Mikromechanisch weit abstimmbare Oberflächen-emittierende Laserdioden für Sensoranwendungen (English shop)

Preview

Table of Contents, Datei (54 KB)
Extract, Datei (240 KB)

Wavelength-tunable lasers are the key components of future optical communication systems. Moreover, a broadly and continuously tunable laser with a highly pure emission spectrum is a versatile tool for various sensing applications. For example, the emission of greenhouse gases can be monitored by absorption spectroscopy, or the deformation of structures by the interrogation of fiber Bragg gratings (FBG transducers). Owing to their single-mode behavior, low power consumption and compactness, vertical-cavity surface-emitting lasers (VCSELs) are particularly suited for this purpose.

This work presents the design, technology and characterization as well as sensing applications of micromechanically widely tunable VCSELs with wavelengths around 1.55 µm. The developed laser consists of an active optical component (half-VCSEL) and a movable micromechanical component (micro-electro-mechanical system, MEMS), which are combined in a hybrid assembly. By means of electro-thermal actuation, the mirror membrane can be deflected, the air gap enclosed between the two components enlarged, and thus the resonance wavelength continuously tuned.

In a novel cavity design, the elements of the MEMS-VCSEL are decoupled and the tuning range is increased to 60 nm. A curved mirror membrane is employed in order to excite exclusively the desired fundamental mode (Gaussian beam). This yields single-mode output powers of up to 2.8 mW, which represent an international record value, even in comparison with non-tunable VCSELs at this wavelength. Because of the cylindrical cavity geometry, the stability of the polarization is a fundamental problem of VCSELs. By using an elliptical mirror membrane, this symmetry is broken and a linear polarization is deliberately established. A narrow emission linewidth and a high tuning speed are two conflicting properties, since agile mechanical components are subject to thermal noise. By increasing the spring constant (stiffness) of the MEMS, linewidths <40 MHz and tuning with repetition rates >100 Hz can be demonstrated.

Finally, absorption spectroscopy of, among others, carbon monoxide and carbon dioxide as well as FBG measurement technology are demonstrated as two applications in which the unique advantage of the broadband and continuous tunability of the MEMS-VCSEL comes into its own.