JAGGEDNESS FREE C-OFDR FOR OPTICAL COMPONENTS CHARACTERIZATION

R. Passy * Group of Applied Physics

University of Geneva 20 Rue de 1'Ecole de Medecine, 121 1 Geneve 4 - Switzerland

and J.P. von der Weid Center for Telecommunications Studies

Pontificia Universidade Catolica do Rio de Janeiro Rua Marquss de S3o Vicente 225 - Gavea, Rio de Janeiro - Brazil

Ab st ract

High resolution and high sensitivity optical components characterization was performed with a jaggedness free coherent OFDR with mdti-electrode DFB laser. The fading noise was reduced to less than 1 dEi by using a double optical frequency sweep, improving the accuracy of insertion loss measurements. Sensitivities down to -105 dE3 were achieved with minute measurement times.

It is well know that high return losses (reflectivity) produce several effects in optical networks such as crosstalk, conversion of phase noise into laser intensity noise and creating external cavity modes in the laser spectrum which

cts the linearity of the intensity modulation. In optical amplifiers, reflections can also cause lasing that drastically restricts the gain [1,2]. In order to improve the performance of broadband optical networks, the return BQSS of optical components have been then strongly reduced. Passive optical networks also require low loss optical c o ~ ~ n e n ~ due to the great number of network interconnections. Hence, new high sensitivity characterization techniques Rave to be developed in order ?o control the quality of these optical components.

ack reflection detection, Photon Counting OTDR [ 3 ] , Optical Low Coherence Reflectometry (OLCR) [4] tic$ Frequency Domain Reflectometry QFDR [SI have been proposed as return loss measurement

techniques for optical components characterization. Direct detection of the reflected light gives only the total amount ofthe backreflected light and is limited to -75 dE3. It is then impossible to distinguish between the Rayleigh backscattered light intensity and the light reflected by the particular component. In this case for low reflection levels the pigtail length has an influence on the measurement because of the Rayleigh backscattering. Photon counting in 1.55 p n requires low temperature cooling systems to decrease the detector dark current and are still not veqy escient at high resolution. On the other hand, sensitivities down to -160 dB at submillimiter resolution 161 were obtained using the OLCX technique, which is far enough for the present characterization needs . However this technique is limited to a few centimeters inspection range due to the mechanically variable delay line, giving very long acquisition times depending on the length of the pigtail of the device being inspected.

technique consist of anaiyzing the beat signal produced by the optical interference between a fixed reference reflection called local oscillator reflection and different reflections coming from the component

A laser source has its optical frequency swept linearly and the beat frequency is proportional to the (ween the LO and the reflection point while the amplitude of the beat signal is related to the reflection

intensity. The maximum range of the QFDR is limited by the coherence length of the laser whereas the spatial r e s o ~ ~ ~ o n is limited by total optical frequency deviation and the linearity of the frequency sweep. Sensitivities down lo -100 dE3 can be easily obtained with less than mW laser power thanks to the CW operation system.

uted feedback semiconductor lasers are known to have sharp laser lines and the possibility of optical equency control via the laser injection current. The performance of these lasers received a great improvement

of three section semiconductor lasers. Here the linewidth decreased down to a few lity of the order of 2 nanometers free of mode hopping, which are kndamental properties [7]. Observation of the Iiayleigh backscattering with centimeter resolution can be achieved

0-7803-2674-1/95/~E4.00 0 1995 IEEE. 590 ~ ~ ~ O ~ E E t4.TT-S IMQC'95 Proceedings

with these high performance diode lasers, strongly improving the performance of the OFDR for optical component characterization. However, due to a fading noise, the Rayleigh backscattering level jags with amplitudes up to 20 dI3 reducing the accuracy of the measurements [8]. In this work we present measurements of optical components using a jaggedness free OFDR technique.

2. C-OFDR SETUP

Figure 1 shows the coherent OFDR setup. It consists of an all-fiber Michelson interferometer, a tunable diode laser and a photodetector. The optical component under inspection is connected to one of the interferometeir' arms whereas the other provides a reference reflection. The measurements were done with a 20 mW three electrode 1.55 pm DFB laser diode pigtailed using an optical isolator. The linewidth of the laser and the chirp parameter were 8 MHz and 1.1 GHdmA respectively. The optical frequency sweep was performed by modulating the current of the central electrode with a triangular waveform at 50 Hz and 20 mA peak-to-peak. This procedure allowed a %2 GHz optical frequency variation with an excellent linearity, free of mode hopping and with low intensity modulation. We used a shot noise limited detector coupled to a DC-300kHz amplifier. The spectral analysis of the interference signal was done by an spectrum analyzer triggered by the wavefunction generator. Although the optical frequency variation was 22 GHz, we used only 9 GHz of the sweep for the measurements, corresponding to the most linear part of the optical sweep. The linearity of this part of the optical frequency sweep was better than 1%. This is an important parameter because it also limits the resolution of the measurment.

Device under test

LO

FFT Sepctrum Analyrer

Figure I - Coherent OFDR setup

The distance between the optical coupler used as beam splitter was the same for the laser and detector connections, as well as for the reference and test arm connectors. This insures that the beat signal corresponding to these reflections fall at zero frequency. However, multiple reflections will occur, giving rise to spurious peaks which can be strongly attenuated if angular polished connectors are used instead of the FGhPC connectors. An optical isolator was also used between the detector and the interferometer, in order to avoid its strong reflection. The test arm of the interferometer was terminated with an FCFC connector, used as local coscillator. The calibration of the reflectivity scale was done with the Fresnel reflection of the cleaved far end of a fiber connected to the test arm.

Fading noise in coherent OFDR is produced either by the interference between signals generated by different neighboring scatter points or by the state of polarization of the light reflected from the test ami. The light reflected by a given scattering region beats with the local oscillator, giving rise to a low frequency dectric signal whose frequency is given by the position of the scattering point. The amplitude of this electric signal depends on the intensity of the reflected light as well as on its polarization state, whereas the phase depends on the phase of the reflected light. Because there is a residual birefringence in the fiber, the state of polarization of the light reflected

back from the test arm will depend on the position of the reflecting region [9]. When the state of polarization coincides with the reference reflection the beat signal amplitude is maximum, whereas it is zero when they are orthogonal. Hence, the Rayleigh Backscattering level of a birefringent fiber fluctuates if the resolution of the measurement is greater than the beat length of the fiber, giving rise to a polarization fading noise.

The combination of beat signals generated by neighboring scattering regions also gives rise to fluctuations of the Rayleigh Backscattering level because the random character of the amplitude of the scattered light from each region. Here we have the sum of beat signals with random amplitudes while the phases are slowly varying with the position of the scattering region. This fading noise can be eliminated by varying the central optical frequency during the sweep and averaging the beat frequency spectra. This variation induces phase changes in each low frequency signal entering into the sum within the resolution of the spectrum analyzer, thus averaging out the random fluctuations. This slow frequency variation was done by changing the temperature of the laser. The total temperature variation during 1000 acquisitions was 19'C producing an optical frequency variation of 100 GHz.

3. RESULTS

Figure 2 shows the reflectogram of a 8 meter single mode fiber without (a) and with temperature variation 0). The Rayleigh Backscattering level can be recognized around -85 dB, presenting oscillations with amplitudes up to 20 dB strongly reducing the sensitivity of the system. By applying the double modulation (current and temperature) technique it can be clearly seen a significant reduction of the fading noise down to -2 dE3 peak-to-peak. The reflection peak of -63 dB located at 35 cm is produced by internal reflections within the interferometer which are detected by the system. As mentioned before, they can be eliminated by using APC connectors instead of FCPC connectors.

-70 7 i - - - - - r -80 -90

p. -100

-100 1 '------$

Length [m]

Figure 2 - Reflectogram of a 8 meter single mode fiber (a) without and (b) with double modulation technique

The decay on the Rayleigh backscattering level is not due to the fiber attenuation but due bo the loss on the interference contrast produced by the limited coherence length of the laser. The relation between the linewidth Af ofthe Baser source and the decay of the interference contrast a is:

ca 2 0 m log e

Af= --

where c is the vacuum velocity of the light and n is the fiber group refractive index.

592

100

80

rc 0 0

20

0

Double Modulation

ormal Modulation

- 80 85 90 95 100

Return loss [dB]

Figure 3 - Statistical distribution of reflection levels with single and double modulation of the optical frequency of the laser diode

-60

-70

-80 % U

rc- 8 -90

a 'S -s

8 -100

0 1 2 3 4 5 6 7 8 9

Length [m]

Figure 3 - Measurement of a mechanical splice.

The 1 dB/m decay rate of the Rayleigh Backscattering level observed in figure 2 corresponds to linewidth of 7.3 MWZ, quite close to the 8 MHz linewidth measured with the delayed self homodyne technique. The small difference between the two measurements are compatible with the observed linewidth variations depending on the laser operating conditions.

The Rayleigh Backscattering level of any guided wave device depends on the resolution of the reflectometer. In the case of a normal 1.55 pm single mode fiber, this level is given by R = 10 log[S a AI 1, where a is the Rayleigh Scattering Loss coefficient, S the recapturing factor and A l is the resolution of the measurement. In our case, a resolution of 5 cm, corresponds to the Backscattering level of -85 dB. The noise floor of our set-up is -105 dB, so that with our 1 dB/m coherence decay rate, the Rayleigh Backscattering level will fall under the noise floor around 20 m. This is, in principle, the maximum range attainable for the detection of Rayleigh Backscattering levels. However, the maximum frequency of our set-up is limited to 100 kJ3z so that Rayleigh Backscattering measurements at 20 m are only attainable with 10 cm resolution. In practice, precise measurements should only be considered above -10 dB from the noise floor, so that a practical limit for our set-up is -10 m.

The reduction of the fading noise by the double modulation technique can be better visualized by the statistical diagram presented in figure 3. Here we present the statistical distribution of reflection levels in dB and the effect of the double modulation technique. In this distribution, the linear decay due to the coherence length of the laser was compensated, so that all points were normalized to constant contrast and maximum coherence. As it can be clearly seen, we obtain a reduction in the width of the distribution from 2 .5 dB with the normal technique to 0.25 dB with the double modulation technique. We also observe a -3 dB shift of the mean value towards higher reflection levels. The reason for this shift, which was systematically observed in the different measurements, is still unknown.

As an application of our system, we measured the reflectogram of a mechanical splice, shown in figure 3. T&e reflection peak located around 3.5 meters represents the return loss of the splice. The FMWH of this reflection peak was 16 cm in contrast with the 5 cm obtained without fading noise reduction. This spatial resolution loss was produced by the variation of the chirp parameter of the laser with the temperature. Because the laser chirp parameter slightly depends on the temperature, the calibration factor of the frequency-to-lenght conversion varies during the averaging process, giving rise to a loss in the resolution. The splice insertion loss of 1.84 dB was calculated by using the fixed slope method [lo].

4. CONCLUSION

We have presented a jaggedness free coherent OFDR measurements. The fading noise was strongly reduced by using the double modulation technique thus improving the accuracy for insertion loss measurements. A loss on the spatial resolution was observed due to the chirp parameter dependence with the temperature. A spatial resolution of less than 5 cm were obtained without fading noise reduction being more appropriate for high resolution return loss measurements. Sensitivity better than -100 dB were achieved even with fading noise reduction. The range of our measuring system is limited to -10 m by the coherence length of the laser

5. ACKNOWLEDGMENTS

The authors are grateful to the Swiss telecom and Telebras for financial support.

* Present address: Center for Telecommunications Studies, Pontificia Universidade Catolica do Rio de Janeiro Rua Marques de S o Vicente 225 - Gave,, Rio de Janeiro - Brazil

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