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A monolithic optical-frequency comb generator based on multisection quaternary/quaternary eight-quantum-well InP-InGaAsP material is presented. Findings reveal the production of a number of lines with spacing exactly equal to the modulation frequency, differential phase noise between adjacent lines of -82 dBc/Hz at 1-kHz offset and a potential comb spectrum width of up to 2 THz.

We demonstrate the generation of continuous wave terahertz (THz) frequency radiation from photomixers fabricated on both Fe-doped InGaAs and Fe-doped InGaAsP, grown by metal-organic chemical vapor deposition. The photomixers were excited using a pair of distributed Bragg reflector lasers with emission around 1550 nm, and THz radiation was emitted over a bandwidth of greater than 2.4 THz. Two InGaAs and four InGaAsP wafers with different Fe doping concentrations were investigated, with the InGaAs material found to outperform the InGaAsP in terms of emitted THz power. The dependencies of the emitted power on the photomixer applied bias, incident laser power, and material doping level were also studied. [ABSTRACT FROM AUTHOR]

We report recent advances in photonic–electronic integration developed in the European research project L3MATRIX. The aim of the project was to demonstrate the basic building blocks of a co-packaged optical system. Two-dimensional silicon photonics arrays with 64 modulators were fabricated. Novel modulation schemes based on slow light modulation were developed to assist in achieving an efficient performance of the module. Integration of DFB laser sources within each cell in the matrix was demonstrated as well using wafer bonding between the InP and SOI wafers. Improved semiconductor quantum dot MBE growth, characterization and gain stack designs were developed. Packaging of these 2D photonic arrays in a chiplet configuration was demonstrated using a vertical integration approach in which the optical interconnect matrix was flip-chip assembled on top of a CMOS mimic chip with 2D vertical fiber coupling. The optical chiplet was further assembled on a substrate to facilitate integration with the multi-chip module of the co-packaged system with a switch surrounded by several such optical chiplets. We summarize the features of the L3MATRIX co-package technology platform and its holistic toolbox of technologies to address the next generation of computing challenges. [ABSTRACT FROM AUTHOR]

A photonic wireless bridge operating at a carrier frequency of 250 GHz is proposed and demonstrated. To mitigate the phase noise of the free-running lasers present in such a link, the tone-assisted carrier recovery is used. Compared to the blind phase noise compensation (PNC) algorithm, this technique exhibited penalties of 0.15 and 0.46 dB when used with aggregated Lorentzian linewidths of 28 and 359 kHz, respectively, and 20 GBd 16-quadrature amplitude modulation (QAM) signals. The wireless bridge is also demonstrated in a wavelength division multiplexing (WDM) scenario, where five optical channels are generated and sent to the Tx remote antenna unit (RAU). In this configuration, the full band from 224 to 294 GHz is used. Finally, a 50 Gbit/s transmission is achieved with the proposed wireless bridge in single channel configuration. The wireless transmission distance is limited to 10 cm due to the low power emitted by the uni-travelling carrier photodiode used in the experiments. However, link budget calculations based on state-of-the-art THz technology show that distances > 1000 m can be achieved with this approach. [ABSTRACT FROM AUTHOR]

A series of AlxGa(1−x)As ternary alloys were grown by molecular beam epitaxy (MBE) at the technologically relevant composition range, x < 0.45, and characterized using spectroscopic ellipsometry to provide accurate refractive index values in the wavelength region below the bandgap. Particular attention is given to O-band and C-band telecommunication wavelengths around 1.3 µm and 1.55 µm, as well as at 825 nm. MBE gave a very high accuracy for grown layer thicknesses, and the alloys' precise compositions and bandgap values were confirmed using high-resolution x-ray diffraction and photoluminescence, to improve the refractive index model fitting accuracy. This work is the first systematic study for MBE-grown AlxGa(1−x)As across a wide spectral range. In addition, we employed a very rigorous measurement-fitting procedure, which we present in detail. [ABSTRACT FROM AUTHOR]

We demonstrate experimentally an optoelectronic oscillator (OEO) in which high side-mode suppression is achieved by cascading a phase modulator-based single passband tunable microwave photonic (MWP) filter with an optoelectronic feedback loop-based infinite impulse response (IIR) MWP filter. The OEO provides an RF oscillation that can be tuned from 6.5 GHz to 17.8 GHz with a phase noise lower than -103 dBc/Hz. Experimental results show that inclusion of the IIR section leads to a 20 dB reduction of phase noise close to the carrier and an increase of 10 dB in side mode suppression, compared to the equivalent OEO without an IIR section. The OEO was used to drive an optical frequency comb generator to generate a THz signal at 242.6 GHz by optical heterodyning; inclusion of the IIR section increases suppression of the side modes neighboring the THz carrier. A radio over fiber link was then implemented at a 242.6 GHz carrier frequency, with transmission of a 24 Gbps signal over 40 km of fiber and a 30 cm wireless path at a bit error rate below the forward error correction limit. The proposed system may be applied to frequency reconfigurable THz links and radars. [ABSTRACT FROM AUTHOR]

We propose and demonstrate experimentally a photonic THz signal generation technique based on a tunable optoelectronic oscillator (OEO), and its application in a radio over fiber (RoF) link. The OEO's frequency is tuned by varying the bandwidth of a tunable optical bandpass filter (TOBF) that is cascaded with a phase modulator (PM). The resulting tunable microwave photonic filter is used to generate OEO oscillations from 6.58 GHz up to 18.36 GHz (with a phase noise of ≤−103 dBc/Hz at 10 kHz offset from the carrier frequency). The OEO is subsequently used to drive an optical comb, generating 22 comb lines with a frequency spacing of 17.33 GHz covering a bandwidth of 360 GHz within a 20 dB envelope. By selecting two optical comb lines with a wavelength selective switch (WSS) and beating them in a uni-traveling carrier photodiode (UTC-PD), THz signals are generated at 101.5 GHz and 242.6 GHz with phase noise of −90 dBc/Hz and −78 dBc/Hz, respectively at 10 kHz offset from carrier frequency. Tunable mm-wave and THz signals can be generated either by changing the OEO oscillation frequency or the selected comb lines. Using the OEO driven OFCG, we implemented a RoF link at 242.6 GHz with a data rate of 24 Gbps over a wireless distance of 30 cm and with a bit error rate (BER) below the hard decision forward error correction (FEC) limit of 3.8 × 10−3. This method allows the creation of an all-photonic frequency reconfigurable THz signal generator and RoF system. [ABSTRACT FROM AUTHOR]

In this work, we present an approach to modelling III-V lasers on silicon based on a travelling-wave rate equation model with sub-micrometer resolution. By allowing spatially resolved inclusion of individual dislocations along the laser cavity, our simulation results offer new insights into the physical mechanisms behind the characteristics of 980 nm In(Ga)As/GaAs quantum well (QW) and 1.3 μm quantum dot (QD) lasers grown on silicon. We identify two effects with particular importance for practical applications from studying the reduction of the local gain in carrier-depleted regions around dislocation locations and the resulting impact on threshold current increase and slope efficiency at high dislocation densities. First, a large minority carrier diffusion length is a key parameter inhibiting laser operation by enabling carrier migration into dislocations over larger areas, and secondly, increased gain in dislocation-free regions compensating for gain dips around dislocations may contribute to gain compression effects observed in directly modulated silicon-based QD lasers. We believe that this work is an important contribution in creating a better understanding of the processes limiting the capabilities of III-V lasers on silicon in order to explore suitable materials and designs for monolithic light sources for silicon photonics. [ABSTRACT FROM AUTHOR]

We describe fast recovery at a wavelength of 1550 nm in a multiple-quantum-well (MQW) saturable absorber with InGaAsP quaternary wells and barriers using electric-field-induced carrier sweepout. The MQW SA is integrated with a distributed Bragg reflector in a p-type–intrinsic–n-type structure. Pump–probe measurements show that the recovery time can be reduced from >900 to 28 ps by application of a 151 kV/cm sweepout field. We measure the dependence of the recovery time on the energy of the saturating pulse and explain our results in terms of carrier dynamics in the sweepout field. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In future photonic-based THz backhaul links, integrating the optical local oscillator (LO) in a remote antenna unit can be beneficial in terms of optical bandwidth efficiency and higher compatibility with passive optical networks. In such a scenario, two approaches can be used to reduce the high phase noise associated with free-running lasers: 1) baseband (BB) signals & carrier recovery, and 2) single sideband (SSB) signals & envelope detection. In this paper, we compare the performance of the two approaches for various optical LO linewidths using 5 GBd 16-QAM signals. We find that, for a total linewidth wider than 0.55 MHz, the SSB approach yields better results. The superior performance, however, comes at the expense of reducing the net information spectral density (ISD) of the SSB signal by 39% compared to that of the BB signal. However, using signal–signal beat interference-mitigation algorithms, an ISD only 15% lower than the BB signal ISD was sufficient to meet the FEC requirement. Given these results, we believe that the envelope detection of SSB signals is a promising solution to mitigate the phase noise problem of THz links based on free-running lasers, without excessively compromising the spectral efficiency of the system. [ABSTRACT FROM AUTHOR]

We present the first demonstration of a wireless transmission link based on uni-traveling carrier photodiodes (UTC-PDs) as transmitter and receiver. In this demonstration, a UTC-PD was used at the transmitter to generate a 1-Gbps on–off keying data signal at a carrier frequency of 61.3 GHz by heterodyning two modulated optical tones originating from an optical frequency comb system. The generated electrical heterodyne signal was transmitted, using a 25-dBi gain parabolic antenna. An identical antenna was used to detect the signal at the receiver, followed by an optically pumped UTC-PD mixer to down-convert the received RF signal to an intermediate frequency of 6.3 GHz, which was subsequently amplified and acquired by a real-time oscilloscope for offline processing. The recovered data showed an open eye diagram, and a bit error rate of the order of 10–5 was measured. The receiver UTC-PD was characterized in terms of its conversion loss and noise figure (NF), and the overall NF of the receiver was measured at 21.5 dB. [ABSTRACT FROM AUTHOR]

This paper describes an optical phase lock loop (OPLL) implemented as an ultraselective optical frequency comb line filter. The OPLL is based on a photonic integrated circuit (PIC) fabricated for the first time through a generic foundry approach. The PIC contains a distributed Bragg reflector (DBR) laser whose frequency and phase are stabilized by reference to an optical frequency comb generator. The OPLL output is a single-mode DBR laser line; other comb lines and noise at the output of the OPLL filter are attenuated by >58 dB below the peak power of the OPLL-filter output line. The OPLL bandwidth is up to 200 MHz, giving a filter quality factor greater than 1,000,000. The DBR laser can be tuned over 1 THz (8 nm), enabling different comb lines to be selected. Locking to a comb line with a frequency offset precisely selectable between 4 and 12 GHz is also possible. The coherence between the DBR laser and the comb lines is demonstrated by measurements of the heterodyne signal residual phase noise level, which is below −100 dBc/Hz at 5 kHz offset from the carrier. The OPLL-filter output can be up to 6 dB higher than the peak power of the comb line to be isolated by the filter. This optical gain is a unique characteristic which can significantly improve the SNR of communication or spectroscopy systems. This OPLL is envisaged to be used for high purity, tuneable microwave, millimetre-wave, and THz generation. [ABSTRACT FROM AUTHOR]

THz wireless signals have become of interest for future broadband wireless communication. In a scenario where the wireless signals are distributed to many small remote antenna units, this will require systems which allow flexible frequency tuning of the generated THz carrier. In this paper, we demonstrate experimentally the implementation of two tuning methods using an optical frequency comb generator for coherent optical frequency tuning in THz wireless-over-fiber systems. The first method is based on using a photonic integrated circuit optical phase lock loop (OPLL) subsystem implemented as a high quality optical filter for single comb line selection and optical amplification. The OPLL generates an optical carrier, which is frequency and phase stabilized in reference to one of the optical comb lines with a frequency offset precisely selectable between 4 and 12 GHz. The second method is based on optical single sideband suppressed carrier (SSB-SC) modulation from the filtered comb line using an optical IQ modulator. With this technique, it is possible to suppress the other unwanted optical tones by more than 40 dB. This generated optical carrier is then heterodyned with another filtered optical comb line to generate a tuneable and stable THz carrier. The full system implementations for both methods are demonstrated by transmitting THz wireless signal over fiber with 20 Gb/s data in QPSK modulation. The system performance and the quality of the generated THz carrier are evaluated for both methods at different tuned THz carrier frequencies. The demonstrated methods confirm that a high quality tuneable THz carrier can easily be implemented in systems where dynamic frequency allocation is required. [ABSTRACT FROM AUTHOR]

We report the first demonstration of gain-switched optical pulses generated by continuous-wave 1.3 μm InAs/GaAs quantum dot (QD) broad-area lasers directly grown on silicon. The shortest observed pulses have typical durations between 175 and 200 ps with peak output powers of up to 66 mW. By varying the drive current pulsewidth and amplitude systematically, we find that the peak optical power is maximized through sufficiently long high-amplitude drive pulses, whereas shorter drive pulses with high amplitudes yield the narrowest achievable pulses. A three-level rate equation travelling-wave model is used for the simulation of our results in order to gain a first insight into the underlying physics and the laser parameters responsible for the observed behavior. The simulations indicate that a limited gain from the InAs QDs and a very high gain compression factor are the main factors contributing to the increased pulsewidth. As the optical spectra of the tested broad-area QD laser give a clear evidence of multitransverse-mode operation, the laser's dynamic response could be additionally limited by transversal variations of the gain, carrier density, and photon density over the 50 μm wide laser waveguide. [ABSTRACT FROM AUTHOR]

Higher capacity wireless access networks are required to serve the growing demands for mobile traffic and multimedia services. The use of sub-THz carrier frequencies is a potential solution for the increased data demands. This paper proposes and demonstrates experimentally the photonic generation of a multiband signal for sub-THz wireless-over-fiber transmission at up to 100 Gbit/s (20 Gbit/s in each band) using the full spectrum 220– 280 GHz for downlink wireless transmission and an uplink with 10 Gbit/s on-off keying. By using an optical frequency comb generator (OFCG), five optical tones spaced by 15 GHz are selected and split into odd and even optical subcarriers modulated separately using 10 Gbaud quadrature phase shift keying with Nyquist bandwidth achieved by using root raised cosine filtering with 0.01 roll off factor. These optical subcarriers are combined and transmitted over 10 km of fiber to the remote antenna unit (RAU). The optical bands are then filtered and transmitted separately at the RAU in a wireless channel. The received sub-THz band is down-converted to the intermediate frequency and digital signal processing (DSP) is employed at the receiver to measure the bit error ratio (BER). The performance is also evaluated to investigate the impact of the uplink on the downlink optical transmission. The receiver link budget and wireless distance for acceptable BER are also explored. The proposed system aims to distribute sub-band THz signals for short range indoor mobile units. The overall transmission capacity is increased by transmitting it as a multiband, which also reduces the bandwidth requirements on opto-electronic devices. [ABSTRACT FROM PUBLISHER]

The authors report 1.3‐μm InAs/GaAs quantum‐dot (QD) lasers monolithically grown on a Si substrate by optimising the dislocation filter layers (DFLs). InAlAs/GaAs strained layer superlattices (SLSs) have been presented as DFLs in this study. A distinct improvement in the InAs/GaAs QDs was observed when using InAlAs/GaAs SLSs because of the effective filtering of threading dislocations. Consequently, a laser with a threshold current density of 194 A/cm2 at room temperature and an operating temperature as high as 85°C is successfully demonstrated. These results show the potential for integrating III–V QD materials on a Si platform via InAlAs/GaAs SLSs as DFL. [ABSTRACT FROM AUTHOR]

Optical fibre transmission has enabled greatly increased transmission rates with 10 Gb/s common in local area networks. End users find wireless access highly convenient for mobile communication. However, limited spectrum availability at microwave frequencies results in per-user transmission rates limited to much lower values, e.g., 500 Mb/s for 5-GHz band IEEE 802.11ac. Extending the high data-rate capacity of optical fiber transmission to wireless devices requires greatly increased carrier frequencies. This paper will describe how photonic techniques can enable ultrahigh capacity wireless data distribution and transmission using signals at millimeter-wave and TeraHertz (THz) frequencies. [ABSTRACT FROM AUTHOR]

This paper describes the advantages that the introduction of photonic integration technologies can bring to the development of photonic-enabled wireless communications systems operating in the millimeter wave frequency range. We present two approaches for the development of dual wavelength sources for heterodyne-based millimeter wave generation realized using active/passive photonic integration technology. One approach integrates monolithically two distributed feedback semiconductor lasers along with semiconductor optical amplifiers, wavelength combiners, electro-optic modulators and broad bandwidth photodiodes. The other uses a generic photonic integration platform, developing narrow linewidth dual wavelength lasers based on arrayed waveguide gratings. Moreover, data transmission over a wireless link at a carrier wave frequency above 100 GHz is presented, in which the two lasers are free-running, and the modulation is directly applied to the single photonic chip without the requirement of any additional component. [ABSTRACT FROM PUBLISHER]

We present a review of the critical design aspects of monolithically integrated optical phase lock loops (OPLLs). OPLL design procedures and OPLL parameters are discussed. A technique to evaluate the gain of the closed loop operating system is introduced and experimentally validated for the first time. A dual-OPLL system, when synchronised to an optical frequency comb generator without any prior filtering of the comb lines, allows generation of high spectral purity signals at any desired frequency from several GHz up to THz range. Heterodyne phase locking was achieved at a continuously tuneable offset frequency between 2 and 6 GHz. Thanks to the photonic integration, small dimensions, and custom-made electronics, the propagation delay in the loop was less than 1.8 ns, allowing good phase noise performance with OPLLs based on lasers with linewidths less than a few MHz. The system demonstrates the potential for photonic integration to be applied in various microwave photonics applications where narrow-bandwidth tuneable optical filters with amplification functionality are required. [ABSTRACT FROM PUBLISHER]

The realization of semiconductor lasers on Si substrates will enable the fabrication of complex optoelectronic circuits. This will permit the creation of the long-dreamed chip-to-chip and system-to-system optical interconnects. This paper reports recent developments in our work on InAs/GaAs quantum-dot (QD) lasers monolithically grown on Si, Ge, and Ge-on-Si (Ge/Si) substrates. A thin AlAs nucleation layer (NL) was first investigated for the growth of InAs/GaAs QDs on Si substrates. The AlAs NL enables more defects to be confined in the interface between the GaAs epitaxial layer and Si substrate, and hence leads to higher photoluminescence intensity for InAs/GaAs QDs. Room-temperature lasing at 1.29 μm with a threshold current density of 650 A/cm ^2 was demonstrated with the use of an AlAs NL. The growth of InAs/GaAs QDs on Ge and Ge/Si substrates was further studied. A low threshold current density of ∼200 A/cm ^2 for 1-mm long QD lasers has been demonstrated for QD lasers grown on Ge substrates by using Ga prelayer technique. This growth technique has also been explored for Ge/Si substrates. Room-temperature lasing at 1.28 μm with threshold current density of ∼164 A/cm ^2 and lasing operation up to 84 °C has been demonstrated for a 3-mm long device. [ABSTRACT FROM PUBLISHER]

We present a novel technique for optoelectronic frequency down-conversion of millimeter-wave signals generated from photomixers. The mixing element proposed here is a uni-traveling carrier (UTC) photodiode employing a traveling wave design, originally fabricated for the generation of millimeter-wave signals. For fundamental mixing at a frequency of 100 GHz, a conversion gain of -\ 32∼dB was measured, representing a significant improvement on previously published results. When the device was operated as a subharmonic mixer, an additional loss of 20, 27, and 39 dB was measured for second-, third- and fourth-harmonic mixing, respectively, for the same level of RF input power (0 dBm). A nonlinear dependence of the IF signal on the optically generated signal was measured. From subharmonic mixing measurements, a flat intermediate frequency (IF) response was found over a wide range of frequencies, limited mainly by the IF electronic components. [ABSTRACT FROM AUTHOR]