Large-core Multimode Fiber Research Articles

Q-switched temperature fiber sensing device based on graded index and D-shaped multimode fiber

In the paper, a Q-switched temperature sensing device is proposed and verified, which consists of a modulator based on a large core multimode fiber and a graded index multimode optical fiber (LMF-GIMF), and a sensing unit formed by sandwiching a D-shaped multimode fiber (MMF) between two single mode fibers (SMF). Utilizing the modulator to transform continuous light into pulsed laser, a stable Er3+-doped Q-switched laser with a repetition frequency of 23.04 kHz and a modulation depth of 7.47 % has been achieved. Under its pumping, the SMF-MMF-SMF sensing structure performed thermal detection with a sensitivity of −81.5 pm/℃ as temperature varies in the range of 25 °C to 65 °C. The temperature sensing device based on Q-switching can be used as an independent pulse laser to realize the switch between the light source and the sensing device. At the same time, the sensing device can provide real-time feedback according to the change of the central wavelength, which allows for immediate adjustments in the operational state, enhancing the flexibility and responsiveness. In addition, the characteristics of the sensor based on Q-switched laser can reduce thermal damage and extends the service life of the sensor. As far as we know, this is the first report on Q-switched temperature sensing based on the LMF-GIMF modulator. The proposed sensing device has the advantages of high signal-to-noise ratio, low operating threshold, minimal thermal damage, and serves as an independent Q-switched laser source, which can be applied to pulse laser measurement with high precision and time resolution in the future.

Large core multimode fiber with high tolerance to coupling misalignment and dust contamination in intra-vehicle networks

This paper introduces an optical power ring model for calculating the efficiency of fiber coupling. The model obtains more accurate results than the Gaussian model. This model compares the coupling efficiency of large core and standard multimode fiber and demonstrates that the tolerance of the large-core multimode fiber to the coupling misalignment and dust contamination is higher than that of the standard multimode fiber.

Displacement sensor based on a large-core hollow fiber and specklegram analysis

The research enterprise towards achieving high-performance hollow-core photonic crystal fibers has led to impressive advancements in the latest years. Indeed, using this family of fibers becomes nowadays an overarching strategy for building a multitude of optical systems ranging from beam delivery devices to optical sources and sensors. In most applications, an effective single-mode operation is desired and, as such, the fiber microstructure or the light launching setups are typically designed for achieving such a behavior. Alternatively, one can identify the use of large-core multimode hollow-core fibers as a promising avenue for the development of new photonic devices. Thus, in this manuscript, we propose and demonstrate the utilization of a large-core tubular-lattice fiber for accomplishing a speckle-based displacement sensor, which has been built up by inserting and suitably dislocating a single-mode fiber inside the void core of the hollow fiber. The work reported herein encompasses both simulation and experimental studies on the evolution of the multimode intensity distributions within the device as well as the demonstration of a displacement sensor with an estimated resolution of 0.7 μm. We understand that this investigation identifies a new opportunity for the employment of large-core hollow fibers within the sensing framework hence widening the gamut of applications of this family of fibers.

Designation of Pump-Signal Combiner with Negligible Beam Quality Degradation for a 15 kW Tandem-Pumping Fiber Amplifier

In this paper, the fabrication method of a pump/signal (6 + 1) × 1 combiner based on a large-core (48 μm) multimode signal fiber is introduced. Since the signal fiber is not tapered in the production, and an effective feedback alignment method is adopted during the splice process, the degradation ratio of the M2 value of the signal light is only about 5% after passing through the beam combiner. In addition, with the help of a home-made beam combiner, a counter-directional tandem-pumping amplifier is built. The maximum output power of the amplifier is 15.31 kW with the slope efficiency of 83.2%. The temperature rise coefficient of the home-made combiner is 3.2 °C/kW and the backward isolation degree is more than 36 dB from each pump pigtail. Both test results prove the outstanding potential of the pump-signal combiner in high-power laser applications.

Flexible chalcogenide glass large-core multimode fibers for hundred-watt-level mid-infrared 2-5 µm laser transmission

The rapidly-developed high-power mid-infrared 2-5 µm laser technology requires a compact, flexible low-loss glass fiber for power delivery or laser generation. With the broadest bandwidth of low-loss transmission window in mid-infrared region amongst all mid-infrared glass fibers, chalcogenide glass fiber is the best candidate covering the whole 2-5 µm range. Multi-hundred-watt high-power delivery for 5.4-µm CO laser was previously demonstrated in a multimode chalcogenide fiber with a 1-mm-diameter large core, at the cost of giving up one of the most desirable fiber advantages, the flexibility. Indeed, chalcogenide glass fibers with decent flexibility have never exhibited hundred-watt-level power transmitting capability in the 2-5 µm range. In this paper, we have experimentally demonstrated 100-watt-level power transmission in multimode As2S3 chalcogenide fibers, using a customized high-power 2-µm thulium doped silica fiber laser source. With effective forced cooling, the multimode As2S3 fiber with 200 µm core diameter can resist incident laser power of 120 W and deliver transmitted power of 63 W. Nano-sized scattering center related laser damage mechanism and the cylindrical heat transfer model have been proposed to explain the high-power damage process of chalcogenide glass fibers. The calculation is in good agreement with the experiments. It is promising to further enhance the transmitted power above 100 W in flexible chalcogenide glass large-core fibers.

Do-it-yourself three-dimensional large core multimode fiber splitters through a consumer-grade 3D printer

We report the design, fabrication and characterization of 3D large core (1 mm) multimode fiber splitters using a low-cost stereolithography 3D printer. The results were accomplished for symmetrical 1 × 2 and 1 × 4 splitters, where the angle between the output arms was varied from 10 to 180°, showing good uniformity between the splitting ratios. Asymmetrical 1 × 2 splitters were also studied to achieve different splitting ratios. This was done by fixing one arm at a specific angle, while varying the other. Results were quite satisfactory, paving the way for simple and customizable manufacturing of passive optical elements.

Large core multimode fiber with high bandwidth and high connector tolerance for broadband short distance communications

We propose a large core multimode fiber with 100-µm core diameter for short distance communication that is compatible with existing transceivers designed for 50-µm diameter core multimode fibers. The fiber exhibits a bandwidth that is over four times higher than the 50-µm OM4 fiber, low bending loss, and large connector offset tolerance. We have studied the bandwidth capability through both detailed modeling and experiments. A prototype fiber was fabricated, and it achieves 24 GHz km peak modal bandwidth in the 850 nm window, and the wavelength window with bandwidth above 5 GHz km is more than 100 nm. The bending losses of the fiber measured at 850 and 1300 nm with an encircled flux launch condition exceed the requirements for bending insensitive multimode fibers. The fiber has much higher connector offset tolerance compared to conventional 50-µm diameter core fibers as has been exhibited by modeling and experimental study. The additional loss caused by up to 10-µm connector offset is only 0.3 dB. Transmission experiments using 25 G SR and 100 G shortwave wavelength division multiplexing transceivers over such a fiber show error-free performance over 500 and 300 m, respectively. The proposed large core multimode fiber can address versatile needs in short distance communication including the bandwidth for high data rate transmission and a wide wavelength window for wavelength multiplexed transmission. The large connector tolerance and low bending loss can also enable low cost, compact connectivity solutions with passive alignment and ease of handling.

Effect of lateral error on the coupling efficiency and beam quality of gaussian beam launched into large-core fiber

We study the effect of lateral error on the coupling efficiency and beam quality of a Gaussian beam coupled into a large-core multimode fiber. The formula for the effect of lateral error on coupling efficiency is derived in detail and verified with both simulations and experiments. The effect of lateral error on beam quality is also analyzed with the use of ZEMAX and verified with our experimental setup. The results show that lateral error can change the path of the light coupled into the fiber to such a large extent that it influences the power distribution of the Gaussian beam.

Research on the influence of alignment error on coupling efficiency and beam quality for Gaussian beam to multimode fiber

Abstract The effect of alignment error on the coupling efficiency and beam quality of a Gaussian beam coupled into a large-core multimode fiber is studied in this paper. The equations for evaluating the effect of alignment error on the coupling efficiency are derived separately, and verified with the method of simulation. The calculation and simulation results obtained are highly consistent. In the same way, the effects of alignment error on the beam power distribution are discussed. The results show that the lateral error will change the path of partial light to a large extent, and have a greater influence on the power distribution of the Gaussian beam than the longitudinal error and angular error do.

Impact of modulation bandwidth on multiplexing using principal modes in MMF links.

Multimode fibers (MMFs) are widely used for short fiber links. However, the data rates through MMFs is limited owing to modal dispersion. The so-called "principal modes" (PMs) permit transmission and multiplexing through the MMFs without modal dispersion for small modulation bandwidths. For larger modulation bandwidths, however, they lose their dispersion-free nature. In this paper, we model the impact of modulation bandwidth and mode coupling strength on the performance of PMs. We develop a simulator that characterizes the dispersion and cross-talk of the PMs of few-mode and large-core graded-index MMFs with mode-dependent losses (MDL). Simulations reveal that for fibers without MDL, for modulation frequencies beyond 10 GHz diminishes the PMs' frequency response by more than 1 dB for 100 m in large-core MMF links and 10 km few-mode fiber links. With MDL, simulations reveal that for modulation bandwidths beyond 2 GHz diminishes the frequency response by 3 dB for a 1 km few-mode fiber and by more than 4 dB for a 1 km large-core multimode fiber. While multiplexing using PMs in large-core MMFs with MDL, we find that for modulation bandwidths beyond 3 GHz, the cross-talk is 20 dB in 1 km large-core MMF links, thereby limiting system performance.

An All-Fiber Fan-Out Device for Varying Twin-Core Fiber Types

An all-fiber twin-core fiber (TCF) fan-out device is proposed and experimentally demonstrated. The device is capable of simultaneously accessing optical signals from both cores for a variety of TCF types. The device was fabricated via bi-tapering of a fiber bundle consisting of two parallel large-core multimode fibers, each of which was tapered-spliced with single mode fibers. A commercial fiber fusion splicer was used in the device fabrication process for both tapering and splicing. A series of experiments were conducted in which two different TCF types, featuring different inter-core distances, were connected to test the functionality of the proposed device. The results demonstrate successful accessing of optical signals from the two TCF cores with a low crosstalk of –38.6 dB. The proposed all-fiber TCF fan-out device could be further developed for applications in novel fiber sensors or advanced optical communication systems.

Quasi-homogeneous partial coherent source modeling of multimode optical fiber output using the elementary source method

Multimode fibers (MMF) have many applications in illumination, spectroscopy, sensing and even in optical communication systems. In this work, we present a model for the MMF output field assuming the fiber end as a quasi-homogenous source. The fiber end is modeled by a group of partially coherent elementary sources, spatially shifted and uncorrelated with each other. The elementary source distribution is derived from the far field intensity measurement, while the weighting function of the sources is derived from the fiber end intensity measurement. The model is compared with practical measurements for fibers with different core/cladding diameters at different propagation distances and for different input excitations: laser, white light and LED. The obtained results show normalized root mean square error less than 8% in the intensity profile in most cases, even when the fiber end surface is not perfectly cleaved. Also, the comparison with the Gaussian–Schell model results shows a better agreement with the measurement. In addition, the complex degree of coherence, derived from the model results, is compared with the theoretical predictions of the modified Van Zernike equation showing very good agreement, which strongly supports the assumption that the large core MMF could be considered as a quasi-homogenous source.

Fiber cavities with integrated mode matching optics

In fiber based Fabry-Pérot Cavities (FFPCs), limited spatial mode matching between the cavity mode and input/output modes has been the main hindrance for many applications. We have demonstrated a versatile mode matching method for FFPCs. Our novel design employs an assembly of a graded-index and large core multimode fiber directly spliced to a single mode fiber. This all-fiber assembly transforms the propagating mode of the single mode fiber to match with the mode of a FFPC. As a result, we have measured a mode matching of 90% for a cavity length of ~400 μm. This is a significant improvement compared to conventional FFPCs coupled with just a single mode fiber, especially at long cavity lengths. Adjusting the parameters of the assembly, the fundamental cavity mode can be matched with the mode of almost any single mode fiber, making this approach highly versatile and integrable.

Lens-free all-fiber probe with an optimized output beam for optical coherence tomography.

A high-efficiency lensless all-fiber probe for optical coherence tomography (OCT) is presented. The probe is composed of a segment of large-core multimode fiber (MMF), a segment of tapered MMF, and a length of single-mode fiber (SMF). A controllable output beam can be designed by a simple adjustment of its probe structure parameters (PSPs), instead of the selection of fibers with different optical parameters. A side-view probe with a diameter of 340μm and a rigid length of 6.37mm was fabricated, which provides an effective imaging range of ∼0.6 mm with a full width at half-maximum beam diameter of less than 30μm. The insertion loss of the probe was measured to be 0.81dB, ensuring a high sensitivity of 102.25dB. Satisfactory images were obtained by the probe-based OCT system, demonstrating the feasibility of the probe for endoscopic OCT applications.

Power Flow in a Large-Core Multimode Fiber under External Perturbation and its Applications

Large core optical multimode fiber provides benefits such as a large light-coupling tolerance, easy handling, and delivery of higher light power without undesirable nonlinear effects. In this research, we exploit the effects of external perturbation on the power flow within the large core fiber and present two relevant applications, namely a perturbation sensor and a doughnut beam tuner. Since conventional multimode fiber power flow model does not take into consideration the perturbation effect, we modify the power flow model so that the influence of time varying perturbation can be theoretically analyzed. Based on our theory, we further conduct the numerical simulation and experiments on these two applications. For the fiber vibration sensor, the proposed numerical model shows that the sensor sensitivity depends on the intensity profile of the launched beam and also the higher-order harmonics that were not reported previously can become interferences to affect the signal. For the beam tuner application, we prove both theoretically and experimentally that the doughnut intensity profile at the fiber output can be tuned in real-time by applying external perturbations to the fiber. We expect that the results can be useful to further exploit the external perturbation on large core fiber in various applications.

Analysis and design of a high power laser interrupter for MEMS based safety and arming systems

A high power laser energy interrupter is investigated and developed. The micromachined laser interrupter is a critical component of the MEMS based safety and arming (S&A) system, which acts as a switch to selectively couple optical power between input and output fibers using direct coupling technique. The switching of high power laser energy is demonstrated by taking advantage of direct thermal actuation of cantilevered large-core multimode fibers. Models describing the combined electrothermal and thermomechanical characteristics of the thermal actuators are developed with temperature-dependent parameters. The temperature distribution of the actuator is investigated, and the influence of the actuator’s geometrical parameters on its force and displacement is analyzed. Actuator displacement versus drive current is found to be in good agreement with model predictions. Switching time is 19 ms from the safety to the arming state with a maximum operational frequency of 34 Hz. Optical efficiency is predicted and a series of tests are conducted to measure transmission efficiency of the laser interrupter. The optical efficiency is found to be about 92.7% with a maximum power transfer of 2780 mW from a 3000 mW input, and the channel isolation is 57 dB.

An All-Fiber-Optic Combined System of Noncontact Photoacoustic Tomography and Optical Coherence Tomography.

We propose an all-fiber-based dual-modal imaging system that combines noncontact photoacoustic tomography (PAT) and optical coherence tomography (OCT). The PAT remotely measures photoacoustic (PA) signals with a 1550-nm laser on the surface of a sample by utilizing a fiber interferometer as an ultrasound detector. The fiber-based OCT, employing a swept-source laser centered at 1310 nm, shares the sample arm of the PAT system. The fiber-optic probe for the combined system was homemade with a lensed single-mode fiber (SMF) and a large-core multimode fiber (MMF). The compact and robust common probe is capable of obtaining both the PA and the OCT signals at the same position without any physical contact. Additionally, the MMF of the probe delivers the short pulses of a Nd:YAG laser to efficiently excite the PA signals. We experimentally demonstrate the feasibility of the proposed dual-modal system with a phantom made of a fishing line and a black polyethylene terephthalate fiber in a tissue mimicking solution. The all-fiber-optic system, capable of providing complementary information about absorption and scattering, has a promising potential in minimally invasive and endoscopic imaging.

Tapered structure based all-fiber probe for endoscopic optical coherence tomography

A tapered structure based all-fiber endoscopic probe for endoscopic optical coherence tomography (OCT) is presented in this paper. The designation and fabrication of a miniaturized high-performance probe are critical in endoscopic OCT. Compared to the conventional fiber-lens structure based endoscopic probe, the all-fiber probe has a prominent edge in size and flexibility. Due to its lower beam-divergence, the large core multi-mode fiber makes a better fit than a general single mode fiber does when utilized to replace the micro lens in a conventional endoscopic probe as the imaging component. Furthermore, a tapered fiber is introduced as a transition section between the single mode fiber and the large core multi-mode fiber in order to enhance the light transmission efficiency and reduce the rigid length of the probe simultaneously. First, in order to obtain an optimal performance, optical simulation software(Rsoft) is adopted to determine the probe's proper lengths of the tapered section and the large core multi-mode fiber. Second, the all-fiber structure based endoscopic probe is fabricated by means of large core multi-mode fiber tapering, cutting and fusing processes. The beam characterization and insertion loss of the fabricated probe are measured experimentally The probe itself is 250 m, and after covering with a stainless steel protective tube, its outer diameter becomes 325 m. The rigid length of the probe is about 1 cm, which is more flexible and easier for inserting into curved blood vessels. The insertion loss of the probe is measured to be about 0.3 dB. To the best of our knowledge, it is the lowest among all of the all-fiber endoscopic probes. Finally, the probe is integrated with a custom-built swept-source optical coherence tomography system. Imaging of human fingertip and ex-vivo chicken trachea is conducted to demonstrate the key performance parameters of our probe. The effective imaging range of the probe is up to 800 microns in air without the help of any extra mechanism to expand its depth of focus. The probe offers a compact, efficient and flexible candidate for endoscopic optical coherence tomography, which is promising in cardiovascular investigations.

Corrugated Fiber Grating for Detection of Lead Ions in Water

Optical fiber fluorescence sensors are widely used for chemical sensing. Intrinsic fluorescence sensors such as evanescent-wave sensors can be used for surface sensing while extrinsic sensors are used for bulk sensing. In intrinsic/extrinsic fluorescence sensors, typically the side-wall/end-face of a single/multimode fiber is used. In this paper, we introduce a novel intrinsic optical fiber fluorescence sensor which uses the side-wall of a large-core multimode fiber to collect the fluorescence from the bulk of samples. The method is to use a corrugated fiber grating, fabricated by a CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser, to couple radiating modes to the core modes. Using the sidewall of a fiber helps to fabricate a multitarget or a distributed sensor. We also report the sensing of Lead ions with concentration as low as few ppb in water using a fluorescence turn-on polymer as a chemical probe.

Sensitivity improvement of optical-fiber temperature sensor with solid cladding material based on multimode interference

We have developed a simple, high-sensitivity optical-fiber temperature sensor based on multimode interference (MMI). The fabricated MMI structure comprises three segmented fibers: a single-mode fiber (SMF); a large-core multimode fiber (MMF), whose outer surface is coated with a temperature-sensitive material; and another SMF. Fluoroacrylate and silicone rubber are tested as temperature-sensitive cladding materials. The silicone rubber coating exhibits a large shift in interference wavelength with temperature, producing a very fine temperature resolution as low as 0.01 °C.