In this work, electrically-injected microdisk lasers with diameter varied from 15 to 31μm based on an InAs/InGaAs QD
active region have been fabricated and tested in continuous wave regime. At room temperature, lasing is achieved at
wavelength around 1.26…1.27 μm with threshold current density about 900 A/cm2. Specific series resistance is
estimated to be about 10-4 Ohm•cm2. The lasers were tested at elevated temperatures. Lasing is achieved up to 100°C
with threshold current of 13.8mA and lasing wavelength of 1304nm in device with 31μm diameter. To the best of our
knowledge, this is the highest CW lasing temperature and the longest lasing wavelength ever reported for injection QD
microdisk/microring lasers on GaAs substrates. Emission spectrum demonstrates single-mode lasing with side mode
suppression ration of 24dB and dominant mode linewidth of 35pm. The far field radiation pattern demonstrates two
narrow maxima off the disk plane. A combination of device characteristics achieved (low threshold, long wavelength,
operation at elevated temperatures) makes them suitable for application in future optoelectronic circuits for optical
interconnect systems.
Tremendous efforts have been developed for multi-Tbps over ultra-long distance and metro and access optical networks. With the exponential increase demand on data transmission, storage and serving, especially the 5G wireless access scenarios, the optical Internet networking has evolved to data-center based optical networks pressuring on novel and economical access transmission systems. This paper reports (1) Experimental platforms and transmission techniques employing band-limited optical components operating at 10G for 100G based at 28G baud. Advanced modulation formats such as PAM-4, DMT, duo-binary etc are reported and their advantages and disadvantages are analyzed so as to achieve multi-Tbps optical transmission systems for access inter- and intra- data-centered-based networks; (2) Integrated multi-Tbps combining comb laser sources and micro-ring modulators meeting the required performance for access systems are reported. Ten-sub-carrier quantum dot com lasers are employed in association with wideband optical intensity modulators to demonstrate the feasibility of such sources and integrated micro-ring modulators acting as a combined function of demultiplexing/multiplexing and modulation, hence compactness and economy scale. Under the use of multi-level modulation and direct detection at 56 GBd an aggregate of higher than 2Tbps and even 3Tbps can be achieved by interleaved two comb lasers of 16 sub-carrier lines; (3) Finally the fundamental designs of ultra-compacts flexible filters and switching integrated components based on Si photonics for multi Tera-bps active interconnection are presented. Experimental results on multi-channels transmissions and performances of optical switching matrices and effects on that of data channels are proposed.
Tunable continuous wave (CW) green light generation between 517 nm and 538 nm at room-temperature has been
demonstrated from a frequency-doubled broadly tunable quantum well (QW) external-cavity fiber-coupled diode laser
by use of an uncoated periodically poled potassium titanyl phosphate (PPKTP) crystal waveguide crystal. Green light at
530 nm with maximum conversion efficiency of 14.8% and output power of 12.88 mW has been generated using a
PPKTP crystal waveguide with the cross-sectional area of 3x5μm2. The possibility of tunable second harmonic
generation in the PPKTP crystal waveguides with the cross-sectional areas of 4x4μm2 and 2x6μm2 was also investigated.
Orange light with maximum conversion efficiency exceeding 10% and CW output power of 12.04 mW, 10.45 mW and 6.24 mW has been generated at 606, 608, and 611 nm, respectively, from a frequency-doubled InAs/GaAs quantum-dot external-cavity diode laser by use of a periodically-poled KTP waveguides with different cross-sectional areas. The wider waveguide with the cross-sectional area of 4×4 μm2 demonstrated better results in comparison with the narrower waveguides (3×5 μm2 and 2×6 μm2) which corresponded to lower coupling efficiency. Additional tuning of second harmonic light (between 606 and 614 nm) with similar conversion efficiency was possible by changing the crystal temperature.
We demonstrate a compact all-room-temperature picosecond laser source broadly tunable in the visible spectral region between 600 nm and 627 nm. The tunable radiation is obtained by frequency-doubling of a tunable quantum-dot external-cavity mode-locked laser in a periodically-poled KTP multimode waveguide. In this case, utilization of a significant difference in the effective refractive indices of the high- and low-order modes enables to match the period of poling in a very broad wavelength range. The maximum achieved second harmonic output peak power is 3.25 mW at 613 nm for 71.43 mW of launched pump peak power at 1226 nm, resulting in conversion efficiency of 4.55%.
Spectral and power characteristics of QD stripe lasers operating in two-state lasing regime have been studied in a wide range of operation conditions. It was demonstrated that neither self-heating nor increase of the homogeneous broadening are responsible for quenching of the ground-state lasing beyond the two-state lasing threshold. It was found that difference in electron and hole capture rates strongly affects light-current curve. Modulation p-type doping is shown to enhance the peak power of GS lasing transition. Microring and microdisk structures (D = 4-9 μm) comprising 1.3 μm InAs/InGaAs quantum dots have been fabricated and studied by μ-PL and NSOM. Ground-state lasing was achieved well above root temperature (up to 380 K). Effect of inner diameter on threshold characteristics was evaluated.
We demonstrate an instantaneous all-optical manipulation of optical absorption at the ground state of InGaAs/GaAs
quantum dots (QDs) via a quantum-confined Stark effect (QCSE) induced by the electric field of incident THz pulses
with peak electric fields reaching 200 kV/cm in the free space. As a result, a THz signal with the full bandwidth of 3
THz can be directly encoded onto an optical signal probing the ground state absorption in QDs, resulting in the encoded
temporal features as fast as 460 fs. The optical absorption modulation at highest THz fields reaches about 30% of the
total optical absorption in QDs at the ground state. The dependency of electro-absorption modulation depth on the peak
THz field is found to be strongly nonlinear, as expected from the QCSE. From this dependency we conclude that the
dominant contribution to the observed electro-absorption modulation in our sample is made by the overall optical
absorption quenching via a reduction of the overlap integral and hence the probability of inter-band transition, rather
than by the Stark shift of the QD absorption peak away from the spectrum of the optical probe. As expected from the
three-dimensional geometry of a QD, the THz QCSE was found to be independent of the polarization of the THz field.
The instantaneous nature of THz QCSE in QDs enables femtosecond all-optical switching at very high repetition rates.
This allowed us to demonstrate the potential for applications in THz-range wireless communication systems with the
data rate of at least 0.5 Tbit/s.
We present high average power femtosecond VECSELs based on both quantum dot (QD) and quantum well (QW) gain
with extremely low dispersion. 1.05 W in 784-fs pulses could be achieved from a QD-VECSEL modelocked by a QDSESAM
with fast recovery dynamics. A similar QW-gain structure modelocked by the same SESAM enabled stable
480-fs with an average output power of 300 mW at a repetition rate of 7 GHz. Furthermore, we investigated repetition
rate scaling by changing the cavity length. We demonstrated fundamentally modelocked pulses over a tuning range from
6.5 GHz to 11.3 GHz. Without any realignment of the cavity over the whole tuning range, the pulse duration remained
nearly constant around 625 fs (±3.5%) while the output power was 169 mW (±6%). The center wavelength changed
only about ±0.2 nm around 963.8 nm. A tunable repetition rate can be of interest for various metrology application such
as optical sampling by laser cavity tuning.
In this work the use of two identical QD SOAs to enhance the performance of swept laser system for OCT applications is
discussed, resulting in an increase in bandwidth up to 94nm. The combination of GaAs based QD SOAs and InP based
QW SOAs for realizing broad bandwidth sources for OCT system is described. For the swept laser source a 154nm
spectral bandwidth from 1193nm to 1347nm and an average power of 8mW is obtained and for the filtered ASE source a
225 nm bandwidth is demonstrated.
Using quantum well gain materials, ultrafast VECSELs have achieved higher output powers (2.1 W) and shorter pulses (60 fs) than any other semiconductor laser. Quantum dot (QD) gain materials offer a larger inhomogeneously broadened bandwidth, potentially supporting shorter pulse durations. We demonstrate the first femtosecond QD-based VECSEL using a QD-SESAM for modelocking, obtaining 63 mW at 3.2 GHz in 780-fs pulses at 960 nm. In continuous wave operation we obtained 5.2 W using an intra-cavity diamond heat spreader, which has been the highest output power from a QD-VECSEL so far. Further power scaling is thus expected also for modelocked operation.
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.
F. Hopfer, A, Mutig, G. Fiol, M. Kuntz, S. Mikhrin, I. Krestnikov, D. Livshits, A. Kovsh, C. Bornholdt, V. Shchukin, N. Ledentsov, V. Gaysler, N. Zakharov, P. Werner, D. Bimberg
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.
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.
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.
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.
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.
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.
In this paper we present the calculation of local mirror facet overheating of the SCH laser diode active region. It is shown that optical strength of the mirror facet prior to optical damage depends on optical confinement factor ((Gamma) -factor) and thickness of the active region layer adjacent to surface (`dead layer') where nonradiative recombination rate is much higher. Detailed investigations of local mirror facet temperature and optical strength of mirror facet in both Al-containing (AlGaAs/GaAs, InGaAs/AlGaAs/GaAs) and Al-free (InGaAsP/GaAs) single quantum well laser diodes were carried out. Experimental results are in good agreement with calculations. It is shown that optical strength of mirror facet for laser heterostructure can be derived from the behavior of local mirror facet overheating of laser diodes.
Spectra of some ridge-waveguide lasers grown by metal-organic chemical vapor deposition (MOCVD) undergo a reversible transformation at a certain value of drive current -- usually from 5 to 10 thresholds. When the current is increased past this point, the spectrum abruptly widens and its amplitude drops correspondingly. In the widened spectrum a structure with period equal to longitudinal mode separation can be seen. We call this effect `spectral collapse.' The effect seems to be typical for ridge-waveguide lasers with ternary active regions and independent of active region strain. Data on `collapse' in both cw and pulsed modes at different temperatures suggest its connection with active region overheating. The intensity noise versus current dependence for some of the lasers reveals two peaks, one near the threshold and the other near the `spectral collapse' point. This led us to a suggestion that the `collapse' can be explained by nonlinear mode interaction. Some LPE-grown InGaAsP/GaAs lasers of similar design also exhibit spectral collapse while other samples from the same wafers do not, which may be evidence of a competition between nonlinear effects that cause spectral collapse and continuous widening of the spectrum with current due to spinodal decomposition in quaternary active region.
We investigate high-power InGaAsP/GaAs (0.77 - 0.83 micrometers ) buried heterostructure lasers grown by LPE technique. A redistribution of the output power in the far field pattern from higher-order modes into the fundamental mode was observed with a temperature increase in the range of 10 degree(s) - 70 degree(s)C. A theoretical model taking into account the affect of boundary recombination velocity on the mesa walls on the carrier concentration profile in the active region is proposed. Significant rise of the boundary recombination velocity with temperature was confirmed experimentally by comparing the temperature dependences of quantum efficiency in buried and stripe-contact (without mesa walls) lasers fabricated from the same wafers. As a result of these investigations, we propose a new laser design in which the carrier concentration profile is similar to that in a heated device. A narrow contact mesa stripe laser permits us to concentrate most of the pumping current in the middle of active region and, hence, to increase the overlap of the carrier concentration profile with the fundamental mode intensity. The optimal dimensions for single-mode laser were calculated.
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