![]() ![]() At the same time, if the seed pulse from the oscillator is interrupted at the input of an amplifier, Q-switching can result, which invariably leads to optical damage on a fiber end face.Ī mechanism to prevent feedback from entering ultrafast fiber oscillators, and to protect the subsequent chain of amplifiers, is essential. In contrast, solid-state mode-locked oscillators can still operate with many times that amount of feedback. Otherwise, noise that disturbs the spectral content can inhibit mode-locking or even cause Q-switching– the power that is fed back must be smaller than the ASE that pulses build from this often means that an oscillator requires << 60 dB of feedback to remain stably mode-locked. But the oscillator can only remain stable with feedback on the microwatt (μW) level or less. Amplifiers are required to achieve performance targets, and often use large-mode area (LMA) amplifiers producing 1W to even up to more than 100W. Practically speaking, whenever a laser system is designed using a mode-locked fiber oscillator as the pulse source, the oscillator typically provides only mW level output while system requirements can go to the 10s of Watts and beyond. Examples of these designs can be seen in Figures 1 and 2. Two typical configurations are 1) an optical Master Oscillator Power Amplifier (MOPA) scheme for ps pulses and often with hybrid bulk-fiber design, and 2) a Chirped Pulse Amplifier (CPA) scheme for fs pulses. Generating pulses below 10 picoseconds (ps) and into the femtosecond (fs) range, ultrafast laser systems can be designed for a range of operating regimes, depending upon target pulse energies and repetition rates. The relatively smaller size of the beam at the end of a fiber, compared to bulk gain material, leads to higher intensity at the end face, and creates scenarios where fiber systems can reach the damage threshold at lower optical powers. Small signal, single-pass gains of ~ 20 dB or more are typical in optical fibers, as opposed to the relatively lower gain of ~ 5 dB in bulk doped materials. Optical feedback is caused by back-reflections off of downstream optics or by amplified spontaneous emission (ASE) from amplifier stages, and it is amplified by the high gain in doped optical fibers. Because fiber is a high gain medium, any light that is inadvertently injected into the ultrafast oscillator, as well as into amplifiers, can degrade and potentially irreversibly affect system performance by causing instability (at best) or damage (at worst). However, there are design issues inherent to fiber-based oscillators and amplifiers. Rare-earth doped optical fiber has become a more widespread medium for ultrafast laser systems in both fiber-only and hybrid (fiber and bulk) lasers. Different ways of design of Faraday isolator for 1kW average power with isolation ratio about 30 dB are discussed.1 The Effect of Feedback on Femtosecond Fiber Laser SystemsĪ little (optical) feedback can go a long way. Both schemes allow to increase isolation ratio up to 100 times in comparison to the traditional scheme. The polarization distortions which a beam undertakes while passing the first rotator will be partially compensated in the second rotator. The idea of compensating depolarization is to replace one 45 degree Faraday rotator by two 22.5 degrees rotators and a reciprocal optical element between them. In order to suppress the self-induced depolarization tow novel optical schemes was suggested and realized in experiment. It results in spatial nonuniform distribution of temperature giving rise to two effects which reduce the isolation ratio: the temperature dependence of Verdet constant and birefringence ratio is sum of two terms which represent these two effects and the last phenomena is more efficient. The physical reason for the self- induced depolarization is absorption of laser radiation. However, the depolarization can significantly limit the isolation ratio. Although many investigations are devoted to the thermal lens effect, polarization contamination has not been discussed in detail. A lot of laser applications require propagation of extremely high average power radiation through Faraday isolators. ![]()
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