Micromachined mirror assembly having reflective layers on both sides

1. A micromachined mirror assembly, comprising:

a mirror-base layer; and

a first reflective layer on a top surface of the mirror-base layer and a second reflective layer on a bottom surface of the mirror-base layer, wherein:

the first reflective layer is reflective to incident light of the micromachined mirror assembly,

the first reflective layer and the second reflective layer are made of a same material and have same dimensions, and

the first reflective layer and the second reflective layer are each externally exposed at a respective surface facing away from the mirror-base layer.

2. The micromachined mirror assembly of claim 1 , wherein the first and the second reflective layers are each made of at least one of metal and a distributed Bragg reflector that have same thermal expansion coefficient and same Young's modulus.

3. The micromachined mirror assembly of claim 2 , wherein the first and second reflective layers include aluminum, gold, or a combination thereof.

4. The micromachined mirror assembly of claim 1 , wherein the first and second reflective layers each has a same thickness and a total area.

5. The micromachined mirror assembly of claim 1 , wherein a stress gradient through the first reflective layer, the mirror-base layer, and the second reflective layer is equal to zero along a direction perpendicular to the top surface of the mirror-base layer.

6. The micromachined mirror assembly of claim 1 , wherein the mirror-base layer includes silicon.

7. The micromachined mirror assembly of claim 1 , wherein at least one of the top surface and the bottom surface includes more than one portions, the respective top surfaces of the more than one portions being positioned at different elevations along a direction perpendicular to the respective surface.

8. An optical sensing system, comprising:

a transmitter configured to scan a laser beam towards an object, wherein the transmitter comprises a micromachined mirror assembly that outputs the laser beam, the micromachined mirror assembly comprising a rotatable mirror that comprises:

a mirror-base layer; and

a first reflective layer on a top surface of the mirror-base layer and a second reflective layer on a bottom surface of the mirror-base layer, wherein:

the first reflective layer is reflective to incident light of the micromachined mirror assembly,

the first reflective layer and the second reflective layer are made of a same material and have same dimensions, and

the first reflective layer and the second reflective layer are each externally exposed at a respective surface facing away from the mirror-base layer; and

a receiver configured to detect a returned laser from the object.

9. The optical sensing system of claim 8 , wherein the first and the second reflective layers are each made of at least one of metal and a distributed Bragg reflector that have same thermal expansion coefficient and same Young's modulus.

10. The optical sensing system of claim 9 , wherein the first and second reflective layers include aluminum, gold, or a combination thereof.

11. The optical sensing system of claim 8 , wherein the first and second reflective layers each has a same thickness and a total area.

12. The optical sensing system of claim 8 , wherein a stress gradient through the first reflective layer, the mirror-base layer, and the second reflective layer is equal to zero along a direction perpendicular to the top surface of the mirror-base layer.

13. The optical sensing system of claim 8 , wherein the mirror-base layer includes silicon.

14. The optical sensing system of claim 8 , wherein at least one of the top surface and the bottom surface includes more than one portions, the respective top surfaces of the more than one portions being positioned at different elevations along a direction perpendicular to the respective surface.

15. The optical sensing system of claim 8 , further comprising an additional rotatable mirror and a stationary mirror, wherein

the rotatable mirror receives a first laser signal and reflects the first light signal towards the stationary mirror to form a second laser signal;

the stationary mirror receives the second laser signal and reflects the second laser signal towards the additional rotatable mirror to form a third laser signal; and

the additional rotatable mirror receives the third laser signal and reflects the third laser signal as the laser beam.

16. The optical system of claim 15 , wherein the additional rotatable mirror is identical to the rotatable mirror.

17. A transmitter in an optical sensing system, comprising:

a scanner configured to scan a laser beam towards an object, wherein the scanner comprises a micromachined mirror assembly that comprises a rotatable mirror configured to receive and reflect a laser signal as the laser beam, the rotatable mirror comprising:

a mirror-base layer; and

a first reflective layer on a top surface of the mirror-base layer and a second reflective layer on a bottom surface of the mirror-base layer, wherein:

the first reflective layer is reflective to incident light of the micromachined mirror assembly,

the first reflective layer and the second reflective layer are made of a same material and have same dimensions, and

the first reflective layer and the second reflective layer are each externally exposed at a respective surface facing away from the mirror-base layer.

18. The transmitter of claim 17 , wherein the first and the second reflective layers are each made of at least one of metal and a distributed Bragg reflector that have same thermal expansion coefficient and same Young's modulus.

19. The transmitter of claim 18 , wherein the first and second reflective layers include aluminum, gold, or a combination thereof.

20. The transmitter of claim 17 , wherein the first and second reflective layers each has a same thickness and a total area.