Active tapered double-clad fiber with low birefringence

Area of Science:

• Optics and Photonics
• Fiber Lasers
• Materials Science

Background:

• High-power fiber amplifiers are crucial for various applications.
• State of polarization (SOP) drift due to heating under intense cladding pumping is a significant challenge in active tapered fibers.
• Existing polarization-maintaining (PANDA) and non-PM fibers exhibit birefringence dependent on pump power, leading to SOP instability.

Purpose of the Study:

• To investigate and mitigate the state of polarization (SOP) drift in active tapered double-clad fibers caused by thermal effects during high-intensity cladding pumping.
• To develop and characterize a novel active double-clad fiber with low intrinsic birefringence for improved polarization stability.
• To demonstrate the performance of this new fiber in a high-power fiber amplifier system.

Main Methods:

• Experimental investigation of SOP and degree of polarization (DOP) variations in PANDA-type and regular non-PM Yb-doped tapered double-clad fibers under varying pump power.
• Fabrication and characterization of an Yb-doped spun tapered double-clad fiber (sT-DCF) with ultra-low intrinsic birefringence (1.45×10⁻⁸).
• Evaluation of sT-DCF performance in an all-fiber picosecond master-oscillator power-amplifier (MOPA) system.

Main Results:

• Birefringence in active fibers was found to be highly dependent on launched pump power.
• The novel sT-DCF demonstrated significantly more stable SOP and DOP compared to PANDA-type and non-PM tapered fibers as pump power increased.
• SOP drift in sT-DCF was reduced by nearly an order of magnitude, while DOP drift was comparable to PANDA-type fibers and an order of magnitude better than non-PM fibers.
• The sT-DCF MOPA system achieved 50 W average power, 34 dB gain, and delivered 50 ps pulses at 1040 nm with excellent beam quality.

Conclusions:

• Active double-clad fibers with low intrinsic birefringence, such as the developed sT-DCF, effectively suppress SOP drift caused by thermal effects in high-power cladding-pumped systems.
• The spun tapered double-clad fiber offers a promising solution for stable, high-power fiber amplification.
• The demonstrated sT-DCF based MOPA system highlights the practical viability of this approach for generating high-quality, high-power picosecond pulses.