Regulación mediada por el complejo srebp-scap-insig

The summary of the results I achieved is presented in this section.

- Step hollow optical fibers : Chapter 3 presented the feasibility of step HOF for

high power fiber sources. The step HOF consists of the ring-core around the air

hole in the center. It has the fundamental mode cut-off at a finite wavelength

due to the negative dielectric volume provided by the air hole. It thus acts as a

distributed fiber waveguide filter. However, its fundamental mode cut-off

sharpness is not good enough to suppress unwanted stimulated emission

because of the slow dependency of effective indices on wavelength. It provides

the large bending loss at a broad wavelength range near the fundamental mode

cut-off. In spite of this, the ring-core structure of the step HOF provides the

relatively large ratio of core and cladding, which can improve the pump

absorption in the cladding pumping configuration. An Er:Yb-doped step HOF

laser was first demonstrated. The maximum output power was 2.5 W at 1544

nm when the pump power of 12.6 W was launched. The slope efficiency was

dependent on the core thickness, which also determines the fundamental mode

cut-off wavelength. In the small core thickness regime, the efficiency at 1550

nm was quite low (~ 8% w.r.t the launched pump power for a 125 µm inner cladding fiber) and Yb co-lasing at 1060 nm became significant because of the

low gain at the Er emission band. This was due to the induced loss at 1550 nm

and low signal modal overlap factor near the fundamental mode cut-off

wavelength. Otherwise, in the large core thickness, the efficiency was improved

(η=29% w.r.t the absorbed pump power at 250 µm outer cladding diameter fiber ). Contrary to the improved efficiency, the large hole and step core HOF

degrades the beam quality (M2 value = 5.2) because the higher order mode can easily be excited in the large hole and large core fiber. Otherwise, in the small

core fiber, the beam quality factor was improved (M2 value for 125 µm inner cladding fiber = 1.5 and M2 value for 150 µm inner cladding fiber = 2.4). In this experiment, the operation at the shorter wavelength (<1530 nm) was not

realized because the Yb gain reached a high enough level to lase before Er gain

at a shorter wavelength did. For the operation of the shorter wavelength (S-

band) in Er3+-doped fiber, Yb gain should be suppressed if Er:Yb-doped fiber is used. In order to avoid this, the pure erbium doped fiber is a better choice

because no suppression of the Yb-gain at the shorter wavelength is required.

- Nd:Al-doped depressed clad hollow optical fiber : The DCHOF consists of

an air hole at the center of the fiber, a ring-shaped core surrounding the central

air hole, a depressed refractive index of first cladding and a second cladding. In

the first section, the modal characteristics and bending loss properties of the

DCHOF were investigated. Based on these results, the fiber was designed for

experimentally. It provided the maximum output power of 3.3 W with the 41%

slope efficiency with respect to the launched pump power. The laser was

tunable from 917 nm to 936 nm. The beam quality factor was M2 value of 1.05 when the hole at the fiber output end was collapsed. Although aluminosilicate

host enhances the solubility limit of Nd-ions, but at the same time, favours 1060

nm emission. However, this showed the efficient suppression of the stimulated

emission at 1060 nm. It produced similar results as with the Nd:Al-doped W-

type fiber presented in reference [8]. However, in the Q-switched configuration,

the DCHOF generated a much higher energy than the W-type fiber. This is due

to the large core area of the DCHOF. Nd:Al-doped DCHOF produced 133 µJ pulse energy at 5 kHz repetition rate. The pulse width was 172 ns and the

average output power was 647 mW. This corresponds to ~750 W of peak power.

Then, using the Q-switched Nd:Al-doped DCHOF sources, the blue light was

generated by frequency doubling. The average output power of the beam at 927

nm was 902 mW at a 5 kHz repetition rate and the pulse energy was 176 µJ, where the pulse width was 140 ns. The corresponding peak power was 1.3 kW.

The SHG crystal used for this was bismuth borate (BiB3O6). The maximum

blue power was 50 mW at 463.5 nm. The conversion efficiency of the crystal

was 10.8%, which was low due to the un-polarized characteristic of the output

beam, at 927 nm, from the DCHOF. However, through a simple frequency

doubling process, the relatively high blue power was achieved due to the high

peak power of the laser output at 927 nm. This shows the possibility of the

power-scaling of the blue. In reference [24], the elliptical HOF provided high

core. It will be useful for polarization-maintaining fiber. Thus, for the second

harmonic high conversion efficiency, the elliptical DCHOF can be considered.

- Yb:Al-doped depressed clad hollow optical fiber : In chapter 5, the DCHOF

structure was used for generating high power 980 nm laser sources in Yb:Al-

doped DCHOF. It is much more challenging to generate 980 nm fiber souces

because of the very high GSA in Yb3+-doped fibers at 980 nm. In order to overcome the GSA, the very high pump absorption is required. Furthermore,

the wavelength space between 980 nm and ~1030 nm, where it should be

suppressed, is too narrow to be sharply separated. Eventually, the induced

bending loss affects the 980 nm loss. In order to solve this, more sophisticated

fiber control is required. In this chapter, the bending loss effect on 980 nm,

depending on the fiber parameter, were presented and then using a Yb:Al-doped

DCHOFs, the 980 nm operation was demonstrated. The inner cladding diameter

was 120 µm. It provided 3.1 W output power with an M2 value of 1.09. The slope efficiency was 34% with respect to the launched pump power. In order to

scale up the output power by reducing the laser threshold, the relatively small

inner cladding (90 µm and 80 µm diameter) Yb:Al-doped DCHOFs was used. They have nearly the same core structure as the 120 µm inner cladding diameter fiber. The output power was increased to 7.5 W for 90 µm and 9.2 W for 80 µm because of the reduction of the laser threshold. However, the beam quality was

significantly degraded due to the cladding mode lasing. The power leaked out

from the core to the inner cladding by relatively high bending loss. Because of

that, the significant power in the inner cladding causes lasing at 980 nm. In

particular, this cladding mode lasing became significant in the small inner