Kilometric optical fiber interferometer

Kilometric optical fiber interferometerLaurent Delage and François Reynaud

Equipe optique, Institut de Recherche en Communications Optiques et Micro-ondes (IRCOM), Unité Mixte deRecherche 6615, 123, avenue A. Thomas, 87060 Limoges Cedex France

[email protected] and [email protected]

Abstract: We report on a preliminary experimental study of aninterferometer built with two 500 meters long arms made of polarizationmaintaining optical fibers. The control of the field polarization state alongthe single-mode fiber arms enables to measure fringe contrast up to 93%with a laser source emitting a 1290nm carrier wavelength. Tests achievedwith broadband spectrum exhibit dispersion differential effect resultingfrom fiber inhomogeneities. Partial compensation of this effect is achievedintroducing additional fiber pieces on one arm. 2001 Optical Society of AmericaOCIS codes:(060.2310) Fiber optics, (120.3180) Interferometry

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(C) 2001 OSA 10 September 2001 / Vol. 9, No. 6 / OPTICS EXPRESS 267#34870 - $15.00 US Received July 30, 2001; Revised August 29, 2001

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1. Introduction

In a synthesized aperture implemented for stellar interferometry applications, the use ofoptical fiber to carry the light beams from telescopes to the mixing station is a goodalternative to the conventional device consisting of vacuum or air tubes and mirror trains[1,2]. Main advantages of fiber use come from their high transmission of light flux, theflexibility of these waveguides and spatial filtering which reduces the effects of theatmospheric turbulence and others instrument optical aberrations [3,4].

Nevertheless, to preserve the optical field coherence all along the fiber, various effectsmust be compensated or controlled. First, the dispersive properties of fibers lead to differentialeffects reducing the fringe visibility contrast for interferometric applications [5,6]. Second, thehigh sensitivity of fibers to thermal and mechanical perturbations induces difficulties tocontrol the polarization state [7,8] but also compels to control the fiber optical length with ametrological device [9,10]. Experimentally, for an interferometric device including three 25-m-long highly birefringent (HB) fiber arms, full control of optical path stability and correctionof dispersion and polarization differential effects have been achieved. It led to the observationof well-contrasted fringes with a randomly polarized source radiating a broadband spectrum(650-850nm) [11]. On the other hand, the various techniques developed at the IRCOMInstitute led to the implementation of an all guided interferometer using fiber delay line tosynchronize the fields and fiber couplers for the beam recombination [12].

At the present time, several projects plan to connect elementary telescopes of a long-baseline interferometer over hundreds meters such as OHANA project [13-15]. Then, wepurpose to experimentally investigate the potentiality of silica fiber to propagate coherentlylight beams over kilometric spans.

2. General Description of the Experiment

The experimental set up is shown in the figure 1 and consists of a Mach-Zehnderinterferometer. Each interferometric arm is implemented with a 500 meter long HB1250polarization maintaining optical fiber with a λc=1090nm cut-off wavelength and a 1mm beat-length. With a very similar fiber, the dispersion at 1270 nm is in the range of -10 ps/nm.km.The stress-applying part of this fiber is ‘Bow-Tie’ type with an extinction ration in the rangeof 20dB. These two fiber arms are came from the same fiber roll in order to minimize alldifferential effects. The manufacturer matches the two sections of fiber in geometrical lengthwith twenty centimeters accuracy.

Our interferometer can be alternately fed by two light sources. The first one is a laserdiode emitting a 1290nm mean wavelength. The coherence length of this source can bemodified by adjusting the driving current. We obtain either a narrow spectrum with 0.1nmspectral bandwidth or, under the laser threshold, a broadband emission with a 30nm spectralbandwidth. The second source consists of a LED with a 140nm spectral bandwidth around1270nm. These wavelengths have been intentionally selected with a view to taking advantageof the low chromatic dispersion effects of silica fibers in these spectral domains. For otherwavelength, the material dispersion increases. It could involve difficulties to locate the zerogroup delay between the two fiber arms because the fast degradation of fringe contrast if thedispersion characteristics of the two fiber sections are not accurately matched. These varioussources are not spatially coherent. We insert a standard single-mode fiber between the sourceand the collimator focal plane to avoid reducing the measured fringe visibility by spatialcoherence effects. The emerging beam is linearly polarized with a polarizer. After passingthrough the collimator, the optical field is partly collected by two microscope objectivesacting as telescopes and located in the same wave front. These apertures feed the fiber inputs


taking care to align the linearly polarized optical field with the fast axis of the two inputsections of HB fibers. Each fiber output is placed on a beam collimating assembly including amicroscope objective.

Fig. 1. Overall layout of the interferometric experiment

One of the interferometric fiber arms has a 4-m fiber length wound over a 60-mm-diameter cylindrical piezoelectric transducer (PZT). This modulator generates an optical pathmodulation used to temporally scan the fringes. The full stroke of this fiber stretcher is in therange of 100µm. In addition, an optical fiber delay line [6,12] has been implemented with a12cm total stroke to control the differential group delay in the interferometer with an accurateresolution. This fiber delay line consists of seven meters of HB fiber wound and glued on arubber rim. A mechanical process drives the rubber rim radial expansion so that the fiber isuniformly stretched. The total geometrical length of the first fiber arm of the interferometer isclose to 500m. The second interferometric arm includes 500m of HB fiber and an air delaybased on the output beam collimating assemblies motion by steps of 10cm. A beam splitterachieves the recombination of the linearly polarized output beams. This interferometricmixing is launched in a standard single-mode fiber to perform a spatial filtering that cancelsthe influence of beams overlapping quality on the fringe visibility contrast. The emergingbeam is focused on an InGaAs monopixel detector.

3. Experimental Results

In a first experimental step, we have characterized the cross-coupling between the twoprincipal polarization modes for each interferometric arm. The following relation defines theextinction ratio in decibel:


]yI)yIx(I10.Log[η ++++==== (1)

Where Ix is the intensity of excited mode and Iy the parasitic intensity coupled to thesecond polarization mode. For the two interferometric arms, this ratio was better than 15dbthroughout the experiment. It will lead to a 5% theoretical loss of fringe contrast.

In the second experimental step, we have looked at the fringe position by canceling thegroup delay between the two 500m interferometric arms. For this purpose, the laser diode isused under the threshold leading a short coherence length. We have scanned the fringe patternby using the fiber PZT modulator. The equalization of the group delay was achieved byactuating the fiber delay line placed in the first interferometric arm for various positions of theoutput collimating assembly of the second fiber arm. The fringes have been found with a 20cm air delay. If we consider homogeneous fiber, the corresponding difference in geometricallength between the two interferometric arms would be 13cm. The flux on each recombinedbeams are alternately measured to take the photometric effects on the fringe visibilitymeasurements into account. When the laser operates in the narrow spectral bandwidth mode,the measured contrast is equal to 92%. With this source, the differential chromatic dispersioneffects are not significant. Consequently, this high level of visibility contrast demonstrates thegood control of the polarization status of the propagating waves. The fringe contrastsmeasured with large spectrum sources i.e. 30nm and 120nm are close to 24% and 11%respectively. These low contrasts observed with a broadband spectrum source can be justifiedby a strong chromatic dispersion effect between the two fiber arms. The inhomogeneity of thewaveguide structure can mainly explain this differential effect of dispersion over this spectraldomain where the material dispersion is near zero [5].

The third experimental step was based on the visibility contrast improvement with abroadband spectrum source by adding various HB single-mode fiber pieces in one of the arms.To search the least dispersive configuration, we alternately plug a fiber sample on eachinterferometric arm and we measure the fringe contrast evolution. We equalize the two opticalpaths of the interferometer by adjusting the air delay in order to roughly cancel the equivalentair path generated by the additional fiber sample. Then we accurately retrieve the fringepacket around the zero group delay by continuously actuating the optical fiber delay line.Figure 2 reports on the different visibility measurements got for various spectral bandwidthsas a function of the equivalent air path associated to additional fiber samples. The geometricallengths of these additional fiber samples varied between 0.5m and 7.2m. The weak dispersionof the silica fiber material around 1300nm justifies the use of large geometrical length ofadditional sample in order to compensate the dispersion between the two interferometric fiberarms.

With the narrow spectrum source, the fringe visibility is close to 93% with a 1.3%standard deviation whatever the additional fiber sample. These high levels of contrast proveonce again the good control of the polarization state all along the propagation in theinterferometer. The visibility measurement deviation is mainly due to the large fluctuations ofthe intensity that is not monitored in real time. The maximum of contrast observed with thetwo broadband sources are 69% and 42% for a 30nm and 140nm spectral bandwidthrespectively. These contrasts are obtained with a 5.2m long additional fiber sample. Owing tothe fact that the field polarization state are mastered, the degradation of the visibility contrastaccording to the spectral bandwidth can be explain by the variable dispersion between the twofiber arms resulting from inhomogeneities.


Fig. 2. Fringe contrast measurements versus the equivalent air path generated by variousadditional fiber samples for three diverse spectral bandwidths : 0.1nm (green diamond), 30nm

(red square) and 140nm (blue cross).

The experimental results show that the differential dispersion effects cannot be cancelledonly by adjusting the difference in optical length between the two fibers. Consequently anadditional experimental study is necessary to achieve a spectral analysis to determine thedifferential chromatic dispersion evolution.

4. Conclusion

We have experimentally demonstrated the possibility to obtain well-contrasted fringes in akilometric interferometer. The use of polarization maintaining fiber enables to control thepolarization state leading to high contrast measurement with a laser source. Nevertheless, thedispersion inhomogeneity of the fibers induces fringe contrast reduction for a source with abroadband spectrum. A partial compensation allowed us to minimize this effect, the contrastreaching 42% for a 140nm spectral bandwidth around 1270nm.

To continue this study, we plan an experimental investigation to characterize thedifferential dispersion evolution according to the various additional fiber sections. A spectralanalysis of the interferometric mixing using the channeled spectrum method [16] will allow toaccurately measure the differential effect of chromatic dispersion i.e. second and third orderterm of the phase shift versus frequency. Only this analysis will allow modelling thedispersion characteristics and the corresponding evolution of fringe contrast. Moreover, weschedule a study on the metrological system devoted to control the optical path difference inthe frame of a long-baseline interferometer with kilometric arms. This interferometric devicecan be a good solution to perform the connection of several elementary telescopes such as inthe OHANA project or in the frame of large spatial interferometer.


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Kilometric optical fiber interferometer