Ohd-Rike Spectrometer Using Pulsed Lasers: a Step Towards Quantum Limited Detection

The detailed design, construction and utilization of a sensitive spectrometer which utilizes the high peak power available with pulsed lasers is described. The spectrometer runs under the control of computer through fast CAMAC interface. The design of spectrometer and advanced development of the technique is original to this laboratory. The spectrometer is built on Optically Heterodyne version of RIKE (Raman Induced Kerr Effect) which is a non-linear polarization technique. In this experiment one highly polarized pulsed probe beam is overlapped with high intensity and circularly polarized pump beam in a non-linear medium. The non-linearity (chi ^{(3)}, chi^ {(5)}etc.) created in the medium induces birefringence that generate a polarization component of probe laser which is orthogonal to the original polarization. The magnitude of birefringence peaks when difference between pump and probe frequency is tuned to the resonance of the medium. The orthogonal component is transmitted through the analyzing polarizer which is adjusted to block the original polarization of the probe beam. The transmitted intensity (signal) is detected by the semi-conductor detector. Signal levels of the order of 10^{-8} of the probe laser intensity can be detected. To enhance the strength of weak signal, the signal heterodyne with local oscillator at the detector. With optimum value of the local oscillator intensity the major sources of noise at the detector are: (a) Quantum noise in the detector; (b) Pulse-to-Pulse fluctuations in the local oscillator intensity. The fluctuation noise at the detector is removed on pulse-to-pulse basis when a twin reference beam which fluctuate in phase and by the same amount as the local oscillator is subtracted from the net signal at the detector. The high peak power of pulsed laser and heterodyning increase the signal. The only noise at the detector is the quantum noise. Thus, the signal to noise ratio which measures the sensitivity is greatly enhanced. The additional problems which arise because of high intensity and extremely low level (10^ {-8}) of polarization extinction of the special purpose polarizers employed in the experiment, are discussed. Low pressure and high resolution spectrums of commonly available gases are recorded. The effect of above saturating laser fields on OHD-RIKE signal and lineshape is determined. Work also describes the computer controlled precision tuning of pulsed dye laser. This laser is demonstrated to be continuously tunable with resolution ~ 0.003^circA ( ~0.013cm^{-1} ) over ~10^ circA (~40cm ^{-1}) and has capability of such tuning over the whole dye curve. Design construction and computer control through fast CAMAC interface of high speed pulsed wavemeter which is developed from Fabry-Perot etalons is also described. The accuracy of this device is few parts in 10^8 . This device is used to monitor the tuning of pulsed (as well as CW) lasers. The computing time for absolute wave-number is less than the inter pulse period of 10 Hz repetition rate lasers. The overall experimental number of few parts in 10^8 about precision and sensitivity in this spectrometer make this experiment very difficult to practice. It is a tough experiment.