Chapter 1: Introduction to fiber lasers including the history of development, design of double clad fiber allowing pumping by high power laser diode for higher power-scaling. Besides, several pumping schemes including side-pumping and end-pumping are introduced. Finally the motivation for this thesis and the achievement are presented.
Chapter 2: In this chapter, the properties and importance of Yb ions and Er/Yb ions are introduced firstly. Afterwards a continuous wave tunable Yb fiber and an Er/Yb codoped fiber by use of thin film filters were demonstrated.
Chapter 3: In this chapter, I pay my attention to passively Q-switched fiber lasers. A fiber laser was Q-switch passively by Cr4+:YAG crystal. Besides, an analytical model was demonstrated to optimizing an external Q-switch, which consisted of the saturable absorber, re-imagining focus lens, and a high reflection mirror. The influences of the parameters of the external Q-switch were discussed.
Chapter 4: In this chapter I demonstrated an Yb-doped and an Er/Yb codoped fiber laser by AlGaInAs based semiconductor periodic multi quantum wells, respectively. The pulse energies are to date the highest ones for both in the Yb-doped and Er/Yb codoped passively Q-switched fiber lasers.
Chapter 5: In this chapter, I demonstrated fiber lasers Q-switched by an AO Q-switch, hybrid Q-switch, and an AO Q-switch with polarization control respectively. The influences of a saturable absorber and polarization control on the performance of Q-switched fiber lasers by AO Q-switch were discussed.
Chapter 6: This is the final chapter in this thesis and I concluded all the results here.
Besides, outlook for the fiber lasers are discussed.
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Chapter 2 Continuous-wave tunable Yb doped and Er/Yb codoped fiber lasers
2.1 Yb doped and Er/Yb codoped silica fiber lasers
Ytterbium (Yb) and Erbium (Er) are both rare earth elements of the lanthanide series.
They are often used as doping materials in many hosts such as YAG, YVO4, and various glasses such as silicate, borate, fluoride, and phosphate glasses. Here we qualitatively focus on silica glasses based fibers only because silica glass is the most popular and mature material for fabrication of fibers. Besides, we would also observe that both the dopants are suitable for tunable operation thanks to their broad emission spectrum.
2.1.1 Yb doped silica fiber
Ytterbium is the one of the lanthanons element with an atomic number of 70. The electronic configuration of an ytterbium atom is [kr]4d104f145s25p66s2 while the outermost 6s and one of 4f electrons are removed when it is ionized as a trivalent ion.
The optical active orbital 4f electrons are hence partly shielded by 5s and 5p shells.
The main energy levels concerning the optical transitions of ytterbium ions (Yb3+) are shown in Fig. 2.1. The 4f13 electrons possess only two energy levels where the higher, or say excited level is 2F5/2 and the lower one is 2F7/2. Both of them have split into three or four fine stark levels due to several reasons.
The L-S coupling (Russell-Saunders coupling) mainly determined the energy structure for that the optical electrons are shielded partly by 5s and 5p filled orbital.
The shielding effect has optical transitions of Yb3+ doped materials almost the same.
The factors affecting finer split energy levels include local field of hosts, thermal phonons, electronic and magnetic interactions with each individual ions in its vicinity, and resonant interaction with neighboring Yb ions if the doping concentration is sufficiently high [1]. The calculations of fine structures are complicated and are not discussed in this thesis.
Yb3+ ions are three or quasi-three stark levels structure, which means that they may have slight lower optical transition efficiency than the four levels. However the development of high power diode lasers help to efficiently depopulate the ions in ground level to the excited state. Besides there are some advantages making Yb attractive dopants in fiber gain medium:
A. Low quantum defect:
The simple energy levels structures also reveal that the usual pump wavelengths are close to the radiation showing the quantum defect is quite small, reducing thermal effect at high power operation.
B. Weak quench effect:
Some detrimental effects such as excited stimulated absorption (ESA), concentration quench, and upconversion are weak (still exist actually) due to no other 4f energy level of Yb doped gain medium [2], thus a high doping levels even for 10,000 ppm wt. is achievable in Yb3+ doped fiber.
C. Broad emission region:
Fig. 2.1. Yb3+ energy level diagram in silica fibers. The red line indicates the absorption line and the blue ones indicate emission line.
2F7/2
2F5/2
0.91~
0.98μm
0.98~
1.18 μm
Broad optical emission and absorption profile as shown in Fig. 2.2 [2]
showing Yb3+ a potential gain medium in amplifier, tunable lasers, and ultrafast laser.
The stimulated emission and absorption cross-section of Yb doped silicate glass are shown in Fig. 1.2 as well, where the host was germanosilicate glass. The profiles of the optical absorption and emission cross sections are nearly mirror symmetry. The spontaneous emission extends from 0.9µm to 1.1µm continuously due to the strong homogeneous and inhomogeneous broadening. On the other hand the absorption band ranges from 0.8µm–1µm, which indicates a laser-diode or a Ndlaser could be as a pump source. The fluorescence lifetime of a pure silicate glass based Yb3+ medium is around 1.5 ms [2]. Usually some elements such as Ge, Al, or K are codoped to reduce concentration quench and the lifetime is reduced [2].
2.1.2 Er/Yb codoped silica fiber
Er3+-dope materials are attractive for their useful gain region in 1.5~1.6 µm, as shown
Fig. 2.2. Emission and absorption cross section of ytterbium ion. Solid: absorption cross section; dash: emission cross section. [2]
in Fig. 2.3. For high power output higher doping concentration is necessary due to the absorption of erbium ion is impractically low. A problem is followed that the reduction of gain and pump efficiency due to quench effects such as ion-pairing [3, 4]
and upconversion, and the latter would be enhanced if ion-pairing is present [5]. It has been showed that Er3+-ion pairs would act like a saturable absorber inducing self-pulsing effect in Er-doped fiber lasers at high paired ions level and that did reduce optical efficiency. Afterward several methods were proposed to improve those detrimental effects. An Er-doped fiber laser was shown theoretically and demonstrated simultaneously operating at a stable CW output utilizing a pump wavelength at 1.51µm instead of at 0.98µm [6,7], where self-pulsing was suppressed by a dumping (resonant pumping) of Er3+-ions from excited state to ground state.
However the dumping effect significantly decreases pumping efficiency. Other methods to prevent ion-pairing includes low-temperature deposition technique or co-doping with Al or P; however they does not eliminate upconversion among the uniformly distributed ions. In addition, an efficient and practical method to solve this problem is co-doping with ytterbium to absorb the pump light around 980nm and then transferred to Er-ion nonradiatively. The proposal is based on the properties as the following:
A. The ionic radius of Yb3+ (85.8 pm)is close to that of Er3+ (88.1 pm) [8]. It is
Fig. 2.3. Emission and absorption cross section of Erbium ion.
possible to surround each Er3+ ion with several Yb3+ ions to facilitate energy transfer process, especially ytterbium tends to cluster as a high concentration doping level.
B. As mentioned before Yb3+-ion owns a simple electronic level, which means the sensitizer does not results in other detrimental effect.
These features increase the efficiency of energy transfer from Yb3+ ion to Er3+
ion. The pump energy is nonradiatively transferred from Yb3+-ion to Er3+-ion as shown in Fig 2.3. The pump light is absorbed mainly by Yb3+-ion (partially by Er3+-ion) and the electrons of the ground level 2F7/2 are stimulated to excited level
2F5/2, and due to clustering the excited Yb3+-ions have a possibility to transfer their energy to their neighboring Er3+ ion via elastic collision to indirectly excite, or pump the electrons at ground state 4I15/2 to excited state 4I11/2. It is worthwhile noting a back-transfer process is seldom cause of the lifetime of 4I11/2 is quite short and for high-efficient Yb:Er codoped fibers and by the aid of codoping of phosphate and aluminum [9-14].
An important issue is ratio of the concentration for Yb and Er. As we mentioned above an Er3+ ion is expected to be surrounded by several Yb3+ ions, which obviously
Yb3+ Donor Er3+ Acceptor
2
F
7/22
F
5/24
I
15/24
I
13/24
I
11/20.98μm
1.5~1.6 μm
Yb3+ Donor Er3+ Acceptor
2
F
7/22
F
5/24
I
15/24
I
13/24
I
11/20.98μm
1.5~1.6 μm
Fig. 2.4. Plot of Yb sensitized Er doped fibers. The energy is transferred nonradiatively from the excited state of Yb3+ ion to the metastable state of Er3+ ion to enhance pump absorption.
indicating a much higher concentration of Yb3+ ion than that of Er3+ ion. Nevertheless a too high concentration of Yb would waste pump energy and reduce the energy transfer efficiency, as well as the gain coefficient. The ratio of the concentration NYb/NEr is recommended to be between 4 and 20 [15]. Besides, some imperfections exist and are not understood yet so far. For example, isolated Yb3+-ions would radiate 1.1-µm emission without transferring energy to Er3+-ions and non-uniformly distributed concentration is harmful to energy-transferring as well.
2.2 All-dielectric thin film Fabry-Pérot filters 2.2.1 Introduction to dielectric thin film filters
Among the optical filters there are several types based on different principles such as absorbing glass filter and color filter are based on wavelength-dependent absorption, Lyot filters [16] are based on wavelength-dependent polarization change, and prisms are based on wavelength-dependent diffraction. On the other hand, interference effect based filters including etalon, Mach-Zehnder interferometers, acousticoptic filters [17-18], FBGs [19-20], VBGs [21-22], and AWGs [23]. They provide wavelength-selection and are well-developed and commercialized.
Dielectric thin film filter or multilayer interference filter is also based on interference effect and offer another approach for filtering. They are the first filter type to be widely deployed in wavelength division multiplexing (WDM) systems in the 1990s [24-25]. The technology has also been applied to a number of important optical network applications such as gain-flattening filters (GFFs), high performance band splitter-pump WDMs for erbium-doped fiber amplifiers (EDFAs), and wideband splitting filters for separating bands of channels. Thin film filter possess the merits of very low temperature coefficient, long stability, and small losses of chromatic dispersion and polarization-related dispersion. Thin film filter consists of an alternating sequence of layers of transparent dielectric material of high and low refractive indices deposited on a substrate.
The candidates for substrates are usually BK7 glass and fused silica and the
coating materials are often categorized two types. One is oxides including SiO2, TiO2, and Al2O3, the other is fluorides including MgF2, La F3, and Al F3. The fabrications of the thin films are based on the techniques such as electron beam deposition, ion-assisted deposition (IAD), and ion-beam sputtering (IBS) deposition. The technology is very flexible; AR, narrow bandpass, wide bandpass, edge, gain flattening, dispersion compensation, and other filters can be designed for applications.
2.2.2 All-dielectric Fabry-Pérot filters
In this section I pay my attention to narrow bandpass type thin film filter which realizes a narrowband transmission to be an efficient wavelength-narrowing element for broadband lasers. Figure 2.1 shows the structure of a typical bandpass dielectric thin film filter. The structure of is based on Fabry-Perot (FP) cavity type and is formed by a space region sandwiched between two highly-reflecting multilayer stacks deposited on a glass substrate. The basic design formula for the more commonly used all-dielectric version is:
Substrate | (LH)m (2L)n (HL)m |air , (2.1) where m is the number of periods in the HRs; n is the order of the spacer.
The highly-reflecting stacks are composed of multi alternating sequence of high and low refractive indices with optical thickness of quarter-wave of central wavelength. The optical high spacer thickness at the centre wavelength for the first order (n = 1) is one halfwave, for the second order (m = 2) two halfwaves, etc. In a FP filter, only a small fraction of light normally penetrates the first reflector, but at certain resonant wavelengths, the light intensity builds up in the spacer layer until a significant fraction close to 100% of the input light is transmitted. The transmittance of thin film filter can be expressed as:
( )
max 1 sin2 int( )
12
T θ T F φ φout δ θ
⎡ ⎛ + ⎞⎤−
= ⎢⎣ + ⎜⎝ − ⎟⎠⎥⎦ , (2.1)
where
Rint and Rout are the reflectance at incidence and output side of filter respectively; Ta
and Tb are the reflectance at incidence and output side of filter respectively;
int and out
φ φ are the phase change at incidence surface and output surface; ns is the refractive index of the filter; and ds is the thickness of the filter.
The maximum transmission Tmax would equal unity only if all the materials do not absorb the light inelastically or the transmittance of the incidence side is equal to that of the output one. The highest transmission occurs at the total phase difference is mπ for λ=λp, the central wavelength, i.e.
λ/4 layers λ/2 λ/4 layers
Reflector spacer Reflector substrate
λ/4 layers λ/2 λ/4 layers
Fig. 2.5. The structure of a typical thin film Fabry-Perot filter.
where λ0 is the measured central wavelength at normal incidence. Hence the central wavelength of a FP filter is a function of the angle of incidence, which meaning it could be adopted as a tunable wavelength selector for optical measurement or wavelength tuning in a laser.
2.3 Tunable ytterbium doped fiber lasers
The wavelength selection in this section is achieved by an all-dielectric FP bandpass filter owning a bandwidth of approximately 1.24 THz or 5nm at 1100nm. In this part we demonstrated an efficient tunable Yb doped fiber laser with the FP bandpass filter.
The efficiency from a free running lasing with a bandwidth of >10 nm to a narrow linewidth lasing with a linewidth < 1 nm can exceed 96%. By tilting the incidence angle, the lasing wavelength can be tuned from 1540 nm to 1100 nm with a tuning
The efficiency from a free running lasing with a bandwidth of >10 nm to a narrow linewidth lasing with a linewidth < 1 nm can exceed 96%. By tilting the incidence angle, the lasing wavelength can be tuned from 1540 nm to 1100 nm with a tuning