Showing papers in "IEEE Journal of Quantum Electronics in 1976"

Switched directional couplers with alternating ΔΒ

Abstract: Coupled waveguide structures with sections of alternating phase mismatch are proposed as switched directional coupler configurations in which complete conversion of light from one guide to the other can be achieved by an electrical adjustment. These structures can be used to make electrooptic switches and amplitude modulators in integrated optics form with improved conversion and on-off ratios. Couplers with 2, 3, 4, and N sections of alternating phase mismatch are analyzed, and diagrams describing their switching characteristics are given.

Antisymmetric taper of distributed feedback lasers

Abstract: In a distributed feedback (DFB) laser with spatial index modulation, an antisymmetric taper of the feedback parameter, k , removes the threshold degeneracy, which is characteristic of uniform structures, and leads to one mode of particularly low threshold. Exact solutions are presented for the special case of an antisymmetric step of k . An approximate perturbation method is developed which gives simple expressions for the threshold gains, and external Q 's of some tapered distributed feedback structures. The method is tested against two exact solutions, the uniform, and the stepped- k DFB laser. It is shown that the threshold of the stepped- k DFB laser is the lowest of any structure with an antisymmetric taper and a prescribed maximum value of |k| .

Dependence of the 4 F 3/2 → 4 I 11/2 induced-emission cross section for Nd 3+ on glass composition

Abstract: Systematic studies of spectral intensities using the Judd-Ofelt (J-O) theory for crystal-field-induced electric-dipole transitions demonstrate that Nd-doped glasses can be tailored for specific laser applications by proper selection of the glass constituents. Variations with glass composition are reported for the following properties: induced-emission cross section, peak fluorescence wavelength, effective fluorescence bandwidth, radiative lifetime, transition probabilities, and fluorescence branching ratios. Both glass network formers and network-modifier ions were changed. In these studies the induced-emission cross section for the Nd3+,4 F_{3/2} \rightarrow 4 I_{11/2} laser transition was varied by more than a factor of 4 by changing the glass host.

Correction to "Analysis of curved optical waveguides by conformal transformation"

Abstract: The method of conformal transformations is applied to the analysis of waveguide bends. Equivalent structures are obtained that permit solution by traditional methods of optical waveguide analysis. Losses associated with both curvature and with the transition from straight to curved guides are discussed and simple first-order expressions that describe the dependence of the losses on waveguide parameters are derived.

Parameter ranges for CW passive mode locking

Abstract: CW passive mode locking of a homogeneously broadened laser is considered. In the coordinate plane, whose abscissa is proportional to the small-signal saturable absorber loading, and whose ordinate is proportional to the small-signal gain, the following regimes are laid out: 1) steady-state single-pulse mode-locking solutions; 2) stability against relaxation oscillations; 3) self-starting of mode locking. The assumption is made that CW mode locking can be obtained only for a choice of parameters for which all three regimes overlap. We require further that the overlap regime be reached by a monotonic increase of small-signal gain (pumping), without passing outside regime 2). Under these conditions one may state requirements on the system parameters for the obtainment of single-pulse mode locking by a saturable absorber. The analysis explains why it has been impossible to mode lock passively the CW Nd:YAG laser, but passive mode locking of the CW dye-laser system is possible.

Analysis of grating-coupled radiation in GaAs:GaAlAs lasers and waveguides - I

Abstract: Grating-coupled radiation in GaAs:GaAlAs lasers and waveguides is analyzed. A general formulation is developed for arbitrary-shaped gratings which need not be small in size. Two methods are used to solve the resulting equations in the case of rectangular-shaped gratings. The first is a perturbation technique and the second is iterative in nature. The iterative procedure converges to a numerical exact solution in many cases of practical interest and indicates that the perturbation results are quite accurate. Curves are presented for radiated power from traveling waves as a function of grating tooth height, tooth width, refractive index, waveguide thickness, and period for rectangular gratings in heterostructure waveguiding geometries. It is shown that radiation is not a monotonically increasing function of tooth height, but rather maxima occur when the teeth are half the optical wavelength in the material. Also, in particular geometries with an air:GaAs grating interface, radiated power of a mode can exceed 100 cm-1.

Experimental and theoretical investigation of the nitrogen laser

Abstract: We have studied several types of flat-plate-transmission-line pulse-excited nitrogen lasers. Experiments were carried over a wide variety of parameters such as laser-tube pressure, charging voltage, laser-tube inductance, transmission-line impedance, and others. A theory of the nitrogen laser is presented which treats the laser as an integrated electrooptical system. The theory is based upon the known macroscopic properties of nitrogen discharges along with the known electron impact ionization and excitation cross sections for the N 2 molecule. The theoretical predictions are in very good agreement with the experimental observations. It is suggested that this type of theoretical approach should be applicable to fast high-power discharges in many other gases.

Injection locking and mode selection in TEA-CO 2 laser oscillators

Abstract: The injection of a master oscillator signal in a high-power TEA-CO 2 laser is analyzed and a dynamic model is formulated to represent the interaction. Based on the competition between the injected signal and the spontaneous emission, the model describes the transient evolution of the different field amplitudes and phases together with their effect on the inversion. A study over a wide range of injection levels and detuning frequencies clearly indicates three distinct regions of operation: a spontaneous oscillation zone, a mode-selection zone, and a frequency-locking zone. The main predictions of the model are compared with the results obtained with an experimental injection apparatus that assures adequate control of the TEA laser cavity length and that provides means for measuring the frequency of the output pulse. While the first two zones are directly observed, it is experimentally established that, at injection levels up to 5 W/cm2, the frequency-locking zone does not exceed, as predicted, the 3-MHz resolution limit of the apparatus.

Radiation loss coefficients of asymmetric dielectric waveguides with shallow sinusoidal corrugations

Abstract: Kiselev [10] has solved the boundary equations for an asymmetric slab waveguide with a shallow sinusoidal corrugation, and derived explicit, closed expressions for the TE radiation loss coefficients of various waveguide geometries. For many practical applications in integrated optics, the Rayleigh assumption justifying the boundary-matching procedure and the approximations of linear perturbation analysis employed in this derivation are both satisfied. We have extended this procedure to TM modes and found that the expressions for both TE and TM modes in all of the geometries so treated agree identically with those derived by coupled-mode theory. The dependence of the TM radiation loss coefficient in each waveguide geometry on the grating period, radiation angle, refractive index profile, and mode polarization is illustrated graphically, and compared with its TE counterpart.

Nonlinearities in the emission characteristics of stripe-geometry (AlGa)As double-heterostructure junction lasers

Abstract: Significant changes in the optical and electrical properties of stripe-geometry (AlGa)As double-heterostructure junction lasers have been observed to accompany a nonlinearity in the current dependence of the lasing emission. Over the nonlinear range, the optical field of the lasing emission shifts continuously with current toward one boundary of the active stripe. Simultaneously, effective gain saturation is lost and additional transverse modes are excited. The increasing gain is believed to occur primarily in spatial regions where the optical intensity is reduced as a result of the transverse motion of the field. Additional observations suggest that this transverse motion results from an interaction between the intense lasing field and the active medium. Consequently, gain depletion is discussed as a possible cause for the transverse instability. Implications of this model for a qualitative understanding of the improved output performance recently obtained from lasers with narrow stripes are also considered.

Suppression of the relaxation oscillation in the modulated output of semiconductor lasers

Abstract: Practical rates of direct pulse modulation of semiconductor lasers have so far been limited to below several hundred megahertz, owing to serious distortion in the output signal caused by the relaxation oscillation of the light intensity. Based on theoretical analysis of the dynamic properties of lasing under constant injection of external radiation into the cavity modes, light injection is proposed as a method for suppressing the relaxation oscillation. The effectiveness of this method has been confirmed in a series of preliminary experiments which employed GaAs injection lasers both as the modulated laser and as the external source of the injected radiation.

Derivative measurements of the current-voltage characteristics of double-heterostructure injection lasers

Abstract: Measurements of the electrical characteristics of double-heterostructure (DH) stripe-geometry AlGaAs junction lasers are reported. We show that the electrical characteristics of these lasers are adequately represented by a resistor in series with a p-n junction characterized by a classical diode equation, I = I_{s} [\exp (q V_{j}/\etakT) - 1] . Devices have been measured with almost ideal junction characteristics with both the saturation current, I s , and the exponential parameter, η, constant, while in other diodes both η and I s exhibit a strong current dependence. Measurement of the first derivative dV/dI and the product IdV/dI is shown to be an especially useful technique for electrical characterization, yielding accurate and direct determination of the series resistance, the exponential junction parameter, and the laser threshold current. The observation of lasing threshold from an electrical measurement, instead of from the more usual optical measurement, is discussed in terms of the saturation behavior of the junction voltage at and above threshold.

A canonical formulation for analyzing multielement unstable resonators

Abstract: A simple but general formulation for the unified analysis of many different varieties of unstable optical resonators is obtained by expressing the round-trip Huygens' integral for the resonator in terms of the round-trip paraxial ray matrix. Complicated multielement resonators, ring resonators, and unstable resonators with variable-reflectivity output mirrors are thereby all reduced to a single equivalent collimated-beam diffraction problem. Essential assumptions in the analysis are that all elements in the resonator obey paraxial ray theory and that there is only one significant output coupler or limiting aperture per round trip inside the resonator. The only parameters needed to describe an arbitrary unstable resonator in this formulation are the round-trip magnification M and either the equivalent Fresnel number N eq or else an equivalent collimated Fresnel number N_{c} = [2M^{2}/(M^{2} - 1)] N_{eq} , plus for variable-reflectivity mirrors a specification of the mirror reflectivity profile.

TM-mode coupling coefficients in guided-wave distributed feedback lasers

Abstract: An equation is derived for TM-mode coupling coefficients in guided-wave distributed feedback (DFB) lasers. Results for GaAs:GaAlAs double-heterostructure lasers with rectangular, triangular, and sawtooth-shaped teeth, various Bragg diffraction orders, and transverse mode numbers are presented for TM modes and compared with those for TE modes. Generally, but not always, coupling coefficients for TE modes exceed those for TM modes.

Analysis of grating-coupled radiation in GaAs:GaAlAs lasers and waveguides - II: Blazing effects

Abstract: Grating-coupled radiation in lasers and waveguides is analyzed for trapezoidal-shaped grating teeth which includes all triangular shapes. A perturbation technique is used which involves calculating the modes in a geometry with an inhomogeneous refractive index profile in the grating region. Curves are presented for radiated power from traveling waves as a function of blazing angle, grating tooth height, refractive index, and grating period. It is shown that the ratio of power radiated into the outer media depends greatly on tooth shape and size. By judiciously selecting parameters this ratio can exceed 50:1.

Continuous-wave CH 3 F waveguide laser at 496 µm: Theory and experiment

Abstract: The kinetic processes controlling the conversion of CO 2 pump radiation into far-infrared (FIR) radiation in optically pumped waveguide lasers are discussed. It is found that pump saturation and excited-state FIR absorption play major roles in limiting the conversion. Good agreement between theory and experiment was obtained over a wide range of pump powers, gas pressures, and mirror reflectivities. For a well-optimized system, power-conversion efficiencies of 0.2-0.6 percent can be realistically achieved for CH 3 F.

Analysis of injection locking in pulsed dye laser systems

Abstract: A pulsed dye laser system with a narrow-band signal injected into the cavity is considered. The injection starts prior to the excitation pulse. It is shown that during a transient period, injection locking can be achieved for a much wider range of parameters than under conventional CW conditions. The analysis is based on a set of coupled rate equations for the population densities and photon fluxes at all wavelengths. These are solved numerically with appropriate initial and boundary conditions. Analytical approximations, which are helpful for understanding the behavior of such systems, are presented. Both straight, two-mirror cavities, and ring laser cavities are discussed. The temporal evolution of the spectral components at all wavelengths is described. The range of tunability in this mode of operation and its dependence on various parameters, such as pulse length, cavity dimensions, and injection intensity are discussed.

Volume ultraviolet preionization from bare sparks

Abstract: Volumetric photoionization in CO 2 -laser gas mixtures by UV emission from bare sparks was studied in two ways: by spectroscopic emission and absorption measurements, and by direct collection of photoionized electrons. The physical processes were understood sufficiently to permit prediction of initial electron distributions in arbitrary geometries of CO 2 , N 2 , He gas mixtures containing known concentrations of ionizable impurity.

Infrared saturation spectroscopy in p-type germanium

Abstract: We investigate spectral changes induced by high-power CO 2 laser radiation in the direct intervalence band absorption of p-type germanium. Using a model of inhomogeneously broadened two-level systems, we deduce a room-temperature saturation intensity of 4 MW/ cm2from published observations of saturation of this absorption. The applicability of the two-level model to transitions between two electronic bands is discussed. We relate the linear absorption coefficient to the saturation intensity and the effective population difference, where the latter is obtained in a spherical band approximation. The result yields a value of 0.55 ps for the geometric mean of the phase and energy relaxation times. The meaning of these times is explained for our system, and quantitative agreement is found with published data on optical and acoustical lattice scattering probabilities. Specifically, we find a very short phase relaxation time of 0.09 ps. This allows the prediction of a burnt-hole spectral width of 57 cm-1(HWHM) at small saturation. In a preliminary experiment, we have observed the modulation of a CW CO2laser beam by intense nanosecond pulses of a second CO2laser offset in frequency by up to 140 cm-1.

Second-harmonic generation in KB 5 O 8 · 4H 2 O from 217.1 to 315.0 nm

Abstract: Second-harmonic generation (SHG) has been observed in KB 5 O 8 ċ 4H 2 O (KB5) between 217.1 and 315.0 nm by angle tuning in a single crystal using a single cut. A conversion efficiency of 9.2 percent was observed for type I noncritical phase matching at 217.1 nm for a peak power of 15 kW at 434.2 nm. The nonlinear coefficients d 31 and d 32 are estimated to be approximately 1.1 \times 10^{-10} ESU ( 4.0 \times 10^{-25} m/V) and 0.08 \times 10^{-10} ESU ( 0.29 \times 10^{-25} m/V), respectively.

Pump saturation in molecular far-infrared lasers

Abstract: Precise absorption measurements on vibration-rotation transitions pumping far-infrared (FIR) laser transitions in HCOOH, CH 3 OH, CH 3 I and CH 3 F were carried out using a frequency-stabilized and intensity-controlled CO 2 laser. Linear absorption coefficients and saturation intensities as a function of pressure are obtained. With the help of a rate equation model it is concluded that within the pressure range relevant for CW FIR lasers the rate of deexcitation of pumped molecules is limited by rotational relaxation. For CH 3 F, limitation by diffusion of excited molecules to the cell wall is observed.

Compact high-power N 2 laser: Circuit theory and design

Abstract: Based on measurements of the transient voltage across the cavity of a transversely excited N 2 laser an equivalent circuit is developed which provides better understanding of the contribution of all individual components to maximum peak power. Consideration of the design criteria established guarantees about one million light pulses with more than 1 MW from simple high-efficiency construction.

A 16-µm source for laser isotope enrichment

Abstract: A potentially efficient, high-energy 16-μm source is proposed based on stimulated rotational Raman scattering in H 2 or D 2 using a TEA CO 2 laser source. The calculated pump energy threshold for rotational Raman scattering on the S (0) 354.33 cm-1Raman line of H 2 at 77 K and 1 atm pressure is 3.1 J.

A theoretical analysis for gigabit/second pulse code modulation of semiconductor lasers

Abstract: Investigation of the rate equations of a semiconductor laser suggests that bit rates of 3-4 Gbit/s can be achieved. Delay, ringing transients, and charge-storage effects can be removed by adjusting the dc-bias current and the peak and width of the current pulse to values prescribed by simple analytical expressions. Also, simple approximate formulas for the light pulse maximum, width, delay, and integrated values are given.

Lasing characteristics of distributed-feedback GaAs-GaAlAs diode lasers with separate optical and carrier confinement

Abstract: The lasing characteristics of separate-confinement-heterostructure (SCH-structure) distributed-feedback (DFB) diode lasers are examined theoretically and experimentally. Wave propagation in five-layer SCH waveguides is analyzed to estimate such parameters as the lasing wavelength, coupling constant, and external quantum efficiency. Spectral and modal behavior are studied in the experiment and compared with the theoretical predictions. Diodes are shown to lase in a single longitudinal mode with a definite polarization. Spectral width is about 300 MHz just above the threshold, and becomes wider with increased excitation level. An output power of 40 mW with an external quantum efficiency of 5 percent is obtained under CW operation.

Theory and operation of high-power CW and long-pulse dye lasers

Abstract: A complete theoretical model for CW and long-pulse dye lasers is developed. The effects of excited singlet-state absorption of pump and laser fight and triplet absorption of pump and laser light are included. Expressions are derived for gain, actual dye transmission, and output power. Experimental work is presented to verify the theory and to obtain values of excited-state absorption cross sections for rhodamine 6G by matching experimental to theoretical curves.

Substrate radiation losses in GaAs heterostructure lasers

Abstract: Double-heterostructure (DH) diode lasers with a thin Ga 1-x Al x As layer between the active GaAs region and the GaAs substrate or superstrate are analyzed. In these devices power flows through the thin Ga 1-x Al x As layer and is radiated into the substrate or superstrate. Three methods for computing the laser thresholds are developed and compared. The first is an analytic perturbation technique, which yields accurate results in many cases of practical interest. The second and third are rapidly convergent numerical iteration techniques. The former utilizes overlap integrals to compute absorption losses and thresholds; the latter includes all losses and gains directly in the formulation. We show that conventional DH diode lasers can be designed with thick active GaAs layers and still achieve lowest order TE-mode operation. These devices will produce better collimated, higher power output beams than do similar devices with thinner active regions. Transverse-mode control is achieved because all higher order modes have increased penetration through the thin Ga 1-x Al x As , and therefore exhibit inereased radiation losses into the substrate or superstrate. A design example is included in which it is shown that with proper choice of the Ga 1-x Al x As-layer thickness the TE 0 -mode threshold increases by 5 percent compared with a 110-percent increase in the TE 1 threshold. These results are virtually independent of the substrate power-absorption coefficient. Threshold current densities are computed for a set of diodes studied experimentally by Casey and Panish and the results are shown to be in excellent agreement.

Theory and experiments on grating couplers for thin-film waveguides

Abstract: A theoretical and experimental study is made of the grating couplers for thin-film waveguides. It is shown that the perturbation series expansion of the field based upon the Green's function approach can actually be summed to all orders to give an expression for the efficiency of grating couplers of arbitrary thickness. The results obtained by this theory are in good agreement with the experimental results obtained. It is also shown that the grating coupling efficiency depends strongly on the grating profile.

Effects of nonlinear propagation on laser focusing properties

Abstract: In a high-power laser system both the nonlinear growth of small-scale spatial irregularities on a pulse (beam breakup) and the accumulation of self-induced phase-front distortion on the spatial envelope of the pulse (whole-beam self-focusing) alter the focusing properties of the system output. We present experimental results which show simultaneously and in detail the influence of these two effects on the spatial and temporal profiles of the focused output of a simple system. in an extreme case the focused on-axis intensity is observed to be reduced by beam breakup to 20 percent of the value obtained in the absence of breakup. Diffraction code calculations of whole-beam self-focusing, which in these experiments tends to enhance the focused on-axis intensity, are found to agree closely with the measured results and demonstrate that whole-beam self-focusing effects depend strongly on the plane being observed.

Radiation losses in distributed feedback lasers and longitudinal mode selection

Abstract: We show that seemingly symmetric longitudinal modes symmetrical distributed feedback (DFB) lasers utilizing higher orc Bragg reflection actually have differing thresholds and resonant wavelength shifts from the Bragg condition as a result of power radiated by the grating.