The semiconductor optical amplifier (SOA) is a versatile component that can be deployed to meet the expanding applications associated with the introduction of additional functionalities at the optical level in wavelength division multiplexed systems. The future network requires low cost, small footprint, directly controllable amplification throughout the different application layers from long haul through to metro; the intrinsic size and integration capability advantages will ensure that the SOA plays a key role in this evolution. In multi-wavelength gating/amplification applications the gain dynamics, oscillating at timescales comparable to that of the data which is being amplified, introduce issues of pattern dependent waveform distortion (patterning) in single channel, and inter-channel cross-talk in multi-wavelength cases which require management through careful SOA design and understanding of the network application scenarios. In this paper, an optical linear amplifier (OLA) architecture with the unique capability to provide variable gain whilst maintaining linear operation at high output saturation powers will be described. Initial characterisation results for the OLA will be presented.
The Linear Optical Amplifier (LOA) is a chip-based amplifier that was developed to meet the optical amplifier requirements for emerging optical networks. The requirements include operating at wide range of bit rates and protocols, varying number of channels, as well as reduced cost and size. In this work, LOA performance characteristics are measured and its linearity characterized under both static and dynamic switching environments. The LOA performance is also shown in three key applications: transmitter boost, receiver pre-amplification, and in an in-line, cascaded metropolitan ring.
We present an improvement of an analog optical transmission performance by reducing both the nonlinearity and the periodic RF carrier suppression. The proposed technique takes advantage of the gain saturation characteristic of semiconductor optical amplifier (SOA) to reduce the intrinsic nonlinearity of electroabsorption-modulated laser (EML) and the negative chirp characteristic of SOA and EML to expand the transmission length and operating frequency. Furthermore, RF signal gain was also obtained through optical signal amplification.
Ultrafast optical time-division multiplexing (OTDM) networks have the potential to provide truly flexible bandwidth-on-demand at burst rates in excess of 100 Gbit/s for high-end users, high-speed video servers, terabyte media banks, supercomputers, and aggregates of lower speed users. Because 100 Gbit/s channel rates exceed the current speed available from electronics, functions such as slot or packet synchronization, header address comparison, and data rate conversion at OTDM packet routers or network receiver nodes must be achieved using all-optical techniques. Interferometric logic gates based on gain and index nonlinearities in semiconductor optical amplifiers (SOAs) are of particular interest due to their compact size, low latency, low required switching pulse energies, and potential for large-scale integration. One challenge for SOA-based optical switching is gain saturation that leads to pattern-dependent amplitude modulation at the switch output. We demonstrate pulse-position modulation as a viable means for mitigating carrier-induced amplitude patterning and use this data format to implement optical switches capable of stable operation at 100 Gbit/s data rates with low switching energies. We also show that semiconductor-based optical logic gates can be cascaded together to achieve advanced functionality for ultrafast system applications. As an example, we will present our recent implementation of a synchronous OTDM network testbed capable of fully loaded packet transmission. We demonstrate receiver functionality with multi-layered independent all-optical logic to achieve packet self-synchronization, multiple-bit address comparison, and data demultiplexing at channel speeds exceeding 100 Gbit/s.
Vertical-cavity semiconductor optical amplifiers (VCSOAs) are interesting devices because of their small form factor, potential low manufacturing cost, high coupling efficiency to optical fiber, and polarization independent gain. In this paper, an overview of the properties and possible applications of long-wavelength VCSOAs is given. We present general design rules and analyze how the mirror reflectivity affects the properties of the VCSOA. Experimental results of reflection-mode VCSOAs operating at 1.3-μm wavelength are presented. The devices were fabricated using InP-GaAs wafer bonding and were optically pumped by a 980-nm laser diode. These VCSOAs have demonstrated the highest fiber-to-fiber gain (17 dB), as well as the highest saturation output power (-3.5 dBm) of any long-wavelength VCSOA to date. We have also used these VCSOAs for optical preamplification at 10 Gb/s. Using an 11-dB gain VCSOA, the sensitivity of a regular PIN detector was increased by 7 dB resulting in a receiver sensitivity of -26.2 dBm.
Both thanks to their capability to operate as multifunctional devices, their compatibility with numerous optical passive or active devices and their potentially low cost, Semiconductor Optical Amplifiers (SOAs) have demonstrated their attractiveness for lightwave communication systems and future integrated photonic circuits. In this context, we have studied in detail their photodetection functionality. The photodetection realization is very attractive with a SOA since it can be achieved in-line, in the amplification regime enabling good responsivity and without optical withdrawal. In this paper, we report our latest results about the in-line photodetection performances of Multielectrode SOAs (MSOAs), showing that they can be of great usefulness to optimize the reception and to perform fast in-line photodetection. The dynamic measurement experimental results, obtained by using a bielectrode SOA (300μm-200μm) and by detecting on its rear contact, show a bandwidth of about 1,2GHz which gives an idea of MSOAs potentiality. The latter has been investigated in this work by simulation especially for the long MSOAs and the results reveal that the bandwidth can significantly be enhanced, by simultaneously increasing the SOA total length and reducing the detection contact length, allowing a high speed photodetection on a wide band of frequency of about 7GHz centered on 4GHz with a 2mm long component having a 100μm rear contact length.
Development toward a low-cost 10Gbit/s module is presented. We focus on four key issues for transmission lasers:
1) Optical sources with integrated laser and modulator often involve four our more separate growths. We demonstrate a strain-compensated AlGaIn-As-based monolithic source with uses a single overgrowth. EMLs with a complex-coupled DFB grating provide high sidemode suppression ration (SMSR>50dB), high single-mode yield, and excellent immunity to optical reflection.
2) A new method for characterizing complex-coupled gratings allows us to determine the gain-grating component of DFBs (or EMLs) beyond threshold. Gain grating strength will drop with spatial holeburning. This method allows one to measure gain-grating strength at any power.
3) We investigate the effect of reflection-induced chirp. Theory and experiment show that EMLs and other integrated photonic sources can be 100-1000 times more resistant to feedback if a strong (κrL=6) grating is used instead of the type of gratings typically used. This supports the growing trend that isolator-free transmission lasers are practical. It also permits integration of amplifiers or other elements with the laser.
4) Chirp-free transmission is non-optimum for long-span transmission. A number of different types of fiber currently exist, and EMLS can be made with the capability of adjustable chirp. Thus, a single EML can suit each of these fiber types (e.g. SMF-28, LEAF, zero-dispersion, and MetroCor). We investigate the effects of dispersion on non-return-to-zero (NRZ) data from these EMLs. Bit error ratios (BER) are theoretically and experimentally analyzed for optimum chirp for long spans without dispersion-compensation.
We analyze the high-temperature continuous-wave performance of 1.3 micron AlGaInAs/InP laser diodes grown by digital alloy molecular beam epitaxy. Commercial laser software is utilized that self-consistently combines quantum well bandstructure and gain calculations with two-dimensional simulations of carrier transport, wave guiding, and heat flow. Excellent agreement between simulation and measurements is obtained by careful adjustment of material parameters in the model. Joule heating is shown to be the main heat source; quantum well recombination heat is almost compensated for by Thomson cooling. Auger recombination is the main carrier loss mechanism at lower injection current. Vertical electron escape into the p-doped InP cladding dominates at higher current and it causes the thermal power roll-off. Self-heating and optical gain reduction are the triggering mechanisms behind the leakage escalation.
We investigate In1-xGaxAs1-yPy and In1-x-yGaxAlyAs quantum-well (QW) lasers for 1.55 μm telecommunication applications and compare their temperature dependence both theoretically and experimentally. Under steady-state (DC) electric bias, the gain and intrinsic absorption losses are measured based on the well-known Hakki-Paoli method from below threshold to threshold. The photon lifetime is obtained from this measurement. A comprehensive theoretical gain model with the realistic band structure including the valence band mixing and many-body effect is then used to fit the experimentally obtained modal gain spectra and extract the carrier density, and thus, the differential gain. We then carried out the high-speed microwave modulation measurement, and the experimental modulation response curves are fitted by the theory and the important parameters such as the differential gain and Κ factor are obtained. These high-speed differential gain agrees very well with the value obtained from the steady-state direct optical gain measurement. New experimental data on the temperature-dependent characteristics of two compressively strained lasers are then presented. The comparison of two material systems will be important to design high-bandwidth high-performance semiconductor lasers in order to meet requirements of 1.55 μm telecommunication applications.
A novel method to measure the optical modulation response of laser diodes that uses as the modulation source the output of a femtosecond optical parametric oscillator (OPO) is described. The femtosecond OPO generates a train of ~ 150 fs pulses tunable between 1.03 and 1.35 μm with an average power of 12 mW at a repetition rate of 81 MHz. With such a narrow pulse a rich frequency spectrum of flat intensity distribution that easily surpasses the 2000 GHz 3 dB-bandwidth is obtained. To perform modulation response measurements the OPO is selectively tuned to modulate the carrier population in either the well or separate confinement region of the laser diode. Modulation traces obtained with this method in 1.3 μm InAsP lasers are presented and compared with those obtained from electrical modulation at the same operating conditions.
Direct modulation of the injected current still represents an attractive and inexpensive technique for encoding information in the output of a semiconductor laser. The growing requirements in volume of information to be transmitted and their conflict with the progressive, and rapid, degradation of the signal at high speeds have made this simple technical solution less and less attractive. A brief analysis of the sources of the problem is offered. A viable and inexpensive way of improving DM's performance is discussed. High quality signals are predicted at data transmission speeds that exceed by over an order of magnitude those obtainable with DM.
A high-brightness semiconductor diode laser design, which utilizes a slab-coupled optical waveguide region to achieve several potentially important advances in performance, is described and experimentally demonstrated using simple rib waveguide quantum well structures. These lasers operate in a large, low-aspect-ratio, lowest-order spatial mode, which can be butt coupled to a single-mode fiber with very high coupling efficiency. The acronym used for this new type of structure is SCOWL, taken from "slab-coupled optical waveguide laser". Initial results on 1.3μmm InGaAsP/InP and 980-nm AlGaAs/InGaAs SCOWLs are presented.
A new approach for InP-based long-wavelength VCSELs based on buried tunnel junctions: the buried-tunnel-junction (BTJ) VCSEL is reviewed. Excellent cw laser performance has been demonstrated for BTJ-VCSELs in the 1.55μm wavelength range, such as sub-mA threshold currents, 0.9 V threshold voltage (at λ=1.55μm), operation voltages below 1.4V, 10-70 Ω series resistance, differential efficiencies >25%, up to more than 7mW optical output power, >100°C cw operation, stable polarization and single-mode operation with SSR of the order 50 dB. Also, recent achievements on high-speed long-wavelength VCSELs are reported.
We have produced GaAs-based quantum-dot edge-emitting lasers operating at 1.16 μm with record-low transparency current, high output power, and high internal quantum efficiencies. We have also realized GaAs-based quantum-dot lasers emitting at 1.3 μm, both high-power edge emitters and low-power surface emitting VCSELs. We investigated the ultrafast dynamics of quantum-dot semiconductor optical amplifiers. The dephasing time at room temperature of the ground-state transition in semiconductor quantum dots is around 250 fs in an unbiased amplifier, decreasing to below 50 fs when the amplifier is biased to positive net gain. We have further measured gain recovery times in quantum dot amplifiers that are significantly lower than in bulk and quantum-well semiconductor optical amplifiers. This is promising for future demonstration of quantum dot devices with high modulation bandwidth.
This paper explores the development of cascade semiconductor lasers for communications applications. Both interband and intersubband cascade emission devices are examined theoretically and experimentally. The motivation for cascade sources in both high fidelity and high bandwidth applications is presented. The ability to transmit signals with lower signal loss and improved noise performance is verified by measurements on a model systems consisting of series coupled DFB lasers.
We present ultra-widely tunable micro-cavity devices realized by micro-opto-electro-mechanical system (MOEMS) technology. We modeled, fabricated and characterized 1.55μm micromachined optical filter and VCSEL devices capable of wide, monotonic and kink-free tuning by a single control parameter. Our vertical cavity devices comprise single or multiple horizontal air-gaps in the dielectric and InP-based material system. Distributed Bragg mirrors with multiple air-gaps are implemented. Due to the high refractive index contrast between air (n=1) and InP (n=3.17) only 3 periods are sufficient to guarantee a reflectivity exceeding 99.8% and offer an enormous stop-band width exceeding 500nm. Unlike InGaAsP/InP or dielectric mirrors they ensure short penetration depth of the optical intensity field in the mirrors and low absorption values. Stress control of the suspended membrane layers is of outmost importance for the fabrication of MOEMS devices. By controlling the stress we are able to fabricate InP membranes which are extremely thin (357nm thickness) and at the same time flat (radius of curvature above 5mm). Micromechanical single parametric actuation is achieved by both, thermal and electrostatic actuation. Filter devices with a record tuning over 127nm with 7.3V are presented.
We present a new method for fabrication of tunable InGaAsP-InP single mode lasers without epitaxial overgrowth. These devices show the advantage of a considerably simplified fabrication process compared to conventional tunable laser types. The lasers comprise an active Bragg reflector integrated with an uncorrugated separately pumped gain region. To overcome the extensive and expensive overgrowth step we realized a surface grating on both sides of the ridge mesa, which provides DFB operation. By adjusting the current through the Bragg reflector, the wavelength can be tuned between 1590.8 nm and 1595.2 nm. A maximum of 11 wavelength channels with an average spacing of ~0.5 nm and a constant optical output power of ~0.5 mW are addressable.