MEMS control system gain normalization
A method and system for providing gain normalization in a MEMS control system and/or device includes moving a MEMS structure from a first position wherein light is redirected from a first input port to an output port to a second position wherein light is redirected from a second input port to the output port, dithering an orientation of the MEMS structure, monitoring an intensity of the dithered light and providing a feedback signal in dependence upon the monitored intensity, and using the feedback signal, determining a control loop gain for active alignment of the MEMS structure. Gain normalization is achieved by applying a fitting function, which is split into a gain control loop gain part and a mechanical part.
1. An optical system comprising:
a MEMS structure movable between a first position in which light is redirected from a first input port to an output port and a second position in which light is redirected from a second input port to the output port, the first input port spatially separated from the second input port;
a MEMS actuator for moving the MEMS structure between the first and second positions, and for dithering an orientation of the MEMS structure about the first and second positions;
a detector optically coupled to the output port for monitoring an intensity of the dithered light and for providing a feedback signal in dependence upon the monitored intensity; and
a controller for receiving the feedback signal from the detector, for determining a control loop gain used in active alignment of the MEMS structure, for applying a first normalization function to the control loop gain, and for applying a second normalization function to a dither magnitude,
wherein the first normalization function is selected in dependence on the second normalization function,
wherein the first input port transmits light from a first laser in a laser array, and the second input port transmits light from a second laser in the laser array,
wherein the output port transmits light into an input waveguide of an optical modulator, and
wherein the feedback signal is shared between four separate control loops, a first control loop for actively aligning the MEMS structure in a first dimension, a second control loop for actively aligning the MEMS structure in a second other dimension, a third control loop for controlling the bias of the optical modulator, and a fourth control loop for controlling a driver voltage of the optical modulator, the control loop gain corresponding to a gain provided in the first control loop.
2. An optical system according to claim 1 , wherein the feedback signal is AC coupled.
3. An optical system according to claim 1 , wherein the first and second normalization functions are determined using fixed point calculations.
4. An optical system according to claim 3 , wherein the first and second normalization functions are limited to a square term.
5. An optical system according to claim 3 , wherein each of the first and second normalization functions is calculated from
Normalization
(
LSBs
)
=
3
,
200
,
000
(
LSBs
/
200
)
2
+
140
+
130
.
6. An optical system according to claim 1 , wherein the MEMS structure is an electrostatically actuated two-axis MEMS mirror.
7. An optical system according to claim 6 , wherein the detector is a photodiode integrated with the optical modulator.
8. An optical system according to claim 7 , comprising a collimating lens for collimating light transmitted from the laser array to the MEMS minor and a focusing lens for focusing collimated light redirected from the MEMS minor to the input waveguide of the optical modulator.
9. An optical system according to claim 8 , comprising a first tap for redirecting a portion of the light redirected from the MEMS mirror to a wavelength detector, and a second tap for redirecting light another portion of the light redirected from the MEMS mirror to a quadrant detector.
10. An optical system according to claim 1 , wherein a same dither voltage provides different dither magnitudes for the first and second positions, and wherein the second normalization function is selected to reduce the difference in the dither magnitudes.
11. An optical system according to claim 1 , wherein each of the first and second normalization functions is selected to compensate for non-linearity in MEMS actuation, the non-linearity including a non-linear relationship between MEMS deflection angle and applied deflection voltage.
12. An optical system according to claim 1 , wherein each of the first and second normalization functions is a curve corresponding to the square-root of total dither compensation.
13. An optical system according to claim 1 , wherein each of the first and second normalization functions is selected to provide only partial gain normalization.