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II-VI compounds are promising materials for the fabrication of room-temperature terahertz devices due to their beneficial properties like as type-I conduction band alignment, high breakdown field strength (~331 kV/cm for ZnSe vs. ~80 kV/cm for GaAs), and higher values of the conduction band offset (1.5 eV for BeSe/ZnSe vs. 0.7 eV for AlAs/GaAs). In this paper we report on numerical study of the resonant tunneling transport in ZnBeSe/ZnSe/ZnBeSe symmetric and asymmetric resonant tunneling diodes (RTDs). The negative differential resistance feature is observed in the current-voltage characteristics of the ZnSe-based RTDs. It is found that the maximum of peak-to-valley ratio (PVR) of the current density is equal to 6.025 and 7.144 at 150 K, and to 1.120 and 1.105 at 300 K for the symmetric and asymmetric RTDs, respectively. The effect of barrier heights on the frequency and output power performance of RTD devices are studied and discussed.

In this paper an on-chip device capable of wavelength-selective generation of vortex beams is demonstrated. The device is realized by integrating a spiral phase-plate onto a MEMS tunable Fabry-Perot filter. This vortex-MEMS filter, being capable of functioning simultaneously in wavelength and orbital angular momentum (OAM) domains at around 1550 nm, is considered as a compact, robust and cost-effective solution for simultaneous OAM- and WDM optical communications. Experimental spectra for azimuthal orders 1, 2 and 3 show OAM state purity >92% across 30 nm wavelength range. A demonstration of multi-channel transmission is carried out as a proof of concept.

In this work, we have used a tunable VCSEL for high-speed optical data transmission. To obtain wide tunability, a MEMS-DBR is surface micromachined onto a short-cavity high-speed VCSEL operating at 1550 nm. Ultra-wide continuous tuning is realized with electro-thermal actuation of the MEMS with built-in stress gradient within SiOx/SiNy dielectric layers. The MEMS-VCSEL operates in single-mode with SMSR > 40 dB across the entire tuning range. Quasi-error-free transmission of direct-modulation at record 15 Gbps is reported for 20 nm tuning, showing the potential towards the standard requirements for the SFP+ modules in the tail-ends of the WDM transmission system.

With the use of SiO2/SiC based movable MEMS-DBR, the continuous tuning range of electrically pumped MEMS-VCSEL can be extended to > 140 nm. The high refractive index contrast of Δn > 1 between SiO2 and SiC reduces the needed number of layers (11 layers) and broadens the spectral width of the reflectivity (448nm for R > 99.5 %) by more than a factor of two compared to the material system SiO2/Si3N4 (23 layers / 216nm for R > 99.5 %), which has been used for the current world record continuous tuning range of 100nm of an electrically pumped MEMS-VCSEL. The smaller number of needed DBR-layers enables a significant reduction of the overall mirror thickness, which enables a further miniaturization of the device and thus an increase of the free spectral range (FSR), the ultimate limit for continuous wavelength tuning. In this paper we evaluate the performance advantages of using SiO2/SiC based MEMS-DBR for tunable VCSEL by using Transfer-matrix method simulations.

This paper experimentally investigates power budget extension configurations for WDM NG-PONs. Differential Phase Shift Keying (DPSK), and Differential Quadrature Phase Shift Keying (DQPSK) are considered. The budget enhancement techniques are based on Semiconductor Optical Amplifier (SOA). The paper thoroughly studies power budget enhancement for the two modulation formats and shows that the proposed configurations comply with current standards such as XG-PON1.

As demand for higher bandwidth is drastically increasing, bandwidth efficiency is going to be an issue in Passive Optical Networks (PONs). Moreover, network operators plan to reduce the number of central offices (COs) while extending the reach of the optical links, this enables them to reduce the deployment costs. This paper demonstrates the advantages of advanced modulation formats and a suitable configuration for the power budget enhancement of hybrid Wavelength Division Multiplexing-Time Division Multiplexing-Passive Optical Networks (WDM-TDM-PONs). The proposed technique can offer higher data rate, better bandwidth efficiency, and large number of customers. Differential (Quadrature) Phase Shift Keying (DQPSK) signals and Orthogonal Frequency Division Multiplexing (OFDM) will be considered. Simulations are performed using different modulation formats to evaluate the behavior of the proposed PON extender. Finally, Transmission of 1 Tbps WDM/TDM-OFDMPON over 60 km optical link is presented here. The simulation results, prove that 1600 users can be covered with 40 Gbps peak data rate.

In this paper, we demonstrate for the first time the far-field experimental results and the linewidth characteris- tics for widely tunable surface-micromachined micro-electro-mechanical system (MEMS) vertical-cavity surface- emitting lasers (VCSELs) operating at 1550 nm. The fundamental Gaussian mode emission is confirmed by optimizing the radius of curvature of top distributed Bragg reflector (DBR) membrane and by choosing an ap- propriate diameter of circular buried tunnel junctions (BTJs) so that only the fundamental Gaussian mode can sustain. For these VCSELs, a mode-hop free continuous tuning over 100 nm has already been demonstrated, which is achieved by electro-thermal tuning of the MEMS mirror. The fiber-coupled optical power of 2mW over the entire tuning range has been reported. The singlemode laser emission has more than 40 dB of side-mode suppression ratio (SMSR). The smallest linewidth achieved with these of MEMS tunable VCSELs is 98MHz which is one order of magnitude higher than that of fixed-wavelength VCSELs.

A mode hop free single mode tuning < 90nm at 40°C and 45nm at 70°C is demonstrated with a MEMS tunable VCSEL for the first time. The device shows a fiber-coupled output power of 2.9mW at 20°C and 0.5mW at 70°C. The side mode suppression ratio is larger than 40 dB over the entire tuning and temperature range of up to 70°C. The presented technology is cost effective and thus capable for mass production. It is applicable for tuneable VCSELs operating in different wavelength regimes, which are limited by the absorption of the DBR materials only.

Chirp-managed lasers (CML) are demonstrated as simple low-cost transmitter with high tolerance to chromatic dispersion. This manuscript proposes the use of CML as cost-effective downstream (DS) transmitters for next generation access networks. The laser chirp, which is the main drawback limiting the transmission performance of directly modulated lasers, is now used to generate differential phase-shift keying (DPSK) modulation format by direct modulation. The network architecture using CML as downstream DPSK transmitter is proposed. Bit­ error-rate measurement showed that an optical power budget of 36 dB could be obtained with direct phase-shift­ keying modulation ofCML which proves that the proposed solution is a strong candidate for future WDM-PONs. Budget-extended WDM-PON configuration is also demonstrated using Saturated Collision Amplifier, which is an amplification scheme that uses SOA saturation in order to maximize the output power and minimize the ASE noise and the polarization sensitivity. The extension scheme is demonstrated for four-wavelength 10 Gbit/s unidirectional downstream configuration with 60-dB maximum total optical budget for each wavelength.

This paper shows optical power extension techniques for 10 Gbit/s per A. channel for WDM-PON DPSK systems. The scheme is based on semiconductor optical amplifiers. We present 56.5 dB total optical power budget enhancement using our configuration for the downstream scenario. The setup is cost effective in terms of optical components. Only one DLI (Delay Line Interferometer) is used to convert DPSK signals to OOK. We present experimentally as well as through simulations that our scheme has better performance than a single SOA as a power budget extender.

Long-wavelength VCSELs with emission wavelengths beyond 1.3 μm have seen a remarkable progress over the last decade. This success has been accomplished by using highly advanced device concepts which effectively overcome the fundamental technological drawbacks related with long-wavelength VCSELs such as inferior thermal properties and allow for the realization of lasers with striking device performance. In this presentation, we will give an overview on the state of the technology for long-wavelength VCSELs in conjunction with their opportunities in applications for optical sensing. While VCSELs based on InP are limited to maximum emission wavelengths around 2.3 μm, even longer wavelengths up to the mid-infrared range beyond 3 μm can be achieved with VCSELs based on GaSb. For near-infrared InP-based VCSELs, the output characteristics include sub-mA threshold currents, up to several milliwatts of singlemode output power and ultralow power consumption. New concepts for widely tunable VCSELs with tuning ranges up to 100 nm independent from the material system for the active region are also presented. Today, optical sensing by Tunable Diode Laser Spectroscopy is a fast emerging market. Gas sensing systems are used for a wide range of applications such as industrial process control, environmental monitoring and safety applications. With their inherent and compared to other laser types superior properties including enhanced current tuning rates, wavelength tuning ranges, modulation frequencies and power consumption, long-wavelength VCSELs are regarded as key components for TDLS applications.

We report the investigation of the state of polarization (SOP) of a tunable vertical-cavity surface-emitting laser (VCSEL) operating near 850 nm with a mode-hop free single-mode tuning range of about 12 nm and an amplitude modulation bandwidth of about 5 GHz. In addition, the effect of a sub-wavelength grating on the device and its influence on the polarization stability and polarization switching has been investigated. The VCSEL with an integrated sub-wavelength grating shows a stable SOP with a polarization mode suppression ratio (PMSR) more than 35 dB during the tuning.

We present surface micro-machined tunable vertical-cavity surface-emitting lasers (VCSELs) operating around 1550nm with tuning ranges up to 100nm and side mode suppression ratios beyond 40 dB. The output power reaches 3.5mW at 1555 nm. The electro-thermal and the electro-statical actuation of a micro electro-mechanical system (MEMS) movable distributed Bragg reflector (DBR) membrane increases/decreases the cavity length which shifts the resonant wavelength of the cavity to higher/lower values. The wavelength is modulated with 200 Hz/120 kHz. Both tuning mechanisms can be used simultaneously within the same device. The newly developed surface micro-machining technology uses competitive dielectric materials for the MEMS, deposited with low temperature plasma enhanced chemical vapor deposition (PECVD), which is cost effective and capable for on wafer mass production.

Widely tunable vertical cavity surface emitting lasers (VCSEL) are of high interest for optical communications, gas spectroscopy and fiber-Bragg-grating measurements. In this paper we present tunable VCSEL operating at wavelength around 850 nm and 1550 nm with tuning ranges up to 20 nm and 76 nm respectively. The first versions of VCSEL operating at 1550 nm with 76 nm tuning range and an output power of 1.3mW were not designed for high speed modulation, but for applications where only stable continious tuning is essential (e.g. gas sensing). The next step was the design of non tunable VCSEL showing high speed modulation frequencies of 10 GHz with side mode supression ratios beyond 50 dB. The latest version of these devices show record output powers of 6.7mW at 20 °C and 3mW at 80 °C. The emphasis of our present and future work lies on the combination of both technologies. The tunable VCSEL operating in the 850 nm-region reaches a modulation bandwidth of 5.5GHz with an output power of 0.8mW.

A nonlinear optical loop mirror with a bidirectional attenuator has been used for regeneration of return-to-zero differential phase-shift-keyed (RZ-DPSK) signals. A 2.5 ps, 10 Gb/s signal with amplitude fluctuations of 28 % was regenerated with a negative power penalty of 2 dB practically back to the quality of the undistorted reference signal. Parameters limiting system performance and optimization possibilities will be discussed.

Due to the doubling of the internet traffic every twelve month and upgrading existing optical metro-, regio- and long haul transport networks, the migration from existing networks toward high speed optical networks with channel data rates up to 100 Gbit/s/λ is one of the most important questions today and in the near future. Current WDM Systems in photonic networks are commonly operated at linerates of 2.5 and 10 Gbit/s/λ and major carriers already started the deployment of 40 Gbit/s/λ services. Due to the inherent increase of the bandwidth per channel, limitations due to linear and non-linear transmission impairments become stronger resulting in a highly increased complexity of link engineering, potentially increasing the operational expenditures (OPEX). Researchers, system vendors and -operators focus on investigations, targeting the relaxation of constraints for 100 Gbit/s transmission to find the most efficient upgrade strategies. The approaches towards increased robustness against signal distortions are the transmission of the 100 Gbit/s data signals via multiple fibers, wavelength, subcarriers or the introduction of more advanced modulation formats. Different modulation schemes and reduced baud rates show strongly different optical WDM transmission characteristics. The choice of the appropriate format does not only depend on the technical requirements, but also on economical considerations as an increased transmitter- and receiver-complexity will drive the transponder price. This article presents investigations on different approaches for the upgrade of existing metro-/ regio and long haul transport networks. The robustness against the main degrading physical effects and economy of scale are considered for different mitigation strategies.

With increasing line rates system tolerances due to both, chromatic dispersion and dispersion slope, become more restrictive. By means of numerical simulations we investigate the influence of temperature fluctuations in cascaded multi-section dispersion-compensated standard single-mode fiber SMF based transmission systems. Eye opening penalty (EOP) for different modulation formats - non-return-to-zero NRZ as well as return-to-zero RZ with duty cycles varying from 0.2 to 0.6 and for different signal power levels varying from -9 to 3 dBm at a line rate of 160 Gbit/s are calculated. Performance degradation due to temperature-induced dispersion variations is investigated for three different scenarios: a) the temperature of the installed terrestrial fiber-optic cable changes, b) the temperature of the temperature-controlled indoor dispersion compensating module DCM changes, c) combined effects.