1. HOMEOSTASIS DEL COLESTEROL
1.2. Captación del colesterol de las lipoproteínas
In most Yb-doped alumino-silicate fiber lasers, the output wavelength is ~1.1 µm with the lowest threshold, as mentioned before, whilst the Yb-doped phospho-silicate fiber
laser can produce the laser output at a relatively shorter wavelength, which is not as
efficient as aluminosilicate host due to the limited solubility of Yb ions in the
phosposilicate host. However, the DCHOF is available to generate a laser output at the
shorter wavelength, ≤ 1.06µm by suppressing the amplified stimulated emission (ASE) at 1.1 µm using the fundamental mode cut-off. Even Raman gain at the 1st Stokes wavelength can be reduced due to the induced loss by the fundamental mode cut-off when the cut-off
wavelength is located properly between the signal and 1st order stoke wavelength [39] and the power scaling up to multi kW using single mode doped core is possible without the
The same preform as used in the previous section (LF239) was drawn to a fiber
with 170 µm inner-cladding diameter (F643-LF239) in order to move the fundamental mode cut-off to a slightly longer wavelength, and coated with a low-index polymer outer
cladding which provided a nominal inner-cladding NA of 0.48. The core comprised a 6.2
µm Yb-doped ring (NA ~ 0.07) around an air hole of 18µm diameter, and a depressed ring in the inner cladding of thickness 14 µm (NA ~ 0.08). From these fiber parameters, the modal characteristics, using the equation (4-3) in the previous chapter 4, were numerically
analyzed. Figure 5-16 (a) shows the effective indexes changes of LP01 mode depending on
the wavelength of such a fiber. The fundamental, LP01, mode cut-off is ~1.15 µm. Thus the
guided core mode does not exist beyond this wavelength. Experimentally, the cut-off
wavelength was measured with a white light transmission measurement, as shown in figure
5-16 (b), and the result is in good agreement with our modeling.
Based on the transmission spectrum, the induced loss by the fundamental mode cut-
off is around 10 dB/m, while the loss at 1040 nm was not significantly affected by the
fundamental mode cut-off. Although the bending radius was as small as 5 cm, the induced
0.7 0.8 0.9 1.0 1.1 1.2 1.4572 1.4574 1.4576 1.4578 1.4580 E ff e c ti v e i n d e x ( a .u .) Wavelength (µµµµm) LP01 mode LP11 mode 800 900 1000 1100 1200 -90 -80 -70 -60 800 1000 1200 -80 -60 Wavelength (nm) T ra n s m it te d p o w e r (d B m )
Figure 5-16. (a) Effective indexes changes of each guided mode in DCHOF (b) Spectrum of transmitted light from a tungsten filament lamp (fiber length : 1 m)
Not bent
Bending radius : 5cm
LP01 mode cut-off
(a) (b)
bending loss was 1 dB/m, which is much smaller than before. The Yb-doped DCHOF was
pumped by 975 nm multimode diode stack lasers (Laserline source with 500 W output)
through a combination of collimating lenses and dichroic mirrors. Figure 5-17 shows the
laser configuration for Yb:Al-doped DCHOF. 5 m long fiber was used and a simple laser
cavity was formed between perpendicularly cleaved end facets of the fiber, providing 4%
Fresnel reflections. Dichroic mirrors (high reflection at 980 nm, high transmission at 1030
nm) were used to separate signal and pump beams. The output power was monitored at
both sides with power meters. The operational pump absorption was 2 dB/m.
Figure 5-18 shows the laser output characteristics. The output power reached 59.1
W with a slope efficiency of 81% with respect to the launched pump power and 85% with
respect to the absorbed pump power, with a central lasing wavelength of 1046nm. The
emission at longer wavelength with low threshold was suppressed by the LP01 mode cut-
off, instead, the shorter wavelength of 1046 nm was lased in this cavity. The mode field at
the longer wavelength is not confined to the doped core and exists as the cladding mode,
which is caused to significantly reduce the overlap with the doped core significantly and
thus the gain at longer wavelengths is not enough to generate a laser. In practice, the
overlap factor at 1046nm was 61%, while it is 3% at 1080 nm according to the calculation.
In a narrow wavelength range of 34 nm, the overlap factor is reduced by a factor of 20,
Pump
@ 975 nm
Yb-doped DCHOF (5m) Laser output Residual pump Dichroic mirror HT: 975 nm HR: 1030 – 1150 nm Dichroic mirror HT: 975 nm HR: 1030 - 1150 nmFigure 5-17. Laser configuration for Yb-doped DCHOF . HR: high reflectivity, HT: high transmission.
Perpendicularly cleaved
which means that the gain at longer wavelengths can be suppressed by a factor of 20 and it
was possible due to the steep LP01 cut-off characteristic of DCHOF. In this case, the laser
wavelength of 1046nm is quite near the fundamental mode cut-off. It can be expected that
a high loss is employed at the signal wavelength due to the fundamental mode cut-off.
However, the laser at the shorter wavelength can be achievable with such high efficiency
without any additional loss by the LP01 mode-cut-off. In addition, only the LP01 mode is
guided in this ring core at the laser wavelength of 1046nm, where the LP11 mode is cut-off
based on the modal calculation in figure 5-16 and thus, the output beam quality is expected
to be a diffraction-limited single mode, as shown in the previous results [94]. Moreover, as
the emission at longer wavelength is filtered out from the core, SRS, the main constraint on
power scaling, can be suppressed by the induced loss of the fundamental mode cut-off. In
practice, the suppression of SRS was demonstrated using W-type fiber, which has a similar
cut-off characteristic and will be discussed in details in chapter 6.
The Yb:Al-doped DCHOF thus looks promising to generate lasers over all the
emission bands of Yb ions in a silicate glass host.
1000 1020 1040 1060 1080 -80 -60 -40 -20 O u tp u t (d B m ) Wavelength (nm) (b) 0 10 20 30 40 50 60 70 0 10 20 30 40 50 60 O u tp u t p o w e r (W )
Absorbed pump power (W) Measured data Linear fit
(a)
Figure 5-18. (a) Laser output characteristics of Yb-doped DCHOF at 1046nm, (b) Laser output spectrum (OSA resolution : 2 nm)
5-3. Summary
In this chapter, an Yb:Al-doped silica DCHOF, designed with non-zero
fundamental mode cut-off, for high-power cladding-pumped 980 nm laser operation was
demonstrated. In addition, laser operation at the other shorter wavelength (1046 nm) in the
Yb:Al-doped silica DCHOF was also presented. Such fibers act as short-pass filters that
allow unwanted longer wavelengths to be suppressed.
Firstly, the requirements of a cladding-pumped 980 nm fiber laser were analyzed,
in particular the need to suppress competing emission at 1030 nm and longer wavelengths.
Then the ability of DCHOFs of different designs to satisfy these requirements was
investigated. It was found that even though the 980 nm emission in the Yb-system is quite
close to the competing emission, which normally dominates in a cladding-pumped fiber, it
is still possible to design the DCHOF to efficiently suppress the competing emission at the
longer wavelength. Experimentally, 3.1 W of output power was obtained in a nearly
diffraction limited beam (M2 1.09) in a fiber with a 120 µm inner cladding diameter, with a laser threshold of 9.2 W and a slope efficiency of 34%. Although, the large inner cladding
facilitates pumping with a low brightness pump diodes, but it leads to a high 980 nm laser
threshold. The output power increased to 7.5 W when the inner cladding diameter was
reduced to 90 µm, while keeping the core parameters constant, as a result of a lower threshold. However, at the same time, the beam quality degraded to an M2-value of 2.7. This is due to cladding-mode lasing in the thinner fiber with its larger overlap between the
cladding-modes and the core. In an improved fiber design, cladding-modes can be
suppressed by a 980 nm absorber in the cladding. The relatively low slope efficiency is
A cladding-diameter of 90 – 100 µm is still compatible with standard low-cost diode sources. Our initial result suggests that the DCHOF approach can be used for low-
cost single-mode 980 nm laser sources, with a few watts of output power, pumped by
single-emitter multimode diodes with ~ 10 W of output power. It would allow for scaling
up the power beyond 10 W with commercially available multi-diode and multi-emitter
diode sources including diode bars.
Secondly, through a simple laser characterization of the Yb:Al-doped DCHOF, the
suppression of the gain at the longer wavelength side in Yb emission bands was verified.
The DCHOF structure was useful to suppress the undesired emission at the longer
wavelength when the fiber was properly designed. 59.1W of the output power with 85%
slope efficiency with respect to the absorbed pump power at a shorter wavelength, 1046nm,
was demonstrated, by filtering out the emission at the longer wavelength (~ 1100 nm). In
addition, the high pump absorption in DCHOF was achieved due to the ring core. The
output beam is expected to be single-mode based on our modal calculation. Therefore, the
Yb-doped DCHOF shows promising prospects to scale up the output power in a single