Development of bottom-emitting 1300-nm vertical-cavity surface-emitting lasers

We present experimental results on the development of bottom-emitting GaInNAs vertical-cavity surface-emitting lasers (VCSELs) operating at wavelengths near 1300 nm. This development effort is based on the modification of oxide-apertured top-emitting structures to allow emission through the GaAs substrate. Similar device performance was seen in both the top- and bottom-emitting structures. Single-mode output powers (adjusted for substrate absorption) of /spl sim/0.75 mW, with threshold currents of 1.3 mA, were achieved with /spl sim/3.5-/spl mu/m aperture diameters. Larger multimode devices exhibited a maximum adjusted output power of 2.2 mW. To the best of our knowledge, these are the first bottom-emitting flip-chip compatible 1300-nm VCSELs fabricated with GaInNAs-GaAs active regions.     

Temperature-sensitivity analysis of 1360-nm dilute-nitride quantum-well lasers

The N-content of an InGaAsN quantum-well (QW) laser is found to dramatically affect the temperature sensitivity of the current injection efficiency (/spl eta//sub inj/) and material gain parameter (g/sub oJ/). The increased temperature sensitivity of /spl eta//sub inj/ and g/sub oJ/ of InGaAsN QW lasers with increasing N-content leads to a significant increase in their temperature sensitivity of threshold current and external differential quantum efficiency. Increasing the N-content of the InGaAsN QW potentially results in a reduction of the heavy hole confinement, which may account for the increased temperature sensitivity of the current injection efficiency.     

1300-nm horizontal-cavity surface-emitting BH-DFB lasers for uncooled operation

We present a novel type of 1300-nm horizontal-cavity surface-emitting buried heterostructure distributed feedback (DFB) lasers showing high optical output power and uncooled direct modulation capability of 7.5 Gb/s. These lasers can be fabricated and tested using on-wafer techniques only, so the overall fabrication costs are considerably lower than with conventional edge-emitting DFB lasers.     

1300-nm pulsed fiber lasers mode-locked by purified carbon nanotubes

For the first time, we demonstrate a novel passively mode-locked fiber laser operating at 1300 nm using purified single-walled carbon nanotubes (CNTs) as a saturable absorber. The saturable absorber incorporates diameter-controlled CNTs with peak absorption /spl sim/1300 nm, guaranteeing mode-locking over the same wavelength region. The ring laser uses praseodymium-doped fiber as a gain medium. The pulse repetition rate is 3.18 MHz, and the spectral half-width is 0.15 nm. Dual-wavelength mode-locking is also demonstrated with a channel spacing of 1.1 nm.     

The role of carrier transport on the current injection efficiency of InGaAsN quantum-well lasers

A theoretical and experimental study demonstrates that the current injection efficiency of InGaAsN quantum-well (QW) lasers can be significantly affected by carrier transport in the separate confinement heterostructure (SCH) region. An offset QW design is utilized to show the impact of hole transport on the temperature dependence of the external differential quantum efficiency and above threshold injection efficiency. A reduction of the current injection efficiency is found for structures which have significant hole transport times in the SCH.     

High-modal gain 1300-nm In(Ga)As-GaAs quantum-dot lasers

A semiconductor laser containing seven InAs-InGaAs stacked quantum-dot (QD) layers was grown by molecular beam epitaxy. Shallow mesa ridge-waveguide lasers with stripe width of 120 /spl mu/m were fabricated and tested. A high modal gain of 41 cm/sup -1/ was obtained at room temperature corresponding to a modal gain of /spl sim/6 cm/sup -1/ per QD layer, which is very promising to enable the realization of 1.3-/spl mu/m ultrashort cavity devices such as vertical-cavity surface-emitting lasers. Ground state laser action was achieved for a 360-/spl mu/m-cavity length with as-cleaved facets. The transparency current density per QD layer and internal quantum efficiency were 13 A/cm/sup 2/ and 67%, respectively.     

High-performance 980-nm strained-layer GaInAs-GaInAsP-GaInP quantum-well lasers grown by all solid-source molecular-beam epitaxy

High-performance strained-layer GaInAs-GaInAsP-GaInP separate-confinement quantum-well lasers emitting at /spl lambda/=980 nm were grown by all solid-source molecular-beam epitaxy. Valved cracker cells were employed to generate group-V beam fluxes. Fabricated ridge-waveguide lasers exhibited stable, kink-free, single-mode operation up to 260 mW. A maximum output power of 550 mW was achieved. Complete thermal roll-over tests were done tens of times without any sign of degradation for p-side up-mounted lasers. Preliminary lifetime tests for over 4500 h at 150-mW power level indicate that these aluminum-free pump lasers are very reliable sources for pumping light into erbium-doped fiber amplifiers.     

Effects of doping in the active region of 630-nm band GaInP-AlGaInPtensile-strained quantum-well lasers

We theoretically analyze 630-nm band GaInP-AlGaInP tensile-strained quantum-well (QW) lasers with doping in the active region. The radiative current can be significantly reduced by introducing n-type doping in the active region. However, this advantage is reduced by the increase of the leakage current. As a result, the threshold current is reduced and the emission wavelength is shortened for multiquantum-well (MQW) lasers by n-type doping. But for single-quantum-well (SQW) lasers, because of the large increase of the leakage current, the threshold current increases with n-type doping     

Temperature dependence of electrical and optical modulationresponses of quantum-well lasers

We present theory and experiment for high-speed optical injection in the absorption region of a quantum-well laser and compare the results with those of electrical injection including the carrier transport effect. We show that the main difference between the two responses is the low-frequency roll-off. By using both injection methods, we obtain more accurate and consistent measurements of many important dynamic laser parameters, including the differential gain, carrier lifetime, K factor, and gain compression factor. Temperature-dependent data of the test laser are presented which show that the most dominant effect is the linear degradation of differential gain and injection efficiency with increasing temperature. While the K-factor is insensitive to temperature variation for multiple-quantum-well lasers, we find that the carrier capture time and nonlinear gain suppression coefficient decreases as temperature increases     

Gain, index variation, and linewidth-enhancement factor in 980-nm quantum-well and quantum-dot lasers

An experimental comparative study of the gain, index variation, and linewidth enhancement factor in 980-nm quantum-well (QW) and quantum-dot (QD) lasers structures, designed for high power applications, is presented. The gain spectra of the QW lasers at high injection level revealed three different transition energies, with a low linewidth enhancement factor (/spl sim/1.2) for E2HH2 transitions. Similar values for the linewidth enhancement factor, ranging between 2.5 and 4.5, were found for QW and QD devices, when comparing at similar values of the peak gain. This result is attributed to the contribution of excited state transitions in the measured QD lasers.     

OC-48 capable InGaAsN vertical cavity lasers

A selectively oxidised InGaAsN/GaAs three quantum well vertical cavity laser (VCSEL) demonstrated continuous wave (CW) lasing with a single-mode output power of 0.749 mW at 1266 nm. This is the first reported InGaAsN VCSEL capable of meeting the power and wavelength requirements for both OC-48 SR and OC-48 IR-1 compliant links. The VCSEL uses two low absorption n-type GaAs/AlGaAs distributed Brag reflectors and a tunnel junction to achieve current injection into the active region. A multimode version of the VCSEL had a output power of 1.43 mW at 1.26 μm. CW lasing continued up to temperatures as high as 107°C. The VCSEL material was grown by solid source molecular beam epitaxy with an RF nitrogen plasma source     

Gain spectra of quantum-well lasers

The gain spectrum and its sensitivity to carrier density is calculated for a model quantum-well heterostructure semiconductor laser for a range of quantum-well widths. The gain spectra, especially for narrow wells, show better mode-to-mode gain discrimination than for the equivalent bulk laser. Good carrier confinement helps obtain this desirable feature.     

Impedance characteristics of quantum-well lasers

We derive theoretical expressions for the impedance of quantum-well lasers below and above threshold based on a simple rate equation model. These electrical laser characteristics are shown to be dominated by purely electrical parameters related to carrier capture/transport and carrier re-emission. The results of on-wafer measurements of the impedance of high-speed In_0.35Ga_0.65 As/GaAs multiple-quantum-well lasers are shown to be in good agreement with this simple model, allowing us to extract the effective carrier escape time and the effective carrier lifetime, and to estimate the effective carrier capture/transport time     

Temperature sensitivity of injection-locked vertical-cavity surface-emitting lasers

The dynamics of injection-locked vertical-cavity surface-emitting lasers (VCSELs) are studied as a function of temperature. The temperature dependence of the slave VCSEL's parameters is used in a rate-equation analysis and parametric maps in the injection strength K- and frequency detuning /spl omega/-planes are calculated in order to investigate the temperature dependence of the system's stability. We demonstrate that, as we increase temperature for the range where the linewidth enhancement factor /spl alpha/ starts to stabilize, approximately 10 K above the temperature of where the minimum of the threshold carrier density occurs, the locking region tends to be suppressed and the nonlinearities to grow due to the increase of the relaxation resonance frequency /spl omega//sub R/ and the total loss rate /spl Gamma//sub 0/. Below that range, the opposite route is followed due to the enhanced value of the linewidth enhancement factor /spl alpha/, and the results are sensitive to the intraband relaxation time /spl tau/. It is finally concluded that, to take advantage of the stable locking region and to avoid the nonlinearities, it is better for the VCSEL device to have a minimum carrier density of 40 K-50 K below room temperature, thus allowing a good operating tolerance in the range /spl plusmn/20 K around room temperature.     

CW technique for measurement of linewidth enhancement factor: application to 735-nm tensile-strained GaAsP quantum-well lasers

We present a procedure for determining the linewidth enhancement factor (/spl alpha/ parameter) in semiconductor lasers under continuous-wave (CW) operation. It is based on the measurement of the amplified spontaneous emission spectra, with a proper correction of thermal effects. The method is applied to 735-nm tensile strained GaAsP-AlGaAs quantum-well lasers and it is validated by comparing CW results, after correcting thermal effects, with pulsed measurements. The results show a low value of the /spl alpha/ parameter attributed to the tensile strain.     

Optical properties of metamorphic hybrid heterostuctures for vertical-cavity surface-emitting lasers operating in the 1300-nm spectral range

The possibility of fabricating hybrid metamorphic heterostructures for vertical-cavity surfaceemitting lasers working in the 1300-nm spectral range is demonstrated. The metamorphic semiconductor part of the heterostructure with a GaAs/AlGaAs distributed Bragg reflector and an active region based on InAlGaAs/InGaAs quantum wells is grown by molecular-beam epitaxy on a GaAs (100) substrate. The top dielectric mirror with a SiO₂/Ta₂O₅ distributed Bragg reflector is formed by magnetron sputtering. The spectra of the room-temperature microphotoluminescence of these vertical-cavity surface-emitting laser heterostructures are studied under 532-nm excitation in the power range of 0–70 mW (with a focused-beam diameter of ~1 μm). The superlinear dependence of the photoluminescence intensity on the excitation power, narrowing of the photoluminescence peaks, and a change in the modal composition may be indications of lasing. The results obtained give evidence that the technology of the metamorphic growth of heterostructures on GaAs substrates can be used for the fabrication of vertical-cavity surface-emitting lasers working in the 1300- nm spectral range.     

Resonant tunneling injection quantum-well lasers

We report, for the first time, the observation of resonant tunneling induced negative differential characteristics in the light-current (L-I) and light-voltage (L-V) curves of a single-quantum-well semiconductor laser. Broad-stripe lasers have exhibited CW threshold current density of 50 A/cm² at 77 K, with a sharp decrease in output light power at 68 A/cm², which corresponds to a resonant point in the current-voltage (I-V) characteristics of the device. Peak to valley ratios of more than 1.5:1 were observed in the L-I and L-V curves     

Linewidth enhancement in quantum-well lasers

We have developed explicit approximations for the Hnewidth enhancement factor in quantum-well lasers. These simple expressions represent a quick and easy means of calculating the linewidth enhancement factor under a variety of operating conditions, and help us to understand the physics influencing this important parameter.     

Wavelength references for 1300-nm wavelength-division multiplexing

We have conducted a study of potential wavelength calibration references for use as both moderate-accuracy transfer standards and high-accuracy National Institute of Standards and Technology (NIST) internal references in the 1280-1320-nm wavelength-division-multiplexing region. We found that most atomic and molecular absorption lines in this region are not ideal for use as wavelength references owing to factors such as weak absorption, complex spectra, or special requirements (for example, frequency-doubling or excitation with an additional light or discharge source). We have demonstrated one of the simpler schemes consisting of a tunable diode laser stabilized to a Doppler-broadened methane absorption line. By conducting a beat-note comparison of this reference to a calcium-based optical frequency standard, we measured the methane line center with an expanded uncertainty (2/spl sigma/) of /spl plusmn/2.3 MHz. This methane-stabilized laser now serves as a NIST internal reference.     

Monolithic colliding-pulse mode-locked quantum-well lasers

Integration of the whole mode-locked laser onto a single piece of semiconductor offers a number of advantages, including total elimination of optical alignment processes, improved mechanical stability, and the generation of short optical pulses at much higher repetition frequencies. Semiconductor laser processing technologies were used to implement the colliding-pulse mode-locking (CPM) scheme, which is known to effectively shorten the pulses and increase stability, on a miniature monolithic semiconductor cavity. The principles of and recent progress in monolithic CPM quantum-well lasers are reviewed