LCoS-based mode shaper for few-mode fiber

Design of a bidirectional TM01(TE01)-LP01 mode converter with a metasurface-on-fiber

Mode conversion is crucial for coupling a light source to a desired waveguide. While traditional mode converters such as fiber Bragg gratings and long-period fiber gratings exhibit high transmission and conversion efficiency, the mode conversion of two orthogonal polarizations remains challenging. Here, we present a bidirectional metasurface mode converter that can convert the transverse electric (TE)01 or transverse magnetic (TM)01 mode to the fundamental mode (LP01) with orthogonal polarization, and vice versa. The mode converter is located on a facet of a few-mode fiber and connected to a single mode fiber. Through simulations, we find that 99.9% of the TM01 or TE01 mode is converted into the x- or y-polarized LP01 mode, and that 99.96% of the x- or y-polarized LP01 mode is converted to the TM01 or TE01 mode. Furthermore, we expect a high transmission of over 84.5% for all mode conversions, up to 88.7% for TE01 to y-polarized LP01 conversion.

All-fiber LP01-LP11 ultra-broadband mode converters based on T-superimposed long period gratings in PCF

In order to cover the bandwidth of optical fiber communication, a LP₀₁-LP₁₁ ultra-broadband mode converter based on triple superimposed long period grating in PCF is proposed and demonstrated. The transmission spectra of the D-SLPG with gratings pitches and the T-SLPG were simulated and analyzed. The simulation results on the D-SLPG indicate that the 3 dB bandwidth of the D-SLPG is more than 1.5 times than the 3 dB bandwidth of the independent LPG and the 3 dB bandwidth of T-SLPG approaches 2.6 times as much as the independent LPG. In the experiment, the mode converter based on PCF-T-SLPG covers the wavelength of S + C + L with 3 dB bandwidth of 121 nm from 1498 nm to 1619 nm. In addition, the mode converter based on PCF-T-SLPG can accomplish ultra-broadband transmission in any wavelength by adjusting the period of gratings.

Mode converter with C+L band coverage based on the femtosecond laser inscribed long period fiber grating

The mode converter (MC) is one of the key components for mode division multiplexing (MDM). Here, we experimentally demonstrate an all-fiber long period grating (LPG) based MC inscribed in the few-mode fiber (FMF) with the line-by-line femtosecond laser irradiation technique. Experimental characterization results agree well with the theoretical calculations, and a clear mode evolution from the LP₀₁ mode to the LP₁₁ mode is observed with the tuning of the operation wavelength. An average mode conversion efficiency of more than 90% and an average insertion loss of less than 5 dB, together with a polarization-dependent loss of less than 3 dB, are achieved over the C+L band with a good repeatability. The proposed MC based on the LPG inscribed into the FMF has the advantages of mode scalability, compact size, and wideband operation, which is desired for the wavelength division multiplexing (WDM) and MDM hybrid transmission.

Polarization-maintaining fiber composed of an elliptical ring core and two circular air holes

In this paper, a new polarization-maintaining fiber (PMF) composed of an elliptical ring core and two circular air holes is exploited. The PMF could support 10 guided modes with refractive index differences $\Delta {n_{{\rm eff}}}$Δn_eff of all the adjacent modes larger than ${{10}^{ - 4}}$10^-4. Moreover, the optimum parameters of the PMF covering the entire $C + L$C+L band are obtained. Last, the chromatic dispersion $D$D of the designed elliptical ring core fiber, the confinement loss $\alpha $α, and the power fraction $\chi $χ of the two-air core are calculated. The calculation results show that $D$D, $\alpha $α, and $\chi $χ are compatible with traditional step-index fiber, and the designed elliptical ring core fiber is suitable for mode-division multiplexing and optical fiber sensors.

Wavefront superposition method for accurate and efficient mode conversion

A wavefront superposition (WS) method is proposed for accurate and efficient mode conversion in mode-division multiplexing transmission. The WS method converts an input beam to the WS state, which is composed of the conversion target and radiation modes of a few-mode fiber. The appropriate weighting for the modal component of the WS state enables more efficient conversion than the conventional method in which the output beam consists only of the conversion target. Further, since the components of the radiation modes in the output are eliminated by the mode-filtering property of the few-mode fiber, no modal crosstalk occurs in the WS method. We examine the conversion performance of the WS method by a numerical simulation for the mode-multiplexing numbers 3, 6, 10, and 15. The WS method shows a 2.4 dB higher efficiency than the conventional method, while maintaining an extremely low modal crosstalk (less than -80  dB), even when the number of multiplexed modes is 15.

Multi-channel mode converter based on a modal interferometer in a two-mode fiber

In this Letter, we propose a multi-channel mode converter with the concept of a modal interferometer in a two-mode fiber (TMF). Two lateral stress points in a TMF function as in-line fiber mode couplers to construct the modal interferometer, and both transmission spectra and near-field patterns confirm that the LP₀₁ mode is successfully converted into an LP₁₁ mode at the multiple channels. The measured mode conversion efficiency almost completely follows the theoretical tendency. Finally, the mode conversion is realized at 20 channels in the C+L wavelength band with conversion efficiency up to 99.5% and insertion loss as low as 0.6 dB. Furthermore, the channel spacing can be freely tailored by adjusting the distance between two stress points.

Ion-exchanged binary phase plates for mode multiplexing in graded-index optical fibers

Few-mode graded-index optical fibers are being increasingly relevant in spatial division multiplexing. Likewise, multi-region binary phase plates are fundamental elements for some mode multiplexing schemes in such optical fibers. In this work, we propose a coupling configuration for mode multiplexing based on collimating-focusing graded-index lenses together with binary phase plates, and calculate, in a fully analytical and quasi-analytical way, the theoretical conversion efficiencies and crosstalks between the fields generated by such plates and the Laguerre-Gaussian modes. These modes describe, directly or by a linear combination of them, the first optical modes of many graded-index optical fibers. The results obtained provide both the illumination conditions of the plates and their optical characteristics. The fabrication of the plates is made by using ion-exchange technology and their optical characterization by beam profilometry. The experimental conversion efficiencies are in agreement with the theoretical results.

Ultra-broadband mode converters based on length-apodized long-period waveguide gratings

We propose an ultra-broadband mode converter based on the structure of a length-apodized long-period grating, where π-phase shifts are introduced at strategic locations of the grating profile. Using a 3-section length-apodized grating structure, we design and fabricate an LP₀₁-LP_11a and an LP₀₁-LP_11b mode converter with a sidewall grating and a surface grating formed along a polymer channel waveguide, respectively. The fabricated LP₀₁-LP_11a and LP₀₁-LP_11b mode converters provide a conversion efficiency higher than 99% over a bandwidth of ~120 nm and ~150 nm, respectively, or a conversion efficiency higher than 90% over a bandwidth of ~180 nm and ~300 nm, respectively. The transmission characteristics of these devices are weakly sensitive to polarization and temperature variations. These mode converters can find applications in ultra-broadband mode-division-multiplexing transmission systems based on few-mode fibers and the design principle can be applied to general grating-based mode-coupling devices for a wide range of applications.

High-order mode conversion based on adiabatical mode evolution for mode division multiplexing applications

Mode conversion based on adiabatical mode evolution in a two-core configuration is investigated. The configuration can convert all the launching modes to higher-order modes from one port and convert all the launching modes to lower-order modes from another port. Mode conversion between the two degenerated high-order modes is also demonstrated numerically. The mode conversion feature is only dependent on the relationship between the effective mode indices of the two cores in the configuration, which shows the characteristics of high flexibility and large fabrication tolerance.

All-fiber spatial rotation manipulation for radially asymmetric modes

We propose and experimentally demonstrate spatial rotation manipulation for radially asymmetric modes based on two kinds of polarization maintaining few-mode fibers (PM-FMFs). Theoretical finding shows that due to successful suppression of both polarization and spatial mode coupling, the spatial rotation of radially asymmetric modes has an excellent linear relationship with the twist angle of PM-FMF. Both elliptical core and panda type FMFs are fabricated, in order to realize manageable spatial rotation of LP₁₁ mode within ±360° range. Finally, we characterize individual PM-FMF based spatial orientation rotator and present comprehensive performance comparison between two PM-FMFs in terms of insertion loss, temperature sensitivity, linear polarization maintenance, and mode scalability.

Interferometric space-mode multiplexing based on binary phase plates and refractive phase shifters

A Mach-Zehnder interferometer (MZI) that includes in an arm either a reflective image inverter or a Gouy phase shifter (RGPS) can (de)multiplex many types of modes of a few mode fiber without fundamental loss. The use of RGPSs in combination with binary phase plates for multiplexing purposes is studied for the first time, showing that the particular RGPS that shifts π the odd modes only multiplexes accurately low order modes. To overcome such a restriction, we present a new exact refractive image inverter, more compact and flexible than its reflective counterpart. Moreover, we show that these interferometers remove or reduce the crosstalk that the binary phase plates could introduce between the multiplexed modes. Finally, an experimental analysis of a MZI with both an approximated and an exact refractive image inverter is presented for the case of a bimodal multiplexing. Likewise, it is proven experimentally that a RGPS that shifts π/2 demultiplexes two odd modes which can not be achieved by any image inverter.

Advanced Spatial-Division Multiplexed Measurement Systems Propositions-From Telecommunication to Sensing Applications: A Review

The concepts of spatial-division multiplexing (SDM) technology were first proposed in the telecommunications industry as an indispensable solution to reduce the cost-per-bit of optical fiber transmission. Recently, such spatial channels and modes have been applied in optical sensing applications where the returned echo is analyzed for the collection of essential environmental information. The key advantages of implementing SDM techniques in optical measurement systems include the multi-parameter discriminative capability and accuracy improvement. In this paper, to help readers without a telecommunication background better understand how the SDM-based sensing systems can be incorporated, the crucial components of SDM techniques, such as laser beam shaping, mode generation and conversion, multimode or multicore elements using special fibers and multiplexers are introduced, along with the recent developments in SDM amplifiers, opto-electronic sources and detection units of sensing systems. The examples of SDM-based sensing systems not only include Brillouin optical time-domain reflectometry or Brillouin optical time-domain analysis (BOTDR/BOTDA) using few-mode fibers (FMF) and the multicore fiber (MCF) based integrated fiber Bragg grating (FBG) sensors, but also involve the widely used components with their whole information used in the full multimode constructions, such as the whispering gallery modes for fiber profiling and chemical species measurements, the screw/twisted modes for examining water quality, as well as the optical beam shaping to improve cantilever deflection measurements. Besides, the various applications of SDM sensors, the cost efficiency issue, as well as how these complex mode multiplexing techniques might improve the standard fiber-optic sensor approaches using single-mode fibers (SMF) and photonic crystal fibers (PCF) have also been summarized. Finally, we conclude with a prospective outlook for the opportunities and challenges of SDM technologies in optical sensing industry.

Generation of linearly polarized orbital angular momentum modes in a side-hole ring fiber with tunable topology numbers

A refractive index (RI) tunable functional materials infiltrated side-hole ring fiber (SHRF) is proposed to generate 10 LP OAM states with 6 topology numbers. On the basis of perturbation theory, the basis of the SHRF is demonstrated to be the LP modes. After a fixed propagation distance of 0.03 m, 0.009 m and 0.012 m, the phase difference between the odd and even LP₁₁x, LP₂₁x,y, LP₃₁x,y modes in the SHRF accumulate to ± π/2 respectively with na ranging from 1.412 to 1.44. Correspondingly, the output states are OAM _{± 1}x, OAM _{± 2}x,y, OAM _{± 3}x,y with a bandwidth of 380 nm, 100 nm and 80 nm respectively. The proposed fiber is easy to be fabricated with the mature fiber drawing technology and could facilitate the realization of all fiber based OAM system.

Mode converter based on the long-period fiber gratings written in the two-mode fiber

We demonstrate the fabrication of long-period fiber gratings (LPFGs) written in the two-mode fiber (TMF) by CO₂ laser. Both uniform and tilted LPFGs were fabricated to provide the light coupling between LP₀₁ mode and LP₁₁ mode with a coupling efficiency of more than 99%. The writing efficiency and the bandwidth of the LPFG mode converter can be adjusted by changing the tilt angle of the tilted TMF-LPFGs. The torsion sensitivity of conventional and tilted LPFG mode converters were measured to be 0.37 nm/(rad/m) and 0.50 nm/(rad/m), respectively. Two orthogonal vector modes (the HEeven 21and HEodd 21 modes) and corresponding orbital angular momentum state were successfully obtained at the resonance wavelength. The proposed LPFG mode converter could be used as not only a high efficiency wavelength tunable mode converter in the mode division multiplexing system but also a high sensitive torsion sensor in the field of optical sensing.

Adaptive mode control based on a fiber Bragg grating

We experimentally demonstrated adaptive control of linearly polarized (LP) modes in a two-mode fiber. Our method is based on a stepwise adaptive optics algorithm, with feedback determined by the relative magnitude of optical power reflected by a fiber Bragg grating and the transmitted power. Selective excitations of the LP01 and LP11 modes are experimentally shown.

Wavelength-selective switch with direct few mode fiber integration

The first realization of a wavelength-selective switch (WSS) with direct integration of few mode fibers (FMF) is fully described. The free-space optics FMF-WSS dynamically steers spectral information-bearing beams containing three spatial modes from an input port to one of nine output ports using a phase spatial light modulator. Sources of mode dependent losses (MDL) are identified, analytically analyzed and experimentally confirmed on account of different modal sensitivities to fiber coupling in imperfect imaging and at spectral channel edges due to mode clipping. These performance impacting effects can be reduced by adhering to provided design guidelines, which scale in support of higher spatial mode counts. The effect on data transmission of cascaded passband filtering and MDL build-up is experimentally investigated in detail.

Selective mode multiplexer based on phase plates and Mach-Zehnder interferometer with image inversion function

We propose a novel mode multiplexer based on phase plates followed by a Mach-Zehnder interferometer (MZI) with image inversion. After the higher-order modes are selectively converted from fundamental linear-polarized (LP) modes by the phase plates, the converted modes are coupled without fundamental loss using MZI with image inversion, in which the original spatial pattern and inverted pattern of the optical signal are interfered. Our scheme is also applicable to the coupling of degenerated LP modes such as LP(11a) and LP(11b). First, we numerically and experimentally evaluate the performance of the mode converter based on phase plates. The mode converter is suitable as long as the five LP modes such as LP(01), LP(11ab) and LP(21ab) are sustained in a few-mode fiber (FMF), although the crosstalk due to excitation of undesirable modes is unavoidable when the higher-order modes over LP(02) are sustained in FMF. Next, we develop and characterize the proposed mode multiplexers based on phase plates and MZIs with image inversion. The insertion loss is suppressed to around 3 dB for mode multiplexing of LP(11a) and LP(11b). Using a fabricated mode multiplexer for LP(31a) and LP(31b), we measure the bit-error rate performance of single-polarization mode-multiplexed quadrature-phase shift keying optical signals.

Dynamic operation of optical fibres beyond the single-mode regime facilitates the orientation of biological cells

The classical purpose of optical fibres is delivery of either optical power, as for welding, or temporal information, as for telecommunication. Maximum performance in both cases is provided by the use of single-mode optical fibres. However, transmitting spatial information, which necessitates higher-order modes, is difficult because their dispersion relation leads to dephasing and a deterioration of the intensity distribution with propagation distance. Here we consciously exploit the fundamental cause of the beam deterioration—the dispersion relation of the underlying vectorial electromagnetic modes—by their selective excitation using adaptive optics. This allows us to produce output beams of high modal purity, which are well defined in three dimensions. The output beam distribution is even robust against significant bending of the fibre. The utility of this approach is exemplified by the controlled rotational manipulation of live cells in a dual-beam fibre-optical trap integrated into a modular lab-on-chip system.

Transmitting spatial information through optical fibres is difficult because scalar high-order modes deteriorate. Here, the authors counter deterioration using adaptive optics to excite vectorial modes, achieving high-quality beams robust against fibre bending and use those to rotate cells in a laser trap.

Performance evaluation of selective mode conversion based on phase plates for a 10-mode fiber

We numerically and experimentally evaluate the performance of higher-order mode conversion based on phase plates for 10-mode fibers (10MFs). The phase plates have the phase jump of π between multiple planes, which match the phase patterns of linearly polarized (LP) modes of 10MF. First, we numerically investigate the effects of the fabrication errors such as the phase-difference error and the slope in the phase jump of the phase plate. The simulation results for the mode conversion to LP11 indicate that such errors make the spatial pattern of the converted beam asymmetric. In order to maintain the symmetric pattern, the phase-difference error is required to be less than ± 2%, and the ratio of the slope width to the input beam waist should be suppressed to be less than 0.05. Next, we calculate the coupling power efficiencies of the excitation of LP modes in 10MF when the converted beams after the phase plate are launched into 10MF using a lens. As the calculation results, highly accurate adjustment of the input beam waist is required to suppress the crosstalk due to coupling of undesirable LP modes by less than -20 dB. For mode excitation of LP11 or LP12, crosstalk of more than -20 dB is not avoidable even if the input beam waist is carefully adjusted. In contrast, the crosstalk for the mode excitation of LP21 or LP31 is easily suppressed to be less than -20 dB without careful adjustment of the input beam waist. These results suggest that phase plates are not applicable to mode conversion to LP11 and LP12 in 10MF while they are suitable for conversion to LP02, LP21 and LP31. Finally, we experimentally demonstrate conversion from LP01 to LP21 and LP31 modes in 10MF using phase plates. We obtain nearly ideal LP21 and LP31 modes with the small crosstalk due to the coupling of the other undesirable LP modes.

Adaptive control of waveguide modes using a directional coupler

Using adaptive optics (AO) and a directional coupler, we demonstrate adaptive control of linearly polarized (LP) modes in a two mode fiber. The AO feedback is provided by the coupling ratio of the directional coupler, and does not depend on the spatial profiles of optical field distributions. As a proof of concept demonstration, this work confirms the feasibility of using AO and all fiber devices to control the waveguide modes in a multimode network in a quasi-distributed manner.

Adaptive control of waveguide modes in a two-mode-fiber

We experimentally demonstrate an adaptive-optics-based approach that allows selective excitation of waveguide modes and their mixtures in a two-mode fiber (TMF). A phase-only spatial light modulator is used for wavefront control, using feedback signals provided by the correlation between the experimentally measured field distribution and the desired mode profiles. Experimental results show the optical field within the TMF can be shaped to be pure linearly polarized (LP) modes or their combinations. Analysis shows selective mode excitation can be achieved using only 5 × 5 independent phase blocks. With proper feedback signals, this method should enable one to precisely control the optical field within any multimode fiber or other types of waveguides in real time.

Design of add-drop multiplexer based on multi-core optical fibers for mode-division multiplexing

A multi-core fiber coupler is proposed to extract one of the modes in a few-mode optical fiber from a light beam, leaving the other modes undisturbed, and allowing a new signal to be retransmitted on that mode. Selective coupling of higher-order modes from a few-mode optical fiber can be realized by increasing the coupling length difference of the modes in the fiber using the multi-core configuration. Low cross-talk and wide bandwidth operation are realized owing to the fact that only one mode can be effectively coupled.

Rapid complex mode decomposition of vector beams by common path interferometry

We have used common path interferometry for rapid determination of the electric field and complex modal content of vector beams, which have spatially-varying polarization. We combine a reference beam with a signal beam prior to a polarization beam splitter for stable interferograms that preserve intermodal phase shifts even in noisy environments. Interferometric decomposition into optical modes (IDIOM) provides a direct, sensitive measure of the complete electric field, enabling rapid modal decomposition and is ideally suited to single-frequency laser sources. We apply the technique to beams exiting optical fibers that support up to 10 modes. We also use the technique to characterize the fibers by determining a scattering matrix that transforms an input superposition of modes into an output superposition. Furthermore, because interferograms are linear in the field, this technique is very sensitive and can accurately reconstruct beams with signal-to-noise << 1.