Adjustable ring mode fiber laser and laser welding stability evaluation method thereof

Abstract

The present application relates to the technical field of fiber laser, and proposes a tunable ring mode fiber laser and a laser welding stability evaluation method thereof, and whether the tunable ring mode fiber laser can realize stable industrial processing is judged based on the Raman light power density and the signal light power density of the tunable ring mode fiber laser. Based on the laser welding stability evaluation method of the tunable ring mode fiber laser given in the present application, the skilled person in the art can optimize the stimulated Raman scattering effect in the tunable ring mode fiber laser, so that good and stable industrial processing effect can be achieved.

Description

TECHNICAL FIELD

[0001] The present application mainly relates to the technical field of fiber lasers, in particular to a tunable ring mode fiber laser and a laser welding stability evaluation method thereof. BACKGROUND

[0002] Fiber lasers have the characteristics of convenient integration, good conversion efficiency and high beam quality, and are widely used in industrial processing fields such as laser welding and laser cutting. Laser welding, as an advanced technology, has the advantages of fast processing speed, narrow heat-affected zone, high melting power intensity, etc. and is applied to industrial manufacturing fields such as automobile body manufacturing, rail transportation manufacturing, aerospace manufacturing and ship manufacturing.

[0003] However, in the process of laser welding, various materials, especially aluminum alloy and copper materials, will inevitably be contacted. Due to the high reflectivity of these materials, it prevents the material from absorbing laser energy. The emergence of the tunable ring mode fiber laser further provides a method to cope with this difficulty, which changes the absorption rate of the material by changing the temperature of the material, and achieves good welding effect by reasonably matching the ring laser power and the center laser power, and improves the quality of the welding surface.

[0004] However, there are various nonlinear effects in the tunable ring mode fiber laser, including stimulated Raman scattering effect. At present, there is no relevant report published on the influence of stimulated Raman scattering effect on laser welding stability, which limits the further optimization direction of the tunable ring mode fiber laser, which has an adverse effect on future manufacturing industry. SUMMARY

[0005] In view of the technical problems existing in the prior art, the present application provides a tunable ring mode fiber laser and a laser welding stability evaluation method thereof.

[0006] To achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows:

[0007] On the one hand, the present application provides a tunable ring mode fiber laser, which comprises a center beam module, an outer ring beam module, a beam combiner and a fiber end cap, the center beam module and the outer ring beam module are combined by the beam combiner, the output fiber of the beam combiner is connected with the fiber end cap to realize the output of the light beam, the center beam module is used for outputting center signal light and coupling the output center signal light into the core of the beam combiner and outputting through the core of the output fiber of the beam combiner, the outer ring beam module is used for outputting outer ring signal light and coupling the output outer ring signal light into the annular cladding outside the core of the beam combiner and outputting through the annular cladding of the output fiber of the beam combiner. Further, the signal light power density of the tunable ring mode fiber laser satisfies W / µm 2 , the Raman light power density satisfy , so the tunable ring mode fiber laser can realize stable industrial processing.

[0008] Further, the output fiber of the combiner comprises, from inside to outside, a center core, a core cladding, an annular cladding, an outer cladding, and a coating layer.

[0009] In another aspect, a welding stability evaluation method of a tunable ring mode fiber laser is provided, comprising:

[0010] Obtaining the Raman light power, the center signal light power, and the outer ring signal light power in the output laser of the tunable ring mode fiber laser;

[0011] Determining the Raman light power density according to the Raman light power, the center core radius of the output fiber of the combiner, and the numerical aperture of the center core;

[0012] Determining the signal light power density according to the center signal light power, the outer ring signal light power, the outer radius of the annular cladding of the output fiber of the combiner, the inner radius of the annular cladding, and the numerical aperture of the annular cladding;

[0013] Judging whether the signal light power density satisfies W / µm 2 whether the Raman light power density satisfies If yes, the tunable ring mode fiber laser can realize stable industrial processing.

[0014] Further, the Raman light power density is calculated by the following formula:

[0015]

[0016] wherein is the center core radius of the output fiber of the combiner, is the numerical aperture of the center core of the output fiber of the combiner, is the Raman light power.

[0017] Further, the signal light power density is calculated by the following formula:

[0018]

[0019] wherein is the outer radius of the annular cladding of the output fiber of the combiner, is the inner radius of the annular cladding of the output fiber of the combiner, is the numerical aperture of the annular cladding of the output fiber of the combiner, central signal light power, outer ring signal light power.

[0020] In another aspect, the above-mentioned tunable ring mode fiber laser is applied to industrial processing, such as laser welding.

[0021] Compared with the prior art, the technical effects of the present application are:

[0022] The present application provides an exact evaluation method, which judges whether the signal light power density of the tunable ring mode fiber laser satisfies W / µm 2 , the Raman light power density satisfies , and further judges whether the tunable ring mode fiber laser can realize stable industrial processing. Further, under the satisfaction of the above-mentioned relationship, with the increase of the output power of the tunable ring mode fiber laser, the welding penetration will uniformly increase with the increase of the output power. If the above-mentioned relationship is not satisfied, with the increase of the output power of the tunable ring mode fiber laser, the penetration will obviously decrease after increasing to a certain value, which indicates that stable industrial processing cannot be realized.

[0023] Based on the laser welding stability evaluation method of the tunable ring mode fiber laser provided by the present application, the technical personnel in the field can optimize the stimulated Raman scattering effect in the tunable ring mode fiber laser, so that good and stable industrial processing effect can be achieved. BRIEF DESCRIPTION OF DRAWINGS

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings in the following description only show some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor on the basis of the drawings shown.

[0025] Figure 1 It is a structural schematic diagram of the tunable ring mode fiber laser in an embodiment;

[0026] Figure 2 It is a structural schematic diagram of the central beam module in an embodiment;

[0027] Figure 3 It is a structural schematic diagram of the outer ring beam module in an embodiment;

[0028] Figure 4 It is a structural schematic diagram of the output optical fiber of the beam combiner in an embodiment;

[0029] Figure 5 It is a structural schematic diagram of the output optical fiber of the beam combiner in an embodiment; and dissatisfaction The penetration depth curve obtained by using a tunable ring-mode fiber laser for copper welding, in which... Figure 5 (a) to satisfy The penetration depth curve obtained by using a ring-mode fiber laser for copper welding. Figure 5 (b) is not satisfied The penetration depth curve obtained by using an adjustable ring-mode fiber laser for copper welding. Detailed Implementation

[0030] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] Reference Figure 1 , Figure 2 , Figure 3 and Figure 4 One embodiment provides a tunable ring-mode fiber laser, including a central beam module 1, an outer ring beam module 2, a combiner 26, and an fiber end cap 27. The central beam module 1 and the outer ring beam module 2 are combined by the combiner 26. The output fiber of the combiner 26 is connected to the fiber end cap 27 to achieve beam output. The central beam module 1 is used to output a central signal light and couple the output central signal light into the core of the combiner, and then output it through the core of the output fiber of the combiner 26. The outer ring beam module 2 is used to output an outer ring signal light and couple the output outer ring signal light into the annular cladding surrounding the core of the combiner 26, and then output it through the annular cladding of the output fiber of the combiner 26. Further, the signal light power density of the tunable ring-mode fiber laser... satisfy W / µm2, Raman power density satisfy .

[0032] Reference Figure 4 The output optical fiber of the combiner 26 includes, from the inside out, a central fiber core 29, a fiber core cladding 30, an annular cladding 31, an outer cladding 32, and a coating layer 33.

[0033] The central beam module 1 adopts an integrated oscillation-amplification structure, while the outer ring beam module 2 uses a conventional oscillation structure. Both modules utilize large-mode-gain fiber to reduce stimulated Raman scattering. Based on the laser welding stability evaluation method for tunable ring-mode fiber lasers provided in the aforementioned embodiment, the signal power density of the tunable ring-mode fiber laser is determined. satisfy W / µm 2 , Raman light power density satisfy , with lower output Raman, can be stable industrial processing, such as for laser welding.

[0034] Specifically, the center beam module 1 is composed of a first cladding power stripper 3, a first signal light reflection fiber grating 4, a first double-clad ytterbium-doped fiber 5, a first output coupling fiber grating 6, a second double-clad ytterbium-doped fiber 7, a first (N+1) × 1 reverse combiner 8, N semiconductor pump sources, and a second cladding power stripper 15, N=6.

[0035] The first cladding power stripper 3 is connected to the first signal light reflection fiber grating 4, the first signal light reflection fiber grating 4 is fused to the first double-clad ytterbium-doped fiber 5 between the first signal light reflection fiber grating 4 and the first output coupling fiber grating 6, the other end of the first output coupling fiber grating 6 is fused to the second double-clad ytterbium-doped fiber 7, the second double-clad ytterbium-doped fiber 7 is connected to the signal input fiber of the first (N+1) × 1 reverse combiner 8, the N pump input fibers of the first (N+1) × 1 reverse combiner 8 are respectively connected to one semiconductor pump source, and the six semiconductor pump sources are respectively the first semiconductor pump source 9, the second semiconductor pump source 10, the third semiconductor pump source 11, the fourth semiconductor pump source 12, the fifth semiconductor pump source 13, and the sixth semiconductor pump source 14. The signal output fiber of the first (N+1) × 1 reverse combiner 8 is connected to one end of the second cladding power stripper 15, and the other end of the second cladding power stripper 15 is connected to the first combiner arm of the combiner 26 input end. The center beam module 1 can reach an ultimate output power of more than 4kW.

[0036] The first signal light reflection fiber grating 4 has a reflectivity of 99.5% for the signal light in the center beam module 1, and the first output coupling fiber grating 6 has a reflectivity of 6% for the signal light in the center beam module 1.

[0037] The outer ring light beam module 2 is composed of a third cladding power stripper 16, a second signal light reflection fiber grating 17, a third double-clad ytterbium-doped fiber 18, a second output coupling fiber grating 19, a second (N+1) x 1 reverse combiner 20, a semiconductor pump source, and a fourth cladding power stripper 25. The third cladding power stripper 16 is connected to the second signal light reflection fiber grating 17. The third double-clad ytterbium-doped fiber 18 is fused between the second signal light reflection fiber grating 17 and the second output coupling fiber grating 19. The other end of the second output coupling fiber grating 19 is connected to the signal input fiber of the second (N+1) x 1 reverse combiner 20. The n pump input fibers of the second (N+1) x 1 reverse combiner 20 are respectively connected to one semiconductor pump source, and n is equal to 4. The four semiconductor pump sources are the seventh semiconductor pump source 21, the eighth semiconductor pump source 22, the ninth semiconductor pump source 23, and the tenth semiconductor pump source 24. The signal output fiber of the second (N+1) x 1 reverse combiner 20 is connected to one end of the fourth cladding power stripper 25. The other end of the fourth cladding power stripper 25 is connected to the second combiner arm of the combiner 26 input end. The reflectivity of the second signal light reflection fiber grating 17 to the signal light in the outer ring light beam module 2 is 99.5%. The reflectivity of the second output coupling fiber grating 19 to the signal light in the outer ring light beam module 2 is 6%. The outer ring light beam module 2 can reach an ultimate output power of more than 2kW.

[0038] The central light beam module 1 and the outer ring light beam module 2 are combined by the combiner 26. The output fiber of the combiner 26 is connected to the fiber end cap 27 to realize light beam output. The output fiber of the combiner 26 comprises, from inside to outside, a central core 29, a core cladding 30, an annular cladding 31, an outer cladding 32, and a coating layer 33. The ultimate output power of the tunable ring mode fiber laser can reach about 6kW. The laser output power of the tunable ring mode fiber laser , wherein is the Raman light power, is the central signal light power, is the outer ring signal light power.

[0039] The output fiber structure of the combiner 26 is shown in Figure 4 , which comprises, from inside to outside, a central core 29, a core cladding 30, an annular cladding 31, an outer cladding 32, and a coating layer 33. The diameter of the central core 29 is 25µm, the numerical aperture is 0.01≤NA1≤0.12, the diameter of the core cladding 30 is 50µm, the diameter of the annular cladding 31 is 75µm, the numerical aperture is 0.1≤NA2≤0.3, the diameter of the outer cladding 32 is 280µm, and the diameter of the coating layer 33 is 480µm.

[0040] The output wavelength of the first semiconductor pump source 9, the second semiconductor pump source 10, the third semiconductor pump source 11, the fourth semiconductor pump source 12, the fifth semiconductor pump source 13 and the sixth semiconductor pump source 14 is 915nm; the output wavelength of the seventh semiconductor pump source 21, the eighth semiconductor pump source 22, the ninth semiconductor pump source 23 and the tenth semiconductor pump source 24 is 976nm.

[0041] The first double-clad doped-ytterbium fiber 5 in the center beam module 1 and the second double-clad doped-ytterbium fiber 7 are both large-mode-area doped-ytterbium fibers, and the third double-clad doped-ytterbium fiber 18 in the outer ring beam module 2 is also a large-mode-area doped-ytterbium fiber.

[0042] The first signal light reflection fiber grating 4 and the second signal light reflection fiber grating 17 have high reflection to signal light 1080nm, and the reflectivity is about 99.5%.

[0043] The first output coupling fiber grating 6 and the second output coupling fiber grating 19 have partial reflection to signal light 1080nm, and the reflectivity is about 6%.

[0044] The first (N+1)×1 reverse combiner 8 has very small insertion loss, and the pump efficiency to 915nm is about 98%.

[0045] The working wavelength of the first cladding power stripper 3 and the third cladding power stripper 16 is 800-1200nm, the stripped power is 100W, and the stripping ratio is 20dB.

[0046] The working wavelength of the second cladding power stripper 15 and the fourth cladding power stripper 25 is 800-1200nm, the maximum bearing power is 600W, and the stripping efficiency is greater than 97%.

[0047] Based on the above embodiment, the adjustable ring mode fiber laser is provided, the structure is as shown in Figure 1 、 2 , 3 and 4, the device parameter design is the same as above, the maximum output power of the adjustable ring mode fiber laser reaches 5.9kW, the output Raman light ratio reaches 0.004%, the Raman light power reaches 0.236W, the outer ring signal light power reaches 2.1kW, and the center signal light power reaches 3.799kW. Since the center core 29 diameter of the signal output fiber of the combiner 26 is 25µm, the numerical aperture NA1 is 0.06, the Raman light power density =0.1335W / µm 2 . The outer ring inner diameter of the combiner 26 is 50µm, the outer diameter is 75µm, the numerical aperture NA2 is 0.22, and the signal light power density =2319W / µm 2 Raman power density of tunable ring-mode fiber lasers Signal optical power density satisfy W / µm 2 Raman power density satisfy The tunable ring-mode fiber laser that meets the above conditions is applied to a practical industrial processing scenario, namely copper welding, and the weld penetration depth is measured. The results are as follows: Figure 5 As shown in (a), from Figure 5 (a) It can be seen that the welding penetration depth increases uniformly with the increase of the output power of the adjustable ring mode fiber laser.

[0048] As Figure 5 (a) A comparative example of a corresponding tunable ring-mode fiber laser, wherein the tunable ring-mode fiber laser provided in the comparative example employs, as shown in the example... Figure 3 The conventional oscillation structure shown serves as the center beam module and the outer ring beam module; that is, both the center beam module and the outer ring beam module employ... Figure 3 The conventional oscillation structure shown is identical to the one described above; all other settings are the same as those described above. Figure 5 (a) The corresponding tunable ring-mode fiber lasers have the same setup, and the structure and parameters of the combiner are also the same. The tunable ring-mode fiber laser in the comparative example has a maximum output power P of 6.1 kW, an output Raman light ratio of 0.03%, and an output Raman light power of The outer ring signal optical power reached 5.49W. Up to 2.1kW, center signal optical power Reaching 3.994kW, Raman optical power density =3.11W / µm2, signal optical power density =2277W / µm2, signal power density of tunable ring-mode fiber laser satisfy W / µm2, Raman power density Not satisfied , will not be satisfied A tunable ring-mode fiber laser was applied to copper welding, and the weld penetration depth was measured. The results are as follows: Figure 5 As shown in (b), from Figure 5 (b) It can be seen that as the power of the tunable ring mode fiber laser increases, the welding penetration depth will decrease significantly after reaching a certain value, which indicates that stable industrial processing cannot be achieved.

[0049] Another embodiment provides a method for evaluating the stability of laser welding using a tunable ring-mode fiber laser, comprising:

[0050] Obtain the Raman light power, the center signal light power and the outer ring signal light power in the output laser of the tunable ring mode fiber laser.

[0051] Determine the Raman light power density according to the Raman light power, the center core radius of the output fiber of the combiner and the numerical aperture of the center core

[0052]

[0053] wherein the center core radius of the output fiber of the combiner, the numerical aperture of the center core of the output fiber of the combiner, the Raman light power.

[0054] Determine the signal light power density according to the center signal light power, the outer ring signal light power, the outer radius of the annular cladding of the output fiber of the combiner, the inner radius of the annular cladding of the output fiber of the combiner and the numerical aperture of the annular cladding of the output fiber of the combiner

[0055]

[0056] wherein the outer radius of the annular cladding of the output fiber of the combiner, the inner radius of the annular cladding of the output fiber of the combiner, the numerical aperture of the annular cladding of the output fiber of the combiner, the center signal light power, the outer ring signal light power.

[0057] Judge whether the signal light power density satisfies W / µm 2 , the Raman light power density satisfies If yes, the tunable ring mode fiber laser can realize stable industrial processing.

[0058] The technical features of the above embodiments can be combined in any manner. In order to make the description concise, all possible combinations of the technical features in the above embodiments are not described, however, as long as the combinations of the technical features do not exist contradictions, they should be considered as the scope of the present disclosure.

[0059] ​​The above embodiments only express several implementation ways of the present application, and the description is more specific and detailed, but it should not be understood as a limitation to the scope of the application. It should be pointed out that for ordinary skilled in the art, several modifications and improvements can be made without departing from the concept of the present application, which are all within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the appended claims.

[0060] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A tunable ring-mode fiber laser, characterized in that, The system includes a central beam module (1), an outer ring beam module (2), a combiner (26), and an optical fiber end cap (27). The central beam module (1) and the outer ring beam module (2) are combined by the combiner (26). The output optical fiber of the combiner (26) is connected to the optical fiber end cap (27) to achieve beam output. The central beam module (1) is used to output the central signal light and couple the output central signal light into the core of the combiner and output it through the core of the output optical fiber of the combiner (26). The outer ring beam module (2) is used to output the outer ring signal light and couple the output outer ring signal light into the annular cladding around the core of the combiner (26) and output it through the annular cladding of the output optical fiber of the combiner (26). The signal light power density of the tunable ring mode fiber laser is... satisfy Raman power density satisfies At that time, the tunable ring-mode fiber laser can achieve stable laser welding. The Raman optical power density is determined based on the Raman optical power, the radius of the central core of the output fiber of the combiner, and the numerical aperture of the central core. The signal optical power density is determined based on the central signal optical power, the outer ring signal optical power, the outer radius of the ring cladding of the output fiber of the combiner, the inner radius of the ring cladding, and the numerical aperture of the ring cladding.

2. The tunable ring-mode fiber laser according to claim 1, characterized in that, The output fiber of the combiner (26) includes, from the inside out, a central fiber core (29), a fiber core cladding (30), an annular cladding (31), an outer cladding (32), and a coating layer (33).

3. The tunable ring-mode fiber laser according to claim 1 or 2, characterized in that, The central beam module (1) consists of a first cladding power stripper (3), a first signal light reflection fiber grating (4), a first double-clad ytterbium-doped fiber (5), a first output coupling fiber grating (6), a second double-clad ytterbium-doped fiber (7), a first (N+1)×1 reverse combiner (8), N semiconductor pump sources, and a second cladding power stripper (15). The first cladding power stripper (3) is connected to the first signal light reflection fiber grating (4). The first signal light reflection fiber grating (4) and the first output coupling fiber grating (6) are fused together with the first double-clad ytterbium-doped fiber (5). The other end of the first output coupling fiber grating (6) is fused together with the second double-clad ytterbium-doped fiber (7). The second double-clad ytterbium-doped fiber (7) is connected to the signal input fiber of the first (N+1)×1 reverse combiner (8). The N pump input fibers of the first (N+1)×1 reverse combiner (8) are respectively connected to a semiconductor pump source. The signal output fiber of the first (N+1)×1 reverse combiner (8) is connected to one end of the second cladding power stripper (15). The other end of the second cladding power stripper (15) is connected to the first combiner arm at the input end of the combiner (26).

4. The tunable ring-mode fiber laser according to claim 3, characterized in that, The first signal light reflecting fiber grating (4) has a reflectivity of 99.5% for the signal light in the central beam module (1), and the first output coupling fiber grating (6) has a reflectivity of 6% for the signal light in the central beam module (1).

5. The tunable ring-mode fiber laser according to claim 4, characterized in that, The outer ring beam module (2) consists of a third cladding power stripper (16), a second signal light reflection fiber grating (17), a third double-clad ytterbium-doped fiber (18), a second output coupling fiber grating (19), a second (N+1)×1 reverse beam combiner (20), a semiconductor pump source, and a fourth cladding power stripper (25). The third cladding power stripper (16) is connected to the second signal light reflection fiber grating (17). The second signal light reflection fiber grating (17) and the second output coupling fiber grating (19) are fused together with a third double-clad ytterbium-doped fiber (18). The other end of the second output coupling fiber grating (19) is connected to the signal input fiber of the second (N+1)×1 reverse combiner (20). The n pump input fibers of the second (N+1)×1 reverse combiner (20) are respectively connected to a semiconductor pump source, where n is less than or equal to N. The signal output fiber of the second (N+1)×1 reverse combiner (20) is connected to one end of the fourth cladding power stripper (25). The other end of the fourth cladding power stripper (25) is connected to the second combiner arm at the input end of the combiner (26).

6. The tunable ring-mode fiber laser according to claim 5, characterized in that, The second signal light reflecting fiber grating (17) has a reflectivity of 99.5% for the signal light in the outer ring beam module (2), and the second output coupling fiber grating (19) has a reflectivity of 6% for the signal light in the outer ring beam module (2).

7. The tunable ring-mode fiber laser according to claim 1, 2, 4, 5, or 6, characterized in that, Raman optical power density is calculated using the following formula. : in The output fiber of the combiner has a central core radius. The numerical aperture of the central core of the output fiber of the combiner. This represents the Raman optical power.

8. The tunable ring-mode fiber laser according to claim 7, characterized in that, The signal optical power density is calculated using the following formula. : in The outer radius of the annular cladding of the output fiber of the combiner. The inner radius of the annular cladding of the output fiber of the combiner. The numerical aperture of the annular cladding of the output fiber of the combiner. For the center signal optical power, This represents the optical power of the outer loop signal.

9. The method for evaluating the laser welding stability of an tunable ring-mode fiber laser as described in claim 1, 2, 4, 5, or 6, characterized in that, include: To obtain the Raman optical power, center signal optical power, and outer ring signal optical power in the output laser of a tunable ring-mode fiber laser; The Raman power density is determined based on the Raman optical power, the radius of the core of the output fiber of the combiner, and the numerical aperture of the core. The signal power density is determined based on the central signal optical power, the outer ring signal optical power, the outer radius of the ring cladding of the output fiber of the combiner, the inner radius of the ring cladding, and the numerical aperture of the ring cladding. Determine signal optical power density Does it meet the requirements? Raman power density Does it meet the requirements? If these conditions are met, the tunable ring-mode fiber laser can achieve stable laser welding.

10. The method according to claim 9, characterized in that, Raman optical power density is calculated using the following formula. : in The output fiber of the combiner has a central core radius. The numerical aperture of the central core of the output fiber of the combiner. This represents the Raman optical power.

11. The method according to claim 10, characterized in that, The signal optical power density is calculated using the following formula. : in The outer radius of the annular cladding of the output fiber of the combiner. The inner radius of the annular cladding of the output fiber of the combiner. The numerical aperture of the annular cladding of the output fiber of the combiner. For the center signal optical power, This represents the optical power of the outer loop signal.

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