DD-IEC-PAS-61300-3-43-2006.pdf

DRAFT FOR DEVELOPMENT DD IEC/PAS 61300-3-43: 2006 Fibre optic interconnecting devices and passive components Basic test and measurement procedures Part 3-43: Examinations and measurements Mode transfer function measurement for fibre optic sources ICS 33.180.20 ? Licensed Copy: London South Bank University, London South Bank University, Thu Dec 20 02:51:59 GMT+00:00 2007, Uncontrolled Copy, (c) BSI DD IEC/PAS 61300-3-43:2006 This Draft for Development was published under the authority of the Standards Policy and Strategy Committee on 30 November 2007 © BSI 2007 ISBN 978 0 580 56659 2 National foreword This Draft for Development is the UK implementation of IEC/PAS 61300-3-43:2006. This publication is not to be regarded as a British Standard. It is being issued in the Draft for Development series of publications and is of a provisional nature. It should be applied on this provisional basis, so that information and experience of its practical application can be obtained. A PAS is a Technical Specification not fulfilling the requirements for a standard, but made available to the public and established in an organization operating under a given procedure. A review of this Draft for Development will be carried out not later than 3 years after its publication. Notification of the start of the review period, with a request for the submission of comments from users of this Draft for Development, will be made in an announcement in the appropriate issue of Update Standards. According to the replies received, the responsible BSI Committee will judge whether the validity of the PAS should be extended for a further 3 years or what other action should be taken and pass their comments on to the relevant international committee. Observations which it is felt should receive attention before the official call for comments will be welcomed. These should be sent to the Secretary of the responsible BSI Technical Committee at British Standards House, 389 Chiswick High Road, London W4 4AL. The UK participation in its preparation was entrusted by Technical Committee GEL/86, Fibre optics, to Subcommittee GEL/86/2, Fibre optic interconnecting devices and passive components. A list of organizations represented on this committee can be obtained on request to its secretary. This publication does not purport to include all the necessary provisions of a contract. Users are responsible for its correct application. Amendments issued since publication Amd. No. DateComments Licensed Copy: London South Bank University, London South Bank University, Thu Dec 20 02:51:59 GMT+00:00 2007, Uncontrolled Copy, (c) BSI PUBLICLY AVAILABLE SPECIFICATION IEC PAS 61300-3-43 Pre-Standard First edition 2006-02 Fibre optic interconnecting devices and passive components Basic test and measurement procedures Part 3-43: Examinations and measurements Mode transfer function measurement for fibre optic sources Reference number IEC/PAS 61300-3-43:2006(E) DD IEC/PAS 61300-3-43:2006 Licensed Copy: London South Bank University, London South Bank University, Thu Dec 20 02:51:59 GMT+00:00 2007, Uncontrolled Copy, (c) BSI CONTENTS 1 Scope 3 2 Normative references .3 3 General description 3 4 Theory .3 4.1 Alternative method.5 4.2 Mode power distribution.5 4.3 Relative power distribution.6 4.4 Constraints7 5 Apparatus.8 5.1 General.8 5.2 Test sample.8 5.3 Sample positioning device .8 5.4 Optical system.8 5.5 Camera .8 5.6 Video digitizer .9 5.7 Calibration.9 6 Procedure 9 6.1 Mounting and aligning the sample9 6.2 Optimization 9 6.3 Acquiring the data .10 7 Calculations .10 7.1 Background level subtraction. 10 7.2 Location of the centroid of the intensity profile10 7.3 Differentiating the intensity profile11 7.4 Computing the MTF .12 8 Results.13 Annex A (normative) Launch modal distribution for multimode attenuation measurements14 Bibliography 16 2 DD IEC/PAS 61300-3-43:2006 Licensed Copy: London South Bank University, London South Bank University, Thu Dec 20 02:51:59 GMT+00:00 2007, Uncontrolled Copy, (c) BSI FIBRE OPTIC INTERCONNECTING DEVICES AND PASSIVE COMPONENTS BASIC TEST AND MEASUREMENT PROCEDURES Part 3-43: Examinations and measurements Mode transfer function measurement for fibre optic sources 1 Scope This part of IEC 61300 describes the method for measuring the mode transfer function (MTF) to be used in characterizing the launch conditions for measurements of attenuation and or return loss of multimode passive components, according to IEC 61300-1 and IEC 61300-3-4. The MTF may be measured at the operational wavelengths. 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. IEC 61300-1, Fibre optic interconnecting devices and passive components Basic test and measurement procedures Part 1: General and guidance IEC 61300-3-4, Fibre optic interconnecting devices and passive components Basic test and measurement procedures Part 3-4: Examination and measurements Attenuation 3 General description The modal distribution launched into multimode fibre can vary widely with different light sources. This variation in launched modal distribution can result in significant differences in measured attenuation in the same component. The MTF test method gives information about the launched modal distribution (LMD) condition in a measured component. The MTF test method is based on a measurement of the near-field intensity distribution in the fibre 1, 21. 4 Theory For a fibre with a power-law index profile n(r), given by 5 , 0 1 21)( = a r nrn 1 a r (1) where a is the fibre core radius; is the profile factor ( = 2 for a parabolic profile); is the relative index difference, given by 1 Figures in square brackets refer to the Bibliography. 3 DD IEC/PAS 61300-3-43:2006 Licensed Copy: London South Bank University, London South Bank University, Thu Dec 20 02:51:59 GMT+00:00 2007, Uncontrolled Copy, (c) BSI 2 1 2 2 2 1 2n nn = (2) where n1 is the index at the fibre centre; n2 is the cladding index. The near-field intensity profile in the fibre I(r) may be determined from an integration of the mode transfer function MTF, , in the fibre, as follows (ignoring constants): () = a r MTFrId).()( (3) where is the normalized propagation constant; r/a is the normalized radial position. Differentiating both sides gives the MTF as follows (ignoring constants): () a rr r rI MTF = = 1 1 d )(d )( (4) The MTF is usually plotted in terms of the principal mode number, m, divided by the maximum principal mode number, M, where 2 )2( 2 )2( + = = a r M m (5) The term, m/M, is usually referred to as the relative mode number, or the normalized mode number. The maximum principle mode number, M, is given by + = an M 2 2 1 (6) A typical normalized MTF plot is shown in Figure 1, where it can be seen, in this example, that normalized mode numbers up to about 0,6 are equally filled and higher order modes are progressively less well-filled. 4 DD IEC/PAS 61300-3-43:2006 Licensed Copy: London South Bank University, London South Bank University, Thu Dec 20 02:51:59 GMT+00:00 2007, Uncontrolled Copy, (c) BSI 0 0.25 0.5 0.75 1 00.20.40.60.81 relative mode group number relative mode power Normalized mode number 0,75 0,5 0,25 0,2 0,40,60,8 Normalized MTF Figure 1 Example of normalized MTF 4.1 Alternative method If the profile factor, , in equation (4) is not known, then an alternative expression for MTF can be used. It is known 3 that, in a fully filled fibre (i.e., MTF = 1 for all mode numbers), the near-field intensity profile, Io, is the same shape as the square of the refractive index profile, n(r)2. Furthermore, the term r -1 in equation (4) is equal (ignoring constants) to the differential of n(r)2 and so equation (4) can be rewritten as: ()2 d)(d 1 d )(d )( a ro rrIr rI MTF = = (7) where a value of = 2 has been assumed in order to compute values for the normalized mode number. Thus the MTF is equal to the ratio of the derivative of the intensity profile under test to the derivative of the intensity profile of the same fibre under fully filled conditions. 4.2 Mode power distribution For graded index multimode fibre the number of discrete modes in a particular mode group is proportional to the principal mode number. Thus, higher-order-mode groups contain more modes and, therefore, will carry more light if all the modes are equally excited. This can be represented by the MPD, defined as: mmMTFmMPD=)()( (8) Because of this relationship of modes within mode groups, the MPD transform effectively displays the relative power in the mode groups. 5 DD IEC/PAS 61300-3-43:2006 Licensed Copy: London South Bank University, London South Bank University, Thu Dec 20 02:51:59 GMT+00:00 2007, Uncontrolled Copy, (c) BSI An example of a normalized MPD is shown in Figure 2, where it can be seen, in this case, that the peak power level occurs around 0,65 of the normalized mode number. 0 0.25 0.5 0.75 1 00.20.40.60.81 relative mode group number relative mode power Normalized mode number 0,75 0,5 0,25 0,2 0,40,60,8 Normalized MPD Figure 2 Example of normalized MPD 4.3 Relative power distribution The relative power distribution (RPD) is another way of expressing MTF data. It is defined as: = 1 µ d)()µ(mmMTFRPD (9) where µ is a variable principle mode number, required for the integration, that takes on values from 1 to 0 (m is the principle mode number). The RPD represents the area under the MTF curve as the mode number is progressively reduced from unity and is, therefore, a measure of the cumulative distribution of power starting at the highest mode number. An example of a normalized RPD is shown in Figure 3. 6 DD IEC/PAS 61300-3-43:2006 Licensed Copy: London South Bank University, London South Bank University, Thu Dec 20 02:51:59 GMT+00:00 2007, Uncontrolled Copy, (c) BSI 0 0.25 0.5 0.75 1 00.20.40.60.81 relative mode group number relative mode power Normalized mode number 0,75 0,5 0,25 0,2 0,40,60,8 Normalized RPD Figure 3 Example of normalized RPD 4.4 Constraints The MTF measurement method described herein is only valid under certain conditions, as follows. Modes within a mode group carry the same power. There are random phases between the propagating modes. It has been found 4 that both these conditions can simultaneously be met if the line-width of the source is sufficiently broad, leading to the so-called “mode-continuum approximation”, given by: Nka o 2 (10) where is the optical wavelength; k0 is 2/; N is the group index, given by: d d 1 1 n nN= (11) Typically, for a 50 µm core diameter fibre, with 0,21 numerical aperture, then 0,5 nm at 850 nm and 1,0 nm at 1 300 nm satisfy this condition. If the source line-width does not meet this criterion then interference between propagating modes may take place, resulting in “speckle” in the near-field image. The method can, however, still be applied to such sources by gently shaking, or somehow agitating, the fibre under test so as to cause a temporal averaging of the speckle pattern. In this case, it is important to ensure the near-field is azimuthally symmetric. This can be achieved by checking that the MTFs measured at 45° intervals around the fibre coincide with each other 5. 7 DD IEC/PAS 61300-3-43:2006 Licensed Copy: London South Bank University, London South Bank University, Thu Dec 20 02:51:59 GMT+00:00 2007, Uncontrolled Copy, (c) BSI 5 Apparatus 5.1 General The apparatus is essentially a video microscope where a near-field image of the end of the fibre under test is formed on the surface of camera, such as a CCD or CMOS camera, by an optical system. The camera image is then digitized by a video digitizer and transferred to a computer for analysis and data presentation. 5.2 Test sample The test sample consists of a multimode patch cord attached to a light source. It should be recognized that the mode distribution at the output of the patch cord is a product of both the launch conditions of the source and of the patch cord itself. The resultant MTF is, therefore, not a parameter of either the light source or the patch cord individually but rather of the combination, including the particular conditions under which the patch cord is disposed, such as bend radius. 5.3 Sample positioning device A positioning device is required to ensure that the end of the patch cord under test is located on the optical axis of the instrument and also in the correct axial position to give a well- focused image on the camera. For this purpose, an XYZ manipulation stage may be used or, preferably, a suitable connector receptacle mounted axially with the optics. An example is a standard 2,5 mm ferrule receptacle which is able to accommodate several connector types, such as FC, ST and SC. In this case, the XY positioning of the patch cord is well-defined and only a focusing adjustment is required. 5.4 Optical system The optical system comprises magnifying optics to produce an image of the fibre end on the camera. To optimize measurement resolution, it is recommended that the optical magnification be chosen so that the image of the fibre core fills a reasonable proportion of the camera. Typically, this might be between 20 % and 50 % of the vertical extent of the camera. The numerical aperture of the imaging system shall be greater than the numerical aperture of the fibre under test. A means of illuminating the end face of the fibre in reflection may also be provided, such as a beam splitter and an LED source positioned between the focusing lens and the camera. Neutral density (ND) filters may also be provided to control the amount of light reaching the camera. 5.5 Camera A high-quality camera having demonstrable geometrical uniformity and intensity linearity shall be used. The pixel size of the camera, picsize, shall be sufficiently small compared with the magnified near-field image as to be less than the system diffraction limits by a factor of 2, given by: NA Mag picsize 2 61, 0 (12) where Mag is the system magnification; NA is the numerical aperture of the fibre. 8 DD IEC/PAS 61300-3-43:2006 Licensed Copy: London South Bank University, London South Bank University, Thu Dec 20 02:51:59 GMT+00:00 2007, Uncontrolled Copy, (c) BSI For example, if Mag = 20, NA = 0,21, = 850 nm then the picsize is 24 µm. It is recommended, however, that the camera pixel size is much smaller this. In this example, the corresponding pixel size at the fibre would be equal to picsize divided by Mag, which is equal to 1,2 µm. 5.6 Video digitizer The video digitize