Fiber Lasers and Ultrashort Pulses

Er-Yb all-fiber laser with a repetition rate for ultrashort pulses of 300 MHz

• A short cavity passively mode-locked erbium-ytterbium fiber laser was optimized
• The cavity used a composite active fiber and a hybrid isolator-coupler-multiplexer
• Aerosol-synthesized single-walled carbon nanotubes were placed in the cavity to achieve mode-locking
• Pulse repetition rate increased from 100 to 298 MHz during length optimization
• At the highest repetition rate, pulses were 3.6 ps and average output power was 1.6 mW

Fiber Lasers for Research

• Fiber lasers are reliable tools in medicine, biology, and materials research
• Passive mode-locked (ML) fiber lasers are widely used for ultrashort pulse sources (USPs) in optical ADCs, materials processing, and supercontinuum generation
• Several passive ML strategies exist, including nonlinear polarization rotation, nonlinear loop mirrors, saturable absorbers (graphene, topological insulators, carbon nanotubes, SESAMs)
• Compact fiber lasers with sub-GHz and GHz USP repetition rates are in demand for applications in terahertz radiation sources, broadband telecommunication, high-resolution microscopy, and more
• High repetition rates can be achieved by reducing cavity length or using harmonic mode-locking

Short Cavity Fiber Lasers

• Short cavity lasers use Fabry-Perot (FP) cavities, which can be challenging with standard active fibers due to their low amplification
• Heavily doped phosphate fibers offer higher gain and thus shorter cavity lengths, enabling higher repetition rates
• However, these are susceptible to air degradation and require specialized splicing
• Ring cavities are an alternative to FP cavities, offering better all-fiber design stability, reduced losses, and better connectivity with standard optical components

Composite Erbium-Ytterbium-Doped Fiber (EYDF)

• Employ a phosphate glass core with a high concentration of Er/Yb ions and a silica glass cladding
• The "rod-in-tube" fabrication technique is used
• Improved thermomechanical properties and moisture stability compared to silicate glasses.
• Low content of residual OH-groups

Mode-Locking using SWCNTs

• Polymer-free thin films of aerosol-synthesized single-walled carbon nanotubes (SWCNTs) were used as a saturable absorber
• This avoids the degradation issues of CNTs in polymer matrices
• The composite fiber, hybrid element, and unique SWCNTs were successfully used to achieve a pulse repetition rate exceeding 250 MHz in an all-fiber ring cavity

Experimental Setup and Results

• Laser cavity length reduction from 1.93 m to 0.67 m enabled higher repetition rates
• At 298.5 MHz repetition rate, pulse duration of 2.3 ps, output power 1.64 mW, and a very high signal-to-noise ratio of 70 dB were achieved
• Harmonic mode-locking regimes were observed (e.g., doubled and tripled) but weren't stably measurable
• Different wavelengths (e.g. 1535nm, 1542 nm) were obtained
• The results show a strong potential for compact high-pulse repetition rate ultrashort pulse sources, using these composite fibers along with appropriate components, like the hybrid component, and SWCNTs.

This quiz explores the advancements in erbium-ytterbium fiber lasers, particularly focusing on their optimization for ultrashort pulse generation at high repetition rates. It discusses the technologies used, including mode-locking strategies and materials like carbon nanotubes that enhance laser performance. Suitable for students in optics and laser technology.