Method of making sub-surface photoalterations in a material

1. A method of photoaltering tissue of a human eye in a scan region using a pulsed laser beam, the method comprising the steps of:

selecting a first pulse energy and a focal point separation of successive pulses of the pulsed laser beam based on a relationship of pulse energy, and focal point separation combinations enabling layer separation of the tissue by photoalteration, the relationship having a slope and having a distinct change in the slope, the distinct change in the slope associated with a second pulse energy on the relationship, the first pulse energy being equal to or less than the second pulse energy;

selecting a focal point depth using the first pulse energy and the focal point separation such that the pulsed laser beam does not cause a gas to rupture a surface of the tissue, the focal point depth being a sub-surface depth; and

scanning the pulsed laser beam along the scan region at the first pulse energy, the focal point depth, and the focal point separation.

2. The method of claim 1 , wherein the scanning step comprises scanning the pulsed laser beam with the operating focal point separation between about 1 μm and about 8 μm.

3. The method of claim 1 , wherein the scan region underlies a surface layer of the tissue, and wherein the scanning step comprises at least partially separating the surface layer from the tissue.

4. The method of claim 1 further comprising, prior to the selecting step, determining the relationship based on a minimum pulse energy for layer separation of the tissue.

5. The method of claim 1 , wherein the relationship has a first region and a second region adjacent to the first region, the first region associated with a first slope, the second region associated with a second slope, and wherein the method further comprises, prior to the step of selecting a first pulse energy and a focal point separation, comparing a difference between the first slope and the second slope with a pre-determined value to determine the distinct change in the slope.

6. The method of claim 1 , wherein the scanning step comprises scanning the pulsed laser beam at a repetition rate between about 30 MHz and about 1 GHz.

7. The method of claim 1 , wherein the scanning step comprises scanning the pulsed laser beam with the operating pulse energy at less than about 800 nanojoules.

8. A method of separating a surface layer from a tissue of a human eye via photoalteration by a pulsed laser beam, the method comprising:

determining a relationship for the tissue based on laser pulse energies enabling layer separation of the tissue through photoalteration as a function of focal point separations of successive pulses of the pulsed laser beam within the tissue, the relationship having a slope;

identifying a pulse energy value from the relationship corresponding to a distinct change of the slope;

generating the pulsed laser beam, each pulse of the pulsed laser beam having an energy less than or equal to the pulse energy value; and

scanning the pulsed laser beam along a region within the tissue with an operating focal point separation corresponding with the pulse energy value, the region underlying the surface layer, wherein scanning comprises minimizing a focal point depth of the pulsed laser beam such that the pulsed laser beam does not cause a gas to rupture a surface of the tissue, the focal point depth based on the pulse energy value and the operating focal point separation.

9. The method of claim 8 , wherein the determining step comprises measuring a minimum pulse energy for layer separation in the tissue material at each of a plurality of laser pulse focal point separations.

10. The method of claim 8 , wherein the determining step comprises measuring a sufficient pulse energy for a desired quality of layer paration in the tissue at each of a plurality of laser pulse focal point separations.

11. The method of claim 8 , wherein the scanning step comprises scanning the beam of laser pulses along the region within the tissue material in a pattern providing substantially consistent focal point separations between the laser pulses.

12. A system for sub-surface photoalteration of a tissue of a human eye, the apparatus comprising:

a laser configured to produce a pulsed laser beam having a first pulse energy, a sub-surface focal point depth, and a focal point separation of successive pulses of the pulsed laser beam; and

a controller coupled to the laser, the controller configured to:

determine the first pulse energy and the focal point separation based on a relationship of pulse energy and focal point separation combinations enabling layer separation of the tissue by photoalteration, the relationship having a slope and having a distinct change in the slope, the distinct change in the slope associated with a second pulse energy on the relationship, the first pulse energy being equal to or less than the second pulse energy; and

determine the focal point depth at the first pulse energy and the focal point separation such that the pulsed laser beam at the focal point depth does not cause a gas to rupture a surface of the tissue.

13. The system of claim 12 , wherein the pulsed laser beam has a pulse frequency selected from a range of about 30 MHz to about 1 GHz.

14. The system of claim 12 , wherein the first pulse energy is less than or equal to about 800 nanojoules.

15. The system of claim 12 , wherein the first focal point separation is between about 1 μm and about 8 μm.

16. The system of claim 12 , wherein the laser is further configured to scan the pulsed laser beam along a scan region of the material to at least partially separate a surface layer from the tissue, the scan region underlying the surface layer of the tissue.

17. The system of claim 12 , wherein the relationship is based on a minimum pulse energy for layer separation of the tissue by photoalteration.