Laser Bandwidth Research Articles

Phase modulation failsafe system for multi-kJ lasers based on optical heterodyne detection.

Amplification of the transverse scattered component of stimulated Brillouin scattering (SBS) can contribute to optical damage in the large aperture optics of multi-kJ lasers. Because increased laser bandwidth from optical phase modulation (PM) can suppress SBS, high energy laser amplifiers are injected with PM light. Phase modulation distributes the single-frequency spectrum of a master oscillator laser among individual PM sidebands, so a sufficiently high modulation index β can maintain the fluence for all spectral components below the SBS threshold. To avoid injection of single frequency light in the event of a PM failure, a high-speed PM failsafe system (PMFS) must be employed. Because PM is easily converted to AM, essentially all PM failsafes detect AM, with the one described here employing a novel configuration where optical heterodyne detection converts PM to AM, followed by passive AM power detection. Although the PMFS is currently configured for continuous monitoring, it can also detect PM for pulse durations ≥2 ns and could be modified to accommodate shorter pulses. This PMFS was deployed on the Z-Beamlet Laser (ZBL) at Sandia National Laboratories, as required by an energy upgrade to support programs at Sandia's Z Facility such as magnetized liner inertial fusion. Depending on the origin of a PM failure, the PMFS responds in as little as 7 ns. In the event of an instantaneous failure during initiation of a laser shot, this response time translates to a 30-50 ns margin of safety by blocking a pulse from leaving ZBL's regenerative amplifier, which prevents injection of single frequency light into the main amplification chain. The performance of the PMFS, without the need for operator interaction, conforms to the principles of engineered safety.

Performance of quantum-dot-based tunnel-injection lasers: A theoretical analysis

Tunnel-injection lasers promise advantages in the modulation bandwidth and temperature stability in comparison with conventional laser designs. In this paper, we present results of a microscopic theory for laser properties of tunnel-injection devices and a comparison with a conventional quantum-dot laser structure. In general, the modulation bandwidth of semiconductor lasers is affected by the steady-state occupations of electrons and holes via the presence of spectral hole burning. For tunnel-injection lasers with InGaAs quantum dot emitting at an telecom wavelength of 155 μm, we demonstrate that the absence of spectral hole burning favors this concept over conventional quantum-dot based lasers.

Compensation of non-linear bandwidth broadening by laser chirping in Thomson sources

A new laser chirping prescription is derived by means of the phase-stationary method for an incident Gaussian laser pulse in conjunction with a Liénard-Wiechert calculation of the scattered radiation flux and spectral brilliance. This particularly efficient laser chirp has been obtained using the electric field of the laser and for electrons and radiation on axis. The frequency modulation is somewhat reduced with respect to that proposed in the previous literature, allowing the application of this procedure to lasers with larger values of the parameter a0. Numerical calculations have been performed using mildly focused and narrow bandwidth laser pulses, confirming a larger efficiency of the chirp prescription here introduced. The chirp efficiency has been analysed as a function of the laser parameter and focusing.

Mitigation of cross-beam energy transfer in inertial-confinement-fusion plasmas with enhanced laser bandwidth

Cross-beam energy transfer (CBET) is a significant energy-loss mechanism in directly driven inertial-confinement-fusion (ICF) targets. One strategy for mitigating CBET is to increase the bandwidth of the laser light, thereby disrupting the resonant three-wave interactions that underlie this nonlinear scattering process. Here, we report on numerical simulations performed with the wave-based code lpse that show a significant reduction in CBET for bandwidths of 2-5 THz (corresponding to a normalized bandwidth of 0.2%-0.6% at a laser wavelength of 351nm) under realistic plasma conditions. Such bandwidths are beyond those available with current high-energy lasers used for ICF, but could be achieved using stimulated rotation Raman scattering in diatomic gases like nitrogen.

Simultaneous distance measurement at multiple wavelengths using the intermode beats from a femtosecond laser coherent supercontinuum

The intermode beats generated by direct detection of a mode-locked femtosecond laser represent inherent high-quality and high-frequency modulations suitable for electro-optical distance measurement (EDM). This approach has already been demonstrated as a robust alternative to standard long-distance EDM techniques. However, we extend this idea to intermode beating of a wideband source obtained by spectral broadening of a femtosecond laser. We aim at establishing a technological basis for accurate and flexible multiwavelength distance measurement. Results are presented from experiments using beat notes at 1 GHz generated by two bandpass-filtered regions from both extremes of a coherent supercontinuum ranging from 550 to 1050 nm. The displacement measurements performed simultaneously on both colors on a short-distance setup show that noise and coherence of the wideband laser are adequate for achieving accuracies of about 0.01 mm on each channel with a potential improvement by accessing higher beat notes. Pointing and power instabilities have been identified as dominant sources of systematic deviations. Nevertheless, the results demonstrate the basic feasibility of the proposed technique. We consider this a promising starting point for the further development of multiwavelength EDM enabling increased accuracy over long distances through dispersion-based integral refractivity compensation and for remote surface material probing along with distance measurement in laser scanning.

All-fiber radially/azimuthally polarized lasers based on mode coupling of tapered fibers.

We demonstrate a mode converter with an insertion loss of 0.36dB based on mode coupling of tapered single-mode and two-mode fibers, and realize all-fiber flexible cylindrical vector lasers at 1550nm. Attributing to the continuous distribution of a tangential electric field at taper boundaries, the laser is switchable between the radially and azimuthally polarized states by adjusting the input polarization. In the temporal domain, the operation is controllable among continuous-wave, Q-switched, and mode-locked statuses by changing the saturable absorber or pump strength. The duration of Q-switched radially/azimuthally polarized laser spans from 10.4/10.8 to 6/6.4μs at the pump range of 38 to 58mW, while that of the mode-locked pulse varies from 39.2/31.9 to 5.6/5.2ps by controlling the laser bandwidth. The proposed laser combines the features of a cylindrical vector beam, a fiber laser, and an ultrafast pulse, providing a special and cost-effective source for practical applications.

Coherent dynamics in cavity femtochemistry: Application of the multi-configuration time-dependent Hartree method

The photochemistry of a molecular ensemble coupled to a resonance cavity and triggered by a femtosecond laser pulse is investigated from a real-time, quantum dynamics perspective with the multi-configuration time-dependent Hartree method. Coherent excitation of a superposition of electronic states in the ensemble leads to superradiant energy transfer to the cavity characterized by quadratic scaling with the number of molecules. Electronic decoherence associated with loss of nuclear wave packet overlap among those states destroys superradiant energy transfer, returning to a linear regime. For equal pump laser conditions, the photoexcitation probability per molecule decreases with increase of the number of molecules inside the cavity. This is caused by a loss of resonance condition of the laser with the bright electronic-photonic states of the coupled cavity-ensemble system. Increase of the laser bandwidth restores the energy transferred per molecule and the trigger probability remains independent of the number of molecules in the cavity.

Excitation of positronium: from the ground state to Rydberg levels

Following bombardment of a mesoporous silica sample by positrons ejected from a two-stage buffer gas trap, ortho-positronium (o-Ps) was emitted into vacuum with an efficiency of around 27%–28%. This ensemble, with a density close to 1011 m−3, was then irradiated by multiple solid state-based laser beams up to 50 ns after positron implantation. Tunable over a wide range (230–2200 nm), the lasers have enabled excitation of the o-Ps Lyman-α transition, followed by subsequent excitation and, if desired, ionisation to vacuum. Excitation from the 2P state to intermediate states in the principal quantum number, , range has been achieved with efficiencies , whilst the excitation efficiency of ground state o-Ps to the 2P state of is currently limited by a mismatch between the Doppler broadening of the 1S-2P transition and the 225 GHz laser bandwidth at 243 nm.

3000 W tandem pumped all-fiber laser based on domestic fiber

In recent years, high power fiber laser has received great attention, leading to wide applications in numerous fields such as industry, biology and relevant research. Nevertheless, the output power of typical diode pumped fiber laser is limited by the thermal effect and brightness of pump source. Owing to the low quantum deficit, the tandem pumping employing ytterbium-doped fiber lasers (YDFLs) as the pumping source can effectively reduce the thermal issue and achieve high power output. With the much lower absorption coefficient at 1018 nm than at 976 nm, longer gain fiber is necessary in tandem pumped configuration to sufficiently absorb pump light, which in turn induces a more severe nonlinear effect such as the stimulated Raman scattering, bringing in more challenges in laser configuration design. In this paper, we demonstrate an all-fiber laser under master oscillator power amplifier configuration based on tandem pumping with domestic gain fiber produced by China Electronics Technology Group Corporation No. 46 Research Institute. The diameters of the core and inner cladding of the Yb3+ doped double cladding fiber are 25 m and 250 m, respectively. The modified chemical vapor deposition method with gas-solution co-doping method is adopted so that the fiber has a more uniform distribution of Yb ions, larger absorption cross section and higher absorption coefficient (0.41 dB/m@1018 nm). In the amplifier stage, a 40-m-long gain fiber is pumped by fourteen 1018 nm fiber lasers with a maximum total output power of 3511 W. A 67.8 W 1080 nm seed is amplified to 3079 W with a corresponding slope efficiency of 85.9%. The beam quality factor M2 is measured to be 2.14. In addition, no stimulated Raman scattering is found in output spectrum and the 3 dB band width of output laser is measured to be 1.4 nm. To the best of our knowledge, this marks the highest result ever reported for tandem pumping based on domestic gain fiber. Taking stimulated Raman scattering into account, the rate equations are built to calculate the properties and power evolution in the fiber amplifier. The numerical results are in good agreement with the experiment results. Besides, based on heat conduction equation, heat power density in the fiber core is analyzed, showing that the tandem pumping has great advantages in heat management and a huge potential to reach a higher power compared with the method of direct pumping. The theoretical and experimental results show that with ever-maturing fiber manufacturing technology, domestic fiber is capable of withstanding laser power as high as 3 kilowatts. Meanwhile, domestic fiber may achieve a higher output power by increasing the pump power, optimizing the gain fiber length and improving the cooling condition.

Time delay characteristics and bandwidth of chaotic laser from semiconductor laser

The center wavelength of the distribution feedback semiconductor laser is about 1550 nm, and it is in the lowest loss window of the optical fiber communication. A distribution feedback semiconductor laser (DFB-SL) can generate wideband chaotic signals under external disturbances such as optical feedback, optical injection, etc. Thus, due to the simple structure, DFB-SLs with the optical feedback are widely applied to many fields, including information security, lasers radar, and physical entropy sources for generating physical random numbers. However, optical feedback can cause weak periodicity in chaotic signals from the semiconductor laser, and increase the time delay characteristics of chaotic laser, moreover reduce the quality of random numbers generated by using chaotic signals. Meanwhile, to meet the needs of the current high speed and large capacity communication, the DFB-SL, which can generate wideband chaotic laser with low time delay characteristics, has received wide attention and become a hot research subject.In this paper, we present a new scheme for suppressing the time delay characteristics and investigating the bandwidth (BW) of chaotic signals from the semiconductor laser. In this scheme, we build a system that is a distribution feedback semiconductor laser with double phase modulated optical feedback (DFB-SL-DPMOF). In this system, two phase modulators driven by the pseudorandom signals are respectively added to the two optical feedback cavities to eliminate the weak periodicity of the generated chaotic signals. For this system, we numerically investigate the influence of the system parameter, such as the delay time, feedback coefficient, etc., on the time delay characteristic of the chaotic laser. In this paper, the time delay characteristic of chaotic signal is expressed by the maximum value of the time delay signature (TDS) peak of the autocorrelation function curve. Then, to illuminate the effectiveness of this system, other two systems, i.e., DFB-SL with double optical feedback (DFB-SL-DOF) and DFB-SL with single phase modulated optical feedback (DFB-SL-SPMOF) are considered. We study the suppression effect of the system on the TDS among DFB-SL-DPMOF, DFB-SL-DOF and DFB-SL-SPMOF. For these three systems, we give and analyze the simulation curves of the time delay characteristic values with the feedback coefficient and the pumping factor respectively. The results indicate that our proposed scheme has the best suppression effect. Moreover, we numerically investigate the BW of chaotic signals from DFB-SL-DPMOF based on the parameter conditions suppressing TDS effectively. The results show that BW becomes large with the pumping factor and feedback coefficient increasing, and the maximum BW value of the obtained chaotic laser is about 7.2 GHz. Therefore the effectiveness of the presented scheme is numerically clarified. And the conclusions of this paper are useful for applying the chaotic laser to the secure communication field.

正弦调频下大带宽激光调频连续波测距技术

正弦调频下大带宽激光调频连续波测距技术

Schlieren imaging for the determination of the radius of an excited rubidium column

AWAKE develops a new plasma wakefield accelerator using the CERN SPS proton bunch as a driver Muggli et al. (2017). The proton bunch propagates through a 10m long rubidium plasma, induced by an ionizing laser pulse. The co-propagation of the laser pulse with the proton bunch seeds the self modulation instability of the proton bunch that transforms the bunch to a train with hundreds of bunchlets which drive the wakefields. Therefore the plasma radius must exceed the proton bunch radius. Schlieren imaging is proposed to determine the plasma radius on both ends of the vapor source. We use Schlieren imaging to estimate the radius of a column of excited rubidium atoms. A tunable, narrow bandwidth laser is split into a beam for the excitation of the rubidium vapor and for the visualization using Schlieren imaging. With a laser wavelength very close to the D2 transition line of rubidium (λ≈780nm), it is possible to excite a column of rubidium atoms in a small vapor source, to record a Schlieren signal of the excitation column and to estimate its radius. We describe the method and show the results of the measurement.

Influence of Carbon Nanotubes Saturable Absorbers Diameter on Mode-Locking Erbium-Doped Fiber Laser Performance

We present a study about the influence of saturable absorbers single-wall carbon nanotubes (CNT) diameter on the passive mode-locking Erbium doped fiber laser performance. We used several samples of CNT with different mean diameters (from 0.8 to 1.4 nm) and distributions to fabricated CNT/polymer films. Films with 1.0 nm mean diameter CNT showed best results in terms of larger laser bandwidths and shorter pulse durations, compared to the other CNT samples used in our experiments. This result is due to the fact that 1.0 nm diameter CNT absorption energy gap (∼0.8 eV) is close to the photon energy of operating Erbium-doped fiber laser wavelength (∼1557 nm). Laser bandwidths up to 8.7 nm and pulse durations as short as 0.55 ps were obtained.

Spectral and far-field broadening due to stimulated rotational Raman scattering driven by the Nike krypton fluoride laser.

Stimulated rotational Raman scattering (SRRS) in the ultraviolet region (λ=248 nm) has been observed at the Nike laser over extended propagation paths in air during high power operation. Although this phenomenon is not significant for standard operating configurations at Nike, broadening of the laser spectrum and far-field focal profiles has been observed once the intensity-path length product exceeds a threshold of approximately 1 TW/cm. This paper presents experimental results and a new theoretical evaluation of these effects. The observations suggest that significantly broader spectra can be achieved with modest degradation of the final focal distribution. These results point to a possible path for enhanced laser-target coupling with the reduction of laser-plasma instabilities due to broad laser bandwidth produced by the SRRS.

Toward High-Power All-Fiber 2–5 μm Supercontinuum Generation in Chalcogenide Step-Index Fiber

In this paper, a high-power spectrally flat 2-5 μm supercontinuum (SC) laser is reported in a piece of As2S3 chalcogenide step-index fiber. A fluoride fiber-based SC laser which spans from 2-4.2 μm is used as the pump light. Two pump coupling systems between the fluoride and As2S3 fibers, one based on aspheric lens coupling and the other using all-fiber mechanical splicing are investigated. With the setup of aspheric lens coupling, the obtained SC laser has a 10 dB spectral bandwidth of 3000 nm spanning from 2050 to 5050 nm, corresponding to an average power of 57.6 mW. While with the all-fiber mechanical splicing, the 10 dB spectral bandwidth of the SC laser shrinks slightly, spanning from 2140 to 4980 nm range, but the average output power is further scaled to a record power of 97.1 mW. It is found that the dominant nonlinearities for continuous spectral broadenings in the As2 S3 fiber are self-phase modulation and stimulated Raman scattering in the normal dispersion regime. This paper presents an all-fiber mid-infrared SC laser with spectrum beyond 5 μm for the first time. It also offers a practical solution for robust and reliable high power 2-5 μm broadband fiber laser.

Wide bandwidth and flat multiwavelength Brillouin-erbium fiber laser.

A wide bandwidth and flat multiwavelength Brillouin-erbium fiber laser is demonstrated experimentally. In the proposed laser setup, the combination of a Brillouin mirror with feedback and a ring cavity with four-wave mixing assistance is realized. The efficiency of Brillouin Stokes lines generation is enhanced by the feedback-based Brillouin mirror structure. The effect of four-wave mixing in highly nonlinear fiber increases the generation of Brillouin Stokes lines in a wider bandwidth. The laser lines over 16 nm bandwidth (i.e 200 channels) within 4.65 dB power difference are obtained. The generated laser lines span from 1534 to 1550 nm with wavelength spacing of 0.08 nm and optical signal-to-noise ratio of at least 15 dB. The laser can also be freely tuned over 32 nm and is stable with power fluctuations of 0.7 dB over 1 hour duration.

TiO<sub>2</sub>-Doped Single-Mode Fiber as Active Material for Raman Lasers

The properties of TiO2-doped fiber are considered as optoelectronic material in our work. The advantages of such fiber have been studied with the aim of its application to active medium in Raman fiber lasers. The comparison of spontaneous Raman spectra and corresponding gain profiles in TiO2-doped and GeO2-doped fiber is presented. Raman gain profiles were obtained over a broad spectral range of Stokes shifted frequencies up to 1400 cm-1(42 THz). The spectral decomposition using multimode Gaussian components has been performed for both Raman gain profiles. High accuracy analytic form of Raman gain profile of TiO2-doped fiber is obtained using 12 components. The pump power of Raman gain threshold is introduced as the function of wavelength within the telecommunication windows for both fiber types. Our spectroscopic analysis allows presenting the numerical results on lasing bandwidth and Raman gain threshold inTiO2-doped single-mode fiber. It has been shown that the lasing bandwidth in TiO2-doped fiber may be almost twice wider than the lasing bandwidth in standard GeO2-doped fiber.

Single-mode Nd:YAG laser with transverse diode pumping and multiloop self-pumped phase-conjugate cavity

A high-power compact repetitively pulsed Nd:YAG laser with transverse diode pumping and multiloop self-pumped phase-conjugate cavity is proposed. Pulse trains with an energy of 1.5 J and beam quality parameter of M 2 ≤ 1.2 are generated at a divergence of 0.4 mrad and luminance of 5 × 1014 W/(cm2 sr). The peak power of single-frequency pulses is greater than 15 MW at an energy of 170 mJ and a laser bandwidth of 300 MHz.

Ultrabroadband 2D electronic spectroscopy with high-speed, shot-to-shot detection.

Two-dimensional electronic spectroscopy (2DES) is an incisive tool for disentangling excited state energies and dynamics in the condensed phase by directly mapping out the correlation between excitation and emission frequencies as a function of time. Despite its enhanced frequency resolution, the spectral window of detection is limited to the laser bandwidth, which has often hindered the visualization of full electronic energy relaxation pathways spread over the entire visible region. Here, we describe a high-sensitivity, ultrabroadband 2DES apparatus. We report a new combination of a simple and robust setup for increased spectral bandwidth and shot-to-shot detection. We utilize 8-fs supercontinuum pulses generated by gas filamentation spanning the entire visible region (450 - 800 nm), which allows for a simultaneous interrogation of electronic transitions over a 200-nm bandwidth, and an all-reflective interferometric delay system with angled nanopositioner stages achieves interferometric precision in coherence time control without introducing wavelength-dependent dispersion to the ultrabroadband spectrum. To address deterioration of detection sensitivity due to the inherent instability of ultrabroadband sources, we introduce a 5-kHz shot-to-shot, dual chopping acquisition scheme by combining a high-speed line-scan camera and two optical choppers to remove scatter contributions from the signal. Comparison of 2D spectra acquired by shot-to-shot detection and averaged detection shows a 15-fold improvement in the signal-to-noise ratio. This is the first direct quantification of detection sensitivity on a filamentation-based ultrabroadband 2DES apparatus.

Iterative algorithm for phase noise estimation in coherent optical DFT-S OFDM systems

An iterative algorithm is proposed and investigated for phase noise estimation of discrete Fourier transform spread (DFT-S) orthogonal frequency division multiplexing (OFDM). Compared with common phase estimation (CPE), significant improvement of tolerance to laser bandwidth is obtained and 2.7 dB improvement at BER of 3.8×10−3 is found using the CPE-iter scheme in 103.3 Gb/s coherent polarization-division-multiplexed DFT-S OFDM system over 480-km transmission through simulation.