The Ondax PicoPulse™ VHG (volume holographic grating) pulse stretchers/compressors are based on Bragg diffraction from volume gratings where the grating period varies along the length of the component (so-called chirped grating). These VHGs replace conventional fiber gratings and diffraction gratings to provide a more compact and robust solution.

The functionality of VHG chirped pulse compression gratings is identical to that of chirped Fiber Bragg Gratings (FBG) with the important addition that the power handling of VHG chirped gratings is many orders of magnitude higher because of the free space operation allowing a large input beam size.

The principle of operation is the following: an input collimated beam is incident on the PicoPulse™ VHG stretcher/compressor (see figure left). The diffracted beam is counter-propagating (a small angle can be chosen to select the diffracted beam from the input beam). In the figure on the left, the red spectral component of the pulse is diffracted first and the blue spectral component of the pulse is diffracted last at the back-end of the VHG. In a linear chirp VHG, the period is smoothly linearly varying. A single chirped VHG can be used as a stretcher or compressor by simply interchanging the input facet by 180 degrees.

Ondax’s breakthrough technology produces chirped volume grating devices with near perfect beam quality over a wide range of temperature. The figure below illustrates the excellent beam quality of the diffracted beam measured after 1.5 meters of free space propagation at three temperatures, 13, 35 and 60 degrees Celsius. Compared to the input beam size, The PicoPulse&trade introduces less than 0.7% beam change in the X direction and less than 2% in the Y direction. Typically the stretched beam can be coupled into single mode fiber with coupling efficiencies larger than 70%.

The excellent beam quality of the stretched pulse enables high efficiency coupling into a fiber amplifier or near perfect mode matching in a bulk amplifier. The figure below shows the excellent beam quality of a sub-picosecond pulse duration after the steps of pulse stretching, amplification and compression with the PicoPulse&trade chirped VHG.

Ondax started manufacturing VHG chirped gratings for DWDM dispersion compensation in 2001 and has since perfected the manufacturing process of chirped gratings that produces very low spatial chirp across the clear aperture and no "stitching" errors in the chirp. The material is a silicate glass optimized to work with low loss and scatter above 400nm.

Power Handling: the glass material is resistant to average powers in excess of several kW/cm2 at 1064nm and peak power in excess of 170MW/cm2 peak power (fluence of 3.8 J/cm2) at 1064nm.

The FWHM spectral bandwidth Δλ of the VHG stretcher/compressor is related to the length, L, of the VHG and the chirp rate R by Δλ= R*L, where the chirp rate is in units of [nm/mm] and the length in units of [mm]. For example, a 20mm long VHG with a chirp rate of 0.05nm/mm yields a FWHM bandwidth of 1nm. Sometimes, the chirp rate is given in units of [ps/nm]. The conversion between the two units is: R [ps/nm] = (2*n/c)/R[nm/mm], where n is the index of refraction (1.5) and c the speed of light. From the numerical example above the equivalent chirp rate in [ps/nm] is 200. The total delay in [ps] is obtained by multiplying the chirp rate in [ps/nm] by the FWHM spectral bandwidth.

Call 626.357.9600 or e-mail requests to sales@ondax.com.