Highly index-sensitive optical devices including long period fiber gratings

A long period fiber grating (LPG) device is formed to exhibit a “turn-around-point” (TAP) in a phase matching curve when the group velocities of two propagating modes are matched. When the grating period of the LPG is selected to coincide with the TAP, a large tuning bandwidth is formed. This device has been found to be highly sensitive to changes in the refractive index of the ambient surrounding the LPG (recognizing a change in refractive index as low as 10−4), allowing the device to be used as a sensor for trace elements in the atmosphere. The ability of the TAP LPG to modify the intensity of a propagating optical signal as a function of changes in the refractive dielectric of a surround material also allows for this device to be used as an all-fiber high speed optical signal modulator.

TECHNICAL FIELD

The present invention relates to long period fiber gratings (LPGs) and, more particularly, to “turn around point” (TAP) LPGs configured to exhibit an arbitrary bandwidth and capable of recognizing changes in ambient refractive index on the order of 10−4or lower.

BACKGROUND OF THE INVENTION

Tunable attenuators are ubiquitous in fiber optic systems, as a result of the wide variety of applications they enable. In particular, tunable attenuators may be used to dynamically alter the input power at an amplifier node of an optical communication system, where this functionality is necessary to maintain constant gain and low noise figures. The desired time scales for such variable optical attenuators ranges from minutes to sub-microseconds. Alternatively, devices that can provide rapid attenuation changes in a fiber optic line (at rates of, for example, sub-microseconds to picoseconds) may be utilized to realize high speed modulators for encoding data in a communication system. On the other hand, devices whose attenuation changes in response to varying ambient conditions (such as, for example, outside temperature, pressure, chemical compositions, etc.) are of immense importance to sensor technologies employing fiber optic detection schemes. In particular, devices with high sensitivity to the ambient refractive index of chemical compositions would be attractive for realizing sensors used for identifying trace gases or chemicals in the atmosphere.

Long period fiber gratings (LPGs) are mode conversion devices that have been used extensively to provide components that offer wavelength-selective attenuation in a wavelength division multiplexed (WDM) communication system. Dynamic tuning of the spectral characteristics of LPGs has been achieved by a variety of techniques. For example, LPGs that couple the core mode to a cladding mode can be tuned dynamically by modulating the refractive index of an outer or inner cladding material that is interrogated by a cladding mode of the fiber. The refractive index of such cladding materials can be varied by temperature, the electro-optic effect or some nonlinear optical effect, depending on the nature of the cladding material used. Alternatively, the LPGs may be mechanically strained to change the refractive index, using, for example, piezo-electric packages, simple motion control housings or magnetically latchable materials. The tuning mechanisms described above serve to shift the spectral response of LPGs from one center wavelength to another. While these techniques are useful for tuning the wavelength-selective attenuation in a fiber optic system, they are not sensitive enough to enable detection of small ambient changes in refractive index due to trace gases in the ambient environment, or to provide high speed modulation (on the order of 100 Ghz or higher). Typically, an ambient refractive index change (Δn) of at least 0.01 is required to achieve “complete” tuning (“complete” being defined as a tunable device that changes state from fully transparent to >20 dB attenuation). To date, means of increasing the grating sensitivity have concentrated on etching fibers down to very small outer diameters (OD) to obtain a cladding mode that is significantly more sensitive to ambient index changes. The highest sensitivity reported to date is on the order of 2×10−3, from fibers etched to an outer diameter of 32 μm. Since trace elements typically change ambient indices by less than a factor of 10−4, and high speed electro-optic elements yield index changes on the order of 5×10−4, the etching fiber arrangement remains inadequate. Additionally, a fiber with an outer diameter of only 32 μm would be significantly unreliable as a result of the diminished strength of thin fibers.

In addition, the tuning that is most desirable for dynamic filters is tuning of the strength (loss) of the coupling, not tuning of the resonant wavelength itself. The prior art is replete with “tuning” arrangements that alter the resonant wavelength, but very little has been reported on providing the ability to tune the strength of the coupled signal. One prior art article by V. Grubsky et al., entitled “Long-period fiber gratings with variable coupling for real-time sensing applications”, appearing inOptics Letters, Vol. 25, p. 203 (2000), discloses an arrangement for broadening the bandwidth of LPGs by coupling to a higher-order cladding mode. In this arrangement, greater than 50 nm coupling was achieved, albeit with weak coupling strengths. Grubsky et al. provided the coupling strength tuning using either ambient temperature or applied mechanical strain. While this arrangement achieved coupling strength tuning, the sensitivity of the response is considered to be too low for use in high speed modulation or trace element detection schemes. In the Grubsky et al. arrangement, the sensitivity was fixed by the cladding mode order chosen to induce coupling. Moreover, the wavelength of coupling was constrained by the specific cladding mode that afforded tunable attenuation.

Thus, there exists a remaining need in the art for a fiber grating device that can offer strong tunable attenuation, preferably over bandwidths exceeding 20 nm, whose coupling strength is tuned by small changes in the refractive index of the active material. The active material may either be a coating whose index changes are due to incorporation of trace elements in the atmosphere, or an electro-optic or nonlinear-optic material that changes index as a result of an applied electric field or optical radiation. A practical device would also allow operation at any desired wavelength without impacting the sensitivity, since the desired wavelength of operation would be different for various applications.

SUMMARY OF THE INVENTION

The need remaining in the prior art is addressed by the present invention, which relates to long period fiber gratings (LPGs) and, more particularly, to “turn-around-point” (TAP) LPGs configured to exhibit an arbitrary bandwidth and capable of recognizing changes in ambient refractive index on the order of 10−4or lower. In particular, if a fiber waveguide is engineered to yield two modes with identical group velocities, a broadband spectrum is obtained in which the strength (or loss), rather than the resonant wavelength, varies when tuned. This approach yields strong broadband loss-filters, in which the LPG couples the core mode to a specific higher order cladding mode whose group velocity equals that of the core mode. Such gratings are thus referred to as “turn-around-point” (TAP) LPGs.

In accordance with the present invention, it has been found that by matching the group velocities of one or more propagating signal modes to form a TAP LPG, the grating properties of the LPG become extremely sensitive to the refractive index of the outer cladding (e.g., “ambient”) material. Since there exist a number of techniques well-known in the art to adjust an optical signal group velocity, it is possible to create a TAP LPG whose refractive index sensitivity can be arbitrarily adjusted, while independently selecting the operational wavelength range of interest.

An advantage of this inventive TAP LPG is that it facilitates construction of a device with arbitrary sensitivity to the ambient refractive index, and can do so at any desired wavelength of operation. The index sensitivities that are achievable with this inventive arrangement are such that trace elements in the atmosphere may be sensed, or the inventive device can be combined with high-speed electro-optic nonlinear optical or refracto-optic materials to realize a high-speed fiber optic modulator.

Other and further advantages of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.

DETAILED DESCRIPTION

An exemplary long period grating (LPG)-based device10of the present invention is illustrated inFIG. 1. LPG device10comprises a single mode fiber with a core area12and a cladding region14surrounding core area12. As shown, cladding region14is defined as comprising an outer diameter D and a refractive index denoted ncl. A “coating”/ambient region16is illustrated as surrounding device10, where coating/ambient region16may comprise organic materials such as polymers, inorganic materials such as semiconductors or glasses, or a gaseous ambient such as air containing a trace gas or chemical substance, where the latter is of great concern with respect to finding a suitable detector. A grating18of a predetermined period Λ is written in core area12as shown. In accordance with the present invention, the material comprising cladding region14is selected such that ncl(the refractive index of cladding region14) will be greater than ncoat, the refractive index of coating/ambient region16. In forming device10, the outer diameter D of cladding region14needs to be carefully chosen, along with the period Λ of grating18in order to provide the desired ambient index sensitivity, high-speed operation or wideband filtering, as desired.

FIG. 2illustrates the phase matching curve20for an LPG that couples the fundamental mode propagating in core area12to the higher order mode propagating in cladding region14, both modes propagating at the same group velocity in accordance with the present invention. Indeed, this matching of group velocity results in curve20exhibiting a turn-around-point (TAP)21at the wavelength (and period) where the slope of curve20is zero. Referring toFIG. 2, the grating period for the LPG is illustrated as horizontal line22, where this particular arrangement has a grating period Λ of 146 μm. It has previously been demonstrated that when the LPG grating period is chosen to couple the phase matching curve at the TAP, large bandwidth mode coupling is achieved. Phase matching curve20ofFIG. 2is one such arrangement, defined hereinafter as exhibiting the “TAP resonance condition”.FIG. 3illustrates the exemplary large bandwidth coupling in curve30, which illustrates a typical spectrum of light remaining in the fundamental mode (propagating in core area12) after the higher-order cladding mode is stripped out for the TAP resonance condition of curve20inFIG. 2. For the sake of comparison, a conventional, prior art spectrum32associated with a narrowband LPG is also shown inFIG. 3. It is apparent that bandwidth improvements by a factor of one hundred or more can be achieved when modes exhibiting the TAP resonance condition (i.e., matching group velocities) are selected for coupling.

Referring back toFIG. 2, additional phase matching curves24and26are shown, where these curves are associated with fibers having a progressively higher refractive index coating region16(i.e., increasing ncoat), while exhibiting the same outer diameter D and grating period Λ. As shown, curve24is separated from the TAP resonance condition grating value (line22) by a first distance y1and curve26is separated from the TAP resonance condition grating value (line22) by a second distance y2.FIG. 4contains the spectra associated with all three phase matching curves20,24and26ofFIG. 2, where spectrum40ofFIG. 4is associated with curve20and is identical to spectrum30ofFIG. 3. Continuing, spectrum42ofFIG. 4is associated with curve24ofFIG. 2and spectrum44ofFIG. 4is associated with curve26ofFIG. 2. As is evident fromFIG. 4, slight changes in the coating refractive index lead to significant changes in the coupling strength (loss) of the grating. It is to be noted that while the strength of the coupling changes, the spectral shape remains nominally the same. This is in contrast to conventional prior art narrowband LPGs, where the tuning shifts the resonant wavelength of the spectrum.

The origin of the coupling effect/refractive index sensitivity relationship of the present invention can best be understood by realizing that the coupling efficiency, η, of an LPG (i.e., the factor that determines the amount of light coupled by the LPG) is defined by:η=(κ⁢⁢L)2·sin2⁡((κ⁢⁢L)2+(δ⁢⁢L)2)(κ⁢⁢L)2+(δ⁢⁢L)2,
where κ is the coupling coefficient, which is proportional to the amount of index change induced in the fiber, L is the physical length of the grating and δ is a detuning parameter, defined as:δ=12⁢([β01-β0,12]-2⁢πΛ),
where λ is the propagating wavelength, Λ is the period of the grating, and β01and β0,12are the propagating constants of the fundamental and an exemplary higher-order mode, respectively. Thus, in light of the definition of TAP from above (where the group velocities of the modes are equal), a resonance occurs and strong coupling is achieved when the condition δ=0 is satisfied. The coupling efficiency relation, in the above equation, indicates that the coupling strength decreases with a Sinc function dependence as δ deviates from zero. The detuning parameter relation represents the resonance condition for an LPG, where phase matching curve20ofFIG. 2is indeed a curve of this function/condition, measuring wavelength λ as a function of grating period Λ when δ=0. From these two equations, it can be deduced that for phase matching curves24and26, δ is greater than 0 at all wavelengths. This implies, therefore, that the coupling strength is less than optimal for all wavelengths along curves24and26. Additionally, it is apparent fromFIG. 2that δ becomes progressively larger as the curves move further away from the TAP resonance condition. Likewise, the corresponding spectra ofFIG. 4are illustrated as evidencing progressively weaker coupling as 6 increases (which, in turn, increases as the coating refractive index increases).

Thus, in accordance with the present invention, the coupling strength of an LPG can be deduced by inspecting the phase matching curve and associated line defining the period of the grating. One such example is illustrated inFIG. 5. As shown, curve50represents an exemplary phase matching curve for a particular LPG, where in this case the LPG has a grating period Λ of approximately 112.1 μm (indicated by horizontal line52inFIG. 5). The relative coupling strength at any wavelength is then proportional to the length of a line54connecting phase matching curve50to grating period line52. It is to be understood that this relationship is strictly true for only Gaussian apodized gratings, and is approximately true for uniform gratings. In conclusion, therefore, the coupling strength of this new class of gratings may be changed without significantly perturbing their spectral shapes.

As was shown inFIG. 2and can be explained in the context ofFIG. 5, changing the refractive index of the “coating” (which may be, as mentioned above, the ambient atmosphere, such as air containing a trace gas) results in shifting the phase matching curve of a TAP LPG, thus resulting in varying the coupling strength. Indeed, the sensitivity to ncoatis determined by the proximity of the grating period to a TAP, where inFIG. 5the grating period is illustrated by line52and the TAP of curve50is illustrated by minima56. The grating period can then be matched to TAP56for any coating refractive index ncoatby one of at least two techniques: (1) the grating period Λ can be physically changed, such as by strain or temperature; or (2) the cladding outer dimension D can be changed.FIG. 6illustrates a first phase matching curve60associated with an LPG exhibiting a TAP condition between the fundamental order core area mode and higher-order cladding mode for an LPG fiber with an outer diameter D=125 μm and ncoat=l, with a grating period Λ of 145.9 μm (horizontal line62inFIG. 6), illustrating in this case a TAP resonance condition at a wavelength λ of approximately 1550 nm. When the refractive index of the coating increases to a value of 1.43, the phase matching curve shifts downward away from this resonant condition, as illustrated by curve64inFIG. 6. The LPG fiber is returned to resonance in this case by changing the diameter D of the fiber. In particular, the cladding layer (such as layer14) is etched such that D is reduced to 124.5 μm. Lowering the outer diameter raises the phase matching curve, as shown by curve66inFIG. 6, to recover the TAP resonance condition. Thus, in accordance with the present invention, a TAP condition and a broadband, strength-tunable resonance may be obtained for any ambient refractive index value.

Further, it is also possible to write a grating with a predetermined period Λ associated with the “TAP condition” (i.e., slope equals zero) of a give phase matching curve. This aspect of the present invention is particularly useful in constructing a device with arbitrary sensitivity to the coating index ncoatand is a result of the fact that the propagation constants of the cladding modes are critically dependent on the refractive index value of the coating, as shown below:ⅆβⅆncoat∝ncoat(ncl2-ncoat2)3/2

This relation clearly shows that the rate of change of the propagation constant of the cladding mode increases monotonically with the refractive index of the coating material, that is, as ncoatapproaches the silica-cladding index, ncl. Since it was previously shown that a rapid change in the propagation constants of either mode leads to rapid changes in attenuation level for the TAP LPG device, it is now evident that the LPG attenuation also becomes increasingly sensitive to changes in the refractive index of the “coating”, particularly as the refractive index approaches the value associated with the silica cladding. This effect, combined with the fact that a TAP condition can be obtained for any desired coating index value, as discussed above in association withFIG. 6, allows for the construction of an LPG device whose attenuation level can be made arbitrarily sensitive to the outside/ambient “coating” refractive index.

FIGS. 7(a) and7(b) illustrate two sets of broadband spectra obtained for devices that are identical in all respects, except for the “coating” refractive index values and outer cladding diameter D, where the cladding diameter D is adjusted in each case to arrive at the TAP resonance condition for each refractive index value. Referring toFIG. 7(a), it is shown that an index change of approximately 5×10−3is required when the ambient coating index is approximately 1.4325 (to obtain a 25 dB attenuation change). Looking at the spectra ofFIG. 7(b), it is shown that an index change of only 4×10−4is required when the ambient/coating refractive index is 1.44856.FIG. 8contains a graph charting the experimentally observed monotonic decrease in required index change for ncoatrequired to induce a 25 dB strength change in the TAP LPG of the present invention as a function of the ambient refractive index. From this relationship, it is clear that the TAP LPG device of the present invention can be configured to achieve arbitrarily high sensitivities, such as those required for high-frequency modulation.

It is to be noted that while the increased sensitivity to changes in ambient index as illustrated inFIGS. 7(a) and7(b) were obtained by adjusting the cladding outer diameter D, the same effect can be observed by adjusting the grating period of the actual grating written in the core area of the LPG to coincide with the zero slope point of the phase matching curve. More generally, the ability to obtain a TAP resonance condition for any arbitrary ambient index configuration may be enabled by one of several mechanisms including, but not limited to, modifying the grating period, inducing a DC voltage-based index change, changing the ambient temperature, changing the strain on the fiber grating, nominal etching of the fiber cladding, or any combination of the above.

As mentioned above, there are many practical applications for an LPG device that offers variable coupling (or attenuation) levels with sensitivity to changes in the ambient refractive index. In particular, an LPG device of the present invention may be formed as shown inFIG. 9to include a coating material15surrounding cladding layer14, where coating material15induces efficient absorption of trace elements/chemicals in specific compounds or ambient gases that are desired to be detected. The incorporation of these trace elements thus changes the refractive index of coating15, thereby changing the transmitted intensity through the device over a relatively broad wavelength range. Since the attenuation level changes over such a broad range, an LED17with a center wavelength approximately equal to the TAP wavelength, denoted λTAP, and a simple photodetector19coupled to the output of device10, yields a sensor whose detected intensity changes with relatively small changes in the refractive index of coating15. Since this device can be made arbitrarily sensitive to index changes (using any of the methods defined above), extremely small levels of contamination can be sensed.

The cladding diameter D and coating refractive index ncoatcan be adjusted, as discussed above, to yield an LPG device with an approximate 25 dB transmission change for coating index changes of 2×10−4or less. Since electro-optic polymers can offer fast transmission rates (e.g., approximately 300 GHz speed) but only relatively small refractive index changes (less than 5×10−4), a TAP LPG device of the present invention can be used to implement a high speed, polarization insensitive, all-fiber-based data modulator.FIG. 10illustrates one exemplary modulator100, including a relatively thin electrode110disposed to surround cladding layer14. The material of thin electrode110is selected to be relatively transparent to the optical field of the cladding mode induced by the LPG, where the compound indium-tin-oxide (ITO) may be used. ITO is a conductor that is transparent to 1550 nm light and is thus an optimal candidate. Gold or silver may also be used, inasmuch as both of these materials that are opaque to 1550 nm light. In any case, the thickness of conducting electrode110is minimized so as to not perturb the optical field of the cladding mode. A layer of electro-optic material112is disposed to surround thin electrode110, as shown inFIG. 10, with an outer electrode114disposed over electro-optic material112. In operation, a continuous wave (CW) optical signal is coupled to the input of modulator100, where the wavelength of the CW signal is selected, in combination with the grating period of the device, to exhibit the TAP condition. An electrical modulating data input signal is applied between electrodes114and112, the voltage modulation changing the value of ncoat, the refractive index of electro-optic material112. The modulation of this refractive index thus results in modulating the coupling strength of the propagating optical signal, providing a high speed modulated optical output signal.

It is to be understood that although the above-described embodiments illustrated the phase matching “turn around point” as associated with a group velocity matching between the fundamental mode and a higher-order cladding mode, a TAP condition can be formed between any two (or more) matched modes, whether core-propagating modes or cladding-propagating modes. In this context, the choice of the kind of LPG used to fabricate the TAP LPG device of the present invention depends upon the preferred mode(s) of choice. For example, it may be preferable to use symmetric gratings (such as UV-induced LPGs) for coupling between the fundamental mode (LP01) and a symmetric mode (such as the LP0,12mode). In contrast, it may be preferable to use asymmetric gratings (such as microbend LPGs induced by pressing corrugated surfaces on the fiber, or by acousto-optic excitation) for coupling the fundamental mode with an anti-symmetric mode (such as the LP1,12mode) of the fiber. Indeed, the subject matter of the present invention is intended to be limited only by the scope of the claims appended hereto.