WO2009040465A2

WO2009040465A2 - An optical amplifier

Info

Publication number: WO2009040465A2
Application number: PCT/FI2008/050513
Authority: WO
Inventors: Simo Tammela; Kalle YLÄ-JARKKO
Original assignee: Corelase Oy
Priority date: 2007-09-28
Filing date: 2008-09-17
Publication date: 2009-04-02
Also published as: FI20075685A0; WO2009040465A3

Abstract

The invention relates to an optical amplifier (10) using a multimode optical fiber (7), comprising a pump laser (1), an optical fiber (7), forming a waveguide and having at least partially a radius of curvature for causing losses for high order modes, connected to the optical amplifier (1). In accordance with the invention the fiber (7) has at least on a part of its length a continuous change of the bending plane in three dimensions such that the resulting effective refractive index of the waveguide is rocking in two or more directions.

Description

An optical amplifier

The present invention relates to an optical amplifier in accordance with the preamble of claim 1.

The present invention relates also to a method in connection with an optical amplifier.

In the prior art multimode core double cladding active fibers have been used for making substantially single mode amplifiers and problems relating to suppressing undesired modes have been presented. Especially US-patent 6496301 describes a uniform bending or spooling of the fiber. In this solution the undesired modes are coupled to cladding modes. The coupling of the light from the cladding modes back into the core, the resonance effect, is not solved. This limits the achievable discrimination, output beam quality, efficiency and gain.

With this solution, however, the problem relating to the resonance effects between the core modes and cladding modes in double cladding fibers can not be solved in a satisfactory way.

The object of the present invention is to improve the discrimination between the fundamental and higher order core modes. This is achieved mainly by reducing the back coupling from the cladding modes to core modes, also called as resonance effect, in double cladding fibers.

The closest mode to the fundamental mode (LPOl) is asymmetric mode (LPl 1). The optical power of the LPl 1 mode is in two side lobes, there is no power in the center. There are actually two LPl 1 modes, one mode having lobes situated horizontally and the second having them situated vertically. If the bending is done only in one plane it is difficult to remove the both LPl 1 modes. This can be done by using two coils with 90 degree difference as shown in US-patent 6496301. However, the resonance effect limits in this solution the achievable discrimination between the fundamental mode and the undesirable higher order modes; it is not enough for high gain amplifiers. Helical cladding modes are un-desirable modes in double cladding fiber. The optical powers of these cladding modes travel in a helical route in the cladding and these modes have very little, if any, optical power in the center part of the fiber where the doped core exist. The pump light in these modes can not be absorbed by the core and thus the energy in these modes are carrying can not be used for amplification. Several patents describe the ways to solve this problem; many of them describe how to break the circular symmetry of the cladding geometry. Unfortunately as the circular symmetry of the cladding is broken also the mechanical strength of the fiber is worse and especially the splicing of these non- circular fibers becomes difficult.

In accordance with the invention the fiber is coiled in such a manner that the plane where the fiber is curved / bent continuously changes thus removing the existence of 'horizontal' and 'vertical' higher order anti symmetric core LPl 1 modes, the undesired core modes are coupled into a large group of cladding modes thus removing the resonance effect and simultaneously causing coupling between the cladding modes that result strong diffusion to the helical modes.

As the distance of the waveguide gets longer at the outer edge of the waveguide it is equal to the situation where the refractive index increases at the outer edge of the waveguide and the waveguide is kept straight. Thus there is a way to transform the effect of the bending to the refractive index of the optical waveguide when it is bent by tilting the refractive index of the waveguide. When the bending radius is changed it is equal that the refractive index of the waveguide is rocking in one direction.

In one preferred embodiment of the invention the fiber is coiled in such a manner that the refractive index of the fiber rocks in two directions

This can be achieved e.g., by bending the fiber in such a way that the bending plane and the resulting effective refractive index of the waveguide is rocking in two directions.

More specifically, the apparatus in accordance with invention is characterized by what is stated in the characterizing portion of Claim 1. More specifically, the method in accordance with invention is characterized by what is stated in the characterizing portion of Claim 13.

The following benefits may be obtained by the invention.

The discrimination between fundamental mode and higher order core modes can be improved by eliminating the resonance effects between the core modes and cladding modes. This improves the beam quality (less power at higher order modes), the efficiency can be improved (less gain for the un-wanted modes that are coupled to cladding), the tolerance for the active fiber core diameter and NA are larger.

The helical modes in the cladding are mixed with other modes and thus the absorption of the pump light carried with these modes is improved. This improves the efficiency by increasing the pump power which will be used for amplification

Further, the degeneration of the circular polarization modes as well as both the horizontal and vertical higher order modes can be removed.

This method can be applied also for making a mode filter in a multimode fiber in which the un-wanted higher order modes are removed from the core by bending the multimode fiber as described in the characterizing portion of Claim 12.

In the following, the invention is examined on the basis of an example of an embodiment according to the accompanying drawings.

Figure 1 shows schematically one optical amplifier in accordance with the invention.

Figure 2 shows schematically another optical amplifier in accordance with the invention.

Figure 3 shows as a perspective view one embodiment how to spool an optical fiber around a cylindrical body in accordance with the invention. Figure 4 shows as a perspective view one embodiment how to spool an optical fiber spirally on a curved body in accordance with the invention.

Figure 5 shows as a perspective view one embodiment how to spool an optical fiber spirally on a corrugated body in accordance with the invention.

Figure 6 shows a cross section of a typical optical fiber in accordance with the invention.

Figures 7a-7d show behaviour of the fundamental mode and the first higher order modes (LPl 1 modes) in the fiber when the fiber is bent and the bending plane has rocked 90 degrees from the beginning.

Figures 8a-8h show the behaviour of fundamental mode, the first higher order core modes (LPl 1 modes) and schematically the emerging cladding modes in the fiber when the fiber is bent and the bending plane is rocking so that the bending plane has turned 90 degree from the beginning.

Figure 9a shows as a top view another embodiment of the invention how to spool an optical fiber spirally on a corrugated body in accordance with the invention.

Figure 9b shows side view of the figure 9a.

Figure 10 shows as a perspective view one embodiment of the invention with a toroid body.

Figure 11 shows as a perspective view a tapering, helically spooled fiber in accordance with the invention.

Figure 12a shows as a top view another embodiment of the invention how to spool an optical fiber spirally on a corrugated body in accordance with the invention.

Figure 12b shows side view of the figure 12a. Figure 13a shows as a top view another embodiment of the invention how to spool an optical fiber spirally on a curved body in accordance with the invention.

Figure 13b shows side view of the figure 13 a.

Figure 14a shows as a top view another embodiment of the invention how to spool an optical fiber spirally and in a sine wave format on a curved body in accordance with the invention.

Figure 14b shows side view of the figure 14a.

In accordance with figure 1 the optical amplifier 10 typically comprises a pump laser 1 and signal source 5, which are combined optically by a combiner 2. Combiner 2 is further connected to an optical fiber 7, which is spooled on a cylindrical body 6, forming a fiber spool 3. Finally, the signal is output 4 at the end of the fiber 7. In order to achieve continuous bending of the fiber along its entire length on the cylinder 6, a sine form bending is performed on the cylinder surface of the body 6. By this way the fiber 7 bends all the time along the length of the cylinder 6 surface in three dimensions. By this topology the sine wave form of the fiber takes care of bending in first two dimensions and the cylinder 6 surface takes care of the third dimension. It should be emphasized that the cross- section of the cylinder 6 is not necessarily circular, although it can be. Also the radius of the cylinder can vary, see figure 11. In optimized situation the bending caused by the cylinder 6 should follow the sine form bending of the fibers in a manner that keeps the total fiber bending radius the same everywhere. In accordance with the invention instead of the sine wave form also waveforms close to sine or cosine can be used as well as combinations of continuous curvature and non-continuous curvatures.

Figure 2 shows the amplifier 10 in a laser configuration in which the laser cavity is formed by placing the amplifier 10 in between a mirror 5 operating as a high reflector in place of the signal source 5 of figure 1 and a mirror 4 operating as an output coupler. Figure 3 shows the fiber spool 3 with its cylindrical body 6 in more detail. The fiber 7 is wound around the cylindrical surface of the body such that the continuous fiber 7 forms a sine wave figure on the cylinder surface. The form of the bending does not have to be pure sine waveform, the most important thing is the overall design of the changing bending direction of the fiber. Other examples will describe this in more detail. In the embodiment of figure 3 the waist (circumference) of the cylinder 3 is an integer multiplier of the bending pitch λ of the sine wave. The amplitude of the sine wave is defined so that the minimum radius of the sine wave equals roughly the radius (=d/2) of the cylinder 3. In practice the diameter of the cylinder d is approximately 100 - 180 millimeters. These dimensions are suitable for a fiber with total diameter of 400 μm and core diameter of 20 μm.

Figure 4 shows another embodiment, where the fiber 7 is spooled spirally on a curved body 6. Here the radius of curvature of the body 6 is approximately 50 - 100 mm and the diameter of the fiber bends on the surface are in the range of 100 - 200 mm when the core diameter of the fiber is 20 μm.

In figure 5 the fiber spool 3 is formed with an expanded spiral spooling of the fiber 7 on a corrugated body 6.

Figure 6 shows the principle structure of the fiber 7, comprising of a core 9 and a first cladding 8 surrounding it. Second cladding 10 surrounds further the first cladding 8. The refractive index decreases from the core 9 to the second cladding 10 such that the step of change in the refractive index is greater between first cladding 8 and second cladding 10 than between core 9 and the first cladding 8.

Typically there are two basic solutions for pumping the fiber 7, either the pump light is coupled into the core 9 or alternatively the pump light is coupled into the first cladding layer 8. Usually the latter principle is used because the pump laser costs depend heavily on the pump fiber core diameter. Typical bending radius for a 20 μm core fiber with NA of 0.06 - 0.08 varies froml5 mm up to 120 mm. The pitch length (the distance where plane of the bending of the fiber are orthogonal) can be from 10 mm 400 mm depending on the mechanical design and the gain / length in the fiber.

The basic dimensioning and the properties of the suitable materials are known in the prior art textbooks and for example in the US-patent 6496301 mentioned earlier.

Figures 7a-7d explain in connection with figure 3, how the higher order modes are leaked to the cladding when fiber is bent at position 0 the higher order anti symmetric mode at plane Y in figure 7b leaks to the cladding but not the same order of the mode at plane X. After half wave λ/2 of the spool 3 the plane of the bending of the fiber has changed so that the mode at plane X in figure 7c is now leaking and not the mode at plane Y in figure 7d.

Figures 8a-8h show that as the curvature plane rocks the higher order core modes couple into a set of cladding modes instead of a few of them.

The figures 8a and 8b correspond to figures 7a and 7b at position 0 of figure 3. Figures 8g and 8h respectively correspond to figures 7b and 7b at position λ/2 of the spool 3 of figure 3. Figures 8 c- 8 f represent the intermediate positions between positions 0 and λ/2 of the spool 3 of figure 3.

The rocking of the curvature plane removes the resonance behavior of the core mode leaking into cladding modes. This also enhances the mode coupling of the cladding modes and thus further increases the diffusion of the light between the cladding modes.

In the following more detailed consideration about the key feature of the invention, how to bend the fiber 7 optimally.

By selecting the rocking pitch and the modulation (height) in such a way that the modulation of the refractive index remains adiabatic for the fundamental mode while being non-adiabatic for the higher order modes, it is possible to substantially increase the coupling of the higher order core modes into cladding modes while retaining the fundamental mode un-coupled into cladding modes in contrary to the well known uniform bending or spooling of the fiber described in US-patent 6496301. As the curvature plane rocks the higher order core modes couple into a larger set of cladding modes. This removes the resonance behavior where the optical powers in these cladding modes are coupling back into the unwanted higher order core modes. This also enhances the mode coupling between the cladding modes and thus further increases the diffusion of the light between the cladding modes. This would not happen without the rocking of the curvature plane in two directions and would lead to coupling of higher order core modes to only of a few of cladding modes. The optical power of this set of a few cladding modes would start to couple back into core modes. This resonance effect then would limit the achievable discrimination between the un-wanted higher order core modes and the fundamental mode.

The increased coupling of the higher order modes into cladding modes by bending the fiber can be explained by looking into the propagation constants of the different modes within the fiber core. The propagation constant of the higher order modes are closer to the propagation constants of the cladding modes and thus the criterion for the slow variation of the waveguide for the adiabatic waveguide modulation is tighter than that of the fundamental mode. By bending the fiber this difference is further increased as the propagation modes of the cladding modes increase and come closer to the propagation modes of the core modes. By choosing the modulation of the refractive index profile (that is due to the change of bending) in such a manner that it is adiabatic for the fundamental mode there is very little coupling of the fundamental mode into other modes. Additionally, as the refractive index modulation is non-adiabatic for the higher order core modes the coupling between the higher order / asymmetric / core modes and the cladding modes is enhanced.

These principles of adiabatic transformations are described in detail in following prior art publication:

A.W. Snyder and J.D. Love:" Optical Waveguide Theory", page 408 - 409 paragraph 19-1 Fields of Local Modes and page 409 - 410 paragraph 19-2 Criterion for slow variation. The two dimensional rocking of the index profile increases the coupling between the cladding modes and thus also couples the helical cladding modes into modes having larger cross-section with the core in the center of the fiber. This improves the absorption of the pump light that is coupled in the cladding.

The rocking motion is preferentially continuous in both the planes along the fiber length with a constant pitch and amplitude. Alternatively pitch and amplitude can be varied and/or the rocking motion can be divided in three or more phases, see figures 12 and 13. The amplitude is determined by the applicable radius of the curvature corresponding to the desired output beam quality as defined by the fiber core diameter and core numerical aperture, for example on the same manner as in US6496301. Basic rule of thumb is that to achieve operation close to the fundamental mode the chosen radius of curvature is smaller in order to have higher discrimination between the fundamental and higher order modes. In case the operation of the amplifier is desired to be multimode the radius of curvature is increased.

In figures 9a and 9b is presented a combination of sinusoidal modulation of the fiber 7 on the surface 6 and in the fiber spiral spool. The pitch on the modulation is the same both on the surface 6 and on the fiber 7, but there is a λ/4 phase shift between the modulations.

In figure 10 is presented a helical fiber 7 formed on a toroid body 6. The rocking of the refractive index profile is optimum when the pitch is equal to pi*diameter of the toroid body 6.

In figure 11 is presented a tapering helically spooled fiber 7. The bending radius of the cylinder 6 is constantly decreasing throughout the cylinder length, i.e. di>d₂.

Figures 12a and 12b represent a slight modification of the embodiment of figures 9a and 9b such that the sine wave form is not used in spooling. Spooling is made only spirally. Figures 13a and 13b represent a slight modification of the embodiment of figures 12a and 12b such that the body 6 is not corrugated but comprises a single curved projecting part instead of the corrugations.

Figures 14a and 14b represent a combination of the body 6 of figures 13a and 13 b and the spooling of fiber 7 in an essentially similar way as in figures 9a and 9b.

In the following further examples of the plane having two dimensional features in accordance with the present invention:

- Circular double cladding fiber where the coupling of the pump light to the core is increased by two-dimensional coiling.

- Tapered fiber amplifier/laser where the diameter of the core increases adiabatically (for the fundamental mode) towards the end of the fiber

- Tapered fiber amplifier/laser where the diameter of the core &cladding increases adiabatically (for the fundamental mode) towards the end of the fiber

- Tapered fiber amplifier/laser where the diameter of the cladding increases adiabatically (for the fundamental mode) towards the end of the fiber

Claims

Claims:

1. An optical amplifier (10) using a multimode optical fiber (7), comprising

- a pump laser (1), - an optical fiber (7), forming a waveguide and having at least partially a radius of curvature for causing losses for high order modes, connected to the optical amplifier (1),

characterized in that

- the fiber (7) has at least on a part of its length a continuous change of the bending plane in three dimensions such that the resulting effective refractive index of the waveguide is rocking in two or more directions.

2. An optical amplifier in accordance with claim 1, characterized in that the continuous change of the bending plane is implemented by attaching the fiber (7) on a three- dimensional curved body (6) such that the fiber (7) itself forms continuously changing pattern on the curved surface of the body (6).

3. An optical amplifier in accordance with claim 1, characterized in that the continuous change of the bending plane is divided in three or more parts wherein the bending plane is changed from one section to the other.

4. An optical amplifier in accordance with claim 1 characterized in that the amplified output is substantially in the fundamental mode.

5. An optical amplifier in accordance with claim 1 characterized in that the amplified output is multimoded.

6. An optical amplifier in accordance with claim 1, characterized in that the continuous change of the bending plane of the fiber (7) is formed by spooling the fiber (7) essentially in a periodic sine wave type form on a cylindrical body (6).

7. An optical amplifier in accordance with claim 1, characterized in that the continuous change of the bending plane of the fiber (7) is formed by spooling the fiber (7) in spiral form on a curved body (6).

8. An optical amplifier in accordance with claim 1, characterized in that the continuous change of the bending plane of the fiber (7) is formed by spooling the fiber (7) in spiral form on a corrugated body (6).

9. An optical amplifier in accordance with claim 1, characterized in that with a cylindrical body (3) the circumference of the cylinder (3) is an integer multiplier of the bending pitch λ of the sine wave and the amplitude of the sine wave is defined such that the minimum radius of the sine wave equals roughly half of the diameter (d) of the cylinder (3), for a fiber (7) with total diameter of 400 μm and core diameter of 20 μm.

10. An optical amplifier (10) accordance with claim 1, characterized by using a cladding pumped optical fiber (7) including o a core (9) having a first index of refraction, o a multimode first cladding layer (8) to receive the pump light having a second refractive index that is less than said first index of refraction, and o a second cladding layer (10) including material having a third refractive index of refraction lower than the said second index of refraction.

11. An optical amplifier (10) accordance with claim 1, characterized by using a core pumped optical fiber (7).

12. An optical fiber (7) forming a waveguide and having at least partially a radius of curvature for causing losses for high order modes,

characterized in that the fiber (7) has at least on a part of its length a continuous change of the bending plane in three dimensions such that the resulting effective refractive index of the waveguide is rocking in two or more directions.

13. A method in an optical amplifier (10) using a multimode optical fiber (7), comprising steps for

- forming optical energy by a pump laser (1),

- forming a waveguide by an optical fiber (7), having at least partially a radius of curvature for causing losses for high order modes, connected to the optical amplifier (1),

characterized in

- changing continuously the bending plane of the fiber (7) at least on a part of its length in three dimensions such that the resulting effective refractive index of the waveguide is rocking in two or more directions.

14. A method in an optical amplifier in accordance with claim 13, characterized in that the continuous change of the bending plane is implemented by attaching the fiber (7) on a three-dimensional curved body (6) such that the fiber (7) itself forms continuously changing pattern on the curved surface of the body (6).

15. A method in an optical amplifier in accordance with claim 13, characterized in dividing the continuous change of the bending plane in three or more parts wherein the bending plane is changed from one section to the other.

16. A method in an optical amplifier in accordance with claim 13, characterized in forming the amplified output in substantially in fundamental mode.

17. A method in an optical amplifier in accordance with claim 13, characterized in forming the amplified output in multimode.

18. A method in an optical amplifier in accordance with claim 13, characterized in forming the continuous change of the bending plane of the fiber (7) by spooling the fiber (7) essentially in sine wave form on a cylindrical body (6).

19. A method in an optical amplifier in accordance with claim 13, characterized in forming the continuous change of the bending plane of the fiber (7) by spooling the fiber (7) in spiral form on a curved body (6).

20. A method in an optical amplifier in accordance with claim 13, characterized in forming the continuous change of the bending plane of the fiber (7) by spooling the fiber (7) in spiral form on a corrugated body (6).

21. A method in an optical amplifier in accordance with claim 13, characterized in that with a cylindrical body (3) the circumference of the cylinder (3) is dimensioned as an integer multiplier of the bending pitch λ of the sine wave and the amplitude of the sine wave is defined such that the minimum radius of the sine wave equals roughly half of the diameter (d) of the cylinder (3), for a fiber (7) with total diameter of 400 μm and core diameter of 20 μm.

22. A method in an optical amplifier (10) accordance with claim 13, characterized by using a cladding pumped optical fiber (7) including o a core (9) having a first index of refraction, o a multimode first cladding layer (8) to receive the pump light having a second refractive index that is less than said first index of refraction, and o a second cladding layer (10) including material having a third refractive index of refraction lower than the said second index of refraction.

23. A method in an optical amplifier (10) accordance with claim 13, characterized by using a core pumped optical fiber (7).