![]() When selecting an isolator, there are several criteria to keep in mind: the incident beam size, the incident optical power on the isolator, the required transmitted power for the next stage, and the amount of isolation required.įind the Coherent Faraday Isolator that’s best suited for your requirements or shop online now.ĭispersion is a significant issue for ultrafast lasers, one that can affect the pulse duration and therefore, the peak power, of ultrashort pulses. Containing low absorption, high damage threshold optics, our Faraday Rotators and Isolators are ideally suited for use with average power levels of up to 50W for ultrafast laser systems. A Faraday Isolator protects laser oscillators and laser amplifiers from the deleterious effects of back reflections. When placed between crossed polarizers, a Faraday Rotator becomes a Faraday isolator. The main component in an optical isolator is the Faraday Rotator, which is made of a magneto-optic material with special properties the Faraday Rotator operates by rotating the plane of polarized light 45 degrees in the forward direction and an additional 45 degrees of non-reciprocal rotation in the reverse direction, while maintaining the light’s linear polarization. Fortunately, there is a solution to the problems arising from optical feedback the use of Coherent Optical Isolators.Ģ Using a Faraday Isolator to Protect against FeedbackĪ Faraday Isolator is an optical device that provides high transmission of signal light in the forward direction, but strongly blocks light from the reverse direction. 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. 1 The Effect of Feedback on Femtosecond Fiber Laser SystemsĪ little (optical) feedback can go a long way.
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