Computer simulation and noise analysis of the system performance of 1.55-µm single-frequency semiconductor lasers

A theoretical model for the noise analysis of the system performance of 1.55-μm single-frequency semiconductor lasers is presented. Computer simulations are used to analyze the role of various noise sources in a 1.7-Gbit/s transmission experiment where the data was transmitted over 69 km using a 1.56-μm distributed-feedback laser. The bit-error-rate curves generated from numerical simulations agree well with the results of the transmission experiment. The relative contributions of various noise sources in limiting the system performance are discussed and compared. In particular, we consider circuit noise, shot noise, laser intensity noise, mode-partition noise, parasitic reflections, and the frequency chirp.     

Laser pumped molecular lasers--Part I: Theory

The density matrix formalism describing the interaction between a three level molecular system and two nearly resonant laser fields of arbitrary intensity is extended to include the molecularM-level degeneracy. Expressions for gain on the two possible relative polarizations of a laser pumped molecular laser are derived and its saturation behavior discussed. Numerical results for the 385 μm transition in D2O and 496 μm transition in CH3F are presented.     

Real-time measurements of semiconductor laser spectra

A technique for measuring the instantaneous spectrum of a semiconductor laser is described. Spectral fluctuations of several different devices, including three different length channel-substrate buried-crescent lasers emitting at 1.52 μm were examined. The shortest (100-μm) device showed a trend towaxds single-mode operation, and it is believed that devices of up to 70-μm length should emit predominantly in one longitudinal mode. Mode selection effects, thought to be due to reflections from the monitor diode, were observed in a 1.3-μm packaged laster. The technique has also been used to measure correlations between successive pulses, and between spectral width and mean position.     

Lateral analysis of quasi-index-guided injection lasers: Transition from gain to index guiding

In double-heterostructure stripe-geometry semiconductor lasers an effective lateral index stepDelta n_{L}over the stripe region can be induced through evanescent-field coupling. Such a quasi-index-guided device exhibits a transition from the gain-guided to the index-guided regime whenDelta n_{L}is progressively increased. Using parameters appropriate to a 1.3-μm InGaAsP laser, the transition is shown to occur aroundDelta n_{L} sim 5 times 10^{-3}. The exact value ofDelta n_{L}depends on the extent of carrier-induced antiguiding. In the transition region the threshold current decreases rapidly, the lateral mode contracts, and the far field changes from a twin-lobe to a single-lobe pattern. Our analysis suggests that a quasi-index-guided device operates most efficiently for values ofDelta n_{L}at which the index-guided regime is just approached. With a further increase ofDelta n_{L}, the mismatch between the gain and mode profiles leads to lower differential quantum efficiencies. Among other structures, the analysis is applicable to a ridge waveguide laser. For a 1.3-μm laser the optimumDelta n_{L}can be obtained using 0.2-μm-thick cladding layers for a 0.2-μm thick active layer.     

Fluorescence and oscillation characteristics of LiNdP4O12lasers at 1.317 µm

Spectroscopic data and laser oscillation characteristics of the 1.317 μ line in lithium neodymium tetraphosphate (LNP) are reported. A stimulated emission cross section of this transition was spectroscopically determined as7.1 times 10^{-20}cm², which corresponds to 1/4.5 of that at the 1.047-μm transition. Cross-section temperature dependence, laser cavity loss, threshold versus crystal length, and threshold temperature dependence were measured experimentally. Since resonant loss was negligible at the 1.317 μm line, room temperature threshold is lower than that at 1.047-μm in the case of long crystal, and threshold temperature dependence is weaker than that at 1.047 μm. A miniaturized LNP laser, using a graded index fiber as a focusing medium, is also shown. LED pump intensity required to obtain a constant output is compared for 1.047- and 1.317-μm wavelengths. The 1.317-μm line seems to be useful as miniaturized optical sources in optical communication systems, since required intensity is around several W/cm²for the side pump, and the wavelength corresponds to the most transparent band of ultra low-loss optical fibers.     

Dynamic Stark effect in CH3F and other optically pumped lasers

The dynamic Stark effect in optically pumped lasers is discussed. Javan's theory of three-level masers is extended and elucidated by transforming his expressions into new forms in which all of the major terms can be deduced from a modified energy-level diagram through simple physical arguments. The gain spectra of high-power 496-μm CH3F lasers are calculated from the theory. Contrary to previous interpretations, the wide emission bandwidth at pump intensities above 30 kW/cm²is shown to be primarily due to the dynamic Stark effect onK= 1, 2,and 3 components. It is found that to obtain a narrow emission bandwidth the pump intensity should be kept below 10 kW/cm².     

Gain of high-pressure CO2lasers

Gain formulas are shown for the 10.4- and 9.4-μm CO2bands for the high-pressure broadening case to point out a new regime of operation for this laser. Computer-simulated results of this gain are presented as a function of wavenumber, with density equivalent pressure and population inversion ratio as parameters. The results show gain curves that suggest continuously tunable lasers over the entire 9.4- and 10.4-μm bands with no rotational structure.     

Stark cell noise reduction of a CO2laser beam

A technique is demonstrated for reducing the rapid intensity fluctuations in the beam of a single frequency, polarized CO2laser tuned to theP(20)transition at 10.6 μm. The method uses a Stark-tunable absorption line in NH2D to compensate for the fluctuations of the laser output. Substantial reductions in laser beam intensity noise have been obtained. The method may be applied to other lines of the CO2laser and indeed to other types of lasers.     

Far infrared frequency synthesis with stabilized CO2lasers: Accurate measurements of the water vapor and methyl alcohol laser frequencies

A far infrared (FIR) frequency synthesis technique using saturated-absorption stabilized CO2lasers and a point-contact diode has been used to measure frequencies of a number of strong CW H2O, D2O, and CH3OH laser lines. The first frequency measurements of the 79-μm H2O, the 73- and 108-μm D2O, and 11 CO2-pumped CW¹²CH2¹⁶OH laser lines are reported. This measurement is the first demonstration of the general usefulness of CO2lasers for accurate synthesis of FIR frequencies.     

On the performance of selenium rich lead-salt heterostructure lasers with remote p-n junction

In order to increase the operating temperature of lasers emitting in the mid-IR, various lead chalcogenide diode lasers have been fabricated and examined: Pb0.95Sn0.05Se/PbSe and PbSe/PbS double heterostructure and funnel-type Pb1-xEuxSe lasers (the Eu content in the cladding layers increased steadily fromx = 0.01near the active layer up tox = 0.03). All the lasers were grown by molecular beam epitaxy on PbSe substrates. The p-n junctions were located not on the (lattice mismatched) interfaces, but at various distances (2000-6000 A) within the confinement layer. The highest operation temperatures (CW operation up to 165 K at 5.3 μm and pulsed operation up to 220 K at 4.4 μm) were attained by the PbSe/Pb1-xEuxSe funnel-type laser. The experimental results were compared to theoretical computer-assisted calculations, which accounted in a self-consistent way for the distributions of light, charge carriers, and temperature within the lasers. The agreement between theory and measurements for the temperature dependence of the threshold current and wavelength and for maximal operation temperatures was quite good. We found that the main contributions to the current come from interface recombination and from overbarrier leakage of nonconfined carriers (both minority and majority). Ways to improve the lasers are discussed.     

Rotational level competition in CO2lasers

Competition effects between rotational levels of the rotation-vibration band of CO2at 10.6 μ have been investigated in both traveling-wave and standing-wave CO2lasers operated in a single mode and single frequency. In a ring laser, Doppler shift and gain proportionality as a function of gas flow can be used to generate a gain anisotropy as a function of frequency so that the ring laser operates as a unidirectional oscillator. Over a narrow frequency interval, two rotational levels can be made to oscillate with oppositely directed traveling waves with an intensity crossover between the two Doppler centers. In this way, a discriminant can be derived that allows frequency stabilization to 5 parts in 10¹²in frequency. In standing-wave lasers of high-frequency stability, the rotational level competition can be observed by synchronous detection of a low-frequency variation of the heterodyne beat frequency signal of two lasers. The competition effects are due to intensity-dependent anomalous dispersion arising from saturation.     

1.3-µm BH laser performance at microwave frequencies

We have studied the performance of In1-xGaxAsyP1-yburied heterostructure (BH) 1.3-μm lasers in the microwave range. This study consisted of small-signal, large-signal, and digital pseudo-random word evaluation of these lasers. The small-signal study pointed out the impact of the oxide stripe capacitance on the laser response at microwave frequencies. The large-signal study uncovered basic laser non-linearities that affect the temporal response, spectral broadening, and wavelength chirp. Finally, digital pseudo-random word tests performed at 1.7 Gbit/s indicated that in spite of these inherent laser nonlinearities, the 1.3-μm BH lasers performed well enough to be considered as promising sources for gigabit optical communication systems.     

Electronic- and phonon-terminated laser emission from Ho3+in BaY2F8

Infrared spontaneous and stimulated emission from Ho³⁺in BaY2F8is reported. In addition to the familiar⁵I_{7} rightarrow⁵I8transition at 2 μ,⁵F_{5} rightarrow⁵I5emission at 2.4μ and⁵I_{6} rightarrow⁵I7emission at 2.9μ are discussed. There are several unusual features of the 2-μ laser emission. At room temperature, phonon-terminated laser emission is observed at 2.171 μ. At 77 K a complex CW laser output is observed in a wavelength interval lying on the shoulder of a fluorescence line. The complex output is attributed to oscillation in transverse modes of the resonator. Oscillation is not observed in the strongest emission line, despite a large terminal state splitting of 310 cm^-1. These results are explained on the basis of a theory developed earlier for transition metal ion lasers. The validity of the model is supported by demonstrating the tunability through loss modulation predicted by theory. The observation of these effects is made possible by the very low internal scattering loss in the crystals. The⁵F_{5} rightarrow⁵I5laser lines near 2.4 μ represent relatively low gain transitions with pulse durations limited by accumulation in a longer lived terminal state. The dynamics of laser emission indicate the possible absence of thermal equilibrium in the excited state. For the 2.9-μ transition the bottleneck posed by a longer lived terminal state may be eliminated by the addition of Eu³⁺or Pr³⁺, but laser emission could not be obtained.     

High-speed optical pulse transmission at 1.29-µm wavelength using low-loss single-mode fibers

Optical-fiber transmission experiments in the 1.3-μm wavelength region are reported. GaInAsP/InP double-heterostructure semiconductor laser emitting at 1.293 μm is modulated directly in nonreturn-to-zero (NRZ) codes at digit rates tanging from 100 Mbit/s to 1.2 Gbit/s. Its output is transmitted through low-loss GeO2-doped single-mode silica fibers in 11-km lengths. Transmitted optical signals are detected by a high-speed Ge avalanche photodiode. Overall loss of the 11-km optical fibers, including 11 splices, is 15.5 dB at 1.3 μm. Average received optical power levels necessary for 10^-9error rate are -39.9 dBm at 100 Mbit/s and -29.1 dBm at 1.2 Gbit/s. In the present system configuration, the repeater spacing is limited by loss rather than dispersion. It seems feasible that a more than 30 km repeater spacing at 100 Mbit/s and a more than 20 km even at 1.2 Gbit/s can be realized with low-loss silica fiber cables, whose loss is less than 1 dB/km. Distinctive features and problems associated with this experimental system and constituent devices are discussed.     

445 Mbit/s transmission field experiment at 1.55-µm wavelength region over 80 km using undersea optical-fiber cable

New transmission equipment employing a 1.55-μm distributed feedback laser diode (DFB-LD) to overcome fiber dispersion has been tested at environmental conditions using 1.3-μm zero-dispersion fiber cable on the undersea section of route F-400M. The DFB-LD's dynamic spectrum characteristics, in relation to power penalty, were examined and a suitable laser prebias control level was obtained. Field experimental transmission lines operated error free for a two-month period, and applicability to 1.55-μm 445-Mbit/s systems of over 100 km was shown.     

Far-infrared raman laser with high intensity laser pumping

Theoretical and experimental results are presented for a pulsed far-infrared (FIR) molecular gas laser with high intensity laser pumping. In these FIR lasers, high intensity pumping is found to produce stimulated Raman emission at very large offsets (up to 30 GHz) from resonance with the intermediate state. A theoretical, density matrix model is developed for these lasers to account for simultaneous Raman emission on rotational levels in the ground and excited vibrational states (double Raman resonance). This theoretical approach is necessary in the case of off-resonant, high intensity pumping. Theory predicts the FIR emission frequency, the FIR laser gain, and the pump threshold intensity as a function of pump laser frequency. Experimental results are obtained onP-,Q-, andR-branch transitions in¹²CH3F and¹³CH3F using a single-mode, grating tuned CO2TEA pump laser with an intensity of up to 40 MW/cm². Good agreement is obtained between theory and experiment for the observed values of FIR emission frequency and pump threshold intensity. These results indicate that a widely tunable (150-1200 mum), pulsed FIR CH3F laser could be constructed with a tunable, multiatmospheric CO2pump laser of modest power (about 2-5 MW).     

Absorption coefficient, transition probability, and collision-broadening frequency of dimethylether at He-Xe laser 3.51-µ wavelength

The pressure dependence of the absorption coefficient of dimethylether (DME) gas at He-Xe laser 3.508-μ wavelength is measured. The experimental results are in fairly good agreement with theory. A collision-broadening frequency of1.6 pm 0.1 times 10^{8}second^-1at 15 torr, 293°K, and a transition lifetime of 2.2 ± 0.2 seconds are obtained. A saturation intensity of about 2 mW/cm²for 1 torr of DME is also estimated.     

385 µmD2O laser linewidth measurements to -60 dB

First linewidth measurements over a 60 dB dynamic range of a pulsed, high-power, optically pumped far infrared laser are presented. These measurements were made possible by using a 385 μmD2O laser with an N2O absorption filter and a sensitive heterodyne receiver. Studying a 385 μmD2O laser oscillator we find that the low-level linewidth (<-20 dB) can be explained by the ac Stark effect due to the high-power pump field. Also, we have not observed any frequency pulling of the main Raman emission frequency due to the strong pump and FIR laser fields.     

1.5-µm λ/4-shifted InGaAsP/InP DFB lasers

A single wavelength light source in 1.5-μm range was developed using InGaAsP/InPlambda/4-shifted distributed feedback (DFB) semiconductor heterostructures. Superiority of thelambda/4-shifted DFB structure in terms of stability of the main mode at the Bragg wavelength was shown theoretically, in which the threshold, the output, and the polarization characteristics were taken into account. Alambda/4-shifted corrugated grating was made by a newly developed negative and positive photoresists technique. Buried heterostructure (BH) diode lasers with nonreflective window ends were fabricated and highly stable single-mode operation with a low threshold was obtained reproducibly. Direct modulation properties and life-tests results indicated that thelambda/4-shifted DFB lasers could be a reliable single-mode light source in a long span lightwave transmission system in 1.5-μm range.