Passively mode-coupled femtosecond laser

• US 6,618,423 B1
• Filed: 02/20/2002
• Issued: 09/09/2003
• Est. Priority Date: 05/21/1999
• Status: Expired due to Term

• Current Assignee: Laser Quantum GmbH (Novanta Inc.)
• Original Assignee: Laser Quantum GmbH (Novanta Inc.)
• Inventors: Bartels, Albrecht, Dekorsy, Thomas, Kurz, Heinrich
• Primary Examiner(s): Ip, Paul
• Assistant Examiner(s): Vy, Hung T
• Application Number:
  US09/979,165
• Time in Patent Office: 566 Days
• Field of Search: 372/18, 372/102, 372/100, 372/107, 372/94, 359/326
• US Class Current: 372/94
• CPC Class Codes: H01S 2301/08 Generation of pulses with special temporal shape or frequency spectrum
• CPC Class Codes: H01S 3/0057 Temporal shaping, e.g. pulse compression, frequency chirping soliton generation and propagation G02F1/3513, H01S3/063 and H01S3/108
• CPC Class Codes: H01S 3/08 Construction or shape of optical resonators or components thereof
• CPC Class Codes: H01S 3/0811 incorporating a dispersive element, e.g. a prism for wavelength selection
• CPC Class Codes: H01S 3/0813 Configuration of resonator
• CPC Class Codes: H01S 3/083 Ring lasers fibre ring lasers H01S3/06791
• CPC Class Codes: H01S 3/1112 Passive mode locking
• CPC Class Codes: H01S 3/1305 Feedback control systems
• CPC Class Codes: H01S 3/1625 titanium
• CPC Class Codes: H01S 3/1636 Al2O3 (Sapphire)

Abstract

The invention relates to a passive mode locked femtosecond laser whose ring resonator comprises the elements: a) a laser-active element 1, b) at least a dielectric mirror 2 having a negative group velocity dispersion GVD such that for a continuous part of the optical spectrum amplifieable by the laser-active element the sum of the group velocity dispersion of the mirror 2 and the positive group velocity dispersion of the laser-active element 1 is negative, i.e. $\sum _n{\mathrm{GVD}}_n<0$

c) two concave mirrors 21 and 22 which are spatial adjacent next to the laser-active element and which are orientated with their concave surfaces towards the laser-active element 1 and d) an optical output coupler 3. The optical path length in the resonator is below 60 cm, advantageously below 30 cm, in particular below 15 cm, the focus length f21 and f22 of the concave mirrors 21 and 22 are below 3 cm, advantageously below 2 cm, in particular below or equal to 1,5 cm, such that a high repetition generation of femtosecond pulses is achievable.

Further the invention relates to a continuous tuneable high repetition rate femtosecond laser, the method of use thereof for high repetition generation of femtosecond pulses and a method for tuning thereof.

Claims

1. Passive mode locked femtosecond laser having a ring resonator comprising:

a laser-active element having a group velocity dispersion;

a prismatic element having a group velocity dispersion;

at least one dielectric mirror, having a negative group velocity dispersion (GVD_η

) for reflection;

two concave mirrors which are spatially arranged immediately adjacent to the laser-active element and are oriented with their concave surfaces towards the laser-active element; and

an optical output coupler,

wherein the ring resonator has an optical path and each of said laser-active element, prismatic element and mirrors is arranged along the optical path, and

wherein for a contiguous part of the optical spectrum capable of being amplified by the laser-active element, the value of the negative group velocity dispersion associated with the at least one dielectric mirror is greater than the combined value of the positive group velocity dispersion associated with the laser active element and the prismatic element,

characterized in that

the optical path of the resonator is below 120 cm,

the focus length of each of the concave mirrors, which are arranged adjacent next to the laser-active element is below 3 cm,

whereby a high repetition generation of femtosecond pulses is attainable and one of (i) said at least one dielectric mirror or (ii) the output coupler or (iii) a further mirror is tiltable at a tilt angle upon an axis wherein by adjusting the tilt angle in interplay with the spatial dispersion of the prismatic element, a wavelength is adjustable for which the resonator supports laser activity.

2. The laser according to claim 1, wherein the optical path length of the resonator is below 60 cm.

3. The laser according to claim 1, wherein the optical path length of the resonator is below 30 cm.

4. The laser according to claim 1, wherein the optical path length of the resonator is below 15 cm.

5. The laser according to claim 1, wherein the focus length of each of the concave mirrors is below 2 cm.

6. The laser according to claim 1, wherein the focus length of each of the concave mirrors is below 1.5 cm.

7. The laser according to claim 1, characterized in that the laser-active element is also self-focusing element.

8. The laser according to claim 1, characterized in that the resonator is adapted in interplay with the non-linear element, such that pulsed laser activity allows a higher energy output from the laser-active element as compared to continuous laser activity.

9. The laser according to claim 1, characterized in that the prismatic element has a surface and a surface of exit adapted to have an optical beam incident on the prism surface under condition of minimal deflection wherein the optical beam is incident on the prism surface at a Brewster angle for the wavelength amplifiable by the laser-active medium and escapes at the Brewster angle from the surface of exit.

10. The laser according to claim 1, characterized in that the prismatic element has a prism surface of exit adapted to have an optical beam incident on the prism surface at a Brewster angle for the wavelength amplifiable by the laser-active medium, wherein the optical beam escapes substantially at a right angle from the surface of exit.

11. The laser according to claim 10, characterized in that an antireflection coating for the wavelength amplifiable by the laser-active medium is applied to the surface of exit.

17. The laser according to claim 1 or 12, characterized in that both focus lengths of the concave mirrors which are adjacent to the laser-active elements are substantially equal in value.

18. The laser according to claim 1 or 12, characterized in that the distance of the concave mirrors to each other, which are adjacent to the laser-active element is smaller than the sum of their focus lengths.

19. The laser according to claim 18, characterized in that the distance of the laser-active element from a first concave mirror differs by more than 2% from the distance of the laser-active element from a second concave mirror.

20. The laser according to claim 19, wherein the distance differs by more than 50%.

21. The laser according to claim 19, wherein the distance differs by more than 10%.

22. The laser according to claim 1 or 12, characterized in that the distance of the concave mirrors to each other which are adjacent to the laser-active element is greater than the sum of focus lengths of the concave mirrors, and a ring aperture is located in the ring resonator.

23. The laser according to claim 1 or 12, characterized in that the laser-active element substantially comprises at least one of Ti:

Saphir, Cr;

LiSAF, Cr;

Forsterite, Cr;

LiSGaF, CR;

LiCAF and Yb;

YAG.

24. The laser according to claim 1 or 12, comprising a frequency doubled Nd:

YVO₄₋

, Yb;

YVO₄₋

, Nd;

YAG- or Yb;

YAG-laser used as a pump laser.

25. The laser according to claim 1 or 12 characterized in that the degree of outcoupling of the optical output coupler is below 5%.

26. The laser according to claim 25, wherein the degree of outcoupling is below 3%.

27. The laser according to claim 25, wherein the degree of outcoupling is below or equal to 2%.

28. The laser according to claim 1 or 12, characterized in that the output coupler is configured as a partial reflecting mirror.

29. The laser according to claim 1 or 12, characterized in that all mirrors have a negative group velocity dispersion (GVD).

30. The laser according to claim 1 or 12, characterized in that only one mirror has a negative group velocity dispersion (GVD).

31. The laser according to claim 1 or 12, characterized in that a ring resonator is compensated for astigmatism.

32. The laser according to claim 1 or 12, characterized in that all elements of a ring resonator are mechanically arranged on a common mounting platform.

33. The laser according to claim 1 or 12 characterized in that two beams are reflected by each of the concave mirrors and an angle enclosed between the beaus amounts between 15°

and 25°

.

12. Passive mode locked femtosecond laser having a ring resonator comprising:

a laser-active element having a group velocity dispersion;

at least one dielectric mirror having a negative group velocity dispersion (GVD_η

) for reflection;

two concave mirrors which are spatially arranged immediately adjacent to the laser-active element and are oriented with their concave surfaces towards the laser-active element; and

an optical output coupler;

wherein the ring resonator has an optical path and each of said laser-active element and mirrors is arranged along the optical path, and

wherein for a contiguous part of the optical spectrum capable of being amplified by the laser-active element, the value of the negative group velocity dispersion associated with the at least one dielectric mirror is greater than the value of the positive group velocity dispersion associated with the laser active element,

characterized in that

the optical path of the resonator is below 60 cm, and

the focus length of each of the concave mirrors which are arranged immediately adjacent to the laser-active element is below 3 cm,

whereby a high repetition generation of femtosecond pulses is attainable.

13. The laser according to claim 12, wherein the optical path length of the resonator is below 30 cm.

14. The laser according to claim 12, wherein the optical path length of the resonator is below 15 cm.

15. The laser according to claim 12, wherein the focus length of each of the concave mirrors is below 2 cm.

16. The laser according to claim 12, wherein the focus length of each of the concave mirrors is below 1.5 cm.

34. A method for obtaining a generation of femtosecond pulses from and turning a passive mode locked femtosecond laser having a ring resonator comprising a laser-active element having a group velocity dispersion, a prismatic element having a group velocity dispersion, at least one dielectric mirror having a negative group velocity dispersion (GVD_η

) for reflection, two concave mirror which are spatially arranged immediately adjacent to the laser-active element and are oriented with respective concave surfaces towards the laser-active element, and an optical output coupler, wherein the ring resonator has an optical path and each of said laser-active element, prismatic element and mirrors is arranged along the optical path, and wherein for a contiguous part of the optical spectrum capable of being amplified by the laser-active element, the value of the negative group velocity dispersion associated with the at least one dielectric mirror is greater than the combined value of the positive group velocity dispersion associated with the laser active element and the prismatic element, characterized in that the optical path of the resonator is below 120 cm, the focus length of each of the concave mirrors, which are arranged adjacent next to the laser-active element is below 3 cm, where

in a first step a high repetition generation of femtosecond pulses is obtained and

in a second step one of (i) said at least one dielectric mirror or (ii) the output coupler or (iii) a further mirror is tiltable at a tilt angle upon an axis wherein by adjusting the tilt angle in interplay with the spatial dispersion of the prismatic element, a wavelength is adjusted for which the resonator supports laser activity.

35. The method according to claim 34, characterized in that the passive mode coupled femtosecond laser is operated at a pulse repetition rate exceeding 500 MHz.

36. The method according to claim 34, characterized in that the passive mode femtosecond laser is operated at a pulse repetition rate exceeding 1 GHz.

37. The method according to claim 34, characterized in that the passive mode femtosecond laser is operated at a pulse repetition rate exceeding 2 GHz.

38. The method according to claim 34, characterized in that the passive mode femtosecond laser is operated at a pulse repetition rate exceeding 10 GHz.

39. A method for obtaining a generation of femtosecond pulses from a passive mode locked femtosecond laser having a ring resonator comprising a laser-active element having a group velocity dispersion, at least one dielectric mirror having a negative group velocity dispersion (GVD_η

) for reflection, two concave mirrors which are spatially arranged immediately adjacent to the laser-active element and are oriented with respective concave surfaces towards the laser-active element and an optical output coupler, wherein the ring resonator has an optical path and each of said laser-active element and mirrors is arranged along the optical path, and wherein for a contiguous part of the optical spectrum capable of being amplified by the laser-active element, the value of the negative group velocity dispersion associated with the at least one dielectric mirror is greater than the value of the positive group velocity dispersion associated with the laser active element, characterized in that the optical path of the resonator is below 60 cm, and the focus length of each of the concave mirrors which are arranged immediately adjacent to the laser-active element is below 3 cm, where

a high repetition generation of femtosecond pulses is attained.

40. The method according to claim 39, characterized in that the passive mode coupled femtosecond laser is operated at a pulse repetition rate exceeding 500 MHz.

41. The method according to claim 39, characterized in that the passive mode femtosecond laser is operated at a pulse repetition rate exceeding 1 GHz.

42. The method according to claim 39, characterized in that the passive mode femtosecond laser is operated at a pulse repetition rate exceeding 2 GHz.

43. The method according to claim 39, characterized in that the passive mode femtosecond laser is operated at a pulse repetition rate exceeding 10 GHz.

44. Passive mode coupled femtosecond laser having a ring resonator comprising:

a laser-active element having a group velocity dispersion;

two concave mirrors which are spatially arranged immediately adjacent to the laser-active element and are oriented with their concave surfaces towards the laser-active element;

at least one dielectric mirror receiving a reflection from at least one of said concave mirror, having a negative group velocity dispersion for reflection and wherein for a contiguous part of the optical spectrum capable of being amplified by the laser-active element, the value of the negative group velocity dispersion associated with the at least one dielectric mirror is greater than the value of the positive group velocity dispersion associated with the laser active element; and

an optical output coupler receiving a reflection from at least one of said at least one concave mirror and dielectric mirror,

whereby a high repetition generation of femtosecond pulses is obtained.

45. Passive mode coupled femtosecond laser having a ring resonator comprising:

a laser-active element having a group velocity dispersion,

two concave mirrors which are spatially arranged immediately adjacent to tee laser-active element and are oriented with their concave surfaces towards the laser-active element;

at least one dielectric mirror, having a negative group velocity dispersion for reflection and wherein for a contiguous part of the optical spectrum capable of being amplified by the laser-active element, the value of the negative group velocity dispersion associated with the at least one dielectric mirror is greater than the value of the positive group velocity dispersion associated with the laser active element; and

an optical output coupler,

wherein the ring resonator has an optical path and each of said laser-active element, mirrors and coupler is arranged along the optical path, and

whereby a high repetition generation of femtosecond pulses is obtained.

46. Passive mode coupled femtosecond laser having a ring resonator comprising:

a laser-active element having a group velocity dispersion;

a prismatic element having a group velocity dispersion,

two concave mirrors which are spatially arranged immediately adjacent to the laser-active element and are oriented with their concave surfaces towards the laser-active element;

at least one dielectric mirror, having a negative group velocity dispersion for reflection and wherein for a contiguous part of the optical spectrum capable of being amplified by the laser-active element, the value of the negative group velocity dispersion associated with the at least one dielectric mirror is greater than the value of the positive group velocity dispersion associated with the laser active element; and

an optical output coupler,

wherein the ring resonator has an optical path and each of said laser-active element, mirrors and coupler is arranged along the optical path, and

whereby a high repetition generation of femtosecond pulses is obtained, and one of said at least one dielectric mirror, output coupler, or a further mirror is tiltable at a tilt angle upon an axis wherein by adjusting the tilt angle in interplay with the spatial dispersion of the prismatic element, a wavelength is adjusted for which the resonator supports laser activity.

47. Passive mode coupled femtosecond laser having a ring resonator comprising:

a laser-active element having a group velocity dispersion;

two concave mirrors which are spatially arranged immediately adjacent to the laser-active element and are oriented with their concave surfaces towards the laser-active element;

at least one dielectric moor receiving a reflection from at least one of said concave mirror, and a prismatic element receiving a reflection from the at least one dielectric mirror, wherein the at least one dielectric mirror has a negative group velocity dispersion for reflection and wherein for a contiguous part of the optical spectrum capable of being amplified by the laser-active element, the value of the negative group velocity dispersion associated with the at least one dielectric mirror is greater than the combined value of the positive group velocity dispersion associated with the laser active element and at least one prismatic element; and

an optical output coupler receiving a reflection from at least of one of said concave mirror and at least one dielectric mirror,

whereby a high repetition generation of femtosecond pulses is obtained, and one of said at least one dielectric mirror, output coupler, or a further mirror is tiltable at a tilt angle upon an axis wherein by adjusting the tilt angle in interplay with the spatial dispersion of the prismatic element, a wavelength is adjusted for which the resonator supports laser activity.