M. Stubenrauch

Comparison of dynamic properties of ground- and excited-state emission in p-doped InAs/GaAs quantum-dot lasers

The dynamic properties of ground- and excited-state emission in InAs/GaAs quantum-dot lasers operating close to 1.31 μm are studied systematically. Under low bias conditions, such devices emit on the ground state, and switch to emission from the excited state under large drive currents. Modification of one facet reflectivity by deposition of a dichroic mirror yields emission at one of the two quantum-dot states under all bias conditions and enables to properly compare the dynamic properties of lasing from the two different initial states. The larger differential gain of the excited state, which follows from its larger degeneracy, as well as its somewhat smaller nonlinear gain compression results in largely improved modulation capabilities. We demonstrate maximum small-signal bandwidths of 10.51 GHz and 16.25 GHz for the ground and excited state, respectively, and correspondingly, large-signal digital modulation capabilities of 15 Gb/s and 22.5 Gb/s. For the excited state, the maximum error-free bit rate i...

Temperature-Dependent Characteristics of Single-Mode InAs Submonolayer Quantum-Dot Lasers

Temperature-dependent lasing characteristics of transverse single-mode GaAs-based InAs submonolayer (SML) quantum-dot (QD) lasers are investigated. The SML QD lasers, when operated under continuous wave (cw) conditions up to 65°C, exhibit a high characteristic temperature (<i>T</i>₀) of around 101 K. The temperature-dependent net modal gain is also studied using the Hakki-Paoli method, yielding a narrow gain spectrum width of 15.6 meV at 20°C where the peak gain is 9.6 cm^-1. The lasers exhibit a low loss of -6 to -9 cm^-1 in the temperature range of 15°C to 65°C.

Spontaneous emission study on 1.3 µm InAs/InGaAs/GaAs quantum dot lasers

True spontaneous emission (TSE) measurements on InAs/InGaAs/GaAs quantum dot (QD) lasers have been performed as a function of injection current and cavity length. For each laser, TSE from both the ground state (GS) transition and the excited state (ES) transition has been analyzed. It is found that Auger processes are the major nonradiative recombination (NR) processes for both the GS and ES transitions. In particular, for the first time, the existence of Auger like NR features in ES transitions has been experimentally demonstrated. In addition, obvious competition for carriers between the ES transition and the GS transition has been observed in TSE analysis. Furthermore, the QD lasers cavity length has a strong effect on the NR process in GS transitions, due to GS gain saturation. Therefore, when analyzing the NR processes in operating QD lasers, gain saturation due to cavity length limits should be properly considered.

15 Gb/s index-coupled distributed-feedback lasers based on 1.3 μm InGaAs quantum dots

The static properties and large-signal modulation capabilities of directly modulated p-doped quantum-dot distributed-feedback lasers are presented. Based on pure index gratings the devices exhibit a side-mode-suppression ratio of 58 dB and optical output powers up to 34 mW. Assisted by a broad gain spectrum, which is typical for quantum-dot material, emission wavelengths from 1290 nm to 1310 nm are covered by the transversal and longitudinal single-mode lasers fabricated from the same single wafer. Thus, these lasers are ideal devices for on-chip wavelength division multiplexing within the original-band according to the IEEE802.3ba standard. 10 Gb/s data transmission across 30 km of single mode fiber is demonstrated. The maximum error-free data rate is found to be 15 Gb/s.

Nonlinear pulse propagation in a quantum dot laser

We investigate the nonlinear propagation of an ultra-short, 150 fs, optical pulse along the waveguide of a quantum dot (QD) laser operating above threshold. We demonstrate that among the various nonlinear processes experienced by the propagating pulse, four-wave mixing (FWM) between the pulse and the two oscillating counter-propagating cw fields of the laser is the dominant one. FWM has two important consequences. One is the creation of a spectral hole located in the vicinity of the cw oscillating frequency. The width of the spectral hole is determined by an effective carrier and gain relaxation time. The second is a modification of the shape of the trailing edge of the pulse. The wave mixing involves first and second order processes which result in a complicated interaction among several fields inside the cavity, some of which are cw while the others are time varying, all propagating in both directions. The nonlinear pulse propagation is analyzed using two complementary theoretical approaches. One is a semi-analytical model which considers only the wave mixing interaction between six field components, three of which propagate in each direction (two cw fields and four time-varying signals). This model predicts the deformation of the tail of the output signal by a secondary idler wave, produced in a cascaded FWM process, which co-propagates with the original injected pulse. The second approach is a finite-difference time-domain simulation, which considers also additional nonlinear effects, such as gain saturation and self-phase modulation. The theoretical results are confirmed by a series of experiments in which the time dependent amplitude and phase of the pulse after propagation are measured using the cross-frequency-resolved optical gating technique.

25 Gbit/s differential phase-shift-keying signal generation using directly modulated quantum-dot semiconductor optical amplifiers

Error-free generation of 25-Gbit/s differential phase-shift keying (DPSK) signals via direct modulation of InAs quantum-dot (QD) based semiconductor optical amplifiers (SOAs) is experimentally demonstrated with an input power level of −5 dBm. The QD SOAs emit in the 1.3-μm wavelength range and provide a small-signal fiber-to-fiber gain of 8 dB. Furthermore, error-free DPSK modulation is achieved for constant optical input power levels from 3 dBm down to only −11 dBm for a bit rate of 20 Gbit/s. Direct phase modulation of QD SOAs via current changes is thus demonstrated to be much faster than direct gain modulation.

Quantum-dot based distributed feedback lasers and electro-absorption modulators for datacom applications

The unique properties of quantum dots (QDs) as gain material for semiconductor lasers result in advantageous characteristics like high temperature stability, low noise, low feedback sensitivity and low chirp. Their low internal losses make QD based devices suitable for high-speed telecommunication applications. Increase of data transmission rate by wavelength division multiplexing techniques requires temperature stable longitudinal single-mode transmitters. We present 1.3 μm index-coupled QD distributed feedback (DFB) lasers with p-doped active region showing wide temperature independent operation, high side-mode suppression ratio (>; 55 dB) and quantum efficiencies up to 40%. The data rate achieved by direct modulation is exceeded by using QD electro-absorption modulators (EAMs) based on the Quantum-Confined Stark Effect. Presently we achieve a 3 dB-bandwidth of 17 GHz and extinction ratios of 18 dB. Monolithic integration of both, DFB and EAM, into a single two-section device provides an inexpensive transmitter unit suitable for metropolitan area network applications.

40 GBd D(Q)PSK and OOK amplification using o-band quantum-dot semiconductor optical amplifiers

Error-free (bit-error ratio < 10−9) generation and amplification of phase-coded signals are presented with a symbol rate up to 40 GBd using quantum-dot (QD) based semiconductor optical amplifiers (SOA) emitting at a wavelength of 1.3 µm. Phase-coded signal generation is demonstrated via direct modulation of a QD SOA exhibiting a linear fiber-to-fiber gain of 8 dB. In comparison to gain modulation, the phase modulation of the QD SOAs via the current is found to be much faster. Various symbol rates from 10 to 25 GBd and input power levels from −11 dBm to 3 dBm are investigated. The amplification of a 40 GBd differential (quadrature) phase-shift keying (D(Q)PSK) signal is investigated using QD SOAs exhibiting a linear fiber-to-fiber gain of 26 dB. The input-power dynamic range is determined and discussed, based on bit-error ratio and error-vector magnitude measurements using differential detection and coherent detection receivers. Finally, investigations of the influence of a 40 GBd OOK neighboring channel (5 nm grid) on a 40 GBd DQPSK channel in dependence of the input power levels of both signals are presented.

1.31 µm quantum-dot hybrid mode-locked lasers for optical time-division multiplexing

We report on jitter reduction and frequency tuning ability of 1.31 μm 40 GHz quantum-dot mode-locked lasers with the main focus on hybrid mode locking. The frequency of the external electrical source is usually chosen to be close to the passive frequency of the mode-locked laser, which is increasingly difficult for high frequencies far beyond 40 GHz. Sub-harmonic mode locking is demonstrated, showing a significant jitter reduction down to 381 fs and a locking range of 6.6 MHz. 80 Gb/s on-off keying data transmission based on optical time-division multiplexing is demonstrated using a quantum-dot mode-locked laser module.