The present invention relates to optical fiber devices and, more particularly, to optical microfiber devices including at least one inner enclosed cavity within the microfiber.
Various types of optical fiber devices and components have emerged for a wide range of optical fiber communication and sensor applications. To improve optical fiber system simplicity and efficiency as well as reducing cost, it is desirable to have a versatile fiber in-line device that is easily integrated into a fiber-based system and is capable of performing multiple functions. Current fiber in-line devices include fiber gratings, photonic crystal fibers (PCF), and microfibers. Fiber gratings, including fiber Bragg gratings (FBG) and long period fiber gratings (LPFG), are typically formed by introducing a periodic refractive index (RI) or geometric structure modulation in a small section of the fiber length. Since its resonant wavelength is determined by the grating period and the effective RI of the fiber, which can be adjusted by various means such as strain, temperature, and RI of the surrounding medium, many communication and sensing functions can be achieved using fiber gratings. PCFs exhibit a periodic microstructure along the whole fiber length, which enables a different light guiding mechanism than conventional optical fibers. Microfibers have a small size but a large evanescent field for the guided light, which makes them sensitive to a surrounding medium.
However, there is a need in the art for structure which can perform multiple functions and operate in a variety of optical modes. Such structures could be used for improved multifunction optical sensors having enhanced sensitivity.
The present invention provides photonic devices that include in-line microfiber optical fiber sensors. An exemplary enclosed cavity is positioned completely within the optical microfiber. An input light beam travelling within the optical fiber is split into two portions and passes through both the enclosed cavity and a remaining portion of the optical fiber that is not occupied by the enclosed cavity. The recombination of the two portions of light that propagate through the remaining portion of the optical fiber and through the cavity has a light intensity that can be related to an external factor such as temperature or surrounding refractive index. Alternatively, the combined spectrum can be related to strain or bend within the microfiber and the device can be used to sense changes in refractive index, temperature, strain and bending.
Strain can further be measured using the whispering gallery mode (WGM) resonator properties of the cavity and a second optical fiber carrying light that is evanescently coupled into the cavity followed by spectral analysis.
The microfiber optical fiber device is fabricated by providing a first optical fiber having a cladding layer and a core layer and cleaving the first optical fiber to expose an end surface. Micromachining by femtosecond (fs) laser ablation forms one or more micro-hole(s) positioned on the exposed end surface of the first optical fiber. A second optical fiber is cleaved to expose an end surface and the end surface of the first optical fiber having the microhole formed therein is fused to the end surface of the second optical fiber. The fused structure is heated and drawn to form a microfiber region having a diameter on the order of microns in the narrowed waist region. Within the microfiber region is an elongated cavity formed from the microhole micromachined in the first optical fiber. Depending on the application of the device, more than one cavity can be formed within one optical fiber by varying the operational parameters of the fs laser. The position of said cavity can also be varied to be positioned along a central axis or off-center with respect to a central axis of the formed microfiber, the position being determined by the position of the micromachining at the exposed end surface of the optical fiber.
a)-2(d) schematically depict a method of fabricating a microfiber with a cavity according to an embodiment of the present invention.
a) depicts a photomicrograph of a micro-hole fabricated by fs laser ablation at the center of a cleaved single mode fiber end facet.
b) shows a hollow sphere formed after fusion splicing.
c) is an SEM image of a cross section of the hollow sphere of
d) depicts a microfiber with an inner air-cavity. Inset is the cross section view of the microfiber.
a) is a photomicrograph of an optical device used in refractive index measurement.
b) depicts a transmission spectra evolution with external refractive index.
c) shows the variation of dip wavelength with refractive index.
d) is a photomicrograph of an optical device used in axial strain measurement.
e) is a transmission spectra evolution with axial strain.
f) depicts the variation of dip wavelength with axial strain.
g) is a photomicrograph of an optical device used in high temperature measurement.
h) is a transmission spectra evolution with ambient temperature.
i) depicts the variation of dip wavelength with temperature.
j) is a photomicrograph of an optical device used in bend measurement.
k) is a transmission spectra evolution with curvature ratio between 0.4 and 2.1 m−1.
l) depicts the variation of dip wavelength with curvature.
a) depicts a two-fiber system based on a microfiber with inner air-cavity.
b) shows spectra evolution with axial strain.
c) depicts the variation of dip wavelength with increasing strain.
a) depicts an optical microfiber having two inner air-cavities formed in the fusion splicing with another cleaved SMF.
b) is a side view of waist region of the microfiber with two inner air-cavities.
c) is a cross-sectional view of the morphology of waist region of the microfiber with two inner air-cavities.
d) is the measurement of polarization dependent loss (PDL) of the microfiber device at different wavelengths.
a) is a cross-sectional view of the microfiber with a deviated air-cavity.
b) is a cross-sectional view of the microfiber with three symmetrical air-cavities.
c) is a photomicrograph showing cascaded microfibers with multiple inner air-cavities.
Turning to the drawings in detail,
Note that although cavity 30 is depicted in
The optical devices described above have a number of applications such as sensing applications. Below, detailed explanations of the fabrication and operation of these devices is provided.
a) shows the cleaved fiber end facet with a micro-hole fabricated by fs laser ablation. After fusion splicing with another section of single mode fiber (SMF), a hollow sphere-like void is formed in the SMF, its microscope side view and scanning electron-microscope (SEM) image of a cross section view are displayed in
where I represents the intensity of the interference signal, λ is the wavelength, L is the cavity length, Δn=nwall−nhole denotes the effective refractive index (RI) difference of the two interference arms, nwall and nhole are the effective RI of the silica wall mode and the air-cavity mode respectively. When the phase term satisfies the condition
where m is an integer, the intensity dip appears at the wavelength
To test the system response to RI change, the device was immersed into the RI liquid (from Cargille Laboratories) with the RI value of 1.33 at room temperature and the temperature coefficient of 3.37×10−4/° C. The liquid RI value was changed by varying its temperature. The dip wavelength shift with the RI change can be derived from Eq. (2) as
where δn denotes the change in the effective RI of silica wall mode.
In the axial strain measurement, a 30 μm diameter microfiber with inner cavity of ˜1.9 mm in length and ˜12 μm in diameter as shown in
From Eq. (2), the wavelength shift due to the change of axial strain can be expressed as
where δLs is the change in cavity length and δns denotes the change in the effective RI of the silica wall mode, induced by the increased axial strain. The experimental results obtained indicate that for the size of the microfiber and its inner cavity employed, the effective RI, δns, plays the dominant role in determining the dip wavelength and a blue shift of dip wavelength corresponds to an increase of axial strain.
High temperature sensing capability of the device was investigated by use of a tube furnace (CARBOLITE MTF 12/38/250).
h) demonstrates the transmission spectra with offset at different temperatures. A fringe dip at ˜1510 nm at room temperature was found to experience a red shift with the increase of temperature.
where δLT is the change of inner cavity length induced by material thermal-expansion and δnT denotes the change in effective RI of the silica wall mode, due to thermal-optical effect. The thermal-optical effect plays the dominant role as the thermo-optic coefficient (7.8×10−6) in silica is larger than thermal expansion coefficient (4.1×10−7).
j) shows the microscopic image of the sample used in the bending measurement, in which the microfiber diameter is ˜20 μm and the inner cavity length and diameter are ˜1.1 mm and ˜7 μm, respectively. The spectra obtained at different curvatures are shown in
a) illustrates a strain measurement system including an optical sensor 10 with an inner cavity 30. Light is evanescently coupled in the sensor 10 by a second microfiber 50 perpendicularly crossing and in intimate contact with cavity-containing microfiber 10. The second microfiber 60 optically communicates with a broadband source (BBS) and an optical spectrum analyzer (OSA) with the resolution of 10 pm. A spectrum corresponding to the microfiber 10 having a diameter of ˜16 μm and cavity wall 20 thickness of ˜2 μm is displayed in the inset of
b) shows the spectra obtained at different axial strain values, and a blue shift of the dip wavelength can be observed. For the dip wavelength at ˜1636 nm, its variation with axial strain corresponding to the device 10, a 16 μm diameter microfiber without inner air-cavity and a 16 μm diameter microfiber with inner air-cavity are demonstrated in
As shown in the above embodiments, by fabricating an air-cavity inside a microfiber, a variety of optical sensors can be formed. In particular an extremely small fiber interferometer can be created. In such a device, the unique features of microfibers are effectively used to create highly sensitive Mach-Zehnder interferometers or multiple fiber sensor systems, thus providing versatile optical fiber sensing applications.
A modified device configuration with two parallel inner air-cavities is created in microfiber for polarization maintaining (PM) fiber use. Initially, femtosecond laser is used to ablate two similar-size holes of ˜15 μm in diameter and ˜100 μm in depth. Both holes are ˜25 μm distance away from the fiber core and positioned in symmetry. After being splicing with another cleaved SMF tip with fusing current of 17.0 mA and fusing duration of 2.2 s, two parallel inner air-cavities with similar size and shape are simultaneously formed, as shown
The polarization dependent loss (PDL) of such microfiber device is measured by an Agilent 81910A photonic all-parameter analyzer. In
In
In
c) is a photomicrograph of a cascaded microfiber with inner air-cavity. It is known that by cascading the fiber MZIs, the bandwidth of the output spectrum can be further widened and the extinction ratio of the device will be enhanced. Thus, it will be an easy way to adjust the output spectrum by cascading a number of such microfiber MZIs with suitable size.
While the foregoing invention has been described with respect to various embodiments, such embodiments are not limiting. Numerous variations and modifications would be understood by those of ordinary skill in the art. Such variations and modifications are considered to be included within the scope of the following claims.