| Home / lib / E_Engineering / EO_Optical devices / | ||
Fermann, Galvanauskas, Sucha (eds.). Ultrafast Lasers.. Technology and Applications (M.Dekker2002)(ISBN 0824708415)(797s)_EO_.pdf |
|
Size 11.4Mb Date Apr 28, 2005 |
Table 1.
ML technique l0 Pav,out 600 mW 80.5 MHz 300 mW Highly chirped 1.4 ps output pulses, externally compressed down to 200 fs 250 MHz frep 814 nm 1.4 ps 150 fs tp Remarks Ref....
Dispersive SESAMs Negative dispersion can also be obtained from semiconductor saturable absorber mirrors (SESAMs) (see Sec. 1.4.3) with specially modified designs. The simplest option is to use a GTI-like structure (see earlier in this section) (Kopf et al., 1996a). A double-chirped dispersive semiconductor mirror has also been demonstrated (Paschotta et al., 1999b), and a saturable absorber could be integrated into such a device....
where g is the power gain at the center frequency, M is the modulation strength (2M ¼ peak-to-peak variation of power transmission), fm is the modulation frequency, and Dfg is the FWHM gain bandwidth. The peak gain g is slightly higher than the cavity loss l (for minimum modulator loss) according to...
which is a reasonable approximation in many cases. The response is then characterized by the parameter Isat, called the saturation intensity, and the unsaturated loss q0. For pulses with a given peak intensity Ip, an average value of q can be calculated that represents the effective loss for the pulse. Figure 2 shows this quantity as a function of the normalized peak intensity for Gaussian and solition (sech2) pulses, together with qðIp Þ. We see that the pulse form has little influence on the average loss. The behavior of a slow saturable absorber is described by the differential equation
*Note that some authors define q as the amplitude (instead of intensity) loss coefficient, which is one-half the value used here....
For ultrashort pulses with multigigahertz repetition rate, laser designs are required that are very different from those described in the previous sections. We discussed such designs with up to 77 GHz repetition rate in Section 1.4.3. Another type of laser in this domain is described in the next section. 1.5.5 Passively Modelocked Optically Pumped S e m i c on d u c t or L a s e r s...
2.1 INTRODUCTION Soon after the first demonstration of ultrashort pulses, it appeared that although the pulse duration was very short, its peak power was still in the kilowatt range, too small to be used in most nonlinear optics experiments. Amplifiers were designed to increase the energy of the pulse, and multigigawatt pulses were obtained in 1982 (Fork et al., 1982; Migus et al., 1982). At that time, femtosecond lasers were based on dyes, and the pulses were amplified by simply sending them into a series of dye cells pumped by a nanosecond green laser. The efficiency was in general below 1%, and the pulse was hidden in a large amplified spontaneous emission (ASE) pedestal. In the meantime people were trying to decrease the duration of the pulse produced by a large-scale Nd:glass system. The main difficulties were linked to damage in the amplifier chain due to the very high peak power of the amplified short pulses. A new era started when Mourou and coworkers introduced the concept of chirped pulse amplification in optics (Strickland and Mourou, 1985; Maine et al., 1988). This idea had been developed during World War II for radar but had never been applied to lasers. The peak power of short pulses quickly increased to the terawatt level. The final stage of this evolution was reached when femtosecond pulses were obtained in Ti:sapphire (Spence et al., 1991), opening the way for ultrashort and ultra-intense pulses. The first amplification of 100 fs pulses in a solid-state amplifier (Squier et al., 1991) and the first demonstration of a kilohertz repetition rate amplifier (Salin et al., 1991) followed this discovery by only a few months. Although great progress has been made in the performance of these systems (see Fig. 1), nowadays high-intensity laser chains are still based on concepts developed
61...
This very simple formula is valid for only small input fluences. As soon as the input fluence reaches a level comparable to that of the saturation fluence Jsat of the amplifier, one must use a more complicated expression derived by Frantz and Nodvick (1963) that is valid only for a four-level system: Jout ¼ Eout =S ¼ Jsat log½g0 ½expðJin =Jsat Þ À 1 þ 1 ð 4Þ...
Table 1 Saturation Fluence of Typical Amplifier Media Used in Short-Pulse Amplification Amplifier medium Dyes Excimers Nd:YAG Ti:Al2O3 Cr:LiSAF Nd:glass Yb:glass Alexandrite Jsat $1 mJ=cm2 $1 mJ=cm2 0.5 J=cm2 1 J=cm2 at 800 nm 5 J=cm2 at 830 nm 5 J=cm2 100 J=cm2 22 J=cm2...
Figure 8 shows an example of the gain curve of Ti:sapphire as well as the spectrum of a 10 fs pulse. The 65 nm spectrum of this very short pulse seems to fit easily in the gain curve, but a simple calculation shows that this spectrum is reduced to a mere 32 nm after amplification by a factor of 106. Indeed, the actual gain of the amplifier (ratio between the output and the input) is not the parameter of importance. Gain narrowing is related to the total gain delivered by the amplifier even if part of this gain has been used to cancel the optical loss of the amplifier. The total gain of a regenerative amplifier with a 10% loss per pass and a small signal gain of 2 is typically 107 for an amplification factor of 106. In that sense, multipass amplifiers have higher gain per pass and lower loss and do exhibit lower gain narrowing than regenerative amplifiers for the same amplification factor. Because gain narrowing was a severe limitation to the amplification of ultrashort pulses, several solutions have been proposed. The basic idea is to flatten the gain curve. By far the most effective way is to include in the amplifier a filter whose minimum transmission corresponds to the maximum of the amplifier gain curve (including the mirror’s reflectivity curve) and whose spectral shape resembles that of the gain. Different setups have been proposed and successfully implemented (Barty et al., 1996). Ultrabroad spectra have been obtained using thin air gap Fabry-Perot interferometers (see Fig. 9). The main difficulty is to stabilize these etalons against thermal drift. A more robust solution consists of a thin birefringent filter (Bagnoud...
and Salin, 2000). These filters are easily implemented in regenerative amplifiers, and their use has been called regenerative gain filtering. This technique has made it possible to produce spectra as broad as 120 nm, and further improvement is now limited by the bandwidth of mirrors and polarizers....
| © 2007 eKnigu | ||