Integrated Laser Sources Research Articles

Frequency stabilization of C-band semiconductor lasers through a SiN photonic integrated circuit

Integrated semiconductor lasers represent essential building blocks for integrated optical components and circuits and their stability in frequency is fundamental for the development of numerous frontier applications and engineering tasks. When dense optical circuits are considered, the stability of integrated laser sources can be impaired by the thermal cross-talk generated by the action of neighboring components, leading to a deterioration of the long-term system performance (on the scale of seconds). In this work we show the design and the experimental characterization of a silicon nitride photonic integrated circuit (PIC) that is able to frequency stabilize 16 semiconductor lasers, simultaneously. A stabilized 50 GHz-spaced two-channel system is demonstrated through the detection of the related beating note and the stability of the resulting waveform is characterized via the use of artificially induced thermal cross-talk stimuli.

On-Chip Lasers for Silicon Photonics

With the growing trend in the information industry, silicon photonics technology has been explored in both academia and industry and utilized for high-bandwidth data transmission. Thanks to the benefits of silicon, such as high refractive index contrast with its oxides, low loss, substantial thermal–optical effect, and compatibility with CMOS, a range of passive and active photonic devices have been demonstrated, including waveguides, modulators, photodetectors, and lasers. The most challenging aspect remains to be the on-chip laser source, whose performance is constrained by the indirect bandgap of silicon. This review paper highlights the advancements made in the field of integrated laser sources on the silicon photonics platform. These on-chip lasers are classified according to their gain media, including V semiconductors, III–V semiconductors, two-dimensional materials, and colloidal quantum dots. The methods of integrating these lasers onto silicon are also detailed in this review.

Hybrid integrated Si3N4 external cavity laser with high power and narrow linewidth.

We have designed and fabricated a hybrid integrated laser source with full C-band wavelength tunability and high-power output. The external cavity laser is composed of a gain chip and a dual micro-ring narrowband filter integrated on the silicon nitride photonic chip to achieve a wavelength tuning range of 55 nm and a SMSR higher than 50 dB. Through the integration of the semiconductor optical amplifier in the miniaturized package, the laser exhibits an output power of 220 mW and linewidth narrower than 8 kHz over the full C-band. Such a high-power, narrow-linewidth laser diode with a compact and low-cost design could be applied whenever coherence and interferometric resolutions are needed, such as silicon optical coherent transceiver module for space laser communication, light detection and ranging (LiDAR).

Heterogeneous integration of III–V semiconductor lasers on thin-film lithium niobite platform by wafer bonding

Thin-film lithium niobate (TFLN) photonic integrated circuits (PICs) have emerged as a promising integrated photonics platform for the optical communication, microwave photonics, and sensing applications. In recent years, rapid progress has been made on the development of low-loss TFLN waveguides, high-speed modulators, and various passive components. However, the integration of laser sources on the TFLN photonics platform is still one of the main hurdles in the path toward fully integrated TFLN PICs. Here, we present the heterogeneous integration of InP-based semiconductor lasers on a TFLN PIC. The III–V epitaxial layer stack is adhesively bonded to a TFLN waveguide circuit. In the laser device, the light is coupled from the III–V gain section to the TFLN waveguide via a multi-section spot size converter. A waveguide-coupled output power above 1 mW is achieved for the device operating at room temperature. This heterogeneous integration approach can also be used to realize on-chip photodetectors based on the same epitaxial layer stack and the same process flow, thereby enabling large-volume, low-cost manufacturing of fully integrated III–V-on-lithium niobate systems for next-generation high-capacity communication applications.

Gesn Alloys: From Optical to Electrical Pumped Lasers

Internet of Things applications, including medical diagnostics, imaging and industrial process controls, pollution monitoring or real-time motion control, push the extension of the Si photonics towards mid-infrared range.In recent years, group IV SiGeSn-based optoelectronic devices including light emitting, detectors, modulators, and CMOS devices have been developed. The main advantage of SiGeSn is the bandgap tunability in the range between 1.5 to 5 µm via Si and Sn compositions. Remarkably, the GeSn binary alloy becomes a direct bandgap semiconductor at a relative low Sn content of about 7-8 at.% opening the quest for the realization of monolithically integrated laser sources on Si. In this direction, from the first laser emission proof in 2015 remarkable progress has been done. Supported by epitaxial growth success, processing technology, and cavity design, the laser emission of optically pumped was continuously increased from 80K to 270K. The main role here was played by increasing the Sn content up a concentration of 17 at.%.The next major improvement came from layer transfer technology allowing defect engineering and heat management. Their combination made possible room temperature lasing. However, the real life application requires low laser threshold. Here the use of multiple quantum wells based on SiGeSn/GeSn heterostructures and strain engineering allow a remarkable threshold reduction to values of about 1 kA/cm2. Interestingly, modeling shows that the very epitaxial challenging of having high Sn contents can be reduced by band structure engineering via introducing tensile strain and, in addition, larger gains are possible compared with high-Sn strain free layers. The optical laser spectrum at 0 °C for a GeSn layer under 1 % biaxial tensile strain is shown in Fig 1a.The next step is the realization of an electrical pumped GeSn laser. The first data indicate that both waveguide and microdisk cavities can be electrically pumped leading to low temperature laser emission. The effect of the geometry and strain relaxation of under-etched cavities on the laser emission is under study. Figure 1b shows an exemplary spectrum and current-in light-out (LI) curve for a device with a SiGeSn/GeSn heterostructure taken at 50 K. Figure 1

Integrated Lasers on Silicon at Communication Wavelength: A Progress Review

AbstractWith the emerging trend of big data and internet of things, silicon (Si) photonics technology has been developed and applied for high‐bandwidth data transmission. Contributed by the advantages of Si including high refractive index, low loss, significant thermal‐optical effect, and CMOS compatibility, various functional photonic devices have been demonstrated. One exception is the laser source, which is limited by the indirect bandgap of Si. Researchers have come up with different ways to make lasers on Si. This review presents the development progress of integrated laser sources on Si, with the focus on the wavelength regimes for communication. These lasers are categorized based on their gain media, including III–V semiconductor‐based laser, germanium/germanium tin‐based laser, Raman‐based laser, and rare‐earth‐doped laser. The laser performance and characteristics are summarized in tables for comparison. The key results are discussed. Future perspectives on the development of integrated lasers on Si have also been provided.

Kerr optical parametric oscillation in a photonic crystal microring for accessing the infrared.

Continuous wave optical parametric oscillation (OPO) provides a flexible approach for accessing mid-infrared wavelengths between 2 µm and 5 µm, but operation at these wavelengths has not yet been integrated into silicon nanophotonics. Typically, a Kerr OPO uses a single transverse mode family for pump, signal, and idler modes, and relies on a delicate balance to achieve normal (but close-to-zero) dispersion near the pump and the requisite higher-order dispersion needed for phase- and frequency-matching. Within integrated photonics platforms, this approach results in two major problems. First, the dispersion is very sensitive to geometry, so that small fabrication errors can have a large impact. Second, the device is susceptible to competing nonlinear processes near the pump. In this Letter, we propose a flexible solution to infrared OPO that addresses these two problems by using a silicon nitride photonic crystal microring (PhCR). The frequency shifts created by the PhCR bandgap enable OPO that would otherwise be forbidden. We report an intrinsic optical quality factor up to (1.2 ± 0.1)×106 in the 2-µm band, and use a PhC ring to demonstrated an OPO with a threshold dropped power in the cavity of (90 ± 20) mW, with the pump wavelength at 1998 nm, and the signal and idler wavelengths at 1937 nm and 2063 nm, respectively. We further discuss how to extend the OPO spectral coverage in the mid-infrared. These results establish the PhCR OPO as a promising route for integrated laser sources in the infrared.

Modeling of the Bandgap Distribution in Bi-Axially Strained Germanium Crossbeam for Laser Applications

High tensile strained Ge cavities in crossbeam are promising for the development of integrated laser sources on Si. However, the optimization of such cavities remains more challenging than the uniaxial beams. Indeed, the spatial distributions of both the optical field and the material gain have to be simultaneously defined by the nuances of complex cavity geometry. In this work, we simulate spatial distribution of the bandgap in bi-axially strained Ge crossbeams. Starting from stress map calculations, we achieved the bandgaps mapping for all possible suspended strain configuration (i.e ϵ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">xx</sub> , ϵ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">yy</sub> and ϵ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">xy</sub> ), when merged with bandgaps modeling. A trade-off is evidenced between membrane orientation and arm curvature to reach the largest possible volume of amplifying material. Our work highlights the fundamental importance of these two parameters for Ge crossbeam design and presents a relevant modeling tool to define suitable cross designs for laser applications.

Self-oscillating optical frequency comb generation based on the gain-switching dual-tone semiconductor laser

A novel method to generate the optical frequency comb (OFC) based on the gain-switching integrated semiconductor laser is proposed in this paper. In the system, by modulating the core device, i.e., dual-tone monolithic integrated semiconductor laser (DT-MISL), which is biased appropriately, with an enough large amplitude RF signal, it would be pushed into the gain-switching state and thus various OFCs with distinct comb lines can be generated. The superior advantage here is that the RF modulation signal is offered by the local oscillator, which can be originally generated after the photodetector based on the DT-MISL itself. In this case, no additional external microwave driver is required and thus the OFC generator is fully self-oscillating. To my best of knowledge, it is the first try based on the integrated laser source under the gain-switching state. Compared to other discrete systems, this compact structure is potential to develop for photonic integration. In the experiment, one discernible OFC with 114.448-GHz bandwidth and 17 comb lines within 10-dB amplitude variation is obtained, indicating the feasibility of this scheme. In addition, the tunability and the application of the OFCs in the generation of the multi-carrier signals are discussed at last.© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Subwavelength-Polarized Quasi-Two-Dimensional Perovskite Single-Mode Nanolaser.

When approaching the subwavelength or deep subwavelength scale, there is a fundamental trade-off between the ultimate shrinking size and the performance for miniaturized lasers. Herein, to overcome this trade-off, we investigated the excitonic gain nature of quasi-two-dimensional (quasi-2D) perovskites and revealed that both singlet excitons and polarons would make nearly the entire contribution within ∼50 ps to a high net gain of 558 cm-1. Inspired by the gain characteristic, we successfully shrank the quasi-2D perovskites laser to the subwavelength scale using only a layer of ultraviolet glue and a glass substrate in the vertical dimension. In spite of the compact and simple cavity structure, single-mode lasing with a highly linear polarization degree of 81% and a quality factor of 1635 was achieved. The extremely short cavity, excellent lasing performance, and simple structure of the quasi-2D perovskite laser are expected to provide insights into next-generation integrated laser sources.

Ultrafast control of fractional orbital angular momentum of microlaser emissions

On-chip integrated laser sources of structured light carrying fractional orbital angular momentum (FOAM) are highly desirable for the forefront development of optical communication and quantum information–processing technologies. While integrated vortex beam generators have been previously demonstrated in different optical settings, ultrafast control and sweep of FOAM light with low-power control, suitable for high-speed optical communication and computing, remains challenging. Here we demonstrate fast control of the FOAM from a vortex semiconductor microlaser based on fast transient mixing of integer laser vorticities induced by a control pulse. A continuous FOAM sweep between charge 0 and charge +2 is demonstrated in a 100 ps time window, with the ultimate speed limit being established by the carrier recombination time in the gain medium. Our results provide a new route to generating vortex microlasers carrying FOAM that are switchable at GHz frequencies by an ultrafast control pulse.

Integrated Raman Laser: A Review of the Last Two Decades.

Important accomplishments concerning an integrated laser source based on stimulated Raman scattering (SRS) have been achieved in the last two decades in the fields of photonics, microphotonics and nanophotonics. In 2005, the first integrated silicon laser based upon SRS was realized in the nonlinear waveguide. This breakthrough promoted an intense research activity addressed to the realization of integrated Raman sources in photonics microstructures, like microcavities and photonics crystals. In 2012, a giant Raman gain in silicon nanocrystals was measured for the first time. Starting from this impressive result, some promising devices have recently been realized combining nanocrystals and microphotonics structures. Of course, the development of integrated Raman sources has been influenced by the trend of photonics towards the nano-world, which started from the nonlinear waveguide, going through microphotonics structures, and finally coming to nanophotonics. Therefore, in this review, the challenges, achievements and perspectives of an integrated laser source based on SRS in the last two decades are reviewed, side by side with the trend towards nanophotonics. The reported results point out promising perspectives for integrated micro- and/or nano-Raman lasers.

Comb Line Multiplication in an InP Integrated Photonic Circuit Based on Cascaded Modulators

We report the broadband characterization of an indium phosphide monolithically integrated optical frequency comb generator based on cascaded modulators. The device consists of a dual-drive Mach–Zehnder modulator (MZM), two cascaded phase modulators, and is also equipped with an integrated laser source, resulting in an extremely compact device, tunable both in wavelength and comb line frequency. Comb generation is shown in the range 4 to 8 GHz, and comb line multiplication is demonstrated, for the first time in an integrated device, exploiting the dual-drive configuration of the MZM. In particular, by feeding the MZM upper and lower arms with two synchronous sinusoidal RF signals, where one signal frequency is multiple of the others. At the repetition rate of 1 GHz, this technique allows to obtain up to 55 comb lines within a 3 dB power variation. A characterization of the comb phase noise when line multiplication is performed is also reported.

Polymer Lasing in a Periodic-Random Compound Cavity.

Simultaneous distributed feedback (DFB) lasing and linear polarized random lasing are observed in a compound cavity, which consists of a grating cavity and a random cavity. The grating cavity is fabricated by interference lithography. A light-emitting polymer doped with silver nanoparticles is spin-coated on the grating, forming a random cavity. DFB lasing and random lasing occur when the periodic-random compound cavity is optically pumped. The directionality and polarization of the random laser are modified by the grating structure. These results can potentially be used to design integrated laser sources.

Compact narrow-linewidth optical negative feedback laser with Si optical filter

A compact narrow-linewidth semiconductor laser is demonstrated by hybridly integrating an optical filter constructed from Si waveguides with a single-mode semiconductor laser, whose spectral linewidth is narrowed through optical negative feedback. We design the optimum structure of a reflective ring filter for an optical negative feedback laser, and a narrow spectral linewidth of 160 kHz is realized by the hybridly integrated laser source whose effective device length is less than 1.5 mm.

Monolithically Integrated Perovskite Semiconductor Lasers on Silicon Photonic Chips by Scalable Top-Down Fabrication.

Metal-halide perovskites are promising lasing materials for the realization of monolithically integrated laser sources, the key components of silicon photonic integrated circuits (PICs). Perovskites can be deposited from solution and require only low-temperature processing, leading to significant cost reduction and enabling new PIC architectures compared to state-of-the-art lasers realized through the costly and inefficient hybrid integration of III-V semiconductors. Until now, however, due to the chemical sensitivity of perovskites, no microfabrication process based on optical lithography (and, therefore, on existing semiconductor manufacturing infrastructure) has been established. Here, the first methylammonium lead iodide perovskite microdisc lasers monolithically integrated into silicon nitride PICs by such a top-down process are presented. The lasers show a record low lasing threshold of 4.7 μJcm-2 at room temperature for monolithically integrated lasers, which are complementary metal-oxide-semiconductor compatible and can be integrated in the back-end-of-line processes.

Room temperature continuous-wave nanolaser diode utilized by ultrahigh-Q few-cell photonic crystal nanocavities

Few-cell point-defect photonic crystal (PhC) nanocavities (such as LX and H1 type cavities), have several unique characteristics including an ultra-small mode volume (Vm), a small device footprint advantageous for dense integration, and a large mode spacing advantageous for high spontaneous-emission coupling coefficient (β), which are promising for energy-efficient densely-integratable on-chip laser light sources enhanced by the cavity QED effect. To achieve this goal, a high quality factor (Q) is essential, but conventional few-cell point-defect cavities do not have a sufficiently high Q. Here we adopt a series of modified designs of LX cavities with a buried heterostructure (BH) multi-quantum-well (MQW) active region that can achieve a high Q while maintaining their original advantages and fabricate current-injection laser devices. We have successfully observed continuous-wave (CW) lasing in InP-based L1, L2, L3 and L5 PhC nanocavities at 23°C with a DC current injection lower than 10 μA and a bias voltage lower than 0.9 V. The active volume is ultra-small while maintaining a sufficiently high confinement factor, which is as low as ~10-15 cm3 for a single-cell (L1) nanocavity. This is the first room-temperature current-injection CW lasing from any types of few-cell point-defect PhC nanocavities (LX or H1 types). Our report marks an important step towards realizing a nanolaser diode with a high cavity-QED effect, which is promising for use with on-chip densely integrated laser sources in photonic networks-on-chip combined with CMOS processors.

Germanium-based integrated photonics from near- to mid-infrared applications

Abstract Germanium (Ge) has played a key role in silicon photonics as an enabling material for datacom applications. Indeed, the unique properties of Ge have been leveraged to develop high performance integrated photodectors, which are now mature devices. Ge is also very useful for the achievement of compact modulators and monolithically integrated laser sources on silicon. Interestingly, research efforts in these domains also put forward the current revolution of mid-IR photonics. Ge and Ge-based alloys also present strong advantages for mid-infrared photonic platform such as the extension of the transparency window for these materials, which can operate at wavelengths beyond 8 μm. Different platforms have been proposed to take benefit from the broad transparency of Ge up to 15 μm, and the main passive building blocks are now being developed. In this review, we will present the most relevant Ge-based platforms reported so far that have led to the demonstration of several passive and active building blocks for mid-IR photonics. Seminal works on mid-IR optical sensing using integrated platforms will also be reviewed.

3-D Hybrid Laser Integration for Silicon Photonics

Silicon photonics is poised to revolutionize information transfer and will profoundly impact a range of industries. Yet integrated laser sources remain a bottleneck for silicon photonics. This year, we demonstrated a 3-D hybrid laser integration approach that is scalable, low cost and reliable, with superior thermal performance.

Thermal Management of Monolithic Versus Heterogeneous Lasers Integrated on Silicon

Thermal management is one of the most important issues for today's microelectronics design. Photonics is expected to bring significant advantages to future beyond-CMOS microprocessors in terms of heat dissipation with the use of optical interconnects but thermal management of integrated laser sources remains an issue. In this paper, we investigate the thermal properties of various III-V semiconductor laser sources integrated on silicon. Monolithic (through GaSb epitaxy) and heterogeneous (through InP/SOI bonding) integration approaches are studied with both ridge and microresonators laser geometries. It is first shown that the presence of dislocations in monolithically integrated structures, as determined by transmission electron microscopy, have no impact on thermal conductivities. Thermal mappings of the devices are then computed. It is demonstrated that heterogeneous lasers are thermally more insulated from the surrounding CMOS than monolithic lasers (especially at the back side), which avoids photonic-induced hot spots formation on the chip, but can degrade the overall laser performances.