Reflectivity-modulated grating mirror
The invention relates to vertical cavity lasers (VCL) incorporating a reflectivity-modulated grating mirror ( 1 ) for modulating the laser output. A cavity is formed by a bottom mirror ( 4 ), an active region ( 3 ), and an outcoupling top grating mirror ( 1 ) formed by a periodic refractive index grating region in a layer structure comprising a p- and a n-doped semiconductor layer with an electrooptic material layer ( 12 ) arranged there between. The grating region comprises a grating structure formed by periodic perforations to change the refractive index periodically in directions normal to the oscillation axis. A modulated voltage ( 91 ) is applied in reverse bias between the n- and p-doped layers to modulate the refractive index of the electrooptic material layer ( 12 ) and thereby the reflectivity spectrum of the grating mirror ( 1 ). The reflectivity of the grating mirror ( 1 ) can be modulated between a reflectivity with little or no out coupling and a reflectivity with normal out coupling, wherein lasing in the VCL is supported at both the first and the second reflectivity. As the out coupling mirror modulates the output, the lasing does not need to be modulated, and the invention provides the advantage of lower power consumption at high modulation speeds.
1. A vertical cavity laser comprising:
a cavity comprising:
a reflector formed on a substrate, the reflector comprising:
an active material configured to generate light in response to an applied current; and
a reflecting region;
a reflectivity modulated grating minor comprising:
a p-doped semiconductor layer;
an n-doped semiconductor layer; and
an electrooptic material layer arranged between the p-doped semiconductor layer and the n-doped semiconductor layer, wherein the p-doped semiconductor layer, the n-doped semiconductor layer, and the electrooptic material layer form a 1D or 2D grating structure having a plurality of perforations formed therein so that a refractive index changes periodically or nonperiodically
a direction normal to said oscillation axis; and
a gap formed between the first reflector and the reflectivity modulated grating mirror,
wherein the cavity is adapted to support light oscillation between the reflecting region of the first reflector and the reflectivity modulated grating mirror along an oscillation axis normal to the substrate; and
electric contacts in electrical connection to the p-doped semiconductor layer and to the n-doped semiconductor layer configured to apply electrical bias to the electrooptic material layer wherein the p-doped semiconductor layer and the n-doped semiconductor layer act as electric contacts for the electrooptic layer to change a refractive index of the electrooptic material in response to the applied electrical bias.
2. The vertical cavity laser according to claim 1 , wherein the cavity is selected to support lasing in the vertical cavity laser at a predetermined wavelength.
3. The vertical cavity laser according to claim 2 , wherein the electrooptic material layer is configured to, upon application of a first and a second reverse bias voltage between the p-doped semiconductor layer and the n-doped semiconductor layer of the reflectivity modulated grating mirror, provide respective first and second reflectivity spectra with different first and second reflectivities at the predetermined wavelength.
4. The vertical cavity laser according to claim 2 , wherein the reflector is a grating mirror made in a Si layer of a SOI wafer and having a reflectivity of at least 99.9% at the predetermined wavelength.
5. The vertical cavity laser according to claim 1 , wherein the electrooptic material layer is a quantum well semiconductor structure.
6. The vertical cavity laser according to claim 1 , wherein the electrooptic material layer comprises a type-II heterojunction.
7. An optical interconnect comprising the vertical cavity laser according to claim 1 configured to provide a light source.
8. A method for modulating the light emission from a vertical cavity laser comprising:
providing a vertical cavity laser having a cavity, the cavity comprising:
a reflector formed on a substrate, the reflector comprising:
an active material configured to generate light in response to an applied current; and
a reflecting region;
a reflectivity modulated grating mirror, the reflectivity modulated grating mirror comprising:
a p-doped semiconductor layer;
an n-doped semiconductor layer;
an electrooptic material layer arranged between the p-doped semiconductor layer and the n-doped semiconductor layer, the p-doped semiconductor layer, the electrooptic material layer and the n-doped semiconductor layer forming a 1D or 2D grating structure having a plurality of perforations formed therein so that a refractive index of the reflectivity modulated grating mirror changes periodically or nonperiodically in a direction normal to an oscillation axis of the vertical cavity laser; and
a gap formed between the reflector and the reflectivity modulated grating mirror,
wherein the cavity is adapted to support light oscillation between the reflecting region of the first reflector and the reflectivity modulated grating mirror along an oscillation axis normal to the substrate;
initiating laser action in the vertical cavity laser at a predetermined wavelength; and
applying a modulated reverse bias voltage between the n-doped semiconductor layer and the p-doped semiconductor layer of the reflectivity modulated grating minor to modulate the refractive index of the electrooptic material layer to modulate a reflectivity spectrum of the grating mirror between at least a first and a second reflectivity, providing different first and second reflectivities at the predetermined wavelength, respectively, wherein lasing in the vertical cavity laser is supported at both the first and the second reflectivity.
9. The method according to claim 8 , further comprising maintaining the laser action in the vertical cavity laser continuously during the modulation of the reflectivity spectrum of the reflectivity modulated grating mirror.
10. The method according to claim 8 , wherein an electrical bias to an active material of the vertical cavity laser is not modulated during the modulation voltage to the reflectivity modulated grating minor.
11. The method according to claim 8 , further comprising receiving one or more digitally-modulated electric signals and performing the modulation of the modulated reverse bias voltage between the p-doped semiconductor layer and the n-doped semiconductor layer according to the digital modulation of the electric signals so as to imply the same modulation onto the reflectivity of the reflectivity modulated grating mirror and thereby to the optical output signal of the vertical cavity laser.
12. The method according to claim 8 , wherein the first reflectivity is in the interval of 99-99.5%, and wherein the second reflectivity is at least 99.7%.
13. The method according to claim 8 , wherein the electrooptic material layer, the modulated reverse bias voltages, and the predetermined wavelength are selected so that the modulation of the modulated reverse bias voltage predominantly modulates the refractive index in the electrooptic material layer while absorption is substantially small.
14. The method according to claim 8 , wherein the electrooptic material layer comprises a quantum well semiconductor structure, type-II heterojunction, or other structure, wherein the refractive index modulation in the electrooptic material layer is a result of the quantum-confined Stark effect (QCSE) or other effect and is thereby wavelength dependent; and wherein the electrooptic material layer, the modulated reverse bias voltages, and the predetermined wavelength are selected so that the modulation of the modulated reverse bias voltage predominantly modulates the refractive index in the electrooptic material layer via QCSE or other effect while an absorption is at least substantially small.