In this paper, we demonstrate a high-efficiency, short-cavity heterogeneously integrated C-band DFB laser on a Si waveguide realized using adhesive bonding. First, simulation results regarding the integrated cavity design are discussed. In order to decrease the optical loss inside the cavity, we designed a configuration where the optical mode inside the laser cavity is predominantly confined to the Si waveguide underneath. Then, the fabrication technology of the demonstrated device is explained. Finally, we discuss the measured static and dynamic characteristics of the integrated laser. Up to 13% wall plug efficiency is achieved for a 200 μm long DFB laser diode at 20 ⁰C. Up to two times 6 mW of optical power is coupled into the silicon waveguide and more than 44 dB side-mode suppression ratio is obtained. In addition, the dynamic characteristics of the device are demonstrated by non-return-to-zero on-off keying modulation at 20 Gb/s and the transmission over a 2 km long optical fiber.
Micro-transfer-printing (µTP) enables the intimate integration of diverse non-inherent functionalities on a target substrate and hence allows for the realization of complex photonic integrated circuits (PICs) with small footprint. By employing a polydimethylsiloxane (PDMS) elastomeric stamp with an array of posts, a large number of micro-components can be integrated on a target wafer in one transfer printing operation, which leads to substantial cost reduction of the resulting PICs. This paper discusses the use of µTP for the realization of III-V lasers on Si and SiN PICs and summarizes the recent progress that has been made in this field.
An optical backplane based on Wavelength Division Multiplexing (WDM) for onboard data and signal handling is introduced. It is a tunable transmitter fixed receiver architecture incorporating an NxN Arrayed Waveguide Grating (AWG) element for passive data routing between the nodes. In conjunction with star couplers both unicast and multicast capabilities are offered. The control plane has been implemented on a high-speed FPGA and a four-node demonstrator has been built. Bit-Error-Rate (BER) versus power incident on the receiver, employing three different AWGs, has been measured at a data rate of 10Gbps per link. A total switching time of 500ns has been achieved, leading to more than 95% efficiency with packet lengths greater than 10KBytes.
In the paper, we elaborate our recent work on monolithic (by epitaxial growth) and heterogeneous (by adhesive bonding) integration techniques that may pave the path to the final solutions of IIIV lasers on silicon in different scenarios. In the case of on-chip optical interconnects, a large number of IIIV lasers with high integration density are highly demanded. By using a buffer-less selective growth technique, we are able to grow submicron-sized InP waveguides directly on silicon. All the dislocations are confined at the interface between Si and InP, which leads to the successful demonstration of a distributed feedback (DFB) laser array with good uniformity. Thanks to the minimized buffer layer thickness (20 nm) and the standard top-down laser process flow, it is possible to demonstrate very high integration density of IIIV lasers on silicon. Recently, by growing InGaAs/InP heterostructures on the virtual lattice-matched InP-on-Si template, we are able to achieve room-temperature lasing at communication wavelength range.
On the other hand, the relatively mature bonding based heterogeneous integration technology has been well developed over the last decade, and the integration of various laser configurations on silicon lead to more system level demonstrations. Here, we present our recent work on IIIV-on-Si mode-locked lasers. Thanks to the extremely low silicon waveguide loss, we are able to achieve record-low repetition rate of 1GHz, with an extremely low RF linewidth (sub-kHz). Such devices are promising for applications such as spectroscopy, microwave photonics etc.
We review recent theoretical and experimental work on InP membrane microdisk lasers heterogeneously integrated on SOI and coupled to a Si bus waveguide. After a general introduction on the fabrication and the operation principles, we will describe various improvements in the fabrication technology. This includes improvements in the yield of the bonding of the InP die on the SOI die and in the controllability of the bonding layer thickness, as well as an optimization of the alignment of the microdisk with respect to the silicon waveguide and some proposals for better heat sinking and loss reduction. Improvement in the alignment and the bonding has led to interesting results on the uniformity in device characteristics. In a second part, unidirectional behaviour and reflection sensitivity will be briefly discussed. Theoretical, numerical and experimental results will be shown about the unidirectional behavior and it will be explained how unidirectional microdisk lasers can be a lot less sensitive to external reflections than other lasers. We will also show how such lasers can be used as optical signal regenerators that can work with low optical input powers and that have small power consumption. We will end with a description of demonstrations of optical interconnects based on heterogeneously integrated microdisk lasers and heterogeneously integrated photodetectors. Optical interconnects on chip have been demonstrated at 10 Gb/s. An epitaxial layer stack that contains both the laser and the detector structure has been used for this purpose.
We present analytical, numerical and experimental results about the unidirectional behaviour of semiconductor ring
or disk lasers in which the coupling from the clockwise (CW) mode to the counterclockwise (CCW) mode is different
from the coupling from the counterclockwise to the clockwise mode. The theoretical and numerical results show
different regimes, depending on the gain suppression in the active layer. At very low power, the ratio of the powers of
the CW and CCW modes depends mainly on the ratio of the coupling constants, while at high power gain suppression is
dominant and the ratio of the powers depends on the ratio of the gain suppression and the weakest coupling coefficient.
Our analytical expressions for the unidirectionality are in excellent agreement with numerical results obtained using
coupled rate equations. Some experimental results are given for a microdisk laser coupled to a bus waveguide, on one
side of which an almost 100% reflecting Bragg grating is designed.
Polymer has been considered to be an ideal material option for integrated photonics devices. To measure these devices,
normally the route of horizontal coupling is chosen to couple the light into or out of the polymer waveguide. Due to the
relatively low refractive index, implementing the surface grating coupler in this material system remains to be a
challenge. In this paper, we present a polymer based surface grating coupler. Rather than expensive CMOS fabrication,
the device is fabricated through a simple and fast UV based soft imprint technique utilizing self-developed low loss
polymer material. The coupling efficiency is enhanced by embedding a thin Si3N4 layer between the waveguide core and
under cladding layer. Around -19.8dB insertion loss from single-mode fiber (SMF) to single-mode fiber is obtained for a
straight waveguide with grating coupler at each end. If collected with multi-mode fiber (MMF), it can be reduced to
around -17.3dB. The 3dB bandwidth is 32nm centered at 1550nm. The proposed surface grating coupler and its easy
fabrication method would be attractive for practical applications.
Integrated waveguide microwave photonic filters (MPFs) have the potential to bring down volume, weight, and power consumption of signal processing equipment besides the common advantages of discrete-component-based MPFs. A polysiloxane-liquid polymer-based optical wave-guide microring resonator was designed and fabricated by a simple ultraviolet-based soft-imprint technology, with which the quasi-single-sideband filtering for the 10 to 22 GHz microwave signal was realized and 20 Mbps quadrature phase shift keying signal carried by 14.35 GHz microwave transmission over a 25 km single mode fiber was demonstrated.
Planar integrated optical biosensors are becoming more and more important as they facilitate label-free and real time
monitoring biosensing with high sensitivity. In this paper, the systematic research on one kind of optical biosensor,
based on a resonant principle in a polymer ring resonator, will be presented. Reduced footprint and high sensitivity are
advantages of this kind of biosensor. Rather than expensive CMOS fabrication, the device with high performance is
fabricated through a simple UV based soft imprint technique utilizing self-developed low loss polymer material. The
measurement results for the bulk sensing of a NaCl solution and the surface sensing of a minimal amount of avidin
molecules in a buffered solution will be presented.
Optical interconnect and optical packet switching systems could take advantage of small footprint, low power lasers and
optical logic elements. Microdisk lasers, with a diameter below 10μm and fabricated in InP membranes with a high
index contrast, offer this possibility at the telecom wavelengths. The lasers are fabricated using heterogeneous
integration of InP membranes on silicon-on-insulator (SOI) passive waveguide circuits, which allows to combine the
active elements with compact, high-index contrast passive elements. The lasing mode in such microdisk lasers is a
whispering gallery mode, which can be either in the clockwise (CW) or counter clockwise direction (CCW) or in both.
The coupling to the SOI wire waveguides is through evanescent coupling. Predefined, unidirectional operation can be
achieved by terminating the SOI wires at one end with Bragg gratings. For all-optical flip-flops, the laser operation must
be switchable between CW and CCW, using short optical pulses. Unidirectional operation in either direction is only
possible if the coupling between CW and CCW direction is very small, requiring small sidewall surface roughness, and if
the gain suppression is sufficiently large, requiring large internal power levels. All-optical flip-flops based on microdisk
lasers with diameter of 7.5μm have been demonstrated. They operate with a CW power consumption of a few mW and
switch in 60ps with switching energies as low as 1.8fJ. Operation as all-optical gate has also been demonstrated. The
surface roughness is limited through optimized etching of the disks and the large internal power is obtained through good
heat sink.
HISTORIC aims to develop and test complex photonic integrated circuits containing a relatively large number
of digital photonic elements for use in e.g. all-optical packet switching. These photonic digital units are alloptical
flip-flops based on ultra compact laser diodes, such as microdisk lasers and photonic crystal lasers.
These lasers are fabricated making use of the heterogeneous integration of InP membranes on top of silicon
on insulator (SOI) passive optical circuits. The very small dimensions of the lasers are, at least for some
approaches, possible because of the high index contrast of the InP membranes and by making use of the
extreme accuracy of CMOS processing.
All-optical flip-flops based on heterogeneously integrated microdisk lasers with diameter of 7.5μm have
already been demonstrated. They operate with a CW power consumption of a few mW and can switch in 60ps
with switching energies as low as 1.8 fJ. Their operation as all-optical gate has also been demonstrated.
Work is also on-going to fabricate heterogeneously integrated photonic crystal lasers and all-optical flip-flops
based on such lasers. A lot of attention is given to the electrical pumping of the membrane InP-based photonic
crystal lasers and to the coupling to SOI wire waveguides. Optically pumped photonic crystal lasers coupled
to SOI wires have been demonstrated already.
The all-optical flip-flops and gates will be combined into more complex photonic integrated circuits,
implementing all-optical shift registers, D flip-flops, and other all-optical switching building blocks.
The possibility to integrate a large number of photonic digital units together, but also to integrate them with
compact passive optical routers such as AWGs, opens new perspectives for the design of integrated optical
processors or optical buffers. The project therefore also focuses on designing new architectures for such
optical processing or buffer chips.
Recent progress in research on polymer photonics is reviewed in this paper, including new concepts of polymer-based
photonic materials, components and devices. Novel polymer photonic materials developed in our photonic research
group, polysiloxanes (named as PSQ-Ls), are reported, including two kinds of PSQ-Ls, named as PSQ-LL and PSQ-LH.
These polymer photonic materials are of a liquid and can be cured by UV light irradiation or by heat. The
characterization of the optical films and waveguides based on the novel polymer materials, including refractive index,
birefringence, optical loss and thermal stability, is given in detail. By blending PSQ-LL and PSQ-LH, the refractive
indexes can be tuned linearly from 1.4482 to 1.5212 at 1310nm and from 1.4478 to 1.5198 at 1550nm. The birefringence
is below 0.0005 with the variation of PSQ-LL content. These materials exhibit low optical losses of 0.31dB/cm at a
wavelength of 1310nm and 0.70dB/cm at 1550nm, and high thermal stability with 1% decomposition temperatures of
297°C (in air) and 340°C (in N2) for PSQ-LH, and 313°C (in air) and 370°C (in N2) for PSQ-LL. Optical waveguide
components such as micro-ring resonators and waveguide gratings based on PSQ-Ls are fabricated by
photolithography-etching method and by UV imprint technology, respectively. The experimental measurements show
that the polymer-based micro-ring resonators exhibit an excellent resonant filtering function. Potential applications of the
polymer-based micro-ring resonators for optical communications and optical sensing are discussed.
All-optical flip-flops draw more and more attention as potential parts of all-optical packet or burst switching schemes.
Recently several schemes for such all-optical flip-flops have been proposed, e.g. mutually coupled laser diodes, Mach-Zehnder interferometers with fed back output and ring lasers. All flip-flops are based on bistable behaviour and on the
possible switching between the two stable states using short optical pulses.
Previously, we have shown numerically that a DFB laser diode in which a CW signal is injected can exhibit a bistability
in the laser output power and the amplification of the input power. A condition for this bistability was that the injected
light is not reflected inside the laser diode. The wavelength of the injected light must therefore be not too close to the
Bragg wavelength and the laser must be AR coated.
In this contribution, we present additional modelling results as well as some experimental results for λ/4-shifted DFBs.
We show numerically that for a certain bias current the bistability only occurs when the carrier lifetime and the series
resistance of the laser diode are not too small, such that sufficient spatial hole burning can exist inside the laser.
Experimental results show that bistability is observed in some lasers but not in others, something which may be related to
the series resistance and carrier lifetime.
We will also discuss the dynamic, all-optical flip-flop operation, which is possible by injecting short pulses on either
facet of the laser.
A wavelength division multiplexing (WDM)-based optical backplane architecture is introduced. It is a tunable transmitter fixed receiver (TT-FR) architecture incorporating an N×N arrayed waveguide grating (AWG) element for passive data routing between the nodes, which in conjunction with star couplers, offers both unicast and multicast capabilities. The data and control plane of the network are implemented on a high-speed field programmable gate array (FPGA), and a four-node demonstrator is built up. Three different types of AWG routing elements implemented in different technologies are employed, and bit error rate (BER) versus incident power on the receiver measurements are presented for a data rate of 10 Gbps per link. A total switching time as low as 500 ns is achieved, permitting packet switching operation with more than 95% efficiency when the packet length is greater than 10 kbytes.
We investigate the significance of secondary effects caused by free carrier accumulation and subsequent heating on the nonlinear behaviour of ultrasmall Silicon-On-Insulator ring resonators. All-optical bistability based on thermal dispersion was experimentally obtained for an input power of only 0.28mW. At higher powers, pulsating behaviour was observed which is problematic for the stability of thermal memory and switching operations. Using free carrier dispersion, we also demonstrate all-optical wavelength conversion with a pulse length of 10 ns, indicating that bitrates of 0.1 Gb/s are feasible. Also here, the presence of unstable pulsations was observed, leading to significant errors in the converted data pattern.
Tunable laser diodes with a tuning range of several tens of nanometers are generally being acknowledged as key components for future generation optical networks. However, all presently available devices suffer from several serious drawbacks. The most well-known issue is the time-consuming calibration procedure that has to be carried out for every single device.
Recently the so-called sampled or superstructure grating tunable twin-guide or (S)SG-TTG laser diode has been suggested to overcome some of the prevailing problems. In this paper we will present tuning characteristics of first devices and discuss the influence of facet reflections on the tuning behaviour.
The widely tunable twin-guide laser diodes operate around 1.55 μm and have a continuous tuning range of ~ 2 nm. However, by utilizing Vernier-effect tuning, the overall quasi-continuous tuning range is extended to 28 nm. Within this tuning range, five supermodes can be continuously tuned without the occurrence of any mode hops. The side-mode suppression ratio is kept between 25 and 37 dB.
Widely tunable lasers are generally considered as the transmitters of future WDM optical communications. Electronically tunable edge-emitting laser diodes are of particular interest as they can switch the wavelength in tens of nanoseconds and thus offer great potential for new networking concepts such as optical packet or burst switching, label switching, bandwidth on demand, ... In this paper we discuss new concepts for such widely tunable laser diodes which are studied in the framework of the European IST project NEWTON (NEw Widely Tunable laser diodes for Optical Networks).
Widely tunable lasers are generally considered as key components of future optical communication networks. However, practically all widely tunable lasers that have been fabricated so far suffer from drawbacks, like elaborate calibration procedures that are required for each specific device, low output powers, and limited direct modulation capabilities.
To overcome the aforementioned issues, the sampled or superstructure grating tunable twin-guide or (S)SG-TTG laser diode has been suggested recently. In this paper we will focus on the operation principle, the fabrication, and performance of the first widely tunable twin-guide laser diodes.
The devices operate at ~ 1.55 µm wavelength. By means of Vernier effect tuning, the continuous tuning range of ~ 2 nm is extended to an overall tuning range of 28 nm. Within this tuning range, five supermodes are useable and can be continuously tuned without any mode hops. The side-mode suppression ratio remains between 25 and 37 dB over the whole tuning range. Without any tuning currents applied, a maximum output power of 12 mW has been achieved.
In this paper we theoretically investigate the influence of the injection of a so-called holding beam - a CW beam at the transparency wavelength of the gain medium on the direct modulation properties of semiconductor lasers. Both the small-signal and the large-signal behaviour are investigated. The influence of the holding beam is studied using a simple rate equation model. From the solution of the rate equations it follows that the injection of the holding beam doesn't increase the modulation bandwidth or resonance frequency itself, but that it results in a significant increase of the damping of the relaxation oscillations. Furthermore, it is shown that the chirp (both the adiabatic and dynamic chirp) will be significantly reduced. A few numerical examples - calculated with our CLADISS software - are given to show that the modulation response at not too high bias levels can be significantly improved when a holding beam is applied. For direct, large signal modulation, the injection of a holding beam leads to significantly reduced transient effects and reduced chirping, even at low bias.
A novel multi-longitudinal-mode rate-equations description of the Fabry-Perot type semiconductor laser is presented. The model includes gain dynamics among the longitudinal modes due to e.g. spatial hole burning.
A multimode model is necessary to describe the behavior observed in
a twin stripe diode laser. We will use a single-stripe version of the
device to calibrate the parameters enabling the model to be used in
the description and analysis of the twin-stripe lasers.
Our report focuses on the strong kink found in the P-I output of an asymmetric twin stripe semiconductor laser. A multi longitudinal mode model is used to describe the system. The model allows for asymmetric coupling of the two lasers and also accounts for multi longitudinal effects within and in-between the lasers.
This paper gives an overview of the European ACTS project AC329-ACTUAL. This project was carried out during 1998 and 1999. The stated main objective of the project was: TO sue the state-of-the-art technology on widely tunable lasers available within the Consortium, to bring this to full maturity on a subsystem level and to demonstrate that wavelength agile transmitters based on widely tunable lasers can open the way to the next generation WDM and wavelength switching systems. This paper will give a description of the result from the project.
A novel Optical Decision Circuit (ODC) based on a Mach- Zehnder Interferometer with a Gain Clamped Semiconductor Optical Amplifier in each arm is proposed. Both its static and dynamic response from simulation results demonstrate that the ODC exhibits excellent reshaping capabilities. Also the high speed response of the ODC has been investigated.
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