High power femtosecond laser with adjustable repetition rate
Designs and techniques for constructing and operating femtosecond pulse lasers are provided. One example of a laser engine includes an oscillator that generates and outputs a beam of femtosecond seed pulses, a stretcher-compressor that stretches a duration of the seed pulses, and an amplifier that receives the stretched seed pulses, amplifies an amplitude of selected stretched seed pulses to create amplified stretched pulses, and outputs a laser beam of amplified stretched pulses back to the stretcher-compressor that compresses their duration and outputs a laser beam of femtosecond pulses. The amplifier includes a dispersion controller that compensates a dispersion of the amplified stretched pulses, making the repetition rate of the laser adjustable between procedures or according to the speed of scanning. The laser engine can be compact with a total optical path of less than 500 meters, and have a low number of optical elements, e.g. less than 50.
1. A variable repetition rate laser engine, comprising:
an oscillator that generates and outputs a beam of femtosecond seed pulses;
a stretcher-compressor that stretches a duration of the seed pulses; and
an amplifier that
receives the stretched seed pulses from the stretcher-compressor,
amplifies an amplitude of selected stretched seed pulses to create amplified stretched pulses, and
outputs a laser beam of amplified stretched pulses; wherein
the stretcher-compressor
receives the laser beam of amplified stretched pulses,
compresses a duration of the amplified stretched pulses, and
outputs a laser beam of femtosecond pulses with a pulse duration of less than 1,000 femtoseconds; and
the amplifier comprises a dispersion compensator that reduces a dispersion of the amplified stretched pulses, caused by optical elements of the amplifier and an electro-optical switchable polarizer, configured to operate at a repetition rate in the range of 50 kHz-1 MHz; wherein
the laser engine outputs a laser beam with an output power greater than 0.1 W.
2. The laser engine of claim 1 , wherein:
the oscillator is a diode pumped fiber oscillator.
3. The laser engine of claim 1 , wherein:
the oscillator outputs transform-limited seed pulses.
4. The laser engine of claim 1 , wherein:
the oscillator generates the beam with a seed-pulse-duration of less than 1,000 femtoseconds.
5. The laser engine of claim 1 , wherein:
the oscillator outputs the beam with a seed pulse repetition rate in the range of 10-100 MHz.
6. The laser engine of claim 1 , wherein:
the stretcher-compressor comprises a chirped volume Bragg grating.
7. The laser engine of claim 1 , wherein:
the stretcher-compressor comprises a photothermal refractive glass.
8. The laser engine of claim 1 , wherein:
the stretcher-compressor stretches a duration of the femtosecond seed pulses by a factor greater than 10.
9. The laser engine of claim 1 , wherein:
the stretcher-compressor stretches a duration of the femtosecond seed pulses to a stretched duration of 1,000-200,000 femtoseconds.
10. The laser engine of claim 1 , wherein:
the laser engine does not contain a tunable stretcher-compressor.
11. The laser engine of claim 1 , comprising:
a polarizer and a λ/4 plate between the oscillator and the stretcher-compressor that redirects the beam of stretched seed pulses toward the amplifier.
12. The laser engine of claim 1 , comprising:
a Faraday isolator that
receives the beam of stretched seed pulses from the stretcher-compressor,
outputs the beam of stretched seed pulses toward the amplifier,
receives the laser beam of amplified stretched pulses from the amplifier,
outputs the laser beam of amplified stretched pulses towards a compressor port of the stretcher-compressor, and
isolates the oscillator from the laser beam of amplified stretched pulses.
13. The laser engine of claim 1 , wherein:
the dispersion compensator introduces a dispersion opposite in sign to a dispersion introduced by an optical element of the amplifier.
14. The laser engine of claim 13 , wherein:
the dispersion introduced by the dispersion compensator is essentially equal in magnitude and opposite in sign to a dispersion introduced within one roundtrip by the optical elements of the amplifier other than the dispersion compensator.
15. The laser engine of claim 1 , wherein the dispersion compensator comprises:
at least one of a chirped mirror, a chirped fiber, a chirped grating, a prism, or a chirped transmissive optical element.
16. The laser engine of claim 1 , the amplifier comprising:
a gain material that amplifies the amplitude of the selected stretched seed pulses;
two end-mirrors that define a resonant cavity; and
two folding mirrors that fold a resonant optical pathway inside the amplifier, wherein
at least one of the two end-mirrors and the two folding mirrors is a chirped mirror.
17. The laser engine of claim 16 , wherein:
the chirped mirror introduces a negative dispersion to the amplified stretched pulses.
18. The laser engine of claim 1 , wherein:
the laser engine is configured to output the laser beam
with a first repetition rate, and
subsequently with a different second repetition rate with essentially the same alignment of all optical elements of the laser engine.
19. The laser engine of claim 18 , wherein:
the first repetition rate and the second repetition range fall within the range of 50 kHz-1 MHz.
20. The laser engine of claim 1 , wherein:
the amplifier is configured to have a number of roundtrips of the amplified stretched pulses in the amplifier changed when a repetition rate is changed while keeping an optical setup of the amplifier unchanged.
21. The laser engine of claim 1 , wherein:
the amplifier has an end-mirror-to-end-mirror folded optical pathway of less than 1 meter.
22. The laser engine of claim 1 , wherein the amplifier is one of:
a cavity dumped regenerative amplifier, a chirped pulse amplifier or a Q-switched amplifier.
23. The laser engine of claim 1 , wherein:
the electro-optical switchable polarizer is positioned in an optical pathway between end-mirrors and is configured to select stretched pulses by switching between
a polarization-adjusting state in which the switchable polarizer adjusts a polarization of the amplified stretched pulses; and
a polarization-non-adjusting state in which the switchable polarizer essentially does not adjust the polarization of the amplified stretched pulses.
24. The laser engine of claim 23 , comprising:
a high voltage power-switch that controls the switchable polarizer to switch from the polarization-non-adjusting state to the polarization-adjusting state with a rise time of less than 5 nanoseconds.
25. The laser engine of claim 1 , wherein:
the laser engine changes a first repetition rate of the laser beam of femtosecond pulses to a second repetition rate within 1-120 seconds.
26. The laser engine of claim 1 , wherein:
the laser engine changes a first repetition rate of the laser beam of femtosecond pulses to a second repetition rate within a changing time in the range of 1 μs-1 s.
27. The laser engine of claim 1 , wherein:
the femtosecond pulses of the laser beam have an energy in the range of 1-100 μJ/pulse.
28. The laser engine of claim 1 , wherein:
the laser engine is part of an ophthalmic surgical system.
29. The laser engine of claim 1 , wherein:
the dispersion compensator is configured to compensate a group delay dispersion in the range of 5,000-20,000 fs 2 .
30. The laser engine of claim 1 , wherein:
the variable repetition rate laser engine is configured to vary the repetition rate, following a rate-variation-protocol, by more than 10%.
31. A method of generating a laser beam with a variable repetition rate laser engine, the method comprising the steps of:
generating a beam of seed pulses with duration less than 1000 femtoseconds with an oscillator;
stretching a duration of the seed pulses with a pulse stretcher;
amplifying an amplitude of selected stretched seed pulses with an amplifier to generate amplified stretched pulses;
compressing a duration of the amplified stretched pulses to below 1,000 femtoseconds with a pulse compressor;
outputting a laser beam of femtosecond pulses with an electro-optical switchable polarizer with a first repetition rate in the range of 50 kHz-1 MHz and with a pulse duration less than 1,000 femtoseconds;
changing the repetition rate from the first repetition rate to a second repetition rate in the range of 50 kHz-1 MHz without essentially changing an optical setup of the laser engine; and
outputting the laser beam of femtosecond pulses with the second repetition rate and with a pulse duration less than a 1,000 femtoseconds, wherein
the amplifying comprises utilizing a dispersion compensator in the amplifier to reduce a dispersion of the amplified stretched pulses caused by an optical component of the amplifier; wherein
the laser engine outputs a laser beam with an output power greater than 0.1 W.
32. The method of claim 31 , wherein the reducing the dispersion step comprises:
introducing a compensating dispersion by at least one chirped mirror in the amplifier, wherein
the compensating dispersion is essentially equal in magnitude and opposite in sign to a dispersion introduced by all optical elements of the amplifier other than the dispersion compensator per roundtrip.
33. The method of claim 31 , the changing the repetition rate step comprising:
changing a number of roundtrips in the amplifier.
34. The method of claim 31 , wherein:
the stretching step and the compressing step are executed by the same stretcher-compressor.
35. The method of claim 31 , comprising:
outputting the laser beam with the second repetition rate within 1-120 seconds after having finished the outputting the laser beam with the first repetition rate.
36. The method of claim 31 , comprising:
changing the repetition rate from the first repetition rate to the second repetition rate in a changing time in the range of 1 μs-1 s.
37. A variable repetition rate laser engine, comprising:
an oscillator that generates a pulsed light beam with a pulse duration of less than 1000 femtoseconds;
a stretcher-compressor that stretches the duration of the pulses of the light beam; and
an amplifier that amplifies an amplitude of the stretched light pulses to generate amplified stretched pulses and comprises a dispersion compensator that at least partially compensates a dispersion introduced by optical elements of the amplifier, wherein
the stretcher-compressor
compresses a duration of the amplified stretched pulses, and
outputs a beam of laser pulses; and
the laser engine is operable to output the beam of laser pulses with an electro-optical switchable polarizer
with a first repetition rate in the 50 kHz-1 MHz range and
subsequently with a second repetition rate in the 50 kHz-1 MHz range, utilizing essentially the same alignment of all optical elements of the laser engine,
a duration of the outputted laser pulses being less than 1000 femtoseconds for the first and the second repetition rates; wherein
the laser engine outputs a laser beam with an output power greater than 0.1 W.
38. The laser engine of claim 37 , the amplifier comprising:
a switchable polarizer between end-mirrors of the amplifier that switches between
a state in which the switchable polarizer adjusts a polarization of the amplified stretched pulses; and
a state in which the switchable polarizer does not adjust the polarization of the amplified stretched pulses
with a rise time of less than 5 nanoseconds.
39. The laser engine of claim 37 , wherein:
the laser engine switches between the first repetition rate and the second repetition rate in a time less than 60 seconds.