This chapter takes the readers from the modest beginning of light pulses of millisecond duration emitted by a ruby laser to ultrafast lasers that are capable of twinkling light for barely a million billionth of a second, i.e., 10⁻¹⁵ s or a femtosecond. A deeper understanding of laser dynamics and cavity physics in conjunction with stimulated emission has led to the conceptualization of techniques such as Q-switching, cavity dumping, modelocking, and colloidal pulse modelocking. The implementation of these techniques in the operation of a laser has allowed pulse compression to gradually progress through micro-, nano-, and up to the point of femtosecond and even beyond. A flash of light lasting for about a femtosecond may appear to be ridiculously inconsequential, but it can, as a matter of fact, accomplish seemingly unthinkable tasks. It can function as a scalpel of extraordinary sharpness, offering a surgeon the luxury of performing a high-precision job involving delicate organs such as the eye and heart or allowing a photochemist to take a snapshot of the intermediate species formed during a chemical reaction defeating the lightning speed of their formation. A femtosecond pulse is a communication engineer’s dream, as it presents seemingly limitless bandwidth for data stuffing. However, the power of the ultrashort pulse straight out of the laser is barely a kilowatt and can hardly be boosted by letting it pass through a light amplifier, as its growing power will trigger irreversible optical damage to the amplifier itself. Chap.9 explains how the implementation of chirped pulse amplification (CPA) technique helps overcome this seemingly insurmountable problem. The femtosecond pulse is first stretched manyfold to bring down its power, allowing its amplification without the risk of destroying any amplifying material, and then compressed back to its original temporal state. This makes it possible for a kilowatt ultrashort light pulse to coolly leap to the level of terawatt or even beyond. The CPA technique basically added a new dimension to the laser technology and led to the emergence of an area of research known in the common parlance as “Extreme Science with Extreme Light.”

1. 1.

   Power, as we know, is the rate at which energy is delivered. Clearly therefore an ultrahigh power laser will be capable of delivering either enormously large energy over a modest length of time or a modest amount of energy over an extremely short duration.

2. 2.

   The diffraction of light into multiple orders as it interacts with an acoustic wave is called Raman-Nath effect [29]. There is yet another kind of acousto-optic modulator called the Bragg modulator [30] that diffracts light only in one direction. The Bragg modulator assumes importance when the light diffracted from the central beam constitutes the laser output as in the case of partial cavity dumping to be covered in a latter section of this chapter.

3. 3.

   There is yet another effect called Kerr effect or quadratic electro-optic effect where the change in refractive index is proportional to the square of the electric field. Kerr effect is, however, generally much weaker than Pockels effect.

4. 4.

   Birefringence signifies a phenomenon where the refractive index of a material becomes dependent of the polarization of light passing through it. Such materials are called birefringent.

5. 5.

   A polarizer, as we have seen earlier in Chap.2, is an optical device that passes light on one polarization and blocks light on an orthogonal polarization.

6. 6.

   The interference of the two counterpropagating waves, although not explicitly mentioned here but inevitable in a bidirectional ring cavity, results in manyfold increase of light intensity at the nodal plane. This, in turn, results in even faster modulation of loss yielding better synchronization, higher stability, and shorter pulses.

7. 7.

   Please note that 100 fs or 100x10⁻¹⁵ second is equivalent to 10⁻¹x10⁻⁶x10⁻⁶ second.

8. 8.

   Certain nonlinear materials, with intensity dependent r.i., behave like a lens and progressively focuses an intense beam of light with nonuniform spatial intensity profile, such as a Gaussian beam, passing through it. Self- focusing is a nonlinear optical phenomenon and will be described in detail in V-II of this book.

9. 9.

   A pulse stretcher basically is a dispersive element like a prism or grating that spreads the constituent colors of the femtosecond pulse spatially. Once the color components are spatially separated, they can be made to travel different distances and thus stretched in time. A compressor is again a dispersive element that negates the effect of stretcher and recombines the individual colors both in time and space. The literature is quite rich on CPA, and the interested readers may refer to the literature for developing a thorough understanding on the working of a pulse stretcher or compressor.

Authors and Affiliations

1. Laser and Plasma Technology Division, Bhabha Atomic Research Centre, Mumbai, India

   Dhruba J. Biswas

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