Monolithically integrated multiwavelength sampled grating DBR lasers for dense WDM applications

Wavelength tunable and directly modulated distributed Bragg reflector (DBR) lasers with butt-joint technology are designed, fabricated and characterized. The DBR laser consists of a gain section and a DBR section. To increase the electrical isolation between the gain section and the DBR section, parts of a p-doped material in the isolation region are etched off selectively. Over 2 kΩ isolation resistance is realized ultimately without the need of ion implantation, which simplifies the fabrication process. The laser exhibits high speed modulation with a large tunable range. The 3 dB direct modulation bandwidth of the device is over 8 GHz in a 12 nm tunable range. This widely tunable DBR laser with the simple structure is promising as a colorless light source for the next-generation time and wavelength division multiplexed passive optical network (TWDM-PON) systems.

We demonstrate a distributed Bragg reflector (DBR) waveguide laser array by using an Ag film ion-exchange technique. We achieve an output power of 11 mW at 1540 nm for a coupled pump power of 145 mW, with a threshold of 60 mW and slope efficiency of 13%. The thin film ion exchange produces the highest index change possible at the surface, due to the ion exchange technique. Hence a wide array of wavelengths can be implemented in a single chip. We demonstrate a lasing wavelength range of 2.1 nm (1548.6 to 1550.7 nm) in an array with a single grating. Since the index change due to our process is large, we can fine tune the wavelengths of the array to fall on International Telecommunication Union (ITU) grid wavelengths by annealing. We demonstrate fine tuning of the wavelength for a channel from 1540.2 to 1540 nm, ITU specified wavelength, by annealing.

We describe near optimum near field holography grating masks patterned by e-beam lithography and a distributed Bragg reflector (DBR) multiwavelength laser array fabricated using near field printing with this mask. Grating pitches in the array ranged from 242.861 to 243.750 nm in 0.127 nm steps. Data on the pitch precision and pitch adjustment is presented. Use of a conventional UV source rather than laser illumination both greatly simplified the printing process and eliminated coherent artifacts from the printed gratings. Chemically etched InP test gratings are shown to be extremely ‘‘clean’’ in appearance and low in edge roughness. DBR laser arrays designed with 100 GHz frequency separation were processed using the mask described. The measured frequency separation was 99 GHz which could be further adjusted with a tuning section of the four-section laser design. Characterization of a similar grating mask containing 16 wavelengths with similar pitch increments is also described.

We developed integrated multiwavelength lasers with waveguide couplers using a novel selective metalorganic vapor-phase epitaxy (MOVPE) technique. The lasing wavelength was controlled by modulating the effective refractive index of distributed Bragg reflector (DBR) waveguides. Array lasing wavelengths were distributed from 1548.0 to 1553.5 nm with an average spacing of 1.8 nm. Output power exceeding 100 /spl mu/W was obtained for each DBR laser. The propagation loss of the waveguide coupler was about 16 dB for inner ports and 20 dB for outer ports with a 6-dB splitting loss.

Threshold current density and differential quantum efficiency are analyzed for a surface emitting (SE) lambda /4-shifted distributed feedback (DFB) laser consisting of alternating active and passive layers with a distributed Bragg reflector (DBR). It is shown that the threshold current density can be reduced by using the DFB action induced by alternating active and passive layers in the active region of the SE DFB laser structure, as compared to SE DBR lasers with a homogeneous active region. It is also shown that the differential quantum efficiency of the DFB laser with a DBR can be higher than that of conventional DBR lasers without increasing the threshold current density. >

It was about 125 years ago that the light bulb was commercialized by Thomas Edison. No doubt a brilliant invention at the time, today its low power conversion efficiency is one of the reasons why lighting in the western world has such high energy consumption. Thus, the potential for saving energy is enormous in this area. The introduction of halogen, discharge and fluorescent lamps has lead to certain efficiency improvements, however more than half of the energy is still lost as heat. Light-emitting diodes (LEDs) are very promising candidates for high efficiency light sources, with modern devices showing internal quantum efficiencies of virtually 100 %. However, due to the high refractive index of the commonly used semiconductor materials it is very difficult to have a large extraction efficiency; in a standard cubic geometry most of the internally emitted light is trapped inside the device due to total internal reflection. Several methods have been developed in order to circumvent this problem, either by optimizing the device geometry in order to increase the escape cone or by incorporating a resonant structure in order to force the internal emission into the existing escape cone. The latter approach is called microcavity LED (MCLED) or resonant cavity LED (RCLED). In a MCLED the spontaneous internal emission is controlled by placing the emitter inside an optical cavity with a thickness of the order of its emitting wavelength. The resulting interference effects increase the part of the emission that can be extracted. Contrary to the other approaches this is possible without changing the device geometry and thus without additional costly back-end processing steps. The control of the farfield radiation pattern makes these devices particularly interesting for high brightness applications, which demand highly directional emitters, such as for printing, bar code reading, large area displays and optical communication. The extraction efficiency of a MCLED is inversely proportional to the effective cavity length. An ideal cavity, allowing an extraction efficiency close to unity, consists of a low refractive index material and has an optical length of λ/2. In contrast to this, to obtain high internal quantum efficiencies it is necessary to use high index cavities with an optical length of at least λ. It should be noted, that the large penetration depth of the optical field in the semiconductor-based distributed Bragg reflectors (DBRs) leads to a significant increase of the effective cavity length and thus further reduces the achievable extraction efficiencies. In this thesis novel concepts to reduce effective cavity lengths and therefore increase extraction efficiencies are implemented into standard MCLED structures. The phaseshift cavity principle whilst maintaining the electrical properties of a standard A cavity achieves optical properties approaching that of a λ/2 cavity. The use of AlOx instead of AlAs as the, low refractive index component in the DBRs leads to smaller penetration depths and a concomitant reduction of the effective cavity length. A similar effect can be obtained by combining a resonant cavity with a thin-film structure. Thanks to these design improvements, the external quantum efficiency of different types of MCLEDs was increased. Near infrared emitting InGaAs/GaAs MCLEDs including a phase-shift cavity were realized, as both bottom and top emitting structures. The external quantum efficiencies achieved for emission into air were 18 and 19 %, respectively. With the additional incorporation of an oxide based bottom DBR, the efficiency of top emitting near infrared MCLEDs was further increased to 28 %. Red emitting AlGaInP-based structures are not compatible with the phase-shift cavity principle. However the beneficial effect of the implementation of an oxide DBR is greater at their wavelength rather than in the near infrared. Thus with preliminary red emitting GaInP/AlGaInP MCLEDs containing a bottom oxide DBR external quantum efficiencies of 12% could be achieved. Unfortunately, the incorporation of an oxide DBR significantly complicates the device design and the device fabrication. These problems can be avoided by combining the resonant cavity with a thin-film structure instead. Initial non-optimized red emitting thin-film MCLEDs were realized by OSRAM Opto Semiconductors and characterized in this work. They show external quantum efficiencies of 23 % and 18 % with and without encapsulation, respectively. It is assumed that a significant fraction of the high external quantum efficiency is due to a strong photon recycling effect in these devices. Simulations presented in this thesis show that the theoretical limits for the MCLEDs discussed above are slightly higher than the values obtained, encouraging further device optimization. The thin-film MCLEDs seem to hold the biggest potential for high efficiency emission from MCLEDs, independent of the wavelength range of emission.

A preliminary design for a broadband, multiple quantum well (MQW), distributed Bragg reflector (DBR), electronically tunable laser diode array has been developed for implementation into optical memory configurations. The proposed MQW DBR diode array and a waveguide hologram with a blazed grating and/or holographic diffraction grating will be integrated into a photonic random optical memory access (PROMAC) device. PROMAC is a low inertia, low power, sub-nanosecond, parallel (e.g. 64 bit wide), wavelength addressing device which exploits free space optical interconnects. The MQW DBR laser diode array will enhance the performance of PROMAC by offering advantages in speed, spectral resolution, holographic separation and scan efficiency. An increase in speed as well as a decrease in power consumption can also be realized. Thus, increased storage densities and faster access rates can be achieved.

This paper presents a theoretical and experimental study of nonlinear effects in distributed Bragg reflector (DBR)-type tunable lasers, which are responsible for asymmetric spectral or power characteristics, and for hysteresis phenomena. We propose an optimized DBR semiconductor optical amplifier design that suppresses these effects, making the control simpler and more reliable, while maintaining the same performances as a DBR (16 nm tuning, 40 dB side mode suppression ratio, 20 mW coupled output power). Aiming to reach C-band tuning range with only one Bragg current control and reduced nonlinear effects, a new double Bragg DBR laser is proposed.

A novel magnetic field sensor using distributed Bragg reflector (DBR) fiber laser by Ampere’s force effect is proposed and experimentally demonstrated. The key sensing element, that is the dual-polarization DBR fiber laser, is fixed on the middle part of two copper plates which carry the current. Ampere’s force is applied onto the coppers due to an external magnetic field generated by a DC solenoid. Thus, the lateral force from the coppers is converted to a corresponding beat frequency signal shift produced by the DBR laser. The electric current sensing is also realized by the same configuration and same principle simultaneously in an intuitive manner. Good agreement between the theory calculation and the experimental results is obtained, which shows a good linearity. This sensor’s sensitivity to the magnetic field and to the electric current finally reaches ~258.92 kHz/mT and ~1.08727 MHz/A, respectively.

A theoretical model for two- and three-section tunable distributed Bragg reflector (DBR) lasers is presented. The static tuning properties are studied in terms of threshold current, linewidth, oscillation frequency, and output power. Regions of continuous tuning for three-section DBR lasers are presented. Different routes for continuous tuning, when the injection currents to the passive sections are varied, are discussed and the limitations on the continuous tuning range are clarified. By proper control of these currents, a tuning range of approximately 400 GHz is predicted at 1.55 mu m wavelength, which is in good agreement with the experimental results published thus far.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

An 8-wavelength distributed Bragg reflector (DBR) array for narrow channel wavelength division multiplexing (WDM) has been fabricated with a new technique for printing first-order Bragg gratings using a phase mask and a conventional incoherent source. All the distributed gratings were printed in a single photolithographic step with a slightly modified mask aligner. The DBR's excellent wavelength control for channels separated by as little as 0.8 nm is described. Many advanced photonic devices relying on gratings like quarter-wave shifted distributed feedback (DFB) lasers and wavelength division multiplexing (WDM) components can potentially be manufactured with this technique in a simple and cost-effective way. >

We demonstrate a widely tunable distributed Bragg reflector (DBR) laser operating at 1.8-µm, in which the DBR section was butt-jointed InGaAsP (λ = 1.45 μm) material. Through current and temperature tuning, a widely tuning range of over 11 nm with a side mode suppression ratio (SMSR) higher than 30 dB is obtained. Utilizing this DBR laser, the water and methane detection experiment has been successfully implemented, which illustrates the potential capacity of such DBR laser as the light source used for multispecies gas sensing. The work also illustrates that the butt-joint active-passive integration technology developed for the InGaAsP quantum-wells (QWs) can be successfully applied in the InGaAs QWs.

A key component for high-density and ultra-broad-band intra-board chip-to-chip signal transmission is an integrated-optic coupler of several-micron aperture coincident with the emission diameter of typical single-transverse-mode VCSEL. A new type of coupler named cavity-resonator integrated grating input/output coupler (CRIGIC), recently proposed by some of the authors, has remarkable features of small aperture and vertical coupling with high coupling efficiency. CRIGIC consists of one grating coupler (GC) between two distributed Bragg reflectors (DBRs) with phase-shifting spaces. In an output coupling of a guided wave to a radiation wave, an incident forward guided wave is partially reflected by the front DBR, partially coupled out by GC to radiation waves, and reflected by the rear DBR to backward guided wave. A phase-shifting space between GC and the rear DBR is determined so that the backward guided wave from the rear DBR is radiated by GC in phase with the radiation of the forward guided wave. The coupling efficiency of the front DBR and another phase-shifting space between GC and the front DBR are chosen so that the transmission of the backward guided wave from GC is canceled by the reflection of the forward guided wave by the front DBR. Thus CRIGIC generates neither transmission nor reflection, providing 100% radiation even if GC has too small aperture to give high efficiency in a usual one-way propagation scheme. In order to demonstrate the operation principle of CRIGIC, a simple structure of 20 μm aperture length was designed, fabricated and characterized. The out-coupled power from CRIGIC was estimated to be 80% of that from conventional GC of 500 μm coupling length, indicating almost 20 times miniaturization in aperture was achieved. Integration of CRIGIC and a different-guided-mode-coupling DBR was discussed to construct a free-space-wave add/drop multi-/demultiplexer (ADM) in a thin film waveguide for an application to WDM optical-interconnect system in package. Eight-wavelength multiplexing within 20 nm wavelength range was considered. ADMs were designed and their performance was simulated theoretically.

The distributed Bragg reflectors (DBRs) are important optical components in a wide range of applications, such as Fabry-Perot optical resonators, hyperspectral sensors, micro-cavity structures, and waveguide lasers. In this thesis, we have developed, designed, and fabricated highly reflecting, and mechanically stable DBRs comprised of electron-beam deposited zinc selenide (ZnSe) and barium fluoride (BaF2) thin films and explored its use in optical filtering and hyperspectral sensing applications. Most published results are in the short-wavelength infrared (SWIR) and near-infrared (NIR) with limited results in the mid-wavelength infrared (MWIR). In this work, we have fabricated ZnSe/BaF2-based DBRs in the MWIR region. The fabricated DBRs showed a central wavelength of 4 µm and reflection exceeding 93%. A Fabry-Perot optical filter, that is comprised of two parallel DBR mirrors with half-wavelength optical cavity thickness was used to partially transmit and reflect the incident light at the target wavelength. The assembled fixed-cavity Fabry-Perot optical filters achieved a full-width half maximum (FWHM) of ~ 450 nm. We further showed that the FWHM of the optical filters can be reduced by ~ 50 % with the addition of nano-thick silicon monoxide (SiO) interface grading layer between every pair of the quarter-wave optical thick (QWOT) layers in the stack. The SiO interface grading layer acts as an anti-reflection coating layer to reduce reflection loss in the DBR multilayer structure. The SiO interface grading layer, ~ 80 nm thick, also has the potential to stop atomic inter-diffusion between the layers to minimize free carrier absorption loss. The principal thesis goal is to design and assemble fixed-cavity interface-graded Fabry-Perot optical filters (with SiO interfacial layers) that optimized optical transmission and spectral resolution. The thesis also proposes a low-cost micro-cavity structure that can be formed by embedding two-dimensional atomic crystals, such as black phosphorous into a DBR cavity structure. The proposed design can be used to increase absorption in 2D materials, which has the potential to increase the efficiency of next generation MWIR photodetectors and infrared sensors.

Tunable optical sources used in dense wavelength division multiplexed (DWDM) systems, such as distributed Bragg reflector (DBR) lasers, are highly susceptible to aging-induced wavelength drift. Without the stabilizing influence of gain clamping in the tuning section, the tendency for a tunable source to drift is greater than that of a fixed wavelength source, such as a distributed feedback or electroabsorption modulated laser. Aging of tuning sections can cause a free-running tunable source to wander out of an allotted channel band or, in a severe case, to mode hop. Wavelength drift in a tunable source is mitigated by closed loop feedback, but with time, aging still distorts the multi-dimensional channel map of optimized operating conditions. Assessing the potential for a tunable source to age is essential to ensuring device and system reliability. Using a novel technique, we assess aging in an electro-absorption modulated, distributed Bragg reflector (EADBR) laser using change of Bragg wavelength as a measure of increase of non-radiative recombination.