28 Gb/s unequalized PAM3 modulation of an 850 nm VCSEL for next-generation datacommunication links

An 850 nm vertical-cavity surface-emitting laser is modulated at 28 Gb/s using pulseamplitude modulation with three levels. Unequalized transmission over 100 m of OM3 MMF is demonstrated, with advantages over NRZ and PAM4 modulation. © OSA 2012.

32 Gb/s multilevel modulation of an 850 nm VCSEL for next-generation datacommunication standards

An 850 nm vertical-cavity surface-emitting laser is modulated at 32 Gb/s using pulseamplitude modulation with four levels. Transmitter predistortion generates an optimized modulation waveform, which requires a receiver bandwidth of only 15 GHz. © OSA/ CLEO 2011.

Chirp-enhanced direct modulation of a monolithic sub-Terahertz dual laser transmitter

A technique enabling 10 Gbps data to be directly modulated onto a monolithic sub-THz dual laser transmitter is proposed. As a result of the laser chirp, the logical zeros of the resultant sub-THz signal have a different peak frequency from that of the logical ones. The signal extinction ratio is therefore enhanced by suppressing the logical zeros with a filter stage at the receiver. With the aid of the chirp-enhanced filtering, an improved extinction ratio can be achieved at moderate modulation current. Hence, 10 GHz modulation bandwidth of the transmitter is predicted without the need for external modulators. In this paper, we demonstrate the operational principle by generating an error-free (bit error rate less than 10-9) 100 Mbps Manchester encoded signal with a centre frequency of 12 GHz within the bandwidth of an envelope detector, whilst direct modulation of a 100 GHz signal at data rates of up to 10 Gbps is simulated by using a transmission line model. This work could be a key technique for enabling monolithic sub-THz transmitters to be readily used in high speed wireless links. © 2013 IEEE.

Gigabit/s modulation of twin-electrode high-brightness tapered laser with high modulation efficiency

Simultaneous high modulation speed and high modulation efficiency operation of a two-electrode tapered laser is reported. 1Gb/s direct data modulation is achieved with 68mA applied current swing for a 0.95W output optical modulation amplitude. © 2009 Optical Society of America.

100 Gb/s PAM4-CAP2 real-time modulation of a single optical source for next-generation datacommunication links

100 Gb/s PAM4-CAP2 modulation is demonstrated for next-generation datacommunication links. Simulation studies indicate a power budget advantage of 2.5 dBo relative to PAM8 modulation. A real-time experimental demonstration is performed. © OSA 2014.

Design and fabrication of polarization-independent micro-ring resonators

Origin of polarization sensitivity of photonic wire waveguides (PWWs) is analysed and the effective refractive indices of two different polarization states are calculated by the three-dimensional full-vector beam propagation method. We find that PWWs are polarization insensitive if the distribution of its refractive index is uniform and the cross section is square. An MRR based on such a polarization-insensitive PWW is fabricated on an 8-inch silicon-on-insulator wafer using 248-nm deep ultraviolet lithography and reactive ion etching. The quasi-TE mode is resonant at 1542.25 nm and 1558.90 nm, and the quasi-TM mode is resonant at 1542.12 nm and 1558.94 nm. The corresponding polarization shift is 0.13 nm at the shorter wavelength and 0.04 nm at the longer wavelength. Thus the fabricated device is polarization independent. The extinction ratio is larger than 10 dB. The 3 dB bandwidth is about 2.5 nm and the Qvalue is about 620 at 1558.90 nm.

Analysis of the additional modulation of semiconductor lasers using optical heterodyne techniques

In this letter, the power spectrum of a cooled distributed feedback laser module is measured using the self-heterodyne technique. Periodical oscillation peaks have been observed in the measurement. Further investigation shows that the additional modulation signal is coupled from the thermal electric cooler (TEC) controller to the laser driver, and then applied to the laser diode. The additional modulation can be eliminated by properly isolating the laser driving source from the TEC controller.

The electronic structure of strained ZnO/MgxZn1-xO superlattices and the influence of polarization

Based on the effective-mass model, the lower energies of the electron and the hole of ZnO/MgxZn1-xO superlattices are calculated. Because of the mismatch of the lattice constant between the ZnO well and the MgxZn1-xO barrier, piezoelectric and spontaneous polarization exist in ZnO/MgxZn1-xO superlattices and a macroscopical internal electric held is found when well width L-w >4 nm and Mg concentration x > 0.2. The parameters of ZnO/MgxZn1-xO superlattices such as lattice constant, band offset, etc. are also proposed. Through calculations, we found the internal electric field can change the lowest energies of the electron and hole to 105.4 and 85.1 meV when well width L-w up to 70 angstrom, which will influence the electronic and optical properties of ZnO/MgxZn1-xO superlattices greatly, while the Rashba effect from the internal electric field is so small that it can be neglected. The ground state exciton energies with different Mg concentration x are also calculated by variational method, our results are very close to the experimental results when Mg concentration x <= 0.3. (C) 2008 Elsevier B.V. All rights reserved.

Effects of polarization field on formation of two-dimensional electron gas in (0001) and (11(2)over-bar0) plane AlGaN/GaN heterostructures

Photoluminescence (PL) and temperature-dependent Hall effect measurements were carried out in (0001) and (11 (2) over bar0) AlGaN/GaN heterostructures grown on sapphire substrates by metalorganic chemical vapor deposition. There are strong spontaneous and piezoelectric electric fields (SPF) along the growth orientation of the (0001) AlGaN/GaN heterostructures. At the same time there are no corresponding SPF along that of the (1120) AlGaN/GaN. A strong PL peak related to the recombination between two-dimensional electron gas (2DEG) and photoexcited holes was observed at 3.258 eV at room temperature in (0001) AlGaN/GaN heterointerfaces while no corresponding PL peak was observed in (11 (2) over bar0). The existence of a 2DEG was observed in (0001) AlGaN/GaN multi-layers with a mobility saturated at 6000 cm(2)/V s below 80 K, whereas a much lower mobility was measured in (11 (2) over bar0). These results indicated that the SPF was the main element to cause the high mobility and high sheet-electron-density 2DEG in AlGaN/GaN heterostructures. (C) 2004 Elsevier B.V. All rights reserved.

Simulation of polarization effects in AlGaN/GaN heterojunction

A method for introducing polarization effects in the simulation of GaN-based heterojunction devices is proposed. A delta doping layer is inserted at the interface of heterojunction and the ionized donors or acceptors act as polarization induced fixed charges. Thus polarization effects can be taken into account in a traditional device simulator. Ga-face and N-face single AlGaN/GaN heterostructures are simulated, and the simulation results show that carrier confinement takes place only in the former structure while not in the latter one. The sheet density of free electrons at the interface of Ga-face AlGaN/GaN increases with the Al composition and the thickness of AlGaN. The consistence of simulation results with the experiments and calculations reported elsewhere shows that this method can effectively introduce polarization effects in the simulation of GaN-based heterojunction devices.

Effect of interaction between periodic delta-doping in both well and barrier layers on modulation of superlattice band structure

The modulation of superlattice band structure via periodic delta-doping in both well and barrier layers have been theoretically investigated, and the importance of interaction between the delta-function potentials in the well layers and those in the barrier layers on SL band structure have been revealed. It is pointed out that the energy dispersion relation Eq. (3) given in [G. Ihm, S.K. Noh, J.I. Lee, J.-S. Hwang, T.W. Kim, Phys. Rev. B 44 (1991) 6266] is an incomplete one, as the interaction between periodic delta-doping in both well and barrier layers had been overlooked. Finally, we have shown numerically that the electron states of a GaAs/Ga0.7Al0.3As superlattice can be altered more efficiently by intelligent tuning the two delta-doping's positions and heights. (c) 2007 Elsevier B.V. All rights reserved.

Occupation modulation of higher subbands in a three-barrier tunnelling structure with a magnetic field

We verify that the magnetic suppression of intersubband LO or LA phonon scattering can give rise to a noticeable nonthermal occupation in higher-lying subbands. This is clearly determined by the relative intensity ratio of the interband photoluminescence spectra for the E-2 - HH1 and E-1 - HH1 transitions. The observed phenomenon may provide an effective method to control the intersubband scattering rate, which is a key factor of the so-called quantum cascade lasers. This is helpful for the population inversion between both the subbands in quantum wells.

Impact of polarization on quasi-two-dimensional exciton and barrier-width dependence of the exciton associated transition in wurtzite III-V nitride quantum wells

Excitonic states in AlxGa1-xN/GaN quantum wells (QWs) are studied within the framework of effective-mass theory. Spontaneous and piezoelectric polarizations are included and their impact on the excitonic states and optical properties are studied. We witnessed a significant blue shift in transition energy when the barrier width decreases and we attributed this to the redistribution of the built-in electric field between well layers and barrier layers. For the exciton the binding energies, we found in narrow QWs that there exists a critical value for barrier width, which demarcates the borderline for quantum confinement effect and the quantum confined Stark effect. Exciton and free carrier radiative lifetimes are estimated by simple argumentation. The calculated results suggest that there are efficient non-radiative mechanisms in narrow barrier QWs. (C) 2002 Elsevier Science Ltd. All rights reserved.