Reviews and Original Research ============================== This reading guide separates broad reviews from selected early papers and original demonstrations. It does not assert that every listed paper was the first invention of its technology. Authors, publication venues, years, and identifiers were checked against publisher or author-institution records. See :doc:`access` for exactly which full texts were obtained and which summaries rely on abstracts or bibliographic records. Download the :download:`BibTeX bibliography ` for the 15 papers below. Patent records are listed separately in :doc:`patents`. Recommended reviews ------------------- Start with Fan for the overall combining problem, Stachowiak for physical fiber components, and Birks for mode capacity and lanterns. Smit and van Dam cover wavelength routing; Singh covers planar circuits; Lei and colleagues focus on side-pump combiners. Agarwal provides the historical overview of fiber-coupler constructions; Marchetti and colleagues cover the separate fiber-to-chip interface problem. .. _fiber-review-fan: R1. Overall beam combining: Fan, 2005 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ T. Y. Fan, **Laser beam combining for high-power, high-radiance sources**, *IEEE Journal of Selected Topics in Quantum Electronics* **11** (3), 567–577 (2005). `DOI: 10.1109/JSTQE.2005.850241 `_. **Read for:** the power-versus-brightness distinction and the trade-offs between spectral and coherent combining. This is the broadest starting point here for the many-input problem; it is not a catalog of every fiber splitter. The `MIT Lincoln Laboratory record `_ provides the author-hosted publication summary. A supplied full-text PDF was checked for the combining requirements, coherent architectures, and spectral/coherent comparison in sections I–III and V. .. _fiber-review-stachowiak: R2. Fiber power-combiner construction: Stachowiak, 2018 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Dorota Stachowiak, **High-Power Passive Fiber Components for All-Fiber Lasers and Amplifiers Application—Design and Fabrication**, *Photonics* **5** (4), article 38 (2018). `DOI: 10.3390/photonics5040038 `_. **Read for:** tapered fused bundles, pump-only and pump/signal designs, mode-field adaptors, fabrication constraints, and high-power performance. The `open-access publisher article `_ is the most directly relevant review here for building or specifying physical all-fiber combiners. .. _fiber-review-birks: R3. Single-mode/multimode mapping: Birks et al., 2015 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ T. A. Birks, I. Gris-Sánchez, S. Yerolatsitis, S. G. Leon-Saval, and R. R. Thomson, **The photonic lantern**, *Advances in Optics and Photonics* **7** (2), 107–167 (2015). `DOI: 10.1364/AOP.7.000107 `_. **Read for:** adiabatic transitions between an array of SM cores and one MM core, mode-number matching, losses, and applications. An `author manuscript is available on arXiv `_ with a `direct PDF `_. .. _fiber-review-smit: R4. WDM/AWG routing: Smit and van Dam, 1996 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ M. K. Smit and C. van Dam, **PHASAR-based WDM-devices: principles, design and applications**, *IEEE Journal of Selected Topics in Quantum Electronics* **2** (2), 236–250 (1996). `DOI: 10.1109/2944.577370 `_. **Read for:** phased-array wavelength multiplexer/demultiplexer operation, design, and routing applications. It explains how wavelength selectivity changes the combining problem. The `author-university repository record `_ links a published version; an alternative `repository PDF endpoint `_ is included for access. .. _fiber-review-singh: R5. Planar-circuit background: Singh, 1999 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ B. R. Singh, **Silica Based Planar Lightwave Circuits: Status, Perspective and Future**, *IETE Journal of Research* **45** (5–6), 345–353 (1999). `DOI: 10.1080/03772063.1999.11416120 `_. **Read for:** branching, switching, filtering, fabrication, and integration in silica PLCs. The supplied full text describes cascaded Y-branches or directional couplers for 1-by-N splitters, along with wavelength-routing circuits. Treat its performance examples as historical. The journal year is 1999, despite the publisher's later online-posting date. .. _fiber-review-lei: R6. Side-pump combiners: Lei et al., 2020 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Chengmin Lei, Zhixian Li, Hui Zhang, Zilun Chen, and Jing Hou, **Taper-fused side pump combiner for all-fiber lasers and amplifiers: A review**, *Optics & Laser Technology* **130**, article 106353 (2020). `DOI: 10.1016/j.optlastec.2020.106353 `_. **Read for:** side-pump fabrication, performance, design parameters, and power-scaling challenges. The supplied full text was consulted for input NA sensitivity, taper geometry, backward coupling, and signal-beam degradation. Section 4 explains why a continuous signal core still needs careful control of heating and mechanical deformation during fabrication. .. _fiber-review-agarwal: R7. Historical fiber-coupler overview: Agarwal, 1987 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Arun K. Agarwal, **Review of optical fiber couplers**, *Fiber and Integrated Optics* **6** (1), 27–53 (1987). `DOI: 10.1080/01468038708219344 `_. **Read for:** a broad historical catalog of fused tapers, transmission and reflection stars, micro-optic and grating splitters, offset and mixer constructions, polished couplers, and early SM devices. The supplied PDF contains the complete 27-page article plus a publisher cover. Figures 1–2 give its construction taxonomy; Figures 3–16 illustrate the historical implementations. The characterization discussion on pages 45–47 explains why launch-dependent MM devices need more than a single integrated power measurement to predict cascaded behavior. Pages 47–49 distinguish SM evanescent coupling from the MM taper description. This is particularly useful for the **one-fiber-to-many** side of the survey. Treat manufacturing and performance comparisons as historical, not current specifications. Check the author's loss definition before comparing a quoted value with per-branch insertion loss. The journal year is **1987**; 2006 is the publisher's online-posting date, and 2013 is the download date printed on the supplied cover. .. _fiber-review-marchetti: R8. Fiber-to-chip interfaces: Marchetti et al., 2019 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Riccardo Marchetti, Cosimo Lacava, Lee Carroll, Kamil Gradkowski, and Paolo Minzioni, **Coupling strategies for silicon photonics integrated chips [Invited]**, *Photonics Research* **7** (2), 201–239 (2019). `DOI: 10.1364/PRJ.7.000201 `_. **Read for:** interfacing fibers to a silicon photonic splitter, AWG, or other integrated circuit. The supplied 39-page review covers fiber types and mode-field matching (section 2), edge coupling and inverse tapers (section 3), grating coupling (sections 4–5), and packaging, alignment, microlenses, and photonic wire bonds (section 6). Table 1 compares coupling efficiency, polarization dependence, bandwidth, alignment tolerance, and mode-field diameter. Its notes distinguish **experiments from simulations** and the different loss thresholds used for bandwidth and tolerance. A fiber-to-chip coupler transfers a field between a fiber and a waveguide; it is not by itself a power splitter or a many-source combiner. For an integrated device, budget the input and output interfaces separately from on-chip splitting or routing. Section 6 also explains why fiber-array alignment and adhesive curing can limit assembled performance even when the bare coupling structure is efficient. The review is about silicon photonics, not a qualification of silica PLCs or high-power fiber packages. Selected original papers ------------------------- .. _fiber-original-kawasaki1977: O1. Multimode fused-fiber access coupler, 1977 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ B. S. Kawasaki and K. O. Hill, **Low-loss access coupler for multimode optical fiber distribution networks**, *Applied Optics* **16** (7), 1794–1795 (1977). `DOI: 10.1364/AO.16.001794 `_. **Why it matters:** an early original MM coupler experiment. The supplied two-page paper describes twisted fibers fused and pulled into twin biconical tapers, with 0.1–0.2 dB excess loss for the better devices. It also discusses conversion to cladding-guided light, recapture by expanding cores, and mode mixing. The related technical disclosure is :ref:`fiber-patent-kawasaki`. .. _fiber-original-kawasaki1981: O2. Single-mode fused biconical taper, 1981 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ B. S. Kawasaki, K. O. Hill, and R. G. Lamont, **Biconical-taper single-mode fiber coupler**, *Optics Letters* **6** (7), 327–328 (1981). `DOI: 10.1364/OL.6.000327 `_. **Why it matters:** an early original SM coupler report. The supplied full text describes a 30-device evaluation: a typical good device had 0.5 dB excess loss; the best had 0.2 dB with equal splitting. Its test fiber supported higher-order modes at the measurement wavelengths, so the authors deliberately launched predominantly the fundamental mode. The paper also measures wavelength-dependent branching ratios. .. _fiber-original-hibino: O3. Fiber-connected PLC splitter modules, 1994 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Y. Hibino, F. Hanawa, N. Takato, H. Nakagome, Y. Inoue, T. Miya, and M. Yamaguchi, **Silica-based PLC splitters which satisfy reliability requirements for more than 5000 hours**, *Conference on Optical Fiber Communication*, 1994 OSA Technical Digest Series, volume 4, paper PD7 (1994). `DOI: 10.1364/OFC.1994.PD7 `_. **Why it matters:** an original report addressing 1-by-8 and 1-by-32 fiber-connected splitter modules and packaging reliability. These details are supported by the publisher abstract; the complete reliability data were not obtained. .. _fiber-original-smit: O4. Planar phased-array wavelength component, 1988 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ M. K. Smit, **New focusing and dispersive planar component based on an optical phased array**, *Electronics Letters* **24** (7), 385–386 (1988). `DOI: 10.1049/el:19880260 `_. **Why it matters:** an early phased-array dispersive component relevant to AWG wavelength routing. See the `author-university record `_ and its `repository PDF `_. .. _fiber-original-kozlov: O5. All-fiber laser beam combination, 1999 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ V. A. Kozlov, J. Hernández-Cordero, and T. F. Morse, **All-fiber coherent beam combining of fiber lasers**, *Optics Letters* **24** (24), 1814–1816 (1999). `DOI: 10.1364/OL.24.001814 `_. **Why it matters:** the supplied full text describes a cleaved half of an active-fiber fused coupler serving as the common output mirror of two erbium-fiber laser cavities. Tuning a Bragg grating produced injection locking; the reported spectra were limited by the analyzer's 0.05 nm resolution. Cleave position and fusion geometry affected spatial combination. This is a coupled-resonator experiment; adapting it to separately packaged lasers or an output delivery pigtail requires further design. .. _fiber-original-leonsaval: O6. Early photonic-lantern demonstration, 2005 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ S. G. Leon-Saval, T. A. Birks, J. Bland-Hawthorn, and M. Englund, **Multimode fiber devices with single-mode performance**, *Optics Letters* **30** (19), 2545–2547 (2005). `DOI: 10.1364/OL.30.002545 `_. **Why it matters:** the supplied full text demonstrates a grating filter between two 19-port transitions. Its measured off-notch transmission was only 3.4%: the authors intentionally did not match the 19 SM ports to the roughly 710 supported MM spatial modes. The result establishes the filtering concept and supports the mode-count argument; efficient matched devices are developed in the later R3 review. .. _fiber-original-morizur: O7. Programmable spatial transformation, 2010 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Jean-François Morizur, Lachlan Nicholls, Pu Jian, Seiji Armstrong, Nicolas Treps, Boris Hage, Magnus Hsu, Warwick Bowen, Jiri Janousek, and Hans-A. Bachor, **Programmable unitary spatial mode manipulation**, *Journal of the Optical Society of America A* **27** (11), 2524–2531 (2010). `DOI: 10.1364/JOSAA.27.002524 `_. **Why it matters:** theory and an experimental programmable converter using successive phase manipulations and propagation, relevant to MPLC mode multiplexing. The `downloaded author preprint `_ uses the slightly different title *Programmable unitary spatial modes manipulation*. Its experimental section distinguishes single-input mode-conversion measurements from simulations of general multimode transformations.