A singlemode-multimode-singlemode (SMS) fiber structure consists of a short section of multimode fiber fusion- spliced between two SMS fibers. The mechanism underpinning the operation of an SMS fiber structure is multimode interference and associated self-imaging. SMS structures can be used in a variety of optical fiber systems but are most commonly used as sensors for a variety of parameters, ranging from macro-world measurands such as temperature, strain, vibration, flow rate, RI and humidity to the micro-world with measurands such as proteins, pathogens, DNA and specific molecules. While traditional SMS structures employ a short section of standard multimode fiber, a large number of structures have been investigated and demonstrated over the last decade involving the replacement of the multimode fiber section with alternatives such as a hollow core fiber or a tapered fiber. The objective of replacing the multimode fiber has most often been to allow sensing of different measurands or to improve sensitivity. In this paper, several different categories of SMS fiber structures, including traditional SMS, modified SMS and tapered SMS fiber structures are discussed with some theoretical underpinning and reviews of a wide variety of sensing examples and recent advances. The paper then summarizes and compares the performances of a variety of sensors which have been published under a number of headings. The paper concludes by considering the challenges faced by SMS based sensing schemes in terms of their deployment in real world applications and discusses possible future developments of SMS fiber sensors.

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Singlemode-Multimode-Singlemode Fiber Structures for Sensing Applications—A Review

We experimentally demonstrate for the first time, to the best of our knowledge, an all-fiber passively mode-locked laser operation based on the nonlinear multimode interference of step-index multimode fiber. Such a structure couples the light in and out of the multimode fiber via single-mode fibers, and its physical mechanisms for saturable absorption have been analyzed theoretically based on the third-order nonlinear Kerr effect of multimode fiber. Using the nonlinear multimode interference structure with 48.8 mm length step-index multimode fiber, the modulation depth has been measured to be ∼5%. The passively mode-locked laser output pulses have a central wavelength of 1596.66 nm, bandwidth of 2.18 nm, pulsewidth of ∼625  fs, and fundamental repetition rate of 8.726 MHz. Furthermore, the influence of total cavity dispersion on the optical spectrum, pulse width, and output power is investigated systematically by adding different lengths of single-mode fiber and dispersion compensation fiber in the laser cavity.

All-fiber passively mode-locked laser using nonlinear multimode interference of step-index multimode fiber

A label-free fiber optic biosensor for Salmonella Typhimurium detection

A U-shaped double-side polished plastic optical fiber (POF) is demonstrated as a liquid refractive index (RI) sensor. The refractive index of glycerinum solutions is identified by the intensity detection on the bending and evanescent wave loss change. Heat treatment and mechanical polishing are adopted to form the symmetrical side-polished POF probe. The processing parameters are experimentally optimized on the power transmittance. The sensitivity of 1541%/RIU (Refractive Index Unit) can be obtained with a resolution of 5.35 × 10−4 in the scope of 1.33–1.39. The favorable temperature characteristic is proved to offer stable RI sensing from 20 to 50 °C. This simple POF sensor has potentials in low-cost visible light intensity RI detection.

https://www.mdpi.com/1424-8220/20/18/5253/pdf

Refractive Index Sensor Based on Double Side-Polished U-Shaped Plastic Optical Fiber.

Supercontinuum (SC) generation directly from a random fiber laser (RFL) structure is limited in spectrum span and output power so far. Investigations on wavelength range improvement of SC generated in RFL are analyzed and discussed. The experimental results show that cascaded four wave mixing (FWM) and passive modulation of pump light can explain the appearance of visible components and pulse performance in time domain respectively. To the best of our knowledge, it is the first time a SC covering visible and near-infrared range with 20-dB bandwidth of more than 660 nm is generated directly from a RFL with average output power of 3.4 W, and the spectrum spanning from 600 nm to 1700nm. This work proves that a RFL can be a novel visible to near-infrared SC generation method which has a great potential in various applications.

Random fiber laser directly generates visible to near-infrared supercontinuum.

We analyze the transmission of a Single mode - Multimode -Multimode (SMm) fiber structure with the aim of exciting a single radial mode in the second multimode fiber. We show that by appropriate choice of the length of the central multimode fiber one can obtain > 90% of the total core power in a chosen mode. We also discuss methods of removing undesirable cladding and radiation modes and estimate tolerances for practical applications.

Single LP(0,n) mode excitation in multimode fibers.

We present a single-mode–multimode–multimode (SMm) fiber structure with a few interesting device applications. Unlike the single-mode–multimode–single-mode (SMS) structure, SMm has the unique feature of more than one mode in the output fiber. A detailed physical understanding of the transmission properties, and the differences from the SMS structure, is presented. The device can be used to excite selected modes in the output multimode fiber (MMF). This can be used to reduce the number of modes in high-speed MMF applications using an all-fiber structure instead of bulk optics. In yet another possible application, we show a way of designing sensitive refractive index sensors for measurement in different RI ranges.

Multimode interference devices with single-mode–multimode–multimode fiber structure

Single-mode–multimode–multimode (SMm) fiber structure provides an all-fiber solution for exciting a single radial mode in a multimode fiber (MMF). The proposed SMm design essentially exploits the interfering modes of the first MMF for exciting a single mode in the second. In this paper, the sensitivity of single-mode quality to small variations in the fiber parameters and geometrical misalignments is studied in detail. Different methods are suggested to mitigate the mode quality degradation caused by errors in the fiber parameters. An estimate of the acceptable geometrical misalignments in the structure is also given for achieving a high-quality single mode from the device.

Single-mode-multimode-multimode device: sensitivity of the single mode to the fiber parameters and geometrical misalignments

We demonstrate an efficient ultra-broadband supercontinuum generation in telecom-grade optical fiber by pumping Q-switched subnanosecond laser pulses (0.77 ns) at 1064 nm in the normal dispersion region of the fiber. The fiber supports several spatial modes at this pump wavelength. Multiple sidebands with six Raman stokes spanning over more than 1100 nm (starting well below 600 nm and extending beyond 1700 nm) are generated using very low input pump power (average 57 mW). Theoretical analysis shows that cascaded Raman and cascaded intermodal four-wave mixing processes can account for the generation of multiple sidebands. We also demonstrate that the modal composition of the input pump profile can provide an extra degree of freedom in tailoring the multiple Raman peaks in the output spectrum.

Cascaded Raman and Intermodal Four-Wave Mixing in Conventional Non-Zero Dispersion-Shifted Fiber for Versatile Ultra-Broadband Continuum Generation

We show an all-fiber multimode interference method for achieving pre-determined free-space propagation of multimode optical beams exiting from the fiber end-facet. This method provides independent control over the relative amplitudes and the relative phase difference between the LP0,1 and LP0,2 modes of the output few-mode fiber, ensuring predictable free-space propagation behavior. We also show that the LP0,1 beam suffers greater unpredictability in the free-space propagation as compared to the LP0,2 beam if the phase difference between the two modes is not controlled. The negative effect of mode-mixing can be avoided if both the modes are in-phase at the fiber end-facet. We also demonstrate significant features in the free-space propagation behavior for different optical beams, justifying the theory.

Nitin Bhatia

Papers

Single LP(0,n) mode excitation in multimode fibers.

Multimode interference devices with single-mode–multimode–multimode fiber structure

Single-mode-multimode-multimode device: sensitivity of the single mode to the fiber parameters and geometrical misalignments

Cascaded Raman and Intermodal Four-Wave Mixing in Conventional Non-Zero Dispersion-Shifted Fiber for Versatile Ultra-Broadband Continuum Generation

Design of an all-fiber multimode-interference-based multimode optical beam generator: theory and experiment