Electronic tuning of LEC lasers

We have seen in Chapter 6 that a conventional semiconductor laser does not emit light in a single longitudinal mode. In general, the mode closest to the gain peak is most intense, and a few percent of the output power is carried by other longitudinal modes lying close to the gain peak. Furthermore, even when these side modes are reasonably suppressed under CW operation, their power content increases significantly when the laser is pulsed rapidly. Direct modulation of a semiconductor laser at frequencies in the gigahertz range is commonly employed in optical communication systems. In the presence of chromatic dispersion, the unwanted side modes limit the information transmission rate by reducing the fiber bandwidth. It is therefore desirable to devise means for a semiconductor laser to emit light predominantly in a single longitudinal mode even under high-speed modulation. Such Lasers are referred to as single-frequency or single-longitudinal-mode Lasers and in view of their potential application in optical communication systems were studied extensively during the 1980s.1 They were used in commercial lightwave systems by 1990.

Single longitudinal mode (SLM) lasers are important tools for many scientific and commercial applications. SLM operation can be achieved in distributed-feedback (DFB) lasers based on Bragg structures. Semiconductor waveguide DFB lasers are well-established devices, but their output power is limited to a few hundred milliwatts. DFB lasers have also been demonstrated in dielectric waveguides. However, in this case the output power was even lower. Here we present the first monolithic Yb:YAG DFB laser. The waveguide and the DFB structure were fabricated in the volume of an Yb:YAG crystal by ultrafast laser inscription. The DFB laser delivered 2W of output power at a slope efficiency of 61% in SLM operation under pumping with an optically pumped semiconductor laser. This power level outperforms previously demonstrated dielectric DFB waveguide lasers by nearly an order of magnitude. Our approach paves the way for compact, robust, and highly efficient high-power SLM laser sources.

The effects of nonlinear gain on the single longitudinal mode are analyzed by means of multilongitudinal mode and multitransverse mode rate equations and compared with the linear gain calculations and experments. It is shown that further suppression of the third order nonlinear gain on nonlasing modes always serves to accelerate the process of the single longitudinal mode operation and that results obtained with long cavity length are entirely contrary to those where only linear gain is considered. The experiments also show that single longitudinal mode lasers with even larger output power can be realized with long cavity length; and, with further proper reduction of the stripe width, single mode (single frequency) operation can be realized. But the results obtained from both gain considerations indicate that single longitudinal mode operation will be facilitated if the active layer is not too thin and the end face reflectivity is increased.

In this paper, we propose a novel approach to enhance the single longitudinal mode (SLM) yield for uniform grating distributed feedback (DFB) laser diode by breaking the dual-mode degeneracy. By introducing a horn ridged waveguide (HRW) in replace of the conventional straight ridge waveguide (RW), we create an effectively chirped grating as well as a longitudinally varying modal gain. As such, the field distribution along the laser cavity is different for the two possibly lasing longitudinal modes on each side of the Bragg stopband, and the field having a larger overlap with the gain becomes the only dominant lasing mode. We have optimized the ridge width of HRW-DFB lasers with different normalized coupling coefficients (κ L ) in terms of the SLM yield. Our experimental results show that the SLM yield of the HRW-DFB laser with κ L of 2.9 can reach 65% as opposed to 19% obtained by the conventional RW-DFB lasers fabricated on the same wafer.

We propose a new kind of distributed feedback (DFB) semiconductor laser based on the sampled Moire grating (SMG) integrated with a grating reflector (GR). Coupling coefficient along the laser cavity can be controlled, and $\pi $ phase shift ( $\pi $ -PS) can be introduced by SMG. The GR, which is a uniform sampled grating, can provide light reflection instead of high reflection (HR) facet coating. Thus, the front and rear facets are both coated with anti-reflection coatings (AR/AR) to avoid the influence of random grating phase at the HR facet. Therefore, more output power, better single longitudinal mode (SLM) operation, and higher wavelength precision can be simultaneously achieved, compared with traditional DFB lasers with AR/HR facet coatings. As an example, an 8-channel DFB laser array based on the proposed structure has been simulated. The proposed laser may benefit the applications requiring both high output power and wavelength precision, such as wavelength division multiplexing (WDM) transmission systems and gas sensors.

Single-mode optical fiber communication systems demand lasers to operate in a single longitudinal and fundamental transverse mode. Therefore, various single-mode lasers have recently been studied and fabricated(1), and the stable single-mode operation for a laser under high speed modulation has been realized by means of optical injection locking(2). This paper describes a twin-mesa GaAs-(AlGa)As laser integrated with a passive waveguide, which can be controllablly ocsillated in a single longitudinal and fundamental transverse mode with a Δλ of about 1-1.6 Å.

The operating characteristics of distributed feedback (DFB) InGaAsP lasers are reviewed from the point of view of their applications in optical communication systems. The wavelengths and the threshold gains of the allowed longitudinal modes of a DFB laser are obtained using the coupled-wave theory after incorporating both the facet reflections and the grating-induced distributed feedback. The experimental results are presented for 1.55 μm DFB semiconductor lasers. In particular, we discuss the temperature dependence of the threshold current and the emission wavelength, the modulation response, the frequency chirp, and the linewidth of the single longitudinal mode selected by the DFB mechanism.

5G brings many improvements to cellular networks in terms of performance, such as lower latency, improved network efficiency, and higher throughput, making it an attractive candidate for many applications. One such domain is industrial applications that may require real-time guarantees to transmit time-critical control messages. Assuming the immense number of devices exchanging data in support of Massive Machine-Type Communications (mMTC) applications, the capability of the cellular infrastructure to handle a large number of real-time transmissions may be inadequate. For such cases, there exists an acute desire to reduce any overheads as much as possible in order to guarantee certain deadlines. One such target is the Domain Name System (DNS) service, for which queries precede almost every new network request. This incorporates additional communication delays based on the response time, which in turn is affected by the proximity of the DNS server. While bringing DNS service to the edge has been touted as a logical solution, its integration with 5G systems is still challenging. This is due to the inability to access the DNS query information at the application layer since the User Equipment (UE) traffic is tunneled through to the core network. To this end, we propose a novel approach that can identify DNS queries at the base stations through Software-Defined Networking (SDN) capabilities. Specifically, we develop an SDN controller which is used to identify and extract DNS queries at the base station and handle the query at the edge without going through the 5G core network. This approach was implemented in a virtualized 5G network, in which we demonstrate that it is feasible and can potentially bring significant performance gains, especially in the case of mMTC applications.

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.

Single-mode semiconductor lasers are desirable for fiber-optic communications because their narrow spectrum translates into minimal pulse spreading in a dispersive fiber. Unfortunately, simple Fabry-Perot semiconductor lasers tend to oscillate in several longitudinal modes, particularly under high-frequency modulation. To combat this problem, more complicated laser structures have been proposed, including distributed feedback lasers, distributed Bragg reflectors, and coupled cavity lasers.1 These last are of particular interest due to their relative ease of fabrication. Thus far, most work has focused on longitudinally coupled lasers. In these devices, the gap between the lasers plays a critical role in determining gain selectivity, and it must be altered mechanically. Recently, we demonstrated operation of a laterally coupled laser pair, in which the coupling between two lasers of dissimilar lengths occurs via the evanescent fields of the individual lasers.2 In this paper we solve for the threshold gains of a laterally coupled cavity (LC2) laser and present experimental verification of single-mode operation.

For a semiconductor laser oscillating predominantly in a single longitudinal mode, the reflection feedback from the far end of an optical fiber generates closely spaced side modes corresponding to the external-cavity longitudinal modes. Using a simple model, it is shown that power fluctuations associated with these feedback-induced side modes can decrease the receiver sensitivity over its shot-noise-limited value for a given bit error rate. Since fiber dispersion plays a minor role, the reflection-induced power penalty is likely to occur for all fiber lengths commonly employed in optical communication systems.

A three phase shifted (3PS) distributed feedback (DFB) semiconductor laser based on Reconstruction-Equivalent-Chirp (REC) technique is experimentally demonstrated for the first time. The simulation results show that the performances of the equivalent 3PS DFB semiconductor laser are nearly the same as that of the true 3PS laser. However, it only changes the μm-level sampling structures but the seed grating is uniform. So, its cost of fabrication is low. The measurement results exhibit its good single longitudinal mode (SLM) operation even at high bias current and surrounding temperature.

An intracavity, second-harmonic generation, tunable, dual-frequency, passively Q-switched Nd:YAG laser based on a T-resonator configuration with polarization splitting is proposed, whose frequency difference could be doubled in comparison with fundamental lasing. The Nd:YAG, Cr4 + : YAG, and potassium titanyl phosphate (KTP) crystals were set at the shared arm, which could considerably reduce thermal fluctuation and pulse timing jitter between dual-frequency lasers. One birefringent filter consisting of a polarized beam splitter and a half-wave plate (HWP) is placed in each divided arm to select their single longitudinal mode. As a result, the p-polarized and s-polarized passively Q-switched components of 532 nm are simultaneously operated, whose power easily reaches to same and frequency is beneficial to tune theoretically throughout the whole gain bandwidth. The main characteristics of the power, longitudinal mode selection, and the pulse have been tested experimentally. Moreover, the frequency difference of the dual-frequency laser at 532 nm has been widely tuned from 9.6 to 117 GHz, by slightly adjusting the tilt angles of the HWPs. We offer a simple and widely tunable source with potential for portable frequency reference applications in absolute-distance interferometry, terahertz-wave generation, and other fields.

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. >

We have recently reported optically-pumped lasing in an InGaP/InAlGaP strained quantum well visible vertical cavity surface emitting laser (VCSEL) with both AlAs/AlGaAs distributed Bragg reflectors (DBRs)~ and with InAlPDnAlGaP DBRs2. We have also studied the critical issues for electrical injection in these promising devices and now report the demonstration of the first visible VCSEL diodes. At room temperature with pulsed excitation, the devices lase in a single longitudinal mode within 639.1 nm to 660.7 nm, depending on wafer position. The structures were grown by metalorganic vapor-phase epitaxy (MOVPE) on GaAs substrates and consist of an InAlGaP optical cavity active region surrounded by AlGaAs DBRs. The bottom Si-doped DBR consists of 55.5 periods of alternating AlAs and Al,Gal-,As (x = 0.5) quarter-wave layers with x = 0.75 barrier reduction layers at each interface to reduce series resistance. An identical 36 period C-doped top coupling DBR is used and includes a (p+) GaAs metal contact layer. The InAlGaP optical cavity contains three In0.56Gao.& strained quantum wells in a step graded-barrier separate confinement configuration. Device injection efficiency was optimized by studying the performance of edge-emitting lasers with InAlGaP active regions and AlGaAs DBR cladding layers. Gain-guided etched post test devices were fabricated. A top annulus contact defines emitting diameters of 10 pm and 20 pm with a 5 pm radial thickness, thus, a large percentage of the injected current does not directly contribute to lasing but to device heating. Despite this, lasing wavelengths from 639.1 nm to 660.7 nm were measured, on unrotated wafers, at room temperature without heat-sinking. The pulsed excitation was - 100 ns pulses at 10 kHz to IMHz. Many devices continued lasing with up to a 40% duty cycle at 1MHz. The larger devices typically have a threshold of 30 mA at 2.7 V. The peak output power is 3.38 mW with a resistance of less than 15 0. Substantially improved performance is expected with more sophisticated device processing techniques. This is the first demonstration of InAlGaP visible VCSEL diodes. This work opens the door for advanced device applications such as plastic fiber communications, arrays for displays, interconnects, and holographic memory, and some scanning applications in place of HeNe lasers.