Second-harmonic enhancement using a self-locked 0.63-µ He-Ne laser

Relative second-harmonic power generated in a non-linear crystal has been measured for a multimode He-Ne 0.63-μ laser source operating unlocked and in two different types of spontaneous locking. For the same average laser power the harmonic power was 2.15 times larger forc/2Lpulsing than forc/Lpulsing, in good agreement with a predicted value of 2.18. The harmonic enhancement forc/2Lpulsing compared with unlocked operation was 3.31, considerably lower than the analytic estimate of 4.2, indicating that significant interaction was present between the modes of the unlocked laser.     

He-Ne laser frequencies near 2.4 µm and their application to hydrogen fluoride detection

He-Ne lasers can provide three frequencies near 2.4 μm, one of which coincides with the1R(5)frequency of HF at 4173.98 cm^-1. Either of the other frequencies may serve as a reference for absorbance measurements. One of these, at 4169.33 cm^-1, is a new lasing transition not previously observed (4d'_{4}-3P_{9}). Operating conditions are discussed.     

Low-threshold InGaAsP ridge waveguide lasers at 1.3 µm

The ridge waveguide configuration is shown to provide reliable low-threshold fundamental-transverse-mode lasers that are readily fabricated. Two variants are described: in the simple ridge laser, the 1.3 μm bandgap active layer is sandwiched between InP layers and in the cladded ridge, the active layer is surrounded by 1.1 μm bandgap InGa AsP. Thresholds as low as 34 mA and efficiencies as high as 66 percent are observed. Output power is linear to more than 12 mW. Several lasers have been operated at 30°C for over 1500 h without measurable degradation. Selected lasers exhibit stabilized longitudinal mode behavior over extended temperature and current ranges. The potential manufacturability of this device is its most attractive feature.     

Spectral behavior of 1.5 µm BIG-DBR-DSM lasers

Direct modulation properties of BIG-DBR-DSM lasers were investigated by measuring both the time-averaged and the time-resolved spectra. A high side-mode-suppression ratio of more than 30 dB was obtained from time-averaged measurements with the modulation frequency in the range of 250 MHz-2 GHz. The dynamic wavelength shift at - 3 dB level obtained from both time-averaged and time-resolved measurements was less than 2 Å. Time-resolved measurements showing an oscillatory behavior of the dynamic wavelength shift with both red and blue shifts were also observed. Calculations based on a numerical solution of the rate equations, which agreed quantitatively with the measurement, indicate a line width enhancement factor estimated to lie between 3.5 and 5.5.     

Amplifying medium characteristics in optimized 190 µm/195 µm DCN waveguide lasers

Extensive study of the CW, dischaxge-pumped, DCN laser operating on the (22⁰0-09¹0) 190 and 195 μm lines has been undertaken in order to develop a new source of radiation suitable for plasma diagnostics. The optimum values of the parameters of the discharge, the unsaturated gain, and the saturation intensity are given as a function of the tube diameter. Scaling laws predicting the maximum output power as a function of the tube dimensions and of the cavity loss are established. Such lasers compare favorably with optically pumped submillimeter lasers, since 250 mW are available from a 3 m long, 5 cm diameter waveguide DCN laser.     

Multiphoton ionization of rare gases at 1.06µ and 0.53µ

The interaction between an intense focused beam of optical photons and matter in gaseous form at low pressure (10^-3torr) brings into play some strongly nonlinear processes. These multiphoton processes occur through the simultaneous absoprtion of several quanta by an atom that may be thus either excited or ionized. The orders of nonlinearity of the interaction of a multimodeQ-switched laser beam with rare gas atoms were measured with laser intensities up to 10¹³W ċcm^-2at 1.06μ and up to 10¹²W ċcm^-2at 0.53μ. The energy of the number of quanta corresponding to the order of nonlinearity is always close to the energy of an atomic level. The results seem to emphasize the particularly important role performed by bound states during the ionization process. Thus a two-stage ionization process seems far more probable than a single direct transition between the ground state and the continuum spectrum. Experimental values of multiphoton ionization probabilities are also given after having precisely determined the spatiotemporal intensity distribution function.     

Ultra-High speed InGaAsP/InP DFB lasers emitting at 1.3 µm wavelength

A bandwidth of 13 GHz has been attained in a 1.3 μm DFB laser at 25 °C. A mesa structure was introduced to reduce the parasitic capacitance and the lasing wavelength was detuned from the gain peak to increase the differential gain. This bandwidth is the widest so far reported in 1.3 μm DFB lasers.     

Simple technique for the detection of oscillation at 3.39 µ in a He-Ne laser operating at 632.8 nm

A simple technique is reported for determining if oscillation is occurring simultaneously at 3.39 μ and 632.8 nm in a He-Ne laser. It relies on observation of modulation of the output power at 632.8 nm when a variable path length element is introduced into the optical cavity of the laser. The technique is applicable to other gas laser systems where competition for upper level populations occurs between transitions at widely differing wavelengths.     

Band-to-band Auger processes in 1.3 µm InGaAsP semiconductor lasers

Within the context of the four-band Kane model, we have analyzed the contributions of pure collision and phonon-assisted CHSH and CHCC Auger processes to the nonradiative recombination lifetime in highly excited InGaAsP. Our method incorporates refined expressions for the wavefunction overlap integrals and spectral density functions, and avoids approximations to Fermi and phase-space factors. Our results are in good agreement with recent experiments, despite a rather large uncertainty associated with the unavailability of precise values for certain band-structure parameters.     

Gain measurements in 1.3 µm InGaAsP-InP double heterostructure lasers

The net gain per unit length (G) versus current (I) is measured at various temperatures for 1.3 μm InGaAsP-InP double heterostructure lasers.Gis found to vary linearly with the currentIat a given temperature. The gain bandwidth is found to decrease with decreasing temperature. The lasing photon energy decreases at 0.325 meV/K with increasing temperature. Also, the slopedG/dIat the lasing photon energies decreases with increasing temperature. This decrease is more rapid forT > sim210K. This faster decrease is consistent with the observed higher temperature dependence of threshold (low T0at high temperatures) of 1.3 μm InGaAsP lasers. A carrier loss mechanism, due to Auger recombination, also predicts thatdG/dIshould decrease much faster with increasing temperature at high temperatures. We also find that the slopedG/dIdecreases slowly with increasing temperature for a GaAs laser, which is consistent with the observed temperature dependence of threshold of these lasers.     

1.5 µm range InGaAsP/InP distributed feedback lasers

Lasing characteristics of InGaAsP/InP distributed feedback (DFB) lasers in the 1.5 μm range were studied theoretically and experimentally. Wave propagation in five-layer DFB waveguides were analyzed to estimate the effect of the structural parameters on threshold conditions. A brief consideration on designing a low threshold laser and its lasing wavelength was made. DFB buried heterostructure lasers with fundamental grating emitting at 1.53 μm were prepared by liquid phase epitaxial techniques. CW operation was confirmed in the temperature rangeof -20° to 58°C, and a CW threshold current was as low as 50 mA at room temperature. A stable single longitudinal mode operation was observed both in dc condition and in modulated condition by a pseudorandom pulse current at 500 Mbits/s. No significant increase in the threshold current was observed after 1400 h continuous CW operation at 20°C.     

CW electrooptical properties of InGaAsP(λ = 1.3 µm) buried-heterostructure lasers

The fabrication procedure, electrical properties, optical-bean characteristics, spectral characteristics, and temperature dependence of emission wavelength and threshold of InGaAsP buried-heterostructure (BH) lasers emitting at 1.3 μm are described. The dimensional requirements for fundamental-transverse mode operation have been determined. BH devices are characterized by low threshold currents, fundamental transverse mode operation, linear light output, and narrow spectral width. For 380 μm long devices threshold currents of 40 mA, slope efficiencies of 18 percent, forward resistance of 5 Ω, and T0values of 75 K have been attained.     

Distributed feedback InGaAsP/InP lasers with window region emitting at 1.5 µm range

Distributed feedback (DFB) InGaAsP/InP lasers with a window region formed at an end of the corrugated DFB region were made in order to overcome the problems inherent in the previous structures of DFB lasers with cleaved, sawed, etched, or AR-coated facets, or with an unexcited corrugation region. The window structure DFB lasers showed linear current versus output (I-L) curves, in contrast to those with a hysteresis or a kink appearing in a DFB laser with an unexcited region. Suppression of Fabry-Perot (F-P) resonances due to the two facets were sufficient enough to keep a single longitudinal mode property by DFB up to high excitation level. CW operation up to 65°C was achieved at the 1.5 μm wavelength range. Axial modes concerning the corrugated resonator were measured at about the threshold current. A stop band of a DFB laser was clearly observed with two dominant modes and much smaller submodes, which almost agreed with the axial modes predicted from a basic DFB theory.     

Comparison of the electrical characteristics and threshold temperature dependences of λ ~ 1.3 µm and λ ~ 1.6 µm stripe-geometry InGaAsP DH lasers

Light-current and derivative voltage-current characteristics are compared for dielectric-isolated stripe-geometry InGaAsP DH lasers operating under both pulsed and continuous excitation at wavelengths near 1.3 and 1.6 μm. At both wavelengths the lasers have comparable threshold temperature dependences, and both exhibit similar kinks and light jumps in their characteristics. The low T0values observed (sim50- 70K) cannot be attributed to drift-current losses.     

Frequency response of 1.3µm InGaAsP high speed semiconductor lasers

The frequency response of a group of 1.3 μm InGaAsP vapor-phase-regrown buried heterostructure lasers of various cavity lengths is analyzed by fitting the measured response curves. The dependence of resonant frequency f0and damping rateGammaon bias power is determined. The differential gain coefficient for InGaAsP is determined as3.5 times 10^{-16}cm². The damping rate is found to be proportional to the square of the resonant frequency with a proportionality factor which is independent of device geometry and facet reflectivity. The existence of such a universal relationship betweenGammaand f0and the observed magnitude of the damping rate is explained by the interband relaxation model of nonlinear gain.     

InGaAsP ridge waveguide distributed feedback lasers operating near 1.55 µm

Fabrication and performance of ridge waveguide distributed feedback lasers grown by liquid phase epitaxy for operation in the 1.50-1.58 μm spectral region have been studied. These lasers incorporate first- and second-order diffraction gratings written by the electron beam lithography and optical holography and defined by wet chemical etching and novel ion-beam-milling techniques. Ridge waveguides were formed by post-regrowth processing and heteroepitaxial ridge overgrowth. Distributed feedback ridge lasers were characterized by room temperature CW threshold currents as low as 40 mA, two facet external quantum efficiencies of up to 40 percent and stable transverse mode operation up to the output power of 10 mW. In strongly coupled devices, even with a cleaved resonator, the Bragg mode intensity exceeds that of the residual Fabry-Perot modes by a factor of 4000:1. Stable, single longitudinal mode operation was obtained under modulation rates as high as 4 GHz and error free transmission experiments over 60-km lengths of single mode fiber reproducibly performed at data rates as high as 2 Gbit/s.     

MOVPE-grown 1.5 µm distributed feedback lasers on corrugated InP substrates

A single-step low-pressure metalorganic vapor phase epitaxy (MOVPE) was applied to the fabrication of 1.5 μm InGaAsP/InP distributed feedback laser diodes on corrugated InP substrates, accompanied by LPE for buried heterostructure formation. High probability of single longitudinal mode operation was obtained due to the uniformity of the active layer thickness. A typical threshold current was 35 mA with both facets cleaved. A maximum output power of up to 27 mW was also obtained under single longitudinal mode operation with anti-reflective/cleaved facet configuration. The laser diode had high spectral stability under high-frequency direct modulation of 1.4 GHz. Results of initial aging tests (APC of 5 mW at 25°C for longer than 3800 h) have shown no degradation in driving currents. It is found that low-pressure MOVPE is favorable for epitaxial growth on corrugated substrates.     

Reliability in InGaAsP/InP buried heterostructure 1.3 µm lasers

A study was conducted of aging-induced Sn whisker growth at the surface of Au-Sn bonding solder layers around a laser chip, as well as metallurgical reactions, especially in p-side down lasers, at the interface of the solder and laser crystal just below the active layer. These phenomena cause electrical shorts in InGaAsP/InP laser diodes, which occur suddenly in devices operated for long periods without any previous symptoms having appeared in their aging characteristics. To completely eliminate such failures, a novel assembling method in which chips were mounted p-side up on semiinsulating SiC submounts using Pb-Sn solder, was applied to InGaAsP/InP buried heterostructure (BH) lasers emitting at 1.3 μm. BH lasers assembled by this method do not suffer any shorting failure even after 8000 h operation under 60°C and 5 mW/facet output conditions. The small degradation rates obtained, for example, 3 percent/kh (median) at 60°C, certify the reliability of these improved lasers. As long as the stripe width of the active layer was optimized to be in the range of1.5-2.5 mum necessary for obtaining kink-free light output versus current properties, no detrimental changes in laser characteristics, including transverse and longitudinal modes and dynamic output response, were observed in aged lasers. In this paper, the long-term degradation modes observed are presented, and possible causes are discussed.