Total internal reflection (TIR) coupler and method for side-coupling pump light into a fiber

• US 6,529,318 B1
• Filed: 08/30/2001
• Issued: 03/04/2003
• Est. Priority Date: 08/30/2001
• Status: Active Grant

• Current Assignee: NP Photonics Incorporated
• Original Assignee: NP Photonics Incorporated
• Inventors: Kaneda, Yushi, Jiang, Shibin, Mendes, Sergio Brito
• Primary Examiner(s): Hellner, Mark
• Application Number:
  US09/943,257
• Time in Patent Office: 551 Days
• Field of Search: 359/341.3, 359/341.32, 372/6, 372/70
• US Class Current: 359/341.32
• CPC Class Codes: H01S 3/094003 the pumped medium being a fibre
• CPC Class Codes: H01S 3/094019 Side pumped fibre, whereby pump light is coupled laterally into the fibre via an optical component like a prism, or a grating, or via V-groove coupling

Abstract

A total internal reflection (TIR) coupler for side coupling pump light into a fiber for use in an amplifier or laser is mounted on a flat surface of the fiber'"'"'s inner cladding. The TIR coupler has a reflecting surface that forms an angle of taper a with the inner cladding, which is effective to reflect pump light at a preselected angle of incidence θ_inc and satisfy a TIR condition at its reflecting surface for folding pump light into the fiber. The pump light is launched into the fiber at an angle that also satisfies a TIR condition for guiding pump light inside the inner cladding.

Claims

1. A pump coupler for use in an amplifier or laser, comprising:

A fiber having a core and an inner cladding with at least one flat surface; and

A total internal reflection (TIR) coupler in optical contact with the inner cladding'"'"'s flat surface for a length L and having a reflecting surface that forms an angle of taper α

with said inner cladding, said TIR coupler being effective to reflect pump light at a preselected angle of incidence θ

_inc for the principal ray and satisfy a TIR condition at its reflecting surface for folding pump light into the fiber, wherein said pump light also satisfies a TIR condition for guiding pump light inside the inner cladding.

2. The pump coupler of claim 1, wherein the TIR coupler is further characterized by an input coupling condition for efficiently input coupling the pump light into the fiber and an output coupling condition for avoiding output coupling pump light back out of the fiber, the length L of the TIR coupler being effective to satisfy the input and output coupling conditions.

3. The pump coupler of claim 2, wherein the TIR coupler is effective to receive pump light substantially normal to the fiber so that the angle of incidence θ

_inc at the reflecting surface is substantially equal to the angle of taper α

.

4. The pump coupler of claim 3, wherein said inner cladding has a diameter W, the refractive indices of the TIR coupler, the inner cladding and an outer cladding are n_coupler, n_clad and n_ext respectively, and said pump light is incident on the reflecting surface a distance D from the starting point of the taper as projected onto the fiber, said folding and guiding TIR conditions and said input and output coupling conditions being as follows:

5. The pump coupler of claim 3, wherein said pump light has a beam width d, which modifies the input and output coupling conditions as follows:

6. The pump coupler of claim 4, wherein said TIR coupler and said inner cladding are index matched so that n_coupler=n_clad and θ

_L=θ

_i.

7. The pump coupler of claim 4, wherein said TIR coupler is mounted on said inner cladding towards one end of the fiber, said inner cladding having a second flat surface towards the other end of the fiber, further comprising a second TIR coupler in optical contact with the second flat surface to partially dump and partially recycle pump light that has traveled through the fiber.

8. The pump coupler of claim 4, wherein said fiber includes a segment of passive fiber having an undoped core surrounded by the inner cladding, which is optically coupled to a segment of active fiber having a doped core surrounded by the inner cladding, said TIR coupler being mounted on said passive fiber to fold the pump light into said segment of passive fiber and guide the pump light through and excite the entire length of the doped core in the segment of active fiber.

9. The pump coupler of claim 8, wherein the passive and active fibers comprise a phosphate glass including by weight percentages,

P₂O₅ from 30 to 80 percent,

L₂O₃ from 5 to 30 percent,

MO from 5 to 30 percent,

wherein L₂O₃ is selected from the transition metal oxide group consisting of Al₂O₃, B₂O₃, Y₂O₃, La₂O₃, and mixtures thereof, and MO is selected from the alkaline earth oxide group consisting of BaO, BeO, MgO, SrO, CaO, ZnO, PbO and mixtures thereof, and wherein the active fiber'"'"'s doped core is doped with

Er₂O₃ from 1.5 to 12 weight percent, and

Yb₂O₃ from 0.5 to 30 weight percent.

10. The pump coupler of claim 9, wherein the TIR coupler is effective to reflect multi-mode pump light, the active fiber'"'"'s doped core being doped with Yb₂O₃ from 5 to 30 weight percent.

11. The pump coupler of claim 4, further comprising a substrate on which the fiber is mounted, said TIR coupler'"'"'s angle of taper α

is an interior angle substantially equal to θ

_i/2, said pump light originating from the back side of said substrate so that the pump light traverses through the substrate and the fiber prior to illuminating the TIR coupler'"'"'s reflecting surface at an angle of incidence θ

_inc substantially equal to θ

_i/2.

12. The pump coupler of claim 11, further comprising an anti-reflection coating on the backside-of the substrate in the path of the pump light.

13. The pump coupler of claim 12, wherein the angle of incidence of the pump light on the anti-reflection coating is substantially zero.

14. The pump coupler of claim 4, wherein said TIR coupler'"'"'s angle of taper α

is an exterior angle that is substantially equal to (π

−

θ

_i)/2, said pump light originating from the same side of the fiber as the TIR coupler so that the pump light illuminates the TIR coupler'"'"'s reflecting surface at an angle of incidence θ

_inc substantially equal to (π

−

θ

_i)/2.

15. The pump coupler of claim 14, wherein said TIR coupler has a second surface substantially parallel to said fiber, further comprising an anti-reflection coating on said second surface in the path of the pump light.

16. The pump coupler of claim 15, wherein the angle of incidence of the pump light on the anti-reflection coating is substantially zero.

17. The pump coupler of claim 4, wherein the fiber'"'"'s core is undoped so that the pump light is folded into the fiber in an undoped region.

18. The pump coupler of claim 1, further comprising a plurality of said fibers, each fiber having a core and an inner cladding with at least one flat surface, said TIR coupler being mounted on the fibers'"'"' flat surfaces and effective to fold pump light into each of said fibers simultaneously.

19. An active gain media for use in an amplifier or laser, comprising:

A passive fiber having an undoped core and an inner cladding with at least one flat surface;

An active fiber optically coupled to said passive fiber, said active fiber having a doped core formed of an active media and an inner cladding; and

A total internal reflection (TIR) coupler in optical contact with the flat surface in the passive fiber for a length L and having a reflecting surface that forms an angle of taper α

with said passive fiber, said TIR coupler being effective to reflect pump light at a preselected angle of incidence θ

_inc for the principal ray and satisfy a TIR condition at its reflecting surface for folding pump light into the passive fiber, wherein said pump light also satisfies TIR conditions for guiding pump light inside both the passive and active fibers'"'"' inner claddings, said pump light interacting with the entire length of the active fiber'"'"'s doped core to excite the active media and provide gain to an optical signal in said doped core.

20. The active gain media of claim 19, wherein the TIR coupler is further characterized by an input coupling condition for efficiently input coupling the pump light into the fiber and an output coupling condition for avoiding output coupling pump light back out of the fiber, the length L of the TIR coupler being effective to satisfy the input and output coupling conditions.

21. The active gain media of claim 20, wherein the TIR coupler is effective to receive pump light substantially normal to the fiber so that the angle of incidence θ

_inc at the reflecting surface is substantially equal to the angle of taper α

.

22. The active gain media of claim 21, wherein said inner cladding has a diameter W, the refractive indices of the TIR coupler, the inner cladding of the active fiber and an outer cladding around its doped core are n_coupler, n_clad and n_ext respectively and said pump light is incident on the reflecting surface a distance D from the starting point of the taper as projected onto the fiber, said folding and guiding TIR conditions and said input and output coupling conditions being as follows:

23. The active gain media of claim 22, wherein said pump light has a beam width d, which modifies the input and output coupling conditions as follows:

24. The active gain media of claim 22, wherein the inner claddings of the passive and active fiber and the TIR coupler are index matched so that n_coupler=n_clad and θ

_L=θ

_i.

25. The active gain media of claim 22, wherein said active fiber has at least one flat surface towards the end of the active fiber that is not coupled to the passive fiber, further comprising a second TIR coupler in optical contact with the flat surface on the active fiber to partially dump and partially recycle pump light that has traveled through the active fiber.

26. The active gain media of claim 22, wherein said TIR coupler'"'"'s angle of taper α

is an exterior angle that is substantially equal to (π

−

θ

_i)/2, said pump light originating from the same side of the passive fiber as the TIR coupler so that the pump light illuminates the TIR coupler'"'"'s reflecting surface at an angle of incidence θ

_inc substantially equal to (π

−

θ

_i)/2.

27. The active gain media of claim 19, wherein the passive and active fibers comprise a phosphate glass including by weight percentages,

P₂O₅ from 30 to 80 percent,

L₂O₃ from 5 to 30 percent,

MO from 5 to 30 percent,

wherein L₂O₃ is selected from the transition metal oxide group consisting of Al₂O₃, B₂O₃, Y₂O₃, La₂O₃, and mixtures thereof, and MO is selected from the alkaline earth oxide group consisting of BaO, BeO, MgO, SrO, CaO, ZnO, PbO and mixtures thereof, and wherein the active fiber'"'"'s doped core is doped with

Er₂O₃ from 1.5 to 12 weight percent, and

Yb₂O₃ from 0.5 to 30 weight percent.

28. The active gain media of claim 27, wherein the TIR coupler is effective to reflect multi-mode pump light, the active fiber'"'"'s doped core being doped with Yb₂O₃ from 5 to 30 weight percent.

29. A fiber amplifier, comprising:

An input port for receiving an optical signal;

A passive phosphate glass fiber having an undoped core and an inner cladding around said core with at least one flat surface that together confine the optical signal to propagate through the undoped core;

An active phosphate glass fiber optically coupled to said passive fiber, said active fiber having a core co-doped with Erbium and Ytterbium and an inner cladding that together confine the optical signal to propagate through the doped core;

A total internal reflection (TIR) coupler in optical contact with and index matched to said passive fiber for a length L along the flat surface of the passive fiber'"'"'s inner cladding and having a reflecting surface that forms an angle of taper α

with said inner cladding; and

A pump source positioned to inject pump light substantially normal to the passive fiber and onto the TIR coupler'"'"'s reflecting surface at an angle of incidence θ

_inc, for the principal ray,

said TIR coupler being effective to reflect the principal ray of the pump light and satisfy a TIR condition at its reflecting surface and input and output coupling conditions at an interface between the TIR coupler and the inner cladding for folding pump light into the passive fiber, wherein said pump light also satisfies TIR conditions for guiding pump light inside both the passive and active fibers'"'"' inner claddings, said pump light interacting with the entire length of the active fiber'"'"'s co-doped core to excite the active media and provide gain to the optical signal.

30. The fiber amplifier of claim 29, wherein said inner cladding has a diameter W, the refractive indices of the TIR coupler, the inner cladding of the active fiber and an outer cladding around its co-doped core are n_coupler, n_clad and n_ext respectively, and said pump source is positioned a distance D from the starting point of the taper as projected onto the fiber, said folding and guiding TIR conditions and said input and output coupling conditions being as follows:

31. The fiber amplifier of claim 30, wherein said pump light has a beam width d, which modifies the input and output coupling conditions as follows:

32. The fiber amplifier of claim 31, wherein the inner claddings of the passive and active fiber and the TIR coupler are index matched so that n_coupler=n_clad and θ

_L=θ

_i.

33. The fiber amplifier of claim 30, wherein said TIR coupler'"'"'s angle of taper α

is an exterior angle that is substantially equal to (π

−

θ

_i)/2, said pump source being positioned on the same side of the passive fiber as the TIR coupler so that the pump light illuminates the TIR coupler'"'"'s reflecting surface at an angle of incidence θ

_inc substantially equal to (π

−

θ

_i)/2.

34. The fiber amplifier of claim 29, wherein the passive and active phosphate glass fibers comprise by weight percentages,

P₂O₅ from 30 to 80 percent,

L₂O₃ from 5 to 30 percent,

MO from 5 to 30 percent,

wherein L₂O₃ is selected from the transition metal oxide group consisting of Al₂O₃, B₂O₃, Y₂O₃, La₂O₃, and mixtures thereof, and MO is selected from the alkaline earth oxide group consisting of BaO, BeO, MgO, SrO, CaO, ZnO, PbO and mixtures thereof, and wherein the active fiber'"'"'s doped core is doped with

Er₂O₃ from 1.5 to 12 weight percent, and

Yb₂O₃ from 0.5 to 30 weight percent.

35. The fiber amplifier of claim 34, wherein the pump source is a multi-mode pump source and the active fiber'"'"'s doped core is doped with Yb₂O₃ from 5 to 30 weight percent.

36. A method for side coupling pump light into a fiber, comprising:

Providing a fiber having a core and an inner cladding with at least one flat surface;

Mounting a total internal reflection (TIR) coupler in optical contact with the inner cladding'"'"'s flat surface for a length L, said TIR coupler having a reflecting surface that forms an angle of taper α

with said inner cladding; and

Directing pump light onto said TIR coupler at a preselected angle of incidence θ

_inc for the principal ray, said TIR coupler being effective to satisfy a TIR condition at its reflecting surface for folding pump light into the fiber so that said pump light also satisfies a TIR condition for guiding pump light inside the inner cladding.

37. The method of claim 36, wherein the TIR coupler is further characterized by an input coupling condition for efficiently input coupling the pump light into the fiber and an output coupling condition for avoiding output coupling pump light back out of the fiber, the method further comprising preselecting the length L of the TIR coupler to be effective to satisfy the input and output coupling conditions.

38. The method of claim 37, further comprising directing the pump light substantially normal to the fiber so that the angle of incidence θ

_inc at the reflecting surface is substantially equal to the angle of taper α

.

39. The method of claim 38, wherein said inner cladding has a diameter W, the refractive indices of the TIR coupler, the inner cladding and an outer cladding are n_coupler, n_clad and n_ext respectively, and said pump light is incident on the reflecting surface a distance D from the starting point of the taper as projected onto the fiber, further comprising preselecting the TIR Coupler'"'"'s angle of taper α

and length L satisfy the folding and guiding TIR conditions and said input and output coupling conditions given as follows;

40. The method of claim 39, wherein said pump light has a beam width d, further comprising preselecting the TIR Coupler'"'"'s angle of taper α

and length L to satisfy the modified input and output coupling conditions given as follows;

41. The method of claim 39, further comprising index matching said TIR coupler and said inner cladding so that n_coupler=n_clad and θ

_L=θ

_i.

42. The method of claim 36, wherein said TIR coupler is mounted on said inner cladding towards one end of the fiber, said inner cladding having a second flat surface towards the other end of the fiber, further comprising mounting a second TIR coupler in optical contact with the second flat surface to partially dump and partially recycle pump light that has traveled through the fiber.

43. The method of claim 36, wherein the step of providing said fiber includes:

providing a segment of passive fiber having an undoped core surrounded by the inner cladding;

providing a segment of active fiber having a doped core surrounded by the inner cladding; and

optically coupling the segment of active fiber to the segment of passive fiber,

said TIR coupler being mounted on said passive fiber to fold the pump light into said segment of passive fiber and guide the pump light through and excite the entire length of the doped core in the segment of active fiber.

44. The method of claim 43, wherein the steps of providing the passive and active fibers comprises:

Drawing the passive and active fibers from a phosphate glass including by weight percentages,

P₂O₅ from 30 to 80 percent,

L₂O₃ from 5 to 30 percent,

MO from 5 to 30 percent,

wherein L₂O₃ is selected from the transition metal oxide group consisting of Al₂O₃, B₂O₃, Y₂O₃, La₂O₃, and mixtures thereof, and MO is selected from the alkaline earth oxide group consisting of BaO, BeO, MgO, SrO, CaO, ZnO, PbO and mixtures thereof, and

doping the active fiber'"'"'s doped core with

Er₂O₃ from 1.5 to 12 weight percent, and

Yb₂O₃ from 0.5 to 30 weight percent.

45. The method of claim 44, wherein the TIR coupler is effective to reflect multi-mode pump light, the active fiber'"'"'s doped core being doped with Yb₂O₃ from 5 to 30 weight percent.

46. The method of claim 39, further comprising:

Mounting the fiber on a substrate;

Preselecting said TIR coupler'"'"'s angle of taper α

as an interior angle substantially equal to θ

_i/2; and

Directing said pump light from the back side of said substrate so that the pump light traverses through the substrate and the fiber prior to illuminating the TIR coupler'"'"'s reflecting surface at an angle of incidence θ

_inc substantially equal to θ

_i/2.

47. The method of claim 39, comprising:

Preselecting said TIR coupler'"'"'s angle of taper α

as an exterior angle that is substantially equal to (π

−

θ

_i)/2; and

Directing said pump light originating from the same side of the fiber as the TIR coupler so that the pump light illuminates the TIR coupler'"'"'s reflecting surface at an angle of incidence θ

_inc substantially equal to (π

−

θ

_i)/2.

48. The method of claim 36, further comprising:

Providing a plurality of said fibers, each fiber having a core and an inner cladding with at least one flat surface, and

Mounting the TIR coupler on the fibers'"'"' flat surfaces to fold pump light into each of said fibers simultaneously.