An overview is given about the recent improvement in 1.55 μm QD lasers for direct modulation. Based on improved QD epitaxy, which reduces the inhomogeneous size distribution, record values in small signal modulation bandwidth of more than 15 GHz and in digital modulation of up to 35 GBit/s were obtained. Due to the high modal gain and robust ground state transition, the temperature dependence of the laser performance could be very much improved with characteristic temperatures of T0 = 125 K and T1 near to 400 K. This allow a temperature stable modulation bandwidth between 15-60 °C of (14 +/- 1) GHz sufficient for 25 GBit/s digital modulation.
An overview is given about the recent improvement in 1.5 μm QD lasers for direct modulation. Based on improved QD epitaxy with a reduced inhomogeneous size distribution, record values in small signal modulation bandwidth of more than 15 GHz and in digital modulation of up to 36 GBit/s were obtained. Due to the high modal gain and robust ground state transition, the temperature dependence of the laser performance could be very much improved with characteristic temperatures of T0 = 125 K and T1 near to 400 K. Also the impact of the temperature on the digital modulation speed will be discussed.
The ever-growing need for higher data rates is a driving force for the implementation of higher order coherent communication formats. A key element in coherent detection is the local oscillator (LO) of the decoding unit. This device has to provide coherent light with a narrow linewidth in order to distinguish between different phase and amplitude states of the incoming signal. As predicted by theory, a drastic linewidth reduction is expected from quantum dot (QD) laser materials by the quasi zero-dimensional nature of the gain function. The impact of different gain materials consisting of different numbers of QD layers on the linewidth of distributed feedback (DFB) lasers was investigated and shows an unambiguous dependence on the layer design. Intrinsic linewidths as low as 110 kHz could be determined.
Due to the discrete density of states distribution and spatial localization of carriers in quantum dot (QD) material, the
dynamics should be strongly enhanced in comparison to quantum well material. Based on improved 1.5 μm
InAs/InGaAlAs/InP QD gain material short cavity ridge waveguide lasers were fabricated. Devices with cavity, lengths
of 230 to 338 μm with high reflection coatings on the backside exhibit record value for any QD laser in small and large
signal modulation performance with up to 15 GHz and 36 GBit/s, respectively, obtained at 14 °C. Due to the high
temperature stability of threshold current and external differential efficiency, the lasers exhibit also nearly constant
modulation bandwidth between 14-60 °C.
The direct laser modulation bandwidth can be extended substantially by introducing a supplementary photon-photon resonance (PPR) at a higher frequency than the carrier-photon resonance (CPR). The paper presents a modified rate equation model that takes into account the PPR by treating the longitudinal confinement factor as a dynamic variable. The conditions required for obtaining a strong PPR and an enhancement of the small-signal modulation bandwidth are analyzed and experimental results confirming the model are presented. Since the small-signal modulation bandwidth may not be indicative of the large-signal modulation capability, particularly in case of a small-signal modulation response with substantial variations across the bandwidth, we have also analyzed the influence of the PPR-enhanced small-signal modulation response shape on the large-signal modulation capability as well as the methods that can be employed to flatten the small-signal modulation transfer function between the CPR and PPR.
Semiconductor optical amplifiers based on quantum dots show small-signal cross-gain modulation bandwidths exceeding
40 GHz. In large signal operation wavelength conversion at 80 Gb/s over 10 nm is presented. Two section mode-locked
lasers at 40 GHz yield ultra-low jitter of 200 fs in hybrid operation. Optical feedback presents an alternative way to
effectively reduce the jitter and opens up the possibility to extract a microwave signal, having the same properties as the
optical pulse comb, from the absorber section.
The conventional distributed feedback and distributed Bragg reflector edge-emitting lasers employ buried gratings,
which require two or more epitaxial growth steps. By using lateral corrugations of the ridge-waveguide as surface
gratings the epitaxial overgrowth is avoided, reducing the fabrication complexity, increasing the yield and reducing the
fabrication cost. The surface gratings are applicable to different materials, including Al-containing ones and can be easily
integrated in complex device structures and photonic circuits. Single-contact and multiple contact edge-emitting lasers
with laterally-corrugated ridge waveguide gratings have been developed both on GaAs and InP substrates with the aim to
exploit the photon-photon resonance in order to extend their direct modulation bandwidth. The paper reports on the
characteristics of such surface-grating-based lasers emitting both at 1.3 and 1.55 μm and presents the photon-photon
resonance extended small-signal modulation bandwidth (> 20 GHz) achieved with a 1.6 mm long single-contact device
under direct modulation. Similarly structured devices, with shorter lengths are expected to exceed 40 GHz small-signal
modulation bandwidth under direct modulation.
To combine low-cost fabrication and high-speed data communication like 100 GBit/s, multi-section DBR lasers are
developed with nanoimprint compatible surface defined gratings. This laser design has the potential to enhance the
modulation bandwidth by exciting a higher order optical mode, the so-called photon-photon resonance (PPR). ICP-RIE
etching was used to transfer the e-beam exposed surface pattern in one step into the semiconductor. High aspect ratios of
> 1:15, vertical trenches with a width of about 140 nm and an etch depth of > 2 μm were obtained for the lateral gratings.
Three-section DBR lasers are fabricated on an MOVPE grown 1.5 μm InP laser material exhibiting CW threshold
currents of 94 mA for a 0.9 mm long device. A side mode suppression ratio of > 50 dB could be achieved demonstrating
a high enough coupling strength of the lateral gratings. The influence of different operation conditions (currents,
temperature) and dependence on the grating period on threshold current and emission wavelength are studied and will be
discussed in this paper. First high frequency measurements in operation conditions without PPR enhancement show a -
3dB bandwidth of about 15 GHz.
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
Semiconductor optical amplifiers (SOAs) based on nanostructure gain media such as quantum dots (QD) and quantum dashes (QDASH) have several basic characteristics which offer significant performance improvements over commonly used quantum well (QW) or bulk amplifiers. Among these are broadband optical gain bandwidth (which is two to three times broader than that of QW/bulk gain media), fast gain dynamics, large saturation powers, and low α parameter and population inversion factor. Originally, these properties have been demonstrated for QD/QDASH SOAs operating at 1000 nm and 1300 nm. However, it is imperative that QD/QDASH SOAs operating at 1550 nm be materialized in order for them to have the expected impact on fiber-optic communication. Operation at 1550 nm has been achieved using InAs / InP QD and QDASH laser structures. In this paper the unique gain and noise properties of InAs / InP QDASH SOAs operating at 1550 nm will be presented. Specifically, cross-gain-modulation, four-wave-mixing and chirp measurements which explore the complex spectral cross relaxation dynamics of these SOAs will be described and highlighted in the context of simultaneous, distortionless, high bit-rate multiwavelength data amplification, as well as wideband / high-speed optical signal processing applications. Also, an experimental study of the gain and noise in saturated QDASH SOAs will be described together with a theoretical analysis comprising both coherent and incoherent gain phenomena. The impact of the partially inhomogeneously broadened gain spectrum, fast population pulsation dynamics, α parameter and wetting layer density of states on the noise characteristics will be discussed.
We address basic issues related to the dynamical properties of low dimensionality semiconductor lasers. We concentrate on quantum dash lasers but many of the properties hold, with very few changes, to quantum dot lasers. We formulate a self consistent semiclassical model for a multimode laser field which interacts with an inhomogeneously broadened assembly of quantum dashes. The key advantage of this model is the fact that it spectrally resolves the gain and multimode laser field. We show that the differential gain and the nonlinear damping can not be optimized simultaneously.
Semiconductor lasers and amplifiers were developed based on self-assembled quantum-dot gain material. This paper gives an overview about the recent work on GaAs- and InP-based quantum-dot devices mainly dedicated for telecom applications. The major advantage of quantum-dot like gain material, i.e. the possibility to tailor the spectral and spatial gain properties of an amplifying material, was used to optimize different device aspects, like low threshold current, broad band amplification or low temperature sensitivity. High performance GaAs-based continuous wave (cw) operating quantum-dot lasers could be fabricated with threshold currents of about 2 mA (L = 400 μm). Single mode emitting devices with emission wavelengths > 1.3 μm were realized by laterally coupled feedback gratings with threshold currents below 5 mA, output powers > 5 mW and cw operation temperatures up to 85 °C. Modulation frequencies of up to 7.5 GHz were obtained for standard device structures. For long wavelength telecom applications quantum-dot like material with dash geometry was developed on InP substrates with basic properties in the transition region between quantum-dot and -wire systems. A very large tuning range of the emission wavelength between 1.2 and 2.0 μm (room temperature) was obtained which allow the realization of material with ultra-wide gain bandwidth. Quantum-dash laser structures reaches threshold current densities < 1 kA/cm2. Ridge waveguide lasers with a cavity length of 1.9 mm show cw threshold currents of about 100 mA and maximum output powers > 40 mW per facet. With 300 μm long facet coated devices cw threshold currents of 23 mA and maximum operation temperatures in pulsed mode of 130 °C were achieved. Semiconductor optical amplifiers were fabricated by using broad band quantum-dash material. For a 1.9 mm long device, up to 22 dB gain was obtained with a three times larger spectral range than in comparable quantum well devices. High speed nearly pattern free signal amplification up to 10 GBit/s could be demonstrated and wavelength conversion experiments were performed.