Low Sensitivity to Optical Feedback and Optical Injection of Discrete Mode Lasers

Abstract

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...

Robust single frequency index-patterned laser design using a Fourier design method

“…Index-patterned Fabry-Pérot lasers [1,2] offer a simple regrowth-free path to single-mode laser operation, requiring only a gain source, a lithographically defined perturbative refractive index pattern and reflective facets to function. In addition, the lasers have unique properties including narrow linewidth [3] and are robust against feedback [4]. The concept has recently been applied for Silicon-photonics integrated lasers [5].…”

“…The concept has recently been applied for Silicon-photonics integrated lasers [5]. The refractive perturbation can be achieved with a shallow slot or pit inserted in the waveguide [4,6]. The spectral content of the index-patterned laser comes from sculpting the gain profile of the individual longitudinal modes, for which a wide range of index patterns have been used [3][4][5].…”

Discrete mode laser diodes with very narrow linewidth emission

“…For standard commercial DMLDs such as those used here with Lcav=350µm and high facet reflectivities (√R 1 √R 2 ≅ 0.5), the cold cavity linewidth is ∆ν cavity ≅ 28 GHz. While no explicit gain peak detuning (gain peak λ = 1546 nm) of the laser emission wavelength was designed for the 5 DM to reduce the α parameter (∆ν ∝ 1 + α 2 ), the DM multiple quantum well active region was fabricated in the InGaAlAs material system where due to its higher differential gain, α is about 2-3 compared to 5 for an InGaAsP-based DFB.…”