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High-power tunable Er,Yb ribbon fiber laser
Optics Communications 285 (2012) 1362–1365
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
High-power tunable Er,Yb ribbon fiber laser J.W. Kim a, b,⁎, J.K. Sahu a, W.A. Clarkson a a b
Optoelectronics Research Center, University of Southampton, Southampton, SO17 1BJ, UK Department of Applied Physics, Hanyang University, Ansan, Gyeonggi-do 426–791, Republic of Korea
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 17 September 2011 Received in revised form 15 November 2011 Accepted 19 November 2011 Available online 3 December 2011 Keywords: Er,Yb fiber laser Ribbon fiber Tunable laser operation Temperature distribution
A high-power Er,Yb double-clad ribbon fiber laser pumped by a 9-diode-bar pump module is reported. The laser yielded 102 W of continuous-wave output at 1566 nm for a launched pump power of 244 W, corresponding to a slope efficiency of ~44% with respect to launched pump power. Tunable operation was achieved using a simple external feedback cavity with a diffraction grating and the operating wavelength could be tuned from 1533 nm to 1567 nm. Temperature distribution in the ribbon fiber geometry and prospects of power scaling will be discussed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Cladding-pumped fiber lasers and amplifiers have achieved dramatic progress in power scaling over the last decade due to its excellent properties such as high gain, excellent heat management, good beam quality, etc. [1,2]. For efficient high-power laser operation, the choice of diode pump source and pump launching scheme are very important for overall laser design since these determine not only the maximum output power, but also impact on the fiber design, laser efficiency and flexibility in mode of operation. Therefore the use of a high-brightness diode-pump source in conjunction with a pump launching scheme that does not significantly degrade the pump brightness on launching is desirable for many high power fiber laser configurations. Due to the highly elongated beam profile of high-power multi-emitter diode sources, a re-formatting scheme must be used to equalize the Mx2 and My2 values (e.g. a beam shaper [3]) to allow efficient pump launching into a conventional circular fiber. However, the beam shaping of the pump beam often induces a loss in brightness and pump power, resulting in a reduction of the maximum launched pump power. The maximum pump power Ppmax that can be launched into a double-clad fiber from a multi-emitter diode bar can be estimated from [1]:
P p max ≈
2 P s N ηcoupling M 2x M2y
π ric θNA γ λp
!2 ð1Þ
where Ps is the power available from a constituent single emitter diode, Mx2 and My2 are the beam qualities for the single-emitter diode after ⁎ Corresponding author. Tel.: + 82 31 4005482; fax: + 82 31 4005457. E-mail address: [email protected] (J.W. Kim). 0030-4018/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2011.11.059
fast-axis collimation, N is the number of multi-emitter sources, ηcoupling is the coupling efficiency through the pump beam conditioning and combining optics and into the active fiber, ric is the radius of the innercladding, θNA is the arcsine (NA), γ is a scaling factor (b1) to account for the need to slightly underfill the fiber's aperture and NA to avoid damage to the outer-coating and mounting components and λp is the pump wavelength. As can be seen, the maximum pump power which can be launched is inversely proportional to the brightness, i.e. the product of the beam quality, Mx2My2, and hence a loss in brightness due to a beam shaping optics, (e.g. ~2.4 using a two-mirror beam shaper [3]) decreases the maximum available pump power limiting the overall laser performance. Furthermore, this intermediate beam shaping optics suffers from an increase of complexity and the cost [3,4]. Here we describe an alternative laser configuration based on a double-clad fiber design with a ribbon-shaped inner-cladding and a high-brightness multi-bar pump source to avoid the need for beam shaping. Ribbon fibers offer an improved launching scheme over conventional circular fiber designs since the ribbon geometry with a highly elongated rectangular inner-cladding allows efficient launching of pump light into the fiber without having to resort to complicated beam re-formatting schemes to equalize the M2 parameters in orthogonal planes [5]. Furthermore, the ribbon fiber has the advantage of easier heat-sinking due to its geometry resulting in improved heat management. In addition, we employed a multi-diode-bar pump module, which was an arrangement of diode-bars and optical components, allowing a more efficient coupling of the pump light into the fiber due to the improved beam quality. We have applied this configuration to an Er, Yb co-doped double-clad fiber laser yielding a maximum output power of 102 W at 1566 nm for a launched pump power of 244 W in a free-running laser configuration. Efficient tunable laser operation was also demonstrated over 34 nm from 1533 nm to 1567 nm
J.W. Kim et al. / Optics Communications 285 (2012) 1362–1365
a
Diode-bar @ 975nm x
Fast-axis and slow-axis collimation
y
z
Mirror
Diode-bar @ 975nm
Spatially-multiplexed beam
b
Fig. 1. (a) Schematic diagram of a 9-diode-bar pump module and (b) the cross-section of an Er,Yb fiber.
employing a simple external cavity design. Simple modeling to calculate the temperature distribution in the ribbon fiber was undertaken and has shown that the ribbon geometry can enhance the cooling efficiency by at least ~20% in comparison to a circular fiber geometry. 2. Experimental setup The multi-bar pump module used in our experiments was constructed in-house. It employed a modular architecture comprising nine 30% fill-factor 40 W diode-bars at 975 nm. Each bar was mounted on a water-cooled copper heat-sink and was collimated in the fast and slow directions (i.e. perpendicular and parallel to the array) using a fast-axis collimating lens and a slow-axis lens array respectively. The
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collimated output beams from each diode-bar were then re-directed by an arrangement of mirrors (as shown in Fig. 1) to mimic the optical output from a diode-stack. This multi-bar scheme offers improved reliability in output performance over diode-stacks since it does not require a micro-channel cooling system causing failure mechanisms such as channel blockage or corrosion of the micro-channel. This approach also has the attraction that beam pointing errors derived from imperfect alignment of the fast-axis collimating lenses can be corrected by the steering mirrors yielding a much better final beam quality. The pump module produced a maximum power of ~330 W, corresponding to an overall collection/transmission efficiency of ~92%. The M2 parameters for the combined output beams were measured to be Mx2 ~340 (parallel to the diode junction) and My2 ~50 (perpendicular to the diode junction). The diode pump beams were focused and launched into the front facet of the ribbon fiber with the aid of a dichroic mirror with high reflectivity (>99.5% at 45°) at the pump wavelength, and high transmission (>98%) at 1530–1570 nm to allow efficient extraction of the Er,Yb fiber laser output. The experimental setup for the Er,Yb ribbon fiber laser (EYRFL) is shown in Fig. 2. The double-clad Er,Yb co-doped ribbon fiber used in our experiments was pulled from a rectangular preform fabricated in-house [4]. The ribbon fiber had an Er,Yb-doped phospho-silicate core of ~31 μm diameter and 0.22 NA, surrounded by a pure silica ribbon-shaped inner-cladding with transverse dimensions of ~840 μm and ~280 μm (Fig. 1(b)). The inner-cladding was coated with a low refractive index polymer outer-cladding to produce a high numerical aperture (0.49 NA) waveguide for the pump. By a single aspheric lens of 25 mm focal length antireflection coated at the pump wavelength, the combined diode pump beams could be focused to a beam with transverse dimensions of ~700 μm and ~210 μm and hence most of the pump power was launched into the fiber. A fiber length of ~4 m was selected for free-running operation of the EYRFL since the measured absorption coefficient at the pump wavelength was ~4.3 dB/m. Both end sections of the fiber were carefully mounted in water-cooled aluminum heat sinks maintained at 17 °C to prevent thermal damage to the fiber coating due to unlaunched pump power and heat generated in the core due to quantum defect heating. Feedback for laser oscillation was provided by the Fresnel reflection from a perpendicularly-cleaved fiber end facet at the output end and a simple external cavity comprising an anti-reflection coated collimating lens of 8 mm focal length and a plane mirror with high reflectivity (>99.5%) at 1.5–1.65 μm and high transmission (>95%) at 940– 970 nm. Tunable operation of the EYRFL was performed employing a simple external cavity design (Fig. 2(b)) comprising an antireflection coated collimating lens of focal length 120 mm and a simple replica
Pump source at 975 nm HR @ 1.5-1.6 µm AR @ ~1 µm
f = 25 mm
Output
a
Er, Yb ribbon fiber
f = 8 mm
f = 120 mm
AR @ 1.5-1.6 µm HR @ 975 nm
b
Fig. 2. Schematic diagram of the Er,Yb ribbon fiber laser for (a) free-running configuration and (b) external cavity configuration for tunable operation.
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J.W. Kim et al. / Optics Communications 285 (2012) 1362–1365
120
y
Output power (W)
100 80
2rc
60
b
40
b/2 a
a/2
20
x
Fig. 5. Geometry of the modeled ribbon fiber.
0 0
50
100
150
200
250
300
Launched pump power (W) Fig. 3. Laser output power versus launched pump power for the Er,Yb ribbon fiber laser.
diffraction grating (600 lines/mm). The grating was blazed for wavelength at 1.65 μm with an average reflectivity of ~85% for both light polarisations and was aligned in the Littrow configuration to provide wavelength selective feedback. The fiber end facet nearest the grating was angle-polished at ~ 14° to suppress the feedback. A shorter fiber length of ~2.7 m was selected for the tunable operation.
3. Results and discussion
lasing wavelength could be tuned from 1533 to 1567 nm with power levels in the range of 45–51 W and the linewidth of ~ 0.4 nm (FWHM). 3.2. Temperature distribution analysis In order to investigate the benefit of the ribbon fiber geometry in heat management, temperature distribution was calculated using the simple heat transfer equation. For simplicity, we assume that the fiber has the single cladding structure with the square-shaped core since the core size is much smaller than the cladding size (Fig. 5). We also assume that the core and the cladding have the same thermal conductivity k. The heat transfer equation in the rectangular coordinate [7] is
3.1. Laser output characteristics Under free-running laser operation, the EYRFL reached threshold at a launched pump power of ~ 7 W and produced 102 W of output at 1566 nm for a launched pump power of 244 W, corresponding to an average slope efficiency (with respect to launched power) of ~ 44% (see Fig. 3). The beam quality factors (M 2) of the output beam along the horizontal and vertical directions were measured to be 4.2 and 4.5 respectively, which were not affected by the highly elongated inner-cladding shape. The power fluctuation of the laser output was less than 3% and no self-pulsing was observed at all power levels. Up to the maximum pump power, there was no reduction in slope efficiency due to parasitic lasing on the Yb 3 + transition at ~ 1 μm [6]. In a tunable laser configuration, the EYRFL could produce 51.5 W of output at 1548 nm for 156 W of launched pump power. The threshold pump power (launched) was ~ 11 W and the slope efficiency with respect to launched pump power was ~ 37%. The laser output power as a function of operating laser wavelength is shown in Fig. 4. The
∂2 T ∂2 T þ k 2 ¼ −Q d 2 ∂x ∂y
ð2Þ
where Qd is the deposit heat density in the core of the fiber:
Qd ¼
8 < :
Q 4r 2c L 0
a a −r c bxb þ r c 2 2
ð3Þ where Q is the total deposit heat along the fiber, rc is the transverse dimension of the core, a and b are the transverse dimensions of the cladding, and L is the length of the fiber. Assuming the heat transfer coefficient between the fiber and the heat sink is infinite and hence temperature on the surfaces of the fiber is kept constant, T0, the boundary conditions are given by ð4Þ
The solution for the heat transfer Eq. (2) can be expressed as follows:
50
T−T o ¼
40
∞ X ∞ X
Anm sin
n¼1 m¼1
nπ x mπ y sin : a b
ð5Þ
Substituting Eq. (5) in Eq. (2) yields,
30
nπ x mπ y sin ¼ −Q d Bnm sin a b n¼1 m¼1
∞ X ∞ X
20 1530
b b −r c byb þ r c 2 2 elsewhere and
T ð0; yÞ ¼ T ða; yÞ ¼ T o ; T ðx; 0Þ ¼ T ðx; bÞ ¼ T o :
60
Output power (W)
k
1540
1550
1560
ð6Þ
1570
Wavelength (nm) Fig. 4. Tunable Er,Yb ribbon fiber laser output power versus operating wavelength.
Bnm ¼ −k Anm π
2
n 2 m2 : þ a b
ð7Þ
J.W. Kim et al. / Optics Communications 285 (2012) 1362–1365
Fig. 6. Calculated temperature distribution in the ribbon fiber (solid line) and the circular fiber (dotted line) as a function of radial coordinate.
Integrating Eq. (6) over (0, a) and (0, b) after multiplying sin (nπ x/a) sin (mπ y/b) yields, Bnm ¼ −
nπ mπ nπ r mπ r 4Q 1 c c sin sin sin sin : 2 2 ⋅ nm ⋅ 2 2 a b π rc L
ð8Þ
Substituting Eqs. (8) and (7) in Eq. (5), we obtain the following analytical expression for the temperature distribution in the ribbon fiber: T−T o ¼
∞ ∞ X X 4 Q 1 4 2 ⋅n m n¼1 m¼1 kπ r c L mπ nπ rc mπ rc nπ x mπ y sin nπ 2 sinh 2 sin a i sin b sin : sin ⋅ n2 m2 a b þ b a
ð9Þ
The temperature of the circular-cladding fiber can be calculated from the following expression [8] . 8 > > <
2 Q r r −2 ln ic þ 1− ; 4π kL r ic r oc T−T o ¼ Q r > > : −2 ln ; 4π kL r oc
r b r ic
ð10Þ
1365
This discrepancy is attributed to the assumption of a pure silica cladding structure and an infinite heat transfer coefficient between the fiber and the heat sink. Therefore the actual temperature increase of the fiber will be much higher than the calculated owing to the polymer outercladding with poor thermal conductivity (~0.1 W/mK) resulted in poor heat extraction from the fiber to the heat sink [1]. However, if the thickness of the polymer cladding and the heat transfer coefficient at the boundary are same for both of the fibers, the ratio of temperature increase would be the same, i.e. the temperature increase in the ribbon fiber is ~20% smaller than that in the circular fiber. Furthermore, since the ribbon fiber has much better thermal contact with the rectangular-groove of the heat sink compared to that of a circular fiber in a typical V-groove type heat sink, much more efficient extraction of the deposit heat can be achieved with the ribbon fiber geometry and hence the ribbon fiber is beneficial to a more robust high power laser operation. 4. Conclusion We have demonstrated the efficient, high-power operation of an erbium–ytterbium co-doped ribbon fiber laser pumped by a 9-diodebar pump module. Owing to the good beam quality of the pump module and the rectangular cladding geometry of the ribbon fiber, up to 95% of launching efficiency could be easily obtained using only a single focusing lens. The Er,Yb ribbon fiber laser produced 102 W of continuous-wave output at 1566 nm in a beam with a beam-quality factor (M 2) of b 4.5 for a launched pump power of 244 W with slope efficiency of 44% with respect to launched pump power. Efficient tunable operation was also demonstrated from 1533 nm to 1567 nm. By simple analysis, it was shown that the heat extraction efficiency for the ribbon fiber geometry can be enhanced ~ 20% at least over the conventional circular fiber geometry. Therefore, the combination of a ribbon fiber geometry and multidiode-bar pump module offers high power laser operation with enhanced reliability and simplicity and further scaling of the output power should be possible with the use of higher power diode-bars or increase of emitter density.
r ic b r b r oc
where ric and roc are the radii of the core and the cladding respectively. In order to compare the temperature distribution of the ribbon fiber to the circular fiber, it is assumed that the areas of the core and the cladding for the circular fiber are same with those for the ribbon fiber. The ribbon fiber we used has the transverse dimensions of 840 μm and 280 μm for the cladding and hence the corresponding radius, roc, for the circular fiber is 274 μm. The transverse dimension of the square core for the ribbon fiber, rc, can be calculated in the same way, which is 13.7 μm. If 100 W of the heat is deposited in the fiber of length 4 m (thermal conductivity k of the pure silica : 1.4 W/mK), it is clearly seen in Fig. 6 that the highest temperature of the ribbon fiber is ~20% lower than that of the circular fiber, which proves improved heat management capacity in the ribbon geometry. The highest calculated temperature in the center is only ~10 °C higher than that of the heat sink, which is much smaller than the real temperature increase of the fiber.
Acknowledgment This work was funded by the Electro-Magnetic Remote Sensing (EMRS) Defence Technology Center, established by the UK Ministry of Defence. References [1] D.J. Richardson, J.H. Nilsson, W.A. Clarkson, Journal of the Optical Society of America B 27 (2010) B63. [2] A. Tünnermann, T. Schreiber, J. Limpert, Applied Optics 49 (2010) F71. [3] W.A. Clarkson, D.C. Hanna, Optics Letters 21 (1996) 375. [4] S. Bonora, P. Villoresi, Journal of Optics A: Pure and Applied Optics 9 (2007) 441. [5] L.J. Cooper, P. Wang, R.B. Williams, J.K. Sahu, W.A. Clarkson, Optics Letters 30 (2005) 2906. [6] D.Y. Shen, J.K. Sahu, W.A. Clarkson, Optics Express 13 (2005) 4916. [7] J.P. Holman, Heat Transfer, 6th ed. McGraw Hill Higher Education, 2009 Chap. 1. [8] A.K. Cousins, IEEE Journal of Quantum Electronics 28 (1992) 1057.
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
High-power tunable Er,Yb ribbon fiber laser J.W. Kim a, b,⁎, J.K. Sahu a, W.A. Clarkson a a b
Optoelectronics Research Center, University of Southampton, Southampton, SO17 1BJ, UK Department of Applied Physics, Hanyang University, Ansan, Gyeonggi-do 426–791, Republic of Korea
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 17 September 2011 Received in revised form 15 November 2011 Accepted 19 November 2011 Available online 3 December 2011 Keywords: Er,Yb fiber laser Ribbon fiber Tunable laser operation Temperature distribution
A high-power Er,Yb double-clad ribbon fiber laser pumped by a 9-diode-bar pump module is reported. The laser yielded 102 W of continuous-wave output at 1566 nm for a launched pump power of 244 W, corresponding to a slope efficiency of ~44% with respect to launched pump power. Tunable operation was achieved using a simple external feedback cavity with a diffraction grating and the operating wavelength could be tuned from 1533 nm to 1567 nm. Temperature distribution in the ribbon fiber geometry and prospects of power scaling will be discussed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Cladding-pumped fiber lasers and amplifiers have achieved dramatic progress in power scaling over the last decade due to its excellent properties such as high gain, excellent heat management, good beam quality, etc. [1,2]. For efficient high-power laser operation, the choice of diode pump source and pump launching scheme are very important for overall laser design since these determine not only the maximum output power, but also impact on the fiber design, laser efficiency and flexibility in mode of operation. Therefore the use of a high-brightness diode-pump source in conjunction with a pump launching scheme that does not significantly degrade the pump brightness on launching is desirable for many high power fiber laser configurations. Due to the highly elongated beam profile of high-power multi-emitter diode sources, a re-formatting scheme must be used to equalize the Mx2 and My2 values (e.g. a beam shaper [3]) to allow efficient pump launching into a conventional circular fiber. However, the beam shaping of the pump beam often induces a loss in brightness and pump power, resulting in a reduction of the maximum launched pump power. The maximum pump power Ppmax that can be launched into a double-clad fiber from a multi-emitter diode bar can be estimated from [1]:
P p max ≈
2 P s N ηcoupling M 2x M2y
π ric θNA γ λp
!2 ð1Þ
where Ps is the power available from a constituent single emitter diode, Mx2 and My2 are the beam qualities for the single-emitter diode after ⁎ Corresponding author. Tel.: + 82 31 4005482; fax: + 82 31 4005457. E-mail address: [email protected] (J.W. Kim). 0030-4018/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2011.11.059
fast-axis collimation, N is the number of multi-emitter sources, ηcoupling is the coupling efficiency through the pump beam conditioning and combining optics and into the active fiber, ric is the radius of the innercladding, θNA is the arcsine (NA), γ is a scaling factor (b1) to account for the need to slightly underfill the fiber's aperture and NA to avoid damage to the outer-coating and mounting components and λp is the pump wavelength. As can be seen, the maximum pump power which can be launched is inversely proportional to the brightness, i.e. the product of the beam quality, Mx2My2, and hence a loss in brightness due to a beam shaping optics, (e.g. ~2.4 using a two-mirror beam shaper [3]) decreases the maximum available pump power limiting the overall laser performance. Furthermore, this intermediate beam shaping optics suffers from an increase of complexity and the cost [3,4]. Here we describe an alternative laser configuration based on a double-clad fiber design with a ribbon-shaped inner-cladding and a high-brightness multi-bar pump source to avoid the need for beam shaping. Ribbon fibers offer an improved launching scheme over conventional circular fiber designs since the ribbon geometry with a highly elongated rectangular inner-cladding allows efficient launching of pump light into the fiber without having to resort to complicated beam re-formatting schemes to equalize the M2 parameters in orthogonal planes [5]. Furthermore, the ribbon fiber has the advantage of easier heat-sinking due to its geometry resulting in improved heat management. In addition, we employed a multi-diode-bar pump module, which was an arrangement of diode-bars and optical components, allowing a more efficient coupling of the pump light into the fiber due to the improved beam quality. We have applied this configuration to an Er, Yb co-doped double-clad fiber laser yielding a maximum output power of 102 W at 1566 nm for a launched pump power of 244 W in a free-running laser configuration. Efficient tunable laser operation was also demonstrated over 34 nm from 1533 nm to 1567 nm
J.W. Kim et al. / Optics Communications 285 (2012) 1362–1365
a
Diode-bar @ 975nm x
Fast-axis and slow-axis collimation
y
z
Mirror
Diode-bar @ 975nm
Spatially-multiplexed beam
b
Fig. 1. (a) Schematic diagram of a 9-diode-bar pump module and (b) the cross-section of an Er,Yb fiber.
employing a simple external cavity design. Simple modeling to calculate the temperature distribution in the ribbon fiber was undertaken and has shown that the ribbon geometry can enhance the cooling efficiency by at least ~20% in comparison to a circular fiber geometry. 2. Experimental setup The multi-bar pump module used in our experiments was constructed in-house. It employed a modular architecture comprising nine 30% fill-factor 40 W diode-bars at 975 nm. Each bar was mounted on a water-cooled copper heat-sink and was collimated in the fast and slow directions (i.e. perpendicular and parallel to the array) using a fast-axis collimating lens and a slow-axis lens array respectively. The
1363
collimated output beams from each diode-bar were then re-directed by an arrangement of mirrors (as shown in Fig. 1) to mimic the optical output from a diode-stack. This multi-bar scheme offers improved reliability in output performance over diode-stacks since it does not require a micro-channel cooling system causing failure mechanisms such as channel blockage or corrosion of the micro-channel. This approach also has the attraction that beam pointing errors derived from imperfect alignment of the fast-axis collimating lenses can be corrected by the steering mirrors yielding a much better final beam quality. The pump module produced a maximum power of ~330 W, corresponding to an overall collection/transmission efficiency of ~92%. The M2 parameters for the combined output beams were measured to be Mx2 ~340 (parallel to the diode junction) and My2 ~50 (perpendicular to the diode junction). The diode pump beams were focused and launched into the front facet of the ribbon fiber with the aid of a dichroic mirror with high reflectivity (>99.5% at 45°) at the pump wavelength, and high transmission (>98%) at 1530–1570 nm to allow efficient extraction of the Er,Yb fiber laser output. The experimental setup for the Er,Yb ribbon fiber laser (EYRFL) is shown in Fig. 2. The double-clad Er,Yb co-doped ribbon fiber used in our experiments was pulled from a rectangular preform fabricated in-house [4]. The ribbon fiber had an Er,Yb-doped phospho-silicate core of ~31 μm diameter and 0.22 NA, surrounded by a pure silica ribbon-shaped inner-cladding with transverse dimensions of ~840 μm and ~280 μm (Fig. 1(b)). The inner-cladding was coated with a low refractive index polymer outer-cladding to produce a high numerical aperture (0.49 NA) waveguide for the pump. By a single aspheric lens of 25 mm focal length antireflection coated at the pump wavelength, the combined diode pump beams could be focused to a beam with transverse dimensions of ~700 μm and ~210 μm and hence most of the pump power was launched into the fiber. A fiber length of ~4 m was selected for free-running operation of the EYRFL since the measured absorption coefficient at the pump wavelength was ~4.3 dB/m. Both end sections of the fiber were carefully mounted in water-cooled aluminum heat sinks maintained at 17 °C to prevent thermal damage to the fiber coating due to unlaunched pump power and heat generated in the core due to quantum defect heating. Feedback for laser oscillation was provided by the Fresnel reflection from a perpendicularly-cleaved fiber end facet at the output end and a simple external cavity comprising an anti-reflection coated collimating lens of 8 mm focal length and a plane mirror with high reflectivity (>99.5%) at 1.5–1.65 μm and high transmission (>95%) at 940– 970 nm. Tunable operation of the EYRFL was performed employing a simple external cavity design (Fig. 2(b)) comprising an antireflection coated collimating lens of focal length 120 mm and a simple replica
Pump source at 975 nm HR @ 1.5-1.6 µm AR @ ~1 µm
f = 25 mm
Output
a
Er, Yb ribbon fiber
f = 8 mm
f = 120 mm
AR @ 1.5-1.6 µm HR @ 975 nm
b
Fig. 2. Schematic diagram of the Er,Yb ribbon fiber laser for (a) free-running configuration and (b) external cavity configuration for tunable operation.
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J.W. Kim et al. / Optics Communications 285 (2012) 1362–1365
120
y
Output power (W)
100 80
2rc
60
b
40
b/2 a
a/2
20
x
Fig. 5. Geometry of the modeled ribbon fiber.
0 0
50
100
150
200
250
300
Launched pump power (W) Fig. 3. Laser output power versus launched pump power for the Er,Yb ribbon fiber laser.
diffraction grating (600 lines/mm). The grating was blazed for wavelength at 1.65 μm with an average reflectivity of ~85% for both light polarisations and was aligned in the Littrow configuration to provide wavelength selective feedback. The fiber end facet nearest the grating was angle-polished at ~ 14° to suppress the feedback. A shorter fiber length of ~2.7 m was selected for the tunable operation.
3. Results and discussion
lasing wavelength could be tuned from 1533 to 1567 nm with power levels in the range of 45–51 W and the linewidth of ~ 0.4 nm (FWHM). 3.2. Temperature distribution analysis In order to investigate the benefit of the ribbon fiber geometry in heat management, temperature distribution was calculated using the simple heat transfer equation. For simplicity, we assume that the fiber has the single cladding structure with the square-shaped core since the core size is much smaller than the cladding size (Fig. 5). We also assume that the core and the cladding have the same thermal conductivity k. The heat transfer equation in the rectangular coordinate [7] is
3.1. Laser output characteristics Under free-running laser operation, the EYRFL reached threshold at a launched pump power of ~ 7 W and produced 102 W of output at 1566 nm for a launched pump power of 244 W, corresponding to an average slope efficiency (with respect to launched power) of ~ 44% (see Fig. 3). The beam quality factors (M 2) of the output beam along the horizontal and vertical directions were measured to be 4.2 and 4.5 respectively, which were not affected by the highly elongated inner-cladding shape. The power fluctuation of the laser output was less than 3% and no self-pulsing was observed at all power levels. Up to the maximum pump power, there was no reduction in slope efficiency due to parasitic lasing on the Yb 3 + transition at ~ 1 μm [6]. In a tunable laser configuration, the EYRFL could produce 51.5 W of output at 1548 nm for 156 W of launched pump power. The threshold pump power (launched) was ~ 11 W and the slope efficiency with respect to launched pump power was ~ 37%. The laser output power as a function of operating laser wavelength is shown in Fig. 4. The
∂2 T ∂2 T þ k 2 ¼ −Q d 2 ∂x ∂y
ð2Þ
where Qd is the deposit heat density in the core of the fiber:
Qd ¼
8 < :
Q 4r 2c L 0
a a −r c bxb þ r c 2 2
ð3Þ where Q is the total deposit heat along the fiber, rc is the transverse dimension of the core, a and b are the transverse dimensions of the cladding, and L is the length of the fiber. Assuming the heat transfer coefficient between the fiber and the heat sink is infinite and hence temperature on the surfaces of the fiber is kept constant, T0, the boundary conditions are given by ð4Þ
The solution for the heat transfer Eq. (2) can be expressed as follows:
50
T−T o ¼
40
∞ X ∞ X
Anm sin
n¼1 m¼1
nπ x mπ y sin : a b
ð5Þ
Substituting Eq. (5) in Eq. (2) yields,
30
nπ x mπ y sin ¼ −Q d Bnm sin a b n¼1 m¼1
∞ X ∞ X
20 1530
b b −r c byb þ r c 2 2 elsewhere and
T ð0; yÞ ¼ T ða; yÞ ¼ T o ; T ðx; 0Þ ¼ T ðx; bÞ ¼ T o :
60
Output power (W)
k
1540
1550
1560
ð6Þ
1570
Wavelength (nm) Fig. 4. Tunable Er,Yb ribbon fiber laser output power versus operating wavelength.
Bnm ¼ −k Anm π
2
n 2 m2 : þ a b
ð7Þ
J.W. Kim et al. / Optics Communications 285 (2012) 1362–1365
Fig. 6. Calculated temperature distribution in the ribbon fiber (solid line) and the circular fiber (dotted line) as a function of radial coordinate.
Integrating Eq. (6) over (0, a) and (0, b) after multiplying sin (nπ x/a) sin (mπ y/b) yields, Bnm ¼ −
nπ mπ nπ r mπ r 4Q 1 c c sin sin sin sin : 2 2 ⋅ nm ⋅ 2 2 a b π rc L
ð8Þ
Substituting Eqs. (8) and (7) in Eq. (5), we obtain the following analytical expression for the temperature distribution in the ribbon fiber: T−T o ¼
∞ ∞ X X 4 Q 1 4 2 ⋅n m n¼1 m¼1 kπ r c L mπ nπ rc mπ rc nπ x mπ y sin nπ 2 sinh 2 sin a i sin b sin : sin ⋅ n2 m2 a b þ b a
ð9Þ
The temperature of the circular-cladding fiber can be calculated from the following expression [8] . 8 > > <
2 Q r r −2 ln ic þ 1− ; 4π kL r ic r oc T−T o ¼ Q r > > : −2 ln ; 4π kL r oc
r b r ic
ð10Þ
1365
This discrepancy is attributed to the assumption of a pure silica cladding structure and an infinite heat transfer coefficient between the fiber and the heat sink. Therefore the actual temperature increase of the fiber will be much higher than the calculated owing to the polymer outercladding with poor thermal conductivity (~0.1 W/mK) resulted in poor heat extraction from the fiber to the heat sink [1]. However, if the thickness of the polymer cladding and the heat transfer coefficient at the boundary are same for both of the fibers, the ratio of temperature increase would be the same, i.e. the temperature increase in the ribbon fiber is ~20% smaller than that in the circular fiber. Furthermore, since the ribbon fiber has much better thermal contact with the rectangular-groove of the heat sink compared to that of a circular fiber in a typical V-groove type heat sink, much more efficient extraction of the deposit heat can be achieved with the ribbon fiber geometry and hence the ribbon fiber is beneficial to a more robust high power laser operation. 4. Conclusion We have demonstrated the efficient, high-power operation of an erbium–ytterbium co-doped ribbon fiber laser pumped by a 9-diodebar pump module. Owing to the good beam quality of the pump module and the rectangular cladding geometry of the ribbon fiber, up to 95% of launching efficiency could be easily obtained using only a single focusing lens. The Er,Yb ribbon fiber laser produced 102 W of continuous-wave output at 1566 nm in a beam with a beam-quality factor (M 2) of b 4.5 for a launched pump power of 244 W with slope efficiency of 44% with respect to launched pump power. Efficient tunable operation was also demonstrated from 1533 nm to 1567 nm. By simple analysis, it was shown that the heat extraction efficiency for the ribbon fiber geometry can be enhanced ~ 20% at least over the conventional circular fiber geometry. Therefore, the combination of a ribbon fiber geometry and multidiode-bar pump module offers high power laser operation with enhanced reliability and simplicity and further scaling of the output power should be possible with the use of higher power diode-bars or increase of emitter density.
r ic b r b r oc
where ric and roc are the radii of the core and the cladding respectively. In order to compare the temperature distribution of the ribbon fiber to the circular fiber, it is assumed that the areas of the core and the cladding for the circular fiber are same with those for the ribbon fiber. The ribbon fiber we used has the transverse dimensions of 840 μm and 280 μm for the cladding and hence the corresponding radius, roc, for the circular fiber is 274 μm. The transverse dimension of the square core for the ribbon fiber, rc, can be calculated in the same way, which is 13.7 μm. If 100 W of the heat is deposited in the fiber of length 4 m (thermal conductivity k of the pure silica : 1.4 W/mK), it is clearly seen in Fig. 6 that the highest temperature of the ribbon fiber is ~20% lower than that of the circular fiber, which proves improved heat management capacity in the ribbon geometry. The highest calculated temperature in the center is only ~10 °C higher than that of the heat sink, which is much smaller than the real temperature increase of the fiber.
Acknowledgment This work was funded by the Electro-Magnetic Remote Sensing (EMRS) Defence Technology Center, established by the UK Ministry of Defence. References [1] D.J. Richardson, J.H. Nilsson, W.A. Clarkson, Journal of the Optical Society of America B 27 (2010) B63. [2] A. Tünnermann, T. Schreiber, J. Limpert, Applied Optics 49 (2010) F71. [3] W.A. Clarkson, D.C. Hanna, Optics Letters 21 (1996) 375. [4] S. Bonora, P. Villoresi, Journal of Optics A: Pure and Applied Optics 9 (2007) 441. [5] L.J. Cooper, P. Wang, R.B. Williams, J.K. Sahu, W.A. Clarkson, Optics Letters 30 (2005) 2906. [6] D.Y. Shen, J.K. Sahu, W.A. Clarkson, Optics Express 13 (2005) 4916. [7] J.P. Holman, Heat Transfer, 6th ed. McGraw Hill Higher Education, 2009 Chap. 1. [8] A.K. Cousins, IEEE Journal of Quantum Electronics 28 (1992) 1057.