Single-mode VCSELs are gaining increasing importance for both, high end spectroscopy applications (e.g. Tunable Diode Laser Absorption Spectroscopy = TDLAS) as well as consumer applications like optical engines in modern PC input devices (e.g. mouse). Single-mode VCSELs do have unique characteristics like ideal circular symmetric Gaussian beam, small beam divergence, high side-mode suppression ratio, single polarized highly coherent emission, no mode hopping over the entire operation range, and very small power dissipation. The typical semiconductor device is as small as 250 x 250 µm² in footprint.
Especially for spectroscopy their linear and monotonic tuning of wavelength with ambient temperature as well as internal heating allows for precise adjustment of the emission wavelength and comfortably sweeping the emission wavelength across absorption lines of e.g. gases. Fig. 1 shows a very compact system of single-mode VCSEL, Thermo Electric Cooler (TEC), thermistor, and ESD protection diode on a small standard 5 pin TO46 header (only 5 mm in diameter). The small thermal load of the VCSEL and the small overall thermal mass allows the use of a tiny TEC with minimized electrical power for precise temperature control of the device and thus emission wavelength. The emission wavelength can be tuned by +/- 2 nm.
Today’s dominant application of single-mode VCSELs in spectroscopy is oxygen sensing in gas mixtures. According absorption lines are located spectrally around 760 nm. The high resolution optical spectrum in Fig. 2 clearly shows a nice single-mode behavior with more than 30 dB side-mode suppression ratio at center wavelength of 762 nm and 0.5 mW laser output power. Corresponding LIV characteristics are depicted in Fig. 3. Typical operation point is 1.6 mA laser current at 2.1 V applied voltage, resulting in less than 3 mW dissipated power.
Fig. 2: Optical spectrum of a 760 nm single-mode VCSEL designed for oxygen sensing by TDLAS.
Fig. 3: LIV characteristics of a 760 nm single-mode VCSEL. Dissipated power is less than 3 mW in cw operation.
Whereas 30 dB side-mode suppression ratio is crucial for spectroscopic applications, lower production cost is a key to enter consumer markets like optical input devices for PCs. The first application served by single-mode VCSELs in this field of technology is the optical mouse where 850 nm wavelength is used because of advantageous material quality and good match to standard detector technology. The presence of higher order transverse modes, which are still suppressed by more than 10 dB at 1 mW output power indicates a slightly increased active diameter. Typical data of optical spectrum and LIV are given in Fig. 4 and Fig. 5, respectively.
Fig. 4: Optical spectrum of a low costs 850 nm single-mode VCSEL.
Fig. 5: LIV characteristics of a 850 nm single-mode VCSEL providing up to 1 mW output power at less than 2 mA laser current.
Single-mode VCSELs have been recognized in the early 1990s as scientifically interesting exotic lasers, very difficult to manufacture. Especially for the so called second generation oxide confined VCSELs the control of the oxide aperture, that defines both, current and optical field, is challenging. By improving the control of technology both performance and yields have improved significantly and thus opened markets as described above for this superior laser source.
