Thermally Expanded Core Research Articles

Ultrahigh-Resolution and Large-Dynamic-Range Temperature Sensor Based on Fiber-Optic EFPI Cavity

A fiber-optic extrinsic Fabry–Perot interferometric (EFPI) temperature sensor with extremely high resolution and large dynamic range is proposed and demonstrated. The sensor uses a thermally expanded core (TEC) fiber as the first reflective surface of the interference cavity to focus the beam to improve the finesse of the interference fringes, which enables extremely high cavity length resolution and large dynamic range with a multivalley averaging method based on dynamic estimation of the fringe order. Using a borosilicate capillary as the temperature-sensitizing material, a cavity length sensitivity of 79.25 nm/°C as well as a good linearity were obtained in the temperature range of 10 °C–100 °C. A temperature resolution of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$7.5\times 10^{-{5}}\,\,^{\circ }\text{C}$ </tex-math></inline-formula> was demonstrated. The extremely high-temperature resolution with a large dynamic range has an important application value in the monitoring of weak temperature changes in deep sea and underground fluids.

Bidirectional transmission of a collimator with double-combined collimating lenses and thermally expanded core fibers.

The collimator is an essential part of the fiber optic rotary joint design. This study proposes the Large-Beam Fiber Collimator (LBFC) with a double collimating lens and a Thermally Expanded Core (TEC) fiber structure. The transmission model is constructed based on the defocusing telescope structure. The effects of TEC fiber's mode field diameter (MFD) on the coupling loss are investigated by deriving the loss function for the influence of collimator mismatch error and implementing it on a fiber Bragg grating temperature sensing system. The experimental results show that the coupling loss decreases with the increase of the MFD of TEC fiber, while the coupling loss is less than 1dB when the mode field diameter is greater than 14µm. TEC fibers can reduce the effect of angular deviation. Considering the coupling efficiency and deviation, the preferred mode field diameter for the collimator is 20µm. The proposed LBFC enables bidirectional transmission of optical signals for temperature measurement.

Ultrafast soliton delivered by miniaturized mode-locker with MoTe2 and core-expanded fiber.

Thermally expanded core (TEC) technology is an effective method of high-power fiber lasers. Miniaturization is also a major challenge for high-power lasers. We have proposed a miniaturized mode-locker based on TEC fiber and MoTe2-polyvinyl alcohol (PVA) film. The proposed mode-locker is consisting of two TEC ferrules, a piece of MoTe2-PVA film and a ceramic sleeve. The length of the proposed device is about 20 mm, and its outer diameter is about 2 mm. The relations between heating time, heating temperature, and mode field diameter (MFD) have been numerically simulated. The bending loss with respect to MFD has also been analyzed. The simulation results have revealed the trade-off relation between maximal tolerable intensity and low cavity loss, which means that there is an optimal MFD corresponding to optimal heating time and heating temperature. The proposed mode-locker has been applied in an integrated fiber laser, which has emitted ultrafast soliton with 3 times intensity larger than that of conventional sandwiched-type saturable absorber. The proposed mode-locker and fiber laser will find important applications in laser processing, laser ranging, and optical communication.

Connector-style hollow-core fiber interconnections.

To go beyond the fundamental limits imposed by latency, nonlinearity, and laser damage threshold in silica glass fibers, the hollow-core fiber (HCF) technique has been intensively investigated for decades. Recent breakthroughs in ultralow-loss HCF clearly imply that long-haul applications of HCF in communications and lasers are going to appear. Nevertheless, up to now, the HCF technique as a whole is still hampered by the limited length of a single span and the lack of HCF-based functional devices. To resolve these two issues, it is of importance to develop ultralow-loss and plug-and-play HCF interconnections. In this work, we report on HCF interconnections with the lowest-ever insertion losses (0.10 dB for HCF to standard single-mode fiber (SMF) and 0.13 dB for HCF to itself in the 1.5 µm waveband) and in a pluggable means. Two fiber mode-field adapters, one based on a graded-index multi-mode fiber (GIF) and the other utilizing a thermally expanded core (TEC) SMF, have been tested and compared. An extra insertion loss arising from imperfect refractive index distribution in a commercial GIF is observed. Our HCF interconnections also realize a back-reflection of <-35 dB over a 100 nm bandwidth as well as other critical metrics in favor of practical applications. Our technique is viable for any type of HCF.

Multicore-Fiber Receptacle With Compact Fan-In/Fan-Out Device for SDM Transceiver Applications

We describe a new multicore fiber (MCF) receptacle for space division multiplexing (SDM) based transceiver applications, in which we integrated a compact fiber-bundle type fan-in/fan-out device for directly connecting MCF to LDs and PDs. We design the length of the fan-out fibers taking their bending stress into account to achieve a configuration with a smaller footprint. With the proposed receptacle, we develop a 100 Gb/s SDM transmitter based on a four-channel, 1.3 μm membrane laser array on Si, in which spot-size convertor (SSC) using inverse taper InP and SiON waveguides is integrated. Since the core size of SiON waveguide is much smaller than that of MCF, we utilize different fibers with optimized modal fields at each connection point and introduce thermally-expanded-core (TEC) fusion splicing to realize a low-loss connection between them. We achieve a pluggable connection between the lasers and 125 μm-cladding 4-core MCF in an LC connector. The developed receptacle maintained satisfactory laser characteristics including sufficient output power and clear eye openings for directly modulated 28 Gb/s NRZ signals. We also achieve an error-free transmission at a bit-error rate of less than 10−12 with a 500-m MCF transmission link for all 4 channels with simultaneous 4-channel operation.

高功率高耦合效率光纤模场匹配器

To improve the coupling efficiency for a Large-mode-area Fiber(LMAF)and a Single-mode Fiber(SMF),a Thermally Expanded Core(TEC)technique was used to expand the core of SMF to implement the mode field adaptation for the LMA double clad fiber 15/130μm and the SMF 6/125μm.The transmission loss of the Mode Field Adaptor(MFA)made by TEC processing was analyzed theoretically,and a homemade MFA and a commercial MFA was respectively measured through a selfbuilt fiber laser source with a 1 064 nm central wavelength.Experiment results show that the TEC technique dramatically improves the coupling efficiency of the MFA between the LMA and the SMF.In contract with the commercial MFA,the homemade MFA is capable of a higher operating power,better heat treatment capacity and better reliability.The homemade MFA offers the operating power up to 100 W,and accumulated working time up to 120 min in an air-cooled condition.Furthermore,its single-mode output to be 73 W,and heat treatment capacity to be 27 W.The improvement of theperformance of MFA meets the needs of the development of high-power fiber components and are suitable for achieving all-fiber-integration systems of fiber lasers.

Mode-field adaptor between large-mode-area fiber and single-mode fiber based on fiber tapering and thermally expanded core technique.

We report a method to fabricate a mode-field adaptor (MFA) for a large-mode-area (LMA) fiber and single-mode fibers (SMFs) based on fiber tapering and the thermally expanded core (TEC) technique. Through adiabatically tapering a LMA fiber by an oxygen-hydrogen flame, the mode-field diameter is decreased effectively with nearly no extra loss, which makes it possible to realize mode-field adaptation between the LMA fiber and SMFs. Both numerical calculation and experimental results show it is workable to realize fundamental mode propagation in LMA fiber and get a low insertion loss by this method. For a LMA fiber (d=25 μm, NA=0.06) and a SMF (d=4 μm, NA=0.2), a low loss with less than 0.4 dB has been obtained both in the forward and backward directions experimentally.

Mode field adaptation between single-mode fiber and large mode area fiber by thermally expanded core technique

The main source of light transmission loss between single-mode fiber (SMF) and large mode area (LMA) fiber is from the mode field mismatch. Thermally expanded core (TEC) technique was introduced to realize mode field adaptation between them. The SMF was heated by a H2–O2 flame to increase the mode field diameter (MFD) which can result in a mode field match with LMA. Both numerical calculation and experiment results show it is workable to realize mode field adaptation and get a low transmission loss by this method. For HI1060 (d=5μm, NA=0.14) and LMA-GDF-15/130(d=15μm, NA=0.08), a low-loss with 0.4dB was obtained for SMF–LMA–SMF from 6.4dB with TEC processing experimentally.

Improving fiber to waveguide coupling efficiency by use of a highly germanium-doped thermally expanded core fiber

Possessing a gradient refractive index (RI), thermally expanded core (TEC) fibers have been studied to be able to improve coupling efficiency of waveguide-fiber or LD-fiber according to the qualitative analysis of geometrical optics. The experiments show that the mode field radius (MFR) expansion of single-mode fiber with highly Germanium-doped (HGD) is more effective than that of conventional single-mode fiber (SMF). The coupling efficiency of a fabricated TEC fiber with HGD can enhance 5%–6% compared with that of a conventional-SMF. So the TEC fiber with HGD is a good candidate for the coupling of waveguide-fiber or LD-fiber.

Polarization-Insensitive Wideband OSNR Monitoring Using Thermally Expanded Core Fiber

We propose the in-band optical signal-to-noise-ratio (OSNR) monitoring method using a Mach-Zehnder interferometer. This interferometer is implemented by inserting a piece of Yb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped optical fiber (YDF) between two strands of thermally expanded core (TEC) fibers: the former to give optical phase delay to one of the two arms (core and cladding modes of the YDF) and the latter to give two arm paths. By using TEC fibers to split the input signal and combine again, the fringe pattern necessary for OSNR monitoring shows polarization independence and wideband characteristics. The phase of the optical signal propagating along the core mode of YDF is changed by flowing a laser light of 976 nm to the core of YDF. Compared with the OSNR measured by optical spectrum analyzer (OSA), the errors are less than 0.5 dB over the OSNR range of 20 dB. Also, the dynamic range showed over 20 dB at the wavelength range of 1541-1566 nm.

Novel color combiner composed of red, green, and blue laser diodes and a thermally expanded core fiber waveguide

We propose a new simple structure-based color combiner with red, green, and blue (RGB) laser diodes (LDs) and a thermally expanded core (TEC) fiber waveguide. As the color combiner used the LDs as optical sources, the total volume of the color combiner could be minimized as 3.84 cc and the output powers of RGB optical sources in the color combiner could be obtained efficiently as 10.324, 13.290, and 10.150 mW, respectively. A TEC fiber waveguide was used as a medium for color combination of RGB in the color combiner. As the TEC fiber waveguide was used as a key component for color combination, we could observe the color combiner had the degree of uniformity and the matching accuracy was good. The measured degree of uniformity and matching accuracy of RGB beams were 121 μm (in large diameter), 56 μm (in small diameter), and 109 μm (on x-axis), 20 μm (on y-axis), respectively.

Bending Effects of Thermally-Expanded-Core Fiber and Its Application as Variable Optical Attenuator

This study examined the bending effects of a thermally-expanded-core (TEC) fiber for an application as an in-line variable optical attenuator (VOA). The effects of the structural parameters of the TEC fiber and the refractive index of the external medium surrounding the clad layer on the bending loss are experimentally investigated and the device structure and fabrication process of the VOA are presented. The desired attenuation is obtained by mechanical bending of the TEC fiber. A wide range of attenuation exceeding 55 dB and low excess loss of <0.2 dB at 1550 nm are achieved.

Reflectometric measurements of thermally expanded core area

Reflectometric measurements of thermally expanded core area In this work an analysis method of one-way optical time domain reflectometer (OTDR) measurements has been presented. This method uniquely confirms mode field radii matching in diffusion transit area of the thermally expanded core (TEC) of thermally connected single mode telecommunication fibers. A comparison of reflectometric measurements with theoretical calculations of losses in TEC areas has been demonstrated.

Tunable comb-filter using thermally expanded core fiber and ytterbium doped fiber and its application to multi-wavelength fiber laser

We constructed a novel tunable comb-filter by splicing a commercial Yb 3+ doped fiber (YDF) between 2 thermally expanded core (TEC) fibers at their core-expanded ends. This structure forms an all-fiber Mach–Zehnder interferometer due to the interference between the core mode and the cladding modes of the YDF. The interference fringes shifts to the longer wavelength region as the YDF is pumped by a 980 nm laser diode (LD). We then demonstrated application of this newly proposed tunable comb-filter to a SOA-based ring laser and obtained multi-wavelength lasing. The lasing wavelengths can be also tuned by pumping the YDF in the proposed comb-filter.

Enhancing alignment tolerance of silicon waveguide by using a wide grating coupler

We fabricate a 20 um wide grating coupler for a single-mode thermally-expanded-core (TEC) fiber, in order to enhance positional tolerance in alignment. The minimal coupling loss is measured at 5 dB per facet and the optical 3 dB bandwidth is measured at 40 nm. The 3 dB alignment tolerance is measured at +-7.5 microm in horizontal direction and +290 microm in vertical direction. The 1 dB alignment tolerance is measured at +-4.2 microm in horizontal direction and +125 microm in vertical direction. The alignment tolerance is enhanced twice in horizontal direction and four times in vertical direction, compared with the coupling of a standard single-mode fiber to a standard 10 microm wide grating coupler which is also fabricated in this experiment.

Coupling characteristics of laser diodes to high numerical aperture thermally expanded core fibers

We numerically calculate the coupling loss coefficients of Laser Diode (LD) to high numerical aperture (HNA) thermally expanded core (TEC) fiber coupling in terms of lateral, longitudinal and angular misalignments. The propagating fields in the HNA-TEC fiber are obtained by Galerkin’s method and the coupling loss coefficients are calculated by the overlap integral technique. Our numerical results are compared with recently available experimental data. It is demonstrated that, in HNA-TEC fibers the lateral and longitudinal tolerances were effectively enlarged in comparison with those of conventional SMF-28. It is also found that TEC fibers are good candidates for free-space LD-to-fiber optical coupling scheme with long working distance. However, the angular tolerance of the HNA-TEC fiber is lower than that of the SMF-28.

Characteristics of side-polished thermally expanded core fiber and its application as a band-edge filter with a high cut-off property

Characteristics of the side-polished thermally expanded core (TEC) fiber have been investigated theoretically and experimentally. The effect of core expanding on the transmission of the side-polished TEC fiber is predicted theoretically and demonstrated experimentally. The side-polished TEC fiber covered with an external medium whose chromatic dispersion is much different from the fiber materials, is applied to a band-edge filter with a high cut-off. The relationship between the expanded core diameter and the performance of the band-edge filter was measured and discussed.

광섬유 수동정렬을 위한 단일 모드 대형 코어 폴리머 광도파로

정력 오차 허용 범위를 증가 시켜서 효과적인 수동정렬이 가능토록 하기 위한 단일 모드 대형 코어 폴리머 광도파로를 구현하였다. 대형 코어 광도파로는 TEC(thermally expanded core) 광섬유의 모드와 일치하는 큰 도파모드를 가진다. 이로 인해 광섬유와 도파로의 정렬오차로 인한 모드 결합 손실을 줄일 수 있게 된다. 코어와 클래딩의 굴절률 차이가 5 <TEX>${\times}$</TEX> <TEX>$10^{-4}$</TEX> 인 폴리머 재료를 이용하여 25 <TEX>${\times}$</TEX> 25 <TEX>$\mu\textrm{m}$</TEX><TEX>$^2$</TEX> 크기의 정사각형 광도파로를 제작하였으며 도파모드 관측결과 단일모드로 동작함을 확인하였다. 이와 같이 두꺼운 형태의 광도파로 구조 제작을 위하여 자외선 경화를 이용한 인젝션 몰딩(injection molding) 공정을 사용하였다. 제작된 광도파로 소자를 TEC 광섬유와 정렬연결을 할 때 정렬오차가 4.5 <TEX>$\mu\textrm{m}$</TEX> 까지 증가하더라도 삽입손실 증가는 0.5 dB 이하고 유지됨을 확인하였다. To increase the tolerance for passive fiber alignment, a single mode polymer waveguide with a large core structure is demonstrated. The large core waveguide is designed to have a mode profile comparable to that of a thermally expanded core (TEC) fiber, and it can be connected to a high-contrast waveguide through an adiabatic transition taper structure. From a waveguide with a rectangular core of 25 <TEX>${\times}$</TEX> 25 <TEX>${\mu}{\textrm}{m}$</TEX><TEX>$^2$</TEX>, a single mode propagation is observed when the index contrast is as low as 0.0005. A UV-cured injection molding method is used to fabricate the thick core structure. Due to the large mode size, the insertion loss of the device is below 0.5 dB until the lateral displacement of the TEC fiber is 4.5 <TEX>${\mu}{\textrm}{m}$</TEX>. The low insertion loss is important for reproducible passive alignment.

Fabrication of large-core single-mode polymer waveguide connecting to a thermally expanded core fiber for increased alignment tolerance

To increase the fiber alignment tolerance for the passive alignment with a high-contrast waveguide, a single-mode polymer waveguide with a large core structure is proposed. The large core waveguide is designed to have a mode profile comparable to that of a thermally expanded core (TEC) fiber, and it can be connected to a high-contrast waveguide through an adiabatic transition taper structure. From a waveguide with a rectangular core of 25×25 μm2, a single-mode propagation is observed when the index contrast is as low as 0.0005. A UV-cured injection molding method is used to fabricate the thick core structure. Due to the large mode size, the insertion loss of the device is below 0.5 dB until the lateral displacement of the TEC fiber is 4.5 μm, that is important for reproducible passive alignment.

Lens-free in-line optical isolators

We constructed a lens-free in-line optical isolator by embedding an isolator chip in a thermally expanded core (TEC) fiber without complicated optical alignment. The chip consists of two pairs of rutile wedges and garnet plates. We fabricated the TEC fibers by heating a single-mode fiber with a 200-mum outer diameter; low loss and a spot diameter as large as 49 mum were successfully obtained. The fabricated isolator has excellent optical properties, an insertion loss of 0.45 dB, and an isolation of more than 50 dB at a 1.55-mum wavelength, which confirms the usefulness of the integration techniques.