INDUSTRY LASERS
be locked down in place on the silicon photonics chip, as any additional movement would reduce the proportion of light coupled into the silicon photonics waveguide. Doing this is a slow, costly process that is not acceptable for many silicon photonics applications.
An etch facet wafer loaded on an automatic wafer test station
leading to increases in the dielectric volume and stress in the dielectric. Slow separation of the dielectric from the facet then occurs to expose the semiconductor, which degrades in this environment.
What’s more, most cleaved facet lasers are impaired by a discontinuity between the waveguide dielectric and the dielectrics on the facets. So, to avoid degradation, a hermetic package is added. It is worth noting that the dielectrics, which are deposited with an electron-beam evaporator, do offer some protection from the external environment for the facets that they coat. However, in general, their quality is insufficient to provide protection for any extended period of time in a non-hermetic environment.
In comparison, with our lasers that have facets formed by etching rather than cleaving, it is possible to provide continuous coverage of the waveguide surface and the facets with a dielectric. This coating may be applied by plasma-enhanced CVD, which can produce high quality films that are impermeable to moisture. Thanks to this, etched facet lasers do not require a hermetic package. This claim is supported by our study, which revealed that etched facet lasers can successfully operate in 85 percent relative humidity and 85°C for at least 5000 hours, without the need for a hermetic package.
One opportunity for significant cost reductions associated with the fabrication of photonic circuits is to switch from active alignment to passive alignment – it can be up to ten times cheaper. With active alignment, the laser is powered up and moved around until sufficient light is coupled into the silicon photonics waveguide. Once that is achieved, this emitter must
Figure 4. Hybrid silicon lasers offer alignment-free coupling of a laser into a silicon wavguide
Waveguide dimensions
Silicon waveguides have a range of dimensions, and this has a big impact on how difficult it is to couple light into them. In general, there are two types of silicon photonics waveguides: large ones with dimensions of around 3 µm, which are used by Mellanox (formerly Kotura); and those that are 400 nm or narrower, used by IBM and others.
Figure 3. A cleaved fact laser can couple light into a narrow waveguide via a grating coupler
60
www.compoundsemiconductor.net March 2014
Large waveguides are well matched to the mode size of a typical InP-based laser. Thanks to this, it is relatively easy to realise efficient, direct coupling of the laser into these large silicon photonics waveguides. Edge-emitting lasers, as opposed to their surface-emitting cousins, are well suited to coupling light into these types of waveguides. Shrink the dimensions of the waveguide, however, and the situation changes dramatically. In this regime, some form of mode convertor is often employed to aid the coupling of laser light into the silicon photonics waveguide. One option is to use an adiabatic taper. Ideally, the light from the laser has a narrow beam divergence and
Although turning to passive alignment slashes costs, even under the best circumstances the facets of a cleaved facet InP-based laser can only be positioned to within ±2µm of the desired location. Depending on the dimensions of the waveguide – which are discussed shortly – this may not be good enough.
With our InP-based laser that has facets formed by etching, this situation is markedly different. In this case the facets are lithographically defined, so it is possible to know their location relative to an alignment mark or a fuducial to within 0.1 µm, which is more than sufficient (see Figure 1).
Note that with a hybrid silicon laser, there are no alignment issues, because the glass glue process removes the need to align the InP gain wafer to the silicon photonics wafer.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160 |
Page 161 |
Page 162 |
Page 163 |
Page 164