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We describe a method of tailoring the laser spectrum of Fabry-Pérot (FP) laser diodes using a spatially varying refractive index. The cavity geometry is determined by a perturbative transmission matrix calculation of the threshold gain of the longitudinal modes and solution of the corresponding inverse problem. Single-mode FP lasers with predetermined wavelength, and twocolor FP lasers with predetermined mode spacing in the terahertz region are designed and experimentally demonstrated.

Extremely low-cost semiconductor lasers are key components for a wide range of applications, including sensors and optical communications [1]. Manufacturing method and packaging have a decisive impact on costs. As a consequence, cost reduction must be based on a simplification of the fabrication and/or the packaging of the laser, such as direct coupling of the laser to the fiber [2]. However, most single-mode semiconductor lasers, like DFB lasers, require the introduction of an optical isolator between the laser and the coupling fiber in order to avoid alteration of their performances due to optical feedback [3]. Although, external optical feedback resistant DFB and VCSEL lasers were developed and recently demonstrated [4], [5], their fabrication cost remains high. The main drawback of VCSEL lasers is due to the high dependence of their emitting wavelength on the variations in thickness of the structure's epitaxial layers [6]. Such stringent fabrication conditions lead to an increase of their cost. On the other hand, fabrication of DFB lasers requires both extremely high resolution lithography and two or more epitaxial growth steps. The latter requirement particularly affects the reliability of DFB lasers, increasing their price.In this communication, we demonstrate the low sensitivity to both external optical feedback and external optical injection of a new type of extremely low cost single-mode lasers, called "Discrete Mode" (DM) lasers. The concept of these lasers was built on the work of DeChiaro who demonstrated that the emission spectrum of a laser diode can be altered by introducing one or more artificial index perturbations in the laser cavity [7]. DM lasers are then obtained from ridge waveguide Fabry Perot (FP) lasers, in which the effective refractive index of the lasing mode has been perturbed. The perturbation of this index can be achieved by etching features into the ridge waveguide such that each feature has a small overlap with the transverse field profile of the unperturbed mode. Most of the light in the laser cavity is therefore unaffected by these perturbations whereas a proportion of the propagating light is reflected at the boundaries between the perturbed and the unperturbed sections. It has been demonstrated that suitable positioning of these interfaces allows the mirror loss spectrum of a FP laser to be manipulated in order to achieve single longitudinal mode emission (by reducing the threshold gain of an individual cavity mode below that of the other cavity modes) [8]. Note that the fabrication process does not involve epitaxial re-growth and requires only standard optical lithography. These two facts mean that these devices are cheaper to manufacture than DFB lasers.The devices under study in this communications are a DFB and a DM laser, both commercially available and packaged in fiberised hermetically sealed packages. The DFB laser is characterised by an emission wavelength of 1545 nm, a threshold current of 9.9 mA and has an efficiency of 0.096 W/A. The DM laser has an emission wavel...

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We demonstrate the existence of an unusual and useful high-brightness guided mode of a multimode AlGaAs/GaAs heterostructure compound slab waveguide. This mode has a narrow near-field single lobe confined to the low-index regions of the waveguide. This mode was selectively probed by optically exciting quantum wells optimally placed in the waveguide. By pumping in a stripe geometry, lasing is observed above a threshold of 80 kW/cm2 indicating efficient lasing in the highest-order waveguide mode. The near-field emission pattern of the waveguide was imaged to provide a direct measurement of the intensity profile of the higher-order mode.

A new class of semiconductor laser, termed Discrete Mode (DM) laser, has been introduced [1] which provides a lowcost manufacturable technology for high speed stable single mode communications without optical isolation, and which can achieve wide wavelength tunability [2]. Here we demonstrate that, in addition, this class of laser exhibits ultra-narrow sub MHz linewidth emission such as is necessary for numerous applications in optical communications and sensors, without introducing severe levels of complexity into the laser design which is of paramount importance for devices to be of practical commercial use. We also compare the spectral performance of commercial DM and DFB lasers.Distributed feedback (DFB) lasers and vertical cavity surface emitting lasers (VCSELs) require very stringent fabrication conditions for narrow linewidth operation and neither device is readily able to achieve linewidths below 1 MHz. DM lasers are obtained by etching small numbers of index perturbing slot features into otherwise conventional ridge waveguide FP lasers in order emit light in a single mode at a predetermined wavelength [1]. In these lasers the optical feedback is provided mainly by the facets of the device with the allowed longitudinal modes being determined by the distance between the facets and the average refractive index of the cavity. The slot features physically introduce a spatially varying effective index perturbation on top of the constant average effective index of the unperturbed cavity. This manifests itself in the Fourier domain as a wavelength selective loss spectrum. Suitable positioning of these slots therefore allows the mirror loss spectrum of an FP laser to be manipulated in order to achieve stable single longitudinal mode emission by reducing the threshold gain of an individual cavity mode below that of all other cavity modes [1].The devices used in this work were asymmetrically coated ridge waveguide Fabry Perot lasers which incorporated etched slot features and emitting around X = 1.55 ptm. The asymmetric coating serves to redirect light that would normally exit from the rear facet of the laser to the front facet as is common in FP laser design. The active region of the devices consisted of a strained compensated InAlGaAs MQW structure. The ridge waveguide and slot features were made using only a combination of conventional dry and wet etching surface processing techniques. Regrowth was not used in any stage of device fabrication. Additionally, the slot features do not impinge on the active region [1]. The cavity length of the devices was 350pm and the ridge width was approximately 2.5ptm. To achieve the desired spectrum the positions of the etched slot features were generated using an analytical procedure.