## 3 Optical Properties
### Microdisk Lasers Based on QWDs
#### 3.5.2 Dynamic Characteristics and Data Transmission  
Presently main anticipated application of MD lasers is associated with ultrasmall light sources for optical data transmission between individual optical circuits and inside one circuit. Such an application requires efficient high-frequency modulation. Significant progress has been made in the development of InP-based microlasers based on QWs. A 7.5 \(\upmu\)m in diameter circular MD laser bonded onto a silicon-on-insulator waveguide showed the 3-dB bandwidth of 3.5 GHz [81]. High-speed direct-modulation with a 3-dB bandwidth of 11.7 GHz and open eye diagrams at 10 Gbit/s were demonstrated for a 1550 nm InGaAsP/InP QW microdisk with a diameter of 7.5 \(\upmu\)m bonded to silicon [82]. Small signal modulation (SSM) with a resonance frequency (\(f_{\mathrm{R}}\)) of 12.5 GHz was realized for an AlGaInAs/InP circular microlaser with a radius of 10 \(\upmu\)m at 290 K. Furthermore, \(f_{\mathrm{R}}\) = 6.9 GHz, as well as clear eye diagrams at 12.5 Gbit/s, were observed for a 15-\(\upmu\)m radius circular AlGaInAs/InP microlaser with active region based on six compressively strained QWs [83]. SSM with a 3-dB bandwidth exceeding 10 GHz was realized for a 30 \(\upmu\)m microspiral AlGaInAs/InP disk based on eight QWs [84]. An open eye diagram at 15 Gbit/s with a bias current of 90 mA at a stage temperature of 15 \({}^{\circ}\)C was demonstrated [84]. More recently SSM response with 3-dB bandwidth up to 20 GHz was demonstrated for the AlGaInAs/InP microdisk laser with active region based on 6 QWs with a radius of 7 \(\upmu\)m surrounded by BCB-cladding layer [85]. Eye diagrams at the bit rates of 20, 25, and 30 Gbit/s were also measured at the temperature of 14 \({}^{\circ}\)C [85]. In addition to applications in optical integrated circuits, MD lasers may become a cost-effective alternative for sophisticated and expensive distributed feedback lasers and vertical cavity surface-emitting laser for data transmission over intermediate and short distances. MD lasers have the inherent structural advantage of avoidance any kind of reflector either in the form of a cleaved facet or a diffraction grating. In paper [86] the authors report on the properties of directly modulated active-passive integrated 1570 nm InP-based microring lasers with an active region comprising six InGaAs QWs fabricated using full wafer bonding techniques. Bit rates up to 7 Gb/s were achieved for a 50 km singlemode fiber transmission. The disadvantage of InP-based microlasers is caused by small energy offsets and low thermal conductivity [87], which results in a significant deterioration of their characteristics with temperature increase. For instance, in reference [85], it was observed that when the temperature rises from 14 to 40 \({}^{\circ}\)C, the resonance frequency value drops from 14.9 to about 8.3 GHz. However, dynamic characteristics of much more temperature stable GaAs-based MD lasers are poorly studied. 1 Gbit/s data transmission has been realized using a 6.5 \(\upmu\)m in diameter MD laser based on In\({}_{0.5}\)Ga\({}_{0.5}\)As QDs [88]. It should be mentioned that in data transmission experiments by using InP-based microlasers, spectral filtering, and amplification of the lasing emission was utilized. The use of optical filters and amplifiers in photonic integrated circuit is obviously not desirable as it significantly increases the number of elements and fabrication complexity. In this section, we describe the results on small-signal modulation and data transmission by MD lasers based on QWDs. These experiments were carried out without amplification of the lasing emission. Moreover, the microlasers were neither externally cooled nor temperature stabilized. The microdisk design used for small signal modulation and data transmission experiment was the same as described in the previous section, but the planarizing layer was used. For high-frequency experiments, a ground-signal-ground (GSG) radio-frequency probe was used.