Measurement of gain saturation coefficients in strained-layermultiple quantum-well distributed feedback lasers

Gain saturation coefficients of unstrained- and strained-layer multiple-quantum-well lasers were measured experimentally. These coefficients were higher in lasers that had compressive strain in their active-layer wells: 2.45×10^-17 cm³ with unstrained wells and 12.6×10^-17 cm³ with strained wells. The higher gain saturation coefficient in lasers with strained active-layer wells is related to their higher linear TE mode gain coefficient. The linearity factor (K factor) between a laser's damping constant and the square of the laser's resonant frequency decreased slightly with the introduction of the strain in the laser's active layer wells. This factor, however, took the value of about 0.2×10^-9 s for each of these lasers     

Intervalence band absorption coefficient measurements in bulk layer, strained and unstrained multiquantum well 1.55 mu m semiconductor lasers

Measurements are reported of intervalence band absorption (IVBA) coefficient k/sub 0/ in three types of active layer structure: bulk, unstrained (MQW) and strained multiquantum wells. The IVBA measurements are performed by observing the spontaneous emission from the uncleaved facets of DCPBH Fabry-Perot lasers. k/sub 0/=(3.7+or-0.3)*10/sup -17/ cm/sup 2/ is obtained for bulk, (1.4+or-0.2)*10/sup -16/ cm/sup 2/ for MQW unstrained and (3.5+or-0.3)*10/sup -17/ cm/sup 2/ for strained MQW structures.<>     

A new InGaAs/InGaAsP delta -strained multiple-quantum-well laser grown by chemical-beam epitaxy

The authors propose and demonstrate a delta -strained multiple-quantum-well laser in which the quantum well is composed of a thin strained layer ( approximately AA In/sub x/Ga/sub 1-x/As) sandwiched by lattice-matched (In/sub 0.53/Ga/sub 0.47/As) layers. A threshold current density of 510 A/cm/sup 2 /was obtained from broad-area lasers with four delta -strained quantum wells and a cavity length of 3 mm, with an emission wavelength near 1.55 mu m. The use of a delta -strained quantum well provides an additional degree of freedom in optimizing the amount of strain and thickness of the active layer in improving the device performance.<>     

Minimal group refractive index dispersion and gain evolution in ultra-broad-band quantum cascade lasers

The group refractive index dispersion in ultra-broad-band quantum cascade (QC) lasers has been determined using Fabry-Perot spectra obtained by operating the lasers in continuous wave mode below threshold. In the wavelength range of 5-8 /spl mu/m, the global change of the group refractive index is as small as +8.2 /spl times/ 10/sup -3/ /spl mu/m/sup -1/. Using the method of Hakki and Paoli (1975), the subthreshold gain of the lasers has furthermore been measured as a function of wavelength and current. At the wavelength of best performance, 7.4 /spl mu/m, a modal gain coefficient of 16 cm/spl middot/kA/sup -1/ at threshold and a waveguide loss of 18 cm/sup -1/ have been estimated. The gain evolution confirms an earlier assumption that cross-absorption restricted laser action to above 6 /spl mu/m wavelength.     

Vertical-cavity surface-emitting lasers in the 8-/spl mu/m midinfrared spectral range with continuous-wave and pulsed emission

Continuous-wave (CW) as well as pulsed-laser emission from a midinfrared (/spl lambda/=7.92 /spl mu/m) IV-VI vertical-cavity surface-emitting laser at 1.8 K is presented. The high-finesse microcavity, containing PbSe as an active medium, was optically pumped with a carbon monoxide laser at a wavelength of 5.28 /spl mu/m (1894 cm/sup -1/) in either CW or Q-switched mode. The maximum achieved CW power was 4.8 mW and pulsed peak powers were up to 23 W. Linewidths are considerably narrower than 0.10 cm/sup -1/, corresponding to 0.6 nm.     

Novel method using white spectral probe signals to measure Brillouin gain spectra of pure silica core fibres

The beat spectrum of white spectral probe signals and a pump laser are used to directly give the Brillouin gain spectrum on an RF spectrum analyser. The measured peak gain coefficient is about 2.3*10/sup -11/ m/W and the Brillouin linewidth is 29 MHz which can be further broadened by the additional strain distribution.<>     

Linewidth enhancement in distributed-feedback semiconductor lasers

It is demonstrated that the linewidth enhancement in distributed-feedback (DFB) laser diodes can differ significantly from (1+ alpha /sup 2/), where alpha is Henry's linewidth enhancement factor. The influence of the relevant laser parameters, the detuning of the gain peak wavelength, a pi /2 phase shift and a reflecting end facet on the linewidth enhancement is discussed with respect to an improved DFB laser design.<>     

InAs-InAlGaAs quantum dot DFB lasers based on InP [001]

The InAs-InAlGaAs quantum dot (QD) lasers with the InAlGaAs-InAlAs material system were fabricated on distributed feedback (DFB) grating structures on InP [001]. The single-mode operation of InAs-InAlGaAs QD DFB lasers in continuous-wave mode was successfully achieved at the emission wavelength of 1.564 /spl mu/m at room temperature. This is the first observation on the InP-based QD lasers operating around the emission wavelength window of 1.55 /spl mu/m. The threshold current density of the InAs-InAlGaAs QD DFB laser with a cavity length of 1 mm and a ridge width of 3 /spl mu/m, in which one of the cleaved facets was coated with 95% high-reflection, was 1.23 kA/cm/sup 2/ (176 A/cm/sup 2/ for single QD layer). The sidemode suppression ratio value of the QD DFB laser was as high as 42 dB at the driving current of 100 mA.     

Realization of dot DFB lasers

Dot lasers with first-, second- and third-order gain coupled distributed feedback (DFB) gratings have been realized by low damage dry etching in combination with wet chemical etching and epitaxial over-growth. This technique allows above room temperature (RT) operation of dot DFB lasers with dot diameters down to 85 nm. The laser spectra show the expected emission of gain coupled DFB lasers. Threshold current densities between 1.1 kA/cm/sup 2/ and 2.6 kA/cm/sup 2/ could be obtained depending on size of the active region. An improvement in T/sub 0/ could be demonstrated comparing 0-D/1-D/2-D lasers on the same wafer. Based on the dot grating geometry improvement of the side mode suppression ratio (SMSR) was observed for broad-area dot DFB lasers.     

Measurements of the polarization dependence of the gain of strained multiple quantum well InGaAs-InP lasers

The polarization-dependent gain spectra of both tensile and compressive strain multiple-quantum-well (MQW) In/sub x/Ga/sub 1-x/As-InP lasers in a relatively large strain regime are presented. The results show that MQW lasers with tensile strain and an In concentration as low as 43% in the wells lase in a pure transverse magnetic (TM) mode rather than a transverse electric (TE) mode with a gain difference of 60-70 cm/sup -1/ at all the injection currents investigated. The peak gain for the TE mode is shifted toward shorter wavelengths from that of the TM mode, indicating that the emission is principally due to light hole-electron transition. The differential gain of the TM mode is about 1.5 times higher than that of the TE mode operation. Opposite phenomena were observed in the compressive strained MQW lasers.<>     

High-speed 1.5-μm compressively strained multi-quantum wellself-aligned constricted mesa DFB lasers

A great improvement in the high-speed characteristics for compressively strained multi-quantum-well (MQW) distributed-feedback (DFB) lasers with self-aligned constricted mesa structures is described. Negative wavelength detuning is an important factor in making possible the extraction of potential advantages for the compressively strained MQW DFB lasers. A 17-GHz bandwidth, which is the highest among the 1.5-μm MQW DFB lasers, is demonstrated. A wavelength chirp width of 0.42 nm at 10 Gb/s is obtained due to a reduced linewidth enhancement factor that has a magnitude of less than 2. Nonlinear damping K factor in a DFB laser with 45-nm negative detuning has drastically decreased to 0.13 ns, about half of that for unstrained MQW lasers. This is mainly due to an enhanced differential gain as large as 6.9×10 ^-12 m³/s. The estimated intrinsic maximum bandwidth is 68 GHz     

High breakdown voltage AlAs/InGaAs quantum barrier varactor diodes

Data are presented on single quantum barrier AlAs/In/sub 0.53/Ga/sub 0.47/As varactor diodes intended as submillimetre wavelength frequency multipliers that exhibit extremely high breakdown voltage and excellent capacitance modulation characteristics. Record breakdown voltages as high as 12 V were achieved with a composite 50AA/50AA/50AA thick In/sub 0.52/Al/sub 0.48//AlAs/In/sub 0.52/Al/sub 0.48/As barrier sandwiched between 3000 AA(1.2*10/sup 17/ cm/sup -3/) In/sub 0.53/Ga/sub 0.47/As depletion regions.<>     

Laser gain and current density in a disordered AlGaAs/GaAs quantum well

The laser gain and current density at room temperature are analysed for disordered (interdiffusion induced) Al/sub 0.3/Ga/sub 0.7/As/GaAs single quantum well structures at a carrier injection level of 4*10/sup 12/ cm/sup -2/. The results show that both the peak gain and current density remain about the same strength during the initial stages of disordering, and provide a bandwidth of 55 nm.<>     

Detuning adjustable multiwavelength MQW-DFB laser array grown byeffective index/quantum energy control selective area MOVPE

A multiwavelength MQW DFB laser array with novel structure Is described. Oscillation wavelength and gain peak wavelength were simultaneously controlled on the same epitaxial wafer by using modulated grown thicknesses of selectively grown InGaAs/InGaAsP/InP MQW active waveguides. The laser array with constant-pitch built-in corrugation fabricated by a simple DFB laser process demonstrated 10.1 nm controllable range for lasing wavelength and 45 nm for gain peak wavelengths, with uniform lasing properties and narrow spectral linewidths. The technique is attractive for light sources used in WDM/FDM applications     

Direct interferometric measurements of quantum-confined Stark effect in InAs/GaAs short period strained-layer superlattice MQWs

The electrorefraction within and below the band edge of multiple (50) InAs/GaAs short period strained-layer superlattice quantum wells has been measured. An index change of 4*10/sup -3/ was measured near the excitonic resonance with an applied electric field of 27 kV/cm, corresponding to a quadratic electro-optic coefficient of -3*10/sup -13/ cm/sup 2//V/sup 2/. These results agree with predictions from the Kramers-Kronig relations based on measurements of electroabsorption.<>     

Linewidth study of InAs-InGaAs quantum dot distributed feedback lasers

The linewidth of laterally loss-coupled distributed feedback (DFB) lasers based on InAs quantum dots (QDs) embedded in an InGaAs quantum well (QW) is investigated. Narrow linewidth operation of QD devices is demonstrated. A linewidth-power product less than 1.2 MHz /spl middot/ mW is achieved in a device of 300-/spl mu/m cavity length for an output power up to 2 mW. Depending on the gain offset of the DFB modes from the QD ground state gain peak, linewidth rebroadening or a floor is observed at a cavity photon density of about 1.2-2.4/spl times/10/sup 15/ cm/sup -3/, which is much lower than in QW lasers. This phenomenon is attributed to the enhanced gain compression observed in QDs.     

Monolithic integration of InAs photodiode and GaAs MESFET

InAs pn diodes were grown in wells pre-etched in GaAs substrates. Despite the large lattice mismatch of 7.2% between GaAs and InAs, good photodiode characteristics were obtained with 77 K resistance area products of 70 Omega cm/sup 2/ and a peak detectivity of 1.25*10/sup 11/ cm square root (Hz)/W at 2.95 mu m wavelength. GaAs MESFETs were fabricated next to the embedded detectors demonstrating for the first time the feasibility of the monolithic integration of InAs photodiodes and GaAs electronic circuits.<>     

Heterostructure bipolar transistor employing carbon-doped base grown with trimethyl-Ga and arsine

The investigation of an AlGaAs/GaAs HBT in which a heavily carbon doped base is grown by chemical beam epitaxy using trimethyl-Ga is reported. A planar technique which reduces surface recombination has been employed for selectively contacting the base region. A base width of 1000 AA and a high doping level of 7*10/sup 19/ cm/sup -3/ is used. The sheet resistance of the base is less than 100 Omega / Square Operator . This transistor has a maximum current gain of 25 at a current density of 1.3*10/sup 3/ A/cm/sup 2/.<>     

Numerical analysis of static wavelength shift for DFB lasers with longitudinal mode spatial hole burning

The static wavelength shift induced by longitudinal mode spatial hole burning is analyzed numerically for lambda /4-shifted DFB lasers. The effective Bragg wavelength at each bias level is introduced to clarify the contribution of nonuniformity in carrier density distribution to the lasing wavelength shift. It is shown that the wavelength shift is caused by two separate factors: by the position-dependent deviation and by the average value in the exact N/sub eq/ distribution. The former factor induces both red- and blue-shifted tuning due to the nonuniformity itself in carried density distribution, while the latter results in blue-shifted tuning due to the increase in modal gain.<>     

Selfaligned AlGaAs/GaAs HBT grown by MOMBE

The very low parasitic resistance n-p-n GaAs/AlGaAs heterojunction bipolar transistors (HBT) grown by metal organic molecular beam epitaxy (MOMBE) using all gaseous source dopants are reported. The carbon and tin dopants were introduced through the uses of trimethygallium (TMGa) and tetraethyltin (TESn). To achieve the low parasitics, the graded InGaAs emitter cap layer was doped with tin to 5*10/sup 19/ cm/sup -3/ and the doping level in the subcollector was 3*10/sup 18/ cm/sup -3/. The emitter and collector sheet resistances were 25 Omega / Square Operator and 10 Omega / Square Operator , respectively. The 800 AA thick base layer was carbon doped to a level of 7*10/sup 19/ cm/sup -3/. The base contact resistance and sheet resistance were 0.1 Omega mm and 180 Omega / Square Operator , respectively. With a thin AlGaAs surface passivation layer for the emitter-base junction, the common emitter DC current gain was maintained up to 25, even for 2*5 mu m/sup 2/ emitter size devices. The unity short circuit current gain cutoff frequency f/sub T/, and maximum oscillation frequency f/sub max/, were 48 and 63 GHz, respectively.<>