Intensity fluctuations in longitudinal modes of a nearly single-mode AlGaAs laser

The behaviour of the relative intensity noise in each longitudinal mode of a CW DH AlGaAs laser exhibiting mode jumps is observed and theoretically explained. The stabilisation of the amplitude fluctuations of the main mode due to the gain saturation which could be expected when the injection current increases is cancelled by the shift of the maximum of the gain spectrum.     

Proposed cross grating single-mode DFB laser

A new two-dimensional laser using a cross grating has been proposed. The calculations of the threshold gain and coupling constant show a relation between these parameters similar to the one-dimensional case, and also a uniform mode giving rise to radiation normal to the surface is shown to exist for a two-dimensional cross grating. These results suggest single-mode surface emission lasers with very little beam divergence     

Influence of spontaneous frequency fluctuations on lineshape of semiconductor laser modulated by a pseudorandom pumping current

The lineshape of a single-mode semiconductor laser is studied numerically for fast pseudorandom current modulation. We analyze the contribution of frequency fluctuations originated from spontaneous emission to the laser spectrum. The expression is derived for the nonstationary phase increment determining the influence of frequency noise on the lineshape. The obtained results show that spectral characteristics of laser radiation substantially depends on the spontaneous emission factor as well as on parameters of modulation.     

Mode partition noise of nearly single-mode semiconductor lasersmodulated at GHz rates

A new method to calculate very low errors due to mode partition noise in nearly single-mode semiconductor lasers has been developed. The laser is intensity modulated at GHz rates. The method allows us to obtain the error for a periodic sequence of input bits as a function of the threshold gain difference for different values of the bias current, above and below threshold, injected current in the ON state and modulation frequency. It is shown that bounds for the error under pseudorandom modulation conditions can be obtained from the periodic and gain switching sequences. The required threshold gain difference for an error of 10^-9 is calculated, and it is shown to increase with the current in the ON state. We also observe pattern effects at the output of the laser, for bias current above threshold, due do the pseudorandom sequence of input bits. For a special value of the bias current slightly below threshold, these patterns effects are suppressed and the error is independent of the modulation frequency and of the modulation regime     

Light coupling from a semiconductor laser into a polarisation-maintaining single-mode fibre

For laser to polarisation-maintaining fibre coupling we propose the insertion of a short piece of normal single-mode fibre. As a result a power coupling loss of 2.8 dB is obtained, whereas additional crosspolarisation is very small compared with the unwanted polarisation excitation of the laser itself.     

The single-mode condition for semiconductor rib waveguides withlarge cross section

A comparison of the single-mode condition for semiconductor rib waveguides with large cross section obtained by the effective index method and the corrected formula proposed by Soref et al. [see IEEE Journal of Quantum Electronics, vol. 27, p. 1971, 1991] with experimental data is carried out. It is found that the effective index method is in a better agreement with Rickman's experimental data than the modified version. Moreover a stronger condition is proposed for single-mode design purposes     

GaInAsP semiconductor laser amplifiers for single-mode fiber communications

Gain, polarization sensitivity, saturation power, and noise characteristics of quaternary semiconductor laser amplifiers of the Fabry-Perot (FP) and traveling-wave (TW) types are reviewed. The status of antireflection coatings for TW amplifiers is presented. New results concerning the polarization sensitivity and output saturation power of a 1.5-μm buried-heterostructure (BH) amplifier are reported. A theoretical model is presented concerning the influence of the waveguide structure on the maximum internal gain of a CW-operating 1.5- μm BH amplifier, including thermal effects, and a comparison of this model with recent experimental results is reported. The influence of nonresonant losses on the noise factor of 1.5-μm amplifiers is discussed.     

Gain saturation dependence on signal wavelength in a travelling-wave semiconductor laser amplifier

Gain saturation dependence on signal wavelength in a 1.5 ?m InGaAsP travelling-wave semiconductor laser amplifier is experimentally studied. Saturation output powers were 6.9dBm, 8.3dBm and 10.6dBm for signal wavelengths of 1490 nm, 1510 nm and 1540 nm, respectively. This saturation dependence on wavelength, i.e. higher saturation output power for longer signal wavelength, results from signal gain peak shift towards longer wavelength with increasing input power.     

On single-mode injection laser structures for reduced output fluctuations

This paper explores the possibilities of two principles for reducing the fluctuations in power output of the main mode of an injection laser: 1) including within the laser cavity a section of larger bandgap than the energy of the photons generated; and 2) use of very small wavelength-selective loss differences between the main mode and the side modes. Transient response and steady-state fluctuations are calculated from a rate-equation model. New approximate analytical expressions are given to make possible the calculation of 1) the critical gain-loss term and 2) the consequent laser characteristics. It is shown that a very small side-mode loss difference increases the coupling between the carrier density and the main-mode photons. This produces very fast transient response. It is also shown that the larger bandgap region smoothens the output power and enhances the main-mode power at constant driving current. These results give promise of a low-power-level laser capable of multigigabit direct modulation     

Design of a miniature lens for semiconductor laser to single-mode fiber coupling

A design method for a miniature optical lens tipped on a single-mode fiber end to improve power coupling from a semiconductor laser is described. The lens shapes studied are hemispherical, hemicylindrical, and hemiellipsoidal. The optimum coupling conditions and the obtained coupling efficiency are represented in terms of laser beam and fiber parameters. Preferable ranges of hemispherical and hemicylindrical lenses are classified according to the laser and fiber mode spot sizes. Fiber axis offset and tilt effects on coupling efficiency are also studied.     

Transient frequency shift in a single-mode quasi-continuousdiode-pumped Nd:YAG laser

The transient frequency shift of a quasi-continuous single-mode frequency-doubled diode-pumped Nd:YAG ring laser has been measured using a confocal spherical Fabry-Perot. At a rate of 50 Hz, a quasi-linear shift of 5.4 MHz at the fundamental 1.06-μm wavelength has been measured during the 150-μs laser pulse duration. The doubled frequency, which is obtained by an intracavity LBO crystal, shows also a similar but doubled linear shift. This observed frequency shift is interpreted as due to thermal effects in the rod and fits well a modeling of the pulsed laser diode pumping. Introducing intracavity compensation, a piezoceramic transducer mounted cavity mirror, has limited the observed shift to less than ±1 MHz. Such a quasi-continuous Nd:YAG single-mode and frequency stabilized device should be a very suitable single-mode laser source to pump optical parametric oscillator systems     

Effects of gain saturation in semiconductor laser amplifier links

The effects of gain saturation due to amplified spontaneous emission in systems comprising cascaded semiconductor laser amplifiers and attenuators are investigated. The influence of the relative spatial distribution in the link of gain and loss is analyzed numerically, and conclusions regarding noise optimal structures are drawn. It is shown that the performance of links with gain saturation can be quite different from the ideal, unsaturated case     

High-power nearly diffraction-limited AlGaAs-InGaAs semiconductor slab-coupled optical waveguide laser

Beam-quality measurements on the output of a 915-nm AlGaAs-InGaAs-GaAs slab-coupled optical waveguide laser (SCOWL) are reported. This device had a nearly circular mode (3.8 /spl mu/m by 3.4 /spl mu/m 1/e/sup 2/ widths in the near-field) and was capable of a single-ended continuous-wave output power of greater than 1 W. Measurements of M/sup 2/ indicate that the SCOWL output beam is nearly diffraction-limited in both directions with M/sub x//sup 2/ /spl sim/ M/sub y//sup 2/ /spl sim/ 1.1 over the entire range of output powers measured.     

Delayed-self-heterodyne measurement of laser frequency fluctuations

The two-sample (or Allan) variance of frequency fluctuations was used for laser frequency-noise characterization at Fourier frequencies below 1 MHz. The delayed self-heterodyne technique was used for the variance measurement. Analytical calculations ensure that just twice the variance values were observed at τ<0.3τ_d, where τ is the averaging time for the frequency measurement and τ_d is the delay time. The variance measurements of 1.5-μm distributed Bragg reflector and distributed feedback laser diodes were demonstrated at 6.0×10^-7⩽τ⩽1.0×10^-4 [s] by using an 81.5-km fiber delay line. Observed flicker fluctuations (or 1/ f noise) are discussed     

Characteristics of a hemispherical microlens for coupling between a semiconductor laser and single-mode fiber

A hemispherical microlens is fabricated on the end of a single-mode fiber by an electric arc discharge technique. It improves coupling efficiency between InGaAsP lasers with buried heterostructure geometry and single-mode fiber. The lowest coupling loss of 2.9 dB is achieved with the optimum lens radius of 8.5 μm. This loss is 4.4 dB lower than that with a butt joint. Experimental results of coupling efficiency and alignment tolerances in coupling with different lens radii in the range of3.5-17 mum are discussed in detail. The results are in good agreement with theoretical values derived by Gaussian beam and paraxial ray approximations.     

Gain saturation in traveling-wave ridge waveguide semiconductor laser amplifiers

The saturation behavior of traveling-wave ridge waveguide diode amplifiers is studied both numerically and analytically. It is shown that an 'effective saturation power' can be used to characterize the saturation of these devices by up to 4 dB. The effective saturation power, which is sensitive to device geometry, carrier density, and operating wavelength, is useful in the design of the ridge waveguide optical amplifiers.<>     

Polarization fluctuations in a single-mode optical fiber

Polarization fluctuation characteristics of single-mode optical fiber output are examined theoretically and experimentally. A fiber model for studying these characteristics from the perspective of probability on the Poinare sphere is proposed. The relationship of the degree of polarization fluctuation to time and fiber length is shown theoretically. Measurements of submarine and underground cables and long-haul fibers up to 270 km in length generally support the proposed model     

端面反射率对半导体激光器单模工作性能的影响

本文利用半导体激光器多模速率方程组的隐式解析解，从理论上了端面反射率对半导体激光器单模工作性能的影响，结果表明，在相同的输出功率下，端面反射率越低，单模工作性能越差，与Ｅｔｔｅｎｂｅｒｇ等人通过实验研究所得的结论相同。