Controlling the position of the focal point of a laser beam

Abstract

In one embodiment, a system for controlling the position of a focal point of a laser beam includes a beam expander, a scanner, an objective lens, and a computer. The beam expander controls the focal point of the laser beam and includes a mirror and an expander optical device. The mirror has a surface curvature that can be adjusted to control the z-position of the focal point. The expander optical device directs the laser beam toward the mirror and receives the laser beam reflected from the mirror. The scanner receives the laser beam from the beam expander and manipulates the laser beam to control the x-y position of the focal point. The objective lens receives the laser beam from the scanner and directs the beam toward a target. The computer receives a depth command and sets operating parameters to control the surface curvature of the mirror according to the depth command.

Description

[Technical field]

[0001] The present disclosure relates generally to laser systems, and more particularly to systems and methods for controlling the position of a focal point of a laser beam. [Background technology]

[0002] In ophthalmic laser surgery, it is important to make precise cuts. For example, flaps in laser in situ keratomileusis (LASIK) are typically created close enough to Bowman's layer to avoid trauma caused by pulling back the flap, but far enough away from Bowman's layer to avoid tearing the layer, and thus the flap is cut to a depth of about 80 micrometers (μm) to 500 μm, e.g., about 120 μm. As another example, the lenticule removed in a small incision lenticule extraction (SMILE) procedure leaves a curvature intended to provide refractive correction to the cornea, and thus the lenticule must be precisely cut. Thus, to enable consistent, high-quality results, the focus of the laser beam must be controllable within an accuracy of a few micrometers. Summary of the Invention [Means for solving the problem]

[0003] In one embodiment, a system for controlling the position of a focal point of a laser beam includes a beam expander, a scanner, an objective lens, and a computer. The beam expander controls the focal point of the laser beam and includes a mirror and an expander optical device. The mirror has a surface curvature that can be adjusted to control the z-position of the focal point. The expander optical device directs the laser beam toward the mirror and receives the laser beam reflected from the mirror. The scanner receives the laser beam from the beam expander and steers the laser beam to control the x-y position of the focal point. The objective lens receives the laser beam from the scanner and directs the beam toward a target. The computer receives a depth command and sets operating parameters to control the surface curvature of the mirror according to the depth command.

[0004] In one embodiment, a method for controlling a position of a focal point of a laser beam directed at a target includes receiving a depth command at a computer, where the computer sets operating parameters to control a surface curvature of a mirror according to the depth command. A beam expander includes a mirror and an expander optical device and receives the laser beam. The mirror has a surface curvature that can be adjusted to control a z-position of the focal point. The expander optical device directs the laser beam towards the mirror, which reflects the laser beam with a surface curvature that controls the z-position of the focal point. The expander optical device receives the laser beam reflected from the mirror. A scanner receives the laser beam from the beam expander and steers the laser beam to control the xy-position of the focal point. An objective lens receives the laser beam from the scanner and directs the beam towards the target.

[0005] Embodiments of the systems and methods may include any one, two or more of the following features in any suitable combination.

[0006] the polarizer is configured to receive a laser beam with a first linear polarization and direct the laser beam with the first linear polarization towards the waveplate and to receive a laser beam with a second linear polarization and pass the laser beam with the second linear polarization.

[0007] The waveplate is configured to receive a laser beam with a first linear polarization from the polarizer, direct the laser beam towards the mirror, receive the laser beam reflected from the mirror, and convert the laser beam reflected from the mirror to a second linear polarization that is rotated relative to the first linear polarization.

[0008] The difference between the first and second polarization is 90 degrees.

[0009] The waveplate includes a quarter waveplate configured to receive a laser beam with a first linear polarization from the polarizer, convert the laser beam from the first linear polarization to a circular polarization, direct the laser beam towards a mirror, receive the laser beam reflected from the mirror, and convert the laser beam reflected from the mirror from the circular polarization to a second linear polarization.

[0010] The waveplate includes a half-waveplate configured to receive a laser beam with a first linear polarization from the polarizer, rotate the first linear polarization by 45 degrees, direct the laser beam towards the mirror, receive the laser beam reflected from the mirror, and rotate the polarization of the laser beam reflected from the mirror by 45 to a second linear polarization.

[0011] The collimator is configured to collimate the laser beam and direct the laser beam towards the scanner.

[0012] The depth command specifies the z position for the focus.

[0013] The depth command is a command to increase or decrease the focal point's current z position.

[0014] Embodiments of the present disclosure will now be described in greater detail, by way of example only, with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0015] [Figure 1] FIG. 1 shows an embodiment of a laser system that can control the position of the focal point of a laser beam directed at a target. [Figure 2A-B] 2A and 2B show cross-sectional views of a mirror according to an embodiment of the present invention. [Figure 2C] 2A and 2B show an embodiment of a mirror, and FIG. 2C shows an example of an electrode plate of the mirror. [Diagram 3] FIG. 3 illustrates a method for controlling the position of the focal point of a laser beam directed at a target that may be performed by the system of FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016]

[0023] Referring now to the description and drawings, example embodiments of the disclosed devices, systems, and methods are shown in detail. As will be apparent to those skilled in the art, the disclosed embodiments are illustrative and do not encompass all possible embodiments.

[0017] 1 shows an embodiment of a laser system 10 that can control the position of a focal point of a laser beam directed at a target 24. Laser system 10 includes a beam expander 20 that expands the laser beam. Beam expander 20 has a mirror 22 that has a surface curvature that can be changed by applying a voltage to electrodes on mirror 22. By changing the curvature, the z-position of the focal point of the laser beam is changed, thereby changing the depth of the laser beam within target 24.

[0018] In the illustrated embodiment, laser system 10 includes a laser 26 and a beam expander 20, coupled as shown. Beam expander 20 includes a mirror 22 and an optical device 30, which includes a polarizer 32, a wave plate 34, and a collimator 36, coupled as shown. Laser system 10 also includes a scanner 40, an objective lens 42, control electronics 44, and a computer 46, coupled as shown. Laser system 10 may be used to perform ophthalmic procedures on a portion of an eye (e.g., the cornea of ​​a human or animal eye) or a test material that mimics that portion (e.g., poly(methyl methacrylate) (PMMA)).

[0019] According to an example of a method that may be used with the laser system 10, the computer 46 receives a depth command and sets one or more operating parameters for the control electronics 44 to control the surface curvature of the mirror according to the depth command. The laser 26 generates a laser beam, which may be linearly polarized. The polarizer 32 of the beam expander 20 receives and reflects the linearly polarized laser beam. The waveplate 34 receives the laser beam and converts the laser beam from linear polarization to circular polarization. The mirror 22 receives the laser beam and reflects the laser beam with a surface curvature that is controlled according to the depth command. The waveplate 34 receives the reflected laser beam and converts the laser beam reflected from the mirror from circular polarization to linear polarization, which is then rotated relative to the linear polarization at which the laser beam was previously received. The polarizer 32 passes the laser beam with the rotated linear polarization. The collimator 36 collimates the laser beam and directs it towards the scanner 40. The scanner 40 steers the laser beam to control the xy position of the focal spot. The objective lens 42 directs the laser beam towards the target 24 .

[0020] To aid in the description of the embodiments, an xyz coordinate system for the laser system is described. The direction of the laser beam as it approaches the target defines the z-axis. The z-axis in turn defines the xy plane. A "z position" refers to a point on the z-axis, and an xy position refers to a point on the xy plane. The placement of the x- and y-axes on the xy plane may be selected in any suitable manner. For example, if the target 24 is a patient's eye, the x-axis or y-axis may be parallel to the patient's vertical axis. The origin of the z-axis may be selected in any suitable manner. For example, if the target 24 is an eye, the origin may be the front surface of the eye, which may or may not be in contact with the patient's interface.

[0021] To aid in the description of the embodiments, optical devices are described. Optical devices are devices that control (e.g., reflect, refract, filter, transmit (or pass), and / or polarize) light. The devices may be made of any suitable material that controls light in a designed manner, such as glass, crystal, metal, or semiconductor. Examples of optical devices include lenses, mirrors, prisms, optical filters, wave guides, wave plates, expanders, collimators, splitters, gratings, and polarizers.

[0022] Example components of system 10 may be as follows: Laser 24 is a device that produces an intense beam of coherent monochromatic light by stimulated emission of photons from excited atoms or molecules. The laser beam may have any suitable wavelength, for example, in the infrared (IR) or ultraviolet (UV) range. The pulses of the laser beam may have pulse widths in any suitable range, for example, in the nanosecond, picosecond, femtosecond, or attosecond range.

[0023] The beam expander 20 is one or more optical devices that expand the diameter of the laser beam and control the focus of the laser beam. In the illustrated embodiment, the beam expander 20 includes a mirror 22 and an optical device 30. The mirror 22 is an optical device that reflects light, e.g., a laser beam. In the illustrated embodiment, the mirror 22 has a surface curvature that can be adjusted to control the z-position of the focus of the laser beam. The mirror 22 may be any suitable deformable mirror, e.g., a Revibro Optics deformable mirror. The mirror 22 is described in more detail with reference to Figures 2A and 2B.

[0024] The expander optical device 30 is an optical device that directs the laser beam toward the mirror 22 and receives the laser beam reflected from the mirror 22. In the illustrated embodiment, the optical device 30 includes a polarizer 32, a wave plate 34, and a collimator 36. The polarizer 32 is an optical filter that transmits light of a specific polarization while blocking light of other polarizations. It can convert light of undefined or mixed polarizations into light with a single polarization state (linear, circular, or elliptical). In the illustrated embodiment, the polarizer 32 reflects the laser beam with a first linear polarization toward the wave plate 34 and transmits the laser beam with a second linear polarization. The relationship between the first and second polarizations can be selected such that the polarizer 32 can direct the beam toward the mirror 22 as reflected by the mirror 22 (e.g., through the wave plate 34) and then pass the reflected beam (e.g., through the wave plate 34). In the illustrated embodiment, the waveplate 34 changes the linear polarization of the light beam by 90 degrees, so the difference between the first and second polarizations is 90 degrees.

[0025] Waveplate 34 is an optical device that changes the polarization state of light traveling through it. Waveplate 34 may be any suitable waveplate, such as a quarter waveplate that converts linearly polarized light to circularly polarized light or vice versa, or a half waveplate that rotates linearly polarized light by 45 degrees. In the illustrated embodiment, waveplate 34 is a quarter waveplate that receives a laser beam with a first linear polarization from polarizer 32, converts the laser beam from the first linear polarization to circular polarization, and directs the laser beam towards mirror 22. Waveplate 34 then receives the laser beam reflected from mirror 22 and converts the laser beam reflected from the mirror from circular polarization to a second linear polarization that is rotated relative to the first linear polarization. In the illustrated embodiment, waveplate 34 changes the original linear polarization of the light beam by 90 degrees.

[0026] In another embodiment, waveplate 34 is a half-wave plate that receives the laser beam with a first linear polarization from polarizer 32, rotates the polarization of the laser beam by 45 degrees, and directs the laser beam towards mirror 22. Waveplate 34 then receives the laser beam reflected from mirror 22 and rotates the polarization of the laser beam by 45 degrees to a second linear polarization that is rotated relative to the first linear polarization. In an embodiment, waveplate 34 changes the original linear polarization of the light beam by 90 degrees.

[0027] The collimator 36 is an optical device that creates a substantially parallel or nearly parallel beam of light or radiation. After being reflected by the mirror 22, the beam exhibits strong divergence. Thus, the collimator 36 collimates the laser beam and directs the laser beam to the scanner 40.

[0028] The laser system 10 also includes a scanner 40, an objective lens 42, control electronics 44, and a computer 46. The laser beam scanner 40 is one or more optical devices that control the direction of the laser beam to control the xy position of the focal spot. To deflect the laser beam laterally, the scanner 40 may have a pair of galvanometrically driven scanner mirrors that tilt about mutually perpendicular axes. In the illustrated embodiment, the scanner 40 receives the laser beam from the beam expander 20 and steers the laser beam to control the xy position of the focal spot. The objective lens 42 receives the laser beam from the scanner 40 and directs the beam to the target 24.

[0029] The computer 46 receives the depth command and adjusts one or more operating parameters of the control electronics 44 to control the surface curvature of the mirror 22 in accordance with the depth command. The depth command describes the desired z position of the laser beam, including the depth of the focal spot into the target 24. The command may be based on user input or commands of the software program. For example, a user may input a particular depth of the focal spot on or into the target 24, and the command may reflect that depth. As another example, the software program may instruct the computer 46 to vary the z position (and possibly the xy position) of the focal spot to generate photodisruption on or in the target 24 according to a specified pattern. As another example, the software program may vary the focal spot within a range to find a particular effect (e.g., highest reflectivity) within the range.

[0030] Central electronics 44 receives instructions from computer 46 and applies voltages to mirror 22 to change the curvature of mirror 22 according to the instructions. In one embodiment, central electronics 44 includes an electrical amplifier that receives a low voltage (e.g., 0-10V) signal and provides a high voltage signal (e.g., 0-400V) to operate mirror 22. To control aberrations, the amplifier may control multiple zones with a low voltage signal for each zone.

[0031] Figures 2A-2C show an embodiment of mirror 22. Figures 2A and 2B show cross-sectional views of mirror 22. Mirror 22 may have any suitable diameter, for example, 2-4 mm. The focal length of mirror 22 may be varied over any suitable range, for example, 65 mm to infinity.

[0032] The mirror 22 includes a housing 50, an electrode plate 52, a membrane 54, and a connector 56 coupled as shown. The housing 50 provides structural support for the components of the mirror 22 and may be made of any suitable rigid material. The membrane 54 may include a flexible material, such as a polymer. The membrane 54 has a reflective surface 57, which is the surface that reflects the laser beam in the system 10. The reflective surface 57 may have any suitable diameter, for example, 3-5 mm, and may be a metal, for example, aluminum, gold, or silver. The connector 56 is used to apply a voltage between the membrane 54 and the electrode plate 52 and may be any suitable conductive material.

[0033] 2C shows an example of electrode plate 52. Electrode plate 52 includes electrodes 58 (58a-58e) that, when activated, control the curvature of reflective surface 57. Electrode plate 52 may have any suitable number of electrodes 58 of any suitable shape. In an example, five electrodes 58 form five concentric circles. However, electrode plate 52 may have more or fewer electrodes 58 of any suitable shape, e.g., circular, elliptical, or square.

[0034] FIG. 2B shows when a voltage is applied between membrane 54 and electrode plate 52, the curvature of surface 57 changes in response to electrostatic actuation. Computer 46 sends instructions to control operating parameters of control electronics 44 to control the surface curvature of mirror 22. The operating parameters may operate to apply a particular voltage to a particular electrode 58. For example, the value of the parameter for electrode 58a may specify a particular voltage to apply to electrode 58a. Any suitable voltage may be applied. In this embodiment, the voltage may be in the range of 0 to 400V.

[0035] In the illustrated example, computer 46 instructs control electronics 44 to apply a first voltage to electrode 58a that attracts membrane 54 towards plate 52, and a second voltage to electrode 58b that repels membrane 54 from plate 52. In another example, computer 46 instructs control electronics 44 to apply voltages to electrodes 58a-58e that attract membrane 54 towards plate 52 to result in surface 57 having a concave shape. The deflection may be greater in the center. Thus, the voltage on electrode 58e may be greater than the voltage on electrodes 58a-58d, which may be greater than the voltage on electrodes 58a-58c, and so on.

[0036] The operating parameters for the surface curvature that results in a particular z position of the focal point may be determined in any suitable manner. For example, the curvature may be varied when the laser beam is applied to the test material to determine the curvature that corresponds to the z position within the material. The curvature may be decreased to move the focal point closer to the objective lens 42 curvature and / or increased to move the focal point away from the objective lens 42. The operating parameter values ​​that result in the corresponding z position are recorded for use by computer 46.

[0037] 3 shows a method of controlling the position of the focal point of a laser beam directed at a target 24 that may be performed by the system 10 of FIG. 1. The method begins at step 108, where the computer 46 receives a depth command and sets operating parameters to control the surface curvature of the mirror according to the depth command. At step 110, the beam expander 20 receives a laser beam with a first linear polarization from the laser 26. The beam expander 20 includes a mirror 22 and an expander optical device 30, which includes a polarizer 32 and a wave plate 34. At step 112, the polarizer 32 reflects the laser beam with the first linear polarization toward the wave plate 34.

[0038] In step 114, the waveplate 34 directs the laser beam towards the mirror 22. If the waveplate 34 is a quarter waveplate, the waveplate 34 converts the laser beam from a first linear polarization to a circular polarization. If the waveplate 34 is a half waveplate, the waveplate 34 rotates the first linear polarization by 45 degrees.

[0039] Mirror 22 has a surface curvature that is controlled to reflect the laser beam in step 116 to provide a focal point of the laser beam at a predetermined z-position. Mirror 22 operates as described with reference to Figures 2A-2C. Mirror 22 reflects the beam towards waveplate 34.

[0040] In step 118, the wave plate 34 converts the laser beam to a second linear polarization that is rotated relative to the first linear polarization. If the wave plate 34 is a quarter wave plate, the wave plate 34 converts the laser beam reflected from the mirror 22 from a circular polarization to the second linear polarization. If the wave plate 34 is a half wave plate, the wave plate 34 rotates the polarization of the laser beam reflected from the mirror 22 by 45 degrees to the second linear polarization.

[0041] In step 120, the laser beam with the second linear polarization passes through polarizer 32. In step 122, scanner 40 steers the laser beam to control the xy position of the focal point. In step 124, objective lens 42 directs the beam towards target 24. The method ends.

[0042] A component (e.g., a computer) of the systems and devices disclosed herein may include an interface, logic, and / or memory, any of which may include hardware and / or software. An interface may receive input to the component, provide output from the component, and / or process the input and / or output. Logic may perform the operations of the component, e.g., execute instructions to generate output from input. Logic may be one or more computers, or a processor, such as one or more microprocessors (e.g., chips present in a computer). Logic may be computer-executable instructions encoded in a memory that may be executed by a computer, such as a computer program or software. A memory may store information and may include one or more tangible, non-transitory, computer-readable, computer-executable storage media. Examples of memory include computer memory (e.g., random access memory (RAM) or read-only memory (ROM)), mass storage media (e.g., hard disks), removable storage media (e.g., compact discs (CDs) or digital video discs (DVDs)), and network storage (e.g., servers or databases).

[0043] Although the present disclosure has been described with respect to certain embodiments, modifications of the embodiments (such as substitutions, additions, modifications, or omissions) will be apparent to those skilled in the art. Thus, modifications may be made to the embodiments without departing from the scope of the present invention. For example, modifications may be made to the systems and devices disclosed herein. Components of the systems and devices may be integrated or separated, and operations of the systems and devices may be performed by more, fewer, or other components. As another example, modifications may be made to the methods disclosed herein. The methods may include more, fewer, or other steps, and the steps may be performed in any suitable order. According to aspect (1), there is provided a system for controlling a position of a focal point of a laser beam directed at a target, comprising: a beam expander configured to control the focus of the laser beam, a mirror having a surface curvature that can be adjusted to control the z-position of the focal spot along a z-axis of an xyz coordinate system, the z-axis being defined by the laser beam; and directing said laser beam towards said mirror; a beam expander including one or more expander optical devices configured to receive the laser beam reflected from the mirror; receiving the laser beam from the beam expander; a scanner configured to steer the laser beam to control an x-y position of the focal point in an x-y plane defined by the z-axis; receiving the laser beam from the scanner; an objective lens configured to direct the beam towards the target; Receive depth command, a computer configured to set one or more operating parameters to control the surface curvature of the mirror in accordance with the depth command; The system comprises: According to aspect (2), the expander optical device is receiving the laser beam with a first linear polarization; directing the laser beam with the first linear polarization towards a waveplate; receiving the laser beam with a second linear polarization; A polarizer configured to pass the laser beam with the second linear polarization. According to aspect (3), the wave plate is receiving the laser beam with the first linear polarization from the polarizer; directing said laser beam towards said mirror; receiving the laser beam reflected from the mirror; The laser beam is configured to convert the laser beam reflected from the mirror into the second linear polarization that is rotated relative to the first linear polarization. According to aspect (4), the difference between the first polarization and the second polarization is 90 degrees. According to aspect (5), the wave plate is receiving the laser beam with the first linear polarization from the polarizer; converting the laser beam from the first linear polarization to a circular polarization; directing said laser beam towards said mirror; receiving the laser beam reflected from the mirror; and a quarter wave plate configured to convert the laser beam reflected from the mirror from the circular polarization to the second linear polarization. According to aspect (6), the wave plate is receiving the laser beam with the first linear polarization from the polarizer; Rotating the first linearly polarized light by 45 degrees; directing said laser beam towards said mirror; receiving the laser beam reflected from the mirror; and a half-wave plate configured to rotate the polarization of the laser beam reflected from the mirror by 45 degrees to the second linear polarization. According to aspect (7), the expander optical device is Collimating the laser beam; It is configured to direct the laser beam towards the scanner. According to aspect (8), a depth command specifies the z position for the focal point. According to aspect (9), the depth command is a command to increase or decrease the z position of the focal point from the current z position. According to an aspect (10), there is provided a method for controlling a position of a focal point of a laser beam directed at a target, comprising the steps of: Receiving, at the computer, a depth command; setting one or more operating parameters to control a surface curvature of the mirror in accordance with said depth command; receiving the laser beam at a beam expander including the mirror and one or more expander optical devices, the mirror having a surface curvature that can be adjusted to control a z-position of the focal point along a z-axis of an x-y-z coordinate system, the z-axis being defined by the laser beam; directing the laser beam towards the mirror with the one or more expander optical devices; reflecting the laser beam with the mirror off the surface curvature to control the z position of the focal point; receiving the laser beam reflected from the mirror at the one or more expander optical devices; receiving the laser beam from the beam expander in a scanner; steering the laser beam to control an xy position of the focal spot in an xy plane defined by the z-axis; receiving the laser beam from the scanner at an objective lens; directing the beam towards the target; The method includes: According to an embodiment (11), the one or more expander optical devices include a polarizer; directing the laser beam towards the mirror with the one or more expander optical devices; receiving the laser beam with a first linear polarization at the polarizer; directing the laser beam with the first linear polarization towards a wave plate; receiving the laser beam reflected from the mirror at the one or more expander optical devices; receiving the laser beam with a second linear polarization; passing the laser beam with the second linear polarization; Further includes: According to aspect (12), the present invention provides a method for detecting a polarization of a laser beam having a first linear polarization from a polarizer at the wave plate; directing the laser beam towards the mirror; receiving the laser beam at the waveplate reflected from the mirror; converting the laser beam reflected from the mirror into the second linear polarization rotated relative to the first linear polarization; Further includes: According to aspect (13), the wave plate includes a quarter wave plate, receiving the laser beam with the first linear polarization from the polarizer at the waveplate; converting the laser beam from the first linear polarization to a circular polarization; converting the laser beam reflected from the mirror into the second linear polarization rotated with respect to the first linear polarization by the wave plate; The method further includes converting the laser beam reflected from the mirror from the circular polarization to the second linear polarization. According to aspect (14), the wave plate includes a half-wave plate, receiving the laser beam with the first linear polarization from the polarizer at the waveplate; further comprising rotating the first linear polarization by 45 degrees; converting the laser beam reflected from the mirror into the second linear polarization rotated with respect to the first linear polarization by the wave plate; The method further includes rotating the polarization of the laser beam reflected from the mirror by 45 degrees to become the second linear polarization.

Claims

1. A system configured for performing an ophthalmic procedure, comprising: a beam expander configured to control the focus of the laser beam, a mirror having a surface curvature that can be adjusted to control a z-position of the focal point along a z-axis of an xyz coordinate system, the z-axis being defined by the laser beam, the mirror including a membrane having a reflective surface and an electrode plate disposed opposite the membrane and including a plurality of electrodes for applying a voltage between the membrane and the electrode plate; and directing said laser beam towards said mirror; a beam expander including one or more expander optical devices configured to receive the laser beam reflected from the mirror; receiving the laser beam from the beam expander; a scanner configured to steer the laser beam to control an x-y position of the focal point in an x-y plane defined by the z-axis; receiving the laser beam from the scanner; an objective lens configured to direct the laser beam towards the target; Receive depth command, a computer configured to set one or more operating parameters to control the surface curvature of the mirror in accordance with the depth command; A system comprising:

2. The expander optical device comprises: receiving the laser beam with a first linear polarization; directing the laser beam with the first linear polarization towards a waveplate; receiving the laser beam with a second linear polarization; The system of claim 1 including a polarizer configured to pass the laser beam with the second linear polarization.

3. The wave plate is receiving the laser beam with the first linear polarization from the polarizer; directing said laser beam towards said mirror; receiving the laser beam reflected from the mirror; 3. The system of claim 2 configured to convert the laser beam reflected from the mirror to the second linear polarization that is rotated relative to the first linear polarization.

4. The system of claim 2 , wherein the difference between the first linear polarization and the second linear polarization is 90 degrees.

5. The wave plate is receiving the laser beam with the first linear polarization from the polarizer; converting the laser beam from the first linear polarization to a circular polarization; directing said laser beam towards said mirror; receiving the laser beam reflected from the mirror; 4. The system of claim 3, further comprising a quarter wave plate configured to convert the laser beam reflected from the mirror from the circular polarization to the second linear polarization.

6. The wave plate is receiving the laser beam with the first linear polarization from the polarizer; Rotating the first linearly polarized light by 45 degrees; directing said laser beam towards said mirror; receiving the laser beam reflected from the mirror; 4. The system of claim 3, further comprising a half-wave plate configured to rotate the first linear polarization of the laser beam reflected from the mirror by 45 degrees to the second linear polarization.

7. The expander optical device comprises: Collimating the laser beam; The system of claim 1 , comprising a collimator configured to direct the laser beam towards the scanner.

8. The system of claim 1, wherein the depth command specifies the z position for the focal point.

9. The system of claim 1, wherein the depth command is a command to increase or decrease the z position of the focal point from its current z position.

10. A method for a computer used to perform an ophthalmic procedure to control the position of a focal point of a laser beam directed at a target, comprising: receiving a depth command from the computer; The computer sets one or more operating parameters to control a surface curvature of the mirror according to the depth command, the one or more operating parameters being operative to apply a voltage between a plurality of electrodes of an electrode plate and a membrane including a reflective surface; receiving the laser beam with a beam expander including the mirror and one or more expander optical devices, the mirror having a surface curvature that can be adjusted to control a z-position of the focal point along a z-axis of an x-y-z coordinate system, the z-axis being defined by the laser beam; the one or more expander optical devices directing the laser beam towards the mirror; the mirror reflecting the laser beam off the surface curvature to control the z position of the focal point; the one or more expander optical devices receiving the laser beam reflected from the mirror; a scanner receiving the laser beam from the beam expander; the scanner steering the laser beam to control an x-y position of the focal spot in an x-y plane defined by the z-axis; an objective lens receiving the laser beam from the scanner; said objective lens directing said laser beam towards said target; A method comprising:

11. the one or more expander optical devices include a polarizer; the one or more expander optical devices directing the laser beam towards the mirror; the polarizer receiving the laser beam with a first linear polarization; directing the laser beam with the first linear polarization towards a waveplate; the one or more expander optical devices receiving the laser beam reflected from the mirror; receiving the laser beam with a second linear polarization; passing the laser beam with the second linear polarization; The method of claim 10 further comprising:

12. the waveplate receiving the laser beam with the first linear polarization from the polarizer; the waveplate directing the laser beam towards the mirror; the waveplate receiving the laser beam reflected from the mirror; the waveplate converts the laser beam reflected from the mirror into the second linear polarization rotated relative to the first linear polarization; The method of claim 11 further comprising:

13. the wave plate comprises a quarter wave plate; the waveplate receiving the laser beam with the first linear polarization from the polarizer; converting the laser beam from the first linear polarization to a circular polarization; the wave plate converts the laser beam reflected from the mirror into the second linear polarization rotated with respect to the first linear polarization, The method of claim 12 , further comprising converting the laser beam reflected from the mirror from the circular polarization to the second linear polarization.

14. the wave plate comprises a half wave plate; the waveplate receiving the laser beam with the first linear polarization from the polarizer; rotating the first linear polarization by 45 degrees; the wave plate converts the laser beam reflected from the mirror into the second linear polarization rotated with respect to the first linear polarization, 13. The method of claim 12, further comprising rotating the first linear polarization of the laser beam reflected from the mirror by 45 degrees to become the second linear polarization.

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