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Multi-way optical fibre connectors for astronomy

Dionne M. Haynes*ab, Roger Haynesab, William Ramboldab, Michael Goodwina, Ed J. Pennya a Anglo-Australian Observatory, Sydney, NSW, Australia; b Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany ABSTRACT Instrumentation concepts with many thousands of multi-mode optical fibres are now being proposed in response to the science community’s request for highly multiplexed instruments. Such instruments will require a fibre connector system that can deliver excellent optical performance and reliability. We report on the viability of commercially available fibre connectors for use in current and future multimode optical fibre based astronomical instrumentation. Two of the most promising multi-way fibre connectors were investigated and their optical performance characterised. We outline the two fibre connector technologies and report on the characterisation of their focal ratio degradation, throughput and wavelength fringing performance. Keywords: Optical fibre connector, fibre connector, multi-fibre connector, multi-way fibre connector, optical fibre instrumentation, fibre cable design, focal ration degradation, FRD, wavelength fringing, fibre throughput, multiplexed fibre instruments.

1. INTRODUCTION Over the past two decades, various fibre connector systems have been investigated1 and implemented2, often with limited success, in multi-mode optical fibre based instruments. Due to the complexity of current and proposed future fibre based astronomical instruments, there is an increasing demand for modularity, manufacturing and operational simplicity, and hence the need to identify a cost effective, high performance, robust, multi-way optical fibre connector technology. The telecommunications and defence industry have been using multi-fibre connectors for many years with great success for standard Telco fibres i.e. 125µm cladding. These industries have the finances and facilities to develop multi-fibre connector technology delivering reliable, robust and cost effective devices that can now be purchased off the shelf. We selected two multi-way commercially available connectors : the US Conec MTP® and the Deutch MC5 MilSpec . We report preliminary wavelength fringing, FRD and throughput results. . For comparison we also tested two single fibre connectors of type FC and ST which are commonly used in laboratory test equipment. Wavelength fringing and FRD are not typically considered or relevant to telecommunications applications but are critical for high performance mutltimode astronomical applications. FRD, wavelength fringing and throughput all impact the total system efficiency. If wavelength fringing is present and unstable between calibration exposures its impact can be catastrophic and the astronomy data can be unusable Note: This work was carried out at the Anglo-Australian Observatory as part of the instrument science technology transfer studies. Authors D. Haynes, R. Haynes and W. Rambold have since moved to the Astrophysikalisches Institut Potsdam.

*[email protected]; phone +49 (0)331 7499-674; fax +49 (0)331 7499-209; www.aip.de

Modern Technologies in Space- and Ground-based Telescopes and Instrumentation, edited by Eli Atad-Ettedgui, Dietrich Lemke, Proc. of SPIE Vol. 7739, 773946 · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.856805

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2. FIBRE CONNECTORS 2.1 Background The US Conec brand MTP connector is based on mechanical transfer ferrule technology. The MTP® (mechanical transfer push-on) is US Conec’s trademarked enhanced-performance version of the MPO (multi-fibre push-on connector). The MTP® is currently available in 4, 8, 12, 24 and 72 fibre densities for multimode (50µm and 62.5µm core 125µm clad) fibre3. The MTP® ferrule is a monolithic, high precision, moulded component and custom ferrules can be made to order for a given fibre count and fibre outer diameter. The Deutsch MC5 Military Specification connector uses individual ceramic ferrule and alignment sleeve technology to provide high and repeatable optical performance. They are available with fibre densities of 1, 2, 4, 6, 8, 18, and 30 for standard Telco fibre core sizes from 5 to 200µm, custom ferrule bore sizes are also available on request4. Table 1 Manufacturer data for dry connection3,4

Connector

Typical insertion loss

Durability

Operating temperature

Standard MTP® MTP Elite® MC5

0.2dB (4.5% loss) 0.1dB (2.3% loss) 0.25dB (5.5% loss)

< 0.2dB change 1000 remates < 0.2dB change 1500 remates

-40ºC to +80ºC -40ºC to +80ºC -65ºC to +150ºC

The insertion loss is measured in dB and defined as: 10 log [incident power/transmitted power]. Note that the insertion loss is fibre and polishing process dependant. Most optical fibres used in astronomical instruments have an outer diameter which requires a non-standard ferrule bore size in the commercially available connectors. Both the multi-way connector types offer custom ferrules sizes but at a significant additional setup and tooling cost, both in money as well as time, however once the initial outlay is made the costs are similar to the standard Telco connector. Early in the project it was decided that an effective method to test the connector’s viability was to purchase fibre commonly used in astronomy with an outer diameter to match the standard ferrule bore size. The fibre used in the following connector tests was Polymicro FIP 100µm core, 110µm cladding, and 124µm polyimide buffer. Table 2 Fibre connector standard ferrule bore diameters

Connector type

Ferrule bore diameter (μm)

Standard MTP®

127 ± 1

MTP Elite®

125 ± 1

MC5

125 ± 2

FC

125 ± 2

ST

125 ± 2

We assembled and tested MTP® connectors containing 12 fibres and an MC5 connector capable of holding 30 fibres. 2.2 Assembly and polishing After extensive investigation and consultation, two epoxies were selected for the connector tests: Epotek 353ND and Trabond F113SC. These epoxies are commonly used for fibre connector assemblies in the telecommunication industry because of their low shrinkage and hardness properties. Both epoxies are designed to cure at high temperature (typically > 60ºC) over a short period of time (typically < 1 hour); however they also cure very effectively at room temperature (~22 ºC) over a period of 1-2 days. We were advised by the connector manufactures to out-gas the epoxy in order to reduce or eliminate the bubbles formed during the mixing of part A and B before the epoxy is applied to the fibre connector. Bubbles in the epoxy can cause

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additional stress induced FRD on curing, as well as fibre lateral and piston alignment errors during curing. Epoxy outgassing is becoming increasing common practice for fibre applications in astronomy. Table 3 Connectors, ferrules and polishing jigs used in tests

Connector

Ferrule

Hand polishing jig

MTP® (12 fibre)

MC5 MilSpec (30 fibre)

FC (1 fibre)

ST (1 fibre)

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MTP®, FC, ST and MC5-MilSpec connnectors weree assembled using u both Epotek and Trabbond epoxies, and cured at a both room tem mperature andd oven tempeerature. Ovenn curing of Ep potek epoxy reequired 70ºC for two hourss and Trabondd 60ºC for one hour. At room m temperaturee (22ºC) the Trabond T took one day to fully cure and thhe Epotek too ok two days too fully cure. We W found the Epotek 353N ND epoxy eassier to apply than the Trabbond F113FC C because 1) Epotek E is lesss viscous than Trabond, 2) Epotek is a clear c yellow color c making it easy to seee the alignment v-grooves in the MTP® ® connector onn assembly whhereas the Traabond is a veery dark blue (almost blackk) in colour, 3) Epotek has a pot life of approx. two hours h whereass than Trabondd only has a pot p life of thirtty minutes. Thhe disadvantaage to using Ep potek is that it i wicks up the fibres more readily. r Precauutions must bee taken to preevent this from m occurring ottherwise it wiill cause stresss induced FRD D. One possiblle way to conttrol wicking is i the applicattion of a wettiing agent appllied to the fibres behind thee glue area of fibre f ferrule. All of the coonnectors werre hand polished using custom designed d polishing jiigs and wet laapping films. Starting withh 12µm lapping film then, 3µm lapping film, 1µm laapping film and a finishing on 0.3µm lappping film. Table T 3 showss images of eacch connector type t with ferruules and correesponding poliishing jig.

3 EXPER 3. RIMENTAL L DETAIL LS put and FRD D. Three experiimental measuurements werre made for each connecttor type: wavvelength fringging, throughp Reference fibbres prepared and measuredd using the sam me methods were w used to reemove instrum mental effects and provide a reference meaasurement in order o to determ mine the relattive contributiion from the connector. 3.1 Experim mental setup for f wavelengtth fringing an nd throughpu ut

Test fibre and connector Fibre clamp Phottodiode

Powerr meter

Figure 1 Diagram D of the optical apparattus used to measure the waveleength fringing and a throughput of the connecto ors

We used the same front end e of the expperimental setup for both fringing and throughput m measurements and used twoo different setuups at the outpput end. For thhe fringing meeasurement the light from thhe output endd of the fibre was w collimatedd and then connverged to foccus and alignned on the cenntre of the en nd-face of thee spectrographh fibre. For th he throughpuut measurementt, the fibre outtput end was coupled c to a phhotodiode witth power meteer.

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The source fibre was 50µm core and the spectrograph fibre was 200µm core. Both are ocean optics UV-VIS fibres. We used Nikon camera lenses to collimate and converge the beam. The front end lenses have a focal length of 50mm and Fratio of 1.8 and the back end lenses have a focal length of 50mm and F-ratio of 1.4. We used an Ando white light source (400 – 1800nm) however it was necessary to use Schott filters GG455 and BG18 to give a pass band of 450 – 630nm in order to reduce the effects of lens aberrations. An adjustable iris was used to set the input F-ratio to f/4. All fibre ends had to be polished to take these measurements as the inconsistent geometry and surface roughness of the cleaved ends caused inconsistent coupling and scattering losses. 3.2 Experimental setup for laser FRD The parallel laser beam technique5,6 uses a collimated laser beam to inject light into the fibre at selected angles (F-ratios). The change in FRD is measured from the far field radial profile width on output. This technique is highly sensitive to small changes in FRD and is commonly used as a rapid diagnostic technique. The experimental set-up for the collimated laser beam experiment is shown in Figure 2

Fibre input

Spatial filter and collimator

HeNe laser

x-y-z stage and rotation stage Fibre Screen

Re-imaging lens

CCD camera

Fibre output Figure 2 Diagram of the experimental apparatus used to image the fibre far field light distribution (FRD distribution)

A 633nm HeNe laser was injected into the fibre mounted on an x-y-z-θ stage set to an 8º (f/3.6) launch angle to simulate the F-ratio at the AAT prime focus. The fibre output far-field pattern was projected onto a semi-transparent screen which was re-imaged with a f/2 lens onto the CCD camera (a Peltier cooled SBIG ST-7, 765 × 510 pixels), giving 11 pixels per radial degree. All fibre ends were cleaved for the FRD tests so that the only stress induced FRD (microbending) in the system was that caused by the connector and epoxy. 3.3 Data reduction To obtain high signal-to-noise ratio for the spectrum data we took the average of 100 x one second exposures for each fibre. Any wavelength fringing introduced at the light source or spectrograph was removed by dividing all connector spectrums with the reference spectrum (fibre with no connector).

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For each FRD far-field measurement, we extracted a radial profile by first determining the centre of the annulus and then integrating (0 to 2π) the light falling within annuli of varying radii, starting at a radius of two pixels and incrementing by one pixel out to the edge of the frame. The resultant far field radial profile can be represented by a Voigt function in the presence of fibre end-face scattering. The Gaussian component of the Voigt represents the diffraction and modal diffusion process and the Lorentzian component represents the fibre end-face scattering2. We used the resultant far field radial profile to measure the increase in FRD due to modal diffusion introduced by the connector and epoxy by fitting a Gaussian function to the core of the radial profile from which the change in angular FWHM was calculated. As a comparison and demonstration of change in performance of the fibre instrument we also use the far field radial profile to estimate the encircled energy on output for a given F-ratio. Using Matlab software we calculated the fractional encircled energy (EE) within a solid angle cone (f-ratio) for a particular pupil function with a specified f-ratio and central obstruction ratio (e.g. 2dF instrument on the AAT prime focus f/3.6 with f/9 central obstruction). The EE was calculated using the averaged radial intensity FRD profile measured at the fibre output where the fibre input laser incident angle was set to 8 degrees (f-ratio ~ 3.6). The EE calculation involved convolving the measured FRD profile with the pupil function and then performing 2-D integration (simplified to 1-D integration assuming radial symmetry). This method has the advantage of quantifying FRD with a simple experimental setup allowing an efficient comparison between different fibre experiments and is less subject to systematics from beam alignment error. 3.4 Quantification method As a reference, three fibres where prepared in the same fashion as the connector fibres, but without a connector. Wavelength fringing, throughput and FRD measurements of all three reference fibres were taken in order to calibrate the connector data. In this paper, we discuss the additional encircled energy and throughput losses introduced by the connector system. Reference fibre 1 had both input and output ends cleaved and was used to calibrate the FRD data. Reference fibre 2 had both input and output ends mounted in stainless steel ferrules (using epotek epoxy cured at room temperature) and polished end-faces. Reference fibre 2 was used to calibrate the throughput and fringing data for the MC5 Milspec, FC and ST connectors. The third reference (Reference fibre slit) was composed of 12 fibres mounted in a MTP® ferrule at both ends (using Epotek epoxy and cured at room temperature) with polished end-faces. Reference fibre slit was used to calibrate the throughput and fringing data of the MTP® connectors. 3.5 Performance requirements of connector system The fibre connectors were being considered for three development projects at the AAO namely WFMOS1, HERMES7 and MANIFEST8. The specific requirements varied with each system but can broadly described by the following •

The fibre connector throughput should be maximized over the operating wavelength range (e.g. greater than 90%)



The fibre connector should minimally impact the FRD of the system (e.g. less than 10% reduction in encircled energy coupled into the spectrograph collimator)



The fibre connector wavelength fringing should be minimized and stable (e.g. less than 5% peak-to-peak and stable to less than 1% over the period of a night)

Note: An operational lifetime of greater than ten years was assumed for the three applications considered; this would be around 500 mating cycles. In order to maintain performance, careful management of the ingress of dust and dirt would be necessary and should be an important connector design parameter. For example MTP® connectors and adaptors have dust protection design features.

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4. RESULTS 4.1 Wavelength fringing Wavelength fringing is the result of interference fringing caused by an air gap between the ends of the fibres producing a Fabry-Perot etalon9. An air gap can be created by the polishing process whereby the fibre and ferrule are not flat or from dust/dirt on the fibre and ferrule. In Figure 3 - Figure 8 we show a sample of wavelength fringing plots for each connector type when the connection was dry (dark blue trace) and when the connection was coupled with and optical gel (red trace) to eliminate the impact of the air gap. We used index matching gel with a refractive index of 1.456 (@ 633nm). The x-axis on the plots is the wavelength from 460nm to 620nm and the y-axis is the normalized intensity.

Figure 3 Wavelength fringing plots for connector MTP®1. From left to right; best, typical and worst

Figure 4 Wavelength fringing plots for connector MTP®2. From left to right; best, typical and worst

Figure 5 Wavelength fringing plots for connector MTP®3. From left to right; best, typical and worst

Figure 6 Wavelength fringing plots for connector MTP Elite®. From left to right; best, typical and worst

Figure 7 Wavelength fringing plots for connector MC5.

Figure 8 Wavelength fringing plots Left; FC connector and Right; ST connector

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Figure 3-Figure 8 show that all gel coupled connections have no obvious fringing with the possible exception of Figure 8 which looks to have low order fringing for both gel and dry connections. All the fibre connectors performed within spec ( 90%) for a dry connection. MTP1 shows a very high throughput for the dry connection indicating no air gap and good fibre to fibre alignment. The throughput of all the other connectors improved with the application of gel indicating the presence of Fresnel losses (air gap) in the dry connection. All the connectors perform within spec (>90%) when gel coupled.

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4.3 Focal Ratio Degradation A sample of the laser FRD results for each type of connector as well as different epoxies and curing methods in shown in Table 5. We measured the angular change in the FWHM (Δ θFWHM º) of the Gaussian component of the far field radial profile relative to reference fibre 1 for an input f-ratio of f/3.6. The angular change represented an increase in FRD due to induced microbending from the epoxy and connector. We also calculated the encircled energy within an f-ratio of f/3.15 corresponding to the collimator speed in AAOmega instrument. Table 5 Laser FRD results. Epoxy type A= Epotek 353ND and epoxy type B=Trabond F113SC. For oven curing OT1=6773ºC, OT2=57-63ºC and room temperature curing RT=19-23ºC.

Connector type/fibre

Epoxy

Epoxy curing method

Δ θFWHM º

F/3.15 encircled energy%

MTP 1/a

A

2 hrs @ OT1

2.1

93.8

MTP 1/b

A

2 hrs @ OT1

2.1

94.0

MTP 1/c

A

2 hrs @ OT1

2.6

92.2

MTP 2/a

A

2 days @ RT

2.7

90.0

MTP 2/b

A

2 days @ RT

1.1

99.5

MTP 2/c

A

2 days @ RT

2.8

86.5

MTP 3/a

B

1 day @ RT

2.3

93.8

MTP 3/b

B

1 day @ RT

1.4

101.4

MTP 3/c

B

1 day @ RT

2.1

96.1

MTP 3/d

B

1 day @ RT

1.6

97.5

MTP Elite/a

A

2 days @ RT

1.9

92.4

MTP Elite/b

A

2 days @ RT

1.7

97.3

MTP Elite/c

A

2 days @ RT

1.7

96.0

MTP Elite/d MC5/a

A A

2 days @ RT 2 days @ RT

1.9 2.0

95.4 96.6

MC5/b

B

1 day @ RT

2.4

95.8

MC5/c

B

1hr @ OT2

3.2

92.1

FC

B

1hr @ OT2

4.4

87.2

ST 1

B

1 day @ RT

1.3

94.8

ST 2

B

1hr @ OT2

4.8

88.0

Notes rapid epoxy cure and scattering from cleaves rapid epoxy cure and scattering from cleaves rapid epoxy cure and scattering from cleaves excess epoxy and scattering from cleaves minimal epoxy and good cleaves excess epoxy and scattering from cleaves scattering from cleaves minimal epoxy and minimal scattering (improved cleaves) excess epoxy and scattering from cleaves minimal epoxy and good cleaves scattering from cleaves minimal epoxy and good cleaves minimal epoxy , scattering from cleaves scattering from cleaves excess epoxy excess epoxy and scattering from cleaves rapid epoxy cure rapid epoxy cure and excess epoxy minimal epoxy but scattering from cleaves rapid epoxy cure and excess epoxy

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The Laser FRD results shown in Table 5 indicate high temperature curing of epoxies can induces more FRD, i.e. oven cure ΔθFWHM > room temperature cure ΔθFWHM. The likely cause is greater epoxy shrinkage during a high temperature rapid cure, which stresses the fibre and induces additional FRD. To minimize epoxy stress we therefore recommend room temperature curing for a minimum of 1-2 days. In addition to the impact of curing temperature, the FRD data for MTP2 (cured at room temperature) shows elevated levels of FRD for fibres (a) and (c). These fibres were seen to have excess epoxy at the ferrule and the additional stress induced from this excess is the source of the increased FRD. The FC connector also has high levels of stress induced FRD, likely a result of a combination of oven curing the epoxy and excess epoxy at the back of the ferrule. The design of the FC, ST and MC5 ferrules makes it difficult to control the amount of epoxy applied. MTP2 fibre (b) has a ΔθFWHM of 1.1º and f/3.15 encircled energy of 99.5% relative to reference fibre 1 indicating minimal stress from epoxy and minimal scattering from cleaved ends. MTP2 (b), MTP3 (b), MTP (d), MTP Elite (b), MTP Elite (c), MC5 (a) are all well within the spec (EE > 90% at collimator f-ratio). This is a typical result if the application of epoxy is minimized and cured at room temperature. Contributions to FRD can be from scattering at the fibre end-faces2, significantly impacting the FRD encircled energy, however this can be effectively managed with high quality polished fibre ends. It is normal practice to polish fibre ends in astronomy however we employed cleaved fibre ends for this test because we needed to measure the stress induced FRD caused by the connector only. Care was taken to ensure high quality cleaves but despite these attempts there was still enough variation of end-face roughness to impact the encircled energy results. An example of improved cleave quality i.e. little end-face surface roughness is shown for MTP3 fibre (b) in Table 5, this fibre has less end-face scattering than reference fibre 1.

5. SUMMARY Three standard astronomical fibre instrumentation tests were performed on two types of multi-fibre connectors, the US Conec MTP® connector and the Deutsch, MC5 MilSpec connector. The tests performed were wavelength fringing, throughput and FRD. The tests were done for both dry and gel coupled connections. We show that it is possible for both the MTP® and MC5 connector to have no wavelength fringing for a dry connection if the fibre ends and ferrule are polished flat across all fibres. Connectors that did show fringing with a dry connection were cured when coupled with an index matching gel. The performance requirements for the connector system were: wavelength fringing 90% and FRD

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