Mode-Locked Laser
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Introduction
A laser in which multiple phase-locked longitudinal modes oscillate simultaneously. Unlike a typical CW Laser, a Mode-Locked Laser emits a series of short pulses at a steady repetition rate. In the frequency domain, a mode-locked laser puts out a series of spectral lines separated by the laser's repetition rate, and spanning a range inversely proportional to the (temporal) pulse length. A Frequency Comb Laser is based on the operation of a mode-locked laser with pulse lengths that are short enough that the output spans an octave (in the frequency domain).
Basic Operation
Although a laser is usually thought of as source of a single, well defined, continuous wave, it is not at all difficult to induce multi-mode oscillation in a laser cavity (in fact, great lengths are often taken to avoid this behavior.) What is required is a laser gain medium with a sufficiently broad gain-bandwidth, a means of enforcing pulsed operation (over CW operation), and a mechanism for phase-locking the different longitudinal modes.
Active Medium
As the minimum pulse length of mode-locked laser is limited by the gain-bandwidth of the active medium, good performance can only be obtained for a few materials. The most popular gain medium for mode-locked lasers is Ti:S, due largely to it's magically enormous gain-bandwidth product. Dye Lasers are also used.
Pulsed Operation
As stated above, mode-locking requires an cavity architecture which provides a higher gain for optical pulses than for a CW beam. The most popular mechanisms for enforcing pulsed operation is to employ a gain material that exhibits a Kerr Effect, whereby the material index of refraction increases linearly with the optical intensity. Since the cross-sectional beam (or pulse) profile inside of an optical cavity is Gaussian, the Kerr effect will imprint a Gaussian index profile on the material so that it will behave like a lens, focusing the beam. As this lensing effect will be more pronounced for higher intensities, pulsed operation (which results in higher peak intensities) may be enforced by carefully placing an intracavity aperture -- low intensity (i.e. cw) operation will be prohibited through loss. In addition to providing the laser gain, Ti:S is also used as the Kerr-lens material for almost all mode-locked lasers. Another method of enforcing pulsed operation is to encorporate a saturable absorber into the cavity: since pulsed operation results in high peak intensities (relative to CW operation), the saturable absorber will create less fractional loss for mode-locked operation when compared to cw operation.
Dispersion Compensation
As a pulse propagate through the gain medium it will experience a certain amount of group velocity dispersion, i.e. the spreading of the pulse do to the different phase velocities of the frequency components. For example, Ti:S will generally display normal dispersion (as opposed to anomolous dispersion) such that lower frequencies will travel faster than higher frequencies. In order to compensate for this, a prism pair is typically incorporated into the cavity architecture. One prism acts to spatially separate the pulse's spectral components, while the other prism is positioned such that lower frequencies will pass through a greater optical path length than higher frequencies. If the optical path length difference is carefully tuned (and actively adjusted using a rotatable mirror) the dispersion from the gain medium may be cancelled to third order. In addition to, or instead of, the prism pair compensation, "chirped" dielectric mirrors may be used for the dispersion compensation. The reflective surfaces of these mirrors must be carefully constructed so that they add a frequency dependent phase shift which cancels the dispersion of the gain medium.
Phase-Locking
Phase locking the modes of a pulsed laser requires not only cavity length stabilization, but also active dispersion compensation. In the frequency domain, an overall cavity length change translates into a change in the spacing between successive frequency components and an overall shift. To independently control these two parameters (a requirement for the frequency components to be phase coherent), active control over the cavity length and the dispersion compensation are necessary.
References
S.T. Cundiff and J. Ye, Rev. Mod. Phys., 75, 325 (2003).
