Generation of Ultrashort Laser Pulses at Wavelengths

Abstract

A method for generating pulsed laser radiation in the spectral range from 860 nm to 1000 nm is disclosed, including the steps of generating pulsed laser radiation in the spectral range from 1500 nm to 1600 nm, preferably at a wavelength of 1560 nm; shifting the wavelength of the pulsed laser radiation to a longer wavelength of at least 1720 nm, and preferably to 1840 nm; amplifying the wavelength-shifted pulsed laser radiation in a Thulium-doped gain medium so that the Thulium-doped gain medium is pumped in an in-band pumping scheme; and frequency-doubling the amplified wavelength-shifted pulsed laser radiation. A laser system suitable for practicing the method is also disclosed.

Claims

1. A method for generating pulsed laser radiation in the spectral range from 860 nm to 1000 nm, the method comprising the steps of: generating pulsed laser radiation in the spectral range from 1500 nm to 1600 nm, preferably at a wavelength of 1560 nm; shifting the wavelength of the pulsed laser radiation to a longer wavelength of at least 1720 nm; amplifying the wavelength-shifted pulsed laser radiation in a Thulium-doped gain medium, so that the Thulium-doped gain medium is pumped in an in-band pumping scheme; and frequency-doubling the amplified wavelength-shifted pulsed laser radiation.

2. The method of claim 1, wherein the pulsed laser radiation is generated in the spectral range from 1500 nm to 1600 nm by a mode-locked Erbium fiber laser with a pulse duration of less than 1 ps.

3. The method of claim 1, wherein the pulsed laser radiation is amplified and spectrally broadened prior to shifting the wavelength.

4. The method of claim 1, wherein the pulsed laser radiation is spectrally broadened by self-phase-modulation after the step of shifting the wavelength such that the spectral bandwidth of the wavelength-shifted pulsed laser radiation is more than 60 nm.

5. The method of claim 1, wherein the wavelength-shifted laser pulses are stretched to a pulse duration of at least 10 ps prior to amplification in the Thulium-doped gain medium.

6. The method of claim 4, wherein the wavelength-shifted laser pulses are compressed to a pulse duration of less than 1 ps after amplification in the Thulium-doped gain medium.

7. The method of claim 1, wherein the Thulium-doped gain medium is a single-clad optical fiber.

8. The method of claim 7, wherein the single-clad optical fiber is core-pumped by a pump laser.

9. The method of claim 8, wherein the pump laser is an Erbium/Ytterbium fiber laser emitting at an average pump power of at least 500 mW.

10. A laser system for generating pulsed laser radiation in the spectral range from 860 nm to 1000 nm, comprising: a seed laser (1) for generating pulsed laser radiation in the spectral range from 1500 nm to 1600 nm; an optical fiber section shifting the wavelength of the pulsed laser radiation to a longer wavelength of at least 1720 nm; an optical amplifier (4) comprising an Thulium-doped gain medium which amplifies the wavelength-shifted pulsed laser radiation; a pump laser (5) for pumping the Thulium-doped gain medium in an in-band pumping scheme; and a frequency-multiplier (7) converting the wavelength of the amplified wavelength-shifted pulsed laser radiation to the spectral range from 900 nm to 950 nm.

11. The laser system of claim 10, wherein the seed laser (1) is a mode-locked erbium fiber laser generating the pulsed laser radiation in the spectral range from 1500 nm to 1600 nm by a mode-locked Erbium fiber laser with a pulse duration of less than 1 ps.

12. The laser system of claim 10, further comprising an optical boosting amplifier (2), which receives the pulsed laser radiation from the seed laser (1) and boosts the average power of the pulsed laser radiation to more than 25 mW.

13. The laser system of any one of claim 10, further comprising a pulse stretcher stretching the wavelength-shifted laser pulses to a pulse duration of at least 10 ps prior to amplification in the Thulium-doped gain medium.

14. The laser system of claim 13, further comprising a pulse compressor (6) compressing the wavelength-shifted laser pulses to a pulse duration of less than 1 ps after amplification in the Thulium-doped gain medium.

15. The laser system of any one of claim 10, comprising a further optical fiber section spectrally broadening the wavelength-shifted pulsed laser radiation to a spectral bandwidth of at least 60 nm through self-phase-modulation.

16. The laser system claim 10, wherein the Thulium-doped gain medium is a single-clad optical fiber.

17. The laser system of claim 10, further comprising an Erbium/Ytterbium fiber laser as the pump laser (5) which core-pumps the single-clad optical fiber at an average pump power of at least 500 mW.

18. The laser system of claim 10, wherein the frequency-multiplier (7) is a periodically poled lithium niobate crystal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 shows a system a laser system according to the invention as a block diagram.

DETAILED DESCRIPTION OF SEVERAL EXAMPLE EMBODIMENTS

[0026] FIG. 1 schematically shows an embodiment of a laser system according to the invention as a block diagram.

[0027] The laser system comprises an Erbium-doped fiber laser oscillator 1 as a seed laser which emits a stable output pulse train at a fixed repetition rate of, e.g., 80 MHz. The seed laser 1 generates soliton pulses of a duration of 500 fs centered at a wavelength of 1560 nm. Mode-locking is achieved by a semiconductor saturable absorber mirror (not depicted).

[0028] A consecutive Erbium-doped fiber amplifier 2 receives the laser pulses from the seed laser 1 and boosts their energy such that the average power of the amplified pulse train is on the order of 100 mW.

[0029] The amplified pulses are wavelength-shifted through the Raman-Effect in a Raman unit 3 to a wavelength of 1840 nm in several meters of PM1550 fiber. The central wavelength of the wavelength-shifted seed pulses is tunable by the pump power of the boosting amplifier 2.

[0030] The wavelength-shifted laser pulses are then spectrally broadened in a SPM unit 4 such that the spectral bandwidth of the wavelength-shifted pulsed laser radiation is more than 60 nm, preferably more than 100 nm. The spectral broadening takes place self-phase-modulation (SPM) in a section of optical fiber having normal dispersion. The SPM unit 4 further comprises a dispersive fiber grating as a stretcher for stretching the laser pulses to a pulse duration of about 100 ps.

[0031] The stretched laser pulses are then amplified in a Thulium-doped fiber amplifier 5. The single-clad active fiber of the Thulium-doped fiber amplifier 4 is core-pumped by a 5 W pump laser 6 (Erbium/Ytterbium fiber laser) at 1570 nm in an in-band pumping scheme. The average power of the amplified laser pulses at 1840 nm at the output of the Thulium-doped fiber amplifier is 2-5 W.

[0032] The laser pulses are then re-compressed in a pulse compressor 7 comprising a dispersive grating and a prism to a pulse duration of less than 100-110 fs.

[0033] Finally, the compressed laser pulses are frequency-doubled in a periodically poled lithium niobate crystal 8. The laser pulses at the output 9 of the depicted laser system have an average power of more than 1 W at a wavelength of 920 nm with a pulse duration of 80-100 fs.

[0034] The foregoing specification is provided for illustrative purposes only, and is not intended to describe all possible aspects of the present invention. Moreover, while the invention has been shown and described in detail with respect to several exemplary embodiments, those of skill in the pertinent arts will appreciate that minor changes to the description and various other modifications, omissions and additions may be made without departing from the scope thereof.