Nanolasers and nanoLEDs are seen as potential sources for low-power optical interconnects. The enhancement of the spontanous emission rate (Purcell effect) related to the small volume has been perceived as a key aspect in the operation of these devices. The fundamental aspects of size scaling in practical nanolasers and nanoLEDs will be discussed in this talk. Firstly, experimental results on nanoLEDs coupled to waveguides on Si will be presented. The effect of surface recombination will be discussed, together with promising passivation methods. In the second part of the talk, a simple theoretical model based on rate equations will be used to investigate the ultimate limits to scaling. In this model, spontaneous and stimulated emission are treated on the same footing, leading to a consistent treatment of the rate enhancement due to the decreasing volume. The analysis shows that Purcell enhancement of spontaneous emission plays a limited role in practical structures, due to the unavoidable linewidth broadening, while the related volume dependence of the stimulated emission rate has a key impact on nanolaser dynamics.
In this paper a generic monolithic photonic integration technology platform and tunable laser devices for gas sensing applications at 2 μm will be presented. The basic set of long wavelength optical functions which is fundamental for a generic photonic integration approach is realized using planar, but-joint, active-passive integration on indium phosphide substrate with active components based on strained InGaAs quantum wells. Using this limited set of basic building blocks a novel geometry, widely tunable laser source was designed and fabricated within the first long wavelength multiproject wafer run. The fabricated laser operates around 2027 nm, covers a record tuning range of 31 nm and is successfully employed in absorption measurements of carbon dioxide. These results demonstrate a fully functional long wavelength photonic integrated circuit that operates at these wavelengths. Moreover, the process steps and material system used for the long wavelength technology are almost identical to the ones which are used in the technology process at 1.5μm which makes it straightforward and hassle-free to transfer to the photonic foundries with existing fabrication lines. The changes from the 1550 nm technology and the trade-offs made in the building block design and layer stack will be discussed.
In this paper we report on the progress in the development of modelocked ring lasers that are integrated on a single chip in the InP/InGaAsP material system. With the current optical integration technology it is possible to integrate quantum well optical amplifiers, phase modulators and passive optical components such as waveguides, splitters and spectral filters as standardized building blocks on a single chip. Using such standardized components a number of passively modelocked ring laser devices have been realized in a standardized fabrication process. Results from a few of these devices are presented here. We have observed a record width of the frequency comb from a modelocked quantum well ring laser operating at a 20 GHz repetition rate. The optical coherent comb is centered around 1542 nm and has a 3 dB bandwidth of 11.5 nm. A minimum pulse width of 900 fs was observed. A second device that is highlighted is a modelocked ring laser with a 2.5 GHz repetition rate. Its 33 mm long cavity is fitted onto a chip of 2.2x1.9 mm2. One of the goals of this work is to make such designs available in device libraries for use in more complex integrated optical systems using standardized technology platforms.
In Europe a number of technology platforms for generic integration are being created for photonic integrated circuits (PICs); in Silicon, in passive dielectrics, and in Indium Phosphide. Such platforms are on the brink of commercialization, they offer a range of calibrated building blocks from which application specific PICs can be built and allow simplified, reduced cost access to a standardised technology, but presently only InP based platforms allow the integration of optical gain blocks; the essential feature of a semiconductor laser. The wavelength is constrained by the platform, usually C-band, but in the near future we expect other wavelengths in the 1.3μm-2.0μm range will be addressed. A frozen platform technology may not seem an ideal starting point for novel laser research but for what may be appear to be lost in epitaxial and process flexibility, much more is gained through a new-found ability to build up complex circuits quickly to deliver new and interesting laser based functionality. Building blocks such as reflectors (a distributed Bragg reflector (DBR) or a multimode interference reflector (MIR)), an amplifier section, and passive waveguides, can be built up by designers into integrated semiconductor lasers of a wide variety of types. This ready integration of novel sources with other circuit functionality can address a wide range of applications in telecoms, datacoms, and fibre based sensing systems. In this paper we describe a number of recent developments on generic InP-based platforms ranging from the fabrication of simple Fabry-Perot lasers, through tuneable DBR lasers, multi-wavelength comb lasers, picosecond pulse lasers and ring lasers.
A new photonic integration technique is presented, based on the use of an indium phosphide membrane on top of a silicon chip. This can provide electronic chips (CMOS) with an added optical layer (IMOS) for resolving the communication bottleneck. A major advantage of InP is the possibility to integrate passive and active components (SOAs, lasers) in a single membrane. In this paper we describe progress achieved in both the passive and active components. For the passive part of the circuit we succeeded to bring the propagation loss of our circuits close to the values obtained with silicon; we achieved propagation loss as low as 3.3 dB/cm through optimization of the lithography and the introduction of C60 (fullerene) in an electro resist. Further we report the smallest polarisation converter reported for membrane waveguides ( <10 μm) with low-loss (< 1 dB from 1520- 1550 nm), > 95% polarisation conversion efficiency over the whole C-band and tolerant fabrication. We also demonstrate an InP-membrane wavelength demultiplexer with a loss of 2.8 dB, a crosstalk level of better than 18 dB and a uniformity over the 8 channels of better than 1.2 dB. For the integration of active components we are testing a twin guide integration scheme. We present our design based on optical and electrical simulations and the fabrication techniques.
In this paper we show that using a DVS-BCB adhesive bonding process compact heterogeneously integrated III-V/silicon single mode lasers can be realized. Two new designs were implemented: in a first design a multimode interferometer coupler (MMI) – ring resonator combination is used to provide a comb-like reflection spectrum, while in a second design a triplet-ring reflector design is used to obtain the same. A broadband silicon Bragg grating reflector is implemented on the other side of the cavity. The III-V optical amplifier is heterogeneously integrated on the 400nm thick silicon waveguide layer, which is compatible with high-performance modulator designs and allows for efficient coupling to a standard 220nm high index contrast silicon waveguide layer. In order to make the optical coupling efficient, both the III-V waveguide and the silicon waveguide are tapered, with a tip width of the III-V waveguide of around 500nm. The III-V thin film optical amplifier is implemented as a 3μm wide mesa etched through to the n-type InP contact layer. In this particular device implementation the amplifier section was 500μm long. mW-level waveguide coupled output power at 20°C and a side mode suppression ratio of more than 40dB is obtained.
Application Specific Photonic Integrated Circuits (ASPICs) are considered key elements to make photonic systems or subsystems cheap and ubiquitous. ASPICs still are several orders of magnitude more expensive than their microelectronic counterpart: ASICS, which has restricted their application to a few niche markets. A novel approach in photonic integration is emerging that will reduce the R&D costs of ASPICs by more than an order of magnitude. It will bring the application of ASPICs that integrate complex and advanced photonic functionality on a single chip within reach for a large number of small and larger companies and initiate a breakthrough in the application of Photonic ICs. In this paper the process is explained. A significant number of designs has been realized the last 4 years, for a variety of applications in telecoms, datacoms, medical and sensing, from parties all over the world.
Integrated spot size converters (SSCs) are key components for efficient coupling between Photonic Integrated Circuits (PICs) and fibre-arrays. We report a compact SSC which is suitable for integration into dense arrays with a pitch down to 25 μm and compatible with our generic InP-based platform technology, which supports integration of SOAs and Electro Optical Modulators with a range of passive components. The small pitch supports coupling tens of on-chip optical waveguide ports to fiber arrays via a low-loss dielectric interposer chip. The density allows the design of a customized optical bus between the InP PIC and the interposer chip. The dielectric chip may simply expand to the pitch of a fiber array but also contain low-loss passive circuitry like delay-lines, high Q-filters and multiplexers. The latter enables the formation of a hybrid integration platform with our InP-based technology. Efficient coupling is obtained by adiabatically transforming the sub-micron modes of the InP waveguides to the 3 μm diameter mode of the interposer. We tested our SSCs by coupling to a lensed fibre with a mode field diameter of 2.5 μm. Coupling losses were found to be as low as 0.6 dB per fiber chip coupling for device lengths of a few 100 μm. We also measured the crosstalk from one input port to output ports adjacent to the targeted output port. We present simple design rules for reducing the crosstalk to neighbouring output ports below -50 dB. The quality and uniformity of the SSCs is demonstrated by fabrication of an 8 x 8 AWG demultiplexer between two SSC arrays placed at input and output ports. We measured an insertion loss between fibres of 4 dB for the central channel of the AWG, which is record low for an InP-based device.
An understanding of parasitic effects is essential to maximize the performance of a Photonic Integrated Circuit (PIC). Using
a circuit simulator, we are able to model mode conversion at the interface between straight and bent waveguides, parasitic
reflections in multi-mode interference couplers (MMIs), interference between multiple modes, residual facet reflections,
and reflections at junctions between components. Even though these effects are usually low in intensity, around -20 dB to
-30 dB from the main signal level, they can still have a strong influence on the circuit performance. This is because the
mentioned parasitic effects are coherent with the desired signal and interference between them is therefore a field effect.
By analyzing three different circuits, and comparing the results to measurements, we show that these effects need to be
carefully managed in order to ensure circuit performance. The circuits we investigate are a Fabry-Perot cavity, a Mach-
Zehnder interferometric structure, and a Michelson interferometer. Especially residual reflections coming from angled
facets and back-reflections in MMIs are shown to be the main parasitic effects in the investigated circuits.
In this paper we present recent results obtained in the area of monolithically integrated modelocked semiconductor laser systems using generic InP based integration platform technology operating around 1550nm. Standardized components defined in this technology platform can be used to design and realize short pulse lasers and optical pulse shapers. This makes that these devices can be realized on wafers that can contain many other devices. In the area of short pulse lasers we report design studies based on measured optical amplifier performance data. This work has the ultimate goal to establish a library of widely applicable short pulse laser designs. Such lasers can include components for e.g. wavelength control. An important boundary condition on the laser design is that the laser can be located anywhere on the InP chip. In the area of pulse shaping we report on a 20 channel monolithic pulse shaper capable of phase and amplitude control in each channel. Special attention is given to the calibration of the phase modulator and amplifier settings. Pulse compression and manipulation of pulse generated from modelocked semiconductor lasers is demonstrated using a 40 GHz quantum dash modelocked laser.
Brillouin scattering can be used for determining strain distributions along fibers. This is a convenient way to monitor the structural integrity of large constructions. However, the high cost of the optical circuitry has prevented wide use of this technique. We report on a design study on an integrated optical circuit for a Brillouin optical time domain reflection readout unit with low-cost potential. The circuit contains narrow linewidth tuneable distributed Bragg reflector (DBR) lasers, photodiodes with an optical mixer for coherent detection, a 10-bit digitally switched delay line for frequency tuning, and a switching fabric that allows three modes of operation. Such a circuit can be realized through one of the upcoming foundries in the field of photonic integration.
In this paper we investigate options for monolithically integrated multiwavelength transmitters in indium phosphidebased
materials. In particular, we focus on transmitters that use arrayed waveguide gratings as wavelength selective
elements. The multiwavelength lasers that simultaneously emit on different wavelength channels are crucial in Fiber-tothe-
Home systems because they increase the bandwidth and the transmission capacity of such optical networks.
In this paper we report the design, fabrication, simulation and characterization of a novel discretely
tunable laser based on filtered feedback. This Integrated Filtered-Feedback Tunable Laser (IFF-TL) device
combines a simple and robust switching algorithm with good wavelength stability. It consists of a Fabry-Perot
laser with deeply-etched broadband DBR mirrors. Single mode operation is achieved by using feedback from an
integrated filter. This filter contains an AWG wavelength router and an SOA gate array. A rate equation model
predicts that a properly designed device can switch within 1 ns, while characterization measurements show a
value of only 4 ns. The fast switching and reduced control complexity makes the device very promising for various
advanced applications in optical telecommunication networks.
Arrayed waveguide grating (AWG) devices play a crucial role in wavelength division multiplexing (WDM) networks
and links. AWGs are key building blocks in multi-wavelength receivers and transmitters, wavelength routers, add-drop
multiplexers and optical crossconnects. AWG size becomes a critical issue when they are used in higher complexity
photonic integrated circuits. The last years have shown a steady reduction of AWG device dimensions in silica-on-silicon,
silicon-on-insulator and InP-based technologies. Extremely compact AWGs with good performance are feasible and allow
for a significant reduction in cost, when integrated with other components in photonic integrated circuits.
The importance of polarization manipulation is increased as optical fiber systems evolve to higher data rates. Photonic integrated circuits should be insensitive to the state of polarization of the light emanating from fibers if they are to be used as detectors, add-drop mutiplexers or cross connects. Either all the integrated components have to be polarization independent or only one polarization direction should be used. For either method, a compact polarization converter is useful. A model is developed for the mode propagation in the waveguide of the slanted side polarization converter based on double hetero structures. The model is extended to include polarization dependent absorption and mismatch. Polarization converters on different double heterostructures (with and without quantum wells) have been designed and are realized by contact optical lithography. The performance of the realized converters is well described with the model.
In this paper an overview is given of the results we have obtained at the COBRA Research Institute in our work on
passively modelocked semiconductor lasers operating in the 1.5 μm wavelength region. Most results concern
modelocked ring lasers that are realized monolithically in the InP/InGaAsP materials system as well as simulations using
lumped element and traveling wave type models. The experimental results show that the ring lasers appear as the more
stable type of lasers. The modeling results show the importance of using a symmetrical configuration in the ring laser for
stable operation. Most recent results on linear modelocked quantum dot lasers at 1.5 μm indicate the improvements
possible using these materials.
A tunable Mach-Zehnder wavelength duplexer has been realized based on P-i-n-N InGaAsP/InP. It has been
made polarization insensitive by proper wafer layer stack and proper waveguide geometry. The layer stack for
the duplexer was tested first with a waveguide phase shifter, which resulted in up to 36°/(V•mm) phase shifting
efficiency for TE polarization, which is slightly more efficient than the most efficient phase shifter reported to
date in bulk InP at 1.55 μm, and with much lower transmission loss[1]. The transmission loss was measured to
be 4 dB/cm (5 dB/cm) for TE (TM) polarized light, for 2 μm wide shallowly etched waveguides, which is rather
low compared to other reported high efficiency phase shifters for this material system. With this layerstack, we
designed a Mach-Zehnder (MZ) duplexer with narrow, 1.5 μm wide, deeply etched phase shifters that meet the
polarization insensitivity requirement. The measurement results showed that the phase shifting efficiency of this
narrow and deeply etched duplexer is up to 34°/(V•mm) for both TE and TM polarization, and the transmission
loss of this 1.5 μm wide waveguide is about 10 dB/cm for both TE and TM polarization. This is also the first
reported deeply etched narrow phase shifter with high phase shifting efficiency and relatively low loss.
Lasing and sharp line emission in the 1.55-μm wavelength region is demonstrated from ensembles and single InAs
quantum dots (QDs) embedded in InGaAsP on InP (100) by metalorganic vapor phase epitaxy (MOVPE). Wavelength tuning of the QDs is achieved through the insertion of ultra-thin (1-2 monolayers) GaAs interlayers underneath the
InAs QDs. To increase the active volume widely-stacked QD layers are identically reproduced. Closely-stacked QDs
reveal unpolarized emission from the cleaved side due to vertical electronic coupling which is important for polarization
insensitive semiconductor optical amplifiers. Fabry-Perot narrow ridge-waveguide lasers implementing five layers of
widely-stacked QDs as gain medium operate in continuous wave mode at room temperature with low threshold current,
low transparency current density of 6 A/cm2 per QD layer, and low loss of 4.2 cm-1, which are accompanied by a 80 nm
wide gain spectrum. Device performance does not suffer from sidewall recombination in deeply-etched QD lasers which
possess similar threshold currents as shallowly-etched ones and do not deteriorate with time. This allows the fabrication
of mono-mode and compact devices with small bending radii, as demonstrated by the operation of a QD ring laser with
40-GHz free spectral range. Micro-PL of single QDs exhibits sharp exciton-biexciton emission around 1.55 μm
persisting to temperatures above 70 K; the prerequisite for single photon sources working at liquid nitrogen temperature
for fiber-based quantum information and cryptography systems.
We present some progress in the field of optical signal processing that could be utilized in all-optical packet switching. We demonstrate error-free 160 Gb/s optical wavelength conversion employing a single semiconductor optical amplifier. The gain recovery time of the semiconductor optical amplifier is greater than 90 ps. Assisted by an optical bandpass filter, an effective recovery time of 3 ps is achieved in the wavelength converter, which ensures 160 Gb/s operation. This optical wavelength converter can be controlled by a monolithically integrated optical flip-flop memory to route 80 Gb/s data-packets all-optically. The routing is realized without electronic control. The integrated optical flip-flop is based on two-coupled lasers, exhibits single-mode operation, has 35 dB contrast ratio between the states and switches state in about 2 ns. We demonstrate that the integrated flip-flop is able to control the optical wavelength converter up to 160 Gb/s. The system is capable of routing 80 Gb/s data packets with duration of 35 ns, separated by 15 ns of guard time.
In this paper we first present a brief overview of our work on indium phoshide integrated optical circuits. Integrated circuits can be produced that contain active components such as optical amplifiers and passive component such as waveguides, arrayed waveguide gratings and phase modulators. With this set of components complete laser systems can be designed and realized on a chip. Then we will present in what way our integration technology can be used to generate and utilize ultrafast optical pulses. Issues concerning the realization, operation and future developments will be discussed.
Arrayed waveguide gratings and MMI couplers are key components in Photonic Integrated Circuits. However, mass application of PICs still has to come. The road to such a broad application is reviewed.
In this paper we present a simulation strategy for the accurate prediction of the functionality of an InP based opto-electronic modulator. The device is composed by an InP-InGaAsP p-i-n diode embedded in a rib waveguide and a Mach-Zehnder interferometer. Finite Element Analysis for both semiconductor and optical equations solution is exploited. We present numerical results indicating that with a 2 mm-long device a reverse bias of 11 V is needed for a 180° phase shift.
In this paper we present a simulation strategy for the accurate prediction of the functionality of an InP based optoelectronic modulator. The device is composed of an InP/InGaAsP p-i-n diode embedded in a rib waveguide and a Mach-Zehnder interferometer. Finite Element Analysis for both semiconductor and optical equations solution is exploited. The presented numerical results, indicating a reverse bias voltage of 5.5 V for a 180° phase shift in a 2 mm-long device, are confirmed by measured data. Transient simulation predicts that this structure is suitable for 40 Gbit/s operation.
Ridge waveguides with smooth and vertical sidewalls are essential in photonic circuits. We have investigated waveguide realization with reactive ion etching of InP and InP-based structures using a SiNx in a Cl2/H2/CH4 chemistry in an ICP plasma. Depending on ICP power and RF power, etching rates can be obtained from 200 nm/min up to > 2μm/min. A maximum etching selectivity of InP vs SiNx of 12 was obtained at 2000 W of ICP power. Deep etched waveguides, fabricated in an InP/InGaAsP double heterostructure, show typical losses of 2 dB/cm. This low value shows the potential of ICP technique in the fabrication of photonic circuits.
In this paper we present a scheme for integration of a number of key devices for WDM-networks based on three components. The elementary components, such as phased array demultiplexers, multi mode interference (MMI)-couplers and Mach-Zehnder Interferometer optical switches, are explained and experimental results are presented on integrated Multiwavelength (MW) add-drop multiplexers, MW-optical crossconnects. The integration of optical amplifiers and detectors to MW-sources and MW-receivers is briefly discussed.
This paper describes the implementation and investigation of an all-optical amplified ring network with Phased Array based optical add/drop multiplexers (OADMs). From crosstalk analysis follows that an OADM with a foldback-structure and 1 X 2-switches has an outstanding crosstalk performance. From the investigation of the dynamic behavior of Erbium doped fiber amplifiers (EDFAs) in a ring configuration we found that EDFAs in a ring-configuration require a faster gain-control when compared to a cascade-configuration.
A scattering matrix oriented CAD tool is presented for design and simulation of photonic integrated circuits. In a scattering matrix approach each component is represented by a scattering matrix which describes the signal transfer between the different ports of the component. These components are usually waveguides. As an example of a more complicated component simulation of an optical PHASAR demultiplexer will be described and the potential of the approach will be illustrated in simulating an integrated add-drop multiplexer and comparing the results with measurement data of a realized device.
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