A Horn Ridge Waveguide DFB Laser for High Single Longitudinal Mode Yield

Single longitudinal mode operation of laterally coupled distributed feedback (DFB) laser diodes (LDs) based on GaN containing 10th-order surface Bragg gratings with V-shaped grooves is demonstrated using i-line stepper lithography and inductively coupled plasma etching. A threshold current of 82 mA, a slope efficiency of 1.7 W/A, a single peak emission at 404.5 nm with a full width at half maximum of 0.04 nm and a side mode suppression ratio of > 23 dB at an output power of about 46 mW were achieved under pulsed operation. The shift of the lasing wavelength of DFB LDs with temperature was around three times smaller than that of conventional ridge waveguide LDs.

A key element in any optical system is the light source and semi-conductor lasers have become the most attractive candidates for many applications due to their small size, low drive voltages and high output powers. However, their linewidths need to be reduced for operation at ever-increasing modulation rates. Monolithic laser designs such as the distributed feedback (DFE) and distributed Bragg reflector (DER) produce line-narrowed outputs by including an internal grating within the laser cavity [1). If the cavity is extended beyond the active semi-conductor chip then this long external cavity arrangement, with a reflector at the end, can produce far greater line-width reductions provided that a mechanism is included for selection of a single longitudinal lasing mode [2). Applications for tunable LEC lasers have increased dramatically in recent years and now extend from local oscillators in coherent detection schemes to testing of optical components such as filters and amplifiers and they might in future also be employed in sophisticated fibre optic sensor systems. Various designs have been proposed for mechanical arrangements which would enable wide-range tuning whilst maintaining a single longitudinal mode [3-5) but, in practice, suffer from backlash, hysteresis and slow operation which make them non—ideal for field operation. In the present work we seek an improved control system which will enable wavelength tuning to be performed with the ease and consistency of the electronic tuning schemes.

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.

We demonstrate a stable single longitudinal mode Tm:YAG ceramic laser operating at a single wavelength of 2012.1 nm. The single-longitudinal-mode Tm:YAG ceramic laser is obtained by employing volume Bragg grating as one cavity mirror and inserting a Fabry—Perot etalon into the laser cavity. A maximum single longitudinal mode output power of 165 mW with a slope efficiency of 6.6% is achieved under the pump power of 5.02 W. The laser has a beam quality of M2 = 1.18 at the maximum single longitudinal mode output power.

A statistical yield analysis is presented for gain- and index-coupled distributed feedback (DFB) laser structures, allowing a comparison of their single longitudinal mode (SLM) yield capabilities. For the yield calculations, the threshold gain difference and the longitudinal spatial hole burning (SHB) are taken into account. By comparing the experimental and theoretical yield of index-coupled DFB lasers, the significance of SHB for correct yield predictions is illustrated. For the purpose of comparison, yield calculations for various lambda /4-shifted DFB lasers (with low facet reflectivities) are presented. The most emphasis, however, is on partly gain-coupled DFB lasers. Estimations of practical gain coupling coefficient values for gain and for loss gratings are discussed. >

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.

Single longitudinal mode (SLM) distributed feedback (DFB) lasers with a linewidth lower than a few kHz find applications in many coherent detection systems. In this paper, we proposed and experimentally demonstrated a novel method to compress the linewidth of a SLM DFB laser by utilizing a dual-cavity feedback structure (DCFS). The DCFS first provides optical self-injection feedback to compress the laser linewidth, and then the two feedback lengths are carefully optimized to achieve SLM output via the Vernier principle and the suppression of modes overlapping between two cavities. The linewidthes of 1 MHz and 200 kHz were successfully compressed to ~2.7 and 1.5 kHz with a side mode suppression ratio of 38 and 45 dB, respectively. The stability of the DCFS output power can be controlled within ~0.21%. Our method provides a simple, effective, low cost way to achieve DFB linewidth compression, which will greatly improve the performance of coherent detection systems using DFB laser as sources.

Distributed feedback (DFB) semiconductor laser arrays operating at precisely controlled wavelengths are important components for wavelength division multiplexing networks. Recently, precise wavelength control has been realized by the reconstruction-equivalent-chirp (REC) technique based on sampled Bragg gratings (SBGs) [1]. However, the effective coupling coefficient, κ, of a sampled grating is necessarily reduced substantially from that of a uniform grating, compromising the single longitudinal mode (SLM) performance of DFB lasers. To overcome this, designs of SBGs with phase shifted grating sections have been proposed and demonstrated in fibre lasers. In these structures, the (not required) zeroth-order mode is suppressed while the index modulation experienced by the nonzeroth-order channels is enhanced, the ±1st order channels being of particular interest [2]. Here, for the first time, we apply a combination of π-phase shifted gratings with the REC technique to DFB diode lasers. Using a single electron beam lithography (EBL) step we have demonstrated an increased effective coupling coefficient κ and have fabricated an eight-channel laser array with a spacing of 100 GHz (∼0.8 nm @1550 nm). For the same channel spacing using conventional 0th order gratings (Fig. 1(a)), a resolution of about 0.125 nm would be required, which is beyond the typical resolution limit of 0.5 nm of EBL machines.

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.

It is 21 years since the first passive optical network (PON) was standardized as an asynchronous transfer mode passive optical network (APON) with same optical distribution network scheme as we know in current networks. A lot of PON networks were standardized in the following years and became an important part of telecommunication. The general principles of these PON networks are described in many papers and books, but only a little information about used lasers is available. The aim of this tutorial is to describe lasers used in PON networks and principles of their operation. The paper describes the principles of single longitudinal mode (SLM), multi longitudinal mode (MLM), distributed-feedback (DFB), and Fabry–Pérot (FP) lasers. Furthermore, the lasers are compared by their usage in optical line termination (OLT) for passive optical networks. The second part of this tutorial deals with activation process of optical network unit. The described principle is the same for connection of a new customer or blackout scenario. The end unit is not able to communicate until reach the operational state; each state is defined with physical layer operation and administration and maintenance (PLOAM) messages sequence and their processing.

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.

We report high slope efficiency and low noise characteristics in a distributed feedback (DFB) laser lasing at 1.3 /spl mu/m with narrow beam divergence by employment of a laterally tapered active stripe over the whole cavity. This tapered structure is designed to realize narrow beam divergence, low threshold current, and high longitudinal mode selectivity. The fabricated tapered-active-stripe DFB lasers demonstrated of 9.20/spl times/13.40 and a record slope efficiency, for a narrow beam DFB laser, of 0.59 mW/mA. The temperature characteristics from -10/spl deg/C to 85/spl deg/C shows high output power at high temperature, stable single longitudinal mode oscillation and stable far-field patterns. Furthermore, for the first time ever, a low relative intensity noise characteristic of under -155 dB/Hz has been realized when butt-coupling into a single-mode fiber.

To our knowledge, this is the first report of a monolithically integrated distributed feedback (DFB) semiconductor laser array based on reconstruction-equivalent-chirp (REC) technology. A laser bar with 30 different lasers is obtained, lasing at 30 different wavelengths under single longitudinal mode. The typical threshold is about 40mA to 60mA. The typical slope efficiency is about 0.07 mW/mA to .13 mW/mA. Tested under the injection current of 100mA, the side mode suppression ratios (SMSR) range from 24.9dB to 46.8 dB, with an average of 37.2dB. The proposed method is presented in hoping to make a positive contribution to large-scale photonics integrated circuits (PIC) research for the next-generation fiber-optic networks.

The development of fiber optic interferometric sensor systems has created new demands for single mode semiconductor diode lasers with low output noise power. Recently, several single longitudinal and transverse mode GaAlAs laser diodes have become available. Several experimenters have reported the deleterious effects of optical feedback resulting from fiber optic coupling on the laser's mode structure and high frequency noise spectrum.1-3 Our interest for interferometric sensors is in the low frequency noise, up to a few kilohertz. We report here the first such low frequency noise measurements in fiber coupled and uncoupled single mode diode lasers. Additionally, we also show that the use of index matching fluid can be used to greatly reduce optical feedback and its associated low frequency noise, upon coupling into an optical fiber.

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.