Analysis of radiation mode effects on oscillating properties of DFBlasers

The effects of radiation mode on the oscillating properties of distributed feedback (DFB) lasers with second-order corrugations are analyzed for designing a new type of DFB laser. A formulation based on the transfer matrix technique is applied to calculating Streifer's ζ-terms added to the coupled-wave equations. These terms represent the effects of radiation and evanescent modes. This formulation greatly simplifies the analysis of distributed resonance along multilayered waveguide structures with arbitrary-shaped second-order corrugations. The effects of vertical resonance are also incorporated into the formulation. Various types of DFB lasers with phase-shifted second-order corrugations are analyzed using this method. It is found that the phase shift and the blaze of the corrugations greatly affect the longitudinal mode selectivity. A new phase-shift DFB laser structure with two complementary blazing regions connected at the shift is proposed. It is demonstrated that this structure has small radiation loss resulting in low-threshold performance despite employing second-order corrugations     

RIN transfer measurement and modeling in dual-order Raman fiber amplifiers

A numerical analysis is presented that models the transfer of relative intensity noise (RIN) from the first- and second-order pump lasers to the signal radiation in dual-order Raman fiber amplifiers. Measurements are presented of the first- and second-order RIN transfer functions for co- and counter-propagating fiber amplifiers. The second-order RIN transfer function is similar to that found in single-order Raman fiber amplifiers and the first-order transfer function is approximately 15 dB less than the second-order transfer function. The impact of the RIN transfer from the first- and second-order pump lasers to the signal radiation on the system performance is examined and estimates for the required pump laser RIN levels are presented.     

Dispersion-induced composite second-order distortion at 1.5 μm

Experimental and theoretical results have shown that the composite second-order (CSO) nonlinearity of 1.5-μm AM analog laser links is inadvertently affected by the coupling of laser chirp with fiber dispersion in regular single-mode fiber. To counteract the dispersion effect, it is shown that dispersion-shifted fiber can be employed in place of the regular fiber. Other possible ways to reduce distortion are to use lasers with reduced chirp, presumably with a multiquantum-well laser structure, or lasers with well-controlled spatial hole burning. Short fiber spans (⩽5 km) for applications providing loop distribution in conjunction with fiber amplifiers are feasible without the use of countermeasures     

Axial profile of grating coupled radiation from second-order DFBlasers with phase shifts

Axial profiles of the grating-coupled radiation field emitted in a direction normal to the surface of distributed-feedback (DFB) lasers with phase-shifted second-order corrugation are theoretically analyzed. The profiles are calculated for nonreflecting DFB lasers with the following phase shifts: λ/4 shift, two λ/8 shifts, and two 3λ/8 shifts. It is demonstrated that the radiation field can be controlled by changing the phase shifts. This result suggests a new method for modulating the surface emission of a DFB laser     

Stability of self-pulsing due to Hopf bifurcation in semiconductor lasers

A formulation is given of the stability criterion for oscillations arising at Hopf bifurcations in semiconductor lasers. A second-order perturbation expansion is utilized to provide nonlinear rate equations for the laser. The rate equations contain terms whose second-order partial derivatives are required in the definition of stability parameters. Using these parameters, a simple statement of the stability criterion is possible. Numerical calculations are included, using typical device parameters, and show the existence of both stable and unstable Hopf oscillations.     

Effects of radiation loss on the performance of second-order DFBsemiconductor lasers

The effects of radiation loss on the performance of second-order distributed-feedback (DFB) semiconductor lasers with a symmetrical grating are analyzed systematically. The threshold gains of the two lowest modes, the differential quantum efficiency, and the spectral linewidth are calculated as a function of the complex coupling coefficient. The comparison is also made between a conventional DFB and a quarter-wave phase-shifted DFB laser. It is shown that the effects of radiation loss are different in these two laser types. In addition, a simple approximate expression for threshold gain is derived. By way of example, a DFB laser with a rectangular grating is analyzed. It is shown that the performance of a second-order DFB laser is not sensitive to the grating pitch over a fairly wide range, provided that it is close to the pitch which gives the maximum coupling coefficient     

太赫兹二级分布反馈量子级联激光器中的模式竞争与功率特性

结合实验和理论计算研究了太赫兹二级分布反馈量子级联激光器中的模式竞争和功率特性.研究表明,激光器在整个动力学范围内均稳定地工作在横向及纵向的基模.横向基模的产生原因是脊条两侧的吸收边界有效提高了高阶横模的损耗.纵向基模的产生原因主要是谐振腔内基模与高阶纵模的电磁场分布交叠较大,并且频率接近,从而有效避免了增益在空间和频域的烧孔效应.此外,激光器的辐射效率随分布反馈光栅长度的增加而减小,导致只有在特定的光栅长度才能获得最大的输出功率.该工作有助于高性能单模太赫兹激光器及锁相激光器阵列的研制.     

Laser Diode-Pumped Organic Semiconductor Lasers Utilizing Two-Dimensional Photonic Crystal Resonators

Two-dimensional photonic crystal lasers based on the small molecule organic semiconductor tris-(8-hydroxyquinoline) aluminum (Alq ₃) doped with 4-Dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) are optically pumped with a conventional low-cost pulsed (In)GaN laser diode. We compare photonic crystal resonators providing first- and second-order distributed feedback and find threshold values of 1.9 and 3.2kW/cm², respectively. Such inorganic-organic hybrid laser systems open up a way to inexpensive, tunable, and all solid-state lasers in the full visible wavelength range     

Fundamental second-order nonlinear distortions in analog AM CATVtransport systems based on single frequency semiconductor lasers

The author analyzes the fundamental second-order nonlinearities and their composite distortions in lightwave analog AM CATV transport systems based on single frequency semiconductor lasers. The nonlinear interaction between the photons and the electrons in the laser, the nonlinear dynamics of laser spatial hole burning, and the nonlinear interaction of fiber chromatic dispersion and laser frequency chirping are strongly dependent on the CATV carrier frequencies. The author found that the overall distortions increase for higher carrier frequencies and longer fiber spans. The distortion levels, however, are within the critical requirements for current and future CATV transport systems     

Reduction of laser chirp in 1.5 ?m DFB lasers by modulation pulse shaping

The dynamic linewidth of 1.5 ?m ridge waveguide DFB lasers is shown to be reduced by shaping the pulse of the laser modulating waveform. Pulse shaping is performed by a second-order network designed to cancel the small-signal laser resonance. Results demonstrate a dynamic linewidth reduction from 1.4 ? to 0.55 ? FWHM for a 500 ps pulse.     

Above-Threshold Spectrum of the Radiation Field in Surface-Emitting DFB Lasers

This paper addresses the above-threshold analysis of the amplified spontaneous emission spectrum of the radiation field in surface-emitting distributed feedback (DFB) lasers with a second-order grating. To the best of the authors' knowledge, this paper is the first report in this regard. The analysis takes advantage of the time- and frequency-domain approaches. The essence of this method is obtaining the spectrum of the laser power using the fields in the frequency domain, whereas the above-threshold carrier distribution inside the cavity is obtained by the finite-difference time-domain approach. The use of this approach can be justified by the fact that the wave equations in the time and frequency domains are related by the Fourier transform. Applying this method to the surface-emitting DFB lasers with a second-order grating, the authors will demonstrate that excitation of the radiation field by the interference between the counter-propagating waves inside the cavity provides additional filtering mechanism, which makes the side-mode suppression ratio of the power emitted from the surface different than that from the edge. More importantly, it is shown that the interesting features of a properly designed quarter-wave phase-shifted surface-emitting DFB lasers with a second-order grating can be exploited in the design of transmitters for optical communications     

Analysis of terahertz surface emitting quantum-cascade lasers

An analysis of surface-emitting terahertz quantum-cascade lasers operating at wavelengths near 100 /spl mu/m is presented. The devices use distributed feedback through second-order Bragg metal gratings to generate strong emission of radiation normal to the laser surface. The analysis is based on coupling between the exact Floquet-Bloch eigenmodes of infinite periodic structures in finite length devices. The results show performance of surface-emitting terahertz lasers comparable to edge-emitting devices, with high radiative efficiencies and low threshold gains. Using phase-shifts in the grating, high-quality single-lobe beams in the farfield are obtained.     

Highly monomode W1 waveguide square lattice photonic crystal lasers

Lasing of W1 square lattice-based photonic crystal lasers on the substrate approach is analyzed. A second-order distributed feedback highly monomode behavior is observed. A comprehensive two-dimensional and three-dimensional finite-difference time-domain computation analysis shows that this characteristic arises from the lattice geometry. Single-mode lasers can be obtained using this geometry, opening the way to the realization of monomode laser arrays.     

RIN transfer suppression technique for dual-order Raman pumping schemes

An optical technique is demonstrated for suppressing the relative intensity noise (RIN) transfer from a high-power second-order pump laser to the signal radiation in a copropagating dual-order Raman pumping scheme. RIN transfer suppression is accomplished by intensity-modulating a low-power semiconductor laser diode first-order pump. Measurements are presented demonstrating suppression over the entire electronic bandwidth and -20-dB suppression spanning a 23-nm optical bandwidth. This technique may facilitate the use of high-power fiber lasers in copropagating dual-order distributed Raman amplifiers.     

Theoretical analysis of modulation response and second-orderharmonic distortion in vertical-cavity surface-emitting lasers

A rate-equation model is developed, with the consideration of size effects, to analyze the steady state and dynamic behavior of index-guided vertical-cavity surface-emitting lasers. The size dependence of spatial hole burning, cavity loss, as well as thermal resistance of device cavity are taken into account. Using this model, the influence of size effects on the amplitude modulation response and second-order harmonic distortion are studied. It is found that a laser with a small core radius exhibits better modulation response and less harmonic distortion than that of a large waveguide device, however, there is a tradeoff between the output power and modulation efficiency of the lasers     

Second-harmonic generation in GaAs-based quantum-cascade lasers grown on <100> substrates

Emission of coherent light at 5.1 /spl mu/m wavelength from GaAs-based quantum cascade lasers is reported. This was achieved by integrating nonlinear cascades with large second-order susceptibility in the active regions of the laser.     

Dynamic analysis of radiation and side-mode suppression in asecond-order DFB laser using time-domain large-signal traveling wavemodel

In this paper, we have developed a relatively simple algorithm to calculate the large-signal dynamic response of DFB lasers by solving the time-dependent coupled wave equations directly in the time domain. The spontaneous emission noise, longitudinal variations of carrier (hole burning) and photon densities as well as that of the refractive index are taken into consideration. To demonstrate the power of this straightforward algorithm, the model shows how the side-mode suppression ratio in devices with high κL and a λ/4: phase shift is significantly affected by the radiation in the second-order DFB laser. The time-dependent radiation pattern in grating-coupled surface-emitting lasers is also calculated for the first time     

Transmission characteristics of DFB laser modules for analogapplications

Analog transmission characteristics of highly linear 1.3- and 1.55-μm distributed feedback (DFB) laser modules are described. To obtain high-power and high-performance DFB lasers, the bias dependence of distortion characteristics is discussed. The carrier-to-noise ratio, the composite second-order distortion, and the composite triple beat of the DFB laser module which is developed for analog CATV applications are presented. The analog transmission characteristics, such as CNR degradation, after fiber transmission, dispersion-induced distortion, and the influence of reflection, have been made clear. The stable high linearity of the DFB laser also has been confirmed by the composite distortion measurement under an accelerated aging test     

40-mW 650-nm distributed feedback lasers

Using a buried second-order grating in a single-mode GaInP-AlInP laser structure, we obtain single-spatial- and longitudinal-mode operation at 652 nm. Over 40-mW continuous wave (CW) is obtained at room temperature (RT), with efficiencies and far fields comparable to conventional Fabry-Perot lasers. The devices are continuously tunable with current and temperature and exhibit no mode hops, and thus should be useful for applications where short wavelength and longitudinal mode stability is required, such as spectroscopy, interferometry, or optical storage     

Temporal coherence properties of picosecond pulses generated by GaAlAs semiconductor lasers for directly modulated and frequency stabilized optical communication systems

The temporal coherence properties of picosecond pulses from GaAlAs semiconductor lasers have been studied by measuring first- and second-order autocorrelation functions with a Michelson interferometer. The time-bandwidth product and chirp have been evaluated for several different laser structures and frequency stabilization schemes. Time-bandwidth products from 1.2 for a CSP laser to 3.6 for a TJS laser were obtained with operating conditions representative of high-speed communication systems. Computer simulations based on the rate equations have been used to model the temporal coherence during the pulse and they show good agreement with our measurements. The results show that it is important to select the appropriate laser structure and stabilization scheme in applications where the temporal coherence is significant for system performance.