Robert Rossbach

Quantum key distribution using quantum dot single-photon emitting diodes in the red and near infrared spectral range

We report on in-lab free space quantum key distribution (QKD) experiments over 40cm distance using highly efficient electrically driven quantum dot single-photon sources emitting in the red as well as near-infrared spectral range. In the case of infrared emitting devices, we achieve sifted key rates of 27.2kbits 1 (35.4kbits 1 ) at a quantum bit error rate (QBER) of 3.9% (3.8%) and a g (2) (0) value of 0.35 (0.49) at moderate (high) excitation. The

Analog Modulation of 650-nm VCSELs

Small-signal properties of 650-nm vertical-cavity surface-emitting lasers (VCSELs) with different oxide aperture sizes were measured. A small diameter VCSEL of 3.5 mum has a maximum resonance frequency of 5.7 GHz. The photon density determines the maximum resonance frequency. Modeling also indicates higher photon densities in the small VCSEL due to better thermal behavior

Analysis of the modulation behavior of red VCSELs

This talk focuses on the high frequency characteristics of red VCSELs. After a short description of important fabrication issues the modulation behaviour of GaInP surface emitting lasers is discussed on the basis of the laser rate equations. The influence of the geometric dimensions of the laser structure and of the operating conditions is investigated. From the S-parameter analysis a modulation coefficient of 3 GHz/(mA)1/2 for VCSELs with a 7 µm aperture and a differential gain of 1.15•10-16 cm2 are deduced. A more detailed analysis reveals, that the modulation behaviour of red VCSELs nearly solely depends on their photon density inside the quantum wells as expected from the rate equations. These results imply that for a certain range of geometries diffusion and diffraction have a second order influence on the high frequency characteristics of red VCSELs. The K-factor analysis indicates very short carrier transfer and relaxation times around 5 ps and a maximum frequency of 25 GHz. Large signal modulation issues such as the properties of the eye diagram are also addressed. From the device characteristics it is concluded that the GaInP-VCSEL is suitable for data communication applications. Low cost fabrication makes the red VCSEL an attractive candidate for both automotive and high-speed data communication.

Red surface emitters: powerful and fast

Vertical cavity surface emitting lasers (VCSEL) in the GaInP/AlGaInP material system have experienced a rapid development in their short history. In general lasers from that material system are suitable for a huge number of applications beginning with TV lasers and high power lasers for edge emitters, continuing with optical data storage, medical applications as well as data communication in cars, air planes, offices and between computers as application field for VCSELs. Especially automotive applications show the highest requirements on a laser with respect to operation temperature and power. In this talk we draw out the problems of the material system AlGaInP and its implications for laser applications. We discuss the epitaxial and technological solutions to overcome at least a part of these inherent problems. We will discuss the possible power that we can expect from VCSELs emitting in the range between 650 nm to 670 nm. We got from our lasers 5 mW, CW @ RT, 670nm and 2.5mW, CW@RT, 650 nm. We emphasize the role of doping, Bragg mirror grading, suitable detuning of cavity mode and gain, and optimisation of the contact layer and control of the oxide aperture in the VCSEL structure to get improved operation characteristics at higher temperatures. From the analysis of high frequency measurements, we could evaluate modulation bandwidths between 4 GHz and 10 GHz. The application of polyimide as a dielectric isolation material shows the potential to obtain modulation bandwidths beyond 10 GHz. For the intrinsic modulation bandwidth we get a value of 25 GHz, which is near the value edge emitters show. A more detailed discussion on photon lifetimes and carrier transport times will be given in the talk. Red light emitting VCSELS driven with short current pulses showed laser emission up to + 160°C case temperature. Thus, a CW operation up to +120°C can be expected after further improvement of power generation (decrease of series resistance) and heat spreading (optimized contacts and mounting). From these characteristics we can conclude that AlGaInP-surface emitting lasers have a real potential as low cost lasers for automotive applications as we all as data communication applications up to 10 GHz.

Red VCSEL for automotive applications

In this paper we discuss the problems of the AlGaInP material system and its consequences for the laser applications in vertical-cavity surface-emitting lasers (VCSEL). The epitaxial and technological solutions to overcome at least parts of the inherent problems were presented. Measured power-current curves of 660nm AlGaInP-based oxide-confined VCSEL are compared with calculated data by a cylindrical heat dissipation model to improve heat removal out of the device. Pulsed lasing operation of a 670nm VCSEL at +120°C heat sink temperature is demonstrated, where we exceeded 0.5mW and at +160°C still 25μW output power were achieved. We also studied the modulation bandwidth of our devices and achieved 4GHz and calculations lead to a maximum possible intrinsic -3dB frequency of 25GHz.

5 GHz modulation of 650 nm VCSEL

Vertical-cavity surface-emitting laser (VCSEL) emitting at 650 nm are promising candidates as transmitter in optical data communication systems based on polymer optical fibres (POF) because of a narrow minimum in transmission loss of POF at this wavelength. So far, small-signal resonance frequencies fr have been reported for 670 nm devices of 7 GHz [1] and for 656 nm VCSEL of 3.75 GHz [2], respectively. In this paper we discuss theoretically and experimentally the influence of heat dissipation on the modulation behaviour of red 650 nm VCSEL.

Generation of UV laser light via intra-cavity frequency doubling of an AlGaInP-VECSEL

In recent years, vertical external cavity surface-emitting lasers (VECSELs) became an important category of power-scalable semiconductor lasers in a wide range of applications. Examples can be found in many fields of work as biophotonics, television or projectors, spectroscopy or lithography. Using external cavities and optical excitation, VECSELs exhibit high continuous-wave (cw) output power and near-diffraction-limited beam quality with a TEM00 Gaussian beam profile [1]. With an external cavity, the possibility of utilizing intra-cavity optical elements arise. Here, filters and frequency conversion crystals could be used intra-cavity to expand the spectral range of the laser by wavelength tuning or harmonic generation of coherent laser light.

High-frequency electrically driven quantum dot single-photon source

Compact and efficient single-photon sources are key components for several future applications, e.g in quantum cryptography, random number generators, and for a future standard of optical brightness. To date, commercial single-photon detectors provide highest sensitivity in the red spectral range and single-photon based technologies such as quantum communication benefit from their low detector dark count rates. Thus, the red spectral range is suited for free space communication or via polymer optical fibers in last mile networks.

Low threshold electrically pumped red emitting InP/Al 0.20 GaInP quantum dot vertical microcavity laser

Using quantum dots (QDs) as gain medium for semiconductor laser, theory has predicted excellent properties such as low laser thresholds or broad gain spectra [1]. Especially microdisk structures and microcavities, for example vertical-cavity surface-emitting laser (VCSEL), provide very low threshold operation in the submilliampere regime [2] and smallest sensitivity of wavelength and threshold current changes with temperature if QDs were used [3]. For various applications, as for emitters for on-board interconnects or polymer optical fiber communication, it is preferable to operate at short wavelength, especially in the red spectral range.

Temperature Dependent Photoluminescence Measurements of Single InP Quantum Dots

In this work, we have investigated single InP quantum dots by way of photoluminescence measurements. The lower energy dots show emission from excited states at moderate excitation powers while those emitting at higher energies do not even at high power densities. Temperature dependent measurements were carried out with the objective of understanding this difference and to develop a better understanding of the carrier escape from the dots at elevated temperatures. We observed a strong correlation between the electronic level spacings and the activation energies obtained using an arrhenius model, which thereby indicates that the carriers escape via higher lying levels.