Laser hardened materials evaluation laboratory (LHMEL)

For the past 28 years, the Laser Hardened Materials Evaluation Laboratory (LHMEL) at the Wright-Patterson Air Force Base, OH, has worked with CO 2 lasers capable of producing continuous energy up to 150 kW. These lasers are used in a number of advanced materials processing applications that require accurate spatial energy measurements of the laser. Conventional non-electronic methods are not satisfactory for determining the spatial energy profile. This paper describes continuing efforts in qualifying the new method in which a continuous, real-time electronic spatial energy profile can be obtained for very high power, (VHP) CO 2 lasers.

The objective of this program is the design and development of a millimeter-wave device to produce 200 kW of continuous-wave power at 60 GHz. The device, which will be a gyrotron oscillator, will be compatible with power delivery to an electron-cyclotron plasma. Smooth control of rf power output over a 17 dB range is required, and the device should be capable of operation into a severe time-varying rf load mismatch. An interim program goal is to demonstrate operation at 100 ms pulse widths by 31 December 1981. During this quarter, progress was made in the areas of tube, solenoid, and facility construction. Two magnetron injection gun assemblies were completed, and one was partially evaluated as a gun tester. The superconducting solenoid was assembled, but a vacuum leak prevented early delivery in March. Construction began on the gyrotron test facility by enclosing the area and providing bake-out and vacuum processing utilities.

Several mini-TWTs (traveling-wave tubes) were recently power-combined in the band 2.0 to 8.0 GHz to achieve 250 W of continuous-wave (CW) power at better than 90% combining efficiency. Graceful degradation was demonstrated by successively turning off each of the TWTs. The combining device, known as the spatial field power combiner, is especially suited for high-average-power applications. This approach has the potential for achieving CW powers in excess of 1 kW over multioctave frequency bands up to 20 GHz. This work focuses on a four-way combiner, and the results obtained in combining mini-TWTs are discussed. >

Introduction It is perhaps not surprising that using extremely high power and short pulse duration pump sources leads to dramatic nonlinear processes in optical fibres; in contrast, the generation of a supercontinuum from a continuous pump wave of relatively meagre power is at first sight, astounding. Yet supercontinua spanning over 1000 nm have been generated with pump powers of a few tens of watts – orders of magnitude lower than pulse pumped systems. The key to continuous wave (CW) supercontinuum generation is the utilisation of modulation instability (MI). This is inherent to any anomalously dispersive, nonlinear medium, and has been observed in a wide range of systems. This instability can enable the creation of the extremes of peak power and pulse duration necessary for dramatic nonlinear processes to occur, even from very low power CW pump lasers. But although MI from CW pump lasers was observed in the 1980s by Itoh et al. (1989), other factors required for efficient continuum generation were missing, causing another decade to pass before such results were obtained. A full review of experimental results will be presented later, but for reference, some examples of continuous wave supercontinua are shown in Fig. 8.1, which illustrate the high spectral power and the spectral smoothness and flatness which are characteristic of CW continuum generation. Although significantly different from the physical mechanisms involved in ultra-short – femtosecond based – supercontinuum generation, the basic physical processes underpinning CW continuum generation are the same as for longer pump pulses (greater than a few picoseconds) and the observations and conclusions developed in the 1980s surrounding these type of sources apply to the CW regime.

The issues associated with processing the outputs of an integrating acoustooptical spectrum analyzer (AOSA) to detect and identify the frequency and power of continuous wave (CW) signals are considered. The performance of the optimum detection strategy is presented, along with the Cramer-Rao bound on the performance of estimators of the frequency, and the performance of a simple peak-based frequency estimator. The performance of the maximum likelihood (ML) estimator of the power level is also presented. The results provide the behavior of the system as a function of the product of the aperture time and photodetector spacing amongst other parameters.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High power, 0.81-μm-emitting, semiconductor diode lasers are used as pump sources for Nd:YAG solid-state lasers. Devices (1-mm-long) consisting of a InGaAsP/In0.5(Ga0.9Al0.1)0.5P/In0.5(Ga0.5Al0.5)0.5P laser structure provide a threshold-current density, Jth, of 290 A/cm2 and a relatively high threshold-current characteristic temperature, T0 (140 K). Uncoated diode lasers (1.2-mm-long) have a maximum continuous wave output power of 5 W (both facets) at 20 °C. The internal power density at catastrophic optical mirror damage (COMD), P̄COMD, is determined to be 9.1 MW/cm2; that is, 1.8 times that for GaAs-active layer, Al-free, uncoated devices. Coated, InGaAsP-active devices are expected to have P̄COMD=18 MW/cm2, more than twice the P̄COMD of AlGaAs-active, 0.81-μm-emitting devices with the same emitting aperture. Therefore, 0.81-μm-emitting, InGaAsP-active diode lasers should operate reliably at powers at least twice those of AlGaAs-based devices with the same contact-stripe geometry.

Power amplifiers (PAs) using HEMTs with aluminum nitride (AlN) buffer layers promise superior thermal performance over the current generation of GaN-buffer HEMT PAs. However, the continuous-wave (CW) output power densities from AlN-buffer HEMTs have been limited to <6 W/mm so far. This work presents the advancement in the large-signal RF performance of AlN-buffer HEMTs with a 2× improvement in CW output powers. AlN-buffer HEMTs with 180 nm gate length and 2 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i>m source-drain distance fabricated on 100 mm SiC substrates, show oncurrents of 1.1 A/mm and breakdown voltage ∼100 V. On-wafer load-pull power sweep measurements conducted at 10 GHz demonstrated a maximum CW power output (P<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">out</sub>) of 5.5 W/mm at 28 V and 11.7 W/mm at 50 V bias. At 50 V, the peak PAE of 49 % had an associated Pout 11.32 W/mm and linear gain of 9.35 dB. This significant power enhancement in AlN-buffer HEMT performance, coupled with its intrinsic thermal advantage over GaN-buffer HEMTs, paves the way for adoption in critical RF applications.

Diode lasers emitting single-frequency, diffraction limited beams at a continuous-wave (CW) optical power of several Watts are required for many applications including frequency conversion, free-space communications, and pumping of fibre lasers and fibre amplifiers. The spectral region around 980 nm is of particular interest for the pumping of erbium and ytterbium doped fibres as well as for the second harmonic generation of laser radiation at 488 nm replacing Ar ion lasers in material analytics. One of the most promising devices is the monolithically integrated master-oscillator power-amplifier (MOPA), where either a distributed Bragg reflector (DBR) laser or a distributed feedback (DFB) laser acting as the master oscillator and a straight or tapered gain-region power-amplifier are combined on a single chip. Here, a MOPA consisting of a DBR laser and a tapered amplifier is demonstrated which emits a CW power of more than 10 W at 977 nm in a nearly diffraction limited beam and narrow spectral bandwidth of 40 pm. To the authors' knowledge, the 10 W reported here is the highest nearly diffraction-limited single-frequency power ever demonstrated from a single-chip diode laser at any wavelength.

AIGaN/GaN HEMTs have positioned themselves as strong candidates for future radar applications requiring high power at microwave frequencies. In the context of studying the reliability of RF High Power Amplifiers (HPA) in their real environment, a study of the effect of electromagnetic stress on AIGaN/GaN HEMTs performances is presented in this paper. A near field setup is used to disturb with electromagnetic field a device under test (DUT). A comparative study between the injection of CW (Continuous Wave) and pulsed RF power are presented. Degradations in DC and power characteristics are observed. Thermal and trapping processes induce them. Pulsed RF power allows reducing the effect of self-heating.

The two-stage (TS) multibeam klystron (MBK) technology has recently attracted significant research attention due to its compactness and high-efficiency (HE) performance. However, there is still a lack of scientific research on the beam optics for such microwave power sources integrated with a postacceleration (PA) gap. In this article, a comprehensive optics study based on the newly developed 2-D optics code CGUN is conducted for the first time to demonstrate the most critical steps in the optics design process, by adopting the 400-MHz TS MBK for the future circular collider (FCC) as an example. Two specific challenges arise in this TS MBK, which are studied in this article, and solutions are given. First, due to the combination of slow electrons, impedance change from individual beamtubes into common volume, and the mild decay of the magnetic field, there are possible reflected electrons at the collector entrance. This requires an increase in the beam voltage to 80 kV, beyond the requirements from considering the output gap alone, as well as tighter control on bouncing electrons. The beam scalloping is also found to be highly sensitive to the position of the PA gap and magnetic field, which later demonstrates that large gap length and magnetic field are required. Final all-in-one particle-in-cell (PIC) simulations of this klystron equipped with this special optics design demonstrate that the specification of 1.2-MW continuous wave (CW) power is practically attainable with an efficiency of 77.5% and without the presence of reflected electrons at any point in the whole circuit.

The coagulation effect of a continuous-wave neodymium-YAG (Nd-YAG) laser and of a high-power (500 W) pulsed (3–500 ms) microcomputer-controlled Nd-YAG laser are compared in rat liver in vivo. In a series of 68 animals, surface temperature was measured with an infrared camera, and necrosed volume was assessed histologically 24 h after laser irradiation. The high-power pulsed Nd-YAG laser presents two interesting features. First, it gives a better control of surface temperature. Second, a controlled sequence of high-power short pulses produces a predictable coagulated volume which can be comparable to that obtained with continuous-wave Nd-YAG or argon lasers.

This manuscript reviews the present status of 'commercial- grade,' state-of-the-art planar, batch-fabricable, vertical- cavity surface-emitting lasers (VCSELs). Commercial-grade performance on all fronts for high-speed data communications is clearly established. In discussing the 'present,' we focus on the entrenched proton-implanted AlGaAs-based (emitting near 850 nm) technology. Renditions of this VCSEL design exist in commercial products and have enabled numerous application demonstrations. Our designs more than adequately meet producibility, performance, and robustness stipulations. Producibility milestones include greater than 99% device yield across 3-in-dia metal-organic vapor phase epitaxy (MOVPE)-grown wafers and wavelength operation across greater than 100-nm range. Progress in performance includes the elimination of the excessive voltage-drop that plagued VCSELs as recently as 2 to 3 years ago. Threshold voltages as low as V<SUB>th</SUB> equals 1.53 V (and routinely less than 1.6 V) are now commonplace. Submilliamp threshold currents (I<SUB>th</SUB> equals 0.68 mA) have even been demonstrated with this planar structure. Moreover, continuous wave (cw) power P<SUB>cw</SUB> greater than 59 mW and respectable wall-plug efficiencies ((eta) <SUB>wp</SUB> equals 28%) have been demonstrated. VCSEL robustness is evidenced by maximum cw lasing temperature T equals 200 degrees Celsius and temperature ranges of 10 K to 400 K and minus 55 degrees Celsius to 155 degrees Celsius on a single VCSEL. These characteristics should enable great advances in VCSEL-based technologies and beckon the notion that 'commercial-grade' VCSELs are viable in cryogenic and avionics/military environments. We also discuss what the future may hold in extensions of this platform to different wavelengths, increased integration, and advanced structures. This includes low-threshold, high- speed, single-mode VCSELs, hybrid VCSEL transceivers, and self-pulsating VCSELs.

A computer simulation model of the nonstationary Doppler ultrasound signals arising from pulsatile blood flow is presented. The model uses sinusoidal components that are modulated by a power spectral density function that varies over the cardiac cycle. An empirical model consisting of two exponential functions is used to represent both the continuous wave and pulsed Doppler power spectral density for normal carotid arteries. It is shown that the spectrogram speckle patterns of the synthesized Doppler signals compare very well with those clinically recorded.

We have developed a single-frequency tapered laser emitting at a wavelength of 1.55 pm. The ridge waveguide section enables only one lateral mode, while the DBR structure provides single ffequency operation. This tapered laser has 20 dl3 side-mode suppression at continuous wave (CW) power levels up to 0.6 W. 80 'YO of the power from this device remains in the central lobe of the far-field at a power level of 0.5 W. A Gaussian contact over the tapered section matches the current distribution to the lateral optical profile reducing self-focusing and beam instability effects. We are also developing both distributed feedback (DFB) and distributed Bragg reflector (DBR) angled-stripe semiconductor laser designs for high power. To obtain power levels approaching 10 W, we are using a lenslike waveguide that causes divergence along the lateral direction along with a second order grating to provide outcoupling along the surface of the device.

Clinical protocols are recommended in device guidelines outlined for treating many diseases on empirical basis. However, effects of low-intensity infrared lasers at fluences used in clinical protocols on DNA are controversial. Excitation of endogenous chromophores in tissues and free radicals generation could be described as a consequence of laser used. DNA lesions induced by free radicals cause changes in DNA structure, chromatin organization, ploidy degrees and cell death. In this work, we investigated whether low-intensity infrared laser therapy could alter the fibroblasts nuclei characteristics and induce DNA fragmentation. Tendons of <i>Wistar</i> rats were exposed to low-intensity infrared laser (830 nm), at different fluences (1, 5 and 10 J/cm²), in continuous wave (power output of 10mW, power density of 79.6 mW/cm²). Different frequencies were analyzed for the higher fluence (10 J/cm²), at pulsed emission mode (2.5, 250 and 2500 Hz), with the laser source at surface of skin. Geometric, densitometric and textural parameters obtained for Feulgen-stained nuclei by image analysis were used to define nuclear phenotypes. Significant differences were observed on the nuclear phenotype of tendons after exposure to laser, as well as, high cell death percentages was observed for all fluences and frequencies analyzed here, exception 1 J/cm² fluence. Our results indicate that low-intensity infrared laser can alter geometric, densitometric and textural parameters in tendon fibroblasts nuclei. Laser can also induce DNA fragmentation, chromatin lost and consequently cell death, using fluences, frequencies and emission modes took out from clinical protocols.