Studying focal ratio degradation of optical fibres with a core size of 50 μm for astronomy

Mon. Not. R. Astron. Soc. 356, 1079–1087 (2005) doi:10.1111/j.1365-2966.2004.08536.x Studying focal ratio degradation of optical fibres with a core size of 50 µm for astronomy A. C. Oliveira,1
L. S. de Oliveira1
and J. B. dos Santos2
1 Laboratório Nacional de Astrofisica, Rua Estados Unidos 154, Bairro das Nações, CEP 37.500.000, Itajubá, MG, Brazil 2 Instituto de Fı́sica de São Carlos/USP, Av. Trabalhador São Carlense 400, Caixa Postal 369, CEP 13.560-970, São Carlos, SP, Brazil Accepted 2004 October 20. Received 2004 October 19; in original form 2004 February 3 ABSTRACT Key words: instrumentation: miscellaneous – instrumentation: spectrographs. 1 INTRODUCTION Mode-dependent loss mechanisms are the causes of focal ratio degradation (FRD) in optical fibres, and are not often addressed by manufacturers. Mode-dependent losses can be divided into two basic mechanisms. The first is waveguide scattering, which causes transfer of energy into lossy modes by variations of the core diameter along the length of the fibre. The second is mechanical deformation. Mechanical deformation is a change of the geometry of the fibre away from a straight cylinder. Large-scale bending, or macrobending, is where the radius of curvature of the bend is very large in comparison to the core diameter. On the other hand, microbends are deformations of the cylindrical core shape which are small compared to the fibre diameter (Ramsey 1988). It is well known that mechanical deformation causes FRD by the formation of microbends in the fibre (Clayton 1989). FRD is a non-conservation of étendue (or optical entropy) such that the focal ratio is broadened by propagation in the fibre. When mounting the fibre, the appropriate epoxy and tubing should be selected, and general care must be taken to minimize mechanical stress and avoid additional FRD. FRD may be a source of scattered light in spectrographs fed by optical fibres from an integral field unit (IFU). The GMOS IFU has 1500 fibres with a core size of 67 µm and uses an output lenslet to minimize the throughput loss due to focal degradation (Murray et al. 2002). The situation is worst when it is necessary to remove the
E-mail: (ACO); (LSdO); (JBdS)
C 2004 RAS acrylate buffer of the fibres. Sometimes this operation is necessary to build a linear slit with a reduced centre-to-centre distance for the fibres, such as the SPIRAL (Kenworthy, Parry & Taylor 2001) and Eucalyptus (Cesar et al. 2002) IFUs. The removal of the acrylate buffer reduces the protection to the cladding and the core, so that the optical fibre may be exposed to high stress. During optical tests of the Eucalyptus IFU, the slitlet blocks were identified as a source of FRD. This may be a result of the close packing of the fibres on the slitlet resulting in mechanical stress. 2 M E A S U R E M E N T O F F O C A L R AT I O D E G R A DAT I O N Astronomers recognized early the necessity of performing measurements of FRD. The published literature is quite informative in this regard (Angel et al. 1977; Barden et al. 1980, 1981; Gray 1983; Lund & Enard 1983; Powell 1983; Ramsey & Huenemoerder 1986). Measurements of the FRD are reasonably straightforward. The methods used by Powell (1983) and others (Lund & Enard 1983; Guerin & Felenbok 1988) are a clear improvement on the early technique employed by Angel et al. (1977) and Barden et al. (1981). Most of the FRD measurements to date are relative in nature; that is, they assume that all light is transmitted at some lower limit of the output f -ratio. The measurements of Powell (1983) are an exception to this as they are absolute. While relative measurements are quite adequate for many purposes, they can be misleading when assessing the fibre performance at relatively fast f -ratios ( f /# < 3.0) or looking at small core fibres. An apparatus that measures the amount Along with the spectral attenuation properties, the focal ratio degradation (FRD) properties of optical fibres are the most important for instrumental applications in astronomy. We present a special study about the FRD of optical fibres with a core size of 50 µm to evaluate the effects of stress when mounting the fibre. Optical fibres like this were used to construct the Eucalyptus integral field unit. This fibre is very susceptible to the FRD effects, especially after the removal of the acrylate buffer. This operation is sometimes necessary to allow close packing of the fibres at the input to the spectrograph. Without the acrylate buffer, the protection of the cladding and core of the fibre may be easily damaged. In the near future, fibres of this size will be used to build the Southern Observatory for Astronomical Research (SOAR) integral field unit spectrograph (SIFS) and other instruments. It is important to understand the correct procedure which minimizes any increase in FRD during the construction of the instrument. 1080 A. C. Oliveira, L. S. de Oliveira and J. B. dos Santos of light output from the fibre relative to that input with a given mode distribution is described by Ramsey (1988). The scheme, originally designed by Barden while he was at Penn State, compares the light in the input beam to that emanating from the fibre by way of a simple 90◦ flip of two separate mirrors. In this work we have used the same method used by Lee, Haynes & Skeen (2001) and originally developed by Carrasco & Parry (1994). This method does not use mirrors to provide a reference and produces relative measurements. This type of result can be used for comparative analyses and is sufficient for a study of the effects of stress on the optical fibres. Figure 2. Schematic diagram of the optical fibre. 3 S U M M A RY O F F I B R E I S S U E S 3.1 Types of fibre tested 3.2 Mounting of the fibres The optical fibres are generally mounted in some form of connector and sometimes in special structures. The Eucalyptus IFU has two Table 1. Specification of the Eucalyptus IFU fibre. Type of fibre Length Numerical aperture Core Cladding Inner buffer (polyimide) Outer buffer (acrylate) High OH− 2m 0.22 ± 0.02 50 µm 60 µm 70 µm 200 µm Figure 3. Schematic diagram of the slit block with the optical fibres aligned. types of termination: input microhole arrays and output slit blocks. We are especially interested in evaluating the construction of the slit blocks. During optical tests of the Eucalyptus IFU, the slit blocks were identified to be a source of FRD. This may be a result of the close packing of fibres on the slit block and the utilization of the Araldite epoxy, resulting in mechanical stress. A dual buffered fibre (Fig. 2) was chosen for a number of reasons. However, to allow closer packing of the fibres at the output slit, the outer acrylate buffer can be readily removed so that the fibres can be assembled with a spacing determined by the inner polyamide buffer, as shown in Fig. 3. The blocks for the Eucalyptus slit were made with brass. However the block tests were made with q (...truncated)