We demonstrate a monostatic LiDAR based on an InP photonic integrated optical phased array (OPA). The system utilizes an OPA with on-chip amplification which transmits and receives light simultaneously through an array of eight end-fire waveguide antennas. The OPA is capable of a 4.6° angular resolution and a 41° field of view. The on-chip amplifiers provide up to 21.5dB gain in a 1465-1600nm wavelength range. We show proof-of-principle FMCW (frequency modulated continuous wave) sensing through the monostatic OPA. The system relies on the frequency modulation with up to 10GHz frequency excursion of an external optically isolated DFB laser, which allows the simultaneous detection of range and velocity. The measurements were performed with a reflective target located ~2m away from the OPA, by varying the target position and velocity of 30 cm and ±5cm/s respectively. To the best of our knowledge, we demonstrated the first monostatic FMCW LiDAR implementation on an integrated InP OPA.
An InP integrated widely tunable laser is investigated for the use as a swept source in optical coherence tomography (OCT) applications. The laser is realized on a generic integration technology platform. It consists of a gain medium and a bandpass filter with 3 cascaded asymmetric Mach-Zehnder interferometers. The additional presence of a balanced Mach-Zehnder modulator as variable out-coupler is instrumental to increase the laser tuning range to 90 nm between 1480 and 1570 nm but can add to additional filtering effects in the laser cavity. In this work, we propose an optimized control strategy for the wavelength calibration of this widely tunable laser source, for a stepwise wavelength scan that is suitable for OCT. The aim is to obtain a wavelength scan with at least 1000 of 10 GHz equally spaced optical frequencies, having uniform power around 100 µW and 1 GHz accuracy. The control strategy is based on the a-priori knowledge of the coarse and the medium filter tuning and on an optimization of the fine filter tuning and the longitudinal cavity mode tuning that can be frequently updated. In this way, the calibration of the laser system can be kept sufficiently accurate and stability of the scan quality can be ensured. With this strategy, 10 GHz spaced optical lasing frequencies are obtained over 30 nm making the calibrated laser suitable as an OCT source
We demonstrate the first on-chip laser frequency comb based on hybrid integration with low-loss Si3N4 waveguide circuits. The laser comprises an InP diode amplifier of which a small fraction is reverse biased for passive locking, while a Si3N4 feedback waveguide extends the optical cavity to a roundtrip length of 15 cm. The generated comb densely covers a 25 nm broad spectrum, at a 3 dB level, with more than 1600 comb-lines at 2 GHz spacing. With such properties, hybrid integrated diode lasers show great promise for widespread use in applications such as integrated microwave photonics or metrology.
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We present results on the investigation of the dynamics of wavelength switching in a monolithically integrated widely tunable semiconductor ring laser for application in swept source optical coherence tomography. In this application wavelength switching within several tens of nanoseconds is desirable to reduce motion blur artefacts during imaging. The device under test is realized in an InGaAsP/InP platform, operates around 1530 nm wavelength and has been shown to have a tuning range over 50 nm. Both measurements and simulations of the wavelength switching behavior of the laser are presented. Tuning is achieved using voltage controlled electro-refractive phase modulators with a response faster than 1 GHz and negligible residual thermal tuning. The fastest switching strategy, of the three that we compare in our simulations, is shown to be the one that relies on rapid power-off of the origin wavelength. The longitudinal cavity mode position, that has an important impact on the switching time and switching stability, is shown to be hard to predict after switching due to gain-phase coupling in the amplifier.
Photonic integration technologies allow for fabrication of on-chip laser sources and systems that provide functionalities for applications beyond telecommunications, such as sensing, healthcare, millimeter and terahertz generation and quantum technologies. New applications impose a different range of demands regarding performance of such semiconductor laser sources. All characteristics of the optical output signal, output power, wavelength tuning range and mechanism, long and short term stability as well as the energy footprint have to be considered. Monolithic integration technologies on indium phosphide substrates natively support an on-chip combination of active and passive functions that enable development of a new class semiconductor lasers with complex cavities. Such lasers can be tailored to achieve optimum performance with respect to a specific application. A number of single frequency, tunable laser sources in form of photonic integrated circuits for applications in gas sensing, optical coherence tomography, millimeter and terahertz generation and quantum applications have been developed at Eindhoven University of Technology. Ongoing research and development activities that address challenges related to addressable wavelength bands, wavelength tuning and stability imposed by specific applications are enabled by mature generic monolithic technology on indium phosphide. In parallel to those efforts, extensive research works towards expansion of accessible wavelength bands. Tunable and mode-locked leaser geometries and challenges related to unique performance expectations are presented.
Step-wise tuning of a monolithically integrated widely tunable continuous wave semiconductor ring laser is investigated, for application in Fourier domain optical coherence tomography (OCT). The device operates around 1530 nm and was realized on an InP generic photonic integration technology platform. The laser is tuned using voltage-controlled electrooptic phase modulators with <100 μW thermal dissipation, which reduces time dependent thermal effects in the filter. Here we present a calibration method with progressively finer wavelength control steps and discuss the limits of wavelength accuracy and repeatability with respect to OCT requirements. It is shown that thermal effects due to light absorption in the phase modulators have a negligible effect on the tuning of the laser for six out of seven phase modulators. To bring the thermal dissipation of the seventh phase modulator in line with the others a design change is proposed. Wavelength switching dynamics are investigated with a numerical model of the laser. A simulation based on this model shows that it takes around 50 ns from the wavelength switching instant to establish a single mode operation with side mode suppression ratio of 30 dB.
Photonic reservoir computing uses recent advances in machine learning, and in particular the reservoir computing algorithm, to carry out complex computations optically. Experimental demonstrations with performance comparable to state of the art digital implementations have been reported. However, most experiments so far were based on sequential processing using time-multiplexing. Parallel architectures promise considerable speedup. Recently, a reservoir computing architecture based on frequency parallelism was proposed by our laboratory, and a preliminary demonstration was carried out using optical fibres. In this system the reservoir is linear and the nonlinearity is provided by readout photodiodes. Here, we study in simulation an implementation of this frequency parallel architecture on an InP chip using a generic integration platform. This would dramatically reduce the footprint and cost of the reservoir. The input signal is encoded by modulating the frequency comb produced by a mode locked laser with a repetition rate of 10GHz. The update rate of the input is 2.5GHz. The reservoir, an active cavity with a time delay of 0.4ns, contains a phase modulator which is driven by a 10GHz RF signal, and a semiconductor amplifier to compensate the losses in the cavity. Readout is carried out by measuring the intensity of individual frequency combs and linearly combining them. We performed time domain simulations on a standard channel equalization task. The simulation takes in to account the phase and amplitude noise of the laser source, and the amplifier noise. The power leakage between neighboring channels at the de-multiplexer is also included. To evaluate the system performance, noise is added as a global parameter on the input signal to assess the SNR requirements. Simulation results show that the laser phase noise is far more important that other types of noise, hence the laser source design/operation should be optimized to achieve low phase noise comb.
RF frequency downconverters are of key importance in communication satellites. Classically, this is implemented using an electronic mixer. In this paper we explore the use of photonic technology to realize the same functionality. The potential advantages of such an approach compared to the classical microwave solutions are that it is lighter weight, has lower power consumption and can be made smaller if photonic technology is used. An additional advantage is the fact that the optical local oscillator (LO) reference can easily be transported over longer distances than the equivalent LO signal in the microwave domain due to the large bandwidth and low loss and dispersion of optical fiber. Another big advantage is that one can envision the use of short pulse trains as the LO – starting off from a sinusoidal RF reference – in order to exploit subsampling. Subsampling avoids the need for high frequency LO references, which is especially valuable if a downconversion over several 10s of GHz is required. In this paper we present the operation principle of such a photonic frequency downconverter and describe the performance of the developed micro-photonic building blocks required for this functionality. These micro-photonic building blocks are implemented on a III-V semiconductor-on-silicon photonic platform. The components include a micro-photonic hybridly modelocked laser, a 30GHz electroabsorption modulator and an intermediate frequency (1.5GHz) photodetector.
In this paper an overview is presented of results obtained with mode-locked semiconductor laser systems that are monolithically integrated using a standardized photonic integration platform based on InP. The laser systems are operating around 1550nm. In this technology platform the basic components that form the laser circuits such as amplifiers, passive waveguides and filters, as well as the semiconductor processing are standardized. Several of the possibilities that such a standardized technology offer are demonstrated by a number of examples of realized devices such as low repetition rate mode-locked lasers, a stabilized comb system and a wide frequency comb source.
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 cost-effective solution to provide higher data rates in wireless communication system is to push carrier wave
frequencies into millimeter wave (MMW) range, where the frequency bands within the E-band and F-band have been
allocated. Photonics is a key technology to generate low phase noise signals, offering methods of generating continuous
MMW with varying performance in terms of frequency bandwidth, tunability, and stability.
Recently, we demonstrated for the first time of our knowledge the generation of a 95-GHz signal by optical heterodyning
of two modes from different channels of a monolithically integrated arrayed waveguide grating multi-wavelength laser
(AWGL). The device uses an arrayed waveguide grating (AWG) as an intra-cavity filter. With up to 16-channel sources
with independent amplifiers and a booster amplifier on the common waveguide, the laser cavity is formed between
cleaved facets of the chip. The two wavelengths required for optical heterodyning are generated activating
simultaneously two channel SOAs and the Boost amplifier.
In this work, we analyze the effect on the dual-wavelength operation of the Boost SOA, which is shared by two
wavelengths. Mapping the optical spectrum, sweeping the two channel and Boost bias currents, we show the interaction
among the different SOAs two find the regions of dual wavelength operation. The size of dual wavelength operation
region depends greatly on the Boost SOA bias level. Initial results of a numerical model of the AWGL will be also
presented, in which a digital filter is used to implement the AWG frequency behavior.
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.
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.
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.
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.
The technique of operating picosecond modelocked laser systems quasi-CW is presented. Higher modelocked output power results at the expense of reduced effective pulse repetition rate. Passively modelocked quasi-CW lasers at 1.0 and 1.3 μm with high 'on-time' powers (~65W) are described. Active stabilization of these systems is demonstrated via negative feedback using an intracavity loss modulator. Stabilization suppresses relaxation-oscillation driven spiking on reaching threshold and prevents Q-switching of the laser caused by the presence of a saturable absorber for modelocking. In this way, the usable parameter range of a saturable absorber for stable CW modelocking is extended such that stable Q-switch free modelocking of a quasi-CW laser with pulse repetition rates over 900MHz, is possible. Numerical modeling confirms the flexible nature of stabilization via negative feedback applied to unstable laser systems incorporating saturable absorbers.
Preliminary investigations into the potential for automatic spatial-mode optimization in tehrmally distorted soild-sate lasers using deformable mirrors are presented. A 37 element adaptive optic mirror has been used intracavity to control the oscillation mode profile of a diode-laser pumped Nd:YVO4 laser. Spatial mode and output power optimization are demonstrated by closed loop computer control of the deformable mirror using a modified hill-climbing algorithm.
The use of semiconductor saturable absorbers has emerged as an enabling technology in modern passively modelocked laser systems. Their application to high power picosecond lasers, most notably Nd-doped lasers, has produced systems with average power levels of a few tens of watts. In this paper, the development of these laser systems to the 100W level and above will be outlined.
A high damage threshold, strain compensated, double quantum well InGaAs saturable Bragg reflector has been developed and successfully used to modelock a high average power, all- solid-state Nd:YVO4 laser. A methodology for obtaining single transverse mode oscillation at high output powers was coupled with a `lens relay' approach to access practical cavity configurations. Ultrashort pulses of 21 ps duration were recorded at a repetition rate of 90 MHz and a diffraction limited average output power of greater than 20 W. By extending the laser resonator to give a pulse repetition rate of 36 MHz, a pulse duration of 25 ps was recorded and the Q-switching instability could be eliminated for all output power levels. In this configuration the peak power and pulse energy were in excess of 24 kW and 0.6 (mu) J respectively.
We report on a side-pumped and passively mode-locked all- solid-state laser. The laser consists of an astigmatically compensated resonator with a saturable Bragg reflector to achieve mode locking and a Brewster-cut Nd:YVO4 rod, which is side-pumped by a diode-laser bar. At 17 W of pump power a fundamental-mode average output power of 4.4 W was attained. Pulses as short as 33 ps have been measured at pulse repetition rates of 235 MHz and 440 MHz. When synchronously pumping an optical parametric oscillator (OPO), these pulse durations lead to wide cavity length detuning tolerances and a comparatively narrow spectral bandwidth of < 15 GHz which is very suitable for applications such as molecular spectroscopy and pollutant detection. A pump depletion of 78% and 1 W of signal output power between 1461 nm and 1601 nm were obtained from an OPO based on periodically poled lithium niobate.
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