Chirped mirrors

Fig. 1. Schematic of experimental setup. %J2 - 800 nm wave-plate SP 2-mm sapphire plate PI, 2 45° quartz prisms P3 69° quartz prism, the distance from P3 to the NOPA crystal is 80 cm CM1, 2 ultrabroadband chirped mirrors GR 300 lines/mm ruled diffraction grating (Jobin Yvon) SM spherical mirror, R=-400 mm BS1, 2 chromium-coated d=0.5 mm quartz beam splitters. SHG crystal 0.4-mm 0=29° BBO (EKSMA) NOPA crystal 1-mm 0=31.5° BBO (Casix) SHG FROG crystal 0=29° BBO wedge plate d=5- -20 pm (EKSMA). Spherical mirrors around NOPA crystal are R=-200 mm Thick arrows on the left indicate the data flow from the pulse diagnostic setup (SHG FROG) and the feedback to the flexible mirror.

The beam was passed through a pulse picker (4). In order to compensate pulse broadening ( chirp ) introduced by the pulse picker and to precompensate that caused by other bulky optics (lenses, polarizers etc.), the beam was passed through a group velocity dispersion compensation line (5), consisting of a pair of Brewster prisms and a mirror [20], The temporal width of the pump and probe pulses was checked with a fringe-resolved... 

Experiments were performed using a visible optical parametric amplifier based on noncollinear phase-matching in /3-barium borate, followed by a pulse compressor using chirped dielectric mirrors. This optical source provides ultrabroadband pulses, with bandwidth extending from 500 to 720 nm, compressed to an almost transform-limited duration of 5-6 fs. The pump-... 

Figure 5. (A) Scheme of two-photon laser scanning microscope (1) Ti Sa laser, 100 fs, 80 MHz, 750-980 nm, 1.6W 800 nm (TSUNAMI, Spectra Physics), (2) pre-chirp, (3) beam multiplexer, (4) scanning mirrors, (5) microscope (Olympus IX 71, XLUMPLFL20XW, WD = 2 mm, NA = 0.95), (6) fluorescent foci in sample, (7) filter wheel/spectrograph (SpectraPro 2300i, Acton Research Corporation)/spectrometer (home built), (8) back illuminated EMCCD camera (IXON BV887ECS-UVB, Andor Technology), (9) dichroic mirror (2P-Beamsplitter 680 DCSPXR, Chroma). (B) Experimental setup of two-photon laser scanning microscope.

The maximum incident pump power was 3.97 W. The folding mirrors, i.e., Ml and M2, had a 100 mm radius of curvature (ROC) and AR-coated below 1000 nm and HR-coated above 1020 nm. The laser beam was focused onto a semiconductor saturable absorption mirror (SESAM) by a concave mirror M3 with ROC = 50 mm. The chirped mirror pair, i.e., CMl and CM2, had a group-delay dispersion of -2000 fs per one round trip. The OC had a 1 % transmittance. 

The development of broadband saturable semiconductor absorber mirrors and of dispersion-engineered chirped multilayer dielectric mirrors has allowed the realization of self-starting ultrashort laser pulses, which routinely reach sub-10 fs pulsewidths and peak powers above the megawatt level. 

Chirped mirror only Bragg wavelength chirped... 

Double-chirped mirror Bragg wavelength and coupling chirped... 

Fig. 6.32 Chirped mirrors (a) Bragg mirror with no chirp (b) simple chirped mirror for one wavelength (c) double-chirped mirror with matching sections to avoid residual reflections [695]...

Instead of varying the thicknesses of the layers, one can also produce chirped mirrors by smoothly altering the indices of refraction and the difference between them (nr - i) (Fig. 6.33). 

These mirrors may be regarded as one-dimensional holograms that are generated when a chirped and an unchirped laser pulse from opposite directions are superimposed in a medium where they generate a refractive index pattern proportional to their total intensity [693]. When a chirped pulse is reflected by such a hologram, it becomes compressed, similar to the situation with phase-conjugated mirrors. 

In practice, such mirrors are produced by evaporation techniques controlled by a corresponding computer program. In Fig. 6.33 the variation of the refractive index for the different dielectric layers is shown for a mirror with negative GDD, and in Fig. 6.34 the reflectivity and the group delay is plotted as a function of wavelength for mirrors with negatively and positively chirped graded-index profiles. In combi-... 

Fig. 6.33 Refractive index profile of a discrete-valued chirped dielectric mirror...

In Fig. 6.34 the measured group delay dispersion (GDD) is shown as realized with a double-chirped mirror, compared with the wanted one. This illustrates that for the spectral range 1000-1200 nm the match is quite good while for X < 1000 nm and X > 1200 nm still large deviations appear [693b, 693c]. 

Another alternative for the generation of ultrafast pulses is the passive mode locking by fast semiconductor saturable absorbers in front of chirped mirrors (Fig. 6.35) in combination with Kerr lens mode locking [694]. The recovery time of the saturable absorber must be generally faster then the laser pulse width. This is provided by KLM, which may be regarded as artificial saturable absorber that is as fast as the Kerr nonlinearity following the laser intensity. Since the recovery time in a semi-... 

Fig. 6.34 Reflectivity R, realized group delay despersion (GDD) and wanted GDD as a function of X for a chirped mirror [693b]...

Fig. 6.36 Setup of the double-Z cavity for an ultrashort pulse Ti sapphire laser. The two prisms Pi and 2 and eight bounces on double-chirped mirrors M2-M6 provide flat dispersion. A second focus in a BK7-plate (P) leads to enhanced SPM, and the laser generates significantly wider spectra [695]...

A combination of saturable semiconductor media in front of a chirped mirror and KLM techniques can be used for reliable operation of sub-10 fs pulses. 

A possible setup for generating the shortest pulses without pulse compression is shown in Fig. 6.36. It consists of five chirped mirrors M2-M6 and two prisms Pi, P2-... 

There are several ways of generating short pulses. One is based on mode-locking lasers which have gain media with a broad special range. While the first experiments relied on dye lasers or Nd YAG-lasers, the Ti Sapphire laser has now become the most attractive choice. Some of the most commonly used materials are listed in Table 6.1 and [756]. Laser pulses down to 4 fs have been demonstrated using Ken-lens mode-locking and chirped mirrors. 

Fig. 11.29. (a) Refractive index profile of a discrete-valued chirped dielectric mirror (b) group delay of chirped mirrors with positively dashed) and negatively solid line) chirped graded index profile. The dotted curve corresponds to the discrete index profile of (a) [11.64]... 

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