High-power Tm3+-doped all-fiber laser operating at 1908 nm by a master oscillator power amplifier configuration with narrow spectral linewidth

We demonstrate a diode-pump Tm3+-doped all-fiber laser operating at 1908 nm based on a master oscillator power amplifier (MOPA) configuration. In our work, 152 W of laser output power is generated by a total incident pump power of 434 W at 790 nm, corresponding to the total optical efficiency of 35%. The laser wavelength is 1908.29 nm. To the best of our knowledge, it is the highest output power reached around 1908 nm with such a narrow linewidth of 0.18 nm based on a MOPA configuration.

Intrinsic polarization control in rectangular GaN nanowire lasers.

We demonstrate intrinsic, linearly polarized lasing from single GaN nanowires using cross-sectional shape control. A two-step top-down fabrication approach was employed to create straight nanowires with controllable rectangular cross-sections. A clear lasing threshold of 444 kW cm(-2) and a narrow spectral line width of 0.16 nm were observed under optical pumping at room temperature, indicating the onset of lasing. The polarization was along the short dimension (y-direction) of the nanowire due to the higher transverse confinement factors for y-polarized transverse modes resulting from the rectangular nanowire cross-section. The results show that cross-sectioned shape control can enable inherent control over the polarization of nanowire lasers without additional environment requirements, such as placement onto lossy substrates.

Evolution of the Single-Nanocrystal Photoluminescence Linewidth with Size and Shell: Implications for Exciton-Phonon Coupling and the Optimization of Spectral Linewidths.

The optimization of photoluminescence spectral linewidths in semiconductor nanocrystal preparations involves minimizing both the homogeneous and inhomogeneous contributions to the ensemble spectrum. Although the inhomogeneous contribution can be controlled by eliminating interparticle inhomogeneities, far less is known about how to synthetically control the homogeneous, or single-nanocrystal, spectral linewidth. Here, we use solution photon-correlation Fourier spectroscopy (S-PCFS) to measure how the sample-averaged single-nanocrystal emission linewidth of CdSe core and core/shell nanocrystals change with systematic changes in the size of the cores and the thickness and composition of the shells. We find that the single-nanocrystal linewidth at room temperature is heavily influenced by the nature of the CdSe surface and the epitaxial shell, which have a profound impact on the internal electric fields that affect exciton-phonon coupling. Our results explain the wide variations, both experimental and theoretical, in the magnitude and size dependence in previous reports on exciton-phonon coupling in CdSe nanocrystals. Moreover, our findings offer a general pathway for achieving the narrow spectral linewidths required for many applications of nanocrystals.

Plasmon resonance and perfect light absorption in subwavelength trench arrays etched in gallium-doped zinc oxide film

Near-perfect light absorption in subwavelength trench arrays etched in highly conductive gallium-doped zinc oxide films was experimentally observed in the mid infrared regime. At wavelengths corresponding to the resonant excitation of surface plasmons, up to 99% of impinging light is efficiently trapped and absorbed in the periodic trenches. Scattering cross sectional calculations reveal that each individual trench acts like a vertical split ring resonator with a broad plasmon resonance spectrum. The coupling of these individual plasmon resonators in the grating structure leads to enhanced photon absorption and significant resonant spectral linewidth narrowing. Ellipsometry measurements taken before and after device fabrication result in different permittivity values for the doped zinc oxide material, indicating that localized annealing occurred during the plasma etching process due to surface heating. Simulations, which incorporate a 50 nm annealed region at the zinc oxide surface, are in a good agreement with the experimental results.

High-power lasers for directed-energy applications.

In this article, we review and discuss the research programs at the Naval Research Laboratory (NRL) on high-power lasers for directed-energy (DE) applications in the atmosphere. Physical processes affecting propagation include absorption/scattering, turbulence, and thermal blooming. The power levels needed for DE applications require combining a number of lasers. In atmospheric turbulence, there is a maximum intensity that can be placed on a target that is independent of the initial beam spot size and laser beam quality. By combining a number of kW-class fiber lasers, scientists at the NRL have successfully demonstrated high-power laser propagation in a turbulent atmosphere and wireless recharging. In the NRL experiments, four incoherently combined fiber lasers having a total power of 5 kW were propagated to a target 3.2 km away. These successful high-power experiments in a realistic atmosphere formed the basis of the Navy's Laser Weapon System. We compare the propagation characteristics of coherently and incoherently combined beams without adaptive optics. There is little difference in the energy on target between coherently and incoherently combined laser beams for multi-km propagation ranges and moderate to high levels of turbulence. Unlike incoherent combining, coherent combining places severe constraints on the individual lasers. These include the requirement of narrow power spectral linewidths in order to have long coherence times as well as polarization alignment of all the lasers. These requirements are extremely difficult for high-power lasers.

Generation of 130 W narrow-linewidth high-peak-power picosecond pulses directly from a compact Yb-doped single-stage fiber amplifier

We report a compact, 130-W single-stage master oscillator power amplifier with a high peak power of 51.3 kW and a narrow spectral linewidth of 0.1 nm. The seed source is a single-mode, passively mode-locked solid-state laser at 1064 nm with an average power of 2 W. At a repetition rate of 73.5 MHz, the pulse duration is 30 ps. After amplification, it stretches to 34.5 ps. The experiment enables the optical-to-optical conversion efficiency to reach 75%. To the best of our knowledge, this is the first report of such a high-power, narrow spectral linewidth, high peak power picosecond-pulse fiber amplifier based on a continuous-wave, mode-locked solid-state seeding laser. No amplified spontaneous emission and stimulated Raman scattering were observed when the pump was increased.

25-W Monolithic Spectrally Stabilized 975-nm Minibars for Dense Spectral Beam Combining

Low-fill-factor laser bars composed of five narrow-stripe broad-area lasers (30 $\mu \text{m} \,\, \times \,\, 6000~\mu \text{m}$ ) with monolithic spectral stabilization are presented. Each laser on the bar emits at a unique wavelength from 970 to 980 nm with a channel spacing of 2.5 nm. Narrow spectral line width $\kappa \text{L}~\sim $ 0.2, which is shown to be sufficient to suppress lasing in Fabry-Perot modes for narrow line operation across all wavelength channels. However, additional optical scattering losses of the gratings are found to limit output power and efficiency. The final realized minibars deliver 25-W continuous wave output power with a conversion efficiency >40% and operate with a beam parameter product of $\sim 1.6$ mm mrad per single emitter, twofold improved in comparison with typical 90- $\mu \text{m}$ wide broad area laser. Such laser minibars are an attractive source for brilliant dense spectral beam combining.

Annular-Shaped Emission from Gallium Nitride Nanotube Lasers

Annular-shaped lasing emission is demonstrated from gallium nitride nanotubes fabricated using a two-step top-down technique. By optically pumping, we observe a clear threshold of 1055 kW/cm2, a narrow spectral linewidth of 0.19 nm, and guided emission from the nanotubes. Lasing is also demonstrated in a liquid environment, with an approximate doubling in threshold observed. The nanotube lasers could be of interest for optical nanofluidic applications or applications benefiting from a hollow beam shape. More generally, the results indicate that cross-sectional shape control can be employed to manipulate the properties of nanolasers.

Mid-infrared laser emission from Fe:ZnSe cladding waveguides

The authors present a mid-IR depressed cladding waveguide laser in Fe:ZnSe. The laser produced a maximum output power of 76 mW at 4122 nm and laser thresholds as low as 154 mW were demonstrated. This represents a 44% reduction in threshold power compared with the bulk laser system demonstrated in this paper. The waveguide laser was found to have a narrow spectral linewidth of 6 nm FHWM compared to the 50 nm typical of bulk Fe:ZnSe lasers.

Strong Acoustic Phonon Localization in Copolymer-Wrapped Carbon Nanotubes.

Understanding and controlling exciton-phonon interactions in carbon nanotubes has important implications for producing efficient nanophotonic devices. Here we show that laser vaporization-grown carbon nanotubes display ultranarrow luminescence line widths (120 μeV) and well-resolved acoustic phonon sidebands at low temperatures when dispersed with a polyfluorene copolymer. Remarkably, we do not observe a correlation of the zero-phonon line width with (13)C atomic concentration, as would be expected for pure dephasing of excitons with acoustic phonons. We demonstrate that the ultranarrow and phonon sideband-resolved emission spectra can be fully described by a model assuming extrinsic acoustic phonon localization at the nanoscale, which holds down to 6-fold narrower spectral line width compared to previous work. Interestingly, both exciton and acoustic phonon wave functions are strongly spatially localized within 5 nm, possibly mediated by the copolymer backbone, opening future opportunities to engineer dephasing and optical bandwidth for applications in quantum photonics and cavity optomechanics.