Fiber laser having optical resonator comprising tapered element
A laser system for generating optical pulses at an operating wavelength of the laser system. The system has an optical resonator comprising first and second reflectors, and a tapered optical fiber disposed between the first and second reflectors. The tapered optical fiber has a core which has a tapered input section which tapers from single mode to multimode at the laser operating wavelength, an inner section of substantially constant diameter capable of supporting multiple modes at the laser operating wavelength and a tapered output section which tapers from a first diameter to a second diameter that is smaller than the first diameter.
1. A laser system for generating optical pulses at an operating wavelength of the laser system, the system having an optical resonator comprising:
first and second reflectors; and
a tapered optical fiber disposed between the first and second reflectors and having a core which has a tapered input section which tapers from single mode to multimode at the laser operating wavelength, an inner section of substantially constant diameter capable of supporting multiple modes at the laser operating wavelength and a tapered output section which tapers from a first diameter to a second diameter that is smaller than the first diameter.
2. The laser system according to claim 1 , wherein the tapered output section of the core tapers from a diameter capable of supporting multiple modes to single mode at the laser operating wavelength.
3. The laser system according to claim 1 , wherein the taper of the tapered input section is such that the single mode guidance is maintained during the transition to the inner section so that substantially only the fundamental mode propagates through the inner section.
4. The laser system according to claim 1 , wherein the gain medium of the laser system is comprised of the tapered optical fiber.
5. The laser system according to claim 1 , wherein the gain medium comprises a length of rare earth doped (RED) optical fiber separate from the tapered optical fiber.
6. The laser system according to claim 1 , wherein the tapered optical fiber comprises at least 80% of the length of the optical resonator.
7. The laser system according to claim 1 , wherein the repetition rate of the generated optical pulses is no more than 10 MHz.
8. The laser system according to claim 1 , wherein the second reflector is a semiconductor saturable absorber mirror and one or more free space optical elements are disposed between the tapered output section and the semiconductor saturable absorber mirror.
9. The laser system according to claim 1 , wherein the tapered optical fiber is composed of a single continuous piece of fiber.
10. The laser system according to claim 1 , wherein the length of the tapered optical fiber is greater than about 5 meters.
11. The laser system according to claim 1 , wherein the length of the tapered optical fiber is greater than about 10 meters.
12. The laser system according to claim 1 , wherein the tapered input section and/or the tapered output section include a nonlinear taper profile or alternatively wherein the tapered input section and/or the tapered output section include a substantially exponential taper profile.
13. The laser system according to claim 1 , wherein the length of the tapered input section and the tapered output section combined is no more than 10% of the total length of the tapered optical fiber.
14. The laser system according to claim 1 , wherein the inner section of the core of the tapered optical fiber has a refractive index (RI) profile which is substantially constant.
15. The laser system according to claim 1 , wherein the inner section of the core of the tapered optical fiber has a refractive index (RI) profile taken relative to silica wherein the percentage RI variation of the core RI according to the formula [(maximum RI−minimum RI)/(2×minimum RI)]×100 is no greater than about 20%.