METHOD OF AND APARATUS FOR MEASURING THE TRANSVERSE MOMENTS OF THE ELECTROMAGNETIC FIELD ASSOCIATED WITH AN OPTICAL BEAM

Abstract: A method and an apparatus are provided wherein a light beam is passed through a slit (7) oscillating with re­ spect to the beam, and a periodic signal is obtained from the beam portion traversing the slit and is split into its spectral components. Moment values are derived from the values of the amplitudes of such components.

Description: The present invention relates to methods of and apparatus for optical measurements, and provides a method of and an apparatus for measuring the transverse moments of the electromagnetic field associated with an optical beam. The term "moment" is used herein in its statistical meaning: For a given statistical distribution, the 1st order moment gives the mean value, the 2nd order moment the root mean square, the 3rd order moment the skewness (asymmetry) which increases as the moment increases, the 4th order moment the so-called Kurtosis (degree of heart-shaping of the distribution). The physical meaning of the moments depend on the nature of the quantity observed: In the particular case of the electromagnetic field transmitted through a waveguide the moments give for instance information about the field confinement within the guide. Preferably, but not exclusively, the invention can be applied to spot size evaluation in an optical fibre, i.e. to measurement of the 2nd order moment of the near or far output field of such a fibre. It is known that the generic n-th order moment (n being an integer and positive number) of the electromagnetic field of an optical beam is given by the relation: where I(r) is the near field or far field beam intensity at distance r from the axis. More particularly, the square root W₀ of the 2nd order moment, i.e. the root mean square of the spatial distribution of the electromagnetic field of the beam (or of the field at the output of an optical fibre, in the preferred application) represents the beam spot-size. The knowledge of w₀ is important for the knowledge of the geometric dimensions of the field, which gives information both as to the collimation of and as to the power distribution in the beam. In the particular case of optical fibres (to which reference will be made hereinafter since the invention has been mainly developed for applications in that domain), spot-size data provides information on propagation within the fibre and on splice losses; such information is indispensable when using optical fibres in a telecommunications system. Even more particularly, spot-size W₀ both in the near and in the far field characterizes the properties of monomode fibres: in fact for such fibres splice and bending losses, and cabling losses due to microbending, can be obtained from these parameters. The variation of spot-size with wavelength indicates the cut off wavelength of the first higher order mode, as well as the fibre dispersion. A number of different techniques have been proposed for spot-size measurements in optical fibres. One is described by R. Yamauchi, T. Murayama, Y. Kikuchi, Y. Sugawara and K. Inada in the paper "Spot-Sizes of Single Mode Fibres With a Noncircular Core" presented at the Fourth International Conference on Integrated Optics and Optical Fibre Communication (IOOC'83, Tokyo, Japan, 27-30 June 1983, Paper 28A2-3, pages 39 and ff.). In this method spot size is obtained by determining the value of I at the fibre output by near field intensity scanning and then by directly applying relation (1), with n=2. This method can be used for measuring moments of any order. Since the integration interval extends to infinity, but, beyond a certain distance from the beam axis, intensity I will be masked by measurement noise, the method can introduce some significant errors into the value obtained. In addition, radial scanning is inherently complex. According to other methods a Gaussian distribution is assumed for the function representing I and quantities are measured which can be correlated to spot size by means of formulae, which are valid only if the hypothesis of a Gaussian field is satisfied. Examples of such methods are described in the papers: "Direct Method of Determining Equivalent-Step-Index Profiles for Multimode Fibres" by C.A. Millar, Electronics Letters, Vol. 17, No, 13, 25 June 1981, pp. 458 and ff., and "Fundamental Mode Spot-Size Measurement in Single-Mode Optical Fibres" by F. Alard, L. Jeunhomme, P. Sansonetti, Electronics Letters, Vol. 17, N. 25, 10 December 1981, pp. 958 and ff. Since the hypothesis of Gaussian field applies only in very particular cases, the measurements obtained by these methods present an intrinsic uncertainty which is difficult to quantify. To overcome these disadvantages, the inventors have sought to provide a method allowing the accurate determination of transverse moments of any order, and in particular of the spot-size, with no need for complicated measurements requiring radial scanning of the beam intensity, nor numeric computation of integrals, nor application of approximation hypotheses as to field distribution. The inventors started from the consideration that usually in measurements on optical beams the signal emitted from the source is modulated and only the electrical signal component at a frequency equal to the modulation frequency is extracted by measurement instruments. More particularly the investigations made by the inventors have established that the beam moments depend on the higher harmonics of a periodic signal obtained by a suitable scanning of the modulated signal. The essential features of the method and of the apparatus will become apparent from claims 1 and 9. Preferred features are described in claims 2-8 and 10-16 for the method and the apparatus, respectively. For a better understanding reference is made to the annexed drawing in which: Fig. 1 shows an apparatus according to the invention for carrying out measurements on a beam outgoing from a fibre; Fig. 2 and 3 are partial views of two alternatives. In such drawings, electrical connections are represented in heavy lines. In Fig. 1 light from a source 1 is concentrated by a suitable optical system 2 (e.g. a microscope objective) at the input of a fibre 3. Fibre end portion 3a comprising the output face, is mounted on a device 4 which makes it oscillate orthogonally to its axis. Device 4 can be an electro-acoustical transducer or a piezoelectric device, and is connected to a generator 13 of oscillations at the frequency desired for the fibre end motion. Such frequency can range for instance between the tens and the thousands of MHz, depending on the features of the measurement electronics. The light leaving fibre 3 is collected by a second optical system 5 which forms a near field image of the fibre output face on a screen 6. The position of the image, and hence of the screen, depends on the position of the fibre output face with respect to the optical system. If said face were at infinite distance, the image would be in the focal plane. The screen 6 presents a longitudinal slit 7, perpendicular to directions of oscillation x and of propagation z. Width s of slit 7 must be somewhat smaller than the spot size: For instance, in case of measurement on a monomode fibre, the spot size can be of the order of 3 um and slit 7 may have a width of about 1 um. Owing to the oscillation of fibre 3 also the image oscillates at the same frequency and thus slit 7 localizes instant by instant a different portion of the image. The light having passed through slit 7 is collected by a further optical system 8 by which it is focused on a photodetector 9. The photodetector output signal is a periodic signal, consisting of pulses having a frequency equal to the oscillation frequency and a shape depending on field distribution. Such output signal is fed to a sampler 10 which samples it at intervals equal to twice its period and for a time equal to the pulse duration, so as to eliminate the even pulses. The respective circuit can consist e. g. of a monostable with high input impendance controlling an analog switch placed in series with the photodetector. The same effect can be obtained by modulating the beam emitted by the source by a square wave that has a frequency half the frequency of the oscillatory motion, in such a way as to periodically annul, or substantially annul, the beam for a time equal to a period of the oscillatory motion. The signal outgoing from circuit 10 is sent to a spectrum analyzer or another equivalent device supplying the various spectral components of the signal, which depend on the field moments, as will be seen hereinafter. A computing system 12 computes the values of such moments and more particularly the spot size. By eliminating optical system 5, i.e. by collecting the beam outgoing from fibre 3 direct on screen 6, the described apparatus can be used for far-field moment measurements. Mathematical considerations whereupon the invention is based will be now explained, supposing for sake of simplicity that the beam has a radial symmetry. If the fibre end portion oscillates according to the law x=A · sin ωT (with ω=2πf), indicating by I(x,y) the intensity (in the near or far field as the case may be) of the radiation outgoing from the fibre, and by s the slit width, the intensity of the radiation outgoing from the slit 7 (with good approximation, for s rather small) is the signal outgoing from photodetector 9 will be V(t) = K I'(x)= K I'(A sin ωt), K being a proportionality constant. Since I has radial symmetry, function I'(x) is clearly an even function and signal V(t) can be represented by the following Fourier series where coefficients V n are given by relations: T = 2π/ω being the period of signal V(t). The integral at the numerator of (3) is calculated between -T/4 and T/4, since the presence of device 10 makes V(t) null in the intervals between - T/2 and - T/4 and between T/4 and T/2. By computing the denominator of (3) and passing from integration variable t to spatial variable x , the following relations are obtained for V1, V3: where <x²> indicates the expression ∫I'(x) x² dx/∫I'(x) dx It is easily deduced that: and, taking into account that the field distribution in the two directions x,y is identical, where <r²> = ∫I(r) r² dr/∫I(r) dr It is immediate to pass from these relations to those valid for the most general case, expressed by (1), substituting values r cos  and r sin  for x and y. W o is immediately obtained from value <r²>. To obtain higher order moments, the harmonics of V(t) higher than the third one are to be considered. More particularly, to obtain the 4th order moment, V5 is to be determined, which depends on <x⁴> according to relation: wherefrom Taking into account that x = r cos  simple passages show that <r⁴> = 8/3 <x⁴>. To carry out the measurements with the described device, screen 6 is located so that slit 7 is at the centre of the near or far field image. Generator 13 is then energized: the image displaces with respect to the slit and the portion localized by the slit is collected by the photodetector and supplied through circuit 10 to spectrum analyser 11, which supplies the values of V1, V3, V5... From said values computing system 12 obtains the spot size and the values of any other desired moment, by applying the relations above. It is evident that what described has been given only by way of non-limiting example and that modifications and variations are possible without going out of the scope of the invention. More particularly, vibrating device 4 can be associated with screen 6 instead of being associated with the end portion of fibre 3; for near field measurements, device 4 can be associated with a cylindrical lens 14 (Fig. 2), which is placed between fibre 3 and the screen, and lets through a beam whose diameter basically corresponds to the width of slit 7. The lens can consist of an optical fibre trunk. In a variant of the measurement device, shown in Fig. 3, the spectrum analyzer can be replaced by a lock-in amplifier 15. Such amplifier needs for its operation a signal with the same frequency as the harmonic of V(t) whose value is being calculated: generator 13 will be replaced in this case by a generator 16 at frequency N · f (with N maximum order of the moment to be measured) associated with suitable frequency dividers, schematized by block 17 and supplying signals with frequency f, 3f, 5f... A switch 18 allows amplifier 15 to be fed with the signal necessary for the measurement. It is to be noted that the arrangement of screen 6 shown in Fig. 3 is that necessary to far field measurements and that transducer 4 has been shown associated with screen 6. For measurements on a beam not guided by a fibre, screen 6 or optical system 5 will directly collect the light emitted from source 1.

Claims: A method of measuring the transverse moments of the electromagnetic field associated with an optical beam, characterized in that : said beam is made to pass through a slit (7), perpendicular to the beam propagation direction; a scanning of the beam is effected through the slit (7), generating a relative oscillatory motion between the beam and the slit in a direction (x) perpendicular to the slit and to the propagation direction of the beam; a periodic electrical signal is generated consisting of pulses which occur at a frequency half the frequency of the oscillatory motion, and have amplitude and shape dependent on the intensity distribution in the beam fraction which has passed through the slit; the periodic signal is split into its spectral components; the amplitude values of such components are measured; the moment values are derived from the amplitude values measured. A method as claimed in Claim 1, characterized in that said optical beam is a beam transmitted by a monomode optical fibre (3) whose spot size is to be measured. A method according to claim 1 or 2, characterized in that said optical beam is transmitted to the slit (7) by a cylindrical lens (14), with axis parallel to the slit, emitting a beam with a diameter basically corresponding to the slit width. A method according to any preceding claim, characterized in that said oscillatory motion is obtained by making the end portion (3a) of the fibre (3) vibrate orthogonally to its axis. A method as claimed in any one of claims 1 to 3, characterized in that said oscillatory motion is obtained by making a screen (6) vibrate, which screen is located in a plane perpendicular to the propagation direction of the beam and contains the slit (7). A method as claimed in claim 3, characterized in that said oscillatory motion is obained by making said cylindrical lens (14) vibrate. A method as claimed in any preceding claim, charaterized in that said periodic signal is obtained by generating first an intermediate perodic signal consisting of pulses which occur at a frequency equal to the frequency of the oscillatory motion and by sampling then said intermediate signal at intervals equal to twice the period of said signal and for a time equal to a pulse duration Method as claimed in any of claim 1 to 6, characterized in that said periodic signal is obtained by modulating the light beam with a square wave having frequency equal to half the frequency of the oscillatory motion. Apparatus for measuring the transverse moments of the electromagnetic field associated with an optical beam, comprising a source (1) of the optical beam, means (9) for converting the light beam into an electrical signal, and a computing system (12), characterized in that it further comprises: a screen (6) perpendicular to the direction of propagation of the beam and provided with a slit (7); means (4, 13; 4, 16, 17) for generating a relative oscillatory motion between the slit (7) and the optical beam; means for generating at the output of said converting means (9), a periodic signal consisting of pulses which occur at a frequency half that of the oscillatory motion and have shape and amplitude dependent on the electromagnetic field distribution in the beam fraction which has traversed the slit; means (11, 15) for splitting the periodic signal into its spectral components and for supplying the amplitude values of such components to the computing system (12), which obtains the moment values from said amplitude values. Apparatus as claimed in claim 9, characterized in that a mono-mode optical fibre (3) is located between the source (1) and the screen (6), and the means (11) for splitting the periodic signal supply the computing system (12) with the amplitude values of the fundamental component and of the third harmonic, the computing system (12) obtaining from such values the spot size of the beam outgoing from the fibre (3). Apparatus as claimed in claim 9 or 10, characterized in that a cylindrical lens (14) with axis parallel to the slit (7) is located upstream the screen (6) and sends the slit (7) a beam with a diameter basically corresponding to the slit width. Apparatus as claimed in claim 10 or 11 if referred to claim 10, characterized in that said means (4, 13; 4, 16, 17) generating the oscillatory motion are associated with the fibre (3) and cause the vibration of a fibre end portion (3a) orthogonally to its axis. Apparatus as claimed in any of claims 9 to 11, characterized in that the means (4, 13; 4, 16, 17) generating the oscillatory motion are associated with the screen (6). Apparatus as claimed in claim 11, characterized in that the means (4, 13; 4, 16, 17) generating the oscillatory motion are associated with the cylindrical lens (14) and cause its vibration, orthogonally to its axis, in a plane parallel to the screen (6). Apparatus as claimed in any one of claims 9 to 14, characterized in that the means for generating the periodic signal comprises a sampler (10) connected to the output of the converting means (9), for sampling the output signal of the converting means at intervals equal to twice the period of the oscillatory motion and for a time equal to a pulse duration. Apparatus as claimed in any of claims 9 to 14, characterized in that the means for generating the periodic signal comprise a source of a square wave with frequency half that of the oscillatory motion associated to the source (1) of the optical beam for modulating said beam.

Inventors: COPPA, GIANNI, DI VITA, PIETRO
Applicants: CSELT CENTRO STUDI E LABORATORI TELECOMUNICAZIONI S.P.A.
Application year: 1985
Main classification: G01J1
IPC codes: G01J1, G01M11, G01J4