Diffraction gratings can be used as the critical optical element used to stretch and recompress pulses in the application of chirped pulse amplification. The most popular example of chirped pulse amplification is described generically in the diagram below. Titanium Sapphire lasers can produce <50 fs laser pulses that if amplified in this form would damage the optics and electro-optics of the amplifier. By stretching the pulses the peak power (energy per second) is reduced and the limitations of amplification can be reduced. Clearly the benefits of ultrashort pulses need to be reclaimed and the pulse compression process allows the amplified laser pulse to be reduced in temporal width close to its original value.

The diffraction grating is the critical element for this process. The diffraction grating disperses the light such that the component colors of the Titanium Saphire laser output are delayed relative to each other; the pulse becomes "chirped". The recompression process is the final manipulation of the beam before it becomes incident on its targeted application. It is this process that ultimately determines the spatial mode of the beam and also the final output energy of the pulse.

Features and Benefits

Most chirped pulse amplifiers are limited by the efficiency of the grating in the compressor and not by the grating in the stretcher. Thus it may be important to specify pulse stretcher or pulse compressor when requesting a quotation for a grating. Newport Corporation will base many quotations on the tightness of the specifications and in general terms gratings of higher efficiency are more difficult to manufacture.

Output efficiency of a given compressor is typically based on a 4-pass geometry. Therefore if the efficiency of a grating in one pass is 90% then the efficiency of the compressor itself cannot be higher than 65.6%.

The gratings in a CPA system are used in specrograph mode. The incident angle is fixed. On request Newport Corporation will add efficiency measurements to your grating for left, center, and right positions of the grating over a wavelength range of choice at a given fixed incidence angle. Efficiency measurements allow the customer to sort gratings accordingly to ensure that the best grating can be used in the compressor to enable maximum efficiency from your CPA.

Master MR 136 is a unique offering for 1200 g/mm grating compressors. The efficiency stays very constant as a function of wavelength with a fixed incident angle geometry. Most holographic gratings with this groove frequency cannot fundamentally provide this feature. Therefore the output pulsewidth of the compressor may not be optimized, reducing peak-power. MR136 is a recommended grating for 1200 g/mm compression systems.

Wavefront of the grating is very important. Poor wavefront can be caused by imperfect groove parallelism/ linearity (master grating dependent) or by poor quality substrates (substrate dependent) Newport gratings come with a standard ?/4 P-V specification over the clear aperture of the grating. Phase Measuring Interferometer plots can be provided on request for your gratings. Better wavefronts lead to better spatial mode qualities which, for example, lead to better focusing and hence increased efficiency in the optical parametric amplification (OPA).

Test data for efficiency and wavefront on your garting allows easy trouble-shooting when power drops in your CPA. Remember that CPA's are complicated systems with many different sources of misalignment/low output power. To be able to rule out specific optics as a source of poor performance is truly valuable for system maintenance and time/resource management.

Scratches and digs contribute to poor mode quality for chirped pulse amplifiers. We will not ship a grating unless it passes our strict and tight inspection standards. Please ask for a copy of this standard for CPA gratings. Ruling streaks may occur in some of our larger ruled gratings used for this application. Ruling streaks are unfortunately a fact of life in the ruling of any large diffraction grating master, and once on the master will be transferred to each full-size replica.

Size is critical in terms of damage threshold for very high intensity lasers. All diffraction gratings when exposed to a high enough level of photon flux will be damaged. Thus by increasing the surface area the photon flux can be reduced. This is a perfect solution if the diffraction grating can be scaled up accordingly. Newport Corporation offers large holographic gratings of up to 102 mm x 140 mm (e.g.5338) clear aperture and large ruled gratings (e.g. MR136) of up to 156 x 208 mm in clear aperture. Of course any size up to these maximum ruled areas can be accommodated and there is no penalty in wavefront for selecting a smaller grating cut down from a larger submaster tool.

Gold coated replica gratings typically offer high efficiency over a broad range of wavelengths. A fourier transform limited pulse will have its temporal width determined by the bandwidth of the compressor. The broader the wavelength range of high efficiency, the shorter the pulse! Gold coated gratings are the only option for ultrashort Titanium Sapphire pulse compression systems.

Gratings carrying Rowland ghosts may affect the temporal and spatial mode output of your compressor. All holographic gratings are free of Rowland Ghosts. MR136 is free of Rowland ghosts that would affect the compressor output.

Grating-to-grating consistency is imperative in a multi grating compression system. Master gratings cannot be recorded with the same consistency that is inherent to replicas taken from the same master grating. Even excelent tolerancing in the master ruling process cannot remove groove frequency variation from grating to grating and for this reason we recommend the use of replica gratings for CPA's in general.