The output beam quality of a Q-switched Nd:glass slablaser

The authors have constructed and tested a flashlamp pumped, Q -switched, Nd:glass zigzag slab laser. The thermally induced optical distortion through the slab is minimized by uniform pumping and cooling and the use of corrective pump shields at the slab ends. The laser spatial output for Q-switched resonators has been measured and modeled. It is shown that a large aperture planar oscillator has an output divergence many times above the diffraction limit. Operation as a one-dimensional unstable resonator in the wide direction of the slab allows the efficient extraction of energy in a high-quality beam. Near-diffraction-limited laser output of 5 J at 4 Hz is achieved with a resonator that includes an intracavity telescope to correct for residual defocusing in the thin direction of the slab     

Characterization of an ultrahigh peak powerXeF(C→A) excimer laser system

The gain characteristics of an electron-beam pumped XeF(C→A) excimer amplifier operating in the blue-green spectral region were investigated for several laser pulse lengths. Saturation energy densities of 50 and 80 mJ/cm² were measured for injected laser pulse durations of 250 fs and ~100 ps, respectively. A gain bandwidth of 60 nm was observed with ~100-ps pulse injection. Using an optimized unstable resonator design, the laser amplifier has produced 275-mJ pulses with a pulse duration of 250 fs and a 2.5 times diffraction-limited beam quality, making the XeF(C→A) amplifier the first compact laser system in the visible spectral region to reach peak powers at the terawatt level     

A 5 kW CW CO2 laser using a novel negative-branchunstable resonator with a phase-unifying output coupler

A 5 kW CW CO₂ laser using a negative-branch unstable resonator (M=-1.5) is proposed and characterized experimentally. The resonator consists of a stepwise variable reflecting output coupler, called the phase-unifying output coupler, and a total reflector. A 5 kW CW laser beam with diffraction limited quality (2&thetas;=0.45 mrad) is obtained and affected by the focal point in the resonator. The misalignment angle to reduce the laser power to 95% is improved by a factor of 19 compared with a positive-branch unstable resonator (M=1.5) with a phase-unifying output coupler at the same resonator length     

Output energy characteristics of optimally pumped rotating mirrorQ-switch lasers

The author develops a simple analytical formula for the key laser resonator and Q-switch parameters of an optimally pumped rotating mirror Q-switch laser. The analytical expression developed relates the maximum attainable single pulse output energy to the laser parameters such as rotating mirror speed, output mirror reflectivity, length of the resonator, and the absorption cross section of the laser media on the basis of the experimentally measured mechanical Q-switch loss form. Good agreement between theory and the output performance of a test Nd-glass laser has been demonstrated     

Optimization of an injection-controlled excimer laser guided byanalytical modeling

The application of an analytical model describing the injection control of pulsed laser systems is successfully demonstrated for the design of a scaled XeF(C→A) excimer laser system. Enhancements to an earlier version o the model which improve the treatment of spatial beam overlap and saturation, unpumped volume, intracavity losses, and a noninteger number of roundtrips in the unstable resonator are described. These result in the accurate simulation of injection-controlled laser performance over a wide range of unstable resonator magnifications, mirror spacings, and intracavity optical losses. Excellent agreement between calculated and experimentally observed energies and temporal profiles of the injection-controlled laser output was obtained     

Development of EM field in lasers with rotating mirrorQ-switch

A fast-Fourier-transform (FFT) computational scheme based on a Fourier expansion technique is developed to calculate the development of an electromagnetic (EM) field from spontaneous noise inside a loaded rotating mirror Q-switched laser. A rate equations analysis is also carried out, using the FFT-calculated dependence of the diffraction loss on the fixed Q-switch mirror tilt angle. The computational results are compared with the output characteristics of an eye-safe (λ=1.54 μm) erbium glass laser. The experiments are in good qualitative and quantitative agreement with the predictions of the FFT mode. The model reveals short-duration intense spatial regions, as well as two types of temporal modulations of the laser output pulses. These modulations are attributed to the inhomogeneous development of the laser field in the longitudinal and transverse laser resonator direction. These irregularities must be taken into account when calculating eye-safety levels of rotating mirror Q-switched lasers     

Time-resolved spectroscopic study of high-pressure self-sustaineddischarge-pumped atomic xenon lasers

To explore the lasing kinetics of UV-preionized, self-sustained discharge-pumped atomic xenon (5d→6p) lasers, the time-resolved spectroscopy of the laser output from the multiline laser resonator is reported. The dilutents used were Ar and He. Increasing Xe concentration shortened the 1.73-μm laser pulse duration and decreased the total (multiline) laser output energy, because increased Xe metastable state population contributes to the increase of the 6p state population (lower laser level) by electron-impact excitation and radiation trapping during discharge pumping. High-excitation-rate pumping resulted in the decrease of the laser output power of 1.73- and 2.63-μm lines. Increasing the total gas pressure leads to high-efficiency operation due to modest-excitation-rate pumping at high pressures     

5 GHz high-temperature-superconductor resonators with high Q and low power dependence up to 90 K

The authors have fabricated high-temperature superconducting films made of TlBaCaCuO (2212) and YBaCuO (123) by postdeposition annealing techniques on (100) LaAlO₃ substrates. These films, especially the TlBaCaCuO(2212), exhibit high temperature operation, high Q (low surface resistance), and low power dependence. Both types of films have measured surface resistances which are better than 1/10 that of copper to 20 GHz. Microstrip resonators with a fundamental resonance frequency of 5 GHz were fabricated from these materials. The performance of the best resonator at 90 K (loaded Q>20000 at 5 GHz) was 50 times better than an analogous copper resonator (also measured at 90 K) and can handle more than 10 W of peak power in the resonator with only a small degradation of the Q. In addition, the shift of the resonator frequencies with temperature was fit to a two-fluid model     

Ferromagnetic tridisk-coupled resonator and magnetically tunablestripline Y circulator

A ferromagnetic tri-disk-coupled (TDC) resonator is constructed by placing three YIG ferrite disks mutually attached on the circular center conductor. The EM fields in one of the disks is described in terms of expanded circular harmonics with an approximated transformation of derivative, and the EM fields in a TDC resonator is synthesized from the respective constituent fields of the disks, by sum-transformation, regarding three eigenjunctures of the TEC resonator. The synthesized fields include coefficients A, B, and D, which are described as functions of the radis ratio r₀ /r and azimuthal angle φ. The special case in which A=D=0 is treated. Two conditional equations of perfect Y circulation are calculated. Magnetically tunable operation is theoretically analyzed using the conditional curves, and the magnetically tunable operation is examined experimentally with theoretical analysis     

A novel multilevel coherent optical system: 4-quadrature signaling

A novel multilevel coherent optical system is proposed. It is based on the exploitation of the property that the electromagnetic field propagating in a single-mode optical fiber can be represented by a four-dimensional vector whose components are the phase and quadrature terms of the two polarization components of the electrical field. This allows a wider use of the resources of the electromagnetic field for information transmission in order to obtain a spectrally efficient modulation format with a limited end. The net performance gain with respect to multilevel amplitude and phase modulation (N-APK) and N-PSK increases with an increase in the number of levels N. For instance, for N=32 the gain is 1.6 and 7.7 dB with respect to N-APK and N-PSK systems. The effect of laser phase noise on the system performance is evaluated     

Wavelength dependence of 1/f noise in the light output oflaser diodes: an experimental study

The optical power emitted by a monomode GaAlAs laser is filtered with a monochromator. The 1/f noise in the filtered emission is found to be directly dependent on the noncoherent emission, such as S_pαP_nc^m. Here s_p is the spectral density of the 1/f fluctuations, P_nc is the average noncoherent power, m=3/2 under spontaneous emission, and m=4 in the superradiation and laser regions. Study of the 1/f noise in the optical power in a band centered at the laser wavelength and with variable bandwidth shows three operating regions. (1) LED region (at low currents): the fluctuations with a 1/f spectrum are uncorrelated in wavelength. (2) Superradiation region (at currents close to the threshold): the fluctuations are correlated. (3) Laser region: the 1/f noise apparently is dominated by noncoherent emission within a small optical band around the laser wavelength     

Influence of free carrier plasma effect on carrier-inducedrefractive index change for quantum-well lasers

The influence of the free carrier component due to the plasma effect on carrier-induced refractive index change and its dependency on polarization for multiple-quantum-well (MQW) and bulk lasers are experimentally studied. The ratios of the component to the total index change, R_fc, are 0.6, 0.4, and 0.1 for 1.3-μm MQW, 1.3-μm bulk, and 0.8-μm MQW lasers, respectively. The TM/TE polarization ratios of the component, R_TMTE/, are 0.8 and 0.3 for 1.3-μm MQW and 0.8-μm MQW lasers. The relationship between the index change and the carrier overflow (to barrier and separate confinement heterostructure layers) for MQW lasers is also discussed. Large R_fc and R_TMTE/ for the 1.3-μm MQW laser result from the carrier overflow     

Analysis of laser diodes as transmitter and preamplifiers in timecompression multiplexing systems

It is proposed that a laser diode used as a transmitter in the transmission mode be used as a low bias current preamplifier in the receiving mode for time compression multiplexing transmission. By adopting the low bias current approach, the preamplifier is wavelength and polarization independent. The maximum bias current is determined to be about 0.5 I_th (I_th=threshold current) by experiments. A transmission experiment at 3.5 Mb/s reveals that about 4 dB of improvement in receiver sensitivity can be obtained by using a laser preamplifier at I=0.54 I_th when compared to an unbiased laser. The receiver sensitivity of the proposed configuration (laser amplifier at low bias current and photodiode) is estimated by considering the laser preamplifier noise and circuit noise. The laser preamplifier noise is calculated by treating the laser as a traveling wave amplifier with highly reflecting facets     

Seasat over-land scatterometer data. I. Global overview of theKu-band backscatterer coefficients

Statistics on the backscatter coefficient σ⁰ from the Ku-band Seasat-A Satellite Scatterometer (SASS) collected over the world's land surfaces are presented. This spaceborne scatterometer provided data on σ⁰ between latitude 80° S and 80° N at incidence angles up to 70°. The global statistics of vertical (V) and horizontal (H) polarization backscatter coefficients for 10° bands in latitude are presented for incidence angles between 20° and 70° and compared with the Skylab and ground spectrometer results. Global images of the time-averaged V polarization σ⁰ at a 45° incidence angle and its dependence on the incidence angle are presented and compared to a generalized map of the terrain type. Global images of the differences between the V an H polarization backscatter coefficients are presented and discussed. The most inhomogeneous region, which contains the deserts of North Africa and the Arabian Peninsula, is studied in greater detail and compared with the terrain type     

Repetitively pulsed operation of an injection-controlled high-powerXeF(C→A) excimer laser

The operation at a 1-Hz repetition frequency of an injection-controlled electron-beam-pumped XeF(C→A) excimer laser system is reported. A compact, halogen-compatible, closed flow loop incorporating a transverse inline fan was used for gas circulation. In single-laser-shot operation, the timing between an electron beam and the injection dye laser was carefully adjusted to obtain an optimum laser pulse energy stability. An improved output-laser energy of 1.2 J per pulse with an intrinsic efficiency of 1.1% at 486.8 nm was achieved with a large-aperture unstable resonator. Interferograms taken during and after an electron-beam pump pulse to determine the minimum optical cavity recovery time of this device indicate that stable laser output energy performance at repetition rates of up to 25 Hz could be achieved with the present flow loop     

Dielectric resonator filters with wide stopbands

Dielectric resonators used in filter networks present a model spectrum with undesired, or spurious, resonances in close proximity to the desired one. Through the use of evanescent mode bandpass irises tuned to the filter center frequency, the resonator spurious modes are suppressed, resulting in N-section filters with stopbands clean to at least (N-1) times the individual iris stopband levels. The problem of achieving a wide stopband is reduced to the problems of realizing the resonating capacitance required in an evanescent bandpass filter and computing the junction susceptance occurring at the interface between a larger evanescent section (the resonator enclosure) and the smaller iris opening. The technique results in high-Q resonator filters with stopbands clean to at least -55 dBc, out to at least 1.7 times the filter center frequency     

Modeling the nonlinearity of superconducting strip transmissionlines

The use of the Ginzburg-Landau (GL) theory to predict the nonlinear behavior in a superconducting stripline resonator as a function of input current is reported. A method for calculating the nonlinear inductance and the fractional change in the resonant frequency (Δf/f) of a stripline resonator is presented. By solving the GL equations inside the superconducting strip, the spatial variation of the number density of superconducting electrons and, hence, the spatial variation of the magnetic penetration depth are obtained for different values of input current. First, an infinite parallel plate transmission line is considered where the one-dimensional GL equations are solved. The two-dimensional case of a stripline is then considered. Nonlinear inductances are calculated as functions of input current for different superconducting striplines. Comparisons of the calculated Δf/f with measurements for YBa₂Cu₃O_7-x stripline resonators show excellent agreement     

Dependence of transient gain on gas-flow velocity and dischargecurrent in the FAF CO2 laser amplifier

Transient behaviors in which the output power amplified through the fast-axial-flow (FAF) CO₂ laser amplifier decreases temporally after initiating the discharge are discussed. The output power becomes stable within a few minutes, i.e. the transient time. These phenomena should strongly depend on input irradiance, plasma length, gas-flow velocity v, and discharge current I _dis. The small signal gain γ₀ is formulated as a function of v and I_dis and the discharge time for a partially-homogeneously-broadened, slightly saturated gain medium at individual vibrational-rotational transitions     

Feedback phenomena in a semiconductor laser induced by distantreflectors

Multiple pass resonances in the noise spectrum and low-frequency self-oscillations in asymmetric geometrical configurations are described. The double peaked structure of the external cavity resonance f_c (at c/2L), the `subharmonic cascading' from f_c, and the generation of subharmonics of a modulated injection current are detailed. The exact field intensity distribution in the passive external resonator is shown to command these phenomena which reveal intermingled spatial and dynamical effects. The different behaviors are related to the characteristic curves of the laser. The authors have found that when the external cavity laser operates in its unstable regime, a locking of the otherwise randomly distributed intensity drops can occur giving a narrow resonance in the noise spectrum. In addition, based on the authors' observations, adapted rate equations are built starting from the Lang and Kobayashi model. Results show an adequate match between theory and experiments for the detailed microwave spectra, multiple pass resonances of the external cavity, and low-frequency resonances     

Output characteristics of a compact 1 J carbon dioxide laser with aGaussian reflectivity resonator

The authors describe the performance of a compact (41-cm-long) e-beam sustained atmospheric pressure CO₂ laser using a Gaussian reflectivity mirror in an unstable resonator configuration. The device operated on a single longitudinal and transverse mode (W _1L=12.9 mm) at an output energy of around 1 J. The frequency characteristics were studied and acoustic effects were observed as a result of the near-threshold operation