Letter
Vol. 42, No. 20 / October 15 2017 / Optics Letters
4219
Point-by-point fabrication and characterization of sapphire fiber Bragg gratings SHUO YANG,* DI HU,
AND
ANBO WANG
Center for Photonics Technology, Department of Electrical and Computer Engineering, Virginia Tech, Blacksburg, Virginia 24061, USA *Corresponding author:
[email protected] Received 22 August 2017; revised 15 September 2017; accepted 19 September 2017; posted 22 September 2017 (Doc. ID 305378); published 13 October 2017
The Letter reports the inscription of fiber Bragg gratings (FBGs) in a single-crystal sapphire optical fiber via a point-by-point method by 780 nm infrared-femtosecond laser pulses. Compared to phase mask exposure, the use of the point-by-point method for the inscription provides a flexible way to fabricate sapphire FBGs and to make wavelength division multiplexing in sapphire fiber more practicable. The multiplexing of three cascade gratings is demonstrated, and their performance up to 1400°C is tested. The permanent enhancement of reflectivity by a factor of about 5 after heat treatment and the nearly linear temperature response with a slope of 25.8 pm/°C are demonstrated. © 2017 Optical Society of America OCIS codes: (060.3735) Fiber Bragg gratings; (060.2370) Fiber optics sensors; (140.3390) Laser materials processing. https://doi.org/10.1364/OL.42.004219
Fiber Bragg gratings (FBGs) are important sensing elements in many applications. Many silica fiber FBG-based sensors have been reported in the past few decades for various applications [1,2]. One of the limitations to a silica-glass-based-FBG is its use in high-temperature environments. Silica-glass-based FBGs fabricated by ultraviolet-photosensitized refractive change may be bleached out at temperatures higher than 200°C–300°C [2]. With additional heat treatment called thermal regeneration, the temperature range can be leveled up to 1295°C [3,4]. Later, silica-glass-based FBGs fabricated with femtosecond laser pulse induced refractive index change were demonstrated, and they can operate up to 1200°C [5,6]. Although many efforts have been made to further push the temperature limit of silica glassbased FBGs, the softening point of the silica glass (∼1330°C) limits the ultimate operating temperature for such sensors. However, there are still many applications that desire FBGbased sensors for higher temperature environments, such as temperature or strain measurement in gas turbines or furnaces. Owing to its high melting point (∼2040°C), chemical stability and optical transparency, a single-crystal sapphire fiber is attractive for the construction of sensors for various harsh environments, including ultrahigh temperatures [7–10]. FBGs fabricated in sapphire fiber by femtosecond laser have been 0146-9592/17/204219-04 Journal © 2017 Optical Society of America
demonstrated for sensing applications up to 1745°C [11]. The reason for using a femtosecond laser for the FBG fabrication is that it provides high peak intensity, which enables a multi-photon process with sufficient efficiency for a permanent change of refractive index. The most common techniques to fabricate an FBG are phase mask exposure and point-by-point inscription [2]. The phase mask technique uses either the interference pattern directly located behind a phase mask or a phase mask as a beam splitting element to form an interference pattern with an additional interferometer. The whole grating structure is formed at the same time with this method. Conversely, for the point-bypoint approach, the laser is focused into the core of a fiber to induce a localized refractive index change point; then the grating is created by scanning the fiber so that the points are periodically distributed along the fiber. The advantage of using the point-by-point inscription over the phase mask method is its inherent flexibility. A range of grating structures fabricated in both single-mode and multimode silica fiber via this method has been reported, and it is quite easy to implement wavelength division multiplexing (WDM) [12–15]. Until now, to the best of our knowledge, only the phase mask method has been reported for the fabrication of FBGs in single-crystal sapphire fibers, including both direct phase mask exposure [16,17] and a phase-mask-based Talbot interferometer [18]. The first method can only inscribe FBGs in sapphire fiber with single Bragg wavelength per fabrication due to the fixed pitch of the phase mask. Although the second approach provides better geometrical flexibility, the extremely short pulse width requires the path difference between the two arms of the interferometer to be tuned within a few micrometers to achieve good overlapping of the laser pulses and thus, requires a fabrication setup with high stability and accuracy. In addition, the grating length in both methods is also limited by the laser beam size. This Letter reports the sapphire fiber FBGs inscribed by a point-by-point method with a 780 nm infrared-femtosecond (IR-fs) laser. The phase matching condition for a FBG is given by [2] mλBragg 2neff Λ; (1) where m is the order of the grating, λBragg is the Bragg wavelength, neff is the effective refractive index of the reflected
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mode, and Λ is the pitch of the grating. Since the refractive index of sapphire fiber is about 1.745 at 1550 nm, the pitch of the first few orders of the grating with a Bragg wavelength lying in the C-band is smaller than 2 μm. Thus, a sufficiently small laser focal area is necessary, which requires the use of a high magnification lens. The femtosecond laser used in the fabrication is a Q-switched Ti: sapphire IR-fs pulsed laser (Coherent Libra series) with a 780 nm emission wavelength, ∼100 fs pulse width, and 3 mm beam waist linearly polarized Gaussian beam. A Nikon oil-immersion objective (100×, NA 1.25) was selected for the fabrication. The size of the focal area is estimated to be about 0.2 μm in diameter via w λf ∕πw0 , where λ is the wavelength of the laser in the medium, f is the focal length of the lens, and w0 is the input beam waist. Figure 1 describes the setup of FBG inscription in a sapphire fiber by a point-by-point method. In the fabrication, a 40 cm long single-crystal sapphire fiber with a 125 μm diameter and a c-axis as the axial direction (MicroMaterial Inc.) were mounted on an assembled five-axis translation stage (Aerotech ABL10050L-LN and Newport 562 Series), and one of its flat faces was tuned to be normal to the incident beam. The fiber was placed on a flat surface and clamped on both sides to ensure the stability. A half-wave plate and a linear polarizer were implemented to attenuate and control the polarization of the laser beam. The pulse energy was carefully tuned to exceed the damage threshold in sapphire, but not induce cracks. Due to the birefringence nature of sapphire, the polarization of the incident beam was tuned to be perpendicular to the optic axis of the sapphire fiber to reduce the extraordinary components which, in this case, was perpendicular to the fiber. Then the fiber moved along the axial direction at a constant speed (v s ) and, in the meantime, laser pulses with a constant repetition rate (f rep ) were focused into the fiber to induce localized refractive index change. A refractive index oil matching the sapphire was applied for the oil-immersion objective to reduce the beam distortion at the interface. By tuning the relation between the moving speed and the repetition rate, a desired pitch of the
Letter inscribed FBG can be easily altered. The length of the FBG can be adjusted by controlling the total number of the laser pulses. In this fabrication, the moving speed was 0.88 mm/s, and the repetition rate of the laser was 500 Hz, which induces an FBG with 1.776 μm pitch. The speed accuracy of the stage was less than 10 nm, guaranteeing the uniformity of the pitch. This FBG corresponds to the fourth-order grating for the lowest guided mode at a wavelength of 1550 nm. The grating length was 2 mm and, thus, the total fabrication time for this FBG only requires ∼2.5 s. On the other hand, a transmission microscopy system using the same objective was implemented to monitor the fabrication in real time (see Fig. 1). Figure 2 shows the microscopic images of the inscribed FBG in the center of a 125 μm diameter single-crystal sapphire fiber, and the cross-sectional area is about 1.81 μm in width and 3.28 μm in depth. Characterization of the inscribed FBG was conducted with the setup described in Fig. 3(a). A superluminescent light-emitting diode (SLED) with a center wavelength of 1565 nm and a bandwidth of 80 nm (Thorlabs S5FC1005P) was used as the light source. A step-index 105/125 μm multimode silica fiber was chosen as the lead-in fiber because it can excite a sufficient number of modes into the sapphire fiber, which gives us a full characterization of the reflection spectrum of the inscribed FBG. A customized 2 × 2 3 dB 105/125 μm multimode fiber coupler was used here. The idle port of the coupler was tied into a small knot and then dipped into a refractive index matching gel to prevent the end reflection. For further reducing the background reflection, one end of the sapphire fiber was connected to the lead-in fiber with FC/APC butt coupling, while the other end was polished to 7 deg and dipped into a refractive index matching gel. The spectrum of the reflected light was recorded by an optical spectrum analyzer (ANDO AQ-6315A). The reflection spectrum of the inscribed FBG illustrated in Fig. 2 is shown in Fig. 3(b). Due to the high modal volume of this sapphire fiber, the reflective spectrum has a broad peak with ripples on the top, and its full width at half maximum is about 6 nm. By taking only the SLED power and the insert
Camera Lens
780 nm fs laser
Dichroic Mirror HWP
LP 100X NA 1.25
Pulse Chain,, frep Clamps
Index Matching Oil (n = 1.75)
= vs/f / rep Glass Slide Moving, vs
Illumination
5-axis Translation Stage
Fig. 1. Scheme for the setup and procedure of FBG inscription in a sapphire fiber via the point-by-point method. (fs, femtosecond; HWP, half-wave plate; LP, linear polarizer. The laser pulse energy 500 nJ).
Fig. 2. Microscopic images of the inscribed fourth-order FBG in 125 μm diameter sapphire fiber from (a) a top view and (b) a side view.
Letter
Vol. 42, No. 20 / October 15 2017 / Optics Letters
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loss of the 2 × 2 coupler into consideration, the reflectivity of the grating was estimated to be ∼0.6%. The actual reflectivity is higher than this value because there is connection loss at the silica-sapphire interface. This reflectivity is enough for many sensing applications, especially for those that need to multiplex a large number of weak (
