Introduction

Due to the fast growing communication need, the required capacity of the optical fiber network has been more than doubled every year. And the technology break through in dry fiber fabrication opens the possibility for fiber bandwidth all the way from 1.3 to 1.6 μm. The fast increasing demand of communication capacity results in the emergence of wavelength division multiplexing (WDM) technology, enabling tens or even hundreds of channels with different wavelengths transmitted simultaneously on an optical fiber [1.1]. In consequence, it necessitates the requirement of broadband spectral characteristics of all the optical components used in the optical network systems.

Broadband light source and optical amplifier are the key components in WDM networks. Table 1.1 summaries the various techniques for generating broadband emission light source and optical amplifier. Edge emitting light emission diode (EELED) and superluminescent diode (SLD) used the multiple quantum well to cover the broadband gain with designated pump current [1.2]. The current dependent gain spectra limit the application in optical communication system. A supercontinuum technique used an amplified pulse laser by an Erbium-doped fiber firstly. Then the pulse laser is broadened with a dispersive fiber due to the self phase modulation of Kerr effect [1.3]. High cost and complicated configuration are the disadvantages.

Rare-earth-element doped silicate fibers are the most common optical fiber light sources. They are highly efficient and mature techniques already [1.4-1.11], but the gain bandwidth cannot fully cover the whole 1.3 to 1.6 μm bandwidth with a single fiber amplifier. Cr⁴⁺:YAG is an attractive gain medium for broadband light source and optical amplifier because of its ultra-broadband emission spectra from 1253 to 1530 nm. Besides, the 0.9~1.2 μm optical pumping absorption range for Cr⁴⁺:YAG covers available 910~1060-nm broad-stripe diode laser with generous wavelength tolerance.

As shown in Table 1.1, the end pump scheme with laser diode (LD) leads to a compact, cost-effective package. The crystalline host offers higher thermal conductivity and hence a higher power handling capability than glass fibers.

Table 1.1. Techniques of broadband emission light sources [1.2-1.11].

Technique Generating scheme Emission range (nm)

EELED or SLD 1300~1580

Supercontinuum 1420~1700

Er³⁺ doped fiber 1530~1610

Tm³⁺ doped fiber 1480~1510

Pr³⁺ doped fiber 1280~1360

Cr⁴⁺:YAG crystal fiber 1253~1530

P N 6nm

8.7nm In_0.57Ga_0.33As_0.72P_0.2

In_0.53Ga_0.47A

laser Pulse

Dispersive fiber EDFA

RE³⁺ ions doped Pump Laser

Pump/Signal Multiplexer

LD

Cr⁴⁺:YAG crystal fiber

This dissertation contains four main chapters. In chapter 2, the properties of Cr⁴⁺:YAG are introduced. Based on the absorption and emission spectra of Cr⁴⁺:YAG, precise energy-level diagram was obtained. Furthermore, we established distributed numerical model for the amplified spontaneous emission (ASE) and optical amplifier, and lumped model for laser, respectively. In chapter 3, a well known growth technique for crystal fibers, the laser heated pedestal growth (LHPG) method [1.12]

was introduced. The fiber diameter was difficult to be reduced to below 30 μm that was limited by the few-tens of microns focal spot size of the laser used in the LHPG system. To further reduce the core size, a co-drawing LHPG (CDLHPG) technique was developed [1.13]. A double-clad crystal fiber (DCF) structure is formed with an additional inner cladding layer made of fused silica and YAG mixture. The fiber core diameter is reduced during the formation of the inner cladding layer, and can be as small as 10 μm. However, the fiber core diameter is very non-uniform because the core size is very sensitive to the power stability of the heating laser (< 0.5% power fluctuation in our case) during the growth process, especially when the core diameter

is small. As a result, the 10-μm-core fiber has about 60% peak-to-peak core variation typically. So a sapphire tube assisted CDLHPG technique cooperated with power feedback controller was developed to solve the difficulty [1.14]. A 10-μm-core Cr⁴⁺:YAG DCF was successfully fabricated, and fulfilled the adiabatic propagation criterion. In chapter 4, we used X-ray diffraction to measure and compare the crystal quality of the Cr⁴⁺:YAGs grown by Czochralski (CZ) and LHPG methods. The micro structures in core and inner cladding of Cr⁴⁺:YAG DCF were also characterized with high-resolution transmission electron microscopy (HRTEM). The core region still remained crystalline structure in <111> orientation and the inner cladding had γ-Al2O3 nano-crystals surrounded with mixture of YAG and silica. The major compositions and Cr doping concentrations were also measured by electron probe micro-analyzer (EPMA) to analyze the influence of growth parameters. The fluorescence of Cr³⁺ and Cr⁴⁺ doped in YAG and mixture of YAG and silica were mapped to analyze the relation between compositions. Propagation loss of the DCF was also measured and discussed in the subsection. The loss was improved from around 0.6 dB/cm for the single crystal fiber to 0.02 dB/cm for the uniform-core DCF.

In chapter 5, the ASE and optical gain were measured and data were used to determine the cross sections of absorption, emission, and excited-state absorptions (ESAs) of pump and signal. A knotty difficulty for pump ESA loss perplexed the optical performance. Cladding pump scheme and shifting of the pump wavelength was discussed to suppress the pump ESA loss and improve the optical performance.

In addition, an ultra-low threshold (2.5 mW) Cr⁴⁺:YAG DCF laser was demonstrated with 6.9% slope efficiency. The ASE of Cr⁴⁺:YAG DCF was used to be the light source of optical coherence tomography (OCT). A theoretical-limit axial resolution of 3.5 μm was demonstrated. Finally, in chapter 6 conclusions and future improvements are described.