Spatial Light Modulator with Structured Mirror Surfaces

• US 20090303571A1
• Filed: 04/24/2009
• Published: 12/10/2009
• Est. Priority Date: 04/24/2008
• Status: Active Grant

• Current Assignee: Micronic Laser Systems AB (Mycronic AB)
• Original Assignee: Micronic Laser Systems AB (Mycronic AB)
• Inventors: Sandstrom, Torbjorn
• Granted Patent: US 8,077,377 B2
• Time in Patent Office: Days
• US Class Current: 359/291
• CPC Class Codes: G02B 26/06 for controlling the phase of light G02B26/08 takes precedence , measuring optical phase difference G01J9/00
• CPC Class Codes: G02B 26/0808 by means of one or more diffracting elements
• CPC Class Codes: G02B 26/0841 the reflecting element being moved or deformed by electrostatic means
• CPC Class Codes: G02B 5/1809 with pitch less than or comparable to the wavelength
• CPC Class Codes: G02B 5/1861 Reflection gratings characterised by their structure, e.g. step profile, contours of substrate or grooves, pitch variations, materials G02B5/1809, G02B5/1828, G02B5/1833, G02B5/1838 and G02B5/1847 take precedence
• CPC Class Codes: G03F 7/70291 Addressable masks, e.g. spatial light modulators [SLMs], digital micro-mirror devices [DMDs] or liquid crystal display [LCD] patterning devices

Abstract

The invention relates to methods to improve SLMs, in particular to reflecting micromechanical SLMs, for applications with simple system architecture, high precision, high power handling capability, high throughput, and/or high optical processing capability. Applications include optical data processing, image projection, lithography, image enhancement, holography, optical metrology, coherence and wavefront control, and adaptive optics. A particular aspect of the invention is the achromatization of diffractive SLMs so they can be used with multiple wavelengths sequentially, simultaneously or as a result of spectral broadening in very short pulses.

Claims

1. A spatial light modulator (an SLM) with a plurality of off-axis stepped surface mirrors, adapted to relay coherent or partially coherent illumination with predetermined resulting wave characteristics of the relayed illumination, including:

an array of tilting mirrors, each mirror having at least one essentially flat reflecting surface and a tilt axis;

surface-structure over the essentially flat reflecting surface of at least some mirrors among the array of tilting mirrors, the surface-structured mirrors having at least one step that divides the reflecting surface into a plurality of reflecting surfaces and creates a phase difference between parts of the relayed illumination from the plurality of reflecting surfaces;

wherein the at least one step is off-axis, relative to the tilt axis.

2. The SLM of claim 1, further including:

the surface-structured mirrors having off-axis steps effective to compensate for variation in wavelength of the illumination, resulting in varying wave characteristics.

3. The SLM of either claim 1 or 2, further including:

the relayed illumination having a center wavelength; and

the phase difference between parts of the relayed illumination from the plurality of reflecting surfaces being larger than 270 degrees relative to the center wavelength.

4. The SLM of either claim 1 or 2, further including:

the relayed illumination having a center wavelength; and

the phase difference between the parts of the relayed illumination being approximately n*360 degrees relative to the center wavelength, where n is a positive, non-zero.

5. The SLM of either claim 1 or 2, wherein:

calculated derivatives of phase of the relayed illumination having opposite signs on different sides of the step.

6. The SLM of claim 1, further including:

the surface-structured mirrors having off-axis steps effective to produce a predetermined polarization state of the relayed illumination.

7. The SLM of any of claims 1 or 6 further including:

the surface structured mirrors having at least one partial reflecting surface formed from micro structures with a lateral dimension being smaller than half a wavelength in regular one-dimensional grids.

8. The SLM of any of claims 1 or 6, further including:

having microstructures that reflect two different incident polarizations with different phases.

9. The SLM of any of claims 1 or 6, further including:

having microstructures that reflect two different incident polarizations with different magnitudes.

10. The SLM of any of claims 1 or 6, further including:

reflecting two different incident polarisations with a phase difference, said phase difference being different on two sides of the tilt axis.

11. The SLM of claim 1, further including:

the surface-structured mirrors having off-axis steps effective to produce a predetermined trajectory of intensity and phase in the relayed illumination through a range of mirror tilts, the predetermined trajectory being different from trajectories produced by mirrors without off-axis steps.

12. A spatial light modulator (an SLM) with a plurality of phase offset pixel groups, adapted to relay coherent or partially coherent illumination with fully complex intensity and phase modulation of the relayed illumination from the pixel groups, including:

an array of tilting mirrors, the mirrors having at least one essentially flat reflecting surface and a tilt axis;

groups of mirrors among the array of tilting mirrors, individual mirrors in the groups arranged to have differing heights producing phase offsets among the individual mirrors in the groups;

wherein driving the individual mirrors in the groups of mirrors produces a fully complex intensity and phase modulation of the relayed illumination from the groups.

13. The SLM of claim 12, further including:

at least two mirrors in a group having a phase step of (1+2n)*lambda/4 (n being a non-negative integer), the two mirrors having different phase offsets relative to an axis of the illumination.

14. The SLM of claim 13, further including:

producing any complex reflection constant within a unit circle while limiting travel to values smaller or equal to a quarter of the wavelength at the edge of the mirrors.

15. A spatial light modulator (an SLM) with a plurality of deep-stepped mirrors, adapted to relay coherent or partially coherent illumination with reduced operational sensitivity to illumination wavelength, including:

an array of tilting mirrors, the mirrors having at least one essentially flat reflecting surface and a tilt axis;

deep-stepped mirrors among the array of tilting mirrors, the deep-stepped mirrors having at least one step that divides the reflecting surface into a plurality of reflecting surfaces and creates a height difference more than 135 degrees, relative to a center wavelength of the illumination, between adjacent reflecting surfaces on a particular mirror;

whereby dependence of diffraction effects resulting from mirror operation on illumination wavelength is reduced

16. A spatial light modulator (SLM) with a plurality of polarization-modifying microstructures applied to pixel groups, adapted to relay coherent or partially coherent illumination with fully complex intensity and phase modulation of the relayed illumination from the pixel groups, including:

an array of tilting mirrors, the mirrors having at least one essentially flat reflecting surface and a tilt axis;

groups of mirrors among the array of tilting mirrors, individual mirrors in the groups arranged to have differing polarization-producing microstructures on surfaces of the individual mirrors among the individual mirrors in the groups;

wherein driving the individual mirrors in the groups of mirrors produces a controllable polarization state of the relayed illumination from the groups.

17. The SLM of claim 16, further including:

Steps dividing at least one essentially flat reflecting surface into partial surfaces with different heights chosen to reduce the influence on the intensity and phase of the relayed radiation by the incident wavelength.

18. Method for producing the steps and structures by nanoimprinting. Support exists in the description.

19. A spatial light modulator (an SLM) with a plurality of mirrors, adapted to modify phase and/or polarization of and to relay coherent or partially coherent illumination, including:

a mirror array of tilting mirrors that control intensity of the relayed illumination by diffraction effects, the mirrors having at least one essentially flat reflecting surface and a tilt axis; and

individual mirrors or groups of mirrors having a predetermined difference in surface structure (that is, a difference in height and/or surface microstructure) among areas on the individual mirrors or among mirrors in the groups of mirrors;

wherein the mirrors with the predetermined difference in surface structure are adapted to be driven individually or in groups to produce a combination of intensity and of (phase and/or polarization) that cannot be achieved by either an array of tilting mirrors that have a single flat reflecting surface or by an array of tilting mirrors that have a stepped reflecting surface with a step coincident with the tilt axis.

20. A method of producing steps and structures by nanoimprinting on mirrors in an array of micromirrors, the method including:

nano-imprinting a UV-hardening compound on a polymer film with a depth pattern from a template;

hardening the UV-hardening compound;

depositing a reflecting film over the hardened compound; and

processing the reflecting film to form an array of tilting micromirrors.