774

Laser Physics, Vol. 8, No. 3, 1998, pp. 774–781.

Original Text Copyright © 1998 by Astro, Ltd.Copyright © 1998 by

åÄàä ç‡Û͇

/Interperiodica Publishing (Russia).

INTRODUCTION

The research of high-power fiber lasers for indus-trial applications to be described here is the by-prod-ucts of pure-scientific research for gravitational wavedetection. For the last eight years, we developed theultrahigh frequency stabilized lasers for gravitationalwave detection. During these developments, we re-understood that the frequency and phase control are rel-atively easy because the laser cavity has a clear bound-ary in the longitudinal direction. On the other hand, thespatial mode control in high-power lasers has an intrin-sic difficulty, due to the lack of boundary condition intransverse mode control. Optical fibers and waveguidesare exceptional devices that can control the propagatingand generating mode of laser beams. In various kinds ofindustrial applications, they use the high power densityof laser beams without any spectral control. In suchcases the fiber lasers have a great potential if highpower density does not induce serious losses by nonlin-ear scattering and absorption. This means high-powerfiber laser should be a cw fiber laser.

WHAT IS A HIGH-POWER FIBER LASER?

First of all, we define the future image of high-power fiber lasers. In most of high-power laser applica-tions like welding, cutting, and drilling, the essentialperformance of lasers is the high power density andeasy manipulations. The importance of fiber deliveringsystems is widely understood from the view of combi-nation to the robot techniques. The flexibility and smallinertia are key points for the high speed processing.Many people want to develop the fiber delivering systemby using high-power all-solid-state lasers. However, itshould be mentioned that, if we replace the pumpingsource flash lamps to LDs, the beam quality problem isnot solved principally. The beam quality of high-powersolid-state lasers is not good enough, even in the LD-pumping systems. Another point is that the simple fiber

cannot create the high-quality beams without seriouspower loss, because the beam brightness is conservative.It means that an optical fiber is a good mode selector orspatial filter. However, the fiber lasers with an active core[1, 2] compress the photon flux from clad pumping dur-ing the fiber propagation with large magnitude of ampli-fication as shown in Fig. 1. The maximum output densityshould be almost the same level of simple fibers, becauseit is determined by the damage threshold or nonlinearscattering losses in fiber propagation.

From the application view, the future image of laserprocessing factory is the fiber delivering systems likeelectric power delivering systems as shown in Fig. 2.Fiber delivering system allows the separation betweenthe processing unit to power generating unit. So, thefuture laser power generator should he composed of thelarge number of laser units which can deliver the fiber-coupled outputs. Another important device in this pic-ture is the high-power switching devices and connect-ing devices in the fiber power lines.

Future of High-Power Fiber Lasers

K. Ueda and A. Liu

Institute of Laser Science, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, 182 Japan

e-mail: ueda@ils.uec.ac.jpReceived January 13, 1998

Abstract

—Possible designs of high-power fiber lasers are discussed in this paper related to the future powerdelivering system in laser material processing factories. High efficiency operation of 73% was demonstrated byclad pumping scheme using homogeneous absorption regime of Nd

3+

-doped rectangular double clad fibers. Thepossibility of solar pumping without any concentrator was evaluated using the power scaling law of pumpingto lasing. Fiber-embedded lasing disk and tube are proposed for the high-power laser that produces more than1 kW from a single core of 50

µ

m in diameter.

Fiber lasers can compress theenergy density in space

Nd or Yb doped single mode fiber

LD array

Ld-pumpingHigh qualitylaser output

M = A1/A2 = (D1/D2)

2

A1

A2

Fig. 1.

Fiber lasers convert the mode and compress the spa-tial power density.

FIBRE AND WAVEGUIDELASERS

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FUTURE OF HIGH-POWER FIBER LASERS 775

SCALING PHYSICS OF SOLID-STATE LASERS FOR GRAVITATIONAL WAVE DETECTION

AND FIBER LASERS

We developed frequency-stabilized solid-statelasers [3, 5] for gravitational wave detection in theseseven years. The final goal of sensitivity of gravita-tional wave antenna needs the quantum limited noise ofkW-class single-mode lasers. Such a requirement can-not be achieved by the simple extension of traditionaldesign of solid-state lasers. At first, we discovered thescaling law of cooling efficiency, because the maxi-mum output power is limited by the cooling power.Assuming constant volume, the cooling efficiency isdescribed by the volume to surface ratio. The coolingscaling shown in Fig. 3 gave us two kinds of extremecase: one is the thin disk we called an active mirror, andthe other is the thin rod. The ideal thin rod in the solid-state lasers is the fiber lasers. It means that the fiberlasers have a great potential for high-power lasers.

Another scaling law is the pumping scaling. Theend-pumping regime is wide spread for the LD pump-ing, because of the high beam quality of output. Thereason of the high-quality beams is that the pumpingbeam excites the smaller volume of active medium thanthe fundamental mode volume of the resonator. This isa great advantage of end-pumping regime, but the totalvolume of pumping region is limited by the product ofthe pumping area and the penetration depth. So, the endpumping is good for small power lasers, but we needthe larger pumping area of side-pumping regime.According to these concepts, we developed the newgeometry named a VPS (virtual point source) pumpingsystem [7–11] for the high-power laser oscillator ofoutput level of 105 W. This is similar to the large scalefiber laser. In the next section, we examine the basicadvantage of fiber lasers using a simple and fundamen-tal physics.

BASIC EQUATION OF LASER OSCILLATORS

The basic equation of laser oscillator is described as

(1)

where

I

±

is the intensity in the cavity in propagating andcounterpropagating modes,

I

s

is the saturation intensity,

g

0

is the small-signal gain coefficient,

α

is the loss coef-ficient. From (1), the following relation between coun-terpropagating beams is derived easily:

(2)

This means that the product of intensities of counter-propagating beams is always constant. So, we can

dI±

dz--------±

g0

1 I+ I–+( )/Is+----------------------------------- α–⎝ ⎠

⎛ ⎞ I±,=

dI+I–

dz------------ I+

dI–

dz------- I–

dI+

dz--------+ 0.= =

2D LD array

Laser processing factory

Solenoidal fiber laser tube

Clean laboratory

Fiber power linesfor laser processing factory

Fig. 2.

Future image of a laser processing factory with fiber delivering lines from central laser power plants.

High aspectedge cooling

Constant volumelaser rod End cooled

S2

L

V

Low aspect thin film

S1

b

Fig. 3.

Scaling on cooling efficiency of solid-state lasers.

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define the intensity parameter of laser resonators as fol-lows:

(3)

where

I

s

is the saturation intensity of active medium.We do not describe the details of these equations, butgive the final result of analytical solutions here. Theintensity distribution of laser beams in the resonator ispresented as

(3)

where

γ

≡

g

0

/

α

is the gain-to-loss ratio,

R

a

and

R

b

is theeffective reflectivity, including the surface loss of thelaser rod. As a result, the output power is described as

(4)

Thus, the output power

I

out

is determined by

I

s

,

δ

, and

R

out

, where

δ

is a complicated function of the otherparameters as shown in (3). But, the most importantthing we mention here, is that the performance of thesteady-state condition of laser oscillation has an analyt-ical solution. This analytical solution describes the las-ing performance basically.

The maximum intensity of the laser beam is deter-mined by the zero gain condition

g

–

α

= 0 as follows:

(5)

So, the gain-to-loss ratio is most important for high-power lasers. A fiber laser has the lowest loss factoramong the solid-state lasers. Equation (5) shows that

I+I– δIs2,=

αz RaRblnγ

γ 1–( )2 4δ–-----------------------------------–=

×δ/Ra p+–

δRb p+–---------------------------

δRb p––

δ/Ra p––--------------------------

⎝ ⎠⎜ ⎟⎛ ⎞

,ln

p±12--- γ 1– γ 1–( )2 4δ–±( ),=

Iout Isδ

Rout--------- 1 Rout–( ).=

Imax Is

g0

α----- 1–⎝ ⎠

⎛ ⎞ Is γ 1–( ).= =

the 10

×

larger gain is equivalent to the 1/10 smallerloss. Larger gain means the higher pumping density.Thus, smaller loss coefficient is the best way to gener-ate the high power output by low pumping power den-sity.

Again, we check the detail of the loss coefficient.The loss coefficient contains the various kinds oflosses, absorption, and scattering within the cavity. In atypical solid-state laser, the scattering loss on the sur-face of the rod is large, and the laser cavity loses theinternal power every round trip from the output coupler.Such internal loss mechanisms make it difficult toachieve the ideal condition of laser oscillation. On theother hand, the fiber laser is long enough to neglect theother cavity losses except the coupling loss because ofits extremely low loss coefficient. Furthermore, themode control capability of fiber propagation is good,because the fiber has a clear boundary between the coreand clad that can determine the cut-off frequency andthe spatial mode.

SPATIAL HOLE BURNING IN THE ABSORPTION OF CLAD PUMPING

The laser oscillation of fiber lasers is easy if theyabsorb the pumping power efficiently, because thegain–length product

g

0

L

is quite large, due to the longactive length. The most critical problem in the fiberlasers is the absorption efficiency [12, 13] in the cladpumping geometry. In a typical coaxial fiber, there aretwo kinds of optical rays, a meridional mode, and askew mode. The pumping beams in the meridionalmode pass the center core in every trip, but the beamsin the skew mode cannot reach the core, because theyhave constant distances from the center as shown inFig. 4. The injected pumping power from the end isabsorbed first in the meridional mode by the effectiveabsorption coefficient of

(6)

where

α is the absorption coefficient of laser material,r0 and R is the radius of core and clad, respectively. Butthe meridional mode is soon exhausted.

Assuming the uniform input into the clad, the prob-ability of meridional mode is described by

(7)

This means the available part of pumping power is

almost less than 1.3 . Such spatial hole burning is

attributed to the energy decoupling between the spatialmodes. So, we can call it as a homogeneous probabilityin the spatial broadening. Most of the pumping poweris not available, unless the mode conversion is not

αeff αr0

R----,=

P2π---

r0

R---- 1

r0

R----⎝ ⎠

⎛ ⎞2

–r0

R----sin

1–+ .=

rR---

Modeconversion

Meridional mode Skew mode

Inhomogeneousbroadening

Homogeneousbroadening

R R

Fig. 4. Spatial hole burning effect of clad pumping is the bigproblem for double-clad fibers with circular first clad.

LASER PHYSICS Vol. 8 No. 3 1998

FUTURE OF HIGH-POWER FIBER LASERS 777

effective. The mode conversion effects using the bentfiber are investigated intensively in Germany.

What is the ideal absorption in fiber lasers? If theabsorption in space is homogeneous, the injected pumppower is deposited in the fiber core completely withoutsaturation. Is it possible to develop such a homoge-neous fiber laser? We investigated the rectangular cladfor pumping.

As shown in Fig. 5, the injected beam at the arbi-trary point A(x0, y0) of the clad area is shifted to thepoint B(x0, y1) after the round trip. The spatial shift perevery round trip is expressed by

(8)

where a and b are the parameters of rectangular area,and θ is the incident angle in this plane. The spatial shift∆y is constant and independent to the coordinates of theinput point. Thus, all the incident beams are equivalent,so that the absorption coefficient for every pumpingbeam is equal. This is the evidence of the 100% homo-geneous absorption in a rectangular clad fiber pumping.In the case of homogeneous absorption in rectangular-clad fiber, the effective absorption coefficient is givenby the following equation:

(9)

where N is the number of cores, Acore and Aclad are thecross sections of fiber core and clad, respectively.

According to (9), the output of a single-fiber laserdoes not depend on the core diameter in the optimizedclad pumping geometry. When we use a large corediameter, the effective absorption efficiency shown in(9) should be large. Large absorption coefficient intro-duces the short penetration depth so that the effectivefiber length becomes shorter. As a result, the outputpower of fiber lasers is determined by the active vol-ume, not by core diameter. It means that the incoherentaddition of multiple-fiber lasers is most promising toachieve kW-class output.

OUTPUT PERFORMANCE OF RECTANGULAR CLAD FIBER LASERS

The experimental demonstration of LD-pumpedfiber laser was carried out by Nd3+-doped rectangular-double-clad fiber (RDCF) lasers made of fused silica[14, 15]. The core of 16 µm diameter was Al- and Nd-doped with a dopant of 0.4 wt % Al and 0.5 wt % Nd.The dimension of the silica first clad was 235 × 590 µm,and the NA for the core propagation is 0.2. For the con-finement of pumping beams in the first clad, the lowindex (1.378) and low optical loss UV-curable polymercladding was fabricated as the outer clad. The NA forthe pumping beams was 0.48. The calculated effectiveabsorption factor (αAcore /Aclad = 120 dB/m × 1.44 ×

∆y y1 y0– 2 a θ b–tan( ),= =

αeff NαAcore

Aclad-----------,=

10–3 = 172 dB/km) agreed well with the measured valueof 169 dB/km. The RDCF laser was 52 m long. For thelasing experiments, a fiber-coupled LD with a maxi-mum output power of 10 W was used. The output diam-eter and NA were 0.4 mm and 0.4, respectively. Result-ant absorption in clad pumping scheme was measuredto be 90% in a single-pass excitation.

The output characteristics of RDCF lasers areshown in Fig. 6. RDCF lasers generate the outputpower with low threshold and high efficiency. When weset the dichroic mirror with 95% reflection at 1.06 µmand 97% transmission at 805 nm, the output power islinearly dependent to the launched power with a slopeefficiency of 58.6%, and a threshold of about 200 mW.The maximum output in this one-end pumping wasmeasured to be 1.86 W. In other experiment, we dem-onstrated the maximum output of 2.85 W using thetwo-end pumping scheme. The RDCF lasers generateequal power output from both ends, even if we removethe dichroic mirror at the front end. Due to the large

a

b

A(x0, y0)

B(x0, y1)

θ

Fig. 5. Pumping beams shift with the constant rate everyround trip over the clad area.

2.0

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

1.5

1.0

0.5

Length 50 mCore Ø 0.015 mmClad 0.24*0.6 mmNd doped 0.5 wt%

Po = 1.86 WPp = 3.48 W

Fiber-coupled LD0.4 mm

NA = 0.5

With HR1.06 µm

73%58.6%

PumpWithout HR

36.6%

Pump

Pth = 0.2 W

Launched power, W

Output power, W

Fig. 6. Output performance of RDCF lasers in one- and two-end output.

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LASER PHYSICS Vol. 8 No. 3 1998

UEDA, LIU

gain–length product, the small Fresnel reflection atboth ends of fiber makes an effective feedback for thelaser oscillation. The measured slope efficiency forone-end output was 36.6% It means the efficiency oftwo-end output is larger than 73%. This is close to thequantum efficiency of Nd3+ ions of 78%. We analyzedthese performance using the basic equation.

On account of the pumping profile of clad pumpingthat has a nearly exponential distribution from thepumping end, the intensity profiles of counterpropagat-ing beams were calculated for one-end output and two-end outputs in Fig. 7. In spite of the gain distribution,the output powers from two ends are exactly equal, andthe sum of the two-end outputs is 10% larger than theone-end output with 100% reflection at the other end.This is because the effective gain-to-loss ratiodecreased in a double pass, due to the gain saturation.Nonsaturable loss like absorption and scattering deter-mines the laser efficiency.

FIBER LASERS IN SPACE

There are many programs of satellite networksaround the world. The first generation of satellite net-work will be constructed by the microwave network.However, optical networks in space is the most promis-ing candidate for high-capacity network in the nextcentury, as shown in Fig. 8. In Japan, we are planningto demonstrate the high-speed optical communicationsbetween satellites. Fiber lasers have several advantageslike single transverse mode operation, spatial resolu-tion, light weight, and the laser resonator insensitive tothe external fluctuation. In this paper, we propose anidea of solar-pumped fiber laser [16–18] for our future.We define the solar pumping as the solar power pump-ing in the natural flux density, because it should be

equal to the flash lamp pumping if we use the large fluxconcentrator like mirrors. Natural flux density is as lowas 1 kW/m2, and the available region of spectrum istypically less than 10%. So, the solar-pumped lasermeans the laser by very low pumping flux density.

Low-flux pumping means the low gain. So, thesolar-pumped laser should be composed of the low-lossmaterial. Fiber lasers are best in this point. Power fluxscaling was studied. We compared the power compres-sion ratio of several kinds of pumping geometry asshown in Fig. 9. Typical LD-pumped YAG laser has anend-pump geometry. In this case, the pumping area issmaller than the fundamental mode volume for thehigh-quality laser output. So, the power scaling definedby the area ratio of pumping input and laser output isless than unity. The clad pumping scheme has a largepower scaling of about 1000. If we realize the sidepumping of fiber lasers with kilometer size, the power

scaling ratio M is as large as M = ≈ , where L

and d are the length and diameter of fiber core, respec-tively. Assuming the very long fiber with L = 5 km andsingle-mode fiber d = 5 km, the magnification factor M

4dL

πd2---------- L

d---

7

100 20 30 40 50

1

2

3

4

5

6

Loss coeff. 20 dB/km

5.5 W

R1 = R2 = 0.04

R1 = 1, R2 = 0.04

Lossy DCF

Position along fiber, m

Power, W

Fig. 7. Sum of the two-end output is always larger than thesingle-end output.

Compactsingle mode lasers

Ultra-precisionoptical control

Ultra-precisionadaptive optics

Earth

Optical super-highway by laser communication in spacelossless, turbulence free, dispersion free,

super high speed signal & energy transport

Fig. 8. Future image of optical superhighway by laser com-munication in space.

End pumpingM < 1

Laser output

Solar power

Side pumpingM = 1000000000

Clad pumpingM = 1000

Laser output

Fig. 9. Large scaling ratio of pumping to lasing power den-sity is the basic physics for low flux density pumping likesolar power lasers.

LASER PHYSICS Vol. 8 No. 3 1998

FUTURE OF HIGH-POWER FIBER LASERS 779

is as large as 109. Such a large magnification factor inthe power scaling is essential for the low pump ratelasers, because the output power density is determinedby the saturation intensity.

Side pumping of fiber lasers has never been pro-posed in the realistic sense, because the absorption effi-ciency is too low. If the absorption efficiency is lessthan 10–4, such a low efficiency laser is no use in a com-mercial application. However, the solar power is free incharge fortunately, and we do not worry about theabsorption efficiency as shown in Fig. 10. The gaincoefficient does not depend on the pumping efficiency,but pumping density per unit volume. So, if solar fluxcan deposit the power at the pumping density thatinduces the small-signal gain larger than the loss coef-ficient, the side-pumped fiber laser can oscillate. Thethreshold pumping of 200 mW for our 52-m-longRDCF laser corresponds to the pumping density ofabout 1 mW/m as a zero-net-gain condition for 100–4%reflection cavity. We evaluated the realistic conditionon effective power flux and spectral efficiency, the gainand loss coefficient, and the coupling conditions of cav-ity by using the computer simulation. At this moment,the solar-pumped fiber laser doped by Nd3+ ion cannotreach the threshold condition immediately. The combi-nation of absorption and emission cross sections shouldbe one order of magnitude larger to reach the thresholdpumping density. To overcome the one order of magni-tude larger efficiency, the basic research on the photo-synthesized technique by codoping ions, and the reso-nant transfer between synthesized ions to lasing ions isessential. The clustering technique to control the effec-tive distance between ions will be available in the futureto enhance such an energy transfer without concentra-tion quenching.

If we can solve the problem for enhancing theabsorption and emission in the fiber lasers, quite uniquefeature of lasers will be developed. Flexible and verythin fibers are available for weaving the clothes. Thus,lasing T-shirts and space suits and other applications inspace like Fig. 11 might be realized in future. Thesenew images of lasers will open the new generation oflaser applications on the ground of the earth.

SIDE PUMPING OF FIBER-EMBEDDED DISK USING LD ARRAYS

If we apply the LDs for the side-pumping scheme,we need high absorption efficiency. We discussedabove that the absorption efficiency is too low in theside pumping of the fiber lasers. The reason of the lowabsorption efficiency is due to the optical thickness of5 µm core. We proposed the fiber-embedded disk [19,20] to increase the optical thickness by the multipleabsorption. The long fiber laser with a single core isfabricated into the optical disk by the spiral mode.Assuming that the aspect ratio of core to clad is onlytwo, the cross section of the fiber-embedded disk has aperiodic structure composed of the core and the clad.

So, the effective absorption coefficient is only a half ofthe bulk material. From the view of the laser physics,the discrete distribution of active media does not affectthe absorption efficiency. Of course, such a structuredisturbs the optical propagation in the radial direction.The fluctuation of refractive index between core andclad induces the scattering in principle. But, fortu-nately, the typical value of difference in refractive indi-ces between core and clad is so small to be able toneglect the beam deflection by the core. The fiber corecan control the laser beams to propagate in the fibermode in a manner of full saturation. So, there is no

Sun

Solar power

No power deliveryto the sun

5 micron core

Absorptionefficiencies

very low

Fig. 10. The absorption coefficient of side-pumped fiberlasers is very low. But, fortunately, the solar power is chargefree and stable in space.

Solarpower

T-Shirt

CD

Fiber embeddingspace suit

Lasing output

Solarpower

Fig. 11. Solar-pumped fibers will be available for new styleof lasers.

Inlet

LD array

Fiber laser disk

Outlet

Cooling unit

Fig. 12. The basic design of fiber-embedded disk fabricatedby the high-power LD arrays around the disk.

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LASER PHYSICS Vol. 8 No. 3 1998

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interaction between multiple cores in the radial direc-tion of the lasing disk. The basic design of the fiber-embedded lasing disk is shown in Fig. 12. The pumpingbeams emitted from the LDs fabricated on the edge ofthe lasing disk are confined in this disk with reflectingcoatings on the upper and lower sides. The upper andlower surfaces are also available for the cooling of thefiber disk by cooling fluid.

The advanced technique of pumping the fiber disksis proposed to be the photon-storage ring pumping. Theinjection ports of pumping beams has a certain angle inthis version described in Fig. 13. The injected pumpingbeams propagate along the fiber-embedded ring in amanner of circulation. This appears to be somethinglike a storage ring for the synchrotron orbital radiation.So, we can call this version a photon-storage ringpumping, because the pumping beams are confined inthe small volume of ring-shaped large-aperturewaveguide. The photon-storage ring consists of a largenumber of cores in the cross section. Such an obliquepumping is similar to the multicore clad pumping inprinciple. So, the effective absorption coefficient iscontrolled to be the designed penetration length usingthe relation of (9). Although the physics of absorptionscheme is the multicore clad pumping, the photon-stor-age ring pumping has no limitation of core diameterand active volume. This is because the pumping beamsare effectively injected at every turn of the fiber ring.Thus, the photon-storage ring pumping has a greatpotential to generate more than kW output from singlecore of 50 µm.

STACKED DISK AND BOBBIN SYSTEM FOR LASER POWER PLANTS

The lasing fiber disk described above is a stand-alone lasing system. For much a higher power systemlike laser power plants for material processing factory,the multistacking fiber-disk system like Fig. 11 ispromising. The most simple extension of the fiber diskis the multistacking of the fiber disk into the single unit.In this case, the fiber belt composed of the multicorefibers is wound into the thick disk. The ratio betweenthe active volume to the input surface of pumping keepsconstant, so that the output power in the summation offiber ends is linear to the number of stacking. Accord-ing to the simple scaling law, the output increases lin-early to the thickness of the multistacked disk. Thelarge extension model of this multistacked fiber diskapproaches the fiber-embedded tube. The large numberof fibers are wound on the bobbin. Such a type of fibertube is similar to the crystalline tube laser, except thatit cannot generate the output in the direction of tubeaxis. The pumping geometry for the fiber tube is almostthe same as in the side pumping of solid-state lasers likeglass lasers. But the cooling efficiency is also similar tothe glass tube lasers. From the cooling scaling, the mul-tistacked but thin disk should be better. On the otherhand, the fiber-tube laser has another advantage that thepumping source, the conventional two-dimensional LDarrays, and flash lamps are also available to excite thefiber tube.

CONCLUSION

In this paper, the physics of the absorption processof double-clad fiber lasers was studied in detail.According to the analysis, we demonstrated the high-efficiency and high-power output performance of fiberlasers by using an RDCF laser. The slope efficiencyfrom LD pumping to laser output from a 16 µm corewas measured to be larger than 73% with low thresholdpower of 200 mW. The very challenging idea of theside-pumping scheme of fiber lasers was investigated inthe relation to the solar pumping of lasers in space. Thecritical scaling of the pump to lasing intensity wasdeveloped and gave a result that the low-loss fiberlasers are the most promising candidates of solar-pumped laser without power concentrators. The con-cept of the side pumping of fiber lasers was applied tothe LD- and lamp-pumping geometry. The fiber-embedded disk and tube are proposed for our futurelaser power plants. In principle, there are no seriousproblems of such new type of lasers.

We are now developing a wide variety of designs forour new style of laser technologies.

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LD arrayCirculating power

Multi-layer fiber

Photon storage ring excitation laser disc

Fig. 13. Photon-storage ring pumping is based on the multi-core clad pumping geometry.

2D LD array

Fig. 14. Stacked disk and bobbin system for laser power sta-tion.

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FUTURE OF HIGH-POWER FIBER LASERS 781

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