Broadband Laser Research Articles

Efficient enhancement of bismuth near infrared luminescence by the co-doping of tantalum in GYAP crystal.

Bi/Ta double doped Gd0.1Y0.9AlO3 (Bi/Ta:GYAP) near-infrared (NIR) laser crystal was successfully grown. The codoping of Ta5+ was demonstrated to effectively enhance the NIR fluorescence emission in Bi/Ta:GYAP crystal for the first time. The introduced Ta5+ ion can induce the change of valence states from Bi3+ to Bi+ based on the charge compensation mechanism, bringing about the enhanced NIR fluorescence emission. The three NIR fluorescence emission peaks centering at 1005, 1195, and 1280 nm were ascribed to three different Bi+ active centers in Bi/Ta:GYAP crystal. These results suggest that the Bi/Ta:GYAP crystal may have potential applications in NIR broadband lasers.

High-sensitive absorption measurement in transparent isotropic dielectrics with time-resolved photothermal common-path interferometry.

We propose a time-resolved photothermal common-path interferometry scheme at fast heating in the absence of heat diffusion and employ it to measure absorption in Suprasil 311 silica glass (2.8·10-6 cm-1) at a wavelength of 1071nm and continual absorption in laboratory air (2.9·10-8 cm-1) for a signal-to-noise ratio of 100/1. The absorption was measured at a thermally induced phase incursion of less than 0.1rad in a heating beam, which guaranteed correct calibration. To calibrate this scheme, we developed a theory of diffraction on deformations taking into account the stresses arising in an inhomogeneous temperature field. This allowed us to use a standard glass K8 for calibration. The low level of noise and time resolution of pulsed signals allowed the distinguishing of the contributions of Kerr and striction nonlinearities to absorption measurements in Suprasil 311 silica glass and enabled the observance of the time evolution of strictional deformations. Additionally, an anomalous temporal development of the absorption of broadband laser radiation in atmospheric air at 2.9·10-8 cm-1 has been revealed.

Systematic spectral shifts in the mid-infrared spectroscopy of aerosols.

Infrared spectroscopy in the spectral fingerprint region from 6.5 to 20 µm has been applied for decades to identify vapor- and condensed-phase chemicals with high confidence. By employing a unique broadband laser operating from 7.2- to 11.5-µm we show that, in this region, wavelength-dependent Mie-scattering effects substantially modulate the underlying chemical absorption signature, undermining the ability of conventional infrared absorption spectroscopy to identify aerosolized liquids and solids. In the aerosol studied, Mie theory predicts that the positions of spectroscopic features will blue-shift by up to 200 nm, and this behavior is confirmed by experiment, illustrating the critical importance of considering Mie contributions to aerosol spectroscopy in the mid infrared. By examining the spectroscopy of light scattered from an aerosol of the chemical diethyl phthalate, we demonstrate excellent agreement with a Mie-scattering model informed by direct measurements of the particle-size-distribution and complex refractive index.

Ultra-Bright Rhodamines with Sulfobutylether-β-Cyclodextrin: A Viable Supramolecular Dye Laser in Aqueous Medium.

Although aqueous dye lasers are much sought after, they have been of no practical use, as laser dyes show a strong tendency for aggregation in water, thus diminishing their optical output. Contributing towards this shortcoming, we studied the noncovalent interactions of two prominent laser dyes, namely, rhodamine 6G and rhodamine B, with a water soluble macrocyclic host, sulfobutylether-β-cyclodextrin (SBE7 βCD). Spectral changes in the absorption and fluorescence behavior of dyes in presence of the SBE7 βCD host indicated adequate complex formation between dye and host (K∼104 M-1 ). A combination of various photophysical parameters evaluated from measurements such as Job plot, changes in the fluorescence lifetime/anisotropy values, and favorable thermodynamic parameters from isothermal titration calorimetric measurements adjudicated a 1 : 1 stoichiometric complex formation between dye and SBE7 βCD host. Consequently, SBE7 βCD prevents dye aggregation/adsorption and present rhodamine dyes in their monomeric forms with enhanced fluorescence yield and brightness. These vital parameters were utilized to optimize and demonstrate cost-effective supramolecular broad-band and narrow-band aqueous dye laser systems with improved lasing efficiencies (∼25 % higher for the SBE7 βCD : RhB system and ∼10 % higher for SBE7 βCD : Rh6G system), better beam profile, and enhanced durability compared to the respective dyes in optically matched ethanol solutions.

Benefits of Unidirectional Design Based on Decoupled Transmitters and Receivers in Tackling Traffic Asymmetry for Elastic Optical Networks

We consider a unidirectional design approach to tackle the traffic asymmetry in an elastic optical network (EON). We decouple a bidirectional transponder into a pair of separate unidirectional transmitters and unidirectional receivers, and consider different multi-flow transmitters that use either an array of laser diodes or a broadband laser source with filters to generate multiple sub-carriers. To evaluate the benefit of this unidirectional design, we have considered the routing and spectrum allocation optimization problem for the EON. An integer linear programming model and an efficient heuristic algorithm based on spectrum window planes have been developed. Simulation results show that the proposed unidirectional design approach can significantly improve capacity utilization and minimize network cost compared to one based on bidirectional symmetric traffic flows. It is also efficient for network scenarios with spatial traffic asymmetry and where multi-period network planning is required.

Propagation of the phase-modulated femtosecond pulses through the optically-dense quasi-resonant medium

It is known that ultrashort pulse coherent propagation through the resonant absorbing medium leads to the pulse splitting and pulse train generation. First of these pulses is the input pulse and the following pulses are resonant medium response. It is also known that such a propagation mode leads to the «spectrum condensation» effect, at which broad-band input femtosecond laser emission, as it leaves opically-dense medium, centralized in the vicinity of the resonance absorption lines. It’s interesting to clear up the dependence of a condensation effect on an input femtosecond pulse phase modulation value (chirp).

Mid-infrared supercontinuum generation in fluoride fiber amplifiers: current status and future perspectives

The quest for a compact and efficient broadband laser source able to probe the numerous fundamental molecular absorption lines in the mid-infrared (3–8 µm) for various applications has been going on for more than a decade. While robust commercial fiber-based supercontinuum (SC) systems have started to appear on the market, they still exhibit poor energy conversion into the mid-infrared (typically under 30%) and are generally not producing wavelengths exceeding 4.7 µm. Here, we present an overview of the results obtained from a novel approach to SC generation based on spectral broadening inside of an erbium-doped fluoride fiber amplifier seeded directly at 2.8 µm, allowing mid-infrared conversion efficiencies reaching up to 95% and spectral coverage approaching the transparency limit of ZrF4 (4.2 µm) and InF3 (5.5 µm) fibers. The general concept of the approach and the physical mechanisms involved are presented alongside the various configurations of the system to adjust the output characteristics in terms of spectral coverage and output power for different applications.

Wavelength-limited optical accordion.

We demonstrate a method to create dynamic optical lattices with lattice constant tunable down to the optical wavelength limit. The periodicity of 1D lattice is to be adjusted by rotating the incoming direction of one of the two interfering laser beams with its fiber port. The relative phase between the stationary and rotating lasers are stabilized with a heterodyne phase-lock loop (Ma et al, Opt. Lett. 19, 1777, 1994), by reflecting part of the rotating laser beam back from a cylindrical mirror near the experiment. Our preliminary demonstration shows tuning of lattice constant λ2sinθ/2, limited by our imaging resolution, between θ = 3° and 20°, with stable and tunable phase. The results can be extended to achieve lattice constant tuning range from ∼ 10λ down to λ/2. We discuss extension of the demonstrated scheme for improved vibration suppression, and for lattice utilizing broadband lasers. Finally we propose a 2D accordion lattice design for quantum gas experiments.

Charge compensation effects of Yb3+ on the Bi+: near-infrared emission in PbF2 crystal.

The use of Yb3+ codoping for enhancing the near-infrared (NIR) emission in Bi, Yb codoped PbF2 crystal was investigated for the first time, to the best of our knowledge. The NIR and visible fluorescence emission properties of the as-grown crystals were investigated in detail. It was found that the Yb3+ ion can act as an effective charge compensated ion to bring about the conversion from Bi2+ to Bi+, enhancing the NIR fluorescence emission in Bi:PbF2 crystal. Moreover, the NIR fluorescence emission peaking at around 1090 and 1485nm was demonstrated to be two different Bi+ related centers, Bi(I) and Bi(II), respectively. These results suggest that Bi, Yb codoped PbF2 crystal may become an attractive gain material for developing NIR broadband lasers under pumps of different wavelengths.

Amplification combination of Er3+ and Tm3+ emissions in titanium-diffused lithium niobate strip waveguide

Erbium(Er)/thulium(Tm)-codoped active lithium niobate(LiNbO3) channel waveguides were fabricated by titanium diffusion following erbium/thulium co-diffusion doping in an X-/Z-cut LiNbO3 substrate with an initial congruent/Li-deficient composition. We show that the waveguide is in an initial congruent or Li-deficient composition environment, remains LiNbO3 crystalline phase, is monomode at the 1.5 µm wavelength region under either the transverse electric or the transverse magnetic polarization, and has a loss < 0.5 dB/cm. The X-cut/Z-propagation scheme can effectively suppress the photorefractive effect, while the Z-cut/Y-propagation scheme cannot. Codoping with thulium effectively promotes the resistance to optical damage. An optical gain of 0.8/0.1 dB/cm is attained for the 1547(erbium)/1450(thulium) nm small-signal, showing that erbium/thulium codoping enables to combine the emissions of the two ions and realize the S+C+L broadband amplification or lasing by using 980 and 795 nm laser diodes as the pump sources.

A systematic study of the microstructure and laser characteristics of Pr3+-doped lithium lutetium fluoride

Rare-earth-doped fluoride crystals have been widely investigated owing to their broadband laser and medical applications. The most promising is praseodymium (Pr3+)-doped lithium lutetium fluoride (LiLuF4) crystals. However, knowledge of the microstructure and local interactions of Pr3+-doped LiLuF4 is still insufficient to determinate the electric and magnetic dipole transition mechanisms. To solve this key problem, we present a systematic study of the ground-state structure of Pr3+-doped LiLuF4 using the CALYPSO methodology combined with density functional theory. The results uncover a stable structure with a P4¯ space group, where the Pr3+ ions occupy the Lu3+ sites. Subsequent crystal-field calculations are carried out using our developed WEPMD method. A very small RMS of 2.41 cm−1 is obtained for the first time. In the electric and magnetic dipole transitions of the Pr3+-doped LiLuF4 crystals, we find some important fluorescence transitions. In the visible spectral range, two intense transitions, 1D2 → 3H4 (600 nm) and 1I6 → 3F4 (682 nm), are predicted to provide good laser action. A potential emission channel occurs at 1003 nm, originating from the 1D2 → 3F4 transition in the NIR region. These significant findings provide potential candidates for future laser action and outline a new systematic approach to studying other laser materials.

Broadband pulsed difference frequency generation laser source centered 3326 nm based on ring fiber lasers

A broadband pulsed mid-infrared difference frequency generation (DFG) laser source based on MgO-doped congruent LiNbO3 bulk is experimentally demonstrated, which employs a homemade pulsed ytterbium-doped ring fiber laser and a continuous wave erbium-doped ring fiber laser to act as seed sources. The experimental results indicate that the perfect phase match crystal temperature is about 74.5[Formula: see text]C. The maximum spectrum bandwidth of idler is about 60 nm with suitable polarization states of fundamental lights. The central wavelength of idlers varies from 3293 nm to 3333 nm over the crystal temperature ranges of 70.4–76[Formula: see text]C. A jump of central wavelength exists around crystal temperature of 72[Formula: see text]C with variation of about 30 nm. The conversion efficiency of DFG can be tuned with the crystal temperature and polarization states of fundamental lights.

Validating data analysis of broadband laser ranging.

Broadband laser ranging combines spectral interferometry and a dispersive Fourier transform to achieve high-repetition-rate measurements of the position of a moving surface. Telecommunications fiber is a convenient tool for generating the large linear dispersions required for a dispersive Fourier transform, but standard fiber also has higher-order dispersion that distorts the Fourier transform. Imperfections in the dispersive Fourier transform significantly complicate the ranging signal and must be dealt with to make high-precision measurements. We describe in detail an analysis process for interpreting ranging data when standard telecommunications fiber is used to perform an imperfect dispersive Fourier transform. This analysis process is experimentally validated over a 27-cm scan of static positions, showing an accuracy of 50 μm and a root-mean-square precision of 4.7 μm.

Interferometer-based high-accuracy white light measurement of neutral rubidium density and gradient at AWAKE

The AWAKE experiment requires an automated online rubidium (Rb) plasma density and gradient diagnostic for densities between 1 and 10⋅1014 cm−3. A linear density gradient along the plasma source at the percent level may be useful to improve the electron acceleration process. Because of full laser ionization of Rb vapor to Rb+ within a radius of 1 mm, the plasma density equals the vapor density. We measure the Rb vapor densities at both ends of the source, with high precision using, white light interferometry. At either source end, broadband laser light passes a remotely controlled Mach–Zehnder interferometer built out of single mode fibers. The resulting interference signal, influenced by dispersion in the vicinity of the Rb D1 and D2 transitions, is dispersed in wavelength by a spectrograph. Fully automated Fourier-based signal conditioning and a fit algorithm yield the density with an uncertainty between the measurements at both ends of 0.11 to 0.46% over the entire density range. These densities used to operate the plasma source are displayed live in the control room.

Remote sensing of methane with OSAS-lidar on the 2ν3 band Q-branch: Experimental proof

Optical sensors based on absorption spectroscopy play a central role in the detection and monitoring of atmospheric trace gases. We here present for the first time the experimental demonstration of OSAS-Lidar on the remote sensing of CH4 in the atmosphere. This new methodology, the OSAS-Lidar, couples the Optical Similitude Absorption Spectroscopy (OSAS) methodology with a light detection and ranging device. It is based on the differential absorption of spectrally integrated signals following Beer Lambert-Bouguer law, which are range-resolved. Its novelty originates from the use of broadband laser spectroscopy and from the mathematical approach used to retrieve the trace gas concentration. We previously applied the OSAS methodology in laboratory on the 2ν3 methane absorption band, centered at the 1665 nm wavelength and demonstrated that the OSAS-methodology is almost independent from atmospheric temperature and pressure. In this paper, we achieve an OSAS-Lidar device capable of observing large concentrations of CH4 released from a methane source directly into the atmosphere. Comparison with a standard in-situ measurement device shows that the path-integrated concentrations retrieved from OSAS-Lidar methodology exhibit sufficient sensitivity (2 000 ppm m) and observational time resolution (1 s) to remotely sense methane leaks in the atmosphere. The coupling of OSAS-lidar with a wind measurement device opens the way to monitor time-resolved methane flux emissions, which is important in regards to future climate mitigation involving regional reduction of CH4 flux emissions.

Temporal coherence study of four-wave mixing products with and without the laser cavity effect.

In this work, we have studied the temporal coherence property of a four-wave mixing (FWM)-assisted continuous-wave erbium-doped fiber ring laser source. To understand the temporal coherence characteristics of FWM, we have analyzed the visibility of their interference pattern with the help of a Mach-Zehnder-based fiber interferometer. The visibility of FWM sidebands is compared with respect to the input pumps, with and without the cavity effect. The stand-alone FWM without any cavity effect has a visibility comparable to its input coherent pumps, while the FWM in the presence of the cavity effect has a poorer visibility due to the intra-cavity noise arising from the amplified spontaneous emission, which is not entirely annulled in broadband lasers. The study also confirms that the visibility/temporal coherence property of FWM sidebands does not depend on the efficiency of their generation.

RIN Mitigation and Transmission Performance Enhancement With Forward Broadband Pump

We demonstrate that using a broadband, first order, and coherent pump laser enables effective and efficient forward-pumped distributed Raman amplification for long-haul transmission systems, thanks to the simultaneous suppression of ASE noise and RIN-related penalty. We show in both experiments and simulation that this scheme extends the reach of 10 × 120 Gb/s DP-QPSK WDM transmission by a minimum of 50%, compared with low RIN Bi-doped fibre laser and other commercially available pump lasers. Moreover, it requires very low forward pump power, and maintains uniform/symmetric signal power distribution which allows effective nonlinearity compensation.

Dynamics of a broad-band quantum cascade laser: from chaos to coherent dynamics and mode-locking.

The dynamics of a multimode quantum cascade laser, are studied in a model based on effective semiconductor Maxwell-Bloch equations, encompassing key features for the radiation-medium interaction such as an asymmetric frequency dependent gain and refractive index as well as the phase-amplitude coupling provided by the linewidth enhancement factor. By considering its role and that of the free spectral range, we find the conditions in which the traveling wave emitted by the laser at the threshold can be destabilized by adjacent modes, thus leading the laser emission towards chaotic or regular multimode dynamics. In the latter case our simulations show that the field oscillations are associated to self-confined structures which travel along the laser cavity, bridging mode-locking and solitary wave propagation. In addition, we show how a RF modulation of the bias current leads to active mode-locking yielding high-contrast, picosecond pulses. Our results compare well with recent experiments on broad-band THz-QCLs and may help in the understanding of the conditions for the generation of ultrashort pulses and comb operation in mid-IR and THz spectral regions.

宽带激光熔覆铁基非晶涂层的微观组织及形成机制

宽带激光熔覆铁基非晶涂层的微观组织及形成机制

Multiplex CARS imaging with spectral notch shaped laser pulses delivered by optical fibers.

We present an experimental demonstration of single-pulse coherent anti-Stokes Raman spectroscopy (CARS) using a spectrally shaped broadband laser that is delivered by an optical fiber to a sample at its distal end. The optical fiber consists of a fiber Bragg grating component to serve as a narrowband notch filter and a combined large-mode-area fiber to transmit such shaped ultrashort laser pulses without spectral distortion in a long distance. Experimentally, our implementation showed a capability to measure CARS spectra of various samples with molecular vibrations in the fingerprint region. Furthermore, CARS imaging of poly(methyl methacrylate) bead samples was carried out successfully under epi-CARS geometry in which backward-scattered CARS signals were collected into a multimode optical fiber. A compatibility of single-pulse CARS scheme with fiber optics, verified in this study, implies a potential for future realization of compact all-fiber CARS spectroscopic imaging systems.