Self-interference of long-period fibre grating and its ... - IEEE Xplore

Self-interference of long-period fibre grating and its application as temperature sensor B.H. Lee and J. Nishii The authors describe self-interferencefringes created in a single long-period fibre grating (LPG) formed in a double-cladding fibre, one end of which is coated with a metal film. The temperature-induced fiinge-shift in the reflection spectrum of the proposed LPG device was measured to be 0.055nmi"C with 0.9"C RMS deviation. Applications of such devices as filters and fme temperature sensors are expected. Introduction: The LPG (long-period fibre grating) is known as a loss device, where the coupled cladding modes are generally absorbed or scattered at the cladding-coating boundary [I]. By removing the coating layer of the fibre and inserting an identical LPG in series, the coupled cladding mode can be recoupled to the core mode. The in-series LPG has a sinusoidal transmission spectrum and has been used as a filter and a sensor [2 - 41. However, making an identical LPG pair is not simple, and furthermore, only the transmission spectrum can be used because a conventional LPG has no appreciable reflection [I]. In this Letter, we present a self-interfering LPG (SLLPG) device, which gives a sinusoidal reflection spectrum. To implement the device, we form a metal coating on one end surface of the fibre that embeds a single conventional LPG. The reflection spectrum of the proposed device is the same as the transmission spectrum of an LPG pair, which is composed of two identical LPGs with the corresponding grating separation. As a practical application, the temperature of an oven is measured by monitoring the shifts in the fringes formed by the proposed LPG device. Owing to the narrow bandwidth of the fringe, the sensor device based on the SILPG inherently gives a better resolution than that based on a conventional LPG [5, 61.

Theory: The formation of interference fringes in an LPG pair has been analysed by successively using the standard coupled mode equations [4, 71. The same analysis can be used to study the properties of the SILPG composed of an LPG and a fibre reflector. A beam passing through an LPG and being reflected by the reflector will see the same LPG and form self-interference. 'fierefore, we can obtain a sinusoidal reflection spectrum that is the same as the transmission spectrum of the ideal LPG pair, the separation of which is twice the distance from the centre of the LPG to the reflector. The schematic configuration of the proposed LPG device is depicted in Fig. 1.

Experiments: An LPG was made in a hydrogen loaded DSC (dual shape core) type DSF (dispersion shifted fibre) fibre (Mitsubishi) by illuminating a KrF excimer laser beam through an amplitude ~ and a 20" length. The mask. The grating had 5 0 0 periodicity intensity of the grating was controlled to have -3dB loss at the centre of the stop-band positioned at - 1 . 5 5 ~ .One end of the fibre embedding the LPG was cleaved and coated with silver using a DC sputter in order to make a reflector. The coating thickness was estimated to be > 50 nm, enough to neglect the skin effect of a thin metal film. The distance from the centre of the LPG to the cleaved end was -30".

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Fig. 2 Reflection spectrum of SILPG measured at room temperature

LPG had 5 0 0 p periodicity and 20" centre to reflector was 30"

length; distance from grating

As shown in Fig. 2, the reflection spectrum of the SILPG has a series of interference fringes in each stop-band. However, the contrast of the fringes worsens with the order of the corresponding stop-band, which results from the interaction with the coating on the side of the fibre and from the poorly cleaved surface. Because of instrument limitations, the coating on the fibre side was inevitable from the coating of the end surface; furthermore, the coating thickness on the side was not uniform. Chips or cracks on the cleaved surface, which might be induced by the fibre cleaver, have a deleterious effect on the reflection of the cladding modes. The fringes in the first stop-band have rather good contrast; the corresponding cladding mode is considered to be distributed strongly near the centre of the fibre, where the cleaved surface has rather good optical quality.

overt

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I holler

cz:silver Eg

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ma

-

(-

. aXspeczng x

+ h,ff 1 ") L dT

(1)

where L is the length between the centre of the LPG and the is the fringe spacing, and Anerris the differential reflector, AXspaerng effective index.

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In the case of low grating strength, from the analysis used for an LPG pair [7] and by performing some mathematical manipulations, the thermally induced fringe-shift of an SILPG is obtained as dT

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oven temperature,%

Fig. 1 Schematic diagram of setup for temperature measurements using SILPG Fibre end coated with silver film to make reflector

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Fig. 3 Temperature induced fringe-shift of SILPG Shifts of several fringes located near band centres of first two stopbands were plotted best fitting linear curves

For temperature sensor applications, the SILPG was placed in an oven and the reflection spectrum was measured while the oven temperature was varied between 75 and 145°C. The shifts in the fAges in the first two stop-bands are plotted in Fig. 3 against oven temperature. To avoid the skew effect at the band edge, only several fringes near the centre of each stop-band were measured. The solid lines of the Figure show the best fit linear curves, from which we can obtain the thermal coefficient of the fringe-shift to be -0.055rmd"C. From eqn. 1 the contribution of the thermal expansion of the fibre length was calculated to be O.3pd0C, less

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than 1% of the total shift. The fringe spacing (4.4nm for the fringe in the first band) was measured from Fig. 2, and the differential effective index was obtained from the mode condition of the LPG [l] (An, = UA, where A is the grating periodicity). The thermal expansion coefficient of fused silica (0.5 x 1Ck6/OC)was used in the calculation. Therefore, we can conclude that most of the fringeshift was contributed by the thermal variation of the differential effective index that is calculated to be dArzJ3T = 0.3 x 1Ck6from eqn. 1.

4OGbit/s 1.55p.mpin-HEMT photoreceiver monolithically integrated on 3in GaAs substrate V. Hurm, W. Benz, W. Bronner, A. Hulsmann, T. Jakobus, K. Kohler, A. Leven, M. Ludwig, B. Raynor, J. Rosenzweig, M. Schlechtweg a n d A. Thiede 36.5GHi bandwidth, 1 . 5 5 ~wavelength pin-HEMT photoreceiver with a distributed amplifier has been monolithically integrated on a 3in GaAs substrate using a 0.15pm gate-length pseudomorphic HEMT process. The pin photodiode has a responsivity of 0.34A/W. Clearly-opened eye diagrams for a 4OGbitk optical data stream have been demonstrated.

A

Discussion: Owing to instrument limitations, we could not obtain the spectrum of the proposed device with 100% fringe contrast. The fringe contrast is considered to be improved by polishing the cleaved surface and by removing the unwanted coating on the fibre side. We note that no appreciable change was observed in the relative phase between the fringe and its embedding band within the 70°C dynamic range. The RMS deviation of the measured temperature from the applied temperature is calculated to be 0.9.C. Most of the deviation resulted from the error in reading the oven temperature, M.5"C; a conventional mercury thermometer was used. The oven was initially heated up to 150"C, and then the fringe-shifts were measured while the oven was cooled down. The maximum cooling rate was 1.5"C/min. The different locations and the different thermal masses between the thermometer and the SILPG are considered to induce non-negligible measurement error. Future improvements in the apparatus are expected to allow better sensor performance.

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Introduction: Monolithic integrated photoreceivers for 1.55pn wavelength are usually realised on I&' substrates. An alternative is to grow the absorbing InGaAs layer on a GaAs substrate to take advantage of a well-established GaAs-based electronic device technology. Recently we reported the fist 2OGbit/s 1 . 5 5 MSM~ HEMT photoreceiver grown on GaAs, combining an InGaAs MSM photodiode and 0 . 3 gate-length ~ AlGaAdGaAs HEMTs [I, 21. Furthermore, we manufactured the first 1OGbiUs 1 . 5 5 ~ pin-HEMT photoreceiver grown on GaAs using the same HEMT process [l, 31. We have now integrated the InGaAspin photodiode in our 0 . 1 5 gate-length ~ AlGaAslInGaAdGaAs pseudomorphic HEMT process. Here we present the fist 40GbiUs 1 . 5 5 pin~ HEMT photoreceiver grown on GaAs. pin-PD airbridge

Conclusion: We have demonstrated that a conventional LPG could give a sinusoidal reflection spectrum by forming a reflector on one end of the embedding fibre. The beams coupled by an LPG are reflected by the reflector and then recoupled by the same LPG, which gives self-interference fringes. The fringe formed in the first stop-band of a double-cladding fibre showed good fringe contrast. Most of the thermal induced fringe-shift is caused by variations in the differential effective index, and was measured to be O.O55nm/"C with a 0.9"C RMS deviation within a dynamic range of 75145°C. The proposed device is expected to be used as a filter for WDM telecommunications and as a sensor for f i e temperature measurements. 0 IEE 1998

15 September 1998

Electronics Letters Online No: 19981420

B.H. Lee and J. Nishii (Osaka National Research Institute, AIST, I-831 Midorigaoka, Ikeda, Osaka 563-8577, Japan)

References VENGSARKAR, A.M., LEMAIRE, P.J., JUDKINS, J.B., BHATIA, V., ERDOGAN, T., and SIPE, J.E.: 'Long-period fiber gratings as band-

rejection filters', J. Lightwave Technol., 1996, 14, pp. 58-64 TALLONE, L., BOSCHIS, L., COGNOLATO, L., EMELLI, E., RICCARDI, E.,

and ROSSOTTO, 0.: 'Narrow-band rejection filters through fabrication of in-series long-period gratings'. Proc. Conf. Optical Fiber Commun., 1997, p. 175 GU, x.J.:'Wavelength-division multiplexing isolation fiber filter and light source using cascaded long-period fiber gratings', Opt. Lett., 1998, 23, pp. 509-510 DIANOV, E.M., VASILIEV, s.A., KURKOV, AS., MEDVEDKOV, o.I., and PROTOPOPOV, v.N.: 'In-fiber Mach-Zehnder interferometer based on a pair of long-period gratings'. Proc. European Conf. Optical Commun., 1996, pp. 65-68 BHATIA, v., and VENGSARKAR, A.M.: 'Optical fiber long-period grating sensors', Opt. Lett., 1996, 21, pp. 692-694 PATRICK, H.J., WILLIAMS, G.M.,KERSEY, A.D., PEDRAZZANI, J.R., and VENGSARKAR, A.M.: 'Hybrid fiber Bragg gratingilong period fiber grating sensor for strainltemperature discrimination', IEEE Photonics Technol Lett., 1996, 8, pp. 1223-1225 LEE, B.H., and NISHII, J.: 'Bending sensitivity of in-series long-period fiber gratings', Opt. Lett., 1998, (in press)

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Fig. 1 Cross-section of InGaAs pin photodiode integrated with pseudomorphic GaAs-based H E M T

Design and fabrication: In our approach, the vertical structure is grown on the semi-insulating 3in GaAs substrate by a single molecular beam epitaxy run without growth interruption. As shown in Fig. 1, the AlGaAslInGaAdGaAs HEMT layers are grown first, followed by the 600nm thick buffer of compositionally graded AlGaInAs to accommodate the lattice mismatch between the GaAs substrate and the InGaAs photodiode layers. The pin photodiode consists of three layers: a 300nm thick n+-layer, a 400nm undoped absorption layer, and a 300nm p+-layer [3]. The photodiode mesa is structured by a combination of non-selective wet etch processes and a selective dry etch process which stops at the AlGaAs etch-stop layer above the HEMT layers. The HEMT layer structure is designed for best operation at zero gate-source bias. The 0 . 1 5 ~gate-length pseudomorphic HEMTs with a 12nm thick Iq.ZSGq.75A~ channel have an extrinsic transconductance g, of 745mS/mm, a current-gain cutoff frequency f T of 90GHz, and a maximum oscillation frequencyf,, of 15OGHz. Our process enables the fabrication of NiCr thin f i resistors, MIM capacitors, and airbridges. The interconnection between different devices is achieved through two levels of metallisation. The manufactured photoreceiver includes a four-stage distributed amplifer and a pin photodiode with a circular light-sensitive area of diameter l o p (Fig. 2). The photodiode is surface-illuminated through an anti-reflection coating which is also used as the dielectric layer in the MIM capacitors. Each stage of the distributed amplifier consists of a cascode pair of HEMTs with 9 0 gate width, embedded in 70Q coplanar transmission lines (Fig. 3).

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