KEYWORDS: Radio over Fiber, Gain switching, Modulation, Laser frequency, Frequency combs, Modulation frequency, Pulse signals, Laser systems engineering, Photonic integrated circuits, Optical switching
A Delay Differential equation model of a bidirectionally coupled two section comb source is investigated. The model consists of a single mode Primary Laser (PL) bidirectionally coupled to a single mode Secondary Laser (SL). Optical Frequency Combs (OFCs) are then generated in the SL via gain switching. An investigation into the impact of this gain switching frequency on the comb generated in each laser is then performed. Stable, well defined combs are obtained for modulation frequencies close to the Relaxation Oscillation Frequency (ROF) of the SL whilst poorly-defined, likely chaotic combs are produced when gain switching close to the ROF of the PL.
Electro-absorption modulators operate based on the quantum-confined Stark effect (QCSE). In quantum well and existing quantum dot (QD) structures, the use of top and bottom contacts allows application of an electric field along the growth direction. In this work, we theoretically analyse the QCSE in QD structures, investigating whether a lateral field orientation provides an appreciable QCSE sufficient to implement lateral QD-EAMs suitable for integration in photonic integrated circuits. We focus on InAs/GaAs QD structures close to 1300 nm, showing how the dot dimensions and built-in piezoelectric potential impact the calculated absorption spectrum as a function of applied lateral field.
A regrowth-free single-mode laser that is made using standard UV photolithography is reported. The laser achieves a single-mode side-mode suppression ratio of 37 dB, linewidth of 450 kHz, and tunes across 2.9 nm and is suitable for monolithic integration as a distributed feedback replacement, due to a large free spectral range of 60 nm.
In this paper, we demonstrate a regrowth-free monolithically integrated photonics circuit which consists of a tunable, single-frequency, low-linewidth 1x2 multimode-interferometer laser diode (MMILD) and two waveguide devices connected with the two arms of the laser and electrically isolated by deep etched slots. In this photonic integrated circuit (PIC), the 1x2 MMILD showed similar tuning and single frequency performance as the discrete 1x2 MMILD, with more than 30dB side mode suppression ratio (SMSR). The integrated waveguide devices could be used as semiconductor optical amplifiers, photodiodes or electro-absorption modulators, thus the integrated circuit could be used for different functions. The integrability of the MMI lasers indicates its potential applicability in more advanced low-cost topological PICs.
We present a vision for the hybrid integration of advanced transceivers at 1.3 μm wavelength, and the progress done towards this vision in the EU-funded RAPIDO project. The final goal of the project is to make five demonstrators that show the feasibility of the proposed concepts to make optical interconnects and packet-switched optical networks that are scalable to Pb/s systems in data centers and high performance computing. Simplest transceivers are to be made by combining directly modulated InP VCSELs with 12 μm SOI multiplexers to launch, for example, 200 Gbps data into a single polymer waveguide with 4 channels to connect processors on a single line card. For more advanced transceivers we develop novel dilute nitride amplifiers and modulators that are expected to be more power-efficient and temperatureinsensitive than InP devices. These edge-emitting III-V chips are flip-chip bonded on 3 μm SOI chips that also have polarization and temperature independent multiplexers and low-loss coupling to the 12 μm SOI interposers, enabling to launch up to 640 Gbps data into a standard single mode (SM) fiber. In this paper we present a number of experimental results, including low-loss multiplexers on SOI, zero-birefringence Si waveguides, micron-scale mirrors and bends with 0.1 dB loss, direct modulation of VCSELs up to 40 Gbps, ±0.25μm length control for dilute nitride SOA, strong band edge shifts in dilute nitride EAMs and SM polymer waveguides with 0.4 dB/cm loss.
The packaging of high speed Photonic Integrated Circuits (PICs) should maintain the electrical signal integrity. The standard packaging of high speed PICs relies on wire bonds. This is not desirable because wire bonds degrade the quality of the electrical signal. The research presented in this paper proposes to replace wire bonds with an interposer with multilevel transmission lines. By attaching the PIC by flip chip onto the interposer, the use of wire bonds is avoided. The main concern for designing an interposer with multilevel transmission lines is the vertical transition, which must be designed to avoid return and radiation losses. In this paper, a novel design of a high speed vertical transition for Low Temperature Co-fired Ceramic (LTCC) is presented. The proposed vertical transition is simpler than others recently published in the literature, due to eliminating the need for additional ceramic layers or air cavities. A LTCC board was fabricated with several variations of the presented transition to find the optimal dimensions of the structure. The structures were fabricated then characterized and have a 3 dB bandwidth of 37 GHz and an open eye diagram at 44 Gbps. A full wave electromagnetic simulation is described and compared with good agreement to the measurements. The results suggest that an LTCC board with this design can be used for 40 Gbps per channel applications. Keywords: Photonics packaging, Low Temperature Co-Fired Ceramics.
Designing photonic integrated circuits (PICs) with packaging in mind is important since this impacts the performance of the final product. In coherent optical communication applications there are a large number of DC and RF lines that need routed to connect the PIC to the outer packaging. These RF lines should be impedance matched to the devices, isolated from each other, low loss and protected against electromagnetic interference (EMI) over the frequency range of interest to achieve the performance required for the application. Multilevel low temperature co-fired ceramic (LTCC) boards can be used as a carrier board connecting the PIC to the packaging due to its good RF performance, machinability, compatibility with hermetic sealing, and ability to integrate drivers into the board. Flexibility with layer numbers enables additional layers for shielding against electromagnetic interference or increased space for routing electrical connections. In this paper the design, simulations, and measured results for a set of 4 phase matched transmission lines in LTCC that would be used with an IQ MZM are presented. The measured 3dB bandwidth for a set of four phase matched transmission lines for an IQ MZM was measured to be 19.8 GHz.
In this paper, we present a novel 1x2 multi-mode-interferometer-Fabry-Perot (MMI-FP) laser diode, which demonstrated tunable single frequency operation with more than 30dB side mode suppression ratio (SMSR) and a tuning range of 25nm in the C and L bands, as well as a 750 kHz linewidth. These lasers do not require material regrowth and high resolution gratings; resulting in a simpler process that can significantly increase the yield and reduce the cost.
To compensate for velocity mismatch in travelling wave opto-electronic devices, the microwave velocity of the propagating RF signal is reduced by introducing capacitively loaded elements. For high speed operation, these elements must be electrically isolated from one another, which is typically achieved by using ion-implantation to render the p-doped material non-conducting. We propose and demonstrate through optical and electrical simulations that ion-implantation can be avoided by using a quasi-shallow etch to electrically isolate the capacitive elements. High isolation can be achieved using such an etch without introducing additional losses to the propagating optical signal.
In this paper, we demonstrate a novel InGaAlAs/InGaAlAs quantum well multimode-interferometer-Fabry-Perot laser
diode (MMI-FP LD) in which a 1x3 multimode interferometer is inserted into the conventional FP laser waveguide to
generate single wavelength emission. The designed and fabricated laser diode shows a single longitudinal mode lasing
with side mode suppression ratio (SMSR) of 25dBm at a wavelength of 1567nm with driving current of 170mA and can
be tuned over a certain range by adjusting the driving current. A laser diode incorporating a 1x3 MMI and three single
mode waveguide outputs is also proposed which could be potentially used to generate a 3-channel single longitudinal
mode coherent source using injection locking. The simple structure of this single longitudinal mode laser significantly
eases the fabrication processing enabling an increase in the yield and a reduction in the cost compared with the
traditional single mode lasers.
In this paper, a single facet slotted Fabry-Perot (FP) laser is demonstrated to provide tunable, single mode operation and
has been monolithically integrated into a photonic integrated circuit (PIC) with semiconductor optical amplifiers and a
multimode interference coupler. These lasers are designed by incorporating slots into the ridge of traditional FP cavity
lasers to achieve single mode output, integrability and tunability. With the feature size of the slots around 1μm, standard
photolithographic techniques can be used in the fabrication of the devices. This provides a time and cost advantage in
comparison to ebeam or holographic lithography as used for defining gratings in distributed feedback (DFB) or
distrusted Bragg reflector (DBR) lasers, which are typically used in PICs. The competitive integrable single mode laser
also enables the PIC to be fabricated using only one epitaxial growth and one etch process as is done with standard FP
lasers. This process simplicity can reduce the cost and increase the yield.
In this paper, the composition profiles within intermixed AlInGaAs-based multiple quantum wells structure are analyzed
by secondary ion mass spectrometry and the bandgap blue shift is found to be mainly attributed to the interdiffusion of In
and Ga between the quantum wells and barriers. Based on these results, AlInGaAs-based single quantum well structures
with various compressive strain (CS) levels are then investigated and we report an enhancement of the bandgap shift by
increasing the compressive strain level in the SQW. For instance, at an annealing temperature of 850°C, the
photoluminescence blue shift can reach more than 110 nm for the sample with 1.2%-CS SQW, but only 35 nm with
0.4%-CS SQW. The indium composition ratios are designed to be 0.59 and 0.71 for the 0.4% and 1.2%-CS quantum
wells, respectively, as opposed to 0.53 for the lattice-matched barrier. This relatively larger atomic compositional
gradient between the CS quantum well and barrier is expected to facilitate the atomic interdiffusion and lead to the more
pronounced bandgap shift.
The photodiode (PD) is a key component in optical transmission and optical measurement systems which receive optical
signals and convert them into electric signals. High speed, high responsivity, high power and low dark current are
desirable attributes of the PD in these applications, but also a simple fabrication process for high yield and low cost is
essential for industry production. In this paper, an undercut-air-bridge high speed InGaAs/InP PIN structural photodiode
is presented. By utilizing the crystal orientation dependent wet etching of InP material and designing the arms of the
bridge with proper angle, the air bridge was easily obtained, which greatly eased the fabrication. The fabricated devices
with 120μm×3μm ridge waveguides work robustly up to 30GHz in the measurements and potentially faster with
optimized material.
The packaging aspect of the high-speed TO laser module has been investigated in this paper. A conventional TO 56
package is employed for compact and low cost high-speed applications. In the TO header, a special designed RF
substrate is developed to minimize the RF reflection and insert loss. The influence of the feedthrough and the TO leads
on signal transmission is analyzed using the electromagnetic (EM) method. A testing interface PCB based on the tapered
coplanar waveguide (CPW) transmission line is proposed, and simulation results indicate that the coaxial type TO
package has an insert loss of 1.5dB at 10GHz. In order to improve the electronic performance of the TO module, the
equivalent circuit is built and a bandwidth compensation circuit is introduced, and results show that the 3dB bandwidth
of the TO package can extend to 16.3GHz.
Photodiode (PD) is a key component in optical transmission and optical measurement systems. In this paper, we present
the design and fabrication of traveling-wave edge-coupled Unitraveling Carrier (UTC) PD. The fabricated UTC PD with
40μm×5μm waveguide shows 3dB bandwidth 13GHZ and 32GHz under 0 biases and -1V respectively. In parallel, PIN
PD was also fabricated for comparison and only shows 4GHz and 18GHz under same bias conditions. This indicates the
UTC PD is superior to the PIN PD for higher speed operation, especially in application of system without power supply.
We report on quantum well intermixing of AlInGaAs-MQWs using the impurity-free vacancy diffusion method with
dielectric capping layers which has potential for realization of photonic integrated circuits. The extent of the bandgap
shifts with respect to different dielectric capping layers and alloy temperatures are investigated. The intermixing inhibitor
and promoter are then integrated using combination of SiO2 and SiNx dielectric capping layers which shows a
differential photoluminescence wavelength more than 110 nm. Based on this developed intermixing technique, we have
fabricated AlInGaAs-InP based material stripe lasers emitting at two different wavelength ranges centered at 1519 nm
and 1393 nm respectively. Characterizations including the current-voltage and electroluminescence measurements show
that the integration of two-bandgaps can be achieved and furthermore a differential wavelength in lasing spectra up to
120 nm is demonstrated.
An investigation into the carrier and spectral dynamics of a 1.55 μm Buried Tunnel Junction (BTJ) VCSEL
was carried out by examining the emission spectra under high resolution and the voltage across the junction
as polarisation resolved light from a tunable laser source was injected into the cavity. The VCSEL combines
an epitaxial InGaAlAs distributed Bragg reflector with a Si/ZnS dielectric reflector and an oval shaped BTJ
leading to a predominantly single transverse polarisation mode and laser linewidths as low as 20 MHz. Around
lasing threshold and injecting into the primary mode, the voltage required to maintain the current drops due to
stimulated emission and a consequent reduction in the carrier density. Locking behaviour associated with this
characteristic is measured with increased input power. Voltage drops as large as 6 mV are measured. Above
threshold, injection locking is measured in addition to features associated with the relaxation oscillations of the
carriers.
We present an efficient laser welding assembling sequence for minimizing the misalignment from the postweld shift in butterfly packages during fiber-to-semiconductor laser coupling. The misalignment from the postweld shift that arises during the welding of a nickel clip and metal ferrule can be compensated by an optimal initial shift prior to Nd:YAG laser welding and by a mechanical adjustment prior to the laser hammering process. As a result, the number of laser hammering sequences required to compensate for misalignment are significantly reduced.
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