Yu. M. Shernyakov

High power temperature-insensitive 1.3 µm InAs/InGaAs/GaAs quantum dot lasers

We report on GaAs-based broad area (100 µm) 1.3 µm quantum dot (QD) lasers with high CW output power (5 W) and wall-plug efficiency (56%). The reliability of the devices has been demonstrated beyond 3000 h of CW operation at 0.9 W and 40 °C heat sink temperature with 2% degradation in performance. P-doped QD lasers with a temperature-insensitive threshold current (T0 > 650 K) and differential efficiency (T1 = infinity) up to 80 °C have been realized.

Continuous-wave operation of long-wavelength quantum-dot diode laser on a GaAs substrate

Continuous-wave operation near 1.3 /spl mu/m or a diode laser based on self-organized quantum dots (QDs) on a GaAs substrate is demonstrated. Multiple stacking of InAs QD planes covered by thin InGaAs layers allows us to prevent gain saturation and achieve long-wavelength lasing with low threshold current density (90-105 A/cm/sup 2/) and high output power (2.7 W) at 17/spl deg/C heatsink temperature. It is thus confirmed that QD lasers of this kind are potential candidates to substitute InP-based lasers in optical fiber systems.

Long-wavelength lasing from multiply stacked InAs/InGaAs quantum dots on GaAs substrates

An InAs quantum dot (QD) array covered by a thin InGaAs layer was used as the active region of diode lasers grown by molecular beam epitaxy on GaAs substrates. The wavelength of the ground-state transition in such heterostructures is in the 1.3 μm range. In the laser based on the single layer of QDs, lasing proceeds via the excited states due to insufficient gain of the ground level. Stacking of three QD planes prevents gain saturation and results in a low threshold (85 A/cm2 in broad-area 1.9-mm-long stripe) long-wavelength (1.25 μm) lasing at room temperature via the QD ground state with relatively high differential efficiency (>50%).

35GHz mode-locking of 1.3μm quantum dot lasers

35GHz passive mode-locking of 1.3μm (InGa)As∕GaAs quantum dot lasers is reported. Hybrid mode-locking was achieved at frequencies up to 20GHz. The minimum pulse width of the Fourier-limited pulses was 7ps with a peak power of 6mW. Low uncorrelated timing jitter below 1ps was found in cross correlation experiments. High-frequency operation of the lasers was eased by a ridge waveguide design that includes etching through the active layer.

High-power continuous-wave operation of a InGaAs/AlGaAs quantum dot laser

A 1 W continuous-wave laser operation via the ground state of vertically coupled InGaAs quantum dots (VCQDs) in an AlGaAs matrix is demonstrated. VCQDs are directly revealed in transmission electron microscopy images of the laser structure. Ninety-six percent internal quantum efficiency is realized. The laser gain maximum shifts significantly with drive current towards higher photon energies in agreement with the relatively broad size distribution of VCQDs.

Gain characteristics of quantum dot injection lasers

Gain characteristics of injection lasers based on self-organized quantum dots (QDs) were studied experimentally for two systems: InGaAs QDs in an AlGaAs matrix on a GaAs substrate and InAs QDs in an InGaAs matrix on an InP substrate. A ground-to-excited state transition was observed with increasing threshold gain. An empirical equation is proposed to fit the current density dependence of the QD gain. This fitting equation is shown to be valid for both the ground and excited state lasing in the systems under study in the 77-300 K temperature range. The effect of QD surface density on gain characteristics is calculated analytically.

Low-threshold injection lasers based on vertically coupled quantum dots

We have fabricated and studied injection lasers based on vertically coupled quantum dots (VECODs). VECODs are self-organized during successive deposition of several sheets of (In,Ga)As quantum dots separated by thin GaAs spacers. VECODs are introduced in the active region of a GaAs-A1GaAs GRIN SCH lasers. Increasing the number of periods (N) in the VECOD leads to a remarkable decrease in threshold current density ( ~ 100 A/cm 2 at 300 K for N = 10). Lasing proceeds via the ground state of the quantum dots (QD) up to room temperature. Placing the QD array into an external AIGaAs--GaAs quantum well allows us to extend the range of thermal stability of threshold current density (To = 350 K) up to room temperature. Using (In,Ga)As-(A1,Ga)As VECODs in combination with high temperature growth of emitter and waveguide layers results in further reduction of threshold current density (60-80 A/cm 2, 300 K) and increase in internal quantum efficiency (70%). Room temperature continuous wave operation (light output 160 mW per mirror) and lasing via the states of QDs up to I = (6-7) Ith have been demonstrated.

Gain and threshold characteristics of long wavelength lasers based on InAs/GaAs quantum dots formed by activated alloy phase separation

Experimental and theoretical study was made of injection lasers based on InAs/GaAs quantum dots (QDs) formed by the activated alloy phase separation and emitting at about 1.3 /spl mu/m. Electroluminescence and gain spectra were investigated. The maximum modal gain is measured experimentally using two different techniques. Threshold current densities as low as 22 A cm/sup -2/ per QD sheet were achieved. A step-like switch from ground- to excited-state transition lasing was observed with an increasing cavity loss. The characteristic temperatures for a sample with four cleaved sides and a 2-mm long stripe device at 300 K were 140 and 83 K, respectively. Single lateral-mode continuous-wave (CW) operation with the maximum output power of 210 mW was realized. Threshold characteristics of a laser were simulated taking into account radiative recombination in QDs, the wetting layer, and the optical confinement layer. The dependence of the threshold current density on the cavity length was shown to be extremely sensitive to the QD-array parameters determining the maximum gain for ground- and excited-state transitions and to the waveguide design. Our analysis reveals that nonradiative recombination channels may play an important role in the laser operation.

Direct modulation and mode locking of 1.3 μm quantum dot lasers

We report 7 GHz cut-off frequency, 2.5 and 5 Gb s−1 eye pattern measurements upon direct modulation of 1.3 μm quantum dot lasers grown without incorporating phosphorus in the layers. Passive mode-locking is achieved from very low frequencies up to 50 GHz and hybrid mode-locking is achieved up to 20 GHz. The minimum pulse width of the Fourier-limited pulses at 50 GHz is 3 ps, with an uncorrelated timing jitter below 1 ps. The lasers are optimized for high frequency operation by a ridge waveguide design that includes etching through the active layer and ridge widths down to 1 μm. The far-field shape for 1 μm is close to circular with a remaining asymmetry of 1.2.

Spectrotemporal response of 1.3 μm quantum-dot lasers

We report the spectrotemporal measurements on 1.3 μm quantum-dot lasers with picosecond time resolution. The relaxation oscillations of the various mode groups monitored separately are identical and thus allow us to examine the spectrally integrated transient of the laser pulse for its characteristics. A modulation bandwidth of 2.3 GHz at room temperature is determined, demonstrating the potential of high-speed operation of these devices at wavelengths relevant for optical data transmission. The differential gain at room temperature was measured to be g′=1×10−15 cm2, the gain compression factor is e=1×10−15 cm3.