Optical pulse compression

The idea of spectral broadening of optical pulses by self phase-modulation in optical fibers with subsequent pulse compression represented a breakthrough for achieving pulse widths of only a few femtoseconds. The method is based on the following principle  

When an optical pulse with the spectral amplitude distribution E(a ) propagates through a medium with refractive index n(o ), its time profile will change because the group velocity 

For d w/dk 0 the velocity differs for the different frequency components of the pulse which means that the shape of the pulse will change during its propagation through the medium (Fig. 11.20a). For negative dispersion (dn/dA 0), for example, the red wavelengths have a larger velocity than the blue ones, i.e. the pulse becomes spatially broader. 

If the optical pulse of a mode-locked laser is focussed into an optical fiber, the intensity I becomes very high. The amplitude of the forced oscillations which the electrons in the fiber material perform under the influence of the optical field, increases with the field amplitude and the refractive index becomes intensity dependent 

The linear dispersion no(A) causes a spatial broadening, the intensity-dependent refractive index nglO) a spectral broadening. The spatial broadening of the pulse (which corresponds to a broadening of its time profile) is proportional to the length of the fiber and depends on the spectral width Aw of the pulse and on its intensity. 

For the time profile of a pulse with half-width r(L) after travelling over a path-length L through the dispersive medium one obtains from (6.15) 

Is the dispersion length. After a pathlength L = the pulse-width has broadened 

A quantitative description starts from the wave equation for the pulse envelope [11.56,11.57] 

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Treacy E B 1969 Optical pulse compression with diffraction gratings IEEE J. Quantum. Electron. 5 454-8... 

To carry out a spectroscopy, that is the structural and dynamical determination, of elementary processes in real time at a molecular level necessitates the application of laser pulses with durations of tens, or at most hundreds, of femtoseconds to resolve in time the molecular motions. Sub-100 fs laser pulses were realised for the first time from a colliding-pulse mode-locked dye laser in the early 1980s at AT T Bell Laboratories by Shank and coworkers by 1987 these researchers had succeeded in producing record-breaking pulses as short as 6fs by optical pulse compression of the output of mode-locked dye laser. In the decade since 1987 there has only been a slight improvement in the minimum possible pulse width, but there have been truly major developments in the ease of generating and characterising ultrashort laser pulses. 

Another method uses optical pulse compression in optical fibers, where the intensity-dependent part of the refractive index causes a frequency chirp and increases the spectral profile and the time duration of the pulse. Subsequent pulse compression, achieved with a pair of optical gratings or by using prisms, leads to a drastic shortening of the pulse. 

E. B. Treacy, Optical pulse compression with diffraction gratings. IEEE J. Quantum Electron. 5, 454 (1969)... 

In this brief description of contemporary approaches to ultrafast spectroscopy, we will focus specifically on some of the newer techniques which can produce tunable pump and probe pulses of duration 10 s Linear and nonlinear optical pulse compression... 

Fork R L, Brito Cruz C H, Becker P C and Shank C V 1987 Compression of optical pulses to six femtoseconds by using cubic phase compensation Qpt. Lett. 12 483-5... 

Kuhl J and Heppner J 1986 Compression of femtosecond optical pulses with dielectric multilayer interferometers IEEE J. Quantum. Electron. 22 182-5... 

This technique will allow compression of a 100-femtosecond pulse down to 12 femtoseconds or even to 8 femtoseconds. (A femtosecond is a millionth of a billionth of a second or 1 x 10-15 s.) Pulse compression can be used to study chemical reactions, particularly intermediate states, at very high speeds. Alternatively, these optical pulses can be converted to electrical pulses to study electrical phenomena. This aspect, of course, is of great interest to people in the electronics industry because of their concern with the operation of high-speed electronic devices. It also is of great interest to people who are trying to understand the motion of biological objects such as bacteria. 

P. Loza-Alvarez, D.T. Reid, P. Faller, M. Ebrahimzadeh, W. Sibbett, H. Karlsson, and F. Laurell, Simultaneous femtosecond-pulse compression and second-harmonic generation in aperiodically poled KTiOP04, Optics Letters 24(15), 1071-1073 (1999). 

M.A. Arbore, O. Marco, and M.M. Fejer, Pulse compression during second-harmonic generation in aperiodic quasi-phase-matching gratings. Optics Letters 22(12), 865-867 (1997). 

Khaydarov, J. D., Andrews, J. H., and Singer, K. D. 1994. Pulse compression in a synchronously pumped optical parametric oscillator from group-velocity mismatch. Opt. Lett. 19 831-33. 

Verluise, R, Laude, V., Cheng, Z., Spiehnann, C., and Toumois, P. 2000. Amplitude and phase control of ultrashort pulses by use of an acousto-optic programmable dispersive filter Pulse compression and shaping. Opt. Lett. 25(8) 575-77. 

D. Strickland, G. Mourou, Compression of amplified chirped optical pulses, Optics Communications 56, 219 (1985)... 

Pulse compression in optical fibers to remove distortion (Na, Cs)... 

In conclusion, it is worth reiterating that the anomalous absorption effects described here may be manifest in any experiments that employ sufficiently high-intensity broadband radiation. To this extent, anomalies may be observable in experiments not specifically involving USES light. In particular, the continued advances in techniques of laser pulse compression have now resulted in the production of femtosecond pulses only a few optical cycles in duration (Knox et al. 1985 Brito Cruz et al. 1987 Fork et al. 1987) which necessarily have a very broad frequency spread, as the time/energy uncertainty principle shows. Thus, mean-frequency absorption may have a wider role to play in the absorption of femtosecond pulses. If this is correct, it raises further questions over the suitablity of absorption-based techniques for their characterization. 

Until recently, the pulses used in those experiments were the shortest optical pulses characterized. The transition-state spectroscopy of Zewail and Bernstein [16,17,18,19, 20,21 and 22] exploited an amplified CPM laser after frequency doubling and/or continuum generation. The chemical systems that were most easily studied, however, were those that could be stimulated either by the 620 nm output of the CPM directly or after frequency doubling to 310 nm. In addition, the CPM laser and its contemporary, more tunable alternative, the pulse-compressed, synchronously pumped dye laser [H], were tools that could be effectively used only by researchers with extensive backgrounds in lasers and optics. 

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