Quantum Dot Laser Diodes Research Articles

Quantum-dot laser diode design optimized to reach the maximum power conversion efficiency

A model is developed that allows the design of lasers based on multilayer quantum dot (QD) arrays to be optimized so as to reach the maximum conversion efficiency at a preset optical output power level. It is shown that, for the present-day state of the art in QD lasers for the 1.2–1.3 μm wavelength range, the wall-plug efficiency can exceed 70% in a broad range of optical output powers.

InGaAs–GaAs Quantum-Dot Mode-Locked Laser Diodes: Optimization of the Laser Geometry for Subpicosecond Pulse Generation

The effect of cavity geometry on both pulsewidth and pulse output power is systematically investigated for a monolithic mode-locked quantum-dot laser diode. Subpicosecond pulse generation is demonstrated, and trends in pulsewidth and pulse output power are correlated to the ratio of the gain-to-absorber section lengths. A three-fold reduction in the pulsewidth, from 2.3 ps to 800 fs, is observed for a change in the gain-to-absorber section length ratio of 14 : 1 to 3 : 1. In addition, an associated five-fold increase in average power and 14-fold increase in peak power are also observed.

Low Noise Performance of Passively Mode-Locked 10-GHz Quantum-Dot Laser Diode

We present a detailed investigation of the radio-frequency (RF) linewidth and noise performance of a 10-GHz packaged monolithic two-section quantum-dot mode-locked laser diode. Record low 500-Hz RF linewidths are reported under passive mode-locking operation, and the single sideband phase noise spectra and associated timing jitter performance under both passive and hybrid mode-locking is investigated.

Passively Mode-Locked 4.6 and 10.5 GHz Quantum Dot Laser Diodes Around 1.55 μm With Large Operating Regime

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Passive mode-locking in two-section InAs/InP quantum dot laser diodes operating at wavelengths around 1.55 <formula formulatype="inline"><tex Notation="TeX">$\mu$</tex></formula>m is reported. For a 4.6-GHz laser, a large operating regime of stable mode-locking, with RF-peak heights of over 40 dB, is found for injection currents of 750 mA up to 1.0 A and for values of the absorber bias voltage of 0 V down to −3 V. Optical output spectra are broad, with a bandwidth of 6–7 nm. However, power exchange between different spectral components of the laser output leads to a relatively large phase jitter, resulting in a total timing jitter of around 35 ps. In a 4-mm-long, 10.5-GHz laser, it is shown that the operating regime of stable mode-locking is limited by the appearance of quantum dot excited state lasing, since higher injection current densities are necessary for these shorter lasers. The output pulses are stretched in time and heavily up-chirped with a value of 16–20 ps/nm. This mode of operation can be compared to Fourier domain mode-locking. The lasers have been realized using a fabrication technology that is compatible with further photonic integration. This makes such lasers promising candidates for, e.g., a coherent multiwavelength source in a complex photonic chip. </para>

Quantum dot laser diode with low threshold and low internal loss

Data are presented demonstrating that a low-threshold quantum dot laser diode can achieve very low internal optical loss. The broad-area laser diode operates at the wavelength 1.22 µm and delivers 2 W of power from a 1.6 cm-long cavity with uncoated facets, with a lasing threshold current density of 10.4 A/cm2. The laser diode&apos;s internal waveguide loss is extracted from cavity length measurements to be ∼0.25 cm−1. The interdependence of threshold current density and internal optical loss is discussed.

InGaAs Quantum-Dot Mode-Locked Laser Diodes

This paper presents a selection of recent advances on two-section passively mode-locked InGaAs-based quantum-dot laser diodes. Pulse generation is demonstrated for repetition rates ranging from 310 MHz to 240 GHz, and with pulse durations ranging from the picosecond to the sub-400 fs regime. Mode-locking trends in these devices are discussed, and device performance improvements in terms of pulse duration, output power, and noise properties are presented. Design rules for reducing the pulse duration, increasing the output power, and improving noise performance are outlined. Implementation of tapered waveguide structures yields significant performance improvements, allowing the simultaneous achievement of ultrashort, Fourier-limited pulse generation with low amplitude noise, low timing jitter, and narrow RF linewidths.

양자점 레이저 다이오드의 식각 깊이에 따른 접합온도 측정

순방향 전압-온도 (forward voltage-temperature)법을 이용하여 양자점 레이저 다이오드의 접합온도를 측정하였다. 식각 깊이가 깊은 mesa 구조의 경우 입력전류에 대한 접합온도의 증가율은 0.05 K/mA인 반면, 식각 깊이가 낮은 mesa 구조의 경우 0.07 K/mA로서 상대적으로 높게 측정되었다. 깊은 mesa 구조에서의 상대적으로 낮은 접합온도 증가율은 mesa 측면 방향으로의 열확산 효과 때문인 것으로 설명된다. Junction temperature of quantum dot laser diodes is investigated by utilizing forward voltage-temperature method. In the case of ridge type laser diodes with deep mesa the increasing rate of junction temperature to current is about 0.05 K/mA, while in the case of shallow mesa the increasing rate is about 0.07 K/mA. It is explained that the relatively low increasing rate in the deep mesa results from the heat expansion to the lateral direction of mesa.

Junction Temperature Measurement of InAs Quantum-Dot Laser Diodes by Utilizing Voltage–Temperature Method

Junction temperature of InAs quantum-dot laser diodes (LDs) is measured by utilizing a forward voltage- temperature method. Although the forward voltage decrease with junction temperature increment is low, the linear relation between forward voltage and temperature clearly occurs. It is found that SiNx is more useful than SiO2 to keep junction temperature low. Injection current to shift the lasing wavelength from ground to excited state is increased over 200 mA by using SiNx instead of SiO2 as the insulating layer. As a result, ground state optical power is doubled.

Process‐free estimation of threshold current density of InAs quantum dot laser

AbstractWe have associated threshold current density of 1.3 μm InAs quantum dot (QD) laser diodes with the photoluminescence (PL) peak intensity of as‐grown wafers. These InAs QD laser wafers with stacking structure of 5 and 10 QD layers were grown by molecular beam epitaxy. PL measurements have carried out by high power excitation at room temperature. The PL peak intensity from ground state of InAs QDs has been inversely proportional to the threshold current density. Additionally, enhancement of PL peak intensity by increase in stacked QD layers has been corresponded to increase in maximum modal gain of QD laser. These results indicate that PL characterization by high power excitation is useful to estimate quality of as‐grown wafers without fabrication of QD laser diode. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Quantum dot diode lasers for optical communication systems

Basic technological and physical aspects of injection lasers based on arrays of self-organised InAs/InGaAs quantum dots on GaAs substrates for optical communication systems in the 1.2–1.3 μm spectral range are considered. The possibility of simultaneous lasing at a great nimber of longitudinal modes at a high power level and low noise is demonstrated. The use of these lasers in wavelength-division-multiplexing systems based on the spectral separation of the laser output spectrum is substantiated.

Optical Characteristics and the Linewidth Enhancement Factor Measured from InAs/GaAs Quantum Dot Laser Diodes

We report the optical characteristics and the linewidth enhancement factor (alpha-factor) of InAs/GaAs quantum dot (QD) laser diodes (LDs) with a 5-mum-wide stripe and 1-mm-long cavity. Continuous wave (CW) operation of the laser yielded a kink-free output power of 160 mW with an external efficiency of 0.35 W/A. The threshold current was 28 mA and the lasing occurred at 1286 nm, solely at the ground state (GS) up to a current of 345 mA, without switching to the excited state (ES). Then, to estimate the alpha-factor of the LDs, we investigated the differential gain and wavelength shift versus the current. The differential gain was 2.37 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> /mA at 1286 nm, and the wavelength shift was 0.00046 nm/mA, which resulted in an alpha-factor of 0.057 at 1286 nm. The alpha-factor decreased with increasing wavelength. To the best of our knowledge, this is the lowest value of the alpha-factor that has been reported so far, indicating that our QD-LD has excellent beam quality under high-power operation due to the elimination of the filamentation.

Multimode Lasing Characteristics of Quantum Dot Lasers due to Inhomogeneously Broadened Gain

The growth condition for In(Ga)As/InGaAsP/InP quantum dots (QDs) has been optimized and room temperature lasing has been obtained from QD laser diodes (LDs) emitting at 1.55 m. However, it is observed that many new lasing modes keep appearing at longer wavelength with increase in injection current, and the output power of the new modes at longer wavelength gets much stronger than that of the mode that started to lase rst. This behavior is explained as having been caused both by an inhomogeneously broadened gain of QD LDs where the gains at di erent wavelengths keep increasing even though the gain of the primary mode is clamped at the threshold value and by a Gaussian-like distribution of the magnitudes of density of states due to a statistical distribution of the sizes of the grown QDs.

Very-low-threshold current density continuous-wave quantum-dot laser diode

Data are presented showing that a quantum-dot laser diode can achieve a continuous-wave room-temperature threshold current density as low as 11.7 A/cm2. The broad-area laser diode operates at a wavelength of 1.22 µm and delivers a power level of 0.42 W with 40% maximum slope efficiency in p-up mounting without heatsinking. Spectral measurements indicate the onset of stimulated emission at ∼6 A/cm2.

InGaAs 양자점 레이저 다이오드와 양자우물 레이저 다이오드의 특성 비교

분자선 에피택시(molecular beam epitaxy, MBE)로 성장된 InGaAs 양자점 레이저 다이오드(quantum dot laser diode, QD-LD)와 InGaAs 양자우물 레이저 다이오드(quantum well laser diode, QW-LD)의 특성을 비교하였다. 펄스 입력전류 하에서 문턱전류밀도(threshold current density, <TEX>$J_{th}$</TEX>), 특성온도(characteristic temperature, <TEX>$T_0$</TEX>), 온도에 따른 발진파장의 변화도(<TEX>$d{\lambda}/dT$</TEX>)를 측정한 결과, 양자우물 레이저 다이오드는 <TEX>$J_{th}\;=\;322\;A/cm^2,\;T_0\;=\;55.2\;K,\;d{\lambda}/dT\;=\;0.41\;nm/^{\circ}C$</TEX>로 측정되었으며, 양자점 레이저 다이오드는 <TEX>$J_{th}\;=\;116\;A/cm^2,\;T_0\;=\;81.8\;K,\;d{\lambda}/dT\;=\;0.33\;nm/^{\circ}C$</TEX>로 측정되었다. 양자점 레이저 다이오드는 양자우물 레이저 다이오드와 비교하였을 때, 문턱전류밀도 및 발진 광 파워가 상대적으로 우수한 결과를 보여주었다. We have investigated the lasing characteristics of the InGaAs quantum dot laser diode (QD-LD) and InGaAs quantum well laser diode (QW-LD) operated at the 980 nm wavelength range. The 980-nm lasers are used as a pumping source for a erbium-doped fiber amplifier (EDFA) and it shows high efficiency in long-haul optical fiber network. We have compared the threshold current density, the characteristic temperature, the optical power and the internal efficiency of QD-LD and QW-LD under a pulsed current condition. The QD-LD shows superior performances to the QW-LD. Further optimization of a LD structure is expected to the superior performances of a QD-LD.

Theoretical and experimental investigations on temperature induced wavelength shift of tapered laser diodes based on InGaAs∕GaAs quantum dots

The influence of the taper losses on the temperature induced wavelength shift of tapered quantum dot laser diodes emitting at 920nm was investigated. The experimentally obtained results of the taper losses were compared with simulations carried out by a self-consistent, complete steady-state electrical, thermal, and optical model. A good agreement between theory and experiment was found. The device geometry with a length of 3mm and a front facet width of 35μm yielded the best wavelength stability of 0.14nm∕K and exhibits an output power of more than 900mW with a beam-propagation factor M2 of 2.4.

High-Temperature Continuous-Wave Operation of GaInAsN–GaAs Quantum-Dot Laser Diodes Beyond 1.3 $\mu$m

Internal laser parameters and characteristic temperature T0 of a GaInAsN-GaAs quantum-dot (QD) laser emitting above 1350 nm were determined. Continuous-wave operation of GaInAsN-GaAs QD ridge waveguide laser diodes with an emission wavelength of 1355 nm at room temperature (RT) is reported for the first time. Low threshold currents of 16 mA are achieved at RT. Ground state laser emission is observed up to temperatures of 75degC with a resulting laser emission wavelength of 1395 nm.

Thermal Analysis of InAs Quantum Dot Laser Diodes with an Additional Au Layer on p-Metal

The thermal characteristics of laser diodes (LDs) are analyzed by using a finite element method. A change in the junction temperature depending on the thickness of the additional Au layer, which might be deposited or plated on the p-contact metal of the LD, has been observed. As the Au layer thickness increases, the junction temperature of the LDs, which are epi-up mounted on a Cu sub-mount, goes down drastically and then saturates. On the other hand, the junction temperature of LDs epi-down mounted on a Cu sub-mount shows relatively little variation with the Au thickness, although it is lowered by more than 30 ◦C at an electrical input power of 1.2 W compared to that of epi-up mounted LDs. We measure the junction temperatures of InAs quantum dot (QD) LDs and compare them with simulation results. The junction temperatures increased at a rate of 86 ◦C/W and 48 ◦C/W for epi-up and epi-down mounting with an additional Au layer of 0.2 μm, respectively.

Wavelength Bistability in Two-Section Mode-Locked Quantum-Dot Diode Lasers

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We report a two-section mode-locked quantum-dot laser with an emission wavelength that is bistable with respect to applied bias on the saturable absorber region. The two stable lasing wavelengths for this device are about 1173 and 1166 nm with a power contrast ratio of over 30 dB. The largest switchable wavelength range is 7.7 nm. The optical power and pulsewidth (6.5 ps) are almost identical in the two lasing modes under optimized conditions. The operation of this laser can be explained by the interplay of the spectral-hole burning and the quantum-confined Stark effect. </para>

Near-Field Emission Properties of a Self-Formed InAs Quantum Dot Laser Diode by Using Near-Field Scanning Optical Microscopy

Near-field scanning optical microscopy studies of a self-formed InAs quantum dot laser diode (QDLD) at room-temperature were performed. We measured the near-field electroluminescence spectra and the spectrally resolved near-field scanning images emitted from a QDLD with a spatial resolution of ∼100 nm. We show that these data have direct implications for device performance associated with the injection current.

Growth of InAs bilayer quantum dots for long-wavelength laser emission on GaAs

We reported the growth of asymmetric InAs bilayer quantum dots (QD) on GaAs substrate by molecular beam epitaxy. A combination of different InAs growth rates (GR) for the two QD layers was used, respectively, to increase QD density and extend photoluminescence (PL) emission wavelength. A high GR used for the first layer guarantees a high QD density; while a low GR used for the second layer helps to extend the PL emission wavelength due to the dot size increase, which was confirmed by transmission electron microscopy and PL measurements. It was found that this kind of QDs is very sensitive to post-growth annealing procedure due to the dot size increase. By appropriately choosing the growth parameters, InAs bilayer QD laser diodes embedded in a conventional AlGaAs/GaAs waveguide structure were demonstrated. The devices yielded the lowest ever reported threshold current density of 134 A/cm 2 in the range of 1.5 μm wavelength for GaAs-based lasers.