Scope for an optical circulator in metrology

Abstract

An optical device is described that maintains undamped circulation of a single short light pulse in a closed loop. A circulator based on the latest fiber-optic components can be used as a frequency measure in the radio range and also for frequency measurement of distances, measurement of single pulse lengths, and so on.At the start of the 1970s, one of us proposed a model for an optical circulator, namely a device that maintains undamped circulation with the velocity of light for a short light pulse in a closed optical ring of a certain optical length. Such a circulator can handle various tasks in physics and metrology [1][2][3]. However, the suggestions remained unrealized at the time because of the lack of optical components with appropriate characteristics.Later, papers appeared [4-11] on optoelectronic oscillators similar to this circulator in which a feedback loop was employed containing a semiconductor diode or gas laser, a modulator for the current in the diode laser (or modulator for the gas laser intensity), a photocell, a linear electronic amplifier, and an optical delay (or an amplifier with time delay). The generators described in [4][5][6][7][8][9][10][11] differed from an optical circulator in that they were based on automodulation of continuous-wave laser radiation. Experiments with them showed how far their performance is restricted by the parameters of the electronic units (noise, response rates, frequency characteristics, sensitivity, and so on). This showed that the methods proposed in [1-3] could not be realized effectively on the basis of these optoelectronic generators.Recent advances in fiber optics and in single-mode fiber light guides with exceptionally low losses (0.15-0.2 dB/km at wavelengths of 1.3-1.6 #m [12]) can be used in conjunction with single-phase ultrawide-band fiber-optic amplifiers [13, 14] and other fiber-optic components to construct an optical circulator. We therefore briefly consider an optical circulator and describe some of its possibilities.The circulator operation can be understood from Fig. 1. We assume that a single short light pulse from the laser 1 passes through the beam splitters 2 into the closed loop formed by the beam splitter 2, prism 3, and mirror 4. Let the optical length of the loop be L o. If we assume that there is no energy loss from the light pulse in the loop, and if the propagation time in one transit of the closed loop exceeds the length of the pulse, one gets a pulse circulating with a period T O = L0/c and frequency f0 = To-1 = c/L0 ' in which c is the mean integral value for the group velocity of the pulse in the loop.However, there is an energy loss in the splitter 2 (minimum of 50%) and also on reflection from mirror 4 and passage through the prism 3. If the losses are not compensated, the circulation will cease after a few periods. To maintain continuous circulation, there is the optical amplifier (power amplifier) 5 as shown in Fig. 1.This circular poses two topics connected with the following: 1) providing an optical circuit of sufficien...