Method for measuring and suppressing repetition rate phase noise of femtosecond laser using optical fiber delay line

1. A method of measuring a phase noise of a repetition rate of a femtosecond laser, the method comprising:

generating a combined signal using a first wavelength element and a second wavelength element of an optical pulse train generated by the femtosecond laser, which is mode-locked;

guiding the combined signal to a first path and a second path, delaying a signal of the first path, shifting a frequency, allowing a signal of the second path to interfere with the signal of the first path, and outputting an interfering combined signal;

dividing the interfering combined signal into a first interference signal corresponding to the first wavelength element and a second interference signal corresponding to the second wavelength element;

converting the first interference signal to a first radio frequency signal and the second interference signal to a second radio frequency signal using a photoelectric device; and

detecting a baseband signal including a frequency noise of the repetition rate from the first radio frequency signal and the second radio frequency signal using a mixer.

2. The method of claim 1 , further comprising:

obtaining the first wavelength element and the second wavelength element from the optical pulse train using a dense wavelength division multiplexing (DWDM) filter.

3. The method of claim 1 , further comprising:

amplifying the combined signal using an erbium-doped fiber amplifier (EDFA).

4. The method of claim 1 , wherein the first path includes a dispersion compensated fiber configured to delay the signal of the first path and an acousto-optic modulator (AOM) configured to shift the frequency of the signal of the first path.

5. The method of claim 4 , wherein the dispersion compensated fiber has a length of at least 1 kilometer.

6. The method of claim 1 , wherein a Faraday rotator mirror (FRM) is disposed at each end of the first path and the second path.

7. The method of claim 1 , wherein the converting comprises:

obtaining the first radio frequency signal by allowing the first interference signal to pass through a first photodiode, a first band pass filter (BPF), and a first amplifier; and

obtaining the second radio frequency signal by allowing the second interference signal to pass through a second photodiode, a second BPF, and a second amplifier.

8. The method of claim 1 , further comprising:

outputting a feedback signal by allowing the baseband signal to pass through a loop filter.

9. The method of claim 8 , further comprising:

providing the feedback signal for a piezoelectric element in the femtosecond laser.

10. The method of claim 8 , further comprising:

measuring the frequency noise of the repetition rate at a Fourier frequency being greater than or equal to a lock bandwidth from the baseband signal and measuring the frequency noise of the repetition rate at the Fourier frequency being within the lock bandwidth from the feedback signal using a spectrum analyzer; and

calculating the phase noise of the repetition rate based on the measured frequency noise of the repetition rate.

11. A method of suppressing a phase noise of a repetition rate of a femtosecond laser, the method comprising:

generating a combined signal using a first wavelength element and a second wavelength element of an optical pulse train generated by the femtosecond laser, which is mode-locked;

guiding the combined signal to a first path and a second path, delaying a signal of the first path, shifting a frequency, allowing a signal of the second path to interfere with the signal of the first path, and outputting an interfering combined signal;

dividing the interfering combined signal into a first interference signal corresponding to the first wavelength element and a second interference signal corresponding to the second wavelength element;

converting the first interference signal to a first radio frequency signal and the second interference signal into a second radio frequency signal using a photoelectric device;

detecting a baseband signal including a frequency noise of the repetition rate from the first radio frequency signal and the second radio frequency signal using a mixer;

obtaining a first feedback signal by allowing the baseband signal to pass through a first loop filter and obtaining a second feedback signal by allowing the baseband signal to pass through a second loop filter; and

providing the first feedback signal for a piezoelectric element in the femtosecond laser and providing the second feedback signal for an electro-optic modulator in the femtosecond laser.

12. The method of claim 11 , further comprising:

obtaining the first wavelength element and the second wavelength element from the optical pulse train using a dense wavelength division multiplexing (DWDM) filter.

13. The method of claim 11 , further comprising:

amplifying the combined signal using an erbium-doped fiber amplifier (EDFA).

14. The method of claim 11 , wherein the first path includes a dispersion compensated fiber configured to delay the signal of the first path and an acousto-optic modulator (AOM) configured to shift the frequency of the signal of the first path.

15. The method of claim 14 , wherein the dispersion compensated fiber has a length of at least 1 kilometer.

16. The method of claim 11 , wherein a Faraday rotator mirror (FRM) is disposed at each end of the first path and the second path.

17. The method of claim 11 , wherein the converting comprises:

obtaining the first radio frequency signal by allowing the first interference signal to pass through a first photodiode, a first band pass filter (BPF), and a first amplifier; and

obtaining the second radio frequency signal by allowing the second interference signal to pass through a second photodiode, a second BPF, and a second amplifier.