Calibrating the position of the focal point of a laser beam

1. A system for calibrating a position of a focal point of a laser beam, comprising:

a laser configured to generate the laser beam;

a plurality of focusing optics configured to:

direct the focal point of the laser beam along a z-axis towards a zero-surface, the zero-surface corresponding to a zero-plane; and

receive at least a portion of the laser beam reflected by the zero-surface;

one or more detector optics configured to:

receive the reflected portion from the focusing optics; and

direct the reflected portion towards a two-photon absorption (TPA) detector;

the TPA detector configured to:

sense a peak intensity of the reflected portion, the peak intensity indicating a proximity of the position of the focal point of the laser beam to the zero-surface; and

generate a signal representing the peak intensity of the reflected portion; and

a computer configured to determine whether the position of the focal point of the laser beam is calibrated in response to the signal representing the peak intensity.

2. The system of claim 1 , the computer configured to:

determine whether the peak intensity is a maximum peak intensity; and

if the peak intensity is the maximum peak intensity, determine that the position of the focal point of the laser beam is substantially at the zero-surface.

3. The system of claim 1 , the computer configured to:

determine whether the peak intensity is a maximum peak intensity; and

if the peak intensity is not the maximum peak intensity, adjust the plurality of focusing optics to direct the focal point of the laser beam to a different point of the z-axis.

4. The system of claim 1 , the computer configured to repeat the following until the peak intensity is a maximum peak intensity:

determine whether the peak intensity is a maximum peak intensity; and

if the peak intensity is not the maximum peak intensity, adjust the plurality of focusing optics to direct the focal point of the laser beam to a different point of the z-axis.

5. The system of claim 1 , the computer configured to:

adjust the plurality of focusing optics to direct the focal point of the laser beam along a plurality of larger intervals to locate a general region of the zero-surface; and

adjust the plurality of focusing optics to direct the focal point of the laser beam along a plurality of smaller intervals of the general region to determine the location of the zero-surface.

6. The system of claim 1 , the computer configured to:

generate from the signal a graph representing the peak intensity of the reflected portion.

7. The system of claim 1 , the plurality of focusing optics comprising a beam expander, a scanner, and an objective.

8. The system of claim 1 , wherein the one or more detector optics comprise a polarizer and a quarter-waveplate:

the polarizer configured to transmit the laser beam with a first linear polarization to the quarter-waveplate;

the quarter-waveplate configured to:

convert the laser beam from the first linear polarization to a circular polarization; and

convert the reflected portion from the circular polarization to a second linear polarization;

the polarizer further configured to deflect the reflected portion with the second linear polarization towards the TPA detector.

9. The system of claim 1 , wherein the one or more detector optics comprise a polarizer and a combination comprising a half-waveplate and a Faraday rotator:

the polarizer configured to transmit the laser beam with a first linear polarization to the combination;

the combination configured to:

rotate the linear polarization of the laser beam 0 degrees and rotate the first linear polarization of the reflected portion 90 degrees to a second linear polarization; or

rotate the linear polarization of the laser beam 90 degrees and rotate the first linear polarization of the reflected portion 0 degrees to a second linear polarization; and

the polarizer further configured to deflect the reflected portion with the second linear polarization towards the TPA detector.

10. A method for calibrating a position of a focal point of a laser beam, comprising:

generating a laser beam;

directing, by a plurality of focusing optics, the focal point of the laser beam along a z-axis towards a zero-surface, the zero-surface corresponding to a zero-plane;

receiving at least a portion of the laser beam reflected by the zero-surface;

receiving, by one or more detector optics, the reflected portion from the focusing optics;

directing the reflected portion towards a two-photon absorption (TPA) detector;

sensing, by the TPA detector, a peak intensity of the reflected portion, the peak intensity indicating a proximity of the position of the focal point of the laser beam to the zero-surface;

generating a signal representing the peak intensity of the reflected portion; and

determining, by a computer, whether the position of the focal point of the laser beam is calibrated in response to the signal representing the peak intensity.

11. The method of claim 10 , further comprising:

determining, by the computer, whether the peak intensity is a maximum peak intensity; and

when the peak intensity is the maximum peak intensity, determining that the position of the focal point of the laser beam is substantially at the zero-surface.

12. The method of claim 10 , further comprising:

determining, by the computer, whether the peak intensity is a maximum peak intensity; and

when the peak intensity is not the maximum peak intensity, adjusting the plurality of focusing optics to direct the focal point of the laser beam to a different point of the z-axis.

13. The method of claim 10 , further comprising repeating, by the computer, the following until the peak intensity is a maximum peak intensity:

determining whether the peak intensity is a maximum peak intensity; and

when the peak intensity is not the maximum peak intensity, adjusting the plurality of focusing optics to direct the focal point of the laser beam to a different point of the z-axis.

14. The method of claim 10 , further comprising:

adjusting, by the computer, the plurality of focusing optics to direct the focal point along a plurality of larger intervals to locate a general region of the zero-surface; and

adjusting the plurality of focusing optics to direct the focal point of the laser beam along a plurality of smaller intervals of the general region to determine the location of the zero-surface.

15. The method of claim 10 , the computer configured to:

generating, by the computer, from the signal a graph representing the peak intensity of the reflected portion.