Patent GB-994210-A (GB)
Title: Optical travelling wave parametric devices
IPC: G02F1/39
Filed: 1963-06-25
Abstract: 994,210. Optical apparatus. INTERNATIONAL BUSINESS MACHINES CORPORATION. June 25, 1963 [June 28,1962], No. 25141/63. Heading G2J. [Also in Division H3] A parametric light amplifier or harmonic generator comprises a non-linear dielectric medium which supports a fundamental optical frequency and desired harmonic frequencies by providing matched wave propagation velocities for the plurality of waves. Harmonic generators. In Fig. 1 and 1a an optical fibre 12 made of a non linear dielectric medium is located at an angle # to light radiation of frequency f1 from an optical source 10, the light being focused by a lens 16 on to one end of the fibre. The harmonic, e.g. f 3  = 2f 1 , is radiated from the other end of the fibre and is detected by 14 after collimation by a lens 18. The fibre 12 is made up of a core 20 and cladding 22, the respective refractive indexes nf,n c being such that n c < nf. Either the core or cladding, or both may be of non-linear dielectric material such as quartz, potassium dl-hydrogen phosphate, tri-glycine sulphate or other similar optical crystal. An alternative arrangement is described, Fig. 5, in which a pair of parallel reflective plates 70, 72 of the Fabry-Perot type are positioned at the ends of a non-linear dielectric medium, 74, the arrangement forming a resonant structure 68 tuned to the fundamental frequency f1. The plate 70 is irradiated by preferably coherent light from a source 76 of frequency f1, and the plate 72 is designed to pass waves of frequency 2f1 = f3 to a suitable detector 78. The dielectric material is chosen to be such that its index of refraction which varies non-linearly with the optical frequency has the same value at frequencies f1 and f3, Fig. 6 (not shown), and the non-linear crystalline axis is arranged such as to minimise the rate of change of the dielectric constant with respect to the electric field intensity produced by the electromagnetic radiations within the medium. Parametric light amplifiers. In Fig. 2 an optical fibre 12 has one end exposed to a weak signal of frequency f1 from an optical source 26 and to a pumping frequency f3 = 2f, from an intense coherent optical source 24, lenses 28, 30 being provided for focusing. The fibre is arranged to be such that a momentum balance is maintained for waves at frequencies f1 and f3 so that mixing of the two frequencies occurs. Consequently an amplified idler frequency f2 is emitted from the other fibre end and applied to detector 14, the frequency relationship being f 1  + f 2  f 3 . As in this arrangement f 3  = 2f 1 , then the amplified idler frequency is f1. It is stated that the desired phase velocities determining the frequency relation is obtained by suitable choice of the indices of refraction of the fibre materials and by adjustment of the angle # . As an alternative the arrangement may operate as a down converter, the frequency relationship then being f 1 -f 2  = f 3 . Such an arrangement is shown in Fig. 3, in which a microwave is produced in one end 38 of a flared rectangular waveguide 34 by irradiating one end of a fibre 42 of non-linear dielectric material with two optical frequencies of slightly different values. As shown the optical sources 32, 50 may respectively comprise a ruby laser and a thermally or otherwise detuned ruby laser, the two emitted waves being focussed by lenses 48, 52 and a half silvered mirror 54. The other end of the fibre is provided with a suitable optical radiation absorber 44. A dielectric material 46 surrounds the fibre and extends into the flared portion 40 of the waveguide. The arrangement serves as a detector if one laser provides a frequency modulated output. A modified microwave down converter is shown in Fig. 4 in which the optical fibre 42 and surrounding dielectric medium 46 is located in a cavity 56 tuned to the microwave frequency required. First and second optical reflectors 58, 60 are disposed at opposite ends of the fibre 42 so as to form an interferometer of the Fabry-Perot type. The cavity 56 includes an iris 62 for the incident light waves and an aperture 64 which couples the microwave energy to a waveguide 66. Fig. 7 shows how the Fig. 5 arrangement may be used as an up or down converter; optical source 88 providing an intense and preferably coherent output at frequency f1 and source 90 providing either intense coherent radiations or functioning as a weak signal source of frequency f2 = f1/2. In Fig. 7 the plate 82 is highly reflective to waves of frequency f1 and transparent to frequency f2, while plate 84 is highly reflective to waves of frequency f1 and transparent to frequency f3, the equation f1 + f2 = f3 applying to the arrangement, The plates may be omitted when the radiations applied to the non- linear dielectric medium 86 have an intense electric field. The optical source 90 and detector 92 are so positioned relative to the axis of the dielectric medium that the angular dispositions #2 and #3 of waves f2 and f3 satisfy a momentum vector diagram, Fig. 4 (not shown), involving frequencies f1,f2 and f3. Specification 793, 799 is referred to.