Diode-pumped 2-μm lasers for the microjoule-per-pulse regime and beyond

The relative merits of coherent and noncoherent laser radars are reviewed. An information theoretic approach is used to investigate the tradeoffs between degrees of freedom (resolution) and signal-to-noise ratio in maximizing information in radar images. Similar tradeoffs between carrier-to-noise ratio and degrees of freedom are obtained for maximizing the signal-to-noise ratio in detection and estimation problems. It is shown that under some conditions noncoherent laser radars provide better performance than coherent radars, while under other conditions, the reverse is true.

2-μm wavelength-switchable Tm/Ho composite laser

A simple analytical model of a laser radar's subtended irradiance probability-density-function has been developed for both direct detection and coherent detection laser radar. The vacuum speckle irradiance statistics are developed following Goodman's M parameter treatment for direct detection ladar and also by setting the M-parameter equal to one (negative-exponential power statistics) for coherent laser radar. A turbulence M parameter is then computed using the round-trip aperture averaging analyses of Gudimetla and Holmes based on the Rytov-variance parameter computation over an atmospheric path of interest. The vacuum M parameter and the turbulence M parameter are then combined to form an effective M parameter. This effective M parameter is used in an analytically simple gamma distribution probability-density-function for the laser radar's subtended irradiance. We will show excellent agreement with the more analytically complicated two-parameter K-distribution from the literature. We will also indicate how one may include the turbulence scintillation in addition to the fundamental vacuum speckle, with increasing levels of turbulence to determine ladar performance.

We demonstrate a 2.1 μm Tm/Ho composite laser via diffusion-bonding the Tm-doped and Ho-doped YAG crystals into a single bulk structure, which facilitates the direct use of common AlGaAs diode lasers (LD) for an efficient and compact Ho laser source at room temperature. Locking the pump wavelength at absorption peak of 784.9 nm of the Tm-doped region of the composite gain medium, maximum output power of 6 W at 2122 nm was obtained with a slope efficiency (SE) of 40.1% and conversion efficiency (CE) of 33.6% from absorbed LD power to Ho laser, which is comparable in efficiency with the 1.9 μm LD resonantly pumped Ho lasers. Above 1.6 W Ho laser power at operation temperature range from 9 °C to 27 °C could also be realized by side-pumping absorption band of the Tm-doped region at 808 nm, which indicates a 40 nm broad pump wavelength range for the Ho laser at room temperatures. Via broad-band coating the output coupler with the same transmittance from 2000 nm to 2100 nm, synchronous Tm laser and Ho laser oscillation at both 2.1 μm and 2 μm was observed, where signal of the Tm laser was faded finally with the increased pump power. Furthermore, thermal lens and temperature distribution of the composite gain medium is analyzed basing on a proposed thermal model recently.

Ultrafast laser systems with pulse durations in the femto-and picosecond regime have proven their potential in several material processing applications. So far, a lot of these applications have been limited by the costly and difficult operation of the laser systems. We report on a new generation of powerful ultra-compact laser oscillators combining ’cavity - dumping’ with the proven techniques of direct diode-pumping and passive semiconductor saturable absorber mirror (SESAM) modelocking. Without using complex amplifier schemes these lasers generate femto-and picosecond short pulses with energies in the µJ range at tunable repetition rates up to 1 MHz. We demonstrate the use of the laser for fabrication of photonic devices in bulk glass. The combination of high pulse energies at high repetition rates together with a short pulse duration and good beam quality opens up new dimensions in high speed micro processing.Ultrafast laser systems with pulse durations in the femto-and picosecond regime have proven their potential in several material processing applications. So far, a lot of these applications have been limited by the costly and difficult operation of the laser systems. We report on a new generation of powerful ultra-compact laser oscillators combining ’cavity - dumping’ with the proven techniques of direct diode-pumping and passive semiconductor saturable absorber mirror (SESAM) modelocking. Without using complex amplifier schemes these lasers generate femto-and picosecond short pulses with energies in the µJ range at tunable repetition rates up to 1 MHz. We demonstrate the use of the laser for fabrication of photonic devices in bulk glass. The combination of high pulse energies at high repetition rates together with a short pulse duration and good beam quality opens up new dimensions in high speed micro processing.

The Massachusetts Institute of Technology Lincoln Laboratory has developed and tested the most advanced, high power, coherent CO2 laser radar ever built. The Firepond imaging laser radar is part of a complex of sophisticated radar systems at Millstone Hill, which is located in Westford, Massachusetts. The radar site includes the L band Millstone Hill space surveillance radar and the X band Haystack wideband imaging radar. A photograph of the Millstone Hill radar site is shown in Figure 1. The CO2 laser radar utilizes the 13C16O2 isotope to avoid the effects of atmospheric attenuation and dispersion caused by atmospheric 12C16O2. The laser radar uses linear frequency modulated waveforms to create two dimensional, range-Doppler images using the principles of inverse synthetic aperture radar (ISAR). The waveform agility of the laser radar allows a variety of radar functions to be served including acquisition and tracking.

Large-scale offshore wind turbine blades need careful dimensional inspection at the production stage. This paper aims to establish an accurate measurement technique using Coherent Laser Radar technology combined with B-Spline point generation and alignment. Through varying the Degrees of Freedom (DoF), used for data point transformation, within the Spatial Analyser software package, erroneous inspection results generated by unconstrained blade flexing can be eradicated. The paper concludes that implementing a single B-Spline point generation and alignment method, whilst allowing transformation with DoF in X, Y and Rz, provides confidence to wind turbine blade manufacturers that inspection data is accurate. The experimental procedure described in this paper can also be applied to the precision inspection of other large-scale non-rigid, unconstrained objects.

We have characterized the energy-dependent X-ray variability properties of the Seyfert 1 galaxy NGC 3783 using archival XMM-Newton and Rossi X-Ray Timing Explorer data. The high-frequency fluctuation power spectral density function (PSD) slope is consistent with flattening toward higher energies. Light-curve cross-correlation functions yield no significant lags, but peak coefficients generally decrease as energy separation of the bands increases on both short and long timescales. We have measured the coherence between various X-ray bands over the temporal frequency range of 6 × 10-8-1 × 10-4 Hz; this range includes the temporal frequency of the low-frequency PSD break tentatively detected by Markowitz et al. and includes the lowest temporal frequency over which coherence has been measured in any active galactic nucleus to date. Coherence is generally near unity at these temporal frequencies, although it decreases slightly as energy separation of the bands increases. Temporal frequency-dependent phase lags are detected on short timescales; phase lags are consistent with increasing as energy separation increases or as temporal frequency decreases. All of these results are similar to those obtained previously for several Seyfert galaxies and stellar mass black hole systems. Qualitatively, these results are consistent with the variability models of Kotov et al. and Lyubarskii, wherein the X-ray variability is due to inwardly propagating variations in the local mass accretion rate.

A methodology for the estimation of ladar cross sections from high-resolution image data of geometrically complex targets is presented. Coherent CO(2) laser radar was used to generate high-resolution amplitude imagery of a UC-8 Buffalo test aircraft at a range of 1.3 km at nine different aspect angles. The average target ladar cross section was synthesized from these data and calculated to be sigma(T) = 15.4 dBsm, which is similar to the expected microwave radar cross sections. The aspect angle dependence of the cross section shows pronounced peaks at nose on and broadside, which are also in agreement with radar results. Strong variations in both the mean amplitude and the statistical distributions of amplitude with the aspect angle have also been observed. The relative mix of diffuse and specular returns causes significant deviations from a simple Lambertian or Swerling II target, especially at broadside where large normal surfaces are present.

Diode-pumped alkali lasers (DPALs) have drawn much attention since they were proposed in 2001. The narrow-linewidth DPAL can be potentially applied in the fields of coherent communication, laser radar, and atomic spectroscopy. In this study, we propose a novel protocol to narrow the width of one kind of DPAL, diode-pumped rubidium vapor laser (DPRVL), by use of an injection locking technique. A kinetic model is first set up for an injection-locked DPRVL with the end-pumped configuration. The laser tunable duration is also analyzed for a continuous wave (CW) injection-locked DPRVL system. Then, the influences of the pump power, power of a master laser, and reflectance of an output coupler on the output performance are theoretically analyzed. The study should be useful for design of a narrow-linewidth DPAL with the relatively high output.

In conventional imaging laser radar, the resolution of target is constrained by the diffraction-limited, which includes the beamwidth of the laser in the target plane and the telescope's aperture. Synthetic aperture imaging Ladar (SAIL) is an imaging technique which employs aperture synthesis with coherent laser radar, the resolution is determined by the total frequency spread of the source and is independent of range, so can achieve fine resolution in long range. Ray tracing is utilized here to obtain two-dimensional scattering properties from three-dimensional geometric model of actual target, and range-doppler algorithm is used for synthetic aperture process in laser image simulation. The results show that the SAIL can support better resolution.

A low average power, pulsed, solid-state, 1.06-micron coherent laser radar (CLR) for range and velocity measurements of atmospheric and hard targets has been developed. The system has been operating at a field test site near Boulder, CO since September, 1988. Measurements have been taken on moving targets such as atmospheric aerosol particles, belt sanders, spinning disks, and various stationary targets. The field measurements have shown that this system exhibits excellent velocity measurement performance. A fast-tuning CW Nd:YAG oscillator has also been developed which has a frequency tuning range of greater than 30 GHz (which spans a target radial velocity range of over 16 km/s) and a tuning speed greater than 30 GHz/ms.

${\mathrm{Cr}}^{2+}$ - and ${\mathrm{Fe}}^{2+}$ -doped $\mathrm{Zn}\mathrm{Se}$ crystals are laser materials with phonon-broadened absorption and emission spectra, which provide broadband laser gain in the 2- to 5-$\text{\ensuremath{\mu}}\mathrm{m}$ wavelength range. While ${\mathrm{Cr}}^{2+}\text{:ZnSe}$ can be directly pumped with high-power Er, Ho, or Tm lasers, no such possibility exists for ${\mathrm{Fe}}^{2+}\text{:ZnSe}$. To this end, electronic excitation transfer between ${\mathrm{Cr}}^{2+}$ and ${\mathrm{Fe}}^{2+}$ ions in codoped ZnSe is investigated as an alternative excitation process in photo luminescence (PL) experiments with sub-10-ns temporal resolution. For a wide range of ion concentrations, we observe nonexponential decays of ${\mathrm{Cr}}^{2+}$ PL on a microsecond time scale, a 60-ns rise time of ${\mathrm{Fe}}^{2+}$ PL at high doping densities, and a prolonged decay of ${\mathrm{Fe}}^{2+}$ PL due to the temporal characteristics of excitation transfer over a range of interionic distances. Using a multirate equation model, the transfer process is analyzed on length scales up to 30 nm and compared to the established continuum model approach. The analysis reveals an unexpectedly efficient excitation transfer from ${\mathrm{Cr}}^{2+}$ to ${\mathrm{Fe}}^{2+}$ ions with an enhancement of the excitation transfer rates by up to a factor of 5 in comparison to resonant dipole-dipole coupling. The enhancement is assigned to (multi)phonon-assisted excitation transfer, in analogy to the phonon-mediated efficient radiationless decay of the excited ${\mathrm{Fe}}^{2+}$ state. As nonradiative losses and excitation transfer show different temperature scaling, a cryogenic temperature regime is found that promises overall efficiencies above 50%, making ${\mathrm{Fe}}^{2+}:{\mathrm{Cr}}^{2+}\text{:ZnSe}$ a much more viable alternative to parametric conversion schemes in the midinfrared range.

Coherent Laser Radar is a powerful remote sensing tool, which can be applied to range-finding, target discrimination, vibrometric monitoring, air pollution monitoring, aircraft wake-vortex and clear-air turbulence analysis. A high power, highly efficient, near diffraction limited and highly reliable pulsed coherent laser source is a key sub-system required in a coherent Lidar sensor. When humans are involved, eye safe laser emission is also typically required. Therefore a highly efficient fiber laser system based on a coherent Master-Oscillator followed by a chain of Erbium (EDFA) and Erbium co-doped with Ytterbium fiber amplifiers (EYDFA) is ideally suited for this application suite. In this paper, we are presenting an all polarization-maintaining fiber architecture and experimental results on such a high peak power fiber laser system allowing for versatile modulation strategies at a wavelength of 1563nm commensurate with a clear atmospheric transmission window and eye-safe operation. The system is constituted by three amplification stages, all based on Polarization-Maintaining fiber. With 660ns and 20Kpps, over 500W peak power pulses have been experimentally demonstrated with near diffraction limited performance with this all PM fiber system.

While synoptic surveys in the optical and at high energies have revealed a rich discovery phase space of slow transients, a similar yield is still awaited in the radio. Majority of the past blind surveys, carried out with radio interferometers, have suffered from a low yield of slow transients, ambiguous transient classifications, and contamination by false positives. The newly-refurbished Karl G. Jansky Array (Jansky VLA) offers wider bandwidths for accurate RFI excision as well as substantially-improved sensitivity and survey speed compared with the old VLA. The Jansky VLA thus eliminates the pitfalls of interferometric transient search by facilitating sensitive, wide-field, and near-real-time radio surveys and enabling a systematic exploration of the dynamic radio sky. This thesis aims at carrying out blind Jansky VLA surveys for characterizing the radio variable and transient sources at frequencies of a few GHz and on timescales between days and years. Through joint radio and optical surveys, the thesis addresses outstanding questions pertaining to the rates of slow radio transients (e.g. radio supernovae, tidal disruption events, binary neutron star mergers, stellar flares, etc.), the false-positive foreground relevant for the radio and optical counterpart searches of gravitational wave sources, and the beaming factor of gamma-ray bursts. The need for rapid processing of the Jansky VLA data and near-real-time radio transient search has enabled the development of state-of-the-art software infrastructure. This thesis has successfully demonstrated the Jansky VLA as a powerful transient search instrument, and it serves as a pathfinder for the transient surveys planned for the SKA-mid pathfinder facilities, viz. ASKAP, MeerKAT, and WSRT/Apertif.