Adjustable Diffractive Filter for High Power Lasers

Adjustable narrow band holographic reflective filter with the absolute diffraction efficiency about 95% has been recorded in the volume of photo-thermo-refractive glass Attenuation up to 20 times of transmitted beam was observed. Advantages of holographic filters in inorganic glass for laser power attenuation are discussed. © 2003 Optical Society of America OCIS codes: (230.1480) Bragg reflectors, (050.7330) Volume holographic gratings Bragg gratings are the key components in various optical systems applied in optical communication, lasers, medicine, aerospace, etc., because of their high spectral selectivity. To realize all advantages of volume holograms, it is necessary to use the materials allowing an achievement of high diffraction efficiency coupled with low optical losses. These properties are demonstrated in optical fibers where Bragg gratings are used for the multiplexing of closely spaced spectral channels (DWDM technology). They are fabricated by the UV irradiation of fiber through a phase mask or by interference patterns [1]. However, this process requires large dosage of UV irradiation, and it is restricted by one-dimensional geometry. The advantages of Bragg gratings recorded in photo-thermo-refractive (PTR) glass are very low dosage of UV exposure in the range of hundreds of millijoules, real three-dimensional geometry of grating, very high stability, i.e. heating up to 400°C and illumination by optical radiation do not degrade their characteristics. High efficient transmission gratings with efficiency more than 95%have been recorded in PTR glass [2]. An important feature of diffractive elements in PTR glass is their high laser-induced damage threshold that is equal to 10 J/cm for I ns pulses of Nd laser at 1058 nm [3]. Mathematical modeling of different configurations of transmitting and reflecting Bragg gratings has shown that the optimal approach is the use of Bragg mirror having narrow spectral selectivity and wide angular selectivity. The last Figure 1. Beam profiles of original Nd:Yag laser (a) and attenuated on 85% by Bragg mirror a b feature of the adjustable filter enables the uniformity of attenuation both in near and far fields. The multicomponent silicate photosensitive glass was used as a media for Bragg grating recording [2]. The Bragg grating formation is a two-step process: the UV irradiation followed by thermal treatment. Samples were exposed to the interference pattern of a He-Cd laser at 325 nm with corresponding spatial frequency. The narrow band holographic mirrors were recorded for visible and IR regions. The high efficient Bragg mirrors were recorded and tested in visible (633 nm) and IR region (1064 nm ─1100 nm). The beam profile of attenuated laser beam was not distorted by Bragg mirrors. Figure 1 presents the beam profile of original Nd:Yag laser and attenuated beam when 85% of incident beam reflected out by Bragg mirror. The spectral selectivity of less then 0.6 nm FWHM of recorded mirrors was measured at tunable Yb:doped fiber glass laser (~1100 nm). The transmittance function of Bragg mirror is shown on figure 2 versus incident angels. Wide acceptance angels ~4°of Bragg mirror allow its usage for applications where laser beam quality is very poor i.e. laser radars, high power lasers and etc. The stability of high efficient Bragg gratings was tested at 100 kW/cm at 1100 nm by CW laser. No heating of sample or decaying of diffraction efficiency and shifting of the Bragg angle were observed. Thus, the robust and compact adjustable filter based on three-dimensional Bragg grating recorded in PTR glass is created. Such filter has unique for the holographical element properties and could serve not only as adjustable filter but as narrow band filter as well.