High power 808nm semiconductor lasers are widely used for pumping neodymium-doped yttrium aluminum garnet (Nd:YAG) crystal to produce high-brightness lasing at 1064nm. In addition, there are growing interest to use such high power 808nm lasers in the field of automotive infra-red (IR) illumination and medical aesthetic treatment. Vertical-cavity surface-emitting lasers (VCSELs) have emerged as a promising candidate and attracted increased interests for those applications, due to their combined advantages of high efficiency, low diverging circular beam, narrow emission spectrum with reduced temperature sensitivity, low-cost manufacturability, simpler coupling optics, and increased reliability, especially at high temperatures. They can emit very high power with very high power density as they can be conveniently configured into large two-dimensional arrays and modules of arrays. We report recent development on such high-power, high-efficiency 808nm VCSELs with industrial leading ~55% power conversion efficiency (PCE). Top emitting VCSELs were grown by MOCVD and processed into single devices and 2D arrays using selective wet oxidation process and substrate removal technique for efficient current confinement and heat removal. Peak PCE of 51% and peak power of 800W were achieved from 5x5mm array, corresponding to peak power density of ~4kW/cm2. Pumped with new generation of 2.3kW VCSEL module, Q-switched laser pulse energy at 1064nm reached 46.9mJ, more than doubled from previously reported results.
A compact passively Q-switched Nd:YAG laser was end-pumped by a water-cooled 808 nm vertical-cavity surface-emitting laser (VCSEL) pump module comprising four high power, high brightness VCSEL chips with a combined 10 mm diameter circular emitting area and 2.3 kW total peak power, resulting in 47 mJ laser pulse energy at 1064 nm with 16% optical efficiency at 15 Hz repetition frequency. A laser package comprising an air-cooled 1.6 kW VCSEL pump module produced 37 mJ laser pulse energy, while more than 13 mJ laser pulse energy was demonstrated in a bench-top experiment with a very compact laser set-up using a single 5 mm x 5 mm VCSEL chip.
Quantum dot-based diode comb lasers can provide a single multi-channel-laser source for short-reach, high-speed WDM interconnects. In this paper, we review the technology and demonstrate for the first time a 15 channel, low RIN comb laser with 80 GHz channel spacing. We show that each of the Fabry-Perot (FP) modes can be externally modulated at 10 Gb/s or all modes directly modulated, at 3.2 Gb/s so far. The latter indicates that the comb laser may be an ideal broadband light source in WDM-PON applications. We further demonstrate that the whole comb laser spectrum can be amplified by a quantum dot SOA without increasing relative density noise (RIN) of the individual channels. The small signal amplification factor was measured up to 30dB and the saturated output power was as high as 15 dBm.
Quantum dot-based diode comb lasers enable a single multi-channel-laser source for short-reach, high-speed WDM
interconnects. In this paper, we demonstrate for the first time a 15 channel low RIN comb laser with 80 GHz channel
spacing. We show that all the FP modes can be simultaneously directly modulated simply by modulating the pump
current at 3.2 Gb/s, which indicates that the comb laser may be an ideal broadband light source in WDM-PON
applications.We demonstrate that the whole comb laser spectrum can be amplified by a quantum dot SOA without
increasing the relative intensity noise (RIN). Small signal amplification factor was measured as high as 30 dB and the
saturated output power was as high as 15 dBm.
980 nm VCSELs based on sub-monolayer growth show for 20 Gbit/s large signal modulation clearly open eyes without
adjustment of the driving conditions between 25 and 120 °C. To access the limiting mechanism for the modulation
bandwidth, a temperature dependent small signal analysis is carried out on these devices. Single mode devices are
limited by damping, whereas multimode devices are limited by thermal effects, preventing higher photon densities in the
cavity.
High-channel-count WDM will eventually be used for short reach optical interconnects since it maximizes link bandwidth and efficiency. An impediment to adoption is the fact that each WDM wavelength currently requires its own DFB laser. The alternative is a single, multi-wavelength laser, but noise, size and/or expense make existing options impractical. In contrast, a new low-noise, diode comb laser based on InAs/GaAs quantum dots provides a practical and timely alternative, albeit in the O-band. Samples are being evaluated in short reach WDM development systems. Tests show this type of Fabry-Perot laser permits >10 Gb/s error-free modulation of 10 to over 50 separate channels, as well as potential for 1.25 Gb/s direct modulation. The paper describes comb laser requirements, noise measurements for external and direct modulation, O-band issues, transmitter photonic circuitry and components, future CMP applications, and optical couplers that may help drive down packaging costs to below a dollar.
We report on edge-emitting InAs/GaAs quantum dot laser promising as multiple wavelength light source for dense
wavelength-division-multiplexing systems in future generation of silicon photonic integrated circuits. Broad and flat gain
spectrum of quantum dots as well as pronounced gain saturation effect facilitate simultaneous lasing via a very large
number of longitudinal modes with uniform intensity distribution (comb spectrum). A very broad lasing spectrum of
about 75 nm in the 1.2-1.28 μm wavelength range with a total output power of 750 mW in single lateral mode regime is
achieved by intentional inhomogeneous broadening of ground state transition peak and contribution of lasing via excited
state transitions. Average spectral power density exceeds 10 mW/nm. A bit error rate less than 10-13 is demonstrated for
ten spectrally filtered and externally modulated at 10 Gb/s Fabry-Perot modes owing to a low (<0.3% in the 0.001-10 GHz range) relatively intensity noise of each individual mode. This result shows aptitude of a multimode quantum dot
laser for high bandwidth wavelength-division-multiplexing systems.
Low transparency current density and improved temperature stability with a large characteristic temperature
T0 > 650 K up to 80 °C are demonstrated for 1.3 μm MBE grown InGaAs quantum dot (QD) edge emitting
lasers. Digital modulation with an open eye pattern up to 12 Gb/s at room temperature and bit error rate below
10-12 for 10 Gb/s modulation was realized for this wavelength. Semiconductor optical amplifiers based on
InGaAs QD gain media achieved a chip gain of 26 dB. A conventionally doped semiconductor DBR QD-VCSEL
containing 17 p-modulation doped QD layers demonstrated a cw output power of 1.8 mW and a
differential efficiency of 20 % at 20 °C. The maximum -3dB modulation bandwidth at 25 °C was 3 GHz. First
MOCVD-grown QD-VCSELs with selectively oxidized DBRs and 9 QD-layers were realized, emitting at 1.1
μm. A cw multimode output power of 1.5 mW, 6 mW in pulsed operation, and an cw external efficiency of 45 %
were achieved at 20 °C. The minimum threshold current of a device with 2 μm aperture was 85 μA.
A. Ramdane, A. Martinez, S. Azouigui, D.-Y. Cong, K. Merghem, A. Akrout, C. Gosset, G. Moreau, F. Lelarge, B. Dagens, J.-G. Provost, A. Accard, O. Le Gouezigou, I. Krestnikov, A. Kovsh, M. Fischer
This paper presents recent progress in the field of semiconductor lasers based on self-assembled quantum dots grown
either on GaAs or InP substrates.
Quantum dot (QD) based lasers are attracting a lot of interest owing to their remarkable optoelectronic properties that
result from the three dimensional carrier confinement. They are indeed expected to exhibit much improved performance
than that of quantum well devices. Extremely low threshold currents as well as high temperature stability have readily
been demonstrated in the InAs/GaAs material system.
The unique properties of quantum dot based active layers such as broad optical gain spectrum, high saturation output
power, ultrafast gain dynamics and low loss are also very attractive for the realization of mode-locked lasers.
Recent results in the field of directly modulated InAs/GaAs lasers emitting in the 1.3 μm window are discussed. We
report in particular on temperature independent linewidth enhancement factor (or Henry factor αH) up to 85°C. This is a
key parameter which determines many laser dynamic properties. Optical feedback insensitive operation of specifically
band-gap engineered devices, compatible with high bit rate isolator-less transmission is also reported at 1.55 μm.
Monolithic mode locked lasers based on InAs/InP quantum dashes have been investigated for 1.55 μm applications. Subpicosecond
pulse generation at very high repetition rates (> 100 GHz) is reported for self-pulsating one-section Fabry
Perot devices. Specific applications based on these compact pulse generators including high bit rate clock recovery are
discussed.
N. Ledentsov, F. Hopfer, A. Mutig, V. Shchukin, A. V. Savel'ev, G. Fiol, M. Kuntz, V. A. Haisler, T. Warming, E. Stock, S. S. Mikhrin, A. R. Kovsh, C. Bornholdt, H. Eisele, M. Dähne, N. D. Zakharov, P. Werner, D. Bimberg
Advanced types of QD media allow an ultrahigh modal gain, avoid temperature depletion and gain saturation effects,
when used in high-speed quantum dot (QD) vertical-cavity surface-emitting lasers (VCSELs). An anti-guiding VCSEL
design reduces gain depletion and radiative leakage, caused by parasitic whispering gallery VCSEL modes. Temperature
robustness up to 100°C for 0.96 - 1.25 &mgr;m range devices is realized in the continuous wave (cw) regime. An open eye
20 Gb/s operation with bit error rates better than 10-12 has been achieved in a temperature range 25-85°C without current
adjustment. A different approach for ultrahigh-speed operation is based on a combination of the VCSEL section,
operating in the CW mode with an additional section of the device, which is electrooptically modulated under a reverse
bias. The tuning of a resonance wavelength of the second section, caused by the electrooptic effect, affects the
transmission of the system. The second cavity mode, resonant to the VCSEL mode, or the stopband edge of the second
Bragg reflector can be used for intensity modulation. The approach enables ultrahigh speed signal modulation. 60GHz
electrical and ~35GHz optical (limited by the photodetector response) bandwidths are realized.
Diode lasers supply high power densities at wavelengths from 635-nm to 2000-nm, with different applications enabled
by providing this power at different wavelengths. As the range of available wavelengths broadens, many novel medical
and atmospheric applications are enabled. Traditional quantum well lasers provide high performance in the range 635-
nm to 1100-nm range for GaAs-based devices and 1280-nm to 2000-nm for InP, leaving a notable gap in the 1100 to
1280-nm range. There are many important medical and sensing applications in this range and quantum dots produced
using Stranski-Krastanow self-organized MBE growth on GaAs substrates provide an alternative high performance
solution. We present results confirming broad area quantum dot lasers can deliver high optical powers of 16-W per
emitter and high power conversion efficiency of 35% in this wavelength range. In addition, there are growing
applications for high power sources in wavelengths > 1500-nm. We present a brief review of our current performance
status in this wavelength range, both with conventional quantum wells in the 1500-nm to 2500-nm range and MOCVD
grown quantum cascade lasers for wavelengths > 4000-nm. At each wavelength, we review the designs that deliver this
performance, prospects for increased performance and the potential for further broadening the availability of novel
wavelengths for high power applications.
980 nm vertical-cavity surface-emitting laser based on sub-monolayer growth of quantum dots show at 25 and 85°C for 20 Gb/s without current adjustment clearly open eyes and error free operation with bit error rates better than 10-12. For these multimode lasers the small signal modulation bandwidth decreases only from 15 GHz at 25°C to 13 GHz at 85°C. Single mode devices demonstrate at 20°C a small signal modulation bandwidth of 16.6 GHz with 0.8 mW optical output power and a record high modulation current efficiency factor of 19 GHz/mA1/2.
Nanotechnology is a driver for novel opto-electronic devices and systems. Nanosemiconductors like quantum dots allow controlled variation of fundamental electronic and optical properties by changing the size and shape of the nanostructures. This applies directly to self-organized quantum dots which find a versatile use in many kinds of photonic devices.
Wavelength tunability, decreased laser threshold, scalability of gain by stacking quantum dot layers, low linewidth enhancement factor and temperature stability are consequences of three-dimensional carrier confinement in semiconductor quantum dots. Directly modulated lasers using quantum dots offer further advantages like strongly damped relaxation oscillations yielding low patterning effects in digital data transmission. Quantum dot mode-locked lasers feature a broad gain spectrum leading to ultra-short pulses with sub-ps width and a low alpha factor for low-chirp. Thereby, optical comb generators for the future 100G Ethernet are feasible. Semiconductor optical amplifiers based on quantum dots show advantages as compared to classical ones: broad bandwidth due to the inhomogeneous quantum dot size distribution, ultrafast gain recovery for high-speed amplification and small patterning in optical data transmission. We present our most recent results on temperature stable 10 Gb/s, 23°-70°C direct modulation of lasers, ultrafast 80 GHz and short 710 fs optical pulse combs with mode-locked lasers and semiconductor optical amplifiers showing ultrafast amplification of these optical combs as well as error-free 40 Gb/s data modulation, all based on a quantum dot gain medium.
Quantum dot (QD) diode lasers attract currently much attention due to their ability to emit light in the advanced near-
infrared region at extraordinarily low threshold current densities. A vertical-cavity surface emitting laser (VCSEL),
having a superior beam quality, improved temperature stability, low threshold current, and cost-effective planar
fabrication, is also an attractive device variant. Here we discuss the state of the art of these lasers intended for the use in
1.3-μm fiber-optic communications. The discussion is centered on an InAs/GaAs semiconductor QD system. Basic
issues of the QD synthesis in the system are addressed. The achievement of the control over the 1.3-μm QD emission is
demonstrated. Both, wide-stripe and single-mode edge-emitting lasers are described. The lasers designed have a very low
threshold current density, high differential efficiency, and a high output power. Narrow-stripe 1.3-μm QD lasers generate
in a single mode, have a record-low threshold current, and produce the continuous-wave (CW) power output in excess of
100 mW. Also, we report on QD VCSELs emitting at 1.3 μm. The design of their cavity and active region are described.
The room-temperature CW output power of these lasers is as high as 2 mW. Both, the edge- and surface-emitting lasers
satisfy the demands of the fiber optical communication technology.
Through absorber length optimisation, sub-picosecond pulse generation and low timing jitter are demonstrated in a 20GHz passively mode-locked quantum-dot laser diode. Pulse-widths as low as 800fs and timing jitter performance of 390fs (20kHz-50MHz) are achieved.
We have performed a systematic study of structural and optical properties of Quantum dot (QDs) lasers based on InAs/InGaAs quantum dots grown on GaAs substrates emitting in the 1.3 - 1.5 μm range. 1.3 μm range QD lasers are grown using GaAs as matrix material. It is shown that the lasers, grown with large number of QD stacks are metamorphic, with plastic relaxation occurring through the formation of misfit dislocations. Thus, 1.3 μm QD lasers with large number of stacks grown without strain compensation are metamorphic. Another type of defects is related to local dislocated clusters, which are the most dangerous. When proper optimization of the growth conditions is carried out, including a selective thermal etching off of statistically formed dislocated clusters through the defect-reduction technique (DRT), no significant impact of misfit dislocations on the degradation robustness is observed. In uncoated devices a high cw single mode power of ~700 mW is realized limited by thermal roll-over, which is not affected by 500 h ageing at room temperature. At elevated temperatures the main degradation mechanism revealed is catastrophic optical mirror damage (COMD). When the facet are passivated, the devices show the extrapolated operation lifetime in excess of 106 h at 40°C at ~100 mW cw single mode output power. Longer wavelength (1.4 - 1.5 μm) devices are grown on metamorphic (In,Ga,Al)As layers deposited on GaAs substrates. In this case, the plastic relaxation occurs through formation of both misfit and threading dislocations. The latter kill the device performance. Using DRT in this case enables blocking of threading dislocation with growth of QDs in defect-free upper layers. DRT is realized by selective capping of the defect-free areas and high-temperature etching of nano-holes at the non-capped regions near the dislocation. The procedure results in etching of holes and is followed by fast lateral overgrowth with merger of the growth fronts. If the defect does not propagate into the upper layer when the hole is capped, the upper layers become defect-free. Lasers based on this approach exhibited emission wavelength in the 1.4 -1.5 μm range with a differential quantum efficiency of about ~50%. The narrow-stripe lasers operate in a single transverse mode and withstand continuous current density above 20 kA cm-2 without degradation. A maximum continuous-wave output power of 220 mW limited by thermal roll-over is obtained. No beam filamentation was observed up to the highest pumping levels. Narrow stripe devices with as-cleaved facets are tested for 60°C (800 h) and 70°C (200 h) on-chip temperature. No noticeable degradation has been observed at 50 mW cw single mode output power. This shows the possibility of degradation-robust devices on foreign substrates. The technology opens a way for integration of various III-V materials and may target degradation-free lasers on silicon for further convergence of computing and communications.
The characteristics of p-doped 1.1 μm and 1.3 μm self-assembled In(Ga)As quantum dot lasers grown by molecular beam epitaxy have been studied. With optimum p-doping, we demonstrate quantum dot lasers with zero-temperature dependence of the threshold current (T0 = ∞) and the output slope efficiency. These characteristics are explained through a self-consistent model that includes temperature-dependent Auger recombination in the quantum dots. With tunnel injection, we measure greatly enhanced -3dB frequency response, 25 GHz and 11 GHz in 1.1 μm and 1.3 μm tunnel injection quantum dot lasers, respectively. These devices also exhibit near zero α-parameters and extremely small chirp (< 0.2 Å), in addition to temperature insensitive operation.
The Tilted Cavity (TC) concept has been proposed to combine advantages of edge- and surface-emitting lasers (detectors, amplifiers, switches, etc.). Tilted Cavity Lasers (TCL) enable wavelength-stabilized high-power edge and surface emitters (TCSEL) in low-cost single-epitaxial step design. The concept covers numerous applications including mode-locked TCL for light speed control, dispersion and linewidth engineering, GaN-based light-emitters, electrooptic wavelength tunable devices, and other applications. Presently, wavelength stabilized TC operation is realized between -200°C and 70°C in broad TCL diodes with cleaved facets based on quantum dots (QDs). The spectral width is below 0.6 nm in broad area 100 μm-wide-stipe devices. The far fields are: 4° (lateral) and 42° (vertical). Wavelength-stabilized 1.16 μm and 1.27 μm edge-emitting QD TCL lasers are demonstrated. Quantum well TCL demonstrate high-temperature operation up to 240°C with a low threshold, high temperature stability and improved wavelength stability. The tilted cavity approach can also be applied in wavelength-optimized photodetectors, switches, semiconductor optical amplifiers, including multi-channel devices, in optical fibers, in photodetectors, in light-emitting diodes and in many other applications. Moreover, microelectronic devices based on similar tilted angle resonance phenomena in quantum wells and superlattices can be realized in electron- or hole-wavefunction-engineered structures, thus, merging the fields of nanophotonics and nanoelectronics. The tilted cavity concept can be further complimented by lateral patterning and (or) processing of three-dimensional photonic crystal structures further extending horizons of modern optoelectronics.
Two approaches to realize the VCSEL devices based on GaAs substrates are investigated. The first approach utilizes InGaAs quantum wells with dilute nitride to extend the bandgap toward long wavelenegth. The second approach utilizes InAs/InGaAs quantum dots based on Stranski and Krastanov growth mode with confinement and strain combined to adjust the bandgap to 1.3 μm wavelength. High quality epitaxial layers with low threshold have been achieved with MBE and MOCVD. VCSEL performances that have been achieved are: Multimode operation at 1.303 μm with slope efficiency of 0.15 W/A (0.2 W/A), and maximum power of 1 mW (4 mW) for room temperature CW (pulse) operation have been achieved with MBE-grown In GaAaN active regions. Room temperature, CW single mode operation with SMSR > 40 dB at 1.303 μm has also been achieved with a slope efficiency of 0.17 W/A and maximum power of 0.75 mW also with MBE-grown InGaAaN active regions. In addition, MOCVD grown has also achieved a performance at 1.29 μm with slope efficiency, 0.066 W/A, and maximum power, 0.55 mW. VCSELs with 9 layers of quantum dots and all-semiconductor DBRs also achieved lasing at 1.3 μm.
Quantum dot (QDs) heterostructures structurally represent tiny 3D insertions of a narrow bandgap material, coherently embedded in a wide-bandgap single-crystalline matrix. The QDs are produced by conventional epitaxial techniques applying self-organized growth and behave electronically as artificial atoms. Strain-induced attraction of QDs in different rows enables vertically-coupled structures for polarization, lifetime and wavelength control. Overgrowth with ternary or quaternary alloy materials allows controllable increase in the QD volume via the island-activated alloy phase separation. Repulsive forces during overgrowth of QDs by a matrix material enable selective capping of coherent QDs, keeping the defect regions uncapped for their subsequent selective evaporation. Low-threshold injection lasing is achieved up to 1350 nm wavelength at 300K using InAs-GaAs QDs. 8 mW VCSELs at 1.3 μm with doped DBRs are realized. Edge-emitters demonstrate 10 GHz bandwidth up to 70°C without current adjustment. VCSELs show ~4 GHz relaxation oscillation frequency. QD lasers demonstrate above 3000 h of CW operation at 1.5 W at 45°C heat sink temperature without degradation. The defect reduction technique (DRT) applied to thick layers enables realization of defect-free structures on top of dislocated templates. Using of DRT metamorphic buffer layers allowed 7W GaAs-based QD lasers at 1500 nm.
We analyse the sensitivity of quantum dot semiconductor lasers to optical feedback. While bulk and quantum well semiconductor lasers are usually extremely unstable when submitted to back reflection, quantum dot semiconductor lasers exhibit a reduced sensitivity. Using a rate equation approach, we show that this behaviour is the result of a relatively low but nonzero line-width enhancement factor and strongly damped relaxation oscillations.
Extensive mode-locking investigations are performed in InGaAs/InAs/GaAs quantum dot (QD) lasers. Monolithic mode-locked lasers are fabricated using QD material systems grown by MOCVD and MBE techniques and emitting at 1.1μm and 1.3μm, respectively. The mode-locking performance is evaluated using a variety of laser designs, with various ridge waveguide geometries, cavity and absorber lengths. Passive and hybrid mode-locking are studied and compared in 3.9mm long devices emitting at 1.1μm and operating at a repetition rate of 10GHz. Using 2.1mm long devices emitting at 1.3μm, 18GHz passive mode locking with 10ps Fourier transform limited pulses is demonstrated. This confirms the potential of quantum dot laser for low chirp, short optical pulse generation. Preliminary investigation of the timing jitter of QD passively mode-locked lasers and the behaviour of the QD absorber are also presented. Finally, we report 36GHz passive mode-locking with 6ps optical pulse obtained using 1.1mm long QD lasers emitting at 1.3μm.
MBE growth of high quality diluted Nitride materials have been investigated. Photoluminescence intensity of high nitrogen content InGaAsN/GaAs SQW can be improved significantly by decreasing the growth temperature due to suppressd phase separation of InGaAsN alloy. The longest room temperature PL peak wavelength obtained in this study is 1.59 μm by increasing the nitrogen composition up to 5.3%. High performance ridge-waveguide InGaAsN/GaAs single quantum well lasers at wavelength 1.3 μm have been demonstrated. Threshold current density of 0.57 KA/cm2 was achieved for the lasers with a 3-μm ridge width and a 2-mm cavity length. Slope efficiencies of 0.67 W/A was obtained with 1 mm cavity length. The cw kink-free output power of wavelength 1.3 μm single lateral mode laser is more than 200 mW, and the maximum total wallplug efficiency of 29% was obtained. Furthermore, monolithic MBE-grown vertical cavity surface emitting lasers (VCSELs) on GaAs substrate with an active region based on InGaAsN/GaAs double quantum wells emitting at 1304 nm with record threshold current density below 2 KA/cm2 also have been demonstrated. The CW output power exceeds 1 mW with an initial slope efficiency of 0.15 W/A. Such low threshold current density indicates the high quality of InGaAsN/GaAs QW active region.
Tilted Cavity Laser (TCL) is developed that combines advantages of a high power operation of an edge-emitting semiconductor diode laser and wavelength-stabilized operation of a surface emitting laser. A TCL emits laser light in a tilted optical mode that propagates effectively at a certain tilt angle to the p-n junction. Designed TCL comprises a high-finesse cavity into which an active region is placed and at least one multilayer interference reflector (MIR). The cavity and the MIR are designed such that the spectral position of the reflectivity dip of the cavity and the position of the stopband reflectivity maximum of the MIR coincides at one tilt angle of a tilted optical mode, and draw apart as the angle deviates from the optimum value. As a result, the leakage loss of the optical modes to the substrate is minimum at the optimum wavelength and increases dramatically as the wavelength deviates from the optimum one. This ensures the stabilization of the wavelength of the emitted laser light. Both quantum well (QW) and quantum dot (QD) TCLs have been fabricated on the basis of GaAs/GaAlAs waveguides. QW TCL using InGaAs QW as the active region and operating at 1000-1100 nm reveals the temperature shift of the lasing wavelength 0.2 nm/K. QW TCL operates up to and above 210°C with the differential efficiency 20%. QD TCL using InAs QD overgrown by InGaAs alloy as the active region and operating at 1100-1200 nm reveals the temperature shift of the lasing wavelength 0.165 nm/K. These shifts are significantly slower than the shift for a conventional edge-emitting semiconductor diode laser. The QD TCL shows an output power 2W in a pulsed mode. Characteristic temperature of the threshold current measured at and below room temperature (T0) is 150 K.
The molecular beam epitaxy of self-assembled quantum dots (QDs) has reached a level such that the principal advantages of QD lasers can now be fully realized. We overview the most important recent results achieved to date including excellent device performance of 1.3 μm broad area and ridge waveguide lasers (Jth<150A/cm2, Ith=1.4 mA, differential efficiency above 70%, CW 300 mW single lateral mode operation), suppression of non-linearity of QD lasers, which results to improved beam quality, reduced wavelength chirp and sensitivity to optical feedback. Effect of suppression of side wall recombination in QD lasers is also described. These effects give a possibility to further improve and simplify processing and fabrication of laser modules targeting their cost reduction. Recent realization of 2 mW single mode CW operation of QD VCSEL with all-semiconductor DBR is also presented. Long-wavelength QD lasers are promising candidate for mode-locking lasers for optical computer application. Very recently 1.7-ps-wide pulses at repetition rate of 20 GHz were obtained on mode-locked QD lasers with clear indication of possible shortening of pulse width upon processing optimization. First step of unification of laser technology for telecom range with QD-lasers grown on GaAs has been done. Lasing at 1.5 μm is achieved with threshold current density of 0.8 kA/cm2 and pulsed output power 7W.
We report our results on InGaNAs/GaAs vertical-cavity surface-emitting lasers (VCSELs) for fiber-optic applications in the 1.3 μm range. The epitaxial structures were grown on (100) GaAs substrates by MBE or MOCVD. The nitrogen composition of the InGaNAs/GaAs quantum-well (QW) active region is 0 to 0.02. Long-wavelength (up to 1.3 μm) room-temperature continuous-wave (RT CW) lasing operation was achieved for MBE and MOCVD-grown VCELs. For MOCVD-grown devices with n- and p-doped distributed Bragg reflectors (DBRs), a maximum optical output power of 0.74 mW was measured for In0.36Ga0.64N0.006As0.994/GaAs VCSELs. The MBE-grown devices were made with intracavity structure. Top-emitting multi-mode 1.3 μm In0.35Ga0.65N0.02As0.98/GaAs VCSELs with 1mW output power have been achieved under RT CW operation. Emission characteristics of InGaNAs/GaAs VCSELs were measured and analyzed.
We analyse the sensitivity of quantum dot semiconductor lasers to optical. While bulk and quantum well semiconductor lasers are usually extremely unstable when submitted to back reflection, quantum dot semiconductor lasers exhibit a reduced sensitivity. Using a rate equation approach, we show that this behaviour is the result of a relatively low but nonzero line-width enhancement factor and of strongly damped relaxation oscillations.
In this work we present a detailed study of picosecond optical pulse generation using high-repetition rate mode-locked quantum dot lasers. MOCVD-grown quantum dot lasers emitting at 1.1μm and MBE-grown quantum dot lasers emitting at 1.3μm are investigated. Passive mode-locking at 10GHz, 18GHz and 36GHz with pulse widths in the 6-12ps range are reported. Hybrid mode-locking is demonstrated at 10GHz, showing a significant improvement in the RF spectral characteristics when compared with passive mode-locking. A timing jitter of 600fs (2.5MHz to 50MHz) is measured in the 18GHz passively mode-locked laser. Autocorrelation techniques are used to characterise the high repetition rate mode-locked lasers as well as the time-bandwidth product of the optical pulses. Fourier-transform
limited pulses are obtained from passively mode-locked QD lasers.
1.3 μm GaAs-based quantum dot (QD) lasers demonstrate parameters improved over InP-based devices. They exhibit lower threshold current densities and losses, higher differential efficiencies and improved temerature stability. Highspeed operation is demonstrated. Reduced linewidth enhancement factor advantageous for low-chirp operation makes it possible to suppress dramatically filamentation effects destroying lateral far-field pattern. GaAs-based QD 1.3 μm VCSEL with 8 μm oxide aperture wavelength emits up to 1.2 mW CW multimode.
Quantum dot (QD) is one of the most perspective candidates to be used as an active region of temperature-insensitive 1.3-micron GaAs based lasers for optical networks. However, the limited optical gain achievable in QD ground state hindered their practical use. In this work we have demonstrated that using of high number of QDs stacks grown under proper conditions by MBE is an effective way to considerably increase the optical gain of QD lasers. Ridge waveguide laser diodes with width of 2.7 μm and 4.5 μm based on various numbers of QD layers (N=2, 5, 10) were fabricated and studied in this work. Ultra-low threshold current of 1.43 mA was achieved for 2-stack QD. Regime of simultaneous lasing at ground- and excited-states was discovered. This effect was accounted for the finite time of carriers capture to the ground-state in QD. Multi-stack QD structures enabled to maintain continuous work ground-state lasing up to the current density of 10 kA = 100xJth. Enhanced optical gain allowed us to unite very high differential efficiency (>75%) with low threshold current (<100 A/cm2) and characteristic temperature (T0>100K). For example, laser diode of 1-mm cavity length has shown single mode output power of 100mW at operating current of 195 mA and at high operation power demonstrated insensibility to the changes of temperature. The combination of parameters achieved is quite competitive to all technologies currently used for 1.3-micron lasers including traditional InP-based lasers and makes QD gain medium very promising for VCSEL and telecom laser applications.
Ilja Soshnikov, Boris Volovik, Alexey Kovsh, Alexey Zhukov, Anrei Tsatsul'nikov, O. Gorbenko, N. Ledentsov, Victor Ustinov, P. Werner, N. Zakharov, D. Gerthsen
Heterostructures with submonolayer insertions attract interest due to optical properties and to possibility of the studying of the self-organization effect. Besides, investigation of SML heterostructures is retarded as consequence of problems of the structural and composition characterization. The situation is changed on account of development of quantitative methods for HREM image analysis. There formation of superlattice heterostructures with InAs submonolayer insertions in GaAs matrix and them optical properties are investigated in the work.
Development of submonolayer deposition technique can offer significant flexibility in creation of strained heterostructures of different types and material systems. It was found that under certain growth conditions the deposition of InAs insertions of less than 1 monolayer (ML) thickness in GaAs matrix forms so-called sub-monolayer quantum dots (SML QDs). The energy spectrum of these QDs can be varied over a wide range by tuning the InAs coverage and the thickness of GaAs spacers. Stranski-Krastanow (In,Ga)As QDs (SK QDs), which have been investigated in more details, have proved theoretically predicted lower threshold current density of 26 A/cm2 in compare with QW lasers. However, strong size variation of SK QDs in combination with the relatively low sheet density leads to low peak gain achievable in the ground state. This problem is the reason of typically low efficiency of SK QD-based lasers. Due to higher gain, SML QDs have proved their potential for high power laser application. In this presentation we report on further progress in the technology of SML QD lasers demonstrating high output power (6W) from 100-μm-wide laser diode emitting at 0.94 μm. High power QW-based lasers of the state-of-the-art performance are also presented for comparison.
A series of narrow emission lines (halfwidth 0.5 - 2 meV) corresponding to quantum-dot-like compositional fluctuations have been observed in low temperature near-field photoluminescence spectra of GaAsN and InGaAsN alloys. The estimation of the size, density, and nitrogen excess of individual compositional fluctuations (clusters) using scanning near-field magneto-spectroscopy reveals phase-separation effects in the distribution of nitrogen. We found a strong effect of In on the exciton g-factor in InGaAsN alloys.
Optical properties of GaAsN/GaAs heterostructures with different N contents grown by molecular-beam epitaxy were investigated. We show that under the certain grows reigmes the optical properties of the GaAsN layers are determined by recombination via localized states which is due to composition fluctuation. An increase in the N concentration leads to increase in composition fluctuation and, correspondingly, to increase in energy of localized states. Thermal annealing reduces nonuniformity distribution of nitrogen atoms. In short-period GaAsN/GaAs superlattice the effects of phase separation can be enhanced.
Recent results on molecular-beam epitaxy growth of the quantum dot InGaAs/GaAs heterostructures for long-wavelength lasers on GaAs substrates are presented. As a result of optimization of the growth procedure for active region and emitter layers low-threshold current density (45 - 80 A/cm2) long-wavelength (1.27 - 1.3 μm) laser diodes may be fabricated with high reproducibility.
The development of 1.3 micron VCSELs is currently considered to give a strong impulse for a wide use of ultra-fast local area networks. In the present work we discuss MBE growth and characteristics of InAs/GaAs quantum dot (QD) lasers, we also give characteristics of 1.3 micron QD VCSELs grown on GaAs and compare them with those of 1.3 micron InGaAsN/GaAs QW VCSELs. Overgrowing the InAs quantum dot array with thin InGaAs layer allows us to achieve 1.3 micron emission. Long stripe lasers showed low threshold current density (<100 A/cm2), high differential efficiency (>50%), and low internal loss (1-2 cm-1). Maximum continuous wave (CW) output power for wide stripe lasers was as high as 2.7 W and 110 mW for single mode devices. Uncoated broad area lasers showed no visible degradation of characteristics during 450 hours (60C, ambient environment). 1.3 micron InGaAsN/GaAs QW VCSELs are characterized by higher optical loss and lower differential efficiency than QD VCSELs. Due to high gain in the active region QW VCSELS demonstrate high output power (1 mW). QW VCSELs show extremely low internal round-trip optical loss (<0.05%), low threshold currents (<2 mA), high differential efficiency (40%) and output power (600 microW).
Vladimir Aleshkin, Alexander Andronov, A. Antonov, E. Demidov, Alexander Dubinov, Vladimir Gavrilenko, Dmitry Revin, B. Zvonkov, N. Zvonkov, E. Uskova, Leonid Vorobjev, D. Firsov, S. Danilov, Ilya Titkov, V. Shalygin, Alexey Zhukov, Alexey Kovsh, Victor Ustinov
Discussion of ways to achieve mid and far IR intraband lasing just by lateral electric field carrier (electron or hole) heating in multiple quantum well (MQW) structures is given. It is argued that the Gunn diodes are low frequency indirect transition lasers based on hot electron population inversion arising under electron intervalley transfer. In the MQW structures direct optical transitions exist while hot carrier population inversion can be achieved due to inter-valley/real space transfer. The two MQW structures are considered in this work: GaAs/AlAs and GaAs/InGaAs systems. In the first the hot electron (Gamma) -X intervalley/real space transfer from GaAs layers to AlAs layers provides population inversion while in the second the inversion can arise due to interlevel/interlayer transfer. Evaluations via the Monte-Carlo simulation of the hot electron phenomena in some of the structures are given and observation of the hot carrier phenomena of the type (including far and mid IR emission and absorption) are presented. Consideration of the appropriate laser design which provides also a way to cope with the low frequency (Gunn type) current oscillations is given.
We report on a GaAs/AlAs superlattice detector as a novel direct detector and autocorrelator for THz radiation. It is based on a doped wide-miniband GaAs/AlAs superlattice, with submonolayer AlAs barrier layers; the superlattice is operated at room temperature. THz radiation, generated by a free-electron laser and a mode locked p-Ge laser, was coupled into the superlattice via a corner cube antenna system. THz-irradiation of the biased superlattice resulted in a current reduction, which was monitored. The direct detector showed a fast response (20 ps, limited by the electronic circuit) and was robust against intense radiation pulses (peak power 10 kW). The responsivity was 100 times higher than the responsivity of detectors of comparable risetime and comparable robustness. Intense THz radiation caused a complete suppression of the current through the superlattice. This is the basis of the superlattice autocorrelator. The superlattice autocorrelator could resolve picosecond radiation pulses.
The physical mechanism for creation of intraband population inversion between levels of quantum dots under injection of electron-hole pairs of suggested. The method is based on employment of generation of interband radiation providing fast depopulation of quantum dot ground level. Spontaneous far-IR radiation from diode laser structures with InGaAs/AlGaAs quantum dots connected with intraband hole and/or electron transitions between levels of size quantization in quantum dots was found and investigated for the first time. Spontaneous far-IR radiation is observed only under simultaneous generation of stimulated near-IR radiation connected with interband carrier transitions. Far- IR emission is observed also from laser structures with InGaAs/GaAs quantum wells. Intensity of this radiation is about of order less then intensity of radiation from structures with quantum dots. Qualitative explanations of phenomena observed are proposed.
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