WO2001005004A1

WO2001005004A1 - Wavelength tunable, polarization stable mode-locked fiber laser

Info

Publication number: WO2001005004A1
Application number: PCT/US2000/019170
Authority: WO
Inventors: Hong Lin
Original assignee: Calmar Optcom, Inc.
Priority date: 1999-07-14
Filing date: 2000-07-14
Publication date: 2001-01-18
Also published as: AU7699600A

Abstract

A pulsed tunable fiber laser is described with a laser cavity comprising a wavelength tuning unit (15) forming a first terminus of the laser cavity, a saturable absorber unit (11) forming a second terminus of the laser cavity, and a single-mode rare-earth doped fiber section (13) connected between the wavelength tuning unit (15) and the saturable absorber unit (13), and a coupler (12) connected to the single-mode rare-earth doped fiber section (13) providing a conduit for pumping energy to the single-mode rare-earth doped fiber section (13). Both the wavelength tuning unit (15) has a rotatable reflective grating (45) to select the output Faraday rotator (34, 43), and the wavelength tuning unit (15) has a rotatable reflective grating (45) to select the output wavelength.

Description

WAVELENGTH TUNABLE, POLARIZATION STABLE MODE-LOCKED FIBER LASER

CROSS-REFERENCES TO RELATED APPLICATIONS This patent application claims priority from Provisional Patent Application

No. 60/143,704, filed July 14, 1999, which provisional patent application is incorporated herein by reference.

BACKGROUND OF THE INVENTION The present invention is related to fiber lasers, and in particular, to fiber lasers with outputs which are tunable and stably polarized.

In WDM (Wavelength Division Multiplexing) fiber optic networks, optical signals are sent at predetermined wavelengths over optical fibers. Each predetermined wavelength forms a communication channel in the network and the wavelength (or frequency) of the optical signal is used to control the destination of the signal through the network. An advanced version of WDM networks is the DWDM (Dense Wavelength Division Multiplexing) network in which the number of wavelength channel is increased by reducing the channel wavelength separation. In DWDM networks, the communication channels are separated by 100GHz, as set by the ITU (International Telecommunications Union) and the fiber optic industry is already assuming an unofficial channel separation of 50 GHz for advanced DWDM networks.

In these WDM networks, referring to both WDM and DWDM networks, modulated optical signals from a laser source are used. Such a laser source is typically created from a semiconductor laser diode in which a laser cavity is formed in a semiconductor substrate with regions of differing indices of refractivity. Compact size, stability, reliability and good performance parameters are some of the advantages of a semiconductor laser.

A recent candidate for a laser source is the fiber laser. In a fiber laser, a laser cavity is formed with an optical fiber doped with some rare-earth element. The doped optical fiber forms a lasing medium which is energized by a pump laser (typically a semiconductor laser diode) at a predetermined wavelength. The energized optical fiber lases at a wavelength of lower energy. Besides the advantages of good stability, high reliability, and low noise output, the fiber laser has the additional advantage of higher power than a semiconductor laser diode; and if the fiber laser is pulsed, the fiber laser has a short pulsewidth for high pulse rates and, consequently, higher data rates. However, to obtain optimum performance, typically a fiber laser has polarization-maintaining fibers and components to avoid the variations in the polarization state of the light in a conventional single-mode fiber. Nonetheless, polarization-maintaining fibers and components are expensive.

The present invention provides for a pulsed fiber laser which has high performance, a polarization stable output and which is also tunable. Costs are relatively low.

SUMMARY OF THE INVENTION The present invention provides for a fiber laser having a laser cavity having a wavelength tuning unit forming a first terminus of the laser cavity, a saturable absorber unit forming a second terminus of the laser cavity, a single-mode, rare-earth doped fiber section connected between the wavelength tuning unit and the saturable absorber unit, and a coupler connected to the single-mode, rare-earth doped fiber section providing a conduit for pumping energy to the single-mode, rare-earth doped fiber section. The wavelength tuning unit reflects light received from the remainder of the laser cavity back into the remainder of said laser cavity polarized and at a selected wavelength. The saturable absorber unit has a reflecting element, a 45° Faraday rotator, and a saturable absorber element. The reflecting element, Faraday rotator and the saturable element are arranged so that light received from the remainder of the laser cavity is reflected into the remainder of the laser cavity with amplitude-dependent losses, and any polarization of light reflected from said saturable absorber is orthogonal to any polarization of light entering the saturable absorber unit.

For lower costs, the wavelength tuning unit comprises a polarization- maintaining component and the saturable absorber unit, the rare-earth doped fiber section, and the coupler comprise single-mode optical fiber components.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a diagram of one embodiment of a fiber laser according to the present invention; Fig. 2 is a diagram of another embodiment of a fiber laser according to the present invention; and

Figs. 3 A and 3B are plots of the outputs of the fiber laser of Fig. 1 at different selected wavelengths.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS In accordance with one embodiment of the present invention, a pulsed fiber laser which is tunable and which has a defined polarized output is illustrated in Fig. 1. The fiber laser has two sections, a polarization-maintaining (PM) section 20 and a nonpolarization-maintaining section 10 which is enclosed by a dotted line. The nonPM section 10 has optical fiber sections interconnecting a saturable absorber unit 11 and a WDM (wavelength division multiplexer) 12 which is also connected to a pump laser 18. The WDM 12 is also connected to one end of an erbium-doped fiber section 13. The optical fiber sections in the nonPM section 10 are single-mode optical fibers and the components in the section 10 are for single-mode optical fibers.

The PM section 20 has a polarization-maintaining coupler 14 which is connected to the other end of the erbium-doped fiber section 13. The coupler 14 is also connected to a polarization-maintaining wavelength tuning unit 15. Also connected to the coupler 14 are two polarization-maintaining isolators 16 and 17 which are each connected to output connectors 19 and 21 respectively. The optical fiber sections in this section 20 are polarization-maintaining fibers.

The saturable absorber unit 11 is formed with a reflective mirror 31 , a semiconductor saturable absorber 32 with its active layer attached to the mirror 31, a focusing lens 33, a 45° Faraday rotator 34 and a single-mode fiber collimator 35, such as a quarter-pitch GRIN lens. The focusing lens 33 focuses the collimated light from the collimator 35 to the active layer of the saturable absorber 32 and the surface of the mirror 32. The saturable absorber 32 provides amplitude-dependent losses for the light traveling back and forth in the laser cavity, which is formed by the optical path in the fiber laser between the saturable absorber unit 11 and the tuning unit 15. This creates the pulsing operation in the fiber laser.

The polarization-maintaining wavelength tuning unit 15 has a polarization- maintaining fiber connector 41, a focusing lens 42, a 45° Faraday rotator 43, and a Reflective grating 44. The end of the connector 41 is angled to reduce undesirable reflection. The grating 44 is mounted on an angular rotational stage 45 which adjusts the angle of the grating 44 for wavelength tuning. The grating 44 selects a lasing wavelength (and also a lasing polarization) in the laser cavity.

In the drawings the short lines perpendicular to the optical fiber sections represent splices. All fiber connections are formed by fiber splicing techniques, preferably fusion splicing. The single-mode section 10 uses single-mode fiber splices and the PM section 20 uses polarization-maintaining splices. For connections between single- mode and polarization-maintaining fibers, single-mode accuracy is used. Since most commercial fusion splicers do not have automatic computer programs to handle splicing between single-mode and polarization-maintaining fibers, splicing is manually adjusted. The non-PM section 10 of the Fig. 1 fiber laser operates for optimum operation of the gain-inducing section 13. Assuming, for example, that the light passing from the PM coupler 14 into the section 13 is in one polarization state, the light is collimated by the collimator 35 in the saturable absorber unit 11 after passing through the WDM 12. The collimated light is rotated 45° in a rotational direction by the Faraday rotator 34 and focused by the lens 33 upon the mirror 31. After being reflected, the light is refocused into collimated light by the lens 33 and rotated another 45° by the Faraday rotator 34 in the same direction. The polarization state of the light heading from the WDM 12 and the erbium-doped section 13 is orthogonal to the polarization state of the light passing into the erbium-doped fiber section 13 from the PM coupler 14. The orthogonality of the two polarization states automatically cancels out any birefringence effects which might be induced by the erbium-doped section 13 (and WDM 12). Such effects cause undesirable instability in frequency, polarization, and power in the output of the fiber laser. Light leaving the erbium-doped section 13 passes into the PM section 20.

Light passes through the PM coupler 14 to the wavelength tuning unit 15. In the unit 15 light leaves the PM connector 41 and is focused by the lens 42 which focuses the light from the connector end on the surface of the grating 44, such as a grating of 600 grooved lines per millimeter. The magnification of the lens 42 on the grating 44 influences the bandwidth of the output wavelength of the fiber laser. The greater the number of grooves receiving and reflecting the focused light, the more narrow the bandwidth. The bandwidth in turn influences the pulsewidth of the laser output. The direction of the grooves of the grating 44 polarizes the light in the PM section 20 and for optimum operation, the slow axis (or fast axis) of the PM fiber connector 41 is set at 45° with respect to the direction of the grooves of the grating 44.

The fiber sections and components of the PM section 20 maintain the state of polarization of the light traveling through the section to the non-PM section 10 where the light is amplified, as described previously. The polarization state is stable in the laser cavity and the light is passively mode-locked. The output connectors 19 and 21 are exit ports for the polarization stable output of the fiber laser. The connected polarization- maintaining isolators 16 and 17 ensure that unwanted external light signals are not transmitted through the coupler 14 into the laser cavity. The resulting fiber laser has picosecond and femtosecond pulses with wavelength tunability and adjustment-free operation. The desired output wavelength and power level is selected by adjusting the angular position of the grating 44 and the power of the pump laser 18. A tuning range of 1525-1567nm has been obtained. Figs. 3A and 3B are plots of the optical spectrum of a tunable pulsed fiber laser according to the present invention. In Fig. 3 A the center wavelength is 1560nm, and in Fig. 3B the center wavelength is 1530.6nm. The high performance of the fiber laser is obtained with only a minimal amount of polarization-maintaining fibers and components to keep costs relatively low.

It should be noted that the substitutions of materials and components can be made in the fiber laser according to the present invention. For example, other rare- earth elements, such as ytterbium, neodymium, holmium, thulium, or combinations of these elements may be used in place of erbium as dopants in the gain-inducing section 13. Of course, the output wavelengths and power would be different. A more complex arrangement for the gain-inducing section 13 is a double clad gain fiber in which the signal to be amplified propagates in a doped (rare earth elements Er, Yb, Er/Yb) single- mode central core and the pump energy propagates in a larger multimode core which surrounds the single-mode core. In place of the wavelength tuning unit described above, an interference transmission filter, a polarizing element and mirror can be used. Wavelength tuning is performed by changing the angle of the interference filter with respect to the incident light. The polarizing element polarizes the light reflected by the mirror.

Another embodiment of the present invention is a tunable fiber laser illustrated in Fig. 2. The fiber laser uses all single-mode components. The fiber laser has a saturable absorber unit 51, a single-mode WDM 52, a single-mode Er-doped fiber section 53, a single-mode fiber coupler 54 and a single-mode wavelength tuning unit 55. A pump laser 58 provides the energy to the Er-doped fiber section 53. Two single-mode optical isolators 56 and 57 are connected to the coupler 54; two output connectors 59 and 62 are respectively connected to the isolators 56 and 57. The saturable absorber unit 51 is constructed as described with respect to the Fig. 1 fiber laser and the wavelength tuning unit 55 is the same as the wavelength tuning unit in Fig. 1, except the components are for single-mode.

The operation of the components of the Fig. 2 fiber laser is similar to that of the fiber laser of Fig. 1. It should be noticed, though, that the Fig. 2 arrangement of components is slightly different. The gain-inducing components, i.e., the Er-doped fiber section 53, the coupler 54 and the pump laser 58, are closer to the wavelength tuning unit than that in the Fig. 1 arrangement. In the Fig. 2 arrangement, the randomness in the polarization of the light traveling through fiber section 53 is reduced by proximity to the polarized light from the grating 74 in the wavelength tuning unit 55. This helps to reduce undesired mode hopping and improves the operation of the section 53.

The fiber laser, which is tunable, produces single-mode fiber pulsed output light which does not have a well-defined polarization, in contrast to the fiber laser of Fig. 1. Therefore, while the description above provides a full and complete disclosure of the preferred embodiments of the present invention, various modifications, alternate constructions, and equivalents will be obvious to those with skill in the art. Thus, the scope of the present invention is limited solely by the metes and bounds of the appended claims.

Claims

WHAT IS CLAIMED IS:

1. In a fiber laser, a laser cavity comprising a wavelength tuning unit forming a first terminus of said laser cavity, said wavelength tuning unit reflecting light received from a remainder of said laser cavity back into said remainder of said laser cavity polarized and at a selected wavelength; a saturable absorber unit forming a second terminus of said laser cavity, said saturable absorber unit having a reflecting element; a 45° Faraday rotator; a saturable absorber element, said reflecting element, Faraday rotator and said saturable element arranged so that light received from said remainder of said laser cavity is reflected into said remainder of said laser cavity with amplitude-dependent losses, any polarization of light reflected from said saturable absorber is orthogonal to any polarization of light entering said saturable absorber unit; a rare-earth doped fiber section connected between said wavelength tuning unit and said saturable absorber unit; and a wavelength division multiplexer connected to said single-mode, rare- earth doped fiber section providing a conduit for pumping energy to said single-mode, rare-earth doped fiber section.

2. The fiber laser of claim 1 wherein said wavelength tuning unit comprises a polarization-maintaining component.

3. The fiber laser of claim 1 wherein said saturable absorber unit, said single-mode, rare-earth doped fiber section, and said coupler comprise single-mode optical fiber components.

4. The fiber laser of claim 2 wherein said wavelength tuning unit comprises a polarization-maintaining coupler connected to said remainder of said laser cavity; a rotatable reflective grating for reflecting light of a selected wavelength from said polarization-maintaining coupler back to polarization-maintaining coupler, said wavelength selected by said rotation; a lens focusing said light from said polarization-maintaining coupler upon said Reflective grating and refocusing light reflected by said reflective grating back to said polarization-maintaining coupler; and a 45° Faraday rotator in the light path between said polarization- maintaining coupler and said reflective grating.

5. The fiber laser of claim 4 wherein said polarization-maintaining coupler has fast and slow axes of indices of refraction, said reflective grating having grooves which are aligned 45° to one of said axes of said polarization-maintaining fiber.

6. The fiber laser of claim 3 wherein said reflecting element of said saturable absorber unit comprises a mirror, said saturable absorber element comprises a semiconductor having an active layer attached to said mirror; and said saturable absorber unit further comprises a collimator connected to said remainder of said laser cavity and arranged to direct light from said aid remainder of said laser cavity to said mirror; and a lens in the path between said mirror and said collimator for focusing light from said collimator to said mirror and refocusing light reflected by said mirror back to said collimator.

7. The fiber laser of claim 1 wherein said 45° Faraday rotator in said saturable absorber unit is arranged between said collimator and said lens.

8. The fiber laser of claim 1 further comprising a pump laser connected to said wavelength division multiplexer for providing pumping energy.

9. The fiber laser of claim 1 wherein said laser cavity further comprises a coupler, said coupler connected to at least one output port for said light in said laser cavity.

10. The fiber laser of claim 9 wherein said coupler comprises a polarization-maintaining coupler.

11. The fiber laser of claim 9 further comprising an optical isolator connected between said output port and said coupler.

12. The fiber laser of claim 9 wherein said coupler is connected to two output ports, each output port connected to an optical isolator between said output port and said coupler.

13. A fiber laser comprising a laser pump; a single-mode wavelength division multiplexer having first, second and third terminals, said first terminal connected to said laser pump; a saturable absorber unit connected to said second terminal of a single- mode wavelength division multiplexer, said saturable absorber unit reflecting light received through said second terminal back through said second terminal; a single-mode rare-earth doped fiber section having a first end and a second end, said first end connected to said third terminal of said single-mode wavelength division multiplexer; a polarization-maintaining coupler having first, second, and third terminals, said first terminal connected to said second end of said a single-mode rare- earth doped fiber section; a polarization-maintaining wavelength tuning unit connected to said second terminal of said a polarization-maintaining coupler; and a first output port connected to said third terminal of said polarization- maintaining coupler through a first optical isolator; said first output port and said first optical isolator comprising polarization-maintaining components.

14. The fiber laser of claim 13 wherein said polarization-maintaining coupler has a fourth terminal, and further comprising a second output port connected to said fourth terminal of said polarization-maintaining coupler through a second optical isolator; said second output port and said second optical isolator comprising polarization- maintaining components.

15. A fiber laser comprising a laser pump; a single-mode wavelength division multiplexer having first, second and third terminals, said first terminal connected to said laser pump; a single-mode rare-earth doped fiber section having a first end and a second end, said first end connected to said second terminal of said single-mode wavelength division multiplexer; a single-mode wavelength tuning unit connected to said second end of said a single-mode rare-earth doped fiber section; a single-mode coupler having first, second, and third terminals, said first terminal connected to said third terminal of said single-mode wavelength division multiplexer; a saturable absorber unit connected to said second terminal of a single- mode coupler, said saturable absorber unit reflecting light received through said second terminal back through said second terminal; and a first output port connected to said third terminal of said single-mode coupler through a first optical isolator; said first output port and said first optical isolator comprising single-mode components.

16. The fiber laser of claim 15 wherein said single-mode coupler has a fourth terminal, and further comprising a second output port connected to said fourth terminal of said single-mode coupler through a second optical isolator; said second output port and said second optical isolator comprising single-mode components.