Optical pulse train generator
An optical pulse train generator 10 includes beat light generation means 11 for generating a beat light 21 having a predetermined repeated frequency, a highly nonlinear fiber 12 for generating a pulse train 22 formed by adding a side mode to the beat light 21 by a four-wave mixing (FWM), and a band pass filter (BPF) 13 for adjusting a power balance of the side mode of the pulse train 22 so as to shape the frequency spectrum.
1. An optical pulse train generator comprising:
beat light generation means for combining two continuous lights having a frequency difference corresponding to a predetermined repetition frequency to generate a beat light having the predetermined repetition frequency;
side mode generation means for generating a side mode with respect to the beat light based on a nonlinear effect; and
spectrum shaping means for adjusting a power ratio of the side mode generated by the side mode generation means and the beat light,
wherein an in-phase pulse train is generated.
2. The optical pulse train generator according to claim 1 , wherein the beat light generation means generates an unbalanced beat light by providing a difference between powers of the two continuous lights and combining the continuous lights.
3. The optical pulse train generator according to claim 1 , wherein a ratio of amplitudes of the two continuous lights is set to fall within the range of 0.82 to 0.88.
4. The optical pulse train generator according to claim 1 , wherein the beat light generation means includes:
two laser oscillators that output laser beams having a frequency difference corresponding to the repetition frequency;
a 3-dB coupler that combines the two laser beams to output the beat light;
a first optical amplifier that amplifies the beat light; and
a first band pass filter that suppresses ASE (Amplified Spontaneous Emission) noise generated in the first optical amplifier.
5. The optical pulse train generator according to claim 1 , wherein the side mode generation means is a four-wave mixing in an optical fiber.
6. The optical pulse train generator according to claim 5 , wherein the optical fiber is a highly nonlinear fiber.
7. The optical pulse train generator according to claim 5 , wherein two or more fibers are connected with each other in the optical fiber and an isolator is interposed between the fibers connected with each other.
8. The optical pulse train generator according to claim 1 , wherein the spectrum shaping means is a second band pass filter.
9. The optical pulse train generator according to claim 8 , wherein the second band pass filter has a central frequency that is different from a central frequency of an input optical pulse train by a predetermined width.
10. The optical pulse train generator according to claim 1 , further comprising an anomalous dispersion medium on an outlet side of a band pass filter, the anomalous dispersion medium performing chirp compensation with respect to the optical pulse train having the shaped frequency spectrum.
11. The optical pulse train generator according to claim 10 , wherein the anomalous dispersion medium is a single-mode fiber.
12. The optical pulse train generator according to claim 10 , further comprising an optical pulse compressor on an outlet side of the anomalous dispersion medium.
13. The optical pulse train generator according to claim 12 , wherein the optical pulse compressor is configured by alternately connecting a nonlinear medium and the anomalous dispersion medium.
14. The optical pulse train generator according to claim 13 , further comprising an isolator at any one or more positions between the nonlinear medium and the anomalous dispersion medium.
15. The optical pulse train generator according to claim 12 , comprising a second optical amplifier and a third band pass filter between the anomalous dispersion medium and the optical pulse compressor.
16. The optical pulse train generator according to claim 4 , comprising a filter that allows line spectral components alone of the optical pulse train to be transmitted therethrough at any one or more positions on an outlet side of the 3-dB coupler or/and a downstream side of the 3-dB coupler.
17. The optical pulse train generator according to claim 16 , wherein the filter is a Fabry-Perot filter.
18. The optical pulse train generator according to claim 10 , further comprising optical time-division multiplexers whose quantity is determined based on the repetition frequency of the beat light and a required repetition frequency on the outlet side of the anomalous dispersion medium.
19. The optical pulse train generator according to claim 18 , wherein the optical pulse compressor is further provided between the anomalous dispersion medium and the optical time-division multiplexers.
20. The optical pulse train generator according to claim 1 , wherein a central frequency of the pulse train is even-fold of the repetition frequency.