Low-loss propagation in Cr^4+:YAG double-clad crystal fiber fabricated by sapphire tube assisted CDLHPG technique

K Y Huang,K Y Hsu,P Y Chen,W J Zhuo,D Y Jheng,P S Yeh,S L Huang

doi:10.1364/oe.16.012264

Abstract

Cr4+:YAG double-clad crystal fiber with an uniform 10-microm core was fabricated by using a sapphire tube as a heat capacitor to stabilize the power fluctuation of the CO2 laser in the co-drawing laser-heated pedestal growth system. The uniformity of the fiber core showed a factor of 3 improvement compared to that without the use of sapphire tube. The variation of the core diameter is within the +/-1.35-degree adiabatic criterion and has a autocorrelation length of 1.7 mm. The measured propagation loss is only 0.02 dB/cm. The sapphire tube also reduces the vertical temperature gradient during the crystal fiber growth process so the 10-microm crystal core exhibits a smooth perimeter. The sapphire tube assisted system can be applied to the growth of many other optical crystal materials.

Highlights

Cr4+:YAG crystal has been widely used as the gain medium for tunable solid-state lasers in the near infrared region and as the saturable absorber medium for passively Q-switched lasers due to its large pump absorption cross-section [1,2]
To improve the core uniformity of small-core crystal fiber, we propose, for the first time, to use the thermal capacitance effect in a similar way to suppress the power fluctuation of heating laser by incorporating a sapphire tube in the co-drawing LHPG (CDLHPG) system
For the case of the 10-μm core, the core variation was reduced from 58% to 17% by using the sapphire tube in the growth system

Summary

Introduction

Cr4+:YAG crystal has been widely used as the gain medium for tunable solid-state lasers in the near infrared region and as the saturable absorber medium for passively Q-switched lasers due to its large pump absorption cross-section [1,2]. A double-clad crystal fiber (DCF) structure is formed with the formation of an inner cladding layer made of fused silica and YAG mixture. To improve the core uniformity of small-core crystal fiber, we propose, for the first time, to use the thermal capacitance effect in a similar way to suppress the power fluctuation of heating laser by incorporating a sapphire tube in the CDLHPG system. A few hundred microwatts of ASE was obtained by a 10-μm-core and 6.4-cm-long Cr4+:YAG DCF Such broadband light source with a 3-dB bandwidth of 265 nm is useful for optical fiber communications as well as optical coherence tomography. This technique can be applied to the growth of other materials for better uniformity

Sapphire tube assisted CDLHPG fiber growth process

Core uniformity and propagation loss

ASE generation

Coupling efficiency with SMF28

Conclusion