The biggest barrier for many scientific and commercial applications in communications, spectroscopy, life sciences, food safety, biomedicine as well as industrial metrology is the lack of appropriate sources and detectors of THz radiation with enough power and sensitivity with small footprints and portability. Currently available photonic based THz systems have already demonstrated great potential in terms of high tunability, standard room temperature operation, and signal quality, however they are still suffering from many drawbacks, such as big size equipment (needs an optical table), mechanical disturbance (additional to noise and alignment), high power consumption (electrical and optical), and low flexibility system (each application needs a new setup). We therefore propose a new THz system platform, aimed to overcome all the above drawbacks, based on photonic integrated circuits (PICs) and nanotechnology.
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.
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.
The integration of ultra-wideband (UWB)-over- ber into passive optical network (PON) is of great interest as it bene ts the high bandwidth capability from optical network technologies and the high exibility from wireless network technologies. The later can only be done with a reasonable cost when a universal optical transmitter, which is capable of generation both UWB and PON signals, is available. Direct modulation of semiconductor laser was demonstrated to be suitable for high bit-rate PON systems, however the generation of UWB signals by this technique is still challenging. Using the chirp properties of directly modulated semiconductor lasers, UWB signals are generated. Di erent UWB signal waveforms and polarities are obtained. The received electrical spectra conform to the requirements of indoor UWB systems.
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.
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.
We present surface micro-machined micro-electro mechanical-system (MEMS) tunable vertical-cavity surfaceemitting
lasers (VCSEL) with rectangular and triangular shaped quantum wells (QWs) emitting around 1:95 μm
predestined for broadband tunable diode laser absorption spectroscopy. The VCSELs show single-mode operation
and high side-mode suppression-ratio SMSR < 50 dB within the whole tuning range of 50nm and 35 nm, the
fibre-coupled optical power of 1:0mW and 1:76mW and the threshold current of 2:5mA and 2:0mA for the
rectangular and triangular shaped QWs respectively. The 3 dB modulation frequency of the MEMS is 110 Hz.
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.
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.
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.
We present a micro electro-mechanical system (MEMS) tunable vertical-cavity surface-emitting laser (VCSEL) emitting
around 1.55 μm with single-mode output power of >2.5mW and high side-mode suppression-ratio (SMSR) of >50dB
over the entire tuning range of >50nm. The small-signal modulation technique (S21) has been used to study intrinsic and
parasitic influences on the modulation response of the device. Additionally, the static characteristics as well as electrical
and thermal design of the device are discussed with respect to its high-speed modulation behavior. The tunable laser
shows 3-dB direct modulation frequencies above 6.4 GHz.
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.
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.
Self-phase modulation in a highly nonlinear fiber (HNLF) had been demonstrated to have regeneration capabilities when
associated with shifted filtering. In this paper, we show how a saturated semiconductor optical amplifier (SOA) can
enhance this concept by optimally transferring the signal power to the shifted-filter's side of the spectrum. We also
provide a detailed optimization analysis of the filter's detuning and bandwidth by means of numerical simulations. In
this study, we consider an amplitude and phase noise degraded return to zero differential binary phase shift keyed signal
(RZ-DBPSK).
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.
Photonic balancing - a scheme where logically opposite pulses derived from the two outputs of a delay-line demodulator
for phase shift keyed (PSK) signals counter-propagate in the saturated regime of a semiconductor optical amplifier
(SOA) - has been proven to enhance the receiver performance, e.g. in terms of decreased optical signal-to-noise-ratio
(OSNR) requirements for a given target bit error ratio (BER). Here, we extend the photonic balancing scheme towards a
new concept for a regenerative amplifier targeted at extending the reach and/or the number of subscribers in passive
optical networks (PON) in order to support major operators' plans to reduce the number of central offices and access
areas by approximately 90%. For a given target BER, we demonstrate experimentally (a) an 8-dB higher post-amplifier
loss tolerance, (b) an extended feeder line length (75 km) combined with high splitting ratio (10 layers) for a preamplified
version, and (c) high input power variation tolerance (> 30 dB burst-to-burst) in upstream direction as needed
for highly asymmetric tree structures.
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.
We present an optical receiver for RZ-DPSK signals that use photonic balancing. Photonic balancing is achieved through
pulse counter-propagation and collision in a saturated SOA. We explain the principles of photonic balancing and show
how it can lead to an improvement in RZ-DPSK detection by 3 dB, similar to electrical balancing. We also show how
this scheme can be used as a Mamyshev-type regenerator.
This paper investigates serial 100 Gbit/s PM-RZ-DQPSK transmission in the presence of perturbations from neighboring
10 Gbit/s NRZ, 40 Gbit/s RZ-DPSK, and 40 Gbit/s RZ-DQPSK DWDM channels. It addresses the need to outline
upgrade paths of current hybrid DWDM systems equipped with 10 and 40 Gbit/s line cards towards 100 Gbit/s for
remaining channels. A numerical simulation approach is used to evaluate the signal quality of the central probe for
various DWDM channel constellations and power levels.
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.
We generated 2 nJ, ~690 fs pulses with 10 MHz repetition rate from a linear cavity mode-locked Er3+-doped fiber laser with a fiber taper embedded in carbon nanotubes (CNTs)/polymer composite. Evanescent
field out of the taper section can interact with CNTs to see saturation of absorption. With the fiber based
saturable absorber this laser has simple and robust all-fiber configuration comparing to traditional linear
cavity mode-locked lasers with semiconductor saturable absorbers. In addition, we have demonstrated a
mode-locked ring laser, with a similar saturable absorber, by using an ion-exchanged Er3+-Yb3+-codoped
planar waveguide as the gain medium.
In this paper, we propose and experimentally demonstrate a pressure sensor based on birefringent single mode
fiber FP cavity using optical heterodyne. The proof of concept device consists of a light source, a polarizer controller,
a modulator, a RF generator, a single mode fiber Fabry-Perot cavity, a strain inspector, an erbium doped fiber amplifier,
a filter, a polarizer, an optical spectrum analyzer, and a digital communication analyzer. The dynamic range of the
proposed sensor is explored. The results demonstrate the new concept of fiber pressure sensors and the technical
feasibility for pressure measurements.
Future transparent networks require optical regeneration. The regeneration process of amplitude-modulated signals
consists of three key stages: re-amplification, re-shaping, and re-timing. In this Paper we briefly review various optical
clock recovery methods and propose a polarization insensitive low-cost scheme capable of resolving timing information
simultaneously for multiple wavelength channels. We also discuss the prospects of low-cost volume production with
integrated optics and especially with Si-photonics.
Sophisticated modulation formats like phase shift keying (PSK) as discussed for high-speed fiber-optic transmission systems operating at 40 Gbit/s and beyond, cause new challenges for clock recovery. Whereas conventional return-to-zero on-off keyed (RZ-OOK) modulated signals provide proper clock tones which can be used to recover the clock signal, the additional phase modulation (RZ-PSK) changes the spectral composition of the signal and weakens or suppresses the clock tones. This effect is bit pattern dependent as can be seen from a simple example: If all the pulses in an RZ-PSK signal are in phase, the result is equivalent to an ordinary RZ on-off-keying (OOK) signal with a strong carrier and clock tones. If the pulses in a sequence oscillate in phase by &pgr;, the result is equivalent to the well-known carrier-suppressed return-to-zero signal (CS-RZ) where the carrier is suppressed and where clock tones are differently spaced compared to RZ-OOK. In this paper we present results of the simulation of the different cases taking into account realistic bit sequences and analyze the results with special emphasis on the influence the effect has on clock recovery.
Current WDM Systems in optical networks are commonly operated at 2.5 and 10 Gbit/s per wavelength. First transmission systems, offering the possibility of using 40 Gbit/s/λ transponders, are now commercially available. Transaction of various field trials over the last years indicates remarkable interest for this systems among the network operators. Potential further stages of systems at higher channel data rates e.g. 80, 100 or 160 Gbit/s/λ thus attracted more and more attention in the R&D community. Existing concepts like broadband dispersion compensation, distributed Raman amplification, bandwidth efficient and impairment tolerant modulation formats have been applied, but also new circumstances and physical impairments have to be considered, which are negligible at lower data rates. For data rates above 40 Gbit/s chromatic dispersion causes pulses to broaden extremely rapidly, so that transmission behaviour can be regarded as "quasi-linear". We will present a comprehensive overview to what extend system reach limits can be stressed for 160 Gbit/s/λ data rates when different parameters such as fiber type (standard single-mode fiber SMF and non-zero dispersion fiber NZDSF), modulation format (return to zero RZ, carrier suppressed return to zero CS-RZ, intensity modulated differential phase shift keying IM DPSK), different dispersion compensation schemes and signal power levels are optimized. Further, the benefit from using balanced instead of single ended receiver is investigated for IM DPSK.
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.
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