Mode Field Mismatch Research Articles

Monolithic Fiber Lasers Combining Active PCF With Bragg Gratings in Conventional Single-Mode Fibers

Novel monolithic fiber laser architectures utilizing large mode area (LMA) photonic crystal fiber (PCF) and fiber Bragg gratings (FBG) in conventional single-mode fibers (SMF) are presented. The main challenge is to address high cavity losses arising from the intrinsic 18-fold mode-field mismatch between the SMF and the active LMA PCF. Employing an all-fiber, robust and reproducible mode-field matching approach based on graded-index multimode fibers, we numerically and experimentally demonstrate that the SMF-to-LMA PCF coupling can be more than three-fold improved. This MFA approach is further implemented in monolithic fiber laser cavities combining FBGs in SMF and active LMA PCF. We demonstrate that cavity losses can be significantly mitigated when using appropriate MFAs resulting in a substantial increase of the laser output performances.

Gaussian-Like Mode Field Generated in a Seven-Core Photonic Crystal Fiber for Low Loss Splicing by Air Hole Collapse Technique

Multicore photonic crystal fiber is an important research direction to realize high-power supercontiuum. We report two post processing methods based on air hole collapse technique to generate Gaussian-like mode field in a seven-core photonic crystal fiber (SCPCF) for low loss splicing. By heating and collapsing all air holes in the meantime, the mode field of SCPCF becomes more concentrated and the valleys between separated core modes become much flatter. Another method to generate Gaussian-like mode field is selectively collapsing the central air holes between the seven cores, which can form an integrated larger core. Theoretical simulation results are confirmed by experiment and both methods are used to decrease mode field mismatch between SCPCF and double clad fiber with splicing losses as low as 0.46 and 0.22 dB, respectively.

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.

High temperature fiber sensor with high sensitivity based on core diameter mismatch

We report a simple fiber sensor for measurement of high temperature with high sensitivity. The sensing head is a multimode-single mode-multimode (MM-SM-MM) fiber configuration formed by splicing a section of uncoated single mode fiber (SMF) with two short sections of multimode fibers (MMF) whose core is composed of pure silica. Because of the mode-field mismatch at the splicing points of the SMF with 2 sections of MMFs, as well as index matching between the core of the MMF and the cladding of the SMF, optical power from the lead-in fiber can be partly coupled to the cladding modes of the SMF through the MMF. The cladding modes of the SMF then re-coupled to the lead-out fiber, in the same fashion. Due to the effective index difference between the core and cladding modes, an interference pattern in the transmission spectrum of the proposed device was obtained. The interference pattern was found to shift to the longer wavelength region with respect to temperature variation. The temperature sensor can measure temperature stably up to more than 900 degrees C with sensitivity of 0.088 nm/ degrees C.

Automation of Active Quadrangular Pyramid-Shaped Fiber Endface Polishing Method

A high-power laser diode (LD) has an elliptical mode field at the endface of its laser diodes. A mode field mismatch between the high-power LD and single-mode fibers (SMFs) results in low coupling efficiency. Accordingly, various methods for coupling between the LD with the higher aspect ratio and SMFs have been employed. Among these approaches, the quadrangular pyramid-shaped fiber endface (QPSFE), which has the capability to control both axial curvatures, can easily match the far field of the high-power LD. However, a QPSFE, which was fabricated by polishing techniques, is hard to employ because of the special shape involved and the need for high precision. This paper presents a novel method for automating QPSFE polishing based on a thorough system analysis and using an online image inspection system. The system successfully supported automatic polishing with an encouraged fiber tip offset that ensures high coupling efficiency. Such an automatic polishing system can also be applied to any other type of fiber endface.

Low-loss waveguides written with a femtosecond laser for flexible interconnection in a planar light-wave circuit

We describe a low-loss single-mode waveguide in planar light-wave circuit (PLC) glass doped with boron and phosphorus, which is more difficult to write than pure-silica glass. The written waveguide has a rectangular core, a symmetric near-field pattern, and a propagation loss of 0.35 dB/cm. The loss that originates from the mode-field mismatch between the mode-field diameters of the written and the PLC waveguides is less than 0.1 dB/point. In addition, we successfully connected PLC waveguides with a 500-microm-long waveguide written with a laser. The laser-written waveguide can flexibly connect PLC waveguides with a low coupling loss.