Theory of spontaneous emission noise in multisection semiconductor lasers

Calculations of spontaneous emission noise in semiconductor lasers are mainly based on a fundamental theory developed by Henry in 1986, which is useful for simple systems, together with a formulation in terms of transfer matrices by Makino and others, which facilitates application of the theory to more complicated multisection systems. The aim of this review is to present a unified account of this theoretical work in a transparent form intended to encourage its further use in complex systems. The opportunity is taken to strengthen the existing theory by including the effects of differing optical wave vectors in different sections and the consequent reflections at interfaces, which are important in some applications. Sample calculations are presented for a range of systems with one, two, three, and four sections and the predictions compared with other theoretical and experimental results.     

Fourier series expansion method for gain measurement from amplified spontaneous emission spectra of Fabry-Perot semiconductor lasers

A gain measurement technique, based on Fourier series expansion of periodically extended single fringe of the amplified spontaneous emission spectrum, is proposed for Fabry-Perot semiconductor lasers. The underestimation of gain due to the limited resolution of the measurement system is corrected by a factor related to the system response function. The standard deviations of the gain-reflectivity product under low noise conditions are analyzed for the Fourier series expansion method and compared with those of the Hakki-Paoli method and Cassidy's method. The results show that the Fourier series expansion method is the least sensitive to noise among the three methods. The experiment results obtained by the three methods are also presented and compared.     

Theory and experiment on the amplified spontaneous emission fromdistributed-feedback lasers

The amplified spontaneous emission (ASE) of a strained quantum-well distributed feedback (DFB) laser biased below laser threshold is used to extract the gain and refractive index spectra in a systematic manner. A modified Hakki-Paoli method is used to obtain the gain and differential gain spectra. The refractive index change due to carrier injection is obtained from the shift of the Fabry-Perot peaks in the ASE spectrum. The measured ASE spectrum, gain, refractive index change, and linewidth enhancement factor are then compared with our theoretical model for strained quantum-well lasers. Our model takes into account the realistic band structure and uses the material and quantum-well dimensions directly in the calculation of the electronic and optical properties. The theory agrees very well with the experiment     

Theoretical investigation on semiconductor lasers with passivewaveguides

Semiconductor lasers with vertically integrated passive waveguides are theoretically studied using the coupled mode theory and exact calculation. Formulas for the threshold current density and the far-field patterns are derived. The physical concepts of the modulation of the beam divergence by passive waveguides are given. The exact calculated results show that the beam divergence can be greatly improved by paying a price of only a slight increase of the threshold current density. The operation mode selection is discussed. Attention is also paid to the appearance of side lobes for very narrow far-field patterns. Discussions are given for device design     

Nonuniform emission characteristics of electron-beam-pumped dislocation-free GaAs lasers

Filamentary and other nonuniform emission characteristics have been observed from electron-beam-pumped dislocation-free GaAs lasers. X-ray topographs taken before and after bombardment showed relatively few dislocations present in the as-grown material and no dislocation introduced at the power levels used for electron-beam excitation.     

Measurement of spontaneous emission factor for vertical-cavitysurface-emitting semiconductor lasers

Using a method based on measurement of modulation frequency harmonics, we have determined experimentally the spontaneous emission factors of GaAs-AlGaAs QW vertical cavity surface-emitting semiconductor lasers with square windows of widths 10, 15, and 30 μm. The values obtained are of the order 10^-4 and scale as the reciprocal of the window width. We have also assessed the reliability and accuracy of the modulation frequency harmonic measurement technique by comparison with the prior technique based on curve fitting to the light-current characteristic     

The symmetry of the amplified spontaneous emission spectrum incomplex-coupled DFB lasers

We present relations between facet reflections and coupling coefficient for the amplified spontaneous emission (ASE) spectrum of a complex-coupled, an index-coupled, and a gain-coupled distributed feedback (DFB) laser to have a mirror image with respect to the Bragg wavelength. We show that the ASE spectrum of a complex-coupled DFB laser cannot be symmetric in general with respect to the Bragg wavelength. The index-coupled and gain-coupled DFB lasers can have symmetric ASE spectrum for some values of the phase of the facet reflection coefficient that are determined     

Influence of electron energy on the characteristics of electron-beam-pumped semiconductor lasers

The influence of electron energy on the characteristics of electron-beam-pumped GaAs lasers is studied experimentally. Increases in the electron energy lead to increases in output power and decreases in beam divergence. Above radiation-damage threshold, a degradation of the laser output with time is observed due to bulk defects created by the beam. These defects are partially self-annealing if radiation is stopped.     

Transfer matrix analysis of the amplified spontaneous emission ofDFB semiconductor laser amplifiers

Analytical expressions are derived for the amplified spontaneous emission of a DFB (distributed-feedback) semiconductor laser amplifier with reflective cavity ends. The analysis is extended to a multisection DFB structure including a phase-shifted DFB semiconductor laser amplifier. It is shown that the spontaneous emission power per unit frequency bandwidth emitted from one facet is proportional to the transmission gain and to a quantity which at threshold becomes the inverse of the differential quantum efficiency of the other facet. The analysis is applied to two practical cases: (1) calculation of emission spectra of a DFB semiconductor laser biased below the threshold, and (2) assessment of the signal-to-noise ratio performance of DFB semiconductor laser amplifiers     

Improved performance due to suppression of spontaneous emission in tensile-strain semiconductor lasers

It is shown that application of biaxial tension to the active region of a bulk or quantum well semiconductor laser can significantly enhance TM gain compared to TE gain and reduce the threshold current density, due to suppression of spontaneous emission polarised in the growth plane of the laser structure. The differential gain is enhanced compared to unstrained structures and a larger peak gain can be achieved than in comparable structures under biaxial compression.<>     

半导体激光放大器的理论与实验研究

从理论与实验上研究了1.3μm的法布里-珀罗放大器(FP-SLA)与行波放大器(TW-SLA).分析了半导体激光器的增益特性,指出了“准”行波放大器的优越性.     

Wavelength tuning of the stimulated emission from GaAs and CdSe electron-beam-pumped lasers

The wavelength at which the stimulated output from a number of electron-beam-pumped semiconductor lasers peaks is a function of the time after the start of the pumping pulse. In general, the output intensity reaches a maximum at short wavelengths considerably before it peaks at the longest. Detailed observations of the wavelength shifts of the peak output as a function of both time and pumping current are reported here for both CdSe and GaAs laser crystals. It is found that the rate of shifting of the wavelength peak (tuning rate) as a function of electron-beam pumping current is very similar in both functional form and magnitude in the two materials. Furthermore, the behavior of the tuning rate is dependent on whether or not lasing action occurs in the crystal under examination. For a crystal in which lasing action is obtained, the tuning rate saturates for pumping current densities above threshold. In contrast, for a nonlasing crystal, the tuning rate is a linearly increasing function of the pumping current density. The implications of these results are discussed with reference to previously reported work concerning shifts with respect to the band edge of the stimulated emission from semiconductor lasers.     

Analysis of the spontaneous emission rate of multiple-phase-shift distributed feedback semiconductor lasers

A general expression for the spontaneous emission rate of multiple-phase-shift DFB semiconductor lasers is given using the transfer matrix based on the Green's function method. The spontaneous emission rate for coupled phase-shift DFB lasers is calculated.<>     

Optical preamplifier using optical modulation of amplified spontaneous emission in saturated semiconductor optical amplifier

This paper demonstrates a novel optical preamplifier using optical modulation of amplified spontaneous emission (ASE) emitted from a saturated semiconductor optical amplifier (SOA). Requirements on optical alignments and antireflection coating for SOAs can be relaxed and the elimination of an optical filter gives us a large tolerance of an input light wavelength in the proposed optical preamplifier. A small-signal gain of a fabricated preamplifier was over 13.5 dB for an input power of below -20 dBm. An optical gain bandwidth was over 60 nm. We measured the small-signal response of the optically modulated ASE. The 3 dB bandwidths at SOA bias currents of 200, 300, and 400 mA were 5.8, 12.6, and 16.5 GHz, respectively. We also investigated improvements in receiver sensitivities with the proposed optical preamplifier. Our calculation shows a possibility of 10 dB improvement in receiver sensitivities by using the optical preamplifier at 10 Gb/s. The measured receiver sensitivity was -22.7 dBm at 10 Gb/s with the optical preamplifier, which is corresponding to an improvement of 2.5 dB in the receiver sensitivity. Further improvements of the receiver sensitivity can be expected by optimizing the structure of SOAs for saturating ASE.     

Widely tunable S-band fiber-ring lasers and broadband amplified spontaneous emission sources with thulium-doped fluoride fibers

This paper reports our investigation on widely tunable fiber-ring lasers and broadband amplified spontaneous emission (ASE) light sources with thulium-doped fluoride fibers (TDFs). The experimental results for thulium-doped optical fiber amplifiers will also be discussed. We achieved widely tunable fiber lasers that had tuning ranges of 94 nm with 1550 nm + 1050 nm pump configurations and of 85 nm with 1550 nm + 1400 nm pump configurations, respectively. It was observed that the lasers oscillated under multilongitudinal-mode operation with linewidth of 400 MHz, signal-to-source noise ratio of 60 dB, and power stability of 0.003 dB for a laser output of 1 mW. A broadband ASE source with a power density of more than -15 dBm/nm between 1465 and 1535 nm was also demonstrated with a novel hybrid configuration by using both TDF and erbium-doped fiber.     

All-optical logic NXOR based on semiconductor optical amplifiers with the effect of amplified spontaneous emission

The performance of all-optical logic NXOR gate based on semiconductor optical amplifiers Mach-Zehnder interferometer(SOAs-MZI)is simulated.The effects of amplified spontaneous emission(ASE)and the input pulse energy on the system’s quality factor are studied.For the parameters used,the all-optical logic gates using SOAs are capable of operating at speed of 80Gbit/s.     

Theoretical considerations of time delays in semiconductor lasers

Calculations have been made of time delays in semiconductor lasers, using a rate-equation approach to describe the transient behavior of photon and electron populations. Three possible models were considered in order to try to explain the delays: a) saturable absorption via conduction-band tail states in equilibrium with the band, with the lasing emission occurring via transitions from the band states; b) saturable absorption via conduction-band tail states not in equilibrium with the band; and c) absorption due to loss of waveguiding as a result of hole injection into the n-type regions of diode lasers. It was found that only the model b) was capable of satisfactorily accounting for the known properties of time delays.     

Reduced spontaneous emission factor in quantum-well lasers

Spatial and spectral mode measurements on gain-guided GaAs multiquantum-well lasers yield greatly reduced spontaneous emission K-factors in lasing modes as compared to conventional lasers. Small K-factors lead to narrow far fields and to nearly single longitudinal mode operation in gain-guided quantum-well lasers