Ultrafast Fiber Lasers
Frequently Asked Questions
Ultrashort fiber lasers are mode-locked lasers that generate light pulses. The phases of the different modes present in the cavity are fixed or “locked” such that these waves interfere constructively in a periodic way, thus, giving rise to peaks in intensity known as pulses. These seed pulses can then be amplified through one or more stages to generate a train of amplified ultrafast laser pulses.
Ultrafast fiber lasers are characterized by large amplification bandwidth which allows generating very short pulses of down to 100 fs in duration. Their compact construction offers robustness, ease of maintenance and resilience against thermal fluctuations. Also important to note is the exceptional beam quality offered by ultrafast fiber amplifiers.
Mode locking is a technique widely used in ultrafast lasers. The modes of a laser (standing waves) are characterized by random phases. When they interfere with each other they generate a continuous beam. However, by fixing the phase difference between the different modes, they start interfering constructively periodically which results in sharp peaks of intensity or what is known as a pulse.
There are 2 types of mode-locking devices: active and passive. Active mode-locking can be achieved using acousto-optic modulators, electro-optic modulators, or integrated optic modulators. On the other hand, passive mode-locking can be implemented using a saturable absorber. In most cases, this component is a saturable absorber mirror.
The output of a CW laser is a continuous wave radiation, whereas ultrashort lasers generate a train of ultrashort pulses separated from each other by a delay defined by their repetition rate. Ultrashort lasers contain within their cavities a mode-locking device that can be active (optical modulator) or passive (saturable absorber). In the absence of this device, the laser light coming out of the cavity would be continuous.
In the laser industry, the Strehl ratio is a measure of the beam quality, and it is the ratio of the peak irradiance to the peak irradiance of an ideal laser. A laser with a 0.8 Strehl ratio is considered a pretty decent beam quality.
In recent years, ultrafast fiber lasers have made into the domain of laser medicine. The pulse duration of ultrashort lasers allows for very short exposure times. This protects tissue from high energy levels which could otherwise cause thermal damage. The laser-tissue interaction with ultrashort pulses is much less thermal in nature and this allows for fast recovery and minimally invasive approach. While the output characteristics of ultrafast regenerative and fiber amplifiers are similar, the latter allows for much more compact construction, robustness and reliability – characteristics that are essential for surgical applications.
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