Optical Phase Conjugation Technique for Laser Beam Delivery System

Phase Conjugate MirrorThe Laser Science & Technology Centre (LASTEC) , Delhi, has developed a Phase Conjugate (PC) Mirror via stimulated brillouin scattering (SBS) technique employing carbon disulphide as the nonlinear medium, under its project for Development of Optical Phase Conjugation Technique for Laser Beam Delivery System. This PC mirror gives a reflectivity of 60- 70 per cent at a wavelength of 1.06 Ám with a pump beam energy of 200 mJ in 12.5 ns pulse width. The reflectivity of this mirror increases with the pump beam energy. However, the fidelity of PC wave (which gives the quality of wavefront reversal) in the above mirror degraded with the increase in pump beam energy .This drop in fidelity at higher energies is due to the onset of other nonlinear phenomena, e.g., optical breakdown, thermal heating and self-focussing etc., which disrupts the SBS process.

To overcome this problem, the development of two-cell scheme was carried out, Original beam, Distorted beam, SBS corrected beamwherein one cell acts like an
oscillator and other like an amplifier. The leading edge of the  pump beam initially generates a PC wave from the SBS oscillator which then interferes in the SBS amplifier cell with the input pump beam and scatters the energy in the backward direction and restricts the energy going into the oscillator. With this technique, the original PC beam is generated in the oscillator cell at low energy thus ensuring high fidelity which is then amplified in the amplifier cell. By employing two cells in the SBS-PCM, the PC fidelity is improved from 40-70percent@200mJ pump energy.

During the propagation of high energy laser beam through the atmosphere, the turbulent nature of atmosphere distorts the laser beam wavefront, which in turn reduces the power density reaching onto the target. Non-linear optical phase conjugation technique based on SHS is currently under investigation around the world to auto point the high energy laser beam onto stationary and moving targets in real time and also to simultaneously correct aberrations generated by the turbulent atmosphere. The distortions introduced by the medium can be corrected and the properties of the original beam of high optical quality can be preserved by generating the phase conjugate replica of the distorted wavefront, which faithfully carries all the distortions introduced by the medium but in reverse sense and exactly retraces its path in the opposite direction. While returning through the medium, the phase conjugate wave compensates the distortion originally produced and the beam reaching the target is thus free from degradation. These non-linear optical techniques can thus dispense with a sophisticated laser beam delivery mechanism.