Electro-absorption modulator
An electro-absorption modulator 100 including a quantum well 102 configured to provide a variable electromagnetic absorption spectrum in response to an applied electric field, wherein the quantum well is a type-II quantum well comprising two material layers, each material layer having a graded material composition, such that the potential in each material layer varies as a function of position.
1 . An electro-absorption modulator comprising:
an input configured to receive an incoming beam of electromagnetic radiation;
a quantum well configured to: (i) provide a variable electromagnetic absorption spectrum (ii) to receive and modulate the incoming beam of electromagnetic radiation in response to an electric field applied to the quantum well, and (iii) generate a modulated beam of electromagnetic radiation, and
an output configured to pass the modulated beam from the electro-absorption modulator,
wherein the quantum well is a type-II quantum well comprising two material layers, wherein a first material layer of the two material layers has a first material composition and a second material layer of the two material layers has a second material composition which differs from the first material composition, and
wherein the first material composition and the second composition are each graded such that a potential in each of the two material layers varies as a function of position.
2 . The electro-absorption modulator of claim 1 , wherein the potential in one or more of the two material layers is parabolic as a function of position.
3 . The electro-absorption modulator of claim 1 , wherein the potential in one or more of the two material layers is triangular as a function of position.
4 . The electro-absorption modulator of claim 1 , wherein the first material layer is formed of AlInAs and the second material layer is formed of InAsP, and
the second material layer is directly adjacent the first material layer.
5 . The electro-absorption modulator of claim 4 , wherein the first material layer of AlInAs has a thickness in a range 15 nm to 25 nm.
6 . The electro-absorption modulator of claim 4 , wherein the second material layer of InAsP has a thickness in a range 10 nm to 20 nm.
7 . The electro-absorption modulator of claim 4 , wherein in the first material layer the proportion of Al is graded in a range of 0.5 to 0.3.
8 . The electro-absorption modulator of claim 1 , wherein in the second material layer arsenic (As) is in a graded proportion in a range 0.4 to 0.2.
9 . The electro-absorption modulator of claim 1 , wherein the first material layer is formed of InGaAs and the second material layer is formed of GaAsSb, and
the second material layer is directly adjacent the first material layer.
10 . The electro-absorption modulator of claim 9 , wherein the first material layer of InGaAs has a thickness in a range 15 nm to 25 nm.
11 . The electro-absorption modulator of claim 9 , wherein the second material layer of GaAsSb has a thickness in a range 10 nm to 20 nm.
12 . The electro-absorption modulator of claim 9 , wherein in the first material layer a proportion of In is graded in a range 0.35 to 0.55.
13 . The electro-absorption modulator of claim 9 , wherein in the second material layer the proportion of As is graded in a range 0.4 to 0.6.
14 . The electro-absorption modulator of claim 1 , wherein the quantum well is in a quantum well region comprising a plurality of type-II quantum wells, wherein the type-II quantum wells are adjacent each other to provide a multiple quantum well structure.
15 . A modulating retroreflector comprising the electro-absorption modulator of claim 1 .
16 . A free-space optical communications system, comprising the modulating retroreflector of claim 15 .
17 . The free-space optical communications system of claim 16 , wherein the modulating retroreflector is located on an unmanned aerial vehicle.
18 . The electro-absorption modulator of claim 1 , wherein the first material composition is a first group of chemical elements and a second material composition is a second group of chemical elements that differ from the first group.
19 . The electro-absorption modulator of claim 18 , wherein relative proportions of the chemical elements of the first group of chemical elements varies as a function of position within the first material layer, and wherein relative proportions of chemical elements in the second group of chemical elements varies as a function of position within the second material layer.