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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 10 — May. 20, 2013
  • pp: 11794–11807
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All-optical, thermo-optical path length modulation based on the vanadium-doped fibers

Ziga Matjasec, Stanislav Campelj, and Denis Donlagic  »View Author Affiliations


Optics Express, Vol. 21, Issue 10, pp. 11794-11807 (2013)
http://dx.doi.org/10.1364/OE.21.011794


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Abstract

This paper presents an all-fiber, fully-optically controlled, optical-path length modulator based on highly absorbing optical fiber. The modulator utilizes a high-power 980 nm pump diode and a short section of vanadium-co-doped single mode fiber that is heated through absorption and a non-radiative relaxation process. The achievable path length modulation range primarily depends on the pump’s power and the convective heat-transfer coefficient of the surrounding gas, while the time response primarily depends on the heated fiber’s diameter. An absolute optical length change in excess of 500 µm and a time-constant as short as 11 ms, were demonstrated experimentally. The all-fiber design allows for an electrically-passive and remote operation of the modulator. The presented modulator could find use within various fiber-optics systems that require optical (remote) path length control or modulation.

© 2013 OSA

1. Introduction

Optical-path length (OPL) control and modulation are frequently required within various optical systems such as low-coherence interferometers [1

1. G. Beheim, “Remote displacement measurement using a passive interferometer with a fiber-optic link,” Appl. Opt. 24(15), 2335–2340 (1985). [CrossRef] [PubMed]

7

7. D. Donlagic and E. Cibula, “An all-fiber scanning interferometer with a large optical path length difference,” Opt. Lasers Eng. 43(6), 619–623 (2005). [CrossRef]

], optical coherence tomography (OCT) [8

8. D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory, C. A. Puliafito, and J. G. Fujimoto, “Optical coherence tomography,” Science 254(5035), 1178–1181 (1991). [CrossRef] [PubMed]

11

11. W. Y. Oh, B. E. Bouma, N. Iftimia, R. Yelin, and G. J. Tearney, “Spectrally-modulated full-field optical coherence microscopy for ultrahigh-resolution endoscopic imaging,” Opt. Express 14(19), 8675–8684 (2006). [CrossRef] [PubMed]

], microwave photonics systems [12

12. J. Tratnik, L. Pavlovic, B. Batagelj, P. Lemut, P. Ritosa, M. Ferianis, and M. Vidmar, “Fiber length compensated transmission of 2998.01 MHz RF signal with femtosecond precision,” Microw. Opt. Technol. Lett. 53(7), 1553–1555 (2011). [CrossRef]

], coherence optical communications [13

13. W. Z. Li and J. P. Yao, “Investigation of photonically assisted microwave frequency multiplication based on external modulation,” IEEE Trans. Microw. Theory Tech. 58(11), 3259–3268 (2010). [CrossRef]

], etc.

Whilst small changes in optical path length can be efficiently achieved through electro-optic modulation, larger changes exceeding tens of micrometers mostly remain within the domain of mechanical-scanning devices. Moving mirrors [1

1. G. Beheim, “Remote displacement measurement using a passive interferometer with a fiber-optic link,” Appl. Opt. 24(15), 2335–2340 (1985). [CrossRef] [PubMed]

4

4. X. M. Zhang, Y. X. Liu, H. Bae, C. Pang, and M. Yu, “Phase modulation with micromachined resonant mirrors for low-coherence fiber-tip pressure sensors,” Opt. Express 17(26), 23965–23974 (2009). [CrossRef] [PubMed]

] and fibers wound onto piezoelectriczirconate-lead titane transducers (PZT) [5

5. H. S. Choi, H. F. Taylor, and C. E. Lee, “High-performance fiber-optic temperature sensor using low-coherence interferometry,” Opt. Lett. 22(23), 1814–1816 (1997). [CrossRef] [PubMed]

, 6

6. B. T. Meggitt, C. J. Hall, and K. Weir, “An all fibre white light interferometric strain measurement system,” Sens. Actuators A Phys. 79(1), 1–7 (2000). [CrossRef]

] are traditionally used within such systems. The technology of moving mirrors was perfected for OCT applications [11

11. W. Y. Oh, B. E. Bouma, N. Iftimia, R. Yelin, and G. J. Tearney, “Spectrally-modulated full-field optical coherence microscopy for ultrahigh-resolution endoscopic imaging,” Opt. Express 14(19), 8675–8684 (2006). [CrossRef] [PubMed]

], however such systems are bulky, complex (expensive), and incompatible with, for example, vibration reach environments, and have limited life-time service spans. PZT systems exhibit similar limitations further including high temperature sensitivity and hysteresis effects. Modulation of optical-fiber temperature was also used in the past as a method for OPL modulation. Electrical heating has been reported either by driving an electrical current through a metalized fiber’s coating [14

14. B. J. White, J. P. Davis, L. C. Bobb, H. D. Krumboltz, and D. C. Larson, “Optical-fiber thermal modulator,” J. Lightwave Technol. 5(9), 1169–1175 (1987). [CrossRef]

], or by the mounting of fibers onto a Peltier-stack [7

7. D. Donlagic and E. Cibula, “An all-fiber scanning interferometer with a large optical path length difference,” Opt. Lasers Eng. 43(6), 619–623 (2005). [CrossRef]

]. The response times of these modulators are, however, relatively long and require direct electrical driving. None of these systems can be controlled electrically passively through, for example, longer section of an optical fiber.

This paper presents a thermo-optic, all-fiber OPL modulator that relies on direct heating of the fiber’s core. The controlled heating of the core is achieved by introducing an optically-absorbing dopant into the fiber’s core and the application of a high-powered pump diode. Analysis of fiber length, dopant concentration, fiber diameter, and fibers’ environmental parameters are provided, to allow for optimization of such a modulator’s performance. The proposed concept allows for remote and electrically fully-passive electrical operation of the modulator through a single fiber. This opens-up the possibility for the designing various fiber-optic systems, where the controlled optical path and electronics’ control system are spatially dislocated. A similar principle was used in the past, to create fiber anemometers [15

15. S. Gao, A. P. Zhang, H. Y. Tam, L. H. Cho, and C. Lu, “All-optical fiber anemometer based on laser heated fiber Bragg gratings,” Opt. Express 19(11), 10124–10130 (2011). [CrossRef] [PubMed]

] (optical path length change was limited in this case).

2. Optical path-length modulation using optical heating of the fiber core

Heat-generation within doped optical fibers due to the exposure to the high pump powers is a phenomenon well-known in fiber lasers and fiber amplifiers [16

16. C. J. Koester and E. Snitzer, “Amplification in a fiber laser,” Appl. Opt. 3(10), 1182–1186 (1964). [CrossRef]

]. Heat-generation leads to the rise in the fiber’s temperature that degrades laser/amplifier systems’ performance, and has been thus extensively studied in the past [17

17. D. C. Hanna, M. J. McCarthy, and P. J. Suni, “Thermal considerations in longitudinally pumped fiber and miniature bulk lasers,” Proc. SPIE 1171, 160–167 (1990). [CrossRef]

21

21. M. Gorjan, M. Marincek, and M. Copic, “Pump absorption and temperature distribution in erbium-doped double-clad fluoride-glass fibers,” Opt. Express 17(22), 19814–19822 (2009). [CrossRef] [PubMed]

]. The in-fiber generated heat can however be used for the effective modulation and/or control of OPL (optical path length changes throughout the paper are referred to vacuum length change) in single-mode optical-fiber systems.

A steady-state analysis of core-heated optical-fiber was performed in [19

19. D. C. Brown and H. J. Hoffman, “Thermal, stress, and thermo-optic effects in high average power double-clad silica fiber lasers,” IEEE J. Quantum Electron. 37(2), 207–217 (2001). [CrossRef]

]. An expression was derived that correlates the temperature in the fibers’ center T0, the fibers’ surrounding temperature Ts, and the core-generated thermal power density Q0:
T0=Ts+Q0a24ks[1+2ln(ba)u+2ksbhw],
(1)
where h denotes the convective heat-transfer coefficient of the fluid surrounding the fiber, ks is the thermal conductivity of the silica, and a and b are the (heated) core and fiber outer radius, respectively. Since b>>a in the case of the single-mode fiber, the temperature in the fiber center T0 can be used as a good approximation for the determining the fiber’s core temperature.

There are two extreme cases predicted by Eq. (1): in the first case, the second term of Eq. (1) (denoted as u) prevails. This happens mainly when the fiber’s outer radius b is very large. In the second case, the third term of Eq. (1) (denoted as w) prevails, which makes the convective heat-transfer a main mechanism that governs the steady-state fiber-core temperature. This third term of Eq. (1) prevails when:
h<ksblnba.
(2)
By assuming typical values for an optical-fiber ks = 1.38 W/mK, 2b = 125 µm and 2a = 9 µm, this condition can be approximately stated as h<8400 W/m2K.

The convective heat-transfer coefficient for an infinite horizontal cylindrical body immersed into the fluid can be described as [22

22. F. P. Incropera, D. P. Dewitt, T. L. Bergman, and A. S. Lavine, “Introduction to convection,” in Fundamentals of heat and mass transfere, J. Hayton, ed. (John Wiley & Sons, 2007).

]:
h=kf×Nu2b,
(3)
where kf represents the thermal conductivity of the surrounding fluid, 2b is the cylinder (fiber) diameter, and Nu the Nusselt number. The Nusselt number depends on geometrical factors and the surrounding fluid’s properties [22

22. F. P. Incropera, D. P. Dewitt, T. L. Bergman, and A. S. Lavine, “Introduction to convection,” in Fundamentals of heat and mass transfere, J. Hayton, ed. (John Wiley & Sons, 2007).

]:
Nu={0.6+0.387Ra1/6(1+(0.559/Pr)9/16)8/27}2,
(4)
where Gr, Pr, and Ra are Grashof, Prandtl, and Rayleigh numbers respectively, defined as:
Gr=8b3ρf2gΔTffβfμf2,Pr=μfcfkf,andRa=GrPrforRa1012,
(5)
with ρf representing the fluid density, g gravitational acceleration, ΔTff the temperature difference between the fluid and the fiber surface, βf the fluid thermal expansion coefficient, µf the fluid viscosity, and cf the fluid’s specific heat.

By assuming typical values for air at 20°C, fiber diameter of 125 µm, and a temperature difference between the fluid and the fiber surface of 100 K, h corresponds to about 125 W/m2K. This value is more than the order of magnitude below the limit imposed by condition (2), which indicates the dominance of the third term in Eq. (1). Thus the remaining two terms within the brackets of Eq. (1) (e.g. factor 1 and u) can be neglected. Furthermore, by merging (3) and (1) we can express the approximate fiber-core temperature change ΔTfc caused by fiber-core heating as:

ΔTfcT0TsQ0a2kfNu.
(6)

Q0 is, in general, a function of longitudinal coordinate x (due to the optical pump-power decay along the fiber length) and the local thermal-power density Q(x), which can be obtained from the local thermal-power dPT generated within the local active fiber-core volume dV:
Q(x)=dPTdV=1SdPTdx,
(7)
where S represents the fiber-core cross-section (e.g. πa2).

The thermal power dPT generated over fiber length dx is equal to the absorbed optical power dPO, e.g. dPT(x) = -dPO(x). Since the absorbed optical power can be expressed as dPO(x) = -αPO(x)dx, the Eq. (7) can be re-written as:
Q(x)=αPO(x)S=αPO(0)eαxS,
(8)
where α represents the fiber attenuation coefficient, and PO(0) the input optical pump-power.

OPL change due to the fiber heating is mainly caused by fiber’s refractive index n change, while the contribution due to the fiber’s thermal expansion can be neglected (dn/dT for fused silica corresponds to 10−5 K−1 and coefficient of thermal expansion corresponds to 5·10−7 K−1, which makes contribution of thermal expansion about 50 times smaller than contribution due to refractive index change). Thus, the OPL change over fiber distance dx can be approximated as:

d(OPL)=(ndndT)ΔTfcdx.
(9)

The total OPL change is obtained by inserting (6) and (8) into (9), and integrating the obtained expression over the heated fiber length L:

ΔOPL=0L(ndndT)a2kfNuαPO(0)Seαxdx=PO(0)πkfNu(ndndT)(1eαL).
(10)

Here we assumed that Nu was a constant, which further allowed for a closed-form solution of the integral. Nu also depended on ΔTff, however this dependence is limited, as discussed further below, and could thus be neglected without seriously compromising the validly of the final approximation for OPL change.

Since the optical pump-power represents a major cost in potential thermo-optic modulation systems, it is meaningful to choose a length of fiber that absorbs most of the pump-power, e.g. the optimum length of the fiber should exceed, for example, L>3/α. In such a case the term in the brackets on the right approaches 1 and the final expression for OPL-change reduces to
ΔOPL=PO(0)kf1πNu(ndndT).
(11)
Because dn/dT and n are the fundamental characteristics of fused silica, the pump-power P0(0) and thermal conductivity kf of the surrounding fluid are the only controllable parameters that profoundly affect the OPL modulation range.

It should also be stressed that Nu depends on fiber diameter, fiber surface temperature, thermal conductivity, and other fluid properties, as indicated by Eq. (4). In general, an increase in fiber diameter and/or increase in fiber surface temperature results in an increase of Nu, while a rise in the thermal conductivity of the fluid reduces Nu. These dependences are, however, limited and the Nu does not change significantly even over the broad range of these parameters’ variations. Figure 1(a)
Fig. 1 (a) Nusselt numbers depending on the fiber diameter and temperature differences between the fiber surface and surrounding gas, for air and xenon. (b) Nusselt numbers as a function of the temperature difference between fiber surface temperature and surrounding gas temperature for 125 µm fiber in air (an increase in temperature difference from 25°C to 200°C causes less than 10% change in Nusselt number).
was obtained from Eq. (4) and demonstrates a few typical dependences of Nu on several relevant parameters within practical ranges of interest.

While Nu, in general, depends on ΔTff, this dependence is small, as shown in Fig. 1(a) and Fig. 1(b). For example, Nu changes by less than 10% within a temperature difference ΔTff span of between 25°C and 200°C. Assuming the Nusselt number to be constant the derivations of Eqs. (10) and (11) were therefore justified. Thus, this assumption did not significantly reduce the overall validity for both the obtained expressions.

Finally, Eq. (11) was evaluated for typical input parameters. The Nusselt number was determined from Fig. 1(a) for ΔTff = 250°C. The results are shown in Table 1

Table 1. Predicted OPL and time constants for fibers with diameters of 125 µm and 20 µm in air and xenon, with pump optical power of P(0) = 0.5 W.

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(fourth column) and predict an achievable OPL modulation range of between 100 µm and 600 µm for a 0.5 W pump source.

The maximum permitted upper-pump power is limited by fiber degradation due to the core overheating. Since the pump-power decays along the doped fiber length, the highest temperature is reached within the vicinity of the splice between the lead-in and doped fibers. Since the potential for glass devitrification and crystallization can occur above about Tmax = 800°C [23

23. A. H. Rose, “Devitrification in annealed optical fiber,” J. Lightwave Technol. 15(5), 808–814 (1997). [CrossRef]

], the upper pump power limit POMAX(0) can be assessed by merging Eqs. (6) and (8), and by setting x = 0:

POMAX(0)(TMAXTs)πkfNuα.
(12)

Equation (12) is an approximation, valid for fibers with low α. When a short section of fiber with high α is spliced to the (low absorption) lead-in fiber, the thermal conductance of the fiber distributes generated heat along the fiber’s longitudinal direction (including into the lead-fiber), and thus reduces the fiber’s peak temperature at the splice, which actually increases the allowable power to above the limit predicted by (12).

Further analysis relates to the dynamic performance of the proposed system. The analysis performed in [18

18. M. K. Davis, M. J. F. Digonnet, and R. H. Pantell, “Thermal effects in doped fibers,” J. Lightwave Technol. 16(6), 1013–1023 (1998). [CrossRef]

] shows that self-heated fiber can be represented as a multi-order system composed of a set of first order blocks. The time-constants of individual first-order blocks are given by [18

18. M. K. Davis, M. J. F. Digonnet, and R. H. Pantell, “Thermal effects in doped fibers,” J. Lightwave Technol. 16(6), 1013–1023 (1998). [CrossRef]

]:
τm=ρscsksb2pm2,
(13)
where ρs represents silica specific gravity, cs silica specific heat, and pm the coefficient calculated by
pmJ1(pm)=hbksJ0(pm),
(14)
where J0(pm) and J1(pm) are Bessel functions. The dominant pole of such a multi-order system is obtained by setting m = 1, which yieldsp12hb/ks. The dominate pole time-constant can thus be written as:
τ1=ρscs2bh.
(15)
By using (3), Eq. (15) can be rewritten as:

τ1=ρscsb2kf×Nu.
(16)

The Eq. (16) was evaluated for typical input parameters. The results are shown in Table 1 (the last column on the right). The time-constant of the self-heated fiber OPL modulation system can thus be effectively decreased by reducing the fiber’s outer diameter or increasing the thermal conductivity of the surrounding medium.

In conclusion, the fiber diameter and thermal conductivity of the environment surrounding the fiber are two parameters that can be used to manipulate and optimize the proposed OPL modulator’s performance. Reduction in the thermal conductivity of the surrounding fluid increases the OPL modulation range, but however at the expense of a prolonged system’s time response, whilst the reduction in fiber diameter mainly improves dynamic performance with an additional, but limited positive impact on the achievable OPL modulation range. Thus, a fast and long modulation range OPL modulator, in general, requires thin fiber immersed in the low-thermally conductive gas.

3. Vanadium-doped fibers and their properties relevant for thermo-optic modulation

A dopant that causes high optical absorption at low concentrations and provides a non-radiative relaxation is required in order to maximize the conversion of optical pump power into heat. An ideal dopant should promote absorption around the existing (and cost effective) pump laser wavelengths (e.g. around 980 nm), while any increase in absorption around the transmission/signal wavelengths (e.g. within the 1300-1600 nm range) should be as low as possible to allow for direct incorporation of dopant into the fiber core.

Amongst the considered dopants, transition metals, such as vanadium, are of special interest. While the effect of vanadium-doping on the absorption in silica glass has been investigated for bulk samples [24

24. P. C. Schultz, “Optical absorption of the transition elements in vitreous silica,” J. Am. Ceram. Soc. 57(7), 309–313 (1974). [CrossRef]

], only very limited work has been performed on vanadium-doped fibers (VDF) [25

25. M. K. Davis and M. J. F. Digonnet, “Measurements of thermal effects in fibers doped with cobalt and vanadium,” J. Lightwave Technol. 18(2), 161–165 (2000). [CrossRef]

].

The fibers produced during this investigation were obtained by modified chemical vapor deposition (MCVD) and vanadium precursor introduction by flash vaporization technology (FVT) [26

26. B. Lenardic, M. Kveder, H. Guillon, and S. Bonnafous, “Fabrication of specialty optical fibers using flash vaporization method,” Proc. SPIE 7134, 71341K, 71341K-11 (2008). [CrossRef]

]. FVT uses the instantaneous vaporization of solvent. The system uses high-pressure injectors to inject the precursor’s solution mixed with carrier gas into a heated vaporization cell in the form of a fine aerosol. The solvent evaporates before the aerosol touches the vaporization cell’s walls. The precursor remains in the form of fine aerosol. This makes the flow of precursor vapor very uniform towards the reaction and deposition zone, which is a precondition for the deposition of uniform layers along a substrate tube. FVT was described in detail in [26

26. B. Lenardic, M. Kveder, H. Guillon, and S. Bonnafous, “Fabrication of specialty optical fibers using flash vaporization method,” Proc. SPIE 7134, 71341K, 71341K-11 (2008). [CrossRef]

]. The precursor chemical used for vanadium-doping was vanadium-oxoizopropoxide - VO(OC3H7)3 dissolved in chloroform. Various concentrations of VO(OC3H7)3 within chloroform were used to obtain different concentrations of vanadium ions in the produced fibers. The fibers’ cores were also co-doped with germania in order to obtained a refractive index profile compatible with standard single-mode fiber (SMF). Three experimentally-produced vanadium-doped fibers (VDFs) using different precursor chemical concentrations are listed in Table 2

Table 2. Experimentally-produced vanadium fibers.

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. Since meaningful attenuations were obtained at those vanadium contractions that were too low to be determined by conventional methods such as energy-dispersive X-ray spectroscopy, initial concentrations of VO(OC3H7)3 in chloroform are given for the reference. The absorption of each VDF was measured at three different wavelengths, using a cut-back method.

The full spectral absorption from 800 nm to 1600 nm was measured on 10 cm of Fiber 2, from Table 2 and is presented in Fig. 2
Fig. 2 Spectral attenuation for 10 cm long section of Fiber 2 from Table 2.
.

The experimental configuration used for determining the OPL modulation range is illustrated in Fig. 3
Fig. 3 Measure set-up, based on a Michelson interferometer for OPL variation determination.
. The set-up utilized a 3 dB coupler at 1550 nm to form a fiber-optic Michelson interferometer at 1550 nm. One of the interferometer’s arms contained a 980/1550 nm wavelength-division multiplexing (WDM) coupler that was further connected to VDF under test (all tested sections of VDFs, mentioned in this manuscript, were uncoated). This allowed for selective coupling of the optical pump-power into VDF under test. A 1550 nm distributed feedback (DFB) laser source, a detector, sampling oscilloscope, and a personal computer were further added to monitor OPL variation within the interferometer set-up. Additionally, a 980 nm blocking filter was introduced onto a path leading to the detector in order to fully remove any remaining pump power (simply, a 10 cm-long section of Fiber 2 from Table 2 was used for this purpose). A 500 mW single-mode-fiber-coupled 980 nm pump laser was periodically switched on and off to induce a fiber heating/cooling cycle, whilst fringe counting was performed with the help of an oscilloscope and personal computer (PC) for measuring OPL variation.

Furthermore, interferometer was inserted into the hermetically-sealed vessel to allow for further investigation of gas composition and pressure impact during the OPL variation cycle.

In the first series of tests, VDF with different absorption coefficients and different lengths were tested, but the length of VDF was always chosen in such a way as to provide about 95% absorption of the total pump power within VDF (e.g. L = 3α). The results are shown in Fig. 4
Fig. 4 OPL change versus time for various doped fibers from Table 2 (125 µm).
and indicate a minor dependence of OPL variation on the fiber length and absorption coefficients when equal optical power is absorbed within the fiber. This result is consistent with the analysis from section 2 and indicates the possibility of choosing relatively arbitrary combinations of fiber lengths and dopant concentrations when building an OPL modulator. It should be stressed, however, that shortening of the heated fiber, whilst increasing vanadium concentrations, increases the peak fiber core temperature, which then limits the maximum allowable pump-power, as already analyzed in section 2 (highly doped VDF can be melted or otherwise damaged at the splice between VDF and the lead-in fiber when applying too high power-pump laser). A rough estimation of the maximum allowable optical power for a given absorption coefficient can be obtained from Eq. (12). Path-length modulation is also linear using pump-power, as indicated in Fig. 5
Fig. 5 Linear dependence of modulated optical path and pump-laser optical power for a 125 µm, 10 cm-long Fiber 2 from Table 2.
. All the above experiments were performed in air. In order to modify the thermal conductivity of the fiber’s environment, the vessel was filled with different gases. Fiber 2 from the Table 2 with a length corresponding to 10 cm was used during this test. Air, argon, and xenon gases were introduced into the vessel under atmospheric pressure and the OPL variation was observed during excitation by the pump-diode. Furthermore, OPL variation was also observed using a vessel evacuated to 50 mBar (typical technical low vacuum range), and to high vacuum (< 0.02 mBar). Comparisons between all five responses are shown in Fig. 6
Fig. 6 OPL change versus time for 125 µm, 10 cm-long section of Fiber 2 from Table 2 in different fiber atmospheres.
and Table 3

Table 3. Time constants and achieved OPL modulation ranges for 10 cm-long section of Fiber 2 from Table 2.

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. Xenon atmosphere, which had the lowest thermal conductivity, increased OPL variation by about 2.5 times in comparison with the air. This is somewhat below those theoretical predictions presented in section 2, which can be attributed to Nusselt number increase. This increase can be attributed to an increase in the temperature of the fiber’s surface in the low thermal conductivity atmosphere and temperature depended changes in gas parameters that influence the Nusselt number, such as for example gas viscosity and density. Reduction of air pressure to the technical low vacuum range had a comparable effect on the OPL variation to that of the argon atmosphere, under atmospheric pressures. Significant increase in OPL modulation can however be achieved by the application of a high vacuum, but such a solution is impractical in most applications as it usually requires an active vacuum maintenance process (for example continuous pumping using a turbo-molecular pump). In conclusion, xenon atmosphere can be relatively easily applied when designing a practical modulator to extend the range of OPL variation.

Reducing the thermal conductivity of the gas increases the time-constant, as shown by Fig. 6. The measured time-constants for 125 µm fibers were: 460 ms in air, 605 ms in argon, 1070 ms in xenon, all at atmospheric pressures, and 600 ms and 2360 ms in air at reduced absolute pressures corresponding to 50 mBar and 0.02 mBar respectively.

Further tests included observing the OPL modulation range and the dynamic behavior of the system when using reduced diameter fibers. The first measurement was performed by using a 10 cm-long section of VDF, Fiber 2 from Table 2, which was etched in hydrofluoric acid (HF) to reduce its outer diameter to 20 µm. The results are shown in Fig. 7
Fig. 7 OPL change versus time for 20 µm, 10 cm-long section of Fiber 2 from Table 2 in different fiber gasses.
. The second measurement was performed by using a 1 cm-long section of Fiber 3 from Table 2, which was progressively etched in HF to reduce its outer diameter to different values. Table 4

Table 4. Dynamic response and OPL modulation range as a function of fiber diameter for 1 cm-long Fiber 3 from Table 2 at atmospheric pressure.

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shows the time responses for these reduced size diameters’ fibers. Fiber diameter reduction significantly shortened the time-constant. A time-constant as low as 11 ms was achieved for 15 µm outer-diameter-VDF in air.

Figure 8
Fig. 8 Continuous modulation of OPL using 20 µm outer diameter, 10 cm-long section of Fiber 2 from Table 2, with 0.5 W of pump-laser optical power; (a) in air (b) in xenon.
demonstrates the continuous modulation of OPL achieved by the application of a 20 μm outer diameter VDF (10 cm-long section of Fiber 2 from Table 2) and a 0.5 W pump laser. Since the OPL variation is produced during heating-up and cooling-down cycles, the OPL scanning rate is actually doubled in respect to the pump laser’s modulation frequency. The achieved OPL variation was similar for air and xenon atmospheres, i.e. 60 µm and 65 µm, when the pump laser modulation frequency corresponded to 50 Hz (100 scans/s). At pump laser modulation frequency of 10 Hz (20 scans/s), the OPL variation corresponded to 170 µm in air and 260 µm in xenon. Reduction of laser modulation frequency to 2 Hz (4 scans/s) increased the OPL modulation range to 300 µm and 500 µm in air and xenon, respectively. Apparently, a low thermal conductivity atmosphere only has a marginal effect on the OPL modulation range when the system is operated at high scanning rates. However xenon atmosphere has a profound effect on the achievable OPL when the scanning rate is reduced. Finally, since Michelson interferometer configuration is often used and applied within many practical systems requiring OPL modulation, the OPL modulation can be, in such cases straightforwardly doubled by interferometer configuration, leading to about a 1 mm OPL modulation range when using widely-available commercial 0.5 W pump lasers.

4. Application of thermo-optic OPL modulation for the interrogation of white-light interferometric sensors

This section demonstrates an application of the proposed OPL modulator for interrogating low-coherence fiber-optic sensors. The system consists of a sensor (interrogated Michelson interferometer), a resolving Michelson interferometer that further contains the VDF in one of its arms (10 cm of Fiber 2 from Table 2), a low coherence source (superluminescent diode - SLED) at 1550 nm, a 1310 nm DFB diode, a 0.5 W 980 nm single-mode fiber coupled pump diode, a 1310/1550 WDM coupler, a 980/1310/1550 nm coupler, a 980 nm blocking filter (section of VDF), a standard 4-port circulator (at 1550 nm), two detectors, a hermetic case encompassing the resolving interferometer’s fibers, and an electronics control and signal processing sub-system. The set-up is shown in Fig. 9
Fig. 9 A complete low-coherence fiber-optic measurement system based on thermo-optic scanning: optical part of the system operating at 1550 nm acts as low coherent interferometer, whilst part of the system operating at 1310 nm provides reference path length measures.
. The coupler that formed the resolving interferometer was custom-made by Gould Inc. USA and had the following split-ratios: 50:50 at 1550 nm, 99:1 at 980 nm and 60:40 at 1310 nm (ideally the split ratio at 1310 nm should also be 50:50, but only limited coupler optimization was performed). This coupler splits and recombines the delayed waves at 1550 nm and 1310 nm, whilst it selectively directs the pump-power into the interferometer’s arm containing VDF. The hermetic case containing the resolving interferometer’s fibers was filled with xenon under atmospheric pressure. 10 cm-long VDF (Fiber 2 from Table 2) was etched to reduce the outer diameter to 20 µm.

Detector 1 receives low coherence optical radiation emitted by SLED, after passing consecutively through both the sensor and resolver interferometers. Detector 2 receives high-coherence optical radiation generated by a 1310 nm DFB source after passing through the resolver interferometer only. During cyclical switching of the 980 nm pump diode, signals from both detectors are recorded and processed. Modulation of the resolver interferometer’s OPL generates well-defined, constant amplitude, a high-coherence fringe pattern at detector 2, whilst low-coherence fringe pattern is generated at detector 1. The 1310 nm fiber-optic branch of the system (e.g. the 1310 DFB diode, the 1310 nm and 1310/1550 couplers, the 980 filter, and the detector 2) provides a precise (reference) signal for measuring OPL change during its modulation.

Figure 10
Fig. 10 Low-coherence interferogram obtained at detector 1 and reference trace at detector 2: (a) Interrogated sensor OPL difference was set to 832 µm, scanning rate was 2 scans/s. (b) Interrogated sensor OPL difference was set to 492 µm, scanning rate was 2 scans/s. (c) Interrogated sensor OPL difference was set to 308 µm, scanning rate was 2 scans/s. (d) Interrogated sensor OPL difference was set to 158 µm, scanning rate was 20 scans/s (high-speed scanning).
shows typical signals recorded at both detectors for various sensors’ interferometer optical path length differences. Since Michelson configuration doubles the OPL modulation range, this demonstration system was able to provide scanning of up to about 1 mm of OPL difference. While the OPL change versus time is non-linear due to the exponentially-asymptotic temperature swing of the core’s temperature, replacement of on-off current switching with properly shaped continuous pump diode drive-current could be used to provide linear OPL sweep if so desired.

5. Conclusion

This paper investigated the possibilities and limitations for the realization of an all-fiber, all-optically controlled, thermo-optical path length modulator. The optical path length modulation of the single-mode fiber was obtained by direct heating of the fiber’s core through the doping of the core by vanadium ions, and the application of a high-power pump-laser. The produced vanadium-doped fibers exhibited high absorption at 980 nm, whilst the absorption remained relatively low at telecommunication wavelengths (e.g. 1300 and particularly 1550 nm), which makes such fibers especially suitable for thermo-optic fiber device design. Vanadium-doped fibers were produced by the utilization of a conventional MCVD process and flash-vaporization technology.

The theoretical analysis, further confirmed by experimental investigation, showed that the optical path length modulation between 100 μm and 1 mm can be typically achieved by proper design of the fiber, the modulation system, and the application of commercially-available pump laser diodes (e.g. diodes intended for use in telecommunication optical amplifiers). The achievable OPL modulation range depends considerably on the thermal conductivity of the gas surrounding the fiber and available pump laser power. Other factors, such as fiber diameter and other properties of surrounding fluid also influence OPL modulation range, however to lesser extent. Thus application of low thermal conductivity gas, such as xenon, can be used to attain extended OPL modulation ranges. Typical response time (time constant) of the core heated fiber depends on fiber diameter, thermal conductivity and other properties of the surrounding fluid. With significant reduction of fiber outer diameter, e.g. down to 15 µm, we were able to achieve response times as low as 11 ms in air, whilst for 125 µm fibers these times were typically within the range of 500 ms. An optimum OPL modulator should thus utilize reduced diameter fiber encapsulated within a low thermally conductive atmosphere. Maximum absorption per unit length and consequently minimum length of the heated fibers are determined by input pump power, which also determines achievable OPL modulation range. Fiber length and absorption coefficient have little effect on the achievable OPL modulation range as long as a sufficient length of the fiber is used to absorb most of the available pump-power.

The presented system was used within a low-coherence fiber-optic-sensor integration system; however other uses, such as new classes of thermo-optic sensors, adjustable delay lines, optical coherence tomography, and similar applications might benefit from the presented concept in the future. Since the loss in silica fiber is below 2 dB/km at 980 nm, full remotely-operated-thermo-optic, all-fiber devices are also possible.

Acknowledgments

This work was supported by Slovenian Public Research agency under J2-3623 and P2-0368.

References and links

1.

G. Beheim, “Remote displacement measurement using a passive interferometer with a fiber-optic link,” Appl. Opt. 24(15), 2335–2340 (1985). [CrossRef] [PubMed]

2.

Y. J. Rao and D. A. Jackson, “Recent progress in fibre optic low-coherence interferometry,” Meas. Sci. Technol. 7(7), 981–999 (1996). [CrossRef]

3.

L. B. Yuan, Q. B. Li, Y. J. Liang, J. Yang, and Z. H. Liu, “Fiber optic 2-D sensor for measuring the strain inside the concrete specimen,” Sens. Actuators A Phys. 94(1-2), 25–31 (2001). [CrossRef]

4.

X. M. Zhang, Y. X. Liu, H. Bae, C. Pang, and M. Yu, “Phase modulation with micromachined resonant mirrors for low-coherence fiber-tip pressure sensors,” Opt. Express 17(26), 23965–23974 (2009). [CrossRef] [PubMed]

5.

H. S. Choi, H. F. Taylor, and C. E. Lee, “High-performance fiber-optic temperature sensor using low-coherence interferometry,” Opt. Lett. 22(23), 1814–1816 (1997). [CrossRef] [PubMed]

6.

B. T. Meggitt, C. J. Hall, and K. Weir, “An all fibre white light interferometric strain measurement system,” Sens. Actuators A Phys. 79(1), 1–7 (2000). [CrossRef]

7.

D. Donlagic and E. Cibula, “An all-fiber scanning interferometer with a large optical path length difference,” Opt. Lasers Eng. 43(6), 619–623 (2005). [CrossRef]

8.

D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory, C. A. Puliafito, and J. G. Fujimoto, “Optical coherence tomography,” Science 254(5035), 1178–1181 (1991). [CrossRef] [PubMed]

9.

A. F. Fercher, W. Drexler, C. K. Hitzenberger, and T. Lasser, “Optical coherence tomography - principles and applications,” Rep. Prog. Phys. 66(2), 239–303 (2003). [CrossRef]

10.

Y. Nakamura, S. Makita, M. Yamanari, M. Itoh, T. Yatagai, and Y. Yasuno, “High-speed three-dimensional human retinal imaging by line-field spectral domain optical coherence tomography,” Opt. Express 15(12), 7103–7116 (2007). [CrossRef] [PubMed]

11.

W. Y. Oh, B. E. Bouma, N. Iftimia, R. Yelin, and G. J. Tearney, “Spectrally-modulated full-field optical coherence microscopy for ultrahigh-resolution endoscopic imaging,” Opt. Express 14(19), 8675–8684 (2006). [CrossRef] [PubMed]

12.

J. Tratnik, L. Pavlovic, B. Batagelj, P. Lemut, P. Ritosa, M. Ferianis, and M. Vidmar, “Fiber length compensated transmission of 2998.01 MHz RF signal with femtosecond precision,” Microw. Opt. Technol. Lett. 53(7), 1553–1555 (2011). [CrossRef]

13.

W. Z. Li and J. P. Yao, “Investigation of photonically assisted microwave frequency multiplication based on external modulation,” IEEE Trans. Microw. Theory Tech. 58(11), 3259–3268 (2010). [CrossRef]

14.

B. J. White, J. P. Davis, L. C. Bobb, H. D. Krumboltz, and D. C. Larson, “Optical-fiber thermal modulator,” J. Lightwave Technol. 5(9), 1169–1175 (1987). [CrossRef]

15.

S. Gao, A. P. Zhang, H. Y. Tam, L. H. Cho, and C. Lu, “All-optical fiber anemometer based on laser heated fiber Bragg gratings,” Opt. Express 19(11), 10124–10130 (2011). [CrossRef] [PubMed]

16.

C. J. Koester and E. Snitzer, “Amplification in a fiber laser,” Appl. Opt. 3(10), 1182–1186 (1964). [CrossRef]

17.

D. C. Hanna, M. J. McCarthy, and P. J. Suni, “Thermal considerations in longitudinally pumped fiber and miniature bulk lasers,” Proc. SPIE 1171, 160–167 (1990). [CrossRef]

18.

M. K. Davis, M. J. F. Digonnet, and R. H. Pantell, “Thermal effects in doped fibers,” J. Lightwave Technol. 16(6), 1013–1023 (1998). [CrossRef]

19.

D. C. Brown and H. J. Hoffman, “Thermal, stress, and thermo-optic effects in high average power double-clad silica fiber lasers,” IEEE J. Quantum Electron. 37(2), 207–217 (2001). [CrossRef]

20.

N. A. Brilliant and K. Lagonik, “Thermal effects in a dual-clad ytterbium fiber laser,” Opt. Lett. 26(21), 1669–1671 (2001). [CrossRef] [PubMed]

21.

M. Gorjan, M. Marincek, and M. Copic, “Pump absorption and temperature distribution in erbium-doped double-clad fluoride-glass fibers,” Opt. Express 17(22), 19814–19822 (2009). [CrossRef] [PubMed]

22.

F. P. Incropera, D. P. Dewitt, T. L. Bergman, and A. S. Lavine, “Introduction to convection,” in Fundamentals of heat and mass transfere, J. Hayton, ed. (John Wiley & Sons, 2007).

23.

A. H. Rose, “Devitrification in annealed optical fiber,” J. Lightwave Technol. 15(5), 808–814 (1997). [CrossRef]

24.

P. C. Schultz, “Optical absorption of the transition elements in vitreous silica,” J. Am. Ceram. Soc. 57(7), 309–313 (1974). [CrossRef]

25.

M. K. Davis and M. J. F. Digonnet, “Measurements of thermal effects in fibers doped with cobalt and vanadium,” J. Lightwave Technol. 18(2), 161–165 (2000). [CrossRef]

26.

B. Lenardic, M. Kveder, H. Guillon, and S. Bonnafous, “Fabrication of specialty optical fibers using flash vaporization method,” Proc. SPIE 7134, 71341K, 71341K-11 (2008). [CrossRef]

OCIS Codes
(060.2270) Fiber optics and optical communications : Fiber characterization
(060.2280) Fiber optics and optical communications : Fiber design and fabrication
(060.2300) Fiber optics and optical communications : Fiber measurements
(060.2370) Fiber optics and optical communications : Fiber optics sensors
(060.2400) Fiber optics and optical communications : Fiber properties
(060.4080) Fiber optics and optical communications : Modulation
(060.5060) Fiber optics and optical communications : Phase modulation
(140.6810) Lasers and laser optics : Thermal effects
(230.4110) Optical devices : Modulators

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: March 21, 2013
Revised Manuscript: April 26, 2013
Manuscript Accepted: April 29, 2013
Published: May 7, 2013

Citation
Ziga Matjasec, Stanislav Campelj, and Denis Donlagic, "All-optical, thermo-optical path length modulation based on the vanadium-doped fibers," Opt. Express 21, 11794-11807 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-10-11794


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References

  1. G. Beheim, “Remote displacement measurement using a passive interferometer with a fiber-optic link,” Appl. Opt.24(15), 2335–2340 (1985). [CrossRef] [PubMed]
  2. Y. J. Rao and D. A. Jackson, “Recent progress in fibre optic low-coherence interferometry,” Meas. Sci. Technol.7(7), 981–999 (1996). [CrossRef]
  3. L. B. Yuan, Q. B. Li, Y. J. Liang, J. Yang, and Z. H. Liu, “Fiber optic 2-D sensor for measuring the strain inside the concrete specimen,” Sens. Actuators A Phys.94(1-2), 25–31 (2001). [CrossRef]
  4. X. M. Zhang, Y. X. Liu, H. Bae, C. Pang, and M. Yu, “Phase modulation with micromachined resonant mirrors for low-coherence fiber-tip pressure sensors,” Opt. Express17(26), 23965–23974 (2009). [CrossRef] [PubMed]
  5. H. S. Choi, H. F. Taylor, and C. E. Lee, “High-performance fiber-optic temperature sensor using low-coherence interferometry,” Opt. Lett.22(23), 1814–1816 (1997). [CrossRef] [PubMed]
  6. B. T. Meggitt, C. J. Hall, and K. Weir, “An all fibre white light interferometric strain measurement system,” Sens. Actuators A Phys.79(1), 1–7 (2000). [CrossRef]
  7. D. Donlagic and E. Cibula, “An all-fiber scanning interferometer with a large optical path length difference,” Opt. Lasers Eng.43(6), 619–623 (2005). [CrossRef]
  8. D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory, C. A. Puliafito, and J. G. Fujimoto, “Optical coherence tomography,” Science254(5035), 1178–1181 (1991). [CrossRef] [PubMed]
  9. A. F. Fercher, W. Drexler, C. K. Hitzenberger, and T. Lasser, “Optical coherence tomography - principles and applications,” Rep. Prog. Phys.66(2), 239–303 (2003). [CrossRef]
  10. Y. Nakamura, S. Makita, M. Yamanari, M. Itoh, T. Yatagai, and Y. Yasuno, “High-speed three-dimensional human retinal imaging by line-field spectral domain optical coherence tomography,” Opt. Express15(12), 7103–7116 (2007). [CrossRef] [PubMed]
  11. W. Y. Oh, B. E. Bouma, N. Iftimia, R. Yelin, and G. J. Tearney, “Spectrally-modulated full-field optical coherence microscopy for ultrahigh-resolution endoscopic imaging,” Opt. Express14(19), 8675–8684 (2006). [CrossRef] [PubMed]
  12. J. Tratnik, L. Pavlovic, B. Batagelj, P. Lemut, P. Ritosa, M. Ferianis, and M. Vidmar, “Fiber length compensated transmission of 2998.01 MHz RF signal with femtosecond precision,” Microw. Opt. Technol. Lett.53(7), 1553–1555 (2011). [CrossRef]
  13. W. Z. Li and J. P. Yao, “Investigation of photonically assisted microwave frequency multiplication based on external modulation,” IEEE Trans. Microw. Theory Tech.58(11), 3259–3268 (2010). [CrossRef]
  14. B. J. White, J. P. Davis, L. C. Bobb, H. D. Krumboltz, and D. C. Larson, “Optical-fiber thermal modulator,” J. Lightwave Technol.5(9), 1169–1175 (1987). [CrossRef]
  15. S. Gao, A. P. Zhang, H. Y. Tam, L. H. Cho, and C. Lu, “All-optical fiber anemometer based on laser heated fiber Bragg gratings,” Opt. Express19(11), 10124–10130 (2011). [CrossRef] [PubMed]
  16. C. J. Koester and E. Snitzer, “Amplification in a fiber laser,” Appl. Opt.3(10), 1182–1186 (1964). [CrossRef]
  17. D. C. Hanna, M. J. McCarthy, and P. J. Suni, “Thermal considerations in longitudinally pumped fiber and miniature bulk lasers,” Proc. SPIE1171, 160–167 (1990). [CrossRef]
  18. M. K. Davis, M. J. F. Digonnet, and R. H. Pantell, “Thermal effects in doped fibers,” J. Lightwave Technol.16(6), 1013–1023 (1998). [CrossRef]
  19. D. C. Brown and H. J. Hoffman, “Thermal, stress, and thermo-optic effects in high average power double-clad silica fiber lasers,” IEEE J. Quantum Electron.37(2), 207–217 (2001). [CrossRef]
  20. N. A. Brilliant and K. Lagonik, “Thermal effects in a dual-clad ytterbium fiber laser,” Opt. Lett.26(21), 1669–1671 (2001). [CrossRef] [PubMed]
  21. M. Gorjan, M. Marincek, and M. Copic, “Pump absorption and temperature distribution in erbium-doped double-clad fluoride-glass fibers,” Opt. Express17(22), 19814–19822 (2009). [CrossRef] [PubMed]
  22. F. P. Incropera, D. P. Dewitt, T. L. Bergman, and A. S. Lavine, “Introduction to convection,” in Fundamentals of heat and mass transfere, J. Hayton, ed. (John Wiley & Sons, 2007).
  23. A. H. Rose, “Devitrification in annealed optical fiber,” J. Lightwave Technol.15(5), 808–814 (1997). [CrossRef]
  24. P. C. Schultz, “Optical absorption of the transition elements in vitreous silica,” J. Am. Ceram. Soc.57(7), 309–313 (1974). [CrossRef]
  25. M. K. Davis and M. J. F. Digonnet, “Measurements of thermal effects in fibers doped with cobalt and vanadium,” J. Lightwave Technol.18(2), 161–165 (2000). [CrossRef]
  26. B. Lenardic, M. Kveder, H. Guillon, and S. Bonnafous, “Fabrication of specialty optical fibers using flash vaporization method,” Proc. SPIE7134, 71341K, 71341K-11 (2008). [CrossRef]

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