Modular construction kit for fiber coupling and lens attach

The full time-jitter response of a single-photon detector can make a significant contribution to the quantum bit error rate (QBER) of high repetition rate quantum key distribution (QKD) implementations. Although there have been studies into understanding the contribution for single-mode optical fiber coupled single-photon detectors, the contribution of larger, multimode core diameters to the QBER have not been explored in detail. With the growing importance of free-space QKD, which typically use multimode fibers to reduce coupling loss, it is vitally important to understand how the multimode fiber coupling will impact the total QBER. This work studies the impact of the time-jitter contribution to QBER when coupling a commercial off-the-shelf silicon single-photon avalanche diode with various multimode fibers while simulating operating at 1 GHz with empirical measurements taken at 1 MHz repetition rate. It was found that step-index multimode fibers can significantly increase the QBER, while graded-index fibers can provide an QBER contribution similar to a single-mode fiber. The results highlight that there is a significant benefit in using graded index multimode fibers for a free-space QKD receiver, particularly for high repetition rate applications.

We investigate the mode propagation and coupling in multimode fibers, based on modeling the wave propagation through the coupled power equation. Using this model, the power evolution of the various propagating modes in fibers can be visualized and the impact of mode-dependent loss and mode coupling can be quantitatively analyzed.

For several decades silica-based optical fibres have been used for telecommunication and sensor purposes. The single-mode fibre is frequently employed in long-distance networks, whereas the multi-mode fibre is the preferred means of signal transport in campus and in-building networks. Because of the huge bandwidth of optical fibres in comparison to its electrical wireless and copper-based counterparts, the demand for optical fibres keeps increasing. In a competitive market, fibre manufacturers aim to produce ever better fibres that are as cheap and easy to employ as possible. As fibre research, development and manufacturing is a mature discipline, improvements in fibre design can only be achieved through the construction of robust, accurate and efficient numerical fibre models for the computation of those quantities that determine the behaviour of the fibre. We have developed a modular software code, based on Maxwell’s equations, to compute these quantities in a vectorial full-wave way for both single-mode and multi-mode optical fibres. Key is the refractive-index profile, or, more specifically, the dopant profile, as it defines the propagation, splicing and bending-loss characteristics of the fibre. For the single-mode fibre, the fibre quantities that we have concentrated on are dispersion, dispersion slope, mode-field diameter, effective area, bending loss, effective and theoretical cut-off wavelength and MAC-value. We highlight one fibre quantity in particular, viz. the computation of the bending loss in a single-mode fibre. Many approximate modelling techniques have been developed to estimate this loss in a fast way. Our numerical scheme, however, is the first rigorous one, as we have performed a vectorial full-wave analysis of the bent optical fibre. In this context, triple integrals involving products of Bessel functions with large, complex order and argument appear. Due to cancellations in the pertaining computation, a high relative accuracy is needed for the computation of each product. As a result, it takes weeks on a contemporary computer to compute the bending loss as a function of the radius of curvature. We have used the vectorial full-wave bending-loss results to determine the most appropriate approximate method. Subsequently, we have extended that approximate method to compute the bending losses of higher-order modes, since the required effective cut-off wavelength depends on the bending loss of the first higher-order mode. The selected approximate method has been used in the ensuing bending-loss calculations. Since the fibre properties are often conflicting, it is a challenging task to adapt the radial dopant profile to meet a set of predefined design goals. A design goal is a combination of desired values for (some of) the aforementioned fibre quantities, and can mathematically be translated into a cost function. The minimisation of this cost function provides us with the optimal dopant profile for that specific set. For the single-mode fibre, we have performed this minimisation for piecewise-linear profiles, by employing various global and gradient-based local optimisation strategies to speed up the design step considerably. Frequently,these optimisation strategies lead to counter-intuitive dopant profile designs that could not have been contrived otherwise. We have selected a deliberate mix of several optimisation routines and have compared their performances. Perhaps the most important conclusion is that there still appears to be room for improvement in the design of the radial dopant profile of commercially available fibres. For the multi-mode fibre, vectorial full-wave optimisation is not feasible yet because of the long computation times for the large number of propagating modes. Still, our numerical scheme allows for a manual fine-tuning of the popular power-law profile to minimise differential mode delay. Further, we have included mode coupling and differential mode attenuation in our model to obtain intensity patterns that match closely with measurements. We have also analysed the influence of profile variations, e.g. on-axis dips and kinks, on the intensity pattern. A selective excitation of different mode groups in a multi-mode fibre, offers the possibility to create several independent transmission channels, and thus a higher information capacity. Recently, the feasibility of this so-called mode group diversity multiplexing technique has been demonstrated. Simulations provide us with a means to better understand its operation and possibly increase its efficiency. The channel separation may be enhanced by employing a lens between the fibre and the detector, which is called mode-selective spatial filtering. Our numerical simulations of a mode group diversity multiplexing link, with and without mode-selective spatial filtering, are in agreement with the measurements. The above discussion makes clear that the developed software code has a wide range of applicability. Moreover, it is built in a modular way and thus extensions, like the inclusion of more fibre quantities or different profile dopants, are straightforward.

Multimode and multicore optical fibers are pivotal for spatial division multiplexing, a key technology for future high-capacity optical communication systems. A critical transmission characteristic of these fibers is modal dispersion, which is made random by unpredictable perturbations and manufacturing imperfections. While the statistical properties of modal dispersion are well-explored, the underlying local phenomenon of mode coupling—its primary cause—has received comparatively less attention. Accurate modeling of mode coupling is essential for a deeper understanding of both the linear and nonlinear propagation dynamics in these fibers and for advancing their practical implementation. This paper provides a comprehensive review of mode coupling in multimode and multicore fibers, highlighting aspects of general validity and conducting an in-depth analysis of bending and twisting—the two most common perturbations affecting deployed fibers. The results reveal significant similarities between the mode coupling behaviors of multimode and multicore fibers, along with some remarkable differences.

There is a trend toward increasing use of single mode transmission, particularly in telecommunications where high data bit rates are transmitted for long distances. Inter-connections of multimode fibers can be made in a number of ways, using ferrules, v-grooves, elastomeric splices, etc. However, the connection of single mode fibers, which have core diameters of 4 to 13 μm, requires more precise alignment than do the multimode fibers having core diameters of 50 μm or more. At TRW, we have adapted the four rod alignment guide concept for single mode fiber inter-connections. The principle of this OPTAGUIDE* alignment guide is presented. The single mode connectors and splices use the four rod scheme with an index matching material to eliminate or reduce the losses incurred through fiber end roughness or angularity. We are able to produce demountable connectors for 80/4.4 pm fibers having typical insertion losses of 1.0dB. The main factors in obtaining this result are the naturally precise fiber alignment provided by the alignment guide, and the ability of several manufacturers to maintain tight diametral and core offset tolerances. The single mode OPTALIGN* SM Connectors have been subjected to performance and environmental tests including repeated matings, temperature cycle and vibration. The results of these tests are described in this paper. A feature of the OPTALIGN* SM Connectors is the relative ease and speed of attachment to fiber optic cable in the field, without the use of epoxy or polishing procedures. The alignment guide concept has also been applied to permanent single mode splices. The splicing procedure is simple to perform in the field without expensive or delicate equipment. Construction and assembly procedures of the demountable connectors and permanent splices will be described with the aid of diagrams and photographs.

We present a compact and lightweight 1.5 μm lidar using a free-running single-photon detector (SPD) based on a multi-mode fiber (MMF) coupling InGaAs/InP negative feedback avalanche diode. The ultimate light detection sensitivity of SPD highly reduces the power requirement of the laser, whilst the enhanced collection efficiency due to MMF coupling significantly reduces the volume and weight of telescopes. We develop a specific algorithm for the corrections of errors caused by the SPD and erbium-doped fiber amplifier to extract accurate backscattering signals. We also perform a comparison between single-mode fiber (SMF) coupling and MMF coupling in the lidar receiver, and the results show that the collection efficiency with MMF coupling is five times higher than that with SMF coupling. In order to validate the functionality, we use the lidar system for the application of cloud detection. The lidar system exhibits the ability to detect both the cloud base height and the thickness of multi-layer clouds to an altitude of 12 km with a temporal resolution of 1 s and a spatial resolution of 15 m. Due to the advantages of compactness and lightweight, our lidar system can be installed on unmanned aerial vehicles for wide applications in practice.

We theoretically investigate a twist-related speckle variation in a graded-index multimode fiber. The simulation results show that by increasing the offset splicing distance between a single-mode fiber and a multimode fiber, the speckle pattern tends to be more chaotic. Also, the fiber length is a crucial parameter that affects speckle distribution, and the speckle pattern varies periodically along the torsional-less multimode fiber. By exerting twist on a multimode fiber, the speckle pattern revolves. The rotational angle ascends linearly with a twist angle and this tendency is not affected by an offset distance and a fiber length. The theoretic results disclose a twist-related coupling in the graded-index multimode fiber, provide a novel twist sensing method and show potential application in a high-power laser and an imaging field.

This paper reports, what is believed to be the first statistical analysis of restricted launch schemes. This is achieved by a detailed theoretical study of a large range of perturbed multimode optical fibers (MMF's) that are believed to be representative of those currently in use. It is shown that radial "offset" launches allow a robust and reliable solution to bandwidth collapse that occurs when standard laser sources are used with multimode fibers. Using such an approach, it is shown that gigabit per second link lengths using the offset launch technique can be increased to 550 m at an operating wavelength of 1300 nm using 62.5-/spl mu/m diameter MMF. These link lengths are greater than those achievable with conventional techniques. The approach is also used to indicate the tolerance on alignment precision required for successful operation and to suggest a simple measurement for the assessment of link performance. Finally, the use of angled launching into MMF is shown to allow enhanced operating bandwidths.

Волоконно-оптичні системи є одним з найбільш перспективних напрямків при розробці фізичної основи інформаційного простору. При реалізації оптоволоконних ліній зв'язку часто виникає необхідність узгодження оптоелектронних компонентів з різними поперечними перетинами випромінюючих і приймаючих областей і різними числовими апертурами. Варіантом такого завдання може бути з'єднання одномодового і багатомодового волокон або випромінювачів і фотоприймачів з волокнами різних типів. Одним з методів такого узгодження є створення каскадних волокон, в яких лінзове волокно і кілька ділянок градієнтних волокон інтегровано в одномодове волокно. Предметом дослідження є технології елементів волоконно-оптичних систем передачі. Метою даної роботи є створення компонентів волоконно-оптичних ліній зв’язку, що дозволяють підвищувати ефективність вводу випромінювання від джерела в оптичних світловод, зокрема одномодовий. Для досягнення поставленої мети необхідно вирішення наступних завдань: розглянути фактори, що впливають на втрати енергії при генерації і вводу випромінювання джерела в оптичне волокно, а також розробити і впровадити систему узгоджуючих елементів і дати послідовність дій для ефективного вводу випромінювання джерела в одномодове волокно. Висновки. Запропонована технічна і технологічна реалізація процесу робить можливим вводити більшу потужність в оптоелектронні лінії передачі інформації, що підвищує ефективність вводу випромінювання до 20 дБ. Визначення точності розташування сполучної площини в пристроях узгодження та взаємного розміщення випромінювачів та оптичного волокна дає можливість знизити втрати в з'єднанні до 0,1 дБ, передавати інформацію з більшою надійністю на більші відстані без регенерації і зберегти працездатність волоконно-оптичної системи в умовах впливу механічних, кліматичних і радіаційних факторів

In the present work, a compact all-fiber plasmonic focusing beam generator with single and multiple spots is proposed and demonstrated in a conventional multimode fiber. Here, the focusing beam generator is composed of air slit arrays perforated through the gold films deposited on the end facet of a multimode fiber. The array of nanoscale slits with varying widths is used to modulate phase distribution of the focused light. An all-fiber focusing beam generator provides many advantages, such as self-alignment, high flexibility, lower insert loss, and easy portability, which is of importance to realize optical trapping, micromanipulation, beam shaping, and fiber integrated devices.

We introduce a new drift model for slow environmental perturbation affecting modal coupling in optical multi-mode fibers. This model preserves the mode coupling strength asymmetries characteristic to mode pairs of different mode groups while decorrelating the fiber transmission matrix. Existing drift models were derived for the strong coupling regime, in which case the coupling matrix elements are identically distributed, and not suitable for the weak to intermediate coupling regime as shown here. The models' impact on the inherent crosstalk characteristic of a fiber were evaluated for all linear coupling regimes and for fibers with up to 42 spatial and polarization modes. Moreover, transmission performance of 32 GBd 16-QAM (per polarization mode) over the dynamic channel is studied considering singular value decomposition (SVD) pre-coding for the multiple-input multiple-output multi-mode fiber channel. The impact of slow drift on channel equalization performance is evaluated in terms of residual crosstalk. A large discrepancy is observed for fiber channels in the weak and intermediate coupling regimes, while converging in the strong coupling regime. Furthermore, we show that 2 × 1 multiple-input single-output equalizers can be sufficient to compensate for the residual crosstalk in the weak to intermediate linear coupling regime and achieve optimal performance.

Typical high power broad-area semiconductor lasers exhibit a highly astigmatic beam profile. However, many applications require a homogenous and circular symmetric beam. Thus coupling into circular multimode optical fibers is often employed. The strip-like astigmatic output of the diode laser underfills the circular multimode fiber, thus a decrease in beam quality occurs after fiber coupling due to mode mixing inside the optical fiber. This Letter presents a 3D integrated optics approach to shape the output of a broad-area laser diode. Ultrafast laser inscription is utilized to create a pair of photonic lanterns connected back to back inside a glass chip that captures and shapes the output of a commercial 976nm wavelength broad-area laser diode with 95μm emitter width. Compared to coupling to a 105μm diameter, 0.15 numerical aperture step-index multimode fiber, the photonic chip-based approach results in a 13× higher beam quality and 7× greater brightness.

Mode coupling in multimode step-index plastic-clad silica fibers with corrugated surfaces

Mode coupling, caused by random variations of core index or random irregularities of the fiber wall, influences the transmission characteristics of multimode optical fibers in a complicated way. The effects of mode coupling in multimode optical fibers have been reported using coupled power equations or power flow equations, and the good agreement between theoretical and experimental results provides further evidence that the power flow equation is a useful description of the power distribution in a multimode optical fiber. From these situations, it would be useful to develop general means to analyze power flow equations. In this paper, a means applicable to any coupling mechanism for treating the effects of mode coupling is given together with formal solutions.

We developed a compact fiber-coupled high-power diode-laser unit with optical output power up to 40 W cw, coupled into a multimode fiber with 600 micrometers core diameter and NA 0.22. This diode-laser unit is suitable for pumping solid-sate or fiber lasers as well as for material processing. Essential part is a novel beam-shaping system with compact size, high flexibility and low alignment requirements, which uses a pair of micro step-mirrors. The whole unit fits into a housing of approximately 110 X 100 X 90 mm.