Thermally Expanded Core Fiber Research Articles

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.

Low loss and high performance interconnection between standard single-mode fiber and antiresonant hollow-core fiber

We demonstrate halving the record-low loss of interconnection between a nested antiresonant nodeless type hollow-core fiber (NANF) and standard single-mode fiber (SMF). The achieved interconnection loss of 0.15 dB is only 0.07 dB above the theoretically-expected minimum loss. We also optimized the interconnection in terms of unwanted cross-coupling into the higher-order modes of the NANF. We achieved cross-coupling as low as −35 dB into the LP_{11} mode (the lowest-loss higher-order mode and thus the most important to eliminate). With the help of simulations, we show that the measured LP_{11} mode coupling is most likely limited by the slightly imperfect symmetry of the manufactured NANF. The coupling cross-talk into the highly-lossy LP_{02} mode (>2000 dB/km in our fiber) was measured to be below −22 dB. Furthermore, we show experimentally that the anti-reflective coating applied to the interconnect interface reduces the insertion loss by 0.15 dB while simultaneously reducing the back-reflection below −40 dB over a 60 nm bandwidth. Finally, we also demonstrated an alternative mode-field adapter to adapt the mode-field size between SMF and NANF, based on thermally-expanded core fibers. This approach enabled us to achieve an interconnection loss of 0.21 dB and cross-coupling of −35 dB into the LP_{11} mode.

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.

All-optical signal gating using ytterbium-doped fiber and a pair of thermally expanded core fibers

We report all-optical signal gating using ytterbium-doped fiber (YDF) and a pair of thermally expanded core fibers (TECFs). Splicing the YDF with two TECFs at their expanded-core ends, we obtained an interference pattern in the transmission spectrum of the device. When pumped by a laser diode at 980 nm, the interference fringes shift to longer wavelength due mainly to the resonantly enhanced nonlinearity of the YDF. Consequently, signals within the interference pattern can be gated or blocked all optically. Full gating of cw signals at 1564.21 and 1564.54 nm were obtained using a pump power of about 38.44 mW.

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.

Characteristics of thermally expanded core fiber

Thermally expanded core (TEC) fiber is expected to reduce fiber-to-fiber and fiber-to-laser diode connection loss. This paper describes the characteristics of TEC fiber theoretically and experimentally. We reveal theoretically that when fabricating TEC fiber the mode field diameter (MFD) is enlarged more effectively by increasing the heating temperature rather than the heating time. In the 1300-1600/spl deg/C temperature range with heating times between 0 and 60 min, it is necessary to control the temperature accurately so that no deviation from the target temperature is more than /spl plusmn/30/spl deg/C. This is in order to ensure that any connection loss caused by MFD mismatch is less than 0.1 dB. We show experimentally that the propagation loss of TEC fiber is dependent on the heating region and wavelength by using a micro burner with a propane/oxygen flame. Based on the relationship between the loss characteristics and the expanded MFD, we suggest a method for nondestructively measuring the MFD in TEC fibers.

Polarization-independent in-line optical isolator with lens-free configuration

A fiber-embedded polarization-independent optical isolator that consists of an isolator chip, and thermally expanded core (TEC) fibers, and that has no lens elements was developed. The chip was made of birefringent crystal plates, rutile polarization-splitters, a quartz half-wave plate, and a Faraday rotation garnet, with the chip embedded into a TEC fiber. By utilizing the proper orientation flat formed on the edge of each plate, alignment of these plates was simplified. Measured backward and forward losses at a wavelength of 1.55 mu m are >40 dB and approximately 2.5 dB, respectively, which agree with predicted values. It is also shown that the characteristics would be dramatically improved by utilizing new polarization splitters instead of rutile plates. Experimental results support the promise of constructing fiber-integrated isolators which then enable the mass-production of inline isolators. >