Simultaneous Measurements Of Gain And Spontaneous Emission Noise In Rf Excited CO 2 Amplifying Media

Abstract

SIMULTANEOUS MEASUREMENTS OF GAIN AND SPONTANEOUS EMISSION NOISE INRF EXCITED CO2 AMPLIFYING MEDIAP E Jackson and D R HallDepartment of Applied Physics, University of Hull, UKR M Jenkins and J M VaughanRoyal Signals and Radar Establishment, Malvern, UK.AbstractWe report the results of studies intended to characterise the amplifying behaviour ofrf- excited CO2 discharge modules in the context of assessing their potential application asoptical pre -amplifiers. The dependences of the small -signal gain coefficient and theamplified spontaneous emission noise power on rf input power density have been measuredsimultaneously for various gas mixtures and pressures.IntroductionThere is much current interest in the performance of optical amplifiers for CO2 lidarapplications (1,2,3), where a weak return signal would benefit from pre- detectionamplification with an increase in signal -to -noise ratio. A low noise, high gain, widebandoptical pre -amplifier may revive direct detection techniques and enhance optical heterodynetechniques.One of the fundamental processes in an amplifying medium is the spontaneous emissionof radiation by the molecules in the discharge volume. The higher the gain of the activemedium, the higher (other conditions being equal) the spontaneous emission power (2).Furthermore, the spontaneous emission power increases rapidly with increasing amplifierbandwidth (higher gas pressures), while the available gain decreases (2,3). Consequentlythere are conflicting requirements in the design of optical amplifiers. There are howeveroptimum operating conditions for particular applications and noise reduction systems (4)which optimise the amplifier signal -to -noise ratio by taking advantage of the non -coherenceof the spontaneous emission noise, its random polarisation and its broad spectralcomposition.In this paper we describe an experimental technique developed to characterise theamplifying behaviour of rf- excited CO2 discharge modules. In particular the dependence ofthe small -signal gain coefficient and amplified spontaneous emission (ASE) noise power ongas mixture and pressure, rf excitation frequency and power density are beingsimultaneously measured for various discharge geometries (both waveguide and non -waveguide), as part of a continuing activity in optical remote sensing, ranging,velocimetry and optical radar.ExperimentalThe experimental apparatus is shown schematically in figure 1. The linearly polarisedprobe beam was derived from a GTE Sylvania 948 CO2 laser in which a stable discharge in 8torr, 3He:1N2:1CO2 was maintained at -12 kV and - 5 mA. Analysis of the output spectrumshowed that several rotational lines near 1O.6Ft.m would oscillate. The laser could betuned to oscillate on a single rotational line by varying the cavity length with a piezo-electric transducer (PZT), while intra -line stability was achieved with a Lansing lock -instabiliser. This applied a dither bias signal at 512 Hz to the PZT and continuouslyadjusted the PZT DC bias for maximum laser output power monitored by phase sensitivedetection with a pyro- electric detector in a feed -back configuration. Long term stabilitywas aided by using a heat exchanger in a closed circuit water cooling system to maintainthe quartz laser cavity tube at 15 °C. Measurements of the DC loop gain and the drift inthe voltage applied to the PZT to maintain lock -in stabilisation over a 1 Hr periodsuggested a long term stability of 3 MHz or 1:108 while a power stability of 1:32 over 30min was measured with a Coherent power meter. Scanning- rotating mirror beam profilemeasurements revealed a high TEM00 mode content. The unoptimised gas mixture gave anoutput power of typically 2 -4 W cw.The frequency stability of the probe laser is of prime importance when measuring thegain characteristics of CO2 amplifiers because of the strong frequency dependence of gainwhich is especially prevalent in low pressure discharges. For example, a discharge in a3He:1CO2:1N2 gas mixture is pressure broadened at a rate of - 5 MHz /torr limited to aminimum Doppler broadened value of -50 MHz. Hence an unstabilised Doppler broadened probelaser beam amplified in a low pressure discharge may experience a detuned gain diminished