Current communication networks needs to keep up with the exponential growth of today’s internet traffic, and telecommunications industry is looking for radically new integrated photonics components for new generation optical networks. We at National Research Council (NRC) Canada have successfully developed nanostructure InAs/InP quantum dot (QD) coherence comb lasers (CCLs) around 1.55 m. Unlike uniform semiconductor layers in most telecommunication lasers, in these QD CCLs light is emitted and amplified by millions of semiconductor QDs less than 60 nm in diameter. Each QD acts like an isolated light source acting independently of its neighbours, and each QD emits light at its own unique wavelength. The end result is a QD CCL is more stable and has ultra-low timing jitter. But most importantly, a single QD CCL can simultaneously produce 50 or more separate laser beams at distinct wavelengths over the telecommunications C-band. Utilizing those unique properties we have put considerable effort well to design, grow and fabricate InAs/InP QD gain materials. After our integrated packaging and using electrical feedback-loop control systems, we have successfully demonstrated ultra-low intensity and phase noise, frequency-stabilized integrated QD CCLs with the repetition rates from 10 GHz to 100 GHz and the total output power up to 60 mW at room temperature. We have investigated their relative intensity noises, phase noises, RF beating signals and other performance of both filtered individual channel and the whole CCLs. Those highly phase-coherence comb lasers are the promising candidates for flexible bandwidth terabit coherent optical networks and signal processing applications.
The gain media of the quantum dot lasers consist of InAs QDs in an InGaAsP matrix on an InP substrate. The
quantum dot lasers have different free spacing ranges (FSRs) corresponding to Fabry-Pérot (F-P) cavity lengths. A
silicon ring resonator and a QD laser have been combined to form comb laser. The output characteristics of the
combined comb laser were investigated. The measured FSR was about 2.8nm and the extinction ratio was about
10(dB) when the FSR of the QD laser was about 0.4nm and the FSR of the ring resonator was about 0.47nm. The
experimental results show that the ring resonator had a strong control on the FSR and extinction ratio of the comb
Linewidth enhancement factor (LEF) of InAs/InP quantum dot (QD) multi-wavelength lasers (MWL) emitting at 1.53 μm
are investigated both above and below threshold. Above threshold, LEFs at three different wavelengths around the gain
peak by injection locking technique are obtained to be 1.63, 1.37 and 1.59, respectively. Then by Hakki-Paoli method LEF
is found to decrease with increased current and shows a value of less than 1 below threshold. These small LEF values have
confirmed our InAs/InP QDs are perfect gain materials for laser devices around 1.5 μm.
We have designed and fabricated a quantum dot (QD) gain medium which consists of InAs QDs in an InGaAsP matrix
on an InP substrate. By using these InAs/InP QD layers, we have generated femtosecond (fs) pulses with pulse duration
of 295 fs from a single-section monolithic Fabry-Perot (F-P) cavity at the repetition rate of 50 GHz around 1560 nm
wavelength range without any external pulse compression. The average output power is 40.1 mW at the injection current
of 200 mA. Optical signal-to-noise ratio (OSNR) of the proposed QD mode-locked laser (QD-MLL) is up to 50 dB. The
lasing threshold current and the external differential quantum efficiency are 23 mA and 30 %, respectively. And the
mode beating linewidth was measured to be less than 20 KHz. We have interpreted that several nonlinear optical effects
related to interaction of QD excitons with intracavity laser fields could create nonlinear dispersion to compensate
intracavity linear dispersion. So total dispersion is minimized and four-wave mixing (FWM) is dramatically enhanced
within QD F-P cavity. If spectral bandwidth is broad enough, tens or hundreds of longitudinal modes would lase and
their phases would be locked together through FWM process. Eventually a train of fs pulses with a repetition rate
corresponding to cavity round-trip time is generated.
Single-mode laser diodes on GaSb substrates were developed using InGaAsSb/AlGaAsSb triple quantum well
active regions grown by molecular beam epitaxy. The devices were fabricated
using lateral Cr gratings, with a grating pitch designed to coincide with a strong absorption feature of HF gas, deposited adjacent to a dry-etched narrow ridge waveguide.
High sidemode suppression was achieved, and in 20°C continuous-wave operation, devices with a 400μm-long cavity provided 4.5mW total output power
at the 2396nm target wavelength.
Anti-reflection and high-reflection facet coatings exhibited no deleterious effects on the laser tunability or mode quality, thus allowing
the preferential extraction of output power from a single laser facet.
We have demonstrated a novel approach to achieve a stable multi-wavelength laser system (MWLS) which is making
use of a quantum dot semiconductor optical amplifier (QD SOA) as a highly birefringence material and an optical
polarizer at the same time. Both the channel frequency spacing and the central lasing wavelength of the QD MWLS can
be accurately set by using the desired-designed QD SOA with the certain operation conditions and by setting the
polarization controller properly. The detailed working principles and the experimental results have been reported in this
paper. The proposed QD MWLS technology can be used for characterizing the intrinsic properties of the QD
semiconductor waveguide materials that could also be used for spectral narrowing of a laser system. We have
experimentally confirmed that the QD SOA is highly inhomogeneous gain material as compared with QW SOA.
We have demonstrated femtosecond pulses from a passive single-section monolithic InAs/InP quantum-dot (QD)
semiconductor laser with the active length of 456 µm and the ridge width of 2.5 µm in the C-band wavelength range
from 1528 nm to 1565 nm. The transform-limited Gaussian-pulses are generated at the 92-GHz repetition rate with the
312-fs pulse duration without any pulse compression scheme. The average output power is larger than 13.2 mW for the
injection current of 60 mA. And the lasing threshold current and external differential quantum efficiency are 17.2 mA
and 38%, respectively. The mode-beating linewidth of the proposed QD mode-locked laser (MLL) was measured less
than 20 KHz. We have interpreted that four-wave-mixing (FWM) process and other nonlinear effects within the QD
waveguide gain materials make the major contributions to lock the phase the longitudinal modes of the QD Fabry-Perot
cavity together to achieve this strong self-pulsation process.
We have designed, fabricated and characterized self-assembled InAs/InGaAsP QD-waveguide devices around 1.55 μm.
In order to obtain optimal performance, we have investigated several QD-based semiconductor optical amplifiers
(SOAs) / lasers with different core geometry and doped profiles. To make the fair comparison between QD-SOA and
QW-SOA, InAs/InGaAsP QW-SOAs with the same structure and the doped profiles have been designed and
characterized. The experimental results indicate the QD-SOA is much better than QW-SOA in term of optical spectral
bandwidth, temperature sensitivity and output power stability. The
3-dB and 10-dB bandwidths of the amplified
spontaneous emission (ASE) spectra of the QD-SOA are 150 nm and 300 nm around 1520 nm. By using CW pump and
probe signals we have demonstrated a non-degenerated four-wave mixing (ND-FWM) process and the experimental
results indicate that the asymmetry of the FWM conversion efficiencies is eliminated by using the QD-SOA. To make
use of the inhomogeneous broadening which is one of the specific properties of QD waveguide devices, we have
designed and investigated the QD-based multi-wavelength semiconductor laser. A stable multi-wavelength laser output
with a 93-channel multi-wavelength laser with maximum channel intensity non-uniformity of 3-dB were demonstrated
on the basis of a single InAs/InGaAsP QD F-P cavity chip. All channels were ultra-stable because of the inhomogeneous
gain broadening due to statistically distributed sizes and geometries of self-assembled QDs.
The room-temperature 1.55 &mgr;m continuous-wave (CW) operation of single-lateral mode GaInNAsSb ridge
waveguide lasers grown on GaAs is reported.
Detailed measurements of the light output power and spectral properties were used to assess the device characteristics
as a function of applied current and temperature in both CW and pulsed operation. An exemplary, 3&mgr;×750&mgr;m,
device with a 92% high-reflectivity back facet coating exhibited a record low CW threshold current of 63~mA, with a peak output power of 15~mW.
High-resolution modal gain spectra were extracted from amplified spontaneous emission measurements yielding the
internal loss (8.0~cm-1, transparency current (50~mA) and the wavelength dependence of the differential gain.
The latter was used with careful measurements of the Fabry-Perot mode shift with injection current to determine
the linewidth enhancement factor of 2.8 at the transparency current. The first measurement of intrinsic modulation frequency in 1.55 &mgr;m GaInNAsSb lasers is
reported, based on the observed relative intensity noise (RIN). The RIN measurements indicate a maximum modulation frequency of 7.2~GHz,
which is a promising result for future telecommunications applications.
The properties of a 1.3μm GaInNAs Double Quantum Well (QW) ridge waveguide (RWG) laser have been systematically studied for GaAs based uncooled long wavelength lasers. The threshold current, transparency current, optical gain, internal loss and quantum efficiency characteristics were assessed by light-current (L-I) measurement using devices with different geometries. Measurements of gain spectra versus injection current and temperature were taken and used to analyze GaInNAs as an active material in terms of gain, loss and transparency. The experimental observations are discussed. The results are compared with those obtained from lasers made by other conventional materials. The high characteristic temperature (T0=155K from 20°C to 75°C) and comparable stimulated emission to InP based lasers offer the promise of application as a light source for low cost data communication systems.
Self-assembled quantum dot (QD) Semiconductor Optical Amplifiers (SOAs) are believed to have faster carrier recovery times than conventional multiple quantum well, or bulk SOAs. It is therefore of interest to study the carrier dynamics of QD SOAs to assess their potential as ultrafast nonlinear devices for switching and signal processing. In this work we report experimental characterization of the ultrafast carrier dynamics of a novel InAs/InGaAsP self-assembled QD SOA with its peak gain in the important 1.55 μm telecommunications wavelength range. The temporal dynamics are measured with a heterodyne pump-probe technique with 150 fs resolution. The measurements show carrier heating dynamics with lifetimes of 0.5-2.5 ps, and a 13.2 ps gain recovery, making the device a promising candidate for ultrafast switching applications. The results are compared to previous reports on QD amplifiers operating in the 1.3 μm and 1.1 μm spectral regions. This report represents the first study of the temporal dynamics of a QD SOA operating at 1.55 μm.
High hydrostatic pressure can be used for wavelength tuning of semiconductor laser diodes in a wide spectral range. Coupling the laser with external grating leads to wavelength tuning within the gain spectrum (i.e. in a narrower range than with pressure) but allows for a narrow emission line and nearly continuous tuning (mode-hop free if anti-reflecting coating is applied). Here we demonstrate a combination of pressure and external-resonator tuning for the GaInNAs laser emitting at 1343 nm at ambient conditions. Using the specially designed liquid pressure cell working up to 20 kbar we shift the emission down to 1170 nm while the external grating (used in Littrow configuration) allows for fine tuning in the ~10 nm range (at each pressure).
Transparency current density (Jtr) was studied in GaInNAs ridge waveguide lasers. The devices employ Ga1-xInxNyAs1-y multiple quantum wells and were grown on GaAs substrates using solid-source molecular beam epitaxy (MBE) with an RF plasma cell. The transparency current density is sensitive to material quality: defects, traps and other sources of non-radiative recombination. It is also dependent on the rate of thermionic emission from quantum wells. Wavelength, polarization and temperature dependence of transparency carrier density of annealed material was studied. Record low transparency carrier densities of 20 and 90 A/cm2/well were observed (for TM and TE polarizations) in devices based on GaInNAs material designed for emission at 1340 nm after optimized rapid thermal annealing. This low value of Jtr confirms the excellent quality of the GaInNAs material and demonstrates that GaInNAs lasers with excellent material properties can be grown for long wavelength applications provided appropriate annealing is applied. It is believed that the low transparency current density is a unique feature of GaInNAs and is due to the band structure and band alignment of the material system.
Progress in optical switching technology currently faces several major obstacles. One of these is high power consumption, which quickly multiplies in cascaded switch configurations. As well, many implementations have long switching speeds and large footprints. An improved compact 1x2 digital optical switch (DOS) in InGaAsP/InP is presented, with experimental results compared to numerical modeling. The Y-junction waveguide switch operation is based on reconfiguration of output waveguide arms by carrier injection at the electrodes. We present experimental results of DOS fabricated with InGaAsP cores having bandgaps of 1.2um, 1.3um, and 1.4um. The results are compared with calculations of refractive index change versus carrier concentration in the different InGaAsP alloys. Additionally, wafer layer structure and waveguide parameters were redesigned to decrease, respectively, power dissipation and optical confinement. Switching current is significantly reduced from greater than 100 mA to about 20 mA, which not only provides power savings, but also results in less thermal overshoot in the switched optical pulse. The DOS has a measured switching contrast ratio of better than 12.5 dB, with a transition time of less than 5ns. Polarization dependence of switching contrast is also explored.
A compact Y-junction waveguide switch with electrically reconfigurable output waveguide arms is demonstrated in InGaAsP/InP. Simulations indicate that the plasma effect or the thermo-optic effect can be used as the active switching mechanism, as corroborated by experimental tests. For the plasma effect the induced index change under the electrode, Δn, is negative. The Y-junction device has a measured switch contrast ratio ~ 20 dB, with a response time of ~ 5 ns. Using the thermo-optic effect Δn is positive and the observed contrast ratio is better than 10 dB. The highly localized nature of the thermal gradient in these devices yields thermo-optic switching into the hundred of nanoseconds range, several orders of magnitude faster than the overall thermal response time. This is the fastest thermo-optic switch reported to date. Fabrication of these switches, and in particular the use of O+-ion implantation to provide electrical isolation of the waveguide branches, is described.
We demonstrate Digital Optical Switches (DOS) on InP based on carrier-injection and on the quantum-confined Stark effect (QCSE). The active waveguide core is composed of either a double-heterostructure of InGaAsP for carrier-injection or a InGaAs-InGaAsP for reverse bias operation. O-Ion implantation was employed to isolate the branches of the DOS instead of the usual isolation by etching employed elsewhere.
Silica-based photonic integrated circuits (PICs) have been making major advances and are finding increasing applications in optical communications, networking, and signal processing. For the next generation of photonic integrated circuits, it is desirable to add more functionality as well as to increase the integration level. This would involve introducing a variety of heterogenous materials and devices on the same substrates, using a monolithic and/or hybrid integration method. In this paper we describe the results of our efforts of developing/incorporating new functions to the silica-based integrated circuits. 1) Optical amplifiers, suitable for monolithic integration with other guided-optic devices, are promising as loss-compensating devices for photonic integrated circuits. 2) Silicon is the most commonly used substrate for silica-based PICs. A novel method has been developed for forming 2D waveguides on silicon substrates, utilizing the photoelastic effect in Si induced by thin-film stress. This method does not require any separate guiding layer nor etching of silicon, and therefore is expected to increase the flexibility in designing/implementing advanced PICs on Si. 3) Ferroelectric materials possess various functional properties and are expected to play an important role in advanced PICs. The major challenges and progress are discussed in achieving monolithic integration of functional films, such as PZT, on silica and Si substrates.
We report stress-induced channel waveguides formed in epitaxial BaTiO3 films. BaTiO3 epitaxial films (doped with and without erbium) were grown on MgO (001) single- crystal substrates using rf magnetron sputtering. In the channel waveguides developed, the lateral confinement of light is achieved via the photoelastic effect in BaTiO3 induced by thin-film stress. As a stress-inducing film, a 0.5- micrometer-thick SiO2 film was sputter-deposited on top of a 3.0-micrometer-thick BaTiO3 film with a 7 to 10- micrometer-wide window opening. The fabricated structures were characterized in terms of their guided mode profiles at 1.3 and 1.55 micrometer wavelength. The measurement result clearly shows both the lateral and vertical confinement of light in the channel region. The stress distribution in the channel structure was calculated by solving the coupled equations that describe the elastomechanical and piezoelectric effects in the ferroelectric material. The refractive index changes were than calculated taking into account both the photoelastic and electro-optic effects of BaTiO3. The simulation results show a good agreement with the measurement results. The waveguide structure developed in this work does not require etching of BaTiO3, and is expected to be useful as a simple and economical method for forming channel waveguides with other ferroelectric films as well.
In this work, we proposed a new waveguide structure that can be formed on bulk semiconductor substrates without requiring any epitaxial or separate cladding layers for vertical confinement of light. In the proposed structure, vertical confinement of light is achieved via a photoelastic effect induced by thin-film stress, and lateral confinement is obtained by a semiconductor mesa or a photoelastic effect itself. We have carried out numerical analyses on the stress distribution, dielectric constant changes, and mode profiles at 1.3 micrometer or 1.55 micrometer wavelength in GaAs or Si. The results show that the proposed structure can support guided modes with the amount of stress that can be obtained from typical thin-film/semiconductor interfaces. To demonstrate the proposed waveguide concept, we fabricated the photoelastic waveguides on bulk GaAs substrate. The fabricated structures were characterized in terms of their guided mode profiles, using ga 1.3 micrometer wavelength semiconductor laser as a light source. Both the vertical and the horizontal profiles were obtained, and the results show a good agreement with the simulation results, thus confirming the proposed concept.