8 - Lasers
from Part III - Active Components
Published online by Cambridge University Press: 05 April 2015
Summary
This chapter discusses one of the most challenging aspects of silicon photonics, namely the laser. It is highly desirable to have a silicon-compatible material that can provide optical emission and optical gain, for light sources (lasers, LEDs) and for on-chip optical amplifiers. Silicon is an indirect-band semiconductor, hence very inefficient at light generation. The common semiconductors used to make lasers at wavelengths where silicon is transparent (> 1.1 µm), such as In P-based compounds, have a crystal-lattice constant that is much larger than silicon, hence are difficult to grow on silicon. Germanium is the closest-matched material and has a smaller band gap than silicon; however, it is also an indirect-band semiconductor.
Present-day multi-project wafer (MPW) foundries (see Section 1.5.5) do not provide monolithic or hybrid-integrated lasers, and users rely on external lasers. While edge and grating couplers have both seen improvements in coupling efficiency, the lack of an on-chip source limits the potential applications of these chips. Laser integration is not widely available, and in turn the design of lasers for silicon photonics is an evolving research area. While there are design methodologies for the various laser-integration approaches described below, these depend on the type of approach, hence silicon photonic laser design is not a typical “text-book” topic.
This chapter describes the challenges associated with integrating lasers and optical amplifiers on the silicon photonics platform. It begins with the easiest method of getting light on the chip – namely using external lasers. We then discuss approaches with increasing level of difficulty: co-packaging, epitaxial bonding (hybrid lasers), monolithic growth, and germanium lasers.
External lasers
One approach for silicon photonic systems is to consider the laser as an optical power supply [1], similar to electrical power supplies, both delivering a constant amplitude.
There are several advantages for keeping the laser off-chip.
(1) Thermal management is simplified by keeping the laser off-chip. Lasers are typically 10%–30% efficient, hence an on-chip laser would be a contributing heat source; the heat output is 3–10 times higher than the optical power of the laser itself.
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- Silicon Photonics DesignFrom Devices to Systems, pp. 295 - 310Publisher: Cambridge University PressPrint publication year: 2015
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