OCT swept laser with high coherence signal extraction
View Patent ↗An optical coherence tomography system utilizes an optical swept laser that has improved coherence length in the swept optical signal. This is accomplished using an intra-cavity element that extracts the tunable optical signal at the optimal location within the laser's resonant cavity. Generally this location is between the intracavity tuning element and the cavity's gain element so that light coming from the tuning element is extracted. In general in lasers, the gain element adds noise and chirp and this degrades the tunable optical signal's coherence length.
1. A swept laser that generates a swept optical signal, the swept laser comprising:
a laser cavity in which the swept optical signal is generated;
a tuning element for a controlling an optical frequency of the swept optical signal;
a gain element for amplifying light in the laser cavity; and
an optical signal extraction element located in the laser cavity between the tuning element and the gain element for coupling the swept optical signal from the laser cavity after being filtered by the tuning element but before amplification by the gain element.
2. The swept laser of claim 1 , wherein the laser cavity is a linear cavity and the signal extraction element is located downstream of the tuning element but upstream of the gain element.
3. The swept laser of claim 1 , wherein the signal extraction element is a beam splitter.
4. The swept laser of claim 1 , wherein the tuning element is a Fabry Perot tunable filter.
5. The swept laser of claim 4 , further comprising quarter wave plates on either side of the tuning element to rotate the polarization of the optical signal within the laser cavity so that light transmitted through the tunable filter has a polarization that is appropriate for amplification by the gain element whereas light that is rejected by the tunable filter has a polarization that is orthogonal to the polarization at which the gain element amplifies light.
6. The swept laser of claim 1 , wherein the gain element is a reflective semiconductor optical amplifier.
7. The swept laser of claim 1 , further comprising a low coherence signal extraction port through which a lower coherence version of the swept optical signal is provided.
8. The swept laser of claim 1 , wherein the laser cavity is a ring cavity and the swept optical signal is directed to pass through the tuning element a second time before being coupled from the cavity by the optical extraction element.
9. The swept laser of claim 8 , wherein the signal extraction element is located downstream of the tuning element but upstream of the gain element.
10. The swept laser of claim 1 , wherein the tuning element controls the optical frequency of the swept optical signal to sweep through a scan band of greater than 50 nanometers at greater than 5 nanometers per microsecond.
11. An optical coherence tomography system, comprising:
an interferometer that combines a swept optical signal from a sample and from a reference path to generate an interference signal;
a swept laser that generates the swept optical signal, the swept laser comprising a laser cavity, a gain element or amplifying light in the laser cavity, and an optical signal extraction element located within the laser cavity for coupling the swept optical signal from the laser cavity prior to amplification by the gain element; and
a detection system that detects the interference signal.
12. The system of claim 11 , wherein the laser cavity is a linear cavity and the signal extraction element is located downstream of a tuning element within the laser cavity but upstream of the gain element.
13. The system of claim 12 , wherein the signal extraction element is a beam splitter.
14. The system of claim 11 , further comprising a tunable filter within the laser cavity.
15. The system of claim 14 , further comprising quarter wave plates on either side of the tuning element to rotate the polarization of the optical signal within the laser cavity so that light transmitted through the tunable filter has a polarization that is appropriate for amplification by the gain element whereas light that is rejected by the tunable filter has a polarization that is orthogonal to the polarization at which the gain element amplifies light.
16. The system of claim 11 , wherein the gain element is a reflective semiconductor optical amplifier.
17. The system of claim 11 , further comprising a low coherence signal extraction port that generates a lower coherence version of the swept optical signal.
18. The system of claim 11 , wherein the laser cavity is a ring cavity and the swept optical signal is directed to pass through a tunable element a second time before being coupled from the cavity by the optical extraction element.
19. The system of claim 11 , wherein the signal extraction element is located downstream of a tuning element within the laser cavity but upstream of the gain element.
20. The system of claim 11 , wherein the swept laser sweeps through a scan band of greater than 50 nanometers at greater than 5 nanometers per microsecond.