New Concept: the EOM Tilted Wave Laser
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For applications in access networks, single mode
single wavelength devices operating in the 1300 nm range are needed
for single mode fiber transmission at intermediate distances. The
existing advanced solutions based on distributed feedback InP-based
lasers combined with external modulators are far too expensive to
enable scalable future-proof economic solutions for 40Gb/s+
applications. The proposed Tilted Wave Laser (TWL) concept bridges the
gap, being comparable in cost to VCSELs. On one side it provides the
performance characteristic of DFB devices. On the other side it can be
easily integrated vertically with the EO modulator section and, thus,
operate at ultrahigh speeds. There are two key features, which enable
the functionality of the EOM TWL:
1) Wavelength stabilization: The all epitaxial TWL design enables wavelength-stabilized operation, which is a must both for proper single mode single wavelength DFB-class operation, needed for coupling to single mode fiber and avoiding wavelength dispersion effects, and also a prerequisite for intensity modulation using the electro-optical effect.
2) Vertical Integration: Applying the concept of high-order “tilted” mode gives an opportunity to couple vertically the two key sections of the device: the TWL section, providing wavelength-stabilized lasing and operating at a mode angle beyond the angle of the total internal reflection at the facet, and the light output section, which enables light outcoupling through a facet, where the coupling efficiency of this section is electro-optically controlled through the refractive index modulation.
The most straightforward approach of wavelength stabilization in TWLs is based on coupling the leaky waveguide cavity to a transparent substrate. Light emitted from the active medium leaks to the substrate, reflected back towards the waveguide, undergoes constructive or destructive interference with the light propagating in the planar waveguide. Therefore, for a given angle of light propagation in the substrate only certain resonant cavity wavelengths are possible, which fit to the phase matching condition. The smaller the propagation angles of the light in the substrate, the larger the spectral mode splitting. Dielectric and metal multilayer structures may be used to tune the wavelength, improve wavelength temperature stabilization. On-wafer etching may enable on-chip DFB-class multi-wavelength arrays.
After fabricating a TWL, the vertically-integrated EOM BR concept will be applied to such a laser. The all-epitaxial coupled cavity edge-emitting EOM BR TWL will operate in such a way that only one cavity will serve as an exit cavity (the EOM cavity). The second cavity (the TWL cavity) will operate in a high-order vertical mode with an effective mode angle beyondthe angle of the total internal reflection at the facet surface. EO modulation of the coupling strength between the two cavities will modulate the output power. Thus, combination of the wavelength stabilization and EO modulation approach provides a key synergetic advantage and enables a new generation of ultrahigh-speed edge-emitters.
A schematic approach of future integration of the TWL laser and EO modulator is presented in the figure. The mode tilt angle is chosen to be beyond the angle of the total internal reflection. Thus, no light can come out of the waveguide. For light outcoupling an additional narrow waveguide is epitaxially integrated into the structure, to enable a small fraction of the light to undergo a diffraction outcoupling through this layer. The coupling strength between the thin waveguide and the TWL section is controlled by the EO-modulator media, which is introduced either directly into the narrow coupled waveguide, or adjusted in its vicinity. Now the coupling efficiency can be tuned and the EO-modulated TWL can be realized. As a first step, a 980 nm device will be realized and the design optimization will be performed at this wavelength. In a second phase the design will be adopted for 1300 nm wavelength to address applications in medium range LAN and access networks (RoF, PON).