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Nano photonics

In this field we investigate the use of semiconductor nanostructures in novel photonic devices. These novel devices are mode coupled lasers, semiconductor amplifiers, vertical emitting lasers, emitter of single and crossed photons on the basis of quantum dots for data communications and encryption. The development of special synchronizable femto-seconds laser systems for components of future THz-data networks and busses is another focus of our research.

Edge emitter

Semiconductor lasers with quantum dots

Abbildung 1: Schematischer Querschnitt eines Kantenemitters.

Two types of lasers are used for optical data transmission: edge emitters and surface emitters. Edge emitters are easier to produce and provide higher optical output power than surface emitters but their wavers need to be divided into pieces of several mm², the single laser diodes. That is why testing and packaging of these devices into modules is expensive.

The layer structure of the laser diodes is grown epitactically (with MOVCD) using different semiconductor materials. The structure has to fulfill two tasks: it has to guide light (“optical waveguide“) and it has to be a pn-junction for injection and recombination of charge carriers. The standard structure fulfilling both tasks is the double-heterostructure (nobel prize 2000 Prof. Zh. Alferov) with separate optical wave guiding. The quantum dots (QDs) are placed into the active zone of the semiconductor laser.

The edge emitters mirrors' which form the laser cavity are achieved by cleaving the semiconductor wafer along a crystal axis. The reflectivity of these crystal facets depends on the index of refraction of the semiconductor material. For GaAs this reflectivity is about 32%. In some cases the facets are additionally coated with highly reflecting or anti reflecting layers.


Directly modulated lasers

Abbildung 3: Prinzip der digitalen optischen Datenübertragung mit einem direktmodulierten Laser.

For transmission of digital optical data in optical fiber networks digital electrical signals have to be transformed into optical signals (light pulses). This fundamental process takes place in a semiconductor laser or semiconductor-LED. Figure 3 schematically shows the electrical signal (left) and the resulting optical signal (right). In this case, the modulation is achieved by directly modulating the laser diodes pump current.

Mode-coupled quantum dot semiconductor lasers (generation of short pulses)

Abbildung 4: Methoden zur Kurzpulserzeugung mit Halbleiterlasern.
Abbildung 5: Prinzip der optischen Datenübertragung mit einem modengekoppelten Laser.

There are a few complementary methods for generation of short optical pulses (fs – ps) with semiconductor lasers (SL), as shown in Figure 4.

The quality switching (Q-Switching, QS) is based on controlled manipulation of the cavity losses of the laser diode after the electrical pulse. This can be realized with a saturable absorber section (passive QS) or by an externally controlled loss section (an electrically controlled absorber section for example, active QS). The sudden reduction of the cavity loss leads to a situation in which the laser is driven above threshold and emits an optical pulse of a length depending among other factors on the number of charge carriers stored in the laser diode.

Mode-coupling is based on the phase-locked coupling of the longitudinal modes of a laser diode and the resulting modal interference, which generates a pulse train of shortest optical pulses. Active mode-coupling is realized with external electrical modulation. Passive and hybrid mode-coupling requires an absorber section (electrically controlled) similar to Q-switching.

For data transmission with high bit rates sources of short optical pulses are used as comb generator for the frequency range above 5 Ghz. For this a modulator is connected in series with a mode-coupled laser transferring the electrical data stream into an optical data stream. (see: Figure 5)

The rate of data transmission depends on the laser's mode-coupling frequency. For applications as wavelength- and time-multiplexes the shortness, temporal stability (jitter) and spectral width of the optical pulses plays an important role.

In principle with mode-coupled lasers optical pulses with pulse widths below 1 ps can be realized. In order to minimize the effects of dispersion in the glass fiber and the interaction of different wavelength channels, “fourier limited” optical pulses are aspired. In this case the product of spectral width a temporal length, connected by the nature of light, is minimal.

Mode-coupled lasers are particularly suitable for applications requiring optical pulse width in the range of pico seconds and high repetition rates above 5 Ghz.

Compared to mode-coupled lasers semiconductor lasers used in the q-switch modus possess the advantage of a variable repetition rate. However the minimal pulse width is larger than those of mode-coupled lasers.


Optical amplifiers

Schematische Darstellung eines Quantenpunkt-SOA mit Glasfaserkopplung.

Optical amplifiers are used to amplify continuous and dynamic optical signals. There are different types of optical amplifiers, for example based on doped glass fibers (erbium doped optical amplifiers, EDFA) or based on semiconductor material. The advantages of semiconductor optical amplifiers (SOAs) are small size and reduced production costs. In addition they are easily to be combined with other optoelectronic components. In particular in the so called O-band (1260 nm to 1360 nm) there are hardly any commercially available and cheap alternatives to SOAs.

The design of a SOA is similar to a semiconductor laser. The semiconductor gain material is electrically pumped to achieve amplification by stimulated emission. Because of special processing of the material it can be achieved that the light will be guided in a waveguide.

In contrast to a laser lasing shall be suppressed. If light is coupled from outside into the SOA, it will be amplified (stimulated emission) during a single transition, without feedback through reflection at the facets (travelling wave amplifier). The suppression of the resonator is realized by anti-reflection coating of the facets. Light injection is realized by special glass fibers (tapers). They are also used to couple the amplified light, after transition trough the SOA, back into a glass fiber.

A field of application for SOAs is optical data communication networks. High-speed data transmissions for internet applications for example are nowadays realized optically via glass fiber networks. Mainly two wavelength bands (1.3 µm and 1.5 µm) are used, which lie in the minimum of dispersion and absorption of conventional glass fibers.

This workgroup investigates SOAs for the wavelength range of 1.3 µm which shall be used in metropolitan area networks (MAN). These networks consist of a sender, probably several dividers to reach different receivers, the receivers and the interconnected amplifiers.


High Brightness Lasers

Modellliertes Nahfeld eines Laser-Barrens aus 9 Einzelemittern
Verlauf des Brechungsindex eines PBC-Wellenleiters (links). Der rechte Teil zeigt ein elektronenmikroskopisches Bild einer epitaktisch gewachsenen Laserstruktur.

Lasers have a wide range of applications, its application fields expand constantly nowadays. Because of their superior properties compared to other types of lasers, it would be desirable to cover additional areas of applications by semiconductor laser diodes. A drawback here is the still not sufficient beam quality of today's high-power diode laser modules especially at high output powers, which typically consist of one or more multimode broad area lasers.
Bars of broad area lasers lasers reach output powers in excess of 100 watts per bar, wherein the beam quality is very small particularly in lateral direction with beam parameter products around 500 mm*mrad. However, a high brightness in addition to high output power is absolutely essential for many applications. In particular, direct applications such as welding, cutting and drilling is only possible with high brightness, and thus a good focusing of the laser beam is needed. A maximum brightness and thus an optimum focusing ability at a given performance is achieved by single-mode emission with a Gaussian beam profile, any deviation from the Gaussian shape, however, leads to a broadening of the beam.

Narrow stripe lasers in which the laser stripe is so narrow that no high order modes can appear, provide single-mode emission. Here, however, the maximum output power is limited by the occurrence of catastrophic optical mirror damage due to high optical power density at the facet. However, increasing the optical field by simply extending the waveguide can result in the occurrence of higher order modes, which leads to a reduction of brightness. PBC lasers (PBC: Photonic Band Crystal) have a vertically very broad waveguide and thus a very large expansion of the vertical near-field, which achieves both extremely narrow far-field divergences as well as very high total output power. The appearance of higher vertical modes is suppressed by growth of alternating layers with different refractive index in the waveguide, acting as a photonic crystal. PBC laser so far showed extremely narrow vertical beam divergences of down to 5 ° and very high single-mode output power up to 10.7 W in pulsed operation.

Surface emitters


Abbildung 2: Simulation eines Feldes von VCSELn. Die unteren und oberen Bragg-Spiegel sind deutlich zu erkennen.

A „vertical cavity surface emitting laser“ (VCSEL) as the name says is a laser emitting from the surface. In VCSEL-structures two bragg mirrors, underneath and above the active zone, form the cavity of the laser. VCSELs are bonded from above. That is why in VCSEL production there is no need of cleaving the wafer allowing parallel production of many devices.

QD-lasers have unique features:

  • extremely low threshold current densities
  • reduced temperature dependence of the threshold current
  • increased radiation resistance and suppressed degradation of facets
  • reduced alpha-factor
  • total suppression of transversal fundamental mode filamentation
  • improvement of stability regarding external optical feedback

Nowadays conventional high frequency laser modules with fiber connection need a stabilization of temperature of the laser diode realized with a Peltier element, a temperature sensor and external temperature controller and power supply. Due to the reduced temperature dependence of quantum dot lasers in the temperature range from -40°C to 80°C such temperature stabilization can be unnecessary in some cases. This reduces module packaging costs and current consumption.

Single photon emitters

Abb.7: Ein EPE, dessen Struktur auf dem VCSEL-Konzept aufsetzt. Über eine strompfadbegrenzende Apertur oberhalb eines QPes wird der Einzel-QP-Betrieb realisiert.

One of the most important challenges of the communication branch is to improve the safety of data transmission. Actual secure data transmission is based on the application of encryption algorithms. Their keys are transmitted unencrypted via conventional channels. Because of this secret bugging operations are possible. In the same time decoding procedures keep up with encryption because of growing computing power and more efficient algorithms. The only real secure solution consists of realizing quantum cryptographic data transmission procedures. Quantum cryptography is superior to conventional encryption algorithms because it theoretically assures absolute protection against bugging-operations. That is why there is a high demand on small, efficient quantum cryptographic devices which are plug and play capable. The central device of a quantum cryptographic emitter unit is a single photon emitter (SPE) with defined polarization of the photons. Concerning the properties of polarization two different approaches are promising. One is single photons with defined linear polarization, the other cascades of polarization entangled photons. Central element of a SPE is a quantized system with discrete energy levels. Therefore isolated atoms, molecules and quantum dots can be used. Quantum dots have an important advantage: in contrast to isolated atoms non-resonant excitement of the quantum dot's discrete energy levels is possible. By embedding quantum dots in an appropriate semiconductor structure electrically driven devices can be realized. The most often used approach to a semiconductor SPE very much resembles a VCSEL-structure as shown in figure 7. In contrast to a VCSEL only single quantum dots shall contribute to electrically triggered emission of single photons. Therefore only a single quantum dot shall be pumped electrically which can be realized by restricting the current path.

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