Optical fibers with very broad-band gain have aroused great interest in developing tunable miniature lasers and amplifiers, which can be used in telecommunications with wider gain bandwidth than commercial erbium-doped fiber amplifiers and others reported rare earth (RE)-doped fiber devices. During the past decade, the fast increasing demand of communication capacity results in the emergence of wavelength division multiplexing (WDM) technology. In consequence, it arises the stringent requirement of spectral characteristics of all the optical components used in the optical fiber networking systems. Cr4+ doped Yttrium aluminum garnet (YAG) has a strong spontaneous emission that can generate near-infrared emission from 1.2 to 1.6 µm. This broadband emission just covers the low-loss window of low-water-peak optical fiber through out optical communication bands. Such broadband characteristics offer unprecedented one-for-all convenience, flexibility, and simplicity to multi-band component manufacturing as ultra broadband laser source, amplifier, etc [1.1-1.7]. For example, the wavelength of room temperature tunable lasers cover the visible and mid-IR spectra range, including Ti:sapphire (790-1100 nm), Cr:alexandrite (600-810 nm), Cr:LiSAF or Cr:LiCAF (750-950 nm), Cr4+:Mg2SiO4 (forsterite) (1100-1300 nm), Cr4+:YAG (1200-1600 nm).
Various types of Cr4+-doped garnet span the spectral range of 1200-1800 nm [1.8].
For optical fiber amplifier, silica-based erbium-doped fiber amplifier (EDFA) provides gain in the C-band (1530-1565 nm) [1.9-1.10] and L-band (1570-1605 nm) [1.4] even in the S-band (1450-1520 nm) [1.11]. The other types of EDFAs are fluoride- [1.12] and tellurite- [1.13] based EDFAs. Thulium-doped fiber amplifiers can give gain in the S-band. The thulium ions can be doped in fluoride [1.14-1.15], tellurite [1.16] or silica glass [1.17].Praseodymium-doped fiber amplifiers can be operated in the O-band [1.18]. Among them, the gain bandwidth of Cr4+-doped fiber amplifier (CDFA) can fully cover the whole ranges as shown in Fig. 1.1. Therefore, Cr4+:YAG crystal was selected for study in this dissertation. Pulling bulk crystal into a fiber with a diameter of several dozen µm is useful in optical communications, which is due to its structure similarity to that of silica fiber [1.19]. Moreover, fiber
Fluoride
1200 1300 1400 1500 1600 1700
0
configuration can confine pump light into a small cross-section area with a high energy density for a long distance.
Fig. 1.1. PDFA, TDFA, EDFA and HDFA denote praseodymium, thulium, erbium and holmium-doped fiber amplifiers. These amplifiers can produce gains in the O band, S band, C band and U band, respectively. The scripts, -F, -T, and -S, denote fluoride, tellurite, and silica-based fiber amplifiers, respectively. CDFA can fully cover the whole bands.
It is well-known that when Cr ions are doped into YAG crystal with a dominant ionic state of +3, Cr3+ can substitute Al3+at the octahedral sites. Selected divalent cations were evaluated for their effectiveness as co-dopant ions in Cr:YAG to enhance the Cr4+ concentration. The most suitable co-dopants are Ca2+ and Mg2+, which divalent ions can serve as efficient charge compensators.
Cr4+:YAG rod have been grown by Czochralski method [1.20-1.21], floating-zone growth [1.20], pulsed laser deposition [1.22], and the laser-heated pedestal growth (LHPG) method [1.23]. For Cr4+:YAG crystal fiber, the LHPG
O E S C L U
method was adopted in my dissertation. There are many advantages of LHPG method [1.24], including crucible free, high-speed growth, and small core diameter. It is easy to adjust the growth conditions and dopant concentrations for use as an ultra broadband amplified spontaneous emission (ASE). Because of the out-diffusion and evaporation of Cr2O3 during LHPG process, the Cr4+ concentration decreases substantially, which results in reduced signal gain and ASE power. For this reason, improving the Cr4+ charge compensation efficiency is first challenge for ASE power.
Sugimoto, et al. have been shown that as much as 6% of Cr ions can exist in a tetravalent coordination state with bulk crystal by Czochralski method [1.25]. But after diameter-reduction in 70 µm during LHPG method, the ratio of Cr4+ to Cr is decreased to less than 1% [1.26]. Therefore, enhancement of Cr4+ concentration is very important for ASE.
Ishibashi, et al. adopted a new experimental method to adjust the dopant concentration in the YAG crystal. They investigated that using electron beam gum to perimeter deposit Cr2O3 and CaO on 700-µm-diameter YAG crystal fiber as shown in Fig. 3.21 [1.23]. The experimental result showed that there is a small correlation between the deposited thickness and the ions concentration for calcium. The correlation is unclear for chromium. They suppose that most Cr ions are vaporized from the surface of the source rod by condition heat and then incorporated into the molten zone of the crystal. Under such conditions, small change in gas flow or focus laser power would make large differences in the resulting dopant concentrations.
Fig. 1.2. Schematic illustration of the LHPG method using a deposited undoped YAG rod as the source material. First CaO and then Cr2O3 are deposited [1.23].
In this dissertation, we use electron gun to deposit CaO/Cr2O3 or MgO/Cr2O3 on the source rod circumference of the Ca,Cr:Y3Al5O12 single crystal fiber to replenish the divalent ions. They were then grown to 70 µm in diameter, and followed by post-growth thermal annealing during oxygen atmosphere for enhancing the Cr4+
concentration.
This dissertation contains four main chapters. In chapter 2, the properties of Cr4+:YAG will be introduced. The charge compensation efficiency and the evolution of several Cr oxidation states will also be described. In chapter 3, the fabrication processes and optical characterization of Cr4+:YAG by the LHPG method will be discussed. The laser scanning confocal microscopy (LSCM) will be used to measure the concentrations and distributions of octahedral Cr3+ (
Cr
oct3+) and tetrahedral Cr4+ ions (Cr ) in Cr:YAG crystal fibers. In chapter 4, for Ca,Cr:YAG or Mg,Ca,Cr:YAG tetr4+ crystal fibers, there are several Cr oxidation states in YAG structure, including Cr3+ in octahedral sites, Cr4+ in octahedral (Cr ) and tetrahedral sites. After Ca,Cr:YAG or oct4+ Mg,Ca,Cr:YAG crystal fibers were grown with oxygen or nitrogen annealing treatment, the Cr4+ ion concentration in tetrahedral site is analyzed. The LSCM wasemployed for identifying
Cr
oct3+ andCr
tetr4+ ions. We characterized the concentrations of Cr oxidation states, analyzed the Cr valence change, and the main reasons why+ 4
Cr concentrations are usually limited to below 7% of the total Cr ions.tetr
Since the concentration of Cr4+ ions in 66-µm Cr4+:YAG crystal fiber is 10-fold decreased than that in bulk crystal, in chapter 5 the enhancement of Cr4+ concentration was demonstrated by electron gun deposition of divalent-ion oxide (CaO or MgO) on the source rod circumference of the Ca,Cr:YAG crystal fiber followed by re-growth.
And the perimeter deposited charge compensators of Ca2+ and Mg2+ ions were compared to see which one is better for increasing the concentrations of Cr4+ ion in oxygen atmosphere environment. Scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM) were employed to study the defects in Ca,Cr:YAG or Mg,Ca,Cr:YAG crystal fibers. The qualities of Ca2+ or Mg2+ enhanced Ca,Cr:YAG single crystal fiber were analyzed to find which one causes less propagation loss and is more suitable for developing amplified spontaneous emitter.