Mode-locked lasers (MLLs) and semiconductor optical amplifiers (SOAs) based on quantum dot (QD) gain material will
impact the development of next generation networks like the 100Gb/s Ethernet. Hybrid mode-locked lasers consisting of
a monolithic two section device presently already generate picosecond pulse trains at 40 GHz with an extremely low
jitter in the range of 200 fs under optimum operating conditions. A detailed chirp analysis which is prerequisite for
optical time division multiplexing applications is presented. QD SOAs are showing superior performance for linear
amplification as well as nonlinear signal processing. Wavelength conversion via cross-gain modulation is shown to have
a small signal bandwidth beyond 40 GHz under high bias current injection. This makes QD SOAs much superior to
Low transparency current density and improved temperature stability with a large characteristic temperature
T<sub>0</sub> > 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<sup>-12</sup> 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.
Recent results on GaAs-based high-speed mode-locked quantum dot (QD) lasers and optical amplifiers with an operation
wavelength centered at 1290 nm are reviewed and their complex dependence on device and operating parameters is
discussed on the basis of experimental data obtained with integrated fiber-based QD device modules.
Hybrid and passive mode-locking of QD lasers with repetition frequencies between 5 and 80 GHz, sub-ps pulse widths,
ultra-low timing jitter down to 190 fs, high output peak power beyond 1 W and suppression of Q-switching are reported,
showing the large potential of this class of devices for O-band optical fiber applications.
Results on cw and dynamical characterization of quantum dot semiconductor optical amplifiers are presented. QD
amplifiers exhibit a close-to-ideal noise figure of 4 dB and demonstrate multi-wavelength amplification of three CWDM
wavelengths simultaneously. Modelling of QD polarization dependence shows that it should be possible to achieve
polarization insensitive SOAs using vertically coupled QD stacks. Amplification of ultra-fast 80 GHz optical combs and
bit-error-free data signal amplification at 40 Gb/s with QD SOAs show the potential for their application in future 100
Gb Ethernet networks.
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.
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.
Universal self-organisation on surfaces of semiconductors upon deposition of a few non-lattice-matched monolayers using MOCVD or MBE lead to the formation of quantum dots. Their electronic and optical properties are closer to those of atoms than of solids.
We have demonstrated for QD-lasers a record low transparency current density of 6A/cm<sup>2</sup> per dot layer at 1.16 μm, high-power of 12W, an internal quantum efficiency of 98%, and an internal loss below 1.5 cm-1. Relaxation oscillations indicate the potential for cut-off frequencies larger than 10 GHz.
GaAs-based QD-lasers emitting at 1.3 μm exhibit output power of 5 W and single transverse mode operation up to 300 mW. At 1.5 μm again an output power of 5 W has been obtained for first devices showing a transparency current of 700 A/cm<sup>2</sup>.
Single mode lasers at 1.16 and 1.3 μm show no beam filamentation, reduced M<sup>2</sup>, sensitivity to optical feedback by 30 db and α-parameter as compared to quantum well lasers.
Passive mode locking of 1.3 μm lasers up to 20 GHz is obtained.
Thus GaAs-lasers can now replace InP-based ones at least in the range up to 1.3 µm, probably up to 1.55 μm.
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.