Zig-zag active-mirror laser with cryogenic Yb^3+:YAG/YAG composite ceramics

Zig-zag active-mirror laser with cryogenic

Yb3+

:YAG/YAG composite ceramics

Hiroaki Furuse,1,*

Junji Kawanaka,2 Noriaki Miyanaga,

2 Taku Saiki,

1 Kazuo Imasaki,

1

Masayuki Fujita,1 Kenji Takeshita,

3 Shinya Ishii,

3 and Yasukazu Izawa

1

1Institute for Laser Technology, 2-6 Yamada-oka, Suita, Osaka, Japan 2Institute of Laser Engineering, Osaka University, 2-6 yamada-oka, Suita, Osaka, Japan

3Mitsubishi Heavy Industries, 16-5 Konan 2-Chome, Minato-ku, Tokyo, Japan

*[email protected]

Abstract: We report on a novel amplifier configuration concept for a 10

kW laser system using a zig-zag optical path based on a cryogenic Yb:YAG

Total-Reflection Active-Mirror (TRAM) laser. The laser material is a

compact composite ceramic, in which three Yb:YAG TRAMs are combined

in series to increase the output power. Output powers of up to 214 W with a

slope efficiency of 63% have been demonstrated for CW operation, even at

a quite low pump intensity of less than 170 W/cm2. Further scaling could

achieve output powers of more than 10 kW.

©2011 Optical Society of America

OCIS codes: (140.3480) Lasers, diode-pumped; (140.3580) Lasers, solid-state.

References and links

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kW Coherently Combined Slab MOPAs,” in Conference on Lasers and Electro-Optics, Technical Digest

(Optical Society of America, 2009), paper CThA1.

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Top. Quantum Electron. 13(3), 598–609 (2007).

5. H. Furuse, J. Kawanaka, K. Takeshita, N. Miyanaga, T. Saiki, K. Imasaki, M. Fujita, and S. Ishii, “Total-reflection active-mirror laser with cryogenic Yb:YAG ceramics,” Opt. Lett. 34(21), 3439–3441 (2009).

6. T. Y. Fan, D. J. Ripin, R. L. Aggarwal, J. R. Ochoa, B. Chann, M. Tilleman, and J. Spitzberg, “Cryogenic Yb3+-

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599 (2005).

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#136878 - $15.00 USD Received 19 Oct 2010; revised 14 Jan 2011; accepted 21 Jan 2011; published 25 Jan 2011(C) 2011 OSA 31 January 2011 / Vol. 19, No. 3 / OPTICS EXPRESS 2448

1. Introduction

Diode-pumped solid-state lasers with high average powers of more than 10 kW are interesting

candidates for novel industrial applications, such as material processing. Nowadays 16 kW

CW and 400 W Q-switched Yb:YAG thin-disk lasers are commercially available [1]. 50 kW

multi-mode and 10 kW single-mode fiber lasers were developed [2]. In addition, a laser

system with 105 kW in CW has been demonstrated by a coherent beam combining technology

with seven Nd:YAG slab amplifiers [3].

The thin-disk laser concept is one of the most promising designs for high average power

lasers. As the disk is thinner than 200 μm, a temperature rise of the disk is small. With a high

diameter-thickness aspect ratio and a uni-directional heat flow along the disk axis, the radial

temperature gradient is ideally negligible, reducing the wavefront distortion dramatically.

Also, power scaling can be achieved by enlarging the beam aperture size [4], but is ultimately

limited by Amplified Spontaneous Emission (ASE) effects. The complicated optics for the

multi-pass pump is, however, indispensable to absorb the pump power efficiently. The

necessary applied high-reflection (HR) coating on the back side of the thin-disk introduces an

additional thermal insulation, which can increase the experienced thermal stress within the

gain medium.

To confront these problems, we applied two approaches [5]. One was to use an Yb:YAG

gain medium at low temperature. Thermal properties of Yb:YAG (such as thermal

conductivity k, thermal expansion (1/L) dL/dT, and the thermo-optic effect dn/dT) are

considerably improved at low temperature [6–8]. These improvements allow the use of a

thicker Yb:YAG disk to absorb most of the pump power using simple pumping optics. An

additional advantage is the four-level laser system at low temperature compared to the quasi-

three-level system at room temperature. Also, the emission cross section increases at low

temperature. A high laser gain is therefore obtainable even at a lower pump intensity [9,10].

By using a cryogenic Yb:YAG, high average power operation has been extensively researched

[11–14]. The other approach was to use a Total-Reflection Active-Mirror (TRAM)

configuration [5]. The TRAM uses total reflection on the bottom surface instead of the HR-

coating. The laser gain material can be directly cooled by a coolant without additional

temperature rise at a HR-coating. In addition, a spatial separation of input and output surfaces

improves the optically induced damage threshold in pulse operation by reducing a locally

increased electric field due to optical interference.

In our previous work on the cryogenic Yb:YAG TRAM [5], the laser oscillator generated

a high output power of 273 W with an optical efficiency of 65% and a slope efficiency of

72%. Enlarging the laser aperture can increase the output power, similar to the thin-disk laser.

Another way for power-scaling is to increase the pump power (pump intensity). These two

ways are finally limited by ASE loss and the temperature rise of the active layer of the

TRAM. For further power-scaling, serial amplification with multiple TRAM units is

reasonable.

In this paper, based on the multiple TRAMs concept as a laser amplifier, we demonstrate a

laser oscillator. A long, monolithic Yb:YAG/YAG composite ceramics, combining three

TRAMs in series, is used. Both pump and laser beams propagate along a zig-zag optical path

in the ceramics, consequently the composite ceramics is called ZiZa-AM (Zig-Zag Active-

Mirror). Although our laser diode power limited the pump power to 470 W, a high output

power of 214 W and a high optical efficiency of 50% are obtained.

2. Zig-Zag Active-Mirror for 10 kW

Figure 1 shows a ZiZa-AM configuration to achieve a 10 kW output power, which was

fabricated by Konoshima Chemical Co., Ltd. The ZiZa-AM is a composite ceramics with an

elongated trapezoidal YAG prism and three Yb:YAG layers. Top and bottom faces are 68 x

34 and 85 x 34 mm2, respectively. The height of the prism is 14.7 mm. Two slope surfaces at


a skew-angle of 60 degrees were antireflection (AR) coated at the laser wavelength of

1030 nm. Three 9.8 at.% Yb:YAG layers were attached to the YAG prism, each with cross

section of 34 x 34 mm2. The pump and laser beams are incident at right angles on the slope of

the YAG prism, and reflected at the outer surfaces of the Yb:YAG layers, forming a zig-zag

optical path. In the ZiZa-AM, the thickness of the layers were designed to satisfy the

following requirements: (1) each temperature rise is almost same for all layers and is less than

15 K at the total absorbed pump power of 15 kW, (2) more than 95% of the pump beam can

be totally absorbed. The absorption can be estimated by the equation of exp(2αd/cosθ),

where α is the absorption coefficient, d is the Yb:YAG thickness, and θ is the slope angle. The

temperature rise in a Yb:YAG layer along the heat flow was roughly evaluated by the

equation of ΔT = ηIabsd/k, where η is the rate of the generated heat power against the absorbed

pump power, Iabs is the absorbed pump intensity, and k is the thermal conductivity. We used

the following parameters in the calculations: α = 12.7 cm1

, θ = 60 degrees, η = 10%, and k =

20 W/mK. The pump beam diameter is 15 mm.

In Table 1, thickness and pump condition for each layer are summarized for a 10 kW laser

system. Yb1, Yb2 and Yb3 denote the first, second and third Yb:YAG layers from the

incident slope face, respectively. There are small differences in the corresponding layer

thickness compared the designed and manufactured ZiZa-AM. In this design, a gain loss due

to the ASE was not considered because of the limited doping concentration of 9.8 at.%. To

suppress the ASE loss, optimization of doping concentration and/or absorbing cladding (e.g.

Cr4+

:YAG) would be necessary.

Fig. 1. Schematic of the zig-zag active-mirror sample.

Table 1. Specifications of thickness and absorption for designed and manufactured ZiZa-

AM. The estimated temperature rise at an incident pump power of 15 kW and a pump

beam diameter of 15 mm are also listed

Laser gain medium Yb1 Yb2 Yb3

Designed

Thickness (μm) 120 170 300

Absorption (%) 45.6 31.4 17.9

Iabs (W/cm2) 1,940 1,330 760

Temperature rise (K) 11.6 11.3 11.4

Manufactured

Thickness (μm) 100 190 290

Absorption (%) 39.8 37.3 17.7

Iabs (W/cm2) 1,690 1,580 750

Temperature rise (K) 8.5 15.0 10.9

3. Experimental and discussion

Figure 2 shows the experimental setup for the ZiZa-AM laser oscillator. A photograph of the

manufactured ZiZa-AM sample is also shown in Fig. 2. The sample was put into a liquid-

nitrogen cryostat equipped with two AR-coated windows. Indium wires were used as sealant

between the liquid nitrogen and vacuum. A 500 W, 940 nm fiber-coupled laser diode (LD)

was used as pump source. Both pump and laser beams are totally reflected on the bottom faces

of Yb:YAG layers. They reveal elliptical dimension onto Yb:YAG layers due to the


60 degrees angle of incidence. The pumping beam was focused onto the central Yb:YAG

layer (Yb2) and the pump beam diameters on Yb1, Yb2 and Yb3 were approximately 9 mm,

8 mm and 9 mm, respectively. The laser cavity was V-shaped with a flat dichroic mirror

(DM), flat output coupler (OC) and a lens. The focal length of the lens was f = 1000 mm. To

estimate the laser gain, several output couplers with the reflectivity of 93.6, 84.6, 72.0, 63.5,

and 42.5% were used. From a transmission experiment, the total absorption of incident pump

power was evaluated to be about 94.8%. By considering the transmittance of optics, the total

absorbed pump power of this ZiZa-AM can be estimated to be 431 W.

Fig. 2. Schematic of the ZiZa-AM power oscillator.

The output power Pout as a function of absorbed pump power Pabs is shown in Fig. 3. The

spatial mode of the output was multi-mode with a circular profile. Maximum output power of

214 W was obtained using an OC reflectivity of 72%. The optical efficiency and slope

efficiency against the absorbed pump power are 49.7% and 62.7%, respectively. These results

are lower compared with our previous experimental results with a single TRAM laser. This is

because the absorbed pump intensity is quite low. The absorbed pump powers for each

Yb:YAG layer are estimated to be about 181, 170, and 80 W. The absorbed pump intensities

are evaluated respectively to be 142, 169 and 63 W/cm2. The optimum slope efficiency can be

found experimentally by varying the output coupler reflectivities over wide range. This is

done with a set of different output couplers. However, as the cavity length is approximately

1.5 times longer than in our previous experiments, diffraction losses can be accounted for a

reduction of slope efficiency.

The laser output power shows a linear increase even at the maximum pump power, which

is limited by the laser diode. Therefore, the temperature rise of Yb:YAG layers is estimated to

be low. Both the laser power and the optical efficiency could be improved with a further

increase in pump power. Using this ZiZa-AM, one can expect that the maximum pump power

can reach 15 kW with a pump beam diameter of 15 mm, as shown in Table 1. In Fig. 3, the

lasing threshold is also shown as a function of the mirror reflectivity to evaluate the laser gain

and the resonator loss based on the Findlay and Clay method [15]. This measurement reveals

a loss in the resonator of δ = 3.6%, and the slope of the fitting line of 0.004. Thus, we can

obtain the relation 2g0l = 0.004 x Pabs (W1

). The small signal gain G = exp(g0l) can be

estimated to be 2.36 when Pabs is 431 W.


Fig. 3. (a) Output powers of the Zig-Zag Active-Mirror laser as a function of absorbed diode pump power, and (b) the laser threshold power.

To verify the obtained gain of the ZiZa-AM amplifier, we performed direct measurements

of the Small Signal Gain (SSG), shown in Fig. 4. We used a 1 W CW, linearly polarized

single-mode fiber laser as a seed source (1029.4-nm center wavelength, 0.2 nm FWHM). A

dichroic mirror DM1 (Rmax@1030 nm, Tmax@940 nm) between the cryostat and the focusing

optics combines the seed and pump beams. After amplification, DM2 and DM3 were used to

separate the amplified laser beam from transmitted pump beam, and the output power was

measured using a power meter. In Fig. 5, the obtained SSG is shown as a function of the

absorbed pump power together with calculations. The circles and red dotted line represent the

experimental and theoretical results, respectively.

In the calculation at 77 K, an emission cross section of σemi = 1.3 x 1019

cm2, a

fluorescence lifetime of τf = 1 ms, and an absorption coefficient of α = 12.7 cm1

were used. A

detailed discussion of the gain can be found in Ref [5]. As seen in Fig. 5, there is a good

agreement between the experimental results and the calculations when Pabs < 200 W. This

shows that the temperatures of Yb:YAG layers do not rise so high to transform to a quasi-

three level laser system. By using the inclination of the dotted line in Fig. 3(b), the SSG at Pabs

= 200 W is evaluated to be 1.49 (g0l = 0.4), which is consistent with Fig. 5.

However, there is a discrepancy between the experimental results and the calculation for

Pabs > 200 W. The temperature rise in the Yb:YAG layers can be considered small, as the

layers are thin and the absorbed pump intensity in each of the layers is low. Therefore, we

attribute the observed reduced SSG values to ASE and parasitic oscillations in radial direction

on the Yb layer, rather than to thermal effects. In fact, the ratio between the ASE length lASE =

D/cosθ and the thickness of the Yb layer d is very large, where D is the pump beam diameter

and θ is the angle of incidence. Then the ASE gain g0lASE for each layer at Pabs = 200 W can

be calculated to be 7.28 (Yb1), 4.02 (Yb2), and 1.12 (Yb3), although the spatially averaged

gain along the laser pass g0l is 0.16, 0.19, and 0.07, respectively. For simplicity we assumed

the gain of Yb1 to be constant for Pabs > 200 W, as indicated by the blue dashed line in Fig. 5,

as it is the highest value of all the g0lASE. Using this approximation, we obtained a reasonably

good agreement with the experiments. The calculations will be more close to the experimental

results by investigating the ASE condition in detail and considering the gain suppression

within the other layers.

Fig. 4. Schematic of the single-pass amplifier.


Fig. 5. The small signal gain as a function of the absorbed pump power. The calculated blue

line corresponds to the case when ASE suppressed and the gain is constant on Yb1 disk.

To investigate the thermal effect on the beam profile, such as thermal lensing and thermal

birefringence of the ZiZa-AM laser, we measured the spatial beam quality and the degree of

polarization (DOP) at maximum pump power. Figure 6(a) shows the near-field (NFP) and far-

field patterns (FFP) of the amplified laser beam together with M2-fit data in the x and y

transverse dimensions at Pabs = 400 W. NFP was observed by relaying the beam image on the

DM2 to a charge-coupled device. The spatial beam propagation factor of the laser was

characterized by the beam propagation method. The beam was focused using a 500-mm lens,

and the beam radii were measured around the waist using a delay stage. Then, beam

propagation factor was evaluated by fitting a hyperbola to the measured data using the least

squares method. The beam diameters (1/e2) of NFP and FFP were about 5.0 mm and 180 μm,

respectively. A typical fit of beam diameter data to a calculated hyperbola with M2 = 1.0 is

also shown in Fig. 6(b).

As can be seen in Fig. 6(b), hyperbolic fits were quite good for measuring the M2 factor in

both directions when Pabs = 400 W although the ZiZa-AM laser oscillator generated multi-

transverse mode. Inasmuch as the position of the beam waist at Pabs = 400 W was nearly the

same as that with no pumping action, the thermal lens effect is negligible.

Fig. 6. (a) Amplified beam profiles of near-field (NFP) and far-field (FFP) patterns at absorbed

pump power of 400 W. Beam diameters (1/e2) of NFP and FFP are about 5.0 mm and 180 μm, respectively. (b) M2-fit data in x and y transverse dimensions. The solid curves are calculated

results for M2 = 1.0.

The DOP was measured using a λ/2 plate and a Glan laser polarizer. The DOP is

determined by DOP = Pt / (Pt + Pr) relation, where Pt and Pr are transmitted and reflected


output powers from the polarizer, respectively. Figure 7 shows the DOP as a function of

absorbed pump power. As shown in Fig. 7, the DOP of the ZiZa-AM laser maintains the

linear polarization state with increasing absorbed pump power. At Pabs = 400 W a maximum

DOP of ~98% has been measured. Therefore, thermally induced birefringence in ZiZa-AM

can be neglected.

Fig. 7. The degree of polarization (DOP) as a function of absorbed pump power.

In the beam profile and polarization measurements, we found that the thermal effects in

the ZiZa-AM are very low. This result suggests that the ZiZa-AM laser is a compact high

average power amplifier possessing a good beam quality at several hundred watt of output

power. In the case of an amplifier, however, the effect of ASE and parasitic lasing in radial

direction on the Yb:YAG is a key issue. For higher power operation, it is necessary to avoid

this impact.

To suppress the parasitic lasing and minimize ASE while maintaining a larger aperture, a

lower gain coefficient (lower doping concentration) is required. In our future efforts to

develop a more advanced laser source, we will optimize not only the thickness but also the Yb

concentration of each Yb:YAG disk. The maximum amplified power can be further increased

by optimizing the amplifier specifications, as enlarging the size and increasing the number of

disks inside the ZiZa-AM for symmetrical pumping from both sides. A conceptual design

about this is shown in Fig. 8. The entire surface of the Yb:YAG disks (6.8 x 6.8 cm2) is

pumped, therefore, the ASE length is the diagonal size of the disk (lASE = 9.6 cm). For

example, the total pump power of about 40 kW from both ends of the slab may be possible

with low ASE gain (g0lASE < 3) when the doping concentration and thickness of Yb1 are 0.3

at.% and 2.6 mm. We expect that for an amplifier system with more than 10 kW output

power, a high efficiency and a good beam quality will be achieved out of this ZiZa-AM laser

concept.

Fig. 8. Conceptual amplifier design for more than 10 kW output power. Cr:YAG is used as an

absorber of spontaneous emission to avoid ASE effects.

4. Conclusions

In conclusion, we presented a multiple-TRAM laser using a zig-zag optical path in a

monolithic composite ceramic, called “ZiZa-AM”-laser. Using this concept as a CW

oscillator, we demonstrated 214 W output power with a 50% optical efficiency and 62% slope

efficiency, although the pump intensities were quite low. We have estimated that by using a


more powerful pump source and sufficient cooling, a maximum output power of 10 kW would

be possible. As a cooling method we consider a liquid-nitrogen re-circulating flow system.

We studied the small signal gain both experimentally and theoretically. Reasonably good

agreement between theoretical predictions and experimental results was obtained. The

amplified beam profile showed a very good beam quality. The thermal lensing and thermal

birefringence effects could be neglected under present experimental conditions. We believe

that over a 10 kW amplifier system with high efficiency and good beam quality will be also

achieved from this ZiZa-AM laser concept.

Acknowledgments

The authors wish to express their appreciation and thank to Dr. Daniel Albach and Dr. Haik

Chosrowjan for reading and valuable comment for the manuscript. They also wish to thank to

Dr. Shinji Motokoshi for coating the laser material.


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Zig-zag active-mirror laser with cryogenic Yb^3+:YAG/YAG composite ceramics