Direct determination of main fibre Bragg grating parameters using OLCR

P.Y. Fonjallaz H.G. Limberger R.P. Salathe Ch. Zimmer H.H. Gilgen

Indexing t e r m Fibre gratings, Reflectometry

Abstract: Optical low coherence reflectometry (OLCR) is applied to the characterisation of homogeneous Bragg gratings written in single- mode optical fibres with a UV laser. This tech- nique allows for the measurement of the spatial distribution of the field reflection coefficient within the fibre witha spatial resolution of 9 pm in the fibre material. A grating with a high spatial defin- ition has been measured and its length precisely determined. The amplitude of the index modula- tion was then obtained by the absolute signal level just after the entrance of the grating on the cali- brated OLCR spectrum. Thus, this method gives a direct information of the main fibre Bragg grating parameters, the length and the index modulation.

1 Introduction

Monomode optical fibres are photosensitive: a per- manent change of the refractive index can be induced by the absorption of ultraviolet or blue light [l]. This effect has mainly been observed in monomode fibres as it is generally linked to the presence of the germanium dopant of these fibres. The photoinduced modification of the refractive index is supposed to be initiated by the break- ing of 'wrong bonds' between Ge and Si atoms [2, 31. These bonds are present because of an oxygen deficiency during the fabrication of the preforms. A periodic varia- tion of the refractive index can be induced in the core of a monomode optical fibre by exposing it laterally to the interference fringes of a laser emitting around 240nm [4]. The index modulation occurs as a result of the modulation of the intensity thus forming a grating usable as a Bragg reflector.

Fibre Bragg gratings normally are characterised by measuring the wavelength spectrum. In the following, another method of characterisation is applied; called optical low coherence reflectometry (OLCR). OLCR is essentially the application of a Michelson interferometer using a broadband light source [S, 61. The spatial dis-

0 IEE, 1994 Paper 99951 (E3, E13), first rmived 7th July and in revised form 12th October 1993 P.Y. Fonjallaz, H.G. Limberger, and R.P. Salathi: are with the Labor- atoin d'Optique Appliquk, Ecole Polytechnique Ftdbrale, 1015 Lau- sanne, Switzerland Ch. h e r and H.H. Gilgen are with the Technical Center, Swiss €TI', 3000 Bern 29, Switzerland

IEE Proc.-Optoelectron., Vol. 141, No. 2, April 1994

tribution of the field reflection coefficient of a grating placed in one arm can be measured by displacing the mirror of the reference arm and monitoring the inter- ference signal. The spatial resolution of the measurement is determined by the coherence length of the LED source used.

2 Experiments

The setup used for the fabrication of gratings is rep- resented in Fig. 1. The laser system, a frequency-doubled

Pellin- Broca prism residual

blue

doubled

dye laser

I focusing optics I

Fig. 1 Experimental setupfor gratings formation

XeCl excimer pumped dye laser, is characterised by an emission wavelength at 240 nm, a pulse duration of 20 ns and pulse energies up to 2mJ. The coherence length is increased to about 20cm by inserting an etalon in the

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oscillator of the dye laser. The output beam still consists of a large proportion of blue light which has to be elim- inated using a Pellin-Broca prism to avoid damage to the 240 nm-suited coatings. The geometry of the beam is adjusted to match that of the fibre. First, the beam is enlarged by a beam expander. It is then focused perpen- dicularly to the fibre axis and energy densities per pulse of up to 200 mJ/cm2 are obtained. The expansion of the beam enables the selection of its central part by placing a mask directly in front of the fibre to be irradiated. Such a top-hat irradiation intensity profile allows for the forma- tion of gratings with high spatial definition.

The interference fringes are created with a very compact interferometer based on the Lloyd's mirror prin- ciple. A dielectric mirror is placed at right angles to the irradiated fibre, almost touching it. The beam is centered at the point of intersection of the fibre and the plane of the mirror. Half of the beam will hence cross the other half and the result is a series of fringes in the region of the intersection perpendicular to the fibre axis. This type of interferometer offers the advantage that the position of the fringes depends only on the relative position of the fibre and the mirror which are mechanically very stable. Phase difference instabilities, induced by the fluctuations of the refractive index of air, may create a problem in the Mach-Zehnder type interferometer, but are strongly reduced in this Lloyd configuration. The resonance wave- lengths of the gratings are selected by rotating the mirror-fibre assembly around an axis perpendicular to the fibre-beam plane.

To detect the formation of the gratings and to spec- trally characterise them, an entirely fibre-guided measure- ment setup is used. An edge-emitting pigtailed LED emitting at 1.3 pm is connected to a 3 dB coupler. The light reflected back through the coupler by a grating is analysed with a spectrometer of 1 m focal length whose maximal resolution is 16 pm (0.16 A). To detect the for- mation of a grating, the resonance wavelength is first determined and the spectrometer input slit is then com- pletely opened to monitor all of the reflected light from the grating. The increase in light reflection on grating for- mation can hence be controlkd online. A transmission mcasurement is performed by connecting the LED to the arm in which the grating is placed. In this manner, the reflection spectrum can be calibrated as losses at the res- onance wavelength are negligible.

The OLCR measurement is realised using a Michelson interferometer (Fig. 2). The central part of this interfer- ometer is a 3 dB coupler for 1.3 pm [7]. At one side of the coupler an edge-emitting LED and an InGaAs photodiode are connected. At the other side, the two interferometer arms are to be found. In the measurement arm, the grating to be characterised is incorporated. The reference arm consists of collimating optics and an alu- minium mirror which is mounted on a motorised trans- lation stage. Because of the low coherence length of the source, the interference signal is only detectable for light reflected in both arms at virtually identical optical dis- tances. By displacing the reference mirror, information about the spatial distribution of thc field reflection coeffi- cient in the measurement arm is obtained. In the refer- ence arm, a piezoelectric-driven phase modulation (MOD in Fig. 2) is inserted. The mirror displacenent is done stepwise. At each step, the amplitude of the interference signal is monitored using a lock-in amplifier.

The resolution of the OLCR setup used is obtained by a measurement of the reflection at a carefully polished fibre end [7]. A signal FWHM value of 13 pm in air was

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measured which gives a value of L, = 9 pm in the fibre. This corresponds to the half of the coherence length of the light source defined with a 50% fringe visibility

L2 n AI L, = 0.882 ~

coupler H I 1 mod

reference Fig. 2 LED: light emitting diode: PD photodiode; CO. coupler; MOD: modulator

Experimental setup /or OLCR measurements

AA is the FWHM of the LED source and is 54 nm. L, is half of the value of L, owing to the back and forth path of the light within the interferometer arms.

3 Results

A very short grating has been formed in a standard depressed cladding PCVD fibre. The irradiation condi- tions were as follows. A pulse energy of 0.2 mJ, which gives a fluence per pulse of 30 mJ/cm2, a total fluence of 21 kJ/cm2 and a pulse repetition rate of 100 Hz.

Fig. 3 shows the result of an OLCR measurement of this grating. On the x-axis is depicted the relative posi-

0016r

reference mirror position, m

Fig. 3 mirror position in uir

tion of the reference mirror in units of optical distance in air. The y-axis represents the interference signal normal- ised in such a way that it corresponds to the field reflec- tion coefficient in the measurement arm. The grating is clearly defined with two steep slopes at its entrance and exit. The rising interval at the entrance, defined as the distance on which the signal inreases from 10 to 90% of its maximal value, is equal to 44 pm in the fibre. For a grating with a perfectly homogeneous index modulation, one expects this rising interval to be almost identical to the spatial resolution of the OLCR measurement. The

Measured field reflection coefficient r as Junction of reference

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larger measured interval is due to diffraction on the edge of the mask which was placed more than 100 pm from the fibre. After the initial rapid rise the signal decreases slowly. This might be due to small period and amplitude inhomogeneities of the index modulation. At the exit of the grating, the signal decreases in an interval which has a width of 36 pm, probably also increased by diffraction effects. The FWHM of this field reflection coefficient dis- tribution is taken as a definition of the grating length. It corresponds to L = FWHM/n = 330 pm within the fibre, where the FWHM is measured in air (mirror position) and n is the refractive index for which a value of 1.45 was taken.

Fig. 4 represents the OLCR signal defined as 20 times the ten-based logarithm of the field reflection coefficient.

reference mirror positan. rnm Fig. 4 in air OLCR ugnal = 20 log,&)

Measured OLCR signal asfunction of reference mirror position

This shows the high sensitivity of this measurement tech- nique. Signals as low as - 100 dB can easily be detected. This corresponds to a field reflectivity of and index changes below An interference signal is still present even after the physical length of the grating. Multiple oscillations similar to those in a DFB structure lead to contributions which appear in positions corresponding to longer optical paths. These oscillations are more numer- ous and decrease slower in gratings with a higher/ coupling coefficient and length product [SI. From the absolute level of the interference signal, the amplitude of the index modulation can be obtained. The maximum field reflection coefficient of about 1.2 k 0.1 x is found at the input of the grating. Since there are no multiple oscillations at this position, the detected signal is due to reflections in a zone whose extension corresponds approximately to the spatial resolution L, of the interferometer. This allows for the calculation of the index change An by applying the coupled-mode theory which gives an expression of the field reflectivity

r = tanh (7 x Ani LR)

where A,, is the resonant wavelength in air and q is the confinement factor of the fundamental mode for which a value of 0.83 is taken. As r is very small, eqn. 2 can be approximated by n AnqLR/Ao. The error is less than 1% for r < 0.1735. An index change of 6.6 f 0.5 x can hence be calculated. This calculation is only applicable when the resonance wavelength of the grating and the

IEE Proc.-Optoelectron., Vol. 141, No. 2, April 1994

LED spectrum are centred (here both are centred at 1.29 pm). In this case, the total LED spectrum is reson- ant, because the bandwidth of this small part of the grating of a length L, is larger than that of the LED.

The mean index change of the grating can however be more precisely determined by comparing measured and calculated wavelength reflection spectra [SI. The meas- ured spectrum is symmetrical and sidelobes are visible (Fig. 5, dotted line). The low reflectivity of this grating,

1282 1284 1286 1288 1290 1292

wavelength, nm

Fig. 6 0-0 measured

~ calculated

Measured and calculated refection spectra

only 16%, determined by a transmission measurement, is due to the very short grating length. The bandwidth of 3.2 nm, defined as the interval between the first minima, is very large as expected.

Theoretical reflection spectra are calculated by varying the index change while the grating length determined by the OLCR measurement is kept constant. Because it is determined on the whole reflection spectrum, this value has to correspond more accurately to the mean index modulation amplitude within the grating than the value directly obtained from the OLCR spectrum on a limited portion. The adjustment between the measured and cal- culated spectra gives a value of 6.1 x which will be taken as a reference. The theoretical spectrum obtained is shown in Fig. 5 as a solid line.

The relative difference between both determined values of the index change is only 8%. This indicates that the OLCR measurement is sufficient for obtaining the main parameters of a homogeneous Bragg grating. The modu- lation index is simply proportional to the measured field reflection coefficient at the entrance of the grating, but the latter has to be carefully calibrated. It is also possible to evaluate the maximal spectral reflectivity R,, of the grating using only the OLCR information

R,,, = tanh2( E) (3)

Here we obtain R,,, = 17% which agrees well with the measured value and confirms that the definition taken for L, was appropriate.

A dynamic range of more than 110dB, as in this experiment, is not necessary. An index change of lo-’ gives an OLCR signal of about - 75 dB. Thus, a dynamic range of 80 dB is sufficient even for the determination of the length of a well defined grating. Such a dynamic range is relatively easy to obtain with standard LED sources and detection systems.

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4 Conclusion

OLCR measurements have been used to characterise a fibre Bragg grating with a spatial resolution of 9 pm in the fibre material. The grating was formed by exposing a fibre to the fringes of a UV laser beam. The interference pattern has been generated using a compact and stable configuration based on the Lloyd’s mirror principle. Only the central part of the beam was selected by placing a mask in front of the irradiated fibre. The OLCR measure- ment allowed for the precise determination of the loca- tion and the length of the grating. The absolute OLCR signal level was calibrated to represent the field reflection coefficient, and was used to evaluate the amplitude of index modulation within the grating. The value obtained was only 8% higher than a reference value of 6.1 x deduced by a more precise method. It has been shown that OLCR measurements give the main fibre Bragg grating parameters in a relatively direct way. The dynamic range of the OLCR setup used was about 110 dB which allows for the detection of index changes lower than 10 6 .

5 References

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