Method and apparatus for measuring temporal response characteristics of digital mirror devices

• US 20040042000A1
• Filed: 08/29/2002
• Published: 03/04/2004
• Est. Priority Date: 08/29/2002
• Status: Active Grant

• Current Assignee: Texas Instruments, Inc.
• Original Assignee: Texas Instruments, Inc.
• Inventors: Pan, Kun Cindy, Mehrl, David Joseph, Strumpell, Mark Henry, Carr, Rand Derek
• Granted Patent: US 7,095,494 B2
• Time in Patent Office: Days
• US Class Current: 356/218
• CPC Class Codes: G01M 11/005 Testing of reflective surfaces, e.g. mirrors
• CPC Class Codes: G02B 26/0833 the reflecting element being a micromechanical device, e.g. a MEMS mirror, DMD G02B26/0825 takes precedence; micromechanical devices in general B81B

Abstract

A method and system for measuring the temporal response of a micromirror array to a variety of driving signals. A micromirror array is illuminated with a coherent light source so that a diffraction pattern is reflected from the micromirror array. One or more photodetectors are aligned with spots of light in the diffraction pattern that correspond to orders of the diffraction pattern. Diffraction pattern theory predicts that the intensity of these spots of light will vary as the tilt angle of the micromirrors is changed. Thus, by measuring the relative intensity of the spots of light as the micromirror array is provided with a variety of driving signals, many performance characteristics of the micromirror array can be measured. Some of these characteristics include the impulse response, the forced resonant frequency (i.e. the natural frequency), the damped resonant frequency, the quality factor of the micromirror response, the damping factor of the micromirror response, and the frequency transfer function. According to another aspect of the invention, the electromechanical compliance of the micromirrors in the micromirror array can also be measured. It is further contemplated that all of these measurements can be localized to specific regions on the surface of the micromirror array so that the variance of different characteristics across its surface can be analyzed. Another aspect of the disclosed invention is the measurement of the tilt angle of the micromirror array at a non-biased state.

Claims

1. A method of measuring a response characteristic of a micromirror array, the method comprising:

illuminating a micromirror array with a coherent light source so as to generate a reflected diffraction pattern;

aligning a photodetector with a spot of light of the diffraction pattern, wherein the spot of tight corresponds to an order of the diffraction pattern;

applying a dc bias to the micromirror array so that an amount of light directed to the photodetector is at a maximum;

applying a periodic driving signal to the micromirror array; and

measuring the intensity of the spot of light with the photodetector for a period of time after the periodic signal has been applied to the micromirror device.

2. A method according to claim 1, further comprising:

illuminating a plurality of locations on the surface of the micromirror array; and

measuring the intensity of the spot of light at each of the plurality of locations.

3. A method according to claim 1, further comprising determining a transfer function associated with the micromirror array based upon the measurements of the intensity of the spot of light by the photodetector.

4. A method according to claim 1, further comprising determining a frequency of the driving signal at which the amplitude of a response signal measured by the photodetector is at a maximum.

5. A method according to claim 4, further comprising identifying a set of half-power points corresponding the response signal.

6. A method according to claim 5, further comprising identifying a damping factor corresponding to the response signal.

7. A method according to claim 6, further comprising identifying a damped resonant frequency corresponding to the response signal.

8. A method of measuring an impulse response characteristic of a micromirror array, the method comprising:

illuminating a micromirror array with a coherent light source so as to generate a reflected diffraction pattern;

aligning a photodetector with a spot of light of the diffraction pattern, wherein the spot of light corresponds to an order of the diffraction pattern;

applying a dc bias to the micromirror array so that the micromirrors are disposed at an operating point so that the amplitude of the impulse response curve measured by the photodetector is at a maximum;

applying a impulse signal to the micromirror array; and

measuring the intensity of the spot of light with the photodetector for a period of time after the impulse signal has been applied to the micromirror device.

9. A method according to claim 8 further comprising measuring the frequency of the response signal generated by the photodetector during the period of time after the impulse signal.

10. A method according to claim 8 further comprising measuring the amplitude of the response signal generated by the photodetector during the period of time after the impulse signal.

11. A method of measuring a forced resonant frequency characteristic of a micromirror array, the method comprising:

illuminating a micromirror array with a coherent light source so as to generate a reflected diffraction pattern;

aligning a photodetector with a spot of light of the diffraction pattern, wherein the spot of light corresponds to an order of the diffraction pattern;

applying a periodic driving signal to the micromirror array so that a periodic response signal corresponding to the fluctuating intensity of the spot of light is measured by the photodetector;

applying a DC bias to the micromirror array so that an amplitude of the response signal measured by the photodetector is at a maximum; and

measuring the intensity of the spot of light with the photodetector as the frequency of the periodic driving signal is swept over a desired range.

12. A method according to claim 11, further comprising identifying a frequency of the driving signal at which the amplitude of the response signal is at a maximum.

13. A method according to claim 12, further comprising identifying a set of half-power points corresponding the response signal.

14. A method according to claim 13, further comprising identifying a damping factor corresponding to the response signal.

15. A method according to claim 14, further comprising identifying a damped resonant frequency corresponding to the response signal.

16. A method according to claim 15 wherein the measuring, sweeping and identifying operations are performed by a computer system.

17. A method of measuring an electromechanical compliance characteristic of a micromirror array, the method comprising:

illuminating a micromirror array with a coherent light source so as to generate a reflected diffraction pattern;

aligning a photodetector with a spot of light of the diffraction pattern, wherein the spot of light corresponds to an order of the diffraction pattern;

applying a periodic driving signal to the micromirror array so that a response signal corresponding to the intensity of the spot of light is measured by the photodetector;

applying a DC bias to the micromirror array so that an amplitude of the response signal measured by the photodetector is at a maximum;

measuring the amplitude of the response signal; and

determining a relative electromechanical compliance value corresponding to the ratio of an amplitude of the driving signal to the amplitude of the response signal.

18. A method according to claim 17, further comprising:

illuminating a plurality of locations on the surface of the micromirror array; and

measuring the relative electromechanical compliance at each of the plurality of locations.

19. A method of measuring a non-biased response characteristic of a micromirror array, the method comprising:

illuminating a micromirror array with a coherent light source so as to generate a reflected diffraction pattern;

aligning a first photodetector with a first spot of light of the diffraction pattern corresponding to an order of the diffraction pattern;

applying a first dc bias to the micromirror array so that an amplitude of a response signal measured by the first photodetector is at a maximum;

applying a second dc bias to the micromirror array so that the micromirrors in the micromirror array are landed;

releasing the micromirrors in the micromirror array from the landed position by removing the second dc bias; and

measuring the intensity of the first spot of light with the first photodetector for a first period of time after the micromirrors have been released from the landed position.

20. A method according to claim 19, further comprising:

aligning a second photodetector with a second spot of light of the diffraction pattern corresponding to an order of the diffraction pattern; and

measuring the intensity of the second spot of light with the second photodetector for the first period of time.

21. A method according to claim 20, further comprising:

normalizing response curves corresponding to the measured intensity of the first and second spots of light, respectively, so that the final steady state response corresponds to zero; and

determining the non-biased tilt angle of the micromirror array by analyzing when the response curves are at zero.

22. A system adapted to measure the temporal response characteristics of a micromirror array, the system comprising:

a function generator adapted to provide a-driving signal and a sync signal;

a dc power supply adapted to provide a bias voltage;

a signal summer adapted combine the driving signal with the bias voltage and provide a combined signal to the micromirror array;

a coherent light source adapted to illuminate the micromirror array with a coherent light beam so as to generate a reflected diffraction pattern;

at least one photodetector aligned with a spot of light of the diffraction pattern that corresponds to an order of the diffraction pattern, wherein the scope is adapted to generate a response signal corresponding to the intensity of the spot of light; and

a scope adapted to receive the response signal from the at least one photodetector and adapted to receive the sync signal from the function generator.

23. A system according to claim 22, further comprising:

a dual phase lock-in amplifier adapted to provide a periodic signal at a frequency that can be swept through a desired range; and

a computer adapted to control the operations of the dual phase lock-in amplifier and monitor the combined signal provided by the signal summer.