External Differential Quantum Efficiency Research Articles

Analysis of the characteristic temperatures of (Ga,In)(N,As)/GaAs laser diodes

The characteristic temperatures of the threshold current density, T0, and external differential quantum efficiency, T1, of a series of (Ga,In)(N,As)/GaAs quantum well (QW) laser diodes are measured in the wavelength range from 1 to 1.5 µm. It is found that both T0 and T1 strongly decrease with increasing lasing wavelength. The origin of this degradation is shown to be, in the case of T0, mostly dominated by a decrease in the transparency current density characteristic temperature, an increase in the optical losses and a decrease in the modal gain. The degradation of T1 is mainly due to the increase in the optical losses. The effective carrier recombination lifetime in the QW is shown to decrease from 1.2 to 0.2 ns with N content up to 2%, in good agreement with previous reports that link this low lifetime to non-radiative monomolecular recombination through defects in the QW. Carrier leakage is ruled out as the dominant process degrading T0 and T1 on the basis of the temperature dependence of the effective carrier recombination lifetime.

312-fs pulse generation from a passive C-band InAs/InP quantum dot mode-locked laser

For the first time, we report femtosecond pulses from a passive single-section InAs/InP quantum-dot (QD) mode-locked laser (MLL) with the active length of 456 microm and ridge width of 2.5 microm at the C-band wavelength range. Without any external pulse compression, the transform-limited Gaussian-pulses are generated at the 92 GHz repetition rate with the 312 fs pulse duration, which is the shortest pulse from any directly electric-pumping semiconductor MLLs to our best knowledge. The lasing threshold injection current and external differential quantum efficiency are 17.2 mA and 38%, respectively. We have also investigated the working principles of the proposed QD MLLs.

Measurements of Cavity-Length-Dependent Internal Differential Quantum Efficiency and Internal Optical Loss in Laser Diodes

We measured the cavity length dependence of the internal differential quantum efficiency ηint and the internal optical loss αint in 1500-nm-wavelength laser diodes (LDs). By evaluating αint directly from gain/absorption spectra for various injection current densities and measuring the external differential quantum efficiency ηext, we obtained the ηint and αint values of all the LDs with different cavity lengths. The obtained ηint and αint respectively showed strong and weak cavity length dependences, and were both very different from those derived by the widely used method of plotting ηext-1 against the cavity length.

Analysis of 2-D Surface-Emitting ROW-DFB Semiconductor Lasers for High-Power Single-Mode Operation

A detailed analysis of 2D surface-emitting active-photonic-crystal (APC) lasers composed of a resonant-optical-waveguide (ROW) array and Pi-phaseshifted second-order distributed feedback/distributed Bragg reflector (DFB/DBR) gratings, made atop the ROW-array elements, is performed. Coupled-mode theory and the transfer-matrix method are used for analysis in the longitudinal direction, and 2D rigorous array modeling is used for analysis in the lateral direction. The gratings, beside their usual functions (i.e., feedback and outcoupling), act as strong lateral-mode (i.e., array-mode) selectors by exploiting the array-modes' different field-intensity overlap with the element regions, at the in-phase-mode resonance. The in-phase array mode is strongly favored to lase at and around its (lateral) resonance due both to better longitudinal-field overlap with the active-grating (i.e., DFB) region <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">and </i> to much lower interelement loss than the other array modes. The loss in the interelement regions that suppresses lasing of the nonresonant modes is due to strong absorption to both the metal contact (i.e., Au) as well as to the semiconductor/metal grating layers (i.e., GaAs-Au) in the element regions. The intermodal discrimination can be further enhanced by introducing strong free-carrier absorption in the interelement regions. For 20-element arrays with 700 mum/600 mum DFB/DBR gratings, emitting at 0.98 mum and of 100-mum-wide lateral dimension, the intermodal discrimination, DeltaJ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> , is high (~500 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) at the in-phase-mode resonance, and > 70 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> as the regrown interelement-region thickness t is varied (during fabrication) over a 0.02-mum range. The external differential quantum efficiency reaches 58%, and the guided-field intensity profile is virtually uniform in both the lateral and longitudinal directions. For 40-element devices, the projected CW power in a single, diffraction-limited lobe is 2.4 W at 10 x threshold. For 20-element arrays with intentionally introduced free-carrier absorption in the interelement regions, DeltaJ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> is very high (~6500 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) at the in-phase-mode resonance, and >70 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> as t is varied over a 0.035-mum range. The projected single-mode, diffraction-limited continuous-wave power, from 40-element devices at 10x threshold, is similar: 2.4 W.

Performance improvement of InGaAs quantum well distributed Bragg reflector laser with curved grating

AbstractPerformance improvement is performed for an InGaAs quantum well DBR laser consisting of a ridge active channel and a curved grating such that single‐mode oscillation with high efficiency and high spectral purity can be realized. The waveguide absorption loss is substantially reduced by selectively disordering the quantum well in the DBR region. The curved DBR grating is optimized so that a high reflectivity and sharp wavelength selectivity are obtained simultaneously. In addition, the external differential quantum efficiency is enhanced by optimizing the reflectivity at the output end with a low‐reflection coating. In this approach, a single‐mode oscillation is realized with a high external differential quantum efficiency of 0.79 and a side mode suppression ratio of 50 dB. In the laser with a window structure near the output end, resistance to catastrophic optical damage is significantly improved and the maximum output is 294 mW. © 2007 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 90(8): 1– 7, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjb.20348

Narrow ridge waveguide high power single mode 1.3-μm InAs/InGaAs ten-layer quantum dot lasers

Ten-layer InAs/In0.15Ga0.85As quantum dot (QD) laser structures have been grown using molecular beam epitaxy (MBE) on GaAs (001) substrate. Using the pulsed anodic oxidation technique, narrow (2 μm) ridge waveguide (RWG) InAs QD lasers have been fabricated. Under continuous wave operation, the InAs QD laser (2 × 2,000 μm2) delivered total output power of up to 272.6 mW at 10 °C at 1.3 μm. Under pulsed operation, where the device heating is greatly minimized, the InAs QD laser (2 × 2,000 μm2) delivered extremely high output power (both facets) of up to 1.22 W at 20 °C, at high external differential quantum efficiency of 96%. Far field pattern measurement of the 2-μm RWG InAs QD lasers showed single lateral mode operation.

Effect of inter-level relaxation and cavity length on double-state lasing performance of quantum dot lasers

Double-state lasing phenomena are easily observed in self-assembled quantum dot (QD) lasers. The effect of inter-level relaxation rate and cavity length on the double-state lasing performance of QD lasers is investigated on the basis of a rate equation model. Calculated results show that, for a certain cavity length, the ground state (GS) lasing threshold current increases almost linearly with the inter-level relaxation lifetime. However, as the relaxation rate becomes slower, the ratio of excited state (ES) lasing threshold current over the GS one decreases, showing an evident exponential behavior. A relatively feasible method to estimate the inter-level relaxation lifetime, which is difficult to measure directly, is provided. In addition, fast inter-level relaxation is favorable for the GS single-mode lasing, and leads to lower wetting layer (WL) carrier occupation probability and higher QD GS capture efficiency and external differential quantum efficiency. Besides, the double-state lasing effect strongly depends on the cavity length.

Temperature sensitivity of InGaAs quantum-dot lasers grown by MOCVD

Temperature-dependent cavity length studies have been performed on multiple stack strain compensated InGaAs quantum-dot (QD) active region broad stripe laser structures grown by metal-organic chemical vapor deposition. The characteristic temperature coefficients of the threshold current density (T/sub 0/) and external differential quantum efficiency (T/sub 1/) were calculated from variable temperature measurements. The correlation of the T/sub 0/,T/sub 1/ values and the extracted values of the characteristic temperature coefficients of the transparency current density, material gain, injection efficiency, and internal loss (T/sub tr/,T/sub g0/,T/sub /spl eta/inj/,T/sub /spl alpha/i/) from the temperature-dependent study is discussed. The T/sub 1/ values are higher than 400 K for five-stack QD laser structures, comparable values to conventional quantum-well (QW) laser structures. T/sub 0/ values are lower than 100 K. Extracted material gain parameters are found to increase with increasing temperature for the three-stack QD structure, and are nearly temperature independent for the five-stack structure, different to that observed in InGaAs QW lasers.

External cavity InAs∕InP quantum dot laser with a tuning range of 166nm

We have studied the tuning behavior of an external cavity laser in Littrow configuration using antireflection/high-reflection coated InAs∕InGaAsP∕InP quantum dot laser diodes as the amplifying element. Adding the coatings improves the performance of the setup, and the tunability of the external cavity laser output has been increased up to 166nm. Detailed investigations have revealed that laser diode length and width influence the magnitude of the tuning range. Furthermore, the external differential quantum efficiency is systematically increasing as the external cavity laser wavelength is decreasing. These characteristics are discussed in terms of energy levels available in the inhomogeneous broadening of the self-assembled quantum dots.

High-efficiency native-oxide-passivated high-index-contrast ridge waveguide lasers

A GaAs-based high-index-contrast ridge waveguide laser is fabricated using a self-aligned process of deep dry etching plus oxygen-enhanced wet thermal oxidation of low Al-content AlGaAs. Lasers operating at /spl lambda/=813 nm (CW, 300 K) with external differential quantum efficiencies as high as 78% are demonstrated, indicating effective passivation of the directly-oxidised etched active region sidewall surface.

Impact of N on the lasing characteristics of GaInNAs∕GaAs quantum well lasers emitting from 1.29 to 1.52μm

The origin of the degradation with N of the threshold current density (Jth) and external differential quantum efficiency (ηd) of 1.29 to 1.52μmGaInNAs∕GaAs laser diodes is analyzed. Adding N to InGaAs leads to a ∼25% reduction of the carrier injection efficiency and thus to an increase of Jth and a decrease of ηd. This effect is likely related to carrier recombination losses in the barriers and is independent of the N content. The optical absorption losses and the internal transparency current density are found to increase with N content, accounting for the rest of the degradation in Jth. Modeling of the transparency carrier and radiative current densities identifies the increase of the defect-related recombination coefficient in GaInNAs as the dominant effect leading to the N dependence of Jtr.

High-efficiency InGaAs QW distributed Bragg reflector laser with curved grating for squeezed light generation

A high-efficiency InGaAs quantum-well curved distributed Bragg reflector (DBR) laser was designed for squeezed light generation and fabricated using a selective-area quantum-well disordering technique to reduce absorption loss in the DBR grating region. Single-mode lasing with side-mode suppression ratio of 50 dB was obtained, and an external differential quantum efficiency as high as 0.71 was achieved under continuous-wave operation. Using balanced detection technique, the laser intensity noise relative to the standard quantum limit (SQL) was measured. Ultra-low noise characteristic of 0.8 dB below the standard quantum limit was achieved by wideband constant-current driving of the laser. This is the first demonstration of single-mode squeezed light generation by a monolithic semiconductor laser at room temperature.

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.

Quantum efficiency of ZnO nanowire nanolasers

Crystalline ZnO nanowires were grown on sapphire and silicon substrates using pulsed-laser deposition. The optical properties of nanowire nanolasers, including their absolute light emission intensity and external and internal quantum efficiencies were experimentally determined. The external differential quantum efficiency was measured to be as high as 60% for lasing ZnO nanowires of 7.5μm in length, compared to a value of approximately 10% for photoluminescence. The absolute light emission intensity for individual nanowires was found to be in the vicinity of 0.1mW. By measuring the dependence of external differential quantum efficiency on the cavity length, the internal quantum efficiency of ZnO nanowire nanolasers was determined to be about 85%.

Fabrication and characterization of a planarized vertical-cavity surface-emitting laser by using the silicon oxide

In this article, an alternative method is presented to fabricate a planar-type oxide-confined 850-nm vertical-cavity surface-emitting laser (VCSEL). The threshold voltage, threshold current, light output power, external differential quantum efficiency, emission spectrum, and dynamic response of VCSELs planarized with a silicon oxide (SiOx) have been evaluated. These devices exhibit excellent static characteristics, including a threshold voltage (Vth) of 2.05V corresponding to a threshold current of 0.88mA, a minimum threshold current of 0.7mA near 60°C, a maximum output power of 4.28mW at 11mA, a maximum external differential quantum efficiency (ηex) of 43% just above threshold, and an operation temperature beyond 130°C. In addition, the transverse modes of the device initially are low-order, while high-order modes appear at elevated current levels. The fundamental transverse mode at the longest wavelength increases with injected current with a redshift of 0.49nm∕mA due to the Joule effect. Since the thermal resistance of the VCSEL with a SiOx buried layer is less than that of device without it, the VCSEL with the buried layer displays less redshift and better performance. Finally, the VCSEL with a SiOx buried layer shows a clear eye-opening feature as operating at 2.488Gbit∕s with a bias current of 2mA. Further increasing the current level, the device can work at the maximum bit rate of 8Gbit∕s and a bias current of 3.7mA.

Silicon oxide-planarized single-mode 850-nm VCSELs with TO package for 10 Gb/s data transmission

We report on an alternative method to fabricate a high-efficiency planar-type oxide-confined 850-nm vertical-cavity surface-emitting lasers (VCSELs). The planarized process of VCSELs was to use the silicon oxide as the buried layer. As a result, these devices with an oxidized aperture of 3 μm in diameter exhibit a single-transverse mode behavior throughout the operation current range. In addition, the static characteristics of VCSELs at 300 K include a threshold current of 0.52mA corresponding to a threshold voltage of 2.2 V, a maximum single transverse-mode light output power of 1.13 mW at 4.5 mA, and an external differential quantum efficiency of 35%. On the other hand, this TO-packaged planar-type 850-nm VCSEL for back-to-back test shows a wide open along with symmetric eye diagram and could also pass the 10 Gb/s mask as operating at 10.3 Gb/s and 4 mA. Furthermore, the VCSEL can still keep the eye diagram open and symmetric after the 66-m multi-mode fiber transmission and has a power penalty of 6.6 dB because of fiber dispersion for 10.3 Gb/s data rate at a bit error rate of 10/sup -11/. These results confirm the excellent high-speed performance of SiO/sub x/-planarized VCSELs as compared to the polyimide-planarized VCSELs.

Recombination, gain, band structure, efficiency, and reliability of 1.5-μm GaInNAsSb/GaAs lasers

We present temperature-dependent measurements of the local Z parameter, which reflects the dominant recombination processes. At room temperature, Z increases with current from 1.2 at low bias up to a threshold value, Zth, of only 2.0, demonstrating the existence of significant amounts of monomolecular recombination. At elevated temperatures, Zth rises above 2.5. We calculate the laser’s band diagram and estimate 35 meV of band-gap renormalization at threshold through polarization-resolved measurements of exciton peaks in the absorption spectrum and inflection points of the absorption, spontaneous emission, and gain spectra. The small effective valence-band barrier height of 115 meV leads to hole leakage due to thermionic emission and carrier spillover into the barriers. This could explain the increase in Zth at elevated temperatures and decrease in internal quantum efficiency from 57% at 15 °C to 47% at 75 °C. We also analyze the spontaneous emission and gain spectra below and above threshold. The spontaneous emission clamped rather weakly at threshold, which also partially explains the low internal efficiency at room temperature. We present initial reliability measurements of over 100 h at 10-mW continuous-wave power from a single facet. After 50 h of life testing, the threshold current increased by 79% and the external differential quantum efficiency decreased by 39% of their respective pretest values. By comparing the local Z parameter versus current-density curves before and after life testing, we have identified an increase in monomolecular recombination as the main source of degraded device performance.

External cavity quantum dot tunable laser through 1.55 μm

The optical performance of InAs/InGaAsP quantum dot (QD) lasers grown on (1 0 0) InP was studied for three different material structures. The most efficient QD laser structure, having a threshold current of ∼107 mA and an external differential quantum efficiency of 9.4% at room temperature, was used to form the active region of a grating-coupled external cavity tunable laser. A tuning range of 110 nm was demonstrated, which was mainly limited by the mirror and internal losses of the uncoated laser diode. Rapid state-filling of the QDs was also demonstrated by observing the evolution of the spectra with increasing injected current.

Characteristics of InGaAsN/GaAsN quantum well lasers emitting in the 1.4-μm regime

To improve the performance of metalorganic chemical vapor deposition (MOCVD)-grown long wavelength InGaAsN quantum well (QW) diode lasers emitting beyond 1.3 μm, a detailed examination of the growth parameters was performed, including DMHy/V ratio, QW growth temperature, choice of barrier material and thermal annealing temperature. This study reveals that a growth temperature in the 530–540 °C range is preferred in order to improve nitrogen incorporation and prevent degradation of the material luminescence. Increasing the DMHy/V ratio is found to be the preferred method to achieve wavelength extension. Utilization of GaAsN barrier layers, instead of GaAs barriers, suppresses the spectral blue-shift of the quantum well luminescence after thermal annealing treatment. Under optimized growth conditions, InGaAsN diode lasers emitting at 1.378 μm are realized with a threshold current density of 661 A/cm 2 and external differential quantum efficiency of 34%. Lasing wavelengths as long as 1.41 μm with a threshold current density of 1.93 kA/cm 2 are also demonstrated, representing the longest wavelength InGaAsN QW lasers realized by MOCVD.

Low-threshold and high-efficiency operation of 1.5 µm distributed reflector laser with DFB grating and Q-wire DBR sections

A low-threshold and high-efficiency distributed reflector laser consisting of active distributed feedback and quantum-wire distributed Bragg reflector sections has been demonstrated. A threshold current Ith as low as 2.5 mA ( jth=190 A/cm2), an external differential quantum efficiency from the front facet of 36%, and a submode suppression ratio of 54 dB at a bias current of twice the threshold current were obtained for an active section length of 300 µm with a stripe width of 4.4 µm.